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Cell_Biology_Alberts. underlying mechanism is poorly understood, but the external exposure of phosphatidylserine is likely to depend on caspase cleavage of some protein involved in phospholipid distribution in the membrane. A variety of soluble “bridging” proteins interact with the exposed phosphatidylserine on the apoptotic cell. These bridging proteins also interact with specific receptors on the surface of a neighboring cell or macrophage, triggering cytoskeletal and other changes that initiate the engulfment process. Macrophages do not phagocytose healthy cells in the animal—despite the fact that healthy cells normally expose some phosphatidylserine on their surfaces. Healthy cells express signal proteins on their surface that interact with inhibitory receptors on macrophages that block phagocytosis. Thus, in addition to expressing cell-surface signals such as phosphatidylserine that stimulate phagocytosis, apoptotic cells must lose or inactivate these “don’t eat me” signals that block phagocytosis.


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Cell_Biology_Alberts. Either Excessive or Insufficient Apoptosis Can Contribute to Disease There are many human disorders in which excessive numbers of cells undergo apoptosis and thereby contribute to tissue damage. Among the most dramatic examples are heart attacks and strokes. In these acute conditions, many cells die by necrosis as a result of ischemia (inadequate blood supply), but some of the less affected cells die by apoptosis. It is hoped that, in the future, drugs that block apoptosis—such as specific caspase inhibitors—will prove useful in saving such cells. There are other conditions where too few cells die by apoptosis. Mutations in mice and humans, for example, that inactivate the genes that encode the Fas death receptor or the Fas ligand prevent the normal death of some lymphocytes, causing these cells to accumulate in excessive numbers in the spleen and lymph glands. In many cases, this leads to autoimmune disease, in which the lymphocytes react against the individual’s own tissues.


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Cell_Biology_Alberts. Decreased apoptosis also makes an important contribution to many tumors, as cancer cells often regulate their apoptotic program abnormally. The Bcl2 gene, for example, was first identified in a common form of lymphocyte cancer in humans, where a chromosome translocation causes excessive production of the Bcl2 protein; indeed, Bcl2 gets its name from this B cell lymphoma. The high level of Bcl2 protein in the lymphocytes that carry the translocation promotes the development of cancer by inhibiting apoptosis, thereby prolonging lymphocyte survival and increasing their number; it also decreases the cells’ sensitivity to anticancer drugs, which commonly work by causing cancer cells to undergo apoptosis.


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Cell_Biology_Alberts. Similarly, the gene encoding the tumor suppressor protein p53 is mutated in about 50% of human cancers so that it no longer promotes apoptosis or cell-cycle arrest in response to DNA damage. The lack of p53 function therefore enables the cancer cells to survive and proliferate even when their DNA is damaged; in this way, the cells accumulate more mutations, some of which make the cancer more malignant (discussed in Chapter 20). As many anticancer drugs induce apoptosis (and cell-cycle arrest) by a p53-dependent mechanism (discussed in Chapters 17 and 20), the loss of p53 function also makes cancer cells less sensitive to these drugs.


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Cell_Biology_Alberts. If decreased apoptosis contributes to many cancers, then we might be able to treat those cancers with drugs that stimulate apoptosis. This line of thinking has recently led to the development of small chemicals that interfere with the function of anti-apoptotic Bcl2 family proteins such as Bcl2 and BclXL. These chemicals bind with high affinity to the hydrophobic groove on anti-apoptotic Bcl2 family proteins, blocking their function in essentially the same way that BH3-only proteins do (Figure 18–13). The intrinsic pathway of apoptosis is thereby stimulated, which in certain tumors increases the amount of cell death.


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Cell_Biology_Alberts. Most human cancers arise in epithelial tissues such as those in the lung, intestinal tract, breast, and prostate. Such cancer cells display many abnormalities in their behavior, including a decreased ability to adhere to the extracellular matrix and to one another at specialized cell–cell junctions. In the next chapter, we discuss the remarkable structures and functions of the extracellular matrix and cell junctions.


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Cell_Biology_Alberts. Animal cells can activate an intracellular death program and kill themselves in a controlled way when they are irreversibly damaged, no longer needed, or are a threat to the organism. In most cases, these deaths occur by apoptosis: the cells shrink, condense, and frequently fragment, and neighboring cells or macrophages rapidly phagocytose the cells or fragments before there is any leakage of cytoplasmic contents. Apoptosis is mediated by proteolytic enzymes called caspases, which cleave specific intracellular proteins to help kill the cell. Caspases are present in all nucleated animal cells as inactive precursors. Initiator caspases are activated when brought into proximity in activation complexes: once activated, they cleave and thereby activate downstream executioner caspases, which then cleave various target proteins in the cell, producing an amplifying, irreversible proteolytic cascade.


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Cell_Biology_Alberts. Cells use at least two distinct pathways to activate initiator caspases and trigger a caspase cascade leading to apoptosis: the extrinsic pathway is activated by extracellular ligands binding to cell-surface death receptors; the intrinsic pathway is activated by intracellular signals generated when cells are stressed. Each pathway uses its own initiator caspases, which are activated in distinct activation complexes: in the extrinsic pathway, the death receptors recruit caspase-8 via adaptor proteins to form the DISC; in the intrinsic pathway, cytochrome c released from the intermembrane space of mitochondria activates Apaf1, which assembles into an apoptosome and recruits and activates caspase-9.


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Cell_Biology_Alberts. Intracellular Bcl2 family proteins and IAP proteins tightly regulate the apoptotic program to ensure that cells kill themselves only when it benefits the animal. Both anti-apoptotic and pro-apoptotic Bcl2 family proteins regulate the intrinsic pathway by controlling the release of mitochondrial intermembrane proteins, while IAP proteins inhibit activated caspases and promote their degradation.


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Cell_Biology_Alberts. Figure 18–13 How the chemical ABT737 inhibits anti-apoptotic Bcl2 family proteins. As shown in Figure 18–10B, an apoptotic signal results in activation of Bh3-only proteins, which interact with a long hydrophobic groove in anti-apoptotic Bcl2 family proteins, thereby preventing them from blocking apoptosis. Using the crystal structure of the groove, the drug shown in (A), called ABt-737, was designed and synthesized to bind tightly in the groove, as shown for the anti-apoptotic Bcl2 family protein, BclXL, in (B). By inhibiting the activity of these proteins, the drug promotes apoptosis in any cell that depends on them for survival. (pDB code: 2YXJ.) how many forms of programmed cell death exist? What are the underlying mechanisms and benefits of each? thousands of caspase substrates have been identified. Which ones are the critical proteins that must be cleaved to trigger the major cell remodeling events underlying apoptosis?


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Cell_Biology_Alberts. thousands of caspase substrates have been identified. Which ones are the critical proteins that must be cleaved to trigger the major cell remodeling events underlying apoptosis? how did the intrinsic pathway of apoptosis evolve, and what is the advantage of having mitochondria play such a central role in regulating apoptosis? how are “don’t eat me” signals eliminated or inactivated during apoptosis to allow the cells to be phagocytosed? Which statements are true? Explain why or why not. 18–1 In normal adult tissues, cell death usually balances cell division. 18–2 Mammalian cells that do not have cytochrome c should be resistant to apoptosis induced by DNA damage. Discuss the following problems.


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Cell_Biology_Alberts. 18–2 Mammalian cells that do not have cytochrome c should be resistant to apoptosis induced by DNA damage. Discuss the following problems. 18–3 One important role of Fas and Fas ligand is to mediate the elimination of tumor cells by killer lymphocytes. In a study of 35 primary lung and colon tumors, half the tumors were found to have amplified and overexpressed a gene for a secreted protein that binds to Fas ligand. How do you suppose that overexpression of this protein might contribute to the survival of these tumor cells? Explain your reasoning.


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Cell_Biology_Alberts. 18–4 Development of the nematode Caenorhabditis elegans generates exactly 959 somatic cells; it also produces an additional 131 cells that are later eliminated by apoptosis. Classical genetic experiments in C. elegans isolated mutants that led to the identification of the first genes involved in apoptosis. Of the many mutations affecting apoptosis in the nematode, none have ever been found in the gene for cytochrome c. Why do you suppose that such a central effector molecule in apoptosis was not found in the many genetic screens for “death” genes that have been carried out in C. elegans? 18–5 Imagine that you could microinject cytochrome c into the cytosol of wild-type mammalian cells and of cells that were doubly defective for Bax and Bak. Would you expect one, both, or neither type of cell to undergo apoptosis? Explain your reasoning.


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Cell_Biology_Alberts. 18–6 In contrast to their similar brain abnormalities, newborn mice deficient in Apaf1 or caspase-9 have distinctive abnormalities in their paws. Apaf1-deficient mice fail to eliminate the webs between their developing digits, whereas caspase-9-deficient mice have normally formed digits (Figure Q18–1). If Apaf1 and caspase-9 function in the same apoptotic pathway, how is it possible for these deficient mice to differ in web-cell apoptosis?


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Cell_Biology_Alberts. Figure Q18–1 Appearance of paws in Apaf1–/– and Casp9–/– newborn mice relative to normal newborn mice (problem +/+ –/– 18–6). (From h. Yoshida et al., Cell 94:739– Casp9 750, 1998. With permission from Elsevier.) 18–7 When human cancer cells are exposed to ultraviolet (UV) light at 90 mJ/cm2, most of the cells undergo apoptosis within 24 hours. Release of cytochrome c from mitochondria can be detected as early as 6 hours after exposure of a population of cells to UV light, and it continues to increase for more than 10 hours thereafter. Does this mean that individual cells slowly release their cytochrome c over this time period? Or, alternatively, do individual cells release their cytochrome c rapidly but with different cells being triggered over the longer time period?


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Cell_Biology_Alberts. To answer this fundamental question, you have fused the gene for green fluorescent protein (GFP) to the gene for cytochrome c, so that you can observe the behavior of individual cells by confocal fluorescence microscopy. In cells that are expressing the cytochrome c–GFP fusion, fluorescence shows the punctate pattern typical of mitochondrial proteins. You then irradiate these cells with UV light and observe individual cells for changes in the punctate pattern. Two such cells (outlined in white) are shown in Figure Q18–2A and B. Release of cytochrome c–GFP is detected as a change from a punctate to a diffuse pattern of fluorescence. Times after UV exposure are indicated as hours:minutes below the individual panels. Which model for cytochrome c release do these observations support? Explain your reasoning. 10:09 10:15 17:10 17:18


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Cell_Biology_Alberts. 10:09 10:15 17:10 17:18 Figure Q18–2 time-lapse video fluorescence microscopic analysis of cytochrome c–GFp release from mitochondria of individual cells (problem 18–7). (A) Cells observed for 6 minutes, 10 hours after UV irradiation. (B) Cells observed for 8 minutes, 17 hours after UV irradiation. One cell in (A) and one in (B), each outlined in white, have released their cytochrome c–GFp during the time frame of the observation, which is shown as hours:minutes below each panel. (From


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Cell_Biology_Alberts. J.C. Goldstein et al., Nat. Cell Biol. 2:156–162, 2000. With permission from Macmillan publishers Ltd.) 18–8 Fas ligand is a trimeric, extracellular protein that binds to its receptor, Fas, which is composed of three identical transmembrane subunits (Figure Q18–3). The binding of Fas ligand alters the conformation of Fas so that it binds an adaptor protein, which then recruits and activates caspase-8, triggering a caspase cascade that leads to cell death. In humans, the autoimmune lymphoproliferative syndrome (ALPS) is associated with dominant mutations in Fas that include point mutations and C-terminal 1034 Chapter 18: Cell Death truncations. In individuals that are heterozygous for such mutations, lymphocytes do not die at their normal rate and accumulate in abnormally large numbers, causing a variety of clinical problems. In contrast to these patients, individuals that are heterozygous for mutations that eliminate Fas expression entirely have no clinical symptoms.


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Cell_Biology_Alberts. A. Assuming that the normal and dominant forms of Fas are expressed to the same level and bind Fas ligand equally, what fraction of Fas–Fas ligand complexes on a lymphocyte from a heterozygous ALPS patient would be expected to be composed entirely of normal Fas subunits? B. In an individual heterozygous for a mutation that eliminates Fas expression, what fraction of Fas–Fas ligand complexes would be expected to be composed entirely of normal Fas subunits? C. Why are the Fas mutations that are associated with ALPS dominant, while those that eliminate expression of Fas are recessive? Crawford ED & Wells JA (2011) Caspase substrates and cellular remodeling. Annu. Rev. Biochem. 80, 1055–1087. Czabotar pE, Lessene G, Strasser A et al. (2014) Control of apoptosis by the BCL-2 protein family: implications for physiology and therapy. Nat. Rev. Mol. Cell Biol. 15, 49–63.


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Cell_Biology_Alberts. Czabotar pE, Lessene G, Strasser A et al. (2014) Control of apoptosis by the BCL-2 protein family: implications for physiology and therapy. Nat. Rev. Mol. Cell Biol. 15, 49–63. Danial NN & korsmeyer SJ (2004) Cell death: critical control points. Cell 116, 205–219. Elliott Mr & ravichandran kS (2010) Clearance of apoptotic cells: implications in health and disease. J. Cell Biol. 189, 1059–1070. Ellis rE, Yuan JY & horvitz rA (1991) Mechanisms and functions of cell death. Annu. Rev. Cell Biol. 7, 663–698. Fadok VA & henson pM (2003) Apoptosis: giving phosphatidylserine recognition an assist—with a twist. Curr. Biol. 13, r655–r657. Green Dr (2011) Means to an End: Apoptosis and Other Cell Death Mechanisms. Cold Spring harbor, New York: Cold Spring harbor Laboratory press. Jacobson MD, Weil M & raff MC (1997) programmed cell death in animal development. Cell 88, 347–354. Jiang X & Wang X (2004) Cytochrome C-mediated apoptosis. Annu. Rev. Biochem. 73, 87–106.


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Cell_Biology_Alberts. Jacobson MD, Weil M & raff MC (1997) programmed cell death in animal development. Cell 88, 347–354. Jiang X & Wang X (2004) Cytochrome C-mediated apoptosis. Annu. Rev. Biochem. 73, 87–106. kerr JF, Wyllie Ah & Currie Ar (1972) Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics. Brit. J. Cancer 26, 239–257. kumar S (2007) Caspase function in programmed cell death. Cell Death Differ. 14, 32–43. Figure Q18–3 The binding of trimeric Fas ligand to Fas (problem 18–8). Lavrik I, Golks A & krammer ph (2005) Death receptor signaling. J. Cell Sci. 118, 265–267. Lessene G, Czabotar pE & Colman pM (2008) BCL-2 family antagonists for cancer therapy. Nat. Rev. Drug Discov. 7, 989–1000. Mace pD & riedl SJ (2010) Molecular cell death platforms and assemblies. Curr. Opin. Cell Biol. 22, 828–836. Nagata S (2005) DNA degradation in development and programmed cell death. Annu. Rev. Immunol. 23, 853–875.


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Cell_Biology_Alberts. Nagata S (2005) DNA degradation in development and programmed cell death. Annu. Rev. Immunol. 23, 853–875. raff MC (1999) Cell suicide for beginners. Nature 396, 119–122. tait SW & Green Dr (2013) Mitochondrial regulation of cell death. Cold Spring Harb. Perspect. Biol. 5, a008706. Vanden Berghe t, Linkermann A, Jouan-Lanhouet S et al. (2014) regulated necrosis: the expanding network of non-apoptotic cell death pathways. Nat. Rev. Mol. Cell Biol. 15, 135–147. Vousden kh (2005) Apoptosis. p53 and pUMA: a deadly duo. Science 309, 1685–1686. Willis SN & Adams JM (2005) Life in the balance: how Bh3-only proteins induce apoptosis. Curr. Opin. Cell Biol. 17, 617–625. Yuan S & Akey CW (2013) Apoptosome structure, assembly, and procaspase activation. Structure 21, 501–515. Cell Junctions and the Extracellular Matrix


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Cell_Biology_Alberts. Yuan S & Akey CW (2013) Apoptosome structure, assembly, and procaspase activation. Structure 21, 501–515. Cell Junctions and the Extracellular Matrix Of all the social interactions between cells in a multicellular organism, the most fundamental are those that hold the cells together. Cells may be linked by direct interactions, or they may be held together within the extracellular matrix, a complex network of proteins and polysaccharide chains that the cells secrete. By one means or another, cells must cohere if they are to form an organized multicellular structure that can withstand and respond to the various external forces that try to pull it apart.


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Cell_Biology_Alberts. The mechanisms of cohesion govern the architecture of the body—its shape, its strength, and the arrangement of its different cell types. The making and breaking of the attachments between cells and the modeling of the extracellular matrix govern the way cells move within the organism, guiding them as the body grows, develops, and repairs itself. Attachments to other cells and to extracellular matrix control the orientation and behavior of the cell’s cytoskeleton, thereby allowing cells to sense and respond to changes in the mechanical features of their environment. Thus, the apparatus of cell junctions and the extracellular matrix is critical for every aspect of the organization, function, and dynamics of multicellular structures. Defects in this apparatus underlie an enormous variety of diseases.


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Cell_Biology_Alberts. The key features of cell junctions and the extracellular matrix are best illustrated by considering two broad categories of tissues that are found in all animals (Figure 19–1). Connective tissues, such as bone or tendon, are formed from an extracellular matrix produced by cells that are distributed sparsely in the matrix. It is the matrix—rather than the cells—that bears most of the mechanical stress to which the tissue is subjected. Direct attachments between one cell and another are relatively rare, but the cells have important attachments to the matrix. These cell–matrix junctions link the cytoskeleton to the matrix, allowing the cells to move through the matrix and monitor changes in its mechanical properties.


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Cell_Biology_Alberts. In epithelial tissues, such as the lining of the gut or the epidermal covering of the skin, cells are tightly bound together into sheets called epithelia. The extracellular matrix is less pronounced, consisting mainly of a thin mat called the basal lamina (or basement membrane) underlying the sheet. Within the epithelium, cells are attached to each other directly by cell–cell junctions, where cytoskeletal filaments are anchored, transmitting stresses across the interiors of the cells, from mechanical stresses are transmitted from cell to cell by cytoskeletal flaments anchored to cell–matrix and cell–cell adhesion sites extracellular matrix directly bears mechanical stresses of tension and compression adhesion site to adhesion site. The cytoskeleton of epithelial cells is also linked to the basal lamina through cell–matrix junctions.


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Cell_Biology_Alberts. Figure 19–2 provides a closer view of epithelial cells to illustrate the major types of cell–cell and cell–matrix junctions that we will discuss in this chapter. The diagram shows the typical arrangement of junctions in a simple columnar epithelium such as the lining of the small intestine of a vertebrate. Here, a single layer of tall cells stands on a basal lamina, with the cells’ uppermost surface, or apex, free and exposed to the extracellular medium. On their sides, or lateral surfaces, the cells make junctions with one another. Two types of anchoring junctions link the cytoskeletons of adjacent cells: adherens junctions are anchorage sites for actin filaments; desmosomes are anchorage sites for intermediate filaments. Two additional types of anchoring junctions link the cytoskeleton of the epithelial cells to the basal lamina: actin-linked cell–matrix junctions anchor actin filaments to the matrix, while hemidesmosomes anchor intermediate filaments to it.


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Cell_Biology_Alberts. Figure 19–1 Two main ways in which animal cells are bound together. In connective tissue, the main stress-bearing component is the extracellular matrix. In epithelial tissue, it is the cytoskeletons of the cells themselves, linked from cell to cell by adhesive junctions. Cell–matrix attachments bond epithelial tissue to the connective tissue beneath it. adherens junction connects actin flament bundle in one cell with that in the next cell desmosome connects intermediate flaments in one cell to those in the next cell gap junction allows the passage of small water-soluble molecules from cell to cell Figure 19–2 A summary of the various cell junctions found in a vertebrate epithelial cell, classified according to their primary functions.


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Cell_Biology_Alberts. Figure 19–2 A summary of the various cell junctions found in a vertebrate epithelial cell, classified according to their primary functions. In the most apical portion of the cell, the relative positions of the junctions are the same in nearly all vertebrate epithelia. the tight junction occupies the most apical position, followed by the adherens junction (adhesion belt) and then by a special parallel row of desmosomes; together these form a structure called a junctional complex. Gap junctions and additional desmosomes are less regularly organized. two types of cell-matrix anchoring junctions tether the basal surface of the cell to the basal lamina. the drawing is based on epithelial cells of the small intestine.


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Cell_Biology_Alberts. Figure 19–3 Transmembrane adhesion proteins link the cytoskeleton to extracellular structures. the external linkage may be either to other cells (cell–cell junctions, mediated typically by cadherins) or to extracellular matrix (cell–matrix junctions, mediated typically by integrins). the internal linkage to the cytoskeleton is generally indirect, via intracellular adaptor proteins, to be discussed later. Two other types of cell–cell junction are shown in Figure 19–2. Tight junctions hold the cells closely together near the apex, sealing the gap between the cells and thereby preventing molecules from leaking across the epithelium. Near the basal end of the cells are channel-forming junctions, called gap junctions, that create passageways linking the cytoplasms of adjacent cells.


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Cell_Biology_Alberts. Each of the four major anchoring junction types depends on transmembrane adhesion proteins that span the plasma membrane, with one end linking to the cytoskeleton inside the cell and the other end linking to other structures outside it (Figure 19–3). These cytoskeleton-linked transmembrane proteins fall neatly into two superfamilies, corresponding to the two basic kinds of external attachment. Proteins of the cadherin superfamily chiefly mediate attachment of cell to cell (Movie 19.1). Proteins of the integrin superfamily chiefly mediate attachment of cells to matrix. There is specialization within each family: some cadherins link to actin and form adherens junctions, while others link to intermediate filaments and form desmosomes; likewise, some integrins link to actin and form actinlinked cell–matrix junctions, while others link to intermediate filaments and form hemidesmosomes (Table 19–1).


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Cell_Biology_Alberts. There are some exceptions to these rules. Some integrins, for example, mediate cell–cell rather than cell–matrix attachment. Moreover, there are other types of cell adhesion molecules that can provide transient cell–cell attachments more flimsy than anchoring junctions, but sufficient to stick cells together in special circumstances. We begin the chapter with a discussion of the major forms of cell–cell junctions. We then consider in turn the extracellular matrix of animals, the structure and function of integrin-mediated cell–matrix junctions, and, finally, the plant cell wall, a special form of extracellular matrix.


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Cell_Biology_Alberts. Cell–cell junctions come in many forms and can be regulated by a variety of mechanisms. The best understood and most common are the two types of cell– cell anchoring junctions, which employ cadherins to link the cytoskeleton of one cell with that of its neighbor. Their primary function is to resist the external forces that pull cells apart. The epithelial cells of your skin, for example, must remain tightly linked when they are stretched, pinched, or poked. Cell–cell anchoring junctions must also be dynamic and adaptable, so that they can be altered or rearranged when tissues are remodeled or repaired, or when there are changes in the forces acting on them. In this section, we focus primarily on the cadherin-based anchoring junctions. We then briefly describe tight junctions and gap junctions. Finally, we consider the more transient cell–cell adhesion mechanisms employed by some cells in the bloodstream. Cadherins Form a Diverse Family of adhesion Molecules


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Cell_Biology_Alberts. Cadherins Form a Diverse Family of adhesion Molecules Cadherins are present in all multicellular animals whose genomes have been analyzed. They are also present in the choanoflagellates, which can exist either as free-living unicellular organisms or as multicellular colonies and are thought to be representatives of the group of protists from which all animals evolved. Other eukaryotes, including fungi and plants, lack cadherins, and they are also absent from bacteria and archaea. Cadherins therefore seem to be part of the essence of what it is to be an animal.


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Cell_Biology_Alberts. The cadherins take their name from their dependence on Ca2+ ions: removing Ca2+ from the extracellular medium causes adhesions mediated by cadherins to come apart. The first three cadherins to be discovered were named according to the main tissues in which they were found: E-cadherin is present on many types of epithelial cells; N-cadherin on nerve, muscle, and lens cells; and P-cadherin on cells in the placenta and epidermis. All are also found in other tissues. These and other classical cadherins are closely related in sequence throughout their extracellular and intracellular domains.


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Cell_Biology_Alberts. There are also a large number of nonclassical cadherins that are more distantly related in sequence, with more than 50 expressed in the brain alone. The nonclassical cadherins include proteins with known adhesive function, such as the diverse protocadherins found in the brain, and the desmocollins and desmogleins that form desmosomes (see Table 19–1). Other family members are involved primarily in signaling. Together, the classical and nonclassical cadherin proteins constitute the cadherin superfamily (Figure 19–4), with more than 180 members in humans. Anchoring junctions between cells are usually symmetrical: if the linkage is to actin in the cell on one side of the junction, it will be to actin in the cell on the other side. In fact, the binding between cadherins is generally homophilic (liketo-like, Figure 19–5): cadherin molecules of a specific subtype on one cell bind to cadherin molecules of the same or closely related subtype on adjacent cells.


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Cell_Biology_Alberts. The spacing between the cell membranes at an anchoring junction is precisely defined and depends on the structure of the participating cadherin molecules. All the members of the superfamily, by definition, have an extracellular portion consisting of several copies of the extracellular cadherin (EC) domain. Homophilic binding occurs at the N-terminal tips of the cadherin molecules—the cadherin domains that lie furthest from the membrane. These terminal domains each form a knob and a nearby pocket, and the cadherin molecules protruding from opposite cell membranes bind by insertion of the knob of one domain into the pocket of the other (Figure 19–6A).


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Cell_Biology_Alberts. Each cadherin domain forms a more-or-less rigid unit, joined to the next cadherin domain by a hinge. Ca2+ ions bind to sites near each hinge and prevent it from flexing, so that the whole string of cadherin domains behaves as a rigid and slightly curved rod. When Ca2+ is removed, the hinges can flex, and the structure becomes floppy (Figure 19–6B). At the same time, the conformation at the N-terminus is thought to change slightly, weakening the binding affinity for the matching cadherin molecule on the opposite cell.


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Cell_Biology_Alberts. Unlike receptors for soluble signal molecules, which bind their specific ligand with high affinity, cadherins (and most other cell–cell adhesion proteins) typically bind to their partners with relatively low affinity. Strong attachments result from the formation of many such weak bonds in parallel. When binding to oppositely oriented partners on another cell, cadherin molecules are often clustered side-to-side with many other cadherin molecules on the same cell (Figure 19–6C). The strength of this junction is far greater than that of any individual intermolecular bond, and yet regulatory mechanisms can easily disassemble the junction by separating the molecules sequentially, just as two pieces of fabric can be joined strongly by Velcro and yet easily peeled apart from the sides. A similar “Velcro principle” also operates at cell–cell and cell–matrix adhesions formed by other types of transmembrane adhesion proteins. Figure 19–4 The cadherin superfamily.


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Cell_Biology_Alberts. Figure 19–4 The cadherin superfamily. the diagram shows some of the diversity among cadherin superfamily members. these proteins all have extracellular portions containing multiple copies of the extracellular cadherin domain (green ovals). In the classical cadherins of vertebrates there are 5 of these domains, and in desmogleins and desmocollins there are 4 or 5, but some nonclassical cadherins have more than 30. the intracellular portions are more varied, reflecting interactions with a wide variety of intracellular ligands, including signaling molecules and adaptor proteins that connect the cadherin to the cytoskeleton. In some cases, such as t-cadherin, a transmembrane domain is not present and the protein is attached to the plasma membrane by a glycosylphosphatidylinositol (GpI) anchor. the differently colored motifs in Fat, Flamingo, and ret represent conserved domains that are also found in other protein families.


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Cell_Biology_Alberts. Figure 19–5 Homophilic versus heterophilic binding. Cadherins in general bind homophilically; some other cell adhesion molecules, discussed later, bind heterophilically. 38.5 nm hinge regions


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Cell_Biology_Alberts. Ca2+ of cell 1 of cell 2 membrane membrane < 0.05 mM Ca2+ (B) plasma membrane of cell 1 plasma membrane of cell 2 Figure 19–6 Cadherin structure and function. (a) the extracellular region of a classical cadherin contains five copies of the extracellular cadherin domain (see Figure 19–4) separated by flexible hinge regions. Ca2+ ions (red dots) bind in the neighborhood of each hinge, preventing it from flexing. as a result, the extracellular region forms a rigid, curved structure as shown here. to generate cell–cell adhesion, the cadherin domain at the N-terminal tip of one cadherin molecule binds the N-terminal domain from a cadherin molecule on another cell. the structure was determined by x-ray diffraction of the crystallized C-cadherin extracellular region. (B) In the absence of Ca2+, increased flexibility in the hinge regions results in a floppier molecule that is no longer oriented correctly to interact with a cadherin on another cell—and adhesion fails. (C) at a typical


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Cell_Biology_Alberts. Ca2+, increased flexibility in the hinge regions results in a floppier molecule that is no longer oriented correctly to interact with a cadherin on another cell—and adhesion fails. (C) at a typical cell–cell junction, an organized array of cadherin molecules functions like Velcro to hold cells together. Cadherins on the same cell are thought to be coupled by side-to-side interactions between their N-terminal head regions, resulting in a linear array like the alternating green and light green cadherins on the lower cell shown here. these arrays are thought to interact with similar linear arrays on an adjacent cell (blue cadherin molecules, top cell). the linear arrays on one cell are perpendicular to those on the other cell, as indicated by the red arrows. Multiple perpendicular arrays on both cells interact to form a tight-knit mat of cadherin proteins. (a, based on t.J. Boggon et al., Science 296:1308–1313, 2002; C, based on O.J. harrison et al. Structure 19:244–256, 2011.)


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Cell_Biology_Alberts. Cadherin-Dependent Cell–Cell adhesion Guides the Organization of Developing tissues Cadherins form specific homophilic attachments, explaining why there are so many different family members. Cadherins are not like glue, making cell surfaces generally sticky. Rather, they mediate highly selective recognition, enabling cells of a similar type to stick together and to stay segregated from other types of cells.


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Cell_Biology_Alberts. Selectivity in the way that animal cells consort with one another was first demonstrated in the 1950s, long before the discovery of cadherins, in experiments in which amphibian embryos were dissociated into single cells. These cells were then mixed up and allowed to reassociate. Remarkably, the dissociated cells often reassembled into structures resembling those of the original embryo (Figure 19–7). These experiments, together with numerous more recent experiments, reveal that selective cell–cell recognition systems make cells of the same differentiated tissue preferentially adhere to one another.


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Cell_Biology_Alberts. Figure 19–7 Sorting out. Cells from different layers of an early amphibian embryo will sort out according to their origins. In the classical experiment shown here, mesoderm cells (green), neural plate cells (blue), and epidermal cells (red) have been disaggregated and then reaggregated in a random mixture. they sort out into an arrangement reminiscent of a normal embryo, with a “neural tube” internally, epidermis externally, and mesoderm in between. (Modified from p.L. townes and J. holtfreter, J. Exp. Zool. 128:53–120, 1955. With permission from Wiley-Liss.)


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Cell_Biology_Alberts. Cadherins play a crucial part in these cell-sorting processes during development. The appearance and disappearance of specific cadherins correlate with steps in embryonic development where cells regroup and change their contacts to create new tissue structures. In the vertebrate embryo, for example, changes in cadherin expression are seen when the neural tube forms and pinches off from the overlying ectoderm: neural tube cells lose E-cadherin and acquire other cadherins, including N-cadherin, while the cells in the overlying ectoderm continue to express E-cadherin (Figure 19–8A and B). Then, when the neural crest cells migrate away from the neural tube, these cadherins become scarcely detectable, and another cadherin (cadherin 7) appears that helps hold the migrating cells together as loosely associated cell groups (Figure 19–8C). Finally, when the cells aggregate to form a ganglion, they switch on expression of N-cadherin again. If N-cadherin is artificially overexpressed in the


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Cell_Biology_Alberts. loosely associated cell groups (Figure 19–8C). Finally, when the cells aggregate to form a ganglion, they switch on expression of N-cadherin again. If N-cadherin is artificially overexpressed in the emerging neural crest cells, the cells fail to escape from the neural tube.


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Cell_Biology_Alberts. Studies with cultured cells further support the idea that the homophilic binding of cadherins controls these processes of tissue segregation. In a line of cultured fibroblasts called L cells, for example, cadherins are not expressed and the cells do not adhere to one another. When these cells are transfected with DNA encoding E-cadherin, E-cadherins on one cell bind to E-cadherins on another, resulting in cell–cell adhesion. If L cells expressing different cadherins are mixed together, they sort out and aggregate separately, indicating that different cadherins preferentially bind to their own type (Figure 19–9A), mimicking what happens when cells derived from tissues that express different cadherins are mixed together. A similar segregation of cells occurs if L cells expressing different amounts of the same cadherin are mixed together (Figure 19–9B). It therefore seems likely that both qualitative and quantitative differences in the expression of cadherins have a role in organizing


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Cell_Biology_Alberts. amounts of the same cadherin are mixed together (Figure 19–9B). It therefore seems likely that both qualitative and quantitative differences in the expression of cadherins have a role in organizing tissues.


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Cell_Biology_Alberts. Figure 19–8 Changing patterns of cadherin expression during construction of the vertebrate nervous system. the figure shows cross sections of the early chick embryo, as the neural tube detaches from the ectoderm and then as neural crest cells detach from the neural tube. (a, B) Immunofluorescence micrographs showing the developing neural tube labeled with antibodies against (a) E-cadherin (blue) and (B) N-cadherin (yellow). (C) as the patterns of gene expression change, the different groups of cells segregate from one another according to the cadherins they express. (Micrographs courtesy of Miwako Nomura and Masatoshi takeichi.) cell expressing high level of E-cadherin cell expressing low level of E-cadherin (B)


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Cell_Biology_Alberts. Figure 19–9 Cadherin-dependent cell sorting. Cells in culture can sort themselves out according to the type and level of cadherins they express. this can be visualized by labeling different populations of cells with dyes of different colors. (a) Cells expressing N-cadherin sort out from cells expressing E-cadherin. (B) Cells expressing high levels of E-cadherin sort out from cells expressing low levels of E-cadherin. the cells expressing high levels adhere more strongly and end up internally. Epithelial–Mesenchymal transitions Depend on Control of Cadherins


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Cell_Biology_Alberts. Epithelial–Mesenchymal transitions Depend on Control of Cadherins The assembly of cells into an epithelium is a reversible process. By switching on expression of adhesion molecules, dispersed unattached mesenchymal cells, such as fibroblasts, can come together to form an epithelium. Conversely, epithelial cells can change their character, disassemble, and migrate away from their parent epithelium as separate cells. Such epithelial–mesenchymal transitions play an important part in normal embryonic development; the origin of the neural crest is one example. These transitions depend in part on transcription regulatory proteins called Slug, Snail, and Twist. Increased expression of Twist, for example, converts epithelial cells to a mesenchymal character, and switching it off does the opposite. Twist exerts its effects, in part, by inhibiting expression of cadherins, including E-cadherin, that hold epithelial cells together.


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Cell_Biology_Alberts. Epithelial–mesenchymal transitions also occur as pathological events during adult life, in cancer. Most cancers originate in epithelia, but become dangerously prone to spread—that is, malignant—only when the cancer cells escape from the epithelium of origin and invade other tissues. Experiments with malignant breast cancer cells in culture show that blocking expression of Twist can convert the cells back toward a nonmalignant character. Conversely, by forcing Twist expression, one can make normal epithelial cells undergo an epithelial–mesenchymal transition and behave like malignant cells. Mutations that disrupt the production or function of E-cadherin are often found in cancer cells and are thought to help make them malignant. Catenins Link Classical Cadherins to the actin Cytoskeleton


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Cell_Biology_Alberts. Catenins Link Classical Cadherins to the actin Cytoskeleton The extracellular domains of cadherins mediate homophilic binding at adherens junctions. The intracellular domains of typical cadherins, including all classical and some nonclassical ones, interact with filaments of the cytoskeleton: actin at adherens junctions and intermediate filaments at desmosomes (see Table 19–1). These cytoskeletal linkages are essential for efficient cell–cell adhesion, as cadherins that lack their cytoplasmic domains cannot stably hold cells together.


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Cell_Biology_Alberts. The linkage of cadherins to the cytoskeleton is indirect and depends on adaptor proteins that assemble on the cytoplasmic tail of the cadherin. At adherens junctions, the cadherin tail binds two such proteins: β-catenin and a distant relative called p120-catenin; a third protein called α-catenin interacts with β-catenin and recruits a variety of other proteins to provide a dynamic linkage to actin filaments (Figure 19–10). At desmosomes, cadherins are linked to intermediate filaments through other adaptor proteins, including a β-catenin-related protein called plakoglobin, as we discuss later.


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Cell_Biology_Alberts. In their mature form, adherens junctions are enormous protein complexes containing hundreds to thousands of cadherin molecules, packed into dense, regular arrays that are linked on the extracellular side by lateral interactions between cadherin domains, as we discussed earlier (see Figure 19–6C). On the cytoplasmic side, a complex network of catenins, actin regulators, and contractile actin bundles holds the cluster of cadherins together and links it to the actin cytoskeleton. Assembling a structure of this complexity is not a simple task, and it involves a complex sequence of events controlled by the actin-regulatory proteins discussed in Chapter 16. The general features of the assembly process are summarized in Figure 19–11. adherens Junctions respond to Forces Generated by the actin Cytoskeleton


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Cell_Biology_Alberts. adherens Junctions respond to Forces Generated by the actin Cytoskeleton Most adherens junctions are linked to contractile bundles of actin filaments and non-muscle myosin II. These junctions are therefore subjected to pulling forces generated by the attached actin. The pulling forces are important for junction assembly and maintenance: disruption of myosin activity, for example, results in Figure 19–10 The linkage of classical cadherins to actin filaments. the cadherins are coupled indirectly to actin filaments through an adaptor protein complex containing p120-catenin, β-catenin, and α-catenin. Other proteins, including vinculin, associate with α-catenin and help provide the linkage to actin. β-Catenin has a second, and very important, function in intracellular signaling, as we discuss in Chapter 15 (see Figure 15–60). For clarity, this diagram does not show the cadherin of the adjacent cell in the junction.


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Cell_Biology_Alberts. (A) (B) recruitment of more (C) contractile actin and the disassembly of many adherens junctions. Furthermore, the contractile forces acting on a junction in one cell are balanced by contractile forces at the junction of the opposite cell, so that no cell pulls others toward it and thereby disrupts the uniform distribution of cells in the tissue.


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Cell_Biology_Alberts. We do not understand the mechanisms responsible for maintaining this balance. Adherens junctions seem to sense the forces acting on them and modify local actin and myosin behavior to balance the forces on both sides of the junction. Evidence for these mechanisms comes from studies of pairs of cultured mammalian cells connected by adherens junctions. If contractile activity in one cell is increased experimentally, the adherens junctions linking the two cells increase in size, and the contractile activity of the second cell increases to match that of the first—resulting in a balance of forces across the junction. These and other experiments suggest that adherens junctions are not simply passive sites of protein–protein binding but are dynamic tension sensors that regulate their behavior in response to changing mechanical conditions. This ability to transduce a mechanical signal into a change in junctional behavior is an example of mechanotransduction. We will see later that it is also


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Cell_Biology_Alberts. response to changing mechanical conditions. This ability to transduce a mechanical signal into a change in junctional behavior is an example of mechanotransduction. We will see later that it is also important at cell–matrix junctions.


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Cell_Biology_Alberts. The mechanotransduction at cell–cell junctions is thought to depend, at least in part, on proteins in the cadherin complex that alter their shape when stretched by tension. The protein α-catenin, for example, is stretched from a folded to an extended conformation when contractile activity increases at the junction. The unfolding exposes a cryptic binding site for another protein, vinculin, which promotes the recruitment of more actin to the junction (Figure 19–12). By mechanisms such as this, pulling on a junction makes it stronger. Furthermore, as noted above, pulling on a junction in one cell will increase the contractile force generated in the attached cell.


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Cell_Biology_Alberts. In some cell types, actin contractility reduces cell–cell adhesion, particularly if large forces are involved. Large actin-based contractile forces might, in some tissues, pull sufficiently hard on the edges of cell–cell adhesions to peel them apart, particularly if contraction is coupled to additional regulatory mechanisms that weaken the adhesion. This mechanism might be important in certain forms of tissue remodeling during development, as we describe next. tissue remodeling Depends on the Coordination of actin-Mediated Contraction With Cell–Cell adhesion Adherens junctions are an essential part of the machinery for modeling the shapes of multicellular structures in the animal body. By indirectly linking the actin filaments in one cell to those in its neighbors, they enable the cells in the tissue to use their actin cytoskeletons in a coordinated way.


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Cell_Biology_Alberts. Adherens junctions occur in various forms. In many nonepithelial tissues, they appear as small punctate or linear attachments that connect the cortical Figure 19–11 Assembly of an adherens junction. (a) assembly begins when two unattached epithelial cell precursors explore their surroundings with membrane protrusions, generated by local nucleation of actin networks. When the cells make contact, small cadherin and catenin clusters take shape at the contact sites and associate with actin, leading to activation of the small monomeric Gtpase rac (not shown), an important actin regulator (see Figure 16–85). (B) rac promotes additional actin protrusions in the vicinity, expanding the size of the contact zone and thereby promoting further recruitment of cadherins and their associated catenin proteins.


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Cell_Biology_Alberts. (C) Eventually, rac is inactivated and replaced by the related Gtpase rho (not shown), which shifts actin remodeling toward the assembly of linear, contractile filament bundles. rho also promotes the assembly of myosin II filaments that associate with bundles of actin filaments to generate contractile activity. this contractile activity generates tension that stimulates further actin recruitment and expansion of the junction, in part through the mechanisms illustrated in Figure 19–12.


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Cell_Biology_Alberts. actin filaments beneath the plasma membranes of two interacting cells. In heart muscle, they anchor the actin bundles of the contractile apparatus and act in parallel with desmosomes to link the contractile cells end-to-end. But the prototypical examples of adherens junctions occur in epithelia, where they often form a continuous adhesion belt (or zonula adherens) just beneath the apical face of the epithelium, encircling each of the interacting cells in the sheet (Figure 19–13). Within each cell, a contractile bundle of actin filaments and myosin II lies adjacent to the adhesion belt, oriented parallel to the plasma membrane and tethered to bundle of actin flaments lateral plasma membranes of adjacent epithelial cells


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Cell_Biology_Alberts. Figure 19–12 Mechanotransduction in an adherens junction. (a) Cell–cell junctions are able to sense increased tension and respond by strengthening their actin linkages. tension sensing is thought to depend in part on α-catenin (see Figure 19–10). (B) When actin filaments are pulled from within the cell by non-muscle myosin II, the resulting force unfolds a domain in α-catenin, thereby exposing an otherwise hidden binding site for the adaptor protein vinculin. Vinculin then promotes additional actin recruitment, strengthening the linkages between the junction and the cytoskeleton.


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Cell_Biology_Alberts. Figure 19–13 Adherens junctions between epithelial cells in the small intestine. these cells are specialized for absorption of nutrients; at their apex, facing the lumen of the gut, they have many microvilli (protrusions that increase the absorptive surface area). the adherens junction takes the form of an adhesion belt, encircling each of the interacting cells. Its most obvious feature is a contractile bundle of actin filaments running along the cytoplasmic surface of the junctional plasma membrane. the actin filament bundles are tethered by intracellular proteins to cadherins, which bind to cadherins on the adjacent cell. In this way, the actin filament bundles in adjacent cells are tied together. For clarity, this drawing does not show most of the other cell–cell and cell–matrix junctions of epithelial cells (see Figure 19–2).


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Cell_Biology_Alberts. it by the cadherins and their associated intracellular adaptor proteins. The actin– myosin bundles are thus linked, via the cadherins, into an extensive transcellular network. Coordinated contraction of this network provides the motile force for a fundamental process in animal morphogenesis—the folding of epithelial cell sheets into tubes, vesicles, and other related structures (Figure 19–14).


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Cell_Biology_Alberts. The coordination of cell–cell adhesion and actin contractility is beautifully illustrated by cellular rearrangements that occur early in the development of the fruit fly Drosophila melanogaster. Soon after gastrulation, the outer epithelium of the embryo is elongated by a process called germ-band extension, in which the cells converge inward toward the dorsal–ventral axis and extend along the anterior–posterior axis (Figure 19–15). Actin-dependent contraction along specific cell boundaries is coordinated with a loss of specific adherens junctions to allow cells to insert themselves between other cells (a process called intercalation), resulting in a longer and narrower epithelium. The mechanisms underlying the loss of adhesion along specific cell boundaries are not clear, but they depend in part on increased degradation of β-catenin, due to its phosphorylation by a protein kinase that is localized specifically at those boundaries.


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Cell_Biology_Alberts. Desmosomes are structurally similar to adherens junctions but contain specialized cadherins that link to intermediate filaments instead of actin filaments. Their main function is to provide mechanical strength. Desmosomes are important Figure 19–14 The folding of an epithelial sheet to form an epithelial tube. the oriented contraction of the bundles of actin and myosin filaments running along adhesion belts causes the epithelial cells to narrow at their apex and helps the epithelial sheet to roll up into a tube. an example is the formation of the neural tube in early vertebrate development (see Figure 19–8).


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Cell_Biology_Alberts. Figure 19–15 Remodeling of cell–cell adhesions in embryonic Drosophila epithelium. Depicted at left is a group of cells in the outer epithelium of a Drosophila embryo. During germ-band extension, cells converge toward each other (middle) on the dorsal–ventral axis and then extend (right) along the anterior–posterior axis. the result is intercalation: cells that were originally far apart along the dorsal–ventral axis (dark green) are inserted between the cells (light green) that separated them. these rearrangements depend on the spatial regulation of actin–myosin contractile bundles, which are localized primarily at the vertical cell boundaries (red, left). Contraction of these bundles is accompanied by removal of E-cadherin (not shown) at the same cell boundaries, resulting in shrinkage and loss of adhesion along the vertical axis (middle). New cadherin-based adhesions (blue, right) then form and expand along horizontal boundaries, resulting in extension of the cells in the


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Cell_Biology_Alberts. and loss of adhesion along the vertical axis (middle). New cadherin-based adhesions (blue, right) then form and expand along horizontal boundaries, resulting in extension of the cells in the anterior–posterior dimension.


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Cell_Biology_Alberts. 0.5 µm 100 nm


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Cell_Biology_Alberts. Figure 19–16 Desmosomes. (a) the structural components of a desmosome. On the cytoplasmic surface of each interacting plasma membrane is a dense plaque composed of a mixture of intracellular adaptor proteins. a bundle of keratin intermediate filaments is attached to the surface of each plaque. transmembrane nonclassical cadherins bind to the plaques and interact through their extracellular domains to hold the adjacent membranes together. (B) Some of the molecular components of a desmosome. Desmoglein and desmocollin are nonclassical cadherins. their cytoplasmic tails bind plakoglobin (γ-catenin) and plakophilin (a distant relative of p120-catenin), which in turn bind to desmoplakin. Desmoplakin binds to the sides of intermediate filaments, thereby tying the desmosome to these filaments. (C) an electron micrograph of desmosome junctions between three epidermal cells in the skin of a baby mouse. (D) part of the same tissue at higher magnification, showing a single desmosome, with


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Cell_Biology_Alberts. (C) an electron micrograph of desmosome junctions between three epidermal cells in the skin of a baby mouse. (D) part of the same tissue at higher magnification, showing a single desmosome, with intermediate filaments attached to it. (C and D, from W. he, p. Cowin and D.L. Stokes, Science 302:109–113, 2003. With permission from aaaS.) in vertebrates but are not found, for example, in Drosophila. They are present in most mature vertebrate epithelia and are particularly plentiful in tissues that are subject to high levels of mechanical stress, such as heart muscle and the epidermis, the epithelium that forms the outer layer of the skin.


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Cell_Biology_Alberts. Figure 19–16A shows the general structure of a desmosome, and Figure 19–16B shows some of the proteins that form it. Desmosomes typically appear as buttonlike spots of adhesion, riveting the cells together (Figure 19–16C). Inside the cell, the bundles of ropelike intermediate filaments that are anchored to the desmosomes form a structural framework of great tensile strength (Figure 19–16D), with linkage to similar bundles in adjacent cells, creating a network that extends throughout the tissue (Figure 19–17). The particular type of intermediate filaments attached to the desmosomes depends on the cell type: they are keratin filaments in most epithelial cells, for example, and desmin filaments in heart muscle cells.


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Cell_Biology_Alberts. The importance of desmosomes is demonstrated by some forms of the potentially fatal skin disease pemphigus. Affected individuals make antibodies against one of their own desmosomal cadherin proteins. These antibodies bind to and disrupt the desmosomes that hold their epidermal cells (keratinocytes) together. This results in a severe blistering of the skin, with leakage of body fluids into the loosened epithelium. Sheets of epithelial cells enclose and partition the animal body, lining all its surfaces and cavities, and creating internal compartments where specialized processes occur. The epithelial sheet seems to be one of the inventions that lie at the origin of animal evolution, diversifying in a huge variety of ways but retaining an organization based on a set of conserved molecular mechanisms.


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Cell_Biology_Alberts. Essentially all epithelia are anchored to other tissue on one side—the basal side—and free of such attachment on their opposite side—the apical side. A basal lamina lies at the interface with the underlying tissue, mediating the attachment, while the apical surface of the epithelium is generally bathed by extracellular fluid. Thus, all epithelia are structurally polarized, and so are their individual cells: the basal end of a cell, adherent to the basal lamina below, differs from the apical end, exposed to the medium above. Correspondingly, all epithelia have at least one function in common: they serve as selective permeability barriers, separating the fluid that permeates the tissue on their basal side from fluid with a different chemical composition on their apical side. This barrier function requires that the adjacent cells be sealed together by tight junctions, so that molecules cannot leak freely across the cell sheet.


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Cell_Biology_Alberts. The epithelium of the small intestine provides a good illustration of tight-junction structure and function (see Figure 19–2). This epithelium has a simple columnar structure; that is, it consists of a single layer of tall (columnar) cells. These are of several differentiated types, but the majority are absorptive cells, specialized for uptake of nutrients from the internal cavity, or lumen, of the gut. The absorptive cells have to transport selected nutrients across the epithelium from the lumen into the extracellular fluid on the other side. From there, these nutrients diffuse into small blood vessels to provide nourishment to the organism. This transcellular transport depends on two sets of transport proteins in the plasma membrane of the absorptive cell. One set is confined to the apical surface of the cell (facing the lumen) and actively transports selected molecules into the cell from the gut. The other set is confined to the basolateral (basal and lateral) surfaces of the cell,


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Cell_Biology_Alberts. surface of the cell (facing the lumen) and actively transports selected molecules into the cell from the gut. The other set is confined to the basolateral (basal and lateral) surfaces of the cell, and it allows the same molecules to leave the cell by passive transport into the extracellular fluid on the other side of the epithelium. For this transport activity to be effective, the spaces between the epithelial cells must be tightly sealed, so that the transported molecules cannot leak back into the gut lumen through these spaces (Figure 19–18). Moreover, the transport proteins must be correctly distributed in the plasma membranes: the apical transporters must be delivered to the cell apex and must not be allowed to drift to the basolateral membrane, and the basolateral transporters must be delivered to and remain in the basolateral membrane. Tight junctions, besides sealing the gaps between the cells, also function as “fences” that help prevent apical or basolateral proteins from


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Cell_Biology_Alberts. be delivered to and remain in the basolateral membrane. Tight junctions, besides sealing the gaps between the cells, also function as “fences” that help prevent apical or basolateral proteins from diffusing into the wrong region.


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Cell_Biology_Alberts. The sealing function of tight junctions is easy to demonstrate experimentally: a low-molecular-weight tracer added to one side of an epithelium will generally not pass beyond the tight junction (Figure 19–19). This seal is not absolute, however. Although all tight junctions are impermeable to macromolecules, their permeability to ions and other small molecules varies. Tight junctions in the epithelium lining the small intestine, for example, are 10,000 times more permeable to inorganic ions, such as Na+, than the tight junctions in the epithelium lining the urinary bladder. The movement of ions and other molecules between epithelial cells is called paracellular transport, and tissue-specific differences in transport rates generally result from differences in the proteins that form tight junctions. tight Junctions Contain Strands of transmembrane adhesion proteins


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Cell_Biology_Alberts. tight Junctions Contain Strands of transmembrane adhesion proteins When tight junctions are visualized by freeze-fracture electron microscopy, they are seen as a branching network of sealing strands that completely encircles the Figure 19–17 Desmosomes, hemidesmosomes, and the intermediate filament network. the keratin intermediate filament networks of adjacent cells—in this example, epithelial cells of the small intestine—are indirectly connected to one another through desmosomes, and to the basal lamina through hemidesmosomes.


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Cell_Biology_Alberts. plasma membranes of adjacent cells apical end of each cell in the epithelial sheet (Figure 19–20A and B). In conventional electron micrographs, the outer leaflets of the two interacting plasma membranes are tightly apposed where sealing strands are present (Figure 19–20C). Each sealing strand is composed of a long row of transmembrane homophilic adhesion proteins embedded in each of the two interacting plasma membranes. The extracellular domains of these proteins adhere directly to one another to occlude the intercellular space (Figure 19–21).


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Cell_Biology_Alberts. The main transmembrane proteins forming these strands are the claudins, which are essential for tight-junction formation and function. Mice that lack the claudin-1 gene, for example, fail to make tight junctions between the cells in the epidermal layer of the skin; as a result, the baby mice lose water rapidly by evaporation through the skin and die within a day after birth. Conversely, if nonepithelial cells such as fibroblasts are artificially caused to express claudin genes, they (A) (B) 0.5 µm 0.5 µm


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Cell_Biology_Alberts. Figure 19–18 The role of tight junctions in transcellular transport. For clarity, only the tight junctions are shown. transport proteins are confined to different regions of the plasma membrane in epithelial cells of the small intestine. this segregation permits a vectorial transfer of nutrients across the epithelium from the gut lumen to the blood. In the example shown, glucose is actively transported into the cell by Na+-driven glucose transporters at its apical surface, and it leaves the cell through passive glucose transporters in its basolateral membrane. tight junctions are thought to confine the transport proteins to their appropriate membrane domains by acting as diffusion barriers, or “fences,” within the lipid bilayer of the plasma membrane; these junctions also block the backflow of glucose from the basal side of the epithelium into the gut lumen (see Movie 11.2).


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Cell_Biology_Alberts. Figure 19–19 The role of tight junctions in allowing epithelia to serve as barriers to solute diffusion. (a) the drawing shows how a small extracellular tracer molecule added on one side of an epithelium is prevented from crossing the epithelium by the tight junctions that seal adjacent cells together. adherens junctions and other cell junctions are not shown for clarity. (B) Electron micrographs of cells in an epithelium in which a small, extracellular, electron-dense tracer molecule has been added to either the apical side (on the left) or the basolateral side (on the right). the tight junction blocks passage of the tracer in both directions. (B, courtesy of Daniel Friend.) (A) lateral plasmaridges of transmembrane focal connection focal connection plasma membrane 50 nm0.5 µm intestinal lumenmicrovilli (C)(B) tightjunctioncell 1 cell 2


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Cell_Biology_Alberts. Figure 19–20 The structure of a tight junction between epithelial cells of the small intestine. the junctions are shown (a) schematically, (B) in a freeze-fracture electron micrograph, and (C) in a conventional electron micrograph. In (B), the plane of the micrograph is parallel to the plane of the membrane, and the tight junction appears as a band of branching sealing strands that encircle each cell in the epithelium (see Figure 19–21a). In (C), the junction is seen in cross section as a series of focal connections between the outer leaflets of the two interacting plasma membranes, each connection corresponding to a sealing strand in cross section. (B and C, from N.B. Gilula, in Cell Communication [r.p. Cox, ed.], pp. 1–29. New York: Wiley, 1974.) will form tight-junctional connections with one another. Normal tight junctions also contain a second major transmembrane protein called occludin, which is not essential for the assembly or structure of the tight junction but is important


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Cell_Biology_Alberts. with one another. Normal tight junctions also contain a second major transmembrane protein called occludin, which is not essential for the assembly or structure of the tight junction but is important for limiting junctional permeability. A third transmembrane protein, tricellulin, is required to seal cell membranes together and prevent transepithelial leakage at the points where three cells meet.


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Cell_Biology_Alberts. The claudin protein family has many members (24 in humans), and these are expressed in different combinations in different epithelia to confer particular permeability properties on the epithelial sheet. They are thought to form paracellular pores—selective channels allowing specific ions to cross the tight-junctional barrier, from one extracellular space to another. A specific claudin found in kidney epithelial cells, for example, is needed to let Mg2+ pass between the cells of the kidney tubules so that this ion can be resorbed from the urine into the blood. A mutation in the gene encoding this claudin results in excessive loss of Mg2+ in the urine.


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Cell_Biology_Alberts. Like the cadherin molecules of an adherens junction, the claudins and occludins of a tight junction interact with each other on their extracellular sides to promote junction assembly. Also as in adherens junctions, the organization of adhesion proteins in a tight junction depends on additional proteins that bind the cytoplasmic side of the adhesion proteins. The key organizational proteins at tight junctions are the zonula occludens (ZO) proteins. The three major members of the ZO family—ZO-1, ZO-2, and ZO-3—are large scaffold proteins that provide a structural support on which the tight junction is built. These intracellular molecules cell cell 0.3 µm 12 consist of strings of protein-binding domains, typically including several PDZ domains—segments about 80 amino acids long that can recognize and bind the C-terminal tails of specific partner proteins (Figure 19–22). One domain of these scaffold proteins can attach to a claudin protein, while others can attach to occludin or the actin


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Cell_Biology_Alberts. and bind the C-terminal tails of specific partner proteins (Figure 19–22). One domain of these scaffold proteins can attach to a claudin protein, while others can attach to occludin or the actin cytoskeleton. Moreover, one molecule of scaffold protein can bind to another. In this way, the cell can assemble a mat of intracellular proteins that organizes and positions the sealing strands of the tight junction.


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Cell_Biology_Alberts. The tight-junctional network of sealing strands usually lies just apical to adherens and desmosome junctions that bond the cells together mechanically; the whole assembly is called a junctional complex (see Figure 19–2). The parts of this junctional complex depend on each other for their formation. For example, anticadherin antibodies that block the formation of adherens junctions also block the formation of tight junctions. Tight junctions block the passageways through the gaps between epithelial cells, preventing extracellular molecules from leaking from one side of an epithelium to the other. Another type of junctional structure has a radically different function: it bridges gaps between adjacent cells so as to create direct channels from the Figure 19–21 A model of a tight junction.


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Cell_Biology_Alberts. Figure 19–21 A model of a tight junction. (a) the sealing strands hold adjacent plasma membranes together. the strands are composed of transmembrane proteins that make contact across the intercellular space and create a seal. (B) the molecular composition of a sealing strand. the major extracellular components of the tight junction are members of a family of proteins with four transmembrane domains. One of these proteins, claudin, is the most important for the assembly and structure of the sealing strands, whereas the related protein occludin has the less critical role of determining junction permeability. the two termini of these proteins are both on the cytoplasmic side of the membrane, where they interact with large scaffolding proteins that organize the sealing strands and link the tight junction to the actin cytoskeleton (not shown here, but see Figure 19–22).


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Cell_Biology_Alberts. Figure 19–22 Scaffold proteins at the tight junction. the scaffold proteins ZO-1, ZO-2, and ZO-3 are concentrated beneath the plasma membrane at tight junctions. Each of the proteins contains multiple protein-binding domains, including three pDZ domains, an Sh3 domain, and a GK domain, linked together like beads on a flexible string. these domains enable the proteins to interact with each other and with numerous other partners, as indicated here, to generate a tightly woven protein network that organizes the sealing strands of the tight junction and links them to the actin cytoskeleton. Scaffold proteins with similar structure help organize other junctional complexes, including those at neural synapses. cytoplasm of one to that of the other. These channels are called gap junctions.


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Cell_Biology_Alberts. cytoplasm of one to that of the other. These channels are called gap junctions. Gap junctions are present in most animal tissues, including connective tissues as well as epithelia and heart muscle. Each gap junction appears in conventional electron micrographs as a patch where the membranes of two adjacent cells are separated by a uniform narrow gap of about 2–4 nm (Figure 19–23). The gap is spanned by channel-forming proteins, of which there are two distinct families, called the connexins and the innexins. Connexins are the predominant gap-junction proteins in vertebrates, with 21 isoforms in humans. Innexins are found in the gap junctions of invertebrates.


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Cell_Biology_Alberts. Gap junctions have a pore size of about 1.4 nm, which allows the exchange of inorganic ions and other small water-soluble molecules, but not of macromolecules such as proteins or nucleic acids (Figure 19–24). An electric current injected into one cell through a microelectrode causes an electrical disturbance in the neighboring cell, due to the flow of ions carrying electric charge through gap junctions. This electrical coupling via gap junctions serves an obvious purpose in tissues containing electrically excitable cells: action potentials can spread rapidly from cell to cell, without the delay that occurs at chemical synapses. In vertebrates, for example, electrical coupling through gap junctions synchronizes the contractions of heart muscle cells as well as those of the smooth muscle cells responsible for the peristaltic movements of the intestine. Gap junctions also occur in many tissues whose cells are not electrically excitable. In principle, the sharing of small metabolites and


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Cell_Biology_Alberts. responsible for the peristaltic movements of the intestine. Gap junctions also occur in many tissues whose cells are not electrically excitable. In principle, the sharing of small metabolites and ions provides a mechanism for coordinating the activities of individual cells in such tissues and for smoothing out random fluctuations in small-molecule concentrations in different cells.


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Cell_Biology_Alberts. a Gap-Junction Connexon Is Made of Six transmembrane Connexin Subunits Connexins are four-pass transmembrane proteins, six of which assemble to form a hemichannel, or connexon. When the connexons in the plasma membranes of two cells in contact are aligned, they form a continuous aqueous channel that connects the two cell interiors (Figure 19–25). A gap junction consists of many such connexon pairs in parallel, forming a sort of molecular sieve. Not only does this sieve provide a communication channel between cells, but it also provides a form of cell–cell adhesion that supplements the cadherinand claudin-mediated adhesions we discussed earlier.


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Cell_Biology_Alberts. Figure 19–23 Gap junctions as seen in the electron microscope. (a) thin-section and (B) freeze-fracture electron micrographs of a large and a small gap-junction plaque between fibroblasts in culture. In (B), each gap junction is seen as a cluster of homogeneous intramembrane particles. Each intramembrane particle corresponds to a connexon (see Figure 19–25). (From N.B. Gilula, in Cell Communication [r.p. Cox, ed.], pp. 1–29. New York: Wiley, 1974.) MW1001000500020,000


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Cell_Biology_Alberts. MW1001000500020,000 Figure 19–24 Determining the size of a gap-junction channel. When fluorescent molecules of various sizes are injected into one of two cells coupled by gap junctions, molecules with a molecular weight (MW) of less than about 1000 daltons can pass into the other cell, but larger molecules cannot. thus, the coupled cells share their small molecules (such as inorganic ions, sugars, amino acids, nucleotides, vitamins, and the intracellular signaling molecules cyclic aMp and inositol trisphosphate) but not their macromolecules (proteins, nucleic acids, and polysaccharides). 1.5 nm in diameter (C) 1.4 nm


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Cell_Biology_Alberts. 1.5 nm in diameter (C) 1.4 nm Gap junctions in different tissues can have different properties because they are formed from different combinations of connexins, creating channels that differ in permeability and regulation. Most cell types express more than one type of connexin, and two different connexin proteins can assemble into a heteromeric connexon, with its own distinct properties. Moreover, adjacent cells expressing different connexins can form intercellular channels in which the two aligned half-channels are different (see Figure 19–25B).


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Cell_Biology_Alberts. Like conventional ion channels (discussed in Chapter 11), individual gap-junction channels do not remain open all the time; instead, they flip between open and closed states. These changes are triggered by a variety of stimuli, including the voltage difference between the two connected cells, the membrane potential of each cell, and various chemical properties of the cytoplasm, including the pH and concentration of free Ca2+. Some subtypes of gap junctions can also be regulated by extracellular signals such as neurotransmitters. We are only just beginning to understand the physiological functions and structural basis of these various gating mechanisms.


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Cell_Biology_Alberts. Each gap-junctional plaque is a dynamic structure that can readily assemble, disassemble, or be remodeled, and it can contain a cluster of a few to many thousands of connexons (see Figure 19–23B). Studies with fluorescently labeled connexins in living cells show that new connexons are continually added around the periphery of an existing junctional plaque, while old connexons are removed from the middle of it and destroyed (Figure 19–26). This turnover is rapid: the connexin molecules have a half-life of only a few hours.


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Cell_Biology_Alberts. The mechanism of removal of old connexons from the middle of the plaque is not known, but the route of delivery of new connexons to its periphery seems clear: they are inserted into the plasma membrane by exocytosis, like other integral membrane proteins, and then diffuse in the plane of the membrane until they bump into the periphery of a connexon plaque and become trapped. This has a corollary: the plasma membrane away from the gap junction should contain connexons—hemichannels—that have not yet paired with their counterparts on another cell. It is thought that these unpaired hemichannels are normally held in a closed conformation, preventing the cell from losing its small molecules by


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Cell_Biology_Alberts. Figure 19–25 Gap junctions. (a) a drawing of the interacting plasma membranes of two adjacent cells connected by gap junctions. Each lipid bilayer is shown as a pair of red sheets. protein assemblies called connexons (green), each of which is formed by six connexin subunits, penetrate the apposed lipid bilayers. two connexons join across the intercellular gap to form a continuous aqueous channel connecting the two cells. (B) the organization of connexins into connexons, and connexons into intercellular channels. the connexons can be homomeric or heteromeric, and the intercellular channels can be homotypic or heterotypic. (C) the high-resolution structure of a homomeric gap-junction channel, determined by x-ray crystallography of human connexin 26. In this view, we are looking down on the pore, formed from six connexin subunits. the structure illustrates the general features of the channel and suggests a pore size of about 1.4 nm, as predicted from studies of gap-junction permeability


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Cell_Biology_Alberts. formed from six connexin subunits. the structure illustrates the general features of the channel and suggests a pore size of about 1.4 nm, as predicted from studies of gap-junction permeability with molecules of various sizes (see Figure 19–24). (pDB code: 2ZW3.)


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Cell_Biology_Alberts. Figure 19–26 Connexin turnover at a gap junction. Cells were transfected with a slightly modified connexin gene, coding for a connexin with a short amino acid tag containing four cysteines in the sequence Cys-Cys-X-X-Cys-Cys (where X denotes an arbitrary amino acid). this tetracysteine tag can bind strongly to certain small fluorescent dye molecules, which can be added to the culture medium and will readily enter cells by diffusing across the plasma membrane. In the experiment shown, a green dye was added first to label all the connexin molecules in the cells, and the cells were then washed and incubated for 4 or 8 hours. at the end of this time, a red dye was added to the medium and the cells were washed again and fixed. Connexin molecules already present at the beginning of the experiment are labeled green (and take up no red dye because their tetracysteine tags are already saturated with green dye), while connexins synthesized subsequently, during the 4or 8-hour incubation, are


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Cell_Biology_Alberts. are labeled green (and take up no red dye because their tetracysteine tags are already saturated with green dye), while connexins synthesized subsequently, during the 4or 8-hour incubation, are labeled red. the fluorescence images show gap junctions between pairs of cells treated in this way. the central part of the gap-junction plaque is green, indicating that it consists of old connexin molecules, while the periphery is red, indicating that it consists of connexins synthesized during the previous 4 or 8 hours. the longer the time of incubation, the smaller the green central patch of old molecules, and the larger the peripheral ring of new molecules that have been recruited to replace the old ones. (From G. Gaietta et al., Science 296:503–507, 2002. With permission from aaaS.) leakage through them. But there is also evidence that in some circumstances they can open and serve as channels for the release of small signal molecules.


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Cell_Biology_Alberts. In plants, plasmodesmata perform Many of the Same Functions as Gap Junctions The tissues of a plant are organized on different principles from those of an animal. This is because plant cells are imprisoned within tough cell walls composed of an extracellular matrix rich in cellulose and other polysaccharides, as we discuss later. The cell walls of adjacent cells are firmly cemented to those of their neighbors, which eliminates the need for anchoring junctions to hold the cells in place. But a need for direct cell–cell communication remains. Thus, plant cells have only one class of intercellular junctions, plasmodesmata. Like gap junctions, they directly connect the cytoplasms of adjacent cells.


Cell_Biology_Alberts

Cell_Biology_Alberts. In plants, the cell wall between a typical pair of adjacent cells is at least 0.1 μm thick, and so a structure very different from a gap junction is required to mediate communication across it. Plasmodesmata solve the problem. With a few specialized exceptions, every living cell in a higher plant is connected to its living neighbors by these structures, which form fine cytoplasmic channels through the intervening cell walls. As shown in Figure 19–27A, the plasma membrane of one cell is continuous with that of its neighbor at each plasmodesma, which connects the cytoplasms of the two cells by a roughly cylindrical channel with a diameter of 20–40 nm.


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Cell_Biology_Alberts. Running through the center of the channel in most plasmodesmata is a narrower cylindrical structure, the desmotubule, which is continuous with elements of the smooth endoplasmic reticulum (ER) in each of the connected cells (Figure 19–27B–D). Between the outside of the desmotubule and the inner face of the cylindrical channel formed by plasma membrane is an annulus of cytosol through which small molecules can pass from cell to cell. As each new cell wall is assembled during the cytokinesis phase of cell division, plasmodesmata are created within it. They form around elements of smooth ER that become trapped across the developing cell plate (discussed in Chapter 17). They can also be inserted de novo through preexisting cell walls, where they are commonly found in dense clusters called pit fields. When no longer required, plasmodesmata can be removed.


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Cell_Biology_Alberts. In spite of the radical difference in structure between plasmodesmata and gap junctions, they seem to function in remarkably similar ways. Evidence obtained by injecting tracer molecules of different sizes suggests that plasmodesmata allow the passage of molecules with a molecular weight of less than about 800, which cytoplasm plasmodesmataplasma membrane lining plasmodesma, connecting two adjacent cells smooth endoplasmic reticulum desmotubule 100 nm cell walls of adjacent plant cells Figure 19–27 Plasmodesmata. (a) the cytoplasmic channels of plasmodesmata pierce the plant cell wall and connect cells in a plant together. (B) Each plasmodesma is lined with plasma membrane that is common to the two connected cells. It usually also contains a fine tubular structure, the desmotubule, derived from smooth endoplasmic reticulum.


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Cell_Biology_Alberts. (C) Electron micrograph of a longitudinal section of a plasmodesma from a water fern. the plasma membrane lines the pore and is continuous from one cell to the next. Endoplasmic reticulum and its association with the central desmotubule can also be seen. (D) a similar plasmodesma seen in cross section. (C and D, from r. Overall, J. Wolfe and B.E.S. Gunning, in protoplasma 111, pp. 134–150. heidelberg: Springer-Verlag, 1982.) is similar to the molecular-weight cutoff for gap junctions. As with gap junctions, transport through plasmodesmata is regulated. Dye-injection experiments, for example, show that there can be barriers to the movement of even low-molecularweight molecules between certain cells, or groups of cells, that are connected by apparently normal plasmodesmata; the mechanisms that restrict communication in these cases are not understood. Selectins Mediate transient Cell–Cell adhesions in the Bloodstream


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Cell_Biology_Alberts. Selectins Mediate transient Cell–Cell adhesions in the Bloodstream We now complete our overview of cell–cell junctions and adhesion by briefly describing some of the more specialized adhesion mechanisms used in some tissues. In addition to those we have already discussed, at least three other super-families of cell–cell adhesion proteins are important: the integrins, the selectins, and the adhesive immunoglobulin (Ig) superfamily members. We shall discuss integrins in more detail later: their main function is in cell–matrix adhesion, but a few of them mediate cell–cell adhesion in specialized circumstances. Ca2+ dependence provides one simple way to distinguish among these classes of adhesion proteins experimentally. Selectins, like cadherins and integrins, require Ca2+ for their adhesive function; Ig superfamily members do not.


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Cell_Biology_Alberts. Selectins are cell-surface carbohydrate-binding proteins (lectins) that mediate a variety of transient cell–cell adhesion interactions in the bloodstream. Their main role, in vertebrates at least, is in governing the traffic of white blood cells into normal lymphoid organs and any inflamed tissues. White blood cells lead a nomadic life, roving between the bloodstream and the tissues, and this necessitates special adhesive behavior. The selectins control the binding of white blood cells to the endothelial cells lining blood vessels, thereby enabling the blood cells to migrate out of the bloodstream into a tissue.


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Cell_Biology_Alberts. Each selectin is a transmembrane protein with a conserved lectin domain that binds to a specific oligosaccharide on another cell (Figure 19–28A). There are at least three types: L-selectin on white blood cells, P-selectin on blood platelets and on endothelial cells that have been locally activated by an inflammatory response, and E-selectin on activated endothelial cells. In a lymphoid organ, such as a lymph (C) 0.1 µm node or the spleen, the endothelial cells express oligosaccharides that are recognized by L-selectin on lymphocytes, causing the lymphocytes to loiter and become trapped. At sites of inflammation, the roles are reversed: the endothelial cells switch on expression of selectins that recognize the oligosaccharides on white blood cells and platelets, flagging the cells down to help deal with the local emergency. Selectins do not act alone, however; they collaborate with integrins, which strengthen the binding of the blood cells to the endothelium. The cell–cell adhesions


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Cell_Biology_Alberts. help deal with the local emergency. Selectins do not act alone, however; they collaborate with integrins, which strengthen the binding of the blood cells to the endothelium. The cell–cell adhesions mediated by both selectins and integrins are heterophilic—that is, the binding is to a molecule of a different type: selectins bind to specific oligosaccharides on glycoproteins and glycolipids, while integrins bind to specific Ig-family proteins.


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Cell_Biology_Alberts. Selectins and integrins act in sequence to let white blood cells leave the bloodstream and enter tissues (Figure 19–28B). The selectins mediate a weak adhesion because the binding of the lectin domain of the selectin to its carbohydrate ligand is of low affinity. This allows the white blood cell to adhere weakly and reversibly to the endothelium, rolling along the surface of the blood vessel, propelled by the flow of blood. The rolling continues until the blood cell activates its integrins. As we discuss later, these transmembrane molecules can be switched into an adhesive conformation that enables them to latch onto specific macromolecules external to the cell—in the present case, proteins on the surfaces of the endothelial cells. Once it has attached in this way, the white blood cell escapes from the bloodstream into the tissue by crawling out of the blood vessel between adjacent endothelial cells.


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Cell_Biology_Alberts. Members of the Immunoglobulin Superfamily Mediate Ca2+-Independent Cell–Cell adhesion The chief endothelial cell proteins that are recognized by the white blood cell integrins are called ICAMs (intercellular cell adhesion molecules) or VCAMs (vascular cell adhesion molecules). They are members of another large and ancient family of cell-surface molecules—the immunoglobulin (Ig) superfamily. These contain one or more extracellular Ig-like domains that are characteristic of antibody molecules. They have many functions outside the immune system that are unrelated to immune defenses.


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Cell_Biology_Alberts. While ICAMs and VCAMs on endothelial cells both mediate heterophilic binding to integrins, many other Ig superfamily members appear to mediate homophilic binding. An example is the neural cell adhesion molecule (NCAM), which is expressed by various cell types, including most nerve cells, and can take different forms, generated by alternative splicing of an RNA transcript produced from a single gene (Figure 19–29). Some forms of NCAM carry an unusually large


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Cell_Biology_Alberts. Figure 19–28 The structure and function of selectins. (a) the structure of p-selectin. the selectin attaches to the actin cytoskeleton through adaptor proteins that are still poorly characterized. (B) how selectins and integrins mediate the cell–cell adhesions required for a white blood cell to migrate out of the bloodstream into a tissue. First, selectins on endothelial cells bind to oligosaccharides on the white blood cell, so that it becomes loosely attached and rolls along the vessel wall. then the white blood cell activates a cell-surface integrin called LFa1, which binds to a protein called ICaM1 (belonging to the Ig superfamily) on the membrane of the endothelial cell. the white blood cell adheres to the vessel wall and then crawls out of the vessel by a process that requires another immunoglobulin superfamily member called pECaM1 (or CD31), not shown (Movie 19.2). EGF, epidermal growth factor.


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Cell_Biology_Alberts. quantity of sialic acid (with chains containing hundreds of repeating sialic acid units). By virtue of their negative charge, the long polysialic acid chains can interfere with cell adhesion (because like charges repel one another); thus, these forms of NCAM can serve to inhibit adhesion, rather than cause it.


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Cell_Biology_Alberts. A cell of a given type generally uses an assortment of different adhesion proteins to interact with other cells, just as each cell uses an assortment of different receptors to respond to the many soluble extracellular signal molecules in its environment. Although cadherins and Ig superfamily members are frequently expressed on the same cells, the adhesions mediated by cadherins are much stronger, and they are largely responsible for holding cells together, segregating cell collectives into discrete tissues, and maintaining tissue integrity. Molecules such as NCAM seem to contribute more to the fine-tuning of these adhesive interactions during development and regeneration, playing a part in various specialized adhesive phenomena, such as that discussed for blood cells and endothelial cells. Thus, while mutant mice that lack N-cadherin die early in development, those that lack NCAM develop relatively normally but show some mild abnormalities in the development of certain specific


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Cell_Biology_Alberts. cells. Thus, while mutant mice that lack N-cadherin die early in development, those that lack NCAM develop relatively normally but show some mild abnormalities in the development of certain specific tissues, including parts of the nervous system.


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Cell_Biology_Alberts. In epithelia, as well as in some other types of tissue, cells are directly attached to one another through strong cell–cell adhesions, mediated by transmembrane proteins called cadherins, which are anchored intracellularly to the cytoskeleton. Cadherins generally bind to one another homophilically: the head of one cadherin molecule binds to the head of a similar cadherin on an opposite cell. This selectivity enables mixed populations of cells of different types to sort out from one another according to the specific cadherins they express, and it helps to control cell rearrangements during development. The “classical” cadherins at adherens junctions are linked to the actin cytoskeleton by intracellular adaptor proteins called catenins. These form an anchoring complex on the intracellular tail of the cadherin molecule, and are involved not only in physical anchorage but also in the detection of and response to tension and other regulatory signals at the junction.


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Cell_Biology_Alberts. Tight junctions seal the gaps between cells in epithelia, creating a barrier to the diffusion of molecules across the cell sheet and also helping to separate the populations of proteins in the apical and basolateral plasma membrane domains of the epithelial cell. Claudins are the major transmembrane proteins forming gap junctions. Intracellular scaffold proteins organize the claudins and other junctional proteins into a complex protein network that is linked to the actin cytoskeleton. Figure 19–29 Two members of the Ig superfamily of cell–cell adhesion molecules. NCaM is expressed on neurons and many other cell types, and mediates homophilic binding. ICaM is expressed on endothelial cells and some other cell types and binds heterophilically to an integrin on white blood cells. Both NCaM and ICaM are glycoproteins, but their attached carbohydrate chains are not shown.


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Cell_Biology_Alberts. The cells of many animal tissues are coupled by gap junctions, which take the form of plaques of clustered connexons, which usually allow molecules smaller than about 1000 daltons to pass directly from the inside of one cell to the inside of the next. Cells connected by gap junctions share many of their inorganic ions and other small molecules and are therefore chemically and electrically coupled.


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Cell_Biology_Alberts. Three additional classes of transmembrane adhesion proteins mediate more transient cell–cell adhesion: selectins, immunoglobulin (Ig) superfamily members, and integrins. Selectins are expressed on white blood cells, blood platelets, and endothelial cells; they bind heterophilically to carbohydrate groups on cell surfaces, helping to mediate the adhesive interactions between these cells. Ig superfamily proteins also play a part in these interactions, as well as in many other adhesive processes; some of them bind homophilically, some heterophilically. Integrins, though they mainly serve to attach cells to the extracellular matrix, can also mediate cell– cell adhesion by binding to specific Ig superfamily proteins.


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Cell_Biology_Alberts. Tissues are not made up solely of cells. They also contain a remarkably complex and intricate network of macromolecules constituting the extracellular matrix. This matrix is composed of many different proteins and polysaccharides that are secreted locally and assembled into an organized meshwork in close association with the surfaces of the cells that produce them.