Dissociation of cell contacts

Because compatible cells achieve stable interactions by forming a dynamic equilibrium between adhesive and LM-uptake reactions (Figures 2 .5 and 2 .6), this finely balanced symmetry between two cells is a precondition for multicellular growth and development and, consequently, is used by multicellular organisms to monitor tissue integrity Given that interactive models describe cell-cell interactions as a tug-of-war, where neighboring cells attempt to engage in mutual uptake reactions, the question is how do individual cells detach from neighbors when they divide or acquire a new developmental fate? Unilateral destabi-lization of adhesive membrane proteins or reduction of LM-uptake forces is not an option, because this would lead to a reduction in surface tension and subsequent phagocytosis by the neighboring cell In fact, the uptake of apoptotic cells by adjacent epithelial cells in nema-todes (Horvitz and Reddien, 2004) could be depicted in this context. Changes in membrane composition or adhesive properties of apoptotic cells are possible factors that alter the balance of forces and drive uptake reactions, leading to phagocytosis of dead cells (Fadok and Chimini, 2001) . Note that this process occurs without a need for specific signals other than a unilateral change in balance of forces between the two neighboring cells

Thus the only way to release single cells from tissue attachments is to mutually change adhesive connections on the cell interface of all adjacent cells One approach is for a single cell to secrete proteases into the intercellular space that cut relevant adhesion proteins, thereby simultaneously removing the attachments to neighboring cells This may be






Figure 2.7 Schematic depiction of a dynamic relationship between adhesive and uptake reactions . A shift in balance toward internalization of receptors in phagocytosis or endocytosis reactions can deplete the cell surface of receptors leading to loss of adhesive abilities realized in Notch-regulated cell fate determination, where a single epithelial cell is detached from neighboring cells by proteolytic enzymes (Fortini, 2001) These proteases are locally restricted by lateral inhibition processes to the adjacent intercellular space, which allows the secreting cell to round up while neighboring cells retain most of their epithelial connections (Figure 2 . 8, dots depict proteases) . Another mechanism involves the release of soluble counter-adhesion proteins, such as matricellular proteins (Greenwood and Murphy, 1998) into the intercellular space One of the crucial properties of counter-adhesion proteins may be their ability to form strong LM-assemblies, thereby forcing the membrane-anchored adhesive receptors to internalize from the cell surface of adjacent cells against cytoplasmic receptor-anchorage (Figure 2 . 8, dots depict matricellular proteins) . In this context, counter-adhesion proteins, such as thrombospondin, can be viewed as adhesive proteins that form LM-complexes strong enough to internalize receptors against strong cytoplasmic anchorage The outcome of both mechanisms is the detachment of individual cells from epithelial or other tissue connections to undergo cell divisions or cell migrations

Thus, the coexistence of different organisms can be perceived by two strategies: the adoption of cell surface properties that allow a balance of forces between the two organisms, or by a coexistence that is based on lack of cellular interactions due to detachment While there are no clear examples of the former mechanisms in insects, certain trophic interactions in plants, such as mycorrhizal symbionts and parasitic mistletoe, may involve direct membrane alignments to exchange nutrients

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