MIT Scientists Work to Unlock Mystery of Cell Suicide

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Researchers at the Massachusetts Institute of Technology (MIT) figure they have moved a step closer to understanding the mechanisms of cell suicide. By studying the Drosophila fruit fly, they have discovered a gene called Drosophila caspase-1 (DCP-1) which is critical to causing programmed cell death similar to that in mammals. Their findings will appear in the January 24 issue of the journal Science.

"Every cell in the body has the same cell-death engine that is encoded by genes. A central question is how this engine is started at the right time and in the right place," said Professor Hermann Steller, co-author of the Science paper. Professor Steller has dual appointments in MIT's Department of Biology and Department of Brain and Cognitive Sciences. He also is an associate investigator at the Howard Hughes Medical Institute. Postdoctoral fellows Zhiwei Song and Kimberly McCall, both of MIT's Department of Brain and Cognitive Sciences, are co-authors of the paper.


To date, much study has been devoted to cell death in the nematode Carnorhabditis elegans, a small soil-living worm whose development can be analyzed at the level of single cells. About four years ago, Professor Robert Horvitz, an MIT biology professor, and his colleagues at MIT discovered that cell death in the nematode required an unusual type of protease encoded by a gene called CED-3 (CEll Death abnormal-3). Proteases are enzymes that chop up other proteins, thus irreversibly destroying them. Subsequently, a large number of related proteases were identified in mammals, including humans, named "caspases."


In his research, Professor Steller has questioned why there are so many caspases when the activation of merely one of them is sufficient to kill a cell. He and his colleagues also are trying to understand how caspases are activated in response to death-inducing stimuli.

"The identification and characterization of a caspase in Drosophila represents an important step towards answering these mysteries," Professor Steller said.

In Drosophila, as in mammals, the cell suicide program can be activated by many different stimuli, including a variety of distinct developmental signals, hormones and radiation or other cellular damage.

Professor Steller said different death-inducing signals lead to the expression of a gene named reaper, which signals cells that it is time to die. The reaper gene and two closely linked apoptotic activator genes -- hid (head involution defective) and grim -- kill a cell by activating CED-3-like proteases such as the DCP-1 protease.

Professor Steller said that the DCP-1 protease is structurally and biochemically similar to the CED-3 protease and has all the properties of a cell death caspase.



Apoptosis, a Greek word for "falling away," is one of the most active topics in cell biology today because of its implications in treating disease. Apoptosis is a tidy way for cells to quietly commit suicide, their debris being absorbed by macrophages, cells that are specifically designed to sweep clean the body. If cells fail to commit suicide, a person may get cancer or auto-immune diseases such as lupus.

The steps cells take to commit suicide are complex, but they die by essentially dismantling into pieces. During apoptosis, the nucleus and the cytoplasm shrink and chromosomal DNA in the cell nucleus fragments. The dying cell fragments are quickly eaten by macrophages or by neighboring cells. The whole process can take as little as 40 minutes. Professor Steller describes the death as a fast, efficient cleanup process.

Apoptosis is very different than a violent cell death called necrosis, which is caused by massive injury that causes a cell to swell, rupture and spill its contents, often causing inflammation.

Cell suicide is necessary to health as a proofreading and correction mechanism, Professor Steller said. It begins soon after fertilization and is very active during embryo development. Later in life cells commit suicide when they are damaged or injured by ultraviolet light, ionizing radiation, peroxide or other chemicals, excessive heat or viral infection. Cells die in large numbers in our thymus and whenever cells proliferate, for example, during wound healing and regeneration. Apoptosis also kills brain cells in neurodegenerative disorders such as Alzheimer's and Parkinson's diseases or in strokes and as people age.

The basic machinery to carry out apoptosis appears to be the same in animals as diverse as worms, flies, and humans. It also appears to be present in essentially all mammalian cells at all times. But the decision between life and death for a cell is influenced by many different signals that originate from within or outside a cell.

To better understand the role of DCP-1, Professor Steller and his colleagues performed "knock out" studies, or tests in which DCP-1 was eliminated from cells. They found that the lack of DCP-1 in Drosophila caused larvae to die and also caused tumors.


Professor Steller said the researchers also hope to learn more about whether caspases play other important developmental roles. He said a better understanding of the precise function of caspases and their mode of activation should ultimately allow for the development of drugs to control apoptosis for therapeutic purposes in a variety of diseases, including cancer, AIDS and neurodegenerative disorders.


Topics: Biology


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