Every minute of the day, millions of cells in the human body commit a form of suicide called programmed cell death or apoptosis. Although this is a very normal and crucial process that cleanses the body of unneeded or damaged cells and helps prevent diseases such as cancer, scientists are just beginning to piece together how programmed cell death happens.
Researchers at MIT believe 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 appeared in the January 24 issue of 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 the Departments of Biology and 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 brain and cognitive sciences, are co-authors of the paper.
Scientists found evidence for the existence of a cell suicide mechanism more than a decade ago. But genes involved in programmed cell death have only been cloned in the past few years.
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 of Biology Robert Horvitz and his MIT colleagues 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, destroying them. Subsequently, a large number of related proteases were identified in mammals, including humans, named "caspases."
At least some of these caspases are active in cell death in mammals. However, because humans have at least 11 such caspases, it has been difficult to determine the precise role that any single caspase has in cell death. Drosophila, by contrast, has fewer caspases (three are known to date), making it easier for scientists to study their function.
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 are also 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," he 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. He 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.
Because reaper, hid and grim provide a crucial link between different death-inducing signaling pathways and DCP-1 activation, it should eventually be possible to deduce the precise mechanism by which specific cell suicide stimuli activate the cell death program, Professor Steller said.
In addition, since there are at least two more related caspases in Drosophila, it is now possible to study whether they function in a cascade, in parallel pathways or if they have redundant functions. Research already indicates that different Drosophila caspases have distinct but somewhat overlapping functions.
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, after which their debris is 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 autoimmune 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 neighboring cells. The whole process can take as little as 40 minutes. Professor Steller describes the death as a fast, efficient cleanup
Apoptosis is very different from a violent cell death called necrosis, which results from a 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 the thymus and whenever cells proliferate-for example, during wound healing and regeneration. Apoptosis also kills brain cells in neuro-degenerative 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 "knockout" 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.
"The flies don't develop normally, so this is the first indication that it is an essential gene," he said. "The second thing we found is that even without the DCP-1 gene, there is still cell death. This indicates that other death caspases in Drosophila can partially substitute for its loss of function. It also appears that DCP-1 may have a novel function that is independent and distinct from its role in cell death."
The researchers also hope to learn more about whether caspases play other important developmental roles, Professor Steller 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 neuro-degenerative disorders, he added.
The research was funded by the Howard Hughes Medical Institute and the American Cancer Society.
A version of this article appeared in MIT Tech Talk on January 29, 1997.