The two most common forms of cell death are necrosis and apoptosis. Apoptosis, sometimes called programmed cell death, is a normal and necessary part of animal and plant development. For instance, the formation of fingers and toes in a fetus requires the apoptotic removal of the tissue between them. Apoptosis is also a means of eliminating cells that threaten the integrity of an organism, like those infected by a virus or containing damaged DNA. In contrast, necrosis is unexpected cell death that can be induced by a number of physical and chemical events including heat shock, radiation, toxins, hypoxia (lack of oxygen) and blood loss. The protein SRP-6 is believed to help cells survive in the face of these stressors, thereby protecting them from necrosis. The study was carried out in Caenorhabditis elegans (or C. elegans), a transparent worm having close to 1,000 cells that serves as a model organism commonly used to study biological phenomena.
"For years, we believed that cell death related to a catastrophic insult, such as a stroke or heart attack that deprives tissue of oxygen, couldn't really be treated, so we focused on strategies to prevent further damage by restoring blood flow as quickly as possible with clot busters and surgery," said Gary Silverman, the study's senior author. "But our research indicates that necrosis can be interrupted and possibly repaired, even after the injury process is well underway. This insight has exciting implications for the management of heart disease, stroke and neurological illnesses."
SRP-6, which enhances survival by blocking necrosis, belongs to a large class of proteins called serine protease inhibitors or serpins having over 1,500 representatives in nature, 37 of which are found in humans, and which regulate protein degradation pathways by inhibiting protein-degrading enzymes. SRP-6 is believed to limit the damage caused by protein-degrading enzymes that are released by lysosomes-tiny bag-like structures in cells-in response to stressors like heat, toxins, etc. Normally, lysosomes in cells are involved in digesting both worn-out cellular entities like proteins, DNA and carbohydrates, and invading pathogens like viruses and bacteria. SRP-6, the authors of the work believe, not only prevents lysosomes from releasing their content but also inactivates any degrading enzymes released.
The role of SRP-6 in preventing necrosis was discovered serendipitously by Cliff Luke, primary author of the work. The team was initially interested in studying the role of serpins in preventing protein degradation, and in controlling blood clotting and inflammation. To this end, they designed worms that lacked the gene responsible for producing SRP-6. To their disappointment, the "knocked out" worms appeared no different from the normal worms. However, Luke noticed that, unlike normal worms, the SRP-6 deficient worms died rapidly when washed in room temperature water instead of the saline solution usually used. On further investigation the team found the cause of death to be necrosis due to an adverse stress response to the water-unlike normal worms, the knocked out worms couldn't adjust the volume of fluid in their cells, and so their cells burst open. Moreover, in these worms, the lysosomes burst and released their contents when placed in water, leading to cell death. Similar observations were made when the worms were subjected to other stressors like heat shock and oxygen deprivation. In addition, unlike normal worms, the worms lacking SRP-6 couldn't repair lysosomes. Taking all these observations into account, and after performing additional experiments, the group concluded that SRP-6 protects cells against lysosome-induced necrosis.
Speaking about the significance of their work, Cliff Luke remarked, "There are a lot of diseases associated with cell necrosis, such as stroke, neurodegenerative diseases and NEC (necrotizing enterocolitis, caused due to a bacterial infection), and now we know that the pathway to necrosis is much more systematic than we once thought it was. With further study, we may be able to identify targets of intervention to halt the necrotic progression in some of these diseases and possibly even prevent them." Moreover, it may be possible in the future to deprive cancerous cells of serpins and thereby target them for termination.
Caroline Goutte, a professor of biology at the College, finds the study to be "a very cool story, and once again shows how incredibly powerful a genetic approach can be to figuring out the function of a protein." She added, "The authors use my personal favorite model system (C. elegans) in which it is relatively straight forward to knock out a gene and analyze its phenotype (physical traits). Notice that the srp-6 mutant worms [srp-6 refers to the gene and SRP-6 refers to the corresponding protein] are completely normal under normal growing conditions ... this is a great example of how a gene may not appear to be 'essential' until you thoroughly poke and prod your organism."
"By moving to this well-studied model system, the authors are able to boot-strap our understanding of this well-conserved family of proteins into a functional context and make some very interesting connections with other known cellular pathways," she continued.
Professor of Biology Patrick Williamson, however, cautions that the work is still in its preliminary stage, and that further investigation needs to be conducted to determine how well necrosis can be controlled. "The basic finding is certainly interesting-it suggests that there is a protease of some kind (unidentified, and thus only theoretical) whose activity is dangerous to cells in the absence of srp-6, especially if the cells are stressed," he said. "However, the ideas that srp-6 is a 'regulator,' or that there is a 'core stress response pathway' of any kind, or that there is 'necrotic cell death routine,' lysosomal dependent or otherwise, or even that the primary function of srp-6 is to inhibit this dangerous protease, are all essentially made up out of thin air. Once the relevant protease is identified, localized, inactivated and its target proteins identified, the significance of the srp-6 result will be much clearer. Lots of interesting work to do, but it's still pretty early days."