‘We live because we constantly die’
Most of the time, body cells die a silent, programmed death, but sometimes they burst like a balloon while loudly asking immune cells for help. Biomedical scientist Vishva Dixit unravels the underlying processes of the various forms of cell death, laying the foundation for treatments of various diseases such as cancer, Alzheimer’s, and sepsis. For his research, he received the Dr A.H. Heineken Prize for Medicine.
Every second, a million cells in our body die, amounting to one kilogram of dead cells per day. Our body rids itself of these cells because they are old, damaged or mutated and replaces them with new cells. This cleansing process is essential to life, but it walks a thin line. If too few cells die, mutated cells accumulate, which can lead to cancer. When more cells die than our bodies can replace, it leads to degenerative diseases such as Alzheimer’s. Vishva Dixit, vice-president of Early Discovery Research at the South San Francisco biotechnology company, Genentech, a member of the Roche Group, studies the mechanism responsible for the process of cell death. Because the better we understand it, the better we can understand diseases such as cancer and Alzheimer’s.
‘We live because we constantly die’, Dixit says. ‘For a long time, it was a great mystery exactly how our cells were made to die, and what the nature of the assassin was.’ He entered the field of cell death research at an interesting time. In the early 1990s, researchers discovered a ‘death receptor’ on the cell surface, activation of which ultimately led to cell death. But how exactly was not at all clear. In addition, American biologist Robert Horvitz discovered a gene that played an important role in programmed cell death in the C. elegans worm, which won him the Nobel Prize in 2002. For many years, it was unclear what the gene’s precise function was, but eventually it was discovered to code for a protease: a so-called ‘molecular scissors’. Once activated, these scissors shred the contents of the cell, causing the cell to die. ‘We began to wonder whether such a protease was involved in other organisms, including humans,’ says Dixit. ‘So, we did an experiment with a protein that is produced during infection with the cowpox virus. This protein inhibits the family of proteases. When we stimulated the death receptor in the presence of this inhibitor, the cells did not die, but without the inhibitor they did. From this we could conclude that a protease from this family had to play a role in cell death.
Years later, they identified the specific molecular scissors that cut up the cell. They called it Yama, after the Hindu god of death. Today, this protease is known by the somewhat more boring name of ‘caspase-3’. But a big question remained: how does the death receptor activate the molecular scissors? ‘The receptor had no obvious way of signalling. It looked like nothing we had ever seen,’ says Dixit. He eventually discovered that there were a few intermediate steps: the receptor eventually activates the molecular scissors via various intermediary adaptors. The knowledge of this signalling pathway forms the basis of several pioneering treatments that can be found in hospitals today. For example, this knowledge has been used to improve immunotherapy in cancer patients, by ensuring that immune cells initiate this chain reaction, killing cancer cells.
The kilo of cells that die in your body every day doesn’t hurt a bit. But in addition to this ‘silent’ form of cell death – apoptosis – there is also ‘loud’ cell death: necrosis. This occurs, for example, in the case of a cut, infection or burn. Dying cells alert immune cells to come and clear away the danger: they trigger an inflammatory response to destroy, for example, the pathogens. Dixit discovered that cells have different sensors for specific forms of danger, but that they trigger the inflammatory process in the same way.
These findings are important for treating sterile inflammation – inflammation without a pathogen, such as a bacterium, virus or parasite. ‘Sterile inflammation is at the heart of many first world diseases,’ says Dixit. ‘For example, they are behind atherosclerosis (hardening of the arteries), which leads to heart attacks. Furthermore, sterile inflammation accelerates degenerative diseases such as arthritis and Alzheimer’s disease. Our dream is to find a way to inhibit sterile inflammation without inhibiting the inflammatory response as a whole, because that makes you very susceptible to pathogens.’
Because damage to the cell membrane triggers sterile inflammation, you can target the receptor that detects that damage, known as NLRP3. For a long time, it was thought that it would not be possible to develop inhibitors for that receptor specifically. ‘But Mohamed Lamkanfi, a former postdoctoral researcher in my lab, demonstrated in 2017 that this was indeed possible,’ says Dixit. ‘He used analogues of sulphonylurea, a class of molecules used in the treatment of diabetes. These inhibitors were not very powerful; he needed a very high concentration. But because he demonstrated that it is possible, other researchers entered the field to develop more powerful inhibitors. And today, there are a number of NLRP3 inhibitors in clinical trials for the treatment of sterile inflammation.’
Sometimes there is a pathogen involved, to which the body reacts far too violently. This is the case with sepsis (blood poisoning). Bacteria such as Salmonella or E. coli cause an extremely severe inflammatory response. ‘Seven million people die of sepsis every year,’ says Dixit. ‘And despite all efforts, there is still no effective treatment.’ But there is hope: Dixit discovered that there is an alternative signalling pathway through which sepsis occurs. This opens the door to new methods of treatment.
In his research, Dixit frequently uses mice that have been genetically modified so that they lack one specific gene. By comparing them to mice that do have the gene, you can find out in which processes the gene plays a role. But in 2011, he accidentally discovered that mice widely used in the research field, on which hundreds of papers are based, were missing not one, but two genes. They not only lacked the gene for caspase-1, an enzyme that triggers an inflammatory response, but also the gene for another enzyme: caspase-11. ‘All the conclusions drawn about caspase-1 could just as easily apply to caspase-11,’ says Dixit. ‘We began to wonder what caspase-11 does, and modified mice so that they only lacked the gene for caspase-11. We found that these mice did not respond to a class of bacteria that includes salmonella and E. coli. So, we could conclude that these bacteria activate caspase-11, but we did not yet know how. The big surprise was that one specific molecule in the bacteria was responsible for this: LPS (lipopolysacharide). And this was the very molecule that was already known to be responsible for the majority of septic shock.’
The American immunologist Bruce Beutler had previously identified another receptor that responded to LPS and it was thought to mediate sepsis, for which he was awarded the Nobel Prize in 2011. But shortly thereafter Dixit, in 2013, discovered that there is another pathway to sepsis. In fact, he and his research group showed that this pathway is much more important, at least in mice. ‘If you can develop an inhibitor for caspase-11 or one of the proteins in the cascade, then you have a new opportunity to find a treatment for this disease,’ says Dixit. ‘My fondest hope is that we can find drugs that act on this pathway, reducing the disastrous effects of sepsis on mankind.’
Dixit is currently studying in greater detail what necrotic cell death looks like. ‘We are taught that this cell death is like a water balloon that you stick a needle into: it bursts, and the liquid is hurled in all directions. We recently discovered that a membrane protein, NINJ1, greatly accelerates this explosive process. The big mystery is: why? My hypothesis is that pathogens such as intracellular bacteria and viruses are rapidly ejected out of the cell so that immune cells can promptly clear them away. Moreover, in doing so, in one fell swoop, the bursting cell denies intracellular bacteria and viruses of the home they need to replicate. This is just a hypothesis; I could be completely wrong. But that’s the wonderful thing about research – you get to ponder things like that and then find out if you’re right.’
Vishva Dixit (Kisii, Kenya, 1956) studied medicine at the University of Nairobi in Kenya. In 1981, he moved to the United States for a medical residency training programme at Washington University in St. Louis. In 1986, he was hired as faculty by the pathology department of the University of Michigan, where he was appointed a professor in 1995. Beginning in 1997, he held various positions at the biotech company, Genentech. He has been vice president of the Early Discovery Research Department since 2005. Between 1999 and 2008, he was also an adjunct professor of pharmaceutical chemistry at the University of California, San Francisco. Dixit has more than 50 patents to his name.
Vishva Dixit studies how our bodies regulate the process of programmed cell death (apoptosis). He discovered which enzymes are involved in this process and how they activate each other in a chain reaction. This knowledge is used, among other things, to improve immunotherapy for cancer patients, with immune cells initiating this chain reaction, killing cancer cells. Dixit also studies necrosis, the cell death that occurs, for example, after a cut, burn or infection. Dying cells alert immune cells to come and clear away the danger: they trigger an inflammatory response. His findings have the potential to lead to improved treatment of arthritis, Alzheimer’s, and sepsis, among others.
Every two years, Heineken Prizes are awarded to five renowned international scientists and one artist. In 1964, Alfred Heineken established the Dr H.P. Heineken Prize for Biochemistry and Biophysics as a tribute to his father. Later, Heineken Prizes for the Arts, Medicine, Environmental Sciences, Historical Sciences, and Cognitive Sciences followed.