Graduate School Life

Starving Well

BY RICHARD KESSIN, PH.D.,
ASSOCIATE DEAN FOR GRADUATE AFFAIRS

IT WILL NOT DO TO IRRITATE PROFESSOR GAUTIER. NO, NO, NO. Especially if he is on your thesis committee. When he says, "Drop that project or you will be a graduate student until you are 40," a student should listen. Salute even. But insubordination is the stuff of which good Ph.D. students are made. And when the next thesis committee meeting rolls around, both the student, Turgay Tekinay, and the adviser, in this case the author, hope that Jean Gautier will be convinced.
It all concerns a peculiar problem how cells survive starvation, slowly digesting their guts until better times. The process is called autophagy (literally, self-eating) and it is something all cells can do, autophagy having evolved in the distant past before the time of multicellular animals. Not to be able to induce autophagy causes death at the first pang of hunger. And so Mr. Tekinay came to be studying autophagy in a peculiar amoeba that normally lives in the soil. It is called Dictyostelium discoideum and although it diverged from the line that led to animals perhaps as much as a billion years ago, surprisingly, its mechanism of autophagy is quite similar to that of humans. And it is a lot easier to study.
These amoebae are normally like macrophages, consuming bacteria at a furious clip, but when they run out of soil bacteria to eat and sense starvation, they all come together and form a fruiting body with a ball of spores held on a thin stalk. Spores are dormant and survive. It takes 24 hours and a lot of energy and raw material to make a fruiting body.
The amoebae are forced to digest their membranes and mitochondria everything except the nucleus. A lot about how they do this is a mystery although years of study in yeast and mammalian cells have given us an outline, if not the full roster of players.
Turgay's interest was to find genes that control the onset of autophagy and coordinate it with the development of the Dictyostelium amoebae. He did not make the assumption that all the interesting autophagy genes had been described in work on budding yeast. He already had another perfectly viable project, having to do with the regulation of a known autophagy gene, which would have given him a Ph.D. But when the intriguing new mutant showed up in a genetic screen, he quietly refused to abandon it despite the best advice of the thesis committee.
At first all he knew was that the mutant died when it was starved a sign of an autophagy mutant but no proof that it was a new one. Then he recovered the gene and sequenced it. The gene is novel and is present in animals, but not in yeast. Perhaps autophagy is more complicated than our colleagues working on baker's yeast think it is.

What was more intriguing was the electron microscopy of starving cells. If he starved wild-type cells for 24 hours they looked awful their cytoplasms were full of empty spaces. Mitochondria were rare. Membranes were gone. But the cells were alive and if you fed them again, giving them bacteria to eat, they recovered and grew, with nearly 100 percent efficiency.
The new mutant, on the other hand, looked just like well-fed cells do, despite the fact that it had been starving for 24 hours. It was chock-a-block full of mitochondria, vesicles, membranes, and glycogen. There was just one problem: The cells were dead.
OPPOSITE PAGE: Turgay Tekinay and his mentor, Richard H. Kessin, who is also the associate dean for graduate affairs. More on this organism can be found on the lab Web site, reached by typing Kessin into the CUMC search engine. ABOVE, TOP: Autophagy depends on a mysterious body called the Preautophagosomal Structure (PAS). When cells starve, this body produces a double-membrane vesicle that gradually folds around cellular contents and eventually closes. The closed vesicles fuse with lysosomes that contain the digestive enzymes of the cell, the contents are degraded, and raw materials are recycled. ABOVE, BOTTOM: Starving cells digest their contents. They have empty areas that have been cleared by autophagosomes, but feed them and they recover. The new mutant (on the right) is full of cytoplasm and organelles. It looks well fed, but in the absence of the new gene product it cannot mobilize autophagy and cannot survive. N marks the nucleus, M the mitochondria.
Turgay is now busy using all the tools of biochemistry and genetics to find out how the new gene product acts to regulate autophagy, but let me point out the clinical interest of the process. Autophagy is the only way that the cell has to survive starvation, but it is also the only way that the cell has to remove large aggregates, such as those present in various neurological diseases, including Parkinsonism and Alzheimer's disease. In mice that have been engineered to have Huntington's disease because of long polyglutamine repeats produced in their neurons, inducing autophagy with rapamycin can delay the onset of symptoms. We might expect that the inappropriate induction of autophagy would contribute to wasting diseases and we are beginning to see some evidence for this idea. Finally, we have considerable evidence that autophagy is essential in the defense against bacterial infection and is an important part of the innate immune system.
These are the reasons that we must know all of the components of autophagy and how to regulate them to our advantage. It is also why we need insubordinate graduate students.

ART COURTESY OF MARY WU AND GRANT OTTO

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