// Aug 14, 2018
Peter Attia, who was our very first guest on STEM-Talk, describes David Sabatini’s discovery of mTOR as one of his two favorite science stories.
Today, Dr. David Sabatini joins us and gives us a first-hand account of how his research into rapamycin in 1994 as a graduate student led him to the discovery of mTOR, which we now know is a critical regulator of cellular growth.
Our interview with David delves into his continuing research into mTOR, which has led to promising opportunities for the development of new treatments for debilitating diseases such as cancer, diabetes and neurological disorders. He also discusses mTOR’s role in healthspan and lifespan.
David is a molecular cell biologist who, according to Reuters News Service, is on the short list for a Nobel Prize. David is on the faculty at MIT and heads up the Sabatini Lab at the Whitehead Institute.
In today’s episode, we discuss:
• Rapamycin, a macrolide antibiotic discovered in the soil of Easter Island
• David’s discovery of mTOR while a grad student at Johns Hopkins
• mTOR’s role as one of the major growth pathways in the body
• mTOR’s role as a nutrient sensor
• How mTOR inhibiton has become one of the hottest topics in longevity research
• mTOR’s role in diseases, especially its connection to cancer
• The role of RAG GTPases as key mTOR mediators
• Protein intake and downstream mTOR activation
• Research into ketogenic diets effect on longevity and healthspan
• Whether David would take rapamycin as a means to enhance his longevity
• And much, much more
Show notes:[00:03:32] David talks about growing up in New York and having parents who immigrated to the United States from Argentina. [00:04:00] Dawn asks what David was like as a kid. [00:04:59] Dawn asks David about his decision to attend Brown University. [00:05:56] David talks about his decision to become a scientist and the time he spent in the lab of Al Dahlberg [00:06:53] Ken mentions that after his time at Brown, David headed off for Johns Hopkins to work in Sol Snyder’s lab, a professor known particularly for the work he and his colleagues did on the opioid receptor. Ken asks what drew David to Sol’s lab. [00:08:25] David talks about how as graduate student at Johns Hopkins in the M.D./Ph.D. program, he began trying to understand the molecular mechanism of rapamycin, a macrolide antibiotic discovered in the soil of Easter Island. Rapamycin was known as a potent antifungal, immunosuppressive with anti-tumorigenic properties. That research led David to the major discovery in 1994 of the protein to which rapamycin binds, now referred to as the mechanistic target of rapamycin, or mTOR. [00:11:46] Dawn asks David to give a high-level definition and overview of what mTOR does. [00:13:44] Dawn asks why the “m” in mTOR went from standing for “mammalian” to “mechanistic.” [00:14:11] Ken mentions that we now know mTOR is one of the major growth pathways in the body that is responsible for growth in both a positive sense and a pathologic sense. He goes on to mention that mTOR acts as a major switch between catabolism and anabolism, and asks David to explain why both of these processes are essential for survival. [00:16:10] Dawn asks how the two different mTOR protein complexes, mTORC1 and mTORC2, differ with regards to their activation and downstream function. [00:17:40] Dawn asks David about his decision to join the faculty atMIT and embark on a research-focused career there, starting his own lab at the Whitehead Institute rather than following the clinical path arising from his M.D. [00:20:50] Ken asks about how nutrients and other inputs are sensed and integrated by the mTOR complexes, given how one of the most fascinating aspects about mTORC1 is its role as a nutrient sensor. [00:23:46] Ken asks why both nutrients and growth factors are required to activate mTORC1. [00:25:54] Dawn mentions her interest in the connection of mTOR to aging, mentioning that mTOR inhibition through rapamycin or its analogues is currently one of the hottest topics in longevity research. She asks why mTOR inhibition appears to be life-extending? [00:30:38] Ken asks what the risks are of excess catabolism, when inhibiting mTOR pharmacologically, in terms of both health and longevity. [00:32:09] Dawn asks if there is data in humans suggesting that suppressing mTOR will extend longevity and healthspan, either pharmacologically, genetically or through diet and fasting. [00:34:23] Dawn asks where mTOR is made, both in terms of the individual cell, as well as specific tissues in the body. [00:35:02] Ken asks if there is a significant difference between the mTOR signaling in organs like the liver, skeletal muscle, and the brain. [00:37:07] Ken asks if tissue specific inhibition of mTOR is possible. [00:38:29] Dawnmentions how it is becoming clear that mTOR plays a role in a number of diseases, most notably is its connection to cancer. She goes on to say that in 2008, David’s team published a highly-cited paper in Science that described the role of Rag GTPases, key mTOR mediators, that sense the amino acid input of a cell. She asks David to discuss this discovery and about the importance of these enzymes in disease processes like cancer? [00:42:27] Ken mentions that protein intake and downstream mTOR activation is often said to be associated with dysregulated cell growth (i.e. cancer). He goes on to ask if there is there sufficient evidence to suggest that high levels of protein or amino acid intake are necessary or sufficient to produce clinically-meaningful cancers? [00:43:59] Dawn comments on the epidemic of type-2 diabetes in the United States, going on to say that the mTOR pathway is a known suppressor of the insulin-signaling pathway. She asks if mTOR modulators have the potential to make an impact in the diabetes world? [00:45:28] Ken asks if the potential negative effects of protein intake on mTOR activation and longevity might be mitigated in the presence of lower-blood glucose or insulin signaling, given the overlap between the insulin and IGF-1 signaling pathways. [00:46:31] Dawn opens the question by mentioning that late last year, the journal Cell Metabolism published two excellent papers related to ketogenic diets and the extension of healthspan and lifespan. She goes on to mention that the paper from Keith Baar’s group at UC Davis titled “A Ketogenic Diet Extends Longevity and Healthspan in Adult Mice” was of particular interest to several people at IHMC. The findings showed that in mouse models, researchers have seen extended longevity, cognitive protection, cancer reduction, improved strength and coordination, and immune rejuvenation. The paper from the team at UC Davis, showed a 13% increase in median life span for the mice on a high fat vs high carb diet. Ken jumps in and mentions that from his perspective the most important aspect is that those mice retained their quality of health well into later life. He mentions that it was particularly interesting that they found the ketogenic diet increasedprotein acetylation levels and regulated mTORC1signaling in a tissue-dependent manner. Specifically, the ketogenic diet increasedmTOR signaling in skeletal muscle and inhibited mTORC1 signaling in the liver. [00:48:59] Ken mentions that an interesting paper from a year or two ago showed that ketones, particularly BHB, greatly increased IGF-1 sensitivity in the muscle. [00:50:59] Dawn asks what the role of mTOR is in neurodegenerative diseases such as Alzheimer’s or Parkinson’s disease. [00:53:02] Dawn mentions that mTORC1 responds to intracellular and environmental stresses that are incompatible with growth, including hypoxia. She asks about the effect of hypoxia on mTORC1. [00:53:51] Dawn mentions that the amino acid, methionine, is widely appreciated to have interesting effects on animal physiology, and that diets low in methionine increase longevity and overall health. She goes on to mention a paper David and a group of his colleagues published in “Science” that described a potential molecular link between the effects of methionine restriction and mTOR1. She asks David to give a summary of that paper and his findings. [00:55:27] Ken mentions that several human pathologies are linked to mTOR hyperactivation such as TSC and epilepsy due to gene deletions. He goes on to ask about the consequences of gene deletion-induced mTOR hypoactivation and if it would it confer a longevity benefit. [00:57:02] Dawn comments on how a few years ago, David joined Peter Attia, our very first guests on STEM-Talk, and a few other friends to take a trip to Easter Island to explore the place where rapamycin was discovered. She asks if David could talk briefly about the story of the initial discovery and isolation of rapamycin on Easter Island, and how, at one point, it was only in Suren Sehgal’s hands and almost lost. [00:59:17] Dawn mentions how David has previously written about how caloric restriction might produce the best balance of mTORC1 inhibition and overall organismal health, asking David to expand on that idea since caloric restriction does not consistently produce life extension in animal models. [01:01:14] Dawn asks if rather than restricting nutrients in mass with caloric restriction, might it be possible to manipulate individual nutrients to extend human life. [01:02:11] Ken mentions that in mammalian cells, inhibition of mTOR results in reduced cell size as a result of reduced protein synthesis. He goes on to ask what the downstream effects are of this, and if it results in a specific phenotype when mammals are exposed to mTOR inhibitors. [01:03:40] Following up, Dawn comments that one might speculate that chronic inhibition of mTOR in humans would likely degrade a patient’s health and cause fragility due to immunosuppression and loss of muscle. She asks if intermittent use of low-dose rapamycin would help avoid these effects and some of the widely observed side effects such as mouth sores. [01:05:11] Ken asks if David would hypothesize about intermittent use of low-dose rapamycin and its potential to activate autophagy in a meaningful way. [01:06:04] Dawn mentions a story about David on the Whitehead Institute website that makes the point that rapamycin appears to have all the makings of a magic bullet for treating diseases involving the mTOR pathway. David, however, was quoted as saying “we’re not there yet”. [01:07:08] Ken asks if we know how long mTOR inhibition takes with rapamycin before mTORC2 is inhibited. [01:08:59] Ken mentions that one hears ofphysicians prescribing rapamycin off label for older patients. He goes on to say that there aren’t sufficient studies to justify this behavior, asking David’s for his thoughts on this. [01:10:13] Ken comments on how the captive mouse studies, where the subjects are fed a bad diet, is not too far removed from human life in cities in many ways. [01:11:06] Dawn asks David what evidence would have to be there to get him to consider taking rapamycin as a means for enhancing his own longevity. [01:12:31] Dawn closes the interview by mentioning that Davidwrote a piece for the Proceedings of the National Academy of Sciences Journal titled, “Twenty-Five Years of mTOR.” Toward the end of that article David wrote that his friends in the field laugh at him whenever he says he’s done with mTOR, that there’s nothing left to discover. Dawn then quotes what David had to say about this: “Whenever I feel like calling it quits, the laboratory conveniently makes a great discovery that piques our interest in a new facet of the pathway.” And while, mTOR may not regulate everything, he went on to say, there are enough mysteries in how it senses everything to keep him and his lab occupied for the foreseeable future. [01:13:07] Dawn closes the interview by asking David to describe some of these mysteries he and his colleagues are exploring.