TORwards a Victory Over Aging

For over two thousand years, tales of a so-called “Fountain of Youth” have reflected the human desire to extend life span and health span. The discovery that calorie restriction (CR) can extend the life span of rodents and even non-human primates has excited the imagination and provided valuable insights into the aging process, yet is of limited clinical use due to the inability of most people to adhere to such an abstemious diet. A decade ago, the National Institute on Aging Interventions Testing Program identified rapamycin, an inhibitor of the protein kinase mTOR (mechanistic Target Of Rapamycin) purified from bacteria discovered on Easter Island in the 1970s, as capable of extending the life span of mice (1). While not the proverbial fountain of youth, the ability of rapamycin to extend life span when treatment was started in middle age was a clear demonstration that developing pharmaceuticals to extend human life span might truly be possible. Since then, numerous studies from multiple labs have demonstrated that rapamycin can extend the life span of mice, even if given transiently or intermittently, with short-term rapamycin treatment also protecting or rejuvenating rodent tissues (2). In this special issue, we have collected a series of primary papers and reviews at the cutting edge of research into understanding the potent biological effects of rapamycin and how these can be translated into effective therapeutic options for age-related disease.

The mTOR protein kinase is found in two discrete complexes, mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), that are responsive to different environmental stimuli and hormonal cues, and which affect independent downstream signaling pathways by phosphorylation of different substrates. mTORC1 is acutely sensitive to amino acids and glucose, and inhibition of mTORC1, either pharmaceutically or genetically, extends life span in many species. Deletion of TOR1 is epistatic with CR in yeast (3), and thus it has been suggested that mTORC1 may be a critical effector of CR. Following this rationale, rapamycin, which acutely inhibits mTORC1, has been proposed as a CR mimetic. Here, Unnikrishnan and colleagues review the evidence for this proposition, concluding on the basis of their molecular impact and on the life span of some mouse models that rapamycin likely is not a mimic of CR, though both may affect many of the same molecular pathways (4).

Rapamycin has a number of side effects in humans, including metabolic and immunological, that have limited enthusiasm for its widespread utilization in humans for diseases of aging. Lamming and colleagues discovered in 2012 that at least some of the side effects of prolonged rapamycin treatment were mediated not by inhibition of mTORC1, but by “off-target” inhibition of mTORC2, and they proposed that mTORC1-specific inhibition could potentially minimize or avoid the side effects of prolonged rapamycin treatment (5). Here, Dumas and Lamming review recent progress in selectively inhibiting mTORC1 using intermittent dosing regimens, dietary strategies, and novel compounds (6). Emblematic of how rapidly progress is being made in these areas, two new mTORC1-selective inhibitors have been identified since publication of the review (7,8).

Low-protein diets promote longevity and metabolic health in rodents as well as humans (9–12), perhaps in part by reducing the activity of mTORC1 (12,13), which is exquisitely sensitive to amino acids. The three branched-chain amino acids (BCAAs), leucine, isoleucine, and valine, are strong agonists of mTORC1 signaling, and BCAA levels are associated with diabetes and insulin resistance in rodents and humans (14). The BCAAs, but not other essential amino acids, have a unique impact on metabolic health and glucose homeostasis in mice (9,15). Here, Juricic and colleagues find that restriction of BCAAs extends the life span of flies and ameliorates age-related pathologies in flies; however, restriction of three other essential amino acids had equivalent geroprotective effects and also inhibited mTORC1 (16). The authors suggest that dietary level of the most limiting essential amino acids, rather than the identity of those amino acids, is what inhibits mTORC1 and promotes longevity and health. If these results also apply to mammals, diets which restrict limiting essential amino acids could prove to be geroprotective.

The molecular mechanisms by which rapamycin extends life span are unknown, but as mTOR is a key regulator of both protein translation and autophagy, it has been thought that rapamycin may promote proteostasis (17). Two studies in this issue dissected how proteostasis is modified by rapamycin alone or by a combination of both rapamycin and metformin (18,19). The combination of these two molecules was recently shown to extend life span in mice to a greater extent than treatment with rapamycin alone (20). In one study, Wolff and colleagues show in a skeletal muscle cell culture model that, contrary to their expectations, both treatments actually slowed protein breakdown relative to untreated cells (18). While rapamycin was found to promote proteostasis, cells co-treated with metformin did not have improved proteostasis including no significant impact on autophagy, suggesting that improved proteostasis may not be responsible for the beneficial effects of rapamycin on health. In a second study, Reid and colleagues addressed a similar question in vivo; that is, does rapamycin, or co-treatment of rapamycin and metformin, affect proteostasis in the brain of mice (19)? Using an 8-week intervention started either in young or older adult mice, the authors found effects of these treatments on protein synthesis rates that were dependent on both sex and age at time of intervention. Interestingly, the molecular inhibition of mTOR signaling with rapamycin was blunted when treatment was begun in older mice. As shown previously, such later-life intervention with rapamycin is sufficient to extend mouse life span (1,21), raising the intriguing possibility that the extent to which mTOR inhibition slows aging may vary between tissues.

A well-established effect of rapamycin treatment in mice is glucose intolerance resulting in part from hepatic insulin resistance (5,22). However, the effect of rapamycin on organismal insulin sensitivity, as measured by the response of blood glucose to exogenous insulin has been more controversial, with groups alternately demonstrating that rapamycin improves, has no effect on, or impairs organismal insulin sensitivity (5,23–25). Interpreting these varied results has been difficult, as strain, sex, and rapamycin dosing protocols differ greatly between the studies. In this issue, Reifsnyder and colleagues survey the effect of diet-delivered rapamycin on glucose homeostasis in nine inbred strains, and demonstrate that genetic background influences the metabolic response to rapamycin (26). The results highlight the importance of genetic background in studies involving rapamycin, and suggest that a precision medicine approach may be important in the clinical use of mTOR inhibitors as geroprotectors.

While the role of mTOR (and the effect of rapamycin and rapamycin analogs (rapalogs) has now been studied in the context of several age-related pathologies, relatively little attention has been paid to bone aging. Osteoporosis is a widespread condition with significant health and economic impacts for the aged. One issue at the root of osteoporosis is fat infiltration-mediated lipotoxicity in the bone marrow. In this issue, Al Saedi and colleagues tested whether rapamycin, through the induction of autophagy, could protect osteoblasts from such lipotoxicity (27). They show that rapamycin protected osteoblasts in culture against apoptosis caused by high concentrations of palmitic acid, likely through induction of autophagy, or even more specifically mitophagy. These data are then offered in support of a potentially novel therapeutic approach to osteoporosis.

Lastly, while it is still unclear how relevant genetic mouse models of “accelerated aging” are to “normal” aging (28,29), progeroid mouse models are clearly appropriate as models of specific age-related diseases. In this issue, Lee and colleagues review the existing literature on the role of mTORC1 in the etiology of disease in such mouse models and the extent to which mTOR inhibition may prevent or alleviate pathology (30). In their discussion of models of neurodegeneration, cancer, metabolic dysfunction, and cellular dysregulation, they thoroughly categorize the benefits and limitations of the models as well as the potential for mTOR inhibition as a translational approach to treat the diseases which they model. Overall, this discussion highlights how findings in these mouse models may lead to the development of potential therapeutic treatments not only for rare diseases, but also common pathologies of aging.

In summary, this special issue highlights the extent to which a seminal finding from the NIA’s Interventions Testing Program 10 years ago has made a dramatic impact on the direction of aging research as well as our fundamental understanding of the aging process. In these primary papers and reviews, this issue confirms the breadth to which science driven by the desire to understand the geroprotective powers of rapamycin continues to influence aging research. The continuous expansion of our knowledge regarding how age-related conditions are influenced by mTOR signaling in many species and disease models, and the continued pursuit of safe, efficacious ways to reduce mTORC1, signaling confirm the value that persists in exploring the regulation of aging by mTOR.

Conflict of Interest

D.W.L. has received funding from, and is a scientific advisory board member of, Aeovian Pharmaceuticals, which seeks to develop novel, selective mTOR inhibitors for the treatment of various diseases.