The effect of rapamycin, an anti-aging drug, on the muscle, was puzzling for a decade. It seemed paradoxical that rapamycin, an inhibitor of mTOR component 1 (mTORC1), prevents muscle loss and sarcopenia. Yet, this is well established [1-5]. How is that possibly possible? The nutrient-sensing mTOR pathway increases protein synthesis and cellular mass growth. Furthermore, physical exercise, which causes muscle hypertrophy, activates mTOR. So, rapamycin, an inhibitor of mTOR component 1 (mTORC1), must seemingly prevent muscle gain.
A recent study challenged a dogma that the muscle size depends on mTORC1 . It showed that mTORC1 is dispensable for muscle size in adult mice . (We will discuss this important paper later). This may explain why rapamycin does not cause muscle loss. But still, how rapamycin can cause muscle gain.
Hyperplasia and hypertrophy
The mechanism of muscle gain is extremely complex; it’s not just protein synthesis. We should distinguish the muscle as an organ (the muscle) and muscle cells or muscle fibers. The size of the muscle is a multiplication of a number of fibers and their size. Exercise causes both hyperplasia (an increased number) and hypertrophy (an increased size) of cellular fibers. In other words, it increases the number of fibers, not only their size [7-10].
Hyperplasia depends on functional satellite (stem) muscle cells. In addition, Satellite cells can fuse with pre-existing fibers to increase their size (hypertrophy). Rapamycin rejuvenates satellite cells, or prevents senescence; thus, contributing to both hyperplasia and hypertrophy of the fibers. Second, rapamycin can prevent age-related loss (death) of muscle fibers associated with hyper-active mTOR. This may explain why immobilization-caused muscle atrophy in the elderly is poorly reversible: lost fibers cannot be later replaced by senescent satellite (stem) cells.
The muscle can be atrophic even if muscle fibers are hypertrophic when their number is low.
Rapamycin suppresses conversion from quiescence to senescence
In resting cells, chronic and excessive mTOR activation causes a cellular conversion from quiescence to senescence, a process known as geroconversion . Geroconversion is accompanied by cellular hypertrophy and hyperfunction (such as SASP). At the end stages, however, mTOR-dependent cellular hyperfunction and hypertrophy can eventually result in cell exhaustion and even cell death, as it is observed with beta-cells in diabetes. Thus hypertrophy and hyperfunction may eventually be shifted to tissue atrophy and loss of function. For example, activation of mTORC1 contributes to muscle atrophy, and rapamycin mitigates denervation-induced atrophy .
Geroconversion leads to permanent loss of the proliferative potential [12, 13]. The proliferative potential is the ability to restart proliferation on demand such as exercise, fiber loss and damage. The proliferative potential is a feature of quiescent cells . Rapamycin prevents geroconversion from quiescence to senescence. Thus, rapamycin maintains the proliferative potential, by keeping cells quiescent [12, 13].
Growth factors (GF) activate mTOR. Increased GF signaling in aged satellite cells leads to loss of quiescence and regenerative ability, satellite (stem) cell depletion . Senescent stem cells cannot form new muscle fibers, neither increase their size. Geroconversion leads to exhaustion of the stem cell pool. Geriatric muscle stem cells switch reversible quiescence into senescence [16, 17]. Rapamycin prevents loss of quiescence, restores autophagy (catabolism) and reverses senescence in geriatric satellite cells . mTORC1 signaling is increased in senescent muscle stem cells and impairs their regenerative function ,. Rapamycin promotes autophagy, regenerative potential, decreased apoptosis and senescence. mTORC1 is hyperactivated in sarcopenic muscle, and its inhibition of mTORC1 counteracts sarcopenia .
In sum, we can suggest four mechanisms of how rapamycin can prevent sarcopenia: (a) Rapamycin may prevent death of hypertrophic muscle cells that in immobilization and obesity, (b) prevent senescence and exhaustion of satellite stem cell, potentially allowing muscle to regenerate, (d) increase size fiber size, by preserving satellite cells that may then fuse with muscle fibers (c) sensitize fibers and satellite cells to signals, originated by physical exercise. Overactivated mTOR, via a feedback loop, blocks signaling in muscle cells , causing signal resistance, including insulin-resistance. Basal mTORC1 hyper-function in the elderly contributes to insulin resistance and resistance of skeletal muscle growth to exercise .
In fact, when mTOR is chronically activated, it cannot be induced by any stimuli further, and it is the induction that is most important for the growth program. Rapamycin “cleans” these pathways from feedback blocks, potentially rendering cells (such as stem and “healing” cells) responsive on demand, as suggested in 2008 .
Just as geroconversion is a continuation of cellular growth, organismal aging and its diseases are a continuation of developmental growth . Thus, both geroconversion/aging and growth are mTOR-dependent . Sarcopenia is associated with hyper-activated mTOR in the muscle  as an example for hyperfunction theory of aging [23, 25]. Note that mTOR is also hyper-activated in obesity, which is associated with sarcopenia .
In simpler words, chronic and excessive mTOR activation caused by nutrients and insulin may lead to muscle loss. When basal levels of mTOR activity are low, then acute mTOR activation, caused by exercise leads to muscle hypertrophy. But still, something was missing to explain why rapamycin does not counteract anabolic stimuli. For example, Rapamycin does not inhibit growth of skeletal muscles caused by androgens . A new study provided a missing link, demonstrating that mTORC1 is not actually needed to maintain muscle size and function in adult animals . Ham et al concluded that “complete inhibition of mTORC1 signaling in fully grown muscle leads to metabolic and morphological changes without inducing muscle atrophy even after 5 months. Maintenance of muscle size does not require mTORC1 signaling”. Even further, rapamycin inhibits only some, but not all, functions of mTORC1 : some activities of mTORC1 are relatively resistant to therapeutic concentrations of rapamycin .
Now we may discuss how to combine rapamycin treatment and physical exercise for maximum benefit; how to decrease obesity and achieve maximum weight loss without loss of lean mass; how to maximize anti-aging effects of rapamycin by physical exercise; how to achieve desirable weight loss by combining rapamycin, exercise and diet; how to potentiate physical exercise with rapamycin to increase the muscle. A potentially powerful combination – exercise, rapamycin and diet – needs to be investigated. I hope to continue this discussion soon…
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