mTOR-regulated senescence and autophagy during reprogramming of somatic cells to pluripotency: a roadmap from energy metabolism to stem cell renewal and aging.

Molecular controllers of the number and function of tissue stem cells may share common regulatory pathways for the nuclear reprogramming of somatic cells to become induced Pluripotent Stem Cells (iPSCs). If this hypothesis is true, testing the ability of longevity-promoting chemicals to improve reprogramming efficiency may provide a proof-of-concept validation tool for pivotal housekeeping pathways that limit the numerical and/or functional decline of adult stem cells. Reprogramming is a slow, stochastic process due to the complex and apparently unrelated cellular processes that are involved. First, forced expression of the Yamanaka cocktail of stemness factors, OSKM, is a stressful process that activates apoptosis and cellular senescence, which are the two primary barriers to cancer development and somatic reprogramming. Second, the a priori energetic infrastructure of somatic cells appears to be a crucial stochastic feature for optimal successful routing to pluripotency. If longevity-promoting compounds can ablate the drivers and effectors of cellular senescence while concurrently enhancing a bioenergetic shift from somatic oxidative mitochondria toward an alternative ATP-generating glycolytic metabotype, they could maximize the efficiency of somatic reprogramming to pluripotency. Support for this hypothesis is evidenced by recent findings that well-characterized mTOR inhibitors and autophagy activators (e.g., PP242, rapamycin and resveratrol) notably improve the speed and efficiency of iPSC generation. This article reviews the existing research evidence that the most established mTOR inhibitors can notably decelerate the cellular senescence that is imposed by DNA damage-like responses, which are somewhat equivalent to the responses caused by reprogramming factors. These data suggest that fine-tuning mTOR signaling can impact mitochondrial dynamics to segregate mitochondria that are destined for clearance through autophagy, which results in the loss of mitochondrial function and in the accelerated onset of the glycolytic metabolism that is required to fuel reprogramming. By critically exploring how mTOR-regulated senescence, bioenergetic infrastructure and autophagy can actively drive the reprogramming of somatic cells to pluripotency, we define a metabolic roadmap that may be helpful for designing pharmacological and behavioral interventions to prevent or retard the dysfunction/exhaustion of aging stem cell populations.