Investigating the effects of calorie restriction on mitochondrial biology in adult and aged skeletal muscles

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With aging, skeletal muscle progressively loose mass and function, a process termed sarcopenia. Previous investigations, including ours, have shown that sarcopenia involves dysfunction of mitochondria, intracellular organelles playing key roles in muscle biology. These aging-related dysfunctions include (i) an impaired mitochondrial bioenergetics in vivo, caused at the cellular level by a decreased mitochondrial capacity to sense and respond to energy demand, or "affinity for ADP"; and (ii) an increase in mitochondrial-mediated cell death via apoptosis, caused by a sensitized mitochondrial permeability transition pore (mPTP). This proposal builds upon these recent discoveries, and uses one of the most efficient interventions to attenuate sarcopenia - calorie restriction (CR) - to investigate the role of mitochondria in sarcopenia. CR, which consists in limiting food intake per day, is believed to mediate its protective "anti-aging" effects by impacting mitochondrial function. However, many of the specific effects of CR on mitochondrial biology, especially in aged muscle, remain unclear or unknown. **In particular, three fundamental aspects of mitochondrial biology that have important implications for our understanding of sarcopenia require further work. First, whether CR affects mitochondrial affinity for ADP and mPTP function in adult skeletal muscles, and thus prevents the aging-related impairment in these parameters is unknown. Second, it is now well established that mitochondria form a dynamic network able to undergo fusion and fission events, where changes in mitochondrial morphology impact mitochondrial function, and vice versa. To date, the effects of aging on mitochondrial morphology and dynamics remain unclear, and those of CR are unexplored. Third, emerging evidence indicates that skeletal muscle aging might be associated with an impaired capacity to remove damaged mitochondria through the normal process of mitophagy, causing the accumulation of dysfunctional mitochondria. The effects of CR on mitophagic processes in aged skeletal muscles have currently never been investigated. **The present research program was designed to address these issues by (i) defining the effects of CR on crucial indices of mitochondrial function in adult and aged skeletal muscles, (ii) determining if the impact of aging and CR on mitochondrial function is linked to changes mitochondrial morphology/dynamics and (iii) defining if the protective effects of CR on muscle and mitochondrial function involve optimization of mitophagic processes. Methods that will be used include a non-disruptive approach where muscle cells are permeabilized, giving us access to morphologically intact mitochondria in their intracellular environment for functional measures; a quantitative electron microscopy approach enabling precise quantification of the mitochondrial network in 3-dimension; detailed muscle fiber-type analysis to reinforce our interpretation and contextualize our results; and dynamic flux assessments for quantifying mitophagy.**The proposed research program will generate new fundamental knowledge on how aging affects understudied aspects of mitochondrial biology. In addition, it will shed new light on the mechanisms by which CR impacts mitochondrial biology in skeletal muscle, and thereby exerts its protective effects in the context of sarcopenia. My research program will therefore enhance our current understanding of the mechanisms underlying sarcopenia. It will also provide novel knowledge on the regulation of mitochondrial biology - linking function, morphology and quality control processes - in skeletal muscle. Ultimately, the novel knowledge could find applications in the prevention of sarcopenia and other adverse events affecting skeletal muscle.