Myonuclear dynamics during skeletal muscle aging

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Project Summary/Abstract Sarcopenia is a devastating skeletal muscle condition that occurs in advanced age and due to various chronic conditions. Despite the widespread prevalence of sarcopenia there are no treatment options and the mechanisms underlying this process are not completely understood. A major hallmark of skeletal muscle aging is a reduction in myofiber size, which can be controlled by the hundreds of myonuclei within a single myofiber. Myonuclei are accrued during development, and new nuclei can also be added in the adult through cellular fusion of muscle stem cells (MuSCs). The presence of hundreds of nuclei and the need to add more has led to questions if the pre-existing myonuclei are at their transcriptional ceiling and thus require the myofiber to add new nuclei for adaptations. To begin to understand the requirement for myonuclei to maintain muscle size, we generated a unique mouse model that allows titration of myonuclear numbers and utilized strategies to track specific myonuclear populations. Our recent studies showed that myonuclear numbers ultimately determine size of myofibers, but that myonuclei possess a transcriptional reserve capacity to increase biosynthetic output and maintain larger cytoplasmic volumes. While the compensatory adaptations in mice with reduced nuclear numbers were advantageous during development, they were associated with evidence of accelerated aging and muscle loss, leading to the hypothesis that loss of functional gene copy numbers is a contributor to sarcopenia. Indeed, by utilizing snRNA-seq technology, we detected altered myonuclear populations during mouse aging suggesting dysregulated transcription, which could be one mechanism to explain a reduction in gene copy numbers. In addition to altered transcription, another mechanism for reductions of gene copy numbers is if myonuclei are lost from the syncytium and not replaced by MuSC fusion, and it is known that MuSCs have reduced activity in aged muscle. Based on these preliminary data, we will utilize unique models and myonuclear tracking systems, to uncover the transcriptional reserve in myonuclei of aging myofibers, elicited either through dysregulated transcriptional profiles or myonuclear loss, and elucidate the link between such myonuclear infidelity and the development of sarcopenia. Specifically, we propose to: 1) understand the molecular and cellular consequences of reductions in myonuclear number during aging 2) molecularly dissect the mechanisms of activation of myonuclear transcriptional reserve during development and aging 3) determine if myonuclei turnover during homeostasis, aging, and atrophy. Successful completion of these studies will provide unique insight into the myonuclear control of sarcopenia and provide new knowledge that will identify new therapeutic strategies to combat muscle loss.