PROJECT SUMMARY/ABSTRACT The objectives of this proposal are to (1) obtain the experimental skills and career training necessary to develop a translational research program investigating mechanisms underlying skeletal muscle wasting and (2) generate the data necessary to determine the feasibility of targeting senescent cells to restore muscle size and function. Sarcopenia is a debilitating age-related skeletal muscle wasting syndrome associated with poor quality of life and high health care utilization. The etiology of sarcopenia is not fully understood, and there are currently no effective treatments. Identifying the processes mediating sarcopenia is critical for developing pharmacologic therapies. Senescent cells accumulate with age and at the anatomical sites of disease. Thus, they are regarded as a logical therapeutic target. Indeed, in mice, the targeted elimination of senescent cells improves function and parameters of health in multiple tissues. Our preliminary data show (1) senescent post- mitotic muscle fibers accumulate in sarcopenic mice, (2) the cyclin-dependent kinase inhibitor p21 is a critical regulator of muscle fiber senescence, and (3) a subset of myonuclei within multi-nucleated muscle fibers are uniquely vulnerable to senescence induction. I will use mouse models that allow for muscle fiber-specific labeling and modulation of p21 to advance our understanding for muscle fiber senescence and examine the direct contribution of senescence to muscle dysfunction. In Aim 1, I will overexpress p21 in muscle fibers in vivo and apply well-validated markers of senescence together with transcriptome-wide analyses to determine the core properties of muscle fiber senescence and identify regulatory processes that may serve as therapeutic targets. In Aim 2, I will use gain-of-function and loss-of-function p21 models and assess measures of muscle health, function, and size to determine the clinical relevance of senescent cell burden in skeletal muscle. In Aim 3, I will use myonuclear identification and isolation techniques together multiomic profiling to examine differences between senescent and non-senescent subsets of myonuclei to gain new insight into the factors regulating myonuclear senescence. The K99 phase will be conducted at Mayo Clinic and will focus on obtaining mentored training in methods required to complete the proposed aims. The R00 phase will be conducted in my independent lab and will focus on analyzing mouse tissues and data, publishing findings, and developing an R01 application based on these results. This proposal synergizes new skills in advanced senescent cell identification, imaging, bioinformatics, and drug target discovery techniques to create a research trajectory that is distinct from my mentors’ foci. This work will produce a robust foundation for an independent research career elucidating cellular mechanisms of skeletal muscle dysfunction with the aim of translating these findings into therapies to improve human health and well-being.

PROJECT NARRATIVE Sarcopenia is a debilitating age-related skeletal muscle wasting syndrome associated with poor quality of life and high health care utilization. Identifying the mechanisms underpinning sarcopenia is critical for developing pharmacologic therapies to enhance late-life health and function. The proposed work will interrogate the extent to which cellular senescence, a central aging mechanism, mediates skeletal muscle dysfunction and thus, may reveal a new druggable target for treatment or prevention of sarcopenia.