Abstract: Apelin Signaling Pathway In Muscle Regeneration And Rejuvenation PI: Madelaine Romain How extracellular signals and cell-to-cell communication dysregulation affect tissue homeostasis during aging are still poorly understood. The age associated muscle disease, sarcopenia, affects more than 60% of people over 80 years of age and results in mobility disorders and a significantly increased risk of mortality. Identifying dysregulated biological mechanisms involved in the etiology of sarcopenia is critical to develop therapeutic treatments to limit muscle atrophy. Recently, the expression of the apelin peptide has been shown to decline with age, correlating with an increased inflammation, senescence, and degeneration of the muscle tissue. Interestingly, treatments with the apelin peptide improve muscle regeneration and muscular function in aged animals, indicating a role in muscle regeneration and rejuvenation. However, the molecular and cellular mechanisms underlying apelin function are largely unknown, and the apelin dependent cellular crosstalk between muscle stem cells (MuSC), endothelial cells, and immune cells contributing to enhanced muscle regeneration are uncharacterized. Using the zebrafish as a model system, we will combine novel pharmacological genetic approaches to perform state of the art apelin loss and gain of function studies and use single cell RNA sequencing to identify downstream effectors of apelin that limit the impacts of aging. We propose to test the function of apelin as an anti-aging and pro-regenerative signaling peptide and decipher cell type specific functions of apelin during muscle aging and regeneration. This COBRE project includes two specific aims. During the first aim, we will use a recently discovered chemical agonist of the apelin receptors and newly generated Cre/Lox zebrafish transgenic lines to determine how chronic and genetically-induced muscle-derived apelin affects the hallmarks of muscle aging (atrophy, senescence, inflammation…). Novel CRISPR/Cas-9 zebrafish mutants will also us allow to test the function of the apelin receptor activation in MuSC, endothelial cells and macrophages to decipher the cell type-specific apelin functions during muscle aging and regeneration. Using state of the art single cell RNA-sequencing, aim 2 will reveal a set of genes regulating MuSC transition into proliferation after injury. Taking advantage of the apelin receptors mutant fish, this transcriptomic analysis should also lead to the identification of regenerative factors underlying apelin functions in MuSC-dependent regenerative response. Successful completion of these aims will define the role of the apelin signaling in multiple different cell types during muscle aging and reveal molecular and cellular therapeutic targets to alleviate or reverse age-associated sarcopenia.