Project Summary Tissue function declines with age which has deleterious effects that can be mitigated by exercise. The molecular causes of functional decline with age and the extent to which tissue aging can be mitigated by exercise is unclear. The reversal of cells to a younger epigenetic and phenotypic age is controlled by Yamanaka factors (OCT3/4, SOX2, KLF4, and MYC, or “OKSM”). In skeletal muscle, the most voluminous tissue in the body, MYC is the only Yamanaka factor induced by exercise. MYC also becomes less responsive to exercise with advancing age. The purpose of this proposal is to examine the role that MYC plays in skeletal muscle functional, metabolic, cellular, and molecular plasticity throughout the lifespan. To answer our research questions, we developed a mouse model that allows for pulsatile control of MYC specifically in skeletal muscle fibers simultaneous with fluorescent labeling of muscle fiber nuclei (myonuclei) for purification and downstream analyses. We also developed a novel murine model of voluntary exercise for aged mice. We will perform: 1) functional, bioenergetic, and cellular analyses, 2) single myonuclear RNA-sequencing, and 3) global DNA methylation and methylation clock analyses in myonuclei after weekly pulsatile MYC induction and exercise throughout the lifespan. We hypothesize that MYC induction in muscle will mimic functional and cellular aspects of exercise adaptation throughout the lifespan and amplify the effects of exercise training. MYC will mediate youthfulness at several molecular levels, including biological aging determined by DNA methylation “clock” age. Furthemore, we hypothesize that MYC induction late in life is sufficient to reverse molecular and cellular aspects of muscle aging. Our experiments will provide fundamental information on the role of MYC in skeletal muscle and its capacity for epigenetic and transcriptional age reversal in myonuclei. We expect that our innovative approaches and comprehensive hypothesis-driven -omics analyses will serve as a foundation for understanding skeletal muscle mass regulation with aging, and provide new directions for exploring what mediates the age-defying effects of exercise.

Project Narrative Exercise is a powerful stimulus for attenuating age-associated tissue decline, but the molecular mechanisms behind this are incompletely understood. Using muscle functional, bioenergetic, and cellular measures as well as nucleus-specific DNA methylation aging analysis and single myonuclear RNA-sequencing, the purpose of this proposal is to provide fundamental information on what mediates skeletal muscle molecular reprogramming toward youthfulness. Combined with a conditional and inducible muscle-specific gene expression model and a new murine exercise training approach, we hypothesize that the exercise-responsive Yamanaka factor MYC plays a central role in promoting a youthful tissue profile during aging and can be leveraged to improve muscle health throughout the lifespan.