Regulation of Myonuclear Turnover Dynamics in Multinucleated Muscle Cells

Project Summary Skeletal muscle is comprised of post-mitotic, multinucleated cells that are long-lived and must be maintained for the duration of an organism’s lifespan. Effective tissue maintenance requires the turnover of damaged proteins and organelles, including the nucleus. Recent findings clearly demonstrate that muscle stem cells continue to fuse into muscle fibers throughout adulthood, yet the number of myonuclei remains constant across the lifespan, suggesting that there must be an opposing mechanism to degrade myonuclei. However, no mechanism to selectively target and degrade a myonucleus has yet been identified, leading to the longstanding debate over whether entire myonuclei can be degraded and turned over. The project's overarching goal is to define the rate of myonuclear turnover in adult skeletal muscles, identify the mechanisms by which myonuclei are degraded, and unravel the targeting complexes and cellular pathways involved in this process. The central hypothesis is that accumulation of unrepairable nuclear damage leads to myonuclei undergoing turnover in adult muscle fibers through ‘nucleophagy’. This hypothesis stems from our preliminary data using a unique mouse model that exhibits significant myonuclear and DNA damage due to a loss-of-function in lamin A and C proteins. Using this model, our preliminary data demonstrates that nuclear damage results in enhanced myonuclear turnover, with damaged myonuclei being enriched in autophagic markers, but not apoptotic markers. The project comprises three specific aims: firstly, determining the rate of myonuclear turnover across different muscles and whether this rate changes with physical activity and aging; secondly, exploring whether myonuclear damage triggers turnover, utilizing transgenic mouse models to induce myonuclear-specific damage; and finally, investigating the mechanism behind myonuclear degradation, with a focus on nucleophagy. Findings from this project could fundamentally change how we think about myonuclear and genomic maintenance during skeletal muscle aging, with additional relevance for the long-term effectiveness of gene therapy treatments for muscular diseases.