Antagonistic control of myofiber size and muscle protein quality control by the ubiquitin ligase UBR4 during aging

Sarcopenia is a degenerative condition that consists in the age-induced atrophy and functional decline of skeletal muscle cells (myofibers). A common hypothesis is that inducing myofiber hypertrophy should also reinstate myofiber contractile function but such model has not been extensively tested. Here, we find that the levels of the ubiquitin ligase UBR4 increase in skeletal muscle with aging, and that muscle-specific UBR4 loss rescues age-associated myofiber atrophy in mice. However, UBR4 promotes proteolysis by the proteasome and consequently UBR4 loss reduces the muscle specific force and accelerates the decline in muscle protein quality that occurs with aging in mice. Similarly, hypertrophic signaling induced via muscle-specific loss of UBR4 and of several other ubiquitin ligases consistently compromises muscle function and shortens lifespan in Drosophila by reducing protein quality control. Altogether, these findings indicate that ubiquitin ligases regulate in opposite fashion myofiber size and muscle protein quality control during aging.