Distinct roles of UVRAG and EGFR signaling in skeletal muscle homeostasis.

OBJECTIVE: Autophagy is a physiological self-eating process that can promote cell survival or activate cell death in eukaryotic cells. In skeletal muscle, it is important in the maintenance of muscle mass and function that is critical to sustain mobility and regulate metabolism. UV radiation resistance-associated gene (UVRAG) regulates early stages of autophagy and autophagosome maturation, while also playing a key role in endosomal trafficking. This study investigated the essential in vivo role of UVRAG in skeletal muscle biology.

METHODS: To determine the role of UVRAG in skeletal muscle in vivo, we generated muscle specific UVRAG knock-out mice using the cre-loxP system driven by Myf6 promoter that is exclusively expressed in skeletal muscle. Myf6- Cre+ UVRAGfl/fl (M-UVRAG-/- ) mice were compared to littermate Myf6-Cre+ UVRAG+/+ (M-UVRAG+/+ ) controls under basal conditions on normal chow diet. Body composition, muscle function and mitochondria morphology were assessed in muscles of WT and KO mice at 24 weeks of age.

RESULTS: M-UVRAG-/- mice developed accelerated sarcopenia and impaired muscle function compared to M-UVRAG+/+ littermates at 24 weeks of age. Interestingly, these mice displayed improved glucose tolerance and increased energy expenditure likely related to up-regulated Fgf21, a marker of muscle dysfunction. Skeletal muscle of M-UVRAG-/- mice showed altered mitochondrial morphology with increased mitochondrial fission, as well as EGFR accumulation reflecting defects in endosomal trafficking. To determine whether increased EGFR signaling had a causal role in muscle dysfunction, mice were treated with an EGFR inhibitor, gefitinib, which partially restored markers of muscle and mitochondrial deregulation. Conversely, constitutively active EGFR transgenic expression in UVRAG deficient muscle led to further detrimental effects with non-overlapping distinct defects in muscle function, with EGFR activation affecting muscle fiber type whereas UVRAG deficiency impaired mitochondrial homeostasis.

CONCLUSIONS: Our results show that both UVRAG and EGFR signaling are critical in the maintenance of muscle mass and function with distinct mechanisms in the differentiation pathway.