| Skeletal muscle wasting can be a fatal complication of many diseases, such as cancer, AIDS and neuromuscular disorders. During wasting, the ubiquitin-proteasome system (UPS) is the primary pathway for the catabolism of myofibrillar proteins. Many studies have explored the importance of enzymes mediating the conjugation of ubiquitin to muscle proteins. However, the role of deubiquitinating enzymes in skeletal muscle wasting is poorly understood. Our laboratory previously identified USP19 as a deubiquitinating enzyme which is upregulated in response to atrophic stimuli in vivo and is capable of indirectly regulating the expression of myofibrillar proteins in muscle cells in vitro. However, the role of USP19 during skeletal muscle wasting in vivo remains unexplored. To address this question, I have characterized the phenotype of USP19 KO mice and determined the effects of denervation induced wasting of hindlimb muscles of USP19 KO mice by measuring various anatomical and structural parameters. Denervation stimulus was chosen to minimize number of animals used since control and treated limbs are within the same animal. USP19 KO mice had slightly heavier gastrocnemius muscle (GAS) mass than WT mice, with a trend towards a larger fiber size and higher protein content. After denervation, the skeletal muscle mass of KO mice is 30% heavier than that of denervated WT mice in both the tibialis anterior (TA) and GAS muscle. This significant sparing is correlated with KO having 30% more protein than WT in GAS muscles, after denervation. The level of the myofibrillar protein tropomyosin was significantly higher in denervated KO GAS than in denervated WT GAS, consistent with previous USP19 siRNA studies in rat skeletal muscle cells. Although, structural analysis of denervated GAS fiber reveals a small but significant sparing in the fiber size of KO mice as compared to WT mice, this difference only accounts for 5% of the mass sparing observed in KOs. Loss of neural stimulation in muscle is known to induce apoptosis during atrophy. My biochemical analysis showed that denervation results in less DNA loss in KO GAS muscles when compared to WT GAS muscles. In addition, there was a decrease in the levels of apoptotic markers, caspase-3 and Poly-ADP-Ribose-Polymerase (PARP) cleavage, in denervated KO GAS as compared to denervated WT GAS. Taken together, this evidence suggests that the muscle sparing occurs via a novel mechanism, where USP19 deletion promotes skeletal fiber survival after loss of neuronal stimulation. Therefore, USP19 could be a therapeutic target in the treatment of muscle wasting disorders. |
