Role of UNC45B in the regulation of skeletal muscle mass

Background Sarcopenia, the loss of skeletal muscle mass with aging, is a very common syndrome. Skeletal muscle accounts for a large percentage of the body mass and serves as a locomotive organ, and its loss of function increases the risk of disability, poor quality of life, and death. Therefore, understanding the mechanisms of sarcopenia is important to improve the quality of life in the elderly. It has been reported that the myosin chaperone UNC45B decreases with age in male rats and humans. Therefore, it is possible that the decrease in UNC45B is one of the causes of sarcopenia by increasing myosin misfolding, but the role of UNC45B in the skeletal muscle mass regulation is relatively unknown. Purpose To investigate the effects of knockdown (KD) and overexpression (OE) of UNC45B on myosin content and skeletal muscle mass. Hypothesis UNC45B KD decreases skeletal muscle mass, while OE increases skeletal muscle mass. Methods Young male C57BL/6J mice were divided into KD or OE groups. Mice of the KD group were injected with adeno-associated virus 6 (AAV6) encoding shRNA targeting UNC45B in the right triceps surae muscle, while mice of the OE group were injected with AAV6 encoding UNC45B. In both groups, control AAV6 vectors were injected into the left triceps surae muscle. The gastrocnemius and soleus muscles were excised 2 weeks after AAV6 injection. Result The weight of the gastrocnemius muscle increased with UNC45B KD but not with UNC45B OE. On the other hand, both UNC45B KD and OE did not change the weight of the soleus muscle. UNC45B KD, but not UNC45B OE, increased molecular chaperones HSP70 and HSP90 in the gastrocnemius muscle. Neither UNC45B KD nor OE altered myosin content in the gastrocnemius and soleus muscles. Muscle protein synthesis (measured by the SUnSET method) was increased in the gastrocnemius muscle with UNC45B KD but not with UNC45B OE. In both KD and OE, there was no change in the amount of p62, LC3-I, and LC3-II, which are related to autophagy in the gastrocnemius and soleus muscle. Discussion In this study, UNC45B KD increased HSP70 and HSP90. These compensatory responses might have contributed to maintaining muscle protein degradation and increasing muscle protein synthesis and muscle mass. In contrast, there was no significant change in the amount of myosin and its chaperones by the OE of UNC45B. The reason may be that UNC45B is sufficiently expressed for myosin in skeletal muscle of young mice and therefore overexpression of it has little effect on skeletal muscle. Conclusion UNC45B does not play a major role in the regulation of myosin content and skeletal muscle mass in young mice.