UNC45B Reduction With Aging: A Myofiber-Intrinsic Promoting Factor for Sarcopenia

Skeletal muscle mass and force decline with age, and the loss of muscle force precedes muscle atrophy. However, the underlying mechanisms remain unclear. Here, we investigated the role of the myosin co-chaperone, uncoordinated mutant number-45 myosin chaperone B (UNC45B), in regulating muscle mass and force. UNC45B expression decreased in mouse gastrocnemius muscle with age, particularly at 24 months old, and adeno-associated virus vector-mediated knockdown of Unc45b in 3-month-old mouse triceps surae muscle first reduced plantar flexor torque and then decreased gastrocnemius muscle mass. In addition, Unc45b knockdown in the triceps surae muscle resulted in lower bone mineral density. While maximum Ca²⁺-activated force in mechanically skinned fibers was not affected by Unc45b knockdown, Unc45b knockdown decreased the ratio of depolarization-induced force to the maximum Ca²⁺-activated force. We established tamoxifen-inducible skeletal muscle-specific Unc45b knockout (Unc45b imKO) mice to investigate whether the muscle atrophy and weakness due to the loss of Unc45b impacts metabolism and behavior. We found that Unc45b imKO reduced muscle mass and force at a whole-body level, but did not influence systemic glucose tolerance, insulin sensitivity, or the respiratory exchange ratio. However, Unc45b imKO mice reduced the amount of deeper non-rapid eye movement sleep, locomotor activity, and body temperature during the sleep phase. We conclude that UNC45B is essential for maintaining fast-twitch muscle mass and muscle force. In addition, Unc45b deficiency-mediated muscle loss is also associated with bone fragility, decreased body temperature, and impaired sleep quality.