Overcoming muscle atrophy in a hibernating mammal despite prolonged disuse in dormancy: proteomic and molecular assessment

Prolonged disuse of skeletal muscle causes significant loss of myofibrillar contents, muscle tension, and locomotory capacity. However, hibernating mammals like bats appear to deviate from this trend. Although low functional demands during winter dormancy has been implicated as a factor contributing to reduced muscle loss, the precise mechanism that actively prevents muscle atrophy remains unclear. We explored proteomic and molecular assessments of bat muscle to test a hypothesis that expression levels of major myofibrillar proteins are retained during hibernation, with periodic arousals utilized as a potential mechanism to prevent disuse atrophy. We examined changes in myofibrillar contents and contractile properties of the pectoral or biceps brachii muscles of the bat Murina leucogaster in summer active (SA), hibernation (HB) and early phase of arousal (AR) states. We found the bat muscles did not show any sign of atrophy or tension reduction over the 3-month winter dormancy. Levels of most sarcomeric and metabolic proteins examined were maintained through hibernation, with some proteins (e.g., actin and voltage dependent anion channel 1) 1.6- to 1.8-fold upregulated in HB and AR compared to SA. Moreover, expression levels of six heat shock proteins (HSPs) including glucose-regulated protein 75 precursor were similar among groups, while the level of HSP70 was even 1.7-fold higher in HB and AR than in SA. Thus, considering the nature of arousal with strenuous muscle shivering and heat stress, upregulation or at least balanced regulation of the chaperones (HSPs) would contribute to retaining muscle properties during prolonged disuse of the bat.