Proteostasis of Heat Shock Protein HSP90 in Skeletal Muscles of the Long-Tailed Ground Squirrel during Hibernation

Changes in the content of heat shock protein 90 (HSP90) in m. soleus (contains mainly fibers expressing the “slow” isoform I MyHC) and m. gastrocnemius (contains mainly fibers expressing the “fast” isoforms II MyHC) of a true hibernant, the long-tailed ground squirrel (Urocitellus undulatus), during different periods of the annual cycle, summer activity (seasonal control), hypothermia/winter torpor, and winter (interbout) activity, were studied. It was found that despite the development of atrophic changes that were more pronounced in the “fast” m. gastrocnemius, the content of HSP90 in both muscles did not change throughout the hibernation period. The role of HSP90 in maintaining the stability of the titin giant sarcomeric protein molecules during the periods of the animal’s entry into and exit from hypothermia, when the activity of calpain proteases increased due to the increased content of Ca²⁺ in the cytosol of muscle cells, as well as during hypothermia, when the activity of calpains most likely was not completely inhibited, was discussed. During the winter/interbout activity, when there was an increased titin turnover in the striated ground squirrel muscles, a constant content of HSP90 was apparently necessary for the correct folding of newly synthesized titin molecules and their embedding into sarcomeres, as well as for the removal of improperly folded and old titin molecules/fragments, as well as other proteins. Thus, HSP90 proteostasis in skeletal muscles of the long-tailed ground squirrel could contribute to maintaining a stable level of titin and, possibly, other sarcomeric proteins during hibernation, which, in turn, would contribute to maintaining a highly ordered sarcomeric structure and the necessary level of contractile muscle activity in different phases of the hibernation–wakefulness cycle.