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.

Hibernators like bats show only marginal muscle atrophy during prolonged hibernation. The current study was designed to test the hypothesis that hibernators use periodic arousal to increase protein anabolism that compensates for the continuous muscle proteolysis during disuse. To test this hypothesis, we investigated the effects of 3-month hibernation (HB) and 7-day post-arousal torpor (TP) followed by re-arousal (RA) on signaling activities in the pectoral muscles of summer-active (SA) and dormant Murina leucogaster bats. The bats did not lose muscle mass relative to body mass during the HB or TP-to-RA period. For the first 30-min following arousal, the peak amplitude and frequency of electromyographic spikes increased 3.1- and 1.4-fold, respectively, indicating massive myofiber recruitment and elevated motor signaling during shivering. Immunoblot analyses of whole-tissue lysates revealed several principal outcomes: (1) for the 3-month HB, the phosphorylation levels of Akt1 (p-Akt1) and p-mTOR decreased significantly compared to SA bats, but p-FoxO1 levels remained unaltered; (2) for the TP-to-RA period, p-Akt1 and p-FoxO1 varied little, while p-mTOR showed biphasic oscillation; (3) proteolytic signals (i.e., atrogin-1, MuRF1, Skp2 and calpain-1) remained constant during the HB and TP-to-RA period. These results suggest that the resistive properties of torpid bat muscle against atrophy might be attained primarily by relatively constant proteolysis in combination with oscillatory anabolic activity (e.g., p-mTOR) corresponding to the frequency of arousals occurring throughout hibernation.

In order to elucidate muscle functional changes by acute reloading, contractile and fatigue properties of the rat soleus muscle were investigated at three weeks of hindlimb suspension and the following 1 hr, 5 hr, 1 d, and 2 weeks of reloading. Compared to age-matched controls, three weeks of unloading caused significant changes in myofibrillar alignments, muscle mass relative to body mass (-43%), normalized tension (-35%), shortening velocity (+143%), and response times. Further significant changes were not observed during early reloading, because the transitional reverse process was gradual rather than abrupt. Although most of the muscle properties returned to the control level after two weeks of reloading, full recovery of the tissue would require more than the two-week period. Delayed recovery due to factors such as myofibrillar arrangement and fatigue resistance was apparent, which should be considered for rehabilitation after a long-term spaceflight or bed-rest.

Skeletal muscle undergoes a significant reduction in tension upon unloading. To explore intracellular signalling mechanisms underlying this phenomenon, we investigated twitch tension, the ratio of actin/myosin filaments, and activities of key signalling molecules in rat soleus muscle during a 3-week hindlimb suspension and 2-week reloading. Twitch tension and myofilament ratio (actin/myosin) gradually decreased during unloading but progressively recovered to initial levels during reloading. To study the involvement of stress-responsive signalling proteins during these changes, the activities of protein kinase C alpha (PKCalpha) and three mitogen-activated protein kinases (MAPKs)--c-Jun NH2-terminal kinase (JNK), extracellular signal-regulated protein kinase (ERK), and p38 MAPK--were examined using immunoblotting and immune complex kinase assays. PKCalpha phosphorylation correlated positively with the tension (Pearson's r = 0.97, P < 0.001) and the myofilament ratio (r = 0.83, P < 0.01) over the entire unloading and reloading period. Treatment of the soleus muscle with a PKC activator resulted in a similar paralleled increment in both PKCalpha phosphorylation and the alpha-sarcomeric actin expression. The three MAPKs differed in the pattern of activation in that JNK activity peaked only for the first hours of reloading, whereas ERK and p38 MAPK activities remained elevated during reloading. These results suggest that PKCalpha may play a pivotal role in converting loading stress to intracellular changes in contractile proteins that determine muscle tension. Differential activation of MAPKs may also help alleviate muscle damage, modulate energy transport and/or regulate the expression of contractile proteins upon altered loading.

Fine control of protein expression must be crucial for hibernators to promote energy conservation and survival during harsh winters. This study was aimed at investigating seasonal proteomic plasticity in order to evaluate physiological relevance of proteomic adjustments in the brain of the Murina leucogaster bat in three functional states: summer-active (SA), torpor (TR), and early phase of arousal (AR). Our two-dimensional electrophoresis and immunoblotting analyses revealed that 74% of identified neuronal, synaptic, metabolic, and stress proteins maintained stable levels throughout the three states. Proteins associated with axonal outgrowth and synaptic transmission (e.g., dihydropyrimidinase related protein-2 and N-ethylmaleimide sensitive fusion protein) and heme catabolism (biliverdin reductase B) were generally downregulated in TR and upregulated in AR. The levels of molecular chaperones such as heat shock protein 70 and glucose-regulated protein 78 remained unchanged over the three states. In parallel, glucose and lactate concentrations were relatively low in TR, whereas the glucose concentration was low but the lactate level was high in AR, implying metabolic stress due to arousal. These findings suggest that seasonal proteomic variability was observed mainly in proteins that functioned to regulate neural network, antioxidant activity, and neuroprotection in the hibernator brain.