A Temporal Examination of Cytoplasmic Ca2 + Levels, Sarcoplasmic Reticulum Ca2 + Levels, and Ca2 + -Handling-Related Proteins in Different Skeletal Muscles of Hibernating Daurian Ground Squirrels

Zhe Wang,Yun-Fang Gao,Jie Zhang,Xiu-Feng Ma,Shen-Hui Xu,Hui Chang,Hui-Ping Wang,Xin Peng

doi:10.3389/fphys.2020.562080

Abstract

To explore the possible mechanism of the sarcoplasmic reticulum (SR) in the maintenance of cytoplasmic calcium (Ca2+) homeostasis, we studied changes in cytoplasmic Ca2+, SR Ca2+, and Ca2+-handling proteins of slow-twitch muscle (soleus, SOL), fast-twitch muscle (extensor digitorum longus, EDL), and mixed muscle (gastrocnemius, GAS) in different stages in hibernating Daurian ground squirrels (Spermophilus dauricus). Results showed that the level of cytoplasmic Ca2+ increased and SR Ca2+ decreased in skeletal muscle fiber during late torpor (LT) and inter-bout arousal (IBA), but both returned to summer active levels when the animals aroused from and re-entered into torpor (early torpor, ET), suggesting that intracellular Ca2+ is dynamic during hibernation. The protein expression of ryanodine receptor1 (RyR1) increased in the LT, IBA, and ET groups, whereas the co-localization of calsequestrin1 (CSQ1) and RyR1 in GAS muscle decreased in the LT and ET groups, which may increase the possibility of RyR1 channel-mediated Ca2+ release. Furthermore, calcium pump (SR Ca2+-ATPase 1, SERCA1) protein expression increased in the LT, IBA, and ET groups, and the signaling pathway-related factors of SERCA activity [i.e., β-adrenergic receptor2 protein expression (in GAS), phosphorylation levels of phospholamban (in GAS), and calmodulin kinase2 (in SOL)] all increased, suggesting that these factors may be involved in the up-regulation of SERCA1 activity in different groups. The increased protein expression of Ca2+-binding proteins CSQ1 and calmodulin (CaM) indicated that intracellular free Ca2+-binding ability also increased in the LT, IBA, ET, and POST groups. In brief, changes in cytoplasmic and SR Ca2+ concentrations, SR RyR1 and SERCA1 protein expression levels, and major RyR1 and SERCA1 signaling pathway-related factors were unexpectedly active in the torpor stage when metabolic functions were highly inhibited.

Highlights

Calcium (Ca2+) is homeostatically controlled in mammals (Thomas et al, 1996)
Our results showed that the co-localization levels of SLN and sarco/endoplasmic reticulum Ca2+ ATPase isoform 1 (SERCA1) did not change in the three skeletal muscles in any group, suggesting that SLN might not be involved in the regulation of SERCA activity during these periods
The cytoplasmic Ca2+ level in skeletal muscle fiber increased, whereas the sarcoplasmic reticulum (SR) Ca2+ level decreased during late torpor

Summary

Introduction

Prolonged skeletal muscle disuse (e.g., during spaceflight, hindlimb unloading, and bed rest) can lead to disturbance of intracellular Ca2+ homeostasis, mainly exhibited by cytoplasmic Ca2+ overload (Ingalls et al, 2001; Wu et al, 2012; Hu et al, 2017). Cytoplasmic Ca2+ overload can activate the calpain protein degradation system and promote skeletal muscle protein degradation, which is an important mechanism leading to skeletal muscle atrophy (Goll et al, 2003; Gao et al, 2018). Studies on the potential mechanisms involved in Ca2+ homeostasis in the skeletal muscle fibers of hibernators are of great significance for revealing the mechanism controlling disuse-induced skeletal muscle atrophy

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Results

Discussion

Conclusion