Abstract
S100A1, a Ca2+-binding protein of the EF-hand type, is most highly expressed in striated muscle and has previously been shown to interact with the skeletal muscle sarcoplasmic reticulum (SR) Ca2+ release channel/ryanodine receptor (RyR1) isoform. However, it was unclear whether S100A1/RyR1 interaction could modulate SR Ca2+ handling and contractile properties in skeletal muscle fibers. Since S100A1 protein is differentially expressed in fast- and slow-twitch skeletal muscle, we used saponin-skinned murine Musculus extensor digitorum longus (EDL) and Musculus soleus (Soleus) fibers to assess the impact of S100A1 protein on SR Ca2+ release and isometric twitch force in functionally intact permeabilized muscle fibers. S100A1 equally enhanced caffeine-induced SR Ca2+ release and Ca2+-induced isometric force transients in both muscle preparations in a dose-dependent manner. Introducing a synthetic S100A1 peptide model (devoid of EF-hand Ca2+-binding sites) allowed identification of the S100A1 C terminus (amino acids 75-94) and hinge region (amino acids 42-54) to differentially enhance SR Ca2+ release with a nearly 3-fold higher activity of the C terminus. These effects were exclusively based on enhanced SR Ca2+ release as S100A1 influenced neither SR Ca2+ uptake nor myofilament Ca2+ sensitivity/cooperativity in our experimental setting. In conclusion, our study shows for the first time that S100A1 augments contractile performance both of fast- and slow-twitch skeletal muscle fibers based on enhanced SR Ca2+ efflux at least mediated by the C terminus of S100A1 protein. Thus, our data suggest that S100A1 may serve as an endogenous enhancer of SR Ca2+ release and might therefore be of physiological relevance in the process of excitation-contraction coupling in skeletal muscle.
Highlights
S100A1, a Ca2؉-binding protein of the EF-hand type, is most highly expressed in striated muscle and has previously been shown to interact with the skeletal muscle sarcoplasmic reticulum (SR) Ca2؉ release channel/ryanodine receptor (RyR1) isoform
Our study shows for the first time that S100A1 augments contractile performance both of fast- and slow-twitch skeletal muscle fibers based on enhanced SR Ca2؉ efflux at least mediated by the C terminus of S100A1 protein
RyR is the major channel for Ca2ϩ release from intracellular stores to cause an increase in myoplasmic Ca2ϩ concentration, resulting in muscle contraction
Summary
RyR, release channel/ryanodine receptor; skeletal muscle isoform; SR, sarcoplasmic reticulum; Soleus, M. soleus; EDL, M. extensor digitorum longus; aa, amino acid; ANOVA, analysis of variance; N, N-terminal; H, hinge region; C, C-terminal; pCa, Ϫlog[Ca2ϩ](M); SERCA, SR Ca2ϩ-ATPase. Based on novel insights gathered from structural analysis of S100A1 protein [14], a S100A1 peptide model consisting of the region amino acids 2–16 (Fig. 1C, N-terminal (N)), amino acids 42–54 (Hinge-region (H)), and amino acids 75–94 (C-terminal (C)) devoid of Ca2ϩ-binding motifs was synthesized to gain further insight into structurefunction relationship of S100A1/RyR1 interaction as well as to exclude adverse Ca2ϩ-buffering effects by the native protein [15, 16]. Further testing of single S100A1 domains identified the hydrophobic C-terminal extension (aa 75–94) as well as the hinge region (aa 42–54) to differentially affect SR function These effects are apparently based on enhanced SR Ca2ϩ release as S100A1 neither influenced SR Ca2ϩ uptake nor myofilament Ca2ϩ sensitivity/cooperativity in skeletal muscle fibers in our experimental setting. Our data suggest a putative physiological role for S100A1 to serve as an endogenous enhancer of SR Ca2ϩ release in skeletal muscle
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