Defective EC coupling is partially responsible for impaired contractibility in hindlimb muscle of Stac3 knockout mice

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The SH3 and cysteine rich domain 3 (Stac3) gene was recently found to be exclusively expressed in skeletal muscle and essential for skeletal muscle contraction and postnatal life in mice. Data from Stac3 mutant diaphragm suggested that impaired muscle contractility was due to defective EC coupling. In this study, we compared the contractility of hindlimb muscle of embryonic day 18.5 Stac3 knockout and control (heterozygous) mice. Twitch responses were evident in control but undetectable in Stac3 mutant muscle. In response to frequent electrical stimulation, the maximum tension generated in Stac3 mutant muscle was ~20% of control. High [K + ]‐induced maximum tension in Stac3 mutant muscle was ~29% of control. The maximum tensions induced by caffeine and 4‐chloro‐m‐cresol in Stac3 mutant muscle were ~55% and ~58% of control, respectively. These data confirm a role for Stac3 in EC coupling, but also suggest that defective EC coupling is only partially responsible for impaired contractility in Stac3 mutant muscle. Myosin‐ATPase and NADH‐tetrazolium reductase staining revealed that hindlimb muscle of Stac3 mutant mice contained 60% more slow fiber‐like fibers than control. qPCR analyses revealed that fast muscle‐prevalent mRNAs were less abundant in Stac3 mutant muscle than in control. These and previously discovered structural abnormalities (centrally located myonuclei and streaming Z lines) in Stac3 mutant muscle further suggest that Stac3 may regulate skeletal muscle contraction through mechanisms in addition to mediating EC coupling.

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