Structural Adaptation of the Excitation–Contraction Coupling Apparatus in Calsequestrin1-Null Mice during Postnatal Development

SIMPLE SUMMARY: The efficiency of the contractile machinery in muscle fibers is affected by the adequate development of skeletal muscle. CASQ1 knockout impairs the correct assembly of the excitation–contraction coupling apparatus and functional properties. In this study, to obtain further clues on CASQ1 functions, we characterized the ultrastructural alterations and examined the maturation of calcium release units, as well as the involvement of different junctional proteins in juxtaposition of the membrane system in skeletal muscle fibers during development from birth to the adult age. In the calcium release units, junctional proteins directly linked to CASQ1 exhibit a modified expression pattern during postnatal development, resulting in a delayed maturation process. In summary, our findings demonstrate that CASQ1 is essential for the full maturation of calcium release units during postnatal development. The disruption caused by its absence affects the correct assembly and coordination of junctional proteins, resulting in impaired Transverse-tubule biogenesis and altered muscle morphology. This study enhances our understanding of the intricate mechanisms underlying calcium release units formation and highlights the critical role of CASQ1 in this process. ABSTRACT: The precise arrangement and peculiar interaction of transverse tubule (T-tubule) and sarcoplasmic reticulum (SR) membranes efficiently guarantee adequate contractile properties of skeletal muscle fibers. Fast muscle fibers from mice lacking calsequestrin 1 (CASQ1) are characterized by the profound ultrastructural remodeling of T-tubule/SR junctions. This study investigates the role of CASQ1, an essential component of calcium release units (CRUs), in the postnatal development of muscle fibers. By using CASQ1-knockout mice, we examined the maturation of CRUs and the involvement of different junctional proteins in the juxtaposition of the membrane system. Our morphological investigation of both wild-type (WT) and CASQ1-null extensor digitorum longus (EDL) fibers, from 1 week to 4 months of age, yielded noteworthy findings. Firstly, we observed that the absence of CASQ1 hindered the full maturation of CRUs, despite the correct localization of key junctional components (ryanodine receptor, dihydropyridine receptor, and triadin) to the junctional SR in adult animals. Furthermore, analysis of protein expression profiles related to T-tubule biogenesis and organization (junctophilin 1, amphiphysin 2, caveolin 3, and mitsugumin 29) demonstrated delayed progression in their expression during postnatal development in the absence of CASQ1, suggesting the impaired maturation of CRUs. The absence of CASQ1 directly impacts the proper assembly of CRUs during development and influences the expression and coordination of other proteins involved in T-tubule biogenesis and organization.