Sarcomeric Alpha‐actinin is a Myotendinous Junction Component in Neonatal Mice

Abstract

Myotendinous junctions (MTJs) are cell adhesion sites where new sarcomeres are added in series during longitudinal muscle growth. Currently, little is known about the developmental dynamics of this specialized region of the muscle cell. This study compared the ultrastructure and composition of MTJs from rapidly-growing neonatal mice, and adult mice whose muscles have reached a steady-state length. At neonatal MTJs membrane exhibits greatly reduced folding, myofibrils are immature and interspersed with unorganized cytoplasm, and the tendon is densely populated with mononucleated cells, most likely fibroblasts, many of which are in close association with the muscle cell. Immunolocalization of the nebulin-related protein N-RAP, which is localized exclusively to the terminal cytoskeletal filament bundles at MTJs in adult skeletal muscle, is also restricted to these sites in neonatal muscle. Vinculin and dystrophin were co-localized subjacent to the membrane, also at both adult and neonatal MTJs. Interestingly, sarcomeric α-actinin was localized to MTJ adhesion plaques in neonates, but was not detected at the adult MTJ. The variable presence of sarcomeric α-actinin at MTJs suggests that muscle fibers differentially regulate actin-membrane interactions during postnatal growth, either in response to developmental stage-specific differences in mechanical stress, or to accommodate serial sarcomere addition in neonates. Supported by a grant from the American Heart Association, and by the Dean of the UMKC School of Biological Sciences.