Abnormal features in skeletal muscle from mice lacking mitsugumin29

Physiological roles of the members of the synaptophysin family, carrying fo ur transmembrane segments and being basically distributed on intracellular membranes including synaptic vesicles, have not been established yet. Recen tly, mitsugumin29 (MG29) was identified as a novel member of the synaptophy sin family from skeletal muscle. MG29 is expressed in the junctional membra ne complex between the cell surface transverse (T) tubule and the sarcoplas mic reticulum (SR), called the triad junction, where the depolarization sig nal is converted to Ca2+ release from the SR. In this study, we examined bi ological functions of MG29 by generating knockout mice. The MG29-deficient mice exhibited normal health and reproduction but were slightly reduced in body weight. Ultrastructural abnormalities of the membranes around the tria d junction were detected in skeletal muscle from the mutant mice, i.e., swo llen T tubules, irregular SR structures, and partial misformation of triad junctions. In the mutant muscle, apparently normal tetanus tension was obse rved, whereas twitch tension was significantly reduced. Moreover, the mutan t muscle showed faster decrease of twitch tension under Ca2+-free condition s. The morphological and functional abnormalities of the mutant muscle seem to be related to each other and indicate that MG29 is essential for both r efinement of the membrane structures and effective excitation-contraction c oupling in the skeletal muscle triad junction. Our results further imply a role of MG29 as a synaptophysin family member in the accurate formation of junctional complexes between the cell surface and intracellular membranes.