EXPRESSION AND LOCALIZATION OF THE PHOSPHOGLUCOMUTASE-RELATED CYTOSKELETAL PROTEIN, ACICULIN, IN SKELETAL-MUSCLE

Recently, a 60/63 kDa cytoskeletal protein, highly homologous to the g lycolytic enzyme phosphoglucomutase (PGM 1), was isolated from smooth muscle tissue and shown to localize in various adherens-type junctions of muscle and some nonmuscle cells. Since this protein, tentatively n amed 'aciculin', was enriched in muscle tissues and cells, we have att empted to study its expression and localization during myodifferentiat ion. C2C12 mouse myoblasts did not express any aciculin before cell fu sion in culture. Immediately after cell fusion aciculin became detecta ble and its content continued to rise during myotube maturation. In ea rly myotubes aciculin appeared first at cell tips and was predominantl y localized to focal adhesions of immature myotubes. As myotubes matur ed in culture, aciculin became associated with growing myofibrils, and finally was found redistributed in striations, corresponding to sarco mere Z-discs. Immunoblotting showed that aciculin content in chicken b reast skeletal muscle remained very low until day 11 of embryogenesis, but significantly increased in late prenatal and early postnatal deve lopment. By immunofluroescence, aciculin was not revealed in thigh ske letal muscle of day 11 chicken embryos, but was prominently localized at myotendinous junctions in thigh muscle of day 16 embryos. Myotendin ous junctions appeared to be major sites of aciculin accumulation in d eveloping and mature skeletal muscle fibers in vivo, suggesting some r ole for this protein in thin filament-membrane interactions and, poten tially, in force transmission at these cell-matrix contacts. In adult skeletal muscle faint aciculin staining appeared at the sarcolemma and as striations in register with Z-discs. Since the protein was not ide ntified in glycerinated myofibrils but was localized to striations in C2C12 myotubes and within the limited areas on skeletal muscle tissue sections, we conclude that aciculin is a component of skeletal muscle costameres. In cultured C2C12 myotubes we found some codistribution of aciculin with clusters of acetylcholine receptors, suggesting its pre sence at neuromuscular junctions. However, we did not detect any signi ficant concentration of aciculin at neuromuscular junctions in both em bryonic and adult skeletal muscle. Taken together, our data show that aciculin expression in skeletal muscle is differentiation-dependent an d upregulated during muscle development, and that this novel cytoskele tal protein is a component of various cell-matrix adherens junctions i n muscle cells.