ALPHA(V) AND ALPHA(3) INTEGRIN SUBUNITS ARE ASSOCIATED WITH MYOFIBRILS DURING MYOFIBRILLOGENESIS

The development of the myofibrillar apparatus in skeletal muscle is a process in which transmembrane linkages with adhesion molecules are im plicated, Integrins are one class of transmembrane adhesion receptors which appear to mediate these interactions. Two prominent linkages are at the myotendinous junction (MTJ), which resides at the ends of the cell and connects myofibrils to the tendon, and the costameres, which encircle the girth of the cell and connect the Z-disks to the sarcolem ma, In this study we report that the alpha(v) integrin subunit is a pr ominent component of the costamere. The alpha(v) subunit is present in itially on developing myotubes in a diffuse staining pattern with some concentration along nascent myofibrils, However, it appears in a stri ated pattern at the costamere and inconsistently at the M-line followi ng the striation of alpha-actinin and titin but before that of desmin, Its recruitment to preformed striation suggests that it is incorporat ed into a pre-existing structure. The presence of alpha(v) in the cost amere points to a role in lateral myofibrillar anchorage, In addition, we find that the alpha(3) subunit is transiently associated with myof ibrils along portions of their lengths and at their ends during myofib rillogenesis, The alpha(3) subunit staining shows a novel localization and junctional structure. As myofibrils become striated the alpha(3) integrin dissociates from the localized pattern and becomes diffuse, T his suggests a possible role in the stabilization of nascent myofibril s prior to striation, Antibody-induced perturbation of adhesion mediat ed by the integrin beta(1) subunit in developing myotubes inhibits ass embly of the sarcomeric architecture. The observations presented here also contribute to an emerging theme, where different integrins occupy unique junctional regions on muscle.