αv and α3 integrin subunits are associated with myofibrils during myofibrillogenesis

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Abstract
The development of the myofibrillar apparatus in skeletal muscle is a process in which transmembrane linkages with adhesion molecules are implicated. 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 sarcolemma. In this study we report that the αv integrin subunit is a prominent component of the costamere. The αv subunit is present initially on developing myotubes in a diffuse staining pattern with some concentration along nascent myofibrils. However, it appears in a striated pattern at the costamere and inconsistently at the M-line following the striation of α-actinin and titin but before that of desmin. Its recruitment to preformed striation suggests that it is incorporated into a pre-existing structure. The presence of αv in the costamere points to a role in lateral myofibrillar anchorage. In addition, we find that the α3 subunit is transiently associated with myofibrils along portions of their lengths and at their ends during myofibrillogenesis. The α3 subunit staining shows a novel localization and junctional structure. As myofibrils become striated the α3 integrin dissociates from the localized pattern and becomes diffuse. This suggests a possible role in the stabilization of nascent myofibrils prior to striation. Antibody-induced perturbation of adhesion mediated by the integrin β1 subunit in developing myotubes inhibits assembly of the sarcomeric architecture. The observations presented here also contribute to an emerging theme, where different integrins occupy unique junctional regions on muscle.