CAVEOLIN-3 ASSOCIATES WITH DEVELOPING T-TUBULES DURING MUSCLE DIFFERENTIATION

Caveolae, flask-shaped invaginations of the plasma membrane, are parti cularly abundant in muscle cells. We have recently cloned a muscle-spe cific caveolin, termed caveolin-3, which is expressed in differentiate d muscle cells. Specific antibodies to caveolin-3 were generated and u sed to characterize the distribution of caveolin-3 in adult and differ entiating muscle. In fully differentiated skeletal muscle, caveolin-3 was shown to be associated exclusively with sarcolemmal caveolae. Loca lization of caveolin-3 during differentiation of primary cultured musc le cells and development of mouse skeletal muscle in vivo suggested th at caveolin-3 is transiently associated with an internal membrane syst em. These elements were identified as developing transverse-(T)-tubule s by double-labeling with antibodies to the alpha(1) subunit of the di hydropyridine receptor in C2C12 cells. Ultrastructural analysis of the caveolin-3-labeled elements showed an association of caveolin-3 with elaborate networks of interconnected caveolae, which penetrated the de pths of the muscle fibers. These elements, which formed regular reticu lar structures, were shown to be surface-connected by labeling with ch olera toxin conjugates. The results suggest that caveolin-3 transientl y associates with T-tubules during development and may be involved in the early development of the T-tubule system in muscle.