Impact of alpha-skeletal actin but not alpha-cardiac actin on myoblast morphology

Muscle differentiation involves a profound change in cell cytoarchitecture. This is accompanied by extensive isoform replacement in which the major non-muscle isoforms of the actin filament system are replaced by their muscle isoform counterparts. We have tested whether the sequential expression of the actin isoforms is functionally significant by precociously expressing the two striated muscle actins (alpha-skeletal and alpha-cardiac) in mouse myoblasts. The human alpha-skeletal and alpha-cardiac actin genes were transfected into mouse C2 myoblasts and clones expressing the human genes at the highest level were identified. Expression of the human alpha-skeletal actin gene was low with the highest mRNA level found to be 4% of that in adult human skeletal muscle. Clones expressing alpha-cardiac actin accumulated the mRNA up to 13% of the level of alpha-skeletal actin in adult human skeletal muscle. Despite the low level of alpha-skeletal actin expression, myoblasts transfected with this gene displayed a profound decrease in cell spreading. In contrast, alpha-cardiac actin had no impact on cell spreading. Neither alpha-skeletal actin nor alpha-cardiac actin had any impact on the total actin protein pool nor on the levels of the high molecular weight tropomyosins. The organisation of actin and tropomyosin into stress fibres was similar between transfected and control cells. We conclude that precocious expression of alpha-skeletal actin, but not alpha-cardiac actin, compromises myoblast morphology but not the ability of the cell to assemble stress-fibre-like structures.