MEROSIN AND LAMININ IN MYOGENESIS - SPECIFIC REQUIREMENT FOR MEROSIN IN MYOTUBE STABILITY AND SURVIVAL

Laminin (laminin-l; alpha 1-beta 1-gamma 1) is known to promote myobla st proliferation. fusion, and myotube formation. Merosin (laminin-2 an d -4; alpha 2-beta 1/beta 2-gamma 1) is the predominant laminin varian t in skeletal muscle basement membranes: genetic defects affecting its structure or expression are the causes of some types of congenital mu scular dystrophy. However, the precise nature of the functions of mero sin in muscle remain unknown. We have developed an in vitro system tha t exploits human RD and mouse C2C12 myoblastic cell lines and their cl onal variants to study the roles of merosin and laminin in myogenesis. In the parental cells, which fuse efficiently to multinucleated myotu bes, merosin expression is upregulated as a function of differentiatio n while laminin expression is downregulated. Cells from fusion-deficie nt clones do not express either protein, but laminin or merosin added to the culture medium induced their fusion. Clonal variants which fuse , but form unstable myotubes, express laminin but not merosin. Exogeno us merosin converted these myotubes to a stable phenotype, while lamin in had no effect. Myotube instability was corrected most efficiently b y transfection of the merosin-deficient cells with the merosin alpha 2 chain cDNA. Finally, merosin appears to promote myotube stability by preventing apoptosis. Hence, these studies identify novel biological f unctions for merosin in myoblast fusion and muscle cell survival; furt hermore, these explain some of the pathogenic events observed in conge nital muscular dystrophy caused by merosin deficiency and provide in v itro models to further investigate the molecular mechanisms of this di sease.