Differential inhibition of myoblast fusion.

The aggregation and fusion of myoblasts in the presence of either metabolic inhibitors or alterations in the incubation medium or under conditions which result in structural changes in the cells was studied using previously described assays for the intercellular interactions of myoblasts in suspension [Knudsen, K. A., and Horwitz, A. F. (1977). Develop. Biol.58, 328]. These perturbations inhibit myoblast fusion differently. For example, energy poisons, prior trypsin or glutaraldehyde treatment, and inhibitors of protein or cholesterol synthesis all inhibit the Ca²⁺-mediated myoblast aggregation. In contrast, whereas myoblasts aggregate in the presence of 20 mM Mg²⁺, these aggregates are dispersed, even after 1–2 hr, with EDTA or trypsin. Furthermore, enriching the fatty acyl chains in elaidate or prior incubation of the myoblasts in the presence of cytochalasin B or colchicine results in aggregates which, after 1–2 hr, are dispersed by trypsin but not by EDTA. Aggregates of unaltered, control myoblasts, on the other hand, begin to show resistance to dispersion by trypsin after these times. These observations support the suggestion that multinucleate cell formation results from a sequence of events. The influence of these perturbations on cellular aggregation also provides some initial, tentative insight into the molecular mechanism of myoblast fusion. Recognition (calcium-mediated aggregate formation) appears to be mediated by a protein(s) that is turning over during the period of fusion competence, while membrane union (formation of aggregates resistant to dispersion by trypsin) most likely involves the direct participation of membrane lipid.