Increased membrane fluidity precedes fusion of muscle cells.

Abstract

EMBRYONIC skeletal muscle cell cultures are highly suitable for demonstrating the dynamic role of the plasma membrane in cellular differentiation. A primary event in the overt development of muscle in culture is fusion of the plasma membranes of neighbouring mononucleated myoblasts to form multinucleated myotubes1,2. Previous studies of muscle fusion have focused on ultrastructural3 and electrophysiological4 aspects, the requirement for Ca2+ (ref. 5), and the importance of fusion for the subsequent biosynthesis of muscle-specific proteins5. But, understanding of this process has been hampered by the absence of reliable markers for the initiation of myoblast fusion4. In addition, the importance of the lipid properties of the cell membrane in regulation of the onset of muscle fusion has not been unambiguously demonstrated in spite of the well-established role of lipids in other cases of membrane fusion6. We examined the possibility that fusion of myoblasts is regulated by changes in the lipid fluidity of the cell membrane, as measured by the rotational diffusion of a fluorescent probe. We report here that, as a primary step in myogenesis in culture, mononucleated myoblasts undergo a sharp decrease in membrane microviscosity, and shortly afterwards fuse rapidly to form multinucleated myotubes. The microviscosity remains at a minimal value during the period of rapid cell fusion. The subsequent post-fusional differentiation of the muscle cell is accompanied by the regeneration of membrane rigidity.