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 myotubes^1,2. 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.