Reorganization of membrane cholesterol during membrane fusion in myogenesis in vitro: a study using the filipin-cholesterol complex.

Using filipin and freeze-fracture electron microscopy, we examined the distribution of membrane cholesterol during the fusion of myoblasts in vitro. The early stages of fusion were characterized by the depletion of cholesterol from the membrane apposition sites, at which the plasma membranes of two adjacent cells were in close contact. At first, filipin-cholesterol complexes were absent from the plasma membrane of one cell only and were distributed homogeneously on the membrane of the other cell. Eventually, both of the closely apposed membranes became almost completely free the filipin-cholesterol complexes. Membrane fusion took place at several points within the filipin-cholesterol complex-free areas. In later stages, the cytoplasms of the fusing cells became confluent by fenestration of the plasma membranes formed with the filipin-cholesterol complex-free regions. Our observations suggest that membrane cholesterol is reorganized at these fusion sites and that fusion initiated by the juxtaposition of the cholesterol-free areas of each plasma membrane of the adjacent cells.