Membrane events involved in myoblast fusion.

Abstract

Myoblast fusion has been studied in cultures of chick embryonic muscle utilizing ultrastructural techniques. The multinucleated muscle cells (myotubes) are generated by the fusion of two plasma membranes from adjacent cells, apparently by forming a single bilayer that is particle-free in freeze-fracture replicas. This single bilayer subsequently collapses, and cytoplasmic continuity is established between the cells. The fusion between the two plasma membranes appears to take place primarily within particle-free domains (probably phospholipid enriched), and cytoplasmic unilamellar, particle-free vesicles are occasionally associated with these regions. These vesicles structurally resemble phospholipid vesicles (liposomes). They are present in normal myoblasts, but they are absent in certain fusion-arrested myoblast populations, such as those treated with either 5-bromodeoxyuridine (BUdR), cycloheximide (CHX), or phospholipase C (PLC). The unilamellar, particle-free vesicles are present in close proximity to the plasma membranes, and physical contact is observed frequently between the vesicle membrane and the plasma membrane. The regions of vesicle membrane-plasma membrane interaction are characteristically free of intramembrane particles. A model for myoblast fusion is presented that is based on an interpretation of these observations. This model suggests that the cytoplasmic vesicles initiate the generation of particle-depleted membrane domains, both being essential components in the fusion process. KEY WORDS freeze-fracture membrane fusion myoblast fusion myogenesis plasma membranes unilamellar vesiclesThe multinucleated muscle fiber is generated during myogenesis by fusion of mononucleated myoblasts. Several ultrastructural studies (13,21,35,40) provided the basis for the current appreciation of the fusion process: cytoplasmic continuity is established between two fusing myoblasts as a result of the partial disappearance of the plasma membranes. Consequently, many of the studies related to myoblast fusion were focused on the plasma membrane and its involvement in the process; for example, there have been a number of studies on myoblast fusion that have been focused on the cell surface membrane proteins. The rationale for these studies has been that fusion must be regulated and carried out by cell surface membrane components, and the obvious candidate is a protein or group of proteins (3,5,9,11,24,35,36,39). Moreover, two processes that may be fundamentally different-(a) recognition between cells and (b) membrane fusionhave not been distinguished in most of these studies. These two processes occur independently in a wide variety of cellular systems; for example, recognition may be an important process in many differentiating multicellular systems where membrane fusion is not a consequence (23). Con-J. CELL BIOLO6Y 9 The Rockefeller University Press.

The Endocytic Recycling Protein EHD2 Interacts with Myoferlin to Regulate Myoblast Fusion

“…Topologically, the process of myoblast fusion is not well understood but has been observed that transient vesicular structures appear at the sites of membrane fusion (6). Our data suggest that these vesicles may derive from the endocytic recycling compartment.…”

“…In mammalian cells additional proteins that regulate myoblast fusion have been characterized, as well as many proteins implicated in myoblast differentiation (2)(3)(4). In both vertebrate and invertebrate systems, ultrastructural analysis of fusing myoblasts reveal the presence of vesicles accumulating at the sites of membrane fusion (5,6). Because vesicle fusion is a common feature of most cells, it is possible that aspects of the myoblast fusion machinery utilize pathways more commonly used by other cell types for intracellular trafficking.…”

Calcium-sensitive Phospholipid Binding Properties of Normal and Mutant Ferlin C2 Domains

“…These prefusion myoblasts were distinguished from quiescent myoblasts by their elongated appearance and an accumulation of myoferlin-containing vesicles. The fusion of myoblasts to myotubes is known to involve vesicular structures (35). Similarly, the repair of torn surface membranes uses a number of mechanisms that may also include vesicular structures (36).…”

Differentiation of skeletal muscle cells in culture.

“…Ultrastructural studies have suggested that fusion is the result of the partial disappearance of the plasma membrane between two adjoining cells (63,75,118). An increase in membrane fluidity precedes myoblast fusion (51,104), and perturbations which are expected to alter fluidity, alter the recognition and fusion processes (68,131).…”