Increased membrane fluidity precedes fusion of muscle cells

Undifferentiated and differentiated L6 myoblast plasma membranes. I: Comparison of the morphology of plasma membrane vesiculation and the factors influencing the vesiculation process

Monolayer cultures of a variety of cell lines can be induced to shed plasma membrane vesicles by exposure to 25 mM formaldehyde--2 mM dithiothreitol. An analysis of the process of membrane shedding can provide a method to probe the general characteristics of the plasma membrane in different cell populations. We have, therefore, analyzed membrane shedding in L6 myoblasts at different stages of differentiation. The results show that the morphology of membrane shedding is comparable in undifferentiated or differentiated L6 myoblasts, as are the factors that influence the process of membrane shedding. L6 myoblasts at various stages of differentiation have similar requirements for metabolic energy, physiological temperature, pH, and monovalent and divalent cations.

Plasma membrane phospholipid, cholesterol and fatty acyl composition of differentiated and undifferentiated L6 myoblasts

The lipid composition of plasma membranes isolated from differentialted and undifferentiated L6 myoblasts have been compared. In general, the plasma membranes of differentiated L6 myoblasts have a higher cholesterol to phospholipid molar ratio than plasma membranes of undifferentiated cells. Differentiated L6 myoblasts have increased relative amounts of phosphatidylethanolamine and phosphatidylcholine ine in their plasma membrane and a decreased relative amount of sphingomyelin when compared with the plasma membranes of undifferentiated myoblasts. In addition, preliminary results show that differentiated L6 myoblasts plasma membrane phospholipid shows differences in the fatty acyl composition, specifically there appears to be relatively more 17:0 and 24:1 and less 16:1 and 18:1 than in plasma membrane phospholipids of undifferentiated L6 myoblasts. These observations indicate that significant changes in plasma membrane lipid composition occur during myoblast differentiation. The role that changes in lipid composition play in control of cellular differentiation, however, remains to be elucidated.

Changes in membrane dynamics associated with myogenic cell fusion

Changes in membrane dynamic properties associated with membrane fusion are studied employing in vitro myoblast fusion as a model system. We utilize a microscopic fluorescence relaxation approach which makes feasible the study of local variations in membrane dynamics within surface subdomains of single intact cells. Studies of the average rotational mobility of the fluorescent probe-1-anilino-naphthalene-8-sulfonate by this technique indicate that myoblast fusion activity is preceded by a generalized increased in membrane fluidity and that areas of cell contact between fusing cells exhibit higher fluidity and polarity, locally, than non-fusion regions.