Developmental regulation of neuraminidase-sensitive lectin-binding glycoproteins during myogenesis of rat L6 myoblasts

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Abstract
Intact monolayers of L6 myoblasts were treated with neuraminidase, with the aim of selectively removing sialic acid residues of cell-surface glycoproteins. Neuraminidase treatment unmasked binding sites for Ricinus communis agglutinin I and peanut agglutinin, thus allowing the identification of the major binding proteins for these lectins. For R. communis agglutinin I these neuraminidase-sensitive glycoproteins had apparent MW values of 136,000, 115,000, 87,000, 83,000 and 49,000. For peanut agglutinin, the major neuraminidase-sensitive glycoproteins had apparent MW values of 200,000, 136,000, 87,000 and 83,000. Highly reproducible, developmentally regulated, changes in the lectin-binding capacity of certain of these glycoproteins as L6 myoblasts differentiated into myotubes were found . Coincident with myoblast fusion there was a coordinate decrease in R. communis agglutinin I binding by glycoproteins of apparent MW of 136,000 and 49,000. There was also a coordinate shift in mobility of the broad band of glycoprotein, centered at an apparent MW of 115,000 in myoblasts, to a new average apparent MW of 107,000 in mid-fusion cultures and myotube cultures. Peanut agglutinin binding by the major protein of apparent MW 136,000 also decreased at the mid-fusion stage of myogenesis, and was barely detectable in 7-day-old fused cultures. These developmentally regulated changes in neuraminidase-sensitive glycoproteins were all inhibited by growth of myoblasts in 6.4 .mu.M-5-bromo-2''-deoxyuridine, indicating that they are associated with myoblast differentiation. An increase in fibronectin was seen in midfusion cultures, which was not inhibited by growth of myoblasts in 5-bromo-2''-deoxyuridine. This initial increase in fibronectin is, therefore, unlikely to be directly related to myoblast fusion or differentiation.