Characterization of glycosylphosphatidylinositol (GPI)-anchored NCAM on mouse skeletal muscle cell line C2C12: the structure of the GPI glycan and release during myogenesis.

The mouse myoblast cell line C2C12 constitutively expressed 160-kDa transmembrane NCAM isoform and 135-kDa GPI-anchored isoform before differentiation. During differentiation into multinucleated myotubes, the cells newly expressed 150-kDa GPI-anchored isoform and the level of 135-kDa GPI-anchored isoform increased. Structural analysis of the GPI glycan of NCAM, which was purified from C2C12 myotubes after metabolic labeling with [3H]inositol, was performed by sequential exoglycosidase digestion and Wistaria floribunda agglutinin-agarose column chromatography. The core GPI glycan structure, Man alpha 1-2Man alpha-Man alpha-GlcNH2-myoInositol, was conserved and variations were observed in additional mannose and N-acetylgalactosamine residues. Structural analysis of the GPI glycans of the two GPI-anchored isoforms, GPI-NCAM 135 and GPI-NCAM 150, showed the enhanced attachment of the N-acetylgalactosamine residue to the GPI glycan core of GPI-NCAM 150. These GPI-anchored NCAM isoforms were released from C2C12 cells during the myoblast differentiation. Release of GPI-anchored NCAMs was observed when C2C12 cells were cultured in a serum-free medium, and inositol but not inositol phosphate was detected after nitrous acid deamination of the released NCAM. These results suggest that the GPI-anchored NCAM was released from the cell surface by the action of an endogeneous phospholipase D.