Polysialic acid attached to the neural cell adhesion molecule (NCAM) is thought to play a critical role in development. NCAM in muscle tissue contains a muscle-specific domain (MSD) to which mucin type O-glycans are attached. In the present study, using the C2C12 myoblast system, we show that NCAM containing MSD is increasingly expressed on the cell surface as myotubes form. Polysialic acid is primarily attached to N-glycans of NCAM, and polysialylated NCAM is expressed on the outer surface of myotube bundles. By transfecting cDNAs encoding wild type and mutant forms of NCAM, we found that NCAM containing MSD facilitates myoblast fusion, and this effect is diminished by mutating O-glycosylation sites at MSD. By contrast, forced expression of polysialic acid in early differentiation stages reduces myotube formation and delays the expression of NCAM containing the MSD domain. Strikingly, inhibition of polysialic acid synthesis by antisense DNA approach induced differentiation in both human rhabdomyosarcoma cells, which overexpress polysialic acid, and C2C12 cells. These results indicate that polysialic acid and mucin type O-glycans on NCAM differentially regulate myoblast fusion, playing critical roles in muscle development.During mammalian embryonic development, muscle development is initiated once mesenchymal cells commit to become myoblasts. Myoblasts then align and fuse, forming multinucleated myotubes, and mature myotubes are bundled together, forming sarcolemma. In parallel, each myotube starts contraction, receives innervation from motor and sensory neurons, and eventually forms muscle tissue. In this process, one of the most intriguing steps is myoblast fusion. Myoblast fusion consists of multiple steps: expression of the fusion competent phenotype, specific myoblast-myoblast recognition and alignment, cell adhesion and electrical coupling, and fusion of lipid bilayer membrane (1). It has been reported that anti-NCAM 1 antibody inhibits aggregation of fusion-competent chick embryo myoblasts (2), indicating that cell-cell adhesion via NCAM apparently initiates myoblast fusion. In addition, integrins, N-cadherin, VCAM, and VLA4 are thought to be involved (see Ref. 3 and references herein).NCAM is found in several isoforms due to alternative mRNA splicing of more than 19 exons. In mammalian brain, the glycosylphosphatidylinositol (GPI)-anchored 120-kDa isoform and transmembrane 140-and 180-kDa isoforms are primarily expressed. In muscles, three isoforms have been reported: a GPIanchored 125-kDa isoform and transmembrane 140-and 155-kDa isoforms (4). Muscle-specific domain (MSD) was identified in the cDNA of the 125-kDa NCAM isoform obtained from human skeletal muscle (5). MSD is encoded by four different exons, MSD1a, MSD1b, MSD1c, and codon AAG. The fulllength MSD sequence consists of 35 amino acids inserted between two fibronectin type III domains of NCAM. MSD insertion is observed only in skeletal muscle and heart and not in other tissues. O-Glycosylation of MSD was shown by peanut agglutinin (PNA) lecti...