NCAM polypeptides in heart development: association with Z discs of forms that contain the muscle-specific domain

Previous studies of neural cell adhesion molecule (NCAM) cDNAs have revealed an alternatively spliced set of small exons (12A, 12B, 12C, and 12D) that encode a region in the extracellular portion of the molecule known as the muscle-specific domain (MSD). The entire MSD region can be expressed in skeletal muscle, heart, and skin; only exons 12A and 12D have been found in brain. These studies did not reveal which NCAM polypeptides contain the MSD region or the immunohistochemical distribution of these NCAM molecules. To address these questions, we prepared antibodies against the oligopeptides encoded by exons 12A and 12B and by exons 12C and 12D, and we used these antibodies to study the forms of NCAM containing the MSD region expressed during embryonic chicken heart development. These antibodies recognize certain forms of NCAM found in the heart, but they do not recognize brain NCAM. In the heart, each of the splice variants of NCAM (large cytoplasmic domain, small cytoplasmic domain, and small surface domain) that differ in their mode of attachment to the plasma membrane or in the size of their cytoplasmic domain is expressed in a form that contains and in a form that lacks the MSD region. No microheterogeneity is observed in the size of NCAM molecules containing the MSD region, even at the level of cyanogen bromide fragments, suggesting that exons 12A-D are expressed as a single unit. Depending on the site and the stage of development, the percent of NCAM molecules containing the MSD region can vary from nearly 0 to 100%. In general, this percentage increases during development. In immunohistochemical studies of hearts from stage 18 embryos, forms of NCAM containing the MSD region colocalized with Z discs. No other adhesion molecules were found in this distribution at this early stage of development. Studies on isolated cells in vitro demonstrate that the colocalization with Z discs of NCAM molecules containing the MSD region does not depend on cell-cell contact, and they raise the possibility that this form of NCAM is involved in cell-extracellular matrix interactions. The association of NCAM molecules containing the MSD region with Z discs suggests that this form of NCAM is involved in early myofibrillogenesis.