Smooth muscle cells transiently express NCAM

NCAM (neural cell adhesion molecule) polypeptides were first detected on neuronal cells and were subsequently found to be expressed at least transiently by a number of other cell types including skeletal and cardiac but not smooth muscle. We report here that rat smooth muscle expresses NCAM in vitro and transiently in vivo. Using a monoclonal antibody 3F4 which reacts with most rat NCAM polypeptides, NCAM was found on the surface of cultured rat aortic smooth muscle lines A10 and A7r5 and mouse smooth muscle like line BC₃H1 in abundances equal to or greater than those of cardiac muscle, skeletal muscle, and neuronal cell lines. The major NCAM polypeptide of muscle cells was M_r = 140 kDa with lesser amounts of the 120 kDa form. Consistent with these results, a major NCAM mRNA of 6.7 kb was detected in Northern analyses with lesser amounts of the 4.3 and 2.9 kb mRNA size classes. The relative abundance of NCAM mRNA was similar in RNA prepared from smooth muscle A7r5 cells, L6 skeletal muscle cells, and 9-day-old rat brain. NCAM was distributed across the entire surface of cultured smooth muscle cells in a highly punctate manner. Cryostat sections of rat aorta, intestine and bladder were examined by immunofluorescence to determine if NCAM is also expressed on smooth muscle in vivo. In each case NCAM was found to be transiently expressed by the smooth muscle cells of these tissues. Highest NCAM levels were observed at embryonic day 17 which then declined to undetectable levels in tissues from adults. These results extend previous observations to indicate all muscle types transiently express NCAM in development.