ACTIVITY-DEPENDENT MOBILIZATION OF THE ADHESION MOLECULE POLYSIALIC NCAM TO THE CELL-SURFACE OF NEURONS AND ENDOCRINE-CELLS

The alpha-2,8-linked sialic acid polymer (PSA) on the neural cell adhe sion molecule (NCAM) is an important regulator of cell surface interac tions. We have examined the translocation of PSA-NCAM to the surface o f cultured cortical neurons and insulin secreting beta cells under dif ferent conditions of cell activity. Endoneuraminidase N, an enzyme tha t specifically cleaves PSA chains, was used to remove pre-existing PSA from the plasma membrane and the re-expression of the molecule was mo nitored by immunocytochemistry. Punctate IPSA immunostaining was resto red on the surface of 68% of neurons within 1 h. This recovery was alm ost completely prevented by tetrodotoxin, suggesting that spontaneous electrical activity is required. K+ depolarization (50 mM) allowed rec overy of PSA surface staining in the presence of tetrodotoxin and this effect required the presence of extracellular Ca2+. Rapid redistribut ion of PSA-NCAM to the surface of beta cells was observed under condit ions that stimulate insulin secretion. Ca2+ channel inhibition decreas ed both PSA-NCAM expression and insulin secretion to control, non-stim ulated levels. Finally, subcellular fractionation of an insulin-secret ing cell line showed that the secretory vesicle fraction is highly enr iched in PSA-NCAM. These results suggest that PSA-NCAM can be transloc ated to the cell surface via regulated exocytosis. Taken together, our results provide unprecedented evidence linking cell activity and PSA- NCAM, expression, and suggest a mechanism for rapid modulation of cell surface interactions.