Activity-dependent mobilization of the adhesion molecule polysialic NCAM to the cell surface of neurons and endocrine cells.

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Abstract
The alpha‐2,8‐linked sialic acid polymer (PSA) on the neural cell adhesion molecule (NCAM) is an important regulator of cell surface interactions. We have examined the translocation of PSA‐NCAM to the surface of cultured cortical neurons and insulin secreting beta cells under different conditions of cell activity. Endoneuraminidase N, an enzyme that specifically cleaves PSA chains, was used to remove pre‐existing PSA from the plasma membrane and the re‐expression of the molecule was monitored by immunocytochemistry. Punctate PSA immunostaining was restored on the surface of 68% of neurons within 1 h. This recovery was almost completely prevented by tetrodotoxin, suggesting that spontaneous electrical activity is required. K+ depolarization (50 mM) allowed recovery of PSA surface staining in the presence of tetrodotoxin and this effect required the presence of extracellular Ca2+. Rapid redistribution of PSA‐NCAM to the surface of beta cells was observed under conditions that stimulate insulin secretion. Ca2+ channel inhibition decreased both PSA‐NCAM expression and insulin secretion to control, non‐stimulated levels. Finally, subcellular fractionation of an insulin‐secreting cell line showed that the secretory vesicle fraction is highly enriched in PSA‐NCAM. These results suggest that PSA‐NCAM can be translocated 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.