INTRACELLULAR LOCATION, TEMPORAL EXPRESSION, AND POLYSIALYLATION OF NEURAL CELL-ADHESION MOLECULE IN ASTROCYTES IN PRIMARY CULTURE

Neural cell adhesion molecules (NCAMs) constitute a group of cell surf ace glycoproteins that control cell-cell interactions and play importa nt morphoregulatory roles in the developing and regenerating nervous s ystem. NCAMs exist in a variety of isoforms differing in the cytoplasm ic domain and/or their content in sialic acid. The highly sialylated f orm (PSA-NCAM) is expressed by neurons, whereas it is believed that th e less sialylated NCAM forms are synthesised by astrocytes. Moreover, little is known about the molecular sequence of the events that contri bute to its expression at the cell surface. Here we report that during the proliferation of cortical astrocytes, at 4 days in primary cultur e, these cells expressed PSA-NCAM as well as NCAM 180. Then, during ce ll differentiation these isoforms progressively disappeared and the NC AM 140 became predominant. By immunofluorescence and immunocytochemist ry studies we also show that PSA-NCAM and NCAM are first observed in s mall cytoplasmic spots or vesicles, located in or near the Golgi appar atus, as demonstrated by their co-localization with labelled wheat ger m agglutinin (WGA) in this cell organelle. Thereafter, immunostained c ytoplasmic NCAM gradually disappeared and became detectable at the cel l surface of differentiating astrocytes. We also describe for the firs t time sialyltransferase activity in these cells and report that the l evels of this activity correlated with the decrease in PSA-NCAM expres sion during the differentiation of astrocytes. These results will cont ribute to our understanding of the PSA and NCAM intracellular transpor t pathways and their expression at the cell surface. Moreover, the pre sence of PSA-NCAM in astrocytes suggests their possible role in nerve branching, fasciculation, and synaptic plasticity. GLIA 24:415-427, 19 98. (C) 1998 Wiley-Liss, Inc.