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 surface glycoproteins that control cell-cell interactions and play important morphoregulatory roles in the developing and regenerating nervous system. NCAMs exist in a variety of isoforms differing in the cytoplasmic domain and/or their content in sialic acid. The highly sialylated form (PSA-NCAM) is expressed by neurons, whereas it is believed that the less sialylated NCAM forms are synthesised by astrocytes. Moreover, little is known about the molecular sequence of the events that contribute to its expression at the cell surface. Here we report that during the proliferation of cortical astrocytes, at 4 days in primary culture, these cells expressed PSA-NCAM as well as NCAM 180. Then, during cell differentiation these isoforms progressively disappeared and the NCAM 140 became predominant. By immunofluorescence and immunocytochemistry studies we also show that PSA-NCAM and NCAM are first observed in small cytoplasmic spots or vesicles, located in or near the Golgi apparatus, as demonstrated by their co-localization with labelled wheat germ agglutinin (WGA) in this cell organelle. Thereafter, immunostained cytoplasmic NCAM gradually disappeared and became detectable at the cell surface of differentiating astrocytes. We also describe for the first time sialyltransferase activity in these cells and report that the levels of this activity correlated with the decrease in PSA-NCAM expression during the differentiation of astrocytes. These results will contribute to our understanding of the PSA and NCAM intracellular transport pathways and their expression at the cell surface. Moreover, the presence of PSA-NCAM in astrocytes suggests their possible role in nerve branching, fasciculation, and synaptic plasticity.