EVIDENCE FOR A ROLE FOR CALCIUM-IONS IN THE DEPHOSPHORYLATION OF GLIAL FIBRILLARY ACIDIC PROTEIN (GFAP) IN IMMATURE HIPPOCAMPAL SLICES AND IN ASTROCYTE CULTURES FROM THE RAT

Evidence was sought for a role for Ca2+ in the dephosphorylation of th e astrocyte marker glial fibrillary acidic protein (GFAP) in immature hippocampal slices. Although previous work showed that the main phosph atase dephosphorylating GFAP in this preparation is a Ca2+-independent type 1 enzyme, a role for Ca2+ was suggested by the observation that the incorporation of [P-32]phosphate into GFAP in immature slices is i nhibited by external Ca2+. This inhibition is strikingly different to the situation in mature slices where GFAP phosphorylation is completel y dependent on Ca2+. Pure astrocyte cultures were probed by immunoblot ting for the presence of the Ca2+-dependent phosphatase calcineurin. A n enzyme content, amounting to about 2% of that found in fresh hippoca mpal tissue, was detected for both the catalytic (alpha) and regulator y (beta) subunits. The direct or indirect association of calcineurin w ith GFAP was suggested by observations showing that FK506, a specific inhibitor of calcineurin, increased the phosphorylation state of GFAP in immature slices and of GFAP and vimentin in astrocyte cultures. (C) 1997 Elsevier Science B.V.