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

Abstract

Evidence was sought for a role for Ca 2+ in the dephosphorylation of the astrocyte marker glial fibrillary acidic protein (GFAP) in immature hippocampal slices. Although previous work showed that the main phosphatase dephosphorylating GFAP in this preparation is a Ca 2+-independent type 1 enzyme, a role for Ca 2+ was suggested by the observation that the incorporation of [ 32P]phosphate into GFAP in immature slices is inhibited by external Ca 2+. This inhibition is strikingly different to the situation in mature slices where GFAP phosphorylation is completely dependent on Ca 2+. Pure astrocyte cultures were probed by immunoblotting for the presence of the Ca 2+-dependent phosphatase calcineurin. An enzyme content, amounting to about 2% of that found in fresh hippocampal tissue, was detected for both the catalytic (α) and regulatory (β) subunits. The direct or indirect association of calcineurin with 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.