Control of the phosphorylation of the astrocyte marker glial fibrillary acidic protein (GFP) in the immature rat hippocampus by glutamate and calcium ions: possible key factor in astrocytic plasticity

The present review describes recent research on the regulation by glutamate and Ca2+ of the phosphorylation state of the intermediate filament protein of the astrocytic cytoskeleton, glial fibrillary acidic protein (GFAP), in immature hippocampal slices. The results of this research are discussed against a background of modern knowledge of the functional importance of astrocytes in the brain and of the structure and dynamic properties of intermediate filament proteins. Astrocytes are now recognized as partners with neurons in many aspects of brain function with important roles in neural plasticity. Site-specific phosphorylation of intermediate filament proteins, including GFAP, has been shown to regulate the dynamic equilibrium between the polymerized and depolymerized state of the filaments and to play a fundamental role in mitosis. Glutamate was found to increase the phosphorylation state of GFAP in hippocampal slices from rats in the post-natal age range of 12-16 days in a reaction that was dependent on external Ca2+. The lack of external Ca2+ in the absence of glutamate also increased GFAP phosphorylation to the same extent. These effects of glutamate and Ca2+ were absent in adult hippocampal slices, where the phosphorylation of GFAP was completely Ca2+ -dependent. Studies using specific agonists of glutamate receptors showed that the glutamate response was mediated by a G protein-linked group II metabotropic glutamate receptor (mGluR). Since group II mGluRs do not act by liberating Ca2+ from internal stores, it is proposed that activation of thereceptor by glutamate inhibits Ca2+ entry into the astrocytes andconsequently down-regulates a Ca2+-dependent dephosphorylationcascade regulating the phosphorylation state of GFAP. The functional significance of these results may be related to the narrow developmental window when the glutamate response is present. In the rat brain this window corresponds to the period of massive synaptogenesis during which astrocytes are known to proliferate. Possibly, glutamate liberated from developing synapses during this period may signal an increase in the phosphorylation state of GFAP and a consequent increase in the number of mitotic astrocytes.

Properties of phosphorylated glial fibrillary acidic protein in brain slices of guinea pig, mouse and rat

1. Brain micro-slices from guinea pig, mouse and rat were incubated in Krebs-Ringer Na- HEPES buffered medium containing [32P]-phosphate and characterized by immunoblotting with polyclonal antibody to glial fibrillary acidic protein (GFAP). 2. GFAP presented small differences in two-dimensional electrophoretic mobility. 3. The phosphorylation of GFAP was dependent on Ca2+ in the incubation medium in adult animals. 4. Both the immunocontent and level of phosphorylation of GFAP were higher in hippocampus than in cerebral cortex of all the three species