Glial fibrillary acidic protein and induced differentiation of glia in vitro.

Glial fibrillary acidic protein (GFAP), a protein largely limited to astrocytes, is abundant and uniformly distributed in spindle-shaped human astrocytoma cells in early explants in culture but becomes predominantly perinuclear as the cells assume a "flat" irregular (epithelioid) shape. GFAP moves from the predominantly perinuclear site to the periphery of the cells and into the developing processes of those cells in which differentiation has been induced by serum starvation in minimum essential medium (MEM). This redistribution of GFAP does not occur in a small percentage of cells, specifically those in which low serum concentration fails to induce morphologic differentiation. In Eagle's basal medium (BME), which, combined with serum starvation, fails to induce differentiation, there is no shift of GFAP. Spontaneously differentiated cells in media wtih 10% serum, differ morphologically from cells with induced differentiation; in the former the cell bodies are smaller and the processes better developed and longer. GFAP in the spontaneously differentiated cells is distributed throughout the cytoplasm and the processes and is more abundant than in cells with induced differentiation. The results suggest the following: (1) GFAP redistribution plays a role in glial differentiation and process formation. (2) There are differences between spontaneous and induced differentiation. (3) There is some critical difference between MEM and BME in the induction of differentiation. (4) A heterogeneous expression of GFAP is implied in the variable staining of cells grown from different primary tumors. Also it is hypothesized that GFAP may play an inhibitory role in highly plastic movements of astrocytes but not in extension and retraction movements of processes.