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GFAP-deficient astrocytes are capable of stellation in vitro when cocultured with neurons and exhibit a reduced amount of intermediate filaments and an increased cell saturation density.

Glial fibrillary acidic protein (GFAP) is an intermediate filament protein predominantly expressed in cells of astroglial origin. To allow for the study of the biological functions of GFAP we have previously generated GFAP-negative mice by gene targeting [Pekny et al. (1995) EMBO J. 14, 1590-1598]. Astrocytes in culture, similar to reactive astrocytes in vivo, express three intermediate filament proteins: GFAP, vimentin, and nestin. Using primary astrocyte-enriched cultures from GFAP-negative mice, we now report on the effect of GFAP absence on (i) the synthesis of other intermediate filament proteins in astrocytes, (ii) intermediate filament formation, (iii) astrocyte process formation (stellation) in response to neurons in mixed cerebellar astrocyte/neuron cultures, and (iv) saturation cell density in vitro. GFAP-/- astrocytes were found to produce both nestin and vimentin. At the ultrastructural level, the amount of intermediate filaments as revealed by transmission electron microscopy was reduced in GFAP-/- astrocytes compared to that in GFAP+/+ astrocytes. GFAP-/- astrocytes retained the ability to form processes in response to neurons in mixed astrocyte/neuron cultures from the cerebellum. GFAP-/- astrocyte-enriched primary cultures exhibited an increased final cell saturation density. The latter leads us to speculate that the loss of GFAP expression observed focally in a proportion of human malignant gliomas may reflect tumor progression toward a more rapidly growing and malignant phenotype.