DISRUPTED GLIAL FIBRILLARY ACIDIC PROTEIN NETWORK IN ASTROCYTES FROM VIMENTIN KNOCKOUT MICE

Glial fibrillary acidic protein (GFAP) is an intermediate filament pro tein expressed predominantly in astrocytes. The study of its expressio n in the astrocyte lineage during development and in reactive astrocyt es has revealed an intricate relationship with the expression of vimen tin, another intermediate filament protein widely expressed in embryon ic development. These findings suggested that vimentin could be implic ated in the organization of the GFAP network. To address this question , we have examined GFAP expression and network formation in the recent ly generated vimentin knockout (Vim(-)) mice. We show that the GFAP ne twork is disrupted in astrocytes that normally coexpress vimentin and GFAP, e.g., those of the corpus callosum or the Bergmann glia of cereb ellum. Furthermore, Western blot analysis of GFAP protein content in t he cerebellum suggests that posttranslational mechanisms are implicate d in the disturbance of GFAP network formation. The role of vimentin i n this process was further suggested by transfection of Vim(-) culture d astrocytes with a vimentin cDNA, which resulted in the normal assemb ly of the GFAP network. Finally, we examined GFAP expression after sta b wound-induced astrogliosis. We demonstrate that in Vim(-) mice, reac tive astrocytes that normally express both GFAP and vimentin do not ex hibit GFAP immunoreactivity, whereas those that normally express GFAP only retain GFAP immunoreactivity. Taken together, these results show that in astrocytes, where vimentin is normally expressed with GFAP, th e GFAP fails to assemble into a filamentous network in the absence of vimentin. In these cells, therefore, vimentin appears necessary to sta bilize GFAP filaments and consequently the network formation.