Current studies of the central nervous system (CNS) are assigning an increasing number of activities to astrocytes, many of which are potentially relevant to stroke. However, nearly all of these suggested functions are based on observed correlations, and many of these were made on cultured cells, whose properties may differ from those in vivo. As an alternative approach to understanding astrocyte function, we are studying transcriptional regulation of the human gene encoding glial fibrillary acidic protein (GFAP), the major component of astrocyte intermediate filaments. By studying GFAP transcription, insights may be gained into mechanisms governing development, reaction to injury, and cell specificity. A second goal is to use identified astrocyte-specific transcription elements to direct expression of other genes in astrocytes. This enables testing of the roles of specific factors in CNS function, and may produce disease models. Transcriptional studies have focused primarily on identifying factors that act at a consensus AP-1 site that is essential for GFAP transcription. Since proteins encoded by the jun and fos proto-oncogene families are known to modulate transcription via AP-1 sites, their presence in a GFAP expressing astrocytic cell line was examined. Analyses included gel mobility shift assays, """"""""shift Westerns"""""""" and detection of the specific mRNAs by Northern analysis. Preliminary results show a correlation between GFAP transcription and the presence of c-Jun, JunD, and Fra-2, but not with JunB, c-Fos, FosB or Fra-1. Several results have been obtained with mice carrying transgenes driven by GFAP regulatory regions. One study has provided additional support for our previous suggestion that astrocytes in different regions of the CNS use different regulatory elements of the GFAP gene to control its expression. Another has shown that overexpression of TGF-beta1 in astrocytes produces a severe, communicating hydrocephalus whose penetrance is dependent on the genetic background of the host. A third study has demonstrated the importance of astrocytes in development of the CNS. Mice carrying the herpes simplex virus thymidine kinase (TK) gene were treated with gancyclovir, an anti-herpes drug that is innocuous unless converted to a toxic product by the TK. Treated mice were ataxic, and displayed complete cerebellar disruption, accompanied by a reduction in the amount and organization of myelin.
