Cell specific expressions of the murine CD21 gene.

The effect of histone acetylation on the expression of CD21 in non-expressing T and expressing B lymphocytes was analyzed. The deacetylase inhibitors Sodium Butyrate and Trichostatin A, which cause histone hyperacetylation, specifically activated the CD21 gene in nonexpressing cells in a time and dose dependent fashion. These data suggest a model of cell-specific deacetylase activity that represses CD21 gene transcription. Replacing the CD21 gene promoter with the SV-40 promoter resulted in the loss of this cell specific control. Further delineation of intronic regulatory elements by fragmentation also resulted in the loss of cell specific gene expression, suggesting that multiple CD21 promoter and intronic elements interact for appropriate CD21 gene expression. Data obtained from electrophoretic mobility shift assays and DNaseI hypersensitivity assays revealed that multiple DNA binding factors are present in B and T cell extracts, with a minority demonstrating B cell specificity. However, CD21 regulatory elements were sensitive to DNaseI treatment only in B cells until the histone acetylation status of the gene was altered. These data suggest that chromatin remodeling facilitates cell specific CD21 gene expression by modulating access of transcription factors to CD21 regulatory elements. EMSA competition and supershift analyses revealed the formation of many complexes at or near mouse/human homology intronic (MHi) regions that included YY1, Oct1 and HDAC. B cell specific complexes contained YY1, Oct1 and NFAT. Oct1 and YY1 also independently bound sequences outside MHi regions. Treatment of B cells with Cyclosporin A, which inhibits NFAT nuclear translocation and DNA binding, specifically repressed CD21 gene expression in a time and dose dependent manner. Functional data revealed that no single MHi region conferred cell specific reporter gene expression, but rather the entire CD21 regulatory element was required to confer cell specific gene expression. These data demonstrate the formation of repeating regulatory modules that bind intronic regulatory elements, all of which are required to coordinately control the cell specific expression of the murine CD21 gene. We propose a model in which YY1 and Oct1 recruit HDAC to multiple sites in the CD21 intronic regulatory element in nonexpressing cells. NFAT displaces HDAC from these complexes in expressing cells.