Genomic organization and restricted expression of the human Mona/Gads gene suggests regulation by two specific promoters

Spi-1 and Spi-B control the expression of the Grap2 gene in B cells

The Ets family members Spi-1 and Spi-B have been implicated in the regulation of genes important for B cell antigen receptor (BCR) signaling. Mice deficient in Spi-B exhibit reduced B cell proliferation in response to BCR cross-linking and impaired T cell-dependent immune responses. This defect is exacerbated in the presence of Spi-1 haplo-insufficiency (Spi1+/- SpiB-/-). Tyrosine phosphorylation and calcium mobilization induced by BCR engagement is diminished in Spi1+/- SpiB-/- B lymphocytes, although many key BCR signaling proteins are expressed, suggesting that Spi-1 and Spi-B regulate expression of additional, unidentified signaling molecules. We now demonstrate that expression of the adaptor protein Grap2 is impaired in Spi1+/- SpiB+/- and Spi1+/- SpiB-/- B lymphocytes. Analysis of two alternate murine Grap2 promoters revealed a functionally important Spi-1 and Spi-B DNA binding element located in the downstream promoter. Ectopic expression of Grap2 in Grap2-deficient B cells reduced the recruitment of BLNK to Igalpha and the phosphorylation of specific substrates. Regulation of BLNK recruitment was dependent upon the Grap2 proline-rich domain, while modulation of phosphorylation was dependent upon both the proline-rich and SH2 domains. These data indicate that Spi-1 and Spi-B directly regulate the expression of Grap2 and that Grap2 functions to modulate BCR signaling, but that reduced Grap2 expression is unlikely to account for the BCR signaling defects observed in Spi1+/- SpiB-/- B cells.

Characterization of promoter elements directing Mona/Gads molecular adapter expression in T and myelomonocytic cells: involvement of the AML-1 transcription factor

Monocytic adaptor (Mona, also called Gads) is a molecular adaptor implicated in T cell activation and macrophage differentiation. The objective of this study was to identify elements regulating specific expression of Mona/Gads in human T cell and myelomonocytic cell lines. We first confirmed that the -2000 to +150 genomic region relative to the Mona gene transcription start site is sufficient to direct specific reporter gene expression in T cell lines, Jurkat, and MOLT-4 and in the immature myeloid cell lines, KG1a and RC2A. Deletion analysis and electrophoresis mobility shift assay identified several cis regulatory elements: overlapping initiator sequences, one interferon response factor-2 (IRF-2)-binding site at position -154, one GC box recognized by Sp1 and Sp3 at position -52, and two acute myeloid leukemia (AML)-1 binding sites at positions -70 and -13. Site-directed mutagenesis experiments indicated a key role of AML-1 for driving Mona expression in T cells and myeloid cells, and involvement of Sp1/Sp3 and IRF-2 transcription factors to modulate Mona expression in a cell-specific manner.

Characterization of the promoter controlling Mona/Gads expression in the megakaryocytic lineage

Mona/grb2 related adapter downstream of shc is a molecular adapter expressed in platelets, T lymphocytes and myelomonocytic cells. Using human hematopoietic cell lines, we have previously shown that lineage-specific Mona expression is achieved through the production of two transcripts (named 1A and 1B) differing by their 5' untranslated region (5'UTR). Thus, platelets and megakaryocytic cell lines K562 and HEL (Human Erythro-Leukemia) specifically express 1B messenger RNA (mRNA). We report here characterization of the (-2031/+72) genomic region relative to the putative transcription start site of 1B mRNA. We show this region is sufficient to ensure specific reporter gene expression in megakaryocytic cell lines, and that most promoter activity is contained in the (-225/+72) fragment. Electro-mobility shift assay and mutational analyses indicated that GATA-1 and a yet unidentified E-26 family member transcription factor are required for 1B (-2031/+72) promoter activity. Thus, Mona 1B promoter exhibits typical features of megakaryocyte-specific promoters.