The intersection of BMP signaling and GATA transcription factors
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Bone Morphogenetic Proteins (BMPs) are members of the TGF-Beta superfamily of secreted morphogens that are implicated in an array of biological phenomena. It is generally thought that BMPs exert their functions by activating a signal transduction pathway that ultimately leads to the translocation of Smad proteins to the nucleus, followed by gene activation of target promoters. One of the major challenges in understanding the function of BMPs is determining how activation of this pleiotropic pathway results in tissue-specific gene activation. As has been shown for numerous signal transduction pathways, appropriate gene expression ultimately results from a combination of regulation at two levels: cis (pathway specific response elements in target reporters) and trans (signal regulated transcription factors). Using two candidate target promoters, the zebrafish GATA-2 gene and the Xenopus Myosin Light Chain 2 (MLC2) gene, we characterized cis and trans mechanisms utilized by the BMP-4 signal transduction pathway.;The GATA-2 zinc finger transcription factor was previously shown to be induced by BMP-4 and is thought to be downstream of a BMP mediated ventral signaling pathway. We confirmed and extended this analysis by studying the regulation of a zebrafish GATA-2 reporter construct during Xenopus embryogenesis. This construct can be activated by both endogenous and exogenous BMP signaling. Furthermore, this activation is recapitulated with Smad1 and is blocked by Smad6, indicating that activation of the GATA-2 promoter is Smad dependent. Deletion analysis identified a 68 basepair BMP-4 response element (BRE) which is required for induction. Scanning mutagenesis identified an octamer cis element within this region that is bound by Xenopus Oct-1 and is required for BMP-4 mediated activation. This region is necessary, but not sufficient, for BMP-4 activation, as heterologous promoter studies identified an additional 50 basepair element that is required for full activation. These results suggest that POU domain transcription factors modulate BMP signaling at the transcriptional level to regulate target promoters.;Since GATA-2 is regulated by BMP-4 and most likely functions as a downstream mediator of this pathway, we wondered whether one of the downstream functions of GATA factors could be to integrate BMP signaling at target promoters. GATA-4 is a member of this family that is expressed throughout cardiogenesis, as well as in a variety of other tissues. BMP signaling is required at multiple steps during cardiac development, and we were interested in determining whether GATA-4 might play a role in mediating its cardiogenic effects. We demonstrate that GATA-4 and Smad1 directly interact, consistent with our hypothesis that GATA-4 can modulate BMP signaling during cardiac development. A one hundred amino acid domain in the carboxy-terminus of Smad1 is required for this interaction. These results raised the possibility that this regulation occurs at the level of transcription. (Abstract shortened by UMI.).
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