Our general long-term objective is to understand the transcriptional mechanisms that control cell-type diversity within the nervous system. To pursue this objective we are investigating cis and trans transcriptional control of genes encoding neuronal nicotinic receptors. These genes encode subunits that can be assembled into a variety of functionally distinct heteromeric excitatory ligand-gated ion channels. Expression patterns of these genes indicates that different heteromers are expressed in adrenal chromaffin cells, peripheral ganglia, retina, and throughout the brain. The focus of our research is a cluster of nAchR genes, beta4, alpha3, and alpha5 that encode subunits assembled into a single receptor subtype in ganglia and possibly central neurons. The basic question driving our research is how are members of this cluster coordinately controlled to generate requisite overlapping patterns of expression for heteromer assembly? Coexpression and the clustered organization suggest that these genes are subject to control by shared cis elements. However, expression patterns of these genes are not entirely concordant and therefore individual genes in the cluster are likely to be controlled by independent cis elements as well. We have identified independent promoters adjacent to the alpha3 and beta4 genes as well as a potential enhancer within the beta4/alpha3 intergenic region. Our interest now is to investigate these cis elements in nAchR expressing PC12 cells to define their functional properties and in transgenic animal to determine their role in neural- specific expression of these nAchR genes. We have also identified trans- acting factors that modulate alpha3 and beta4 promoter activity. The zinc- finger protein Sp1 or an Sp1-related factor transactivates the alpha3 promoter via a G+A-rich motif positioned adjacent to the alpha3 transcription start site region. Sp1 belongs to a differentially expressed gene family and therefore one goal is to identify Sp1 family members that are expressed in PC12 cells and to assess their function in nAchR transcription. Toward this coal we have found that the Sp1-related factor, Sp4, is coexpressed with Sp1 in PC12 cells. Thus we will investigate the expression and function of this second zinc-finger in nAchR gene transcription. We will also extend these studies to the beta4 promoter in order to determine whether alpha3 and beta4 are coordinately controlled by these proteins. We have discovered that a POU-domain transcription factor, SCIP/Tst-1/Oct-6, is a potent and specific activator of alpha3. This represents the first cellular gene identified that is positively modulated by SCIP and it raises the possibility that SCIP controls cholinoceptive phenotype in neurons. Our recent studies in PC12 cells indicate that alpha3 regulation by SCIP is cell-type specific and suggest that activation occurs via a novel mechanism. We are interested in using PC12 cells as a neural model to investigate the potential alternative mechanism of SCIP action on alpha3 and other promoters. Together the proposed work will lead to a better understanding of how cholinergic transmitter systems are built and more generally will help to provide a clear view of the control of gene expression in neurons. Ultimately, these studies are likely to establish a foundation for future investigations of the role of aberrant gene control in specific neurological disorders.
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