Browsing by Subject "Regulatory elements"

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Regulatory elements controlling the expression of OR37 genes
(2007) Zhang, Yongquan; Breer, Heinz
The genes of the OR37 family are clustered in two loci (cluster I and cluster II) on mouse chromosome 4. These genes encode distinct olfactory receptors (ORs) which are characterised by an insertion of six amino acids in the third extracellular loop and moreover, these receptor types are only expressed in cells which are segregated in a small patch on the central nasal turbinate. As first steps to unravel the molecular basis of this unique topographic expression pattern previous studies have led to the identification of highly conserved sequence motifs including an olf-1 site in the putative promoter region of these genes and subsequently several transcription factors were identified which did bind to these sites. However, it remained elusive if an interaction between the transcription factors and the putative promoter sites may have functional implications. Therefore, a heterologous system was employed to assess the consequence of an interaction between the putative promoters and the transcription factors. HEK 293 cells were cotransfected with a reporter gene under the control of putative mOR37 promoter regions and an expression vector based gene encoding the transcription factor. The expression rate of the reporter gene was monitored by measuring luciferase activity. It was found that the three O/E transcription factors (O/E-1, O/E-2 and O/E-4) induced significant activation of the mOR37 promoters; in addition, it was observed that the putative promoters of other OR genes were also activated, suggesting that the O/E proteins may play a general role in the regulation of OR gene expression. Mutagenesis experiments revealed that the effects of O/E proteins were dependent on the presence of an olf-1 site within the promoter region. For the transcription factor Lhx-2 it was found that not all but only promoters of distinct OR-genes were affected. For the mOR37 promoters a simultaneous action of O/E protein and Lhx-2 elicited an increase of reporter gene expression. The data indicate that the putative mOR37 promoters could drive gene expression in the presence of the crucial transcription factors in this heterologous system. In order to explore to what extent the promoter may contribute to the characteristic topographic expression pattern of the mOR37 genes in vivo, a mOR37C transgene which included the coding exon and the putative promoter, was randomly inserted into the mouse genome. Seven lines were obtained; in all lines the transgene was specifically expressed in olfactory sensory neurons (OSN). In six lines the transgene expression was restricted to the central patch of the olfactory turbinates, typical for the OR37 genes. In one line (line 7) the transgene was also expressed in OSNs ectopically positioned outside the patch within the medial zone. It was found that the transgene was expressed in a mutually exclusive manner and from only one allele. The axons of OSNs expressing the transgene co-converged in the same glomerulus with the axons from neurons expressing the endogenous gene. In line #7 the formation of ectopic glomeruli was observed. The number of OSNs expressing the transgene varied considerably among lines; these differences were independent from the copy number of the transgene. The data indicate that the short putative promoters, most likely the conserved motifs, were sufficient to drive the OR37 gene expression in a tissue specific way and most aspects of the OR37 gene expression were mimicked by the transgene; however, considerable differences between certain lines suggested additional regulatory elements, such as a locus control region (LCR). Since regulatory elements for gene transcription, such as promoters, enhancers and LCRs, appear to be conserved across species, a comparative approach was utilized to search for the LCR-like element for the OR37 locus by sequence alignment across distantly related mammals. A segment of 270 base pairs located 137 Kb upstream of OR37 cluster I was found to be highly conserved between mouse, human, dog and opossum. It was not associated with an exon of any known gene and was highly correlated with OR37 cluster I rather than with the neighboring genes, since the flanking genes did not show syntenic conservation in the opossum genome. A homologous counterpart for this segment was found downstream of the OR37 cluster II locus; an alignment of the cluster II sequence across species identified the conservation of this counterpart. Examination for relevant motifs in this segment and comparison with the conserved H element revealed two common transcription factor binding sites, at least one of them is known to be essential for generating DNase I hypersensitive sites in the LCR of the beta globin gene locus. Further studies are required to evaluate a possible role of this conserved segment in the regulation of the OR37 gene expression.