Molecular cloning and characterization of chicken orphan nuclear receptor cTR2

Molecular cloning, expression analysis and transcript localization of testicular orphan nuclear receptor 2 in the male catfish, Clarias batrachus

Testicular receptor 2 (TR2; also known as Nr2c1) is one of the first orphan nuclear receptors identified and known to regulate various physiological process with or without any ligand. In this study, we report the cloning of full length nr2c1 and its expression analysis during gonadal development, seasonal testicular cycle and after human chorionic gonadotropin (hCG) induction. In addition, in situ hybridization (ISH) was performed to localize nr2c1 transcripts in adult testis and whole catfish (1day post hatch). Tissue distribution and gonadal ontogeny studies revealed high expression of nr2c1 in developing and adult testis. Early embryonic stage-wise expression of nr2c1 seems to emphasize its importance in cellular differentiation and development. Substantial expression of nr2c1 during pre-spawning phase and localization of nr2c1 transcripts in sperm/spermatids were observed. Significant upregulation after hCG induction indicate that nr2c1 is under the regulation of gonadotropins. Whole mount ISH analysis displayed nr2c1 expression in notochord indicating its role in normal vertebrate development. Taken together, our findings suggest that nr2c1 may have a plausible role in the testicular and embryonic development of catfish.

SmTR2/4, a Schistosoma mansoni homologue of TR2/TR4 orphan nuclear receptor

cDNA clones encoding a Schistosoma mansoni homologue of the TR2/TR4 group of nuclear receptors, SmTR2/4, were identified by screening an adult female worm cDNA library. SmTR2/4 is a 1,943 amino acid protein, the largest member of the TR2/TR4 group of nuclear receptors and also the largest nuclear receptor reported to date. SmTR2/4 retains a typical domain organisation of nuclear receptors exhibiting 69-77% sequence identity in the DNA binding domain and 16-22% sequence identity in the ligand binding domain compared with its orthologues. SmTR2/4 contains a large A/B domain and hinge region. SmTR2/4 also contains a 100 amino acid F domain, which is absent from its orthologues. SmTR2/4 mRNA is expressed in every stage of the S. mansoni life cycle, exhibiting an elevated expression level in cercariae. Western blot analysis identified two forms of SmTR2/4 protein in adult worms. Our in vitro DNA binding assay showed that SmTR2/4 binds to the DR-3 consensus hormone response element, suggesting a functional conservation among the TR2/TR4 group members in terms of DNA binding specificity. A yeast-based transactivation assay demonstrated that the A/B domain, F domain and N-terminal part of the hinge region in SmTR2/4, when tethered to a GAL4 DNA binding domain, exhibited an autonomous transcription activation function.

Gene Silencing by Nuclear Orphan Receptors

Nuclear orphan receptors represent a large and diverse subgroup in the nuclear receptor superfamily. Although putative ligands for these orphan members remain to be identified, some of these receptors possess intrinsic activating, inhibitory, or dual regulatory functions in development, differentiation, homeostasis, and reproduction. In particular, gene-silencing events elicited by chicken ovalbumin upstream promoter-transcription factors (COUP-TFs); dosage-sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome, gene 1 (DAX-1); germ cell nuclear factor (GCNF); short heterodimer partner (SHP); and testicular receptors 2 and 4 (TR2 and TR4) are among the best characterized. These orphan receptors are critical in controlling basal activities or hormonal responsiveness of numerous target genes. They employ multiple and distinct mechanisms to mediate target gene repression. Complex cross-talk exists between these orphan receptors at their cognate DNA binding elements and an array of steroid?nonsteroid hormone receptors, other transcriptional activators, coactivators and corepressors, histone modification enzyme complexes, and components of basal transcriptional components. Therefore, perturbation induced by these orphan receptors at multiple levels, including DNA binding activities, receptor homo- or heterodimerization, recruitment of cofactor proteins, communication with general transcriptional machinery, and changes at histone acetylation status and chromatin structures, may contribute to silencing of target gene expression in a specific promoter or cell-type context. Moreover, the findings derived from gene-targeting studies have demonstrated the significance of these orphan receptors' function in physiologic settings. Thus, COUP-TFs, DAX-1, GCNF, SHP, and TR2 and 4 are known to be required for multiple physiologic and biologic functions, including neurogenesis and development of the heart and vascular system steroidogenesis and sex determination, gametogenesis and embryonic development, and cholesterol?lipid homeostasis.