Characterization of the repressor function of the nuclear orphan receptor retinoid receptor-related testis-associated receptor/germ cell nuclear factor

The expanding roles of Nr6a1 in development and evolution

The Nuclear Receptor (NR) family of transcriptional regulators possess the ability to sense signalling molecules and directly couple that to a transcriptional response. While this large class of proteins are united by sequence and structural homology, individual NR functional output varies greatly depending on their expression, ligand selectivity and DNA binding sequence specificity. Many NRs have remained somewhat enigmatic, with the absence of a defined ligand categorising them as orphan nuclear receptors. One example is Nuclear Receptor subfamily 6 group A member 1 (Nr6a1), an orphan nuclear receptor that has no close evolutionary homologs and thus is alone in subfamily 6. Nonetheless, Nr6a1 has emerged as an important player in the regulation of key pluripotency and developmental genes, as functionally critical for mid-gestational developmental progression and as a possible molecular target for driving evolutionary change in animal body plan. Here, we review the current knowledge on this enigmatic nuclear receptor and how it impacts development and evolution.

Nr6a1 controls Hox expression dynamics and is a master regulator of vertebrate trunk development

The vertebrate main-body axis is laid down during embryonic stages in an anterior-to-posterior (head-to-tail) direction, driven and supplied by posteriorly located progenitors. Whilst posterior expansion and segmentation appears broadly uniform along the axis, there is developmental and evolutionary support for at least two discrete modules controlling processes within different axial regions: a trunk and a tail module. Here, we identify Nuclear receptor subfamily 6 group A member 1 (Nr6a1) as a master regulator of trunk development in the mouse. Specifically, Nr6a1 was found to control vertebral number and segmentation of the trunk region, autonomously from other axial regions. Moreover, Nr6a1 was essential for the timely progression of Hox signatures, and neural versus mesodermal cell fate choice, within axial progenitors. Collectively, Nr6a1 has an axially-restricted role in all major cellular and tissue-level events required for vertebral column formation, supporting the view that changes in Nr6a1 levels may underlie evolutionary changes in axial formulae.

Nuclear receptors in neural stem/progenitor cell homeostasis

In the central nervous system, embryonic and adult neural stem/progenitor cells (NSCs) generate the enormous variety and huge numbers of neuronal and glial cells that provide structural and functional support in the brain and spinal cord. Over the last decades, nuclear receptors and their natural ligands have emerged as critical regulators of NSC homeostasis during embryonic development and adult life. Furthermore, substantial progress has been achieved towards elucidating the molecular mechanisms of nuclear receptors action in proliferative and differentiation capacities of NSCs. Aberrant expression or function of nuclear receptors in NSCs also contributes to the pathogenesis of various nervous system diseases. Here, we review recent advances in our understanding of the regulatory roles of steroid, non-steroid, and orphan nuclear receptors in NSC fate decisions. These studies establish nuclear receptors as key therapeutic targets in brain diseases.

Ubiquitination of nuclear receptors

Nuclear receptors (NRs) are cellular proteins, which upon ligand activation, act to exert regulatory control over transcription and subsequent expression. Organized via systemic classification into seven subfamilies, NRs partake in modulating a vast expanse of physiological functions essential for maintenance of life. NRs display particular characteristics towards ubiquitination, the process of addition of specific ubiquitin tags at appropriate locations. Orchestrated through groups of enzymes harboring a diverse array of specialized structural components, the ubiquitination process emphatically alters the fate or downstream effects of NRs. Such influence is especially prominent in transcriptional processes such as promoter clearing for optimization and degradation pathways eliminating or recycling targeted proteins. Ultimately, the ubiquitination of NRs carries significant implications in terms of generating pathological clinical manifestations. Increasing evidence from studies involving patients and disease models suggests a role for ubiquitinated NRs in virtually every organ system. This supports the broad repertoire of roles that NRs play in the body, including modulatory conductors, facilitators, responders to external agents, and critical constituents for pharmacological or biological interventions. This review aims to cover relevant background and mechanisms of NRs and ubiquitination, with a focus towards elucidating subsequent pathophysiology and therapeutics in clinical disorders encompassing such ubiquitinated NRs.

Role of Nuclear Receptors in Central Nervous System Development and Associated Diseases

The nuclear hormone receptor (NHR) superfamily is composed of a wide range of receptors involved in a myriad of important biological processes, including development, growth, metabolism, and maintenance. Regulation of such wide variety of functions requires a complex system of gene regulation that includes interaction with transcription factors, chromatin-modifying complex, and the proper recognition of ligands. NHRs are able to coordinate the expression of genes in numerous pathways simultaneously. This review focuses on the role of nuclear receptors in the central nervous system and, in particular, their role in regulating the proper development and function of the brain and the eye. In addition, the review highlights the impact of mutations in NHRs on a spectrum of human diseases from autism to retinal degeneration.

Revisiting the role of GCNF in embryonic development

GCNF (NR6A1) is essential for embryonic development. GCNF belongs to the nuclear receptor (NR) gene family, it is distantly related to other NRs and is the only member of subfamily 6. As the ligand for GCNF has not been identified, GCNF is designated an orphan nuclear receptor. GCNF has been found to be a transcriptional repressor, through specific binding to DR0 response elements, which is found in the Oct4 proximal promoter for example. GCNF is expressed widely in early mouse embryos, and later in the developing nervous system. GCNF knockout mouse embryos die around E10.5. GCNF is required for the restriction of Oct4 expression to primordial germ cells after gastrulation. GCNF is expressed in ES/EC cells and during their differentiation, and has been reported to be required for pluripotency gene repression during retinoic acid (RA)-induced mES cell differentiation. GCNF can interact with DNA methylation proteins, and is suggested to recruit DNA methylation complexes to repress and silence Oct4 expression. Nuclear receptor regulation in embryonic development is a complex process, as different nuclear receptors have overlapping and distinct functions. In-depth exploration of GCNF function and mechanism of action will help to comprehensively understand the nuclear receptor regulation in embryonic development.

Expression screening of cancer/testis genes in prostate cancer identifies NR6A1 as a novel marker of disease progression and aggressiveness

BACKGROUND

Cancer/Testis (CT) genes are expressed in male gonads, repressed in most healthy somatic tissues and de-repressed in various somatic malignancies including prostate cancers (PCa). Because of their specific expression signature and their associations with tumor aggressiveness and poor outcomes, CT genes are considered to be useful biomarkers and they are also targets for the development of new anti-cancer immunotherapies. The aim of this study was to identify novel CT genes associated with hormone-sensitive prostate cancer (HSPC), and castration-resistant prostate cancer (CRPC).

METHODS

To identify novel CT genes we screened genes for which transcripts were detected by RNA profiling specifically in normal testis and in either HSPC or CRPC as compared to normal prostate and 44 other healthy tissues using GeneChips. The expression and clinicopathological significance of a promising candidate--NR6A1--was examined in HSPC, CRPC, and metastatic site samples using tissue microarrays.

RESULTS

We report the identification of 98 genes detected in CRPC, HSPC and testicular samples but not in the normal controls. Among them, cellular levels of NR6A1 were found to be higher in HSPC compared to normal prostate and further increased in metastatic lesions and CRPC. Furthermore, increased NR6A1 immunoreactivity was significantly associated with a high Gleason score, advanced pT stage and cancer cell proliferation.

CONCLUSIONS

Our results show that cellular levels of NR6A1 are correlated with disease progression in PCa. We suggest that this essential orphan nuclear receptor is a potential therapeutic target as well as a biomarker of PCa aggressiveness.

The ligand binding domain of GCNF is not required for repression of pluripotency genes in mouse fetal ovarian germ cells

In mice, successful development and reproduction require that all cells, including germ cells, transition from a pluripotent to a differentiated state. This transition is associated with silencing of the pluripotency genes Oct4 and Nanog. Interestingly, these genes are repressed at different developmental timepoints in germ and somatic cells. Ovarian germ cells maintain their expression until about embryonic day (E) 14.5, whereas somatic cells silence them much earlier, at about E8.0. In both somatic cells and embryonic stem cells, silencing of Oct4 and Nanog requires the nuclear receptor GCNF. However, expression of the Gcnf gene has not been investigated in fetal ovarian germ cells, and whether it is required for silencing Oct4 and Nanog in that context is not known. Here we demonstrate that Gcnf is expressed in fetal ovarian germ cells, peaking at E14.5, when Oct4 and Nanog are silenced. However, conditional ablation of the ligand-binding domain of Gcnf using a ubiquitous, tamoxifen-inducible Cre indicates that Gcnf is not required for the down-regulation of pluripotency genes in fetal ovarian germ cells, nor is it required for initiation of meiosis and oogenesis. These results suggest that the silencing of Oct4 and Nanog in germ cells occurs via a different mechanism from that operating in somatic cells during gastrulation.

Minireview: the diverse roles of nuclear receptors in the regulation of embryonic stem cell pluripotency

Extensive research has been devoted to the goal of understanding how a single cell of embryonic origin can give rise to every somatic cell type and the germ cell lineage, a hallmark defined as "pluripotency." The aggregate of this work supports fundamentally important roles for the gene transcription networks inherent to the pluripotent cell. Transcription networks have been identified that are both required for pluripotency, as well as sufficient to reprogram somatic cells to a naive pluripotent state. Several members of the nuclear receptor (NR) superfamily of transcription factors have been identified to play diverse roles in the regulation of pluripotency. The ligand-responsive nature of NRs coupled with the abundance of genetic models available has led to a significant advance in the understanding of NR roles in embryonic stem cell pluripotency. Furthermore, the presence of a ligand-binding domain may lead to development of small molecules for a wide range of therapeutic and research applications, even in cases of NRs that are not known to respond to physiologic ligands. Presented here is an overview of NR regulation of pluripotency with a focus on the transcriptional, proteomic, and epigenetic mechanisms by which they promote or suppress the pluripotent state.

Differential recruitment of methyl CpG-binding domain factors and DNA methyltransferases by the orphan receptor germ cell nuclear factor initiates the repression and silencing of Oct4

The pluripotency gene Oct4 encodes a key transcription factor that maintains self-renewal of embryonic stem cell (ESC) and is downregulated upon differentiation of ESCs and silenced in somatic cells. A combination of cis elements, transcription factors, and epigenetic modifications, such as DNA methylation, mediates Oct4 gene expression. Here, we show that the orphan nuclear receptor germ cell nuclear factor (GCNF) initiates Oct4 repression and DNA methylation by the differential recruitment of methyl-CpG binding domain (MBD) and DNA methyltransferases (Dnmts) to the Oct4 promoter. When compared with wild-type ESCs and gastrulating embryos, Oct4 repression is lost and its proximal promoter is significantly hypomethylated in retinoic acid (RA)-differentiated GCNF^−/− ESCs and GCNF^−/− embryos. Efforts to characterize mediators of GCNF's repressive function and DNA methylation of the Oct4 promoter identified MBD3, MBD2, and de novo Dnmts as GCNF interacting factors. Upon differentiation, endogenous GCNF binds to the Oct4 proximal promoter and differentially recruits MBD3 and MBD2 as well as Dnmt3A. In differentiated GCNF^−/− ESCs, recruitment of MBD3 and MBD2 as well as Dnmt3A to Oct4 promoter is lost and subsequently Oct4 repression and DNA methylation failed to occur. Hypomethylation of the Oct4 promoter is also observed in RA-differentiated MBD3^−/− and Dnmt3A^−/− ESCs, but not in MBD2^−/− and Dnmt3B^−/− ESCs. Thus, recruitment of MBD3, MBD2, and Dnmt3A by GCNF links two events: gene-specific repression and DNA methylation, which occur differentially at the Oct4 promoter. GCNF initiates the repression and epigenetic modification of Oct4 gene during ESC differentiation. Stem Cells 2011;29:1041–1051

Orphan nuclear receptors as targets for drug development

Orphan nuclear receptors regulate diverse biological processes. These important molecules are ligand-activated transcription factors that act as natural sensors for a wide range of steroid hormones and xenobiotic ligands. Because of their importance in regulating various novel signaling pathways, recent research has focused on identifying xenobiotics targeting these receptors for the treatment of multiple human diseases. In this review, we will highlight these receptors in several physiologic and pathophysiologic actions and demonstrate how their functions can be exploited for the successful development of newer drugs.

Identification of networks of co-occurring, tumor-related DNA copy number changes using a genome-wide scoring approach

Tumorigenesis is a multi-step process in which normal cells transform into malignant tumors following the accumulation of genetic mutations that enable them to evade the growth control checkpoints that would normally suppress their growth or result in apoptosis. It is therefore important to identify those combinations of mutations that collaborate in cancer development and progression. DNA copy number alterations (CNAs) are one of the ways in which cancer genes are deregulated in tumor cells. We hypothesized that synergistic interactions between cancer genes might be identified by looking for regions of co-occurring gain and/or loss. To this end we developed a scoring framework to separate truly co-occurring aberrations from passenger mutations and dominant single signals present in the data. The resulting regions of high co-occurrence can be investigated for between-region functional interactions. Analysis of high-resolution DNA copy number data from a panel of 95 hematological tumor cell lines correctly identified co-occurring recombinations at the T-cell receptor and immunoglobulin loci in T- and B-cell malignancies, respectively, showing that we can recover truly co-occurring genomic alterations. In addition, our analysis revealed networks of co-occurring genomic losses and gains that are enriched for cancer genes. These networks are also highly enriched for functional relationships between genes. We further examine sub-networks of these networks, core networks, which contain many known cancer genes. The core network for co-occurring DNA losses we find seems to be independent of the canonical cancer genes within the network. Our findings suggest that large-scale, low-intensity copy number alterations may be an important feature of cancer development or maintenance by affecting gene dosage of a large interconnected network of functionally related genes.

It is generally accepted that a normal cell has to acquire multiple mutations in order to become a malignant tumor cell. Considerable effort has been invested in finding single genes involved in tumor initiation and progression, but relatively little is known about the constellations of cancer genes that effectively collaborate in oncogenesis. In this study we focus on the identification of co-occurring DNA copy number alterations (i.e., gains and losses of pieces of DNA) in a series of tumor samples. We describe an analysis method to identify DNA copy number mutations that specifically occur together by examining every possible pair of positions on the genome. We analyze a dataset of hematopoietic tumor cell lines, in which we define a network of specific DNA copy number mutations. The regions in this network contain several well-studied cancer related genes. Upon further investigation we find that the regions of DNA copy number alteration also contain large networks of functionally related genes that have not previously been linked to cancer formation. This might illuminate a novel role for these recurrent DNA copy number mutations in hematopoietic malignancies.

Germ cell nuclear factor directly represses the transcription of peroxisome proliferator-activated receptor delta gene

Germ cell nuclear factor (GCNF) is a transcription factor that can repress gene transcription and plays an important role during spermatogenesis. Peroxisome proliferator-activated receptor delta (PPARdelta) is a nuclear hormone receptor belonging to the steroid receptor superfamily. It can activate the expression of many genes, including those involved in lipid metabolism. In this report, we showed that GCNF specifically interacts with PPARdelta promoter. Overexpression of GCNF in African green monkey SV40-transformed kidney fibroblast COS7 cells and mouse embryo fibroblast NIH 3T3 cells represses the activity of PPARdelta promoter. The mutation of GCNF response element in PPARdelta promoter relieves the repression in NIH 3T3 cells and mouse testis. Moreover, we showed that GCNF in nuclear extracts of mouse testis is able to bind to PPARdelta promoter directly. We also found that GCNF and PPARdelta mRNA were expressed with different patterns in mouse testis by in situ hybridization. These results suggested that GCNF might be a negative regulator of PPARdelta gene expression through its direct interaction with PPARdelta promoter in mouse testis.

0610009K11Rik, a testis-specific and germ cell nuclear receptor-interacting protein

Using an in silico approach, a putative nuclear receptor-interacting protein 0610009K11Rik was identified in mouse testis. We named this gene testis-specific nuclear receptor-interacting protein-1 (Tnrip-1). Tnrip-1 was predominantly expressed in the testis of adult mouse tissues. Expression of Tnrip-1 in the testis was regulated during postnatal development, with robust expression in 14-day-old or older testes. In situ hybridization analyses showed that Tnrip-1 is highly expressed in pachytene spermatocytes and spermatids. Consistent with its mRNA expression, Tnrip-1 protein was detected in adult mouse testes. Immunohistochemical studies showed that Tnrip-1 is a nuclear protein and mainly expressed in pachytene spermatocytes and round spermatids. Moreover, co-immunoprecipitation analyses showed that endogenous Tnrip-1 protein can interact with germ cell nuclear receptor (GCNF) in adult mouse testes. Our results suggest that Tnrip-1 is a testis-specific and GCNF-interacting protein which may be involved in the modulation of GCNF-mediated gene transcription in spermatogenic cells within the testis.

Fine mapping of a swine quantitative trait locus for number of vertebrae and analysis of an orphan nuclear receptor, germ cell nuclear factor (NR6A1)

The number of vertebrae in pigs varies and is associated with meat productivity. Wild boars, which are ancestors of domestic pigs, have 19 vertebrae. In comparison, European commercial breeds have 21-23 vertebrae, probably owing to selective breeding for enlargement of body size. We previously identified two quantitative trait loci (QTL) for the number of vertebrae on Sus scrofa chromosomes (SSC) 1 and 7. These QTL explained an increase of more than two vertebrae. Here, we performed a map-based study to define the QTL region on SSC1. By using three F2 experimental families, we performed interval mapping and recombination analyses and defined the QTL within a 1.9-cM interval. Then we analyzed the linkage disequilibrium of microsatellite markers in this interval and found that 10 adjacent markers in a 300-kb region were almost fixed in European commercial breeds. Genetic variation of the markers was observed in Asian local breeds or wild boars. This region encoded an orphan nuclear receptor, germ cell nuclear factor (NR6A1, formerly known as GCNF), which contained an amino acid substitution (Pro192Leu) coincident with the QTL. This substitution altered the binding activity of NR6A1 to its corepressors, nuclear receptor-associated protein 80 (RAP80) and nuclear receptor corepressor 1 (NCOR1). In addition, somites of mouse embryos demonstrated expression of NR6A1 protein. Together, these results suggest that NR6A1 is a strong candidate for one of the QTL that influence number of vertebrae in pigs.

International Union of Pharmacology. LXVI. Orphan nuclear receptors

Half of the members of the nuclear receptors superfamily are so-called "orphan" receptors because the identity of their ligand, if any, is unknown. Because of their important biological roles, the study of orphan receptors has attracted much attention recently and has resulted in rapid advances that have helped in the discovery of novel signaling pathways. In this review we present the main features of orphan receptors, discuss the structure of their ligand-binding domains and their biological functions. The paradoxical existence of a pharmacology of orphan receptors, a rapidly growing and innovative field, is highlighted.

Germ cell nuclear factor is a repressor of CRIPTO-1 and CRIPTO-3

The pluripotency of embryonic stem and embryonic carcinoma cells is maintained by the expression of a set of "stemness" genes. Whereas these genes are down-regulated upon induction of differentiation, the germ cell nuclear factor (GCNF) is transiently up-regulated and represses several pluripotency genes. CRIPTO-1, a co-receptor for the morphogen nodal, is strongly expressed in undifferentiated cells and is rapidly down-regulated during retinoic acid-induced differentiation. Although CRIPTO-1 is expressed at very low levels in adult tissues under normal conditions, it is found highly expressed in a broad range of tumors, where it acts as a potent oncogene. We show that expression of CRIPTO-1 is directly repressed by GCNF during differentiation of the human teratocarcinoma cell line, NT2. GCNF bound to a DR0 element of the CRIPTO-1 promoter in vitro, as shown by electrophoretic mobility shift assays, and in vivo, as demonstrated by chromatin immunoprecipitation. Reporter gene assays demonstrated that GCNF-mediated repression of the CRIPTO-1 promoter is dependent upon the DR0 site. Overexpression of GCNF in NT2 cells resulted in repression of CRIPTO-1 transcription, whereas expression of the transcription-activating fusion construct GCNF-VP16 led to an induction of the CRIPTO-1 gene and prevented its retinoic acid-induced down-regulation. Furthermore, we demonstrated that CRIPTO-3, a processed pseudogene of CRIPTO-1 on the X chromosome, is expressed in undifferentiated NT2 cells and is regulated by GCNF in parallel to CRIPTO-1. Thus, our study supports the hypothesis of GCNF playing a central role during differentiation of stem cells by repression of stem cell-specific genes.

Loss of orphan nuclear receptor GCNF function disrupts forebrain development and the establishment of the isthmic organizer

The isthmic organizer, which is located at the midbrain-hindbrain boundary, is important for midbrain development. The mechanism by which the development of the organizer is initiated and maintained is not well understood. Inactivation of the gene encoding the orphan nuclear receptor, GCNF, diminishes the expression of secreted signaling molecules, Fgf8 and Wnt1, the paired box genes Pax2/5, En1/2, and homeodomain transcription factor Gbx2; all of which are essential for isthmic organizer function. In addition, full neuronal differentiation is not observed in the midbrain region of GCNF-/- embryos. Increased cell death may contribute to the loss of midbrain structure in GCNF-/- embryos. These results indicate that GCNF is required for establishment of the isthmic organizer, thereby regulating the midbrain development.

Evolutionary trace-based peptides identify a novel asymmetric interaction that mediates oligomerization in nuclear receptors

Germ cell nuclear factor (GCNF) is an orphan nuclear receptor that plays important roles in development and reproduction, by repressing the expression of essential genes such as Oct4, GDF9, and BMP15, through binding to DR0 elements. Surprisingly, whereas recombinant GCNF binds to DR0 sequences as a homodimer, endogenous GCNF does not exist as a homodimer but rather as part of a large complex termed the transiently retinoid-induced factor (TRIF). Here, we use evolutionary trace (ET) analysis to design mutations and peptides that probe the molecular basis for the formation of this unusual complex. We find that GCNF homodimerization and TRIF complex formation are DNA-dependent, and ET suggests that dimerization involves key functional sites on both helix 3 and helix 11, which are located on opposing surfaces of the ligand binding domain. Targeted mutations in either helix of GCNF disrupt the formation of both the homodimer and the endogenous TRIF complex. Moreover, peptide mimetics of both of these ET-determined sites inhibit dimerization and TRIF complex formation. This suggests that a novel helix 3-helix 11 heterotypic interaction mediates GCNF interaction and would facilitate oligomerization. Indeed, it was determined that the endogenous TRIF complex is composed of a GCNF oligomer. These findings shed light on an evolutionarily selected mechanism that reveals the unusual DNA-binding, dimerization, and oligomerization properties of GCNF.

Reciprocal regulation of the mouse protamine genes by the orphan nuclear receptor germ cell nuclear factor and CREMtau

Germ cell nuclear factor (GCNF) is a member of the nuclear receptor superfamily, which is expressed in the adult predominantly in the male and female germ cells. In the male, GCNF is expressed in spermatogenic cells. GCNF binds as a homodimer to direct repeat response elements of the consensus half-site sequence, AGGTCA, with 0 bp spacing (DR0). Using this information, a search of genomic databases was performed to identify candidate GCNF responsive, spermatogenic-specific, genes that contain DR0 sequences. The mouse protamine genes are the strongest candidates identified to date, as they are post-meiotically expressed in round spermatids and contain DR0 elements in their proximal promoters. Previous work has shown that both recombinant and endogenous GCNF bind to DR0 elements in the mouse protamine 1 and 2 (Prm 1 and Prm 2) promoters with high affinity and specificity. The present work shows that in transient transfection assays in GC-1 and JEG-3 cells, co-transfection of a GCNF-VP16 expression plasmid with reporter plasmids containing either the wild type Prm 1 or Prm 2 promoter established that GCNF-VP16 is able to regulate transcription from both promoters in a DR0-dependent manner. Wild type GCNF, in contrast, acts as a repressor of basal transcription on both the Prm 1 and Prm 2 promoters in a DR0-dependent manner. Furthermore, CREMtau activation of these promoters is also repressed by wild-type GCNF, indicating that GCNF also acts as a repressor of activated transcription. GCNF therefore defines a novel nuclear receptor-signaling pathway that may regulate a subset of genes involved in the terminal differentiation process of spermatogenesis, exemplified by the protamines.

Germ Cell Nuclear Factor Relieves cAMP-response Element Modulator τ-mediated Activation of the Testis-specific Promoter of Human Mitochondrial Glycerol-3-phosphate Dehydrogenase

Mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) is an essential component of the glycerol phosphate shuttle that transfers reduction equivalents from the cytosol into the mitochondrion. Within the testis, immunohistological analysis localized human mGPDH to late spermatids and to the midpiece of spermatozoa. The expression of human mGPDH is regulated by two somatic promoters, and here, we describe a third testis-specific promoter of human mGPDH. The usage of this testis-specific promoter correlates with the expression of a shortened mGPDH transcript of approximately 2.4 kb in length, which is solely detectable from testicular RNA. Within the testis-specific promoter, we detected a cAMP-response element (CRE) site at -51, which binds the testis-specific transcriptional activator CRE modulator tau (CREMtau) in electrophoretic mobility shift assays. This recognition site overlaps with a nuclear receptor binding half-site at -49, which binds the testis-specific transcriptional repressor germ cell nuclear factor (GCNF). Both factors compete for binding to the same DNA response element. Ectopic expression of CREMtau in HepG2 cells activated a promoter-driven luciferase construct in transient transfection experiments. Additional cotransfection of GCNF relieved this activity, suggesting a down-regulation of CREMtau-mediated activation by GCNF. This effect was preserved by introducing the CRE/nuclear receptor-binding element into a heterologous promoter context. Our data suggest a down-regulation of CREMtau-mediated gene expression by GCNF, which might be a general regulation mechanism for several postmeiotically expressed genes with a temporal expression peak during early spermatid development.

The hinge-helix 1 region of peroxisome proliferator-activated receptor gamma1 (PPARgamma1) mediates interaction with extracellular signal-regulated kinase 5 and PPARgamma1 transcriptional activation: involvement in flow-induced PPARgamma activation in endothelial cells

Peroxisome proliferator-activated receptors (PPAR) are ligand-activated transcription factors that form a subfamily of the nuclear receptor gene family. Since both flow and PPARgamma have atheroprotective effects and extracellular signal-regulated kinase 5 (ERK5) kinase activity is significantly increased by flow, we investigated whether ERK5 kinase regulates PPARgamma activity. We found that activation of ERK5 induced PPARgamma1 activation in endothelial cells (ECs). However, we could not detect PPARgamma phosphorylation by incubation with activated ERK5 in vitro, in contrast to ERK1/2 and JNK, suggesting a role for ERK5 as a scaffold. Endogenous PPARgamma1 was coimmunoprecipitated with endogenous ERK5 in ECs. By mammalian two-hybrid analysis, we found that PPARgamma1 associated with ERK5a at the hinge-helix 1 region of PPARgamma1. Expressing a hinge-helix 1 region PPARgamma1 fragment disrupted the ERK5a-PPARgamma1 interaction, suggesting a critical role for hinge-helix 1 region of PPARgamma in the ERK5-PPARgamma interaction. Flow increased ERK5 and PPARgamma1 activation, and the hinge-helix 1 region of the PPARgamma1 fragment and dominant negative MEK5beta significantly reduced flow-induced PPARgamma activation. The dominant negative MEK5beta also prevented flow-mediated inhibition of tumor necrosis factor alpha-mediated NF-kappaB activation and adhesion molecule expression, including vascular cellular adhesion molecule 1 and E-selectin, indicating a physiological role for ERK5 and PPARgamma activation in flow-mediated antiinflammatory effects. We also found that ERK5 kinase activation was required, likely by inducing a conformational change in the NH(2)-terminal region of ERK5 that prevented association of ERK5 and PPARgamma1. Furthermore, association of ERK5a and PPARgamma1 disrupted the interaction of SMRT and PPARgamma1, thereby inducing PPARgamma activation. These data suggest that ERK5 mediates flow- and ligand-induced PPARgamma activation via the interaction of ERK5 with the hinge-helix 1 region of PPARgamma.

The expression level of the orphan nuclear receptor GCNF (germ cell nuclear factor) is critical for neuronal differentiation

The germ cell nuclear factor (GCNF) is essential for normal embryonic development and gametogenesis. To test the prediction that GCNF is additionally required for neuronal differentiation, we used the mouse embryonal carcinoma cell line PCC7-Mz1, which represents an advantageous model to study neuronal cells from the stage of fate choice until the acquirement of functional competence. We generated stable transfectants that express gcnf sense or antisense RNA under the control of a tetracycline-regulated promoter. After retinoic acid-induced withdrawal from the cell cycle, sense clones developed a neuron network with changed properties, and the time course of neuron maturation was shortened. Consistent with these data, differentiation of neuronal precursor cells was impaired in antisense cultures. This involved a delay in 1) the down-regulation of nestin, a marker for undifferentiated neuroepithelial cells and stem cells of the central nervous system, and 2) up-regulation of the somatodendritic protein microtubule-associated protein 2 and the synaptic vesicle protein synaptophysin. Neuronal cells in the antisense cultures acquired functional competence, although with a significant delay. Our data propose that the level of GCNF is critical for differentiation and maturation of neuronal precursor cells.

Selective LXXLL peptides antagonize transcriptional activation by the retinoid-related orphan receptor RORgamma

The retinoid-related orphan receptor gamma (RORgamma) has been shown to function as a positive regulator of transcription in many cell lines. Transcriptional activation by nuclear receptors involves recruitment of co-activators that interact with receptors through their LXXLL motifs (NR box). In this study, we analyze the interaction of RORgamma with the co-activator SRC1 and use a series of LXXLL-containing peptides to probe for changes in the conformation of the co-activator interaction surface of the RORgamma LBD. We demonstrate that the H3-4/H12 co-activator interaction surface of RORgamma displays a selectivity for LXXLL peptides that is distinct from those of other nuclear receptors. LXXLL peptides that interacted with RORgamma efficiently antagonized RORgamma-mediated transcriptional activation. Mutations E502Q and Y500F in H12, and K334A, Q347A, and I348D in H3 and H4 of RORgamma, severely impact the recruitment of LXXLL peptides. The effects of these mutations are consistent with predictions made on the basis of the structure of the RORgamma(LBD) derived through homology modeling. These peptide antagonists provide a useful tool to analyze the conformation changes in the RORgamma(LBD) and to study RORgamma receptor signaling.

Post-meiotic gene products as targets for male contraception

Post-meiotic stages of male germ cell maturation represent an interesting target system for the development of novel male contraceptive agents. In the human, these stages represent a period of only about 16 days differentiation, and thus targeting these cells would represent a contraceptive approach with a relatively rapid onset and equivalent recovery. Results from the Human Genome Project suggest that these cells also express a high number of very specific transcripts, though whether all of these are functional and/or essential for sperm differentiation and function requires more research. Until recently, however, these haploid stages were relatively inaccessible to molecular research because of the lack of appropriate model systems and methods. This situation has recently improved, with several new techniques involving manipulation of primary cells and seminiferous tubules, germ cell transplantation and the development of new immortalized cell-lines. Also, new biochemical approaches are yielding more information about haploid-specific transcription factors, such as GCNF. It is therefore to be expected that soon several new targets for a potential post-meiotic male contraceptive will become available for pharmaceutical development.

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.

The function of Xenopus germ cell nuclear factor (xGCNF) in morphogenetic movements during neurulation

The germ cell nuclear factor (GCNF, NR6A1) is a nuclear orphan receptor first described in the mouse testis and subsequently identified as an essential transcription factor in vertebrate embryogenesis. Here, we analyze the phenotype of Xenopus embryos after depletion of embryonic GCNF (xEmGCNF) protein using a specific morpholino antisense oligonucleotide. Morphological defects after xEmGCNF knockdown became obvious from neurulation onward. Among the abnormalities observed, defective formation of the neural tube and a short and curved main body axis were the most remarkable traits. Histological analysis, lineage tracing of injected blastomeres, and Keller sandwich explants revealed that xEmGCNF function is required for different patterns of cell intercalation during neurulation and consequently for the sequence of morphogenetic movements leading to formation of the neural tube. Further characterization of the phenotype at the molecular level showed an abnormal distribution of the extracellular matrix protein fibronectin and a reduction in the expression level of the integrin subunits alpha5 and alpha6, the limiting components of the laminin and fibronectin receptors, respectively. We propose integrin-mediated cell-matrix interaction as a process that requires xEmGCNF function and provides, in concert with cadherins-mediated cell-cell interactions, a molecular basis for morphogenetic cell movements during neurulation.

Analysis of germ cell nuclear factor transcripts and protein expression during spermatogenesis

Germ cell nuclear factor (GCNF), an orphan receptor in the nuclear receptor superfamily, is expressed predominantly in developing germ cells in the adult mouse. Two Gcnf transcripts (7.4 and 2.1 kilobase [kb]) encoded by a single copy gene are expressed in the testis of several mammalian species. To identify features that regulate Gcnf expression, we characterized the structure and sequence of the mouse gene and its two transcripts and determined the expression profile of the GCNF protein during spermatogenesis. Genomic fragments spanning part of the 5'-untranslated region (UTR), the coding sequence, and the complete 3'-UTR (approximately 80 kb) were isolated and sequenced. The 3'-UTRs of the two transcripts are quite distinct. The 7.4 kb transcript, which appears earlier in spermatogenesis, has a very long 3'-UTR of 4451 nucleotides. In contrast, the 2.1 kb transcript, which is expressed predominantly during the haploid phase of spermatogenesis, has a 3'-UTR that is only 202 nucleotides in length. Additional analyses indicate that both transcripts share the same coding region and are associated with polysomes. A single GCNF protein band was detected in testis extracts by Western blotting with a specific antiserum. Immunohistochemical analysis showed that GCNF is localized in the nuclei of pachytene spermatocytes and round spermatids. GCNF is first detectable in early pachytene spermatocytes (stage II) and is continuously expressed until spermatids begin to elongate in stage IX. Although GCNF is generally distributed throughout the nucleus, it is particularly prominent in heterochromatic regions at some stages and in condensed chromosomes undergoing the meiotic divisions. This expression profile suggests that GCNF plays a role in transcriptional regulation during meiosis and the early haploid phase of spermatogenesis.

The germ cell nuclear factor is required for retinoic acid signaling during Xenopus development

The germ cell nuclear factor (GCNF, NR6A1) is a nuclear orphan receptor that functions as a transcriptional repressor and is transiently expressed in mammalian carcinoma cells during retinoic acid (RA) induced neuronal differentiation. During Xenopus laevis development, the spatiotemporal expression pattern of embryonic GCNF (xEmGCNF) suggests a role in anteroposterior specification of the neuroectoderm. Here, we show that RA treatment of Xenopus embryos enhances xEmGCNF expression. Moreover, we present evidence for the relevance of this finding in the context of primary neurogenesis and hindbrain development. During early development of the central nervous system, RA signals promote posterior transformation of the neuroectoderm and increase the number of cells undergoing primary neurogenesis. Our loss-of-function analyses using a xEmGCNF-specific morpholino antisense oligonucleotide indicate that xEmGCNF is required for the effect of RA on primary neurogenesis. This may be caused by transcriptional regulation of the gene encoding the RA-degrading enzyme CYP26, since this gene is derepressed after depletion of xEmGCNF and an antimorph of xEmGCNF directly activates transcription of CYP26, also in absence of protein synthesis. The effect of xEmGCNF knockdown on hindbrain patterning is similar to conditions of reduced RA signaling, which may be caused by a reduction of RAR gamma expression specifically in the presumptive hindbrain.

Mutation of a putative nuclear receptor binding site abolishes activity of the nestin midbrain enhancer

Regional differences in gene expression are critical to the proper development of specialized cell types in the nervous system. The ventral midbrain is the prominent source of dopaminergic neurons, which are affected in Parkinson's disease. We have recently identified a gene regulatory element that is specifically active in ventral midbrain neuroepithelium of developing embryos. This 204-bp transcriptional enhancer is conserved within the second intron of mammalian nestin genes and contains a putative binding site for a protein of the nuclear receptor family. Our present study shows, by mutagenesis and reporter gene assay in transgenic mice, that this site is essential for enhancer function in the developing midbrain. The characterization of regulatory sites and transcription factors with specific activity in the ventral midbrain provides insight into the molecular mechanisms by which neural progenitor cells become specified towards particular neuronal differentiation pathways.

SF1 polymorphisms in the mouse and steroidogenic potential

ACTH-resistance in four mutant derivatives of a mouse adrenocortical tumor cell line results from a defect that reduces the activity of steroidogenic factor-1 (SF1) thereby preventing expression of the ACTH receptor and other SF1-dependent genes. The SF1 genes from these mutants contain a sequence difference that changes an Ala to Ser at codon 172. Steroidogenic factor-1(S172) represents a polymorphism rather than a spontaneous mutation since the two forms of SF1, SF1(A172), and SF1(S172), can be traced to the hybrid mouse strain (C57L/J x A/HeJ) from which the original adrenal tumor was derived. The SF1(S172) allele is amplified in three of the four mutant clones together with the neighboring genes germ cell nuclear factor and LIM homeobox2. The two forms of SF1 had only modest differences in transcriptional activity in reporter gene assays, suggesting that the SF1 polymorphism per se is not directly responsible for the loss of mc2r expression. Rather, ACTH resistance in this family of adrenocortical tumor cell mutants may be due to a closely linked gene on the SF1(S172) allele. Mouse strains with reportedly high steroidogenic capacity (C57Bl/6J, C57Bl/10J) also have the SF1(A172) allele while mouse strains with low steroidogenic capacity (C3H/HeJ, DBA/2J) have the SF1(S172) allele. These latter observations suggest that the two SF1 alleles also may be markers of steroidogenic potential among mouse strains.

RAP80, a novel nuclear protein that interacts with the retinoid-related testis-associated receptor

In this study, we describe the characterization of a novel nuclear protein, referred to as RAP80. The RAP80 cDNA was cloned from a human testis cDNA library and encodes a 719-amino acid protein containing two potential CX(2)CX(11)HX(3)C-type zinc finger motifs at its carboxyl-terminal region. Analysis of its genomic structure revealed that the RAP80 gene covers more than 90 kb and consists of 15 exons and 14 introns. Fluorescence in situ hybridization mapped the RAP80 gene to human chromosome 5q35. RAP80 mRNA is expressed in many human tissues, but its expression is particularly high in testis. In situ hybridization showed that RAP80 is highly expressed in germ cells of mouse testis but is not differentially regulated during spermatogenesis. Confocal microscopy showed that RAP80 is localized to the nucleus, where it is distributed in a speckled pattern. Deletion analysis showed that a bipartite nuclear localization signal at the amino terminus is important in mediating nuclear transport of RAP80. Monohybrid analysis showed that RAP80 might function as an active repressor of transcription. Mammalian two-hybrid analysis demonstrated that RAP80 was able to interact with the retinoid-related testis-associated receptor (RTR), an orphan receptor that has been implicated in the control of embryonic development and spermatogenesis. Pull-down analysis showed that RAP80 and RTR physically interact in vitro. Deletion and point mutation analyses revealed that part of the hinge domain of RTR is required for this interaction. RAP80 is able to inhibit the interaction of RTR with the co-repressor N-CoR likely by competing with N-CoR for RTR binding. Our results suggest that RAP80 may be functioning as a modulator of RTR signaling.

Mouse germline restriction of Oct4 expression by germ cell nuclear factor

The POU-domain transcription factor Oct4 is essential for the maintenance of the mammalian germline. In this study, we show that the germ cell nuclear factor (GCNF), an orphan nuclear receptor, represses Oct4 gene activity by specifically binding within the proximal promoter. GCNF expression inversely correlates with Oct4 expression in differentiating embryonal cells. GCNF overexpression in embryonal cells represses Oct4 gene and transgene activities, and we establish a link to transcriptional corepressors mediating repression by GCNF. In GCNF-deficient mouse embryos, Oct4 expression is no longer restricted to the germ cell lineage after gastrulation. Our studies suggest that GCNF is critical in repressing Oct4 gene activity as pluripotent stem cells differentiate and in confining Oct4 expression to the germline.

Genomic structure of the gene for mouse germ-cell nuclear factor (GCNF). II. Comparison with the genomic structure of the human GCNF gene

BACKGROUND

Germ-cell nuclear factor (GCNF, NR6AI) is an orphan nuclear receptor. Its expression pattern suggests it functions during embryogenesis, in the placenta and in germ-cell development. Mouse GCNF cDNA codes for a protein of 495 amino acids, whereas the four reported human cDNA variants code for proteins of 454 to 480 amino acids. Apart from this size difference, there is sequence conservation of up to 98.7%. To elucidate the genomic structure that gives rise to the different human GCNF mRNAs, the sequence information of the human GCNF locus is compared to the previously reported structure of the mouse locus.

RESULTS

The genomic structures of the mouse and human GCNF genes are highly conserved. The comparison reveals that the shorter human protein results from skipping the 45 base-pair third exon. Three different human isoforms - GCNF-1, GCNF-2a and GCNF-2b - are generated by differential usage of alternative splice acceptor sites of the fourth and the seventh exon.

CONCLUSION

By homology with the mouse gene, 11 GCNF coding exons can be defined on human chromosome 9. All human GCNF cDNAs identified so far are, however, derived from mRNAs generated by splicing the fourth to the second exon. Although the genomic sequence is highly conserved, the analysis suggests that alternative splicing generates a higher complexity of human GCNF isoforms compared with the situation in the mouse.