FOR, a novel orphan nuclear receptor related to farnesoid X receptor

Enterohepatic and non-canonical roles of farnesoid X receptor in controlling lipid and glucose metabolism

Farnesoid X receptor (FXR) is a nuclear receptor that transcriptionally regulates bile acid homeostasis along with nutrient metabolism. In addition to the gastrointestinal (GI) tract, FXR expression has been widely noted in kidney, adrenal gland, pancreas, adipose, skeletal muscle, heart, and brain. Except for the liver and gut, the relevance of FXR signaling in metabolism in other tissues remains poorly understood. This review examines the classical and non-canonical tissue-specific roles of FXR in regulating, lipids, and glucose homeostasis under normal and diseased states. FXR activation has been reported to be protective against cholestasis, nonalcoholic fatty liver disease (NAFLD), nonalcoholic steatohepatitis (NASH), type 2 diabetes, cardiovascular and kidney diseases. Several ongoing clinical trials are investigating FXR ligands as a therapeutic target for primary biliary cholangitis (PBC) and NASH, which substantiate the significance of FXR signaling in modulating metabolic processes. This review highlights that FXR ligands, albeit an attractive therapeutic target for treating metabolic diseases, tissue-specific modulation of FXR may be the key to overcoming some of the adverse clinical effects.

Liver Specification in the Absence of Cardiac Differentiation Revealed by Differential Sensitivity to Wnt/β Catenin Pathway Activation

Embryonic precursors of liver and heart, whilst not sharing cellular origin, develop in close proximity through a dynamic series of inductive signaling events. During gastrulation anterior endoderm (AE) provides cardiogenic signals that act on adjacent mesoderm, resulting in induction of cardiac precursors. Subsequently cardiogenic mesoderm generates a FGF signal that acts on adjacent AE to induce foregut organ specification. Additional signals such as BMP and Wnt provide further information required for liver specification. Most findings on liver specification were derived from mouse explant studies as well as experiments with Xenopus and zebrafish embryos. To address some of the limitations of these models, here we used two complementary ex vivo models based on Xenopus embryos: pluripotent animal cap explants expressing Gata4 transcription factor and conjugates of gastrula-stage AE with animal caps (AC). We show that in these models liver specification is not sensitive to Wnt signaling manipulation, in contrast to the requirement for Wnt antagonism shown in vivo. FGF pathway is not necessary for Gata4-induced liver specification in animal cap explants but is required for prolonged period in sandwiches of AE and AC. In contrast, BMP signaling is shown to be essential for Gata4-induced liver specification. Our findings may have implications for research on liver differentiation from embryonic stem cells.

Retinoic acid-activated Ndrg1a represses Wnt/β-catenin signaling to allow Xenopus pancreas, oesophagus, stomach, and duodenum specification

How cells integrate multiple patterning signals to achieve early endoderm regionalization remains largely unknown. Between gastrulation and neurulation, retinoic acid (RA) signaling is required, while Wnt/β-catenin signaling has to be repressed for the specification of the pancreas, oesophagus, stomach, and duodenum primordia in Xenopus embryos. In attempt to screen for RA regulated genes in Xenopus endoderm, we identified a direct RA target gene, N-myc downstream regulated gene 1a (ndrg1a) that showed expression early in the archenteron roof endoderm and late in the developing pancreas, oesophagus, stomach, and duodenum. Both antisense morpholino oligonucleotide mediated knockdown of ndrg1a in Xenopus laevis and the transcription activator-like effector nucleases (TALEN) mediated disruption of ndrg1 in Xenopus tropicalis demonstrate that like RA signaling, Ndrg1a is specifically required for the specification of Xenopus pancreas, oesophagus, stomach, and duodenum primordia. Immunofluorescence data suggest that RA-activated Ndrg1a suppresses Wnt/β-catenin signaling in Xenopus archenteron roof endoderm cells. Blocking Wnt/β-catenin signaling rescued Ndrg1a knockdown phenotype. Furthermore, overexpression of the putative Wnt/β-catenin target gene Atf3 phenocopied knockdown of Ndrg1a or inhibition of RA signaling, while Atf3 knockdown can rescue Ndrg1a knockdown phenotype. Lastly, the pancreas/stomach/duodenum transcription factor Pdx1 was able to rescue Atf3 overexpression or Ndrg1a knockdown phenotype. Together, we conclude that RA activated Ndrg1a represses Wnt/β-catenin signaling to allow the specification of pancreas, oesophagus, stomach, and duodenum progenitor cells in Xenopus embryos.

Effects of exposure to 17α-ethynylestradiol during sexual differentiation on the transcriptome of the African clawed frog (Xenopus laevis)

Exposure to estrogens during the period of sexual differentiation is known to adversely affect the development of testes in African clawed frogs (Xenopus laevis), but little is known about molecular changes that coincide with the development of altered phenotypes. Therefore, the transcriptome-level effects of exposure to 17α-ethynylestradiol (EE2) during sexual differentiation of X. laevis were evaluated by use of Illumina sequencing coupled with RNA-Seq expression analysis. Overall, a number of processes were affected by 17α-ethynylestradiol, including steroid biosynthesis, thyroid hormone signaling and metabolism, testicular development, and spermatogenesis. Some of the altered pathways, such as thyroid hormone signaling and testicular development, could be linked with biological effects on metamorphosis and gonadal phenotypes, respectively, that were observed in frogs that were exposed to 17α-ethynylestradiol throughout metamorphosis and the early postmetamorphic period. Thus, early changes at the transcriptome-level were predictive of pathologies that did not manifest until later in development. To validate the quantitative capacity of RNA-Seq, a subset of transcripts identified to have altered abundances in individuals exposed to 17α-ethynylestradiol was also evaluated by use of quantitative polymerase chain reaction (qPCR). While small sample sizes (n = 3) limited the ability to draw conclusions pertaining to differences in qPCR-derived abundances of transcripts between control and exposed tadpoles, there was a significant relationship (r(2) = 0.78) between fold-changes for RNA-Seq and qPCR.

Prolonged FGF signaling is necessary for lung and liver induction in Xenopus

BACKGROUND

FGF signaling plays numerous roles during organogenesis of the embryonic gut tube. Mouse explant studies suggest that different thresholds of FGF signaling from the cardiogenic mesoderm induce lung, liver, and pancreas lineages from the ventral foregut progenitor cells. The mechanisms that regulate FGF dose in vivo are unknown. Here we use Xenopus embryos to examine the hypothesis that a prolonged duration of FGF signaling from the mesoderm is required to induce foregut organs.

RESULTS

We show that both mesoderm and FGF signaling are required for liver and lung development in Xenopus; formally demonstrating that this important step in organ induction is conserved with other vertebrate species. Prolonged contact with the mesoderm and persistent FGF signaling through both MEK and PI3K over an extended period of time are required for liver and lung specification. Inhibition of FGF signaling results in reduced liver and lung development, with a modest expansion of the pancreas/duodenum progenitor domain. Hyper-activation of FGF signaling has the opposite effect expanding liver and lung gene expression and repressing pancreatic markers. We show that FGF signaling is cell autonomously required in the endoderm and that a dominant negative FGF receptor decreases the ability of ventral foregut progenitor cells to contribute to the lung and liver buds.

CONCLUSIONS

These results suggest that the liver and lungs are specified at progressively later times in development requiring mesoderm contact for different lengths of time. Our data suggest that this is achieved at least in part through prolonged FGF signaling. In addition to providing a foundation for further mechanistic studies on foregut organogenesis using the experimental advantages of the Xenopus system, these data have implications for the directed differentiation of stem cells into foregut lineages.

Homeoprotein hhex-induced conversion of intestinal to ventral pancreatic precursors results in the formation of giant pancreata in Xenopus embryos

Liver and ventral pancreas develop from neighboring territories within the endoderm of gastrulae. ventral pancreatic precursor 1 (vpp1) is a marker gene that is differentially expressed in a cell population within the dorsal endoderm in a pattern partially overlapping with that of hematopoietically expressed homeobox (hhex) during gastrulation. In tail bud embryos, vpp1 expression specifically demarcates two ventral pancreatic buds, whereas hhex expression is mainly restricted to the liver diverticulum. Ectopic expression of a critical dose of hhex led to a greatly enlarged vpp1-positive domain and, subsequently, to the formation of giant ventral pancreata, putatively by conversion of intestinal to ventral pancreatic precursor cells. Conversely, antisense morpholino oligonucleotide-mediated knockdown of hhex resulted in a down-regulation of vpp1 expression and a specific loss of the ventral pancreas. Furthermore, titration of hhex with a dexamethasone-inducible hhex-VP16GR fusion construct suggested that endogenous hhex activity during gastrulation is essential for the formation of ventral pancreatic progenitor cells. These observations suggest that, beyond its role in liver development, hhex controls specification of a vpp1-positive endodermal cell population during gastrulation that is required for the formation of the ventral pancreas.

Evolution of promiscuous nuclear hormone receptors: LXR, FXR, VDR, PXR, and CAR

Nuclear hormone receptors (NHRs) are transcription factors that work in concert with co-activators and co-repressors to regulate gene expression. Some examples of ligands for NHRs include endogenous compounds such as bile acids, retinoids, steroid hormones, thyroid hormone, and vitamin D. This review describes the evolution of liver X receptors α and β (NR1H3 and 1H2, respectively), farnesoid X receptor (NR1H4), vitamin D receptor (NR1I1), pregnane X receptor (NR1I2), and constitutive androstane receptor (NR1I3). These NHRs participate in complex, overlapping transcriptional regulation networks involving cholesterol homeostasis and energy metabolism. Some of these receptors, particularly PXR and CAR, are promiscuous with respect to the structurally wide range of ligands that act as agonists. A combination of functional and computational analyses has shed light on the evolutionary changes of NR1H and NR1I receptors across vertebrates, and how these receptors may have diverged from ancestral receptors that first appeared in invertebrates.

The evolution of farnesoid X, vitamin D, and pregnane X receptors: insights from the green-spotted pufferfish (Tetraodon nigriviridis) and other non-mammalian species

Background

The farnesoid X receptor (FXR), pregnane X receptor (PXR), and vitamin D receptor (VDR) are three closely related nuclear hormone receptors in the NR1H and 1I subfamilies that share the property of being activated by bile salts. Bile salts vary significantly in structure across vertebrate species, suggesting that receptors binding these molecules may show adaptive evolutionary changes in response. We have previously shown that FXRs from the sea lamprey (Petromyzon marinus) and zebrafish (Danio rerio) are activated by planar bile alcohols found in these two species. In this report, we characterize FXR, PXR, and VDR from the green-spotted pufferfish (Tetraodon nigriviridis), an actinopterygian fish that unlike the zebrafish has a bile salt profile similar to humans. We utilize homology modelling, docking, and pharmacophore studies to understand the structural features of the Tetraodon receptors.

Results

Tetraodon FXR has a ligand selectivity profile very similar to human FXR, with strong activation by the synthetic ligand GW4064 and by the primary bile acid chenodeoxycholic acid. Homology modelling and docking studies suggest a ligand-binding pocket architecture more similar to human and rat FXRs than to lamprey or zebrafish FXRs. Tetraodon PXR was activated by a variety of bile acids and steroids, although not by the larger synthetic ligands that activate human PXR such as rifampicin. Homology modelling predicts a larger ligand-binding cavity than zebrafish PXR. We also demonstrate that VDRs from the pufferfish and Japanese medaka were activated by small secondary bile acids such as lithocholic acid, whereas the African clawed frog VDR was not.

Conclusions

Our studies provide further evidence of the relationship between both FXR, PXR, and VDR ligand selectivity and cross-species variation in bile salt profiles. Zebrafish and green-spotted pufferfish provide a clear contrast in having markedly different primary bile salt profiles (planar bile alcohols for zebrafish and sterically bent bile acids for the pufferfish) and receptor selectivity that matches these differences in endogenous ligands. Our observations to date present an integrated picture of the co-evolution of bile salt structure and changes in the binding pockets of three nuclear hormone receptors across the species studied.

Bile salts of vertebrates: structural variation and possible evolutionary significance

Biliary bile salt composition of 677 vertebrate species (103 fish, 130 reptiles, 271 birds, 173 mammals) was determined. Bile salts were of three types: C(27) bile alcohols, C(27) bile acids, or C(24) bile acids, with default hydroxylation at C-3 and C-7. C(27) bile alcohols dominated in early evolving fish and amphibians; C(27) bile acids, in reptiles and early evolving birds. C(24) bile acids were present in all vertebrate classes, often with C(27) alcohols or with C(27) acids, indicating two evolutionary pathways from C(27) bile alcohols to C(24) bile acids: a) a 'direct' pathway and b) an 'indirect' pathway with C(27) bile acids as intermediates. Hydroxylation at C-12 occurred in all orders and at C-16 in snakes and birds. Minor hydroxylation sites were C-1, C-2, C-5, C-6, and C-15. Side chain hydroxylation in C(27) bile salts occurred at C-22, C-24, C-25, and C-26, and in C(24) bile acids, at C-23 (snakes, birds, and pinnipeds). Unexpected was the presence of C(27) bile alcohols in four early evolving mammals. Bile salt composition showed significant variation between orders but not between families, genera, or species. Bile salt composition is a biochemical trait providing clues to evolutionary relationships, complementing anatomical and genetic analyses.

C/EBPalpha initiates primitive myelopoiesis in pluripotent embryonic cells

The molecular mechanisms that underlie the development of primitive myeloid cells in vertebrate embryos are not well understood. Here we characterize the role of cebpa during primitive myeloid cell development in Xenopus. We show that cebpa is one of the first known hematopoietic genes expressed in the embryo. Loss- and gain-of-function studies show that it is both necessary and sufficient for the development of functional myeloid cells. In addition, we show that cebpa misexpression leads to the precocious induction of myeloid cell markers in pluripotent prospective ectodermal cells, without the cells transitioning through a general mesodermal state. Finally, we use live imaging to show that cebpa-expressing cells exhibit many attributes of terminally differentiated myeloid cells, such as highly active migratory behavior, the ability to quickly and efficiently migrate toward wounds and phagocytose bacteria, and the ability to enter the circulation. Thus, C/EPBalpha is the first known single factor capable of initiating an entire myelopoiesis pathway in pluripotent cells in the embryo.

GATA4 and GATA5 are essential for heart and liver development in Xenopus embryos

Background

GATA factors 4/5/6 have been implicated in the development of the heart and endodermal derivatives in vertebrates. Work in zebrafish has indicated that GATA5 is required for normal development earlier than GATA4/6. However, the GATA5 knockout mouse has no apparent embryonic phenotype, thereby questioning the importance of the gene for vertebrate development.

Results

In this study we show that in Xenopus embryos GATA5 is essential for early development of heart and liver precursors. In addition, we have found that in Xenopus embryos GATA4 is important for development of heart and liver primordia following their specification, and that in this role it might interact with GATA6.

Conclusion

Our results suggest that GATA5 acts earlier than GATA4 to regulate development of heart and liver precursors, and indicate that one early direct target of GATA5 is homeobox gene Hex.

Ligand specificity and evolution of liver X receptors

Liver X receptors (LXRs) are key regulators of lipid and cholesterol metabolism in mammals. Little is known, however, about the function and evolution of LXRs in non-mammalian species. The present study reports the cloning of LXRs from African clawed frog (Xenopus laevis), Western clawed frog (Xenopus tropicalis), and zebrafish (Danio rerio), and their functional characterization and comparison with human and mouse LXRs. Additionally, an ortholog of LXR in the chordate invertebrate Ciona intestinalis was cloned and functionally characterized. Ligand specificities of the frog and zebrafish LXRs were very similar to LXRalpha and LXRbeta from human and mouse. All vertebrate LXRs studied were activated robustly by the synthetic ligands T-0901317 and GW3965 and by a variety of oxysterols. In contrast, Ciona LXR was not activated by T-0901317 or GW3965 but was activated by a limited number of oxysterols, as well as some androstane and pregnane steroids. Pharmacophore analysis, homology modeling, and docking studies of Ciona LXR predict a receptor with a more restricted ligand-binding pocket and less intrinsic disorder in the ligand-binding domain compared to vertebrate LXRs. The results suggest that LXRs have a long evolutionary history, with vertebrate LXRs diverging from invertebrate LXRs in ligand specificity.

Evolution of the bile salt nuclear receptor FXR in vertebrates

Bile salts, the major end metabolites of cholesterol, vary significantly in structure across vertebrate species, suggesting that nuclear receptors binding these molecules may show adaptive evolutionary changes. We compared across species the bile salt specificity of the major transcriptional regulator of bile salt synthesis, the farnesoid X receptor (FXR). We found that FXRs have changed specificity for primary bile salts across species by altering the shape and size of the ligand binding pocket. In particular, the ligand binding pockets of sea lamprey (Petromyzon marinus) and zebrafish (Danio rerio) FXRs, as predicted by homology models, are flat and ideal for binding planar, evolutionarily early bile alcohols. In contrast, human FXR has a curved binding pocket best suited for the bent steroid ring configuration typical of evolutionarily more recent bile acids. We also found that the putative FXR from the sea squirt Ciona intestinalis, a chordate invertebrate, was completely insensitive to activation by bile salts but was activated by sulfated pregnane steroids, suggesting that the endogenous ligands of this receptor may be steroidal in nature. Our observations present an integrated picture of the coevolution of bile salt structure and of the binding pocket of their target nuclear receptor FXR.

The farnesoid X receptor FXRalpha/NR1H4 acquired ligand specificity for bile salts late in vertebrate evolution

The nuclear receptor FXRalpha (NR1H4) plays a pivotal role in maintaining bile salt and lipid homeostasis by functioning as a bile salt sensor in mammals. In contrast, FXRbeta (NR1H5) from mouse is activated by lanosterol and does not share common ligands with FXRalpha. To further elucidate FXR ligand/receptor and structure/function relationships, we characterized a FXR gene from the marine skate, Leucoraja erinacea, representing a vertebrate lineage that diverged over 400 million years ago. Phylogenetic analysis of sequence data indicated that skate Fxr (sFxr) is a FXRbeta. There is an extra sequence in the middle of the sFxr ligand binding domain (LBD) compared with the LBD of FXRalpha. Luciferase reporter assays demonstrated that sFxr responds weakly to scymnol sulfate, bile salts, and synthetic FXRalpha ligands, in striking difference from human FXRalpha (hFXRalpha). Interestingly, all-trans retinoic acid was capable of transactivating both hFXRalpha and sFxr. When the extra amino acids in the sFxr LBD were deleted and replaced with the corresponding sequence from hFXRalpha, the mutant sFxr gained responsiveness to ursodeoxycholic acid, GW4064, and fexaramine. Surprisingly, chenodeoxycholic acid antagonized this activation. Together, these results indicate that FXR is an ancient nuclear receptor and suggest that FXRalpha may have acquired ligand specificity for bile acids later in evolution by deletion of a sequence from its LBD. Acquisition of this property may be an example of molecular exploitation, where an older molecule is recruited for a new functional role.

Repression of Wnt/beta-catenin signaling in the anterior endoderm is essential for liver and pancreas development

The liver and pancreas are specified from the foregut endoderm through an interaction with the adjacent mesoderm. However, the earlier molecular mechanisms that establish the foregut precursors are largely unknown. In this study, we have identified a molecular pathway linking gastrula-stage endoderm patterning to organ specification. We show that in gastrula and early-somite stage Xenopus embryos, Wnt/beta-catenin activity must be repressed in the anterior endoderm to maintain foregut identity and to allow liver and pancreas development. By contrast, high beta-catenin activity in the posterior endoderm inhibits foregut fate while promoting intestinal development. Experimentally repressing beta-catenin activity in the posterior endoderm was sufficient to induce ectopic organ buds that express early liver and pancreas markers. beta-catenin acts in part by inhibiting expression of the homeobox gene hhex, which is one of the earliest foregut markers and is essential for liver and pancreas development. Promoter analysis indicates that beta-catenin represses hhex transcription indirectly via the homeodomain repressor Vent2. Later in development, beta-catenin activity has the opposite effect and enhances liver development. These results illustrate that turning Wnt signaling off and on in the correct temporal sequence is essential for organ formation, a finding that might directly impact efforts to differentiate liver and pancreas tissue from stem cells.

Identification of intermediates in the bile acid synthetic pathway as ligands for the farnesoid X receptor

Bile acid synthesis from cholesterol is tightly regulated via a feedback mechanism mediated by the farnesoid X receptor (FXR), a nuclear receptor activated by bile acids. Synthesis via the classic pathway is initiated by a series of cholesterol ring modifications and followed by the side chain cleavage. Several intermediates accumulate or are excreted as end products of the pathway in diseases involving defective bile acid biosynthesis. In this study, we investigated the ability of these intermediates to activate human FXR. In a cell-based reporter assay and coactivator recruitment assays in vitro, early intermediates possessing an intact cholesterol side chain were inactive, whereas 26- or 25-hydroxylated bile alcohols and C27 bile acids were highly efficacious ligands for FXR at a level comparable to that of the most potent physiological ligand, chenodeoxycholic acid. Treatment of HepG2 cells with these precursors repressed the rate-limiting cholesterol 7alpha-hydroxylase mRNA level and induced the small heterodimer partner and the bile salt export pump mRNA, indicating the ability to regulate bile acid synthesis and excretion. Because 26-hydroxylated bile alcohols and C27 bile acids are known to be evolutionary precursors of bile acids in mammals, our findings suggest that human FXR may have retained affinity to these precursors during evolution.

Differential Role of the Loop Region between Helices H6 and H7 within the Orphan Nuclear Receptors Small Heterodimer Partner and DAX-1

The orphan nuclear receptors small heterodimer partner (SHP) and dosage-sensitive sex-reversal adrenal hypoplasia congenital (AHC) critical region on the X chromosome gene 1 (DAX-1) contain extra amino acids between helices H6 and H7 of LBD, and here we investigated a possible role of these additional amino acids. Transient transfection assay demonstrated that, in contrast to wild type, in mutant SHP Delta128-139 deletion of 12 extra amino acids in H6-H7 failed to repress the transactivity of orphan nuclear receptors such as estrogen receptor-related receptor-gamma, hepatocyte nuclear factor 4alpha, and constitutive androstane receptor. Interestingly, yeast two-hybrid and glutathione-S-transferase pull-down assays demonstrated that wild-type and SHP Delta128-139 have similar abilities to interact with estrogen receptor-related receptor-gamma, hepatocyte nuclear factor 4alpha, and constitutive androstane receptor. Unexpectedly, in wild-type DAX-1 and mutant DAX-1 Delta338-362, deletion of 25 extra amino acids in H6-H7 had no significant difference in the interaction and repression of steroidogenic factor 1 transactivation. Mutant SHP that contains DAX-1 extra amino acids or polyalanine stretch in H6-H7 showed indistinguishable pattern of repression from wild-type SHP. Interestingly, the swapped SHP mutant with DAX-1 extra amino acids interacted with EID-1 (E1A-like inhibitor of differentiation 1), which is characterized as an SHP-interacting corepressor. However, interaction between SHP Delta128-139 and EID-1 was significantly diminished. Moreover, SHP-mediated repression of constitutive androstane receptor transactivation was significantly released by down-regulation of EID-1 expression with EID-1 small interfering RNA. The present study suggests that H6-H7 loop regions of SHP and DAX-1 play a different role in the repression of nuclear receptor transactivation.

Crustacean retinoid-X receptor isoforms: distinctive DNA binding and receptor-receptor interaction with a cognate ecdysteroid receptor

We have identified cDNA clones that encode homologs of the ecdysteroid receptor (EcR) and retinoid-X receptor (RXR)/USP classes of nuclear receptors from the fiddler crab Uca pugilator (UpEcR and UpRXR). Several UpRXR cDNA splicing variants were found in coding regions that could potentially influence function. A five-amino acid (aa) insertion/deletion is located in the "T" box in the hinge region. Another 33-aa insertion/deletion is found inside the ligand-binding domain (LBD), between helix 1 and helix 3. Ribonuclease protection assays (RPA) showed that four UpRXR transcripts [UpRXR(+5+33), UpRXR(-5+33), UpRXR(+5-33) and UpRXR(-5-33)] were present in regenerating limb buds. UpRXR(-5+33) was the most abundant transcript present in regenerating limb buds in both early blastema and late premolt growth stages. Expression vectors for these UpRXR variants and UpEcR were constructed, and the proteins expressed in E. coli and in vitro expression systems. The expressed crab nuclear receptors were then characterized by electrophoretic mobility shift assay (EMSA) and glutathione S-transferase (GST) pull down experiments. EMSA results showed that UpEcR/UpRXR(-5+33) heterocomplexes bound with a series of hormone response elements (HREs) including eip28/29, IRper-1, DR-4, and IRhsp-1 with appreciable affinity. Competition EMSA also showed that the affinity decreased as sequence composition deviated from a perfect consensus element. Binding to IRper-1 HREs occurred only if the heterodimer partner UpRXR contained the 33-aa LBD insertion. UpRXR lacking both the 5-aa and 33-aa insertion bound to a DR-1G HRE in the absence of UpEcR. The results of GST-pull down experiments showed that UpEcR interacted only with UpRXR variants containing the 33-aa insertion, and not with those lacking the 33-aa insertion. These in vitro receptor protein-DNA and receptor protein-protein interactions occurred in the absence of hormone (20-hydroxyecdysone and 9-cis retinoid acid, 9-cis RA). Transactivation studies using a hybrid UpEcR ligand-binding domain construct and UpRXR (+/-33) ligand-binding domain constructs also showed that the 33-aa insertion was indispensable in mediating ecdysteroid stimulated transactivation.

Active repression of organizer genes by C-terminal domain of PV.1

PV.1, a homeotic protein, ventralizes dorsal mesoderm and inhibits neuralization by mediating BMP-4 signaling in Xenopus embryo. In our previous report antimorphic PV.1 causes a secondary axis by inducing the ectopic organizer. We analyzed the structure of this transcription factor through domain level assessment. In a phenotype-inducing test, half of the N-terminus at the N-terminal side was unessential for inducing ventralization of embryos. We examined the transacting activity of several regions of PV.1 utilizing GAL4 hybrid system. The C-terminal region/GAL4DBD (DNA binding domain) exhibited strong repressive activity on a reporter gene (operator/promoter/reporter; Gal4-TK-luc) as much as the whole polypeptide/GAL4DBD, whereas the N-terminal region/GAL4DBD showed only modest repression. The results suggest that PV.1 functions as a transcriptional repressor and this repressive activity is localized mostly to the C-terminal region. Additional characterizations of N- and C-terminus with respect to the effects on the expression of other genes are described.

Induction of cardiomyocytes by GATA4 in Xenopus ectodermal explants

The earliest step in heart formation in vertebrates occurs during gastrulation, when cardiac tissue is specified. Dorsoanterior endoderm is thought to provide a signal that induces adjacent mesodermal cells to adopt a cardiac fate. However, the nature of this signalling and the precise role of endoderm are unknown because of the close proximity and interdependence of mesoderm and endoderm during gastrulation. To better define the molecular events that underlie cardiac induction, we have sought to develop a simple means of inducing cardiac tissue. We show that the transcription factor GATA4, which has been implicated in regulating cardiac gene expression, is sufficient to induce cardiac differentiation in Xenopus embryonic ectoderm (animal pole) explants, frequently resulting in beating tissue. Lineage labelling experiments demonstrate that GATA4 can trigger cardiac differentiation not only in cells in which it is present, but also in neighbouring cells. Surprisingly, cardiac differentiation can occur without any stable differentiation of anterior endoderm and is in fact enhanced under conditions in which endoderm formation is inhibited. Remarkably, cardiac tissue is formed even when GATA4 activity is delayed until long after explants have commenced differentiation into epidermal tissue. These findings provide a simple assay system for cardiac induction that may allow elucidation of pathways leading to cardiac differentiation. Better knowledge of the pathways governing this process may help develop procedures for efficient generation of cardiomyocytes from pluripotent stem cells.

Cholestatic syndromes

Further insights into the cellular and molecular mechanisms underlying hepatobiliary transport function and its regulation now permit a better understanding of the pathogenesis and treatment options of cholestatic liver diseases. Identification of the molecular basis of hereditary cholestatic syndromes will result in an improved diagnosis and management of these conditions. New insights into the pathogenesis of extrahepatic manifestations of cholestasis (eg, pruritus) have facilitated new treatment strategies. Important new studies have been published about the pathogenesis, clinical features, diagnosis, and treatment of primary biliary cirrhosis, primary sclerosing cholangitis, cholestasis of pregnancy, total parenteral nutrition-induced cholestasis, drug-induced cholestasis, and viral cholestatic syndromes.

Identification of farnesoid X receptor beta as a novel mammalian nuclear receptor sensing lanosterol

Nuclear receptors are ligand-modulated transcription factors. On the basis of the completed human genome sequence, this family was thought to contain 48 functional members. However, by mining human and mouse genomic sequences, we identified FXRbeta as a novel family member. It is a functional receptor in mice, rats, rabbits, and dogs but constitutes a pseudogene in humans and primates. Murine FXRbeta is widely coexpressed with FXR in embryonic and adult tissues. It heterodimerizes with RXRalpha and stimulates transcription through specific DNA response elements upon addition of 9-cis-retinoic acid. Finally, we identified lanosterol as a candidate endogenous ligand that induces coactivator recruitment and transcriptional activation by mFXRbeta. Lanosterol is an intermediate of cholesterol biosynthesis, which suggests a direct role in the control of cholesterol biosynthesis in nonprimates. The identification of FXRbeta as a novel functional receptor in nonprimate animals sheds new light on the species differences in cholesterol metabolism and has strong implications for the interpretation of genetic and pharmacological studies of FXR-directed physiologies and drug discovery programs.