The nuclear hormone receptor Ftz-F1 is a cofactor for the Drosophila homeodomain protein Ftz

Identification and characterization of a novel fushi tarazu factor 1 (FTZ-F1) nuclear receptor in Schistosoma mansoni

Fushi-tarazu factor-1 (FTZ-F1) is an orphan nuclear receptor involved in gene regulation of various developmental processes and physiological activities. We identified a new member of ftz-f1 gene in Schistosoma mansoni, termed Smftz-f1alpha. The Smftz-f1alpha gene has a complex structure with 15 exons interrupted by 14 introns. It encodes an unusually long SmFTZ-F1alpha protein of 1892 amino acids containing all the modular domains found in nuclear receptors. The DNA-binding domain (DBD) of SmFTZ-F1alpha is conserved and most similar to those of human and mouse FTZ-F1 orthologues, exhibiting a 76% identity. The ligand-binding domain (LBD) is less conserved than the DBD; it shares more diverse identity scores in different regions ranging from 23% to 42% in region II and 28% to 72% in region III. A conserved activation function-2 (AF-2) sequence is present in the SmFTZ-F1alpha LBD. This protein also contains a long hinge region (1027 aa) and an F region (220 aa) at the carboxyl end. Phylogenetic analysis suggests that SmFTZ-F1alpha is the orthologue of Drosophila FTZ-F1alpha and vertebrate NR5 members. Western blot analysis of a schistosome extract identified two proteins, one with a size (206 kDa) predicted by the SmFTZ-F1alpha cDNA sequence and a smaller component of 120 kDa. Smftz-f1alpha is expressed throughout the schistosome life cycle with the highest expression in the egg stage. SmFTZ-F1alpha mRNA is widely distributed in adult worms but does not appear in vitelline cells of female worms. SmFTZ-F1alpha localizes to a variety of tissues but is most abundant in the testis of the male and the ovary of female worms. Our results suggest that SmFTZ-F1alpha plays a role in regulating schistosome development and sexual differentiation similar to other FTZ-F1 family members.

Computational Identification of Ftz/Ftz-F1 downstream target genes

Hox genes encode DNA binding transcription factors that regulate the body plans of metazoans by regulating the expression of downstream target 'realizator genes' that direct morphogenesis and growth. Although some Hox target genes have been identified, the code used by Hox proteins to select regulatory targets remains elusive. This failure is due, in part, to the overlapping and promiscuous DNA binding potential of different Hox proteins. The identification of cofactors that modulate Hox DNA binding specificity suggested that target site selection is specified by composite binding sites in the genome for a Hox protein plus its cofactor. Here we have made use of the fact that the DNA binding specificity of the Drosophila Hox protein Fushi Tarazu (Ftz) is modulated by interaction with its partner, the orphan nuclear receptor Ftz-F1, to carry out a computational screen for genomic targets. At least two of the first 30 potential target genes--apontic (apt) and sulfated (Sulf1)--appear to be bona fide targets of Ftz and Ftz-F1. apt is expressed in stripes within the Ftz domain, but posterior to engrailed (en) stripes, suggesting a parasegmental border-independent function of ftz. Ftz/Ftz-F1 activate Sulf1 expression in blastoderm embryos via composite binding sites. Sulf1 encodes a sulfatase thought to be involved in wingless (Wg) signaling. Thus, in addition to regulating en, Ftz and Ftz-F1 coordinately and directly regulate different components of segment polarity pathways in parallel.

The HMG-box protein Lilliputian is required for Runt-dependent activation of the pair-rule gene fushi-tarazu

lilliputian (lilli), the sole Drosophila member of the FMR2/AF4 (Fragile X Mental Retardation/Acute Lymphoblastic Leukemia) family of transcription factors, is widely expressed with roles in segmentation, cellularization, and gastrulation during early embryogenesis with additional distinct roles at later stages of embryonic and postembryonic development. We identified lilli in a genetic screen based on the suppression of a lethal phenotype that is associated with ectopic expression of the transcription factor encoded by the segmentation gene runt in the blastoderm embryo. In contrast to other factors identified by this screen, lilli appears to have no role in mediating either the establishment or maintenance of engrailed (en) repression by Runt. Instead, we find that Lilli plays a critical role in the Runt-dependent activation of the pair-rule segmentation gene fushi-tarazu (ftz). The requirement for lilli is distinct from and temporally precedes the Runt-dependent activation of ftz that is mediated by the orphan nuclear receptor protein Ftz-F1. We further describe a role for lilli in the activation of Sex-lethal (Sxl), an early target of Runt in the sex determination pathway. However, lilli is not required for all targets that are activated by Runt and appears to have no role in activation of sloppy paired (slp1). Based on these results we suggest that Lilli plays an architectural role in facilitating transcriptional activation that depends both on the target gene and the developmental context.

Activation of the Blue Opsin Gene in Cone Photoreceptor Development by Retinoid-Related Orphan Receptor β

Color vision requires the expression of opsin photopigments with different wavelength sensitivities in retinal cone photoreceptors. The basic color visual system of mammals is dichromatic, involving differential expression in the cone population of two opsins with sensitivity to short (S, blue) or medium (M, green) wavelengths. However, little is known of the factors that directly activate these opsin genes and thereby contribute to the S or M opsin identity of the cone. We report that the orphan nuclear receptor RORβ (retinoid-related orphan receptor β) activates the S opsin gene (Opn1sw) through binding sites upstream of the gene. RORβ lacks a known physiological ligand and activates the Opn1sw promoter modestly alone but strongly in synergy with the retinal cone-rod homeobox factor (CRX), suggesting a cooperative means of enhancing RORβ activity. Comparison of wild-type and mutant lacZ reporter transgenes showed that the RORβ-binding sites in Opn1sw are required for expression in mouse retina. RORβ-deficient mice fail to induce S opsin appropriately during postnatal cone development. Photoreceptors in these mice also lack outer segments, indicating additional functions for RORβ in photoreceptor morphological maturation. The results identify Opn1sw as a target gene for RORβ and suggest a key role for RORβ in regulating opsin expression in the color visual system.

Molecular cloning and tissue distribution of SF-1-related orphan receptors during sexual maturation in female goldfish

The steroidogenic factor (SF)-1 gene is one of a number of orphan nuclear receptors, which is a key transcriptional regulator in vertebrate reproduction. We have isolated the SF-1 homologue cDNA from the goldfish pituitary and designed primers for SF-1 on the basis of the highly conserved regions of various known SF-1 superfamily genes. SF-1 cDNA contained 1,948 nucleotides including an open reading frame predicted to encode a protein of 503 amino acids. The distribution pattern of SF-1 in a variety of tissues during sexual maturation in female goldfish was also examined by RT-PCR. Significant variations in the relative expression of SF-1 were observed in different tissues in immature and mature female goldfish. SF-1 transcript in pituitary was significantly higher than other tissues tested in immature and mature female goldfish. Lower expression of SF-1 was observed in the liver but was not detected in brain and ovary of the immature female goldfish. Presence of SF-1 was the predominant expression in the pituitary and brain of mature female goldfish. Also, in the mature female goldfish, a weak transcript was detected in liver and ovary. Interestingly, RT-PCR analysis revealed that the expression of SF-1 became higher in the brain and weaker in the liver in maturing female goldfish. Thus, SF-1 may be regulated in goldfish brain and/or liver. Thus is also tissue-specific distribution of SF-1 during sexual maturation in female goldfish.

Cooperative transcription activation by Nurr1 and Pitx3 induces embryonic stem cell maturation to the midbrain dopamine neuron phenotype

Midbrain dopamine (DA) neurons play a central role in the regulation of voluntary movement, and their degeneration is associated with Parkinson's disease. Cell replacement therapies, and in particular embryonic stem (ES) cell-derived DA neurons, offer a potential therapeutic venue for Parkinson's disease. We sought to identify genes that can potentiate maturation of ES cell cultures to the midbrain DA neuron phenotype. A number of transcription factors have been implicated in the development of midbrain DA neurons by expression analyses and loss-of-function knockout mouse studies, including Nurr1, Pitx3, Lmx1b, Engrailed-1, and Engrailed-2. However, none of these factors appear sufficient alone to induce the mature midbrain DA neuron phenotype in ES cell cultures in vitro, suggesting a more complex regulatory network. Here we show that Nurr1 and Pitx3 cooperatively promote terminal maturation to the midbrain DA neuron phenotype in murine and human ES cell cultures.

Over-expression and purification of isotopically labeled recombinant ligand-binding domain of orphan nuclear receptor human B1-binding factor/human liver receptor homologue 1 for NMR studies

The human hepatitis B virus enhancer II B1 binding factor (hB1F), which regulates the expression of hepatitis B virus genes, is identified as a nuclear receptor. It regulates several liver-specific genes and plays an important role in the bile acid biosynthesis pathway. A significantly optimized protocol has been worked out to prepare 15N and/or 13C-labeled hB1F ligand-binding domain in minimal medium with high yields for NMR studies. Under the various conditions optimized for the purification of His6-hB1F ligand-binding domain, the yield of the purified protein is estimated to be 25-30 mg from 0.5 L of M9 minimal media. Electrospray ionization mass spectrometry data confirm the correctness of the primary sequence. Dynamic light scattering experiment proves that the protein exists as a monomeric form. In addition, the circular dichroism results show that the protein has a well-regulated secondary structure and a high alpha-helical content in ammonium bicarbonate buffer at 20 degrees C and pH 7.4. Finally, uniformly 15N-labeled protein is characterized by a TROSY-HSQC spectrum, and the dispersion of 15N-1H cross-peaks in the spectrum indicates the presence of well-ordered and properly folded protein as a monomer.

Aryl hydrocarbon receptor activation by cAMP vs. dioxin: divergent signaling pathways

Even before the first vertebrates appeared on our planet, the aryl hydrocarbon receptor (AHR) gene was present to carry out one or more critical life functions. The vertebrate AHR then evolved to take on functions of detecting and responding to certain classes of environmental toxicants. These environmental pollutants include polycyclic aromatic hydrocarbons (e.g., benzo[a]pyrene), polyhalogenated hydrocarbons, dibenzofurans, and the most potent small-molecular-weight toxicant known, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin). After binding of these ligands, the activated AHR translocates rapidly from the cytosol to the nucleus, where it forms a heterodimer with aryl hydrocarbon nuclear translocator, causing cellular responses that lead to toxicity, carcinogenesis, and teratogenesis. The nuclear form of the activated AHR/aryl hydrocarbon nuclear translocator complex is responsible for alterations in immune, endocrine, reproductive, developmental, cardiovascular, and central nervous system functions whose mechanisms remain poorly understood. Here, we show that the second messenger, cAMP (an endogenous mediator of hormones, neurotransmitters, and prostaglandins), activates the AHR, moving the receptor to the nucleus in some ways that are similar to and in other ways fundamentally different from AHR activation by dioxin. We suggest that this cAMP-mediated activation may reflect the true endogenous function of AHR; disruption of the cAMP-mediated activation by dioxin, binding chronically to the AHR for days, weeks, or months, might be pivotal in the mechanism of dioxin toxicity. Understanding this endogenous activation of the AHR by cAMP may help in developing methods to counteract the toxicity caused by numerous environmental and food-borne toxic chemicals that act via the AHR.

Nuclear receptor NHR-25 is required for cell-shape dynamics during epidermal differentiation in Caenorhabditis elegans

Epithelial cell shape changes underlie important events in animal development. During the postembryonic life of the nematode Caenorhabditis elegans, stem epidermal seam cells lose and actively renew mutual adherens junction contacts after each asymmetric division that separates them. The seam cell contacts are important for epidermal differentiation, but what regulates the cell-shape changes that restore them is unknown. Here, we show that NHR-25, a transcription factor of the nuclear receptor family, is expressed in the seam cells and is necessary for these cells to elongate and reach their neighbors after the asymmetric divisions. A failure to do so, caused by nhr-25 RNA interference, compromises the subsequent fate of seam-cell anterior daughters. Unexpectedly, the lack of cell-cell contacts does not prevent a unique seam cell to produce a neuroblast, even though a homeotic gene (mab-5) that normally prevents the neuroblast commitment is ectopically expressed in the absence of nhr-25 function. Seam cells lacking mutual contacts display reduced expression of a Fat-like cadherin marker cdh-3::gfp. Although some seam cells retain the ability to fuse at the final larval stage, the resulting syncytium shows gaps and bifurcations, translating into anomalies in cuticular ridges (alae) produced by the syncytium. nhr-25 RNAi markedly enhances branching of the alae caused by a mutant cuticular collagen gene rol-6. Silencing of nhr-25 also disturbs epidermal ultrastructure, which is probably the cause of compromised cuticle secretion and molting. Cell shape dynamics and molting thus represent distinct roles for NHR-25 in epidermal development.

A screen for genes that interact with the Drosophila pair-rule segmentation gene fushi tarazu

The pair-rule gene fushi tarazu (ftz) of Drosophila is expressed at the blastoderm stage in seven stripes that serve to define the even-numbered parasegments. ftz encodes a DNA-binding homeodomain protein and is known to regulate genes of the segment polarity, homeotic, and pair-rule classes. Despite intensive analysis in a number of laboratories, how ftz is regulated and how it controls its targets are still poorly understood. To help understand these processes, we conducted a screen to identify dominant mutations that enhance the lethality of a ftz temperature-sensitive mutant. Twenty-six enhancers were isolated, which define 21 genes. All but one of the mutations recovered show a maternal effect in their interaction with ftz. Three of the enhancers proved to be alleles of the known ftz protein cofactor gene ftz-f1, demonstrating the efficacy of the screen. Four enhancers are alleles of Atrophin (Atro), the Drosophila homolog of the human gene responsible for the neurodegenerative disease dentatorubral-pallidoluysian atrophy. Embryos from Atro mutant germ-line mothers lack the even-numbered (ftz-dependent) engrailed stripes and show strong ftz-like segmentation defects. These defects likely result from a reduction in Even-skipped (Eve) repression ability, as Atro has been shown to function as a corepressor for Eve. In this study, we present evidence that Atro is also a member of the trithorax group (trxG) of Hox gene regulators. Atro appears to be particularly closely related in function to the trxG gene osa, which encodes a component of the brahma chromatin remodeling complex. One additional gene was identified that causes pair-rule segmentation defects in embryos from homozygous mutant germ-line mothers. The single allele of this gene, called bek, also causes nuclear abnormalities similar to those caused by alleles of the Trithorax-like gene, which encodes the GAGA factor.

Nuclear receptors--a perspective from Drosophila

Nuclear receptors are ancient ligand-regulated transcription factors that control key metabolic and developmental pathways. The fruitfly Drosophila melanogaster has only 18 nuclear-receptor genes - far fewer than any other genetic model organism and representing all 6 subfamilies of vertebrate receptors. These unique attributes establish the fly as an ideal system for studying the regulation and function of nuclear receptors during development. Here, we review recent breakthroughs in our understanding of D. melanogaster nuclear receptors, and interpret these results in light of findings from their evolutionarily conserved vertebrate homologues.

Convergent genetic programs regulate similarities and differences between related motor neuron classes in Caenorhabditis elegans

How do genetic programs create features common to a specific cell or tissue type while generating modifications necessary for functional diversification? We have addressed this question using the nematode Caenorhabditis elegans. The dorsal D (DD) and ventral D (VD) motorneurons (mns), referred to collectively as the D mns, compose a cross-inhibitory network that contributes to the animal's sinuous locomotion. The D mns share a number of structural and functional features, but are distinguished from one another by their synaptic patterns and the expression of a neuropeptide gene. Our findings suggest that the similarities and differences are generated at the transcriptional level. UNC-30 contains a homeodomain and activates structural and functional genes expressed in both classes. UNC-55 is a nuclear receptor expressed in the VD mns that is necessary for generating features that distinguish the two classes of D mns from one another. In unc-55 mutants, the VD mns adopt the DD mn synaptic pattern and peptide expression profile. Conversely, ectopic expression of unc-55 in the DD mns causes them to adopt VD mn features. The promoter of the neuropeptide gene expressed in the DD mns contains putative binding sites for both UNC-30 and UNC-55; alteration of these sites suggests that UNC-55 represses the ability of UNC-30 to activate a subset of genes that are expressed in the DD mns but not in the VD mns. Thus UNC-55 acts as a switch for the features that distinguish these two functionally related classes of mns.

Cofactor-Interaction Motifs and the Cooption of a Homeotic Hox Protein into the Segmentation Pathway of Drosophila melanogaster

Some Drosophila Hox-complex members, including the segmentation gene fushi tarazu (Dm-ftz), have nonhomeotic functions. Characteristic expression in other arthropods supports an ancestral homeotic role for ftz, indicating that ftz function changed during arthropod evolution. Dm-Ftz segmentation function depends on interaction with ftz-F1 via an LXXLL motif and homeodomain N-terminal arm. Hox proteins interact with the cofactor Extradenticle (Exd) via their YPWM motif. Previously, we found that Dm-ftz mediates segmentation but not homeosis, whereas orthologs from grasshopper (Sg-ftz) and beetle (Tc-Ftz), both containing a YPWM motif, have homeotic function. Tc-Ftz, which unlike Sg-Ftz contains an LXXLL motif, displays stronger segmentation function than Sg-Ftz. Cofactor-interaction motifs were mutated in Dm-Ftz and Tc-Ftz and effects were evaluated in Drosophila to assess how these motifs contributed to Ftz evolution. Addition of YPWM to Dm-Ftz confers weak homeotic function, which is increased by simultaneous LXXLL mutation. LXXLL is required for strong segmentation function, which is unimpeded by the YPWM, suggesting that acquisition of LXXLL specialized Ftz for segmentation. Strengthening the Ftz/Ftz-F1 interaction led to degeneration of the YPWM and loss of homeotic activity. Thus, small changes in protein sequence can result in a qualitative switch in function during evolution.

Unique functional properties of a member of the Fushi Tarazu-Factor 1 family from Schistosoma mansoni

SmFtz-F1 (Schistosoma mansoni Fushi Tarazu-Factor 1) belongs to the Ftz-F1 subfamily of nuclear receptors, but displays marked structural differences compared with its mammalian homologues SF-1 (steroidogenic factor-1) or liver receptor homologue-1. These include a long F domain (104 amino acids), an unusually large hinge region (133 amino acids) and a poorly conserved E-domain. Here, using Gal4 constructs and a mammalian two-hybrid assay, we have characterized the roles of these specific regions both in the transcriptional activity of the receptor and in its interactions with cofactors. Our results have shown that, although the AF-2 (activation function-2) region is the major activation function of the receptor, both the F and D domains are essential for AF-2-dependent activity. Modelling of SmFtz-F1 LBD (ligand-binding domain) and structure-guided mutagenesis allowed us to show the important role of helix H1 in maintaining the structural conformation of the LBD, and suggested that its autonomous transactivation activity, also observed with SF-1, is fortuitous. This strategy also allowed us to study an eventual ligand-dependence for this orphan receptor, the predicted three-dimensional models suggesting that the SmFtz-F1 LBD contains a large and well-defined ligand-binding pocket sealed by two arginine residues orientated towards the interior of the cavity. Mutation of these two residues provoked a loss of transcriptional activity of the receptor, and strongly reduced its interaction with SRC1 (steroid receptor cofactor-1), suggesting a ligand-dependent activity for SmFtz-F1. Taken together, our results argue for original and specific functional activities for this platyhelminth nuclear receptor.

LRH-1: an orphan nuclear receptor involved in development, metabolism and steroidogenesis

The liver receptor homolog-1 (LRH-1; NR5A2) and steroidogenic factor-1 (SF-1; NR5A1) are two orphan members of the Ftz-F1 subfamily of nuclear receptors. LRH-1 is expressed in tissues derived from endoderm, including intestine, liver and exocrine pancreas, as well as in the ovary. In these tissues, LRH-1 plays a predominant role in development, reverse cholesterol transport, bile-acid homeostasis and steroidogenesis. SF-1 expression is confined to steroidogenic tissues and the hypothalamo-pituitary-adrenal axis, where it is involved in the control of development, differentiation, steroidogenesis and sexual determination. In this article, we will review data concerning the structure, regulation and function of LRH-1. These data highlight structural similarities between LRH-1 and other Ftz-F1 members but also underscore important functional differences, assigning to LRH-1 a unique position among nuclear receptors.

The Caenorhabditis elegans nuclear receptor gene nhr-25 regulates epidermal cell development

The development of the epidermis of Caenorhabditis elegans involves cell fusion, migration, and differentiation events. To understand the mechanisms underlying these processes, we characterized the roles of NHR-25, a member of the nuclear receptor family of transcription factors. The NHR-25 homologs Ftz-F1 in Drosophila and SF-1 in mammals are involved in various biological processes, including regulation of patterning during development, reproduction, metabolism, metamorphosis, and homeostasis. Impairment of nhr-25 activity leads to severe phenotypes in embryos and many postembryonic tissues. Further analysis has indicated that nhr-25 activity is required for the proper development, including cell-cell fusion, of several epidermal cell types, such as the epidermal syncytial, seam, and Pn.p cells. Our results also suggest that nhr-25 is likely to regulate cell-cell junctions and/or fusion. In a subset of Pn.p cells, called vulval precursor cells, nhr-25 acts collaboratively with the lin-39 Hox gene in regulating vulval cell differentiation. Additionally, our data suggest that nhr-25 may also function with another Hox gene, nob-1, during embryogenesis. Overall, our results indicate that nhr-25 plays an integral role in regulating cellular processes of epidermal cells.

Transcriptional control in the segmentation gene network of Drosophila

The segmentation gene network of Drosophila consists of maternal and zygotic factors that generate, by transcriptional (cross-) regulation, expression patterns of increasing complexity along the anterior-posterior axis of the embryo. Using known binding site information for maternal and zygotic gap transcription factors, the computer algorithm Ahab recovers known segmentation control elements (modules) with excellent success and predicts many novel modules within the network and genome-wide. We show that novel module predictions are highly enriched in the network and typically clustered proximal to the promoter, not only upstream, but also in intronic space and downstream. When placed upstream of a reporter gene, they consistently drive patterned blastoderm expression, in most cases faithfully producing one or more pattern elements of the endogenous gene. Moreover, we demonstrate for the entire set of known and newly validated modules that Ahab's prediction of binding sites correlates well with the expression patterns produced by the modules, revealing basic rules governing their composition. Specifically, we show that maternal factors consistently act as activators and that gap factors act as repressors, except for the bimodal factor Hunchback. Our data suggest a simple context-dependent rule for its switch from repressive to activating function. Overall, the composition of modules appears well fitted to the spatiotemporal distribution of their positive and negative input factors. Finally, by comparing Ahab predictions with different categories of transcription factor input, we confirm the global regulatory structure of the segmentation gene network, but find odd skipped behaving like a primary pair-rule gene. The study expands our knowledge of the segmentation gene network by increasing the number of experimentally tested modules by 50%. For the first time, the entire set of validated modules is analyzed for binding site composition under a uniform set of criteria, permitting the definition of basic composition rules. The study demonstrates that computational methods are a powerful complement to experimental approaches in the analysis of transcription networks.

Functional conservation of interactions between a homeodomain cofactor and a mammalian FTZ-F1 homologue

Nuclear receptors are master regulators of metazoan gene expression with crucial roles during development and in adult physiology. Fushi tarazu factor 1 (FTZ-F1) subfamily members are ancient orphan receptors with homologues from Drosophila to human that regulate diverse gene expression programs important for developmental processes, reproduction and cholesterol homeostasis in an apparently ligand-independent manner. Thus, developmental and tissue-specific cofactors may be particularly important in modulating the transcriptional activities of FTZ-F1 receptors. In Drosophila, the homeodomain protein Fushi tarazu acts as a cofactor for FTZ-F1 (NR5A3), leading to the hypothesis that a similar type of homeodomain cofactor-nuclear receptor relationship might exist in vertebrates. In this study, we have identified and characterized the homeodomain protein Prox1 as a co-repressor for liver receptor homologue 1 (LRH1/NR5A2), a master regulator of cholesterol homeostasis in mammals. Our study suggests that interactions between LRH1 and Prox1 may fulfil roles both during development of the enterohepatic system and in adult physiology of the liver.

The fetoprotein transcription factor (FTF) gene is essential to embryogenesis and cholesterol homeostasis and is regulated by a DR4 element

The fetoprotein transcription factor (FTF) gene was inactivated in the mouse, with a lacZ gene inserted inframe into exon 4. LacZ staining of FTF+/- embryos shows that the mFTF gene is activated at initial stages of zygotic transcription. FTF gene activity is ubiquitous at the morula and blastocyst stages and then follows expression patterns indicative of multiple FTF functions in fetal development. FTF-/- embryos die at E6.5-7.5, with features typical of visceral endoderm dysfunction. Adult FTF+/- mice are hypocholesterolemic, and express liver FTF at about 40% of the normal level. Overexpression of liver FTF in transgenic mice indicates in vivo that FTF is an activator of CYP7A1. However, CYP7A1 expression is increased in FTF+/- liver. Gene expression profiles indicate that higher CYP7A1 expression is caused by attenuated liver cell stress signaling. Diet experiments support a model where FTF is quenched both by activated c-Jun, and by SHP as a stronger feedback mechanism to repress CYP7A1. A DR4 element is conserved in the FTF gene promoter and activated by LXR-RXR and TR-RXR, qualifying the FTF gene as a direct metabolic sensor. Liver FTF increases in rats treated with thyroid hormone or a high cholesterol diet. The FTF DR4 element tightens functional links between FTF and LXRalpha in cholesterol homeostasis and can explain transient surges of FTF gene activities during development and FTF levels lower than predicted in FTF+/- liver. The FTF-lacZ mouse establishes a central role for FTF in developmental, nutritive, and metabolic functions from early embryogenesis through adulthood.

Low-level ectopic expression of Fushi tarazu in Drosophila melanogaster results in ftzUal/Rpl-like phenotypes and rescues ftz phenotypes

The protein encoded by the Drosophila pair-rule gene fushi tarazu (ftz) is required for the formation of the even-numbered parasegments. Here we analyze the phenotypes of ectopic expression of FTZ and FTZ protein deletions from the Tubulin alpha1 (Tubalpha1) promoter. Fusion of ftz to the Tubalpha1 promoter resulted in low-level ectopic expression of FTZ relative to FTZ expressed from the endogenous ftz gene. The effects of ectopic expression of four FTZ proteins, FTZ(1-413) (full length wild-type FTZ), FTZ(delta257-316) (a complete deletion of the HD), FTZ(delta101-150) (a deletion that includes the major FTZ-F1 binding site) and FTZ(delta151-209) were determined. Ectopic expression of FTZ(1-413), FTZ(delta257-316) and FTZ(delta101-151) did not result in an anti-ftz phenotype; however, ectopic expression of FTZ(1-413), and FTZ(delta257-316) did result in a ftz(Ual/Rpl)-like phenotype. In addition, low-level ectopic expression of FTZ(1-413) and FTZ(delta257-316) rescued ftz phenotypes. This was an important observation because the even-numbered parasegment pattern of FTZ expression is considered important for normal segmentation. Therefore, the rescue of ftz phenotypes by low-level FTZ expression in all cells of the embryo suggests that the even-numbered parasegment expression pattern of FTZ is not the sole factor restricting FTZ action. Low-level ectopic expression of FTZ(delta151-209) resulted in the anti-ftz phenotype and rescued hypomorphic ftz-f1 phenotypes indicating that FTZ(delta151-209) is a hyperactive FTZ molecule. Therefore, the region encompassing amino acids 151-209 of FTZ is required in some manner for repression of FTZ activity. These results are discussed in relation to the current understanding of the mechanism of FTZ action.

Prox1 is a novel coregulator of Ff1b and is involved in the embryonic development of the zebra fish interrenal primordium

Steroidogenic factor 1 (SF-1) plays an essential role in adrenal development, although the exact molecular mechanisms are unclear. Our previous work established that Ff1b is the zebra fish homologue of SF-1 and that its disruption by antisense morpholinos leads to a complete ablation of the interrenal organ. In this study, results of biochemical analyses suggest that Ff1b and other Ff1 members interact with Prox1, a homeodomain protein. Fine mapping using site-directed mutants showed that this interaction requires an intact Ff1b heptad 9 and AF2, as well as Prox1 NR Box I. In vivo, this physical interaction led to the inhibition of Ff1-mediated transactivation of pLuc3XFRE, indicating that Prox1 acts to repress the transcriptional activity of Ff1b. In situ hybridization demonstrates that prox1 colocalizes with ff1a and ff1b in the liver and interrenal primordia, respectively. Embryos microinjected with prox1 morpholino displayed a consistent partial reduction of 3 eta-Hsd activity in the interrenal organ, while ff1b morpholino led to a disappearance of prox1. Based on these results, we propose that during the course of interrenal organogenesis, Prox1 functions as a tissue-specific coregulator of Ff1b and that the subsequent inhibition of Ff1b activity, after its initial roles in the specification of interrenal primordium, is critical for the maturation of the interrenal organ.

Molecular mechanisms underlying midbrain dopamine neuron development and function

The mesencephalic dopaminergic system is involved in the control of multiple brain functions including movement control and emotion and is of clinical importance because it is implicated in several psychiatric disorders, of which many are considered to have a neurodevelopmental origin. Studies into the developmental pathways of these neurons have led to the identification of the transcription factors En1, Pitx3, Nurr1 and Lmx1b, all shown to be important for the development of the mesencephalic dopaminergic system. In this paper, we discuss the consequences of genetic ablation of essential developmental genes. Furthermore, we discuss the consequences of changes in dopamine homeostasis for the function of the mesencephalic dopaminergic system. Finally, we analyse the potential of the mesencephalic dopaminergic system to adapt to gene dysfunction.

Enhancer sequences influence the role of the amino-terminal domain of bicoid in transcription

Bicoid (Bcd) is a Drosophila melanogaster morphogenetic gradient that controls embryonic patterning by activating target gene expression in a concentration-dependent manner. In this study we describe experiments to determine how different enhancers respond to Bcd distinctively, focusing on two natural Bcd-responsive enhancer elements, hunchback (hb) and knirps (kni). Our results show that, on the hb enhancer element, the amino-terminal domain of Bcd (residues 1 to 91) plays primarily an inhibitory role, whereas on the kni enhancer element this same Bcd domain plays a positive role at low protein concentrations. We further demonstrate that while the amino-terminal domain is largely dispensable for cooperative binding to the hb enhancer element, it is preferentially required for cooperative binding to the kni enhancer element. Alteration of the arrangement of Bcd binding sites in the kni enhancer element reduces the role of the amino-terminal domain in cooperative DNA binding but increases the effectiveness of the self-inhibitory function. In addition, elimination of symmetric pairs of Bcd binding sites in the kni enhancer element reduces both DNA binding and activation by Bcd. We propose that the amino-terminal domain of Bcd is an enhancer-specific switch that contributes to the protein's ability to activate different target genes in distinct manners.

Drosophila homeodomain protein REPO controls glial differentiation by cooperating with ETS and BTB transcription factors

In Drosophila, cell-fate determination of all neuroectoderm-derived glial cells depends on the transcription factor Glial cells missing (GCM), which serves as a binary switch between the neuronal and glial cell fates. Because the expression of GCM is restricted to the early phase of glial development, other factors must be responsible for the terminal differentiation of glial cells. Expression of three transcription factors, Reversed Polarity (REPO), Tramtrack p69 (TTK69) and PointedP1 (PNTP1), is induced by GCM in glial cells. REPO is a paired-like homeodomain protein, expressed exclusively in glial cells, and is required for the migration and differentiation of embryonic glial cells. To understand how REPO functions in glial terminal differentiation, we have analyzed the mechanism of gene regulation by REPO. We show that REPO can act as a transcriptional activator through the CAATTA motif in glial cells, and define three genes whose expression in vivo depends on REPO function. In different types of glial cells, REPO can act alone, or cooperate with either TTK69 or PNTP1 to regulate different target genes. Coordination of target gene expression by these three transcription factors may contribute to the diversity of glial cell types. In addition to promoting glial differentiation, we found that REPO is also necessary to suppress neuronal development, cooperating with TTK69. We propose that REPO plays a key role in both glial development and diversification.

Evolution of transcription factor function

Functional assays in Drosophila melanogaster with orthologous transcription factors from other species suggest that changes in the protein-coding sequence may play a larger role in the evolution of transcription factor pathways than was previously believed. Interestingly, recent studies provide evidence that changes in transcription factor protein sequence can affect the regulation of only a subset of target genes, even in the same cells of a developing animal.

A DNA-binding-independent pathway of repression by the Drosophila Runt protein

DNA-binding proteins are important for regulating gene expression during development. It is widely assumed that this regulation involves sequence-specific DNA binding by these transcription factors to cognate cis-regulatory sequences of their downstream target genes. However, studies in both the Drosophila and the mouse model systems have provided examples in which the DNA-binding activity of a transcription factor is not essential for in vivo function. Using a system that allows for quantitative analysis of gene function in the Drosophila embryo, we have discovered a DNA-binding-independent activity of Runt, the founding member of the RUNX family of transcriptional regulators. Examination of the in vivo potency of a DNA-binding-defective form of Runt reveals differential requirements for DNA binding in the regulation of different downstream target genes. DNA binding is not required for establishing repression of the odd-numbered stripes of the segment polarity gene engrailed, but does contribute to Runt's role as a regulator of sloppy-paired, another downstream target gene in the pathway of segmentation. We investigate this DNA-binding-independent pathway using a genetic screen for dose-dependent modifiers of runt activity. These studies reveal that DNA-binding proteins encoded by the tramtrack locus cooperate with Runt to repress engrailed. These results provide new insights into the context-dependent regulatory functions of Runt domain proteins and provide a paradigm for understanding DNA-binding-independent regulation by developmentally important transcription factors.

A functionally conserved member of the FTZ-F1 nuclear receptor family from Schistosoma mansoni

The fushi tarazu factor 1 (FTZ-F1) nuclear receptor subfamily comprises orphan receptors with crucial roles in development and sexual differentiation in vertebrates and invertebrates. We describe the structure and functional properties of an FTZ-F1 from the platyhelminth parasite of humans, Schistosoma mansoni, the first receptor from this family to be characterized in a Lophotrochozoan. It contains a well conserved DNA-binding domain (55-63% identity to other family members) and a poorly conserved ligand-binding domain (20% identity to that of zebrafish FF1a). However, both the ligand domain signature sequence and the activation function 2-activation domain (AF2-AD) are perfectly conserved. Phylogenetic analysis confirmed that SmFTZ-F1 is a member of nuclear receptor subfamily 5, but that it clustered with the Drosophila receptor DHR39 and has consequently been named NR5B1. The gene showed a complex structure with 10 exons and an overall size of 18.4 kb. Two major transcripts were detected, involving alternative promoter usage and splicing of the two 5' exons, but which encoded identical proteins. SmFTZ-F1 mRNA is expressed at all life-cycle stages with the highest amounts in the larval forms (miracidia, sporocysts and cercariae). However, expression of the protein showed a different pattern; low in miracidia and higher in adult male worms. The protein bound the same monomeric response element as mammalian SF-1 (SF-1 response element, SFRE) and competition experiments with mutant SFREs showed that its specificity was identical. Moreover, SmFTZ-F1 transactivated reporter gene transcription from SFRE similarly to SF-1. This functional conservation argues for a conserved biological role of the FTZ-F1 nuclear receptor family throughout the metazoa.

Hox genes and the evolution of the arthropod body plan

In recent years researchers have analyzed the expression patterns of the Hox genes in a multitude of arthropod species, with the hope of understanding the mechanisms at work in the evolution of the arthropod body plan. Now, with Hox expression data representing all four major groups of arthropods (chelicerates, myriapods, crustaceans, and insects), it seems appropriate to summarize the results and take stock of what has been learned. In this review we summarize the expression and functional data regarding the 10 arthropod Hox genes: labial proboscipedia, Hox3/zen, Deformed, Sex combs reduced, fushi tarazu, Antennapedia, Ultrabithorax, abdominal-A, and Abdominal-B. In addition, we discuss mechanisms of developmental evolutionary change thought to be important for the emergence of novel morphological features within the arthropods.

BetaFTZ-F1 dependent and independent activation of Edg78E, a pupal cuticle gene, during the early metamorphic period in Drosophila melanogaster

Insect metamorphosis is a developmentally important event for formation of adult structures from larval imaginal cells, and it is controlled by the ecdysteroid hormone. At the onset of metamorphosis, both the cuticle gene Edg78E and the transcription factor betaFTZ-F1 are expressed during the mid- to late prepupal period after a large ecdysteroid pulse. Edg78E mRNA is inducible by premature expression of betaFTZ-F1 and the Edg78E expression level is reduced in an ftz-f1 mutant. Using a transgenic fly reporter assay, a 1.2 kb promoter region of the Edg78E gene has been identified, which was sufficient for appropriate temporally and spatially specific expression of the reporter gene LacZ. Within the promoter region, two betaFTZ-F1 binding sites are present and disruption of these sites reduced the expression level of the reporter gene. LacZ expression levels were dramatically reduced in the head and thorax regions but not affected in the abdominal region, suggesting that betaFTZ-F1 is required for high-level Edg78E expression specifically in the head and thorax regions. The findings suggest that betaFTZ-F1 is a regulator for temporal gene expression at the onset of metamorphosis, and that complex mechanisms regulate the temporal and spatial regulation of gene expression during metamorphosis.

Specificity of Distalless repression and limb primordia development by abdominal Hox proteins

In Drosophila, differences between segments, such as the presence or absence of appendages, are controlled by Hox transcription factors. The Hox protein Ultrabithorax (Ubx) suppresses limb formation in the abdomen by repressing the leg selector gene Distalless, whereas Antennapedia (Antp), a thoracic Hox protein, does not repress Distalless. We show that the Hox cofactors Extradenticle and Homothorax selectively enhance Ubx, but not Antp, binding to a Distalless regulatory sequence. A C-terminal peptide in Ubx stimulates binding to this site. However, DNA binding is not sufficient for Distalless repression. Instead, an additional alternatively spliced domain in Ubx is required for Distalless repression but not DNA binding. Thus, the functional specificities of Hox proteins depend on both DNA binding-dependent and -independent mechanisms.

Segmentation gene product Fushi tarazu is an LXXLL motif-dependent coactivator for orphan receptor FTZ-F1

Orphan receptors for whom cognate ligands have not yet been identified form a large subclass within the nuclear receptor superfamily. To address one aspect of how they might regulate transcription, we analyzed the mode of interaction between the Drosophila orphan receptor FTZ-F1 (NR5A3) and a segmentation gene product Fushi tarazu (FTZ). Strong interaction between these two factors was detected by use of the mammalian one- and two-hybrid interaction assays without addition of ligand. This interaction required the AF-2 core and putative ligand-binding domain of FTZ-F1 and the LXXLL motif of FTZ. The requirement of these elements was further confirmed by examination of their target gene expression in Drosophila embryos and observation of a cuticle phenotype in transgenic fly lines that express mutated factors. In Drosophila cultured cells, FTZ is required for FTZ-F1 activation of a FTZ-F1 reporter gene. These results reveal a resemblance in the mode of interaction between FTZ-F1 and FTZ and that of nuclear receptor-coactivator and indicate that direct interaction is required for regulation of gene expression by FTZ-F1. Thus, we propose that FTZ may represent a category of LXXLL motif-dependent coactivators for nuclear receptors.

Drosophila fushi tarazu. a gene on the border of homeotic function

BACKGROUND

Hox genes specify cell fate and regional identity during animal development. These genes are present in evolutionarily conserved clusters thought to have arisen by gene duplication and divergence. Most members of the Drosophila Hox complex (HOM-C) have homeotic functions. However, a small number of HOM-C genes, such as the segmentation gene fushi tarazu (ftz), have nonhomeotic functions. If these genes arose from a homeotic ancestor, their functional properties must have changed significantly during the evolution of modern Drosophila.

RESULTS

Here, we have asked how Drosophila ftz evolved from an ancestral homeotic gene to obtain a novel function in segmentation. We expressed Ftz proteins at various developmental stages to assess their potential to regulate segmentation and to generate homeotic transformations. Drosophila Ftz protein has lost the inherent ability to mediate homeosis and functions exclusively in segmentation pathways. In contrast, Ftz from the primitive insect Tribolium (Tc-Ftz) has retained homeotic potential, generating homeotic transformations in larvae and adults and retaining the ability to repress homothorax, a hallmark of homeotic genes. Similarly, Schistocerca Ftz (Sg-Ftz) caused homeotic transformations of antenna toward leg. Primitive Ftz orthologs have moderate segmentation potential, reflected by weak interactions with the segmentation-specific cofactor Ftz-F1. Thus, Ftz orthologs represent evolutionary intermediates that have weak segmentation potential but retain the ability to act as homeotic genes.

CONCLUSIONS

ftz evolved from an ancestral homeotic gene as a result of changes in both regulation of expression and specific alterations in the protein-coding region. Studies of ftz orthologs from primitive insects have provided a "snap-shot" view of the progressive evolution of a Hox protein as it took on segmentation function and lost homeotic potential. We propose that the specialization of Drosophila Ftz for segmentation resulted from loss and gain of specific domains that mediate interactions with distinct cofactors.

Evolution of Ftz protein function in insects

The Drosophila gene fushi tarazu (ftz) encodes a homeodomain-containing transcriptional regulator (Ftz) required at several stages during development. Drosophila melanogaster ftz (Dm-ftz) is first expressed in seven stripes defining alternate parasegments of the embryo--a "pair-rule" segmentation function [1, 2]. It is then expressed in specific neural precursor cells in the central nervous system and finally in the developing hindgut [3]. An Orthopteran ortholog of ftz (Sg-ftz, formally Dax) has been isolated from the grasshopper Schistocerca gregaria [4]. The pattern of Sg-ftz expression in Schistocerca embryos suggests that some developmental roles of the ftz gene are likely to be conserved between these two species (e.g., CNS functions) while others may have diverged (e.g., segmentation functions). To test whether the function of the Ftz protein itself differs between these two species, here we compare the functions of Sg-Ftz and Dm-Ftz proteins by expressing both in Drosophila embryos. Sg-ftz mimics only poorly several segmentation roles of Dm-ftz (engrailed activation, wingless repression, and embryonic cuticle transformation). However, the two proteins are similarly active in the rescue of a CNS-specific ftz mutant. These findings argue that this ftz CNS function is mediated by conserved parts of the protein, while efficient pair-rule function requires sequences present specifically in the Drosophila protein.

The nuclear receptor Ftz-F1 and homeodomain protein Ftz interact through evolutionarily conserved protein domains

The Drosophila homeodomain protein Fushi Tarazu (Ftz) and its partner, the orphan receptor Ftz-F1, are members of two distinct families of DNA binding transcriptional regulators. Ftz and Ftz-F1 form a novel partnership in vivo as a Hox/orphan receptor heterodimer. Here we show that the murine Ftz-F1 ortholog SF-1 functionally substitutes for Ftz-F1 in vivo, rescuing the defects of ftz-f1 mutants. This finding identified evolutionarily conserved domains of Ftz-F1 as critical for activity of this receptor in vivo. These domains function, at least in part, by mediating direct protein interactions with Ftz. The Ftz-F1 DNA binding domain interacts strongly with Ftz and dramatically facilitates the binding of Ftz to target DNA. This interaction is augmented by a second interaction between the AF-2 domain of Ftz-F1 and the N-terminus of Ftz via an LRALL sequence in Ftz that is reminiscent of LXXLL motifs in nuclear receptor coactivators. We propose that Ftz-F1 serves as a cofactor for Ftz by facilitating the selection of target sites in the genome that contain Ftz/Ftz-F1 composite binding sites. Ftz, on the other hand, influences Ftz-F1 activity by interacting with its AF-2 domain in a manner that mimics a nuclear receptor coactivator.

Characterization of the genomic structure and tissue-specific promoter of the human nuclear receptor NR5A2 (hB1F) gene

The human homologue of the Drosophila melanogaster orphan nuclear receptor fushi tarazu factor 1 (Ftz-F1), NR5A2 (hB1F), was initially identified as a regulatory factor that binds and activates enhancer II of hepatitis B virus. NR5A2 (hB1F) is expressed specifically in pancreas and liver, playing important roles in the regulation of several liver-specific genes. A detailed analysis on the genomic structure and promoter activity will greatly promote future studies on the function of the NR5A2 (hB1F) gene. In this report, a bacterial artificial chromosome clone and several phage clones covering the NR5A2 (hB1F) gene were isolated and the complete genomic sequence was obtained. Alignment of different cDNAs of the NR5A2 (hB1F) gene with the genomic sequence facilitated the delineation of its structural organization, which spans over 150 kb and consists of eight exons interrupted by seven introns. RT-PCR and 3'-RACE revealed that utilization of two polyadenylation signals results in the 3.8 and 5.2 kb transcripts that were observed previously. The transcription start site of the NR5A2 (hB1F) gene was mapped downstream of a canonical TATA box. An upstream fragment containing binding sites for several liver-specific and ubiquitous transcription factors exhibits hepatocyte-specific promoter activity. Transient transfections indicated that hepatocyte nuclear factors HNF1 and HNF3beta could activate NR5A2 (hB1F) promoter.

Irx4 forms an inhibitory complex with the vitamin D and retinoic X receptors to regulate cardiac chamber-specific slow MyHC3 expression

The slow myosin heavy chain 3 gene (slow MyHC3) is restricted in its expression to the atrial chambers of the heart. Understanding its regulation provides a basis for determination of the mechanisms controlling chamber-specific gene expression in heart development. The observed chamber distribution results from repression of slow MyHC3 gene expression in the ventricles. A binding site, the vitamin D response element (VDRE), for a heterodimer of vitamin D receptor (VDR) and retinoic X receptor alpha (RXR alpha) within the slow MyHC3 promoter mediates chamber-specific expression of the gene. Irx4, an Iroquois family homeobox gene whose expression is restricted to the ventricular chambers at all stages of development, inhibits AMHC1, the chick homolog of quail slow MyHC3, gene expression within developing ventricles. Repression of the slow MyHC3 gene in ventricular cardiomyocytes by Irx4 requires the VDRE. Unlike VDR and RXR alpha, Irx4 does not bind directly to the VDRE. Instead two-hybrid and co-immunoprecipitation assays show that Irx4 interacts with the RXR alpha component of the VDR/RXR alpha heterodimer and that the amino terminus of the Irx4 protein is required for its inhibitory action. These observations indicate that the mechanism of atrial chamber-specific expression requires the formation of an inhibitory protein complex composed of VDR, RXR alpha, and Irx4 that binds at the VDRE inhibiting slow MyHC3 expression in the ventricles.

The identification of Prx1 transcription regulatory domains provides a mechanism for unequal compensation by the Prx1 and Prx2 loci

Transcription regulatory domains of the Prx1a and Prx1b homeoproteins were analyzed in transient transfection assays using artificial promoters as well as an established downstream target promoter (tenascin-c). Activation and repression domains were detected in their common amino end. In the carboxyl end of Prx1a an activation domain and an inhibition/masking region (OAR domain) were detected. The Prx1b isoform, generated by alternative splicing, does not contain these carboxyl activation or inhibition domains. Instead, the data demonstrate that the carboxyl tail of Prx1b contains a potent repressor region. This difference in the carboxyl tail accounts for a 45-fold difference observed in transcription regulatory activity between Prx1a and Prx1b. The data also support the likelihood that this difference between Prx1a and Prx1b is higher in the presence of still undetermined cofactors. DNA binding affinities of Prx1a, Prx1b, and a series of truncation mutants were also examined. The carboxyl tail of Prx1a, which inhibited transcription activation in the transfection assays, also inhibited DNA binding. These differences in biochemical function between Prx1a and Prx1b, as well as the recently described activities of Prx2, provide a mechanism for the unequal compensation between the Prx1 and Prx2 loci.

The developmental and molecular biology of genes that subdivide the body of Drosophila

During the past decade, much progress has been made in understanding how the adult fly is built. Some old concepts such as those of compartments and selector genes have been revitalized. In addition, recent work suggests the existence of genes involved in the regionalization of the adult that do not have all the features of selector genes. Nevertheless, they generate morphological distinctions within the body plan. Here we re-examine some of the defining criteria of selector genes and suggest that these newly characterized genes fulfill many, but not all, of these criteria. Further, we propose that these genes can be classified according to the domains in which they function. Finally, we discuss experiments that address the molecular mechanisms by which selector and selector-like gene products function in the fly.

Transcriptional regulation of cytoskeletal functions and segmentation by a novel maternal pair-rule gene, lilliputian

Transcriptional control during early Drosophila development is governed by maternal and zygotic factors. We have identified a novel maternal transcriptional regulator gene, lilliputian (lilli), which contains an HMG1 (AT-hook) motif and a domain with similarity to the human fragile X mental retardation FMR2 protein and the AF4 proto-oncoprotein. Embryos lacking maternal lilli expression show specific defects in the establishment of a functional cytoskeleton during cellularization, and exhibit a pair-rule segmentation phenotype. These mutant phenotypes correlate with markedly reduced expression of the early zygotic genes serendipity alpha, fushi tarazu and huckebein, which are essential for cellularization and embryonic patterning. In addition, loss of lilli in adult photoreceptor and bristle cells results in a significant decrease in cell size. Our results indicate that lilli represents a novel pair-rule gene that acts in cytoskeleton regulation, segmentation and morphogenesis.

FTZ-Factor1 and Fushi tarazu interact via conserved nuclear receptor and coactivator motifs

To activate transcription, most nuclear receptor proteins require coactivators that bind to their ligand-binding domains (LBDs). The Drosophila FTZ-Factor1 (FTZ-F1) protein is a conserved member of the nuclear receptor superfamily, but was previously thought to lack an AF2 motif, a motif that is required for ligand and coactivator binding. Here we show that FTZ-F1 does have an AF2 motif and that it is required to bind a coactivator, the homeodomain-containing protein Fushi tarazu (FTZ). We also show that FTZ contains an AF2-interacting nuclear receptor box, the first to be found in a homeodomain protein. Both interaction motifs are shown to be necessary for physical interactions in vitro and for functional interactions in developing embryos. These unexpected findings have important implications for the conserved homologs of the two proteins.

The orphan nuclear receptor NOR-1 interacts with the homeobox containing protein Six3

Neuron-derived orphan receptor 1 (NOR-1) is a member of the NGFI-B subfamily within the nuclear receptor superfamily. In order to identify cofactors that associate with NOR-1 in the fetal forebrain, we tested a yeast two-hybrid system with the NOR-1 cDNA fragment lacking a transactivating domain as a bait. By screening of the rat fetal brain embryonic day 17 library, a rat homologue of Six3 was identified as an associated protein. We demonstrated that NOR-1 interacted with Six3 in yeast and in vitro, and the association was required for the DNA binding and AF2 domains of NOR-1. Regarding the other members of the family (NGFI-B and RNR-1), association with Six3 was not observed in yeast. In addition, cotransfection experiments with Six3 and NOR-1 indicated that Six3 had a negative activity against the transactivation by NOR-1 through the NBRE response element in a dose-dependent manner. The overlap in expression of NOR-1 and Six3 was mainly detected in the rat fetal forebrain on embryonic day 18. Thereafter, the expression of both genes diminished rapidly. These results suggest that a dimer consisting of a homeobox containing protein Six3 and transcriptional factor NOR-1 might regulate gene expression during the late stage of the fetal forebrain development. This study provides, after the association of Ftz and Ftz-F1 in Drosophila, another example of a dimer formation of a homeobox protein and an orphan nuclear receptor.

Temporally restricted expression of transcription factor betaFTZ-F1: significance for embryogenesis, molting and metamorphosis in Drosophila melanogaster

FTZ-F1, a member of the nuclear receptor superfamily, has been implicated in the activation of the segmentation gene fushi tarazu during early embryogenesis of Drosophila melanogaster. We found that an isoform of FTZ-F1, betaFTZ-F1, is expressed in the nuclei of almost all tissues slightly before the first and second larval ecdysis and before pupation. Severely affected ftz-f1 mutants display an embryonic lethal phenotype, but can be rescued by ectopic expression of betaFTZ-F1 during the period of endogenous betaFTZ-F1 expression in the wild type. The resulting larvae are not able to molt, but this activity is rescued again by forced expression of betaFTZ-F1, allowing progression to the next larval instar stage. On the other hand, premature expression of betaFTZ-F1 in wild-type larvae at mid-first instar or mid-second instar stages causes defects in the molting process. Sensitive periods were found to be around the time of peak ecdysteroid levels and slightly before the start of endogenous betaFTZ-F1 expression. A hypomorphic ftz-f1 mutant that arrests in the prepupal stage can also be rescued by ectopic, time-specific expression of betaFTZ-F1. Failure of salivary gland histolysis, one of the phenotypes of the ftz-f1 mutant, is rescued by forced expression of the ftz-f1 downstream gene BR-C during the late prepupal period. These results suggest that betaFTZ-F1 regulates genes associated with ecdysis and metamorphosis, and that the exact timing of its action in the ecdysone-induced gene cascade is important for proper development.

Target selectivity of bicoid is dependent on nonconsensus site recognition and protein-protein interaction

We describe experiments to compare the activities of two Drosophila homeodomain proteins, Bicoid (Bcd) and an altered-specificity mutant of Fushi tarazu, Ftz(Q50K). Although the homeodomains of these proteins share a virtually indistinguishable ability to recognize a consensus Bcd site, only Bcd can activate transcription from natural enhancer elements when assayed in both yeast and Drosophila Schneider S2 cells. Our analysis of chimeric proteins suggests that both the homeodomain of Bcd and sequences outside the homeodomain contribute to its ability to recognize natural enhancer elements. We further show that, unlike the Bcd homeodomain, the Ftz(Q50K) homeodomain fails to recognize nonconsensus sites found in natural enhancer elements. The defect of a chimeric protein containing the homeodomain of Ftz(Q50K) in place of that of Bcd can be preferentially restored by converting the nonconsensus sites in natural enhancer elements to consensus sites. Our experiments suggest that the biological specificity of Bcd is determined by combinatorial contributions of two important mechanisms: the nonconsensus site recognition function conferred by the homeodomain and the cooperativity function conferred primarily by sequences outside the homeodomain. A systematic comparison of different assay methods and enhancer elements further suggests a fluid nature of the requirements for these two Bcd functions in target selection.

NFkappa B interacts with serum amyloid A3 enhancer factor to synergistically activate mouse serum amyloid A3 gene transcription

We had previously identified a distal regulatory element (DRE) in the mouse serum amyloid A3 (SAA3) promoter that functions as a cytokine-inducible transcription enhancer. Within this DRE, three functional elements interact with CCAAT/enhancer-binding protein (C/EBP) and SAA3 enhancer factor (SEF) transcription factors. In this study, we show that cotransfection of the SEF expression plasmid with an SAA3/luciferase reporter resulted in 3-5-fold activation of the SAA3 promoter. When SEF-transfected cells were further stimulated with conditioned medium or interleukin-1, SAA3 promoter activity was dramatically increased, suggesting that SEF may cooperate functionally with other interleukin-1-inducible transcription factors to synergistically up-regulate SAA3 gene transcription. Indeed, cotransfection of SEF and NFkappaBp65 expression DNAs resulted in synergistic activation of the SAA3 promoter. Intriguingly, no consensus NFkappaB-binding site was found in the SAA3 promoter region; rather a putative NFkappaB-binding sequence with 3-base pair mismatches was identified in the DRE. When this sequence was used in an electrophoretic mobility shift assay, it interacted with NFkappaBp50, albeit with binding affinities that were several hundredfold lower than that with the consensus NFkappaB probe. Functional cooperation between SEF and NFkappaB was further strengthened by the finding that SEF and NFkappaB formed stable cytokine-inducible protein-protein complexes. Finally, despite its weak binding, mutation of this NFkappaB-binding site nevertheless dramatically reduced both NFkappaBp65- and cytokine-mediated induction of SAA3 promoter. Therefore, the molecular basis for the functional synergy between SEF and NFkappaB may, in part, be the ability of SEF to recruit NFkappaB through physical interactions that lead to enhancement or stabilization of NFkappaB binding to the SAA3 promoter element.

Genetic pathways in the developmental specification of hypothalamic neuropeptide and midbrain catecholamine systems

The neuropeptide concept concerns the diverse and broad physiological functions of neuropeptides in behavioral adaptation. Neuropeptides like vasopressin and corticotropin-releasing hormone can coordinate multiple brain functions due to the anatomical organization of the neurons producing them. The cell bodies are focally positioned in the hypothalamus and send long-reaching efferents to limbic and brainstem areas. Likewise, midbrain dopamine systems coordinate emotional behaviors and movement control by specific connectivity of neurons in the midbrain to limbic and striatal centers, respectively. The fundament of the functions of these signalling molecules is laid out during development when transmitter identity and connectivity are specified. This is a highly controlled process involving multiple transcription factors and growth factors acting together in genetic pathways. Here, the genetic pathways enrolling in developing vasopressin, corticotropin-releasing hormone, and midbrain dopamine neurons are discussed.

The conserved nuclear receptor Ftz-F1 is required for embryogenesis, moulting and reproduction in Caenorhabditis elegans

BACKGROUND

Nuclear receptors are essential players in the development of all metazoans. The nematode Caenorhabditis elegans possesses more than 200 putative nuclear receptor genes, several times more than the number known in any other organism. Very few of these transcription factors are conserved with components of the steroid response pathways in vertebrates and arthropods. Ftz-F1, one of the evolutionarily oldest nuclear receptor types, is required for steroidogenesis and sexual differentiation in mice and for segmentation and metamorphosis in Drosophila.

RESULTS

We employed two complementary approaches, direct mutagenesis and RNA interference, to explore the role of nhr-25, a C. elegans ortholog of Ftz-F1. Deletion mutants show that nhr-25 is essential for embryogenesis. RNA interference reveals additional requirements throughout the postembryonic life, namely in moulting and differentiation of the gonad and vulva. All these defects are consistent with the nhr-25 expression pattern, determined by in situ hybridization and GFP reporter activity.

CONCLUSIONS

Our data link the C. elegans Ftz-F1 ortholog with a number of developmental processes. Significantly, its role in the periodical replacement of cuticle (moulting) appears to be evolutionarily shared with insects and thus supports the monophyletic origin of moulting.