Functional interactions between Drosophila bHLH/PAS, Sox, and POU transcription factors regulate CNS midline expression of the slit gene

Zebrafish pou5f1/pou2, homolog of mammalian Oct4, functions in the endoderm specification cascade

pou5f1, also known as Oct4, is required to establish the pluripotent cell population necessary for embryogenesis in mouse. Additional roles during development, including endoderm formation, have been proposed. In zebrafish, the zygotic pou5f1/pou2 mutant spiel ohne grenzen (spg) shows neural plate patterning defects and reduced endoderm at the tailbud stage. To investigate the function of maternal and early zygotic pou5f1 expression, we rescued zygotic spg(m793) mutants by injecting pou5f1 mRNA at the one-cell stage and raised them into fertile homozygous spg(m793) adults that mate to produce maternal-zygotic spg (MZspg) mutant embryos. Although neurectoderm, mesoderm, and germ cells develop in MZspg mutants, gastrulation is delayed and proceeds abnormally. Further, MZspg mutants do not maintain expression of sox32/casanova, express little or no sox17, and fail to develop endodermal tissue. Constitutively active Nodal receptor TARAM-A or sox32 overexpression induces ubiquitous sox17 expression in wild-type embryos, but not in MZspg mutants. Overexpression of a Pou5f1-VP16 activator fusion protein can rescue gastrulation and endodermal tissues in MZspg mutants. We propose that pou5f1 plays an activating role in zebrafish endodermal development, where it maintains sox32 expression during gastrulation and acts with sox32 to induce sox17 expression in endodermal precursor cells.

Contribution of the Per/Arnt/Sim (PAS) Domains to DNA Binding by the Basic Helix-Loop-Helix PAS Transcriptional Regulators

The basic helix-loop-helix (bHLH) PAS transcriptional regulators control critical developmental and metabolic processes, including transcriptional responses to stimuli such as hypoxia and environmental pollutants, mediated respectively by hypoxia inducible factors (HIF-alpha) and the dioxin (aryl hydrocarbon) receptor (DR). The bHLH proteins contain a basic DNA binding sequence adjacent to a helix-loop-helix dimerization domain. Dimerization among bHLH.PAS proteins is additionally regulated by the PAS region, which controls the specificity of partner choice such that HIF-alpha and DR must dimerize with the aryl hydrocarbon nuclear translocator (Arnt) to form functional DNA binding complexes. Here, we have analyzed purified bacterially expressed proteins encompassing the N-terminal bHLH and bHLH.PAS regions of Arnt, DR, and HIF-1alpha and evaluated the contribution of the PAS domains to DNA binding in vitro. Recovery of functional DNA binding proteins from bacteria was dramatically enhanced by coexpression of the bHLH.PAS regions of DR or HIF-1alpha with the corresponding region of Arnt. Formation of stable protein-DNA complexes by DR/Arnt and HIF-1alpha/Arnt heterodimers with their cognate DNA sequences required the PAS A domains and exhibited KD values of 0.4 nM and approximately 50 nM, respectively. In contrast, the presence of the PAS domains of Arnt had little effect on DNA binding by Arnt homodimers, and these bound DNA with a KD of 45 nM. In the case of the DR, both high affinity DNA binding and dimer stability were specific to its native PAS domain, since a chimera in which the PAS A domain was substituted with the equivalent domain of Arnt generated a destabilized protein that bound DNA poorly.

Multiple roles for u-turn/ventral veinless in the development of Drosophila PNS

Most of the cells in the embryonic peripheral nervous system (PNS) of Drosophila are born in their final location. One known exception is the group of lateral chordotonal organs (lch5) whose precursors form in a dorsal position, yet the mature organs are located in the lateral PNS cluster. Mutations in the u-turn (ut) locus perturb the localization of lch5 neurons and result in a 'dorsal chordotonals' phenotype. We show that ut is allelic to ventral veinless (vvl), also known as drifter. VVL, a POU-domain transcription factor, has been shown to participate in the development of tracheae and CNS in the embryo, and in wing development in the adult; however, its role in PNS development has not been described. Characterization of the 'dorsal chordotonals' phenotype of vvl mutant embryos revealed that in the absence of VVL, cell fates within the lch5 lineage are determined properly and the entire organ is misplaced. Based on the positions of lch5 cells relative to each other in mutant embryos, and in normal embryos at different developmental stages, we propose a two-step model for lch5 localization. lch5 organs must first rotate to assume a correct polarity and are then stretched ventrally to their final position. In this process, VVL function is required in the ectoderm and possibly in the lch5 organs too. VVL is also expressed in developing external sensory organs in the embryo and in the adult. In the embryo, loss of VVL function results in increased apoptosis in specific es organs. Analysis of vvl mutant clones in adults revealed a requirement for VVL in the control of cell number within the bristle lineage.

Irx4-mediated regulation of Slit1 expression contributes to the definition of early axonal paths inside the retina

Although multiple axon guidance cues have been discovered in recent years, little is known about the mechanism by which the spatiotemporal expression patterns of the axon guidance cues are regulated in vertebrates. We report that a homeobox gene Irx4 is expressed in a pattern similar to that of Slit1 in the chicken retina. Overexpression of Irx4 led to specific downregulation of Slit1 expression, whereas inhibition of Irx4 activity by a dominant negative mutant led to induction of Slit1 expression, indicating that Irx4 is a crucial regulator of Slit1 expression in the retina. In addition, by examining axonal behavior in the retinas with overexpression of Irx4 and using several in vivo assays to test the effect of Slit1, we found that Slit1 acts positively to guide the retinal axons inside the optic fiber layer (OFL). We further show that the regulation of Slit1 expression by Irx4 is important for providing intermediate targets for retinal axons during their growth within the retina.

The zinc finger transcription factor Gfi1, implicated in lymphomagenesis, is required for inner ear hair cell differentiation and survival

Gfi1 was first identified as causing interleukin 2-independent growth in T cells and lymphomagenesis in mice. Much work has shown that Gfi1 and Gfi1b, a second mouse homolog, play pivotal roles in blood cell lineage differentiation. However, neither Gfi1 nor Gfi1b has been implicated in nervous system development, even though their invertebrate homologues, senseless in Drosophila and pag-3 in C. elegans are expressed and required in the nervous system. We show that Gfi1 mRNA is expressed in many areas that give rise to neuronal cells during embryonic development in mouse, and that Gfi1 protein has a more restricted expression pattern. By E12.5 Gfi1 mRNA is expressed in both the CNS and PNS as well as in many sensory epithelia including the developing inner ear epithelia. At later developmental stages, Gfi1 expression in the ear is refined to the hair cells and neurons throughout the inner ear. Gfi1 protein is expressed in a more restricted pattern in specialized sensory cells of the PNS, including the eye, presumptive Merkel cells, the lung and hair cells of the inner ear. Gfi1 mutant mice display behavioral defects that are consistent with inner ear anomalies, as they are ataxic, circle, display head tilting behavior and do not respond to noise. They have a unique inner ear phenotype in that the vestibular and cochlear hair cells are differentially affected. Although Gfi1-deficient mice initially specify inner ear hair cells, these hair cells are disorganized in both the vestibule and cochlea. The outer hair cells of the cochlea are improperly innervated and express neuronal markers that are not normally expressed in these cells. Furthermore, Gfi1 mutant mice lose all cochlear hair cells just prior to and soon after birth through apoptosis. Finally, by five months of age there is also a dramatic reduction in the number of cochlear neurons. Hence, Gfi1 is expressed in the developing nervous system, is required for inner ear hair cell differentiation, and its loss causes programmed cell death.

Formation of neuroblasts in the embryonic central nervous system of Drosophila melanogaster is controlled by SoxNeuro

Sox proteins form a family of HMG-box transcription factors related to the mammalian testis determining factor SRY. Sox-mediated modulation of gene expression plays an important role in various developmental contexts. Drosophila SoxNeuro, a putative ortholog of the vertebrate Sox1, Sox2 and Sox3 proteins, is one of the earliest transcription factors to be expressed pan-neuroectodermally. We demonstrate that SoxNeuro is essential for the formation of the neural progenitor cells in central nervous system. We show that loss of function mutations of SoxNeuro are associated with a spatially restricted hypoplasia: neuroblast formation is severely affected in the lateral and intermediate regions of the central nervous system, whereas ventral neuroblast formation is almost normal. We present evidence that a requirement for SoxNeuro in ventral neuroblast formation is masked by a functional redundancy with Dichaete, a second Sox protein whose expression partially overlaps that of SoxNeuro. Genetic interactions of SoxNeuro and the dorsoventral patterning genes ventral nerve chord defective and intermediate neuroblasts defective underlie ventral and intermediate neuroblast formation. Finally, the expression of the Achaete-Scute gene complex suggests that SoxNeuro acts upstream and in parallel with the proneural genes.

Novel behavioral and developmental defects associated with Drosophila single-minded

In Drosophila, the development of the midline cells of the embryonic ventral nerve cord depends on the function of the bHLH-PAS transcription factor Single-minded (Sim). The expression domain of sim, however, is also found anterior and posterior to the developing ventral cord throughout the germ band. Indeed, mutations in sim were identified based on their characteristic cuticle phenotype. Eight abdominal segments (A1-A8) can be easily seen in the larval cuticle, while three more can be identified during embryogenesis. Cells located in A8-A10 give rise to the formation of the genital imaginal discs, and a highly modified A11 segment gives rise to the anal pads that flank the anus. sim is expressed in all these segments and is required for the formation of both the anal pads and the genital imaginal discs. A new temperature-sensitive sim allele allowed an assessment of possible postembryonic function(s) of sim. Reduction of sim function below a 50% threshold leads to sterile flies with marked behavioral deficits. Most mutant sim flies were only able to walk in circles. Further analyses indicated that this phenotype is likely due to defects in the brain central complex. This brain region, which has previously been implicated in the control of walking behavior, expresses high levels of nuclear Sim protein in three clusters of neurons in each central brain hemisphere. Additional Sim localization in the medullary and laminar neurons of the optic lobes may correlate with the presence of ectopic axon bundles observed in the optic lobes of sim mutant flies.

Evidence for differential and redundant function of the Sox genesDichaeteandSoxNduring CNS development inDrosophila

Group B Sox-domain proteins encompass a class of conserved DNA-binding proteins expressed from the earliest stages of metazoan CNS development. In all higher organisms studied to date, related Group B Sox proteins are co-expressed in the developing CNS; in vertebrates there are three (Sox1, Sox2 and Sox3) and in Drosophila there are two (SoxNeuro and Dichaete). It has been suggested there may be a degree of functional redundancy in Sox function during CNS development. We describe the CNS phenotype of a null mutation in the Drosophila SoxNeuro gene and provide the first direct evidence for both redundant and differential Sox function during CNS development in Drosophila. In the lateral neuroectoderm, where SoxNeuro is uniquely expressed, SoxNeuro mutants show a loss or reduction of achaete expression as well as a loss of many correctly specified lateral neuroblasts. By contrast, in the medial neuroectoderm, where the expression of SoxNeuro and Dichaete overlaps, the phenotypes of both single mutants are mild. In accordance with an at least partially redundant function in that region, SoxNeuro/Dichaete double mutant embryos show a severe neural hypoplasia throughout the central nervous system, as well as a dramatic loss of achaete expressing proneural clusters and medially derived neuroblasts. However, the finding that Dichaete and SoxN exhibit opposite effects on achaete expression within the intermediate neuroectoderm demonstrates that each protein also has region-specific unique functions during early CNS development in the Drosophila embryo.

Matching SOX: partner proteins and co-factors of the SOX family of transcriptional regulators

SOX transcription factors perform a remarkable variety of important roles in vertebrate development, either activating or repressing specific target genes through interaction with different partner proteins. Surprisingly, these interactions are often mediated by the conserved, DNA-binding HMG domain, raising questions as to how each factor's specificity is generated. We propose a model whereby non-HMG domains may influence partner protein selection and/or binding stability.

Drosophila Morgue is an F box/ubiquitin conjugase domain protein important for grim-reaper mediated apoptosis

In Drosophila melanogaster, apoptosis is controlled by the integrated actions of the Grim-Reaper (Grim-Rpr) and Drosophila Inhibitor of Apoptosis (DIAP) proteins (reviewed in refs 1 4). The anti-apoptotic DIAPs bind to caspases and inhibit their proteolytic activities. DIAPs also bind to Grim-Rpr proteins, an interaction that promotes caspase activity and the initiation of apoptosis. Using a genetic modifier screen, we identified four enhancers of grim-reaper-induced apoptosis that all regulate ubiquitination processes: uba-1, skpA, fat facets (faf), and morgue. Strikingly, morgue encodes a unique protein that contains both an F box and a ubiquitin E2 conjugase domain that lacks the active site Cys required for ubiquitin linkage. A reduction of morgue activity suppressed grim-reaper-induced cell death in Drosophila. In cultured cells, Morgue induced apoptosis that was suppressed by DIAP1. Targeted morgue expression downregulated DIAP1 levels in Drosophila tissue, and Morgue and Rpr together downregulated DIAP1 levels in cultured cells. Consistent with potential substrate binding functions in an SCF ubiquitin E3 ligase complex, Morgue exhibited F box-dependent association with SkpA and F box-independent association with DIAP1. Morgue may thus have a key function in apoptosis by targeting DIAP1 for ubiquitination and turnover.

ThejingZn-finger transcription factor is a mediator of cellular differentiation in theDrosophilaCNS midline and trachea

We establish that the jing zinc-finger transcription factor plays an essential role in controlling CNS midline and tracheal cell differentiation. jing transcripts and protein accumulate from stage 9 in the CNS midline, trachea and in segmental ectodermal stripes. JING protein localizes to the nuclei of CNS midline and tracheal cells implying a regulatory role during their development. Loss of jing-lacZ expression in homozygous sim mutants and induction of jing-lacZ by ectopic sim expression establish that jing is part of the CNS midline lineage. We have isolated embryonic recessive lethal jing mutations that display genetic interactions in the embryonic CNS midline and trachea, with mutations in the bHLH-PAS genes single-minded and trachealess, and their downstream target genes (slit and breathless). Loss- and gain-of-function jing is associated with defects in CNS axon and tracheal tubule patterning. In jing homozygous mutant embryos, reductions in marker gene expression and inappropriate apoptosis in the CNS midline and trachea establish that jing is essential for the proper differentiation and survival of these lineages. These results establish that jing is a key component of CNS midline and tracheal cell development. Given the similarities between JING and the vertebrate CCAAT-binding protein AEBP2, we propose that jing regulates transcriptional mechanisms in Drosophila embryos and promotes cellular differentiation in ectodermal derivatives.

Differential activities of murine single minded 1 (SIM1) and SIM2 on a hypoxic response element. Cross-talk between basic helix-loop-helix/per-Arnt-Sim homology transcription factors

The basic helix-loop-helix/Per-Arnt-Sim homology (bHLH/PAS) protein family comprises a group of transcriptional regulators that often respond to a variety of developmental and environmental stimuli. Two murine members of this family, Single Minded 1 (SIM1) and Single Minded 2 (SIM2), are essential for postnatal survival but differ from other prototypical family members such as the dioxin receptor (DR) and hypoxia-inducible factors, in that they behave as transcriptional repressors in mammalian one-hybrid experiments and have yet to be ascribed a regulating signal. In cell lines engineered to stably express SIM1 and SIM2, we show that both are nuclear proteins that constitutively complex with the general bHLH/PAS partner factor, ARNT. We report that the murine SIM factors, in combination with ARNT, attenuate transcription from the hypoxia-inducible erythropoietin (EPO) enhancer during hypoxia. Such cross-talk between coexpressed bHLH/PAS factors can occur through competition for ARNT, which we find evident in SIM repression of DR-induced transcription from a xenobiotic response element reporter gene. However, SIM1/ARNT, but not SIM2/ARNT, can activate transcription from the EPO enhancer at normoxia, implying that the SIM proteins have the ability to bind hypoxia response elements and affect either activation or repression of transcription. This notion is supported by co-immunoprecipitation of EPO enhancer sequences with the SIM2 protein. SIM protein levels decrease with hypoxia treatment in our stable cell lines, although levels of the transcripts encoding SIM1 and SIM2 and the approximately 2-h half-lives of each protein are unchanged during hypoxia. Inhibition of protein synthesis, known to occur in cells during hypoxic stress in order to decrease ATP utilization, appears to account for the fall in SIM levels. Our data suggest the existence of a hypoxic switch mechanism in cells that coexpress hypoxia-inducible factor and SIM proteins, where up-regulation and activation of hypoxia-inducible factor-1alpha is concomitant with attenuation of SIM activities.

The Sox-domain containing geneDichaete/fish-hookacts in concert withvndandindto regulate cell fate in theDrosophilaneuroectoderm

In the Drosophila embryonic central nervous system, neural stem cells, called neuroblasts, acquire fates in a position-specific manner. Recent work has identified a set of genes that functions along the dorsoventral axis to enable neuroblasts that develop in different dorsoventral domains to acquire distinct fates. These genes include the evolutionarily conserved transcription factors ventral nerve cord defective and intermediate neuroblasts defective, as well as the Drosophila EGF receptor. We show that the Sox-domain-containing gene Dichaete/fish-hook also plays a crucial role to pattern the neuroectoderm along the DV axis. Dichaete is expressed in the medial and intermediate columns of the neuroectoderm, and mutant analysis indicates that Dichaete regulates cell fate and neuroblast formation in these domains. Molecular epistasis tests, double mutant analysis and dosage-sensitive interactions demonstrate that during these processes, Dichaete functions in parallel with ventral nerve cord defective and intermediate neuroblasts defective, and downstream of EGF receptor signaling to mediate its effect on development. These results identify Dichaete as an important regulator of dorsoventral pattern in the neuroectoderm, and indicate that Dichaete acts in concert with ventral nerve cord defective and intermediate neuroblasts defective to regulate pattern and cell fate in the neuroectoderm.

Drosophila single-minded represses gene transcription by activating the expression of repressive factors

The Drosophila single-minded gene controls CNS midline cell development by both activating midline gene expression and repressing lateral CNS gene expression in the midline cells. The mechanism by which Single-minded represses transcription was examined using the ventral nervous system defective gene as a target gene. Transgenic-lacZ analysis of constructs containing fragments of the ventral nervous system defective regulatory region identified sequences required for lateral CNS transcription and midline repression. Elimination of Single-minded:Tango binding sites within the ventral nervous system defective gene did not affect midline repression. Mutants of Single-minded that removed the DNA binding and transcriptional activation regions abolished ventral nervous system defective repression, as well as transcriptional activation of other genes. The replacement of the Single-minded transcriptional activation region with a heterologous VP16 transcriptional activation region restored the ability of Single-minded to both activate and repress transcription. These results indicate that Single-minded indirectly represses transcription by activating the expression of repressive factors. Single-minded provides a model system for how regulatory proteins that act only as transcriptional activators can control lineage-specific transcription in both positive and negative modes.

Coevolution of HMG domains and homeodomains and the generation of transcriptional regulation by Sox/POU complexes

The highly conserved homeodomains and HMG domains are components of a large number of proteins that play a role in the transcriptional regulation of gene expression during embryogenesis. Both the HMG domain and the homeodomain serve as interfaces for factor interactions with DNA, as well as with other proteins, and it is likely that the high degree of structural and sequence conservation within these domains reflects the conservation of basic aspects of these interactions. Classical HMG domain proteins have an ancient origin, being found in all eukaryotes, and are thought to have given rise to the metazoan-specific class of HMG domain proteins called the Sox proteins. Similarly, the metazoan-specific POU domain proteins are thought to have arisen from genes encoding ancestral homeodomain proteins. In this review, we summarize several examples of different HMG-homeodomain interactions that illustrate not only the ancient origin of each of these protein families, but also their relationship to each other, and discuss how coevolution of HMG and homeodomains may have lead to creation of the specialized Sox/POU protein complexes. Using the FGF-4 gene as an example, we also speculate on how coevolution of regulatory Sox/POU target DNA sequences may have occurred, and how the summation of these changes may have lead to the emergence of new developmental pathways.