X:A ratio, the primary sex-determining signal in Drosophila, is transduced by helix-loop-helix proteins

A drug stabilizable GAL80ds for conditional control of gene expression via GAL4-UAS and CRISPR-Cas9 systems in Drosophila

The binary GAL4-UAS expression system has been widely used in Drosophila to achieve tissue-specific expression of genes. To further allow for simultaneous spatial and conditional control of gene expression in existing GAL4 expression lines backgrounds, temperature and chemical controllable GAL80 variants have been engineered. Here we add a new drug stabilizable GAL80ds variant, by fusing it to a low-background DHFR-22-DD. We first quantify both single (DD-GAL80) and double (DD-GAL80-DD) architectures and show varied background and activation levels. Next, we demonstrate the utility of GAL80ds Drosophila line to regulate a cell death gene ectopically, in a drug-dependent manner, by utilizing an existing tissue-specific GAL4 driver that regulates the expression of a cell death gene under a UAS. Finally, we showcase the usefulness of GAL80ds in tight drug-mediated regulation of a target gene, from an endogenous locus, by utilizing an existing tissue-specific GAL4 to drive the expression of a dead Cas9 variant fused to the transcriptional coactivator nejire, under a UAS and in gRNA lines. Overall, these new GAL80ds lines expand the use of the wide variety of existing tissue-specific GAL4 and gene-specific gRNA lines. This enables conditional control of genes, both ectopically and endogenously, for a broad array of gene expression control applications.

A history of the genetic and molecular identification of genes and their functions controlling insect sex determination

The genetics of the sex determination regulatory cascade in Drosophila melanogaster has a fascinating history, interlinked with the foundation of the Genetics discipline itself. The discovery that alternative splicing rather than differential transcription is the molecular mechanism underlying the upstream control of sex differences in the Drosophila model system was surprising. This notion is now fully integrated into the scientific canon, appearing in many genetics textbooks and online education resources. In the last three decades, it was a key reference point for starting evolutionary studies in other insect species by using homology-based approaches. This review will introduce a very brief history of Drosophila genetics. It will describe the genetic and molecular approaches applied for the identifying and cloning key genes involved in sex determination in Drosophila and in many other insect species. These comparative analyses led to supporting the idea that sex-determining pathways have evolved mainly by recruiting different upstream signals/genes while maintaining widely conserved intermediate and downstream regulatory genes. The review also provides examples of the link between technological advances and research achievements, to stimulate reflections on how science is produced. It aims to hopefully strengthen the related historical and conceptual knowledge of general readers of other disciplines and of younger geneticists, often focused on the latest technical-molecular approaches.

Manipulating Insect Sex Determination Pathways for Genetic Pest Management: Opportunities and Challenges

Sex determination pathways in insects are generally characterised by an upstream primary signal, which is highly variable across species, and that regulates the splicing of a suite of downstream but highly-conserved genes (transformer, doublesex and fruitless). In turn, these downstream genes then regulate the expression of sex-specific characteristics in males and females. Identification of sex determination pathways has and continues to be, a critical component of insect population suppression technologies. For example, "first-generation" transgenic technologies such as fsRIDL (Female-Specific Release of Insects carrying Dominant Lethals) enabled efficient selective removal of females from a target population as a significant improvement on the sterile insect technique (SIT). Second-generation technologies such as CRISPR/Cas9 homing gene drives and precision-guided SIT (pgSIT) have used gene editing technologies to manipulate sex determination genes in vivo. The development of future, third-generation control technologies, such as Y-linked drives, (female to male) sex-reversal, or X-shredding, will require additional knowledge of aspects of sexual development, including a deeper understanding of the nature of primary signals and dosage compensation. This review shows how knowledge of sex determination in target pest species is fundamental to all phases of the development of control technologies.

A chimeric gene paternally instructs female sex determination in the haplodiploid wasp Nasonia

Various primary signals direct insect sex determination. In hymenopteran insects, the presence of a paternal genome is needed to initiate female development. When absent, uniparental haploid males develop. We molecularly and functionally identified the instructor sex-determination gene, wasp overruler of masculinization (wom), of the haplodiploid wasp Nasonia vitripennis This gene contains a P53-like domain coding region and arose by gene duplication and genomic rearrangements. Maternal silencing of wom results in male development of haploid embryos. Upon fertilization, early zygotic transcription from the paternal wom allele is initiated, followed by a timely zygotic expression of transformer (tra), leading to female development. Wom is an instructor gene with a parent-of-origin effect in sex determination.

POLYGENIC MUTATION IN DROSOPHILA MELANOGASTER: ESTIMATES FROM DIVERGENCE AMONG INBRED STRAINS

A highly inbred line of Drosophila melanogaster was subdivided into 25 replicate sublines, which were independently maintained for 100 generations with 10 pairs of unselected flies per generation. The polygenic mutation rate (V_M ) for two quantitative traits, abdominal and sternopleural bristle number, was estimated from divergence among sublines at 10 generation intervals from generations 30-100, and from response of each line to divergent selection after more than 65 generations of mutation accumulation. Estimates of V_M averaged over males and females both from divergence among lines and from response to selection within lines were 3.3 × 10^-3 V_E for abdominal bristles and 1.5 × 10^-3 V_E for sternopleural bristles, where V_E is the environmental variance. The actual rate of production of mutations affecting these traits may be considerably higher if the traits are under stabilizing selection, and if mutations affecting bristle number have deleterious effects on fitness. There was a substantial component of variance for sex × mutant effect interaction and the sublines evolved highly significant mutational variation in sex dimorphism of abdominal bristle number. Pleiotropic effects on sex dimorphism may be a general property of mutations at loci determining bristle number.

A new mode of DNA binding distinguishes Capicua from other HMG-box factors and explains its mutation patterns in cancer

HMG-box proteins, including Sox/SRY (Sox) and TCF/LEF1 (TCF) family members, bind DNA via their HMG-box. This binding, however, is relatively weak and both Sox and TCF factors employ distinct mechanisms for enhancing their affinity and specificity for DNA. Here we report that Capicua (CIC), an HMG-box transcriptional repressor involved in Ras/MAPK signaling and cancer progression, employs an additional distinct mode of DNA binding that enables selective recognition of its targets. We find that, contrary to previous assumptions, the HMG-box of CIC does not bind DNA alone but instead requires a distant motif (referred to as C1) present at the C-terminus of all CIC proteins. The HMG-box and C1 domains are both necessary for binding specific TGAATGAA-like sites, do not function via dimerization, and are active in the absence of cofactors, suggesting that they form a bipartite structure for sequence-specific binding to DNA. We demonstrate that this binding mechanism operates throughout Drosophila development and in human cells, ensuring specific regulation of multiple CIC targets. It thus appears that HMG-box proteins generally depend on auxiliary DNA binding mechanisms for regulating their appropriate genomic targets, but that each sub-family has evolved unique strategies for this purpose. Finally, the key role of C1 in DNA binding also explains the fact that this domain is a hotspot for inactivating mutations in oligodendroglioma and other tumors, while being preserved in oncogenic CIC-DUX4 fusion chimeras associated to Ewing-like sarcomas.

Control of vein network topology by auxin transport

Background

Tissue networks such as the vascular networks of plant and animal organs transport signals and nutrients in most multicellular organisms. The transport function of tissue networks depends on topological features such as the number of networks’ components and the components’ connectedness; yet what controls tissue network topology is largely unknown, partly because of the difficulties in quantifying the effects of genes on tissue network topology. We address this problem for the vein networks of plant leaves by introducing biologically motivated descriptors of vein network topology; we combine these descriptors with cellular imaging and molecular genetic analysis; and we apply this combination of approaches to leaves of Arabidopsis thaliana that lack function of, overexpress or misexpress combinations of four PIN-FORMED (PIN) genes—PIN1, PIN5, PIN6, and PIN8—which encode transporters of the plant signal auxin and are known to control vein network geometry.

Results

We find that PIN1 inhibits vein formation and connection, and that PIN6 acts redundantly to PIN1 in these processes; however, the functions of PIN6 in vein formation are nonhomologous to those of PIN1, while the functions of PIN6 in vein connection are homologous to those of PIN1. We further find that PIN8 provides functions redundant and homologous to those of PIN6 in PIN1-dependent inhibition of vein formation, but that PIN8 has no functions in PIN1/PIN6-dependent inhibition of vein connection. Finally, we find that PIN5 promotes vein formation; that all the vein-formation-promoting functions of PIN5 are redundantly inhibited by PIN6 and PIN8; and that these functions of PIN5, PIN6, and PIN8 are independent of PIN1.

Conclusions

Our results suggest that PIN-mediated auxin transport controls the formation of veins and their connection into networks.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-015-0208-3) contains supplementary material, which is available to authorized users.

Origins of context-dependent gene repression by capicua

Receptor Tyrosine Kinase (RTK) signaling pathways induce multiple biological responses, often by regulating the expression of downstream genes. The HMG-box protein Capicua (Cic) is a transcriptional repressor that is downregulated in response to RTK signaling, thereby enabling RTK-dependent induction of Cic targets. In both Drosophila and mammals, Cic is expressed as two isoforms, long (Cic-L) and short (Cic-S), whose functional significance and mechanism of action are not well understood. Here we show that Drosophila Cic relies on the Groucho (Gro) corepressor during its function in the early embryo, but not during other stages of development. This Gro-dependent mechanism requires a short peptide motif, unique to Cic-S and designated N2, which is distinct from other previously defined Gro-interacting motifs and functions as an autonomous, transferable repressor element. Unexpectedly, our data indicate that the N2 motif is an evolutionary innovation that originated within dipteran insects, as the Cic-S isoform evolved from an ancestral Cic-L-type form. Accordingly, the Cic-L isoform lacking the N2 motif is completely inactive in early Drosophila embryos, indicating that the N2 motif endowed Cic-S with a novel Gro-dependent activity that is obligatory at this stage. We suggest that Cic-S and Gro coregulatory functions have facilitated the evolution of the complex transcriptional network regulated by Torso RTK signaling in modern fly embryos. Notably, our results also imply that mammalian Cic proteins are unlikely to act via Gro and that their Cic-S isoform must have evolved independently of fly Cic-S. Thus, Cic proteins employ distinct repressor mechanisms that are associated with discrete structural changes in the evolutionary history of this protein family.

Understanding the evolution of developmental regulatory mechanisms is a central challenge of biology. Here we uncover a newly evolved mechanism of transcriptional repression by Capicua (Cic), a conserved sensor of Receptor Tyrosine Kinase (RTK) signaling. In Drosophila, Cic patterns the central regions of the embryo by repressing genes induced by Torso RTK signaling at the poles. We show that Cic performs this function by recruiting the Groucho (Gro) corepressor and that this mechanism is an evolutionary innovation of dipteran insects. Indeed, we find that recruitment of Gro depends on a short motif of Cic (N2) specific to dipterans. Strikingly, moreover, the form of Cic that existed before the origin of dipterans is completely inactive in fly embryos, whereas the equivalent form carrying N2 displays significant function. This suggests that evolution of the N2 motif caused a fundamental change in Cic repressor activity, which we propose has enabled the complex roles of Cic, Gro and Torso signaling in fly embryonic patterning. In contrast, Cic functions independently of Gro in other Drosophila tissues and probably also in mammals, where Cic lacks the N2 sequence. Thus, our results illustrate the structural and evolutionary origins of essential functional variations within a highly conserved family of developmental regulators.

Gene overexpression: uses, mechanisms, and interpretation

The classical genetic approach for exploring biological pathways typically begins by identifying mutations that cause a phenotype of interest. Overexpression or misexpression of a wild-type gene product, however, can also cause mutant phenotypes, providing geneticists with an alternative yet powerful tool to identify pathway components that might remain undetected using traditional loss-of-function analysis. This review describes the history of overexpression, the mechanisms that are responsible for overexpression phenotypes, tests that begin to distinguish between those mechanisms, the varied ways in which overexpression is used, the methods and reagents available in several organisms, and the relevance of overexpression to human disease.

The Drosophila STUbL protein Degringolade limits HES functions during embryogenesis

Degringolade (Dgrn) encodes a Drosophila SUMO-targeted ubiquitin ligase (STUbL) protein similar to that of mammalian RNF4. Dgrn facilitates the ubiquitylation of the HES protein Hairy, which disrupts the repressive activity of Hairy by inhibiting the recruitment of its cofactor Groucho. We show that Hey and all HES family members, except Her, interact with Dgrn and are substrates for its E3 ubiquitin ligase activity. Dgrn displays dynamic subcellular localization, accumulates in the nucleus at times when HES family members are active and limits Hey and HES family activity during sex determination, segmentation and neurogenesis. We show that Dgrn interacts with the Notch signaling pathway by it antagonizing the activity of E(spl)-C proteins. dgrn null mutants are female sterile, producing embryos that arrest development after two or three nuclear divisions. These mutant embryos exhibit fragmented or decondensed nuclei and accumulate higher levels of SUMO-conjugated proteins, suggesting a role for Dgrn in genome stability.

Molecular titration and ultrasensitivity in regulatory networks

Protein sequestration occurs when an active protein is sequestered by a repressor into an inactive complex. Using mathematical and computational modeling, we show how this regulatory mechanism (called "molecular titration") can generate ultrasensitive or "all-or-none" responses that are equivalent to highly cooperative processes. The ultrasensitive nature of the input-output response is mainly determined by two parameters: the dimer dissociation constant and the repressor concentration. Because in vivo concentrations are tunable through a variety of mechanisms, molecular titration represents a flexible mechanism for generating ultrasensitivity. Using physiological parameters, we report how details of in vivo protein degradation affect the strength of the ultrasensitivity at steady state. Given that developmental systems often transduce signals into cell-fate decisions on timescales incompatible with steady state, we further examine whether molecular titration can produce ultrasensitive responses within physiologically relevant time intervals. Using Drosophila somatic sex determination as a developmental paradigm, we demonstrate that molecular titration can generate ultrasensitivity on timescales compatible with most cell-fate decisions. Gene duplication followed by loss-of-function mutations can create dominant negatives that titrate and compete with the original protein. Dominant negatives are abundant in gene regulatory circuits, and our results suggest that molecular titration might be generating an ultrasensitive response in these networks.

Maternal Groucho and bHLH repressors amplify the dose-sensitive X chromosome signal in Drosophila sex determination

In Drosophila, XX embryos are fated to develop as females, and XY embryos as males, because the diplo-X dose of four X-linked signal element genes, XSEs, activates the Sex-lethal establishment promoter, SxlPe, whereas the haplo-X XSE dose leaves SxlPe off. The threshold response of SxlPe to XSE concentrations depends in part on the bHLH repressor, Deadpan, present in equal amounts in XX and XY embryos. We identified canonical and non-canonical DNA-binding sites for Dpn at SxlPe and found that cis-acting mutations in the Dpn-binding sites caused stronger and earlier Sxl expression than did deletion of dpn implicating other bHLH repressors in Sxl regulation. Maternal Hey encodes one such bHLH regulator but the E(spl) locus does not. Elimination of the maternal corepressor Groucho also caused strong ectopic Sxl expression in XY, and premature Sxl activation in XX embryos, but Sxl was still expressed differently in the sexes. Our findings suggest that Groucho and associated maternal and zygotic bHLH repressors define the threshold XSE concentrations needed to activate SxlPe and that they participate directly in sex signal amplification. We present a model in which the XSE signal is amplified by a feedback mechanism that interferes with Gro-mediated repression in XX, but not XY embryos.

The creation of sexual dimorphism in the Drosophila soma

Animals have evolved a fascinating array of mechanisms for conducting sexual reproduction. These include producing the sex-specific gametes, as well as mechanisms for attracting a mate, courting a mate, and getting the gametes together. These processes require that males and females take on dramatically different forms (sexual dimorphism). Here, we will explore the problem of how sex is determined in Drosophila, and pay particular attention to how information about sexual identity is used to instruct males and females to develop differently. Along the way, we will highlight new work that challenges some of the traditional views about sex determination. In Drosophila, it is commonly thought that every cell decides its own sex based on its sex chromosome constitution (XX vs. XY). However, we now know that many cell types undergo nonautonomous sex determination, where they are told what sex to be through signals from surrounding cells, independent of their own chromosomal content. Further, it now appears that not all cells even "know" their sex, since key members of the sex determination pathway are not expressed in all cells. Thus, our understanding of how sex is determined, and how sexual identity is used to create sexual dimorphism, has changed considerably.

The tailless nuclear receptor acts as a dedicated repressor in the early Drosophila embryo

Tailless is an orphan nuclear receptor that controls terminal body patterning in Drosophila. Genetic analyses have revealed both positive and negative regulatory interactions of Tailless with various target genes, leading to the idea that, like many other nuclear receptors, Tailless mediates both activation and repression of transcription. In this paper, we have examined the consequences of converting Tailless into an obligate repressor and compared the activities of the resulting protein with those of wild-type Tailless. We find that this repressor form of Tailless behaves like the intact protein in gain- and loss-of-function experiments, being sufficient to support normal embryonic development and establish accurate patterns of gene expression even for positive Tailless targets such as hunchback and brachyenteron. This suggests that Tailless functions exclusively as a transcriptional repressor in the embryo and that the observed positive interactions of Tailless with specific targets are secondary effects involving repression of repressors. We provide evidence that knirps is one such repressor gene acting between Tailless and its indirect positive targets. Finally, our results indicate that Tailless exerts an active mechanism of repression via its ligand-binding domain and that this activity is largely independent of the activation function 2 (AF2) motif characteristic of most nuclear receptors.

An eh1-like motif in odd-skipped mediates recruitment of Groucho and repression in vivo

Drosophila Groucho, like its vertebrate Transducin-like Enhancer-of-split homologues, is a corepressor that silences gene expression in numerous developmental settings. Groucho itself does not bind DNA but is recruited to target promoters by associating with a large number of DNA-binding negative transcriptional regulators. These repressors tether Groucho via short conserved polypeptide sequences, of which two have been defined. First, WRPW and related tetrapeptide motifs have been well characterized in several repressors. Second, a motif termed Engrailed homology 1 (eh1) has been found predominantly in homeodomain-containing transcription factors. Here we describe a yeast two-hybrid screen that uncovered physical interactions between Groucho and transcription factors, containing eh1 motifs, with different types of DNA-binding domains. We show that one of these, the zinc finger protein Odd-skipped, requires its eh1-like sequence for repressing specific target genes in segmentation. Comparison between diverse eh1 motifs reveals a bias for the phosphoacceptor amino acids serine and threonine at a fixed position, and a mutational analysis of Odd-skipped indicates that these residues are critical for efficient interactions with Groucho and for repression in vivo. Our data suggest that phosphorylation of these phosphomeric residues, if it occurs, will down-regulate Groucho binding and therefore repression, providing a mechanism for posttranslational control of Groucho-mediated repression.

The small GTPase Rac plays multiple roles in epithelial sheet fusion—dynamic studies of Drosophila dorsal closure

The coordinated migration and fusion of epithelial sheets is a crucial morphogenetic tool used on numerous occasions during the normal development of an embryo and re-activated as part of the wound healing response. Drosophila dorsal closure, whereby a hole in the embryonic epithelium is zipped closed late in embryogenesis, serves as an excellent, genetically tractable model for epithelial migration. Using live confocal imaging, we have dissected multiple roles for the small GTPase Rac in this process. We show that constitutive activation of Rac1 leads to excessive assembly of lamellipodia and precocious halting of epithelial sweeping, possibly through premature activation of contact-inhibition machinery. Conversely, blocking Rac activity, either by loss-of-function mutations or expression of dominant negative Rac1, disables the assembly of both actin cable and protrusions by epithelial cells. Movies of mutant embryos show that continued contraction of the amnioserosa is sufficient to draw the epithelial edges towards one another, allowing the zipper machinery to bypass non-functioning regions of leading edge. In addition to illustrating the key role of Rac in organization of leading edge actin, loss-of-function mutants also provide substantive proof that Rac acts upstream in the Jun N-terminal kinase (JNK) cascade to direct epithelial cell shape changes during dorsal closure.

Inheritance of most X-linked traits is not dominant or recessive, just X-linked

The existence of X-linked disorders in humans has been recognized for many centuries, based on lessons in religious texts and observations of specific human families (e.g., color blindness or Daltonism). Our modern concepts of Mendelian (including X-linked) inheritance originated just after the turn of the last century. Early concepts of dominance and recessiveness were first used in conjunction with autosomal traits, and then applied to "sex"-linked traits to distinguish X-linked recessive and X-linked dominant inheritance. The former was defined as vertical transmission in which carrier women pass the disorder to affected sons, while the latter was defined as vertical transmission in which daughters of affected males are always affected, transmitting the disorder to offspring of both sexes. However, many X-linked disorders such as adrenoleukodystrophy, fragile X syndrome, and ornithine transcarbamylase deficiency do not fit these rules. We reviewed the literature on 32 X-linked disorders and recorded information on penetrance and expressivity in both sexes. As expected, penetrance and an index of severity of the phenotype (defined in our Methods) were both high in males, while the severity index was low in females. Contrary to standard presentations of X-linked inheritance, penetrance was highly variable in females. Our analysis classified penetrance as high in 28% of the disorders studied, intermediate in 31%, and low in 40%. The high proportion of X-linked disorders with intermediate penetrance is difficult to reconcile with standard definitions of X-linked recessive and dominant inheritance. They do not capture the extraordinarily variable expressivity of X-linked disorders or take into account the multiple mechanisms that can result in disease expression in females, which include cell autonomous expression, skewed X-inactivation, clonal expansion, and somatic mosaicism. We recommend that use of the terms X-linked recessive and dominant be discontinued, and that all such disorders be simply described as following "X-linked" inheritance.

Rho1 regulates signaling events required for proper Drosophila embryonic development

The Rho small GTPase has been implicated in many cellular processes, including actin cytoskeletal regulation and transcriptional activation. The molecular mechanisms underlying Rho function in many of these processes are not yet clear. Here we report that in Drosophila, reduction of maternal Rho1 compromises signaling pathways consistent with defects in membrane trafficking events. These mutants fail to maintain expression of the segment polarity genes engrailed (en), wingless (wg), and hedgehog (hh), contributing to a segmentation phenotype. Formation of the Wg protein gradient involves the internalization of Wg into vesicles. The number of these Wg-containing vesicles is reduced in maternal Rho1 mutants, suggesting a defect in endocytosis. Consistent with this, stripes of cytoplasmic beta-catenin that accumulate in response to Wg signaling are narrower in these mutants relative to wild type. Additionally, the amount of extracellular Wg protein is reduced in maternal Rho1 mutants, indicating a defect in secretion. Signaling pathways downregulated by endocytosis, such as the epidermal growth factor receptor (EGFR) and Torso pathways, are hyperactivated in maternal Rho1 mutants, consistent with a general role for Rho1 in regulating signaling events governing proper patterning during Drosophila development.

Hairy transcriptional repression targets and cofactor recruitment in Drosophila

Members of the widely conserved Hairy/Enhancer of split family of basic Helix-Loop-Helix repressors are essential for proper Drosophila and vertebrate development and are misregulated in many cancers. While a major step forward in understanding the molecular mechanism(s) surrounding Hairy-mediated repression was made with the identification of Groucho, Drosophila C-terminal binding protein (dCtBP), and Drosophila silent information regulator 2 (dSir2) as Hairy transcriptional cofactors, the identity of Hairy target genes and the rules governing cofactor recruitment are relatively unknown. We have used the chromatin profiling method DamID to perform a global and systematic search for direct transcriptional targets for Drosophila Hairy and the genomic recruitment sites for three of its cofactors: Groucho, dCtBP, and dSir2. Each of the proteins was tethered to Escherichia coli DNA adenine methyltransferase, permitting methylation proximal to in vivo binding sites in both Drosophila Kc cells and early embryos. This approach identified 40 novel genomic targets for Hairy in Kc cells, as well as 155 loci recruiting Groucho, 107 loci recruiting dSir2, and wide genomic binding of dCtBP to 496 loci. We also adapted DamID profiling such that we could use tightly gated collections of embryos (2-6 h) and found 20 Hairy targets related to early embryogenesis. As expected of direct targets, all of the putative Hairy target genes tested show Hairy-dependent expression and have conserved consensus C-box-containing sequences that are directly bound by Hairy in vitro. The distribution of Hairy targets in both the Kc cell and embryo DamID experiments corresponds to Hairy binding sites in vivo on polytene chromosomes. Similarly, the distributions of loci recruiting each of Hairy's cofactors are detected as cofactor binding sites in vivo on polytene chromosomes. We have identified 59 putative transcriptional targets of Hairy. In addition to finding putative targets for Hairy in segmentation, we find groups of targets suggesting roles for Hairy in cell cycle, cell growth, and morphogenesis, processes that must be coordinately regulated with pattern formation. Examining the recruitment of Hairy's three characterized cofactors to their putative target genes revealed that cofactor recruitment is context-dependent. While Groucho is frequently considered to be the primary Hairy cofactor, we find here that it is associated with only a minority of Hairy targets. The majority of Hairy targets are associated with the presence of a combination of dCtBP and dSir2. Thus, the DamID chromatin profiling technique provides a systematic means of identifying transcriptional target genes and of obtaining a global view of cofactor recruitment requirements during development.

Drosophila Topors Is a RING Finger-containing Protein That Functions as a Ubiquitin-protein Isopeptide Ligase for the Hairy Basic Helix-Loop-Helix Repressor Protein

Transcriptional repression plays an essential role in many aspects of metazoan development. Drosophila hairy is a primary pair-rule gene encoding a basic helix-loop-helix class transcriptional repressor that is required for proper segmentation. Previous characterization of Hairy-binding proteins has implicated two different classes of histone deacetylase as mediators of Hairy repression. Here, we present the characterization of a novel Hairy-interacting protein (dTopors) that binds specifically to the basic region of Hairy, but does not affect the ability of Hairy to bind DNA. By reducing the gene dose of dtopors, we demonstrate that it acts genetically as an antagonist of Hairy-mediated transcriptional repression. Consistent with this genetic interaction, we show that that recombinant dTopors protein possesses ubiquitin-protein isopeptide ligase activity in vitro and that dTopors mediates Hairy polyubiquitination and can lead to Hairy degradation. This work provides the first evidence that regulated proteolysis of Hairy is required for correct segmentation.

The roles of transcription factors in B lymphocyte commitment, development, and transformation

Studies of normal blood cell development and malignant transformation of hematopoietic cells have shown that the correctly regulated expression of stage- and lineage-specific genes is a key issue in hematopoiesis. Experiments in transgenic mice have defined a number of transcription factors such as SCL/Tal, core-binding factor/acute myeloid leukemia, and c-myb, all crucial for the establishment of definitive hematopoiesis and development of all blood cell lineages. Other regulators such as IKAROS, E47/E2A, early B cell factor, Sox-4, and B cell-specific activator protein (Pax-5) appear crucial, more or less selectively, for B lymphopoiesis, allowing for detailed analysis of the development of this lineage. In addition, several of these transcription factors are found translocated in human tumors, often resulting in aberrant gene expression or production of modified proteins. This article concerns the role of transcription factors in B lymphoid development with special focus on lineage initiation and commitment events but also to some extent on the roles of transcription factors in human B lymphoid malignancies.

Insect basic leucine zipper proteins and their role in cyclic AMP-dependent regulation of gene expression

The cAMP-protein kinase A (PKA) pathway is an important intracellular signal transduction cascade that can be activated by a large variety of stimuli. Activation or inhibition of this pathway will ultimately affect the transcriptional regulation of various genes through distinct responsive sites. In vertebrates, the best- characterized nuclear targets of PKA are the cyclic AMP response element-binding (CREB) proteins. It is now well established that CREB is not only regulated by PKA, but many other kinases can exert an effect as well. Since CREB-like proteins were also discovered in invertebrates, several studies unraveling their physiological functions in this category of metazoans have been performed. This review will mainly focus on the presence and regulation of CREB proteins in insects. Differences in transcriptional responses to the PKA pathway and other CREB-regulating stimuli between cells, tissues, and even organisms can be partially attributed to the presence of different CREB isoforms. In addition, the regulation of CREB appears to show some important differences between insects and vertebrates. Since CREB is a basic leucine zipper (bZip) protein, other insect members of this important family of transcriptional regulators will be briefly discussed as well.

A Theoretical Model for the Regulation of Sex-lethal, a Gene That Controls Sex Determination and Dosage Compensation in Drosophila melanogaster

Cell fate commitment relies upon making a choice between different developmental pathways and subsequently remembering that choice. Experimental studies have thoroughly investigated this central theme in biology for sex determination. In the somatic cells of Drosophila melanogaster, Sex-lethal (Sxl) is the master regulatory gene that specifies sexual identity. We have developed a theoretical model for the initial sex-specific regulation of Sxl expression. The model is based on the well-documented molecular details of the system and uses a stochastic formulation of transcription. Numerical simulations allow quantitative assessment of the role of different regulatory mechanisms in achieving a robust switch. We establish on a formal basis that the autoregulatory loop involved in the alternative splicing of Sxl primary transcripts generates an all-or-none bistable behavior and constitutes an efficient stabilization and memorization device. The model indicates that production of a small amount of early Sxl proteins leaves the autoregulatory loop in its off state. Numerical simulations of mutant genotypes enable us to reproduce and explain the phenotypic effects of perturbations induced in the dosage of genes whose products participate in the early Sxl promoter activation.

Constructing an organ: the Drosophila salivary gland as a model for tube formation

Tubes are required in metazoans to transport the liquids and gases that sustain life. The conservation of molecules and mechanisms involved in tube formation suggests that what we learn by studying simple systems will apply to related processes in higher animals. Studies over the past 10 years have revealed the molecules that specify cell fate in Drosophila salivary gland and the cellular events that mediate tube morphogenesis. Here, we discuss how anterior-posterior and dorsal-ventral patterning information specifies both the position of salivary-gland primordia and how many cells they contain. We examine the transformation of a polarized epithelial sheet into an elongated, unbranched tube, and the intrinsic and extrinsic factors that influence the final position of the salivary gland.

Epithelial tube morphology is determined by the polarized growth and delivery of apical membrane

Formation of tubes of the correct size and shape is essential for viability of most organisms, yet little is understood of the mechanisms controlling tube morphology. We identified a new allele of hairy in a mutagenesis screen and showed that hairy mutations cause branching and bulging of the normally unbranched salivary tube, in part through prolonged expression of huckebein (hkb). HKB controls polarized cell shape change and apical membrane growth during salivary cell invagination via two downstream target genes, crumbs (crb), a determinant of the apical membrane, and klarsicht (klar), which mediates microtubule-dependent organelle transport. In invaginating salivary cells, crb and klar mediate growth and delivery of apical membrane, respectively, thus regulating the size and shape of the salivary tube.

Rho1 interacts with p120ctn and α-catenin, and regulates cadherin-based adherens junction components in Drosophila

Rho GTPases are important regulators of cellular behavior through their effects on processes such as cytoskeletal organization. Here we show interactions between Drosophila Rho1 and the adherens junction components α-catenin and p120ctn. We find that while Rho1 protein is present throughout the cell, it accumulates apically, particularly at sites of cadherin-based adherens junctions. Cadherin and catenin localization is disrupted in Rho1 mutants, implicating Rho1 in their regulation. p120ctn has recently been suggested to inhibit Rho activity through an unknown mechanism. We find that Rho1 accumulates in response to lowered p120ctn activity. Significantly, we find that Rho1 binds directly to α-catenin and p120ctn in vitro, and these interactions map to distinct surface-exposed regions of the protein not previously assigned functions. In addition, we find that both α-catenin and p120ctn co-immunoprecipitate with Rho1-containing complexes from embryo lysates. Our observations suggest that α-catenin and p120ctn are key players in a mechanism of recruiting Rho1 to its sites of action.

Thetransformergene inCeratitis capitataprovides a genetic basis for selecting and remembering the sexual fate

The medfly Ceratitis capitata contains a gene (Cctra) with structural and functional homology to the Drosophila melanogaster sex-determining gene transformer (tra). Similar to tra in Drosophila, Cctra is regulated by alternative splicing such that only females can encode a full-length protein. In contrast to Drosophila, however, where tra is a subordinate target of Sex-lethal (Sxl), Cctra seems to initiate an autoregulatory mechanism in XX embryos that provides continuous tra female-specific function and act as a cellular memory maintaining the female pathway. Indeed, a transient interference with Cctra expression in XX embryos by RNAi treatment can cause complete sexual transformation of both germline and soma in adult flies, resulting in a fertile male XX phenotype. The male pathway seems to result when Cctra autoregulation is prevented and instead splice variants with truncated open reading frames are produced. We propose that this repression is achieved by the Y-linked male-determining factor (M).

Drosophila Sir2 is required for heterochromatic silencing and by euchromatic Hairy/E(Spl) bHLH repressors in segmentation and sex determination

Yeast SIR2 is a NAD+-dependent histone deacetylase required for heterochromatic silencing at telomeres, rDNA, and mating-type loci. We find that the Drosophila homolog of Sir2 (dSir2) also encodes deacetylase activity and is required for heterochromatic silencing, but unlike ySir2, is not required for silencing at telomeres. We show that dSir2 interacts genetically and physically with members of the Hairy/Deadpan/E(Spl) family of bHLH euchromatic repressors, key regulators of Drosophila development. dSir2 is an essential gene whose loss of function results in both segmentation defects and skewed sex ratios, associated with reduced activities of the Hairy and Deadpan bHLH repressors. These results indicate that Sir2 in higher organisms plays an essential role in both euchromatic repression and heterochromatic silencing.

Vertebrate hairy and Enhancer of split related proteins: transcriptional repressors regulating cellular differentiation and embryonic patterning

The basic-helix-loop-helix (bHLH) proteins are a superfamily of DNA-binding transcription factors that regulate numerous biological processes in both invertebrates and vertebrates. One family of bHLH transcriptional repressors is related to the Drosophila hairy and Enhancer-of-split proteins. These repressors contain a tandem arrangement of the bHLH domain and an adjacent sequence known as the Orange domain, so we refer to these proteins as bHLH-Orange or bHLH-O proteins. Phylogenetic analysis reveals the existence of four bHLH-O subfamilies, with distinct, evolutionarily conserved features. A principal function of bHLH-O proteins is to bind to specific DNA sequences and recruit transcriptional corepressors to inhibit target gene expression. However, it is likely that bHLH-O proteins repress transcription by additional mechanisms as well. Many vertebrate bHLH-O proteins are effectors of the Notch signaling pathway, and bHLH-O proteins are involved in regulating neurogenesis, vasculogenesis, mesoderm segmentation, myogenesis, and T lymphocyte development. In this review, we discuss mechanisms of action and biological roles for the vertebrate bHLH-O proteins, as well as some of the unresolved questions about the functions and regulation of these proteins during development and in human disease.

Emc, a negative HLH regulator with multiple functions in Drosophila development

Expression and functional analyses of Emc have demonstrated that it is a prototype for a protein required for multiple processes in development. Initially characterized as a negative regulator of sensory organ development, it was later found to regulate many other developmental processes and cell proliferation. Its ability to block the function of bHLH proteins by forming heterodimers, which are ineffective in DNA binding, accounts for the role of Emc in preventing the acquisition of several cell fates which are under the control of bHLH proteins. However, while maintaining this repressive molecular mechanism, emc also appears to act as a positive regulator of differentiation.

Brinker requires two corepressors for maximal and versatile repression in Dpp signalling

decapentaplegic (dpp) encodes a Drosophila transforming growth factor-beta homologue that functions as a morphogen in the developing embryo and in adult appendage formation. In the wing imaginal disc, a Dpp gradient governs patterning along the anteroposterior axis by inducing regional expression of diverse genes in a concentration-dependent manner. Recent studies show that responses to graded Dpp activity also require an input from a complementary and opposing gradient of Brinker (Brk), a transcriptional repressor protein encoded by a Dpp target gene. Here we show that Brk harbours a functional and transferable repression domain, through which it recruits the corepressors Groucho and CtBP. By analysing transcriptional outcomes arising from the genetic removal of these corepressors, and by ectopically expressing Brk variants in the embryo, we demonstrate that these corepressors are alternatively used by Brk for repressing some Dpp-responsive genes, whereas for repressing other distinct target genes they are not required. Our results show that Brk utilizes multiple means to repress its endogenous target genes, allowing repression of a multitude of complex Dpp target promoters.

Groucho augments the repression of multiple Even skipped target genes in establishing parasegment boundaries

Groucho acts as a co-repressor for several Drosophila DNA binding transcriptional repressors. Several of these proteins have been found to contain both Groucho-dependent and -independent repression domains, but the extent to which this distinction has functional consequences for the regulation of different target genes is not known. The product of the pair-rule gene even skipped has previously been shown to contain a Groucho-independent repression activity. In the Even skipped protein, outside the Groucho-independent repression domain, we have identified a conserved C-terminal motif (LFKPY), similar to motifs that mediate Groucho interaction in Hairy, Runt and Huckebein. Even skipped interacts with Groucho in yeast and in vitro, and groucho and even skipped genetically interact in vivo. Even skipped with a mutated Groucho interaction motif, which abolished binding to Groucho, showed a significantly reduced ability to rescue the even skipped null phenotype when driven by the complete even skipped regulatory region. Replacing this motif with a heterologous Groucho interaction motif restored the rescuing function of Even skipped in segmentation. Further functional assays demonstrated that the Even skipped C terminus acts as a Groucho-dependent repression domain in early Drosophila embryos. This novel repression domain was active on two target genes that are normally repressed by Even skipped at different concentrations, paired and sloppy paired. When the Groucho interaction motif is mutated, repression of each target gene is reduced to a similar extent, with some activity remaining. Thus, the ability of Even skipped to repress different target genes at different concentrations does not appear to involve differential recruitment or function of Groucho. The accumulation of multiple domains of similar function within a single protein may be a common evolutionary mechanism that fine-tunes the level of activity for different regulatory functions.

Interpretation of X chromosome dose at Sex-lethal requires non-E-box sites for the basic helix-loop-helix proteins SISB and daughterless

For Drosophila melanogaster flies, sexual fate is determined by the X chromosome number. The basic helix-loop-helix protein product of the X-linked sisterlessB (sisB or scute) gene is a key indicator of the X dose and functions to activate the switch gene Sex-lethal (Sxl) in female (XX), but not in male (XY), embryos. Zygotically expressed sisB and maternal daughterless (da) proteins are known to form heterodimers that bind E-box sites and activate transcription. We examined SISB-Da binding at Sxl by using footprinting and gel mobility shift assays and found that SISB-Da binds numerous clustered sites in the establishment promoter Sxl(Pe). Surprisingly, most SISB-Da sites at Sxl(Pe) differ from the canonical CANNTG E-box motif. These noncanonical sites have 6-bp CA(G/C)CCG and 7-bp CA(G/C)CTTG cores and exhibit a range of binding affinities. We show that the noncanonical sites can mediate SISB-Da-activated transcription in cell culture. P-element transformation experiments show that these noncanonical sites are essential for Sxl(Pe) activity in embryos. Together with previous deletion analysis, the data suggest that the number, affinity, and position of SISB-Da sites may all be important for the operation of the Sxl(Pe) switch. Comparisons with other dose-sensitive promoters suggest that threshold responses to diverse biological signals have common molecular mechanisms, with important variations tailored to suit particular functional requirements.

Specific genetic interference with behavioral rhythms in Drosophila by expression of inverted repeats

We describe a new experimental technique that allows for a tissue-specific reduction of gene activity in the Drosophila nervous system. On the basis of the observation that certain gene functions can be ubiquitously blocked by injecting double-stranded RNA into Drosophila embryos, we employed a method to interfere with an individual gene function permanently in a predetermined cell type. This was achieved by the formation of an inverted-repeat RNA sequence in the tissue of interest under control of the GAL4/UAS binary expression system. As an example, we show that inverted-repeat-mediated interference with the period gene produces a hypomorphic period phenotype. A selective decrease of period RNA appears to be a component of the cellular response.

Drosophila C-terminal binding protein functions as a context-dependent transcriptional co-factor and interferes with both mad and groucho transcriptional repression

Drosophila C-terminal binding protein (dCtBP) and Groucho have been identified as Hairy-interacting proteins required for embryonic segmentation and Hairy-mediated transcriptional repression. While both dCtBP and Groucho are required for proper Hairy function, their properties are very different. As would be expected for a co-repressor, reduced Groucho activity enhances the hairy mutant phenotype. In contrast, reduced dCtBP activity suppresses it. We show here that dCtBP can function as either a co-activator or co-repressor of transcription in a context-dependent manner. The regions of dCtBP required for activation and repression are separable. We find that mSin3A-histone deacetylase complexes are altered in the presence of dCtBP and that dCtBP interferes with both Groucho and Mad transcriptional repression. Similar to CtBP's role in attenuating E1A's oncogenicity, we propose that dCtBP can interfere with corepressor-histone deacetylase complexes, thereby attenuating transcriptional repression. Hairy defines a new class of proteins that requires both CtBP and Groucho co-factors for proper function.

Sequence and expression analysis of Nhlh1: a basic helix-loop-helix gene implicated in neurogenesis

Nhlh1 is a basic helix-loop-helix (bHLH) gene that has been implicated in mouse neurogenesis. Previous studies have shown it to be expressed in regions in which there are differentiating neurons during late embryonic and fetal development, but detailed studies of the role of Nhlh1 earlier in embryonic development have not been performed. In this paper, we examine the expression of Nhlh1 transcripts at early embryonic stages (E8.5-E10.5), at the onset of neurogenesis, and compare the pattern of expression with that of Islet-1, a marker of postmitotic neurons. We show that Nhlh1 is expressed in early postmitotic neurons but is down-regulated as these cells migrate from the ventricular zone. We have also determined the genomic structure of mouse Nhlh1 and have characterised the promoter sequence, as a first step towards identifying factors that may control Nhlh1 expression. Nhlh1 has been implicated previously as a candidate for the neural tube defect mutant loop-tail (Lp); here, we present sequence and expression data indicating that Nhlh1 is unlikely to be responsible for the Lp mutation.

Groucho/TLE family proteins and transcriptional repression

The Drosophila Groucho (Gro) protein is the prototype for a large family of corepressors, examples of which are found in most metazoans. This family includes the human transducin-like Enhancer of split (TLE) proteins. As corepressors, Gro/TLE family proteins do not bind to DNA directly, but rather are recruited to the template by DNA-bound repressor proteins. Gro/TLE family proteins are required for many developmental processes, including lateral inhibition, segmentation, sex determination, dorsal/ventral pattern formation, terminal pattern formation, and eye development. These proteins are characterized by a conserved N-terminal glutamine-rich domain and a conserved C-terminal WD-repeat domain. The primary role of the glutamine-rich domain is apparently to mediate tetramerization, while the WD-repeat domain may mediate interactions with DNA-bound repressors. The glutamine rich and WD-repeat domains are separated by a less conserved region containing domains that have been implicated in transcriptional repression and nuclear localization. In addition to encoding full-length Gro/TLE family proteins, most metazoan genomes encode truncated family members that contain the N-terminal oligomerization domain, but lack the C-terminal WD-repeat domain. These truncated proteins may negatively regulate full-length Gro/TLE proteins, perhaps by sequestering them in non-productive complexes. Gro/TLE family proteins probably repress transcription by multiple mechanisms. For example, a glycine/proline-rich domain in the central variable region functions to recruit the histone deacetylase Rpd3 to the template. This histone deacetylase then presumably silences transcription by altering local chromatin structure. Other repression domains in Gro may function in a histone deacetylase-independent manner. Many aspects of Gro/TLE protein function remain to be explored, including the possible post-translational regulation of Gro/TLE activity as well as the mechanisms by which Gro/TLE proteins direct repression at a distance.

Transcriptional repression by Pax5 (BSAP) through interaction with corepressors of the Groucho family

Pax5 (BSAP) functions as both a transcriptional activator and repressor during midbrain patterning, B-cell development and lymphomagenesis. Here we demonstrate that Pax5 exerts its repression function by recruiting members of the Groucho corepressor family. In a yeast two-hybrid screen, the groucho-related gene product Grg4 was identified as a Pax5 partner protein. Both proteins interact cooperatively via two separate domains: the N-terminal Q and central SP regions of Grg4, and the octapeptide motif and C-terminal transactivation domain of Pax5. The phosphorylation state of Grg4 is altered in vivo upon Pax5 binding. Moreover, Grg4 efficiently represses the transcriptional activity of Pax5 in an octapeptide-dependent manner. Similar protein interactions resulting in transcriptional repression were also observed between distantly related members of both the Pax2/5/8 and Groucho protein families. In agreement with this evolutionary conservation, the octapeptide motif of Pax proteins functions as a Groucho-dependent repression domain in Drosophila embryos. These data indicate that Pax proteins can be converted from transcriptional activators to repressors through interaction with corepressors of the Groucho protein family.

Overlapping expression of early B-cell factor and basic helix-loop-helix proteins as a mechanism to dictate B-lineage-specific activity of the lambda5 promoter

The basic helix-loop-helix (bHLH) transcription factors are a large group of proteins suggested to control key events in the development of B lymphocytes as well as of other cellular lineages. To examine how bHLH proteins activate a B-lineage-specific promoter, I investigated the ability of E47, E12, Heb, E2-2, and MyoD to activate the lambda5 surrogate light chain promoter. Comparison of the functional capacity of the E2A-encoded E47 and E12 proteins indicated that even though both were able to activate the lambda5 promoter and act in synergy with early B-cell factor (EBF), E47 displayed a higher functional activity than E12. An ability to act in synergy with EBF was also observed for Heb, E2-2, and MyoD, suggesting that these factors were functionally redundant in this regard. Mapping of functional domains in EBF and E47 revealed that the dimerization and DNA binding domains mediated the synergistic activity. Electrophoretic mobility shift assay analysis using the 5' part of the lambda5 promoter revealed formation of template-dependent heteromeric complexes between EBF and E47, suggesting that the synergistic mechanism involves cooperative binding to DNA. These findings propose a unique molecular function for E47 and provide overlapping expression with EBF as a molecular mechanism to direct B-cell-specific target gene activation by bHLH proteins.

Structure, function and evolution of sex-determining systems in Dipteran insects

Nature has evolved an astonishing variety of genetic and epigenetic sex-determining systems which all achieve the same result, the generation of two sexes. Genetic and molecular analyses, mainly performed during the last 20 years, have gradually revealed the mechanisms that govern sexual differentiation in a few model organisms. In this review, we will introduce the sex-determining system of Drosophila and compare the fruitfly to the housefly Musca domestica and other Dipteran insects. Despite the ostensible variety, all these insects use the same basic strategy: a primary genetic signal that is different in males and females, a key gene that responds to the primary signal, and a double-switch gene that eventually selects between two alternative sexual programmes. These parallels, however, do not extend to the molecular level. Except for the double-switch gene doublesex at the end of the cascade, no functional homologies were found between more distantly related insects. In particular, Sex-lethal, the key gene that controls sexual differentiation in Drosophila, does not have a sex-determining function in any other genus studied so far. These results show that sex-determining cascades, in comparison to other regulatory pathways, evolve much more rapidly.

Control of endodermal endocrine development by Hes-1

Development of endocrine cells in the endoderm involves Atonal and Achaete/Scute-related basic helix-loop-helix (bHLH) proteins. These proteins also serve as neuronal determination and differentiation factors, and are antagonized by the Notch pathway partly acting through Hairy and Enhancer-of-split (HES)-type proteins. Here we show that mice deficient in Hes1 (encoding Hes-1) display severe pancreatic hypoplasia caused by depletion of pancreatic epithelial precursors due to accelerated differentiation of post-mitotic endocrine cells expressing glucagon. Moreover, upregulation of several bHLH components is associated with precocious and excessive differentiation of multiple endocrine cell types in the developing stomach and gut, showing that Hes-1 operates as a general negative regulator of endodermal endocrine differentiation.

Mutations in the Rho1 small GTPase disrupt morphogenesis and segmentation during early Drosophila development

Rho GTPases play an important role in diverse biological processes such as actin cytoskeleton organization, gene transcription, cell cycle progression and adhesion. They are required during early Drosophila development for proper execution of morphogenetic movements of individual cells and groups of cells important for the formation of the embryonic body plan. We isolated loss-of-function mutations in the Drosophila Rho1 (Rho1) gene during a genetic screen for maternal-effect mutations, allowing us to investigate the specific roles Rho1 plays in the context of the developing organism. Here we report that Rho1 is required for many early events: loss of Rho1 function results in both maternal and embryonic phenotypes. Embryos homozygous for the Rho1 mutation exhibit a characteristic zygotic phenotype, which includes severe defects in head involution and imperfect dorsal closure. Two phenotypes are associated with reduction of maternal Rho1 activity: the actin cytoskeleton is disrupted in egg chambers, especially in the ring canals and embryos display patterning defects as a result of improper maintenance of segmentation gene expression. Despite showing imperfect dorsal closure, Rho1 does not activate downstream genes or interact genetically with members of the JNK signaling pathway, used by its relatives dRac and dCdc42 for proper dorsal closure. Consistent with its roles in regulating actin cytoskeletal organization, we find that Rho1 interacts genetically and physically with the Drosophila formin homologue, cappuccino. We also show that Rho1 interacts both genetically and physically with concertina, a G(alpha) protein involved in cell shape changes during gastrulation.

Huckebein repressor activity in Drosophila terminal patterning is mediated by Groucho

The Groucho corepressor mediates negative transcriptional regulation in association with various DNA-binding proteins in diverse developmental contexts. We have previously implicated Groucho in Drosophila embryonic terminal patterning, showing that it is required to confine tailless and huckebein terminal gap gene expression to the pole regions of the embryo. Here we reveal an additional requirement for Groucho in this developmental process by establishing that Groucho mediates repressor activity of the Huckebein protein. Putative Huckebein target genes are derepressed in embryos lacking maternal groucho activity and biochemical experiments demonstrate that Huckebein physically interacts with Groucho. Using an in vivo repression assay, we identify a functional repressor domain in Huckebein that contains an FRPW tetrapeptide, similar to the WRPW Groucho-recruitment domain found in Hairy-related repressor proteins. Mutations in Huckebein's FRPW motif abolish Groucho binding and in vivo repression activity, indicating that binding of Groucho through the FRPW motif is required for the repressor function of Huckebein. Taken together with our earlier results, these findings show that Groucho-repression regulates sequential aspects of terminal patterning in Drosophila.

A conserved motif in goosecoid mediates groucho-dependent repression in Drosophila embryos

Surprisingly small peptide motifs can confer critical biological functions. One example is the WRPW tetrapeptide present in the Hairy family of transcriptional repressors, which mediates recruitment of the Groucho (Gro) corepressor to target promoters. We recently showed that Engrailed (En) is another repressor that requires association with Gro for its function. En lacks a WRPW motif; instead, it contains another short conserved sequence, the En homology region 1 (eh1)/GEH motif, that is likely to play a role in tethering Gro to the promoter. Here, we characterize a repressor domain from the Goosecoid (Gsc) developmental regulator that includes an eh1/GEH-like motif. We demonstrate that this domain (GscR) mediates efficient repression in Drosophila blastoderm embryos and that repression by GscR requires Gro function. GscR and Gro interact in vitro, and the eh1/GEH motif is necessary and sufficient for the interaction and for in vivo repression. Because WRPW- and eh1/GEH-like motifs are present in different proteins and in many organisms, the results suggest that interactions between short peptides and Gro represent a widespread mechanism of repression. Finally, we investigate whether Gro is part of a stable multiprotein complex in the nucleus. Our results indicate that Gro does not form stable associations with other proteins but that it may be able to assemble into homomultimeric complexes.

Groucho and dCtBP mediate separate pathways of transcriptional repression in the Drosophila embryo

The precellular Drosophila embryo contains approximately 10 well characterized transcriptional repressors. At least half are short-range repressors that must bind within 100 bp of either upstream activators or the core transcription complex to inhibit (or quench) gene expression. The two long-range repressors can function over distances of 1 kilobase or more to silence transcription. Previous studies have shown that three of the five short-range repressors interact with a common corepressor protein, dCtBP. In contrast, the two long-range repressors, Hairy and Dorsal, recruit a different corepressor protein, Groucho. Hairy also was shown to interact with dCtBP, thereby raising the possibility that Groucho and dCtBP are components of a common corepressor complex. To investigate this issue, we have misexpressed wild-type and mutant forms of Hairy in transgenic embryos. Evidence is presented that Hairy-mediated repression depends on the Groucho interaction sequence (WRPW) but not the weak dCtBP motif (PLSLV) present in the native protein. Conversion of the PLSLV motif into an optimal dCtBP interaction sequence (PLDLS) disrupts the activity of an otherwise normal Hairy protein. These results suggest that dCtBP and Groucho mediate separate pathways of transcriptional repression and that the two proteins can inhibit one another when both bind the same repressor.

Equivalence of the fly orthodenticle gene and the human OTX genes in embryonic brain development of Drosophila

Members of the orthodenticle gene family are essential for embryonic brain development in animals as diverse as insects and mammals. In Drosophila, mutational inactivation of the orthodenticle gene results in deletions in anterior parts of the embryonic brain and in defects in the ventral nerve cord. In the mouse, targeted elimination of the homologous Otx2 or Otx1 genes causes defects in forebrain and/or midbrain development. To determine the morphogenetic properties and the extent of evolutionary conservation of the orthodenticle gene family in embryonic brain development, genetic rescue experiments were carried out in Drosophila. Ubiquitous overexpression of the orthodenticle gene rescues both the brain defects and the ventral nerve cord defects in orthodenticle mutant embryos; morphology and nervous system-specific gene expression are restored. Two different time windows exist for the rescue of the brain versus the ventral nerve cord. Ubiquitous overexpression of the human OTX1 or OTX2 genes also rescues the brain and ventral nerve cord phenotypes in orthodenticle mutant embryos; in the brain, the efficiency of morphological rescue is lower than that obtained with overexpression of orthodenticle. Overexpression of either orthodenticle or the human OTX gene homologs in the wild-type embryo results in ectopic neural structures. The rescue of highly complex brain structures in Drosophila by either fly or human orthodenticle gene homologs indicates that these genes are interchangeable between vertebrates and invertebrates and provides further evidence for an evolutionarily conserved role of the orthodenticle gene family in brain development.

The function of hairy-related bHLH repressor proteins in cell fate decisions

Hairy-related proteins are a distinct subfamily of basic helix-loop-helix (bHLH) proteins that generally function as DNA-binding transcriptional repressors. These proteins act in opposition to bHLH transcriptional activator proteins such as the proneural and myogenic proteins; together, the activator and repressor genes that encode these proteins have co-evolved as a regulatory gene "cassette" or "module" for controlling cell fate decisions. In the development of the Drosophila peripheral nervous system, Hairy-related genes function at multiple steps during neurogenesis, for example, as positional information genes that establish the "prepattern" that controls where "proneural cluster" equivalence groups will form, and later as nuclear effectors of the Notch signaling pathway to "single out" individual precursor cells within the equivalence group. Hairy-related genes also function in the establishment and restriction of other types of equivalence groups, such as those for muscle and Malphigian tubule precursors. This general function in cell fate specification has been conserved from Drosophila to vertebrates and has implications for human disease pathogenesis.