Groucho is required for Drosophila neurogenesis, segmentation, and sex determination and interacts directly with hairy-related bHLH proteins

Capicua DNA-binding sites are general response elements for RTK signaling in Drosophila

RTK/Ras/MAPK signaling pathways play key functions in metazoan development, but how they control expression of downstream genes is not well understood. In Drosophila, it is generally assumed that most transcriptional responses to RTK signal activation depend on binding of Ets-family proteins to specific cis-acting sites in target enhancers. Here, we show that several Drosophila RTK pathways control expression of downstream genes through common octameric elements that are binding sites for the HMG-box factor Capicua, a transcriptional repressor that is downregulated by RTK signaling in different contexts. We show that Torso RTK-dependent regulation of terminal gap gene expression in the early embryo critically depends on Capicua octameric sites, and that binding of Capicua to these sites is essential for recruitment of the Groucho co-repressor to the huckebein enhancer in vivo. We then show that subsequent activation of the EGFR RTK pathway in the neuroectodermal region of the embryo controls dorsal-ventral gene expression by downregulating the Capicua protein, and that this control also depends on Capicua octameric motifs. Thus, a similar mechanism of RTK regulation operates during subdivision of the anterior-posterior and dorsal-ventral embryonic axes. We also find that identical DNA octamers mediate Capicua-dependent regulation of another EGFR target in the developing wing. Remarkably, a simple combination of activator-binding sites and Capicua motifs is sufficient to establish complex patterns of gene expression in response to both Torso and EGFR activation in different tissues. We conclude that Capicua octamers are general response elements for RTK signaling in Drosophila.

Mast cell transcription factors--regulators of cell fate and phenotype

Transcription factors have a key role in mast cell differentiation and response of differentiated mast cells to external stimuli. During differentiation of progenitor cells to mast cells, a role for different GATA transcription factors in combination with PU.1 expression and downregulation of C/EBPα has been described. Notch pathway has been proposed to have a role in mast cell development. The microphthalmia-associated transcription factor expression is upregulated in later stages of mast cells differentiation, but it is not expressed in the closely related basophiles. In differentiated mast cells, there is a role for transcription factors both in determining the specific mast cell phenotype and in the response to immune stimuli such as IgE-Ag. A large number of transcription factors, including AP-1 family proteins, microphthalmia-associated transcription factor and STAT5, are modulated by these stimuli. These transcription factors and related protein modulators form a complex transcription factor network. They can form stimuli regulated specific heterodimers and common inhibitors can move from one protein to another. Transcription factors are the key regulators of mast cell physiology. Modulation of key transcription by such means as the therapeutic siRNA may hopefully allow us to modulate mast cell function, obtaining clinical benefit in a variety of diseases. This article is part of a Special Issue entitled: Mast cells in inflammation.

Identification and characterization of Ixodes scapularis antigens that elicit tick immunity using yeast surface display

Repeated exposure of rabbits and other animals to ticks results in acquired resistance or immunity to subsequent tick bites and is partially elicited by antibodies directed against tick antigens. In this study we demonstrate the utility of a yeast surface display approach to identify tick salivary antigens that react with tick-immune serum. We constructed an Ixodes scapularis nymphal salivary gland yeast surface display library and screened the library with nymph-immune rabbit sera and identified five salivary antigens. Four of these proteins, designated P8, P19, P23 and P32, had a predicted signal sequence. We generated recombinant (r) P8, P19 and P23 in a Drosophila expression system for functional and immunization studies. rP8 showed anti-complement activity and rP23 demonstrated anti-coagulant activity. Ixodes scapularis feeding was significantly impaired when nymphs were fed on rabbits immunized with a cocktail of rP8, rP19 and rP23, a hall mark of tick-immunity. These studies also suggest that these antigens may serve as potential vaccine candidates to thwart tick feeding.

Differential regulation of transcription through distinct Suppressor of Hairless DNA binding site architectures during Notch signaling in proneural clusters

In Drosophila melanogaster, achaete (ac) and m8 are model basic helix-loop-helix activator (bHLH A) and repressor genes, respectively, that have the opposite cell expression pattern in proneural clusters during Notch signaling. Previous studies have shown that activation of m8 transcription in specific cells within proneural clusters by Notch signaling is programmed by a "combinatorial" and "architectural" DNA transcription code containing binding sites for the Su(H) and proneural bHLH A proteins. Here we show the novel result that the ac promoter contains a similar combinatorial code of Su(H) and bHLH A binding sites but contains a different Su(H) site architectural code that does not mediate activation during Notch signaling, thus programming a cell expression pattern opposite that of m8 in proneural clusters.

Transcriptional repression: conserved and evolved features

The regulation of gene expression by transcriptional repression is an ancient and conserved mechanism that manifests itself in diverse ways. Here we summarize conserved pathways for transcriptional repression prevalent throughout all forms of life, as well as indirect mechanisms that appear to have originated in eukaryotes, consistent with the unique chromatin environment of eukaryotic genes. The direct interactions between transcriptional repressors and the core transcriptional machinery in bacteria and archaea are sufficient to generate a sophisticated suite of mechanisms that provide flexible control. These direct interactions contrast with the activity of corepressors, which provide an additional regulatory control in eukaryotes. Their modulation of chromatin structure represents an indirect pathway to downregulate transcription, and their diversity and modulation provide additional complexity suited to the requirements of elaborate eukaryotic repression patterns. New findings indicate that corepressors are not necessarily restricted to generating a single stereotypic output, but can rather exhibit diverse functional responses depending on the context in which they are recruited, providing a hitherto unsuspected additional source of diversity in transcriptional control. Mechanisms within eukaryotes appear to be highly conserved, with novel aspects chiefly represented by addition of lineage-specific corepressor scaffolds that provide additional opportunities for recruiting the same core machinery.

Basic helix-loop-helix transcription factor gene family phylogenetics and nomenclature

A phylogenetic analysis of the basic helix-loop-helix (bHLH) gene superfamily was performed using seven different species (human, mouse, rat, worm, fly, yeast, and plant Arabidopsis) and involving over 600 bHLH genes (Stevens et al., 2008). All bHLH genes were identified in the genomes of the various species, including expressed sequence tags, and the entire coding sequence was used in the analysis. Nearly 15% of the gene family has been updated or added since the original publication. A super-tree involving six clades and all structural relationships was established and is now presented for four of the species. The wealth of functional data available for members of the bHLH gene superfamily provides us with the opportunity to use this exhaustive phylogenetic tree to predict potential functions of uncharacterized members of the family. This phylogenetic and genomic analysis of the bHLH gene family has revealed unique elements of the evolution and functional relationships of the different genes in the bHLH gene family.

WD40 domain divergence is important for functional differences between the fission yeast Tup11 and Tup12 co-repressor proteins

We have previously demonstrated that subsets of Ssn6/Tup target genes have distinct requirements for the Schizosaccharomyces pombe homologs of the Tup1/Groucho/TLE co-repressor proteins, Tup11 and Tup12. The very high level of divergence in the histone interacting repression domains of the two proteins suggested that determinants distinguishing Tup11 and Tup12 might be located in this domain. Here we have combined phylogenetic and structural analysis as well as phenotypic characterization, under stress conditions that specifically require Tup12, to identify and characterize the domains involved in Tup12-specific action. The results indicate that divergence in the repression domain is not generally relevant for Tup12-specific function. Instead, we show that the more highly conserved C-terminal WD40 repeat domain of Tup12 is important for Tup12-specific function. Surface amino acid residues specific for the WD40 repeat domain of Tup12 proteins in different fission yeasts are clustered in blade 3 of the propeller-like structure that is characteristic of WD40 repeat domains. The Tup11 and Tup12 proteins in fission yeasts thus provide an excellent model system for studying the functional divergence of WD40 repeat domains.

Evolution of a genomic regulatory domain: the role of gene co-option and gene duplication in the Enhancer of split complex

The Drosophila Enhancer of split complex [E(spl)-C] is a remarkable complex of genes many of which are effectors or modulators of Notch signaling. The complex contains different classes of genes including four bearded genes and seven basic helix-loop-helix (bHLH) genes. We examined the evolution of this unusual complex by identifying bearded and bHLH genes in the genome sequences of Arthropods. We find that a four-gene E(spl)-C, containing three bHLH genes and one bearded gene, is an ancient component of the genomes of Crustacea and Insects. The complex is well conserved in insects but is highly modified in Drosophila, where two of the ancestral genes of the complex are missing, and the remaining two have been duplicated multiple times. Through examining the expression of E(spl)-C genes in honeybees, aphids, and Drosophila, we determined that the complex ancestrally had a role in Notch signaling. The expression patterns of genes found inserted into the complex in some insects, or that of ancestral E(spl)-C genes that have moved out of the complex, imply that the E(spl)-C is a genomic domain regulated as a whole by Notch signaling. We hypothesize that the E(spl)-C is a Notch-regulated genomic domain conserved in Arthropod genomes for around 420 million years. We discuss the consequence of this conserved domain for the recruitment of novel genes into the Notch signaling cascade.

Groucho-mediated repression may result from a histone deacetylase-dependent increase in nucleosome density

Groucho (Gro) is a Drosophila melanogaster transcriptional corepressor that directly interacts with the histone deacetylase Rpd3. Although previous studies suggest that this interaction is required for repression of Gro-responsive reporters in cultured cells, the in vivo significance of this interaction and the mechanism by which it leads to repression remain largely unexplored. In this study, we show that Gro is partially dependent on Rpd3 for repression, supporting the idea that Rpd3-mediated repression is one mode of Gro-mediated repression. We demonstrate that Gro colocalizes with Rpd3 to the chromatin of a target gene and that this is accompanied by the deacetylation of specific lysines within the N-terminal tails of histones H3 and H4. Gro overexpression leads to wing patterning defects and ectopic repression in the wing disc of transcription directed by the vestigial quadrant enhancer. These effects are reversed by the histone deacetylase inhibitors TSA and HC-Toxin and by the reduction of Rpd3 gene dosage. Furthermore, repression of the vestigial quadrant enhancer is accompanied by a Gro-mediated increase in nucleosome density, an effect that is reversed by histone deacetylase inhibitors. We propose a model in which Gro-mediated histone deacetylation results in increased nucleosome density leading to transcriptional repression.

Notch targets and their regulation

The proteolytic cleavages elicited by activation of the Notch receptor release an intracellular fragment, Notch intracellular domain, which enters the nucleus to activate the transcription of targets. Changes in transcription are therefore a major output of this pathway. However, the Notch outputs clearly differ from cell type to cell type. In this review we discuss current understanding of Notch targets, the mechanisms involved in their transcriptional regulation, and what might underlie the activation of different sets of targets in different cell types.

Evidence that the C-terminal domain (CtD) autoinhibits neural repression by Drosophila E(spl)M8

Analysis of the retinal defects of a CK2 phosphomimetic variant of E(spl)M8 (M8S(159)D) and the truncated protein M8* encoded by the E(spl)D allele, suggest that the nonphosphorylated CtD "autoinhibits" repression. We have investigated this model by testing for inhibition (in "trans") by the CtD fragment in its nonphosphorylated (M8-CtD) and phosphomimetic (M8SD-CtD) states. In N(+) flies, ectopic M8-CtD compromises lateral inhibition, i.e., elicits supernumerary bristles as with loss of N signaling. This antimorphic activity of M8-CtD strongly rescues the reduced eye and/or bristle loss phenotypes that are elicited by ectopic M8SD or wild type M8. Additionally, the severely reduced eye of N(spl)/Y; E(spl)D/+ flies is also rescued by M8-CtD. Rescue is specific to the time and place, the morphogenetic furrow, where "founding" R8 photoreceptors are specified. In contrast, the phosphomimetic M8SD-CtD that is predicted to be deficient for autoinhibition, exhibits significantly attenuated or negligible activity. These studies provide evidence that autoinhibition by the CtD regulates M8 activity in a phosphorylation-dependent manner.

Hijacking HES1: how tumors co-opt the anti-differentiation strategies of quiescent cells

Quiescent and tumor cells share the ability to evade irreversible cell fates. Recent studies have shown that the transcriptional regulator Hairy and Enhancer of Split 1 (HES1) protects quiescent fibroblasts from differentiation or senescence. HES1 is highly expressed in rhabdomyosarcomas, and the inhibition of HES1 restores differentiation in these cells. Pathways that lead to elevated HES1 levels, such as the Notch and Hedgehog pathways, are frequently upregulated in tumors. Compounds that inhibit these pathways induce differentiation and apoptosis in cancer cells and several are in clinical trials. HES1 might repress gene expression in part by recruiting histone deacetylases (HDACs). HDACs inhibit differentiation, whereas histone deacetylase inhibitors (HDACis) induce differentiation or apoptosis in tumors and are also showing promise as therapeutics. Small molecules that directly target HES1 itself were recently identified. Here, we discuss the importance of HES1 function in quiescent and tumor cells. Elucidating the pathways that control quiescence could provide valuable information not only for treating cancer but also other diseases.

Bayesian network analysis of targeting interactions in chromatin

In eukaryotes, many chromatin proteins together regulate gene expression. Chromatin proteins often direct the genomic binding pattern of other chromatin proteins, for example, by recruitment or competition mechanisms. The network of such targeting interactions in chromatin is complex and still poorly understood. Based on genome-wide binding maps, we constructed a Bayesian network model of the targeting interactions among a broad set of 43 chromatin components in Drosophila cells. This model predicts many novel functional relationships. For example, we found that the homologous proteins HP1 and HP1C each target the heterochromatin protein HP3 to distinct sets of genes in a competitive manner. We also discovered a central role for the remodeling factor Brahma in the targeting of several DNA-binding factors, including GAGA factor, JRA, and SU(VAR)3-7. Our network model provides a global view of the targeting interplay among dozens of chromatin components.

Cofactor-activated phosphorylation is required for inhibition of cortical neuron differentiation by Groucho/TLE1

Background

Transcriptional co-repressors of the Groucho/transducin-like Enhancer of split (Gro/TLE) family regulate the expression of a variety of genes and are involved in numerous developmental processes in both invertebrate and vertebrate species. More specifically, Gro/TLE1 participates in mechanisms that inhibit/delay the differentiation of cerebral cortex neural progenitor cells into neurons during mammalian forebrain development. The anti-neurogenic function of Gro/TLE1 depends on the formation of protein complexes with specific DNA-binding transcription factors that engage Gro/TLE1 through WRP(W/Y) sequences. Interaction with those transcription partners results in Gro/TLE1 recruitment to selected DNA sites and causes increased Gro/TLE1 phosphorylation. The physiological significance of the latter event, termed “cofactor-activated phosphorylation,” had not been determined. Therefore, this study aimed at clarifying the role of cofactor-activated phosphorylation in the anti-neurogenic function of Gro/TLE1.

Methods and Principal Findings

A combination of site-directed mutagenesis, mass spectrometry, biochemistry, primary cell culture, and immunocytochemical assays was utilized to characterize point mutations of Ser-286, a residue that is phosphorylated in vivo and is located within the serine/proline-rich (SP) domain of Gro/TLE1. Mutation of Ser-286 to alanine or glutamic acid does not perturb the interaction of Gro/TLE1 with DNA-binding partners, including the basic helix-loop-helix transcription factor Hes1, a prototypical anti-neurogenic WRP(W/Y) motif protein. Ser-286 mutations do not prevent the recruitment of Gro/TLE1 to DNA, but they impair cofactor-activated phosphorylation and weaken the interaction of Gro/TLE1 with chromatin. These effects are correlated with an impairment of the anti-neurogenic activity of Gro/TLE1. Similar results were obtained when mutations of Ser-289 and Ser-298, which are also located within the SP domain of Gro/TLE1, were analyzed.

Conclusion

Based on the positive correlation between Gro/TLE1 cofactor-activated phosphorylation and ability to inhibit cortical neuron differentiation, we propose that hyperphosphorylation induced by cofactor binding plays a positive role in the regulation of Gro/TLE1 anti-neurogenic activity.

Changes in global gene expression during in vitro decidualization of rat endometrial stromal cells

During the preimplantation phase of pregnancy the endometrial stroma differentiates into decidua, a process that implies numerous morphological changes and is an example of physiological transdifferentiation. Here we show that UIII rat endometrial stromal cells cultured in the presence of calf serum acquired morphological features of decidual cells and expressed decidual markers. To identify genes involved in decidualization we compared gene expression patterns of control and decidualized UIII cells using cDNA microarray. We found 322 annotated genes exhibiting significant differences in expression (>3-fold, fold discovery rate (FDR) >0.005), of which 312 have not been previously related to decidualization. Analysis of overrepresented functions revealed that protein synthesis, gene expression, and chromatin architecture and remodeling are the most relevant modified functions during decidualization. Relevant genes are also found in the functional terms differentiation, cell proliferation, signal transduction, and matrix/structural proteins. Several of these new genes involved in decidualization (Csdc2, Trim27, Eef1a1, Bmp1, Wt1, Aes, Gna12, and Men1) are shown to be also regulated in uterine decidua during normal pregnancy. Thus, the UIII cell culture model will allow future mechanistic studies to define the transcriptional network regulating reprogramming of stromal cells into decidual cells.

Her6 regulates the neurogenetic gradient and neuronal identity in the thalamus

During vertebrate brain development, the onset of neuronal differentiation is under strict temporal control. In the mammalian thalamus and other brain regions, neurogenesis is regulated also in a spatially progressive manner referred to as a neurogenetic gradient, the underlying mechanism of which is unknown. Here we describe the existence of a neurogenetic gradient in the zebrafish thalamus and show that the progression of neurogenesis is controlled by dynamic expression of the bHLH repressor her6. Members of the Hes/Her family are known to regulate proneural genes, such as Neurogenin and Ascl. Here we find that Her6 determines not only the onset of neurogenesis but also the identity of thalamic neurons, marked by proneural and neurotransmitter gene expression: loss of Her6 leads to premature Neurogenin1-mediated genesis of glutamatergic (excitatory) neurons, whereas maintenance of Her6 leads to Ascl1-mediated production of GABAergic (inhibitory) neurons. Thus, the presence or absence of a single upstream regulator of proneural gene expression, Her6, leads to the establishment of discrete neuronal domains in the thalamus.

The Notch-responsive transcription factor Hes-1 attenuates osteocalcin promoter activity in osteoblastic cells

Notch signaling plays a key role in osteoblast differentiation. A major transcriptional downstream regulator of this pathway is the helix-loop-helix (HLH) transcription factor Hairy/Enhancer of Split 1 (Hes-1). Here we investigated the function of Hes-1 in osteoblastic cells. Endogenous Hes-1 gene expression decreases during progression of bone cell phenotype development in MC3T3-E1 osteoblasts suggesting that it is a negative regulator of osteoblast differentiation. Forced expression of Hes-1 inhibits osteocalcin (OC) mRNA levels, and luciferase assays indicate that Hes-1 directly represses OC promoter activity. In vitro and in vivo protein/DNA interaction assays reveal that recombinant Hes-1 binds specifically to an E-box in the proximal promoter of the OC gene. Deletion of the Hes-1 WRPW domain (MHes-1) that recruits the co-repressor Groucho abrogates repression of OC promoter activity by Hes-1, but also blocks Hes-1 binding to the promoter. The latter result suggests that exogenous Hes-1 may be recruited to the OC promoter by both protein/DNA and protein/protein interactions. We conclude that the Notch-responsive Hes-1 protein is capable of repressing OC gene transcription in osteoblastic cells through an E-box in the proximal promoter. Hes-1 may contribute to osteoblast growth and differentiation by controlling basal bone-specific transcription directly through interactions with transcriptional regulators that are known to bind to the OC gene promoter.

Groucho corepressor functions as a cofactor for the Knirps short-range transcriptional repressor

Despite the pervasive roles for repressors in transcriptional control, the range of action of these proteins on cis regulatory elements remains poorly understood. Knirps has essential roles in patterning the Drosophila embryo by means of short-range repression, an activity that is essential for proper regulation of complex transcriptional control elements. Short-range repressors function in a local fashion to interfere with the activity of activators or basal promoters within approximately 100 bp. In contrast, long-range repressors such as Hairy act over distances >1 kb. The functional distinction between these two classes of repressors has been suggested to stem from the differential recruitment of the CtBP corepressor to short-range repressors and Groucho to long-range repressors. Contrary to this differential recruitment model, we report that Groucho is a functional part of the Knirps short-range repression complex. The corepressor interaction is mediated via an eh-1 like motif present in the N terminus and a conserved region present in the central portion of Knirps. We also show that this interaction is important for the CtBP-independent repression activity of Knirps and is required for regulation of even-skipped. Our study uncovers a previously uncharacterized interaction between proteins previously thought to function in distinct repression pathways, and indicates that the Groucho corepressor can be differentially harnessed to execute short- and long-range repression.

Hes1 potentiates T cell lymphomagenesis by up-regulating a subset of notch target genes

BACKGROUND

Hairy/Enhancer of Split (Hes) proteins are targets of the Notch signaling pathway and make up a class of basic helix-loop-helix (bHLH) proteins that function to repress transcription. Data from Hes1 deficient mice suggested that Hes1, like Notch1, is necessary for the progression of early T cell progenitors. Constitutive activation of Notch is known to cause T cell leukemia or lymphoma but whether Hes1 has any oncogenic activity is not known.

METHODOLOGY/PRINCIPAL FINDINGS

We generated mice carrying a Hes1 transgene under control of the proximal promote of the lck gene. Hes1 expression led to a reduction in numbers of total thymocytes, concomitant with the increased percentage and number of immature CD8+ (ISP) T cells and sustained CD25 expression in CD4+CD8+ double positive (DP) thymocytes. Hes1 transgenic mice develop thymic lymphomas at about 20 weeks of age with a low penetrance. However, expression of Hes1 significantly shortens the latency of T cell lymphoma developed in Id1 transgenic mice, where the function of bHLH E proteins is inhibited. Interestingly, Hes1 increased expression of a subset of Notch target genes in pre-malignant ISP and DP thymocytes, which include Notch1, Notch3 and c-myc, thus suggesting a possible mechanism for lymphomagenesis.

CONCLUSIONS/SIGNIFICANCE

We have demonstrated for the first time that Hes1 potentiates T cell lymphomagenesis, by up-regulating a subset of Notch target genes and by causing an accumulation of ISP thymocytes particularly vulnerable to oncogenic transformation.

On the mechanism underlying the divergent retinal and bristle defects of M8* (E(spl)D) in Drosophila

Our results, using endogenous mutants and Gal4-UAS driven transgenes, implicate multisite phosphorylation in repression by E(spl)M8. We propose that these phosphorylations occur in the morphogenetic furrow (MF) to reverse an auto-inhibited state of M8, enabling repression of Atonal during R8 specification. Our studies address the paradoxical behavior of M8*, the truncated protein encoded by E(spl)D. We suggest that differences in N signaling in the bristle versus the eye underlie the antimorphic activity of M8* in N(+) (ectopic bristles) and hypermorphic activity in N(spl) (reduced eye). Ectopic M8* impairs eye development (in N(spl)) only during establishment of the atonal feedback loop (anterior to the MF), but is ineffective after this time point. In contrast, a CK2 phosphomimetic M8 lacking Groucho (Gro) binding, M8SDDeltaGro, acts antimorphic in N(+) and suppresses the eye/R8 and bristle defects of N(spl), as does reduced dosage of E(spl) or CK2. Multisite phosphorylation could serve as a checkpoint to enable a precise onset of repression, and this is bypassed in M8*. Additional implications are discussed.

Differential expression of the Enhancer of split genes in the developing Drosophila midgut

The Notch signaling pathway plays an important role during development in animals from worms to humans and pathway components are required for the differentiation of many different cell types. In Drosophila, Su(H) dependent Notch activation up-regulates transcription of the Enhancer of split-Complex (E(spl)-C). The E(spl) genes are known to function during neurogenesis, although expression and genetics studies suggest that they also play roles in the development of other tissues. The majority of the E(spl) genes contain upstream binding sites for Su(H), proneural proteins, and E(spl) bHLH proteins resulting in overlapping expression patterns during embryonic development. However, their expression patterns are quite distinct during later embryonic stages and in larval imaginal discs. In order to characterize expression patterns of the E(spl) genes during development and determine potential mechanisms through which expression is controlled, we examined the expression levels and patterns of the E(spl) genes in the midgut during metamorphosis. Quantitative Reverse Transcriptase-PCR and X-Gal staining results show that the genes have different levels and patterns of expression in the developing midgut. Two ancestral E(spl) genes, malpha and mbeta, are highly expressed and increase significantly at puparium formation, whereas another gene, mgamma, is expressed at low levels and decreases in expression at puparium formation. We also show that mbeta is expressed in cells throughout the midgut, while mgamma is expressed in two small regions. These results provide further evidence that the E(spl) genes function during midgut development and that they are regulated by different factors.

Otx2 induction of the gonadotropin-releasing hormone promoter is modulated by direct interactions with Grg co-repressors

Hormonal communication between the hypothalamus, pituitary, and gonads orchestrates the development and regulation of mammalian reproductive function. In mice, gonadotropin-releasing hormone (GnRH) expression is limited to approximately 1000 neurons that originate in the olfactory placode then migrate to specific positions scattered throughout the hypothalamus. Coordination of the hypothalamic-pituitary-gonadal axis is dependent upon correct migration of GnRH neurons into the hypothalamus followed by the appropriate synthesis and pulsatile secretion of GnRH. Defects in any one of these processes can cause infertility. Recently, substantial progress has been made in identifying transcription factors, and their cofactors, that regulate not only adult expression of GnRH, but also the maturation of GnRH neurons. Here, we show that expression of Otx2, a homeodomain protein required for the formation of the forebrain, is dramatically up-regulated during GnRH neuronal maturation and that overexpression of Otx2 increases GnRH promoter activity in GnRH neuronal cell lines. Furthermore, Otx2 transcriptional activity is modulated by Grg4, a member of the Groucho-related-gene (Grg) family. Using mutational analysis, we show that a WRPW peptide motif within the Otx2 protein is required for physical interaction between Otx2 and Grg4. Without this physical interaction, Grg4 cannot repress Otx2-dependent activation of GnRH gene transcription. Taken together, these data show that Otx2 is important for GnRH expression and that direct interaction between Otx2 and Grg co-repressors regulates GnRH gene expression in hypothalamic neurons.

The homeodomain protein Cux1 interacts with Grg4 to repress p27 kip1 expression during kidney development

The homeodomain protein Cux1 is highly expressed in the nephrogenic zone of the developing kidney where it functions to regulate cell proliferation. Here we show that Cux1 directly interacts with the co-repressor Grg4 (Groucho 4), a known effector of Notch signaling. Promoter reporter based luciferase assays revealed enhanced repression of p27(kip1) promoter activity by Cux1 in the presence of Grg4. Chromatin immunoprecipitation (ChIP) assays demonstrated the direct interaction of Cux1 with p27(kip1) in newborn kidney tissue in vivo. ChIP assays also identified interactions of Cux1, Grg4, HDAC1, and HDAC3 with p27(kip1) at two separate sites in the p27(kip1) promoter. DNAse1 footprinting experiments revealed that Cux1 binds to the p27(kip1) promoter on the sequence containing two Sp1 sites and a CCAAT box approximately 500 bp from the transcriptional start site, and to an AT rich sequence approximately 1.5 kb from the transcriptional start site. Taken together, these results identify Grg4 as an interacting partner for Cux1 and suggest a mechanism of p27(kip1) repression by Cux1 during kidney development.

Groucho binds two conserved regions of LEF-1 for HDAC-dependent repression

Background

Drosophila Groucho and its human Transducin-like-Enhancer of Split orthologs (TLEs) function as transcription co-repressors within the context of Wnt signaling, a pathway with strong links to cancer. The current model for how Groucho/TLE's modify Wnt signaling is by direct competition with β-catenin for LEF/TCF binding. The molecular events involved in this competitive interaction are not defined and the actions of Groucho/TLEs within the context of Wnt-linked cancer are unknown.

Methods

We used in vitro protein interaction assays with the LEF/TCF family member LEF-1, and in vivo assays with Wnt reporter plasmids to define Groucho/TLE interaction and repressor function.

Results

Mapping studies reveal that Groucho/TLE binds two regions in LEF-1. The primary site of recognition is a 20 amino acid region in the Context Dependent Regulatory domain. An auxiliary site is in the High Mobility Group DNA binding domain. Mutation of an eight amino acid sequence within the primary region (RFSHHMIP) results in a loss of Groucho action in a transient reporter assay. Drosophila Groucho, human TLE-1, and a truncated human TLE isoform Amino-enhancer-of-split (AES), work equivalently to repress LEF-1•β-catenin transcription in transient reporter assays, and these actions are sensitive to the HDAC inhibitor Trichostatin A. A survey of Groucho/TLE action in a panel of six colon cancer cell lines with elevated β-catenin shows that Groucho is not able to repress transcription in a subset of these cell lines.

Conclusion

Our data shows that Groucho/TLE repression requires two sites of interaction in LEF-1 and that a central, conserved amino acid sequence within the primary region (F S/T/P/xx y I/L/V) is critical. Our data also reveals that AES opposes LEF-1 transcription activation and that both Groucho and AES repression require histone deacetylase activity suggesting multiple steps in Groucho competition with β-catenin. The variable ability of Groucho/TLE to oppose Wnt signaling in colon cancer cells suggests there may be defects in one or more of these steps.

The fanconi anemia core complex acts as a transcriptional co-regulator in hairy enhancer of split 1 signaling

Mutations in one of the 13 Fanconi anemia (FA) genes cause a progressive bone marrow failure disorder associated with developmental abnormalities and a predisposition to cancer. Although FA has been defined as a DNA repair disease based on the hypersensitivity of patient cells to DNA cross-linking agents, FA patients develop various developmental defects such as skeletal abnormalities, microphthalmia, and endocrine abnormalities that may be linked to transcriptional defects. Recently, we reported that the FA core complex interacts with the transcriptional repressor Hairy Enhancer of Split 1 (HES1) suggesting that the core complex plays a role in transcription. Here we show that the FA core complex contributes to transcriptional regulation of HES1-responsive genes, including HES1 and the cyclin-dependent kinase inhibitor p21(cip1/waf1). Chromatin immunoprecipitation studies show that the FA core complex interacts with the HES1 promoter but not the p21(cip1/waf1) promoter. Furthermore, we show that the FA core complex interferes with HES1 binding to the co-repressor transducin-like-Enhancer of Split, suggesting that the core complex affects transcription both directly and indirectly. Taken together these data suggest a novel function of the FA core complex in transcriptional regulation.

[Progress of studies on bHLH transcription factor families]

bHLH transcription factors are important players in various developmental processes of eukaryotes. They constitute a large family of transcription factors. bHLH family members have been identified in genomes of 20 organisms including 17 animals, two plants, and one yeast. Animal bHLHs are classified into 45 families based on their different functions in the regulation of gene expression. In addition, they are divided into 6 groups according to target DNA elements they bind and their own structural characteristics. Group A consists of 22 families. They mainly regulate neurogenesis, myogenesis and mesoderm formation. Group B consists of 12 families. They mainly regulate cell proliferation and differentiation, sterol metabolism and adipocyte formation, and expression of glucose-responsive genes. Group C has seven families. They are responsible for the regulation of midline and tracheal development, circadian rhythms, and for the activation of gene transcription in response to environmental toxins. Group D has only one family. It forms inactive heterodimers with group A bHLH proteins. Group E has two families, which regulate embryonic segmentation, somitogenesis and organogenesis etc. Group F also has one family. It regulates head development and formation of olfactory sensory neurons etc. This article presents a brief review on progress achieved in studies related to the classification, origination and functions of bHLH transcription factor families.

Position dependent responses to discontinuities in the retinal determination network

The development of any cell and/or tissue is dependent upon interconnections between several signaling pathways and myriad transcription factors. It is becoming more apparent that these inputs are best studied, not as individual components, but rather as elements of a gene regulatory network. Over the last decade several networks governing the specification of single cells, individual organs and entire stages of development have been described. The current incarnations of these networks are the products of the continual addition of newly discovered genetic, molecular and biochemical interactions. However, as currently envisaged, network diagrams may not sufficiently describe the spatial and temporal dynamics that underlie developmental processes. We have conducted a developmental analysis of a sub circuit of the Drosophila retinal determination network. This sub circuit is comprised of three genes, two (sine oculis and dachshund) of which code for DNA binding proteins and one (eyes absent) that encodes a transcriptional co-activator. We demonstrate here that the nature of the regulatory relationships that exist between these three genes changes as retinal development progresses. We also demonstrate that the response of the tissue to the loss of any of these three RD genes is dependent upon the position of the mutant cells within the eye field. Depending upon its location, mutant tissue will either overproliferate itself or will signal to surrounding cells instructing them to propagate and compensate for the eventual loss through apoptosis of the mutant clone. Taken together these results suggest that the complexities of development are best appreciated when spatial and temporal information is incorporated when describing gene regulatory networks.

A role of Pygopus as an anti-repressor in facilitating Wnt-dependent transcription

Wnt/beta-catenin signaling controls animal development and tissue homeostasis, and is also an important cancer pathway. Pygopus (Pygo) is a conserved nuclear Wnt signaling component that is essential for Wingless-induced transcription throughout Drosophila development. It associates with Armadillo/beta-catenin and T cell factor (TCF) through the Legless/BCL9 adaptor, but its molecular function in TCF-mediated transcription is unknown. Here, we use a groucho-null allele to show that Groucho represses Wingless target genes during Drosophila development. Interestingly, groucho pygo double-mutants revealed that Pygo is not obligatory for transcriptional and phenotypic Wingless signaling outputs if the interaction between Groucho and Drosophila TCF is compromised genetically. Pygo function is also non-essential for Wingless outputs in the absence of other transcriptional antagonists of Wingless signaling. This indicates an anti-repressor function of Pygo: we propose that Pygo predisposes Drosophila TCF target genes for rapid Wingless-induced transcription, or that it protects them against premature shut-down.

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.

Context-dependent regulation of Groucho/TLE-mediated repression

Groucho/TLE proteins are global corepressors that are recruited to target promoters by different families of DNA-binding repressors. As these corepressors are widely expressed, the long-standing view had been that Groucho/TLE-mediated repression is regulated solely by the spatial and temporal distribution of partner repressors. It has recently emerged, however, that Groucho/TLE repressor activity is itself regulated, in a signal induced, context-dependent manner. Here we review the essential roles played by Groucho/TLE factors in different cell-signalling processes that illustrate different modes for regulating Groucho/TLE-mediated repression: (i) via the expression of partner repressors; (ii) by competition with coactivators and (iii) through post-translational modifications of Groucho/TLE. We also discuss how the intrinsic properties of repressors can result in differential responses to Groucho/TLE regulation.

Evolution of an insect-specific GROUCHO-interaction motif in the ENGRAILED selector protein

Animal morphology evolves through alterations in the genetic regulatory networks that control development. Regulatory connections are commonly added, subtracted, or modified via mutations in cis-regulatory elements, but several cases are also known where transcription factors have gained or lost activity-modulating peptide motifs. In order to better assess the role of novel transcription factor peptide motifs in evolution, we searched for synapomorphic motifs in the homeotic selectors of Drosophila melanogaster and related insects. Here, we describe an evolutionarily novel GROUCHO (GRO)-interaction motif in the ENGRAILED (EN) selector protein. This "ehIFRPF" motif is not homologous to the previously characterized "engrailed homology 1" (eh1) GRO-interaction motif of EN. This second motif is an insect-specific "WRPW"-type motif that has been maintained by purifying selection in at least the dipteran/lepidopteran lineage. We demonstrate that this motif contributes to in vivo repression of the wingless (wg) target gene and to interaction with GRO in vitro. The acquisition and conservation of this auxiliary peptide motif shows how the number and activity of short peptide motifs can evolve in transcription factors while existing regulatory functions are maintained.

Inhibition of cortical neuron differentiation by Groucho/TLE1 requires interaction with WRPW, but not Eh1, repressor peptides

In both invertebrates and vertebrates, transcriptional co-repressors of the Groucho/transducin-like Enhancer of split (Gro/TLE) family regulate a number of developmental mechanisms, including neuronal differentiation. The pleiotropic activity of Gro/TLE depends on context-specific interactions with a variety of DNA-binding proteins. Most of those factors engage Gro/TLE through two different types of short peptide motifs, the WRP(W/Y) tetrapeptide and the Engrailed homology 1 (Eh1) sequence (FXIXXIL). The aim of this study was to elucidate the contribution of WRP(W/Y) and Eh1 motifs to mammalian Gro/TLE anti-neurogenic activity. Here we describe point mutations within the C-terminal WD40 repeat domain of Gro/TLE1 that do not perturb protein folding but disrupt the ability of Gro/TLE1 to inhibit the differentiation of cerebral cortex neural progenitor cells into neurons. One of those mutations, L743F, selectively blocks binding to Hes1, an anti-neurogenic basic helix-loop-helix protein that harbors a WRPW motif. In contrast, the L743F mutation does not disrupt binding to Engrailed1 and FoxG1, which both contain Eh1 motifs, nor to Tcf3, which binds to the Gro/TLE N terminus. These results demonstrate that the recruitment of transcription factors harboring WRP(W/Y) tetrapeptides is essential to the anti-neurogenic function of Gro/TLE1.

Differential gene expression of the honey bee Apis mellifera associated with Varroa destructor infection

Background

The parasitic mite, Varroa destructor, is the most serious pest of the western honey bee, Apis mellifera, and has caused the death of millions of colonies worldwide. This mite reproduces in brood cells and parasitizes immature and adult bees. We investigated whether Varroa infestation induces changes in Apis mellifera gene expression, and whether there are genotypic differences that affect gene expression relevant to the bee's tolerance, as first steps toward unravelling mechanisms of host response and differences in susceptibility to Varroa parasitism.

Results

We explored the transcriptional response to mite parasitism in two genetic stocks of A. mellifera which differ in susceptibility to Varroa, comparing parasitized and non-parasitized full-sister pupae from both stocks. Bee expression profiles were analyzed using microarrays derived from honey bee ESTs whose annotation has recently been enhanced by results from the honey bee genome sequence. We measured differences in gene expression in two colonies of Varroa-susceptible and two colonies of Varroa-tolerant bees. We identified a set of 148 genes with significantly different patterns of expression: 32 varied with the presence of Varroa, 116 varied with bee genotype, and 2 with both. Varroa parasitism caused changes in the expression of genes related to embryonic development, cell metabolism and immunity. Bees tolerant to Varroa were mainly characterized by differences in the expression of genes regulating neuronal development, neuronal sensitivity and olfaction. Differences in olfaction and sensitivity to stimuli are two parameters that could, at least in part, account for bee tolerance to Varroa; differences in olfaction may be related to increased grooming and hygienic behavior, important behaviors known to be involved in Varroa tolerance.

Conclusion

These results suggest that differences in behavior, rather than in the immune system, underlie Varroa tolerance in honey bees, and give an indication of the specific physiological changes found in parasitized bees. They provide a first step toward better understanding molecular pathways involved in this important host-parasite relationship.

Notch and suppressor of Hairless regulate levels but not patterns of Delta expression in Drosophila

The Notch signal transduction pathway is highly conserved and governs many developmental decisions in metazoans. The ligand Delta, and its receptor Notch, are often expressed in complementary patterns during Drosophila postembryonic development. Notch signaling is thought to play a role in generation of these complementary patterns through feedback mechanisms that regulate Delta and Notch expression. We have examined Delta expression during postembryonic development, following global alteration of Notch-dependent or Su(H)-dependent transcriptional regulation. We find that Notch and Su(H) regulate Delta expression in a manner that varies by context. Surprisingly, we find that wild type Delta expression levels are influenced by Su(H)-dependent mechanisms only in regions of high Delta/low Notch expression. In contrast, Delta expression levels in regions of low Delta/high Notch expression appear to be unaffected by Su(H)-mediated regulation. We conclude that Notch pathway feedback regulation is unlikely to contribute to the generation of complementary patterns in the contexts examined.

Molecular cloning of novel Monad binding protein containing tetratricopeptide repeat domains

We have previously reported that Monad, a novel WD40 repeat protein, potentiates apoptosis induced by tumor necrosis factor-alpha(TNF-alpha) and cycloheximide (CHX). By affinity purification and mass spectrometry, we identified RNA polymerase II-associated protein 3 (RPAP3) as a binding protein of Monad. Overexpression of RPAP3 in HEK 293 potentiated caspase-3 activation and apoptosis induced by TNF-alpha and CHX. In addition, knockdown of RPAP3 by RNA interference resulted in a significant reduction of apoptosis induced by TNF-alpha and CHX in HEK293 and HeLa cells. These results raise the possibility that RPAP3, together with Monad, may function as a novel modulator of apoptosis pathway.

Phylogenetic and expression analysis of the basic helix-loop-helix transcription factor gene family: genomic approach to cellular differentiation

A phylogenetic analysis of seven different species (human, mouse, rat, worm, fly, yeast, and plant) utilizing all (541) basic helix-loop-helix (bHLH) genes identified, including expressed sequence tags (EST), was performed. A super-tree involving six clades and a structural categorization involving the entire coding sequence was established. A nomenclature was developed based on clade distribution to discuss the functional and ancestral relationships of all the genes. The position/location of specific genes on the phylogenetic tree in relation to known bHLH factors allows for predictions of the potential functions of uncharacterized bHLH factors, including EST's. A genomic analysis using microarrays for four different mouse cell types (i.e. Sertoli, Schwann, thymic, and muscle) was performed and considered all known bHLH family members on the microarray for comparison. Cell-specific groups of bHLH genes helped clarify those bHLH genes potentially involved in cell specific differentiation. This phylogenetic and genomic analysis of the bHLH gene family has revealed unique aspects of the evolution and functional relationships of the different genes in the bHLH gene family.

Notch signaling during larval and juvenile development in the polychaete annelid Capitella sp. I

Notch signaling is involved in a large range of developmental processes, and has been functionally implicated in body plan segmentation in two of the three diverse segmented taxa, the vertebrates and arthropods. Here we investigate expression of Notch, Delta, and hes gene homologues during larval and juvenile development in the polychaete annelid Capitella sp. I., a member of the third group of segmented animals. During larval stages, CapI-Notch, CapI-Delta, CapI-hes2, and CapI-hes3 transcripts are initially detected in broad ectodermal domains in future segments as well as in the brain and foregut; later, CapI-Notch, CapI-Delta, and CapI-hes2 transcripts are detected in the presumptive chaetal sacs. In contrast, CapI-hes1 has a segmentally reiterated pattern in a restricted region of the mesoderm in each presumptive segment. CapI-Notch, CapI-Delta, CapI-hes2, and CapI-hes3 and CapI-hes1 are all expressed in the terminal growth zone that generates post-metamorphic segments, however, CapI-hes1 has a non-overlapping complementary expression pattern to that of CapI-Notch and CapI-Delta. CapI-Delta and CapI-Notch transcripts are localized to already formed segments, with posterior boundaries that correlate with the posterior boundary of the nascent segment, while CapI-hes1 lies posterior to CapI-Notch and CapI-Delta. The localization of CapI-Notch, CapI-Delta, and CapI-hes transcripts correlate with areas of rapid cell proliferation in Capitella, which include the brain, foregut, and terminal growth zone.

Analysis of the Tribolium homeotic complex: insights into mechanisms constraining insect Hox clusters

The remarkable conservation of Hox clusters is an accepted but little understood principle of biology. Some organizational constraints have been identified for vertebrate Hox clusters, but most of these are thought to be recent innovations that may not apply to other organisms. Ironically, many model organisms have disrupted Hox clusters and may not be well-suited for studies of structural constraints. In contrast, the red flour beetle, Tribolium castaneum, which has a long history in Hox gene research, is thought to have a more ancestral-type Hox cluster organization. Here, we demonstrate that the Tribolium homeotic complex (HOMC) is indeed intact, with the individual Hox genes in the expected colinear arrangement and transcribed from the same strand. There is no evidence that the cluster has been invaded by non-Hox protein-coding genes, although expressed sequence tag and genome tiling data suggest that noncoding transcripts are prevalent. Finally, our analysis of several mutations affecting the Tribolium HOMC suggests that intermingling of enhancer elements with neighboring transcription units may constrain the structure of at least one region of the Tribolium cluster. This work lays a foundation for future studies of the Tribolium HOMC that may provide insights into the reasons for Hox cluster conservation.

RUNX genes in development and cancer: regulation of viral gene expression and the discovery of RUNX family genes

Mouse embryonal carcinoma (EC) cells, also called teratocarcinoma stem cells, are nonpermissive for polyomavirus growth, whereas differentiated derivatives of the cells are permissive. Mutant viruses capable of growing in EC cells can be isolated. They have genomic alterations within the viral enhancer, which is required for viral gene expression and DNA replication. This viral regulatory region was considered as a potential probe for mouse cell differentiation. The 24-bp-long A element within the enhancer was identified as a minimum element, which also shows a lower activity in EC cells compared with the differentiated cells. Transcription factors PEA1/AP1, PEA2/PEBP2, and PEA3/ETS were identified as A element-binding proteins. All of them are absent in EC cells and induced to be expressed when the cells are differentiated. Although PEBP2 has a weaker transactivation activity compared with other two, it is essential for the enhancer function of the A element. Purification and cDNA cloning revealed that PEBP2 has two subunits, DNA-binding alpha (PEBP2alpha) and non-DNA-binding beta (PEBP2beta). PEBP2alpha was found to be highly homologous to a Drosophila segmentation gene, runt, and a human gene AML1 that was identified as a part of the fusion gene, AML1/ETO (MTG8) generated by t(8;21) chromosome translocation associated with acute myelogenous leukemia (AML). Core-binding factor (CBF), which interacts with a murine retrovirus enhancer, was found to be identical to PEBP2. runt, PEBP2alpha and AML1 are now termed RUNX family, which are involved in cell specification during development. There are three mammalian RUNX genes, RUNX1, RUNX2, and RUNX3. RUNX1 is essential for generation of hematopoietic stem cells and is involved in human leukemia. RUNX2 is essential for skeletal development and has an oncogenic potential. RUNX3 is expressed in wider ranges of tissues and has multiple roles. Among others, RUNX3 is a major tumor suppressor of gastric and many other solid tumors.

Spreading of a corepressor linked to action of long-range repressor hairy

Transcriptional repressor proteins play key roles in the control of gene expression in development. For the Drosophila embryo, the following two functional classes of repressors have been described: short-range repressors such as Knirps that locally inhibit the activity of enhancers and long-range repressors such as Hairy that can dominantly inhibit distal elements. Several long-range repressors interact with Groucho, a conserved corepressor that is homologous to mammalian TLE proteins. Groucho interacts with histone deacetylases and histone proteins, suggesting that it may effect repression by means of chromatin modification; however, it is not known how long-range effects are mediated. Using embryo chromatin immunoprecipitation, we have analyzed a Hairy-repressible gene in the embryo during activation and repression. When inactivated, repressors, activators, and coactivators cooccupy the promoter, suggesting that repression is not accomplished by the displacement of activators or coactivators. Strikingly, the Groucho corepressor is found to be recruited to the transcribed region of the gene, contacting a region of several kilobases, concomitant with a loss of histone H3 and H4 acetylation. Groucho has been shown to form higher-order complexes in vitro; thus, our observations suggest that long-range effects may be mediated by a "spreading" mechanism, modifying chromatin over extensive regions to inhibit transcription.

Phylogenetic footprinting analysis in the upstream regulatory regions of the Drosophila enhancer of split genes

During Drosophila development Suppressor of Hairless [Su(H)]-dependent Notch activation upregulates transcription of the Enhancer of split-Complex [E(spl)-C] genes. Drosophila melanogaster E(spl) genes share common transcription regulators including binding sites for Su(H), proneural, and E(spl) basic-helix-loop-helix (bHLH) proteins. However, the expression patterns of E(spl) genes during development suggest that additional factors are involved. To better understand regulators responsible for these expression patterns, recently available sequence and annotation data for multiple Drosophila genomes were used to compare the E(spl) upstream regulatory regions from more than nine Drosophila species. The mgamma and mbeta regulatory regions are the most conserved of the bHLH genes. Fine analysis of Su(H) sites showed that high-affinity Su(H) paired sites and the Su(H) paired site plus proneural site (SPS + A) architecture are completely conserved in a subset of Drosophila E(spl) genes. The SPS + A module is also present in the upstream regulatory regions of the more ancient mosquito and honeybee E(spl) bHLH genes. Additional transcription factor binding sites were identified upstream of the E(spl) genes and compared between species of Drosophila. Conserved sites provide new understandings about E(spl) regulation during development. Conserved novel sequences found upstream of multiple E(spl) genes may play a role in the expression of these genes.

TOPLESS mediates auxin-dependent transcriptional repression during Arabidopsis embryogenesis

The transcriptional response to auxin is critical for root and vascular development during Arabidopsis embryogenesis. Auxin induces the degradation of AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) transcriptional repressors, freeing their binding partners, the AUXIN RESPONSE FACTOR (ARF) proteins, which can activate transcription of auxin response genes. We show that TOPLESS (TPL) can physically interact with IAA12/BODENLOS (IAA12/BDL) through an ETHYLENE RESPONSE FACTOR (ERF)-associated amphiphilic repression (EAR) motif. TPL can repress transcription in vivo and is required for IAA12/BDL repressive activity. In addition, tpl-1 can suppress the patterning defects of the bdl-1 mutant. Direct interaction between TPL and ARF5/MONOPTEROS, which is regulated by IAA12/BDL, results in a loss-of-function arf5/mp phenotype. These observations show that TPL is a transcriptional co-repressor and further our understanding of how auxin regulates transcription during plant development.

The Groucho/TLE/Grg family of transcriptional co-repressors

The Groucho family of co-repressor proteins are essential for development and may also have a role in some human cancers.

Multiple RTK pathways downregulate Groucho-mediated repression in Drosophila embryogenesis

RTK pathways establish cell fates in a wide range of developmental processes. However, how the pathway effector MAPK coordinately regulates the expression of multiple target genes is not fully understood. We have previously shown that the EGFR RTK pathway causes phosphorylation and downregulation of Groucho, a global co-repressor that is widely used by many developmentally important repressors for silencing their various targets. Here, we use specific antibodies that reveal the dynamics of Groucho phosphorylation by MAPK, and show that Groucho is phosphorylated in response to several RTK pathways during Drosophila embryogenesis. Focusing on the regulation of terminal patterning by the Torso RTK pathway, we demonstrate that attenuation of Groucho's repressor function via phosphorylation is essential for the transcriptional output of the pathway and for terminal cell specification. Importantly, Groucho is phosphorylated by an efficient mechanism that does not alter its subcellular localisation or decrease its stability; rather, modified Groucho endures long after MAPK activation has terminated. We propose that phosphorylation of Groucho provides a widespread, long-term mechanism by which RTK signals control target gene expression.

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.

BHLH proteins and their role in somitogenesis

The most obvious manifestation of the existence of a segmented, or metameric, body plan in vertebrate embryos is seen during the formation of the somites. Somites are transient embryonic structures formed in a progressive manner from a nonsegmented mesoderm in a highly regulated process called somitogenesis. As development proceeds different compartments are formed within each somite and these progressively follow a variety of differentiation programs to form segmented organs, such as the different bones that make the axial skeleton, body skeletal muscles and part of the dermis. Transcription factors from the basic helix-loop-helix (bHLH) protein family have been described to be implicated in each of the processes involved in somite formation. bHLH proteins are a family of transcription factors characterized by the presence of a DNA binding domain and a dimerization motif that consists of a basic region adjacent to an amphipathic helix, a loop and a second amphipathic helix. In this chapter we will review a number of bHLH proteins known to play a role in somitogenesis.