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

Lateral inhibition in proneural clusters: cis-regulatory logic and default repression by Suppressor of Hairless

Lateral inhibition, wherein a single cell signals to its neighbors to prevent them from adopting its own fate, is the best-known setting for cell-cell communication via the Notch (N) pathway. During peripheral neurogenesis in Drosophila, sensory organ precursor (SOP) cells arise within proneural clusters (PNCs), small groups of cells endowed with SOP fate potential by their expression of proneural transcriptional activators. SOPs use N signaling to activate in neighboring PNC cells the expression of multiple genes that inhibit the SOP fate. These genes respond transcriptionally to direct regulation by both the proneural proteins and the N pathway transcription factor Suppressor of Hairless [Su(H)], and their activation is generally highly asymmetric; i.e. only in the inhibited(non-SOP) cells of the PNC, and not in SOPs. We show that the substantially higher proneural protein levels in the SOP put this cell at risk of inappropriately activating the SOP-inhibitory genes, even without input from N-activated Su(H). We demonstrate that this is prevented by direct `default'repression of these genes by Su(H), acting through the same binding sites it uses for activation in non-SOPs. We show that de-repression of even a single N pathway target gene in the SOP can extinguish the SOP cell fate. Finally, we define crucial roles for the adaptor protein Hairless and the co-repressors Groucho and CtBP in conferring repressive activity on Su(H) in the SOP. Our work elucidates the regulatory logic by which N signaling and the proneural proteins cooperate to create the neural precursor/epidermal cell fate distinction during lateral inhibition.

The Groucho-related gene family regulates the gonadotropin-releasing hormone gene through interaction with the homeodomain proteins MSX1 and OCT1

Gonadotropin-releasing hormone (GnRH) is exclusively expressed in a unique population of hypothalamic neurons that controls reproductive function. GnRH gene expression is highly dynamic. Its transcriptional activity is regulated in a complex spatiotemporal manner during embryonic development and postnatal life. Although a variety of transcription factors have been identified as regulators of GnRH transcription, most are promiscuous in their DNA-binding requirements, and none are solely expressed in GnRH neurons. Their specific activity is probably determined by interactions with distinct cofactors. Here we find that the Groucho-related gene (GRG) family of co-repressors is expressed in a model cell line for the GnRH neuron and co-expresses with GnRH during prenatal development. GRG proteins associate in vivo with the GnRH promoter. Furthermore, GRG proteins interact with two regulators of GnRH transcription, the homeodomain proteins MSX1 and OCT1. Co-transfection experiments indicate that GRG proteins regulate GnRH promoter activity. The long GRG forms enhance MSX1 repression and counteract OCT1 activation of the GnRH gene. In contrast, the short form, GRG5, has a dominant-negative effect on MSX1-dependent repression. Taken together, these data suggest that the dynamic switch between activation and repression of GnRH transcription is mediated by recruitment of the GRG co-regulators.

Fly Six-type homeodomain proteins Sine oculis and Optix partner with different cofactors during eye development

Two members from the Six class of homeobox transcription factors, Sine oculis (SO) and Optix, function during development of the fly visual system. Differences in gain-of-function phenotypes and gene expression suggest that these related factors play distinct roles in the formation of the fly eye. However, the molecular nature of their functional differences remains unclear. In this study, we report the identification of two novel factors that participate in specific partnerships with Sine oculis and Optix during photoreceptor neurons formation and in eye progenitor cells. This work shows that different cofactors likely mediate unique functions of Sine oculis and Optix during the development of the fly eye and that the repeated requirement for SO function at multiple stages of eye development reflects the activity of different SO-cofactor complexes.

The REF-1 Family of bHLH Transcription Factors Pattern C. elegans Embryos through Notch-Dependent and Notch-Independent Pathways

Much of the patterning of early C. elegans embryos involves a series of Notch interactions that occur in rapid succession and have distinct outcomes; however, none of the targets for these interactions have been identified. We show that the REF-1 family of bHLH transcription factors is a major target of Notch signaling in all these interactions and that most examples of Notch-mediated transcriptional repression can be attributed to REF-1 activities. The REF-1 family is expressed and has similar functions in both Notch-dependent and Notch-independent pathways, and this dual mode of deployment is used repeatedly to pattern the embryo. REF-1 proteins are unusual in that they contain two different bHLH domains and lack the distinguishing characteristics of Hairy/Enhancer of Split (HES) bHLH proteins that are Notch targets in other systems. Our results show that the highly divergent REF-1 proteins are nonetheless HES-like bHLH effectors of Notch signaling.

Choice of either beta-catenin or Groucho/TLE as a co-factor for Xtcf-3 determines dorsal-ventral cell fate of diencephalon during Xenopus development

Co-repressor Groucho/Transducin-Like Enhancer of split (TLE) interacts with transcription factors that are expressed in the central nervous system (CNS), and regulates transcriptional activities. In this study, we examined the contribution of Groucho/TLE to CNS development in Xenopus. The functional inhibition of Groucho/TLE using the WRPW motif as a competitor resulted in the conversion of the ventral cell into the dorsal fate in the prospective diencephalon. We also found that the neural plate was expanded laterally without inhibiting neural crest development. In tailbud, the disturbance of trigeminal ganglion development was observed. These observations allow us to conclude that Groucho/TLE plays important roles in the induction and patterning of distinct CNS territories. We found that Xtcf-3 is involved in some of the patterning in these territories. We generated the variant of Xtcf-3, Xtcf-3BDN-, which is suspected to interfere with the interaction between endogenous Groucho/TLE and Xtcf-3. The transcriptional activation of the Xtcf-3-target genes in response to endogenous Wnt/beta-catenin signaling by the overexpression of Xtcf-3BDN- led to a reduction of the ventral diencephalon. This result indicates that transcriptional repression by the Groucho/TLE-Xtcf-3 complex is important for ventral diencephalon patterning. This idea is supported by the finding that the overexpression of the dominant-negative form of Xtcf-3 or axil causes the expansion of the ventral diencephalon. Based on these data, we propose that the localized activation of Wnt/beta-catenin signaling, which converts Tcf from a repressor to an activator, is required for the establishment of dorsal-ventral patterning in the prospective diencephalon.

The DM Domain Protein MAB-3 Promotes Sex-Specific Neurogenesis in C. elegans by Regulating bHLH Proteins

Sexual dimorphism in the nervous system is required for sexual behavior and reproduction in many metazoan species. However, little is known of how sex determination pathways impose sex specificity on nervous system development. In C. elegans, the conserved sexual regulator MAB-3 controls several aspects of male development, including formation of V rays, male-specific sense organs required for mating. Here we show that MAB-3 promotes expression of the proneural protein LIN-32 in V ray precursors by transcriptional repression of ref-1, a member of the Hes family of neurogenic factors. Mutations in ref-1 restore lin-32::gfp expression and normal V ray development to mab-3 mutants, suggesting that ref-1 is the primary target of MAB-3 in the V ray lineage. Proteins related to MAB-3 (DM domain proteins) control sexual differentiation in diverse metazoans. We therefore suggest that regulation of Hes genes by DM domain proteins may be a general mechanism for specifying sex-specific neurons.

Roles of bHLH genes in neural stem cell differentiation

Neural stem cells change their characteristics over time during development: they initially proliferate only and then give rise to neurons first and glial cells later. In the absence of the repressor-type basic helix-loop-helix (bHLH) genes Hes1, Hes3 and Hes5, neural stem cells do not proliferate sufficiently but prematurely differentiate into neurons and become depleted without making the later born cell types such as astrocytes and ependymal cells. Thus, Hes genes are essential for maintenance of neural stem cells to make cells not only in correct numbers but also in full diversity. Hes genes antagonize the activator-type bHLH genes, which include Mash1, Math and Neurogenin. The activator-type bHLH genes promote the neuronal fate determination and induce expression of Notch ligands such as Delta. These ligands activate Notch signaling and upregulate Hes1 and Hes5 expression in neighboring cells, thereby maintaining these cells undifferentiated. Thus, the activator-type and repressor-type bHLH genes regulate each other, allowing only subsets of cells to undergo differentiation while keeping others to stay neural stem cells. This regulation is essential for generation of complex brain structures of appropriate size, shape and cell arrangement.

SB-236057: Critical window of sensitivity study and embryopathy of a potent musculoskeletal teratogen

BACKGROUND

SB-236057 is a potent skeletal teratogen in rodents and rabbits. The study objective was to identify the critical developmental window of compound sensitivity and to characterize the early onset of SB-236057 embryopathy.

METHODS

SB-236057 was orally administered to Sprague Dawley dams at 100 mg/kg/day on days 6-7, 8-11, 12-14, or 15-17 postcoitus (pc). The critical window of sensitivity was identified to occur between days 8-11 pc. Dams were then dosed on days 8-11 pc and embryos were evaluated by histochemical procedures on days 11, 13, or 15 pc.

RESULTS

Axial malformations were evident by day 11 pc. Analysis of the cartilaginous skeleton revealed missing posterior axial skeletal elements. However, only about one-third of the malformed fetuses exhibited obvious rib and vertebrae abnormalities, and none of the affected fetuses exhibited abnormal appendicular skeletal elements. Expression pattern of sonic hedgehog in the notochord and floor plate was not affected, suggesting ventral midline signaling was not disrupted. Histological analysis demonstrated hypoplastic and/or missing musculature in proximity to the ribs and vertebrae. Caspase 3 analysis revealed no increases in apoptotic cells in the musculature. Confocal analysis of the limbs demonstrated truncated peripheral nerve formation and shortening of the appendicular musculature.

CONCLUSIONS

SB-236057 is speculated to alter paraxial mesoderm programming. Many of the skeletal malformations may be caused secondarily from musculature abnormalities, suggesting that the myotome may be particularly sensitive to the compound. Furthermore, the finding that peripheral nerve trajectories were altered along the axis and in the limb suggests that SB-236057 may alter early embryonic signaling pathways necessary for neuronal differentiation/axonal guidance that occur subsequently in embryo-fetal development.

Evidence for a molecular mechanism of teratogenicity of SB-236057, a 5-HT1B receptor inverse agonist that alters axial formation

BACKGROUND

SB-236057 is a potent skeletal teratogen in rodents and rabbits, producing axial and posterior somite malformations in cultured rat embryos. The compound shares some structural similarity to cyclopamine.

METHODS

M13 phage display was used to identify amino acid motifs with binding affinity to SB-236057. A 10 microM SB-236057 solution was administered to cultured day 9 postcoitus rat embryos and real-time PCR was conducted at 6 hr posttreatment to evaluate early transcriptional response of axial development genes. Whole-mount in situ hybridization of selected transcripts was conducted on embryos at 48 hr post-compound administration. The rat-enhancer of split protein 1 (r-esp1) expression-functional characterization was done by transcriptional expression and morpholino antisense approaches.

RESULTS

We identified several amino acid motifs that had high binding affinity to SB-236057-biotin conjugates, one with 100% sequence homology to a region of r-esp1, one of the Groucho homologs transcribed by the enhancer of split complex (En[spl]C). SB-236057 repressed expression of r-esp1 and members of the Notch-En[spl]C pathway. Goosecoid and HNF3-beta, both suspected to associate with Groucho proteins, were also responsive, although expression of another putative binding protein, engrailed-1 (en-1), and other en-1 pathway members was not affected. R-esp1 mRNA was localized along the axis and antisense inhibition produced similar somite malformations as SB-236057 did. At 48 hr post-SB-236057 or post-r-esp1 antisense administration, affected embryos demonstrated unchanged sonic hedgehog (shh) expression, however HNF3-beta expression was either absent, altered, or reduced.

CONCLUSIONS

We present experimental evidence that the mechanism of SB-236057 teratogenicity includes transcriptional alterations to the Notch1-En[spl] pathway. In addition, alterations in HNF3-beta expression were similar to those induced by cyclopamine. The relationships between r-esp1 with Notch1 and shh signaling pathways and potential mechanisms of SB-236057 teratogenicity are also discussed.

Phosphorylation by the DHIPK2 protein kinase modulates the corepressor activity of Groucho

Groucho function is essential for Drosophila development, acting as a corepressor for specific transcription factors that are downstream targets of various signaling pathways. Here we provide evidence that Groucho is phosphorylated by the DHIPK2 protein kinase. Phosphorylation modulates Groucho corepressor activity by attenuating its protein-protein interaction with a DNA-bound transcription factor. During eye development, DHIPK2 modifies Groucho activity, and eye phenotypes generated by overexpression of Groucho differ depending on its phosphorylation state. Moreover, analysis of nuclear extracts fractionated by column chromatography further shows that phospho-Groucho associates poorly with the corepressor complex, whereas the unphosphorylated form binds tightly. We propose that Groucho phosphorylation by DHIPK2 and its subsequent dissociation from the corepressor complex play a key role in relieving the transcriptional repression of target genes regulated by Groucho, thereby controlling cell fate determination during development.

Zebrafish Genetics and Formation of Embryonic Vasculature

The embryonic vasculature develops in a conserved manner in all vertebrates. Endothelial progenitor cells differentiate from mesodermal cells, then migrate and assemble into the dorsal aorta and the cardinal vein. This primitive circulatory loop undergoes sprouting and branching via a two-step navigation mechanism to form the trunk vascular network. Various studies using several model systems have uncovered a number of signaling mechanisms that regulate these complex processes. A genetic approach in zebrafish has led to identification of mutations and molecules that are responsible for specification of endothelial progenitor cells, differentiation of arterial and venous cells, and patterning of the dorsal aorta and intersegmental vessels. These studies highlight the unique utilities and benefits of the zebrafish system for studying development of embryonic blood vessels.

EGFR signaling attenuates Groucho-dependent repression to antagonize Notch transcriptional output

Crosstalk between signaling pathways is crucial for the generation of complex and varied transcriptional networks. Antagonism between the EGF-receptor (EGFR) and Notch pathways in particular is well documented, although the underlying mechanism is poorly understood. The global corepressor Groucho (Gro) and its transducin-like Enhancer-of-split (TLE) mammalian homologs mediate repression by a myriad of repressors, including effectors of the Notch, Wnt (Wg) and TGF-beta (Dpp) signaling cascades. Given that there are genetic interactions between gro and components of the EGFR pathway (ref. 9 and P.H. et al., unpublished results), we tested whether Gro is at a crossroad between this and other pathways. Here we show that phosphorylation of Gro in response to MAPK activation weakens its repressor capacity, attenuating Gro-dependent transcriptional silencing by the Enhancer-of-split proteins, effectors of the Notch cascade. Thus, Gro is a new junction between signaling pathways, enabling EGFR signaling to antagonize transcriptional output by Notch and potentially other Gro-dependent pathways.

Sequences downstream of the bHLH domain of the Xenopus hairy-related transcription factor-1 act as an extended dimerization domain that contributes to the selection of the partners

XHRT1 is a member of the HRT/Hey protein subfamily that are known as Notch effectors. XHRT1 is expressed in the developing floor plate and encodes a basic helix-loop-helix (bHLH) transcription repressor. Here, we show that XHRT1 misexpression in the neural plate inhibits differentiation of neural precursor cells and thus may be important for floor plate cells to prevent them from adopting a neuronal fate. Deletion analysis indicated that inhibition of differentiation by XHRT1 requires the DNA-binding bHLH motif and either the Orange domain or the C-terminal region. XHRT1 could efficiently homodimerize and heterodimerize with hairy proteins. Among those hairy genes, Xhairy2b shows extensive overlap of expression with XHRT1 in floor plate precursors and may be a biologically relevant XHRT1 partner. Dimerization is mediated through both the bHLH and downstream sequences, the Orange domain being particularly important for the efficiency of the interaction. Using chimeric constructs between XHRT1 and the ESR9 bHLH-O protein that does not interact with Xhairy1 and Xhairy2b, we found that both the bHLH domain and downstream sequences of XHRT1 were required for heterodimerization with Xhairy2b, while only the XHRT1 sequences downstream of the Orange domain are required for the interaction with Xhairy1. Together, these results suggest that XHRT1 plays a role in floor plate cell development and highlight the importance of the Orange and downstream sequences in dimerization and in the selection of the bHLH partners.

Groucho-dependent repression by Sloppy-paired 1 differentially positions anterior pair-rule stripes in the Drosophila embryo

The Drosophila body plan is composed of a linear array of cephalic, thoracic, and abdominal segments along the anterior posterior axis. The number and positions of individual segments are established by a transcriptional network comprised of maternal effect, gap, pair-rule, and segment polarity genes. The sloppy-paired (slp) locus contains two genes (slp1 and slp2) that are expressed in overlapping striped patterns in the presumptive thorax and abdomen. Previous studies suggest that these genes function at the pair-rule and segment polarity levels to establish the spacing and polarity of thoracic and abdominal segments. One of these genes (slp1) is also expressed in a broad anterior domain that appears before the striped patterns. There are severe cephalic defects in slp1 mutants, including the complete loss of the mandibular segment, but the molecular roles played by Slp1 in anterior patterning are not clear. Here, we present evidence that the anterior Slp1 domain acts as a gradient to differentially repress the anteriormost stripes of several different pair-rule genes. This repressive gradient contributes to the precise spatial arrangement of anterior pair-rule stripe borders required for expression of the first engrailed stripe and the formation of the mandibular segment. These results suggest that Slp1 functions as a gap gene-like repressor, in addition to its roles at the pair-rule and segment polarity levels of the hierarchy. The Slp1 protein contains a protein motif (EH1) which mediates binding to the transcriptional corepressor Groucho (Gro). We show that this domain is required for Slp1-mediated repression in vivo.

nlz gene family is required for hindbrain patterning in the zebrafish

This study describes the conserved nlz gene family whose members encode unusual zinc finger proteins. In the zebrafish neurectoderm, both nlz1 and the newly isolated nlz2 are expressed in the presumptive hindbrain and midbrain/hindbrain boundary, where expression of nlz1 is dependent on pax2a. In addition, nlz2 is uniquely expressed more anteriorly, in the presumptive midbrain and diencephalon. Overexpression of Nlz proteins during gastrula stages inhibits hindbrain development. In particular, ectopically expressed Nlz1 inhibits formation of future rhombomeres 2 and 3 (r2, r3), whereas neighboring r1 and r4 are not affected. Conversely, simultaneous reduction of Nlz1 and Nlz2 protein function by expression of antisense morpholino-modified oligomers leads to expansion of future r3 and r5, with associated loss of r4. These data indicate that one function of the nlz gene family is to specify or maintain r4 identity, and to limit r3 and r5 during hindbrain formation.

Drosophila CK2 regulates eye morphogenesis via phosphorylation of E(spl)M8

The Notch effector E(spl)M8 is phosphorylated at Ser159 by CK2, a highly conserved Ser/Thr protein kinase. We have used the Gal4-UAS system to assess the role of M8 phosphorylation during bristle and eye morphogenesis by employing a non-phosphorylatable variant (M8SA) or one predicted to mimic the 'constitutively' phosphorylated protein (M8SD). We find that phosphorylation of M8 does not appear to be critical during bristle morphogenesis. In contrast, only M8SD elicits a severe 'reduced eye' phenotype when it is expressed in the morphogenetic furrow of the eye disc. M8SD elicits neural hypoplasia in eye discs, elicits loss of phase-shifted Atonal-positive cells, i.e. the 'founding' R8 photoreceptors, and consequently leads to apoptosis. The ommatidial phenotype of M8SD is similar to that in Nspl/Y; E(spl)D/+ flies. E(spl)D, an allele of m8, encodes a truncated protein known as M8*, which, unlike wild type M8, displays exacerbated antagonism of Atonal via direct protein-protein interactions. In line with this, we find that the M8SD-Atonal interaction appears indistinguishable from that of M8*-Atonal, whereas interaction of M8 or M8SA appears marginal, at best. These results raise the possibility that phosphorylation of M8 (at Ser159) might be required for its ability to mediate 'lateral inhibition' within proneural clusters in the developing retina. This is the first identification of a dominant allele encoding a phosphorylation-site variant of an E(spl) protein. Our studies uncover a novel functional domain that is conserved amongst a subset of E(spl)/Hes repressors in Drosophila and mammals, and suggests a potential role for CK2 during retinal patterning.

Phosphorylation of serine 239 of Groucho/TLE1 by protein kinase CK2 is important for inhibition of neuronal differentiation

Transcriptional corepressors of the Groucho (Gro)/TLE family play important roles during a variety of developmental pathways, including neuronal differentiation. In particular, they act as negative regulators of neurogenesis, together with Hairy/Enhancer of split (Hes) DNA-binding proteins. The interaction with Hes1 leads to Gro/TLE hyperphosphorylation and increased transcription repression activity in mammalian cells, but the underlying molecular mechanisms are poorly characterized. We now show that Gro/TLE1 is phosphorylated in vivo by protein kinase CK2. This phosphorylation occurs at serine 239 within the conserved CcN domain present in all Gro/TLE family members. Mutation of serine 239 into alanine decreases Hes1-induced hyperphosphorylation of Gro/TLE1 and also reduces its nuclear association and transcription repression activity. We demonstrate further that Gro/TLE1 inhibits the transition of cortical neural progenitors into neurons and that its antineurogenic activity is inhibited by a serine-239-alanine mutation but not by a serine-239-glutamate mutation. These results suggest that CK2 phosphorylation of serine 239 of Gro/TLE1 is important for its function during neuronal differentiation.

Capicua integrates input from two maternal systems in Drosophila terminal patterning

In Drosophila, the maternal terminal system specifies cell fates at the embryonic poles via the localised stimulation of the Torso receptor tyrosine kinase (RTK). Signalling by the Torso pathway relieves repression mediated by the Capicua and Groucho repressors, allowing the restricted expression of the zygotic terminal gap genes tailless and huckebein. Here we report a novel positive input into tailless and huckebein transcription by maternal posterior group genes, previously implicated in abdomen and pole cell formation. We show that absence of a subset of posterior group genes, or their overactivation, leads to the spatial reduction or expansion of the tailless and huckebein posterior expression domains, respectively. We demonstrate that the terminal and posterior systems converge, and that exclusion of Capicua from the termini of posterior group mutants is ineffective, accounting for reduced terminal gap gene expression in these embryos. We propose that the terminal and posterior systems function coordinately to alleviate transcriptional silencing by Capicua, and that the posterior system fine-tunes Torso RTK signalling output, ensuring precise spatial domains of tailless and huckebein expression.

Role of the Sc C terminus in transcriptional activation and E(spl) repressor recruitment

Neurogenesis in all animals is triggered by the activity of a group of basic helix-loop-helix transcription factors, the proneural proteins, whose expression endows ectodermal regions with neural potential. The eventual commitment to a neural precursor fate involves the interplay of these proneural transcriptional activators with a number of other transcription factors that fine tune transcriptional responses at target genes. Most prominent among the factors antagonizing proneural protein activity are the HES basic helix-loop-helix proteins. We have previously shown that two HES proteins of Drosophila, E(spl)mgamma and E(spl)m7, interact with the proneural protein Sc and thereby get recruited onto Sc target genes to repress transcription. Using in vivo and in vitro assays we have now discovered an important dual role for the Sc C-terminal domain. On one hand it acts as a transcription activation domain, and on the other it is used to recruit E(spl) proteins. In vivo, the Sc C-terminal domain is required for E(spl) recruitment in an enhancer context-dependent fashion, suggesting that in some enhancers alternative interaction surfaces can be used to recruit E(spl) proteins.

Possible role of Hes5 for the rostrocaudal polarity formation of the tectum

The alar plate of the mesencephalon differentiates into the optic tectum. Retinal fibers project to the tectum topographically in a retinotopic manner. Engrailed (En) is responsible for the tectum polarity formation and regionalization. Former study indicated the presence of the molecule whose expression is repressed by En and that represses the isthmus-related gene expression. To isolate such molecules, we constructed a subtracted library between cDNA population of the normal rostral mesencephalon and of the rostral mesencephalon that misexpresses En2. From the library, we isolated cHes5, a chicken homolog of Drosophila hairy/Enhancer of split. cHes5 begins to be expressed in the rostral part of the E2 mesencephalon, and spreads to caudal mesencephalon by E3. To our expectation, cHes5 expression was repressed by En2. Furthermore, misexpression of cHes5 in the mesencephalon inhibited expression of ephrinA2, a marker of caudal mesencephalon. An active repressor form of Hes5, which is a chimeric molecule of Hes5 and repressor domain of En2, showed a similar but more severe phenotype. The results indicate that Hes5 is regulated by En and is responsible for rostral identity of mesencephalon by repressing ephrinA2.

Notch signaling patterns Drosophila mesodermal segments by regulating the bHLH transcription factor twist

One of the first steps in embryonic mesodermal differentiation is allocation of cells to particular tissue fates. In Drosophila, this process of mesodermal subdivision requires regulation of the bHLH transcription factor Twist. During subdivision, Twist expression is modulated into stripes of low and high levels within each mesodermal segment. High Twist levels direct cells to the body wall muscle fate, whereas low levels are permissive for gut muscle and fat body fate. We show that Su(H)-mediated Notch signaling represses Twist expression during subdivision and thus plays a critical role in patterning mesodermal segments. Our work demonstrates that Notch acts as a transcriptional switch on mesodermal target genes, and it suggests that Notch/Su(H) directly regulates twist, as well as indirectly regulating twist by activating proteins that repress Twist. We propose that Notch signaling targets two distinct 'Repressors of twist' - the proteins encoded by the Enhancer of split complex [E(spl)C] and the HLH gene extra machrochaetae (emc). Hence, the patterning of Drosophila mesodermal segments relies on Notch signaling changing the activities of a network of bHLH transcriptional regulators, which, in turn, control mesodermal cell fate. Since this same cassette of Notch, Su(H) and bHLH regulators is active during vertebrate mesodermal segmentation and/or subdivision, our work suggests a conserved mechanism for Notch in early mesodermal patterning.

her3, a zebrafish member of the hairy-E(spl) family, is repressed by Notch signalling

her3 encodes a zebrafish bHLH protein of the Hairy-E(Spl) family. During embryogenesis, the gene is transcribed exclusively in the developing central nervous system, according to a fairly simple pattern that includes territories in the mesencephalon/rhombencephalon and the spinal cord. In all territories, the her3 transcription domain encompasses regions in which neurogenin 1 (neurog1) is not transcribed, suggesting regulatory interactions between the two genes. Indeed, injection of her3 mRNA leads to repression of neurog1 and to a reduction in the number of primary neurones, whereas her3 morpholino oligonucleotides cause ectopic expression of neurog1 in the rhombencephalon. Fusions of Her3 to the transactivation domain of VP16 and to the repression domain of Engrailed show that Her3 is indeed a transcriptional repressor. Dissection of the Her3 protein reveals two possible mechanisms for transcriptional repression: one mediated by the bHLH domain and the C-terminal WRPW tetrapeptide; and the other involving the N-terminal domain and the orange domain. Gel retardation assays suggest that the repression of neurog1 transcription occurs by binding of Her3 to specific DNA sequences in the neurog1 promoter. We have examined interrelationships of her3 with members of the Notch signalling pathway by the Gal4-UAS technique and mRNA injections. The results indicate that Her3 represses neurog1 and, probably as a consequence of the neurog1 repression, deltaA, deltaD and her4. Moreover, Her3 represses its own transcription as well. Surprisingly, and in sharp contrast to other members of the E(spl) gene family, transcription of her3 is repressed rather than activated by Notch signalling.

Identification of a novel basic helix-loop-helix gene, Heslike, and its role in GABAergic neurogenesis

Neuronal subtype specification depends on multiple transcription factors such as basic helix-loop-helix (bHLH) factors. However, transcription factor codes for most neurons remain to be determined. Here, we report identification of a novel mouse bHLH factor, termed Heslike, that has Hes1-like bHLH domain and transcriptional repressor activity. Heslike is coexpressed with the bHLH factor Mash1 in brain regions that give rise to GABAergic neurons. In the mesencephalon and the caudal diencephalon, coexpression of Heslike and Mash1 is initially restricted to small regions but expanded dorsally from embryonic day 9.5 onward, and this expansion of coexpression is followed by GABAergic neurogenesis. Misexpression of Heslike in mouse embryos generates ectopic GABAergic neurons only from the Mash1(+) region. In contrast, in the mesencephalon and the caudal diencephalon of Mash1-null mice, GABAergic neurons are almost completely missing and, instead, other neurons are generated, although Heslike is still expressed. Furthermore, coexpression of Heslike and Mash1 significantly promotes formation of GABAergic neurons, compared with each gene alone, in neural precursor cell culture. Thus, Heslike or Mash1 alone is not sufficient, but their coexpression may be important for generation of GABAergic neurons. These results suggest that combinations of distinct bHLH factors promote formation of distinct neuronal subtypes, thereby increasing neuronal diversity.

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.

Groucho oligomerization is required for repression in vivo

Drosophila Groucho (Gro) is a member of a family of metazoan corepressors with widespread roles in development. Previous studies indicated that a conserved domain in Gro, termed the Q domain, was required for repression in cultured cells and mediated homotetramerization. Evidence presented here suggests that the Q domain contains two coiled-coil motifs required for oligomerization and repression in vivo. Mutagenesis of the putative hydrophobic faces of these motifs, but not of the hydrophilic faces, prevents the formation of both tetramers and higher order oligomers. Mutagenesis of the hydrophobic faces of both coiled-coil motifs in the context of a Gal4-Gro fusion protein prevents repression of a Gal4-responsive reporter in S2 cells, while mutagenesis of a single motif weakens repression. The finding that the repression directed by the single mutants depends on endogenous wild-type Gro further supports the idea that oligomerization plays a role in repression. Overexpression in the fly of forms of Gro able to oligomerize, but not of a form of Gro unable to oligomerize, results in developmental defects and ectopic repression of Gro target genes in the wing disk. Although the function of several corepressors is suspected to involve oligomerization, these studies represent one of the first direct links between corepressor oligomerization and repression in vivo.

Oncogenic potential of the RUNX gene family: 'overview'

Runt-related (RUNX) gene family is composed of three members, RUNX1/AML1, RUNX2 and RUNX3, and encodes the DNA-binding (alpha) subunits of the Runt domain transcription factor polyomavirus enhancer-binding protein 2 (PEBP2)/core-binding factor (CBF), which is a heterodimeric transcription factor. RUNX1 is most frequently involved in human acute leukemia. RUNX2 shows oncogenic potential in mouse experimental system. RUNX3 is a strong candidate as a gastric cancer tumor suppressor. The beta subunit gene of PEBP2/CBF is also frequently involved in chromosome rearrangements associated with human leukemia. In this Overview, I will summarize how this growing field has been formed and what are the challenging new frontiers for better understanding of the oncogenic potential of this gene family.

The proline-rich homeodomain protein recruits members of the Groucho/Transducin-like enhancer of split protein family to co-repress transcription in hematopoietic cells

The proline-rich homeodomain protein (PRH/Hex) is important in the control of cell proliferation and differentiation and in the regulation of multiple processes in embryonic development. We have shown previously that PRH contains two domains that can independently bring about transcriptional repression. The PRH homeodomain represses transcription by binding to TATA box sequences, whereas the proline-rich N-terminal domain of PRH can repress transcription when attached to a heterologous DNA-binding domain. The Groucho/transducin-like enhancer of split (TLE) family of proteins are transcriptional co-repressors that interact with a number of DNA-bound transcription factors and play multiple roles in development. Here we demonstrate that the proline-rich N-terminal domain of PRH binds to TLE1 in vitro and in yeast two-hybrid assays. We show that PRH and TLE proteins are co-expressed in hematopoietic cells and interact in co-immunoprecipitation assays. We demonstrate that TLE1 increases repression by PRH in transient transfection assays and that titration of endogenous TLE proteins by co-expression of Grg5, a natural trans-dominant negative protein, alleviates transcriptional repression by PRH. Finally, we show that a mutation in the PRH N-terminal domain that blocks the PRH-TLE1 interaction in vitro eliminates co-repression. We discuss these results in terms of the roles of PRH and TLE in cell differentiation and development.

Neurogenic phenotypes induced by RNA interference with bHLH genes of theEnhancer of splitcomplex ofDrosophila melanogaster

The Enhancer of split gene complex [E(spl)-C] of Drosophila melanogaster harbors seven highly related genes encoding transcriptional regulators with a basic helix-loop-helix (bHLH) domain. They are activated by the Notch signaling pathway in order to inhibit proneural gene activity, for example, during neurogenesis in the developing embryo. The E(spl) proteins are at least partly redundant, despite some remarkable differences in their expression patterns. We attempted to address the degree of redundancy by means of RNA interference. We find a quantitative correlation between the degree of a neurogenic phenotype and the number of genes affected. Surprisingly, interference with m3 results in a high rate of mortality which cannot be reproduced by genetic mutation. Most likely, m3 dsRNA interferes with unrelated genes involved in other aspects of embryonic development.

A mitochondrial protein, Bit1, mediates apoptosis regulated by integrins and Groucho/TLE corepressors

A delicate balance of signals regulates cell survival. One set of these signals is derived from integrin-mediated cell adhesion to the extracellular matrix (ECM). Loss of cell attachment to the ECM causes apoptosis, a process known as anoikis. In searching for proteins involved in cell adhesion-dependent regulation of anoikis, we identified Bit1, a mitochondrial protein that is released into the cytoplasm during apoptosis. Cytoplasmic Bit1 forms a complex with AES, a small Groucho/transducin-like enhancer of split (TLE) protein, and induces cell death with characteristics of caspase-independent apoptosis. Cell attachment to fibronectin counteracts the apoptotic effect of Bit1 and AES. Increasing Bit1 expression enhances anoikis, while suppressing the expression reduces it. Thus, we have elucidated an integrin-controlled pathway that is, at least in part, responsible for the cell survival effects of cell-ECM interactions.

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.

Gene regulatory factors in pancreatic development

The intensity of research on pancreatic development has increased markedly in the past 5 years, primarily for two reasons: we now know that the insulin-producing beta-cells normally arise from an endodermally derived, pancreas-specified precursor cell, and successful transplants of islet cells have been performed, relieving patients with type I diabetes of symptoms for extended periods after transplantation. Combining in vitro beta-cell formation from a pancreatic biopsy of a diabetic patient or from other stem-cell sources followed by endocrine cell transplantation may be the most beneficial route for a future diabetes therapy. However, to achieve this, a thorough understanding of the genetic components regulating the development of beta-cells is required. The following review discusses our current understanding of the transcription factor networks necessary for pancreatic development and how several genetic interactions coming into play at the earliest stages of endodermal development gradually help to build the pancreatic organ. Developmental Dynamics 229:176-200, 2004.

Post-transcriptional regulation of the E/Daughterless ortholog HLH-2, negative feedback, and birth order bias during the AC/VU decision in C. elegans

The anchor cell/ventral uterine precursor cell (AC/VU) decision in Caenorhabditis elegans is a canonical example of lin-12/Notch-mediated lateral specification. Two initially equivalent cells interact via the receptor LIN-12 and its ligand LAG-2, so that one becomes the AC and the other a VU. During this interaction, feedback loops amplify a small difference in lin-12 activity, limiting lin-12 transcription to the presumptive VU and lag-2 transcription to the presumptive AC. Here, we find that hlh-2 appears to be required for the VU fate and directly activates lag-2 transcription in the presumptive AC. HLH-2 appears to accumulate selectively in the presumptive AC prior to differential transcription of lin-12 or lag-2, and is therefore the earliest detectable difference between the two cells undergoing the AC/VU decision. The restricted accumulation of HLH-2 to the presumptive AC reflects post-transcriptional down-regulation of HLH-2 in the presumptive VU. Our observations suggest that hlh-2 is regulated as part of the negative feedback that down-regulates lag-2 transcription in the presumptive VU. Finally, we show that the AC/VU decision in an individual hermaphrodite is biased by the relative birth order of the two cells, so that the first-born cell is more likely to become the VU. We propose models to suggest how birth order, HLH-2 accumulation, and transcription of lag-2 may be linked during the AC/VU decision.

Senseless acts as a binary switch during sensory organ precursor selection

During sensory organ precursor (SOP) specification, a single cell is selected from a proneural cluster of cells. Here, we present evidence that Senseless (Sens), a zinc-finger transcription factor, plays an important role in this process. We show that Sens is directly activated by proneural proteins in the presumptive SOPs and a few cells surrounding the SOP in most tissues. In the cells that express low levels of Sens, it acts in a DNA-binding-dependent manner to repress transcription of proneural genes. In the presumptive SOPs that express high levels of Sens, it acts as a transcriptional activator and synergizes with proneural proteins. We therefore propose that Sens acts as a binary switch that is fundamental to SOP selection.

Hes6 promotes cortical neurogenesis and inhibits Hes1 transcription repression activity by multiple mechanisms

Hes1 is a mammalian basic helix-loop-helix transcriptional repressor that inhibits neuronal differentiation together with corepressors of the Groucho (Gro)/Transducin-like Enhancer of split (TLE) family. The interaction of Hes1 with Gro/TLE is mediated by a WRPW tetrapeptide present in all Hairy/Enhancer of split (Hes) family members. In contrast to Hes1, the related protein Hes6 promotes neuronal differentiation. Little is known about the molecular mechanisms that underlie the neurogenic activity of Hes6. It is shown here that Hes6 antagonizes Hes1 function by two mechanisms. Hes6 inhibits the interaction of Hes1 with its transcriptional corepressor Gro/TLE. Moreover, it promotes proteolytic degradation of Hes1. This effect is maximal when both Hes1 and Hes6 contain the WRPW motif and is reduced when Hes6 is mutated to eliminate a conserved site (Ser183) that can be phosphorylated by protein kinase CK2. Consistent with these findings, Hes6 inhibits Hes1-mediated transcriptional repression in cortical neural progenitor cells and promotes the differentiation of cortical neurons, a process that is normally inhibited by Hes1. Mutation of Ser183 impairs the neurogenic ability of Hes6. Taken together, these findings clarify the molecular events underlying the neurogenic function of Hes6 and suggest that this factor can antagonize Hes1 activity by multiple mechanisms.

The Arabidopsis basic/helix-loop-helix transcription factor family

The basic/helix-loop-helix (bHLH) proteins are a superfamily of transcription factors that bind as dimers to specific DNA target sites and that have been well characterized in nonplant eukaryotes as important regulatory components in diverse biological processes. Based on evidence that the bHLH protein PIF3 is a direct phytochrome reaction partner in the photoreceptor's signaling network, we have undertaken a comprehensive computational analysis of the Arabidopsis genome sequence databases to define the scope and features of the bHLH family. Using a set of criteria derived from a previously defined consensus motif, we identified 147 bHLH protein-encoding genes, making this one of the largest transcription factor families in Arabidopsis. Phylogenetic analysis of the bHLH domain sequences permits classification of these genes into 21 subfamilies. The evolutionary and potential functional relationships implied by this analysis are supported by other criteria, including the chromosomal distribution of these genes relative to duplicated genome segments, the conservation of variant exon/intron structural patterns, and the predicted DNA binding activities within subfamilies. Considerable diversity in DNA binding site specificity among family members is predicted, and marked divergence in protein sequence outside of the conserved bHLH domain is observed. Together with the established propensity of bHLH factors to engage in varying degrees of homodimerization and heterodimerization, these observations suggest that the Arabidopsis bHLH proteins have the potential to participate in an extensive set of combinatorial interactions, endowing them with the capacity to be involved in the regulation of a multiplicity of transcriptional programs. We provide evidence from yeast two-hybrid and in vitro binding assays that two related phytochrome-interacting members in the Arabidopsis family, PIF3 and PIF4, can form both homodimers and heterodimers and that all three dimeric configurations can bind specifically to the G-box DNA sequence motif CACGTG. These data are consistent, in principle, with the operation of this combinatorial mechanism in Arabidopsis.

Basic helix-loop-helix factors in cortical development

Transcription factors with bHLH motifs modulate critical events in the development of the mammalian neocortex. Multipotent cortical progenitors are maintained in a proliferative state by bHLH factors from the Id and Hes families. The transition from proliferation to neurogenesis involves a coordinate increase in the activity of proneural bHLH factors (Mash1, Neurogenin1, and Neurogenin2) and a decrease in the activity of Hes and Id factors. As development proceeds, inhibition of proneural bHLH factors in cortical progenitors promotes the formation of astrocytes. Finally, the formation of oligodendrocytes is triggered by an increase in the activity of bHLH factors Olig1 and Olig2 that may be coupled with a decrease in Id activity. Thus, bHLH factors have key roles in corticogenesis, affecting the timing of differentiation and the specification of cell fate.

ELM server: A new resource for investigating short functional sites in modular eukaryotic proteins

Multidomain proteins predominate in eukaryotic proteomes. Individual functions assigned to different sequence segments combine to create a complex function for the whole protein. While on-line resources are available for revealing globular domains in sequences, there has hitherto been no comprehensive collection of small functional sites/motifs comparable to the globular domain resources, yet these are as important for the function of multidomain proteins. Short linear peptide motifs are used for cell compartment targeting, protein-protein interaction, regulation by phosphorylation, acetylation, glycosylation and a host of other post-translational modifications. ELM, the Eukaryotic Linear Motif server at http://elm.eu.org/, is a new bioinformatics resource for investigating candidate short non-globular functional motifs in eukaryotic proteins, aiming to fill the void in bioinformatics tools. Sequence comparisons with short motifs are difficult to evaluate because the usual significance assessments are inappropriate. Therefore the server is implemented with several logical filters to eliminate false positives. Current filters are for cell compartment, globular domain clash and taxonomic range. In favourable cases, the filters can reduce the number of retained matches by an order of magnitude or more.

In vivo analysis of a developmental circuit for direct transcriptional activation and repression in the same cell by a Runx protein

Runx proteins have been implicated in acute myeloid leukemia, cleidocranial dysplasia, and stomach cancer. These proteins control key developmental processes in which they function as both transcriptional activators and repressors. How these opposing regulatory modes can be accomplished in the in vivo context of a cell has not been clear. In this study we use the developing cone cell in the Drosophila visual system to elucidate the mechanism of positive and negative regulation by the Runx protein Lozenge (Lz). We describe a regulatory circuit in which Lz causes transcriptional activation of the homeodomain protein Cut, which can then stabilize a Lz repressor complex in the same cell. Whether a gene is activated or repressed is determined by whether the Lz activator or the repressor complex binds to its upstream sequence. This study provides a mechanistic basis for the dual function of Runx proteins that is likely to be conserved in mammalian systems.

Recent advances in stem cell neurobiology

1. Neural stem cells can be cultured from the CNS of different mammalian species at many stages of development. They have an extensive capacity for self-renewal and will proliferate ex vivo in response to mitogenic growth factors or following genetic modification with immortalising oncogenes. Neural stem cells are multipotent since their differentiating progeny will give rise to the principal cellular phenotypes comprising the mature CNS: neurons, astrocytes and oligodendrocytes. 2. Neural stem cells can also be derived from more primitive embryonic stem (ES) cells cultured from the blastocyst. ES cells are considered to be pluripotent since they can give rise to the full cellular spectrum and will, therefore, contribute to all three of the embryonic germ layers: endoderm, mesoderm and ectoderm. However, pluripotent cells have also been derived from germ cells and teratocarcinomas (embryonal carcinomas) and their progeny may also give rise to the multiple cellular phenotypes contributing to the CNS. In a recent development, ES cells have also been isolated and grown from human blastocysts, thus raising the possibility of growing autologous stem cells when combined with nuclear transfer technology. 3. There is now an emerging recognition that the adult mammalian brain, including that of primates and humans, harbours stem cell populations suggesting the existence of a previously unrecognised neural plasticity to the mature CNS, and thereby raising the possibility of promoting endogenous neural reconstruction. 4. Such reports have fuelled expectations for the clinical exploitation of neural stem cells in cell replacement or recruitment strategies for the treatment of a variety of human neurological conditions including Parkinson's disease (PD), Huntington's disease, multiple sclerosis and ischaemic brain injury. Owing to their migratory capacity within the CNS, neural stem cells may also find potential clinical application as cellular vectors for widespread gene delivery and the expression of therapeutic proteins. In this regard, they may be eminently suitable for the correction of genetically-determined CNS disorders and in the management of certain tumors responsive to cytokines. Since large numbers of stem cells can be generated efficiently in culture, they may obviate some of the technical and ethical limitations associated with the use of fresh (primary) embryonic neural tissue in current transplantation strategies. 5. While considerable recent progress has been made in terms of developing new techniques allowing for the long-term culture of human stem cells, the successful clinical application of these cells is presently limited by our understanding of both (i) the intrinsic and extrinsic regulators of stem cell proliferation and (ii) those factors controlling cell lineage determination and differentiation. Although such cells may also provide accessible model systems for studying neural development, progress in the field has been further limited by the lack of suitable markers needed for the identification and selection of cells within proliferating heterogeneous populations of precursor cells. There is a further need to distinguish between the committed fate (defined during normal development) and the potential specification (implying flexibility of fate through manipulation of its environment) of stem cells undergoing differentiation. 6. With these challenges lying ahead, it is the opinion of the authors that stem-cell therapy is likely to remain within the experimental arena for the foreseeable future. In this regard, few (if any) of the in vivo studies employing neural stem cell grafts have shown convincingly that behavioural recovery can be achieved in the various model paradigms. Moreover, issues relating to the quality control of cultured cells and their safety following transplantation have only begun to be addressed. 7. While on the one hand cell biotechnologists have been quick to realise the potential commercial value, human stem cell research and its clinical applications has been the subject of intense ethical and legislative considerations. The present chapter aims to review some recent aspects of stem cell research applicable to developmental neurobiology and the potential applications in clinical neuroscience.

HES and HERP families: multiple effectors of the Notch signaling pathway

Notch signaling dictates cell fate and critically influences cell proliferation, differentiation, and apoptosis in metazoans. Multiple factors at each step-ligands, receptors, signal transducers and effectors-play critical roles in executing the pleiotropic effects of Notch signaling. Ligand-binding results in proteolytic cleavage of Notch receptors to release the signal-transducing Notch intracellular domain (NICD). NICD migrates into the nucleus and associates with the nuclear proteins of the RBP-Jkappa family (also known as CSL or CBF1/Su(H)/Lag-1). RBP-Jkappa, when complexed with NICD, acts as a transcriptional activator, and the RBP-Jkappa-NICD complex activates expression of primary target genes of Notch signaling such as the HES and enhancer of split [E(spl)] families. HES/E(spl) is a basic helix-loop-helix (bHLH) type of transcriptional repressor, and suppresses expression of downstream target genes such as tissue-specific transcriptional activators. Thus, HES/E(spl) directly affects cell fate decisions as a primary Notch effector. HES/E(spl) had been the only known effector of Notch signaling until a recent discovery of a related but distinct bHLH protein family, termed HERP (HES-related repressor protein, also called Hey/Hesr/HRT/CHF/gridlock). In this review, we summarize the recent data supporting the idea of HERP being a new Notch effector, and provide an overview of the similarities and differences between HES and HERP in their biochemical properties as well as their tissue distribution. One key observation derived from identification of HERP is that HES and HERP form a heterodimer and cooperate for transcriptional repression. The identification of the HERP family as a Notch effector that cooperates with HES/E(spl) family has opened a new avenue to our understanding of the Notch signaling pathway.

The role of Grg4 in tectal laminar formation

Mature chick optic tecta consist of 16 laminae and receive retinal fiber projections in a precise retinotopic manner. Retinal axons arborize in laminae a-f of the SGFS, but do not cross the border between lamina f and g. In order to elucidate molecular mechanisms of tectal laminar formation, we first looked at the migration of tectal postmitotic cells. We found that the migration pattern of postmitotic cells changes around E5 and that late migratory cells intervened laminae that were formed by early migratory cells. The coincident appearance of Grg4 expression in the tectal ventricular layer and the change in migration pattern suggested an important role for Grg4. Clonal misexpression of Grg4 resulted in cells migrating to laminae h-j of the SGFS. Massive misexpression of Grg4 resulted in disruption of laminae that were formed by early migratory cells, in particular lamina g of the SGFS. Application of Grg4 morpholino antisense oligonucleotide or the misexpression of a dominant-negative form of Grg4 exerted the opposite effect. We concluded that Grg4 may direct tectal postmitotic cells to follow a late migratory pathway.

Human Sir2-related protein SIRT1 associates with the bHLH repressors HES1 and HEY2 and is involved in HES1- and HEY2-mediated transcriptional repression

The Hairy-related bHLH proteins function as transcriptional repressors in most cases and play important roles in diverse aspects of metazoan development. Recently, it was shown that the Drosophila bHLH repressor proteins, Hairy and Deadpan, bind to and function with the NAD(+)-dependent histone deacetylase, Sir2. Here we demonstrate that the human Sir2 homologue, SIRT1, also physically associates with the human bHLH repressor proteins, hHES1 and hHEY2, both in vitro and in vivo. Moreover, using the reporter assay, we show that both SIRT1-dependent and -independent deacetylase pathways are involved in the transcriptional repressions mediated by these bHLH repressors. These results indicate that the molecular association between bHLH proteins and Sir2-related proteins is conserved among metazoans, from Drosophila to human, and suggest that the Sir2-bHLH interaction also plays important roles in human cells.

A role for cell cycle-regulated phosphorylation in Groucho-mediated transcriptional repression

Transcriptional corepressors of the Groucho/transducin-like Enhancer of split (Gro/TLE) family are involved in a variety of cell differentiation mechanisms in both invertebrates and vertebrates. They become recruited to specific promoter regions by forming complexes with a number of different DNA-binding proteins thereby contributing to the regulation of multiple genes. To understand how the functions of Gro/TLE proteins are regulated, it was asked whether their ability to mediate transcriptional repression might be controlled by cell cycle-dependent phosphorylation events. It is shown here that activation of p34(cdc2) kinase (cdc2) with okadaic acid is correlated with hyperphosphorylation of Gro/TLEs. Moreover, pharmacological inhibition of cdc2 activity results in Gro/TLE dephosphorylation. In agreement with these findings, a purified cdc2-cyclin B complex can directly phosphorylate Gro/TLEs in vitro. Two separate Gro/TLE domains, the CcN and SP regions, contain sequences that are phosphorylated by cdc2. Deletion of these sequences is correlated with loss of Gro/TLE phosphorylation by cdc2 in vitro and okadaic acid-induced Gro/TLE hyperphosphorylation in vivo. In addition, Gro/TLEs are phosphorylated during the G(2)/M phase of the cell cycle, and this is correlated with a decreased nuclear interaction. Finally, the transcription repression ability of Gro/TLEs is enhanced by pharmacological inhibition of cdc2. Taken together, these results demonstrate that Gro/TLE proteins are phosphorylated as a function of the cell cycle and implicate phosphorylation events occurring during mitosis in the negative regulation of Gro/TLE activity.

Characterization of a novel mammalian Groucho isoform and its role in transcriptional regulation

The Wnt/beta-catenin/Tcf pathway serves important functions in embryonic development and is constitutively activated in human colorectal cancer. The nuclear output of Wnt signaling is mediated by a complex between DNA-binding proteins of the TCF family and the transcriptional coactivator beta-catenin. Groucho proteins act to repress transcriptional activation by beta-catenin-Tcf complexes, probably by interacting directly with Tcf transcription factors. We have identified several splice forms of the mouse Groucho Grg1 gene expressed in the developing intestine. Prominent among these is a novel and abundant isoform, Grg1-S, which we characterize in this report. Grg1-S has highest homology with the TLE family of large Groucho proteins but features only the amino-terminal Q and glycine- and proline-rich domains typical of the Groucho/AES subfamily. Grg1-S is expressed in development and in several adult mouse tissues. Expression in the adult small intestine is highest at the base of the crypts of Lieberkuhn. Grg1-S acts to antagonize beta-catenin activity in Xenopus axis duplication and luciferase reporter assays in mammalian cells. Taken together, these findings suggest that Grg1-S may operate in conjunction with beta-catenin and Tcf factors to regulate vertebrate gut epithelial cell differentiation.

Isolation and characterization of Xenopus Hey-1: a downstream mediator of Notch signaling

Regulation of Notch signaling likely occurs, at least in part, at the level of basic helix-loop-helix (bHLH) transcription factors that function downstream of Suppressor of Hairless (Su(H)) in the Notch pathway. To begin to characterize modulation of Notch signaling during organogenesis, we examined the bHLH transcription factor, XHey-1 (hairy related-1) in early Xenopus laevis embryos. XHey-1 is expressed in numerous tissues during early development including the somites, head, embryonic kidneys, and heart. Importantly, the expression of XHey-1 was significantly altered in response to perturbation of Notch signaling by means of inducible constructs that served to either activate or suppress Notch signaling through Su(H) in a temporally controlled manner.

Anterior repression of a Drosophila stripe enhancer requires three position-specific mechanisms

The striped expression pattern of the pair-rule gene even skipped(eve) is established by five stripe-specific enhancers, each of which responds in a unique way to gradients of positional information in the earlyDrosophila embryo. The enhancer for eve stripe 2(eve 2) is directly activated by the morphogens Bicoid (Bcd) and Hunchback (Hb). As these proteins are distributed throughout the anterior half of the embryo, formation of a single stripe requires that enhancer activation is prevented in all nuclei anterior to the stripe 2 position. The gap genegiant (gt) is involved in a repression mechanism that sets the anterior stripe border, but genetic removal of gt (or deletion of Gt-binding sites) causes stripe expansion only in the anterior subregion that lies adjacent to the stripe border. We identify a well-conserved sequence repeat, (GTTT)4, which is required for repression in a more anterior subregion. This site is bound specifically by Sloppy-paired 1 (Slp1),which is expressed in a gap gene-like anterior domain. Ectopic Slp1 activity is sufficient for repression of stripe 2 of the endogenous eve gene,but is not required, suggesting that it is redundant with other anterior factors. Further genetic analysis suggests that the(GTTT)4-mediated mechanism is independent of the Gt-mediated mechanism that sets the anterior stripe border, and suggests that a third mechanism, downregulation of Bcd activity by Torso, prevents activation near the anterior tip. Thus, three distinct mechanisms are required for anterior repression of a single eve enhancer, each in a specific position. Ectopic Slp1 also represses eve stripes 1 and 3 to varying degrees,and the eve 1 and eve 3+7 enhancers each contain GTTT repeats similar to the site in the eve 2 enhancer. These results suggest a common mechanism for preventing anterior activation of three different eve enhancers.

On the organisation of the regulatory region of the zebrafish deltaD gene

deltaD is one of the four zebrafish Delta homologues presently known. Experimental evidence indicates that deltaD participates in a number of important processes during embryogenesis, including early neurogenesis and somitogenesis, whereby the protein it encodes acts as a ligand for members of the Notch receptor family. In accordance with its functional role, deltaD is transcribed in several domains of mesodermal and ectodermal origin during embryogenesis. We have analysed the organisation of the regulatory region of the deltaD gene using fusions to the reporter gene gfp and germline transgenesis. Cis-regulatory sequences are dispersed over a stretch of 12.5 kb of genomic DNA, and are organised in a similar manner to those in the regulatory region of the Delta-like 1 gene of mouse. Germline transformation using a minigene comprising 10.5 kb of this genomic DNA attached to the 3′ end of a full-length cDNA clone rescues the phenotype of embryos homozygous for the amorphic deltaD mutation after eightAR33. Several genomic regions that drive transcription in mesodermal and neuroectodermal domains have been identified. Transcription in all the neural expression domains, with one exception, is controlled by two relatively small genomic regions, which are regulated by the proneural proteins neurogenin 1 and zash1a/b acting as transcriptional activators that bind to so-called E-boxes. Transcriptional control of deltaD by proneural proteins therefore represents a molecular target for the regulatory feedback loop mediated by the Notch pathway in lateral inhibition.

Keeping a good pathway down: transcriptional repression of Notch pathway target genes by CSL proteins

CSL [CBF-1, Su(H), Lag-1]-type transcription factors are the primary effectors of the Notch pathway, a signal transduction cascade that is essential for the development of all metazoan organisms. Interestingly, CSL proteins were originally classified as transcriptional repressors in vertebrates, but as transcriptional activators in model invertebrate organisms. Resolution of this paradox came with the realization that repression and activation by CSL proteins occurs in both systems and that the switch involves recruitment of distinct co-repressor and co-activator complexes. Although CSL proteins appear to utilize a common co-activator complex of largely similar constitution, recent studies have demonstrated that vertebrate and Drosophila CSL interact with a variety of distinct co-repressor complexes. This review highlights differences in composition and similarities in function of different CSL co-repressor complexes, which actively repress Notch pathway target genes in the absence of Notch pathway activity.

Distinct in vivo requirements for establishment versus maintenance of transcriptional repression

Low-level ectopic expression of the Runt transcription factor blocks activation of the Drosophila melanogaster segmentation gene engrailed (en) in odd-numbered parasegments and is associated with a lethal phenotype. Here we show, by using a genetic screen for maternal factors that contribute in a dose-dependent fashion to Runt-mediated repression, that there are two distinct steps in the repression of en by Runt. The initial establishment of repression is sensitive to the dosage of the zinc-finger transcription factor Tramtrack. By contrast, the co-repressor proteins Groucho and dCtBP, and the histone deacetylase Rpd3, do not affect establishment but instead maintain repression after the blastoderm stage. The distinction between establishment and maintenance is confirmed by experiments with Runt derivatives that are impaired specifically for either co-repressor interaction or DNA binding. Other transcription factors can also establish repression in Rpd3-deficient embryos, which indicates that the distinction between establishment and maintenance may be a general feature of eukaryotic transcriptional repression.