Hairy function as a DNA-binding helix-loop-helix repressor of Drosophila sensory organ formation

Juvenile hormone signal transducer hairy inhibits Krüppel homolog1 expression

Hairy and Krüppel homolog 1 (Kr-h1) are transcriptional repressors that act synergistically to mediate the gene-repressive action of juvenile hormone (JH). However, whether a regulatory relationship exists between Hairy and Kr-h1 remains unclear. In this study, an inhibitory effect of Hairy on Kr-h1 expression was found. Genetic studies in Drosophila have shown that the simultaneous overexpression of Hairy and Kr-h1 can rescue the defective phenotypes caused by the overexpression of a single factor. Reduced expression of Kr-h1 was observed in Hairy-overexpressing flies and cells, whereas the expression levels of Hairy were unaffected in cells with ectopic expression of Kr-h1. The inhibitory effect of Hairy on Kr-h1 expression was found to occur at the transcriptional level, as Hairy bound directly to the B-box within the Kr-h1 promoter via the bHLH motif and recruited the corepressors C-terminal binding protein (CtBP) and Groucho (Gro) through the PLSLV and WRPW motifs, respectively. Our findings revealed a regulatory relationship between two JH response factors, which advances our understanding of the molecular mechanism of JH signaling.

Histone H3K27 methylation-mediated repression of Hairy regulates insect developmental transition by modulating ecdysone biosynthesis

Insect development is cooperatively orchestrated by the steroid hormone ecdysone and juvenile hormone (JH). The polycomb repressive complex 2 (PRC2)-mediated histone H3K27 trimethylation (H3K27me3) epigenetically silences gene transcription and is essential for a range of biological processes, but the functions of H3K27 methylation in insect hormone action are poorly understood. Here, we demonstrate that H3K27 methylation-mediated repression of Hairy transcription in the larval prothoracic gland (PG) is required for ecdysone biosynthesis in Bombyx and Drosophila H3K27me3 levels in the PG are dynamically increased during the last larval instar. H3K27me3 reduction induced by the down-regulation of PRC2 activity via inhibitor treatment in Bombyx or PG-specific knockdown of the PRC2 component Su(z)12 in Drosophila diminishes ecdysone biosynthesis and disturbs the larval-pupal transition. Mechanistically, H3K27 methylation targets the JH signal transducer Hairy to repress its transcription in the PG; PG-specific knockdown or overexpression of the Hairy gene disrupts ecdysone biosynthesis and developmental transition; and developmental defects caused by PG-specific Su(z)12 knockdown can be partially rescued by Hairy down-regulation. The application of JH mimic to the PG decreases both H3K27me3 levels and Su(z)12 expression. Altogether, our study reveals that PRC2-mediated H3K27 methylation at Hairy in the PG during the larval period is required for ecdysone biosynthesis and the larval-pupal transition and provides insights into epigenetic regulation of the crosstalk between JH and ecdysone during insect development.

Mechanisms of Binding Specificity among bHLH Transcription Factors

The transcriptome of every cell is orchestrated by the complex network of interaction between transcription factors (TFs) and their binding sites on DNA. Disruption of this network can result in many forms of organism malfunction but also can be the substrate of positive natural selection. However, understanding the specific determinants of each of these individual TF-DNA interactions is a challenging task as it requires integrating the multiple possible mechanisms by which a given TF ends up interacting with a specific genomic region. These mechanisms include DNA motif preferences, which can be determined by nucleotide sequence but also by DNA’s shape; post-translational modifications of the TF, such as phosphorylation; and dimerization partners and co-factors, which can mediate multiple forms of direct or indirect cooperative binding. Binding can also be affected by epigenetic modifications of putative target regions, including DNA methylation and nucleosome occupancy. In this review, we describe how all these mechanisms have a role and crosstalk in one specific family of TFs, the basic helix-loop-helix (bHLH), with a very conserved DNA binding domain and a similar DNA preferred motif, the E-box. Here, we compile and discuss a rich catalog of strategies used by bHLH to acquire TF-specific genome-wide landscapes of binding sites.

Pen-2 Negatively Regulates the Differentiation of Oligodendrocyte Precursor Cells into Astrocytes in the Central Nervous System

Mutations on γ-secretase subunits are associated with neurologic diseases. Whereas the role of γ-secretase in neurogenesis has been intensively studied, little is known about its role in astrogliogenesis. Recent evidence has demonstrated that astrocytes can be generated from oligodendrocyte precursor cells (OPCs). However, it is not well understood what mechanism may control OPCs to differentiate into astrocytes. To address the above questions, we generated two independent lines of oligodendrocyte lineage-specific presenilin enhancer 2 (Pen-2) conditional KO mice. Both male and female mice were used. Here we demonstrate that conditional inactivation of Pen-2 mediated by Olig1-Cre or NG2-CreERT2 causes enhanced generation of astrocytes. Lineage-tracing experiments indicate that abnormally generated astrocytes are derived from Cre-expressing OPCs in the CNS in Pen-2 conditional KO mice. Mechanistic analysis reveals that deletion of Pen-2 inhibits the Notch signaling to upregulate signal transducer and activator of transcription 3, which triggers activation of GFAP to promote astrocyte differentiation. Together, these novel findings indicate that Pen-2 regulates the specification of astrocytes from OPCs through the signal transducer and activator of transcription 3 signaling.SIGNIFICANCE STATEMENT Astrocytes and oligodendrocyte (OLs) play critical roles in the brain. Recent evidence has demonstrated that astrocytes can be generated from OL precursor cells (OPCs). However, it remains poorly understood what mechanism governs the differentiation of OPCs into astrocytes. In this study, we took advantage of OL lineage cells specific presenilin enhancer 2 (Pen-2) conditional KO mice. We show that deletion of Pen-2 leads to dramatically enhanced astrocyte differentiation from OPCs in the CNS. Mechanistic analysis reveals that deletion of Pen-2 inhibits Hes1 and activates signal transducer and activator of transcription 3 to trigger GFAP activation which promotes astrocyte differentiation. Overall, this study identifies a novel function of Pen-2 in astrogliogenesis from OPCs.

Neurogenesis From Neural Crest Cells: Molecular Mechanisms in the Formation of Cranial Nerves and Ganglia

The neural crest (NC) is a transient multipotent cell population that originates in the dorsal neural tube. Cells of the NC are highly migratory, as they travel considerable distances through the body to reach their final sites. Derivatives of the NC are neurons and glia of the peripheral nervous system (PNS) and the enteric nervous system as well as non-neural cells. Different signaling pathways triggered by Bone Morphogenetic Proteins (BMPs), Fibroblast Growth Factors (FGFs), Wnt proteins, Notch ligands, retinoic acid (RA), and Receptor Tyrosine Kinases (RTKs) participate in the processes of induction, specification, cell migration and neural differentiation of the NC. A specific set of signaling pathways and transcription factors are initially expressed in the neural plate border and then in the NC cell precursors to the formation of cranial nerves. The molecular mechanisms of control during embryonic development have been gradually elucidated, pointing to an important role of transcriptional regulators when neural differentiation occurs. However, some of these proteins have an important participation in malformations of the cranial portion and their mutation results in aberrant neurogenesis. This review aims to give an overview of the role of cell signaling and of the function of transcription factors involved in the specification of ganglia precursors and neurogenesis to form the NC-derived cranial nerves during organogenesis.

Krüppel homologue 1 interacts directly with Hairy and regulates ecdysis in the brown planthopper

Juvenile hormone (JH) plays important roles in the growth and development of insects. JH and its receptor methoprene-tolerant (Met) regulate the expression of transcription factors to control the transcription of downstream genes. The expression of Hairy (Hry) and Krüppel homologue 1 (Kr-h1) is regulated by JH and JH receptors. Hry and Kr-h1 are both crucial in mediating JH signalling. However, whether they interact at the gene level in regulating metamorphosis and whether they interact physically at the protein level remain unknown. We used co-immunoprecipitation, glutathione S-transferase pull-down and RNA interference (RNAi) approaches to study the genetic and biochemical interactions of the two proteins Hry and Kr-h1. The results showed that brown planthopper (Nilaparvata lugens) Hry and Kr-h1 interact directly: Hry binds to the N-terminal of Kr-h1, which includes five zinc-finger domains. The RNAi experiment showed that downregulation of Hry reduced the ratio of ecdysis failure caused by knockdown of Kr-h1, indicating that the downregulation of Hry might mitigate ecdysis failure via the downregulation of Kr-h1. The expression of Hry increased significantly when Kr-h1 was downregulated, whereas it did not change significantly when both were downregulated. Our results suggest that the binding of Hry protein with Kr-h1 prevents the N-terminal five zinc-finger domains from binding with DNA, which in turn inactivates the transcription activator or inhibitor function of Kr-h1. Hry could possibly be used as a target for pesticide applications in the future.

A Myt1 family transcription factor defines neuronal fate by repressing non-neuronal genes

Cellular differentiation requires both activation of target cell transcriptional programs and repression of non-target cell programs. The Myt1 family of zinc finger transcription factors contributes to fibroblast to neuron reprogramming in vitro. Here, we show that ztf-11 (Zinc-finger Transcription Factor-11), the sole Caenorhabditis elegans Myt1 homolog, is required for neurogenesis in multiple neuronal lineages from previously differentiated epithelial cells, including a neuron generated by a developmental epithelial-to-neuronal transdifferentiation event. ztf-11 is exclusively expressed in all neuronal precursors with remarkable specificity at single-cell resolution. Loss of ztf-11 leads to upregulation of non-neuronal genes and reduced neurogenesis. Ectopic expression of ztf-11 in epidermal lineages is sufficient to produce additional neurons. ZTF-11 functions together with the MuvB corepressor complex to suppress the activation of non-neuronal genes in neurons. These results dovetail with the ability of Myt1l (Myt1-like) to drive neuronal transdifferentiation in vitro in vertebrate systems. Together, we identified an evolutionarily conserved mechanism to specify neuronal cell fate by repressing non-neuronal genes.

Spatiotemporal coordination of trophoblast and allantoic Rbpj signaling directs normal placental morphogenesis

The placenta, responsible for the nutrient and gas exchange between the mother and fetus, is pivotal for successful pregnancy. It has been shown that Rbpj, the core transcriptional mediator of Notch signaling pathway, is required for normal placentation in mice. However, it remains largely unclear how Rbpj signaling in different placental compartments coordinates with other important regulators to ensure normal placental morphogenesis. In this study, we found that systemic deletion of Rbpj led to abnormal chorioallantoic morphogenesis and defective trophoblast differentiation in the ectoplacental cone (EPC). Employing mouse models with selective deletion of Rbpj in the allantois versus trophoblast, combining tetraploid aggregation assay, we demonstrated that allantois-expressed Rbpj is essential for chorioallantoic attachment and subsequent invagination of allantoic blood vessels into the chorionic ectoderm. Further studies uncovered that allantoic Rbpj regulates chorioallantoic fusion and morphogenesis via targeting Vcam1 in a Notch-dependent manner. Meanwhile, we also revealed that trophoblast-expressed Rbpj in EPC facilitates Mash2’s transcriptional activity, promoting the specification of Tpbpα-positive trophoblasts, which differentiate into trophoblast subtypes responsible for interstitial and endovascular invasion at the later stage of placental development. Collectively, our study further shed light on the molecular network governing placental development and functions, highlighting the necessity of a spatiotemporal coordination of Rbpj signaling for normal placental morphogenesis.

Spatial regulation of expanded transcription in the Drosophila wing imaginal disc

Growth and patterning are coordinated during development to define organ size and shape. The growth, proliferation and differentiation of Drosophila wings are regulated by several conserved signaling pathways. Here, we show that the Salvador-Warts-Hippo (SWH) and Notch pathways converge on an enhancer in the expanded (ex) gene, which also responds to levels of the bHLH transcription factor Daughterless (Da). Separate cis-regulatory elements respond to Salvador-Warts-Hippo (SWH) and Notch pathways, to bHLH proteins, and to unidentified factors that repress ex transcription in the wing pouch and in the proneural region at the anterior wing margin. Senseless, a zinc-finger transcription factor acting in proneural regions, had a negative impact on ex transcription in the proneural region, but the transcriptional repressor Hairy had no effect. Our study suggests that a complex pattern of ex transcription results from integration of a uniform SWH signal with multiple other inputs, rather than from a pattern of SWH signaling.

Rbpj direct regulation of Atoh7 transcription in the embryonic mouse retina

In vertebrate retinal progenitor cells, the proneural factor Atoh7 exhibits a dynamic tissue and cellular expression pattern. Although the resulting Atoh7 retinal lineage contains all seven major cell types, only retinal ganglion cells require Atoh7 for proper differentiation. Such specificity necessitates complex regulation of Atoh7 transcription during retina development. The Notch signaling pathway is an evolutionarily conserved suppressor of proneural bHLH factor expression. Previous in vivo mouse genetic studies established the cell autonomous suppression of Atoh7 transcription by Notch1, Rbpj and Hes1. Here we identify four CSL binding sites within the Atoh7 proximal regulatory region and demonstrate Rbpj protein interaction at these sequences by in vitro electromobility shift, calorimetry and luciferase assays and, in vivo via colocalization and chromatin immunoprecipitation. We found that Rbpj simultaneously represses Atoh7 transcription using both Notch-dependent and –independent pathways.

Synergistic Binding of bHLH Transcription Factors to the Promoter of the Maize NADP-ME Gene Used in C4 Photosynthesis Is Based on an Ancient Code Found in the Ancestral C3 State

C4 photosynthesis has evolved repeatedly from the ancestral C3 state to generate a carbon concentrating mechanism that increases photosynthetic efficiency. This specialized form of photosynthesis is particularly common in the PACMAD clade of grasses, and is used by many of the world's most productive crops. The C4 cycle is accomplished through cell-type-specific accumulation of enzymes but cis-elements and transcription factors controlling C4 photosynthesis remain largely unknown. Using the NADP-Malic Enzyme (NADP-ME) gene as a model we tested whether mechanisms impacting on transcription in C4 plants evolved from ancestral components found in C3 species. Two basic Helix-Loop-Helix (bHLH) transcription factors, ZmbHLH128 and ZmbHLH129, were shown to bind the C4NADP-ME promoter from maize. These proteins form heterodimers and ZmbHLH129 impairs trans-activation by ZmbHLH128. Electrophoretic mobility shift assays indicate that a pair of cis-elements separated by a seven base pair spacer synergistically bind either ZmbHLH128 or ZmbHLH129. This pair of cis-elements is found in both C3 and C4 Panicoid grass species of the PACMAD clade. Our analysis is consistent with this cis-element pair originating from a single motif present in the ancestral C3 state. We conclude that C4 photosynthesis has co-opted an ancient C3 regulatory code built on G-box recognition by bHLH to regulate the NADP-ME gene. More broadly, our findings also contribute to the understanding of gene regulatory networks controlling C4 photosynthesis.

bHLH transcription factors in neural development, disease, and reprogramming

The formation of functional neural circuits in the vertebrate central nervous system (CNS) requires that appropriate numbers of the correct types of neuronal and glial cells are generated in their proper places and times during development. In the embryonic CNS, multipotent progenitor cells first acquire regional identities, and then undergo precisely choreographed temporal identity transitions (i.e. time-dependent changes in their identity) that determine how many neuronal and glial cells of each type they will generate. Transcription factors of the basic-helix-loop-helix (bHLH) family have emerged as key determinants of neural cell fate specification and differentiation, ensuring that appropriate numbers of specific neuronal and glial cell types are produced. Recent studies have further revealed that the functions of these bHLH factors are strictly regulated. Given their essential developmental roles, it is not surprising that bHLH mutations and de-regulated expression are associated with various neurological diseases and cancers. Moreover, the powerful ability of bHLH factors to direct neuronal and glial cell fate specification and differentiation has been exploited in the relatively new field of cellular reprogramming, in which pluripotent stem cells or somatic stem cells are converted to neural lineages, often with a transcription factor-based lineage conversion strategy that includes one or more of the bHLH genes. These concepts are reviewed herein.

Reciprocal signaling by Wnt and Notch specifies a muscle precursor in the C. elegans embryo

The MS blastomere produces one-third of the body wall muscles (BWMs) in the C. elegans embryo. MS-derived BWMs require two distinct cell-cell interactions, the first inhibitory and the second, two cell cycles later, required to overcome this inhibition. The inductive interaction is not required if the inhibitory signal is absent. Although the Notch receptor GLP-1 was implicated in both interactions, the molecular nature of the two signals was unknown. We now show that zygotically expressed MOM-2 (Wnt) is responsible for both interactions. Both the inhibitory and the activating interactions require precise spatiotemporal expression of zygotic MOM-2, which is dependent upon two distinct Notch signals. In a Notch mutant defective only in the inductive interaction, MS-derived BWMs can be restored by preventing zygotic MOM-2 expression, which removes the inhibitory signal. Our results suggest that the inhibitory interaction ensures the differential lineage specification of MS and its sister blastomere, whereas the inductive interaction promotes the expression of muscle-specifying genes by modulating TCF and β-catenin levels. These results highlight the complexity of cell fate specification by cell-cell interactions in a rapidly dividing embryo.

Hes1: the maestro in neurogenesis

The process of neurogenesis is well orchestrated by the harmony of multiple cues in a spatiotemporal manner. In this review, we focus on how a dynamic gene, Hes1, is involved in neurogenesis with the view of its regulation and functional implications. Initially, we have reviewed the immense functional significance drawn by this maestro during neural development in a context-dependent manner. How this indispensable role of Hes1 in conferring the competency for neural differentiation partly relies on the direct/indirect mode of repression mediated by very specific structural and functional arms of this protein has also been outlined here. We also review the detailed molecular mechanisms behind the well-tuned oscillatory versus sustained expression of this antineurogenic bHLH repressor, which indeed makes it a master gene to implement the elusive task of neural progenitor propensity. Apart from the functional aspects of Hes1, we also discuss the molecular insights into the endogenous regulatory machinery that regulates its expression. Though Hes1 is a classical target of the Notch signaling pathway, we discuss here its differential expression at the molecular, cellular, and/or regional level. Moreover, we describe how its expression is fine-tuned by all possible ways of gene regulation such as epigenetic, transcriptional, post-transcriptional, post-translational, and environmental factors during vertebrate neurogenesis.

Hairy and Groucho mediate the action of juvenile hormone receptor Methoprene-tolerant in gene repression

The arthropod-specific juvenile hormone (JH) controls numerous essential functions. Its involvement in gene activation is known to be mediated by the transcription factor Methoprene-tolerant (Met), which turns on JH-controlled genes by directly binding to E-box-like motifs in their regulatory regions. However, it remains unclear how JH represses genes. We used the Aedes aegypti female mosquito, in which JH is necessary for reproductive maturation, to show that a repressor, Hairy, is required for the gene-repressive action of JH and Met. The RNA interference (RNAi) screen for Met and Hairy in the Aedes female fat body revealed a large cohort of Met- and Hairy-corepressed genes. Analysis of selected genes from this cohort demonstrated that they are repressed by JH, but RNAi of either Met or Hairy renders JH ineffective in repressing these genes in an in vitro fat-body culture assay. Moreover, this JH action was prevented by the addition of the translational inhibitor cycloheximide (CHX) to the culture, indicating the existence of an indirect regulatory hierarchy. The lack of Hairy protein in the CHX-treated tissue was verified using immunoblot analysis, and the upstream regions of Met/Hairy-corepressed genes were shown to contain common binding motifs that interact with Hairy. Groucho (gro) RNAi silencing phenocopied the effect of Hairy RNAi knockdown, indicating that it is involved in the JH/Met/Hairy hierarchy. Finally, the requirement of Hairy and Gro for gene repression was confirmed in a cell transfection assay. Thus, our study has established that Hairy and its cofactor Gro mediate the repressive function of JH and Met.

Regulation of Stem Cell Self-Renewal and Oncogenesis by RNA-Binding Proteins

Throughout their life span, multicellular organisms rely on stem cell systems. During development pluripotent embryonic stem cells give rise to all cell types that make up the organism. After birth, tissue stem cells maintain properly functioning tissues and organs under homeostasis as well as promote regeneration after tissue damage or injury. Stem cells are capable of self-renewal, which is the ability to divide indefinitely while retaining the potential of differentiation into multiple cell types. The ability to self-renew, however, is a double-edged sword; the molecular mechanisms of self-renewal can be a target of malignant transformation driving tumor development and progression. Growing lines of evidence have shown that RNA-binding proteins (RBPs) play pivotal roles in the regulation of self-renewal by modulating metabolism of coding and non-coding RNAs both in normal tissues and in cancers. In this review, we discuss our current understanding of tissue stem cell systems and how RBPs regulate stem cell fates as well as how the regulatory functions of RBPs contribute to oncogenesis.

Rice phytochrome-interacting factor protein OsPIF14 represses OsDREB1B gene expression through an extended N-box and interacts preferentially with the active form of phytochrome B

DREB1/CBF genes, known as major regulators of plant stress responses, are rapidly and transiently induced by low temperatures. Using a yeast one-hybrid screening, we identified a putative Phytochrome-Interacting bHLH Factor (OsPIF14), as binding to the OsDREB1B promoter. bHLH proteins are able to bind to hexameric E-box (CANNTG) or N-box (CACG(A/C)G) motifs, depending on transcriptional activity. We have shown that OsPIF14 binds to the OsDREB1B promoter through two N-boxes and that the flanking regions of the hexameric core are essential for protein-DNA interaction and stability. We also showed that OsPIF14 down-regulates OsDREB1B gene expression in rice protoplasts, corroborating the OsPIF14 repressor activity observed in the transactivation assays using Arabidopsis protoplasts. In addition, we showed that OsPIF14 is indeed a phytochrome interacting factor, which preferentially binds to the active form (Pfr) of rice phytochrome B. This raises the possibility that OsPIF14 activity might be modulated by light. However, we did not observe any regulation of the OsDREB1B gene expression by light under control conditions. Moreover, OsPIF14 gene expression was shown to be modulated by different treatments, such as drought, salt, cold and ABA. Interestingly, OsPIF14 showed also a specific cold-induced alternative splicing. All together, these results suggest the possibility that OsPIF14 is involved in cross-talk between light and stress signaling through interaction with the OsDREB1B promoter. Although in the absence of stress, OsDREB1B gene expression was not regulated by light, given previous reports, it remains possible that OsPIF14 has a role in light modulation of stress responses.

The achaete-scute complex in Diptera: patterns of noncoding sequence evolution

The achaete‐scute complex (AS‐C) has been a useful paradigm for the study of pattern formation and its evolution. achaete‐scute genes have duplicated and evolved distinct expression patterns during the evolution of cyclorraphous Diptera. Are the expression patterns in different species driven by conserved regulatory elements? If so, when did such regulatory elements arise? Here, we have sequenced most of the AS‐C of the fly Calliphora vicina (including the genes achaete, scute and lethal of scute) to compare noncoding sequences with known cis‐regulatory sequences in Drosophila. The organization of the complex is conserved with respect to Drosophila species. There are numerous small stretches of conserved noncoding sequence that, in spite of high sequence turnover, display binding sites for known transcription factors. Synteny of the blocks of conserved noncoding sequences is maintained suggesting not only conservation of the position of regulatory elements but also an origin prior to the divergence between these two species. We propose that some of these enhancers originated by duplication with their target genes.

NLR-associating transcription factor bHLH84 and its paralogs function redundantly in plant immunity

In plants and animals, nucleotide-binding and leucine-rich repeat domain containing (NLR) immune receptors are utilized to detect the presence or activities of pathogen-derived molecules. However, the mechanisms by which NLR proteins induce defense responses remain unclear. Here, we report the characterization of one basic Helix-loop-Helix (bHLH) type transcription factor (TF), bHLH84, identified from a reverse genetic screen. It functions as a transcriptional activator that enhances the autoimmunity of NLR mutant snc1 (suppressor of npr1-1, constitutive 1) and confers enhanced immunity in wild-type backgrounds when overexpressed. Simultaneously knocking out three closely related bHLH paralogs attenuates RPS4-mediated immunity and partially suppresses the autoimmune phenotypes of snc1, while overexpression of the other two close paralogs also renders strong autoimmunity, suggesting functional redundancy in the gene family. Intriguingly, the autoimmunity conferred by bHLH84 overexpression can be largely suppressed by the loss-of-function snc1-r1 mutation, suggesting that SNC1 is required for its proper function. In planta co-immunoprecipitation revealed interactions between not only bHLH84 and SNC1, but also bHLH84 and RPS4, indicating that bHLH84 associates with these NLRs. Together with previous finding that SNC1 associates with repressor TPR1 to repress negative regulators, we hypothesize that nuclear NLR proteins may interact with both transcriptional repressors and activators during immune responses, enabling potentially faster and more robust transcriptional reprogramming upon pathogen recognition.

In plants and animals, NLR immune receptors are utilized to detect pathogen-derived molecules and activate immunity. However, the mechanisms of plant NLR activation remain unclear. Here, we report on bHLH84, which functions as a transcriptional activator. Simultaneously knocking out three closely related bHLH paralogs partially suppresses the autoimmunity of snc1 and compromises RPS4-mediated defense, while overexpression of these close paralogs renders strong autoimmunity, suggesting functional redundancy in the gene family. In planta co-immunoprecipitation revealed interactions between not only bHLH84 and SNC1, but also bHLH84 and RPS4. Therefore bHLH84 family transcription factors associate with these NLRs to activate defense responses, enabling potentially faster and more robust transcriptional reprogramming upon pathogen recognition.

Hes-1 SUMOylation by protein inhibitor of activated STAT1 enhances the suppressing effect of Hes-1 on GADD45α expression to increase cell survival

Background

Hairy and Enhancer of split 1 (Hes-1) is a transcriptional repressor that plays an important role in neuronal differentiation and development, but post-translational modifications of Hes-1 are much less known. In the present study, we aimed to investigate whether Hes-1 could be SUMO-modified and identify the candidate SUMO acceptors on Hes-1. We also wished to examine the role of the SUMO E3 ligase protein inhibitor of activated STAT1 (PIAS1) in SUMOylation of Hes-1 and the molecular mechanism of Hes-1 SUMOylation. Further, we aimed to identify the molecular target of Hes-1 and examine how Hes-1 SUMOylation affects its molecular target to affect cell survival.

Results

In this study, by using HEK293T cells, we have found that Hes-1 could be SUMO-modified and Hes-1 SUMOylation was greatly enhanced by the SUMO E3 ligase PIAS1 at Lys8, Lys27 and Lys39. Furthermore, Hes-1 SUMOylation stabilized the Hes-1 protein and increased the transcriptional suppressing activity of Hes-1 on growth arrest and DNA damage-inducible protein alpha (GADD45α) expression. Overexpression of GADD45α increased, whereas knockdown of GADD45αα expression decreased cell apoptosis. In addition, H₂O₂ treatment increased the association between PIAS1 and Hes-1 and enhanced the SUMOylation of Hes-1 for endogenous protection. Overexpression of Hes-1 decreased H₂O₂-induced cell death, but this effect was blocked by transfection of the Hes-1 triple sumo-mutant (Hes-1 3KR). Overexpression of PIAS1 further facilitated the anti-apoptotic effect of Hes-1. Moreover, Hes-1 SUMOylation was independent of Hes-1 phosphorylation and vice versa.

Conclusions

The present results revealed, for the first time, that Hes-1 could be SUMO-modified by PIAS1 and GADD45α is a novel target of Hes-1. Further, Hes-1 SUMOylation mediates cell survival through enhanced suppression of GADD45α expression. These results revealed a novel role of Hes-1 in addition to its involvement in Notch signaling. They also implicate that SUMOylation could be an important posttranslational modification that regulates cell survival.

The emerging roles of TCF4 in disease and development

Genome-wide association studies have identified common variants in transcription factor 4 (TCF4) as susceptibility loci for schizophrenia, Fuchs' endothelial corneal dystrophy, and primary sclerosing cholangitis. By contrast, rare TCF4 mutations cause Pitt-Hopkins syndrome, a disorder characterized by intellectual disability and developmental delay, and have also been described in patients with other neurodevelopmental disorders. TCF4 therefore sits at the nexus between common and rare disorders. TCF4 interacts with other basic helix-loop-helix proteins, forming transcriptional networks that regulate the differentiation of several distinct cell types. Here, we review the role of TCF4 in these seemingly diverse disorders and discuss recent data implicating TCF4 as an important regulator of neurodevelopment and epithelial-mesenchymal transition.

miR-9a minimizes the phenotypic impact of genomic diversity by buffering a transcription factor

Gene expression has to withstand stochastic, environmental, and genomic perturbations. For example, in the latter case, 0.5%-1% of the human genome is typically variable between any two unrelated individuals. Such diversity might create problematic variability in the activity of gene regulatory networks and, ultimately, in cell behaviors. Using multigenerational selection experiments, we find that for the Drosophila proneural network, the effect of genomic diversity is dampened by miR-9a-mediated regulation of senseless expression. Reducing miR-9a regulation of the Senseless transcription factor frees the genomic landscape to exert greater phenotypic influence. Whole-genome sequencing identified genomic loci that potentially exert such effects. A larger set of sequence variants, including variants within proneural network genes, exhibits these characteristics when miR-9a concentration is reduced. These findings reveal that microRNA-target interactions may be a key mechanism by which the impact of genomic diversity on cell behavior is dampened.

The bristle patterning genes hairy and extramacrochaetae regulate the development of structures required for flight in Diptera

The distribution of sensory bristles on the thorax of Diptera (true flies) provides a useful model for the study of the evolution of spatial patterns. Large bristles called macrochaetes are arranged into species-specific stereotypical patterns determined via spatially discrete expression of the proneural genes achaete–scute (ac–sc). In Drosophila ac-sc expression is regulated by transcriptional activation at sites where bristle precursors develop and by repression outside of these sites. Three genes, extramacrochaetae (emc), hairy (h) and stripe (sr), involved in repression have been documented. Here we demonstrate that in Drosophila, the repressor genes emc and h, like sr, play an essential role in the development of structures forming part of the flight apparatus. In addition we find that, in Calliphora vicina a species diverged from D. melanogaster by about 100 Myr, spatial expression of emc, h and sr is conserved at the location of development of those structures. Based on these findings we argue, first, that the role emc, h and sr in development of the flight apparatus preceded their activities for macrochaete patterning; second, that species-specific variation in activation and repression of ac-sc expression is evolving in parallel to establish a unique distribution of macrochaetes in each species.

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The distribution of sensory bristles is a useful model to study spatial patterns.

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In Drosophila melanogaster the genes emc, h and sr repress bristle formation.

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In D. melanogaster emc and h are essential for flight apparatus development.

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Notably, in Calliphora vicina emc, h and sr are expressed in the flight apparatus.

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We argue that emc, h and sr had an early role in flight apparatus development.

Proneural genes in neocortical development

Neurons, astrocytes and oligodendrocytes arise from CNS progenitor cells at defined times and locations during development, with transcription factors serving as key determinants of these different neural cell fates. An emerging theme is that the transcription factors that specify CNS cell fates function in a context-dependent manner, regulated by post-translational modifications and epigenetic alterations that partition the genome (and hence target genes) into active or silent domains. Here we profile the critical roles of the proneural genes, which encode basic-helix-loop-helix (bHLH) transcription factors, in specifying neural cell identities in the developing neocortex. In particular, we focus on the proneural genes Neurogenin 1 (Neurog1), Neurog2 and Achaete scute-like 1 (Ascl1), which are each expressed in a distinct fashion in the progenitor cell pools that give rise to all of the neuronal and glial cell types of the mature neocortex. Notably, while the basic functions of these proneural genes have been elucidated, it is becoming increasingly evident that tight regulatory controls dictate when, where and how they function. Current efforts to better understand how proneural gene function is regulated will not only improve our understanding of neocortical development, but are also critical to the future development of regenerative therapies for the treatment of neuronal degeneration or disease.

The Drosophila T-box transcription factor Midline functions within the Notch-Delta signaling pathway to specify sensory organ precursor cell fates and regulates cell survival within the eye imaginal disc

We report that the T-box transcription factor Midline (Mid), an evolutionary conserved homolog of the vertebrate Tbx20 protein, functions within the Notch-Delta signaling pathway essential for specifying the fates of sensory organ precursor (SOP) cells. These findings complement an established history of research showing that Mid regulates the cell-fate specification of diverse cell types within the developing heart, epidermis and central nervous system. Tbx20 has been detected in unique neuronal and epithelial cells of embryonic eye tissues in both mice and humans. However, the mechanisms by which either Mid or Tbx20 function to regulate cell-fate specification or other critical aspects of eye development including cell survival have not yet been elucidated. We have also gathered preliminary evidence suggesting that Mid may play an indirect, but vital role in selecting SOP cells within the third-instar larval eye disc by regulating the expression of the proneural gene atonal. During subsequent pupal stages, Mid specifies SOP cell fates as a member of the Notch-Delta signaling hierarchy and is essential for maintaining cell viability by inhibiting apoptotic pathways. We present several new hypotheses that seek to understand the role of Mid in regulating developmental processes downstream of the Notch receptor that are critical for specifying unique cell fates, patterning the adult eye and maintaining cellular homeostasis during eye disc morphogenesis.

Patterning and cell fate in the inner ear: a case for Notch in the chicken embryo

The development of the inner ear provides a beautiful example of one basic problem in development, that is, to understand how different cell types are generated at specific times and domains throughout embryonic life. The functional unit of the inner ear consists of hair cells, supporting cells and neurons, all deriving from progenitor cells located in the neurosensory competent domain of the otic placode. Throughout development, the otic placode resolves into the complex inner ear labyrinth, which holds the auditory and vestibular sensory organs that are innervated in a highly specific manner. How does the early competent domain of the otic placode give rise to the diverse specialized cell types of the different sensory organs of the inner ear? We review here our current understanding on the role of Notch signaling in coupling patterning and cell fate determination during inner ear development, with a particular emphasis on contributions from the chicken embryo as a model organism. We discuss further the question of how these two processes rely on two modes of operation of the Notch signaling pathway named lateral induction and lateral inhibition.

Topology and dynamics of the zebrafish segmentation clock core circuit

By combining biochemical, embryological, and mathematical approaches, this work uncovers an important role for protein-protein interactions in determining the dynamics of the somite-forming segmentation clock in vertebrates.

During vertebrate embryogenesis, the rhythmic and sequential segmentation of the body axis is regulated by an oscillating genetic network termed the segmentation clock. We describe a new dynamic model for the core pace-making circuit of the zebrafish segmentation clock based on a systematic biochemical investigation of the network's topology and precise measurements of somitogenesis dynamics in novel genetic mutants. We show that the core pace-making circuit consists of two distinct negative feedback loops, one with Her1 homodimers and the other with Her7:Hes6 heterodimers, operating in parallel. To explain the observed single and double mutant phenotypes of her1, her7, and hes6 mutant embryos in our dynamic model, we postulate that the availability and effective stability of the dimers with DNA binding activity is controlled in a “dimer cloud” that contains all possible dimeric combinations between the three factors. This feature of our model predicts that Hes6 protein levels should oscillate despite constant hes6 mRNA production, which we confirm experimentally using novel Hes6 antibodies. The control of the circuit's dynamics by a population of dimers with and without DNA binding activity is a new principle for the segmentation clock and may be relevant to other biological clocks and transcriptional regulatory networks.

The segmented pattern of the vertebral column, one of the defining features of the vertebrate body, is established during embryogenesis. The embryo's segments, called somites, form sequentially and rhythmically from head to tail. The periodicity of somite formation is regulated by the segmentation clock, a genetic oscillator that ticks in the posterior-most embryonic tissue: for each tick of the clock, one new bilateral pair of segments is made. The period of the clock appears to determine the number and the length of segments, but what controls this periodicity? In this article, we have investigated the interactions of three transcription factors that form the core of the clock's regulatory circuit, and have measured how the period of segmentation changes when these factors are mutated alone or in combination. We find that these three factors contribute to a “dimer cloud” that contains all possible dimeric combinations; however, only two dimers in this cloud can bind DNA, which allows them to directly regulate the oscillatory gene expression that underpins the periodicity of segment formation. Nevertheless, a mathematical model of the clock's dynamics based on our experimental findings indicates that the non-DNA-binding dimers also influence the stability, and hence the function, of the two DNA-binding dimers controlling the segmentation clock's period. Such involvement of non-DNA-binding dimers is a novel regulatory principle for the segmentation clock, which might also be a general mechanism that operates in other biological clocks.

Robust specification of sensory neurons by dual functions of charlatan, a Drosophila NRSF/REST-like repressor of extramacrochaetae and hairy

Sensory bristle formation in Drosophila is a well-characterized system for studying sensory organ development at the molecular level. The master proneural genes of the achaete-scute (ac-sc) complex, which encode basic-helix-loop-helix (bHLH) transcription factors, are necessary and sufficient for sensory bristle formation. charlatan (chn) was originally identified as a transcriptional activator of ac-sc gene expression through interaction with its enhancer, an activity that promotes sensory bristle development. In contrast, Chn was also identified as a functional homologue of mammalian neuron-restrictive silencing factor or RE1 silencing transcription factor (NRSF/REST), an important transcriptional repressor during vertebrate neurogenesis and stem cell development that acts through epigenetic gene silencing. Here, we report that Chn acts as a repressor of extramacrochaetae (emc) and hairy, molecules that inhibit ac-sc expression. This double-negative mechanism, together with direct activation via the achaete enhancer, increases expression of achaete and ensures robust development of sensory neurons. A mutation in the C-terminal repressor motif of Chn, which causes Chn to lose its repression activity, converted Chn to an activator of emc and hairy, suggesting that Chn is a dual functional regulator of transcription. Because chn-like sequences are found among arthropods, regulation of neuronal development by Chn-like molecules may be widely conserved.

The Drosophila STUbL protein Degringolade limits HES functions during embryogenesis

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

The role of the Notch pathway in healthy and osteoarthritic articular cartilage: from experimental models to ex vivo studies

Osteoarthritis is the most prevalent form of arthritis in the world. With the progressive ageing of the population, it is becoming a major public health problem. The involvement of certain signaling pathways, such as the Notch pathway, during cartilage pathology has been reported. In this review, we report on studies that investigated the expression pattern of the Notch family members in articular cartilage and the eventual involvement of this pathway in the modulation of the physiology and pathology of chondrocytes. Temporal and/or spatial modulation of this signaling pathway may help these cells to synthesize a new functional extracellular matrix and restore the functional properties of the articular cartilage.

The bHLH factor deadpan is a direct target of Notch signaling and regulates neuroblast self-renewal in Drosophila

A defining feature of stem cells is their capacity to renew themselves at each division while producing differentiated progeny. How these cells balance self-renewal versus differentiation is a fundamental issue in developmental and cancer biology. The Notch signaling pathway has long been known to influence cell fate decisions during development. Indeed, there is a great deal of evidence correlating its function with the regulation of neuroblast (NB) self-renewal during larval brain development in Drosophila. However, little is known about the transcription factors regulated by this pathway during this process. Here we show that deadpan (dpn), a gene encoding a bHLH transcription factor, is a direct target of the Notch signaling pathway during type II NB development. Type II NBs undergo repeated asymmetric divisions to self-renew and to produce immature intermediate neural progenitors. These cells mature into intermediate neural progenitors (INPs) that have the capacity to undergo multiple rounds of asymmetric division to self-renew and to generate GMCs and neurons. Our results indicate that the expression of dpn at least in INPs cells depends on Notch signaling. The ectopic expression of dpn in immature INP cells can transform these cells into NBs-like cells that divide uncontrollably causing tumor over-growth. We show that in addition to dpn, Notch signaling must be regulating other genes during this process that act redundantly with dpn.

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.

Implication of HpEts in gene regulatory networks responsible for specification of sea urchin skeletogenic primary mesenchyme cells

The large micromeres of the 32-cell stage of sea urchin embryos are autonomously specified and differentiate into primary mesenchyme cells (PMCs), giving rise to the skeletogenic cells. We previously demonstrated that HpEts, an ets-related transcription factor, plays an essential role in the specification of PMCs in sea urchin embryos. In order to clarify the function of HpEts in the gene regulatory network involved in PMC specification, we analyzed the zygotic expression pattern and the cis-regulatory region of HpEts, and examined the activity of the HpEts protein as a transcription factor. Intron-based PCR reveals that zygotic expression of HpEts starts at the cleavage stage, and that the rate of transcription reaches maximum at the unhatched blastula stage. A series of progressive deletions of the fragments from -4.2 kbp to +1206 bp of the HpEts, which directs PMC-specific expression, caused a gradual decrease in the specificity, implying that coordination of several cis-regulatory elements regulates the expression in PMCs. A minimum cis-element required for the temporal expression is located within a 10 bp from -243 bp to -234 bp. The HpEts protein remains in the cytoplasm of entire embryonic cells in the cleavage stage. At the unhatched blastula stage, the HpEts protein translocates into the nucleus in presumptive PMCs. Transactivation assays demonstrate that the HpEts protein activates a promoter of Spicule Matrix Protein 50 (SM50), which is a target of HpEts, which binds to the regulatory region of SM50.

An arthropod cis-regulatory element functioning in sensory organ precursor development dates back to the Cambrian

Background

An increasing number of publications demonstrate conservation of function of cis-regulatory elements without sequence similarity. In invertebrates such functional conservation has only been shown for closely related species. Here we demonstrate the existence of an ancient arthropod regulatory element that functions during the selection of neural precursors. The activity of genes of the achaete-scute (ac-sc) family endows cells with neural potential. An essential, conserved characteristic of proneural genes is their ability to restrict their own activity to single or a small number of progenitor cells from their initially broad domains of expression. This is achieved through a process called lateral inhibition. A regulatory element, the sensory organ precursor enhancer (SOPE), is required for this process. First identified in Drosophila, the SOPE contains discrete binding sites for four regulatory factors. The SOPE of the Drosophila asense gene is situated in the 5' UTR.

Results

Through a manual comparison of consensus binding site sequences we have been able to identify a SOPE in UTR sequences of asense-like genes in species belonging to all four arthropod groups (Crustacea, Myriapoda, Chelicerata and Insecta). The SOPEs of the spider Cupiennius salei and the insect Tribolium castaneum are shown to be functional in transgenic Drosophila. This would place the origin of this regulatory sequence as far back as the last common ancestor of the Arthropoda, that is, in the Cambrian, 550 million years ago.

Conclusions

The SOPE is not detectable by inter-specific sequence comparison, raising the possibility that other ancient regulatory modules in invertebrates might have escaped detection.

Regulation of tissue-specific expression of SPATULA, a bHLH gene involved in carpel development, seedling germination, and lateral organ growth in Arabidopsis

SPATULA is a bHLH transcription factor that promotes growth of tissues arising from the carpel margins, including the septum and transmitting tract. It is also involved in repressing germination of newly harvested seeds, and in inhibiting cotyledon, leaf, and petal expansion. Using a reporter gene construct, its expression profile was fully defined. Consistent with its known functions, SPT was expressed in developing carpel margin tissues, and in the hypocotyls and cotyledons of germinating seedlings, and in developing leaves and petals. It was also strongly expressed in tissues where no functions have been identified to date, including the dehiscence zone of fruits, developing anthers, embryos, and in the epidermal initials and new stele of root tips. The promoter region of SPT was dissected by truncation and deletion, and two main regions occupied by tissue-specific enhancers were identified. These were correlated with eight regions conserved between promoter regions of Arabidopsis, Brassica oleracea, and Brassica rapa. When transformed into Arabidopsis, the B. oleracea promoter drove expression in reproductive tissues mostly comparable to the equivalent Arabidopsis promoter. There is genetic evidence that SPT function in the gynoecium is associated with the perception of auxin. However, site-directed mutagenesis of three putative auxin-response elements had no detectable effect on SPT expression patterns. Even so, disruption of a putative E-box variant adjacent to one of these resulted in a loss of valve dehiscence zone expression. This expression was also specifically lost in mutants of another bHLH gene INDEHISCENT, indicating that IND may directly regulate SPT expression through this variant E-box.

The complex tale of the achaete-scute complex: a paradigmatic case in the analysis of gene organization and function during development

The achaete-scute gene complex (AS-C) contains four genes encoding transcription factors of the bHLH family, achaete, scute, lethal of scute, and asense located in 40 kb of DNA containing multiple cis-regulatory position-specific enhancers. These genes play a key role in the commitment of epidermal cells toward a neural fate, promoting the formation of both sensory organs in the peripheral nervous system (bristles) of the adult and of neuroblasts in the central nervous system of the embryo. The analysis of the AS-C initially focused on the variations in positional specificity of effects of achaete (ac) and scute (sc) alleles on macrochaete bristle pattern in the Drosophila adult epidermis, and from there it evolved as a key entry point into understanding the molecular bases of pattern formation and cell commitment. In this perspective, we describe how the study of the AS-C has contributed to the understanding of eukaryotic gene organization and the dissection of the developmental mechanisms underlying pattern formation.

A screen for modifiers of notch signaling uncovers Amun, a protein with a critical role in sensory organ development

Notch signaling is an evolutionarily conserved pathway essential for many cell fate specification events during metazoan development. We conducted a large-scale transposon-based screen in the developing Drosophila eye to identify genes involved in Notch signaling. We screened 10,447 transposon lines from the Exelixis collection for modifiers of cell fate alterations caused by overexpression of the Notch ligand Delta and identified 170 distinct modifier lines that may affect up to 274 genes. These include genes known to function in Notch signaling, as well as a large group of characterized and uncharacterized genes that have not been implicated in Notch pathway function. We further analyze a gene that we have named Amun and show that it encodes a protein that localizes to the nucleus and contains a putative DNA glycosylase domain. Genetic and molecular analyses of Amun show that altered levels of Amun function interfere with cell fate specification during eye and sensory organ development. Overexpression of Amun decreases expression of the proneural transcription factor Achaete, and sensory organ loss caused by Amun overexpression can be rescued by coexpression of Achaete. Taken together, our data suggest that Amun acts as a transcriptional regulator that can affect cell fate specification by controlling Achaete levels.

Molecular cloning and characterization of homologs of achaete-scute and hairy-enhancer of split in the olfactory organ of the spiny lobster Panulirus argus

The olfactory organ of the Caribbean spiny lobster Panulirus argus maintains lifelong proliferation and turnover of olfactory receptor neurons (ORNs). Towards examining the molecular basis of this adult neurogenesis, we search for expression of homologs of proneural, neurogenic, and pre-pattern genes in this olfactory organ. We report here a homolog of the proneural Achaete-Scute family, called splash (spiny lobster achaete-scute homolog), and a homolog of the pre-pattern and neurogenic hairy-enhancer of split family, called splhairy (spiny lobster hairy). Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) indicates a molt stage dependence of the levels of expression of splash and splhairy mRNA in the olfactory organ, with higher expression in premolt than in postmolt or intermolt animals, which is positively correlated with rates of neurogenesis. splash and splhairy mRNA are expressed not only in the olfactory organ but also in other tissues, albeit at lower levels, irrespective of molt stage. We conclude that the expression of achaete-scute and hairy-enhancer of split in the proliferation zone of the olfactory organ of spiny lobsters and their enhanced expression in premolt animals suggest that they play a role in the proliferation of ORNs and that their expression in regions of the olfactory organ populated by mature ORNs and in other tissues suggests that they have additional functions.

Redundant mechanisms mediate bristle patterning on the Drosophila thorax

The thoracic bristle pattern of Drosophila results from the spatially restricted expression of the achaete-scute (ac-sc) genes in clusters of cells, mediated by the activity of many discrete cis-regulatory sequences. However, ubiquitous expression of sc or asense (ase) achieved with a heterologous promoter, in the absence of endogenous ac-sc expression, and the activity of the cis-regulatory elements, allows the development of bristles positioned at wild-type locations. We demonstrate that the products of the genes stripe, hairy, and extramacrochaetae contribute to rescue by antagonizing the activity of Sc and Ase. The three genes are expressed in specific but overlapping spatial domains of expression that form a prepattern that allows precise positioning of bristles. The redundant mechanisms might contribute to the robustness of the pattern. We discuss the possibility that patterning in trans by antagonism is ancestral and that the positional cis-regulatory sequences might be of recent origin.

Premature differentiation and aberrant movement of pituitary cells lacking both Hes1 and Prop1

In the pituitary, the transition from proliferating progenitor cell into differentiated hormone producing cell is carefully regulated in a time-dependent and spatially-restricted manner. We report that two targets of Notch signaling, Hes1 and Prop1, are needed to maintain progenitors within Rathke's pouch and for the restriction of differentiated cells to the ventral pituitary. We observed ACTH and alphaGSU producing cells that had prematurely differentiated within Rathke's pouch along with correlated ectopic expression of Mash1 only when both Prop1 and Hes1 were lost. We also discovered that downregulation of N-cadherin expression in cells as they transition from Rathke's pouch to the anterior lobe appears to be essential for their movement. In the Prop1 mutant, cells are trapped in Rathke's pouch and N-cadherin expression remains high. Also, Slug, a marker of epithelial-to-mesenchymal transition, is absent in the dorsal anterior lobe. When Hes1 is lost in the Prop1 mutant, N-cadherin is downregulated and cells are able to exit Rathke's pouch but have lost their migrational cues and form ectopic foci surrounding Rathke's pouch. Our data reveal important overlapping functions of Hes1 and Prop1 in cell differentiation and movement that are critical for pituitary organogenesis.

Promoter-specific co-activation by Drosophila mastermind

Mastermind (Mam) is a co-activator protein of binary complexes consisting of Suppressor of Hairless (Su(H)) and Notch Intracellular Domain (NICD) proteins assembled on cis-regulatory regions of target genes activated by Notch signaling. Current evidence indicates that Mastermind is necessary and sufficient for the formation of a functional Su(H)/NICD/Mam ternary complex on at least one specific architecture of Su(H) binding sites, called the SPS element (Su(H) Paired Sites). However, using transcription assays with a combination of native and synthetic Notch target gene promoters in Drosophila cultured cells, we show here that co-activation of Su(H)/NICD complexes on SPS elements by Mam is promoter-specific. Our novel results suggest this promoter specificity is mediated by additional unknown cis-regulatory elements present in the native promoters that are required for the recruitment of Mam and formation of functional Su(H)/NICD/Mam complexes on SPS elements. Together, the findings in this study suggest Mam is not always necessary and sufficient for co-activation of binary Su(H)/NICD complexes on SPS elements.

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.