Ze’ev Paroush

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-β (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 Given that there are genetic interactions between gro and components of the EGFR pathway (ref. 9 and P.H. et al., unpublished results), we tested whether Gro is at a crossroad between this and other pathways.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.

A MAPK docking site is critical for downregulation of Capicua by Torso and EGFR RTK signaling

Early Drosophila development requires two receptor tyrosine kinase (RTK) pathways: the Torso and the Epidermal growth factor receptor (EGFR) pathways, which regulate terminal and dorsal‐ventral patterning, respectively. Previous studies have shown that these pathways, either directly or indirectly, lead to post‐transcriptional downregulation of the Capicua repressor in the early embryo and in the ovary. Here, we show that both regulatory effects are direct and depend on a MAPK docking site in Capicua that physically interacts with the MAPK Rolled. Capicua derivatives lacking this docking site cause dominant phenotypes similar to those resulting from loss of Torso and EGFR activities. Such phenotypes arise from inappropriate repression of genes normally expressed in response to Torso and EGFR signaling. Our results are consistent with a model whereby Capicua is the main nuclear effector of the Torso pathway, but only one of different effectors responding to EGFR signaling. Finally, we describe differences in the modes of Capicua downregulation by Torso and EGFR signaling, raising the possibility that such differences contribute to the tissue specificity of both signals.

Phospholipase C and termination of G-protein-mediated signalling in vivo

In Drosophila photoreceptors, phospholipase C (PLC) and other signalling components form multiprotein structures through the PDZ scaffold protein INAD. Association between PLC and INAD is important for termination of responses to light; the underlying mechanism is, however, unclear. Here we report that the maintenance of large amounts of PLC in the signalling membranes by association with INAD facilitates response termination, and show that PLC functions as a GTPase-activating protein (GAP). The inactivation of the G protein by its target, the PLC, is crucial for reliable production of single-photon responses and for the high temporal and intensity resolution of the response to light.

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

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

Substrate-dependent control of MAPK phosphorylation in vivo.

Phosphorylation of the mitogen‐activated protein kinase (MAPK) is essential for its enzymatic activity and ability to control multiple substrates inside a cell. According to the current models, control of MAPK phosphorylation is independent of its substrates, which are viewed as mere sensors of MAPK activity. Contrary to this modular view of MAPK signaling, our studies in the Drosophila embryo demonstrate that substrates can regulate the level of MAPK phosphorylation in vivo. We demonstrate that a twofold change in the gene dosage of a single substrate can induce a significant change in the phosphorylation level of MAPK and in the conversion of other substrates. Our results support a model where substrates of MAPK counteract its dephosphorylation by phosphatases. Substrate‐dependent control of MAPK phosphorylation is a manifestation of a more general retroactive effect that should be intrinsic to all networks with covalent modification cycles.

MAPK substrate competition integrates patterning signals in the Drosophila embryo.

Terminal regions of the Drosophila embryo are patterned by the localized activation of the mitogen-activated protein kinase (MAPK) pathway. This depends on the MAPK-mediated downregulation of Capicua (Cic), a repressor of the terminal gap genes. We establish that downregulation of Cic is antagonized by the anterior patterning morphogen Bicoid (Bcd). We demonstrate that this effect does not depend on transcriptional activity of Bcd and provide evidence suggesting that Bcd, a direct substrate of MAPK, decreases the availability of MAPK for its other substrates, such as Cic. Based on the quantitative analysis of MAPK signaling in multiple mutants, we propose that MAPK substrate competition coordinates the actions of the anterior and terminal patterning systems. In addition, we identify Hunchback as a novel target of MAPK phosphorylation that can account for the previously described genetic interaction between the posterior and terminal systems. Thus, a common enzyme-substrate competition mechanism can integrate the actions of the anterior, posterior, and terminal patterning signals. Substrate competition can be a general signal integration strategy in networks where enzymes, such as MAPK, interact with their multiple regulators and targets.

The Capicua repressor – a general sensor of RTK signaling in development and disease

Receptor tyrosine kinase (RTK) signaling pathways control multiple cellular decisions in metazoans, often by regulating the expression of downstream genes. In Drosophila melanogaster and other systems, E-twenty-six (ETS) transcription factors are considered to be the predominant nuclear effectors of RTK pathways. Here, we highlight recent progress in identifying the HMG-box protein Capicua (CIC) as a key sensor of RTK signaling in both Drosophila and mammals. Several studies have shown that CIC functions as a repressor of RTK-responsive genes, keeping them silent in the absence of signaling. Following the activation of RTK signaling, CIC repression is relieved, and this allows the expression of the targeted gene in response to local or ubiquitous activators. This regulatory switch is essential for several RTK responses in Drosophila, from the determination of cell fate to cell proliferation. Furthermore, increasing evidence supports the notion that this mechanism is conserved in mammals, where CIC has been implicated in cancer and neurodegeneration. In addition to summarizing our current knowledge on CIC, we also discuss the implications of these findings for our understanding of RTK signaling specificity in different biological processes.

Groucho Oligomerization Is Required for Repression In Vivo

ABSTRACT 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.

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

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

A Myc–Groucho complex integrates EGF and Notch signaling to regulate neural development

Integration of patterning cues via transcriptional networks to coordinate gene expression is critical during morphogenesis and misregulated in cancer. Using DNA adenine methyltransferase (Dam)ID chromatin profiling, we identified a protein–protein interaction between the Drosophila Myc oncogene and the Groucho corepressor that regulates a subset of direct dMyc targets. Most of these shared targets affect fate or mitosis particularly during neurogenesis, suggesting the dMyc–Groucho complex may coordinate fate acquisition with mitotic capacity during development. We find an antagonistic relationship between dMyc and Groucho that mimics the antagonistic interactions found for EGF and Notch signaling: dMyc is required to specify neuronal fate and enhance neuroblast mitosis, whereas Groucho is required to maintain epithelial fate and inhibit mitosis. Our results suggest that the dMyc–Groucho complex defines a previously undescribed mechanism of Myc function and may serve as the transcriptional unit that integrates EGF and Notch inputs to regulate early neuronal development.