Notch Activation of yan Expression Is Antagonized by RTK/Pointed Signaling in the Drosophila Eye (original) (raw)
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Mechanisms of Development, 2007
The Notch and Epidermal Growth Factor Receptor (EGFR) signaling pathways interact cooperatively and antagonistically to regulate many aspects of Drosophila development, including the eye. How output from these two signaling networks is fine-tuned to achieve the precise balance needed for specific inductive interactions and patterning events remains an open and important question. Previously, we reported that the gene split ends (spen) functions within or parallel to the EGFR pathway during midline glial cell development in the embryonic central nervous system. Here, we report that the cellular defects caused by loss of spen function in the developing eye imaginal disc place spen as both an antagonist of the Notch pathway and a positive contributor to EGFR signaling during retinal cell differentiation. Specifically, loss of spen results in broadened expression of Scabrous, ectopic activation of Notch signaling, and a corresponding reduction in Atonal expression at the morphogenetic furrow. Consistent with Spen's role in antagonizing Notch signaling, reduction of spen levels is sufficient to suppress Notch-dependent phenotypes. At least in part due to loss of Spen-dependent down-regulation of Notch signaling, loss of spen also dampens EGFR signaling as evidenced by reduced activity of MAP kinase (MAPK). This reduced MAPK activity in turn leads to a failure to limit expression of the EGFR pathway antagonist and the ETS-domain transcriptional repressor Yan and to a corresponding loss of cell fate specification in spen mutant ommatidia. We propose that Spen plays a role in modulating output from the Notch and EGFR pathways to ensure appropriate patterning during eye development.
BMC developmental biology, 2004
The receptor protein Notch and its ligand Delta are expressed throughout proneural regions yet non-neural precursor cells are defined by Notch activity and neural precursor cells by Notch inactivity. Not even Delta overexpression activates Notch in neural precursor cells. It is possible that future neural cells are protected by cis-inactivation, in which ligands block activation of Notch within the same cell. The Delta-ubiquitin ligase Neuralized has been proposed to antagonize cis-inactivation, favoring Notch activation. Cis-inactivation and role of Neuralized have not yet been studied in tissues where neural precursor cells are resistant to nearby Delta, however, such as the R8 cells of the eye or the bristle precursor cells of the epidermis. Overexpressed ligands could block Notch signal transduction cell-autonomously in non-neural cells of the epidermis and retina, but did not activate Notch nonautonomously in neural cells. High ligand expression levels were required for cis-ina...
Molecular and Cellular Biology, 1997
The Notch signaling pathway is known to regulate cell fate decisions in a variety of organisms from worms to humans. Although several components of the pathway have been characterized, the actual mechanism and molecular results of signaling remain elusive. We have examined the role of the Notch signaling pathway in the transcriptional regulation of two Drosophila Enhancer of split [E(spl)] genes, whose gene products have been shown to be downstream players in the pathway. Using a reporter assay system in Drosophila tissue culture cells, we have observed a significant induction of E(spl) m␥ and m␦ expression after cotransfection with activated Notch. Characterization of the 5 regulatory regions of these two genes led to the identification of a number of target sites for the Suppressor of Hairless [Su(H)] protein, a transcription factor activated by Notch signaling. We show that Notch-inducible expression of E(spl) m␥ and m␦ both in cultured cells and in vivo is dependent on functional Su(H). Although overexpression of Su(H) augments the level of induction of the reporter genes by activated Notch, Su(H) alone is insufficient to produce high levels of transcriptional activation. Despite the synergy observed between activated Notch and Su(H), the former affects neither the nuclear localization nor the DNA binding activity of the latter.
The role of yan in mediating the choice between cell division and differentiation
Development (Cambridge, England), 1995
An allele of the yan locus was isolated as an enhancer of the Ellipse mutation of the Drosophila epidermal growth factor receptor (Egfr) gene. This yan allele is an embryonic lethal and also fails to complement the lethality of anterior open (aop) mutations. Phenotypic and complementation analysis revealed that aop is allelic to yan and genetically the lethal alleles act as null mutations for the yan gene. Analysis of the lethal alleles in the embryo and in mitotic clones showed that loss of yan function causes cells to overproliferate in the dorsal neuroectoderm of the embryo and in the developing eye disc. Our studies suggest that the role of yan is defined by the developmental context of the cells in which it functions. An important role of this gene is in allowing a cell to choose between cell division and differentiation. The relationship of the Egfr and Notch pathways to this developmental role of yan is discussed.
The role of yan in mediating the choice between cell division and differentiation
Development, 1995
An allele of the yan locus was isolated as an enhancer of the Ellipse mutation of the Drosophila epidermal growth factor receptor (Egfr) gene. This yan allele is an embryonic lethal and also fails to complement the lethality of anterior open (aop) mutations. Phenotypic and complementation analysis revealed that aop is allelic to yan and genetically the lethal alleles act as null mutations for the yan gene. Analysis of the lethal alleles in the embryo and in mitotic clones showed that loss of yan function causes cells to overproliferate in the dorsal neuroectoderm of the embryo and in the developing eye disc. Our studies suggest that the role of yan is defined by the developmental context of the cells in which it functions. An important role of this gene is in allowing a cell to choose between cell division and differentiation. The relationship of the Egfr and Notch pathways to this developmental role of yan is discussed.
Notch activation stimulates transient and selective binding of Su(H)/CSL to target enhancers
Genes & Development, 2007
The CSL [CBF1/Su(H)/Lag2] proteins [Su(H) in Drosophila] are implicated in repression and activation of Notch target loci. Prevailing models imply a static association of these DNA-binding transcription factors with their target enhancers. Our analysis of Su(H) binding and chromatin-associated features at 11 E(spl) Notch target genes before and after Notch revealed large differences in Su(H) occupancy at target loci that correlated with the presence of polymerase II and other marks of transcriptional activity. Unexpectedly, Su(H) occupancy was significantly and transiently increased following Notch activation, suggesting a more dynamic interaction with targets than hitherto proposed.
Mol. Cell. Biol., 2011
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. In Drosophila melanogaster neurogenesis, the proneural basic helix-loop-helix activator (bHLH A) genes are initially expressed in clusters of adjacent cells called "proneural clusters" (Fig. 1A). Although each cell within the proneural cluster has the potential to adopt a neural cell fate, only one cell or a few cells within the cluster become a neural precursor cell (NPC). Subsequently, the expression of both the proneural bHLH A genes and several putative downstream "panneural" target genes are strongly upregulated in the NPC. In contrast, the expression of proneural and panneural gene is not upregulated in the non-NPCs. Notch signaling-mediated lateral inhibition is critical for repression of proneural bHLH A gene expression in the non-NPCs. Several effector genes for the lateral inhibition pathway in proneural clusters are in the Enhancer of split Complex [E(spl)-C]. The E(spl)-C bHLH repressor (bHLH R) genes (m3, m5, m7, m8, m␥, and m␦) are well-characterized effector genes for Notch signaling (4), and the E(spl)-C m4 and m␣ Bearded-like (Brd-like) genes have also been proposed to mediate lateral inhibition (2). The bHLH R proteins can repress proneural gene expression by binding to R sites in proneural gene regulatory regions (33, 34, 40) as well as physically interacting with the proneural proteins and blocking proneural au-toactivation (15, 16). The Brd-like proteins physically interact with the Neuralized panneural protein and modulate intracel-lular processing of the Notch signaling ligand Delta (2). Activation of E(spl)-C gene transcription in proneural clusters is initially inhibited by a "default repression" mechanism that is mediated by the bifunctional protein Suppressor of Hairless [Su(H); also called CSL], which binds to S DNA binding sites (3, 5, 21, 28). In the absence of Notch signaling, Su(H) mediates repression of these genes by recruiting specific corepressors, including Hairless (H), Groucho (Gro), and dCtBP (Fig. 1B) (3, 30). However, once the NPC is established in proneural clusters, the Notch receptor becomes selectively activated in the non-NPCs, and Su(H)-mediated repression of the E(spl)-C genes in the non-NPCs is relieved. This derepres-sion is due to the cleaved Notch intracellular domain (NICD) binding to Su(H) and displacing the corepressor proteins (Fig. 1C). The Su(H)/NICD binary complex then recruits additional coactivators, such as Mastermind (Mam) (5, 21, 24). The resulting ternary complex can also synergistically interact with other transcription factors bound nearby on the DNA (Fig. 1D). For example, synergistic interactions between Notch transcription complexes and bHLH A proteins is critical for strong expression of m8 and several neural E(spl)-C genes in non-NPCs (5, 7, 9). Several E(spl)-C bHLH R and Brd-like genes have cell-specific expression patterns in proneural clusters that are the opposite of the proneural bHLH A genes during Notch signal-ing (Fig. 1A). These opposing expression patterns are programmed by "DNA transcription codes" embedded in regulatory DNA sequences. Transcription codes are the specific combinations and "architectures" (that is, the order, orientation , and spacing) of transcription factor binding sites clustered in small promoter or enhancer regions that program a specific component of the overall expression pattern (26). For example , the m8 model bHLH R gene contains an "SPSϩA" transcription code that mediates synergistic interactions between Su(H)/NICD complexes and bHLH A protein complexes (Fig. 1E) (7). The SPSϩA code contains an SPS element [Su(H) paired site] and at least one A site. The SPS element has a specific, inverted repeat architecture of S sites that is critical for programming Notch-proneural transcriptional synergy on the m8 promoter. The SPS element architecture is also present in vertebrate Notch pathway target genes (1, 20, 32) and can also mediate strong transcriptional synergy with vertebrate ho-mologues to Drosophila proneural bHLH A proteins (7, 25).