Echinoid limits R8 photoreceptor specification by inhibiting inappropriate EGF receptor signalling within R8 equivalence groups

Signaling Pathways Controlling Axonal Wrapping in Drosophila

The rapid transmission of action potentials is an important ability that enables efficient communication within the nervous system. Glial cells influence conduction velocity along axons by regulating the radial axonal diameter, providing electrical insulation as well as affecting the distribution of voltage-gated ion channels. Differentiation of these wrapping glial cells requires a complex set of neuron-glia interactions involving three basic mechanistic features. The glia must recognize the axon, grow around it, and eventually arrest its growth to form single or multiple axon wraps. This likely depends on the integration of numerous evolutionary conserved signaling and adhesion systems. Here, we summarize the mechanisms and underlying signaling pathways that control glial wrapping in Drosophila and compare those to the mechanisms that control glial differentiation in mammals. This analysis shows that Drosophila is a beneficial model to study the development of even complex structures like myelin.

The cell adhesion molecule Echinoid promotes tissue survival and separately restricts tissue overgrowth in Drosophila imaginal discs

The interactions that cells in Drosophila imaginal discs have with their neighbors are known to regulate their ability to survive. In a screen of genes encoding cell surface proteins for gene knockdowns that affect the size or shape of mutant clones, we found that clones of cells with reduced levels of echinoid (ed) are fewer, smaller, and can be eliminated during development. In contrast, discs composed mostly of ed mutant tissue are overgrown. We find that ed mutant tissue has lower levels of the anti-apoptotic protein Diap1 and has increased levels of apoptosis which is consistent with the observed underrepresentation of ed mutant clones and the slow growth of ed mutant tissue. The eventual overgrowth of ed mutant tissue results not from accelerated growth, but from prolonged growth resulting from a failure to arrest growth at the appropriate final size. Ed has previously been shown to physically interact with multiple Hippo-pathway components and it has been proposed to promote Hippo pathway signaling, to exclude Yorkie (Yki) from the nucleus, and restrain the expression of Yki-target genes. We did not observe changes in Yki localization in ed mutant tissue and found decreased levels of expression of several Yorkie-target genes, findings inconsistent with the proposed effect of Ed on Yki. We did, however, observe increased expression of several Yki-target genes in wild-type cells neighboring ed mutant cells, which may contribute to elimination of ed mutant clones. Thus, ed has two distinct functions: an anti-apoptotic function by maintaining Diap1 levels, and a function to arrest growth at the appropriate final size. Both of these are unlikely to be explained by a simple effect on the Hippo pathway.

An interview with Emma Rawlins

Emma Rawlins is a senior Group Leader at the Gurdon Institute, University of Cambridge, where her research focuses on lung development and regeneration. This year, Emma was the recipient of the BSDB Cheryll Tickle Medal, which is awarded to a mid-career female scientist for outstanding achievements in developmental biology. We talked to Emma about her research career, mentorship and how she felt about receiving the Cheryll Tickle Medal.

RASSF8-mediated transport of Echinoid via the exocyst promotes Drosophila wing elongation and epithelial ordering

Cell-cell junctions are dynamic structures that maintain cell cohesion and shape in epithelial tissues. During development, junctions undergo extensive rearrangements to drive the epithelial remodelling required for morphogenesis. This is particularly evident during axis elongation, where neighbour exchanges, cell-cell rearrangements and oriented cell divisions lead to large-scale alterations in tissue shape. Polarised vesicle trafficking of junctional components by the exocyst complex has been proposed to promote junctional rearrangements during epithelial remodelling, but the receptors that allow exocyst docking to the target membranes remain poorly understood. Here, we show that the adherens junction component Ras Association domain family 8 (RASSF8) is required for the epithelial re-ordering that occurs during Drosophila pupal wing proximo-distal elongation. We identify the exocyst component Sec15 as a RASSF8 interactor. Loss of RASSF8 elicits cytoplasmic accumulation of Sec15 and Rab11-containing vesicles. These vesicles also contain the nectin-like homophilic adhesion molecule Echinoid, the depletion of which phenocopies the wing elongation and epithelial packing defects observed in RASSF8 mutants. Thus, our results suggest that RASSF8 promotes exocyst-dependent docking of Echinoid-containing vesicles during morphogenesis.

Smurf Downregulates Echinoid in the Amnioserosa To Regulate Drosophila Dorsal Closure

Drosophila dorsal closure is a morphogenetic movement that involves flanking epidermal cells, assembling actomyosin cables, and migrating dorsally over the underlying amnioserosa to seal at the dorsal midline. Echinoid (Ed)-a cell adhesion molecule of adherens junctions (AJs)-participates in several developmental processes. The disappearance of Ed from the amnioserosa is required to define the epidermal leading edge for actomyosin cable assembly and coordinated cell migration. However, the mechanism by which Ed is cleared from amnioserosa is unknown. Here, we show that Ed is cleared in amnioserosa by both transcriptional and post-translational mechanisms. First, Ed mRNA transcription was repressed in amnioserosa prior to the onset of dorsal closure. Second, the ubiquitin ligase Smurf downregulated pretranslated Ed by binding to the PPXY motif of Ed. During dorsal closure, Smurf colocalized with Ed at AJs, and Smurf overexpression prematurely degraded Ed in the amnioserosa. Conversely, Ed persisted in the amnioserosa of Smurf mutant embryos, which, in turn, affected actomyosin cable formation. Together, our results demonstrate that transcriptional repression of Ed followed by Smurf-mediated downregulation of pretranslated Ed in amnioserosa regulates the establishment of a taut leading edge during dorsal closure.

Regulatory mechanisms of EGFR signalling during Drosophila eye development

EGFR signalling is a well-conserved signalling pathway playing major roles during development and cancers. This review explores what studying the EGFR pathway during Drosophila eye development has taught us in terms of the diversity of its regulatory mechanisms. This model system has allowed the identification of numerous positive and negative regulators acting at specific time and place, thus participating to the tight control of signalling. EGFR signalling regulation is achieved by a variety of mechanisms, including the control of ligand processing, the availability of the receptor itself and the transduction of the cascade in the cytoplasm. Ultimately, the transcriptional responses contribute to the establishment of positive and negative feedback loops. The combination of these multiple mechanisms employed to regulate the EGFR pathway leads to specific cellular outcomes involved in functions as diverse as the acquisition of cell fate, proliferation, survival, adherens junction remodelling and morphogenesis.

A comparative study of Pointed and Yan expression reveals new complexity to the transcriptional networks downstream of receptor tyrosine kinase signaling

The biochemical regulatory network downstream of receptor tyrosine kinase (RTK) signaling is controlled by two opposing ETS family members: the transcriptional activator Pointed (Pnt) and the transcriptional repressor Yan. A bistable switch model has been invoked to explain how pathway activation can drive differentiation by shifting the system from a high-Yan/low-Pnt activity state to a low-Yan/high-Pnt activity state. Although the model explains yan and pnt loss-of-function phenotypes in several different cell types, how Yan and Pointed protein expression dynamics contribute to these and other developmental transitions remains poorly understood. Toward this goal we have used a functional GFP-tagged Pnt transgene (Pnt-GFP) to perform a comparative study of Yan and Pnt protein expression throughout Drosophila development. Consistent with the prevailing model of the Pnt-Yan network, we found numerous instances where Pnt-GFP and Yan adopt a mutually exclusive pattern of expression. However we also observed many examples of co-expression. While some co-expression occurred in cells where RTK signaling is presumed low, other co-expression occurred in cells with high RTK signaling. The instances of co-expressed Yan and Pnt-GFP in tissues with high RTK signaling cannot be explained by the current model, and thus they provide important contexts for future investigation of how context-specific differences in RTK signaling, network topology, or responsiveness to other signaling inputs, affect the transcriptional response.

Drosophila EHBP1 regulates Scabrous secretion during Notch-mediated lateral inhibition

Notch signaling is an evolutionarily conserved pathway that plays a central role in numerous developmental and disease processes. The versatility of the Notch pathway relies on the activity of context-dependent regulators. These include rab11, sec15, arp3 and Drosophila EHBP1 (dEHBP1), which control Notch signaling and cell fate acquisition in asymmetrically dividing mechanosensory lineages by regulating the trafficking of the ligand Delta. Here, we show that dEHBP1 also controls the specification of R8 photoreceptors, as its loss results in the emergence of supernumerary R8 photoreceptors. Given the requirements for Notch signaling during lateral inhibition, we propose that dEHBP1 regulates distinct aspects of Notch signaling in different developmental contexts. We show that dEHBP1 regulates the exocytosis of Scabrous, a positive regulator of Notch signaling. In conclusion, dEHBP1 provides developmental versatility of intercellular signaling by regulating the trafficking of distinct Notch signaling components.

RBF and Rno promote photoreceptor differentiation onset through modulating EGFR signaling in the Drosophila developing eye

The retinoblastoma gene Rb is the prototype tumor suppressor and is conserved in Drosophila. We use the developing fly retina as a model system to investigate the role of Drosophila Rb (rbf) during differentiation. This report shows that mutation of rbf and rhinoceros (rno), which encodes a PHD domain protein, leads to a synergistic delay in photoreceptor cell differentiation in the developing eye disc. We show that this differentiation delay phenotype is caused by decreased levels of different components of the Epidermal Growth Factor Receptor (EGFR) signaling pathway in the absence of rbf and rno. We show that rbf is required for normal expression of Rhomboid proteins and activation of MAP kinase in the morphogenetic furrow (MF), while rno is required for the expression of Pointed (Pnt) and Ebi proteins, which are key factors that mediate EGFR signaling output in the nucleus. Interestingly, while removing the transcription activation function of dE2F1 is sufficient to suppress the synergistic differentiation delay, a mutant form of de2f1 that disrupts the binding with RBF but retains the transcription activation function does not mimic the effect of rbf loss. These observations suggest that RBF has additional functions besides dE2F1 binding that regulates EGFR signaling and photoreceptor differentiation.

Asymmetric distribution of Echinoid defines the epidermal leading edge during Drosophila dorsal closure

Upon loss of a binding partner in apposed tissue, the homophilic cell adhesion protein Echinoid adopts a planar polarized localization, which promotes the planar polarized localization of the planar cell polarity protein Bazooka/Par-3 and targets actomyosin cable assembly to the epidermal leading edge, thus establishing the migration direction of the developing epidermis.

During Drosophila melanogaster dorsal closure, lateral sheets of embryonic epidermis assemble an actomyosin cable at their leading edge and migrate dorsally over the amnioserosa, converging at the dorsal midline. We show that disappearance of the homophilic cell adhesion molecule Echinoid (Ed) from the amnioserosa just before dorsal closure eliminates homophilic interactions with the adjacent dorsal-most epidermal (DME) cells, which comprise the leading edge. The resulting planar polarized distribution of Ed in the DME cells is essential for the localized accumulation of actin regulators and for actomyosin cable formation at the leading edge and for the polarized localization of the scaffolding protein Bazooka/PAR-3. DME cells with uniform Ed fail to assemble a cable and protrude dorsally, suggesting that the cable restricts dorsal migration. The planar polarized distribution of Ed in the DME cells thus provides a spatial cue that polarizes the DME cell actin cytoskeleton, defining the epidermal leading edge and establishing its contractile properties.

Echinoid regulates tracheal morphology and fusion cell fate in Drosophila

Morphogenesis of the Drosophila embryonic trachea involves a stereotyped pattern of epithelial tube branching and fusion. Here, we report unexpected phenotypes resulting from maternal and zygotic (M/Z) loss of the homophilic cell adhesion molecule Echinoid (Ed), as well as the subcellular localization of Ed in the trachea. ed(M/Z) embryos have convoluted trachea reminiscent of septate junction (SJ) and luminal matrix mutants. However, Ed does not localize to SJs, and ed(M/Z) embryos have intact SJs and show normal luminal accumulation of the matrix-modifying protein Vermiform. Surprisingly, tracheal length is not increased in ed(M/Z) mutants, but a previously undescribed combination of reduced intersegmental spacing and deep epidermal grooves produces a convoluted tracheal phenotype. In addition, ed(M/Z) mutants have unique fusion defects involving supernumerary fusion cells, ectopic fusion events and atypical branch breaks. Tracheal-specific expression of Ed rescues these fusion defects, indicating that Ed acts in trachea to control fusion cell fate.

Echinoid regulates Flamingo endocytosis to control ommatidial rotation in the Drosophila eye

Planar cell polarity (PCP) refers to a second polarity axis orthogonal to the apicobasal axis in the plane of the epithelium. The molecular link between apicobasal polarity and PCP is largely unknown. During Drosophila eye development, differentiated photoreceptors form clusters that rotate independently of the surrounding interommatidial cells (ICs). Here, we demonstrate that both Echinoid (Ed), an adherens junction-associated cell adhesion molecule, and Flamingo (Fmi), a PCP determinant, are endocytosed via a clathrin-mediated pathway in ICs. Interestingly, we found that Ed binds the AP-2 adaptor and is required for the internalization of Fmi into ICs. Loss of ed led to increased amounts of Fmi on the cell membrane of non-rotating ICs and also to the misrotation of photoreceptor clusters. Importantly, overexpression of fmi in ICs alone was sufficient to cause misrotation of the adjacent photoreceptor clusters. Together, we propose that Ed, when internalized by AP-2, undergoes co-endocytosis with, and thereby decreases, Fmi levels on non-rotating ICs to permit correct rotation of ommatidial clusters. Thus, co-endocytosis of Ed and Fmi provides a link between apicobasal polarity and PCP.

The cell adhesion molecules Echinoid and Friend of Echinoid coordinate cell adhesion and cell signaling to regulate the fidelity of ommatidial rotation in the Drosophila eye

Directed cellular movements are a universal feature of morphogenesis in multicellular organisms. Differential adhesion between the stationary and motile cells promotes these cellular movements to effect spatial patterning of cells. A prominent feature of Drosophila eye development is the 90 degrees rotational movement of the multicellular ommatidial precursors within a matrix of stationary cells. We demonstrate that the cell adhesion molecules Echinoid (Ed) and Friend of Echinoid (Fred) act throughout ommatidial rotation to modulate the degree of ommatidial precursor movement. We propose that differential levels of Ed and Fred between stationary and rotating cells at the initiation of rotation create a permissive environment for cell movement, and that uniform levels in these two populations later contribute to stopping the movement. Based on genetic data, we propose that ed and fred impart a second, independent, ;brake-like' contribution to this process via Egfr signaling. Ed and Fred are localized in largely distinct and dynamic patterns throughout rotation. However, ed and fred are required in only a subset of cells - photoreceptors R1, R7 and R6 - for normal rotation, cells that have only recently been linked to a role in planar cell polarity (PCP). This work also provides the first demonstration of a requirement for cone cells in the ommatidial rotation aspect of PCP. ed and fred also genetically interact with the PCP genes, but affect only the degree-of-rotation aspect of the PCP phenotype. Significantly, we demonstrate that at least one PCP protein, Stbm, is required in R7 to control the degree of ommatidial rotation.

Retinoblastoma family protein promotes normal R8-photoreceptor differentiation in the absence of rhinoceros by inhibiting dE2F1 activity

The retinoblastoma gene Rb is a prototype tumor suppressor which is conserved in Drosophila. Although much is known about the roles of Rb in cell proliferation and apoptosis, much less is known about how Rb regulates cell differentiation. Inactivation of Drosophila Rb (rbf) exhibited subtle differentiation defects similar to inactivation of Rb in mice, suggesting the existence of redundant mechanisms in the control of cell differentiation. To test this possibility and to characterize the role of Rbf in cell differentiation during retinal development, we carried out a genetic screen and identified a mutation in rhinoceros (rno), which leads to synergistic differentiation defects in conjunction with rbf inactivation. Characterization of an early differentiation defect, the multiple-R8 phenotype, revealed that this phenotype was caused by limiting amounts of Notch signaling due to reduced expression of the Notch ligand, Delta (Dl). Decreasing the gene dosage of Dl enhanced the multiple-R8 phenotype, while increasing the level of Dl suppressed this phenotype. Interestingly, removal of the transcriptional activation of dE2F1 partially restores Dl expression in rbf,rno mutant clones and suppresses the associated differentiation defects, indicating that this differentiation function of RBF is mediated by its regulation of dE2F1 activity.

Two-step selection of a single R8 photoreceptor: a bistable loop between senseless and rough locks in R8 fate

Patterning of sensory organs requires precise regulation of neural induction and repression. The neurocrystalline pattern of the adult Drosophila compound eye is generated by ordered selection of single founder photoreceptors (R8s) for each unit eye or ommatidium. R8 selection requires mechanisms that restrict R8 potential to a single cell from within a group of cells expressing the proneural gene atonal (ato). One model of R8 selection suggests that R8 precursors are selected from a three-cell ;R8 equivalence group' through repression of ato by the homeodomain transcription factor Rough (Ro). A second model proposes that lateral inhibition is sufficient to select a single R8 from an equipotent group of cells called the intermediate group (IG). Here, we provide new evidence that lateral inhibition, but not ro, is required for the initial selection of a single R8 precursor. We show that in ro mutants, ectopic R8s develop from R2,5 photoreceptor precursors independently of ectopic Ato and hours after normal R8s are specified. We also show that Ro directly represses the R8 specific zinc-finger transcription factor senseless (sens) in the developing R2,5 precursors to block ectopic R8 differentiation. Our results support a new model for R8 selection in which lateral inhibition establishes a transient pattern of selected R8s that is permanently reinforced by a repressive bistable loop between sens and ro. This model provides new insight into the strategies that allow successful integration of a repressive patterning signal, such as lateral inhibition, with continued developmental plasticity during retinal differentiation.

Signal integration during development: mechanisms of EGFR and Notch pathway function and cross-talk

Metazoan development relies on a highly regulated network of interactions between conserved signal transduction pathways to coordinate all aspects of cell fate specification, differentiation, and growth. In this review, we discuss the intricate interplay between the epidermal growth factor receptor (EGFR; Drosophila EGFR/DER) and the Notch signaling pathways as a paradigm for signal integration during development. First, we describe the current state of understanding of the molecular architecture of the EGFR and Notch signaling pathways that has resulted from synergistic studies in vertebrate, invertebrate, and cultured cell model systems. Then, focusing specifically on the Drosophila eye, we discuss how cooperative, sequential, and antagonistic relationships between these pathways mediate the spatially and temporally regulated processes that generate this sensory organ. The common themes underlying the coordination of the EGFR and Notch pathways appear to be broadly conserved and should, therefore, be directly applicable to elucidating mechanisms of information integration and signaling specificity in vertebrate systems.

Complex interaction of Drosophila GRIP PDZ domains and Echinoid during muscle morphogenesis

Glutamate receptor interacting protein (GRIP) homologues, initially characterized in synaptic glutamate receptor trafficking, consist of seven PDZ domains (PDZDs), whose conserved arrangement is of unknown significance. The Drosophila GRIP homologue (DGrip) is needed for proper guidance of embryonic somatic muscles towards epidermal attachment sites, with both excessive and reduced DGrip activity producing specific phenotypes in separate muscle groups. These phenotypes were utilized to analyze the molecular architecture underlying DGrip signaling function in vivo. Surprisingly, removing PDZDs 1-3 (DGripDelta1-3) or deleting ligand binding in PDZDs 1 or 2 convert DGrip to excessive in vivo activity mediated by ligand binding to PDZD 7. Yeast two-hybrid screening identifies the cell adhesion protein Echinoid's (Ed) type II PDZD-interaction motif as binding PDZDs 1, 2 and 7 of DGrip. ed loss-of-function alleles exhibit muscle defects, enhance defects caused by reduced DGrip activity and suppress the dominant DGripDelta1-3 effect during embryonic muscle formation. We propose that Ed and DGrip form a signaling complex, where competition between N-terminal and the C-terminal PDZDs of DGrip for Ed binding controls signaling function.

Differential expression of the adhesion molecule Echinoid drives epithelial morphogenesis in Drosophila

Epithelial morphogenesis requires cell movements and cell shape changes coordinated by modulation of the actin cytoskeleton. We identify a role for Echinoid (Ed), an immunoglobulin domain-containing cell-adhesion molecule, in the generation of a contractile actomyosin cable required for epithelial morphogenesis in both the Drosophila ovarian follicular epithelium and embryo. Analysis of ed mutant follicle cell clones indicates that the juxtaposition of wild-type and ed mutant cells is sufficient to trigger actomyosin cable formation. Moreover, in wild-type ovaries and embryos, specific epithelial domains lack detectable Ed, thus creating endogenous interfaces between cells with and without Ed; these interfaces display the same contractile characteristics as the ectopic Ed expression borders generated by ed mutant clones. In the ovary, such an interface lies between the two cell types of the dorsal appendage primordia. In the embryo, Ed is absent from the amnioserosa during dorsal closure, generating an Ed expression border with the lateral epidermis that coincides with the actomyosin cable present at this interface. In both cases, ed mutant epithelia exhibit loss of this contractile structure and subsequent defects in morphogenesis. We propose that local modulation of the cytoskeleton at Ed expression borders may represent a general mechanism for promoting epithelial morphogenesis.

Inhibition of ERK-MAP kinase signaling by RSK during Drosophila development

Although p90 ribosomal S6 kinase (RSK) is known as an important downstream effector of the ribosomal protein S6 kinase/extracellular signal-regulated kinase (Ras/ERK) pathway, its endogenous role, and precise molecular function remain unclear. Using gain-of-function and null mutants of RSK, its physiological role was successfully characterized in Drosophila. Surprisingly, RSK-null mutants were viable, but exhibited developmental abnormalities related to an enhanced ERK-dependent cellular differentiation such as ectopic photoreceptor- and vein-cell formation. Conversely, overexpression of RSK dramatically suppressed the ERK-dependent differentiation, which was further augmented by mutations in the Ras/ERK pathway. Consistent with these physiological phenotypes, RSK negatively regulated ERK-mediated developmental processes and gene expressions by blocking the nuclear localization of ERK in a kinase activity-independent manner. In addition, we further demonstrated that the RSK-dependent inhibition of ERK nuclear migration is mediated by the physical association between ERK and RSK. Collectively, our study reveals a novel regulatory mechanism of the Ras/ERK pathway by RSK, which negatively regulates ERK activity by acting as a cytoplasmic anchor in Drosophila.

Regulating the dynamics of EGF receptor signaling in space and time

The epidermal growth factor receptor (EGFR) signaling cascade represents one of the cardinal pathways that transmits information between cells during development in a broad range of multicellular organisms. Most of the elements that constitute the core EGFR signaling module, as well as a variety of negative and positive modulators, have been identified. Although this molecular pathway is utilized multiple times during development, the spatial and temporal features of its signaling can be modified to fit a particular developmental setting. Recent work has unraveled the various mechanisms by which the EGFR pathway can be modulated.

Cell Adhesion: Sorting out Cell Mixing with Echinoid?

Cadherins control intercellular adhesion in epithelial cells. This property relies on the ability to recruit actin filaments at adherens junctions via beta-catenin and alpha-catenin. A recent study shows that Echinoid, a member of the immunoglobulin domain containing protein family, is a modulator of intercellular adhesion in Drosophila that controls cell sorting.

Echinoid Is a Component of Adherens Junctions That Cooperates with DE-Cadherin to Mediate Cell Adhesion

Echinoid is an immunoglobulin domain-containing transmembrane protein that modulates cell-cell signaling by Notch and the EGF receptors. We show that, in the Drosophila wing disc epithelium, Echinoid is a component of adherens junctions that cooperates with DE-Cadherin in cell adhesion. Echinoid and beta-catenin (a DE-Cadherin interacting protein) each possess a C-terminal PDZ domain binding motif that binds to Bazooka/PAR-3; these motifs redundantly position Bazooka to adherens junctions. Echinoid also links to actin filaments by binding to Canoe/AF-6/afadin. Moreover, interfaces between Echinoid- and Echinoid+ cells, like those between DE-Cadherin- and DE-Cadherin+ cells, are deficient in adherens junctions and form actin cables. These characteristics probably facilitate the strong sorting behavior of cells that lack either of these cell-adhesion molecules. Finally, cells lacking either Echinoid or DE-Cadherin accumulate a high density of the reciprocal protein, further suggesting that Echinoid and DE-Cadherin play similar and complementary roles in cell adhesion.

EGF Receptor Signaling Triggers Recruitment of Drosophila Sense Organ Precursors by Stimulating Proneural Gene Autoregulation

In Drosophila, commitment of a cell to a sense organ precursor (SOP) fate requires bHLH proneural transcription factor upregulation, a process that depends in most cases on the interplay of proneural gene autoregulation and inhibitory Notch signaling. A subset of SOPs are selected by a recruitment pathway involving EGFR signaling to ectodermal cells expressing the proneural gene atonal. We show that EGFR signaling drives recruitment by directly facilitating atonal autoregulation. Pointed, the transcription factor that mediates EGFR signaling, and Atonal protein itself bind cooperatively to adjacent conserved binding sites in an atonal enhancer. Recruitment is therefore contingent on the combined presence of Atonal protein (providing competence) and EGFR signaling (triggering recruitment). Thus, autoregulation is the nodal control point targeted by signaling. This exemplifies a simple and general mechanism for regulating the transition from competence to cell fate commitment whereby a cell signal directly targets the autoregulation of a selector gene.

Negative regulation of ErbB family receptor tyrosine kinases

Receptors of the EGF receptor or ErbB family of growth factor receptor tyrosine kinases are frequently overexpressed in a variety of solid tumours, and the aberrant activation of their tyrosine kinase activities is thought to contribute to tumour growth and progression. Much effort has been put into developing inhibitors of ErbB receptors, and both antibody and small-molecule approaches have exhibited clinical success. Recently, a number of endogenous negative regulatory proteins have been identified that suppress the signalling activity of ErbB receptors in cells. These include intracellular RING finger E3 ubiquitin ligases such as cbl and Nrdp1 that mediate ErbB receptor degradation, and may include a wide variety of secreted and transmembrane proteins that suppress receptor activation by growth factor ligands. It will be of interest to determine the extent to which tumour cells suppress these pathways to promote their progression, and whether restoration of endogenous receptor-negative regulatory pathways may be exploited for therapeutic benefit.

Cell-cycle regulation and cell-type specification in the developing Drosophila compound eye

During nervous system development stem cell daughters must exit the proliferative cycle to adopt specific neural and glial fates and they must do so in the correct positions. Cell proliferation in the central nervous system occurs in neuroepithelia such as the neural retina and the ventricular zones. As cells are assigned specific fates they migrate out of the plane of the epithelium to form higher layers. Recent evidence from the Drosophila compound eye suggests that a novel mode of Ras pathway regulation may be crucial in both cell-cycle exit and neural patterning: "MAP Kinase cytoplasmic hold".

Echinoid mutants exhibit neurogenic phenotypes and show synergistic interactions with the Notch signaling pathway

During neurogenesis in Drosophila, groups of ectodermal cells are endowed with the capacity to become neuronal precursors. The Notch signaling pathway is required to limit the neuronal potential to a single cell within each group. Loss of genes of the Notch signaling pathway results in a neurogenic phenotype: hyperplasia of the nervous system accompanied by a parallel loss of epidermis. Echinoid (Ed), a cell membrane associated Immunoglobulin C2-type protein, has previously been shown to be a negative regulator of the EGFR pathway during eye and wing vein development. Using in situ hybridization and antibody staining of whole-mount embryos, we show that Ed has a dynamic expression pattern during embryogenesis. Embryonic lethal alleles of ed reveal a role of Ed in restricting neurogenic potential during embryonic neurogenesis, and result in a phenotype similar to that of loss-of-function mutations of Notch signaling pathway genes. In this process Ed interacts closely with the Notch signaling pathway. Loss of ed suppresses the loss of neuronal elements caused by ectopic activation of the Notch signaling pathway. Using a temperature-sensitive allele of ed we show, furthermore, that Ed is required to suppress sensory bristles and for proper wing vein specification during adult development. In these processes also, ed acts in close concert with genes of the Notch signaling pathway. Thus the extra wing vein phenotype of ed is enhanced upon reduction of Delta (Dl) or Enhancer of split [E(spl)] proteins. Overexpression of the membrane-tethered extracellular region of Ed results in a dominant-negative phenotype. This phenotype is suppressed by overexpression of E(spl)m7 and enhanced by overexpression of Dl. Our work establishes a role of Ed during embryonic nervous system development, as well as adult sensory bristle specification and shows that Ed interacts synergistically with the Notch signaling pathway.

Senseless represses nuclear transduction of Egfr pathway activation

The Epidermal growth factor receptor (Egfr) pathway controls cell fate decisions throughout phylogeny. Typically, binding of secreted ligands to Egfr on the cell surface initiates a well-described cascade of events that ultimately invokes transcriptional changes in the nucleus. In contrast, the mechanisms by which autocrine effects are regulated in the ligand-producing cell are unclear. In the Drosophila eye, Egfr signaling, induced by the Spitz ligand, is required for differentiation of all photoreceptors except for R8, the primary source of Spitz. R8 differentiation is instead under the control of the transcription factor Senseless. We show that high levels of Egfr activation are incompatible with R8 differentiation and describe the mechanism by which Egfr signaling is actively prevented in R8. Specifically, Senseless does not affect cytoplasmic transduction of Egfr activation, but does block nuclear transduction of Egfr activation through transcriptional repression of pointed, which encodes the nuclear effector of the pathway. Thus, Senseless promotes normal R8 differentiation by preventing the effects of autocrine stimulation by Spitz. An analogous relationship exists between Senseless and Egfr pathway orthologs in T-lymphocytes, suggesting that this mode of repression of Egfr signaling is conserved.

Echinoid synergizes with the Notch signaling pathway in Drosophila mesothorax bristle patterning

echinoid (ed) encodes an immunoglobulin domain-containing cell adhesion molecule that negatively regulates the Egfr signaling pathway during Drosophila photoreceptor development. We show a novel function of Ed, i.e. the restriction of the number of notum bristles that arise from a proneural cluster. Thus, loss-of-function conditions for ed give rise to the development of extra macrochaetae near the extant ones and increase the density of microchaetae. Analysis of ed mosaics indicates that extra sensory organ precursors (SOPs) arise from proneural clusters of achaete-scute expression in a cell-autonomous way. edembryos also exhibit a neurogenic phenotype. These phenotypes suggest a functional relation between ed and the Notch (N) pathway. Indeed,loss-of-function of ed reduces the expression of the N pathway effector E(spl)m8 in proneural clusters. Moreover, combinations of moderate loss-of-function conditions for ed and for different components of the N pathway show clear synergistic interactions manifested as strong neurogenic bristle phenotypes. We conclude that Ed is not essential for, but it facilitates, N signaling. It is known that the N and Egfr pathways act antagonistically in bristle development. Consistently, we find that Ed also antagonizes the bristle-promoting activity of the Egfr pathway, either by the enhancement of N signalling or, similar to the eye, by a more direct action on the Egfr pathway.

Echinoid facilitates Notch pathway signalling during Drosophila neurogenesis through functional interaction with Delta

The Notch intercellular signalling pathway is important throughout development, and its components are modulated by a variety of cellular and molecular mechanisms. Ligand and receptor trafficking are tightly controlled, although context-specific regulation of this is incompletely understood. We show that during sense organ precursor specification in Drosophila, the cell adhesion molecule Echinoid colocalises extensively with the Notch ligand, Delta, at the cell membrane and in early endosomes. Echinoid facilitates efficient Notch pathway signalling. Cultured cell experiments suggest that Echinoid is associated with the cis-endocytosis of Delta, and is therefore linked to the signalling events that have been shown to require such Delta trafficking. Consistent with this, overexpression of Echinoid protein causes a reduction in Delta level at the membrane and in endosomes. In vivo and cell culture studies suggest that homophilic interaction of Echinoid on adjacent cells is necessary for its function.

Echinoid is essential for regulation of Egfr signaling and R8 formation during Drosophila eye development

Precisely regulated Egfr activity is essential for normal development and cell differentiation. We demonstrate that the transmembrane protein Echinoid is required to downregulate Egfr activity in the developing Drosophila eye, ensuring a normal array of R8 photoreceptor neurons. Echinoid is an L1-type transmembrane molecule that is expressed in all cells of the eye imaginal discs and, unlike many other Egfr inhibitors, does not appear to be regulated transcriptionally. Echinoid co-precipitates with Egfr from cultured cells and eye imaginal discs, and Egfr activity promotes tyrosine phosphorylation of Echinoid. These observations suggest that Echinoid inhibits Egfr through direct interactions.