secCl is a cys-loop ion channel necessary for the chloride conductance that mediates hormone-induced fluid secretion in Drosophila.

Organisms use circulating diuretic hormones to control water balance (osmolarity), thereby avoiding dehydration and managing excretion of waste products. The hormones act through G-protein-coupled receptors to activate second messenger systems that in turn control the permeability of secretory epithelia to ions like chloride. In insects, the chloride channel mediating the effects of diuretic hormones was unknown. Surprisingly, we find a pentameric, cys-loop chloride channel, a type of channel normally associated with neurotransmission, mediating hormone-induced transepithelial chloride conductance. This discovery is important because: 1) it describes an unexpected role for pentameric receptors in the membrane permeability of secretory epithelial cells, and 2) it suggests that neurotransmitter-gated ion channels may have evolved from channels involved in secretion.

Determination of EGFR Signaling Output by Opposing Gradients of BMP and JAK/STAT Activity.

A relatively small number of signaling pathways drive a wide range of developmental decisions, but how this versatility in signaling outcome is generated is not clear. In the Drosophila follicular epithelium, localized epidermal growth factor receptor (EGFR) activation induces distinct cell fates depending on its location. Posterior follicle cells respond to EGFR activity by expressing the T-box transcription factors Midline and H15, while anterior cells respond by expressing the homeodomain transcription factor Mirror. We show that the choice between these alternative outputs of EGFR signaling is regulated by antiparallel gradients of JAK/STAT and BMP pathway activity and that mutual repression between Midline/H15 and Mirror generates a bistable switch that toggles between alternative EGFR signaling outcomes. JAK/STAT and BMP pathway input is integrated through their joint and opposing regulation of both sides of this switch. By converting this positional information into a binary decision between EGFR signaling outcomes, this regulatory network ultimately allows the same ligand-receptor pair to establish both the anterior-posterior (AP) and dorsal-ventral (DV) axes of the tissue.

Response to the dorsal anterior gradient of EGFR signaling in Drosophila oogenesis is prepatterned by earlier posterior EGFR activation.

Spatially restricted epidermal growth factor receptor (EGFR) activity plays a central role in patterning the follicular epithelium of the Drosophila ovary. In midoogenesis, localized EGFR activation is achieved by the graded dorsal anterior localization of its ligand, Gurken. Graded EGFR activity determines multiple dorsal anterior fates along the dorsal-ventral axis but cannot explain the sharp posterior limit of this domain. Here, we show that posterior follicle cells express the T-box transcription factors Midline and H15, which render cells unable to adopt a dorsal anterior fate in response to EGFR activation. The posterior expression of Midline and H15 is itself induced in early oogenesis by posteriorly localized EGFR signaling, defining a feedback loop in which early induction of Mid and H15 confers a molecular memory that fundamentally alters the outcome of later EGFR signaling. Spatial regulation of the EGFR pathway thus occurs both through localization of the ligand and through localized regulation of the cellular response.

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

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.

Graded Egfr activity patterns the Drosophila eggshell independently of autocrine feedback.

The pattern of the Drosophila eggshell is determined by the establishment of a complex and stereotyped pattern of cell fates in the follicular epithelium of the ovary. Localized activation of the Epidermal growth factor receptor (Egfr) is essential for this patterning. Modulation of Egfr pathway activity in time and space determines distinct fates at their appropriate locations, but the details of how Egfr signaling is regulated and how the profile of Egfr activity corresponds to cell fate remain unclear. Here we analyze the effect of loss of various Egfr regulators and targets on follicle cell patterning, using a marker for follicle cell fate, and on the mature eggshell phenotype, using a novel eggshell marker. We show, contrary to current patterning models, that feedback regulation of Egfr activity by the autocrine ligand Spitz and the inhibitor Argos is not necessary for patterning. Given the cell-autonomous nature of the mutant phenotypes we observed, we propose instead that the pattern of cell fates is generated by spatial information derived directly from the germline ligand Gurken, without a requirement for subsequent patterning by diffusible Egfr regulators in the follicular epithelium.

Translational repression of gurken mRNA in the Drosophila oocyte requires the hnRNP Squid in the nurse cells.

Establishment of the Drosophila dorsal-ventral axis depends upon the correct localization of gurken mRNA and protein within the oocyte. gurken mRNA becomes localized to the presumptive dorsal anterior region of the oocyte, but is synthesized in the adjoining nurse cells. Normal gurken localization requires the heterogeneous nuclear ribonucleoprotein Squid, which binds to the gurken 3' untranslated region. However, whether Squid functions in the nurse cells or the oocyte is unknown. To address this question, we generated genetic mosaics in which half of the nurse cells attached to a given oocyte are unable to produce Squid. In these mosaics, gurken mRNA is localized normally but ectopically translated during the dorsal anterior localization process, even though the oocyte contains abundant Squid produced by the wild type nurse cells. These data indicate that translational repression of gurken mRNA requires Squid function in the nurse cells. We propose that Squid interacts with gurken mRNA in the nurse cell nuclei and, together with other factors, maintains gurken in a translationally silent state during its transport to the dorsal anterior region of the oocyte. This translational repression is not required for gurken mRNA localization, indicating that the information repressing translation is separable from that regulating localization.

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.

Capicua regulates follicle cell fate in the Drosophila ovary through repression of mirror.

The dorsoventral axis of the Drosophila egg is established by dorsally localized activation of the epidermal growth factor receptor (Egfr) in the ovarian follicular epithelium. Subsequent positive- and negative-feedback regulation generates two dorsolateral follicle cell primordia that will produce the eggshell appendages. A dorsal midline domain of low Egfr activity between the appendage primordia defines their dorsal boundary, but little is known about the mechanisms that establish their ventral limit. We demonstrate that the transcriptional repressor Capicua is required cell autonomously in ventral and lateral follicle cells to repress dorsal fates, and functions in this process through the repression of mirror. Interestingly, ectopic expression of mirror in the absence of capicua is observed only in the anterior half of the epithelium. We propose that Capicua regulates the pattern of follicle cell fates along the dorsoventral axis by blocking the induction of appendage determinants, such as mirror, by anterior positional cues.

Production of gurken in the nurse cells is sufficient for axis determination in the Drosophila oocyte.

The asymmetric localization of gurken mRNA and protein in the developing Drosophila oocyte defines both the anteroposterior and dorsoventral axes of the future embryo. Understanding the origin of these asymmetries requires knowledge of the source of gurken transcripts. During oogenesis most transcripts in the oocyte are produced by the associated nurse cells, but it has been proposed that gurken is an exceptional oocyte-derived transcript. Using a novel application of a standard mitotic recombination technique, we generated mosaic egg chambers in which the nurse cells, but not the oocyte, could produce gurken. Gurken was properly localized in these mosaics and oocyte axial polarity was established normally, indicating that the nurse cells synthesize gurken and that their contribution is sufficient for Gurken function. Our data demonstrate the existence of a mechanism for transport of gurken from the nurse cells and its subsequent localization within the oocyte.