Decoding vectorial information from a gradient: sequential roles of the receptors Frizzled and Notch in establishing planar polarity in the Drosophila eye

Planar Cell Polarity Signaling: Coordinated Crosstalk for Cell Orientation

The planar cell polarity (PCP) system is essential for positioning cells in 3D networks to establish the proper morphogenesis, structure, and function of organs during embryonic development. The PCP system uses inter- and intracellular feedback interactions between components of the core PCP, characterized by coordinated planar polarization and asymmetric distribution of cell populations inside the cells. PCP signaling connects the anterior-posterior to left-right embryonic plane polarity through the polarization of cilia in the Kupffer's vesicle/node in vertebrates. Experimental investigations on various genetic ablation-based models demonstrated the functions of PCP in planar polarization and associated genetic disorders. This review paper aims to provide a comprehensive overview of PCP signaling history, core components of the PCP signaling pathway, molecular mechanisms underlying PCP signaling, interactions with other signaling pathways, and the role of PCP in organ and embryonic development. Moreover, we will delve into the negative feedback regulation of PCP to maintain polarity, human genetic disorders associated with PCP defects, as well as challenges associated with PCP.

The Rap1 small GTPase affects cell fate or survival and morphogenetic patterning during Drosophila melanogaster eye development

The Drosophila melanogaster eye has been instrumental for determining both how cells communicate with one another to determine cell fate, as well as cell morphogenesis and patterning. Here, we describe the effects of the small GTPase Rap1 on the development of multiple cell types in the D. melanogaster eye. Although Rap1 has previously been linked to RTK-Ras-MAPK signaling in eye development, we demonstrate that manipulation of Rap1 activity is modified by increase or decrease of Delta/Notch signaling during several events of cell fate specification in eye development. In addition, we demonstrate that manipulating Rap1 function either in primary pigment cells or in interommatidial cells affects cone cell contact switching, primary pigment cell enwrapment of the ommatidial cluster, and sorting of secondary and tertiary pigment cells. These data suggest that Rap1 has roles in both ommatidial cell recruitment/survival and in ommatidial morphogenesis in the pupal stage. They lay groundwork for future experiments on the role of Rap1 in these events.

Notch-dependent Abl signaling regulates cell motility during ommatidial rotation in Drosophila

A collective cell motility event that occurs during Drosophila eye development, ommatidial rotation (OR), serves as a paradigm for signaling-pathway-regulated directed movement of cell clusters. OR is instructed by the EGFR and Notch pathways and Frizzled/planar cell polarity (Fz/PCP) signaling, all of which are associated with photoreceptor R3 and R4 specification. Here, we show that Abl kinase negatively regulates OR through its activity in the R3/R4 pair. Abl is localized to apical junctional regions in R4, but not in R3, during OR, and this apical localization requires Notch signaling. We demonstrate that Abl and Notch interact genetically during OR, and Abl co-immunoprecipitates in complexes with Notch in eye discs. Perturbations of Abl interfere with adherens junctional organization of ommatidial preclusters, which mediate the OR process. Together, our data suggest that Abl kinase acts directly downstream of Notch in R4 to fine-tune OR via its effect on adherens junctions.

The early history of the eye-antennal disc of Drosophila melanogaster

A pair of eye-antennal imaginal discs give rise to nearly all external structures of the adult Drosophila head including the compound eyes, ocelli, antennae, maxillary palps, head epidermis, and bristles. In the earliest days of Drosophila research, investigators would examine thousands of adult flies in search of viable mutants whose appearance deviated from the norm. The compound eyes are dispensable for viability and perturbations to their structure are easy to detect. As such, the adult compound eye and the developing eye-antennal disc emerged as focal points for studies of genetics and developmental biology. Since few tools were available at the time, early researchers put an enormous amount of thought into models that would explain their experimental observations-many of these hypotheses remain to be tested. However, these "ancient" studies have been lost to time and are no longer read or incorporated into today's literature despite the abundance of field-defining discoveries that are contained therein. In this FlyBook chapter, I will bring these forgotten classics together and draw connections between them and modern studies of tissue specification and patterning. In doing so, I hope to bring a larger appreciation of the contributions that the eye-antennal disc has made to our understanding of development as well as draw the readers' attention to the earliest studies of this important imaginal disc. Armed with the today's toolkit of sophisticated genetic and molecular methods and using the old papers as a guide, we can use the eye-antennal disc to unravel the mysteries of development.

Different strategies by distinct Wnt-signaling pathways in activating a nuclear transcriptional response

The Wnt family of secreted glycolipo-proteins signals through multiple signal transduction pathways and is essential for embryonic development and organ development and homeostasis. The Wnt-pathways are conserved and critical in all metazoans. Wnt signaling pathways comprise the canonical Wnt/β-catenin pathway and several non-canonical signaling branches, of which Wnt-Planar Cell Polarity (PCP) signaling and the Wnt/Calcium pathway have received the most attention and are best understood. nterestingly, all Wnt-pathways have a nuclear signaling branch and also can affect many cellular processes independent of its nuclear transcriptional regulation. Canonical Wnt/β-catenin signaling is the most critical for a nuclear transcriptional response, in both development and disease, yet the mechanism(s) on how the "business end" of the pathway, β-catenin, translocates to the nucleus to act as co-activator to the TCF/Lef transcription factor family still remains obscure. Here we discuss and compare the very different strategies on how the respective Wnt signaling pathways activate a nuclear transcriptional response. We also highlight some recent new insights into how β-catenin is translocated to the nucleus via an IFT-A, Kinesin-2, and microtubule dependent mechanism and how this aspect of canonical Wnt-signaling uses ciliary proteins in a cilium independent manner, conserved between Drosophila and mammalian cells.

Wnt-frizzled planar cell polarity signaling in the regulation of cell motility

The molecular complexes underlying planar cell polarity (PCP) were first identified in Drosophila through analysis of mutant phenotypes in the adult cuticle and the orientation of associated polarized protrusions such as wing hairs and sensory bristles. The same molecules are conserved in vertebrates and are required for the localization of polarized protrusions such as primary or sensory cilia and the orientation of hair follicles. Not only is PCP signaling required to align cellular structures across a tissue, it is also required to coordinate movement during embryonic development and adult homeostasis. PCP signaling allows cells to interpret positional cues within a tissue to move in the appropriate direction and to coordinate this movement with their neighbors. In this review we outline the molecular basis of the core Wnt-Frizzled/PCP pathway, and describe how this signaling orchestrates collective motility in Drosophila and vertebrates. Here we cover the paradigms of ommatidial rotation and border cell migration in Drosophila, and convergent extension in vertebrates. The downstream cell biological processes that underlie polarized motility include cytoskeletal reorganization, and adherens junctional and extracellular matrix remodeling. We discuss the contributions of these processes in the respective cell motility contexts. Finally, we address examples of individual cell motility guided by PCP factors during nervous system development and in cancer disease contexts.

Interaction between Ras and Src clones causes interdependent tumor malignancy via Notch signaling in Drosophila

Cancer tissue often comprises multiple tumor clones with distinct oncogenic alterations such as Ras or Src activation, yet the mechanism by which tumor heterogeneity drives cancer progression remains elusive. Here, we show in Drosophila imaginal epithelium that clones of Ras- or Src-activated benign tumors interact with each other to mutually promote tumor malignancy. Mechanistically, Ras-activated cells upregulate the cell-surface ligand Delta while Src-activated cells upregulate its receptor Notch, leading to Notch activation in Src cells. Elevated Notch signaling induces the transcriptional repressor Zfh1/ZEB1, which downregulates E-cadherin and cell death gene hid, leading to Src-activated invasive tumors. Simultaneously, Notch activation in Src cells upregulates the cytokine Unpaired/IL-6, which activates JAK-STAT signaling in neighboring Ras cells. Elevated JAK-STAT signaling upregulates the BTB-zinc-finger protein Chinmo, which downregulates E-cadherin and thus generates Ras-activated invasive tumors. Our findings provide a mechanistic explanation for how tumor heterogeneity triggers tumor progression via cell-cell interactions.

Integrins are required for synchronous ommatidial rotation in the Drosophila eye linking planar cell polarity signalling to the extracellular matrix

Integrins mediate the anchorage between cells and their environment, the extracellular matrix (ECM), and form transmembrane links between the ECM and the cytoskeleton, a conserved feature throughout development and morphogenesis of epithelial organs. Here, we demonstrate that integrins and components of the ECM are required during the planar cell polarity (PCP) signalling-regulated cell movement of ommatidial rotation in the Drosophila eye. The loss-of-function mutations of integrins or ECM components cause defects in rotation, with mutant clusters rotating asynchronously compared to wild-type clusters. Initially, mutant clusters tend to rotate faster, and at later stages they fail to be synchronous with their neighbours, leading to aberrant rotation angles and resulting in a disorganized ommatidial arrangement in adult eyes. We further demonstrate that integrin localization changes dynamically during the rotation process. Our data suggest that core Frizzled/PCP factors, acting through RhoA and Rho kinase, regulate the function/activity of integrins and that integrins thus contribute to the complex interaction network of PCP signalling, cell adhesion and cytoskeletal elements required for a precise and synchronous 90° rotation movement.

Counter-rotational cell flows drive morphological and cell fate asymmetries in mammalian hair follicles

Organ morphogenesis is a complex process coordinated by cell specification, epithelial-mesenchymal interactions and tissue polarity. A striking example is the pattern of regularly spaced, globally aligned mammalian hair follicles, which emerges through epidermal-dermal signaling and planar polarized morphogenesis. Here, using live-imaging, we discover that developing hair follicles polarize through dramatic cell rearrangements organized in a counter-rotational pattern of cell flows. Upon hair placode induction, Shh signaling specifies a radial pattern of progenitor fates that, together with planar cell polarity, induce counter-rotational rearrangements through myosin and ROCK-dependent polarized neighbour exchanges. Importantly, these cell rearrangements also establish cell fate asymmetry by repositioning radial progenitors along the anterior-posterior axis. These movements concurrently displace associated mesenchymal cells, which then signal asymmetrically to maintain polarized cell fates. Our results demonstrate how spatial patterning and tissue polarity generate an unexpected collective cell behaviour that in turn, establishes both morphological and cell fate asymmetry.

Antagonistic PCP Signaling Pathways in the developing Drosophila eye

In Planar cell polarity (PCP), cells coordinately polarize their cytoskeletons within the plane of the epithelium in which they lie. In most insect epithelia this is indicated by the coordinated projections of the hairs secreted by the ectodermal cells. PCP of this form has been effectively studied in Drosophila, but it has proven difficult to achieve an integrated description of the roles played by the various proteins. In the insect eye, PCP is not evident as the polarization of individual cells, but as the asymmetric arrangements of the cells of the ommatidia. This different form of PCP allows different studies to be performed, and using this system we have detected the action of two antagonistic signaling pathways. Even though antagonistic, the two pathways synergize and cooperate to ensure that the correct arrangement of the cells is achieved. The cooperative use of antagonistic signaling pathways occurs in the polarization of chemotacting cells, and we discuss the possibility that a similar molecular principle may underlie PCP.

AKAP200 promotes Notch stability by protecting it from Cbl/lysosome-mediated degradation in Drosophila melanogaster

AKAP200 is a Drosophila melanogaster member of the “A Kinase Associated Protein” family of scaffolding proteins, known for their role in the spatial and temporal regulation of Protein Kinase A (PKA) in multiple signaling contexts. Here, we demonstrate an unexpected function of AKAP200 in promoting Notch protein stability. In Drosophila, AKAP200 loss-of-function (LOF) mutants show phenotypes that resemble Notch LOF defects, including eye patterning and sensory organ specification defects. Through genetic interactions, we demonstrate that AKAP200 interacts positively with Notch in both the eye and the thorax. We further show that AKAP200 is part of a physical complex with Notch. Biochemical studies reveal that AKAP200 stabilizes endogenous Notch protein, and that it limits ubiquitination of Notch. Specifically, our genetic and biochemical evidence indicates that AKAP200 protects Notch from the E3-ubiquitin ligase Cbl, which targets Notch to the lysosomal pathway. Indeed, we demonstrate that the effect of AKAP200 on Notch levels depends on the lysosome. Interestingly, this function of AKAP200 is fully independent of its role in PKA signaling and independent of its ability to bind PKA. Taken together, our data indicate that AKAP200 is a novel tissue specific posttranslational regulator of Notch, maintaining high Notch protein levels and thus promoting Notch signaling.

AKAP200 belongs to a family of scaffolding proteins best known for their regulation of PKA localization. In this study, we have identified a novel role of AKAP200 in Notch protein stability and signaling. In Drosophila melanogaster, AKAP200’s loss and gain-of-function (LOF/GOF) phenotypes are characteristic of Notch signaling defects. Furthermore, we demonstrated genetic interactions between AKAP200 and Notch. Consistent with this, AKAP200 stabilizes the endogenous Notch protein and limits its ubiquitination. AKAP200 exerts its effects on Notch by antagonizing Cbl-mediated ubiquitination and thus lysosome targeting of Notch. Based on these data, we postulate a novel PKA independent mechanism of AKAP200 to achieve optimal Notch protein levels, with AKAP200 preventing Cbl-mediated lysosomal degradation of Notch.

Tissue morphodynamics: Translating planar polarity cues into polarized cell behaviors

The ability of cells to collectively orient and align their behaviors is essential in multicellular organisms for unidirectional cilia beating, collective cell movements, oriented cell divisions, and asymmetric cell fate specification. The planar cell polarity pathway coordinates a vast and diverse array of collective cell behaviors by intersecting with downstream pathways that regulate cytoskeletal dynamics and intercellular signaling. How the planar polarity pathway translates directional cues to produce polarized cell behaviors is the focus of this review.

Feedback regulation of apical progenitor fate by immature neurons through Wnt7-Celsr3-Fzd3 signalling

Sequential generation of neurons and glial cells during development is critical for the wiring and function of the cerebral cortex. This process requires accurate coordination of neural progenitor cell (NPC) fate decisions, by NPC-autonomous mechanisms as well as by negative feedback from neurons. Here, we show that neurogenesis is protracted and gliogenesis decreased in mice with mutations of genes Celsr3 and Fzd3. This phenotype is not due to gene inactivation in progenitors, but rather in immature cortical neurons. Mutant neurons are unable to upregulate expression of Jag1 in response to cortical Wnt7, resulting in blunted activation of Notch signalling in NPC. Thus, Celsr3 and Fzd3 enable immature neurons to respond to Wnt7, upregulate Jag1 and thereby facilitate feedback signals that tune the timing of NPC fate decisions via Notch activation.

Coordination of planar cell polarity pathways through Spiny-legs

Morphogenesis and physiology of tissues and organs requires planar cell polarity (PCP) systems that orient and coordinate cells and their behaviors, but the relationship between PCP systems has been controversial. We have characterized how the Frizzled and Dachsous-Fat PCP systems are connected through the Spiny-legs isoform of the Prickle-Spiny-legs locus. Two different components of the Dachsous-Fat system, Dachsous and Dachs, can each independently interact with Spiny-legs and direct its localization in vivo. Through characterization of the contributions of Prickle, Spiny-legs, Dachsous, Fat, and Dachs to PCP in the Drosophila wing, eye, and abdomen, we define where Dachs-Spiny-legs and Dachsous-Spiny-legs interactions contribute to PCP, and provide a new understanding of the orientation of polarity and the basis of PCP phenotypes. Our results support the direct linkage of PCP systems through Sple in specific locales, while emphasizing that cells can be subject to and must ultimately resolve distinct, competing PCP signals.

DOI:http://dx.doi.org/10.7554/eLife.09946.001

Animals have many asymmetric organs. Wings, for example, are aerodynamically shaped and have a clear front, back, top and bottom, and even additions to these organs, such as feathers on the wing, often need to be oriented in a specific manner.

This kind of orientation arises when cells divide and grow asymmetrically in a flat plane. The asymmetry is established at the level of single cells when proteins are not equally spread throughout a cell, but rather asymmetrically distributed. Such cells are said to be ‘planar polarized’; and many experiments addressing this so-called planar cell polarity have been conducted in fruit flies, because they can be genetically altered easily. Previous studies have shown that two signaling pathways—called Frizzled and Dachsous-Fat—regulate how individual cells orient themselves within a flat sheet of cells that forms fruit fly’s wing. The two pathways are not independent, but it is unclear how they are linked. In particular, there has been conflicting evidence as to whether the Dachsous-Fat pathway controls the Frizzled pathway or whether the two act in parallel.

Now, Ambegaonkar and Irvine have discovered new roles for a protein that is involved in both pathways, called 'Spiny-legs'. This protein was known to be important in the Frizzled pathway, but, when it was tracked with a fluorescent tag in developing wing cells it also accumulated in areas where two proteins that make up part of the Dachsous-Fat pathway were located. Biochemical experiments showed that both of these proteins (which are called Dachs or Dachsous) could physically interact with Spiny-legs. Ambegaonkar and Irvine therefore deleted the genes for Dachs or Dachsous in fruit flies and observed that Spiny-legs no longer organized itself in the proper way, implying that Dachs and Dachsous control where Spiny-legs goes within cells.

When this analysis was extended to other fruit fly organs, such as the eyes, Ambegaonkar and Irvine found that Dachsous was more important than Dachs for the correct localization of Spiny-legs. Additionally, the Frizzled and Dachsous-Fat pathways seemed to compete for interactions with Spiny-legs. This connection between the two pathways helps to explain how cells behave when several different signals reach them. It also shows how different organs can reuse conserved components of the pathways to make different end products. Future studies should aim to work out the number of systems that polarize cells and how they are connected in different tissues.

DOI:http://dx.doi.org/10.7554/eLife.09946.002

A dynamic intracellular distribution of Vangl2 accompanies cell polarization during zebrafish gastrulation

During vertebrate gastrulation, convergence and extension movements elongate embryonic tissues anteroposteriorly and narrow them mediolaterally. Planar cell polarity (PCP) signaling is essential for mediolateral cell elongation underlying these movements, but how this polarity arises is poorly understood. We analyzed the elongation, orientation and migration behaviors of lateral mesodermal cells undergoing convergence and extension movements in wild-type zebrafish embryos and mutants for the Wnt/PCP core component Vangl2 (Trilobite). We demonstrate that Vangl2 function is required at the time when cells transition to a highly elongated and mediolaterally aligned body. vangl2 mutant cells fail to undergo this transition and to migrate along a straight path with high net speed towards the dorsal midline. Instead, vangl2 mutant cells exhibit an anterior/animal pole bias in cell body alignment and movement direction, suggesting that PCP signaling promotes effective dorsal migration in part by suppressing anterior/animalward cell polarity and movement. Endogenous Vangl2 protein accumulates at the plasma membrane of mesenchymal converging cells at the time its function is required for mediolaterally polarized cell behavior. Heterochronic cell transplantations demonstrated that Vangl2 cell membrane accumulation is stage dependent and regulated by both intrinsic factors and an extracellular signal, which is distinct from PCP signaling or other gastrulation regulators, including BMP and Nodals. Moreover, mosaic expression of fusion proteins revealed enrichment of Vangl2 at the anterior cell edges of highly mediolaterally elongated cells. These results demonstrate that the dynamic Vangl2 intracellular distribution is coordinated with and necessary for the changes in convergence and extension cell behaviors during gastrulation.

Extramacrochaetae functions in dorsal-ventral patterning of Drosophila imaginal discs

One of the seminal events in the history of a tissue is the establishment of the anterior-posterior, dorsal-ventral (D/V) and proximal-distal axes. Axis formation is important for the regional specification of a tissue and allows cells along the different axes to obtain directional and positional information. Within the Drosophila retina, D/V axis formation is essential to ensure that each unit eye first adopts the proper chiral form and then rotates precisely 90° in the correct direction. These two steps are important because the photoreceptor array must be correctly aligned with the neurons of the optic lobe. Defects in chirality and/or ommatidial rotation will lead to disorganization of the photoreceptor array, misalignment of retinal and optic lobe neurons, and loss of visual acuity. Loss of the helix-loop-helix protein Extramacrochaetae (Emc) leads to defects in both ommatidial chirality and rotation. Here, we describe a new role for emc in eye development in patterning the D/V axis. We show that the juxtaposition of dorsal and ventral fated tissue in the eye leads to an enrichment of emc expression at the D/V midline. emc expression at the midline can be eliminated when D/V patterning is disrupted and can be induced in situations in which ectopic boundaries are artificially generated. We also show that emc functions downstream of Notch signaling to maintain the expression of four-jointed along the midline.

Planar cell polarity signaling: coordination of cellular orientation across tissues

Establishment of Planar Cell Polarity (PCP) in epithelia, in the plane of an epithelium, is an important feature of the development and homeostasis of most organs. Studies in different model organisms have contributed a wealth of information regarding the mechanisms that govern PCP regulation. Genetic studies in Drosophila have identified two signaling systems, the Fz/PCP and Fat/Dachsous system, which are both required for PCP establishment in many different tissues in a largely non-redundant manner. Recent advances in vertebrate PCP studies have added novel factors of PCP regulation and also new cellular features requiring PCP-signaling input, including the positioning and orientation of the primary cilium of many epithelial cells. This review focuses mostly on several recent advances made in the Drosophila and vertebrate PCP field and integrates these within the existing PCP-signaling framework.

Switching cell fates in the developing Drosophila eye

The developing Drosophila ommatidium is characterized by two distinct waves of pattern formation. In the first wave, a precluster of five cells is formed by a complex cellular interaction mechanism. In the second wave, cells are systematically recruited to the cluster and directed to their fates by developmental cues presented by differentiating precluster cells. These developmental cues are mediated through the receptor tyrosine kinase (RTK) and Notch (N) signaling pathways and their combined activities are crucial in specifying cell type. The transcription factor Lozenge (Lz) is expressed exclusively in second wave cells. Here, we ectopically supply Lz to precluster cells and concomitantly supply the various RTK/N codes that specify each of three second wave cell fates. We thereby reproduce molecular markers of each of the second wave cell types in precluster cells and draw three inferences. First, we confirm that Lz provides key intrinsic information to second wave cells. We can now combine this with the RTK/N signaling to provide a cell fate specification code that entails both extrinsic and intrinsic information. Second, the reproduction of each second wave cell type in the precluster confirms the accuracy of the RTK/N signaling code. Third, RTK/N signaling and Lz need only be presented to the cells for a short period of time in order to specify their fate.

The Drosophila selectin furrowed mediates intercellular planar cell polarity interactions via frizzled stabilization

Establishment of planar cell polarity (PCP) in a tissue requires coordination of directional signals from cell to cell. It is thought that this is mediated by the core PCP factors, which include cell-adhesion molecules. Here, we demonstrate that furrowed, the Drosophila selectin, is required for PCP generation. Disruption of PCP in furrowed-deficient flies results from a primary defect in Fz levels and cell adhesion. Furrowed localizes at or near apical junctions, largely colocalizing with Frizzled and Flamingo (Fmi). It physically interacts with and stabilizes Frizzled, and it mediates intercellular Frizzled-Van Gogh (Vang)/Strabismus interactions, similarly to Fmi. Furrowed does so through a homophilic cell-adhesion role that is distinct from its known carbohydrate-binding function described during vertebrate blood-cell/endothelial cell interactions. Importantly, the carbohydrate function is dispensable for PCP establishment. In vivo studies suggest that Furrowed functions partially redundantly with Fmi, mediating intercellular Frizzled-Vang interactions between neighboring cells.

Strabismus promotes recruitment and degradation of farnesylated prickle in Drosophila melanogaster planar polarity specification

The core planar polarity proteins are required to specify the orientation of structures that are polarised in the plane of the epithelium. In the Drosophila melanogaster wing, the core proteins localise asymmetrically at either proximal or distal cell edges. Asymmetric localisation is thought to be biased by long-range cues, causing asymmetric complexes to become aligned with the tissue axes. Core proteins are then thought to participate in feedback interactions that are necessary to amplify asymmetry, and in order for such feedback interactions to operate correctly, the levels of the core proteins at junctions must be tightly regulated. We have investigated regulation of the core protein Prickle (Pk) in the pupal wing. The core protein Strabismus (Stbm) is required to recruit Pk into asymmetric complexes at proximal cell ends, and we report here that it also promotes proteasomal degradation of excess Pk, probably via a Cullin-1 dependent process. We also show for the first time that Pk is farnesylated in vivo, and this is essential for Pk function in the wing. Notably, farnesylation of Pk is necessary for it to be recruited into asymmetric complexes and function in feedback amplification, probably by reinforcing weak direct interactions between Stbm and Pk. Furthermore, farnesylation is also required for Stbm to promote proteasomal degradation of Pk. We propose that Stbm recruits farnesylated Pk into asymmetric complexes, but also promotes degradation of excess Pk that would otherwise perturb feedback amplification.

Frizzled receptors signal through G proteins

Frizzled receptors have long been thought to couple to G proteins but biochemical evidence supporting such an interaction has been lacking. Here we expressed mammalian Wnt-Frizzled fusion proteins in Saccharomyces cerevisiae and tested the receptors' ability to activate the yeast mitogen-activated protein kinase (MAPK) pathway via heterotrimeric G proteins. Our results show that Frizzled receptors can interact with Gαi, Gαq, and Gαs proteins, thus confirming that Frizzled functions as a G protein coupled receptor (GPCR). However, the activity level of Frizzled-mediated G protein signaling was much lower than that of a typical GPCR and, surprisingly, was highest when coupled to Gαs. The Frizzled/Gαs interaction was further established in vivo as Drosophila expressing a loss-of-function Gαs allele rescued the photoreceptor differentiation phenotype of Frizzled mutant flies. Together, these data point to an important role for Frizzled as a nontraditional GPCR that preferentially couples to Gαs heterotrimeric G proteins.

Notch and the awesome power of genetics

Notch is a receptor that mediates cell-cell interactions in animal development, and aberrations in Notch signal transduction can cause cancer and other human diseases. Here, I describe the major advances in the Notch field from the identification of the first mutant in Drosophila almost a century ago through the elucidation of the unusual mechanism of signal transduction a little over a decade ago. As an essay for the GENETICS Perspectives series, it is my personal and critical commentary as well as an historical account of discovery.

Hibris, a Drosophila nephrin homolog, is required for presenilin-mediated Notch and APP-like cleavages

Drosophila Hibris (Hbs), a member of the Nephrin Immunoglobulin Super Family, has been implicated in myogenesis and eye patterning. Here, we uncover a role of Hbs in Notch (N) signaling and γ-secretase processing. Loss of hbs results in classical N-signaling-associated phenotypes in Drosophila, including eye patterning, wing margin, and sensory organ specification defects. In particular, hbs mutant larvae display altered γ-secretase-dependent Notch proteolytic processing. Hbs also interacts molecularly and genetically with Presenilin (Psn) and other components of the γ-secretase complex. This Hbs function appears conserved, as mammalian Nephrin also promotes N signaling in mammalian cells. Our data suggest that Hbs is required for Psn maturation. Consistent with its role in Psn processing, Hbs genetically interacts with the Drosophila β-amyloid protein precursor-like (Appl) protein, the homolog of mammalian APP, the cleavage of which is associated with Alzheimer's disease. Thus, Hbs/Nephrin appear to share a general requirement in Psn/γ-secretase regulation and associated processes.

Planar cell polarity controls directional Notch signaling in the Drosophila leg

The generation of functional structures during development requires tight spatial regulation of signaling pathways. Thus, in Drosophila legs, in which Notch pathway activity is required to specify joints, only cells distal to ligand-producing cells are capable of responding. Here, we show that the asymmetric distribution of planar cell polarity (PCP) proteins correlates with this spatial restriction of Notch activation. Frizzled and Dishevelled are enriched at distal sides of each cell and hence localize at the interface with ligand-expressing cells in the non-responding cells. Elimination of PCP gene function in cells proximal to ligand-expressing cells is sufficient to alleviate the repression, resulting in ectopic Notch activity and ectopic joint formation. Mutations that compromise a direct interaction between Dishevelled and Notch reduce the efficacy of repression. Likewise, increased Rab5 levels or dominant-negative Deltex can suppress the ectopic joints. Together, these results suggest that PCP coordinates the spatial activity of the Notch pathway by regulating endocytic trafficking of the receptor.

Ral inhibits ligand-independent Notch signaling in Drosophila

We discovered recently that the Drosophila Ral GTPase regulates Notch signaling and thereby affects cell patterning in the eye. Although Ral functions in the ligand signaling cells, Ral does not stimulate ligand signaling directly. Rather, in cells that express both Notch receptor and ligand, Ral activity promotes a cell to become the signaler by inhibiting Notch receptor activation in that cell. Moreover, Ral inhibits a particular pathway of Notch activation-receptor activation that occurs independent of ligand binding. In this Commentary, we discuss the phenomenon of ligand-independent Notch receptor activation and how this event might be regulated by Ral.

Novel regulators of planar cell polarity: a genetic analysis in Drosophila

Planar cell polarity (PCP) is a common feature of many epithelia and epithelial organs. Although progress has been made in the dissection of molecular mechanisms regulating PCP, many questions remain. Here we describe a screen to identify novel PCP regulators in Drosophila. We employed mild gain-of-function (GOF) phenotypes of two cytoplasmic Frizzled (Fz)/PCP core components, Diego (Dgo) and Prickle (Pk), and screened these against the DrosDel genome-wide deficiency collection for dominant modifiers. Positive genomic regions were rescreened and narrowed down with smaller overlapping deficiencies from the Exelixis collection and RNAi-mediated knockdown applied to individual genes. This approach isolated new regulators of PCP, which were confirmed with loss-of-function analyses displaying PCP defects in the eye and/or wing. Furthermore, knockdown of a subset was also sensitive to dgo dosage or dominantly modified a dishevelled (dsh) GOF phenotype, supporting a role in Fz/PCP-mediated polarity establishment. Among the new "PCP" genes we identified several kinases, enzymes required for lipid modification, scaffolding proteins, and genes involved in substrate modification and/or degradation. Interestingly, one of them is a member of the Meckel-Gruber syndrome factors, associated with human ciliopathies, suggesting an important role for cell polarity in nonciliated cells.

Drosophila Epsin's role in Notch ligand cells requires three Epsin protein functions: the lipid binding function of the ENTH domain, a single Ubiquitin interaction motif, and a subset of the C-terminal protein binding modules

Epsin is an endocytic protein that binds Clathrin, the plasma membrane, Ubiquitin, and also a variety of other endocytic proteins through well-characterized motifs. Although Epsin is a general endocytic factor, genetic analysis in Drosophila and mice revealed that Epsin is essential specifically for internalization of ubiquitinated transmembrane ligands of the Notch receptor, a process required for Notch activation. Epsin's mechanism of function is complex and context-dependent. Consequently, how Epsin promotes ligand endocytosis and thus Notch signaling is unclear, as is why Notch signaling is uniquely dependent on Epsin. Here, by generating Drosophila lines containing transgenes that express a variety of different Epsin deletion and substitution variants, we tested each of the five protein or lipid interaction modules for a role in Notch activation by each of the two ligands, Serrate and Delta. There are five main results of this work that impact present thinking about the role of Epsin in ligand cells. First, we discovered that deletion or mutation of both UIMs destroyed Epsin's function in Notch signaling and had a greater negative impact on Epsin activity than removal of any other module type. Second, only one of Epsin's two UIMs was essential. Third, the lipid-binding function of the ENTH domain was required only for maximal Epsin activity. Fourth, although the C-terminal Epsin modules that interact with Clathrin, the adapter protein complex AP-2, or endocytic accessory proteins were necessary collectively for Epsin activity, their functions were highly redundant; most unexpected was the finding that Epsin's Clathrin binding motifs were dispensable. Finally, we found that signaling from either ligand, Serrate or Delta, required the same Epsin modules. All of these observations are consistent with a model where Epsin's essential function in ligand cells is to link ubiquitinated Notch ligands to Clathrin-coated vesicles through other Clathrin adapter proteins. We propose that Epsin's specificity for Notch signaling simply reflects its unique ability to interact with the plasma membrane, Ubiquitin, and proteins that bind Clathrin.

The frizzled/stan pathway and planar cell polarity in the Drosophila wing

Drosophila has been the key model system for studies on planar cell polarity (PCP). The rich morphology of the insect exoskeleton contains many structures that display PCP. Among these are the trichomes (cuticular hairs) that cover much of the exoskeleton, sensory bristles, and ommatidia. Many genes have been identified that must function for the development of normal PCP. Among these are the genes that comprise the frizzled/starry night (fz/stan) and dachsous/fat pathways. The mechanisms that underlie the function of the fz/stan pathway are best understood. All of the protein products of these genes accumulate asymmetrically in wing cells and there is good evidence that this involves local intercellular signaling between protein complexes on the distal edge of one cell and the juxtaposed proximal edge of its neighbor. It is thought that a feedback system, directed transport, and stabilizing protein-protein interactions mediate the formation of distal and proximal protein complexes. These complexes appear to recruit downstream proteins that function to spatially restrict the activation of the cytoskeleton in wing cells. This leads to the formation of the array of distally pointing hairs found on wings.

Planar cell polarity in Drosophila

In all multicellular organisms, epithelial cells are not only polarized along the apical-basal axis, but also within the epithelial plane, giving cells a sense of direction. Planar cell polarity (PCP) signaling regulates establishment of polarity within the plane of an epithelium. The outcomes of PCP signaling are diverse and include the determination of cell fates, the generation of asymmetric but highly aligned structures, such as the stereocilia in the human inner ear or the hairs on a fly wing, or the directional migration of cells during convergence and extension during vertebrate gastrulation. In humans, aberrant PCP signaling can result in severe developmental defects, such as open neural tubes (spina bifida), and can cause cystic kidneys. In this review, we discuss the basic mechanism and more recent findings of PCP signaling focusing on Drosophila melanogaster, the model organism in which most key PCP components were initially identified.

Principles of planar polarity in animal development

Planar polarity describes the coordinated polarisation of cells or structures in the plane of a tissue. The patterning mechanisms that underlie planar polarity are well characterised in Drosophila, where many events are regulated by two pathways: the 'core' planar polarity complex and the Fat/Dachsous system. Components of both pathways also function in vertebrates and are implicated in diverse morphogenetic processes, some of which self-evidently involve planar polarisation and some of which do not. Here, we review the molecular mechanisms and cellular consequences of planar polarisation in diverse contexts, seeking to identify the common principles across the animal kingdom.

The functions of auxilin and Rab11 in Drosophila suggest that the fundamental role of ligand endocytosis in notch signaling cells is not recycling

Notch signaling requires ligand internalization by the signal sending cells. Two endocytic proteins, epsin and auxilin, are essential for ligand internalization and signaling. Epsin promotes clathrin-coated vesicle formation, and auxilin uncoats clathrin from newly internalized vesicles. Two hypotheses have been advanced to explain the requirement for ligand endocytosis. One idea is that after ligand/receptor binding, ligand endocytosis leads to receptor activation by pulling on the receptor, which either exposes a cleavage site on the extracellular domain, or dissociates two receptor subunits. Alternatively, ligand internalization prior to receptor binding, followed by trafficking through an endosomal pathway and recycling to the plasma membrane may enable ligand activation. Activation could mean ligand modification or ligand transcytosis to a membrane environment conducive to signaling. A key piece of evidence supporting the recycling model is the requirement in signaling cells for Rab11, which encodes a GTPase critical for endosomal recycling. Here, we use Drosophila Rab11 and auxilin mutants to test the ligand recycling hypothesis. First, we find that Rab11 is dispensable for several Notch signaling events in the eye disc. Second, we find that Drosophila female germline cells, the one cell type known to signal without clathrin, also do not require auxilin to signal. Third, we find that much of the requirement for auxilin in Notch signaling was bypassed by overexpression of both clathrin heavy chain and epsin. Thus, the main role of auxilin in Notch signaling is not to produce uncoated ligand-containing vesicles, but to maintain the pool of free clathrin. Taken together, these results argue strongly that at least in some cell types, the primary function of Notch ligand endocytosis is not for ligand recycling.

Ral GTPase promotes asymmetric Notch activation in the Drosophila eye in response to Frizzled/PCP signaling by repressing ligand-independent receptor activation

Ral is a small Ras-like GTPase that regulates membrane trafficking and signaling. Here, we show that in response to planar cell polarity (PCP) signals, Ral modulates asymmetric Notch signaling in the Drosophila eye. Specification of the initially equivalent R3/R4 photoreceptor precursor cells in each developing ommatidium occurs in response to a gradient of Frizzled (Fz) signaling. The cell with the most Fz signal (R3) activates the Notch receptor in the adjacent cell (R4) via the ligand Delta, resulting in R3/R4 cell determination and their asymmetric positions within the ommatidium. Two mechanisms have been proposed for ensuring that the cell with the most Fz activation sends the Delta signal: Fz-dependent transcriptional upregulation in R3 of genes that promote Delta signaling, and direct blockage of Notch receptor activation in R3 by localization of an activated Fz/Disheveled protein complex to the side of the plasma membrane adjacent to R4. Here, we discover a distinct mechanism for biasing the direction of Notch signaling that depends on Ral. Using genetic experiments in vivo, we show that, in direct response to Fz signaling, Ral transcription is upregulated in R3, and Ral represses ligand-independent activation of Notch in R3. Thus, prevention of ligand-independent Notch activation is not simply a constitutive process, but is a target for regulation by Ral during cell fate specification and pattern formation.

Two-color in vivo imaging of photoreceptor apoptosis and development in Drosophila

We report a new two-color fluorescent imaging system to visualize the mosaic adult photoreceptor neurons (PRs) in real-time. Using this method, we examined a collection of 434 mutants and identified genes required for PR survival, planar cell polarity (PCP), patterning and differentiation. We could track the progression of PR degeneration in living flies. By introducing the expression of p35, a caspase inhibitor, we found mutations that specifically activate caspase-dependent death. Moreover, we showed that grh is required in R3 for correct PCP establishment. The "Tomato/GFP-FLP/FRT" method allows high-throughput, rapid and precise identification of survival and developmental pathways in living adult PRs at single-cell resolution.

Stochastic mechanisms of cell fate specification that yield random or robust outcomes

Although cell fate specification is tightly controlled to yield highly reproducible results and avoid extreme variation, developmental programs often incorporate stochastic mechanisms to diversify cell types. Stochastic specification phenomena are observed in a wide range of species and an assorted set of developmental contexts. In bacteria, stochastic mechanisms are utilized to generate transient subpopulations capable of surviving adverse environmental conditions. In vertebrate, insect, and worm nervous systems, stochastic fate choices are used to increase the repertoire of sensory and motor neuron subtypes. Random fate choices are also integrated into developmental programs controlling organogenesis. Although stochastic decisions can be maintained to produce a mosaic of fates within a population of cells, they can also be compensated for or directed to yield robust and reproducible outcomes.

Abelson family kinases regulate Frizzled planar cell polarity signaling via Dsh phosphorylation

Abelson (Abl) family tyrosine kinases have been implicated in cell morphogenesis, adhesion, motility, and oncogenesis. Using a candidate approach for genes involved in planar cell polarity (PCP) signaling, we identified Drosophila Abl (dAbl) as a modulator of Frizzled(Fz)/PCP signaling. We demonstrate that dAbl positively regulates the Fz/Dishevelled (Dsh) PCP pathway without affecting canonical Wnt/Wg-Fz signaling. Genetic dissection suggests that Abl functions via Fz/Dsh signaling in photoreceptor R3 specification, a well-established Fz-PCP signaling readout. Molecular analysis shows that dAbl binds and phosphorylates Dsh on Tyr473 within the DEP domain. This phosphorylation event on Dsh is functionally critical, as the equivalent DshY473F mutant is nonfunctional in PCP signaling and stable membrane association, although it rescues canonical Wnt signaling. Strikingly, mouse embryonic fibroblasts (MEFs) deficient for Abl1 and Abl2/Arg genes also show reduced Dvl2 phosphorylation as compared with control MEFs, and this correlates with a change in subcellular localization of endogenous Dvl2. As in Drosophila, such Abl-deficient MEFs show no change in canonical Wnt signaling. Taken together, our results argue for a conserved role of Abl family members in the positive regulation of Dsh activity toward Fz-Dsh/PCP signaling by Dsh phosphorylation.

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.

Planar cell polarity signaling in the Drosophila eye

Planar cell polarity (PCP) signaling regulates the establishment of polarity within the plane of an epithelium and allows cells to obtain directional information. Its results are as diverse as the determination of cell fates, the generation of asymmetric but highly aligned structures (e.g., stereocilia in the human ear or hairs on a fly wing), or the directional migration of cells during convergent extension during vertebrate gastrulation. Aberrant PCP establishment can lead to human birth defects or kidney disease. PCP signaling is governed by the noncanonical Wnt or Fz/PCP pathway. Traditionally, PCP establishment has been best studied in Drosophila, mainly due to the versatility of the fly as a genetic model system. In Drosophila, PCP is essential for the orientation of wing and abdominal hairs, the orientation of the division axis of sensory organ precursors, and the polarization of ommatidia in the eye, the latter requiring a highly coordinated movement of groups of photoreceptor cells during the process of ommatidial rotation. Here, I review our current understanding of PCP signaling in the Drosophila eye and allude to parallels in vertebrates.

Signaling in the third dimension: the peripodial epithelium in eye disc development

The eye-antennal imaginal disc of Drosophila melanogaster has often been described as an epithelial monolayer with complex signaling events playing out in two dimensions. However, the imaginal disc actually comprises two opposing epithelia (the peripodial epithelium, or PE, and the disc proper, or DP) separated by a lumen to form a sac-like structure. Recent studies expose complex molecular interactions between the PE and the DP, and reveal dynamic communication between the two tissues. Further findings suggest the PE makes important contributions to DP development by acting as a source of signaling molecules as well as cells. Here we summarize those findings and highlight implications for further research.

cis-Inhibition of Notch by endogenous Delta biases the outcome of lateral inhibition

Lateral inhibition mediated by Delta/Notch (Dl/N) signaling is used throughout development to limit the number of initially equivalent cells that adopt a particular fate. Although adjacent cells express both Dl ligand and N receptor, signaling between them ultimately occurs in only one direction. Classically, this has been explained entirely by feedback: activated N can downregulate Dl, amplifying even slight asymmetries in the Dl or N activities of adjacent cells. Here, however, we present an example of lateral inhibition in which unidirectional signaling depends instead on Dl's ability to inhibit N within the same cell, a phenomenon known as cis-inhibition. By genetically manipulating individual R1/R6/R7 photoreceptor precursors in the Drosophila eye, we show that loss of Dl-mediated cis-inhibition reverses the direction of lateral signaling. Based on our finding that Dl in R1/R6s requires endocytosis to trans-activate but not to cis-inhibit N, we reexamine previously published data from other examples of lateral inhibition. We conclude that cis-inhibition generally influences the direction of Dl/N signaling and should therefore be included in standard models of lateral inhibition.

The apical/basal-polarity determinant Scribble cooperates with the PCP core factor Stbm/Vang and functions as one of its effectors

Most tissues display several features of cellular polarization. Besides the ubiquitous epithelial polarization in the Apical-Basal (A/B) axis, many epithelia (and associated organs) display a Planar Cell Polarization (PCP). Recently, a crosstalk between the PCP and A/B polarity determinants has been suggested, i.e. the activity or stability of the PCP factor Frizzled is regulated by the A/B determinants aPKC and Bazooka in the Drosophila eye. We have systematically investigated genetic and physical interactions between the Drosophila A/B factors and the core PCP component Strabismus (Stbm)/Van Gogh (Vang). The A/B determinant Scribble was found to interact both genetically and physically with Stbm/Vang. We demonstrate that Scribble binds Stbm/Vang through its PDZ domain 3 and that it cooperates with Stbm/Vang in PCP establishment. Our data indicate that Scribble, in addition to its role in A/B polarity, has a distinct requirement in PCP establishment in the Drosophila eye and wing. We define a scribble allele that is largely PCP specific. Our data show that Scribble is part of the Stbm/Vang PCP complex and further suggest that it might act as an effector of Stbm/Vang during PCP establishment.

Planar polarization in embryonic epidermis orchestrates global asymmetric morphogenesis of hair follicles

Mammalian body hairs align along the anterior-posterior (A-P) axis and offer a striking but poorly understood example of global cell polarization, a phenomenon known as planar cell polarity (PCP). We have discovered that during embryogenesis, marked changes in cell shape and cytoskeletal polarization occur as nascent hair follicles become anteriorly angled, morphologically polarized and molecularly compartmentalized along the A-P axis. Hair follicle initiation coincides with asymmetric redistribution of Vangl2, Celsr1 and Fzd6 within the embryonic epidermal basal layer. Moreover, loss-of-function mutations in Vangl2 and Celsr1 show that they have an essential role in hair follicle polarization and orientation, which develop in part through non-autonomous mechanisms. Vangl2 and Celsr1 are both required for their planar localization in vivo, and physically associate in a complex in vitro. Finally, we provide in vitro evidence that homotypic intracellular interactions of Celsr1 are required to recruit Vangl2 and Fzd6 to sites of cell-cell contact.

Combinatorial signaling by the Frizzled/PCP and Egfr pathways during planar cell polarity establishment in the Drosophila eye

Frizzled (Fz)/PCP signaling regulates planar, vectorial orientation of cells or groups of cells within whole tissues. Although Fz/PCP signaling has been analyzed in several contexts, little is known about nuclear events acting downstream of Fz/PCP signaling in the R3/R4 cell fate decision in the Drosophila eye or in other contexts. Here we demonstrate a specific requirement for Egfr-signaling and the transcription factors Fos (AP-1), Yan and Pnt in PCP dependent R3/R4 specification. Loss and gain-of-function assays suggest that the transcription factors integrate input from Fz/PCP and Egfr-signaling and that the ETS factors Pnt and Yan cooperate with Fos (and Jun) in the PCP-specific R3/R4 determination. Our data indicate that Fos (either downstream of Fz/PCP signaling or parallel to it) and Yan are required in R3 to specify its fate (Fos) or inhibit R4 fate (Yan) and that Egfr-signaling is required in R4 via Pnt for its fate specification. Taken together with previous work establishing a Notch-dependent Su(H) function in R4, we conclude that Fos, Yan, Pnt, and Su(H) integrate Egfr, Fz, and Notch signaling input in R3 or R4 to establish cell fate and ommatidial polarity.

A new allele uncovers the role of echinus in the control of ommatidial rotation in the Drosophila eye

The precise orientation of the ommatidia in the Drosophila eye is achieved through a specialized process of cell migration taking place in the third-instar eye imaginal disc when ommatidial clusters rotate by 90 degrees. This process is strictly coordinated with the establishment of planar cell polarity (PCP), but it relies on a specific set of genes that control its mechanism independently from PCP signaling. Recently, the epidermal growth factor receptor (EGFR) pathway has been implicated in determining ommatidial rotation. We have isolated a new allele of echinus, a gene known to control the patterning and number of interommatidial cells. We show that echinus displays defects in the rotation of ommatidia that are not evident until mid-pupal stages, and we propose that echinus action is that of opposing EGFR by an unknown mechanism and that this can explain both its influence in ommatidial rotation and lattice programmed cell death (PCD).

Wingless signaling in Drosophila eye development

The secreted morphogen Wingless (Wg) has a variety of functions throughout Drosophila eye development, controlling tissue specification, growth, and patterning. Wg plays a critical role in subdividing the eye imaginal disc into separate primordia that will give rise to the adult retina and the surrounding head capsule. During larval development, wg is expressed in the anterior lateral margins of the eye disc, regions that will give rise to head cuticle; Wg signaling promotes the head fate and prevents these marginal regions from initiating ectopic photoreceptor differentiation. Expression of wg at the dorsal margin is earlier and stronger than at the ventral margin, allowing Wg to contribute to specifying the dorsal domain of the eye disc. Finally, during the pupal stages, wg expression surrounds the entire eye and a concentric gradient of Wg establishes several distinct peripheral retinal cell fates. This chapter reviews these aspects of Wg function and describes how to generate clones of cells mutant for genes encoding components of the Wg signaling pathway in the eye disc and examine their effects on photoreceptor differentiation by immunohistochemistry.

Bedraggled, a putative transporter, influences the tissue polarity complex during the R3/R4 fate decision in the Drosophila eye

The tissue polarity pathway is required for the establishment of epithelial polarity in a variety of vertebrate and invertebrate organs. Core tissue polarity proteins act in a dynamically regulated complex to direct the polarization of the Drosophila eye. We report the identification and characterization of bedraggled (bdg), a novel gene that regulates one output of the tissue polarity pathway--the establishment of the R3/R4 photoreceptor fates. bdg encodes a novel, putative transporter protein and interacts genetically with all of the core polarity genes to influence the specification of the R3 and R4 cell fates. Finally, bdg is required for both viability and the initial stages of imaginal disc development.

Lola regulates cell fate by antagonizing Notch induction in the Drosophila eye

Lola is a transcription repressor that regulates axon guidance in the developing embryonic nervous system of Drosophila. Here, we show that Lola regulates two binary cell fate decisions guided by Notch inductive signaling in the developing eye: the R3-R4 and the R7-cone cell fate choices. Lola is required cell-autonomously in R3 for its specification, and Lola transforms R4 into R3 if overexpressed. Lola also promotes R7 fate at the expense of cone cell fate. Lola antagonizes Notch-dependent gene expression and Notch-dependent fate transformation. Expression analysis shows that Lola is constitutively present in all photoreceptors and cone cells. We propose that when a precursor cell receives a weak Notch inductive signal, it is not sufficiently strong to overcome the constitutive repression of target gene transcription by Lola. A precursor that receives a strong Notch inductive signal expresses target genes despite a constitutive repression by Lola. The predicted consequence of this mechanism is to sharpen a cell's responsiveness to Notch signaling by creating a threshold.

Screen for genetic modifiers of stbm reveals that photoreceptor fate and rotation can be genetically uncoupled in the Drosophila eye

Background

Polarity of the Drosophila compound eye arises primarily as a consequence of two events that are tightly linked in time and space: fate specification of two photoreceptor cells, R3 and R4, and the subsequent directional movement of the unit eyes of the compound eye, or ommatidia. While it is thought that these fates dictate the direction of ommatidial rotation, the phenotype of mutants in the genes that set up this polarity led to the hypothesis that these two events could be uncoupled.

Methodology/Principal Findings

To definitively demonstrate these events are genetically separable, we conducted a dominant modifier screen to determine if genes, when misexpressed, could selectively enhance subclasses of mutant ommatidia in which the direction of rotation does not follow the R3/R4 cell fates, yet not affect the number of ommatidia in which rotation follows the R3/R4 cell fates. We identified a subset of P element lines that exhibit this selective enhancement. We also identified lines that behave in the opposite manner: They enhance the number of ommatidia that rotate in the right direction, but do not alter the number of ommatidia that rotate incorrectly with respect to the R3/R4 fates.

Conclusions/Significance

These results indicate that fate and direction of rotation can be genetically separated, and that there are genes that act between R3/R4 fate specification and direction of ommatidial rotation. These data affirm what has been a long-standing assumption about the genetic control of ommatidial polarity.