The function of the frizzled pathway in the Drosophila wing is dependent on inturned and fuzzy

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.

CPLANE Complex and Ciliopathies

Primary cilia are non-motile organelles associated with the cell cycle, which can be found in most vertebrate cell types. Cilia formation occurs through a process called ciliogenesis, which involves several mechanisms including planar cell polarity (PCP) and the Hedgehog (Hh) signaling pathway. Some gene complexes, such as BBSome or CPLANE (ciliogenesis and planar polarity effector), have been linked to ciliogenesis. CPLANE complex is composed of INTU, FUZ and WDPCP, which bind to JBTS17 and RSG1 for cilia formation. Defects in these genes have been linked to a malfunction of intraflagellar transport and defects in the planar cell polarity, as well as defective activation of the Hedgehog signalling pathway. These faults lead to defective cilium formation, resulting in ciliopathies, including orofacial-digital syndrome (OFDS) and Bardet-Biedl syndrome (BBS). Considering the close relationship, between the CPLANE complex and cilium formation, it can be expected that defects in the genes that encode subunits of the CPLANE complex may be related to other ciliopathies.

Planar cell polarity pathway in kidney development, function and disease

Planar cell polarity (PCP) refers to the coordinated orientation of cells in the tissue plane. Originally discovered and studied in Drosophila melanogaster, PCP is now widely recognized in vertebrates, where it is implicated in organogenesis. Specific sets of PCP genes have been identified. The proteins encoded by these genes become asymmetrically distributed to opposite sides of cells within a tissue plane and guide many processes that include changes in cell shape and polarity, collective cell movements or the uniform distribution of cell appendages. A unifying characteristic of these processes is that they often involve rearrangement of actomyosin. Mutations in PCP genes can cause malformations in organs of many animals, including humans. In the past decade, strong evidence has accumulated for a role of the PCP pathway in kidney development including outgrowth and branching morphogenesis of ureteric bud and podocyte development. Defective PCP signalling has been implicated in the pathogenesis of developmental kidney disorders of the congenital anomalies of the kidney and urinary tract spectrum. Understanding the origins, molecular constituents and cellular targets of PCP provides insights into the involvement of PCP molecules in normal kidney development and how dysfunction of PCP components may lead to kidney disease.

The CPLANE protein Intu protects kidneys from ischemia-reperfusion injury by targeting STAT1 for degradation

Intu is known as a ciliogenesis and planar polarity effector (CPLANE) protein. Although roles for Intu have been reported during embryonic development and in the context of developmental disorders, its function and regulation in adult tissues remain poorly understood. Here we show that ablation of Intu specifically in kidney proximal tubules aggravates renal ischemia-reperfusion injury, and leads to defective post-injury ciliogenesis. We identify signal transducer and activator of transcription 1 (STAT1) as a novel interacting partner of Intu. In vitro, Intu and STAT1 colocalize at the centriole/basal body area, and Intu promotes proteasomal degradation of STAT1. During cell stress, Intu expression preserves cilia length and cell viability, and these actions are antagonized by STAT1 expression. Thus, we propose a role for Intu in protecting cells and tissues after injury by targeting STAT1 for degradation and maintaining primary cilia.

From Planar Cell Polarity to Ciliogenesis and Back: The Curious Tale of the PPE and CPLANE proteins

Why some genes are more popular than others remains an open question, but one example of this phenomenon involves the genes controlling planar cell polarity (PCP), the polarization of cells within a plane of a tissue. Indeed, the so-called 'core' PCP genes such as dishevelled, frizzled, and prickle have been extensively studied both in animal models and by human genetics. By contrast, other genes that influence PCP signaling have received far less attention. Among the latter are inturned, fuzzy, and fritz, but recent work should bring these once obscure regulators into the limelight. We provide here a brief history of planar polarity effector (PPE) and CPLANE (ciliogenesis and planar polarity effector) proteins, discuss recent advances in understanding their molecular mechanisms of action, and describe their roles in human disease.

A Novel Frizzled-Based Screening Tool Identifies Genetic Modifiers of Planar Cell Polarity in Drosophila Wings

Most mutant alleles in the Fz-PCP pathway genes were discovered in classic Drosophila screens looking for recessive loss-of-function (LOF) mutations. Nonetheless, although Fz-PCP signaling is sensitive to increased doses of PCP gene products, not many screens have been performed in the wing under genetically engineered Fz overexpression conditions, mostly because the Fz phenotypes were strong and/or not easy to score and quantify. Here, we present a screen based on an unexpected mild Frizzled gain-of-function (GOF) phenotype. The leakiness of a chimeric Frizzled protein designed to be accumulated in the endoplasmic reticulum (ER) generated a reproducible Frizzled GOF phenotype in Drosophila wings. Using this genotype, we first screened a genome-wide collection of large deficiencies and found 16 strongly interacting genomic regions. Next, we narrowed down seven of those regions to finally test 116 candidate genes. We were, thus, able to identify eight new loci with a potential function in the PCP context. We further analyzed and confirmed krasavietz and its interactor short-stop as new genes acting during planar cell polarity establishment with a function related to actin and microtubule dynamics.

The polarity protein Inturned links NPHP4 to Daam1 to control the subapical actin network in multiciliated cells

Inturned-mediated complex formation of NPHP4 and DAAM1 is important for ciliogenesis and ciliary function in multiciliated cells, presumably because of its requirement for the local rearrangement of actin cytoskeleton.

Motile cilia polarization requires intracellular anchorage to the cytoskeleton; however, the molecular machinery that supports this process remains elusive. We report that Inturned plays a central role in coordinating the interaction between cilia-associated proteins and actin-nucleation factors. We observed that knockdown of nphp4 in multiciliated cells of the Xenopus laevis epidermis compromised ciliogenesis and directional fluid flow. Depletion of nphp4 disrupted the subapical actin layer. Comparison to the structural defects caused by inturned depletion revealed striking similarities. Furthermore, coimmunoprecipitation assays demonstrated that the two proteins interact with each other and that Inturned mediates the formation of ternary protein complexes between NPHP4 and DAAM1. Knockdown of daam1, but not formin-2, resulted in similar disruption of the subapical actin web, whereas nphp4 depletion prevented the association of Inturned with the basal bodies. Thus, Inturned appears to function as an adaptor protein that couples cilia-associated molecules to actin-modifying proteins to rearrange the local actin cytoskeleton.

Retracing the path of planar cell polarity

BACKGROUND

The Planar Cell Polarity pathway (PCP) has been described as the main feature involved in patterning cell orientation in bilaterian tissues. Recently, a similar phenomenon was revealed in cnidarians, in which the inhibition of this pathway results in the absence of cilia orientation in larvae, consequently proving the functional conservation of PCP signaling between Cnidaria and Bilateria. Nevertheless, despite the growing accumulation of databases concerning basal lineages of metazoans, very few information concerning the existence of PCP components have been gathered outside of Bilateria and Cnidaria. Thus, the origin of this module or its prevalence in early emerging metazoans has yet to be elucidated.

RESULTS

The present study addresses this question by investigating the genomes and transcriptomes from all poriferan lineages in addition to Trichoplax (Placozoa) and Mnemiopsis (Ctenophora) genomes for the presence of the core components of this pathway. Our results confirm that several PCP components are metazoan innovations. In addition, we show that all members of the PCP pathway, including a bona fide Strabismus ortholog (Van gogh), are retrieved only in one sponge lineage (Homoscleromorpha) out of four. This highly suggests that the full PCP pathway dates back at least to the emergence of homoscleromorph sponges. Consequently, several secondary gene losses would have occurred in the three other poriferan lineages including Amphimedon queenslandica (Demospongiae). Several proteins were not retrieved either in placozoans or ctenophores leading us to discuss the difficulties to predict orthologous proteins in basally branching animals. Finally, we reveal how the study of multigene families may be helpful to unravel the relationships at the base of the metazoan tree.

CONCLUSION

The PCP pathway antedates the radiation of Porifera and may have arisen in the last common ancestor of animals. Oscarella species now appear as key organisms to understand the ancestral function of PCP signaling and its potential links with Wnt pathways.

Positioning of centrioles is a conserved readout of Frizzled planar cell polarity signalling

Planar cell polarity (PCP) signalling is a well-conserved developmental pathway regulating cellular orientation during development. An evolutionarily conserved pathway readout is not established and, moreover, it is thought that PCP mediated cellular responses are tissue-specific. A key PCP function in vertebrates is to regulate coordinated centriole/cilia positioning, a function that has not been associated with PCP in Drosophila. Here we report instructive input of Frizzled-PCP (Fz/PCP) signalling into polarized centriole positioning in Drosophila wings. We show that centrioles are polarized in pupal wing cells as a readout of PCP signalling, with both gain and loss-of-function Fz/PCP signalling affecting centriole polarization. Importantly, loss or gain of centrioles does not affect Fz/PCP establishment, implicating centriolar positioning as a conserved PCP-readout, likely downstream of PCP-regulated actin polymerization. Together with vertebrate data, these results suggest a unifying model of centriole/cilia positioning as a common downstream effect of PCP signalling from flies to mammals.

Planar cell polarity (PCP) contributes to cellular orientation during development but how this is regulated in Drosophila is unclear. Here, the authors identify Frizzled-PCP signalling as regulating polarised centriole positioning in the wing disc.

Prickle isoforms control the direction of tissue polarity by microtubule independent and dependent mechanisms

Planar cell polarity signaling directs the polarization of cells within the plane of many epithelia. While these tissues exhibit asymmetric localization of a set of core module proteins, in Drosophila, more than one mechanism links the direction of core module polarization to the tissue axes. One signaling system establishes a polarity bias in the parallel, apical microtubules upon which vesicles containing core proteins traffic. Swapping expression of the differentially expressed Prickle isoforms, Prickle and Spiny-legs, reverses the direction of core module polarization. Studies in the proximal wing and the anterior abdomen indicated that this results from their differential control of microtubule polarity. Prickle and Spiny-legs also control the direction of polarization in the distal wing (D-wing) and the posterior abdomen (P-abd). We report here that this occurs without affecting microtubule polarity in these tissues. The direction of polarity in the D-wing is therefore likely determined by a novel mechanism independent of microtubule polarity. In the P-abd, Prickle and Spiny-legs interpret at least two directional cues through a microtubule-polarity-independent mechanism.

Control of vertebrate core planar cell polarity protein localization and dynamics by Prickle 2

Planar cell polarity (PCP) is a ubiquitous property of animal tissues and is essential for morphogenesis and homeostasis. In most cases, this fundamental property is governed by a deeply conserved set of 'core PCP' proteins, which includes the transmembrane proteins Van Gogh-like (Vangl) and Frizzled (Fzd), as well as the cytoplasmic effectors Prickle (Pk) and Dishevelled (Dvl). Asymmetric localization of these proteins is thought to be central to their function, and understanding the dynamics of these proteins is an important challenge in developmental biology. Among the processes that are organized by the core PCP proteins is the directional beating of cilia, such as those in the vertebrate node, airway and brain. Here, we exploit the live imaging capabilities of Xenopus to chart the progressive asymmetric localization of fluorescent reporters of Dvl1, Pk2 and Vangl1 in a planar polarized ciliated epithelium. Using this system, we also characterize the influence of Pk2 on the asymmetric dynamics of Vangl1 at the cell cortex, and we define regions of Pk2 that control its own localization and those impacting Vangl1. Finally, our data reveal a striking uncoupling of Vangl1 and Dvl1 asymmetry. This study advances our understanding of conserved PCP protein functions and also establishes a rapid, tractable platform to facilitate future in vivo studies of vertebrate PCP protein dynamics.

The Drosophila planar polarity gene multiple wing hairs directly regulates the actin cytoskeleton

The evolutionarily conserved frizzled/starry night (fz/stan) pathway regulates planar cell polarity (PCP) in vertebrates and invertebrates. This pathway has been extensively studied in the Drosophila wing, where it is manifested by an array of distally pointing cuticular hairs. Using in vivo imaging we found that, early in hair growth, cells have multiple actin bundles and hairs that subsequently fuse into a single growing hair. The downstream PCP gene multiple wing hairs (mwh) plays a key role in this process and acts to antagonize the actin cytoskeleton. In mwh mutants hair initiation is not limited to a small region at the distal edge of pupal wing cells as in wild type, resulting in multiple hairs with aberrant polarity. Extra actin bundles/hairs are formed and do not completely fuse, in contrast to wild type. As development proceeded additional hairs continued to form, further increasing hair number. We identified a fragment of Mwh with in vivo rescue activity and that bound and bundled F-actin filaments and inhibited actin polymerization in in vitro actin assays. The loss of these activities can explain the mwh mutant phenotype. Our data suggest a model whereby, prior to hair initiation, proximally localized Mwh inhibits actin polymerization resulting in polarized activation of the cytoskeleton and hair formation on the distal side of wing cells. During hair growth Mwh is found in growing hairs, where we suggest it functions to promote the fusion of actin bundles and inhibit the formation of additional actin bundles that could lead to extra hairs.

The planar cell polarity effector protein Wdpcp (Fritz) controls epithelial cell cortex dynamics via septins and actomyosin

Planar cell polarity (PCP) signaling controls polarized behaviors in diverse tissues, including the collective cell movements of gastrulation and the planar polarized beating of motile cilia. A major question in PCP signaling concerns the mechanisms linking this signaling cascade with more general cytoskeletal elements to drive polarized behavior. Previously, we reported that the PCP effector protein Wdpcp (formerly known as Fritz) interacts with septins and is critical for collective cell migration and cilia formation. Here, we report that Wdpcp is broadly involved in maintaining cortical tension in epithelial cells. In vivo 3D time-lapse imaging revealed that Wdpcp is necessary for basolateral plasma membrane stability in epithelial tissues, and we further show that Wdpcp controls cortical septin localization to maintain cortical rigidity in mucociliary epithelial cells. Finally, we show that Wdpcp acts via actomyosin to maintain balanced cortical tension in the epithelium. These data suggest that, in addition to its role in controlling plasma membrane dynamics in collective mesenchymal cell movements, Wdpcp is also essential for normal cell cortex stability during epithelial homeostasis.

Combover/CG10732, a novel PCP effector for Drosophila wing hair formation

The polarization of cells is essential for the proper functioning of most organs. Planar Cell Polarity (PCP), the polarization within the plane of an epithelium, is perpendicular to apical-basal polarity and established by the non-canonical Wnt/Fz-PCP signaling pathway. Within each tissue, downstream PCP effectors link the signal to tissue specific readouts such as stereocilia orientation in the inner ear and hair follicle orientation in vertebrates or the polarization of ommatidia and wing hairs in Drosophila melanogaster. Specific PCP effectors in the wing such as Multiple wing hairs (Mwh) and Rho Kinase (Rok) are required to position the hair at the correct position and to prevent ectopic actin hairs. In a genome-wide screen in vitro, we identified Combover (Cmb)/CG10732 as a novel Rho kinase substrate. Overexpression of Cmb causes the formation of a multiple hair cell phenotype (MHC), similar to loss of rok and mwh. This MHC phenotype is dominantly enhanced by removal of rok or of other members of the PCP effector gene family. Furthermore, we show that Cmb physically interacts with Mwh, and cmb null mutants suppress the MHC phenotype of mwh alleles. Our data indicate that Cmb is a novel PCP effector that promotes to wing hair formation, a function that is antagonized by Mwh.

The proteins encoded by the Drosophila Planar Polarity Effector genes inturned, fuzzy and fritz interact physically and can re-pattern the accumulation of "upstream" Planar Cell Polarity proteins

The frizzled/starry night pathway regulates planar cell polarity in a wide variety of tissues in many types of animals. It was discovered and has been most intensively studied in the Drosophila wing where it controls the formation of the array of distally pointing hairs that cover the wing. The pathway does this by restricting the activation of the cytoskeleton to the distal edge of wing cells. This results in hairs initiating at the distal edge and growing in the distal direction. All of the proteins encoded by genes in the pathway accumulate asymmetrically in wing cells. The pathway is a hierarchy with the Planar Cell Polarity (PCP) genes (aka the core genes) functioning as a group upstream of the Planar Polarity Effector (PPE) genes which in turn function as a group upstream of multiple wing hairs. Upstream proteins, such as Frizzled accumulate on either the distal and/or proximal edges of wing cells. Downstream PPE proteins accumulate on the proximal edge under the instruction of the upstream proteins. A variety of types of data support this hierarchy, however, we have found that when over expressed the PPE proteins can alter both the subcellular location and level of accumulation of the upstream proteins. Thus, the epistatic relationship is context dependent. We further show that the PPE proteins interact physically and can modulate the accumulation of each other in wing cells. We also find that over expression of Frtz results in a marked delay in hair initiation suggesting that it has a separate role/activity in regulating the cytoskeleton that is not shared by other members of the group.

Dachsous-dependent asymmetric localization of spiny-legs determines planar cell polarity orientation in Drosophila

In Drosophila, planar cell polarity (PCP) molecules such as Dachsous (Ds) may function as global directional cues directing the asymmetrical localization of PCP core proteins such as Frizzled (Fz). However, the relationship between Ds asymmetry and Fz localization in the eye is opposite to that in the wing, thereby causing controversy regarding how these two systems are connected. Here, we show that this relationship is determined by the ratio of two Prickle (Pk) isoforms, Pk and Spiny-legs (Sple). Pk and Sple form different complexes with distinct subcellular localizations. When the amount of Sple is increased in the wing, Sple induces a reversal of PCP using the Ds-Ft system. A mathematical model demonstrates that Sple is the key regulator connecting Ds and the core proteins. Our model explains the previously noted discrepancies in terms of the differing relative amounts of Sple in the eye and wing.

Prickle/spiny-legs isoforms control the polarity of the apical microtubule network in planar cell polarity

Microtubules (MTs) are substrates upon which plus- and minus-end directed motors control the directional movement of cargos that are essential for generating cell polarity. Although centrosomal MTs are organized with plus-ends away from the MT organizing center, the regulation of non-centrosomal MT polarity is poorly understood. Increasing evidence supports the model that directional information for planar polarization is derived from the alignment of a parallel apical network of MTs and the directional MT-dependent trafficking of downstream signaling components. The Fat/Dachsous/Four-jointed (Ft/Ds/Fj) signaling system contributes to orienting those MTs. In addition to previously defined functions in promoting asymmetric subcellular localization of 'core' planar cell polarity (PCP) proteins, we find that alternative Prickle (Pk-Sple) protein isoforms control the polarity of this MT network. This function allows the isoforms of Pk-Sple to differentially determine the direction in which asymmetry is established and therefore, ultimately, the direction of tissue polarity. Oppositely oriented signals that are encoded by oppositely oriented Fj and Ds gradients produce the same polarity outcome in different tissues or compartments, and the tissue-specific activity of alternative Pk-Sple protein isoforms has been observed to rectify the interpretation of opposite upstream directional signals. The control of MT polarity, and thus the directionality of apical vesicle traffic, by Pk-Sple provides a mechanism for this rectification.

Planar cell polarity gene mutations contribute to the etiology of human neural tube defects in our population

Neural Tube Defects (NTDs) are congenital malformations that involve failure of the neural tube closure during the early phases of development at any level of the rostro-caudal axis. The planar cell polarity (PCP) pathway is a highly conserved, noncanonical Wnt-Frizzled-Dishevelled signaling cascade, that was first identified in the fruit fly Drosophila. We are here reviewing the role of the PCP pathway genes in the etiology of human NTDs, updating the list of the rare and deleterious mutations identified so far. We report 50 rare nonsynonymous mutations of PCP genes in 54 patients having a pathogenic effect on the protein function. Thirteen mutations that have previously been reported as novel are now reported in public databases, although at very low frequencies. The mutations were private, mostly missense, and transmitted by a healthy parent. To date, no clear genotype-phenotype correlation has been possible to create. Even if PCP pathway genes are involved in the pathogenesis of neural tube defects, future studies will be necessary to better dissect the genetic causes underlying these complex malformations.

Planar cell polarity genes, Celsr1-3, in neural development

flamingo is among the 'core' planar cell-polarity genes, protein of which belongs to a unique cadherin subfamily. In contrast to the classic cadherins, composed of several extracellular cadherin repeats, one transmembrane domain and one cytoplasmic segment linked to catenin binding, Drosophila Flamingo has seven transmembrane segments and a cytoplasmic tail with no catenin-binding sequence. In Drosophila, Flamingo has pleotropic roles in controlling epithelial polarity and neuronal morphogenesis. Three mammalian orthologs of flamingo, Celsr1-3, are widely expressed in the nervous system. Recent work has shown that Celsr1-3 play important roles in neural development, such as in axon guidance, neuronal migration, and cilium polarity. Celsr1-3 single-gene knockout mice exhibit different phenotypes, but there are cooperative interactions among these genes.

Functional modelling of planar cell polarity: an approach for identifying molecular function

BACKGROUND

Cells in some tissues acquire a polarisation in the plane of the tissue in addition to apical-basal polarity. This polarisation is commonly known as planar cell polarity and has been found to be important in developmental processes, as planar polarity is required to define the in-plane tissue coordinate system at the cellular level.

RESULTS

We have built an in-silico functional model of cellular polarisation that includes cellular asymmetry, cell-cell signalling and a response to a global cue. The model has been validated and parameterised against domineering non-autonomous wing hair phenotypes in Drosophila.

CONCLUSIONS

We have carried out a systematic comparison of in-silico polarity phenotypes with patterns observed in vivo under different genetic manipulations in the wing. This has allowed us to classify the specific functional roles of proteins involved in generating cell polarity, providing new hypotheses about their specific functions, in particular for Pk and Dsh. The predictions from the model allow direct assignment of functional roles of genes from genetic mosaic analysis of Drosophila wings.

The PCP effector Fuzzy controls cilial assembly and signaling by recruiting Rab8 and Dishevelled to the primary cilium

During vertebrate development, the PCP pathway controls multiple cellular processes. Loss of the gene for the PCP effector Fuzzy affects formation of primary cilia via mostly unknown mechanisms. We report that Fuzzy localizes to the primary cilia and orchestrates delivery of Rab8 and Dishevelled to the primary cilium; loss of Fuzzy affects cilia-dependent signaling.

The planar cell polarity (PCP) pathway controls multiple cellular processes during vertebrate development. Recently the PCP pathway was implicated in ciliogenesis and in ciliary function. The primary cilium is an apically projecting solitary organelle that is generated via polarized intracellular trafficking. Because it acts as a signaling nexus, defects in ciliogenesis or cilial function cause multiple congenital anomalies in vertebrates. Loss of the PCP effector Fuzzy affects PCP signaling and formation of primary cilia; however, the mechanisms underlying these processes are largely unknown. Here we report that Fuzzy localizes to the basal body and ciliary axoneme and is essential for ciliogenesis by delivering Rab8 to the basal body and primary cilium. Fuzzy appears to control subcellular localization of the core PCP protein Dishevelled, recruiting it to Rab8-positive vesicles and to the basal body and cilium. We show that loss of Fuzzy results in inhibition of PCP signaling and hyperactivation of the canonical WNT pathway. We propose a mechanism by which Fuzzy participates in ciliogenesis and affects both canonical WNT and PCP signaling.

Drosophila ATP6AP2/VhaPRR functions both as a novel planar cell polarity core protein and a regulator of endosomal trafficking

Planar cell polarity (PCP) controls the orientation of cells within tissues and the polarized outgrowth of cellular appendages. So far, six PCP core proteins including the transmembrane proteins Frizzled (Fz), Strabismus (Stbm) and Flamingo (Fmi) have been identified. These proteins form asymmetric PCP domains at apical junctions of epithelial cells. Here, we demonstrate that VhaPRR, an accessory subunit of the proton pump V-ATPase, directly interacts with the protocadherin Fmi through its extracellular domain. It also shows a striking co-localization with PCP proteins during all pupal wing stages in Drosophila. This localization depends on intact PCP domains. Reversely, VhaPRR is required for stable PCP domains, identifying it as a novel PCP core protein. VhaPRR performs an additional role in vesicular acidification as well as endolysosomal sorting and degradation. Membrane proteins, such as E-Cadherin and the Notch receptor, accumulate at the surface and in intracellular vesicles of cells mutant for VhaPRR. This trafficking defect is shared by other V-ATPase subunits. By contrast, the V-ATPase does not seem to have a direct role in PCP regulation. Together, our results suggest two roles for VhaPRR, one for PCP and another in endosomal trafficking. This dual function establishes VhaPRR as a key factor in epithelial morphogenesis.

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.

Patterning skin by planar cell polarity: the multi-talented hair designer

In mammals, the skin can form complex global and local patterns to meet diverse functional requirements in different parts of the body. To date, the fundamental principles that underlie skin patterning remain poorly understood because of the involvement of multiple interacting processes. Genes involved in the planar cell polarity (PCP) signalling pathway, which is capable of polarizing cells within the planar plane of an epithelium, can control the orientation and differentiation of hair follicles, underlining their involvement in skin pattern formation. Here, we summarize recent progress that has been made to understand the PCP signalling pathway and its function in mammalian skin, including its role in hair follicle morphogenesis, ciliogenesis and wound healing. We argue that dissecting PCP signalling in the context of hair follicle formation might reveal many as-yet-undiscovered functions for PCP in the development, homeostasis and regeneration of skin.

Asymmetric protein localization in planar cell polarity: mechanisms, puzzles, and challenges

The polarization of epithelial cells along an axis orthogonal to their apical-basal axis is increasingly recognized for roles in a variety of developmental events and physiological functions. While now studied in many model organisms, mechanistic understanding is rooted in intensive investigations of planar cell polarity (PCP) in Drosophila. Consensus has emerged that two molecular modules, referred to here as the global and core modules, operate upstream of effector proteins to produce morphological PCP. Proteins of the core module develop subcellular asymmetry, accumulating in two groups on opposite sides of cells, consistent with proposed functions in producing cell polarity and in communicating that polarity between neighboring cells. Less clear are the molecular and cell biological mechanisms underlying core module function in the generation and communication of subcellular asymmetry and the relationship between the global and the core modules. In this review, we discuss these two unresolved questions, highlighting important studies and potentially enlightening avenues for further investigation. It is likely that results from Drosophila will continue to inform our views of the growing list of examples of PCP in vertebrate systems.

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.

Canonical Wnt signaling in the visceral muscle is required for left-right asymmetric development of the Drosophila midgut

Many animals develop left-right (LR) asymmetry in their internal organs. The mechanisms of LR asymmetric development are evolutionarily divergent, and are poorly understood in invertebrates. Therefore, we studied the genetic pathway of LR asymmetric development in Drosophila. Drosophila has several organs that show directional and stereotypic LR asymmetry, including the embryonic gut, which is the first organ to develop LR asymmetry during Drosophila development. In this study, we found that genes encoding components of the Wnt-signaling pathway are required for LR asymmetric development of the anterior part of the embryonic midgut (AMG). frizzled 2 (fz2) and Wnt4, which encode a receptor and ligand of Wnt signaling, respectively, were required for the LR asymmetric development of the AMG. arrow (arr), an ortholog of the mammalian gene encoding low-density lipoprotein receptor-related protein 5/6, which is a co-receptor of the Wnt-signaling pathway, was also essential for LR asymmetric development of the AMG. These results are the first demonstration that Wnt signaling contributes to LR asymmetric development in invertebrates, as it does in vertebrates. The AMG consists of visceral muscle and an epithelial tube. Our genetic analyses revealed that Wnt signaling in the visceral muscle but not the epithelium of the midgut is required for the AMG to develop its normal laterality. Furthermore, fz2 and Wnt4 were expressed in the visceral muscles of the midgut. Consistent with these results, we observed that the LR asymmetric rearrangement of the visceral muscle cells, the first visible asymmetry of the developing AMG, did not occur in embryos lacking Wnt4 expression. Our results also suggest that canonical Wnt/β-catenin signaling, but not non-canonical Wnt signaling, is responsible for the LR asymmetric development of the AMG. Canonical Wnt/β-catenin signaling is reported to have important roles in LR asymmetric development in zebrafish. Thus, the contribution of canonical Wnt/β-catenin signaling to LR asymmetric development may be an evolutionarily conserved feature between vertebrates and invertebrates.

Fuz regulates craniofacial development through tissue specific responses to signaling factors

The planar cell polarity effector gene Fuz regulates ciliogenesis and Fuz loss of function studies reveal an array of embryonic phenotypes. However, cilia defects can affect many signaling pathways and, in humans, cilia defects underlie several craniofacial anomalies. To address this, we analyzed the craniofacial phenotype and signaling responses of the Fuz^−/− mice. We demonstrate a unique role for Fuz in regulating both Hedgehog (Hh) and Wnt/β-catenin signaling during craniofacial development. Fuz expression first appears in the dorsal tissues and later in ventral tissues and craniofacial regions during embryonic development coincident with cilia development. The Fuz^−/− mice exhibit severe craniofacial deformities including anophthalmia, agenesis of the tongue and incisors, a hypoplastic mandible, cleft palate, ossification/skeletal defects and hyperplastic malformed Meckel's cartilage. Hh signaling is down-regulated in the Fuz null mice, while canonical Wnt signaling is up-regulated revealing the antagonistic relationship of these two pathways. Meckel's cartilage is expanded in the Fuz^−/− mice due to increased cell proliferation associated with the up-regulation of Wnt canonical target genes and decreased non-canonical pathway genes. Interestingly, cilia development was decreased in the mandible mesenchyme of Fuz null mice, suggesting that cilia may antagonize Wnt signaling in this tissue. Furthermore, expression of Fuz decreased expression of Wnt pathway genes as well as a Wnt-dependent reporter. Finally, chromatin IP experiments demonstrate that β-catenin/TCF-binding directly regulates Fuz expression. These data demonstrate a new model for coordination of Hh and Wnt signaling and reveal a Fuz-dependent negative feedback loop controlling Wnt/β-catenin signaling.

Planar polarity pathway and Nance-Horan syndrome-like 1b have essential cell-autonomous functions in neuronal migration

Components of the planar cell polarity (PCP) pathway are required for the caudal tangential migration of facial branchiomotor (FBM) neurons, but how PCP signaling regulates this migration is not understood. In a forward genetic screen, we identified a new gene, nhsl1b, required for FBM neuron migration. nhsl1b encodes a WAVE-homology domain-containing protein related to human Nance-Horan syndrome (NHS) protein and Drosophila GUK-holder (Gukh), which have been shown to interact with components of the WAVE regulatory complex that controls cytoskeletal dynamics and with the polarity protein Scribble, respectively. Nhsl1b localizes to FBM neuron membrane protrusions and interacts physically and genetically with Scrib to control FBM neuron migration. Using chimeric analysis, we show that FBM neurons have two modes of migration: one involving interactions between the neurons and their planar-polarized environment, and an alternative, collective mode involving interactions between the neurons themselves. We demonstrate that the first mode of migration requires the cell-autonomous functions of Nhsl1b and the PCP components Scrib and Vangl2 in addition to the non-autonomous functions of Scrib and Vangl2, which serve to polarize the epithelial cells in the environment of the migrating neurons. These results define a role for Nhsl1b as a neuronal effector of PCP signaling and indicate that proper FBM neuron migration is directly controlled by PCP signaling between the epithelium and the migrating neurons.

Mutations in the planar cell polarity gene, Fuzzy, are associated with neural tube defects in humans

Neural tube defects (NTDs) are a heterogeneous group of common severe congenital anomalies which affect 1-2 infants per 1000 births. Most genetic and/or environmental factors that contribute to the pathogenesis of human NTDs are unknown. Recently, however, pathogenic mutations of VANGL1 and VANGL2 genes have been associated with some cases of human NTDs. Vangl genes encode proteins of the planar cell polarity (PCP) pathway that regulates cell behavior during early stages of neural tube formation. Homozygous disruption of PCP genes in mice results in a spectrum of NTDs, including defects that affect the entire neural axis (craniorachischisis), cranial NTDs (exencephaly) and spina bifida. In this paper, we report the dynamic expression of another PCP gene, Fuzzy, during neural tube formation in mice. We also identify non-synonymous Fuzzy amino acid substitutions in some patients with NTDs and demonstrate that several of these Fuzzy mutations affect formation of primary cilia and ciliary length or affect directional cell movement. Since Fuzzy knockout mice exhibit both NTDs and defective primary cilia and Fuzzy is expressed in the emerging neural tube, we propose that mutations in Fuzzy may account for a subset of NTDs in humans.

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.

The planar cell polarity pathway in vertebrate epidermal development, homeostasis and repair

The planar cell polarity (PCP) pathway plays a critical role in diverse developmental processes that require coordinated cellular movement, including neural tube closure and renal tubulogenesis. Recent studies have demonstrated that this pathway also has emerging relevance to the epidermis, as PCP signaling underpins many aspects of skin biology and pathology, including epidermal development, hair orientation, stem cell division and cancer. Coordinated cellular movement required for epidermal repair in mammals is also regulated by PCP signaling, and in this context, a new PCP gene encoding the developmental transcription factor Grainyhead-like 3 (Grhl3) is critical. This review focuses on the role that PCP signaling plays in the skin across a variety of epidermal functions and highlights perturbations that induce epidermal pathologies.

Planar cell polarity signaling, cilia and polarized ciliary beating

Planar cell polarity signaling governs a wide array of polarized cell behaviors in animals. Recent reports now show that PCP signaling is essential for the directional beating of motile cilia. Interestingly, PCP signaling acts in a variety of ciliated cell types that use motile cilia to generate directional fluid flow in very different ways. This review will synthesize these recent papers and place them in context with previous studies of PCP signaling in polarized cellular morphogenesis and collective cell movement.

Strange as it may seem: the many links between Wnt signaling, planar cell polarity, and cilia

Cilia are important cellular structures that have been implicated in a variety of signaling cascades. In this review, we discuss the current evidence for and against a link between cilia and both the canonical Wnt/β-catenin pathway and the noncanonical Wnt/planar cell polarity (PCP) pathway. Furthermore, we address the evidence implicating a role for PCP components in ciliogenesis. Given the lack of consensus in the field, we use new data on the control of ciliary protein localization as a basis for proposing new models by which cell type-specific regulation of ciliary components via differential transport, regulated entry and exit, or diffusion barriers might generate context-dependent functions for cilia.

Fuz controls the morphogenesis and differentiation of hair follicles through the formation of primary cilia

Planar cell polarity (PCP) signaling is essential in determining the polarity of cells within the plane of an epithelial sheet. Core PCP genes have been recently shown to control the global polarization of hair follicles in mice. Fuz, a homologue of the Drosophila PCP effector gene, fuzzy, is critical in ciliogenesis in vertebrates, and is required for the development of a wide range of organs in mice. Here, we report that disruption of the Fuz gene in mice severely blocked the development of hair follicles in the skin. In contrast to the loss of hair follicle polarization in mice deficient in core PCP genes, hair follicles in mice lacking the Fuz gene retained their typical anterior-posterior orientation. We show that disruption of Fuz impaired the formation of primary cilia and the hedgehog signaling pathway in the skin. In addition, using skin grafts and skin reconstitution assays we demonstrate that the expression of Fuz is required in both epidermal and dermal cells and that the formation of primary cilia is a cell-autonomous process that does not require cross talk between the epithelia and mesenchymal compartments during hair follicle formation.

The relationship between sonic Hedgehog signaling, cilia, and neural tube defects

The Hedgehog signaling pathway is essential for many aspects of normal embryonic development, including formation and patterning of the neural tube. Absence of the sonic hedgehog (shh) ligand is associated with the midline defect holoprosencephaly, whereas increased Shh signaling is associated with exencephaly and spina bifida. To complicate this apparently simple relationship, mutation of proteins required for function of cilia often leads to impaired Shh signaling and to disruption of neural tube closure. In this article, we review the literature on Shh pathway mutants and discuss the relationship between Shh signaling, cilia, and neural tube defects.

The Drosophila planar polarity proteins inturned and multiple wing hairs interact physically and function together

The conserved frizzled (fz) pathway regulates planar cell polarity in both vertebrate and invertebrate animals. This pathway has been most intensively studied in the wing of Drosophila, where the proteins encoded by pathway genes all accumulate asymmetrically. Upstream members of the pathway accumulate on the proximal, distal, or both cell edges in the vicinity of the adherens junction. More downstream components including Inturned and Multiple Wing Hairs accumulate on the proximal side of wing cells prior to hair initiation. The Mwh protein differs from other members of the pathway in also accumulating in growing hairs. Here we show that the two Mwh accumulation patterns are under different genetic control with the early proximal accumulation being regulated by the fz pathway and the latter hair accumulation being largely independent of the pathway. We also establish recruitment by proximally localized Inturned to be a putative mechanism for the localization of Mwh to the proximal side of wing cells. Genetically inturned (in) acts upstream of mwh (mwh) and is required for the proximal localization of Mwh. We show that Mwh can bind to and co-immunoprecipitate with Inturned. We also show that these two proteins can function in close juxtaposition in vivo. An InMwh fusion protein provided complete rescue activity for both in and mwh mutations. The fusion protein localized to the proximal side of wing cells prior to hair formation and in growing hairs as expected if protein localization is a key for the function of these proteins.

Drosophila Rab23 is involved in the regulation of the number and planar polarization of the adult cuticular hairs

The planar coordination of cellular polarization is an important, yet not well-understood aspect of animal development. In a screen for genes regulating planar cell polarization in Drosophila, we identified Rab23, encoding a putative vesicular trafficking protein. Mutations in the Drosophila Rab23 ortholog result in abnormal trichome orientation and the formation of multiple hairs on the wing, leg, and abdomen. We show that Rab23 is required for hexagonal packing of the wing cells. We found that Rab23 is able to associate with the proximally accumulated Prickle protein, although Rab23 itself does not seem to display a polarized subcellular distribution in wing cells, and it appears to play a relatively subtle role in cortical polarization of the polarity proteins. The absence of Rab23 leads to increased actin accumulation in the subapical region of the pupal wing cells that fail to restrict prehair initiation to a single site. Rab23 acts as a dominant enhancer of the weak multiple hair phenotype exhibited by the core polarity mutations, whereas the Rab23 homozygous mutant phenotype is sensitive to the gene dose of the planar polarity effector genes. Together, our data suggest that Rab23 contributes to the mechanism that inhibits hair formation at positions outside of the distal vertex by activating the planar polarity effector system.

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.

The planar cell polarity effector Fuz is essential for targeted membrane trafficking, ciliogenesis and mouse embryonic development

The planar cell polarity (PCP) signalling pathway is essential for embryonic development because it governs diverse cellular behaviours, and 'core PCP' proteins, such as Dishevelled and Frizzled, have been extensively characterized. By contrast, the 'PCP effector' proteins, such as Intu and Fuz, remain largely unstudied. These proteins are essential for PCP signalling, but they have never been investigated in mammals and their cell biological activities remain entirely unknown. We report here that Fuz mutant mice show neural tube defects, skeletal dysmorphologies and Hedgehog signalling defects stemming from disrupted ciliogenesis. Using bioinformatics and imaging of an in vivo mucociliary epithelium, we established a central role for Fuz in membrane trafficking, showing that Fuz is essential for trafficking of cargo to basal bodies and to the apical tips of cilia. Fuz is also essential for exocytosis in secretory cells. Finally, we identified a Rab-related small GTPase as a Fuz interaction partner that is also essential for ciliogenesis and secretion. These results are significant because they provide new insights into the mechanisms by which developmental regulatory systems such as PCP signalling interface with fundamental cellular systems such as the vesicle trafficking machinery.

A quest for the mechanism regulating global planar cell polarity of tissues

Most epithelial cells, besides their ubiquitous apical-basal polarity, are polarized within the plane of the epithelium, which is called planar cell polarity (PCP). Using Drosophila as a model, meaningful progress has been made in the identification of key PCP factors and the dissection of their intracellular molecular interactions. The long-range, global aspects of coordinated polarization and the overlying regulatory mechanisms that create the initial polarity direction have, however, remained elusive. Several recent publications have outlined potential mechanisms of how the global regulation of PCP might be controlled and how the distinct core factor groups might interact via frizzled, Van Gogh or flamingo. This review focuses on these exciting features and attempts to provide an integrated picture of these recent and novel insights.

Van Gogh and Frizzled act redundantly in the Drosophila sensory organ precursor cell to orient its asymmetric division

Drosophila sensory organ precursor cells (SOPs) divide asymmetrically along the anterior-posterior (a-p) body axis to generate two different daughter cells. Planar Cell Polarity (PCP) regulates the a-p orientation of the SOP division. The localization of the PCP proteins Van Gogh (Vang) and Frizzled (Fz) define anterior and posterior apical membrane domains prior to SOP division. Here, we investigate the relative contributions of Vang, Fz and Dishevelled (Dsh), a membrane-associated protein acting downstream of Fz, in orienting SOP polarity. Genetic and live imaging analyses suggest that Dsh restricts the localization of a centrosome-attracting activity to the anterior cortex and that Vang is a target of Dsh in this process. Using a clone border assay, we provide evidence that the Vang and fz genes act redundantly in SOPs to orient its polarity axis in response to extrinsic local PCP cues. Additionally, we find that the activity of Vang is dispensable for the non-autonomous polarizing activity of fz. These observations indicate that both Vang and Fz act as cues for downstream effectors orienting the planar polarity axis of dividing SOPs.

Molecular dissection of Drosophila Prickle isoforms distinguishes their essential and overlapping roles in planar cell polarity

Prickle-Spiny-Legs (Pk) is an essential component of the planar cell polarity (PCP) pathway, together with Frizzled (Fz) and Dishevelled (Dsh). A role for Pk was proposed to mediate feedback amplification of asymmetric Fz/Dsh activity across cell boundaries, ensuring a single prehair initiates at each distal vertex. Here we show that apical localisation of Pk(Pk) and Pk(Sple) isoforms are mutually independent and regulated by the C-terminal domain. The N-terminus of Pk(Pk) is dispensable for PCP, whereas the unique N-terminal domain of Pk(Sple) contains an additional localisation function, which confers a qualitatively different activity. Our results suggest that endogenous Pk(Pk) and Pk(Sple) can affect each other's function via the C-terminal domain, yet may not form heteromeric complexes. Overexpressing PET domain-deleted Pk variants interferes with a branch of Fz/Dsh signalling that regulates the number of wing hairs, and blocks non-cell-autonomous repolarisation. We infer that Pk(Pk) is sufficient to mediate the intercellular feedback signalling. Significantly, Pk(Pk) but not Pk(Sple) is required for hexagonal cell packing in the pupal wing. We propose that Fz-dependent PCP readout reflects short-range, cell-contact based, interactions between hexagonal cells, rather than a direct response to an as yet unidentified diffusible ligand.

The atypical cadherin Flamingo is required for sensory axon advance beyond intermediate target cells

The Drosophila atypical cadherin Flamingo plays key roles in a number of developmental processes. We have used the sensory nervous system of the Drosophila embryo to shed light on the mechanism by which Flamingo regulates axon growth. flamingo loss of function mutants display a highly penetrant sensory axon stall phenotype. The location of these axon stalls is stereotypic and corresponds to the position of intermediate target cells, with which sensory axons associate during normal development. This suggests that Flamingo mediates an interaction between the sensory neuron growth cones and these intermediate targets, which is required for continued axon advance. Mutant rescue experiments show that Flamingo expression is required only in sensory neurons for normal axon growth. The flamingo mutant phenotype can be partially rescued by expressing a Flamingo construct lacking most of the extracellular domain, suggesting that regulation of sensory axon advance by Flamingo does not absolutely depend upon a homophilic Flamingo-Flamingo interaction or its ability to mediate cell-cell adhesion. Loss of function mutants for a number of key genes that act together with Flamingo in the planar cell polarity pathway do not display the highly penetrant stalling phenotype seen in flamingo mutants.

Planar cell polarity: A bridge too far?

The mechanisms of planar cell polarity are being revealed by genetic analysis. Recent studies have provided new insights into interactions between three proteins involved in planar cell polarity: Flamingo, Frizzled and Van Gogh.

The frizzled extracellular domain is a ligand for Van Gogh/Stbm during nonautonomous planar cell polarity signaling

The Frizzled (Fz) receptor is required cell autonomously in Wnt/beta-catenin and planar cell polarity (PCP) signaling. In addition to these requirements, Fz acts nonautonomously during PCP establishment: wild-type cells surrounding fz(-) patches reorient toward the fz(-) cells. The molecular mechanism(s) of nonautonomous Fz signaling are unknown. Our in vivo studies identify the extracellular domain (ECD) of Fz, in particular its CRD (cysteine rich domain), as critical for nonautonomous Fz-PCP activity. Importantly, we demonstrate biochemical and physical interactions between the FzECD and the transmembrane protein Van Gogh/Strabismus (Vang/Stbm). We show that this function precedes cell-autonomous interactions and visible asymmetric PCP factor localization. Our data suggest that Vang/Stbm can act as a FzECD receptor, allowing cells to sense Fz activity/levels of their neighbors. Thus, direct Fz-Vang/Stbm interactions represent an intriguing mechanism that may account for the global orientation of cells within the plane of their epithelial field.

The multiple-wing-hairs gene encodes a novel GBD-FH3 domain-containing protein that functions both prior to and after wing hair initiation

The frizzled signaling/signal transduction pathway controls planar cell polarity (PCP) in both vertebrates and invertebrates. Epistasis experiments argue that in the Drosophila epidermis multiple wing hairs (mwh) acts as a downstream component of the pathway. The PCP proteins accumulate asymmetrically in pupal wing cells where they are thought to form distinct protein complexes. One is located on the distal side of wing cells and a second on the proximal side. This asymmetric protein accumulation is thought to lead to the activation of the cytoskeleton on the distal side, which in turn leads to each cell forming a single distally pointing hair. We identified mwh as CG13913, which encodes a novel G protein binding domain-formin homology 3 (GBD-FH3) domain protein. The Mwh protein accumulated on the proximal side of wing cells prior to hair formation. Unlike planar polarity proteins such as Frizzled or Inturned, Mwh also accumulated in growing hairs. This suggested that mwh had two temporally separate functions in wing development. Evidence for these two functions also came from temperature-shift experiments with a temperature-sensitive allele. Overexpression of Mwh inhibited hair initiation, thus Mwh acts as a negative regulator of the cytoskeleton. Our data argued early proximal Mwh accumulation restricts hair initiation to the distal side of wing cells and the later hair accumulation of Mwh prevents the formation of ectopic secondary hairs. This later function appears to be a feedback mechanism that limits cytoskeleton activation to ensure a single hair is formed.

Planar polarity genes in the Drosophila wing regulate the localisation of the FH3-domain protein Multiple Wing Hairs to control the site of hair production

The core planar polarity proteins play important roles in coordinating cell polarity, in part by adopting asymmetric subcellular localisations that are likely to serve as cues for cell polarisation by as yet uncharacterised pathways. Here we describe the role of Multiple Wing Hairs (Mwh), a novel formin homology 3 (FH3)-domain protein, which acts downstream of the core polarity proteins to restrict the production of actin-rich prehairs to distal cell edges in the Drosophila pupal wing. Mwh appears to function as a repressor of actin filament formation and, in its absence, ectopic actin bundles are seen across the entire apical surface of cells. We show that the proximally localised core polarity protein Strabismus acts via the downstream effector proteins Inturned, Fuzzy and Fritz to stabilise Mwh in apico-proximal cellular regions. In addition, the distally localised core polarity protein Frizzled positively promotes prehair initiation, suggesting that both proximal and distal cellular cues act together to ensure accurate prehair placement.

Planar cell polarity signaling in vertebrates

Planar cell polarity (PCP) refers to the polarization of a field of cells within the plane of a cell sheet. This form of polarization is required for diverse cellular processes in vertebrates, including convergent extension (CE), the establishment of PCP in epithelial tissues and ciliogenesis. Perhaps the most distinct example of vertebrate PCP is the uniform orientation of stereociliary bundles at the apices of sensory hair cells in the mammalian auditory sensory organ. The establishment of PCP in the mammalian cochlea occurs concurrently with CE in this ciliated epithelium, therefore linking three cellular processes regulated by the vertebrate PCP pathway in the same tissue and emerging as a model system for dissecting PCP signaling. This review summarizes the morphogenesis of this model system to assist the interpretation of the emerging data and proposes molecular mechanisms underlying PCP signaling in vertebrates.