Planar cell polarity in the Drosophila eye is directed by graded Four-jointed and Dachsous expression

Translating cell polarity into tissue elongation

Planar cell polarity, the orientation of single-cell asymmetries within the plane of a multicellular tissue, is essential to generating the shape and dimensions of organs and organisms. Planar polarity systems align cell behavior with the body axes and orient the cellular processes that lead to tissue elongation. Using Drosophila as a model system, significant progress has been made toward understanding how planar polarity is generated by biochemical and mechanical signals. Recent studies using time-lapse imaging reveal that cells engage in a number of active behaviors whose orientation and dynamics translate planar cell polarity into tissue elongation. Here we review recent progress in understanding the cellular mechanisms that link planar polarity to large-scale changes in tissue structure.

The roles of the cadherins Fat and Dachsous in planar polarity specification in Drosophila

Planar polarity is generated through the activity of two groups of proteins, the "core" system and the Fat (Ft)/Dachsous (Ds) system. Although both are conserved from insects to mammals, vertebrate studies into planar polarity have primarily focussed on core planar polarity proteins and have only recently branched into the study of the Ft/Ds system. In Drosophila, however, years of detailed analysis have started to elucidate some of the mechanisms by which Ft/Ds signalling might set up polarity across a tissue, and how this may impact upon core protein-mediated planar polarity. In this review, we discuss the major findings, models, and controversies that have emerged from Drosophila research into the Ft/Ds system, and indicate some areas for further investigation.

Morphogens, nutrients, and the basis of organ scaling

The regulation of organ size is a long-standing problem in animal development. Studies in this area have shown that organ-intrinsic patterning morphogens influence organ size, guiding growth in accordance with positional information. However, organ-extrinsic humoral factors such as insulin also affect organ size, synchronizing growth with nutrient levels. Proliferating cells must integrate instructions from morphogens with those from nutrition so that growth proceeds as a function of both inputs. Coordinating cell proliferation with morphogens and nutrients ensures organs scale appropriately with body size, but the basis of this coordination is unclear. Here, the problem is illustrated using the Drosophila wing--a paradigm for organ growth and size control--and a potential solution suggested.

The roles of maternal Vangl2 and aPKC in Xenopus oocyte and embryo patterning

The Xenopus oocyte contains components of both the planar cell polarity and apical-basal polarity pathways, but their roles are not known. Here, we examine the distribution, interactions and functions of the maternal planar cell polarity core protein Vangl2 and the apical-basal complex component aPKC. We show that Vangl2 is distributed in animally enriched islands in the subcortical cytoplasm in full-grown oocytes, where it interacts with a post-Golgi v-SNARE protein, VAMP1, and acetylated microtubules. We find that Vangl2 is required for the stability of VAMP1 as well as for the maintenance of the stable microtubule architecture of the oocyte. We show that Vangl2 interacts with atypical PKC, and that both the acetylated microtubule cytoskeleton and the Vangl2-VAMP1 distribution are dependent on the presence of aPKC. We also demonstrate that aPKC and Vangl2 are required for the cell membrane asymmetry that is established during oocyte maturation, and for the asymmetrical distribution of maternal transcripts for the germ layer and dorsal/ventral determinants VegT and Wnt11. This study demonstrates the interaction and interdependence of Vangl2, VAMP1, aPKC and the stable microtubule cytoskeleton in the oocyte, shows that maternal Vangl2 and aPKC are required for specific oocyte asymmetries and vertebrate embryonic patterning, and points to the usefulness of the oocyte as a model to study the polarity problem.

Planar cell polarity in kidney development and disease

Planar cell polarity (PCP) describes the coordinated polarization of tissue cells in a direction that is orthogonal to their apical/basal axis. In the last several years, studies in flies and vertebrates have defined evolutionarily conserved pathways that establish and maintain PCP in various cellular contexts. Defective responses to the polarizing signal(s) have deleterious effects on the development and repair of a wide variety of organs/tissues. In this review, we cover the known and hypothesized roles for PCP in the metanephric kidney. We highlight the similarities and differences in PCP establishment in this organ compared with flies, especially the role of Wnt signaling in this process. Finally, we present a model whereby the signal(s) that organizes PCP in the kidney epithelium, at least in part, comes from the adjacent stromal fibroblasts.

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.

Pointing in the right direction: new developments in the field of planar cell polarity

Planar cell polarity (PCP) is observed in an array of developmental processes that involve collective cell movement and tissue organization, and its disruption can lead to severe developmental defects. Recent studies in flies and vertebrates have identified new functions for PCP as well as new signalling components, and have proposed new mechanistic models. However, despite this progress, the search to simplify principles of understanding continues and important mechanistic uncertainties still pose formidable challenges.

Characterization of a Dchs1 mutant mouse reveals requirements for Dchs1-Fat4 signaling during mammalian development

The Drosophila Dachsous and Fat proteins function as ligand and receptor, respectively, for an intercellular signaling pathway that regulates Hippo signaling and planar cell polarity. Although gene-targeted mutations in two mammalian Fat genes have been described, whether mammals have a Fat signaling pathway equivalent to that in Drosophila, and what its biological functions might be, have remained unclear. Here, we describe a gene-targeted mutation in a murine Dachsous homolog, Dchs1. Analysis of the phenotypes of Dchs1 mutant mice and comparisons with Fat4 mutant mice identify requirements for these genes in multiple organs, including the ear, kidney, skeleton, intestine, heart and lung. Dchs1 and Fat4 single mutants and Dchs1 Fat4 double mutants have similar phenotypes throughout the body. In some cases, these phenotypes suggest that Dchs1-Fat4 signaling influences planar cell polarity. In addition to the appearance of cysts in newborn kidneys, we also identify and characterize a requirement for Dchs1 and Fat4 in growth, branching and cell survival during early kidney development. Dchs1 and Fat4 are predominantly expressed in mesenchymal cells in multiple organs, and mutation of either gene increases protein staining for the other. Our analysis implies that Dchs1 and Fat4 function as a ligand-receptor pair during murine development, and identifies novel requirements for Dchs1-Fat4 signaling in multiple organs.

Two frizzled planar cell polarity signals in the Drosophila wing are differentially organized by the Fat/Dachsous pathway

The regular array of distally pointing hairs on the mature Drosophila wing is evidence for the fine control of Planar Cell Polarity (PCP) during wing development. Normal wing PCP requires both the Frizzled (Fz) PCP pathway and the Fat/Dachsous (Ft/Ds) pathway, although the functional relationship between these pathways remains under debate. There is strong evidence that the Fz PCP pathway signals twice during wing development, and we have previously presented a Bidirectional-Biphasic Fz PCP signaling model which proposes that the Early and Late Fz PCP signals are in different directions and employ different isoforms of the Prickle protein. The goal of this study was to investigate the role of the Ft/Ds pathway in the context of our Fz PCP signaling model. Our results allow us to draw the following conclusions: (1) The Early Fz PCP signals are in opposing directions in the anterior and posterior wing and converge precisely at the site of the L3 wing vein. (2) Increased or decreased expression of Ft/Ds pathway genes can alter the direction of the Early Fz PCP signal without affecting the Late Fz PCP signal. (3) Lowfat, a Ft/Ds pathway regulator, is required for the normal orientation of the Early Fz PCP signal but not the Late Fz PCP signal. (4) At the time of the Early Fz PCP signal there are symmetric gradients of dachsous (ds) expression centered on the L3 wing vein, suggesting Ds activity gradients may orient the Fz signal. (5) Localized knockdown or over-expression of Ft/Ds pathway genes shows that boundaries/gradients of Ft/Ds pathway gene expression can redirect the Early Fz PCP signal specifically. (6) Altering the timing of ds knockdown during wing development can separate the role of the Ft/Ds pathway in wing morphogenesis from its role in Early Fz PCP signaling.

Planar Cell Polarity (PCP) describes the orientation of a cell within the plane of a cell layer. The precise control of PCP has been shown to be vital for normal development in both vertebrates and invertebrates, and failures of PCP have been implicated in human disease. Studies in the fruit fly Drosophila have identified two genetic pathways, the Frizzled and Fat/Dachsous pathways, that are required to organize PCP, although the functional relationship between the two pathways remains unresolved. We have previously proposed a model of Frizzled pathway activity in the Drosophila wing that invokes two consecutive Frizzled signaling events oriented in different directions. The Early and Late Fz PCP signals use different isoforms of the Prickle protein. The goal of this study was to define the activity of the Fat/Dachsous pathway in the context of our Frizzled signaling model. Our results suggest that the Fat/Dachsous pathway has a different functional relationship with each of the Frizzled signaling events. Specifically, we find that by altering Fat/Dachsous pathway activity, we can reorient the Early Frizzled signal without affecting the Late Frizzled signal. This suggests that the functional relationship between the Fat/Dachsous pathway and the Frizzled pathway can vary, even between consecutive Frizzled signaling events within the same set of cells.

Modeling the control of planar cell polarity

A growing list of medically important developmental defects and disease mechanisms can be traced to disruption of the planar cell polarity (PCP) pathway. The PCP system polarizes cells in epithelial sheets along an axis orthogonal to their apical-basal axis. Studies in the fruitfly, Drosophila, have suggested that components of the PCP signaling system function in distinct modules, and that these modules and the effector systems with which they interact function together to produce emergent patterns. Experimental methods allow the manipulation of individual PCP signaling molecules in specified groups of cells; these interventions not only perturb the polarization of the targeted cells at a subcellular level, but also perturb patterns of polarity at the multicellular level, often affecting nearby cells in characteristic ways. These kinds of experiments should, in principle, allow one to infer the architecture of the PCP signaling system, but the relationships between molecular interactions and tissue-level pattern are sufficiently complex that they defy intuitive understanding. Mathematical modeling has been an important tool to address these problems. This article explores the emergence of a local signaling hypothesis, and describes how a local intercellular signal, coupled with a directional cue, can give rise to global pattern. We will discuss the critical role mathematical modeling has played in guiding and interpreting experimental results, and speculate about future roles for mathematical modeling of PCP. Mathematical models at varying levels of inhibition have and are expected to continue contributing in distinct ways to understanding the regulation of PCP signaling.

Planar polarization of the atypical myosin Dachs orients cell divisions in Drosophila

Tissues can grow in a particular direction by controlling the orientation of cell divisions. This phenomenon is evident in the developing Drosophila wing epithelium, where the tissue becomes elongated along the proximal-distal axis. We show that orientation of cell divisions in the wing requires planar polarization of an atypical myosin, Dachs. Our evidence suggests that Dachs constricts cell-cell junctions to alter the geometry of cell shapes at the apical surface, and that cell shape then determines the orientation of the mitotic spindle. Using a computational model of a growing epithelium, we show that polarized cell tension is sufficient to orient cell shapes, cell divisions, and tissue growth. Planar polarization of Dachs is ultimately oriented by long-range gradients emanating from compartment boundaries, and is therefore a mechanism linking these gradients with the control of tissue shape.

Hippo signaling: growth control and beyond

The Hippo pathway has emerged as a conserved signaling pathway that is essential for the proper regulation of organ growth in Drosophila and vertebrates. Although the mechanisms of signal transduction of the core kinases Hippo/Mst and Warts/Lats are relatively well understood, less is known about the upstream inputs of the pathway and about the downstream cellular and developmental outputs. Here, we review recently discovered mechanisms that contribute to the dynamic regulation of Hippo signaling during Drosophila and vertebrate development. We also discuss the expanding diversity of Hippo signaling functions during development, discoveries that shed light on a complex regulatory system and provide exciting new insights into the elusive mechanisms that regulate organ growth and regeneration.

Planar cell polarity signaling: the developing cell's compass

Cells of many tissues acquire cellular asymmetry to execute their physiologic functions. The planar cell polarity system, first characterized in Drosophila, is important for many of these events. Studies in Drosophila suggest that an upstream system breaks cellular symmetry by converting tissue gradients to subcellular asymmetry, whereas a downstream system amplifies subcellular asymmetry and communicates polarity between cells. In this review, we discuss apparent similarities and differences in the mechanism that controls PCP as it has been adapted to a broad variety of morphological cellular asymmetries in various organisms.

Planar cell polarity: keeping hairs straight is not so simple

The fur on a cat's back, the scales on a fish, or the bristles on a fly are all beautifully organized, with a high degree of polarization in their surface organization. Great progress has been made in understanding how individual cell polarity is established, but our understanding of how cells coordinate their polarity in forming coherent tissues is still fragmentary. The organization of cells in the plane of the epithelium is known as planar cell polarity (PCP), and studies in the past decade have delineated a genetic pathway for the control of PCP. This review will first briefly review data from the Drosophila field, where PCP was first identified and genetically characterized, and then explore how vertebrate tissues become polarized during development.

Atypical cadherins Dachsous and Fat control dynamics of noncentrosomal microtubules in planar cell polarity

How global organ asymmetry and individual cell polarity are connected to each other is a central question in studying planar cell polarity (PCP). In the Drosophila wing, which develops PCP along its proximal-distal (P-D) axis, we previously proposed that the core PCP mediator Frizzled redistributes distally in a microtubule (MT)-dependent manner. Here, we performed organ-wide analysis of MT dynamics by introducing quantitative in vivo imaging. We observed MTs aligning along the P-D axis at the onset of redistribution and a small but significant excess of + ends-distal MTs in the proximal region of the wing. This characteristic alignment and asymmetry of MT growth was controlled by atypical cadherins Dachsous (Ds) and Fat (Ft). Furthermore, the action of Ft was mediated in part by PAR-1. All these data support the idea that the active reorientation of MT growth adjusts cell polarity along the organ axis.

A feed-forward circuit linking wingless, fat-dachsous signaling, and the warts-hippo pathway to Drosophila wing growth

The secreted morphogen Wingless promotes Drosophila wing growth by fueling a wave front of Fat-Dachsous signaling that recruits new cells into the wing primordium.

During development, the Drosophila wing primordium undergoes a dramatic increase in cell number and mass under the control of the long-range morphogens Wingless (Wg, a Wnt) and Decapentaplegic (Dpp, a BMP). This process depends in part on the capacity of wing cells to recruit neighboring, non-wing cells into the wing primordium. Wing cells are defined by activity of the selector gene vestigial (vg) and recruitment entails the production of a vg-dependent “feed-forward signal” that acts together with morphogen to induce vg expression in neighboring non-wing cells. Here, we identify the protocadherins Fat (Ft) and Dachsous (Ds), the Warts-Hippo tumor suppressor pathway, and the transcriptional co-activator Yorkie (Yki, a YES associated protein, or YAP) as components of the feed-forward signaling mechanism, and we show how this mechanism promotes wing growth in response to Wg. We find that vg generates the feed-forward signal by creating a steep differential in Ft-Ds signaling between wing and non-wing cells. This differential down-regulates Warts-Hippo pathway activity in non-wing cells, leading to a burst of Yki activity and the induction of vg in response to Wg. We posit that Wg propels wing growth at least in part by fueling a wave front of Ft-Ds signaling that propagates vg expression from one cell to the next.

Under normal conditions, animals and their various body parts grow until they achieve a genetically predetermined size and shape—a process governed by secreted organizer proteins called morphogens. How morphogens control growth remains unknown. In Drosophila, wings develop at the larval stage from wing primordia. Recently, we discovered that the morphogen Wingless promotes growth of the Drosophila wing by inducing the recruitment of neighboring cells into the wing primordium. Wing cells are defined by the expression of the “selector” gene vestigial. Recruitment depends on the capacity of wing cells to send a short-range, feed-forward signal that allows Wingless to activate vestigial in adjacent non-wing cells. Here, we identify the molecular components and circuitry of the recruitment process. We define the protocadherins Fat and Dachsous as a bidirectional ligand-receptor system that is controlled by vestigial to generate the feed-forward signal. Further, we show that the signal is transduced by the conserved Warts-Hippo tumor suppressor pathway via activation of its transcriptional effector Yorkie. Finally, we propose that Wingless propels wing growth by fueling a wave front of Fat-Dachsous signaling and Yorkie activity that propagates vestigial expression from one cell to the next.

When pathways collide: collaboration and connivance among signalling proteins in development

Signal transduction pathways interact at various levels to define tissue morphology, size and differentiation during development. Understanding the mechanisms by which these pathways collude has been greatly enhanced by recent insights into how shared components are independently regulated and how the activity of one system is contextualized by others. Traditionally, it has been assumed that the components of signalling pathways show pathway fidelity and act with a high degree of autonomy. However, as illustrated by the Wnt and Hippo pathways, there is increasing evidence that components are often shared between multiple pathways and other components talk to each other through multiple mechanisms.

Four-jointed modulates growth and planar polarity by reducing the affinity of dachsous for fat

The Drosophila genes fat (ft) and dachsous (ds) encode large atypical cadherins that collaborate to coordinately polarize cells in the plane of the epithelium (planar cell polarity) and to affect growth via the Warts/Hippo pathway. Ft and Ds form heterodimeric bridges that convey polarity information from cell to cell. four-jointed (fj) is a modulator of Ft/Ds activity that acts in a graded fashion in the abdomen, eye, and wing. Genetic evidence indicates that Fj acts via Ds and/or Ft, and here we demonstrate that Fj can act independently on Ds and on Ft. It has been reported that Fj has kinase activity and can phosphorylate a subset of cadherin domains of both Ft and Ds in vitro. We have used both cell and in vitro assays to measure binding between Ft and Ds. We find that phosphorylation of Ds reduces its affinity for Ft in both of these assays. By expressing forms of Ds that lack the defined phosphorylation sites or have phosphomimetic amino acids at these positions, we demonstrate that effects of Fj on wing size and planar polarity can be explained by Fj phosphorylating these sites.

Modulation of fat:dachsous binding by the cadherin domain kinase four-jointed

In addition to quantitative differences in morphogen signaling specifying cell fates, the vector and slope of morphogen gradients influence planar cell polarity (PCP) and growth. The cadherin Fat plays a central role in this process. Fat regulates PCP and growth through distinct downstream pathways, each involving the establishment of molecular polarity within cells. Fat is regulated by the cadherin Dachsous (Ds) and the protein kinase Four-jointed (Fj), which are expressed in gradients in many tissues. Previous studies have implied that Fat is regulated by the vector and slope of these expression gradients. Here, we characterize how cells interpret the Fj gradient. We demonstrate that Fj both promotes the ability of Fat to bind to its ligand Ds and inhibits the ability of Ds to bind Fat. Consequently, the juxtaposition of cells with differing Fj expression results in asymmetric Fat:Ds binding. We also show that the influence of Fj on Fat is a direct consequence of Fat phosphorylation and identify a phosphorylation site important for the stimulation of Fat:Ds binding by Fj. Our results define a molecular mechanism by which a morphogen gradient can drive the polarization of Fat activity to influence PCP and growth.

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.

Gradients and the specification of planar polarity in the insect cuticle

In addition to specifying cell fate, there is a wealth of evidence that molecular gradients are also primarily responsible for specifying cell polarity, particularly in the plane of epithelial sheets ("planar polarity"). The first compelling evidence of a role for gradients in specifying planar polarity came from transplantation experiments in the insect cuticle. More recent molecular genetic analyses in the fruit fly Drosophila have begun to give insights into the molecular nature of the gradients involved, and how they are interpreted at the cellular level.

Herding Hippos: regulating growth in flies and man

Control of cell number requires the coordinate regulation of cell proliferation and cell death. Studies in both the fly and mouse have identified the Hippo kinase pathway as a key signaling pathway that controls cell proliferation and apoptosis. Several studies have implicated the Hippo pathway in a variety of cancers. Recent studies have also revealed a role for the Hippo pathway in the control of cell fate decisions during development. In this review, we will cover the current model of Hippo signaling in development. We will explore the differences between the Hippo pathway in invertebrates and mammals, and focus on recent advances in understanding how this conserved pathway is regulated.

Drosophila lowfat, a novel modulator of Fat signaling

The Fat-Hippo-Warts signaling network regulates both transcription and planar cell polarity. Despite its crucial importance to the normal control of growth and planar polarity, we have only a limited understanding of the mechanisms that regulate Fat. We report here the identification of a conserved cytoplasmic protein, Lowfat (Lft), as a modulator of Fat signaling. Drosophila Lft, and its human homologs LIX1 and LIX1-like, bind to the cytoplasmic domains of the Fat ligand Dachsous, the receptor protein Fat, and its human homolog FAT4. Lft protein can localize to the sub-apical membrane in disc cells, and this membrane localization is influenced by Fat and Dachsous. Lft expression is normally upregulated along the dorsoventral boundary of the developing wing, and is responsible for elevated levels of Fat protein there. Levels of Fat and Dachsous protein are reduced in lft mutant cells, and can be increased by overexpression of Lft. lft mutant animals exhibit a wing phenotype similar to that of animals with weak alleles of fat, and lft interacts genetically with both fat and dachsous. These studies identify Lft as a novel component of the Fat signaling pathway, and the Lft-mediated elevation of Fat levels as a mechanism for modulating Fat signaling.

The skinny on Fat: an enormous cadherin that regulates cell adhesion, tissue growth, and planar cell polarity

Fat is an extremely large atypical cadherin involved in the regulation of cell adhesion, tissue growth, and planar cell polarity (PCP). Recent studies have begun to illuminate the mechanisms by which Fat performs these functions during development. Fat relays signals to the Hippo pathway to regulate tissue growth, and to PCP proteins to regulate tissue patterning. In this review we briefly cover the historical data demonstrating that Fat regulates tissue growth and tissue patterning, and then focus on advances in the past three years illuminating the mechanisms by which Fat controls growth and planar polarity in flies and mammals.

Progress and challenges in understanding planar cell polarity signaling

During development, epithelial cells in some tissues acquire a polarity orthogonal to their apical-basal axis. This polarity, referred to as planar cell polarity (PCP), or tissue polarity, is essential for the normal physiological function of many epithelia. Early studies of PCP focused on insect epithelia (Lawrence, 1966 [1]), and the earliest genetic analyses were carried out in Drosophila (Held et al., 1986; Gubb and Garcia-Bellido, 1982 [2,3]). Indeed, most of our mechanistic understanding of PCP derives from the ongoing use of Drosophila as a model system. However, a range of medically important developmental defects and physiological processes are under the control of PCP mechanisms that appear to be at least partially conserved, driving considerable interest in studying PCP both in Drosophila and in vertebrate model systems. Here, I present a model of the PCP signaling mechanism based on studies in Drosophila. I highlight two areas in which our understanding is deficient, and which lead to current confusion in the literature. Future studies that shed light on these areas will substantially enhance our understanding of the fascinating yet challenging problem of understanding the mechanisms that generate PCP.

Processing and phosphorylation of the Fat receptor

The Drosophila tumor suppressors fat and discs overgrown (dco) function within an intercellular signaling pathway that controls growth and polarity. fat encodes a transmembrane receptor, but post-translational regulation of Fat has not been described. We show here that Fat is subject to a constitutive proteolytic processing, such that most or all cell surface Fat comprises a heterodimer of stably associated N- and C-terminal fragments. The cytoplasmic domain of Fat is phosphorylated, and this phosphorylation is promoted by the Fat ligand Dachsous. dco encodes a kinase that influences Fat signaling, and Dco is able to promote the phosphorylation of the Fat intracellular domain in cultured cells and in vivo. Evaluation of dco mutants indicates that they affect Fat's influence on growth and gene expression but not its influence on planar cell polarity. Our observations identify processing and phosphorylation as post-translational modifications of Fat, correlate the phosphorylation of Fat with its activation by Dachsous in the Fat-Warts pathway, and enhance our understanding of the requirement for Dco in Fat signaling.

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.

Phosphorylation of the tumor suppressor fat is regulated by its ligand Dachsous and the kinase discs overgrown

The Drosophila tumor suppressor gene fat encodes a large cadherin that regulates growth and a form of tissue organization known as planar cell polarity (PCP). Fat regulates growth via the Hippo kinase pathway, which controls expression of genes promoting cell proliferation and inhibiting apoptosis (reviewed in). The Hippo pathway is highly conserved and is implicated in the regulation of mammalian growth and cancer development. Genetic studies suggest that Fat activity is regulated by binding to another large cadherin, Dachsous (Ds). The tumor suppressor discs overgrown (dco)/Casein Kinase I delta/epsilon also regulates Hippo activity and PCP. The biochemical nature of how Fat, Ds, and Dco interact to regulate these pathways is poorly understood. Here we demonstrate that Fat is cleaved to generate 450 kDa and 110 kDa fragments (Fat(450) and Fat(110)). Fat(110) contains the cytoplasmic and transmembrane domain. The cytoplasmic domain of Fat binds Dco and is phosphorylated by Dco at multiple sites. Importantly, we show Fat forms cis-dimers and that Fat phosphorylation is regulated by Dachsous and Dco in vivo. We propose that Ds regulates Dco-dependent phosphorylation of Fat and Fat-associated proteins to control Fat signaling in growth and PCP.

Is left-right asymmetry a form of planar cell polarity?

Consistent left-right (LR) patterning is a clinically important embryonic process. However, key questions remain about the origin of asymmetry and its amplification across cell fields. Planar cell polarity (PCP) solves a similar morphogenetic problem, and although core PCP proteins have yet to be implicated in embryonic LR asymmetry, studies of mutations affecting planar polarity, together with exciting new data in cell and developmental biology, provide a new perspective on LR patterning. Here we propose testable models for the hypothesis that LR asymmetry propagates as a type of PCP that imposes coherent orientation onto cell fields, and that the cue that orients this polarization is a chiral intracellular structure.

Planar cell polarity signaling: from fly development to human disease

Most, if not all, cell types and tissues display several aspects of polarization. In addition to the ubiquitous epithelial cell polarity along the apical-basolateral axis, many epithelial tissues and organs are also polarized within the plane of the epithelium. This is generally referred to as planar cell polarity (PCP; or historically, tissue polarity). Genetic screens in Drosophila pioneered the discovery of core PCP factors, and subsequent work in vertebrates has established that the respective pathways are evolutionarily conserved. PCP is not restricted only to epithelial tissues but is also found in mesenchymal cells, where it can regulate cell migration and cell intercalation. Moreover, particularly in vertebrates, the conserved core PCP signaling factors have recently been found to be associated with the orientation or formation of cilia. This review discusses new developments in the molecular understanding of PCP establishment in Drosophila and vertebrates; these developments are integrated with new evidence that links PCP signaling to human disease.

Do the protocadherins Fat and Dachsous link up to determine both planar cell polarity and the dimensions of organs?

Most, perhaps all cells in epithelial sheets are polarized in the plane of the sheet. This type of polarity, referred to as planar cell polarity (PCP), can be expressed in the orientation of cilia and stereocilia, in oriented outgrowths such as hairs, in the plane of cell division, in directed cell movement and possibly in the orientation of axon extension. Another popular area in current research is growth: there is an attempt to find systems that fix the shape and size of organs. Although both polarity and growth are subject to overall control by morphogen gradients, the mechanisms of this control are almost completely unknown. Here we discuss recent evidence for a 'steepness hypothesis' that links these two apparently disconnected features of animal development.

Cell packing influences planar cell polarity signaling

Some epithelial cells display asymmetry along an axis orthogonal to the apical-basal axis, referred to as planar cell polarity (PCP). A Frizzled-mediated feedback loop coordinates PCP between neighboring cells, and the cadherin Fat transduces a global directional cue that orients PCP with respect to the tissue axes. The feedback loop can propagate polarity across clones of cells that lack the global directional signal, although this polarity propagation is error prone. Here, we show that, in the Drosophila wing, a combination of cell geometry and nonautonomous signaling at clone boundaries determines the correct or incorrect polarity propagation in clones that lack Fat mediated global directional information. Pattern elements, such as veins, and sporadic occurrences of irregular geometry are obstacles to polarity propagation. Hence, in the wild type, broad distribution of the global directional cue combines with a local feedback mechanism to overcome irregularities in cell packing geometry during PCP signaling.

Is anisotropic propagation of polarized molecular distribution the common mechanism of swirling patterns of planar cell polarization?

Mutations in multiple planar cell polarity (PCP) genes can cause swirling patterns indicated by whorls and tufts of hairs in the wings and the abdomen of Drosophila and in the skin of vertebrates. Damaged global directional cue caused by mutations in four-jointed, fat, and dachsous, impaired cellular hexagonal packing caused by mutations in frizzled, or weakened intracellular signaling caused by mutations in disheveled, inturned, and prickle all make hair patterns globally irregular yet locally aligned, and in some cases, typically swirling. Why and how mutations in different genes all lead to swirling patterns is unexplored. Although the mechanisms of molecular signaling remain unclear, the features of molecular distribution are evident-most PCP molecules develop the polarized distribution in cells and this distribution can be induced by intercellular signaling. Does this suggest something fundamental to swirling patterns beyond the particular functions of genes, proteins, and signaling? A simple model indeed indicates this. Disregarding detailed molecular interactions, the induced polarization of molecular distribution in an epithelial cell can be modeled as the induced polarization of positive and negative charge distribution in a dielectric molecule. Simulations reveal why and how mutations in different genes all lead to swirling patterns, and in particular, the conditions for generating typical swirling patterns. The results show that the anisotropic propagation of polarized molecular distribution may be the common mechanism of swirling patterns caused by different mutations. They also suggest that at the cell level, as at the molecular level, a simple mechanism can generate complex and diverse patterning phenotypes in different molecular contexts. The similarity between the induced polarization and its propagation in both the epithelial cells and the dielectric molecules also interestingly suggests some commonalities between pattern formation in the biological and physical systems.

Boundaries of Dachsous Cadherin activity modulate the Hippo signaling pathway to induce cell proliferation

The conserved Hippo tumor suppressor pathway is a key signaling pathway that controls organ size in Drosophila. To date a signal transduction cascade from the Cadherin Fat at the plasma membrane into the nucleus has been discovered. However, how the Hippo pathway is regulated by extracellular signals is poorly understood. Fat not only regulates growth but also planar cell polarity, for which it interacts with the Dachsous (Ds) Cadherin, and Four-jointed (Fj), a transmembrane kinase that modulates the interaction between Ds and Fat. Ds and Fj are expressed in gradients and manipulation of their expression causes abnormal growth. However, how Ds and Fj regulate growth and whether they act through the Hippo pathway is not known. Here, we report that Ds and Fj regulate Hippo signaling to control growth. Interestingly, we found that Ds/Fj regulate the Hippo pathway through a remarkable logic. Induction of Hippo target genes is not proportional to the amount of Ds or Fj presented to a cell, as would be expected if Ds and Fj acted as traditional ligands. Rather, Hippo target genes are up-regulated when neighboring cells express different amounts of Ds or Fj. Consistent with a model that differences in Ds/Fj levels between cells regulate the Hippo pathway, we found that artificial Ds/Fj boundaries induce extra cell proliferation, whereas flattening the endogenous Ds and Fj gradients results in growth defects. The Ds/Fj signaling system thus defines a cell-to-cell signaling mechanism that regulates the Hippo pathway, thereby contributing to the control of organ size.

Morphogen control of wing growth through the Fat signaling pathway

Organ growth is influenced by organ patterning, but the molecular mechanisms that link patterning to growth have remained unclear. We show that the Dpp morphogen gradient in the Drosophila wing influences growth by modulating the activity of the Fat signaling pathway. Dpp signaling regulates the expression and localization of Fat pathway components, and Fat signaling through Dachs is required for the effect of the Dpp gradient on cell proliferation. Juxtaposition of cells that express different levels of the Fat pathway regulators four-jointed and dachsous stimulates expression of Fat/Hippo pathway target genes and cell proliferation, consistent with the hypothesis that the graded expression of these genes contributes to wing growth. Moreover, uniform expression of four-jointed and dachsous in the wing inhibits cell proliferation. These observations identify Fat as a signaling pathway that links the morphogen-mediated establishment of gradients of positional values across developing organs to the regulation of organ growth.

The Fat and Warts signaling pathways: new insights into their regulation,mechanism and conservation

A cassette of cytoplasmic Drosophila tumor suppressors, including the kinases Hippo and Warts, has recently been linked to the transmembrane tumor suppressor Fat. These proteins act within interconnected signaling pathways, the principal functions of which are to control the growth and polarity of developing tissues. Recent studies have enhanced our understanding of the basis for signal transduction by Fat and Warts pathways, including the identification of a DNA-binding protein at the end of the pathway, have established the conservation of Fat and Warts signaling from flies to mammals,and have given us new insights into their regulation and biological functions.

Loss of Fat4 disrupts PCP signaling and oriented cell division and leads to cystic kidney disease

Tissue organization in Drosophila is regulated by the core planar cell polarity (PCP) proteins Frizzled, Dishevelled, Prickle, Van Gogh and Flamingo. Core PCP proteins are conserved in mammals and function in mammalian tissue organization. Recent studies have identified another group of Drosophila PCP proteins, consisting of the protocadherins Fat and Dachsous (Ds) and the transmembrane protein Four-jointed (Fj). In Drosophila, Fat represses fj transcription, and Ds represses Fat activity in PCP. Here we show that Fat4 is an essential gene that has a key role in vertebrate PCP. Loss of Fat4 disrupts oriented cell divisions and tubule elongation during kidney development, leading to cystic kidney disease. Fat4 genetically interacts with the PCP genes Vangl2 and Fjx1 in cyst formation. In addition, Fat4 represses Fjx1 expression, indicating that Fat signaling is conserved. Together, these data suggest that Fat4 regulates vertebrate PCP and that loss of PCP signaling may underlie some cystic diseases in humans.

Planar cell polarity: one or two pathways?

In multicellular organisms, cells are polarized in the plane of the epithelial sheet, revealed in some cell types by oriented hairs or cilia. Many of the underlying genes have been identified in Drosophila melanogaster and are conserved in vertebrates. Here we dissect the logic of planar cell polarity (PCP). We review studies of genetic mosaics in adult flies - marked cells of different genotypes help us to understand how polarizing information is generated and how it passes from one cell to another. We argue that the prevailing opinion that planar polarity depends on a single genetic pathway is wrong and conclude that there are (at least) two independently acting processes. This conclusion has major consequences for the PCP field.

Planar Polarity and Tissue Morphogenesis

Planar polarity is a global, tissue-level phenomenon that coordinates cell behavior in a two-dimensional plane. The Frizzled/planar cell polarity (PCP) and anterior-posterior (AP) patterning systems for planar polarity operate in a variety of cell types and provide direction to cells with different morphologies and behaviors. These two systems involve different sets of proteins but both use directional cues provided locally by communication between neighboring cells. This review describes our current understanding of the mechanisms that transmit directional signals from cell to cell and compares the strategies for generating global systems of spatial information in stationary and dynamic cell populations.

Asymmetric distribution of prickle-like 2 reveals an early underlying polarization of vestibular sensory epithelia in the inner ear

Vestibular hair cells have a distinct planar cell polarity (PCP) manifest in the morphology of their stereocilia bundles and the asymmetric localization of their kinocilia. In the utricle and saccule the hair cells are arranged in an orderly array about an abrupt line of reversal that separates fields of cells with opposite polarity. We report that the putative PCP protein Prickle-like 2 (Pk2) is distributed in crescents on the medial sides of vestibular epithelial cells before the morphological polarization of hair cells. Despite the presence of a line of polarity reversal, crescent position is not altered between hair cells of opposite polarity. Frizzled 6 (Fz6), a second PCP protein, is distributed opposite Pk2 along the lateral side of vestibular support cells. Similar to Pk2, the subcellular localization of Fz6 does not differ between cells located on opposite sides of the line of reversal. In addition, in Looptail/Van Gogh-like2 mutant mice Pk2 is distributed asymmetrically at embryonic day 14.5 (E14.5), but this localization is not coordinated between adjacent cells, and the crescents subsequently are lost by E18.5. Together, these results support the idea that a conserved PCP complex acts before stereocilia bundle development to provide an underlying polarity to all cells in the vestibular epithelia and that cells on either side of the line of reversal are programmed to direct the kinocilium in opposite directions with respect to the polarity axis defined by PCP protein distribution.

Frizzled/PCP signalling: a conserved mechanism regulating cell polarity and directed motility

Signalling through Frizzled (Fz)/planar cell polarity (PCP) is a conserved mechanism that polarizes cells along specific axes in a tissue. Genetic screens in Drosophila melanogaster pioneered the discovery of core PCP factors, which regulate the orientation of hairs on wings and facets in eyes. Recent genetic evidence shows that the Fz/PCP pathway is conserved in vertebrates and is crucial for disparate processes as gastrulation and sensory cell orientation. Fz/PCP signalling depends on complex interactions between core components, leading to their asymmetric distribution and ultimately polarized activity in a cell. Whereas several mechanistic aspects of PCP have been uncovered, the global coordination of this polarization remains debated.

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.

Cadherins in development: cell adhesion, sorting, and tissue morphogenesis

Tissue morphogenesis during development is dependent on activities of the cadherin family of cell-cell adhesion proteins that includes classical cadherins, protocadherins, and atypical cadherins (Fat, Dachsous, and Flamingo). The extracellular domain of cadherins contains characteristic repeats that regulate homophilic and heterophilic interactions during adhesion and cell sorting. Although cadherins may have originated to facilitate mechanical cell-cell adhesion, they have evolved to function in many other aspects of morphogenesis. These additional roles rely on cadherin interactions with a wide range of binding partners that modify their expression and adhesion activity by local regulation of the actin cytoskeleton and diverse signaling pathways. Here we examine how different members of the cadherin family act in different developmental contexts, and discuss the mechanisms involved.

Developmental expression of the chick four-jointed homologue

Four-jointed is a type II transmembrane protein that is thought to be cleaved to give rise to a secreted protein. In Drosophila, four-jointed controls outgrowth, vein patterning, and bristle polarity in the developing limb together with the polarity of the ommatidia in the developing eye. In Drosophila and mice, Fj is regulated by notch signaling. Here, we have determined the expression of the chick four-jointed (fjx) homologue during embryonic development. We show that fjx is expressed in the limb bud; facial primordia; the proliferating zone of the lens, feather buds, the neural tube; and neural crest derivatives such as the dorsal root ganglia. Analysis of the fjx expression in the developing limb bud showed that initially fjx is expressed throughout the limb bud, but as the limb develops, highest levels of fjx transcripts are found distally. However, by stage 27, fjx expression is predominantly found in the central core of the limb bud. Finally, fjx expression becomes confined to the developing tendons, ligaments, articular cartilage, and arteries but not the veins. Comparison with scleraxis (scx), a marker of tendons and ligaments, revealed that they are coexpressed in the majority of tendons but that fjx is expressed after scx, when the tendons have begun to differentiate. These data suggest that fjx has two roles during limb development: the first controlling outgrowth and the second tissue differentiation.

Shaping the mammalian auditory sensory organ by the planar cell polarity pathway

The human ear is capable of processing sound with a remarkable resolution over a wide range of intensity and frequency. This ability depends largely on the extraordinary feats of the hearing organ, the organ of Corti and its sensory hair cells. The organ of Corti consists of precisely patterned rows of sensory hair cells and supporting cells along the length of the snail-shaped cochlear duct. On the apical surface of each hair cell, several rows of actin-containing protrusions, known as stereocilia, form a "V"-shaped staircase. The vertices of all the "V"-shaped stereocilia point away from the center of the cochlea. The uniform orientation of stereocilia in the organ of Corti manifests a distinctive form of polarity known as planar cell polarity (PCP). Functionally, the direction of stereociliary bundle deflection controls the mechanical channels located in the stereocilia for auditory transduction. In addition, hair cells are tonotopically organized along the length of the cochlea. Thus, the uniform orientation of stereociliary bundles along the length of the cochlea is critical for effective mechanotransduction and for frequency selection. Here we summarize the morphological and molecular events that bestow the structural characteristics of the mammalian hearing organ, the growth of the snail-shaped cochlear duct and the establishment of PCP in the organ of Corti. The PCP of the sensory organs in the vestibule of the inner ear will also be described briefly.

Wnt, Hedgehog and junctional Armadillo/beta-catenin establish planar polarity in the Drosophila embryo

To generate specialized structures, cells must obtain positional and directional information. In multi-cellular organisms, cells use the non-canonical Wnt or planar cell polarity (PCP) signaling pathway to establish directionality within a cell. In vertebrates, several Wnt molecules have been proposed as permissible polarity signals, but none has been shown to provide a directional cue. While PCP signaling components are conserved from human to fly, no PCP ligands have been reported in Drosophila. Here we report that in the epidermis of the Drosophila embryo two signaling molecules, Hedgehog (Hh) and Wingless (Wg or Wnt1), provide directional cues that induce the proper orientation of Actin-rich structures in the larval cuticle. We further find that proper polarity in the late embryo also involves the asymmetric distribution and phosphorylation of Armadillo (Arm or β-catenin) at the membrane and that interference with this Arm phosphorylation leads to polarity defects. Our results suggest new roles for Hh and Wg as instructive polarizing cues that help establish directionality within a cell sheet, and a new polarity-signaling role for the membrane fraction of the oncoprotein Arm.

Genes affecting cell competition in Drosophila

Cell competition is a homeostatic mechanism that regulates the size attained by growing tissues. We performed an unbiased genetic screen for mutations that permit the survival of cells being competed due to haplo-insufficiency for RpL36. Mutations that protect RpL36 heterozygous clones include the tumor suppressors expanded, hippo, salvador, mats, and warts, which are members of the Warts pathway, the tumor suppressor fat, and a novel tumor-suppressor mutation. Other hyperplastic or neoplastic mutations did not rescue RpL36 heterozygous clones. Most mutations that rescue cell competition elevated Dpp-signaling activity, and the Dsmurf mutation that elevates Dpp signaling was also hyperplastic and rescued. Two nonlethal, nonhyperplastic mutations prevent the apoptosis of Minute heterozygous cells and suggest an apoptosis pathway for cell competition . In addition to rescuing RpL36 heterozygous cells, mutations in Warts pathway genes were supercompetitors that could eliminate wild-type cells nearby. The findings show that differences in Warts pathway activity can lead to competition and implicate the Warts pathway, certain other tumor suppressors, and novel cell death components in cell competition, in addition to the Dpp pathway implicated by previous studies. We suggest that cell competition might occur during tumor development in mammals.