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

Epithelial polarization by the planar cell polarity complex is exclusively non-cell autonomous

For cells to polarize collectively along a tissue plane, asymmetrically localized planar cell polarity (PCP) complexes must form intercellular contacts between neighboring cells. Yet, it is unknown whether asymmetric segregation of PCP complexes requires cell-cell contact, or if cell autonomous, antagonistic interactions are sufficient for polarization. To test this, we generated mouse chimeras consisting of dual PCP-reporter cells mixed with unlabeled cells that cannot form PCP bridges. In the absence of intercellular interactions, PCP proteins failed to polarize cell autonomously. Rather, PCP-mediated contacts along single cell-cell interfaces were sufficient to sort PCP components to opposite sides of the junction, independent of junction orientation. Thus, intercellular binding of PCP complexes is the critical step that initiates sorting of opposing PCP complexes to generate asymmetry.

Cryo-EM structure and oligomerization of the human planar cell polarity core protein Vangl1

Vangl is a planar cell polarity (PCP) core protein essential for aligned cell orientation along the epithelial plane perpendicular to the apical-basal direction, which is important for tissue morphogenesis, development and collective cell behavior. Mutations in Vangl are associated with developmental defects, including neural tube defects (NTDs), according to human cohort studies of sporadic and familial cases. The complex mechanisms underlying Vangl-mediated PCP signaling or Vangl-associated human congenital diseases have been hampered by the lack of molecular characterizations of Vangl. Here, we show biochemical and structural evidence that human Vangl1 oligomerizes as dimers of trimers, and that the dimerization of trimers promotes binding to the PCP effector Prickle1 (Pk1) in vitro. Mapping of human disease-associated point mutations suggests potential pathological mechanisms and paves the way for future studies on the importance of lipid binding, central vestibule and oligomerization of Vangl, thereby providing insights into the molecular mechanisms of the PCP signaling pathway.

The bidirectional relationship between cilia and PCP signaling pathway core protein Vangl2

Vangl2, a core component of the PCP signaling pathway, serves as a scaffold protein on the cell membrane, playing a crucial role in organizing protein complexes. Cilia, dynamic structures on the cell surface, carry out a wide range of functions. Research has highlighted a bidirectional regulatory interaction between Vangl2 and cilia, underscoring their interconnected roles in cellular processes. This relationship is demonstrated by the localization of Vangl2 at the base and proximal regions of cilia, where it plays essential roles in ciliary positioning, asymmetric distribution, and ciliogenesis. In contrast, the absence of cilia can disrupt Vangl2-mediated signal transduction processes. This review offers a narrative review of recent research on Vangl2's function in cilia and examines the regulatory effects of cilia on Vangl2-mediated signaling.

Centriole Translational Planar Polarity in Monociliated Epithelia

Ciliated epithelia are widespread in animals and play crucial roles in many developmental and physiological processes. Epithelia composed of multi-ciliated cells allow for directional fluid flow in the trachea, oviduct and brain cavities. Monociliated epithelia play crucial roles in vertebrate embryos, from the establishment of left-right asymmetry to the control of axis curvature via cerebrospinal flow motility in zebrafish. Cilia also have a central role in the motility and feeding of free-swimming larvae in a variety of marine organisms. These diverse functions rely on the coordinated orientation (rotational polarity) and asymmetric localization (translational polarity) of cilia and of their centriole-derived basal bodies across the epithelium, both being forms of planar cell polarity (PCP). Here, we review our current knowledge on the mechanisms of the translational polarity of basal bodies in vertebrate monociliated epithelia from the molecule to the whole organism. We highlight the importance of live imaging for understanding the dynamics of centriole polarization. We review the roles of core PCP pathways and of apicobasal polarity proteins, such as Par3, whose central function in this process has been recently uncovered. Finally, we emphasize the importance of the coordination between polarity proteins, the cytoskeleton and the basal body itself in this highly dynamic process.

A Van Gogh/Vangl tyrosine phosphorylation switch regulates its interaction with core Planar Cell Polarity factors Prickle and Dishevelled

Epithelial tissues can be polarized along two axes: in addition to apical-basal polarity they are often also polarized within the plane of the epithelium, known as planar cell polarity (PCP). PCP depends upon the conserved Wnt/Frizzled (Fz) signaling factors, including Fz itself and Van Gogh (Vang/Vangl in mammals). Here, taking advantage of the complementary features of Drosophila wing and mouse skin PCP establishment, we dissect how Vang/Vangl phosphorylation on a specific conserved tyrosine residue affects its interaction with two cytoplasmic core PCP factors, Dishevelled (Dsh/Dvl1-3 in mammals) and Prickle (Pk/Pk1-3). We demonstrate that Pk and Dsh/Dvl bind to Vang/Vangl in an overlapping region centered around this tyrosine. Strikingly, Vang/Vangl phosphorylation promotes its binding to Prickle, a key effector of the Vang/Vangl complex, and inhibits its interaction with Dishevelled. Thus phosphorylation of this tyrosine appears to promote the formation of the mature Vang/Vangl-Pk complex during PCP establishment and conversely it inhibits the Vang interaction with the antagonistic effector Dishevelled. Intriguingly, the phosphorylation state of this tyrosine might thus serve as a switch between transient interactions with Dishevelled and stable formation of Vang-Pk complexes during PCP establishment.

Vangl-dependent Wnt/planar cell polarity signaling mediates collective breast carcinoma motility and distant metastasis

BACKGROUND

In light of the growing appreciation for the role of collective cell motility in metastasis, a deeper understanding of the underlying signaling pathways will be critical to translating these observations to the treatment of advanced cancers. Here, we examine the contribution of Wnt/planar cell polarity (Wnt/PCP), one of the non-canonical Wnt signaling pathways and defined by the involvement of the tetraspanin-like proteins Vangl1 and Vangl2, to breast tumor cell motility, collective cell invasiveness and mammary tumor metastasis.

METHODS

Vangl1 and Vangl2 knockdown and overexpression and Wnt5a stimulation were employed to manipulate Wnt/PCP signaling in a battery of breast cancer cell lines representing all breast cancer subtypes, and in tumor organoids from MMTV-PyMT mice. Cell migration was assessed by scratch and organoid invasion assays, Vangl protein subcellular localization was assessed by confocal fluorescence microscopy, and RhoA activation was assessed in real time by fluorescence imaging with an advanced FRET biosensor. The impact of Wnt/PCP suppression on mammary tumor growth and metastasis was assessed by determining the effect of conditional Vangl2 knockout on the MMTV-NDL mouse mammary tumor model.

RESULTS

We observed that Vangl2 knockdown suppresses the motility of all breast cancer cell lines examined, and overexpression drives the invasiveness of collectively migrating MMTV-PyMT organoids. Vangl2-dependent RhoA activity is localized in real time to a subpopulation of motile leader cells displaying a hyper-protrusive leading edge, Vangl protein is localized to leader cell protrusions within leader cells, and actin cytoskeletal regulator RhoA is preferentially activated in the leader cells of a migrating collective. Mammary gland-specific knockout of Vangl2 results in a striking decrease in lung metastases in MMTV-NDL mice, but does not impact primary tumor growth characteristics.

CONCLUSIONS

We conclude that Vangl-dependent Wnt/PCP signaling promotes breast cancer collective cell migration independent of breast tumor subtype and facilitates distant metastasis in a genetically engineered mouse model of breast cancer. Our observations are consistent with a model whereby Vangl proteins localized at the leading edge of leader cells in a migrating collective act through RhoA to mediate the cytoskeletal rearrangements required for pro-migratory protrusion formation.

Tissue flow regulates planar cell polarity independently of the Frizzled core pathway

Planar cell polarity (PCP) regulates the orientation of external structures. A core group of proteins that includes Frizzled forms the heart of the PCP regulatory system. Other PCP mechanisms that are independent of the core group likely exist, but their underlying mechanisms are elusive. Here, we show that tissue flow is a mechanism governing core group-independent PCP on the Drosophila notum. Loss of core group function only slightly affects bristle orientation in the adult central notum. This near-normal PCP results from tissue flow-mediated rescue of random bristle orientation during the pupal stage. Manipulation studies suggest that tissue flow can orient bristles in the opposite direction to the flow. This process is independent of the core group and implies that the apical extracellular matrix functions like a "comb" to align bristles. Our results reveal the significance of cooperation between tissue dynamics and extracellular substances in PCP establishment.

The conserved Pelado/ZSWIM8 protein regulates actin dynamics by promoting linear actin filament polymerization

Actin filament polymerization can be branched or linear, which depends on the associated regulatory proteins. Competition for actin monomers occurs between proteins that induce branched or linear actin polymerization. Cell specialization requires the regulation of actin filaments to allow the formation of cell type-specific structures, like cuticular hairs in <i>Drosophila</i>, formed by linear actin filaments. Here, we report the functional analysis of CG34401/<i>pelado</i>, a gene encoding a SWIM domain-containing protein, conserved throughout the animal kingdom, called ZSWIM8 in mammals. Mutant <i>pelado</i> epithelial cells display actin hair elongation defects. This phenotype is reversed by increasing actin monomer levels or by either pushing linear actin polymerization or reducing branched actin polymerization. Similarly, in hemocytes, Pelado is essential to induce filopodia, a linear actin-based structure. We further show that this function of Pelado/ZSWIM8 is conserved in human cells, where Pelado inhibits branched actin polymerization in a cell migration context. In summary, our data indicate that the function of Pelado/ZSWIM8 in regulating actin cytoskeletal dynamics is conserved, favoring linear actin polymerization at the expense of branched filaments.

Inter-growth cone communications mediated by planar cell polarity pathway in axon guidance

The emergence of exquisitely organized axonal projections is one of the greatest wonders of nervous system development. In addition to growing along stereotyped directions, axons join one another as they extend to form highly organized projections. Axon-axon interactions are essential for axon guidance during nervous system wiring. Axonal growth cones recognize cell surface guidance cues on axons and either grow along the axons or away from the axons. However, it is less well understood whether and how the growth cones communicate with each other and, if so, what do these interactions mean. Recent studies from our lab provided direct evidence that the growth cones do interact with each other during axon pathfinding. And this interaction is regulated by highly regulated protein-protein interactions among components of the planar cell polarity pathway. The disruption of these interactions lead to guidance defects and disorganization of axons. We propose that these local inter-growth cone PCP signaling reinforces and increases the sensitivity of the growth cone response to shallow Wnt gradients to turn in a precise and organized fashion.

Alignment of the cell long axis by unidirectional tension acts cooperatively with Wnt signalling to establish planar cell polarity

Planar cell polarity (PCP) is the aligned cell polarity within a tissue plane. Mechanical signals are known to act as a global cue for PCP, yet their exact role is still unclear. In this study, we focused on PCP in the posterior neuroectoderm of Xenopus laevis and investigated how mechanical signals regulate polarity. We reveal that the neuroectoderm is under a greater tension in the anterior-posterior direction and that perturbation of this tension causes PCP disappearance. We show that application of uniaxial stretch to explant tissues can control the orientation of PCP and that cells sense the tissue stretch indirectly through a change in their shape, rather than directly through detection of anisotropic tension. Furthermore, we reveal that PCP is most strongly established when the orientation of tissue stretch coincides with that of diffusion of locally expressed Wnt ligands, suggesting a cooperative relationship between these two PCP regulators.

Vangl as a Master Scaffold for Wnt/Planar Cell Polarity Signaling in Development and Disease

The establishment of polarity within tissues and dynamic cellular morphogenetic events are features common to both developing and adult tissues, and breakdown of these programs is associated with diverse human diseases. Wnt/Planar cell polarity (Wnt/PCP) signaling, a branch of non-canonical Wnt signaling, is critical to the establishment and maintenance of polarity in epithelial tissues as well as cell motility events critical to proper embryonic development. In epithelial tissues, Wnt/PCP-mediated planar polarity relies upon the asymmetric distribution of core proteins to establish polarity, but the requirement for this distribution in Wnt/PCP-mediated cell motility remains unclear. However, in both polarized tissues and migratory cells, the Wnt/PCP-specific transmembrane protein Vangl is required and appears to serve as a scaffold upon which the core pathway components as well as positive and negative regulators of Wnt/PCP signaling assemble. The current literature suggests that the multiple interaction domains of Vangl allow for the binding of diverse signaling partners for the establishment of context- and tissue-specific complexes. In this review we discuss the role of Vangl as a master scaffold for Wnt/PCP signaling in epithelial tissue polarity and cellular motility events in developing and adult tissues, and address how these programs are dysregulated in human disease.

Frizzled3 inhibits Vangl2-Prickle3 association to establish planar cell polarity in the vertebrate neural plate

The orientation of epithelial cells in the plane of the tissue, known as planar cell polarity (PCP), is regulated by interactions of asymmetrically localized PCP protein complexes. In the Xenopus neural plate, Van Gogh-like2 (Vangl2) and Prickle3 (Pk3) proteins form a complex at the anterior cell boundaries, but how this complex is regulated in vivo remains largely unknown. Here, we use proximity biotinylation and crosslinking approaches to show that Vangl2-Pk3 association is inhibited by Frizzled3 (Fz3, also known as Fzd3), a core PCP protein that is specifically expressed in the neuroectoderm and is essential for the establishment of PCP in this tissue. This inhibition required Fz3-dependent Vangl2 phosphorylaton. Consistent with our observations, the complex of Pk3 with nonphosphorylatable Vangl2 did not polarize in the neural plate. These findings provide evidence for in vivo regulation of Vangl2-Pk3 complex formation and localization by a Frizzled receptor.

Conserved and Divergent Principles of Planar Polarity Revealed by Hair Cell Development and Function

Planar polarity describes the organization and orientation of polarized cells or cellular structures within the plane of an epithelium. The sensory receptor hair cells of the vertebrate inner ear have been recognized as a preeminent vertebrate model system for studying planar polarity and its development. This is principally because planar polarity in the inner ear is structurally and molecularly apparent and therefore easy to visualize. Inner ear planar polarity is also functionally significant because hair cells are mechanosensors stimulated by sound or motion and planar polarity underlies the mechanosensory mechanism, thereby facilitating the auditory and vestibular functions of the ear. Structurally, hair cell planar polarity is evident in the organization of a polarized bundle of actin-based protrusions from the apical surface called stereocilia that is necessary for mechanosensation and when stereociliary bundle is disrupted auditory and vestibular behavioral deficits emerge. Hair cells are distributed between six sensory epithelia within the inner ear that have evolved unique patterns of planar polarity that facilitate auditory or vestibular function. Thus, specialized adaptations of planar polarity have occurred that distinguish auditory and vestibular hair cells and will be described throughout this review. There are also three levels of planar polarity organization that can be visualized within the vertebrate inner ear. These are the intrinsic polarity of individual hair cells, the planar cell polarity or coordinated orientation of cells within the epithelia, and planar bipolarity; an organization unique to a subset of vestibular hair cells in which the stereociliary bundles are oriented in opposite directions but remain aligned along a common polarity axis. The inner ear with its complement of auditory and vestibular sensory epithelia allows these levels, and the inter-relationships between them, to be studied using a single model organism. The purpose of this review is to introduce the functional significance of planar polarity in the auditory and vestibular systems and our contemporary understanding of the developmental mechanisms associated with organizing planar polarity at these three cellular levels.

The EMT activator ZEB1 accelerates endosomal trafficking to establish a polarity axis in lung adenocarcinoma cells

Epithelial-to-mesenchymal transition (EMT) is a transcriptionally governed process by which cancer cells establish a front-rear polarity axis that facilitates motility and invasion. Dynamic assembly of focal adhesions and other actin-based cytoskeletal structures on the leading edge of motile cells requires precise spatial and temporal control of protein trafficking. Yet, the way in which EMT-activating transcriptional programs interface with vesicular trafficking networks that effect cell polarity change remains unclear. Here, by utilizing multiple approaches to assess vesicular transport dynamics through endocytic recycling and retrograde trafficking pathways in lung adenocarcinoma cells at distinct positions on the EMT spectrum, we find that the EMT-activating transcription factor ZEB1 accelerates endocytosis and intracellular trafficking of plasma membrane-bound proteins. ZEB1 drives turnover of the MET receptor tyrosine kinase by hastening receptor endocytosis and transport to the lysosomal compartment for degradation. ZEB1 relieves a plus-end-directed microtubule-dependent kinesin motor protein (KIF13A) and a clathrin-associated adaptor protein complex subunit (AP1S2) from microRNA-dependent silencing, thereby accelerating cargo transport through the endocytic recycling and retrograde vesicular pathways, respectively. Depletion of KIF13A or AP1S2 mitigates ZEB1-dependent focal adhesion dynamics, front-rear axis polarization, and cancer cell motility. Thus, ZEB1-dependent transcriptional networks govern vesicular trafficking dynamics to effect cell polarity change.

The way in which metastatic tumour cells control endocytic vesicular trafficking networks to establish a front-rear polarity axis that facilitates motility remains unclear. Here, the authors show that the EMT activator ZEB1 influences vesicular trafficking dynamics to execute cell polarity change.

Bridging the gap between non-canonical and canonical Wnt signaling through Vangl2

Non-canonical Wnt signaling (encompassing Wnt/PCP and WntCa²⁺) has a dual identity in the literature. One stream of research investigates its role in antagonizing canonical Wnt/β-catenin signaling in cancer, typically through Ca²⁺, while the other stream investigates its effect on polarity in development, typically through Vangl2. Rarely do these topics intersect or overlap. What has become clear is that Wnt5a can mobilize intracellular calcium stores to inhibit Wnt/β-catenin in cancer cells but there is no evidence that Vangl2 is involved in this process. Conversely, Wnt5a can independently activate Vangl2 to affect polarity and migration but the role of calcium in this process is also limited. Further, Vangl2 has also been implicated in inhibiting Wnt/β-catenin signaling in development. The consensus is that a cell can differentiate between canonical and non-canonical Wnt signaling when presented with a choice, always choosing non-canonical at the expense of canonical Wnt signaling. However, these are rare events in vivo. Given the shared resources between non-canonical and canonical Wnt signaling it is perplexing that there is not more in vivo evidence for cross talk between these two pathways. In this review we discuss the intersection of non-canonical Wnt, with a focus on Wnt/PCP, and Wnt/β-catenin signaling in an attempt to shed some light on pathways that rarely meet at a crossroads in vivo.

Prickle promotes the formation and maintenance of glutamatergic synapses by stabilizing the intercellular planar cell polarity complex

Whether there exists a common signaling mechanism that assembles all glutamatergic synapses is unknown. We show here that knocking out Prickle1 and Prickle2 reduced the formation of the PSD-95–positive glutamatergic synapses in the hippocampus and medial prefrontal cortex in postnatal development by 70–80%. Prickle1 and Prickle2 double knockout in adulthood lead to the disassembly of 70 to 80% of the postsynaptic-density(PSD)-95–positive glutamatergic synapses. PSD-95–positive glutamatergic synapses in the hippocampus of Prickle2E8Q/E8Q mice were reduced by 50% at postnatal day 14. Prickle2 promotes synapse formation by antagonizing Vangl2 and stabilizing the intercellular complex of the planar cell polarity (PCP) components, whereas Prickle2 E8Q fails to do so. Coculture experiments show that the asymmetric PCP complexes can determine the presynaptic and postsynaptic polarity. In summary, the PCP components regulate the assembly and maintenance of a large number of glutamatergic synapses and specify the direction of synaptic transmission.

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.

Apical Restriction of the Planar Cell Polarity Component VANGL in Pancreatic Ducts Is Required to Maintain Epithelial Integrity

Cell polarity is essential for the architecture and function of numerous epithelial tissues. Here, we show that apical restriction of planar cell polarity (PCP) components is necessary for the maintenance of epithelial integrity. Using the mammalian pancreas as a model, we find that components of the core PCP pathway, such as the transmembrane protein Van Gogh-like (VANGL), become apically restricted over a period of several days. Expansion of VANGL localization to the basolateral membranes of progenitors leads to their death and disruption of the epithelial integrity. VANGL basolateral expansion does not affect apico-basal polarity but acts in the cells where Vangl is mislocalized by reducing Dishevelled and its downstream target ROCK. This reduction in ROCK activity culminates in progenitor cell egression, death, and eventually pancreatic hypoplasia. Thus, precise spatiotemporal modulation of VANGL-dependent PCP signaling is crucial for proper pancreatic morphogenesis.

How do the Fat-Dachsous and core planar polarity pathways act together and independently to coordinate polarized cell behaviours?

Planar polarity describes the coordinated polarization of cells within the plane of a tissue. This is controlled by two main pathways in Drosophila: the Frizzled-dependent core planar polarity pathway and the Fat–Dachsous pathway. Components of both of these pathways become asymmetrically localized within cells in response to long-range upstream cues, and form intercellular complexes that link polarity between neighbouring cells. This review examines if and when the two pathways are coupled, focusing on the Drosophila wing, eye and abdomen. There is strong evidence that the pathways are molecularly coupled in tissues that express a specific isoform of the core protein Prickle, namely Spiny-legs. However, in other contexts, the linkages between the pathways are indirect. We discuss how the two pathways act together and independently to mediate a diverse range of effects on polarization of cell structures and behaviours.

LRRK2 mediates axon development by regulating Frizzled3 phosphorylation and growth cone-growth cone communication

Axon-axon interactions are essential for axon guidance during nervous system wiring. However, it is unknown whether and how the growth cones communicate with each other while sensing and responding to guidance cues. We found that the Parkinson's disease gene, leucine-rich repeat kinase 2 (LRRK2), has an unexpected role in growth cone-growth cone communication. The LRRK2 protein acts as a scaffold and induces Frizzled3 hyperphosphorylation indirectly by recruiting other kinases and also directly phosphorylates Frizzled3 on threonine 598 (T598). In LRRK1 or LRRK2 single knockout, LRRK1/2 double knockout, and LRRK2 G2019S knockin, the postcrossing spinal cord commissural axons are disorganized and showed anterior-posterior guidance errors after midline crossing. Growth cones from either LRRK2 knockout or G2019S knockin mice showed altered interactions, suggesting impaired communication. Intercellular interaction between Frizzled3 and Vangl2 is essential for planar cell polarity signaling. We show here that this interaction is regulated by phosphorylation of Frizzled3 at T598 and can be regulated by LRRK2 in a kinase activity-dependent way. In the LRRK1/2 double knockout or LRRK2 G2019S knockin, the dopaminergic axon bundle in the midbrain was significantly widened and appeared disorganized, showing aberrant posterior-directed growth. Our findings demonstrate that LRRK2 regulates growth cone-growth cone communication in axon guidance and that both loss-of-function mutation and a gain-of-function mutation (G2019S) cause axon guidance defects in development.

Molecular mechanisms that regulate export of the planar cell-polarity protein Frizzled-6 out of the endoplasmic reticulum

Planar cell polarity (PCP) is a process during which cells are polarized along the plane of the epithelium and is regulated by several transmembrane signaling proteins. After their synthesis, these PCP proteins are delivered along the secretory transport pathway to the plasma membrane, where they perform their physiological functions. However, the molecular mechanisms that regulate PCP protein transport remain largely unclear. Here, we found that the delivery of a PCP protein, Frizzled-6, to the cell surface is regulated by two conserved polybasic motifs: one located in its first intracellular loop and the other in its C-terminal cytosolic domain. We observed that the polybasic motif of Frizzled is also important for its surface localization in the Drosophila wing. Results from a mechanistic analysis indicated that Frizzled-6 packaging into vesicles at the endoplasmic reticulum (ER) is regulated by a direct interaction between the polybasic motif and the Glu-62 and Glu-63 residues on the secretion-associated Ras-related GTPase 1A (SAR1A) subunit of coat protein complex II (COPII). Moreover, we found that newly synthesized Frizzled-6 is associated with another PCP protein, cadherin EGF LAG seven-pass G-type receptor 1 (CELSR1), in the secretory transport pathway, and that this association regulates their surface delivery. Our results reveal insights into the molecular machinery that regulates the ER export of Frizzled-6. They also suggest that the association of CELSR1 with Frizzled-6 is important, enabling efficient Frizzled-6 delivery to the cell surface, providing a quality control mechanism that ensures the appropriate stoichiometry of these two PCP proteins at cell boundaries.

Gastrulation and germ layer formation in the sea anemone Nematostella vectensis and other cnidarians

Among the basally branching metazoans, cnidarians display well-defined gastrulation processes leading to a diploblastic body plan, consisting of an endodermal and an ectodermal cell layer. As the outgroup to all Bilateria, cnidarians are an interesting group to investigate ancestral developmental mechanisms. Interestingly, all known gastrulation mechanisms known in Bilateria are already found in different species of Cnidaria. Here I review the morphogenetic processes found in different Cnidaria and focus on the investigation of the cellular and molecular mechanisms in the sea anemone Nematostella vectensis, which has been a major model organism among cnidarians for evolutionary developmental biology. Many of the genes involved in germ layer specification and morphogenetic processes in Bilateria are also found active during gastrulation of Nematostella and other cnidarians, suggesting an ancestral role of this process. The molecular analyses indicate a tight link between gastrulation and axis patterning processes by Wnt and FGF signaling. Interestingly, the endodermal layer displays many features of the mesodermal layer in Bilateria, while the pharyngeal ectoderm has an endodermal expression profile. Comparative analyses as well as experimental studies using embryonic aggregates suggest that minor differences in the gene regulatory networks allow the embryo to transition relatively easily from one mode of gastrulation to another.

Percolation in a reduced equilibrium model of planar cell polarity

Planar cell polarity (PCP) is a biological phenomenon where a large number of cells get polarized and coordinatedly align in a plane. PCP is an example of self-organization through local and global interactions between cells. In this work, we have used a lattice-based spin model for PCP that mimics the alignment of cells through local interactions. We have investigated the equilibrium behavior of this model. In this model, alignment of cells leads to the formation of clusters of aligned cells, and such clustering exhibits percolation transition. Even though the alignment of a cell in this model depends upon its neighbors, finite-size scaling analysis shows that this model belongs to the universality class of simple two-dimensional random percolation.

Frizzled3 and Frizzled6 Cooperate with Vangl2 to Direct Cochlear Innervation by Type II Spiral Ganglion Neurons

Type II spiral ganglion neurons provide afferent innervation to outer hair cells of the cochlea and are proposed to have nociceptive functions important for auditory function and homeostasis. These neurons are anatomically distinct from other classes of spiral ganglion neurons because they extend a peripheral axon beyond the inner hair cells that subsequently makes a distinct 90 degree turn toward the cochlear base. As a result, patterns of outer hair cell innervation are coordinated with the tonotopic organization of the cochlea. Previously, it was shown that peripheral axon turning is directed by a nonautonomous function of the core planar cell polarity (PCP) protein VANGL2. We demonstrate using mice of either sex that Fzd3 and Fzd6 similarly regulate axon turning, are functionally redundant with each other, and that Fzd3 genetically interacts with Vangl2 to guide this process. FZD3 and FZD6 proteins are asymmetrically distributed along the basolateral wall of cochlear-supporting cells, and are required to promote or maintain the asymmetric distribution of VANGL2 and CELSR1. These data indicate that intact PCP complexes formed between cochlear-supporting cells are required for the nonautonomous regulation of axon pathfinding. Consistent with this, in the absence of PCP signaling, peripheral axons turn randomly and often project toward the cochlear apex. Additional analyses of Porcn mutants in which WNT secretion is reduced suggest that noncanonical WNT signaling establishes or maintains PCP signaling in this context. A deeper understanding of these mechanisms is necessary for repairing auditory circuits following acoustic trauma or promoting cochlear reinnervation during regeneration-based deafness therapies.SIGNIFICANCE STATEMENT Planar cell polarity (PCP) signaling has emerged as a complementary mechanism to classical axon guidance in regulating axon track formation, axon outgrowth, and neuronal polarization. The core PCP proteins are also required for auditory circuit assembly, and coordinate hair cell innervation with the tonotopic organization of the cochlea. This is a non-cell-autonomous mechanism that requires the formation of PCP protein complexes between cochlear-supporting cells located along the trajectory of growth cone navigation. These findings are significant because they demonstrate how the fidelity of auditory circuit formation is ensured during development, and provide a mechanism by which PCP proteins may regulate axon outgrowth and guidance in the CNS.

Experimental and Theoretical Evidence for Bidirectional Signaling via Core Planar Polarity Protein Complexes in Drosophila

In developing tissues, sheets of cells become planar polarized, enabling coordination of cell behaviors. It has been suggested that "signaling" of polarity information between cells may occur either bidirectionally or monodirectionally between the molecules Frizzled (Fz) and Van Gogh (Vang). Using computational modeling we find that both bidirectional and monodirectional signaling models reproduce known non-autonomous phenotypes derived from patches of mutant tissue of key molecules but predict different phenotypes from double mutant tissue, which have previously given conflicting experimental results. Furthermore, we re-examine experimental phenotypes in the Drosophila wing, concluding that signaling is most likely bidirectional. Our modeling suggests that bidirectional signaling can be mediated either indirectly via bidirectional feedbacks between asymmetric intercellular protein complexes or directly via different affinities for protein binding in intercellular complexes, suggesting future avenues for investigation. Our findings offer insight into mechanisms of juxtacrine cell signaling and how tissue-scale properties emerge from individual cell behaviors.

Wnt/PCP Signaling Contribution to Carcinoma Collective Cell Migration and Metastasis

Our understanding of the cellular mechanisms governing carcinoma invasiveness and metastasis has evolved dramatically over the last several years. The previous emphasis on the epithelial-mesenchymal transition as a driver of the migratory properties of single cells has expanded with the observation that carcinoma cells often invade and migrate collectively as adherent groups. Moreover, recent analyses suggest that circulating tumor cells within the vasculature often exist as multicellular clusters and that clusters more efficiently seed metastatic lesions than single circulating tumor cells. While these observations point to a key role for collective cell migration in carcinoma metastasis, the molecular mechanisms driving collective tumor cell migration remain to be discerned. Wnt/PCP (planar cell polarity) signaling, one of the noncanonical Wnt signaling pathways, mediates collective migratory events such as convergent extension during developmental processes. Wnt/PCP signaling components are frequently dysregulated in solid tumors, and aberrant pathway activation contributes to tumor cell migratory properties. Here we summarize key studies that address the mechanisms by which Wnt/PCP signaling mediate collective cell migration in developmental and tumor contexts. We emphasize Wnt/PCP component localization within migrating cells and discuss how component asymmetry may govern the spatiotemporal control of downstream cytoskeletal effectors to promote collective cell motility.

A theoretical framework for planar polarity establishment through interpretation of graded cues by molecular bridges

Planar polarity is a widespread phenomenon found in many tissues, allowing cells to coordinate morphogenetic movements and function. A common feature of animal planar polarity systems is the formation of molecular bridges between cells, which become polarised along a tissue axis. We propose that these bridges provide a general mechanism by which cells interpret different forms of tissue gradients to coordinate directional information. We illustrate this using a generalised and consistent modelling framework, providing a conceptual basis for understanding how different mechanisms of gradient function can generate planar polarity. We make testable predictions of how different gradient mechanisms can influence polarity direction.

Summary: This Hypothesis uses a theoretical framework to explore how molecular bridges provide a general mechanism to interpret different forms of tissue gradients to establish planar polarity.

The polarity protein VANG-1 antagonizes Wnt signaling by facilitating Frizzled endocytosis

Signaling that instructs the migration of neurons needs to be tightly regulated to ensure precise positioning of neurons and subsequent wiring of the neuronal circuits. Wnt-Frizzled signaling controls neuronal migration in metazoans, in addition to many other aspects of neural development. We show that Caenorhabditis elegans VANG-1, a membrane protein that acts in the planar cell polarity (PCP) pathway, antagonizes Wnt signaling by facilitating endocytosis of the Frizzled receptors. Mutations of vang-1 suppress migration defects of multiple classes of neurons in the Frizzled mutants, and overexpression of vang-1 causes neuronal migration defects similar to those of the Frizzled mutants. Our genetic experiments suggest that VANG-1 facilitates Frizzled endocytosis through β-arrestin2. Co-immunoprecipitation experiments indicate that Frizzled proteins and VANG-1 form a complex, and this physical interaction requires the Frizzled cysteine-rich domain. Our work reveals a novel mechanism mediated by the PCP protein VANG-1 that downregulates Wnt signaling through Frizzled endocytosis.

Functional dissection of the N-terminal extracellular domains of Frizzled 6 reveals their roles for receptor localization and Dishevelled recruitment

The Frizzled (FZD) proteins belong to class F of G protein-coupled receptors (GPCRs) and are essential for various pathways involving the secreted lipoglycoproteins of the wingless/int-1 (WNT) family. A WNT-binding cysteine-rich domain (CRD) in FZDs is N-terminally located and connected to the seven transmembrane domain-spanning receptor core by a linker domain that has a variable length in different FZD homologs. However, the function and importance of this linker domain are poorly understood. Here we used systematic mutagenesis of FZD6 to define the minimal N-terminal domain sufficient for receptor surface expression and recruitment of the intracellular scaffold protein Dishevelled (DVL). Further, we identified a triad of evolutionarily conserved cysteines in the FZD linker domain that is crucial for receptor membrane expression and recruitment of DVL. Our results are in agreement with the concept that the conserved cysteines in the linker domain of FZDs assist with the formation of a common secondary structure in this region. We propose that this structure could be involved in agonist binding and receptor activation mechanisms that are similar to the binding and activation mechanisms known for other GPCRs.

Wingless Signaling: A Genetic Journey from Morphogenesis to Metastasis

This FlyBook chapter summarizes the history and the current state of our understanding of the Wingless signaling pathway. Wingless, the fly homolog of the mammalian Wnt oncoproteins, plays a central role in pattern generation during development. Much of what we know about the pathway was learned from genetic and molecular experiments in Drosophila melanogaster, and the core pathway works the same way in vertebrates. Like most growth factor pathways, extracellular Wingless/Wnt binds to a cell surface complex to transduce signal across the plasma membrane, triggering a series of intracellular events that lead to transcriptional changes in the nucleus. Unlike most growth factor pathways, the intracellular events regulate the protein stability of a key effector molecule, in this case Armadillo/β-catenin. A number of mysteries remain about how the "destruction complex" destabilizes β-catenin and how this process is inactivated by the ligand-bound receptor complex, so this review of the field can only serve as a snapshot of the work in progress.

Planar Cell Polarity Signaling in Mammalian Cardiac Morphogenesis

The mammalian heart is the first organ to form and is critical for embryonic survival and development. With an occurrence of 1%, congenital heart defects (CHDs) are also the most common birth defects in humans, and major cause of childhood morbidity and mortality (Hoffman and Kaplan in J Am Coll Cardiol 39(12):1890-1900, 2002; Samanek in Cardiol Young 10(3):179-185, 2000). Understanding how the heart forms will not only help to determine the etiology and to design diagnostic and therapeutic approaches for CHDs, but may also provide insight into regenerative medicine to repair injured adult hearts. Mammalian heart development requires precise orchestration of growth, differentiation, and morphogenesis to remodel a simple linear heart tube into an intricate, four-chambered heart with properly connected pulmonary artery and aorta, a structural basis for establishing the pulmonary and systemic circulation. Here we will review the recent advance in our understanding of how the planar cell polarity pathway, a highly conserved morphogenetic engine in vertebrates, regulates polarized morphogenetic processes to contribute to both the arterial and venous poles development of the heart.

Coordinated directional outgrowth and pattern formation by integration of Wnt5a and Fgf signaling in planar cell polarity

Embryonic morphogenesis of a complex organism requires proper regulation of patterning and directional growth. Planar cell polarity (PCP) signaling is emerging as a crucial evolutionarily conserved mechanism whereby directional information is conveyed. PCP is thought to be established by global cues, and recent studies have revealed an instructive role of a Wnt signaling gradient in epithelial tissues of both invertebrates and vertebrates. However, it remains unclear whether Wnt/PCP signaling is regulated in a coordinated manner with embryonic patterning during morphogenesis. Here, in mouse developing limbs, we find that apical ectoderm ridge-derived Fgfs required for limb patterning regulate PCP along the proximal-distal axis in a Wnt5a-dependent manner. We demonstrate with genetic evidence that the Wnt5a gradient acts as a global cue that is instructive in establishing PCP in the limb mesenchyme, and that Wnt5a also plays a permissive role to allow Fgf signaling to orient PCP. Our results indicate that limb morphogenesis is regulated by coordination of directional growth and patterning through integration of Wnt5a and Fgf signaling.

HMGA2 enhances 5-fluorouracil chemoresistance in colorectal cancer via the Dvl2/Wnt pathway

Drug resistance is one of the main hurdles to overcome for the improvement of cancer patient survival. However, the underlying mechanisms remain largely unknown, and therapeutic options are limited. Here, we demonstrate a strong correlation between HMGA2 expression and chemosensitivity to 5-fluorouracil (5-FU), a widely used first-line systemic chemotherapy regimen for colorectal cancer (CRC) patients. Overexpression of HMGA2 enhances chemoresistance to 5-FU of CRC both in vitro and in vivo. Further experiments indicate that HMGA2 directly binds to the promoter of Dvl2 and induces its transcription, which leads to increased activation of the Wnt/β-catenin pathway. Taken together, our data suggest that HMGA2 enhances the chemoresistance to 5-FU in CRC via activating the Dvl2/Wnt pathway. Therefore, HMGA2 may serve as a predictive biomarker and a potential therapeutic target in CRC.

From instruction to output: Wnt/PCP signaling in development and cancer

Planar cell polarity (PCP) provides positional information to direct tissue patterning and morphogenesis. While much of the molecular detail of the pathway has been delineated in Drosophila, ensuing studies have shown considerable conservation of both the components and mechanisms of signaling in vertebrates. A recognized feature of PCP is the asymmetric localization of components that translates a global directional cue into a polarized downstream output. Here we discuss recent advances in the PCP field, from the organization of these asymmetric complexes to their upstream directional regulation by Wnt ligands. We also discuss the impact of Wnt/PCP signaling in disease and more specifically an emerging role in cancer progression.

Trans-endocytosis of Planar Cell Polarity Complexes during Cell Division

To coordinate epithelial architecture with proliferation, cell polarity proteins undergo extensive remodeling during cell division [1-3]. A dramatic example of polarity remodeling occurs in proliferative basal cells of mammalian epidermis whereupon cell division, transmembrane planar cell polarity (PCP) proteins are removed from the cell surface via bulk endocytosis [4]. PCP proteins form intercellular complexes, linked by Celsr1-mediated homophilic adhesion, that coordinate polarity non-autonomously between cells [5, 6]. Thus, the mitotic reorganization of PCP proteins must alter not only proteins intrinsic to the dividing cell but also their interacting partners on neighboring cells. Here, we show that intercellular Celsr1 complexes that connect dividing cells with their neighbors remain intact during mitotic internalization, resulting in an uptake of Celsr1 protein from interphase neighbors. Trans-internalized Celsr1 carries with it additional core PCP proteins, including the posteriorly enriched Fz6 and anteriorly enriched Vangl2. Cadherin-mediated homophilic adhesion is necessary for trans-endocytosis, and adhesive junctional PCP complexes appear to be destined for degradation upon internalization. Surprisingly, whereas Fz6 and Vangl2 both internalize in trans, Vangl2 proteins intrinsic to the dividing cell remain associated with the plasma membrane. Persistent Vangl2 stabilizes Celsr1 and impedes its internalization, suggesting that dissociation of Vangl2 from Celsr1 is a prerequisite for Celsr1 endocytosis. These results demonstrate an unexpected transfer of PCP complexes between neighbors and suggest that the Vangl2 population that persists at the membrane during cell division could serve as an internal cue for establishing PCP in new daughter cells.

Wnt-induced Vangl2 phosphorylation is dose-dependently required for planar cell polarity in mammalian development

Planar cell polarity (PCP) is an evolutionarily conserved essential mechanism that provides directional information to control and coordinate polarized cellular and tissue behavior during embryonic development. Disruption of PCP leads to severe morphological defects in vertebrates and its dysregulation results in a variety of human diseases such as neural tube defects and skeletal dysplasia. PCP is governed by a set of highly conserved core proteins that are asymmetrically localized at the cell surface throughout the polarized tissues. The uniform directionality of PCP is established by global cues, such as Wg/Wnt signaling gradients that break the original symmetrical localization of core PCP proteins including Vang/Vangl and Fz/Fzd. However, the exact mechanism remains elusive. In this study, we found that Vangl2 phosphorylation, which was previously identified to be induced by Wnt5a signaling, is required for Vangl2 functions in mammalian PCP in multiple tissues. The in vivo activities of Vangl2 are determined by its phosphorylation level. Phospho-mutant Vangl2 exhibits dominant negative effects, whereas Vangl2 with reduced phosphorylation is hypomorphic. We show that Vangl2 phosphorylation is essential for its uniform polarization pattern. Moreover, serine/threonine kinases CK1ɛ and CK1δ are redundantly required for Wnt5a-induced Vangl2 phosphorylation. Dvl family members are also required for Wnt5a-induced Vangl2 phosphorylation by enhancing the interaction of CK1 and Vangl2. These findings demonstrate that induction of Vangl protein phosphorylation plays an essential role in transducing Wnt5a signaling to establish PCP in mammalian development, suggesting a phosphorylation-regulated "Vangl activity gradient" model in addition to the well-documented "Fz activity gradient" model in Wnt/PCP signaling.

Bimodal regulation of Dishevelled function by Vangl2 during morphogenesis

Convergent extension (CE) is a fundamental morphogenetic mechanism that underlies numerous processes in vertebrate development, and its disruption can lead to human congenital disorders such as neural tube closure defects. The dynamic, oriented cell intercalation during CE is regulated by a group of core proteins identified originally in flies to coordinate epithelial planar cell polarity (PCP). The existing model explains how core PCP proteins, including Van Gogh (Vang) and Dishevelled (Dvl), segregate into distinct complexes on opposing cell cortex to coordinate polarity among static epithelial cells. The action of core PCP proteins in the dynamic process of CE, however, remains an enigma. In this report, we show that Vangl2 (Vang-like 2) exerts dual positive and negative regulation on Dvl during CE in both the mouse and Xenopus. We find that Vangl2 binds to Dvl to cell-autonomously promote efficient Dvl plasma membrane recruitment, a pre-requisite for PCP activation. At the same time, Vangl2 inhibits Dvl from interacting with its downstream effector Daam1 (Dishevelled associated activator of morphogenesis 1), and functionally suppresses Dvl → Daam1 cascade during CE. Our finding uncovers Vangl2-Dvl interaction as a key bi-functional switch that underlies the central logic of PCP signaling during morphogenesis, and provides new insight into PCP-related disorders in humans.

Planar Cell Polarity Effector Fritz Interacts with Dishevelled and Has Multiple Functions in Regulating PCP

The Planar cell Polarity Effector (PPE) genes inturned, fuzzy, and fritz are downstream components in the frizzled/starry night signaling pathway, and their function is instructed by upstream Planar Cell Polarity (PCP) core genes such as frizzled and dishevelled. PPE proteins accumulate asymmetrically in wing cells and function in a protein complex mediated by direct interactions between In and Frtz and In and Fy. How the PCP proteins instruct the accumulation of PPE protein is unknown. We found a likely direct interaction between Dishevelled and Fritz and Dishevelled and Fuzzy that could play a role in this. We previously found that mild overexpression of frtz rescued a weak in allele. To determine if this was due to extra Frtz stabilizing mutant In or due to Frtz being able to bypass the need for In we generate a precise deletion of the inturned gene (in^PD). We found that mild overexpression of Fritz partially rescued in^PD, indicating that fritz has In independent activity in PCP. Previous studies of PPE proteins used fixed tissues, and did not provide any insights into the dynamic properties of PPE proteins. We used CRISPR/Cas9 genome editing technology to edit the fritz gene to add a green fluorescent protein tag. fritz^mNeonGreen provides complete rescue activity and works well for in vivo imaging. Our data showed that Fritz is very dynamic in epidermal cells and preferentially distributed to discrete membrane subdomains (“puncta”). Surprisingly, we found it in stripes in developing bristles.

Zebrafish models of non-canonical Wnt/planar cell polarity signalling: fishing for valuable insight into vertebrate polarized cell behavior

Planar cell polarity (PCP) coordinates the uniform orientation, structure and movement of cells within the plane of a tissue or organ system. It is beautifully illustrated in the polarized arrangement of bristles and hairs that project from specialized cell surfaces of the insect abdomen and wings, and pioneering genetic studies using the fruit fly, Drosophila melanogaster, have defined a core signalling network underlying PCP. This core PCP/non-canonical Wnt signalling pathway is evolutionarily conserved, and studies in zebrafish have helped transform our understanding of PCP from a peculiarity of polarized epithelia to a more universal cellular property that orchestrates a diverse suite of polarized cell behaviors that are required for normal vertebrate development. Furthermore, application of powerful genetics, embryonic cell-transplantation, and live-imaging capabilities afforded by the zebrafish model have yielded novel insights into the establishment and maintenance of vertebrate PCP, over the course of complex and dynamic morphogenetic events like gastrulation and neural tube morphogenesis. Although key questions regarding vertebrate PCP remain, with the emergence of new genome-editing technologies and the promise of endogenous labeling and Cre/LoxP conditional targeting strategies, zebrafish remains poised to deliver fundamental new insights into the function and molecular dynamic regulation of PCP signalling from embryonic development through to late-onset phenotypes and adult disease states. WIREs Dev Biol 2017, 6:e267. doi: 10.1002/wdev.267 For further resources related to this article, please visit the WIREs website.

Planar cell polarity in development and disease

Planar cell polarity (PCP) is an essential feature of animal tissues, whereby distinct polarity is established within the plane of a cell sheet. Tissue-wide establishment of PCP is driven by multiple global cues, including gradients of gene expression, gradients of secreted WNT ligands and anisotropic tissue strain. These cues guide the dynamic, subcellular enrichment of PCP proteins, which can self-assemble into mutually exclusive complexes at opposite sides of a cell. Endocytosis, endosomal trafficking and degradation dynamics of PCP components further regulate planar tissue patterning. This polarization propagates throughout the whole tissue, providing a polarity axis that governs collective morphogenetic events such as the orientation of subcellular structures and cell rearrangements. Reflecting the necessity of polarized cellular behaviours for proper development and function of diverse organs, defects in PCP have been implicated in human pathologies, most notably in severe birth defects.

Boundary propagation of planar cell polarity is robust against cell packing pattern

Planar cell polarity is an important property of epithelial tissue. The boundary propagation model was proposed as the mechanism of PCP induction, while it has been doubted whether it can induce PCP on wide tissue. Using simulation, a set of proteins can be shown to induce PCP, but it does not explain why and how the set can induce PCP. In this study, we made theoretical model and simulation model to explore when and how the boundary propagation induce PCP. We incorporated multipolar cell in our model. Intracellular interactions have been thought to amplify polarity of a cell, but we propose instead that they are to keep a cell-cell interface polarized, and bipolarity of cell is obtained as a result. We show that the boundary propagation can propagate polarity as long as average size of local cell group is constant and levels of PCP proteins are balanced in every cell. Therefore, this model provide an explanation for PCP induction on a tissue with multiple cell types.

Wnt proteins can direct planar cell polarity in vertebrate ectoderm

The coordinated orientation of cells across the tissue plane, known as planar cell polarity (PCP), is manifested by the segregation of core PCP proteins to different sides of the cell. Secreted Wnt ligands are involved in many PCP-dependent processes, yet whether they act as polarity cues has been controversial. We show that in Xenopus early ectoderm, the Prickle3/Vangl2 complex was polarized to anterior cell edges and this polarity was disrupted by several Wnt antagonists. In midgastrula embryos, Wnt5a, Wnt11, and Wnt11b, but not Wnt3a, acted across many cell diameters to orient Prickle3/Vangl2 complexes away from their sources regardless of their positions relative to the body axis. The planar polarity of endogenous Vangl2 in the neuroectoderm was similarly redirected by an ectopic Wnt source and disrupted after depletion of Wnt11b in the presumptive posterior region of the embryo. These observations provide evidence for the instructive role of Wnt ligands in vertebrate PCP.

DOI:http://dx.doi.org/10.7554/eLife.16463.001

Wnt-Frizzled/planar cell polarity signaling: cellular orientation by facing the wind (Wnt)

The establishment of planar cell polarity (PCP) in epithelial and mesenchymal cells is a critical, evolutionarily conserved process during development and organogenesis. Analyses in Drosophila and several vertebrate model organisms have contributed a wealth of information on the regulation of PCP. A key conserved pathway regulating PCP, the so-called core Wnt-Frizzled PCP (Fz/PCP) signaling pathway, was initially identified through genetic studies of Drosophila. PCP studies in vertebrates, most notably mouse and zebrafish, have identified novel factors in PCP signaling and have also defined cellular features requiring PCP signaling input. These studies have shifted focus to the role of Van Gogh (Vang)/Vangl genes in this molecular system. This review focuses on new insights into the core Fz/Vangl/PCP pathway and recent advances in Drosophila and vertebrate PCP studies. We attempt to integrate these within the existing core Fz/Vangl/PCP signaling framework.

Cells must express components of the planar cell polarity system and extracellular matrix to support cytonemes

Drosophila dorsal air sac development depends on Decapentaplegic (Dpp) and Fibroblast growth factor (FGF) proteins produced by the wing imaginal disc and transported by cytonemes to the air sac primordium (ASP). Dpp and FGF signaling in the ASP was dependent on components of the planar cell polarity (PCP) system in the disc, and neither Dpp- nor FGF-receiving cytonemes extended over mutant disc cells that lacked them. ASP cytonemes normally navigate through extracellular matrix (ECM) composed of collagen, laminin, Dally and Dally-like (Dlp) proteins that are stratified in layers over the disc cells. However, ECM over PCP mutant cells had reduced levels of laminin, Dally and Dlp, and whereas Dpp-receiving ASP cytonemes navigated in the Dally layer and required Dally (but not Dlp), FGF-receiving ASP cytonemes navigated in the Dlp layer, requiring Dlp (but not Dally). These findings suggest that cytonemes interact directly and specifically with proteins in the stratified ECM.

Planar cell polarity: global inputs establishing cellular asymmetry

Many tissues develop coordinated patterns of cell polarity that align with respect to the tissue axes. This phenomenon refers to planar cell polarity (PCP) and is controlled by multiple conserved PCP modules. A key feature of PCP proteins is their asymmetric localization within the tissue plane, whose orientation is guided by global directional cues. Here, we highlight current models and recent findings on the role of tissue-level gradients, local organizer signals, and mechanical forces in establishing the global patterns of PCP.

Frizzled-Induced Van Gogh Phosphorylation by CK1ε Promotes Asymmetric Localization of Core PCP Factors in Drosophila

Epithelial tissues are polarized along two axes. In addition to apical-basal polarity, they are often polarized within the plane of the epithelium, so-called Planar Cell Polarity (PCP). PCP depends upon Wnt/Frizzled (Fz) signaling factors, including Fz itself and Van Gogh (Vang/Vangl). We sought to understand how Vang interaction with other core PCP factors affects Vang function. We find that Fz induces Vang phosphorylation in a cell-autonomous manner. Vang phosphorylation occurs on conserved N-terminal serine/threonine residues, is mediated by CK1ε/Dco, and is critical for polarized membrane localization of Vang and other PCP proteins. This regulatory mechanism does not require Fz signaling through Dishevelled and thus represents a cell-autonomous upstream interaction between Fz and Vang. Furthermore, this signaling event appears to be related to Wnt5a-mediated Vangl2 phosphorylation during mouse limb patterning and may thus be a general mechanism underlying Wnt-regulated PCP establishment.

PCP Signaling between Migrating Neurons and their Planar-Polarized Neuroepithelial Environment Controls Filopodial Dynamics and Directional Migration

The planar cell polarity (PCP) pathway is a cell-contact mediated mechanism for transmitting polarity information between neighboring cells. PCP “core components” (Vangl, Fz, Pk, Dsh, and Celsr) are essential for a number of cell migratory events including the posterior migration of facial branchiomotor neurons (FBMNs) in the plane of the hindbrain neuroepithelium in zebrafish and mice. While the mechanism by which PCP signaling polarizes static epithelial cells is well understood, how PCP signaling controls highly dynamic processes like neuronal migration remains an important outstanding question given that PCP components have been implicated in a range of directed cell movements, particularly during vertebrate development. Here, by systematically disrupting PCP signaling in a rhombomere-restricted manner we show that PCP signaling is required both within FBMNs and the hindbrain rhombomere 4 environment at the time when they initiate their migration. Correspondingly, we demonstrate planar polarized localization of PCP core components Vangl2 and Fzd3a in the hindbrain neuroepithelium, and transient localization of Vangl2 at the tips of retracting FBMN filopodia. Using high-resolution timelapse imaging of FBMNs in genetic chimeras we uncover opposing cell-autonomous and non-cell-autonomous functions for Fzd3a and Vangl2 in regulating FBMN protrusive activity. Within FBMNs, Fzd3a is required to stabilize filopodia while Vangl2 has an antagonistic, destabilizing role. However, in the migratory environment Fzd3a acts to destabilize FBMN filopodia while Vangl2 has a stabilizing role. Together, our findings suggest a model in which PCP signaling between the planar polarized neuroepithelial environment and FBMNs directs migration by the selective stabilization of FBMN filopodia.

Planar cell polarity (PCP) is a common feature of many animal tissues. This type of polarity is most obvious in cells that are organized into epithelial sheets, where PCP signaling components act to orient cells in the plane of the tissue. Although, PCP is best understood for its function in polarizing stable epithelia, PCP is also required for the dynamic process of cell migration in animal development and disease. The goal of this study was to determine how PCP functions to control cell migration. We used the migration of facial branchiomotor neurons in the zebrafish hindbrain, which requires almost the entire suite of PCP core components, to address this question. We present evidence that PCP signaling within migrating neurons, and between migrating neurons and cells of their migratory environment promote migration by regulating filopodial dynamics. Our results suggest that broadly conserved interactions between PCP components control the cytoskeleton in motile cells and non-motile epithelia alike.

Cell adhesion molecule control of planar spindle orientation

Polarized epithelial cells align the mitotic spindle in the plane of the sheet to maintain tissue integrity and to prevent malignant transformation. The orientation of the spindle apparatus is regulated by the immobilization of the astral microtubules at the lateral cortex and depends on the precise localization of the dynein-dynactin motor protein complex which captures microtubule plus ends and generates pulling forces towards the centrosomes. Recent developments indicate that signals derived from intercellular junctions are required for the stable interaction of the dynein-dynactin complex with the cortex. Here, we review the molecular mechanisms that regulate planar spindle orientation in polarized epithelial cells and we illustrate how different cell adhesion molecules through distinct and non-overlapping mechanisms instruct the cells to align the mitotic spindle in the plane of the sheet.

Identification of p62/SQSTM1 as a component of non-canonical Wnt VANGL2-JNK signalling in breast cancer

The non-canonical Wnt/planar cell polarity (Wnt/PCP) pathway plays a crucial role in embryonic development. Recent work has linked defects of this pathway to breast cancer aggressiveness and proposed Wnt/PCP signalling as a therapeutic target. Here we show that the archetypal Wnt/PCP protein VANGL2 is overexpressed in basal breast cancers, associated with poor prognosis and implicated in tumour growth. We identify the scaffold p62/SQSTM1 protein as a novel VANGL2-binding partner and show its key role in an evolutionarily conserved VANGL2–p62/SQSTM1–JNK pathway. This proliferative signalling cascade is upregulated in breast cancer patients with shorter survival and can be inactivated in patient-derived xenograft cells by inhibition of the JNK pathway or by disruption of the VANGL2–p62/SQSTM1 interaction. VANGL2–JNK signalling is thus a potential target for breast cancer therapy.

Defects in non-canonical Wnt/planar cell polarity signalling have recently been linked to breast cancer aggressiveness. Puvirajesinghe et al. identify VANGL2, p62/SQSTM1 and JNK as important players in this pathway which may be amenable to therapeutic intervention in breast cancer.

Polarity Determinants in Dendritic Spine Development and Plasticity

The asymmetric distribution of various proteins and RNAs is essential for all stages of animal development, and establishment and maintenance of this cellular polarity are regulated by a group of conserved polarity determinants. Studies over the last 10 years highlight important functions for polarity proteins, including apical-basal polarity and planar cell polarity regulators, in dendritic spine development and plasticity. Remarkably, many of the conserved polarity machineries function in similar manners in the context of spine development as they do in epithelial morphogenesis. Interestingly, some polarity proteins also utilize neuronal-specific mechanisms. Although many questions remain unanswered in our understanding of how polarity proteins regulate spine development and plasticity, current and future research will undoubtedly shed more light on how this conserved group of proteins orchestrates different pathways to shape the neuronal circuitry.