De-Li Shi
email: de-li.shi@upmc.fr

Planar cell polarity regulators in asymmetric organogenesis during development and disease

Keywords: planar cell polarity, Wnt/PCP signaling, left-right asymmetry, heart and gut morphogenesis, lung and kidney branching, inner ear hair cell orientation, limb outgrowth

The phenomenon of planar cell polarity is critically required for a myriad of morphogenetic processes in metazoan and is accurately controlled by several conserved modules. Six "core" proteins, including Frizzled, Flamingo (Celsr), Van Gogh (Vangl), Dishevelled, Prickle, and Diego (Ankrd6), are major components of the Wnt/planar cell polarity pathway. The Fat/Dchs protocadherins and the Scrib polarity complex also function to instruct cellular polarization. In vertebrates, all these pathways are essential for tissue and organ morphogenesis, such as neural tube closure, left-right symmetry breaking, heart and gut morphogenesis, lung and kidney branching, stereociliary bundle orientation, and proximal-distal limb elongation. Mutations in planar polarity genes are closely linked to various congenital diseases. Striking advances have been made in deciphering their contribution to the establishment of spatially oriented pattern in developing organs and the maintenance of tissue homeostasis. The challenge remains to clarify the complex interplay of different polarity pathways in organogenesis and the link of cell polarity to cell fate specification. Interdisciplinary approaches are also important to understand the roles of mechanical forces in coupling cellular polarization and differentiation. This review outlines current advances on planar polarity regulators in asymmetric organ formation, with the aim to identify questions that deserve further investigation.

Introduction

Planar cell polarity (PCP), initially studied in the retina and the wing of insects Oncopeltus fasciatus and Drosophila melanogaster [START_REF] Lawrence | The determination of polarity in the developing insect retina[END_REF][START_REF] Gubb | A genetic analysis of the determination of cuticular polarity during development in Drosophila melanogaster[END_REF], refers to coordinated cellular orientation within the plane of an epithelium or a tissue. This fascinating process exerts wide functions in organizing the asymmetric cellular choreography and the spatially oriented pattern during tissue and organ morphogenesis, contributing to, for example, the architectural beauty of the Drosophila compound eye and the mammalian cochlea.

The phenomenon of PCP is mostly regulated by a set of evolutionarily conserved proteins, including Frizzled receptors, Flamingo (Celsr1-3 in vertebrates), Dishevelled (Dvl1-3 in vertebrates), Diego (Ankrd6 in vertebrates), Van Gogh (Vangl1-2 in vertebrates), and Prickle (Prickle1-3 in vertebrates). These "core" PCP proteins transduce Wnt/PCP signaling to regulate cytoskeletal rearrangements and polarized cell behaviors in a variety of morphogenetic processes [START_REF] Wallingford | Planar cell polarity and the developmental control of cell behavior in vertebrate embryos[END_REF][START_REF] Yang | Wnt-Frizzled/planar cell polarity signaling: cellular orientation by facing the wind (Wnt)[END_REF][START_REF] Henderson | Planar cell polarity in organ formation[END_REF]. Increasing evidence suggests that several vertebrate Wnts, such as Wnt5a and Wnt11, are also important components of the Wnt/PCP pathway, although they are not considered as "core" PCP proteins.

There are also other conserved protein complexes that function as important PCP regulators in Drosophila and vertebrates. The heteromeric protocadherins Fat4 and Dchs1 (dachsous cadherin-related 1) represent the second PCP pathway (Fat/Dchs module) that is regulated by the Golgi resident transmembrane kinase Four-jointed or Fj [START_REF] Blair | Big roles for Fat cadherins[END_REF]. The Scrib (Scrb1 or Scribble1) polarity complex, originally identified as a regulator of apico-basal cell polarity, consists of Scrib, Dlg (Discs-large) and Lgl (Lethal-giant larvae) proteins [START_REF] Milgrom-Hoffman | Regulation of cellular and PCP signalling by the Scribble polarity module[END_REF]. These pathways exert broad activity in cellular signaling and cytoskeletal organization. They function in concert with or independently of the "core" Wnt/PCP pathway to regulate cell polarity. Two additional systems, the Fat2/Lar (leukocyte antigen-related receptor tyrosine phosphatase) and Toll-8/Cirl (adhesion G protein-coupled receptor) pathways, have been recently shown to instruct some Drosophila PCP processes, but their functions in vertebrates merit future investigation [START_REF] Lavalou | In search of conserved principles of planar cell polarization[END_REF].

The "core" PCP pathway, the Fat/Dchs polarity module and the Scrib complex are critically involved in tissue and organ morphogenesis, from the emergence of organ primordia to terminal organogenesis and tissue homeostasis. In vertebrates, the functions of PCP proteins (herein collectively referred to as regulators of PCP-dependent cell behaviors) have been well documented in many morphogenetic processes, such as neural tube closure, embryonic left-right symmetry breaking, heart and gut morphogenesis, lung and kidney branching, orientation of inner ear hair cells, and proximal-distal limb bud elongation [START_REF] Yang | Wnt-Frizzled/planar cell polarity signaling: cellular orientation by facing the wind (Wnt)[END_REF][START_REF] Henderson | Planar cell polarity in organ formation[END_REF]. Mutations of PCP genes impair organ development and are closely linked to various congenital anomalies, including neural tube defects [START_REF] Wang | Update on the role of the non-canonical Wnt/planar cell polarity pathway in neural tube defects[END_REF], laterality disorders [START_REF] Grimes | Left-right patterning: Breaking symmetry to asymmetric morphogenesis[END_REF], hearing deficits (May-Simera and Kelley, 2012), lung and kidney diseases [START_REF] Vladar | Noncanonical Wnt planar cell polarity signaling in lung development and disease[END_REF][START_REF] Torban | Planar cell polarity pathway in kidney development, function and disease[END_REF], as well as limb abnormalities [START_REF] Gao | Planar cell polarity in vertebrate limb morphogenesis[END_REF]. Importantly, PCP proteins regulate common cellular processes underlying the outgrowth and elongation of tissue primordia, such as convergent extension (CE) movements driven by oriented cell division and intercalation [START_REF] Wallingford | Planar cell polarity and the developmental control of cell behavior in vertebrate embryos[END_REF]. Therefore, their dysfunctions can lead to multiple phenotypes, highlighting a general importance in organogenesis. There are many excellent reviews focusing on PCP functions in specific aspects of morphogenesis, but a more comprehensive analysis of PCP regulators during development of different organs that display PCP-dependent cellular organization is beneficial for understanding their complex interplay in the establishment of cell polarity. This review attempts to present past achievements and latest advances of PCP protein functions in asymmetric organogenesis, with the aim to identify challenges in deciphering the extraordinary process of asymmetry formation.

Wnt signaling pathways and planar cell polarity protein complexes

Vertebrate Wnt pathways can be divided into three branches based on the activation of different biological readouts (Fig. 1A). Wnt/ß-catenin or canonical Wnt signaling induces target gene transcription and cell fate specification by stabilizing ß-catenin through inhibition of its destruction complex. Non-canonical Wnt signaling includes Wnt/PCP and Wnt/Ca 2+ branches that function independently of ß-catenin. Wnt/PCP signaling controls cell polarity by regulating cytoskeletal rearrangements and/or transcriptional responses. It functions through several well-characterized planar polarity effector (PPE) proteins, such as Daam1 (Dishevelled-associated activator of morphogenesis 1), Rho family of small GTPases and Jun N-terminal kinase (JNK), but also via a few less well-studied ciliogenesis and planar polarity effector (CPLANE) proteins including Intu, Fuz and Wdpcp [START_REF] Adler | From planar cell polarity to ciliogenesis and back: The curious tale of the PPE and CPLANE proteins[END_REF]. Wnt/Ca 2+ signaling triggers intracellular calcium flux to induce actin polymerization and NFAT (nuclear factor of activated Tcells)-mediated target gene transcription. Dvl family proteins function in all Wnt pathway branches through distinct domains including DIX, PDZ and DEP [START_REF] Shi | Decoding Dishevelled-mediated Wnt signaling in vertebrate early development[END_REF]. Their conserved C-terminus PDZ domain-binding motif can modulate Wnt/ß-catenin and Wnt/PCP signaling through interaction with the PDZ domain [START_REF] Lee | Conformational change of Dishevelled plays a key regulatory role in the Wnt signaling pathways[END_REF].

The "core" PCP proteins form two separate complexes that are distributed on opposite cell borders within the tissue plane (Fig. 1B,C). Frizzled (Fzd), Dvl and Ankrd6 reside on one side of the cell, while Vangl and Prickle localize to the opposite side. Celsr is present on both sides and forms homodimers between adjacent cells to propagate polarity information across cells. This asymmetric distribution of "core" PCP proteins is a hallmark of planar polarization and reflects the coordinated cellular orientation during tissue or organ morphogenesis. Vertebrate Wnt5a signaling gradient can provide global cues to instruct the asymmetric localization of "core" PCP proteins in several well-documented PCP-dependent processes, such as the proximal-distal development of limb bud and the anteroposterior (AP) polarization of node cells [START_REF] Gao | Wnt signaling gradients establish planar cell polarity by inducing Vangl2 phosphorylation through Ror2[END_REF][START_REF] Minegishi | A Wnt5 activity asymmetry and intercellular signaling via PCP proteins polarize node cells for left-right symmetry breaking[END_REF]. Similarly, Fat and Dchs heteromeric protocadherins are localized to opposite sides of adjacent cells (Fig. 1B), and they may function either as a ligand or as a receptor for the other to mediate cell interaction [START_REF] Blair | Big roles for Fat cadherins[END_REF]. Through a multitude of biochemical, functional and genetic interactions, PCP proteins regulate non-canonical Wnt signaling and make an important contribution to the spatiotemporal organization of cellular activities in a variety of tissues and organs (Table 1). The following sections will detail their implications in asymmetric organ morphogenesis during development and disease.

Early embryonic left-right asymmetry

Left-right asymmetry, either external or internal, is a common feature in animals. Although vertebrate embryos are seemingly symmetrical, they already display asymmetric gene expression at early stages of development. This is critical for the asymmetric formation of internal organ primordia and their subsequent morphogenesis to generate left-right differences in sizes, shapes and anatomical locations. Transient structures formed during early development, such as the posterior gastrocoel roof plate (GRP) in Xenopus, the Kupffer's vesicle (KV) in zebrafish, the Hensen's node in chick and the node in mice, constitute the left-right organizer (LRO) involved in breaking the bilateral symmetry across the mediolateral plane (Fig. 2A,B). They initiate left-right asymmetry by providing instructive signals mostly through cilia-mediated directional fluid flow [START_REF] Axelrod | Planar cell polarity signaling in the development of left-right asymmetry[END_REF][START_REF] Little | Right, left and cilia: How asymmetry is established[END_REF]. Wnt/PCP signaling acts in the LRO to coordinate the orientation of individual cells and multicellular structures with respect to the embryonic axes.

Subsequently, signals generated in a manner that is both dependent and independent on motile cilia create a gradient of Nodal protein to activate left-sided expression of the Nodal-Lefty-Pitx2 network [START_REF] Grimes | Left-right patterning: Breaking symmetry to asymmetric morphogenesis[END_REF][START_REF] Axelrod | Planar cell polarity signaling in the development of left-right asymmetry[END_REF]. Thus, differential gene activation and repression establish left-right embryonic polarity that will influence the asymmetric morphogenesis of organ primordia.

Dvl, Vangl and Prickle are important for cilia-dependent asymmetric fluid flow. In node cells, Vangl1, Vangl2 and Prickle2 are localized to the anterior side, while Dvl2 and Dvl3 are enriched at the posterior side [START_REF] Antic | Planar cell polarity enables posterior localization of nodal cilia and left-right axis determination during mouse and Xenopus embryogenesis[END_REF][START_REF] Hashimoto | Planar polarization of node cells determines the rotational axis of node cilia[END_REF][START_REF] Grimes | Left-right patterning: Breaking symmetry to asymmetric morphogenesis[END_REF]). Wnt5a and Wnt5b, which are expressed posteriorly relative to the node, form a diffusible gradient and provide instructive signals to induce the asymmetric localization of Vangl1 and Prickle2, thus polarizing node cells along the AP axis [START_REF] Minegishi | A Wnt5 activity asymmetry and intercellular signaling via PCP proteins polarize node cells for left-right symmetry breaking[END_REF]. The asymmetric localization of "core" PCP proteins then restricts the posterior positioning of ciliary basal bodies at the dome-shaped apical surfaces of node cells and promotes the posterior tilting of cilia by coordinating the asymmetric distribution of microtubules and actomyosin networks [START_REF] Sai | Planar cell polarity-dependent asymmetric organization of microtubules for polarized positioning of the basal body in node cells[END_REF]. Subsequently, cilia-driven leftward fluid flow across the node initiates left-right asymmetry.

Knockout of Dvl2 and Dvl3 impairs the posterior location of ciliary basal bodies [START_REF] Hashimoto | Planar polarization of node cells determines the rotational axis of node cilia[END_REF]. Loss of Vangl1 and Vang2 disrupts the posterior orientation of primary motile cilia in the LRO, resulting in defective expression of Nodal and Pitx2 on the left side [START_REF] Borovina | Vangl2 directs the posterior tilting and asymmetric localization of motile primary cilia[END_REF][START_REF] Antic | Planar cell polarity enables posterior localization of nodal cilia and left-right axis determination during mouse and Xenopus embryogenesis[END_REF][START_REF] Song | Planar cell polarity breaks bilateral symmetry by controlling ciliary positioning[END_REF]. Consistent with their association into a complex, Prickle1 and Prickle2 interact with Vangl2 to regulate its anterior localization in node cells [START_REF] Minegishi | A Wnt5 activity asymmetry and intercellular signaling via PCP proteins polarize node cells for left-right symmetry breaking[END_REF], while Prickle3 shows reciprocal interactions with Vangl2 for the anterior localization in GRP cells to promote growth and posterior positioning of motile cilia [START_REF] Chu | Prickle3 synergizes with Wtip to regulate basal body organization and cilia growth[END_REF]. Downstream of "core" PCP proteins, JNK activity is required for modulating ciliogenesis and cilia length in the zebrafish KV [START_REF] Derrick | Sequential action of jnk genes establishes the embryonic left-right axis[END_REF]. Altogether, these observations reveal an important implication of Wnt/PCP signaling in the orientation of motile cilia within the LRO. Thus, the AP polarity generated by the asymmetric distribution of PCP proteins can be translated into left-right asymmetry through the directional fluid flow.

The unconventional Myosin1D (Myo1D) also functions to shape the cilia-driven directional fluid flow in the LRO, although unlikely being a component of the Wnt/PCP pathway. It interacts with Vangl2 to initiate left-right axis formation in zebrafish and Xenopus embryos [START_REF] Juan | Myosin1D is an evolutionarily conserved regulator of animal left-right asymmetry[END_REF][START_REF] Tingler | A conserved role of the unconventional Myosin 1d in laterality determination[END_REF]. In the zebrafish KV, Myo1D antagonizes the activity of Vangl2 to restrict the localization of posteriorly pointing cilia at the anterior region and anteriorly pointing cilia at the posterior region, thereby establishing a circular geometry of fluid flow [START_REF] Juan | Myosin1D is an evolutionarily conserved regulator of animal left-right asymmetry[END_REF]. Thus, the cilia-dependent function of Myo1D acts in concert with Wnt/PCP signaling to break left-right symmetry.

Asymmetric cardiac morphogenesis

The rightward looping of the heart primordium contributes to determine the relative position of cardiac chambers and is the first event of asymmetric organogenesis [START_REF] Desgrange | Left-right asymmetry in heart development and disease: forming the right loop[END_REF]. It is initiated by the transformation of the cardiac tube into a loop (Fig. 2C). Abnormal left-right patterning is intimately associated with congenital heart diseases, such as X-linked heterotaxy caused by mutations of the ZIC3 transcription factor. Recent evidence suggests that Zic3 regulates the expression of PCP genes and is required for Dvl phosphorylation during node morphogenesis. It also genetically interacts with PCP genes to establish the left-right heart asymmetry [START_REF] Bellchambers | Loss of Zic3 impairs planar cell polarity leading to abnormal leftright signaling, heart defects and neural tube defects[END_REF]. Therefore, there is a possibility that dysregulation of PCP proteins may represent an underlying mechanism of ZIC3 mutations in congenital disorders associated with heterotaxy.

Wnt/PCP signaling critically regulates heart morphogenesis after looping, particularly directional cell movements from the second heart field (SHF) during outflow tract (OFT) development [START_REF] Henderson | Vang-like 2 and noncanonical Wnt signaling in outflow tract development[END_REF]. Wnt5a, Wnt5b and Wnt11 are important for modulating extracellular matrix composition, cytoskeletal rearrangements, actomyosin contractility, and cell adhesion during SHF deployment and heart tube remodeling [START_REF] Zhou | Modulation of morphogenesis by noncanonical Wnt signaling requires ATF/CREB familymediated transcriptional activation of TGFbeta2[END_REF][START_REF] Sinha | Loss of Wnt5a disrupts second heart field cell deployment and may contribute to OFT malformations in DiGeorge syndrome[END_REF][START_REF] Merks | Planar cell polarity signalling coordinates heart tube remodelling through tissue-scale polarisation of actomyosin activity[END_REF]. Loss of Wnt5a in mice leads to OFT malformations due to impaired cell deployment in the SHF [START_REF] Sinha | Loss of Wnt5a disrupts second heart field cell deployment and may contribute to OFT malformations in DiGeorge syndrome[END_REF]. Moreover, Wnt5a functions through Daam1 in OFT morphogenesis by promoting the AP elongation of the SHF [START_REF] Li | Planar cell polarity signaling regulates polarized second heart field morphogenesis to promote both arterial and venous pole septation[END_REF].

Dvl family members collectively contribute to OFT morphogenesis. Loss of Dvl1 and Dvl2 impairs SHF deployment and OFT lengthening by disrupting actin polymerization and filopodia formation [START_REF] Sinha | Disheveled mediated planar cell polarity signaling is required in the second heart field lineage for outflow tract morphogenesis[END_REF]. Homozygous Looptail mice, which carry a single nucleotide mutation in the Vangl2 gene that is predicted to produce a malfunctional Vangl2 protein, show defects in the polarization of myocardial cells likely due to reduced activity of RhoA and ROCK1 (Rho-associated kinase 1). As a consequence, myocardializing cells fail to extend cellular protrusions into the OFT cushion, resulting in abnormal muscularization of the proximal outlet septum [START_REF] Phillips | Vangl2 acts via RhoA signaling to regulate polarized cell movements during development of the proximal outflow tract[END_REF]. Conditional knockout of Vangl2 indicates that it is solely required within the SHF to promote lengthening of the OFT by regulating cellular polarity in the distal outflow wall [START_REF] Ramsbottom | Vangl2-regulated polarisation of second heart field-derived cells is required for outflow tract lengthening during cardiac development[END_REF]. Similarly, loss of Prickle1 in mice leads to shortened OFT due to absence of polarized cell orientation in the SHF [START_REF] Gibbs | Prickle1 mutation causes planar cell polarity and directional cell migration defects associated with cardiac outflow tract anomalies and other structural birth defects[END_REF]. Conditional knockout of Daam1 in the myocardium prevents cardiomyocytes from extending protrusions into the OFT [START_REF] Ajima | DAAM1 and DAAM2 are co-required for myocardial maturation and sarcomere assembly[END_REF]. Disruption of the CPLANE protein Wdpcp in mice also prevents polarized migration of cardiomyocytes to invade the OFT cushion and causes OFT septation defects [START_REF] Cui | Wdpcp, a PCP protein required for ciliogenesis, regulates directional cell migration and cell polarity by direct modulation of the actin cytoskeleton[END_REF]. Thus, Wnt/PCP signaling coordinates directional cell migration in the SHF to promote heart morphogenesis.

PCP proteins are also involved in other aspects of heart development. In mice, Fzd4 is required for arterial and arteriolar formation. It cooperates with Dvl3 to regulate vascular cell proliferation and migration through microtubule stabilization and cellular polarization [START_REF] Descamps | Frizzled 4 regulates arterial network organization through noncanonical Wnt/planar cell polarity signaling[END_REF]. Fzd2 and Fzd7 are redundantly required for CE movements to promote closure of the ventricular septum [START_REF] Yu | Frizzled 2 and frizzled 7 function redundantly in convergent extension and closure of the ventricular septum and palate: evidence for a network of interacting genes[END_REF]. Vangl2 interacts with RhoA and ROCK in the formation of the coronary vasculature through a non-autonomous manner, likely by regulating deposition of fibronectin in the subepicardial space for the migration of epicardially-derived cells [START_REF] Phillips | Non-cell-autonomous roles for the planar cell polarity gene Vangl2 in development of the coronary circulation[END_REF]. Combined loss of Daam1 and Daam2 in mice causes severe cardiac abnormalities including ventricular noncompaction, ventricular septal defects, irregular sarcomere assembly, and impaired myocardial maturation [START_REF] Li | Dishevelled-associated activator of morphogenesis 1 (Daam1) is required for heart morphogenesis[END_REF][START_REF] Ajima | DAAM1 and DAAM2 are co-required for myocardial maturation and sarcomere assembly[END_REF]. However, it is unclear whether these defects are all direct consequences of disrupted Wnt/PCP signaling.

Gut looping and elongation

The left-right asymmetry of the developing gut arises through an important morphogenetic process known as gut looping. This event has been shown to rely on physical forces generated by differential growth rates between the gut and the dorsal mesentery that connects the gut to the body wall [START_REF] Savin | On the growth and form of the gut[END_REF]. The earliest left-right asymmetric gut morphology is manifested by the leftward curvature of the stomach (Fig. 2D). In the chick embryo, Nodal-induced expression of Pitx2 maintains left-side identity of the early gut tube [START_REF] Huycke | Chick midgut morphogenesis[END_REF][START_REF] Grzymkowski | The twists and turns of left-right asymmetric gut morphogenesis[END_REF]. Wnt/PCP signaling acts downstream of Pitx2 to coordinate asymmetric processes of the digestive system, including gut looping, intestine elongation, villification of the intestinal epithelium, stem cell lineage segregation, and maintenance of epithelial homeostasis.

Several PCP proteins display asymmetric expression in the chicken dorsal mesentery, which exhibits polarized cellular behaviors with extensive membrane contacts between adjacent mesenchymal cells on the left side. Fzd4 and Daam2 show Pitx2-dependent left-sided expression, and Daam2 is required for cell adhesion necessary for mesenchymal condensation in the dorsal mesentery [START_REF] Welsh | Integration of left-right Pitx2 transcription and Wnt signaling drives asymmetric gut morphogenesis via Daam2[END_REF]. Although Wnt5a does not display asymmetric expression, its activity on the right side is antagonized by the right-sided expression of secreted Frizzled-related proteins (sFRPs). Therefore, the asymmetric activation of Wnt5a-Fzd4-Daam2 pathway triggers leftward tilt of the midgut [START_REF] Welsh | Integration of left-right Pitx2 transcription and Wnt signaling drives asymmetric gut morphogenesis via Daam2[END_REF].

Intestine elongation critically requires functional PCP signaling. In mice, Wnt5a is expressed in the gut mesenchyme. Constitutive knockout of Wnt5a disrupts midgut elongation by preventing post-mitotic daughter cells from re-intercalating into the epithelial layer [START_REF] Cervantes | Wnt5a is essential for intestinal elongation in mice[END_REF] as well as from extending filopodia for basal return of their nuclei during interkinetic nuclear migration [START_REF] Wang | Radial WNT5A-guided post-mitotic filopodial pathfinding is critical for midgut tube elongation[END_REF]. Ror2 and Ryk differentially regulate midgut elongation through dynamic spatiotemporal interactions with Wnt5a. Mesenchyme-derived Ror2 functions before the early phase of midgut elongation, while both epithelial and mesenchymal Ryk contributes to midgut elongation throughout the early phase [START_REF] Wang | RYK-mediated filopodial pathfinding facilitates midgut elongation[END_REF]. Vangl2 regulates several polarized cell behaviors in the gut epithelium to couple epithelial morphogenesis with gut tube elongation. In mice, it functions with Wnt5a for oriented cell division along the rostrocaudal axis to increase fore-stomach length [START_REF] Matsuyama | Sfrp controls apicobasal polarity and oriented cell division in developing gut epithelium[END_REF]. In Xenopus, Vangl2 is enriched at the apical and anterior surfaces of radially elongated gut endoderm cells; it coordinates microtubule organization, cell adhesion and endodermal cell shape changes to promote gut elongation and lumen formation [START_REF] Dush | Vangl2 coordinates cell rearrangements during gut elongation[END_REF]. At late stages of intestine elongation, the Fat4/Dchs1 module acts in parallel with Vangl2 to drive mesenchymal cell clustering necessary for epithelial folding during villus formation, while Wnt5a functions upstream of Fat4 and acts as a chemoattractant to guide directional migration of mesenchymal cells [START_REF] Rao-Bhatia | Hedgehog-activated Fat4 and PCP pathways mediate mesenchymal cell clustering and villus formation in gut development[END_REF].

The gene encoding cilia and flagella associated protein 126, Cfap126 (also known as Flattop or Fltp), is transcriptionally activated during Wnt/PCP acquisition in ciliated cells, thus representing a target of Wnt/PCP signaling [START_REF] Gegg | Flattop regulates basal body docking and positioning in mono-and multiciliated cells[END_REF]. Cfap126 can function as a PPE, and its transient expression in Lgr5 + intestinal stem cells regulates lineage priming and cell cycle exit at the base of the crypt, contributing to the differentiation of Paneth and enteroendocrine cells [START_REF] Böttcher | Non-canonical Wnt/PCP signalling regulates intestinal stem cell lineage priming towards enteroendocrine and Paneth cell fates[END_REF]. In the adult, Wnt5a is expressed in FoxL1 + telocytes at the crypt-villus junction regions and is required for homeostatic renewal of the intestinal epithelium [START_REF] Shoshkes-Carmel | Subepithelial telocytes are an important source of Wnts that supports intestinal crypts[END_REF]. These data highlight an essential role of Wnt/PCP signaling in intestinal stem cell lineage segregation and directly link cell polarity to cell fate specification.

Lung branching morphogenesis

The highly elaborated and polarized architecture of the respiratory system emerges from reciprocal interactions between lung epithelium and mesenchyme (Fig. 3A). Lung branching morphogenesis involves repetitive formation of new buds, epithelial sprouting, and tube elongation to generate an arborized airway network. This process is also dependent on several aforementioned asymmetric cell behaviors. PCP proteins regulate different aspects of lung morphogenesis, and their dysfunctions are linked to lung diseases, such as emphysema, a chronic obstructive pulmonary disease, also known as COPD [START_REF] Vladar | Noncanonical Wnt planar cell polarity signaling in lung development and disease[END_REF].

In the mouse embryo, Wnt5a is mainly expressed in the mesenchyme of branch points and functions through Ror2 to synchronize radial polarization of mesenchymal cells for tube elongation along the AP axis [START_REF] Kishimoto | Synchronized mesenchymal cell polarization and differentiation shape the formation of the murine trachea and esophagus[END_REF]. It also activates the Ror2-Vangl2 cascade in both lung epithelium and mesenchyme to coordinate alveolar formation through cytoskeletonmediated cell shape changes of alveolar type I cells and migration of myofibroblasts. Moreover, Wnt5a and Vangl2 show reduced expression in lung tissues of human patients with bronchopulmonary dysplasia and emphysema, suggesting a possible implication of these genes in lung diseases [START_REF] Li | WNT5a-ROR signaling is essential for alveologenesis[END_REF][START_REF] Zhang | A mammalian Wnt5a-Ror2-Vangl2 axis controls the cytoskeleton and confers cellular properties required for alveologenesis[END_REF]. Fzd2 regulates epithelial cell shape changes to promote branch point formation through RhoA-induced apical localization of phosphorylated myosin light chain 2 [START_REF] Kadzik | Wnt ligand/Frizzled 2 receptor signaling regulates tube shape and branch-point formation in the lung through control of epithelial cell shape[END_REF]. Celsr1 and Vangl2 are enriched at the basal/luminal surface of branching lung epithelial buds. They regulate cytoskeletal remodeling likely via ROCK to maintain epithelial architecture, thus their mutations lead to a reduced number of epithelial branches (Yates et al., 2010b). Tracheal and alveolar epithelial cells from heterozygous Looptail mice show highly disrupted actomyosin networks and abnormal focal adhesions, which are correlated with decreased RhoA activity and reduced YAP signaling, suggesting that Vangl2 may function as a regulator of mechanotransduction [START_REF] Cheong | The planar polarity component VANGL2 is a key regulator of mechanosignaling[END_REF]. Scrib is required for lumen morphogenesis. It maintains tight junction integrity and epithelial cohesion through interaction with Wnt/PCP signaling by modulating the localization of Celsr1 and Vangl2 in lung epithelium [START_REF] Yates | Scribble is required for normal epithelial cell-cell contacts and lumen morphogenesis in the mammalian lung[END_REF].

Airway motile cilia present distinct orientations and highly coordinated movements, which are important for mucociliary clearance. During ciliogenesis, asymmetrically localized PCP complexes first polarize epithelial cells along the proximal (oral) to distal axis, and then provide polarity cues to orient ciliary basal bodies in the proximal direction [START_REF] Vladar | Airway epithelial homeostasis and planar cell polarity signaling depend on multiciliated cell differentiation[END_REF]. In airway epithelia, Fzd3, Fzd6, Dvl1 and Dvl3 are enriched at the proximal side of multiciliated cells; Vangl1, Vangl2 and Prickle2 are distributed at the distal side [START_REF] Vladar | Microtubules enable the planar cell polarity of airway cilia[END_REF][START_REF] Vladar | Airway epithelial homeostasis and planar cell polarity signaling depend on multiciliated cell differentiation[END_REF]; Dvl2 and Ankrd6 are localized to the base of cilia [START_REF] Vladar | Microtubules enable the planar cell polarity of airway cilia[END_REF]. Loss of PCP protein expression or localization leads to mis-orientations of airway cilia and chronic inflammatory lung diseases [START_REF] Vladar | Microtubules enable the planar cell polarity of airway cilia[END_REF][START_REF] Vladar | Airway epithelial homeostasis and planar cell polarity signaling depend on multiciliated cell differentiation[END_REF], suggesting an essential role of PCP signaling in maintaining lung homeostasis during fetal and post-natal development. Indeed, conditional knockout of Vangl1 or Prickle2 in mice impairs tracheal epithelial regeneration [START_REF] Vladar | Airway epithelial homeostasis and planar cell polarity signaling depend on multiciliated cell differentiation[END_REF]; reducing the activity of Vangl2 causes tissue damage as observed in emphysema and impairs adult lung function [START_REF] Poobalasingam | Heterozygous Vangl2(Looptail) mice reveal novel roles for the planar cell polarity pathway in adult lung homeostasis and repair[END_REF]. Knockout of Cfap126 mostly reduces the diameter of terminal lung bronchioles, leading to constricted distal airways in the post-natal lung [START_REF] Gegg | Flattop regulates basal body docking and positioning in mono-and multiciliated cells[END_REF].

Wnt/PCP signaling also coordinates the development of pulmonary airway and vasculature. Wnt5a interacts with Ror2 to direct avian pulmonary vasculogenesis by regulating VEGF signaling through fibronectin expression in the mesenchyme [START_REF] Loscertales | Chick pulmonary Wnt5a directs airway and vascular tubulogenesis[END_REF].

Pericytes are perivascular cells that promote vessel maturation. Loss of Wnt5a and Fzd7 impairs endothelium-pericyte interactions during pulmonary angiogenesis by disrupting the motility and polarity of pericytes, which may contribute to pulmonary arterial hypertension [START_REF] Yuan | Loss of endothelium-derived Wnt5a is associated with reduced pericyte recruitment and small vessel loss in pulmonary arterial hypertension[END_REF].

Since PCP proteins are critically involved in lung development, homeostasis and disease, it will be important to further decipher the mechanisms by which they orchestrate stage-specific cellular activities, ranging from epithelial-mesenchymal interactions in branching morphogenesis to the maturation of multiciliated cells and the development of respiratory vasculature.

Kidney development

The morphogenesis of the early urinary tract involves reciprocal interactions between the Wolffian duct-derived ureteric bud and its adjacent metanephric mesenchyme. Strikingly, the ureteric bud undergoes reiterative branching morphogenesis with distal elements engaged in directed cell migration, oriented cell division, and cell intercalation. These PCP-dependent cellular behaviors play important roles in ureteric bud branching, tubular elongation, and tubule diameter establishment [START_REF] Carroll | The kidney and planar cell polarity[END_REF]. Different polarity pathways are critically implicated in kidney development and potentially linked to various genetic disorders, such as congenital malformations of the kidney and urinary tract (CAKUT) and glomerular diseases [START_REF] Torban | Planar cell polarity pathway in kidney development, function and disease[END_REF].

It is well documented that Wnt ligands are essential in kidney morphogenesis. Pioneer studies have demonstrated a requirement of Wnt4 for mesenchymal to epithelial transformation that results in epithelialization of the ureteric bud [START_REF] Stark | Epithelial transformation of metanephric mesenchyme in the developing kidney regulated by Wnt-4[END_REF][START_REF] Kispert | Wnt-4 is a mesenchymal signal for epithelial transformation of metanephric mesenchyme in the developing kidney[END_REF]. Wnt5a-Ror2 signaling regulates epithelial tubular formation from the ureteric bud. Loss of Wnt5a or Ror2 in mice impairs the positioning of metanephric mesenchyme and causes its aberrant interaction with the Wolffian duct, resulting in duplicated ureters and kidneys [START_REF] Nishita | Role of Wnt5a-Ror2 signaling in morphogenesis of the metanephric mesenchyme during ureteric budding[END_REF][START_REF] Yun | Non-canonical Wnt5a/Ror2 signaling regulates kidney morphogenesis by controlling intermediate mesoderm extension[END_REF]. Wnt9b acts through Rho/JNK signaling to regulate CE movements and maintain polarized cell divisions in the epithelium during early stages of kidney morphogenesis, thus inhibition of Wnt9b-mediated signaling affects epithelial polarity and causes increased tubule diameter [START_REF] Karner | Wnt9b signaling regulates planar cell polarity and kidney tubule morphogenesis[END_REF]. Wnt11 derived from the ureteric branch tip promotes the stable attachment of nephron progenitors to the epithelial tip by regulating their polarity and motile behaviors, thereby maintaining nephrogenic niche integrity [START_REF] O'brien | Wnt11 directs nephron progenitor polarity and motile behavior ultimately determining nephron endowment[END_REF].

Several "core" PCP proteins clearly display asymmetric localization in developing renal tubule epithelia along the proximal-distal axis. In both the collecting duct and proximal tubules, Vangl1 and Vangl2 are accumulated at the proximal side, while Fzd3 and Fzd6 are distributed at the distal side, suggesting that they are involved in the polarization of renal tubules [START_REF] Kunimoto | Disruption of core planar cell polarity signaling regulates renal tubule morphogenesis but is not cystogenic[END_REF]. Functional analyses indicate that other "core" PCP proteins are also important for renal tubule morphogenesis. Mice deficient for Fzd4 and Fzd8 show renal hypoplasia, with delayed growth, branching and proliferation of the ureteric epithelium [START_REF] Ye | Genetic mosaic analysis reveals a major role for frizzled 4 and frizzled 8 in controlling ureteric growth in the developing kidney[END_REF]. However, these phenotypes may also result from disrupted Wnt/ß-catenin signaling. Perturbation of Dvl function in Xenopus disrupts rosette-based CE movements driven by mediolaterally oriented cell intercalations, resulting in shorter and wider kidney tubules [START_REF] Lienkamp | Vertebrate kidney tubules elongate using a planar cell polarity-dependent, rosette-based mechanism of convergent extension[END_REF]. Vangl2 regulates morphogenetic processes in the ureteric bud and metanephric mesenchyme.

Homozygous Looptail mice show defective ureteric branching and glomerular maturation but with normal ureteric bud formation, consistent with impaired CE movements (Yates et al., 2010a).

Prickle1 is required for tubule morphogenesis by regulating the distribution of actin cytoskeleton in the ureteric bud; its loss of function impairs cell arrangements in the collecting duct and renal tubules, resulting in abnormal tubule shape [START_REF] Liu | Null and hypomorph Prickle1 alleles in mice phenocopy human Robinow syndrome and disrupt signaling downstream of Wnt5a[END_REF]. Celsr1 genetically interacts with Vangl2 to regulate rostrocaudal patterning of renal tubules and maturation of glomeruli; it also promotes ureteric tree growth during early stages of kidney development but prevents tubule overgrowth at late stages [START_REF] Brzóska | Planar cell polarity genes Celsr1 and Vangl2 are necessary for kidney growth, differentiation, and rostrocaudal patterning[END_REF]. Consistent with these functions, mutations of CELSR1 are associated with renal disorders, including unilateral renal agenesis, hydronephrosis, and hydroureter [START_REF] Brzóska | Planar cell polarity genes Celsr1 and Vangl2 are necessary for kidney growth, differentiation, and rostrocaudal patterning[END_REF]. Therefore, it will be important to understand the molecular and cellular mechanisms underlying the stage-specific functions of Celsr1 in kidney development.

Effectors of Wnt/PCP signaling are implicated in different aspects of renal tubule development. In Xenopus and zebrafish pronephros, Daam1 may function through Rho GTPase to regulate cytoskeletal rearrangements for proper tubulogenesis and tubule morphology [START_REF] Miller | Pronephric tubulogenesis requires Daam1-mediated planar cell polarity signaling[END_REF]. It also colocalizes with E-cadherin at cell-cell contact regions in the Xenopus nephric primordium to ensure intercellular adhesion and epithelial tissue organization during CE movements [START_REF] Krneta-Stankic | The Wnt/PCP formin Daam1 drives cell-cell adhesion during nephron development[END_REF]. In mice, Fuz appears to regulate ureteric bud branching morphogenesis through cilia-dependent and -independent pathways, raising the possibility that it may play a role in ciliogenesis [START_REF] Wang | Loss of planar cell polarity effector Fuzzy causes renal hypoplasia by disrupting several signaling pathways[END_REF].

Fat and Dchs heteromeric protocadherins are important for cell interactions in the developing kidney. Loss of Fat4 disrupts polarized cell behaviors and leads to cyst formation [START_REF] Saburi | Loss of Fat4 disrupts PCP signaling and oriented cell division and leads to cystic kidney disease[END_REF]. Fat1 cooperates with Fat4 in renal tubular elongation [START_REF] Saburi | Functional interactions between Fat family cadherins in tissue morphogenesis and planar polarity[END_REF].

Importantly, the interaction between Fat4 and Dchs1 is necessary for ureteric bud branching and tubule diameter establishment. Dchs1 is specifically expressed in the cap (condensing) mesenchyme that coalesces around the ureteric bud, while Fat4 is only present in stromal cells that surround the cap mesenchyme (Fig. 3B). These expression patterns establish a crosstalk whereby Fat4 interacts with Dchs1 to regulate self-renewal and differentiation of ureteric epithelial progenitors through Hippo, Notch or FGF signaling [START_REF] Das | Stromal-epithelial crosstalk regulates kidney progenitor cell differentiation[END_REF][START_REF] Bagherie-Lachidan | Stromal Fat4 acts non-autonomously with Dchs1/2 to restrict the nephron progenitor pool[END_REF]. Fat4/Dchs1-mediated stroma-to-cap signaling may also function to polarize the cap mesenchyme for proper nephrogenesis [START_REF] Mao | Fat4/Dchs1 signaling between stromal and cap mesenchyme cells influences nephrogenesis and ureteric bud branching[END_REF]. There is also a complex interplay between Fat family protocadherins and other signaling pathways during kidney development. In zebrafish pronephros, Fat1 functions with Scrib in Hippo signaling through regulation of YAP1.

Depletion of Fat1 and Scrib causes severe cyst formation, suggesting that Scrib may couple Fat1 with the Hippo pathway to coordinate cellular polarization and growth [START_REF] Skouloudaki | Scribble participates in Hippo signaling and is required for normal zebrafish pronephros development[END_REF].

In mice, Fat4 can act non-autonomously to prevent ectopic ureteric bud formation and kidney duplication by fine-tuning the signaling function of nephric duct-expressed RET, a GNDF receptor that regulates cell rearrangements in ureteric bud morphogenesis [START_REF] Zhang | FAT4 fine-tunes kidney development by regulating RET signaling[END_REF]. Thus, Fat4 plays a critical role in the cap mesenchyme to control the nephron progenitor pool.

Wnt/PCP signaling is also important for proper organization of podocytes, which are highly specialized and polarized glomerular visceral epithelial cells that are essential for maintaining the correct function of the glomerular filtration barrier to prevent proteinuria. Podocyte-specific loss of Vangl2 in mice causes abnormal glomerular maturation in fetal kidneys and increases the susceptibility of glomerular injury in the adults [START_REF] Rocque | Deficiency of the planar cell polarity protein Vangl2 in podocytes affects glomerular morphogenesis and increases susceptibility to injury[END_REF]. Thus, Vangl2 play an important role to promote glomerular development and protect glomerular injury.

The improved understanding of PCP proteins in kidney morphogenesis helps to clarify the link between Wnt/PCP signaling and chronic kidney failure. Nephron, the functional unit of the kidney, uses numerous ciliated epithelial tubules to reabsorb nutrients and concentrate waste products. Dysfunctions of primary cilia contribute to the pathogenesis of polycystic kidney disease (PKD). As Wnt/PCP signaling, PKD genes encoding proteins mainly localized to primary cilia are also required for oriented cell divisions and CE movements in developing renal tubules, inferring that defective Wnt/PCP signaling may contribute to PKD as well. However, recent studies indicate that mutations of PKD genes disrupt renal tubule morphogenesis independently of Wnt/PCP signaling and that the asymmetric localization of PCP proteins remains intact in adult cystic kidney [START_REF] Kunimoto | Disruption of core planar cell polarity signaling regulates renal tubule morphogenesis but is not cystogenic[END_REF]. Moreover, collecting duct-specific loss of Vangl1 and Vangl2 in mice impairs oriented cell divisions and CE movements, leading to abnormal tubule diameter in pre-natal renal tubules but not cyst formation in post-natal development [START_REF] Kunimoto | Disruption of core planar cell polarity signaling regulates renal tubule morphogenesis but is not cystogenic[END_REF][START_REF] Derish | Differential role of planar cell polarity gene Vangl2 in embryonic and adult mammalian kidneys[END_REF]. Therefore, PCP proteins play an important role in the accurate control of tubule lumen diameter. Nevertheless, it remains to be determined in more detail how they regulate early and late stages of kidney epithelial morphogenesis and how they are linked to other kidney diseases.

Inner ear development and hair cell polarity

The inner ear exhibits marvelous features of PCP-dependent asymmetric morphogenesis, making it an attractive system for studying PCP regulation and function. Mammalian inner ear consists of a snail-shaped cochlea required for hearing and a vestibular system necessary for maintaining balance. Cochlear extension and development of the spiral organ of Corti from a thicker and shorter primordium involve cell intercalations at the luminal surface and directed migration of hair cells toward the apex [START_REF] Driver | Cell migration, intercalation and growth regulate mammalian cochlear extension[END_REF]. These highly regulated cellular processes promote cochlea growth and contribute to the spatial distribution of sensory cells.

Thus, the organ of Corti forms one row of inner hair cells (IHCs) and three rows of outer hair cells (OHC1, OHC2, and OHC3), which are separated by specialized supporting cells. Based on their locations with respect to the center of the cochlear spiral, IHCs are referred to as neural/medial (or proximal), while OHCs are described as abneural/lateral (or distal). Strikingly, both cochlear and vestibular hair cells display uniformly oriented stereociliary bundles on their apical surfaces.

Wnt/PCP signaling functions to promote cochlea growth and organize the highly choreographed orientation of stereociliary bundles.

In cochlear and vestibular epithelia of the developing mouse embryo, PCP proteins display striking asymmetric localization in both sensory and supporting cells in a pattern that is tightly linked to hair cell polarity (Fig. 4A). Fzd3, Fzd6 and Dvl1 accumulate at the neural boundary of IHCs and OHCs, opposite to the site of stereociliary bundle formation, while Dvl2 and Dvl3 are apparently enriched at the abneural edge [START_REF] Najarro | Dual regulation of planar polarization by secreted Wnts and Vangl2 in the developing mouse cochlea[END_REF]. Ankrd6 localizes to the neural edge [START_REF] Jones | Ankrd6 is a mammalian functional homolog of Drosophila planar cell polarity gene diego and regulates coordinated cellular orientation in the mouse inner ear[END_REF]. Celsr1 is likely enriched both at the neural side of hair cells and at the abneural side of supporting cells [START_REF] Duncan | Celsr1 coordinates the planar polarity of vestibular hair cells during inner ear development[END_REF]. Analysis by stimulated emission depletion microscopy indicates that Vangl2 is present at the abneural side of supporting cells [START_REF] Giese | Gipc1 has a dual role in Vangl2 trafficking and hair bundle integrity in the inner ear[END_REF]. Prickle2 is enriched at the neural side of vestibular hair cells before the formation of stereocilia, and Vangl2 maintains this asymmetric distribution [START_REF] Deans | Asymmetric distribution of prickle-like 2 reveals an early underlying polarization of vestibular sensory epithelia in the inner ear[END_REF].

Dysfunctions of PCP proteins affect cochlear outgrowth and hair cell polarity. Wnt5a alone is not sufficient for hair cell polarization in the developing cochlea, but multiple Wnt ligands collectively contribute to this process [START_REF] Najarro | Dual regulation of planar polarization by secreted Wnts and Vangl2 in the developing mouse cochlea[END_REF]. During cochlear extension to form the organ of Corti, Dvl1, Dvl2 and Vangl2 regulate cell intercalation that drives unidirectional elongation of the primordium. Loss of their functions interferes with outgrowth and morphogenesis of the cochlea, resulting in abnormal patterning of the cochlear duct and defective uniform hair bundle orientation [START_REF] Montcouquiol | Identification of Vangl2 and Scrb1 as planar polarity genes in mammals[END_REF][START_REF] Wang | Regulation of polarized extension and planar cell polarity in the cochlea by the vertebrate PCP pathway[END_REF]. Combined loss of Fzd3 and Fzd6 mostly affects hair bundle orientation in the organ of Corti [START_REF] Wang | The role of Frizzled3 and Frizzled6 in neural tube closure and in the planar polarity of inner ear sensory hair cells[END_REF]. In mice, missense mutations that may reduce the structural integrity of Celsr1 protein disrupt the earliest stages of hair cell polarity in the cochlea and lead to mis-oriented stereociliary bundles between vestibular hair cells associated with defective vestibular behaviors [START_REF] Curtin | Mutation of Celsr1 disrupts planar polarity of inner ear hair cells and causes severe neural tube defects in the mouse[END_REF].

Knockout of Fat4 mildly affects hair bundle orientation by mostly leads to defects in cochlear extension and organization of OHCs, which are exacerbated by reducing the dosage of Fat1 [START_REF] Saburi | Functional interactions between Fat family cadherins in tissue morphogenesis and planar polarity[END_REF].

Different PCP genes show genetic interactions in cochlear extension and stereociliary bundle orientation, likely due to functional redundancy and interdependent asymmetric localization of their protein products. In particular, there is evidence showing Vangl2 interaction with other PCP proteins in auditory hair cells by utilizing the Looptail mutant. Inhibiting the secretion of Wnt ligands, including Wnt5a, in a heterozygous Looptail background aggravates defects in cochlear extension and sensory hair cell polarization [START_REF] Najarro | Dual regulation of planar polarization by secreted Wnts and Vangl2 in the developing mouse cochlea[END_REF]. Loss of Fzd1 or Fzd2 causes mis-orientations of hair cells only when Vangl2 activity is also reduced [START_REF] Yu | Frizzled 1 and frizzled 2 genes function in palate, ventricular septum and neural tube closure: general implications for tissue fusion processes[END_REF]. Although Ankrd6 mutant mice show disrupted uniform orientation of hair cells in the utricle, only Ankrd6 and Vangl2 compound mutants display defective hair cell polarity and patterning in the cochlea [START_REF] Jones | Ankrd6 is a mammalian functional homolog of Drosophila planar cell polarity gene diego and regulates coordinated cellular orientation in the mouse inner ear[END_REF]. Fat4 cooperates with Vangl2 to regulate cochlear elongation and sensory epithelium patterning [START_REF] Saburi | Functional interactions between Fat family cadherins in tissue morphogenesis and planar polarity[END_REF]. There are also physical and genetic interactions between Scrib and Vangl2 in the planar polarization of stereociliary bundles [START_REF] Montcouquiol | Identification of Vangl2 and Scrb1 as planar polarity genes in mammals[END_REF][START_REF] Montcouquiol | Asymmetric localization of Vangl2 and Fz3 indicate novel mechanisms for planar cell polarity in mammals[END_REF]. It is possible that the Looptail mutation contributes partially to enhance hair cell phenotypes because the resulting malfunctional Vangl2 protein can interfere with the proper localization of other PCP complexes.

During post-natal development, loss of Vangl2 has mild effects on the orientation of auditory stereociliary bundles and on hearing loss, but mostly causes profound changes in the shape and distribution of supporting cells that are necessary for cochlear amplification mediated by OHCs [START_REF] Copley | Postnatal refinement of auditory hair cell planar polarity deficits occurs in the absence of Vangl2[END_REF]. Thus, it will be of interest to determine the post-natal requirement of other PCP proteins in maintaining hearing function.

Intriguingly, PCP proteins can function in parallel to instruct hair cell orientation. Zebrafish lateral line neuromasts also consist of mechanosensory hair cells and support cells. Loss of Vangl2 directly leads to PCP-dependent defects in hair cells. However, loss of Wnt11 or Fzd7 indirectly affects hair cell polarity by disrupting the alignment of support cells, suggesting a contribution of support cells in sensory cell orientation [START_REF] Navajas Acedo | PCP and Wnt pathway components act in parallel during zebrafish mechanosensory hair cell orientation[END_REF]. Interestingly, supporting cells can also dictate hair cell polarization in the mammalian cochlea. The evolutionarily conserved transmembrane receptor protein tyrosine kinase 7 (PTK7) functions as a vertebrate-specific PCP regulator. Disruption of PTK7 mostly affects stereociliary bundle orientation in OHC3 [START_REF] Lu | PTK7/CCK-4 is a novel regulator of planar cell polarity in vertebrates[END_REF]. Mechanistically, PTK7 acts in parallel with Wnt/PCP signaling to polarize hair cells by promoting junctional localization of myosin II near the apical surfaces of supporting cells [START_REF] Lee | PTK7 regulates myosin II activity to orient planar polarity in the mammalian auditory epithelium[END_REF]. Thus, PTK7 restricts the abneural positioning of kinocilia by exerting local tension at the neural edge of hair cells.

Cochlear OHCs are innervated by type II spiral ganglion neurons, which project peripheral axons that extend beyond the IHCs and then turn 90° toward the cochlear base. Disruption of Wnt secretion suggests that non-canonical Wnts are necessary to trigger Wnt/PCP signaling during axon guidance [START_REF] Ghimire | Frizzled3 and frizzled6 cooperate with Vangl2 to direct cochlear innervation by type II spiral ganglion neurons[END_REF]. Fzd3 and Fzd6 are present at the basolateral boundary between adjacent supporting cells along the trajectory of axonal growth cones; they promote the asymmetric localization of Vangl2 and function redundantly to direct cochlear innervation [START_REF] Ghimire | Frizzled3 and frizzled6 cooperate with Vangl2 to direct cochlear innervation by type II spiral ganglion neurons[END_REF]. Consequently, Vangl2 is enriched at intercellular junctions between cochlear supporting cells and is non-autonomously required for axon turning [START_REF] Ghimire | A non-autonomous function of the core PCP protein VANGL2 directs peripheral axon turning in the developing cochlea[END_REF]. Although it is unclear how Prickle1 and Prickle2 regulate hair cell polarity, Prickle1 is required for neurite outgrowth of type II spiral ganglion neurons (Yang et al., 2017a).

Together, these observations demonstrate an essential role for Wnt/PCP signaling in hair cell innervation.

Generally, dysfunctions of PCP proteins affect the uniform orientation of stereociliary bundles between neighboring hair cells, but individual hair cells still retain polarized stereocilia (Fig. 4B). This is because there are independent but nevertheless interconnected mechanisms coordinating hair cell polarity. Intrinsic or intracellular PCP instructs hair bundle polarization within a hair cell, while tissue-level or intercellular PCP establishes the uniform orientation of hair cells. Indeed, several cytoskeletal regulatory proteins, such as Inscuteable (Insc), Partner of Inscuteable (Pins), Cfap126 and Wdpcp, have been shown to control or restrict positioning of the tubulin-based kinocilium and arrowhead-shaped distribution of actin-rich stereocilia [START_REF] Ezan | Primary cilium migration depends on G-protein signalling control of subapical cytoskeleton[END_REF][START_REF] Tarchini | A molecular blueprint at the apical surface establishes planar asymmetry in cochlear hair cells[END_REF][START_REF] Cui | Wdpcp, a PCP protein required for ciliogenesis, regulates directional cell migration and cell polarity by direct modulation of the actin cytoskeleton[END_REF][START_REF] Gegg | Flattop regulates basal body docking and positioning in mono-and multiciliated cells[END_REF]. Therefore, they can function to establish intrinsic hair bundle polarity. However, the emergence of asymmetric hair bundle position in the hair cell apical surface is tightly coupled with long-range orientation cues in the tissue. Evidence is accumulating that G protein signaling functions to establish intrinsic asymmetry and interpret intercellular PCP signaling. Accordingly, inhibition of Gαi function leads to randomized positioning of kinocilia and mis-orientations of OHCs [START_REF] Tarchini | A molecular blueprint at the apical surface establishes planar asymmetry in cochlear hair cells[END_REF]. Daple, a guanine nucleotide exchange factor and a Dvl-binding protein, mediates Wnt-stimulated heterotrimeric G protein and phosphoinositide 3-kinase signaling to regulate both microtubuledependent eccentric kinocilium positioning and asymmetric localization of PCP proteins [START_REF] Siletti | Daple coordinates organ-wide and cell-intrinsic polarity to pattern inner-ear hair bundles[END_REF][START_REF] Malt | Wnts regulate planar cell polarity via heterotrimeric G protein and PI3K signaling[END_REF]. Therefore, a complex interplay between intracellular and intercellular PCP signaling coordinates the acquisition of hair cell polarity.

Proximal-distal limb outgrowth and elongation

Vertebrate limb buds arise from the lateral plate mesoderm and its overlying ectoderm at the presumptive forelimb and hindlimb locations. Dynamic formation of cellular protrusion and biased orientation of cell division are major driving forces that elongate the limb bud along its proximal-distal axis [START_REF] Boehm | The role of spatially controlled cell proliferation in limb bud morphogenesis[END_REF][START_REF] Wyngaarden | Oriented cell motility and division underlie early limb bud morphogenesis[END_REF]. Wnt/PCP signaling plays an essential role in regulating these directional cell behaviors [START_REF] Barrow | Wnt/planar cell polarity signaling: an important mechanism to coordinate growth and patterning in the limb[END_REF][START_REF] Gao | Planar cell polarity in vertebrate limb morphogenesis[END_REF].

Early studies show that Wnt5a is expressed in the apical ectoderm ridge (AER) and distal mesenchyme of the limb bud as a gradient along the proximal-distal axis [START_REF] Gavin | Expression of multiple novel Wnt-1/int-1-related genes during fetal and adult mouse development[END_REF][START_REF] Dealy | Wnt-5a and Wnt-7a are expressed in the developing chick limb bud in a manner suggesting roles in pattern formation along the proximodistal and dorsoventral axes[END_REF]. Similarly, Vangl2 protein also displays a proximal-distal gradient of asymmetric distribution [START_REF] Gao | Wnt signaling gradients establish planar cell polarity by inducing Vangl2 phosphorylation through Ror2[END_REF].

Wnt5a plays an important role in establishing cell polarity for limb outgrowth (Fig. 4C).

Mice mutant for Wnt5a exhibit reduced individual skeletal elements, with absence of distal phalanges [START_REF] Yamaguchi | A Wnt5a pathway underlies outgrowth of multiple structures in the vertebrate embryo[END_REF]. Analysis of cell behaviors by living imaging in mouse embryos at E10.5 demonstrate that Wnt5a deficiency essentially disrupts the orientation of mesenchymal cell movements and divisions toward the overlying ectoderm [START_REF] Gros | WNT5A/JNK and FGF/MAPK pathways regulate the cellular events shaping the vertebrate limb bud[END_REF].

Conversely, implantation of Wnt5a-soaked beads into the lateral plate mesoderm of chick embryos reveals that Wnt5a acts as a chemoattractant in the emerging limb bud to instruct cellular polarization necessary for limb outgrowth [START_REF] Wyngaarden | Oriented cell motility and division underlie early limb bud morphogenesis[END_REF]. Moreover, locally oriented cell shape changes and biased cell division planes are conserved cellular mechanisms in vertebrate limb morphogenesis [START_REF] Wyngaarden | Oriented cell motility and division underlie early limb bud morphogenesis[END_REF].

Wnt5a promotes the interaction between Ror2 and Vangl2 to establish the asymmetric localization of Vangl2 in limb chondrocytes [START_REF] Gao | Wnt signaling gradients establish planar cell polarity by inducing Vangl2 phosphorylation through Ror2[END_REF]. It acts through Ror2 to induce a proximal-distal gradient of Vangl2 phosphorylation in the mesenchyme, setting up a higher Vangl2 activity in most distal cells [START_REF] Gao | Wnt signaling gradients establish planar cell polarity by inducing Vangl2 phosphorylation through Ror2[END_REF]. This phosphorylation is mediated by casein kinase 1 and functions to polarize cellular behaviors of limb chondrocytes in a dose-dependent manner (Yang et al., 2017b). Wnt5a and Vangl2 also show genetic interaction in limb skeletal development. Reducing Wnt5a dosage in Looptail mice not only aggravates defects in distal limb skeletal elements but also causes shortened long bones in the limb, reminiscent of Robinow syndrome and brachydactyly type B [START_REF] Wang | Disruption of PCP signaling causes limb morphogenesis and skeletal defects and may underlie Robinow syndrome and brachydactyly type B[END_REF]. Therefore, a Wnt5a gradient provides global cues, which are interpreted by Ror2 and Vangl2, to coordinate directional cellular behaviors for proximal-distal limb elongation. Interestingly, Wnt5a can also generate mechanical signals to trigger digit elongation likely through ROCK-mediated actomyosin contractility [START_REF] Parada | Mechanical feedback defines organizing centers to drive digit emergence[END_REF]. Loss of Wnt5a disrupts anisotropic active stresses and CE movements in developing digits, leading to absence of digit-organizing centers (also known as phalange-forming regions) and digit formation [START_REF] Parada | Mechanical feedback defines organizing centers to drive digit emergence[END_REF]. Ror2 has been shown to regulate phalange development mediated by the digit-organizing center [START_REF] Witte | Receptor tyrosine kinase-like orphan receptor 2 (ROR2) and Indian hedgehog regulate digit outgrowth mediated by the phalanxforming region[END_REF]. Thus, Wnt5a may interact at least partially with Ror2 to combine mechanical cues and molecular signals in digit emergence.

Other PCP proteins also contribute to limb outgrowth. Ryk regulates Wnt/PCP signaling and limb elongation in part by promoting Vangl2 stability [START_REF] Andre | The Wnt coreceptor Ryk regulates Wnt/planar cell polarity by modulating the degradation of the core planar cell polarity component Vangl2[END_REF]. Dvl family proteins are required for Wnt5a-induced Vangl2 phosphorylation by facilitating the interaction of casein kinase 1 and Vangl2 (Yang et al., 2017b). Mice deficient for Prickle1 show loss of one phalangeal segment on digits 2-5 in both forelimbs and hindlimbs [START_REF] Yang | Prickle1 stunts limb growth through alteration of cell polarity and gene expression[END_REF][START_REF] Liu | Null and hypomorph Prickle1 alleles in mice phenocopy human Robinow syndrome and disrupt signaling downstream of Wnt5a[END_REF]. Thus, biochemical and functional interactions between PCP proteins are essential for proper limb morphogenesis.

Concluding remarks

PCP proteins contribute to the highly choreographed cellular organization in many morphogenetic processes. Extensive research has greatly advanced our understanding on their implications in controlling asymmetric organogenesis and tissue homeostasis. Accumulating evidence also suggest that dysfunctions of PCP proteins are linked to a variety of human diseases. Nevertheless, there are still many intriguing questions that deserve further investigation.

Although Wnt ligands can provide global cues to initiate cell polarity in several PCPdependent processes, it remains to be determined whether molecular pathways act in concert with other orientation cues to instruct and establish PCP features. Interestingly, a recent research shows that the orientation of tissue stretch cooperates with Wnt signaling gradient to align PCP in Xenopus neuroectoderm [START_REF] Hirano | Alignment of cell long axis by unidirectional tension acts cooperatively with Wnt signalling to establish PCP[END_REF]. Since mechanical constraints act as important cues in planar polarization, it will be important to further develop interdisciplinary approaches for understanding the interplay of PCP signaling and mechanotransduction in morphogenesis.

Functional gene disruptions combined with in vivo live imaging and manipulation of mechanical state should help to push the field forward.

Another important aspect is to understand how cell polarity can be translated to cell fate specification. These two processes are clearly interconnected in several contexts, such that molecular asymmetry provided by PCP proteins helps to promote cell type-specific differentiation.

For example, cell polarity cues are coupled with cell lineage segregation during intestinal stem cell self-renewal and differentiation [START_REF] Böttcher | Non-canonical Wnt/PCP signalling regulates intestinal stem cell lineage priming towards enteroendocrine and Paneth cell fates[END_REF]. Deciphering the interaction between molecular signals and mechanical forces in the coupling of cell fate decision and morphogenetic movements also represents a significant challenge. In this regard, it is noteworthy that Wnt5a signaling functions in a mechanical feedback that links specification and elongation of developing digits [START_REF] Parada | Mechanical feedback defines organizing centers to drive digit emergence[END_REF].

The formation of separately localized "core" PCP protein complexes is important to create cellular polarization. What are the exact roles of each complex in the coordination of polarized cellular behaviors? How do they converge to regulate cell polarity? Similarly, it is unclear how and when the "core" PCP pathway and the Fat/Dchs polarity module coordinate to regulate polarized cellular behaviors. In Drosophila, it appears that they may be coupled by specific Prickle isoforms through regulation of microtubule plus-end bias [START_REF] Strutt | How do the Fat-Dachsous and core planar polarity pathways act together and independently to coordinate polarized cell behaviours?[END_REF]. Whether this is conserved in vertebrates awaits future investigation. Therefore, future research using both invertebrate and vertebrate PCP systems will certainly bring important breakthrough for some of these issues and provide the link between PCP protein dysfunctions and human diseases. PCP proteins display asymmetric localization in bronchial epithelium composed of multiciliated cells and other cell types (yellow) as well as in distal lung epithelium. B: Schematic of ureteric bud development in E11 mouse embryos shows the interaction between the Wolffian ductderived ureteric bud and its adjacent metanephric mesenchyme (stroma and cap mesenchyme).

PCP proteins act in the mesenchyme and/or in the ureteric bud to promote ureteric bud branching, tubular elongation, and tubule diameter establishment. The Fat/Dchs polarity module mediates stroma-to-cap mesenchyme signaling in nephrogenesis.

Fig. 1 .

 1 Fig. 1. Wnt signaling branches and asymmetric localization of PCP protein complexes. A: Wnt/ß-

Fig. 2 .

 2 Fig. 2. Left-right organizers and asymmetric organogenesis. A: In the zebrafish KV, Vangl2 and

Fig. 3 .

 3 Fig. 3. PCP regulators in lung and kidney morphogenesis. A: At the earliest stage of lung

Fig. 4 .

 4 Fig. 4. Wnt/PCP signaling in stereociliary bundle orientation and proximal-distal limb elongation.

Acknowledgments

I would like to apologize for being unable to include many studies in the discussion due to space limitations. This work was supported by grants from the National Natural Science Foundation of China (grant number 32070813), the French Muscular Dystrophy Association (AFM-Téléthon grant number 23545), the Centre National de la Recherche Scientifique (CNRS), and the Sorbonne University.

Conflict of interest

The authors declare no competing interests. [START_REF] Saburi | Loss of Fat4 disrupts PCP signaling and oriented cell division and leads to cystic kidney disease[END_REF][START_REF] Das | Stromal-epithelial crosstalk regulates kidney progenitor cell differentiation[END_REF][START_REF] Mao | Fat4/Dchs1 signaling between stromal and cap mesenchyme cells influences nephrogenesis and ureteric bud branching[END_REF][START_REF] Bagherie-Lachidan | Stromal Fat4 acts non-autonomously with Dchs1/2 to restrict the nephron progenitor pool[END_REF]Zhang et