Planar cell polarity in the inner ear: how do hair cells acquire their oriented structure?

Radial polarity in the first cranial neuromast of selected teleost fishes

The neuromast is a sensory structure of the lateral line system in aquatic vertebrates, which consists of hair cells and supporting cells. Hair cells are mechanosensory cells, generally arranged with bidirectional polarity. Here, we describe a neuromast with hair cells arranged radially instead of bidirectionally in the first cranial neuromast of four teleost species: red seabream (Pagrus major), spotted halibut (Verasper variegatus), brown sole (Pseudopleuronectes herzensteini), and marbled sole (Pseudopleuronectes yokohamae). In these four species, this polarity was identified only in the first cranial neuromast, where it appeared at the rostral edge of the otic vesicle before hatching. We investigated the initial appearance and fate of this unique neuromast using scanning electron microscopy. We also assessed characteristics of radial neuromast pertaining to morphogenesis, development, and innervation using a vital fluorescent marker and immunohistochemistry in V. variegatus. The kinocilium initially appears at the center of each hair cell, then moves to its outer perimeter to form radial polarity by around 7 days postfertilization. However, hair cells arranged radially disappear about 15 days after hatching. This is followed by the appearance of bidirectionally arranged hair cells, indicating that polarity replacement from radial to bidirectional has occurred. In P. herzensteini, both afferent and efferent synapses between the nerve fibers and hair cells were observed by transmission electron microscopy, suggesting that radial neuromast is functional. Our discovery suggests that neuromasts with radial polarity could enable larval fish to assimilate multiaxial stimuli during this life stage, potentially assisting them in detecting small water vibrations or water pressure changes.

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.

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.

Planar cell polarity in organ formation

The planar cell polarity (PCP) pathway controls a variety of morphological events across many species. During embryonic development, the PCP pathway regulates coordinated behaviour of groups of cells to direct morphogenetic processes such as convergent extension and collective cell migration. In this review we discuss the increasingly prominent role of the PCP pathway in organogenesis, focusing on the lungs, kidneys and heart. We also highlight emerging evidence that PCP gene mutations are associated with adult diseases.

Domineering non-autonomy in Vangl1;Vangl2 double mutants demonstrates intercellular PCP signaling in the vertebrate inner ear

The organization of polarized stereociliary bundles is critical for the function of the inner ear sensory receptor hair cells that detect sound and motion, and these cells present a striking example of Planar Cell Polarity (PCP); the coordinated orientation of polarized structures within the plane of an epithelium. PCP is best understood in Drosophila where the essential genes regulating PCP were first discovered, and functions for the core PCP proteins encoded by these genes have been deciphered through phenotypic analysis of core PCP gene mutants. One illuminating phenotype is the domineering non-autonomy that is observed where abrupt disruptions in PCP signaling impacts the orientation of neighboring wild type cells, because this demonstrates local intercellular signaling mediated by the core PCP proteins. Using Emx2-Cre to generate an analogous mutant boundary in the mouse inner ear, we disrupted vertebrate PCP signaling in Vangl1;Vangl2 conditional knockouts. Due to unique aspects of vestibular anatomy, core PCP protein distribution along the mutant boundary generated in the utricle resembles the proximal side of vang mutant clones in the Drosophila wing, while the boundary in the saccule resembles and the distal side. Consistent with these protein distributions, a domineering non-autonomy phenotype occurs along the Emx2-Cre boundary in the mutant utricle that does not occur in the saccule. These results further support the hypothesis that core PCP function is conserved in vertebrates by demonstrating intercellular PCP signaling in the sensory epithelia of the mouse ear.

A synthetic planar cell polarity system reveals localized feedback on Fat4-Ds1 complexes

The atypical cadherins Fat and Dachsous (Ds) have been found to underlie planar cell polarity (PCP) in many tissues. Theoretical models suggest that polarity can arise from localized feedbacks on Fat-Ds complexes at the cell boundary. However, there is currently no direct evidence for the existence or mechanism of such feedbacks. To directly test the localized feedback model, we developed a synthetic biology platform based on mammalian cells expressing the human Fat4 and Ds1. We show that Fat4-Ds1 complexes accumulate on cell boundaries in a threshold-like manner and exhibit dramatically slower dynamics than unbound Fat4 and Ds1. This suggests a localized feedback mechanism based on enhanced stability of Fat4-Ds1 complexes. We also show that co-expression of Fat4 and Ds1 in the same cells is sufficient to induce polarization of Fat4-Ds1 complexes. Together, these results provide direct evidence that localized feedbacks on Fat4-Ds1 complexes can give rise to PCP.

A quantitative approach to understanding vertebrate limb morphogenesis at the macroscopic tissue level

To understand organ morphogenetic mechanisms, it is essential to clarify how spatiotemporally-regulated molecular/cellular dynamics causes physical tissue deformation. In the case of vertebrate limb development, while some of the genes and oriented cell behaviors underlying morphogenesis have been revealed, tissue deformation dynamics remains incompletely understood. We here introduce our recent work on the reconstruction of tissue deformation dynamics in chick limb development from cell lineage tracing data. This analysis has revealed globally-aligned anisotropic tissue deformation along the proximo-distal axis not only in the distal region but also in the whole limb bud. This result points to a need, as a future challenge, to find oriented molecular/cellular behaviors for realizing the observed anisotropic tissue deformation in both proximal and distal regions, which will lead to systems understanding of limb morphogenesis.

Quantitative Analysis of Supporting Cell Subtype Labeling Among CreER Lines in the Neonatal Mouse Cochlea

Four CreER lines that are commonly used in the auditory field to label cochlear supporting cells (SCs) are expressed in multiple SC subtypes, with some lines also showing reporter expression in hair cells (HCs). We hypothesized that altering the tamoxifen dose would modify CreER expression and target subsets of SCs. We also used two different reporter lines, ROSA26 ^tdTomato and CAG-eGFP, to achieve the same goal. Our results confirm previous reports that Sox2 ^CreERT2 and Fgfr3-iCreER ^T2 are not only expressed in neonatal SCs but also in HCs. Decreasing the tamoxifen dose did not reduce HC expression for Sox2 ^CreERT2 , but changing to the CAG-eGFP reporter decreased reporter-positive HCs sevenfold. However, there was also a significant decrease in the number of reporter-positive SCs. In contrast, there was a large reduction in reporter-positive HCs in Fgfr3-iCreER ^T2 mice with the lowest tamoxifen dose tested yet only limited reduction in SC labeling. The targeting of reporter expression to inner phalangeal and border cells was increased when Plp-CreER ^T2 was paired with the CAG-eGFP reporter; however, the total number of labeled cells decreased. Changes to the tamoxifen dose or reporter line with Prox1 ^CreERT2 caused minimal changes. Our data demonstrate that modifications to the tamoxifen dose or the use of different reporter lines may be successful in narrowing the numbers and/or types of cells labeled, but each CreER line responded differently. When the ROSA26 ^tdTomato reporter was combined with any of the four CreER lines, there was no difference in the number of tdTomato-positive cells after one or two injections of tamoxifen given at birth. Thus, tamoxifen-mediated toxicity could be reduced by only giving one injection. While the CAG-eGFP reporter consistently labeled fewer cells, both reporter lines are valuable depending on the goal of the study.

Lectins selectively label cartilage condensations and the otic neuroepithelium within the embryonic chicken head

Cartilage morphogenesis during endochondral ossification follows a progression of conserved developmental events. Cells are specified towards a prechondrogenic fate and subsequently undergo condensation followed by overt differentiation. Currently available molecular markers of prechondrogenic and condensing mesenchyme rely on common regulators of the chondrogenic program that are not specific to the tissue type or location. Therefore tissue-specific condensations cannot be distinguished based on known molecular markers. Here, using the chick embryo model, we utilized lectin labeling on serial sections, demonstrating that differential labeling by peanut agglutinin (PNA) and Sambucus nigra agglutinin (SNA) successfully separates adjacently located condensations in the proximal second pharyngeal arch. PNA selectively labels chick middle ear columella and basal plate condensation, whereas SNA specifically marks extracolumella and the ventro-lateral part of the otic capsule. We further extended our study to examine lectin-binding properties of the different parts of the inner ear epithelium, neural tube and notochord. Our results show that SNA labels the auditory and vestibular hair cells of the inner ear, whereas PNA specifically recognizes the statoacoustic ganglion. PNA is also highly specific for the floor plate of the neural tube. Additionally, wheat germ agglutinin (WGA) labels the basement membrane of the notochord and is a marker of the apical-basal polarity of the cochlear duct. Overall, this study indicates that selective lectin labeling is a promising approach to differentiate between contiguously located mesenchymal condensations and subregions of epithelia globally during development.

Role of Wnt and Notch signaling in regulating hair cell regeneration in the cochlea

Sensory hair cells in the inner ear are responsible for sound recognition. Damage to hair cells in adult mammals causes permanent hearing impairment because these cells cannot regenerate. By contrast, newborn mammals possess limited regenerative capacity because of the active participation of various signaling pathways, including Wnt and Notch signaling. The Wnt and Notch pathways are highly sophisticated and conserved signaling pathways that control multiple cellular events necessary for the formation of sensory hair cells. Both signaling pathways allow resident supporting cells to regenerate hair cells in the neonatal cochlea. In this regard, Wnt and Notch signaling has gained increased research attention in hair cell regeneration. This review presents the current understanding of the Wnt and Notch signaling pathways in the auditory portion of the inner ear and discusses the possibilities of controlling these pathways with the hair cell fate determiner Atoh1 to regulate hair cell regeneration in the mammalian cochlea.

Making sense of Wnt signaling-linking hair cell regeneration to development

Wnt signaling is a highly conserved pathway crucial for development and homeostasis of multicellular organisms. Secreted Wnt ligands bind Frizzled receptors to regulate diverse processes such as axis patterning, cell division, and cell fate specification. They also serve to govern self-renewal of somatic stem cells in several adult tissues. The complexity of the pathway can be attributed to the myriad of Wnt and Frizzled combinations as well as its diverse context-dependent functions. In the developing mouse inner ear, Wnt signaling plays diverse roles, including specification of the otic placode and patterning of the otic vesicle. At later stages, its activity governs sensory hair cell specification, cell cycle regulation, and hair cell orientation. In regenerating sensory organs from non-mammalian species, Wnt signaling can also regulate the extent of proliferative hair cell regeneration. This review describes the current knowledge of the roles of Wnt signaling and Wnt-responsive cells in hair cell development and regeneration. We also discuss possible future directions and the potential application and limitation of Wnt signaling in augmenting hair cell regeneration.

Are accessory hearing structures linked to inner ear morphology? Insights from 3D orientation patterns of ciliary bundles in three cichlid species

Background

Cichlid fishes show considerable diversity in swim bladder morphology. In members of the subfamily Etroplinae, the connection between anterior swim bladder extensions and the inner ears enhances sound transmission and translates into an improved hearing ability. We tested the hypothesis that those swim bladder modifications coincide with differences in inner ear morphology and thus compared Steatocranus tinanti (vestigial swim bladder), Hemichromis guttatus (large swim bladder without extensions), and Etroplus maculatus (intimate connection between swim bladder and inner ears).

Methodology and results

We applied immunostaining together with confocal imaging and scanning electron microscopy for the investigation of sensory epithelia, and high-resolution, contrast-enhanced microCT imaging for characterizing inner ears in 3D, and evaluated otolith dimensions. Compared to S. tinanti and H. guttatus, inner ears of E. maculatus showed an enlargement of all three maculae, and a particularly large lacinia of the macula utriculi. While our analysis of orientation patterns of ciliary bundles on the three macula types using artificially flattened maculae uncovered rather similar orientation patterns of ciliary bundles, interspecific differences became apparent when illustrating the orientation patterns on the 3D models of the maculae: differences in the shape and curvature of the lacinia of the macula utriculi, and the anterior arm of the macula lagenae resulted in an altered arrangement of ciliary bundles.

Conclusions

Our results imply that improved audition in E. maculatus is associated not only with swim bladder modifications but also with altered inner ear morphology. However, not all modifications in E. maculatus could be connected to enhanced auditory abilities, and so a potential improvement of the vestibular sense, among others, also needs to be considered. Our study highlights the value of analyzing orientation patterns of ciliary bundles in their intact 3D context in studies of inner ear morphology and physiology.

Inner ear supporting cells: rethinking the silent majority

Sensory epithelia of the inner ear contain two major cell types: hair cells and supporting cells. It has been clear for a long time that hair cells play critical roles in mechanoreception and synaptic transmission. In contrast, until recently the more abundant supporting cells were viewed as serving primarily structural and homeostatic functions. In this review, we discuss the growing information about the roles that supporting cells play in the development, function and maintenance of the inner ear, their activities in pathological states, their potential for hair cell regeneration, and the mechanisms underlying these processes.

Planar polarity: A new player in both lung development and disease

The clinical burden of both adult and neonatal lung disease worldwide is substantial; in the UK alone, respiratory disease kills one in four people. It is increasingly recognized that genes and pathways that regulate lung development, may be aberrantly activated in disease and/or reactivated as part of the lungs' intrinsic repair mechanisms. Investigating the genes and signaling pathways that regulate lung growth has led to significant insights into the pathogenesis of congenital and adult lung disease. Recently, the planar cell polarity (PCP) pathway has been shown to be required for normal lung development, and data suggests that this signaling pathway is also involved in the pathogenesis of some lung diseases. In this review, we summarize current evidence indicating that the PCP pathway is required for both lung development and disease.

Principles of planar polarity in animal development

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

Compartmentalized Notch signaling sustains epithelial mirror symmetry

Bilateral symmetric tissues must interpret axial references to maintain their global architecture during growth or repair. The regeneration of hair cells in the zebrafish lateral line, for example, forms a vertical midline that bisects the neuromast epithelium into perfect mirror-symmetric plane-polarized halves. Each half contains hair cells of identical planar orientation but opposite to that of the confronting half. The establishment of bilateral symmetry in this organ is poorly understood. Here, we show that hair-cell regeneration is strongly directional along an axis perpendicular to that of epithelial planar polarity. We demonstrate compartmentalized Notch signaling in neuromasts, and show that directional regeneration depends on the development of hair-cell progenitors in polar compartments that have low Notch activity. High-resolution live cell tracking reveals a novel process of planar cell inversions whereby sibling hair cells invert positions immediately after progenitor cytokinesis, demonstrating that oriented progenitor divisions are dispensable for bilateral symmetry. Notwithstanding the invariably directional regeneration, the planar polarization of the epithelium eventually propagates symmetrically because mature hair cells move away from the midline towards the periphery of the neuromast. We conclude that a strongly anisotropic regeneration process that relies on the dynamic stabilization of progenitor identity in permissive polar compartments sustains bilateral symmetry in the lateral line.

Developmental expression of the actin depolymerizing factor ADF in the mouse inner ear and spiral ganglia

Hair cells, the inner ear's sensory cells, are characterized by tens to hundreds of actin-rich stereocilia that form the hair bundle apparatus necessary for mechanoelectrical transduction. Both the number and length of actin filaments are precisely regulated in stereocilia. Proper cochlear and vestibular function also depends on actin filaments in nonsensory supporting cells. The formation of actin filaments is a dynamic, treadmill-like process in which actin-binding proteins play crucial roles. However, little is known about the presence and function of actin binding molecules in the inner ear, which set up, and maintain, actin-rich structures and regulate actin turnover. Here we examined the expression and subcellular location of the actin filament depolymerizing factor (ADF) in the cochlea and vestibular organs. By means of immunocytochemistry and confocal microscopy, we analyzed whole-mount preparations and cross-sections in fetal and postnatal mice (E15-P26). We found a transient ADF expression in immature hair cells of the organ of Corti, the utricle, and the saccule. Interestingly, the stereocilia were not labeled. By P26, ADF expression was restricted to supporting cells. In addition, we localized ADF in presynaptic terminals of medio-olivocochlear projections after hearing onset. A small population of spiral ganglion neurons strongly expressed ADF. Based on their relative number, peripheral location within the ganglion, smaller soma size, and coexpression of neurofilament 200, we identified these cells as Type II spiral ganglion neurons. The developmentally regulated ADF expression suggests a temporally restricted function in the stereocilia and, thus, a hitherto undescribed role of ADF.

Mapping of Wnt, frizzled, and Wnt inhibitor gene expression domains in the avian otic primordium

Wnt signaling activates at least three different pathways involved in development and disease. Interactions of secreted ligands and inhibitors with cell-surface receptors result in the activation or regulation of particular downstream intracellular cascades. During the developmental stages of otic vesicle closure and beginning morphogenesis, the forming inner ear transcribes a plethora of Wnt-related genes. We report expression of 23 genes out of 25 tested in situ hybridization probes on tissue serial sections. Sensory primordia and Frizzled gene expression share domains, with Fzd1 being a continuous marker. Prospective nonsensory domains express Wnts, whose transcripts mainly flank prosensory regions. Finally, Wnt inhibitor domains are superimposed over both prosensory and nonsensory otic regions. Three Wnt antagonists, Dkk1, SFRP2, and Frzb are prominent. Their gene expression patterns partly overlap and change over time, which adds to the diversity of molecular microenvironments. Strikingly, prosensory domains express Wnts transiently. This includes: 1) the prosensory otic region of high proliferation, neuroblast delamination, and programmed cell death at stage 20/21 (Wnt3, -5b, -7b, -8b, -9a, and -11); and 2) sensory primordia at stage 25 (Wnt7a and Wnt9a). In summary, robust Wnt-related gene expression shows both spatial and temporal tuning during inner ear development as the otic vesicle initiates morphogenesis and prosensory cell fate determination.

Left-right asymmetry in the chick embryo requires core planar cell polarity protein Vangl2

Consistent left-right patterning is a fascinating and biomedically important problem. In the chick embryo, it is not known how cells determine their position (left or right) relative to the primitive streak, which is required for subsequent asymmetric gene expression cascades. We show that the subcellular localization of Vangl2, a core planar cell polarity (PCP) protein, is consistently polarized, giving cells in the blastoderm a vector pointing toward the primitive streak. Moreover, morpholino-mediated loss-of-function of Vangl2 by electroporation into chicks at very early stages randomizes the normally left-sided expression of Sonic hedgehog. Strikingly, Vangl2 morpholinos also induce a desynchronization of asymmetric gene expression within the left and right domains of Hensen's node. These data reveal the existence of polarized planar cell polarity protein localization in gastrulating chick and demonstrate that the PCP pathway is functionally required for normal asymmetry in the chick upstream of Sonic hedgehog. These data suggest a new and widely applicable class of models for the spread and coordination of left-right patterning information in the embryonic blastoderm.

Regulation of planar cell polarity by Smurf ubiquitin ligases

Planar cell polarity (PCP) is critical for morphogenesis in metazoans. PCP in vertebrates regulates stereocilia alignment in neurosensory cells of the cochlea and closure of the neural tube through convergence and extension movements (CE). Noncanonical Wnt morphogens regulate PCP and CE in vertebrates, but the molecular mechanisms remain unclear. Smurfs are ubiquitin ligases that regulate signaling, cell polarity and motility through spatiotemporally restricted ubiquitination of diverse substrates. Here, we report an unexpected role for Smurfs in controlling PCP and CE. Mice mutant for Smurf1 and Smurf2 display PCP defects in the cochlea and CE defects that include a failure to close the neural tube. Further, we show that Smurfs engage in a noncanonical Wnt signaling pathway that targets the core PCP protein Prickle1 for ubiquitin-mediated degradation. Our work thus uncovers ubiquitin ligases in a mechanistic link between noncanonical Wnt signaling and PCP/CE.

The irre cell recognition module (IRM) proteins

One of the most challenging problems in developmental neurosciences is to understand the establishment and maintenance of specific membrane contacts between axonal, dendritic, and glial processes in the neuropils, which eventually secure neuronal connectivity. However, underlying cell recognition events are pivotal in other tissues as well. This brief review focuses on the pleiotropic functions of a small, evolutionarily conserved group of proteins of the immunoglobulin superfamily involved in cell recognition. In Drosophila, this protein family comprises Irregular chiasm C/Roughest (IrreC/Rst), Kin of irre (Kirre), and their interacting protein partners, Sticks and stones (SNS) and Hibris (Hbs). For simplicity, we propose to name this ensemble of proteins the irre cell recognition module (IRM) after the first identified member of this family. Here, we summarize evidence that the IRM proteins function together in various cellular interactions, including myoblast fusion, cell sorting, axonal pathfinding, and target recognition in the optic neuropils of Drosophila. Understanding IRM protein function will help to unravel the epigenetic rules by which the intricate neurite networks in sensory neuropils are formed.

Perspectives and open problems in the early phases of left-right patterning

Embryonic left-right (LR) patterning is a fascinating aspect of embryogenesis. The field currently faces important questions about the origin of LR asymmetry, the mechanisms by which consistent asymmetry is imposed on the scale of the whole embryo, and the degree of conservation of early phases of LR patterning among model systems. Recent progress on planar cell polarity and cellular asymmetry in a variety of tissues and species provides a new perspective on the early phases of LR patterning. Despite the huge diversity in body-plans over which consistent LR asymmetry is imposed, and the apparent divergence in molecular pathways that underlie laterality, the data reveal conservation of physiological modules among phyla and a basic scheme of cellular chirality amplified by a planar cell polarity-like pathway over large cell fields.

Comprehensive Wnt-related gene expression during cochlear duct development in chicken

The avian cochlear duct houses both a vestibular and auditory sensory organ (the lagena macula and basilar papilla, respectively), which each have a distinct structure and function. Comparative mRNA in situ hybridization mapping conducted over the time course of chicken cochlear duct development reveals that Wnt-related gene expression is concomitant with various developmental processes such as regionalization, convergent extension of the cochlear duct, cell fate specification, synaptogenesis, and the establishment of planar cell polarity. Wnts mostly originate from nonsensory tissue domains, whereas the sensory primordia preferentially transcribe Frizzled receptors, suggesting that paracrine Wnt signaling predominates in the cochlear duct. Superimposed over this is the strong expression of two secreted Frizzled-related Wnt inhibitors that tend to show complementary expression patterns. Frzb (SFRP3) is confined to the nonsensory cochlear duct and the lagena macula, whereas SFRP2 is maintained in the basilar papilla along with Fzd10 and Wnt7b. Flanking the basilar papilla are Wnt7a, Wnt9a, Wnt11, and SFRP2 on the neural side and Wnt5a, Wnt5b, and Wnt7a on the abneural side. The lateral nonsensory cochlear duct continuously expresses Frzb and temporarily expresses Wnt6 and SFRP1. Characteristic for the entire lagena is the expression of Frzb; in the lagena macula are Fzd1, Fzd7, and Wnt7b, and in the nonsensory tissues are Wnt4 and Wnt5a. Auditory hair cells preferentially express Fzd2 and Fzd9, whereas the main receptors expressed in vestibular hair cells are Fzd1 and Fzd7, in addition to Fzd2 and Fzd9.

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

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

Mechanism of murine epidermal maintenance: cell division and the voter model

The dynamics of a genetically labeled cell population may be used to infer the laws of cell division in mammalian tissue. Recently, we showed that in mouse tail skin, where proliferating cells are confined to a two-dimensional layer, cells proliferate and differentiate according to a simple stochastic model of cell division involving just one type of proliferating cell that may divide both symmetrically and asymmetrically. Curiously, these simple rules provide excellent predictions of the cell population dynamics without having to address the cells' spatial distribution. Yet, if the spatial behavior of cells is addressed by allowing cells to diffuse at random, one deduces that density fluctuations destroy tissue confluence, implying some hidden degree of spatial regulation of cell division. To infer the mechanism of spatial regulation, we consider a two-dimensional model of cell fate that preserves the overall population dynamics. By identifying the resulting behavior with a three-species variation of the voter model, we predict that proliferating cells in the basal layer should cluster. Analysis of empirical correlations of cells stained for proliferation activity confirms that the expected clustering behavior is indeed seen in nature. As well as explaining how cells maintain a uniform two-dimensional density, these findings present an interesting experimental example of voter-model statistics in biology.

Primary cilia in planar cell polarity regulation of the inner ear

Primary cilia are essential components of diverse cellular processes. Many of the requirements can be linked to the apparent signaling function of primary cilia. Recent studies have also uncovered a role for primary cilia in planar cell polarity (PCP) signaling. PCP refers to the coordinated orientation of cells along an axis parallel to the plane of the cell sheet. In vertebrates, the inner ear sensory organs display distinctive forms of PCP. One of the inner ear PCP characteristics is the coordinated positioning of a primary cilium eccentrically in every sensory hair cell within each organ. The inner ear, therefore, provides an opportunity to explore the cellular role of primary cilia in PCP signaling. In this chapter, we will introduce the PCP of the inner ear sensory organs, describe the conserved mechanism underlying the establishment of the planar polarity axis in invertebrates and vertebrates, and highlight a unique requirement for primary cilia in PCP regulation in vertebrates. Additionally, we will discuss a potentially ubiquitous role for cilia in cellular polarization in general.

Planar cell polarity: one or two pathways?

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

Tissue/planar cell polarity in vertebrates: new insights and new questions

This review focuses on the tissue/planar cell polarity (PCP) pathway and its role in generating spatial patterns in vertebrates. Current evidence suggests that PCP integrates both global and local signals to orient diverse structures with respect to the body axes. Interestingly, the system acts on both subcellular structures, such as hair bundles in auditory and vestibular sensory neurons, and multicellular structures, such as hair follicles. Recent work has shown that intriguing connections exist between the PCP-based orienting system and left-right asymmetry, as well as between the oriented cell movements required for neural tube closure and tubulogenesis. Studies in mice, frogs and zebrafish have revealed that similarities, as well as differences, exist between PCP in Drosophila and vertebrates.

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

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

Order from disorder: Self-organization in mammalian hair patterning

Hairs, feathers, and scales normally exhibit precise orientations with respect to the body axes. In Frizzled6 (Fz6)(-/-) mice, the global orientation of hair follicles is disrupted, leading to waves, whorls, and tufts, each comprising many hundreds of hairs. By analyzing the orientation of developing hair follicles, we observed that the nearly parallel arrangement of wild-type (WT) hairs arises from fields of imperfectly aligned follicles, and that the Fz6(-/-) hair patterns arise from fields of grossly misoriented or randomly oriented follicles. Despite their large size, both mutant and WT hair follicles display a remarkable and unexpected plasticity, reorienting on a time scale of days in what seems to be a self-organized refinement process. The essential features of this process can be studied with a simple cellular automata model in which a local consensus "rule" acts iteratively to bias each hair's orientation in favor of the average orientation of its neighbors. These experiments define two systems for hair orientation: a global orienting system that acts early in development and is Fz6-dependent, and a local self-organizing system that acts later and is Fz6 independent.

A two-step mechanism underlies the planar polarization of regenerating sensory hair cells

The restoration of planar cell polarity is an essential but poorly understood step toward physiological recovery during sensory-organ regeneration. Investigating this issue in the lateral line of the zebrafish, we found that hair cells regenerate in pairs along a single axis established by the restricted localization and oriented division of their progenitors. By analyzing mutants lacking the planar-polarity determinant Vangl2, we ascertained that the uniaxial production of hair cells and the subsequent orientation of their hair bundles are controlled by distinct pathways, whose combination underlies the establishment of hair-cell orientation during development and regeneration. This mechanism may represent a general principle governing the long-term maintenance of planar cell polarity in remodeling epithelia.

Planar cell polarity, ciliogenesis and neural tube defects

Cilia are microtubule-based protrusions that are found on the surface of most vertebrate cells. Long studied by cell biologists, these organelles have recently caught the attention of developmental biologists and human geneticists. In this review, I will discuss recent findings suggesting a link between cilia and the planar cell polarity signaling cascade. In particular, I will focus on how this interaction may influence the process of neural tube closure and how these results may be relevant to our understanding of common human birth defects in which neural tube closure is compromised.

Two separate molecular systems, Dachsous/Fat and Starry night/Frizzled, act independently to confer planar cell polarity

Planar polarity is a fundamental property of epithelia in animals and plants. In Drosophila it depends on at least two sets of genes: one set, the Ds system, encodes the cadherins Dachsous (Ds) and Fat (Ft), as well as the Golgi protein Four-jointed. The other set, the Stan system, encodes Starry night (Stan or Flamingo) and Frizzled. The prevailing view is that the Ds system acts via the Stan system to orient cells. However, using the Drosophila abdomen, we find instead that the two systems operate independently: each confers and propagates polarity, and can do so in the absence of the other. We ask how the Ds system acts; we find that either Ds or Ft is required in cells that send information and we show that both Ds and Ft are required in the responding cells. We consider how polarity may be propagated by Ds-Ft heterodimers acting as bridges between cells.

Hair cell development: commitment through differentiation

The perceptions of sound, balance and acceleration are mediated through the vibration of stereociliary bundles located on the lumenal surfaces of mechanosensory hair cells located within the inner ear. In mammals, virtually all hair cells are generated during a relatively brief period in embryogenesis with any subsequent hair cell loss leading to a progressive and permanent loss of sensitivity. In light of the importance of these cells, considerable effort has been focused on understanding the molecular genetic pathways that regulate their development. The results of these studies have begun to elucidate the signaling molecules that regulate several key events in hair cell development. In particular, significant progress has been made in the understanding of hair cell commitment, survival and differentiation. In addition, several aspects of the development of the stereociliary bundle, including its elongation and orientation, have recently been examined. This review will summarize results from each of these developmental events and describe the molecular signaling pathways involved.

The role of Frizzled3 and Frizzled6 in neural tube closure and in the planar polarity of inner-ear sensory hair cells

In the mouse, Frizzled3 (Fz3) and Frizzled6 (Fz6) have been shown previously to control axonal growth and guidance in the CNS and hair patterning in the skin, respectively. Here, we report that Fz3 and Fz6 redundantly control neural tube closure and the planar orientation of hair bundles on a subset of auditory and vestibular sensory cells. In the inner ear, Fz3 and Fz6 proteins are localized to the lateral faces of sensory and supporting cells in all sensory epithelia in a pattern that correlates with the axis of planar polarity. Interestingly, the polarity of Fz6 localization with respect to the asymmetric position of the kinocilium is reversed between vestibular hair cells in the cristae of the semicircular canals and auditory hair cells in the organ of Corti. Vangl2, one of two mammalian homologs of the Drosophila planar cell polarity (PCP) gene van Gogh/Strabismus, is also required for correct hair bundle orientation on a subset of auditory sensory cells and on all vestibular sensory cells. In the inner ear of a Vangl2 mutant (Looptail; Lp), Fz3 and Fz6 proteins accumulate to normal levels but do not localize correctly at the cell surface. These results support the view that vertebrates and invertebrates use similar molecular mechanisms to control a wide variety of PCP-dependent developmental processes. This study also establishes the vestibular sensory epithelium as a tractable tissue for analyzing PCP, and it introduces the use of genetic mosaics for determining the absolute orientation of PCP proteins in mammals.

Dishevelled genes mediate a conserved mammalian PCP pathway to regulate convergent extension during neurulation

The planar cell polarity (PCP) pathway is conserved throughout evolution, but it mediates distinct developmental processes. In Drosophila, members of the PCP pathway localize in a polarized fashion to specify the cellular polarity within the plane of the epithelium, perpendicular to the apicobasal axis of the cell. In Xenopus and zebrafish, several homologs of the components of the fly PCP pathway control convergent extension. We have shown previously that mammalian PCP homologs regulate both cell polarity and polarized extension in the cochlea in the mouse. Here we show, using mice with null mutations in two mammalian Dishevelled homologs, Dvl1 and Dvl2, that during neurulation a homologous mammalian PCP pathway regulates concomitant lengthening and narrowing of the neural plate, a morphogenetic process defined as convergent extension. Dvl2 genetically interacts with Loop-tail, a point mutation in the mammalian PCP gene Vangl2, during neurulation. By generating Dvl2 BAC (bacterial artificial chromosome) transgenes and introducing different domain deletions and a point mutation identical to the dsh1 allele in fly, we further demonstrated a high degree of conservation between Dvl function in mammalian convergent extension and the PCP pathway in fly. In the neuroepithelium of neurulating embryos, Dvl2 shows DEP domain-dependent membrane localization, a pre-requisite for its involvement in convergent extension. Intriguing, the Loop-tail mutation that disrupts both convergent extension in the neuroepithelium and PCP in the cochlea does not disrupt Dvl2 membrane distribution in the neuroepithelium, in contrast to its drastic effect on Dvl2 localization in the cochlea. These results are discussed in light of recent models on PCP and convergent extension.

Expression of mouse dchs1, fjx1, and fat-j suggests conservation of the planar cell polarity pathway identified in Drosophila

The dachsous (ds), fat (ft), and four-jointed (fj) genes have been identified in Drosophila as part of a signaling pathway that regulates planar cell polarity (PCP). A homologous PCP signaling pathway has also been identified in vertebrates, but nothing is known thus far about the conservation of Ds/Ft/Fj signaling. Here we analyzed and compared for the first time the expression patterns of all ds, ft and fj homologs in the mouse. During embryogenesis, expression analysis was performed by RNA in situ hybridization and in adult organs by real time PCR. As in Drosophila, we detected a complementary expression of fjx1 and dchs1 in organs like kidney, lung, and intestine. The ubiquitous expression of ft in several tissues in Drosophila appears to be split into an epithelial expression of fat1/fat3 and a mesenchymal expression of fat-j. These data are compatible with a conservation and sub-functionalization of the Drosophila Ds, Fj, and Fat signaling in higher vertebrates.

Cilia and disease

Cilia are classified according to their microtubule components as 9+2 (motile) and 9+0 (primary) cilia. Disruption of 9+2 cilia, which move mucus across respiratory epithelia, leads to rhinitis, sinusitis and bronchiectasis. Approximately half of the patients with primary ciliary dyskinesia (PCD) have situs inversus, providing a link between left-right asymmetry and cilia. 9+0 cilia at the embryonic node are also motile and involved in establishing left-right asymmetry. Most 9+0 cilia, however, act as antennae, sensing the external environment. Defective 9+0 cilia of principal cells of the nephron cause cystic diseases of the kidney. In the rods and cones of the retina, photoreceptor discs and visual pigments are synthesized in the inner segment and transported to the distal outer segment through a narrow 9+0 connecting cilium; defects in this process lead to retinitis pigmentosa. Although the function of primary cilia in some organs is being elucidated, in many other organs they have not been studied at all. It is probable that many more cilia-related disorders remain to be discovered.

Regulation of polarized extension and planar cell polarity in the cochlea by the vertebrate PCP pathway

The mammalian auditory sensory organ, the organ of Corti, consists of sensory hair cells with uniformly oriented stereocilia on the apical surfaces and has a distinct planar cell polarity (PCP) parallel to the sensory epithelium. It is not certain how this polarity is achieved during differentiation. Here we show that the organ of Corti is formed from a thicker and shorter postmitotic primordium through unidirectional extension, characteristic of cellular intercalation known as convergent extension. Mutations in the PCP pathway interfere with this extension, resulting a shorter and wider cochlea as well as misorientation of stereocilia. Furthermore, parallel to the homologous pathway in Drosophila melanogaster, a mammalian PCP component Dishevelled2 shows PCP-dependent polarized subcellular localization across the organ of Corti. Taken together, these data suggest that there is a conserved molecular mechanism for PCP pathways in invertebrates and vertebrates and indicate that the mammalian PCP pathway might directly couple cellular intercalations to PCP establishment in the cochlea.

Planar polarity of hair cells in the chick inner ear is correlated with polarized distribution of c-flamingo-1 protein

Hair cells of the vertebrate inner ear are directional mechanosensors: they have a polarity, defined by a vector in the plane of the sensory epithelium. It has been suggested that this polarity might be controlled by genes homologous to those that control planar cell polarity (PCP) in Drosophila, and vertebrate homologues of the Drosophila PCP genes Van Gogh/strabismus and flamingo/starry night are indeed essential for normal hair cell PCP. The underlying molecular mechanism is unclear, however. Although the PCP protein Flamingo shows a polarized intracellular distribution in the fly, it is unknown whether this is necessary for its function. Here, we describe the expression pattern of a flamingo homologue, c-flamingo-1 (c-fmi-1), in the developing chick ear and show that its protein product, like that of flamingo in the fly, has a polarized distribution in each hair cell, defining an axis that corresponds to the structural PCP axis. This conservation between fly and vertebrate suggests that the polarized protein localization is functionally important. In the basilar papilla, the same localization is seen in supporting cells also, suggesting that supporting cells are cryptically polarized, despite having no overt structural polarity; they may thus participate in PCP signal transmission across the sensory patch.

Directional cell migration establishes the axes of planar polarity in the posterior lateral-line organ of the zebrafish

The proper orientation of mechanosensory hair cells along the lateral-line organ of a fish or amphibian is essential for the animal's ability to sense directional water movements. Within the sensory epithelium, hair cells are polarized in a stereotyped manner, but the mechanisms that control their alignment relative to the body axes are unknown. We have found, however, that neuromasts can be oriented either parallel or perpendicular to the anteroposterior body axis. By characterizing the strauss mutant zebrafish line and by tracking labeled cells, we have demonstrated that neuromasts of these two orientations originate from, respectively, the first and second primordia. Furthermore, altering the migratory pathway of a primordium reorients a neuromast's axis of planar polarity. We propose that the global orientation of hair cells relative to the body axes is established through an interaction between directional movement by primordial cells and the timing of neuromast maturation.

Cell interactions and planar polarity in the abdominal epidermis ofDrosophila

The integument of the Drosophila adult abdomen bears oriented hairs and bristles that indicate the planar polarity of the epidermal cells. We study four polarity genes, frizzled (fz), prickle (pk), Van gogh/strabismus(Vang/stbm) and starry night/flamingo (stan/fmi),and note what happens when these genes are either removed or overexpressed in clones of cells. The edges of the clones are interfaces between cells that carry different amounts of gene products, interfaces that can cause reversals of planar polarity in the clone and wild-type cells outside them. To explain,we present a model that builds on our earlier picture of a gradient of X, the vector of which specifies planar polarity and depends on two cadherin proteins, Dachsous and Fat. We conjecture that the X gradient is read out,cell by cell, as a scalar value of Fz activity, and that Pk acts in this process, possibly to determine the sign of the Fz activity gradient.

We discuss evidence that cells can compare their scalar readout of the level of X with that of their neighbours and can set their own readout towards an average of those. This averaging, when it occurs near the edges of clones,changes the scalar response of cells inside and outside the clones, leading to new vectors that change polarity. The results argue that Stan must be present in both cells being compared and acts as a conduit between them for the transfer of information. And also that Vang assists in the receipt of this information. The comparison between neighbours is crucial, because it gives the vector that orients hairs – these point towards the neighbour cell that has the lowest level of Fz activity.

Recently, it has been shown that, for a limited period shortly before hair outgrowth in the wing, the four proteins we study, as well as others, become asymmetrically localised in the cell membrane, and this process is thought to be instrumental in the acquisition of cell polarity. However, some results do not fit with this view – we suggest that these localisations may be more a consequence than a cause of planar polarity.

Wnt-3a and Dvl induce neurite retraction by activating Rho-associated kinase

Dvl is a key protein that transmits the Wnt signal to the canonical beta-catenin pathway and the noncanonical planar cell polarity (PCP) pathway. We studied the roles of Rho-associated kinase (Rho-kinase), which is activated by Dvl in the PCP pathway of mammalian cells. The expression of Dvl-1, Wnt-1, or Wnt-3a activated Rho-kinase in COS cells, and this activation was inhibited by the Rho-binding domain of Rho-kinase. The expression of Dvl-1 in PC12 cells activated Rho and inhibited nerve growth factor (NGF)-induced neurite outgrowth. This inhibition was reversed by a Rho-kinase inhibitor but not by a c-Jun N-terminal kinase inhibitor. Dvl-1 also inhibited serum starvation-dependent neurite outgrowth of N1E-115 cells, and expression of the Rho-binding domain of Rho-kinase reversed this inhibitory activity of Dvl-1. Dvl-1 mutants that did not activate Rho-kinase did not inhibit the neurite outgrowth of N1E-115 cells. Furthermore, the purified Wnt-3a protein activated Rho-kinase and inhibited the NGF-dependent neurite outgrowth of PC12 cells. Wnt-3a-dependent neurite retraction was also prevented by a Rho-kinase inhibitor and a Dvl-1 mutant that suppresses Wnt-3a-dependent activation of Rho-kinase. These results suggest that Wnt-3a and Dvl regulate neurite formation through Rho-kinase and that PC12 and N1E-115 cells are useful for analyzing the PCP pathway.

Strategies for replacing lost cochlear hair cells

The auditory sensory epithelium is a mosaic composed of sensory (hair) cells and several types of non-sensory (supporting) cells. All these cells are highly differentiated in their structure and function. Mosaic epithelia (and other complex tissues) are generally formed by differentiation of distinct and specialized cell types from common progenitors. Most types of epithelial tissues maintain a population of undifferentiated (basal) cells which facilitate turnover (renewal) and repair, but this is not the case for the organ of Corti in the cochlea. Therefore, when cochlear hair cells are lost they cannot be replaced. Consequently, sensorineural hearing loss is permanent. In designing therapy for sensorineural deafness, the most important task is to find a way to generate new cochlear hair cells to replace lost cells.

Frizzled signalling and cell polarisation in Drosophila and vertebrates

A key aspect of animal development is the appropriate polarisation of different cell types in the right place at the right time. Such polarisation is often precisely coordinated relative to the axes of a tissue or organ, but the mechanisms underlying this coordination are still poorly understood. Nevertheless, genetic analysis of animal development has revealed some of the pathways involved. For example, a non-canonical Frizzled signalling pathway has been found to coordinate cell polarity throughout the insect cuticle, and recent work has implicated an analogous pathway in coordinated polarisation of cells during vertebrate development. This review discusses recent findings regarding non-canonical Frizzled signalling and cell polarisation.

Forced activation of Wnt signaling alters morphogenesis and sensory organ identity in the chicken inner ear

Components of the Wnt signaling pathway are expressed in the developing inner ear. To explore their role in ear patterning, we used retroviral gene transfer to force the expression of an activated form of beta-catenin that should constitutively activate targets of the canonical Wnt signaling pathway. At embryonic day 9 (E9) and beyond, morphological defects were apparent in the otic capsule and the membranous labyrinth, including ectopic and fused sensory patches. Most notably, the basilar papilla, an auditory organ, contained infected sensory patches with a vestibular phenotype. Vestibular identity was based on: (1) stereociliary bundle morphology; (2) spacing of hair cells and supporting cells; (3) the presence of otoliths; (4) immunolabeling indicative of vestibular supporting cells; and (5) expression of Msx1, a marker of certain vestibular sensory organs. Retrovirus-mediated misexpression of Wnt3a also gave rise to ectopic vestibular patches in the cochlear duct. In situ hybridization revealed that genes for three Frizzled receptors, c-Fz1, c-Fz7, and c-Fz10, are expressed in and adjacent to sensory primordia, while Wnt4 is expressed in adjacent, nonsensory regions of the cochlear duct. We hypothesize that Wnt/beta-catenin signaling specifies otic epithelium as macular and helps to define and maintain sensory/nonsensory boundaries in the cochlear duct.

Ringing in the new ear: resolution of cell interactions in otic development

The vertebrate inner ear is a marvel of structural and functional complexity, which is all the more remarkable because it develops from such a simple structure, the otic placode. Analysis of inner ear development has long been a fascination of experimental embryologists, who sought to understand cellular mechanisms of otic placode induction. More recently, however, molecular and genetic approaches have made the inner ear a useful model system for studying a much broader range of basic developmental mechanisms, including cell fate specification and differentiation, axial patterning, epithelial morphogenesis, cytoskeletal dynamics, stem cell biology, neurobiology, physiology, etc. Of course, there has also been tremendous progress in understanding the functions and processes peculiar to the inner ear. The goal of this review is to recount how historical approaches have shaped our understanding of the signaling interactions controlling early otic development; to discuss how new findings have led to fundamental new insights; and to point out new problems that need to be resolved in future research.

Prickle and Strabismus form a functional complex to generate a correct axis during planar cell polarity signaling

Frizzled (Fz) signaling regulates the establishment of planar cell polarity (PCP). The PCP genes prickle (pk) and strabismus (stbm) are thought to antagonize Fz signaling. We show that they act in the same cell, R4, adjacent to that in which the Fz/PCP pathway is required in the Drosophila eye. We demonstrate that Stbm and Pk interact physically and that Stbm recruits Pk to the cell membrane. Through this interaction, Pk affects Stbm membrane localization and can cause clustering of Stbm. Pk is also known to interact with Dsh and is thought to antagonize Dsh by affecting its membrane localization. Thus our data suggest that the Stbm/Pk complex modulates Fz/Dsh activity, resulting in a symmetry-breaking step during polarity signaling.

Mutation of Celsr1 disrupts planar polarity of inner ear hair cells and causes severe neural tube defects in the mouse

We identified two novel mouse mutants with abnormal head-shaking behavior and neural tube defects during the course of independent ENU mutagenesis experiments. The heterozygous and homozygous mutants exhibit defects in the orientation of sensory hair cells in the organ of Corti, indicating a defect in planar cell polarity. The homozygous mutants exhibit severe neural tube defects as a result of failure to initiate neural tube closure. We show that these mutants, spin cycle and crash, carry independent missense mutations within the coding region of Celsr1, encoding a large protocadherin molecule [1]. Celsr1 is one of three mammalian homologs of Drosophila flamingo/starry night, which is essential for the planar cell polarity pathway in Drosophila together with frizzled, dishevelled, prickle, strabismus/van gogh, and rhoA. The identification of mouse mutants of Celsr1 provides the first evidence for the function of the Celsr family in planar cell polarity in mammals and further supports the involvement of a planar cell polarity pathway in vertebrate neurulation.