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

Junctional force patterning drives both positional order and planar polarity in the auditory epithelia

Tissue function depends on the precise organisation of the constituent cells. In the cochlea, the fidelity of hearing depends on mechanosensory hair cells being consistently surrounded by supporting cells. In addition to this positional order, auditory sensitivity depends crucially on planar cell polarity. This is characterised by the alignment of the orientation of eccentrically placed hair bundles on each hair cell. These two levels of order emerge simultaneously despite the cellular fluxes that occur during cochlear development. However, the link between tissue-scale cellular rearrangements and intrinsic cellular mechanisms remains unknown. By combining experimental and theoretical approaches, we find a precise force patterning underpinning positional order and planar cell polarity. This occurs through the modulation of the levels and phospho-type of the regulatory light chain of non-muscle myosin II at specific cell-cell junctions of the auditory epithelium. We propose that the control of junctional mechanics is vital for the organisation of multi-cell-type epithelia.

Non-canonical Wnt pathway expression in the developing mouse and human retina

The non-canonical Wnt pathway is an evolutionarily conserved pathway essential for tissue patterning and development across species and tissues. In mammals, this pathway plays a role in neuronal migration, dendritogenesis, axon growth, and synapse formation. However, its role in development and synaptogenesis of the human retina remains less established. In order to address this knowledge gap, we analyzed publicly available single-cell RNA sequencing (scRNAseq) datasets for mouse retina, human retina, and human retinal organoids over multiple developmental time points during outer retinal maturation. We identified ligands, receptors, and mediator genes with a putative role in retinal development, including those with novel or species-specific expression, and validated this expression using fluorescence in situ hybridization (FISH). By quantifying outer nuclear layer (ONL) versus inner nuclear layer (INL) expression, we provide evidence for the differential expression of certain non-canonical Wnt signaling components in the developing mouse and human retina during outer plexiform layer (OPL) development. Importantly, we identified distinct expression patterns of mouse and human FZD3 and WNT10A, as well as previously undescribed expression, such as for mouse Wnt2b in Chat+ starburst amacrine cells. Human retinal organoids largely recapitulated the human non-canonical Wnt pathway expression. Together, this work provides the basis for further study of non-canonical Wnt signaling in mouse and human retinal development and synaptogenesis.

Emx2 lineage tracing reveals antecedent patterns of planar polarity in the mouse inner ear

The planar polarized organization of hair cells in the vestibular maculae is unique because these sensory organs contain two groups of cells with oppositely oriented stereociliary bundles that meet at a line of polarity reversal (LPR). EMX2 is a transcription factor expressed by one hair cell group that reverses the orientation of their bundles, thereby forming the LPR. We generated Emx2-CreERt2 transgenic mice for genetic lineage tracing and demonstrate Emx2 expression before hair cell specification when the nascent utricle and saccule constitute a continuous prosensory domain. Precursors labeled by Emx2-CreERt2 at this stage give rise to hair cells located along one side of the LPR in the mature utricle or saccule, indicating that this boundary is first established in the prosensory domain. Consistent with this, Emx2-CreERt2 lineage tracing in Dreher mutants, where the utricle and saccule fail to segregate, labels a continuous field of cells along one side of a fused utriculo-saccular-cochlear organ. These observations reveal that LPR positioning is pre-determined in the developing prosensory domain, and that EMX2 expression defines lineages of hair cells with oppositely oriented stereociliary bundles.

Diversity matters - extending sound intensity coding by inner hair cells via heterogeneous synapses

Our sense of hearing enables the processing of stimuli that differ in sound pressure by more than six orders of magnitude. How to process a wide range of stimulus intensities with temporal precision is an enigmatic phenomenon of the auditory system. Downstream of dynamic range compression by active cochlear micromechanics, the inner hair cells (IHCs) cover the full intensity range of sound input. Yet, the firing rate in each of their postsynaptic spiral ganglion neurons (SGNs) encodes only a fraction of it. As a population, spiral ganglion neurons with their respective individual coding fractions cover the entire audible range. How such "dynamic range fractionation" arises is a topic of current research and the focus of this review. Here, we discuss mechanisms for generating the diverse functional properties of SGNs and formulate testable hypotheses. We postulate that an interplay of synaptic heterogeneity, molecularly distinct subtypes of SGNs, and efferent modulation serves the neural decomposition of sound information and thus contributes to a population code for sound intensity.

The dark kinase STK32A regulates hair cell planar polarity opposite of EMX2 in the developing mouse inner ear

The vestibular maculae of the inner ear contain sensory receptor hair cells that detect linear acceleration and contribute to equilibrioception to coordinate posture and ambulatory movements. These hair cells are divided between two groups, separated by a line of polarity reversal (LPR), with oppositely oriented planar-polarized stereociliary bundles that detect motion in opposite directions. The transcription factor EMX2 is known to establish this planar polarized organization in mouse by regulating the distribution of the transmembrane receptor GPR156 at hair cell boundaries in one group of cells. However, the genes regulated by EMX2 in this context were previously not known. Using mouse as a model, we have identified the serine threonine kinase STK32A as a downstream effector negatively regulated by EMX2. Stk32a is expressed in hair cells on one side of the LPR in a pattern complementary to Emx2 expression in hair cells on the opposite side. Stk32a is necessary to align the intrinsic polarity of the bundle with the core planar cell polarity (PCP) proteins in EMX2-negative regions, and is sufficient to reorient bundles when ectopically expressed in neighboring EMX2-positive regions. We demonstrate that STK32A reinforces LPR formation by regulating the apical localization of GPR156. These observations support a model in which bundle orientation is determined through separate mechanisms in hair cells on opposite sides of the maculae, with EMX2-mediated repression of Stk32a determining the final position of the LPR.

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.

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

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

Canonical Wnt Signaling Pathway on Polarity Formation of Utricle Hair Cells

As part of the inner ear, the vestibular system is responsible for sense of balance, which consists of three semicircular canals, the utricle, and the saccule. Increasing evidence has indicated that the noncanonical Wnt/PCP signaling pathway plays a significant role in the development of the polarity of the inner ear. However, the role of canonical Wnt signaling in the polarity of the vestibule is still not completely clear. In this study, we found that canonical Wnt pathway-related genes are expressed in the early stage of development of the utricle and change dynamically. We conditionally knocked out β-catenin, a canonical Wnt signaling core protein, and found that the cilia orientation of hair cells was disordered with reduced number of hair cells in the utricle. Moreover, regulating the canonical Wnt pathway (Licl and IWP2) in vitro also affected hair cell polarity and indicated that Axin2 may be important in this process. In conclusion, our results not only confirm that the regulation of canonical Wnt signaling affects the number of hair cells in the utricle but also provide evidence for its role in polarity development.

New insights into regulation and function of planar polarity in the inner ear

Acquisition of cell polarity generates signaling and cytoskeletal asymmetry and thus underpins polarized cell behaviors during tissue morphogenesis. In epithelial tissues, both apical-basal polarity and planar polarity, which refers to cell polarization along an axis orthogonal to the apical-basal axis, are essential for epithelial morphogenesis and function. A prime example of epithelial planar polarity can be found in the auditory sensory epithelium (or organ of Corti, OC). Sensory hair cells, the sound receptors, acquire a planar polarized apical cytoskeleton which is uniformely oriented along an axis orthogonal to the longitudinal axis of the cochlear duct. Both cell-intrinsic and tissue-level planar polarity are necessary for proper perception of sound. Here we review recent insights into the novel roles and mechanisms of planar polarity signaling gained from genetic analysis in mice, focusing mainly on the OC but also with some discussions on the vestibular sensory epithelia.

PCP and Wnt pathway components act in parallel during zebrafish mechanosensory hair cell orientation

Planar cell polarity (PCP) plays crucial roles in developmental processes such as gastrulation, neural tube closure and hearing. Wnt pathway mutants are often classified as PCP mutants due to similarities between their phenotypes. Here, we show that in the zebrafish lateral line, disruptions of the PCP and Wnt pathways have differential effects on hair cell orientations. While mutations in the PCP genes vangl2 and scrib cause random orientations of hair cells, mutations in wnt11f1, gpc4 and fzd7a/b induce hair cells to adopt a concentric pattern. This concentric pattern is not caused by defects in PCP but is due to misaligned support cells. The molecular basis of the support cell defect is unknown but we demonstrate that the PCP and Wnt pathways work in parallel to establish proper hair cell orientation. Consequently, hair cell orientation defects are not solely explained by defects in PCP signaling, and some hair cell phenotypes warrant re-evaluation.

Planar cell polarity (PCP) regulates hair cell orientation in the zebrafish lateral line. Here, the authors show that mutating Wnt pathway genes (wnt11f1, fzd7a/b, and gpc4) causes concentric hair cell patterns not regulated by PCP, thus showing PCP/Wnt pathway genes have different consequences on hair cell orientation.

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

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

Microtubules are required for the maintenance of planar cell polarity in monociliated floorplate cells

The asymmetric localization of planar cell polarity (PCP) proteins is essential for the establishment of many planar polarized cellular processes, but the mechanisms that maintain these asymmetric distributions remain poorly understood. A body of evidence has tied oriented subapical microtubules (MTs) to the establishment of PCP protein polarity, yet recent studies have suggested that the MT cytoskeleton is later dispensable for the maintenance of this asymmetry. As MTs underlie the vesicular trafficking of membrane-bound proteins within cells, the requirement for MTs in the maintenance of PCP merited further investigation. We investigated the complex interactions between PCP proteins and the MT cytoskeleton in the polarized context of the floorplate of the zebrafish neural tube. We demonstrated that the progressive posterior polarization of the primary cilia of floorplate cells requires not only Vangl2 but also Fzd3a. We determined that GFP-Vangl2 asymmetrically localizes to anterior membranes whereas Fzd3a-GFP does not polarize on anterior or posterior membranes but maintains a cytosolic enrichment at the base of the primary cilium. Vesicular Fzd3a-GFP is rapidly trafficked along MTs primarily toward the apical membrane during a period of PCP maintenance, whereas vesicular GFP-Vangl2 is less frequently observed. Nocodazole-induced loss of MT polymerization disrupts basal body positioning as well as GFP-Vangl2 localization and reduces cytosolic Fzd3a-GFP movements. Removal of nocodazole after MT disruption restores MT polymerization but does not restore basal body polarity. Interestingly, GFP-Vangl2 repolarizes to anterior membranes and vesicular Fzd3a-GFP dynamics recover after multiple hours of recovery, even in the context of unpolarized basal bodies. Together our findings challenge previous work by revealing an ongoing role for MT-dependent transport of PCP proteins in maintaining both cellular and PCP protein asymmetry during development.

A non-autonomous function of the core PCP protein VANGL2 directs peripheral axon turning in the developing cochlea

The cochlea is innervated by neurons that relay sound information from hair cells to central auditory targets. A subset of these are the type II spiral ganglion neurons, which have nociceptive features and contribute to feedback circuits providing neuroprotection in extreme noise. Type II neurons make a distinctive 90° turn towards the cochlear base to synapse with 10-15 outer hair cells. We demonstrate that this axon turning event requires planar cell polarity (PCP) signaling and is disrupted in Vangl2 and Celsr1 knockout mice, and that VANGL2 acts non-autonomously from the cochlea to direct turning. Moreover, VANGL2 is asymmetrically distributed at intercellular junctions between cochlear supporting cells, and in a pattern that could allow it to act directly as an axon guidance cue. Together, these data reveal a non-autonomous function for PCP signaling during axon guidance occurring in the tissue that is innervated, rather than the navigating growth cone.