Specializations of intercellular junctions are associated with the presence and absence of hair cell regeneration in ears from six vertebrate classes

Genetic and pharmacologic alterations of claudin9 levels suffice to induce functional and mature inner hair cells

Hearing loss is the most common form of sensory deficit. It occurs predominantly due to hair cell (HC) loss. Mammalian HCs are terminally differentiated by birth, making HC loss challenging to replace. Here, we show the pharmacogenetic downregulation of Cldn9, a tight junction protein, generates robust supernumerary inner HCs (IHCs) in mice. The ectopic IHC shared functional and synaptic features akin to typical IHCs and were surprisingly and remarkably preserved for at least fifteen months >50% of the mouse's life cycle. In vivo, Cldn9 knockdown using shRNA on postnatal days (P) P2-7 yielded analogous functional ectopic IHCs that were equally durably conserved. The findings suggest that Cldn9 levels coordinate embryonic and postnatal HC differentiation, making it a viable target for altering IHC development pre- and post-terminal differentiation.

Comparative biology of the amniote vestibular utricle

The sensory epithelia of the auditory and vestibular systems of vertebrates have shared developmental and evolutionary histories. However, while the auditory epithelia show great variation across vertebrates, the vestibular sensory epithelia appear seemingly more conserved. An exploration of the current knowledge of the comparative biology of the amniote utricle, a vestibular sensory epithelium that senses linear acceleration, shows interesting instances of variability between birds and mammals. The distribution of sensory hair cell types, the position of the line of hair bundle polarity reversal and the properties of supporting cells show marked differences, likely impacting vestibular function and hair cell regeneration potential.

Spherical harmonics analysis reveals cell shape-fate relationships in zebrafish lateral line neuromasts

Cell shape is a powerful readout of cell state, fate and function. We describe a custom workflow to perform semi-automated, 3D cell and nucleus segmentation, and spherical harmonics and principal components analysis to distill cell and nuclear shape variation into discrete biologically meaningful parameters. We apply these methods to analyze shape in the neuromast cells of the zebrafish lateral line system, finding that shapes vary with cell location and identity. The distinction between hair cells and support cells accounted for much of the variation, which allowed us to train classifiers to predict cell identity from shape features. Using transgenic markers for support cell subpopulations, we found that subtypes had different shapes from each other. To investigate how loss of a neuromast cell type altered cell shape distributions, we examined atoh1a mutants that lack hair cells. We found that mutant neuromasts lacked the cell shape phenotype associated with hair cells, but did not exhibit a mutant-specific cell shape. Our results demonstrate the utility of using 3D cell shape features to characterize, compare and classify cells in a living developing organism.

Transcriptomic Analysis Identifies Candidate Genes for Differential Expression during Xenopus laevis Inner Ear Development

Background

The genes involved in inner ear development and maintenance of the adult organ have yet to be fully characterized. Previous genetic analysis has emphasized the early development that gives rise to the otic vesicle. This study aimed to bridge the knowledge gap and identify candidate genes that are expressed as the auditory and vestibular sensory organs continue to grow and develop until the systems reach postmetamorphic maturity.

Methods

Affymetrix microarrays were used to assess inner ear transcriptome profiles from three Xenopus laevis developmental ages where all eight endorgans comprise mechanosensory hair cells: larval stages 50 and 56, and the post-metamorphic juvenile. Pairwise comparisons were made between the three developmental stages and the resulting differentially expressed X. laevis Probe Set IDs (Xl-PSIDs) were assigned to four groups based on differential expression patterns. DAVID analysis was undertaken to impart functional annotation to the differentially regulated Xl-PSIDs.

Results

Analysis identified 1510 candidate genes for differential gene expression in one or more pairwise comparison. Annotated genes not previously associated with inner ear development emerged from this analysis, as well as annotated genes with established inner ear function, such as oncomodulin, neurod1, and sp7. Notably, 36% of differentially expressed Xl-PSIDs were unannotated.

Conclusions

Results draw attention to the complex gene regulatory patterns that characterize Xenopus inner ear development, and underscore the need for improved annotation of the X. laevis genome. Outcomes can be utilized to select candidate inner ear genes for functional analysis, and to promote Xenopus as a model organism for biomedical studies of hearing and balance.

Atomic force microscopy-mediated mechanobiological profiling of complex human tissues

Tissue mechanobiology is an emerging field with the overarching goal of understanding the interplay between biophysical and biochemical responses affecting development, physiology, and disease. Changes in mechanical properties including stiffness and viscosity have been shown to describe how cells and tissues respond to mechanical cues and modify critical biological functions. To quantitatively characterize the mechanical properties of tissues at physiologically relevant conditions, atomic force microscopy (AFM) has emerged as a highly versatile biomechanical technology. In this review, we describe the fundamental principles of AFM, typical AFM modalities used for tissue mechanics, and commonly used elastic and viscoelastic contact mechanics models to characterize complex human tissues. Furthermore, we discuss the application of AFM-based mechanobiology to characterize the mechanical responses within complex human tissues to track their developmental, physiological/functional, and diseased states, including oral, hearing, and cancer-related tissues. Finally, we discuss the current outlook and challenges to further advance the field of tissue mechanobiology. Altogether, AFM-based tissue mechanobiology provides a mechanistic understanding of biological processes governing the unique functions of tissues.

On the value of diverse organisms in auditory research: From fish to flies to humans

Historically, diverse organisms have contributed to our understanding of auditory function. In recent years, the laboratory mouse has become the prevailing non-human model in auditory research, particularly for biomedical studies. There are many questions in auditory research for which the mouse is the most appropriate (or the only) model system available. But mice cannot provide answers for all auditory problems of basic and applied importance, nor can any single model system provide a synthetic understanding of the diverse solutions that have evolved to facilitate effective detection and use of acoustic information. In this review, spurred by trends in funding and publishing and inspired by parallel observations in other domains of neuroscience, we highlight a few examples of the profound impact and lasting benefits of comparative and basic organismal research in the auditory system. We begin with the serendipitous discovery of hair cell regeneration in non-mammalian vertebrates, a finding that has fueled an ongoing search for pathways to hearing restoration in humans. We then turn to the problem of sound source localization - a fundamental task that most auditory systems have been compelled to solve despite large variation in the magnitudes and kinds of spatial acoustic cues available, begetting varied direction-detecting mechanisms. Finally, we consider the power of work in highly specialized organisms to reveal exceptional solutions to sensory problems - and the diverse returns of deep neuroethological inquiry - via the example of echolocating bats. Throughout, we consider how discoveries made possible by comparative and curiosity-driven organismal research have driven fundamental scientific, biomedical, and technological advances in the auditory field.

Rack1 regulates cellular patterning and polarity in the mouse cochlea

Rack1 features seven WD40 repeats that fold into a multifaceted scaffold used to build signaling complexes in a context-dependent manner. Previous in vitro studies have revealed associations between Rack1 and many other proteins. Rack 1 is required for establishing planar cell polarity (PCP) in zebrafish and Xenopus. However, any molecular role of Rack1 in protein complexes or polarity regulation remains unclear. Here, we show that Rack1 is an essential gene in mice. Conditional knockout of Rack1 shortened the cochlear duct and induced cellular patterning defects characteristic of defective convergent extension (this PCP process is mediated by cellular junctional remodeling in the developing cochlear epithelium). Also, cochlear hair cells were no longer uniformly oriented in Rack1 conditional knockout mutants. Rack1 was enriched in the cellular cortices of sensory hair cells. In Rack1-deficient cochleae, E-cadherin expression at the cellular boundaries was greatly reduced. Together, the findings reveal a molecular role of Rack1 in PCP signaling that likely involves modulation of E-cadherin levels at the adherens junctions of the plasma membrane.

The heterogeneity of mammalian utricular cells over the course of development

BACKGROUND

The inner ear organ is a delicate tissue consisting of hair cells (HCs) and supporting cells (SCs).The mammalian inner ear HCs are terminally differentiated cells that cannot spontaneously regenerate in adults. Epithelial non-hair cells (ENHCs) in the utricle include HC progenitors and SCs, and the progenitors share similar characteristics with SCs in the neonatal inner ear.

METHODS

We applied single-cell sequencing to whole mouse utricles from the neonatal period to adulthood, including samples from postnatal day (P)2, P7 and P30 mice. Furthermore, using transgenic mice and immunostaining, we traced the source of new HC generation.

RESULTS

We identified several sensory epithelial cell clusters and further found that new HCs arose mainly through differentiation from Sox9+ progenitor cells and that only a few cells were produced by mitotic proliferation in both neonatal and adult mouse utricles. In addition, we identified the proliferative cells using the marker UbcH10 and demonstrated that in adulthood the mitotically generated HCs were primarily found in the extrastriola. Moreover, we observed that not only Type II, but also Type I HCs could be regenerated by either mitotic cell proliferation or progenitor cell differentiation.

CONCLUSIONS

Overall, our findings expand our understanding of ENHC cell fate and the characteristics of the vestibular organs in mammals over the course of development.

Cochlear Development; New Tools and Approaches

The sensory epithelium of the mammalian cochlea, the organ of Corti, is comprised of at least seven unique cell types including two functionally distinct types of mechanosensory hair cells. All of the cell types within the organ of Corti are believed to develop from a population of precursor cells referred to as prosensory cells. Results from previous studies have begun to identify the developmental processes, lineage restrictions and signaling networks that mediate the specification of many of these cell types, however, the small size of the organ and the limited number of each cell type has hampered progress. Recent technical advances, in particular relating to the ability to capture and characterize gene expression at the single cell level, have opened new avenues for understanding cellular specification in the organ of Corti. This review will cover our current understanding of cellular specification in the cochlea, discuss the most commonly used methods for single cell RNA sequencing and describe how results from a recent study using single cell sequencing provided new insights regarding cellular specification.

The regenerative capacity of neonatal tissues

It is well established that humans and other mammals are minimally regenerative compared with organisms such as zebrafish, salamander or amphibians. In recent years, however, the identification of regenerative potential in neonatal mouse tissues that normally heal poorly in adults has transformed our understanding of regenerative capacity in mammals. In this Review, we survey the mammalian tissues for which regenerative or improved neonatal healing has been established, including the heart, cochlear hair cells, the brain and spinal cord, and dense connective tissues. We also highlight common and/or tissue-specific mechanisms of neonatal regeneration, which involve cells, signaling pathways, extracellular matrix, immune cells and other factors. The identification of such common features across neonatal tissues may direct therapeutic strategies that will be broadly applicable to multiple adult tissues.

Stiffening of Circumferential F-Actin Bands Correlates With Regenerative Failure and May Act as a Biomechanical Brake in the Mammalian Inner Ear

The loss of inner ear hair cells causes permanent hearing and balance deficits in humans and other mammals, but non-mammals recover after supporting cells (SCs) divide and replace hair cells. The proliferative capacity of mammalian SCs declines as exceptionally thick circumferential F-actin bands develop at their adherens junctions. We hypothesized that the reinforced junctions were limiting regenerative responses of mammalian SCs by impeding changes in cell shape and epithelial tension. Using micropipette aspiration and atomic force microscopy, we measured mechanical properties of utricles from mice and chickens. Our data show that the epithelial surface of the mouse utricle stiffens significantly during postnatal maturation. This stiffening correlates with and is dependent on the postnatal accumulation of F-actin and the cross-linker Alpha-Actinin-4 at SC-SC junctions. In chicken utricles, where SCs lack junctional reinforcement, the epithelial surface remains compliant. There, SCs undergo oriented cell divisions and their apical surfaces progressively elongate throughout development, consistent with anisotropic intraepithelial tension. In chicken utricles, inhibition of actomyosin contractility led to drastic SC shape change and epithelial buckling, but neither occurred in mouse utricles. These findings suggest that species differences in the capacity for hair cell regeneration may be attributable in part to the differences in the stiffness and contractility of the actin cytoskeletal elements that reinforce adherens junctions and participate in regulation of the cell cycle.

Vestibular Deficits in Deafness: Clinical Presentation, Animal Modeling, and Treatment Solutions

The inner ear is responsible for both hearing and balance. These functions are dependent on the correct functioning of mechanosensitive hair cells, which convert sound- and motion-induced stimuli into electrical signals conveyed to the brain. During evolution of the inner ear, the major changes occurred in the hearing organ, whereas the structure of the vestibular organs remained constant in all vertebrates over the same period. Vestibular deficits are highly prevalent in humans, due to multiple intersecting causes: genetics, environmental factors, ototoxic drugs, infections and aging. Studies of deafness genes associated with balance deficits and their corresponding animal models have shed light on the development and function of these two sensory systems. Bilateral vestibular deficits often impair individual postural control, gaze stabilization, locomotion and spatial orientation. The resulting dizziness, vertigo, and/or falls (frequent in elderly populations) greatly affect patient quality of life. In the absence of treatment, prosthetic devices, such as vestibular implants, providing information about the direction, amplitude and velocity of body movements, are being developed and have given promising results in animal models and humans. Novel methods and techniques have led to major progress in gene therapies targeting the inner ear (gene supplementation and gene editing), 3D inner ear organoids and reprograming protocols for generating hair cell-like cells. These rapid advances in multiscale approaches covering basic research, clinical diagnostics and therapies are fostering interdisciplinary research to develop personalized treatments for vestibular disorders.

A cell-type-specific atlas of the inner ear transcriptional response to acoustic trauma

Noise-induced hearing loss (NIHL) results from a complex interplay of damage to the sensory cells of the inner ear, dysfunction of its lateral wall, axonal retraction of type 1C spiral ganglion neurons, and activation of the immune response. We use RiboTag and single-cell RNA sequencing to survey the cell-type-specific molecular landscape of the mouse inner ear before and after noise trauma. We identify induction of the transcription factors STAT3 and IRF7 and immune-related genes across all cell-types. Yet, cell-type-specific transcriptomic changes dominate the response. The ATF3/ATF4 stress-response pathway is robustly induced in the type 1A noise-resilient neurons, potassium transport genes are downregulated in the lateral wall, mRNA metabolism genes are downregulated in outer hair cells, and deafness-associated genes are downregulated in most cell types. This transcriptomic resource is available via the Gene Expression Analysis Resource (gEAR; https://umgear.org/NIHL) and provides a blueprint for the rational development of drugs to prevent and treat NIHL.

YAP Mediates Hair Cell Regeneration in Balance Organs of Chickens, But LATS Kinases Suppress Its Activity in Mice

Loss of sensory hair cells causes permanent hearing and balance deficits in humans and other mammals, but for nonmammals such deficits are temporary. Nonmammals recover hearing and balance sensitivity after supporting cells proliferate and differentiate into replacement hair cells. Evidence of mechanical differences between those sensory epithelia and their supporting cells prompted us to investigate whether the capacity to activate YAP, an effector in the mechanosensitive Hippo pathway, correlates with regenerative capacity in acceleration-sensing utricles of chickens and mice of both sexes. After hair cell ablation, YAP accumulated in supporting cell nuclei in chicken utricles and promoted regenerative proliferation, but YAP remained cytoplasmic and little proliferation occurred in mouse utricles. YAP localization in supporting cells was also more sensitive to shape change and inhibition of MST1/2 in chicken utricles than in mouse utricles. Genetic manipulations showed that in vivo expression of the YAP-S127A variant caused robust proliferation of neonatal mouse supporting cells, which produced progeny that expressed hair cell markers, but proliferative responses declined postnatally. Expression of YAP-5SA, which more effectively evades inhibitory phosphorylation, resulted in TEAD-dependent proliferation of striolar supporting cells, even in adult utricles. Conditional deletion of LATS1/2 kinases abolished the inhibitory phosphorylation of endogenous YAP and led to striolar proliferation in adult mouse utricles. The findings suggest that damage overcomes inhibitory Hippo signaling and facilitates regenerative proliferation in nonmammalian utricles, whereas constitutive LATS1/2 kinase activity suppresses YAP-TEAD signaling in mammalian utricles and contributes to maintaining the proliferative quiescence that appears to underlie the permanence of sensory deficits.SIGNIFICANCE STATEMENT Loud sounds, ototoxic drugs, infections, and aging kill sensory hair cells in the ear, causing irreversible hearing loss and balance deficits for millions. In nonmammals, damage evokes shape changes in supporting cells, which can divide and regenerate hair cells. Such shape changes are limited in mammalian ears, where supporting cells develop E-cadherin-rich apical junctions reinforced by robust F-actin bands, and the cells fail to divide. Here, we find that damage readily activates YAP in supporting cells within balance epithelia of chickens, but not mice. Deleting LATS kinases or expressing YAP variants that evade LATS-mediated inhibitory phosphorylation induces proliferation in supporting cells of adult mice. YAP signaling eventually may be harnessed to overcome proliferative quiescence that limits regeneration in mammalian ears.

EGF and a GSK3 Inhibitor Deplete Junctional E-cadherin and Stimulate Proliferation in the Mature Mammalian Ear

Sensory hair cell losses underlie the vast majority of permanent hearing and balance deficits in humans, but many nonmammalian vertebrates can fully recover from hearing impairments and balance dysfunctions because supporting cells (SCs) in their ears retain lifelong regenerative capacities that depend on proliferation and differentiation as replacement hair cells. Most SCs in vertebrate ears stop dividing during embryogenesis; and soon after birth, vestibular SCs in mammals transition to lasting quiescence as they develop massively thickened circumferential F-actin bands at their E-cadherin-rich adherens junctions. Here, we report that treatment with EGF and a GSK3 inhibitor thinned the circumferential F-actin bands throughout the sensory epithelium of cultured utricles that were isolated from adult mice of either sex. That treatment also caused decreases in E-cadherin, β-catenin, and YAP in the striola, and stimulated robust proliferation of mature, normally quiescent striolar SCs. The findings suggest that E-cadherin-rich junctions, which are not present in the SCs of the fish, amphibians, and birds which readily regenerate hair cells, are responsible in part for the mammalian ear's vulnerability to permanent balance and hearing deficits.SIGNIFICANCE STATEMENT Millions of people are affected by hearing and balance deficits that arise when loud sounds, ototoxic drugs, infections, and aging cause hair cell losses. Such deficits are permanent for humans and other mammals, but nonmammals can recover hearing and balance after supporting cells regenerate replacement hair cells. Mammalian supporting cells lose the capacity to proliferate around the time they develop unique, exceptionally reinforced, E-cadherin-rich intercellular junctions. Here, we report the discovery of a pharmacological treatment that thins F-actin bands, depletes E-cadherin, and stimulates proliferation in long-quiescent supporting cells within a balance epithelium from adult mice. The findings suggest that high E-cadherin in those supporting cell junctions may be responsible, in part, for the permanence of hair cell loss in mammals.

Molecular markers for corneal epithelial cells in larval vs. adult Xenopus frogs

Corneal Epithelial Stem Cells (CESCs) and their proliferative progeny, the Transit Amplifying Cells (TACs), are responsible for maintaining the integrity and transparency of the cornea. These stem cells (SCs) are widely used in corneal transplants and ocular surface reconstruction. Molecular markers are essential to identify, isolate and enrich for these cells, yet no definitive CESC marker has been established. An extensive literature survey shows variability in the expression of putative CESC markers among vertebrates; being attributed to species-specific variations, or other differences in developmental stages of these animals, approaches used in these studies and marker specificity. Here, we expanded the search for CESC markers using the amphibian model Xenopus laevis. In previous studies we found that long-term label retaining cells (suggestive of CESCs and TACs) are present throughout the larval basal corneal epithelium. In adult frogs, these cells become concentrated in the peripheral cornea (limbal region). Here, we used immunofluorescence to characterize the expression of nine proteins in the corneas of both Xenopus larvae and adults (post-metamorphic). We found that localization of some markers change between larval and adult stages. Markers such as p63, Keratin 19, and β1-integrin are restricted to basal corneal epithelial cells of the larvae. After metamorphosis their expression is found in basal and intermediate layer cells of the adult frog corneal epithelium. Another protein, Pax6 was expressed in the larval corneas, but surprisingly it was not detected in the adult corneal epithelium. For the first time we report that Tcf7l2 can be used as a marker to differentiate cornea vs. skin in frogs. Tcf7l2 is present only in the frog skin, which differs from reports indicating that the protein is expressed in the human cornea. Furthermore, we identified the transition between the inner, and the outer surface of the adult frog eyelid as a key boundary in terms of marker expression. Although these markers are useful to identify different regions and cellular layers of the frog corneal epithelium, none is unique to CESCs or TACs. Our results confirm that there is no single conserved CESC marker in vertebrates. This molecular characterization of the Xenopus cornea facilitates its use as a vertebrate model to understand the functions of key proteins in corneal homeostasis and wound repair.

Junctional E-cadherin/p120-catenin Is Correlated with the Absence of Supporting Cells to Hair Cells Conversion in Postnatal Mice Cochleae

Notch inhibition is known to generate supernumerary hair cells (HCs) at the expense of supporting cells (SCs) in the mammalian inner ear. However, inhibition of Notch activity becomes progressively less effective at inducing SC-to-HC conversion in the postnatal cochlea and balance organs as the animal ages. It has been suggested that the SC-to-HC conversion capacity is inversely correlated with E-cadherin accumulation in postnatal mammalian utricles. However, whether E-cadherin localization is linked to the SC-to-HC conversion capacity in the mammalian inner ear is poorly understood. In the present study, we treated cochleae from postnatal day 0 (P0) with the Notch signaling inhibitor DAPT and observed apparent SC-to-HC conversion along with E-cadherin/p120ctn disruption in the sensory region. In addition, the SC-to-HC conversion capacity and E-cadherin/p120ctn disorganization were robust in the apex but decreased toward the base. We further demonstrated that the ability to regenerate HCs and the disruption of E-cadherin/p120ctn concomitantly decreased with age and ceased at P7, even after extended DAPT treatments. This timing is consistent with E-cadherin/p120ctn accumulation in the postnatal cochleae. These results suggest that the decreasing capacity of SCs to transdifferentiate into HCs correlates with E-cadherin/p120ctn localization in the postnatal cochleae, which might account for the absence of SC-to-HC conversion in the mammalian cochlea.

Development and regeneration of vestibular hair cells in mammals

Vestibular sensation is essential for gaze stabilization, balance, and perception of gravity. The vestibular receptors in mammals, Type I and Type II hair cells, are located in five small organs in the inner ear. Damage to hair cells and their innervating neurons can cause crippling symptoms such as vertigo, visual field oscillation, and imbalance. In adult rodents, some Type II hair cells are regenerated and become re-innervated after damage, presenting opportunities for restoring vestibular function after hair cell damage. This article reviews features of vestibular sensory cells in mammals, including their basic properties, how they develop, and how they are replaced after damage. We discuss molecules that control vestibular hair cell regeneration and highlight areas in which our understanding of development and regeneration needs to be deepened.

Hearing sensitivity differs between zebrafish lines used in auditory research

Zebrafish are increasingly used in auditory studies, in part due to the development of several transgenic lines that express hair cell-specific fluorescent proteins. However, it is largely unknown how transgene expression influences auditory phenotype. We previously observed reduced auditory sensitivity in adult Brn3c:mGFP transgenic zebrafish, which express membrane-bound green fluorescent protein (GFP) in sensory hair cells. Here, we examine the auditory sensitivity of zebrafish from multiple transgenic and background strains. We recorded auditory evoked potentials in adult animals and observed significantly higher auditory thresholds in three lines that express hair cell-specific GFP. There was no obvious correlation between hair cell density and auditory thresholds, suggesting that reduced sensitivity was not due to a reduction in hair cell density. FM1-43 uptake was reduced in Brn3c:mGFP fish but not in other lines, suggesting that a mechanotransduction defect may be responsible for the auditory phenotype in Brn3c animals, but that alternate mechanisms underlie the increased AEP thresholds in other lines. We found reduced prepulse inhibition (a measure of auditory-evoked behavior) in larval Brn3c animals, suggesting that auditory defects develop early in this line. We also found significant differences in auditory sensitivity between adults of different background strains, akin to strain differences observed in mouse models of auditory function. Our results suggest that researchers should exercise caution when selecting an appropriate zebrafish transgenic or background strain for auditory studies.

Supervillin Is a Component of the Hair Cell's Cuticular Plate and the Head Plates of Organ of Corti Supporting Cells

The organ of Corti has evolved a panoply of cells with extraordinary morphological specializations to harness, direct, and transduce mechanical energy into electrical signals. Among the cells with prominent apical specializations are hair cells and nearby supporting cells. At the apical surface of each hair cell is a mechanosensitive hair bundle of filamentous actin (F-actin)-based stereocilia, which insert rootlets into the F-actin meshwork of the underlying cuticular plate, a rigid organelle considered to hold the stereocilia in place. Little is known about the protein composition and development of the cuticular plate or the apicolateral specializations of organ of Corti supporting cells. We show that supervillin, an F-actin cross-linking protein, localizes to cuticular plates in hair cells of the mouse cochlea and vestibule and zebrafish sensory epithelia. Moreover, supervillin localizes near the apicolateral margins within the head plates of Deiters’ cells and outer pillar cells, and proximal to the apicolateral margins of inner phalangeal cells, adjacent to the junctions with neighboring hair cells. Overall, supervillin localization suggests this protein may shape the surface structure of the organ of Corti.

Characterizing human vestibular sensory epithelia for experimental studies: new hair bundles on old tissue and implications for therapeutic interventions in ageing

Balance disequilibrium is a significant contributor to falls in the elderly. The most common cause of balance dysfunction is loss of sensory cells from the vestibular sensory epithelia of the inner ear. However, inaccessibility of inner ear tissue in humans severely restricts possibilities for experimental manipulation to develop therapies to ameliorate this loss. We provide a structural and functional analysis of human vestibular sensory epithelia harvested at trans-labyrinthine surgery. We demonstrate the viability of the tissue and labeling with specific markers of hair cell function and of ion homeostasis in the epithelium. Samples obtained from the oldest patients revealed a significant loss of hair cells across the tissue surface, but we found immature hair bundles present in epithelia harvested from patients >60 years of age. These results suggest that the environment of the human vestibular sensory epithelium could be responsive to stimulation of developmental pathways to enhance hair cell regeneration, as has been demonstrated successfully in the vestibular organs of adult mice.

Macrophage recruitment and epithelial repair following hair cell injury in the mouse utricle

The sensory organs of the inner ear possess resident populations of macrophages, but the function of those cells is poorly understood. In many tissues, macrophages participate in the removal of cellular debris after injury and can also promote tissue repair. The present study examined injury-evoked macrophage activity in the mouse utricle. Experiments used transgenic mice in which the gene for the human diphtheria toxin receptor (huDTR) was inserted under regulation of the Pou4f3 promoter. Hair cells in such mice can be selectively lesioned by systemic treatment with diphtheria toxin (DT). In order to visualize macrophages, Pou4f3-huDTR mice were crossed with a second transgenic line, in which one or both copies of the gene for the fractalkine receptor CX3CR1 were replaced with a gene for GFP. Such mice expressed GFP in all macrophages, and mice that were CX3CR1(GFP/GFP) lacked the necessary receptor for fractalkine signaling. Treatment with DT resulted in the death of ∼70% of utricular hair cells within 7 days, which was accompanied by increased numbers of macrophages within the utricular sensory epithelium. Many of these macrophages appeared to be actively engulfing hair cell debris, indicating that macrophages participate in the process of 'corpse removal' in the mammalian vestibular organs. However, we observed no apparent differences in injury-evoked macrophage numbers in the utricles of CX3CR1(+/GFP) mice vs. CX3CR1(GFP/GFP) mice, suggesting that fractalkine signaling is not necessary for macrophage recruitment in these sensory organs. Finally, we found that repair of sensory epithelia at short times after DT-induced hair cell lesions was mediated by relatively thin cables of F-actin. After 56 days recovery, however, all cell-cell junctions were characterized by very thick actin cables.

Sensory hair cell death and regeneration in fishes

Sensory hair cells are specialized mechanotransductive receptors required for hearing and vestibular function. Loss of hair cells in humans and other mammals is permanent and causes reduced hearing and balance. In the early 1980’s, it was shown that hair cells continue to be added to the inner ear sensory epithelia in cartilaginous and bony fishes. Soon thereafter, hair cell regeneration was documented in the chick cochlea following acoustic trauma. Since then, research using chick and other avian models has led to great insights into hair cell death and regeneration. However, with the rise of the zebrafish as a model organism for studying disease and developmental processes, there has been an increased interest in studying sensory hair cell death and regeneration in its lateral line and inner ears. Advances derived from studies in zebrafish and other fish species include understanding the effect of ototoxins on hair cells and finding otoprotectants to mitigate ototoxin damage, the role of cellular proliferation vs. direct transdifferentiation during hair cell regeneration, and elucidating cellular pathways involved in the regeneration process. This review will summarize research on hair cell death and regeneration using fish models, indicate the potential strengths and weaknesses of these models, and discuss several emerging areas of future studies.

Characterization of a unique cell population marked by transgene expression in the adult cochlea of nestin-CreER(T2)/tdTomato-reporter mice

Hair cells in the adult mammalian cochlea cannot spontaneously regenerate after damage, resulting in the permanency of hearing loss. Stem cells have been found to be present in the cochlea of young rodents; however, there has been little evidence for their existence into adulthood. We used nestin-CreER(T2)/tdTomato-reporter mice to trace the lineage of putative nestin-expressing cells and their progeny in the cochleae of adult mice. Nestin, an intermediate filament found in neural progenitor cells during early development and adulthood, is regarded as a multipotent and neural stem cell marker. Other investigators have reported its presence in postnatal and young adult rodents; however, there are discrepancies among these reports. Using lineage tracing, we documented a robust population of tdTomato-expressing cells and evaluated these cells at a series of adult time points. Upon activation of the nestin promoter, tdTomato was observed just below and medial to the inner hair cell layer. All cells colocalized with the stem cell and cochlear-supporting-cell marker Sox2 as well as the supporting cell and Schwann cell marker Sox10; however, they did not colocalize with the Schwann cell marker Krox20, spiral ganglion marker NF200, nor glial fibrillary acidic acid (GFAP)-expressing supporting cell marker. The cellular identity of this unique population of tdTomato-expressing cells in the adult cochlea of nestin-CreER(T2)/tdTomato mice remains unclear; however, these cells may represent a type of supporting cell on the neural aspect of the inner hair cell layer.

How to bury the dead: elimination of apoptotic hair cells from the hearing organ of the mouse

Hair cell death is a major cause of hearing impairment. Preservation of surface barrier upon hair cell loss is critical to prevent leakage of potassium-rich endolymph into the organ of Corti and to prevent expansion of cellular damage. Understanding of wound healing in this cytoarchitecturally complex organ requires ultrastructural 3D visualization. Powered by the serial block-face scanning electron microscopy, we penetrate into the cell biological mechanisms in the acute response of outer hair cells and glial-like Deiters' cells to ototoxic trauma in vivo. We show that Deiters' cells function as phagocytes. Upon trauma, their phalangeal processes swell and the resulting close cellular contacts allow engulfment of apoptotic cell debris. Apical domains of dying hair cells are eliminated from the inner ear sensory epithelia, an event thought to depend on supporting cells' actomyosin contractile activity. We show that in the case of apoptotic outer hair cells of the organ of Corti, elimination of their apices is preceded by strong cell body shrinkage, emphasizing the role of the dying cell itself in the cleavage. Our data reveal that the resealing of epithelial surface by junctional extensions of Deiters' cells is dynamically reinforced by newly polymerized F-actin belts. By analyzing Cdc42-inactivated Deiters' cells with defects in actin dynamics and surface closure, we show that compromised barrier integrity shifts hair cell death from apoptosis to necrosis and leads to expanded hair cell and nerve fiber damage. Our results have implications concerning therapeutic protective and regenerative interventions, because both interventions should maintain barrier integrity.

Dynamic gene expression by putative hair-cell progenitors during regeneration in the zebrafish lateral line

Hearing loss is most commonly caused by the destruction of mechanosensory hair cells in the ear. This condition is usually permanent: Despite the presence of putative hair-cell progenitors in the cochlea, hair cells are not naturally replenished in adult mammals. Unlike those of the mammalian ear, the progenitor cells of nonmammalian vertebrates can regenerate hair cells throughout life. The basis of this difference remains largely unexplored but may lie in molecular dissimilarities that affect how progenitors respond to hair-cell death. To approach this issue, we analyzed gene expression in hair-cell progenitors of the lateral-line system. We developed a transgenic line of zebrafish that expresses a red fluorescent protein in the presumptive hair-cell progenitors known as mantle cells. Fluorescence-activated cell sorting from the skins of transgenic larvae, followed by microarray-based expression analysis, revealed a constellation of transcripts that are specifically enriched in these cells. Gene expression analysis after hair-cell ablation uncovered a cohort of genes that are differentially regulated early in regeneration, suggesting possible roles in the response of progenitors to hair-cell death. These results provide a resource for studying hair-cell regeneration and the biology of sensory progenitor cells.

Responses to cell loss become restricted as the supporting cells in mammalian vestibular organs grow thick junctional actin bands that develop high stability

Sensory hair cell (HC) loss is a major cause of permanent hearing and balance impairments for humans and other mammals. Yet, fish, amphibians, reptiles, and birds readily replace HCs and recover from such sensory deficits. It is unknown what prevents replacement in mammals, but cell replacement capacity declines contemporaneously with massive postnatal thickening of F-actin bands at the junctions between vestibular supporting cells (SCs). In non-mammals, SCs can give rise to regenerated HCs, and the bands remain thin even in adults. Here we investigated the stability of the F-actin bands between SCs in ears from chickens and mice and Madin-Darby canine kidney cells. Pharmacological experiments and fluorescence recovery after photobleaching (FRAP) of SC junctions in utricles from mice that express a γ-actin-GFP fusion protein showed that the thickening F-actin bands develop increased resistance to depolymerization and exceptional stability that parallels a sharp decline in the cell replacement capacity of the maturing mammalian ear. The FRAP recovery rate and the mobile fraction of γ-actin-GFP both decreased as the bands thickened with age and became highly stabilized. In utricles from neonatal mice, time-lapse recordings in the vicinity of dying HCs showed that numerous SCs change shape and organize multicellular actin purse strings that reseal the epithelium. In contrast, adult SCs appeared resistant to deformation, with resealing responses limited to just a few neighboring SCs that did not form purse strings. The exceptional stability of the uniquely thick F-actin bands at the junctions of mature SCs may play an important role in restricting dynamic repair responses in mammalian vestibular epithelia.

Sensational placodes: neurogenesis in the otic and olfactory systems

For both the intricate morphogenetic layout of the sensory cells in the ear and the elegantly radial arrangement of the sensory neurons in the nose, numerous signaling molecules and genetic determinants are required in concert to generate these specialized neuronal populations that help connect us to our environment. In this review, we outline many of the proteins and pathways that play essential roles in the differentiation of otic and olfactory neurons and their integration into their non-neuronal support structures. In both cases, well-known signaling pathways together with region-specific factors transform thickened ectodermal placodes into complex sense organs containing numerous, diverse neuronal subtypes. Olfactory and otic placodes, in combination with migratory neural crest stem cells, generate highly specialized subtypes of neuronal cells that sense sound, position and movement in space, odors and pheromones throughout our lives.

Postnatal refinement of auditory hair cell planar polarity deficits occurs in the absence of Vangl2

The distinctive planar polarity of auditory hair cells is evident in the polarized organization of the stereociliary bundle. Mutations in the core planar cell polarity gene Van Gogh-like 2 (Vangl2) result in hair cells that fail to properly orient their stereociliary bundles along the mediolateral axis of the cochlea. The severity of this phenotype is graded along the length of the cochlea, similar to the hair cell differentiation gradient, suggesting that an active refinement process corrects planar polarity phenotypes in Vangl2 knock-out (KO) mice. Because Vangl2 gene deletions are lethal, Vangl2 conditional knock-outs (CKOs) were generated to test this hypothesis. When crossed with Pax2-Cre, Vangl2 is deleted from the inner ear, yielding planar polarity phenotypes similar to Vangl2 KOs at late embryonic stages except that Vangl2 CKO mice are viable and do not have craniorachischisis like Vangl2 KOs. Quantification of planar polarity deficits through postnatal development demonstrates the activity of a Vangl2-independent refinement process that rescues the planar polarity phenotype within 10 d of birth. In contrast, the Pax2-Cre;Vangl2 CKO has profound changes in the shape and distribution of outer pillar cell and Deiters' cell phalangeal processes that are not corrected during the period of planar polarity refinement. Auditory brainstem response analyses of adult mice show a 10-15 dB shift in auditory threshold, and distortion product otoacoustic emission measurements indicate that this mild hearing deficit is of cochlear origin. Together, these data demonstrate a Vangl2-independent refinement mechanism that actively reorients auditory stereociliary bundles and reveals an unexpected role of Vangl2 during supporting cell morphogenesis.

A historical to present-day account of efforts to answer the question: "what puts the brakes on mammalian hair cell regeneration?"

Hearing and balance deficits often affect humans and other mammals permanently, because their ears stop producing hair cells within a few days after birth. But production occurs throughout life in the ears of sharks, bony fish, amphibians, reptiles, and birds allowing them to replace lost hair cells and quickly recover after temporarily experiencing the kinds of sensory deficits that are irreversible for mammals. Since the mid 1970s, researchers have been asking what puts the brakes on hair cell regeneration in mammals. Here we evaluate the headway that has been made and assess current evidence for alternative mechanistic hypotheses that have been proposed to account for the limits to hair cell regeneration in mammals.