Oncomodulin identifies different hair cell types in the mammalian inner ear

Establishment of planar cell polarity is coupled to regional cell cycle exit and cell differentiation in the mouse utricle

Sensory hair cells are coordinately oriented within each inner ear sensory organ to exhibit a particular form of planar cell polarity (PCP) necessary for mechanotransduction. However, the developmental events associated with establishing PCP in the vestibule are unclear, hindering data interpretation and employment of the vestibule for PCP studies. Herein, we investigated PCP of the mouse vestibular organs. We further characterised cell cycle exit, cell differentiation, and PCP establishment in the utricle. We found that hair cells formed first in the striolar and medial extrastriolar (MES) regions of the utricle at embryonic day 11.5 (E11.5), while cells in the lateral extrastriolar region (LES) mostly formed at E13.5. Cell differentiation was initiated in the striolar region, which expanded first toward the MES, then to the LES by E15.5. The polarity of hair cells was established at birth along a putative line of polarity reversal (LPR), lateral to the striolar region. Core PCP protein Vangl2 emerged in the cell boundaries since E11.5, while cell intrinsic polarity protein Gαi3 appeared at E12.5, then polarized to the bare zone of individual hair cell at E13.5. These findings provide a blueprint of the developmental events associated with establishing PCP in the utricle.

Distinct capacity for differentiation to inner ear cell types by progenitor cells of the cochlea and vestibular organs

Disorders of hearing and balance are most commonly associated with damage to cochlear and vestibular hair cells or neurons. Although these cells are not capable of spontaneous regeneration, progenitor cells in the hearing and balance organs of the neonatal mammalian inner ear have the capacity to generate new hair cells after damage. To investigate whether these cells are restricted in their differentiation capacity, we assessed the phenotypes of differentiated progenitor cells isolated from three compartments of the mouse inner ear - the vestibular and cochlear sensory epithelia and the spiral ganglion - by measuring electrophysiological properties and gene expression. Lgr5⁺ progenitor cells from the sensory epithelia gave rise to hair cell-like cells, but not neurons or glial cells. Newly created hair cell-like cells had hair bundle proteins, synaptic proteins and membrane proteins characteristic of the compartment of origin. PLP1⁺ glial cells from the spiral ganglion were identified as neural progenitors, which gave rise to neurons, astrocytes and oligodendrocytes, but not hair cells. Thus, distinct progenitor populations from the neonatal inner ear differentiate to cell types associated with their organ of origin.

Oncomodulin, an EF-Hand Ca2+ Buffer, Is Critical for Maintaining Cochlear Function in Mice

Oncomodulin (Ocm), a member of the parvalbumin family of calcium binding proteins, is expressed predominantly by cochlear outer hair cells in subcellular regions associated with either mechanoelectric transduction or electromotility. Targeted deletion of Ocm caused progressive cochlear dysfunction. Although sound-evoked responses are normal at 1 month, by 4 months, mutants show only minimal distortion product otoacoustic emissions and 70-80 dB threshold shifts in auditory brainstem responses. Thus, Ocm is not critical for cochlear development but does play an essential role for cochlear function in the adult mouse.

SIGNIFICANCE STATEMENT

Numerous proteins act as buffers, sensors, or pumps to control calcium levels in cochlear hair cells. In the inner ear, EF-hand calcium buffers may play a significant role in hair cell function but have been very difficult to study. Unlike other reports of genetic disruption of EF-hand calcium buffers, deletion of oncomodulin (Ocm), which is predominately found in outer hair cells, leads to a progressive hearing loss after 1 month, suggesting that Ocm critically protects hearing in the mature ear.

Adenosine Triphosphate (ATP) Inhibits Voltage-Sensitive Potassium Currents in Isolated Hensen's Cells and Nifedipine Protects Against Noise-Induced Hearing Loss in Guinea Pigs

Background

There is increasing evidence that adenosine triphosphate (ATP), a well-known neurotransmitter and neuromodulator in the central nervous system, plays an important role as an extracellular chemical messenger in the cochlea.

Material/Methods

Using a whole-cell recording technique, we studied the effects of ATP on isolated Hensen’s cells, which are supporting cells in the cochlea, to determine if they are involved in the transduction of ions with hair cells.

Results

ATP (0.1–10 μM) reduced the potassium current (I_K⁺) in the majority of the recorded Hensen’s cells (21 out of 25 cells). An inward current was also induced by high concentrations of ATP (100 μM to 10 mM), which was reversibly blocked by 100 μM suramin (a purinergic antagonist) and blocked by nifedipine (an L-type calcium channel blocker). After the cochleas were perfused with artificial perilymph solutions containing nifedipine and exposed to noise, the amplitude increase in the compound action potential (CAP) threshold and the reduction in cochlear microphonics was lower than when they were exposed to noise alone.

Conclusions

Our results suggest that ATP can block I_K⁺ channels at a low concentration and induce an inward Ca²⁺ current at high concentrations, which is reversed by purinergic receptors. Nifedipine may have a partially protective effect on noise-induced hearing loss (NIHL).

Single-cell RNA-Seq resolves cellular complexity in sensory organs from the neonatal inner ear

In the inner ear, cochlear and vestibular sensory epithelia utilize grossly similar cell types to transduce different stimuli: sound and acceleration. Each individual sensory epithelium is composed of highly heterogeneous populations of cells based on physiological and anatomical criteria. However, limited numbers of each cell type have impeded transcriptional characterization. Here we generated transcriptomes for 301 single cells from the utricular and cochlear sensory epithelia of newborn mice to circumvent this challenge. Cluster analysis indicates distinct profiles for each of the major sensory epithelial cell types, as well as less-distinct sub-populations. Asynchrony within utricles allows reconstruction of the temporal progression of cell-type-specific differentiation and suggests possible plasticity among cells at the sensory–nonsensory boundary. Comparisons of cell types from utricles and cochleae demonstrate divergence between auditory and vestibular cells, despite a common origin. These results provide significant insights into the developmental processes that form unique inner ear cell types.

Heterogeneous sensory epithelia of the inner ear are difficult to study owing to the few cells that can be isolated. Here the authors provide insight into the developmental processes underlying the formation of these cells by single-cell RNA-Seq.

RFX transcription factors are essential for hearing in mice

Sensorineural hearing loss is a common and currently irreversible disorder, because mammalian hair cells (HCs) do not regenerate and current stem cell and gene delivery protocols result only in immature HC-like cells. Importantly, although the transcriptional regulators of embryonic HC development have been described, little is known about the postnatal regulators of maturating HCs. Here we apply a cell type-specific functional genomic analysis to the transcriptomes of auditory and vestibular sensory epithelia from early postnatal mice. We identify RFX transcription factors as essential and evolutionarily conserved regulators of the HC-specific transcriptomes, and detect Rfx1,2,3,5 and 7 in the developing HCs. To understand the role of RFX in hearing, we generate Rfx1/3 conditional knockout mice. We show that these mice are deaf secondary to rapid loss of initially well-formed outer HCs. These data identify an essential role for RFX in hearing and survival of the terminally differentiating outer HCs.

A study of whirlin isoforms in the mouse vestibular system suggests potential vestibular dysfunction in DFNB31-deficient patients

The DFNB31 gene plays an indispensable role in the cochlea and retina. Mutations in this gene disrupt its various isoforms and lead to non-syndromic deafness, blindness and deaf-blindness. However, the known expression of Dfnb31, the mouse ortholog of DFNB31, in vestibular organs and the potential vestibular-deficient phenotype observed in one Dfnb31 mutant mouse (Dfnb31(wi/wi)) suggest that DFNB31 may also be important for vestibular function. In this study, we find that full-length (FL-) and C-terminal (C-) whirlin isoforms are expressed in the vestibular organs, where their stereociliary localizations are similar to those of developing cochlear inner hair cells. No whirlin is detected in Dfnb31(wi/wi) vestibular organs, while only C-whirlin is expressed in Dfnb31(neo/neo) vestibular organs. Both FL- and C-whirlin isoforms are required for normal vestibular stereociliary growth, although they may play slightly different roles in the central and peripheral zones of the crista ampullaris. Vestibular sensory-evoked potentials demonstrate severe to profound vestibular deficits in Dfnb31(neo/neo) and Dfnb31(wi/wi) mice. Swimming and rotarod tests demonstrate that the two Dfnb31 mutants have balance problems, with Dfnb31(wi/wi) mice being more affected than Dfnb31(neo/neo) mice. Because Dfnb31(wi/wi) and Dfnb31(neo/neo) mice faithfully recapitulate hearing and vision symptoms in patients, our findings of vestibular dysfunction in these Dfnb31 mutants raise the question of whether DFNB31-deficient patients may acquire vestibular as well as hearing and vision loss.

Lgr5+ cells regenerate hair cells via proliferation and direct transdifferentiation in damaged neonatal mouse utricle

Recruitment of endogenous progenitors is critical during tissue repair. The inner ear utricle requires mechanosensory hair cells (HCs) to detect linear acceleration. After damage, non-mammalian utricles regenerate HCs via both proliferation and direct transdifferentiation. In adult mammals, limited transdifferentiation from unidentified progenitors occurs to regenerate extrastriolar Type II HCs. Here we show that HC damage in neonatal mouse utricle activates the Wnt target gene Lgr5 in striolar supporting cells. Lineage tracing and time-lapse microscopy reveal that Lgr5+ cells transdifferentiate into HC-like cells in vitro. In contrast to adults, HC ablation in neonatal utricles in vivo recruits Lgr5+ cells to regenerate striolar HCs through mitotic and transdifferentiation pathways. Both Type I and II HCs are regenerated, and regenerated HCs display stereocilia and synapses. Lastly, stabilized ß-catenin in Lgr5+ cells enhances mitotic activity and HC regeneration. Thus Lgr5 marks Wnt-regulated, damage-activated HC progenitors and may help uncover factors driving mammalian HC regeneration.

Efferent innervation of turtle semicircular canal cristae: comparisons with bird and mouse

In the vestibular periphery of nearly every vertebrate, cholinergic vestibular efferent neurons give rise to numerous presynaptic varicosities that target hair cells and afferent processes in the sensory neuroepithelium. Although pharmacological studies have described the postsynaptic actions of vestibular efferent stimulation in several species, characterization of efferent innervation patterns and the relative distribution of efferent varicosities among hair cells and afferents are also integral to understanding how efferent synapses operate. Vestibular efferent markers, however, have not been well characterized in the turtle, one of the animal models used by our laboratory. Here we sought to identify reliable efferent neuronal markers in the vestibular periphery of turtle, to use these markers to understand how efferent synapses are organized, and to compare efferent neuronal labeling patterns in turtle with two other amniotes using some of the same markers. Efferent fibers and varicosities were visualized in the semicircular canal of red-eared turtles (Trachemys scripta elegans), zebra finches (Taeniopygia guttata), and mice (Mus musculus) utilizing fluorescent immunohistochemistry with antibodies against choline acetyltransferase (ChAT). Vestibular hair cells and afferents were counterstained using antibodies to myosin VIIa and calretinin. In all species, ChAT labeled a population of small diameter fibers giving rise to numerous spherical varicosities abutting type II hair cells and afferent processes. That these ChAT-positive varicosities represent presynaptic release sites were demonstrated by colabeling with antibodies against the synaptic vesicle proteins synapsin I, SV2, or syntaxin and the neuropeptide calcitonin gene-related peptide. Comparisons of efferent innervation patterns among the three species are discussed.

Utricular afferents: morphology of peripheral terminals

The utricle provides critical information about spatiotemporal properties of head movement. It comprises multiple subdivisions whose functional roles are poorly understood. We previously identified four subdivisions in turtle utricle, based on hair bundle structure and mechanics, otoconial membrane structure and hair bundle coupling, and immunoreactivity to calcium-binding proteins. Here we ask whether these macular subdivisions are innervated by distinctive populations of afferents to help us understand the role each subdivision plays in signaling head movements. We quantified the morphology of 173 afferents and identified six afferent classes, which differ in structure and macular locus. Calyceal and dimorphic afferents innervate one striolar band. Bouton afferents innervate a second striolar band; they have elongated terminals and the thickest processes and axons of all bouton units. Bouton afferents in lateral (LES) and medial (MES) extrastriolae have small-diameter axons but differ in collecting area, bouton number, and hair cell contacts (LES >> MES). A fourth, distinctive population of bouton afferents supplies the juxtastriola. These results, combined with our earlier findings on utricular hair cells and the otoconial membrane, suggest the hypotheses that MES and calyceal afferents encode head movement direction with high spatial resolution and that MES afferents are well suited to signal three-dimensional head orientation and striolar afferents to signal head movement onset.

Notch signaling limits supporting cell plasticity in the hair cell-damaged early postnatal murine cochlea

In mammals, auditory hair cells are generated only during embryonic development and loss or damage to hair cells is permanent. However, in non-mammalian vertebrate species, such as birds, neighboring glia-like supporting cells regenerate auditory hair cells by both mitotic and non-mitotic mechanisms. Based on work in intact cochlear tissue, it is thought that Notch signaling might restrict supporting cell plasticity in the mammalian cochlea. However, it is unresolved how Notch signaling functions in the hair cell-damaged cochlea and the molecular and cellular changes induced in supporting cells in response to hair cell trauma are poorly understood. Here we show that gentamicin-induced hair cell loss in early postnatal mouse cochlear tissue induces rapid morphological changes in supporting cells, which facilitate the sealing of gaps left by dying hair cells. Moreover, we provide evidence that Notch signaling is active in the hair cell damaged cochlea and identify Hes1, Hey1, Hey2, HeyL, and Sox2 as targets and potential Notch effectors of this hair cell-independent mechanism of Notch signaling. Using Cre/loxP based labeling system we demonstrate that inhibition of Notch signaling with a γ- secretase inhibitor (GSI) results in the trans-differentiation of supporting cells into hair cell-like cells. Moreover, we show that these hair cell-like cells, generated by supporting cells have molecular, cellular, and basic electrophysiological properties similar to immature hair cells rather than supporting cells. Lastly, we show that the vast majority of these newly generated hair cell-like cells express the outer hair cell specific motor protein prestin.

In-depth proteomic analysis of mouse cochlear sensory epithelium by mass spectrometry

Proteomic analysis of sensory organs such as the cochlea is challenging due to its small size and difficulties with membrane protein isolation. Mass spectrometry in conjunction with separation methods can provide a more comprehensive proteome, because of the ability to enrich protein samples, detect hydrophobic proteins, and identify low abundant proteins by reducing the proteome dynamic range. GELFrEE as well as different separation and digestion techniques were combined with FASP and nanoLC-MS/MS to obtain an in-depth proteome analysis of cochlear sensory epithelium from 30-day-old mice. Digestion with LysC/trypsin followed by SCX fractionation and multiple nanoLC-MS/MS analyses identified 3773 proteins with a 1% FDR. Of these, 694 protein IDs were in the plasmalemma. Protein IDs obtained by combining outcomes from GELFrEE/LysC/trypsin with GELFrEE/trypsin/trypsin generated 2779 proteins, of which 606 additional proteins were identified using the GELFrEE/LysC/trypsin approach. Combining results from the different techniques resulted in a total of 4620 IDs, including a number of previously unreported proteins. GO analyses showed high expression of binding and catalytic proteins as well as proteins associated with metabolism. The results show that the application of multiple techniques is needed to provide an exhaustive proteome of the cochlear sensory epithelium that includes many membrane proteins. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium with the data set identifier PXD000231.

Beyond generalized hair cells: molecular cues for hair cell types

Basic helix-loop-helix (bHLH) transcription factors (TFs) are crucial for inner ear neurosensory development. The proneural TF Atoh1 regulates the differentiation of hair cells (HCs) whereas Neurog1 and Neurod1 regulate specification and differentiation of neurons, respectively, but also affect HC development. Expression of Delta and Jagged ligands in nascent HCs and Notch receptors in supporting cells induce supporting cell differentiation through the regulation of neurogenic bHLH TFs (such as Hes1, Hes5) and suppression of limited Atoh1 expression. In sensorineural hearing loss, HCs are lost followed by supporting cells and progressive degeneration of neurons, at least in rodents. Regaining complete hearing may require reconstituting the organ of Corti from scratch, including the two types of HCs, inner and outer hair cells with the precise sorting of two types of afferent (type I and II) and efferent (lateral and medial olivo-cochlear) innervation. We review effects of bHLH TF dosage and their cross-regulation to differentiate HC types in the organ of Corti. We categorize findings of specific gene expressions in HCs: 1. as markers without meaning for the regeneration task, 2. as stabilizers who are needed to maintain or complete differentiation, and 3. as decision-making genes, expressed and acting early enough to be useful in this process. Only one TF has been characterized that fits the last aspect: Atoh1. We propose that temporal and intensity variations of Atoh1 are naturally modulated to differentiate specific types of HCs. Importantly, the molecular means to modify the Atoh1 expression are at least partially understood and can be readily implemented in the attempts to regenerate specific types of HCs.

Age-dependent in vivo conversion of mouse cochlear pillar and Deiters' cells to immature hair cells by Atoh1 ectopic expression

Unlike nonmammalian vertebrates, mammals cannot convert inner ear cochlear supporting cells (SCs) into sensory hair cells (HCs) after damage, thus causing permanent deafness. Here, we achieved in vivo conversion of two SC subtypes, pillar cells (PCs) and Deiters' cells (DCs), into HCs by inducing targeted expression of Atoh1 at neonatal and juvenile ages using novel mouse models. The conversion only occurred in ∼10% of PCs and DCs with ectopic Atoh1 expression and started with reactivation of endogenous Atoh1 followed by expression of 11 HC and synaptic markers, a process that took approximately 3 weeks in vivo. These new HCs resided in the outer HC region, formed stereocilia, contained mechanoelectrical transduction channels, and survived for >2 months in vivo; however, they surprisingly lacked prestin and oncomodulin expression and mature HC morphology. In contrast, adult PCs and DCs no longer responded to ectopic Atoh1 expression, even after outer HC damage. Finally, permanent Atoh1 expression in endogenous HCs did not affect prestin expression but caused cell loss of mature HCs. Together, our results demonstrate that in vivo conversion of PCs and DCs into immature HCs by Atoh1 is age dependent and resembles normal HC development. Therefore, combined expression of Atoh1 with additional factors holds therapeutic promise to convert PCs and DCs into functional HCs in vivo for regenerative purposes.

Comparison of phenotypes between different vangl2 mutants demonstrates dominant effects of the Looptail mutation during hair cell development

Experiments utilizing the Looptail mutant mouse, which harbors a missense mutation in the vangl2 gene, have been essential for studies of planar polarity and linking the function of the core planar cell polarity proteins to other developmental signals. Originally described as having dominant phenotypic traits, the molecular interactions underlying the Looptail mutant phenotype are unclear because Vangl2 protein levels are significantly reduced or absent from mutant tissues. Here we introduce a vangl2 knockout mouse and directly compare the severity of the knockout and Looptail mutant phenotypes by intercrossing the two lines and assaying the planar polarity of inner ear hair cells. Overall the vangl2 knockout phenotype is milder than the phenotype of compound mutants carrying both the Looptail and vangl2 knockout alleles. In compound mutants a greater number of hair cells are affected and changes in the orientation of individual hair cells are greater when quantified. We further demonstrate in a heterologous cell system that the protein encoded by the Looptail mutation (Vangl2^S464N) disrupts delivery of Vangl1 and Vangl2 proteins to the cell surface as a result of oligomer formation between Vangl1 and Vangl2^S464N, or Vangl2 and Vangl2^S464N, coupled to the intracellular retention of Vangl2^S464N. As a result, Vangl1 protein is missing from the apical cell surface of vestibular hair cells in Looptail mutants, but is retained at the apical cell surface of hair cells in vangl2 knockouts. Similarly the distribution of Prickle-like2, a putative Vangl2 interacting protein, is differentially affected in the two mutant lines. In summary, we provide evidence for a direct physical interaction between Vangl1 and Vangl2 through a combination of in vitro and in vivo approaches and propose that this interaction underlies the dominant phenotypic traits associated with the Looptail mutation.

Vestibular hair cells and afferents: two channels for head motion signals

Vestibular epithelia of the inner ear detect head motions over a wide range of amplitudes and frequencies. In mammals, afferent nerve fibers from central and peripheral zones of vestibular epithelia form distinct populations with different response dynamics and spike timing. Central-zone afferents are large, fast conduits for phasic signals encoded in irregular spike trains. The finer afferents from peripheral zones conduct more slowly and encode more tonic, linear signals in highly regular spike trains. The hair cells are also of two types, I and II, but the two types do not correspond directly to the two afferent populations. Zonal differences in afferent response dynamics may arise at multiple stages, including mechanoelectrical transduction, voltage-gated channels in hair cells and afferents, afferent transmission at calyceal and bouton synapses, and spike generation in regular and irregular afferents. In contrast, zonal differences in spike timing may depend more simply on the selective expression of low-voltage-activated ion channels by irregular afferents.

The postnatal accumulation of junctional E-cadherin is inversely correlated with the capacity for supporting cells to convert directly into sensory hair cells in mammalian balance organs

Mammals experience permanent impairments from hair cell (HC) losses, but birds and other non-mammals quickly recover hearing and balance senses after supporting cells (SCs) give rise to replacement HCs. Avian HC epithelia express little or no E-cadherin, and differences in the thickness of F-actin belts at SC junctions strongly correlate with different species' capacities for HC replacement, so we investigated junctional cadherins in human and murine ears. We found strong E-cadherin expression at SC-SC junctions that increases more than sixfold postnatally in mice. When we cultured utricles from young mice with γ-secretase inhibitors (GSIs), striolar SCs completely internalized their E-cadherin, without affecting N-cadherin. Hes and Hey expression also decreased and the SCs began to express Atoh1. After 48 h, those SCs expressed myosins VI and VIIA, and by 72 h, they developed hair bundles. However, some scattered striolar SCs retained E-cadherin and the SC phenotype. In extrastriolar regions, the vast majority of SCs also retained E-cadherin and failed to convert into HCs even after long GSI treatments. Microscopic measurements revealed that the junctions between extrastriolar SCs were more developed than those between striolar SCs. In GSI-treated utricles as old as P12, differentiated striolar SCs converted into HCs, but such responses declined with age and ceased by P16. Thus, temporal and spatial differences in postnatal SC-to-HC phenotype conversion capacity are linked to the structural attributes of E-cadherin containing SC junctions in mammals, which differ substantially from their counterparts in non-mammalian vertebrates that readily recover from hearing and balance deficits through hair cell regeneration.