Large-scale annotated dataset for cochlear hair cell detection and classification

Our sense of hearing is mediated by cochlear hair cells, of which there are two types organized in one row of inner hair cells and three rows of outer hair cells. Each cochlea contains 5-15 thousand terminally differentiated hair cells, and their survival is essential for hearing as they do not regenerate after insult. It is often desirable in hearing research to quantify the number of hair cells within cochlear samples, in both pathological conditions, and in response to treatment. Machine learning can be used to automate the quantification process but requires a vast and diverse dataset for effective training. In this study, we present a large collection of annotated cochlear hair-cell datasets, labeled with commonly used hair-cell markers and imaged using various fluorescence microscopy techniques. The collection includes samples from mouse, rat, guinea pig, pig, primate, and human cochlear tissue, from normal conditions and following in-vivo and in-vitro ototoxic drug application. The dataset includes over 107,000 hair cells which have been identified and annotated as either inner or outer hair cells. This dataset is the result of a collaborative effort from multiple laboratories and has been carefully curated to represent a variety of imaging techniques. With suggested usage parameters and a well-described annotation procedure, this collection can facilitate the development of generalizable cochlear hair-cell detection models or serve as a starting point for fine-tuning models for other analysis tasks. By providing this dataset, we aim to give other hearing research groups the opportunity to develop their own tools with which to analyze cochlear imaging data more fully, accurately, and with greater ease.

Deciphering the genetic interactions between Pou4f3, Gfi1, and Rbm24 in maintaining mouse cochlear hair cell survival

Mammals harbor a limited number of sound-receptor hair cells (HCs) that cannot be regenerated after damage. Thus, investigating the underlying molecular mechanisms that maintain HC survival is crucial for preventing hearing impairment. Intriguingly, Pou4f3-/- or Gfi1-/- HCs form initially but then rapidly degenerate, whereas Rbm24-/- HCs degenerate considerably later. However, the transcriptional cascades involving Pou4f3, Gfi1, and Rbm24 remain undescribed. Here, we demonstrate that Rbm24 expression is completely repressed in Pou4f3-/- HCs but unaltered in Gfi1-/- HCs, and further that the expression of both POU4F3 and GFI1 is intact in Rbm24-/- HCs. Moreover, by using in vivo mouse transgenic reporter assays, we identify three Rbm24 enhancers to which POU4F3 binds. Lastly, through in vivo genetic testing of whether Rbm24 restoration alleviates the degeneration of Pou4f3-/- HCs, we show that ectopic Rbm24 alone cannot prevent Pou4f3-/- HCs from degenerating. Collectively, our findings provide new molecular and genetic insights into how HC survival is regulated.

Therapeutic potential of salidroside in preserving rat cochlea organ of corti from gentamicin-induced injury through modulation of NRF2 signaling and GSK3β/NF-κB pathway

Salidroside (SAL) is a phenol glycoside compound found in plants of the Rhodiola genus which has natural antioxidant and free radical scavenging properties. SAL are able to protect against manganese-induced ototoxicity. However, the molecular mechanism by which SAL reduces levels of reactive oxygen species (ROS) is unclear. Here, we established an in vitro gentamicin (GM) ototoxicity model to observe the protective effect of SAL on GM-induced hair cells (HC) damage. Cochlear explants of postnatal day 4 rats were obtained and randomly divided into six groups: two model groups (treatment with 0.2 mM or 0.4 mM GM for 24 h); two 400 μmol/L SAL-pretreated groups pretreatment with SAL for 3 h followed by GM treatment (0.2 mM or 0.4 mM) for 24 h; 400 μmol/L SAL group (treatment with SAL for 24 h); control group (normal cultured cochlear explants). The protective effects of SAL on GM-induced HC damage, and on mRNA and protein levels of antioxidant enzymes were observed. HC loss occurred after 24 h of GM treatment. Pretreatment with SAL significantly reduced GM-induced OHC loss. In cochlear tissues, mRNA and protein levels of NRF2 and HO-1 were enhanced in the GM alone group compared with the SAL pretreatment GM treatment group. SAL may protect against GM-induced ototoxicity by regulating the antioxidant defense system of cochlear tissues; SAL can activate NRF2/HO-1 signaling, inhibit NF-κB activation, activate AKT, and increase inhibitory phosphorylation of GSK3β to decrease GSK3 activity, all of which exert antioxidant effects.

Expression of Atoh1, Gfi1, and Pou4f3 in the mature cochlea reprograms nonsensory cells into hair cells

Mechanosensory hair cells of the mature mammalian organ of Corti do not regenerate; consequently, loss of hair cells leads to permanent hearing loss. Although nonmammalian vertebrates can regenerate hair cells from neighboring supporting cells, many humans with severe hearing loss lack both hair cells and supporting cells, with the organ of Corti being replaced by a flat epithelium of nonsensory cells. To determine whether the mature cochlea can produce hair cells in vivo, we reprogrammed nonsensory cells adjacent to the organ of Corti with three hair cell transcription factors: Gfi1, Atoh1, and Pou4f3. We generated numerous hair cell-like cells in nonsensory regions of the cochlea and new hair cells continued to be added over a period of 9 wk. Significantly, cells adjacent to reprogrammed hair cells expressed markers of supporting cells, suggesting that transcription factor reprogramming of nonsensory cochlear cells in adult animals can generate mosaics of sensory cells like those seen in the organ of Corti. Generating such sensory mosaics by reprogramming may represent a potential strategy for hearing restoration in humans.

An essential signaling cascade for avian auditory hair cell regeneration

Hearing loss is a chronic disease affecting millions of people worldwide, yet no restorative treatment options are available. Although non-mammalian species can regenerate their auditory sensory hair cells, mammals cannot. Birds retain facultative stem cells known as supporting cells that engage in proliferative regeneration when surrounding hair cells die. Here, we investigated gene expression changes in chicken supporting cells during auditory hair cell death. This identified a pathway involving the receptor F2RL1, HBEGF, EGFR, and ERK signaling. We propose a cascade starting with the proteolytic activation of F2RL1, followed by matrix-metalloprotease-mediated HBEGF shedding, and culminating in EGFR-mediated ERK signaling. Each component of this cascade is essential for supporting cell S-phase entry in vivo and is integral for hair cell regeneration. Furthermore, STAT3-phosphorylation converges with this signaling toward upregulation of transcription factors ATF3, FOSL2, and CREM. Our findings could provide a basis for designing treatments for hearing and balance disorders.

MSC-derived exosomes protect auditory hair cells from neomycin-induced damage via autophagy regulation

BACKGROUND

Sensorineural hearing loss (SNHL) poses a major threat to both physical and mental health; however, there is still a lack of effective drugs to treat the disease. Recently, novel biological therapies, such as mesenchymal stem cells (MSCs) and their products, namely, exosomes, are showing promising therapeutic potential due to their low immunogenicity, few ethical concerns, and easy accessibility. Nevertheless, the precise mechanisms underlying the therapeutic effects of MSC-derived exosomes remain unclear.

RESULTS

Exosomes derived from MSCs reduced hearing and hair cell loss caused by neomycin-induced damage in models in vivo and in vitro. In addition, MSC-derived exosomes modulated autophagy in hair cells to exert a protective effect. Mechanistically, exogenously administered exosomes were internalized by hair cells and subsequently upregulated endocytic gene expression and endosome formation, ultimately leading to autophagy activation. This increased autophagic activity promoted cell survival, decreased the mitochondrial oxidative stress level and the apoptosis rate in hair cells, and ameliorated neomycin-induced ototoxicity.

CONCLUSIONS

In summary, our findings reveal the otoprotective capacity of exogenous exosome-mediated autophagy activation in hair cells in an endocytosis-dependent manner, suggesting possibilities for deafness treatment.

GelMA/PEDOT:PSS Composite Conductive Hydrogel-Based Generation and Protection of Cochlear Hair Cells through Multiple Signaling Pathways

Recent advances in cochlear implantology are exemplified by novel functional strategies such as bimodal electroacoustic stimulation, in which the patient has intact low-frequency hearing and profound high-frequency hearing pre-operatively. Therefore, the synergistic restoration of dysfunctional cochlear hair cells and the protection of hair cells from ototoxic insults have become a persistent target pursued for this hybrid system. In this study, we developed a composite GelMA/PEDOT:PSS conductive hydrogel that is suitable as a coating for the cochlear implant electrode for the potential local delivery of otoregenerative and otoprotective drugs. Various material characterization methods (e.g., 1H NMR spectroscopy, FT-IR, EIS, and SEM), experimental models (e.g., murine cochlear organoid and aminoglycoside-induced ototoxic HEI-OC1 cellular model), and biological analyses (e.g., confocal laser scanning microscopy, real time qPCR, flow cytometry, and bioinformatic sequencing) were used. The results demonstrated decent material properties of the hydrogel, such as mechanical (e.g., high tensile stress and Young's modulus), electrochemical (e.g., low impedance and high conductivity), biocompatibility (e.g., satisfactory cochlear cell interaction and free of systemic toxicity), and biosafety (e.g., minimal hemolysis and cell death) features. In addition, the CDR medicinal cocktail sustainably released by the hydrogel not only promoted the expansion of the cochlear stem cells but also boosted the trans-differentiation from cochlear supporting cells into hair cells. Furthermore, hydrogel-based drug delivery protected the hair cells from oxidative stress and various forms of programmed cell death (e.g., apoptosis and ferroptosis). Finally, using large-scale sequencing, we enriched a complex network of signaling pathways that are potentially downstream to various metabolic processes and abundant metabolites. In conclusion, we present a conductive hydrogel-based local delivery of bifunctional drug cocktails, thereby serving as a potential solution to intracochlear therapy of bimodal auditory rehabilitation and diseases beyond.

Compensation in neuro-system related to age-related hearing loss

BACKGROUND

Age-related hearing loss (ARHL) is a major cause of chronic disability among the elderly. Individuals with ARHL not only have trouble hearing sounds, but also with speech perception. As the perception of auditory information is reliant on integration between widespread brain networks to interpret auditory stimuli, both auditory and extra-auditory systems which mainly include visual, motor and attention systems, play an important role in compensating for ARHL.

OBJECTIVES

To better understand the compensatory mechanism of ARHL and inspire better interventions that may alleviate ARHL.

METHODS

We mainly focus on the existing information on ARHL-related central compensation. The compensatory effects of hearing aids (HAs) and cochlear implants (CIs) on ARHL were also discussed.

RESULTS

Studies have shown that ARHL can induce cochlear hair cell damage or loss and cochlear synaptopathy, which could induce central compensation including compensation of auditory and extra-auditory neural networks. The use of HAs and CIs can improve bottom-up processing by enabling 'better' input to the auditory pathways and then to the cortex by enhancing the diminished auditory signal.

CONCLUSIONS

The central compensation of ARHL and its possible correlation with HAs and CIs are current hotspots in the field and should be given focus in future research.

Noise-induced damage in the zebrafish inner ear endorgans: evidence for higher acoustic sensitivity of saccular and lagenar hair cells

The three otolithic endorgans of the inner ear are known to be involved in sound detection in different teleost fishes, yet their relative roles for auditory-vestibular functions within the same species remain uncertain. In zebrafish (Danio rerio), the saccule and utricle are thought to play key functions in encoding auditory and vestibular information, respectively, but the biological function of the lagena is not clear. We hypothesized that the zebrafish saccule serves as a primary auditory endorgan, making it more vulnerable to noise exposure, and that the lagena might have an auditory function given its connectivity to the saccule and the dominant vestibular function of the utricle. We compared the impact of acoustic trauma (continuous white noise at 168 dB for 24 h) between the sensory epithelia of the three otolithic endorgans. Noise treatment caused hair cell loss in both the saccule and lagena but not in the utricle. This effect was identified immediately after acoustic treatment and did not increase 24 h post-trauma. Furthermore, hair cell loss was accompanied by a reduction in presynaptic activity measured based on ribeye b presence, but mainly in the saccule, supporting its main contribution for noise-induced hearing loss. Our findings support the hypothesis that the saccule plays a major role in sound detection and that the lagena is also acoustically affected, extending the species hearing dynamic range.

Ultrastructure of noise-induced cochlear synaptopathy

Acoustic overexposure can eliminate synapses between inner hair cells (IHCs) and auditory nerve fibers (ANFs), even if hair-cell function recovers. This synaptopathy has been extensively studied by confocal microscopy, however, understanding the nature and sequence of damage requires ultrastructural analysis. Here, we used focused ion-beam scanning electron microscopy to mill, image, segment and reconstruct ANF terminals in mice, 1 day and 1 week after synaptopathic exposure (8-16 kHz, 98 dB SPL). At both survivals, ANF terminals were normal in number, but 62% and 53%, respectively, lacked normal synaptic specializations. Most non-synapsing fibers (57% and 48% at 1 day and 1 week) remained in contact with an IHC and contained healthy-looking organelles. ANFs showed a transient increase in mitochondrial content (51%) and efferent innervation (34%) at 1 day. Fibers maintaining synaptic connections showed hypertrophy of pre-synaptic ribbons at both 1 day and 1 week. Non-synaptic fibers were lower in mitochondrial content and typically on the modiolar side of the IHC, where ANFs with high-thresholds and low spontaneous rates are normally found. Even 1 week post-exposure, many ANF terminals remained in IHC contact despite loss of synaptic specializations, thus, regeneration efforts at early post-exposure times should concentrate on synaptogenesis rather than neurite extension.

The proper timing of Atoh1 expression is pivotal for hair cell subtype differentiation and the establishment of inner ear function

Atoh1 overexpression is essential for hair cell (HC) regeneration in the sensory epithelium of mammalian auditory and vestibular organs. However, Atoh1 overexpression alone cannot induce fully mature and functional HCs in the mammalian inner ear. In the current study, we investigated the effect of Atoh1 constitutive overexpression in native HCs by manipulating Atoh1 expression at different developmental stages. We demonstrated that constitutive overexpression of Atoh1 in native vestibular HCs did not affect cell survival but did impair vestibular function by interfering with the subtype differentiation of HCs and hair bundle development. In contrast, Atoh1 overexpression in cochlear HCs impeded their maturation, eventually leading to gradual HC loss in the cochlea and hearing dysfunction. Our study suggests that time-restricted Atoh1 expression is essential for the differentiation and survival of HCs in the inner ear, and this is pivotal for both hearing and vestibular function re-establishment through Atoh1 overexpression-induced HC regeneration strategies.

3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (HMGCR) protects hair cells from cisplatin-induced ototoxicity in vitro: possible relation to the activities of p38 MAPK signaling pathway

The 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (HMGCR) gene encodes rate-limiting enzyme in cholesterol biosynthesis, which is related to cell proliferation and mitochondrial function. The present study was designed to explore the expression of HMGCR in murine cochlear hair cells and HEI-OC1 cells and the possible mechanisms underpinning the actions of HMGCR in cisplatin-induced ototoxicity, with special attention given to p38 mitogen-activated protein kinase (MAPK) activities in vitro. The expressions of HMGCR, p-p38, cleaved caspase-3 and LC3B was measured by immunofluorescence and western blot. JC-1 staining and MitoSOX Red were used to detect mitochondria membrane potential (MMP) and reactive oxygen species (ROS) levels respectively. The apoptosis of auditory cells was assessed by TUNEL staining and flow cytometry. Protein levels of bcl2/bax and beclin1 were examined by western blot. We found that HMGCR was widely expressed in the auditory cells, of both neonatal mice and 2-month-old mice, in cytoplasm, nucleus and stereocilia. Moreover, 30 μM cisplatin elicited the formation of ROS, which, in turn, led to HMGCR reduction, activating p38 kinase-related apoptosis and autophagy in auditory cells. Meanwhile, co-treatment with ROS scavenger at a concentration of 2 mM, N-acetyl-L-cysteine (NAC), could alleviate the aforementioned changes. In addition, HMGCR silencing resulted in higher p38 MAPK-mediated apoptosis and autophagy under cisplatin injury. Taken together, we demonstrate that, for the first time, that HMGCR is expressed in the cochlear. Furthermore, HMGCR exerts protective benefit on auditory cells against cisplatin-mediated injury stimulated by ROS, culminating in regulation of p38 MAPK-dependent apoptosis and autophagy.

Fish hearing revealed: Do we understand hearing in critical fishes and marine tetrapods

Hearing evolved in lampreys with a frequency range of 50-200 Hz. This hearing range is comparable to that of elasmobranchs, most non-teleosts, and lungfish. Elasmobranchs most likely use the saccule and the papilla neglecta (PN) for hearing. In non-teleosts and teleosts, lungfish, and certain tetrapods the saccule is the likely sensor for sound reception while the lagena and the PN are important for gravistatic sensing. Coelacanth and most tetrapods have a basilar papilla (BP) for hearing. In coelacanth and tetrapods, the hair cells of the BP are in contact with a basilar and a tectorial membrane. These membranes transmit mechanical vibrations. A cochlear aqueduct (CA) provides a connection between the cerebrospinal fluid that has a sodium rich space in coelacanth and tetrapods while the potassium rich endolymph is known in vertebrates. A unique feature is known in basic sarcopterygians, the intracranial joint, that never developed in actinopterygians and has been lost in lungfish and tetrapods. The BP in coelacanths is thought to generate pressure with the intracranial joint that will be transmitted to the CA. Lungs or a swim bladder are not forming in Chondrichthyes, structures that have a major impact on hearing in teleosts and tetrapods.

CFTR potentiator ivacaftor protects against noise-induced hair cell loss by increasing Nrf2 and reducing oxidative stress

Over-production of reactive oxygen species (ROS) in the inner ear can be triggered by a variety of pathological events identified in animal models after traumatic noise exposure. Our previous research found that inhibition of the AMP-activated protein kinase alpha subunit (AMPKα) protects against noise-induced cochlear hair cell loss and hearing loss by reducing ROS accumulation. However, the molecular pathway through which AMPKα exerts its antioxidative effect is still unclear. In this study, we have investigated a potential target of AMPKα and ROS, cystic fibrosis transmembrane conductance regulator (CFTR), and the protective effect against noise-induced hair cell loss of an FDA-approved CFTR potentiator, ivacaftor, in FVB/NJ mice, mouse explant cultures, and HEI-OC1 cells. We found that noise exposure increases phosphorylation of CFTR at serine 737 (p-CFTR, S737), which reduces wildtype CFTR function, resulting in oxidative stress in cochlear sensory hair cells. Pretreatment with a single dose of ivacaftor maintains CFTR function by preventing noise-increased p-CFTR (S737). Furthermore, ivacaftor treatment increases nuclear factor E2-related factor 2 (Nrf2) expression, diminishes ROS formation, and attenuates noise-induced hair cell loss and hearing loss. Additionally, inhibition of noise-induced AMPKα activation by compound C also diminishes p-CFTR (S737) expression. In line with these in-vivo results, administration of hydrogen peroxide to cochlear explants or HEI-OC1 cells increases p-CFTR (S737) expression and induces sensory hair cell or HEI-OC1 cell damage, while application of ivacaftor halts these effects. Although ivacaftor increases Nrf2 expression and reduces ROS accumulation, cotreatment with ML385, an Nrf2 inhibitor, abolishes the protective effects of ivacaftor against hydrogen-peroxide-induced HEI-OC1 cell death. Our results indicate that noise-induced sensory hair cell damage is associated with p-CFTR. Ivacaftor has potential for treatment of noise-induced hearing loss by maintaining CFTR function and increasing Nrf2 expression for support of redox homeostasis in sensory hair cells.

Mapping oto-pharyngeal development in a human inner ear organoid model

Inner ear development requires the coordination of cell types from distinct epithelial, mesenchymal and neuronal lineages. Although we have learned much from animal models, many details about human inner ear development remain elusive. We recently developed an in vitro model of human inner ear organogenesis using pluripotent stem cells in a 3D culture, fostering the growth of a sensorineural circuit, including hair cells and neurons. Despite previously characterizing some cell types, many remain undefined. This study aimed to chart the in vitro development timeline of the inner ear organoid to understand the mechanisms at play. Using single-cell RNA sequencing at ten stages during the first 36 days of differentiation, we tracked the evolution from pluripotency to various ear cell types after exposure to specific signaling modulators. Our findings showcase gene expression that influences differentiation, identifying a plethora of ectodermal and mesenchymal cell types. We also discern aspects of the organoid model consistent with in vivo development, while highlighting potential discrepancies. Our study establishes the Inner Ear Organoid Developmental Atlas (IODA), offering deeper insights into human biology and improving inner ear tissue differentiation.

Nonextensive realizations in interacting ion channels: Implications for mechano-electrical transducer mechanisms

We propose a theoretical model to investigate the thermodynamics of single and coupled two-state ion channels, associated with mechanoelectrical transduction (MET) and hair cell biophysics. The modeling was based on the Tsallis nonextensive statistical mechanics. The choice for a nonextensive framework in modeling ion channels is encouraged on the fact that we take into account the presence of interactions or long-range correlations in the dynamics of single and coupled ion channels. However, the basic assumptions that support Boltzmann-Gibbs statistics, traditionally used to model ion channel dynamics, state that the system is formed by independent or weakly interacting elements. Despite being well studied in many biological systems, the literature has not yet addressed the study of both entropy and mutual information related to isolated or physically interacting pairs of MET channels. Inspired by hair cell biophysics, we show how the presence of nonextensivity, or subadditivity and superadditivity modulates the nonextensive entropy and mutual information as functions of stereocilia displacements. We also observe that the magnitude of the interaction between the two channels, given by a nonextensive parameter, influences the amplitude of the nonextensive joint entropy and mutual information as functions of the hair cell displacements. Finally, we show how nonextensivity regulates the current versus displacement curve for a single and a pair of interacting two-state channels. The present findings shed light on the thermodynamic process involved in the molecular mechanisms of the auditory system.

Expression of the human usherin c.2299delG mutation leads to early-onset auditory loss and stereocilia disorganization

Usher syndrome (USH) is the leading cause of combined deafness and blindness, with USH2A being the most prevalent form. The mechanisms responsible for this debilitating sensory impairment remain unclear. This study focuses on characterizing the auditory phenotype in a mouse model expressing the c.2290delG mutation in usherin equivalent to human frameshift mutation c.2299delG. Previously we described how this model reproduces patient's retinal phenotypes. Here, we present the cochlear phenotype, showing that the mutant usherin, is expressed during early postnatal stages. The c.2290delG mutation results in a truncated protein that is mislocalized within the cell body of the hair cells. The knock-in model also exhibits congenital hearing loss that remains consistent throughout the animal's lifespan. Structurally, the stereocilia bundles, particularly in regions associated with functional hearing loss, are disorganized. Our findings shed light on the role of usherin in maintaining structural support, specifically in longer inner hair cell stereocilia, during development, which is crucial for proper bundle organization and hair cell function. Overall, we present a genetic mouse model with cochlear defects associated with the c.2290delG mutation, providing insights into the etiology of hearing loss and offering potential avenues for the development of effective therapeutic treatments for USH2A patients.

Sox9 Inhibits Cochlear Hair Cell Fate by Upregulating Hey1 and HeyL Antagonists of Atoh1

It is widely accepted that cell fate determination in the cochlea is tightly controlled by different transcription factors (TFs) that remain to be fully defined. Here, we show that Sox9, initially expressed in the entire sensory epithelium of the cochlea, progressively disappears from differentiating hair cells (HCs) and is finally restricted to supporting cells (SCs). By performing ex vivo electroporation of E13.5-E14.5 cochleae, we demonstrate that maintenance of Sox9 expression in the progenitors committed to HC fate blocks their differentiation, even if co-expressed with Atoh1, a transcription factor necessary and sufficient to form HC. Sox9 inhibits Atoh1 transcriptional activity by upregulating Hey1 and HeyL antagonists, and genetic ablation of these genes induces extra HCs along the cochlea. Although Sox9 suppression from sensory progenitors ex vivo leads to a modest increase in the number of HCs, it is not sufficient in vivo to induce supernumerary HC production in an inducible Sox9 knockout model. Taken together, these data show that Sox9 is downregulated from nascent HCs to allow the unfolding of their differentiation program. This may be critical for future strategies to promote fully mature HC formation in regeneration approaches.

β-Catenin transcriptional activity is required for establishment of inner pillar cell identity during cochlear development

The mammalian cochlea is composed of sensory hair cells as well as multiple different types of non-sensory supporting cells. Pillar cells are one type of supporting cell that form the tunnel of Corti and include two morphologically and functionally distinct subtypes: inner pillar cells (IPCs) and outer pillar cells (OPCs). The processes of specification and differentiation of inner versus outer pillar cells are still unclear. Here, we show that β-Catenin is required for establishing IPC identity in the mammalian cochlea. To differentiate the transcriptional and adhesion roles of β-Catenin in establishing IPC identity, we examined two different models of β-Catenin deletion; one that deletes both transcriptional and structural functions and one which retains cell adhesion function but lacks transcriptional function. Here, we show that cochleae lacking β-Catenin transcriptional function lost IPCs and displayed extranumerary OPCs, indicating its requirement for establishing IPC identity. Overexpression of β-Catenin induced proliferation within IPCs but not ectopic IPCs. Single-cell transcriptomes of supporting cells lacking β-Catenin transcriptional function show a loss of the IPC and gain of OPC signatures. Finally, targeted deletion of β-Catenin in IPCs also led to the loss of IPC identity, indicating a cell autonomous role of β-Catenin in establishing IPC identity. As IPCs have the capacity to regenerate sensory hair cells in the postnatal cochlea, our results will aid in future IPC-based hair cell regeneration strategies.

The protective role of Wnt3a in peroxynitrite-induced damage of cochlear hair cells in vitro

OBJECTIVE

To investigate the effect of peroxynitrite on the cultured cochlear hair cells of C57BL/6 P3 mice in vitro as well as the role of Wnt3a, as an activator of the canonical Wnt signaling pathway, underlying the action of such an oxidative stress.

METHODS

The in vitro primary cultured cochlear hair cells were subjected to l00 μM peroxynitrite and l00 μM peroxynitrite +25 ng/mL Wnt3a for 24 h, the cell survival and morphological changes were examined by immunofluorescence and transmission electron microscopy.

RESULTS

The number of surviving hair cells was significantly reduced in the 100 μM peroxynitrite group, while it was significantly higher in the Wnt3a + peroxynitrite treated group compared with the peroxynitrite treated group. The transmission electron microscopy showed that exposure to peroxynitrite induced a dramatic decrease in the number of mitochondria and severely disrupted mitochondrial ultrastructure, while Wnt3a clearly diminished the disruption of mitochondrial structure and preserved a higher number of mitochondria.

CONCLUSION

These results indicated that peroxynitrite could cause oxidative damage to the cochlear hair cells, and low concentrations of Wnt3a has a protective effect against oxidative damage.

LEVEL OF EVIDENCE

Level 2.

Dissociation between video head impulse test and caloric test: a marker of menière's disease? - A systematic review and meta-analysis

OBJECTIVE

To analyze, by means of a systematic review and meta-analysis, the proportion of patients with Meniere's disease who have altered caloric test and vHIT, as well as to determine the prevalence of altered caloric test and normal vHIT dissociation in the diagnosis of Meniere's disease.

METHODS

The literature search had no restriction regarding the period of publication on the following indexed data platforms: PubMed, PubMed PMC, BVS-Bireme, Web of Science, Embase and Cochrane Library. Articles that evaluated patients with Meniere's disease who underwent caloric test and vHIT were included. Two researchers independently conducted the analysis of the articles, promoting the selection and capture of data, following the recommendations of the PRISMA method, and complying with the criteria for articles inclusion and exclusion defined in the research protocol. In case of disagreement during the selection process, a third researcher was included for analysis.

RESULTS

From a total of 427 initial studies, the researchers selected 12 articles, published between 2014 and 2021, with a total of 708 patients evaluated, with a mean age of 52.72 years old. The prevalence of patients with Meniere's disease with altered caloric reflex test was 64% (95% CI 57%‒71%), while the prevalence of altered vHIT was only 28% (95% CI 16%-40%). The prevalence of the altered caloric test + normal vHIT dissociation was 47% (95% CI 37%-57%).

CONCLUSION

The video head impulse test and the caloric test are valuable tools for vestibular assessment. The dissociation of findings between these two tests in patients with Meniere's disease was more prevalent in this meta-analysis and may be a result of the tonotopy of specialized hair cells in the ampullary crest. The prevalence of altered caloric test was 64% and anormal vHIT was 28%. The dissociation caloric asymmetry and normal vHIT was observed in 47% of the patients.

LEVEL OF EVIDENCE: 1

Key genes and potential drugs in age-related hearing loss: transcriptome analysis of cochlear hair cells in old mice

This study aimed to dig new molecular mechanisms and medications for age-related hearing loss (ARHL or presbycusis) by extracting common results of publicly available datasets. Based on five datasets (GSE153882, GSE121856, GSE98070, GSE45026, and GSE98071) in studies of cochlear hair cells, we explored the interrelationships among presbycusis-related genes, including gene interactions, enrichment analysis, miRNA-mRNA matching pairs, and potential new drugs. Together, there were 25 common increased mRNAs. A total of 183 drugs can simultaneously target 11 of these mRNAs. In the interaction network, hub genes included: Cbln1, Prl, Mpp6 and Gh. Meanwhile, there were 74 common decreased mRNAs. The hub genes include Cdkn1a, Egr1, and Ctgf. After de-duplication, the 25 common increased mRNAs had 946 matched miRNAs, with 34 decreased ones; and the 74 decreased mRNAs had 1164 matched miRNAs, with 26 increased ones. Between the inhibitors of increased mRNAs and enhancers of decreased mRNAs, there were 26 common drugs. Besides, we discovered six key genes that may play a crucial role in the onset of presbycusis. In conclusion, by jointly analyzing multiple datasets, we found 25 common increased mRNAs (e.g., Cbln1, Prl, Mpp6 and Gh) and 74 common decreased mRNAs (Cdkn1a, Egr1, and Ctgf), as well as 34 potential therapeutic miRNAs and 26 pathogenic miRNAs, and three candidate drugs (calcitriol, diclofenac, and diethylstilbestrol). They may provide new targets and strategies for mechanistic and therapeutic studies in ARHL.

Early radial positional information in the cochlea is optimized by a precise linear BMP gradient and enhanced by SOX2

Positional information encoded in signaling molecules is essential for early patterning in the prosensory domain of the developing cochlea. The sensory epithelium, the organ of Corti, contains an exquisite repeating pattern of hair cells and supporting cells. This requires precision in the morphogen signals that set the initial radial compartment boundaries, but this has not been investigated. To measure gradient formation and morphogenetic precision in developing cochlea, we developed a quantitative image analysis procedure measuring SOX2 and pSMAD1/5/9 profiles in mouse embryos at embryonic day (E)12.5, E13.5, and E14.5. Intriguingly, we found that the pSMAD1/5/9 profile forms a linear gradient up to the medial ~ 75% of the PSD from the pSMAD1/5/9 peak in the lateral edge during E12.5 and E13.5. This is a surprising activity readout for a diffusive BMP4 ligand secreted from a tightly constrained lateral region since morphogens typically form exponential or power-law gradient shapes. This is meaningful for gradient interpretation because while linear profiles offer the theoretically highest information content and distributed precision for patterning, a linear morphogen gradient has not yet been observed. Furthermore, this is unique to the cochlear epithelium as the pSMAD1/5/9 gradient is exponential in the surrounding mesenchyme. In addition to the information-optimized linear profile, we found that while pSMAD1/5/9 is stable during this timeframe, an accompanying gradient of SOX2 shifts dynamically. Last, through joint decoding maps of pSMAD1/5/9 and SOX2, we see that there is a high-fidelity mapping between signaling activity and position in the regions that will become Kölliker's organ and the organ of Corti. Mapping is ambiguous in the prosensory domain precursory to the outer sulcus. Altogether, this research provides new insights into the precision of early morphogenetic patterning cues in the radial cochlea prosensory domain.

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.

Autophagy proteins are essential for aminoglycoside-induced hearing loss

Aminoglycosides (AGs) are widely used to treat severe infections. However, systemically administered AGs preferentially kill cochlear hair cells, resulting in irreversible hearing loss. Recently, we found that AGs bind to RIPOR2 and trigger its rapid translocation in cochlear hair cells. Reducing RIPOR2 expression entirely prevents AG-induced hair cell death and subsequent hearing loss in mice. Next using yeast two-hybrid screening, we found that RIPOR2 interacts with GABARAP, a key macroautophagy/autophagy pathway protein. Following AG treatment, RIPOR2 colocalizes with GABARAP and regulates the activation of autophagy. Remarkably, reducing the expression of GABARAP, or another key autophagy protein MAP1LC3B/LC3B, entirely prevents AG-induced hair cell death and subsequent hearing loss in mice. Furthermore, we found that AGs activate the autophagy pathway specific to mitochondria. Reducing the expression of PINK1 or PRKN/parkin, two key mitophagy proteins, protects hair cells against AG toxicity. Thus, our findings demonstrated that RIPOR2-mediated autophagic dysfunction is essential for AG-induced hearing loss and provided potential therapeutic strategies for preventing AG toxicity.

Valproic Acid Inhibits Progressive Hereditary Hearing Loss in a KCNQ4 Variant Model through HDAC1 Suppression

Genetic or congenital hearing loss still has no definitive cure. Among genes related to genetic hearing loss, the potassium voltage-gated channel subfamily Q member 4 (KCNQ4) is known to play an essential role in maintaining ion homeostasis and regulating hair cell membrane potential. Variants of the KCNQ4 show reductions in the potassium channel activity and were responsible for non-syndromic progressive hearing loss. KCNQ4 has been known to possess a diverse variant. Among those variants, the KCNQ4 p.W276S variant produced greater hair cell loss related to an absence of potassium recycling. Valproic acid (VPA) is an important and commonly used histone deacetylase (HDAC) inhibitor for class I (HDAC1, 2, 3, and 8) and class IIa (HDAC4, 5, 7, and 9). In the current study, systemic injections of VPA attenuated hearing loss and protected the cochlear hair cells from cell death in the KCNQ4 p.W276S mouse model. VPA activated its known downstream target, the survival motor neuron gene, and increased acetylation of histone H4 in the cochlea, demonstrating that VPA treatment directly affects the cochlea. In addition, treatment with VPA increased the KCNQ4 binding with HSP90β by inhibiting HDAC1 activation in HEI-OC1 in an in vitro study. VPA is a candidate drug for inhibiting late-onset progressive hereditary hearing loss from the KCNQ4 p.W276S variant.

AAV-Net1 facilitates the trans-differentiation of supporting cells into hair cells in the murine cochlea

Mechanosensitive hair cells (HCs) in the cochlear sensory epithelium are critical for sound detection and transduction. Mammalian HCs in the cochlea undergo cytogenesis during embryonic development, and irreversible damage to hair cells postnatally is a major cause of deafness. During the development of the organ of Corti, HCs and supporting cells (SCs) originate from the same precursors. In the neonatal cochlea, damage to HCs activates adjacent SCs to act as HC precursors and to differentiate into new HCs. However, the plasticity of SCs to produce new HCs is gradually lost with cochlear development. Here, we delineate an essential role for the guanine nucleotide exchange factor Net1 in SC trans-differentiation into HCs. Net1 overexpression mediated by AAV-ie in SCs promoted cochlear organoid formation and HC differentiation under two and three-dimensional culture conditions. Also, AAV-Net1 enhanced SC proliferation in Lgr5-EGFPCreERT2 mice and HC generation as indicated by lineage tracing of HCs in the cochleae of Lgr5-EGFPCreERT2/Rosa26-tdTomatoloxp/loxp mice. We further found that the up-regulation of Wnt/β-catenin and Notch signaling in AAV-Net1-transduced cochleae might be responsible for the SC proliferation and HC differentiation. Also, Net1 overexpression in SCs enhanced SC proliferation and HC regeneration and survival after HC damage by neomycin. Taken together, our study suggests that Net1 might serve as a potential target for HC regeneration and that AAV-mediated gene regulation may be a promising approach in stem cell-based therapy in hearing restoration.

Promoting TFEB nuclear localization with curcumin analog C1 attenuates sensory hair cell injury and delays age-related hearing loss in C57BL/6 mice

Sensory hair cell (HC) injuries, especially outer hair cell (OHC) loss, are well-documented to be the primary pathology of age-related hearing loss (AHL). Recent studies have demonstrated that autophagy plays an important role in HC injury in the inner ear. In our previous works, a decline in autophagy levels and HC loss were found to occur simultaneously in the inner ears of aged C57BL/6 mice, and the administration of rapamycin promoted autophagy levels, which reduced OHC loss and delayed AHL, but the underlying mechanism of autophagy in AHL has not been well elucidated. Transcription factor EB (TFEB), an autophagy regulator and the downstream target of mammalian target of rapamycin (mTOR), is involved in the pathological development of neurodegenerative disease. This study would address the link between autophagy and TFEB in aged C57BL/6 mouse cochleae and clarify the effect of the TFEB activator curcumin analog C1 (C1) in aged cochleae. Decreased TFEB nuclear localization (p = 0.0371) and autophagy dysfunction (p = 0.0273) were observed in the cochleae of aged C57BL/6 mice that exhibited AHL, HCs loss and HCs senescence. Treatment with C1 promoted TFEB nuclear localization and restored autophagy, subsequently alleviating HC injury and delaying AHL. The protective effect of C1 on HEI-OC1 cells against autophagy disorder and aging induced by D-galactose was abolished by chloroquine, which is one of the commonly used autophagy inhibitors. Overall, our results demonstrated that the capacity to perform autophagy is mediated by the nuclear localization of TFEB in aged C57BL/6 mouse cochleae. C1 promotes the nuclear localization of TFEB, subsequently alleviating HC injury and delaying AHL by restoring the impaired autophagy function. TFEB may serve as a new therapeutic target for AHL treatment.

Aucubin protects mouse cochlear hair cells from cisplatin-induced ototoxicity via activation of the PI3K/AKT/STAT3 pathway

Cisplatin is commonly used to treat cancers and is associated with a significant risk of irreversible sensorineural hearing loss. However, no effective preventive strategies are available for cisplatin-induced HL. Therefore, significant efforts have been made to discover new drugs protecting cochlear hair cells from cisplatin-induced damage. We found that a new phytochemical, aucubin, attenuated cisplatin-induced apoptosis, the production of reactive oxygen species, and mitochondrial dysfunction in House Ear Institute Organ of Corti 1 cells and cochlear hair cells. Moreover, aucubin attenuated cisplatin-induced sensorineural hearing loss and hair cells loss in vivo. Furthermore, RNA sequencing analysis revealed that the otoprotective effects of aucubin were mainly mediated by increased STAT3 phosphorylation via the PI3K/AKT pathway. Inhibition of the STAT3 signaling pathway with the inhibitor S3I-201 or siRNA disrupted the protective effects of aucubin on cisplatin-induced apoptosis. In conclusion, we identified an otoprotective effect of aucubin. Therefore, aucubin could be used to prevent cisplatin-induced ototoxicity.

Precise alternating cellular pattern in the inner ear by coordinated hopping intercalations and delaminations

The mammalian hearing organ, the organ of Corti, is one of the most organized tissues in mammals. It contains a precisely positioned array of alternating sensory hair cells (HCs) and nonsensory supporting cells. How such precise alternating patterns emerge during embryonic development is not well understood. Here, we combine live imaging of mouse inner ear explants with hybrid mechano-regulatory models to identify the processes that underlie the formation of a single row of inner hair cells (IHCs). First, we identify a previously unobserved morphological transition, termed "hopping intercalation," that allows cells differentiating toward IHC fate to "hop" under the apical plane into their final position. Second, we show that out-of-row cells with low levels of the HC marker Atoh1 delaminate. Last, we show that differential adhesion between cell types contributes to straightening of the IHC row. Our results support a mechanism for precise patterning based on coordination between signaling and mechanical forces that is likely relevant for many developmental processes.

Fbxo2CreERT2: A new model for targeting cells in the neonatal and mature inner ear

The mammalian inner ear contains six sensory patches that allow detection of auditory stimuli as well as movement and balance. Much research has focused on the organ of Corti, the sensory organ of the cochlea that detects sound. Unfortunately, these cells are difficult to access in vivo, especially in the mature animal, but the development of genetically modified mouse models, including Cre/Lox mice, has improved the ability to label, purify or manipulate these cells. Here, we describe a new tamoxifen-inducible CreER mouse line, the Fbxo2CreERT2 mouse, that can be used to specifically manipulate cells throughout the cochlear duct of the neonatal and mature cochlear epithelium. In vestibular sensory epithelia, Fbxo2CreERT2-mediated recombination occurs in many hair cells and more rarely in supporting cells of neonatal and adult mice, with a higher rate of Fbxo2CreERT2 induction in type 1 versus type 2 hair cells in adults. Fbxo2CreERT2 mice, therefore, are a new tool for the specific manipulation of epithelial cells of the inner ear and targeted manipulation of vestibular type 1 hair cells.

Lateral line ablation by ototoxic compounds results in distinct rheotaxis profiles in larval zebrafish

The zebrafish lateral line is an established model for hair cell organ damage, yet few studies link mechanistic disruptions to changes in biologically relevant behavior. We used larval zebrafish to determine how damage via ototoxic compounds impact rheotaxis. Larvae were treated with CuSO4 or neomycin to disrupt lateral line function then exposed to water flow stimuli. Their swimming behavior was recorded on video then DeepLabCut and SimBA software were used to track movements and classify rheotaxis behavior, respectively. Lateral line-disrupted fish performed rheotaxis, but they swam greater distances, for shorter durations, and with greater angular variance than controls. Furthermore, spectral decomposition analyses confirmed that lesioned fish exhibited ototoxic compound-specific behavioral profiles with distinct changes in the magnitude, frequency, and cross-correlation between fluctuations in linear and angular movements. Our observations demonstrate that lateral line input is needed for fish to hold their station in flow efficiently and reveals that commonly used lesion methods have unique effects on rheotaxis behavior.

Cochlear hair cell innervation is dependent on a modulatory function of Semaphorin-3A

BACKGROUND

Proper connectivity between type I spiral ganglion neurons (SGNs) and inner hair cells (IHCs) in the cochlea is necessary for conveying sound information to the brain in mammals. Previous studies have shown that type I SGNs are heterogeneous in form, function and synaptic location on IHCs, but factors controlling their patterns of connectivity are not well understood.

RESULTS

During development, cochlear supporting cells and SGNs express Semaphorin-3A (SEMA3A), a known axon guidance factor. Mice homozygous for a point mutation that attenuates normal SEMA3A repulsive activity (Sema3aK108N ) show cochleae with grossly normal patterns of IHC innervation. However, genetic sparse labeling and three-dimensional reconstructions of individual SGNs show that cochleae from Sema3aK108N mice lacked the normal synaptic distribution of type I SGNs. Additionally, Sema3aK108N cochleae show a disrupted distribution of GLUA2 postsynaptic patches around the IHCs. The addition of SEMA3A-Fc to postnatal cochleae led to increases in SGN branching, similar to the effects of inhibiting glutamate receptors. Ca2+ imaging studies show that SEMA3A-Fc decreases SGN activity.

CONCLUSIONS

Contrary to the canonical view of SEMA3A as a guidance ligand, our results suggest SEMA3A may regulate SGN excitability in the cochlea, which may influence the morphology and synaptic arrangement of type I SGNs.

Characterizing the Relationship Between Reflection and Distortion Otoacoustic Emissions in Normal-Hearing Adults

Otoacoustic emissions (OAEs) arise from one (or a combination) of two basic generation mechanisms in the cochlea: nonlinear distortion and linear reflection. As a result of having distinct generation processes, these two classes of emissions may provide non-redundant information about hair-cell integrity and show distinct sensitivities to cochlear pathology. Here, we characterize the relationship between reflection and distortion emissions in normal hearers across a broad frequency and stimulus-level space using novel analysis techniques. Furthermore, we illustrate the promise of this approach in a small group of individuals with mild-moderate hearing loss. A "joint-OAE profile" was created by measuring interleaved swept-tone stimulus-frequency OAEs (SFOAEs) and 2f1-f2 distortion-product OAEs (DPOAEs) in the same ears using well-considered parameters. OAE spectra and input/output functions were calculated across five octaves. Using our specific recording protocol and analysis scheme, SFOAEs in normal hearers had higher levels than did DPOAEs, with the most pronounced differences occurring at the highest stimulus levels. Also, SFOAE compression occurred at higher stimulus levels (than did DPOAE compression) and its growth in the compressed region was steeper. The diagnostic implications of these findings and the influence of the measurement protocol on both OAEs (and on their relationship) are discussed.

Reducing Taperin Expression Restores Hearing in Grxcr2 Mutant Mice

Recessive mutations in GRXCR2 cause deafness in both humans and mice. In Grxcr2 null hair cells, the sensory receptors for sound in the inner ear, stereocilia are disorganized. Reducing the expression of taperin, a protein that interacts with GRXCR2 at the base of stereocilia, corrects the morphological defects of stereocilia and restores hearing in Grxcr2 null mice. To further validate this finding, this study generated two novel taperin mutant mouse lines that exhibit progressive hearing loss. Then Grxcr2 null mice were crossed with one of these taperin mutant mice. The following morphological analysis revealed that reducing taperin expression indeed corrected stereocilia morphological abnormalities in Grxcr2 null mice. Functional analysis further confirmed that reducing taperin expression partially restored hearing in Grxcr2 null mice.

WNT Activation and TGFβ-Smad Inhibition Potentiate Stemness of Mammalian Auditory Neuroprogenitors for High-Throughput Generation of Functional Auditory Neurons In Vitro

Hearing loss affects over 460 million people worldwide and is a major socioeconomic burden. Both genetic and environmental factors (i.e., noise overexposure, ototoxic drug treatment and ageing), promote the irreversible degeneration of cochlear hair cells and associated auditory neurons, leading to sensorineural hearing loss. In contrast to birds, fish and amphibians, the mammalian inner ear is virtually unable to regenerate due to the limited stemness of auditory progenitors, and no causal treatment is able to prevent or reverse hearing loss. As of today, a main limitation for the development of otoprotective or otoregenerative therapies is the lack of efficient preclinical models compatible with high-throughput screening of drug candidates. Currently, the research field mainly relies on primary organotypic inner ear cultures, resulting in high variability, low throughput, high associated costs and ethical concerns. We previously identified and characterized the phoenix auditory neuroprogenitors (ANPGs) as highly proliferative progenitor cells isolated from the A/J mouse cochlea. In the present study, we aim at identifying the signaling pathways responsible for the intrinsic high stemness of phoenix ANPGs. A transcriptomic comparison of traditionally low-stemness ANPGs, isolated from C57Bl/6 and A/J mice at early passages, and high-stemness phoenix ANPGs was performed, allowing the identification of several differentially expressed pathways. Based on differentially regulated pathways, we developed a reprogramming protocol to induce high stemness in presenescent ANPGs (i.e., from C57Bl6 mouse). The pharmacological combination of the WNT agonist (CHIR99021) and TGFβ/Smad inhibitors (LDN193189 and SB431542) resulted in a dramatic increase in presenescent neurosphere growth, and the possibility to expand ANPGs is virtually limitless. As with the phoenix ANPGs, stemness-induced ANPGs could be frozen and thawed, enabling distribution to other laboratories. Importantly, even after 20 passages, stemness-induced ANPGs retained their ability to differentiate into electrophysiologically mature type I auditory neurons. Both stemness-induced and phoenix ANPGs resolve a main bottleneck in the field, allowing efficient, high-throughput, low-cost and 3R-compatible in vitro screening of otoprotective and otoregenerative drug candidates. This study may also add new perspectives to the field of inner ear regeneration.

Rescue of autosomal dominant hearing loss by in vivo delivery of mini dCas13X-derived RNA base editor

Programmable RNA editing tools enable the reversible correction of mutant transcripts, reducing the potential risk associated with permanent genetic changes associated with the use of DNA editing tools. However, the potential of these RNA tools to treat disease remains unknown. Here, we evaluated RNA correction therapy with Cas13-based RNA base editors in the myosin VI p.C442Y heterozygous mutation (Myo6C442Y/+) mouse model that recapitulated the phenotypes of human dominant-inherited deafness. We first screened several variants of Cas13-based RNA base editors and guide RNAs (gRNAs) targeting Myo6C442Y in cultured cells and found that mini dCas13X.1-based adenosine base editor (mxABE), composed of truncated Cas13X.1 and the RNA editing enzyme adenosine deaminase acting on RNA 2 deaminase domain variant (ADAR2ddE488Q), exhibited both high efficiency of A > G conversion and low frequency of off-target edits. Single adeno-associated virus (AAV)-mediated delivery of mxABE in the cochlea corrected the mutated Myo6C442Y to Myo6WT allele in homozygous Myo6C442Y/C442Y mice and resulted in increased Myo6WT allele in the injected cochlea of Myo6C442Y/+ mice. The treatment rescued auditory function, including auditory brainstem response and distortion product otoacoustic emission up to 3 months after AAV-mxABE-Myo6 injection in Myo6C442Y/+ mice. We also observed increased survival rate of hair cells and decreased degeneration of hair bundle morphology in the treated compared to untreated control ears. These findings provide a proof-of-concept study for RNA editing tools as a therapeutic treatment for various semidominant forms of hearing loss and other diseases.

Modeling Active Non-Markovian Oscillations

Modeling noisy oscillations of active systems is one of the current challenges in physics and biology. Because the physical mechanisms of such processes are often difficult to identify, we propose a linear stochastic model driven by a non-Markovian bistable noise that is capable of generating self-sustained periodic oscillation. We derive analytical predictions for most relevant dynamical and thermodynamic properties of the model. This minimal model turns out to describe accurately bistablelike oscillatory motion of hair bundles in bullfrog sacculus, extracted from experimental data. Based on and in agreement with these data, we estimate the power required to sustain such active oscillations to be of the order of 100  k_{B}T per oscillation cycle.

Single-cell RNA-sequencing of zebrafish hair cells reveals novel genes potentially involved in hearing loss

Hair cells play key roles in hearing and balance, and hair cell loss would result in hearing loss or vestibular dysfunction. Cellular and molecular research in hair cell biology provides us a better understanding of hearing and deafness. Zebrafish, owing to their hair cell-enriched organs, have been widely applied in hair cell-related research worldwide. Similar to mammals, zebrafish have inner ear hair cells. In addition, they also have lateral line neuromast hair cells. These different types of hair cells vary in morphology and function. However, systematic analysis of their molecular characteristics remains lacking. In this study, we analyzed the GFP+ cells isolated from Tg(Brn3c:mGFP) larvae with GFP expression in all hair cells using single-cell RNA-sequencing (scRNA-seq). Three subtypes of hair cells, namely macula hair cell (MHC), crista hair cell (CHC), and neuromast hair cell (NHC), were characterized and validated by whole-mount in situ hybridization analysis of marker genes. The hair cell scRNA-seq data revealed hair cell-specific genes, including hearing loss genes that have been identified in humans and novel genes potentially involved in hair cell formation and function. Two novel genes were discovered to specifically function in NHCs and MHCs, corresponding to their specific expression in NHCs and MHCs. This study allows us to understand the specific genes in hair cell subpopulations of zebrafish, which will shed light on the genetics of both human vestibular and cochlear hair cell function.

[Research progress of the regulation of cochlear sensitivity to noise by circadian rhythm]

High level noise can damage cochlear hair cells, auditory nerve and synaptic connections between cochlear hair cells and auditory nerve, resulting in noise-induced hearing loss (NIHL). Recent studies have shown that animal cochleae have circadian rhythm, which makes them different in sensitivity to noise throughout the day. Cochlear circadian rhythm has a certain relationship with brain-derived neurotrophic factor and glucocorticoids, which affects the degree of hearing loss after exposure to noise. In this review, we summarize the research progress of the regulation of cochlear sensitivity to noise by circadian rhythm and prospect the future research direction.

Fast adaptation of cooperative channels engenders Hopf bifurcations in auditory hair cells

Since the pioneering work of Thomas Gold, published in 1948, it has been known that we owe our sensitive sense of hearing to a process in the inner ear that can amplify incident sounds on a cycle-by-cycle basis. Called the active process, it uses energy to counteract the viscous dissipation associated with sound-evoked vibrations of the ear's mechanotransduction apparatus. Despite its importance, the mechanism of the active process and the proximate source of energy that powers it have remained elusive, especially at the high frequencies characteristic of amniote hearing. This is partly due to our insufficient understanding of the mechanotransduction process in hair cells, the sensory receptors and amplifiers of the inner ear. It has been proposed previously that cyclical binding of Ca2+ ions to individual mechanotransduction channels could power the active process. That model, however, relied on tailored reaction rates that structurally forced the direction of the cycle. Here we ground our study on our previous model of hair-cell mechanotransduction, which relied on cooperative gating of pairs of channels, and incorporate into it the cyclical binding of Ca2+ ions. With a single binding site per channel and reaction rates drawn from thermodynamic principles, the current model shows that hair cells behave as nonlinear oscillators that exhibit Hopf bifurcations, dynamical instabilities long understood to be signatures of the active process. Using realistic parameter values, we find bifurcations at frequencies in the kilohertz range with physiological Ca2+ concentrations. The current model relies on the electrochemical gradient of Ca2+ as the only energy source for the active process and on the relative motion of cooperative channels within the stereociliary membrane as the sole mechanical driver. Equipped with these two mechanisms, a hair bundle proves capable of operating at frequencies in the kilohertz range, characteristic of amniote hearing.

Grxcr1 regulates hair bundle morphogenesis and is required for normal mechanoelectrical transduction in mouse cochlear hair cells

Tasmanian devil (tde) mice are deaf and exhibit circling behaviour. Sensory hair cells of mutants show disorganised hair bundles with abnormally thin stereocilia. The origin of this mutation is the insertion of a transgene which disrupts expression of the Grxcr1 (glutaredoxin cysteine rich 1) gene. We report here that Grxcr1 exons and transcript sequences are not affected by the transgene insertion in tde homozygous (tde/tde) mice. Furthermore, 5'RACE PCR experiments showed the presence of two different transcripts of the Grxcr1 gene, expressed in both tde/tde and in wild-type controls. However, quantitative analysis of Grxcr1 transcripts revealed a significantly decreased mRNA level in tde/tde mice. The key stereociliary proteins ESPN, MYO7A, EPS8 and PTPRQ were distributed in hair bundles of homozygous tde mutants in a similar pattern compared with control mice. We found that the abnormal morphology of the stereociliary bundle was associated with a reduction in the size and Ca2+-sensitivity of the mechanoelectrical transducer (MET) current. We propose that GRXCR1 is key for the normal growth of the stereociliary bundle prior to the onset of hearing, and in its absence hair cells are unable to mature into fully functional sensory receptors.

High-resolution imaging of the mouse-hair-cell hair bundle by scanning electron microscopy

Scanning electron microscopy (SEM) allows cell surface imaging at a sub-nanometric resolution. However, the sample requires a specific preparation to sustain the high vacuum of the SEM and be electrically conductive. The sample preparation consists of dissection, fixation, dehydration, metal coating, and tissue mounting. Here we provide a comprehensive protocol to perform SEM on the mouse’s inner ear, and image the hair bundles at high resolution. Hair bundles are the force-sensitive organelles located at the apical surface of hair cells.

For complete details on the use and execution of this protocol, please refer to Trouillet et al. (2021).

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Histology and dissection of the mouse’s inner ear sensory epithelium

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Sample preparation for scanning electron microscopy of the hair cells

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Allows imaging of the mechanotransduction organelle and the tip links

Acupuncture Attenuates Ototoxicity Induced by Gentamicin in Mice

CONTEXT

Presbycusis is age-related, progressive, and symmetrical hearing loss in both ears. Acupuncture can play a vital role in the diagnosis and treatment of deafness, but its functional mechanism is still not entirely clear.

OBJECTIVE

The study intended to explore acupuncture's protective effects and mechanism of treatment in addressing ototoxicity induced by gentamicin (GM) in aged mice.

DESIGN

The research team designed an animal study, and a mouse model of ototoxicity induced by GM was established.

SETTING

The study took place in Nanchong Central Hospital, Sichuan, China.

ANIMALS

The animals were 48 male, Kunming mice, with sixteen being three months old and 32 being 18 month old.

INTERVENTION

The three-month-old mice were randomly assigned to a control group (n = 8) and a GM group (n = 8). The 18-month-old mice were randomly divided into four groups with eight mice each: a positive control group; a negative control group, the GM group; and two intervention groups, the acupuncture + GM group and the drug + GM group. The GM groups were intraperitoneally injected with 100 mg/kg daily of GM for 10 consecutive days. The acupuncture + GM group received acupuncture, and the drug + GM group was injected intraperitoneally with Genadol.

OUTCOME MEASURES

The effects of GM induction and treatment with acupuncture or a drug on the numbers of auditory cochlear hair cells were evaluated via an auditory test and cell staining. A real-time polymerase chain reaction (PCR) was performed for gene detection. The levels of superoxide dismutase (SOD), malondialdehyde (MDA), and glutathione (GSH) were measured.

RESULTS

The aged mice were susceptible to GM ototoxicity. After acupuncture, the threshold of the auditory brainstem response and the number of cochlear hair cells increased significantly. Acupuncture inhibited oxidative stress via the nuclear factor erythroid-derived factor 2-related factor 2 (NRF2) signaling pathway in the mice.

CONCLUSIONS

The data demonstrated that acupuncture can alleviate GM ototoxicity via the NRF2 signaling pathway, providing important support for acupuncture in treatment of GM ototoxicity.

Methods to Investigate Cell Polarity of Inner Ear

Hair cells in cochlea and vestibular organs depend on the coordinated cell polarity to perform the normal auditory or balance function. The mouse inner ear is one of the ideal model to study planar cell polarity. In this chapter, we introduce a series of general experimental methods for studying planar cell polarity in the inner ear. The approaches presented here are also applicable to other organs with particular polarity phenotypes.

Intermodulation distortions from an array of active nonlinear oscillators

Coupling is critical in nonlinear dynamical systems. It affects the stabilities of individual oscillators as well as the characteristics of their response to external forces. In the auditory system, the mechanical coupling between sensory hair cells has been proposed as a mechanism that enhances the inner ear's sensitivity and frequency discrimination. While extensive studies investigate the effects of coupling on the detection of a sinusoidal signal, the role of coupling underlying the response to a complex tone remains elusive. In this study, we measured the acoustic intermodulation distortions (IMDs) produced by the inner ears of two frog species stimulated simultaneously by two pure tones. The distortion intensity level displayed multiple peaks across stimulus frequencies, in contrast to the generic response from a single nonlinear oscillator. The multiple-peaked pattern was altered upon varying the stimulus intensity or an application of a perturbation tone near the distortion frequency. Numerical results of IMDs from a chain of coupled active nonlinear oscillators driven by two sinusoidal forces reveal the effects of coupling on the variation profile of the distortion amplitude. When the multiple-peaked pattern is observed, the chain's motion at the distortion frequency displays both a progressive wave and a standing wave. The latter arises due to coupling and is responsible for the multiple-peaked pattern. Our results illustrate the significance of mechanical coupling between active hair cells in the generation of auditory distortions, as a mechanism underlying the formation of in vivo standing waves of distortion signals.

Evaluation of the hair cell regeneration and claudin b and phoenix gene expression during exposure to low concentrations of cadmium and zinc in early developing zebrafish larvae

Zebrafish possess hair cells on the body surface similar to that of mammals inner hear, in particular in the neuromasts, and due to its ability in regenerating damaged hair cells, is regularly used as a powerful animal model to study in vivo cytotoxicity. Among the factors leading to hair cell disruption, metal ions are of particular concern since they are important environmental pollutants. To date, several studies on zebrafish hair cell regeneration after metal exposure exist, while no data on regeneration during continuous metal exposure are available. In the present study, neuromast hair cell disruption and regeneration were assessed in zebrafish larvae for the first time during zinc (Zn) and cadmium (Cd) continuous exposure and a visual and molecular approach was adopted. Fluorescent vital dye DASPEI was used to assess hair cell regeneration and the gene expression of claudin b (cldnb) and phoenix (pho), was analyzed. Metallotionein-2 (mt2) gene expression was used as standard molecular marker of metal toxicity and confirmed the higher toxicity of Cd compared to Zn. In addition, Cd caused a delay in hair cell regeneration compared to Zn. Molecular analysis showed cldnb gene expression increased in relation to the metal concentrations used, confirming the involvement of this gene in hair cell regeneration. On the contrary, a dramatic decrease of pho gene expression was observed in Cd exposed groups, suggesting a negative impact of Cd on pho expression, thus negatively interfering with hair cell regeneration in zebrafish larvae exposed to this metal.

Understanding the differentiation and epigenetics of cochlear sensory progenitors in pursuit of regeneration

PURPOSE OF REVIEW

Sensory hair cells (HCs) of the inner ear are responsible for our ability to hear and balance. Loss of these cells results in hearing loss. Stem cell replacement and in situ regeneration have the potential to replace lost HCs. Newly discovered contributions of transcription factor regulatory networks and epigenetic mechanisms in regulating HC differentiation and regeneration are placed into context of the literature.

RECENT FINDINGS

A wealth of new data has helped to define cochlear sensory progenitors in their developmental trajectories. This includes transcription factor networks, epigenetic manipulations, and cochlear HC subtype specification.

SUMMARY

Understanding how sensory progenitors differ and how HC subtypes arise will substantially inform efforts in hearing restoration.

Insights into the molecular mechanisms regulating mammalian hair cell regeneration

PURPOSE OF REVIEW

To give an overview of recent advances in mammalian auditory hair cell regeneration.

RECENT FINDINGS

Supporting cells act as progenitors to regenerate hair cells in the prehearing mammalian cochlea but not in the mature cochlea. To overcome this developmental obstacle, manipulation of multiple genes and intracellular pathways has been investigated, which has obtained promising data. This review focuses on recent advances in auditory hair cell regeneration, including synergic gene regulation associated with Atoh1 and Notch signaling, epigenetics, and functional recovery of regenerated hair cells. Co-manipulation of genes critical for hair cell development and cell cycle re-entry, including Atoh1, Isl1, Pou4f3, Gata3, Gfi1, P27kip1, RB, Myc, and Notch-signaling genes, has generated hair cell-like cells in the adult cochlea both in vitro and in vivo. The epigenetic mechanism has been studied in hair cell development and regeneration. Regeneration of hair cell function has a very limited progress, which lacks in-vitro and in-vivo electrophysiology data.

SUMMARY

Regeneration of adult auditory hair cells remains a major challenge. Manipulation of multiple genes and pathways together with epigenetic regulation might potentially regenerate functional hair cells in the adult mammalian cochlea.