ABR/DPOAE publications

Adenylyl cyclase 6 (AC6) synthesizes second messenger cAMP in G protein-coupled receptor (GPCR) signaling. In cochlear hair cells, AC6 distribution relies on an adhesion GPCR, ADGRV1, which is associated with Usher syndrome (USH), a condition of combined hearing and vision loss. ADGRV1 is a component of the USH type 2 (USH2) protein complex in hair cells and photoreceptors. However, the role of AC6 in the inner ear and retina has not been explored. Here, we found that AC6 distribution in hair cells depends on the USH2 protein complex integrity. Several known AC6 regulators and effectors, which were previously reported to participate in ADGRV1 signaling in vitro, are localized to the stereociliary compartments that overlap with AC6 distribution in hair cells. In young AC6 knockout ( Adcy6 −/− ) mice, the activity of cAMP-dependent protein kinase, but not Akt kinase, is altered in cochleas, while both kinases are normal in vestibular organs. Adult Adcy6 −/− mice however exhibit normal hearing function. AC6 is expressed in mouse retinas but rarely in photoreceptors. Adcy6 −/− mice have slightly enhanced photopic but normal scotopic vision. Therefore, AC6 may participate in the ADGRV1 signaling in hair cells but AC6 is not essential for cochlear and retinal development and maintenance.

Dyslexia is a reading and spelling disorder due to neurodevelopmental abnormalities and is occasionally found to be accompanied by hearing loss, but the reason for the associated deafness remains unclear. This study finds that knockout of the dyslexia susceptibility 1 candidate 1 gene ( Dyx1c1 −/− ) in mice, the best gene for studying dyslexia, causes severe hearing loss, and thus it is a good model for studying the mechanism of dyslexia‐related hearing loss (DRHL). This work finds that the Dyx1c1 gene is highly expressed in the mouse cochlea and that the spontaneous electrical activity of inner hair cells and type I spiral ganglion neurons is altered in the cochleae of Dyx1c1 −/− mice. In addition, primary ciliary dyskinesia‐related phenotypes such as situs inversus and disrupted ciliary structure are seen in Dyx1c1 −/− mice. In conclusion, this study gives new insights into the mechanism of DRHL in detail and suggests that Dyx1c1 may serve as a potential target for the clinical diagnosis of DRHL.

The ankle‐link complex (ALC) consists of USH2A, WHRN, PDZD7, and ADGRV1 and plays an important role in hair cell development. At present, its architectural organization and signaling role remain unclear. By establishing Adgrv1 Y6236fsX1 mutant mice as a model of the deafness‐associated human Y6244fsX1 mutation, the authors show here that the Y6236fsX1 mutation disrupts the interaction between adhesion G protein‐coupled receptor V subfamily member 1 (ADGRV1) and other ALC components, resulting in stereocilia disorganization and mechanoelectrical transduction (MET) deficits. Importantly, ADGRV1 inhibits WHRN phosphorylation through regional cAMP‐PKA signaling, which in turn regulates the ubiquitination and stability of USH2A via local signaling compartmentalization, whereas ADGRV1 Y6236fsX1 does not. Yeast two‐hybrid screening identified the E3 ligase WDSUB1 that binds to WHRN and regulates the ubiquitination of USH2A in a WHRN phosphorylation‐dependent manner. Further FlAsH‐BRET assay, NMR spectrometry, and mutagenesis analysis provided insights into the architectural organization of ALC and interaction motifs at single‐residue resolution. In conclusion, the present data suggest that ALC organization and accompanying local signal transduction play important roles in regulating the stability of the ALC.

Background Mesenchymal stem cell (MSC) transplantation is a promising therapeutic approach for noise-induced hearing loss (NIHL). As the indispensable role of apoptosis in MSC transplantation was raised, the benefits of MSC-derived apoptotic vesicles (apoVs) in several disease models have been proved. However, whether apoVs benefit in NIHL have not been studied yet. Methods Female CBA/J mice and HEI-OC1 cells were used in this study. Flow cytometry, nanoparticle tracking analysis (NTA) and transmission electron microscopy (TEM) were used to characterize apoVs. Proteomic analysis was used to identify function proteins in apoVs. Immunofluorescence was used to reveal distribution pattern. Auditory brainstem response (ABR) test was used to measure the effect of apoVs treatment. DCFH-DA staining and MitoSOX staining were used to indicate oxidative damage. Western-blot and qRT-PCR were used to study the signaling pathways. Results We found that apoVs can be endocytosed by hair cells through systemic administration. Importantly, apoVs administration effectively attenuated NIHL and reduced hair cell loss by resisting oxidative damage in vivo. Further, apoVs application activated forkhead box o3 (FOXO3a)—mitochondrial superoxide dismutase 2(SOD2) pathway, which may relate to signal transduction and activators of transcription 3 (STAT3) in apoVs. Conclusions These findings uncovered the role of apoVs in preventing NIHL and resisting oxidative damage, indicating that apoVs is a promising way for inner ear delivery and a prospective cell-free therapy for NIHL.

Hypothesis Microneedle-mediated intracochlear injection through the round window membrane (RWM) will facilitate intracochlear delivery, not affect hearing, and allow for full reconstitution of the RWM within 48 hours. Background We have developed polymeric microneedles that allow for in vivo perforation of the guinea pig RWM and aspiration of perilymph for diagnostic analysis, with full reconstitution of the RWM within 48 to 72 hours. In this study, we investigate the ability of microneedles to deliver precise volumes of therapeutics into the cochlea and assess the subsequent consequences on hearing. Methods Volumes of 1.0, 2.5, or 5.0 μL of artificial perilymph were injected into the cochlea at a rate of 1 μL/min. Compound action potential (CAP) and distortion product otoacoustic emission were performed to assess for hearing loss (HL), and confocal microscopy was used to evaluate the RWM for residual scarring or inflammation. To evaluate the distribution of agents within the cochlea after microneedle-mediated injection, 1.0 μL of FM 1–43 FX was injected into the cochlea, followed by whole mount cochlear dissection and confocal microscopy. Results Direct intracochlear injection of 1.0 μL of artificial perilymph in vivo , corresponding to about 20% of the scala tympani volume, was safe and did not result in HL. However, injection of 2.5 or 5.0 μL of artificial perilymph into the cochlea produced statistically significant high-frequency HL persisting 48 hours postperforation. Assessment of RWMs 48 hours after perforation revealed no inflammatory changes or residual scarring. FM 1–43 FX injection resulted in distribution of the agent predominantly in the basal and middle turns. Conclusion Microneedle-mediated intracochlear delivery of small volumes relative to the volume of the scala tympani is feasible, safe, and does not cause HL in guinea pigs; however, injection of large volumes induces high-frequency HL. Injection of small volumes of a fluorescent agent across the RWM resulted in significant distribution within the basal turn, less distribution in the middle turn, and almost none in the apical turn. Microneedle-mediated intracochlear injection, along with our previously developed intracochlear aspiration, opens the pathway for precision inner ear medicine.

Physiological and psychoacoustic studies of the medial olivocochlear reflex (MOCR) in humans have often relied on long duration elicitors (>100 ms). This is largely due to previous research using otoacoustic emissions (OAEs) that found multiple MOCR time constants, including time constants in the 100s of milliseconds, when elicited by broadband noise. However, the effect of the duration of a broadband noise elicitor on similar psychoacoustic tasks is currently unknown. The current study measured the effects of ipsilateral broadband noise elicitor duration on psychoacoustic gain reduction estimated from a forward-masking paradigm. Analysis showed that both masker type and elicitor duration were significant main effects, but no interaction was found. Gain reduction time constants were ∼46 ms for the masker present condition and ∼78 ms for the masker absent condition (ranging from ∼29 to 172 ms), both similar to the fast time constants reported in the OAE literature (70–100 ms). Maximum gain reduction was seen for elicitor durations of ∼200 ms. This is longer than the 50-ms duration which was found to produce maximum gain reduction with a tonal on-frequency elicitor. Future studies of gain reduction may use 150–200 ms broadband elicitors to maximally or near-maximally stimulate the MOCR.

Cochlear and vestibular hair cells are highly specialized sensory receptors for hearing and balance. Here, we report a serendipitous identification of a hair-cell-specific organelle in neonatal mouse inner ear, which we name "apicosome." The apicosome is ∼500 nm in diameter and shows itinerant nature and transient appearance during development in cochlear hair cells. In contrast to cochlear hair cells, the apicosome persists in vestibular hair cells even in adult. The timing of apicosome translocation and disappearance in cochlear hair cells during development is correlated with kinocilium development and maintenance. The apicosome is not seen in supporting cells despite the fact that nascent supporting cells have microvilli and a primary cilium. Interestingly, transdifferentiated hair cells from supporting cells also contain apicosome, suggesting that it is unique to hair cells. Thus, our study identifies a previously undescribed organelle in hair cells and lays the foundation for further characterization of this specialized structure.

Objective. Peripheral neural signals recorded during neuromodulation therapies provide insights into local neural target engagement and serve as a sensitive biomarker of physiological effect. Although these applications make peripheral recordings important for furthering neuromodulation therapies, the invasive nature of conventional nerve cuffs and longitudinal intrafascicular electrodes (LIFEs) limit their clinical utility. Furthermore, cuff electrodes typically record clear asynchronous neural activity in small animal models but not in large animal models. Microneurography, a minimally invasive technique, is already used routinely in humans to record asynchronous neural activity in the periphery. However, the relative performance of microneurography microelectrodes compared to cuff and LIFE electrodes in measuring neural signals relevant to neuromodulation therapies is not well understood. Approach. To address this gap, we recorded cervical vagus nerve electrically evoked compound action potentials (ECAPs) and spontaneous activity in a human-scaled large animal model—the pig. Additionally, we recorded sensory evoked activity and both invasively and non-invasively evoked CAPs from the great auricular nerve. In aggregate, this study assesses the potential of microneurography electrodes to measure neural activity during neuromodulation therapies with statistically powered and pre-registered outcomes ( https://osf.io/y9k6j ). Main results. The cuff recorded the largest ECAP signal ( p < 0.01) and had the lowest noise floor amongst the evaluated electrodes. Despite the lower signal to noise ratio, microneurography electrodes were able to detect the threshold for neural activation with similar sensitivity to cuff and LIFE electrodes once a dose-response curve was constructed. Furthermore, the microneurography electrodes recorded distinct sensory evoked neural activity. Significance. The results show that microneurography electrodes can measure neural signals relevant to neuromodulation therapies. Microneurography could further neuromodulation therapies by providing a real-time biomarker to guide electrode placement and stimulation parameter selection to optimize local neural fiber engagement and study mechanisms of action.

Hearing loss is a hallmark of aging, typically initially affecting the higher frequencies. In echolocating bats, the ability to discern high frequencies is essential. However, nothing is known about age-related hearing loss in bats, and they are often assumed to be immune to it. We tested the hearing of 47 wild Egyptian fruit bats by recording their auditory brainstem response and cochlear microphonics, and we also assessed the cochlear histology in four of these bats. We used the bats’ DNA methylation profile to evaluate their age and found that bats exhibit age-related hearing loss, with more prominent deterioration at the higher frequencies. The rate of the deterioration was ∼1 dB per year, comparable to the hearing loss observed in humans. Assessing the noise in the fruit bat roost revealed that these bats are exposed to continuous immense noise—mostly of social vocalizations—supporting the assumption that bats might be partially resistant to loud noise. Thus, in contrast to previous assumptions, our results suggest that bats constitute a model animal for the study of age-related hearing loss.

Tinnitus is an unpleasant symptom characterized by detective hearing without the actual sound input. Despite numerous studies elucidating a variety of pathomechanisms inducing tinnitus, the pathophysiology of tinnitus is not fully understood. The genes that are closely associated with this subtype of the auditory hallucination that could be utilized as potential treatment targets are still unknown. In this study, we explored the transcriptional profile changes of the auditory cortex after noise-induced tinnitus in rats using high throughput sequencing and verification of the detected genes using quantitative PCR (qPCR). Tinnitus models were established by analyzing startle behaviors through gap pre-pulse inhibition (PPI) of the acoustic startle. Two hundred and fifty-nine differential genes were identified, of which 162 genes were up-regulated and 97 genes were down-regulated. Analysis of the pathway enrichment indicated that the tinnitus group exhibited increased gene expression related to neurodegenerative disorders such as Huntington’s disease and Amyotrophic lateral sclerosis. Based on the identified genes, networks of protein–protein interaction were established and five hub genes were identified through degree rank, including Fos, Nr4a1, Nr4a3, Egr2, and Egr3. Therein, the Fos gene ranked first with the highest degree after noise exposure, and may be a potential target for the modulation of noise-induced tinnitus.

During developmental critical periods (CPs), early-life stress (ELS) induces cognitive deficits and alters neural circuitry in regions underlying learning, memory, and attention. Mechanisms underlying critical period plasticity are shared by sensory cortices and these higher neural regions, suggesting that sensory processing may also be vulnerable to ELS. In particular, the perception and auditory cortical (ACx) encoding of temporally-varying sounds both mature gradually, even into adolescence, providing an extended postnatal window of susceptibility. To examine the effects of ELS on temporal processing, we developed a model of ELS in the Mongolian gerbil, a well-established model for auditory processing. In both male and female animals, ELS induction impaired the behavioral detection of short gaps in sound, which are critical for speech perception. This was accompanied by reduced neural responses to gaps in auditory cortex, the auditory periphery, and auditory brainstem. ELS thus degrades the fidelity of sensory representations available to higher regions, and could contribute to well-known ELS-induced problems with cognition. SIGNIFICANCE STATEMENT In children and animal models, early-life stress (ELS) leads to deficits in cognition, including problems with learning, memory, and attention. Such problems could arise in part from a low-fidelity representation of sensory information available to higher-level neural regions. Here, we demonstrate that ELS degrades sensory responses to rapid variations in sound at multiple levels of the auditory pathway, and concurrently impairs perception of these rapidly-varying sounds. As these sound variations are intrinsic to speech, ELS may thus pose a challenge to communication and cognition through impaired sensory encoding.

Prolonged or excessive exposure to noise can lead to hearing loss, tinnitus and hypersensitivity to sound. The effects of noise exposure on main excitatory and inhibitory neurotransmitter systems in auditory pathway have been extensively investigated. However, little is known about aberrant changes in neuromodulator systems caused by noise exposure. In the current study, we exposed 2-month-old mice to a narrow band noise at 116 dB SPL for 6 h or sham exposure, assessed auditory brainstem responses as well as examined the expression of serotonin reuptake transporter (SERT) in the cochlear nucleus (CN), inferior colliculus (IC), and primary auditory cortex (Au1) using immunohistochemistry. We found that noise exposure resulted in a significant increase in hearing thresholds at 4, 8, 16, 24, and 32 kHz, as well as led to a significant reduction of SERT in dorsal cochlear nucleus (DCN), dorsal IC (ICd), external IC (ICe), and Au1 layers I-IV. This reduction of SERT in these subregions of central auditory system was partially recovered 15 or 30 days after noise exposure. Furthermore, we examined efficacy of resveratrol (RSV) on hearing loss and loss of SERT induced by noise exposure. The results demonstrated that RSV treatment significantly attenuated threshold shifts of auditory brainstem responses and loss of SERT in DCN, ICd, ICe, and Au1 layers I-IV. These findings show that noise exposure can cause hearing loss and subregion-specific loss of SERT in the central auditory system, and RSV treatment could attenuate noise exposure-induced hearing loss and loss of SERT in central auditory system.

Background Phosphatidylserine is translocated to the inner leaflet of the phospholipid bilayer membrane by the flippase function of type IV P-tape ATPase (P4-ATPase), which is critical to maintain cellular stability and homeostasis. Transmembrane protein 30A (TMEM30A) is the β-subunit of P4-ATPase. Loss of P4-ATPase function causes sensorineural hearing loss and visual dysfunction in human. However, the function of TMEM30A in the auditory system is unclear. Methods P4-ATPase subtype expression in the cochlea was detected by immunofluorescence staining and quantitative real-time polymerase chain reaction (qRT-PCR) at different developmental stages. Hair cell specific TMEM30A knockout mice and wild-type littermates were used for the following functional and morphological analysis. Auditory function was evaluated by auditory brainstem response. We investigated hair cell and stereocilia morphological changes by immunofluorescence staining. Scanning electron microscopy was applied to observe the stereocilia ultrastructure. Differentially expressed transcriptomes were analyzed based on RNA-sequencing data from knockout and wild-type mouse cochleae. Differentially expressed genes were verified by qRT-PCR. Results TMEM30A and subtypes of P4-ATPase are expressed in the mouse cochlea in a temporal-dependent pattern. Deletion of TMEM30A in hair cells impaired hearing onset due to progressive hair cell loss. The disrupted kinocilia placement and irregular distribution of spectrin-α in cuticular plate indicated the hair cell planar polarity disruption in TMEM30A deletion hair cells. Hair cell degeneration begins at P7 and finishes around P14. Transcriptional analysis indicates that the focal adhesion pathway and stereocilium tip-related genes changed dramatically. Without the TMEM30A chaperone, excessive ATP8A2 accumulated in the cytoplasm, leading to overwhelming endoplasmic reticulum stress, which eventually contributed to hair cell death. Conclusions Deletion of TMEM30A led to disrupted planar polarity and stereocilia bundles, and finally led to hair cell loss and auditory dysfunction. TMEM30A is essential for hair cell polarity maintenance and membrane homeostasis. Our study highlights a pivotal role of TMEM30A in the postnatal development of hair cells and reveals the possible mechanisms underlying P4-ATPase-related genetic hearing loss.

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.

Purpose Cisplatin is a widely used and effective chemotherapeutic agent for most solid malignant tumors. However, cisplatin-induced ototoxicity is a common adverse effect that limits the therapeutic efficacy of tumors in the clinic. To date, the specific mechanism of ototoxicity has not been fully elucidated, and the management of cisplatin-induced ototoxicity is also an urgent challenge. Recently, some authors believed that miR34a and mitophagy played a role in age-related and drug-induced hearing loss. Our study aimed to explore the involvement of miR-34a/DRP-1-mediated mitophagy in cisplatin-induced ototoxicity. Methods In this study, C57BL/6 mice and HEI-OC1 cells were treated with cisplatin. MiR-34a and DRP-1 levels were analyzed by qRT‒PCR and western blotting, and mitochondrial function was assessed via oxidative stress, JC-1 and ATP content. Subsequently, we detected DRP-1 levels and observed mitochondrial function by modulating miR-34a expression in HEI-OC1 cells to determine the effect of miR-34a on DRP-1-mediated mitophagy. Results MiR-34a expression increased and DRP-1 levels decreased in C57BL/6 mice and HEI-OC1 cells treated with cisplatin, and mitochondrial dysfunction was involved in this process. Furthermore, the miR-34a mimic decreased DRP-1 expression, enhanced cisplatin-induced ototoxicity and aggravated mitochondrial dysfunction. We further verified that the miR-34a inhibitor increased DRP-1 expression, partially protected against cisplatin-induced ototoxicity and improved mitochondrial function. Conclusion MiR-34a/DRP-1-mediated mitophagy was related to cisplatin-induced ototoxicity and might be a novel target for investigating the treatment and protection of cisplatin-induced ototoxicity.

Spatial hearing remains one of the major challenges for bilateral cochlear implant (biCI) users, and early deaf patients in particular are often completely insensitive to interaural time differences (ITDs) delivered through biCIs. One popular hypothesis is that this may be due to a lack of early binaural experience. However, we have recently shown that neonatally deafened rats fitted with biCIs in adulthood quickly learn to discriminate ITDs as well as their normal hearing litter mates, and perform an order of magnitude better than human biCI users. Our unique behaving biCI rat model allows us to investigate other possible limiting factors of prosthetic binaural hearing, such as the effect of stimulus pulse rate and envelope shape. Previous work has indicated that ITD sensitivity may decline substantially at the high pulse rates often used in clinical practice. We therefore measured behavioral ITD thresholds in neonatally deafened, adult implanted biCI rats to pulse trains of 50, 300, 900 and 1800 pulses per second (pps), with either rectangular or Hanning window envelopes. Our rats exhibited very high sensitivity to ITDs at pulse rates up to 900 pps for both envelope shapes, similar to those in common clinical use. However, ITD sensitivity declined to near zero at 1800 pps, for both Hanning and rectangular windowed pulse trains. Current clinical cochlear implant (CI) processors are often set to pulse rates ≥ 900 pps, but ITD sensitivity in human CI listeners has been reported to decline sharply above ~ 300 pps. Our results suggest that the relatively poor ITD sensitivity seen at > 300 pps in human CI users may not reflect the hard upper limit of biCI ITD performance in the mammalian auditory pathway. Perhaps with training or better CI strategies good binaural hearing may be achievable at pulse rates high enough to allow good sampling of speech envelopes while delivering usable ITDs.

Ménière’s disease, a multifactorial disorder of the inner ear, is characterized by severe vertigo episodes and hearing loss. Although the role of immune responses in Ménière’s disease has been proposed, the precise mechanisms remain undefined. Here, we show that downregulation of serum/glucocorticoid-inducible kinase 1 is associated with activation of NLRP3 inflammasome in vestibular-resident macrophage-like cells from Ménière’s disease patients. Serum/glucocorticoid-inducible kinase 1 depletion markedly enhances IL-1β production which leads to the damage of inner ear hair cells and vestibular nerve. Mechanistically, serum/glucocorticoid-inducible kinase 1 binds to the PYD domain of NLRP3 and phosphorylates it at Serine 5, thereby interfering inflammasome assembly. Sgk −/− mice show aggravated audiovestibular symptoms and enhanced inflammasome activation in lipopolysaccharide-induced endolymphatic hydrops model, which is ameliorated by blocking NLRP3. Pharmacological inhibition of serum/glucocorticoid-inducible kinase 1 increases the disease severity in vivo. Our studies demonstrate that serum/glucocorticoid-inducible kinase 1 functions as a physiologic inhibitor of NLRP3 inflammasome activation and maintains inner ear immune homeostasis, reciprocally participating in models of Ménière’s disease pathogenesis.

Evidence shows that females have increased supra-threshold peripheral auditory processing compared to males. This is indicated by larger auditory brainstem responses (ABR) wave I amplitude, which measures afferent spiral ganglion neuron (SGN)-auditory nerve synchrony. However, the underlying molecular mechanisms of this sex difference are mostly unknown. We sought to elucidate sex differences in ABR wave I amplitude by examining molecular markers known to affect synaptic transmission kinetics. Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) mediate fast excitatory transmission in mature SGN afferent synapses. Each AMPAR channel is a tetramer composed of GluA2, 3, and 4 subunits ( Gria2, 3 , and 4 genes), and those lacking GluA2 subunits have larger currents, are calcium-permeable, and have faster gating kinetics. Moreover, alternatively spliced flip and flop isoforms of each AMPAR subunit affect channel kinetics, having faster kinetics those AMPARs containing Gria3 and Gria4 flop isoforms. We hypothesized that SGNs of females have more fast-gating AMPAR subunit mRNA than males, which could contribute to more temporally precise synaptic transmission and increased SGN synchrony. Our data show that the index of Gria3 relative to Gria2 transcripts on SGN was higher in females than males (females: 48%; males: 43%), suggesting that females have more SGNs with higher Gria3 mRNA relative to Gria2 . Analysis of the relative abundance of the flip and flop alternatively spliced isoforms showed that females have a 2-fold increase in fast-gating Gria3 flop mRNA, while males have more slow-gating (2.5-fold) of the flip . We propose that Gria3 may in part mediate greater SGN synchrony in females. Significance Statement: Females of multiple vertebrate species, including fish and mammals, have been reported to have enhanced sound-evoked synchrony of afferents in the auditory nerve. However, the underlying molecular mediators of this physiologic sex difference are unknown. Elucidating potential molecular mechanisms related to sex differences in auditory processing is important for maintaining healthy ears and developing potential treatments for hearing loss in both sexes. This study found that females have a 2-fold increase in Gria3 flop mRNA, a fast-gating AMPA-type glutamate receptor subunit. This difference may contribute to greater neural synchrony in the auditory nerve of female mice compared to males, and this sex difference may be conserved in all vertebrates.

The mechanoelectrical transduction (MET) channel of cochlear hair cells is gated by the tip link, but the mechanisms that establish the exquisite force sensitivity of this MET channel are not known. Here, we show that the tetraspan lipoma HMGIC fusion partner-like 5 (LHFPL5) directly couples the tip link to the MET channel. Disruption of these interactions severely perturbs MET. Notably, the N-terminal cytoplasmic domain of LHFPL5 binds to an amphipathic helix in TMC1, a critical gating domain conserved between different MET channels. Mutations in the amphipathic helix of TMC1 or in the N-terminus of LHFPL5 that perturb interactions of LHFPL5 with the amphipathic helix affect channel responses to mechanical force. We conclude that LHFPL5 couples the tip link to the MET channel and that channel gating depends on a structural element in TMC1 that is evolutionarily conserved between MET channels. Overall, our findings support a tether model for transduction channel gating by the tip link.

The peripheral auditory system is influenced by the medial olivocochlear (MOC) and middle ear muscle (MEM) reflexes. When elicited by contralateral acoustic stimulation (CAS), these reflexes reduce cochlear amplification (MOC reflex) and limit low-frequency transmission through the middle ear (MEM reflex). The independent roles of these reflexes on auditory physiology and perception are difficult to distinguish. The amplitude of the cochlear microphonic (CM) is expected to increase or decrease when the MOC and MEM reflexes are elicited by CAS, respectively, which could lead to a straightforward interpretation of what reflex is dominant for a given CAS level. CM and ear canal sound pressure level (SPL) were measured for a 500 Hz, 90 dB SPL probe in the presence of contralateral broadband noise (CBBN) for levels ranging from 45–75 dB SPL. In most subjects, CM amplitude increased for CBBN levels of 45 and 55 dB SPL, while no change in ear canal SPL was observed, consistent with eliciting the MOC reflex. Conversely, CM amplitude decreased, and ear canal SPL increased in the presence of 65 and 75 dB SPL CBBN, consistent with eliciting the MEM reflex. A CM-based test of the MOC reflex may facilitate detection of MEM effects and the assessment of adults with cochlear hearing loss.