AIF translocation into nucleus caused by Aifm1 R450Q mutation: generation and characterization of a mouse model for AUNX1

Mutations in AIFM1, encoding for apoptosis-inducing factor (AIF), cause AUNX1, an X-linked neurologic disorder with late-onset auditory neuropathy (AN) and peripheral neuropathy. Despite significant research on AIF, there are limited animal models with the disrupted AIFM1 representing the corresponding phenotype of human AUNX1, characterized by late-onset hearing loss and impaired auditory pathways. Here, we generated an Aifm1 p.R450Q knock-in mouse model (KI) based on the human AIFM1 p.R451Q mutation. Hemizygote KI male mice exhibited progressive hearing loss from P30 onward, with greater severity at P60 and stabilization until P210. Additionally, muscle atrophy was observed at P210. These phenotypic changes were accompanied by a gradual reduction in the number of spiral ganglion neuron cells (SGNs) at P30 and ribbons at P60, which coincided with the translocation of AIF into the nucleus starting from P21 and P30, respectively. The SGNs of KI mice at P210 displayed loss of cytomembrane integrity, abnormal nuclear morphology, and dendritic and axonal demyelination. Furthermore, the inner hair cells and myelin sheath displayed abnormal mitochondrial morphology, while fibroblasts from KI mice showed impaired mitochondrial function. In conclusion, we successfully generated a mouse model recapitulating AUNX1. Our findings indicate that disruption of Aifm1 induced the nuclear translocation of AIF, resulting in the impairment in the auditory pathway.

Dopaminergic and cholinergic innervation in the mouse cochlea after noise-induced or age-related synaptopathy

Cochlear synaptopathy, the loss of or damage to connections between auditory-nerve fibers (ANFs) and inner hair cells (IHCs), is a prominent pathology in noise-induced and age-related hearing loss. Here, we investigated if degeneration of the olivocochlear (OC) efferent innervation is also a major aspect of the synaptopathic ear, by quantifying the volume and spatial organization of its cholinergic and dopaminergic components, using antibodies to vesicular acetylcholine transporter (VAT) and tyrosine hydroxylase (TH), respectively. CBA/CaJ male mice were examined 1 day to 8 months after a synaptopathic noise exposure, and compared to unexposed age-matched controls and unexposed aged mice at 24-28 months. In normal ears, cholinergic lateral (L)OC terminals were denser in the apical half of the cochlea and on the modiolar side of the inner hair cells (IHCs), where ANFs of low-spontaneous rate are typically found, while dopaminergic terminals were more common in the basal third of the cochlea and, re the IHC axes, were offset towards the habenula with respect to cholinergic terminals. The noise had only small and transient effects on the density of LOC innervation, its spatial organization around the IHC axes, or the extent to which TH and VAT signal were colocalized. The synaptopathic noise also had relatively small and transient effects on cholinergic innervation density in the outer hair cell (OHC) area, which normally peaks in the 16 kHz region and falls monotonically towards higher and lower frequencies. In contrast, in the aged ears, there was massive degeneration of OHC efferents, especially in the apical half of the cochlea, where there was also significant loss of OHCs. In the IHC area, there was significant loss of cholinergic terminals in both apical and basal regions and of dopaminergic innervation in the basal half. Furthermore, the cholinergic terminals in the aged ears spread from their normal clustering near the IHC basolateral pole, where the ANF synapses are found, to positions up and down the IHC somata and regions of the neuropil closer to the habenula. This apparent migration was most striking in the apex, where the hair cell pathology was greatest, and may be a harbinger of impending hair cell death.

The effects of noise-induced hair cell lesions on cochlear electromechanical responses: A computational approach using a biophysical model

A biophysically inspired signal processing model of the human cochlea is deployed to simulate the effects of specific noise-induced inner hair cell (IHC) and outer hair cell (OHC) lesions on hearing thresholds, cochlear compression, and the spectral and temporal features of the auditory nerve (AN) coding. The model predictions were evaluated by comparison with corresponding data from animal studies as well as human clinical observations. The hearing thresholds were simulated for specific OHC and IHC damages and the cochlear nonlinearity was assessed at 0.5 and 4 kHz. The tuning curves were estimated at 1 kHz and the contributions of the OHC and IHC pathologies to the tuning curve were distinguished by the model. Furthermore, the phase locking of AN spikes were simulated in quiet and in presence of noise. The model predicts that the phase locking drastically deteriorates in noise indicating the disturbing effect of background noise on the temporal coding in case of hearing impairment. Moreover, the paper presents an example wherein the model is inversely configured for diagnostic purposes using a machine learning optimization technique (Nelder-Mead method). Accordingly, the model finds a specific pattern of OHC lesions that gives the audiometric hearing loss measured in a group of noise-induced hearing impaired humans.

Inner hair cell dysfunction in Klhl18 mutant mice leads to low frequency progressive hearing loss

Age-related hearing loss in humans (presbycusis) typically involves impairment of high frequency sensitivity before becoming progressively more severe at lower frequencies. Pathologies initially affecting lower frequency regions of hearing are less common. Here we describe a progressive, predominantly low-frequency recessive hearing impairment in two mutant mouse lines carrying different mutant alleles of the Klhl18 gene: a spontaneous missense mutation (Klhl18lowf) and a targeted mutation (Klhl18tm1a(KOMP)Wtsi). Both males and females were studied, and the two mutant lines showed similar phenotypes. Threshold for auditory brainstem responses (ABR; a measure of auditory nerve and brainstem neural activity) were normal at 3 weeks old but showed progressive increases from 4 weeks onwards. In contrast, distortion product otoacoustic emission (DPOAE) sensitivity and amplitudes (a reflection of cochlear outer hair cell function) remained normal in mutants. Electrophysiological recordings from the round window of Klhl18lowf mutants at 6 weeks old revealed 1) raised compound action potential thresholds that were similar to ABR thresholds, 2) cochlear microphonic potentials that were normal compared with wildtype and heterozygous control mice and 3) summating potentials that were reduced in amplitude compared to control mice. Scanning electron microscopy showed that Klhl18lowf mutant mice had abnormally tapering of the tips of inner hair cell stereocilia in the apical half of the cochlea while their synapses appeared normal. These results suggest that Klhl18 is necessary to maintain inner hair cell stereocilia and normal inner hair cell function at low frequencies.

The role of oxidative stress in the susceptibility of noise-impaired cochleae to synaptic loss induced by intracochlear electrical stimulation

Intracochlear electrical stimulation (ES) generated by cochlear implants (CIs) is used to activate auditory nerves to restore hearing perception in deaf subjects and those with residual hearing who use electroacoustic stimulation (EAS) technology. Approximately 1/3 of EAS recipients experience loss of residual hearing a few months after ES activation, but the underlying mechanism is unknown. Clinical evidence indicates that the loss is related to the previous history of noise-induced hearing loss (NIHL). In this report, we investigated the impact of intracochlear ES on oxidative stress levels and synaptic counts in inner hair cells (IHCs) of the apical, middle and basal regions of guinea pigs with normal hearing (NH) and NIHL. Our results demonstrated that intracochlear ES with an intensity of 6 dB above the thresholds of electrically evoked compound action potentials (ECAPs) could induce the elevation of oxidative stress levels, resulting in a loss of IHC synapses near the electrodes in the basal and middle regions of the NH cochleae. Furthermore, the apical region of cochleae with NIHL were more susceptible to synaptic loss induced by relatively low-intensity ES than that of NH cochleae, resulting from the additional elevation of oxidative stress levels and the reduced antioxidant capability throughout the whole cochlea.

Roles of Bak and Sirt3 in Paraquat-Induced Cochlear Hair Cell Damage

Paraquat, a superoxide generator, can damage the cochlea causing an ototoxic hearing loss. The purpose of the study was to determine if deletion of Bak, a pro-apoptotic gene, would reduce paraquat ototoxicity or if deletion of Sirt3, which delays age-related hearing loss under caloric restriction, would increase paraquat ototoxicity. We tested these two hypotheses by treating postnatal day 3 cochlear cultures from Bak^±, Bak^-/-, Sirt3^±, Sirt3^-/-, and WT mice with paraquat and compared the results to a standard rat model of paraquat ototoxicity. Paraquat damaged nerve fibers and dose-dependently destroyed rat outer hair cells (OHCs) and inner hair cells (IHCs). Rat hair cell loss began in the base of the cochlea with a 10 μM dose and as the dose increased from 50 to 500 μM, the hair cell loss increased near the base of the cochlea and spread toward the apex of the cochlea. Rat OHC losses were consistently greater than IHC losses. Unexpectedly, in all mouse genotypes, paraquat-induced hair cell lesions were maximal near the apex of the cochlea and minimal near the base. This unusual damage gradient is opposite to that seen in paraquat-treated rats and in mice and rats treated with other ototoxic drugs. However, paraquat always induced greater OHC loss than IHC loss in all mouse strains. Contrary to our hypothesis, Bak deficient mice were more vulnerable to paraquat ototoxicity than WT mice (Bak^-/- > Bak^± > WT), suggesting that Bak plays a protective role against hair cell stress. Also, contrary to expectation, Sirt3-deficient mice did not differ significantly from WT mice, possibly due to the fact that Sirt3 was not experimentally upregulated in Sirt3-expressing mice prior to paraquat treatment. Our results show for the first time a gradient of ototoxic damage in mice that is greater in the apex than the base of the cochlea.

Contrasting mechanisms for hidden hearing loss: Synaptopathy vs myelin defects

Hidden hearing loss (HHL) is an auditory neuropathy characterized by normal hearing thresholds but reduced amplitudes of the sound-evoked auditory nerve compound action potential (CAP). In animal models, HHL can be caused by moderate noise exposure or aging, which induces loss of inner hair cell (IHC) synapses. In contrast, recent evidence has shown that transient loss of cochlear Schwann cells also causes permanent auditory deficits in mice with similarities to HHL. Histological analysis of the cochlea after auditory nerve remyelination showed a permanent disruption of the myelination patterns at the heminode of type I spiral ganglion neuron (SGN) peripheral terminals, suggesting that this defect could be contributing to HHL. To shed light on the mechanisms of different HHL scenarios observed in animals and to test their impact on type I SGN activity, we constructed a reduced biophysical model for a population of SGN peripheral axons whose activity is driven by a well-accepted model of cochlear sound processing. We found that the amplitudes of simulated sound-evoked SGN CAPs are lower and have greater latencies when heminodes are disorganized, i.e. they occur at different distances from the hair cell rather than at the same distance as in the normal cochlea. These results confirm that disruption of heminode positions causes desynchronization of SGN spikes leading to a loss of temporal resolution and reduction of the sound-evoked SGN CAP. Another mechanism resulting in HHL is loss of IHC synapses, i.e., synaptopathy. For comparison, we simulated synaptopathy by removing high threshold IHC-SGN synapses and found that the amplitude of simulated sound-evoked SGN CAPs decreases while latencies remain unchanged, as has been observed in noise exposed animals. Thus, model results illuminate diverse disruptions caused by synaptopathy and demyelination on neural activity in auditory processing that contribute to HHL as observed in animal models and that can contribute to perceptual deficits induced by nerve damage in humans.

Electrophysiological assessment and pharmacological treatment of blast-induced tinnitus

Tinnitus, the phantom perception of sound, often occurs as a clinical sequela of auditory traumas. In an effort to develop an objective test and therapeutic approach for tinnitus, the present study was performed in blast-exposed rats and focused on measurements of auditory brainstem responses (ABRs), prepulse inhibition of the acoustic startle response, and presynaptic ribbon densities on cochlear inner hair cells (IHCs). Although the exact mechanism is unknown, the “central gain theory” posits that tinnitus is a perceptual indicator of abnormal increases in the gain (or neural amplification) of the central auditory system to compensate for peripheral loss of sensory input from the cochlea. Our data from vehicle-treated rats supports this rationale; namely, blast-induced cochlear synaptopathy correlated with imbalanced elevations in the ratio of centrally-derived ABR wave V amplitudes to peripherally-derived wave I amplitudes, resulting in behavioral evidence of tinnitus. Logistic regression modeling demonstrated that the ABR wave V/I amplitude ratio served as a reliable metric for objectively identifying tinnitus. Furthermore, histopathological examinations in blast-exposed rats revealed tinnitus-related changes in the expression patterns of key plasticity factors in the central auditory pathway, including chronic loss of Arc/Arg3.1 mobilization. Using a formulation of N-acetylcysteine (NAC) and disodium 2,4-disulfophenyl-N-tert-butylnitrone (HPN-07) as a therapeutic for addressing blast-induced neurodegeneration, we measured a significant treatment effect on preservation or restoration of IHC ribbon synapses, normalization of ABR wave V/I amplitude ratios, and reduced behavioral evidence of tinnitus in blast-exposed rats, all of which accorded with mitigated histopathological evidence of tinnitus-related neuropathy and maladaptive neuroplasticity.

THOC1 deficiency leads to late-onset nonsyndromic hearing loss through p53-mediated hair cell apoptosis

Apoptosis of cochlear hair cells is a key step towards age-related hearing loss. Although numerous genes have been implicated in the genetic causes of late-onset, progressive hearing loss, few show direct links to the proapoptotic process. By genome-wide linkage analysis and whole exome sequencing, we identified a heterozygous p.L183V variant in THOC1 as the probable cause of the late-onset, progressive, non-syndromic hearing loss in a large family with autosomal dominant inheritance. Thoc1, a member of the conserved multisubunit THO/TREX ribonucleoprotein complex, is highly expressed in mouse and zebrafish hair cells. The thoc1 knockout (thoc1 mutant) zebrafish generated by gRNA-Cas9 system lacks the C-startle response, indicative of the hearing dysfunction. Both Thoc1 mutant and knockdown zebrafish have greatly reduced hair cell numbers, while the latter can be rescued by embryonic microinjection of human wild-type THOC1 mRNA but to significantly lesser degree by the c.547C>G mutant mRNA. The Thoc1 deficiency resulted in marked apoptosis in zebrafish hair cells. Consistently, transcriptome sequencing of the mutants showed significantly increased gene expression in the p53-associated signaling pathway. Depletion of p53 or applying the p53 inhibitor Pifithrin-α significantly rescued the hair cell loss in the Thoc1 knockdown zebrafish. Our results suggested that THOC1 deficiency lead to late-onset, progressive hearing loss through p53-mediated hair cell apoptosis. This is to our knowledge the first human disease associated with THOC1 mutations and may shed light on the molecular mechanism underlying the age-related hearing loss.

Knock-In Mice with Myo3a Y137C Mutation Displayed Progressive Hearing Loss and Hair Cell Degeneration in the Inner Ear

Myo3a is expressed in cochlear hair cells and retinal cells and is responsible for human recessive hereditary nonsyndromic deafness (DFNB30). To investigate the mechanism of DFNB30-type deafness, we established a mouse model of Myo3a kinase domain Y137C mutation by using CRISPR/Cas9 system. No difference in hearing between 2-month-old Myo3a mutant mice and wild-type mice was observed. The hearing threshold of the ≥6-month-old mutant mice was significantly elevated compared with that of the wild-type mice. We observed degeneration in the inner ear hair cells of 6-month-old Myo3a mutant mice, and the degeneration became more severe at the age of 12 months. We also found structural abnormality in the cochlear hair cell stereocilia. Our results showed that Myo3a is essential for normal hearing by maintaining the intact structure of hair cell stereocilia, and the kinase domain plays a critical role in the normal functions of Myo3a. This mouse line is an excellent model for studying DFNB30-type deafness in humans.

Persistent hair cell malfunction contributes to hidden hearing loss

Noise exposures that result in fully reversible changes in cochlear neural threshold can cause a reduced neural output at supra-threshold sound intensity. This so-called "hidden hearing loss" has been shown to be associated with selective degeneration of high threshold afferent nerve fiber-inner hair cell (IHC) synapses. However, the electrophysiological function of the IHCs themselves in hidden hearing loss has not been directly investigated. We have made round window (RW) measurements of cochlear action potentials (CAP) and summating potentials (SP) after two levels of a 10 kHz acoustic trauma. The more intense acoustic trauma lead to notch-like permanent threshold changes and both CAP and SP showed reductions in supra-threshold amplitudes at frequencies with altered thresholds as well as from fully recovered regions. However, the interpretation of the results in normal threshold regions was complicated by the likelihood of reduced contributions from adjacent regions with elevated thresholds. The milder trauma showed full recovery of all neural thresholds, but there was a persistent depression of the amplitudes of both CAP and SP in response to supra-threshold sounds. The effect on SP amplitude in particular shows that occult damage to hair cell transduction mechanisms can contribute to hidden hearing loss. Such damage could potentially affect the supra-threshold output properties of surviving primary afferent neurons.

Comparison of Three Types of Mesenchymal Stem Cells (Bone Marrow, Adipose Tissue, and Umbilical Cord-Derived) as Potential Sources for Inner Ear Regeneration

In this review, we compared the potential of mesenchymal stem cells derived from bone marrow, adipose tissue and umbilical cord as suitable sources for regeneration of inner ear hair cells and auditory neurons. Our intensive literature search indicates that stem cells in some of adult mammalian tissues, such as bone marrow, can generate new cells under physiological and pathological conditions. Among various types of stem cells, bone marrow-derived mesenchymal stem cells are one of the most promising candidates for cell replacement therapy. Mesenchymal stem cells have been reported to invade the damaged area, contribute to the structural reorganization of the damaged cochlea and improve incomplete hearing recovery. We suggest that bone marrow-derived mesenchymal stem cells would be more beneficial than other mesenchymal stem cells.

Disruption of SorCS2 reveals differences in the regulation of stereociliary bundle formation between hair cell types in the inner ear

Behavioural anomalies suggesting an inner ear disorder were observed in a colony of transgenic mice. Affected animals were profoundly deaf. Severe hair bundle defects were identified in all outer and inner hair cells (OHC, IHC) in the cochlea and in hair cells of vestibular macular organs, but hair cells in cristae were essentially unaffected. Evidence suggested the disorder was likely due to gene disruption by a randomly inserted transgene construct. Whole-genome sequencing identified interruption of the SorCS2 (Sortilin-related VPS-10 domain containing protein) locus. Real-time-qPCR demonstrated disrupted expression of SorCS2 RNA in cochlear tissue from affected mice and this was confirmed by SorCS2 immuno-labelling. In all affected hair cells, stereocilia were shorter than normal, but abnormalities of bundle morphology and organisation differed between hair cell types. Bundles on OHC were grossly misshapen with significantly fewer stereocilia than normal. However, stereocilia were organised in rows of increasing height. Bundles on IHC contained significantly more stereocilia than normal with some longer stereocilia towards the centre, or with minimal height differentials. In early postnatal mice, kinocilia (primary cilia) of IHC and of OHC were initially located towards the lateral edge of the hair cell surface but often became surrounded by stereocilia as bundle shape and apical surface contour changed. In macular organs the kinocilium was positioned in the centre of the cell surface throughout maturation. There was disruption of the signalling pathway controlling intrinsic hair cell apical asymmetry. LGN and Gαi3 were largely absent, and atypical Protein Kinase C (aPKC) lost its asymmetric distribution. The results suggest that SorCS2 plays a role upstream of the intrinsic polarity pathway and that there are differences between hair cell types in the deployment of the machinery that generates a precisely organised hair bundle.

Styrene enhances the noise induced oxidative stress in the cochlea and affects differently mechanosensory and supporting cells

Experimental and human investigations have raised the level of concern about the potential ototoxicity of organic solvents and their interaction with noise. The main objective of this study was to characterize the effects of the combined noise and styrene exposure on hearing focusing on the mechanism of damage on the sensorineural cells and supporting cells of the organ of Corti and neurons of the ganglion of Corti. The impact of single and combined exposures on hearing was evaluated by auditory functional testing and histological analyses of cochlear specimens. The mechanism of damage was studied by analyzing superoxide anion and lipid peroxidation expression and by computational analyses of immunofluorescence data to evaluate and compare the oxidative stress pattern in outer hair cells versus the supporting epithelial cells of the organ of Corti. The oxidative stress hypothesis was further analyzed by evaluating the protective effect of a Coenzyme Q₁₀ analogue, the water soluble Q_ter, molecule known to have protective antioxidant properties against noise induced hearing loss and by the analysis of the expression of the endogenous defense enzymes. This study provides evidence of a reciprocal noise-styrene synergism based on a redox imbalance mechanism affecting, although with a different intensity of damage, the outer hair cell (OHC) sensory epithelium. Moreover, these two damaging agents address preferentially different cochlear targets: noise mainly the sensory epithelium, styrene the supporting epithelial cells. Namely, the increase pattern of lipid peroxidation in the organ of Corti matched the cell damage distribution, involving predominantly OHC layer in noise exposed cochleae and both OHC and Deiters' cell layers in the styrene or combined exposed cochleae. The antioxidant treatment reduced the lipid peroxidation increase, potentiated the endogenous antioxidant defense system at OHC level in both exposures but it failed to ameliorate the oxidative imbalance and cell death of Deiters' cells in the styrene and combined exposures. Current antioxidant therapeutic approaches to preventing sensory loss focus on hair cells alone. It remains to be seen whether targeting supporting cells, in addition to hair cells, might be an effective approach to protecting exposed subjects.

Role of p19ink4d in the pathogenesis of hearing loss

This study aimed to investigate the p19 expression in cisplatin-treated rats and the role of p19 in the degeneration of inner ear cells. It also searched for p19 gene alterations in patients with profound sensorineural deafness. P19ink4d is essential for the postmitotic maintenance of hair cells. It is presumed that a mutation in the functional homolog of p19 or a disturbance in its regulated expression can be the underlying cause of hearing loss. Experiments were conducted on male and female Sprague-Dawley rats (aged 6-7 weeks, 280-320 g) with thresholds of auditory brainstem responses <30 dB in the sound pressure level, and signs of middle ear infection were used for the experiment. For clinical evaluation, 400 children (age less than 13 years) from unrelated families with severe or profound sensorineural hearing loss (SNHL) were recruited at the second Xiangya Hospital of Central South University between 2005 and 2013, and genomic DNA for deafness gene analysis was obtained from peripheral blood samples of the patients and their lineal relatives. It was found that the p19 expression increased over time in the inner ear cells after cisplatin administration, but the p19 mRNA and protein levels significantly decreased in rats with manifested hearing loss induced by cisplatin. However, no mutation existed within the coding exons of p19 in the patients with profound sensorineural deafness. To conclude, the results support the concept that p19 may play an important role in the ototoxic effects of cisplatin and is probably involved in the pathogenesis of hearing loss.

[Effect of low dosage of ciplatin on the shape and otoferlin in cochlea inner hair cells]

OBJECTIVE

To establish the stable and efficient hearing damage model by using low dosage of cispla tin, and investigate the mechanism.

METHOD

C57 mice were divided into 7 groups (every group, n = 8), the first group was the control group, the others were separately intraperitoneally injected with different dosages of cispla tin for different time. We measured the auditory brainstem response (ABR) of the mice, and obtained the basal coil of organ Corti. We observed the shape of inner hair cells (IHC) by staining AgNO3 and marked otoferlin in the IHC by immunofluorescence,successively sliced by laser confocal microscopy. The RNA fragments were amplified by RT-PCR.

RESULT

After cisplatin administration for four days, the thresholds of the ABR improved in 1.5 mg/kg and 3.0 mg/kg group, and compared with the control group, the ABR thresholds improved in each group with ciplatin administration for seven days. With the same dosage, the ABR threshold of the 0.75 mg/kg x 7 d group was higher than 0.75 mg/kg x 4 d group, and there was no time-effect relationship existing in other groups with different dosage. The otoferlin was less expressed 3.0 mg/kg groups than the control group. However, the oto ferlin expressed in the 1.5 mg/kg groups were almost the same as the control group. The alteration of the IHC was observed most remarkablely in 3.0 mg/kg x 7 d group.

CONCLUSION

Low dosage of cisplatin can impair the hearing, and the expression of otoferlin may involve in the underlying mechanism.

[The expression of miR-183 family in the pathogenesis and development of noise-induced deafness]

OBJECTIVE

To detect the expression variation of microRNA-183 family in cochlea of animal model characterized by noise-induced deafness at various time points, and to explore the mechanisms responsible for noise-induced deafness.

METHOD

Fifty mice were randomly divided into 5 groups. In the experimental group, 40 mice were exposed to 2-4 kHz narrow band noise at 100 dB SPL 6h per day for 3 consecutive days. The rest 10 mice served as the control group without receiving any noise. Auditory brainsterm response (ABR) were examined at the 1st, 7th, 14th and 28th day compaired with the ABR before the experiment,to confirm noise lead to the permanent threshold shift. The pathological damage processes of hair cell were detected by the basilar membrane stretched techniques. Real-time reverse transcriptase-polymerase chain reaction (qRT-PCR) was apply to quantify the expression of microRNA183 family members. Statistical analysis was performed by the SPSS 17.0 software.

RESULT

The hearing of mice in the experimental group was significantly less than that in the control group. In the experimental group, the hearing of mice exposed to noise were markedly less when compared with the one exposure to null-noise. The hearing in the 1st day group was least among experimental groups, and the followed one was mice in the 7th day group. No statistical difference were observed between the 14th and 28th day groups (P > 0.05). The results of surface preparation showed that the outer hair cells were chaotic, deformational, and their number decreased is time-dependent. The missing of the outer hair cells occurred mainly in the first and second rows, while the inner hair cells were not pronouncedly missing. The qRT-PCR showed that the expressions of the three genes (miR-183/96/182)in the 1st day and 7th day group with exposure to noise were less than in the control group (P < 0.01), while no significant difference was found between 1st day and 7th day group (P > 0.05). The expressions rised in the 14th day experimental groups, whereas the 28th day group's expressions of the three genes decreased markedly which were more than that in the 1st day and 7th day group (P < 0.01).

CONCLUSION

After noise exposure for some time, the expressions of miRNA-183 family members have significant changes in animal model with noise-induced deafness, which indicated that the miRNA183 family members may play important roles in the pathogenesis and development of noise-induced deafness.

Ribbon synapse plasticity in the cochleae of Guinea pigs after noise-induced silent damage

Noise exposure at low levels or low doses can damage hair cell afferent ribbon synapses without causing permanent threshold shifts. In contrast to reports in the mouse cochleae, initial damage to ribbon synapses in the cochleae of guinea pigs is largely repairable. In the present study, we further investigated the repair process in ribbon synapses in guinea pigs after similar noise exposure. In the control samples, a small portion of afferent synapses lacked synaptic ribbons, suggesting the co-existence of conventional no-ribbon and ribbon synapses. The loss and recovery of hair cell ribbons and post-synaptic densities (PSDs) occurred in parallel, but the recovery was not complete, resulting in a permanent loss of less than 10% synapses. During the repair process, ribbons were temporally separated from the PSDs. A plastic interaction between ribbons and postsynaptic terminals may be involved in the reestablishment of synaptic contact between ribbons and PSDs, as shown by location changes in both structures. Synapse repair was associated with a breakdown in temporal processing, as reflected by poorer responses in the compound action potential (CAP) of auditory nerves to time-stress signals. Thus, deterioration in temporal processing originated from the cochlea. This deterioration developed with the recovery in hearing threshold and ribbon synapse counts, suggesting that the repaired synapses had deficits in temporal processing.

A new mutation of the Atoh1 gene in mice with normal life span allows analysis of inner ear and cerebellar phenotype in aging

Atoh1 is a transcription factor that regulates neural development in multiple tissues and is conserved among species. Prior mouse models of Atoh1, though effective and important in the evolution of our understanding of the gene, have been limited by perinatal lethality. Here we describe a novel point mutation of Atoh1 (designated Atoh1^trhl) underlying a phenotype of trembling gait and hearing loss. Histology revealed inner ear hair cell loss and cerebellar atrophy. Auditory Brainstem Response (ABR) and Distortion Product Otoacoustic Emission (DPOAE) showed functional abnormalities in the ear. Normal lifespan and fecundity of Atoh1^trhlmice provide a complementary model to facilitate elucidation of ATOH1 function in hearing,central nervous system and cancer biology.

Effect of metabolic presbyacusis on cochlear responses: a simulation approach using a physiologically-based model

In the presented model, electrical, acoustical, and mechanical elements of the cochlea are explicitly integrated into a signal transmission line where these elements convey physiological interpretations of the human cochlear structures. As a result, this physiologically-motivated model enables simulation of specific cochlear lesions such as presbyacusis. The hypothesis is that high-frequency hearing loss in older adults may be due to metabolic presbyacusis whereby age-related cellular/chemical degenerations in the lateral wall of the cochlea cause a reduction in the endocochlear potential. The simulations quantitatively confirm this hypothesis and emphasize that even if the outer and inner hair cells are totally active and intact, metabolic presbyacusis alone can significantly deteriorate the cochlear functionality. Specifically, in the model, as the endocochlear potential decreases, the transduction mechanism produces less receptor current such that there is a reduction in the battery of the somatic motor. This leads to a drastic decrease in cochlear amplification and frequency sensitivity, as well as changes in position-frequency map (tuning pattern) of the cochlea. In addition, the simulations show that the age-related reduction of the endocochlear potential significantly inhibits the firing rate of the auditory nerve which might contribute to the decline of temporal resolution in the aging auditory system.

Mutations in KARS, encoding lysyl-tRNA synthetase, cause autosomal-recessive nonsyndromic hearing impairment DFNB89

Previously, DFNB89, a locus associated with autosomal-recessive nonsyndromic hearing impairment (ARNSHI), was mapped to chromosomal region 16q21-q23.2 in three unrelated, consanguineous Pakistani families. Through whole-exome sequencing of a hearing-impaired individual from each family, missense mutations were identified at highly conserved residues of lysyl-tRNA synthetase (KARS): the c.1129G>A (p.Asp377Asn) variant was found in one family, and the c.517T>C (p.Tyr173His) variant was found in the other two families. Both variants were predicted to be damaging by multiple bioinformatics tools. The two variants both segregated with the nonsyndromic-hearing-impairment phenotype within the three families, and neither mutation was identified in ethnically matched controls or within variant databases. Individuals homozygous for KARS mutations had symmetric, severe hearing impairment across all frequencies but did not show evidence of auditory or limb neuropathy. It has been demonstrated that KARS is expressed in hair cells of zebrafish, chickens, and mice. Moreover, KARS has strong localization to the spiral ligament region of the cochlea, as well as to Deiters' cells, the sulcus epithelium, the basilar membrane, and the surface of the spiral limbus. It is hypothesized that KARS variants affect aminoacylation in inner-ear cells by interfering with binding activity to tRNA or p38 and with tetramer formation. The identification of rare KARS variants in ARNSHI-affected families defines a gene that is associated with ARNSHI.

Diaphanous homolog 3 (Diap3) overexpression causes progressive hearing loss and inner hair cell defects in a transgenic mouse model of human deafness

We previously demonstrated that a mutation in the 5′ untranslated region of Diaphanous homolog 3 (DIAPH3) results in 2 to 3-fold overexpression of the gene, leading to a form of delayed onset, progressive human deafness known as AUNA1 (auditory neuropathy, nonsyndromic, autosomal dominant, 1). To investigate the mechanism of deafness, we generated two lines of transgenic mice overexpressing Diap3, the murine ortholog of DIAPH3, on an FVB/NJ background. Line 771 exhibits a relatively mild 20 dB hearing loss at 12 kHz at 4 and 8 weeks of age, progressing to 40 dB and 60 dB losses at 16 and 24 weeks, respectively, at 12 and 24 kHz. Line 924 shows no hearing loss at 4 or 8 weeks, but manifests 35 and 50 dB threshold shifts at 16 and 24 weeks, respectively, at both 12 and 24 kHz. Notably, mice from the two transgenic lines retain distortion product otoacoustic emissions, indicative of normal cochlear outer hair cell (OHC) function despite elevation of auditory thresholds. Scanning electron microscopy of the organ of Corti demonstrates striking anomalies of the inner hair cell (IHC) stereocilia, while OHCs are essentially intact. Over time, IHCs of both lines develop elongated stereocilia that appear fused with neighboring stereocilia, in parallel to the time course of hearing loss in each line. Furthermore, we observe significant reduction in the number of IHC ribbon synapses over 24 weeks in both lines, although this reduction does not correlate temporally with onset and progression of hearing loss or stereociliary anomalies. In summary, overexpression of wild-type Diap3 in two lines of transgenic mice results in hearing loss that recapitulates human AUNA1 deafness. These findings suggest an essential role of Diap3 in regulating assembly and/or maintenance of actin filaments in IHC stereocilia, as well as a potential role at the IHC ribbon synapse.

Immunoelectron microscopic analysis of neurotoxic effect of glutamate in the vestibular end organs during ischemia

CONCLUSION

The excessive glutamate released from the type I and type II hair cells and the supporting cells injure the bouton-type endings and the nerve chalices in 30 min ischemia, and neuronal damage of glutamate was slight in 10 min ischemia.

OBJECTIVE

In the present study, we investigated by means of post-embedding immunoelectron microscopic analysis whether neuronal damage in the vestibular end organs is associated with the change of cellular glutamate concentration during ischemia.

METHODS

Transient local anoxia (10 min, 30 min) of guinea pig inner ear was induced by pressing the left labyrinthine artery. The right sides were used as controls. The morphological changes of the vestibular end organs and the areal gold particle densities representing glutamate were compared in the ischemia side and the control side.

RESULTS

The areal gold particle densities of the type I and type II hair cells and the supporting cells in the ischemic side were lower than those of the control side. There were no remarkable morphological changes compared to the control side in 10 min ischemia. In 30 min ischemia, the bouton-type endings were swollen and intercellular spaces between the type I hair cells and the nerve chalices were enlarged.

Screening for chemicals that affect hair cell death and survival in the zebrafish lateral line

The zebrafish lateral line is an efficient model system for the evaluation of chemicals that protect and damage hair cells. Located on the surface of the body, lateral line hair cells are accessible for manipulation and visualization. The zebrafish lateral line system allows rapid screens of large chemical libraries, as well as subsequent thorough evaluation of interesting compounds. In this review, we focus on the results of our previous screens and the evolving methodology of our screens for chemicals that protect hair cells, and chemicals that damage hair cells using the zebrafish lateral line.

The protective effect of conditioning on noise-induced hearing loss is frequency-dependent

We compared the extent of temporary threshold shift (TTS) and hair cell loss following high level 4 kHz noise exposure with those preconditioned with moderate level 1 and 4 kHz octave band noise. Fifteen Male albino guinea pigs (300- 350 g in weight) were randomly allocated into three groups: those exposed to 4 kHz octave band noise at 102 dB SPL (group 1, n=5); those conditioned with 1 kHz octave band noise at 85 dB SPL, 6 hours per day for 5 days, then exposed to noise (group 2, n=5); those conditioned with 4 kHz octave band noise at 85 dB SPL, then exposed to noise (group 3, n=5). An hour and one week after noise exposure, threshold shifts were evaluated by auditory-evoked brainstem response (ABR) and then animals were euthanized for histological evaluation. We found that TTS and cochlear damage caused by noise exposure were significantly reduced by 1 kHz and 4 kHz conditioning (P<0.001). We also showed that 4 kHz protocol attenuates noise- induced TTS but no significant TTS reduction occurred by 1 kHz conditioning. Both protocol protected noise-induced cochlear damage. We concluded that lower tone conditioning could not protect against higher tone temporary noise-induced hearing loss, thus conditioning is a local acting and frequency-dependent phenomenon.

Anti-epileptic drugs delay age-related loss of spiral ganglion neurons via T-type calcium channel

Loss of spiral ganglion neurons is a major cause of age-related hearing loss (presbycusis). Despite being the third most prevalent condition afflicting elderly persons, there are no known medications to prevent presbycusis. Because calcium signaling has long been implicated in age-related neuronal death, we investigated T-type calcium channels. This family is comprised of three members (Ca(v)3.1, Ca(v)3.2, and Ca(v)3.3), based on their respective main pore-forming alpha subunits: α1G, α1H, and α1I. In the present study, we report a significant delay of age-related loss of cochlear function and preservation of spiral ganglion neurons in α1H null and heterozygous mice, clearly demonstrating an important role for Ca(v)3.2 in age-related neuronal loss. Furthermore, we show that anticonvulsant drugs from a family of T-type calcium channel blockers can significantly preserve spiral ganglion neurons during aging. To our knowledge, this is the first report of drugs capable of diminishing age-related loss of spiral ganglion neurons.

New insights into glutamate ototoxicity in cochlear hair cells and spiral ganglion neurons

CONCLUSION

Excess glutamate (Glu) exposure (20 mM) in the cochlear perilymph affects the physiological function of outer hair cells (OHCs) within a 2 h period and induces apoptosis in the modiolus spiral ganglion neurons (SGNs) in an apoptosis-inducing factor (AIF)-dependent manner.

OBJECTIVES

To determine whether high-dose Glu affects the function of OHCs and whether it induces AIF- and caspase-3-dependent apoptosis in the cochlear SGNs.

METHODS

Perilymphatic perfusions of Glu (20 mM) and artificial perilymph (AP) solutions were performed in adult guinea pig cochleae. Both cochlear microphonics (CM) and electrical auditory brainstem response (eABR) were measured before and 2 h after perfusions. The hair cell morphologies were examined using transmission electron microscopy. The expression of two apoptotic indicators, AIF and caspase-3, was examined 8 h after perfusions.

RESULTS

In contrast to AP perfusions, the perfusion of 20 mM Glu caused significant reduction in the CM and eABR amplitudes. Inner hair cells (IHCs) after Glu perfusion were deformed and exhibited vacuolization in the postsynaptic region, whereas the OHC system appeared unaffected. AIF expression was detected in the nuclei of SGNs 8 h after Glu exposure, but the expression of caspase-3 was not shown in any cochlear tissues.

Effects of age and sex on the expression of estrogen receptor alpha and beta in the mouse inner ear

CONCLUSION

Estrogen receptor (ER) alpha and beta were expressed in the inner ear, and expression decreased with increasing age. ERalpha may alter cochlear and vestibular sensory transduction, and ERbeta may have a neuroprotective function in the inner ear.

OBJECTIVE

Expression of ERalpha and ERbeta in the mouse inner ear and its alterations with sex and aging were analyzed.

MATERIALS AND METHODS

Male and female CBA/J mice aged 8 weeks and 24 months were used. The localization and the intensity of ERalpha and ERbeta immunoreactivity in the inner ear of young and old mice of both sexes were investigated by immunohistochemistry.

RESULTS

ERalpha and ERbeta were co-expressed in the inner ear, i.e. in the nuclei of stria vascularis, outer and inner hair cells, spiral ganglion cells and vestibular ganglion cells, vestibular dark cells and endolymphatic sac. Strial marginal cells, outer hair cells and type II ganglion cells showed less expression of ERalpha. No gender- or age-related difference was noted in the expression pattern of ERalpha or ERbeta, but fluorescence intensity of ERalpha was stronger in young female mice than in young male mice. In contrast, ERbeta revealed no significant difference. In the old mice, fluorescence intensities of both ERalpha and ERbeta were significantly decreased in both sexes.

Detecting dead regions using psychophysical tuning curves: A comparison of simultaneous and forward masking

A dead region (DR) is a region of the cochlea where there are no functioning inner hair cells and/or neurones. We compared the edge frequencies, fe, of DRs estimated using four methods: the TEN(HL) test; psychophysical tuning curves (PTCs) measured in simultaneous masking (320-Hz wide noise masker) using a 'fast' method (sweeping masker) and a 'classical' method; and PTCs measured in forward masking (sinusoidal masker) using a 'classical' method. Fourteen subjects with high-frequency DRs were tested. For measurement of PTCs, the signal frequency was chosen to fall inside the DR; the tip frequencies of the PTCs were taken as indicating the values of fe. The values of fe obtained from the PTCs in forward and simultaneous masking (both fast and classical methods) were similar and were usually close to, but somewhat above, the values of fe estimated from the TEN(HL) test. Fast PTCs measured in simultaneous masking are recommended for use in clinical practice, as they give a precise estimate of fe and are quick to administer.

Psychophysical evidence of damaged active processing mechanisms in Belgian Waterslager Canaries

Belgian Waterslager canaries (BWC), bred for a distinct low-pitched song, have an inherited high-frequency hearing loss associated with hair cell abnormalities. Hair cells near the abneural edge of the papilla, which receive primarily efferent innervation in normal birds, are among the most severely affected. These cells are thought to support nonlinear active processing in the avian ear, though the mechanisms are poorly understood. Here we present psychophysical evidence that suggests degraded active processing in BWC compared to normal-hearing non-BWC. Critical ratios, psychophysical masking patterns and phase effects on masking by harmonic complexes were measured in BWC and non-BWC using operant conditioning procedures. Critical ratios were much larger in BWC than in non-BWC at high frequencies. Psychophysical tuning curves derived from the masking patterns for BWC were broadened at high frequencies. BWC also showed severely reduced phase effects on masking by harmonic complexes compared to non-BWC. As has been hypothesized previously for hearing-impaired humans, these results are consistent with a loss of active processing mechanisms in BWC.

[Effect of the spiral ganglion cell and nerve fiber of rat cochlea in vitro to hypoxia]

OBJECTIVE

To establish a practical model for Wistar rat cochlea organ culturing in vitro, and to observe the growing status in hypoxia of the spiral ganglion cell and nerve fiber.

METHOD

We used an in vitro hypoxia model and dissociated cultures of the basal membrane from the cochlea of 3-day-old Wistar rats. And put them in incubator (37 degrees C, 90% N2, 5% CO2, 5% O2) to hypoxia culture for different times. The culture were Immunofluorescence dyed and count the number of the spiral ganglion cell and the cell density in unit area (24 mm x 36 mm), and observe the morph of nerve fiber under the confocal microscope, the results were compared with controls.

RESULT

Hypoxia early (6 h) nerve fiber appear edema, spiral ganglion cell didn't change compared with controls; nerve fiber appear break and disintegration and the spiral ganglion cell decrease in 12 hours culturing, and the cell density in unit area had remarkable difference compared with control (P < 0.01). Hypoxia leads to the cell density decrease in a time-dependent manner, the longer of cultures times in hypoxia, the heavier of damage in spiral ganglion cell and nerve fiber. Twelve hours culturing, and the cell density in unit area had remarkable difference compared with control (P < 0.01). Hypoxia leads to the cell density decrease in a time-dependent manner, the longer of cultures times in hypoxia, the heavier of damage in spiral ganglion cell and nerve fiber.

CONCLUSION

The study findings suggest that hypoxia makes the spiral ganglion cell and nerve fiber damage of culturing in vitro, and nerve fiber more susceptible than spiral ganglion cell for hypoxia.

Ginkgo Biloba Extract (EGb 761) Protects Against Cisplatin-Induced Ototoxicity in Rats

HYPOTHESIS

A standardized Ginkgo biloba extract, EGb 761, may have protective effect against cisplatin-induced ototoxicity in rats.

BACKGROUND

Cisplatin-induced ototoxicity is a major dose-limiting side effect in anticancer chemotherapy. Cisplatin-induced ototoxicity has been correlated to depletion of the cochlear antioxidant system and increased lipid peroxidation. EGb 761 contains potent antioxidants capable of scavenging free radicals, inhibiting nitric oxide synthesis, reducing lipid peroxidation, and protecting against apoptosis. The purpose of this study was to investigate the effect of EGb 761 on cisplatin-induced ototoxicity in rats.

METHODS

Male Wistar rats were divided into four groups and were treated as follows: 1) vehicle control; 2) cisplatin (13 mg/kg, intraperitoneally) plus vehicle; 3) EGb 761 (200 mg/kg, intraperitoneally); and 4) EGb 761 plus cisplatin. Auditory brainstem responses (ABRs) were measured pretreatment and 72 hours posttreatment, and threshold shifts were analyzed. Endocochlear potentials (EPs) were also obtained at 72 hours posttreatment. Cochleae were harvested and processed for scanning electron microscopy after completion of auditory testing.

RESULTS

Cisplatin-treated rats showed significant ABR threshold shifts across all frequencies (click, and 2-, 4-, 8-, 16-, and 32-kHz tones) compared with each of the other groups (p < 0.001). Rats treated with EGb 761 plus cisplatin did not show significant ABR threshold shifts (p > 0.05). Similarly, the EPs of cisplatin-treated rats were decreased significantly approximately 50% in comparison with the other groups (p < 0.001). The EPs of EGb 761 plus cisplatin-treated rats were decreased less than 20% compared with vehicle control group or the EGb 761 only group (p < 0.01). The scanning electron microscopy observation indicated severe outer hair cell loss in the basal turn of cochleae of cisplatin-treated rats, whereas outer hair cells remained intact in the rats treated with EGb 761 plus cisplatin.

CONCLUSION

These results demonstrate that EGb 761 protects against cisplatin-induced ototoxicity.

Whirler mutant hair cells have less severe pathology than shaker 2 or double mutants

MYOSIN XV is a motor protein that interacts with the PDZ domain-containing protein WHIRLIN and transports WHIRLIN to the tips of the stereocilia. Shaker 2 (sh2) mice have a mutation in the motor domain of MYOSIN XV and exhibit congenital deafness and circling behavior, probably because of abnormally short stereocilia. Whirler (wi) mice have a similar phenotype caused by a deletion in the third PDZ domain of WHIRLIN. We compared the morphology of Whrn (wi/wi) and Myo15 (sh2/sh2) sensory hair cells and found that Myo15 (sh2/sh2) have more frequent pathology at the base of inner hair cells than Whrn (wi/wi), and shorter outer hair cell stereocilia. Considering the functional and morphologic similarities in the phenotypes caused by mutations in Myo15 and Whrn, and the physical interaction between their encoded proteins, we used a genetic approach to test for functional overlap. Double heterozygotes (Myo15 (sh2/+), Whrn (wi/+)) have normal hearing and no increase in hearing loss compared to normal littermates. Single and double mutants (Myo15 (sh2/sh2), Whrn (wi/wi)) exhibit abnormal persistence of kinocilia and microvilli, and develop abnormal cytoskeletal architecture. Double mutants are also similar to the single mutants in viability, circling behavior, and lack of a Preyer reflex. The morphology of cochlear hair cell stereocilia in double mutants reflects a dominance of the more severe Myo15 (sh2/sh2) phenotype over the Whrn (wi/wi) phenotype. This suggests that MYOSIN XV may interact with other proteins besides WHIRLIN that are important for hair cell maturation.

N-Acetyl l-cysteine does not protect against premature age-related hearing loss in C57BL/6J mice: A pilot study

A compound capable of preventing age-related hearing loss would be very useful in an aging population. N-acetyl-L-cysteine (L-NAC) has been shown to be protective against noise exposure, a condition that leads to increased oxidative stress. Not withstanding environmental factors, there is evidence that age-related hearing loss (AHL) in the mouse is linked to more than one genetic loci and, by extension, in humans. Our hypothesis is that AHL defect results in increased sensitivity to oxidative stress and L-NAC would be able to protect the hearing of a mouse model of pre-mature AHL, the C57BL/6J (B6) mouse strain. L-NAC was added to the regular water bottle of B6 mice (experimental group) and available ad lib. The other group received normal tap water. Hearing was tested monthly by the ability to generate the auditory brainstem response (ABR). After the final ABR test, mice were sacrificed by an overdose of Avertin, ears were harvested and hair cell loss was quantified. There was no difference in ABR thresholds or in histopathology between the control group and the group receiving L-NAC in their drinking water. In contrast to the protective effects of L-NAC against noise-induced hearing loss, the lack of protective effect in this study may be due to (i) the dosage level; (ii) the duration of treatment; (iii) the biochemical mechanisms underlying age-induced hearing loss; or (iv) how the mouse metabolizes L-NAC.

Cell division and maintenance of epithelial integrity in the deafened auditory epithelium

Quiescence is among the hallmarks of the sensory epithelium of the cochlea. When auditory sensory cells (hair cells) degenerate they are not replaced, and therefore hearing loss is permanent. Cochlear hair cells are susceptible to several types of lesions, including aminoglycoside antibiotics. The application of the aminoglycoside neomycin in the inner ear mimics cases of severe hair cell loss and leads to collapse of the cochlear epithelium. We now report that in mature guinea pig cochleae injected with neomycin, the remaining nonsensory cells undergo robust proliferative response. p27( Kip1), an inhibitor of cell cycle in the cochlea, was present in nondividing cells and absent during mitosis. Dividing cells retained their tight junction complexes and maintained the structural confluence of the auditory epithelium during cell division. The plane of mitosis was invariably parallel to the luminal surface. These results indicate that the flat epithelium of the cochlea can down-regulate p27( Kip1) and divide after a severe lesion and suggest that the cell divisions assist in maintaining the epithelial confluence throughout the cochlea. Presence of mitosis in the tissue presents therapeutic opportunities for gene transfer and stem cells therapies.

The role of deferoxamine in the prevention of gentamicin ototoxicity: a histological and audiological study in guinea pigs

CONCLUSION

The addition of deferoxamine to gentamicin seems to confer partial functional and histological protection to the cochlea.

OBJECTIVE

Aminoglycosides are known ototoxic agents. The toxicity occurs via an activation process involving the formation of an iron-gentamicin complex with free radical production. Iron chelation will supposedly limit this toxic effect. This study aimed to determine the possible cochleoprotective role of deferoxamine on the ototoxic effect of gentamicin.

MATERIALS AND METHODS

Sixty healthy active guinea pigs, weighing 400-600 g, with an average age of 6 months were used. They were divided into three groups. Group 1 received intramuscular gentamicin 8 mg/kg/day, group 2 received gentamicin 8 mg/kg/day and deferoxamine 150 mg/kg twice daily for 19 days and group 3 served as a control. All animals had a baseline measurement of distortion product oto-acoustic emissions. At the end of 33 days they were submitted to another measurement and then the animals were sacrificed and their cochleas were examined histologically by light and transmission electron microscopy.

RESULTS

In group 1 the mean amplitude post-injection ranged from 5.83 dB at 1001 Hz to 22.33 dB at 6348 Hz. In the deferoxamine + gentamicin group the mean amplitude post-injection ranged from 5.10 dB at 1001 Hz, to 24.45 dB at 6348 Hz. This was statistically significant. At 4004, 5042 and 6348 Hz group 2 showed less histological damage than group 1.

Hematopoietic stem cells prevent hair cell death after transient cochlear ischemia through paracrine effects

Transplantation of hematopoietic stem cells (HSCs) is regarded to be a potential approach for promoting repair of damaged organs. Here, we investigated the influence of hematopoietic stem cells on progressive hair cell degeneration after transient cochlear ischemia in gerbils. Transient cochlear ischemia was produced by extracranial occlusion of the bilateral vertebral arteries just before their entry into the transverse foramen of the cervical vertebra. Intrascalar injection of HSCs prevented ischemia-induced hair cell degeneration and ameliorated hearing impairment. We also showed that the protein level of glial cell line-derived neurotrophic factor (GDNF) in the organ of Corti was upregulated after cochlear ischemia and that treatment with HSCs augmented this ischemia-induced upregulation of GDNF. A tracking study revealed that HSCs injected into the cochlea were retained in the perilymphatic space of the cochlea, although they neither transdifferentiated into cochlear cell types nor fused with the injured hair cells after ischemia, suggesting that HSCs had therapeutic potential possibly through paracrine effects. Thus, we propose HSCs as a potential new therapeutic strategy for hearing loss.

Imaging the living inner ear using intravital confocal microscopy

Confocal laser scanning microscopy permits detailed visualization of structures deep within thick fluorescently labeled specimen. This makes it possible to investigate living cells inside intact tissue without prior chemical sample fixation and sectioning. Isolated guinea pig temporal bones have previously been used for confocal experiments in vitro, but tissue deterioration limits their use to a few hours after the death of the animal. In order to preserve the cochlea in an optimal functional and physiological condition, we have developed an in vivo model based on a confocal microscopy approach. Using a ventral surgical approach, the inner ear is exposed in deeply anaesthetized, tracheotomized, living guinea pigs. To label the inner ear structures, scala tympani is perfused via an opening in the basal turn, delivering tissue culture medium with fluorescent vital dyes (RH 795 and calcein AM). An apical opening is made in the bony shell of cochlea to enable visualization using a custom-built objective lens. Intravital confocal microscopy, with preserved blood and nerve supply, may offer an important tool for studying auditory physiology and the pathology of hearing loss. After acoustic overstimulation, shortening and swelling of the sensory hair cells were observed.

Distribution of focal lesions in the chinchilla organ of Corti following exposure to a 4-kHz or a 0.5-kHz octave band of noise

An octave band of noise (OBN) delivers fairly uniform acoustic energy over a specific range of frequencies. Above and below this range, energy is at least 30 dB SPL less than that within the OBN. When the ear is exposed to an OBN, hair-cell loss often occurs outside the octave band. The frequency location of hair-cell loss is evident when the percent distance from the apex of focal lesions is analyzed. Focal lesions involve substantial loss of outer hair cells (OHCs) only, inner hair cells (IHCs) only, or both OHCs and IHCs (i.e., combined lesions) in a specific region of the organ of Corti (OC). Data sets were assembled from our permanent collection of noise-exposed chinchillas as follows: (1) the sum of exposure duration and recovery time was less than or equal to 11 d; (2) the exposure level was less than or equal to 108 dB SPL; and (3) focal lesions were less than 1.5mm in length. The data sets included a variety of exposures ranging from high-level, short duration to moderate-level, moderate duration. The center of each focal lesion was expressed as percent distance from the OC apex. Means, standard deviations and medians were calculated for focal-lesion size resulting from exposure to a 4-kHz or a 0.5-kHz OBN. Histograms were then constructed from the percent-location data using 2.0% bins. For the 4-kHz OBN, 5% of the lesions were in the apical half of the OC and 95% were in the basal half. The mean lesion size was 1.68% of total OC length for OHC and combined focal lesions and 0.42% for IHC focal lesions. Most OHC and combined lesions occurred in the 5-7-kHz region, at and just above the upper edge of the OBN. Clusters of lesions were also found around 8 and 12 kHz. A cluster was present at and just below the lower edge of the OBN, as well as another in the 1.5-kHz region. For the 0.5-kHz OBN, 34% of the lesions were in the apical half of the OC and 66% were in the basal half. The mean lesion size was 0.93% for OHC and combined focal lesions and 0.32% for IHC focal lesions. OHC and combined focal-lesion distribution showed clusters at 0.25, 0.75 and 1.5 kHz in the apical half of the OC. In the basal half, the distribution of focal lesions was similar to that seen with the 4-kHz OBN (r=0.54). With both OBNs, most IHC focal lesions occurred in the basal half of the OC. High resolution power spectrum analysis of each OBN and non-invasive tests for harmonics and distortion products in a chinchilla were performed to look for exposure energy above and below the OBN. No energy was found that could explain the OC damage.

Electrophysiological correlates of progressive sensorineural pathology in carboplatin-treated chinchillas

Carboplatin produces progressive damage to auditory nerve fibers, spiral ganglion neurons (SGNs) and inner hair cells (IHC) in the chinchilla cochlea but leaves outer hair cells intact. Within 1 h after injection, many afferent terminals beneath IHCs and myelin lamellae surrounding SGN processes are vacuolated. One day after injection, approximately half of the nerve fibers are missing. IHCs are intact at 2 days, but 20-30% are missing at 3 days. We studied the electrophysiological correlates of this progressive morphological damage by recording cochlear microphonics (CM), distortion product otoacoustic emissions (DPOAE), summating potentials (SP), compound action potentials (CAP) and midbrain evoked potentials (IC-EVP) before and 1 h, 12 h, 1 days, 3 days, 5 days, 7 days and 14 days after carboplatin injection (75 mg/kg IP) in four chinchillas. CM and DPOAEs tended to be unchanged or enhanced. CAP and SP showed little change until Day 3, when amplitudes were reduced in all animals and CAP thresholds were elevated by 9 dB; amplitudes declined further between Days 3 and 5 but not thereafter. IC-EVP amplitudes decreased on Days 3 or 5 but thresholds were relatively unchanged. All animals showed some recovery of IC-EVP between Days 7 and 14, including one with 70% enhancement on Day 14. The results indicate that threshold and amplitude measures fail to detect peripheral pathology until some relatively high threshold level of damage has been exceeded. This has important implications for monitoring peripheral damage and interpreting electrophysiological test results in animals and humans.

Reduction of inhibition in the inferior colliculus after inner hair cell loss

Effects of carboplatin-induced partial loss of inner hair cells on rate-level functions of neurons in the central nucleus of the inferior colliculus of the same chinchillas before and 1-3 months after carboplatin treatment were examined. Partial inner hair cell loss caused only a small elevation of response thresholds but induced a significant increase in the proportion of monotonic and a significant decrease in the proportion of non-monotonic rate-level functions at the characteristic frequency of 210 multiunits. This indicates that inhibition in the inferior colliculus is reduced after sensory deafferentation. We conclude that some of the functional changes reported in the auditory cortex after peripheral deafferentation are already realized at the level of the inferior colliculus.

The role of toxicokinetics in xylene-induced ototoxicity in the rat and guinea pig

In the rat, some aromatic solvents cause a high level of ototoxicity that is characterized by damage to outer hair cells in the cochlea, which results in irreversible hearing loss. However, there is a vast difference in their potency. Among the three isomers of xylene, only para-xylene has been shown to be ototoxic in the rat. Moreover, all the species do not show the same susceptibility to ototoxic solvents, the rat being the most susceptible and the guinea pig seeming resistant to this ototoxic effect. The objective of the study was to determine whether toxicokinetic factors could explain the differences in ototoxicity observed among the three isomers of xylene in the rat and the species-dependent ototoxicity in the rat and the guinea pig. Blood and brain concentrations of each isomer were monitored in Sprague-Dawley rats treated orally by gastric intubation with a single dose or a 10 day-repeated treatment of 8.47 mmol/kg (an ototoxic dosage for para-xylene) of each isomer. Moreover, histology of the cochlea was carried out and the toxicokinetics of meta-xylene was monitored in rats treated with a single dose or a 10 day-repeated treatment of 16.94 mmol/kg meta-xylene, a non-ototoxic isomer. Similarly, histology of the cochlea was carried out and the toxicokinetics of para-xylene was followed in guinea pigs treated by gavage with a single dose or a 10 day-repeated treatment of 8.47 mmol/kg para-xylene. Finally, the blood and brain concentrations of para-xylene were measured in both the rats and the guinea pigs after a 4-h exposure to 1800 ppm of para-xylene. Among the three isomers studied, para-xylene yielded the highest blood and brain concentrations in the acutely and repeatedly exposed rats. When given a high dosage of meta-xylene (16.94 mmol/kg), the rats showed blood and brain concentrations of meta-xylene in the same order as those obtained with 8.47 mmol/kg para-xylene, but no outer hair cell loss was observed. No outer hair cell loss was observed in the guinea pigs treated with para-xylene. Whatever the exposure pattern, the blood and brain concentrations of para-xylene in the rats were 3.1-9.5 times higher than those measured in the guinea pigs. These results indicate that toxicokinetic factors cannot explain the differences in ototoxicity observed with the three isomers in the rat. However, they suggest that the differences in susceptibility to para-xylene observed between the rats and the guinea pigs might be due to toxicokinetic factors.

Low-dose, long-term caroverine administration attenuates impulse noise-induced hearing loss in the rat

CONCLUSION

Physiological and morphological assessments indicated that low-dose and long-term caroverine delivery might be a new approach to protect against impulse noise-induced hearing loss.

BACKGROUND

Although the exact mechanisms by which impulse noise causes hearing loss are still unclear, there is accumulating evidence that increased reactive oxygen species (ROS) production and excessive glutamate released from the inner hair cells lead to hair cell loss and consequently hearing loss. Caroverine is an antagonist of two glutamate receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors in the inner ear, as well as an antioxidant.

MATERIALS AND METHODS

In this study, caroverine was delivered subcutaneously using an osmotic pump. This kind of delivery has the advantage, via continuous, long-term and low dose drug administration, of avoiding systemic side effects.

RESULTS

It was shown that caroverine could significantly protect the cochlea against impulse noise trauma.

Therapeutic efficacy of intra-cochlear administration of methylprednisolone after acoustic trauma caused by gunshot noise in guinea pigs

The therapeutic efficacy of cochlear infusion of methylprednisolone (MP) after an impulse noise trauma (170dB SPL peak) was evaluated in guinea pigs. The compound action potential threshold shifts were measured over a 14 days recovery period after the gunshot exposure. For each animal, one of the cochlea was perfused directly into the scala tympani with MP during 7 days via a mini-osmotic pump, whereas the other cochlea was not pump-implanted. The functional study of hearing was supplemented by histological analysis. Forty eight hours after the trauma, significant differences between auditory threshold shifts in the implanted and non-implanted ears were observed for frequencies above 8kHz. At day 7, the difference was significant for only one frequency and no difference was observed after 14 days recovery. Cochleograms showed that the hair cell losses were significantly lower in the MP treated ears. This work indicates that direct infusion of MP into perilymphatic space accelerates hearing recovery, reduces hair cell losses after impulse noise trauma but does not limit permanent threshold shifts.

Synaptic alterations at inner hair cells precede spiral ganglion cell loss in aging C57BL/6J mice

Hearing deficits have often been associated with loss of or damage to receptor hair cells and/or degeneration of spiral ganglion cells. There are, however, some physiological abnormalities that are not reliably attributed to loss of these cells. The afferent synapse between radial fibers of spiral ganglion neurons and inner hair cells (IHCs) emerges as another site that could be involved in transmission abnormalities. We tested the hypothesis that the structure of these afferent terminals would differ between young animals and older animals with significant hearing loss. Afferent endings and their synapses were examined by transmission electron microscopy at approximately 45% distance from the basal end of the cochlea in 2-3 month-old and 8-12 month-old C57BL/6J mice. The number of terminals in older animals was reduced by half compared to younger animals. In contrast, there was no difference in the density of SGCs between the age groups. Older animals featured enlarged terminals and mitochondria and enlarged postsynaptic densities and presynaptic bodies. These morphological changes may be a combination of pathologic, adaptive and compensatory responses to sensory dysfunction. Improved knowledge of these processes is necessary to understand the role of afferent connectivity in dysfunction of the aging cochlea.

A model of peripherally developing hearing loss and tinnitus based on the role of hypoxia and ischemia

The incidence of sensorineural hearing loss often caused by direct damage to the cochlear hair cells is by far more frequent and more serious than disorders affecting the external ear or the middle ear. Mechanisms that are discussed to be relevant for the genesis of tinnitus and acquired hearing impairment are hair cell loss, signal transduction disturbances in the region of the outer and inner hair cells and the spiral ganglion, impairment of cochlear blood flow, mechanical disturbance, and hypoxia and ischemia. The present model surveys the possible cellular and molecular biological causes of peripherally developing hearing loss and tinnitus. In particular, the paper discusses the roles of hypoxia and ischemia in the cochlea and in the etiology of the neurosensory types of tinnitus. Peripheral origins of hearing disturbances and tinnitus may be: (a) damage to the stereocilia and the tip links, (b) dysfunction of potassium channels or (c) modification of the glutamate release. Moreover, the hypoxia inducible factor-1 may have an important role to play as a key transcription factor in the cells' adaptation to hypoxia and ischemia. An impairment of the cochlear blood flow may be induced by the expression of target genes like nitrogen monoxide synthase and endothelin-1 resulting in tinnitus. The paper discusses consequences resulting from the present model for the medical treatment of peripherally developing tinnitus and hearing loss.

Localization of NKCC1 in the cochlea and morphology of the cochlea in NKCC1-knockout mice

The distribution of the Na-K-2Cl co-transporter (NKCC1) in the cochlear K+ cycling pathway in cochlea and cochlear histological changes in the NKCC1 knockout mice were investigated. By using immunohistochemistry and toluidine blue staining, the localization of NKCC1 in cochlea of the C57BL/6J mice and the cochlear histological changes in the NKCC1 knockout mice were observed. It was found that the NKCC1 was expressed mainly in the stria marginal cells and the fibrocytes in the inferior portion of the spiral ligament in the adult C57BL/6J mice. Subpopulation of the fibrocytes in the suprastrial region and the limbus was also moderately immunoreactive. While in the cochlea of the NKCC1 knockout mice, Reissner's membrane was collapsed and scala media disappeared, accompanied with the loss of inner hair cells, outer hair cells and the support cells. The tunnel of Corti was often absent. All the findings suggested the localization of NKCC1 in the cochlea was closely correlated with cochlear K+ cycling. Loss of NKCC1 led to the destruction of the cochlear structures, and subsequently influenced the physiological function of cochlea.

Deletion of SLC19A2, the high affinity thiamine transporter, causes selective inner hair cell loss and an auditory neuropathy phenotype

Mutations in the gene coding for the high-affinity thiamine transporter Slc19a2 underlie the clinical syndrome known as thiamine-responsive megaloblastic anemia (TRMA) characterized by anemia, diabetes, and sensorineural hearing loss. To create a mouse model of this disease, a mutant line was created with targeted disruption of the gene. Cochlear function is normal in these mutants when maintained on a high-thiamine diet. When challenged with a low-thiamine diet, Slc19a2-null mice showed 40-60 dB threshold elevations by auditory brainstem response (ABR), but only 10-20 dB elevation by otoacoustic emission (OAE) measures. Wild-type mice retain normal hearing on either diet. Cochlear histological analysis showed a pattern uncommon for sensorineural hearing loss: selective loss of inner hair cells after 1-2 weeks on low thiamine and significantly greater inner than outer hair cell loss after longer low-thiamine challenges. Such a pattern is consistent with the observed discrepancy between ABR and OAE threshold shifts. The possible role of thiamine transport in other reported cases of selective inner hair cell loss is considered.

Cochlear compression in listeners with moderate sensorineural hearing loss

Psychophysical estimates of basilar membrane (BM) responses suggest that normal-hearing (NH) listeners exhibit constant compression for tones at the characteristic frequency (CF) across the CF range from 250 to 8000 Hz. The frequency region over which compression occurs is broadest for low CFs. This study investigates the extent that these results differ for three hearing-impaired (HI) listeners with sensorineural hearing loss. Temporal masking curves (TMCs) were measured over a wide range of probe (500-8000 Hz) and masker frequencies (0.5-1.2 times the probe frequency). From these, estimated BM response functions were derived and compared with corresponding functions for NH listeners. Compressive responses for tones both at and below CF occur for the three HI ears across the CF range tested. The maximum amount of compression was uncorrelated with absolute threshold. It was close to normal for two of the three HI ears, but was either slightly (at CFs < or =1000 Hz) or considerably (at CFs > or =4000 Hz) reduced for the third ear. Results are interpreted in terms of the relative damage to inner and outer hair cells affecting each of the HI ears. Alternative interpretations for the results are also discussed, some of which cast doubts on the assumptions of the TMC-based method and other behavioral methods for estimating human BM compression.