Hair ; Hair Cells, Auditory, Inner ; Humans ; Regeneration ; Wound Healing

Hearing loss has become increasingly prevalent and causes considerable disability, thus gravely burdening the global economy. Irreversible loss of hair cells is a main cause of sensorineural hearing loss, and currently, the only relatively effective clinical treatments are limited to digital hearing equipment like cochlear implants and hearing aids, but these are of limited benefit in patients. It is therefore urgent to understand the mechanisms of damage repair in order to develop new neuroprotective strategies. At present, how to promote the regeneration of functional hair cells is a key scientific question in the field of hearing research. Multiple signaling pathways and transcriptional factors trigger the activation of hair cell progenitors and ensure the maturation of newborn hair cells, and in this article, we first review the principal mechanisms underlying hair cell reproduction. We then further discuss therapeutic strategies involving the co-regulation of multiple signaling pathways in order to induce effective functional hair cell regeneration after degeneration, and we summarize current achievements in hair cell regeneration. Lastly, we discuss potential future approaches, such as small molecule drugs and gene therapy, which might be applied for regenerating functional hair cells in the clinic.
Infant, Newborn ; Humans ; Hair Cells, Auditory, Inner/physiology* ; Ear, Inner/physiology* ; Hair Cells, Auditory/physiology* ; Regeneration/genetics* ; Stem Cells

Hair Cells, Auditory, Inner ; cytology ; Synapses ; physiology ; ultrastructure

Hair Cells, Auditory, Inner ; Humans ; Synapses ; Vestibulocochlear Nerve Diseases

BACKGROUNDPrevious studies have suggested that primary degeneration of hair cells causes secondary degeneration of spiral ganglion neurons (SGNs), but the effect of SGN degeneration on hair cells has not been studied. In the adult mouse inner ear ouabain can selectively and permanently induce the degeneration of type 1 SGNs while leaving type 2 SGNs, efferent fibers, and sensory hair cells relatively intact. This study aimed to investigate the dynamic changes in hair cell ribbon synapse induced by loss of SGNs using ouabain application to the round window niche of adult mice.

METHODSIn the analysis, 24 CBA/CAJ mice aged 8-10 weeks, were used, of which 6 normal mice were used as the control group. After ouabain application in the round window niche 6 times in an hour, ABR threshold shifts at least 30 dB in the three experimental groups which had six mice for 1-week group, six for 1-month group, and six for 3-month group. All 24 animals underwent function test at 1 week and then immunostaining at 1 week, 1 month, and 3 months.

RESULTSThe loss of neurons was followed by degeneration of postsynaptic specializations at the afferent synapse with hair cells. One week after ouabain treatment, the nerve endings of type 1 SGNs and postsynaptic densities, as measured by Na/K ATPase and PSD-95, were affected but not entirely missing, but their partial loss had consequences for synaptic ribbons that form the presynaptic specialization at the synapse between hair cells and primary afferent neurons. Ribbon numbers in inner hair cells decreased (some of them broken and the ribbon number much decreased), and the arrangement of the synaptic ribbons had undergone a dynamic reorganization: ribbons with or without associated postsynaptic densities moved from their normal location in the basal membrane of the cell to a more apical location and the neural endings alone were also found at more apical locations without associated ribbons. After 1 month, when the neural postsynaptic densities had completed their degeneration, most ribbons were lost and the remaining ribbons had no contact with postsynaptic densities; after 3 months, the ribbon synapses were gone except for an occasional remnant of a CtBP2-positive vesicle. Hair cells were intact other than the loss of ribbons (based on immunohistochemistry and DPOAE).

CONCLUSIONThese findings define the effect of SGN loss on the precise spatiotemporal size and location of ribbons and the time course of synaptic degeneration and provide a model for studying plasticity and regeneration.

Animals ; Female ; Hair Cells, Auditory ; cytology ; physiology ; Hair Cells, Auditory, Inner ; cytology ; physiology ; Mice ; Mice, Inbred CBA ; Synapses ; physiology

OBJECTIVE: To investigate the expression of adaptin-2(AP-2) in mice cochlea and to discuss the probable role in the endocytosis of hair cells. METHOD: Laser scanning confocal microscopy and immune-fluroscence histochemistry were performed in this study. RESULT: In mature mice cochlea, the immunoreactivity for AP-2 was found in the inner hair cells cytoplasm. This protein mainly expressed in the hair cells basal part and nearby the ribbon synapse. CONCLUSION AP-2 protein mainly expressed in the hair cells synaptic activity zone , which suggested that AP-2 could play an important role in the synaptic vesicle endocytosis. This finding built the foundation for the further research involved in the physiological and pathological role of AP-2.
Adaptor Protein Complex alpha Subunits ; metabolism ; Animals ; Cochlea ; Hair Cells, Auditory ; Hair Cells, Auditory, Inner ; metabolism ; Mice ; Microscopy, Confocal ; Synapses

BACKGROUND AND OBJECTIVES: Cochlear dead region (CDR) is a region in the cochlear where hearing loss has occurred due to damage to the inner hair cells and/or neurons. Recently, a subjective test involving a pure-tone test in the presence of threshold-equalizing noise (TEN) was introduced to identify CDR. However, for uncooperative patients, such a subjective method would be unsuitable and objective methods would be needed instead to detect CDR. The acoustic change complex (ACC) is an evoked potential elicited by changes in the ongoing sound. In this study, we developed an objective method of identifying CDR by combining ACC response with a TEN test, namely the TEN-ACC test, and investigated its feasibility in normal-hearing listeners. SUBJECTS AND METHOD: Ten normal-hearing subjects participated in this study. All subjects underwent both behavioral TEN test and electrophysiological TEN-ACC test. The stimuli for the TEN-ACC test consisted of TEN and embedded pure tones with different frequencies/signals to noise ratios (SNRs). To identify the thresholds, the range SNR of stimulation was varied from 0 to 20 dB, in stages of 4 dB. RESULTS: The ACC responses of all subjects who participated in this study were well elicited by stimuli developed for the TEN-ACC test. We confirm that the pure-tones embedded in TEN elicited the objective ACC response. CONCLUSION: The results of this study suggest that the novel TEN-ACC test can be applied to evoke ACC in normal-hearing listeners. Future research should incorporate hearing-impaired listeners to determine the feasibility of the TEN-ACC test as an objective method to identify CDR.
Acoustics* ; Evoked Potentials ; Hair Cells, Auditory, Inner ; Hearing Loss ; Humans ; Methods ; Neurons ; Noise

Sodium salicylate is an anti-inflammatory medication with a side-effect of tinnitus. Here, we used mouse cochlear cultures to explore the effects of salicylate treatment on cochlear inner hair cells (IHCs). We found that IHCs showed significant damage after exposure to a high concentration of salicylate. Whole-cell patch clamp recordings showed that 1-5 mmol/L salicylate did not affect the exocytosis of IHCs, indicating that IHCs are not involved in tinnitus generation by enhancing their neuronal input. Instead, salicylate induced a larger peak amplitude, a more negative half-activation voltage, and a steeper slope factor of Ca²⁺ current. Using noise analysis of Ca²⁺ tail currents and qRT-PCR, we further found that salicylate increased the number of Ca²⁺ channels along with Ca_V1.3 expression. All these changes could act synergistically to enhance the Ca²⁺ influx into IHCs. Inhibition of intracellular Ca²⁺ overload significantly attenuated IHC death after 10 mmol/L salicylate treatment. These results implicate a cellular mechanism for tinnitus generation in the peripheral auditory system.
Animals ; Calcium ; Exocytosis ; Hair Cells, Auditory, Inner ; Mice ; Sodium Salicylate/pharmacology* ; Tinnitus/chemically induced*

Recent studies have revealed great functional and structural heterogeneity in the ribbon-type synapses at the basolateral pole of the isopotential inner hair cell (IHC). This feature is believed to be critical for audition over a wide dynamic range, but whether the spatial gradient of ribbon morphology is fine-tuned in each IHC and how the mitochondrial network is organized to meet local energy demands of synaptic transmission remain unclear. By means of three-dimensional electron microscopy and artificial intelligence-based algorithms, we demonstrated the cell-wide structural quantification of ribbons and mitochondria in mature mid-cochlear IHCs of mice. We found that adjacent IHCs in staggered pairs differ substantially in cell body shape and ribbon morphology gradient as well as mitochondrial organization. Moreover, our analysis argues for a location-specific arrangement of correlated ribbon and mitochondrial function at the basolateral IHC pole.
Animals ; Artificial Intelligence ; Cochlea/metabolism* ; Hair Cells, Auditory, Inner ; Mice ; Mitochondria ; Synapses/metabolism*

OBJECTIVETo investigate the expression of Dynamin subtypes in inner hair cell (IHC) of mice, and to discuss their possible roles in age-related hearing loss.

METHODSAuditory brainstem response (ABR) was recorded from the Kunming mice on postnatal 3 weeks (young), 10 weeks (adult), and 16 months (aged), 10 mice in each group. The expression of each Dynamin isoforms in the hair cells of the cochlea was observed by immunofluorescence staining and confocal microscope, and the transcription level of Dynamin subtypes mRNA was detected in qRT-PCR. Data analysis using SPSS 18 software.

RESULTSABR threshold showed no significant difference between the group of young and adult (t = -5.273, P = 0.076), but the threshold of the aged group increased comparing with young group (t = -8.365, P = 0.000), and adult group (t = -6.191, P = 0.000). All subtypes expressed in the inner hair cell of mice, of which Dynamin-1 and 2 expressed in the whole inner hair cell in the group of young and adult. In the aged group, Dynamin-1 was lost beneath the nucleus, and Dynamin-2 only be found near the nucleus. In addition, Dynamin-3 was scattered in the region of the basal part of the cells beneath the nucleus and near the spiral ganglion. The qRT-PCR revealed that mRNA of Dynamin-1 reduced with age (F = 10.410, P = 0.011), mRNA of Dynamin-2 increased to a peak in the adult group and then reduced with age (F = 24.575, P = 0.000). Meanwhile, mRNA of Dynamin-3 was not be detected.

CONCLUSIONSAll subtypes of Dynamin express in IHC. The expression of Dynamin-1 and 2 is up-regulated during maturity, which might alter the endocytosis of IHC; and the disorder of endocytosis might modulate the synaptic transmission of IHC. Whether Dynamin-3 plays a role in inner hair cells remains unclear because of the low expression.

Aging ; metabolism ; Animals ; Cochlea ; metabolism ; Dynamins ; metabolism ; Evoked Potentials, Auditory, Brain Stem ; Hair Cells, Auditory, Inner ; metabolism ; Mice