Relative time course of degeneration of different cochlear structures in the CD/1 mouse model of accelerated aging

The Stria Vascularis: Renewed Attention on a Key Player in Age-Related Hearing Loss

Age-related hearing loss (HL), or presbycusis, is a complex and heterogeneous condition, affecting a significant portion of older adults and involving various interacting mechanisms. Metabolic presbycusis, a type of age-related HL, is characterized by the dysfunction of the stria vascularis, which is crucial for maintaining the endocochlear potential necessary for hearing. Although attention on metabolic presbycusis has waned in recent years, research continues to identify strial pathology as a key factor in age-related HL. This narrative review integrates past and recent research, bridging findings from animal models and human studies, to examine the contributions of the stria vascularis to age-related HL. It provides a brief overview of the structure and function of the stria vascularis and then examines mechanisms contributing to age-related strial dysfunction, including altered ion transport, changes in pigmentation, inflammatory responses, and vascular atrophy. Importantly, this review outlines the contribution of metabolic mechanisms to age-related HL, highlighting areas for future research. It emphasizes the complex interdependence of metabolic and sensorineural mechanisms in the pathology of age-related HL and highlights the importance of animal models in understanding the underlying mechanisms. The comprehensive and mechanistic investigation of all factors contributing to age-related HL, including cochlear metabolic dysfunction, remains crucial to identifying the underlying mechanisms and developing personalized, protective, and restorative treatments.

Development of cochlear spiral ligament fibrocytes of the common marmoset, a nonhuman model animal

Spiral ligament fibrocytes generate potassium gradients, which hair cells require to convert mechanical sound waves into electrical palsy. Together with the stria vascularis, they regulate endolymph electrolyte homeostasis. Developing spiral ligament fibrocytes and generating endocochlear potential with an appropriate endolymph ion composition are essential for hearing. Understanding spiral ligament fibrocyte development is useful for studying age-related and genetic hearing loss, as well as for regenerative therapy and cochlear immunology. Despite interspecies differences, most studies of cochlear development have been conducted in rodent models due to the difficulty of using human fetal samples. This study investigated the cochlear development of spiral ligament fibrocytes in a small New World monkey species, the common marmoset (Callithrix jacchus). We examined the developmental expression of specific genes in spiral ligament fibrocytes, including those essential for the generation of endolymphatic potential. Our results showed that this animal model of spiral ligament fibrocyte development is similar to that of humans and is a suitable alternative for the analysis of human cochlear development. The time course established in this study will be useful for studying the primate-specific developmental biology of the inner ear, which may lead to novel treatment strategies for human hearing loss.

Cochlear Function in Adults Exposed to Severe Acute Respiratory Syndrome Coronavirus 2

INTRODUCTION

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a viral infection with a wide variety of symptoms, including fever, coughing, sneezing, fatigue, and a loss of taste and smell. Moreover, there are some recent studies that investigate the effects of SARS-CoV-2 on hearing and auditory performance. With this current study, we investigate the early effects of the coronavirus disease on hair cells in the cochlea.

METHODS

In the current study, there were 25 subjects (17 females, 8 males) who tested positive for polymerase chain reaction on nasopharyngeal swabs. They had reported normal auditory functions and no history of otology before SARS-CoV-2. To determine auditory functions, pure-tone audiometry, otoacoustic emissions (OAE) tests, and threshold equalizing noise (TEN) tests were used.

RESULTS

Although the hearing thresholds increased at higher frequencies, they were within normal limits according to four-frequency pure-tone averages. All participants had normal OAE, and there were no detected dead regions for any of the subjects. Even so, there were significant increases in hearing thresholds in TEN.

CONCLUSION

There is no cochlear dysfunction discovered by OAE and TEN in SARS-CoV-2-affected individuals. Nonetheless, the increase in hearing thresholds at higher frequencies, other than the pure-tone average frequencies detected by TEN, and the decrease in the presence of detected OAE could be related to deterioration in the basal part of the cochlea.

A Gap-Junction Mutation Reveals That Outer Hair Cell Extracellular Receptor Potentials Drive High-Frequency Cochlear Amplification

Cochlear amplification enables the enormous dynamic range of hearing through amplifying cochlear responses to low- to moderate-level sounds and compressing them to loud sounds. Amplification is attributed to voltage-dependent electromotility of mechanosensory outer hair cells (OHCs) driven by changing voltages developed across their cell membranes. At low frequencies, these voltage changes are dominated by intracellular receptor potentials (RPs). However, OHC membranes have electrical low-pass filter properties that attenuate high-frequency RPs, which should potentially attenuate amplification of high-frequency cochlear responses and impede high-frequency hearing. We made in vivo intracellular and extracellular electrophysiological measurements from the organ of Corti of male and female mice of the CBA/J strain, with excellent high-frequency hearing, and from the CD-1 mouse strain, which has sensitive hearing below 12 kHz but loses high-frequency hearing within a few weeks postpartum. The CD-1 mouse strain was transfected with an A88V mutation of the connexin 30 gap-junction protein. By blocking the action of the GJ protein to reduce input resistance, the mutation increased the OHC extracellular RP (ERP) magnitude and rescued high-frequency hearing. However, by increasing the organ of Corti resistance, the mutation rescued high-frequency hearing through preserving the OHC extracellular RP (ERP) magnitude. We measured the voltage developed across the basolateral membranes of OHCs, which controls their electromotility, for low- to high-frequency sounds in male and female mice of the CD-1 strain that expressed the A88V mutation. We demonstrate that ERPs, not RPs, drive OHC motility and cochlear amplification at high frequencies because at high frequencies, ERPs are not frequency attenuated, exceed RPs in magnitude, and are appropriately timed to provide cochlear amplification.SIGNIFICANCE STATEMENT Cochlear amplification, which enables the enormous dynamic range of hearing, is attributed to voltage-dependent electromotility of the mechanosensory outer hair cells (OHCs) driven by sound-induced voltage changes across their membranes. OHC intracellular receptor potentials are electrically low-pass filtered, which should hinder high-frequency hearing. We measured the intracellular and extracellular voltages that control OHC electromotility in vivo in a mouse strain with impaired high-frequency hearing. A gap-junction mutation of the strain rescued high-frequency hearing, increased organ of Corti resistance, and preserved large OHC extracellular receptor potentials but reduced OHC intracellular receptor potentials and impaired low-frequency hearing. We concluded intracellular potentials drive OHC motility at low frequencies and extracellular receptor potentials drive OHC motility and cochlear amplification at high frequencies.

A High-Fat Diet Induces Low-Grade Cochlear Inflammation in CD-1 Mice

There is growing evidence for a relationship between gut dysbiosis and hearing loss. Inflammatory bowel disease, diet-induced obesity (DIO), and type 2 diabetes have all been linked to hearing loss. Here, we investigated the effect of a chronic high-fat diet (HFD) on the development of inner ear inflammation using a rodent model. Three-week-old CD-1 (Swiss) mice were fed an HFD or a control diet for ten weeks. After ten weeks, mouse cochleae were harvested, and markers of cochlear inflammation were assessed at the protein level using immunohistochemistry and at the gene expression level using quantitative real-time RT-PCR. We identified increased immunoexpression of pro-inflammatory biomarkers in animals on an HFD, including intracellular adhesion molecule 1 (ICAM1), interleukin 6 receptor α (IL6Rα), and toll-like-receptor 2 (TLR2). In addition, increased numbers of ionized calcium-binding adapter molecule 1 (Iba1) positive macrophages were found in the cochlear lateral wall in mice on an HFD. In contrast, gene expression levels of inflammatory markers were not affected by an HFD. The recruitment of macrophages to the cochlea and increased immunoexpression of inflammatory markers in mice fed an HFD provide direct evidence for the association between HFD and cochlear inflammation.

Electrical and Immunohistochemical Properties of Cochlear Fibrocytes in 3D Cell Culture and in the Excised Spiral Ligament of Mice

Fibrocyte degeneration in the cochlear lateral wall is one possible pathology of age-related metabolic hearing loss (presbycusis). Within the lateral wall fibrocytes play a role in potassium recycling and maintenance of the endocochlear potential. It has been proposed that cell replacement therapy could prevent fibrocyte degeneration in the CD/1 mouse model of hearing loss. For this to work, the replacement fibrocytes would need to take over the structural and physiological role of those lost. We have grown lateral wall fibrocytes from neonatal CD/1 mice in a 3D-collagen gel culture with the aim of assessing their functional similarity to native lateral wall fibrocytes, the latter in a slice preparation and in excised spiral ligament pieces. We have compared cultured and native fibrocytes using both immuno-labelling of characteristic proteins and single cell electrophysiology. Cultured fibrocytes exhibited rounded cell bodies with extending processes. They labelled with marker antibodies targeting aquaporin 1 and calcium-binding protein S-100, precluding an unambiguous identification of fibrocyte type. In whole-cell voltage clamp, both native and cultured fibrocytes exhibited non-specific currents and voltage-dependent K⁺ currents. The non-specific currents from gel-cultured and excised spiral ligament fibrocytes were partially and reversibly blocked by external TEA (10 mM). The TEA-sensitive current had a mean reversal potential of + 26 mV, suggesting a permeability sequence of Na⁺  > K⁺. These findings indicate that 3D-cultured fibrocytes share a number of characteristics with native spiral ligament fibrocytes and thus might represent a suitable population for transplantation therapy aimed at treating age-related hearing loss.

The effects of substrate composition and topography on the characteristics and growth of cell cultures of cochlear fibrocytes

Spiral ligament fibrocytes of the cochlea play homoeostatic roles in hearing and their degeneration contributes to hearing loss. Culturing fibrocytes in vitro provides a way to evaluate their functional characteristics and study possible therapies for hearing loss. We investigated whether in vivo characteristics of fibrocytes could be recapitulated in vitro by modifying the culture substrates and carried out proof of concept studies for potential transplantation of culture cells into the inner ear. Fibrocytes cultured from 4 to 5-week old CD/1 mice were grown on 2D substrates coated with collagen I, II, V or IX and, after harvesting, onto or into 3D substrates (hydrogels) of collagen I alone or mixed collagen I and II at a 1:1 ratio. We also assessed magnetic nanoparticle (MNP) uptake. Cell counts, immunohistochemical and ultrastructural studies showed that fibrocytes grown on 2D substrates proliferated, formed both small spindle-shaped and large flat cells that avidly took up MNPs. Of the different collagen coatings, only collagen II had an effect, causing a reduced size of the larger cells. On hydrogels, the cells were plump/rounded with extended processes, resembling native cells. They formed networks over the surface and became incorporated into the gel. In all culture formats, the majority co-expressed caldesmon, aquaporin 1, S-100 and sodium potassium ATPase, indicating a mixed or uncharacterised phenotype. Time-course experiments showed a decrease to ∼50% of the starting population by 4d after seeding on collagen I hydrogels, but better survival (∼60%) was found on collagen I + II gels, whilst TEM revealed the presence of apoptotic cells. Cells grown within gels additionally showed necrosis. These results demonstrate that fibrocytes grown in 3D recapitulate in vivo morphology of native fibrocytes, but have poorer survival, compared with 2D. Therefore hydrogel cultures could be used to study fibrocyte function and might also offer avenues for cell-replacement therapies, but need more optimization for therapeutic use. Fibrocyte function could be modified using MNPs in combination, for example, with gene transfection.

Forgotten Fibrocytes: A Neglected, Supporting Cell Type of the Cochlea With the Potential to be an Alternative Therapeutic Target in Hearing Loss

Cochlear fibrocytes are a homeostatic supporting cell type embedded in the vascularized extracellular matrix of the spiral ligament, within the lateral wall. Here, they participate in the connective tissue syncytium that enables potassium recirculation into the scala media to take place and ensures development of the endolymphatic potential that helps drive current into hair cells during acoustic stimulation. They have also been implicated in inflammatory responses in the cochlea. Some fibrocytes interact closely with the capillaries of the vasculature in a way which suggests potential involvement, together with the stria vascularis, also in the blood-labyrinth barrier. Several lines of evidence suggests that pathology of the fibrocytes, along with other degenerative changes in this region, contribute to metabolic hearing loss (MHL) during aging that is becoming recognized as distinct from, and potentially a precursor for, sensorineural hearing loss (SNHL). This pathology may underlie a significant proportion of cases of presbycusis. Some evidence points also to an association between fibrocyte degeneration and Ménière’s disease (MD). Fibrocytes are mesenchymal; this characteristic, and their location, make them amenable to potential cell therapy in the form of cell replacement or genetic modification to arrest the process of degeneration that leads to MHL. This review explores the properties and roles of this neglected cell type and suggests potential therapeutic approaches, such as cell transplantation or genetic engineering of fibrocytes, which could be used to prevent this form of presbycusis or provide a therapeutic avenue for MD.

Early Hearing Loss upon Disruption of Slc4a10 in C57BL/6 Mice

The unique composition of the endolymph with a high extracellular K⁺ concentration is essential for sensory transduction in the inner ear. It is secreted by a specialized epithelium, the stria vascularis, that is connected to the fibrocyte meshwork of the spiral ligament in the lateral wall of the cochlea via gap junctions. In this study, we show that in mice the expression of the bicarbonate transporter Slc4a10/Ncbe/Nbcn2 in spiral ligament fibrocytes starts shortly before hearing onset. Its disruption in a C57BL/6 background results in early onset progressive hearing loss. This hearing loss is characterized by a reduced endocochlear potential from hearing onset onward and progressive degeneration of outer hair cells. Notably, the expression of a related bicarbonate transporter, i.e., Slc4a7/Nbcn1, is also lost in spiral ligament fibrocytes of Slc4a10 knockout mice. The histological analysis of the spiral ligament of Slc4a10 knockout mice does not reveal overt fibrocyte loss as reported for Slc4a7 knockout mice. The ultrastructural analysis, however, shows mitochondrial alterations in fibrocytes of Slc4a10 knockout mice. Our data suggest that Slc4a10 and Slc4a7 are functionally related and essential for inner ear homeostasis.

OHC-TRECK: A Novel System Using a Mouse Model for Investigation of the Molecular Mechanisms Associated with Outer Hair Cell Death in the Inner Ear

Outer hair cells (OHCs) are responsible for the amplification of sound, and the death of these cells leads to hearing loss. Although the mechanisms for sound amplification and OHC death have been well investigated, the effects on the cochlea after OHC death are poorly understood. To study the consequences of OHC death, we established an OHC knockout system using a novel mouse model, Prestin-hDTR, which uses the prestin promoter to express the human diphtheria toxin (DT) receptor gene (hDTR). Administration of DT to adult Prestin-hDTR mice results in the depletion of almost all OHCs without significant damage to other cochlear and vestibular cells, suggesting that this system is an effective tool for the analysis of how other cells in the cochlea and vestibula are affected after OHC death. To evaluate the changes in the cochlea after OHC death, we performed differential gene expression analysis between the untreated and DT-treated groups of wild-type and Prestin-hDTR mice. This analysis revealed that genes associated with inflammatory/immune responses were significantly upregulated. Moreover, we found that several genes linked to hearing loss were strongly downregulated by OHC death. Together, these results suggest that this OHC knockout system is a useful tool to identify biomarkers associated with OHC death.

The connexin 30 A88V mutant reduces cochlear gap junction expression and confers long-term protection against hearing loss

Mutations in the genes that encode the gap junction proteins connexin 26 (Cx26, encoded by GJB2) and Cx30 (GJB6) are the leading cause of hereditary hearing loss. That said, the Cx30 p.Ala88Val (A88V) mutant causes Clouston syndrome, but not hearing loss. Here, we report that the Cx30-A88V mutant, despite being toxic to inner ear-derived HEI-OC1 cells, conferred remarkable long-term protection against age-related high frequency hearing loss in Cx30^A88V/A88V mice. During early development, there were no overt structural differences in the cochlea between genotypes, including a normal complement of hair cells; however, the supporting cell Cx30 gap junction plaques in mutant mice were reduced in size. In adulthood, Cx30^A88V/A88V mutant mice had a reduction of cochlear Cx30 mRNA and protein, yet a full complement of hair cells. Conversely, the age-related high frequency hearing loss in Cx30^+/+ and Cx30^+/A88V mice was due to extensive loss of outer hair cells. Our data suggest that the Cx30-A88V mutant confers long-term hearing protection and prevention of hair cell death, possibly via a feedback mechanism that leads to the reduction of total Cx30 gap junction expression in the cochlea.

Cumulative mitochondrial activity correlates with ototoxin susceptibility in zebrafish mechanosensory hair cells

Mitochondria play a prominent role in mechanosensory hair cell damage and death. Although hair cells are thought to be energetically demanding cells, how mitochondria respond to these demands and how this might relate to cell death is largely unexplored. Using genetically encoded indicators, we found that mitochondrial calcium flux and oxidation are regulated by mechanotransduction and demonstrate that hair cell activity has both acute and long-term consequences on mitochondrial function. We tested whether variation in mitochondrial activity reflected differences in the vulnerability of hair cells to the toxic drug neomycin. We observed that susceptibility did not correspond to the acute level of mitochondrial activity but rather to the cumulative history of that activity.

Transient Abnormalities in Masking Tuning Curve in Early Progressive Hearing Loss Mouse Model

Damage to cochlear outer hair cells (OHCs) usually affects frequency selectivity in proportion to hearing threshold increase. However, the current clinical heuristics that attributes poor hearing performance despite near-normal auditory sensitivity to auditory neuropathy or “hidden” synaptopathy overlooks possible underlying OHC impairment. Here, we document the part played by OHCs in influencing suprathreshold auditory performance in the presence of noise in a mouse model of progressive hair cell degeneration, the CD1 strain, at postnatal day 18–30 stages when high-frequency auditory thresholds remained near-normal. Nonetheless, total loss of high-frequency distortion product otoacoustic emissions pointed to nonfunctioning basal OHCs. This “discordant profile” came with a huge low-frequency shift of masking tuning curves that plot the level of interfering sound necessary to mask the response to a probe tone, against interfering frequency. Histology revealed intense OHC hair bundle abnormalities in the basal cochlea uncharacteristically associated with OHC survival and preserved coupling with the tectorial membrane. This pattern dismisses the superficial diagnosis of “hidden” neuropathy while underpinning a disorganization of cochlear frequency mapping with optimistic high-frequency auditory thresholds perhaps because responses to high frequencies are apically shifted. The audiometric advantage of frequency transposition is offset by enhanced masking by low-frequency sounds, a finding essential for guiding rehabilitation.

Connexin-Mediated Signaling in Nonsensory Cells Is Crucial for the Development of Sensory Inner Hair Cells in the Mouse Cochlea

Mutations in the genes encoding for gap junction proteins connexin 26 (Cx26) and connexin 30 (Cx30) have been linked to syndromic and nonsyndromic hearing loss in mice and humans. The release of ATP from connexin hemichannels in cochlear nonsensory cells has been proposed to be the main trigger for action potential activity in immature sensory inner hair cells (IHCs), which is crucial for the refinement of the developing auditory circuitry. Using connexin knock-out mice, we show that IHCs fire spontaneous action potentials even in the absence of ATP-dependent intercellular Ca²⁺ signaling in the nonsensory cells. However, this signaling from nonsensory cells was able to increase the intrinsic IHC firing frequency. We also found that connexin expression is key to IHC functional maturation. In Cx26 conditional knock-out mice (Cx26^Sox10-Cre), the maturation of IHCs, which normally occurs at approximately postnatal day 12, was partially prevented. Although Cx30 has been shown not to be required for hearing in young adult mice, IHCs from Cx30 knock-out mice exhibited a comprehensive brake in their development, such that their basolateral membrane currents and synaptic machinery retain a prehearing phenotype. We propose that IHC functional differentiation into mature sensory receptors is initiated in the prehearing cochlea provided that the expression of either connexin reaches a threshold level. As such, connexins regulate one of the most crucial functional refinements in the mammalian cochlea, the disruption of which contributes to the deafness phenotype observed in mice and DFNB1 patients.

SIGNIFICANCE STATEMENT The correct development and function of the mammalian cochlea relies not only on the sensory hair cells, but also on the surrounding nonsensory cells. Although the nonsensory cells have been largely implicated in the general homeostasis in the mature cochlea, their involvement in the initial functional differentiation of the sensory inner hair cells is less clear. Using mutant mouse models for the most common form of congenital deafness in humans, which are knock-outs for the gap-junction channels connexin 26 and connexin 30 genes, we show that defects in nonsensory cells prevented the functional maturation of inner hair cells. In connexin knock-outs, inner hair cells remained stuck at a prehearing stage of development and, as such, are unable to process sound information.

A connexin30 mutation rescues hearing and reveals roles for gap junctions in cochlear amplification and micromechanics

Accelerated age-related hearing loss disrupts high-frequency hearing in inbred CD-1 mice. The p.Ala88Val (A88V) mutation in the gene coding for the gap-junction protein connexin30 (Cx30) protects the cochlear basal turn of adult CD-1Cx30^A88V/A88V mice from degeneration and rescues hearing. Here we report that the passive compliance of the cochlear partition and active frequency tuning of the basilar membrane are enhanced in the cochleae of CD-1Cx30^A88V/A88V compared to CBA/J mice with sensitive high-frequency hearing, suggesting that gap junctions contribute to passive cochlear mechanics and energy distribution in the active cochlea. Surprisingly, the endocochlear potential that drives mechanoelectrical transduction currents in outer hair cells and hence cochlear amplification is greatly reduced in CD-1Cx30^A88V/A88V mice. Yet, the saturating amplitudes of cochlear microphonic potentials in CD-1Cx30^A88V/A88V and CBA/J mice are comparable. Although not conclusive, these results are compatible with the proposal that transmembrane potentials, determined mainly by extracellular potentials, drive somatic electromotility of outer hair cells.

A point mutation in the gap-junction protein connexin 30 stops early onset age-related hearing loss. Here, the authors show that gap junctions contribute to cochlear micromechanics and that cochlear amplification is likely controlled by extracellular potentials in vicinity of the cochlear sensory cells.

Application of Mouse Models to Research in Hearing and Balance

Laboratory mice (Mus musculus) have become the major model species for inner ear research. The major uses of mice include gene discovery, characterization, and confirmation. Every application of mice is founded on assumptions about what mice represent and how the information gained may be generalized. A host of successes support the continued use of mice to understand hearing and balance. Depending on the research question, however, some mouse models and research designs will be more appropriate than others. Here, we recount some of the history and successes of the use of mice in hearing and vestibular studies and offer guidelines to those considering how to apply mouse models.

The Structural Development of the Mouse Dorsal Cochlear Nucleus

The dorsal cochlear nucleus (DCN) is a major subdivision of the mammalian cochlear nucleus (CN) that is thought to be involved in sound localization in the vertical plane and in feature extraction of sound stimuli. The main principal cell type (pyramidal cells) integrates auditory and non-auditory inputs, which are considered to be important in performing sound localization tasks. This study aimed to investigate the histological development of the CD-1 mouse DCN, focussing on the postnatal period spanning the onset of hearing (P12). Fluorescent Nissl staining revealed that the three layers of the DCN were identifiable as early as P6 with subsequent expansion of all layers with age. Significant increases in the size of pyramidal and cartwheel cells were observed between birth and P12. Immunohistochemistry showed substantial changes in synaptic distribution during the first two postnatal weeks with subsequent maturation of the presumed mossy fibre terminals. In addition, GFAP immunolabelling identified several glial cell types in the DCN including the observation of putative tanycytes for the first time. Each glial cell type had specific spatial and temporal patterns of maturation with apparent rapid development during the first two postnatal weeks but little change thereafter. The rapid maturation of the structural organization and DCN components prior to the onset of hearing possibly reflects an influence from spontaneous activity originating in the cochlea/auditory nerve. Further refinement of these connections and development of the non-auditory connections may result from the arrival of acoustic input and experience dependent mechanisms.

Molecular basis of hair cell loss

Mechanisms that lead to the death of hair cells are reviewed. Exposure to noise, the use of ototoxic drugs that damage the cochlea and old age are accompanied by hair cell death. Outer hair cells are often more susceptible than inner hair cells, partly because of an intrinsically greater susceptibility; high frequency cells are also more vulnerable. A common factor in hair cell loss following age-related changes and exposure to ototoxic drugs or high noise levels is the generation of reactive oxygen species, which can trigger intrinsic apoptosis (the mitochondrial pathway). However, hair cell death is sometimes produced via an extracellular signal pathway triggering extrinsic apoptosis. Necrosis and necroptosis also play a role and, in various situations in which cochlear damage occurs, a balance exists between these possible routes of cell death, with no one mechanism being exclusively activated. Finally, the numerous studies on these mechanisms of hair cell death have led to the identification of many potential therapeutic agents, some of which have been used to attempt to treat people exposed to damaging events, although clinical trials are not yet conclusive. Continued work in this area is likely to lead to clinical treatments that could be used to prevent or ameliorate hearing loss.

The role of mitochondria in age-related hearing loss

Age-related hearing loss (ARHL), the hearing loss associated with aging, is a vital problem in present society. The severity of hearing loss is possibly associated with the degeneration of cochlear cells. Mitochondria play a key role in the energy supply, cellular redox homeostasis, signaling, and regulation of programmed cell death. In this review, we focus on the central role of mitochondria in ARHL. The mitochondrial redox imbalance and mitochondrial DNA mutation might collaboratively involve in the process of cochlear senescence in response to the aging stress. Subsequent responses, including alteration of mitochondrial biogenesis, mitophagy, apoptosis and paraptosis, participate in the aging process from different respects.

Subcellular distribution and relative expression of fibrocyte markers in the CD/1 mouse cochlea assessed by semiquantitative immunogold electron microscopy

Spiral ligament fibrocytes function in cochlear homeostasis, maintaining the endocochlear potential by participating in potassium recycling, and fibrocyte degeneration contributes to hearing loss. Their superficial location makes them amenable to replacement by cellular transplantation. Fibrocyte cultures offer one source of transplantable cells, but determining what fibrocyte types they contain and what phenotype transplanted cells may adopt is problematic. Here, we use immunogold electron microscopy to assess the relative expression of markers in native fibrocytes of the CD/1 mouse spiral ligament. Caldesmon and aquaporin 1 are expressed more in type III fibrocytes than any other type. S-100 is strongly expressed in types I, II, and V fibrocytes, and α1Na,K-ATPase is expressed strongly only in types II and V. By combining caldesmon or aquaporin 1 with S-100 and α1Na,K-ATPase, a ratiometric analysis of immunogold density distinguishes all except type II and type V fibrocytes. Other putative markers (creatine kinase BB and connective tissue growth factor) did not provide additional useful analytical attributes. By labeling serial sections or by double or triple labeling with combinations of three antibodies, this technique could be used to distinguish all except type II and type V fibrocytes in culture or after cellular transplantation into the lateral wall.