Pathogenesis Of Age-related Hearing Loss Research Articles

MiR-130b-3p involved in the pathogenesis of age-related hearing loss via targeting PPARγ and autophagy

The study focuses on the underlying regulatory mechanism of age-related hearing loss (ARHL), which results from autophagy dysregulation mediated by miR-130b-3p targeting PPARγ. We constructed miR-130b-3p knockout (antagomir) and PPARγ over-expression (OE-PPARγ) mice model by injecting mmu-miR-130b-3p antagomir and HBAAV2/Anc80-m-Pparg-T2A-mCHerry into the right ear’ round window of each mouse, respectively. In vitro, we introduced oxidative stress within HEI-OC1 cells by H2O2 and exogenously changed the miR-130b-3p and PPARγ levels. MiRNA level was detected by RT-qPCR, proteins by western blotting and immunohistochemistry. Morphology of autophagosomes was observed by electron microscopy. In vivo, the cochlea of aged mice showed higher miR-130b-3p expression and lower PPARγ expression, while exogenous inhibition of miR-130b-3p up-regulated PPARγ expression. Autophagy-related biomarkers expression (ATG5, Beclin-1 and LC3B II/I) decreased in aged mice, which reversely increased after the inhibition of miR-130b-3p. The elevation of PPARγ demonstrated similar effects. Contrarily, exogenous overexpression of miR-130b-3p resulted in the decrease of ATG5, Beclin-1 and LC3B II/I. We created oxidative stress within HEI-OC1 by H2O2, subsequently observed the formation of autophagosomes under electron microscope, so as the elevated cell apoptosis rate and weakened cell viability. MiR-130b-3p/PPARγ contributed to the premature senescence of these H2O2-induced HEI-OC1 cells. MiR-130b-3p regulated HEI-OC1 cell growth by targeting PPARγ, thus leading to ARHL.

Relationship Between Serum Brain-Derived Neurotrophic Factor and Neurotrophin-3 Levels and Hearing Thresholds in Patients With Age-Related Hearing Loss.

Age-related hearing loss (ARHL) is a general term used to describe the sensorineural type of hearing loss occurring in both ears in older adults. Neurotrophins are the most promising candidates for supporting the auditory nerve by increasing neuronal survival. This study aimed to help elucidate the pathophysiology of ARHL by determining whether any relationship exists between brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) levels in serum samples from patients diagnosed with ARHL. Seventy-seven individuals, a study group of 41 patients diagnosed with ARHL, and a control group of 36 participants without hearing loss were evaluated. Serum samples were collected and used to measure serum BDNF and NT-3 levels with the new Nepenthe enzyme-linked immunosorbent assay method. Median pure-tone average results in the 2000, 4000, and 6000 Hz ranges were 52.5 (44.3-67.3) dB HL in the ARHL group and 13.5 (11.1-17.1) dB HL in the control group. The difference was statistically significant (p = .001). Although NT-3 and BDNF levels were both lower in ARHL patients than in participants without hearing loss, only the BDNF levels were significantly (p = .002) lower. Mean left and right ear word recognition scores were also lower in ARHL patients than in control groups. The ARHL group was further divided into two subgroups based on word recognition scores to evaluate significant differences in BDNF and NT-3 levels. No statistically significant difference was observed in BDNF and NT-3 levels between these subgroups. However, there was a significant difference in word recognition scores. Low BDNF levels in the ARHL group suggest that BDNF may play a role in the pathogenesis of ARHL. Patients with low (ARHL1) and high (ARHL2) word recognition scores were compared for the first time in the literature in terms of BDNF and NT-3 levels. However, the results were not statistically significant. This article is a preliminary study and was written to provide guidance for our next comprehensive project.

The cells of the sensory epithelium, and not the stria vascularis, are the main cochlear cells related to the genetic pathogenesis of age-related hearing loss

Age-related hearing loss (ARHL) is a major health concern among the elderly population. It is hoped that increasing our understanding of its underlying pathophysiological processes will lead to the development of novel therapies. Recent genome-wide association studies (GWASs) discovered a few dozen genetic variants in association with elevated risk for ARHL. Integrated analysis of GWAS results and transcriptomics data is a powerful approach for elucidating specific cell types that are involved in disease pathogenesis. Intriguingly, recent studies that applied such bioinformatics approaches to ARHL resulted in disagreeing findings as for the key cell types that are most strongly linked to the genetic pathogenesis of ARHL. These conflicting studies pointed either to cochlear sensory epithelial or to stria vascularis cells as the cell types most prominently involved in the genetic basis of ARHL. Seeking to resolve this discrepancy, we integrated the analysis of four ARHL GWAS datasets with four independent inner-ear single-cell RNA-sequencing datasets. Our analysis clearly points to the cochlear sensory epithelial cells as the key cells for the genetic predisposition to ARHL. We also explain the limitation of the bioinformatics analysis performed by previous studies that led to missing the enrichment for ARHL GWAS signal in sensory epithelial cells. Collectively, we show that cochlear epithelial cells, not stria vascularis cells, are the main inner-ear cells related to the genetic pathogenesis of ARHL.

Excessive processing and acetylation of OPA1 aggravate age-related hearing loss via the dysregulation of mitochondrial dynamics.

The pathogenesis of age-related hearing loss (ARHL) remains unclear. OPA1 is the sole fusion protein currently known to be situated in the inner mitochondrial membrane, which is pivotal for maintaining normal mitochondrial function. While it has already been demonstrated that mutations in OPA1 may lead to hereditary deafness, its involvement in the occurrence and development of ARHL has not been previously explored. In our study, we constructed D-gal-induced senescent HEI-OC1 cells and the cochlea of C57BL/6J mice with a mutated SUMOylation site of SIRT3 using CRISPR/Cas9 technology. We found enhanced L-OPA1 processing mediated by activated OMA1, and increased OPA1 acetylation resulting from reductions in SIRT3 levels in senescent HEI-OC1 cells. Consequently, the fusion function of OPA1 was inhibited, leading to mitochondrial fission and pyroptosis in hair cells, ultimately exacerbating the aging process of hair cells. Our results suggest that the dysregulation of mitochondrial dynamics in cochlear hair cells in aged mice can be ameliorated by activating the SIRT3/OPA1 signaling. This has the potential to alleviate the senescence of cochlear hair cells and reduce hearing loss in mice. Our study highlights the significant roles played by the quantities of long and short chains and the acetylation activity of OPA1 in the occurrence and development of ARHL. This finding offers new perspectives and potential targets for the prevention and treatment of ARHL.

Abnormal Cholesterol Metabolism and Lysosomal Dysfunction Induce Age-Related Hearing Loss by Inhibiting mTORC1-TFEB-Dependent Autophagy.

Cholesterol is a risk factor for age-related hearing loss (ARHL). However, the effect of cholesterol on the organ of Corti during the onset of ARHL is unclear. We established a mouse model for the ARHL group (24 months, n = 12) and a young group (6 months, n = 12). Auditory thresholds were measured in both groups using auditory brainstem response (ABR) at frequencies of 8, 16, and 32 kHz. Subsequently, mice were sacrificed and subjected to histological analyses, including transmission electron microscopy (TEM), H&E, Sudan Black B (SBB), and Filipin staining, as well as biochemical assays such as IHC, enzymatic analysis, and immunoblotting. Additionally, mRNA extracted from both young and aged cochlea underwent RNA sequencing. To identify the mechanism, in vitro studies utilizing HEI-OC1 cells were also performed. RNA sequencing showed a positive correlation with increased expression of genes related to metabolic diseases, cholesterol homeostasis, and target of rapamycin complex 1 (mTORC1) signaling in the ARHL group as compared to the younger group. In addition, ARHL tissues exhibited increased cholesterol and lipofuscin aggregates in the organ of Corti, lateral walls, and spiral ganglion neurons. Autophagic flux was inhibited by the accumulation of damaged lysosomes and autolysosomes. Subsequently, we observed a decrease in the level of transcription factor EB (TFEB) protein, which regulates lysosomal biosynthesis and autophagy, together with increased mTORC1 activity in ARHL tissues. These changes in TFEB and mTORC1 expression were observed in a cholesterol-dependent manner. Treatment of ARHL mice with atorvastatin, a cholesterol synthesis inhibitor, delayed hearing loss by reducing the cholesterol level and maintaining lysosomal function and autophagy by inhibiting mTORC1 and activating TFEB. The above findings were confirmed using stress-induced premature senescent House Ear Institute organ of Corti 1 (HEI-OC1) cells. The findings implicate cholesterol in the pathogenesis of ARHL. We propose that atorvastatin could prevent ARHL by maintaining lysosomal function and autophagy by inhibiting mTORC1 and activating TFEB during the aging process.

Tumor necrosis factor-α mediated inflammation versus apoptosis in age-related hearing loss.

An almost universal phenomenon occurring during aging is a state of chronic, low-grade, sterile inflammation. Inflammation is a crucial contributor to various age-related pathologies and natural processes in aging tissues. Tumor necrosis factor-α (TNF-α), a master regulator of the immune system, plays an important role in the propagation of inflammation. Recent research has found correlations between hearing loss and markers such as TNF-α. However, the intrinsic molecular mechanism by which TNF-α influences aging individuals’ increased risk of hearing loss remains unclear. In this study, we found that TNF-α expression gradually increased with age in DBA/2J mice. We then used recombinant TNF-α to upregulate TNF-α levels in House Ear Institute-Organ of Corti 1 (HEI-OC1) cells and found that low concentrations of TNF-α could activate the nuclear factor kappa B (NF-κB) transcriptional response to mediate hair cell survival, while high concentrations of TNF-α could activate the Caspase-3 cascade to mediate hair cell apoptosis, which preliminarily confirmed that a TNF-α mediated signaling pathway plays an important role in the pathogenesis of age-related hearing loss.

Rapamycin ameliorates age-related hearing loss in C57BL/6J mice by enhancing autophagy in the SGNs

Autophagy plays a pathogenic role in neurodegenerative disease. However, the involvement of autophagy in the pathogenesis of age-related hearing loss (ARHL) remains obscure. Naturally aged C57BL/6J mice were used to identify the role of autophagy in ARHL, and rapamycin, a mammalian target of rapamycin (mTOR) inhibitor, was administered for 34 weeks to explore the potential therapeutic effect of rapamycin in ARHL. We found that the number of autophagosomes and the expression of microtubule-associated protein 1 light chain 3B (LC3B) decreased as the mice aged. The expression of autophagy-related (Atg) proteins, including Beclin1 and Atg5, and the ratio of LC3-II/I was reduced in aged mice, while mTOR activity in aged mice gradually increased. Rapamycin improved the auditory brainstem response (ABR) threshold (at 8, 12, and 24 kHz). Further exploration demonstrated that spiral ganglion neuron (SGN) density was enhanced in response to administration of rapamycin. The rate of apoptosis in the basal turn SGNs was decreased, whereas autophagy activity was increased in the experimental group. Meanwhile, mTOR activity in the experimental group was decreased. Our findings indicate that age-related deficiency in autophagy may lead to increased apoptosis of aged SGNs. Rapamycin enhances autophagy of SGNs by inhibiting mTOR activation, resulting in amelioration of ARHL. Therapeutic strategy targeting autophagy may provide a potential approach for treating ARHL.

Age-related Activation of Cyclic GMP–AMP synthase–Stimulator of Interferon Genes Signaling in the Auditory System is Associated with Presbycusis in C57BL/6J Male Mice

Presbycusis, or age-related hearing loss (ARHL), is primarily associated with sensory or transduction nerve cell degeneration in the peripheral and/or central auditory systems. During aging, the auditory system shows mitochondrial dysfunction and increased inflammatory responses. Mitochondrial dysfunction promotes leakage of mitochondrial DNA (mtDNA) into the cytosol, which activates the cyclic GMP–AMP synthase (cGAS)–stimulator of interferon genes (STING) pathway to induce type I interferon and inflammatory responses. However, whether this pathway is involved in the occurrence and development of ARHL is unknown. This study aimed to determine whether there are age-related changes in the levels of cytosolic mtDNA and cGAS–STING pathway activation in the auditory pathway and to explore their relationship with ARHL. The results showed that cGAS-positive immunoreactive cells were observed in the cochlea, inferior colliculus, and auditory cortex. Levels of cytosolic mtDNA, cGAS, STING, phosphorylated interferon regulatory factor 3, and cytokines were significantly increased in the cochlea, inferior colliculus, and auditory cortex of 6-, 9-, and 12-month-old mice compared with 3-month-old mice. These findings suggested that cytosolic mtDNA may play an important role in the pathogenesis of ARHL by activating cGAS–STING-mediated type I interferon and inflammatory responses.

Upregulation of a nuclear factor-kappa B-interacting immune gene network in mice cochleae with age-related hearing loss.

Epidemiological data suggest that inflammation and innate immunity play significant roles in the pathogenesis of age-related hearing loss (ARHL) in humans. In this mouse study, real-time RT-PCR array targeting 84 immune-related genes revealed that the expressions of 40 genes (47.6%) were differentially regulated with greater than a twofold change in 12-month-old cochleae with ARHL relative to young control mice, 33 (39.3%) of which were upregulated. These differentially regulated genes (DEGs) were involved in functional pathways for cytokine-cytokine receptor interaction, chemokine signaling, TNF signaling, and Toll-like receptor signaling. An NF-κB subunit, Nfkb1, was upregulated in aged cochleae, and bioinformatic analyses predicted that NF-κB would interact with the genomic regulatory regions of eight upregulated DEGs, including Tnf and Ptgs2. In aging cochleae, major proinflammatory molecules, IL1B and IL18rap, were upregulated by 6 months of age and thereafter. Remarkable upregulations of seven immune-related genes (Casp1, IL18r1, IL1B, Card9, Clec4e, Ifit1, and Tlr9) occurred at an advanced stage (between 9 and 12 months of age) of ARHL. Immunohistochemistry analysis of cochlear sections from the 12-month-old mice indicated that IL-18r1 and IL-1B were localized to the spiral ligament, spiral limbus, and organ of Corti. The two NF-κB-interacting inflammatory molecules, TNFα and PTGS2, immunolocalized ubiquitously in cochlear structures, including the lateral wall (the stria vascularis and spiral ligament), in the histological sections of aged cochleae. IBA1-positive macrophages were observed in the stria vascularis and spiral ligament in aged mice. Therefore, inflammatory and immune reactions are modulated in aged cochlear tissues with ARHL.

Expression of α2-Na/K-ATPase in C57BL/6J Mice Inner Ear and Its Relationship with Age-related Hearing Loss.

K+ cycling in the cochlea is critical to maintain hearing. Many sodium-potassium pumps are proved to participate in K+ cycling, such as Na/K-ATPase. The α2-Na/K-ATPase is an important isoform of Na/K-ATPase. The expression of α2-Na/K-ATPase in the cochlea is not clear. In this study, we used C57BL/6 mice as a model of presbycusis and implemented immunohistochemistry staining and quantitative real time-PCR, and the α2-Na/K-ATPase expression pattern was confirmed in the inner ear. It was found α2-Na/K-ATPase was expressed widely in cochlea and its mRNA and protein expression was gradually reduced with aging (4-, 14-, 26- and 48-weeks old mice). We suspected that, the down-regulation of α2-Na/K-ATPase expression might be associated with the remodeling of K+ cycling, degeneration of morphological structure and decrease of hearing function in aging C57 mice. In conclusion, we speculated that the reduction of α2-Na/K-ATPase might play an important role in the pathogenesis of age-related hearing loss.

Degradation and modification of cochlear gap junction proteins in the early development of age-related hearing loss

Age-related hearing loss (ARHL) is the progressive, bilateral loss of high-frequency hearing in elderly people. Mutations in GJB2, encoding the cochlear gap junction protein connexin26 (Cx26), are the most frequent cause of hereditary deafness; however, a common molecular pathology between ARHL and GJB2-related hearing loss has not been reported. Here, we investigated the quantitative change in expression and molecular pathology of Cx26 in ARHL. We used C57BL/6J mice as a model of ARHL. Hearing levels that were evaluated by auditory brainstem response thresholds increased gradually between 4 and 32 weeks of age and increased sharply at 36 weeks. Gap junctions in the cochleae of 4-week-old mice had linear plaques along cell–cell junction sites. In contrast, the cochleae from 32-week-old mice had significantly shorter gap junctions. Severe hair cell loss was not observed during this period. Based on western blotting, Cx26 and connexin30 (Cx30) levels were significantly decreased at 32 weeks compared with 4 weeks.Moreover, Cx26 was more significantly enriched in the hydrophilic fraction at 4 weeks but was more significantly enriched in the hydrophobic fraction at 32 weeks, indicating an age-related conversion of this biochemical property. Thus, the hydrophobic conversion of Cx26 and disruption of gap junction proteins and plaques may be involved in the pathogenesis of ARHL and may occur before severe hair cell degeneration.

Effect of SOD2 methylation on mitochondrial DNA4834-bp deletion mutation in marginal cells under oxidative stress.

Presbycusis, or age-related hearing loss, is a prevalent disease that severely affects the physical and mental health of the elderly. Oxidative stress and mitochondrial (mt)DNA deletion mutation are considered as major factors in the pathophysiology of age-related hearing loss. The 4977-bp deletion in human mtDNA (common deletion, corresponding to the 4834-bp mtDNA deletion in rats) is suggested to be closely associated with the pathogenesis of age-related hearing loss. Superoxide dismutase 2 (SOD2), an isoform of SOD that is exclusively expressed in the intracellular mitochondrial matrix, plays a crucial role in oxidative resistance against mitochondrial superoxide. Previous research has shown that methylation of the promoter region of the SOD2 gene decreased the expression of SOD2 in marginal cells (MCs) extracted from the inner ear of rats subjected to D-galactose-induced mtDNA4834 deletion. However, the relationship between SOD2 methylation and mtDNA4834 deletion under oxidative stress remains to be elucidated. Herein, an oxidative damage model was established in the extracted MCs using hydrogen peroxide (H2O2), which increased the methylation level of SOD2 and the copy number of mtDNA4834 mutation in MCs. Decreasing the methylation level of SOD2 using 5-azacytidine, a DNA methylation inhibitor, reduced oxidative stress and the copy number of mtDNA4834 mutation and inhibited H2O2-induced apoptosis. The present work demonstrates that decreasing the methylation of SOD2 suppresses the mtDNA4834 deletion in MCs under oxidative stress and provides potential insights to the intervention therapy of aging-related hearing loss.

Rs1894720 polymorphism inMIAT increased susceptibility to age-related hearing loss by modulating the activation of miR-29b/SIRT1/PGC-1α signaling.

MIAT may be implicated in the pathogenesis of age-related hearing loss (AHL). This study aimed to clarify the effect of a MIAT signaling pathway on the risk of AHL. Terminal deoxynucleotidyl transferase dUTP nick-end labeling assay, auditory brainstem response (ABR) and quantitative hair cell counts were used to compare the hearing functions in different groups of mice. 5,5,6,6-Tetrachloro-1,1,3,3-tetraethylbenzimidazolylcarbocyanine iodide (JC-1) dye method was used to establish the potential association between mitochondrial dysfunction and aging. Real-time polymerase chain reaction, Western blot analysis, computational analysis, and luciferase assay were conducted to establish a myocardial infarction associated transcript (MIAT) signaling pathway, whose role in the pathogenesis of AHL was further validated by 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide(MTT) assay and flow cytometry. Aged C57BL/6 mice were associated with a more severe level of hair cell loss, while exhibiting a higher ABR threshold at various frequencies as well as a lower percentage of inner/outer hair cells. A reduced mitochondrial membrane potential in the cochleae of aged C57BL/6 mice indicated the presence of mitochondrial dysfunction in these mice. Relative expression of MIAT, Sirtuin1 (SIRT1), and peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) was downregulated in aged mice, with microRNA-29b (miR-29b) being highly expressed. Also, MIAT binds to miR-29b, an inhibitor of SIRT1 expression. The regulatory relationship among MIAT, miR-29b, and SIRT1 was further validated by comparing the differentiated expression of these factors in cells treated with phosphate-buffered saline + H2 O2, a negative control + H2 O2, MIAT + H2 O2 , or H2 O2 + anti-miR-29b. MIAT could elevate the expression of SIRT1/PGC-1α via downregulating miR-29b. And the downregulated SIRT/PGC-1α increased the incidence of AHL via promoting the apoptosis of cochlear hair cells.

Activation of miR-34a impairs autophagic flux and promotes cochlear cell death via repressing ATG9A: implications for age-related hearing loss

Age-related hearing loss is a major unresolved public health problem. We have previously elucidated that the activation of cochlear miR-34a is correlated with age-related hearing loss in C57BL/6 mice. A growing body of evidence points that aberrant autophagy promotes cell death during the development of multiple age-related diseases. The aim of this study was to investigate the role of miR-34a-involved disorder of autophagy in the pathogenesis of age-related hearing loss. Our results showed that miR-34a expression was markedly upregulated in the aging cochlea accompanied with impairment of autophagic flux. In the inner ear HEI-OC1 cell line, miR-34a overexpression resulted in an accumulation of phagophores and impaired autophagosome–lysosome fusion, and led to cell death subsequently. Notably, autophagy-related protein 9A (ATG9A), an autophagy protein, was significantly decreased after miR-34a overexpression. Knockdown of ATG9A inhibited autophagy flux, which is similar to the effects of miR-34a overexpression. Moreover, ursodeoxycholic acid significantly rescued miR-34a-induced HEI-OC1 cell death by restoring autophagy activity. Collectively, these findings increase our understanding of the biological effects of miR-34a in the development of age-related hearing loss and highlight miR-34a as a promising therapeutic target for its treatment.

Downregulation of inwardly rectifying potassium channel 5.1 expression in C57BL/6J cochlear lateral wall.

Age-related hearing loss (AHL) is one of the most common sensory disorders among elderly persons. The inwardly rectifying potassium channel 5.1 (Kir5.1) plays a vital role in regulating cochlear K(+) circulation which is necessary for normal hearing. The distribution of Kir5.1 in C57BL/6J mice cochleae, and the relationship between the expression of Kir5.1 and the etiology of AHL were investigated. Forty C57BL/6J mice were randomly divided into four groups at 4, 12, 24 and 52 weeks of age respectively. The location of Kir5.1 was detected by immunofluorescence technique. The mRNA and protein expression of Kir5.1 was evaluated in mice cochleae using real-time polymerase-chain reactions (RT-PCR) and Western blotting respectively. Kir5.1 was detected in the type II and IV fibrocytes of the spiral ligament in the cochlear lateral wall of C57BL/6J mice. The expression levels of Kir5.1 mRNA and protein in the cochleae of aging C57BL/6J mice were down-regulated. It was suggested that the age-related decreased expression of Kir5.1 in the lateral wall of C57BL/6J mice was associated with hearing loss. Our results indicated that Kir5.1 may play an important role in the pathogenesis of AHL.

Age-related change in the expression of NKCC1 in the cochlear lateral wall of C57BL/6J mice

Conclusions: Na+-K+-2Cl– co-transporter isoform 1 (NKCC1) mRNA and protein decrease with increasing age in the cochlear lateral wall of C57BL/6J (C57) mice. The down-regulation of NKCC1 may influence the K+ transport efficiency and the homeostasis of ion transport cells, and cause the irreversible damage of cochlear cells in old C57 mice. Our results indicate that NKCC1 may play an important role in the pathogenesis of age-related hearing loss (AHL). Objectives: The aim of the present study was to investigate the relationship between the functional expression of NKCC1 transporter and the etiology of AHL. Methods: C57 mice were used and randomly divided into four groups according to age (4 weeks, 14 weeks, 26 weeks, and 52 weeks). Immunofluorescence technique, quantitative real-time PCR, and western blot were applied to detect the expression of NKCC1 in the cochlear lateral wall of C57 mice at the various ages. Results: In all four groups, the expression of NKCC1 was observed in the stria vascularis and type II fibrocytes of the spiral ligament. Also, the expression of NKCC1 appeared to decrease with age at both the transcriptional level and the protein level.

The role of mtDNA mutations in the pathogenesis of age-related hearing loss in mice carrying a mutator DNA polymerase γ

Mitochondrial DNA (mtDNA) mutations may contribute to aging and age-related diseases. Previously, we reported that accumulation of mtDNA mutations is associated with age-related hearing loss in mice carrying a mutator allele of the mitochondrial Polg DNA polymerase. To elucidate the role of mtDNA mutations in the pathogenesis of age-related hearing loss or presbycusis, we performed large scale gene expression analysis to identify mtDNA mutation-responsive genes and biological process categories associated with mtDNA mutations by comparing the gene expression patterns of cochlear tissues from 9-month-old mitochondrial mutator and control mice. mtDNA mutations were associated with transcriptional alterations consistent with impairment of energy metabolism, induction of apoptosis, cytoskeletal dysfunction, and hearing dysfunction in the cochlea of aged mitochondrial mutator mice. TUNEL staining and caspase-3 immunostaining analysis demonstrated that the levels of apoptotic markers were significantly increased in the cochleae of mitochondrial mutator mice compared to age-matched controls. These observations support a new model of how mtDNA mutations impact cochlear function whereby accumulation of mtDNA mutations lead to mitochondrial dysfunction, an associated impairment of energy metabolism, and the induction of an apoptotic program. The data presented here provide the first global assessment at the molecular level of the pathogenesis of age-related disease in mitochondrial mutator mice and reveal previously unrecognized biological pathways associated with mtDNA mutations.

Serotonin 2B receptor: Upregulated with age and hearing loss in mouse auditory system

Serotonin (5-HT) is a monoamine neurotransmitter. Serotonin may modulate afferent fiber discharges in the cochlea, inferior colliculus (IC) and auditory cortex. Specific functions of serotonin are exerted upon its interaction with specific receptors; one of those receptors is the serotonin 2B receptor. The aim of this study was to investigate the differences in gene expression of serotonin 2B receptors with age in cochlea and IC, and the possible correlation between gene expression and functional hearing measurements in CBA/CaJ mice. Immunohistochemical examinations of protein expression of IC in mice of different age groups were also performed. Gene expression results showed that serotonin 2B receptor gene was upregulated with age in both cochlea and IC. A significant correlation between gene expression and functional hearing results was established. Immunohistochemical protein expression studies of IC showed more serotonin 2B receptor cells in old mice relative to young adult mice, particularly in the external nucleus. We conclude that serotonin 2B receptors may play a role in the pathogenesis of age-related hearing loss.