Mitochondrial DNA common deletion increases susceptibility to noise-induced hearing loss in a mimetic aging rat model

Regulation of PPARγ/CPT-1 expression ameliorates cochlear hair cell injury by regulating cellular lipid metabolism and oxidative stress

This study aimed to investigate the protective effects of PPARγ/CPT-1 regulation on cisplatin-induced cochlear hair cell injury. The viability, apoptosis and mitochondrial membrane potential of cisplatin-induced HEI-OC1 cells were determined by CCK-8 assay, TUNEL and JC-1 staining, respectively. The oxidative stress and lipid metabolism were detected by the assay kits of MDA, ROS, SOD, CAT, TG and FFA. The transfection efficiency of overexpression (OV)-PPARG and OV-CPT1A was examined by RT-qPCR and the expressions of apoptosis- and lipid metabolism-related proteins were detected by western blot. As a result, cisplatin with varying concentrations (5, 10, 30 μM) suppressed the viability, promoted the apoptosis and hindered the mitochondrial function of HEI-OC1 cells, accompanied with up-regulated expressions of Bax and cleaved caspase-3 and down-regulated expression of Bcl-2. The oxidative stress was aggravated and lipid metabolism was inhibited by cisplatin (5, 10, 30 μM) induction, evidenced by the increased levels of MDA, ROS, TG, FFA and the decreased levels of SOD and CAT. Overexpression of PPARG or CPT1A could improve the viability, mitochondrial function, lipid metabolism and suppress the oxidative stress and apoptosis of cisplatin-induced HEI-OC1 cells. In conclusion, up-regulation of PPARG or CPT1A ameliorated cochlear hair cell injury by improving cellular lipid metabolism and inhibiting oxidative stress.

Integrative Functional Transcriptomic Analyses Implicate Shared Molecular Circuits in Sensorineural Hearing Loss

Sensorineural hearing loss (SNHL) is referred to as the most common type of hearing loss and typically occurs when the inner ear or the auditory nerve is damaged. Aging, noise exposure, and ototoxic drugs represent three main causes of SNHL, leading to substantial similarities in pathophysiological characteristics of cochlear degeneration. Although the common molecular mechanisms are widely assumed to underlie these similarities, its validity lacks systematic examination. To address this question, we generated three SNHL mouse models from aging, noise exposure, and cisplatin ototoxicity, respectively. Through constructing gene co-expression networks for the cochlear transcriptome data across different hearing-damaged stages, the three models are found to significantly correlate with each other in multiple gene co-expression modules that implicate distinct biological functions, including apoptosis, immune, inflammation, and ion transport. Bioinformatics analyses reveal several potential hub regulators, such as IL1B and CCL2, both of which are verified to contribute to apoptosis accompanied by the increase of (ROS) in in vitro model system. Our findings disentangle the shared molecular circuits across different types of SNHL, providing potential targets for the broad effective therapeutic agents in SNHL.

mito-TEMPO Attenuates Oxidative Stress and Mitochondrial Dysfunction in Noise-Induced Hearing Loss via Maintaining TFAM-mtDNA Interaction and Mitochondrial Biogenesis

The excessive generation of reactive oxygen species (ROS) and mitochondrial damage have been widely reported in noise-induced hearing loss (NIHL). However, the specific mechanism of noise-induced mitochondrial damage remains largely unclear. In this study, we showed that acoustic trauma caused oxidative damage to mitochondrial DNA (mtDNA), leading to the reduction of mtDNA content, mitochondrial gene expression and ATP level in rat cochleae. The expression level and mtDNA-binding function of mitochondrial transcription factor A (TFAM) were impaired following acoustic trauma without affecting the upstream PGC-1α and NRF-1. The mitochondria-target antioxidant mito-TEMPO (MT) was demonstrated to enter the inner ear after the systemic administration. MT treatment significantly alleviated noise-induced auditory threshold shifts 3d and 14d after noise exposure. Furthermore, MT significantly reduced outer hair cell (OHC) loss, cochlear ribbon synapse loss, and auditory nerve fiber (ANF) degeneration after the noise exposure. In addition, we found that MT treatment effectively attenuated noise-induced cochlear oxidative stress and mtDNA damage, as indicated by DHE, 4-HNE, and 8-OHdG. MT treatment also improved mitochondrial biogenesis, ATP generation, and TFAM-mtDNA interaction in the cochlea. These findings suggest that MT has protective effects against NIHL via maintaining TFAM-mtDNA interaction and mitochondrial biogenesis based on its ROS scavenging capacity.

Disruption of Hars2 in Cochlear Hair Cells Causes Progressive Mitochondrial Dysfunction and Hearing Loss in Mice

Mutations in a number of genes encoding mitochondrial aminoacyl-tRNA synthetases lead to non-syndromic and/or syndromic sensorineural hearing loss in humans, while their cellular and physiological pathology in cochlea has rarely been investigated in vivo. In this study, we showed that histidyl-tRNA synthetase HARS2, whose deficiency is associated with Perrault syndrome 2 (PRLTS2), is robustly expressed in postnatal mouse cochlea including the outer and inner hair cells. Targeted knockout of Hars2 in mouse hair cells resulted in delayed onset (P30), rapidly progressive hearing loss similar to the PRLTS2 hearing phenotype. Significant hair cell loss was observed starting from P45 following elevated reactive oxygen species (ROS) level and activated mitochondrial apoptotic pathway. Despite of normal ribbon synapse formation, whole-cell patch clamp of the inner hair cells revealed reduced calcium influx and compromised sustained synaptic exocytosis prior to the hair cell loss at P30, consistent with the decreased supra-threshold wave I amplitudes of the auditory brainstem response. Starting from P14, increasing proportion of morphologically abnormal mitochondria was observed by transmission electron microscope, exhibiting swelling, deformation, loss of cristae and emergence of large intrinsic vacuoles that are associated with mitochondrial dysfunction. Though the mitochondrial abnormalities are more prominent in inner hair cells, it is the outer hair cells suffering more severe cell loss. Taken together, our results suggest that conditional knockout of Hars2 in mouse cochlear hair cells leads to accumulating mitochondrial dysfunction and ROS stress, triggers progressive hearing loss highlighted by hair cell synaptopathy and apoptosis, and is differentially perceived by inner and outer hair cells.

SOD2 Alleviates Hearing Loss Induced by Noise and Kanamycin in Mitochondrial DNA4834-deficient Rats by Regulating PI3K/MAPK Signaling

Superoxide dismutase 2 (SOD2)-mediated gene therapy has significant protective effects against kanamycin-induced hearing loss and hair cell loss in the inner ear, but the underlying mechanisms are still unclear. Herein, an in vivo aging model of mitochondrial DNA (mtDNA)4834 deletion mutation was established using D-galactose, and the effects of noise or kanamycin on inner ear injury was investigated. Rats subjected to mtDNA4834 mutation via D-galactose administration showed hearing loss characterized by the disruption of inner ear structure (abnormal cell morphology, hair cell lysis, and the absence of the organ of Corti), increased SOD2 promoter methylation, and an increase in the degree of apoptosis. Exposure to noise or kanamycin further contributed to the effects of D-galactose. SOD2 overexpression induced by viral injection accordingly counteracted the effects of noise and kanamycin and ameliorated the symptoms of hearing loss, suggesting the critical involvement of SOD2 in preventing deafness and hearing-related conditions. The PI3K and MAPK signaling pathways were also regulated by noise/kanamycin exposure and/or SOD2 overexpression, indicating that they may be involved in the therapeutic effect of SOD2 against age-related hearing loss.

FOXG1 promotes aging inner ear hair cell survival through activation of the autophagy pathway

Presbycusis is the cumulative effect of aging on hearing. Recent studies have shown that common mitochondrial gene deletions are closely related to deafness caused by degenerative changes in the auditory system, and some of these nuclear factors are proposed to participate in the regulation of mitochondrial function. However, the detailed mechanisms involved in age-related degeneration of the auditory systems have not yet been fully elucidated. In this study, we found that FOXG1 plays an important role in the auditory degeneration process through regulation of macroautophagy/autophagy. Inhibition of FOXG1 decreased the autophagy activity and led to the accumulation of reactive oxygen species and subsequent apoptosis of cochlear hair cells. Recent clinical studies have found that aspirin plays important roles in the prevention and treatment of various diseases by regulating autophagy and mitochondria function. In this study, we found that aspirin increased the expression of FOXG1, which further activated autophagy and reduced the production of reactive oxygen species and inhibited apoptosis, and thus promoted the survival of mimetic aging HCs and HC-like OC-1 cells. This study demonstrates the regulatory function of the FOXG1 transcription factor through the autophagy pathway during hair cell degeneration in presbycusis, and it provides a new molecular approach for the treatment of age-related hearing loss.

Abbreviations: AHL: age-related hearing loss; baf: bafilomycin A1; CD: common deletion; D-gal: D-galactose; GO: glucose oxidase; HC: hair cells; mtDNA: mitochondrial DNA; RAP: rapamycin; ROS: reactive oxygen species; TMRE: tetramethylrhodamine, ethyl ester

Neural presbycusis at ultra-high frequency in aged common marmosets and rhesus monkeys

The aging of the population and environmental noise have contributed to high rates of presbycusis, also known as age-related hearing loss (ARHL). Because mice have a relatively short life span, murine models have not been suitable for determining the mechanism of presbycusis development and methods of diagnosis. Although the common marmoset, a non-human primate (NHP), is an ideal animal model for studying age-related diseases, its auditory spectrum has not been systematically studied. Auditory brainstem responses (ABRs) from 38 marmosets of different ages demonstrated that auditory function correlated with age. Hearing loss in geriatric common marmosets started at ultra-high frequency (>16 kHz), then extended to lower frequencies. Despite age-related deterioration of ABR threshold and amplitude in marmosets, outer hair cell (OHC) function remained stable at all ages. Spiral ganglion neurons (SGNs), which are the first auditory neurons in the auditory system, were found to degenerate distinctly in aged common marmosets, indicating that neural degeneration caused presbycusis in these animals. Similarly, age-associated ABR deterioration without loss of OHC function was observed in another NHP, rhesus monkeys. Audiometry results from these two species of NHP suggested that NHPs were ideal for studying ARHL and that neural presbycusis at high frequency may be prevalent in primates.

SIRT3 promotes auditory function in young adult FVB/nJ mice but is dispensable for hearing recovery after noise exposure

Noise-induced hearing loss (NIHL) affects millions of people worldwide and presents a large social and personal burden. Pharmacological activation of SIRT3, a regulator of the mitochondrial oxidative stress response, has a protective effect on hearing thresholds after traumatic noise damage in mice. In contrast, the role of endogenously activated SIRT3 in hearing recovery has not been established. Here we tested the hypothesis that SIRT3 is required in mice for recovery of auditory thresholds after a noise exposure that confers a temporary threshold shift (TTS). SIRT3-specific immunoreactivity is present in outer hair cells, around the post-synaptic regions of inner hair cells, and faintly within inner hair cells. Prior to noise exposure, homozygous Sirt3-KO mice have slightly but significantly higher thresholds than their wild-type littermates measured by the auditory brainstem response (ABR), but not by distortion product otoacoustic emissions (DPOAE). Moreover, homozygous Sirt3-KO mice display a significant reduction in the progression of their peak 1 amplitude at higher frequencies prior to noise exposure. After exposure to a single sub-traumatic noise dose that does not permanently reduce cochlear function, compromise cell survival, or damage synaptic structures in wild-type mice, there was no difference in hearing function between the two genotypes, measured by ABR and DPOAE. The numbers of hair cells and auditory synapses were similar in both genotypes before and after noise exposure. These loss-of-function studies complement previously published gain-of-function studies and help refine our understanding of SIRT3’s role in cochlear homeostasis under different damage paradigms. They suggest that SIRT3 may promote spiral ganglion neuron function. They imply that cellular mechanisms of homeostasis, in addition to the mitochondrial oxidative stress response, act to restore hearing after TTS. Finally, we present a novel application of a biomedical statistical analysis for identifying changes between peak 1 amplitude progressions in ABR waveforms after damage.

The effect and mechanism of 19S proteasome PSMD11/Rpn6 subunit in D-Galactose induced mimetic aging models

Regulating proteasome activity is a potent therapeutic aspect of age-related hearing loss, which has been proven to protect neurons from age-related damaging. PSMD11, subunit of the 19S proteasome regulatory particle, is known to mainly up-regulate proteasome activity and prolong aging. However, the mechanism of PSMD11 in age-related hearing loss has not been deeply explored. In the present study, we explore the function and mechanism of PSMD11 protecting neurons in d-Galactose (D-Gal) mimetic aging models. Age-related pathologies were detected by Taq-PCR, ABR, Transmission electron microscopy, toluidine blue and β-galactosidase staining. The relative expressions of the proteins were explored by Western blotting, oxyblot, immunoprecipitation and immunofluorescence. Flow cytometry was used to manifest the oxidative state. We discovered that proteasome activity was impaired with aging, and that ROS and toxic protein accumulated in D-Gal induced aging models. PSMD11 changed with aging, and was associated with the metabolism of proteasome activity in the D-Gal treated models. Moreover, the knockdown or overexpression of PSMD11 was sufficient to change the oxidative state caused by D-Gal. Our results also demonstrated that PSMD11 could bond to AMPKα1/2 in the auditory cortex and PC12 cells, and AMPKα2 but not AMPKα1 was efficient to regulate the function of PSMD11. Deeper insights into the mechanisms of regulating PSMD11 for the anti-aging process are needed, and may offer novel therapeutic methods for central presbycusis.

The nuclear transcription factor FoxG1 affects the sensitivity of mimetic aging hair cells to inflammation by regulating autophagy pathways

Inflammation is a self-defense response to protect individuals from infection and tissue damage, but excessive or persistent inflammation can have adverse effects on cell survival. Many individuals become especially susceptible to chronic-inflammation-induced sensorineural hearing loss as they age, but the intrinsic molecular mechanism behind aging individuals' increased risk of hearing loss remains unclear. FoxG1 (forkhead box transcription factor G1) is a key transcription factor that plays important roles in hair cell survival through the regulation of mitochondrial function, but how the function of FoxG1 changes during aging and under inflammatory conditions is unknown. In this study, we first found that FoxG1 expression and autophagy both increased gradually in the low concentration lipopolysaccharide (LPS)-induced inflammation model, while after high concentration of LPS treatment both FoxG1 expression and autophagy levels decreased as the concentration of LPS increased. We then used siRNA to downregulate Foxg1 expression in hair cell-like OC-1 cells and found that cell death and apoptosis were significantly increased after LPS injury. Furthermore, we used d-galactose (D-gal) to create an aging model with hair cell-like OC-1 cells and cochlear explant cultures in vitro and found that the expression of Foxg1 and the level of autophagy were both decreased after D-gal and LPS co-treatment. Lastly, we knocked down the expression of Foxg1 under aged inflammation conditions and found increased numbers of dead and apoptotic cells. Together these results suggest that FoxG1 affects the sensitivity of mimetic aging hair cells to inflammation by regulating autophagy pathways.

Chronic occupational noise exposure: Effects on DNA damage, blood pressure, and serum biochemistry

Noise levels experienced by industrial workers may cause both auditory and non-auditory harmful effects. We have studied the effects of chronic industrial noise exposure on DNA damage, blood pressure, and serum biochemistry in factory workers. Male workers (109 individuals) in three parts of a food factory in Shahroud, Iran were enrolled as the exposed group and male office workers (123 individuals) were the unexposed control group. Noise exposure was measured (dosimetry) and the comet assay was used to evaluate DNA damage in peripheral blood mononuclear cells. Glutathione peroxidase (GPx) and ceruloplasmin (Cp) levels were measured in serum samples. GPx levels, systolic and diastolic blood pressure, and DNA damage were significantly higher in the exposed group than in the control group. However, ceruloplasmin levels were not significantly different. Based on multivariate linear regression analysis, noise exposure was the most important predictor of GPx levels, systolic and diastolic blood pressure, and DNA damage.

Age-associated decline in Nrf2 signaling and associated mtDNA damage may be involved in the degeneration of the auditory cortex: Implications for central presbycusis

Central presbycusis is the most common sensory disorder in the elderly population, however, the underlying molecular mechanism remains unclear. NF‑E2‑related factor 2 (Nrf2) is a key transcription factor in the cellular response to oxidative stress, however, the role of Nrf2 in central presbycusis remains to be elucidated. The aim of the present study was to investigate the pathogenesis of central presbycusis using a mimetic aging model induced by D‑galactose (D‑gal) in vivo and in vitro. The degeneration of the cell was determined with transmission electron microscopy, terminal deoxynucleotidyl transferase‑mediated deoxyuridine 5'‑triphosphate nick‑end labeling staining, and senescence‑associated β‑galactosidase staining. The expression of protein was detected by western blotting and immunofluorescence. The quantification of the mitochondrial DNA (mtDNA) 4,834‑base pair (bp) deletion and mRNA was detected by TaqMan quantitative polymerase chain reaction (qPCR) and reverse transcription‑qPCR respectively. Cell apoptosis and intracellular ROS in vitro were determined with flow cytometry. The levels of nuclear Nrf2, and the mRNA levels of Nrf2‑regulated antioxidant genes, were downregulated in the auditory cortex of aging rats, which was accompanied by an increase in 8‑hydroxy‑2'‑deoxyguanosine formation, an accumulation of mtDNA 4,834‑bp deletion, and neuron degeneration. In addition, oltipraz, a typical Nrf2 activator, was found to protect cells against D‑gal‑induced mtDNA damage and mitochondrial dysfunction by activating Nrf2 target genes in vitro. It was also observed that activating Nrf2 with oltipraz inhibited cell apoptosis and delayed senescence. Taken together, the data of the present study suggested that the age‑associated decline in Nrf2 signaling activity and the associated mtDNA damage in the auditory cortex may be implicated in the degeneration of the auditory cortex. Therefore, the restoration of Nrf2 signaling activity may represent a potential therapeutic strategy for central presbycusis.

Reduced Connexin26 in the Mature Cochlea Increases Susceptibility to Noise-Induced Hearing Lossin Mice

Connexin26 (Cx26, encoded by GJB2) mutations are the most common cause of non-syndromic deafness. GJB2 is thought to be involved in noise-induced hearing loss (NIHL). However, the role of Cx26 in NIHL is still obscure. To explore the association between Cx26 and NIHL, we established a Cx26 knockdown (KD) mouse model by conditional knockdown of Cx26 at postnatal day 18 (P18), and then we observed the auditory threshold and morphologic changes in these mice with or without noise exposure. The Cx26 KD mice did not exhibit substantial hearing loss and hair cell degeneration, while the Cx26 KD mice with acoustic trauma experienced higher hearing loss than simple noise exposure siblings and nearly had no recovery. Additionally, extensive outer hair cell loss and more severe destruction of the basal organ of Corti were observed in Cx26 KD mice after noise exposure. These data indicate that reduced Cx26 expression in the mature mouse cochlea may increase susceptibility to noise-induced hearing loss and facilitate the cell degeneration in the organ of Corti.