Glucose Protects Cochlear Hair Cells Against Oxidative Stress and Attenuates Noise-Induced Hearing Loss in Mice

Characteristics of spatial protein expression in the mouse cochlear sensory epithelia: Implications for age-related hearing loss

Hair cells in the cochlear sensory epithelia serve as mechanosensory receptors, converting sound into neuronal signals. The basal sensory epithelia are responsible for transducing high-frequency sounds, while the apex handles low-frequency sounds. Age-related hearing loss predominantly affects hearing at high frequencies and is indicative of damage to the basal sensory epithelia. However, the precise mechanism underlying this site-selective injury remains unclear. In this study, we employed a microscale proteomics approach to examine and compare protein expression in different regions of the cochlear sensory epithelia (upper half and lower half) in 1.5-month-old (normal hearing) and 6-month-old (severe high-frequency hearing loss without hair cell loss) C57BL/6J mice. A total of 2,386 proteins were detected, and no significant differences in protein expression were detected in the upper half of the cochlear sensory epithelia between the two age groups. The expression of 20 proteins in the lower half of the cochlear sensory epithelia significantly differed between the two age groups (e.g., MATN1, MATN4, and AQP1). Moreover, there were 311 and 226 differentially expressed proteins between the upper and lower halves of the cochlear sensory epithelia in 1.5-month-old and 6-month-old mice, respectively. The expression levels of selected proteins were validated by Western blotting. These findings suggest that the spatial differences in protein expression within the cochlear sensory epithelia may play a role in determining the susceptibility of cells at different sites of the cochlea to age-related damage.

L-shaped association of triglyceride glucose index and sensorineural hearing loss: results from a cross-sectional study and Mendelian randomization analysis

Background

The association between the sensorineural hearing loss (SNHL) and triglyceride-glucose (TyG) index remains inadequately understood. This investigation seeks to elucidate the connection between the TyG index and SNHL.

Methods

In this cross-sectional study, we utilized datasets sourced from the National Health and Nutrition Examination Survey (NHANES). A comprehensive analysis was conducted on 1,851 participants aged 20 to 69, utilizing complete audiometry data from the NHANES database spanning from 2007 to 2018. All enrolled participants had accessible hearing data, and the average thresholds were measured and calculated as both low-frequency pure-tone average and high-frequency pure-tone average. Sensorineural hearing loss (SNHL) was defined as an average pure tone of 20 dB or higher in at least one better ear. Our analysis involved the application of multivariate linear regression models to examine the linear relationship between the TyG index and SNHL. To delineate any non-linear associations, we utilized fitted smoothing curves and conducted threshold effect analysis. Furthermore, we conducted a two-sample Mendelian randomization (MR) study, leveraging genetic data from genome-wide association studies (GWAS) on circulating lipids, blood glucose, and SNHL. The primary analytical method for the MR study was the application of the inverse-variance-weighted (IVW) approach.

Results

In our multivariate linear regression analysis, a substantial positive correlation emerged between the TyG index and SNHL [2.10 (1.80-2.44), p < 0.0001]. Furthermore, using a two-segment linear regression model, we found an L-shaped relationship between TyG index, fasting blood glucose and SNHL with an inflection point of 9.07 and 94 mg/dL, respectively. Specifically, TyG index [3.60, (1.42-9.14)] and blood glucose [1.01, (1.00-1.01)] concentration higher than the threshold values was positively associated with SNHL risk. Genetically determined triglyceride levels demonstrated a causal impact on SNHL (OR = 1.092, p = 8.006 × 10-4). In addition, blood glucose was found to have a protective effect on SNHL (OR = 0.886, p = 1.012 × 10-2).

Conclusions

An L-shaped association was identified among the TyG index, fasting blood glucose, and SNHL in the American population. TyG index of more than 9.07 and blood glucose of more than 94 mg/dL were significantly and positively associated with SNHL risk, respectively.

Norepinephrine protects against cochlear outer hair cell damage and noise-induced hearing loss via α2A-adrenergic receptor

BACKGROUND

The cochlear sympathetic system plays a key role in auditory function and susceptibility to noise-induced hearing loss (NIHL). The formation of reactive oxygen species (ROS) is a well-documented process in NIHL. In this study, we aimed at investigating the effects of a superior cervical ganglionectomy (SCGx) on NIHL in Sprague-Dawley rats.

METHODS

We explored the effects of unilateral and bilateral Superior Cervical Ganglion (SCG) ablation in the eight-ten weeks old Sprague-Dawley rats of both sexes on NIHL. Auditory function was evaluated by auditory brainstem response (ABR) testing and Distortion product otoacoustic emissions (DPOAEs). Outer hair cells (OHCs) counts and the expression of α2A-adrenergic receptor (AR) in the rat cochlea using immunofluorescence analysis. Cells culture and treatment, CCK-8 assay, Flow cytometry staining and analysis, and western blotting were to explore the mechanisms of SCG fibers may have a protective role in NIHL.

RESULTS

We found that neither bilateral nor unilateral SCGx protected the cochlea against noise exposure. In HEI-OC1 cells, H2O2-induced oxidative damage and cell death were inhibited by the application of norepinephrine (NE). NE may prevent ROS-induced oxidative stress in OHCs and NIHL through the α2A-AR.

CONCLUSION

These results demonstrated that sympathetic innervation mildly affected cochlear susceptibility to acoustic trauma by reducing oxidative damage in OHCs through the α2A-AR. NE may be a potential therapeutic strategy for NIHL prevention.

Systemic health effects of noise exposure

Noise, any unwanted sound, is pervasive and impacts large populations worldwide. Investigators suggested that noise exposure not only induces auditory damage but also produces various organ system dysfunctions. Although previous reviews primarily focused on noise-induced cardiovascular and cerebral dysfunctions, this narrow focus has unintentionally led the research community to disregard the importance of other vital organs. Indeed, limited studies revealed that noise exposure impacts other organs including the liver, kidneys, pancreas, lung, and gastrointestinal tract. Therefore, the aim of this review was to examine the effects of noise on both the extensively studied organs, the brain and heart, but also determine noise impact on other vital organs. The goal was to illustrate a comprehensive understanding of the systemic effects of noise. These systemic effects may guide future clinical research and epidemiological endpoints, emphasizing the importance of considering noise exposure history in diagnosing various systemic diseases.

BDNF-enriched small extracellular vesicles protect against noise-induced hearing loss in mice

Noise-induced hearing loss (NIHL) is one of the most prevalent acquired sensorineural hearing loss etiologies and is characterized by the loss of cochlear hair cells, synapses, and nerve terminals. Currently, there are no agents available for the treatment of NIHL because drug delivery to the inner ear is greatly limited by the blood-labyrinth barrier. In this study, we used mesenchymal stem cell-derived small extracellular vesicles (MSC-sEVs) as nanoscale vehicles to deliver brain-derived neurotrophic factor (BDNF) and evaluated their protective effects in a mouse model of NIHL. Following intravenous administration, BDNF-loaded sEVs (BDNF-sEVs) efficiently increased the expression of BDNF protein in the cochlea. Systemic application of sEVs and BDNF-sEVs significantly attenuated noise-induced cochlear hair cell loss and NIHL in CBA/J mice. BDNF-sEVs also alleviated noise-induced loss of inner hair cell ribbon synapses and cochlear nerve terminals. In cochlear explants, sEVs and BDNF-sEVs effectively protected hair cells against H2O2-induced cell loss. Additionally, BDNF-sEVs remarkably ameliorated H2O2-induced oxidative stress, cell apoptosis, and cochlear nerve terminal degeneration. Transcriptomic analysis revealed that many mRNAs and miRNAs were involved in the protective actions of BDNF-sEVs against oxidative stress. Collectively, our findings reveal a novel therapeutic strategy of MSC-sEVs-mediated BDNF delivery for the treatment of NIHL.

Effects of acute exposure to Al2O3-NPs (α and γ) and white noise and their combination on cochlea structure and function in Wistar rats

Hearing loss induced by noise and combinations of factors is a common occupational disease among workers. This study aimed to investigate the impact of acute exposure to white noise and Al2O3 NPs, alone and in combination, on changes in the hearing and structural functions of the cochlea in rats. Thirty-six rats were randomly assigned to one of six groups: Control, acute exposure to white noise, exposure to γ-Al2O3 NPs, exposure to noise plus γ-Al2O3 NPs, exposure to α-Al2O3 NPs, and exposure to the combination of noise plus α-Al2O3 NPs. TTS and PTS were examined using DPOAE, while oxidative index (MDA, GSH-Px), gene expression (NOX3, TGF-ß, CYP1A1), protein expression (ß-Tubulin, Myosin VII), and histopathological changes were examined in the cochlea. The morphology of Al2O3 NPs was examined by TEM. The results of the DPOAE test showed a significant increase in TTS in all groups and an increase in PTS in the groups exposed to noise, γ-Al2O3 NPs, and a combination of noise plus Al2O3 NPs (P < 0.05). In the group exposed to white noise plus Al2O3 NPs, the MDA levels increased, the level of GSH-Px decreased, and the expression percentage of ß-Tubulin and Myosin VII decreased, while the expression of NOX3, TGF-ß, and CYP1A1 (except for the α-Al2O3 NPs group) significantly increased (P < 0.05). Histopathological changes of the cochlea indicated damage to hair and ganglion cells, which was more severe in the combined exposure group. The combined and independent exposure to white noise and Al2O3 NPs damaged hair and ganglion cells for high-frequency perception, affecting the function and structure of the cochlea and leading to TTS and PTS.

PTEN inhibitor bisperoxovanadium protects against noise-induced hearing loss

Studies have shown that phosphatase and tensin homolog deleted on chromosome ten (PTEN) participates in the regulation of cochlear hair cell survival. Bisperoxovanadium protects against neurodegeneration by inhibiting PTEN expression. However, whether bisperoxovanadium can protect against noise-induced hearing loss and the underlying mechanism remains unclear. In this study, we established a mouse model of noise-induced hearing loss by exposure to 105 dB sound for 2 hours. We found that PTEN expression was increased in the organ of Corti, including outer hair cells, inner hair cells, and lateral wall tissues. Intraperitoneal administration of bisperoxovanadium decreased the auditory threshold and the loss of cochlear hair cells and inner hair cell ribbons. In addition, noise exposure decreased p-PI3K and p-Akt levels. Bisperoxovanadium preconditioning or PTEN knockdown upregulated the activity of PI3K-Akt. Bisperoxovanadium also prevented H₂O₂-induced hair cell death by reducing mitochondrial reactive oxygen species generation in cochlear explants. These findings suggest that bisperoxovanadium reduces noise-induced hearing injury and reduces cochlear hair cell loss.

Intrinsic mechanism and pharmacologic treatments of noise-induced hearing loss

Noise accounts for one-third of hearing loss worldwide. Regretfully, noise-induced hearing loss (NIHL) is deemed to be irreversible due to the elusive pathogenic mechanisms that have not been fully elucidated. The complex interaction between genetic and environmental factors, which influences numerous downstream molecular and cellular events, contributes to the NIHL. In clinical settings, there are no effective therapeutic drugs other than steroids, which are the only treatment option for patients with NIHL. Therefore, the need for treatment of NIHL that is currently unmet, along with recent progress in our understanding of the underlying regulatory mechanisms, has led to a lot of new literatures focusing on this therapeutic field. The emergence of novel technologies that modify local drug delivery to the inner ear has led to the development of promising therapeutic approaches, which are currently under clinical investigation. In this comprehensive review, we focus on outlining and analyzing the basics and potential therapeutics of NIHL, as well as the application of biomaterials and nanomedicines in inner ear drug delivery. The objective of this review is to provide an incentive for NIHL's fundamental research and future clinical translation.

SIRT3/GLUT4 signaling activation by metformin protect against cisplatin-induced ototoxicity in vitro

Cisplatin is highly effective for killing tumor cells. However, as one of its side effects, ototoxicity limits the clinical application of cisplatin. The mechanisms of cisplatin-induced ototoxicity have not been fully clarified yet. SIRT3 is a deacetylated protein mainly located in mitochondria, which regulates a variety of physiological processes in cells. The role of SIRT3 in cisplatin-induced hair cell injury has not been founded. In this study, primary cultured cochlear explants exposed to 5 μM cisplatin, as well as OC-1 cells exposed to 10 μM cisplatin, were used to establish models of cisplatin-induced ototoxicity in vitro. We found that when combined with cisplatin, metformin (75 μM) significantly up-regulated the expression of SIRT3 and alleviated cisplatin-induced apoptosis of hair cells. We regulated the expression of SIRT3 to explore the role of SIRT3 in cisplatin-induced auditory hair cell injury. Overexpression of SIRT3 promoted the survival of auditory hair cells and alleviated the apoptosis of auditory hair cells. In contrast, knockdown of SIRT3 impaired the protective effect of metformin and exacerbated cisplatin injury. In addition, we found that the protective effect of SIRT3 may be achieved by regulating GLUT4 translocation and rescuing impaired glucose uptake caused by cisplatin. Our study confirmed that upregulation of SIRT3 may antagonize cisplatin-induced ototoxicity, and provided a new perspective for the study of cisplatin-induced ototoxicity.

Prevention of Noise-Induced Hearing Loss In Vivo: Continuous Application of Insulin-like Growth Factor 1 and Its Effect on Inner Ear Synapses, Auditory Function and Perilymph Proteins

As noise-induced hearing loss (NIHL) is a leading cause of occupational diseases, there is an urgent need for the development of preventive and therapeutic interventions. To avoid user-compliance-based problems occurring with conventional protection devices, the pharmacological prevention is currently in the focus of hearing research. Noise exposure leads to an increase in reactive oxygen species (ROS) in the cochlea. This way antioxidant agents are a promising option for pharmacological interventions. Previous animal studies reported preventive as well as therapeutic effects of Insulin-like growth factor 1 (IGF-1) in the context of NIHL. Unfortunately, in patients the time point of the noise trauma cannot always be predicted, and additive effects may occur. Therefore, continuous prevention seems to be beneficial. The present study aimed to investigate the preventive potential of continuous administration of low concentrations of IGF-1 to the inner ear in an animal model of NIHL. Guinea pigs were unilaterally implanted with an osmotic minipump. One week after surgery they received noise trauma, inducing a temporary threshold shift. Continuous IGF-1 delivery lasted for seven more days. It did not lead to significantly improved hearing thresholds compared to control animals. Quite the contrary, there is a hint for a higher noise susceptibility. Nevertheless, changes in the perilymph proteome indicate a reduced damage and better repair mechanisms through the IGF-1 treatment. Thus, future studies should investigate delivery methods enabling continuous prevention but reducing the risk of an overdosage.

Treatment with the Ferroptosis Inhibitor Ferrostatin-1 Attenuates Noise-Induced Hearing Loss by Suppressing Ferroptosis and Apoptosis

Hair cell death induced by excessive reactive oxygen species (ROS) has been identified as the major pathogenesis of noise-induced hearing loss (NIHL). Recent studies have demonstrated that cisplatin- and neomycin-induced ototoxicity can be alleviated by ferroptosis inhibitors. However, whether ferroptosis inhibitors have a protective effect against NIHL remains unknown. We investigated the protective effect of the ferroptosis inhibitor ferrostatin-1 (Fer-1) on NIHL in vivo in CBA/J mice and investigated the protective effect of Fer-1 on tert-butyl hydroperoxide (TBHP)-induced hair cell damage in vitro in cochlear explants and HEI-OC1 cells. We observed ROS overload and lipid peroxidation, which led to outer hair cell (OHC) apoptosis and ferroptosis, in the mouse cochlea after noise exposure. The expression level of apoptosis-inducing factor mitochondria-associated 2 (AIFM2) was substantially increased following elevation of the expression of its upstream protein P53 after noise exposure. The ferroptosis inhibitor Fer-1was demonstrated to enter the inner ear after the systemic administration. Administration of Fer-1 significantly alleviated noise-induced auditory threshold elevation and reduced the loss of OHCs, inner hair cell (IHC) ribbon synapses, and auditory nerve fibers (ANFs) caused by noise. Mechanistically, Fer-1 significantly reduced noise- and TBHP-induced lipid peroxidation and iron accumulation in hair cells, alleviating ferroptosis in cochlear cells consequently. Furthermore, Fer-1 treatment decreased the levels of TfR1, P53, and AIFM2. These results suggest that Fer-1 exerted its protective effects by scavenging of ROS and inhibition of TfR1-mediated ferroptosis and P53-AIFM2 signaling pathway-mediated apoptosis. Our findings suggest that Fer-1 is a promising drug for treating NIHL because of its ability to inhibit noise-induced hair cell apoptosis and ferroptosis, opening new avenues for the treatment of NIHL.

Nitric oxide protects against cochlear hair cell damage and noise-induced hearing loss through glucose metabolic reprogramming

Nitric oxide (NO) is critically involved in the regulation of a wide variety of physiological and pathophysiological processes. However, the role of NO in the pathogenesis of noise-induced hearing loss (NIHL) is complex and remains controversial. Here we reported that treatment of CBA/J mice with l-arginine, a physiological precursor of NO, significantly reduced noise-induced reactive oxygen species accumulation in outer hair cells (OHCs), attenuated noise-induced loss of OHCs and NIHL consequently. Conversely, pharmacological inhibition of endothelial nitric oxide synthase exacerbated noise-induced loss of OHCs and aggravated NIHL. In HEI-OC1 cells, NO also showed substantial protection against H₂O₂-induced oxidative stress and cytotoxicity. Mechanistically, NO increased S-nitrosylation of pyruvate kinase M2 (PKM2) and inhibited its activity, which thus diverted glucose metabolic flux from glycolysis into the pentose phosphate pathway to increase production of reducing equivalents (NADPH and GSH) and eventually prevented H₂O₂-induced oxidative damage. These findings open new avenues for protection of cochlear hair cells from oxidative stress and prevention of NIHL through NO modulation of PKM2 and glucose metabolism reprogramming.

Fasudil prevents neomycin-induced hair cell damage by inhibiting autophagy through the miR-489/NDP52 signaling pathway in HEI-OC1 cells

Hearing loss is a common sensory disorder that is mainly caused by the loss of hair cells (HCs). Drug-induced deafness, for which there is currently no effective treatment, is mainly caused by the inappropriate use of aminoglycoside antibiotics. Fasudil (Fas), a novel isoquinoline sulfonamide derivative, has exhibited antioxidant abilities in a number of previous studies. The aim of the present study was to investigate the potential effects of Fas against neomycin (Neo)-induced hair cell damage and elucidate the underlying mechanism. Flow cytometry and western blot analysis were used to detect the effects of Fas on cell apoptosis and to determine the expression levels of autophagy-related proteins, LC3B and Beclin 1, induced by Neo. Mitochondrial membrane potential and reactive oxygen species (ROS) levels were detected using fluorescent probes. The effect of Fas on Neo-induced hair cell injury marker, GFP-LC3B, was also examined using the immunofluorescence technique. Fas was found to inhibit Neo-induced mitochondrial autophagy and mitochondrial membrane potential decline, in addition to reducing ROS levels and cell apoptosis caused by Neo treatment. However, Fas failed to inhibit the Neo-induced these above changes in cells with NDP52 overexpression. The putative binding sites of microRNA (miR)-489 on the 3'-untranslated region of nuclear dot protein 52 (NDP52) were predicted using the TargetScan 7.0 online tool, and this association was further verified using a dual-luciferase reporter assay. Moreover, the expression of miR-489 negatively regulated the expression of NDP52. Fas and miR-489 mimic inhibited the Neo-induced mitochondrial autophagy and mitochondrial membrane potential decline, in addition to reducing ROS levels and cell apoptosis. Knockdown of miR-489 expression using a miR-489 inhibitor blocked the inhibitory effects of Fas on the mitochondrial membrane potential, cell apoptosis and ROS production. Therefore, Fas may upregulate the expression of miR-489 to negatively regulate the expression of NDP52 at the post-transcriptional level, which in turn inhibits the activation of mitophagy and cell injury induced by Neo. Thus, Fas may act as a novel therapeutic option in the clinical treatment of hearing loss in the future.

Aligned Organization of Synapses and Mitochondria in Auditory Hair Cells

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.

Noise-Induced Hearing Loss: Updates on Molecular Targets and Potential Interventions

Noise overexposure leads to hair cell loss, synaptic ribbon reduction, and auditory nerve deterioration, resulting in transient or permanent hearing loss depending on the exposure severity. Oxidative stress, inflammation, calcium overload, glutamate excitotoxicity, and energy metabolism disturbance are the main contributors to noise-induced hearing loss (NIHL) up to now. Gene variations are also identified as NIHL related. Glucocorticoid is the only approved medication for NIHL treatment. New pharmaceuticals targeting oxidative stress, inflammation, or noise-induced neuropathy are emerging, highlighted by the nanoparticle-based drug delivery system. Given the complexity of the pathogenesis behind NIHL, deeper and more comprehensive studies still need to be fulfilled.