Identifying microRNAs involved in degeneration of the organ of corti during age-related hearing loss

Dark exposure reduces high-frequency hearing loss in C57BL/6J mice

Plastic changes in the brain are primarily limited to early postnatal periods. Recovery of adult brain plasticity is critical for the effective development of therapies. A brief (1-2 week) duration of visual deprivation (dark exposure, DE) in adult mice can trigger functional plasticity of thalamocortical and intracortical circuits in the primary auditory cortex suggesting improved sound processing. We tested if DE enhances the ability of adult mice to detect sounds. We trained and continuously evaluated the behavioral performance of mice in control and DE conditions using automated home-cage training. Consistent with age-related peripheral hearing loss present in C57BL/6J mice, we observed decreased performance for high-frequency sounds with age, which was reduced by DE. In CBA mice with preserved peripheral hearing, we also found that DE enhanced auditory performance in low and mid frequencies over time compared to the control.

Potential role of modulating autophagy levels in sensorineural hearing loss

In recent years, extensive research has been conducted on the pathogenesis of sensorineural hearing loss (SNHL). Apoptosis and necrosis have been identified to play important roles in hearing loss, but they cannot account for all hearing loss. Autophagy, a cellular process responsible for cell self-degradation and reutilization, has emerged as a significant factor contributing to hearing loss, particularly in cases of autophagy deficiency. Autophagy plays a crucial role in maintaining cell health by exerting cytoprotective and metabolically homeostatic effects in organisms. Consequently, modulating autophagy levels can profoundly impact the survival, death, and regeneration of cells in the inner ear, including hair cells (HCs) and spiral ganglion neurons (SGNs). Abnormal mitochondrial autophagy has been demonstrated in animal models of SNHL. These findings indicate the profound significance of comprehending autophagy while suggesting that our perspective on this cellular process holds promise for advancing the treatment of SNHL. Thus, this review aims to clarify the pathogenic mechanisms of SNHL and the role of autophagy in the developmental processes of various cochlear structures, including the greater epithelial ridge (GER), SGNs, and the ribbon synapse. The pathogenic mechanisms of age-related hearing loss (ARHL), also known as presbycusis, and the latest research on autophagy are also discussed. Furthermore, we underscore recent findings on the modulation of autophagy in SNHL induced by ototoxic drugs. Additionally, we suggest further research that might illuminate the complete potential of autophagy in addressing SNHL, ultimately leading to the formulation of pioneering therapeutic strategies and approaches for the treatment of deafness.

NcRNA: key and potential in hearing loss

Hearing loss has an extremely high prevalence worldwide and brings incredible economic and social burdens. Mechanisms such as epigenetics are profoundly involved in the initiation and progression of hearing loss and potentially yield definite strategies for hearing loss treatment. Non-coding genes occupy 97% of the human genome, and their transcripts, non-coding RNAs (ncRNAs), are widely participated in regulating various physiological and pathological situations. NcRNAs, mainly including micro-RNAs (miRNAs), long-stranded non-coding RNAs (lncRNAs), and circular RNAs (circRNAs), are involved in the regulation of cell metabolism and cell death by modulating gene expression and protein-protein interactions, thus impacting the occurrence and prognosis of hearing loss. This review provides a detailed overview of ncRNAs, especially miRNAs and lncRNAs, in the pathogenesis of hearing loss. We also discuss the shortcomings and issues that need to be addressed in the study of hearing loss ncRNAs in the hope of providing viable therapeutic strategies for the precise treatment of hearing loss.

RNA-seq analysis highlights DNA replication and DNA repair associated with early-onset hearing loss in the cochlea of DBA/2J mice

AIMS

Age-related hearing loss (ARHL) is a significant health concern, and DBA/2J (D2) and C57BL/6 (B6) mouse strains serve as valuable models for its study. B6 mice, harboring a homozygous ahl allele in Cdh23, manifest high-frequency hearing loss at 3 months. In contrast, D2 mice, carrying the R109H variant of the Fascin-2 gene (Fscn2), experience early-onset hearing loss by 3 weeks. Yet, the underlying molecular mechanisms driving early-onset hearing loss in D2 mice remain elusive. This study aimed to identify novel genes and regulatory pathways as therapeutic targets for early deafness.

MAIN METHODS

This study employs RNA-sequencing (RNA-seq) to analyze cochlear mRNA expression at two different ages in D2 and B6 mice, respectively. The differentially expressed genes (DEGs) are uniquely associated with D2 mice by Venn diagram analysis. A protein-protein interaction (PPI) network is further constructed, followed by module analysis utilizing MCODE. Enrichment analysis of GO and KEGG pathways revealed biological functions and molecular pathways. The PPI network and VarElect analysis are conducted for genes within these pathways, facilitating the identification of pivotal genes based on scoring criteria. Subsequently, five genes are meticulously selected and validated through qRT-PCR.

KEY FINDINGS

Notably, 1181 DEGs are uniquely associated with D2 mice by Venn diagram analysis. GO and KEGG pathway enrichment analyses shed light on distinctive pathways in D2 mice, encompassing DNA replication, mismatch repair, base excision repair, and nucleotide excision repair, which are associated with apoptosis. Five genes involved in these pathways were finally selected and validated by qRT-PCR. Their down-regulation with age is consistent with RNA-seq result.

SIGNIFICANCE

Our study underscores the potential implication of down-regulated genes associated with DNA replication and DNA damage repair in the early-onset hearing loss observed in D2 mice.

Research Progress of Aging-related MicroRNAs

Senescence refers to the irreversible state in which cells enter cell cycle arrest due to internal or external stimuli. The accumulation of senescent cells can lead to many age-related diseases, such as neurodegenerative diseases, cardiovascular diseases, and cancers. MicroRNAs are short non-coding RNAs that bind to target mRNA to regulate gene expression after transcription and play an important regulatory role in the aging process. From nematodes to humans, a variety of miRNAs have been confirmed to alter and affect the aging process. Studying the regulatory mechanisms of miRNAs in aging can further deepen our understanding of cell and body aging and provide a new perspective for the diagnosis and treatment of aging-related diseases. In this review, we illustrate the current research status of miRNAs in aging and discuss the possible prospects for clinical applications of targeting miRNAs in senile diseases.

MiR-29a-deficiency causes thickening of the basilar membrane and age-related hearing loss by upregulating collagen IV and laminin

Age-related hearing loss (ARHL) is the most common sensory degenerative disease and can significantly impact the quality of life in elderly people. A previous study using GeneChip miRNA microarray assays showed that the expression of miR-29a changes with age, however, its role in hearing loss is still unclear. In this study, we characterized the cochlear phenotype of miR-29a knockout (miR-29a^-/-) mice and found that miR-29a-deficient mice had a rapid progressive elevation of the hearing threshold from 2 to 5 months of age compared with littermate controls as measured by the auditory brainstem response. Stereocilia degeneration, hair cell loss and abnormal stria vascularis (SV) were observed in miR-29a^-/- mice at 4 months of age. Transcriptome sequencing results showed elevated extracellular matrix (ECM) gene expression in miR-29a^-/- mice. Both Gene Ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis revealed that the key differences were closely related to ECM. Further examination with a transmission electron microscope showed thickening of the basilar membrane in the cochlea of miR-29a^-/- mice. Five Col4a genes (Col4a1-a5) and two laminin genes (Lamb2 and Lamc1) were validated as miR-29a direct targets by dual luciferase assays and miR-29a inhibition assays with a miR-29a inhibitor. Consistent with the target gene validation results, the expression of these genes was significantly increased in the cochlea of miR-29a^-/- mice, as shown by RT-PCR and Western blot. These findings suggest that miR-29a plays an important role in maintaining cochlear structure and function by regulating the expression of collagen and laminin and that the disturbance of its expression could be a cause of progressive hearing loss.

Single-cell transcriptomic atlas of mouse cochlear aging

Progressive functional deterioration in the cochlea is associated with age-related hearing loss (ARHL). However, the cellular and molecular basis underlying cochlear aging remains largely unknown. Here, we established a dynamic single-cell transcriptomic landscape of mouse cochlear aging, in which we characterized aging-associated transcriptomic changes in 27 different cochlear cell types across five different time points. Overall, our analysis pinpoints loss of proteostasis and elevated apoptosis as the hallmark features of cochlear aging, highlights unexpected age-related transcriptional fluctuations in intermediate cells localized in the stria vascularis (SV) and demonstrates that upregulation of endoplasmic reticulum (ER) chaperon protein HSP90AA1 mitigates ER stress-induced damages associated with aging. Our work suggests that targeting unfolded protein response pathways may help alleviate aging-related SV atrophy and hence delay the progression of ARHL.

MiR-181a-5p promotes neural stem cell proliferation and enhances the learning and memory of aged mice

Hippocampal neural stem cell (NSC) proliferation is known to decline with age, which is closely linked to learning and memory impairments. In the current study, we found that the expression level of miR-181a-5p was decreased in the hippocampal NSCs of aged mice and that exogenous overexpression of miR-181a-5p promoted NSC proliferation without affecting NSC differentiation into neurons and astrocytes. The mechanistic study revealed that phosphatase and tensin homolog (PTEN), a negative regulator of the AKT signaling pathway, was the target of miR-181a-5p and knockdown of PTEN could rescue the impairment of NSC proliferation caused by low miR-181a-5p levels. Moreover, overexpression of miR-181a-5p in the dentate gyrus enhanced the proliferation of NSCs and ameliorated learning and memory impairments in aged mice. Taken together, our findings indicated that miR-181a-5p played a functional role in NSC proliferation and aging-related, hippocampus-dependent learning and memory impairments.

Occlusion of two semicircular canals does not disrupt normal hearing in adult mice

Vertigo is a debilitating disease affecting 15–20% of adults worldwide. Vestibular peripheral vertigo is the most common cause of vertigo, often due to Meniere's disease and benign paroxysmal positional vertigo. Although some vertigo symptoms can be controlled by conservative treatment and/or vestibular rehabilitation therapy, these treatments do not work for some patients. Semicircular canal occlusion surgery has proven to be very effective for these patients with intractable vertigo. However, its application is limited due to concern that the procedure will disrupt normal hearing. In this study, we investigated if occlusion of two semicircular canals would jeopardize auditory function by comparing auditory function and hair cell morphology between the surgical and contralateral ears before and after the surgery in a mouse model. By measuring the auditory brainstem response and distortion product otoacoustic emission 4 weeks post-surgery, we show that auditory function does not significantly change between the surgical and contralateral ears. In addition, confocal imaging has shown no hair cell loss in the cochlear and vestibular sensory epithelia, and scanning electron microscopy also indicates normal stereocilia morphology in the surgical ear. More importantly, the endocochlear potential measured from the surgical ear is not significantly different than that seen in the contralateral ear. Our study suggests that occlusion of two semicircular canals does not disrupt normal hearing in the mouse model, providing a basis to extend the procedure to patients, even those with normal hearing, benefitting more patients with intractable vertigo attacks.

Characterizing subcutaneous cortical auditory evoked potentials in mice

Auditory Brainstem Responses (ABRs) are a reliably robust measure of auditory thresholds in the mammalian hearing system and can be used to determine deficits in the auditory periphery. However, because these measures are limited to the lower stages of the auditory pathway, they are insensitive to changes or deficits that occur in the thalamic and cortical regions. Cortical Auditory Evoked Potentials (CAEPs), as longer latency responses, capture information from these regions. However they are less frequently used as a diagnostic tool, particularly in rodent models, due to their inherent variability and subsequent difficult interpretation. The purpose of this study was to develop a consistent measure of subcutaneous CAEPs to auditory stimuli in mice and to determine their origin. To this end, we investigated the effect on the CAEPs recorded in response to different stimuli (noise, click, and tone (16 kHz) bursts), stimulus presentation rates (2/s, 6/s, 10/s) and electrode placements. Recordings were examined for robust CAEP components to determine the optimal experimental paradigm. We argue that CAEPs can measure robust and replicable cortical responses. Furthermore, by deactivating the auditory cortex with lidocaine we demonstrated that the contralateral cortex is the main contributor to the CAEP. Thus CAEP measurements could prove to be of value diagnostically in future for deficits in higher auditory areas.

Connexin 30 deletion exacerbates cochlear senescence and age-related hearing loss

Pathogenic mutations in the Gjb2 and Gjb6 genes, encoding connexin 26 (Cx26) and connexin 30 (Cx30), respectively, have been linked to the most frequent monogenic hearing impairment, nonsyndromic hearing loss, and deafness DFNB1. It is known that Cx26 plays an important role in auditory development, while the role of Cx30 in hearing remains controversial. Previous studies found that partial deletion of Cx26 can accelerate age-related hearing loss (ARHL), a multifactorial complex disorder, with both environmental and genetic factors contributing to the etiology of the disease. Here, we investigated the role of Cx30 in cochlear-aging processes using a transgenic mouse model with total deletion of Cx30 (Cx30 ΔΔ mice), in which Cx30 was removed without perturbing the surrounding sequences. We show that these mice are affected by exacerbated ARHL, with increased morphological cochlear damage, oxidative stress, inflammation, and vascular dysfunctions. Overall, our data demonstrate that Cx30 deletion can be considered a genetic risk factor for ARHL, making cochlear structures more susceptible to aging processes.

Relationship between Nutrient Intake and Hearing Loss According to the Income Level of Working-Aged Adults: A Korean National Health and Nutrition Survey

The relationship between hearing impairment and nutrition has been extensively investigated; however, few studies have focused on this topic in working-age adults by income level. Herein, we aimed to determine the differences in hearing impairment among working-age adults by income level and identify the nutritional factors that affect hearing loss in various socioeconomic groups. Seven-hundred-and-twenty participants had hearing impairment, while 10,130 had normal hearing. After adjustment for propensity score matching, income and smoking status were identified as significant variables. By assessing the relationship between hearing impairment and nutrient intake by income level using multiple regression analyses, significant nutrients differed for each income category. Carbohydrate and vitamin C levels were significant in the low-income group; protein, fat, and vitamin B1 levels were significant in the middle-income group; and carbohydrates were significant in the high-income group. Income was significantly associated with hearing impairment in working-age adults. The proportion of individuals with hearing impairment increased as income decreased. The association between hearing impairment and nutritional intake also differed by income level. Our findings may enable the establishment of health policies for preventing hearing impairment in working-age adults by income level.

Mutation of SLC7A14 causes auditory neuropathy and retinitis pigmentosa mediated by lysosomal dysfunction

Lysosomes contribute to cellular homeostasis via processes including macromolecule degradation, nutrient sensing, and autophagy. Defective proteins related to lysosomal macromolecule catabolism are known to cause a range of lysosomal storage diseases; however, it is unclear whether mutations in proteins involved in homeostatic nutrient sensing mechanisms cause syndromic sensory disease. Here, we show that SLC7A14, a transporter protein mediating lysosomal uptake of cationic amino acids, is evolutionarily conserved in vertebrate mechanosensory hair cells and highly expressed in lysosomes of mammalian cochlear inner hair cells (IHCs) and retinal photoreceptors. Autosomal recessive mutation of SLC7A14 caused loss of IHCs and photoreceptors, leading to presynaptic auditory neuropathy and retinitis pigmentosa in mice and humans. Loss-of-function mutation altered protein trafficking and increased basal autophagy, leading to progressive cell degeneration. This study implicates autophagy-lysosomal dysfunction in syndromic hearing and vision loss in mice and humans.

Hearing Function, Degeneration, and Disease: Spotlight on the Stria Vascularis

The stria vascularis (SV) is a highly vascularized tissue lining the lateral wall of the cochlea. The SV maintains cochlear fluid homeostasis, generating the endocochlear potential that is required for sound transduction. In addition, the SV acts as an important blood-labyrinth barrier, tightly regulating the passage of molecules from the blood into the cochlea. A healthy SV is therefore vital for hearing function. Degeneration of the SV is a leading cause of age-related hearing loss, and has been associated with several hearing disorders, including Norrie disease, Meniere's disease, Alport syndrome, Waardenburg syndrome, and Cytomegalovirus-induced hearing loss. Despite the SV's important role in hearing, there is still much that remains to be discovered, including cell-specific function within the SV, mechanisms of SV degeneration, and potential protective or regenerative therapies. In this review, we discuss recent discoveries elucidating the molecular regulatory networks of SV function, mechanisms underlying degeneration of the SV, and otoprotective strategies for preventing drug-induced SV damage. We also highlight recent clinical developments for treating SV-related hearing loss and discuss future research trajectories in the field.

Characterization of the microRNA transcriptomes and proteomics of cochlear tissue-derived small extracellular vesicles from mice of different ages after birth

The cochlea is an important sensory organ for both balance and sound perception, and the formation of the cochlea is a complex developmental process. The development of the mouse cochlea begins on embryonic day (E)9 and continues until postnatal day (P)21 when the hearing system is considered mature. Small extracellular vesicles (sEVs), with a diameter ranging from 30 to 200 nm, have been considered a significant medium for information communication in both physiological and pathological processes. However, there are no studies exploring the role of sEVs in the development of the cochlea. Here, we isolated tissue-derived sEVs from the cochleae of FVB mice at P3, P7, P14, and P21 by ultracentrifugation. These sEVs were first characterized by transmission electron microscopy, nanoparticle tracking analysis, and western blotting. Next, we used small RNA-seq and mass spectrometry to characterize the microRNA transcriptomes and proteomes of cochlear sEVs from mice at different ages. Many microRNAs and proteins were discovered to be related to inner ear development, anatomical structure development, and auditory nervous system development. These results all suggest that sEVs exist in the cochlea and are likely to be essential for the normal development of the auditory system. Our findings provide many sEV microRNA and protein targets for future studies of the roles of cochlear sEVs.

LncRNA H19 inhibits oxidative stress injury of cochlear hair cells by regulating miR-653-5p/SIRT1 axis

Oxidative stress is one of the important mechanisms of inner ear cell damage, which can lead to age-related hearing loss (ARHL). LncRNA H19 is significantly downregulated in the cochlea of old mouse, however, the role of H19 in the development of ARHL remains unclear. In this study, we aim to investigate the expression and function of H19 in oxidative stress injury of cochlear hair cells induced by HO. RT-qPCR and western blot analysis confirms that HEI-OC1 cells stimulated with HO decreases the expressions of H19 and SIRT1, but increases the expression of miR-653-5p. Overexpression of H19 could increase cell viability, ATP level and mitochondrial membrane potential, but reduce mitochondrial ROS generation and cell apoptosis ratio in HO-stimulated HEI-OC1 cells. MiR-653-5p is a target of H19, which can bind to the 3'-UTR of SIRT1. H19 is found to regulate the expression of SIRT1 through miR-653-5p. Further experiments demonstrates that H19 regulates HEI-OC1 cell viability, ATP level, mitochondrial membrane potential, mitochondrial ROS generation, and cell apoptosis ratio via the miR-653-5p/SIRT1 axis. In conclusion, lncRNA H19 inhibits oxidative stress injury of cochlear hair cells via the miR-653-5p/SIRT1 axis.

Early Noise-Induced Hearing Loss Accelerates Presbycusis Altering Aging Processes in the Cochlea

Several studies identified hearing loss as a risk factor for aging-related processes, including neurodegenerative diseases, as dementia and age-related hearing loss (ARHL). Although the association between hearing impairment in midlife and ARHL has been widely documented by epidemiological and experimental studies, the molecular mechanisms underlying this association are not fully understood. In this study, we used an established animal model of ARHL (C57BL/6 mice) to evaluate if early noise-induced hearing loss (NIHL) could affect the onset or progression of age-related cochlear dysfunction. We found that hearing loss can exacerbate ARHL, damaging sensory-neural cochlear epithelium and causing synaptopathy. Moreover, we studied common pathological markers shared between hearing loss and ARHL, demonstrating that noise exposure can worsen/accelerate redox status imbalance [increase of reactive oxygen species (ROS) production, lipid peroxidation, and dysregulation of endogenous antioxidant response] and vascular dysfunction [increased expression of hypoxia-inducible factor-1alpha (HIF-1α) and vascular endothelial growth factor C (VEGFC)] in the cochlea. Unveiling the molecular mechanisms underlying the link between hearing loss and aging processes could be valuable to identify effective therapeutic strategies to limit the effect of environmental risk factors on age-related diseases.

Evaluating Neurotrophin Signaling Using MicroRNA Perilymph Profiling in Cochlear Implant Patients With and Without Residual Hearing

HYPOTHESIS

MicroRNAs predicted to regulate neurotrophin signaling can be found in human perilymph.

BACKGROUND

Animal and human temporal bone studies suggest that spiral ganglion health can affect cochlear implant (CI) outcomes. Neurotrophins have been identified as a key factor in the maintenance of spiral ganglion health. Changes in miRNAs may regulate neurotrophin signaling and may reflect neurotrophin expression levels.

METHODS

Perilymph sampling was carried out in 18 patients undergoing cochlear implantation or stapedotomy. Expression of miRNAs in perilymph was evaluated using an Agilent miRNA gene chip. Using ingenuity pathway analysis (IPA) software, miRNAs targeting neurotrophin signaling pathway genes present in a cochlear cDNA library were annotated. Expression levels of miRNAs in perilymph were correlated to the patients' preoperative pure-tone average.

RESULTS

Expression of mRNAs coding for neurotrophins and their receptors were identified in tissue obtained from normal human cochlea during skull base surgery. We identified miRNAs predicted to regulate these signaling cascades, including miR-1207-5p, miR-4651, miR-103-3p, miR-100-5p, miR-221-3p, miR-200-3p. There was a correlation between poor preoperative hearing and lower expression of miR-1207 (predicted to regulate NTR3) and miR-4651 (predicted to regulate NTR2). Additionally, miR-3960, miR-4481, and miR-675 showed significant differences in expression level when comparing mild and profound hearing loss patients.

CONCLUSIONS

Expression of some miRNAs that are predicted to regulate neurotrophin signaling in the perilymph of cochlear implant patients vary with the patient's level of residual hearing. These miRNAs may serve as biomarkers for changes in neurotrophin signaling.

Autophagy: A Novel Horizon for Hair Cell Protection

As a general sensory disorder, hearing loss was a major concern worldwide. Autophagy is a common cellular reaction to stress that degrades cytoplasmic waste through the lysosome pathway. Autophagy not only plays major roles in maintaining intracellular homeostasis but is also involved in the development and pathogenesis of many diseases. In the auditory system, several studies revealed the link between autophagy and hearing protection. In this review, we aimed to establish the correlation between autophagy and hair cells (HCs) from the aspects of ototoxic drugs, aging, and acoustic trauma and discussed whether autophagy could serve as a potential measure in the protection of HCs.

Transcription co-factor LBH is necessary for the survival of cochlear hair cells

Hearing loss affects ∼10% of adults worldwide. Most sensorineural hearing loss is caused by the progressive loss of mechanosensitive hair cells (HCs) in the cochlea. The molecular mechanisms underlying HC maintenance and loss remain poorly understood. LBH, a transcription co-factor implicated in development, is abundantly expressed in outer hair cells (OHCs). We used Lbh-null mice to identify its role in HCs. Surprisingly, Lbh deletion did not affect differentiation and the early development of HCs, as nascent HCs in Lbh knockout mice had normal looking stereocilia. The stereocilia bundle was mechanosensitive and OHCs exhibited the characteristic electromotility. However, Lbh-null mice displayed progressive hearing loss, with stereocilia bundle degeneration and OHC loss as early as postnatal day 12. RNA-seq analysis showed significant gene enrichment of biological processes related to transcriptional regulation, cell cycle, DNA damage/repair and autophagy in Lbh-null OHCs. In addition, Wnt and Notch pathway-related genes were found to be dysregulated in Lbh-deficient OHCs. Our study implicates, for the first time, loss of LBH function in progressive hearing loss, and demonstrates a critical requirement of LBH in promoting HC survival in adult mice.

miRNA and mRNA Profiling Links Connexin Deficiency to Deafness via Early Oxidative Damage in the Mouse Stria Vascularis

Pathogenic mutations in the non-syndromic hearing loss and deafness 1 (DFNB1) locus are the primary cause of monogenic inheritance for prelingual hearing loss. To unravel molecular pathways involved in etiopathology and look for early degeneration biomarkers, we used a system biology approach to analyze Cx30^−/− mice at an early cochlear post-natal developmental stage. These mice are a DFNB1 mouse model with severely reduced expression levels of two connexins in the inner ear, Cx30, and Cx26. Integrated analysis of miRNA and mRNA expression profiles in the cochleae of Cx30^−/− mice at post-natal day 5 revealed the overexpression of five miRNAs (miR-34c, miR-29b, miR-29c, miR-141, and miR-181a) linked to apoptosis, oxidative stress, and cochlear degeneration, which have Sirt1 as a common target of transcriptional and/or post-transcriptional regulation. In young adult Cx30^−/− mice (3 months of age), these alterations culminated with blood barrier disruption in the Stria vascularis (SV), which is known to have the highest aerobic metabolic rate of all cochlear structures and whose microvascular alterations contribute to age-related degeneration and progressive decline of auditory function. Our experimental validation of selected targets links hearing acquisition failure in Cx30^−/− mice, early oxidative stress, and metabolic dysregulation to the activation of the Sirt1–p53 axis. This is the first integrated analysis of miRNA and mRNA in the cochlea of the Cx30^−/− mouse model, providing evidence that connexin downregulation determines a miRNA-mediated response which leads to chronic exhaustion of cochlear antioxidant defense mechanisms and consequent SV dysfunction. Our analyses support the notion that connexin dysfunction intervenes early on during development, causing vascular damage later on in life. This study identifies also early miRNA-mediated biomarkers of hearing impairment, either inherited or age related.

Identifying the mechanisms underlying the protective effect of tetramethylpyrazine against cisplatin‑induced in vitro ototoxicity in HEI‑OC1 auditory cells using gene expression profiling

Sensorineural hearing loss is prevalent in patients receiving cisplatin therapy. Tetramethylpyrazine (Tet) and tanshinone IIA (Tan IIA) have protective roles against hearing impairment or ototoxicity. The present study aimed to investigate the molecular mechanisms underlying cisplatin-induced ototoxicity and the protective effect of Tet and Tan IIA against it. House Ear Institute-Organ of Corti 1 auditory cells were treated with titrating doses of Tan IIA, Tet, and cisplatin. In a cell viability assay, cisplatin, Tan IIA and Tet had IC₅₀ values of 42.89 µM, 151.80 and 1.04×10³ mg/l, respectively. Tan IIA augmented cisplatin-induced cytotoxicity. However, Tet concentrations <75 mg/l attenuated cisplatin-induced cytotoxicity and apoptosis. Moreover, RNA sequencing analysis was carried out on auditory cells treated for 30 h with 30 µM cisplatin alone for 48 h or combined with 37.5 mg/l Tet for 30 h. Differentially expressed genes (DEGs) induced in these conditions were identified and examined using Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analysis. Cisplatin increased the expression of genes related to the p53 and FoxO pathways, such as Fas, p21/CDKN1A, and Bcl-2 binding component 3, but decreased the expression of insulin-like growth factor 1 (IGF1), as well as genes in the histone (Hist)1 and Hist2 clusters. Treatment with Tet downregulated FOXO3 and Bcl-2 binding component 3, and increased the expression of IGF1. Moreover, Tet upregulated genes associated with Wnt signaling, but not p53-related genes. Thus, the otoprotective properties of Tet might be mediated by activation of Wnt and IGF1 signaling, and inhibition of FoxO signaling.

Diagnostic and therapeutic applications of genomic medicine in progressive, late-onset, nonsyndromic sensorineural hearing loss

The progressive, late-onset, nonsyndromic, sensorineural hearing loss (PNSHL) is the most common cause of sensory impairment globally, with presbycusis affecting greater than a third of individuals over the age of 65. The etiology underlying PNSHL include presbycusis, noise-induced hearing loss, drug ototoxicity, and delayed-onset autosomal dominant hearing loss (AD PNSHL). The objective of this article is to discuss the potential diagnostic and therapeutic applications of genomic medicine in PNSHL. Genomic factors contribute greatly to PNSHL. The heritability of presbycusis ranges from 25 to 75%. Current therapies for PNSHL range from sound amplification to cochlear implantation (CI). PNSHL is an excellent candidate for genomic medicine approaches as it is common, has well-described pathophysiology, has a wide time window for treatment, and is amenable to local gene therapy by currently utilized procedural approaches. AD PNSHL is especially suited to genomic medicine approaches that can disrupt the expression of an aberrant protein product. Gene therapy is emerging as a potential therapeutic strategy for the treatment of PNSHL. Viral gene delivery approaches have demonstrated promising results in human clinical trials for two inherited causes of blindness and are being used for PNSHL in animal models and a human trial. Non-viral gene therapy approaches are useful in situations where a transient biologic effect is needed or for delivery of genome editing reagents (such as CRISPR/Cas9) into the inner ear. Many gene therapy modalities that have proven efficacious in animal trials have potential to delay or prevent PNSHL in humans. The development of new treatment modalities for PNSHL will lead to improved quality of life of many affected individuals and their families.

MicroRNAs in Noise-Induced Hearing Loss and their Regulation by Oxidative Stress and Inflammation

Noise exposure (NE) has been recognized as one of the causes of sensorineural hearing loss (SNHL), which can bring about irreversible damage to sensory hair cells in the cochlea, through the launch of oxidative stress pathways and inflammation. Accordingly, determining the molecular mechanism involved in regulating hair cell apoptosis via NE is essential to prevent hair cell damage. However, the role of microRNAs (miRNAs) in the degeneration of sensory cells of the cochlea during NE has not been so far uncovered. Thus, the main purpose of this study was to demonstrate the regulatory role of miRNAs in the oxidative stress pathway and inflammation induced by NE. In this respect, articles related to noise-induced hearing loss (NIHL), oxidative stress, inflammation, and miRNA from various databases of Directory of Open Access Journals (DOAJ), Google Scholar, PubMed; Library, Information Science & Technology Abstracts (LISTA), and Web of Science were searched and retrieved. The findings revealed that several studies had suggested that up-regulation of miR-1229-5p, miR-451a, 185-5p, 186 and down-regulation of miRNA-96/182/183 and miR-30b were involved in oxidative stress and inflammation which could be used as biomarkers for NIHL. There was also a close relationship between NIHL and miRNAs, but further research is required to prove a causal association between miRNA alterations and NE, and also to determine miRNAs as biomarkers indicating responses to NE.

Differential Gene Expression Patterns Between Apical and Basal Inner Hair Cells Revealed by RNA-Seq

Tonotopic differences in the structure and physiological function, e.g., synapse number, membrane properties, Ca²⁺ channels, Ca²⁺ dependence of exocytosis and vesicle pool replenishment of inner hair cells (IHCs) along the longitudinal cochlear axis have recently been discovered, suggesting different gene expression patterns of IHCs. To determine whether IHCs present different gene expression patterns along the longitudinal cochlear axis, apical and basal IHCs were collected separately using the suction pipette technique from adult mouse cochleae for RNA-seq and genome-wide transcriptome analysis. We found 689 annotated genes showed more than 2-fold increase in expression. Interestingly, 93.4% of the differentially expressed genes (DEGs) was upregulated in apical IHCs. Although a subset of genes that related to IHC machinery and deafness were found to be differentially expressed, a gradient of gene expression was indeed detected in Ocm, Pvalb, Prkd1, Fbxo32, Nme2, and Sncg, which may play putative roles in the Ca²⁺ buffering and survival regulation. The expression of these genes was validated by real-time quantitative PCR (RT-qPCR) or immunostaining. We conclude that IHCs from different mouse cochlear longitudinal position have different gene expression profiles. Our data might serve as a valuable resource for exploring the molecular mechanisms underlying different biological properties as well as the survival regulation of IHCs.

Generation of a ChATCre mouse line without the early onset hearing loss typical of the C57BL/6J strain

The development of knockin mice with Cre recombinase expressed under the control of the promoter for choline acetyltransferase (ChAT) has allowed experimental manipulation of cholinergic circuits. However, currently available ChAT^Cre mouse lines are on the C57BL/6J strain background, which shows early onset age-related hearing loss attributed to the Cdh23^753A mutation (a.k.a., the ahl mutation). To develop ChAT^Cre mice without accelerated hearing loss, we backcrossed ChAT^IRES-Cre mice with CBA/CaJ mice that have normal hearing. We used genotyping to obtain mice homozygous for ChAT^IRES-Cre and the wild-type allele at the Cdh23 locus (ChAT^Cre,Cdh23WT). In the new line, auditory brainstem response thresholds were ∼20 dB lower than those in 9 month old ChAT^IRES-Cre mice at all frequencies tested (4-31.5 kHz). These thresholds were stable throughout the period of testing (3-12 months of age). We then bred ChAT^Cre,Cdh23WT animals with Ai14 reporter mice to confirm the expression pattern of ChAT^Cre. In these mice, tdTomato-labeled cells were observed in all brainstem regions known to contain cholinergic cells. We then stained the tissue with a neuron-specific marker, NeuN, to determine whether Cre expression was limited to neurons. Across several brainstem nuclei (pontomesencephalic tegmentum, motor trigeminal and facial nuclei), 100% of the tdTomato-labeled cells were double-labeled with anti-NeuN (n = 1896 cells), indicating Cre-recombinase was limited to neurons. Almost all of these cells (1867/1896 = 98.5%) also stained with antibodies against ChAT, indicating that reporter label was expressed almost exclusively in cholinergic neurons. Finally, an average 88.7% of the ChAT+ cells in these nuclei were labeled with tdTomato, indicating that the Cre is expressed in a large proportion of the cholinergic cells in these nuclei. We conclude that the backcrossed ChAT^Cre,Cdh23WT mouse line has normal hearing and expresses Cre recombinase almost exclusively in cholinergic neurons. This ChAT^Cre,Cdh23WT mouse line may provide an opportunity to manipulate cholinergic circuits without the confound of accelerated hearing loss associated with the C57BL/6J background. Furthermore, comparison with lines that do show early hearing loss may provide insight into possible cholinergic roles in age-related hearing loss.

MicroRNAs in acquired sensorineural hearing loss

OBJECTIVE

This review summarises the current literature on the role of microRNAs in presbyacusis (age-related hearing loss) and sudden sensorineural hearing loss.

METHODS

Medline, PubMed, Web of Science and Embase databases were searched for primary English-language studies, published between 2000 and 2017, which investigated the role of microRNAs in the pathogenesis of presbyacusis or sudden sensorineural hearing loss. Quality of evidence was assessed using the National Institutes of Health quality assessment tool.

RESULTS

Nine of 207 identified articles, 6 of good quality, satisfied the review's inclusion criteria. In presbyacusis, microRNAs in pro-apoptotic and autophagy pathways are upregulated, while microRNAs in proliferative and differentiation pathways are downregulated. Evidence for microRNAs having an aetiological role in sudden hearing loss is limited.

CONCLUSION

A shift in microRNA expression, leading to reduced cellular activity and impaired inner-ear homeostasis, may contribute to the pathogenesis of presbyacusis.

Advances in understanding of presbycusis

The pathophysiology of age-related hearing loss (ARHL), or presbycusis, involves a complex interplay between environmental and genetic factors. The fundamental biomolecular mechanisms of ARHL have been well described, including the roles of membrane transport, reactive oxygen species, cochlear synaptopathy, vascular insults, hormones, and microRNA, to name a few. The genetic basis underlying these mechanisms remains under-investigated and poorly understood. The emergence of genome-wide association studies has allowed for the identification of specific groups of genes involved in ARHL. This review highlights recent advances in understanding of the pathogenesis of ARHL, the genetic basis underlying these processes and suggests future directions for research and potential therapeutic avenues.

Modulation of miR-34a/SIRT1 signaling protects cochlear hair cells against oxidative stress and delays age-related hearing loss through coordinated regulation of mitophagy and mitochondrial biogenesis

Mitophagy and mitochondrial biogenesis are 2 pathways that regulate mitochondrial content and metabolism maintaining cellular homeostasis. The imbalance between these opposing processes impairs mitochondrial function and is suggested to be the pathophysiological basis of a variety of neurodegenerative diseases and aging. Here we investigated the role of mitophagy and mitochondrial biogenesis in oxidative damage to the cochlear hair cells and age-related hearing loss. In cultured mouse House Ear Institute-Organ of Corti 1 hair cells, oxidative stress activated mitophagy but inhibited mitochondrial biogenesis and impaired mitochondrial function. Pharmacological inhibition of miR-34a/SIRT1 signaling enhanced mitophagy, mitochondrial biogenesis, and attenuated House Ear Institute-Organ of Corti 1 cell death induced by oxidative stress. In the cochlea of C57BL/6 mice, mitophagy and mitochondrial biogenesis were both upregulated during aging. Long-term supplementation with resveratrol, a SIRT1 activator, not only improved the balance between mitophagy and mitochondrial biogenesis but also significantly reduced age-related cochlear hair cell loss, spiral ganglion neuron loss, stria vascularis atrophy, and hearing threshold shifts in C57BL/6 mice. Moreover, SIRT1 overexpression or miR-34a deficiency both attenuated age-related cochlear hair cell loss and hearing loss in C57BL/6 mice. Our findings reveal that imbalance between mitophagy and mitochondrial biogenesis contributes to cochlea hair cell damage caused by oxidative stress and during aging. Coordinated regulation of these 2 processes by miR-34a/SIRT1 signaling might serve as a promising approach for the treatment of age-related cochlear degeneration and hearing loss.

Association of Nutritional Factors with Hearing Loss

Hearing loss (HL) is a major public health problem. Nutritional factors can affect a variety of diseases, such as HL, in humans. Thus far, several studies have evaluated the association between nutrition and hearing. These studies found that the incidence of HL was increased with the lack of single micro-nutrients such as vitamins A, B, C, D and E, and zinc, magnesium, selenium, iron and iodine. Higher carbohydrate, fat, and cholesterol intake, or lower protein intake, by individuals corresponded to poorer hearing status. However, higher consumption of polyunsaturated fatty acids corresponded to better hearing status of studied subjects. In addition to malnutrition, obesity was reported as a risk factor for HL. In studies of the relationship between middle ear infection and nutrition in children, it was reported that lack of vitamins A, C and E, and zinc and iron, resulted in poorer healing status due to vulnerability to infection. These studies indicate that various nutritional factors can affect hearing. Therefore, considering that multifactorial nutritional causes are responsible, in part, for HL, provision of proper guidelines for maintaining a proper nutritional status is expected to prevent some of the causes and burden of HL.

The microRNA-183/96/182 Cluster is Essential for Stereociliary Bundle Formation and Function of Cochlear Sensory Hair Cells

The microRNA (miR)-183/96/182 cluster plays important roles in the development and functions of sensory organs, including the inner ear. Point-mutations in the seed sequence of miR-96 result in non-syndromic hearing loss in both mice and humans. However, the lack of a functionally null mutant has hampered the evaluation of the cluster’s physiological functions. Here we have characterized a loss-of-function mutant mouse model (miR-183C^GT/GT), in which the miR-183/96/182 cluster gene is inactivated by a gene-trap (GT) construct. The homozygous mutant mice show profound congenital hearing loss with severe defects in cochlear hair cell (HC) maturation, alignment, hair bundle formation and the checkboard-like pattern of the cochlear sensory epithelia. The stereociliary bundles retain an immature appearance throughout the cochlea at postnatal day (P) 3 and degenerate soon after. The organ of Corti of mutant newborn mice has no functional mechanoelectrical transduction. Several predicted target genes of the miR-183/96/182 cluster that are known to play important roles in HC development and function, including Clic5, Rdx, Ezr, Rac1, Myo1c, Pvrl3 and Sox2, are upregulated in the cochlea. These results suggest that the miR-183/96/182 cluster is essential for stereociliary bundle formation, morphogenesis and function of the cochlear HCs.

Cognitive Decline, Dementia, Alzheimer's Disease and Presbycusis: Examination of the Possible Molecular Mechanism

The incidences of presbycusis and dementia are high among geriatric diseases. Presbycusis is the general term applied to age-related hearing loss and can be caused by many risk factors, such as noise exposure, smoking, medication, hypertension, family history, and other factors. Mutation of mitochondrial DNA in hair cells, spiral ganglion cells, and stria vascularis cells of the cochlea is the basic mechanism of presbycusis. Dementia is a clinical syndrome that includes the decline of cognitive and conscious states and is caused by many neurodegenerative diseases, of which Alzheimer’s disease (AD) is the most common. The amyloid cascade hypothesis and tau hypothesis are the two major hypotheses that describe the AD pathogenic mechanism. Recent studies have shown that deposition of Aβ and hyperphosphorylation of the tau protein may cause mitochondrial dysfunction. An increasing number of papers have reported that, on one hand, the auditory system function in AD patients is damaged as their cognitive ability declines and that, on the other hand, hearing loss may be a risk factor for dementia and AD. However, the relationship between presbycusis and AD is still unknown. By reviewing the relevant literature, we found that the SIRT1-PGC1α pathway and LKB1 (or CaMKKβ)-AMPK pathway may play a role in the preservation of cerebral neuron function by taking part in the regulation of mitochondrial function. Then vascular endothelial growth factor signal pathway is activated to promote vascular angiogenesis and maintenance of the blood–brain barrier integrity. Recently, experiments have also shown that their expression levels are altered in both presbycusis and AD mouse models. Therefore, we propose that exploring the specific molecular link between presbycusis and AD may provide new ideas for their prevention and treatment.

Intracellular Regulome Variability Along the Organ of Corti: Evidence, Approaches, Challenges, and Perspective

The mammalian hearing organ is a regular array of two types of hair cells (HCs) surrounded by six types of supporting cells. Along the tonotopic axis, this conserved radial array of cell types shows longitudinal variations to enhance the tuning properties of basilar membrane. We present the current evidence supporting the hypothesis that quantitative local variations in gene expression profiles are responsible for local cell responses to global gene manipulations. With the advent of next generation sequencing and the unprecedented array of technologies offering high throughput analyses at the single cell level, transcriptomics will become a common tool to enhance our understanding of the inner ear. We provide an overview of the approaches and landmark studies undertaken to date to analyze single cell variations in the organ of Corti and discuss the current limitations. We next provide an overview of the complexity of known regulatory mechanisms in the inner ear. These mechanisms are tightly regulated temporally and spatially at the transcription, RNA-splicing, mRNA-regulation, and translation levels. Understanding the intricacies of regulatory mechanisms at play in the inner ear will require the use of complementary approaches, and most probably, a combinatorial strategy coupling transcriptomics, proteomics, and epigenomics technologies. We highlight how these data, in conjunction with recent insights into molecular cell transformation, can advance attempts to restore lost hair cells.

Erythrocyte folate, serum vitamin B12, and hearing loss in the 2003-2004 National Health And Nutrition Examination Survey (NHANES)

BACKGROUND/OBJECTIVES

Studies based on food frequency questionnaires suggest that folate and vitamin B12 intake could protect against hearing loss. We investigated whether erythrocyte folate and serum vitamin B12 levels are independently associated with hearing loss in humans.

SUBJECTS/METHODS

Participants in the 2003-2004 US National Health and Nutrition Examination Survey who had data on hearing, folate, and vitamin B12 levels were included. Pure-tone average (PTA) at 0.5, 1.0, 2.0, and 4.0 kHz was computed for each ear. We used weighted logistic regression to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for the relation between quartiles of folate and vitamin B12, and hearing loss (present if PTA > 25 dB in either ear and absent if PTA ≤ 25 dB in both ears).

RESULTS

Participants (n = 1149) were 20-69 (mean 42) years old and 16.4% had hearing loss in at least one ear. Our data suggest a U-shaped relationship between folate and hearing loss. Compared to the 1st quartile, the ORs (95% CIs) for hearing loss were 0.87 (0.49-1.53), 0.70 (0.49-1.00), and 1.08 (0.61-1.94) for the 2nd, 3rd, and 4th quartile of erythrocyte folate in analyses adjusted for age, sex, vitamin B12, smoking, alcohol use, body mass index, race/ethnicity, exposure to noise, income, and education. Although we observed inverse associations between vitamin B12 and hearing loss, the associations were not statistically significant (P > 0.05).

CONCLUSIONS

Our data show a U-shaped relationship between erythrocyte folate levels and hearing loss, suggesting a need to evaluate whether optimizing blood folate levels could prevent hearing loss.

The Role of MicroRNAs in Environmental Risk Factors, Noise-Induced Hearing Loss, and Mental Stress

SIGNIFICANCE

MicroRNAs (miRNAs) are important regulators of gene expression and define part of the epigenetic signature. Their influence on every realm of biomedicine is established and progressively increasing. The impact of environment on human health is enormous. Among environmental risk factors impinging on quality of life are those of chemical nature (toxic chemicals, heavy metals, pollutants, and pesticides) as well as those related to everyday life such as exposure to noise or mental and psychosocial stress. Recent Advances: This review elaborates on the relationship between miRNAs and these environmental risk factors.

CRITICAL ISSUES

The most relevant facts underlying the role of miRNAs in the response to these environmental stressors, including redox regulatory changes and oxidative stress, are highlighted and discussed. In the cases wherein miRNA mutations are relevant for this response, the pertinent literature is also reviewed.

FUTURE DIRECTIONS

We conclude that, even though in some cases important advances have been made regarding close correlations between specific miRNAs and biological responses to environmental risk factors, a need for prospective large-cohort studies is likely necessary to establish causative roles. Antioxid. Redox Signal. 28, 773-796.

Hearing impairment in MELAS: new prospective in clinical use of microRNA, a systematic review

AIM

To evaluate the feasibility of microRNAs (miR) in clinical use to fill in the gap of current methodology commonly used to test hearing impairment in MELAS patients.

MATERIAL AND METHOD

A literature review was performed using the following keywords, i.e., MELAS, Hearing Loss, Hearing Impairment, Temporal Bone, Otoacustic Emission (OTOAE), Auditory Brain Response (ABR), and microRNA. We reviewed the literature and focused on the aspect of the temporal bone, the results of electrophysiological tests in human clinical studies, and the use of miR for detecting lesions in the cochlea in patients with MELAS.

RESULTS

In patients with MELAS, Spiral Ganglions (SG), stria vascularis (SV), and hair cells are damaged, and these damages affect in different ways various structures of the temporal bone. The function of these cells is typically investigated using OTOAE and ABR, but in patients with MELAS these tests provide inconsistent results, since OTOAE response is absent and ABR is normal. The normal ABR responses are unexpected given the SG loss in the temporal bone. Recent studies in humans and animals have shown that miRs, and in particular miRs 34a, 29b, 76, 96, and 431, can detect damage in the cells of the cochlea with high sensitivity. Studies that focus on the temporal bone aspects have reported that miRs increase is correlated with the death of specific cells of the inner ear. MiR - 9/9* was identified as a biomarker of human brain damage, miRs levels increase might be related to damage in the central auditory pathways and these increased levels could identify the damage with higher sensitivity and several months before than electrophysiological testing.

CONCLUSION

We suggest that due to their accuracy and sensitivity, miRs might help monitor the progression of SNHL in patients with MELAS.

MicroRNAs and Presbycusis

Presbycusis (age-related hearing loss) is the most universal sensory degenerative disease in elderly people caused by the degeneration of cochlear cells. Non-coding microRNAs (miRNAs) play a fundamental role in gene regulation in almost every multicellular organism, and control the aging processes. It has been identified that various miRNAs are up- or down-regulated during mammalian aging processes in tissue-specific manners. Most miRNAs bind to specific sites on their target messenger-RNAs (mRNAs) and decrease their expression. Germline mutation may lead to dysregulation of potential miRNAs expression, causing progressive hair cell degeneration and age-related hearing loss. Therapeutic innovations could emerge from a better understanding of diverse function of miRNAs in presbycusis. This review summarizes the relationship between miRNAs and presbycusis, and presents novel miRNAs-targeted strategies against presbycusis.

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.

Expression Profiling of MicroRNAs in the Inner Ear of Elderly People by Real-Time PCR Quantification

The molecular mechanisms underlying age-related hearing loss are unknown, and currently, there is no treatment for this condition. Recent studies have shown that microRNAs (miRNAs) and age-related diseases are intimately linked, suggesting that some miRNAs may present attractive therapeutic targets. In this study, we obtained 8 human temporal bones from 8 elderly subjects at brain autopsy in order to investigate the expression profile of miRNAs in the inner ear with miRNA arrays. A mean of 478 different miRNAs were expressed in the samples, of which 348 were commonly expressed in all 8 samples. Of these, levels of 16 miRNAs significantly differed between young elderly and old elderly subjects. miRNAs, which play important roles in inner ear development, were detected in all samples, i.e., in both young and old elderly subjects, whether with or without hearing loss. Our results suggest that these miRNAs play important roles not only in development, but also in the maintenance of inner ear homeostasis.

MicroRNAs in Hearing Disorders: Their Regulation by Oxidative Stress, Inflammation and Antioxidants

MicroRNAs (miRs) are small non-coding single-stranded RNAs that bind to their complimentary sequences in the 3′-untranslated regions (3′-UTRs) of the target mRNAs that prevent their translation into the corresponding proteins. Since miRs are strongly expressed in cells of inner ear and play a role in regulating their differentiation, survival and function, alterations in their expression may be involved in the pathogenesis of hearing disorders. Although increased oxidative stress and inflammation are involved in initiation and progression of hearing disorders, it is unknown whether the mechanisms of damage produced by these biochemical events on inner ear cells are mediated by altering the expression of miRs. In neurons and non-neuronal cells, reactive oxygen species (ROS) and pro-inflammatory cytokines mediate their damaging effects by altering the expression of miRs. Preliminary data indicate that a similar mechanism of damage on hair cells produced by oxidative stress may exist in this disease. Antioxidants protect against hearing disorders induced by ototoxic agents or adverse health conditions; however, it is unknown whether the protective effects of antioxidants in hearing disorders are mediated by changing the expression of miRs. Antioxidants protect mammalian cells against oxidative damage by changing the expression of miRs. Therefore, it is proposed that a similar mechanism of protection by antioxidants against stress may be found in hearing disorders. This review article discusses novel concepts: (a) alterations in the expression of miRs may be involved in the pathogenesis of hearing disorders; (b) presents evidence from neurons and glia cells to show that oxidative stress and pro-inflammatory cytokines mediate their damaging effects by altering the expression of miRs; and proposes that a similar mechanism of damage by these biochemical events may be found in hearing loss; and (c) present data to show that antioxidants protect mammalian cells against oxidative by altering the expression of miRs. A similar role of antioxidants in protecting against hearing disorders is put forward. New studies are proposed to fill the gaps in the areas listed above.

Apoptosis in inner ear sensory hair cells

Apoptosis, or controlled cell death, is a normal part of cellular lifespan. Cell death of cochlear hair cells causes deafness; an apoptotic process that is not well understood. Worldwide, 1.3 billion humans suffer some form of hearing loss, while 360 million suffer debilitating hearing loss as a direct result of the absence of these cochlear hair cells (Worldwide Hearing, 2014). Much is known about apoptosis in other systems and in other cell types thanks to studies done since the mid-20th century. Here we review current literature on apoptosis in general, and causes of deafness and cochlear hair cells loss as a result of apoptosis. The family of B-cell lymphoma (Bcl) proteins are among the most studied and characterized. We will review current literature on the Bcl2 and Bcl6 protein interactions in relation to apoptosis and their possible roles in vulnerability and survival of cochlear hair cells.

MiR-34a contributes to diabetes-related cochlear hair cell apoptosis via SIRT1/HIF-1α signaling

Type 2 diabetes (T2DM) has been considered to be associated with a higher likelihood of hearing impairment (HI). However, the molecular mechanisms underlying the association between diabetes and HI are poorly understood. MicroRNAs have recently been demonstrated to be closely associated with hearing loss and considered as promising therapeutic targets. Herein, we investigated whether miR-34a contributes to diabetes-related cochlear hair cell apoptosis and sought to identify the underlying mechanism. The results showed that miR-34a was up-regulated in the cochleas of db/db mice, accompanied by significant hearing threshold elevation and hair cell loss. However, the expression of SIRT1 was significantly down-regulated, while hypoxia-inducible factor-1alpha (HIF-1α) levels were dramatically increased in the cochleas of db/db mice. In addition, in the high-glucose cultured House Ear Institute-Organ of Corti 1 (HEI-OC1) cell line, miR-34a overexpression inhibited sirtuin1 (SIRT1) expression, increased HIF-1α levels and promoted apoptosis. MiR-34a knockdown exerted effects that were diametrically opposed to those observed with overexpression. Interestingly, HIF-1α knockdown almost eliminated the cell apoptosis induced by high glucose levels. We also examined the modulation of HIF-1α expression by SIRT1. The results showed that SIRT1 knockdown further promoted high-glucose-induced HIF-1α expression, while SIRT1 overexpression significantly inhibited HIF-1α level induced by high glucose. These findings point to a new mechanism by which miR-34a exerts its detrimental effects by negatively regulating SIRT1/HIF-1α signaling and provide new therapeutic targets for treating hearing impairment during diabetes.

Insights into inner ear-specific gene regulation: Epigenetics and non-coding RNAs in inner ear development and regeneration

The vertebrate inner ear houses highly specialized sensory organs, tuned to detect and encode sound, head motion and gravity. Gene expression programs under the control of transcription factors orchestrate the formation and specialization of the non-sensory inner ear labyrinth and its sensory constituents. More recently, epigenetic factors and non-coding RNAs emerged as an additional layer of gene regulation, both in inner ear development and disease. In this review, we provide an overview on how epigenetic modifications and non-coding RNAs, in particular microRNAs (miRNAs), influence gene expression and summarize recent discoveries that highlight their critical role in the proper formation of the inner ear labyrinth and its sensory organs. Finally, we discuss recent insights into how epigenetic factors and miRNAs may facilitate, or in the case of mammals, restrict inner ear sensory hair cell regeneration.

Genetics of presbycusis and presbystasis

Presbycusis and presbystasis represent relevant problems of aging, caused by the increase in life expectancy in developed countries. As such, it is advantageous to better understand the physiopathological mechanisms of these age-related inner ear diseases. The hypothesis that presbycusis and presbystasis have a genetic background was proposed some years ago. Several studies (in humans and animals) are available in the literature, and possible genes involved in the physiopathology of both diseases have been identified. The aim of this paper is to present an overview of the information available in the current medical literature on presbycusis and presbystasis.

MicroRNAs: effective elements in ear-related diseases and hearing loss

miRNAs are important factors for post-transcriptional process that controls gene expression at mRNA level. Various biological processes, including growth and differentiation, are regulated by miRNAs. miRNAs have been demonstrated to play an essential role in development and progression of hearing loss. Nowadays, miRNAs are known as critical factors involved in different physiological, biological, and pathological processes, such as gene expression, progressive sensorineural hearing loss, age-related hearing loss, noise-induced hearing loss, cholesteatoma, schwannomas, and inner ear inflammation. The miR-183 family (miR-183, miR-96 and miR-182) is expressed abundantly in some types of sensory cells in inner ear specially mechanosensory hair cells that exhibit a great expression level of this family. The plasma levels of miR-24-3p, miR-16-5p, miR-185-5p, and miR-451a were upregulated during noise exposures, and increased levels of miR-21 have been found in vestibular schwannomas and human cholesteatoma. In addition, upregulation of pro-apoptotic miRNAs and downregulation of miRNAs which promote differentiation and proliferation in age-related degeneration of the organ of Corti may potentially serve as a helpful biomarker for the early detection of age-related hearing loss. This knowledge represents miRNAs as promising diagnostic and therapeutic tools in the near future.

Organ of Corti and Stria Vascularis: Is there an Interdependence for Survival?

Cochlear hair cells and the stria vascularis are critical for normal hearing. Hair cells transduce mechanical stimuli into electrical signals, whereas the stria is responsible for generating the endocochlear potential (EP), which is the driving force for hair cell mechanotransduction. We questioned whether hair cells and the stria interdepend for survival by using two mouse models. Atoh1 conditional knockout mice, which lose all hair cells within four weeks after birth, were used to determine whether the absence of hair cells would affect function and survival of stria. We showed that stria morphology and EP remained normal for long time despite a complete loss of all hair cells. We then used a mouse model that has an abnormal stria morphology and function due to mutation of the Mitf gene to determine whether hair cells are able to survive and transduce sound signals without a normal electrochemical environment in the endolymph. A strial defect, reflected by missing intermediate cells in the stria and by reduction of EP, led to systematic outer hair cell death from the base to the apex after postnatal day 18. However, an 18-mV EP was sufficient for outer hair cell survival. Surprisingly, inner hair cell survival was less vulnerable to reduction of the EP. Our studies show that normal function of the stria is essential for adult outer hair cell survival, while the survival and normal function of the stria vascularis do not depend on functional hair cells.

Cellular mechanisms of noise-induced hearing loss

Exposure to intense sound or noise can result in purely temporary threshold shift (TTS), or leave a residual permanent threshold shift (PTS) along with alterations in growth functions of auditory nerve output. Recent research has revealed a number of mechanisms that contribute to noise-induced hearing loss (NIHL). The principle cause of NIHL is damage to cochlear hair cells and associated synaptopathy. Contributions to TTS include reversible damage to hair cell (HC) stereocilia or synapses, while moderate TTS reflects protective purinergic hearing adaptation. PTS represents permanent damage to or loss of HCs and synapses. While the substrates of HC damage are complex, they include the accumulation of reactive oxygen species and the active stimulation of intracellular stress pathways, leading to programmed and/or necrotic cell death. Permanent damage to cochlear neurons can also contribute to the effects of NIHL, in addition to HC damage. These mechanisms have translational potential for pharmacological intervention and provide multiple opportunities to prevent HC damage or to rescue HCs and spiral ganglion neurons that have suffered injury. This paper reviews advances in our understanding of cellular mechanisms that contribute to NIHL and their potential for therapeutic manipulation.

miR-29b overexpression induces cochlear hair cell apoptosis through the regulation of SIRT1/PGC-1α signaling: Implications for age-related hearing loss

It has been reported that the degeneration of cochlear hair cells is the typical cause of presbycusis (or age-related hearing loss). However, the molecular mechanisms that mediate cochlear hair cell apoptosis are not yet fully understood and there is no effective treatment for this disorder. MicroRNAs (miRNAs or miRs) have been increasingly shown to be associated with age-related diseases and are emerging as promising therapeutic targets. In this study, we investigated whether miR-29b is involved in the degeneration of cochlear hair cells. To examine our hypothesis, nuclear staining and terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) were used to quantify the hair cell counts. RT-qPCR and western blot analysis were used to examine miR-29b/sirtuin 1 (SIRT1)/proliferator-activated receptor-gamma coactivator 1α (PGC-1α) signaling in cochlear hair cells. We found that there was a significant degeneration of cochlear hair cells and a higher expression of miR-29b in aged C57BL/6 mice compared with young mice. There was also an age-related decrease in the expression of SIRT1 and PGC-1α. In the inner ear cell line, HEI-OC1, miR-29b overexpression (by transfection with miR-29b mimic) inhibited SIRT1 and PGC-1α expression, leading to an increase in mitochondrial dysfunction and apoptosis. Moreover, the inhibition of miR-29b (by transfection with miR-29b inhibitor) increased SIRT1 and PGC-1α expression, while it decreased apoptosis. Taken together, our findings support a link between age-related cochlear hair cell apoptosis and miR-29b/SIRT1/PGC-1α signaling, which may present an attractive pharmacological target for the development of novel drugs for the treatment of age-related hearing loss.

MiR-182-5p protects inner ear hair cells from cisplatin-induced apoptosis by inhibiting FOXO3a

Cisplatin is widely used for chemotherapy of a variety of malignancies. However, the clinical application of cisplatin is hampered by the resultant irreversible hearing loss due to hair cell apoptosis. To date, no practical regimen to resolve this has been developed. Meanwhile, the role of microRNA in protecting hair cells from cisplatin-induced apoptosis in the inner ear has not been extensively investigated. In this study, we monitored miR-183, -96, and -182 turnover in the cochlea during cisplatin treatment in vitro. We found that overexpression of miR-182, but not miR-183 and -96, improved hair cell survival after 3 μM cisplatin treatment in vitro. We demonstrated that overexpression of miR-182 repressed the intrinsic apoptotic pathway by inhibiting the translation of FOXO3a. Our study offers a new therapeutic target for alleviating cisplatin-induced hair cell apoptosis in a rapid and tissue-specific manner.