Reduced expression of Connexin26 and its DNA promoter hypermethylation in the inner ear of mimetic aging rats induced by d-galactose

Whole-Genome DNA Methylation Analysis in Age-Related Hearing Loss

BACKGROUND

Presbycusis, also known as age-related hearing loss (ARHL), is the most frequent sensory disability affecting elderly adults worldwide. ARHL is characterized by bilateral, progressive, sensorineural hearing loss that is more pronounced at a high frequency. Conventional factors associated with ARHL include diabetes, hypertension, and a family history of hearing loss. The severity of hearing impairment varies between individuals. The defined causative molecular pathogenesis for ARHL is unknown, thus the identification of underlying pathogenic mechanisms involved in ARHL is imperative for the development of effective therapeutic approaches. Epigenetics is the study of phenotypic changes caused by the modification of gene expression rather than the alteration of a DNA sequence. While it is hypothesized that ARHL could result from undiscovered epigenetic susceptibility, there is a shortage of information on the role that epigenetic modification plays in ARHL. Here we present an investigation on the involvement of DNA methylation in ARHL.

RESULTS

Clinical, audiometric and DNA testing, and high-throughput methylation pattern screening were undertaken for ARHL patients and matched control subjects. Our results demonstrate a strong correlation between patients' hearing measurements and methylation at CpG sites cg1140494 (ESPN) and cg27224823 (TNFRSF25). We identified 136 differentially methylated CpGs that were shared between a high and low audiometric frequency in the patient's cohort. CpG cites in hearing loss candidate genes, KCNQ1, TMEM43, GSTM1, TCF25, and GSR, were found to be highly methylated in presbycusis patients as compared to the controls. A methylation polymerase chain reaction (PCR) assay was used to confirm methylation levels at a specific gene locus in ARHL patients and controls.

CONCLUSIONS

Altered DNA methylation and its impact on gene expression has been implicated in many biological processes. By interrogating the methylation status across the genome of both hearing loss patients and those with normal hearing, our study can help to establish an association between the audiometric patterns and methylation status in ARHL, yielding new avenues for the identification of potential candidate genes for hearing loss.

Regulatory mechanisms of connexin26

Connexins are essential for cellular communication and play a critical role in various physiological processes, including hearing. Connexin26 (Cx26), encoded by the GJB2 gene, is a key component of cochlear gap junctions and is vital for potassium recycling and ATP release-both of which are vital for auditory function. Mutations in GJB2 are the primary cause of sensorineural hearing loss. However, the phenotypic variability observed in individuals with the same mutation suggests the involvement of other complex regulatory factors. While the regulatory mechanisms of Connexin43 have been extensively studied, research on the mechanisms of Cx26 remains limited. This review summarizes the reported regulatory mechanisms of GJB2 from multiple perspectives, both pre- and post-transcription, in an effort to explore ways to regulate connexin expression and provide new insights into gene therapy for diseases caused by alterations in connexin levels.

Degradation of cochlear Connexin26 accelerate the development of age-related hearing loss

The GJB2 gene, encoding Connexin26 (Cx26), is one of the most common causes of inherited deafness. Clinically, mutations in GJB2 cause congenital deafness or late-onset progressive hearing loss. Recently, it has been reported that Cx26 haploid deficiency accelerates the development of age-related hearing loss (ARHL). However, the roles of cochlear Cx26 in the hearing function of aged animals remain unclear. In this study, we revealed that the Cx26 expression was significantly reduced in the cochleae of aged mice, and further explored the underlying molecular mechanism for Cx26 degradation. Immunofluorescence co-localization results showed that Cx26 was internalized and degraded by lysosomes, which might be one of the important ways for Cx26 degradation in the cochlea of aged mice. Currently, whether the degradation of Cx26 in the cochlea leads directly to ARHL, as well as the mechanism of Cx26 degradation-related hearing loss are still unclear. To address these questions, we generated mice with Cx26 knockout in the adult cochlea as a model for the natural degradation of Cx26. Auditory brainstem response (ABR) results showed that Cx26 knockout mice exhibited high-frequency hearing loss, which gradually progressed over time. Pathological examination also revealed the degeneration of hair cells and spiral ganglions, which is similar to the phenotype of ARHL. In summary, our findings suggest that degradation of Cx26 in the cochlea accelerates the occurrence of ARHL, which may be a novel mechanism of ARHL.

The role of epigenetic modifications in sensory hair cell development, survival, and regulation

The cochlea is the sensory organ in the periphery, and hair cells are its main sensory cells. The development and survival of hair cells are highly controlled processes. When cells face intracellular and environmental stimuli, epigenetic regulation controls the structure and function of the genome in response to different cell fates. During sensory hair cell development, different histone modifications can induce normal numbers of functional hair cells to generate. When individuals are exposed to environmental-related hair cell damage, epigenetic modification also plays a significant role in the regulation of hair cell fate. Since mammalian hair cells cannot regenerate, their loss can cause permanent sensorineural hearing loss. Many breakthroughs have been achieved in recent years in understanding the signaling pathways that determine hair cell regeneration, and it is fascinating to note that epigenetic regulation plays a significant role in hair cell regeneration. In this review, we discuss the role of epigenetics in inner ear cell development, survival and regeneration and the significant impact on hearing protection.

UHRF1-induced connexin26 methylation is involved in hearing damage triggered by intermittent hypoxia in neonatal rats

Ubiquitin-like with plant homeodomain and ring finger domains 1 (UHRF1) promotes the maintenance of established patterns of DNA methylation in mammalian cells. Extensive methylation of connexin26 (COX26) during hearing impairment has been demonstrated. The present study aims to determine whether UHRF1 can induce the methylation of COX26 in cochlea damaged by intermittent hypoxia (IH). After the establishment of the cochlear injury model through IH treatment or isolation of the cochlea containing Corti's organ, pathological changes were observed via HE staining. Expressions of COX26 and UHRF1 were detected by quantitative reverse-transcription polymerase chain reaction and Western blot. The effect of COX26 methylation levels was analyzed by methylation-specific PCR (MSP). Phalloidin/immunofluorescence staining was used to observe structural changes. The binding relationship between UHRF1 and COX26 was verified by chromatin immunoprecipitation. IH caused cochlear damage, accompanied by increased methylation of COX26 and expression of UHRF1 in the cochlea of neonatal rats. CoCl₂ treatment caused the loss of cochlear hair cells, downregulation and hypermethylation of COX26, abnormal upregulation of UHRF1, and disordered expressions of apoptosis-related proteins. UHRF1 in cochlear hair cells binds to COX26, and its knockdown upregulated COX26 level. Overexpressed COX26 partially alleviated the CoCl₂-caused cell damage. UHRF1 induces COX26 methylation and aggravates the cochlear damage caused by IH.

Hearing and Ageing

Age-related hearing loss (ARHL), or presbycusis, occurs in most mammals, humans included, with a different age of onset and magnitude of loss. It is associated with two major symptoms: loss of sensitivity to sound, especially for high pitches, and a reduced ability to understand speech in background noise. This phenomenon involves both the peripheral structures of the inner ear and the central acoustic pathways. Several mechanisms have been identified as pro-ageing in the human cochlea. The main one is the oxidative stress. The inner ear physiological degeneration can be affected by both intrinsic conditions, such as genetic predisposition, and extrinsic ones, such as noise exposure. The magnitude of neuronal loss precedes and exceeds that of inner hair cell loss, which is also less important than the loss of outer hair cells. Patients with HL often develop atrophy of the temporal lobe (auditory cortex) and brain gliosis can contribute to the development of a central hearing loss. The presence of white matter hyperintensities (WMHs) on the MRI, which is radiologic representation of brain gliosis, can justify a central HL due to demyelination in the superior auditory pathways. Recently, the presence of WMHs has been correlated with the inability to correctly understand words in elderly with normal auditory thresholds.

Aging and aging-related diseases: from molecular mechanisms to interventions and treatments

Aging is a gradual and irreversible pathophysiological process. It presents with declines in tissue and cell functions and significant increases in the risks of various aging-related diseases, including neurodegenerative diseases, cardiovascular diseases, metabolic diseases, musculoskeletal diseases, and immune system diseases. Although the development of modern medicine has promoted human health and greatly extended life expectancy, with the aging of society, a variety of chronic diseases have gradually become the most important causes of disability and death in elderly individuals. Current research on aging focuses on elucidating how various endogenous and exogenous stresses (such as genomic instability, telomere dysfunction, epigenetic alterations, loss of proteostasis, compromise of autophagy, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, altered intercellular communication, deregulated nutrient sensing) participate in the regulation of aging. Furthermore, thorough research on the pathogenesis of aging to identify interventions that promote health and longevity (such as caloric restriction, microbiota transplantation, and nutritional intervention) and clinical treatment methods for aging-related diseases (depletion of senescent cells, stem cell therapy, antioxidative and anti-inflammatory treatments, and hormone replacement therapy) could decrease the incidence and development of aging-related diseases and in turn promote healthy aging and longevity.

GJB2 and GJB6 gene transcripts in the human cochlea: A study using RNAscope, confocal, and super-resolution structured illumination microscopy

Background

Gap junction (GJ) proteins, connexin26 and 30, are highly prevalent in the human cochlea (HC), where they are involved in transcellular signaling, metabolic supply, and fluid homeostasis. Their genes, GJB2 and GJB6, are both located at the DFNB1 locus on chromosome 13q12. Mutations in GJB2 may cause mild to profound non-syndromic deafness. Here, we analyzed for the first time the various expressions of GJB2 and GJB6 gene transcripts in the different cell networks in the HC using the RNAscope technique.

Materials and methods

Archival paraformaldehyde-fixed sections of surgically obtained HC were used to label single mRNA oligonucleotides using the sensitive multiplex RNAscope^® technique with fluorescent-tagged probes. Positive and negative controls also included the localization of ATP1A1, ATP1A2, and KCNJ10 gene transcripts in order to validate the specificity of labeling.

Results

Confocal and super-resolution structured illumination microscopy (SR-SIM) detected single gene transcripts as brightly stained puncta. The GJB2 and GJB6 gene transcripts were distributed in the epithelial and connective tissue systems in all three cochlear turns. The largest number of GJB2 and GJB6 gene transcripts was in the outer sulcus, spiral ligament, and stria vascularis (SV). Oligonucleotides were present in the supporting cells of the organ of Corti (OC), spiral limbus fibrocytes, and the floor of the scala vestibuli. Multiplex gene data suggest that cells in the cochlear lateral wall contain either GJB2 or GJB6 gene transcripts or both. The GJB6, but not GJB2, gene transcripts were found in the intermediate cells but none were found in the marginal cells. There were no GJB2 or GJB6 gene transcripts found in the hair cells and only a few in the spiral ganglion cells.

Conclusion

Both GJB2 and GJB6 mRNA gene transcripts were localized in cells in the adult HC using RNAscope^®in situ hybridization (ISH) and high resolution microscopy. Generally, GJB6 dominated over GJB2, except in the basal cells. Results suggest that cells may contain either GJB2 or GJB6 gene transcripts or both. This may be consistent with specialized GJ plaques having separate channel permeability and gating properties. A reduction in the number of GJB2 gene transcripts was found in the basal turn. Such information may be useful for future gene therapy.

Intermittent white noise exposure is associated with rat cochleae damage and changes in the gene expression

Background

Noise, a physical factor in most work environments, has many effects on human health. Exposure to excessive noise can modify the expression of associated genes with NIHL. The aim of this study to elucidate changes in expression of GJB2 and SLC26A4 after exposure to intense noise which are the most frequent causing genes to apparent autosomal recessive non-syndromic hearing loss.

Methods

In this experimental and case–control study, 17 male Wistar rats were randomly divided into exposure groups (n = 12) and without exposure (n = 5). First group was exposed to noise (90–120 dB, 70 Hz–16 kHz, 8 h/day) for 3–6 days. Cochlear biopsies performed 1 h and 1 week post-exposure, relative gene expression levels were calculated using $${2}^{-\Delta \Delta Ct}$$ 2 - Δ Δ C t . From each group, one ear was stained by hematoxylin and eosin method for histopathological survey. Real-time PCR technique took place, and gene expression data were normalized by GAPDH gene. One-way ANOVA test was performed with a significance level of 0.05 by GraphPad prism software.

Results

Both GJB2 and SLC26A4 in all groups were down-regulated after exposure compared to their controls. Fold changes in the highest times were related to 1 week after 6 days of exposure, 0.052 and 0.015, respectively. Serious damages occurred in different parts of cochlea, and they were more severe after 6 days and 1-week later.

Conclusion

It is expected that if the hearing threshold tests be performed before/after exposure and considering longer post-exposure times, subsequently, the expression of these genes does not return to basal level, and irrecoverable damage to the cochlea, progressive and irreversible ARNSHL will be expected.

Transcription Factor Reprogramming in the Inner Ear: Turning on Cell Fate Switches to Regenerate Sensory Hair Cells

Non-mammalian vertebrates can restore their auditory and vestibular hair cells naturally by triggering the regeneration of adjacent supporting cells. The transcription factor ATOH1 is a key regulator of hair cell development and regeneration in the inner ear. Following the death of hair cells, supporting cells upregulate ATOH1 and give rise to new hair cells. However, in the mature mammalian cochlea, such natural regeneration of hair cells is largely absent. Transcription factor reprogramming has been used in many tissues to convert one cell type into another, with the long-term hope of achieving tissue regeneration. Reprogramming transcription factors work by altering the transcriptomic and epigenetic landscapes in a target cell, resulting in a fate change to the desired cell type. Several studies have shown that ATOH1 is capable of reprogramming cochlear non-sensory tissue into cells resembling hair cells in young animals. However, the reprogramming ability of ATOH1 is lost with age, implying that the potency of individual hair cell-specific transcription factors may be reduced or lost over time by mechanisms that are still not clear. To circumvent this, combinations of key hair cell transcription factors have been used to promote hair cell regeneration in older animals. In this review, we summarize recent findings that have identified and studied these reprogramming factor combinations for hair cell regeneration. Finally, we discuss the important questions that emerge from these findings, particularly the feasibility of therapeutic strategies using reprogramming factors to restore human hearing in the future.

Heart and liver connexin expression related to the first stage of aging: A study on naturally aged animals

Connexins are membrane-spanning proteins that form membrane channels and hemichannels. They are involved in the cellular communication and in the maintenance of tissue homeostasis. Recent studies in humans and animals have demonstrated that the expression and distribution of Cx43, the most studied connexin, can change during aging. However, the research on the involvement of the other connexins in cardiac and hepatic aging is, at present, still very poor. Hence, the aim of this study is to evaluate the expression of Cx43 and Cx26 in the heart as well as Cx26 and Cx32 in the liver of a rat model that aged naturally, rather than prematurely because of genetic mutations or age-related diseases. The results obtained in the present study have demonstrated that these connexins decrease in rat cardiomyocytes and in rat hepatocytes as they age. This change was revealed only at protein level, as connexin-mRNAs remained unchanged during aging. Moreover, the aged rats showed an increase in body fat, whose subcutaneous layer tended to be higher. Finally, how these changes could represent signs of physiological adaptation in successful aging was discussed.

Recent advancements in understanding the role of epigenetics in the auditory system

Sensorineural deafness in mammals is most commonly caused by damage to inner ear sensory epithelia, or hair cells, and can be attributed to genetic and environmental causes. After undergoing trauma, many non-mammalian organisms, including reptiles, birds, and zebrafish, are capable of regenerating damaged hair cells. Mammals, however, are not capable of regenerating damaged inner ear sensory epithelia, so that hair cell damage is permanent and can lead to hearing loss. The field of epigenetics, which is the study of various phenotypic changes caused by modification of genetic expression rather than alteration of DNA sequence, has seen numerous developments in uncovering biological mechanisms of gene expression and creating various medical treatments. However, there is a lack of information on the precise contribution of epigenetic modifications in the auditory system, specifically regarding their correlation with development of inner ear (cochlea) and consequent hearing impairment. Current studies have suggested that epigenetic modifications influence differentiation, development, and protection of auditory hair cells in cochlea, and can lead to hair cell degeneration. The objective of this article is to review the existing literature and discuss the advancements made in understanding epigenetic modifications of inner ear sensory epithelial cells. The analysis of the emerging epigenetic mechanisms related to inner ear sensory epithelial cells development, differentiation, protection, and regeneration will pave the way to develop novel therapeutic strategies for hearing loss.

Notch1 signaling mediated dysfunction of bone marrow mesenchymal stem cells derived from cyanotic congenital heart disease

Bone marrow mesenchymal stem cells (BMSCs) derived from cyanotic congenital heart disease (CCHD) exhibit deficient multi-lineage differentiation potential due to the abnormal accumulation of D-galactose. However, the underlying mechanisms have not yet been explored. Here, the multi-lineage differentiation potential of the BMSCs from CCHD and non-CCHD (NCHD) patients were assessed. BMSCs from CCHD patients exhibited inferior multi-lineage differentiation potential with reduced Notch1 protein and mRNA level. Bisulfite sequencing PCR results showed the methylation level of Notch1 promoter was raised, which inhibited the binding of NF-Ya. Exposure BMSCs from NCHD patients with D-galactose under hypoxia (4% O₂) decreased the expression of Notch1. And activating Notch1 partially restored the deficient BMSCs of CCHD patients. In conclusion, the impaired multi-lineage differentiation potential of BMSCs from CCHD patients is owing to the decreased Notch1 level with a remarkable hypermethylation in its promoter region. Activated Notch1 signaling pathway could partially restore the deficient BMSCs in the CCHD patients, which may provide a new method on cell therapy in patients with CCHD.

D-Galactose-induced oxidative stress and mitochondrial dysfunction in the cochlear basilar membrane: an in vitro aging model

The cochlear basilar membrane (CBM) contains inner hair cells and outer hair cells that convert sound waves into electrical signals and transmit them to the central auditory system. Cochlear aging, the primary reason of age-related hearing loss, can reduce the signal transmission capacity. There is no ideal in vitro aging model of the CBM. In this study, we cultured the CBM, which was dissected from the cochlea of the C57BL/6 mice 5 days after birth, in a medium containing 20 mg/mL, 40 mg/mL, or 60 mg/mL D-galactose (D-gal). Compared with the control group, the levels of senescence-associated β-galactosidase were increased in a concentration-dependent manner in the CBM of the D-gal groups. In addition, levels of the mitochondrial superoxide and patterns of an age-related mitochondrial DNA3860-bp deletion were significantly increased. The ATP levels and the membrane potential of the mitochondrial were significantly decreased in the CBM of the D-gal groups compared with the control group. Furthermore, in comparison with the control group, damaged hair cell stereocilia and a loss of inner hair cell ribbon synapses were observed in the CBM of the D-gal groups. A loss of hair cells and activation of caspase-3-mediated outer hair cell apoptosis were also observed in the CBM of the high-dose D-gal group. These insults induced by D-gal in the CBM in vitro were similar to the ones that occur in cochlear natural aging in vivo. Thus, we believe that this is a successful in vitro aging model using cultured CBM. These results demonstrate the effects of mitochondrial oxidative damage on presbycusis and provide a reliable aging model to study the mechanisms of presbycusis in vitro.

Presbycusis: An Update on Cochlear Mechanisms and Therapies

Age-related hearing impairment (ARHI), also referred to as presbycusis, is the most common sensory impairment seen in the elderly. As our cochlea, the peripheral organ of hearing, ages, we tend to experience a decline in hearing and are at greater risk of cochlear sensory-neural cell degeneration and exacerbated age-related hearing impairments, e.g., gradual hearing loss, deterioration in speech comprehension (especially in noisy environments), difficulty in the localization sound sources, and ringing sensations in the ears. However, the aging process does not affect people uniformly; nor, in fact, does the aging process appear to be uniform even within an individual. Here, we outline recent research into chronological cochlear age in healthy people, and exacerbated hearing impairments during aging due to both extrinsic factors including noise and ototoxic medication, and intrinsic factors such as genetic predisposition, epigenetic factors, and aging. We review our current understanding of molecular pathways mediating ARHL and discuss recent discoveries in experimental hearing restoration and future prospects.

Metformin attenuates the D‑galactose‑induced aging process via the UPR through the AMPK/ERK1/2 signaling pathways

Age‑related hearing loss, also termed central presbycusis, is a progressive neurodegenerative disease; it is a devastating disorder that severely affects the quality of life of elderly individuals. Substantial evidence has indicated that oxidative stress and associated protein folding dysfunction have a marked influence on neurodegenerative diseases. In this study, we aimed to cells to investigate whether metformin protects against age‑related pathologies and to elucidate the underlying mechanisms; specifically, we focused on the role of unfolded protein response (UPR) via the AMPK/ERK1/2 signaling pathways. For this purpose, the biguanide compound, metformin, a medication widely used in the treatment of type 2 diabetes, was administered to rats in a model of mimetic aging. In addition, senescent PC12 were treated with metformin. Although it has been well established that UPR signaling is activated in response to cellular stress and is associated with the pathogenesis of neuronal deterioration, the detailed functions of the UPR in the auditory cortex remain unclear. We found that metformin treatment markedly affected the UPR and the AMPK/ERK1/2 signaling pathway, and maintained the auditory brainstem response (ABR) threshold during the aging process. The results indicated that the regulation of the UPR and AMPK/ERK1/2 signaling pathway by metformin significantly attenuated hearing loss, cell apoptosis and age‑related neurodegeneration. Reversing these harmful effects through the use of metformin suggests its involvement in restoring the antioxidant status and protein homeostasis related to the underlying pathology of presbycusis. The findings of this study may provide a better approach for the treatment of age‑related neurodegeneration diseases.

Structure and Function of Cochlear Gap Junctions and Implications for the Translation of Cochlear Gene Therapies

Connexins (Cxs) are ubiquitous membrane proteins that are found throughout vertebrate organs, acting as building blocks of the gap junctions (GJs) known to play vital roles in the normal function of many organs. Mutations in Cx genes (particularly GJB2, which encodes Cx26) cause approximately half of all cases of congenital hearing loss in newborns. Great progress has been made in understanding GJ function and the molecular mechanisms for the role of Cxs in the cochlea. Data reveal that multiple types of Cxs work together to ensure normal development and function of the cochlea. These findings include many aspects not proposed in the classic K+ recycling theory, such as the formation of normal cochlear morphology (e.g., the opening of the tunnel of Corti), the fine-tuning of the innervation of nerve fibers to the hair cells (HCs), the maturation of the ribbon synapses, and the initiation of the endocochlear potential (EP). New data, especially those collected from targeted modification of major Cx genes in the mouse cochlea, have demonstrated that Cx26 plays an essential role in the postnatal maturation of the cochlea. Studies also show that Cx26 and Cx30 assume very different roles in the EP generation, given that only Cx26 is required for normal hearing. This article will review our current understanding of the molecular structure, cellular distribution, and major functions of cochlear GJs. Potential implications of the knowledge of cochlear GJs on the design and implementation of translational studies of cochlear gene therapies for Cx mutations are also discussed.

Characterization of GJB2 cis-regulatory elements in the DFNB1 locus

Although most disease-causing variants are within coding region of genes, it is now well established that cis-acting regulatory sequences, depending on 3D-chromatin organization, are required for temporal and spatial control of gene expression. Disruptions of such regulatory elements and/or chromatin conformation are likely to play a critical role in human genetic disease. Hence, recurrent monoallelic cases, who present the most common hereditary type of nonsyndromic hearing loss (i.e., DFNB1), carry only one identified pathogenic allele. This strongly suggests the presence of uncharacterized distal cis-acting elements in the missing allele. Here within, we study the spatial organization of a large DFNB1 locus encompassing the gap junction protein beta 2 (GJB2) gene, the most frequently mutated gene in this inherited hearing loss phenotype, with the chromosome conformation capture carbon copy technology (5C). By combining this approach with functional activity reporter assays and mapping of CCCTC-binding factor (CTCF) along the DFNB1 locus, we identify a novel set of cooperating GJB2 cis-acting elements and suggest a DFNB1 three-dimensional looping regulation model.

Approaches for the study of epigenetic modifications in the inner ear and related tissues

DNA methylation and histone modifications such as methylation, acetylation, and phosphorylation, are two types of epigenetic modifications that alter gene expression. These additions to DNA regulatory elements or to the tails of histones can be inherited or can also occur de novo. Since epigenetic modifications can have significant effects on various processes at both the cellular and organismal level, there has been a rapid increase in research on this topic throughout all fields of biology in recent years. However, epigenetic research is relativity new for the inner ear field, likely due to the limited number of cells present and their quiescent nature. Here, we provide an overview of methods used to detect DNA methylation and histone modifications with a focus on those that have been validated for use with limited cell numbers and a discussion of the strengths and limitations for each. We also provide examples for how these methods have been used to investigate the epigenetic landscape in the inner ear and related tissues.

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.

Protection and Prevention of Age-Related Hearing Loss

Presbycusis is a sensorineural hearing loss caused by hearing system aging and degeneration. The clinical manifestations are progressive bilateral symmetrical hearing loss, and the hearing curve is mostly slope-shaped with high-frequency reduction, sometimes flat. The results of the second national sample survey of disabled persons (2006) showed that the total number of hearing and speech disability in China was 27.8 million, accounting for 34% of the total number of disabled people in China. Among them are people over 60 years old. There are 20.4541 million people with hearing disabilities. There are 9.49 million senile deaf patients, accounting for 34.1% of the total number of hearing disabilities. As society gradually becomes aging, the incidence of presbycusis is getting higher and higher. The study of its pathogenesis is of great significance for the diagnosis, treatment, and prevention of presbycusis. The rapid progress of molecular biology experimental technology has provided us with a new opportunity to fully understand and reveal the presbycusis. In the near future, early diagnosis of presbycusis-related genes and early prevention or delay of the occurrence and development of presbycusis will become a reality.

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

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

CDH23 Methylation Status and Presbycusis Risk in Elderly Women

Introduction: Presbycusis, an age-related hearing impairment (ARHI) disease, is the most common cause for HI in adults worldwide. One of the best candidate genes for ARHI susceptibility is Cadherin 23 (CDH23) which encodes stereocilia tip-links of the inner ear sensory hair cell. Although alterations in the methylation status of CpG dinucleotides across various genes were reported to be associated with HI, methylation changes in CDH23 gene have not been reported previously.

Objectives: This study aimed at investigating whether DNA methylation level of CDH23 gene at intragenic CpG island overlapping an exonic-intronic region at position chr10:73565570-73565827 (GRCh37/hg19) could be risk factor associated with ARHI.

Materials and Methods: We screened for methylation changes in this particular position for CDH23 gene in 50 blood samples of elderly women affected with presbycusis and healthy control cohort. Methylation of CpG sites were assessed using Quantitative methylation-specific PCR (qMSP) following sodium bisulfite DNA conversion chemistry. Methylation levels were normalized against TSH2B reference gene.

Results: DNA methylation analysis for the common CpG islands in CDH23 gene revealed 3.27-folds significant increase (p < 0.0001) in methylation profile for ARHI women as compared to healthy controls with an elevated risk odds ratio (OR) of 2.219 [95% CI 1.071–4.597].

Conclusion: Our study is the first of its kind to prove that higher CpG site methylation levels in CDH23 gene are likely to be associated with ARHI.

Connexin 26 Immunohistochemistry in Temporal Bones With Cochlear Otosclerosis

HYPOTHESIS

Connexin-26 (Cx26) expression is diminished in the spiral ligament of subjects with hearing loss and cochlear otosclerosis (CO).

BACKGROUND

Human temporal bone (HTB) studies have demonstrated that CO is associated with hyalinization of the spiral ligament. We hypothesize that hyalinization is associated with a loss of fibrocytes with a consequent decline in Cx26 expression. Cx26 and Connexin-30 (Cx30) encode gap junction proteins expressed in supporting cells of the organ of Corti, the spiral limbus, stria vascularis, and in fibrocytes of the spiral ligament. These gap junctions are critical for potassium recycling and maintenance of the endocochlear potential. Diminished expression of these proteins would likely be associated with hearing dysfunction.

METHODS

Histopathology and clinical characteristics of 45 HTB specimens with CO and spiral ligament hyalinization were reviewed. Those with sensorineural or mixed hearing loss but normal or near-normal hair cell counts were analyzed with light microscopy, and Cx26-immunoreactive (IR) signal was qualitatively assessed.

RESULTS

H&E staining demonstrated hyalinization in the spiral ligament and loss of type II and type III fibrocytes. Cx26-IR was diminished throughout the cochlea affected with CO compared with normal controls.

CONCLUSIONS

Cx26-IR reduction in the spiral ligament of subjects with CO likely plays a role in hearing loss.

Down-expression of P2RX2, KCNQ5, ERBB3 and SOCS3 through DNA hypermethylation in elderly women with presbycusis

CONTEXT

Presbycusis, an age-related hearing impairment (ARHI), represents the most common sensory disability in adults. Today, the molecular mechanisms underlying presbycusis remain unclear. This is in particular due to the fact that ARHI is a multifactorial complex disorder resulting from several genomic factors interacting with lifelong cumulative effects of: disease, diet, and environment.

OBJECTIVE

Identification of novel biomarkers for presbycusis.

MATERIALS AND METHODS

We selectively ascertained 18 elderly unrelated women lacking environmental and metabolic risk factors. Subsequently, we screened for methylation map changes in blood samples of women with presbycusis as compared to controls, using reduced representation bisulfite sequencing. We focused on hypermethylated cytosine bases located in gene promoters and the first two exons. To elucidate the related gene expression changes, we performed transcriptomic study using gene expression microarray.

RESULTS

Twenty-seven genes, known to be expressed in adult human cochlea, were found in the blood cells to be differentially hypermethylated with significant (p < 0.01) methylation differences (>30%) and down-expressed with fold change >1.2 (FDR <0.05). Functional annotation and qRT-PCR further identified P2RX2, KCNQ5, ERBB3 and SOCS3 to be associated with the progression of ARHI.

DISCUSSION AND CONCLUSION

Down-expressed genes associated with DNA hypermethylation could be used as biomarkers for understanding complex pathogenic mechanisms underlying presbycusis.

Chronic prenatal hypoxia impairs cochlear development, a mechanism involving connexin26 expression and promoter methylation

Chronic prenatal hypoxia is a damaging to fetal development and may have various consequences, including hearing loss. Connexin 26 (Cx26) is one of the major protein subunits required for gap junction formation, and has an important role in maintaining homeostasis in the cochlea and normal hearing. Cx26 mutation and expression abnormality are closely associated with inherited nonsyndromic deafness, but the association between Cx26 and prenatal hypoxia is less established. The present study aimed to examine Cx26 expression and aberrant methylation the Cx26 promoter region in the cochlea from rats exposed to chronic prenatal hypoxia. Hematoxylin and eosin staining demonstrated that the number of hair cells in the organ of Corti were less in the hypoxia group. Reverse transcription-quantitative polymerase chain reaction and western blot analysis revealed that protein and mRNA levels of Cx26 were decreased in the hypoxia group compared with the control group. Further bisulfite sequencing analysis revealed that prenatal hypoxia significantly increased the methylation status of the promoter region of the Cx26 gene. These results demonstrate that chronic prenatal hypoxia caused hearing impairment, and suggest that promoter region hypermethylation and expression downregulation of Cx26 underlie the mechanism of action.

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.

Role of Specificity Protein-1 and Activating Protein-2 Transcription Factors in the Regulation of the Gap Junction Protein Beta-2 Gene in the Epididymis of the Rat

In prepubertal rats, connexin 26 (GJB2) is expressed between adjacent columnar cells of the epididymis. At 28 days of age, when columnar cells differentiate into adult epithelial cell types, Gjb2 mRNA levels decrease to barely detectable levels. There is no information on the regulation of GJB2 in the epididymis. The present study characterized regulation of the Gjb2 gene promoter in the epididymis. A single transcription start site at position −3829 bp relative to the ATG was identified. Computational analysis revealed several TFAP2A, SP1, and KLF4 putative binding sites. A 1.5-kb fragment of the Gjb2 promoter was cloned into a vector containing a luciferase reporter gene. Transfection of the construct into immortalized rat caput epididymal (RCE-1) cells indicated that the promoter contained sufficient information to drive expression of the reporter gene. Deletion constructs showed that the basal activity of the promoter resides in the first −230 bp of the transcriptional start site. Two response elements necessary for GJB2 expression were identified: an overlapping TFAP2A/SP1 site (−136 to −126 bp) and an SP1 site (−50 bp). Chromatin immunoprecipitation (ChIP) and electrophoretic mobility shift assays confirmed that SP1 and TFAP2A were bound to the promoter. ChIP analysis of chromatin from young and pubertal rats indicated that TFAP2A and SP1 binding decreased with age. SP1 and TFAP2A knockdown indicated that SP1 is necessary for Gjb2 expression. DNA methylation did not appear to be involved in the regulation of Gjb2 expression. Results indicate that SP1 and TFAP2A regulate Gjb2 promoter activity during epididymal differentiation in rat.

Regulation of connexin signaling by the epigenetic machinery

Connexins and their channels are involved in the control of all aspects of the cellular life cycle, ranging from cell growth to cell death, by mediating extracellular, intercellular and intracellular communication. These multifaceted aspects of connexin-related cellular signaling obviously require strict regulation. While connexin channel activity is mainly directed by posttranslational modifications, connexin expression as such is managed by classical cis/trans mechanisms. Over the past few years, it has become clear that connexin production is equally dictated by epigenetic actions. This paper provides an overview of the role of major determinants of the epigenome, including DNA methylation, histone acetylation and microRNA species, in connexin expression.