The role of mitochondrial DNA large deletion for the development of presbycusis in Fischer 344 rats

Mitochondrial Dysfunction and Therapeutic Targets in Auditory Neuropathy

Sensorineural hearing loss (SNHL) becomes an inevitable worldwide public health issue, and deafness treatment is urgently imperative; yet their current curative therapy is limited. Auditory neuropathies (AN) were proved to play a substantial role in SNHL recently, and spiral ganglion neuron (SGN) dysfunction is a dominant pathogenesis of AN. Auditory pathway is a high energy consumption system, and SGNs required sufficient mitochondria. Mitochondria are known treatment target of SNHL, but mitochondrion mechanism and pathology in SGNs are not valued. Mitochondrial dysfunction and pharmacological therapy were studied in neurodegeneration, providing new insights in mitochondrion-targeted treatment of AN. In this review, we summarized mitochondrial biological functions related to SGNs and discussed interaction between mitochondrial dysfunction and AN, as well as existing mitochondrion treatment for SNHL. Pharmaceutical exploration to protect mitochondrion dysfunction is a feasible and effective therapeutics for AN.

Increased burden of mitochondrial DNA deletions and point mutations in early-onset age-related hearing loss in mitochondrial mutator mice

Mitochondrial DNA (mtDNA) mutations are thought to have a causal role in a variety of age-related neurodegenerative diseases, including age-related hearing loss (AHL). In the current study, we investigated the roles of mtDNA deletions and point mutations in AHL in mitochondrial mutator mice (Polg^mut/mut) that were backcrossed onto CBA/CaJ mice, a well-established model of late-onset AHL. mtDNA deletions accumulated significantly with age in the inner ears of Polg^mut/mut mice, while there were no differences in mtDNA deletion frequencies in the inner ears between 5 and 17 months old Polg^+/+ mice or 5 months old Polg^+/+ and Polg^mut/mut mice. mtDNA deletions also accumulated significantly in the inner ears of CBA/CaJ mice during normal aging. In contrast, 5 months old Polg^mut/mut mice displayed a 238-fold increase in mtDNA point mutation frequencies in the inner ears compared to age-matched Polg^+/+ mice, but there were no differences in mtDNA point mutation frequencies in the inner ears between 5 and 17 months old Polg^mut/mut mice. Seventeen-month-old Polg^mut/mut mice also displayed early-onset severe hearing loss associated with a significant reduction in neural output of the cochlea, while age-matched Polg^+/+ mice displayed little or no hearing impairment. Consistent with the physiological and mtDNA deletion test result, 17-month-old Polg^mut/mut mice displayed a profound loss of spiral ganglion neurons in the cochlea. Thus, our data suggest that a higher burden of mtDNA point mutations from a young age and age-related accumulation of mtDNA deletions likely contribute to early-onset AHL in mitochondrial mutator mice.

Effects of Ageing on the Mitochondrial Genome in Rat Vestibular Organs

Background:

Deterioration in vestibular function occurs with ageing and is linked to age-related falls. Sensory hair cells located in the inner ear vestibular labyrinth are critical to vestibular function. Vestibular hair cells rely predominantly on oxidative phosphorylation (OXPHOS) for ener-gy production and contain numerous mitochondria. Mitochondrial DNA (mtDNA) mutations and perturbed energy production are associated with the ageing process.

Objective:

We investigated the effects of ageing on mtDNA in vestibular hair and support cells, and vestibular organ gene expression, to better understand mechanisms of age-related vestibular deficits.

Methods:

Vestibular hair and supporting cell layers were microdissected from young and old rats, and mtDNA was quantified by qPCR. Additionally, vestibular organ gene expression was analysed by microarray and gene set enrichment analyses.

Results:

In contrast to most other studies, we found no evidence of age-related mtDNA deletion mu-tations. However, we found an increase in abundance of major arc genes near the mtDNA control re-gion. There was also a marked age-related reduction in mtDNA copy number in both cell types. Ves-tibular organ gene expression, gene set enrichment analysis showed the OXPHOS pathway was down regulated in old animals.

Conclusion:

Given the importance of mtDNA to mitochondrial OXPHOS and hair cell function, our findings suggest the vestibular organs are potentially on the brink of an energy crisis in old animals

Protective Effects of Silymarin Against Age-Related Hearing Loss in an Aging Rat Model

Age-related hearing loss (ARHL) is one of the most common chronic degenerative disorders. Several studies have indicated that supplementation with some antioxidants can slow down the progression of ARHL. Despite several lines of evidence about the potent antioxidant and anti-aging effects of silymarin, its protective effect against ARHL has not evaluated yet. The aim of the current study was to investigate the effects of silymarin in prevention of ARHL in a d-Galactose-induced aging rat model for the first time. 45 male wistar rats aged 3-month old were divided into 5 groups: group 1, 2 and 3 received 500 mg/kg/day d-Gal plus 100, 200 and 300 mg/kg/day silymarin respectively for 8 weeks, placebo group received 500 mg/kg/day d-Gal plus propylene glycol as placebo, and control group received normal saline during this period of time. Auditory brainstem responses were measured at several frequencies (4, 6, 8, 12 and 16 kHz) before and after the intervention. Placebo group and group 3 showed significant ABR threshold increase across frequencies of 4, 6, 16 kHz compared with the other groups (P < 0.05). However, rats treated with silymarin 100 and 200 mg/kg/day plus d-Gal did not show any significant ABR threshold shifts. Similarly, ABR amplitude of P2 at 4, 8 kHz and P1, P4 at 4 kHz in the placebo group and group 3 were decreased significantly compared with other groups (P < 0.05). However, no significant differences are found in ABR absolute and inter-peak latencies between groups (P > 0.05). The findings indicates that silymarin with doses of 100 and 200 mg/kg/day has protective effect against ARHL and it can be supplemented into the diet of older people to slow down the progression of age-related hearing loss.

Role of antioxidants in prevention of age-related hearing loss: a review of literature

Age-related hearing loss (ARHL), also known as presbycusis, is one of the most prevalent chronic degenerative conditions. It is characterized by a decline in auditory function. ARHL is caused by the interaction of multiple factors, including cochlear aging, environment, genetic predisposition, and health comorbidities. The primary pathology of ARHL includes the hair cells loss, stria vascularis atrophy, and loss of spiral ganglion neurons as well as the changes in central auditory pathways. The research to date suggests that oxidative stress and mitochondrial DNA deletion (mtDNA) play a major role in pathophysiology of ARHL. Therefore, similar to other otological conditions, several studies have also showed that antioxidants can slow ARHL, but some also indicate that antioxidant therapy is not a magic elixir that will prevent or treat hearing loss associated with aging completely, but why? All available clinical trials, including animal and human studies, in English language that examined the protective effects of antioxidants against ARHL were reviewed. Materials were obtained by searching ELSEVIER, PubMed, Scopus, Web of knowledge, Google Scholar databases, Clinical trials, and Cochrane database of systematic reviews. Although ARHL has been shown to be slowed by supplementation with antioxidants, particularly in laboratory animals, a few studies have investigated the effect of interventions against ARHL in humans. High-quality clinical trials are needed to investigate if ARHL can be delayed or prevented in humans. However, it seems that targeting several cell-death pathways is better than targeting the only oxidative stress pathway.

Oxidative stress and ROS metabolism via down-regulation of sirtuin 3 expression in Cmah-null mice affect hearing loss

CMP-Neu5Ac hydroxylase (Cmah) disruption caused several abnormalities and diseases including hearing loss in old age. However, underling molecular mechanisms that give rise to age-related hearing loss (AHL) in Cmah-null mouse are still obscure. In this study, Cmah-null mice showed age-related decline of hearing associated with loss of sensory hair cells, spiral ganglion neurons, and/or stria vascularis degeneration in the cochlea. To identify differential gene expression profiles and pathway associated with AHL, we performed microarray analysis using Illumina MouseRef-8 v2 Expression BeadChip and pathway-focused PCR array in the cochlear tissues of Cmah-null mouse. Pathway and molecular mechanism analysis using differentially expressed genes provided evidences that altered biological pathway due to oxidative damage by low expressed antioxidants and dysregulated reactive oxygen species (ROS) metabolism. Especially, low sirtuin 3 (Sirt3) gene expressions in Cmah-null mice decreased both of downstream regulator (Foxo1 and MnSod) and regulatory transcription factor (Hif1αand Foxo3α) gene expression. Taken together, we suggest that down-regulation of Sirt3 expression leads to oxidative stress and mitochondrial dysfunction by regulation of ROS and that it could alter various signaling pathways in Cmah-null mice with AHL.

Expression pattern of oxidative stress and antioxidant defense-related genes in the aging Fischer 344/NHsd rat cochlea

The biological mechanisms that give rise to age-related hearing loss (ARHL) are still poorly understood. However, there is growing recognition that oxidative stress may be an important factor. To address this issue, we measured the changes in the expression of cochlear oxidative stress and antioxidant defense-related genes in young (2 months old), middle-aged (12 months old), and old (21-25 months old) Fischer 344/NHsd (F344/NHsd) rats and compared gene expression changes with ARHL. A quantitative real-time reverse transcription polymerase chain reaction array revealed a significant age-related downregulation of only 1 gene, stearoyl-coenzyme A desaturase 1, and upregulation of 12 genes: 24-dehydrocholesterol reductase; aminoadipate-semialdehyde synthase; cytoglobin; dual oxidase 2; glutathione peroxidase 3; glutathione peroxidase 6; glutathione S-transferase, kappa 1; glutathione reductase; nicotinamide adenine dinucleotide phosphate (NAD(P)H) dehydrogenase, quinone 1; solute carrier Family 38, Member 5; thioredoxin interacting protein; and vimentin. Statistical analyses revealed significant correlations between gene expression and auditory function in 8 genes. Our results identified specific subsets of oxidative stress genes that appear to play an important role in ARHL in the Fischer 344/NHsd rat.

Adenosine kinase inhibition in the cochlea delays the onset of age-related hearing loss

This study was undertaken to determine the role of adenosine signalling in the development of age-related hearing loss (ARHL). We and others have shown previously that adenosine signalling via A(1) receptors is involved in cochlear protection from noise-induced cochlear injury. Here we demonstrate that enhanced adenosine signalling in the cochlea provides partial protection from ARHL in C57BL/6J mice. We targeted adenosine kinase (ADK), the key enzyme in adenosine metabolism, using a treatment regime with the selective ADK inhibitor ABT-702 (1.5mg/kg intraperitoneally twice a week) commencing at the age of three months or six months. This treatment, intended to increase free adenosine levels in the cochlea, was maintained until the age of nine months and hearing thresholds were evaluated monthly using auditory brainstem responses (ABR). At nine months, when C57BL/6J mice normally exhibit significant ARHL, both groups treated with ABT-702 showed lower ABR threshold shifts at 10 and 16kHz compared to control animals receiving the vehicle solution. The better thresholds of the ABT-702-treated mice at these frequencies were supported by increased survival of hair cells in the apical region of the cochlea. This study provides the first evidence that ARHL can be mitigated by enhancing adenosine signalling in the cochlea.

Contribution of common deletion to total deletion burden in mitochondrial DNA from inner ear of d-galactose-induced aging rats

Mitochondrial DNA (mtDNA) mutations, especially deletions, have been suggested to play an important role in aging and degenerative diseases. In particular, the common deletion in humans and rats (4977bp and 4834bp deletion, respectively) has been shown to accumulate with age in post-mitotic tissues with high energetic demands. Among numerous deletions, the common deletion has been proposed to serve as a molecular marker for aging and play a critical role in presbyacusis. However, so far no previous publication has quantified the contribution of common deletion to the total burden of mtDNA deletions in tissues during aging process. In the present study, we established a rat model with various degrees of aging in inner ear induced by three different doses of d-galactose (d-gal) administration. Firstly, multiple mtDNA deletions in inner ear were detected by nested PCR and long range PCR. In addition to the common deletion, three novel mtDNA deletions were identified. All four deletions, located in the major arc of mtDNA, are flanked by direct repeats and involve the cytochrome c oxidase (COX) subunit III gene, encoded by mtDNA. Additionally, absolute quantitative real-time PCR assay was used to detect the level of common deletion and total deletion burden of mtDNA. The quantitative data show that the common deletion is the most frequent type of mtDNA deletions, exceeding 67.86% of the total deletion burden. Finally, increased mtDNA copy number, reduced COX activity and mosaic ultrastructural impairments in inner ear were identified in d-gal-induced aging rats. The increase of mtDNA replication may contribute to the accelerated accumulation of mtDNA deletions, which may result in impairment of mitochondrial function in inner ear. Taken together, these findings suggest that the common deletion may serve as an ideal molecular marker to assess the mtDNA damage in inner ear during aging.

Mitochondrial transcription factor A overexpression and base excision repair deficiency in the inner ear of rats with D-galactose-induced aging

Oxidative damage to mtDNA is associated with excessive reactive oxygen species production. The mitochondrial common deletion (mtDNA 4977-bp and 4834-bp deletion in humans and rats, respectively) is the most typical and frequent form of mtDNA damage associated with aging and degenerative diseases. The accumulation of the mitochondrial common deletion has been proposed to play a crucial role in age-related hearing loss (presbycusis). However, the mechanisms underlying the formation and accumulation of mtDNA deletions are still obscure. In the present study, a rat mimetic aging model induced by D-Gal was used to explore the origin of deletion mutations and how mtDNA repair systems modulate this process in the inner ear during aging. We found that the mitochondrial common deletion was greatly increased and mitochondrial base excision repair capacity was significantly reduced in the inner ear in D-Gal-treated rats as compared with controls. The overexpression of mitochondrial transcription factor A induced by D-Gal significantly stimulated mtDNA replication, resulting in an increase in mtDNA copy number. In addition, an age-related loss of auditory sensory cells in the inner ear was observed in D-Gal-treated rats. Taken together, our data suggest that mitochondrial base excision repair capacity deficiency and an increase in mtDNA replication resulting from mitochondrial transcription factor A overexpression may contribute to the accumulation of mtDNA deletions in the inner ear during aging. This study also provides new insights into the development of presbycusis.

Challenges and Opportunities in Presbycusis

The population aged 65 years and older is increasing at a faster rate than the total population, with predictions that by 2030, 20% of the population will be 65 years or older. In 2006, between 35% and 50% of those aged 65 years or older reportedly had presbycusis, a sensory impairment that contributes to social isolation and loss of autonomy and is associated with anxiety, depression, and cognitive decline. To address these concerns, the Geriatric Committee of the American Academy of Otolaryngology, in conjunction with the Hearing Committee, focused on 3 challenges and opportunities in the management of presbycusis: (1) the financial burden of caring for patients with presbycusis in the face of increasing costs and declining reimbursements; (2) future treatment options arising from improved understanding of the molecular mechanisms underlying presbycusis, and (3) recognition of central presbycusis as a condition commonly superimposed on peripheral age-related hearing loss whose diagnosis and management can improve outcomes.

Duplex real-time PCR assay for quantifying mitochondrial DNA deletions in laser microdissected single spiral ganglion cells

Laser microdissection (LMD) has been used to isolate groups of cells and single cells from numerous tissues. In this chapter, we describe a technique for isolating individual spiral ganglion cells from archival formalin-fixed, celloidin-embedded (FFCE) human temporal bone sections. The DNA isolated from these single cells is suitable for analysis with a duplex real-time polymerase chain reaction (PCR) methodology to quantify the mitochondrial DNA (mtDNA) deletion level present.

Protective roles of alpha-lipoic acid in rat model of mitochondrial DNA4834bp deletion in inner ear

The protective roles of alpha-lipoic acid in the rat model of mitochondrial DNA (mtDNA) 4834bp deletion in inner ear were investigated. Forty female Wistar rats at 4 weeks of age were divided into four groups: group A (D-galactose group, n=10), group B (D-galactose+alpha-lipoic acid group, n=10), group C (alpha-lipoic acid group, n=10), and group D (control group, n=10). Auditory brainstem response (ABR) was used to detect the hearing threshold. Colorimetry was used to analyze activity of superoxide dismutase (SOD) and concentration of malondialdehyde (MDA). The percentage of mtDNA4834bp deletion in inner ear was identified by real-time PCR. There was no significant difference in ABR threshold shift among all groups. The percentage of mtDNA4834bp deletion in group A was higher than that in other groups, but there was no significant difference in percentage of mtDNA4834bp deletion among groups B, C, and D. The activity of SOD in group A was lower than that in other groups. The concentration of MDA in group A was higher than that in other groups. It was concluded that there was no significant hearing loss when the percentage of mtDNA4834bp deletion was lower than 12.5%. Alpha-lipoic acid could prevent the reactive oxygen species (ROS)-induced mtDNA4834bp deletion in inner ear of rats.

Major arc mitochondrial DNA deletions in cytochrome c oxidase-deficient human cochlear spiral ganglion cells

CONCLUSIONS

This study suggests that cytochrome c oxidase subunit 3 (COX 3) expression is diminished in spiral ganglion cells from individuals with presbycusis. In addition to the mitochondrial DNA (mtDNA) common deletion (CD), other deletions involving the mtDNA major arc contribute to the observed deficit in COX 3 expression.

OBJECTIVES

To assess COX 3 deficiency in spiral ganglion cells from individuals with presbycusis and to determine whether deletions other than the CD contribute to this deficiency.

METHODS

COX 3 immunofluorescence staining of archival human temporal bone tissue sections from individuals with presbycusis and from age-matched normal-hearing individuals was performed and the intensity of spiral ganglion cell immunostaining was measured. Single COX 3-deficient spiral ganglion cells were isolated by laser microdissection (LMD) and the DNA was analyzed with duplex real-time PCR assays to assess the CD level and the total mtDNA major arc deletion level.

RESULTS

A statistically significant difference (p = 0.021) in the mean intensity of COX 3 immunofluorescence staining of spiral ganglion cells was observed between individuals with presbycusis and normal-hearing controls. The total mtDNA major arc deletion level was greater than the CD level in COX 3-deficient spiral ganglion cells.

Age-related changes in the central auditory system: comparison of D-galactose-induced aging rats and naturally aging rats

One of the most common complaints among aging individuals is difficulty in understanding speech in a compromised listening environment, such as when background noise is present. Age-related hearing loss (presbycusis) is associated with both peripheral and central neural processing deficits, as it occurs even in those with only a mild peripheral hearing impairment. The current study was designed to investigate potential causative mechanisms of this impairment by using a rat model in which presbycusis is inducible by administration of D-galactose (D-gal). One group of these rats was injected subcutaneously with 150 mg D-gal daily for 8 weeks, while control animals received vehicle only. These groups were compared to naturally aged rats (24 months) that had received no other treatment. Central auditory function of the three groups was evaluated by measuring the auditory brainstem response (ABR) and middle latency response (MLR). A TaqMan real time PCR assay was used to quantify a 4834-bp deletion in the mitochondrial DNA (mtDNA) of the auditory cortex (AC), inferior colliculus (IC) and cochlear nucleus (CN). We assessed changes in lipid peroxidation levels and apoptosis rates, and examined pathological changes corresponding to D-gal-induced aging and natural aging. Both groups of aged rats exhibited delayed ABR latencies (III, IV, V), MLR Pa latency, and I-IV interpeak latency. Moreover, increased mtDNA 4834 bp deletion rates, lipid peroxidation levels, rates of neuronal apoptosis and neurodegenerative changes in the AC, IC and CN were similar among the D-gal induced and NA rats. However, the threshold of ABR in the D-gal group showed no significant change from the control group. These observations suggest that age-related central auditory dysfunction and its corresponding pathological changes are present in both naturally aging rats and the D-gal mimetic aging model. Oxidative stress, large-scale mtDNA 4834 bp deletion, and apoptosis are likely to be involved in the progressive weakening of the central auditory system associated with the aging process.