Attenuation of progressive hearing loss in a model of age-related hearing loss by a heat shock protein inducer, geranylgeranylacetone

Novel insights into mechanisms and therapeutics for presbycusis

Presbycusis, also referred to as age-related hearing loss, poses a substantial burden on both individuals and society. The hallmark of presbycusis is a progressive decrease in auditory sensitivity. Irreversible hearing loss occurs due to the limited regenerative capacity of spiral neurons and peripheral cochlear hair cells (HCs). Although hearing aids and cochlear implantations (CIs) are established approaches for alleviating symptoms of presbycusis, there are currently no preventive or curative measures available. This article provides a comprehensive discussion on the research progress pertaining to the classification, molecular mechanism, genetic susceptibility, as well as the applications and prospects of diverse therapeutic interventions of presbycusis. Building upon these discussions, promising interventions like gene therapy and stem cell (SC) therapy are proposed for their potential value in restoring cochlear function; thus aiming to pave new avenues for prevention and cure of presbycusis.

The Role of Molecular and Cellular Aging Pathways on Age-Related Hearing Loss

Aging, a complex process marked by molecular and cellular changes, inevitably influences tissue and organ homeostasis and leads to an increased onset or progression of many chronic diseases and conditions, one of which is age-related hearing loss (ARHL). ARHL, known as presbycusis, is characterized by the gradual and irreversible decline in auditory sensitivity, accompanied by the loss of auditory sensory cells and neurons, and the decline in auditory processing abilities associated with aging. The extended human lifespan achieved by modern medicine simultaneously exposes a rising prevalence of age-related conditions, with ARHL being one of the most significant. While our understanding of the molecular basis for aging has increased over the past three decades, a further understanding of the interrelationship between the key pathways controlling the aging process and the development of ARHL is needed to identify novel targets for the treatment of AHRL. The dysregulation of molecular pathways (AMPK, mTOR, insulin/IGF-1, and sirtuins) and cellular pathways (senescence, autophagy, and oxidative stress) have been shown to contribute to ARHL. However, the mechanistic basis for these pathways in the initiation and progression of ARHL needs to be clarified. Therefore, understanding how longevity pathways are associated with ARHL will directly influence the development of therapeutic strategies to treat or prevent ARHL. This review explores our current understanding of the molecular and cellular mechanisms of aging and hearing loss and their potential to provide new approaches for early diagnosis, prevention, and treatment of ARHL.

The ubiquitin-proteasome system in normal hearing and deafness

Post-translational modifications of proteins are essential for the proper development and function of many tissues and organs, including the inner ear. Ubiquitination is a highly selective post-translational modification that involves the covalent conjugation of ubiquitin to a substrate protein. The most common outcome of protein ubiquitination is degradation by the ubiquitin-proteasome system (UPS), preventing the accumulation of misfolded, damaged, and excess proteins. In addition to proteasomal degradation, ubiquitination regulates other cellular processes, such as transcription, translation, endocytosis, receptor activity, and subcellular localization. All of these processes are essential for cochlear development and maintenance, as several studies link impairment of UPS with altered cochlear development and hearing loss. In this review, we provide insight into the well-oiled machinery of UPS with a focus on its confirmed role in normal hearing and deafness and potential therapeutic strategies to prevent and treat UPS-associated hearing loss.

Application of New Materials in Auditory Disease Treatment

Auditory diseases are disabling public health problems that afflict a significant number of people worldwide, and they remain largely incurable until now. Driven by continuous innovation in the fields of chemistry, physics, and materials science, novel materials that can be applied to hearing diseases are constantly emerging. In contrast to conventional materials, new materials are easily accessible, inexpensive, non-invasive, with better acoustic therapy effects and weaker immune rejection after implantation. When new materials are used to treat auditory diseases, the wound healing, infection prevention, disease recurrence, hair cell regeneration, functional recovery, and other aspects have been significantly improved. Despite these advances, clinical success has been limited, largely due to issues regarding a lack of effectiveness and safety. With ever-developing scientific research, more novel materials will be facilitated into clinical use in the future.

Inhibition of Geranylgeranylacetone on cholecystokinin-B receptor, BDNF and dopamine D1 receptor induced by morphine

Morphine is the pain releasing and abusing drug. Morphine leads to addiction by activating dopaminergic rewarding system consisted of the ventral tegmental area (VTA) and nucleus accumbens (NAc). Cholecystokinin (CCK) is a gut-brain neuropeptide and involved in morphine dependence. Brain-derived neurotrophic factor (BDNF) is a neurotrophin and plays roles in regulating addiction. Geranylgeranylacetone (GGA) is a medicine of protecting gastric mucosal injury and protecting neurons. Our previous study showed that GGA blocked morphine-induced withdrawal and relapse through inducing thioredoxin 1(Trx1). In this study, we investigated that whether cholecystokinin-B receptor (CCKB receptor) and BDNF were related to GGA inhibition on morphine addiction. At first, we made conditioned place preference (CPP) model and confirmed again that GGA blocked the expression of morphine-CPP in present study. Then, our results showed that morphine increased the expressions of dopamine D1 receptor, tyrosine hydroxylase (TH), CCKB receptor and BDNF in the VTA and NAc in mice, which was inhibited by GGA. These results suggest that CCK and BDNF in dopaminergic systems are associated with the role of GGA blocking morphine-CPP.

The association of heat shock protein genetic polymorphisms with age-related hearing impairment in Taiwan

BACKGROUND

Age-related hearing impairment (ARHI) is a major disability among the elderly population. Heat shock proteins (HSPs) were found to be associated with ARHI in animal studies. The aim of this study was to analyze the associations of single nucleotide polymorphisms (SNPs) of HSP genes with ARHI in an elderly population in Taiwan.

METHODS

Participants ≥65 years of age were recruited for audiometric tests and genetic analyses. The pure tone average (PTA) of the better hearing ear was calculated for ARHI evaluation. The associations of HSPA1L (rs2075800 and rs2227956), HSPA1A (rs1043618) and HSPA1B (rs2763979) with ARHI were analyzed in 146 ARHI-susceptible (cases) and 146 ARHI-resistant (controls) participants.

RESULTS

The "T" allele of HSPA1B rs2763979 showed a decreased risk of ARHI. The "TT" genotype of rs2763979 also showed a decreased risk of ARHI in the dominant hereditary model. For HSPA1L (rs2075800 and rs2227956) and HSPA1A (rs1043618), the haplotype "CAG" was related to a decreased risk of ARHI.

CONCLUSION

These findings suggest that HSP70 polymorphisms are associated with susceptibility to ARHI in the elderly population.

Proteostasis is essential during cochlear development for neuron survival and hair cell polarity

Protein homeostasis is essential to cell function, and a compromised ability to reduce the load of misfolded and aggregated proteins is linked to numerous age-related diseases, including hearing loss. Here, we show that altered proteostasis consequent to Elongator complex deficiency also impacts the proper development of the cochlea and results in deafness. In the absence of the catalytic subunit Elp3, differentiating spiral ganglion neurons display large aggresome-like structures and undergo apoptosis before birth. The cochlear mechanosensory cells are able to survive proteostasis disruption but suffer defects in polarity and stereociliary bundle morphogenesis. We demonstrate that protein aggregates accumulate at the apical surface of hair cells, where they cause a local slowdown of microtubular trafficking, altering the distribution of intrinsic polarity proteins and affecting kinocilium position and length. Alleviation of protein misfolding using the chemical chaperone 4-phenylbutyric acid during embryonic development ameliorates hair cell polarity in Elp3-deficient animals. Our study highlights the importance of developmental proteostasis in the cochlea and unveils an unexpected link between proteome integrity and polarized organization of cellular components.

Geranylgeranylacetone blocks the reinstatement of morphine-conditioned place preference

Morphine is widely used for clinical pain management and induces the dependence. Addiction to morphine is a major public health issue. Geranylgeranylacetone (GGA) is widely used in clinic for treating ulcer. GGA induces expression of thioredoxin-1 (Trx-1) extensively. Trx-1 is a redox regulating protein and plays protecting roles in nervous system. GGA prevents mice against morphine-induced hyperlocomotion, rewarding effect, and withdrawal syndrome. However, whether GGA blocks morphine-conditioned place preference (CPP) reinstatement is still unknown. In the present study, we found that GGA administration blocked the reinstatement of morphine-CPP. The expressions of Trx-1, N-methyl d-aspartate receptor 2B subunit (NR2B), phosphorylated Ca²⁺/calmodulin-dependent protein kinase II (p-CaMKII), phosphorylated extracellular signaling regulated kinases (p-ERK), and phosphorylated cAMP-response element binding protein (p-CREB) were induced in nucleus accumbens (NAc) and hippocampus by morphine or GGA, whereas these proteins were not changed by morphine in GGA-treated mice. Our results indicate that GGA may prevent the reinstatement of morphine-CPP through strengthening the expression of Trx-1 and regulating NR2B/ERK pathway. Thus, we suggest that GGA may be a promising therapeutic candidate for morphine-induced relapse.

Oral administration of geranylgeranylacetone to protect vestibular hair cells

OBJECTIVE

We recently reported that the heat shock response played a major role in the protection of hair cells against stress. Oral administration of the heat shock inducer, geranylgeranylacetone (GGA) protected hair cells against intense noise. In our present study, we investigated the effect of GGA on vestibular hair cell death induced by an aminoglycoside.

METHODS

We used CBA/N mice aged 4-6 weeks. The mice were divided into two groups, GGA and control. Mice in the GGA group were fed a diet containing GGA (0.5%) for 4 weeks, and those in the control group were fed a standard diet. Immunohistochemical analyses for Hsp70 were performed in four animals. The utricles of the remaining animals were cultured in medium for 24h with neomycin to induce hair cell death. After fixation, the vestibular hair cells were immunohistochemically stained against calmodulin, and hair cell survival was evaluated.

RESULTS

The vestibular hair cells of mice in the GGA group expressed Hsp70. In addition, after exposure to neomycin, vestibular hair cell survival was higher in the GGA group than in the control group.

CONCLUSION

Our results demonstrated the oral administration of GGA induced the heat shock response in the vestibule and could protect sensory cells.

HSP105 prevents depression-like behavior by increasing hippocampal brain-derived neurotrophic factor levels in mice

Heat shock proteins (HSPs) are stress-induced chaperones that are involved in neurological disease. Although increasingly implicated in behavioral disorders, the mechanisms of HSP action, and the relevant functional pathways, are still unclear. We examined whether oral administration of geranylgeranylacetone (GGA), a known HSP inducer, produced an antidepressant effect in a social defeat stress model of depression in mice. We also investigated the possible molecular mechanisms involved, particularly focusing on hippocampal neurogenesis and neurotrophic factor expression. In stressed mice, hippocampal HSP105 expression decreased. However, administration of GGA increased HSP105 expression and improved depression-like behavior, induced hippocampal cell proliferation, and elevated brain-derived neurotrophic factor (BDNF) levels in mouse hippocampus. Co-treatment with GGA and the BDNF receptor inhibitor K252a suppressed the antidepressant effects of GGA. HSP105 knockdown decreased BDNF mRNA levels in HT22 hippocampal cell lines and hippocampal tissue and inhibited the GGA-mediated antidepressant effect. These observations suggest that GGA administration is a therapeutic candidate for depressive diseases by increasing hippocampal BDNF levels via HSP105 expression.

Photobiomodulation rescues the cochlea from noise-induced hearing loss via upregulating nuclear factor κB expression in rats

Photobiomodulation (PBM) is a noninvasive treatment that can be neuroprotective, although the underlying mechanisms remain unclear. In the present study, we assessed the mechanism of PBM as a novel treatment for noise-induced hearing loss, focusing on the nuclear factor (NF)-κB signaling pathway. Sprague-Dawley rats were exposed to 1-octave band noise centered at 4kHz for 5h (121dB). After noise exposure, their right ears were irradiated with an 808nm diode laser beam at an output power density of 165mW/cm(2) for 30min a day for 5 consecutive days. Measurement of the auditory brainstem response revealed an accelerated recovery of auditory function in the groups treated with PBM compared with the non-treatment group at 4, 7, and 14 days after noise exposure. Immunofluorescent image analysis for inducible nitric oxide synthase and cleaved caspase-3 showed lesser immunoreactivities in outer hair cells in the PBM group compared with the non-treatment group. However, immunofluorescent image analysis for NF-κB, an upstream protein of inducible nitric oxide synthase, revealed greater activation in the PBM group compared with the naïve and non-treatment groups. Western blot analysis for NF-κB also showed stronger activation in the cochlear tissues in the PBM group compared with the naïve and non-treatment groups (p<0.01, each). These data suggest that PBM activates NF-κB to induce protection against inducible nitric oxide synthase-triggered oxidative stress and caspase-3-mediated apoptosis that occur following noise-induced hearing loss.

Age-related hearing impairment and the triad of acquired hearing loss

Understanding underlying pathological mechanisms is prerequisite for a sensible design of protective therapies against hearing loss. The triad of age-related, noise-generated, and drug-induced hearing loss displays intriguing similarities in some cellular responses of cochlear sensory cells such as a potential involvement of reactive oxygen species (ROS) and apoptotic and necrotic cell death. On the other hand, detailed studies have revealed that molecular pathways are considerably complex and, importantly, it has become clear that pharmacological protection successful against one form of hearing loss will not necessarily protect against another. This review will summarize pathological and pathophysiological features of age-related hearing impairment (ARHI) in human and animal models and address selected aspects of the commonality (or lack thereof) of cellular responses in ARHI to drugs and noise.

Toward therapeutic targets for SCA3: Insight into the role of Machado-Joseph disease protein ataxin-3 in misfolded proteins clearance

Machado-Joseph disease (MJD, also known as spinocerebellar ataxia type 3, SCA3), an autosomal dominant neurological disorder, is caused by an abnormal expanded polyglutamine (polyQ) repeat in the ataxin-3 protein. The length of the expanded polyQ stretch correlates positively with the severity of the disease and inversely with the age at onset. To date, we cannot fully explain the mechanism underlying neurobiological abnormalities of this disease. Yet, accumulating reports have demonstrated the functions of ataxin-3 protein in the chaperone system, ubiquitin-proteasome system, and aggregation-autophagy, all of which suggest a role of ataxin-3 in the clearance of misfolded proteins. Notably, the SCA3 pathogenic form of ataxin-3 (ataxin-3(exp)) impairs the misfolded protein clearance via mechanisms that are either dependent or independent of its deubiquitinase (DUB) activity, resulting in the accumulation of misfolded proteins and the progressive loss of neurons in SCA3. Some drugs, which have been used as activators/inducers in the chaperone system, ubiquitin-proteasome system, and aggregation-autophagy, have been demonstrated to be efficacious in the relief of neurodegeneration diseases like Huntington's disease (HD), Parkinson's (PD), Alzheimer's (AD) as well as SCA3 in animal models and clinical trials, putting misfolded protein clearance on the list of potential therapeutic targets. Here, we undertake a comprehensive review of the progress in understanding the physiological functions of ataxin-3 in misfolded protein clearance and how the polyQ expansion impairs misfolded protein clearance. We then detail the preclinical studies targeting the elimination of misfolded proteins for SCA3 treatment. We close with future considerations for translating these pre-clinical results into therapies for SCA3 patients.

Attenuation of progressive hearing loss in DBA/2J mice by reagents that affect epigenetic modifications is associated with up-regulation of the zinc importer Zip4

Various factors that are important for proper hearing have been identified, including serum levels of zinc. Here we investigated whether epigenetic regulatory pathways, which can be modified by environmental factors, could modulate hearing. RT-PCR detected expression of genes encoding DNA methyltransferase and histone deacetylase (Hdac) in the postnatal as well as adult mouse auditory epithelium. DBA/2J mice, which are a model for progressive hearing loss, were injected subcutaneously with one or a combination of the following reagents: <smallcaps>L</smallcaps>-methionine as a methyl donor, valproic acid as a pan-Hdac inhibitor, and folic acid and vitamin B12 as putative factors involved in age-related hearing loss. The mice were treated from ages 4 to 12 weeks (N ≥ 5), and auditory brainstem response (ABR) thresholds were measured at 8, 16, and 32 kHz. Treatment of the mice with a combination of <smallcaps>L</smallcaps>-methionine and valproic acid (M+V) significantly reduced the increase in the ABR threshold at 32 kHz. Treatment with any of these reagents individually produced no such effect. Microarray analyses detected 299 gene probes that were significantly up- or down-regulated in the cochleae of mice treated with M+V compared with the control vehicle-treated mice. Quantitative RT-PCR confirmed significant up-regulation of a zinc importer gene, Zip4, in the cochleae of mice treated with M+V. Immunohistochemistry demonstrated an intense Zip4 signal in cochlear tissues such as the lateral wall, organ of Corti, and spiral ganglion. Finally, mice treated with the Zip4 inducer (–)-epigallocatechin-3-O-gallate showed a significant reduction in the increase of the ABR threshold at 32 kHz and up-regulation of Zip4 expression in the cochlea. This study suggests that epigenetic regulatory pathways can modify auditory function and that zinc intake in the cochlea via Zip4 mediates maintenance of mammalian hearing.

Auditory nerve synapses persist in ventral cochlear nucleus long after loss of acoustic input in mice with early-onset progressive hearing loss

Perceptual performance in persons with hearing loss, especially those using devices to restore hearing, is not fully predicted by traditional audiometric measurements designed to evaluate the status of peripheral function. The integrity of auditory brainstem synapses may vary with different forms of hearing loss, and differential effects on the auditory nerve-brain interface may have particularly profound consequences for the transfer of sound from ear to brain. Loss of auditory nerve synapses in ventral cochlear nucleus (VCN) has been reported after acoustic trauma, ablation of the organ of Corti, and administration of ototoxic compounds. The effects of gradually acquired forms deafness on these synapses are less well understood. We investigated VCN gross morphology and auditory nerve synapse integrity in DBA/2J mice with early-onset progressive sensorineural hearing loss. Hearing status was confirmed using auditory brainstem response audiometry and acoustic startle responses. We found no change in VCN volume, number of macroneurons, or number of VGLUT1-positive auditory nerve terminals between young adult and older, deaf DBA/2J. Cell-type specific analysis revealed no difference in the number of VGLUT1 puncta contacting bushy and multipolar cell body profiles, but the terminals were smaller in deaf DBA/2J mice. Transmission electron microscopy confirmed the presence of numerous healthy, vesicle-filled auditory nerve synapses in older, deaf DBA/2J mice. The present results suggest that synapses can be preserved over a relatively long time-course in gradually acquired deafness. Elucidating the mechanisms supporting survival of central auditory nerve synapses in models of acquired deafness may reveal new opportunities for therapeutic intervention.

Protective effect of geranylgeranylacetone against methamphetamine-induced neurotoxicity in rat pheochromocytoma cells

BACKGROUND

Methamphetamine is a central nervous system stimulant and is one of the agents most commonly abused by illicit drug users which could induce neuron apoptosis when it is used repeatedly and overdosed. Our previous study demonstrated that geranylgeranylacetone (GGA) was an inducer of thioredoxin-1 (Trx-1) and heat shock protein 70 (Hsp70), which played a cytoprotective role against neurotoxicity.

METHODS

Using the MTT assay, we detected the effect of GGA on cell viability by methamphetamine in rat pheochromocytoma (PC12) cells. Tyrosine hydroxylase, Trx-1, Hsp70, procaspase-9, procaspase-12 and procaspase-3 expression were examined by Western blot analysis. We also detected enzymatic activities of caspase-3 and caspase-9.

RESULTS

We found that GGA protected PC12 cells from apoptosis caused by methamphetamine. Furthermore, GGA reversed the decreases in Trx-1 and Hsp70 by methamphetamine, and prevented the methamphetamine-induced decreases in procaspase-9 and procaspase-3. On the other hand, GGA prevented the methamphetamine-induced increases in the enzymatic activity of caspase-9 and caspase-3. Procaspase-12 was not changed by any treatment.

CONCLUSIONS

These results indicate that GGA protects PC12 cells from methamphetamine-induced toxicity by increasing Trx-1 and Hsp70 and by preventing mitochondria pathway-mediated apoptosis. In summary, GGA may be used as a therapy for neurotoxicity induced by methamphetamine.

Antidepressant effect of geranylgeranylacetone in a chronic mild stress model of depression and its possible mechanism

Depression is a highly debilitating and widely distributed illness in the general population. Geranylgeranylacetone (GGA), a non-toxic anti-ulcer drug, has been reported to have protective effects in the central nervous system. The aim of this study was to determine the antidepressant effect of GGA in a chronic mild stress (CMS) model of depression. We confirmed that CMS in rats caused a reduction in locomotor activity and an increase in the levels of monoamine oxidase-A (MAO-A) and caspase-3 in the hippocampus. GGA treatment reversed stress-induced alterations in locomotor activity and target levels of MAO-A and caspase-3. In addition, GGA treatment induced heat shock protein 70 (Hsp70) expression in the hippocampus. These findings suggest that GGA possesses an antidepressant activity in a CMS model of depression. The activity of GGA in the relief of depression may be mediated via the induction of Hsp70 expression to suppress MAO-A expression and the apoptosis cascade.

Small molecule activators of the heat shock response: chemical properties, molecular targets, and therapeutic promise

All cells have developed various mechanisms to respond and adapt to a variety of environmental challenges, including stresses that damage cellular proteins. One such response, the heat shock response (HSR), leads to the transcriptional activation of a family of molecular chaperone proteins that promote proper folding or clearance of damaged proteins within the cytosol. In addition to its role in protection against acute insults, the HSR also regulates lifespan and protects against protein misfolding that is associated with degenerative diseases of aging. As a result, identifying pharmacological regulators of the HSR has become an active area of research in recent years. Here, we review progress made in identifying small molecule activators of the HSR, what cellular targets these compounds interact with to drive response activation, and how such molecules may ultimately be employed to delay or reverse protein misfolding events that contribute to a number of diseases.

Recent advances in the study of age-related hearing loss: a mini-review

Hearing loss is a common age-associated affliction that can result from the loss of hair cells and spiral ganglion neurons (SGNs) in the cochlea. Although hair cells and SGNs are typically lost in the same cochlea, recent analysis suggests that they can occur independently, via unique mechanisms. Research has identified both environmental and genetic factors that contribute to degeneration of cochlear cells. Additionally, molecular analysis has identified multiple cell-signaling mechanisms that likely contribute to pathological changes that result in hearing deficiencies. These analyses should serve as useful primers for future work, including genomic and proteomic analysis, to elucidate the mechanisms driving cell loss in the aging cochlea. Significant progress in this field has occurred in the past decade. As our understanding of aging-induced cochlear changes continues to improve, our ability to offer medical intervention will surely benefit the growing elderly population.

Geranylgeranylacetone protects mice against morphine-induced hyperlocomotion, rewarding effect, and withdrawal syndrome

There are few efficacious interventions to combat morphine dependence. Thioredoxin-1 (Trx-1) and heat shock protein 70 (Hsp70) are emerging as important modulators of neuronal function. They have been shown to be involved in cellular protective mechanisms against a variety of toxic stressors. This study was designed to investigate the effects of geranylgeranylacetone (GGA), a pharmacological inducer of Trx-1 and Hsp70, on morphine-induced hyperlocomotion, rewarding effect, and withdrawal syndrome. Trx-1 and Hsp70 expression was increased in the frontal cortex, hippocampus, ventral tegmental area, and nucleus accumbens of mice after GGA treatment. GGA administration reduced morphine-induced motor activity and inhibited conditioned place preference. GGA markedly attenuated the morphine-naloxone-induced withdrawal signs, including jumping, rearing, and forepaw tremor. Furthermore, the activation of cAMP-responsive element-binding protein and the expression of ΔFosB and cyclin-dependent kinase 5 were decreased in the nucleus accumbens by GGA treatment after morphine withdrawal. In the nucleus accumbens, GGA enhanced morphine-induced expression of Trx-1 and Hsp70 after morphine withdrawal. These results suggest that strengthening the expression of Trx-1 and Hsp70 in the brain by using noncytotoxic pharmacological inducers may provide a novel therapeutic strategy for morphine dependence. GGA could be a safe and novel therapeutic agent for morphine dependence.

Age-related hearing loss and expression of antioxidant enzymes in BDF1 mice

CONCLUSION

Our data suggest that the changes in expression of antioxidant enzymes may cause age-related hearing loss (AHL).

OBJECTIVES

AHL is an aging process of the inner ear, and oxidant stressors are considered to be one of the leading causes. We investigated the hearing level and expression profile of antioxidant enzymes in aged mice.

METHODS

Mice aged 3, 6, and 11 months were used. Hearing levels of the mice were examined using the auditory brainstem response (ABR). After measuring the ABR threshold, cochleae were dissected. RNA was isolated from the cochleae, and cDNA was synthesized using the retro-transcription enzyme. Expression of the antioxidant enzymes was measured by quantitative real-time PCR.

RESULTS

The ABR thresholds of the BDF1 mice were elevated by 6 months of age. The expression of superoxide dismutase 1 (SOD1) and heme oxygenase 1 (HO1) at 11 months of age significantly decreased compared with that of those at 6 months of age. In contrast, a decrease in the expression level was not observed regarding NAD(P)H-quinone oxidoreductase 1 (NQO1).

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.

Geranylgeranylacetone suppresses noise-induced expression of proinflammatory cytokines in the cochlea

OBJECTIVE

Heat shock transcription factor 1 (HSF1) is a master regulator of heat shock response, and also inhibits expression of inflammatory cytokines directly or indirectly. Here, we examined effects of HSF1 activation on the expression of proinflammatory cytokines in mouse cochlea after exposure to noise.

METHODS

Male CBA/N mice with normal Preyer's reflex were exposed to intense noise for 3h. Three hours after noise exposure, bilateral cochleae were removed and expression of major inflammatory cytokines was examined.

RESULTS

We found that interleukin-6 (IL-6) and interleukin-1β (IL-1β) expression increased significantly after noise exposure, and the expression was suppressed significantly in mice administered with geranylgeranylacetone (GGA), which activates HSF1. Seven days after noise exposure, thresholds for auditory brainstem response were elevated, and GGA administration significantly suppressed this elevation.

CONCLUSION

These results suggest that HSF1-mediated suppression of proinflammatory cytokines in the cochlea by GGA administration could be an important means of inner ear protection.

Salicylate modulates Hsp70 expression in the explanted organ of Corti

Heat shock protein 70 (Hsp70, Hspa1a) is known to play a protective role in the inner ear and in the nervous system. Our recent study demonstrated that the induction of Hsp70 by geldanamycin protected the auditory hair cells against ototoxic insult. Here, using the explanted organ of Corti (OC), we characterized the effect of sodium salicylate on the expression of Hsp70. Using the real-time RT-PCR; after 27 h in standard culture, we observed an increase in the Hsp70 transcript number. After 48 h in culture, the number of Hsp70 transcripts increased further, as compared to the freshly isolated tissues or explant cultured for 27 h. Three hours after the addition of 2.5mM sodium salicylate, the expression of Hsp70 mRNA increased significantly. Interestingly, Hsp70 protein level remained unaffected by the addition of salicylate, as shown by immunoblotting and Hsp70-ELISA. Confocal microscopy imaging demonstrated predominant localization of Hsp70 protein with or without salicylate exposure to the fibrocytes of spiral limbus. Our results suggest that in the OC, explanting process induces expression of Hsp70 in limbal fibrocytes and that this expression can be enhanced by salicylate but only on mRNA and not on the protein level.

Pathogenesis of presbycusis in animal models: a review

Presbycusis is the most common cause of hearing loss in aged subjects, reducing individual's communicative skills. Age related hearing loss can be defined as a progressive, bilateral, symmetrical hearing loss due to age related degeneration and it can be considered a multifactorial complex disorder, with both environmental and genetic factors contributing to the aetiology of the disease. The decline in hearing sensitivity caused by ageing is related to the damage at different levels of the auditory system (central and peripheral). Histologically, the aged cochlea shows degeneration of the stria vascularis, the sensorineural epithelium, and neurons of the central auditory pathways. The mechanisms responsible for age-associated hearing loss are still incompletely characterized. This work aims to give a broad overview of the scientific findings related to presbycusis, focusing mainly on experimental studies in animal models.

Heat shock transcription factors and sensory placode development

The heat shock transcription factor (HSF) family consists of at least three members in mammals and regulates expression of heat shock proteins in response to heat shock and proteotoxic stresses. Especially, HSF1 is indispensable for this response. Members of this family are also involved in development of some tissues such as the brain and reproductive organs. However, we did not know the molecular mechanisms that regulate developmental processes. Involvement of HSFs in the sensory development was implicated by the finding that human hereditary cataract is associated with mutations of the HSF4 gene. Analysis of gene-disrupted mice showed that HSF4 and HSF1 are required for the lens and the olfactory epithelium, respectively. Furthermore, a common molecular mechanism that regulates developmental processes was revealed by analyzing roles of HSFs in the two developmentally-related organs.