Macrophage depletion attenuates degeneration of spiral ganglion neurons in kanamycin-induced unilateral hearing loss model.

Pathological conditions in cochlea, such as ototoxicity, acoustic trauma, and age-related cochlear degeneration, induce cell death in the organ of Corti and degeneration of the spiral ganglion neurons (SGNs). Although macrophages play an essential role after cochlear injury, its role in the SGNs is limitedly understood. We analyzed the status of macrophage activation and neuronal damage in the spiral ganglion after kanamycin-induced unilateral hearing loss in mice. The number of ionized calcium-binding adapter molecule 1 (Iba1)-positive macrophages increased 3 days after unilateral kanamycin injection. Macrophages showed larger cell bodies, suggesting activation status. Interestingly, the number of activating transcription factor 3 (ATF3)-positive-neurons, an indicator of early neuronal damage, also increased at the same timing. In the later stages, the number of macrophages decreased, and the cell bodies became smaller, although the number of neuronal deaths increased. To understand their role in neuronal damage, macrophages were depleted via intraperitoneal injection of clodronate liposome 24 h after kanamycin injection. Macrophage depletion decreased the number of ATF3-positive neurons at day 3 and neuronal death at day 28 in the spiral ganglion following kanamycin injection. Our results suggest that suppression of inflammation by clodronate at early timing can protect spiral ganglion damage following cochlear insult.

Protective Effects of Infliximab Against Kanamycin-Induced Ototoxicity in Rats.

HYPOTHESIS: To examine the protective effects of infliximab (INF) against kanamycin (KM)-induced hearing loss. BACKGROUND: Tumor necrosis factor α blockers can reduce cellular inflammatory reactions and decrease cell death. METHODS: Thirty-six rats with normal hearing were randomly divided into six groups. The first group was injected with 400 mg/kg KM intramuscularly (IM), the second group with 7 mg/kg INF intraperitoneally (IP) and 400 mg/kg KM IM, the third group with 7 mg/kg INF IP and 200 mg/kg KM IM, and the fourth group with 1 mg/kg 6-methylprednisolone (MP) IP and 400 mg/kg KM IM. Group 5 was injected with 1 mg/kg MP IP and 200 mg/kg KM IM, and group 6 with saline IP once. Auditory brain-stem response (ABR) for hearing thresholds was performed on days 7 and 14. From the frozen sections of the cochlea, the area of the stria vascularis, the number of neurons in the spiral ganglion, the fluorescence intensity of hair cells (FIHC), postsynaptic density (PSD), and presynaptic ribbons (PSRs) were calculated. RESULTS: The KM-induced increase in hearing thresholds was detected on the 14th day. Hearing was only preserved in the group treated with INF after low-dose KM exposure but not in the groups that received high-dose KM. The FIHC, excitatory PSD, and PSR were preserved only in the INF-treated group after half-dose KM exposure. In MP groups, FIHC, excitatory PSD, and PSR were significantly lower than in the control group. CONCLUSIONS: Our results support that tumor necrosis factor-based inflammation may play a role in the ototoxicity mechanism.

Hepatocyte growth factor mimetic confers protection from aminoglycoside-induced hair cell death in vitro.

Loss of sensory hair cells from exposure to certain licit drugs, such as aminoglycoside antibiotics, can result in permanent hearing damage. Exogenous application of the neurotrophic molecule hepatocyte growth factor (HGF) promotes neuronal cell survival in a variety of contexts, including protecting hair cells from aminoglycoside ototoxicity. HGF itself is not an ideal therapeutic due to a short half-life and limited blood-brain barrier permeability. MM-201 is a chemically stable, blood-brain barrier permeable, synthetic HGF mimetic that serves as a functional ligand to activate the HGF receptor and its downstream signaling cascade. We previously demonstrated that MM-201 robustly protects zebrafish lateral line hair cells from aminoglycoside ototoxicity. Here, we examined the ability of MM-201 to protect mammalian sensory hair cells from aminoglycoside damage to further evaluate MM-201's clinical potential. We found that MM-201 exhibited dose-dependent protection from neomycin and gentamicin ototoxicity in mature mouse utricular explants. MM-201's protection was reduced following inhibition of mTOR, a downstream target of HGF receptor activation, implicating the activation of endogenous intracellular substrates by MM-201 as critical for the observed protection. We then asked if MM-201 altered the bactericidal properties of aminoglycosides. Using either plate or liquid growth assays we found that MM-201 did not alter the bactericidal efficacy of aminoglycoside antibiotics at therapeutically relevant concentrations. We therefore assessed the protective capacity of MM-201 in an in vivo mouse model of kanamycin ototoxicity. In contrast to our in vitro data, MM-201 did not attenuate kanamycin ototoxicity in vivo. Further, we found that MM-201 was ototoxic to mice across the dose range tested here. These data suggest species- and tissue-specific differences in otoprotective capacity. Next generation HGF mimetics are in clinical trials for neurodegenerative diseases and show excellent safety profiles, but neither preclinical studies nor clinical trials have examined hearing loss as a potential consequence of pharmaceutical HGF activation. Further research is needed to determine the consequences of systemic MM-201 application on the auditory system.

Anti-inflammatory Therapy Protects Spiral Ganglion Neurons After Aminoglycoside Antibiotic-Induced Hair Cell Loss.

Destruction of cochlear hair cells by aminoglycoside antibiotics leads to gradual death of the spiral ganglion neurons (SGNs) that relay auditory information to the brain, potentially limiting the efficacy of cochlear implants. Because the reasons for this cochlear neurodegeneration are unknown, there are no neuroprotective strategies for patients. To investigate this problem, we assessed transcriptomic changes in the rat spiral ganglion following aminoglycoside antibiotic (kanamycin)-induced hair cell destruction. We observed selectively increased expression of immune and inflammatory response genes and increased abundance of activated macrophages in spiral ganglia by postnatal day 32 in kanamycin-deafened rats, preceding significant SGN degeneration. Treatment with the anti-inflammatory medications dexamethasone and ibuprofen diminished long-term SGN degeneration. Ibuprofen and dexamethasone also diminished macrophage activation. Efficacy of ibuprofen treatment was augmented by co-administration of the nicotinamide adenine dinucleotide-stabilizing agent P7C3-A20. Our results support a critical role of neuroinflammation in SGN degeneration after aminoglycoside antibiotic-mediated cochlear hair cell loss, as well as a neuroprotective strategy that could improve cochlear implant efficacy.

Telmisartan Attenuates Kanamycin-Induced Ototoxicity in Rats.

Telmisartan (TM) has been proposed to relieve inflammatory responses by modulating peroxisome proliferator activator receptor-γ (PPARγ) signaling. This study aimed to investigate the protective effects of TM on kanamycin(KM)-induced ototoxicity in rats. Forty-eight, 8-week-old female Sprague Dawley rats were divided into four groups: (1) control group, (2) TM group, (3) KM group, and (4) TM + KM group. Auditory brainstem response was measured. The histology of the cochlea was examined. The protein expression levels of angiotensin-converting enzyme 2 (ACE2), HO1, and PPARγ were measured by Western blotting. The auditory threshold shifts at 4, 8, 16, and 32 kHz were lower in the TM + KM group than in the KM group (all p < 0.05). The loss of cochlear outer hair cells and spiral ganglial cells was lower in the TM + KM group than in the KM group. The protein expression levels of ACE2, PPARγ, and HO1 were higher in the KM group than in the control group (all p < 0.05). The TM + KM group showed lower expression levels of PPARγ and HO1 than the KM group.TM protected the cochlea from KM-induced injuries in rats. TM preserved hearing levels and attenuated the increase in PPARγ and HO1 expression levels in KM-exposed rat cochleae.

Systemic toll-like receptor 9 agonist CpG oligodeoxynucleotides exacerbates aminoglycoside ototoxicity.

The Toll-like receptor (TLR) signaling pathway is the key regulator of the innate immune system in response to systemic infection. Several studies have reported that the systemic TLR4 agonist lipopolysaccharide exacerbates aminoglycoside ototoxicity, but the influence of virus-associated TLR7 and TLR9 signaling cascades on the cochlea is unclear. The present study aimed to investigate the auditory effects of systemic TLR7 and TLR9 agonists during chronic kanamycin treatment. CBA/CaJ mice received the TLR7 agonist gardiquimod or TLR9 agonist CpG oligodeoxynucleotides (ODN) one day before kanamycin injection and on the 5th and 10th days during a 14-day course of kanamycin treatment. We observed that systemic gardiquimod or CpG ODN alone did not affect the baseline auditory brainstem response (ABR) threshold. Three weeks after kanamycin treatment, gardiquimod did not significantly change ABR threshold shifts, whereas CpG ODN significantly increased kanamycin-induced ABR threshold shifts. Furthermore, outer hair cell (OHC) evaluation revealed that CpG ODN reduced distortion product otoacoustic emission amplitudes and increased kanamycin-induced OHC loss. CpG ODN significantly elevated cochlear Irf-7, Tnf-α, Il-1, and Il-6 transcript levels. In addition, an increased number of Iba-1+ cells, which represented activated macrophages, was observed in the cochlea treated with CpG ODN. Our results indicated that systemic CpG ODN exacerbated kanamycin-induced ototoxicity and increased cochlear inflammation. This study implies that patients with underlying virus infection may experience more severe aminoglycoside-induced hearing loss if it occurs.

Deletion of Clusterin Protects Cochlear Hair Cells against Hair Cell Aging and Ototoxicity.

Hearing loss is a debilitating disease that affects 10% of adults worldwide. Most sensorineural hearing loss is caused by the loss of mechanosensitive hair cells in the cochlea, often due to aging, noise, and ototoxic drugs. The identification of genes that can be targeted to slow aging and reduce the vulnerability of hair cells to insults is critical for the prevention of sensorineural hearing loss. Our previous cell-specific transcriptome analysis of adult cochlear hair cells and supporting cells showed that Clu, encoding a secreted chaperone that is involved in several basic biological events, such as cell death, tumor progression, and neurodegenerative disorders, is expressed in hair cells and supporting cells. We generated Clu-null mice (C57BL/6) to investigate its role in the organ of Corti, the sensory epithelium responsible for hearing in the mammalian cochlea. We showed that the deletion of Clu did not affect the development of hair cells and supporting cells; hair cells and supporting cells appeared normal at 1 month of age. Auditory function tests showed that Clu-null mice had hearing thresholds comparable to those of wild-type littermates before 3 months of age. Interestingly, Clu-null mice displayed less hair cell and hearing loss compared to their wildtype littermates after 3 months. Furthermore, the deletion of Clu is protected against aminoglycoside-induced hair cell loss in both in vivo and in vitro models. Our findings suggested that the inhibition of Clu expression could represent a potential therapeutic strategy for the alleviation of age-related and ototoxic drug-induced hearing loss.

Effects of Androgen Receptor Inhibition on Kanamycin-Induced Hearing Loss in Rats.

Megalin has been proposed as an endocytic receptor for aminoglycosides as well as estrogen and androgen. We aimed to investigate the otoprotective effects of antiandrogens (flutamide, FM) on kanamycin (KM)-induced hearing loss in rats. Rats were divided into four groups. The KM group was administered KM (20 mg/kg/day) for 5 days, while the FM group received FM (15 mg/kg/day) for 10 days. In the KM + FM group, KM and FM (15 mg/kg/day) were simultaneously injected for 5 days and then FM was injected for 5 days. Auditory brainstem responses were measured. Western blotting and/or quantitative reverse transcriptase-polymerase chain reaction were performed for megalin, cytochrome P450 1A1 (Cyp1a1), Cyp1b1, metallothionein 1A (MT1A), MT2A, tumor necrosis factor (TNF)-α, caspase 3, and cleaved caspase 3. The FM + KM group showed attenuated auditory thresholds when compared with the KM group at 4, 8, 16, and 32 kHz (all p < 0.05). The KM + FM group showed lower megalin and Cyp1b1 levels than the KM group (all p < 0.05). The KM + FM group revealed lower MT1A, TNFα, and caspase 3 protein levels, compared with those in the KM group (all p < 0.05). Androgen receptor inhibition protects against cochlear injuries in KM-induced hearing loss rats by attenuating megalin expression, revealing anti-inflammatory and anti-apoptotic effects.

Sensorineural hearing loss may lead to dementia-related pathological changes in hippocampal neurons.

Presbycusis contributes to cognitive decline and Alzheimer's disease. However, most research in this area involves clinical observations and statistical modeling, and few studies have examined the relationship between hearing loss and the molecular changes that lead to cognitive dysfunction. The present study investigated whether hearing loss contributes to dementia in the absence of aging and noise using a mouse model of severe bilateral hearing loss induced by kanamycin (1000 mg/kg) and furosemide (400 mg/kg). Immunohistochemistry, silver staining, immunofluorescence analysis, and Western blotting were used to observe pathological changes in different regions of the hippocampus in animals with hearing loss. Changes in the cognitive function of animals with hearing loss were assessed using the Morris water maze test. The results showed that neurons began to degenerate 60 days after hearing loss, and this degeneration was accompanied by structural disorganization and decreased neurogenesis. The level of phosphorylated tau increased over time. Increases in escape latency and distance traveled during the training phase of the Morris water maze test were observed 90 days after hearing loss. Activated microglia and astrocytes with increased levels of inflammatory cytokines, such as tumor necrosis factor-α (TNF-α) and interleukin-1ß (IL-1ß) were detected in the hippocampus. These results suggest that hearing loss alone causes neuronal degeneration, inhibition of neurogenesis, increased tau protein phosphorylation, and increased neuroinflammation in the hippocampus. Early intervention in individuals with hearing loss may reduce the risk of cognitive decline.

SOD2 Alleviates Hearing Loss Induced by Noise and Kanamycin in Mitochondrial DNA4834-deficient Rats by Regulating PI3K/MAPK Signaling.

Superoxide dismutase 2 (SOD2)-mediated gene therapy has significant protective effects against kanamycin-induced hearing loss and hair cell loss in the inner ear, but the underlying mechanisms are still unclear. Herein, an in vivo aging model of mitochondrial DNA (mtDNA)4834 deletion mutation was established using D-galactose, and the effects of noise or kanamycin on inner ear injury was investigated. Rats subjected to mtDNA4834 mutation via D-galactose administration showed hearing loss characterized by the disruption of inner ear structure (abnormal cell morphology, hair cell lysis, and the absence of the organ of Corti), increased SOD2 promoter methylation, and an increase in the degree of apoptosis. Exposure to noise or kanamycin further contributed to the effects of D-galactose. SOD2 overexpression induced by viral injection accordingly counteracted the effects of noise and kanamycin and ameliorated the symptoms of hearing loss, suggesting the critical involvement of SOD2 in preventing deafness and hearing-related conditions. The PI3K and MAPK signaling pathways were also regulated by noise/kanamycin exposure and/or SOD2 overexpression, indicating that they may be involved in the therapeutic effect of SOD2 against age-related hearing loss.

Characterization of EGR-1 Expression in the Auditory Cortex Following Kanamycin-Induced Hearing Loss in Mice.

Deprivation of acoustic input during a critical period leads to abnormal auditory development in humans. The molecular basis underlying the susceptibility of auditory cortex to loss of afferent input remains largely unknown. The transcription factor early growth response-1 (EGR-1) expression in the visual cortex has been shown to be crucial in the formation of vision, but the role of EGR-1 during the process of auditory function formation is still unclear. In this study, we presented data showing that EGR-1 was expressed in the neurons of the primary auditory cortex (A1) in mice. We observed that the auditory deprivation induced by kanamycin during the auditory critical period leads to laminar-specific alteration of neuronal distribution and EGR-1 expression in A1. In addition, MK-801 administration inhibited the expression of EGR-1 in A1 and aggravated the abnormal cortical electric response caused by kanamycin injection. Finally, we showed that the expression of PI3K, the phosphorylation of Akt, as well as the phosphorylation of cAMP-responsive element-binding protein (CREB) were decreased in A1 after kanamycin-induced hearing loss. These results characterized the expression of EGR-1 in A1 in response to the acoustic input and suggested the involvement of EGR-1 in auditory function formation.

miR-182 prevented ototoxic deafness induced by co-administration of kanamycin and furosemide in rats.

Ototoxic drugs may induce auditory sensory hair cell loss and permanent deafness; however, there is still no effective treatments or prevention strategies for this side effect. A recent study found that microRNA182 (miR-182) protected cochlear hair cells from ototoxic drug-induced apoptosis in vitro. However, it remains unclear whether miR-182 can protect drug-induced deafness in vivo. In this study, we overexpressed cochlear miR-182 in Sprague-Dawley rats by trans-round window niche delivery of miR-182 mimics. The rats subsequently received intraperitoneal injections of kanamycin and furosemide to induce acute cochlear outer hair cell death and permanent deafness. Auditory brainstem response tests showed that miR-182 attenuated permanent threshold shifts. Consistent with this result, miR-182 reduced the loss of outer hair cells and missing stereocilia. miR-182 treatment also increased the level of phosphoinositide-3 kinase regulatory subunit p85α in the outer hair cells after co-administration of kanamycin and furosemide. Our findings suggest that miR-182 has powerful protective potential against ototoxic drug-induced acute auditory sensory hair cell loss and permanent deafness.

Generation of new hair cells by DNA methyltransferase (Dnmt) inhibitor 5-azacytidine in a chemically-deafened mouse model.

Regeneration of mature mammalian inner ear hair cells remains to be a challenge. This study aims to evaluate the ability of DNA methyltransferase (Dnmt) inhibitor 5-azacytidine (5-aza) to generate outer hair cells (OHCs) in a chemically-deafened adult mouse model. 5-aza was administrated into the mouse inner ear via the round window. Immunofluorescence was used to examine the expression of hair cell specific proteins following 5-aza treatment. The results showed that in the chemically-deafened mouse cochlea, new OHCs were found post 5-aza treatment, whereas OHCs were completely lost in saline-treated mice. New hair cells expressed multiple hair cell markers included Myosin VIIa, Pou4f3 and Myosin VI. Newly-generated hair cells presented in three cochlear turns and were able to survive for at least six weeks. The effects of new hair cells generation by 5-aza were concentration dependent. Quantitative PCR study indicates that 5-aza may function through Dnmt1 inhibition. The results of this report suggest that the Dnmt inhibitor 5-aza may promote hair cell regeneration in a chemically-deafened mouse model.

Identification and efficacy of glycine, serine and threonine metabolism in potentiating kanamycin-mediated killing of Edwardsiella piscicida.

We previously showed that glucose potentiated kanamycin to kill multidrug-resistant Edwardsiella piscicida through activation of the TCA cycle. However, whether other regulatory mechanism is involved requires further investigation. By quantitative proteomics technology, iTRAQ, we systematically mapped the altered proteins in the presence of glucose and identified 94 differentially expressed proteins. The analysis of the altered proteins by pathways, amino acid biosynthesis and metabolism were enriched. And the most significantly altered eight amino acids tyrosine, phenylalanine, valine, leucine, isoleucine, glycine, serine and threonine were investigated for their potentiation of kanamycin to kill EIB202, where glycine, serine and threonine showed the strongest efficacy than the others. The combinations of glycine and serine or glucose with glycine, serine or threonine had the best effects. Moreover, pyruvate dehydrogenase, α-ketoglutarate dehydrogenase and succinate dehydrogenase activities were increased as well as the proton motive force (PMF) and intracellular kanamycin. Finally, inhibitors that disrupt PMF production abolished the potentiation. These results shed light on the mechanism of how glucose promoting the amino acids biosynthesis and metabolism to potentiate kanamycin to kill antibiotic-resistant bacteria. More importantly, our results suggested that adjusting amino acid biosynthesis and metabolism might be a strategy to become phenotypic resistance to antibiotics in bacteria. SIGNIFICANCE: Tackling antibiotic resistance is an emerging issue in current years. Despite the efforts made toward developing new antibiotics, the progress is still lagged behind expectation. Novel strategies are required. The use of metabolite to revert antibiotic resistant is highly appreciated in recent years due to the less toxicity, more economic and high efficacy. As a continued study of our previous report on glucose potentiating kanamycin to kill antibiotic-resistant bacteria. The current study further expands the previous discovery on the mechanism of how glucose potentiate this effect. This result provides more basis on the action of glucose in reverting antibiotic resistance. And more importantly, we may derive more metabolites other than glucose to manage antibiotic resistance.

Neuronal erythropoietin overexpression is protective against kanamycin-induced hearing loss in mice.

Aminoglycosides have detrimental effects on the hair cells of the inner ear, yet these agents indisputably are one of the cornerstones in antibiotic therapy. Hence, there is a demand for strategies to prevent aminoglycoside-induced ototoxicity, which are not available today. In vitro data suggests that the pleiotropic growth factor erythropoietin (EPO) is neuroprotective against aminoglycoside-induced hair cell loss. Here, we use a mouse model with EPO-overexpression in neuronal tissue to evaluate whether EPO could also in vivo protect from aminoglycoside-induced hearing loss. Auditory brainstem response (ABR) thresholds were measured in 12-weeks-old mice before and after treatment with kanamycin for 15 days, which resulted in both C57BL/6 and EPO-transgenic animals in a high-frequency hearing loss. However, ABR threshold shifts in EPO-transgenic mice were significantly lower than in C57BL/6 mice (mean difference in ABR threshold shift 13.6 dB at 32 kHz, 95% CI 3.8-23.4 dB, p = 0.003). Correspondingly, quantification of hair cells and spiral ganglion neurons by immunofluorescence revealed that EPO-transgenic mice had a significantly lower hair cell and spiral ganglion neuron loss than C57BL/6 mice. In conclusion, neuronal overexpression of EPO is protective against aminoglycoside-induce hearing loss, which is in accordance with its known neuroprotective effects in other organs, such as the eye or the brain.

Genetic disruption of fractalkine signaling leads to enhanced loss of cochlear afferents following ototoxic or acoustic injury.

Cochlear hair cells are vulnerable to a variety of insults like acoustic trauma and ototoxic drugs. Such injury can also lead to degeneration of spiral ganglion neurons (SGNs), but this occurs over a period of months to years. Neuronal survival is necessary for the proper function of cochlear prosthetics, therefore, it is of great interest to understand the mechanisms that regulate neuronal survival in deaf ears. We have recently demonstrated that selective hair cell ablation is sufficient to attract leukocytes into the spiral ganglion, and that fractalkine signaling plays a role in macrophage recruitment and in the survival of auditory neurons. Fractalkine (CX3 CL1), a chemokine that regulates adhesion and migration of leukocytes is expressed by SGNs and signals to leukocytes via its receptor CX3 CR1. The present study has extended the previous findings to more clinically relevant conditions of sensorineural hearing loss by examining the role of fractalkine signaling after aminoglycoside ototoxicity or acoustic trauma. Both aminoglycoside treatment and acoustic overstimulation led to the loss of hair cells as well as prolonged increase in the numbers of cochlear leukocytes. Lack of CX3 CR1 did not affect macrophage recruitment after injury, but resulted in increased loss of SGNs and enhanced expression of the inflammatory cytokine interleukin-1ß, when compared to mice with intact CX3 CR1. These data indicate that the dysregulation of macrophage response caused by the absence of CX3 CR1 may contribute to inflammation-mediated neuronal loss in the deafened ear, suggesting a key role for inflammation in the long-term survival of target-deprived afferent neurons.

Intestinal Stem Cell Pool Regulation in Drosophila.

Intestinal epithelial renewal is mediated by intestinal stem cells (ISCs) that exist in a state of neutral drift, wherein individual ISC lineages are regularly lost and born but ISC numbers remain constant. To test whether an active mechanism maintains stem cell pools in the Drosophila midgut, we performed partial ISC depletion. In contrast to the mouse intestine, Drosophila ISCs failed to repopulate the gut after partial depletion. Even when the midgut was challenged to regenerate by infection, ISCs retained normal proportions of asymmetric division and ISC pools did not increase. We discovered, however, that the loss of differentiated midgut enterocytes (ECs) slows when ISC division is suppressed and accelerates when ISC division increases. This plasticity in rates of EC turnover appears to facilitate epithelial homeostasis even after stem cell pools are compromised. Our study identifies unique behaviors of Drosophila midgut cells that maintain epithelial homeostasis.

c-Jun N-Terminal Phosphorylation: Biomarker for Cellular Stress Rather than Cell Death in the Injured Cochlea.

Prevention of auditory hair cell death offers therapeutic potential to rescue hearing. Pharmacological blockade of JNK/c-Jun signaling attenuates injury-induced hair cell loss, but with unsolved mechanisms. We have characterized the c-Jun stress response in the mouse cochlea challenged with acoustic overstimulation and ototoxins, by studying the dynamics of c-Jun N-terminal phosphorylation. It occurred acutely in glial-like supporting cells, inner hair cells, and the cells of the cochlear ion trafficking route, and was rapidly downregulated after exposures. Notably, death-prone outer hair cells lacked c-Jun phosphorylation. As phosphorylation was triggered also by nontraumatic noise levels and none of the cells showing this activation were lost, c-Jun phosphorylation is a biomarker for cochlear stress rather than an indicator of a death-prone fate of hair cells. Preconditioning with a mild noise exposure before a stronger traumatizing noise exposure attenuated the cochlear c-Jun stress response, suggesting that the known protective effect of sound preconditioning on hearing is linked to suppression of c-Jun activation. Finally, mice with mutations in the c-Jun N-terminal phosphoacceptor sites showed partial, but significant, hair cell protection. These data identify the c-Jun stress response as a paracrine mechanism that mediates outer hair cell death.

Scar formation in mice deafened with kanamycin and furosemide.

In mammals, hair cell loss is irreversible and leads to hearing loss. To develop and test the functioning of different strategies aiming at hair cell regeneration, animal models of sensorineural hearing loss are essential. Although cochleae of these animals should lack hair cells, supporting cells should be preserved forming an environment for the regenerated hair cells. In this study, we investigated how ototoxic treatment with kanamycin and furosemide changes the structure of cochlear sensory epithelium in mice. The study also compared different tissue preparation protocols for scanning electron microscopy (SEM). Cochleae were collected from deafened and nondeafened mice and further processed for plastic mid modiolar sections and SEM. For comparing SEM protocols, cochleae from nondeafened mice were processed using three protocols: osmium-thiocarbohydrazide-osmium (OTO), tannic acid-arginine-osmium, and the conventional method with gold-coating. The OTO method demonstrated optimal cochlear tissue preservation. Histological investigation of cochleae of deafened mice revealed that the supporting cells enlarged and ultimately replaced the lost hair cells forming types 1 and 2 phalangeal scars in a base towards apex gradient. The type 3 epithelial scar, flattened epithelium, has not been seen in analysed cochleae. The study concluded that mice deafened with kanamycin and furosemide formed scars containing supporting cells, which renders this mouse model suitable for testing various hair cell regeneration approaches. Microsc. Res. Tech. 79:766-772, 2016. © 2016 Wiley Periodicals, Inc.

Central Nervous Activity upon Systemic Salicylate Application in Animals with Kanamycin-Induced Hearing Loss--A Manganese-Enhanced MRI (MEMRI) Study.

This study investigated the effect of systemic salicylate on central auditory and non-auditory structures in mice. Since cochlear hair cells are known to be one major target of salicylate, cochlear effects were reduced by using kanamycin to remove or impair hair cells. Neuronal brain activity was measured using the non-invasive manganese-enhanced magnetic resonance imaging technique. For all brain structures investigated, calcium-related neuronal activity was increased following systemic application of a sodium salicylate solution: probably due to neuronal hyperactivity. In addition, it was shown that the central effect of salicylate was not limited to the auditory system. A general alteration of calcium-related activity was indicated by an increase in manganese accumulation in the preoptic area of the anterior hypothalamus, as well as in the amygdala. The present data suggest that salicylate-induced activity changes in the auditory system differ from those shown in studies of noise trauma. Since salicylate action is reversible, central pharmacological effects of salicylate compared to those of (permanent) noise-induced hearing impairment and tinnitus might induce different pathophysiologies. These should therefore, be treated as different causes with the same symptoms.