Gentamicin-induced spiral ganglion cell death: apoptosis mediated by ROS and the JNK signaling pathway

Novel Application of Eupatilin for Effectively Attenuating Cisplatin-Induced Auditory Hair Cell Death via Mitochondrial Apoptosis Pathway

Eupatilin (5,7-dihydroxy-3′,4′,6-trimethoxyflavone) is a pharmacologically active flavone that has been isolated from a variety of medicinal plants and possesses a number of pharmacological properties. This study evaluates the antioxidant and antiapoptotic effects of eupatilin on cisplatin-induced ototoxicity using in vitro and in vivo models including HEI-OC1 cells, cochlear hair cells, and zebrafish. Employing a CCK8 assay and Annexin V-FITC/PI double staining, we found that eupatilin significantly alleviated cisplatin-induced apoptosis and increased hair cell viability. The level of reactive oxygen species (ROS) was evaluated by CellROX green and MitoSOX Red staining. The results showed that eupatilin possesses antioxidant activity. MitoTracker Red staining indicated that eupatilin remarkably decreased mitochondrial damage. Furthermore, we demonstrated that eupatilin protects hair cells from cisplatin-induced damage. Mechanistic studies in cisplatin-induced HEI-OC1 cells revealed that eupatilin promoted Bcl-2 expression, downregulated Bax expression, reversed the increase in caspase-3 and PARP activity, and reduced the expression of phosphorylated p38 and JNK. Our data suggest a novel role for eupatilin as a protective agent against ototoxic drug-induced hair cell apoptosis by inhibiting ROS generation and modulating mitochondrial-related apoptosis.

Mechanism and Prevention of Spiral Ganglion Neuron Degeneration in the Cochlea

Sensorineural hearing loss (SNHL) is one of the most prevalent sensory deficits in humans, and approximately 360 million people worldwide are affected. The current treatment option for severe to profound hearing loss is cochlear implantation (CI), but its treatment efficacy is related to the survival of spiral ganglion neurons (SGNs). SGNs are the primary sensory neurons, transmitting complex acoustic information from hair cells to second-order sensory neurons in the cochlear nucleus. In mammals, SGNs have very limited regeneration ability, and SGN loss causes irreversible hearing loss. In most cases of SNHL, SGN damage is the dominant pathogenesis, and it could be caused by noise exposure, ototoxic drugs, hereditary defects, presbycusis, etc. Tremendous efforts have been made to identify novel treatments to prevent or reverse the damage to SGNs, including gene therapy and stem cell therapy. This review summarizes the major causes and the corresponding mechanisms of SGN loss and the current protection strategies, especially gene therapy and stem cell therapy, to promote the development of new therapeutic methods.

Does Calcium Dobesilate Have Therapeutic Effect on Gentamicin-induced Cochlear Nerve Ototoxicity? An Experimental Study

HYPOTHESIS

The ototoxic effects of aminoglycosides are well known. Gentamicin carries a substantial risk of hearing loss. Gentamicin is widely used to combat life-threatening infections, despite its ototoxic effects. Calcium dobesilate is a pharmacologically active agent used to treat many disorders due to its vasoprotective and antioxidant effects. We investigated the therapeutic role of calcium dobesilate against gentamicin-induced cochlear nerve ototoxicity in an animal model.

METHODS

Thirty-two Sprague Dawley rats were divided into four groups: Gentamicin, Gentamicin + Calcium Dobesilate, Calcium Dobesilate, and Control. Preoperative and postoperative hearing thresholds were determined using auditory brainstem response thresholds with click and 16-kHz tone-burst stimuli. Histological analysis of the tympanic bulla specimens was performed under light and transmission electron microscopy. The histological findings were subjected to semiquantitative grading, of which the results were compared between the groups.

RESULTS

Gentamicin + Calcium Dobesilate group had, on average, 27 dB better click-evoked hearing than Gentamicin group (p < 0.01), whereas the difference was not significant with 16-kHz tone-burst stimuli (p > 0.01). Histologically examining the Control and Calcium Dobesilate groups revealed normal ultrastructural appearances. The Gentamicin group showed the most severe histological alterations including myelin destruction, total axonal degeneration, and edema. The histological evidence of damage was significantly reduced in the Gentamicin + Calcium Dobesilate group compared with the Gentamicin group.

CONCLUSION

Adding oral calcium dobesilate to systemic gentamicin was demonstrated to exert beneficial effects on click-evoked hearing thresholds, as supported by the histological findings.

Hearing loss through apoptosis of the spiral ganglion neurons in apolipoprotein E knockout mice fed with a western diet

Age-related hearing loss (ARHL) is a neurodegenerative disease associated with an aged population. ARHL is influenced by biological factors such as aging, sex difference, and atherosclerosis. The mechanisms of ARHL caused by atherosclerosis have not been previously determined in apolipoprotein E knockout (ApoE KO) male mice. To investigate the onset and cause of the hearing loss, ApoE KO male mice were treated with a western diet (ApoE KO-WD) for 16 weeks. The lipid profile, atherosclerotic plaques throughout the aorta, and auditory brainstem response (ABR) thresholds were measured in the ApoE KO-WD male mice. The expression of S100 calcium-binding protein B (S100B), a neuronal damage biomarker, was also observed. Reactive oxygen species (ROS) and apoptosis rates were detected in the cochlea of the ApoE KO male mice. Atherosclerotic plaques on the aorta and ABR thresholds were significantly increased in the ApoE KO-WD male mice at 24 weeks of age. ABR thresholds had a statistically significant positive correlation with the area of atherosclerotic plaques (r = 0.783, p = 0.013) in male mice at 24 weeks of age. S100B protein expression and the dihydroethidium (DHE) reaction to ROS in the cochlear spiral ganglion neurons (SGNs) were significantly increased in the ApoE KO and ApoE KO-WD male mice. Cells positive for active caspase-3 and terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) in the SGNs were significantly increased in ApoE KO-WD male mice indicating an increased rate of cellular apoptosis. In conclusion, ROS in the SGNs were activated by increased S100B expression in ApoE KO-WD male mice, and this resulted in an increased apoptosis rate. Thus, hearing loss began at 16 weeks in ApoE KO-WD male mice. Our results suggest that the ApoE KO-WD male mice are a suitable animal model for studying ARHL associated with exacerbated atherosclerosis.

A comparison of endolymphatic shunt surgery and intratympanic gentamicin for meniere's disease

OBJECTIVE

To report audiovestibular outcomes following endolymphatic shunt surgery (ELS) and intratympanic gentamicin injections (ITG) in patients with Meniere's disease (MD).

STUDY DESIGN

Retrospective matched cohort study METHODS: Patients with MD refractory to medical management between 2004 and 2017 were reviewed: 44 patients underwent ELS and had outcomes available, while 27 patients underwent ITG and had outcomes available. Mean follow-up durations for the ELS and ITG groups were 39.1 and 43.3 months, respectively. Twenty-six patients from the ELS group and 24 patients from the ITG group were then included in a pretreatment hearing- and age-matched analysis. Main outcome measures were successful control of vertigo, pure-tone average (PTA; 0.5, 1, 2 and 4 kHz), word recognition score (WRS), and treatment complications.

RESULTS

A matched analysis showed vertigo control rates of 73.1% in the ELS group and 66.8% in the ITG group, which were not significantly different (P = .760). The change in PTA following treatment was statistically similar between the ELS group (6.2 dB) and ITG group (4.6 dB) (P = .521), while the change in WRS for the ELS group (+3.9 %) was significantly more favorable than the ITG group (-13.6 %) (P = .046). Chronic post-treatment unsteadiness was reported in 25.0% of the ITG group and was not encountered in the ELS group (P = .009).

CONCLUSION

ELS provided successful vertigo control at least as well as ITG with a lower incidence of audiovestibular complications.

LEVEL OF EVIDENCE

4 Laryngoscope, 130:2455-2460, 2020.

Rapamycin Protects Spiral Ganglion Neurons from Gentamicin-Induced Degeneration In Vitro

Gentamicin, one of the most widely used aminoglycoside antibiotics, is known to have toxic effects on the inner ear. Taken up by cochlear hair cells and spiral ganglion neurons (SGNs), gentamicin induces the accumulation of reactive oxygen species (ROS) and initiates apoptosis or programmed cell death, resulting in a permanent and irreversible hearing loss. Since the survival of SGNs is specially required for cochlear implant, new procedures that prevent SGN cell loss are crucial to the success of cochlear implantation. ROS modulates the activity of the mammalian target of rapamycin (mTOR) signaling pathway, which mediates apoptosis or autophagy in cells of different organs. However, whether mTOR signaling plays an essential role in the inner ear and whether it is involved in the ototoxic side effects of gentamicin remain unclear. In the present study, we found that gentamicin induced apoptosis and cell loss of SGNs in vivo and significantly decreased the density of SGN and outgrowth of neurites in cultured SGN explants. The phosphorylation levels of ribosomal S6 kinase and elongation factor 4E binding protein 1, two critical kinases in the mTOR complex 1 (mTORC1) signaling pathway, were modulated by gentamicin application in the cochlea. Meanwhile, rapamycin, a specific inhibitor of mTORC1, was co-applied with gentamicin to verify the role of mTOR signaling. We observed that the density of SGN and outgrowth of neurites were significantly increased by rapamycin treatment. Our finding suggests that mTORC1 is hyperactivated in the gentamicin-induced degeneration of SGNs, and rapamycin promoted SGN survival and outgrowth of neurites.

Endoplasmic reticulum stress is involved in spiral ganglion neuron apoptosis following chronic kanamycin-induced deafness

Aminoglycoside antibiotics-induced hearing loss is a common sensorineural impairment. Spiral ganglion neurons (SGNs) are first-order neurons of the auditory pathway and are critical for the maintenance of normal hearing. In the present study, we investigated the time-course of morphological changes and the degeneration process of spiral ganglion cells (SGCs) following chronic kanamycin-induced deafness and determined whether the endoplasmic reticulum (ER) stress was involved in the degeneration of SGNs. We detected density changes in SGCs and the expressions of Bip, inositol requirement 1 (IRE1)α, activating transcription factor-6α, p-PERK, p-eIF2α, CHOP, and caspase-12 at each time point after kanamycin treatment. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) staining was also performed. The number of SGC deletions reached ∼50% at the 70th day after kanamycin administration and the ER of most SGCs were dilated. The expression of p-PERK, p-eIF2α, p-IRE1α, Bip, caspase-12, and Chop was significantly unregulated after kanamycin treatment. The number of SGCs that were positive for both TUNEL and caspase-12 increased from day 7 to 28. Taken together, these data demonstrate that ER stress was involved in kanamycin-induced apoptosis of SGNs. Kanamycin-induced SGN apoptosis is mediated, at least in part, by ER stress-induced upregulation of CHOP and caspase-12.

Kanamycin Damages Early Postnatal, but Not Adult Spiral Ganglion Neurons

Although aminoglycoside antibiotics such as kanamycin are widely used clinically to treat life-threatening bacterial infections, ototoxicity remains a significant dose-limiting side effect. The prevailing view is that the hair cells are the primary ototoxic target of aminoglycosides and that spiral ganglion neurons begin to degenerate weeks or months after the hair cells have died due to lack of neurotrophic support. To test the early developmental aspects of this issue, we compared kanamycin-induced hair cell and spiral ganglion pathology in rat postnatal day 3 cochlear organotypic cultures with adult whole cochlear explants. In both adult and postnatal day 3 cultures, hair cell damage began at the base of the cochleae and progressed toward the apex in a dose-dependent manner. In postnatal day 3 cultures, spiral ganglion neurons were rapidly destroyed by kanamycin prior to hair cell loss. In contrast, adult spiral ganglion neurons were resistant to kanamycin damage even at the highest concentration, consistent with in vivo models of delayed SGN degeneration. In postnatal day 3 cultures, kanamycin preferentially damaged type I spiral ganglion neurons, whereas type II neurons were resistant. Spiral ganglion degeneration of postnatal day 3 neurons was associated with upregulation of the superoxide radical and caspase-3-mediated cell death. These results show for the first time that kanamycin is toxic to postnatal day 3 spiral ganglion neurons, but not adult neurons.

Non-autonomous Cellular Responses to Ototoxic Drug-Induced Stress and Death

The first major recognition of drug-induced hearing loss can be traced back more than seven decades to the development of streptomycin as an antimicrobial agent. Since then at least 130 therapeutic drugs have been recognized as having ototoxic side-effects. Two important classes of ototoxic drugs are the aminoglycoside antibiotics and the platinum-based antineoplastic agents. These drugs save the lives of millions of people worldwide, but they also cause irreparable hearing loss. In the inner ear, sensory hair cells (HCs) and spiral ganglion neurons (SGNs) are important cellular targets of these drugs, and most mechanistic studies have focused on the cell-autonomous responses of these cell types in response to ototoxic stress. Despite several decades of studies on ototoxicity, important unanswered questions remain, including the cellular and molecular mechanisms that determine whether HCs and SGNs will live or die when confronted with ototoxic challenge. Emerging evidence indicates that other cell types in the inner ear can act as mediators of survival or death of sensory cells and SGNs. For example, glia-like supporting cells (SCs) can promote survival of both HCs and SGNs. Alternatively, SCs can act to promote HC death and inhibit neural fiber expansion. Similarly, tissue resident macrophages activate either pro-survival or pro-death signaling that can influence HC survival after exposure to ototoxic agents. Together these data indicate that autonomous responses that occur within a stressed HC or SGN are not the only (and possibly not the primary) determinants of whether the stressed cell ultimately lives or dies. Instead non-cell-autonomous responses are emerging as significant determinants of HC and SGN survival vs. death in the face of ototoxic stress. The goal of this review is to summarize the current evidence on non-cell-autonomous responses to ototoxic stress and to discuss ways in which this knowledge may advance the development of therapies to reduce hearing loss caused by these drugs.

Protective effects of the seaweed phlorotannin polyphenolic compound dieckol on gentamicin-induced damage in auditory hair cells

OBJECTIVES

Drug-induced ototoxicity from compounds such as aminoglycosides and platinum can damage the inner ear resulting in hearing loss, tinnitus or balance problems and may be caused by the formation of reactive oxygen species (ROS). Dieckol is a phlorotannin polyphenolic compound with strong antioxidant effects found in edible brown algae. This study investigated the protective effects of dieckol on drug-induced ototoxicity in cochlear cultures obtained from neonatal mice.

METHODS

Cochlear explants were pretreated with dieckol and exposed to gentamicin for 48h. The explants were then fixed and stained with fluorescein isothiocyanate-phalloidin and the intact hair cells counted. The free radical scavenging activity of dieckol was assessed using a 1,1-diphenyl-2-picrylhydrazyl assay. E. coli (Escherichia coli) cultures were used to evaluate the effect of dieckol on the antibiotic activity of gentamicin.

RESULTS

Gentamicin treatment resulted in dose-dependent hair cell loss that was partially protected by dieckol. Moreover, at concentrations >67μM dieckol had significant radical scavenging activity. Dieckol did not compromise the antibiotic effect of gentamicin.

CONCLUSIONS

These findings suggest that dieckol can be used as a therapeutic agent that reduces the damage caused by drug-induced ototoxicity.

Effects of Memantine on Aminoglycoside-Induced Apoptosis of Spiral Ganglion Cells in Guinea Pigs

Objective

To explore whether memantine, an N-methyl-D-aspartate receptor antagonist, exerts a neuroprotective effect against apoptosis of spiral ganglion cells (SGCs) induced by gentamicin.

Study Design

An animal experiment.

Setting

Dong-A University College of Medicine, Busan, Korea.

Subjects and Methods

Gentamicin was injected into the left cochleae of guinea pigs to induce apoptosis of SGCs; the contralateral cochleae served as controls. Memantine was intraperitoneally injected 12 hours and 1 hour prior to gentamicin injection. At 1 week after gentamicin and/or memantine injection, the cochleae were removed and stained with hematoxylin and eosin to evaluate morphologic changes and apoptosis. Western blotting was performed to measure FasL expression and the extent of caspase activation in SGCs.

Results

SGC numbers remained stable after memantine treatment. Western blotting showed that FasL expression and activation of caspases 3, 8, and 9 were reduced in SGCs after memantine treatment.

Conclusion

Memantine attenuated the gentamicin-induced apoptosis of SGCs in guinea pigs. Moreover, memantine may affect Fas-FasL signaling in the receptor-mediated apoptotic pathway and caspase activation involved in the receptor-mediated and mitochondrial apoptotic pathways.

Lysine-specific demethylase 1 inhibitors protect cochlear spiral ganglion neurons against cisplatin-induced damage

Cisplatin is a widely used chemotherapeutic drug, but one of its side effects is ototoxicity. Epigenetic-related drugs, such as lysine-specific demethylase 1 (LSD1) inhibitors, have been reported to protect against cisplatin-induced hair cell loss by preventing demethylation of histone H3K4 (H3K4me2). However, the protective effect of LSD1 inhibitors in spiral ganglion neurons (SGNs) remains unclear. To investigate whether LSD1 inhibitors exert similar protective effects on SGNs, we treated mouse cochlear explant cultures with LSD1 inhibitors (2PCPA, S2101, or CBB1007) together with cisplatin. Low concentrations of cisplatin damaged SGNs much more than high concentrations, and blocking the demethylation of H3K4me2 with LSD1 inhibitors prevented the SGNs from injury. Reactive oxygen species are also involved in the injury process, and LSD1 inhibitors protected SGNs by increasing the expression level of the antioxidant gene Slc7a11 and decreasing the level of the pro-oxidant gene lactoperoxidase (Lpo). Our findings show that LSD1 inhibitors prevent cisplatin-induced SGN loss by regulating the demethylation of H3K4 and preventing increases of reactive oxygen species levels, which might provide a potential therapeutic strategy for cisplatin-induced hearing loss.

Gentamicin alters Akt-expression and its activation in the guinea pig cochlea

Gentamicin treatment induces hair cell death or survival in the inner ear. Besides the well-known toxic effects, the phosphatidylinositol-3 kinase/Akt (PI3K/Akt) pathway was found to be involved in cell protection. After gentamicin application, the spatiotemporal expression patterns of Akt and its activated form (p-Akt) were determined in male guinea pigs. A single dose of 0.1 mL gentamicin (4 mg/ear/animal) was intratympanically injected. The auditory brainstem responses (ABRs) were recorded prior to application and 1, 2 and 7 days afterward. At these three time points the cochleae (n=10 in each case) were removed, transferred to fixative and embedded in paraffin. Seven ears were used as untreated controls. Gentamicin, Akt and p-Akt were identified immunohistochemically in various regions of the cochlea and their staining intensities were quantified on sections using digital image analysis. The application of gentamicin resulted in hearing loss with a concomitant up-regulation of Akt-expression in the organ of Corti and spiral ganglion cells and an additional activation in spiral ganglion cells. At the level of individual ears, clear intracellular correlations were found between Akt- and p-Akt-expression in the stria vascularis and interdental cells and, to a minor extent, in the spiral ligament and the organ of Corti. Furthermore, statistical evidence for the connection between gentamicin up-take and hearing loss was detected. The increase in Akt- and p-Akt-expression in the organ of Corti and spiral ganglion cells indicates a selected response of the cochlea against gentamicin toxicity.

XBP1 mitigates aminoglycoside-induced endoplasmic reticulum stress and neuronal cell death

Here we study links between aminoglycoside-induced mistranslation, protein misfolding and neuropathy. We demonstrate that aminoglycosides induce misreading in mammalian cells and assess endoplasmic reticulum (ER) stress and unfolded protein response (UPR) pathways. Genome-wide transcriptome and proteome analyses revealed upregulation of genes related to protein folding and degradation. Quantitative PCR confirmed induction of UPR markers including C/EBP homologous protein, glucose-regulated protein 94, binding immunoglobulin protein and X-box binding protein-1 (XBP1) mRNA splicing, which is crucial for UPR activation. We studied the effect of a compromised UPR on aminoglycoside ototoxicity in haploinsufficient XBP1 (XBP1^+/−) mice. Intra-tympanic aminoglycoside treatment caused high-frequency hearing loss in XBP1^+/− mice but not in wild-type littermates. Densities of spiral ganglion cells and synaptic ribbons were decreased in gentamicin-treated XBP1^+/− mice, while sensory cells were preserved. Co-injection of the chemical chaperone tauroursodeoxycholic acid attenuated hearing loss. These results suggest that aminoglycoside-induced ER stress and cell death in spiral ganglion neurons is mitigated by XBP1, masking aminoglycoside neurotoxicity at the organismal level.

Cell-specific accumulation patterns of gentamicin in the guinea pig cochlea

Intratympanic gentamicin therapy has become a popular treatment modality for Ménière's disease (MD) through controlled elimination of vertigo spells caused by the balance organ. However, the known ototoxic properties of aminoglycosides lead to cochlear damage. In order to gain more information about cellular preferences for aminoglycoside accumulation within the cochlea, gentamicin was immuno histochemically localized by light microscopy in male guinea pigs 1 and 7 days after intratympanic application (n = 8 ears/incubation time). Differences in the gentamicin-specific cellular storage capacities were quantified by determination of the local immuno staining intensities. Gentamicin was detected in every cochlear cell type, but with spatiotemporal variability. One day after application, an intense staining reaction was found in all cell types except the spiral ganglion cells and the stria vascularis. Six days later, gentamicin staining intensities were additionally reduced in the nerve fibers and the spiral ligament. Statistic analysis revealed strong cellular associations in respect to aminoglycoside accumulation. Furthermore, associations with recorded hearing losses were identified comparing the cellular gentamicin content in the organ of Corti, in the stria vascularis, in the spiral ganglion cells and in fibrocytes of the Limbus. In the lateral wall, clear differences in cellular gentamicin accumulation were found between type I fibrocytes of the spiral ligament compared with basal and intermediate cells of the stria vascularis. This finding was unexpected as these three cell types belong to a well-developed gap-junction system which normally enables unhampered cell communication. Cellular differences in local gentamicin storage capacities, transport processes and inherent diffusion barriers are discussed.

Methionine sulfoxide reductase A, B1 and B2 are likely to be involved in the protection against oxidative stress in the inner ear

The methionine sulfoxide reductase (Msr) family of proteins is a class of repair enzymes that reduce methionine-S (MsrA) or methionine-R (MsrB) sulfoxide to methionine. Recent studies have reported that mutations in the MSRB3 gene cause autosomal recessive hearing loss in humans, and in mice MsrB3 deficiency leads to profound hearing loss due to hair cell apoptosis and stereocilia degeneration. However, apart from MsrB3, studies on Msr proteins in the inner ear have not yet been reported. In this study, we identified and characterized Msr expression in the cochlea and vestibule. First, we confirmed RNA expression levels of Msr family members in the cochlea and vestibule using reverse transcription PCR and detected Msr family members in both tissues. We also conducted immunohistochemical staining to localize Msr family members within the cochlea and vestibule. In the cochlea, MsrA was detected in supporting cells, spiral ligament, spiral limbus, Reissner's membrane and the spiral ganglion. MsrB1 was specifically expressed in hair cells and the spiral ganglion. MsrB2 was noted in the spiral ganglion, tectorial membrane and stria vascularis. In the vestibule, MsrA and MsrB1 were detected in hair cells and the vestibular ganglion, while MsrB2 was restricted to the vestibular ganglion. In this study, we identified distinct distributions of Msr family members in the organ of Corti and hypothesized that MsrA, MsrB1 and MsrB2 protect proteins in the organ of Corti from oxidative stress.

Mechanism of alpha-lipoic acid in attenuating kanamycin-induced ototoxicity

In view of the theory that alpha-lipoic acid effectively prevents cochlear cells from injury caused by various factors such as cisplatin and noise, this study examined whether alpha-lipoic acid can prevent kanamycin-induced ototoxicity. To this end, healthy BALB/c mice were injected subcutaneously with alpha-lipoic acid and kanamycin for 14 days. Auditory brainstem response test showed that increased auditory brainstem response threshold shifts caused by kanamycin were significantly inhibited. Immunohistochemical staining and western blot analysis showed that the expression of phosphorylated p38 mitogen-activated protein kinase and phosphorylated c-Jun N-terminal kinase in mouse cochlea was significantly decreased. The experimental findings suggest that phosphorylated p38 and phosphorylated c-Jun N-terminal kinase mediated kanamycin-induced ototoxic injury in BALB/c mice. Alpha-lipoic acid effectively attenuated kanamycin ototoxicity by inhibiting the kanamycin-induced high expression of phosphorylated p38 and phosphorylated c-Jun N-terminal kinase.

Expression profile of fas-fas ligand in spiral ganglion cells during apoptosis

Objectives

To examine the expression profile of Fas-Fas ligand (FasL) during glutamate (Glu)-induced spiral ganglion cell (SGC) apoptosis.

Methods

Cultured SGCs were treated with 10-mM, 25-mM, and 50-mM concentrations of Glu and incubated for 24 or 48 hours. The expression intensity of FasL, Fas, caspase 3, and morphology of single SGC were evaluated using immunofluorescence staining.

Results

In semiquantitative analysis of the Glu-treated SGC, FasL, and caspase 3 expression intensity were increased with concentration- and time-dependent manner. Fas expression intensity did not change with different concentration at 48 hours. In morphologic analysis of the Glu-treated SGC, number of apoptotic cells were increased with concentration- and time-dependent manner.

Conclusion

FasL was expressed in apoptotic SGCs, suggesting that the Fas-FasL signaling pathway may be involved in the Glu-induced apoptosis of dissociated SGCs.

Intracochlear electrical stimulation suppresses apoptotic signaling in rat spiral ganglion neurons after deafening in vivo

OBJECTIVE

To establish the intracellular consequences of electrical stimulation to spiral ganglion neurons after deafferentation. Here we use a rat model to determine the effect of both low and high pulse rate acute electrical stimulation on activation of the proapoptotic transcription factor Jun in deafferented spiral ganglion neurons in vivo.

STUDY DESIGN

Experimental animal study.

SETTING

Hearing research laboratories of the University of Iowa Departments of Biology and Otolaryngology.

METHODS

A single electrode was implanted through the round window of kanamycin-deafened rats at either postnatal day 32 (P32, n = 24) or P60 (n = 22) for 4 hours of stimulation (monopolar, biphasic pulses, amplitude twice electrically evoked auditory brainstem response [eABR] threshold) at either 100 or 5000 Hz. Jun phosphorylation was assayed by immunofluorescence to quantitatively assess the effect of electrical stimulation on proapoptotic signaling.

RESULTS

Jun phosphorylation was reliably suppressed by 100 Hz stimuli in deafened cochleae of P32 but not P60 rats. This effect was not significant in the basal cochlear turns. Stimulation frequency may be consequential: 100 Hz was significantly more effective than was 5 kHz stimulation in suppressing phospho-Jun.

CONCLUSIONS

Suppression of Jun phosphorylation occurs in deafferented spiral ganglion neurons after only 4 hours of electrical stimulation. This finding is consistent with the hypothesis that electrical stimulation can decrease spiral ganglion neuron death after deafferentation.

Characterization of neuronal cell death in the spiral ganglia of a mouse model of endolymphatic hydrops

HYPOTHESIS

Spiral ganglion neurons (SGN) in the Phex male mouse, a murine model of postnatal endolymphatic hydrops (ELH) undergo progressive deterioration reminiscent of human and other animal models of ELH with features suggesting apoptosis as an important mechanism.

BACKGROUND

Histologic analysis of the mutant's cochlea demonstrates ELH by postnatal Day (P) 21 and SGN loss by P90. The SGN loss seems to occur in a consistent topographic pattern beginning at the cochlear apex.

METHODS

SGN were counted at P60, P90, and P120. Semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR), quantitative PCR, and immunohistochemical analyses of activated caspase-3, caspase-8, and caspase-9 were performed on cochlear sections obtained from mutants and controls. Terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling assay (TUNEL) was carried out on 2 mutants and 2 controls.

RESULTS

Corrected SGN counts in control mice were greater in the apical turn of the cochleae at P90 and P120, respectively (p < 0.01). Increased expression of activated caspase-3, caspase-8, and caspase-9 was seen in the mutant. At later time points, activated caspase expression gradually declined in the apical turns and increased in basal turns of the cochlea. Quantitative and semiquantitative PCR analysis confirmed increased expression of caspase-3, caspase-8, and caspase-9 at P21 and P40. TUNEL staining demonstrated apoptosis at P90 in the apical and basal turns of the mutant cochleae.

CONCLUSION

SGN degeneration in the Phex /Y mouse seems to mimic patterns observed in other animals with ELH. Apoptosis plays an important role in the degeneration of the SGN in the Phex male mouse.

Mechanisms of aminoglycoside ototoxicity and targets of hair cell protection

Aminoglycosides are commonly prescribed antibiotics with deleterious side effects to the inner ear. Due to their popular application as a result of their potent antimicrobial activities, many efforts have been undertaken to prevent aminoglycoside ototoxicity. Over the years, understanding of the antimicrobial as well as ototoxic mechanisms of aminoglycosides has increased. These mechanisms are reviewed in regard to established and potential future targets of hair cell protection.

Blocking caspase-3-dependent pathway preserves hair cells from salicylate-induced apoptosis in the guinea pig cochlea

In the present study, we aim to explore whether the caspase-3-dependent pathway is involved in the apoptotic cell death that occurs in the hair cells (HCs) of guinea pig cochlea following a salicylate treatment. Guinea pigs received sodium salicylate (Na-SA), at a dose of 200 mg·kg(-1)·d(-1) i.p., as a vehicle for 5 consecutive days. In some experiments, N-benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone (zDEVD-FMK), a specific apoptosis inhibitor, was directly applied into the cochlea via the round window niche (RWN) prior to salicylate treatment for determination of caspase-3 activation. Alterations in auditory function were evaluated with auditory brainstem responses (ABR) thresholds. Caspase-3 activity was determined by measuring the proteolytic cleavage product of caspase-3 (N-terminated peptide substrate). DNA fragmentation within the nuclei was examined with a terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) method. Ultrastructure variation in the target cell was assessed by electron microscopy (EM). Salicylate treatment initiated an obvious elevation in ABR thresholds with a maximum average shift of 60 dB sound pressure level (SPL), and caused significant apoptosis in both inner (IHCs) and outer (OHCs) hair cells resulted from an evident increasing in immunoreactivity to caspase-3 protease. Transmission electron microscopy (TEM) displayed chromatin condensation and nucleus margination accompanied by cell body shrinkage in the OHCs, but not in the IHCs. Scanning electron microscopy (SEM) showed breakdown, fusion, and loss in the stereociliary bundles at the apex of OHCs rather than IHCs. zDEVD-FMK pretreatment prior to salicylate injection substantially attenuated an expression of the apoptotic protease and protected HCs against apoptotic death, followed by a moderate relief in the thresholds of ABR, an alleviation in the submicroscopic structure was also identified. In particular, disorientation and insertion in the hair bundles at the apex of OHCs was exhibited though no classic apoptotic change found. The above changes were either prevented or significantly attenuated by zDEVD-FMK. These findings indicate that salicylate could damage cochlear hair cells via inducing apoptosis associated with caspase-3 activation.

Cytomegalovirus-induced sensorineural hearing loss with persistent cochlear inflammation in neonatal mice

Congenital cytomegalovirus (CMV) infection is the leading cause of sensorineural hearing loss (SNHL) in children. During murine (M)CMV-induced encephalitis, the immune response is important for both the control of viral dissemination and the clearance of virus from the brain. While the importance of CMV-induced SNHL has been described, the mechanisms surrounding its pathogenesis and the role of inflammatory responses remain unclear. This study presents a neonatal mouse model of profound SNHL in which MCMV preferentially infected both cochlear perilymphatic epithelial cells and spiral ganglion neurons. Interestingly, MCMV infection induced cochlear hair cell death by 21 days post-infection, despite a clear lack of direct infection of hair cells and the complete clearance of the virus from the cochlea by 14 dpi. Flow cytometric, immunohistochemical, and quantitative PCR analysis of MCMV-infected cochlea revealed a robust and chronic inflammatory response, including a prolonged increase in reactive oxygen species production by infiltrating macrophages. These data support a pivotal role for inflammation during MCMV-induced SNHL.

Effect of Taurine on the antimicrobial efficiency of Gentamicin

CONTEXT

Gentamicin is mainly used in severe infections caused by gram-negatives. However toxicity including nephrotoxicity and ototoxicity is one of the most important complications of its treatment. The production of free radicals seems to be involved in gentamicin toxicity mechanism. Taurine, a major intracellular free β-amino acid, is known to be an endogenous antioxidant. So potentially the co-therapy of taurine and gentamicin would reduce the adverse effects of the antibiotic.

OBJECTIVES

In this study, we wished to know the effect of taurine on the antibiotic capacity of gentamicin.

METHODS

strainsof P. aeruginosa, E. coli, S. aureus and S. epidermidis were used as test organisms. Minimum inhibitory concentrations of gentamicin in the presence and absence of taurine at quantities from 40 to 2 mg/L were determined using macro-dilution method.

RESULTS

MICs were determined in the various concentrations of taurine for bacterial indicators. The MIC values of gentamicin for P. aeruginosa, S. aureus and E. coli remained unchanged in the values of 2.5, 5 and 20 μg/ml respectively in the absence and presences of different concentrations of taurine. The bactericidal activity of gentamicin against S. epidermidis was increased by addition of taurine in the concentrations higher than 6 mg/L.

CONCLUSION

According to our study the antibacterial activity of gentamicin against the indicator microorganisms were not interfere with taurine at selected concentrations. Further in vivo studies are needed to establish if a combination of gentamicin and taurine would have the same effect.