Ouabain-induced apoptosis in cochlear hair cells and spiral ganglion neurons in vitro

Antioxidative Stress-Induced Destruction to Cochlear Cells Caused by Blind Antioxidant Therapy

OBJECTIVE

Verification that blind and excessive use of antioxidants leads to antioxidant stress which exacerbates cochlear cell damage.

STUDY DESIGN

Basic research.

SETTING

The Third Affiliated Hospital of Sun Yat-Sen University.

METHODS

We compared and quantified hair cell-like house ear institute-organ of corti 1 (HEI-OC1) cell density, cell viability, and apoptosis caused by different concentrations of N-acetylcysteine (NAC) via Hoechst staining, Cell Counting Kit 8, Hoechst with propidium iodide staining, and Annexin V with propidium iodide (PI) staining. Apoptosis induced by high concentrations of M40403 and coenzyme Q10 in cochlear explants was analyzed and compared by cochlear dissection and activated caspase 3 labeling.

RESULTS

With the increase of NAC concentration (0-1000 μmol/L), cell density decreased consequently and reached the lowest at 1000 μmol/L (****P ≤ .0001). Cell viability is also declining (**P < .01). The number of Annexin V-fluorescein isothiocyanate-labeled cells and PI-labeled cells increased with increasing NAC concentration after treatment of HEI-OC1 cells for 48 hours. The proportion of apoptotic cells also rose (*P < .05, **P < .01). Cochlear hair cells (HCs) treated with low concentrations of M40403 and coenzyme Q10 for 48 hours showed no damage. When the concentrations of M40403 and coenzyme Q10 were increased (concentrations＞30 μmol/L), HC damage began, followed by a dose-dependent increase in HC loss (*P < .001, **P < .0001). Activated caspase-3 was clearly apparent in cochlear explants treated with 50 μmol/L M40403 and coenzyme Q10 compared with cochlear explants without added M40403 and coenzyme Q10.

CONCLUSION

These experimental results suggest that inappropriate application of antioxidants can cause severe damage to normal cochlear HCs.

OUP accepted manuscript

The spiral ganglion of the cochlea is essential for hearing and contains primary bipolar neurons that relay action potentials generated by mechanosensory hair cells. Injury to spiral ganglion neurons (SGNs) causes permanent hearing loss because these cells have limited regenerative capacity. Establishment of human cell-derived inner ear tissue in vitro could facilitate the development of treatments for hearing loss. Here, we report a stepwise protocol for differentiating human-induced pluripotent stem cells (hiPSCs) into otic organoids that contain SGN-like cells and demonstrate that otic organoids have potential for use as an experimental model of drug-induced neuropathy. Otic progenitor cells (OPCs) were created by 2D culture of hiPSCs for 9 days. Otic spheroids were formed after 2D culture of OPCs for 2 days in a hypoxic environment. Otic organoids were generated by 3D culture of otic spheroids under hypoxic conditions for 5 days and normoxic conditions for a further 30 days or more. The protein expression profile, morphological characteristics, and electrophysiological properties of SGN-like cells in otic organoids were similar to those of primary SGNs. Live-cell imaging of AAV-syn-EGFP-labeled neurons demonstrated temporal changes in cell morphology and revealed the toxic effects of ouabain (which causes SGN-specific damage in animal experiments) and cisplatin (a chemotherapeutic drug with ototoxic adverse effects). Furthermore, a cyclin-dependent kinase-2 inhibitor suppressed the toxic actions of cisplatin on SGN-like cells in otic organoids. The otic organoid described here is a candidate novel drug screening system and could be used to identify drugs for the prevention of cisplatin-induced neuropathy.

RIP3-mediated necroptosis was essential for spiral ganglion neuron damage

To identify the role of RIP3 in ouabain-induced necroptosis and offer clinical implications to prevent spiral ganglion neurons (SGNs) from death, ouabain was applied in SGNs derived from fetal rats and injected into Sprague-Dawley rats to construct injury model in vitro and in vivo, respectively. The necroptosis rate of SGNs was determined by flow cytometry and MTT assays. The protein levels and phosphorylation of RIP3 were evaluated using western blotting and immunofluorescence. SGNs injury was observed using H&E staining and immunofluorescence. The hearing function of rats was evaluated by the auditory brainstem response (ABR) and Distortion Product Otoacoustic Emissions (DPOAE) methods. Ouabain caused dose-dependent necroptosis in SGNs and significant loss of SGNs of the cochlear axis in vivo. RIP3 and pRIP3 were upregulated with SGNs injury promoted, and RIP3 overexpression promoted ouabain-induced necroptosis in SGNs in vitro, which could be suppressed by necrostatin-1. RIP3 knockdown inhibited ouabain-induced necroptosis and reduced the phosphorylation of MLKL but no RIP3-dependent effect on the level of MLKL. RIP3 inhibition in vivo protected rats from ouabain-induced hearing damage with reducing ABR threshold shifts and promoting DPOAE amplitudes, while overexpression of RIP3 enhanced ouabain-induced injury that could be partially reversed by necrostatin-1. A decrease of SGNs density and an upregulation of pRIP3 were observed with RIP3 overexpression, which was in contrast when RIP3 was silenced. Therefore, RIP3 was essential for mediating necroptosis in ouabain-induced SGNs damage. Targeting RIP3 may prevent SGNs from death in clinical practice, and finally help the treatment of sensorineural hearing loss.

Nicotine induced ototoxicity in rat cochlear organotypic cultures

Epidemiological evidence has shown that smoking is associated with an increased risk of hearing loss. However, the underlying mechanisms regarding the impact of nicotine on the cochlea remain unclear. This study aimed to investigate the cytotoxic effects of nicotine on cochlear cells using cultured cochlear basilar membranes. Cochlear basilar membranes were isolated from newborn rats, cultured, and treated with 1–100 ng/mL nicotine for 48 h. Cuticular plates and stereocilia bundle staining were used to evaluate hair cell (HC) loss. Spiral ganglion neuron and acoustic nerve fiber staining were assessed to evaluate cochlear neural injury. Scanning electron microscopy and transmission electron microscopy imaging were employed to examine cochlear ultrastructural changes. Our results showed that compared to spiral ganglia and nerve fibers, HCs are more susceptible to nicotine-induced toxicity. HC loss was more severe in the basal turn than in the middle and apical turns, while nerve fibers and spiral ganglion cells were morphologically maintained. Ultrastructural changes revealed disordered and damaged stereocilia, swelling and decreased mitochondrial density, swelling, and degranulation of the endoplasmic reticulum. Our results suggest that nicotine causes HCs’ degeneration and loss and may have implications for smoking-related hearing loss.

Ouabain Protects Nephrogenesis in Rats Experiencing Intrauterine Growth Restriction and Partially Restores Renal Function in Adulthood

Intrauterine growth restriction (IUGR) is, in general, accompanied by a reduction of the nephron number, which increases the risk of hypertension and renal dysfunction. Studies have revealed that ouabain can partially restore the number of nephrons during IUGR. However, there is limited information regarding the melioration of nephric structure and function. We used maternal malnutrition to induce an IUGR model in rats. Subsequently, we used a mini-pump to administer ouabain to IUGR rats during pregnancy. Male offspring were divided randomly into two groups. One group was fed a normal diet, whereas the other was fed an isocaloric 8% high-salt diet. Maternal malnutrition led to a reduction in the birth weight and number of nephrons in offspring. At the end of a 40-week follow-up period, offspring from the IUGR group had high blood pressure and abnormal excretion of urinary protein; these parameters were exacerbated in offspring fed a high-salt diet. However, ouabain administration during pregnancy could partially restore the number of nephrons in IUGR offspring, normalize blood pressure, and reduce urinary protein excretion, even when challenged with a high-salt diet. Pathology findings revealed that IUGR, particularly following feeding of a high-salt diet, damaged the ultrastructure of glomeruli, but these harmful effects were ameliorated in offspring treated with ouabain. Collectively, our data suggest that ouabain could rescue nephrogenesis in IUGR newborns and protect (at least in part) the structure and function of the kidney during adulthood even when encountering unfavorable environmental challenges in subsequent life.

Bilirubin-induced neurotoxic and ototoxic effects in rat cochlear and vestibular organotypic cultures

Exposure to high levels of bilirubin in hyperbilirubinemia patients and animal models can result in sensorineural deafness. However, the mechanisms underlying bilirubin-induced damage to the inner ear, including the cochlear and vestibular organs, remain unknown. The present analyses of cochlear and vestibular organotypic cultures obtained from postnatal day 3 rats exposed to bilirubin at varying concentrations (0, 10, 50, 100, or 250 μM) for 24 h revealed that auditory nerve fibers (ANFs) and vestibular nerve endings were destroyed even at low doses (10 and 50 μM). Additionally, as the bilirubin dose increased, spiral ganglion neurons (SGNs) and vestibular ganglion neurons (VGNs) exhibited gradual shrinkage in conjunction with nuclei condensation or fragmentation in a dose-dependent manner. The loss of cochlear and vestibular hair cells (HCs) was only evident in explants treated with the highest concentration of bilirubin (250 μM), and bilirubin-induced major apoptosis most likely occurred via the extrinsic apoptotic pathway. Thus, the present results indicate that inner ear neurons and fibers were more sensitive to, and exhibited more severe damage following, bilirubin-induced neurotoxicity than sensory HCs, which illustrates the underlying causes of auditory neuropathy and vestibulopathy in hyperbilirubinemia patients.

A traditional Chinese medicine compound (Jian Er) for presbycusis in a mouse model: Reduction of apoptosis and protection of cochlear sensorineural cells and hearing

Age-related hearing loss (AHL) or presbycusis is steadily increasing due to the overall aging of the Chinese population. Traditional Chinese medicine (TCM) has long been used to prevent and treat deafness, but its effectiveness and mechanism of action are still uncertain. The present study tested a TCM preparation called "Jian Er" in a mouse model of prebycusis.

Cisplatin-induced vestibular hair cell lesion-less damage at high doses

Cisplatin, a widely used anticancer drug, damages hair cells in cochlear organotypic cultures at low doses, but paradoxically causes little damage at high doses resulting in a U-shaped dose-response function. To determine if the cisplatin dose-response function for vestibular hair cells follows a similar pattern, we treated vestibular organotypic cultures with doses of cisplatin ranging from 10 to 1000 μM. Vestibular hair cell lesions progressively increased as the dose of cisplatin increased with maximum damage occurring around 50–100 μM, but the lesions progressively decreased at higher doses resulting in little hair cell loss at 1000 μM. The U-shaped dose-response function for cisplatin-treated vestibular hair cells in culture appears to be regulated by copper transporters, Ctr1, ATP7A and ATP7B, that dose-dependently regulate the uptake, sequestration and extrusion of cisplatin.

Ouabain-induced apoptosis and inhibition of viability of tubulointerstitial cells by regulating NKA/pSrc/pERK/pAkt/pS6k/caspase 3 may contribute to lupus nephritis development

This study aimed to investigate the underlying molecular mechanism of ouabain-induced apoptosis and inhibited viability of tubulointerstitial cells in lupus nephritis (LN). Serum ouabain expression in 21 LN patients and 20 healthy controls (HCs) was detected by ELISA. Na-K-ATPase α1 (NKA) expression in kidney tissue from 21 LN patients and 6 controls who underwent nephrectomy was determined by immunohistochemistry assay. HK-2 cells were treated by ouabain with concentration 0 nM (control), 0.1 nM, 1 nM, 10 nM, 100 nM and 1000 nM for 72 h; MTT assay was performed at 24 h, 48 h and 72 h to detect cells viability, AV/PI assay was conducted at 24 h to detect the cell apoptosis. NKA expression was detected by immunofluorescence assay and western blot (WB) while pSrc, pERK, pAkt, pS6K and caspase 3 expression was detected by WB after 100 nM ouabain treatment. Serum ouabain was elevated in LN patients compared to HCs, while kidney NKA was reduced in LN patients compared with controls. HK-2 cell viability by MTT assay was decreased while cell apoptosis by AV/PI was increased at each time point after ouabain 0.1 nM, 1 nM, 10 nM, 100 nM and 1000 nM treatment compared to control group. Both immunofluorescence and WB disclosed that NKA expression was decreased at 12 h and 24 h compared to 0 h after 100 nM ouabain treatment. Moreover, pSrc, pERK, pAkt, pS6K and caspase 3 expressions were elevated after 100 nM ouabain treatment. In conclusion, ouabain may contribute to LN etiology by inhibiting human proximal tubular cell viability and promoting cells apoptosis through regulating NKA, pSrc, pERK, pAkt, pS6K and caspase 3.

Na+/K+ ATPase activity promotes invasion of endocrine resistant breast cancer cells

Background

The Na⁺/K⁺-ATPase (NKP) is an important ion transporter also involved in signal transduction. Its expression profile is altered in various tumours including that of the breast. We studied the effect of inhibiting NKP activity in non-tumorigenic breast cell line and in estrogen receptor positive and negative breast cancer cells.

Methods

Expression and localization of NKP and downstream signaling molecules were determined by RT-PCR, western blotting and immunofluorescence. Cell proliferation, apoptosis and cell cycle stage were determined using MTT, annexin V and flow cytometry. Cell motility and invasion were determined using wound healing and matrigel assays. Total matrix metalloproteinase (MMP) was determined by a fluorescence-based assay.

Results

NKP was mainly localized on the cell membrane. Its baseline expression and activity were enhanced in breast cancer compared to the non-tumorigenic breast cell line. Ouabain and 3,4,5,6-tetrahydroxyxanthone (TTX) treatment significantly inhibited NKP activity, which significantly reduced cell proliferation, motility, invasion and pH-induced membrane blebbing. EGF stimulation induced internalization of NKP from the cell membrane to the cytoplasm. Ouabain inhibited EGF-induced phosphorylation of Rac/cdc42, profillin, ERK1/2 and P70S6K.

Conclusions

The NKP may offer a novel therapeutic target in breast cancer patients who have developed metastasis, aiming to improve therapeutic outcomes and enhance survival rate.

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.

Dose-dependent effects of ouabain on spiral ganglion neurons and Schwann cells in mouse cochlea

OBJECTIVE

This study aimed in fully investigating the toxicities of ouabain to mouse cochlea and the related cellular environment, and providing an optimal animal model system for cell transplantation in the treatment of auditory neuropathy (AN) and sensorineural hearing loss (SNHL).

METHODS

Different dosages of ouabain were applied to mouse round window. The auditory brainstem responses and distortion product otoacoustic emissions were used to evaluate the cochlear function. The immunohistochemical staining and cochlea surface preparation were performed to detect the spiral ganglion neurons (SGNs), Schwann cells and hair cells.

RESULTS

Ouabain at the dosages of 0.5 mM, 1 mM and 3 mM selectively and permanently destroyed SGNs and their functions, while leaving the hair cells relatively intact. Ouabain at 3 mM resulted in the most severe SGNs loss and induced significant loss of Schwann cells started as early as 7 days and with further damages at 14 and 30 days after ouabain exposure.

CONCLUSIONS

The application of ouabain to mouse round window induces damages of SGNs and Schwann cells in a dose- and time-dependent manner, this study established a reliable and accurate animal model system of AN and SNHL.

Auditory Neuropathy after Damage to Cochlear Spiral Ganglion Neurons in Mice Resulting from Conditional Expression of Diphtheria Toxin Receptors

Auditory neuropathy (AN) is a hearing disorder characterized by normal cochlear amplification to sound but poor temporal processing and auditory perception in noisy backgrounds. These deficits likely result from impairments in auditory neural synchrony; such dyssynchrony of the neural responses has been linked to demyelination of auditory nerve fibers. However, no appropriate animal models are currently available that mimic this pathology. In this study, Cre-inducible diphtheria toxin receptor (iDTR^+/+) mice were cross-mated with mice containing Cre (Bhlhb5-Cre^+/−) specific to spiral ganglion neurons (SGNs). In double-positive offspring mice, the injection of diphtheria toxin (DT) led to a 30–40% rate of death for SGNs, but no hair cell damage. Demyelination types of pathologies were observed around the surviving SGNs and their fibers, many of which were distorted in shape. Correspondingly, a significant reduction in response synchrony to amplitude modulation was observed in this group of animals compared to the controls, which had a Cre⁻ genotype. Taken together, our results suggest that SGN damage following the injection of DT in mice with Bhlhb5-Cre^+/− and iDTR^+/− is likely to be a good AN model of demyelination.

Carbaryl-induced ototoxicity in rat postnatal cochlear organotypic cultures

Carbaryl, a widely used carbamate-based insecticide, is a potent anticholinesterase known to induce delayed neurotoxicity following chronic exposure. However, its potential toxic effects on the cochlea, the sensory organ for hearing that contains cholinergic efferent neurons and acetylcholine receptors on the hair cells (HC) and spiral ganglion neurons has heretofore not been evaluated. To assess ototoxic potential of carbaryl, cochlear organotypic cultures from postnatal day 3 rats were treated with doses of carbaryl ranging from 50 to 500 μM for 48 h up to 96 h. Carbaryl damaged both the sensory HC and spiral ganglion neurons in a dose- and duration-dependent manner. HC and neuronal damage was observed at carbaryl concentrations as low as 50 μM after 96-h treatment and 100 μM after 48-h treatment. Hair cell was greatest in the high frequency basal region of the cochlea and progressively decreased towards the apex consistent with the majority of ototoxic drugs. In contrast, damage to the spiral ganglion neurons was of similar magnitude in the basal and apical regions of the cochlea. Carbaryl damage was characterized by soma shrinkage, nuclear condensation and fragmentation, and blebbing, morphological features of programmed cell death. Carbaryl upregulated the expression of executioner caspase-3 in HC and spiral ganglion neurons indicating that cellular damage occurred primarily by caspase-mediated apoptosis. These results suggest that chronic exposure to carbaryl and other carbamate anticholinesterases may be ototoxic. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 956-969, 2017.

Ouabain-induced changes in MAP kinase phosphorylation in primary culture of rat cerebellar cells

Cardiotonic steroid (CTS) ouabain is a well-established inhibitor of Na,K-ATPase capable of inducing signalling processes including changes in the activity of the mitogen activated protein kinases (MAPK) in various cell types. With increasing evidence of endogenous CTS in the blood and cerebrospinal fluid, it is of particular interest to study ouabain-induced signalling in neurons, especially the activation of MAPK, because they are the key kinases activated in response to extracellular signals and regulating cell survival, proliferation and apoptosis. In this study we investigated the effect of ouabain on the level of phosphorylation of three MAPK (ERK1/2, JNK and p38) and on cell survival in the primary culture of rat cerebellar cells. Using Western blotting we described the time course and concentration dependence of phosphorylation for ERK1/2, JNK and p38 in response to ouabain. We discovered that ouabain at a concentration of 1 μM does not cause cell death in cultured neurons while it changes the phosphorylation level of the three MAPK: ERK1/2 is phosphorylated transiently, p38 shows sustained phosphorylation, and JNK is dephosphorylated after a long-term incubation. We showed that ERK1/2 phosphorylation increase does not depend on ouabain-induced calcium increase and p38 activation. Changes in p38 phosphorylation, which is independent from ERK1/2 activation, are calcium dependent. Changes in JNK phosphorylation are calcium dependent and also depend on ERK1/2 and p38 activation. Ten-micromolar ouabain leads to cell death, and we conclude that different effects of 1-μM and 10-μM ouabain depend on different ERK1/2 and p38 phosphorylation profiles. Copyright © 2016 John Wiley & Sons, Ltd.

The potential use of low-frequency tones to locate regions of outer hair cell loss

Current methods used to diagnose cochlear hearing loss are limited in their ability to determine the location and extent of anatomical damage to various cochlear structures. In previous experiments, we have used the electrical potential recorded at the round window -the cochlear response (CR) -to predict the location of damage to outer hair cells in the gerbil. In a follow-up experiment, we applied 10 mM ouabain to the round window niche to reduce neural activity in order to quantify the neural contribution to the CR. We concluded that a significant proportion of the CR to a 762 Hz tone originated from phase-locking activity of basal auditory nerve fibers, which could have contaminated our conclusions regarding outer hair cell health. However, at such high concentrations, ouabain may have also affected the responses from outer hair cells, exaggerating the effect we attributed to the auditory nerve. In this study, we lowered the concentration of ouabain to 1 mM and determined the physiologic effects on outer hair cells using distortion-product otoacoustic emissions. As well as quantifying the effects of 1 mM ouabain on the auditory nerve and outer hair cells, we attempted to reduce the neural contribution to the CR by using near-infrasonic stimulus frequencies of 45 and 85 Hz, and hypothesized that these low-frequency stimuli would generate a cumulative amplitude function (CAF) that could reflect damage to hair cells in the apex more accurately than the 762 stimuli. One hour after application of 1 mM ouabain, CR amplitudes significantly increased, but remained unchanged in the presence of high-pass filtered noise conditions, suggesting that basal auditory nerve fibers have a limited contribution to the CR at such low frequencies.

Single-Cell Transcriptome Analysis of Developing and Regenerating Spiral Ganglion Neurons

The spiral ganglion neurons (SGNs) of the cochlea are essential for our ability to hear. SGN loss after exposure to ototoxic drugs or loud noise results in hearing loss. Pluripotent stem cell-derived and endogenous progenitor cell types have the potential to become SGNs and are cellular foundations for replacement therapies. Repurposing transcriptional regulatory networks to promote SGN differentiation from progenitor cells is a strategy for regeneration. Advances in the Fludigm C1 workflow or Drop-seq allow sequencing of single cell transcriptomes to reveal variability between cells. During differentiation, the individual transcriptomes obtained from single-cell RNA-seq can be exploited to identify different cellular states. Pseudotemporal ordering of transcriptomes describes the differentiation trajectory, allows monitoring of transcriptional changes and determines molecular barriers that prevent the progression of progenitors into SGNs. Analysis of single cell transcriptomes will help develop novel strategies for guiding efficient SGN regeneration.

Ouabain Enhances ADPKD Cell Apoptosis via the Intrinsic Pathway

Progression of autosomal dominant polycystic kidney disease (ADPKD) is highly influenced by factors circulating in blood. We have shown that the hormone ouabain enhances several characteristics of the ADPKD cystic phenotype, including the rate of cell proliferation, fluid secretion and the capacity of the cells to form cysts. In this work, we found that physiological levels of ouabain (3 nM) also promote programmed cell death of renal epithelial cells obtained from kidney cysts of patients with ADPKD (ADPKD cells). This was determined by Alexa Fluor 488 labeled-Annexin-V staining and TUNEL assay, both biochemical markers of apoptosis. Ouabain-induced apoptosis also takes place when ADPKD cell growth is blocked; suggesting that the effect is not secondary to the stimulatory actions of ouabain on cell proliferation. Ouabain alters the expression of BCL family of proteins, reducing BCL-2 and increasing BAX expression levels, anti- and pro-apoptotic mediators respectively. In addition, ouabain caused the release of cytochrome c from mitochondria. Moreover, ouabain activates caspase-3, a key “executioner” caspase in the cell apoptotic pathway, but did not affect caspase-8. This suggests that ouabain triggers ADPKD cell apoptosis by stimulating the intrinsic, but not the extrinsic pathway of programmed cell death. The apoptotic effects of ouabain are specific for ADPKD cells and do not occur in normal human kidney cells (NHK cells). Taken together with our previous observations, these results show that ouabain causes an imbalance in cell growth/death, to favor growth of the cystic cells. This event, characteristic of ADPKD, further suggests the importance of ouabain as a circulating factor that promotes ADPKD progression.

Cobalt-Induced Ototoxicity in Rat Postnatal Cochlear Organotypic Cultures

Cobalt (Co) is a required divalent metal used in the production of metal alloys, batteries, and pigments and is a component of vitamin B12. Excessive uptake of Co is neurotoxic causing temporary or permanent hearing loss; however, its ototoxic effects on the sensory hair cells, neurons, and support cells in the cochlea are poorly understood. Accordingly, we treated postnatal day 3 rat cochlear organotypic cultures with various doses and durations of CoCl2 and quantified the damage to the hair cells, peripheral auditory nerve fibers, and spiral ganglion neurons (SGN). Five-day treatment with 250 μM CoCl2 caused extensive damage to hair cells and neurons which increased with dose and treatment duration. CoCl2 caused greater damage to outer hair cells than inner hair cells; damage was greatest in the base of the cochlea and decreased towards the base. CoCl2 increased expression of superoxide radical in hair cells and SGNs and SGN loss was characterized by nuclear condensation and fragmentation, morphological features of apoptosis. CoCl2 treatment increased the expression of caspase-3 indicative of caspase-mediated programmed cell death. These results identify hair cells and spiral ganglion neurons as the main targets of Co ototoxicity in vitro and implicate the superoxide radical as a trigger of caspase-mediated ototoxicity.

Polybrene: Observations on cochlear hair cell necrosis and minimal lentiviral transduction of cochlear hair cells

Polybrene is widely used to enhance viral transduction; however, little is known about the utility thereof, in enhancing lentiviral transduction of cochlear cells. In the present study, we examined the cytotoxic effects of polybrene, and the further effects thereof, on lentiviral transduction of cochlear cells, especially sensory hair cells. Cochlear basilar membranes of newborn rats were cultured and treated with 0.1-10 μg/mL polybrene for 24h to explore the potential development of ototoxicity. PI staining and TUNEL detection were used to evaluate necrosis or apoptosis of hair cell. Various doses of lentivirus-GFP were added to cochlear organotypic cultures with safe concentrations of polybrene, incubated for 24h, and cultured (in the absence of the virus and polybrene) for a further 48 h. Transduction efficiencies were evaluated. The results showed that polybrene at 0.1 μg/mL was safe to cochlear cells, and 0.5-10 μg/mL concentration induced hair cell necrosis in a dose-dependent manner. However, supporting cells were not damaged. Lentiviral vectors transduced into cochlear cells and 0.1 μg/mL polybrene enhanced transduction efficiency. However, hair cells were hardly transduced with lentiviral vectors either alone or in the presence of 0.1 μg/mL polybrene. The use of polybrene to aid lentiviral transduction of cochlear hair cells requires further attention.

Standardized surgical approaches to ear surgery in rats

OBJECTIVE

To describe several approaches of ear surgeries for experimental studies in rats.

METHODS

Anesthetized rats were prepared for demonstration of various ear surgery approaches designed to optimize experimental outcomes in studies with specific goals and exposure requirements. The surgical approaches included the posterior tympanum, superior tympanum, inferior tympanum and occipital approaches.

RESULTS

The middle ear cavity and inner ear were successfully exposed from different angles via the mentioned surgical approaches. For example, electrode placement for recording of cochlear bioelectric responses was easily achieved through the posterior tympanum or inferior tympanum approach. Alternatively, drug delivery or gene transfection via round window membrane was most easily accomplished using the posterior tympanum approach. Cochlear perfusion of protective or ototoxic drugs was best performed using the inferior tympanum approach. Ossicular chain interruption to induce a prolonged conductive hearing loss was readily achieved using a superior tympanum approach. Lastly, surgical destruction of the endolymphatic sac to induce experimental endolymphatic hydrops was readily performed via an occipital surgical approach.

CONCLUSION

These standardized surgical approaches can be applied in scientific studies of the ear with different purposes covering electrophysiology, conductive hearing loss, intra-cochlear drug perfusion and experimental studies relevant to Meniere's disease.

Gene transfection mediated by polyethyleneimine-polyethylene glycol nanocarrier prevents cisplatin-induced spiral ganglion cell damage

Polyethyleneimine-polyethylene glycol (PEI-PEG), a novel nanocarrier, has been used for transfection and gene therapy in a variety of cells. In our previous study, we successfully carried out PEI-PEG-mediated gene transfer in spiral ganglion cells. It remains unclear whether PEI-PEG could be used for gene therapy with X-linked inhibitor of apoptosis protein (XIAP) in the inner ear. In the present study, we performed PEI-PEG-mediated XIAP gene transfection in the cochlea of Sprague-Dawley rats, via scala tympani fenestration, before daily cisplatin injections. Auditory brainstem reflex tests demonstrated the protective effects of XIAP gene therapy on auditory function. Immunohistochemical staining revealed XIAP protein expression in the cytoplasm of cells in the spiral ganglion, the organ of Corti and the stria vascularis. Reverse transcription-PCR detected high levels of XIAP mRNA expression in the cochlea. The present findings suggest that PEI-PEG nanocarrier-mediated XIAP gene transfection results in XIAP expression in the cochlea, prevents damage to cochlear spiral ganglion cells, and protects hearing.

Apoptosis-inducing factor and calpain upregulation in glutamate-induced injury of rat spiral ganglion neurons

Spiral ganglion neuron (SGN) damage and apoptosis can lead to noise-induced hearing loss, age-associated hearing loss and, in certain cases, auditory neuropathy. The apoptosis-inducing factor (AIF)-associated pathway may be important in this process. The present study aimed to investigate the expression levels of AIF and calpain in damaged SGNs. Glutamate (Glu) perfusion and cell culture in different concentrations of Glu were performed to damage the SGNs of Sprague-Dawley (SD) rats, with saline water used as a control Different concentrations (5, 10, 20 and 40 mM) of Glu were injected into the cochlear tympanic canal of 18 SD rats, and 10, 20 and 40 mM Glu were added to SGN cultures. Auditory brainstem responses (ABR) were measured prior to and 2 days following the injection of Glu. Immunofluorescent staining was used to detect the SGN damage and the expression levels of AIF and calpain in vivo and in in vitro. Transmission electron microscopy (TEM) was used to measure cell apoptosis and reverse transcription-quantitative polymerase chain reaction was used to analyse the gene expression levels of AIF and calpain in the damaged SGNs. The TEM identified mitochondrial vacuolisation, swelling of the SGN and hetero-chromatin formation. Injection of Glu reduced the number of SGNs and induced apoptosis. AIF was observed to translocate into the nuclei of the SGNs in the 20 and 40 mM Glu groups, and the expression levels of AIF and calpain were markedly upregulated in the modiolus of the Glu-damaged SGNs. The upregulation of AIF and calpain may be important in the process of SGN damage and apoptosis.