Disruption of Gap Junction-Mediated Intercellular Communication in the Spiral Ligament Causes Hearing and Outer Hair Cell Loss in the Cochlea of Mice.

It is well-known that outer hair cell (OHC) loss occurs in the cochlea of animal models of permanent hearing loss induced by intense noise exposure. Our earlier studies demonstrated the production of hydroxynonenal and peroxynitrite, as well as the disruption of gap junction-mediated intercellular communication (GJIC), in the cochlear spiral ligament prior to noise-induced sudden hearing loss. The goal of the present study was to evaluate the mechanism underlying cochlear OHC loss after sudden hearing loss induced by intense noise exposure. In organ of Corti explant cultures from mice, no significant OHC loss was observed after in vitro exposure to 4-hydroxynonenal (a product of lipid peroxidation), H2O2, SIN-1 (peroxynitrite generator), and carbenoxolone (a gap junction inhibitor). Interestingly, in vivo intracochlear carbenoxolone injection through the posterior semicircular canal caused marked OHC and hearing loss, as well as the disruption of gap junction-mediated intercellular communication in the cochlear spiral ligament. However, no significant OHC loss was observed in vivo in animals treated with 4-hydroxynonenal and SIN-1. Taken together, our data suggest that disruption of GJIC in the cochlear lateral wall structures is an important cause of cochlear OHC loss in models of hearing loss, including those induced by noise.

Calpain inhibitor alleviates permanent hearing loss induced by intense noise by preventing disruption of gap junction-mediated intercellular communication in the cochlear spiral ligament.

Our previous studies demonstrated that intense noise-induced hearing loss might be at least in part due to an oxidative stress-induced decrease in the level of gap junction-composing protein connexins in the spiral ligament (SL) of the cochlear lateral wall structures in mice. Further, an in vivo exposure of mice to intense noise activates calpain in the cochlear SL. Based on these studies, we sought to determine whether a calpain inhibitor would prevent an intense noise exposure from causing hearing loss, disruption of gap junction-mediated intercellular communication (GJIC) in the SL. An exposure of mice to intense noise (8-Hz octave band noise, 110-dB sound pressure level, 1h) produced permanent hearing loss and cochlear hair cell death. The results of an ex vivo assay using gap-fluorescence recovery after photobleaching of dissected lateral wall structures revealed that the intense noise disrupted GJIC in the cochlear SL at day-7 post exposure. A prior intracochlear injection of the calpain inhibitor PD150606 significantly abolished this noise-induced hearing loss on days 5 and 7 post exposure. Similarly, PD150606 prevented noise-induced hair cell death and the GJIC disruption on day-7 post exposure. The intense noise temporarily enhanced the gene expression of calpain subtypes Capn1 and Capn2 immediately after exposure. Taken together, our data suggest that calpain inhibitor alleviated the noise-induced hearing loss, at least in part, by preventing disruption of GJIC in the cochlear SL. It possible that calpain inhibitors would be useful as a candidate of therapeutic drugs for sudden sensorineural hearing loss.

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.

Disruption of ion-trafficking system in the cochlear spiral ligament prior to permanent hearing loss induced by exposure to intense noise: possible involvement of 4-hydroxy-2-nonenal as a mediator of oxidative stress.

Noise-induced hearing loss is at least in part due to disruption of endocochlear potential, which is maintained by various K(+) transport apparatuses including Na(+), K(+)-ATPase and gap junction-mediated intercellular communication in the lateral wall structures. In this study, we examined the changes in the ion-trafficking-related proteins in the spiral ligament fibrocytes (SLFs) following in vivo acoustic overstimulation or in vitro exposure of cultured SLFs to 4-hydroxy-2-nonenal, which is a mediator of oxidative stress. Connexin (Cx)26 and Cx30 were ubiquitously expressed throughout the spiral ligament, whereas Na(+), K(+)-ATPase α1 was predominantly detected in the stria vascularis and spiral prominence (type 2 SLFs). One-hour exposure of mice to 8 kHz octave band noise at a 110 dB sound pressure level produced an immediate and prolonged decrease in the Cx26 expression level and in Na+, K(+)-ATPase activity, as well as a delayed decrease in Cx30 expression in the SLFs. The noise-induced hearing loss and decrease in the Cx26 protein level and Na(+), K(+)-ATPase activity were abolished by a systemic treatment with a free radical-scavenging agent, 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl, or with a nitric oxide synthase inhibitor, N(ω)-nitro-L-arginine methyl ester hydrochloride. In vitro exposure of SLFs in primary culture to 4-hydroxy-2-nonenal produced a decrease in the protein levels of Cx26 and Na(+), K(+)-ATPase α1, as well as Na(+), K(+)-ATPase activity, and also resulted in dysfunction of the intercellular communication between the SLFs. Taken together, our data suggest that disruption of the ion-trafficking system in the cochlear SLFs is caused by the decrease in Cxs level and Na(+), K(+)-ATPase activity, and at least in part involved in permanent hearing loss induced by intense noise. Oxidative stress-mediated products might contribute to the decrease in Cxs content and Na(+), K(+)-ATPase activity in the cochlear lateral wall structures.

Systemic dexamethasone for the prevention of cisplatin-induced ototoxicity.

Ototoxicity is a common side effect of cisplatin chemotherapy. This study was undertaken to determine the potential protective effects of a systemic administration of dexamethasone against cisplatin-induced ototoxicity. A prospective controlled trial conducted in an animal model. The setting was Animal care research facilities of the Montreal Children's Hospital Research Institute. An experimental guinea pig model was used. The animals were divided as follows: group 1 (n = 10): 12 mg/kg intraperitoneal (IP) cisplatin, group 2 (n = 14): 15 mg/kg/day dexamethasone IP for 2 days followed by cisplatin 12 mg/kg IP, group 3 (n = 14): 10 mg/kg/day dexamethasone IP for 2 days, on day 3, they received cisplatin 12 mg/kg IP followed by 20 mg/kg/day dexamethasone for 2 days and group 4 (n = 5): 10 ml of saline IP twice a day for 3 days. Auditory brainstem response (ABR) threshold shifts were measured at four frequencies (8, 16, 20 and 25 kHz) for groups 1, 2 and 3. Histological changes in the organ of Corti, the stria vascularis, the spiral ligament and the spiral ganglion neurons as well as scanning electron microscopy for outer hair cells were completed. Immunohistochemistry for tumour necrosis factor-alpha (TNF-α) was performed. ABR threshold shifts were similar in all groups. Histological and scanning electron findings demonstrate that dexamethasone has greater protective effect on the stria vascularis. Systemic dexamethasone administration in a guinea pig model did not provide significant protection against cisplatin-induced ototoxicity. Dexamethasone may be useful in future applications as a complementary treatment.

Functional expression of P2X4 receptor in capillary endothelial cells of the cochlear spiral ligament and its role in regulating the capillary diameter.

The cochlear lateral wall generates the endocochlear potential (EP), which creates a driving force for the hair cell transduction current and is essential for normal hearing. Blood flow at the cochlear lateral wall is critically important for maintaining the EP. The vulnerability of the EP to hypoxia suggests that the blood flow in the cochlear lateral wall is dynamically and precisely regulated to meet the changing metabolic needs of the cochlear lateral wall. It has been reported that ATP, an important extracellular signaling molecule, plays an essential role in regulating cochlear blood flow. However, the cellular mechanism underlying ATP-induced regional blood flow changes has not been investigated. In the current study, we demonstrate that 1) the P2X4 receptor is expressed in endothelial cells (ECs) of spiral ligament (SL) capillaries. 2) ATP elicits a characteristic current through P2X4 on ECs in a dose-dependent manner (EC(50) = 0.16 mM). The ATP current has a reversal potential at ∼0 mV; is inhibited by 5-(3-bromophenyl)-1,3-dihydro-2H-benzofuro[3,2-e]-1,4-diazepin-2-one (5-BDBD), LaCl(3), pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) tetrasodium salt hydrate (PPADS), and extracellular acidosis; and is less sensitive to α,ß-methyleneadenosine 5'-triphosphate (α,ß-MeATP) and 2'- and 3'-O-(4-benzoyl-benzoyl) adenosine 5'-triphosphate (BzATP). 3) ATP elicits a transient increase of intracellular Ca(2+) in ECs. 4) In accordance with the above in vitro findings, perilymphatic ATP (1 mM) caused dilation in SL capillaries in vivo by 11.5%. N(ω)-nitro-l-arginine methyl ester hydrochloride (l-NAME), a nonselective inhibitor of nitric oxide synthase, or 5-BDBD, the specific P2X4 inhibitor, significantly blocked the dilation. These findings support our hypothesis that extracellular ATP regulates cochlear lateral blood flow through P2X4 activation in ECs.

Mechanism underlying the protective effect of tempol and Nω-nitro-L-arginine methyl ester on acoustic injury: possible involvement of c-Jun N-terminal kinase pathway and connexin26 in the cochlear spiral ligament.

There is evidence that reactive oxygen species (ROS) are formed in the cochlea during acoustic injury. However, very little is known about the involvement of ROS signals in the spiral ligament (SL) during such injury. The purpose of this study was to determine the effect of the multifunctional antioxidant tempol and the nitric oxide synthase inhibitor N(ω)-nitro-L-arginine methyl ester (L-NAME) on acoustic injury and the c-Jun N-terminal kinase (JNK) pathway in the SL. Exposure of adult mice to noise (8-kHz octave band, 110-dB SPL for 1 h) produced permanent hearing loss. Noise exposure increased not only the formation of a protein modified by 4-hydroxynonenal and formation of nitrotyrosine, but also the level of phospho-JNK in the SL. Pretreatment with tempol or L-NAME was effective in protecting the noise-exposed animals from hearing loss, as well as in abolishing the noise-induced activation of the JNK signaling pathway. Interestingly, noise exposure caused a dramatic decrease in connexin26 level in the SL. This decrease was prevented by tempol or L-NAME. Taken together, our data suggest that noise-induced hearing loss is due at least in part to ROS / nitric oxide-mediated activation of the JNK pathway and down-regulation of connexin26 in the SL of mice.

A mouse model for degeneration of the spiral ligament.

Previous studies have indicated the importance of the spiral ligament (SL) in the pathogenesis of sensorineural hearing loss. The aim of this study was to establish a mouse model for SL degeneration as the basis for the development of new strategies for SL regeneration. We injected 3-nitropropionic acid (3-NP), an inhibitor of succinate dehydrogenase, at various concentrations into the posterior semicircular canal of adult C57BL/6 mice. Saline-injected animals were used as controls. Auditory function was monitored by measurements of auditory brain stem responses (ABRs). On postoperative day 14, cochlear specimens were obtained after the measurement of the endocochlear potential (EP). Animals that were injected with 5 or 10 mM 3-NP showed a massive elevation of ABR thresholds along with extensive degeneration of the cochleae. Cochleae injected with 1 mM 3-NP exhibited selective degeneration of the SL fibrocytes but alterations in EP levels and ABR thresholds were not of sufficient magnitude to allow for testing functional recovery after therapeutic interventions. Animals injected with 3 mM 3-NP showed a reduction of around 50% in the EP along with a significant loss of SL fibrocytes, although degeneration of spiral ganglion neurons and hair cells was still present in certain regions. These findings indicate that cochleae injected with 3 mM 3-NP may be useful in investigations designed to test the feasibility of new therapeutic manipulations for functional SL regeneration.