Cochleosaccular pathology after perinatal and postnatal asphyxia: histopathologic findings

Apelin-13 prevents apoptosis in the cochlear tissue of noise-exposed rat via Sirt-1 regulation

Noise-induced hearing loss (NIHL) is the second most common cause of acquired hearing loss. Acoustic trauma can cause oxidative damage in the cochlear hair cells (HCs) through apoptotic pathways. Apelin is a newly discovered neuropeptide with neuroprotective effects against the oxidative stress in neurodegenerative disorder. We investigated the preventive effects of apelin-13 on the cochlear HCs and spiral ganglion neurons (SGNs) against acoustic trauma via Sirtuin-1 (Sirt-1) regulation in rats. Animals were assigned to control, control + apelin-13 (50 or 100 μg/kg, ip), and noise exposure groups without any treatment or were administered apelin-13 (50 or 100 μg/kg, ip) and EX-527 (an inhibitor of Sirt-1) prior to each noise session. In the noise groups, 110 dB white noise was applied for 6 h per 5 days. Pre- and post-exposure distortion product otoacoustic emissions (DPOAE) and cochlear superoxide dismutase (SOD) activity were assessed. Western blot evaluated the cochlear protein expressions of Sirt-1, cleaved-caspase-3, Bax, and Bcl-2. Cell apoptosis was detected through TUNEL staining. Immunofluorescence was used to examine expression of HCs and SGNs specific protein. DPOAE level were significantly improved in the noise exposure group receiving 100 μg/kg apelin-13. At high doses, apelin augmented SOD levels in the rat cochlea subjected to noise. Apelin 100 markedly increased Sirt-1, and decreased cleaved- caspase-3 expression as well as Bax/Bcl-2 ratio in the cochlea tissue of noise-exposed rats. These findings suggest the promising therapeutic potential of apelin-13 for the prevention of noise-induced injury to cochlea and hearing loss.

Mitochondrial Dysfunction and Therapeutic Targets in Auditory Neuropathy

Sensorineural hearing loss (SNHL) becomes an inevitable worldwide public health issue, and deafness treatment is urgently imperative; yet their current curative therapy is limited. Auditory neuropathies (AN) were proved to play a substantial role in SNHL recently, and spiral ganglion neuron (SGN) dysfunction is a dominant pathogenesis of AN. Auditory pathway is a high energy consumption system, and SGNs required sufficient mitochondria. Mitochondria are known treatment target of SNHL, but mitochondrion mechanism and pathology in SGNs are not valued. Mitochondrial dysfunction and pharmacological therapy were studied in neurodegeneration, providing new insights in mitochondrion-targeted treatment of AN. In this review, we summarized mitochondrial biological functions related to SGNs and discussed interaction between mitochondrial dysfunction and AN, as well as existing mitochondrion treatment for SNHL. Pharmaceutical exploration to protect mitochondrion dysfunction is a feasible and effective therapeutics for AN.

Is There a Relationship Between Obstructive Sleep Apnea (OSA) and Hearing Loss?

BACKGROUND Obstructive sleep apnea (OSA) is a common disorder with an estimated prevalence in the general population of 2-5%. Its main clinical features are loud snoring and breathing stoppage during sleep. Ischemia could be a consequence of noise-induced hearing loss because cochlear oxygen tension is reduced during and after noise exposure. In this study, we evaluated auditory function in patients affected by OSA and simple snoring. MATERIAL AND METHODS A total of 66 participants (male to female ratio: 40:26) were included in the study, of which 21 were in the control group, 18 were in the simple snoring group, and 27 were in the OSA patient group. Polysomnography and audiometric examination were performed in all participants. RESULTS The mean ages of the participants in the control, simple snoring, and OSA groups were 39.14±9.9, 37.28±8.2, and 41.56±8.99 years, respectively. There were no statistically significant differences among groups regarding age or sex; however, there were statistically significant differences among groups in body mass index, apnea-hypopnea index scores, mean saturation, and duration under 90% saturation. In addition, statistically significant differences were found between the patient group and the control and simple snoring groups concerning the mean saturation, duration under 90% saturation, and the extended high frequency of hearing. CONCLUSIONS These data show that snoring may cause hearing loss at extended high frequencies.

Anomalies of the Ear in the Pierre Robin Triad

Objectives:

The Pierre Robin triad (PRT) consists of micrognathia-retrognathia, glossoptosis, and an oval or cleft palate. The goal of this study was to identify patterns of similarity to and differences from the two previous temporal bone studies of the PRT.

Methods:

Seven children with the PRT (ages, 45 minutes to 2 years; gestational ages, 41 to 43 weeks) were studied. Thirteen temporal bones were decalcified, sectioned at a thickness of 20 μm, and studied by light microscopy.

Results:

Our study demonstrated multiple architectural anomalies involving the entire ear, including abnormal auricles, and anomalies of the ossicles, including abnormal stapes footplates (6/13). All children showed signs of middle ear infection (12/13). Anomalies of the inner ear included aplasia of the lateral semicircular canals (5/13), a large vestibular aqueduct (2/13), and unusually large otoconia (1/13). In the mastoid process there were islands of cartilage in the expected position of Reichert's cartilage (9/13) and dehiscence of the fallopian canal (11/13). Loss of cochlear hair cells was seen in children who had antemortem hypoxia.

Conclusions:

Although the PRT is caused by various genes, most anomalies can be traced to the development of the first and second branchial arches.

Otoprotective effect of recombinant erythropoietin in a model of newborn hypoxic-ischemic encephalopathy

OBJECTIVE

The aim of this study is to test the hypotheses that central auditory pathology as well as inner ear pathology is contributing mechanisms to observed hypoxic-ischemic encephalopathy (HIE) induced hearing loss and that recombinant erythropoietin (rhEPO) will reduce this cellular pathology and attenuate hearing loss.

METHODS

Twenty-eight 7-day Wistar albino rat pups were divided into four groups: Control group (n=8) was given only intraperitoneal saline solution. Sham group (n=5) had only a midline neck incisions without carotid ligation under general anesthesia and administration of intraperitoneal saline solution. HIE group (n=8) and rhEPO treated group (n=7) were subjected to left common carotid artery ligation followed by 2.5h hypoxia exposure to a mixture of 8% oxygen and 92% pure nitrogen. HIE group was injected with intraperitoneal saline solution, while the rhEPO treated group received rhEPO 100 U/kg within the same volume as the saline-alone solution. At the end of the seventh week of age hearing (ABRs) was evaluated in response to clicks, 6 kHz and 8 kHz tone burst stimuli. Animals were sacrificed and both temporal lobes, cochleas and brainstems of the animals were collected. Tissue samples were evaluated with light microscopy, immunohistochemical studies, including TUNEL and caspase-3 stainings, and electron microscopy.

RESULTS

Hearing thresholds were elevated in HIE animals. In rhEPO treated animals, ABR values were similar to controls. HIE caused apoptotic changes in brainstem structures as shown by light microscopy and immunohistochemical methods. Apoptotic changes also were found within the organ of Corti, spiral ganglion cells and neurons of temporal lobe by electron microscopic investigation. In rhEPO animals many of these apoptotic changes were observed, but reduced compared to untreated animals.

CONCLUSIONS

Mechanisms underlying HIE-induced hearing loss are based on apoptosis in inner ear; however central auditory pathway pathology occurs as well, likely contributing to changes in auditory processing and perception of complex signals not reflected by the ABR threshold shifts. For both clinical and basic significance 'rhRPO' is found to reduce those effects.

Role of mitochondrial uncoupling protein 4 in rat inner ear

The uncoupling protein 4 (UCP4) belongs to the mitochondrial anion transporter family. Protein tissue distribution and functions are still a matter of debate. Using an antibody we have previously shown that UCP4 appears in neurons and to a lesser extent in astrocytes of murine neuronal tissue as early as days 12-14 of embryonic development (Smorodchenko et al., 2009). Here we demonstrated for the first time that neurosensory cells such as hair cells of the inner ear and mechanosensitive Merkel cells in skin also express a significant amount of UCP4. We tested the hypothesis about whether UCP4 contributes to the regulation of oxidative stress using the model of oxygen deprivation. For this we compared the protein expression level in freshly isolated explants of organ of Corti, modiolus and stria vascularis from neonatal rats with explants cultured under hypoxia. Western blot analysis revealed that the UCP4 level was not increased under hypoxic conditions, when compared to the mitochondrial outer membrane protein VDAC or to the anti-oxidative enzyme SOD2. We moreover demonstrated that UCP4 expression is differently regulated during postnatal stages and is region-specific. We hypothesized that UCP4 may play an important role in functional maturation of the rat inner ear.

Expression of apoptosis-related genes in the organ of Corti, modiolus and stria vascularis of newborn rats

Cell death in the inner ear tissues is an important mechanism leading to hearing impairment. Here, using microarrays and real-time RT-PCR we analyzed expression of selected apoptosis-related genes in rat's inner ear. We determined the gene expression in tissues freshly isolated from neonatal rats (3-5 days old) and compared it to that of explants cultured for 24 h under normoxic or hypoxic conditions. For the analyses, we used pooled samples of the organ of Corti (OC), modiolus (MOD) and stria vascularis (SV), respectively. We observed region-specific changes in gene expression between the fresh tissues and the normoxic culture. In the OC, expression of the proapoptotic genes caspase-2, caspase-3, caspase-6 and calpain-1 was downregulated. In the MOD, the antioxidative defense SOD-2 and SOD-3 were upregulated. In the SV, caspase-2, caspase-6, calpain-1 and SOD-3 were downregulated and SOD-2 upregulated. We speculate that these changes could reflect survival shift in transcriptome of inner ear explants tissues under in vitro conditions. With the exception of SOD-2, hypoxic culture conditions induced the same changes in gene expression as the normoxic conditions indicating that culture preparation is likely the dominating factor, which modifies the gene expression pattern. We conclude that various culture conditions induce different expression pattern of apoptosis-related genes in the organotypic cochlear cultures, as compared to fresh tissues. This transcriptional pattern may reflect the survival ability of specific tissues and could become a tempting target for a pharmacological intervention in inner ear diseases.

Improved cryosections and specific immunohistochemical methods for detecting hypoxia in mouse and rat cochleae

The present study was undertaken to develop an improved cryoembedding method for analysis of mice and rat cochleae, which permits high-quality cryosections and preserves overall structure and cellular resolution as shown by hematoxylin/eosin staining. The preservation of morphology and antigenicity is mandatory to achieve optimal results. A total of 20 male cd/1 mice and 14 male Sprague-Dawley rats were used in experiments for optimization of preservation, fixative, decalcification, embedding and cryosectioning of cochleae from adult and aged rodents. In addition, a novel immunohistochemical procedure (using Hydroxyprobe-1 kit) was developed for detecting regions of hypoxia in mice and rat cochlea. This method employs a primary fluorescent-conjugated monoclonal antibody directed against pimonidazole protein adducts that are created in hypoxic tissues. Subsequent studies of hypoxia inducible factor-1alpha (HIF-1alpha) by immunofluorescence in the cochlea of these animals were performed in order to confirm that immunochemical detection of pimonidazole protein is representative of a hypoxic environment. We conclude that the present method results in high-quality cryosections of cochlear tissues presenting good anatomical and histological preservation. Furthermore, our optimized procedures provide novel tools for the investigation of neuro-sensory-epithelium in physio-pathological situations associated with hypoxia and/or ischemia, such as inner ear development, plasticity, regeneration and senescence.

Age-related hearing loss in CD/1 mice is associated to ROS formation and HIF target proteins up-regulation in the cochlea

Pathologies of senescence, in particular those of neurosensory organs represent an important health problem. The improvement of the life expectation entails the fast increase of the frequency of the age-related hearing loss (ARHL) in the population. There are numerous factors that contribute to this process, which include altered vascular characteristics, hypoxia/ischemia, genetic mutations and production of reactive oxygen species. We were interested in understanding the mechanisms involved in the cochlear degeneration in a mouse model of ARHL, the cd/1 mice. Since in human, hypoxia/ischemia is an important pathogenetic factor for inner ear disease, the regulation of HIF-1 activity in the cochlea, the presence of radical oxygen species in the cochlea and its subsequent disturbances of cellular signaling cascades were investigated. In this study, we explored auditory function of cd/1 mice at the age of 4, 12 and 24 weeks and correlated it with the presence of oxidative damage in the cochlea, and cochlear HIF-1 responsive target genes regulation, involved in pathways promoting inflammation such as tumor necrosis factor (TNF-alpha), or cell death with the p53 protein, Bax protein and surviving factors with insulin-like growth factor-1 (IGF-1). After implantation of electrodes for auditory nerve acoustic thresholds measurements, we analyzed every cochlea. First, we confirmed that the cd/1 mice presented a characteristic profile of ARHL starting at 12 weeks of age. Then, according to our previous report [Riva, C., Longuet, M., Lucciano, M., Magnan, J., Lavieille, J.P., 2005. Implication of mitochondrial apoptosis in neural degeneration in a murin model for presbyacusis. Rev. Laryngol. Otol. Rhinol. 126 (2), 67-74], we noticed many alterations in the cochlea. Histologically, at 4 weeks, intensive HIF-1alpha expression was detected in the cochlea followed by ROS formation at 12 weeks, which may lead to cochlear degeneration and induction the onset of ARHL in the cd/1 mice model. In the cochlea, while the inner and the outer hair cells remained intact at 4 and 12 weeks, the spiral ganglion was more altered. Moreover, the Schwann cells of the spiral ganglion seemed to be more vulnerable to free radical damage than the neurons and degenerated more rapidly. The mechanisms of degeneration in the spiral ganglion involved a caspase-3 and Bax mediated-apoptosis via p53 protein accumulation. Since oxygen radicals are required for the post-translational stabilization of HIF-1alpha during hypoxia, the tandem " HIF-ROS " induced multiple reactions within the cochlea, like a strong inflammatory response with increased expression of TNF-alpha, and inhibition of neuronal protection mechanisms with repression of IGF-1.

Expression of prestin mRNA in the organotypic culture of rat cochlea

To quantitate in absolute terms the prestin mRNA levels in the explant culture of rat cochlea, we used competitive RT-PCR with a synthetic internal cRNA standard. Prestin gene expression was found at levels of 100 fg specific mRNA/microg total RNA on postnatal day 3, which corresponds to about 300 copies per outer hair cell (OHC) and is indicative of an intermediate level of expression. Two days of culturing resulted in an increase of prestin mRNA levels and in the formation of an apical-basal gradient (p<0.001). To elucidate the variations the prestin mRNA levels undergo as a result of damage to the organ of Corti, we exposed the explant cultures to ischemia and hypoxia. While total RNA was observed to remain unchanged, the numbers of OHCs and the prestin mRNA levels were found to decrease by about 20% and 35%, respectively, compared to normoxia. In conclusion, we showed that the prestin mRNA levels during in vitro development increase and form an apical-basal gradient within 2 days in culture, similar to the postnatal in vivo development. Hypoxia and ischemia result in a decrease of the prestin mRNA level in parallel with OHC loss. The prestin mRNA level can therefore be used as marker of damage to or loss of OHCs.

Susceptibility of the hair cells of the newborn rat cochlea to hypoxia and ischemia

Hypoxia and ischemia are thought to be important pathogenetic factors in bringing about hearing loss. In order to study the effect of these determinants on the loss of inner and outer hair cells (IHCs/OHCs), we used an in vitro hypoxia and ischemia model of the newborn rat cochlea. The specimens of the organ of Corti were exposed either to hypoxia (10-20 mm Hg) or to normoxic glucose deprivation or to both (ischemia) in artificial perilymph for different exposure periods. The number of IHCs and OHCs was counted and the hair cell loss was compared to controls. Normoxic aglycemia did not cause significant hair cell loss as compared to controls. Hypoxia and ischemia led to hair cell loss in a dose-dependent manner, with the loss in the ischemia groups found to be markedly higher than that in the hypoxia groups. Hypoxia resulted in a mean loss of 8% OHC and of 14% IHC after an 8-h exposure. Ischemia increased the loss to 19% OHC and 39% IHC after the same exposure period of 8 h. Our findings suggest that IHCs are more susceptible to hypoxia/ischemia than OHCs.