Expression of the proinflammatory cytokines in cochlear explant cultures: influence of normoxia and hypoxia. Neurosci Lett. 2010;479:249-252. [PMID: 20561939 DOI: 10.1016/j.neulet.2010.05.072]

IGF-1 Mediated Neuroprotective Effects of Olfactory-Derived Mesenchymal Stem Cells on Auditory Hair Cells

IMPORTANCE

Mesenchymal stem cells (MSCs) have the capability of providing ongoing paracrine support to degenerating tissues. Since MSCs can be extracted from a broad range of tissues, their specific surface marker profiles and growth factor secretions can be different. We hypothesized that MSCs derived from different sources might also have different neuroprotective potential.

OBJECTIVE

In this study, we extracted MSCs from rodent olfactory mucosa and compared their neuroprotective effects on auditory hair cell survival with MSCs extracted from rodent adipose tissue.

METHODS

Organ of Corti explants were dissected from 41 cochlea and incubated with olfactory mesenchymal stem cells (OMSCs) and adipose mesenchymal stem cells (AMSCs). After 72 hours, Corti explants were fixed, stained, and hair cells counted. Growth factor concentrations were determined in the supernatant and cell lysate using Enzyme-Linked Immunosorbent Assay (ELISA).

RESULTS

Co-culturing of organ of Corti explants with OMSCs resulted in a significant increase in inner and outer hair cell stereocilia survival, compared to control. Comparisons between both stem cell lines, showed that co-culturing with OMSCs resulted in superior inner and outer hair cell stereocilia survival rates over co-culturing with AMSCs. Assessment of growth factor secretions revealed that the OMSCs secrete significant amounts of insulin-like growth factor 1 (IGF-1). Co-culturing OMSCs with organ of Corti explants resulted in a 10-fold increase in IGF-1 level compared to control, and their secretion was 2 to 3 times higher compared to the AMSCs.

CONCLUSIONS

This study has shown that OMSCs may mitigate auditory hair cell stereocilia degeneration. Their neuroprotective effects may, at least partially, be ascribed to their enhanced IGF-1 secretory abilities compared to AMSCs.

Delivery of therapeutics to the inner ear: The challenge of the blood-labyrinth barrier

Permanent hearing loss affects more than 5% of the world's population, yet there are no nondevice therapies that can protect or restore hearing. Delivery of therapeutics to the cochlea and vestibular system of the inner ear is complicated by their inaccessible location. Drug delivery to the inner ear via the vasculature is an attractive noninvasive strategy, yet the blood-labyrinth barrier at the luminal surface of inner ear capillaries restricts entry of most blood-borne compounds into inner ear tissues. Here, we compare the blood-labyrinth barrier to the blood-brain barrier, discuss invasive intratympanic and intracochlear drug delivery methods, and evaluate noninvasive strategies for drug delivery to the inner ear.

Acute Noise Exposure Is Associated With Intrinsic Apoptosis in Murine Central Auditory Pathway

Noise that is capable of inducing the hearing loss (NIHL) has a strong impact on the inner ear structures and causes early and most obvious pathophysiological changes in the auditory periphery. Several studies indicated that intrinsic apoptotic cell death mechanisms are the key factors inducing cellular degeneration immediately after noise exposure and are maintained for days or even weeks. In addition, studies demonstrated several changes in the central auditory system following noise exposure, consistent with early apoptosis-related pathologies. To clarify the underlying mechanisms, the present study focused on the noise-induced gene and protein expression of the pro-apoptotic protease activating factor-1 (APAF1) and the anti-apoptotic B-cell lymphoma 2 related protein a1a (BCL2A1A) in the cochlear nucleus (CN), inferior colliculus (IC) and auditory cortex (AC) of the murine central auditory pathway. The expression of Bcl2a1a mRNA was upregulated immediately after trauma in all tissues investigated, whereas the protein levels were significantly reduced at least in the auditory brainstem. Conversely, acute noise has decreased the expression of Apaf1 gene along the auditory pathway. The changes in APAF1 protein level were not statistically significant. It is tempting to speculate that the acoustic overstimulation leads to mitochondrial dysfunction and induction of apoptosis by regulation of proapoptotic and antiapoptotic proteins. The inverse expression pattern on the mRNA level of both genes might reflect a protective response to decrease cellular damage. Our results indicate the immediate presence of intrinsic apoptosis following noise trauma. This, in turn, may significantly contribute to the development of central structural deficits. Auditory pathway-specific inhibition of intrinsic apoptosis could be a therapeutic approach for the treatment of acute (noise-induced) hearing loss to prevent irreversible neuronal injury in auditory brain structures and to avoid profound deficits in complex auditory processing.

[Intratympanic injection therapy for therapy refractory acute hearing loss: A safe option for secondary treatment]

BACKGROUND

High-dose corticosteroids are currently recommended for idiopathic sudden sensorineural hearing loss (ISSNHL) treatment. Intratympanic injections (ITI) are of growing importance, especially in cases of therapy resistance. The selection of patients for this procedure in SSNHL has not been adequately examined so far.

STUDY DESIGN

A total of 77 patients with ISSNHL after ineffective systemic pretreatment underwent intratympanic administration of dexamethasone and hyaluronic acid. Improvement after treatment was determined by pure tone audiometry for both ears before and of the treated ear after ITI.

RESULTS

In this study 34 female and 43 male patients with mean age of 57 years showed a pre-ITI hearing loss of 35 dB in the lower frequencies and 69 dB in the higher frequencies. The mean hearing gain was 10 dB and the response rate was 62%. Absolute hearing gain revealed significant improvements at 500 Hz, 1 kHz and 2 kHz. Under inclusion of contralateral thresholds there were hardly any differences up to 4 kHz. In a detailed analysis of responders moderate improvements could be observed even in higher frequencies. Overall, no relevant adverse events occurred.

CONCLUSION

Treatment of ISSNHL resistant to systemic regimens by ITI of steroids provides an option that offers additional prospects of auditory improvement for affected patients. The presented results indicate that these modalities are also valid for patients with pancochlear ISSNHL.

The association between hearing impairment and polymorphisms of genes encoding inflammatory mediators in Japanese aged population

BACKGROUND

Aging process is accompanied by a chronic sub-clinical systemic inflammation. This study aimed to assess the association between hearing impairment and polymorphisms of genes encoding cytokines deeply-committed to the inflammatory response and immune homeostasis in an elderly Japanese population. Data were collected in the Longitudinal Study of Aging surveyed biennially between 1997 and 2010. The participants without any missing information at baseline were 1,957 individuals, and the gross accumulated number of 8,675 subjects (40-89 years of age) was analyzed. Two hearing impairment criteria were taken as the better ear pure-tone average (PTABE) greater than 25 dB and greater than 40 dB. We analyzed cumulative data using generalized estimating equations to investigate the effect of 9 polymorphisms, namely, tumor necrosis factor (TNF) α, rs1800630; TNF receptor super family (TNFRSF) 1B, rs1061624; interleukin (IL)-1A, rs1800587; IL-1B, rs16944; IL-4R, rs1801275; IL-6, rs1800796; IL-10, rs1800872; IL-1 receptor-associated kinase 1 (IRAK1), rs1059702; C reactive protein (CRP), rs1130864.

RESULTS

The odds ratios for the hearing impairment (PTABE >25 dB) risk under additive genetic model were significant in TNF-α rs1800630 and TNFRSF1B rs1061624, which were respectively 1.172 (confidence interval [CI]: 1.005-1.367), 1.211 (CI: 1.053-1.392) in model after adjustment for possible confounders. Using the criterion of PTABE >40 dB as disabling hearing impairment, the association remains significant in TNFRSF1B rs1061624, but not in TNF-α rs1800630. No other polymorphisms showed a significant association.

CONCLUSIONS

The present population-based cohort study demonstrated that TNF-α rs1800630 and TNFRSF1B rs1061624 contributed to the incremental risk of hearing impairment in the elderly. TNF-α and TNF receptor interactions play a pivotal role in the pathogenesis of the inflammatory response, and also cause programmed cell death and cell proliferation. The present observation implied the signalling cascades of TNF were involved in ear aging.

Hearing outcome of early postnatal exposure to hypoxia in Sprague-Dawley rats

Objective: To determine the effect of experimentally induced hypoxia, in the first 10 days of life, on physiological hearing in a Sprague-Dawley rat model. Methods: A prospective, controlled animal study was carried out using 22 male rat pups. The rats in the hypoxic group (n = 12) were reared in hypoxia for the first 10 days of life, and subsequently reared in normoxia, while those in the control group (n = 10) were reared in normoxia for the duration of the experiment. Hearing was assessed using auditory brainstem response testing at approximately 72 days of age. Results: The hypoxia group had higher auditory brainstem response thresholds for all frequencies tested (more pronounced at 16 kHz), compared with controls. Wave I-V inter-peak latencies were more prolonged in the hypoxic rats, while both groups had similar wave I latencies. Conclusion: Chronic postnatal hypoxia induced permanent hearing loss in this Sprague-Dawley rat model. Prolonged wave I-V inter-peak latencies suggested functional abnormality in the central auditory pathway.

Tumor necrosis factor-alpha-mutant mice exhibit high frequency hearing loss

Exogenous tumor necrosis factor-alpha (TNF-α) plays a role in auditory hair cell death by altering the expression of apoptosis-related genes in response to noxious stimuli. Little is known, however, about the function of TNF-α in normal hair cell physiology. We, therefore, investigated the cochlear morphology and auditory function of TNF-α-deficient mice. Auditory evoked brainstem response showed significantly higher thresholds, especially at higher frequencies, in 1-month-old TNF-α(-/-) mice as compared to TNF-α(+/-) and wild type (WT); hearing loss did not progress further from 1 to 4 months of age. There was no difference in the gross morphology of the organ of Corti, lateral wall, and spiral ganglion cells in TNF-α(-/-) mice compared to WT mice at 4 months of age, nor were there differences in the anatomy of the auditory ossicles. Outer hair cells were completely intact in surface preparations of the organ of Corti of TNF-α(-/-) mice, and synaptic ribbon counts of TNF-α(-/-) and WT mice at 4 months of age were similar. Reduced amplitudes of distortion product otoacoustic emissions, however, indicated dysfunction of outer hair cells in TNF-α(-/-) mice. Scanning electron microscopy revealed that stereocilia were sporadically absent in the basal turn and distorted in the middle turn. In summary, our results demonstrate that TNF-α-mutant mice exhibit early hearing loss, especially at higher frequencies, and that loss or malformation of the stereocilia of outer hair cells appears to be a contributing factor.

Noise-induced cochlear inflammation

Hearing loss is the most common sensory disability with considerable social and economic implications. According to recent World Health Organization estimates, 360 million people worldwide suffer from moderate to profound hearing loss. Exposure to excessive noise is one of the major causes of sensorineural hearing loss, secondary only to age-related hearing loss (presbyacusis). Since cochlear tissues have limited abilities of repair and regeneration, this damage can be irreversible, leading to cochlear dysfunction and permanent hearing loss. Recent studies have shown that cochlear inflammation can be induced by noise exposure and contribute to the overall pathogenesis of cochlear injury and hearing loss. The cochlea is separated from the systemic circulation by the blood-labyrinth barrier, which is physiologically similar to the blood-brain barrier of the central nervous system. Because of this feature, the cochlea was originally considered an immunologically privileged organ. However, this postulate has been challenged by the evidence of an inflammatory response in the cochlea in the presence of bacterial or viral pathogens or antigens that can cause labyrinthitis. Although the main purpose of the inflammatory reaction is to protect against invading pathogens, the inflammatory response can also cause significant bystander injury to the delicate structures of the cochlea. The cochlear inflammatory response is characterised by the generation of proinflammatory mediators (cytokines, chemokines and adhesion molecules), and the recruitment of inflammatory cells (leukocytes). Here, we present an overview of the current research on cochlear inflammation, with particular emphasis on noise-induced cochlear inflammation. We also discuss treatment strategies aimed at the suppression of inflammation, which may potentially lead to mitigation of hearing loss.

Molecular and cellular mechanisms of loss of residual hearing after cochlear implantation

OBJECTIVES

We describe the various molecular and cellular pathways that lead to early and delayed loss of residual hearing after cochlear implantation.

METHODS

We performed a systematic review using the Medline database with the key words cochlear implant, residual hearing, inflammation, apoptosis, and necrosis.

RESULTS

The mechanisms underlying the loss of residual hearing after cochlear implantation are multiple. Early hearing loss may be provoked by the surgical access to the inner ear spaces and by trauma caused by insertion of the electrode array. After the initial trauma, an acute inflammatory response promotes elevated levels of cytokines and reactive oxygen species, which in turn promote sensory cell loss by apoptosis, necrosis, and necrosis-like programmed cell death. Treatments that counteract such an inflammatory reaction, production of reactive oxygen species, and apoptosis are effective at preventing hair cell degeneration. However, delayed hearing loss appears to be a consequence of chronic inflammation with development of fibrotic tissue. The mechanisms that lead to fibrosis are poorly understood, and standard antiinflammatory drugs are insufficient for preventing its development.

CONCLUSIONS

Cochlear implantation is followed by an inflammatory response involving several pathways that lead to either short-term or long-term sensory hair cell degeneration. Future studies should focus on revealing the precise molecular mechanisms induced by cochlear implantation to allow the discovery of new targets for the effective prevention and treatment of loss of residual hearing.

Short interfering RNA against Bax attenuates TNFα-induced ototoxicity in rat organ of Corti explants

OBJECTIVE

Evaluate the effectiveness of short interfering RNA against Bax (Bax siRNA) as a treatment for tumor necrosis factor α (TNFα)-induced auditory hair cell (HC) loss in rat organ of Corti (OC) explants.

STUDY DESIGN

Basic science.

SETTING

Basic science laboratory, University of Miami Ear Institute.

METHODS

Organ of Corti explants dissected from 3-day-old rats were cultured in control media, TNFα, and TNFα + Bax siRNA at 0, 48, and 72 hours in vitro. Real-time polymerase chain reaction, enzyme-linked immunosorbent assay, HC viability studies, immunofluorescence, and confocal microscopy were performed. Analysis of variance with post hoc testing was used with P < .05 considered significant.

RESULTS

The TNFα-damaged explants demonstrated significant decreases in viable HCs in the basal turn with corresponding increased levels of Bax gene and protein expression, compared with control explant levels. The levels of viable HCs and Bax gene and protein expression in TNFα + Bax siRNA-treated explants approached levels measured in control explants. Immunolocalization studies showed increased Bax protein expression in basal turn HCs in TNFα-treated explants, whereas control explants and TNFα + Bax siRNA-treated explants had low levels of Bax expression.

CONCLUSION

TNFα initiates the programmed cell death of auditory HCs in OC explants through upregulation of proapoptotic Bax gene and protein expression. Bax siRNA blocks TNFα-induced apoptosis of HCs by decreasing the TNFα-induced levels of Bax mRNA and protein expression in treated explants. Bax siRNA is an effective treatment for TNFα-induced ototoxicity in OC explants in vitro and has great potential to be a therapeutic agent against trauma/inflammation-induced hearing loss.

Resveratrol attenuates CoCl2-induced cochlear hair cell damage through upregulation of Sirtuin1 and NF-κB deacetylation

The goals of this study were to investigate the effects of hypoxia on cochlear hair cell damage, and to explore the role of sirtuin1 in hypoxia-induced hair cell damage. Cochlear organotypic cultures from postnatal day 4 rats were used in this study. Hypoxia was induced by treating cochlear explants with CoCl2. Cochlear cultures were treated with CoCl2 alone or in combination with the sirtuin1 activator resveratrol and the sirtuin1 inhibitor sirtinol. Hair cell damage was identified by phalloidin staining and imaged using scanning electron microscopy. RT-PCR and Western blot analyses were used to detect the expression of sirtuin1 and acetylated nuclear factor-κB (NF-κB). Low concentrations of CoCl2 (100-200 μM) did not cause an obvious change in the number and morphology of hair cells, whereas higher concentrations of CoCl2 (300-400 μM) induced swelling of hair cells, accompanied by cell loss. Increased sirtuin1 expression was induced by CoCl2 at 100 to 200 μM, but not at 400 μM. NF-κB acetylation was significantly increased in explants treated with 400 μM CoCl2. Pretreatment with resveratrol prevented CoCl2-induced hair cell loss and acetylation of NF-κB. The protective effect of resveratrol was significantly reduced by sirtinol. CoCl2 induces hair cell damage in organotypic cochleae cultures. Resveratrol attenuates CoCl2-induced cochlear hair cell damage possibly via activation of sirtuin1, which deacetylates NF-κB.

Exposure of Wistar rats to 24-h psycho-social stress alters gene expression in the inferior colliculus

Recently, we have demonstrated that the exposure of Wistar rats to psycho-social stress results in a transient auditory hypersensitivity. Here, to learn more about modifications occurring in auditory brainstem, we have analyzed gene expression pattern in inferior colliculus using quantitative RT-PCR. As targets, we have chosen genes associated with: neural activity (FBJ osteosarcoma viral oncogene, cFos), hypoxia (nitric oxide synthase inducible, iNos; superoxide dismutase 2, Sod2), neuroprotection (nerve growth factor beta, Ngfb; heat shock factor 1, Hsf1; heat shock protein 70, Hsp70) and inflammation (tumor necrosis factor alpha, Tnfa; tumor necrosis factor alpha receptor, Tnfar; substance P, Sp; cyclooxygenase 2, Cox2). We found that the expression of all genes was modified following stress, as compared to the controls. Immediately after stress, the number of transcripts encoding iNos, Sod2, Hsf1, Ngfb, Tnfa, Tnfar and Sp was significantly increased, suggesting possible modulation during exposure to stressor. Interestingly, we found that expression of Hsf1 and Ngfb at this particular time was left-right asymmetrical: there were more transcripts of both genes found in the left colliculi, as compared to the right colliculi. Three hours post-stress, iNos, Hsf1, Tnfa and Tnfar were still upregulated, Sod2, Ngfb and Sp went back to baseline and Cox2 was upregulated. Six hours post-stress, cFos mRNA became downregulated. The number of Hsp70 mRNA increased 24h post-stress. Except for the reduced number of cFos transcripts, expression of all other genes tested reached the baseline seven days post-stress. Presented results corroborate the concept of auditory system responding to the psycho-social stress. Post-stress changes in the IC gene expression could likely indicate shift from allostasis to homeostasis in the auditory brainstem.

Polymorphisms in genes involved in inflammatory pathways in patients with sudden sensorineural hearing loss

Although the etiology of idiopathic sudden sensorineural hearing loss (SSNHL) remains unclear, the pathologically increased permeability of blood vessels, elucidated by gadolinium-enhanced magnetic resonance imaging (MRI), suggests the involvement of inflammation. Because SSNHL is considered a multifactorial disease, possibly caused by interactions between genetic factors and environmental factors, the authors investigated the associations of polymorphisms of inflammatory mediator genes with susceptibility to SSNHL. The authors compared 72 patients affected by SSNHL and 2010 adults (1010 men and 1000 women; mean age 59.2 years; range 40-79) who participated in the National Institute for Longevity Sciences Longitudinal Study of Aging. Multiple logistic regression was used to obtain odds ratios (ORs) for SSNHL in subjects with polymorphisms in the genes IL-6 C - 572G, IL-4R G1902A, IL-10 A - 592C, TNFα C - 863A, TNFRSF1B G593A, VEGF C936T, VEGF C - 2578A, and VEGF G - 1154A, with adjustment for age, gender, and any history of hypertension, diabetes, or dyslipidemia. The per-allele OR for the risk of SSNHL in subjects bearing IL-6 C - 572G was 1.480 (95% confidence interval [CI], 1.037-2.111) in model 1 (no adjustment), 1.463 (CI, 1.022-2.094) in model 2 (adjusted for age and gender), and 1.460 (CI, 1.016-2.097) in model 3 (adjusted for age, gender, and a history of hypertension, diabetes, or dyslipidemia). Under the dominant model of inheritance, the ORs were 1.734 (CI, 1.080-2.783) in model 1, 1.690 (CI, 1.050-2.721) in model 2, and 1.669 (CI, 1.035-2.692) in model 3. The remaining seven polymorphisms failed to show any associations with the risk of SSNHL. These data need to be confirmed on larger series of patients. In conclusion, the IL-6 C - 572G polymorphism is associated with a risk of SSNHL. Because permeability of blood vessels in the inner ear is frequently increased in patients with SSNHL, inflammation of the inner ear might be involved.

Markers of inflammatory status are associated with hearing threshold in older people: findings from the Hertfordshire Ageing Study

BACKGROUND

Age-related hearing loss is a common disabling condition but its causes are not well understood and the role of inflammation as an influencing factor has received little consideration in the literature.

OBJECTIVE

To investigate the association between inflammatory markers and hearing in community-dwelling older men and women.

DESIGN

Cross-sectional analysis within a cohort study.

SETTING

The Hertfordshire Ageing Study.

PARTICIPANTS

A total of 343 men and 268 women aged 63-74 years on whom data on audiometric testing, inflammatory markers and covariates were available at follow-up in 1995.

MAIN OUTCOME MEASURES

Average hearing threshold level (across 500-4,000 Hz) of the worst hearing ear and audiometric slope in dB/octave from 500 to 4,000 Hz.

RESULTS

Older age, smoking, history of noise exposure and male gender (all P < 0.001) were associated with higher mean hearing threshold in the worse ear in univariate analysis. After adjustment for these factors in multiple regression models, four measures of immune or inflammatory status were significantly associated with hearing threshold, namely white blood cell count (r = 0.13, P = 0.001), neutrophil count (r = 0.13, P = 0.002), IL-6 (r = 0.10, P = 0.05) and C-reactive protein (r = 0.11, P = 0.01). None of the inflammatory markers was associated with maximum audiometric slope in adjusted analyses.

CONCLUSIONS

Markers of inflammatory status were significantly associated with degree of hearing loss in older people. The findings are consistent with the possibility that inflammatory changes occurring with ageing may be involved in age-related hearing loss. Longitudinal data would enable this hypothesis to be explored further.

Salicylate modulates Hsp70 expression in the explanted organ of Corti

Heat shock protein 70 (Hsp70, Hspa1a) is known to play a protective role in the inner ear and in the nervous system. Our recent study demonstrated that the induction of Hsp70 by geldanamycin protected the auditory hair cells against ototoxic insult. Here, using the explanted organ of Corti (OC), we characterized the effect of sodium salicylate on the expression of Hsp70. Using the real-time RT-PCR; after 27 h in standard culture, we observed an increase in the Hsp70 transcript number. After 48 h in culture, the number of Hsp70 transcripts increased further, as compared to the freshly isolated tissues or explant cultured for 27 h. Three hours after the addition of 2.5mM sodium salicylate, the expression of Hsp70 mRNA increased significantly. Interestingly, Hsp70 protein level remained unaffected by the addition of salicylate, as shown by immunoblotting and Hsp70-ELISA. Confocal microscopy imaging demonstrated predominant localization of Hsp70 protein with or without salicylate exposure to the fibrocytes of spiral limbus. Our results suggest that in the OC, explanting process induces expression of Hsp70 in limbal fibrocytes and that this expression can be enhanced by salicylate but only on mRNA and not on the protein level.

Dexamethasone treatment of tumor necrosis factor-alpha challenged organ of Corti explants activates nuclear factor kappa B signaling that induces changes in gene expression that favor hair cell survival

The objective was to determine the role of nuclear factor kappa B (NFκB) in dexamethasone base (DXMb) protection of auditory hair cells from tumor necrosis factor-alpha (TNFα)-induced loss on gene expression and cell signaling levels. Organ of Corti (OC) explants from 3-day-old rats were cultured under one of the following conditions: (1) media only--no treatment; (2) media+TNFα; (3) media+TNFα+DXMb; (4) media+TNFα+DXMb+NFκB-Inhibitor (NFκB-I); or (5) media+TNFα+DXMb+NFκBI-Scrambled control (NFκBI-C). A total of 60 organ of Corti explants (OC) were stained with FITC-Phalloidin after 96 h in culture (conditions 1-5) for hair cell counts and imaging of surface characteristics. A total of 108 OC were used for gene expression studies (i.e. B-actin, Bax, Bcl-2, Bcl-xl, and TNFR1) after 0, 24, or 48 h in vitro (conditions 1-4). A total of 86 OC were cultured (conditions 1-3) for 48 h, 36 of which were used for phosphorylated NFκB (p-NFκB) ELISA studies and 50 for whole mount anti-p-NFκB immunostain experiments. TNFα+DXMb exposed cultures demonstrated significant upregulation in anti-apoptotic Bcl-2 and Bcl-xl genes and downregulation in pro-apoptotic Bax gene expression; DXMb treatment of TNFα explants also lowered the Bax/Bcl-2 ratio and inhibited TNFR1 upregulation. After inhibiting NFκB activity with NFκB-I, the gene expression profile following TNFα+DXMb treatment now mimics that of TNFα-challenged OC explants. The levels of p-NFκB and the degree of nuclear translocation are significantly greater in TNFα+DXMb exposed OC explants than observed in the TNFα and control groups in the middle+basal turns of OC explants. These findings were supported by the results of the hair cell counts and the imaging results obtained from the whole mount OC specimens. DXMb protects against TNFα-induced apoptosis of auditory hair cells in vitro via activation of NFκB signaling in hair cell nuclei, and regulation of the expression levels of anti- and pro-apoptotic genes and a pro-inflammatory gene.

Expression of genes implicated in oxidative stress in the cochlea of newborn rats

Oxidative stress is an important mechanism inducing ototoxicity-, age- and noise-induced hearing loss. To better understand this phenomenon, we examined cochlear tissues for the expression of following genes involved directly or indirectly in the oxidative stress response: glyceraldehyde-3-phosphate dehydrogenase (Gapdh); solute carrier family-2 (facilitated glucose transporter), member-1 (Slc2a1); heme oxygenase-1 (Hmox1); heme oxygenase-2 (Hmox2); inducible nitric oxide synthase-2 (Nos2); transferrin (Tf); transferrin receptor (Tfrc); glutathione S-transferase A3 (Gsta3) and metallothionein-1a (Mt1a). Cochlear tissues were dissected from the p3-p5 Wistar rats, divided into the organ of Corti (OC), modiolus (MOD) and stria vascularis together with spiral ligament (SV + SL) and processed immediately or cultured under normoxic conditions or a short-term, mild hypoxia followed by re-oxygenation. After 24 h, explants were collected and total RNA isolated, transcribed and amplified in the real time RT-PCR. We found all genes listed above expressed in the freshly isolated cochlear tissues. In the OC and MOD, Slc2a1, Tf, and Mt1a were expressed on a lower level than in the SV + SL. In the OC, Hmox1 was expressed on a lower level than in the MOD and SV + SL. Hypoxic and normoxic cultures increased the transcript number of Gapdh, Slc2a1 and Hmox1 in all cochlear tissues. The expression of Nos2, Tf, Gsta3 and Mt1a increased in a tissue-specific manner. In the SV + SL, Mt1a expression decreased after normoxic and hypoxic conditions. Taken together, using real time RT-PCR, our results imply that oxidative stress may be an important component of cochlear injury during the developing period. In spite of the immaturity of the tissue, a differential response of antioxidant enzymes/proteins with respect to the pathway, the expression levels and regions was observed.