Nitric oxide production from cultured spiral ligament fibrocytes: effects of corticosteroids

On the Role of Fibrocytes and the Extracellular Matrix in the Physiology and Pathophysiology of the Spiral Ligament

The spiral ligament in the cochlea has been suggested to play a significant role in the pathophysiology of different etiologies of strial hearing loss. Spiral ligament fibrocytes (SLFs), the main cell type in the lateral wall, are crucial in maintaining the endocochlear potential and regulating blood flow. SLF dysfunction can therefore cause cochlear dysfunction and thus hearing impairment. Recent studies have highlighted the role of SLFs in the immune response of the cochlea. In contrast to sensory cells in the inner ear, SLFs (more specifically type III fibrocytes) have also demonstrated the ability to regenerate after different types of trauma such as drug toxicity and noise. SLFs are responsible for producing proteins, such as collagen and cochlin, that create an adequate extracellular matrix to thrive in. Any dysfunction of SLFs or structural changes to the extracellular matrix can significantly impact hearing function. However, SLFs may prove useful in restoring hearing by their potential to regenerate cells in the spiral ligament.

Genetics of guanylyl cyclase pathways in the cochlea and their influence on hearing

Although hearing loss is the most common sensory deficit in Western societies, there are no successful pharmacological treatments for this disorder. Recent experiments have demonstrated that manipulation of intracellular cyclic guanosine monophosphate (cGMP) concentrations can have both beneficial and harmful effects on hearing. In this review, we will examine the role of cGMP as a key second messenger involved in many aspects of cochlear function and discuss the known functions of downstream effectors of cGMP in sound processing. The nitric oxide-stimulated soluble guanylyl cyclase system (sGC) and the two natriuretic peptide-stimulated particulate GCs (pGCs) will be more extensively covered because they have been studied most thoroughly. The cochlear GC systems are attractive targets for medical interventions that improve hearing while simultaneously representing an under investigated source of sensorineural hearing loss.

The Role of Glia in the Peripheral and Central Auditory System Following Noise Overexposure: Contribution of TNF-α and IL-1β to the Pathogenesis of Hearing Loss

Repeated noise exposure induces inflammation and cellular adaptations in the peripheral and central auditory system resulting in pathophysiology of hearing loss. In this study, we analyzed the mechanisms by which noise-induced inflammatory-related events in the cochlea activate glial-mediated cellular responses in the cochlear nucleus (CN), the first relay station of the auditory pathway. The auditory function, glial activation, modifications in gene expression and protein levels of inflammatory mediators and ultrastructural changes in glial-neuronal interactions were assessed in rats exposed to broadband noise (0.5-32 kHz, 118 dB SPL) for 4 h/day during 4 consecutive days to induce long-lasting hearing damage. Noise-exposed rats developed a permanent threshold shift which was associated with hair cell loss and reactive glia. Noise-induced microglial activation peaked in the cochlea between 1 and 10D post-lesion; their activation in the CN was more prolonged reaching maximum levels at 30D post-exposure. RT-PCR analyses of inflammatory-related genes expression in the cochlea demonstrated significant increases in the mRNA expression levels of pro- and anti-inflammatory cytokines, inducible nitric oxide synthase, intercellular adhesion molecule and tissue inhibitor of metalloproteinase-1 at 1 and 10D post-exposure. In noise-exposed cochleae, interleukin-1β (IL-1β), and tumor necrosis factor α (TNF-α) were upregulated by reactive microglia, fibrocytes, and neurons at all time points examined. In the CN, however, neurons were the sole source of these cytokines. These observations suggest that noise exposure causes peripheral and central inflammatory reactions in which TNF-α and IL-1β are implicated in regulating the initiation and progression of noise-induced hearing loss.

Nitric oxide mediates TNF-α-induced apoptosis in the auditory cell line

OBJECTIVES/HYPOTHESIS

Tumor necrosis factor-alpha (TNF-α) is released in a variety of pathological states in the inner ear. Inducible nitric oxide synthase (iNOS) can be induced by cytokines and other inflammatory factors, and is generally thought to be associated with inflammation and other pathological processes in the cochlea. The purpose of the present study was to reveal that TNF-α could induce apoptosis in the auditory cell line and to investigate the role of nitric oxide (NO) in TNF-α-induced auditory cell death.

STUDY DESIGN

Experimental study.

METHODS

UB-OC1 cells and zebrafish were exposed to TNF-α. Flow cytometry, terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling (TUNEL) assay, assay of mitochondrial membrane potential (MMP), and electron microscopy were used to show that TNF-α could induce apoptosis. Western blot was used to measure iNOS expression and mitogen-activated protein kinase pathway.

RESULTS

Flow cytometric analysis, TUNEL assay, MMP, and electron microscopy all demonstrated that TNF-α could induce apoptosis in UB-OC1 cells. TNF-α significantly increased NO generation and iNOS expression. Pretreatment with iNOS blocker NG-methyl-L-arginine (NMA) attenuated TNF-α-induced cell death and caspase-3 activation. Also, TNF-α treatment increased p-p38 and p-ERK, and pretreatment of NMA reduced this increased expression of p-p38 and p-ERK.

CONCLUSIONS

TNF-α can induce apoptosis in the auditory cell line, and NO production in response to TNF-α is essential for apoptosis.

Nitric oxide--a versatile key player in cochlear function and hearing disorders

Nitric oxide (NO) is a signaling molecule which can generally be formed by three nitric oxide synthases (NOS). Two of them, the endothelial nitric oxide synthase (eNOS) and the neural nitric oxide synthase (nNOS), are calcium/calmodulin-dependent and constitutively expressed in many cell types. Both isoforms are found in the vertebrate cochlea. The inducible nitric oxide synthase (iNOS) is independent of calcium and normally not detectable in the un-stimulated cochlea. In the inner ear, as in other tissues, NO was identified as a multitask molecule involved in various processes such as neurotransmission and neuromodulation. In addition, increasing evidence demonstrates that the NO-dependent processes of cell protection or, alternatively, cell destruction seem to depend, among other things, on changes in the local cochlear NO-concentration. These alterations can occur at the cellular level or within a distinct cell population both leading to an NO-imbalance within the hearing organ. This dysfunction can result in hearing loss or even in deafness. In cases of cochlear malfunction, regulatory systems such as the gap junction system, the blood vessels or the synaptic region might be affected temporarily or permanently by an altered NO-level. This review discusses potential cellular mechanisms how NO might contribute to different forms of hearing disorders. Approaches of NO-reduction are evaluated and the transfer of results obtained from experimental animal models to human medication is discussed.

[Possible molecular mechanisms of spontaneous remission in sudden idiopathic hearing loss]

According to current knowledge, it must be assumed that temporary idiopathic hearing loss and its spontaneous remission are based on mechanical and/or pathological alterations in the inner ear. The causal mechanisms might be based on inter-individual variations. Induced by dose-dependent activators, temporary as well as permanent damage might occur. Sudden hearing loss may be initiated by an increase in the local nitric oxide (NO) concentration. Spontaneous remission, i.e. functional restoration, can be explained by a local decrease in the NO concentration. In this context, regulatory systems such as the gap-junction system, blood vessels or synapses might be affected. In addition, alterations in the hormone level of estrogen and mineralocorticoids, as well as cellular glutathione and vitamin levels, might lead to temporary alterations in the inner ear. Recent experimental findings indicate a role for the shuttle protein Survivin in the spontaneous remission of sudden hearing loss.

Expression of the proinflammatory cytokines in cochlear explant cultures: influence of normoxia and hypoxia

Hearing loss can be induced by a variety of factors including hypoxia and inflammation. Here, we investigated in vitro the effect of hypoxia on the expression of proinflammatory cytokines in the explanted cochlear tissues. Using RT-PCR, we determined the expression of genes encoding IL-1beta, IL-6 and TNF in the organ of Corti (OC), modiolus (MOD) and stria vascularis together with spiral ligament (SV+SL). In addition, using ELISA, we determined the concentration of IL-1beta and IL-6 in the supernatants of explant cultures. We found that the dissection, explanting and consecutive 24-h normoxic culture results in highly increased expression of IL-1beta and IL-6, as compared to the freshly isolated tissues. TNFalpha was upregulated only in the MOD. Interestingly, 24h of hypoxia decreased the number of mRNA encoding IL-1beta and IL-6 and increased the number of mRNA encoding TNFalpha in the SV+SL as compared to normoxia. The concentration of IL-6 measured in the explant tissue culture supernatants was significantly lower in hypoxic than in the normoxic cultures. Our results show that tissue dissection and explanting as well as hypoxia can influence the expression and secretion of proinflammatory cytokines. This implies the presence of tissue-specific regulatory pathways between hypoxia and inflammation in the inner ear.

Issues, indications, and controversies regarding intratympanic steroid perfusion

PURPOSE OF REVIEW

Office-based intratympanic inner ear steroid perfusion (ITPs) treatment for Ménière's disease, autoimmune inner ear disease, and sudden sensorineural hearing loss has been expanding over the past 10-15 years, yet remains controversial. The purpose of this review is to examine the current literature of basic science and human studies of ITPs treatment.

RECENT FINDINGS

Animal studies exist regarding the delivery, distribution, biochemical, and microbiological changes in the inner ear post ITPs. However, few clinical studies exist on ITPs treatment in sudden sensorineural hearing loss and even less in treating Ménière's disease. There are no consistent studies regarding drug delivery methods, type, and concentration of steroids. Moreover, there are no studies comparing ITPs results to the natural history of Ménière's disease.

SUMMARY

ITPs has impacted otology and neurotology practice due to increased utilization. A sound understanding of the basic science and clinical studies is needed to establish long-term efficacy of ITPs in controlling hearing loss in Ménière's disease by comparison to its natural history, as well as, potential application to other disorders.