Significance of spiral ligament fibrocytes with cochlear inflammation

Tracking Lymphatic Drainage Pathways Through Inner Ear Channels: A Systematic Review

The search for potential lymphatic routes through the cochlea, or membranous portions of the inner ear labyrinth, remains a significant challenge. Researchers often focus on lower mammals rather than humans to uncover these pathways. This review aims to delineate the speculated lymphatic routes within the inner ear to date. It follows the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines, conducting a comprehensive search of PubMed, Scopus, Crossref, and Google databases using the terms "inner ear" and "lymph." The problem, intervention, comparison, outcome (PICO) search strategy was employed, and analysis was performed using equation and scope metrics. Articles were screened and filtered using the CADIMA automation tool, resulting in 33 articles being reviewed, of which 23 were selected. Potential lymphatic drainage routes identified include the round window, oval window, scala tympani, spiral limbus, and lateral wall of the cochlea. The vestibular side of Reissner's membrane was noted as a key nodal point for lymphocytes within the inner ear. This review maps the proposed lymphatic networks in the inner ear and highlights existing gaps. It systematically gathers, evaluates, and synthesizes available evidence on the lymphatic pathways of the inner ear, offering valuable insights into their presence, structure, function, and clinical significance.

Immuno-surveillance and protection of the human cochlea

Background

Despite its location near infection-prone areas, the human inner ear demonstrates remarkable resilience. This suggests that there are inherent instruments deterring the invasion and spread of pathogens into the inner ear. Here, we combined high-resolution light microscopy, super-resolution immunohistochemistry (SR-SIM) and synchrotron phase contrast imaging (SR-PCI) to identify the protection and barrier systems in the various parts of the human inner ear, focusing on the lateral wall, spiral ganglion, and endolymphatic sac.

Materials and methods

Light microscopy was conducted on mid-modiolar, semi-thin sections, after direct glutaraldehyde/osmium tetroxide fixation. The tonotopic locations were estimated using SR-PCI and 3D reconstruction in cadaveric specimens. The sections were analyzed for leucocyte and macrophage activity, and the results were correlated with immunohistochemistry using confocal microscopy and SR-SIM.

Results

Light microscopy revealed unprecedented preservation of cell anatomy and several macrophage-like cells that were localized in the cochlea. Immunohistochemistry demonstrated IBA1 cells frequently co-expressing MHC II in the spiral ganglion, nerve fibers, lateral wall, spiral limbus, and tympanic covering layer at all cochlear turns as well as in the endolymphatic sac. RNAscope assays revealed extensive expression of fractalkine gene transcripts in type I spiral ganglion cells. CD4 and CD8 cells occasionally surrounded blood vessels in the modiolus and lateral wall. TMEM119 and P2Y12 were not expressed, indicating that the cells labeled with IBA1 were not microglia. The round window niche, compact basilar membrane, and secondary spiral lamina may form protective shields in the cochlear base.

Discussion

The results suggest that the human cochlea is surveilled by dwelling and circulating immune cells. Resident and blood-borne macrophages may initiate protective immune responses via chemokine signaling in the lateral wall, spiral lamina, and spiral ganglion at different frequency locations. Synchrotron imaging revealed intriguing protective barriers in the base of the cochlea. The role of the endolymphatic sac in human inner ear innate and adaptive immunity is discussed.

Robotic assistance during cochlear implantation: the rationale for consistent, controlled speed of electrode array insertion

Cochlear implants (CI) have revolutionized the treatment of patients with severe to profound sensory hearing loss by providing a method of bypassing normal hearing to directly stimulate the auditory nerve. A further advance in the field has been the introduction of "hearing preservation" surgery, whereby the CI electrode array (EA) is carefully inserted to spare damage to the delicate anatomy and function of the cochlea. Preserving residual function of the inner ear allows patients to receive maximal benefit from the CI and to combine CI electric stimulation with acoustic hearing, offering improved postoperative speech, hearing, and quality of life outcomes. However, under the current paradigm of implant surgery, where EAs are inserted by hand, the cochlea cannot be reliably spared from damage. Robotics-assisted EA insertion is an emerging technology that may overcome fundamental human kinetic limitations that prevent consistency in achieving steady and slow EA insertion. This review begins by describing the relationship between EA insertion speed and generation of intracochlear forces and pressures. The various mechanisms by which these intracochlear forces can damage the cochlea and lead to worsened postoperative outcomes are discussed. The constraints of manual insertion technique are compared to robotics-assisted methods, followed by an overview of the current and future state of robotics-assisted EA insertion.

Association between gut microbiota and sensorineural hearing loss: a Mendelian randomization study

Background

Several recent studies speculated that the gut microbiota is associated with sensorineural hearing loss (SNHL) and proposed the concept of the gut-inner ear axis. However, the causal effect of gut microbiota on SNHL is still unknown. In this study, we performed a two-sample Mendelian randomization (MR) analysis to estimate the causal effect of gut microbiota on SNHL.

Methods

Gut microbiota data were obtained from the largest available genome-wide association study (n = 18,340) conducted by the MiBioGen consortium. The summary statistics of SNHL were obtained from the FinnGen consortium R8 release data (28,310 cases and 302,750 controls). The causal effects were estimated with inverse-variance weighted, MR-Egger, and weighted median. Reverse Mendelian randomization analysis was performed on the bacteria that were found to be associated with SNHL in forward Mendelian randomization analysis. We then performed sensitivity analyses, including Cochran's Q-test, MR-Egger intercept test, MR-PRESSO, cML-MA-BIC, and leave-one-out analysis, to detect heterogeneity and pleiotropy.

Results

The inverse-variance weighted results suggested that Lachnospiraceae (UCG001) had a significant protective effect against SNHL (odds ratio = 0.85, 95% confidence interval: 0.78-0.93, P = 6.99 × 10-4). In addition, Intestinimonas (odds ratio = 0.89, 95% confidence interval: 0.82-0.97, P = 8.53 × 10-3) presented a suggestively protective effect on SNHL. Rikenellaceae (RC9gutgroup) (odds ratio = 1.08, 95% confidence interval: 1.02-1.15, P = 0.01) and Eubacterium (hallii group) (odds ratio = 1.12, 95% confidence interval: 1.00-1.24, P = 0.048) suggestively increase the risk of SNHL. The results of the reverse MR analysis showed that there is no significant causal effect of SNHL on the gut microbiota. No significant heterogeneity of instrumental variables or pleiotropy was detected.

Conclusion

The evidence that the four genera mentioned above are associated with SNHL supports the hypothesis of a gut-inner ear axis. Our study provides microbial markers for the prevention and treatment of SNHL, and further studies are needed to explore the mechanisms of the gut microbiome-inner ear axis in health and diseases.

Spatial architecture of the cochlear immune microenvironment in noise-induced and age-related sensorineural hearing loss

The cochlea encodes sound stimuli and transmits them to the central nervous system, and damage to sensory cells and synapses in the cochlea leads to hearing loss. The inner ear was previously considered to be an immune privileged organ to protect the auditory organ from reactions with the immune system. However, recent studies have revealed the presence of resident macrophages in the cochlea, especially in the spiral ligament, spiral ganglion, and stria vascularis. The tissue-resident macrophages are responsible for the detection, phagocytosis, and clearance of cellular debris and pathogens from the tissues, and they initiate inflammation and influence tissue repair by producing inflammatory cytokines and chemokines. Insult to the cochlea can activate the cochlear macrophages to initiate immune responses. In this review, we describe the distribution and functions of cochlear macrophages in noise-induced hearing impairment and age-related hearing disabilities. We also focus on potential therapeutic interventions concerning hearing loss by modulating local immune responses.

Progression of endolymphatic hydrops and vertigo during treatment in Meniere's disease

BACKGROUND

Patients with Meniere's disease (MD) receive treatment to reduce vertigo.

PURPOSE

To explore the fluctuation of vertigo symptoms and the changes in endolymphatic hydrops (EHs) in MD patients during long-term regular medication.

MATERIALS AND METHODS

We enrolled MD patients who had received two magnetic resonance imaging with intravenous contrast agents.

RESULTS

Of the 42 patients in the study, 18(42.9%) had progressive EHs and 3(7.1%) had remission. The change value of the signal intensity ratio (SIR; cochlear perilymph/cerebellum) on the affected side (1.2) was larger than that on the healthy side (0.9), but there was no statistical difference. Among the 30 patients followed up, two patients had complete control of vertigo, two patients had substantial control, and three patients had worse control. The other 23 patients had insignificant control of vertigo symptoms before and after treatment. The correlation between the progression of cochlear and vestibular hydrops and the improvement of vertigo symptoms in MD patients was not significant.

CONCLUSIONS AND SIGNIFICANCE

In treated patients with MD, EHs may progress or relieve over the long course of the disease. But there was no correlation between the development of EHs and changes in vertigo symptoms.

Serum Fibrinogen Level and Cytokine Production as Prognostic Biomarkers for Idiopathic Sudden Sensorineural Hearing Loss

OBJECTIVES

No clinically useful prognostic factors have been identified for idiopathic sudden sensorineural hearing loss (ISSNHL). The current study therefore sought to identify useful prognostic factors for idiopathic sudden sensorineural hearing loss from blood biomarkers while attempting to classify the pathogenic mechanism and formulate treatment strategies based on these results.

STUDY DESIGN

Prospective cohort study.

SETTING

Tertiary referral center.

METHODS

A total of 47 patients with acute phase ISSNHL were treated with steroid at an initial dose of 1 mg/kg/day and hyperbaric oxygen therapy and followed up for 6 months. Serum fibrinogen levels, peripheral blood mononu- clear cells (PBMCs), and interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α production levels from PBMCs were measured, after which patient's pre- and post- treatment hearing was compared.

RESULTS

In the overall cohort, the mean improvement level, mean recovery rate, and mean fibrinogen level was 20.3 dB, 46.2%, 292.0 mg/mL, respectively. The mean levels of IL-1β, IL-6, and TNF-α produced by peripheral blood mononu- clear cells cultured under lipopolysaccharide stimulation were 318.4, 498.1, and 857.6 pg/mL, respectively. High fibrinogen levels were associated with poor hearing progno- sis. Lipopolysaccharide-stimulated cytokine production by PBMCs did not correlate with hearing changes; however, the prognosis was significantly better in patients with low fibrinogen levels and high IL-1β levels produced by PBMCs than in other patients.

CONCLUSIONS

Our results suggest that patients with simple inflammatory-type ISSNHL responded well to standard therapy. Therefore, serum fibrinogen levels and PBMCs cytokine production may help determine the management of ISSNHL based on its pathogenic mechanism.

Auditory Phenotype and Histopathologic Findings of a Mutant Nlrp3 Expression Mouse Model

Objective

The pathogenesis of hearing loss in autoinflammatory disorders due to activation of the inflammasome remains incompletely understood. Previously no animals expressing mutant Nlrp3 (NOD-, LRR- and pyrin domain-containing protein 3) survived to an age when hearing evaluation was possible due to embryonic lethality. We aimed to establish a novel mouse model that manifests quantifiable hearing loss with other syndromic features due to alteration of Nlrp3 and investigate the audiologic and histopathologic phenotype in the cochlea to clarify how the genetic alterations of NLRP3 could induce autoinflammatory hearing loss.

Methods

To induce inner ear expression of the mutant Nlrp3, Nlrp3^D301NneoR mice were bred with Gfi1^Cre knock-in mice for conditional mutant Nlrp3 activation in the cochlea and hematopoietic cells. Hearing thresholds were measured. Hematoxylin-eosin sections of the cochlea, brain, kidney, and liver were examined under light microscopy. Immunohistochemical analyses using polyclonal anti-NLRP3 antibodies on cochlear whole-mount preparations and frozen sections were performed.

Results

We, for the first time in the literature, established a mouse model that manifests quantifiable hearing loss due to Nlrp3 alteration. ABR recordings of Nlrp3^D301NneoR/+; Gfi1^Cre/+ mice, albeit with limited life expectancy, exhibited severe to profound hearing loss at postnatal day 20 (P20). There was overall overexpression of mutant Nlrp3, and mutant Nlrp3 expression was noted in the spiral prominence, the outer sulcus region (Claudius cells and outer sulcus cells), the organ of Corti, the inner sulcus, and the spiral ganglion neurons in the cochlea. The hematoxylin-eosin sections of Nlrp3^D301NneoR/+; Gfi1^Cre/+ mice cochleae at P12 exhibited a disorganized organ of Corti between the outer hair cells/supporting Deiters' cells and basilar membrane compared with the normal phenotype mice, leading to a collapsed Nuel's space. This morphologic feature gradually returned to normal by P15. Varying degrees of inflammation with lymphocytic infiltrations were observed in the brain, kidney, and liver.

Conclusion

We report the first mutant Nlrp3 overexpression mouse model (Nlrp3^D301NneoR/+; Gfi1^Cre/+) that shows obvious overexpression of Nlrp3 in the cochlea, a transient developmental lag of the cochlea, and severe to profound hearing loss. We expect that this mouse line, which models human autoinflammatory hearing loss, could provide a valuable tool to elucidate the underlying pathogenic mechanism of inflammasome activation-mediated hearing loss.

Age-Related Inflammation and Oxidative Stress in the Cochlea Are Exacerbated by Long-Term, Short-Duration Noise Stimulation

We have previously reported that young adult rats exposed to daily, short-duration noise for extended time periods, develop accelerated presbycusis starting at 6 months of age. Auditory aging is associated with progressive hearing loss, cell deterioration, dysregulation of the antioxidant defense system, and chronic inflammation, among others. To further characterize cellular and molecular mechanisms at the crossroads between noise and age-related hearing loss (ARHL), 3-month-old rats were exposed to a noise-accelerated presbycusis (NAP) protocol and tested at 6 and 16 months of age, using auditory brainstem responses, Real-Time Reverse Transcription-Quantitative PCR (RT-qPCR) and immunocytochemistry. Chronic noise-exposure leading to permanent auditory threshold shifts in 6-month-old rats, resulted in impaired sodium/potassium activity, degenerative changes in the lateral wall and spiral ganglion, increased lipid peroxidation, and sustained cochlear inflammation with advancing age. Additionally, at 6 months, noise-exposed rats showed significant increases in the gene expression of antioxidant enzymes (superoxide dismutase 1/2, glutathione peroxidase 1, and catalase) and inflammation-associated molecules [ionized calcium binding adaptor molecule 1, interleukin-1 beta (IL-1β), and tumor necrosis factor-alpha]. The levels of IL-1β were upregulated in the spiral ganglion and spiral ligament, particularly in type IV fibrocytes; these cells showed decreased levels of connective tissue growth factor and increased levels of 4-hydroxynonenal. These data provide functional, structural and molecular evidence that age-noise interaction contributes to exacerbating presbycusis in young rats by leading to progressive dysfunction and early degeneration of cochlear cells and structures. These findings contribute to a better understanding of NAP etiopathogenesis, which is essential as it affects the life quality of young adults worldwide.

Cochlear and vestibular involvement in children with IgA vasculitis

In this study, our purpose is to evaluate cochlear and vestibular function in juveniles with IgA vasculitis using audiometry, distortion product otoacoustic emissions, and cervical vestibular evoked myogenic potential (cVEMP) tests. Forty children diagnosed with IgA vasculitis from the pediatry clinic and 40 age- and sex-matched healthy children were evaluated with distortion product otoacoustic emissions, audiometry, and cVEMP test in a tertiary hospital. The audiometry average values for both ears of the IgA vasculitis group and the control subjects were compared, and as a result, median 4.7-dB sensorineural hearing loss (SHL) was found for the IgA vasculitis group compared to the control group at 250 Hz and it was statistically significant (p < 0.001). An average of 6.4-dB SHL was detected at 8000 Hz (p < 0.001). There was a statistically significant difference among IgA vasculitis and control groups regarding measurement results of average p1-n1 latency time of both ears (0.9 ms (ms) increase, p = 0.035). In IgA vasculitis patients, the median amplitude difference of both ears' average p1 n1 was found to be 5.6 mV, statistically significantly decreased compared to the control group (p = 0.003).

CONCLUSION

This study, firstly in literature, demonstrated that IgA vasculitis may have association with hearing loss and vestibular dysfunction in children. We think this might be due to autoimmune mechanisms.

WHAT IS KNOWN

• Ig A vasculitis is a leukocytoclastic vasculitis with unknown etiology, involving the skin, joints, gastrointestinal system, kidneys, and rarely other organs. • No study has been reported for the cochlear and vestibular association of Ig A vasculitis in current literature.

WHAT IS NEW

• This study demonstrated that Ig A vasculitis may have association with hearing loss in children. • This study also demonstrated that Ig A vasculitis may have association with vestibular dysfunction in children.

Na/K-ATPase Gene Expression in the Human Cochlea: A Study Using mRNA in situ Hybridization and Super-Resolution Structured Illumination Microscopy

Background

The pervasive Na/K-ATPase pump is highly expressed in the human cochlea and is involved in the generation of the endocochlear potential as well as auditory nerve signaling and relay. Its distribution, molecular organization and gene regulation are essential to establish to better understand inner ear function and disease. Here, we analyzed the expression and distribution of the ATP1A1, ATP1B1, and ATP1A3 gene transcripts encoding the Na/K-ATPase α1, α3, and β1 isoforms in different domains of the human cochlea using RNA in situ hybridization.

Materials and Methods

Archival paraformaldehyde-fixed sections derived from surgically obtained human cochleae were used to label single mRNA gene transcripts using the highly sensitive multiplex RNAscope^® technique. Localization of gene transcripts was performed by super-resolution structured illumination microscopy (SR-SIM) using fluorescent-tagged probes. GJB6 encoding of the protein connexin30 served as an additional control.

Results

Single mRNA gene transcripts were seen as brightly stained puncta. Positive and negative controls verified the specificity of the labeling. ATP1A1 and ATP1B1 gene transcripts were demonstrated in the organ of Corti, including the hair and supporting cells. In the stria vascularis, these transcripts were solely expressed in the marginal cells. A large number of ATP1B1 gene transcripts were found in the spiral ganglion cell soma, outer sulcus, root cells, and type II fibrocytes. The ATP1B1 and ATP1A3 gene transcripts were rarely detected in axons.

Discussion

Surgically obtained inner ear tissue can be used to identify single mRNA gene transcripts using high-resolution fluorescence microscopy after prompt formaldehyde fixation and chelate decalcification. A large number of Na/K-ATPase gene transcripts were localized in selected areas of the cochlear wall epithelium, fibrocyte networks, and spiral ganglion, confirming the enzyme’s essential role for human cochlear function.

MRI With Gadolinium as a Measure of Blood-Labyrinth Barrier Integrity in Patients With Inner Ear Symptoms: A Scoping Review

Objective: Capillaries within the inner ear form a semi-permeable barrier called the blood-labyrinth barrier that is less permeable than capillary barriers elsewhere within the human body. Dysfunction of the blood-labyrinth barrier has been proposed as a mechanism for several audio-vestibular disorders. There has been interest in using magnetic resonance imaging (MRI) with intravenous gadolinium-based contrast agents (GBCA) as a marker for the integrity of the blood labyrinth barrier in research and clinical settings. This scoping review evaluates the evidence for using intravenous gadolinium-enhanced MRI to assess the permeability of the blood-labyrinth barrier in healthy and diseased ears.

Methods: A systematic search was conducted of three databases: PubMed, EMBASE, CINAHL PLUS. Studies were included that used GBCA to study the inner ear and permeability of the blood-labyrinth barrier. Data was collected on MRI protocols used and inner ear enhancement patterns of healthy and diseased ears in both human and animal studies.

Results: The search yielded 14 studies in animals and 53 studies in humans. In healthy animal and human inner ears, contrast-enhanced MRI demonstrated gradual increase in inner ear signal intensity over time that was limited to the perilymph. Signal intensity peaked at 100 min in rodents and 4 h in humans. Compared to controls, patients with idiopathic sudden sensorineural hearing loss and otosclerosis had increased signal intensity both before and shortly after GBCA injection. In patients with Ménière's disease and vestibular schwannoma, studies reported increased signal at 4 h, compared to controls. Quality assessment of included studies determined that all the studies lacked sample size justification and many lacked adequate control groups or blinded assessors of MRI.

Conclusions: The included studies provided convincing evidence that gadolinium crosses the blood-labyrinth barrier in healthy ears and more rapidly in some diseased ears. The timing of increased signal differs by disease. There was a lack of evidence that these findings indicate general permeability of the blood-labyrinth barrier. Future studies with consistent and rigorous methods are needed to investigate the relationship between gadolinium uptake and assessments of inner ear function and to better determine whether signal enhancement indicates permeability for molecules other than gadolinium.

Translational and interdisciplinary insights into presbyacusis: A multidimensional disease

There are multiple etiologies and phenotypes of age-related hearing loss or presbyacusis. In this review we summarize findings from animal and human studies of presbyacusis, including those that provide the theoretical framework for distinct metabolic, sensory, and neural presbyacusis phenotypes. A key finding in quiet-aged animals is a decline in the endocochlear potential (EP) that results in elevated pure-tone thresholds across frequencies with greater losses at higher frequencies. In contrast, sensory presbyacusis appears to derive, in part, from acute and cumulative effects on hair cells of a lifetime of environmental exposures (e.g., noise), which often result in pronounced high frequency hearing loss. These patterns of hearing loss in animals are recognizable in the human audiogram and can be classified into metabolic and sensory presbyacusis phenotypes, as well as a mixed metabolic+sensory phenotype. However, the audiogram does not fully characterize age-related changes in auditory function. Along with the effects of peripheral auditory system declines on the auditory nerve, primary degeneration in the spiral ganglion also appears to contribute to central auditory system aging. These inner ear alterations often correlate with structural and functional changes throughout the central nervous system and may explain suprathreshold speech communication difficulties in older adults with hearing loss. Throughout this review we highlight potential methods and research directions, with the goal of advancing our understanding, prevention, diagnosis, and treatment of presbyacusis.

Carotid imaging changes and serum IL-1β, sICAM-1, and sVAP-1 levels in benign paroxysmal positional vertigo

Benign paroxysmal positional vertigo (BPPV) is the most common cause of vertigo. This study was performed to evaluate serum levels of inflammatory factors and changes in B-mode carotid ultrasound findings in patients with BPPV. The study population consisted of 90 BPPV patients and 90 age- and sex-matched controls. ELISA was used to compare the levels of inflammatory factors, such as interleukin-1β (IL-1β), tumor necrosis factor-alpha (TNF-α), soluble intercellular adhesion molecule-1 (sICAM-1), prostaglandin-E2 (PG-E2), and soluble vascular adhesion protein-1 (sVAP-1), between BPPV patients and controls. In addition, the results of ultrasonographic imaging to determine carotid intima-media thickness (C-IMT), carotid atheromatous plaque, and vertebral artery stenosis were also compared between the BPPV and control groups. Serum levels of IL-1β, sICAM-1, and sVAP-1 were significantly higher in BPPV patients than controls (P < 0.001, P < 0.05, and P < 0.001, respectively). C-IMT and vertebral artery stenosis were significantly different in BPPV patients compared to controls (both P < 0.05). There were no significant relations between other parameters and BPPV. IL-1β, sICAM-1, and sVAP-1 are potentially associated with the pathogenesis of BPPV, and C-ITM and carotid vertebral stenosis may be useful reference imaging findings for the diagnosis of BPPV.

New insights on repeated acoustic injury: Augmentation of cochlear susceptibility and inflammatory reaction resultant of prior acoustic injury

In industrial and military settings, individuals who suffer from one episode of acoustic trauma are likely to sustain another episode of acoustic stress, creating an opportunity for a potential interaction between the two stress conditions. We previously demonstrated that acoustic overstimulation perturbs the cochlear immune environment. However, how the cochlear immune system responds to repeated acoustic overstimulation is unknown. Here, we used a mouse model to investigate the cochlear immune response to repeated stress. We reveal that exposure to an intense noise at 120 dB SPL for 1 h activates the cochlear immune response in a time-dependent fashion with substantial expansion and activation of the macrophage population in the cochlea at 2-days post-exposure. At 20-days post-exposure, the number and pro-inflammatory phenotypes of cochlear macrophages have significantly subsided, but have yet to return to homeostatic levels. Monocytes with anti-inflammatory phenotypes are recruited into the cochlea. With the presence of this residual immune activation, a second exposure to the same noise provokes an exaggerated inflammatory response as evidenced by exacerbated maturation of macrophages. Furthermore, the second noise causes greater sensory cell pathogenesis. Unlike the first noise-induced damage that occurs mainly between 0 and 2 days post-exposure, the second noise-induced damage occurs more frequently between 2 and 20 days post-exposure, the period when secondary damage takes place. These observations suggest that repeated acoustic overstimulation exacerbates cochlear inflammation and secondary sensory cell pathogenesis. Together, our results suggest that the cochlear immune system plays an important role in modulating cochlear responses to repeated acoustic stress.

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.

Forgotten Fibrocytes: A Neglected, Supporting Cell Type of the Cochlea With the Potential to be an Alternative Therapeutic Target in Hearing Loss

Cochlear fibrocytes are a homeostatic supporting cell type embedded in the vascularized extracellular matrix of the spiral ligament, within the lateral wall. Here, they participate in the connective tissue syncytium that enables potassium recirculation into the scala media to take place and ensures development of the endolymphatic potential that helps drive current into hair cells during acoustic stimulation. They have also been implicated in inflammatory responses in the cochlea. Some fibrocytes interact closely with the capillaries of the vasculature in a way which suggests potential involvement, together with the stria vascularis, also in the blood-labyrinth barrier. Several lines of evidence suggests that pathology of the fibrocytes, along with other degenerative changes in this region, contribute to metabolic hearing loss (MHL) during aging that is becoming recognized as distinct from, and potentially a precursor for, sensorineural hearing loss (SNHL). This pathology may underlie a significant proportion of cases of presbycusis. Some evidence points also to an association between fibrocyte degeneration and Ménière’s disease (MD). Fibrocytes are mesenchymal; this characteristic, and their location, make them amenable to potential cell therapy in the form of cell replacement or genetic modification to arrest the process of degeneration that leads to MHL. This review explores the properties and roles of this neglected cell type and suggests potential therapeutic approaches, such as cell transplantation or genetic engineering of fibrocytes, which could be used to prevent this form of presbycusis or provide a therapeutic avenue for MD.

Evidence for a dynorphin-mediated inner ear immune/inflammatory response and glutamate-induced neural excitotoxicity: an updated analysis

Acoustic overstimulation (AOS) is defined as the stressful overexposure to high-intensity sounds. AOS is a precipitating factor that leads to a glutamate (GLU)-induced Type I auditory neural excitotoxicity and an activation of an immune/inflammatory/oxidative stress response within the inner ear, often resulting in cochlear hearing loss. The dendrites of the Type I auditory neural neurons that innervate the inner hair cells (IHCs), and respond to the IHC release of the excitatory neurotransmitter GLU, are themselves directly innervated by the dynorphin (DYN)-bearing axon terminals of the descending brain stem lateral olivocochlear (LOC) system. DYNs are known to increase GLU availability, potentiate GLU excitotoxicity, and induce superoxide production. DYNs also increase the production of proinflammatory cytokines by modulating immune/inflammatory signal transduction pathways. Evidence is provided supporting the possibility that the GLU-mediated Type I auditory neural dendritic swelling, inflammation, excitotoxicity, and cochlear hearing loss that follow AOS may be part of a brain stem-activated, DYN-mediated cascade of inflammatory events subsequent to a LOC release of DYNs into the cochlea. In support of a DYN-mediated cascade of events are established investigations linking DYNs to the immune/inflammatory/excitotoxic response in other neural systems.

Age-Related Changes in Immune Cells of the Human Cochlea

Age-related hearing loss is a chronic degenerative disorder affecting one in two individuals above the age of 75. Current population projections predict a steady climb in the number of older individuals making the search for interventions to prevent or reverse this disorder even more critical. There is growing acceptance that aberrant activity of resident or infiltrating immune cells, such as macrophages, is a major factor contributing to the onset and progression of age-related degenerative diseases. However, how macrophage populations and their functionally-driven morphological characteristics change with age in the human cochlea remains largely unknown. In this study, we employed immunohistochemical approaches along with confocal and super-resolution imaging, three-dimensional reconstructions, and quantitative analysis to determine age-related changes in macrophage numbers and morphology as well as interactions with other cell-types and structures of the auditory nerve and lateral wall in the human cochlea. In the cochlea of human ears from young and middle aged adults those macrophages in the auditory nerve assumed a worm-like structure in contrast to those in the spiral ligament or associated with the dense microvascular network in the stria vascularis which exhibited a highly ramified morphology. Macrophages in both the auditory nerve and cochlear lateral wall showed morphological alterations with age. The population of activated macrophages in the auditory nerve increased in cochleas obtained from older donors. Dual-immunohistochemical staining with macrophage, myelin, and neuronal markers revealed increased interactions of macrophages with the glial and neuronal components of the aged auditory nerve. These findings implicate the involvement of abnormal macrophage-glia interactions in age-related physiological and pathological alterations in the human cochlea. There is clearly a need to further investigate the contribution of macrophage-associated inflammatory dysregulation in human presbyacusis.

Purinergic Signaling and Cochlear Injury-Targeting the Immune System?

Hearing impairment is the most common sensory deficit, affecting more than 400 million people worldwide. Sensorineural hearing losses currently lack any specific or efficient pharmacotherapy largely due to the insufficient knowledge of the pathomechanism. Purinergic signaling plays a substantial role in cochlear (patho)physiology. P2 (ionotropic P2X and the metabotropic P2Y) as well as adenosine receptors expressed on cochlear sensory and non-sensory cells are involved mostly in protective mechanisms of the cochlea. They are implicated in the sensitivity adjustment of the receptor cells by a K⁺ shunt and can attenuate the cochlear amplification by modifying cochlear micromechanics. Cochlear blood flow is also regulated by purines. Here, we propose to comprehend this field with the purine-immune interactions in the cochlea. The role of harmful immune mechanisms in sensorineural hearing losses has been emerging in the horizon of cochlear pathologies. In addition to decreasing hearing sensitivity and increasing cochlear blood supply, influencing the immune system can be the additional avenue for pharmacological targeting of purinergic signaling in the cochlea. Elucidating this complexity of purinergic effects on cochlear functions is necessary and it can result in development of new therapeutic approaches in hearing disabilities, especially in the noise-induced ones.

Evaluation of hearing in pediatric familial Mediterranean fever patients during attack period and attack-free period

OBJECTIVES

Familial Mediterranean fever (FMF) is the most common monogenic autoinflammatory disease worldwide. It usually has a childhood onset and is characterized with recurrent attacks with irregular intervals. Few studies have been performed to investigate hearing in FMF patients ran with various tests and showed different results. In this study, first time in the literature, we aimed to evaluate and compare auditory functions in pediatric FMF patients during the attack periods and attack-free periods to see the possible effects of autoinflammation that was caused by FMF attacks.

METHODS

40 pediatric FMF patients (80 ears) enrolled in the study as study group and 21 matching (42 ears) healthy children as a control group. Blood samples were taken from patients who presented clinical attack symptoms. Control group and FMF patients that were in attack period underwent audiometric evaluation including pure tone audiometry (0.25, 0.5, 1, 2, 4, 8 kHz) and Distortion product otoaoustic emission (DPOAE) (1, 1.4, 2, 2.8, 4 kHz) test. The tests were repeated in attackfree period. Correlations between hearing results and C-reactive protein, erythrocyte sedimentation rate levels, duration of disease, age at disease onset, colchicine usage time, cumulative dosage of colchicine and the number of attacks in the last six months were studied.

RESULTS

Hearing thresholds of FMF patients, compared to control group, were found to be increased at most frequencies. Lower signal/noise ratios (SNR) were detected at most frequencies in the DPAOE test. In the attack period compared to attack-free period, hearing thresholds in the audiometry were found to be increased at some frequencies (p < 0.05). However, this was not supported by the DPOAE test. In correlation analysis, increased colchicine usage time and colchicine cumulative dosage were associated with decreased thresholds in audiogram and increased SNR values in DPOAE. Detailed statistical analyses of all parameters were included in the study.

CONCLUSION

Results demonstrated cochlear involvement in FMF patients. Acute changes in hearing thresholds in the attack period may be suggesting the effect of acute inflammation on cochlea. The difference between the audiometry and otoacoustic emission test results of FMF patients and healthy controls suggests the cumulative effect of recurrent inflammation attacks on cochlea. Positive affect of treatment of the disease with long term colchicine in hearing were also demonstrated.

Differential fates of tissue macrophages in the cochlea during postnatal development

The cochlea contains macrophages. These cells participate in inflammatory responses to cochlear pathogenesis. However, it is not clear how and when these cells populate the cochlea during postnatal development. The current study aims to determine the postnatal development of cochlear macrophages with the focus on macrophage development in the organ of Corti and the basilar membrane. Cochleae were collected from C57BL/6J mice at ages of postnatal day (P) 1 to P21, as well as from mature mice (1-4 months). Macrophages were identified based on their expression of F4/80 and Iba1, as well as their unique morphologies. Two sets of macrophages were identified in the regions of the organ of Corti and the basilar membrane. One set resides on the scala tympani side of the basilar membrane. These cells have a round shape at P1 and start to undergo site-specific differentiation at P4. Apical macrophages adopt a dendritic shape. Middle and basal macrophages take on an irregular shape with short projections. Basal macrophages further differentiate into an amoeboid shape. The other set of macrophages resides above the basilar membrane, either beneath the cells of the organ of Corti or along the spiral vessel of the basilar membrane. As the sensory epithelium matures, these cells undergo developmental death with the phenotypes of apoptosis. Macrophages are also identified in the spiral ligament, spiral limbus, and neural regions. Their numbers decrease during postnatal development. Together, these results suggest a dynamic rearrangement of the macrophage population during postnatal cochlear development.

Epigallocatechin-3-gallate, a prototypic chemopreventative agent for protection against cisplatin-based ototoxicity

Cisplatin-induced ototoxicity is one of the major factors limiting cisplatin chemotherapy. Ototoxicity results from damage to outer hair cells (OHCs) and other regions of the cochlea. At the cellular level, cisplatin increases reactive oxygen species (ROS) leading to cochlear inflammation and apoptosis. Thus, ideal otoprotective drugs should target oxidative stress and inflammatory mechanisms without interfering with cisplatin's chemotherapeutic efficacy. In this study, we show that epigallocatechin-3-gallate (EGCG) is a prototypic agent exhibiting these properties of an effect otoprotective agent. Rats administered oral EGCG demonstrate reduced cisplatin-induced hearing loss, reduced loss of OHCs in the basal region of the cochlea and reduced oxidative stress and apoptotic markers. EGCG also protected against the loss of ribbon synapses associated with inner hair cells and Na+/K+ ATPase α1 in the stria vascularis and spiral ligament. In vitro studies showed that EGCG reduced cisplatin-induced ROS generation and ERK1/2 and signal transducer and activator of transcription-1 (STAT1) activity, but preserved the activity of STAT3 and Bcl-xL. The increase in STAT3/STAT1 ratio appears critical for mediating its otoprotection. EGCG did not alter cisplatin-induced apoptosis of human-derived cancer cells or cisplatin antitumor efficacy in a xenograft tumor model in mice because of its inability to rescue the downregulation of STAT3 in these cells. These data suggest that EGCG is an ideal otoprotective agent for treating cisplatin-induced hearing loss without compromising its antitumor efficacy.

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.

The evaluation of cochlear functions in Familial Mediterranean Fever

Familial Mediterranean Fever (FMF) is a progressive disease characterized by chronic inflammation, which also has negative effects on cochlear functions and hearing levels. We investigated whether the cochlear functions and hearing levels of FMF patients were different than healthy controls and also evaluated the relationship of hearing levels with the age at diagnosis, duration without treatment, and inflammation and lipid parameters in this study. A total of 60 patients diagnosed with FMF and 48 age, gender and body mass index (BMI)-matched healthy controls were included in the study. The hemogram, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and lipid parameters of the subjects were studied and they all underwent pure tone audiometry and Transient evoked otoacoustic emission tests after an otologic examination. The hearing levels of the FMF group were significantly higher than those of the control group. The TEOAE signal/noise (S/N) ratios were similar in both groups. A positive relationship was present between the audiometric test results and the age, BMI, low-density lipoprotein and triglyceride levels and a negative relationship with the high-density lipoprotein levels. A negative relationship was present between the TEOAE S/N ratios and the age of the patients, duration without treatment, lipid parameters, inflammation markers and the creatinine level. FMF patients are exposed to chronic inflammation and this can influence their hearing levels. The age at diagnosis, duration without treatment, chronic inflammation, unfavorable lipid parameters, and obesity can affect hearing tests negatively.

Cross-talk between intestinal epithelial cells and immune cells in inflammatory bowel disease

Inflammatory bowel disease (IBD) involves functional impairment of intestinal epithelial cells (IECs), concomitant with the infiltration of the lamina propria by inflammatory cells. We explored the reciprocal paracrine and direct interaction between human IECs and macrophages (MΦ) in a co-culture system that mimics some aspects of IBD. We investigated the expression of intercellular junctional proteins in cultured IECs under inflammatory conditions and in tissues from IBD patients. IECs establish functional gap junctions with IECs and MΦ, respectively. Connexin (Cx26) and Cx43 expression in cultured IECs is augmented under inflammatory conditions; while, Cx43-associated junctional complexes partners, E-cadherin, ZO-1, and β-catenin expression is decreased. The expression of Cx26 and Cx43 in IBD tissues is redistributed to the basal membrane of IEC, which is associated with decrease in junctional complex proteins' expression, collagen type IV expression and infiltration of MΦ. These data support the notion that the combination of paracrine and hetero-cellular communication between IECs and MΦs may regulate epithelial cell function through the establishment of junctional complexes between inflammatory cells and IECs, which ultimately contribute to the dys-regulation of intestinal epithelial barrier.

Toll-like receptor 4 modulates the cochlear immune response to acoustic injury

Acoustic overstimulation traumatizes the cochlea, resulting in auditory dysfunction. As a consequence of acoustic injury, the immune system in the cochlea is activated, leading to the production of inflammatory mediators and the infiltration of immune cells. However, the molecular mechanisms responsible for initiating these immune responses remain unclear. Here, we investigate the functional role of Toll-like receptor 4 (Tlr4), a cellular receptor that activates the innate immune system, in the regulation of cochlear responses to acoustic overstimulation. Using a Tlr4 knockout mouse model, we examined how Tlr4 deficiency affects sensory cell pathogenesis, auditory dysfunction and cochlear immune activity. We demonstrate that Tlr4 knockout does not affect sensory cell viability under physiological conditions, but reduces the level of sensory cell damage and cochlear dysfunction after acoustic injury. Together, these findings suggest that Tlr4 promotes sensory cell degeneration and cochlear dysfunction after acoustic injury. Acoustic injury provokes a site-dependent inflammatory response in both the organ of Corti and the tissues of the lateral wall and basilar membrane. Tlr4 deficiency affects these inflammatory responses in a site-dependent manner. In the organ of Corti, loss of Tlr4 function suppresses the production of interleukin 6 (Il6), a pro-inflammatory molecule, after acoustic injury. By contrast, the production of inflammatory mediators, including Il6, persists in the lateral wall and basilar membrane. In addition to immune molecules, Tlr4 knockout inhibits the expression of major histocompatibility complex class II, an antigen-presenting molecule, in macrophages, suggesting that Tlr4 participates in the antigen-presenting function of macrophages after acoustic trauma. Together, these results suggest that Tlr4 regulates multiple aspects of the immune response in the cochlea and contributes to cochlear pathogenesis after acoustic injury.

Inhibition of Post-Meningitic Cochlear Injury with Cerebrospinal Fluid Irrigation

OBJECTIVE: Labyrinthitis ossificans, the pathologic ossification of the otic capsule associated with profound deafness and loss of vestibular function occurs frequently as a sequella of bacterial meningitis and subsequent purulent labyrinthitis. Experimentally, in Streptococcus pneumoniae meningitis, it has been shown that a vigorous inflammatory response to teichoic acids in the bacterial cell wall contributes to cochlear damage and subsequent fibrosis and ossification. The hypothesis of this study is that a dilution of concentration of inflammatory mediators through cerebrospinal fluid (CSF) irrigation will lead to a reduction in both inner ear pathology and permanent hearing loss.

STUDY DESIGN AND SETTING: Auditory brainstem response testing was used to determine baseline hearing thresholds in 20 Mongolian gerbils (12 irrigated, 8 sham irrigated animals) at 32 kHz, 16 kHz, 8 kHz, and 4 kHz frequencies. Their thresholds at 14 days and 120 days post-procedure were also obtained. Streptococcus pneumoniae meningitis was induced in both groups of animals by intrathecal (i.t.) injection of bacteria. Both groups received penicillin treatment. Forty-eight hours after inoculation, both groups were implanted with i.t. inflow and outflow catheters. The irrigated group was infused continuously with artificial CSF over 36 hr at a rate of 70 μL/hr and the outflow sampled. The tubing in the sham irrigated group was clamped (without sampling). They were sacrificed at 120 days post-procedure and histomorphometric analysis carried out. The concentration of interleukin 1β (IL-1β) for the CSF samples from the irrigated group were compared to samples collected from an additional control group of 8 non-irrigated meningitic gerbils. IL-1β was chosen to study because it is a potent pro-inflammatory cytokines in bacterial meningitis that is unaffected by the neurosurgical trauma of the experimental protocol.

RESULTS: Twenty animals survived the meningitis (6 irrigation, 6 sham irrigation, 8 non-irrigation meningitic controls). At Days 14 and 120 post-infection, the irrigated animals manifested significantly less hearing loss with a mean loss of 28.82 dB compared to the sham irrigation group mean loss of 40.76 dB ( P < 0.03). The degree of hearing loss in both groups was frequency-dependent with greater loss at higher frequencies (mean loss = 22.4 dB at 32 kHz, 23.0 dB at 16 kHz, 18.6 dB at 8 kHz, and 12.5 dB at 4 kHz). Histomorphometric analysis demonstrated a marked reduction in degeneration of the spiral ligament, spiral ganglion cells, and stria vascularis in experimental animals as compared to controls. Immunohistochemistry showed a significant reduction in IL-β1 concentrations in the irrigated animals compared to the non-irrigated, infected controls ( P < 0.03).

CONCLUSIONS: Irrigation of CSF resulted in a significant reduction in post-meningitic cochlear injury when compared to controls. This model for continuous cerebrospinal fluid irrigation provides a means to evaluate the effects of a dilution of inflammatory mediators on hearing loss and labyrinthitis ossificans after bacterial meningitis.

SIGNIFICANCE: Despite advances in the prevention of meningitis and improved antibiotic treatment, bacterial meningitis continues to have significant associated morbidity. This study provides insight into some of the mechanisms responsible for post-meningitic hearing loss and labyrinthitis ossificans and presents a novel approach to reduce these complications.

EBM rating: C-4

Characterisation of cochlear inflammation in mice following acute and chronic noise exposure

Oxidative stress has been established as the key mechanism of the cochlear damage underlying noise-induced hearing loss, however, emerging evidence suggests that cochlear inflammation may also be a major contributor. This study aimed to improve our understanding of the cochlear inflammatory response associated with acute and chronic noise exposure. C57BL/6 mice were exposed to acute traumatic noise (100 dBSPL, 8-16 kHz for 24 h) and their cochleae collected at various intervals thereafter, up to 7 days. Using quantitative RT-PCR and immunohistochemistry, changes in expression levels of proinflammatory cytokines (TNF-α, IL-1β), chemokines (CCL2) and cell adhesion molecules (ICAM-1) were studied. All gene transcripts displayed similar dynamics of expression, with an early upregulation at 6 h post-exposure, followed by a second peak at 7 days. ICAM-1 immunoexpression increased significantly in the inferior region of the spiral ligament, peaking 24 h post-exposure. The early expression of proinflammatory mediators likely mediates the recruitment and extravasation of inflammatory cells into the noise-exposed cochlea. The occurrence of the latter expression peak is not clear, but it may be associated with reparative processes initiated in response to cochlear damage. Chronic exposure to moderate noise (90 dBSPL, 8-16 kHz, 2 h/day, up to 4 weeks) also elicited an inflammatory response, reaching a maximum after 2 weeks, suggesting that cochlear damage and hearing loss associated with chronic environmental noise exposure may be linked to inflammatory processes in the cochlea. This study thus provides further insight into the dynamics of the cochlear inflammatory response induced by exposure to acute and chronic noise.

Impedance Changes and Fibrous Tissue Growth after Cochlear Implantation Are Correlated and Can Be Reduced Using a Dexamethasone Eluting Electrode

BACKGROUND

The efficiency of cochlear implants (CIs) is affected by postoperative connective tissue growth around the electrode array. This tissue formation is thought to be the cause behind post-operative increases in impedance. Dexamethasone (DEX) eluting CIs may reduce fibrous tissue growth around the electrode array subsequently moderating elevations in impedance of the electrode contacts.

METHODS

For this study, DEX was incorporated into the silicone of the CI electrode arrays at 1% and 10% (w/w) concentration. Electrodes prepared by the same process but without dexamethasone served as controls. All electrodes were implanted into guinea pig cochleae though the round window membrane approach. Potential additive or synergistic effects of electrical stimulation (60 minutes) were investigated by measuring impedances before and after stimulation (days 0, 7, 28, 56 and 91). Acoustically evoked auditory brainstem responses were recorded before and after CI insertion as well as on experimental days 7, 28, 56, and 91. Additionally, histology performed on epoxy embedded samples enabled measurement of the area of scala tympani occupied with fibrous tissue.

RESULTS

In all experimental groups, the highest levels of fibrous tissue were detected in the basal region of the cochlea in vicinity to the round window niche. Both DEX concentrations, 10% and 1% (w/w), significantly reduced fibrosis around the electrode array of the CI. Following 3 months of implantation impedance levels in both DEX-eluting groups were significantly lower compared to the control group, the 10% group producing a greater effect. The same effects were observed before and after electrical stimulation.

CONCLUSION

To our knowledge, this is the first study to demonstrate a correlation between the extent of new tissue growth around the electrode and impedance changes after cochlear implantation. We conclude that DEX-eluting CIs are a means to reduce this tissue reaction and improve the functional benefits of the implant by attenuating electrode impedance.

Molecular regulation of auditory hair cell death and approaches to protect sensory receptor cells and/or stimulate repair following acoustic trauma

Loss of auditory sensory hair cells (HCs) is the most common cause of hearing loss. This review addresses the signaling pathways that are involved in the programmed and necrotic cell death of auditory HCs that occur in response to ototoxic and traumatic stressor events. The roles of inflammatory processes, oxidative stress, mitochondrial damage, cell death receptors, members of the mitogen-activated protein kinase (MAPK) signal pathway and pro- and anti-cell death members of the Bcl-2 family are explored. The molecular interaction of these signal pathways that initiates the loss of auditory HCs following acoustic trauma is covered and possible therapeutic interventions that may protect these sensory HCs from loss via apoptotic or non-apoptotic cell death are explored.

Transforming growth factor β1 inhibition protects from noise-induced hearing loss

Excessive exposure to noise damages the principal cochlear structures leading to hearing impairment. Inflammatory and immune responses are central mechanisms in cochlear defensive response to noise but, if unregulated, they contribute to inner ear damage and hearing loss. Transforming growth factor β (TGF-β) is a key regulator of both responses and high levels of this factor have been associated with cochlear injury in hearing loss animal models. To evaluate the potential of targeting TGF-β as a therapeutic strategy for preventing or ameliorating noise-induced hearing loss (NIHL), we studied the auditory function, cochlear morphology, gene expression and oxidative stress markers in mice exposed to noise and treated with TGF-β1 peptidic inhibitors P17 and P144, just before or immediately after noise insult. Our results indicate that systemic administration of both peptides significantly improved both the evolution of hearing thresholds and the degenerative changes induced by noise-exposure in lateral wall structures. Moreover, treatments ameliorated the inflammatory state and redox balance. These therapeutic effects were dose-dependent and more effective if the TGF-β1 inhibitors were administered prior to inducing the injury. In conclusion, inhibition of TGF-β1 actions with antagonistic peptides represents a new, promising therapeutic strategy for the prevention and repair of noise-induced cochlear damage.

Molecular profile of cochlear immunity in the resident cells of the organ of Corti

BACKGROUND

The cochlea is the sensory organ of hearing. In the cochlea, the organ of Corti houses sensory cells that are susceptible to pathological insults. While the organ of Corti lacks immune cells, it does have the capacity for immune activity. We hypothesized that resident cells in the organ of Corti were responsible for the stress-induced immune response of the organ of Corti. This study profiled the molecular composition of the immune system in the organ of Corti and examined the immune response of non-immune epithelial cells to acoustic overstimulation.

METHODS

Using high-throughput RNA-sequencing and qRT-PCR arrays, we identified immune- and inflammation-related genes in both the cochlear sensory epithelium and the organ of Corti. Using bioinformatics analyses, we cataloged the immune genes expressed. We then examined the response of these genes to acoustic overstimulation and determined how changes in immune gene expression were related to sensory cell damage.

RESULTS

The RNA-sequencing analysis reveals robust expression of immune-related genes in the cochlear sensory epithelium. The qRT-PCR array analysis confirms that many of these genes are constitutively expressed in the resident cells of the organ of Corti. Bioinformatics analyses reveal that the genes expressed are linked to the Toll-like receptor signaling pathway. We demonstrate that expression of Toll-like receptor signaling genes is predominantly from the supporting cells in the organ of Corti cells. Importantly, our data demonstrate that these Toll-like receptor pathway genes are able to respond to acoustic trauma and that their expression changes are associated with sensory cell damage.

CONCLUSION

The cochlear resident cells in the organ of Corti have immune capacity and participate in the cochlear immune response to acoustic overstimulation.

Recognition and control of the progression of age-related hearing loss

Recent breakthroughs have provided notable insights into both the pathogenesis and therapeutic strategies for age-related hearing loss (ARHL). Simultaneously, these breakthroughs enhance our knowledge about this neurodegenerative disease and raise the question of whether the disorder is preventable or even treatable. Discoveries relating to ARHL have revealed a unique link between ARHL and the underlying pathologies. Therefore, we need to better understand the pathogenesis or the mechanism of ARHL and learn how to take full advantage of various therapeutic strategies to prevent the progression of ARHL.

Role of cysteinyl leukotriene signaling in a mouse model of noise-induced cochlear injury

Noise-induced hearing loss is one of the most common types of sensorineural hearing loss. In this study, we examined the expression and localization of leukotriene receptors and their respective changes in the cochlea after hazardous noise exposure. We found that the expression of cysteinyl leukotriene type 1 receptor (CysLTR1) was increased until 3 d after noise exposure and enhanced CysLTR1 expression was mainly observed in the spiral ligament and the organ of Corti. Expression of 5-lipoxygenase was increased similar to that of CysLTR1, and there was an accompanying elevation of CysLT concentration. Posttreatment with leukotriene receptor antagonist (LTRA), montelukast, for 4 consecutive days after noise exposure significantly decreased the permanent threshold shift and also reduced the hair cell death in the cochlea. Using RNA-sequencing, we found that the expression of matrix metalloproteinase-3 (MMP-3) was up-regulated after noise exposure, and it was significantly inhibited by montelukast. Posttreatment with a MMP-3 inhibitor also protected the hair cells and reduced the permanent threshold shift. These findings suggest that acoustic injury up-regulated CysLT signaling in the cochlea and cochlear injury could be attenuated by LTRA through regulation of MMP-3 expression. This study provides mechanistic insights into the role of CysLTs signaling in noise-induced hearing loss and the therapeutic benefit of LTRA.

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.

Blood levels of TNF-α, IL-10, and IL-12 in idiopathic sudden sensorineural hearing loss

OBJECTIVES/HYPOTHESIS

To investigate the blood levels of TNF-α, IL-10, and IL-12 in the idiopathic sudden sensorineural hearing loss patients, and the change of these cytokine levels after treatment.

STUDY DESIGN

Prospective clinical trial.

METHODS

Twenty-three patients with idiopathic sudden sensorineural hearing loss and 20 healthy people were selected as study and control groups. Blood samples for TNF-α, IL-10, and IL-12 were taken before treatment and 6 weeks after treatment. The study group was given combined treatment including dexamethasone, heparin, pentoxifyline, vitamin B1, and B6 for 10 days, and was divided into two groups: treatment responders and treatment nonresponders. The treatment responders group was also divided into three groups according to most accepted criteria for improvement in the literature. Audiograms were taken before treatment and 6 weeks after treatment to determine the response to the treatment.

RESULTS

There was no significant difference between pre- and posttreatment values of IL-10 and IL-12 in all study groups (P > 0.05). There was also no significant difference between pre- and posttreatment values of TNF-α in treatment responders (P > 0.05). Treatment nonresponders had more elevated posttreatment values of TNF-α than pretreatment values (P < 0.05).

CONCLUSION

IL-10 and IL-12 may not play a critical role in idiopathic sudden sensorineural hearing loss. But our data supports the role of TNF-α in the pathophysiology of idiopathic sudden sensorineural hearing loss, and TNF-α receptor blockers may have benefits in these patients.

Progressive familial hearing loss in Muckle-Wells syndrome

CONCLUSION

The age-dependent disease progression requires accelerating diagnosis of Muckle-Wells syndrome (MWS) in order to start treatment as early as possible. The most frequent, but not obligate symptoms are familial fatigue, hearing loss, and arthralgia. The design of further clinical trials should focus on hearing in order to document the long-term effect of anti-interleukin (IL)-1 drugs on hearing preservation.

OBJECTIVES

This paper describes the otologic features of a genetically defined syndrome causing progressive hearing loss by cochlear degeneration. This is the first study reporting the pretreatment otologic presentation of a selected population with familial MWS.

METHODS

A single-center cohort was examined by audiologic and neurotologic methods including pure tone audiograms, vestibular testing, and tinnitus questionnaire. The audiograms of members of the same family were compared to describe the family-specific risk of hearing loss progression.

RESULTS

Nineteen patients (aged 3-72 years) belonging to four families with three different mutations of the NLRP3 gene were examined. Almost all patients (89%, 17/19) demonstrated bilateral sensorineural hearing loss. Hearing loss started in the high frequencies and led to profound deafness in the most severe cases. Even in cases of profound hearing loss the vestibular caloric reactivity was normal. Nearly half of the adults reported intermittent or permanent tinnitus.

Cryopyrin-associated periodic syndromes: otolaryngologic and audiologic manifestations

OBJECTIVE

Cryopyrin-associated periodic syndromes (CAPS) represent a spectrum of CIAS1 gene-mediated autoinflammatory diseases characterized by recurrent systemic inflammation. The clinical spectrum of CAPS varies from mild to severe and includes the syndromes historically described as familial cold autoinflammatory syndrome (FCAS), Muckle-Wells syndrome (MWS), and neonatal-onset multisystem inflammatory disease (NOMID). This article presents the largest cohort of patients with CAPS. The objective is to describe the pathogenesis, otolaryngologic, and audiologic manifestations of CAPS.

STUDY DESIGN

Prospective (2003-2009).

SETTING

National Institutes of Health.

SUBJECTS AND METHODS

Fifty-seven patients with a diagnosis of CAPS were identified (31 NOMID, 11 NOMID/MWS, 9 MWS, and 6 FCAS). Comprehensive data regarding clinical manifestations, audiologic phenotype, and fluid attenuation inversion recovery MRI (FLAIR-MRI) of the brain and inner ear were obtained.

RESULTS

Complete audiologic data obtained on 70% of ears revealed conductive hearing loss in 4 (11%) NOMID ears and mixed hearing loss in 5 (13%) NOMID and 2 (14%) NOMID/MWS ears. Sensorineural hearing loss (SNHL), worse in higher frequencies, was the most common type of hearing loss and was present in 23 (61%) NOMID, 10 (71%) NOMID/MWS, and 4 (33%) MWS ears. All of the patients with FCAS had normal hearing except 2, who had SNHL from 4 to 8 kHz. On FLAIR-MRI sequence, cochlear enhancement was noted in 26 of 29 (90%) NOMID, 6 of 11 (55%) NOMID/MWS, 3 of 9 (33%) MWS, and 1 of 6 (17%) FCAS patients and was significantly associated with the presence of hearing loss. Maxillary sinus hypoplasia and mucosal thickening were found in 39% and 86% of the cohort, respectively.

CONCLUSION

CIAS1 pathway–mediated CAPS is associated with unregulated autoinflammation mediated by interleukin-1 in the cochlea and hearing loss. Timely diagnosis is crucial to initiate early treatment with interleukin-1 receptor antagonists.

Reduced formation of oxidative stress biomarkers and migration of mononuclear phagocytes in the cochleae of chinchilla after antioxidant treatment in acute acoustic trauma

Objective. Inhibition of inflammation and free radical formation in the cochlea may be involved in antioxidant treatment in acute acoustic trauma. Procedure. Chinchilla were exposed to 105 dB sound pressure level octave band noise for 6 hours. One group of chinchilla was treated with antioxidants after noise exposure. Auditory brainstem responses, outer hair cell counts, and immunohistochemical analyses of biomarkers in the cochlea were conducted. Results. The antioxidant treatment significantly reduced hearing threshold shifts, outer hair cell loss, numbers of CD45⁺ cells, as well as 4-hydroxy-2-nonenal and nitrotyrosine formation in the cochlea. Conclusion. Antioxidant treatment may provide protection to sensory cells by inhibiting formation of reactive oxygen and nitrogen products and migration of mononuclear phagocytes in the cochlea. The present study provides further evidence of effectiveness of antioxidant treatment in reducing permanent hearing loss.