CC chemokine receptor 2 is protective against noise-induced hair cell death: studies in CX3CR1(+/GFP) mice

The role of gene mutations and immune responses in sensorineural hearing loss

Sensorineural hearing loss (SNHL) is a prevalent clinical condition primarily attributed to dysfunction within various components of the auditory pathway, spanning from the inner ear to the auditory cortex. Recent research has illuminated immune and inflammation-mediated disorders of the inner ear as critical contributors to SNHL. Disruptions in the equilibrium of inflammatory mediators, chemokines, the complement system, and inflammatory vesicles within the cochlea provoke aberrations in immune cell activity, fostering a chronic pro-inflammatory milieu that detrimentally affects the structural and functional integrity of the inner ear, culminating in hearing impairment. Specific genetic mutations, especially those affecting auditory structures, play an important role in SNHL. These mutations regulate inflammatory mediators and cellular responses, thereby altering the inflammatory dynamics within the cochlea. This review delves into the pathogenesis of sensorineural hearing loss, emphasizing the impact of genetic alterations, immune responses within the inner ear, and inflammatory mediators on auditory function. It highlights the significance of Transmembrane Serine Protease 3 (TMPRSS3) and connexin gene mutations as pivotal genetic elements in SNHL, underscoring the central role of inflammatory responses in cochlear damage. Furthermore, the paper discusses the promise of gene therapy and targeted molecular interventions, underscoring the necessity for continued exploration into the specific actions of various inflammatory agents to refine personalized therapeutic strategies.

Activated tissue-resident macrophages contribute to hair cell insults in noise-induced hearing loss in mice

Macrophages serve as the primary immune cell population and assume a pivotal role in the immune response within the damaged cochleae. Yet, the origin and role of macrophages in response to noise exposure remain controversial. Here, we take advantage of Ccr2RFP/+ Cx3cr1GFP/+ dual-reporter mice to identify the infiltrated and tissue-resident macrophages. After noise exposure, we reveal that activated resident macrophages change in morphology, increase in abundance, and migrate to the region of hair cells, leading to the loss of outer hair cells and the damage of ribbon synapses. Meanwhile, peripheral monocytes are not implicated in the noise-induced hair cell insults. These noise-induced activities of macrophages are abolished by inhibiting TLR4 signaling, resulting in alleviated insults of hair cells and partial recovery of hearing. Our findings indicate cochlear resident macrophages are pro-inflammatory and detrimental players in acoustic trauma and introduce a potential therapeutic target in noise-induced hearing loss.

Chemokine Receptor CCR2 Is Protective toward Outer Hair Cells in Chronic Suppurative Otitis Media

Chronic suppurative otitis media (CSOM) is a neglected disease that afflicts 330 million people worldwide and is the most common cause of permanent hearing loss among children in the developing world. Previously, we discovered that outer hair cell (OHC) loss occurred in the basal turn of the cochlea and that macrophages are the major immune cells associated with OHC loss in CSOM. Macrophage-associated cytokines are upregulated. Specifically, CCL-2, an important member of the MCP family, is elevated over time following middle ear infection. CCR2 is a common receptor of the MCP family and the unique receptor of CCL2. CCR2 knockout mice (CCR2-/-) have been used extensively in studies of monocyte activation in neurodegenerative diseases. In the present study, we investigated the effect of CCR2 deletion on the cochlear immune response and OHC survival in CSOM. The OHC survival rate was 84 ± 12.5% in the basal turn of CCR2+/+ CSOM cochleae, compared with was 63 ± 19.9% in the basal turn of CCR2-/- CSOM cochleae (p ≤ 0.05). Macrophage numbers were significantly reduced in CCR2-/- CSOM cochleae compared with CCR2+/+ CSOM cochleae (p ≤ 0.001). In addition, CCL7 was upregulated, whereas IL-33 was downregulated, in CCR2-/- CSOM cochleae. Finally, the permeability of the blood-labyrinth barrier in the stria vascularis remained unchanged in CCR2-/- CSOM compared with CCR2+/+ CSOM. Taken together, the data suggest that CCR2 plays a protective role through cochlear macrophages in the CSOM cochlea.

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.

Pneumococcal Meningitis Induces Hearing Loss and Cochlear Ossification Modulated by Chemokine Receptors CX3CR1 and CCR2

PURPOSE

Pneumococcal meningitis is a major cause of hearing loss and permanent neurological impairment despite widely available antimicrobial therapies to control infection. Methods to improve hearing outcomes for those who survive bacterial meningitis remains elusive. We used a mouse model of pneumococcal meningitis to evaluate the impact of mononuclear phagocytes on hearing outcomes and cochlear ossification by altering the expression of CX3CR1 and CCR2 in these infected mice.

METHODS

We induced pneumococcal meningitis in approximately 500 C57Bl6 adult mice using live Streptococcus pneumoniae (serotype 3, 1 × 105 colony forming units (cfu) in 10 µl) injected directly into the cisterna magna of anesthetized mice and treated these mice with ceftriaxone daily until recovered. We evaluated hearing thresholds over time, characterized the cochlear inflammatory response, and quantified the amount of new bone formation during meningitis recovery. We used microcomputed tomography (microCT) scans to quantify cochlear volume loss caused by neo-ossification. We also performed perilymph sampling in live mice to assess the integrity of the blood-perilymph barrier during various time intervals after meningitis. We then evaluated the effect of CX3CR1 or CCR2 deletion in meningitis symptoms, hearing loss, macrophage/monocyte recruitment, neo-ossification, and blood labyrinth barrier function.

RESULTS

Sixty percent of mice with pneumococcal meningitis developed hearing loss. Cochlear fibrosis could be detected within 4 days of infection, and neo-ossification by 14 days. Loss of spiral ganglion neurons was common, and inner ear anatomy was distorted by scarring caused by new soft tissue and bone deposited within the scalae. The blood-perilymph barrier was disrupted at 3 days post infection (DPI) and was restored by seven DPI. Both CCR2 and CX3CR1 monocytes and macrophages were present in the cochlea in large numbers after infection. Neither chemokine receptor was necessary for the induction of hearing loss, cochlear fibrosis, ossification, or disruption of the blood-perilymph barrier. CCR2 knockout (KO) mice suffered the most severe hearing loss. CX3CR1 KO mice demonstrated an intermediate phenotype with greater susceptibility to hearing loss compared to control mice. Elimination of CX3CR1 mononuclear phagocytes during the first 2 weeks after meningitis in CX3CR1-DTR transgenic mice did not protect mice from any of the systemic or hearing sequelae of pneumococcal meningitis.

CONCLUSIONS

Pneumococcal meningitis can have devastating effects on cochlear structure and function, although not all mice experienced hearing loss or cochlear damage. Meningitis can result in rapid progression of hearing loss with fibrosis starting at four DPI and ossification within 2 weeks of infection detectable by light microscopy. The inflammatory response to bacterial meningitis is robust and can affect all three scalae. Our results suggest that CCR2 may assist in controlling infection and maintaining cochlear patency, as CCR2 knockout mice experienced more severe disease, more rapid hearing loss, and more advanced cochlear ossification after pneumococcal meningitis. CX3CR1 also may play an important role in the maintenance of cochlear patency.

Macrophage-related immune responses in inner ear: a potential therapeutic target for sensorineural hearing loss

Hearing loss is the most common sensory disorder in human beings. Cochlear sensory cells are the basis of hearing. Cochlear sensory cells suffer from various acute or chronic injuries, such as excessive sound stimulation, ototoxic drugs, and age-related degeneration. In response to these stresses, the cochlea develops an immune response. In recent years, studies have shown that the immune response of the inner ear has been regarded as one of the important pathological mechanisms of inner ear injury. Therapeutic interventions for inflammatory responses can effectively alleviate different types of inner ear injury. As the main immune cells in the inner ear, macrophages are involved in the process of inner ear injury caused by various exogenous factors. However, its specific role in the immune response of the inner ear is still unclear. This review focuses on discusses the dynamic changes of macrophages during different types of inner ear injury, and clarifies the potential role of macrophage-related immune response in inner ear injury.

Identification of immune-related candidate biomarkers in plasma of patients with sporadic vestibular schwannoma

Vestibular schwannoma (VS) is an intracranial tumor arising from neoplastic Schwann cells and typically presenting with hearing loss. The traditional belief that hearing deficit is caused by physical expansion of the VS, compressing the auditory nerve, does not explain the common clinical finding that patients with small tumors can have profound hearing loss, suggesting that tumor-secreted factors could influence hearing ability in VS patients. We conducted profiling of patients' plasma for 66 immune-related factors in patients with sporadic VS (N > 170) and identified and validated candidate biomarkers associated with tumor size (S100B) and hearing (MCP-3). We further identified a nine-biomarker panel (TNR-R2, MIF, CD30, MCP-3, IL-2R, BLC, TWEAK, eotaxin, and S100B) with outstanding discriminatory ability for VS. These findings revealed possible therapeutic targets for VS, providing a unique diagnostic tool that may predict hearing change and tumor growth in VS patients, and may inform the timing of tumor resection to preserve hearing.

Heterogeneity in macrophages along the cochlear spiral in mice: insights from SEM and functional analyses

The susceptibility of sensory cells to pathological conditions differs between the apical and basal regions of the cochlea, and the cochlear immune system may contribute to this location-dependent variability. Our previous study found morphological differences in basilar membrane macrophages between the apical and basal regions of the cochlea. However, the details of this site-dependent difference and its underlying structural and biological basis are not fully understood. In this study, we utilized scanning electron microscopy to examine the ultrastructure of macrophages and their surrounding supporting structures. Additionally, we examined the phagocytic activities of macrophages and the expression of immune molecules in both apical and basal regions of the cochlea. We employed two mouse strains (C57BL/6J and B6.129P-Cx3cr1tm1Litt/J) and evaluated three experimental conditions: young normal (1-4 months), aging (11-19 months), and noise-induced damage (120 dB SPL for 1 h). Using scanning electron microscopy, we revealed location-specific differences in basilar membrane macrophage morphology and surface texture, architecture in mesothelial cell layers, and spatial correlation between macrophages and mesothelial cells in both young and older mice. Observations of macrophage phagocytic activities demonstrated that basal macrophages exhibited greater phagocytic activities in aging and noise-damaged ears. Furthermore, we identified differences in the expression of immune molecules between the apical and basal cochlear tissues of young mice. Finally, our study demonstrated that as the cochlea ages, macrophages in the apical and basal regions undergo a transformation in their morphologies, with apical macrophages acquiring certain basal macrophage features and vice versa. Overall, our findings demonstrate apical and basal differences in macrophage phenotypes and functionality, which are related to distinct immune and structural differences in the macrophage surrounding tissues.

The Stria Vascularis in Mice and Humans Is an Early Site of Age-Related Cochlear Degeneration, Macrophage Dysfunction, and Inflammation

Age-related hearing loss, or presbyacusis, is a common degenerative disorder affecting communication and quality of life for millions of older adults. Multiple pathophysiologic manifestations, along with many cellular and molecular alterations, have been linked to presbyacusis; however, the initial events and causal factors have not been clearly established. Comparisons of the transcriptome in the lateral wall (LW) with other cochlear regions in a mouse model (of both sexes) of "normal" age-related hearing loss revealed that early pathophysiological alterations in the stria vascularis (SV) are associated with increased macrophage activation and a molecular signature indicative of inflammaging, a common form of immune dysfunction. Structure-function correlation analyses in mice across the lifespan showed that the age-dependent increase in macrophage activation in the stria vascularis is associated with a decline in auditory sensitivity. High-resolution imaging analysis of macrophage activation in middle-aged and aged mouse and human cochleas, along with transcriptomic analysis of age-dependent changes in mouse cochlear macrophage gene expression, support the hypothesis that aberrant macrophage activity is an important contributor to age-dependent strial dysfunction, cochlear pathology, and hearing loss. Thus, this study highlights the SV as a primary site of age-related cochlear degeneration and aberrant macrophage activity and dysregulation of the immune system as early indicators of age-related cochlear pathology and hearing loss. Importantly, novel new imaging methods described here now provide a means to analyze human temporal bones in a way that had not previously been feasible and thereby represent a significant new tool for otopathological evaluation.SIGNIFICANCE STATEMENT Age-related hearing loss is a common neurodegenerative disorder affecting communication and quality of life. Current interventions (primarily hearing aids and cochlear implants) offer imperfect and often unsuccessful therapeutic outcomes. Identification of early pathology and causal factors is crucial for the development of new treatments and early diagnostic tests. Here, we find that the SV, a nonsensory component of the cochlea, is an early site of structural and functional pathology in mice and humans that is characterized by aberrant immune cell activity. We also establish a new technique for evaluating cochleas from human temporal bones, an important but understudied area of research because of a lack of well-preserved human specimens and difficult tissue preparation and processing approaches.

Three-dimensional mouse cochlea imaging based on the modified Sca/eS using confocal microscopy

The three-dimensional stria vascularis (SV) and cochlear blood vessel structure is essential for inner ear function. Here, modified Sca/eS, a sorbitol-based optical-clearing method, was reported to visualize SV and vascular structure in the intact mouse cochlea. Cochlear macrophages as well as perivascular-resident macrophage-like melanocytes were detected as GFP-positive cells of the CX3CR1^+/GFP mice. This study's method was effective in elucidating inner ear function under both physiological and pathological conditions.

G protein-coupled receptors in cochlea: Potential therapeutic targets for hearing loss

The prevalence of hearing loss-related diseases caused by different factors is increasing worldwide year by year. Currently, however, the patient’s hearing loss has not been effectively improved. Therefore, there is an urgent need to adopt new treatment measures and treatment techniques to help improve the therapeutic effect of hearing loss. G protein-coupled receptors (GPCRs), as crucial cell surface receptors, can widely participate in different physiological and pathological processes, particularly play an essential role in many disease occurrences and be served as promising therapeutic targets. However, no specific drugs on the market have been found to target the GPCRs of the cochlea. Interestingly, many recent studies have demonstrated that GPCRs can participate in various pathogenic process related to hearing loss in the cochlea including heredity, noise, ototoxic drugs, cochlear structure, and so on. In this review, we comprehensively summarize the functions of 53 GPCRs known in the cochlea and their relationships with hearing loss, and highlight the recent advances of new techniques used in cochlear study including cryo-EM, AI, GPCR drug screening, gene therapy vectors, and CRISPR editing technology, as well as discuss in depth the future direction of novel GPCR-based drug development and gene therapy for cochlear hearing loss. Collectively, this review is to facilitate basic and (pre-) clinical research in this area, and provide beneficial help for emerging GPCR-based cochlear therapies.

Macrophages in the cochlea; an immunological link between risk factors and progressive hearing loss

Macrophages are abundant in the cochlea; however, their role in hearing loss is not well understood. Insults to the cochlea, such as noise or insertion of a cochlear implant, cause an inflammatory response, which includes activation of tissue-resident macrophages. Activation is characterized by changes in macrophage morphology, mediator expression, and distribution. Evidence from other organs shows activated macrophages can become primed, whereby subsequent insults cause an elevated inflammatory response. Primed macrophages in brain pathologies respond to circulating inflammatory mediators by disproportionate synthesis of inflammatory mediators. This signaling occurs behind an intact blood-brain barrier, similar to the blood-labyrinth barrier in the cochlea. Local tissue damage can occur as the result of mediator release by activated macrophages. Damage is typically localized; however, if it is to structures with limited ability to repair, such as neurons or hair cells within the cochlea, it is feasible that this contributes to the progressive loss of function seen in hearing loss. We propose that macrophages in the cochlea link risk factors and hearing loss. Injury to the cochlea causes local macrophage activation that typically resolves. However, in susceptible individuals, some macrophages enter a primed state. Once primed, these macrophages can be further activated, as a consequence of circulating inflammatory molecules associated with common co-morbidities. Hypothetically, this would lead to further cochlear damage and loss of hearing. We review the evidence for the role of tissue-resident macrophages in the cochlea and propose that cochlear macrophages contribute to the trajectory of hearing loss and warrant further study.

Loss of CX3CR1 augments neutrophil infiltration into cochlear tissues after acoustic overstimulation

The cochlea, the sensory organ for hearing, has a protected immune environment, segregated from the systemic immune system by the blood-labyrinth barrier. Previous studies have revealed that acute acoustic injury causes the infiltration of circulating leukocytes into the cochlea. However, the molecular mechanisms controlling immune cell trafficking are poorly understood. Here, we report the role of CX3CR1 in regulating the entry of neutrophils into the cochlea after acoustic trauma. We employed B6.129P-Cx3cr1^tm1Litt /J mice, a transgenic strain that lacks the gene, Cx3cr1, for coding the fractalkine receptor. Our results demonstrate that lack of Cx3cr1 results in the augmentation of neutrophil infiltration into cochlear tissues after exposure to an intense noise of 120 dB SPL for 1 hr. Neutrophil distribution in the cochlea is site specific, and the infiltration level is positively associated with noise intensity. Moreover, neutrophils are short lived and macrophage phagocytosis plays a role in neutrophil clearance, consistent with typical neutrophil dynamics in inflamed non-cochlear tissues. Importantly, our study reveals the potentiation of noise-induced hearing loss and sensory cell loss in Cx3cr1^-/- mice. In wild-type control mice (Cx3cr1^+/+ ) exposed to the same noise, we also found neutrophils. However, neutrophils were present primarily inside the microvessels of the cochlea, with only a few in the cochlear tissues. Collectively, our data implicate CX3CR1-mediated signaling in controlling neutrophil migration from the circulation into cochlear tissues and provide a better understanding of the impacts of neutrophils on cochlear responses to acoustic injury.

The Detrimental and Beneficial Functions of Macrophages After Cochlear Injury

Macrophages are the main intrinsic immune cells in the cochlea; they can be activated and play a complicated role after cochlear injury. Many studies have shown that the number of macrophages and their morphological characteristics within the major cochlear partitions undergo significant changes under various pathological conditions including acoustic trauma, ototoxic drug treatment, age-related cochlear degeneration, selective hair cell (HC) and spiral ganglion neuron (SGN) elimination, and surgery. However, the exact role of these macrophages after cochlear injury is still unclear. Regulating the migration and activity of macrophages may be a therapeutic approach to reduce the risk or magnitude of trauma-induced hearing loss, and this review highlights the role of macrophages on the peripheral auditory structures of the cochlea and elucidate the mechanisms of macrophage injury and the strategies to reduce the injury by regulating macrophage.

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.

Macrophages in Noise-Exposed Cochlea: Changes, Regulation and the Potential Role

Acoustic trauma is an important physical factor leading to cochlear damage and hearing impairments. Inflammation responds to this kind of cochlear damage stress. Macrophages, the major innate immune cells in the cochlea, are important drivers of inflammatory and tissue repair responses after cochlear injury. Recently, studies have shown that after noise exposure, the distribution, phenotype, and the number of cochlear macrophages have significantly changed, and the local environmental factors that shape macrophage differentiation and behavior are also drastically altered. However, the exact role of these immune cells in the cochlea after acoustic injury remains unknown. Here we review the properties of cochlear macrophages both under steady-state conditions and non-homeostatic conditions after cochlear acoustic injury and discuss their potential role in noise-exposed cochlea.

Inflammation associated with noise-induced hearing loss

Inflammation is a complex biological response to harmful stimuli including infection, tissue damage, and toxins. Thus, it is not surprising that cochlear damage by noise includes an inflammatory component. One mechanism by which inflammation is generated by tissue damage is the activation of damage-associated molecular patterns (DAMPs). Many of the cellular receptors for DAMPS, including Toll-like receptors, NOD-like receptors, and DNA receptors, are also receptors for pathogens, and function in the innate immune system. DAMP receptors are known to be expressed by cochlear cells, and binding of molecules released by damaged cells to these receptors result in the activation of cell stress pathways. This leads to the generation of pro-inflammatory cytokines and chemokines that recruit pro-inflammatory leukocytes. Extensive evidence indicates pro-inflammatory cytokines including TNF alpha and interleukin 1 beta, and chemokines including CCL2, are induced in the cochlea after noise exposure. The recruitment of macrophages into the cochlea has also been demonstrated. These provide substrates for noise damage to be enhanced by inflammation. Evidence is provided by the effectiveness of anti-inflammatory drugs in ameliorating noise-induced hearing loss. Involvement of inflammation provides a wide variety of additional anti-inflammatory and pro-resolution agents as potential pharmacological interventions in noise-induced hearing loss.

Circadian Regulation of Cochlear Sensitivity to Noise by Circulating Glucocorticoids

The cochlea possesses a robust circadian clock machinery that regulates auditory function. How the cochlear clock is influenced by the circadian system remains unknown. Here, we show that cochlear rhythms are system driven and require local Bmal1 as well as central input from the suprachiasmatic nuclei (SCN). SCN ablations disrupted the circadian expression of the core clock genes in the cochlea. Because the circadian secretion of glucocorticoids (GCs) is controlled by the SCN and GCs are known to modulate auditory function, we assessed their influence on circadian gene expression. Removal of circulating GCs by adrenalectomy (ADX) did not have a major impact on core clock gene expression in the cochlea. Rather it abolished the transcription of clock-controlled genes involved in inflammation. ADX abolished the known differential auditory sensitivity to day and night noise trauma and prevented the induction of GABA-ergic and glutamate receptors mRNA transcripts. However, these improvements were unrelated to changes at the synaptic level, suggesting other cochlear functions may be involved. Due to this circadian regulation of noise sensitivity by GCs, we evaluated the actions of the synthetic glucocorticoid dexamethasone (DEX) at different times of the day. DEX was effective in protecting from acute noise trauma only when administered during daytime, when circulating glucocorticoids are low, indicating that chronopharmacological approaches are important for obtaining optimal treatment strategies for hearing loss. GCs appear as a major regulator of the differential sensitivity to day or night noise trauma, a mechanism likely involving the circadian control of inflammatory responses.

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.

Interactions between Macrophages and the Sensory Cells of the Inner Ear

Macrophages are present in most somatic tissues, where they detect and attack invading pathogens. Macrophages also participate in many nonimmune functions, particularly those related to tissue maintenance and injury response. The sensory organs of the inner ear contain resident populations of macrophages, and additional macrophages enter the ear after acoustic trauma or ototoxicity. As expected, such macrophages participate in the clearance of cellular debris. However, otic macrophages can also influence the long-term survival of both hair cells and afferent neurons after injury. The signals that recruit macrophages into the injured ear, as well as the precise contributions of macrophages to inner ear pathology, remain to be determined.

Immune cells and non-immune cells with immune function in mammalian cochleae

The cochlea has an immune environment dominated by macrophages under resting conditions. When stressed, circulating monocytes enter the cochlea. These immune mediators, along with cochlear resident cells, organize a complex defense response against pathological challenges. Since the cochlea has minimal exposure to pathogens, most inflammatory conditions in the cochlea are sterile. Although the immune response is initiated for the protection of the cochlea, off-target effects can cause collateral damage to cochlear cells. A better understanding of cochlear immune capacity and regulation would therefore lead to development of new therapeutic treatments. Over the past decade, there have been many advances in our understanding of cochlear immune capacity. In this review, we provide an update and overview of the cellular components of cochlear immune capacity with a focus on macrophages in mammalian cochleae. We describe the composition and distribution of immune cells in the cochlea and suggest that phenotypic and functional characteristics of macrophages have site-specific diversity. We also highlight the response of immune cells to acute and chronic stresses and comment on the potential function of immune cells in cochlear homeostasis and disease development. Finally, we briefly review potential roles for cochlear resident cells in immune activities of the cochlea.

Noise-Induced Dysregulation of Quaking RNA Binding Proteins Contributes to Auditory Nerve Demyelination and Hearing Loss

Noise exposure causes auditory nerve (AN) degeneration and hearing deficiency, though the proximal biological consequences are not entirely understood. Most AN fibers and spiral ganglion neurons are ensheathed by myelinating glia that provide insulation and ensure rapid transmission of nerve impulses from the cochlea to the brain. Here we show that noise exposure administered to mice of either sex rapidly affects myelinating glial cells, causing molecular and cellular consequences that precede nerve degeneration. This response is characterized by demyelination, inflammation, and widespread expression changes in myelin-related genes, including the RNA splicing regulator Quaking (QKI) and numerous QKI target genes. Analysis of mice deficient in QKI revealed that QKI production in cochlear glial cells is essential for proper myelination of spiral ganglion neurons and AN fibers, and for normal hearing. Our findings implicate QKI dysregulation as a critical early component in the noise response, influencing cochlear glia function that leads to AN demyelination and, ultimately, to hearing deficiency.SIGNIFICANCE STATEMENT Auditory glia cells ensheath a majority of spiral ganglion neurons with myelin, protect auditory neurons, and allow for fast conduction of electrical impulses along the auditory nerve. Here we show that noise exposure causes glial dysfunction leading to myelin abnormality and altered expression of numerous genes in the auditory nerve, including QKI, a gene implicated in regulating myelination. Study of a conditional mouse model that specifically depleted QKI in glia showed that QKI deficiency alone was sufficient to elicit myelin-related abnormality and auditory functional declines. These results establish QKI as a key molecular target in the noise response and a causative agent in hearing loss.

Genetic disruption of fractalkine signaling leads to enhanced loss of cochlear afferents following ototoxic or acoustic injury

Cochlear hair cells are vulnerable to a variety of insults like acoustic trauma and ototoxic drugs. Such injury can also lead to degeneration of spiral ganglion neurons (SGNs), but this occurs over a period of months to years. Neuronal survival is necessary for the proper function of cochlear prosthetics, therefore, it is of great interest to understand the mechanisms that regulate neuronal survival in deaf ears. We have recently demonstrated that selective hair cell ablation is sufficient to attract leukocytes into the spiral ganglion, and that fractalkine signaling plays a role in macrophage recruitment and in the survival of auditory neurons. Fractalkine (CX₃ CL1), a chemokine that regulates adhesion and migration of leukocytes is expressed by SGNs and signals to leukocytes via its receptor CX₃ CR1. The present study has extended the previous findings to more clinically relevant conditions of sensorineural hearing loss by examining the role of fractalkine signaling after aminoglycoside ototoxicity or acoustic trauma. Both aminoglycoside treatment and acoustic overstimulation led to the loss of hair cells as well as prolonged increase in the numbers of cochlear leukocytes. Lack of CX₃ CR1 did not affect macrophage recruitment after injury, but resulted in increased loss of SGNs and enhanced expression of the inflammatory cytokine interleukin-1β, when compared to mice with intact CX₃ CR1. These data indicate that the dysregulation of macrophage response caused by the absence of CX₃ CR1 may contribute to inflammation-mediated neuronal loss in the deafened ear, suggesting a key role for inflammation in the long-term survival of target-deprived afferent neurons.

Resolution of Cochlear Inflammation: Novel Target for Preventing or Ameliorating Drug-, Noise- and Age-related Hearing Loss

A significant number of studies support the idea that inflammatory responses are intimately associated with drug-, noise- and age-related hearing loss (DRHL, NRHL and ARHL). Consequently, several clinical strategies aimed at reducing auditory dysfunction by preventing inflammation are currently under intense scrutiny. Inflammation, however, is a normal adaptive response aimed at restoring tissue functionality and homeostasis after infection, tissue injury and even stress under sterile conditions, and suppressing it could have unintended negative consequences. Therefore, an appropriate approach to prevent or ameliorate DRHL, NRHL and ARHL should involve improving the resolution of the inflammatory process in the cochlea rather than inhibiting this phenomenon. The resolution of inflammation is not a passive response but rather an active, highly controlled and coordinated process. Inflammation by itself produces specialized pro-resolving mediators with critical functions, including essential fatty acid derivatives (lipoxins, resolvins, protectins and maresins), proteins and peptides such as annexin A1 and galectins, purines (adenosine), gaseous mediators (NO, H₂S and CO), as well as neuromodulators like acetylcholine and netrin-1. In this review article, we describe recent advances in the understanding of the resolution phase of inflammation and highlight therapeutic strategies that might be useful in preventing inflammation-induced cochlear damage. In particular, we emphasize beneficial approaches that have been tested in pre-clinical models of inflammatory responses induced by recognized ototoxic drugs such as cisplatin and aminoglycoside antibiotics. Since these studies suggest that improving the resolution process could be useful for the prevention of inflammation-associated diseases in humans, we discuss the potential application of similar strategies to prevent or mitigate DRHL, NRHL and ARHL.

Dynamic activation of basilar membrane macrophages in response to chronic sensory cell degeneration in aging mouse cochleae

In the sensory epithelium, macrophages have been identified on the scala tympani side of the basilar membrane. These basilar membrane macrophages are the spatially closest immune cells to sensory cells and are able to directly respond to and influence sensory cell pathogenesis. While basilar membrane macrophages have been studied in acute cochlear stresses, their behavior in response to chronic sensory cell degeneration is largely unknown. Here we report a systematic observation of the variance in phenotypes, the changes in morphology and distribution of basilar membrane tissue macrophages in different age groups of C57BL/6J mice, a mouse model of age-related sensory cell degeneration. This study reveals that mature, fully differentiated tissue macrophages, not recently infiltrated monocytes, are the major macrophage population for immune responses to chronic sensory cell death. These macrophages display dynamic changes in their numbers and morphologies as age increases, and the changes are related to the phases of sensory cell degeneration. Notably, macrophage activation precedes sensory cell pathogenesis, and strong macrophage activity is maintained until sensory cell degradation is complete. Collectively, these findings suggest that mature tissue macrophages on the basilar membrane are a dynamic group of cells that are capable of vigorous adaptation to changes in the local sensory epithelium environment influenced by sensory cell status.

Contributions of Mouse and Human Hematopoietic Cells to Remodeling of the Adult Auditory Nerve After Neuron Loss

The peripheral auditory nerve (AN) carries sound information from sensory hair cells to the brain. The present study investigated the contribution of mouse and human hematopoietic stem cells (HSCs) to cellular diversity in the AN following the destruction of neuron cell bodies, also known as spiral ganglion neurons (SGNs). Exposure of the adult mouse cochlea to ouabain selectively killed type I SGNs and disrupted the blood-labyrinth barrier. This procedure also resulted in the upregulation of genes associated with hematopoietic cell homing and differentiation, and provided an environment conducive to the tissue engraftment of circulating stem/progenitor cells into the AN. Experiments were performed using both a mouse-mouse bone marrow transplantation model and a severely immune-incompetent mouse model transplanted with human CD34⁺ cord blood cells. Quantitative immunohistochemical analysis of recipient mice demonstrated that ouabain injury promoted an increase in the number of both HSC-derived macrophages and HSC-derived nonmacrophages in the AN. Although rare, a few HSC-derived cells in the injured AN exhibited glial-like qualities. These results suggest that human hematopoietic cells participate in remodeling of the AN after neuron cell body loss and that hematopoietic cells can be an important resource for promoting AN repair/regeneration in the adult inner ear.

Immune defense is the primary function associated with the differentially expressed genes in the cochlea following acoustic trauma

Our previous RNA-sequencing analysis of the rat cochlear genes identified multiple biological processes and molecular pathways in the cochlear response to acoustic overstimulation. However, the biological processes and molecular pathways that are common to other species have not been documented. The identification of these common stress processes is pivotal for a better understanding of the essential response of the cochlea to acoustic injury. Here, we compared the RNA-sequencing data collected from mice and rats that sustained a similar, but not identical, acoustic injury. The transcriptome analysis of cochlear genes identified the differentially expressed genes in the mouse and rat samples. Bioinformatics analysis revealed a marked similarity in the changes in the biological processes between the two species, although the differentially expressed genes did not overlap well. The common processes associated with the differentially expressed genes are primarily associated with immunity and inflammation, which include the immune response, response to wounding, the defense response, chemotaxis and inflammatory responses. Moreover, analysis of the molecular pathways showed considerable overlap between the two species. The common pathways include cytokine-cytokine receptor interactions, the chemokine signaling pathway, the Toll-like receptor signaling pathway, and the NOD-like receptor signaling pathway. Further analysis of the transcriptional regulators revealed common upstream regulators of the differentially expressed genes, and these upstream regulators are also functionally related to the immune and inflammatory responses. These results suggest that the immune and inflammatory responses are the essential responses to acoustic overstimulation in the cochlea.

Activation of the antigen presentation function of mononuclear phagocyte populations associated with the basilar membrane of the cochlea after acoustic overstimulation

The immune response is an important component of the cochlear response to stress. As an important player in the cochlear immune system, the basilar membrane immune cells reside on the surface of the scala tympani side of the basilar membrane. At present, the immune cell properties in this region and their responses to stress are not well understood. Here, we investigated the functional role of these immune cells in the immune response to acoustic overstimulation. This study reveals that tissue macrophages are present in the entire length of the basilar membrane under steady-state conditions. Notably, these cells in the apical and the basal sections of the basilar membrane display distinct morphologies and immune protein expression patterns. Following acoustic trauma, monocytes infiltrate into the region of the basilar membrane, and the infiltrated cells transform into macrophages. While monocyte infiltration and transformation occur in both the apical and the basal sections of the basilar membrane, only the basal monocytes and macrophages display a marked increase in the expression of major histocompatibility complex (MHC) II and class II transactivator (CIITA), a MHC II production cofactor, suggesting the site-dependent activation of antigen-presenting function. Consistent with the increased expression of the antigen-presenting proteins, CD4(+) T cells, the antigen-presenting partner, infiltrate into the region of the basilar membrane where antigen-presenting proteins are upregulated. Further pathological analyses revealed that the basal section of the cochlea displays a greater level of sensory cell damage, which is spatially correlated with the region of antigen-presenting activity. Together, these results suggest that the antigen-presenting function of the mononuclear phagocyte population is activated in response to acoustic trauma, which could bridge the innate immune response to adaptive immunity.

Gene expression pattern after insertion of dexamethasone-eluting electrode into the guinea pig cochlea

A cochlear implant is an indispensable apparatus for a profound hearing loss patient. But insertion of the electrode entails a great deal of stress to the cochlea, and may cause irreversible damage to hair cells and related nerve structure. Although damage prevention effects of dexamethasone have been reported, long-term administration is difficult. In this study, we used a dexamethasone-eluting electrode in the guinea pig cochlea, and compared the gene expression after 7 days insertion with that of a normal electrode and non-surgically treated control by microarray. 40 genes were up-regulated 2-fold or more in the normal electrode group compared to the non-surgically treated group. Most of the up-regulated genes were associated with immune response and inflammation. In the dexamethasone-eluting group, compared to the normal electrode group, 7 of the 40 genes were further up-regulated, while 12 of them were down-regulated and there was a tendency to return to the non-surgical condition. 9 genes were down-regulated 2-fold or less with normal electrode insertion, and 4 of the 9 tended to return to the non-surgical condition in the dexamethasone-eluting group. These genes are certainly involved in the maintenance of the physiological functions of the cochlea. Our results indicate that the dexamethasone-eluting electrode will have an effect on the normalization of homeostasis in the cochlea.

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.

Heat shock protein-mediated protection against Cisplatin-induced hair cell death

Cisplatin is a highly successful and widely used chemotherapy for the treatment of various solid malignancies in both adult and pediatric patients. Side effects of cisplatin treatment include nephrotoxicity and ototoxicity. Cisplatin ototoxicity results from damage to and death of cells in the inner ear, including sensory hair cells. We showed previously that heat shock inhibits cisplatin-induced hair cell death in whole-organ cultures of utricles from adult mice. Since heat shock protein 70 (HSP70) is the most upregulated HSP in response to heat shock, we investigated the role of HSP70 as a potential protectant against cisplatin-induced hair cell death. Our data using utricles from HSP70 (-/-) mice indicate that HSP70 is necessary for the protective effect of heat shock against cisplatin-induced hair cell death. In addition, constitutive expression of inducible HSP70 offered modest protection against cisplatin-induced hair cell death. We also examined a second heat-inducible protein, heme oxygenase-1 (HO-1, also called HSP32). HO-1 is an enzyme responsible for the catabolism of free heme. We previously showed that induction of HO-1 using cobalt protoporphyrin IX (CoPPIX) inhibits aminoglycoside-induced hair cell death. Here, we show that HO-1 also offers significant protection against cisplatin-induced hair cell death. HO-1 induction occurred primarily in resident macrophages, with no detectable expression in hair cells or supporting cells. Depletion of macrophages from utricles abolished the protective effect of HO-1 induction. Together, our data indicate that HSP induction protects against cisplatin-induced hair cell death, and they suggest that resident macrophages mediate the protective effect of HO-1 induction.

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.

Resistance to noise-induced hearing loss in 129S6 and MOLF mice: identification of independent, overlapping, and interacting chromosomal regions

Noise-induced hearing loss (NIHL) is a prevalent health risk. Inbred mouse strains 129S6/SvEvTac (129S6) and MOLF/EiJ (MOLF) show strong NIHL resistance (NR) relative to CBA/CaJ (CBACa). In this study, we developed quantitative trait locus (QTL) maps for NR. We generated F1 animals by intercrossing (129S6 × CBACa) and (MOLF × CBACa). In each intercross, NR was recessive. N2 animals were produced by backcrossing F1s to their respective parental strain. The 232 N2-129S6 and 225 N2-MOLF progenies were evaluated for NR using auditory brainstem response. In 129S6, five QTL were identified on chromosomes (Chr) 17, 18, 14, 11, and 4, referred to as loci nr1, nr2, nr3, nr4, and nr5, respectively. In MOLF, four QTL were found on Chr 4, 17, 6, and 12, referred to as nr7, nr8, nr9, and nr10, respectively. Given that NR QTL were discovered on Chr 4 and 17 in both the N2-129S6 and N2-MOLF cross, we generated two consomic strains by separately transferring 129S6-derived Chr 4 and 17 into an otherwise CBACa background and a double-consomic strain by crossing the two strains. Phenotypic analysis of the consomic strains indicated that whole 129S6 Chr 4 contributes strongly to mid-frequency NR, while whole 129S6 Chr 17 contributes markedly to high-frequency NR. Therefore, we anticipated that the double-consomic strain containing Chr 4 and 17 would demonstrate NR across the mid- and high-frequency range. However, whole 129S6 Chr 17 masks the expression of mid-frequency NR from whole 129S6 Chr 4. To further dissect NR on 129S6 Chr 4 and 17, CBACa.129S6 congenic strains were generated for each chromosome. Phenotypic analysis of the Chr 17 CBACa.129S6 congenic strains further defined the NR region on proximal Chr 17, uncovered another NR locus (nr6) on distal Chr 17, and revealed an epistatic interaction between proximal and distal 129S6 Chr 17.

Transcriptome characterization by RNA-Seq reveals the involvement of the complement components in noise-traumatized rat cochleae

Acoustic trauma, a leading cause of sensorineural hearing loss in adults, induces a complex degenerative process in the cochlea. Although previous investigations have identified multiple stress pathways, a comprehensive analysis of cochlear responses to acoustic injury is still lacking. In the current study, we used the next-generation RNA-sequencing (RNA-Seq) technique to sequence the whole transcriptome of the normal and noise-traumatized cochlear sensory epithelia (CSE). CSE tissues were collected from rat inner ears 1d after the rats were exposed to a 120-dB (sound pressure level) noise for 2 h. The RNA-Seq generated over 176 million sequence reads for the normal CSE and over 164 million reads for the noise-traumatized CSE. Alignment of these sequences with the rat Rn4 genome revealed the expression of over 17,000 gene transcripts in the CSE, over 2000 of which were exclusively expressed in either the normal or noise-traumatized CSE. Seventy-eight gene transcripts were differentially expressed (70 upregulated and 8 downregulated) after acoustic trauma. Many of the differentially expressed genes are related to the innate immune system. Further expression analyses using quantitative real time PCR confirmed the constitutive expression of multiple complement genes in the normal organ of Corti and the changes in the expression levels of the complement factor I (Cfi) and complement component 1, s subcomponent (C1s) after acoustic trauma. Moreover, protein expression analysis revealed strong expression of Cfi and C1s proteins in the organ of Corti. Importantly, these proteins exhibited expression changes following acoustic trauma. Collectively, the results of the current investigation suggest the involvement of the complement components in cochlear responses to acoustic trauma.

Adipose tissue-derived stromal cells protect hair cells from aminoglycoside

BACKGROUND

Previous studies have demonstrated the therapeutic paracrine activity of adipose tissue-derived stromal cells (ADSCs). This study aimed to examine the ADSC potential for protecting auditory hair cells from aminoglycoside toxicity via paracrine of multiple growth factors and cytokines.

STUDY DESIGN

Experimental study.

METHODS

We assessed hair cell protection from neomycin toxicity by ADSC-derived factors using an explant culture system, in which cochlear explants and ADSCs were separated by a culture mesh insert to avoid direct contact. We measured the levels of growth factors and cytokines in ADSC culture media using an enzyme-linked immunosorbent assay (ELISA).

RESULTS

Neomycin induced severe degeneration of auditory hair cells in cochlear explants, but co-culture with ADSCs significantly increased the number of surviving hair cells in explants. ELISA analysis revealed that ADSCs secreted insulin-like growth factor-1, nerve growth factor, vascular epithelial growth factor, transforming growth factor β1, monocyte chemotactic protein-1, and most prominently hepatocyte growth factor.

CONCLUSIONS

These findings demonstrate that ADSCs have the capacity to protect auditory hair cells, and can be a useful strategy to develop therapy for deafness in the clinic. The multiple paracrine growth factors and cytokines secreted by ADSCs might be involved in this effect. Laryngoscope, 2011.

Different cellular and genetic basis of noise-related endocochlear potential reduction in CBA/J and BALB/cJ mice

The acute and permanent effects of noise exposure on the endocochlear potential (EP) and cochlear lateral wall were evaluated in BALB/cJ (BALB) inbred mice, and compared with CBA/J (CBA) and C57BL/6 (B6) mice. Two-hour exposure to broadband noise (4-45 kHz) at 110 dB SPL leads to a approximately 50 mV reduction in the EP in BALB and CBA, but not B6. EP reduction in BALB and CBA is reliably associated with characteristic acute cellular pathology in stria vascularis and spiral ligament. By 8 weeks after exposure, the EP in CBA mice has returned to normal. In BALBs, however, the EP remains depressed by an average approximately 10 mV, so that permanent EP reduction contributes to permanent threshold shifts in these mice. We recently showed that the CBA noise phenotype in part reflects the influence of a large effect quantitative trait locus on Chr. 18, termed Nirep (Ohlemiller et al., Hear Res 260:47-53, 2010b). While CBA "EP susceptibility" alleles are dominant to those in B6, examination of (B6 × BALB) F1 hybrid mice and (F1 × BALB) N2 backcross mice revealed that noise-related EP reduction and associated cell pathology in BALBs are inherited in an autosomal recessive manner, and are dependent on multiple genes. Moreover, while N2 mice formed from B6 and CBA retain strong correspondence between acute EP reduction, ligament pathology, and strial pathology, N2s formed from B6 and BALB include subsets that dissociate pathology of ligament and stria. We conclude that the genes and cascades that govern the very similar EP susceptibility phenotypes in BALB and CBA mice need not be the same. BALBs appear to carry alleles that promote more pronounced long term effects of noise on the lateral wall. Separate loci in BALBs may preferentially impact stria versus ligament.

Spiral ligament fibrocyte-derived MCP-1/CCL2 contributes to inner ear inflammation secondary to nontypeable H. influenzae-induced otitis media

Background

Otitis media (OM), one of the most common pediatric infectious diseases, causes inner ear inflammation resulting in vertigo and sensorineural hearing loss. Previously, we showed that spiral ligament fibrocytes (SLFs) recognize OM pathogens and up-regulate chemokines. Here, we aim to determine a key molecule derived from SLFs, contributing to OM-induced inner ear inflammation.

Methods

Live NTHI was injected into the murine middle ear through the tympanic membrane, and histological analysis was performed after harvesting the temporal bones. Migration assays were conducted using the conditioned medium of NTHI-exposed SLFs with and without inhibition of MCP-1/CCL2 and CCR2. qRT-PCR analysis was performed to demonstrate a compensatory up-regulation of alternative genes induced by the targeting of MCP-1/CCL2 or CCR2.

Results

Transtympanic inoculation of live NTHI developed serous and purulent labyrinthitis after clearance of OM. THP-1 cells actively migrated and invaded the extracellular matrix in response to the conditioned medium of NTHI-exposed SLFs. This migratory activity was markedly inhibited by the viral CC chemokine inhibitor and the deficiency of MCP-1/CCL2, indicating that MCP-1/CCL2 is a main attractant of THP-1 cells among the SLF-derived molecules. We further demonstrated that CCR2 deficiency inhibits migration of monocyte-like cells in response to NTHI-induced SLF-derived molecules. Immunolabeling showed an increase in MCP-1/CCL2 expression in the cochlear lateral wall of the NTHI-inoculated group. Contrary to the in vitro data, deficiency of MCP-1/CCL2 or CCR2 did not inhibit OM-induced inner ear inflammation in vivo. We demonstrated that targeting MCP-1/CCL2 enhances NTHI-induced up-regulation of MCP-2/CCL8 in SLFs and up-regulates the basal expression of CCR2 in the splenocytes. We also found that targeting CCR2 enhances NTHI-induced up-regulation of MCP-1/CCL2 in SLFs.

Conclusions

Taken together, we suggest that NTHI-induced SLF-derived MCP-1/CCL2 is a key molecule contributing to inner ear inflammation through CCR2-mediated recruitment of monocytes. However, deficiency of MCP-1/CCL2 or CCR2 alone was limited to inhibit OM-induced inner ear inflammation due to compensation of alternative genes.

Tickets to the brain: role of CCR2 and CX3CR1 in myeloid cell entry in the CNS

Myeloid cells are mediators of central nervous system (CNS) damage and recovery in neuroinflammatory and neurodegenerative disorders. Besides endogenous myelomonocytic cell populations that reside in the brain already during development, newly migrated leukocytes are considered as important disease modulators in the adult brain. Thus, understanding of myeloid cell recruitment is pivotal for manipulating immune cell entry into the CNS and potentially reducing disease burden. Before myeloid cells engraft in the brain, they first tether to and roll on the activated brain endothelium, then they firmly adhere and eventually transmigrate into the damaged brain where they execute effector functions and differentiate into cells with microglia-like features. These steps are mainly regulated by adhesion molecules and by chemokines and their cognate receptors. Due to recent advances in our understanding of monocyte heterogeneity, the interest in chemokine receptors has significantly increased. Among others, the presence of the chemokine receptors CCR2 and CX(3)CR(1) is considered to be critical for both myeloid cell trafficking along inflamed vessels and subsequent accumulation in the brain. Therefore, these molecules present viable targets for therapeutic manipulations of myeloid cells destined for the CNS.

Expression of fractalkine receptor CX3CR1 on cochlear macrophages influences survival of hair cells following ototoxic injury

The role of innate immunity and macrophage recruitment to the inner ear after hair cell injury is a subject where little is known. In this paper, we demonstrate recruitment of monocytes and macrophages to the inner ear after kanamycin. We also examined the effect of fractalkine receptor (CX3CR1) deletion in kanamycin ototoxicity. We observed more functional and structural damage in CX3CR1 null mice compared to wild-type and heterozygous littermates. In order to determine if increased susceptibility to kanamycin resulted from CX3CR1 deletion from cochlear leukocytes, we created bone marrow chimeras by transplanting CX3CR1-null bone marrow into wild-type mice whose native bone marrow was ablated by lethal irradiation. These mice were then treated with kanamycin sulfate. Auditory brainstem responses (ABR), hair cell counts, and numbers of macrophages recruited to the cochlea were recorded in irradiated mice that received either wild-type, CX3CR1 heterozygous, or CX3CR1 knockout bone marrow. A strong correlation was present between numbers of macrophages and hair cell death in recipients transplanted with CX3CR1 null marrow. No correlation between macrophage number and hair cell loss was present in mice transplanted with wild-type or CX3CR1 heterozygous marrow. We suggest that CX3CR1 plays a role in modulating the detrimental effects of cochlear macrophages after kanamycin ototoxicity. Our data point to the possibility that CX3CR1-deficient cochlear macrophages exacerbate kanamycin ototoxicity while CX3CR1-expressing monocytes do not.

CCL2 accelerates microglia-mediated Abeta oligomer formation and progression of neurocognitive dysfunction

Background

The linkages between neuroinflammation and Alzheimer's disease (AD) pathogenesis are well established. What is not, however, is how specific immune pathways and proteins affect the disease. To this end, we previously demonstrated that transgenic over-expression of CCL2 enhanced microgliosis and induced diffuse amyloid plaque deposition in Tg2576 mice. This rodent model of AD expresses a Swedish β-amyloid (Aβ) precursor protein mutant.

Methodology/Principal Findings

We now report that CCL2 transgene expression accelerates deficits in spatial and working memory and hippocampal synaptic transmission in β-amyloid precursor protein (APP) mice as early as 2–3 months of age. This is followed by increased numbers of microglia that are seen surrounding Aβ oligomers. CCL2 does not suppress Aβ degradation. Rather, CCL2 and tumor necrosis factor-α directly facilitated Aβ uptake, intracellular Aβ oligomerization, and protein secretion.

Conclusions/Significance

We posit that CCL2 facilitates Aβ oligomer formation in microglia and propose that such events accelerate memory dysfunction by affecting Aβ seeding in the brain.

RANTES upregulation in the Alzheimer's disease brain: a possible neuroprotective role

Numerous studies demonstrate inflammatory proteins in the brain and microcirculation in Alzheimer's disease (AD) and implicate inflammation in disease pathogenesis. However, emerging literature suggests that neuroinflammation can also be neuroprotective. The chemokine RANTES has been implicated in neurodegenerative diseases including AD. The objectives of this study are to determine the expression of RANTES in AD microvessels, its regulation in endothelial cells and its effects on neuronal survival. Our data show elevated expression of RANTES in the cerebral microcirculation of AD patients. Treatment of neurons in vitro with RANTES results in an increase in cell survival and a neuroprotective effect against the toxicity of thrombin and sodium nitroprusside. Oxidative stress upregulates RANTES expression in rat brain endothelial cells. Developing strategies to augment neuroprotection and diminish inflammatory activation of multifunctional mediators such as RANTES holds promise for the development of novel neuroprotective therapeutics in AD.

Repopulation of cochlear macrophages in murine hematopoietic progenitor cell chimeras: the role of CX3CR1

Cochlear macrophages have been shown to accumulate in the murine cochlea following acoustic trauma. This investigation was performed to determine whether cochlear macrophages could be replaced by donor transplantation of bone marrow precursors. Lethally irradiated C57BL/6 mice were transplanted with hematopoietic precursors from CX3CR1(GFP/GFP) fetal mice. CX3CR1(GFP/GFP) mice express green fluorescent protein (GFP) in monocytes and macrophages and possess no functional CX3CR1. Donor monocytes and macrophages can be easily traced in the wild-type recipient with fluorescent microscopy. We studied mice at 2-16 weeks after transplantation to assess repopulation of cochlear macrophages. A separate group of chimeras was exposed to octave band noise (8-16 kHz for 2 hours) 2 weeks after transplantation to evaluate the migration properties of donor hematopoietic precursors. We found that macrophages derived from donor hematopoietic precursors appeared in cochlea 3-4 weeks after transplantation and increased week by week. Noise exposure induced a massive accumulation of leukocytes, particularly in the spiral ligament of the basal turn. There was no difference between CX3CR1(GFP/GFP) donor/wild-type recipient chimeras and the wild-type donor/wild-type recipient chimeras in hearing thresholds, accumulation of cochlear macrophages, or tissue injury after noise exposure. These data indicate that cochlear macrophages are derived from bone marrow precursors and that they are an exchanging and migratory population. Furthermore, CX3CR1 in hematopoietic precursors is not necessary for macrophage migration into cochlea and when deleted in this cell population, the absence of CX3CR1 does not substantially effect the outcomes after noise.