The Stria Vascularis: Renewed Attention on a Key Player in Age-Related Hearing Loss.

Age-related hearing loss (HL), or presbycusis, is a complex and heterogeneous condition, affecting a significant portion of older adults and involving various interacting mechanisms. Metabolic presbycusis, a type of age-related HL, is characterized by the dysfunction of the stria vascularis, which is crucial for maintaining the endocochlear potential necessary for hearing. Although attention on metabolic presbycusis has waned in recent years, research continues to identify strial pathology as a key factor in age-related HL. This narrative review integrates past and recent research, bridging findings from animal models and human studies, to examine the contributions of the stria vascularis to age-related HL. It provides a brief overview of the structure and function of the stria vascularis and then examines mechanisms contributing to age-related strial dysfunction, including altered ion transport, changes in pigmentation, inflammatory responses, and vascular atrophy. Importantly, this review outlines the contribution of metabolic mechanisms to age-related HL, highlighting areas for future research. It emphasizes the complex interdependence of metabolic and sensorineural mechanisms in the pathology of age-related HL and highlights the importance of animal models in understanding the underlying mechanisms. The comprehensive and mechanistic investigation of all factors contributing to age-related HL, including cochlear metabolic dysfunction, remains crucial to identifying the underlying mechanisms and developing personalized, protective, and restorative treatments.

PARP1 inhibition prevents oxidative stress in age-related hearing loss via PAR-Ca2+-AIF axis in cochlear strial marginal cells.

Studies have highlighted oxidative damage in the inner ear as a critical pathological basis for sensorineural hearing loss, especially the presbycusis. Poly(ADP-ribose) polymerase-1 (PARP1) activation responds to oxidative stress-induced DNA damage with pro-repair and pro-death effects resembling two sides of the same coin. PARP1-related cell death, known as parthanatos, whose underlying mechanisms are attractive research hotspots but remain to be clarified. In this study, we observed that aged rats showed stria vascularis degeneration and oxidative damage, and PARP1-dependent cell death was prominent in age-related cochlear disorganization and dysfunction. Based on oxidative stress model of primary cultured stria marginal cells (MCs), we revealed that upregulated PARP1 and PAR (Poly(ADP-ribose)) polymers are responsible for MCs oxidative death with high mitochondrial permeability transition pore (mPTP) opening and mitochondrial membrane potential (MMP) collapse, while inhibition of PARP1 ameliorated the adverse outcomes. Importantly, the PARylation of apoptosis-inducing factor (AIF) is essential for its conformational change and translocation, which subsequently causes DNA break and cell death. Concretely, the interaction of PAR and truncated AIF (tAIF) is the mainstream in the parthanatos pathway. We also found that the effects of AIF cleavage and release were achieved through calpain activity and mPTP opening, both of which could be regulated by PARP1 via mediation of mitochondria Ca2+ concentration. In conclusion, the PAR-Ca2+-tAIF signaling pathway in parthanatos contributes to the oxidative stress damage observed in MCs. Targeting PAR-Ca2+-tAIF might be a potential therapeutic strategy for the early intervention of presbycusis and other oxidative stress-associated sensorineural deafness.

The cells of the sensory epithelium, and not the stria vascularis, are the main cochlear cells related to the genetic pathogenesis of age-related hearing loss.

Age-related hearing loss (ARHL) is a major health concern among the elderly population. It is hoped that increasing our understanding of its underlying pathophysiological processes will lead to the development of novel therapies. Recent genome-wide association studies (GWASs) discovered a few dozen genetic variants in association with elevated risk for ARHL. Integrated analysis of GWAS results and transcriptomics data is a powerful approach for elucidating specific cell types that are involved in disease pathogenesis. Intriguingly, recent studies that applied such bioinformatics approaches to ARHL resulted in disagreeing findings as for the key cell types that are most strongly linked to the genetic pathogenesis of ARHL. These conflicting studies pointed either to cochlear sensory epithelial or to stria vascularis cells as the cell types most prominently involved in the genetic basis of ARHL. Seeking to resolve this discrepancy, we integrated the analysis of four ARHL GWAS datasets with four independent inner-ear single-cell RNA-sequencing datasets. Our analysis clearly points to the cochlear sensory epithelial cells as the key cells for the genetic predisposition to ARHL. We also explain the limitation of the bioinformatics analysis performed by previous studies that led to missing the enrichment for ARHL GWAS signal in sensory epithelial cells. Collectively, we show that cochlear epithelial cells, not stria vascularis cells, are the main inner-ear cells related to the genetic pathogenesis of ARHL.

Identification of common stria vascularis cellular alteration in sensorineural hearing loss based on ScRNA-seq.

BACKGROUND: The stria vascularis (SV), located in the lateral wall of the cochlea, maintains cochlear fluid homeostasis and mechanoelectrical transduction (MET) activity required for sound wave conduction. The pathogenesis of a number of human inheritable deafness syndromes, age related hearing loss, drug-induced ototoxicity and noise-induced hearing loss results from the morphological changes and functional impairments in the development of the SV. In this study, we investigate the implications of intercellular communication within the SV in the pathogenesis of sensorineural hearing loss (SNHL). We aim to identify commonly regulated signaling pathways using publicly available single-cell transcriptomic sequencing (scRNA-seq) datasets. METHODS: We analyzed scRNA-seq data, which was derived from studying the cochlear SV in mice with SNHL compared to normal adult mice. After quality control and filtering, we obtained the major cellular components of the mouse cochlear SV and integrated the data. Using Seurat's FindAllMarkers and FindMarkers packages, we searched for novel conservative genes and differential genes. We employed KEGG and GSEA to identify molecular pathways that are commonly altered among different types of SNHL. We utilized pySCENIC to discover new specific regulatory factors in SV subpopulation cells. With the help of CellChat, we identified changes in subpopulation cells showing similar trends across different SNHL types and their alterations in intercellular communication pathways. RESULTS: Through the analysis of the integrated data, we discovered new conserved genes to SV specific cells and identified common downregulated pathways in three types of SNHL. The enriched genes for these pathways showing similar trends are primarily associated with the Electron Transport Chain, related to mitochondrial energy metabolism. Using the CellChat package, we further found that there are shared pathways in the incoming signaling of specific intermediate cells in SNHL, and these pathways have common upstream regulatory transcription factor of Nfe2l2. Combining the results from pySCENIC and CellChat, we predicted the transcription factor Nfe2l2 as an upstream regulatory factor for multiple shared cellular pathways in IC. Additionally, it serves as an upstream factor for several genes within the Electron Transport Chain. CONCLUSION: Our bioinformatics analysis has revealed that downregulation of the mitochondrial electron transport chain have been observed in various conditions of SNHL. E2f1, Esrrb, Runx1, Yy1, and Gata2 could serve as novel important common TFs regulating the electron transport chain. Adm has emerged as a potential new marker gene for intermediate cells, while Itgb5 and Tesc show promise as potential new marker genes for marginal cells in the SV. These findings offer a new perspective on SV lesions in SNHL and provide additional theoretical evidence for the same drug treatment and prevention of different pathologies of SNHL.

Reactive Oxygen Species-Related Disruptions to Cochlear Hair Cell and Stria Vascularis Consequently Leading to Radiation-Induced Sensorineural Hearing Loss.

Aims: Radiation-induced sensorineural hearing loss (RISNHL) is one of the major side effects of radiotherapy for head and neck cancers. At present, no effective clinical treatment or prevention is available for RISNHL. This study thus aimed to investigate the cochlear pathology so that the underlying mechanisms of RISNHL may be elucidated, consequently paving the way for potential protective strategies to be developed. Results: Functional and morphological impairment in the stria vascularis (SV) was observed after irradiation (IR), as indicated by endocochlear potential (EP) reduction, hyperpermeability, and SV atrophy. The expression of zonulae occludins-1 was found to have decreased after IR. The loss of outer hair cells (OHCs) occurred later than SV damage. The disruption to the SV and OHCs could be attributed to reactive oxygen species (ROS)-related damage. In addition, EP shifts and the loss of OHCs were reduced when ROS was reduced by N-acetylcysteine (NAC) in C57BL/6 mice, attenuating auditory threshold shifts. Innovation: The damage to the SV was found to occur before OHC loss. ROS-related damage accounted for SV damage and OHC loss. The incidences of SV damage and OHC loss were decreased through ROS modulation by NAC, subsequently preventing RISNHL, suggesting the possible role of NAC as a possible protective agent against RISNHL. Conclusion: The findings from this study suggest oxidative stress-induced early SV injury and late OHC loss to be the key factors leading to RISNHL. NAC prevents IR-induced OHC loss, and attenuates auditory brainstem response and EP shifts by regulating the level of oxidative stress. Antioxid. Redox Signal. 40, 470-491.

Diabetes mellitus and hearing loss.

Diabetes mellitus (DM) is a major disease threatening human health and its incidence is increasing year on year. As a chronic complication of DM, hearing loss mostly occurs undetectably. However, the mechanism of this diabetes-related hearing loss (DRHL) remains unclear and there is no effective clinical treatment. Studies of animal or human pathology show that DM causes damage to the blood vessels, spiral ganglion neurons, afferent nerve fibers, the organ of Corti, and the stria vascularis of the inner ear. In recent years, more advances in pathological research have revealed the possible mechanism of DRHL. In addition, a large number of clinical studies suggest that the duration and severity of DM are closely related to the incidence and severity of DRHL. This review focuses on the relationship between DM and hearing loss. The clinical audiological characteristics of diabetic patients, risk factors for DRHL, typical pathology, and potential interventions of DRHL are summarized. This will help reveal the pathogenesis and intervention approaches for DRHL.

Predicting Atrophy of the Cochlear Stria Vascularis from the Shape of the Threshold Audiogram.

Several lines of evidence have suggested that steeply sloping audiometric losses are caused by hair cell degeneration, while flat audiometric losses are caused by strial atrophy, but this concept has never been rigorously tested in human specimens. Here, we systematically compare audiograms and cochlear histopathology in 160 human cases from the archival collection of celloidin-embedded temporal bones at the Massachusetts Eye and Ear. The dataset included 106 cases from a prior study of normal-aging ears, and an additional 54 cases selected by combing the database for flat audiograms. Audiogram shapes were classified algorithmically into five groups according to the relation between flatness (i.e., SD of hearing levels across all frequencies) and low-frequency pure-tone average (i.e., mean at 0.25, 0.5, and 1.0 kHz). Outer and inner hair cell losses, neural degeneration, and strial atrophy were all quantified as a function of cochlear location in each case. Results showed that strial atrophy was worse in the apical than the basal half of the cochlea and was worse in females than in males. The degree of strial atrophy was uncorrelated with audiogram flatness. Apical atrophy was correlated with low-frequency thresholds and basal atrophy with high-frequency thresholds, and the former correlation was higher. However, a multivariable regression with all histopathological measures as predictors and audiometric thresholds as the outcome showed that strial atrophy was a significant predictor of threshold shift only in the low-frequency region, and, even there, the contribution of outer hair cell damage was larger.SIGNIFICANCE STATEMENT Cochlear pathology can only be assessed postmortem; thus, human cochlear histopathology is critical to our understanding of the mechanisms of hearing loss. Dogma holds that relative damage to sensory cells, which transduce mechanical vibration into electrical signals, versus the stria vascularis, the cellular battery that powers transduction, can be inferred by the shape of the audiogram, that is, down-sloping (hair cell damage) versus flat (strial atrophy). Here we quantified hair cell and strial atrophy in 160 human specimens to show that it is the degree of low-frequency hearing loss, rather than the audiogram slope, that predicts strial atrophy. Results are critical to the design of clinical trials for hearing-loss therapeutics, as current drugs target only hair cell, not strial, regeneration.

Emerging Mechanisms in the Pathogenesis of Menière's Disease: Evidence for the Involvement of Ion Homeostatic or Blood-Labyrinthine Barrier Dysfunction in Human Temporal Bones.

HYPOTHESIS: Analysis of human temporal bone specimens of patients with Menière's disease (MD) may demonstrate altered expression of gene products related to barrier formation and ionic homeostasis within cochlear structures compared with control specimens. BACKGROUND: MD represents a challenging otologic disorder for investigation. Despite attempts to define the pathogenesis of MD, there remain many gaps in our understanding, including differences in protein expression within the inner ear. Understanding these changes may facilitate the identification of more targeted therapies for MD. METHODS: Human temporal bones from patients with MD (n = 8) and age-matched control patients (n = 8) were processed with immunohistochemistry stains to detect known protein expression related to ionic homeostasis and barrier function in the cochlea, including CLDN11, CLU, KCNJ10, and SLC12A2. Immunofluorescence intensity analysis was performed to quantify protein expression in the stria vascularis, organ of Corti, and spiral ganglion neuron (SGN). RESULTS: Expression of KCNJ10 was significantly reduced in all cochlear regions, including the stria vascularis (9.23 vs 17.52, p = 0.011), OC (14.93 vs 29.16, p = 0.014), and SGN (7.69 vs 18.85, p = 0.0048) in human temporal bone specimens from patients with MD compared with control, respectively. CLDN11 (7.40 vs 10.88, p = 0.049) and CLU (7.80 vs 17.51, p = 0.0051) expression was significantly reduced in the SGN. CONCLUSION: The results of this study support that there may be differences in the expression of proteins related to ionic homeostasis and barrier function within the cochlea, potentially supporting the role of targeted therapies to treat MD.

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.

Supporting-cell vs. hair-cell survival in the human cochlea: Implications for regenerative therapies.

Animal studies have shown that the supporting-cells surviving in the organ of Corti after cochlear insult can be transdifferentiated into hair cells as a treatment for sensorineural hearing loss. Clinical trials of small-molecule therapeutics have been undertaken, but little is known about how to predict the pattern and degree of supporting-cell survival based on audiogram, hearing loss etiology or any other metric obtainable pre-mortem. To address this, we systematically assessed supporting-cell and hair cell survival, as a function of cochlear location in 274 temporal bone cases from the archives at the Massachusetts Eye and Ear and compared the histopathology with the audiograms and hearing-loss etiologies. Results showed that supporting-cell survival was always significantly greater in the apical half than the basal half of the cochlea, that inner pillars were more robust than outer pillars or Deiters' cells, and that total replacement of all supporting cells with a flat epithelium was rare outside of the extreme basal 20% of the cochlea. Supporting cell survival in the basal half of the cochlea was better correlated with the slope of the audiogram than with the mean high-frequency threshold per se: i.e. survival was better with flatter audiograms than with steeply down-sloping audiograms. Cochlear regions with extensive hair cell loss and exceptional supporting cell survival were most common in cases with hearing loss due to ototoxic drugs. Such cases also tended to have less pathology in other functionally critical structures, i.e. spiral ganglion neurons and the stria vascularis.

Protective Effects of Infliximab Against Kanamycin-Induced Ototoxicity in Rats.

HYPOTHESIS: To examine the protective effects of infliximab (INF) against kanamycin (KM)-induced hearing loss. BACKGROUND: Tumor necrosis factor α blockers can reduce cellular inflammatory reactions and decrease cell death. METHODS: Thirty-six rats with normal hearing were randomly divided into six groups. The first group was injected with 400 mg/kg KM intramuscularly (IM), the second group with 7 mg/kg INF intraperitoneally (IP) and 400 mg/kg KM IM, the third group with 7 mg/kg INF IP and 200 mg/kg KM IM, and the fourth group with 1 mg/kg 6-methylprednisolone (MP) IP and 400 mg/kg KM IM. Group 5 was injected with 1 mg/kg MP IP and 200 mg/kg KM IM, and group 6 with saline IP once. Auditory brain-stem response (ABR) for hearing thresholds was performed on days 7 and 14. From the frozen sections of the cochlea, the area of the stria vascularis, the number of neurons in the spiral ganglion, the fluorescence intensity of hair cells (FIHC), postsynaptic density (PSD), and presynaptic ribbons (PSRs) were calculated. RESULTS: The KM-induced increase in hearing thresholds was detected on the 14th day. Hearing was only preserved in the group treated with INF after low-dose KM exposure but not in the groups that received high-dose KM. The FIHC, excitatory PSD, and PSR were preserved only in the INF-treated group after half-dose KM exposure. In MP groups, FIHC, excitatory PSD, and PSR were significantly lower than in the control group. CONCLUSIONS: Our results support that tumor necrosis factor-based inflammation may play a role in the ototoxicity mechanism.

Tomodensitometric and histological age-related changes in the normal feline middle and inner ear.

Deafness in cats may be due to acquired causes such as aging. Similar age-related morphological changes in the cochlea have been noted in several animal species. However, little is known about the effects of age on the morphology of the middle and inner ear in cats. The aim of the present study was to compare these structures in middle-aged and geriatric cats using computed tomography and histological morphometric analysis. Data were obtained from 28 cats, aged 3-18 years, with no hearing or neurological disorders. Computed tomography showed an increase in tympanic bulla (middle ear) volume with aging. Histological morphometric analysis revealed thickening of the basilar membrane and atrophy of the stria vascularis (inner ear) in older cats, similar to what has been observed in older humans and dogs. Nevertheless, histological procedures could be improved to provide more data for comparison with different forms of presbycusis in humans.

Single-cell RNA-sequencing of stria vascularis cells in the adult Slc26a4-/- mouse.

BACKGROUND: The primary pathological alterations of Pendred syndrome are endolymphatic pH acidification and luminal enlargement of the inner ear. However, the molecular contributions of specific cell types remain poorly characterized. Therefore, we aimed to identify pH regulators in pendrin-expressing cells that may contribute to the homeostasis of endolymph pH and define the cellular pathogenic mechanisms that contribute to the dysregulation of cochlear endolymph pH in Slc26a4-/- mice. METHODS: We used single-cell RNA sequencing to identify both Slc26a4-expressing cells and Kcnj10-expressing cells in wild-type (WT, Slc26a4+/+) and Slc26a4-/- mice. Bioinformatic analysis of expression data confirmed marker genes defining the different cell types of the stria vascularis. In addition, specific findings were confirmed at the protein level by immunofluorescence. RESULTS: We found that spindle cells, which express pendrin, contain extrinsic cellular components, a factor that enables cell-to-cell communication. In addition, the gene expression profile informed the pH of the spindle cells. Compared to WT, the transcriptional profiles in Slc26a4-/- mice showed downregulation of extracellular exosome-related genes in spindle cells. Immunofluorescence studies in spindle cells of Slc26a4-/- mice validated the increased expression of the exosome-related protein, annexin A1, and the clathrin-mediated endocytosis-related protein, adaptor protein 2. CONCLUSION: Overall, cell isolation of stria vascularis from WT and Slc26a4-/- samples combined with cell type-specific transcriptomic analyses revealed pH-dependent alternations in spindle cells and intermediate cells, inspiring further studies into the dysfunctional role of stria vascularis cells in SLC26A4-related hearing loss.

Dissection of Adult Mouse Stria Vascularis for Single-Nucleus Sequencing or Immunostaining.

Endocochlear potential, which is generated by the stria vascularis, is essential to maintain an environment conducive to appropriate hair cell mechanotransduction and ultimately hearing. Pathologies of the stria vascularis can result in a decreased hearing. Dissection of the adult stria vascularis allows for focused single-nucleus capture and subsequent single-nucleus sequencing and immunostaining. These techniques are used to study stria vascularis pathophysiology at the single-cell level. Single-nucleus sequencing can be used in the setting of transcriptional analysis of the stria vascularis. Meanwhile, immunostaining continues to be useful in identifying specific populations of cells. Both methods require proper stria vascularis dissection as a prerequisite, which can prove to be technically challenging.

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.

The role of the stria vascularis in neglected otologic disease.

The stria vascularis (SV) has been shown to play a critical role in the pathogenesis of many diseases associated with sensorineural hearing loss (SNHL), including age-related hearing loss (ARHL), noise-induced hearing loss (NIHL), hereditary hearing loss (HHL), and drug-induced hearing loss (DIHL), among others. There are a number of other disorders of hearing loss that may be relatively neglected due to being underrecognized, poorly understood, lacking robust diagnostic criteria or effective treatments. A few examples of these diseases include autoimmune inner ear disease (AIED) and/or autoinflammatory inner ear disease (AID), Meniere's disease (MD), sudden sensorineural hearing loss (SSNHL), and cytomegalovirus (CMV)-related hearing loss (CRHL). Although these diseases may often differ in etiology, there have been recent studies that support the involvement of the SV in the pathogenesis of many of these disorders. We strive to highlight a few prominent examples of these frequently neglected otologic diseases and illustrate the relevance of understanding SV composition, structure and function with regards to these disease processes. In this study, we review the physiology of the SV, lay out the importance of these neglected otologic diseases, highlight the current literature regarding the role of the SV in these disorders, and discuss the current strategies, both approved and investigational, for management of these disorders.

Tramadol-induced apoptosis in auditory hair cells of adult male rats.

Reports have emerged on the sudden opioid-induced auditory hearing loss, and the underlying pathology is not fully understood. The present study aimed to determine the mechanism of action of these drugs in the inner ear. For this purpose, 20 rats were treated with 50 mg/kg tramadol daily for three weeks. Next, the stereological and immunohistochemical alteration of the inner hair cells under chronic exposure to tramadol was evaluated. The results revealed that tramadol induced hair cell degeneration and decreased bipolar neurons of the spiral ganglion and the thickness of stria vascularis. Moreover, immunohistochemistry showed that tramadol caused apoptosis in inner hair cells and bipolar neurons. These findings indicate that tramadol induces apoptosis in auditory hair cells, suggesting that tramadol may cause hearing loss and ototoxicity.

Hearing loss caused by CMV infection is correlated with reduced endocochlear potentials caused by strial damage in murine models.

Congenital cytomegalovirus (CMV) infection is a significant cause of neonatal hearing loss. However, at the cochlear level, the anatomical lesions and pathophysiological mechanisms that underlie hearing loss are still not clearly understood. In murine models of CMV infection, we have observed early damage to the capillary networks in stria vascularis, as well as hearing loss manifested in ABR threshold elevations. Our experimental hypothesis is that strial damage causes a reduced endocochlear potential (EP) resulting in impaired haircell activation and consequent hearing loss. We have studied strial damage, EP, and ABR threshold elevations in two mouse models (BALB/c and C57BL6 strains) infected with murine CMV. Neonatal (P3) pups were inoculated with murine CMV (2µl of 200pfu) by intra cerebral injection. Control mice were saline injected. At 6 weeks, ABR thresholds to tonal stimuli at 8, 16 and 32 kHz were determined for each ear. At 8 weeks a sub-group of treated and control animals was prepared for study of cochlear capillary networks using scanning electron microscopy of corrosion cast specimens. In a second group, at 8 weeks, EP measurements from both cochleas were made. We report that in both mouse strains, CMV infection caused capillary loss in the stria vascularis, initially at the cochlear apex, and extending to lower cochlear turns in some subjects. After CMV infection, in both BALB/c and C57BL6 mice, reduced EPs and ABR threshold elevations were observed, and there was a within-animal correlation between loss of EP and ABR threshold elevations across the sound frequencies tested. These results suggest that CMV induced damage to stria vascularis results in EP reduction that is correlated with ABR threshold elevations. Extrapolating to the human condition, we suggest that strial damage and its physiological consequences may contribute to the initial hearing loss in congenital CMV infection. The early involvement of cochlear capillary damage may encourage a focus on therapeutic interventions that can prevent vascular damage, or subsequently promote vascular healing or angiogenesis.

Insights into Presbycusis From the First Temporal Bone Laboratory Within the United States.

The Johns Hopkins Otologic Research Laboratory was founded in 1924 as the first human temporal bone laboratory within the United States. To better understand the contributions of the Johns Hopkins Otologic Research Laboratory to our understanding of presbycusis, we consulted with a medical librarian and archivist to search the Alan Mason Chesney Medical Archives, PubMed, JSTOR, and Johns Hopkins Bulletin for published and unpublished works from the lab. Between 1924 and 1938, Samuel J. Crowe, the Chairman of Otolaryngology, and anatomist Stacy R. Guild amassed a collection of ∼1,800 temporal bones. This collection allowed for an unprecedented period of discovery related to otologic disease. They combined hearing thresholds measured by the recently invented audiometer with new techniques for temporal bone decalcification, sectioning, and staining, and a method for the graphic reconstruction of the cochlea. Crowe and Guild used this unique opportunity to correlate otopathology with hearing and to make the first detailed descriptions of the otopathology of presbycusis. In 1931 and 1934, they observed spiral ganglion neuron and outer hair cell loss in the basal turn of the cochlea in individuals with high-frequency hearing loss. These were the first studies to reveal that stria vascularis degeneration and middle ear pathology were not the most common causes for high-frequency hearing loss. Aside from revealing the primary driving factors of presbycusis, this work provided insight into the tonotopic organization of the cochlea. After initially being recruited to help raise money for the laboratory, medical illustrator Max Brödel used the vertical histologic cross-sections of the cochlea to produce illustrations of the ear. The decision to produce histologic sections in the plane of the superior semicircular canal likely influenced Brödel's illustrations that share a similar orientation and would later become widely circulated. Significant contributions from the Otologic Research Laboratory were also made by Mary Hardy, D.Sc., a woman who has previously received little recognition for her work. The sectioning of temporal bones was stopped in 1938 due to World War II, but much of Crowe's and Guild's work continued into the 1940s until a rift between the two resulted in the temporary closure of the laboratory in 1949. Nearly 100 years after its founding, discoveries from the Johns Hopkins Otologic Research Laboratory remain relevant and emphasize the importance of continued human temporal bone research to improve our understanding and treatment of otologic disease.

Glial-Specific Deletion of Med12 Results in Rapid Hearing Loss via Degradation of the Stria Vascularis.

Mediator protein complex subunit 12 (Med12) is a core component of the basal transcriptional apparatus and plays a critical role in the development of many tissues. Mutations in Med12 are associated with X-linked intellectual disability syndromes and hearing loss; however, its role in nervous system function remains undefined. Here, we show that temporal conditional deletion of Med12 in astrocytes in the adult CNS results in region-specific alterations in astrocyte morphology. Surprisingly, behavioral studies revealed rapid hearing loss after adult deletion of Med12 that was confirmed by a complete abrogation of auditory brainstem responses. Cellular analysis of the cochlea revealed degeneration of the stria vascularis, in conjunction with disorganization of basal cells adjacent to the spiral ligament and downregulation of key cell adhesion proteins. Physiologic analysis revealed early changes in endocochlear potential, consistent with strial-specific defects. Together, our studies reveal that Med12 regulates auditory function in the adult by preserving the structural integrity of the stria vascularis.SIGNIFICANCE STATEMENT Mutations in Mediator protein complex subunit 12 (Med12) are associated with X-linked intellectual disability syndromes and hearing loss. Using temporal-conditional genetic approaches in CNS glia, we found that loss of Med12 results in severe hearing loss in adult animals through rapid degeneration of the stria vascularis. Our study describes the first animal model that recapitulates hearing loss identified in Med12-related disorders and provides a new system in which to examine the underlying cellular and molecular mechanisms of Med12 function in the adult nervous system.