Age-related changes in cochlear endolymphatic potassium and potential in CD-1 and CBA/CaJ mice

Electrical and Immunohistochemical Properties of Cochlear Fibrocytes in 3D Cell Culture and in the Excised Spiral Ligament of Mice

Fibrocyte degeneration in the cochlear lateral wall is one possible pathology of age-related metabolic hearing loss (presbycusis). Within the lateral wall fibrocytes play a role in potassium recycling and maintenance of the endocochlear potential. It has been proposed that cell replacement therapy could prevent fibrocyte degeneration in the CD/1 mouse model of hearing loss. For this to work, the replacement fibrocytes would need to take over the structural and physiological role of those lost. We have grown lateral wall fibrocytes from neonatal CD/1 mice in a 3D-collagen gel culture with the aim of assessing their functional similarity to native lateral wall fibrocytes, the latter in a slice preparation and in excised spiral ligament pieces. We have compared cultured and native fibrocytes using both immuno-labelling of characteristic proteins and single cell electrophysiology. Cultured fibrocytes exhibited rounded cell bodies with extending processes. They labelled with marker antibodies targeting aquaporin 1 and calcium-binding protein S-100, precluding an unambiguous identification of fibrocyte type. In whole-cell voltage clamp, both native and cultured fibrocytes exhibited non-specific currents and voltage-dependent K⁺ currents. The non-specific currents from gel-cultured and excised spiral ligament fibrocytes were partially and reversibly blocked by external TEA (10 mM). The TEA-sensitive current had a mean reversal potential of + 26 mV, suggesting a permeability sequence of Na⁺  > K⁺. These findings indicate that 3D-cultured fibrocytes share a number of characteristics with native spiral ligament fibrocytes and thus might represent a suitable population for transplantation therapy aimed at treating age-related hearing loss.

Prestin derived OHC surface area reduction underlies age-related rescaling of frequency place coding

Outer hair cells (OHC) are key to the mammalian cochlear amplifier, powered by the lateral membrane protein Prestin. In this study, we explored age-related OHC changes and how the changes affected hearing in mouse. OHC nonlinear membrane capacitance measurements revealed that, starting upon completion of postnatal auditory development, a continuous reduction of total Prestin in OHCs accompanied by a significant reduction in their cell surface area. Prestin's density is unaffected by Prestin level drop over the whole age range tested, suggesting that the OHC size reduction is Prestin-dependent. Stereocilia length in aged OHCs remained unchanged but the first row stereocilia on the aged inner hair cells (IHCs) were elongated. Distortion product otoacoustic emission (DPOAE) group delays became longer with aging, suggesting an apical shift in vibration on basilar membrane. Acoustic lesion experiments revealed an apical shift in damage place in old cochleae accompanied by a shallower progression in synaptic damage over a wider frequency range that was indicative of a broader frequency filter. Overall, these findings suggest that in aging cochlea, a shift in frequency place coding could occur due to the changes in cochlear active and passive mechanics. This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.

Translational and interdisciplinary insights into presbyacusis: A multidimensional disease

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

Age-related hearing loss pertaining to potassium ion channels in the cochlea and auditory pathway

Age-related hearing loss (ARHL) is the most prevalent sensory deficit in the elderly and constitutes the third highest risk factor for dementia. Lifetime noise exposure, genetic predispositions for degeneration, and metabolic stress are assumed to be the major causes of ARHL. Both noise-induced and hereditary progressive hearing have been linked to decreased cell surface expression and impaired conductance of the potassium ion channel K_V7.4 (KCNQ4) in outer hair cells, inspiring future therapies to maintain or prevent the decline of potassium ion channel surface expression to reduce ARHL. In concert with K_V7.4 in outer hair cells, K_V7.1 (KCNQ1) in the stria vascularis, calcium-activated potassium channels BK (KCNMA1) and SK2 (KCNN2) in hair cells and efferent fiber synapses, and K_V3.1 (KCNC1) in the spiral ganglia and ascending auditory circuits share an upregulated expression or subcellular targeting during final differentiation at hearing onset. They also share a distinctive fragility for noise exposure and age-dependent shortfalls in energy supply required for sustained surface expression. Here, we review and discuss the possible contribution of select potassium ion channels in the cochlea and auditory pathway to ARHL. We postulate genes, proteins, or modulators that contribute to sustained ion currents or proper surface expressions of potassium channels under challenging conditions as key for future therapies of ARHL.

Endolymphatic Potential Measured From Developing and Adult Mouse Inner Ear

The mammalian inner ear has two major parts, the cochlea is responsible for hearing and the vestibular organ is responsible for balance. The cochlea and vestibular organs are connected by a series of canals in the temporal bone and two distinct extracellular fluids, endolymph and perilymph, fill different compartments of the inner ear. Stereocilia of mechanosensitive hair cells in the cochlea and vestibular end organs are bathed in the endolymph, which contains high K+ ions and possesses a positive potential termed endolymphatic potential (ELP). Compartmentalization of the fluids provides an electrochemical gradient for hair cell mechanotransduction. In this study, we measured ELP from adult and neonatal C57BL/6J mice to determine how ELP varies and develops in the cochlear and vestibular endolymph. We measured ELP and vestibular microphonic response from saccules of neonatal mice to determine when vestibular function is mature. We show that ELP varies considerably in the cochlear and vestibular endolymph of adult mice, ranging from +95 mV in the basal turn to +87 mV in the apical turn of the cochlea, +9 mV in the saccule and utricle, and +3 mV in the semicircular canal. This suggests that ELP is indeed a local potential, despite the fact that endolymph composition is similar. We further show that vestibular ELP reaches adult-like magnitude around post-natal day 6, ~12 days earlier than maturation of cochlear ELP (i.e., endocochlear potential). Maturation of vestibular ELP coincides with the maturation of vestibular microphonic response recorded from the saccular macula, suggesting that maturation of vestibular function occurs much earlier than maturation of hearing in mice.

CD-1db/db mice: A novel type 2 diabetic mouse model with progressive kidney fibrosis

Aims/Introduction

To establish novel therapies to combat diabetic kidney disease, a human disease‐relevant animal model is essential. However, a type 2 diabetic mouse model presenting progressive kidney fibrosis has not yet been established. Kidneys of streptozotocin‐induced diabetic CD‐1 mice showed severe fibrosis compared with other backgrounds of mice associated with the suppression of antifibrotic peptide N‐acetyl‐seryl‐aspartyl‐lysyl‐proline. The BKS background (BKS^db/db) is often utilized for diabetic kidney disease research; the kidney fibrosis in the BKS^db/db phenotype is minimal.

Materials and Methods

We generated CD‐1^db/db mice by backcrossing the db gene into the CD‐1 background, and analyzed phenotypic differences compared with BKS^db/db and CD‐1^db/m mice.

Results

Male CD‐1^db/db mice appeared to have elevated blood glucose levels compared with those of BKS^db/db mice. Fasting insulin levels declined in CD‐1^db/db mice. Plasma cystatin C levels tended to be elevated in CD‐1^db/db mice from 16 to 24 weeks‐of‐age. Male CD‐1^db/db mice showed significantly progressive kidney and heart fibrosis from 16 to 24 weeks‐of‐age when compared with that of age‐matched BKS^db/db mice. The gene expression profile showed fibrogenic program‐associated genes in male CD‐1^db/db mice. Male CD‐1^db/db mice displayed significantly lower urine antifibrotic peptide N‐acetyl‐seryl‐aspartyl‐lysyl‐proline when compared to that of BKS^db/db at 24 weeks‐of‐age. The gene expression of prolyl oligopeptidase, the enzyme essential for antifibrotic peptide N‐acetyl‐seryl‐aspartyl‐lysyl‐proline production from thymosin β4, was significantly lower in the CD‐1 mice. Thymosin β4 levels were also lower in CD‐1 mice.

Conclusions

These results suggest that CD‐1^db/db mice are a novel type 2 diabetic mouse model with progressive kidney and heart fibrosis.

Age-related changes in Na, K-ATPase expression, subunit isoform selection and assembly in the stria vascularis lateral wall of mouse cochlea

Due to the critical role of cochlear ion channels for hearing, the focus of the present study was to examine age-related changes of Na, K-ATPase (NKA) subunits in the lateral wall of mouse cochlea. We combined qRT-PCR, western blot and immunocytochemistry methodologies in order to determine gene and protein expression levels in the lateral wall of young and aged CBA/CaJ mice. Of the seven NKA subunits, only the mRNA expressions of α1, β1 and β2 subunit isoforms were detected in the lateral wall of CBA/CaJ mice. Aging was accompanied by dys-regulation of gene and protein expression of all three subunits detected. Hematoxylin and eosin (H&E) staining revealed atrophy of the cochlear stria vascularis (SV). The SV atrophy rate (20%) was much less than the ∼80% decline in expression of all three NKA isoforms, indicating lateral wall atrophy and NKA dys-regulation are independent factors and that there is a combination of changes involving the morphology of SV and NKA expression in the aging cochlea which may concomitantly affect cochlear function. Immunoprecipitation assays showed that the α1-β1 heterodimer is the selective preferential heterodimer over the α1-β2 heterodimer in cochlea lateral wall. Interestingly, in vitro pathway experiments utilizing cultured mouse cochlear marginal cells from the SV (SV-K1 cells) indicated that decreased mRNA and protein expressions of α1, β1 and β2 subunit isoforms are not associated with reduction of NKA activity following in vitro application of ouabain, but ouabain did disrupt the α1-β1 heterodimer interaction. Lastly, the association between the α1 and β1 subunit isoforms was present in the cochlear lateral wall of young adult mice, but this interaction could not be detected in old mice. Taken together, these data suggest that in the young adult mouse there is a specific, functional selection and assembly of NKA subunit isoforms in the SV lateral wall, which is disrupted and dys-regulated with age. Interventions for this age-linked ion channel disruption may have the potential to help diagnose, prevent, or treat age-related hearing loss.

Mutations in L-type amino acid transporter-2 support SLC7A8 as a novel gene involved in age-related hearing loss

Age-related hearing loss (ARHL) is the most common sensory deficit in the elderly. The disease has a multifactorial etiology with both environmental and genetic factors involved being largely unknown. SLC7A8/SLC3A2 heterodimer is a neutral amino acid exchanger. Here, we demonstrated that SLC7A8 is expressed in the mouse inner ear and that its ablation resulted in ARHL, due to the damage of different cochlear structures. These findings make SLC7A8 transporter a strong candidate for ARHL in humans. Thus, a screening of a cohort of ARHL patients and controls was carried out revealing several variants in SLC7A8, whose role was further investigated by in vitro functional studies. Significant decreases in SLC7A8 transport activity was detected for patient’s variants (p.Val302Ile, p.Arg418His, p.Thr402Met and p.Val460Glu) further supporting a causative role for SLC7A8 in ARHL. Moreover, our preliminary data suggest that a relevant proportion of ARHL cases could be explained by SLC7A8 mutations.

Age-related hearing loss affects about one in three individuals between the ages of 65 and 74. The first symptom is difficulty hearing high-pitched sounds like children’s voices. The disease starts gradually and worsens over time. Changes in the ear, the nerve that connects it to the brain, or the brain itself can cause hearing loss. Sometimes all three play a role. Genetics, exposure to noise, disease, and aging may all contribute. The condition is so complex it is difficult for scientists to pinpoint a primary suspect or develop treatments.

Now, Guarch, Font-Llitjós et al. show that errors in a protein called SLC7A8 cause age-related hearing loss in mice and humans. The SLC7A8 protein acts like a door that allows amino acids – the building blocks of proteins – to enter or leave a cell. This door is blocked in mice lacking SLC7A8 and damage occurs in the part of their inner ear responsible for hearing. As a result, the animals lose their hearing. Next, Guarch, Font-Llitjós et al. scanned the genomes of 147 people from isolated villages in Italy for mutations in the gene for SLC7A8. The people also underwent hearing tests.

Mutations in the gene for SLC7A8 that partially block the door and prevent the flow of amino acids were found in people with hearing loss. Some mutations in SLC7A8 that allow the door to stay open where found in people who could hear. The experiments suggest that certain mutations in the gene for SLC7A8 are likely an inherited cause of age-related hearing loss. It is possible that other proteins that control the flow of amino acids into or out of cells also may play a role in hearing. More studies are needed to see if it is possible to fix errors in the SLC7A8 protein to delay or restore the hearing loss.

Application of Mouse Models to Research in Hearing and Balance

Laboratory mice (Mus musculus) have become the major model species for inner ear research. The major uses of mice include gene discovery, characterization, and confirmation. Every application of mice is founded on assumptions about what mice represent and how the information gained may be generalized. A host of successes support the continued use of mice to understand hearing and balance. Depending on the research question, however, some mouse models and research designs will be more appropriate than others. Here, we recount some of the history and successes of the use of mice in hearing and vestibular studies and offer guidelines to those considering how to apply mouse models.

Sox10 expressing cells in the lateral wall of the aged mouse and human cochlea

Age-related hearing loss (presbycusis) is a common human disorder, affecting one in three Americans aged 60 and over. Previous studies have shown that presbyacusis is associated with a loss of non-sensory cells in the cochlear lateral wall. Sox10 is a transcription factor crucial to the development and maintenance of neural crest-derived cells including some non-sensory cell types in the cochlea. Mutations of the Sox10 gene are known to cause various combinations of hearing loss and pigmentation defects in humans. This study investigated the potential relationship between Sox10 gene expression and pathological changes in the cochlear lateral wall of aged CBA/CaJ mice and human temporal bones from older donors. Cochlear tissues prepared from young adult (1–3 month-old) and aged (2–2.5 year-old) mice, and human temporal bone donors were examined using quantitative immunohistochemical analysis and transmission electron microscopy. Cells expressing Sox10 were present in the stria vascularis, outer sulcus and spiral prominence in mouse and human cochleas. The Sox10⁺ cell types included marginal and intermediate cells and outer sulcus cells, including those that border the scala media and those extending into root processes (root cells) in the spiral ligament. Quantitative analysis of immunostaining revealed a significant decrease in the number of Sox10⁺ marginal cells and outer sulcus cells in aged mice. Electron microscopic evaluation revealed degenerative alterations in the surviving Sox10⁺ cells in aged mice. Strial marginal cells in human cochleas from donors aged 87 and older showed only weak immunostaining for Sox10. Decreases in Sox10 expression levels and a loss of Sox10⁺ cells in both mouse and human aged ears suggests an important role of Sox10 in the maintenance of structural and functional integrity of the lateral wall. A loss of Sox10⁺ cells may also be associated with a decline in the repair capabilities of non-sensory cells in the aged ear.

Compromised potassium recycling in the cochlea contributes to conservation of endocochlear potential in a mouse model of age-related hearing loss

The C57BL/6 strain is considered an excellent model to study age-related hearing loss (AHL). Aging C57BL/6 mice are characterized by profound hearing loss but conservation of the endocochlear potential (EP). Here we show 12-month-old C57BL/6 mice display a notable hearing loss at 4, 8, 16 and 32kHz while the EP is maintained at normal level. Morphological examination shows significant outer hair cells loss in the cochlear basal turn and atrophy of the stria vascularis (SV). Fluorescence immunohistochemical studies reveal that potassium channel KCNJ10 and KCNQ1 expression dramatically decreased in the SV. Concomitant with this, mRNA levels of KCNJ10 and KCNQ1 are also reduced. In addition, three other potassium transporters, including α1-Na,K-ATPase, α2-Na,K-ATPase and NKCC1, reduce their expression at mRNA levels as well. These observations suggest that conservation of the EP in aging C57BL/6 mice is attributable to the SV generating a new balance for potassium influx and efflux at a relatively lower level.

The expression of glutamate aspartate transporter (GLAST) within the human cochlea and its distribution in various patient populations

Glutamate plays an important role in the central nervous system as an excitatory neurotransmitter. However, its abundance can lead to excitotoxicity which necessitates the proper function of active glutamate transporters. The glutamate-aspartate transporter (GLAST) has been shown to exist and function within non-human cochlear specimens regulating the inner ear glutamate concentration. In this study, we examined human cochleas from formalin-fixed celloidin-embedded temporal bone specimens of three different types of patients (Meniere's disease, normal controls, and other otopathologic conditions) and examined the differential expression of GLAST in the spiral ligament of the basal, middle, and apical turns of the cochlea. Immunohistochemical staining was performed with polyclonal antibodies against GLAST and image analysis was carried out with the Image J analysis software. In contrast to other studies with non-human specimens, GLAST was expressed in the spiral ligament fibrocytes but was not detected in the satellite cells of the spiral ganglia or supporting cells of the Organ of Corti in the human cochlea. Our data also showed that GLAST expression significantly differs in the basal and apical turns of the cochlea. Lastly, post-hoc analysis showed a difference in the GLAST immunoreactive area of patients with Meniere's disease when compared to that of patients with other otopathologic conditions-such as presbycusis or ototoxicity. These results may potentially lead to further understanding of different disease states that affect hearing.

Diet-induced obesity exacerbates auditory degeneration via hypoxia, inflammation, and apoptosis signaling pathways in CD/1 mice

The aim of this study was to investigate the mechanisms of diet-induced obesity on hearing degeneration in CD/1 mice. Sixty 4-week-old male CD/1 mice were randomly and equally divided into 2 groups. For 16 weeks, the diet-induced obesity (DIO) group was fed a high fat diet and the control group was fed a standard diet of 13.43 % kcal fat. The morphometry, biochemistry, auditory brainstem response thresholds, omental fat, and histopathology of the cochlea were compared between the beginning and end of the study (4 vs. 20 weeks old). The results show that the body weight, fasting plasma triglyceride concentrations, and omental fat weight were higher in the DIO group than in the control group at the end of experiment. The auditory brainstem response thresholds at high frequencies were significantly elevated in the DIO group compared to those of the control group. Histology studies showed that, compared to the control group, the DIO group had blood vessels with smaller diameters and thicker walls in the stria vascularis at the middle and basal turns of the cochlea. The cell densities in the spiral ganglion and spiral ligament at the basal turn of the cochlea were significantly lower in the DIO group. Immunohistochemical staining showed that hypoxia-induced factor 1 (HIF-1), tumor necrosis factor alpha (TNF-α), nuclear factor kappa B (NF-κB), caspase 3, poly(ADP-ribose) polymerase-1, and apoptosis inducing factor were all significantly more dense in the spiral ganglion and spiral ligament at the basal turn of cochlea in the DIO group. Our results suggest that diet-induced obesity exacerbates hearing degeneration via increased hypoxia, inflammatory responses, and cell loss in the spiral ganglion and spiral ligament and is associated with the activation of both caspase-dependent and -independent apoptosis signaling pathways in CD/1 mice.

Age-related changes of myelin basic protein in mouse and human auditory nerve

Age-related hearing loss (presbyacusis) is the most common type of hearing impairment. One of the most consistent pathological changes seen in presbyacusis is the loss of spiral ganglion neurons (SGNs). Defining the cellular and molecular basis of SGN degeneration in the human inner ear is critical to gaining a better understanding of the pathophysiology of presbyacusis. However, information on age-related cellular and molecular alterations in the human spiral ganglion remains scant, owing to the very limited availably of human specimens suitable for high resolution morphological and molecular analysis. This study aimed at defining age-related alterations in the auditory nerve in human temporal bones and determining if immunostaining for myelin basic protein (MBP) can be used as an alternative approach to electron microscopy for evaluating myelin degeneration. For comparative purposes, we evaluated ultrastructural alternations and changes in MBP immunostaining in aging CBA/CaJ mice. We then examined 13 temporal bones from 10 human donors, including 4 adults aged 38-46 years (middle-aged group) and 6 adults aged 63-91 years (older group). Similar to the mouse, intense immunostaining of MBP was present throughout the auditory nerve of the middle-aged human donors. Significant declines in MBP immunoreactivity and losses of MBP(+) auditory nerve fibers were observed in the spiral ganglia of both the older human and aged mouse ears. This study demonstrates that immunostaining for MBP in combination with confocal microscopy provides a sensitive, reliable, and efficient method for assessing alterations of myelin sheaths in the auditory nerve. The results also suggest that myelin degeneration may play a critical role in the SGN loss and the subsequent decline of the auditory nerve function in presbyacusis.

Subcellular distribution and relative expression of fibrocyte markers in the CD/1 mouse cochlea assessed by semiquantitative immunogold electron microscopy

Spiral ligament fibrocytes function in cochlear homeostasis, maintaining the endocochlear potential by participating in potassium recycling, and fibrocyte degeneration contributes to hearing loss. Their superficial location makes them amenable to replacement by cellular transplantation. Fibrocyte cultures offer one source of transplantable cells, but determining what fibrocyte types they contain and what phenotype transplanted cells may adopt is problematic. Here, we use immunogold electron microscopy to assess the relative expression of markers in native fibrocytes of the CD/1 mouse spiral ligament. Caldesmon and aquaporin 1 are expressed more in type III fibrocytes than any other type. S-100 is strongly expressed in types I, II, and V fibrocytes, and α1Na,K-ATPase is expressed strongly only in types II and V. By combining caldesmon or aquaporin 1 with S-100 and α1Na,K-ATPase, a ratiometric analysis of immunogold density distinguishes all except type II and type V fibrocytes. Other putative markers (creatine kinase BB and connective tissue growth factor) did not provide additional useful analytical attributes. By labeling serial sections or by double or triple labeling with combinations of three antibodies, this technique could be used to distinguish all except type II and type V fibrocytes in culture or after cellular transplantation into the lateral wall.

Physiopathology of the cochlear microcirculation

Normal blood supply to the cochlea is critically important for establishing the endocochlear potential and sustaining production of endolymph. Abnormal cochlear microcirculation has long been considered an etiologic factor in noise-induced hearing loss, age-related hearing loss (presbycusis), sudden hearing loss or vestibular function, and Meniere's disease. Knowledge of the mechanisms underlying the pathophysiology of cochlear microcirculation is of fundamental clinical importance. A better understanding of cochlear blood flow (CoBF) will enable more effective management of hearing disorders resulting from aberrant blood flow. This review focuses on recent discoveries and findings related to the physiopathology of the cochlear microvasculature.

Development of tinnitus in CBA/CaJ mice following sound exposure

Tinnitus, the perception of a sound without an external acoustic source, is a complex perceptual phenomenon affecting the quality of life in 17% of the adult population. Despite its ubiquity and morbidity, the pathophysiology of tinnitus is a work in progress, and there is no generally accepted cure or treatment. Development of a reliable common animal model is crucial for tinnitus research and may advance this field. The goal of this study was to develop a tinnitus mouse model. Tinnitus was induced in an experimental group of mice by an exposure to a loud (116 dB sound pressure level (SPL)) narrow band noise (one octave, centered at 16 kHz) during 1 h under anesthesia. The tinnitus was then assessed behaviorally by measuring gap induced suppression of the acoustic startle reflex. We found that a vast majority of the sound-exposed mice (86%) developed behavioral signs of tinnitus. This was a complex, long lasting, and dynamic process. On the day following exposure, all mice demonstrated signs of acute tinnitus over the entire range of sound frequencies used for testing (10-31 kHz). However, 2-3 months later, a behavioral evidence of tinnitus was evident only at a narrow frequency range (20-31 kHz) representing a presumed chronic condition. Extracellular recordings confirmed a significantly higher rate of spontaneous activity in inferior colliculus neurons in sound-exposed compared to control mice. Surprisingly, unilateral sound exposure suppresses startle responses in mice and they remained suppressed even 3 months post-exposure, whereas auditory brainstem response thresholds were completely recovered during 2 months following exposure. In summary, behavioral evidence of tinnitus can be reliably developed in mice by sound exposure, and tinnitus induction can be assessed by quantifying prepulse inhibition of the acoustic startle reflex.

Fibro-vascular coupling in the control of cochlear blood flow

Background

Transduction of sound in the cochlea is metabolically demanding. The lateral wall and hair cells are critically vulnerable to hypoxia, especially at high sound levels, and tight control over cochlear blood flow (CBF) is a physiological necessity. Yet despite the importance of CBF for hearing, consensus on what mechanisms are involved has not been obtained.

Methodology/Principal Findings

We report on a local control mechanism for regulating inner ear blood flow involving fibrocyte signaling. Fibrocytes in the super-strial region are spatially distributed near pre-capillaries of the spiral ligament of the albino guinea pig cochlear lateral wall, as demonstrably shown in transmission electron microscope and confocal images. Immunohistochemical techniques reveal the inter-connected fibrocytes to be positive for Na+/K+ ATPase β1 and S100. The connected fibrocytes display more Ca²⁺ signaling than other cells in the cochlear lateral wall as indicated by fluorescence of a Ca²⁺ sensor, fluo-4. Elevation of Ca²⁺ in fibrocytes, induced by photolytic uncaging of the divalent ion chelator o-nitrophenyl EGTA, results in propagation of a Ca²⁺ signal to neighboring vascular cells and vasodilation in capillaries. Of more physiological significance, fibrocyte to vascular cell coupled signaling was found to mediate the sound stimulated increase in cochlear blood flow (CBF). Cyclooxygenase-1 (COX-1) was required for capillary dilation.

Conclusions/Significance

The findings provide the first evidence that signaling between fibrocytes and vascular cells modulates CBF and is a key mechanism for meeting the cellular metabolic demand of increased sound activity.

Relative time course of degeneration of different cochlear structures in the CD/1 mouse model of accelerated aging

Presbycusis (age-related hearing loss) can result from various cochlear pathologies. We have studied the time course of degeneration in a mouse that shows accelerated presbycusis, the CD/1 mouse, as a possible model to investigate stem-cell strategies to prevent or ameliorate presbycusic changes. CD/1 mice from 0 to 72 weeks old were examined by light and electron microscopy. Early pathological changes were detected in basal turn spiral ligament fibrocytes and spiral ganglion, but the latter was variable as both satellite cells and neurons were normal in some cochleae. Light microscopic counts in the spiral ligament of 20-week-old mice revealed that of the five main types (types I-V), only type V fibrocytes showed no reduction in numbers compared with 3-week-old animals, and type IV showed the greatest losses. However, all types of fibrocyte showed subtle damage when examined using electron microscopy, in the form of swollen mitochondria, as early as 2 weeks. The extent of mitochondrial damage showed a degree of correspondence with the light microscopic pattern of fibrocyte loss in that types III and IV fibrocytes had the most abnormal mitochondria and type V the least, especially at early stages. By 10-15 weeks, ultrastructural features of fibrocyte damage were similar to longer term changes reported in gerbils. Stria vascularis, spiral ganglion and hair cells showed few consistent early signs of damage but became increasingly affected, lagging behind the fibrocyte damage. Our data suggest that fibrocyte pathology may precede other presbycusic changes; breakdown of homeostatic mechanisms to which they contribute may cause the subsequent degeneration of the hair cells. Overall, there were many similarities to presbycusic changes in other rodents and humans. Therefore, the features of accelerated aging in this mouse make it a suitable model for rapidly assessing possible strategies to prevent or ameliorate presbycusic changes.

Gravity receptor aging in the CBA/CaJ strain: a comparison to auditory aging

The CBA/CaJ mouse strain is commonly used as a control as it has no known genetic mutations affecting the inner ear, maintains hearing sensitivity throughout life, and serves as a background for creating new genetic strains. The purpose of the present study was to characterize the effects of age and gender on gravity receptor function and compare functional changes between auditory and vestibular modalities. Vestibular-evoked potentials (VsEPs), auditory brainstem responses (ABRs), and distortion product otoacoustic emissions were measured in 131 mice. VsEP thresholds deteriorated an average of 0.39 dB re: 1.0 g/ms per month and at the oldest ages (18-23 months old) showed an average loss of 49% of VsEP dynamic range. No significant gender differences were found for VsEPs. ABR thresholds increased by an average of 1.35, 1.38, and 1.15 dB pe SPL per month for ABR stimulus frequencies of 8, 16, and 32 kHz, respectively, demonstrating an average decrease in auditory dynamic range of 25-35% at advanced ages. Both modalities declined with age. Age-related decreases in gravity receptor sensitivity should be considered when using the CBA/CaJ strain for vestibular studies.

Divergent aging characteristics in CBA/J and CBA/CaJ mouse cochleae

Two inbred mouse strains, CBA/J and CBA/CaJ, have been used nearly interchangeably as 'good hearing' standards for research in hearing and deafness. We recently reported, however, that these two strains diverge after 1 year of age, such that CBA/CaJ mice show more rapid elevation of compound action potential (CAP) thresholds at high frequencies (Ohlemiller, Brain Res. 1277: 70-83, 2009). One contributor is progressive decline in endocochlear potential (EP) that appears only in CBA/CaJ. Here, we explore the cellular bases of threshold and EP disparities in old CBA/J and CBA/CaJ mice. Among the major findings, both strains exhibit a characteristic age (∼18 months in CBA/J and 24 months in CBA/CaJ) when females overtake males in sensitivity decline. Strain differences in progression of hearing loss are not due to greater hair cell loss in CBA/CaJ, but instead appear to reflect greater neuronal loss, plus more pronounced changes in the lateral wall, leading to EP decline. While both male and female CBA/CaJ show these pathologies, they are more pronounced in females. A novel feature that differed sharply by strain was moderate loss of outer sulcus cells (or 'root' cells) in spiral ligament of the upper basal turn in old CBA/CaJ mice, giving rise to deep indentations and void spaces in the ligament. We conclude that CBA/CaJ mice differ both quantitatively and qualitatively from CBA/J in age-related cochlear pathology, and model different types of presbycusis.

Chronic reduction of endocochlear potential reduces auditory nerve activity: further confirmation of an animal model of metabolic presbyacusis

Gerbils aged in quiet show a decline of the endocochlear potential (EP) and elevated auditory nerve compound action potential (CAP) thresholds. However, establishing a direct relationship between an age-related reduction in the EP and changes in the activities of primary auditory neurons is difficult owing to the complexity of age-related histological changes in the cochlea. To address this issue, we developed a young gerbil model of "metabolic" presbyacusis that uses an osmotic pump to deliver furosemide into the round window niche for 7 days, resulting in a chronically reduced EP. In this model, the only major histopathologic changes were restricted to the hook region of the cochlea and consisted of loss of strial intermediate cells and massive edema in the lateral wall. The morphological and physiological evidence suggests that the cochlea can adapt to furosemide application over time. The morphology of spiral ganglion cells and hair cells appeared normal throughout the cochlea. CAP responses and EP values in this model are similar to those of quiet-aged ears. The spontaneous activity of single auditory fibers (n = 188) was assessed in 15 young gerbils treated with furosemide for 7 days. The percentage of recorded low-spontaneous rate (SR) fibers at characteristic frequencies (CFs) > or = 6 kHz was significantly lower in furosemide-treated than in control ears. Recovery function tests of CAP responses after prior stimulation also showed a decline in activity of the low-SR population with CFs > or = 6 kHz in the treated cochleas. A similar loss in the activity of low-SR fiber has been previously shown in quiet-aged gerbils. These results suggest that dysfunction of the cochlear lateral wall and subsequent chronic reduction in the EP can directly affect the activity patterns of primary auditory neurons in a manner similar to that seen in aged gerbils.

Quantitative analysis of the expression of the glutamate-aspartate transporter and identification of functional glutamate uptake reveal a role for cochlear fibrocytes in glutamate homeostasis

There are several subtypes of fibrocyte in the spiral ligament and spiral limbus of the cochlea that may contribute to fluid homeostasis. Immunocytochemical data suggest that these fibrocytes possess the glutamate-aspartate transporter, GLAST, as do supporting cells around the hair cells. However, functional glutamate uptake has not been demonstrated in fibrocytes. We used confocal and post-embedding immunogold electron microscopy to confirm that GLAST is expressed in adult fibrocytes of CD-1 mice with a relative expression: spiral limbus fibrocytes>type II>V>IV>I spiral ligament fibrocytes. Because they were sparsely present in most samples, type III fibrocytes were assessed only in one sample where their GLAST levels were similar to type I. Type II, type V and spiral limbus fibrocytes have many fine cellular processes that increase their surface area, those of the latter two coming into direct contact with perilymph, and type V fibrocytes contain the most glutamate. These data imply that glutamate uptake occurs in the fibrocytes. We assessed uptake of D-aspartate (a glutamate analogue) together with GLAST expression immunocytochemically and electrophysiologically. D-aspartate accumulated into GLAST expressing fibrocytes in vitro and evoked currents blockable by the GLAST inhibitor D,L-threo-beta-benzyloxyaspartate (TBOA), similar to those of supporting cells around inner hair cells. Currents were strongest in spiral limbus fibrocytes, progressively lower in type V and type II fibrocytes, and were negligible in type I fibrocytes in accordance with the relative expression levels of GLAST. We conclude that in addition to their known homeostatic functions, fibrocytes, in particular spiral limbus, type II and type V fibrocytes play a role in glutamate homeostasis in the cochlea.

Mechanisms and genes in human strial presbycusis from animal models

Schuknecht proposed a discrete form of presbycusis in which hearing loss results principally from degeneration of cochlear stria vascularis and decline of the endocochlear potential (EP). This form was asserted to be genetically linked, and to arise independently from age-related pathology of either the organ of Corti or cochlear neurons. Although extensive strial degeneration in humans coincides with hearing loss, EPs have never been measured in humans, and age-related EP reduction has never been verified. No human genes that promote strial presbycusis have been identified, nor is its pathophysiology well understood. Effective application of animal models to this issue requires models demonstrating EP decline, and preferably, genetically distinct strains that vary in patterns of EP decline and its cellular correlates. Until recently, only two models, Mongolian gerbils and Tyrp1(B-lt) mice, were known to undergo age-associated EP reduction. Detailed studies of seven inbred mouse strains have now revealed three strains (C57BL/6J, B6.CAST-Cdh23(CAST), CBA/J) showing essentially no EP decline with age, and four strains ranging from modest to severe EP reduction (C57BL/6-Tyr(c-2J), BALB/cJ, CBA/CaJ, NOD.NON-H2(nbl)/LtJ). Collectively, animal models support five basic principles regarding a strial form of presbycusis: 1) Progressive EP decline from initially normal levels as a defining characteristic; 2) Non-universality, not all age-associated hearing loss involves EP decline; 3) A clear genetic basis; 4) Modulation by environment or stochastic events; and 5) Independent strial, organ of Corti, and neural pathology. Shared features between human strial presbycusis, gerbils, and BALB/cJ and C57BL/6-Tyr(c-2J) mice further suggest this condition frequently begins with strial marginal cell dysfunction and loss. By contrast, NOD.NON-H2(nbl) mice may model a sequence more closely associated with strial microvascular disease. Additional studies of these and other inbred mouse and rat models should reveal candidate processes and genes that promote EP decline in humans.

Age-related auditory pathology in the CBA/J mouse

Commercially obtained aged male CBA/J mice presented a complex pattern of hearing loss and morphological changes. A significant threshold shift in auditory brainstem responses (ABR) occurred at 3 months of age at 4 kHz without apparent loss of hair cells, rising slowly at later ages accompanied by loss of apical hair cells. A delayed high-frequency deficit started at 24 kHz around the age of 12 months. At 20-26 months, threshold shifts at 12 and 24 kHz and the accompanying hair cell loss at the base of the cochlea were highly variable with some animals appearing almost normal and others showing large deficits. Spiral ganglion cells degenerated by 18 months in all regions of the cochlea, with cell density reduced by approximately 25%. There was no degeneration of the stria vascularis and the endocochlear potential remained stable from 3 to 25 months of age regardless of whether the animals had normal or highly elevated ABR thresholds. The slow high-frequency hearing loss combined with a modest reduction of ganglion cell density and an unchanged endocochlear potential suggest sensorineural presbycusis. The superimposed early hearing loss at low frequencies, which is not seen in animals bred in-house, may complicate the use of these animals as a presbycusis model.

Mesenchymal stem cell transplantation accelerates hearing recovery through the repair of injured cochlear fibrocytes

Cochlear fibrocytes play important roles in normal hearing as well as in several types of sensorineural hearing loss attributable to inner ear homeostasis disorders. Recently, we developed a novel rat model of acute sensorineural hearing loss attributable to fibrocyte dysfunction induced by a mitochondrial toxin. In this model, we demonstrate active regeneration of the cochlear fibrocytes after severe focal apoptosis without any changes in the organ of Corti. To rescue the residual hearing loss, we transplanted mesenchymal stem cells into the lateral semicircular canal; a number of these stem cells were then detected in the injured area in the lateral wall. Rats with transplanted mesenchymal stem cells in the lateral wall demonstrated a significantly higher hearing recovery ratio than controls. The mesenchymal stem cells in the lateral wall also showed connexin 26 and connexin 30 immunostaining reminiscent of gap junctions between neighboring cells. These results indicate that reorganization of the cochlear fibrocytes leads to hearing recovery after acute sensorineural hearing loss in this model and suggest that mesenchymal stem cell transplantation into the inner ear may be a promising therapy for patients with sensorineural hearing loss attributable to degeneration of cochlear fibrocytes.

EphB2 and ephrin-B2 regulate the ionic homeostasis of vestibular endolymph

The ability to transport cations and anions across epithelia is critical for the regulation of pH, ionic homeostasis, and volume of extracellular fluids. Although the transporters and channels that facilitate ion and water movement across cell membranes are well known, the molecular mechanisms and signal transduction events that regulate these activities remain poorly understood. The Eph family of receptor tyrosine kinases and their membrane-anchored ephrin ligands are well known to transduce bidirectional signals that control axon guidance and other cell migration/adhesion events during development. However, these molecules are also expressed in non-motile epithelial cells, including EphB2 in K(+)-secreting vestibular dark cells and ephrin-B2 in the adjacent transitional cells of the inner ear. Consistent with these expression patterns, mice with cytoplasmic domain mutations that interfere with EphB2 forward signaling or ephrin-B2 reverse signaling exhibit a hyperactive circling (waltzing) locomotion associated with a decreased amount of endolymph fluid that normally fills the vestibular labyrinth. Endolymph is unusual as an extracellular fluid in that it is normally high in K(+) and low in Na(+). Direct measurement of this fluid in live animals revealed significant decreases in K(+) concentration and endolymphatic potential in both EphB2 and ephrin-B2 mutant mice. Our findings provide evidence that bidirectional signaling mediated by B-subclass Ephs and ephrins controls the production and ionic homeostasis of endolymph fluid and thereby provide the first evidence that these molecules can control the activities of mature epithelial cells.

Age-related hearing loss in CD/1 mice is associated to ROS formation and HIF target proteins up-regulation in the cochlea

Pathologies of senescence, in particular those of neurosensory organs represent an important health problem. The improvement of the life expectation entails the fast increase of the frequency of the age-related hearing loss (ARHL) in the population. There are numerous factors that contribute to this process, which include altered vascular characteristics, hypoxia/ischemia, genetic mutations and production of reactive oxygen species. We were interested in understanding the mechanisms involved in the cochlear degeneration in a mouse model of ARHL, the cd/1 mice. Since in human, hypoxia/ischemia is an important pathogenetic factor for inner ear disease, the regulation of HIF-1 activity in the cochlea, the presence of radical oxygen species in the cochlea and its subsequent disturbances of cellular signaling cascades were investigated. In this study, we explored auditory function of cd/1 mice at the age of 4, 12 and 24 weeks and correlated it with the presence of oxidative damage in the cochlea, and cochlear HIF-1 responsive target genes regulation, involved in pathways promoting inflammation such as tumor necrosis factor (TNF-alpha), or cell death with the p53 protein, Bax protein and surviving factors with insulin-like growth factor-1 (IGF-1). After implantation of electrodes for auditory nerve acoustic thresholds measurements, we analyzed every cochlea. First, we confirmed that the cd/1 mice presented a characteristic profile of ARHL starting at 12 weeks of age. Then, according to our previous report [Riva, C., Longuet, M., Lucciano, M., Magnan, J., Lavieille, J.P., 2005. Implication of mitochondrial apoptosis in neural degeneration in a murin model for presbyacusis. Rev. Laryngol. Otol. Rhinol. 126 (2), 67-74], we noticed many alterations in the cochlea. Histologically, at 4 weeks, intensive HIF-1alpha expression was detected in the cochlea followed by ROS formation at 12 weeks, which may lead to cochlear degeneration and induction the onset of ARHL in the cd/1 mice model. In the cochlea, while the inner and the outer hair cells remained intact at 4 and 12 weeks, the spiral ganglion was more altered. Moreover, the Schwann cells of the spiral ganglion seemed to be more vulnerable to free radical damage than the neurons and degenerated more rapidly. The mechanisms of degeneration in the spiral ganglion involved a caspase-3 and Bax mediated-apoptosis via p53 protein accumulation. Since oxygen radicals are required for the post-translational stabilization of HIF-1alpha during hypoxia, the tandem " HIF-ROS " induced multiple reactions within the cochlea, like a strong inflammatory response with increased expression of TNF-alpha, and inhibition of neuronal protection mechanisms with repression of IGF-1.

Cellular correlates of age-related endocochlear potential reduction in a mouse model

Age-related degeneration of cochlear stria vascularis and resulting reduction in the endocochlear potential (EP) are the hallmark features of strial presbycusis, one of the major forms of presbycusis, or age-related hearing loss (ARHL) (Schuknecht, H.F., 1964. Further observations on the pathology of presbycusis. Archives of Otolaryngology 80, 369-382; Schuknecht, H.F., 1993. Pathology of the Ear. Lea and Febiger, Philadelphia; Schuknecht, H.F., Gacek, M.R., 1993. Cochlear pathology in presbycusis. Annals of Otology, Rhinology and Laryngology 102, 1-16). It is unclear whether there are multiple forms of strial ARHL having different sequences of degenerative events and different risk factors. Human temporal bone studies suggest that the initial pathology usually affects strial marginal cells, then spreads to other strial cell types. While inheritance studies support a moderate genetic influence, no contributing genes have been identified. Establishment of mouse models of strial ARHL may promote the identification of underlying genes and gene/environment interactions. We have found that BALB/cJ mice show significant EP reduction by 19 months of age. The reduction only occurs in a subset of animals. To identify key anatomical correlates of the EP reduction, we compared several cochlear lateral wall metrics in BALBs with those in C57BL/6J (B6) mice, which show little EP reduction for ages up to 26 months. Among the measures obtained, marginal cell density and spiral ligament thickness were the best predictors of both the EP decline in BALBs, and EP stability in B6. Our results indicate that the sequence of strial degeneration in BALBs is like that suggested for humans. Additional strain comparisons we have performed suggest that genes governing strial melanin production do not play a role.

Contributions of mouse models to understanding of age- and noise-related hearing loss

Once an oddity, mice have become the most widely used hearing research model. Their value for research in noise-induced hearing loss (NIHL) stems from their high vulnerability to noise and reduced variance of results, made possible by genetic standardization. To research in age-related hearing loss (ARHL), they offer economies of small size and a short lifespan, both of which reduce housing costs. Inbred mouse strains show a wide range of noise sensitivities and rates of hearing loss with age. These can be studied using classical genetic analysis, as well as hypothesis-driven experiments utilizing genetic engineering. Through such investigations, presently 3 loci have been identified to date that contribute to NIHL, 10 that promote ARHL, and at least 6 loci that promote both. The types of genes involved implicate homeostatic and protective mechanisms as key to the appearance of either type of pathology and support a causal link between injury and some apparent ARHL. While the majority of mouse ARHL models examined most closely resemble sensory ARHL, recent work has identified mice possessing the essential characteristics of neural and strial ARHL. Using these models, it should be possible to identify genes and alleles that promote the major forms of ARHL and their combinations.

Pathologic changes of presbycusis begin in secondary processes and spread to primary processes of strial marginal cells

Strial atrophy underlying age-related hearing loss was investigated by ultrastructural comparisons in young and senescent gerbils. In young animals strial marginal cells (MCs) projected primary processes which gave rise to and were connected by numerous ultrathin secondary processes. In 30-36-month-old gerbils, the MC secondary processes degenerated into lamellar or amorphous profiles as the first manifestation of strial atrophy. Some short primary processes shorn of projecting and connecting secondaries coalesced to form mitochondria-filled lobules. Strial involution appeared to progress with transformation of the degenerating processes and lobules into permanent residues of laminated amorphous substance. A second apparently unique form of degeneration was observed in which areas filled with homogeneous granular material replaced the processes that comprise the basal half of the normal MC. An abrupt line of transition separated this structureless degradation product below from the viable upper half of the MC. The terminally involuted stria consisted of MC bodies lining scala media, along with vestigial remnants of MC processes, nearby normal appearing intermediate cells (ICs) and unaltered basal cells. The only age-related change in ICs involved incorporation of melanosomes into very large, matrix-filled lysosomes. A profile of one MC in apparent necrosis provided evidence for an infrequent occurrence of MC death. These data support a progression of pathologic changes beginning with the demise of MC secondary processes and ending with ablation of secondary and primary processes. The initial injury apparently occurs as a result of oxidative self-damage to mitochondria in the MCs primary processes, leading to insufficient ATP for the Na,K-ATPase of the secondary processes. The reduced ATP level may cause cytotoxic alteration of the cytosolic Na(+)/K(+) ratio first in MC secondary processes and later in the primaries, with consequent degeneration of these structures.

Age-related changes in cochlear and brainstem auditory functions in Fischer 344 rats

Auditory function in Fischer 344 (F344) and Long Evans (LE) rats was monitored during their lifespan by evaluating hair cell loss, middle-ear compliance and the recording of otoacoustic emissions and auditory brainstem responses. The results revealed a faster deterioration of hearing function in F344 rats compared with LE rats, resulting in larger hearing threshold shifts, a decrease in the latency and amplitude of click-evoked auditory brainstem responses, diminution of the distortion product otoacoustic emissions and a decrease in middle-ear compliance. However, hair cell loss, observed only at the most basal and apical parts of the organ of Corti, was comparable in older individuals of both rat strains. The results suggest involvement of cochlear (stria vascularis) and extracochlear (middle-ear) pathological changes during ageing. Thus, F344 rats represent a complex mix of conductive hearing loss (with low-frequency threshold shift, declining parameters of the middle-ear admittance and asymmetric otoacoustic emissions) and sensorineural hearing loss (with a decrease in the amplitudes of auditory brainstem response and a high-frequency threshold shift).

Time course of auditory impairment in mice lacking the electroneutral sodium bicarbonate cotransporter NBC3 (slc4a7)

Mice with a targeted disruption of the gene encoding the stilbene-insensitive electroneutral sodium bicarbonate cotransporter (NBC3; slc4a7) exhibit cochlear and retinal degeneration. To establish the progressive nature of sensory cells loss in slc4a7-/- deficient mice, we studied the morphology of cochleas of slc4a7-/- and slc4a7+/+ mice from postnatal day two (P2) to ninety (P90). Cell death was evaluated in slc4a7-/- cochleas using the TUNEL technique and caspase-3 immunoreactivity. The time course of NBC3 expression in the cochlea was assessed by immunohistochemistry using an antibody against NBC3. Between P2 and P8, slc4a7-/- mice cochlea exhibit normal morphology. There was a normal complement of inner and outer hair cells from the hook to the apical region. At P15, slc4a7-/- mice cochlea inner and outer hair cells were still present at the hook region, and vacuoles were seen underneath Hensen's cells. At P21, inner and outer hair cells were degenerated in this region. Between P30 and P90, there was a pronounced loss of hair cells and spiral ganglia neurons. Morphological analysis of the spiral ligament showed a progressive loss of type II and IV fibrocytes beginning at day 21. Transmission electron microscopy observations at P30 and P90 revealed that type II and IV fibrocytes showed shrinkage and vacuolization. In addition, hair cells were deteriorated with evidence of shrinkage and picnotic nuclei. TUNEL staining showed apoptotic cells at P8 in the organ of Corti at the basal region of the cochlea. At P15, caspase-3 immunoreactivity was present in supporting cells of the organ of Corti. NBC3 mild immunoreactivity was detected in the organ of Corti at P11. There was an increase in the expression of NBC3 in the spiral ligament between P17 and P19. From P21 to P90, NBC3 expression was confined to the spiral ligament and inner and outer sulcus cells. The vestibular sensory epithelia from slc4a7-/- mice were normal from P2 to P90. Damage of the sensory epithelia at the high frequency zone of the cochlea suggests that NBC3 may play an important physiological role in this region.

Paget disease and sensorineural hearing loss associated with spiral ligament degeneration

HYPOTHESIS

Previously unreported cystic degeneration of the spiral ligament in cases of Paget disease.

BACKGROUND

About 70% of cases of Paget disease involve the skull, with hearing affected in approximately 50% of these. The hearing impairment may be sensorineural, mixed, or, rarely, only conductive. The etiology and pathogenesis of the hearing loss are not yet understood, and reports in the literature are inconsistent regarding the pathologic changes responsible for sensorineural hearing loss. Of six pairs of temporal bones from patients with Paget disease in the temporal bone collection of a research institution, two pairs have abnormalities not previously associated with sensorineural hearing loss or Paget disease. We report the histopathologic findings in these temporal bones.

METHODS

The temporal bones were fixed in formalin, decalcified in ethylenediaminetetraacetic acid, embedded in celloidin, and sectioned in the horizontal plane at a thickness of 20 microm. Every 10th section was stained with hematoxylin-eosin and mounted on glass slides. The sections were examined by light microscopy.

RESULTS

Cystic degeneration of the spiral ligament, primarily in the basal segment, was found in both cases. Endolymphatic hydrops and a small endolymphatic sac with calcification of the perisaccular tissue were found in one case.

CONCLUSIONS

Cystic degeneration of the spiral ligament has not been previously reported and may be unique to Paget disease. This is consistent with recent literature showing a previously unsuspected role of the spiral ligament in sensorineural hearing loss.

Deafness and cochlear fibrocyte alterations in mice deficient for the inner ear protein otospiralin

In the cochlea, the mammalian auditory organ, fibrocytes of the mesenchymal nonsensory regions play important roles in cochlear physiology, including the maintenance of ionic and hydric components in the endolymph. Occurrence of human deafness in fibrocyte alterations underlines their critical roles in auditory function. We recently described a novel gene, Otos, which encodes otospiralin, a small protein of unknown function that is produced by the fibrocytes of the cochlea and vestibule. We now have generated mice with deletion of Otos and found that they show moderate deafness, with no frequency predominance. Histopathology revealed a degeneration of type II and IV fibrocytes, while hair cells and stria vascularis appeared normal. Together, these findings suggest that impairment of fibrocytes caused by the loss in otospiralin leads to abnormal cochlear physiology and auditory function. This moderate dysfunction may predispose to age-related hearing loss.

Apical-to-basal gradients in age-related cochlear degeneration and their relationship to "primary" loss of cochlear neurons

The predominant conceptual framework for understanding human age-related hearing loss (ARHL, or presbycusis) holds that three different cochlear elements (organ of Corti, afferent neurons, and stria vascularis) can degenerate independently, and exert independent influences on hearing. Within this framework, temporal bones from subjects with ARHL may be classified as exemplifying sensory (referring to organ of Corti), "primary" neural (loss of afferent neurons without loss of their hair cell targets), strial, or mixed ARHL. While there is general agreement as to the types of cochlear cells most affected by aging, there is less agreement about how to classify ARHL, and whether contributions of particular structures to hearing loss can be isolated. The cochlear apex of humans and animals is particularly prone to apparent primary loss of neurons that may represent an aspect of neural ARHL. We recently reported that in 129S6/SvEv mice apical neuronal loss is often accompanied by abnormalities of spiral limbus, pillar cells, and Reissner's membrane (Ohlemiller and Gagnon [2004] J Comp Neurol 469:377-390). We proposed that the initial pathology occurs within limbus, leading to disruption of perilymphatic ion homeostasis, and eventual loss of neurons as one consequence. We have now examined this issue quantitatively in young and old mice of four different strains (129S6/SvEv, CBA/J, C57BL/6, and BALB/c). Abnormalities of apical spiral limbus were found to correlate only weakly with neuronal loss. Strong correlations were found between neuronal loss and abnormalities of both pillar cells and Reissner's membrane, however. Apical neuronal loss and apical-to-basal progression of pathology of limbus, pillar cells, and Reissner's membrane run counter to most reported age-related cochlear trends. Our findings suggest that these changes share a common triggering influence.

Targeting hearing genes in mice

The rate of identification of genes for hearing has clearly outpaced the rate of determination of the functions of these genes' products. The use of transgenic and knock-out mouse models is a powerful approach to the elucidation of gene function in the ear. A large number of gene-targeted mice with auditory defects have recently been created and characterized, and nine independent mouse lines in which Cre recombinase activity begins to be expressed during early embryonic development of the ear or is specifically expressed in hair cells during postnatal development will be useful for ear-specific gene manipulation when combined with mouse lines that have loxP sites flanking the genes of interest. Existing gene-trapped embryonic stem (ES) cells and existing targeting constructs are readily available; new targeting constructs can easily be created by modifying bacterial artificial chromosomes and using them to directly transfect and screen ES cells; and N-ethyl-N-nitrosourea mutagenesis of ES cells can create point mutations in specific genes. To minimize variation in hearing phenotypes and avoid undesired hearing defects, mutant mice in the common gene-targeting background strains (129 and C57BL/6) should be transferred into congenic CBA/CaJ, a strain with "gold standard" normal hearing. Valuable mutant strains can be maintained, distributed, and cryopreserved in one of four NIH-sponsored Mutant Mouse Regional Resource Centers. Targeting hearing genes in mice will provide unprecedented opportunities for collaboration and new directions in the hearing research community.

Age-related hearing loss: the status of Schuknecht’s typology

PURPOSE OF REVIEW

Recent developments in age-related hearing loss (ARHL) are reviewed with an emphasis on their relation to the framework advocated by Schuknecht. More than a classification scheme, Schuknecht's typology incorporates testable hypotheses about the bases of ARHL. Since there is presently no widely accepted competing framework, research in this area should be aimed at supporting, modifying, or replacing Schuknecht scheme. Only recently has our understanding of cellular changes and gene/environment interactions in ARHL achieved the level needed for hypothesis-driven experiments in this area.

RECENT FINDINGS

New findings largely support or amplify aspects of Schuknecht's framework. Consideration of the kinds of cells involved in ARHL has broadened to include more nonsensory and supporting cells. This should provide more complete criteria for comparing models, and for diagnosing particular forms of ARHL. Newly discovered genetic effects and more detailed comparisons have imparted mechanistic significance to the often-noted similarity between sensory ARHL and noise injury. Recent comparative studies, and studies of cell replacement in the cochlear lateral wall, suggest variations in the relation between strial and ligament pathology, and indicate why cell loss occurs during aging. Mouse models carrying mutations affecting processes that may give rise to ARHL are receiving increased attention, even as detailed studies bolster support for mice as valid ARHL models.

SUMMARY

Using Schuknecht's framework as a guide, basic research can now seek to model specific forms of ARHL by combining genetic defects and appropriate environmental conditions. Identification of distinct risk factors for age-related degeneration of organ of Corti, afferent neurons, and stria would verify a key tenet of Schuknecht's scheme, and point the way to interventions.

Cellular correlates of progressive hearing loss in 129S6/SvEv mice

Several strains of mice hear well initially but show progressive sensorineural hearing loss. Affected cochlear cell types include all those known to be affected in human age-related hearing loss (ARHL), or presbycusis. Thus these mice have been offered as models of human ARHL. At present, however, few mouse ARHL models are sufficiently well described to serve as the basis for specific hypotheses about human ARHL. We examined 1-month-old and 15-month-old 129S6/SvEv (129S6) mice and compared them with BALB/cJ and CBA/J mice. Age-related elevation of compound action potential thresholds was interpreted in the light of endocochlear potentials and changes in hair cells, afferent neurons, fibrocytes in spiral limbus and ligament, and supporting cells within the organ of Corti. Aging in 129S6 mice was associated with high-frequency hearing loss. Four components of age-related cochlear degeneration emerged from quantitative analyses, including 1) basal loss of outer hair cells; 2) basal loss of type IV fibrocytes in the spiral ligament; 3) apical loss of fibrocytes in spiral limbus, and 4) anomalies of supporting cells in the cochlear base. Although neuronal loss was not consistently found, two mice showed loss of afferent dendrites and cell bodies in the cochlear apex without inner hair cell loss. Despite multifaceted degeneration, hearing loss in 129S6 mice appears to be best explained by degenerative changes in outer hair cells and in the organ of Corti, conforming to human sensory ARHL. Age-related changes in the apical spiral limbus may promote pathology of the medial organ of Corti and eventual loss of afferent neurons, with possible implications for human neural ARHL.

Exposure of mice to a predator odor increases acoustic startle but does not disrupt the rewarding properties of VTA intracranial self-stimulation

The present investigation assessed the propensity of an acute psychogenic stressor exposure to induce behavioral change in paradigms assessing fear/anxiety (acoustic startle) and motivation/anhedonia (intracranial self-stimulation) in CD-1 mice. In the acoustic startle paradigm, a 10-min exposure of 2-4 month old mice (young adult mice) to fox odor (2,5-dihydro-2,4,5-trimethylthiazoline; TMT) was associated with decreased acoustic startle relative to mice exposed to the control odor, butyric acid (BA), immediately and relative to both saline and BA exposure 24 h following odor exposure in the home cage. In contrast, a 2-min exposure of young adult mice to TMT was associated with an increase in startle relative to saline and BA during the immediate post-odor test session only. In young adult mice a 2-min and a 10-min exposure to BA resulted in a startle profile of mice reminiscent of saline-treated mice. In comparison to young adult mice, a 2-min exposure of mature adult mice (5-7 months old) to TMT enhanced startle for up to 48 h relative to both saline and BA, while a 10-min exposure of mature adult mice to TMT enhanced startle for 168 h post-odor exposure relative to saline-exposed mice only. However, the greatest increase in startle amplitude (i.e. 48 h) was acquired following the 2-min exposure of mature mice to TMT. Among mature adult mice, a 10-min exposure to BA in the home cage eventuated in enhanced startle relative to saline-exposed animals 168 h following odor exposure. In comparison, exposure of mice to 10 min of TMT depressed responding for VTA brain stimulation at the initial 80 Hz frequency, but was ineffective in elevating reward thresholds relative to mice merely exposed to saline. Mice assessed in the ICSS paradigm were approximately 2-4 months old at the time of surgery and 5-7 months old at the completion of testing. These data suggest that acute odor exposure may induce a fear gradient dependent upon the perceived stressor severity and that the resultant anxiety-like effects are dependent on the duration of odor exposure, age of the animals and the temporal interval between odor presentation and behavioral testing. Moreover, the anxiogenic properties of psychogenic stressors can be separated from their anhedonic effects. The implications of these data for clinical psychopathology are discussed.