Adding insult to injury: cochlear nerve degeneration after "temporary" noise-induced hearing loss

Overexposure to intense sound can cause temporary or permanent hearing loss. Postexposure recovery of threshold sensitivity has been assumed to indicate reversal of damage to delicate mechano-sensory and neural structures of the inner ear and no persistent or delayed consequences for auditory function. Here, we show, using cochlear functional assays and confocal imaging of the inner ear in mouse, that acoustic overexposures causing moderate, but completely reversible, threshold elevation leave cochlear sensory cells intact, but cause acute loss of afferent nerve terminals and delayed degeneration of the cochlear nerve. Results suggest that noise-induced damage to the ear has progressive consequences that are considerably more widespread than are revealed by conventional threshold testing. This primary neurodegeneration should add to difficulties hearing in noisy environments, and could contribute to tinnitus, hyperacusis, and other perceptual anomalies commonly associated with inner ear damage.

Sox2 is required for sensory organ development in the mammalian inner ear

Sensory hair cells and their associated non-sensory supporting cells in the inner ear are fundamental for hearing and balance. They arise from a common progenitor, but little is known about the molecular events specifying this cell lineage. We recently identified two allelic mouse mutants, light coat and circling (Lcc) and yellow submarine (Ysb), that show hearing and balance impairment. Lcc/Lcc mice are completely deaf, whereas Ysb/Ysb mice are severely hearing impaired. We report here that inner ears of Lcc/Lcc mice fail to establish a prosensory domain and neither hair cells nor supporting cells differentiate, resulting in a severe inner ear malformation, whereas the sensory epithelium of Ysb/Ysb mice shows abnormal development with disorganized and fewer hair cells. These phenotypes are due to the absence (in Lcc mutants) or reduced expression (in Ysb mutants) of the transcription factor SOX2, specifically within the developing inner ear. SOX2 continues to be expressed in the inner ears of mice lacking Math1 (also known as Atoh1 and HATH1), a gene essential for hair cell differentiation, whereas Math1 expression is absent in Lcc mutants, suggesting that Sox2 acts upstream of Math1.

Pathophysiology of Meniere's syndrome: are symptoms caused by endolymphatic hydrops?

BACKGROUND

The association of Meniere's syndrome with endolymphatic hydrops has led to the formation of a central hypothesis: many possible etiologic factors lead to hydrops, and hydrops in turn generates the symptoms. However, this hypothesis of hydrops as being the final common pathway has not been proven conclusively.

SPECIFIC AIM

To examine human temporal bones with respect to the role of hydrops in causing symptoms in Meniere's syndrome. If the central hypothesis were true, every case of Meniere's syndrome should have hydrops and every case of hydrops should show the typical symptoms.

METHODS

Review of archival temporal bone cases with a clinical diagnosis of Meniere's syndrome (28 cases) or a histopathologic diagnosis of hydrops (79 cases).

RESULTS

All 28 cases with classical symptoms of Meniere's syndrome showed hydrops in at least one ear. However, the reverse was not true. There were 9 cases with idiopathic hydrops and 10 cases with secondary hydrops, but the patients did not exhibit the classic symptoms of Meniere's syndrome. A review of the literature revealed cases with asymptomatic hydrops (similar to the current study), as well as cases where symptoms of Meniere's syndrome existed during life but no hydrops was observed on histology. We also review recent experimental data where obstruction of the endolymphatic duct in guinea pigs resulted in cytochemical abnormalities within fibrocytes of the spiral ligament before development of hydrops. This result is consistent with the hypothesis that hydrops resulted from disordered fluid homeostasis caused by disruption of regulatory elements within the spiral ligament.

CONCLUSION

Endolymphatic hydrops should be considered as a histologic marker for Meniere's syndrome rather than being directly responsible for its symptoms.

Mice homozygous for a targeted disruption of the proto-oncogene int-2 have developmental defects in the tail and inner ear

We derived mice that carry a targeted insertion of a neor gene in the int-2 (Fgf-3) proto-oncogene coding sequences. The mutation was found to be recessive and mice that were homozygous for the insertion did not often survive to adulthood. The mutant mice had defects in the development of the tail and inner ear that could be correlated with disruption of int-2 expression in the posterior primitive streak and hindbrain or otic vesicle. While the tail phenotype was 100% penetrant, we found that the inner ear phenotype had reduced penetrance and variable expressivity. The variable expressivity could not be attributed to variability in the genetic background of the mutant allele or to leaky expression from the mutant allele. Thus, we conclude that even in a uniform genetic background, stochastic variation in the expression of a developmental circuit can result in dramatic differences in phenotypic consequences.

Inner ear defects induced by null mutation of the isk gene

The isk gene is expressed in many tissues. Pharmacological evidence from the inner ear suggests that isk mediates potassium secretion into the endolymph. To examine the consequences of IsK null mutation on inner ear function, and to produce a system useful for examining the role(s) IsK plays elsewhere, we have produced a mouse strain that carries a disrupted isk locus. Knockout mice exhibit classic shaker/waltzer behavior. Hair cells degenerate, but those of different inner ear organs degenerate at different times. Functionally, we show that in mice lacking isk, the strial marginal cells and the vestibular dark cells of the inner ear are unable to generate an equivalent short circuit current in vitro, indicating a lack of transepithelial potassium secretion.

Mutations in the gene encoding pejvakin, a newly identified protein of the afferent auditory pathway, cause DFNB59 auditory neuropathy

Auditory neuropathy is a particular type of hearing impairment in which neural transmission of the auditory signal is impaired, while cochlear outer hair cells remain functional. Here we report on DFNB59, a newly identified gene on chromosome 2q31.1-q31.3 mutated in four families segregating autosomal recessive auditory neuropathy. DFNB59 encodes pejvakin, a 352-residue protein. Pejvakin is a paralog of DFNA5, a protein of unknown function also involved in deafness. By immunohistofluorescence, pejvakin is detected in the cell bodies of neurons of the afferent auditory pathway. Furthermore, Dfnb59 knock-in mice, homozygous for the R183W variant identified in one DFNB59 family, show abnormal auditory brainstem responses indicative of neuronal dysfunction along the auditory pathway. Unlike previously described sensorineural deafness genes, all of which underlie cochlear cell pathologies, DFNB59 is the first human gene implicated in nonsyndromic deafness due to a neuronal defect.

Targeted disruption of the Kvlqt1 gene causes deafness and gastric hyperplasia in mice

The KvLQT1 gene encodes a voltage-gated potassium channel. Mutations in KvLQT1 underlie the dominantly transmitted Ward-Romano long QT syndrome, which causes cardiac arrhythmia, and the recessively transmitted Jervell and Lange-Nielsen syndrome, which causes both cardiac arrhythmia and congenital deafness. KvLQT1 is also disrupted by balanced germline chromosomal rearrangements in patients with Beckwith-Wiedemann syndrome (BWS), which causes prenatal overgrowth and cancer. Because of the diverse human disorders and organ systems affected by this gene, we developed an animal model by inactivating the murine Kvlqt1. No electrocardiographic abnormalities were observed. However, homozygous mice exhibited complete deafness, as well as circular movement and repetitive falling, suggesting imbalance. Histochemical study revealed severe anatomic disruption of the cochlear and vestibular end organs, suggesting that Kvlqt1 is essential for normal development of the inner ear. Surprisingly, homozygous mice also displayed threefold enlargement by weight of the stomach resulting from mucous neck cell hyperplasia. Finally, there were no features of BWS, suggesting that Kvlqt1 is not responsible for BWS.

Kinetics of gentamicin uptake and release in the rat. Comparison of inner ear tissues and fluids with other organs

The kinetics of entry and release of gentamicin was investigated in fluids and tissues of the inner ear of the rat, as well as in renal cortex, and in organs that do not share susceptibility to the toxic effects of aminoglycosides. Various modes of administration were used to achieve different patterns of drug plasma concentrations. Electrophysiological and histological examinations were performed to correlate pharmacokinetics and ototoxicity. Results show that: the uptake of the drug by the inner ear tissues is dose dependent and manifests a rapid saturation kinetics with a concentration plateau of about 1 micrograms/mg of protein. The low ratio of the perilymph and endolymph to plasma concentrations argues against the concept of an accumulation of the drug in the inner ear over drug levels in plasma, which has been considered as the basic mechanism of ototoxicity. In renal cortex, the kinetics appears similar to that of the inner ear but the concentrations achieved are 10-fold higher than in cochlear tissues. In other organs (liver, heart, lung, and spleen), no saturation could be demonstrated within the duration of the experiment. Ototoxicity seems to be related to the penetration of the drug into compartment(s) from which the half-life of disappearance is extremely slow. Rapid uptake, early saturation, and long exposure of the tissues to the drug may account for the development of toxicity in inner ear and kidney.

Eya1 and Six1 are essential for early steps of sensory neurogenesis in mammalian cranial placodes

Eya1 encodes a transcriptional co-activator and is expressed in cranial sensory placodes. It interacts with and functions upstream of the homeobox gene Six1 during otic placodal development. Here, we have examined their role in cranial sensory neurogenesis. Our data show that the initial cell fate determination for the vestibuloacoustic neurons and their delamination appeared to be unaffected in the absence of Eya1 or Six1 as judged by the expression of the basic helix-loop-helix genes, Neurog1 that specifies the neuroblast cell lineage, and Neurod that controls neuronal differentiation and survival. However, both genes are necessary for normal maintenance of neurogenesis. During the development of epibranchial placode-derived distal cranial sensory ganglia, while the phenotype appears less severe in Six1 than in Eya1 mutants, an early arrest of neurogenesis was observed in the mutants. The mutant epibranchial progenitor cells fail to express Neurog2 that is required for the determination of neuronal precursors, and other basic helix-loop-helix as well as the paired homeobox Phox2 genes that are essential for neural differentiation and maintenance. Failure to activate their normal differentiation program resulted in abnormal apoptosis of the progenitor cells. Furthermore, we show that disruption of viable ganglion formation leads to pathfinding errors of branchial motoneurons. Finally, our results suggest that the Eya-Six regulatory hierarchy also operates in the epibranchial placodal development. These findings uncover an essential function for Eya1 and Six1 as critical determination factors in acquiring both neuronal fate and neuronal subtype identity from epibranchial placodal progenitors. These analyses define a specific role for both genes in early differentiation and survival of the placodally derived cranial sensory neurons.

Targeted disruption of the Kcnq1 gene produces a mouse model of Jervell and Lange-Nielsen Syndrome

KCNQ1 encodes KCNQ1, which belongs to a family of voltage-dependent K(+) ion channel proteins. KCNQ1 associates with a regulatory subunit, KCNE1, to produce the cardiac repolarizing current, I(Ks). Loss-of-function mutations in the human KCNQ1 gene have been linked to Jervell and Lange-Nielsen Syndrome (JLNS), a disorder characterized by profound bilateral deafness and a cardiac phenotype. To generate a mouse model for JLNS, we created a line of transgenic mice that have a targeted disruption in the Kcnq1 gene. Behavioral analysis revealed that the Kcnq1(-/-) mice are deaf and exhibit a shaker/waltzer phenotype. Histological analysis of the inner ear structures of Kcnq1(-/-) mice revealed gross morphological anomalies because of the drastic reduction in the volume of endolymph. ECGs recorded from Kcnq1(-/-) mice demonstrated abnormal T- and P-wave morphologies and prolongation of the QT and JT intervals when measured in vivo, but not in isolated hearts. These changes are indicative of cardiac repolarization defects that appear to be induced by extracardiac signals. Together, these data suggest that Kcnq1(-/-) mice are a potentially valuable animal model of JLNS.

Pathology and Pathophysiology of Idiopathic Sudden Sensorineural Hearing Loss

BACKGROUND

The cause and pathogenesis of idiopathic sudden sensorineural hearing loss remain unknown. Proposed theories include vascular occlusion, membrane breaks, and viral cochleitis.

AIMS

To describe the temporal bone histopathology in 17 ears (aged 45-94 yr) with idiopathic sudden sensorineural hearing loss in our temporal bone collection and to discuss the implications of the histopathologic findings with respect to the pathophysiology of idiopathic sudden sensorineural hearing loss.

METHODS

Standard light microscopy using hematoxylin and eosin-stained sections was used to assess the otologic abnormalities.

RESULTS

Hearing had recovered in two ears and no histologic correlates were found for the hearing loss in both ears. In the remaining 15 ears, the predominant abnormalities were as follows: 1) loss of hair cells and supporting cells of the organ of Corti (with or without atrophy of the tectorial membrane, stria vascularis, spiral limbus, and cochlear neurons) (13 ears); 2) loss of the tectorial membrane, supporting cells, and stria vascularis (1 ear); and 3) loss of cochlear neurons only (1 ear). Evidence of a possible vascular cause for the idiopathic sudden sensorineural hearing loss was observed in only one ear. No membrane breaks were observed in any ear. Only 1 of the 17 temporal bones was acquired acutely during idiopathic sudden sensorineural hearing loss, and this ear did not demonstrate any leukocytic invasion, hypervascularity, or hemorrhage within the labyrinth, as might be expected with a viral cochleitis.

DISCUSSION

The temporal bone findings do not support the concept of membrane breaks, perilymphatic fistulae, or vascular occlusion as common causes for idiopathic sudden sensorineural hearing loss. The finding in our one case acquired acutely during idiopathic sudden sensorineural hearing loss as well as other clinical and experimental observations do not strongly support the theory of viral cochleitis.

CONCLUSION

We put forth the hypothesis that idiopathic sudden sensorineural hearing loss may be the result of pathologic activation of cellular stress pathways involving nuclear factor-kappaB within the cochlea.

The use of zebrafish for assessing ototoxic and otoprotective agents

Zebrafish and other fish exhibit hair cells in the lateral-line neuromasts which are structurally and functionally similar to mammalian inner ear hair cells. To facilitate drug screening for ototoxic or otoprotective agents, we report a straightforward, quantitative in vivo assay to determine potential ototoxicity of drug candidates and to screen otoprotective agents in zebrafish larva. In this study, a fluorescent vital dye, DASPEI (2-(4-(dimethylamino)styryl)-N-ethylpyridinium iodide), was used to stain zebrafish hair cells in vivo and morphometric analysis was performed to quantify fluorescence intensity and convert images to numerical endpoints. Various therapeutics, including gentamicin, cisplatin, vinblastine sulfate, quinine, and neomycin, which cause ototoxicity in humans, also resulted in hair cell loss in zebrafish. In addition, protection against cisplatin-induced ototoxicity was observed in zebrafish larva co-treated with cisplatin and different antioxidants including, glutathione (GSH), allopurinol (ALO), N-acetyl l-cysteine (l-NAC), 2-oxothiazolidine-4-carboxylate (OTC) and d-methionine (d-MET). Our data indicate that results of ototoxicity and otoprotection in zebrafish correlated with results in humans, supporting use of zebrafish for preliminary drug screening.

Human adenylate kinase 2 deficiency causes a profound hematopoietic defect associated with sensorineural deafness

Reticular dysgenesis is an autosomal recessive form of human severe combined immunodeficiency characterized by an early differentiation arrest in the myeloid lineage and impaired lymphoid maturation. In addition, affected newborns have bilateral sensorineural deafness. Here we identify biallelic mutations in AK2 (adenylate kinase 2) in seven individuals affected with reticular dysgenesis. These mutations result in absent or strongly decreased protein expression. We then demonstrate that restoration of AK2 expression in the bone marrow cells of individuals with reticular dysgenesis overcomes the neutrophil differentiation arrest, underlining its specific requirement in the development of a restricted set of hematopoietic lineages. Last, we establish that AK2 is specifically expressed in the stria vascularis region of the inner ear, which provides an explanation of the sensorineural deafness in these individuals. These results identify a previously unknown mechanism involved in regulation of hematopoietic cell differentiation and in one of the most severe human immunodeficiency syndromes.

Altered cochlear fibrocytes in a mouse model of DFN3 nonsyndromic deafness

DFN3, an X chromosome-linked nonsyndromic mixed deafness, is caused by mutations in the BRN-4 gene, which encodes a POU transcription factor. Brn-4-deficient mice were created and found to exhibit profound deafness. No gross morphological changes were observed in the conductive ossicles or cochlea, although there was a dramatic reduction in endocochlear potential. Electron microscopy revealed severe ultrastructural alterations in cochlear spiral ligament fibrocytes. The findings suggest that these fibrocytes, which are mesenchymal in origin and for which a role in potassium ion homeostasis has been postulated, may play a critical role in auditory function.

Radiological malformations of the ear in Pendred syndrome

Pendred syndrome comprises the association of severe congenital sensorineural deafness with thyroid pathology. Although it is the commonest form of syndromic hearing loss, the primary genetic defect remains unknown. The variable clinical presentation allied to the difficulty in securing the diagnosis have resulted in relatively poor documentation of the radiological features of this syndrome. We now present data on 40 patients, all complying with strict diagnostic criteria for the disorder, and describe our experience of the prevalence of specific malformations of the inner ear as well as comparing the relative merits of computed tomography (CT) and magnetic resonance imaging (MRI) in the investigation of this inherited condition. Deficiency of the interscalar septum in the distal coils of the cochlea (Mondini deformity) was found to be a common but probably not a constant feature of Pendred syndrome. However, enlargement of the endolymphatic sac and duct in association with a large vestibular aqueduct was present in all 20 patients examined by MRI. We conclude that thin section high resolution MRI on a T2 protocol in the axial and sagittal planes is the imaging investigation of choice.

Meniere's syndrome and endolymphatic hydrops. Double-blind temporal bone study

A systematic double-blind assessment of case histories and histopathologic findings in temporal bones in the collection at the Massachusetts Eye and Ear Infirmary was performed to test the hypothesis that clinical Meniere's syndrome is associated with endolymphatic hydrops demonstrated histopathologically at death. Thirteen of 13 cases of clinical Meniere's syndrome were found to have endolymphatic hydrops not attributable to other causes. However, some patients with idiopathic endolymphatic hydrops did not exhibit clinical Meniere's syndrome as revealed in their medical records. These results challenge the dogma that endolymphatic hydrops per se generates the symptoms of Meniere's syndrome.

Use of laboratory evaluation and radiologic imaging in the diagnostic evaluation of children with sensorineural hearing loss

OBJECTIVE

Laboratory testing and radiologic imaging are commonly used to delineate syndromic from nonsyndromic sensorineural HL (SNHL). The aim of this study was to examine the yield of laboratory tests and radiologic imaging commonly used in the diagnostic evaluation of SNHL in children.

STUDY DESIGN

Retrospective analysis of 114 (54 female, 60 male) consecutively investigated children with SNHL between 1998 and 2000 at a tertiary-care university hospital.

METHODS

Results of routine laboratory testing to assess autoimmunity, blood dyscrasias, endocrine abnormalities, renal function, infection, and cardiac testing were reviewed. Results of radiologic evaluation were also reviewed. In general, computed tomography (CT) was obtained in patients with symmetric SNHL, whereas magnetic resonance imaging (MRI) with or without CT was obtained in asymmetric SNHL.

RESULTS

Laboratory evaluation of the blood did not yield the etiology of SNHL in any patient. Blood tests for autoimmune disease were often positive but did not correlate with clinical disease. Nonspecific elevation of erythrocyte sedimentation rate (ESR) and antinuclear antibody (ANA) was present in 22% of cases. An abnormal electrocardiogram with a prolonged QT interval resulted in the diagnosis of Jervall and Lange-Nielsen syndrome. In the 97 patients who underwent radiologic studies, abnormalities were present in 38 of 97 studies (39%). Isolated inner ear malformations were twice as common as multiple abnormalities with large vestibular aqueducts as the most common isolated finding.

CONCLUSION

In the evaluation of children with unexplained SNHL, routine laboratory evaluation should be reconsidered given its low diagnostic yield. However, radiologic abnormalities of the inner ear are common. Identification of inner ear malformations has direct impact on management of these children, suggesting that all children should undergo radiologic imaging as an integral component of evaluation of SNHL.

The role of Pax2 in mouse inner ear development

The paired box transcription factor, Pax2, is important for cochlear development in the mouse inner ear. Two mutant alleles of Pax2, a knockout and a frameshift mutation (Pax21Neu), show either agenesis or severe malformation of the cochlea, respectively. In humans, mutations in the PAX2 gene cause renal coloboma syndrome that is characterized by kidney abnormalities, optic nerve colobomas and mild sensorineural deafness. To better understand the role of Pax2 in inner ear development, we examined the inner ear phenotype in the Pax2 knockout mice using paint-fill and gene expression analyses. We show that Pax2-/- ears often lack a distinct saccule, and the endolymphatic duct and common crus are invariably fused. However, a rudimentary cochlea is always present in all Pax2 knockout inner ears. Cochlear outgrowth in the mutants is arrested at an early stage due to apoptosis of cells that normally express Pax2 in the cochlear anlage. Lack of Pax2 affects tissue specification within the cochlear duct, particularly regions between the sensory tissue and the stria vascularis. Because the cochlear phenotypes observed in Pax2 mutants are more severe than those observed in mice lacking Otx1 and Otx2, we postulate that Pax2 plays a key role in regulating the differential growth within the cochlear duct and thus, its proper outgrowth and coiling.

Keratitis, ichthyosis, and deafness (KID syndrome): review of the literature and proposal of a new terminology

The so-called KID (keratitis, ichthyosis, deafness) syndrome is a congenital disorder of ectoderm that affects not only the epidermis, but also other ectodermal tissues such as the corneal epithelium and the inner ear. Sixty-one patients who fulfill the criteria for this syndrome were identified in a review of the literature through December 1993. All had cutaneous and auditory abnormalities, and 95% also had ophthalmologic defects. The most frequent clinical features were neurosensory deafness 90%, erythrokeratoderma 89%, vascularizing keratitis 79%, alopecia 79%, and reticulated hyperkeratosis of the palms and soles 41%. All of these findings constitute the major criteria for the diagnosis. The KID acronym does not accurately define this entity since the disorder is not an ichthyosis, because scaling is not the main cutaneous feature and not all patients have keratitis early in the course. We suggest that this syndrome should be included under the general heading of congenital ectodermal defects as a keratodermatous ectodermal dysplasia (KED).

Internal auditory artery infarction: clinicopathologic correlation

OBJECTIVE

To study the pathophysiology of labyrinthine infarction.

BACKGROUND

The syndrome of sudden onset vertigo or hearing loss is commonly attributed to inner ear vascular disease, yet histologic studies of isolated labyrinthine infarction in humans have been rare and have not included a complete examination of the vertebrobasilar vascular system.

METHODS

Temporal bones, brainstem, cerebellum, and the supplying blood vessels were subjected to gross and microscopic postmortem examinations in a 92-year-old woman who had a sudden onset of vertigo and hearing loss in the right ear 7 years before death.

RESULTS

There were prominent atherosclerotic changes at the vertebrobasilar junction, but the internal auditory artery and its branches were patent on both sides. Histologic studies showed degenerative changes in the cochlea and vestibular labyrinth on the right. The posterior canal ampulla and saccular macule were relatively preserved showing partial areas of intact sensory epithelium with underlying nerve fibers. The right vestibulocochlear nerve showed a fibrotic scar and multiple patchy areas of degeneration. These findings are most consistent with a transient period of reduced perfusion of the internal auditory artery.

CONCLUSION

The partial sparing of the inferior vestibular labyrinth may indicate a decreased vulnerability to ischemia because of its better collateral blood supply.

Targeted disruption of otog results in deafness and severe imbalance

Genes specifically expressed in the inner ear are candidates to underlie hereditary nonsyndromic deafness. The gene Otog has been isolated from a mouse subtractive cDNA cochlear library. It encodes otogelin, an N-glycosylated protein that is present in the acellular membranes covering the six sensory epithelial patches of the inner ear: in the cochlea (the auditory sensory organ), the tectorial membrane (TM) over the organ of Corti; and in the vestibule (the balance sensory organ), the otoconial membranes over the utricular and saccular maculae as well as the cupulae over the cristae ampullares of the three semi-circular canals. These membranes are involved in the mechanotransduction process. Their movement, which is induced by sound in the cochlea or acceleration in the vestibule, results in the deflection of the stereocilia bundle at the apex of the sensory hair cells, which in turn opens the mechanotransduction channels located at the tip of the stereo-cilia. We sought to elucidate the role of otogelin in the auditory and vestibular functions by generating mice with a targeted disruption of Otog. In Otog-/- mice, both the vestibular and the auditory functions were impaired. Histological analysis of these mutants demonstrated that in the vestibule, otogelin is required for the anchoring of the otoconial membranes and cupulae to the neuroepithelia. In the cochlea, ultrastructural analysis of the TM indicated that otogelin is involved in the organization of its fibrillar network. Otogelin is likely to have a role in the resistance of this membrane to sound stimulation. These results support OTOG as a possible candidate gene for a human nonsyndromic form of deafness.