Consequences of combined maternal, fetal and persistent postnatal hypothyroidism on the development of auditory function in Tshrhyt mutant mice

Tshrhyt/hyt mutant mice express a point mutation in the gene encoding the thyrotropin receptor, and affected animals are congenitally hypothyroid and profoundly deaf as a consequence when the condition is untreated. In this investigation, a previously unrecognized developmental stage was identified in the hypothyroid, mutant progeny of hypothyroid dams by tracking developmental changes in the auditory brainstem response (ABR). ABR thresholds develop rapidly in normal, euthyroid animals, decreasing as much as 80 dB between P12 (postnatal day 12) and P15, with mature sensitivity being gradually acquired by P18. In contrast, Tshrhyt/hyt mutant mice remained profoundly deaf on P24 and although thresholds improved by approximately 30 dB by P60, residual frequency-dependent deficits of 20-70 dB were observed in animals exhibiting end-stage disease. The rate of threshold improvement in mutant mice was approximately ten times slower than in normal mice. While ABR wave latencies and interpeak intervals decreased early in postnatal life, values decreased over a delayed and protracted time period, reaching adult values well after those of controls attained maturity. As with normal mice, slopes of wave I latency-intensity curves were significantly steeper in immature animals than those observed in adults and decreased during development, but failed to achieve normal adult values and remained significantly steeper than those for controls. Findings reported here suggest that passive aspects of electromechanical transduction achieve maturity in Tshrhyt/hyt progeny of Tshrhyt/hyt mice and that development, limited as it may be, occurs most prominently in the basal half of the cochlea.

Expression pattern and functional characterization of connexin29 in transgenic mice

Using newly generated transgenic mice in which the coding region of the connexin29 (Cx29) gene was replaced by the lacZ reporter gene, we confirmed previous immunochemical results that Cx29 is expressed in Schwann cells, oligodendrocytes and Bergmann glia cells. In addition, we detected lacZ/Cx29 in Schwann cells of the sciatic nerve and in particular of the spiral ganglion in the inner ear, as well as at low abundance in the stria vascularis. Furthermore, we found lacZ/Cx29 expression in nonmyelinating Schwann cells of the adrenal gland, in chondrocytes of intervertebral discs and the epiphysis of developing bones. Electron microscopic analyses of myelin sheaths in the central and peripheral nervous system of Cx29-deficient mice detected no abnormalities. The nerve conduction in the sciatic nerve of adult Cx29-deficient mice and the auditory brain stem response as well as visually evoked potentials in 4- to 10-week-old Cx29-deficient mice were not different from wild-type littermate controls. Thus, in contrast to connexin32 and connexin47, which are also expressed in myelinating cells, Cx29 does not contribute to the function of myelin in adult mice.

Requirement of nicotinic acetylcholine receptor subunit beta2 in the maintenance of spiral ganglion neurons during aging

Age-related hearing loss (presbycusis) is a major health concern for the elderly. Loss of spiral ganglion neurons (SGNs), the primary sensory relay of the auditory system, is associated consistently with presbycusis. The causative molecular events responsible for age-related loss of SGNs are unknown. Recent reports directly link age-related neuronal loss in cerebral cortex with the loss of high-affinity nicotine acetylcholine receptors (nAChRs). In cochlea, cholinergic synapses are made by olivocochlear efferent fibers on the outer hair cells that express alpha9 nAChR subunits and on the peripheral projections of SGNs that express alpha2, alpha4-7, and beta2-3 nAChR subunits. A significantly decreased expression of the beta2 nAChR subunit in SGNs was found specifically in mice susceptible to presbycusis. Furthermore, mice lacking the beta2 nAChR subunit (beta2-/-), but not mice lacking the alpha5 nAChR subunit (alpha5-/-), have dramatic hearing loss and significant reduction in the number of SGNs. Our findings clearly established a requirement for beta2 nAChR subunit in the maintenance of SGNs during aging.

Murine auditory brainstem evoked response: putative two-channel differentiation of peripheral and central neural pathways

Standard noninvasive recordings of the auditory brainstem evoked response (ABR) from a single pair of obliquely oriented electrodes (typically midline vertex referenced to mastoid) confound inherently distinct signals propagating over peripheral and central neural pathways differing in location and spatial orientation. We describe here a technique for recording short-latency auditory evoked potentials that putatively differentiates peripheral and central neural activity in the mouse and rat. The technique involves recording from two orthogonally oriented electrode pairs using fast sample rates (100 k/s) to accurately measure differences in neural timing and waveform morphology. Electrodes oriented in a transverse plane (mastoid-to-mastoid) register an initial positive-going ABR peak (P1T) earlier than a series of peaks recorded from electrodes oriented along the midline (anterior and posterior to the inter-aural line). The absolute P1T latency is consistent with an origin in the primary auditory nerve, while the delayed midline latencies implicate activity farther along central neural pathways. Differences between these latencies (midline minus transverse) provide new and precise measures of central conduction time (CCT), which in one case is as brief as 0.10 ms. Results in wild type (WT) and knockout (KO) mice, as well as rats, show significant differences in absolute latencies as well as CCT.

Different effects of intracochlear sensory and neuronal injury stimulation on expression of synaptic N-methyl-D-aspartate receptors in the auditory cortex of rats in vivo

CONCLUSIONS

The expression of synaptic N-methyl-D-aspartate (NMDA) receptors in the auditory cortex is dynamic and is bidirectionally regulated by auditory activity. Furthermore, the time course of changes in the level of NR2A protein differs after sensory and neuronal injury stimulation, which modulate different changes in synaptic plasticity.

OBJECTIVE

To examine the effects of different types of auditory activity on the expression of synaptic NMDA receptors (NMDARs) in the auditory cortex of rats.

MATERIAL AND METHODS

We prepared synaptosomes from the auditory cortices of postnatal Day 28 ototoxic-deafened Sprague-Dawley rats and postnatal Day 28 Sprague-Dawley rats subjected to noise trauma that were given various treatments and compared them to the synaptosomes of 1-6-week-old normal Sprague-Dawley rats. The expression of different NMDAR subunits in the synaptosomes was investigated by means of Western blotting.

RESULTS

Changes in NR1 and NR2B proteins were not significant during different types of auditory activity. The level of NR2A protein increased remarkably during postnatal development and as a result of electrical intracochlear stimulation, auditory deprivation and noise trauma. Seventy-two h after a 2-h period of sensory electrical intracochlear stimulation, the expression of NR2A protein returned to the level caused by auditory deprivation. Seventy-two h after a 3-h period of noise trauma, elevation of the level of NR2A protein was unchanged. We also confirmed that elevation of the level of synaptic NR2A protein was sensitive to protein synthesis inhibitor and NMDAR antagonist. However, transcription inhibitor had no effect on NR2A protein expression.

Kanamycin ototoxicity in glutamate transporter knockout mice

Glutamate-aspartate transporter (GLAST), a powerful glutamate uptake system, removes released glutamate from the synaptic cleft and facilitates the re-use of glutamate as a neurotransmitter recycling system. Aminoglycoside-induced hearing loss is mediated via a glutamate excitotoxic process. We investigated the effect of aminoglycoside ototoxicity in GLAST knockout mice using the recorded auditory brainstem response (ABR) and number of hair cells in the cochlea. Kanamycin (100 mg/mL) was injected directly into the posterior semicircular canal of mice. Before the kanamycin treatment, there was no difference in the ABR threshold average between the wild-type and knockout mice. Kanamycin injection aggravated the ABR threshold in the GLAST knockout mice compared with the wild-type mice, and the IHC degeneration was more severe in the GLAST knockout mice. These findings suggest that GLAST plays an important role in preventing the degeneration of inner hair cells in aminoglycoside ototoxicity.

Expression of the carrier protein apolipoprotein D in the mouse inner ear

The cochlear portion of the inner ear converts movements produced by sound waves into electrical impulses. Transcripts enriched in the cochlea are likely to have an important role in hearing. In this paper, we report that microarray analyses of the Soares NMIE inner ear library revealed cochlear enriched expression of apolipoprotein D (apoD), a glycoprotein and member of the lipocalin family that transport small hydrophobic ligands. The cochlear enriched expression of Apod was validated by quantitative real time PCR analysis. To investigate the function of apoD in the inner ear the transcript and protein were localised in the cochlea. Apod messenger RNA (mRNA) expression was localised to the spiral ligament and spiral limbus, particularly in the suprastrial and supralimbral regions. The apoD protein was detected in the spiral ligament, spiral limbus and also in the outer hair cells of the organ of Corti. Investigation of cell lines exhibiting characteristics of hair and supporting cells revealed no Apod mRNA expression in these cells. This suggests transport of the protein within the cochlea, followed by internalisation into outer hair cells. The spiral limbus and ligament contain subpopulations of fibrocytes that are intimately involved in regulation of ion balance in the cochlear fluids and type I, II and III fibrocytes of the spiral ligament were all shown to be positive for apoD protein. On the basis of these results it was hypothesised that apoD could be involved in maintaining cochlear fluid homeostasis. To determine whether the apoD gene product was important for normal auditory function the hearing ability of an apoD knockout mouse was tested. The mouse was found to have a hearing threshold that was not significantly different to the control strain.

Heritability and segregation analysis of deafness in U.S. Dalmatians

Hereditary loss of hearing affects many breeds of the domestic dog, but the Dalmatian has the highest prevalence. Approximately 30% are affected in the United States (U.S.) population. It is widely accepted that a relationship exists between deafness and pigmentation in the dog and also in other animals. While the Dalmatian exemplifies this relationship, the genetic origin and mode of inheritance of deafness in this breed are unknown. The goals of this study were to: (1) estimate the heritability of deafness in an extended kindred of U.S. Dalmatians and (2) determine, through complex segregation analysis, whether there is a major segregating locus that has a large effect on the expression of deafness. A kindred of 266 Dalmatians was assembled, of which 199 had been diagnosed using the brainstem auditory evoked response to determine auditory status. Of these, 74.4% (N = 148) had normal hearing, 18.1% (N = 36) were unilaterally deaf, and 7.5% (N = 15) were bilaterally deaf. A heritability of 0.73 was estimated considering deafness a dichotomous trait and 0.75 considering it as a trichotomous trait. Although deafness in the Dalmatian is clearly heritable, the evidence for the presence of a single major gene affecting the disorder is not persuasive.

Characterization of Two Transgene Insertional Mutations at Pirouette, a Mouse Deafness Locus

The mouse mutant 'pirouette' (pi) exhibits profound hearing loss and vestibular defects due to inheritance of a recessive mutation on chromosome 5. Dysfunction has been correlated with defects during maturation of sensory cells in the inner ear. As an initial step in characterizing pirouette at the genetic level, we have localized the candidate interval to a small region on central chromosome 5 by analysis of a congenic strain of pirouette mice. This region exhibits conserved synteny with human chromosome 4 and suggests that pirouette may be a genetic model of the human nonsyndromic deafness disorder DFNB25, which has been localized to 4p15.3-q12. In addition to the original spontaneous pirouette strain, we have identified and characterized 2 additional mouse strains with allelic mutations at the same locus. Analysis of the morphology in each of the 3 pirouette alleles indicated very similar early postnatal alterations in maturation of stereocilia and suggests that the gene affected in pirouette normally plays a role in building or maintaining these structures that are critical for sensory mechanotransduction.

Hearing loss in infantile Pompe's disease and determination of underlying pathology in the knockout mouse

Hearing deficit occurs in several lysosomal storage disorders but has so far not been recognized as a symptom of Pompe's disease (glycogen storage disease type II). We discovered quite unexpectedly 30-90 dB hearing loss in four infants with Pompe's disease, who participated in a study on the safety and efficacy of enzyme replacement therapy. Three other patients with juvenile Pompe's disease did not have this symptom. The ABR (auditory brainstem response) thresholds but not the interpeak latency times were increased. This pointed to middle or inner ear pathology rather than to involvement of the central auditory nervous system. The possible occurrence of cochlear pathology was supported by the absence of oto-acoustic emissions. We investigated this hypothesis in a knockout mouse model of Pompe's disease and found glycogen storage in the inner and outer hair cells of the cochlea, the supporting cells, the stria vascularis, and the spiral ganglion cells. We conclude that cochlear pathology is the most likely cause of hearing loss in infantile Pompe's disease and possibly a characteristic feature of this clinical subtype.

Progressive auditory neuropathy in patients with Leber's hereditary optic neuropathy

OBJECTIVE

To investigate auditory neural involvement in patients with Leber's hereditary optic neuropathy (LHON).

METHODS

Auditory assessment was undertaken in two patients with LHON. One was a 45 year old woman with Harding disease (multiple-sclerosis-like illness and positive 11778mtDNA mutation) and mild auditory symptoms, whose auditory function was monitored over five years. The other was a 59 year old man with positive 11778mtDNA mutation, who presented with a long standing progressive bilateral hearing loss, moderate on one side and severe to profound on the other. Standard pure tone audiometry, tympanometry, stapedial reflex threshold measurements, stapedial reflex decay, otoacoustic emissions with olivo-cochlear suppression, auditory brain stem responses, and vestibular function tests were undertaken.

RESULTS

Both patients had good cochlear function, as judged by otoacoustic emissions (intact outer hair cells) and normal stapedial reflexes (intact inner hair cells). A brain stem lesion was excluded by negative findings on imaging, recordable stapedial reflex thresholds, and, in one of the patients, olivocochlear suppression of otoacoustic emissions. The deterioration of auditory function implied a progressive course in both cases. Vestibular function was unaffected.

CONCLUSIONS

The findings are consistent with auditory neuropathy-a lesion of the cochlear nerve presenting with abnormal auditory brain stem responses and with normal inner hair cells and the cochlear nucleus (lower brain stem). The association of auditory neuropathy, or any other auditory dysfunction, with LHON has not been recognised previously. Further studies are necessary to establish whether this is a consistent finding.

Familial auditory neuropathy

OBJECTIVES/HYPOTHESIS

Auditory neuropathy is a sensorineural hearing disorder characterized by absent or abnormal auditory brainstem responses and normal cochlear outer hair cell function as measured by otoacoustic emission recordings. Many risk factors are thought to be involved in its etiology and pathophysiology. Four Chinese pedigrees with familial auditory neuropathy were presented to demonstrate involvement of genetic factors in the etiology of auditory neuropathy.

STUDY DESIGN

Probands of the above-mentioned pedigrees, who had been diagnosed with auditory neuropathy, were evaluated and followed in the Department of Otolaryngology-Head and Neck Surgery, China People Liberation Army General Hospital (Beijing, China). Their family members were studied, and the pedigree maps established.

METHODS

History of illness, physical examination, pure-tone audiometry, acoustic reflex, auditory brainstem responses, and transient evoked and distortion-product otoacoustic emissions were obtained from members of these families. Some subjects received vestibular caloric testing, computed tomography scan of the temporal bone, and electrocardiography to exclude other possible neuropathic disorders.

RESULTS

In most affected patients, hearing loss of various degrees and speech discrimination difficulties started at 10 to 16 years of age. Their audiological evaluation showed absence of acoustic reflex and auditory brainstem responses. As expected in auditory neuropathy, these patients exhibited near-normal cochlear outer hair cell function as shown in distortion product otoacoustic emission recordings. Pure-tone audiometry revealed hearing loss ranging from mild to profound in these patients. Different inheritance patterns were observed in the four families. In Pedigree I, 7 male patients were identified among 43 family members, exhibiting an X-linked recessive pattern. Affected brothers were found in Pedigrees II and III, whereas in pedigree IV, two sisters were affected. All the patients were otherwise normal without evidence of peripheral neuropathy at the time of writing.

CONCLUSION

Patients with characteristics of nonsyndromic hereditary auditory neuropathy were identified in one large and three smaller Chinese families. Pedigree analysis suggested an X-linked, recessive hereditary pattern in one pedigree and autosomal recessive inheritances in the other three pedigrees. The phenotypes in the study were typical of auditory neuropathy; they were transmitted in different inheritance patterns, indicating clinical and genetic heterogeneity of this disorder. The observed inheritance and clinical audiological findings are different from those previously described for nonsyndromic low-frequency sensorineural hearing loss. This information should facilitate future molecular linkage analyses and positional cloning for the relative genes contributing to auditory neuropathy.

Expression and function of chloride transporters during development of inhibitory neurotransmission in the auditory brainstem

Glycine and GABA, the dominant inhibitory neurotransmitters in the CNS, assume a depolarizing role in early development, leading to increased cytoplasmic Ca2+ levels and action potentials. The effect is thought to be of some significance for maturation. The depolarization is caused by Cl- efflux, and chloride transporters contribute to the phenomenon by raising the intracellular Cl- concentration ([Cl-]i) above equilibrium, thereby generating an outward-directed electrochemical gradient for Cl-. In mature neurons, the [Cl-]i is reduced below equilibrium, thus rendering glycine activity hyperpolarizing. Here, we investigated the temporal expression of the K-Cl cotransporter KCC2 and the Na-K-Cl cotransporter NKCC1 in the lateral superior olive (LSO) of rats and mice. The two cation cotransporters normally extrude and accumulate Cl-, respectively. As evidenced by several methods, KCC2 mRNA was present in LSO neurons during both the depolarizing and hyperpolarizing periods. Western blots confirmed a constant level of KCC2 in the brainstem, and immunohistochemistry showed that the protein is diffusely distributed within neonatal LSO neurons, becoming integrated into the plasma membrane only with increasing age. The glycine reversal potential in KCC2 knock-out mice differed significantly from that determined in wild-type controls at postnatal day 12 (P12) but not at P3, demonstrating that KCC2 is not active in neonates, despite its early presence. NKCC1 mRNA was not detected during the depolarizing phase in the LSO, implying that this transporter does not contribute to the high [Cl-]i. Our results reveal major differences in the development of [Cl-]i regulation mechanisms seen in brainstem versus forebrain regions.

Genetic analyses of the mouse deafness mutations varitint-waddler (Va) and jerker (Espnje)

Genetic studies on spontaneous mouse mutants with hearing defects have provided important insights into the function of genes expressed in inner ear hair cells. Here we report on our genetic analyses of the deaf mutants varitint-waddler (Va) and jerker (Espnje). A high-resolution genetic map localizes VaJ to a 0.14 +/- 0.08 cM region between D3Mit85 and D3Mit259 on distal chromosome 3. By comparative mapping, the human ortholog resides at 1p22.3 between markers D1S3449 and D1S2252. To study the effect of different genetic backgrounds on the hearing phenotype, Espnje and VaJ were crossed to various inbred strains. Auditory-evoked brainstem response tests on F2 progeny demonstrate that expression, inheritance, and penetrance of the hearing phenotype are solely controlled by the mutant allele. To test for a genetic interaction between Espnje and Cdh23v, auditory function was analyzed in double heterozygotes; no significant increases of thresholds of sound pressure levels were observed. The results establish the framework for cloning the Va gene and provide valuable insights into the genetics of deafness mutations in the mouse.

Identification of a monogenic locus (jams1) causing juvenile audiogenic seizures in mice

Epilepsy is a debilitating disease with a strong genetic component. Positional cloning has identified a few genes for rare monogenic epilepsy syndromes; however, the genetics of common human epilepsies are too complex to be analyzed easily by current techniques. Mouse models of epilepsy can further this analysis by eliminating genetic background heterogeneity and enabling the production of sufficient numbers of offspring. Here, we report that Black Swiss mice have a heretofore unrecognized specific susceptibility to audiogenic seizures. These seizures are characterized by wild running, loss of righting reflex, and tonic flexion and extension, and are followed by a postictal period. The susceptibility to these seizures is developmentally regulated, peaking at 21 d of age and nearly disappearing by adulthood. Interestingly, both the susceptibility to seizures and their developmental regulation appear unrelated to hearing thresholds in the Black Swiss strain and backcrossed progeny. Genetic mapping and linkage analysis of hybrid mice localize the seizure gene, jams1 (juvenile audiogenic monogenic seizures), to a 1.6 +/- 0.5 centimorgan (cM) region on mouse chromosome 10, delimited by the gene basigin (Bsg) and marker D10Mit140. Interestingly, the majority of the critical region is syntenic to a region on human chromosome 19p13.3 implicated in a familial form of juvenile febrile convulsions. Cloning the gene for audiogenic seizures in these mice may provide important insight into the fundamental mechanisms for developmentally regulated human epilepsy syndromes.

Mutant ICR mouse, kuru2, manifests hearing impairment and abnormal behavior

BACKGROUND

Establishment of mutant animals presents valuable information on corresponding human diseases. We established a mutant mouse, kuru2, from the previously reported Ascites ICR Mouse.

MATERIALS AND METHODS

Epileptic individuals of Ascites ICR Mouse were mated and maintained with sibling mating. This mouse was characterized by hearing impairment and abnormal behavior such as ataxic gait, disturbance of positional sense, hyperirritability, head-tossing and circling movement.

RESULTS

No detectable auditory brain stem response was evoked from an early stage of life. Abnormal behavior started from 4 to 12 weeks of age. Microscopic examination revealed no major abnormalities in the central nervous system. In the inner ear, the vestibule and cochlea were well developed, however degeneration of the spiral ganglions was observed at a late age. The genetic mode was autosomal recessive.

DISCUSSION

Since this mouse has a distinctive phenotype, the animal may provide an understanding of hereditary hearing impairment and abnormal behavior.

Hearing loss caused by progressive degeneration of cochlear hair cells in mice deficient for the Barhl1 homeobox gene

The cochlea of the mammalian inner ear contains three rows of outer hair cells and a single row of inner hair cells. These hair cell receptors reside in the organ of Corti and function to transduce mechanical stimuli into electrical signals that mediate hearing. To date, the molecular mechanisms underlying the maintenance of these delicate sensory hair cells are unknown. We report that targeted disruption of Barhl1, a mouse homolog of the Drosophila BarH homeobox genes, results in severe to profound hearing loss, providing a unique model for the study of age-related human deafness disorders. Barhl1 is expressed in all sensory hair cells during inner ear development, 2 days after the onset of hair cell generation. Loss of Barhl1 function in mice results in age-related progressive degeneration of both outer and inner hair cells in the organ of Corti, following two reciprocal longitudinal gradients. Our data together indicate an essential role for Barhl1 in the long-term maintenance of cochlear hair cells, but not in the determination or differentiation of these cells.

Retardation of cochlear maturation and impaired hair cell function caused by deletion of all known thyroid hormone receptors

The deafness caused by early onset hypothyroidism indicates that thyroid hormone is essential for the development of hearing. We investigated the underlying roles of the TRalpha1 and TRbeta thyroid hormone receptors in the auditory system using receptor-deficient mice. TRalpha1 and TRbeta, which act as hormone-activated transcription factors, are encoded by the Thra and Thrb genes, respectively, and both are expressed in the developing cochlea. TRbeta is required for hearing because TRbeta-deficient (Thrb(tm1/tm1)) mice have a defective auditory-evoked brainstem response and retarded expression of a potassium current (I(K,f)) in the cochlear inner hair cells. Here, we show that although TRalpha1 is individually dispensable, TRalpha1 and TRbeta synergistically control an extended array of functions in postnatal cochlear development. Compared with Thrb(tm1/tm1) mice, the deletion of all TRs in Thra(tm1/tm1)Thrb(tm1/tm1) mice produces exacerbated and novel phenotypes, including delayed differentiation of the sensory epithelium, malformation of the tectorial membrane, impairment of electromechanical transduction in outer hair cells, and a low endocochlear potential. The induction of I(K,f) in inner hair cells was not markedly more retarded than in Thrb(tm1/tm1) mice, suggesting that this feature of hair cell maturation is primarily TRbeta-dependent. These results indicate that distinct pathways mediated by TRbeta alone or by TRbeta and TRalpha1 together facilitate control over an extended range of functions during the maturation of the cochlea.

Progressive cerebellar, auditory, and esophageal dysfunction caused by targeted disruption of the frizzled-4 gene

Wnt signaling has been implicated in the control of cell proliferation and in synapse formation during neural development, and these actions are presumed to be mediated by frizzled receptors. In this paper we report the phenotype of mice carrying a targeted deletion of the frizzled-4 (fz4) gene. fz4(-/-) mice exhibit three distinct defects: (1) progressive cerebellar degeneration associated with severe ataxia, (2) absence of a skeletal muscle sheath around the lower esophagus associated with progressive esophageal distension and dysfunction, and (3) progressive deafness caused by a defect in the peripheral auditory system unaccompanied by loss of hair cells or other auditory neurons. As assayed using a lacZ knock-in reporter, fz4 is widely expressed within the CNS. In particular, fz4 is expressed in cerebellar Purkinje cells, esophageal skeletal muscle, and cochlear inner hair cells, and the absence of Fz4 in these cells is presumed to account for the fz4(-/-) phenotype. In contrast to the early cell proliferation and patterning effects classically ascribed to Wnts, the auditory and cerebellar phenotypes of fz4(-/-) mice implicate Frizzled signaling in maintaining the viability and integrity of the nervous system in later life.

Deafness Due to Degeneration of Cochlear Neurons in Caspase-3-Deficient Mice

Mice that lack caspase-3, which functions in apoptosis, were generated by gene targeting and shown to undergo hearing loss. The ABR threshold of the caspase-3(-/-) mice was significantly elevated compared to that of caspase-3(+/+) mice at 15 days of age and was progressively elevated further by 30 days. Distortion product otoacoustic emissions were not detectable in caspase-3(-/-) mice at 15 days of age. Caspase-3(-/-) mice exhibited marked degeneration of spiral ganglion neurons and a loss of inner and outer hair cells in the cochlea at 30 days of age, although no such changes were apparent at 15 days. The degenerating neurons manifested features, including cytoplasmic vacuolization, distinct from those characteristic of apoptosis. Spiral ganglion neurons and cochlear hair cells thus appear to require caspase-3 for survival but not for initial development. The mapping of both the human caspase-3 gene and the locus responsible for an autosomal dominant, nonsyndromic form of hearing loss (DFNA24) to chromosome 4q35 suggests that the caspase-3(-/-) mice may represent a model of this human condition.

Audiometric abnormalities in children with Gaucher disease type 3

Exogenous enzyme replacement therapy achieves satisfactory biomedical correction in Gaucher type 1 disease and may halt or reverse neurological progression in type 3, while it does not appear to influence the outcome in type 2. In view of the therapeutic possibilities, early detection and monitoring of type 3 Gaucher disease, as well as evaluation of the effectiveness of enzyme therapy on neuronopathic involvement is necessary. The objective of this study was to evaluate the extent of brainstem disease in children with proven Gaucher type 3, by means of an audiological test battery. We studied 9 patients with Gaucher type 3 disease. The tests included baseline audiometric tests, as well as auditory brainstem evoked responses (ABR), acoustic reflexes and medial olivo-cochlear suppression by contralateral noise tests, that involve overlapping but not identical efferent and afferent pathways and brainstem structures. We found a constellation of abnormalities including bilaterally raised acoustic reflexes, poor medial olivo-cochlear suppression, and very poor brainstem evoked potentials. These abnormalities could be due to a single lesion in the dorsomedial brainstem, or to multiple lesions, and further study is needed to clarify this issue. Combined audiological tests may provide information on the severity of the neurological involvement and should therefore be part of a standard assessment for the diagnosis as well as for long term neurological monitoring of Gaucher type 3 patients.

Hearing loss associated with the modifier of deaf waddler (mdfw) locus corresponds with age-related hearing loss in 12 inbred strains of mice

The modifier of deaf waddler (mdfw) and age-related hearing loss (Ahl) loci were both discovered as inbred strain polymorphisms that affect hearing loss in mice. Both loci map to the same position on chromosome (Chr) 10. The mdfw locus interacts epistatically with the deaf waddler (dfw) mutation on Chr 6, and the Ahl locus is a major contributor to AHL in several inbred strains. To investigate the possibility of allelism, we examined the correspondence of mdfw and Ahl phenotypes among 12 inbred mouse strains. The effects of strain-specific mdfw alleles on hearing loss were assessed in dfw2J/+ F1 hybrids produced from mating BALB-dfw2J/+ mice with mice from each of 12 inbred strains. F1 hybrids were then assessed for hearing by auditory-evoked brainstem response threshold analysis and classified as dfw2J/+ or +/+ by polymerase chain reaction typing. Heterozygosity for dfw2J accelerated hearing loss in F1 hybrids derived from all strains tested, except those produced with the B6.CAST + Ahl congenic strain. dfw2J/+ F1 hybrids derived from parental strains 129P1/ReJ, A/J, BUB/BnJ, C57BR/cdJ, DBA/2J, NOD/LtJ and SKH2/J exhibited a severe hearing loss by 12 weeks of age. Those derived from strains 129T2/SvEmsJ, C3H/HeJ, CBA/CaJ and NON/LtJ exhibited only a slight to intermediate hearing loss at that age. The hearing loss associated with these strain-specific mdfw alleles corresponds with previously determined Ahl allele effects, providing additional evidence that mdfw and Ahl are manifestations of the same gene. A functional relationship therefore may exist between the Ca2+ transporting activity of the dfw gene (Atp2b2) and AHL.

Exacerbation of noise-induced hearing loss in mice lacking the glutamate transporter GLAST

Acoustic overstimulation is one of the major causes of hearing loss. Glutamate is the most likely candidate neurotransmitter for afferent synapses in the peripheral auditory system, so it was proposed that glutamate excitotoxicity may be involved in noise trauma. However, there has been no direct evidence that noise trauma is caused by excessive release of glutamate from the inner hair cells (IHCs) during sound exposure because studies have been hampered by powerful glutamate uptake systems in the cochlea. GLAST is a glutamate transporter highly expressed in the cochlea. Here we show that after acoustic overstimulation, GLAST-deficient mice show increased accumulation of glutamate in perilymphs, resulting in exacerbation of hearing loss. These results suggest that GLAST plays an important role in keeping the concentration of glutamate in the perilymph at a nontoxic level during acoustic overstimulation. These findings also provide further support for the hypothesis that IHCs use glutamate as a neurotransmitter.

Prolongation of brainstem auditory-evoked responses in autistic probands and their unaffected relatives

BACKGROUND

Brain function, as indexed by brain electrical activity, is heritable in humans, and it may be impaired in autism. Autism also has strong genetic determinants, and like all major psychiatric disorders, its complex clinical phenotype renders genetic studies difficult. Innovative strategies focused on alternative biological phenotypes are needed.

METHODS

The early brain auditory-evoked response was assessed in 73 autistic probands and 251 relatives who were compared with 521 normal controls.

RESULTS

We first confirmed in the autistic probands the presence of a slowing in nerve conduction in the auditory system as expressed by the prolongation of early brain auditory-evoked response under the form of I-III interpeak latencies (IPLs). Furthermore, we observed the same I-III IPL prolongation in the unaffected first degree relatives of the autistic probands compared with controls. Despite clear evidence of a coaggregation of autism and I-III IPL prolongation in families, the IPLs did not seem to be the sole liability factor for autism as suggested by the observation of 52% of families in which the autistic proband and relatives showed normal IPLs.

CONCLUSION

A prolongation of the early brain auditory-evoked response IPLs may be a marker for one of several deficits underlying autism and deserves further analysis as a potential alternative phenotype for the disorder.

Complex segregation analysis of deafness in Dalmatians

OBJECTIVE

To use pedigree analysis to evaluate the feasibility of a major locus model for deafness in Dalmatians.

ANIMALS

605 purebred Dalmatians from 42 families.

PROCEDURE

Hearing loss was evaluated through the brainstem auditory-evoked response. Dogs were classified into mutually exclusive categories: normal hearing, unilaterally deaf, or bilaterally deaf. Information was collected on sex, coat color, presence or absence of a color patch at birth, and eye color. Statistical analyses were performed by use of regressive logistic models designed for complex segregation analysis. Genetic correlations among eye color, deafness, and color patch were estimated.

RESULTS

Prevalence of hearing loss was 11% for dogs classified as unilaterally deaf and 5% for dogs that were bilaterally deaf. Complex segregation analysis detected statistical evidence of a single allele with an expected frequency of 0.21 that had an effect on the prevalence of deafness. Results of analyses suggested that this locus cannot completely explain the inheritance and incidence of deafness in Dalmatians. Genetic correlation estimates among deafness, eye color, and color patch revealed strong interrelationships among these characteristics.

CONCLUSIONS AND CLINICAL RELEVANCE

To reduce the incidence of hearing loss in Dalmatians, unilaterally deaf, blue-eyed dogs should not be considered as potential parents.