Loss of the chromatin remodeler CHD7 impacts glial cells and myelination in the mouse cochlear spiral ganglion.

CHARGE syndrome is a multiple anomaly developmental disorder characterized by a variety of sensory deficits, including sensorineural hearing loss of unknown etiology. Most cases of CHARGE are caused by heterozygous pathogenic variants in CHD7, the gene encoding Chromodomain DNA-binding Protein 7 (CHD7), a chromatin remodeler important for the development of neurons and glial cells. Previous studies in the Chd7Gt/+ mouse model of CHARGE syndrome showed substantial neuron loss in the early stages of the developing inner ear that are compensated for by mid-gestation. In this study, we sought to determine if early developmental delays caused by Chd7 haploinsufficiency affect neurons, glial cells, and inner hair cell innervation in the mature cochlea. Analysis of auditory brainstem response recordings in Chd7Gt/+ adult animals showed elevated thresholds at 4 kHz and 16 kHz, but no differences in ABR Wave I peak latency or amplitude compared to wild type controls. Proportions of neurons in the Chd7Gt/+ adult spiral ganglion and densities of nerve projections from the spiral ganglion to the organ of Corti were not significantly different from wild type controls. Inner hair cell synapse formation also appeared unaffected in mature Chd7Gt/+ cochleae. However, histological analysis of adult Chd7Gt/+ cochleae revealed diminished satellite glial cells and hypermyelinated Type I spiral ganglion axons. We characterized the expression of CHD7 in developing inner ear glia and found CHD7 to be expressed during a tight window of inner ear development at the Schwann cell precursor stage at E9.5. While cochlear neurons appear to differentiate normally in the setting of Chd7 haploinsufficiency, our results suggest an important role for CHD7 in glial cells in the inner ear. This study highlights the dynamic nature of CHD7 activity during inner ear development in mice and contributes to understanding CHARGE syndrome pathology.

Noise overstimulation of young adult UMHET4 mice accelerates age-related hearing loss.

Many factors contribute to hearing loss commonly found in older adults. There can be natural aging of cellular elements, hearing loss previously induced by environmental factors such as noise or ototoxic drugs as well as genetic and epigenetic influences. Even when noise overstimulation does not immediately cause permanent hearing loss it has recently been shown to increase later age-related hearing loss (ARHL). The present study further investigated this condition in the UMHET4 mouse model by comparing a small arms fire (SAF)-like impulse noise exposure that has the greatest immediate effect in more apical cochlear regions to a broadband noise (BBN) exposure that has the greatest immediate effect in more basal cochlear regions. Both noise exposures were given at levels that only induced temporary auditory brainstem response (ABR) threshold shifts (TS). Mice were noise exposed at 5 months of age followed by ABR assessment at 6, 12, 18, 21, and 24 months of age. Mice that received the SAF-like impulse noise had accelerated age-related TS at 4 kHz that appeared at 12 months of age (significantly increased compared to no-noise controls). This increased TS at 4 kHz continued at 18 and 21 months but was no longer significantly greater at 24 months of age. The SAF-like impulse noise also induced a significantly greater mean TS at 48 kHz, first appearing at 18 months of age and continuing to be significantly greater than controls at 21 and 24 months. The BBN induced a different pace and pattern of enhanced age-related ABR TS. The mean TS for the BBN group first became significantly greater than controls at 18 months of age and only at 48 kHz. It remained significantly greater than controls at 21 months but was no longer significantly greater at 24 months of age. Results, therefore, show different influences on ARHL for the two different noise exposure conditions. Noise-induced enhancement appears to provide more an acceleration than overall total increase in ARHL.

Effects of Calcitonin-Gene-Related-Peptide on Auditory Nerve Activity.

Calcitonin-gene-related peptide (CGRP) is a lateral olivocochlear (LOC) efferent neurotransmitter. Depression of sound-driven auditory brainstem response amplitude in CGRP-null mice suggests the potential for endogenous CGRP release to upregulate spontaneous and/or sound-driven auditory nerve (AN) activity. We chronically infused CGRP into the guinea pig cochlea and evaluated changes in AN activity as well as outer hair cell (OHC) function. The amplitude of both round window noise (a measure of ensemble spontaneous activity) and the synchronous whole-nerve response to sound (compound action potential, CAP) were enhanced. Lack of change in both onset adaptation and steady state amplitude of sound-evoked distortion product otoacoustic emission (DPOAE) responses indicated CGRP had no effect on OHCs, suggesting the origin of the observed changes was neural. Combined with results from the CGRP-null mice, these results appear to confirm that endogenous CGRP enhances auditory nerve activity when released by the LOC neurons. However, infusion of the CGRP receptor antagonist CGRP (8-37) did not reliably influence spontaneous or sound-driven AN activity, or OHC function, results that contrast with the decreased ABR amplitude measured in CGRP-null mice.

GJB2 gene therapy and conditional deletion reveal developmental stage-dependent effects on inner ear structure and function.

Pathogenic variants in GJB2, the gene encoding connexin 26, are the most common cause of autosomal-recessive hereditary deafness. Despite this high prevalence, pathogenic mechanisms leading to GJB2-related deafness are not well understood, and cures are absent. Humans with GJB2-related deafness retain at least some auditory hair cells and neurons, and their deafness is usually stable. In contrast, mice with conditional loss of Gjb2 in supporting cells exhibit extensive loss of hair cells and neurons and rapidly progress to profound deafness, precluding the application of therapies that require intact cochlear cells. In an attempt to design a less severe Gjb2 animal model, we generated mice with inducible Sox10iCre ERT2 -mediated loss of Gjb2. Tamoxifen injection led to reduced connexin 26 expression and impaired function, but cochlear hair cells and neurons survived for 2 months, allowing phenotypic rescue attempts within this time. AAV-mediated gene transfer of GJB2 in mature mutant ears did not demonstrate threshold improvement and in some animals exacerbated hearing loss and resulted in hair cell loss. We conclude that Sox10iCre ERT2 ;Gjb2 flox/flox mice are valuable for studying the biology of connexin 26 in the cochlea. In particular, these mice may be useful for evaluating gene therapy vectors and development of therapies for GJB2-related deafness.

Rapamycin Added to Diet in Late Mid-Life Delays Age-Related Hearing Loss in UMHET4 Mice.

Our previous study demonstrated rapamycin added to diet at 4 months of age had significantly less age-related outer hair cell loss in the basal half of the cochlea at 22 months of age compared to mice without rapamycin. The present study tested adding rapamycin to diet later in life, at 14 months of age, and added a longitudinal assessment of auditory brain stem response (ABR). The present study used UMHET4 mice, a 4 way cross in which all grandparental strains lack the Cdh23753A allele that predisposes to early onset, progressive hearing loss. UMHET4 mice typically have normal hearing until 16-17 months, then exhibit threshold shifts at low frequencies/apical cochlea and later in more basal high frequency regions. ABR thresholds at 4, 12, 24, and 48 kHz were assessed at 12, 18, and 24 months of age and compared to baseline ABR thresholds acquired at 5 months of age to determine threshold shifts (TS). There was no TS at 12 months of age at any frequency tested. At 18 months of age mice with rapamycin added to diet at 14 months had a significantly lower mean TS at 4 and 12 kHz compared to mice on control diet with no significant difference at 24 and 48 kHz. At 24 months of age, the mean 4 kHz TS in rapamycin diet group was no longer significantly lower than the control diet group, while the 12 kHz mean remained significantly lower. Mean TS at 24 and 48 kHz in the rapamycin diet group became significantly lower than in the control diet group at 24 months. Hair cell counts at 24 months showed large loss in the apical half of most rapamycin and control diet mice cochleae with no significant difference between groups. There was only mild outer hair cell loss in the basal half of rapamycin and control diet mice cochleae with no significant difference between groups. The results show that a later life addition of rapamycin can decrease age-related hearing loss in the mouse model, however, it also suggests that this decrease is a delay/deceleration rather than a complete prevention.

Small Arms Fire-like noise: Effects on Hearing Loss, Gap Detection and the Influence of Preventive Treatment.

A noise-induced loss of inner hair cell (IHC) - auditory nerve synaptic connections has been suggested as a factor that can trigger the progression of maladaptive plastic changes leading to noise-induced tinnitus. The present study used a military relevant small arms fire (SAF)-like noise (50 biphasic impulses over 2.5 min at 152 dB SPL given unilaterally to the right ear) to induce loss (∼1/3) of IHC synaptic ribbons (associated with synapse loss) in rat cochleae with only minor (less than 10%) loss of outer hair cells. Approximately half of the noise-exposed rats showed poorer Gap Detection post-noise, a behavioral indication suggesting the presence of tinnitus. There was significantly greater loss of IHC ribbons in noise-exposed rats with reduced Gap Detection compared to noise-exposed rats retaining normal Gap Detection. We have previously shown systemic administration of piribedil, memantine, and/or ACEMg significantly reduced loss of IHC ribbons induced by a 3 h 4 kHz octave band 117 dB (SPL) noise. The present study examined if this treatment would also reduce ribbon loss from the SAF-like noise exposure and if this would prevent the reduced Gap Detection. As in the previous study, piribedil, memantine, and ACEMg treatment significantly reduced the noise-induced loss of ribbons, such that it was no longer significantly different from normal. However, it did not prevent development of the reduced Gap Detection indication of tinnitus in all treated noise-exposed rats, reducing the incidence but not reaching significance.

Grxcr2 is required for stereocilia morphogenesis in the cochlea.

Hearing and balance depend upon the precise morphogenesis and mechanosensory function of stereocilia, the specialized structures on the apical surface of sensory hair cells in the inner ear. Previous studies of Grxcr1 mutant mice indicated a critical role for this gene in control of stereocilia dimensions during development. In this study, we analyzed expression of the paralog Grxcr2 in the mouse and evaluated auditory and vestibular function of strains carrying targeted mutations of the gene. Peak expression of Grxcr2 occurs during early postnatal development of the inner ear and GRXCR2 is localized to stereocilia in both the cochlea and in vestibular organs. Homozygous Grxcr2 deletion mutants exhibit significant hearing loss by 3 weeks of age that is associated with developmental defects in stereocilia bundle orientation and organization. Despite these bundle defects, the mechanotransduction apparatus assembles in relatively normal fashion as determined by whole cell electrophysiological evaluation and FM1-43 uptake. Although Grxcr2 mutants do not exhibit overt vestibular dysfunction, evaluation of vestibular evoked potentials revealed subtle defects of the mutants in response to linear accelerations. In addition, reduced Grxcr2 expression in a hypomorphic mutant strain is associated with progressive hearing loss and bundle defects. The stereocilia localization of GRXCR2, together with the bundle pathologies observed in the mutants, indicate that GRXCR2 plays an intrinsic role in bundle orientation, organization, and sensory function in the inner ear during development and at maturity.

PlexinA2 Forward Signaling through Rap1 GTPases Regulates Dentate Gyrus Development and Schizophrenia-like Behaviors.

Dentate gyrus (DG) development requires specification of granule cell (GC) progenitors in the hippocampal neuroepithelium, as well as their proliferation and migration into the primordial DG. We identify the Plexin family members Plxna2 and Plxna4 as important regulators of DG development. Distribution of immature GCs is regulated by Sema5A signaling through PlxnA2 and requires a functional PlxnA2 GTPase-activating protein (GAP) domain and Rap1 small GTPases. In adult Plxna2-/- but not Plxna2-GAP-deficient mice, the dentate GC layer is severely malformed, neurogenesis is compromised, and mossy fibers form aberrant synaptic boutons within CA3. Behavioral studies with Plxna2-/- mice revealed deficits in associative learning, sociability, and sensorimotor gating-traits commonly observed in neuropsychiatric disorder. Remarkably, while morphological defects are minimal in Plxna2-GAP-deficient brains, defects in fear memory and sensorimotor gating persist. Since allelic variants of human PLXNA2 and RAP1 associate with schizophrenia, our studies identify a biochemical pathway important for brain development and mental health.

Generation and Characterization of α9 and α10 Nicotinic Acetylcholine Receptor Subunit Knockout Mice on a C57BL/6J Background.

We generated constitutive knockout mouse models for the α9 and α10 nicotinic acetylcholine receptor (nAChR) subunits by derivation from conditional knockouts by breeding with CRE deleter mice. We then backcrossed them onto a C57BL/6J genetic background. In this manuscript, we report the generation of the strains and an auditory phenotypic characterization of the constitutive α9 and α10 knockouts and a double α9α10 constitutive knockout. Although the α9 and α10 nAChR subunits are relevant to a number of physiological measures, we chose to characterize the mouse with auditory studies to compare them to existing but different α9 and α10 nAChR knockouts (KOs). Auditory brainstem response (ABR) measurements and distortion product otoacoustic emissions (DPOAEs) showed that all constitutive mouse strains had normal hearing. DPOAEs with contralateral noise (efferent adaptation measurements), however, showed that efferent strength was significantly reduced after deletion of both the α9 and α10 subunits, in comparison to wildtype controls. Animals tested were 3-8 weeks of age and efferent strength was not correlated with age. Confocal studies of single and double constitutive KOs showed that all KOs had abnormal efferent innervation of cochlear hair cells. The morphological results are similar to those obtained in other strains using constitutive deletion of exon 4 of α9 or α10 nAChR. The results of our physiological studies, however, differ from previous auditory studies using a α9 KO generated by deletion of the exon 4 region and backcrossed onto a mixed CBA/CaJ X 129Sv background.

Selective hair cell ablation and noise exposure lead to different patterns of changes in the cochlea and the cochlear nucleus.

In experimental animal models of auditory hair cell (HC) loss, insults such as noise or ototoxic drugs often lead to secondary changes or degeneration in non-sensory cells and neural components, including reduced density of spiral ganglion neurons, demyelination of auditory nerve fibers and altered cell numbers and innervation patterns in the cochlear nucleus (CN). However, it is not clear whether loss of HCs alone leads to secondary degeneration in these neural components of the auditory pathway. To elucidate this issue, we investigated changes of central components after cochlear insults specific to HCs using diphtheria toxin receptor (DTR) mice expressing DTR only in HCs and exhibiting complete HC loss when injected with diphtheria toxin (DT). We showed that DT-induced HC ablation has no significant impacts on the survival of auditory neurons, central synaptic terminals, and myelin, despite complete HC loss and profound deafness. In contrast, noise exposure induced significant changes in synapses, myelin and CN organization even without loss of inner HCs. We observed a decrease of neuronal size in the auditory pathway, including peripheral axons, spiral ganglion neurons, and CN neurons, likely due to loss of input from the cochlea. Taken together, selective HC ablation and noise exposure showed different patterns of pathology in the auditory pathway and the presence of HCs is not essential for the maintenance of central synaptic connectivity and myelination.

The 133-kDa N-terminal domain enables myosin 15 to maintain mechanotransducing stereocilia and is essential for hearing.

The precise assembly of inner ear hair cell stereocilia into rows of increasing height is critical for mechanotransduction and the sense of hearing. Yet, how the lengths of actin-based stereocilia are regulated remains poorly understood. Mutations of the molecular motor myosin 15 stunt stereocilia growth and cause deafness. We found that hair cells express two isoforms of myosin 15 that differ by inclusion of an 133-kDa N-terminal domain, and that these isoforms can selectively traffic to different stereocilia rows. Using an isoform-specific knockout mouse, we show that hair cells expressing only the small isoform remarkably develop normal stereocilia bundles. However, a critical subset of stereocilia with active mechanotransducer channels subsequently retracts. The larger isoform with the 133-kDa N-terminal domain traffics to these specialized stereocilia and prevents disassembly of their actin core. Our results show that myosin 15 isoforms can navigate between functionally distinct classes of stereocilia, and are independently required to assemble and then maintain the intricate hair bundle architecture.

Ototoxicity-induced loss of hearing and inner hair cells is attenuated by HSP70 gene transfer.

The most common reason for sensorineural deafness is death of hair cells (HCs). Heat shock proteins (HSPs) are molecular chaperones that participate in folding, targeting, and degrading proteins. HSP expression is increased in response to various environmental stresses to protect cells from damage. Here, we tested whether viral-mediated overexpression of HSP70 can protect HCs and hearing from severe ototoxicity (kanamycin and furosemide) in guinea pigs. Adenovirus-HSP70 mCherry (Ad.HSP70-mCherry) was injected to experimental animals and adenovirus-mCherry to controls, 4 days before the ototoxic insult. Hearing thresholds were measured by auditory brainstem response before the insult and again before sacrificing the animals, 14 days after the insult. Epi-fluorescence immunocytochemistry showed that injection of Ad.HSP70-mCherry resulted in mCherry fluorescence in nonsensory cells of the organ of Corti. The ototoxic insult eliminated both outer HCs and inner HCs throughout most of the cochlea of control (adenovirus-mCherry-injected) ears and contralateral (uninjected) ears. Ad.HSP70-mCherry-injected ears exhibited a significant preservation of inner HCs compared to control and contralateral ears, but outer HCs were not protected. Auditory brainstem response thresholds were significantly better in Ad.HSP70-mCherry-injected ears than in control and contralateral ears. Our data show that HSP70 augmentation may represent a potential therapy attenuating ototoxic inner HC loss.

Disruption of lateral olivocochlear neurons with a dopaminergic neurotoxin depresses spontaneous auditory nerve activity.

Neurons of the lateral olivocochlear (LOC) system project from the auditory brainstem to the cochlea, where they synapse on radial dendrites of auditory nerve fibers. Selective LOC disruption depresses sound-evoked auditory nerve activity in the guinea pig, but enhances it in the mouse. Here, LOC disruption depressed spontaneous auditory nerve activity in the guinea pig. Recordings from single auditory nerve fibers revealed a significantly reduced proportion of fibers with the highest spontaneous firing rates (SRs) and an increased proportion of neurons with lower SRs. Ensemble activity, estimated using round window noise, also decreased after LOC disruption. Decreased spontaneous activity after LOC disruption may be a consequence of reduced tonic release of excitatory transmitters from the LOC terminals in guinea pigs.

OTO-201: nonclinical assessment of a sustained-release ciprofloxacin hydrogel for the treatment of otitis media.

HYPOTHESIS: OTO-201 can provide sustained release to the middle ear and effectively treat otitis media, when compared with FDA-approved ciprofloxacin otic drop formulations. BACKGROUND: There is an unmet medical need for antibiotic therapy that can provide a full course of treatment from a single administration by an otolaryngologist at the time of tympanostomy tube placement, obviating the need for twice daily multiday treatment with short-acting otic drops. METHODS: Studies in guinea pigs and chinchillas were conducted. OTO-201 was administered as a single intratympanic injection and compared with the twice daily multi-day treatment with Ciprodex or Cetraxal otic drops. RESULTS: OTO-201 demonstrated sustained release of ciprofloxacin in the middle ear compartment for days to approximately 2 weeks depending on the dose. The substantial C(max) values and steady drug exposure yielded by OTO-201 were in contrast to the pulsatile short lasting exposure seen with Ciprodex and Cetraxal. OTO-201 was also effective in a preclinical chinchilla model of Streptococcus pneumoniae-induced otitis media. The degree of cure was comparable to that afforded by Ciprodex and Cetraxal. There was no evidence of middle or inner ear pathology in guinea pigs treated with OTO-201, unlike Ciprodex and Cetraxal, which both demonstrated mild cochlear ototoxicity. No adverse effects of the poloxamer 407 vehicle were noted. CONCLUSION: Intratympanic injection of OTO-201 constitutes an attractive treatment option to twice daily multiday dosing with ciprofloxacin ear drops for the treatment of otitis media, as evidenced by superior middle ear drug exposure, efficacy in an acute otitis media model, safety of administration, and convenience of a single dose regimen.

Conditioning the cochlea to facilitate survival and integration of exogenous cells into the auditory epithelium.

The mammalian auditory epithelium (AE) cannot replace supporting cells and hair cells once they are lost. Therefore, sensorineural hearing loss associated with missing cells is permanent. This inability to regenerate critical cell types makes the AE a potential target for cell replacement therapies such as stem cell transplantation. Inserting stem cells into the AE of deaf ears is a complicated task due to the hostile, high potassium environment of the scala media in the cochlea, and the robust junctional complexes between cells in the AE that resist stem cell integration. Here, we evaluate whether temporarily reducing potassium levels in the scala media and disrupting the junctions in the AE make the cochlear environment more receptive and facilitate survival and integration of transplanted cells. We used sodium caprate to transiently disrupt the AE junctions, replaced endolymph with perilymph, and blocked stria vascularis pumps with furosemide. We determined that these three steps facilitated survival of HeLa cells in the scala media for at least 7 days and that some of the implanted cells formed a junctional contact with native AE cells. The data suggest that manipulation of the cochlear environment facilitates survival and integration of exogenously transplanted HeLa cells in the scala media.

Hearing dysfunction in heterozygous Mitf(Mi-wh) /+ mice, a model for Waardenburg syndrome type 2 and Tietz syndrome.

The human deafness-pigmentation syndromes, Waardenburg syndrome (WS) type 2a, and Tietz syndrome are characterized by profound deafness but only partial cutaneous pigmentary abnormalities. Both syndromes are caused by mutations in MITF. To illuminate differences between cutaneous and otic melanocytes in these syndromes, their development and survival in heterozygous Microphthalmia-White (Mitf(Mi-wh) /+) mice were studied and hearing function of these mice characterized. Mitf(Mi-wh) /+ mice have a profound hearing deficit, characterized by elevated auditory brainstem response thresholds, reduced distortion product otoacoustic emissions, absent endocochlear potential, loss of outer hair cells, and stria vascularis abnormalities. Mitf(Mi-wh) /+ embryos have fewer melanoblasts during embryonic development than their wild-type littermates. Although cochlear melanocytes are present at birth, they disappear from the Mitf(Mi-wh) /+ cochlea between P1 and P7. These findings may provide insight into the mechanism of melanocyte and hearing loss in human deafness-pigmentation syndromes such as WS and Tietz syndrome and illustrate differences between otic and follicular melanocytes.

Macrophage migration inhibitory factor acts as a neurotrophin in the developing inner ear.

This study is the first to demonstrate that macrophage migration inhibitory factor (MIF), an immune system 'inflammatory' cytokine that is released by the developing otocyst, plays a role in regulating early innervation of the mouse and chick inner ear. We demonstrate that MIF is a major bioactive component of the previously uncharacterized otocyst-derived factor, which directs initial neurite outgrowth from the statoacoustic ganglion (SAG) to the developing inner ear. Recombinant MIF acts as a neurotrophin in promoting both SAG directional neurite outgrowth and neuronal survival and is expressed in both the developing and mature inner ear of chick and mouse. A MIF receptor, CD74, is found on both embryonic SAG neurons and adult mouse spiral ganglion neurons. Mif knockout mice are hearing impaired and demonstrate altered innervation to the organ of Corti, as well as fewer sensory hair cells. Furthermore, mouse embryonic stem cells become neuron-like when exposed to picomolar levels of MIF, suggesting the general importance of this cytokine in neural development.

Induction of heat shock proteins by hyperthermia and noise overstimulation in hsf1 -/- mice.

Diverse cellular and environmental stresses can activate the heat shock response, an evolutionarily conserved mechanism to protect proteins from denaturation. Stressors activate heat shock transcription factor 1 (HSF1), which binds to heat shock elements in the genes for heat shock proteins, leading to rapid induction of these important molecular chaperones. Both heat and noise stress are known to activate the heat shock response in the cochlea and protect it from subsequent noise trauma. However, the contribution of HSF1 to induction of heat shock proteins following noise trauma has not been investigated at the molecular level. We evaluated the role of HSF1 in the cochlea following noise stress by examining induction of heat shock proteins in Hsf1 ( +/- ) control and Hsf1 ( -/- ) mice. Heat stress rapidly induced expression of Hsp25, Hsp47, Hsp70.1, Hsp70.3, Hsp84, Hsp86, and Hsp110 in the cochleae of wild-type and Hsf1 ( +/- ) mice, but not in Hsf1 ( -/- ) mice, confirming the essential role of HSF1 in mediating the heat shock response. Exposure to broadband noise (2-20 kHz) at 106 dB SPL for 2 h produced partial hearing loss. Maximal induction of heat shock proteins occurred 4 h after the noise. In comparison to heat stress, noise stress resulted in lower induced levels of Hsp25, Hsp70.1, Hsp70.3, Hsp86, and Hsp110 in Hsf1 ( +/- ) mice. Induction of these heat shock proteins was attenuated, but not completely eliminated, in Hsf1 ( -/- ) mice. These same noise exposure conditions induced genes for several immediate early transcription factors and maximum induction occurred earlier than for heat shock proteins. Thus, additional signaling pathways and transcriptional regulators that are activated by noise probably contribute to induction of heat shock proteins in the cochlea.

Genetic background of Prop1(df) mutants provides remarkable protection against hypothyroidism-induced hearing impairment.

Hypothyroidism is a cause of genetic and environmentally induced deafness. The sensitivity of cochlear development and function to thyroid hormone (TH) mandates understanding TH action in this sensory organ. Prop1(df) and Pou1f1(dw) mutant mice carry mutations in different pituitary transcription factors, each resulting in pituitary thyrotropin deficiency. Despite the same lack of detectable serum TH, these mutants have very different hearing abilities: Prop1(df) mutants are mildly affected, while Pou1f1(dw) mutants are completely deaf. Genetic studies show that this difference is attributable to the genetic backgrounds. Using embryo transfer, we discovered that factors intrinsic to the fetus are the major contributor to this difference, not maternal effects. We analyzed Prop1(df) mutants to identify processes in cochlear development that are disrupted in other hypothyroid animal models but protected in Prop1(df) mutants by the genetic background. The development of outer hair cell (OHC) function is delayed, but Prestin and KCNQ4 immunostaining appear normal in mature Prop1(df) mutants. The endocochlear potential and KCNJ10 immunostaining in the stria vascularis are indistinguishable from wild type, and no differences in neurofilament or synaptophysin staining are evident in Prop1(df) mutants. The synaptic vesicle protein otoferlin normally shifts expression from OHC to IHC as temporary afferent fibers beneath the OHC regress postnatally. Prop1(df) mutants exhibit persistent, abnormal expression of otoferlin in apical OHC, suggesting delayed maturation of synaptic function. Thus, the genetic background of Prop1(df) mutants is remarkably protective for most functions affected in other hypothyroid mice. The Prop1(df) mutant is an attractive model for identifying the genes that protect against deafness.

A modifier gene alleviates hypothyroidism-induced hearing impairment in Pou1f1dw dwarf mice.

Thyroid hormone has pleiotropic effects on cochlear development, and genomic variation influences the severity of associated hearing deficits. DW/J-Pou1f1dw/dw mutant mice lack pituitary thyrotropin, which causes severe thyroid hormone deficiency and profound hearing impairment. To assess the genetic complexity of protective effects on hypothyroidism-induced hearing impairment, an F1 intercross was generated between DW/J-Pou1f1dw/+ carriers and an inbred strain with excellent hearing derived from Mus castaneus, CAST/EiJ. Approximately 24% of the (DW/J×CAST/EiJ) Pou1f1dw/dw F2 progeny had normal hearing. A genome scan revealed a locus on chromosome 2, named modifier of dw hearing, or Mdwh, that rescues hearing despite persistent hypothyroidism. This chromosomal region contains the modifier of tubby hearing 1 (Moth1) locus that encodes a protective allele of the microtubule-associated protein MTAP1A. DW/J-Pou1f1dw/+ carriers were crossed with the AKR strain, which also carries a protective allele of Mtap1a, and we found that AKR is not protective for hearing in the (DW/J×AKR) Pou1f1dw/dw F2 progeny. Thus, protective alleles of Mtap1a are not sufficient to rescue DW/J-Pou1f1dw/dw hearing. We expect that identification of protective modifiers will enhance our understanding of the mechanisms of hypothyroidism-induced hearing impairment.