Targeting hearing genes in mice

Targeting p53 for neuroinflammation: New therapeutic strategies in ischemic stroke

Ischemic stroke (IS) is characterized by high incidence, high recurrence, and high mortality and places a heavy burden on society and families. The pathological mechanisms of IS are complex, among which secondary neurological impairment mediated by neuroinflammation is considered to be the main factor in cerebral ischemic injury. At present, there is still a lack of specific therapies to treat neuroinflammation. The tumor suppressor protein p53 has long been regarded as a key substance in the regulation of the cell cycle and apoptosis in the past. Recently, studies have found that p53 also plays an important role in neuroinflammatory diseases, such as IS. Therefore, p53 may be a crucial target for the regulation of the neuroinflammatory response. Here, we provide a comprehensive review of the potential of targeting p53 in the treatment of neuroinflammation after IS. We describe the function of p53, the major immune cells involved in neuroinflammation, and the role of p53 in inflammatory responses mediated by these cells. Finally, we summarize the therapeutic strategies of targeting p53 in regulating the neuroinflammatory response after IS to provide new directions and ideas for the treatment of ischemic brain injury.

Vestibular dysfunction in the adult CBA/CaJ mouse after lead and cadmium treatment

OBJECTIVES

The vestibular system allows the perception of position and motion and its dysfunction presents as motion impairment, vertigo and balance abnormalities, leading to debilitating psychological discomfort and difficulty performing daily tasks. Although declines and deficits in vestibular function have been noted in rats exposed to lead (Pb) and in humans exposed to Pb and cadmium (Cd), no studies have directly examined the pathological and pathophysiological effects upon the vestibular apparatus of the inner ear.

METHODS

Eighteen young adult mice were exposed through their drinking water (3 mM Pb, 300 µM Cd, or a control treatment) for 10 weeks. Before and after treatment, they underwent a vestibular assessment, consisting of a rotarod performance test and a novel head stability test to measure the vestibulocolic reflex. At the conclusion of the study, the utricles were analyzed immunohistologically for condition of hair cells and nerve fibers.

RESULTS

Increased levels of Pb exposure correlated with decreased head stability in space; no significant decline in performance on rotarod test was found. No damage to the hair cells or the nerve fibers of the utricle was observed in histology.

CONCLUSIONS

The young adult CBA/CaJ mouse is able to tolerate occupationally-relevant Pb and Cd exposure well, but the correlation between Pb exposure and reduced head stability suggests that Pb exposure causes a decline in vestibular function. © 2016 Wiley Periodicals, Inc. Environ Toxicol, 2016. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 869-876, 2017.

The Time Course of Deafness and Retinal Degeneration in a Kunming Mouse Model for Usher Syndrome

Usher syndrome is a group of autosomal recessive diseases characterized by congenital deafness and retinitis pigmentosa. In a mouse model for Usher syndrome, KM^ush/ush_, discovered in our laboratory, we measured the phenotypes, characterized the architecture and morphology of the retina, and quantified the level of expression of pde6b and ush2a between postnatal (P) days 7, and 56. Electroretinograms and auditory brainstem response were used to measure visual and auditory phenotypes. Fundus photography and light microscopy were used to measure the architecture and morphology of the retina. Quantitative real-time PCR was used to measure the expression levels of mRNA. KM^ush/ush mice had low amplitudes and no obvious waveforms of Electroretinograms after P14 compared with controls. Thresholds of auditory brainstem response in our model were higher than those of controls after P14. By P21, the retinal vessels of KM^ush/ush mice were attenuated and their optic discs had a waxy pallor. The retinas of KM^ush/ush mice atrophied and the choroidal vessels were clearly visible. Notably, the architecture of each retinal layer was not different as compared with control mice at P7, while the outer nuclear layer (ONL) and other retinal layers of KM^ush/ush mice were attenuated significantly between P14 and P21. ONL cells were barely seen in KM^ush/ush mice at P56. As compared with control mice, the expression of pde6b and ush2a in KM^ush/ush mice declined significantly after P7. This study is a first step toward characterizing the progression of disease in our mouse model. Future studies using this model may provide insights about the etiology of the disease and the relationships between genotypes and phenotypes providing a valuable resource that could contribute to the foundation of knowledge necessary to develop therapies to prevent the retinal degeneration in patients with Usher Syndrome.

Retina-specific gene excision by targeted expression of Cre recombinase

The use of Cre recombinase for conditional targeting permits the controlled removal or activation of genes in specific tissues and at specific times of development. The Rho-Cre mice provide an improved tool for studying gene ablation in rod photoreceptor cells. To establish a robust expression of Rho-Cre transgenic mice that would be useful for the study of various protein functions in photoreceptor cells, a total 11,987 kb fragment (pNCHS4 Rho-NLS-cre) containing human rhodopsin promoter was cloned. The Rho-Cre plasmid was digested with EcoR1 and I Ceu-1, and the 9.316 kb fragment containing the hRho promoter and Cre recombinase gel was purified. To generate transgenic mice, the purified DNA fragment was injected into fertilized oocytes according to standard protocols. ROSA26R reported the steady expression of Rho-Cre especially in photoreceptor cells, allowing further excising proteins in rod photoreceptors across the retina. This Rho-Cre transgenic line should thus prove useful as a general deletor line for genetic analysis of diverse aspects of retinopathy.

Conditional gene manipulation: Cre-ating a new biological era

To solve the problem of embryonic lethality in conventional gene knockouts, site-specific recombinase (SSR) systems (Cre-loxP, Flp-FRT, and ΦC31) have been used for tissue-specific gene knockout. With the combination of an SSR system and inducible gene expression systems (tetracycline and tamoxifen), stage-specific knockout and transgenic expression can be achieved. The application of this "SSR+inducible" conditional tool to genomic manipulation can be extended in various ways. Alternatives to conditional gene targeting, such as conditional gene trapping, multipurpose conditional alleles, and conditional gene silencing, have been developed. SSR systems can also be used to construct precise disease models with point mutations and chromosomal abnormalities. With these exciting achievements, we are moving towards a new era in which the whole genome can be manipulated as we wish.

Conditional gene expression in the mouse inner ear using Cre-loxP

In recent years, there has been significant progress in the use of Cre-loxP technology for conditional gene expression in the inner ear. Here, we introduce the basic concepts of this powerful technology, emphasizing the differences between Cre and CreER. We describe the creation and Cre expression pattern of each Cre and CreER mouse line that has been reported to have expression in auditory and vestibular organs. We compare the Cre expression patterns between Atoh1-CreER(TM) and Atoh1-CreER(T2) and report a new line, Fgfr3-iCreER(T2), which displays inducible Cre activity in cochlear supporting cells. We also explain how results can vary when transgenic vs. knock-in Cre/CreER alleles are used to alter gene expression. We discuss practical issues that arise when using the Cre-loxP system, such as the use of proper controls, Cre efficiency, reporter expression efficiency, and Cre leakiness. Finally, we introduce other methods for conditional gene expression, including Flp recombinase and the tetracycline-inducible system, which can be combined with Cre-loxP mouse models to investigate conditional expression of more than one gene.

Outer hair cell-specific prestin-CreERT2 knockin mouse lines

Outer hair cells (OHCs) in the cochlea are crucial for the remarkable hearing sensitivity and frequency tuning. To understand OHC physiology and pathology, it is imperative to use mouse genetic tools to manipulate gene expression specifically in OHCs. Here, we generated two prestin knockin mouse lines: (1) the prestin-CreERT2 line, with an internal ribosome entry site-CreERT2-FRT-Neo-FRT cassette inserted into the prestin locus after the stop codon, and (2) the prestin-CreERT2-NN line, with the FRT-Neo-FRT removed subsequently. We characterized the inducible Cre activity of both lines by crossing them with the reporter lines CAG-eGFP and Ai6. Cre activity was induced with tamoxifen at various postnatal ages and only detected in OHCs, resembling the endogenous prestin expression pattern. Moreover, prestin-CreERT2+/-(heterozygotes) and +/+(homozygotes) as well as prestin-CreERT2-NN+/-mice displayed normal hearing. These two prestin-CreERT2 mouse lines are therefore useful tools to analyze gene function in OHCs in vivo.

Development of tinnitus in CBA/CaJ mice following sound exposure

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

A review of current large-scale mouse knockout efforts

After the successful completion of the human genome project (HGP), biological research in the postgenome era urgently needs an efficient approach for functional analysis of genes. Utilization of knockout mouse models has been powerful for elucidating the function of genes as well as finding new therapeutic interventions for human diseases. Gene trapping and gene targeting are two independent techniques for making knockout mice from embryonic stem (ES) cells. Gene trapping is high-throughput, random, and sequence-tagged while gene targeting enables the knockout of specific genes. It has been about 20 years since the first gene targeting and gene trapping mice were generated. In recent years, new tools have emerged for both gene targeting and gene trapping, and organizations have been formed to knock out genes in the mouse genome using either of the two methods. The knockout mouse project (KOMP) and the international gene trap consortium (IGTC) were initiated to create convenient resources for scientific research worldwide and knock out all the mouse genes. Organizers of KOMP regard it as important as the HGP. Gene targeting methods have changed from conventional gene targeting to high-throughput conditional gene targeting. The combined advantages of trapping and targeting elements are improving the gene trapping spectrum and gene targeting efficiency. As a newly-developed insertional mutation system, transposons have some advantages over retrovirus in trapping genes. Emergence of the international knockout mouse consortium (IKMP) is the beginning of a global collaboration to systematically knock out all the genes in the mouse genome for functional genomic research.

Sox10 promotes the survival of cochlear progenitors during the establishment of the organ of Corti

Transcription factors of the SoxE family are critical players that underlie various embryological processes. However, little is known about their function during inner ear development. Here, we show that Sox10 is initially expressed throughout the otic vesicle epithelium and becomes later restricted to supporting cells as cell differentiation proceeds in the organ of Corti. Morphological analyses of Sox10 mutant mice reveal a significant shortening of the cochlear duct likely resulting from the progressive depletion of cochlear progenitors. While Sox10 appears dispensable for the differentiation and patterning of the inner ear prosensory progenitors, our data support a critical role for this transcription factor in the promotion of their survival. We provide genetic evidences that Sox10, in a concentration-dependant manner, could play a role in the regulation of Jagged1, a gene known to be important for inner ear prosensory development. Together, our results demonstrate that Sox10 regulates the biology of early cochlear progenitors during inner ear development, but, in contrast to neural crest-derived cells, this transcription factor is dispensable for their differentiation. Evidence also suggests that this effect occurs via the activation of the Jagged1 gene.

Essential and synergistic roles of RP1 and RP1L1 in rod photoreceptor axoneme and retinitis pigmentosa

Retinitis pigmentosa 1 (RP1) is a common inherited retinopathy with variable onset and severity. The RP1 gene encodes a photoreceptor-specific, microtubule-associated ciliary protein containing the doublecortin (DCX) domain. Here we show that another photoreceptor-specific Rp1-like protein (Rp1L1) in mice is also localized to the axoneme of outer segments (OSs) and connecting cilia in rod photoreceptors, overlapping with Rp1. Rp1L1-/- mice display scattered OS disorganization, reduced electroretinogram amplitudes, and progressive photoreceptor degeneration, less severe and slower than in Rp1-/- mice. In single rods of Rp1L1-/-, photosensitivity is reduced, similar to that of Rp1-/-. While individual heterozygotes are normal, double heterozygotes of Rp1 and Rp1L1 exhibit abnormal OS morphology and reduced single rod photosensitivity and dark currents. The electroretinogram amplitudes of double heterozygotes are more reduced than those of individual heterozygotes combined. In support, Rp1L1 interacts with Rp1 in transfected cells and in retina pull-down experiments. Interestingly, phototransduction kinetics are normal in single rods and whole retinas of individual or double Rp1 and Rp1L1 mutant mice. Together, Rp1 and Rp1L1 play essential and synergistic roles in affecting photosensitivity and OS morphogenesis of rod photoreceptors. Our findings suggest that mutations in RP1L1 could underlie retinopathy or modify RP1 disease expression in humans.

Unraveling inner ear induction by gene manipulation using Pax2-Cre BAC transgenic mice

One of the biggest drawbacks of conventional mouse knockout techniques in the study of the inner ear is that loss of a gene of interest may cause embryonic lethality before the inner ear develops. Thus, there is a need for an inner ear-specific gene manipulation system for loss- and gain-of-function analysis in the mouse inner ear. We generated a Pax2-Cre BAC transgenic line in which Cre recombinase expression recapitulates Pax2 expression in the presumptive otic ectoderm. Here, we present a brief summary of a recent model of inner ear induction suggested by the results of inner ear-specific gene modification using Pax2-Cre mice.

The practical use of Cre and loxP technologies in mouse auditory research

Gene manipulation, specifically in the hair cells of the inner ear during development and adulthood in mice, is crucial for understanding the physiology of hearing and the pathology of deafness in humans. Recent advances have demonstrated that gene expression can be manipulated in developing mouse hair cells in a spatially and temporally controlled manner. The Cre-loxP system has been widely used for such purposes. Many laboratories, including ours, have developed and characterized transgenic mouse lines that express or induce Cre activity specifically in inner ear hair cells. These Cre lines have been used with high efficiency to inactivate several genes such as Rb in hair cells. Here we discuss the use of these Cre lines in inner ear research with emphasis on practical issues for researchers who are not familiar with these particular techniques but are interested in using these Cre mice and floxed mice to inactivate genes of their interest specifically in inner ear hair cells. We provide detailed protocols for the use of these techniques and reagents. These considerations and protocols can be easily applied to other cell types in the inner ear and other parts of the auditory pathways. Because the NIH Knockout Mouse Project (KOMP) and the European Conditional Mouse Mutant Program (EUCOMM) have initiated plans to create conditional (floxed) knockout strains for every gene in the mouse genome and because numerous Cre-expressing mouse lines have already been created in various systems, including the nervous system, it is our hope that many hearing researchers will benefit from the detailed protocols and practical considerations described in this review.

Normal hearing in alpha-galactosidase A-deficient mice, the mouse model for Fabry disease

Fabry disease (OMIM 301500) is a rare X-linked recessive disorder caused by mutations in the alpha-galactosidase gene (Gla). Loss of Gla activity leads to the abnormal accumulation of glycosphingolipids in lysosomes of predominantly vascular endothelial cells. Clinically the disorder presents with angiokeratomas, clouding of the cornea, and renal, cardiac, and cerebrovascular complications. In addition, there is an increased incidence of sensorineural hearing loss in Fabry patients. In this study, we investigated the loss of alpha-galactosidase A activity on hearing function in Gla-deficient mice (Gla(tm1Kul)). Gla mRNA was readily detected in the cochlea of 2- and 12-month old C57BL/6J and C3HeB/FeJ mice. The targeted allele was introgressed to the normal hearing C3HeB/FeJ strain to eliminate confounding genetic background effects. Auditory brain stem responses (ABR) to click, 8-, 16-, and 32 kHz stimuli measured at regular intervals from animals at the N4 backcross generation and from N4F1 hybrids demonstrated normal hearing in hemizygous and homozygous mutant mice up to 76 weeks of age. By histological criteria, the cyto-architecture of the mutant cochlea showed a normal appearance. The data demonstrate that in the mouse the loss of alpha-galactosidase A activity is genetically or biochemically buffered and not sufficient per se to cause an appreciable degree of hearing impairment.

Detection and differentiation of sensorineural hearing loss in mice using auditory steady-state responses and transient auditory brainstem responses

Sensorineural hearing loss (SNHL) comprises hearing disorders with diverse pathologies of the inner ear and the auditory nerve. To date, an unambiguous phenotypical characterization of the specific pathologies in an affected individual remains impossible. Here, we evaluated the use of scalp-recorded auditory steady-state responses (ASSR) and transient auditory brainstem responses (ABR) for differentiating the disease mechanisms underlying sensorineural hearing loss in well-characterized mouse models. We first characterized the ASSR evoked by sinusoidally amplitude-modulated tones in wild-type mice. ASSR were robustly elicited within three ranges of modulation frequencies below 200 Hz, from 200 to 600 Hz and beyond 600 Hz in most recordings. Using phase information we estimated the apparent ASSR latency to be about 3 ms, suggesting generation in the auditory brainstem. Auditory thresholds obtained by automated and visual analysis of ASSR recordings were comparable to those found with tone-burst evoked ABR in the same mice. We then recorded ASSR and ABR from mouse mutants bearing defects of either outer hair cell amplification (KCNQ4-knockout) or inner hair cell synaptic transmission (Bassoon-mutant). Both mutants showed an increase of ASSR and ABR thresholds of approximately 40 dB versus wild-type when investigated at 8 weeks of age. Mice with defective amplification displayed a steep rise of ASSR and ABR amplitudes with increasing sound intensity, presumably reflecting a strong recruitment of synchronously activated neural elements beyond threshold. In contrast, the amplitudes of ASSR and ABR responses of mice with impaired synaptic transmission grew very little with sound intensity. In summary, ASSR allow for a rapid, objective and frequency-specific hearing assessment and together with ABR and otoacoustic emissions can contribute to the differential diagnosis of SNHL.

Auditory transduction in the mouse

The sensory hair cells of the mammalian cochlea transduce acoustic stimuli into auditory nerve activity. The biomechanical and molecular details of hair cell mechanotransduction are being acquired at an ever-finer level of resolution. In this review, we discuss how selected mouse mutants and transgenic models have contributed to, and will continue to shape, our understanding of the molecular basis of hair cell mechanotransduction. Functional and structural discoveries made originally in hair cells of nonmammalian vertebrates have been further pursued in the mouse inner ear, where transgenic manipulation can be applied to test molecular mechanisms. Additional insights have been obtained from mice bearing mutations in genes underlying deafness in humans. Taken together, these studies emphasize the elegance of mechanotransduction, enlarge the team of molecular players, and begin to reveal the remarkable adaptations that provide the sensitivity and temporal resolution required for mammalian hearing.

Usherin is required for maintenance of retinal photoreceptors and normal development of cochlear hair cells

Usher syndrome type IIA (USH2A), characterized by progressive photoreceptor degeneration and congenital moderate hearing loss, is the most common subtype of Usher syndrome. In this article, we show that the USH2A protein, also known as usherin, is an exceptionally large ( approximately 600-kDa) matrix protein expressed specifically in retinal photoreceptors and developing cochlear hair cells. In mammalian photoreceptors, usherin is localized to a spatially restricted membrane microdomain at the apical inner segment recess that wraps around the connecting cilia, corresponding to the periciliary ridge complex described for amphibian photoreceptors. In sensory hair cells of the cochlea, it is associated transiently with the hair bundles during postnatal development. Targeted disruption of the Ush2a gene in mice leads to progressive photoreceptor degeneration and a moderate but nonprogressive hearing impairment, mimicking the visual and hearing deficits in USH2A patients. These data suggest that usherin is required for the long-term maintenance of retinal photoreceptors and for the development of cochlear hair cells. We propose a model in which usherin in photoreceptors is tethered via its C terminus to the plasma membrane and its large extracellular domain projecting into the periciliary matrix, where they may interact with the connecting cilium to fulfill important structural or signaling roles.

Splice-site A choice targets plasma-membrane Ca2+-ATPase isoform 2 to hair bundles

Localization of mechanotransduction in sensory hair cells to hair bundles requires selective targeting of essential proteins to specific locations. Isoform 2 of the plasma-membrane Ca2+-ATPase (PMCA2), required for hearing and balance, is found exclusively in hair bundles. We determined the contribution of splicing at the two major splicing sites (A and C) to hair-cell targeting of PMCA2. When PMCA2 isoforms were immunoprecipitated from purified hair bundles of rat utricle, 2w was the only site A variant detected; moreover, immunocytochemistry for 2w in rat vestibular and cochlear tissues indicated that this splice form was located solely in bundles. To demonstrate the necessity of the 2w sequence, we transfected hair cells with PMCA2 containing different variants at splice sites A and C. Although native hair bundles exclusively use the 2a form at splice-site C, epitope-tagged PMCA2w/a and PMCA2w/b were both concentrated in bundles, indicating that site C is not involved in bundle targeting. In contrast, PMCA2z/a was excluded from bundles and was instead targeted to the basolateral plasma membrane. Bundle-specific targeting of PMCA2w/a tagged with green fluorescent protein (GFP) was diminished, suggesting that GFP interfered with splice-site A. Together, these data demonstrate that PMCA2w/a is the hair-bundle isoform of PMCA in rat hair cells and that 2w targets PMCA2 to bundles. The 2w sequence is thus the first targeting signal identified for a hair-bundle membrane protein; moreover, the striking distribution of inner-ear PMCA isoforms dictated by selective targeting suggests a critical functional role for segregated pathways of Ca2+ transport.

Genetics of hearing loss: Allelism and modifier genes produce a phenotypic continuum

Recent genetic and genomic studies have greatly advanced our knowledge of the structure and function of genes involved in hearing loss. We are starting to recognize, however, that many of these genes do not appear to follow traditional Mendelian expression patterns and are subject to the effects of allelism and modifier genes. This review presents two genes illustrative of this concept that have varied expression pattern such that they may produce either syndromic or nonsyndromic hearing loss. One of these genes, cadherin 23, produces a spectrum of phenotypic traits, including presbycusis, nonsyndromic prelingual hearing loss (DFNB12), and syndromic hearing loss as part of Usher syndrome (Usher 1D). Missense mutations in CDH23 have been associated with presbycusis and DFNB12, whereas null alleles cause the majority of Usher 1D. Modifier gene products that interact with cadherin 23 also affect the phenotypic spectrum. Similarly, allelsim in the gene encoding wolframin (WFS1) causes either a nonsyndromic dominant low-frequency hearing loss (DFNA6/14/38) or Wolfram syndrome. Missense mutations within a defined region are associated with DFNA6/14/38, while more severe mutations spanning WFS1 are found in Wolfram syndrome patients. The phenotypic spectrum of Wolfram syndrome is also hypothesized to be influenced by modifier genes products. These studies provide increasing evidence for the importance of modifier genes in elucidating the functional pathways of primary hearing loss genes. Characterizing modifier genes may result in better treatment options for patients with hearing loss and define new diagnostic and therapeutic targets.

Absence of hearing loss in a mouse model for DFNA17 and MYH9-related disease: the use of public gene-targeted ES cell resources

Multiple mouse embryonic stem (ES) cell banks expand the capability to characterize functions of genes implicated in human disease and to develop mouse models for the further understanding of disease pathology. Genetic diseases that result in hearing loss can provide insight into causative molecular mechanisms for deafness. We utilized BayGenomics, the public mouse ES cell bank, to identify gene-trapped ES cell lines associated with hearing loss. We identified two gene-trapped ES cell lines specific for the non-muscle myosin heavy chain class IIA or myosin heavy chain IX (Myh9). Inherited mutations in the Myh9 gene have been linked to non-syndromic hereditary hearing impairment DFNA17 as well as 'MYH9-related disease' characterized by macrothrombocytopenia, leukocyte inclusions, and in some patients deafness. Mutant Myh9 mice were derived from one of these ES cell lines that underwent germline transmission for in-depth otological examination. No homozygous mice however were identified at birth, consistent with recently published data describing the embryonic lethality of homozygous mutations in Myh9. We provide evidence that adult heterozygous Myh9 mouse inner ears contain half wild-type levels of Myh9 mRNA. Hearing loss however was not observed in heterozygous Myh9 mice in contrast to human Myh9-related diseases. Aged heterozygous Myh9 mice also did not show signs of cochleosaccular degeneration common in DFNA17. Although inheritance of Myh9 mutations in humans is dominant, we conclude that heterozygous loss of Myh9 is not critical to hearing function in mice by itself.

Strain background effects and genetic modifiers of hearing in mice

Genetic modifiers can be detected in mice by looking for strain background differences in inheritance or phenotype of a mutation. They can be mapped by analyses of appropriate linkage crosses and congenic lines, and modifier genes of large effect can be identified by positional-candidate gene testing. Inbred strains of mice vary widely in onset and severity of age-related hearing loss (AHL), an important consideration when assessing hearing in mutant mice. At least 8 mapped loci and a mitochondrial variant (mt-Tr) are known to contribute to AHL in mouse strains; one locus (ahl) has been identified as a variant of the cadherin 23 gene (Cdh23(753A/G)). This variant also was shown to modify hearing loss associated with the Atp2b2(dfw-2J) and Mass1(frings) mutations. The hearing modifier (Moth1) of tubby (Tub(tub)) mutant mice was shown to be a strain variant of the Mtap1a gene. Human hearing modifiers include DFNM1, which suppresses recessive deafness DFNB26, and a nuclear gene that modulates the severity of hearing loss associated with a mitochondrial mutation. Recently, a variant of the human ATP2B2 gene was shown to exacerbate hearing loss in individuals homozygous for a CDH23 mutation, similar to the Atp2b2(dfw-2J)-Cdh23(753A/G) interaction affecting hearing in mice. Because modifier genes and digenic inheritance are not always distinguishable, we also include in this review several examples of digenic inheritance of hearing loss that have been reported in both mice and humans.

Prospective contributions of transgenic mouse models to central auditory research

Neuroscientists are increasingly embracing mice as a means to address central nervous system questions at a molecular level. Examples abound from sensory systems like olfaction and vision. The use of mice to study central auditory processing, however, has remained relatively limited. In this commentary, I draw on some of the successes from other fields to highlight directions in which mouse models may contribute valuable and otherwise unattainable insights into the neural circuitry and plasticity within central auditory stations. Efforts towards this are beginning and would benefit from increased collaboration to generate useful transgenic mouse models for such studies.

Conditional and inducible gene recombineering in the mouse inner ear

Genetically engineered mice have greatly improved our understanding of gene functions and disease mechanisms. Nevertheless, the traditional knock-out approach has limitations in the overall viability of mutants. The application of the Cre/loxP system in the inner ear can help bypass this difficulty by generation of conditional gene recombineering. However, to do so requires an expression system that allows ear-specific temporally inducible, gene abrogation of one or more of the increasingly available floxed genes. To date, three approaches have been successfully used to create murine inner ear-specific Cre lines: conventional transgenesis, BAC transgenesis, and gene knock-in. Unfortunately, timing of conditional Cre activity does not extend beyond the regulatory range of the gene controlling Cre expression. Rectification of this problem requires the generation of tamoxifen or tetracycline inducible systems in the inner ear. Examination of integrase expression at different loci will facilitate studies on the expression of exogenous transgenes. These genetic applications for the mouse genome will dramatically advance in vivo gene function studies.