Math1 gene transfer generates new cochlear hair cells in mature guinea pigs in vivo. J Neurosci. 2003;23:4395-4400. [PMID: 12805278 DOI: 10.1523/jneurosci.23-11-04395.2003]

Regulated reprogramming in the regeneration of sensory receptor cells

Vision, olfaction, hearing, and balance are mediated by receptors that reside in specialized sensory epithelial organs. Age-related degeneration of the photoreceptors in the retina and the hair cells in the cochlea, caused by macular degeneration and sensorineural hearing loss, respectively, affect a growing number of individuals. Although sensory receptor cells in the mammalian retina and inner ear show only limited or no regeneration, in many nonmammalian vertebrates, these sensory epithelia show remarkable regenerative potential. We summarize the current state of knowledge of regeneration in the specialized sense organs in both nonmammalian vertebrates and mammals and discuss possible areas where new advances in regenerative medicine might provide approaches to successfully stimulate sensory receptor cell regeneration. The field of regenerative medicine is still in its infancy, but new approaches using stem cells and reprogramming suggest ways in which the potential for regeneration may be restored in individuals suffering from sensory loss.

Concise review: induced pluripotent stem cells and lineage reprogramming: prospects for bone regeneration

Bone tissue for transplantation therapies is in high demand in clinics. Osteodegenerative diseases, in particular, osteoporosis and osteoarthritis, represent serious public health issues affecting a respectable proportion of the elderly population. Furthermore, congenital indispositions from the spectrum of craniofacial malformations such as cleft palates and systemic disorders including osteogenesis imperfecta are further increasing the need for bone tissue. Additionally, the reconstruction of fractured bone elements after accidents and the consumption of bone parts during surgical tumor excisions represent frequent clinical situations with deficient availability of healthy bone tissue for therapeutic transplantations. Epigenetic reprogramming represents a powerful technology for the generation of healthy patient-specific cells to replace or repair diseased or damaged tissue. The recent generation of induced pluripotent stem cells (iPSCs) is probably the most promising among these approaches dominating the literature of current stem cell research. It allows the generation of pluripotent stem cells from adult human skin cells from which potentially all cell types of the human body could be obtained. Another technique to produce clinically interesting cell types is direct lineage reprogramming (LR) with the additional advantage that it can be applied directly in vivo to reconstitute a damaged organ. Here, we want to present the two technologies of iPSCs and LR, to outline the current states of research, and to discuss possible strategies for their implementation in bone regeneration.

Towards gene therapy for deafness

Many hearing disorders are associated with the damage or loss of sensory hair cells (HC) which can produce a profound and irreversible deafness. Apoptosis pathway is reported to play an important role leading to rapid expansion of the HC lesion after exposure to intense noise. Furthermore, progress made over the last year in understanding molecular mechanisms involved in the proliferative and regenerative capacity of sensory cells in the mammalian inner ear has raised the possibility that targeted therapies might prevent the loss of these cells and preserve the patient's hearing. A first step towards the successful therapeutic exploitation is a better understanding of the different pathways that control survival and proliferation of sensory cells. In this review, we provide an overview of recent findings concerning the possibility to prevent apoptosis in auditory cells. We also show the current knowledge on the molecular mechanisms involved in the potential regenerative behavior of these cells and the progress of gene therapy to prevent deafness noise-induced.

A review of gene delivery and stem cell based therapies for regenerating inner ear hair cells

Sensory neural hearing loss and vestibular dysfunction have become the most common forms of sensory defects, affecting millions of people worldwide. Developing effective therapies to restore hearing loss is challenging, owing to the limited regenerative capacity of the inner ear hair cells. With recent advances in understanding the developmental biology of mammalian and non-mammalian hair cells a variety of strategies have emerged to restore lost hair cells are being developed. Two predominant strategies have developed to restore hair cells: transfer of genes responsible for hair cell genesis and replacement of missing cells via transfer of stem cells. In this review article, we evaluate the use of several genes involved in hair cell regeneration, the advantages and disadvantages of the different viral vectors employed in inner ear gene delivery and the insights gained from the use of embryonic, adult and induced pluripotent stem cells in generating inner ear hair cells. Understanding the role of genes, vectors and stem cells in therapeutic strategies led us to explore potential solutions to overcome the limitations associated with their use in hair cell regeneration.

Transcriptomic analysis of the zebrafish inner ear points to growth hormone mediated regeneration following acoustic trauma

Background

Unlike mammals, teleost fishes are capable of regenerating sensory inner ear hair cells that have been lost following acoustic or ototoxic trauma. Previous work indicated that immediately following sound exposure, zebrafish saccules exhibit significant hair cell loss that recovers to pre-treatment levels within 14 days. Following acoustic trauma in the zebrafish inner ear, we used microarray analysis to identify genes involved in inner ear repair following acoustic exposure. Additionally, we investigated the effect of growth hormone (GH) on cell proliferation in control zebrafish utricles and saccules, since GH was significantly up-regulated following acoustic trauma.

Results

Microarray analysis, validated with the aid of quantitative real-time PCR, revealed several genes that were highly regulated during the process of regeneration in the zebrafish inner ear. Genes that had fold changes of ≥ 1.4 and P -values ≤ 0.05 were considered significantly regulated and were used for subsequent analysis. Categories of biological function that were significantly regulated included cancer, cellular growth and proliferation, and inflammation. Of particular significance, a greater than 64-fold increase in growth hormone (gh1) transcripts occurred, peaking at 2 days post-sound exposure (dpse) and decreasing to approximately 5.5-fold by 4 dpse. Pathway Analysis software was used to reveal networks of regulated genes and showed how GH affected these networks. Subsequent experiments showed that intraperitoneal injection of salmon growth hormone significantly increased cell proliferation in the zebrafish inner ear. Many other gene transcripts were also differentially regulated, including heavy and light chain myosin transcripts, both of which were down-regulated following sound exposure, and major histocompatability class I and II genes, several of which were significantly regulated on 2 dpse.

Conclusions

Transcripts for GH, MHC Class I and II genes, and heavy- and light-chain myosins, as well as many others genes, were differentially regulated in the zebrafish inner ear following overexposure to sound. GH injection increased cell proliferation in the inner ear of non-sound-exposed zebrafish, suggesting that GH could play an important role in sensory hair cell regeneration in the teleost ear.

Sox2 and Fgf interact with Atoh1 to promote sensory competence throughout the zebrafish inner ear

Atoh1 is required for differentiation of sensory hair cells in the vertebrate inner ear. Moreover, misexpression of Atoh1 is sufficient to establish ectopic sensory epithelia, making Atoh1 a good candidate for gene therapy to restore hearing. However, competence to form sensory epithelia appears to be limited to discrete regions of the inner ear. To better understand the developmental factors influencing sensory-competence, we examined the effects of misexpressing atoh1a in zebrafish embryos under various developmental conditions. Activation of a heat shock-inducible transgene, hs:atoh1a, resulted in ectopic expression of early markers of sensory development within 2h, and mature hair cells marked by brn3c:GFP began to accumulate 9h after heat shock. The ability of atoh1a to induce ectopic sensory epithelia was maximal when activated during placodal or early otic vesicle stages but declined rapidly thereafter. At no stage was atoh1a sufficient to induce sensory development in dorsal or lateral non-sensory regions of the otic vesicle. However, co-misexpression of atoh1a with fgf3, fgf8 or sox2, genes normally acting in the same gene network with atoh1a, stimulated sensory development in all regions of the otic vesicle. Thus, expression of fgf3, fgf8 or sox2 strongly enhances competence to respond to Atoh1.

The α1 subunit of nicotinic acetylcholine receptors in the inner ear: transcriptional regulation by ATOH1 and co-expression with the γ subunit in hair cells

Acetylcholine is a key neurotransmitter of the inner ear efferent system. In this study, we identify two novel nAChR subunits in the inner ear: α1 and γ, encoded by Chrna1 and Chrng, respectively. In situ hybridization shows that the messages of these two subunits are present in vestibular and cochlear hair cells during early development. Chrna1 and Chrng expression begin at embryonic stage E13.5 in the vestibular system and E17.5 in the organ of Corti. Chrna1 message continues through P7, whereas Chrng is undetectable at post-natal stage P6. The α1 and γ subunits are known as muscle-type nAChR subunits and are surprisingly expressed in hair cells which are sensory-neural cells. We also show that ATOH1/MATH1, a transcription factor essential for hair cell development, directly activates CHRNA1 transcription. Electrophoretic mobility-shift assays and supershift assays showed that ATOH1/E47 heterodimers selectively bind on two E boxes located in the proximal promoter of CHRNA1. Thus, Chrna1 could be the first transcriptional target of ATOH1 in the inner ear. Co-expression in Xenopus oocytes of the α1 subunit does not change the electrophysiological properties of the α9α10 receptor. We suggest that hair cells transiently express α1γ-containing nAChRs in addition to α9α10, and that these may have a role during development of the inner ear innervation.

Visualization of intracellular trafficking of Math1 protein in different cell types with a newly-constructed nonviral gene delivery plasmid

BACKGROUND

In recent years, Math1 gene therapy was indicated to be the future therapy for deafness in combination with other growth factors. However, Math1 delivery using adenovirus-mediated gene delivery or electroporation was impractical. The contribution of Math1 in the combined procedure was not clearly elucidated using the existing plasmids. Nonviral gene delivery vectors are expected to be extremely safe and convenient. The present study aimed to construct the pCDNA6.2/C-EmGFP-Math1 plasmid and evaluate its transfection efficiency and intracellular trafficking of Math1 protein corresponding to transcription regulation function.

METHODS

After constructing the pCDNA6.2/C-EmGFP-Math1 expression plasmid, the plasmid was transfected into different cell lines and primary cochlear cells using Lipofectamine 2000. Transfection efficiencies of the plasmid were evaluated. Transfection efficiencies using liposome nanoparticles containing Math1 plasmid were also assessed. Intracellular trafficking of Math1 was monitored using confocal microscopy.

RESULTS

Different cell types can be transfected with high transfection efficiencies by the pcDNA6.2/C-EmGFP-Math1 plasmid using Lipofectamine 2000. Liposome nanoparticles containing the Math1 plasmid expressed the gene with variable efficiencies, depending on the particle size, surface charge and PEGylation status. Unique intracellular trafficking of Math1 was demonstrated in different cell types.

CONCLUSIONS

The newly-constructed plasmid pcDNA6.2/C-EmGFP-Math1 was suitable for nonviral gene delivery of Math1. Unique intracellular trafficking of Math1 with dynamics from the cytoplasm to the nucleus was demonstrated. The modification of mesenchymal stem cells by Math1 gene delivery and by brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor treatments can potentially be applied to cell replacement for the treatment of cochlear spiral ganglion cell loss in deafness.

Regeneration of Hair Cells: Making Sense of All the Noise

Hearing loss affects hundreds of millions of people worldwide by dampening or cutting off their auditory connection to the world. Current treatments for sensorineural hearing loss (SNHL) with cochlear implants are not perfect, leaving regenerative medicine as the logical avenue to a perfect cure. Multiple routes to regeneration of damaged hair cells have been proposed and are actively pursued. Each route not only requires a keen understanding of the molecular basis of ear development but also faces the practical limitations of stem cell regulation in the delicate inner ear where topology of cell distribution is essential. Improvements in our molecular understanding of the minimal essential genes necessary for hair cell formation and recent advances in stem cell manipulation, such as seen with inducible pluripotent stem cells (iPSCs) and epidermal neural crest stem cells (EPI-NCSCs), have opened new possibilities to advance research in translational stem cell therapies for individuals with hearing loss. Despite this, more detailed network maps of gene expression are needed, including an appreciation for the roles of microRNAs (miRs), key regulators of transcriptional gene networks. To harness the true potential of stem cells for hair cell regeneration, basic science and clinical medicine must work together to expedite the transition from bench to bedside by elucidating the full mechanisms of inner ear hair cell development, including a focus on the role of miRs, and adapting this knowledge safely and efficiently to stem cell technologies.

Current status and prospects of gene therapy for the inner ear

Inner ear diseases are common and often result in hearing disability. Sensorineural hearing loss is the main cause of hearing disability. So far, no effective treatment is available although some patients may benefit from a hearing aid equipped with a hearing amplifier or from cochlear implantation. Inner ear gene therapy has become an emerging field of study for the treatment of hearing disability. Numerous new discoveries and tremendous advances have been made in inner ear gene therapy including gene vectors, routes of administration, and therapeutic genes and targets. Gene therapy may become a treatment option for inner ear diseases in the near future. In this review, we summarize the current state of inner ear gene therapy including gene vectors, delivery routes, and therapeutic genes and targets by examining and analyzing publications on inner ear gene therapy from the literature and patent documents, and identify promising patents, novel techniques, and vital research projects. We also discuss the progress and prospects of inner ear gene therapy, the advances and shortcomings, with possible solutions in this field of research.

TAK1 expression in the cochlea: a specific marker for adult supporting cells

Transforming growth factor-β-activated kinase-1 (TAK1) is a mitogen activated protein kinase kinase kinase that is involved in diverse biological roles across species. Functioning downstream of TGF-β and BMP signaling, TAK1 mediates the activation of the c-Jun N-terminal kinase signaling pathway, serves as the target of pro-inflammatory cytokines, such as TNF-α, mediates NF-κβ activation, and plays a role in Wnt/Fz signaling in mesenchymal stem cells. Expression of TAK1 in the cochlea has not been defined. Data mining of previously published murine cochlear gene expression databases indicated that TAK1, along with TAK1 interacting proteins 1 (TAB1), and 2 (TAB2), is expressed in the developing and adult cochlea. The expression of TAK1 in the developing cochlea was confirmed using RT-PCR and immunohistochemistry. Immunolabeling of TAK1 in embryonic, neonatal, and mature cochleas via DAB chromogenic and fluorescent immunohistochemistry indicated that TAK1 is broadly expressed in both the developing otocyst and periotic mesenchyme at E12.5 but becomes more restricted to specific types of supporting cells as the organ of Corti matures. By P1, TAK1 immunolabeling is found in cells of the stria vascularis, hair cells, supporting cells, and Kölliker's organ. By P16, TAK1 labeling is limited to cochlear supporting cells. In the adult cochlea, TAK1 immunostaining is only present in the cytoplasm of Deiters' cells, pillar cells, inner phalangeal cells, and inner border cells, with no expression in any other cochlear cell types. While the role of TAK1 in the inner ear is unclear, TAK1 expression may be used as a novel marker for specific sub-populations of supporting cells.

Neural stem cells secrete factors that promote auditory cell proliferation via a leukemia inhibitory factor signaling pathway

The capacity for perpetual self-renewal is one of the main characteristics of stem cells. Little is known about the effect of embryonic neural stem cell (NSC)-secreted factors on auditory cell proliferation in vitro. In the present work, two auditory cell types were cultured in the presence of NSC-secreted molecules and were evaluated in vitro. Our results demonstrated that both cell viability and cell proliferation were significantly enhanced upon treatment with NSC conditioned medium, which contains significantly elevated levels of leukemia inhibitory factor (LIF) secreted by NSCs. The NSC conditioned medium not only activated the expression of leukemia inhibitory factor receptor in House Ear Institute-organ of Corti 1 cells but also up-regulated the LIF downstream signal transducers and activators of transcription (STAT) 1 and STAT3. Blocking either the LIF signaling pathway with neutralizing antibodies or the downstream Janus kinase (JAK)/STAT pathway with JAK2 inhibitor AG490 resulted in a dose-dependent inhibition of cell proliferation, suggesting that NSC-secreted molecules promote auditory cell survival via the regulatory LIF/JAK/STAT signaling pathway.

Development of gene therapy for inner ear disease: Using bilateral vestibular hypofunction as a vehicle for translational research

Despite the significant impact of hearing and balance disorders on the general population there are currently no dedicated pharmaceuticals that target the inner ear. Advances in molecular biology and neuroscience have improved our understanding of the inner ear allowing the development of a range of molecular targets that have the potential to treat both hearing and balance disorders. One of the principal advantages of the inner ear is that it is accessible through a variety of approaches that would allow a potential to be delivered locally rather than systemically. This significantly broadens the potential medications that can be developed and opens the possibility of local gene delivery as a therapeutic intervention. Several potential clinical targets have been identified including delivery of neurotrophin expressing genes as an adjunct to cochlear implantation, delivery of protective genes to prevent trauma and the development of strategies for regenerating inner ear sensory cells. In order to translate these potential therapeutics into humans we will want to optimize the gene delivery methodology, dosing and activity of the drug for therapeutic value. To this end we have developed a series of adenovectors that efficiently transduce the inner ear. The use of these gene delivery approaches are attractive for the potential of hair cell regeneration after loss induced by trauma or ototoxins. This approach is particularly suited for the development of molecular therapies targeted at the vestibular system given that no device based therapeutic such a cochlear implant available for vestibular loss.

Up-regulation of Nob1 in the rat auditory system with noise-induced hearing loss

The Nob1 gene is assumed to be associated with transcription regulation and may play important roles in mediating some physiological and pathological functions. Here, the rats were randomized equally into experimental group and control group. In experimental group, all subjects were exposed to 4-kHz octave-band noise at 110 dB SPL, 8 h per day for 7 days consecutively. Auditory thresholds were assessed by auditory brainstem response, prior to and 1 h after the cessation of noise exposure. Then, we investigated for the first time the expression of Nob1 in noise-exposed and noise-unexposed rats by quantitative polymerase chain reaction. The distribution of Nob1 in rat cochlea was further examined by immunohistochemistry. The results indicated that the hearing threshold was significantly higher in the noise-injured group than in the uninjured group after noise exposure. Nob1 mRNA was present at higher levels in regions of the noise-injured cochlea. As for noise-exposed rats, Nob1 expression was positive in the inner and outer hair cells of the organ of Corti and spiral ganglion neurons, but it undetectable in the uninjured cochlea. Therefore, Nob1 may play an important role in auditory function following acoustic trauma and can be used as a new target for the treatment of noise-induced hearing loss.

MicroRNAs in hair cell development and deafness

PURPOSE OF REVIEW

The identification of transcriptional activators and repressors of hair cell fates has recently been augmented by the discovery of microRNAs (miRNAs) that can function as post-transcriptional repressors in sensory hair cells.

RECENT FINDINGS

miRNAs are approximately 21-nucleotide single-stranded ribonucleic acids that can each repress protein synthesis of many target genes by interacting with messenger RNA transcripts. A triplet of these miRNAs, the miR-183 family, is highly expressed in vertebrate hair cells, as well as a variety of other peripheral neurosensory cells. Point mutations in one member of this family, miR-96, underlie DFNA50 autosomal deafness in humans and lead to abnormal hair cell development and survival in mice. In zebrafish, overexpression of the miR-183 family induces extra and ectopic hair cells, whereas knockdown reduces hair cell numbers. Genetically engineered mice with a block in miRNA biosynthesis during early ear development, or during hair cell differentiation, reveal the necessity of miRNAs at these crucial time points.

SUMMARY

Because miRNAs can simultaneously down-regulate dozens to perhaps hundreds of transcripts, they will soon be explored as potential therapeutic agents to repair or regenerate hair cells in animal models.

Study of protective effect on rat cochlear spiral ganglion after blast exposure by adenovirus-mediated human β-nerve growth factor gene

OBJECTIVE

To study whether adenovirus-mediated human β-nerve growth factor (Ad-hNGFβ) gene has any protective effect on rat cochlear spiral ganglion after blast exposure.

METHODS

Deafness was induced by blast exposure (172.0 dB) in 20 healthy rats. Seven days after blast exposure, Ad-hNGFβ was infused into the perilymphatic space of 10 animals as the hNGFβ/blast group, and artificial perilymph fluid (APF) was infused into the perilymphatic space of 10 animals as the APF/blast control group. An additional control group consisted of 10 healthy rats which received Ad-hNGFβ target gene with no blast exposure (hNGFβ/control group). Auditory functions were monitored by thresholds of auditory brain stem responses (ABR). At weeks 1, 4, and 8 postoperatively, the animals were killed, and the cochleae were removed for immunohistochemical, hematoxylin and eosin staining study.

RESULTS

The ABR threshold shifts in the hNGFβ/blast group were significantly smaller than that of APF/blast control group. There were no significant differences of the ABR values between before and after operation in the hNGFβ/control group. Expression of Ad-hNGFβ protein was detected in each turn of the cochlea in the first week, with almost equal intensity in all turns. In the fourth week, the reactive intensity decreased. In the eighth week, no reaction was detectable. The results of hematoxylin and eosin stain showed that the number of spiral ganglions in the hNGFβ/blast group was significantly greater than that of the APF/blast control group in the 4th week (P < .01).

CONCLUSION

Adenovirus-mediated human β-nerve growth factor can be expressed at a high level and for a relatively long period in the blast impaired cochlea, suggesting that Ad-hNGFβ has a protective effect on rat cochlear spiral ganglion cells after blast exposure.

Conditional deletion of Atoh1 using Pax2-Cre results in viable mice without differentiated cochlear hair cells that have lost most of the organ of Corti

Atonal homolog1 (Atoh1, formerly Math1) is a crucial bHLH transcription factor for inner ear hair cell differentiation. Its absence in embryos results in complete absence of mature hair cells at birth and its misexpression can generate extra hair cells. Thus Atoh1 may be both necessary and sufficient for hair cell differentiation in the ear. Atoh1 null mice die at birth and have some undifferentiated cells in sensory epithelia carrying Atoh1 markers. The fate of these undifferentiated cells in neonates is unknown due to lethality. We use Tg(Pax2-Cre) to delete floxed Atoh1 in the inner ear. This generates viable conditional knockout (CKO) mice for studying the postnatal development of the inner ear without differentiated hair cells. Using in situ hybridization we find that Tg(Pax2-Cre) recombines the floxed Atoh1 prior to detectable Atoh1 expression. Only the posterior canal crista has Atoh1 expressing hair cells due to incomplete recombination. Most of the organ of Corti cells are lost in CKO mice via late embryonic cell death. Marker genes indicate that the organ of Corti is reduced to two rows of cells wedged between flanking markers of the organ of Corti (Fgf10 and Bmp4). These two rows of cells (instead of five rows of supporting cells) are positive for Prox1 in neonates. By postnatal day 14 (P14), the remaining cells of the organ of Corti are transformed into a flat epithelium with no distinction of any specific cell type. However, some of the remaining organ of Corti cells express Myo7a at late postnatal stages and are innervated by remaining afferent fibers. Initial growth of afferents and efferents in embryos shows no difference between control mice and Tg(Pax2-Cre)::Atoh1 CKO mice. Most afferents and efferents are lost in the CKO mutant before birth, except for the apex and few fibers in the base. Afferents focus their projections on patches that express the prosensory specifying gene, Sox2. This pattern of innervation by sensory neurons is maintained at least until P14, but fibers target the few Myo7a positive cells found in later stages.

Induced endolymphatic flow from the endolymphatic sac to the cochlea in Ménière's disease

OBJECTIVE

The aim of the present study was to verify whether drugs injected into the endolymphatic sac (ES) can reach the cochlea and possibly treat inner ear disorders.

STUDY DESIGN

Prospective cohort study.

SETTING

Tertiary referral center, Otolaryngology Department, University of Verona.

SUBJECTS AND METHODS

Patients with Ménière's disease (MD) who were candidates for ES decompression were selected. Nineteen subjects received dexamethasone (DEX) via injection into the ES. To objectively define whether substances administered into the ES could reach the cochlea, we added gadolinium (GD) in three patients. All subjects had intraoperative electrocorticogram recordings and an audiologic follow-up. The three subjects who underwent injection of the DEX-GD solution were followed-up with magnetic resonance imaging. The audiological data are presented during a follow-up period of 12 months.

RESULTS

Intraoperative electrocochleography recordings revealed no changes in two patients and summating potentials and compound action potential latency and wave-form modifications in all the other subjects. GD distribution was observed from 48 hours to one week after ES injection into the cochlea of the three subjects injected with DEX-GD. GD-related enhancement of inner ear structures lasted more than two weeks in all subjects. Pure tone average results showed hearing improvement of at least 20 dB HL in 42 percent of patients (8 of 19) at the 12-month follow-up. Statistically significant differences emerged between the mean pure tone average of the ES procedure subjects at one and 12 months after surgery (P = 0.0096).

CONCLUSION

This novel approach might reveal new prospects for treating viral, metabolic, autoimmune, and genetic disorders of the cochlea.

Generation and characterization of Atoh1-Cre knock-in mouse line

Atoh1 encodes a basic helix-loop-helix (bHLH) transcription factor required for the development of the inner ear sensory epithelia, the dorsal spinal cord, brainstem, cerebellum, and intestinal secretory cells. In this study, to create a genetic tool for the research on gene function in the ear sensory organs, we generated an Atoh1-Cre knock-in mouse line by replacing the entire Atoh1 coding sequences with the Cre coding sequences. Atoh1(Cre/+) mice were viable, fertile, and displayed no visible defects whereas the Atoh1(Cre/Cre) mice died perinatally. The spatiotemporal activities of Cre recombinase were examined by crossing Atoh1-Cre mice with the R26R-lacZ conditional reporter mice. Atoh1-Cre activities were detected in the developing inner ear, the hindbrain, the spinal cord, and the intestine. In the inner ear, Atoh1-Cre activities were confined to the sensory organs in which lacZ expression is detected in nearly all of the hair cells and in many supporting cells. Thus, Atoh1-Cre mouse line serves as a useful tool for the functional study of genes in the inner ear. In addition, our results demonstrate that Atoh1 is expressed in the common progenitors destined for both hair and supporting cells.

Expression of candidate markers for stem/progenitor cells in the inner ears of developing and adult GFAP and nestin promoter-GFP transgenic mice

Loss of hair cells in the mammalian cochlea leads to permanent sensori-neural hearing loss. Hair cells degenerate and their places are taken by phalangeal scars formed by non-sensory supporting cells. Current data indicate that early postnatal post-mitotic supporting cells can proliferate and differentiate into hair cell-like cells in culture. In this study, we used GFAP and nestin promoter-GFP transgenic mice in combination with other stem cell markers to characterize supporting cell subtypes in the postnatal day-3 (P3) and adult organs of Corti with potential stem/progenitor cell phenotype. In P3 organ of Corti, we show GFAP-GFP signal in all the supporting cell subtypes while the nestin-GFP was restricted to the supporting cells in the inner hair cell area. At this stage, GFAP and selected stem/progenitor markers displayed overlapping expression pattern in the supporting cell population. In the adult, GFAP expression is down-regulated from the supporting cells in the outer hair cell area and nestin expression is down-regulated in the supporting cells of the inner hair cell area. Sox2 and Jagged1 expression is maintained in the mature supporting cells, while Abcg2 was down-regulated in these cells. In contrast, GFAP and Abcg2 expression was up-regulated in the inner sulcus limbal cells outside the mature organ of Corti's area. Using quantitative reverse transcription-PCR, we found a decrease in transcripts for Jagged1 and Sox2 in adult cochleae. Our findings suggest that the loss of regenerative capacity of the adult organ of Corti is related to down-regulation of stem/progenitor key-markers from the mature supporting cells.

Future approaches for inner ear protection and repair

Health care professionals tending to patients with inner ear disease face inquiries about therapy options, including treatments that are being developed for future use but not yet available. The devastating outcome of sensorineural hearing loss, combined with the permanent nature of the symptoms, make these inquiries demanding and frequent. The vast information accessible online and the publicity for breakthroughs in research add to patient requests for access to advanced and innovative therapies, even before these are available for clinical use. This can sometimes be taxing on the health care provider who is in contact with the patients. Here we aim to equip the provider with information about some of the progress made for protective and reparative approaches for treating inner ears.

LEARNING OUTCOMES

(1) Readers will be able to explain why hearing loss is irreversible and common, (2) readers will be able to explain the importance of protective measures and the progress made in discovery and design of novel biological protective molecules, (3) readers will be able to describe reparative approaches currently under investigation (such as tissue engineering), the main difficulties in the design of such therapies and the major hurdles that remain for making novel technologies clinically viable, and (4) readers will be able to explain to their patients some of the progress in developing new treatments without making the promise of imminent clinical use. With this information, readers will be able to guide patients to make better choices for their treatment and to guide students toward research in this exciting field.

The challenge of hair cell regeneration

Sensory hair cells of the inner ear are responsible for translating auditory or vestibular stimuli into electrical energy that can be perceived by the nervous system. Although hair cells are exquisitely mechanically sensitive, they can be easily damaged by excessive stimulation by ototoxic drugs and by the effects of aging. In mammals, auditory hair cells are never replaced, such that cumulative damage to the ear causes progressive and permanent deafness. In contrast, non-mammalian vertebrates are capable of replacing lost hair cells, which has led to efforts to understand the molecular and cellular basis of regenerative responses in different vertebrate species. In this review, we describe recent progress in understanding the limits to hair cell regeneration in mammals and discuss the obstacles that currently exist for therapeutic approaches to hair cell replacement.

Guinea pig adenovirus infection does not inhibit cochlear transfection with human adenoviral vectors in a model of hearing loss

Routine surveillance of guinea pigs maintained within a barrier facility detected guinea pig adenovirus (GPAdV) in sentinel animals. These guinea pigs served as models of induced hearing loss followed by regeneration of cochlear sensory (hair) cells through transdifferentiation of nonsensory cells by using human adenoviral (hAV) gene therapy. To determine whether natural GPAdV infection affected the ability of hAV vectors to transfect inner ear cells, adult male pigmented guinea pigs (n = 7) were enrolled in this study because of their prolonged exposure to GPAdV-seropositive conspecifics. Animals were deafened chemically (n = 2), received an hAV vector carrying the gene for green fluorescent protein (hAV-GFP) surgically without prior deafening (n = 2), or were deafened chemically with subsequent surgical inoculation of hAV-GFP (n = 3). Cochleae were evaluated by using fluorescence microscopy, and GFP expression in supporting cells indicated that the hAV-GFP vector was able to transfect inner ears in GPAdV-seropositive guinea pigs that had been chemically deafened. Animals had histologic evidence of interstitial pneumonia, attributable to prior infection with GPAdV. These findings confirmed that the described guinea pigs were less robust animal models with diminished utility for the overall studies. Serology tests confirmed that 5 of 7 animals (71%) were positive for antibodies against GPAdV at necropsy, approximately 7 mo after initial detection of sentinel infection. Control animals (n = 5) were confirmed to be seronegative for GPAdV with clinically normal pulmonary tissue. This study is the first to demonstrate that natural GPAdV infection does not negatively affect transfection with hAV vectors into guinea pig inner ear cells, despite the presence of other health complications attributed to the viral infection.

Sonic hedgehog (SHH) promotes the differentiation of mouse cochlear neural progenitors via the Math1-Brn3.1 signaling pathway in vitro

Sonic hedgehog (SHH) is essential for the development of the cochlear duct that harbors the organ of Corti. However, little is known about the molecular signaling pathway through which SHH promotes the development of the organ of Corti, especially cochlear sensory epithelial cells. In this study, we demonstrated that SHH contributes to the differentiation of cochlear neural progenitors (CNPs), which are derived from the postnatal day 1 organ of Corti in mice. Addition of SHH to CNPs increased the formation of epithelial cell islands, simultaneously activated the expression of Math1 that is a transcription factor for the initial differentiation of auditory hair cells. The increased expression of Math1 then regulated the promoter activity of Brn3.1, another transcription factor that controls the further differentiation and survival of auditory hair cells. Taken together, our data suggest that SHH plays an important role in the promotion of auditory hair cell differentiation via the Math1-Brn3.1 signaling pathway.

Effects of localized neurotrophin gene expression on spiral ganglion neuron resprouting in the deafened cochlea

A cochlear implant may be used to electrically stimulate spiral ganglion neurons (SGNs) in people with severe sensorineural hearing loss (SNHL). However, these neurons progressively degenerate after SNHL due to loss of neurotrophins normally supplied by sensory hair cells (HCs). Experimentally, exogenous neurotrophin administration prevents SGN degeneration but can also result in abnormal resprouting of their peripheral fibers. This study aimed to create a target-derived neurotrophin source to increase neuron survival and redirect fiber resprouting following SNHL. Adenoviral (Ad) vectors expressing green fluorescent protein (GFP) alone or in combination with brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT3) were injected into the cochlear scala tympani or scala media of guinea-pigs (GPs) deafened via aminoglycosides for 1 week. After 3 weeks, cochleae were examined for gene expression, neuron survival, and the projection of peripheral fibers in response to gene expression. Injection of vectors into the scala media resulted in more localized gene expression than scala tympani injection with gene expression consistently observed within the partially degenerated organ of Corti. There was also greater neuron survival and evidence of localized fiber responses to neurotrophin-expressing cells within the organ of Corti from scala media injections (P < 0.05), a first step in promoting organized resprouting of auditory peripheral fibers via gene therapy.

Transgenic BDNF induces nerve fiber regrowth into the auditory epithelium in deaf cochleae

Sensory organs typically use receptor cells and afferent neurons to transduce environmental signals and transmit them to the CNS. When sensory cells are lost, nerves often regress from the sensory area. Therapeutic and regenerative approaches would benefit from the presence of nerve fibers in the tissue. In the hearing system, retraction of afferent innervation may accompany the degeneration of auditory hair cells that is associated with permanent hearing loss. The only therapy currently available for cases with severe or complete loss of hair cells is the cochlear implant auditory prosthesis. To enhance the therapeutic benefits of a cochlear implant, it is necessary to attract nerve fibers back into the cochlear epithelium. Here we show that forced expression of the neurotrophin gene BDNF in epithelial or mesothelial cells that remain in the deaf ear induces robust regrowth of nerve fibers towards the cells that secrete the neurotrophin, and results in re-innervation of the sensory area. The process of neurotrophin-induced neuronal regeneration is accompanied by significant preservation of the spiral ganglion cells. The ability to regrow nerve fibers into the basilar membrane area and protect the auditory nerve will enhance performance of cochlear implants and augment future cell replacement therapies such as stem cell implantation or induced transdifferentiation. This model also provides a general experimental stage for drawing nerve fibers into a tissue devoid of neurons, and studying the interaction between the nerve fibers and the tissue.

Histone deacetylase inhibition enhances adenoviral vector transduction in inner ear tissue

Adenovirus vectors (AdVs) are efficient tools for gene therapy in many tissues. Several studies have demonstrated successful transgene transduction with AdVs in the inner ear of rodents [Kawamoto K, Ishimoto SI, Minoda R, Brough DE, Raphael Y (2003) J Neurosci 23:4395-4400]. However, toxicity of AdVs [Morral N, O'Neal WK, Rice K, Leland MM, Piedra PA, Aguilar-Cordova E, Carey KD, Beaudet AL, Langston C (2002) Hum Gene Ther 13:143-154.] or lack of tropism to important cell types such as hair cells [Shou J, Zheng JL, Gao WQ (2003) Mol Cell Neurosci 23:169-179] appears to limit their experimental and potential clinical utility. Histone deacetylase inhibitors (HDIs) are known to enhance AdV-mediated transgene expression in various organs [Dion LD, Goldsmith KT, Tang DC, Engler JA, Yoshida M, Garver RI Jr (1997) Virology 231:201-209], but their effects in the inner ear have not been documented. We investigated the ability of one HDI, trichostatin A (TSA), to enhance AdV-mediated transgene expression in inner ear tissue. We cultured neonatal rat macular and cochlear explants, and transduced them with an AdV encoding green fluorescent protein (Ad-GFP) under the control of a constitutive promoter for 24 h. In the absence of TSA, GFP expression was limited, and very few hair cells were transduced. TSA did not enhance transduction when applied at the onset of Ad-GFP transduction. However, administration of TSA during or just after Ad-GFP application increased GFP expression in supporting cells approximately fourfold. Moreover, vestibular hair cell transduction was enhanced approximately sixfold, and that of inner hair cells by more than 17-fold. These results suggest that TSA increases AdV-mediated transgene expression in the inner ear, including the successful transduction of hair cells. HDIs, some of which are currently under clinical trials (Sandor et al., 2002), could be useful tools in overcoming current limitations of gene therapy in the inner ear using Ad-GFP.

Notch-Hes1 pathway contributes to the cochlear prosensory formation potentially through the transcriptional down-regulation of p27Kip1

The Notch signaling pathway has a crucial role in the differentiation of hair cells and supporting cells by mediating "lateral inhibition" via the ligands Delta-like1 (Dll1) and Jagged2 (Jag2) and the effectors Hes1 and Hes5 during mammalian inner ear development. Recently, another Notch ligand, Jagged1 (Jag1)-dependent Notch activation, has been revealed to be important for the determination of the prosensory region in the earlier stage before cell differentiation. However, little is known about the effectors of the Notch pathway in this context. P27(Kip1), a cyclin-dependent kinase inhibitor, is also known to demarcate the prosensory region in the cochlear primordium, which consists of the sensory progenitors that have completed their terminal mitoses. Hes1 reportedly promotes precursor cell proliferation through the transcriptional down-regulation of p27(Kip1) in the thymus, liver, and brain. In this study, we observed Hes1 as a mediator between the Notch signaling pathway and the regulation of proliferation of sensory precursor cells by p27(Kip1) in the developing cochlea. We showed that Hes1, but not Hes5, was weakly expressed at the time of onset of p27(Kip1). The expression pattern of Hes1 prior to cell differentiation was similar to that of activated Notch1. P27(Kip1) was up-regulated and BrdU-positive S-phase cells were reduced in the developing cochlear epithelium of Hes1 null mice. These results suggest that the Notch-Hes1 pathway may contribute to the adequate proliferation of sensory precursor cells via the potential transcriptional down-regulation of p27(Kip1) expression and play a pivotal role in the correct prosensory determination.

Inner ear protection and regeneration: a 'historical' perspective

PURPOSE OF REVIEW

In evaluating strategies to preserve or regenerate the cochlea, understanding the process of labyrinthine injury on a cellular and molecular level is crucial. Examination of inner ear injury reveals mechanism-specific types of damage, often at specific areas within the cochlea. Site-specific interventions can then be considered.

RECENT FINDINGS

The review will briefly summarize the historical perspective of advancements in hearing science through 2006. Areas of research covered include hair cell protection, hair cell regeneration, spiral ganglion cell regeneration, and stria vascularis metabolic regulation.

SUMMARY

The review will briefly summarize the early development of a few such site-specific interventions for inner ear functional rehabilitation, for work done prior to 2006. The outstanding reviews of cutting edge research from this year's and last year's Hearing Science section of Current Opinion in Otolaryngology - Head and Neck Surgery can then be understood and appreciated in a more informed manner.

Regulation of cell fate and patterning in the developing mammalian cochlea

PURPOSE OF REVIEW

A significant proportion of hearing loss and deafness is caused by defects in the structure or function of cells within the organ of Corti. Identification of the molecular factors that regulate the development of this structure should provide valuable insights regarding inner ear formation and the signaling pathways that underlie congenital auditory deficits. In addition, targeted modulation of these same factors could be developed as therapies for hair cell regeneration.

RECENT FINDINGS

Results from experiments using transgenic and mutant mice, as well as in-vitro techniques, have identified genes and signaling pathways that are required to either specify unique auditory cell types, such as hair cells or supporting cells, or to generate the highly ordered cellular pattern that is characteristic for the organ of Corti. In particular, the hedgehog and fibroblast growth factor signaling pathways modulate the formation of the progenitor cells that will give rise to the organ of Corti. SRY-box containing gene 2, a transcription factor that is required for the formation of the cochlear progenitor cell population, has paradoxically been shown to also act as an inhibitor of hair cell development. Finally, the motor protein myosin II regulates extension of the organ of Corti and the alignment of hair cells and supporting cells into ordered rows.

SUMMARY

A better understanding of the signaling pathways that direct different aspects of cochlear development, such as specific of cell fates or cellular patterning, offers the potential to identify new pathways or molecules that could be targeted for therapeutic interventions.

Regeneration of the mammalian inner ear sensory epithelium

PURPOSE OF REVIEW

This review will focus on 'self-repair' of the mammalian inner ear sensory epithelium, including recruiting the in-situ proliferation and differentiation of endogenous cells at the damaged site and the autologous transplantation

RECENT FINDINGS

Self-repair refers to a favorable structural and functional outcome of damaged inner ear sensory epithelium. Our advanced ability of manipulating the fate of inner ear sensory cells makes in-situ proliferation a possible candidate of hearing restoration. A practical alternative of the unavoidable immune rejection is to introduce autologous cells. Ependymal cells, induced pluripotent stem cells, and olfactory neuroepithelial cells have been recognized as promising sources, which will spur ongoing efforts to evaluate these new cell sources for cell replacement therapy.

SUMMARY

Further exploration of the innate advantages of in-situ proliferation and use of novel cell sources for autologous transplantation may serve as rehearsals for clinical trials in the near future.

Automated threshold detection for auditory brainstem responses: comparison with visual estimation in a stem cell transplantation study

BACKGROUND

Auditory brainstem responses (ABRs) are used to study auditory acuity in animal-based medical research. ABRs are evoked by acoustic stimuli, and consist of an electrical signal resulting from summated activity in the auditory nerve and brainstem nuclei. ABR analysis determines the sound intensity at which a neural response first appears (hearing threshold). Traditionally, threshold has been assessed by visual estimation of a series of ABRs evoked by different sound intensities. Here we develop an automated threshold detection method that eliminates the variability and subjectivity associated with visual estimation.

RESULTS

The automated method is a robust computational procedure that detects the sound level at which the peak amplitude of the evoked ABR signal first exceeds four times the standard deviation of the baseline noise. Implementation of the procedure was achieved by evoking ABRs in response to click and tone stimuli, under normal and experimental conditions (adult stem cell transplantation into cochlea). Automated detection revealed that the threshold shift from pre- to post-surgery hearing levels was similar in mice receiving stem cell transplantation or sham injection for click and tone stimuli. Visual estimation by independent observers corroborated these results but revealed variability in ABR threshold shifts and significance levels for stem cell-transplanted and sham-injected animals.

CONCLUSION

In summary, the automated detection method avoids the subjectivity of visual analysis and offers a rapid, easily accessible http://axograph.com/source/abr.html approach to measure hearing threshold levels in auditory brainstem response.

Potential novel drug carriers for inner ear treatment: hyperbranched polylysine and lipid nanocapsules

Aim: Treatment of sensorineural hearing loss could be advanced using novel drug carriers such as hyperbranched polylysine (HBPL) or lipid nanocapsules (LNCs). This study examined HBPL and LNCs for their cellular uptake and possible toxicity in vitro and in vivo as the first step in developing novel nanosized multifunctional carriers. Method: Having incubated HBPL and LNCs with fibroblasts, nanoparticle uptake and cell viability were determined by confocal laser scanning microscopy, fluorescence measurements and neutral red staining. In vivo, electrophysiology, confocal laser scanning microscopy and cytocochleograms were performed for nanoparticle detection and also toxicity studies after intracochlear application. Results: Both nanoparticles were detectable in the fibroblasts’ cytoplasm without causing cytotoxic effects. After in vivo application they were visualized in cochlear cells, which did not lead to a change in hearing threshold or loss of hair cells. Biocompatibility and traceability were demonstrated for HBPL and LNCs. Thus, they comply with the basic requirements for drug carriers for potential application in the inner ear.

Building the world's best hearing aid; regulation of cell fate in the cochlea

In mammals, auditory perception is initially mediated through sensory cells located in a rigorously patterned mosaic of unique cell types located within the coiled cochlea. Identification of the factors that direct multipotent progenitor cells to develop as each of these specialized cell types has the potential to enhance our understanding of the development of the auditory system and to identify potential targets for regenerative therapies. Recent results have identified specific signaling molecules and pathways, including Notch, Hedgehog, Sox2 and Fgfs, that guide progenitor cells to develop first as a sensory precursor, referred to as a prosensory cell, and subsequently as one of the specialized cell types within the sensory mosaic.

Quo vadis, hair cell regeneration?

Hearing loss is a global health problem with profound socioeconomic impact. We contend that acquired hearing loss is mainly a modern disorder caused by man-made noise and modern drugs, among other causes. These factors, combined with increasing lifespan, have exposed a deficit in cochlear self-regeneration that was irrelevant for most of mammalian evolution. Nevertheless, the mammalian cochlea has evolved from phylogenetically older structures, which do have the capacity for self-repair. Moreover, nonmammalian vertebrates can regenerate auditory hair cells that restore sensory function. We will offer a critical perspective on recent advances in stem cell biology, gene therapy, cell cycle regulation and pharmacotherapeutics to define and validate regenerative medical interventions for mammalian hair cell loss. Although these advances are promising, we are only beginning to fully appreciate the complexity of the many challenges that lie ahead.

Gene transfer using bovine adeno-associated virus in the guinea pig cochlea

Gene transfer into the cells of the cochlea is useful for both research and therapy. Bovine adeno-associated virus (BAAV) is a new viral vector with potential for long-term gene expression with little or no side effects. In this study, we assessed transgene expression using BAAV with beta-actin-GFP as a reporter gene, in the cochleae of normal and deafened guinea pigs. We used two different routes to inoculate the cochlea: scala media (SM) or scala tympani (ST). Auditory brainstem response assessments were carried out before inoculation, 7 days after inoculation and immediately before killing, to assess the functional consequences of the treatment. We observed threshold shifts because of the surgical invasion, but no apparent pathology associated with the virus. Fourteen days after the injection, animals were killed and cochleae assessed histologically. Epi-fluorescence showed that BAAV transduced the supporting cells of both normal and deafened animals through SM and ST inoculations. Transgene expression in cells of the membranous labyrinth after ST inoculation is an important outcome because of the greater feasibility of this route for future clinical application. BAAV facilitates efficient transduction of the membranous labyrinth epithelium with minimum pathogenicity and may become clinically applicable for inner ear gene therapy.

New ectopic vestibular hair cell-like cells induced by Math1 gene transfer in postnatal rats

Overexpression of Math1 has been demonstrated to induce robust ectopic new hair cells in greater epithelial ridge (GER) and lesser epithelial ridge (LER) of the postnatal rats' cochlear in vitro. In spite of the similarities between cochlear and vestibular epithelia in origin and structure, no similar results have been reported in the non-sensory region of vestibular epithelia in vitro. In the study, the adenoviral vectors inserted with Math1 gene were constructed to examine their effect on vestibular epithelia in postnatal rats. In vivo, the adenovirus vectors administered in vestibular perilymphatic or endolymphatic space, their transduction efficiency and other indexes are different. We set up a culture construction to simulate and show the differences. In the study, we also developed a new dissection protocol to be quick to harvest the whole maculae and cristae. The new ectopic vestibular hair cell-like cells induced by overexpression of Math1, were found to appear in the non-sensory region of the postnatal rats' vestibular epithelia as observed in the cochlear, and the number of the new cells was different when a different virus administration was simulated in vestibular perilymphatic or endolymphatic space, suggesting that the cells found could have the capability to differentiate into new hair cells. Our study might pave the way for further in vivo studies on vestibular Math1 adenoviral vector gene transfer.

Notch signaling and Hes labeling in the normal and drug-damaged organ of Corti

During the development of the inner ear, the Notch cell signaling pathway is responsible for the specification of the pro-sensory domain and influences cell fate decisions. It is assumed that Notch signaling ends during maturity and cannot be reinitiated to alter the fate of new or existing cells in the organ of Corti. This is in contrast to non-mammalian species which reinitiate Delta 1-Notch1 signaling in response to trauma in the auditory epithelium, resulting in hair cell regeneration through transdifferentiation and/or mitosis. We report immunohistochemical data and Western protein analysis showing that in the aminoglycoside-damaged guinea pig organ of Corti, there is an increase in proteins involved in Notch activation occurring within 24h of a chemical hair cell lesion. The signaling response is characterized by the increased presence of Jagged1 ligand in pillar and Deiters cells, Notch1 signal in surviving supporting cell nuclei, and the absence of Jagged2 and Delta-like1. The pro-sensory bHLH protein Atoh1 was absent at all time points following an ototoxic lesion, while the repressor bHLH transcription factors Hes1 and Hes5 were detected in surviving supporting cell nuclei in the former inner and outer hair cell areas, respectively. Notch pathway proteins peaked at 2 weeks, decreased at 1 month, and nearly disappeared by 2 months. These results indicate that the mammalian auditory epithelium retains the ability to regulate Notch signaling and Notch-dependent Hes activity in response to cellular trauma and that the signaling is transient. Additionally, since Hes activity antagonizes the transcription of pro-sensory Atoh1, the presence of Hes after a lesion may prohibit the occurrence of transdifferentiation in the surviving supporting cells.

Feathers and fins: non-mammalian models for hair cell regeneration

Death of mechanosensory cells in the inner ear results in two profound disabilities: hearing loss and balance disorders. Although mammals lack the capacity to regenerate hair cells, recent studies in mice and other rodents have offered valuable insight into strategies for stimulating hair cell regeneration in mammals. Investigations of model organisms that retain the ability to form new hair cells after embryogenesis, such as fish and birds, are equally important and have provided clues as to the cellular and molecular mechanisms that may block hair cell regeneration in mammals. Here, we summarize studies on hair cell regeneration in the chicken and the zebrafish, discuss specific advantages of each model, and propose future directions for the use of non-mammalian models in understanding hair cell regeneration.

Diverse expression patterns of LIM-homeodomain transcription factors (LIM-HDs) in mammalian inner ear development

LIM-homeodomain transcription factors (LIM-HDs) are essential in tissue patterning and differentiation. But their expression patterns in the inner ear are largely unknown. Here we report on a study of twelve LIM-HDs, by their tempo-spatial patterns that imply distinct yet overlapping roles, in the developing mouse inner ear. Expression of Lmx1a and Isl1 begins in the otocyst stage, with Lmx1a exclusively in the non-sensory and Isl1 in the prosensory epithelia. The second wave of expression at E12.5 includes Lhx3, 5, 9, Isl2, and Lmx1b in the differentiating sensory epithelia with cellular specificities. With the exception of Lmx1a and Lhx3, all LIM-HDs are expressed in ganglion neurons. Expression of multiple LIM-HDs within a cell type suggests their redundant function.