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

NcRNA: key and potential in hearing loss

Hearing loss has an extremely high prevalence worldwide and brings incredible economic and social burdens. Mechanisms such as epigenetics are profoundly involved in the initiation and progression of hearing loss and potentially yield definite strategies for hearing loss treatment. Non-coding genes occupy 97% of the human genome, and their transcripts, non-coding RNAs (ncRNAs), are widely participated in regulating various physiological and pathological situations. NcRNAs, mainly including micro-RNAs (miRNAs), long-stranded non-coding RNAs (lncRNAs), and circular RNAs (circRNAs), are involved in the regulation of cell metabolism and cell death by modulating gene expression and protein-protein interactions, thus impacting the occurrence and prognosis of hearing loss. This review provides a detailed overview of ncRNAs, especially miRNAs and lncRNAs, in the pathogenesis of hearing loss. We also discuss the shortcomings and issues that need to be addressed in the study of hearing loss ncRNAs in the hope of providing viable therapeutic strategies for the precise treatment of hearing loss.

Regeneration of Hair Cells in the Human Vestibular System

The vestibular system is a critical part of the human balance system, malfunction of this system will lead to balance disorders, such as vertigo. Mammalian vestibular hair cells, the mechanical receptors for vestibular function, are sensitive to ototoxic drugs and virus infection, and have a limited restorative capacity after damage. Considering that no artificial device can be used to replace vestibular hair cells, promoting vestibular hair cell regeneration is an ideal way for vestibular function recovery. In this manuscript, the development of human vestibular hair cells during the whole embryonic stage and the latest research on human vestibular hair cell regeneration is summarized. The limitations of current studies are emphasized and future directions are discussed.

Gpr125 Marks Distinct Cochlear Cell Types and Is Dispensable for Cochlear Development and Hearing

The G protein-coupled receptor (GPR) family critically regulates development and homeostasis of multiple organs. As a member of the GPR adhesion family, Gpr125 (Adgra3) modulates Wnt/PCP signaling and convergent extension in developing zebrafish, but whether it is essential for cochlear development in mammals is unknown. Here, we examined the Gpr125 lacZ/+ knock-in mice and show that Gpr125 is dynamically expressed in the developing and mature cochleae. From embryonic day (E) 15.5 to postnatal day (P) 30, Gpr125-β-Gal is consistently expressed in the lesser epithelial ridge and its presumed progenies, the supporting cell subtypes Claudius cells and Hensen's cells. In contrast, Gpr125-β-Gal is expressed transiently in outer hair cells, epithelial cells in the lateral cochlear wall, interdental cells, and spiral ganglion neurons in the late embryonic and early postnatal cochlea. In situ hybridization for Gpr125 mRNA confirmed Gpr125 expression and validated loss of expression in Gpr125 lacZ/lacZ cochleae. Lastly, Gpr125 lacZ/+ and Gpr125 lacZ/ lacZ cochleae displayed no detectable loss or disorganization of either sensory or non-sensory cells in the embryonic and postnatal ages and exhibited normal auditory physiology. Together, our study reveals that Gpr125 is dynamically expressed in multiple cell types in the developing and mature cochlea and is dispensable for cochlear development and hearing.

Simultaneous gentamicin-mediated damage and Atoh1 overexpression promotes hair cell regeneration in the neonatal mouse utricle

Loss of hair cells from vestibular epithelium results in balance dysfunction. The current therapeutic regimen for vestibular diseases is limited. Upon injury or Atoh1 overexpression, hair cell replacement occurs rapidly in the mammalian utricle, suggesting a promising approach to induce vestibular hair cell regeneration. In this study, we applied simultaneous gentamicin-mediated hair cell ablation and Atoh1 overexpression to induce neonatal utricular hair cell formation in vitro. We confirmed that type I hair cells were the primary targets of gentamicin. Furthermore, injury and Atoh1 overexpression promoted hair cell regeneration in a timely and efficient manner through robust viral transfection. Hair cells regenerated with type II characteristics in the striola and type I/II characteristics in non-sensory regions. Rare EdU⁺/myosin7a⁺ cells in sensory regions and robust EdU⁺/myosin7a⁺ signals in ectopic regions indicate that transdifferentiation of supporting cells in situ, and mitosis and differentiation of non-sensory epithelial cells in ectopic regions, are sources of regenerative hair cells. Distinct regeneration patterns in in situ and ectopic regions suggested robust plasticity of vestibular non-sensory epithelium, generating more developed hair cell subtypes and thus providing a promising stem cell-like source of hair cells. These findings suggest that simultaneously causing injury and overexpressing Atoh1 promotes hair cell regeneration efficacy and maturity, thus expanding the understanding of ectopic plasticity in neonatal vestibular organs.

Enhanced Inner-Ear Organoid Formation from Mouse Embryonic Stem Cells by Photobiomodulation

Photobiomodulation (PBM) stimulates different types of stem cells to migrate, proliferate, and differentiate in vitro and in vivo. However, little is known about the effects of PBM on the differentiation of embryonic stem cells (ESCs) toward the otic lineage. Only a few reports have documented the in vitro differentiation of ESCs into inner-ear hair cells (HCs) due to the complexity of HCs compared with other target cell types. In this study, we determined the optimal condition to differentiate the ESCs into the otic organoid using different culture techniques and PBM parameters. The efficiency of organoid formation within the embryoid body (EB) was dependent on the cell density of the hanging drop. PBM, using 630 nm wavelength light-emitting diodes (LEDs), further improved the differentiation of inner-ear hair cell-like cells coupled with reactive oxygen species (ROS) overexpression. Transcriptome analysis showed the factors that are responsible for the effect of PBM in the formation of otic organoids, notably, the downregulation of neural development-associated genes and the hairy and enhancer of split 5 (Hes5) gene, which inhibits the differentiation of prosensory cells to hair cells. These data enrich the current differentiation protocols for generating inner-ear hair cells.

Mimic Cochlear Implant Surgery-Induced Cochlear Infection Fails to Further Damage Auditory Pathway in Deafened Guinea Pigs

Background

Kanamycin and subsequent furosemide administration was applied to the healthy guinea pigs to induce deafness.

Material/Methods

Of the deafened guinea pigs, 10 were further infused with anti-infection procedures (Group B) and the other 10 animals did not undergo anti-infection procedures (Group C). In Group B, the deafened animals were able to restore cochlear and middle ear functions following the anti-infection procedure. In Group C, all animals developed cochlear and middle ear infections.

Results

Compared to the healthy guinea pigs, hair cells and spiral ganglion neurons (SGN) of deafened animals (in Group B and Group C) were severely damaged. SGN density of deafened animals was significantly lower than that of healthy control animals in all ear turns except the basal turn. There was no significant difference between Group B and Group C in SGN density. The average optical density value of neurofilaments of deafened animals was also significantly decreased after the ototoxic drug administration. Notably, the density of the neurons in the cochlear nucleus region (CNR) of the brainstem were not significantly different between the healthy control guinea pigs and deafened animals.

Conclusions

Mimic cochlear implant surgery-induced cochlear infection caused no significant damage to the auditory pathway in ototoxic drug-induced deafened guinea pigs.

Cell proliferation during hair cell regeneration induced by Math1 in vestibular epithelia in vitro

Hair cell regeneration is the fundamental method of correcting hearing loss and balance disorders caused by hair cell damage or loss. How to promote hair cell regeneration is a hot focus in current research. In mammals, cochlear hair cells cannot be regenerated and few vestibular hair cells can be renewed through spontaneous regeneration. However, Math₁ gene transfer allows a few inner ear cells to be transformed into hair cells in vitro or in vivo. Hair cells can be renewed through two possible means in birds: supporting cell differentiation and transdifferentiation with or without cell division. Hair cell regeneration is strongly associated with cell proliferation. Therefore, this study explored the relationship between Math₁-induced vestibular hair cell regeneration and cell division in mammals. The mouse vestibule was isolated to harvest vestibular epithelial cells. Ad-Math₁-enhanced green fluorescent protein (EGFP) was used to track cell division during hair cell transformation. 5-Bromo-2′-deoxyuridine (BrdU) was added to track cell proliferation at various time points. Immunocytochemistry was utilized to determine cell differentiation and proliferation. Results demonstrated that when epithelial cells were in a higher proliferative stage, more of these cells differentiated into hair cells by Math₁ gene transfer. However, in the low proliferation stage, no BrdU-positive cells were seen after Math₁ gene transfer. Cell division always occurred before Math₁ transfection but not during or after Math₁ transfection, when cells were labeled with BrdU before and after Ad-Math₁-EGFP transfection. These results confirm that vestibular epithelial cells with high proliferative potential can differentiate into new hair cells by Math₁ gene transfer, but this process is independent of cell proliferation.

Notch pathway inhibitor DAPT enhances Atoh1 activity to generate new hair cells in situ in rat cochleae

Atoh1 overexpression in cochlear epithelium induces new hair cell formation. Use of adenovirus-mediated Atoh1 overexpression has mainly focused on the rat lesser epithelial ridge and induces ectopic hair cell regeneration. The sensory region of rat cochlea is difficult to transfect, thus new hair cells are rarely produced in situ in rat cochlear explants. After culturing rat cochleae in medium containing 10% fetal bovine serum, adenovirus successfully infected the sensory region as the width of the supporting cell area was significantly increased. Adenovirus encoding Atoh1 infected the sensory region and induced hair cell formation in situ. Combined application of the Notch inhibitor DAPT and Atoh1 increased the Atoh1 expression level and decreased hes1 and hes5 levels, further promoting hair cell generation. Our results demonstrate that DAPT enhances Atoh1 activity to promote hair cell regeneration in rat cochlear sensory epithelium in vitro.

Spatial and Age-Dependent Hair Cell Generation in the Postnatal Mammalian Utricle

Loss of vestibular hair cells is a common cause of balance disorders. Current treatment options for bilateral vestibular dysfunction are limited. During development, atonal homolog 1 (Atoh1) is sufficient and necessary for the formation of hair cells and provides a promising gene target to induce hair cell generation in the mammals. In this study, we used a transgenic mouse line to test the age and cell type specificity of hair cell induction in the postnatal utricle in mice. We found that forced Atoh1 expression in vivo can induce hair cell formation in the utricle from postnatal days 1 to 21, while the efficacy of hair cell induction is progressively reduced as the animals become older. In the utricle, the induction of hair cells occurs both within the sensory region and in cells in the transitional epithelium next to the sensory region. Within the sensory epithelium, the central region, known as the striola, is most subjective to the induction of hair cell formation. Furthermore, forced Atoh1 expression can promote proliferation in an age-dependent manner that mirrors the progressively reduced efficacy of hair cell induction in the postnatal utricle. These results suggest that targeting both cell proliferation and Atoh1 in the utricle striolar region may be explored to induce hair cell regeneration in mammals. The study also demonstrates the usefulness of the animal model that provides an in vivo Atoh1 induction model for vestibular regeneration studies.

Atoh1 expression levels define the fate of rat cochlear nonsensory epithelial cells in vitro

Atonal homolog 1 (Atoh1) is a basic helix-loop-helix transcription factor that is essential for inner ear hair cell differentiation. Previous studies have reported that Atoh1 gene transfer induces the production of ectopic hair cell-like cells (EHCLCs). In the present study, the effect of different Atoh1 expression levels and the duration of EHCLC formation on the lesser epithelial ridge (LER) of cochleae was examined using a human adenovirus serotype 5 (Ad5) vector encoding atoh1 and the reporter gene EGFP. Different Ad5-EGFP-atoh1/Ad5-EGFP virus titers were added to cultured cochlear explants and EHCLCs were detected in the LER at various time points. The results demonstrated that GFP alone did not induce EHCLCs. By contrast, Atoh1 expression induced EHCLCs as early as 2.5–5 days following EGFP-atoh1 infection in the LER and depending upon the viral titer, the number of EHCLCs increased with time. Higher Ad5-EGFP-atoh1 titers induced enhanced Atoh1 expression, resulting in an increase in EHCLCs. Lower Ad5-EGFP-atoh1 titers required more time for EHCLC formation and very low titers of Ad5-EGFP-atoh1 induced only weak Atoh1 expression and did not trigger EHCLC formation. In conclusion, the present study utilized an appropriate Ad5-EGFP-atoh1 titer range to induce Atoh1 expression and the subsequent production of EHCLCs. The results revealed that the Atoh1 expression level defined the fate of LER cells as either EHCLCs or nonsensory epithelial cells. This evidence may provide an important guideline for future studies into gene therapy strategies for the treatment of deafness.

Adenovector-mediated gene delivery to human umbilical cord mesenchymal stromal cells induces inner ear cell phenotype

Hearing is one of our main sensory systems and having a hearing disorder can have a significant impact in an individual's quality of life. Sensory neural hearing loss (SNHL) is the most common form of hearing loss; it results from the degeneration of inner ear sensory hair cells and auditory neurons in the cochlea, cells that are terminally differentiated. Stem cell-and gene delivery-based strategies provide an opportunity for the replacement of these cells. In recent years, there has been an increasing interest in gene delivery to mesenchymal stem cells. In this study, we evaluated the potential of human umbilical cord mesenchymal stromal cells (hUCMSCs) as a possible source for regenerating inner ear hair cells. The expression of Atoh1 induced the differentiation of hUCMSCs into cells that resembled inner ear hair cells morphologically and immunocytochemically, evidenced by the expression of hair cell-specific markers. The results demonstrated for the first time that hUCMSCs can differentiate into hair cell-like cells, thus introducing a new potential tissue engineering and cell transplantation approach for the treatment of hearing loss.

Ectopic hair cell-like cell induction by Math1 mainly involves direct transdifferentiation in neonatal mammalian cochlea

Math1, also known as Atoh1, is a basic helix-loop-helix transcription factor that plays a key role in hair cells (HCs) development. Previous studies have reported that Math1 gene transfer could induce the production of ectopic hair cell-like cells both in vitro and in vivo. Here, we focused on the mechanism of ectopic hair cell-like cellular differentiation from cells in the lateral epithelial ridge (LER) of cochlea with a human adenovirus serotype 5 (Ad5) vector encoding both Math1 and the reporter gene EGFP. Within the Ad5-EGFP-Math1 infection, hair-cell like cells could be detected in the LER. 5'-Bromo-2' deoxyuridine (BrdU) incorporation test results at different time points suggested that LER cells possessed high potential to proliferation, but they could not transdifferentiate into hair cells spontaneously. Almost all of Math1 induced hair cell-like cells were BrdU negative when BrdU incorporation occurred after Math1 expression. In conclusion, Math1 induced hair cell-like cells from LER cells mainly underwent direct trans-differentiation instead of mitosis of LER cells or newly hair cell-like cells.

Atoh1 induces auditory hair cell recovery in mice after ototoxic injury

OBJECTIVES/HYPOTHESIS

To evaluate the ability of the Ad28.gfap.atoh1 to promote hair cell regeneration and hearing recovery in cochlea injured with kanamycin and furosemide.

STUDY DESIGN

In vivo model of hair cell ablation and subsequent treatment with Atoh1.

METHODS

The hair cells of C57BL/6 mice were ablated with systemic administration of kanamycin and furosemide. The left ears were treated with Ad28.gfap.atoh1. The right ears were not treated. Preablation audiograms and distortion product otoacoustic emissions (DPOAEs) were compared to 1- or 2-month postablation studies. Harvested cochleae were examined for histologic evidence of hair cell regeneration and spiral ganglion cell survival.

RESULTS

Delivery of Ad28.gfap.atoh1 results in development of auditory hair cells. There was no recovery of DPOAEs at 1 or 2 months post-treatment. Two months after delivery of Ad28.gfap.atoh1, the left ear exhibited a moderate recovery of hearing at 4 and 8 kHz (P < .01). There was no significant difference at 16 or 32 kHz. One month after treatment, myosin VII-positive immunohistochemical staining can be seen in both the inner and outer hair cells of the treated ear. In the untreated ear, minimal myosin VII-positive debris is seen, with no indication of normal hair cells. Two months after ablation, there is evidence of hair cell recovery on the treated side, whereas the untreated cochlea demonstrates a flattened epithelium. Untreated ears showed decreased spiral ganglion cell density at the basal turn compared to treated ears.

CONCLUSIONS

Ad28.gfap.atoh1 promotes hair cell regeneration in cochlea ablated with kanamycin and furosemide resulting in moderate hearing recovery.

Postnatal development, maturation and aging in the mouse cochlea and their effects on hair cell regeneration

The organ of Corti in the mammalian inner ear is comprised of mechanosensory hair cells (HCs) and nonsensory supporting cells (SCs), both of which are believed to be terminally post-mitotic beyond late embryonic ages. Consequently, regeneration of HCs and SCs does not occur naturally in the adult mammalian cochlea, though recent evidence suggests that these cells may not be completely or irreversibly quiescent at earlier postnatal ages. Furthermore, regenerative processes can be induced by genetic and pharmacological manipulations, but, more and more reports suggest that regenerative potential declines as the organ of Corti continues to age. In numerous mammalian systems, such effects of aging on regenerative potential are well established. However, in the cochlea, the problem of regeneration has not been traditionally viewed as one of aging. This is an important consideration as current models are unable to elicit widespread regeneration or full recovery of function at adult ages yet regenerative therapies will need to be developed specifically for adult populations. Still, the advent of gene targeting and other genetic manipulations has established mice as critically important models for the study of cochlear development and HC regeneration and suggests that auditory HC regeneration in adult mammals may indeed be possible. Thus, this review will focus on the pursuit of regeneration in the postnatal and adult mouse cochlea and highlight processes that occur during postnatal development, maturation, and aging that could contribute to an age-related decline in regenerative potential. Second, we will draw upon the wealth of knowledge pertaining to age related senescence in tissues outside of the ear to synthesize new insights and potentially guide future research aimed at promoting HC regeneration in the adult cochlea.

Canalostomy as a surgical approach for cochlear gene therapy in the rat

This article presents a unique approach for the delivery of gene therapy vectors into the cochlea of the laboratory rat. Mice and guinea pigs are established in vivo models for cochlear gene therapy each of which has distinct advantages and disadvantages. The rat has some of the molecular advantages of a mouse model combined with size advantages for surgical approaches. Vector delivery via cochleostomy or injection through the round window causes concomitant sensorineural hearing loss and is therefore not suitable for studies where the change in hearing is being followed. Compared to the mouse, the rat does not demonstrate easily recognizable landmarks that allow for use of the semicircular canal as an approach to the inner ear. We analyzed sagittal and coronal temporal bone sections of Long Evans rats and identified the bony entrance of the facial nerve as a crucial landmark for canalostomy. Auditory brainstem response and distortion product otoacustic emission measurements revealed minimal differences in the hearing threshold after adenovirus vector application when large volumes of vector were infused to the inner ear. Canalostomy and infusion of adenoviral vectors also resulted in temporary balance disturbance in the rat. Immunohistochemical assessment after delivery of a green fluorescent protein expressing vector showed significant GFP expression in the cochlea.

Functional features of trans-differentiated hair cells mediated by Atoh1 reveals a primordial mechanism

Evolution has transformed a simple ear with few vestibular maculae into a complex three-dimensional structure consisting of nine distinct endorgans. It is debatable whether the sensory epithelia underwent progressive segregation or emerged from distinct sensory patches. To address these uncertainties we examined the morphological and functional phenotype of trans-differentiated rat hair cells to reveal their primitive or endorgan-specific origins. Additionally, it is uncertain how Atoh1-mediated trans-differentiated hair cells trigger the processes that establish their neural ranking from the vestibulocochlear ganglia. We have demonstrated that the morphology and functional expression of ionic currents in trans-differentiated hair cells resemble those of "ancestral" hair cells, even at the lesser epithelia ridge aspects of the cochlea. The structures of stereociliary bundles of trans-differentiated hair cells were in keeping with cells in the vestibule. Functionally, the transient expression of Na⁺ and I(h) currents initiates and promotes evoked spikes. Additionally, Ca²⁺ current was expressed and underwent developmental changes. These events correlate well with the innervation of ectopic hair cells. New "born" hair cells at the abneural aspects of the cochlea are innervated by spiral ganglion neurons, presumably under the tropic influence of chemoattractants. The disappearance of inward currents coincides well with the attenuation of evoked electrical activity, remarkably recapitulating the development of hair cells. Ectopic hair cells underwent stepwise changes in the magnitude and kinetics of transducer currents. We propose that Atoh1 mediates trans-differentiation of morphological and functional "ancestral" hair cells that are likely to undergo diversification in an endorgan-specific manner.

Cochlear gene therapy

PURPOSE OF REVIEW

This review highlights recent advances in cochlear gene therapy over the past several years. Cochlear gene therapy has undergone tremendous advances over the past decade. Beginning with some groundbreaking work in 2005 documenting hair cell regeneration using virally mediated delivery of the mouse atonal 1 gene, gene therapy is now being explored as a possible treatment for a variety of causes of hearing loss.

RECENT FINDINGS

Recent advances in cochlear gene therapy include improved methods of gene delivery with a better delineation of viral vectors that are suitable for this purpose, additional improvements in hair cell regeneration, and directed research toward autoimmune hearing loss, ototoxicity, spiral ganglion survival, and genetic forms of hearing loss.

SUMMARY

If successful, cochlear gene therapy will dramatically alter our ability to treat a variety of forms of acquired and genetic hearing loss.

Inhibition of Notch activity promotes nonmitotic regeneration of hair cells in the adult mouse utricles

The capacity of adult mammals to regenerate sensory hair cells is not well defined. To explore early steps in this process, we examined reactivation of a transiently expressed developmental gene, Atoh1, in adult mouse utricles after neomycin-induced hair cell death in culture. Using an adenoviral reporter for Atoh1 enhancer, we found that Atoh1 transcription is activated in some hair cell progenitors (supporting cells) 3 d after neomycin treatment. By 18 d after neomycin, the number of cells with Atoh1 transcriptional activity increased significantly, but few cells acquired hair cell features (i.e., accumulated ATOH1 or myosin VIIa protein or developed stereocilia). Treatment with DAPT, an inhibitor of γ-secretase, reduced notch pathway activity, enhanced Atoh1 transcriptional activity, and dramatically increased the number of Atoh1-expressing cells with hair cell features, but only in the striolar/juxtastriolar region. Similar effects were seen with TAPI-1, an inhibitor of another enzyme required for notch activity (TACE). Division of supporting cells was rare in any control or DAPT-treated utricles. This study shows that mature mammals have a natural capacity to initiate vestibular hair cell regeneration and suggests that regional notch activity is a significant inhibitor of direct transdifferentiation of supporting cells into hair cells following damage.

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.

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.

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.

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.