Dye Tracking Following Posterior Semicircular Canal or Round Window Membrane Injections Suggests a Role for the Cochlea Aqueduct in Modulating Distribution

Recovery kinetics of dual AAV-mediated human otoferlin expression

Deafness-causing deficiencies in otoferlin (OTOF) have been addressed preclinically using dual adeno-associated virus (AAV)-based approaches. However, timing of transduction, recombination of mRNA, and protein expression with dual hybrid AAV methods methods have not previously been characterized. Here, we have established an ex vivo assay to determine the kinetics of dual-AAV mediated expression of OTOF in hair cells of the mouse utricle. We utilized two different recombinant vectors that comprise DB-OTO, one containing the 5' portion of OTOF under the control of the hair cell-specific Myo15 promoter, and the other the 3' portion of OTOF. We explored specificity of the Myo15 promoter in hair cells of the mouse utricle, established dose response characteristics of DB-OTO ex vivo in an OTOF-deficient mouse model, and demonstrated tolerability of AAV1 in utricular hair cells. Furthermore, we established deviations from a one-to-one ratio of 5' to 3' vectors with little impact on recombined OTOF. Finally, we established a plateau in quantity of recombined OTOF mRNA and protein expression by 14 to 21 days ex vivo with comparable recovery timing to that in vivo model. These findings demonstrate the utility of an ex vivo model system for exploring expression kinetics and establish in vivo and ex vivo recovery timing of dual AAV-mediated OTOF expression.

Disturbance in the protein landscape of cochlear perilymph in an Alzheimer's disease mouse model

Hearing loss is a pivotal risk factor for dementia. It has recently emerged that a disruption in the intercommunication between the cochlea and brain is a key process in the initiation and progression of this disease. However, whether the cochlear properties can be influenced by pathological signals associated with dementia remains unclear. In this study, using a mouse model of Alzheimer's disease (AD), we investigated the impacts of the AD-like amyloid β (Aβ) pathology in the brain on the cochlea. Despite little detectable change in the age-related shift of the hearing threshold, we observed quantitative and qualitative alterations in the protein profile in perilymph, an extracellular fluid that fills the path of sound waves in the cochlea. Our findings highlight the potential contribution of Aβ pathology in the brain to the disturbance of cochlear homeostasis.

Influence of inner ear impedance on middle ear sound transfer functions

Introduction

For experimental studies on sound transfer in the middle ear, it may be advantageous to perform the measurements without the inner ear. In this case, it is important to know the influence of inner ear impedance on the middle ear transfer function (METF). Previous studies provide contradictory results in this regard. With the current study, we investigate the influence of inner ear impedance in more detail and find possible reasons for deviations in the previous studies.

Methods

11 fresh frozen temporal bones were prepared in our study. The factors related to inner ear impedance, including round window membrane stiffness, cochleostomy, cochlea fluid and cochlea destruction were involved in the experimental design. After measuring in the intact specimen as a reference (step 1), the round window membrane was punctured (step 2), then completely removed (step 3). The cochleostomy was performed (step 4) before the cochlear fluid was carefully suctioned through scala tympani (step 5) and scala vestibuli (step 6). Finally, cochlea was destroyed by drilling (step 7). Translational and rotational movement of the stapes footplate were measured and calculated at each step. The results of the steps were compared to quantify the effect of inner ear impedance changing related to the process of cochlear drainage.

Results

As the inner ear impedance decreases from step 1 to 7, the amplitudes of the METF curves at each frequency gradually increase in general. From step 6 on, the measured METF are significantly different with respect to the intact group at high frequencies above 3 kHz. The differences are frequency dependent. However, the significant decrement of rotational motion appears at the frequencies above 4.5 kHz from the step 5.

Conclusion

This study confirms the influence of inner ear impedance on METF only at higher frequencies (≥3 kHz). The rotational motions are more sensitive to the drainage of fluid at the higher frequency. Study results that found no influence of cochlea impedance may be due to incomplete drainage of the cochlea.

Distributional comparison of different AAV vectors after unilateral cochlear administration

The adeno-associated virus (AAV) gene therapy has been widely applied to mouse models for deafness. But, AAVs could transduce non-targeted organs after inner ear delivery due to their low cell-type specificity. This study compares transgene expression and biodistribution of AAV1, AAV2, Anc80L65, AAV9, AAV-PHP.B, and AAV-PHP.eB after round window membrane (RWM) injection in neonatal mice. The highest virus concentration was detected in the injected cochlea. AAV2, Anc80L65, AAV9, AAV-PHP.B, and AAV-PHP.eB transduced both inner hair cells (IHCs) and outer hair cells (OHCs) with high efficiency, while AAV1 transduced IHCs with high efficiency but OHCs with low efficiency. All AAV subtypes finitely transduced contralateral inner ear, brain, heart, and liver compared with the injected cochlea. In most brain regions, the enhanced green fluorescent protein (eGFP) expression of AAV1 and AAV2 was lower than that of other four subtypes. We suggested the cochlear aqueduct might be one of routes for vectors instantaneously infiltrating into the brain from the cochlea through a dye tracking test. In summary, our results provide available data for further investigating the biodistribution of vectors through local inner ear injection and afford a reference for selecting AAV serotypes for gene therapy toward deafness.

Gene Therapy for Inherited Hearing Loss: Updates and Remaining Challenges

Hearing loss stands as the most prevalent sensory deficit among humans, posing a significant global health challenge. Projections indicate that by 2050, approximately 10% of the world's population will grapple with disabling hearing impairment. While approximately half of congenital hearing loss cases have a genetic etiology, traditional interventions such as hearing aids and cochlear implants do not completely restore normal hearing. The absence of biological treatment has prompted significant efforts in recent years, with a strong focus on gene therapy to address hereditary hearing loss. Although several studies have exhibited promising recovery from common forms of genetic deafness in mouse models, existing challenges must be overcome to make gene therapy applicable in the near future. Herein, we summarize the primary gene therapy strategies employed over past years, provide an overview of the recent achievements in preclinical studies for genetic hearing loss, and outline the current key obstacles to cochlear gene therapy.

Selection of viral capsids and promoters affects the efficacy of rescue of Tmprss3-deficient cochlea

Adeno-associated virus (AAV)-mediated gene transfer has shown promise in rescuing mouse models of genetic hearing loss, but how viral capsid and promoter selection affects efficacy is poorly characterized. Here, we tested combinations of AAVs and promoters to deliver Tmprss3, mutations in which are associated with hearing loss in humans. Tmprss3tm1/tm1 mice display severe cochlear hair cell degeneration, loss of auditory brainstem responses, and delayed loss of spiral ganglion neurons. Under the ubiquitous CAG promoter and AAV-KP1 capsid, Tmprss3 overexpression caused striking cytotoxicity in vitro and in vivo and failed to rescue degeneration or dysfunction of the Tmprss3tm1/tm1 cochlea. Reducing the dosage or using AAV-DJ-CAG-Tmprss3 diminished cytotoxicity without rescue of the Tmprss3tm1/tm1 cochlea. Finally, the combination of AAV-KP1 capsid and the EF1α promoter prevented cytotoxicity and reduced hair cell degeneration, loss of spiral ganglion neurons, and improved hearing thresholds in Tmprss3tm1/tm1 mice. Together, our study illustrates toxicity of exogenous genes and factors governing rescue efficiency, and suggests that cochlear gene therapy likely requires precisely targeted transgene expression.

Experimental drugs for the prevention or treatment of sensorineural hearing loss

INTRODUCTION

Sensorineural hearing loss results in irreversible loss of inner ear hair cells and spiral ganglion neurons. Reduced sound detection and speech discrimination can span all ages, and sensorineural hearing rehabilitation is limited to amplification with hearing aids or cochlear implants. Recent insights into experimental drug treatments for inner ear regeneration and otoprotection have paved the way for clinical trials in order to restore a more physiological hearing experience. Paired with the development of innovative minimally invasive approaches for drug delivery to the inner ear, new, emerging treatments for hearing protection and restoration are within reach.

AREAS COVERED

This expert opinion provides an overview of the latest experimental drug therapies to protect from and to restore sensorineural hearing loss.

EXPERT OPINION

The degree and type of cellular damage to the cochlea, the responsiveness of remaining, endogenous cells to regenerative treatments, and the duration of drug availability within cochlear fluids will determine the success of hearing protection or restoration.

Neomycin-Induced Deafness in Neonatal Mice

BACKGROUND

Hearing impairment is a rising public health issue, and current therapeutics fail to restore normal auditory sensation. Animal models are essential to a better understanding of the pathophysiology of deafness and developing therapeutics to restore hearing.

NEW METHODS

Wild-type CBA/CaJ neonatal mice P2-5 were used in this study. Neomycin suspension (500nl of 50 or 100mg/ml) was micro-injected into the endolymphatic space. Cochlear morphology was examined 3 and 7 days after injection; hair cell (HC) loss, supporting cell morphology, and neurite denervation pattern were assessed with whole-mounts. At 2 and 4 weeks post-injection, the spiral ganglion neuron (SGN) density was analyzed with cryostat sections. Audiometric responses were measured with auditory brain response (ABR) at 4 weeks.

RESULTS

Rapid and complete degeneration of the inner and outer HCs occurred as early as 3 days post-injection. Subsequently, time- and dose-dependent degeneration patterns were observed along the axis of the cochlear membranous labyrinth forming a flat epithelium. Likewise, the SGN histology demonstrated significant cell density reduction at 2 and 4 weeks. The ABR threshold measurements confirmed profound deafness at 4 weeks.

COMPARISON WITH EXISTING METHODS

Compared to previously described local and systemic aminoglycoside injections, this method provides a reliable, robust, and rapid deafening model with a single infusion of neomycin in neonatal mice. This model also allows for investigating the effects of inner ear damage during auditory maturation.

CONCLUSIONS

A single injection of neomycin into the endolymphatic space induces robust HC loss and denervation in neonatal mice.

Approaches and Vectors for Efficient Cochlear Gene Transfer in Adult Mouse Models

Inner ear gene therapy using adeno-associated viral vectors (AAVs) in neonatal mice can alleviate hearing loss in mouse models of deafness. However, efficient and safe transgene delivery to the adult mouse cochlea is critical for the effectiveness of AAV-mediated therapy. Here, we examined three gene delivery approaches including posterior semicircular canal (PSCC) canalostomy, round window membrane (RWM) injection, and tubing-RWM+PSCC (t-RP) in adult mice. Transduction rates and survival rates of cochlear hair cells were analyzed, hearing function was recorded, AAV distribution in the sagittal brain sections was evaluated, and cochlear histopathologic images were appraised. We found that an injection volume of 1 μL AAV through the PSCC is safe and highly efficient and does not impair hearing function in adult mice, but local injection allows AAV vectors to spread slightly into the brain. We then tested five AAV serotypes (PHP.eB, IE, Anc80L65, AAV2, and PHP.s) in parallel and observed the most robust eGFP expression in inner hair cells, outer hair cells, and spiral ganglion neurons throughout the cochlea after AAV-Anc80L65 injection. Thus, PSCC-injected Anc80L65 provides a foundation for gene therapy in the adult cochlea and will facilitate the development of inner ear gene therapy.

Analyzing efficacy, stability, and safety of AAV-mediated optogenetic hearing restoration in mice

This longitudinal study revealed stable expression of the fast-gating channelrhodopsin f-Chrimson in cochlear neurons of mice over at least 2 yr upon a single postnatal AAV dosing of the cochlea.

AAV-mediated optogenetic neural stimulation has become a clinical approach for restoring function in sensory disorders and feasibility for hearing restoration has been indicated in rodents. Nonetheless, long-term stability and safety of AAV-mediated channelrhodopsin (ChR) expression in spiral ganglion neurons (SGNs) remained to be addressed. Here, we used longitudinal studies on mice subjected to early postnatal administration of AAV2/6 carrying fast gating ChR f-Chrimson under the control of the human synapsin promoter unilaterally to the cochlea. f-Chrimson expression in SGNs in both ears and the brain was probed in animals aged 1 mo to 2 yr. f-Chrimson was observed in SGNs at all ages indicating longevity of ChR-expression. SGN numbers in the AAV-injected cochleae declined with age faster than in controls. Investigations were extended to the brain in which viral transduction was observed across the organ at varying degrees irrespective of age without observing viral spread-related pathologies. No viral DNA or virus-related histopathological findings in visceral organs were encountered. In summary, our study demonstrates life-long (24 mo in mice) expression of f-Chrimson in SGNs upon single AAV-dosing of the cochlea.

A Single Cisterna Magna Injection of AAV Leads to Binaural Transduction in Mice

Viral-mediated gene augmentation, silencing, or editing offers tremendous promise for the treatment of inherited and acquired deafness. Inner-ear gene therapies often require a safe, clinically useable and effective route of administration to target both ears, while avoiding damage to the delicate structures of the inner ear. Here, we examined the possibility of using a cisterna magna injection as a new cochlear local route for initiating binaural transduction by different serotypes of the adeno-associated virus (AAV2/8, AAV2/9, AAV2/Anc80L65). The results were compared with those following canalostomy injection, one of the existing standard inner ear local delivery routes. Our results demonstrated that a single injection of AAVs enables high-efficiency binaural transduction of almost all inner hair cells with a basal-apical pattern and of large numbers of spiral ganglion neurons of the basal portion of the cochlea, without affecting auditory function and cochlear structures. Taken together, these results reveal the potential for using a cisterna magna injection as a local route for binaural gene therapy applications, but extensive testing will be required before translation beyond mouse models.

Characterizing the Access of Cholinergic Antagonists to Efferent Synapses in the Inner Ear

Stimulation of cholinergic efferent neurons innervating the inner ear has profound, well-characterized effects on vestibular and auditory physiology, after activating distinct ACh receptors (AChRs) on afferents and hair cells in peripheral endorgans. Efferent-mediated fast and slow excitation of vestibular afferents are mediated by α4β2*-containing nicotinic AChRs (nAChRs) and muscarinic AChRs (mAChRs), respectively. On the auditory side, efferent-mediated suppression of distortion product otoacoustic emissions (DPOAEs) is mediated by α9α10nAChRs. Previous characterization of these synaptic mechanisms utilized cholinergic drugs, that when systemically administered, also reach the CNS, which may limit their utility in probing efferent function without also considering central effects. Use of peripherally-acting cholinergic drugs with local application strategies may be useful, but this approach has remained relatively unexplored. Using multiple administration routes, we performed a combination of vestibular afferent and DPOAE recordings during efferent stimulation in mouse and turtle to determine whether charged mAChR or α9α10nAChR antagonists, with little CNS entry, can still engage efferent synaptic targets in the inner ear. The charged mAChR antagonists glycopyrrolate and methscopolamine blocked efferent-mediated slow excitation of mouse vestibular afferents following intraperitoneal, middle ear, or direct perilymphatic administration. Both mAChR antagonists were effective when delivered to the middle ear, contralateral to the side of afferent recordings, suggesting they gain vascular access after first entering the perilymphatic compartment. In contrast, charged α9α10nAChR antagonists blocked efferent-mediated suppression of DPOAEs only upon direct perilymphatic application, but failed to reach efferent synapses when systemically administered. These data show that efferent mechanisms are viable targets for further characterizing drug access in the inner ear.

Transduction Efficiency and Immunogenicity of Viral Vectors for Cochlear Gene Therapy: A Systematic Review of Preclinical Animal Studies

Background: Hearing impairment is the most frequent sensory deficit, affecting 466 million people worldwide and has been listed by the World Health Organization (WHO) as one of the priority diseases for research into therapeutic interventions to address public health needs. Inner ear gene therapy is a promising approach to restore sensorineural hearing loss, for which several gene therapy applications have been studied and reported in preclinical animal studies. Objective: To perform a systematic review on preclinical studies reporting cochlear gene therapy, with a specific focus on transduction efficiency. Methods: An initial PubMed search was performed on April 1st 2021 using the PRISMA methodology. Preclinical in vivo studies reporting primary data regarding transduction efficiency of gene therapy targeting the inner ear were included in this report. Results: Thirty-six studies were included in this review. Transduction of various cell types in the inner ear can be achieved, according to the viral vector used. However, there is significant variability in the applied vector delivery systems, including promoter, viral vector titer, etc. Conclusion: Although gene therapy presents a promising approach to treat sensorineural hearing loss in preclinical studies, the heterogeneity of methodologies impedes the identification of the most promising tools for future use in inner ear therapies.

Microtechnologies for inner ear drug delivery

PURPOSE OF REVIEW

Treatment of auditory dysfunction is dependent on inner ear drug delivery, with microtechnologies playing an increasingly important role in cochlear access and pharmacokinetic profile control. This review examines recent developments in the field for clinical and animal research environments.

RECENT FINDINGS

Micropump technologies are being developed for dynamic control of flow rates with refillable reservoirs enabling timed delivery of multiple agents for protection or regeneration therapies. These micropumps can be combined with cochlear implants with integral catheters or used independently with cochleostomy or round window membrane (RWM) delivery modalities for therapy development in animal models. Sustained release of steroids with coated cochlear implants remains an active research area with first-time-in-human demonstration of reduced electrode impedances. Advanced coatings containing neurotrophin producing cells have enhanced spiral ganglion neuron survival in animal models, and have proven safe in a human study. Microneedles have emerged for controlled microperforation of the RWM for significant enhancement in permeability, combinable with emerging matrix formulations that optimize biological interaction and drug release kinetics.

SUMMARY

Microsystem technologies are providing enhanced and more controlled access to the inner ear for advanced drug delivery approaches, alone and in conjunction with cochlear implants.

The DNA methylation inhibitor RG108 protects against noise-induced hearing loss

Background

Noise-induced hearing loss represents a commonly diagnosed type of hearing disability, severely impacting the quality of life of individuals. The current work is aimed at assessing the effects of DNA methylation on noise-induced hearing loss.

Methods

Blocking DNA methyltransferase 1 (DNMT1) activity with a selective inhibitor RG108 or silencing DNMT1 with siRNA was used in this study. Auditory brainstem responses were measured at baseline and 2 days after trauma in mice to assess auditory functions. Whole-mount immunofluorescent staining and confocal microcopy of mouse inner ear specimens were performed to analyze noise-induced damage in cochleae and the auditory nerve at 2 days after noise exposure.

Results

The results showed that noise exposure caused threshold elevation of auditory brainstem responses and cochlear hair cell loss. Whole-mount cochlea staining revealed a reduction in the density of auditory ribbon synapses between inner hair cells and spiral ganglion neurons. Inhibition of DNA methyltransferase activity via a non-nucleoside specific pharmacological inhibitor, RG108, or silencing of DNA methyltransferase-1 with siRNA significantly attenuated ABR threshold elevation, hair cell damage, and the loss of auditory synapses.

Conclusions

This study suggests that inhibition of DNMT1 ameliorates noise-induced hearing loss and indicates that DNMT1 may be a promising therapeutic target.

Graphical abstract

Developing Fast, Red-Light Optogenetic Stimulation of Spiral Ganglion Neurons for Future Optical Cochlear Implants

Optogenetic stimulation of type I spiral ganglion neurons (SGNs) promises an alternative to the electrical stimulation by current cochlear implants (CIs) for improved hearing restoration by future optical CIs (oCIs). Most of the efforts in using optogenetic stimulation in the cochlea so far used early postnatal injection of viral vectors carrying blue-light activated channelrhodopsins (ChRs) into the cochlea of mice. However, preparing clinical translation of the oCI requires (i) reliable and safe transduction of mature SGNs of further species and (ii) use of long-wavelength light to avoid phototoxicity. Here, we employed a fast variant of the red-light activated channelrhodopsin Chrimson (f-Chrimson) and different AAV variants to implement optogenetic SGN stimulation in Mongolian gerbils. We compared early postnatal (p8) and adult (>8 weeks) AAV administration, employing different protocols for injection of AAV-PHP.B and AAV2/6 into the adult cochlea. Success of the optogenetic manipulation was analyzed by optically evoked auditory brainstem response (oABR) and immunohistochemistry of mid-modiolar cryosections of the cochlea. In order to most efficiently evaluate the immunohistochemical results a semi-automatic procedure to identify transduced cells in confocal images was developed. Our results indicate that the rate of SGN transduction is significantly lower for AAV administration into the adult cochlea compared to early postnatal injection. SGN transduction upon AAV administration into the adult cochlea was largely independent of the chosen viral vector and injection approach. The higher the rate of SGN transduction, the lower were oABR thresholds and the larger were oABR amplitudes. Our results highlight the need to optimize viral vectors and virus administration for efficient optogenetic manipulation of SGNs in the adult cochlea for successful clinical translation of SGN-targeting gene therapy and of the oCI.

A novel intracochlear injection method for rapid drug delivery to vestibular end organs

BACKGROUND

Injection into the inner ear through the round window (RW) or a cochleostomy is a reliable method for delivering drugs or viruses to the cochlea. This method has been less effective for fast deliveries to vestibular end organs.

NEW METHOD

We describe a novel approach for rapid delivery of drugs to the vestibular end organ via the oval window (OW) and scala vestibuli in 1-3 month old C57BL/6 mice. The OW was directly accessed through the external ear canal after ablating the tympanic membrane and middle ear ossicles. A canalostomy in the superior canal provided a low pressure point for faster transit of injected solution from the OW to the vestibular neuroepithelia, allowing for higher rates of injection.

RESULTS

The efficacy of this technique was shown by fast transit times of a colored artificial perilymph from the OW to the utricle and the ampullae of the horizontal and superior canals in ∼2 min. Following injection, the response of the vestibular nerve was preserved, as measured by the vestibular sensory evoked potentials (VsEP).

COMPARISON WITH EXISTING METHODS

Previous studies have used posterior semicircular canals or the RW with canalostomy to gain access to vestibular end organs in mice. The OW with canalostomy, provides the means for high injection rates and fast and reliable delivery of drugs to vestibular hair cells and afferent terminals.

CONCLUSIONS

The presented method for injections through the OW provides rapid delivery of solutions to vestibular end organs without adversely affecting vestibular nerve responses measured by VsEP.

Viral vectors for gene delivery to the inner ear

Gene therapy using virus vectors to treat hereditary diseases has made remarkable progress in the past decade. There are FDA-approved products for ex-vivo gene therapy for diseases such as immunodeficiencies (e.g., SCID), and in vivo gene therapy for a rare blindness and neuro-muscular disease. Gene therapy for hereditary hearing loss has picked up pace in the past five years due to progress in understanding disease gene function as well as the development of better technologies such as adeno-associated virus (AAV) vectors, to deliver nucleic acid to target cells in the inner ear. This review has two major goals. One is to review the state of the art for investigators already working in preclinical cochlear gene therapy. The other is to present the language of vectorology and important considerations for designing and using AAV vectors to inner ear neurobiologists who might use AAV vectors in the cochlea for either therapeutic or basic biological applications.