Adenovirus-mediated overexpression of a gene prevents hearing loss and progressive inner hair cell loss after transient cochlear ischemia in gerbils

Sonoporation of the Round Window Membrane on a Sheep Model: A Safety Study

Sonoporation using microbubble-assisted ultrasound increases the permeability of a biological barrier to therapeutic molecules. Application of this method to the round window membrane could improve the delivery of therapeutics to the inner ear. The aim of this study was to assess the safety of sonoporation of the round window membrane in a sheep model. To achieve this objective, we assessed auditory function and cochlear heating, and analysed the metabolomics profiles of perilymph collected after sonoporation, comparing them with those of the control ear in the same animal. Six normal-hearing ewes were studied, with one sonoporation ear and one control ear for each. A mastoidectomy was performed on both ears. On the sonoporation side, Vevo MicroMarker^® microbubbles (MBs; VisualSonics-Fujifilm, Amsterdam, The Netherlands) at a concentration of 2 × 10⁸ MB/mL were locally injected into the middle ear and exposed to 1.1 MHz sinusoidal ultrasonic waves at 0.3 MPa negative peak pressure with 40% duty cycle and 100 μs interpulse period for 1 min; this was repeated three times with 1 min between applications. The sonoporation protocol did not induce any hearing impairment or toxic overheating compared with the control condition. The metabolomic analysis did not reveal any significant metabolic difference between perilymph samples from the sonoporation and control ears. The results suggest that sonoporation of the round window membrane does not cause damage to the inner ear in a sheep model.

Inner Ear Gene Delivery: Vectors and Routes

Objectives

Current treatments for hearing loss offer some functional improvements in hearing, but do not restore normal hearing. The aim of this review is to highlight recent advances in viral and non-viral vectors for gene therapy and to discuss approaches for overcoming barriers inherent to inner ear delivery of gene products.

Data Sources

The databases used were Medline, EMBASE, Web of Science, and Google Scholar. Search terms were [("cochlea*" or "inner ear" or "transtympanic" or "intratympanic" or "intracochlear" or "hair cells" or "spiral ganglia" or "Organ of Corti") and ("gene therapy" or "gene delivery")]. The references section of resulting articles was also used to identify relevant studies.

Results

Both viral and non-viral vectors play important roles in advancing gene delivery to the inner ear. The round window membrane is one significant barrier to gene delivery that intratympanic delivery methods attempt to overcome through diffusion and intracochlear delivery methods bypass completely.

Conclusions

Gene therapy for hearing loss is a promising treatment for restoring hearing function by addressing innate defects. Recent technological advances in inner ear drug delivery techniques pose exciting opportunities for progress in gene therapy.

Differential deletion of GDNF in the auditory system leads to altered sound responsiveness

Glial-derived neurotrophic factor (GDNF) has been proposed as a potent neurotrophic factor with the potential to cure neurodegenerative diseases. In the cochlea, GDNF has been detected in auditory neurons and sensory receptor cells and its expression is upregulated upon trauma. Moreover, the application of GDNF in different animal models of deafness has shown its capacity to prevent hearing loss and promoted its future use in therapeutic trials in humans. In the present study we have examined the endogenous requirement of GDNF during auditory development in mice. Using a lacZ knockin allele we have confirmed the expression of GDNF in the cochlea including its sensory regions during development. Global inactivation of GDNF throughout the hearing system using a Foxg1-Cre line causes perinatal lethality but reveals no apparent defects during formation of the cochlea. Using TrkC-Cre and Atoh1-Cre lines, we were able to generate viable mutants lacking GDNF in auditory neurons or both auditory neurons and sensory hair cells. These mutants show normal frequency-dependent auditory thresholds. However, mechanoelectrical response properties of outer hair cells (OHCs) in TrkC-Cre GDNF mutants are altered at low thresholds. Furthermore, auditory brainstem wave analysis shows an abnormal increase of wave I. On the other hand, Atoh1-Cre GDNF mutants show normal OHC function but their auditory brainstem wave pattern is reduced at the levels of wave I, III and IV. These results show that GDNF expression during the development is required to maintain functional hearing at different levels of the auditory system.

Virally Mediated Overexpression of Glial-Derived Neurotrophic Factor Elicits Age- and Dose-Dependent Neuronal Toxicity and Hearing Loss

Contemporary cochlear implants (CI) are generally very effective for remediation of severe to profound sensorineural hearing loss, but outcomes are still highly variable. Auditory nerve survival is likely one of the major factors underlying this variability. Neurotrophin therapy therefore has been proposed for CI recipients, with the goal of improving outcomes by promoting improved survival of cochlear spiral ganglion neurons (SGN) and/or residual hair cells. Previous studies have shown that glial-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor, and neurotrophin-3 can rescue SGNs following insult. The current study was designed to determine whether adeno-associated virus vector serotype 5 (AAV-5) encoding either green fluorescent protein or GDNF can transduce cells in the mouse cochlea to express useful levels of neurotrophin and to approximate the optimum therapeutic dose(s) for transducing hair cells and SGN. The findings demonstrate that AAV-5 is a potentially useful gene therapy vector for the cochlea, resulting in extremely high levels of transgene expression in the cochlear inner hair cells and SGN. However, overexpression of human GDNF in newborn mice caused severe neurological symptoms and hearing loss, likely due to Purkinje cell loss and cochlear nucleus pathology. Thus, extremely high levels of transgene protein expression should be avoided, particularly for proteins that have neurological function in neonatal subjects.

Treating hearing disorders with cell and gene therapy

Hearing loss is an increasing problem for a substantial number of people and, with an aging population, the incidence and severity of hearing loss will become more significant over time. There are very few therapies currently available to treat hearing loss, and so the development of new therapeutic strategies for hearing impaired individuals is of paramount importance to address this unmet clinical need. Most forms of hearing loss are progressive in nature and therefore an opportunity exists to develop novel therapeutic approaches to slow or halt hearing loss progression, or even repair or replace lost hearing function. Numerous emerging technologies have potential as therapeutic options. This paper details the potential of cell- and gene-based therapies to provide therapeutic agents to protect sensory and neural cells from various insults known to cause hearing loss; explores the potential of replacing lost sensory and nerve cells using gene and stem cell therapy; and describes the considerations for clinical translation and the challenges that need to be overcome.

A microfluidic reciprocating intracochlear drug delivery system with reservoir and active dose control

Reciprocating microfluidic drug delivery, as compared to steady or pulsed infusion, has unique features which may be advantageous in many therapeutic applications. We have previously described a device, designed for wearable use in small animal models, that periodically infuses and then withdraws a sub-microliter volume of drug solution to and from the endogenous fluid of the inner ear. This delivery approach results in zero net volume of liquid transfer while enabling mass transport of compounds to the cochlea by means of diffusion and mixing. We report here on an advanced wearable delivery system aimed at further miniaturization and complex dosing protocols. Enhancements to the system include the incorporation of a planar micropump to generate reciprocating flow and a novel drug reservoir that maintains zero net volume delivery and permits programmable modulation of the drug concentration in the infused bolus. The reciprocating pump is fabricated from laminated polymer films and employs a miniature electromagnetic actuator to meet the size and weight requirements of a head-mounted in vivo guinea pig testing system. The reservoir comprises a long microchannel in series with a micropump, connected in parallel with the reciprocating flow network. We characterized in vitro the response and repeatability of the planar pump and compared the results with a lumped element simulation. We also characterized the performance of the reservoir, including repeatability of dosing and range of dose modulation. Acute in vivo experiments were performed in which the reciprocating pump was used to deliver a test compound to the cochlea of anesthetized guinea pigs to evaluate short-term safety and efficacy of the system. These advances are key steps toward realization of an implantable device for long-term therapeutic applications in humans.

Axon guidance of sympathetic neurons to cardiomyocytes by glial cell line-derived neurotrophic factor (GDNF)

Molecular signaling of cardiac autonomic innervation is an unresolved issue. Here, we show that glial cell line-derived neurotrophic factor (GDNF) promotes cardiac sympathetic innervation in vitro and in vivo. In vitro, ventricular myocytes (VMs) and sympathetic neurons (SNs) isolated from neonatal rat ventricles and superior cervical ganglia were cultured at a close distance. Then, morphological and functional coupling between SNs and VMs was assessed in response to GDNF (10 ng/ml) or nerve growth factor (50 ng/ml). As a result, fractions of neurofilament-M-positive axons and synapsin-I-positive area over the surface of VMs were markedly increased with GDNF by 9-fold and 25-fold, respectively, compared to control without neurotrophic factors. Pre- and post-synaptic stimulation of β₁-adrenergic receptors (BAR) with nicotine and noradrenaline, respectively, resulted in an increase of the spontaneous beating rate of VMs co-cultured with SNs in the presence of GDNF. GDNF overexpressing VMs by adenovirus vector (AdGDNF-VMs) attracted more axons from SNs compared with mock-transfected VMs. In vivo, axon outgrowth toward the denervated myocardium in adult rat hearts after cryoinjury was also enhanced significantly by adenovirus-mediated GDNF overexpression. GDNF acts as a potent chemoattractant for sympathetic innervation of ventricular myocytes, and is a promising molecular target for regulation of cardiac function in diseased hearts.

Experimental study of transient cochlear ischemia as a cause of sudden deafness

The etiology of sudden deafness or idiopathic sudden sensorineural hearing loss (ISSHL) remains unclear. Over the past 15 years, we have investigated the mechanisms of ischemic-induced hearing loss using a gerbil model of transient cochlear ischemia. In the gerbil, cochlear ischemia can be induced by occluding the bilateral vertebral arteries simultaneously at the neck, because the posterior communicating arteries of the Circle of Willis close spontaneously around 1 mo after birth. When 15 min ischemia was loaded on this animal, permanent hearing loss of about 25 dB and the death of hair cells, especially inner hair cells were induced. These pathological changes were mainly due to lack of an energy source, glutamate excitotoxicity, and the production of free radicals, especially superoxide and nitrous oxide species. Ischemic damage could be prevented by various procedures, such as cooling the cochlea, intratympanic administration of insulin-like growth factor 1 or AM-111 (an anti-apoptotic agent), and systemic administration of prednisolone (steroid), edarabone (free radical scavenger), ginsenoside Rb1 (Kanpo), hematopoietic stem cells, glia-cell derived neurotrophic factor, and liposome-encapsulated hemoglobin (artificial red blood cells). We also found that the cochlea was protected by the ischemic tolerance, indicating that minor cochlear ischemia alleviates or prevents inner ear damage in subsequent severe cochlear ischemia. As ISSHL usually occurs suddenly, with no preceding sign or symptom, we suggest that most ISSHL cases are caused by circulatory disturbance, probably at the stria vascularis.

Gene transfer in inner ear cells: a challenging race

Recent advances in human genomics led to the identification of numerous defective genes causing deafness, which represent novel putative therapeutic targets. Future gene-based treatment of deafness resulting from genetic or acquired sensorineural hearing loss may include strategies ranging from gene therapy to antisense delivery. For successful development of gene therapies, a minimal requirement involves the engineering of appropriate gene carrier systems. Transfer of exogenous genetic material into the mammalian inner ear using viral or non-viral vectors has been characterized over the last decade. The nature of inner ear cells targeted, as well as the transgene expression level and duration, are highly dependent on the vector type, the route of administration and the strength of the promoter driving expression. This review summarizes and discusses recent advances in inner ear gene-transfer technologies aimed at examining gene function or identifying new treatment for inner ear disorders.

Local drug delivery to the inner ear using biodegradable materials

The lack of an effective method of drug delivery has been a considerable obstacle in the development of novel therapeutics for inner ear diseases. However, several strategies have been investigated to achieve drug delivery to the inner ear, particularly for local application. Here, we review recent advances in the development of inner ear drug-delivery systems, focusing on biodegradable materials. Both synthetic and natural biodegradable materials have shown efficacy for inner ear drug delivery, resulting in an attenuation of hearing loss in animal models. We expect the further development of such drug-delivery systems to help translate the findings of experimental studies to clinical applications.

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.

Effect of c-myc on the ultrastructural structure of cochleae in guinea pigs with noise induced hearing loss

Noise over-stimulation may induce hair cells loss and hearing deficit. The c-myc oncogene is a major regulator for cell proliferation, growth, and apoptosis. However, the role of this gene in the mammalian cochlea is still unclear. The study was designed to firstly investigate its function under noise condition, from the aspect of cochlear ultrastructural changes. We had established the adenoviral vector of c-myc gene and delivered the adenovirus suspension into the scala tympani of guinea pigs 4 days before noise exposure. The empty adenoviral vectors were injected as control. Then, all subjects were exposed to 4-kHz octave-band noise at 110dB SPL for 8h/day, 3 days consecutively. Auditory thresholds were assessed by auditory brainstem response, prior to and 7 days following noise exposure. On the seventh days after noise exposure, the cochlear sensory epithelia surface was observed microscopically and the cochleae were taken to study the ultrastructural changes. The results indicated that auditory threshold shift after noise exposure was higher in the ears treated with Ad.EGFP than that treated with Ad.c-myc-EGFP. Stereocilia loss and the disarrangement of outer hair cells were observed, with greater changes found in the Ad.EGFP group. Also, the ultrastructure changes were severe in the Ad.EGFP group, but not obvious in the Ad.c-myc-EGFP group. Therefore, c-myc gene might play an unexpected role in hearing functional and morphological protection from acoustic trauma.

Principles of design and biological approaches for improving the selectivity of cochlear implant electrodes

The perceptual performance of cochlear implant recipients seems to have reached a plateau in recent years. This may be attributable to inadequate neural selectivity of available intracochlear electrodes, caused by current spread and electrode interactions. Attempts to improve electrode selectivity have included manipulating the number and configuration of electrodes that are stimulated at any one time, displacing perilymph from the cochlea to restrict current flow along the cochlea, and reducing the distance between electrodes and neurons. One experimental approach by which the distance between neurons and electrodes may be reduced is to use neurotrophic factors to promote the regeneration of the peripheral dendrites of auditory neurons and guide them towards intracochlear electrodes. The likely requirements of a system for regenerating auditory neurons towards the cochlear electrode include either a stable release of neurotrophin, or transient neurotrophin followed by electrical stimulation; a close proximity of electrode to osseous spiral lamina or a polymer to bridge the gap between the two; guidance signals to attract neurons towards the electrode; patterning of the electrode surface to direct dendrites to electrode contacts and a 'stop' signal to arrest regeneration once the electrode has been reached.

Ischemic tolerance in the cochlea

Ischemic tolerance in the cochlea was investigated in a gerbil model of cochlear ischemia. Transient cochlear ischemia was produced by extracranial occlusion of the bilateral vertebral arteries. The gerbils were divided into two groups; single ischemia group and double ischemia group. In the single ischemia group, animals were subjected to lethal cochlear ischemia for 15 min. In the double ischemia group, animals were subjected to sublethal cochlear ischemia for 2 min at 2 days before lethal ischemia for 15 min. Consequently, sublethal ischemia prevented lethal ischemia-induced hair cell degeneration and ameliorated hearing impairment, suggesting ischemic tolerance in the cochlea.

Inner ear drug delivery for auditory applications

Many inner ear disorders cannot be adequately treated by systemic drug delivery. A blood-cochlear barrier exists, similar physiologically to the blood-brain barrier, which limits the concentration and size of molecules able to leave the circulation and gain access to the cells of the inner ear. However, research in novel therapeutics and delivery systems has led to significant progress in the development of local methods of drug delivery to the inner ear. Intratympanic approaches, which deliver therapeutics to the middle ear, rely on permeation through tissue for access to the structures of the inner ear, whereas intracochlear methods are able to directly insert drugs into the inner ear. Innovative drug delivery systems to treat various inner ear ailments such as ototoxicity, sudden sensorineural hearing loss, autoimmune inner ear disease, and for preserving neurons and regenerating sensory cells are being explored.

Insulin-like growth factor 1 treatment via hydrogels rescues cochlear hair cells from ischemic injury

This study was designed to investigate the protective effects of recombinant human insulin-like growth factor 1 (rhIGF1), applied locally via a hydrogel, against ischemic damage of the cochleae in gerbils. A hydrogel was immersed in rhIGF1 or saline and was applied on the round window membrane 30 min after the ischemia. Local rhIGF1 treatment significantly reduced the elevation of auditory brain responses thresholds at a frequency of 8 kHz on days 1, 4, and 7 after ischemia. A histological analysis revealed increased survival of inner hair cells in the animals treated with rhIGF1 via the hydrogel 7 days after ischemia. These findings showed that local rhIGF1 application using a hydrogel has the potential to protect the cochleae from ischemic injury.

Animal models for target diseases in gene therapy--using DNA and siRNA delivery strategies

Nanoparticles, including lipopolyamines leading to lipoplexes, liposomes, and polyplexes are targeted drug carrier systems in the current search for a successful delivery system for polynucleic acids. This review is focused on the impact of gene and siRNA delivery for studies of efficacy, pharmacodynamics, and pharmacokinetics within the setting of the wide variety of in vivo animal models now used. This critical appraisal of the recent literature sets out the different models that are currently being investigated to bridge from studies in cell lines through towards clinical reality. Whilst many scientists will be familiar with rodent (murine, fecine, cricetine, and musteline) models, few probably think of fish as a clinically relevant animal model, but zebrafish, madake, and rainbow trout are all being used. Larger animal models include rabbit, cat, dog, and cow. Pig is used both for the prevention of foot-and-mouth disease and human diseases, sheep is a model for corneal transplantation, and the horse naturally develops arthritis. Non-human primate models (macaque, common marmoset, owl monkey) are used for preclinical gene vector safety and efficacy trials to bridge the gap prior to clinical studies. We aim for the safe development of clinically effective delivery systems for DNA and RNAi technologies.

Protection against aminoglycoside-induced ototoxicity by regulated AAV vector-mediated GDNF gene transfer into the cochlea

Since standard aminoglycoside treatment progressively causes hearing disturbance with hair cell degeneration, systemic use of the drugs is limited. Adeno-associated virus (AAV)-based vectors have been of great interest because they mediate stable transgene expression in a variety of postmitotic cells with minimal toxicity. In this study, we investigated the effects of regulated AAV1-mediated glial cell line-derived neurotrophic factor (GDNF) expression in the cochlea on aminoglycoside-induced damage. AAV1-based vectors encoding GDNF or vectors encoding GDNF with an rtTA2s-S2 Tet-on regulation system were directly microinjected into the rat cochleae through the round window at 5 x 10(10) genome copies/body. Seven days after the virus injection, a dose of 333 mg/kg of kanamycin was subcutaneously given twice daily for 12 consecutive days. GDNF expression in the cochlea was confirmed and successfully modulated by the Tet-on system. Monitoring of the auditory brain stem response revealed an improvement of cochlear function after GDNF transduction over the frequencies tested. Damaged spiral ganglion cells and hair cells were significantly reduced by GDNF expression. Our results suggest that AAV1-mediated expression of GDNF using a regulated expression system in the cochlea is a promising strategy to protect the cochlea from aminoglycoside-induced damage.

Development of a drug delivery device: using the femtosecond laser to modify cochlear implant electrodes

Animal experiments suggest that pharmacological intervention could possibly enhance cochlear implant performance. One of the key aspects is therefore a drug delivery device for the human inner ear. The objective of this study was to investigate the possibility of using the femtosecond laser for modifying a cochlear implant electrode for the purpose of drug delivery to the cochlea. Using silicone sheets, the best parameters for creating defined channels at calculated diameters were investigated using a femtosecond laser. The results were transferred to a cochlear implant electrode array (Nucleus Contour). The capability of delivering substances through the drilled openings was tested in vitro. By variation of the output of the laser, spot distance, repetition rate, number of cycles and introducing several focus planes, it was possible to drill holes with nearly vertical walls in the silicone sheets. Transferring these data to the cochlear implant electrode resulted in prototypes for drug delivery with various openings along the array. The use of the femtosecond laser allows rapid modification and adaptation of designs to experimental prototypes of cochlear implant electrodes for the purpose of drug delivery to the inner ear.

The potential use of bone marrow stromal cells for cochlear cell therapy

This study investigated the potential of bone-marrow stromal cell transplantation for cell replacement therapy in the cochlea. Bone-marrow stromal cells labeled with enhanced green fluorescent protein were injected into the perilymphatic space of normal cochleae in mice. Histological analysis 2 weeks after transplantation demonstrated that transplanted cells settled within the cochlear tissues, especially in the spiral ligament and the spiral limbus, although most transplants were located in the perilymphatic space. Some of the transplanted cells expressed the cochlear gap-junction protein connexin 26. These findings indicate the potential of bone-marrow stromal cells for delivering therapeutic molecules and for the restoration of cochlear cells, particularly in the spiral ligament and the spiral limbus.

Hematopoietic stem cells prevent hair cell death after transient cochlear ischemia through paracrine effects

Transplantation of hematopoietic stem cells (HSCs) is regarded to be a potential approach for promoting repair of damaged organs. Here, we investigated the influence of hematopoietic stem cells on progressive hair cell degeneration after transient cochlear ischemia in gerbils. Transient cochlear ischemia was produced by extracranial occlusion of the bilateral vertebral arteries just before their entry into the transverse foramen of the cervical vertebra. Intrascalar injection of HSCs prevented ischemia-induced hair cell degeneration and ameliorated hearing impairment. We also showed that the protein level of glial cell line-derived neurotrophic factor (GDNF) in the organ of Corti was upregulated after cochlear ischemia and that treatment with HSCs augmented this ischemia-induced upregulation of GDNF. A tracking study revealed that HSCs injected into the cochlea were retained in the perilymphatic space of the cochlea, although they neither transdifferentiated into cochlear cell types nor fused with the injured hair cells after ischemia, suggesting that HSCs had therapeutic potential possibly through paracrine effects. Thus, we propose HSCs as a potential new therapeutic strategy for hearing loss.

Drug delivery systems for the treatment of sensorineural hearing loss

Sensorineural hearing loss is one of the most common disabilities in our society. Experimentally, many candidates for therapeutic molecules have been discovered. However, the lack of safe and effective methods for drug delivery to the cochlea has been a considerable obstacle to clinical application. Local application of therapeutic molecules into the cochlea has been used in clinic and in animal experiments. Advances in pharmacological technology provide various drug delivery systems via biomaterials, which can be utilized for local drug delivery to the cochlea. Recent studies in the field of otology have demonstrated the potential of synthetic and natural biomaterials for local drug delivery to the cochlea. Although problems still remain to be resolved for clinical application, introduction into clinical practice of these controlled-release systems may be reasonable because of their certain advantages over previous methods.

Sensorineural hearing loss: potential therapies and gene targets for drug development

Recent advances in the developmental biology, genetics and cell biology of the inner ear are guiding research to novel therapeutic modalities - a market currently estimated to be at least US Dollars 10 billion. This article highlights prospects to manipulate the mammalian hearing organ with gene and stem cell delivery to the inner ear to protect, repair or regenerate the hair cells, supporting cells and associated nerves.

Cell-gene delivery of brain-derived neurotrophic factor to the mouse inner ear

Sensorineural hearing loss is a common disability, but treatment options are currently limited to cochlear implants and hearing aids. Studies are therefore being conducted to provide alternative means of biological therapy, including gene therapy. Safe and effective methods of gene delivery to the cochlea need to be developed to facilitate the clinical application of these therapeutic treatments for hearing loss. In this study, we examined the potential of cell-gene therapy with nonviral vectors for delivery of therapeutic molecules into the cochlea. NIH3T3 cells were transfected with the brain-derived neurotrophic factor (Bdnf) gene using lipofection and then transplanted into the mouse inner ear. Immunohistochemistry and Western blotting demonstrated the survival of grafted cells in the cochlea for up to 4 weeks after transplantation. No significant hearing loss was induced by the transplantation procedure. A Bdnf-specific enzyme-linked immunosorbent assay revealed a significant increase in Bdnf production in the inner ear following transplantation of engineered cells. These findings indicate that cell-gene delivery with nonviral vectors may be applicable for the local, sustained delivery of therapeutic molecules into the cochlea.

Advances in inner ear gene therapy: exploring cochlear protection and regeneration

PURPOSE OF REVIEW

To review the application of gene therapy in the inner ear. Gene delivery to the inner ear was first reported in 1996. Since then the field has progressed on multiple fronts.

RECENT DEVELOPMENTS

More diverse and sophisticated vectors are improving the efficiency of delivery to the inner ear. Research is transitioning from the delivery of marker genes to the delivery of therapeutic genes in animal models of inner ear disease. Three distinct areas of research are developing: (1) delivery of genes for protection of spiral ganglion neurons with potential application in cochlear implantation, (2) delivery of genes for protection of hair cells and hearing preservation in degenerative diseases and cochlear insults and (3) the use of gene therapy to transform cells from one phenotype to another and replace lost cells, potentially restoring lost function.

SUMMARY

Currently, no specific drugs are targeted at inner ear disease. The use of gene therapy in the inner ear is being applied in animal models of ototoxicity and ischemia reperfusion injury. Gene therapy can protect the inner ear from damage and even restore function through the regeneration of hair cells.

Substance distribution in a cochlea model using different pump rates for cochlear implant drug delivery electrode prototypes

Several studies using animals have shown the protective effects of neurotrophic factors (NF) on spiral ganglion cells (SGC). This is of particular importance since the number of SGCs is considered to be among the factors defining the efficacy of cochlear implants. A device for local inner ear treatment is therefore of great interest. As described previously, we modified a Contour(TM) cochlear implant electrode, to examine the inbuilt canal to be used for fluid release [Paasche, G., Gibson, P., Averbeck, T., Becker, H., Lenarz, T., Stöver, T., 2003. Technical report: modification of a cochlear implant electrode for drug delivery to the inner ear. Otol. Neurotol. 24, 222-227]. In the present study, three different electrode prototypes with openings of the delivery channel at various locations along the electrode array were examined to determine distribution of dye in a cochlea model over time. We compared dye delivery with: (a) release of the dye at the tip, (b) release of the dye at the tip and the side of the electrode, and (c) release of the dye only at the side of the electrode (6 mm from the tip). A mechanical pump was used to drive the system at pump rates of 100, 10, and 1 microl/h. Dye concentration changes along the length of the whole cochlea were investigated. Mean values for all experimental conditions show that the distribution along the array is fastest with two outlets whereas the distribution via a single outlet at the side of the electrode array is not considered to be sufficient. The established experimental setup provides the possibility of investigating prototypes of a fluid based drug delivery system for the treatment of inner ear pathologies in combination with electrical stimulation.

Keynote review: The auditory system, hearing loss and potential targets for drug development

There is a huge potential market for the treatment of hearing loss. Drugs are already available to ameliorate predictable, damaging effects of excessive noise and ototoxic drugs. The biggest challenge now is to develop drug-based treatments for regeneration of sensory cells following noise-induced and age-related hearing loss. This requires careful consideration of the physiological mechanisms of hearing loss and identification of key cellular and molecular targets. There are many molecular cues for the discovery of suitable drug targets and a full range of experimental resources are available for initial screening through to functional analysis in vivo. There is now an unparalleled opportunity for translational research.

Intratympanic treatment of hearing loss with novel and traditional agents

As knowledge of the cellular and molecular pathophysiology behind otopathologies expands, the possibility exists of preventing sensorineural hearing loss and perhaps reversing the loss. Cellular and molecular mechanisms seem to be similar in hearing loss secondary to aging, drug ototoxicity, noise, or other mechanisms. A final common pathway may hinge upon apoptosis. It is likely that anti-apoptotic factors will increasingly be realized as an important intervention strategy for sensorineural hearing loss. Furthermore, it is also possible that mounting a staged attack at the various regions in the pathway leading to cellular damage using a combination of several protective substances such as steroids, antioxidants, neurotrophic factors, anti-apoptotic compounds, and mitochondrial enhancers may prevent hearing loss and even reverse it in some situations. This article has presented some of the molecular and cellular mechanisms for hearing loss and potential ways of treating them. In theory, the delivery of these medications to the inner ear transtympanically would decrease systemic side effects and be more target specific. Because most of the studies conducted to date have been animal studies, randomized, double-blind, placebo-controlled clinical trials would be necessary before the use of these therapies becomes common practice.

Treatment of peripheral sensorineural hearing loss: gene therapy

Noise, chemicals and genetic defects are all common causes of irreversible hearing loss, which at present have no cure. Gene therapy may soon be utilized in both the protection and the treatment of these exogenous and endogenous sources of hearing loss. Gene therapy technology is rapidly developing and the inner ear is a particularly feasible model for gene therapy. This review outlines our current understanding of the mechanisms behind deafness and prospects for treatment, discusses the inner ear model in detail and reviews the efforts that have been made in inner ear gene therapy. Finally, the proposed next steps will be discussed. The viral mediated delivery of neurotrophins and antioxidants offers imminent promise in preventing and treating exogenous hearing loss and improving cochlear implant therapy.

Stroke and TGF-beta proteins: glial cell line-derived neurotrophic factor and bone morphogenetic protein

Recent studies have indicated that proteins in the transforming growth factor-beta superfamily alter damage induced by various neuronal injuries. Of these proteins, glial cell line-derived neurotrophic factor (GDNF) and bone morphogenetic protein-7 (BMP-7) have unique protective and regenerative effects in stroke animals. Delivery of GDNF or BMP-7 to brain tissue reduced cerebral infarction and improved motor functions in stroke animals. Pretreatment with these factors reduced caspase-3 activity and DNA fragmentation in the ischemic brain region, suggesting that antiapoptotic effects are involved. Beside the protective effects, BMP-7 given after stroke improves locomotor function. These regenerative effects of BMP-7 may involve the enhancement of dendritic growth and remodeling. In this review, we illustrate the neuroprotective and neuroregenerative properties of GDNF and BMP-7 and emphasize their therapeutic potential for stroke.

Sensory hair cell death and regeneration: two halves of the same equation

PURPOSE OF REVIEW

Sensory hair cells are susceptible to ototoxic damage from a variety of sources, including antibiotic treatment. Unfortunately, this often results in permanent hearing and/or balance problems in humans. By understanding how sensory hair cells die in response to aminoglycoside treatment, preventive strategies may be developed. This review will discuss some of the key recent findings in sensory hair cell death and regeneration.

RECENT FINDINGS

Aminoglycosides induce hair cell death through the initiation of apoptosis. Early and late stages of hair cell apoptosis have been defined, and several of the key molecules involved in the cascade have been identified. Moreover, specific inhibitors of apoptosis rescue hair cells from death and preserve function. Hair cell death has been shown to induce regeneration through supporting cell transdifferentiation, proliferation, and new hair cell differentiation in birds and lower vertebrates. Regeneration in the mammalian cochlea does not occur spontaneously, but genetic manipulation of cell cycle genes, induction of new hair cells through gene therapy, and introduction of stem cells into damaged cochleas suggest that repair and replacement of lost hair cells in the organ of Corti may be possible. Finally, continuing investigations of the mouse, zebrafish, and human genomes may one day enable manipulation of the cochlea so that functional regeneration is readily available as a therapeutic intervention.

SUMMARY

The discovery that hair cells can regenerate in birds and other nonmammalian vertebrates has fueled a wide range of studies to find ways to restore hearing and balance in mammals. The demonstration that apoptosis and proliferation are coupled as controlling factors in regeneration and the advent of new approaches such as gene therapy, stem cell transplantation, and genomics may lead to methods for inducing hair cell regeneration and repair in the mammalian cochlear and vestibular systems.