Therapeutic potential of neurotrophins for treatment of hearing loss

Update on treatment options for blast-induced hearing loss

PURPOSE OF REVIEW

The incidence of blast injuries has increased, and the ear is the highest risk organ. Ear injury induced by blast exposure is important in both military and civilian conditions. The permanent hearing loss caused by blast exposure is associated with a decline in the quality of life. In this review, I describe recent therapeutic strategies for each of the ear pathologies caused by blast exposure.

RECENT FINDINGS

For tympanic membrane perforation after blast exposure, basic fibroblast growth factor (bFGF) has been used as a less invasive treatment to repair the tympanic membrane. The closure rates of tympanic membrane perforations treated with bFGF were reported to be comparable to those following conventional tympanoplasty.For sensorineural hearing loss after blast exposure, treatment with neurotrophic factors, such as nerve growth factor (NGF) or neurotrophin-3, antioxidants, and Atoh1 induction have recently been applied, and some of them were considered for clinical application.

SUMMARY

Recent advances of therapeutics for blast-induced hearing loss, based on their pathologies, have been outlined. There are several promising therapeutic approaches for both middle and inner ear disorders after blast exposure; however, further research is needed to establish new treatments for blast-induced hearing dysfunction.

Aminoglycoside Increases Permeability of Osseous Spiral Laminae of Cochlea by Interrupting MMP-2 and MMP-9 Balance

The spiral ganglion neurons (SGNs) located in the Rosenthal's canal of cochlea are essential target for cochlear implant. Previous studies found that the canaliculi perforantes, small pores on the surface of the osseous spiral lamina (OSL) of the scala tympanic (ST) of cochlea, may provide communication between the cochlear perilymph and SGNs. In this study, we found that chronic treatment of aminoglycosides antibiotics, which is well known to cause sensory cell damage in the cochlea, induced significant damage of bone lining cells on the OSLs and increased the permeability of the Rosenthal's canal. The pores among the bone lining cells became significantly wider after chronic treatment of amikacin (100 mg/kg/day for 3-7 days). Injection of Evans Blue in the ST resulted in significant increase in its migration in the modulus in the amikacin-treated cochlea compared to the control ears, suggesting increased permeability of these passages. Treatment of amikacin with oxytetracycline, an inhibitor of matrix metalloproteases (MMPs), significantly reduced the amount of dye migrated from the ST to the modiolus. These results suggest that amikacin enhanced the permeability between the ST and SGNs by increasing MMPs. Aggregating the permeability of the bone lining cells on the OSLs may benefit gene and stem cell delivery to the SGNs in the cochlea.

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

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

Brain-derived nerve growth factor in the treatment of sensorineural hearing loss

OBJECTIVES/HYPOTHESIS

A possible medical treatment for sensorineural hearing loss using brain-derived nerve growth factor (BDNF) was explored. The hypothesis is that direct intracochlear application of BDNF will result in improved hearing.

STUDY DESIGN

Animal research study.

METHODS

Significant hearing loss was created using cisplatin in 11 guinea pigs. One month later, bilateral cochleostomies were performed placing 0.05 microg of BDNF in one cochlea of each animal prior to plugging with connective tissue. The other cochlea served as a control. Auditory brain-stem response (ABR) testing was then carried out for three months at 6,000, 8,000, 12,000, and 24,000 Hz.

RESULTS

ABR thresholds were better in the treated ear for all frequencies. Threshold differences were statistically significantly better two months after treatment (general linear model, repeated measures P = .045).

CONCLUSIONS

Intracochlear application of BDNF may prevent hearing loss.

Neurotrophic factors and neural prostheses: potential clinical applications based upon findings in the auditory system

Spiral ganglion neurons (SGNs) are the target cells of the cochlear implant, a neural prosthesis designed to provide important auditory cues to severely or profoundly deaf patients. The ongoing degeneration of SGNs that occurs following a sensorineural hearing loss is, therefore, considered a limiting factor in cochlear implant efficacy. We review neurobiological techniques aimed at preventing SGN degeneration using exogenous delivery of neurotrophic factors. Application of these proteins prevents SGN degeneration and can enhance neurite outgrowth. Furthermore, chronic electrical stimulation of SGNs increases neurotrophic factor-induced survival and is correlated with functional benefits. The application of neurotrophic factors has the potential to enhance the benefits that patients can derive from cochlear implants; moreover, these techniques may be relevant for use with neural prostheses in other neurological conditions.

Progressive deafness and altered cochlear innervation in knock-out mice lacking prosaposin

After a yeast two-hybrid screen identified prosaposin as a potential interacting protein with the nicotinic acetylcholine receptor (nAChR) subunit alpha10, studies were performed to characterize prosaposin in the normal rodent inner ear. Prosaposin demonstrates diffuse organ of Corti expression at birth, with gradual localization to the inner hair cells (IHCs) and its supporting cells, inner pillar cells, and synaptic region of the outer hair cells (OHCs) and Deiters' cells (DCs) by postnatal day 21 (P21). Microdissected OHC and DC quantitative reverse transcriptase-PCR and immunohistology localizes prosaposin mRNA to DCs and OHCs, and protein predominantly to the apex of the DCs. Subsequent studies in a prosaposin knock-out (KO) (-/-) mouse showed intact but slightly reduced hearing through P19, but deafness by P25 and reduced distortion product otoacoustic emissions from P15 onward. Beginning at P12, the prosaposin KO mice showed histologic organ of Corti changes including cellular hypertrophy in the region of the IHC and greater epithelial ridge, a loss of OHCs from cochlear apex, and vacuolization of OHCs. Immunofluorescence revealed exuberant overgrowth of auditory afferent neurites in the region of the IHCs and proliferation of auditory efferent neurites in the region of the tunnel of Corti. IHC recordings from these KO mice showed normal I-V curves and responses to applied acetylcholine. Together, these results suggest that prosaposin helps maintain normal innervation patterns to the organ of Corti. Furthermore, prosaposin's overlapping developmental expression pattern and binding capacity toward the nAChR alpha10 suggest that alpha10 may also play a role in this function.

Inner ear drug delivery via a reciprocating perfusion system in the guinea pig

Rapid progress in understanding the molecular mechanisms associated with cochlear and auditory nerve degenerative processes offers hope for the development of gene-transfer and molecular approaches to treat these diseases in patients. For therapies based on these discoveries to become clinically useful, it will be necessary to develop safe and reliable mechanisms for the delivery of drugs into the inner ear, bypassing the blood-labyrinthine barrier. Toward the goal of developing an inner ear perfusion device for human use, a reciprocating microfluidic system that allows perfusion of drugs into the cochlear perilymph through a single inlet hole in scala tympani of the basal turn was developed. The performance of a prototype, extracorporeal reciprocating perfusion system in guinea pigs is described. Analysis of the cochlear distribution of compounds after perfusion took advantage of the place-dependent generation of responses to tones along the length of the cochlea. Perfusion with a control artificial perilymph solution had no effect. Two drugs with well-characterized effects on cochlear physiology, salicylate (5 mM) and DNQX (6,7-Dinitroquinoxaline-2,3-dione; 100 and 300 microM), reversibly altered responses. The magnitude of drug effect decreased with distance from the perfusion pipette for up to 10 mm, and increased with dose and length of application.

Intrathecal injection of HVJ-E containing HGF gene to cerebrospinal fluid can prevent and ameliorate hearing impairment in rats

Hearing impairment, which is the most prevalent sensory deficit of human beings, needs a breakthrough in therapeutic technologies. One technology is the usage of a vector system to reach the inner ear, and another is by a therapeutic molecule. Here we developed a novel gene therapy strategy by combining hepatocyte growth factor (HGF) with hemagglutinating virus of Japan envelope (HVJ-E) vector. When HVJ-E containing human HGF gene was injected intrathecally into the cerebrospinal fluid via cisterna magna of rats, the vector reached the inner ear region, and human HGF gene expression was detected in the spiral ganglion cells (SGCs) of the inner ear. Expression of endogenous rat HGF and its receptor, c-Met, was also induced in SGCs by human HGF. Kanamycin treatment results in hearing impairment by inducing degeneration of hair cells (HCs) and apoptosis of SGCs in rats. By HGF gene transfer before kanamycin treatment, both loss of HCs and apoptosis of SGCs were prevented. Furthermore, hearing function, evaluated by auditory brainstem response, was maintained at a normal level. When HGF gene transfer was performed 2 wk after kanamycin treatment, hearing impairment was significantly recovered. These results indicate a novel and effective therapeutic strategy against sensorineural hearing impairment.

Distribution of gentamicin in the guinea pig inner ear after local or systemic application

Uptake and retention of gentamicin by cells in the guinea pig inner ear after a single peritoneal injection or local application on the round window were investigated using immunocytochemistry to localize the drug. The cells that accumulated the drug under the two conditions were the same, but staining for the drug was more intense and was often accompanied by widespread cochlear degeneration following local application. Soon after drug administration by either route, there was diffuse staining for the drug throughout all tissue within the labyrinth, including bone. At later times when distinct cell staining became evident, virtually all cell types were found to be positive, with several cell types staining more darkly for the drug than hair cells, indicating that hair cells were not the most avid in accumulating gentamicin. The infracuticular portion of auditory and vestibular hair cells as well as type III fibrocytes of the spiral ligament were positively stained in almost all cases and these sites were found to be positive for as long as six months post administration. In animals with loss of the organ of Corti, there was unusually intense staining for gentamicin in root cells of the spiral ligament, in marginal cells of the stria vascularis, and in cells of the spiral limbus. Dark staining of surviving cells in cases with overt tissue destruction suggests that variability in the extent of damage caused by the drug was determined more by the degree of its local uptake than by differences in animals' capacities to metabolize the drug systemically. The present results show that gentamicin may damage or destroy all cochlear cells following a single round window application. The findings broaden the scope of our knowledge of cochlear gentamicin uptake and damage and have implications for treatment of patients with vestibular disorders by infusion of aminoglycosides into the middle ear, as well as implications for prospects of rehabilitating patients that have been deafened by aminoglycosides.

Neurotrophin-3 modifies potassium currents in isolated inner hair cells from guinea-pig cochlea

OBJECTIVE

Neurotrophins elicited short-term glutamate release from the presynaptic locus. The aim of this study was to investigate short-term effects of neurotrophin-3 (NT-3) on the membrane current of inner hair cells (IHCs).

METHODS

IHCs were isolated from the guinea-pig cochlea. Membrane currents were measured by conventional whole-cell voltage-clamp recordings. NT-3 was dissolved in a standard external solution and applied to the IHCs under pressure using pipettes.

RESULTS

Six out of eight IHCs demonstrated a suppression of K currents by extracellular application of NT-3. At +60 mV, the amplitudes of the outward current in the control and NT-3 solutions were 5.6+/-1.3 and 4.2+/-1.0 nA, respectively. NT-3 suppression was voltage-independent. One cell showed an immediate suppression with NT-3 and the following potentiation during washing with standard saline.

CONCLUSION

NT-3 suppressed the amplitude of K current on IHCs, suggesting that neurotrophins are capable of potentiating the hair cells' excitability. Neurotrophins may have a therapeutic value in the prevention and treatment of hearing impairment caused by inner ear damage.

Neurotrophin-3 transduction attenuates cisplatin spiral ganglion neuron ototoxicity in the cochlea

Ototoxicity is a major dose-limiting side effect of cisplatin chemotherapy for cancer patients. We previously demonstrated in vitro that herpes simplex type 1 (HSV-1) amplicon-mediated delivery of a neurotrophin-3 (NT-3)/myc chimera protects spiral ganglion neurons (SGNs) in murine cochlear cultures from cisplatin-induced ototoxicity. To extend these findings, a newly constructed amplicon vector (HSVnt-3myc/SV40lac) that expresses the NT-3myc chimera and the Escherichia coli beta-galactosidase (lacZ) reporter gene under separate transcriptional control was initially tested in vitro and then was delivered to the cochlea of aged mice that were subsequently treated with cisplatin. Successful transduction with the new amplicon was observed in vitro as determined by its capacity to infect SGNs and to express NT-3myc mRNA and protein. To determine whether amplicon-directed NT-3myc overexpression could abrogate the ototoxicity in vivo, two groups of aged mice (CBA) were inoculated with HSVnt-3myc/SV40lac or control vector, HSVSV40lac, preceding administration of cisplatin. Cochleas inoculated with HSVnt-3myc/SV40lac harbored significantly greater numbers of surviving SGNs and showed lower incidence of cisplatin-induced apoptosis or necrosis than those injected with the control virus. These data demonstrate that HSV amplicon-mediated NT-3 delivery can attenuate the ototoxic actions of cisplatin in the peripheral auditory system of the aged mouse. The potency of NT-3 in SGN neuroprotection suggests that in vivo neurotrophin-based gene therapy is a promising preventative treatment for chemical-induced hearing disorders, and potentially for hearing degeneration due to normal aging.

Herpes simplex virus type 1-mediated transfer of neurotrophin-3 stimulates survival of chicken auditory sensory neurons

Neurotrophin-3 (NT-3) is one of the most potent stimulators for survival of auditory sensory neurons. Viral transfer of neurotrophins into auditory neurons may offer a route to provide a permanent supply of the growth factor and guarantee their long-term survival. Herpes simplex virus type 1 (HSV-1)-based vectors have demonstrated their effectiveness to transfer genes into peripheral sensory neurons. In the present report, we have produced a HSV-1-based amplicon vector expressing NT-3. This vector efficiently infects isolated auditory neurons and stimulates their survival during distinct developmental stages of the inner ear. Therefore, this vector may present a unique entry point to develop therapies preventing or treating hearing impairment caused by the degeneration of auditory neurons.

Cochlear pathology, sensory cell death and regeneration

Loss of cochlear hair cells leads to permanent hearing loss. Hair cells may degenerate due to hereditary or environmental causes, or a combination of the two. Cochlear supporting cells actively participate in the process of hair cell elimination and scar formation by rapidly expanding and sealing the reticular lamina, the barrier between endolymph and perilymph. This scarring process helps preserve the remaining hair cells and hearing. Anti-apoptotic agents, anti-oxidants and several growth factors have been shown to protect hair cells and hearing against environmental insults. Characterization of the genes that regulate the development of the inner ear and its response to trauma has been helpful in designing strategies for enhancing protection of the inner ear and for inducing hair cell regeneration. This chapter discusses the potential for some of these approaches.

HSV amplicon-mediated neurotrophin-3 expression protects murine spiral ganglion neurons from cisplatin-induced damage

Ototoxicity is a major dose-limiting side effect of cisplatin (DDP) administration due to its propensity to induce destruction of hair cells and neurons in the auditory system. Previous studies demonstrated that TrkC-expressing spiral ganglion neurons (SGN) are protected from the cytotoxic effects of DDP by localized delivery of the trophic factor neurotrophin-3 (NT-3). Successful in vivo implementation of such a therapy requires the development of an efficient gene delivery vehicle for expression of NT-3 within the cochlea. To this end, we constructed a herpes simplex virus (HSV) amplicon vector that expressed a c-Myc-tagged NT-3 chimera (HSVnt-3myc). Helper virus-free vector stocks were initially evaluated in vitro for their capacity to direct expression of NT-3 mRNA and protein. Transduction of cultured murine cochlear explants with HSVnt-3myc resulted in production of NT-3 mRNA and protein up to 3 ng/ml as measured over a 48-h period in culture supernatants. To determine whether NT-3 overexpression could abrogate DDP toxicity, cochlear explants were transduced with HSVnt-3myc or a murine intestinal alkaline phosphatase-expressing control vector, HSVmiap, and then exposed to cisplatin. HSVnt-3myc-transduced cochlear explants harbored significantly greater numbers of surviving SGNs than those infected with control virus. These data demonstrate that amplicon-mediated NT-3 transduction can attenuate the ototoxic action of DDP on organotypic culture. The potency of NT-3 in protecting spiral ganglion neurons from degeneration suggests that in vivo neurotrophin-based gene therapy may be useful for the prevention and/or treatment of hearing disorders.

Role of neurotrophins and lectins in prevention of ototoxicity

Degeneration of hair cells (HC) and/or spiral ganglion neurons (SGN) is a major cause of hearing loss. Postnatal rat cochlear explant cultures are used to study the toxic actions of different classes of ototoxins and to identify molecules that can protect SGN and HC from ototoxic damage. Various ototoxins induce differential damage to HC and/or SGN. While gentamicin preferentially causes HC death, sodium salicylate selectively induces degeneration of SGN. In contrast, cisplatin results in destruction of both SGN and HC. Specific neurotrophins, including NT-4/5, BDNF, and NT-3, greatly protect SGN from all three types of ototoxins. In contrast, NGF and other growth factors have no effect. Of the 51 compounds examined, only concanavalin A (Con A), a lectin molecule, significantly protects HC from gentamicin. A dose-dependent study of Con A shows that maximal protection occurred at 100 nM. Further experiments indicates that preincubation of Con A with gentamicin does not form a complex, and coaddition of Con A and gentamicin to bacterial cultures, such as E. Coli cultures, does not interfere with the antibiotic activity of gentamicin. When the other 21 lectins are examined, Erythrina cristagalli lectin and Detura stramonium lectin also show activity similar to Con A. These findings may help elucidate the mechanisms of ototoxins and suggest that specific neurotrophins and lectins may be of therapeutic value in the prevention of ototoxin-induced hearing loss.