Morphological changes in spiral ganglion cells after intracochlear application of brain-derived neurotrophic factor in deafened guinea pigs

The human neuroprotective placental protein composition suppressing tinnitus and restoring auditory brainstem response in a rodent model of sodium salicylate-induced ototoxicity

The effect of neuroprotective placental protein composition (NPPC) on the suppression of tinnitus and the restoration of the auditory brainstem response (ABR) characteristics was explored in tinnitus-induced rats. The animals were placed into two groups: (1) the study group, rats received sodium salicylate (SS) at the dose of 200 mg/kg twice a day for two weeks, and then 0.4 mg of the NPPC per day, between the 14th and 28th days, (2) the placebo group, rats received saline for two weeks, and then the NPPC alone between the 14th and 28th days. The gap pre-pulse inhibition of the acoustic startle (GPIAS), the pre-pulse inhibition (PPI), and the ABR assessments were performed on animals in both groups three times (baseline, day 14, and 28). The GPIAS value declined after 14 consecutive days of the SS injection, while NPPC treatment augmented the GPIAS score in the study group on the 28th day. The PPI outcomes revealed no significant changes, indicating hearing preservation after the SS and NPPC administrations. Moreover, some changes in ABR characteristics were observed following SS injection, including (1) higher ABR thresholds, (2) lowered waves I and II amplitudes at the frequencies of 6, 12, and 24 kHz and wave III at the 12 kHz, (3) elevated amplitude ratios, and (4) prolongation in brainstem transmission time (BTT). All the mentioned variables returned to their normal values after applying the NPPC. The NPPC use could exert positive therapeutic effects on the tinnitus-induced rats and improve their ABR parameters.

Hepatocyte growth factor mimetic confers protection from aminoglycoside-induced hair cell death in vitro

Loss of sensory hair cells from exposure to certain licit drugs, such as aminoglycoside antibiotics, can result in permanent hearing damage. Exogenous application of the neurotrophic molecule hepatocyte growth factor (HGF) promotes neuronal cell survival in a variety of contexts, including protecting hair cells from aminoglycoside ototoxicity. HGF itself is not an ideal therapeutic due to a short half-life and limited blood-brain barrier permeability. MM-201 is a chemically stable, blood-brain barrier permeable, synthetic HGF mimetic that serves as a functional ligand to activate the HGF receptor and its downstream signaling cascade. We previously demonstrated that MM-201 robustly protects zebrafish lateral line hair cells from aminoglycoside ototoxicity. Here, we examined the ability of MM-201 to protect mammalian sensory hair cells from aminoglycoside damage to further evaluate MM-201's clinical potential. We found that MM-201 exhibited dose-dependent protection from neomycin and gentamicin ototoxicity in mature mouse utricular explants. MM-201's protection was reduced following inhibition of mTOR, a downstream target of HGF receptor activation, implicating the activation of endogenous intracellular substrates by MM-201 as critical for the observed protection. We then asked if MM-201 altered the bactericidal properties of aminoglycosides. Using either plate or liquid growth assays we found that MM-201 did not alter the bactericidal efficacy of aminoglycoside antibiotics at therapeutically relevant concentrations. We therefore assessed the protective capacity of MM-201 in an in vivo mouse model of kanamycin ototoxicity. In contrast to our in vitro data, MM-201 did not attenuate kanamycin ototoxicity in vivo. Further, we found that MM-201 was ototoxic to mice across the dose range tested here. These data suggest species- and tissue-specific differences in otoprotective capacity. Next generation HGF mimetics are in clinical trials for neurodegenerative diseases and show excellent safety profiles, but neither preclinical studies nor clinical trials have examined hearing loss as a potential consequence of pharmaceutical HGF activation. Further research is needed to determine the consequences of systemic MM-201 application on the auditory system.

The Role of BDNF as a Biomarker in Cognitive and Sensory Neurodegeneration

Brain-derived neurotrophic factor (BDNF) has a crucial function in the central nervous system and in sensory structures including olfactory and auditory systems. Many studies have highlighted the protective effects of BDNF in the brain, showing how it can promote neuronal growth and survival and modulate synaptic plasticity. On the other hand, conflicting data about BDNF expression and functions in the cochlear and in olfactory structures have been reported. Several clinical and experimental research studies showed alterations in BDNF levels in neurodegenerative diseases affecting the central and peripheral nervous system, suggesting that BDNF can be a promising biomarker in most neurodegenerative conditions, including Alzheimer's disease, shearing loss, or olfactory impairment. Here, we summarize current research concerning BDNF functions in brain and in sensory domains (olfaction and hearing), focusing on the effects of the BDNF/TrkB signalling pathway activation in both physiological and pathological conditions. Finally, we review significant studies highlighting the possibility to target BDNF as a biomarker in early diagnosis of sensory and cognitive neurodegeneration, opening new opportunities to develop effective therapeutic strategies aimed to counteract neurodegeneration.

Changes in the Electrically Evoked Compound Action Potential over time After Implantation and Subsequent Deafening in Guinea Pigs

The electrically evoked compound action potential (eCAP) is a direct measure of the responsiveness of the auditory nerve to electrical stimulation from a cochlear implant (CI). CIs offer a unique opportunity to study the auditory nerve's electrophysiological behavior in individual human subjects over time. In order to understand exactly how the eCAP relates to the condition of the auditory nerve, it is crucial to compare changes in the eCAP over time in a controlled model of deafness-induced auditory nerve degeneration. In the present study, 10 normal-hearing young adult guinea pigs were implanted and deafened 4 weeks later, so that the effect of deafening could be monitored within-subject over time. Following implantation, but before deafening, most examined eCAP characteristics significantly changed, suggesting increasing excitation efficacy (e.g., higher maximum amplitude, lower threshold, shorter latency). Conversely, inter-phase gap (IPG) effects on these measures - within-subject difference measures that have been shown to correlate well with auditory nerve survival - did not vary for most eCAP characteristics. After deafening, we observed an initial increase in excitability (steeper slope of the eCAP amplitude growth function (AGF), lower threshold, shorter latency and peak width) which typically returned to normal-hearing levels within a week, after which a slower process, probably reflecting spiral ganglion cell loss, took place over the remaining 6 weeks (e.g., decrease in maximum amplitude, AGF slope, peak area, and IPG effect for AGF slope; increase in IPG effect for latency). Our results suggest that gradual changes in peak width and latency reflect the rate of neural degeneration, while peak area, maximum amplitude, and AGF slope reflect neural population size, which may be valuable for clinical diagnostics.

Comparison of response properties of the electrically stimulated auditory nerve reported in human listeners and in animal models

Cochlear implants (CIs) provide acoustic information to implanted patients by electrically stimulating nearby auditory nerve fibers (ANFs) which then transmit the information to higher-level neural structures for further processing and interpretation. Computational models that simulate ANF responses to CI stimuli enable the exploration of the mechanisms underlying CI performance beyond the capacity of in vivo experimentation alone. However, all ANF models developed to date utilize to some extent anatomical/morphometric data, biophysical properties and/or physiological data measured in non-human animal models. This review compares response properties of the electrically stimulated auditory nerve (AN) in human listeners and different mammalian models. Properties of AN responses to single pulse stimulation, paired-pulse stimulation, and pulse-train stimulation are presented. While some AN response properties are similar between human listeners and animal models (e.g., increased AN sensitivity to single pulse stimuli with long interphase gaps), there are some significant differences. For example, the AN of most animal models is typically more sensitive to cathodic stimulation while the AN of human listeners is generally more sensitive to anodic stimulation. Additionally, there are substantial differences in the speed of recovery from neural adaptation between animal models and human listeners. Therefore, results from animal models cannot be simply translated to human listeners. Recognizing the differences in responses of the AN to electrical stimulation between humans and other mammals is an important step for creating ANF models that are more applicable to various human CI patient populations.

mTOR Signaling in BDNF-Treated Guinea Pigs after Ototoxic Deafening

The mammalian target of rapamycin (mTOR) signaling plays a critical role in cell homeostasis, growth and survival. Here, we investigated the localization of the main mTOR signaling proteins in the organ of Corti of normal-hearing and deafened guinea pigs, as well as their possible modulation by exogenously administered brain-derived neurotrophic factor (BDNF) in deafened guinea pigs. Animals were ototoxically deafened by systemic administration of kanamycin and furosemide, and one week later, the right cochleas were treated with gelatin sponge soaked in rhBDNF, while the left cochleas were used as negative controls. Twenty-four hours after treatment, animals were euthanized, and the cochleas were processed for subsequent analysis. Through immunofluorescence, we demonstrated the localization of AKT, pAKT, mTOR, pmTOR and PTEN proteins throughout the cochlea of guinea pigs for the first time, with a higher expression in supporting cells. Moreover, an increase in mTOR immunostaining was observed in BDNF-treated cochleas by means of fluorescence intensity compared to the other groups. Conversely, Western blot analysis showed no significant differences in the protein levels between groups, probably due to dilution of proteins in the neighboring tissues of the organ of Corti. Altogether, our data indicate that mTOR signaling proteins are expressed by the organ of Corti (with a major role for supporting cells) and that the modulation of mTOR may be a protective mechanism triggered by BDNF in the degenerating organ of Corti.

Combined brain-derived neurotrophic factor and neurotrophin-3 treatment is preferred over either one separately in the preservation of the auditory nerve in deafened guinea pigs

Severe hearing loss or deafness is often caused by cochlear hair cell loss and can be mitigated by a cochlear implant (CI). CIs target the auditory nerve, consisting of spiral ganglion cells (SGCs), which degenerate gradually, following hair cell loss. In animal models, it has been established that treatment with the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) reduce SGC degeneration. In this study, we aimed to investigate whether treatment with both BDNF and NT-3 (Cocktail) is superior to treatment with each neurotrophin separately regarding cell preservation and neural responsiveness to electrical stimulation. To this end, deafened guinea pigs received neurotrophic treatment in their right ear via a gelatin sponge on the perforated round window membrane, followed by cochlear implantation 4 weeks later in the same ear for electrophysiological recordings to various stimulation paradigms. Normal-hearing and deafened untreated guinea pigs were included as positive and negative controls, respectively. Substantial SGC loss occurred in all deafened animals. Each of the neurotrophic treatments led to enhanced SGC survival mainly in the basal turn of the cochlea, gradually decreasing toward the apex. The Cocktail treatment resulted in the highest SGC survival in the treated ear, followed by BDNF, with the least protection of SGCs following NT-3 treatment. Survival of the SGC’s peripheral processes (PPs) followed the same trend in response to the treatment. However, survival of SGCs and PPs in the contralateral untreated ears was also highest in the Cocktail group. Consequently, analysis of the ratio between the treated and untreated ears showed that the BDNF group, which showed low SGC survival in the untreated ear, had the highest relative SGC survival of the three neurotrophin-treated groups. Neurotrophic treatment had positive effects in part of the electrically evoked compound action-potential recording paradigms. These effects were only observed for the BDNF or Cocktail treatment. We conclude that treatment with either BDNF or a cocktail of BDNF and NT-3 is preferred to NT-3 alone. Furthermore, since the Cocktail treatment resulted in better electrophysiological responsiveness and overall higher SGC survival than BDNF alone, we are inclined to recommend the Cocktail treatment rather than BDNF alone.

No Protective Effects of Hair Cells or Supporting Cells in Ototoxically Deafened Guinea Pigs upon Administration of BDNF

We investigated whether treatment with brain-derived neurotrophic factor (BDNF), which is known to protect spiral ganglion cells (SGCs), could also protect hair cells (HCs) and supporting cells (SCs) in the organ of Corti of a guinea pig model of sensorineural hearing loss. Hearing loss was induced by administration of kanamycin/furosemide and two BDNF treatments were performed: (1) by gelatin sponge (BDNF-GS) with acute cochlear implantation (CI), and (2) through a mini-osmotic pump (BDNF-OP) with chronic CI. Outer HCs (OHCs), inner HCs (IHCs), Border, Phalangeal, Pillar, Deiters', and Hensen's cells were counted. The BDNF-GS cochleas had significantly fewer OHCs compared to the untreated ones, while the IHC and SC numbers did not differ between treated and untreated cochleas. The BDNF-OP group showed similar cell numbers to the untreated group. SGC packing density was not correlated with the total number of SCs for either BDNF group. Our data suggest that: (1) BDNF does not prevent cell death in the organ of Corti, and that the protection of SGCs could result from a direct targeting by BDNF; (2) BDNF might induce a different function/activity of the remaining cells in the organ of Corti (independently from cell number).

Evaluating Neurotrophin Signaling Using MicroRNA Perilymph Profiling in Cochlear Implant Patients With and Without Residual Hearing

HYPOTHESIS

MicroRNAs predicted to regulate neurotrophin signaling can be found in human perilymph.

BACKGROUND

Animal and human temporal bone studies suggest that spiral ganglion health can affect cochlear implant (CI) outcomes. Neurotrophins have been identified as a key factor in the maintenance of spiral ganglion health. Changes in miRNAs may regulate neurotrophin signaling and may reflect neurotrophin expression levels.

METHODS

Perilymph sampling was carried out in 18 patients undergoing cochlear implantation or stapedotomy. Expression of miRNAs in perilymph was evaluated using an Agilent miRNA gene chip. Using ingenuity pathway analysis (IPA) software, miRNAs targeting neurotrophin signaling pathway genes present in a cochlear cDNA library were annotated. Expression levels of miRNAs in perilymph were correlated to the patients' preoperative pure-tone average.

RESULTS

Expression of mRNAs coding for neurotrophins and their receptors were identified in tissue obtained from normal human cochlea during skull base surgery. We identified miRNAs predicted to regulate these signaling cascades, including miR-1207-5p, miR-4651, miR-103-3p, miR-100-5p, miR-221-3p, miR-200-3p. There was a correlation between poor preoperative hearing and lower expression of miR-1207 (predicted to regulate NTR3) and miR-4651 (predicted to regulate NTR2). Additionally, miR-3960, miR-4481, and miR-675 showed significant differences in expression level when comparing mild and profound hearing loss patients.

CONCLUSIONS

Expression of some miRNAs that are predicted to regulate neurotrophin signaling in the perilymph of cochlear implant patients vary with the patient's level of residual hearing. These miRNAs may serve as biomarkers for changes in neurotrophin signaling.

Amplitude Growth Functions of Auditory Nerve Responses to Electric Pulse Stimulation With Varied Interphase Gaps in Cochlear Implant Users With Ipsilateral Residual Hearing

Amplitude growth functions (AGFs) of electrically evoked compound action potentials (eCAPs) with varying interphase gaps (IPGs) were measured in cochlear implant users with ipsilateral residual hearing (electric-acoustic stimulation [EAS]). It was hypothesized that IPG effects on AGFs provide an objective measure to estimate neural health. This hypothesis was tested in EAS users, as residual low-frequency hearing might imply survival of hair cells and hence better neural health in apical compared to basal cochlear regions. A total of 16 MED-EL EAS subjects participated, as well as a control group of 16 deaf cochlear implant users. The IPG effect on the AGF characteristics of slope, threshold, dynamic range, and stimulus level at 50% maximum eCAP amplitude (level_50%) was investigated. AGF threshold and level_50% were significantly affected by the IPG in both EAS and control group. The magnitude of AGF characteristics correlated with electrode impedance and electrode-modiolus distance (EMD) in both groups. In contrast, the change of the AGF characteristics with increasing IPG was independent of these electrode-specific measures. The IPG effect on the AGF level_50% in both groups, as well as on the threshold in EAS users, correlated with the duration of hearing loss, which is a predictor of neural health. In EAS users, a significantly different IPG effect on level_50% was found between apical and medial electrodes. This outcome is consistent with our hypothesis that the influence of IPG effects on AGF characteristics provides a sensitive measurement and may indicate better neural health in the apex compared to the medial cochlear region in EAS users.

Transplantation of adipose-derived stromal cells protects functional and morphological auditory nerve integrity in a model of cochlear implantation

Cochlear implants are considered the gold standard therapy for subjects with severe hearing loss and deafness. Cochlear implants bypass the damaged hair cells and directly stimulate spiral ganglion neurons (SGNs) of the auditory nerve. Hence, the presence of functional SGNs is crucial for speech perception in electric hearing with a cochlear implant. In deaf individuals, SGNs progressively degenerate due to the lack of neurotrophic support, normally provided by sensory cells of the inner ear. Adipose-derived stromal cells (ASCs) are known to produce neurotrophic factors. In a guinea pig model of sensory hearing loss and cochlear implantation, ASCs were autologously transplanted into the scala tympani prior to insertion of a cochlear implant on one side. Electrically evoked auditory brain stem responses (eABR) were recorded 8 weeks after cochlear implantation. At conclusion of the experiment, the cochleae were histologically evaluated. Compared to untreated control animals, transplantation of ASCs resulted in an increased number of SGNs and their peripheral neurites. In ASC-transplanted animals, mean eABR thresholds were lower and suprathreshold amplitudes larger, suggesting a larger population of intact auditory nerve fibers. Moreover, when compared to controls, amplitude-level functions of eABRs in ASC transplanted animals demonstrated steeper slopes in response to increasing interphase gaps (IPGs), indicative of better functionality of the auditory nerve. In summary, results suggest that transplantation of autologous ASCs into the deaf inner ear may have protective effects on the survival of SGNs and their peripheral processes and may thus contribute to long-term benefits in speech discrimination performance in cochlear implant subjects.

How electrically evoked compound action potentials in chronically implanted guinea pigs relate to auditory nerve health and electrode impedance

This study examined how multiple measures based on the electrically evoked compound action potential (ECAP) amplitude-growth functions (AGFs) were related to estimates of neural [spiral ganglion neuron (SGN) density and cell size] and electrode impedance measures in 34 specific pathogen free pigmented guinea pigs that were chronically implanted (4.9-15.4 months) with a cochlear implant electrode array. Two interphase gaps (IPGs) were used for the biphasic pulses and the effect of the IPG on each ECAP measure was measured ("IPG effect"). When using a stimulus with a constant IPG, SGN density was related to the across-subject variance in ECAP AGF linear slope, peak amplitude, and N1 latency. The SGN density values also help to explain a significant proportion of variance in the IPG effect for AGF linear slope and peak amplitude measures. Regression modeling revealed that SGN density was the primary dependent variable contributing to across-subject variance for ECAP measures; SGN cell size did not significantly improve the fitting of the model. Results showed that simple impedance measures were weakly related to most ECAP measures but did not typically improve the fit of the regression model.

BDNF-mediated preservation of spiral ganglion cell peripheral processes and axons in comparison to that of their cell bodies

Treatment with neurotrophins prevents degeneration of spiral ganglion cells (SGCs) after severe hair cell loss. In a previous study we demonstrated a long-lasting effect with brain-derived neurotrophic factor (BDNF) after cessation of treatment. In that study the survival of the SGC cell bodies was examined. Here we address the question whether their peripheral processes and central processes (axons) were protected by this treatment as well in the cochleas of the aforementioned study. Guinea pigs were deafened by co-administration of kanamycin and furosemide. Two weeks after deafening the right cochleas were implanted with an intracochlear electrode array combined with a cannula connected to an osmotic pump filled with BDNF solution. Four weeks later the treatment was stopped by surgically removing the osmotic pump. At that point, or another four or eight weeks later, the animals were sacrificed for histological analysis. Control groups consisted of normal-hearing animals, and three groups of deafened animals: two-weeks-deaf untreated animals, and six- and fourteen-weeks-deaf sham-treated animals. Cochleas were processed for analysis of: (1) the myelinated portion of peripheral processes in the osseous spiral lamina, (2) the cell bodies in Rosenthal's canal, and (3) axons in the internal acoustic meatus. Packing densities and cross-sectional areas were determined using light microscopy. Up to eight weeks after treatment cessation the numbers of peripheral processes and axons were significantly higher than in untreated cochleas of control animals. Whereas the numbers of cell bodies and axons were similar to those at the start of treatment, the peripheral processes were significantly less well preserved. This smaller protective effect was found mainly in the apical turns. Strategies to prevent SGC degeneration after hair cell loss should consider the differential effects on the various neural elements.

BDNF Outperforms TrkB Agonist 7,8,3'-THF in Preserving the Auditory Nerve in Deafened Guinea Pigs

In deaf subjects using a cochlear implant (CI) for hearing restoration, the auditory nerve is subject to degeneration, which may negatively impact CI effectiveness. This nerve degeneration can be reduced by neurotrophic treatment. Here, we compare the preservative effects of the naturally occurring tyrosine receptor kinase B (TrkB) agonist brain-derived neurotrophic factor (BDNF) and the small-molecule TrkB agonist 7,8,3′-trihydroxyflavone (THF) on the auditory nerve in deafened guinea pigs. THF may be more effective than BDNF throughout the cochlea because of better pharmacokinetic properties. The neurotrophic compounds were delivered by placement of a gelatin sponge on the perforated round window membrane. To complement the histology of spiral ganglion cells (SGCs), electrically evoked compound action potential (eCAP) recordings were performed four weeks after treatment initiation. We analyzed the eCAP inter-phase gap (IPG) effect and measures derived from pulse-train evoked eCAPs, both indicative of SGC healthiness. BDNF but not THF yielded a significantly higher survival of SGCs in the basal cochlear turn than untreated controls. Regarding IPG effect and pulse-train responses, the BDNF-treated animals exhibited more normal responses than both untreated and THF-treated animals. We have thus confirmed the protective effect of BDNF, but we have not confirmed previously reported protective effects of THF with our clinically applicable delivery method.

Expression pattern of brain-derived neurotrophic factor and its associated receptors: Implications for exogenous neurotrophin application

The application of neurotrophins such as brain-derived neurotrophic factor (BDNF) is a promising pharmacological approach in cochlear implant research. Several in vitro and in vivo studies demonstrated that treatment with neurotrophins support the spiral ganglion neuron (SGN) survival and the synapses. Of the more than 40 companies that are working in the field of inner ear therapeutics, only one company is currently advancing BDNF towards clinical translation. Thus, there are no approved clinical therapies with neurotrophins, their precursors or neurotrophin-like substances. For a better understanding of the mechanisms of BDNF in the inner ear, we analysed the expression of mature BDNF (mBDNF), its pro-form proBDNF and their respective receptors the low affinity p75 neurotrophin receptor (p75NTR) and the neurotrophic receptor tyrosine kinase 2 (NTRK2). In the adult murine inner ear, mBDNF is expressed in the inner and outer hair cells (IHC and OHC) of the organ of Corti and in the spiral ganglion of the Rosenthal's canal, whereas proBDNF is only detected in the supporting cells below the OHC. The corresponding receptors NTRK2 and p75NTR are expressed in the spiral ganglion whereof p75NTR is stronger expressed. For more insights in the effects of mBDNF and proBDNF on inner ear specific cells, we treated primary dissociated SGN with different concentrations of mBDNF and proBDNF alone and in combination. Interestingly, treatment with proBDNF is not toxic for SGN but simultaneously not protective. However, combined treatment of mBDNF and proBDNF maintained and perhaps slightly increased the protective effect of mBDNF. Thus, the mixture of mBDNF and proBDNF could be the new direction for the development of BDNF-based therapeutics in cochlear implantation and could represent more precisely the natural environment.

Neurotrophin gene therapy to promote survival of spiral ganglion neurons after deafness

Hearing impairment is a major health and economic concern worldwide. Currently, the cochlear implant (CI) is the standard of care for remediation of severe to profound hearing loss, and in general, contemporary CIs are highly successful. But there is great variability in outcomes among individuals, especially in children, with many CI users deriving much less or even marginal benefit. Much of this variability is related to differences in auditory nerve survival, and there has been substantial interest in recent years in exploring potential therapies to improve survival of the cochlear spiral ganglion neurons (SGN) after deafness. Preclinical studies using osmotic pumps and other approaches in deafened animal models to deliver neurotrophic factors (NTs) directly to the cochlea have shown promising results, especially with Brain-Derived Neurotrophic Factor (BDNF). More recent studies have focused on the use of NT gene therapy to force expression of NTs by target cells within the cochlea. This could provide the means for a one-time treatment to promote long-term NT expression and improve neural survival after deafness. This review summarizes the evidence for the efficacy of exogenous NTs in preventing SGN degeneration after hearing loss and reviews the animal research to date suggesting that NT gene therapy can elicit long-term NT expression in the cochlea, resulting in significantly improved SGN and radial nerve fiber survival after deafness. In addition, we discuss NT gene therapy in other non-auditory applications and consider some of the remaining issues with regard to selecting optimal vectors, timing of treatment, and place/method of delivery, etc. that must be resolved prior to considering clinical application.

AAV-Mediated Neurotrophin Gene Therapy Promotes Improved Survival of Cochlear Spiral Ganglion Neurons in Neonatally Deafened Cats: Comparison of AAV2-hBDNF and AAV5-hGDNF

Outcomes with contemporary cochlear implants (CI) depend partly upon the survival and condition of the cochlear spiral ganglion (SG) neurons. Previous studies indicate that CI stimulation can ameliorate SG neural degeneration after deafness, and brain-derived neurotrophic factor (BDNF) delivered by an osmotic pump can further improve neural survival. However, direct infusion of BDNF elicits undesirable side effects, and osmotic pumps are impractical for clinical application. In this study, we explored the potential for two adeno-associated viral vectors (AAV) to elicit targeted neurotrophic factor expression in the cochlea and promote improved SG and radial nerve fiber survival. Juvenile cats were deafened prior to hearing onset by systemic aminoglycoside injections. Auditory brainstem responses showed profound hearing loss by 16-18 days postnatal. At ~ 4 weeks of age, AAV2-GFP (green fluorescent protein), AAV5-GFP, AAV2-hBDNF, or AAV5-hGDNF (glial-derived neurotrophic factor) was injected through the round window unilaterally. For GFP immunofluorescence, animals were studied ~ 4 weeks post-injection to assess cell types transfected and their distributions. AAV2-GFP immunofluorescence demonstrated strong expression of the GFP reporter gene in residual inner (IHCs), outer hair cells (OHCs), inner pillar cells, and in some SG neurons throughout the cochlea. AAV5-GFP elicited robust transduction of IHCs and some SG neurons, but few OHCs and supporting cells. After AAV-neurotrophic factor injections, animals were studied ~ 3 months post-injection to evaluate neural survival. AAV5-hGDNF elicited a modest neurotrophic effect, with 6 % higher SG density, but had no trophic effect on radial nerve fiber survival, and undesirable ectopic fiber sprouting occurred. AAV2-hBDNF elicited a similar 6 % increase in SG survival, but also resulted in greatly improved radial nerve fiber survival, with no ectopic fiber sprouting. A further study assessed whether AAV2-hBDNF neurotrophic effects would persist over longer post-injection periods. Animals examined 6 months after virus injection showed substantial neurotrophic effects, with 14 % higher SG density and greatly improved radial nerve fiber survival. Our results suggest that AAV-neurotrophin gene therapy can elicit expression of physiological concentrations of neurotrophins in the cochlea, supporting improved SG neuronal and radial nerve fiber survival while avoiding undesirable side effects. These studies also demonstrate the potential for application of cochlear gene therapy in a large mammalian cochlea comparable to the human cochlea and in an animal model of congenital/early acquired deafness.

Ouabain Does Not Induce Selective Spiral Ganglion Cell Degeneration in Guinea Pigs

Round window membrane (RWM) application of ouabain is known to selectively destroy type I spiral ganglion cells (SGCs) in cochleas of several rodent species, while leaving hair cells intact. This protocol has been used in rats and Mongolian gerbils, but observations in the guinea pig are conflicting. This is why we reinvestigated the effect of ouabain on the guinea pig cochlea. Ouabain solutions of different concentrations were placed, in a piece of gelfoam, upon the RWM of the right cochleas. Auditory function was assessed using acoustically evoked auditory brainstem responses (aABR). Finally, cochleas were fixed and processed for histological examination. Due to variability within treatment groups, histological data was pooled and three categories based upon general histological observations were defined: cochleas without outer hair cell (OHC) and SGC loss (Category 1), cochleas with OHC loss only (Category 2), and cochleas with OHC and SGC loss (Category 3). Animals treated with 1 mM or 10 mM ouabain showed shifts in hearing thresholds, corresponding with varying histological changes in their cochleas. Most cochleas exhibited complete outer hair cell loss in the basal and middle turns, while some had no changes, together with either moderate or near-complete loss of SGCs. Neither loss of inner hair cells nor histological changes of the stria vascularis were observed in any of the animals. Cochleas in Category 1 had normal aABRs and morphology. On average, in Category 2 OHC loss was 46.0±5.7%, SGC loss was below threshold, ABR threshold shift was 44.9±2.7 dB, and ABR wave II amplitude was decreased by 17.1±3.8 dB. In Category 3 OHC loss was 68.3±6.9%, SGC loss was 49.4±4.3%, ABR threshold shift was 39.0±2.4 dB, and ABR amplitude was decreased by 15.8±1.6 dB. Our results show that ouabain does not solely destroy type I SGCs in the guinea pig cochlea.

Long-term delivery of brain-derived neurotrophic factor (BDNF) from nanoporous silica nanoparticles improves the survival of spiral ganglion neurons in vitro

Sensorineural hearing loss (SNHL) can be overcome by electrical stimulation of spiral ganglion neurons (SGNs) via a cochlear implant (CI). Restricted CI performance results from the spatial gap between the SGNs and the electrode, but the efficacy of CI is also limited by the degeneration of SGNs as one consequence of SHNL. In the healthy cochlea, the survival of SGNs is assured by endogenous neurotrophic support. Several applications of exogenous neurotrophic supply have been shown to reduce SGN degeneration in vitro and in vivo. In the present study, nanoporous silica nanoparticles (NPSNPs), with an approximate diameter of <100 nm, were loaded with the brain-derived neurotrophic factor (BDNF) to test their efficacy as long-term delivery system for neurotrophins. The neurotrophic factor was released constantly from the NPSNPs over a release period of 80 days when the surface of the nanoparticles had been modified with amino groups. Cell culture investigations with NIH3T3 fibroblasts attest a good general cytocompatibility of the NPSNPs. In vitro experiments with SGNs indicate a significantly higher survival rate of SGNs in cell cultures that contained BDNF-loaded nanoparticles compared to the control culture with unloaded NPSNPs (p<0.001). Importantly, also the amounts of BDNF released up to a time period of 39 days increased the survival rate of SGNs. Thus, NPSNPs carrying BDNF are suitable for the treatment of inner ear disease and for the protection and the support of SGNs. Their nanoscale nature and the fact that a direct contact of the nanoparticles and the SGNs is not necessary for neuroprotective effects, should allow for the facile preparation of nanocomposites, e.g., with biocompatible polymers, to install coatings on implants for the realization of implant-based growth factor delivery systems.

The feasibility of an encapsulated cell approach in an animal deafness model

For patients with profound hearing loss a cochlear implant (CI) is the only treatment today. The function of a CI depends in part of the function and survival of the remaining spiral ganglion neurons (SGN). It is well known from animal models that inner ear infusion of neurotrophic factors prevents SGN degeneration and maintains electrical responsiveness in deafened animals. The purpose with this study was to investigate the effects of a novel encapsulated cell (EC) device releasing neurotrophic factors in the deafened guinea pig.

The results showed that an EC device releasing glial cell line-derived neurotrophic factor (GDNF) or brain-derived neurotrophic factor (BDNF) implanted for four weeks in deafened guinea pigs significantly preserved the SGNs and maintained their electrical responsiveness. There was a significant difference between BDNF and GDNF in favour of GDNF. This study, demonstrating positive structural and functional effects in the deafened inner ear, suggests that an implanted EC device releasing biologically protective substances offers a feasible approach for treating progressive hearing impairment.

Kanamycin Damages Early Postnatal, but Not Adult Spiral Ganglion Neurons

Although aminoglycoside antibiotics such as kanamycin are widely used clinically to treat life-threatening bacterial infections, ototoxicity remains a significant dose-limiting side effect. The prevailing view is that the hair cells are the primary ototoxic target of aminoglycosides and that spiral ganglion neurons begin to degenerate weeks or months after the hair cells have died due to lack of neurotrophic support. To test the early developmental aspects of this issue, we compared kanamycin-induced hair cell and spiral ganglion pathology in rat postnatal day 3 cochlear organotypic cultures with adult whole cochlear explants. In both adult and postnatal day 3 cultures, hair cell damage began at the base of the cochleae and progressed toward the apex in a dose-dependent manner. In postnatal day 3 cultures, spiral ganglion neurons were rapidly destroyed by kanamycin prior to hair cell loss. In contrast, adult spiral ganglion neurons were resistant to kanamycin damage even at the highest concentration, consistent with in vivo models of delayed SGN degeneration. In postnatal day 3 cultures, kanamycin preferentially damaged type I spiral ganglion neurons, whereas type II neurons were resistant. Spiral ganglion degeneration of postnatal day 3 neurons was associated with upregulation of the superoxide radical and caspase-3-mediated cell death. These results show for the first time that kanamycin is toxic to postnatal day 3 spiral ganglion neurons, but not adult neurons.

Encapsulated cell device approach for combined electrical stimulation and neurotrophic treatment of the deaf cochlea

Profound hearing impairment can be overcome by electrical stimulation (ES) of spiral ganglion neurons (SGNs) via a cochlear implant (CI). Thus, SGN survival is critical for CI efficacy. Application of glial cell line-derived neurotrophic factor (GDNF) has been shown to reduce SGN degeneration following deafness. We tested a novel method for local, continuous GDNF-delivery in combination with ES via a CI. The encapsulated cell (EC) device contained a human ARPE-19 cell-line, genetically engineered for secretion of GDNF. In vitro, GDNF delivery was stable during ES delivered via a CI. In the chronic in vivo part, cats were systemically deafened and unilaterally implanted into the scala tympani with a CI and an EC device, which they wore for six months. The implantation of control devices (same cell-line not producing GDNF) had no negative effect on SGN survival. GDNF application without ES led to an unexpected reduction in SGN survival, however, the combination of GDNF with initial, short-term ES resulted in a significant protection of SGNs. A tight fibrous tissue formation in the scala tympani of the GDNF-only group is thought to be responsible for the increased SGN degeneration, due to mechanisms related to an aggravated foreign body response. Furthermore, the fibrotic encapsulation of the EC device led to cell death or cessation of GDNF release within the EC device during the six months in vivo. In both in vitro and in vivo, fibrosis was reduced by CI stimulation, enabling the neuroprotective effect of the combined treatment. Thus, fibrous tissue growth limits treatment possibilities with an EC device. For a stable and successful long-term neurotrophic treatment of the SGN via EC devices in human CI users, it would be necessary to make changes in the treatment approach (provision of anti-inflammatories), the EC device surface (reduced cell adhesion) and the ES (initiation prior to fibrosis formation).

Efr3a Insufficiency Attenuates the Degeneration of Spiral Ganglion Neurons after Hair Cell Loss

Sensorineural hearing loss (SNHL) is caused by an irreversible impairment of cochlear hair cells and subsequent progressive degeneration of spiral ganglion neurons (SGNs). Eighty-five requiring 3 (Efr3) is a plasma membrane protein conserved from yeast to human, and knockout of Efr3a was reported to facilitate the survival of hippocampal newborn neurons in adult mice. Previously, we found Efr3a expression in the auditory neural pathway is upregulated soon after the destruction of hair cells. Here we conducted a time-course analysis of drug-caused damage to hearing ability, hair cells and SGNs in Efr3a knocking down mice (Efr3a^−/+, Efr3a KD) and their wild type littermates. Functional examination showed that both groups of mice suffered from serious hearing loss with a higher level of severity in wild type (WT) mice. Morphologic observation following drugs administration showed that both WT and Efr3a KD mice went through progressive loss of hair cells and SGNs, in association with degenerative changes in the perikarya, intracellular organelles, cell body conformation in SGNs, and the changes of SGNs in WT mice were more severe than in Efr3a KD mice. These beneficial effects of Efr3a KD could be ascribed to an increase in the expression of some neurotrophic factors and their receptors in Efr3a KD mice. Our results indicate that Efr3a insufficiency suppresses drug-caused SNHL neurodegeneration in association with an increase in the expression of some neurotrophic factors and their receptors, which may be targeted in the treatment of neurodegeneration.

Functional Pattern of Increasing Concentrations of Brain-Derived Neurotrophic Factor in Spiral Ganglion: Implications for Research on Cochlear Implants

BACKGROUND:

As previously various studies have suggested application of brain-derived neurotrophic factor (BDNF) may be considered as a promising future therapy for hearing deficits, in particular for the improvement of cochlear neurone loss during cochlear implantation.

AIM:

The present study’s aim was to establish the upper threshold of the concentration of BDNF in Naval Medical Research Institute (NMRI) mice spiral ganglion outgrowth.

METHODS:

Spiral ganglion explants were prepared from post-natal day 4 (p4) (NMRI) mice of both sexes under the approval and guidelines of the regional council of Hearing Research Institute Tubingen.

RESULTS:

Spiral ganglion explants were cultured at postnatal days 4 in the presence of different concentrations of BDNF as described under methods. We chose an age of postnatal day (P4) and concentrations of BDNF 0; 6; 12.5; 25 and 50 ƞg/ml. Averaged neurite outgrowth is measured in 4 different cultures that were treated with different concentrations. Results show that with increasing concentrations of BDNF, the neurite density increases.

CONCLUSION:

The present finding show evidence that BDNF has a clear incremental effect on the number of neurites of spiral ganglia in the prehearing organ, but less on the neurite length. The upper threshold of exogenous BNDF concentration on spiral ganglion explant is 25 ƞg/ml. This means that concentration beyond this level has no further incremental impact. Therefore our suggestion for hydrogel concentration in NMRA mice in future research should be 25 ƞg/ml.

Towards Clinical Application of Neurotrophic Factors to the Auditory Nerve; Assessment of Safety and Efficacy by a Systematic Review of Neurotrophic Treatments in Humans

Animal studies have evidenced protection of the auditory nerve by exogenous neurotrophic factors. In order to assess clinical applicability of neurotrophic treatment of the auditory nerve, the safety and efficacy of neurotrophic therapies in various human disorders were systematically reviewed. Outcomes of our literature search included disorder, neurotrophic factor, administration route, therapeutic outcome, and adverse event. From 2103 articles retrieved, 20 randomized controlled trials including 3974 patients were selected. Amyotrophic lateral sclerosis (53%) was the most frequently reported indication for neurotrophic therapy followed by diabetic polyneuropathy (28%). Ciliary neurotrophic factor (50%), nerve growth factor (24%) and insulin-like growth factor (21%) were most often used. Injection site reaction was a frequently occurring adverse event (61%) followed by asthenia (24%) and gastrointestinal disturbances (20%). Eighteen out of 20 trials deemed neurotrophic therapy to be safe, and six out of 17 studies concluded the neurotrophic therapy to be effective. Positive outcomes were generally small or contradicted by other studies. Most non-neurodegenerative diseases treated by targeted deliveries of neurotrophic factors were considered safe and effective. Hence, since local delivery to the cochlea is feasible, translation from animal studies to human trials in treating auditory nerve degeneration seems promising.

Improved Auditory Nerve Survival with Nanoengineered Supraparticles for Neurotrophin Delivery into the Deafened Cochlea

Cochlear implants electrically stimulate spiral ganglion neurons (SGNs) in order to provide speech cues to severe-profoundly deaf patients. In normal hearing cochleae the SGNs depend on endogenous neurotrophins secreted by sensory cells in the organ of Corti for survival. SGNs gradually degenerate following deafness and consequently there is considerable interest in developing clinically relevant strategies to provide exogenous neurotrophins to preserve SGN survival. The present study investigated the safety and efficacy of a drug delivery system for the cochlea using nanoengineered silica supraparticles. In the present study we delivered Brain-derived neurotrophic factor (BDNF) over a period of four weeks and evaluated SGN survival as a measure of efficacy. Supraparticles were bilaterally implanted into the basal turn of cochleae in profoundly deafened guinea pigs. One ear received BDNF-loaded supraparticles and the other ear control (unloaded) supraparticles. After one month of treatment the cochleae were examined histologically. There was significantly greater survival of SGNs in cochleae that received BDNF supraparticles compared to the contralateral control cochleae (repeated measures ANOVA, p = 0.009). SGN survival was observed over a wide extent of the cochlea. The supraparticles were well tolerated within the cochlea with a tissue response that was localised to the site of implantation in the cochlear base. Although mild, the tissue response was significantly greater in cochleae treated with BDNF supraparticles compared to the controls (repeated measures ANOVA, p = 0.003). These data support the clinical potential of this technology particularly as the supraparticles can be loaded with a variety of therapeutic drugs.

Effects of brain-derived neurotrophic factor (BDNF) on the cochlear nucleus in cats deafened as neonates

Many previous studies have shown significant neurotrophic effects of intracochlear delivery of BDNF in preventing degeneration of cochlear spiral ganglion (SG) neurons after deafness in rodents and our laboratory has shown similar results in developing cats deafened prior to hearing onset. This study examined the morphology of the cochlear nucleus (CN) in a group of neonatally deafened cats from a previous study in which infusion of BDNF elicited a significant improvement in survival of the SG neurons. Five cats were deafened by systemic injections of neomycin sulfate (60 mg/kg, SQ, SID) starting one day after birth, and continuing for 16-18 days until auditory brainstem response (ABR) testing demonstrated profound bilateral hearing loss. The animals were implanted unilaterally at about 1 month of age using custom-designed electrodes with a drug-delivery cannula connected to an osmotic pump. BDNF (94 μg/ml; 0.25 μl/hr) was delivered for 10 weeks. The animals were euthanized and studied at 14-23 weeks of age. Consistent with the neurotrophic effects of BDNF on SG survival, the total CN volume in these animals was significantly larger on the BDNF-treated side than on the contralateral side. However, total CN volume, both ipsi- and contralateral to the implants in these deafened juvenile animals, was markedly smaller than the CN in normal adult animals, reflecting the severe effects of deafness on the central auditory system during development. Data from the individual major CN subdivisions (DCN, Dorsal Cochlear Nucleus; PVCN, Posteroventral Cochlear Nucleus; AVCN, Anteroventral Cochlear Nucleus) also were analyzed. A significant difference was observed between the BDNF-treated and control sides only in the AVCN. Measurements of the cross-sectional areas of spherical cells showed that cells were significantly larger in the AVCN ipsilateral to the implant than on the contralateral side. Further, the numerical density of spherical cells was significantly lower in the AVCN ipsilateral to the implant than on the contralateral side, consistent with the larger AVCN volume observed with BDNF treatment. Together, findings indicate significant neurotrophic effects of intracochlear BDNF infusion on the developing CN.

Organ of Corti explants direct tonotopically graded morphology of spiral ganglion neurons in vitro

The spiral ganglion is a compelling model system to examine how morphological form contributes to sensory function. While the ganglion is composed mainly of a single class of type I neurons that make simple one-to-one connections with inner hair cell sensory receptors, it has an elaborate overall morphological design. Specific features, such as soma size and axon outgrowth, are graded along the spiral contour of the cochlea. To begin to understand the interplay between different regulators of neuronal morphology, we cocultured neuron explants with peripheral target tissues removed from distinct cochlear locations. Interestingly, these "hair cell microisolates" were capable of both increasing and decreasing neuronal somata size, without adversely affecting survival. Moreover, axon characteristics elaborated de novo by the primary afferents in culture were systematically regulated by the sensory endorgan. Apparent peripheral nervous system (PNS)-like and central nervous system (CNS)-like axonal profiles were established in our cocultures allowing an analysis of putative PNS/CNS axon length ratios. As predicted from the in vivo organization, PNS-like axon bundles elaborated by apical cocultures were longer than their basal counterparts and this phenotype was methodically altered when neuron explants were cocultured with microisolates from disparate cochlear regions. Thus, location-dependent signals within the organ of Corti may set the "address" of neurons within the spiral ganglion, allowing them to elaborate the appropriate tonotopically associated morphological features in order to carry out their signaling function. J. Comp. Neurol. 524:2182-2207, 2016. © 2015 Wiley Periodicals, Inc.

Temporary Neurotrophin Treatment Prevents Deafness-Induced Auditory Nerve Degeneration and Preserves Function

After substantial loss of cochlear hair cells, exogenous neurotrophins prevent degeneration of the auditory nerve. Because cochlear implantation, the current therapy for profound sensorineural hearing loss, depends on a functional nerve, application of neurotrophins is being investigated. We addressed two questions important for fundamental insight into the effects of exogenous neurotrophins on a degenerating neural system, and for translation to the clinic. First, does temporary treatment with brain-derived neurotrophic factor (BDNF) prevent nerve degeneration on the long term? Second, how does a BDNF-treated nerve respond to electrical stimulation? Deafened guinea pigs received a cochlear implant, and their cochleas were infused with BDNF for 4 weeks. Up to 8 weeks after treatment, their cochleas were analyzed histologically. Electrically evoked compound action potentials (eCAPs) were recorded using stimulation paradigms that are informative of neural survival. Spiral ganglion cell (SGC) degeneration was prevented during BDNF treatment, resulting in 1.9 times more SGCs than in deafened untreated cochleas. Importantly, SGC survival was almost complete 8 weeks after treatment cessation, when 2.6 times more SGCs were observed. In four eCAP characteristics (three involving alteration of the interphase gap of the biphasic current pulse and one involving pulse trains), we found large and statistically significant differences between normal-hearing and deaf controls. Importantly, for BDNF-treated animals, these eCAP characteristics were near normal, suggesting healthy responsiveness of BDNF-treated SGCs. In conclusion, clinically practicable short-term neurotrophin treatment is sufficient for long-term survival of SGCs, and it can restore or preserve SGC function well beyond the treatment period. Significance statement: Successful restoration of hearing in deaf subjects by means of a cochlear implant requires a healthy spiral ganglion cell population. Deafness-induced degeneration of these cells can be averted with neurotrophic factors. In the present study in deafened guinea pigs, we investigated the long-term effects of temporary (i.e., clinically practicable) treatment with brain-derived neurotrophic factor (BDNF). We show that, after treatment cessation, the neuroprotective effect remains for at least 8 weeks. Moreover, for the first time, it is shown that the electrical responsiveness of BDNF-treated spiral ganglion cells is preserved during this period as well. These findings demonstrate that treatment of the auditory nerve with neurotrophic factors may be relevant for cochlear implant users.

Attenuated infrared neuron stimulation response in cochlea of deaf animals may associate with the degeneration of spiral ganglion neurons

HYPOTHESIS

We hypothesize that degenerated spiral ganglion neurons (SGNs) in guinea pigs reduces auditory brainstem responses evoked by pulsed infrared stimulation.

BACKGROUND

Pulsed infrared laser excitation can directly evoke physiological responses in neuronal and other excitable cells in vivo and in vitro. Laser pulses could benefit patients with cochlear implants to stimulate the auditory system.

METHODS

Pulsed infrared lasers were used to study evoked optical auditory brainstem responses (oABRs) in normal hearing and deafened animals. Aslo, the morphology and anatomy of SGNs in normal hearing and deafened guinea pigs were compared.

RESULTS

By recording oABRs evoked by varying infrared laser pulse durations, it is suggested that degeneration of SGNs in deafened guinea pigs was associated with an elevated oABR threshold and with lower amplitudes. Moreover, oABR threshold decreased while amplitudes increased in both normal hearing and deafened animals as the pulse duration prolonged. Electron microscopy revealed that SGNs in deafened guinea pigs had swollen and vacuolar mitochondria, as well as demyelinated soma and axons.

CONCLUSION

Infrared laser pulses can stimulate SGNs to evoke oABRs in guinea pigs. Deafened guinea pigs have elevated thresholds and smaller amplitude responses, likely a result of degenerated SGNs. Short pulse durations are more suitable to evoke responses in both normal hearing and deafened animals.

Targeted delivery of brain-derived neurotrophic factor for the treatment of blindness and deafness

Neurodegenerative causes of blindness and deafness possess a major challenge in their clinical management as proper treatment guidelines have not yet been found. Brain-derived neurotrophic factor (BDNF) has been established as a promising therapy against neurodegenerative disorders including hearing and visual loss. Unfortunately, the blood–retinal barrier and blood–cochlear barrier, which have a comparable structure to the blood–brain barrier prevent molecules of larger sizes (such as BDNF) from exiting the circulation and reaching the targeted cells. Anatomical features of the eye and ear allow use of local administration, bypassing histo-hematic barriers. This paper focuses on highlighting a variety of strategies proposed for the local administration of the BDNF, like direct delivery, viral gene therapy, and cell-based therapy, which have been shown to successfully improve development, survival, and function of spiral and retinal ganglion cells. The similarities and controversies for BDNF treatment of posterior eye diseases and inner ear diseases have been analyzed and compared. In this review, we also focus on the possibility of translation of this knowledge into clinical practice. And finally, we suggest that using nanoparticulate drug-delivery systems may substantially contribute to the development of clinically viable techniques for BDNF delivery into the cochlea or posterior eye segment, which, ultimately, can lead to a long-term or permanent rescue of auditory and optic neurons from degeneration.

Differential effects of AAV.BDNF and AAV.Ntf3 in the deafened adult guinea pig ear

Cochlear hair cell loss results in secondary regression of peripheral auditory fibers (PAFs) and loss of spiral ganglion neurons (SGNs). The performance of cochlear implants (CI) in rehabilitating hearing depends on survival of SGNs. Here we compare the effects of adeno-associated virus vectors with neurotrophin gene inserts, AAV.BDNF and AAV.Ntf3, on guinea pig ears deafened systemically (kanamycin and furosemide) or locally (neomycin). AAV.BDNF or AAV.Ntf3 was delivered to the guinea pig cochlea one week following deafening and ears were assessed morphologically 3 months later. At that time, neurotrophins levels were not significantly elevated in the cochlear fluids, even though in vitro and shorter term in vivo experiments demonstrate robust elevation of neurotrophins with these viral vectors. Nevertheless, animals receiving these vectors exhibited considerable re-growth of PAFs in the basilar membrane area. In systemically deafened animals there was a negative correlation between the presence of differentiated supporting cells and PAFs, suggesting that supporting cells influence the outcome of neurotrophin over-expression aimed at enhancing the cochlear neural substrate. Counts of SGN in Rosenthal's canal indicate that BDNF was more effective than NT-3 in preserving SGNs. The results demonstrate that a transient elevation in neurotrophin levels can sustain the cochlear neural substrate in the long term.

Diffusion tensor imaging of the auditory nerve in patients with long-term single-sided deafness

A cochlear implant (CI) can restore hearing in patients with profound sensorineural hearing loss by direct electrical stimulation of the auditory nerve. Therefore, the viability of the auditory nerve is vitally important in successful hearing recovery. However, the nerve typically degenerates following cochlear hair cell loss, and the amount of degeneration may considerably differ between the two ears, also in patients with bilateral deafness. A measure that reflects the nerve's condition would help to assess the best of both nerves and decide accordingly which ear should be implanted for optimal benefit from a CI. Diffusion tensor MRI (DTI) may provide such a measure, by allowing noninvasive investigations of the nerve's microstructure. In this pilot study, we show the first use of DTI to image the auditory nerve in five normal-hearing subjects and five patients with long-term profound single-sided sensorineural hearing loss. A specialized acquisition protocol was designed for a 3 T MRI scanner to image the small nerve bundle. The nerve was reconstructed using fiber tractography and DTI metrics - which reflect the nerve's microstructural properties - were computed per tract. Comparing DTI metrics from the deaf-sided with the healthy-sided nerves in patients showed no significant differences. There was a small but significant reduction in fractional anisotropy in both auditory nerves in patients compared with normal-hearing controls. These results are the first evidence of possible changes in the microstructure of the bilateral auditory nerves as a result of single-sided deafness. Our results also indicate that it is too early to assess the degenerative status of the auditory nerve of a subject-specific basis.

The Safety Pharmacology of Auditory Function

Safety pharmacology satisfies a key requirement in the process of drug development. Safety pharmacology studies are required to assess the impact of a new chemical entity (NCE) or biotechnology-derived product for human use on vital systems, such as those subserving auditory function. Safety pharmacology studies accordingly are defined as those studies that investigate the potential undesirable effects of a substance on auditory functions in relation to exposure in and above the therapeutic range. Auditory safety studies should be designed with the primary objective of determining how administration of a compound influences normal hearing. If an effect on hearing is identified, then it is necessary to determine through histopathology the underlying mechanism for the observed hearing loss. Since the auditory system contains a heterogeneous mixture of structural and cellular components that are organized in a very complex and integrated manner, it is necessary to clearly identify the underlying primary mechanism or target of the new chemical entity that produced the hearing loss. This chapter will highlight major components of auditory function with regard to potential opportunities for drug interaction. Aspects of designing ototoxicity studies will be discussed with an emphasis on standards deemed necessary by the US Food and Drug Administration. Additionally, classes of ototoxic compounds and their proposed mechanisms of action are described in depth.

The role of intracochlear drug delivery devices in the management of inner ear disease

INTRODUCTION

Diseases of the inner ear include those of the auditory and vestibular systems, and frequently result in disabling hearing loss or vertigo. Despite a rapidly expanding pipeline of potential cochlear therapeutics, the inner ear remains a challenging organ for targeted drug delivery, and new technologies are required to deliver these therapies in a safe and efficacious manner. In addition to traditional approaches for direct inner ear drug delivery, novel microfluidics-based systems are under development, promising improved control over pharmacokinetics over longer periods of delivery, ultimately with application towards hair cell regeneration in humans.

AREAS COVERED

Advances in the development of intracochlear drug delivery systems are reviewed, including passive systems, active microfluidic technologies and cochlear prosthesis-mediated delivery. This article provides a description of novel delivery systems and their potential future clinical applications in treating inner ear disease.

EXPERT OPINION

Recent progresses in microfluidics and miniaturization technologies are enabling the development of wearable and ultimately implantable drug delivery microsystems. Progress in this field is being spurred by the convergence of advances in molecular biology, microfluidic flow control systems and models for drug transport in the inner ear. These advances will herald a new generation of devices, with near-term applications in preclinical models, and ultimately with human clinical use for a range of diseases of the inner ear.

Deafness induction in mice

HYPOTHESIS

How to induce most efficiently severe sensorineural hearing loss in mice using a single coadministration of an aminoglycoside antibiotic and a loop diuretic?

BACKGROUND

The coadministration of aminoglycosides and a loop diuretic has been widely used to induce hair cell and spiral ganglion cell loss in guinea pigs. However, the development of new treatment strategies against sensorineural hearing loss, such as tissue engineering techniques, requires the use of mouse models. Previous attempts to induce hearing loss in mice have rendered inconsistent results because of resistance to aminoglycoside-induced ototoxicity. Especially inner hair cells seem to be resistant to aminoglycoside-induced ototoxicity.

METHODS

In the present study, we aim to optimize hearing loss in mice, using a single high-dose kanamycin (700 and 1,000 mg/kg) injection followed by a furosemide (100 mg/kg) administration. Although previous studies used intraperitoneal furosemide injections 30 minutes after kanamycin administration, we used intravenous furosemide injections administered within 5 minutes after kanamycin treatment.

RESULTS

Auditory brain stem responses illustrated severe threshold shifts, and histologic analysis showed marked outer hair cell destruction as well as spiral ganglion cell loss. The present protocol results in more severe inner hair cell loss when compared with the results of previous researches.

CONCLUSION

We conclude that severe sensorineural hearing loss can be induced in mice. Moreover, we found that this mouse model can be augmented via the use of rapid intravenous furosemide administrations to maximize inner hair cell loss.

Chronic neurotrophin delivery promotes ectopic neurite growth from the spiral ganglion of deafened cochleae without compromising the spatial selectivity of cochlear implants

Cochlear implants restore hearing cues in the severe-profoundly deaf by electrically stimulating spiral ganglion neurons (SGNs). However, SGNs degenerate following loss of cochlear hair cells, due at least in part to a reduction in the endogenous neurotrophin (NT) supply, normally provided by hair cells and supporting cells of the organ of Corti. Delivering exogenous NTs to the cochlea can rescue SGNs from degeneration and can also promote the ectopic growth of SGN neurites. This resprouting may disrupt the cochleotopic organization upon which cochlear implants rely to impart pitch cues. Using retrograde labeling and confocal imaging of SGNs, we determined the extent of neurite growth following 28 days of exogenous NT treatment in deafened guinea pigs with and without chronic electrical stimulation (ES). On completion of this treatment, we measured the spread of neural activation to intracochlear ES by recording neural responses across the cochleotopically organized inferior colliculus using multichannel recording techniques. Although NT treatment significantly increased both the length and the lateral extent of growth of neurites along the cochlea compared with deafened controls, these anatomical changes did not affect the spread of neural activation when examined immediately after 28 days of NT treatment. NT treatment did, however, result in lower excitation thresholds compared with deafened controls. These data support the application of NTs for improved clinical outcomes for cochlear implant patients.

Auditory-nerve responses to varied inter-phase gap and phase duration of the electric pulse stimulus as predictors for neuronal degeneration

After severe hair cell loss, secondary degeneration of spiral ganglion cells (SGCs) is observed-a gradual process that spans years in humans but only takes weeks in guinea pigs. Being the target for cochlear implants (CIs), the physiological state of the SGCs is important for the effectiveness of a CI. For assessment of the nerve's state, focus has generally been on its response threshold. Our goal was to add a more detailed characterization of SGC functionality. To this end, the electrically evoked compound action potential (eCAP) was recorded in normal-hearing guinea pigs and guinea pigs that were deafened 2 or 6 weeks prior to the experiments. We evaluated changes in eCAP characteristics when the phase duration (PD) and inter-phase gap (IPG) of a biphasic current pulse were varied. We correlated the magnitude of these changes to quantified histological measures of neurodegeneration (SGC packing density and SGC size). The maximum eCAP amplitude, derived from the input-output function, decreased after deafening, and increased with both PD and IPG. The eCAP threshold did not change after deafening, and decreased with increasing PD and IPG. The dynamic range was wider for the 6-weeks-deaf animals than for the other two groups. Excitability increased with IPG (steeper slope of the input-output function and lower stimulation level at the half-maximum eCAP amplitude), but to a lesser extent for the deafened animals than for normal-hearing controls. The latency was shorter for the 6-weeks-deaf animals than for the other two groups. For several of these eCAP characteristics, the effect size of IPG correlated well with histological measures of degeneration, whereas effect size of PD did not. These correlations depend on the use of high current levels, which could limit clinical application. Nevertheless, their potential of these correlations towards assessment of the condition of the auditory nerve may be of great benefit to clinical diagnostics and prognosis in cochlear implant recipients.

Connexin 26 null mice exhibit spiral ganglion degeneration that can be blocked by BDNF gene therapy

Mutations in the connexin 26 gene (GJB2) are the most common genetic cause of deafness, leading to congenital bilateral non-syndromic sensorineural hearing loss. Here we report the generation of a mouse model for a connexin 26 (Cx26) mutation, in which cre-Sox10 drives excision of the Cx26 gene from non-sensory cells flanking the auditory epithelium. We determined that these conditional knockout mice, designated Gjb2-CKO, have a severe hearing loss. Immunocytochemistry of the auditory epithelium confirmed absence of Cx26 in the non-sensory cells. Histology of the organ of Corti and the spiral ganglion neurons (SGNs) performed at ages 1, 3, or 6 months revealed that in Gjb2-CKO mice, the organ of Corti began to degenerate in the basal cochlear turn at an early stage, and the degeneration rapidly spread to the apex. In addition, the density of SGNs in Rosenthal's canal decreased rapidly along a gradient from the base of the cochlea to the apex, where some SGNs survived until at least 6 months of age. Surviving neurons often clustered together and formed clumps of cells in the canal. We then assessed the influence of brain derived neurotrophic factor (BDNF) gene therapy on the SGNs of Gjb2-CKO mice by inoculating Adenovirus with the BDNF gene insert (Ad.BDNF) into the base of the cochlea via the scala tympani or scala media. We determined that over-expression of BDNF beginning around 1 month of age resulted in a significant rescue of neurons in Rosenthal's canal of the cochlear basal turn but not in the middle or apical portions. This data may be used to design therapies for enhancing the SGN physiological status in all GJB2 patients and especially in a sub-group of GJB2 patients where the hearing loss progresses due to ongoing degeneration of the auditory nerve, thereby improving the outcome of cochlear implant therapy in these ears.

Spiral ganglion neuron quantification in the guinea pig cochlea using Confocal Laser Scanning Microscopy compared to embedding methods

Neuron counting in the cochlea is a crucial but time-consuming operation for which various methods have been developed. To improve simplicity and efficiency, we tested an imaging method of the cochlea, and based on Confocal Laser Scanning Microscopy (CLSM), we visualised Rosenthal's Canal and quantified the spiral ganglion neurons (SGN) within. Cochleae of 8 normal hearing guinea pigs and one implanted with a silicone filament were fixed in paraformaldehyde (PFA), decalcified, dehydrated and cleared in Spalteholz solution. Using the tissue's autofluorescence, CLSM was performed at 100 fold magnification generating z-series stacks of about 20 slices of the modiolus. In 5 midmodiolar slices per cochlea the perimeters of the Rosenthal's Canal were surveyed, representative neuron diameters were measured and the neurons first counted manually and then software-assisted. For comparison, 8 normal hearing guinea pig cochleae were embedded in paraffin and examined similarly. The CLSM method has the advantage that the cochleae remain intact as an organ and keep their geometrical structure. Z-stack creation is nearly fully-automatic and frequently repeatable with various objectives and step sizes and without visible bleaching. The tissue shows minimal or no shrinking artefacts and damage typical of embedding and sectioning. As a result, the cells in the cleared cochleae reach an average diameter of 21 μm and a density of about 18 cells/10,000 μm(2) with no significant difference between the manual and the automatical counts. Subsequently we compared the CLSM data with those generated using the established method of paraffin slides, where the SGN reached a mean density of 9.5 cells/10,000 μm(2) and a mean soma diameter of 13.6 μm. We were able to prove that the semi-automatic CLSM method is a simple and effective technique for auditory neuron count. It provides a high grade of tissue preservation and the automatic stack-generation as well as the counter software reduces the effort considerably. In addition this visualisation technique offers the potential to detect the position and orientation of cochlear implants (CI) within the cochlea and tissue growing in the scala tympani around the CI and at the position of the cochleostomy due to the fact that the implant does not have to be removed to perform histology as in case of the paraffin method.

Effects of brain-derived neurotrophic factor (BDNF) and electrical stimulation on survival and function of cochlear spiral ganglion neurons in deafened, developing cats

Both neurotrophic support and neural activity are required for normal postnatal development and survival of cochlear spiral ganglion (SG) neurons. Previous studies in neonatally deafened cats demonstrated that electrical stimulation (ES) from a cochlear implant can promote improved SG survival but does not completely prevent progressive neural degeneration. Neurotrophic agents combined with an implant may further improve neural survival. Short-term studies in rodents have shown that brain-derived neurotrophic factor (BDNF) promotes SG survival after deafness and may be additive to trophic effects of stimulation. Our recent study in neonatally deafened cats provided the first evidence of BDNF neurotrophic effects in the developing auditory system over a prolonged duration Leake et al. (J Comp Neurol 519:1526-1545, 2011). Ten weeks of intracochlear BDNF infusion starting at 4 weeks of age elicited significant improvement in SG survival and larger soma size compared to contralateral. In the present study, the same deafening and BDNF infusion procedures were combined with several months of ES from an implant. After combined BDNF + ES, a highly significant increase in SG numerical density (>50 % improvement re: contralateral) was observed, which was significantly greater than the neurotrophic effect seen with ES-only over comparable durations. Combined BDNF + ES also resulted in a higher density of myelinated radial nerve fibers within the osseous spiral lamina. However, substantial ectopic and disorganized sprouting of these fibers into the scala tympani also occurred, which may be deleterious to implant function. EABR thresholds improved (re: initial thresholds at time of implantation) on the chronically stimulated channels of the implant. Terminal electrophysiological studies recording in the inferior colliculus (IC) revealed that the basic cochleotopic organization was intact in the midbrain in all studied groups. In deafened controls or after ES-only, lower IC thresholds were correlated with more selective activation widths as expected, but no such correlation was seen after BDNF + ES due to much greater variability in both measures.

Filling the silent void: genetic therapies for hearing impairment

The inner ear cytoarchitecture forms one of the most intricate and delicate organs in the human body and is vulnerable to the effects of genetic disorders, aging, and environmental damage. Owing to the inability of the mammalian cochlea to regenerate sensory hair cells, the loss of hair cells is a leading cause of deafness in humans. Millions of individuals worldwide are affected by the emotionally and financially devastating effects of hearing impairment (HI). This paper provides a brief introduction into the key role of genes regulating inner ear development and function. Potential future therapies that leverage on an improved understanding of these molecular pathways are also described in detail.