Brain-derived neurotrophic factor and neurotrophin 3 mRNAs in the peripheral target fields of developing inner ear ganglia

Function of Brain-Derived Neurotrophic Factor in the Vestibular-Cochlear System

Brain-derived neurotrophic factor (BDNF) is essential for the development and functioning of the vestibular system. BDNF promotes the growth, differentiation, and synaptic plasticity of vestibular neurons, ensuring their normal operation and maintenance. According to research, BDNF is pivotal during vestibular compensation, aiding in the recovery of neuron function by remodeling the spontaneous resting potentials of damaged vestibular neurons. Additionally, BDNF exhibits dose-dependent and age-dependent characteristics during vestibular system development, with its deficiencies leading to the degeneration of vestibular neurons. BDNF dynamically interacts with other neurotrophic factors, such as fibroblast growth factor-2 (FGF-2) and glial cell line-derived neurotrophic factor (GDNF), synergistically enhancing neuron survival and functionality. This review outline the function of BDNF in the vestibulocochlear system and explores its potential therapeutic applications, offering fresh perspectives and guidance for future research and treatment of vestibulocochlear system disorders.

Oral Administration of TrkB Agonist, 7, 8-Dihydroxyflavone Regenerates Hair Cells and Restores Function after Gentamicin-Induced Vestibular Injury in Guinea Pig

The causes of vestibular dysfunction include the loss of hair cells (HCs), synapses beneath the HCs, and nerve fibers. 7, 8-dihydroxyflavone (DHF) mimics the physiological functions of brain-derived neurotrophic factor. We investigated the effects of the orally-administered DHF in the guinea pig crista ampullaris after gentamicin (GM)-induced injury. Twenty animals treated with GM received daily administration of DHF or saline for 14 or 28 days (DHF (+) or DHF (-) group; N = 5, each). At 14 days after GM treatment, almost all of the HCs had disappeared in both groups. At 28 days, the HCs number in DHF (+) and DHF (-) groups was 74% and 49%, respectively, compared to GM-untreated control. In the ampullary nerves, neurofilament 200 positive rate in the DHF (+) group was 91% at 28 days, which was significantly higher than 42% in DHF (-). On day 28, the synaptic connections observed between C-terminal-binding protein 2-positive and postsynaptic density protein-95-positive puncta were restored, and caloric response was significantly improved in DHF (+) group (canal paresis: 57.4% in DHF (+) and 100% in DHF (-)). Taken together, the oral administration of DHF may be a novel therapeutic approach for treating vestibular dysfunction in humans.

Extracellular Vesicles in Inner Ear Therapies-Pathophysiological, Manufacturing, and Clinical Considerations

(1) Background: Sensorineural hearing loss is a common and debilitating condition. To date, comprehensive pharmacologic interventions are not available. The complex and diverse molecular pathology that underlies hearing loss may limit our ability to intervene with small molecules. The current review foccusses on the potential for the use of extracellular vesicles in neurotology. (2) Methods: Narrative literature review. (3) Results: Extracellular vesicles provide an opportunity to modulate a wide range of pathologic and physiologic pathways and can be manufactured under GMP conditions allowing for their application in the human inner ear. The role of inflammation in hearing loss with a focus on cochlear implantation is shown. How extracellular vesicles may provide a therapeutic option for complex inflammatory disorders of the inner ear is discussed. Additionally, manufacturing and regulatory issues that need to be addressed to develop EVs as advanced therapy medicinal product for use in the inner ear are outlined. (4) Conclusion: Given the complexities of inner ear injury, novel therapeutics such as extracellular vesicles could provide a means to modulate inflammation, stress pathways and apoptosis in the inner ear.

Therapeutic Application of Mesenchymal Stem Cells for Cochlear Regeneration

Hearing loss is one of the major worldwide health problems that seriously affects human social and cognitive development. In the auditory system, three components outer ear, middle ear and inner ear are essential for the hearing mechanism. In the inner ear, sensory hair cells and ganglion neuronal cells are the essential supporters for hearing mechanism. Damage to these cells can be caused by long-term exposure of excessive noise, ototoxic drugs (aminoglycosides), ear tumors, infections, heredity and aging. Since mammalian cochlear hair cells do not regenerate naturally, some therapeutic interventions may be required to replace the damaged or lost cells. Cochlear implants and hearing aids are the temporary solutions for people suffering from severe hearing loss. The current discoveries in gene therapy may provide a deeper understanding in essential genes for the inner ear regeneration. Stem cell migration, survival and differentiation to supporting cells, cochlear hair cells and spiral ganglion neurons are the important foundation in understanding stem cell therapy. Moreover, mesenchymal stem cells (MSCs) from different sources (bone marrow, umbilical cord, adipose tissue and placenta) could be used in inner ear therapy. Transplanted MSCs in the inner ear can recruit homing factors at the damaged sites to induce transdifferentiation into inner hair cells and ganglion neurons or regeneration of sensory hair cells, thus enhancing the cochlear function. This review summarizes the potential application of mesenchymal stem cells in hearing restoration and combining stem cell and molecular therapeutic strategies can also be used in the recovery of cochlear function.

Effects of Neurotrophin-3 on Intrinsic Neuronal Properties at a Central Auditory Structure

Neurotrophins, a class of growth factor proteins that control neuronal proliferation, morphology, and apoptosis, are found ubiquitously throughout the nervous system. One particular neurotrophin (NT-3) and its cognate tyrosine receptor kinase (TrkC) have recently received attention as a possible therapeutic target for synaptopathic sensorineural hearing loss. Additionally, research shows that NT-3-TrkC signaling plays a role in establishing the sensory organization of frequency topology (ie, tonotopic order) in the cochlea of the peripheral inner ear. However, the neurotrophic effects of NT-3 on central auditory properties are unclear. In this study we examined whether NT-3-TrkC signaling affects the intrinsic electrophysiological properties at a first-order central auditory structure in chicken, known as nucleus magnocellularis (NM). Here, the expression pattern of specific neurotrophins is well known and tightly regulated. By using whole-cell patch-clamp electrophysiology, we show that NT-3 application to brainstem slices does not affect intrinsic properties of high-frequency neuronal regions but had robust effects for low-frequency neurons, altering voltage-dependent potassium functions, action potential repolarization kinetics, and passive membrane properties. We suggest that NT-3 may contribute to the precise establishment and organization of tonotopy in the central auditory pathway by playing a specialized role in regulating the development of intrinsic neuronal properties of low-frequency NM neurons.

In silico analysis of inner ear development using public whole embryonic body single-cell RNA-sequencing data

The inner ear comprises four epithelial domains: the cochlea, vestibule, semicircular canals, and endolymphatic duct/sac. These structures are segregated at embryonic day 13.5 (E13.5). However, these four anatomical structures remain undefined at E10.5. Here, we aimed to identify lineage-specific genes in the early developing inner ear using published data obtained from single-cell RNA-sequencing (scRNA-seq) of embryonic mice. We downloaded 5000 single-cell transcriptome data, named 'auditory epithelial trajectory', from the Mouse Organogenesis Cell Atlas. The dataset was supposed to include otic epithelial cells at E9.5-13.5. We projected the 5000 ​cells onto a two-dimensional space encoding the transcriptional state and visualised the pattern of otic epithelial cell differentiation. We identified 15 clusters, which were annotated as one of the four components of the inner ear epithelium using known genes that characterise the four different tissues. Additionally, we classified 15 clusters into sub-regions of the four inner ear components. By comparing transcriptomes between these 15 clusters, we identified several candidates of lineage-specific genes. Characterising these new candidate genes will help future studies about inner ear development.

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.

Detection of BDNF-Related Proteins in Human Perilymph in Patients With Hearing Loss

The outcome of cochlear implantation depends on multiple variables including the underlying health of the cochlea. Brain derived neurotrophic factor (BDNF) has been shown to support spiral ganglion neurons and to improve implant function in animal models. Whether endogenous BDNF or BDNF-regulated proteins can be used as biomarkers to predict cochlear health and implant outcome has not been investigated yet. Gene expression of BDNF and downstream signaling molecules were identified in tissue of human cochleae obtained from normal hearing patients (n = 3) during skull base surgeries. Based on the gene expression data, bioinformatic analysis was utilized to predict the regulation of proteins by BDNF. The presence of proteins corresponding to these genes was investigated in perilymph (n = 41) obtained from hearing-impaired patients (n = 38) during cochlear implantation or skull base surgery for removal of vestibular schwannoma by nanoscale liquid chromatography coupled to tandem mass spectrometry (nano LC-MS/MS). Analyzed by mass spectrometry were 41 perilymph samples despite three patients undergoing bilateral cochlear implantation. These particular BDNF regulated proteins were not detectable in any of the perilymph samples. Subsequently, targeted analysis of the perilymph proteome data with Ingenuity Pathway Analysis (IPA) identified further proteins in human perilymph that could be regulated by BDNF. These BDNF regulated proteins were correlated to the presence of residual hearing (RH) prior to implantation and to the performance data with the cochlear implant after 1 year. There was overall a decreased level of expression of BDNF-regulated proteins in profoundly hearing-impaired patients compared to patients with some RH. Phospholipid transfer protein was positively correlated to the preoperative hearing level of the patients. Our data show that combination of gene expression arrays and bioinformatic analysis can aid in the prediction of downstream signaling proteins related to the BDNF pathway. Proteomic analysis of perilymph may help to identify the presence or absence of these molecules in the diseased organ. The impact of such prediction algorithms on diagnosis and treatment needs to be established in further studies.

Mapping developmental maturation of inner hair cell ribbon synapses in the apical mouse cochlea

Ribbon synapses of cochlear inner hair cells (IHCs) undergo molecular assembly and extensive functional and structural maturation before hearing onset. Here, we characterized the nanostructure of IHC synapses from late prenatal mouse embryo stages (embryonic days 14-18) into adulthood [postnatal day (P)48] using electron microscopy and tomography as well as optical nanoscopy of apical turn organs of Corti. We find that synaptic ribbon precursors arrive at presynaptic active zones (AZs) after afferent contacts have been established. These ribbon precursors contain the proteins RIBEYE and piccolino, tether synaptic vesicles and their delivery likely involves active, microtubule-based transport pathways. Synaptic contacts undergo a maturational transformation from multiple small to one single, large AZ. This maturation is characterized by the fusion of ribbon precursors with membrane-anchored ribbons that also appear to fuse with each other. Such fusion events are most frequently encountered around P12 and hence, coincide with hearing onset in mice. Thus, these events likely underlie the morphological and functional maturation of the AZ. Moreover, the postsynaptic densities appear to undergo a similar refinement alongside presynaptic maturation. Blockwise addition of ribbon material by fusion as found during AZ maturation might represent a general mechanism for modulating ribbon size.

Biological therapies in otology

Hearing loss is present in millions of people worldwide. Current treatment for patients with severe to profound hearing loss consists of cochlear implantation. Providing the cochlear nerve is intact, patients generally benefit greatly from this intervention, frequently achieving significant improvements in speech comprehension. There are, however, some cases where current technology does not provide patients with adequate benefit. Ongoing research in cell transplantation and gene therapy promises to lead to new developments that will improve the function of cochlear implants. Translation of these experimental approaches is presently at an early stage. This review focuses on the application of biological therapies in severe hearing loss and discusses some of the barriers to translating basic scientific research into clinical reality. We emphasize the application of these novel therapies to cochlear implantation.

Toward Cochlear Therapies

Sensorineural hearing impairment is the most common sensory disorder and a major health and socio-economic issue in industrialized countries. It is primarily due to the degeneration of mechanosensory hair cells and spiral ganglion neurons in the cochlea via complex pathophysiological mechanisms. These occur following acute and/or chronic exposure to harmful extrinsic (e.g., ototoxic drugs, noise...) and intrinsic (e.g., aging, genetic) causative factors. No clinical therapies currently exist to rescue the dying sensorineural cells or regenerate these cells once lost. Recent studies have, however, provided renewed hope, with insights into the therapeutic targets allowing the prevention and treatment of ototoxic drug- and noise-induced, age-related hearing loss as well as cochlear cell degeneration. Moreover, genetic routes involving the replacement or corrective editing of mutant sequences or defected genes are showing promise, as are cell-replacement therapies to repair damaged cells for the future restoration of hearing in deaf people. This review begins by recapitulating our current understanding of the molecular pathways that underlie cochlear sensorineural damage, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. It then guides the reader through to the recent discoveries in pharmacological, gene and cell therapy research towards hearing protection and restoration as well as their potential clinical application.

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

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

A RNAscope whole mount approach that can be combined with immunofluorescence to quantify differential distribution of mRNA

RNAscope® technology provided by Advanced Cell Diagnostics (ACD) allows the detection and evaluation of coinciding mRNA expression profiles in the same or adjacent cells in unprecedented quantitative detail using multicolor fluorescent in situ hybridization (FISH). While already extensively used in thinly sectioned material of various pathological tissues and, to a lesser extent, in some whole mounts, we provide here a detailed approach to use the fluorescent RNAscope method in the mouse inner ear and thick brain sections by modifying and adapting existing techniques of whole mount fluorescent in situ hybridization (WH-FISH). We show that RNAscope WH-FISH can be used to quantify local variation in overlaying mRNA expression intensity, such as neurotrophin receptors along the length of the mouse cochlea. We also show how RNAscope WH-FISH can be combined with immunofluorescence (IF) of some epitopes that remain after proteinase digestion and, to some extent, with fluorescent protein markers such as tdTomato. Our WH-FISH technique provides an approach to detect cell-specific quantitative differences in developing and mature adjacent cells, an emerging issue revealed by improved cellular expression profiling. Further, the presented technique may be useful in validating single-cell RNAseq data on expression profiles in a range of tissue known or suspected to have locally variable mRNA expression levels.

Effect of hyperbaric oxygen on BDNF-release and neuroprotection: Investigations with human mesenchymal stem cells and genetically modified NIH3T3 fibroblasts as putative cell therapeutics

Hyperbaric oxygen therapy (HBOT) is a noninvasive widely applied treatment that increases the oxygen pressure in tissues. In cochlear implant (CI) research, intracochlear application of neurotrophic factors (NTFs) is able to improve survival of spiral ganglion neurons (SGN) after deafness. Cell-based delivery of NTFs such as brain-derived neurotrophic factor (BDNF) may be realized by cell-coating of the surface of the CI electrode. Human mesenchymal stem cells (MSC) secrete a variety of different neurotrophic factors and may be used for the development of a biohybrid electrode in order to release endogenously-derived neuroprotective factors for the protection of residual SGN and for a guided outgrowth of dendrites in the direction of the CI electrode. HBOT could be used to influence cell behaviour after transplantation to the inner ear. The aim of this study was to investigate the effect of HBOT on the proliferation, BDNF-release and secretion of neuroprotective factors. Thus, model cells (an immortalized fibroblast cell line (NIH3T3)–native and genetically modified) and MSCs were repeatedly (3 x – 10 x) exposed to 100% oxygen at different pressures. The effects of HBO on cell proliferation were investigated in relation to normoxic and normobaric conditions (NOR). Moreover, the neuroprotective and neuroregenerative effects of HBO-treated cells were analysed by cultivation of SGN in conditioned medium. Both, the genetically modified NIH3T3/BDNF and native NIH3T3 fibroblasts, showed a highly significant increased proliferation after five days of HBOT in comparison to normoxic controls. By contrast, the number of MSCs was decreased in MSCs treated with 2.0 bar of HBO. Treating SGN cultures with supernatants of fibroblasts and MSCs significantly increased the survival rate of SGN. HBO treatment did not influence (increase / reduce) this effect. Secretome analysis showed that HBO treatment altered the protein expression pattern in MSCs.

Neurotrophins and Proneurotrophins: Focus on Synaptic Activity and Plasticity in the Brain

Neurotrophins have been intensively studied and have multiple roles in the brain. Neurotrophins are first synthetized as proneurotrophins and then cleaved intracellularly and extracellularly. Increasing evidences demonstrate that proneurotrophins and mature neurotrophins exerts opposing role in the central nervous system. In the present review, we explore the role of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT3), and neurotrophin 4 (NT4) and their respective proform in cellular processes related to learning and memory. We focused on their roles in synaptic activity and plasticity in the brain with an emphasis on long-term potentiation, long-term depression, and basal synaptic transmission in the hippocampus and the temporal lobe area. We also discuss new findings on the role of the Val66Met polymorphism on the BDNF propeptide on synaptic activity.

[Biological therapies in otology. German version]

Millions of people worldwide suffer from hearing loss. Current treatment for patients with severe to profound hearing loss consists of cochlear implants. Providing the cochlear nerve is intact, patients generally benefit enormously from this intervention, frequently achieving significant improvements in speech comprehension. There are, however, some cases where current technology does not provide patients with adequate benefit. New therapeutic concepts based on cell transplantation and gene therapy are developing rapidly, at least in the research sector. Compared to the wealth of basic research available in this area, translation of these new experimental approaches into clinical application is presently at a very early stage. The current review focuses on translatable treatment concepts and discusses the barriers that need to be overcome in order to translate basic scientific research into clinical reality. Furthermore, the first examples of clinical application of biological therapies in severe hearing loss are presented, particularly in connection with cochlear implants.

Quantitative Analysis of Supporting Cell Subtype Labeling Among CreER Lines in the Neonatal Mouse Cochlea

Four CreER lines that are commonly used in the auditory field to label cochlear supporting cells (SCs) are expressed in multiple SC subtypes, with some lines also showing reporter expression in hair cells (HCs). We hypothesized that altering the tamoxifen dose would modify CreER expression and target subsets of SCs. We also used two different reporter lines, ROSA26 ^tdTomato and CAG-eGFP, to achieve the same goal. Our results confirm previous reports that Sox2 ^CreERT2 and Fgfr3-iCreER ^T2 are not only expressed in neonatal SCs but also in HCs. Decreasing the tamoxifen dose did not reduce HC expression for Sox2 ^CreERT2 , but changing to the CAG-eGFP reporter decreased reporter-positive HCs sevenfold. However, there was also a significant decrease in the number of reporter-positive SCs. In contrast, there was a large reduction in reporter-positive HCs in Fgfr3-iCreER ^T2 mice with the lowest tamoxifen dose tested yet only limited reduction in SC labeling. The targeting of reporter expression to inner phalangeal and border cells was increased when Plp-CreER ^T2 was paired with the CAG-eGFP reporter; however, the total number of labeled cells decreased. Changes to the tamoxifen dose or reporter line with Prox1 ^CreERT2 caused minimal changes. Our data demonstrate that modifications to the tamoxifen dose or the use of different reporter lines may be successful in narrowing the numbers and/or types of cells labeled, but each CreER line responded differently. When the ROSA26 ^tdTomato reporter was combined with any of the four CreER lines, there was no difference in the number of tdTomato-positive cells after one or two injections of tamoxifen given at birth. Thus, tamoxifen-mediated toxicity could be reduced by only giving one injection. While the CAG-eGFP reporter consistently labeled fewer cells, both reporter lines are valuable depending on the goal of the study.

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.

Gradients of Neurotrophins, Ion Channels, and Tuning in the Cochlea

The mammalian cochlea is exquisitely designed to decompose complex sounds into their component frequencies, accounting in part for the superb auditory discrimination displayed by many species. To perform this task, numerous mechanical and electrical specializations are graded along the length of the cochlea that create a tonotopic map in which sounds of different frequencies produce maximal responses at different cochlear locations. Graded mechanical features include structural changes in the vibratory basilar membrane, on which the hair cell sensory receptors sit, to systematic changes in receptor cell size and stereociliary length. Furthermore, there is growing evidence that frequency specificity does not stop at mechanical and morphological elements in the cochlea, but also extends to the intrinsic electrical profile of the hair cell sensory receptors and the first neural element in the auditory system—the spiral ganglion neurons.

Firing frequency and entrainment maintained in primary auditory neurons in the presence of combined BDNF and NT3

Primary auditory neurons rely on neurotrophic factors for development and survival. We previously determined that exposure to brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) alters the activity of hyperpolarization-activated currents (Ih) in this neuronal population. Since potassium channels are sensitive to neurotrophins, and changes in Ih are often accompanied by a shift in voltage-gated potassium currents (IK), this study examined IK with exposure to both BDNF and NT3 and the impact on firing entrainment during high frequency pulse trains. Whole-cell patch-clamp recordings revealed significant changes in action potential latency and duration, but no change in firing adaptation or total outward IK. Dendrotoxin-I (DTX-I), targeting voltage-gated potassium channel subunits KV1.1 and KV1.2, uncovered an increase in the contribution of DTX-I sensitive currents with exposure to neurotrophins. No difference in Phrixotoxin-1 (PaTX-1) sensitive currents, mediated by KV4.2 and KV4.3 subunits, was observed. Further, no difference was seen in firing entrainment. These results show that combined BDNF and NT3 exposure influences the contribution of KV1.1 and KV1.2 to the low voltage-activated potassium current (IKL). Whilst this is accompanied by a shift in spike latency and duration, both firing frequency and entrainment to high frequency pulse trains are preserved.

Round-window delivery of neurotrophin 3 regenerates cochlear synapses after acoustic overexposure

In acquired sensorineural hearing loss, such as that produced by noise or aging, there can be massive loss of the synaptic connections between cochlear sensory cells and primary sensory neurons, without loss of the sensory cells themselves. Because the cell bodies and central projections of these cochlear neurons survive for months to years, there is a long therapeutic window in which to re-establish functional connections and improve hearing ability. Here we show in noise-exposed mice that local delivery of neurotrophin-3 (NT-3) to the round window niche, 24 hours after an exposure that causes an immediate loss of up to 50% loss of synapses in the cochlear basal region, can regenerate pre- and post-synaptic elements at the hair cell / cochlear nerve interface. This synaptic regeneration, as documented by confocal microscopy of immunostained cochlear sensory epithelia, was coupled with a corresponding functional recovery, as seen in the suprathreshold amplitude of auditory brainstem response Wave 1. Cochlear delivery of neurotrophins in humans is likely achievable as an office procedure via transtympanic injection, making our results highly significant in a translational context.

Neurotrophins and cochlear implants: a solution to sensorineural deafness?

OBJECTIVES

To review current trends for treating sensorineural deafness by enhancing spiral ganglion neuron (SGN) survival using neurotrophins combined with cochlear implants and identify areas for future research and development.

METHODS

A literature search was undertaken on PubMed and Google scholar using terms: neurotrophins, cochlear implants (CIs), and sensorineural to identify the most recent and significant publications. The abstracts were read to identify relevant papers; these were accessed in full and analysed for this review.

RESULTS

Neurotrophins have a known role in cochlear development and the maintenance of SGNs. So far experiments using osmotic pumps to deliver neurotrophins have been successful for short-term enhanced survival of SGN's following aminoglycoside ototoxicity in animal models. They have demonstrated the re-sprouting of radial nerve fibres from SGN's towards the source of delivery. In addition electrical stimulation, gene and cell-based therapy have increased SGN survival to varying degrees.

DISCUSSION

Osmotic pumps carry a high risk of infection therefore CIs coated in a drug containing polymer or hydrogel are a realistic alternative for sustained delivery of neurotrophins. Increased SGN survival combined with neuronal re-growth raises the possibility for CIs to stimulate discrete SGN populations. Unfortunately, the duration of treatment needed for long-term survival still remains unclear and further work is needed. Nevertheless the combination of regenerative medicine to CI technology presents a novel approach to developing CI technology.

Sphingosine 1-phosphate signaling pathway in inner ear biology. New therapeutic strategies for hearing loss?

Hearing loss is one of the most prevalent conditions around the world, in particular among people over 60 years old. Thus, an increase of this affection is predicted as result of the aging process in our population. In this context, it is important to further explore the function of molecular targets involved in the biology of inner ear sensory cells to better individuate new candidates for therapeutic application. One of the main causes of deafness resides into the premature death of hair cells and auditory neurons. In this regard, neurotrophins and growth factors such as insulin like growth factor are known to be beneficial by favoring the survival of these cells. An elevated number of published data in the last 20 years have individuated sphingolipids not only as structural components of biological membranes but also as critical regulators of key biological processes, including cell survival. Ceramide, formed by catabolism of sphingomyelin (SM) and other complex sphingolipids, is a strong inducer of apoptotic pathway, whereas sphingosine 1-phosphate (S1P), generated by cleavage of ceramide to sphingosine and phosphorylation catalyzed by two distinct sphingosine kinase (SK) enzymes, stimulates cell survival. Interestingly S1P, by acting as intracellular mediator or as ligand of a family of five distinct S1P receptors (S1P₁–S1P₅), is a very powerful bioactive sphingolipid, capable of triggering also other diverse cellular responses such as cell migration, proliferation and differentiation, and is critically involved in the development and homeostasis of several organs and tissues. Although new interesting data have become available, the information on S1P pathway and other sphingolipids in the biology of the inner ear is limited. Nonetheless, there are several lines of evidence implicating these signaling molecules during neurogenesis in other cell populations. In this review, we discuss the role of S1P during inner ear development, also as guidance for future studies.

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.

Postnatal expression of neurotrophic factors accessible to spiral ganglion neurons in the auditory system of adult hearing and deafened rats

Spiral ganglion neurons (SGNs) receive input from cochlear hair cells and project from the cochlea to the cochlear nucleus. After destruction of hair cells with aminoglycoside antibiotics or noise, SGNs gradually die. It has been assumed that SGN death is attributable to loss of neurotrophic factors (NTFs) derived from hair cells or supporting cells in the organ of Corti (OC). We used quantitative PCR (qPCR) to assay NTF expression-neurotrophin-3 (NT-3), BDNF, GDNF, neurturin, artemin, and CNTF-in the OC and cochlear nucleus at various ages from postnatal day 0 (P0) to P90 in control hearing and neonatally deafened rats. NT-3, neurturin, and CNTF were most abundant in the postnatal hearing OC; CNTF and neurturin most abundant in the cochlear nucleus. In the OC, NT-3 and CNTF showed a postnatal increase in expression approximately concomitant with hearing onset. In rats deafened by daily kanamycin injections (from P8 to P16), surviving inner hair cells were evident at P16 but absent by P19, with most postsynaptic boutons lost before P16. NT-3 and CNTF, which normally increase postnatally, had significantly reduced expression in the OC of deafened rats, although CNTF was expressed throughout the time that SGNs were dying. In contrast, neurturin expression was constant, unaffected by deafening or by age. CNTF and neurturin expression in the cochlear nucleus was unaffected by deafening or age. Thus, NTFs other than NT-3 are available to SGNs even as they are dying after deafening, apparently conflicting with the hypothesis that SGN death is attributable to lack of NTFs.

Neurotrophin-3 regulates ribbon synapse density in the cochlea and induces synapse regeneration after acoustic trauma

Neurotrophin-3 (Ntf3) and brain derived neurotrophic factor (Bdnf) are critical for sensory neuron survival and establishment of neuronal projections to sensory epithelia in the embryonic inner ear, but their postnatal functions remain poorly understood. Using cell-specific inducible gene recombination in mice we found that, in the postnatal inner ear, Bbnf and Ntf3 are required for the formation and maintenance of hair cell ribbon synapses in the vestibular and cochlear epithelia, respectively. We also show that supporting cells in these epithelia are the key endogenous source of the neurotrophins. Using a new hair cell CreER(T) line with mosaic expression, we also found that Ntf3's effect on cochlear synaptogenesis is highly localized. Moreover, supporting cell-derived Ntf3, but not Bbnf, promoted recovery of cochlear function and ribbon synapse regeneration after acoustic trauma. These results indicate that glial-derived neurotrophins play critical roles in inner ear synapse density and synaptic regeneration after injury.

The function of BDNF in the adult auditory system

The inner ear of vertebrates is specialized to perceive sound, gravity and movements. Each of the specialized sensory organs within the cochlea (sound) and vestibular system (gravity, head movements) transmits information to specific areas of the brain. During development, brain-derived neurotrophic factor (BDNF) orchestrates the survival and outgrowth of afferent fibers connecting the vestibular organ and those regions in the cochlea that map information for low frequency sound to central auditory nuclei and higher-auditory centers. The role of BDNF in the mature inner ear is less understood. This is mainly due to the fact that constitutive BDNF mutant mice are postnatally lethal. Only in the last few years has the improved technology of performing conditional cell specific deletion of BDNF in vivo allowed the study of the function of BDNF in the mature developed organ. This review provides an overview of the current knowledge of the expression pattern and function of BDNF in the peripheral and central auditory system from just prior to the first auditory experience onwards. A special focus will be put on the differential mechanisms in which BDNF drives refinement of auditory circuitries during the onset of sensory experience and in the adult brain. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Inner ear supporting cells: rethinking the silent majority

Sensory epithelia of the inner ear contain two major cell types: hair cells and supporting cells. It has been clear for a long time that hair cells play critical roles in mechanoreception and synaptic transmission. In contrast, until recently the more abundant supporting cells were viewed as serving primarily structural and homeostatic functions. In this review, we discuss the growing information about the roles that supporting cells play in the development, function and maintenance of the inner ear, their activities in pathological states, their potential for hair cell regeneration, and the mechanisms underlying these processes.

Regenerated synapses between postnatal hair cells and auditory neurons

Regeneration of synaptic connections between hair cells and spiral ganglion neurons would be required to restore hearing after neural loss. Here we demonstrate by immunohistochemistry the appearance of afferent-like cochlear synapses in vitro after co-culture of de-afferented organ of Corti with spiral ganglion neurons from newborn mice. The glutamatergic synaptic complexes at the ribbon synapse of the inner hair cell contain markers for presynaptic ribbons and postsynaptic densities. We found postsynaptic density protein PSD-95 at the contacts between hair cells and spiral ganglion neurons in newly formed synapses in vitro. The postsynaptic proteins were directly facing the CtBP2-positive presynaptic ribbons of the hair cells. BDNF and NT-3 promoted afferent synaptogenesis in vitro. Direct juxtaposition of the postsynaptic densities with the components of the preexisting ribbon synapse indicated that growing fibers recognized components of the presynaptic sites. Initiation of cochlear synaptogenesis appeared to be influenced by glutamate release from the hair cell ribbons at the presynaptic site since the synaptic regeneration was impaired in glutamate vesicular transporter 3 mutant mice. These insights into cochlear synaptogenesis could be relevant to regenerative approaches for neural loss in the cochlea.

Challenges for stem cells to functionally repair the damaged auditory nerve

INTRODUCTION

In the auditory system, a specialized subset of sensory neurons are responsible for correctly relaying precise pitch and temporal cues to the brain. In individuals with severe-to-profound sensorineural hearing impairment these sensory auditory neurons can be directly stimulated by a cochlear implant, which restores sound input to the brainstem after the loss of hair cells. This neural prosthesis therefore depends on a residual population of functional neurons in order to function effectively.

AREAS COVERED

In severe cases of sensorineural hearing loss where the numbers of auditory neurons are significantly depleted, the benefits derived from a cochlear implant may be minimal. One way in which to restore function to the auditory nerve is to replace these lost neurons using differentiated stem cells, thus re-establishing the neural circuit required for cochlear implant function. Such a therapy relies on producing an appropriate population of electrophysiologically functional neurons from stem cells, and on these cells integrating and reconnecting in an appropriate manner in the deaf cochlea.

EXPERT OPINION

Here we review progress in the field to date, including some of the key functional features that stem cell-derived neurons would need to possess and how these might be enhanced using electrical stimulation from a cochlear implant.

Beyond generalized hair cells: molecular cues for hair cell types

Basic helix-loop-helix (bHLH) transcription factors (TFs) are crucial for inner ear neurosensory development. The proneural TF Atoh1 regulates the differentiation of hair cells (HCs) whereas Neurog1 and Neurod1 regulate specification and differentiation of neurons, respectively, but also affect HC development. Expression of Delta and Jagged ligands in nascent HCs and Notch receptors in supporting cells induce supporting cell differentiation through the regulation of neurogenic bHLH TFs (such as Hes1, Hes5) and suppression of limited Atoh1 expression. In sensorineural hearing loss, HCs are lost followed by supporting cells and progressive degeneration of neurons, at least in rodents. Regaining complete hearing may require reconstituting the organ of Corti from scratch, including the two types of HCs, inner and outer hair cells with the precise sorting of two types of afferent (type I and II) and efferent (lateral and medial olivo-cochlear) innervation. We review effects of bHLH TF dosage and their cross-regulation to differentiate HC types in the organ of Corti. We categorize findings of specific gene expressions in HCs: 1. as markers without meaning for the regeneration task, 2. as stabilizers who are needed to maintain or complete differentiation, and 3. as decision-making genes, expressed and acting early enough to be useful in this process. Only one TF has been characterized that fits the last aspect: Atoh1. We propose that temporal and intensity variations of Atoh1 are naturally modulated to differentiate specific types of HCs. Importantly, the molecular means to modify the Atoh1 expression are at least partially understood and can be readily implemented in the attempts to regenerate specific types of HCs.

Developmental changes in the responsiveness of rat spiral ganglion neurons to neurotrophic factors in dissociated culture: differential responses for survival, neuritogenesis and neuronal morphology

The way that the development of the inner ear innervation is regulated by various neurotrophic factors and/or their combinations at different postnatal developmental stages remains largely unclear. Moreover, survival and neuritogenesis in deafferented adult neurons is important for cochlear implant function. To address these issues, developmental changes in the responsiveness of postnatal rat spiral ganglion neurons (SGNs) to neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF) and leukemia inhibitory factor (LIF) were examined by using a dissociated cell culture system. SGNs at postnatal day (P) 0, P5 and P20 (young adult) were cultured with the addition of NT-3, BDNF, or LIF or of a combination of NT-3 and BDNF (N + B) or of NT-3, BDNF and LIF (ALL factors). SGNs were analyzed for three parameters: survival, longest neurite length (LNL) and neuronal morphology. At P0, SGNs required exposure to N + B or ALL factors for enhanced survival and the ALL factors combination showed a synergistic effect much greater than the sum of the individual factors. At P5, SGNs responded to a wider range of treatment conditions for enhanced survival and combinations showed only an additive improvement over individual factors. The survival percentage of untreated SGNs was highest at P20 but combinations of neurotrophic factors were no more effective than individual factors. LNL of each SGN was enhanced by LIF alone or ALL factors at P0 and P5 but was suppressed by NT-3, BDNF and N + B at P5 in a dose-dependent manner. The LNL at P20 was enhanced by ALL factors and suppressed by N + B. Treatment with ALL factors increased the proportion of SGNs that had two or more primary neurites in all age groups. These findings suggest that NT-3, BDNF, LIF and their combinations predominantly support different ontogenetic events at different developmental stages in the innervation of the inner ear.

BDNF gene therapy induces auditory nerve survival and fiber sprouting in deaf Pou4f3 mutant mice

Current therapy for patients with hereditary absence of cochlear hair cells, who have severe or profound deafness, is restricted to cochlear implantation, a procedure that requires survival of the auditory nerve. Mouse mutations that serve as models for genetic deafness can be utilized for developing and enhancing therapies for hereditary deafness. A mouse with Pou4f3 loss of function has no hair cells and a subsequent, progressive degeneration of auditory neurons. Here we tested the influence of neurotrophin gene therapy on auditory nerve survival and peripheral sprouting in Pou4f3 mouse ears. BDNF gene transfer enhanced preservation of auditory neurons compared to control ears, in which nearly all neurons degenerated. Surviving neurons in treated ears exhibited pronounced sprouting of nerve fibers into the auditory epithelium, despite the absence of hair cells. This enhanced nerve survival and regenerative sprouting may improve the outcome of cochlear implant therapy in patients with hereditary deafness.

The use of neurotrophin therapy in the inner ear to augment cochlear implantation outcomes

Severe to profound deafness is most often secondary to a loss of or injury to cochlear mechanosensory cells, and there is often an associated loss of the peripheral auditory neural structures, specifically the spiral ganglion neurons and peripheral auditory fibers. Cochlear implantation is currently our best hearing rehabilitation strategy for severe to profound deafness. These implants work by directly electrically stimulating the remnant auditory neural structures within the deafened cochlea. When administered to the deafened cochlea in animal models, neurotrophins, specifically brain derived neurotrophic factor and neurotrophin-3, have been shown to dramatically improve spiral ganglion neuron survival and stimulate peripheral auditory fiber regrowth. In animal models, neurotrophins administered in combination with cochlear implantation has resulted in significant improvements in the electrophysiological and psychophysical measures of outcome. While further research must be done before these therapies can be applied clinically, neurotrophin therapies for the inner ear show great promise in enhancing CI outcomes and the treatment of hearing loss.

Combined application of brain-derived neurotrophic factor and neurotrophin-3 and its impact on spiral ganglion neuron firing properties and hyperpolarization-activated currents

Neurotrophins provide an effective tool for the rescue and regeneration of spiral ganglion neurons (SGNs) following sensorineural hearing loss. However, these nerve growth factors are also potent modulators of ion channel activity and expression, and in the peripheral auditory system brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) have previously been shown to alter the firing properties of auditory neurons and differentially regulate the expression of some potassium channels in vitro. In this study we examined the activity of the hyperpolarization-mediated mixed-cation current (I(h)) in early post-natal cultured rat SGNs following exposure to combined BDNF and NT3. Whole-cell patch-clamp recordings made after 1 or 2 days in vitro revealed no change in the firing adaptation of neurons in the presence of BDNF and NT3. Resting membrane potentials were also maintained, but spike latency and firing threshold was subject to regulation by both neurotrophins and time in vitro. Current clamp recordings revealed an activity profile consistent with activation of the hyperpolarization-activated current. Rapid membrane hyperpolarization was followed by a voltage- and time-dependent depolarizing voltage sag. In voltage clamp, membrane hyperpolarization evoked a slowly-activating inward current that was reversibly blocked with cesium and inhibited by ZD7288. The amplitude and current density of I(h) was significantly larger in BDNF and NT3 supplemented cultures, but this did not translate to a significant alteration in voltage sag magnitude. Neurotrophins provided at 50 ng/ml produced a hyperpolarizing shift in the voltage-dependence and slower time course of I(h) activation compared to SGNs in control groups or cultured with 10 ng/ml BDNF and NT3. Our results indicate that combined BDNF and NT3 increase the activity of hyperpolarization-activated currents and that the voltage-dependence and activation kinetics of I(h) in SGNs are sensitive to changes in neurotrophin concentration. In addition, BDNF and NT3 applied together induce a decrease in firing threshold, but does not generate a shift in firing adaptation.

Ras/p38 and PI3K/Akt but not Mek/Erk signaling mediate BDNF-induced neurite formation on neonatal cochlear spiral ganglion explants

Neurotrophins participate in regulating the survival, differentiation, and target innervation of many neurons, mediated by high-affinity Trk and low-affinity p75 receptors. In the cochlea, spiral ganglion (SG) neuron survival is strongly dependent upon neurotrophic input, including brain-derived neurotrophic factor (BDNF), which increases the number of neurite outgrowth in neonatal rat SG in vitro. Less is known about signal transduction pathways linking the activation of neurotrophin receptors to SG neuron nuclei. In particular, the p38 and cJUN Kinase (JNK), mitogen-activated protein kinase (MAPK) pathways, which participate in JNK signaling in other neurons, have not been studied. We found that inhibition of Ras, p38, phosphatidyl inositol 3 kinase (PI3K) or Akt signaling reduced or eliminated BDNF mediated increase in number of neurite outgrowth, while inhibition of Mek/Erk had no influence. Inhibition of Rac/cdc42, which lies upstream of JNK, modestly enhanced BDNF induced formation of neurites. Western blotting implicated p38 and Akt signaling, but not Mek/Erk. The results suggest that the Ras/p38 and PI3K/Akt are the primary pathways by which BDNF promotes its effects. Activation of Rac/cdc42/JNK signaling by BDNF may reduce the formation of neurites. This is in contrast to our previous results on NT-3, in which Mek/Erk signaling was the primary mediator of SG neurite outgrowth in vitro. Our data on BDNF agree with prior results from others that have implicated PI3K/Akt involvement in mediating the effects of BDNF on SG neurons in vitro, including neuronal survival and neurite extension. However, the identification of p38 and JNK involvement is entirely novel. The results suggest that neurotrophins can exert opposing effects on SG neurons, the balance of competing signals influencing the generation of neurites. This competition could provide a potential mechanism for the control of neurite number during development.

The spiral ganglion: connecting the peripheral and central auditory systems

In mammals, the initial bridge between the physical world of sound and perception of that sound is established by neurons of the spiral ganglion. The cell bodies of these neurons give rise to peripheral processes that contact acoustic receptors in the organ of Corti, and the central processes collect together to form the auditory nerve that projects into the brain. In order to better understand hearing at this initial stage, we need to know the following about spiral ganglion neurons: (1) their cell biology including cytoplasmic, cytoskeletal, and membrane properties, (2) their peripheral and central connections including synaptic structure; (3) the nature of their neural signaling; and (4) their capacity for plasticity and rehabilitation. In this report, we will update the progress on these topics and indicate important issues still awaiting resolution.

Structure and development of cochlear afferent innervation in mammals

In mammals, sensorineural deafness results from damage to the auditory receptors of the inner ear, the nerve pathways to the brain or the cortical area that receives sound information. In this review, we first focused on the cellular and molecular events taking part to spiral ganglion axon growth, extension to the organ of Corti, and refinement. In the second half, we considered the functional maturation of synaptic contacts between sensory hair cells and their afferent projections. A better understanding of all these processes could open insights into novel therapeutic strategies aimed to re-establish primary connections from sound transducers to the ascending auditory nerve pathways.

The molecular basis of making spiral ganglion neurons and connecting them to hair cells of the organ of Corti

The bipolar spiral ganglion neurons apparently delaminate from the growing cochlear duct and migrate to Rosenthal's canal. They project radial fibers to innervate the organ of Corti (type I neurons to inner hair cells, type II neurons to outer hair cells) and also project tonotopically to the cochlear nuclei. The early differentiation of these neurons requires transcription factors to regulate migration, pathfinding and survival. Neurog1 null mice lack formation of neurons. Neurod1 null mice show massive neuronal death combined with aberrant central and peripheral projections. Prox1 protein is necessary for proper type II neuron process navigation, which is also affected by the neurotrophins Bdnf and Ntf3. Neurotrophin null mutants show specific patterns of neuronal loss along the cochlea but remaining neurons compensate by expanding their target area. All neurotrophin mutants have reduced radial fiber growth proportional to the degree of loss of neurotrophin alleles. This suggests a simple dose response effect of neurotrophin concentration. Keeping overall concentration constant, but misexpressing one neurotrophin under regulatory control of another one results in exuberant fiber growth not only of vestibular fibers to the cochlea but also of spiral ganglion neurons to outer hair cells suggesting different effectiveness of neurotrophins for spiral ganglion neurite growth. Finally, we report here for the first time that losing all neurons in double null mutants affects extension of the cochlear duct and leads to formation of extra rows of outer hair cells in the apex, possibly by disrupting the interaction of the spiral ganglion with the elongating cochlea.

Complex primary afferents: What the distribution of electrophysiologically-relevant phenotypes within the spiral ganglion tells us about peripheral neural coding

Spiral ganglion neurons are the first neural element of the auditory system. They receive precise synaptic signals which represent features of sound stimuli encoded by hair cell receptors and they deliver a digital representation of this information to the central nervous system. It is well known that spiral ganglion neurons are selectively responsive to specific sound frequencies, and that numerous structural and physiological specializations in the inner ear increase the quality of this tuning, beyond what could be accomplished by the passive properties of the basilar membrane. Further, consistent with what we know about other sensory systems, it is becoming clear that the parallel divergent innervation pattern of type I spiral ganglion neurons has the potential to encode additional features of sound stimuli. To date, we understand the most about the sub-modalities of frequency and intensity coding in the peripheral auditory system. Work reviewed herein will address the issue of how intrinsic electrophysiological features of the neurons themselves have the potential to contribute to the precision of coding and transmitting information about these two parameters to higher auditory centers for further processing.