Changes in responsiveness of rat spiral ganglion neurons to neurotrophins across age: differential regulation of survival and neuritogenesis

Closing the Gap between the Auditory Nerve and Cochlear Implant Electrodes: Which Neurotrophin Cocktail Performs Best for Axonal Outgrowth and Is Electrical Stimulation Beneficial?

Neurotrophins promote neurite outgrowth of auditory neurons and may help closing the gap to cochlear implant (CI) electrodes to enhance electrical hearing. The best concentrations and mix of neurotrophins for this nerve regrowth are unknown. Whether electrical stimulation (ES) during outgrowth is beneficial or may direct axons is another open question. Auditory neuron explant cultures of distinct cochlear turns of 6-7 days old mice were cultured for four days. We tested different concentrations and combinations of BDNF and NT-3 and quantified the numbers and lengths of neurites with an advanced automated analysis. A custom-made 24-well electrical stimulator based on two bulk CIs served to test different ES strategies. Quantification of receptors trkB, trkC, p75^NTR, and histological analysis helped to analyze effects. We found 25 ng/mL BDNF to perform best, especially in basal neurons, a negative influence of NT-3 in combined BDNF/NT-3 scenarios, and tonotopic changes in trk and p75^NTR receptor stainings. ES largely impeded neurite outgrowth and glia ensheathment in an amplitude-dependent way. Apical neurons showed slight benefits in neurite numbers and length with ES at 10 and 500 µA. We recommend BDNF as a potent drug to enhance the man-machine interface, but CIs should be better activated after nerve regrowth.

A neurotrophic approach to treating hearing loss: Translation from animal models to clinical proof-of-concept

Currently, there are no approved medicines available for the treatment of hearing loss. However, research over the past two decades has contributed to a growing understanding of the pathological mechanisms in the cochlea that result in hearing difficulties. The concept that a loss of the synapses connecting inner hair cells with the auditory nerve (cochlear synaptopathy) contributes to hearing loss has gained considerable attention. Both animal and human post-mortem studies support the idea that these synapses (ribbon synapses) are highly vulnerable to noise, ototoxicity, and the aging process. Their degeneration has been suggested as an important factor in the speech-in-noise difficulties commonly experienced by those suffering with hearing loss. Neurotrophins such as brain derived neurotrophic factor (BDNF) have the potential to restore these synapses and provide improved hearing function. OTO-413 is a sustained exposure formulation of BDNF suitable for intratympanic administration that in preclinical models has shown the ability to restore ribbon synapses and provide functional hearing benefit. A phase 1/2 clinical trial with OTO-413 has provided initial proof-of-concept for improved speech-in-noise hearing performance in subjects with hearing loss. Key considerations for the design of this clinical study, including aspects of the speech-in-noise assessments, are discussed.

Simultaneous treatment with pentoxifylline does not adversely affect the neurotrophic effects of brain-derived neurotrophic factor on spiral ganglion neurons

The hemorheologic drug pentoxifylline is applied for the treatment of sudden sensorineural hearing loss and tinnitus to improve cochlear microcirculation. Recent studies also suggest protective and trophic effects on neuronal cells. Because the preservation of sensorineural structures of the inner ear is fundamental for normal hearing and hearing restoration with auditory prostheses, pentoxifylline and neurotrophic factors such as brain-derived neurotrophic factor (BDNF) are promising candidates to treat degenerative disorders of the inner ear. We used an in-vitro model to determine the neurotrophic effects of these factors on spiral ganglion cells from postnatal rats. Pentoxifylline, alone and in combination with BDNF, was added at various concentrations to the cultured cells. Cells were immunolabeled and analyzed to determine neuronal survival, neurite length, neuronal branching and morphology. Pentoxifylline did not significantly increase or decrease neuronal survival, neurite length and neuronal branching compared to control cultures. Analysis of cellular morphology showed that diverse neuronal subtypes developed in the presence of pentoxifylline. Our data revealed that pentoxifylline did not interfere with the robust neurotrophic effects of BDNF on spiral ganglion neurons when cultured cells were treated with pentoxifylline and BDNF simultaneously. The results of our study do not suggest major neurotrophic effects of pentoxifylline on cultured spiral ganglion neurons. Because pentoxifylline has no detrimental effects on spiral ganglion neurons and does not reduce the effects of BDNF, both agents could be combined to treat diseases of the inner ear provided that future in vivo experiments and clinical studies support these findings.

Age-Dependency of Neurite Outgrowth in Postnatal Mouse Cochlear Spiral Ganglion Explants

Background: The spatial gap between cochlear implants (CIs) and the auditory nerve limits frequency selectivity as large populations of spiral ganglion neurons (SGNs) are electrically stimulated synchronously. To improve CI performance, a possible strategy is to promote neurite outgrowth toward the CI, thereby allowing a discrete stimulation of small SGN subpopulations. Brain-derived neurotrophic factor (BDNF) is effective to stimulate neurite outgrowth from SGNs. Method: TrkB (tropomyosin receptor kinase B) agonists, BDNF, and five known small-molecule BDNF mimetics were tested for their efficacy in stimulating neurite outgrowth in postnatal SGN explants. To modulate Trk receptor-mediated effects, TrkB and TrkC ligands were scavenged by an excess of recombinant receptor proteins. The pan-Trk inhibitor K252a was used to block Trk receptor actions. Results: THF (7,8,3′-trihydroxyflavone) partly reproduced the BDNF effect in postnatal day 7 (P7) mouse cochlear spiral ganglion explants (SGEs), but failed to show effectiveness in P4 SGEs. During the same postnatal period, spontaneous and BDNF-stimulated neurite outgrowth increased. The increased neurite outgrowth in P7 SGEs was not caused by the TrkB/TrkC ligands, BDNF and neurotrophin-3 (NT-3). Conclusions: The age-dependency of induction of neurite outgrowth in SGEs was very likely dependent on presently unidentified factors and/or molecular mechanisms which may also be decisive for the age-dependent efficacy of the small-molecule TrkB receptor agonist THF.

BDNF, NT-3 and Trk receptor agonist monoclonal antibodies promote neuron survival, neurite extension, and synapse restoration in rat cochlea ex vivo models relevant for hidden hearing loss

Neurotrophins and their mimetics are potential treatments for hearing disorders because of their trophic effects on spiral ganglion neurons (SGNs) whose connections to hair cells may be compromised in many forms of hearing loss. Studies in noise or ototoxin-exposed animals have shown that local delivery of NT-3 or BDNF has beneficial effects on SGNs and hearing. We evaluated several TrkB or TrkC monoclonal antibody agonists and small molecules, along with BDNF and NT-3, in rat cochlea ex vivo models. The TrkB agonists BDNF and a monoclonal antibody, M3, had the greatest effects on SGN survival, neurite outgrowth and branching. In organotypic cochlear explants, BDNF and M3 enhanced synapse formation between SGNs and inner hair cells and restored these connections after excitotoxin-induced synaptopathy. Loss of these synapses has recently been implicated in hidden hearing loss, a condition characterized by difficulty hearing speech in the presence of background noise. The unique profile of M3 revealed here warrants further investigation, and the broad activity profile of BDNF observed underpins its continued development as a hearing loss therapeutic.

Myelin-induced inhibition in a spiral ganglion organ culture - Approaching a natural environment in vitro

The performance of a cochlear implant depends on the defined interaction between afferent neurons of the spiral ganglion and the inserted electrode. Neurite outgrowth can be induced by neurotrophins such as brain-derived neurotrophic factor (BDNF) via tropomyosin kinase receptor B (TrkB). However, neurotrophin signaling through the p75 neurotrophin receptor (p75) inhibits neurite outgrowth in the presence of myelin. Organotypic cultures derived from postnatal (P3-5) mice were used to study myelin-induced inhibition in the cochlear spiral ganglion. Neurite outgrowth was analyzed and quantified utilizing an adapted Sholl analysis. Stimulation of neurite outgrowth was quantified after application of BDNF, the selective TrkB agonist 7,8-dihydroxyflavone (7,8-DHF) and a selective inhibitor of the Rho-associated kinase (Y27632), which inhibits the p75 pathway. Myelin-induced inhibition was assessed by application of myelin-associated glycoprotein (MAG-Fc) to stimulate the inhibitory p75 pathway. Inhibition of neurite outgrowth was achieved by the selective TrkB inhibitor K252a. Stimulation of neurite outgrowth was observed after treatment with BDNF, 7,8 DHF and a combination of BDNF and Y27632. The 7,8-DHF-induced growth effects could be inhibited by K252a. Furthermore, inhibition of neurite outgrowth was observed after supplementation with MAG-Fc. Myelin-induced inhibition could be overcome by 7,8-DHF and the combination of BDNF and Y27632. In this study, myelin-induced inhibition of neurite outgrowth was established in a spiral ganglion model. We reveal that 7,8-DHF is a viable novel compound for the stimulation of neurite outgrowth in a myelin-induced inhibitory environment. The combination of TrkB stimulation and ROCK inhibition can be used to overcome myelin inhibition.

Novel High Content Screen Detects Compounds That Promote Neurite Regeneration from Cochlear Spiral Ganglion Neurons

The bipolar spiral ganglion neurons (SGN) carry sound information from cochlear hair cells to the brain. After noise, antibiotic or toxic insult to the cochlea, damage to SGN and/or hair cells causes hearing impairment. Damage ranges from fiber and synapse degeneration to dysfunction and loss of cells. New interventions to regenerate peripheral nerve fibers could help reestablish transfer of auditory information from surviving or regenerated hair cells or improve results from cochlear implants, but the biochemical mechanisms to target are largely unknown. Presently, no drugs exist that are FDA approved to stimulate the regeneration of SGN nerve fibers. We designed an original phenotypic assay to screen 440 compounds of the NIH Clinical Collection directly on dissociated mouse spiral ganglia. The assay detected one compound, cerivastatin, that increased the length of regenerating neurites. The effect, mimicked by other statins at different optimal concentrations, was blocked by geranylgeraniol. These results demonstrate the utility of screening small compound libraries on mixed cultures of dissociated primary ganglia. The success of this screen narrows down a moderately sized library to a single compound which can be elevated to in-depth in vivo studies, and highlights a potential new molecular pathway for targeting of hearing loss drugs.

Neural cell adhesion molecule NrCAM is expressed in the mammalian inner ear and modulates spiral ganglion neurite outgrowth in an in vitro alternate choice assay

Neuron-glial-related cell adhesion molecule (NrCAM) is a neuronal cell adhesion molecule involved in neuron-neuron and neuron-glial adhesion as well as directional signaling during axonal cone growth. NrCAM has been shown to be involved in several cellular processes in the central and peripheral nervous systems, including neurite outgrowth, axonal pathfinding and myelination, fasciculation of nerve fibers, and cell migration. This includes sensory systems such as the eye and olfactory system. However, there are no reports on the expression/function of NrCAM in the auditory system. The aim of the present study was to elucidate the occurrence of NrCAM in the mammalian cochlea and its role in innervation of the auditory end organ. Our work indicates that NrCAM is highly expressed in the developing mammalian cochlea (position consistent with innervation). Moreover, we found that NrCAM, presented in stripe micropatterns, provide directional cues to neonatal rat inner ear spiral ganglion neurites in vitro. Our results are consistent with a role for NrCAM in the pathfinding of spiral ganglion dendrites toward their hair cell targets in the sensory epithelium.

Neural cell adhesion molecule L1 modulates type I but not type II inner ear spiral ganglion neurite outgrowth in an in vitro alternate choice assay

L1, a neural cell adhesion molecule of the immunoglobulin superfamily, is widely expressed in the nervous system and important in axonal outgrowth, guidance, synapse formation, and signaling. Gene deletion studies emphasize the significance of L1 during development of the central nervous system and L1 is crucial for the topographic targeting of retinal axons. In contrast to the brain and retina, the role of L1 in the inner ear is largely unknown. While previous studies have localized L1 in the developing inner ear of the chicken and mouse, its function during the innervation of the cochlea still remains largely unclear. We therefore investigated the functional role of L1 in the mammalian inner ear. Our aim was to determine whether or not L1 can modulate type I and/or type II spiral ganglion neuron outgrowth using an in vitro alternate choice assay. We found that L1, presented in stripe micropatterns, provide directional cues to neonatal rodent type I but not type II inner ear spiral ganglion neurites. The results suggest that L1 may play a role in axonal pathfinding of type I spiral ganglion dendrites toward their inner hair cell targets but not of type II toward the outer hair cells.