Contribution of Macrophages and Fractalkine Towards Degeneration and Repair of Cochlear Synapses

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PROJECT SUMMARY/ABSTRACT Noise trauma can primarily damage the synaptic connections between the inner hair cells and the peripheral axons of the spiral ganglion neurons. Noise-induced synaptopathy is attributed to glutamate excitotoxicity and leads to gradual axonal degeneration and ultimately death of the spiral ganglion neurons. The consequences of loss of synapses and neurons include auditory perceptual dysfunctions leading to difficulty in speech recognition and listening in noisy environments. This type of auditory dysfunction is known as “hidden hearing loss” because it is not readily diagnosed through standard hearing tests. Moreover, absence of spiral ganglion neurons limits the performance of primary therapies for hearing loss such as cochlear implants and future hair cell regeneration strategies. Currently, there are no approved drugs that promote neuron survival or elicit regeneration of lost auditory nerves and replenish their synaptic connections with surviving hair cells. Therefore, it is of great interest to understand the mechanisms for synaptic and neuron degeneration and regeneration for the development of better ototherapeutics. We recently demonstrated that synaptopathic noise trauma is sufficient to recruit macrophages (innate-immune cells) towards the damaged inner hair cell-synaptic region. While the damaged synapses can undergo spontaneous repair however, disruption of fractalkine signaling (by genetic deletion of fractalkine (FKN) receptor CX3CR1 on macrophages) impairs such spontaneous synaptic repair and increases spiral ganglion neuron loss after trauma. These data imply that intact fractalkine signaling is necessary for synaptic repair and neuron survival in the damaged cochlea. Here, we propose to investigate the effect of activation of fractalkine signaling on prevention and repair of loss of synapses and neuron survival following cochlear trauma. Aim 1 will determine whether FKN treatment repairs damaged synapses after noise trauma or excitotoxic insult in mammalian mouse cochlea. Specifically, FKN peptide will be injected either (transtympanically) after synaptopathic noise trauma in vivo or after glutamate- induced excitotoxicity in cochlear explants. The precise contribution of FKN membrane or soluble isoforms towards synaptic repair will be examined. Aim 2 will determine whether FKN treatment reduces degeneration of synapses following noise trauma or glutamate excitotoxicity. We will treat with FKN membrane or soluble isoforms prior to glutamate treatment in ex vivo cochlear explants or prior to noise trauma in vivo (transtympanically). In Aim 3, we will eliminate cochlear macrophages and examine the influence of this intervention on the degree of synaptic degeneration and repair after synaptopathic noise trauma. For each aim, auditory function along with morphometric analyses of hair cell, macrophage, synapse and spiral ganglion neuron counts will be performed. Together, the study design will aid in investigating the effect of macrophages and fractalkine treatment on cochlear synapse degeneration and repair and hearing restoration and may lead to identification of novel fractalkine-based therapeutics for “hidden-hearing loss”.