TEJBEER KAUR, Ph.D. - US grants

? DESCRIPTION (provided by applicant): Noise exposures that cause both permanent threshold shift (with accompanying hair cell loss) and temporary threshold shifts (no evident hair cell loss) also result in rapid and permanent loss of synaptic elements and cochlear nerve terminals. Such injury also leads to degeneration of spiral ganglion (SG) cell bodies, but this occurs over a period of months to years. Neuronal survival is a key determinant of the success of cochlear implants, so it is of great interest to understand the mechanisms that promote neuronal survival after cochlear insults. We have recently discovered that hair cell loss is sufficient to recruit macrophages into the spiral ganglion, and that disruption of signaling between macrophages and afferent neurons (by genetic deletion of fractalkine receptor CX3CR1), leads to reduced macrophage recruitment into the spiral ganglion, and also results in diminished survival of afferent neurons. Here we propose to investigate the role of fractalkine signaling after noise induced hearing loss. We hypothesize that: 1) fractalkine regulates macrophage recruitment into the noise-damaged cochlea, and 2) fractalkine promotes the survival of afferent synapses and neurons after noise injury. Aim 1 will examine the role of fractalkine signaling in neuropathy caused by permanent threshold shift. Specific experiments will assess the effects of genetic disruption of fractalkine signaling on macrophage infiltration, spiral ganglion cell pathology, and auditory function over short (weeks) and long (months) survival periods. Aim 2 will test the hypothesis that macrophages also serve a critical role after noise exposure that causes temporary threshold shift (TTS). We will characterize any migration of macrophages towards inner hair cell-afferent nerve fiber synapse, and determine whether disruption of fractalkine signaling can influence the severity of, or recovery from, noise-induced cochlear synaptopathy and neuropathy. For both aims, we will monitor changes in cochlear function via ABRs, and cochleae will be collected for histological analysis of macrophages, hair cells and afferent neurons, as well as inner hair cell- cochlear nerve terminal synapse number and morphology.

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?.