Manickam, V. N.; Maity, S.; Murali, S. V.; Gawande, D. Y.; Stothert, A. R.; Batalkina, L. V.; Cardona, A.; Kaur, T.

Efficacy of chemokine fractalkine isoforms was evaluated for restoration of loss of inner hair cell ribbon synapses and hearing after noise-induced cochlear synaptopathy (NICS). Previously, we have demonstrated a critical role for fractalkine signaling axis (CX3CL1-CX3CR1) in synaptic repair where in the presence of fractalkine receptor (CX3CR1) expressed by cochlear macrophages, the damaged synapses are spontaneously repaired. Here, we examined whether overexpression of fractalkine ligand (CX3CL1 or FKN) in the form of a peptide is effective in restoring the lost synapses and hearing after NICS. Remarkably, single transtympanic (TT) injection of soluble isoform of FKN (sFKN) peptide at 1 day after synaptopathic noise trauma showed significant recovery of ABR thresholds, ABR peak I amplitudes and ribbon synapses in both FKN-wildtype and knockout mice when compared to mice injected with full length membrane-bound FKN peptide (mFKN). Mechanistically, sFKN peptide treatment increased macrophage numbers in the cochlea and in the absence of those macrophages, sFKN failed to restore loss of synapses and hearing after NICS. Furthermore, sFKN treatment attenuated cochlear inflammation after noise overexposure without altering the expression of CX3CR1. Finally, sFKN peptide was detectable inside the cochlea localized to the sensory epithelium for 24 hours after TT injection. These data provide a robust proof-of-principle that local delivery of an immune factor, sFKN is effective in restoring lost ribbon synapses and hearing after NICS in a macrophage-dependent manner and highlights the potential of sFKN as an immunotherapy for cochlear synaptopathy due to noise or aging.