Role of Neuropilin-1/Semaphorin-3A signaling in the functional and morphological integrity of the cochlea

<div><p><i>Neuropilin-1 (Nrp1)</i> encodes the transmembrane cellular receptor neuropilin-1, which is associated with cardiovascular and neuronal development and was within the peak SNP interval on chromosome 8 in our prior GWAS study on age-related hearing loss (ARHL) in mice. In this study, we generated and characterized an inner ear-specific <i>Nrp1</i> conditional knockout (CKO) mouse line because <i>Nrp1</i> constitutive knockouts are embryonic lethal. <i>In situ</i> hybridization demonstrated weak <i>Nrp1</i> mRNA expression late in embryonic cochlear development, but increased expression in early postnatal stages when cochlear hair cell innervation patterns have been shown to mature. At postnatal day 5, <i>Nrp1</i> CKO mice showed disorganized outer spiral bundles and enlarged microvessels of the stria vascularis (SV) but normal spiral ganglion cell (SGN) density and presynaptic ribbon body counts; however, we observed enlarged SV microvessels, reduced SGN density, and a reduction of presynaptic ribbons in the outer hair cell region of 4-month-old <i>Nrp1</i> CKO mice. In addition, we demonstrated elevated hearing thresholds of the 2-month-old and 4-month-old <i>Nrp1</i> CKO mice at frequencies ranging from 4 to 32kHz when compared to 2-month-old mice. These data suggest that conditional loss of <i>Nrp1</i> in the inner ear leads to progressive hearing loss in mice. We also demonstrated that mice with a truncated variant of <i>Nrp1</i> show cochlear axon guidance defects and that exogenous semaphorin-3A, a known neuropilin-1 receptor agonist, repels SGN axons <i>in vitro</i>. These data suggest that Neuropilin-1/Semaphorin-3A signaling may also serve a role in neuronal pathfinding in the developing cochlea. In summary, our results here support a model whereby Neuropilin-1/Semaphorin-3A signaling is critical for the functional and morphological integrity of the cochlea and that <i>Nrp1</i> may play a role in ARHL.</p>