Genetic polymorphisms in human CX3CR1-mediated macrophage dysregulation are associated with the worsening of hearing loss and cochlear degeneration after noise trauma: a study in a humanized mouse model

Sensorineural hearing loss (SNHL) is characterized by cochlear inflammation, macrophage activation, and degeneration of hair cells, synapses, and neurons. Macrophage-mediated inflammation in the damaged cochlea is regulated via CX3CR1-CX3CL1 signaling, where the fractalkine ligand CX3CL1 serves as a chemotactic and calming signal for macrophage activation. Furthermore, disrupted CX3CR1-CX3CL1 signaling in CX3CR1-KO and CX3CL1-KO mice leads to reduced macrophage numbers, exacerbated inflammation, and loss of hair cells, ribbon synapses, and neurons in the damaged cochlea. Notably, ~ 25% of the human population carries single-nucleotide polymorphisms (SNPs) in the CX3CR1 gene, CX3CR1^I249/M280, which results in a receptor with lower binding affinity for CX3CL1, while most individuals carry the common wild-type CX3CR1^V249/T280 allele. Although these polymorphisms are associated with various CNS neurodegenerative disorders, their impact on SNHL, cochlear degeneration and the macrophage response remains largely unknown. Here, we used a humanized mouse model expressing human CX3CR1 SNPs in lieu of its murine counterpart to investigate the effects of I249/M280 polymorphisms on cochlear function and structure following noise trauma. Young CX3CR1 WT, CX3CR1 KO, and human CX3CR1^I249/M280 mice of both sexes were exposed to a noise level of 93 decibel sound pressure for 2 h at an octave band (8-16 kHz). Cochlear function was assessed prior to exposure and at 1 day and 2 weeks postexposure. Also, the densities of inner and outer hair cells, ribbon synapses, and macrophages in Rosenthal's canal were examined after two weeks of exposure and compared among the three genotypes. We found that at 2 weeks postexposure, hearing thresholds were elevated and input‒output function was impaired in hCX3CR1^I249/M280 and CX3CR1 KO, whereas mice carrying WT alleles showed functional recovery. A significant synaptic loss (~ 30%) in hCX3CR1^I249/M280 and CX3CR1 KO mice was observed relative to that in WT, which exhibited synaptic repair. hCX3CR1^I249/M280 resulted in a ~ 17% loss of outer hair cells, which correlated with reduced otoacoustic emissions in the basal cochlear region. Noise led to increased macrophage numbers in the spiral ganglion and lateral wall of the WT; however, this response was attenuated in the CX3CR1 KO and hCX3CR1^I249/M280 strains. Additionally, macrophages from CX3CR1 KO and hCX3CR1^I249/M280 mice presented altered morphology, increased CD68 expression, and inflammation. Compared with those of mice carrying the CX3CR1 WT or KO allele, young hCX3CR1^I249/M280 mice fostered under ambient noise presented early elevations in hearing thresholds at basal frequencies. Together, these findings reveal that human CX3CR1 variant-mediated macrophage dysregulation strongly correlates with worsening of hearing loss and cochlear degeneration after noise trauma. Our work proposes a novel immune-related genetic polymorphism that may aid in the identification of individuals with increased vulnerability to SNHL.