FK506-mediated prevention of noise-induced hearing loss is associated with attenuated ER stress responses.

Abstract

Noise-induced permanent hearing loss results primarily from the irreversible loss of cochlear sensory hair cells, which do not regenerate in adult mammals. Endoplasmic reticulum (ER) stress may play a major role in noise-induced outer hair cell (OHC) loss. We previously reported that FK506 prevents noise-induced hearing loss (NIHL) in adult CBA/J mice; however, the molecular mechanisms underlying this preventive effect remain incompletely understood. Here, we show that treatment with FK506 attenuates NIHL in adult FVB/NJ mice of both sexes and that this preventive effect is associated with suppression of noise-induced ER stress. Using a permanent threshold shift noise model that reliably induces OHC loss, exploratory RNA-seq analysis identified unfolded protein binding and protein processing among the top enriched pathways associated with FK506-mediated prevention, which guided subsequent validation studies focused on ER-stress-related markers. Noise exposure significantly increased BiP (Grp78) expression and decreased FKBP8 levels in cochlear homogenates within 1-3 h after exposure, whereas FK506 pretreatment prevented these changes. FK506 also blocked CHOP upregulation in basal- turn OHCs, where degeneration typically begins after acoustic trauma. Under physiological conditions, FKBP8 was expressed in sensory hair cells, spiral ganglion neurons, and the stria vascularis and showed predominant colocalization with the ER marker KDEL in OHCs. In HEI-OC1 cells, FK506 similarly prevented thapsigargin-induced increases in BiP and CHOP and the associated decrease in FKBP8 levels, further supporting the association between FK506 treatment and modulation of ER stress. Together, these findings indicate that FK506-mediated prevention of NIHL is associated with suppression of ER stress and preservation of OHC integrity and suggest that FKBP8 may contribute to FK506-associated regulation of ER homeostasis in the cochlea. However, the present data support an association rather than a direct causal role of ER-stress suppression in FK506-mediated prevention of NIHL.