Noise induces Ca2+ signaling waves and Chop/S-Xbp1 expression in the hearing cochlea
Yesai Park, Jiang Li, Noura Ismail Mohamad, Ian R. Matthews, Peu Santra, Elliott H. Sherr, Dylan K. Chan
Yesai Park, Jiang Li, Noura Ismail Mohamad, Ian R. Matthews, Peu Santra, Elliott H. Sherr, Dylan K. Chan
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Research Article Otology

Noise induces Ca2+ signaling waves and Chop/S-Xbp1 expression in the hearing cochlea

Abstract

Exposure to loud noise is a common cause of acquired hearing loss. Disruption of subcellular calcium homeostasis and downstream stress pathways in the endoplasmic reticulum and mitochondria, including the unfolded protein response (UPR), have been implicated in the pathophysiology of noise-induced hearing loss. However, studies on the association between calcium homeostasis and stress pathways have been limited due to limited ability to measure calcium dynamics in mature-hearing, noise-exposed mice. We used a genetically encoded calcium indicator mouse model in which GCaMP6f is expressed specifically in hair cells or supporting cells under control of Myo15Cre or Sox2Cre, respectively. We performed live calcium imaging and UPR gene expression analysis in 8-week-old mice exposed to levels of noise that cause cochlear synaptopathy (98 db sound pressure level [SPL]) or permanent hearing loss (106 dB SPL). UPR activation occurred immediately after noise exposure, and the pattern of UPR activation was dependent on noise level, with the proapoptotic pathway upregulated only after 106 dB noise exposure. Spontaneous calcium transients in hair cells and intercellular calcium waves in supporting cells, which are present in neonatal cochleae, were quiescent in mature-hearing cochleae but reactivated upon noise exposure. Noise exposure of 106 dB was associated with more persistent and expansive intercellular Ca2+ signaling wave activity. These findings demonstrate a strong and dose-dependent association between noise exposure, UPR activation, and changes in calcium homeostasis in hair cells and supporting cells, suggesting that targeting these pathways may be effective to develop treatments for noise-induced hearing loss.

Authors

Yesai Park, Jiang Li, Noura Ismail Mohamad, Ian R. Matthews, Peu Santra, Elliott H. Sherr, Dylan K. Chan

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Figure 3

Ca2+ activity in neonatal cochlear supporting cells.

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Ca2+ activity in neonatal cochlear supporting cells. (A) Live imaging of...
(A) Live imaging of Sox2Cre-GCaMP6f neonatal cochlea. In supporting cells of the inner sulcus (IS) and outer sulcus (OS) of the neonatal cochlea, spontaneous intercellular Ca2+ waves are observed. Time interval between successive images is 1 second. Scale bar: 20 μm. Representative video also shown in Supplemental Video 1. (B) Change in number of Ca2+ peaks per second after drug treatment. Compared with baseline (media only), no change in frequency of Ca2+ peaks is seen with vehicle (VEH, black) or 1 μM thapsigargin (TG, red); 5 μM vanadate (VN, blue) induced increased Ca2+ peak activity. (C–E) Ca2+ decay time with drug treatment. Fluorescence levels (C) and peak decay time at the individual peak (D), or cochlea (E) level demonstrate significant prolongation of return to baseline Ca2+ levels in the presence of TG or VN. In C, peak height is normalized to maximum amplitude for each peak. (F) Change in number of ICS waves per second after drug treatment. No change in frequency of ICS waves is seen with VEH or TG; VN induced more ICS activity. (F and G) ICS wave propagation distance with drug treatment. ICS waves traveled significantly farther in the presence of TG, but not VN, when compared at the individual wave (G) or cochlea (H) level. (I and J) Change in steady-state Ca2+ after drug treatment. (I and J) Fluorescence levels and mean amplitude over a 300-second recording period, representing steady-state Ca2+ level across the entire cochlea, increased after TG and VN, but not VEH application. (B, E, F, H, and J) Data are shown as mean ± SEM, with individual cochlea-level values as dots. Groups were compared with 1-way ANOVA with Dunnett’s test for multiple comparisons against control for each condition, with P values as indicated. Sample sizes tested enabled detection of effect size > 1.98× SD with 80% power. (D and G) Tukey plots (box: first quartile/median/third quartile; whiskers: 10th and 90th percentile; dots: individual points outside the whiskers) representing all peaks (D) or waves (G) measured under the indicated conditions. P values are as indicated for pairwise comparisons versus control (VEH) on 2-tailed unpaired Student’s t test. AU, arbitrary units.