Mitochondrial Calcium Transporters Mediate Sensitivity to Noise-Induced Losses of Hair Cells and Cochlear Synapses.

Xianren Wang,Song Pan,Su-Hua Sha,Kayla Hill,Hu Yuan,Ruosha Lai,Qiaojun Fang,Haishan Long,Yuanping Zhu,Hao Xiong

doi:10.3389/fnmol.2018.00469

Abstract

Mitochondria modulate cellular calcium homeostasis by the combined action of the mitochondrial calcium uniporter (MCU), a selective calcium entry channel, and the sodium calcium exchanger (NCLX), which extrudes calcium from mitochondria. In this study, we investigated MCU and NCLX in noise-induced hearing loss (NIHL) using adult CBA/J mice and noise-induced alterations of inner hair cell (IHC) synapses in MCU knockout mice. Following noise exposure, immunoreactivity of MCU increased in cochlear sensory hair cells of the basal turn, while immunoreactivity of NCLX decreased in a time- and exposure-dependent manner. Inhibition of MCU activity via MCU siRNA pretreatment or the specific pharmacological inhibitor Ru360 attenuated noise-induced loss of sensory hair cells and synaptic ribbons, wave I amplitudes, and NIHL in CBA/J mice. This protection was afforded, at least in part, through reduced cleavage of caspase 9 (CC9). Furthermore, MCU knockout mice on a hybrid genetic CD1 and C57/B6 background showed resistance to noise-induced seizures compared to wild-type littermates. Owing to the CD1 background, MCU knockouts and littermates suffer genetic high frequency hearing loss, but their IHCs remain intact. Noise-induced loss of IHC synaptic connections and reduction of auditory brainstem response (ABR) wave I amplitude were recovered in MCU knockout mice. These results suggest that cellular calcium influx during noise exposure leads to mitochondrial calcium overload via MCU and NCLX. Mitochondrial calcium overload, in turn, initiates cell death pathways and subsequent loss of hair cells and synaptic connections, resulting in NIHL.

Highlights

Dysfunctional buffering of calcium ions in mitochondria or the cytosol is associated with pathological conditions (Williams et al, 2013)
Immunolabeling for mitochondrial calcium uniporter (MCU) on cochlear surface preparations revealed 25% reduction of MCU in outer hair cell (OHC) 72 h after 0.6-μg siRNA delivery compared to untreated controls (Supplementary Figures S1A,B)
The salient finding of this study is that inhibition of MCU via pretreatment with MCU siRNA or the selective MCU inhibitor Ru360 reduced permanent threshold shift (PTS)-noise-induced losses of inner hair cell (IHC) synaptic ribbons, OHCs, and wave I amplitudes, as well as noise-induced hearing loss (NIHL) in adult CBA/J mice

Summary

Introduction

Dysfunctional buffering of calcium ions in mitochondria or the cytosol is associated with pathological conditions (Williams et al, 2013). Dysregulation of cytosolic calcium homeostasis appears to contribute to noise-induced hearing loss (NIHL) This notion is supported by a noise-dependent elevation of calcium levels in sensory hair cells (Maurer et al, 1993; Fridberger et al, 1998; Oliver et al, 2001) and the fact that calcium channel blockers protect from NIHL (Maurer et al, 1993; Fridberger et al, 1998; Heinrich et al, 1999; Oliver et al, 2001; Minami et al, 2004; Shen et al, 2007; Zuo et al, 2008). NCLX has been shown to be involved in neuronal death in a model of Parkinson disease (Gandhi et al, 2009; Palty et al, 2012)

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