Disruption of Hars2 in Cochlear Hair Cells Causes Progressive Mitochondrial Dysfunction and Hearing Loss in Mice

Pengcheng Xu,Hongchao Liu,Qingyue Yuan,Longhao Wang,Jun Xu,Xiuhong Pang,Hu Peng,Huihui Liu,Tao Yang,Hao Wu,Yun Lin

doi:10.3389/fncel.2021.804345

Abstract

Mutations in a number of genes encoding mitochondrial aminoacyl-tRNA synthetases lead to non-syndromic and/or syndromic sensorineural hearing loss in humans, while their cellular and physiological pathology in cochlea has rarely been investigated in vivo. In this study, we showed that histidyl-tRNA synthetase HARS2, whose deficiency is associated with Perrault syndrome 2 (PRLTS2), is robustly expressed in postnatal mouse cochlea including the outer and inner hair cells. Targeted knockout of Hars2 in mouse hair cells resulted in delayed onset (P30), rapidly progressive hearing loss similar to the PRLTS2 hearing phenotype. Significant hair cell loss was observed starting from P45 following elevated reactive oxygen species (ROS) level and activated mitochondrial apoptotic pathway. Despite of normal ribbon synapse formation, whole-cell patch clamp of the inner hair cells revealed reduced calcium influx and compromised sustained synaptic exocytosis prior to the hair cell loss at P30, consistent with the decreased supra-threshold wave I amplitudes of the auditory brainstem response. Starting from P14, increasing proportion of morphologically abnormal mitochondria was observed by transmission electron microscope, exhibiting swelling, deformation, loss of cristae and emergence of large intrinsic vacuoles that are associated with mitochondrial dysfunction. Though the mitochondrial abnormalities are more prominent in inner hair cells, it is the outer hair cells suffering more severe cell loss. Taken together, our results suggest that conditional knockout of Hars2 in mouse cochlear hair cells leads to accumulating mitochondrial dysfunction and ROS stress, triggers progressive hearing loss highlighted by hair cell synaptopathy and apoptosis, and is differentially perceived by inner and outer hair cells.

Highlights

The Ca2+ efficiency triggering exocytosis, quantified as the ratio of capacitance measurement (Cm)/QCa, remain unaltered for both short and prolonged stimulations (Figures 6H,K). These results suggested that the Hars2 conditional knockout (CKO) mice have disrupted inner hair cells (IHCs) synaptic transmission due to reduced calcium influx, which likely contributes to the hearing loss at P30 and is consistent with the decreased supra-threshold amplitude and prolonged latency of the ABR wave I (Figures 2E,F)
We evaluated the proportion of morphologically abnormal mitochondria, defined by swelling, deformation, loss of cristae, and emergence of large intrinsic vacuoles, in IHCs (Figure 7) and outer hair cells (OHC) (Figure 8) of the Hars2 CKO mice by Transmission Electron Microscopy (TEM)
Our results suggested that Hars2 deficiency leads to mitochondrial dysfunction, reduced presynaptic Ca2+ influx current and compromised sustained exocytosis, which in combination likely contributes to the hearing loss of the Hars2 CKO mice prior to the hair cell loss

Summary

Introduction

Hearing loss is the most common sensory disorder affecting approximately 6.1% of the world population (Davis and Hoffman, 2019), which can be caused by excessive noise exposure (Guo L. et al, 2021), ototoxic drugs (He et al, 2017; Li et al, 2018; Liu et al, 2019c, 2021; Zhong et al, 2020), aging (Cheng et al, 2019; Zhou et al, 2020; He et al, 2021), genetic factors (Qian et al, 2020; Hars Disruption in Hair CellsCheng et al, 2021; Fu et al, 2021a; Lv et al, 2021; Zhang S. et al, 2021), and infections (Han et al, 2020; He et al, 2020; Zhang Y. et al, 2021). Autosomal recessive mutations in a number of genes encoding Aminoacyl-tRNA synthetases (ARSs), including HARS2 (Perrault syndrome 2), LARS2 (Perrault syndrome 4), NARS2 (DFNB94), IARS2 (Cataracts, growth hormone deficiency, sensory neuropathy, sensorineural hearing loss, and skeletal dysplasia), PARS2 (Developmental and epileptic encephalopathy 75 including deafness), and KARS (DFNB89) may lead to a variety of syndromic and non-syndromic hearing loss (Konovalova and Tyynismaa, 2013; Oprescu et al, 2017; Wang et al, 2020). Clinical features associated with mutations in mtARS-encoding genes typically include encephalopathy, leukodystrophy, cardiomyopathy, ovarian dysgenesis, and deafness (Figuccia et al, 2021)

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Conclusion