Early downregulation of hair cell (HC)-specific genes in the vestibular sensory epithelium during chronic ototoxicity

Abstract

Exposure of mammals to ototoxic compounds causes hair cell (HC) loss in the vestibular sensory epithelia of the inner ear. In chronic exposure models, this loss often occurs by extrusion of the HC from the sensory epithelium towards the luminal cavity. HC extrusion is preceded by several steps that begin with detachment and synaptic uncoupling of the cells from the afferent terminals of their postsynaptic vestibular ganglion neurons. The purpose of this study was to identify gene expression mechanisms that drive these responses to chronic ototoxic stress. We conducted four RNA-seq experiments that generated five comparisons of control versus treated animals. These involved two species (rat and mouse), two compounds (streptomycin and 3,3-iminodipropionitrile, IDPN), and three time points in our rat/IDPN model. We compared differentially expressed genes and their associated Gene Ontology terms, and several genes of interest were validated by in-situ hybridisation and immunofluorescence analyses. Common and model-unique expression responses were identified. The earliest and most robust common response was downregulation of HC-specific genes, including stereocilium (Atp2b2, Xirp2), synaptic (Nsg2), and ion channel genes (Kcnab1, Kcna10), together with new potential biomarkers of HC stress (Vsig10l2). A second common response across species and compounds was the upregulation of the stress mediator Atf3. Model- or time-restricted responses included downregulation of cell-cell adhesion and mitochondrial ATP synthesis genes, and upregulation of the interferon response, unfolded protein response, and tRNA aminoacylation genes. The present results provide key information on the responses of the vestibular sensory epithelium to chronic ototoxic stress, potentially relevant to other types of chronic stress.