Molecular alterations caused by chronic cochlear depolarization in a mouse model of hearing loss

Abstract

The voltage-gated potassium (K+) channel KCNQ4 is the main re- sponsible for the extrusion of the K+ that enters the cochlear sensory cells upon sound stimulation. Besides, outer hair cells (OHC) excit- ability is under control of the efferent neurons of the Medial Olivo- cochlear (MOC) system. In response to overstimulation, MOC cho- linergic neurons activate the calcium-induced K+ channels BK and SK2, which extrude K+ out of the cell repolarizing the membrane. In- tracellular accumulation of K+ leads to a chronic depolarization that may damage hair cells causing hearing loss (HL). KCNQ4 activity impairment is the main cause of DFNA2, a non-syndromic progres- sive HL. Using a mouse model lacking Kcnq4 (Kcnq4-/-), we report- ed that OHC death begins at the basal turn progressing to the apex in 3-6-week-old (W) animals. We hypothesized that the KCNQ4 ab- sence causes MOC chronic overstimulation leading to activation of death pathways. Using immunofluorescence (IF), we evaluated the MOC terminals and observed a lower synaptic density and mislo- calization of the efferent terminals contacting OHC in 4W Kcnq4-/- mice. In addition, we analyzed by qPCR the gene expression of the efferent components located in the MOC terminals. We detected a ~3.5-fold decrease in the mRNA expression of the nicotinic recep- tor α10 subunit with no changes in the α9 subunit, and a ~8-fold decrease in the mRNA expression of BK and SK2 in 4W Kcnq4-/- animals. Finally, we studied the possible pathways involved in OHC death. By IF, we found an increase of cleaved-caspase 3 expression in the OHC at the basal turn and gene expression analysis by qPCR revealed that the pro-apoptotic Bax transcript was upregulated while anti-apoptotic Bcl2 was downregulated in Kcnq4-/- mice. These re- sults demonstrate an alteration of the efferent transmission in OHC that could contribute to the activation of the apoptotic pathway driv- ing to OHC death.