Impaired Development of the Medial Olivocochlear System in a KCNQ4-Deficient Mouse Model

The medial olivocochlear (MOC) efferent system modulates outer hair cell (OHC) excitability and protects cochlea from overstimulation. Cholinergic activation of α9α10 nicotinic acetylcholine receptors (nAChRs) triggers Ca⁺² influx, activating BK and SK2 Ca⁺²-dependent K⁺ channels, and K⁺ extrusion through KCNQ4 to restore membrane potential. KCNQ4-loss causes chronic depolarization, OHC dysfunction, and hearing loss. Here, we investigated how KCNQ4 deficiency affects cochlear efferent synapse development and organization. Using confocal immunofluorescence, we analyzed efferent innervation in the organ of Corti of Kcnq4^-/- (KO) and Kcnq4^+/+ (WT) mice at 2, 3, 4, and 10 postnatal weeks (W). At 2 W, efferent terminals were similarly distributed between basal and lateral OHC membrane domains in both genotypes. During maturation, WT mice exhibited complete relocation of MOC terminals to the basal domain, whereas KO mice showed delayed maturation, with some terminals laterally displaced up to 10 W. KCNQ4 absence was associated with reduced number and volume of synaptic vesicles per efferent boutons on OHCs. Milder morphometric alterations were observed in efferent boutons within the inner hair cell region. At the molecular level, qPCR revealed downregulation of α10 nAChR subunit, BK, and SK2 transcripts in KO at 4 W, with recovery to 10 W. Despite this recovery, BK protein showed reduced expression, mislocalization, and disorganized synaptic plaques in OHCs. KO also displayed age-dependent upregulation of the calcium-binding proteins calbindin and calretinin, suggesting compensatory responses to altered Ca⁺² homeostasis. Together, these findings demonstrate that KCNQ4 is essential for OHC repolarization, maturation and maintenance of cochlear efferent synapses.