Ba2+ currents in inner and outer hair cells of mice lacking the voltage-dependent Ca2+ channel subunits β3 or β4

Abstract

Voltage-activated Ca2+ channels comprise complexes of a pore-forming CaVα1 and auxiliary subunits Ca Vβ, CaVα2δ and sometimes CaVγ. The intracellular CaVβ subunit assists in trafficking and surface expression of the CaVα1 subunit and can modulate biophysical properties of the Ca2+ channel. Four genes, CaVβ1-4, exist which confer different properties to Ca2+ currents through the various CaVα1 subunits. Ca2+ currents in cochlear inner (IHC) and outer hair cells (OHC) serving synaptic transmission flow predominantly through the L-type CaVα1 subunit CaV1.3, but associated CaVβ subunits are unknown. In the organ of Corti, we found mRNa and protein for all four CaVβ subunits including Ca Vβ2, but clear assignment of the CaVβ1-4 immunolabelling with hair cells or nerve fibers was difficult. We analyzed CaVβ3 knockout (CaVβ3-/-) and Ca Vβ4 mutant mice (CaVβ4lh/lh), which had normal hearing. Recording voltage-activated Ba2+ currents from hair cells of the two mouse models revealed distinct significant changes of cell size and Ba2+ current properties compared with their wild-type controls. Neonatal CaVβ4lh/lh IHCs showed reduced membrane capacitances and changes in the voltage dependence and kinetics of current activation, whereas mature IHCs had reduced peak currents compared with CaVβ4wt, altogether indicating the presence of CaVβ4 in IHCs. Ba2+ currents of Ca Vβ3-/- OHCs showed largely reduced amplitudes, changes in the voltage dependence and kinetics of Ba2+ current activation, and increased inactivation compared with CaVβ3 wt, pointing to a role of CaVβ3 for OHCs. These results indicate that neither CaVβ3 nor CaVβ4 are indispensable for hair cell Ca2+ currents but contribute to the overall current properties.