α2δ2 Controls the Function and Trans-Synaptic Coupling of Cav1.3 Channels in Mouse Inner Hair Cells and Is Essential...

The auxiliary subunit α₂δ2 modulates the abundance and function of voltage-gated calcium channels. Here we show that α₂δ2 mRNA is expressed in neonatal and mature hair cells. A functional α₂δ2-null mouse, the ducky mouse (du), showed elevated auditory brainstem response click and frequency-dependent hearing thresholds. Otoacoustic emissions were not impaired pointing to normal outer hair cell function. Peak Ca²⁺ and Ba²⁺ currents of mature du/du inner hair cells (IHCs) were reduced by 30-40%, respectively, and gating properties, such as the voltage of half-maximum activation and voltage sensitivity, were altered, indicating that Ca_v1.3 channels normally coassemble with α₂δ2 at IHC presynapses. The reduction of depolarization-evoked exocytosis in du/du IHCs reflected their reduced Ca²⁺ currents. Ca²⁺- and voltage-dependent K⁺ (BK) currents and the expression of the pore-forming BKα protein were normal. Ca_v1.3 and Ca_vβ2 protein expression was unchanged in du/du IHCs, forming clusters at presynaptic ribbons. However, the close apposition of presynaptic Ca_v1.3 clusters with postsynaptic glutamate receptor GluA4 and PSD-95 clusters was significantly impaired in du/du mice. This implies that, in addition to controlling the expression and gating properties of Ca_v1.3 channels, the largely extracellularly localized α₂δ2 subunit moreover plays a so far unknown role in mediating trans-synaptic alignment of presynaptic Ca²⁺ channels and postsynaptic AMPA receptors.

Inner hair cells possess calcium channels that are essential for transmitting sound information into synaptic transmitter release. Voltage-gated calcium channels can coassemble with auxiliary subunit α₂δ isoforms 1-4. We found that hair cells of the mouse express the auxiliary subunit α₂δ2, which is needed for normal hearing thresholds. Using a mouse model with a mutant, nonfunctional α₂δ2 protein, we showed that the α₂δ2 protein is necessary for normal calcium currents and exocytosis in inner hair cells. Unexpectedly, the α₂δ2 protein is moreover required for the optimal spatial alignment of presynaptic calcium channels and postsynaptic glutamate receptor proteins across the synaptic cleft. This suggests that α₂δ2 plays a novel role in organizing the synapse.