Ca2+-binding protein 2 inhibits Ca2+-channel inactivation in mouse inner hair cells

Ca2+-binding protein 2 (CaBP2) inhibits the inactivation of heterologously expressed voltage-gated Ca2+ channels of type 1.3 (Ca(V)1.3) and is defective in human autosomal-recessive deafness 93 (DFNB93). Here, we report a newly identified mutation in CABP2 that causes a moderate hearing impairment likely via nonsense-mediated decay of CABP2-mRNA. To study the mechanism of hearing impairment resulting from CABP2 loss of function, we disrupted Cabp2 in mice (Cabp2(LacZ/LacZ)). CaBP2 was expressed by cochlear hair cells, preferentially in inner hair cells (IHCs), and was lacking from the postsynaptic spiral ganglion neurons (SGNs). Cabp2(LacZ/LacZ) mice displayed intact cochlear amplification but impaired auditory brainstem responses. Patch-clamp recordings from Cabp2(LacZ/LacZ) IHCs revealed enhanced Ca2+-channel inactivation. The voltage dependence of activation and the number of Ca2+ channels appeared normal in Cabp2(LacZ/LacZ) mice, as were ribbon synapse counts. Recordings from single SGNs showed reduced spontaneous and sound-evoked firing rates. We propose that CaBP2 inhibits Ca(V)1.3 Ca2+-channel inactivation, and thus sustains the availability of Ca(V)1.3 Ca2+ channels for synaptic sound encoding. Therefore, we conclude that human deafness DFNB93 is an auditory synaptopathy.