The Effect Of Autism Candidate-Gene Mutations In The Voltage-Gated Calcium Channel Beta 2 Subunit On Single Channel Kinetics

The electrophysiological properties of voltage-gated calcium channel (VGCC) complexes are determined by the specific combination of subunit isoforms, i. e. of one pore-forming CaVα1 subunit and the auxiliary subunits β, α2-δ (and γ). Therefore, VGCCs differentially modulate the cellular response to stimuli, which is relevant for neuronal function. Interestingly, the CaVα1C and CaVβ2-subunit genes have been shown to be risk loci for five major psychiatric disorders including autism spectrum disease (ASD) (Lancet (2013) 381:1371). Our group has recently found three missense mutations in the CaVβ2 gene in ASD patients; while two mutants (G167S, S197F) resulted in a retardation of inactivation behavior, one mutant (F240L) accelerated the inactivation of whole-cell Ba2+ currents (PLOS One (2014) 9(4): e95579). In the present study, we performed single channel patch clamp of HEK cells co-transfected with CaVβ2 mutants and CaVα1C. The gating parameters revealed a pronounced biophysical phenotype for all mutations: G167S and S197F persisted longer in an open state by elongating its mean open time (p = 0.005). F240L showed a trend for an increased open probability. Further, the transition rate constants obtained from Markov modelling of the single-channel data were consistent with the observed gating parameters. Here, the Markov model revealed significantly decelerated transition from open to closed state for G167S (p = 0.008) and S197F (p = 0.03). Both G167S and S197F showed slower transition rates suggesting a preference for deeper closed states (for S197F p = 0.04). That would explain why their open probabilities are not increased while mean open times are elongated. We conclude that the three mutations, each exhibiting different biophysical mechanisms, lead to the same outcome: more channel activity.