The Timothy syndrome mutation differentially affects voltage- and calcium-dependent inactivation of CaV1.2 L-type calcium channels

Calcium entry into excitable cells is an important physiological signal, supported by and highly sensitive to the activity of voltage-gated Ca²⁺ channels. After membrane depolarization, Ca²⁺ channels first open but then undergo various forms of negative feedback regulation including voltage- and calcium-dependent inactivation (VDI and CDI, respectively). Inactivation of Ca²⁺ channel activity is perturbed in a rare yet devastating disorder known as Timothy syndrome (TS), whose features include autism or autism spectrum disorder along with severe cardiac arrhythmia and developmental abnormalities. Most cases of TS arise from a sporadic single nucleotide change that generates a mutation (G406R) in the pore-forming subunit of the L-type Ca²⁺ channel Ca_V1.2. We found that the TS mutation powerfully and selectively slows VDI while sparing or possibly speeding the kinetics of CDI. The deceleration of VDI was observed when the L-type channels were expressed with β₁ subunits prominent in brain, as well as β₂ subunits of importance for the heart. Dissociation of VDI and CDI was further substantiated by measurements of Ca²⁺ channel gating currents and by analysis of another channel mutation (I1624A) that hastens VDI, acting upstream of the step involving Gly⁴⁰⁶. As highlighted by the TS mutation, CDI does not proceed to completeness but levels off at ≈50%, consistent with a change in gating modes and not an absorbing inactivation process. Thus, the TS mutation offers a unique perspective on mechanisms of inactivation as well as a promising starting point for exploring the underlying pathophysiology of autism.