Cardiac Sodium Channel Display Coupled Gating

Objective: Mutations in SCN5A, the gene encoding the cardiac sodium channel, have been linked to many inherited cardiac arrhythmias. We have shown the existence of dominant-negative mutations in Brugada Syndrome due to interactions between alpha-subunits. Here we investigated whether cardiac sodium channel alpha-subunits also display coupled gating in addition to interacting. Methods: Biophysical properties were studied by patch-clamp analysis in the whole-cell configuration. Results: We used a non-trafficking mutant which can be rescued with mexilitine, unveiling a +15 mV shift in activation compared to WT. When this non-trafficking mutant was co-expressed with WT, we still saw a shift in activation compared to WT alone, even though the mutant is not conducting. Therefore these results demonstrate that the presence of the mutant, affects the activation of the WT suggesting a gating cooperation between the 2 channels. Then we wanted to address whether the inactivation gating process was also synergic between two sodium channel subunits. We used a channel where the C-terminus was deleted and where inactivation was affected. Expression of this delta-Cter alone showed altered inactivation kinetics. Surprisingly, when co-transfected with the trafficking-competent but gating-deficient mutant channel R878C, the inactivation properties of the mutant were partially rescued. Altogether these data suggest that the presence of the full C-terminal region provided by the nonfunctional channel R878C could rescue the inactivation defect of the delta-Cter channel. Conclusions: Mutations impairing either activation or inactivation of one channel can affect the gating properties of the other WT alpha-subunit demonstrating coupled gating properties between sodium channel alpha-subunits.