Coupling of Slack and Na1.6 sensitizes Slack to quinidine blockade and guides anti-seizure strategy development.

Quinidine has been used as an anticonvulsant to treat patients with KCNT1-related epilepsy by targeting gain-of-function KCNT1 pathogenic mutant variants. However, the detailed mechanism underlying quinidine's blockade against KCNT1 (Slack) remains elusive. Here, we report a functional and physical coupling of the voltage-gated sodium channel Na1.6 and Slack. Na1.6 binds to and highly sensitizes Slack to quinidine blockade. Homozygous knockout of Na1.6 reduces the sensitivity of native sodium-activated potassium currents to quinidine blockade. Na1.6-mediated sensitization requires the involvement of Na1.6's N- and C-termini binding to Slack's C-terminus and is enhanced by transient sodium influx through Na1.6. Moreover, disrupting the Slack-Na1.6 interaction by viral expression of Slack's C-terminus can protect against Slack-induced seizures in mice. These insights about a Slack-Na1.6 complex challenge the traditional view of 'Slack as an isolated target' for anti-epileptic drug discovery efforts and can guide the development of innovative therapeutic strategies for KCNT1-related epilepsy.