Regulation of the voltage-dependent sodium channel Na_V1.1 by AKT1

The voltage-sensitive sodium channel Na_V1.1 plays a critical role in regulating excitability of GABAergic neurons and mutations in the corresponding gene are associated to Dravet syndrome and other forms of epilepsy. The activity of this channel is regulated by several protein kinases. To identify novel regulatory kinases we screened a library of activated kinases and we found that AKT1 was able to directly phosphorylate Na_V1.1. In vitro kinase assays revealed that the phosphorylation site was located in the C-terminal part of the large intracellular loop connecting domains I and II of Na_V1.1, a region that is known to be targeted by other kinases like PKA and PKC. Electrophysiological recordings revealed that activated AKT1 strongly reduced peak Na⁺ currents and displaced the inactivation curve to more negative potentials in HEK-293 cell stably expressing Na_V1.1. These alterations in current amplitude and steady-state inactivation were mimicked by SC79, a specific activator of AKT1, and largely reverted by triciribine, a selective inhibitor. Neurons expressing endogenous Na_V1.1 in primary cultures were identified by expressing a fluorescent protein under the Na_V1.1 promoter. There, we also observed a strong decrease in the current amplitude after addition of SC79, but small effects on the inactivation parameters. Altogether, we propose a novel mechanism that might regulate the excitability of neural networks in response to AKT1, a kinase that plays a pivotal role under physiological and pathological conditions, including epileptogenesis.