Voltage sensor interaction site for a selective small molecule Nav1.1 sodium channel potentiator that enhances firing of fast-spiking interneurons.

Nav1.1 voltage-gated sodium (Nav) channels encoded by the SCN1A gene are critical to high frequency action potential generation in inhibitory interneurons and so play a crucial role in maintaining the excitatory-inhibitory balance in the brain. Rare, often loss-of-function, mutations in the SCN1A gene can lead to a spectrum of developmental and epileptic encephalopathies including Dravet syndrome with common SCN1A variants being risk factors for epilepsy, highlighting the potential for Nav1.1 as a therapeutic target for seizure disorders. Here, we describe a potent small molecule sodium channel potentiator ICA00600232 that exhibits >1000-fold selectivity for human Nav1.1 over all other Nav1.x family members except Nav1.3. Nav1.1 potentiation by ICA00600232 results from a slowing of channel inactivation. Employing both chimeras and single point mutations, we demonstrate that ICA00600232 interacts with the domain 4 voltage sensor region of Nav1.1. Three amino acid residues within the "extracellular" facing regions of the S2 and S3 transmembrane segments are major determinants of Nav1.1 potentiation and subtype selectivity. These same residues are also important for subtype selective inhibitor interactions with Nav1.1, albeit in a different structural conformation. Furthermore, we demonstrate that ICA00600232 at concentrations as low as 10 nM enhances firing frequency in parvalbumin positive fast-spiking interneurons recorded in brain slices from scn1a mice. The current study shows that a small molecule Nav1.1 selective potentiator targeting the fourth voltage sensor can slow inactivation of Nav1.1, increase excitability of parvalbumin positive interneurons and inhibitory tone that could be beneficial in treating developmental and epileptic encephalopathies and other seizure disorders. SIGNIFICANCE STATEMENT: Loss-of-function mutations of Nav1.1 sodium channel genes in human inhibitory interneurons are associated with a spectrum of epilepsies including developmental and epileptic encephalopathies. Potential treatments include enhancement of Nav1.1 activity via administration of small molecule potentiating agents. This study describes a potent subtype selective Nav1.1 channel potentiator that interacts with a region that controls voltage sensitivity and enhances action potential firing in inhibitory interneurons, supporting its potential utility in treating developmental and epileptic encephalopathy associated seizure disorders.