Cellular and behavioral effects of altered NaV1.2 sodium channel ion permeability in Scn2a K1422E mice

Abstract

Genetic variants in SCN2A, encoding the NaV1.2 voltage-gated sodium channel, are associated with a range of neurodevelopmental disorders with overlapping phenotypes. Some variants fit into a framework wherein gain-of-function missense variants that increase neuronal excitability lead to developmental and epileptic encephalopathy, while loss-of-function variants that reduce neuronal excitability lead to intellectual disability and/or autism spectrum disorder with or without co-morbid seizures. One unique case less easily classified using this framework is the de novo missense variant SCN2A-p.K1422E, associated with infant-onset developmental delay, infantile spasms, and features of autism spectrum disorder. Prior structure–function studies demonstrated that K1422E substitution alters ion selectivity of NaV1.2, conferring Ca2+ permeability, lowering overall conductance, and conferring resistance to tetrodotoxin (TTX). Based on heterologous expression of K1422E, we developed a compartmental neuron model incorporating variant channels that predicted reductions in peak action potential speed. We generated Scn2aK1422E mice and characterized effects on neurons and neurological/neurobehavioral phenotypes. Cultured cortical neurons from heterozygous Scn2aK1422E/+ mice exhibited lower current density with a TTX-resistant component and reversal potential consistent with mixed ion permeation. Recordings from Scn2aK1442E/+ cortical slices demonstrated impaired action potential initiation and larger Ca2+ transients at the axon initial segment during the rising phase of the action potential, suggesting complex effects on channel function. Scn2aK1422E/+ mice exhibited rare spontaneous seizures, interictal EEG abnormalities, altered induced seizure thresholds, reduced anxiety-like behavior and alterations in olfactory-guided social behavior. Overall, Scn2aK1422E/+ mice present with phenotypes similar yet distinct from other Scn2a models, consistent with complex effects of K1422E on NaV1.2 channel function.

Altered neurological and neurobehavioral phenotypes in a mouse model of the recurrentKCNB1-p.R306C voltage-sensor variant

“…Susceptibility to seizures induced by the chemoconvulsant flurothyl (Bis(2,2,2-trifluoroethyl) ether, Sigma-Aldrich, St. Louis, MO) was tested in male and female WT, Kcnb1 C/+ and Kcnb1 C/C at P72-90 as previously described (Echevarria-Cooper et al, 2022; Hawkins et al, 2021).…”

“…Baseline locomotor activity was measured in P70-91 male and female WT, at P72-90 as previously described (Echevarria-Cooper et al, 2022;Hawkins et al, 2021).…”

Calcium and activity-dependent signaling in the developing cerebral cortex

“…One such mutation (K1422E) in Na v 1.2 channels renders them calcium-permeable. Scn2a K1442E/+ cortical neurons display larger action potential-evoked calcium transients compared with wild-type neurons, suggesting that this mutation impacts calcium signaling ( Echevarria-Cooper et al, 2022 ). Similarly, potassium channel mutations that alter excitability (e.g.…”

Evaluating the interplay between estrous cyclicity and induced seizure susceptibility inScn2aK1422Emice

“…The variant SCN2A-p.K1422E is associated with infant-onset developmental delay, infantile spasms, and features of ASD. We previously demonstrated that both male and female Scn2a K1422E heterozygous mice (abbreviated as Scn2a E/+ going forward) had a higher threshold for flurothylinduced GTCS compared to WT 6 . Within that initial data set, we also noted that the cumulative distribution of latencies to GTCS in Scn2a E/+ females was significantly different from the other groups 6 .…”