Cellular and behavioral effects of altered Na_V1.2 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 infantile epileptic encephalopathy, while loss-of-function variants that reduce neuronal excitability lead to developmental delay and/or autism spectrum disorder with or without co-morbid seizures. One unique case less easily classified using this binary paradigm 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 that predicted mixed effects on channel function and neuronal activity. We also generated Scn2aK1422E mice and characterized effects on neurons and neurological/neurobehavioral phenotypes. Dissociated neurons from heterozygous Scn2aK1422E/+ mice exhibited a novel TTX-resistant current with a reversal potential consistent with mixed ion permeation. Cortical slice recordings from Scn2aK1422E/+ tissue demonstrated impaired action potential initiation and larger Ca2+ transients at the axon initial segment during the rising phase of the action potential, suggesting mixed effects on channel function. Scn2aK1422E/+ mice exhibited rare spontaneous seizures, interictal EEG abnormalities, altered response to induced seizures, reduced anxiety-like behavior and alterations in olfactory-guided social behavior. Overall, Scn2aK1422E/+ mice present with phenotypes similar yet distinct from Scn2a knockout models, consistent with mixed effects of K1422E on NaV1.2 channel function.

Nav1.2 and BK channel interaction shapes the action potential in the axon initial segment

“…In a heterozygote SCN2A knock‐out mouse, haploinsufficiency of Na v 1.2 is associated with an autistic‐like phenotype attenuated with age (Léna & Mantegazza, 2019), but in another SCN2A +/− model the phenotype is also characterized by epileptic seizures (Miyamoto et al., 2019) consistent with hyperexcitability of L5 pyramidal neurons (Spratt et al., 2021). More recently, a mouse model carrying the K1422A mutation in the SCN2A gene has been generated (Echevarria et al., 2021). This specific mutation is characterized by an increase in Ca 2+ permeability of Na v 1.2 and a mix of phenotypes including rare seizures.…”

Nav1.2 and BK channels interaction shapes the action potential in the axon initial segment

“…In a heterozygote SCN2A knock-out mouse, haploinsufficiency of Na v 1.2 is associated with an autistic-like phenotype attenuated with age [44], but in another SCN2A +/- model the phenotype is also characterized by epileptic seizures [45] consistent with hyperexcitability of L5 pyramidal neurons [46]. More recently, a mouse model carrying the K1422A mutation in the SCN2A gene has been generated [47]. This specific mutation is characterised by an increase in Ca 2+ permeability of Na v 1.2 and a mix of phenotypes including rare seizures.…”

Complex biophysical changes and reduced neuronal firing in anSCN8Avariant associated with developmental delay and epilepsy

“…Moreover, computational modeling enables the simulation of heterozygous genetic conditions, providing a more nuanced understanding of the mutations' effects. Here, To further explore the effects of Na V 1.6 G1625R expression, we modified a wellestablished computational model of a layer V cortical pyramidal cell [27,28,30,[37][38][39]. We optimized a hidden Markov model (HMM) to represent Na V 1.6 channels using an in-house evolutionary algorithm similar to previous studies [33,40].…”