Somatostatin-Positive Interneurons Contribute to Seizures inSCN8AEpileptic Encephalopathy

Abstract

SCN8A epileptic encephalopathy is a devastating epilepsy syndrome caused by mutant SCN8A, which encodes the voltagegated sodium channel Na V 1.6. To date, it is unclear if and how inhibitory interneurons, which express Na V 1.6, influence disease pathology. Using both sexes of a transgenic mouse model of SCN8A epileptic encephalopathy, we found that selective expression of the R1872W SCN8A mutation in somatostatin (SST) interneurons was sufficient to convey susceptibility to audiogenic seizures. Patch-clamp electrophysiology experiments revealed that SST interneurons from mutant mice were hyperexcitable but hypersensitive to action potential failure via depolarization block under normal and seizure-like conditions. Remarkably, GqDREADD-mediated activation of WT SST interneurons resulted in prolonged electrographic seizures and was accompanied by SST hyperexcitability and depolarization block. Aberrantly large persistent sodium currents, a hallmark of SCN8A mutations, were observed and were found to contribute directly to aberrant SST physiology in computational modeling and pharmacological experiments. These novel findings demonstrate a critical and previously unidentified contribution of SST interneurons to seizure generation not only in SCN8A epileptic encephalopathy, but epilepsy in general.

Multifunctional requirements for ERK1/2 signaling in the development of ganglionic eminence derived glia and cortical inhibitory neurons

“…SST mRNA is first detected in immature CINs by mid-embryogenesis, but substantial levels of SST protein are not observed in CINs until early neonatal stages (Batista-Brito et al, 2008; Denaxa et al, 2012; Forloni et al, 1990; D. R. Lee et al, 2022; Neves et al, 2013; Taniguchi et al, 2011). A loss of SST protein has been seen in a number of other neuropathological conditions, but has not been directly assessed in conditions characterized by reduced ERK1/2 signaling (Davies et al, 1980; Fee et al, 2017; Pantazopoulos et al, 2017; Peng et al, 2013; Robbins et al, 1991; Sun et al, 2020; Tripp et al, 2011; Wengert et al, 2021). SST is not only a marker of RS-CINs, but an active neuropeptide with biological functions (Baraban & Tallent, 2004; Grilli et al, 2004; Liguz-Lecznar et al, 2016; Smith et al, 2019; Song et al, 2021; Tereshko et al, 2021).…”

Forebrain epileptiform activity is not required for seizure-induced apnea in a mouse model of Scn8a epilepsy

“…To confirm that CNO does inhibit cortical neuronal activity we recorded individual pyramidal neurons in the motor cortex in vitro and motor cortex ECoG in vivo in Emx1 -Cre mice crossed with LSL-GiDREADD mice to produce GiDREADD Emx 1– Cre ( Figure 3A ). We performed whole-cell current-clamp recordings from layer V pyramidal neurons that were identified by their anatomical location, shape, and firing properties ( Wengert et al, 2019 , 2021a ). Depolarizing current injection steps resulted in generation of APs ( Figure 3B ) that were reversibly inhibited by application of 1 μM CNO ( Figures 3C,D ).…”

“…The mice for experiments testing the role of forebrain excitatory neurons in seizure behaviors ( Figures 4 , 5 ), D/+ mice were crossed with Emx1 -Cre and floxed-GiDREADD mice. Genotyping was performed by Transnetyx, Inc. (Cordova, TN, United States), with methods of each mouse line as previously described ( Wagnon et al, 2015 ; Bunton-Stasyshyn et al, 2019 ; Wengert et al, 2021a ).…”

“…Custom electrocorticogram (ECoG)/electrocardiogram (ECG) headsets (P1 Technologies, Roanoke, VA, United States) were implanted in 6–8-week-old mice using standard aseptic surgical techniques as done previously ( Wengert et al, 2021a , b ; Wenker et al, 2021 , 2022 ). Anesthesia was induced with 5% and maintained with 0.5–3% isoflurane.…”

“…Preparation of acute brain slices for patch-clamp electrophysiology experiments was modified from standard protocols previously described ( Wengert et al, 2021a ). Mice were anesthetized with isoflurane and decapitated.…”