Interneuron axonopathy underlies circuit dysfunction in a mouse model of Dravet syndrome

Children'S Hosp Of Philadelphia, Philadelphia PA

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Abstract

PROJECT SUMMARY Dravet syndrome (DS) is a severe neurodevelopmental disorder affecting 1 in 15,000 children and defined by treatment-resistant epilepsy, intellectual disability, features of autism spectrum disorder (ASD), and high rate of sudden unexplained death (SUDEP). DS is caused by variants in the gene SCN1A encoding the voltage-gated sodium (Na+) channel subunit Nav1.1. Current treatments are palliative, and there is no cure. How variants in SCN1A lead to the circuit-level dysfunction underlying DS remains unclear. This gap in knowledge limits progress toward novel treatments or a cure, which would have enormous lifelong benefits for patients and families. Prior work in DS (Scn1a+/-) mice suggests that loss of Nav1.1 leads to dysfunction of GABAergic inhibitory interneurons (INs) in the cerebral cortex, with the most prominent identified abnormality being impaired action potential generation in a critical subtype known as the parvalbumin-positive fast-spiking GABAergic interneuron (PV-IN). However, we showed during the prior funding period that PV-IN dysfunction is transient, and restricted to a brief time window in early development, with striking recovery of high frequency firing; instead, we identified long-lasting impairment of action potential propagation at PV-IN axons. We discovered that another subset of IN – labeled by vasoactive intestinal peptide (VIP-INs) – express Nav1.1 and are dysfunctional in Scn1a+/- mice in vitro and in vivo; deletion of Scn1a specifically in VIP-INs leads to ASD-linked behavioral abnormalities but without seizure or epilepsy, dissociating these components of the disorder. Such findings refine our mechanistic understanding of DS and have important implications for treatment approaches currently under development such as cell transplantation, RNA therapeutics, and gene therapy. This renewal uses innovative neuroscience approaches including transcriptomic profiling, detailed electrophysiology and anatomy, closed-loop optogenetic manipulation of behavior, and two-photon calcium imaging in vivo to establish the role of impaired action potential propagation along interneuron axons as a key mechanism of DS pathology. Proposed experiments will establish the molecular identity and physiological properties of Na+ channels in PV-IN axons in Scn1a+/- mice vs. controls (Aim 1); assess the fidelity of spike propagation in VIP-IN axons (Aim 2.1) and role of VIP-IN subtypes in features of ASD (Aim 2.2); and move beyond individual cell types to provide an open-source large-scale transcriptomic atlas of the Scn1a+/- mouse brain across development (Aim 3). The outcome of the proposed experiments will set forth a unifying hypothesis as to the pathophysiology of DS, translational knowledge that is critical to furthering the design of targeted therapies. The long-term objective of this line of research is to drive the development of mechanistically informed treatments or an effective cure for human patients with Dravet Syndrome.