PhD defence: Optical Analysis of Nav1.2 Function in Neurons: Physiological Roles and Pathological Implications in SCN2A Loss-of-Function Channelopathies - Fatima Abbas (OPTIMA, LIPhy)

The SCN2A gene encodes the voltage-gated sodium channel Nav1.2, predominantly expressed in the axon initial segment (AIS) of excitatory neurons in the cortex, vital for the generation and back-propagation of action potentials. Pathogenic variants of SCN2A are associated with a wide spectrum of neurodevelopmental disorders, including autism and intellectual disability, commonly linked to Nav1.2 loss-of-function (LoF). In this thesis we investigated the pathophysiology underlying these mutations in two SCN2A LoF mouse models with features of autistic-like behavior compared to the wild-type mouse: a heterozygote knock-out (Scn2a+/-), and a knock-in (Scn2a+/L1314P) carrying a patient-derived missense variant. We used an approach combining whole-cell patch clamp recordings with ultrafast optical imaging of sodium (Na+), calcium (Ca2+), and membrane potential (Vm) in cortical slices to resolve the submillisecond dynamics (50-100 µs scale) of AP generation in layer 5 pyramidal neurons. Our analyses reveal mutation-specific alterations in sodium and calcium influx, together with abnormal modulation of potassium channels, particularly Kv1. Complementary imaging in distal dendritic compartments shows that Nav1.2 defects remodel rather than abolish the ability to generate regenerative calcium signals despite perturbations at the AIS level, indicating the presence of partial regulatory mechanisms in response to axonal alterations. Altogether, these findings shed light on the divergent functional consequences of SCN2A LoF mutations and challenges the widely accepted concept of GoF versus LoF which is applied to classify channelopathies. By combining our cutting-edge techniques with age-specific analyses and targeted pharmacological interventions, we establish a novel approach for physiological phenotyping and drug assessment at the subcellular level, offering valuable insights into complex genotype-phenotype correlations toward therapeutic strategies targeted to specific patterns of Nav1.2 dysfunctions in SCN2A-related disorders.