Pathogenic MTOR somatic variant causing focal cortical dysplasia drives hyperexcitabilityviaoveractivation of neuronal GluN2C NMDA receptors

Objective Genetic variations in proteins of the mechanistic target of rapamycin (mTOR) pathway cause a spectrum of neurodevelopmental disorders often associated with brain malformations and with intractable epilepsy. The mTORopathies are characterized by hyperactive mTOR pathway and comprise tuberous sclerosis complex (TSC) and focal cortical dysplasia (FCD) type II. How hyperactive mTOR translates into abnormal neuronal activity and hypersynchronous network remains to be better understood. Previously, the role of upregulated GluN2C-containing glutamate- gated NMDA receptors (NMDARs) has been demonstrated for germline defects in the TSC genes. Here, we questioned whether this mechanism would expand to other mTORopathies in the different context of a somatic genetic variation of the MTOR protein recurrently found in FCD type II. Methods We used a rat model of FCD created by in utero electroporation of neural progenitors of dorsal telencephalon with expression vectors encoding either the wild-type or the pathogenic MTOR variant (p.S2215F). In this mosaic configuration, patch-clamp whole-cell recordings of the electroporated, spiny stellate neurons and extracellular recordings of the electroporated areas were performed in neocortical slices. Selective inhibitors were used to target mTOR activity and GluN2C- mediated currents. Results Neurons expressing the mutant protein displayed an excessive activation of GluN2C NMDAR-mediated spontaneous excitatory post-synaptic currents. GluN2C-dependent increase in spontaneous spiking activity was detected in the area of electroporated neurons in the mutant condition and was restricted to a critical time-window between postnatal days P9 and P20. Significance Somatic MTOR pathogenic variant recurrently found in FCD type II resulted in overactivation of GluN2C-mediated NMDARs in neocortices of rat pups. The related and time- restricted hyperexcitability was sensitive to subunit GluN2C-specific blockade. Our study suggests that GluN2C-related pathomechanisms might be shared in common by mTOR pathway-related cortical dysplasia. Key points Excessive activation of GluN2C NMDAR-mediated currents in spiny stellate neurons expressing FCD-causing MTOR somatic variation GluN2C-dependent increase in spontaneous spiking activity in rat somatosensory cortex containing mutant MTOR-expressing neurons GluN2C-dependent excessive network activity is time-restricted to a critical period between P9 and P20