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A KCNQ2 variant causing Early Onset Epileptic Encephalopathy increases spontaneous network-driven activity and excitability of pyramidal cells in the layer II/III and V of the motor cortex during a limited period of development

De novo variants in the KCNQ2 gene encoding the Kv7.2 subunit of the voltage-gated potassium Kv7/M channel are the main cause of Early Onset Epileptic Encephalopathy (EOEE) with suppression burst suggesting that this channel plays an important role for proper brain development. Functional analysis of these variants in heterologous cells has shown that most of them are loss of function leading to a reduction of M current. However the cellular mechanism of the neuronal network dysfunctionning is still not known. Here we characterized the electrophysiological properties of developing pyramidal cells of the layer II/III and V and analyzed spontaneous synaptic activity in these layers in motor cortical slices obtained from a recently generated heterozygous knock-in mouse harboring the loss-of-function pathogenic p.T274M variant. Experiments were performed on animals aged one week, three weeks and four-five weeks, and the results were compared with those of pyramidal cells recorded in slices from wild-type mice untreated or treated with the Kv7 channel blocker XE-991. We showed that the variant led to a hyperexcitability of pyramidal cells of layer II/III in cortical slices from animal aged 1 week and 3 weeks and to a level that was similar to the effect of XE-911. In layer V the impact of the variant was observed in slices from animal aged 3 weeks but not earlier and to a level that was lower to the effect of XE-991. However, in cortical slices from animal aged 4-5 weeks electrophysiological properties of pyramidal cells of layers II/III and V were no more affected by the variant but still sensitive to XE-991. The recovery of the electrophysiological responses in knock-in animals was associated with a slight but significant distal shift of the axonal initial segment (AIS) from the soma of pyramidal cells of layer II/III and V. Recordings of spontaneous synaptic activity in these layers revealed the presence of recurrent GABAergic network activities (RGNA) that were mainly observed during the three first postnatal weeks of life and which occurrence and frequency were increased in pyramidal cells of the layer II/III but not of the layer V of the knock-in mouse. There were no significant differences in synaptic activities mediated by GABA and glutamate receptors in cortical slices from animal aged 4-5 weeks. Together our data provided evidences that the heterozygous p.T274M variant impacts the activity of pyramidal cells and probably of Gabaergic interneurons during a limited period of development. Our data also indicated that neurons of the layer II/III are more sensitive to the variant than those located in the layer V in terms of age of onset, neuronal firing and spontaneous synaptic activities. Moreover our data suggest that a compensatory mechanism might take place in the knock-in mice aged 4-5 weeks allowing the recovery of control activity at cellular and network levels and which is associated with a slight displacement of the AIS. Thus, the effect of the variant on neuronal activity is developmentally regulated and is reminiscent to some characteristics of KCNQ2-related EOEE.