Human iPSC-derived microglia sense and dampen hyperexcitability of cortical neurons carrying the epilepsy-associatedSCN2A-L1342P mutation.

Neuronal hyperexcitability is a hallmark of epilepsy. It has been recently shown in rodent models of seizures that microglia, the brain's resident immune cells, can respond to and modulate neuronal excitability. However, how human microglia interact with human neurons to regulate hyperexcitability mediated by an epilepsy-causing genetic mutation found in patients is unknown. The SCN2A gene is responsible for encoding the voltage-gated sodium channel Nav1.2, one of the leading contributors to monogenic epilepsies. Previously, we demonstrated that the recurring Nav1.2-L1342P mutation leads to hyperexcitability in a male donor (KOLF2.1) human-induced pluripotent stem cell (hiPSC)-derived cortical neuron model. Microglia originate from a different lineage (yolk sac) and are not naturally present in hiPSC-derived neuronal cultures. To study how microglia respond to neurons carrying a disease-causing mutation and influence neuronal excitability, we established a coculture model comprising hiPSC-derived neurons and microglia. We found that microglia display increased branch length and enhanced process-specific calcium signal when cocultured with Nav1.2-L1342P neurons. Moreover, the presence of microglia significantly lowered the repetitive action potential firing and current density of sodium channels in neurons carrying the mutation. Additionally, we showed that coculturing with microglia led to a reduction in sodium channel expression within the axon initial segment of Nav1.2-L1342P neurons. Furthermore, we demonstrated that Nav1.2-L1342P neurons release a higher amount of glutamate compared with control neurons. Our work thus reveals a critical role of human iPSC-derived microglia in sensing and dampening hyperexcitability mediated by an epilepsy-causing mutation.