Patients harboring mutations in the PI3K-AKT-MTOR pathway-encoding genes often develop a spectrum of neurodevelopmental disorders including epilepsy. A significant proportion remains unresponsive to conventional anti-seizure medications. Understanding mutation-specific pathophysiology is thus critical for molecularly targeted therapies. We previously determined that mouse models expressing a patient-related activating mutation in PIK3CA, encoding the p110α catalytic subunit of phosphoinositide-3-kinase (PI3K), are epileptic and acutely treatable by PI3K inhibition, irrespective of dysmorphology. Here we report the physiological mechanisms underlying this dysregulated neuronal excitability. In vivo, we demonstrate epileptiform events in the Pik3ca mutant hippocampus. By ex vivo analyses, we show that Pik3ca-driven hyperactivation of hippocampal pyramidal neurons is mediated by changes in multiple non-synaptic, cell-intrinsic properties. Finally, we report that acute inhibition of PI3K or AKT, but not MTOR activity, suppresses the intrinsic hyperactivity of the mutant neurons. These acute mechanisms are distinct from those causing neuronal hyperactivity in other AKT-MTOR epileptic models and define parameters to facilitate the development of new molecularly rational therapeutic interventions for intractable epilepsy.
Mutant hippocampal neurons produce increased epileptiform burst activity.
(A) Flowchart shows acute horizontal brain slicing for whole-cell recording. (B–E) Traces represent silent, tonic, and burst categories of CA1 and CA3 neurons based on spontaneous cellular activity; respective pie charts marked proportion of recorded cells. Mutant CA1 and CA3 exhibited significantly higher proportions of burst-firing cells compared to controls. Significantly fewer tonic-firing cells were observed in mutant CA1. (F,G) Relative to controls, spontaneous tonic spike frequencies were significantly higher in mutant cells; burst frequency was significantly higher in mutant CA1. (H) Representative traces for subtypes of burst firing, namely burst cluster, paroxysmal depolarization shift (PDS) and non-PDS plateau bursts. (I) Proportion of burst subcategories were not overtly different in control and mutant CA1 and CA3 regions; burst clusters were only seen in mutant cells. (J) Representative trace demonstrates how burst duration and inter-burst interval were calculated. (K,L) In both CA1 and CA3, average (avg.) burst duration and inter-burst interval were not significantly different between control and mutant neurons. (M’) Plateau potential shift (PPS) in mutant bursting cells, as depicted in panel (M’), was significantly higher in CA1 but similar in CA3, compared to respective controls. Data is represented as pie charts, % bar graphs and mean ± SEM scatter plots; differences were considered significant at p < 0.05; ns, not significant; PPS, plateau potential shift. Scale bars: 1 s, 20 mV (B,C,J); 0.2 s, 20 mV (H); 0.1 s, 20 mV (M’).
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