Human cerebral organoids (HCOs, also known as mini brains) generated from induced pluripotent stem cells (iPSCs) provide exciting opportunities to study neurological disorders in ways not previously possible from traditional animal, brain slice, or 2D cell culture models commonly used in research. Recent studies have demonstrated that HCOs can reproduce complex neural development pathways and mimic several biomarkers of neurodegenerative disorders. However, it remains uncertain if these HCOs can model the characteristic electrophysiological activities of epilepsy. We have developed a technique to record electrocorticography (ECoG) directly from these HCOs to first characterize their electrographic behavior as a step toward evaluating their potential as a clinically relevant model of epilepsy. We aimed to determine the baseline ECoG characteristics of HCOs and the changes in ECoG characteristics after exposure to common proconvulsants with different modes of action in order to assess their ability to model seizure activity. We recorded ECoG while the HCOs were bathed in normal artificial cerebrospinal fluid (ACSF) and while they were in ACSF solutions containing pentylenetetrazol (PTZ, a GABA A receptor antagonist), kainic acid (KA, a glutamate receptor agonist), or an elevated bath potassium concentration, which induces neuronal hyperexcitability. We observed an increase in spikes and seizure-like activity after high potassium and KA exposure. In contrast, there was no change in activity associated with PTZ exposure. These findings are consistent with the fact that our HCOs are primarily composed of excitatory neurons, with only a minimal number of inhibitory interneurons. Developing reliable brain organoid models of epilepsy is an important next step in the field that will revolutionize studies of precision therapy for epilepsy.

Leigh syndrome (LS), the most common type of pediatric mitochondrial disease, has been associated with loss-of-function mutations in genes that encode for proteins in complex 1 of the electron transport chain. Mutations in a gene called NADH dehydrogenase (ubiquinone) iron sulfur protein 4 (NDUFS4) are linked to LS in several groups of patients. Mice carrying a whole body or central nervous system-specific deletion of this gene develop several symptoms reminiscent of those found in humans. Recent studies in our lab showed that knockout (KO) of Ndufs4 in GABAergic interneurons are the key driver of seizures in the mouse model. In this study, we investigated which subtypes of interneurons are the leading cause of epilepsy. We focused on two major subtypes of GABAergic interneurons: interneurons that express parvalbumin (PV), a high-affinity calcium binding protein, and interneurons that express somatostatin (SST), a neuropeptide. PV interneurons make up 40% of GABAergic cells and have faster spike firing patterns when compared to SST cells. We hypothesize that PV neurons will contribute more to seizure susceptibility than SST interneurons because mitochondrial dysfunction is known to affect cells with high firing rates due to their high energy demand. To test our hypothesis, we generated mice with Ndufs4 KO restricted to PV or SST interneurons using LoxP Cre technology. We evaluated the general health of mice and tested their susceptibility to thermal seizures through behavioral checks and thermal induction tests. We found that Ndufs4 KO in both PV and SST interneurons led to seizure phenotypes. However, this gene KO did not have any detectable effect on body weight, motor activity, breathing patterns, physical appearance, and limb extension. A better understanding of mechanisms underlying epileptic seizures will lead to the development of effective treatments for LS-related epilepsy.