Stage-specific exposure to an activity-permissive medium enhances neuronal activity in oligodendrocyte-enriched cortical organoids.

Oligodendrocyte-enriched cortical organoids (OCOs) are a powerful platform for modeling oligodendrogenesis in a human cellular context. However, neuronal activity is impaired in conventional culture media, limiting assessment of neuronal function in conjunction with oligodendrocyte biology. To address this, we used a modified BrainPhys medium termed neuronal activity medium (NAM) and defined the optimal developmental window for NAM exposure to generate OCOs with robust neuronal activity (NAM-OCOs). Stage-specific exposure to NAM, prior to oligodendrocyte expansion, leads to enhanced structural complexity, as evidenced by increased organoid size, heightened synaptogenesis, and upregulation of transcripts associated with neuronal morphology. Further, NAM-OCOs display increased cellular heterogeneity, with GABAergic interneurons exhibiting enhanced inhibitory synaptic features and a more advanced developmental transcriptional state, while preserving oligodendrocyte development and oligodendrocyte-axon associations. Altogether, our studies demonstrate that stage-specific exposure to an activity-permissive environment enhances neuronal activity, establishing an OCO model which integrates neuronal activity with oligodendrocyte development and differentiation.