Loss of PAX6 alters the excitatory/inhibitory neuronal ratio in human cerebral organoids

The transcription factor PAX6 is a crucial regulator of multiple aspects of embryonic forebrain development. It has well-established roles in the regulation of excitatory and inhibitory neuron development in the embryonic cortex in mice but PAX6’s roles during human forebrain development are less well understood. Using human cerebral organoids, we investigated PAX6’s roles in human neurodevelopment. Homozygous PAX6 mutant ( PAX6 -/- ) organoids were larger than controls and contained increased inhibitory cell types. Excitatory neurons were still generated in PAX6 -/- organoids but they were less mature, and a subset showed dysregulated expression of inhibitory identity genes compared to PAX6 +/+ controls. The inhibitory cells found in PAX6 -/- organoids physically segregated from excitatory neurons and presented a distinct transcriptomic profile when compared to in vivo cortical inhibitory neurons. PAX6 -/- organoids showed a dysregulated cellular response to PTN-PTPRZ1 signalling, which contributed to the observed increase in inhibitory neurons and the consequent altered excitatory to inhibitory neuronal ratio. Summary Statement Human cerebral organoids lacking PAX6 expression show an altered excitatory to inhibitory neuronal ratio, concomitant with altered responses to intercellular PTN-PTPRZ1 signalling.