Proneural genes define ground-state rules to regulate neurogenic patterning and cortical folding

Asymmetric neuronal expansion is thought to drive evolutionary transitions between lissencephalic and gyrencephalic cerebral cortices. We report that Neurog2 and Ascl1 proneural genes together sustain neurogenic continuity and lissencephaly in rodent cortices. Using transgenic reporter mice and human cerebral organoids, we found that Neurog2 and Ascl1 expression defines a continuum of four lineage-biased neural progenitor cell (NPC) pools. Double+ NPCs, at the hierarchical apex, are least lineage restricted due to Neurog2-Ascl1 cross-repression and display unique features of multipotency (more open chromatin, complex gene regulatory network, G2 pausing). Strikingly, selectively eliminating double+ NPCs by crossing Neurog2-Ascl1 split-Cre mice with diphtheria toxin-dependent “deleter” strains locally disrupts Notch signaling, perturbs neurogenic symmetry, and triggers cortical folding. In support of our discovery that double+ NPCs are Notch-ligand-expressing “niche” cells that control neurogenic periodicity and cortical folding, NEUROG2, ASCL1, and HES1 transcript distribution is modular (adjacent high/low zones) in gyrencephalic macaque cortices, prefiguring future folds. [Display omitted] •Neurog2 and Ascl1 proneural gene expression defines four transitional NPC states•Neurog2-Ascl1 cross-repress to block lineage bias of double+ NPCs at hierarchy apex•Double+ NPCs direct uniform neurogenesis via Notch to sustain murine lissencephaly•NEUROG2, ASCL1, and HES1 expression is modular in gyrencephalic macaque cortices Emergence of a gyrencephalic cortex is associated with a break in neurogenic continuity across the cortical germinal zone. Han et al. identify a pool of unbiased neural progenitor cells at a lineage bifurcation point that co-express Neurog2 and Ascl1 and produce Notch ligands to control neurogenic periodicity and cortical folding.