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

SUMMARYTransition from smooth, lissencephalic brains to highly-folded, gyrencephalic structures is associated with neuronal expansion and breaks in neurogenic symmetry. Here we show thatNeurog2andAscl1proneural genes regulate cortical progenitor cell differentiation through cross-repressive interactions to sustain neurogenic continuity in a lissencephalic rodent brain. Usingin vivolineage tracing, we found thatNeurog2andAscl1expression defines a lineage continuum of four progenitor pools, with ‘double+progenitors’ displaying several unique features (least lineage-restricted, complex gene regulatory network, G2pausing). Strikingly, selective killing of double+progenitors using split-Cre;Rosa-DTAtransgenics breaks neurogenic symmetry by locally disrupting Notch signaling, leading to cortical folding. Finally, consistent withNEUROG2andASCL1driving discontinuous neurogenesis and folding in gyrencephalic species, their transcripts are modular in folded macaque cortices and pseudo-folded human cerebral organoids.Neurog2/Ascl1double+progenitors are thus Notch-ligand expressing ‘niche’ cells that control neurogenic periodicity to determine cortical gyrification.HIGHLIGHTSNeurog2 and Ascl1 expression defines four distinct transitional progenitor statesDouble+NPCs are transcriptionally complex and mark a lineage branch pointDouble+NPCs control neurogenic patterning and cortical folding via Notch signalingNeurog2 and Ascl1 expression is modular in folded and not lissencephalic corticeseTOC BLURBEmergence 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 progenitors at a lineage bifurcation point that co-express Neurog2 and Ascl1 and produce Notch ligands to control neurogenic periodicity and cortical folding.