Proneural genes define ground state rules to regulate neurogenic patterning and cortical folding
Asymmetric neuronal expansion is thought to drive evolutionary transitions from lissencephalic to gyrencephalic cerebral cortices. We report that Neurog2 and Ascl1 proneural genes interact to sustain neurogenic continuity and lissencephaly in rodents. 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, selective killing of double+ NPCs using Neurog2-Ascl1 split-Cre mice and three 'deletor' strains breaks neurogenic symmetry by locally disrupting Notch signaling, leading to cortical folding. Consistent with proneural genes driving discontinuous neurogenesis and folding via Notch, NEUROG2, ASCL1 and HES1 transcripts are modular in gyrencephalic macaque cortices. Neurog2/Ascl1 double+ NPCs are thus Notch-ligand expressing 'niche' cells that control neurogenic periodicity and cortical gyrification. Overall design: CD15+ GFP/mCherry negative, single, double positive cortical progenitors were isolated from mouse E12.5 Neurog2Flag-mcherryKI/+;Ascl1GFPKI/+ dorsal telencephalic tissue by Fluorescence-activated cell sorting (FACS). Total RNA from each cell types was extracted by Trizol LS (Life Technologies). RNAseq was performed on each cell type after constructing mRNA libraries using the Illumina TruSeq Stranded mRNA kit. Each library was paired-end sequenced to a length of 2x125 bases. Fastq files were generated from reads passing chastity filter.