Mitotic WNT signalling orchestrates neurogenesis in the developing neocortex

The role of WNT/β‐catenin signalling in mouse neocortex development remains ambiguous. Most studies demonstrate that WNT/β‐catenin regulates progenitor self‐renewal but others suggest it can also promote differentiation. Here we explore the role of WNT/STOP signalling, which stabilizes proteins during G2/M by inhibiting glycogen synthase kinase (GSK3)‐mediated protein degradation. We show that mice mutant for cyclin Y and cyclin Y‐like 1 (Ccny/l1), key regulators of WNT/STOP signalling, display reduced neurogenesis in the developing neocortex. Specifically, basal progenitors, which exhibit delayed cell cycle progression, were drastically decreased. Ccny/l1‐deficient apical progenitors show reduced asymmetric division due to an increase in apical–basal astral microtubules. We identify the neurogenic transcription factors Sox4 and Sox11 as direct GSK3 targets that are stabilized by WNT/STOP signalling in basal progenitors during mitosis and that promote neuron generation. Our work reveals that WNT/STOP signalling drives cortical neurogenesis and identifies mitosis as a critical phase for neural progenitor fate.