Cajal-Retzius fate specification is disrupted by constitutive activation of β-Catenin in hem progenitors

Cajal-Retzius cells (CR cells) are the earliest born neurons in the cerebral cortex, and have been implicated in regulating neuronal migration and development of circuitry. A major source of CR cells is the cortical hem, a signaling center at the dorsal telencephalic midline. The hem functions as the hippocampal organizer via canonical WNT signaling and hem progenitors are therefore exposed to high levels of WNT ligands. We tested whether constitutive stabilization of β-Catenin (gain of function, GOF) in the mouse cortical hem progenitors supports CR cell production. We find that although neurons are produced from the hem, they do not acquire molecular features of CR cell identity. The trajectory of differentiation examined using single-cell transcriptomics reveals that immature CR cells normally display a Tbr2+ stage, which is absent upon β-Catenin GOF. These data indicate that CR progenitors in the hem are sensitive to levels of stabilized β-Catenin and that a Tbr2+ stage may be important for the acquisition of CR cell identity.