Endfoot regeneration restricts radial glial state and prevents translocation into the outer subventricular zone in early mammalian brain development

Neural stem cells, called radial glia, maintain epithelial structure during the early neocortical development. The prevailing view claims that when radial glia first proliferate, their symmetric divisions require strict spindle orientation; its perturbation causes precocious neurogenesis and apoptosis. Here, we show that despite this conventional view, radial glia at the proliferative stage undergo normal symmetric divisions by regenerating an apical endfoot even if it is lost by oblique divisions. We found that the Notch–R-Ras–integrin β1 pathway promotes the regeneration of endfeet, whose leading edge bears ectopic adherens junctions and the Par-polarity complex. However, this regeneration ability gradually declines during the subsequent neurogenic stage and hence oblique divisions induce basal translocation of radial glia to form the outer subventricular zone, a hallmark of the development of the convoluted brain. Our study reveals that endfoot regeneration is a temporally changing cryptic property, which controls the radial glial state and its shift is essential for mammalian brain size expansion.