An unconventional cerebrospinal fluid-derived Semaphorin-signalling regulates apical progenitor dynamics in the developing neocortex

In the embryonic brain, dynamic regulation of apical adhesion is fundamental to generate correct numbers and identity of precursors and neurons. Radial glial cells (RGC) in the cerebral cortex are tightly attached to adjacent neighbours. However, cells committed to differentiate reduce their adhesiveness to detach and settle at distal position from the apical border. Whether diffusible signals delivered from the cerebrospinal fluid (CSF) contribute to the regulation of apical adhesion dynamics remain fully unknown. Here we report that unconventional pre-formed complexes of class3-Semaphorins (Sema) and Neuropilins (Nrp) are released into the cerebrospinal fluid (CSF) from sources including the choroid plexus. Through analysis of mutant mouse models and various ex vivo assays, we propose that two different complexes, Sema3B/Nrp2 and Sema3F/Nrp1, bind to apical endfeet of RGCs, and exert dual regulation of their attachment, nuclei dynamics, that oppositely promotes or inhibits basal progenitor and neuron differentiation. This reveals unexpected contributions of CSF-delivered guidance molecules during cortical development.