The two splice variant forms of Cdc42 exert distinct and essential functions in neurogenesis

The small GTPase cell division cycle 42 (CDC42) plays essential roles in neurogenesis and brain development. Previously, using murine embryonic P19 cells as a model system, we showed that CDC42 stimulates mTOR complex 1 (mTORC1) activity and thereby up-regulates transcription factors required for the formation of neural progenitor cells. However, paradoxically, although endogenous CDC42 is required for both the initial transition of undifferentiated P19 cells to neural progenitors and their ultimate terminal differentiation into neurons, ectopic CDC42 overexpression promotes only the first stage of neurogenesis (i.e. the formation of neuroprogenitors) and not the second phase (differentiation into neurons). Here, using both P19 cells and mouse embryonic stem cells, we resolve this paradox, demonstrating that two splice variants of CDC42, differing only in nine amino acid residues in their very C-terminal regions, play distinct roles in neurogenesis. We found that a CDC42 splice variant that has a ubiquitous tissue distribution, termed here as CDC42u, specifically drives the formation of neuroprogenitor cells, whereas a brain-specific CDC42 variant, CDC42b, is essential for promoting the transition of neuroprogenitor cells to neurons. We further show that the specific roles of CDC42u and CDC42b in neurogenesis are due to their opposing effects on mTORC1 activity. Specifically, CDC42u stimulated mTORC1 activity and thereby induced neuroprogenitor formation, whereas CDC42b worked together with activated CDC42-associated kinase (ACK) in down-regulating mTOR expression and promoting neuronal differentiation. These findings highlight the remarkable functional specificities of two highly similar CDC42 splice variants in regulating distinct stages of neurogenesis.