RATIONALE: Aberrant formation of blood vessels precedes a broad spectrum of vascular complications, however, the cellular and molecular events governing vascular malformations are not yet fully understood.

OBJECTIVE: Here, we investigated the role of CDC42 during vascular morphogenesis and its relative importance for the development of cerebrovascular malformations.

METHODS AND RESULTS: In order to avoid secondary systemic effects often associated with embryonic gene deletion we generated an endothelial-specific and inducible knockout approach to study postnatal vascularization of the mouse brain. Postnatal endothelial-specific deletion of Cdc42 elicits cerebrovascular malformations reminiscent of cerebral cavernous malformations (CCM). At the cellular level, loss of CDC42 function in brain endothelial cells (EC) impairs their sprouting, branching morphogenesis, axial polarity and normal dispersion within the brain tissue. Disruption of CDC42 does not alter EC proliferation, but malformations occur where EC proliferation is most pronounced during brain development - the postnatal cerebellum - indicating that a high naturally occurring EC proliferation provides a permissive state for the appearance of these malformations. Mechanistically, CDC42-depletion in ECs elicited increased MEKK3-MEK5-ERK5 signaling and consequent detrimental overexpression of KLF2 and KLF4, recapitulating the hallmark mechanism for CCM pathogenesis. Through genetic approaches, we demonstrate that the co-inactivation of Klf4 reduces the severity of vascular malformations in Cdc42 mutant mice. Moreover, we show that CDC42 interacts with CCMs, and that CCM3 promotes CDC42 activity in endothelial cells.

CONCLUSIONS: We show that endothelial-specific deletion of Cdc42 elicits CCM-like cerebrovascular malformations, and that CDC42 is engaged in the CCM signaling network to restrain the MEKK3-MEK5-ERK5-KLF2/4 pathway.