LAUSR

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 (cell division cycle 42) during vascular morphogenesis and its relative importance for the development of cerebrovascular malformations. Methods and Results: 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 (CCMs). At the cellular level, loss of CDC42 function in brain endothelial cells (ECs) 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 the 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 (mitogen-activated protein kinase kinase kinase 3)-MEK5 (mitogen-activated protein kinase kinase 5)-ERK5 (extracellular signal–regulated kinase 5) signaling and consequent detrimental overexpression of KLF (Kruppel-like factor) 2 and KLF4, recapitulating the hallmark mechanism for CCM pathogenesis. Through genetic approaches, we demonstrate that the coinactivation 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 ECs. 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.