CDC42 Deletion Elicits Cerebral Vascular Malformations via Increased MEKK3-Dependent KLF4 Expression.

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.

Defective endothelial cell migration in the absence of Cdc42 leads to capillary-venous malformations.

Formation and homeostasis of the vascular system requires several coordinated cellular functions, but their precise interplay during development and their relative importance for vascular pathologies remain poorly understood. Here, we investigated the endothelial functions regulated by Cdc42 and their in vivo relevance during angiogenic sprouting and vascular morphogenesis in the postnatal mouse retina. We found that Cdc42 is required for endothelial tip cell selection, directed cell migration and filopodia formation, but dispensable for cell proliferation or apoptosis. Although the loss of Cdc42 seems generally compatible with apical-basal polarization and lumen formation in retinal blood vessels, it leads to defective endothelial axial polarization and to the formation of severe vascular malformations in capillaries and veins. Tracking of Cdc42-depleted endothelial cells in mosaic retinas suggests that these capillary-venous malformations arise as a consequence of defective cell migration, when endothelial cells that proliferate at normal rates are unable to re-distribute within the vascular network.

VEGF Receptor Tyrosine Kinases: Key Regulators of Vascular Function.

Vascular endothelial growth factor receptor (VEGFR) tyrosine kinases are key regulators of vascular development in vertebrates. Their activation is regulated through a family of secreted glycoproteins, the vascular endothelial growth factors (VEGFs). Expression, proteolytic processing, and diffusion range of VEGF proteins need to be tightly regulated, due to their crucial roles in development. While some VEGFs form concentration gradients across developing tissues and act as morphogenes, others function as inhibitors of receptor activation and downstream signaling. Ligand-induced receptor dimerization leads to activation of the intrinsic tyrosine kinase activity, which results in autophosphorylation of the receptors and in turn triggers the recruitment of interacting proteins as well as the initiation of downstream signaling. Although many biochemical details of VEGFR signaling have been revealed, the in vivo relevance of certain signaling aspects still remains to be demonstrated. Here, we highlight basic principles of VEGFR signaling and discuss its crucial role during development of the vascular system in mammals.