SUMMARYCerebral cavernous malformation (CCM) is a common vascular dysplasia that affects both systemic and CNS blood vessels. Loss of function mutations in the CCM2 gene cause CCM. Here we show that targeted disruption of Ccm2 in mice results in failed lumen formation and early embryonic death through an endothelial cell autonomous mechanism. We demonstrate that CCM2 regulates endothelial cytoskeletal architecture, cell-cell interactions and lumen formation. Heterozygosity at Ccm2, a genotype equivalent to human CCM, results in impaired endothelial barrier function. Because our biochemical studies indicate that loss of CCM2 results in activation of RHOA GTPase, we rescued the cellular phenotype and barrier function in heterozygous mice using simvastatin, a drug known to inhibit Rho GTPases. These data offer the prospect for pharmacologic treatment of a human vascular dysplasia using a widely available and safe drug.

The angiogenic sprout has been compared to the growing axon, and indeed, many proteins direct pathfinding by both structures 1 . The Roundabout (Robo) proteins are guidance receptors with well-established functions in the nervous system2 , 3; however, their role in the mammalian

The innate immune system provides a first line of defense against invading pathogens by releasing multiple inflammatory cytokines, such as interleukin-1β and tumor necrosis factor-α, which directly combat the infectious agent and recruit additional immune responses. This exuberant cytokine release paradoxically injures the host by triggering leakage from capillaries, tissue edema, organ failure, and shock. Current medical therapies target individual pathogens with antimicrobial agents or directly either blunt or boost the host's immune system. We explored a third approach: activating with the soluble ligand Slit an endothelium-specific, Robo4-dependent signaling pathway that strengthens the vascular barrier, diminishing deleterious aspects of the host's response to the pathogen-induced cytokine storm. This approach reduced vascular permeability in the lung and other organs and increased survival in animal models of bacterial endotoxin exposure, polymicrobial sepsis, and H5N1

Slit-Roundabout (Robo) signalling has a well-understood role in axon guidance1 -5. Unlike in the nervous system, however, Slitdependent activation of an endothelial-specific Robo, Robo4, does not initiate a guidance program. Instead, Robo4 maintains the barrier function of the mature vascular

The innate immune response is essential for combating infectious disease. Macrophages and other cells respond to infection by releasing cytokines such as interleukin-1β (IL-1β), which in turn activate a well-described myeloid differentiation factor 88 (MYD88) -mediated, nuclear factor-κB (NF-κB) -dependent transcriptional pathway that results in inflammatory cell activation and recruitment1–4. Endothelial cells, which usually serve as a barrier to the movement of inflammatory cells out of the blood and into tissue, are also critical mediators of the inflammatory response5,6. Paradoxically, the same cytokines vital to a successful immune defense also have disruptive effects on endothelial cell-cell interactions and can trigger degradation of barrier function and dissociation of tissue architecture7–9. The mechanism of this barrier dissolution and its relationship to the canonical NF-κB pathway remains ill defined. Here we show that the direct, immediate, and disruptive effects of IL-1β on endothelial stability are NF-κB independent and are instead the result of signaling via the small GTPase, ADP-ribosylation factor 6 (ARF6), and its activator, ARF nucleotide binding site opener (ARNO). Moreover, we show that ARNO binds directly to the adaptor protein MYD88, and thus propose MYD88-ARNO-ARF6 as a proximal IL-1β signaling pathway distinct from that mediated by NF-κB (Supplementary Fig. 1). Finally, we show that SecinH3, an inhibitor of ARF guanine nucleotide-exchange factors (GEFs) such as ARNO, enhances vascular stability and significantly improves outcomes in animal models of inflammatory arthritis and acute inflammation.