The formation of the vascular network requires a tightly controlled balance of pro-angiogenic and stabilizing signals. Perturbation of this balance can result in dysregulated blood vessel morphogenesis and drive pathologies including cancer. Here, we have identified a novel gene, ARHGAP18, as an endogenous negative regulator of angiogenesis, limiting pro-angiogenic signaling and promoting vascular stability. Loss of ARHGAP18 promotes EC hypersprouting during zebrafish and murine retinal vessel development and enhances tumor vascularization and growth. Endogenous ARHGAP18 acts specifically on RhoC and relocalizes to the angiogenic and destabilized EC junctions in a ROCK dependent manner, where it is important in reaffirming stable EC junctions and suppressing tip cell behavior, at least partially through regulation of tip cell genes, Dll4, Flk-1 and Flt-4. These findings highlight ARHGAP18 as a specific RhoGAP to fine tune vascular morphogenesis, limiting tip cell formation and promoting junctional integrity to stabilize the angiogenic architecture.

Age is the greatest risk factor for cardiovascular disease. In addition, inflammation and age (senescence) have been linked at both the clinical and molecular levels. In general, senescent cells have been described as pro-inflammatory based on their senescence associated secretory phenotype (SASP). However, we have previously shown that senescence induced by overexpression of SENEX (or ARHGAP18), in endothelial cells results in an anti-inflammatory phenotype. We have investigated, at the individual cellular level, the senescent phenotype of endothelial cells following three of the chief signals associated with ageing; oxidative stress, disturbed flow and hypoxia. All three stimuli induce senescence and, based on neutrophil adhesion and expression of the adhesion molecules E-selectin and VCAM-1, a population of senescent cells is seen that is resistant to inflammatory stimuli and thus we define as anti-inflammatory. The proportion of anti-inflammatory cells increases with time but remains stable at approximately 50% by eight days after induction of senescence, suggesting that these are stable phenotypes of endothelial cell senescence. Similar to other senescent cell types, p38MAPK blockade inhibits the development of the pro-inflammatory phenotype but unique to EC, there is a corresponding increase in the number of anti-inflammatory senescent cells. Thus stress-induced senescent endothelial cells display a mosaic of inflammatory phenotypes. The anti-inflammatory population suggests that senescent endothelial cells may have an unique protective role, to inhibit uncontrolled proliferation and to limit the local inflammatory response.

Localization of a regulator of RhoGTPases (ARHGAP18) is important for microtubule stability and endothelial cell function. The localization is demonstrated by advanced imaging and biochemical techniques.