Abstract 206: Small GTPase Rap1 Transmits Mechanical Signals to Control Vascular Tone and Blood Pressure

Abstract

Smooth muscle (SM)-mediated vessel contraction and endothelium-dependent vessel dilation are major mechanisms controlling basal vascular tone. Increased vascular tone leads to arterial hypertension, a risk factor for cardiovascular diseases. Agonist-induced, Ca2+-dependent SM contraction is modulated by intracellular cAMP, which promotes SM relaxation by inhibition of Rho-mediated myosin regulatory light chain (RLC20) phosphorylation and actomyosin contractility. In response to elevated blood flow, endothelium releases dilator substances that relax vascular SM, and in particular defects in production of nitric oxide (NO) lead to hypertension. Rap1 is a small GTPase that integrates signals from multiple receptors and two Rap1 isoforms, Rap1a and Rap1b are ubiquitously expressed. We found global Rap1b deletion leads to cardiac hypertrophy and hypertension and examined the effect of Rap1-deficiency on vascular tone. Using SM and endothelium tissue-restricted Rap1 knockout mice we show that deletion of Rap1, via distinct mechanisms in the two tissues, contributes to elevated vascular tone. In SM Rap1 regulates basal contraction level and mediates cAMP-induced desensitization of contraction. Rap1 suppresses RhoA-mediated RLC20 phosphorylation, Ca+2 sensitization and relaxation, signaling that is further enhanced upon cAMP-dependent activation of Rap1 by Rap1 GEF Epac. In endothelium, we find Rap1 is essential for regulation of NO-dependent vasodilation, as deletion of three of the four Rap1 alleles leads to a significant decrease in NO-dependent vasodilation. Significantly, we find that Rap1 is required for normal shear stress-induced NO release and Rap1-deficency in endothelium leads to elevated blood pressure in mice. Mechanistically, we show that Rap1 is required for transducing signals from the endothelial mechanosensing complex triggering signaling leading to NO production. Because shear stress from flowing blood is the main determinant of NO release, this novel finding positions Rap1 as a key regulator of endothelial function. In conclusion, Rap1 in SM and endothelium, via distinct mechanisms, plays a key role in maintaining normal vascular tone and blood pressure.