Abstract 229: Small GTPase Rap1 deficiency Accelerates Development of Atherosclerosis in ApoE Deficient Mice

Rap1, a ubiquitously expressed small GTPase, integrates signals from multiple receptors and promotes activation and signaling by cell adhesion receptors. In vivo , endothelial (EC) Rap1 is required for normal vessel formation, angiogenesis and dynamic regulation of EC barrier. Two Rap1 isoforms, Rap1A and Rap1B, share 95% identity, and EC deletion of both isoforms leads to embryonic lethality due to cardiovascular defects. We have recently demonstrated that Rap1 is a novel regulator of shear sensing in endothelium and that it is essential for nitric oxide release, and that Rap1 deficiency leads to EC dysfunction. These findings demonstrated that Rap1 plays an essential role in vasoprotective response of endothelium in response to shear stress. The objective of this study is to determine the specific role of Rap1 response to laminar, vasoprotective flow and pro-inflammatory, disturbed flow by examining the effect of EC deletion of Rap1B, the more prominent Rap1 isoform, on progression of atherosclerosis. To investigate the role of Rap1 in progression of atherosclerosis, we crossed apolipoprotein E-deficient (ApoE -/- ) mice with tamoxifen-inducible, EC specific Rap1B-knockout (Cadh5-CreERT +/0 ; Rap1B f/f ) to generate atherogenic EC-Rap1B KO (Athero-Rap1B KO) mice. We hypothesized that Rap1B deficiency will exacerbate progression of atherosclerosis. Athero-Rap1B KO and littermate controls (Cadh5-Cre-negative mice or mice injected with carrier oil only) were fed high fat, western diet (21.2% fat, 0.2% cholesterol) for 16 weeks. No difference in total cholesterol levels were observed between Athero- Rap1B KO and control mice. Atherosclerotic lesion formation was visualized with oil O red and plaque area was quantified in en-face preparations. (n=5-9). We observed increased lesion area in the descending aorta of Athero-Rap1B KO mice (0.7351 ± 0.1065 vs 0.4866 ± 0.09812; p =0.0584, one-tailed t-test, Welch corrected) and a strong trend towards larger lesion area in the total aorta (2.142 ± 0.5058 vs 1.267 ± 0.1718; p=0.0679), and the arch of the aorta (1.489 ± 0.4343 vs 0.7807 ± 0.1152; p=0.0747) compared with control mice. These findings demonstrate that Rap1B deficiency accelerates progression of atherosclerosis and show that Rap1 plays a vasoprotective role in both laminar flow and disturbed flow areas.