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Abstract

Angiopoietin (Angpt)/Tie signaling in microvascular endothelial cells (EC) controls vascular development, remodeling and maturation. Biomarker studies also imply a role of macrovascular Angpt/Tie signaling in the pathogenesis of hypertension and atherosclerosis. Importantly, experimental studies on the role of the Angpt ligands and the Tie receptors in hypertension and atherosclerosis have yielded conflicting results suggesting spatiotemporally context-dependent pro and anti-atherosclerotic/hypertensive functions in different experimental settings. Reports of scattered observations have shown Tie2 expression by vascular smooth muscle cells (VSMC). VSMC contribute to hypertension and atherosclerosis progression either through VSMC hypertrophy and/or switching from a contractile quiescent to a synthetic and activated phenotype. Yet, the functional role of VSMC-expressed Tie2 during these pathological conditions has not been analyzed. Therefore, in this study Tie2 was conditionally deleted in VSMC (Tie2SMC-KO) using a mural cell-specific Sm22α-Cre driver line. These mice were crossed with atherosclerosis-prone ApoE-deficient mice (ApoEKO Tie2SMC-KO). VSMC marker expression did not differ in freshly isolated and lysed aortas, mesenteric and femoral arteries from Tie2+/+ and Tie2SMC-KO mice. Transcriptionally, cultivated Tie2-deficient VSMC show an increased contractile and reduced synthetic phenotype-specific gene expression, suggesting that VSMC from Tie2SMC-KO mice fail to switch towards a synthetic phenotype upon activation. Correspondingly, migration and proliferation was significantly reduced in Tie2-deficient cultured VSMC. Long-term telemetric tracing identified significantly reduced basal systolic blood pressure (SBP) in Tie2SMC-KOmice, indicative of a baseline cardiac phenotype that is independent of Sm22α-driven Tie2 deletion in VSMC. Tie2SMC-KO mice also display a significantly reduced left ventricular posterior wall thickness (LVPWT) and interventricular septum (IVS), which is in line with a reduced cardiomyocyte (CM) cross-sectional area observed in these mice. Notably, GFP-positive cardiomyocytes (CMs) and GFP-positive VSMC were detected in heart and aortic tissue, respectively, obtained from adult Tie2MCM x Rosa26YFP mice, suggesting that Tie2 may be deleted in CMs in the time frame of Sm22α expression in these cells during embryonic development. Furthermore, DOCA-salt-induced hypertension experiments revealed a significant decrease in cardiac size with a slightly reduced blood pressure in Tie2SMC-KO mice. Moreover, AngII-treated Tie2SMC-KO mice demonstrated a decrease in heart rate (HR), cardiac output (CO) and stroke volume (SV) that was compensated over time. Thus, the baseline cardiac phenotype in Tie2SMC-KO mice transiently compromises an adequate cardiac response upon hypertension. Next, ApoEKO Tie2SMC-KO mice were fed a Western-type diet for 14 weeks and showed significantly reduced therosclerotic lesion progression with less VSMC content. Additionally, serum Angpt2 as well as the Angpt2/Angpt1 ratio were significantly increased in ApoEKO Tie2SMC-KO mice. Consistent with the increased systemic Angpt2 levels and the reduced atherosclerosis in ApoEKO Tie2SMC-KO mice, a pro- atherosclerotic phenotype was observed in ApoEKO Angpt2KO double knockout (KO) mice. The data propose a remarkable role of Tie2 receptor in regulating cardiac size and blood pressure, which is most likely a consequence of Sm22α-driven Tie2 deletion in CMs. Moreover, the data identified a cell autonomous function of VSMC-specific Tie2 in controlling VSMC phenotype and function. In the context of atherosclerosis, the data expand and revise the endotheliocentric Tie2 signaling concept to show that mural cell-expressed Tie2 is involved in regulating macrovascular functions. VSMC-expressed Tie2 acts pro-atherosclerotic to control the phenotypic switch towards a proliferative and migratory synthetic VSMC phenotype. The Tie2 ligand Angpt2 acts antiatherosclerotic,which is compatible with an antagonistic mode of action of Angpt2 on Tie2.