Abstract 5296: Receptor Activity-Modifying Protein 2 (RAMP2) is Essential for Angiogenesis and Vascular Integrity
Adrenomedullin (AM), originally identified as a vasodilating peptide, is now recognized to be a pleiotropic vasoactive molecule involved in both the pathogenesis of cardiovascular diseases and circulatory homeostasis. In the present study, we generated and analyzed knockout mice of receptor activity-modifying protein 2 (RAMP2), a small membrane protein and a modulator of G-protein coupled receptor (GPCR), and showed that it is the key determinant of the vascular functions of AM. Like AM−/− embryos, RAMP2−/− embryos died in utero at mid-gestation due to vascular fragility that led to severe systemic edema and hemorrhage. They also showed accumulation of pericardial effusion suggestive of cardiac failure. The expression of AM was upregulated by 5-fold in RAMP2−/−, showing a compensatory response to the absence of the “functional” receptor of AM. Vascular endothelial cells (ECs) in RAMP2−/− embryos were severely deformed and detached from the basement membrane. In addition, there was abnormal thinning of the arterial walls and deformation of their multi-layer structure. Some of the ECs appeared apoptotic. Expression of tight junction, adherence junction and basement membrane molecules by ECs was diminished in RAMP2−/− embryos, leading to paracellular leakage, which likely explains the severe edema and hemorrhage. Unlike their homozygous knockout littermates, RAMP2+/− mice survived until adulthood and were fertile. In RAMP2+/−mice, the aortic expression of RAMP2 was reduced to about half that seen in wild-type mice. RAMP2+/− mice had higher blood pressures than wild-type mice. With acute infusion of AM, RAMP2+/− mice showed a smaller blood pressure response than wild-type mice. In RAMP2+/−mice, reduced RAMP2 expression impaired neovascularization and caused vascular hyperper-meability in the skin and brain edema models. Conversely, ECs overexpressing RAMP2 showed enhanced capillary formation, firmer tight junctions and reduced vascular permeability. Our findings demonstrate that RAMP2 is the key determinant of AM’s vascular functions and RAMP2 is essential for angiogenesis and vascular integrity.