Genetic mouse models elucidate the roles of adrenomedullin in cardiovascular development and physiology

Adrenomedullin (AM) is a highly conserved, secreted 52 amino acid peptide that functions in physiological processes within the nervous, reproductive, and cardiovascular systems. AM is nearly ubiquitously expressed, but most highly expressed from the vasculature, lungs, and heart. During cardiovascular stresses including myocardial infarction, hypertension, renal failure and normal pregnancy, AM serum levels are elevated suggesting that AM functions as a cardioprotective factor. AM signals through a unique paradigm of G-protein coupled receptor (GPCR) signaling in which the affinity of calcitonin receptor-like receptor (CLR=protein, Calcrl=gene) for its ligands, AM or calcitonin gene-related peptide, is dictated by a family of single-pass transmembrane proteins called receptor activity modifying proteins (RAMPs). Ramp2-CLR and Ramp3-CLR specifically bind AM while RAMP1-CLR functions as the CGRP receptor. Research presented in this dissertation aims to further our understanding of AM signaling in mammalian cardiovascular physiology through the utilization of genetic mouse models and in vitro approaches.;I demonstrate that gene-targeted knockout mice of AM, Calcrl , and Ramp2 are embryonic lethal from specific defects in lymphatic endothelial cell (LEC) proliferation resulting in generalized interstitial edema. Within this study, I present a model in which AM signaling components are enriched during LEC differentiation which, in the absence of AM signaling, explains the observed hypoplastic lymphatic vascular development. I also investigated whether a loss in LEC barrier function could have contributed to this phenotype. While I found no difference in the ultra structural features or expression of LEC junctional components in knockout mice, I show that in vivo and in vitro AM stabilizes the LEC barrier and can completely abrogate the highly permeabilizing actions of vascular endothelial growth factor A, supporting an important role of AM signaling in LEC barrier regulation. In a separate study, I generated and characterized a vascular smooth muscle cell-specific Calcrl deficient mouse. These mice survived to adulthood with no defects in the regulation of basal blood pressure or cardiovascular function which may be due to compensation by other hemodynamic mechanisms. Together, these studies have built a solid foundation that will one day benefit future clinical applications of AM in lymphatic and vascular smooth muscle pathologies.