Roles of Fibrillin-1 and Fibronectin in Thoracic Aortopathy - Adhesion Receptor Signaling

Yale University, New Haven CT

Investigators

Abstract

Project Summary – Project 3 Thoracic aortic aneurysms can be driven by variants in genes coding for extracellular matrix (ECM) proteins such as fibrillin-1 (Fbn1), in vascular smooth muscle cell (SMC) cytoskeletal proteins, or their regulators such as Acta2. These genes correspond to components and regulators of the SMC elastic-contractile unit in which elastin fibers are linked to contractile actomyosin filaments through integrins and cytoskeletal linkers that form focal adhesions in the medial SMCs of the aortic wall. In the healthy aorta, changes in wall stress due to blood pressure initiate homeostatic remodeling that restores wall stress close to initial levels; by contrast, when key structural or regulatory components are mutated, wall stress initiates pathological remodeling that expands the aorta and weakens the tissue, eventually resulting in catastrophic mechanical failure in the form of dissection or rupture. Pathological remodeling involves changes in SMC phenotype, with decreased contractile gene expression, induction of degradative programs, and major changes in the composition and organization of the extracellular matrix that feed back to further affect SMC phenotype. Our overarching hypothesis is that the SMC focal adhesions are central to these processes. Focal adhesions are impact by the composition and organization of the ECM, by organization and force generation by contractile actin stress fibers or myofibrils inside the cells, and by tension transmitted across these structures. Published and Preliminary data from my lab show that Fbn1 promotes SMC contractile gene expression, while fibronectin, which is strongly increased in aneurysms, promotes the inflammatory/degradative phenotype and suppresses contractile gene expression. In of these both cases, regulatory effects are exerted through focal adhesion/integrin signaling. For fibronectin, harmful effects involve signaling the cytoplasmic domain of the main fibronectin receptor, integrin α5. Based on these results, we propose a coherent program to elucidate the contribution of focal adhesion signaling to aneurysm development via changes in ECM and contractile proteins. We will leverage this knowledge to identify targets for normalizing focal adhesion signaling and improving patient outcomes. Specific Aim 1 will elucidate the effect of deletion or mutation of Acta2 on focal adhesion signaling. Aim 2 will test whether the interaction between Fbn1 and Magp1/2 promotes SMC contractile gene expression and identify the receptors and mechanisms that mediate cell interactions and regulatory effects. Aims 3 and 4 are based on our finding that mutation of the integrin α5 cytoplasmic domain strongly protects Fbn1 Marfan mice from aneurysm development and fatality. In Aim 3, we will determine the separate contributions of integrin α5 in the intimal, medial, and adventitial layers of the aorta. In Aim 4, we will identify the effector downstream of integrin α5 that mediates harmful effects of fibronectin using both genetic and pharmacological methods. These Aims are highly synergistic and closely collaborative with other Projects and Cores within the PPG. Together, these studies will elucidate the mechano- chemical signaling pathways by which mutations drive thoracic aneurysms and identify novel therapeutic targets.