Parsing Aortic Aneurysms

Abstract

See related article, pages 574–581 Aortic aneurysms are progressive enlargements of the aorta that can lead to life-threatening degenerative changes in the structure of the artery wall including medial dissections and wall rupture. The abdominal aorta is the most common site of aneurysm formation, yet the ascending thoracic aorta can also exhibit aneurysmal expansion with potentially dire outcomes. Ascending aortic aneurysms are frequently associated with connective tissue diseases such as Marfan's syndrome, a disorder of the extracellular matrix protein fibrillin-1.1 Mouse models carrying hypomorphic mutations in fibrillin-1 also develop aortic dilations associated with macrophage infiltration, elastin fragmentation, and enhanced transforming growth factor (TGF)β signaling in the artery wall.2 Aortic aneurysms in fibrillin-1 hypomorphic mice can be prevented by treatment with either anti-TGFβ neutralizing antibodies or with the angiotensin II (Ang II) type 1 (AT1) receptor antagonist losartan.3 Indeed, chronic infusion of Ang II into apoE −/− mice is an effective experimental model to induce aortic aneurysm formation and to study the molecular pathways underlying its pathogenesis.4 Ang II signals through 2 types of angiotensin receptors, AT1 and AT2.5 AT2 receptors have been shown to elicit antiproliferative responses in vascular smooth muscle cells (SMCs), possibly by antagonism of signaling through AT1 receptors.5 In mice, the AT1 receptor exists as 2 subtypes, AT1a and AT1b, and both receptor subtypes are expressed in the artery wall. The receptor that mediates angiotensin-induced contractile responses in vascular smooth muscle is predominantly the AT1b subtype.6 The AT1 receptor subtype that mediates aortic aneurysm formation has been unknown. In a study that appears in this issue of Circulation Research , Rateri et al used a series of mouse models to genetically dissect the process of Ang II–induced aneurysm …