Review TRENDS in Cell Biology Vol.13 No.1 January 2003 51 Extracellular matrix in vascular morphogenesis and disease: structure versus signalq Benjamin S. Brooke, Satyajit K. Karnik and Dean Y. Li Program in Human Molecular Biology and Genetics, Departments of Medicine and Oncological Science, University of Utah School of Medicine, Salt Lake City, UT 84112, USA The vascular system matures during embryonic elastic fibers and smooth-muscle cells, separated by development to form a stable, well-organized tubular interlamellar matrix containing collagens, microfibrils, network. In vivo data have established that the extra- proteoglycans, glycoproteins and ground substance cellular matrix (ECM) is crucial in providing structural (Fig. 1). Finally, the adventitia extends beyond the support to the vascular system. In vitro studies are external elastic lamina and is composed mainly of defining the involvement of ECM–smooth-muscle cell collagen and fibroblasts. Although these ECM molecules signaling in establishing and maintaining the mature have been identified within the vessel wall through tubular structure. However, correlating cell signaling ultrastructural analyses and biochemical studies, their with established structural functions for the ECM and precise interactions are relatively unknown. The com- determining the relative importance of these two roles plex relationship between vascular cells and the ECM in vivo is often difficult. Here, we examine human remainstobedetermined. genetics, murine gene targeting and cell biology to During the past two decades, human genetic studies, better understand the relationship between structural murine knockout models, biomechanical testing and other and signaling roles for the ECM in vascular morphogen- methods have established the importance of the ECM in esis and disease. maintaining the mature tubular structure of the vascular system. These studies indicate that ECM molecules such The assembly and maintenance of a mature tubular as fibrillins, collagens and elastin provide crucial mech- network for blood circulation are crucial during embryo- anical support to the vessel wall during development and nic development. There are three major components of in the mature state. Specifically, these components this network: endothelial cells (ECs), vascular smooth- maintain the competence of the tubular network under muscle cells (VSMCs) and the extracellular matrix hemodynamic pressure [4]. More recently, in vitro studies (ECM). ECs differentiate and form a primordial network of tubes through intussusception, growth and regres- sion. These EC tubes in turn provide signals that lead to the recruitment of VSMCs [1]. The third component consists of ECM molecules produced and organized Vascular smooth by VSMCs within the developing vessel wall. The ECM muscle cell can account for over 50% of the dry weight of the Elastic fiber vasculature and is largely deposited towards the end of Collagen type I development [2]. and type III TheECMofthematurevesselwallisacomplex arrangement of fibrous proteins and associated glyco- Microfibril proteins embedded in a hydrated ground substance of glycosaminoglycans and proteoglycans [3]. These ECM TRENDS in Cell Biology molecules and the vascular cells they associate with are responsible for organizing the vessel wall into three Fig. 1. Simplified schematic representation of mature vessel wall. The media of the discrete layers: the intima, the media and the adven- vessel wall is organized into lamellar units consisting of concentric layers of elastic titia. The intima is composed mostly of ground lamellae, vascular smooth-muscle cells (VSMCs) and interlamellar matrix. Elastic fibers are composed predominantly of elastin, whereas the interlamellar matrix substance separating ECs from the internal elastic includes type-I collagen, type-III collagen and microfibrils such as fibrillin. All of lamina (IEL). The media begins at the IEL and is these extracellular-matrix components provide structural organization to the organized into concentric lamellar units composed of vessel wall through interactions with VSMCs and associated extracellular-matrix elements such as glycoproteins, proteoglycans and glycosaminoglycans. Col- q lagens bind and signal VSMCs via specific matrix receptors. Elastic fibers are This article is the fourth review in our Tube Morphogenesis series linked to VSMCs through a microfibril scaffold composed of fibrillin and microfi- that commenced in the August 2002 issue of TCB. bril-associated glycoproteins. A direct interaction between elastin and VSMCs still Corresponding author: Dean Y. Li ([email protected]). remains to be elucidated. http://ticb.trends.com 0962-8924/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0962-8924(02)00007-7 52 Review TRENDS in Cell Biology Vol.13 No.1 January 2003 and collagen [5,6]. The specific importance of fibrillin-1 microfibrils to vessel-wall structure was discovered by Arterial dilation, dissection and human genetic studies linking mutations in the gene rupture encoding fibrillin-1 (FBN1 ) to Marfan syndrome [7]. Cardiovascular abnormalities are the major source of • Mutations in fibrillin-1 • Mutations in type I morbidity and mortality in Marfan syndrome, and involve collagen dilation of the aortic root with subsequent risk of aortic • Mutations in type III dissection and rupture, and sudden death (Fig. 2). collagen Subsequently, the generation and analysis of two mouse strains with mutations in FBN1 confirmed the crucial role of fibrillin-1 in stabilizing elastic-fiber structure in the mature vessel [8–10]. Both of these mutant mouse strains died postnatally from vessel dissection and rupture (Fig. 2). Specifically, a deficiency of fibrillin-1 appeared to • Mutations in destabilize the elastic-fiber architecture and to make the elastin vessel susceptible to injury from hemodynamic forces and Normal artery inflammation. Finally, human genetic studies and murine gene targeting have failed to reveal a role for fibrillin-2 in vascular morphogenesis or disease [7,11]. Therefore, the in Arterial occlusion TRENDS in Cell Biology vivo data support a specific role for fibrillin-1 in main- taining elastic-fiber structure and integrity in the vessel wall (Table 1). Fig. 2. Pathology of vascular disease resulting from extracellular-matrix gene mutations. There are two main types of pathology associated with vascular dis- The role of fibrillin-1 in cell signaling is less well ease – that in which the arterial lumen becomes occluded and that in which the characterized. Fibrillin-1 polypeptides with RGD vessel wall becomes weakened and subsequently dilates, dissects and ruptures. sequences are recognized by avb3 integrins in vitro, Human genetic studies and murine gene targeting have demonstrated that mutations in vascular extracellular-matrix components can result in pathological and this interaction has been shown to play a role in cell features of each type. Mutations in fibrillin-1, type-I collagen and type-III collagen attachment and adhesion [12] (Fig. 3a). The avb3 lead to arterial dilation, dissection and rupture. By contrast, mutations in elastin lead to arterial occlusion. integrin is known to regulate VSMC activity, including migration, proliferation, adhesion and survival [13,14]. However, avb3 integrins also recognize a wide range of have defined specific signaling interactions between the RGD-containing ECM components, including vitro- ECM and VSMCs of the vessel wall. Together, these nectin, fibronectin and collagen [14]. Mice lacking the experiments implicate the ECM as an integrated scaffold av integrin are embryonic lethal and have multiple that has both structural and signaling functions. However, defects in vasculogenesis, angiogenesis and organo- the relative importance of these two roles is not always genesis, including distended vessels of the perineural clear. Here, we highlight some of the issues to consider plexus [15]. The promiscuous binding of the avintegrin, when reconciling in vitro and in vivo data supporting however, precludes us from making any definite con- structural and signaling roles for ECM in vascular clusions from these in vivo data. morphogenesis and disease. We limit our examination to A model of fibrillin–VSMC signaling was proposed fibrillins, collagens and elastin. based on pathological data showing that VSMCs from fibrillin-1 mutant mice have cytoplasmic changes charac- Fibrillin teristic of synthetic, proliferative cells [10]. This model The fibrillins are a class of extracellular microfibrils that proposes that the loss of fibrillin–VSMC interactions leads associate with elastic fibers in the arterial wall. There are to phenotypic changes in VSMCs from a contractile two highly homologous isoforms: fibrillin-1, encoded by a quiescent state to a synthetic proliferative state. These gene on chromosome 15, and fibrillin-2, encoded by a gene cells subsequently release matrix metalloproteinases that on chromosome 5 [5,6]. Both fibrillin microfibrils are degrade the vessel wall. However, these data are based on known to interact with multiple ECM components within descriptive characterization of mutant mice and need to be the vessel wall, including elastin, vitronectin, fibronectin correlated with in vitro cell signaling experiments. At Table 1. Vascular phenotypes resulting from extracellular-matrix (ECM) gene mutations in humans and mice ECM fibrillar protein Human vascular disease Human vascular phenotype Mouse knockout phenotype Refs Fibrillin-1 Marfan syndrome Arterial dilation, dissection