Multiple cardiovascular defects caused by the absence of alternatively spliced segments of fibronectin

Vascular wall extracellular matrix proteins and vascular diseases

Extracellular matrix proteins form the basic structure of blood vessels. Along with providing basic structural support to blood vessels, matrix proteins interact with different sets of vascular cells via cell surface integrin or non-integrin receptors. Such interactions induce vascular cell de novo synthesis of new matrix proteins during blood vessel development or remodeling. Under pathological conditions, vascular matrix proteins undergo proteolytic processing, yielding bioactive fragments to influence vascular wall matrix remodeling. Vascular cells also produce alternatively spliced variants that induce vascular cell production of different matrix proteins to interrupt matrix homeostasis, leading to increased blood vessel stiffness; vascular cell migration, proliferation, or death; or vascular wall leakage and rupture. Destruction of vascular matrix proteins leads to vascular cell or blood-borne leukocyte accumulation, proliferation, and neointima formation within the vascular wall; blood vessels prone to uncontrolled enlargement during blood flow diastole; tortuous vein development; and neovascularization from existing pathological tissue microvessels. Here we summarize discoveries related to blood vessel matrix proteins within the past decade from basic and clinical studies in humans and animals - from expression to cross-linking, assembly, and degradation under physiological and vascular pathological conditions, including atherosclerosis, aortic aneurysms, varicose veins, and hypertension.

Alternative splicing of endothelial fibronectin is induced by disturbed hemodynamics and protects against hemorrhage of the vessel wall

OBJECTIVE

Abnormally low-flow conditions, sensed by the arterial endothelium, promote aneurysm rupture. Fibronectin (FN) is among the most abundant extracellular matrix proteins and is strongly upregulated in human aneurysms, suggesting a possible role in disease progression. Altered FN splicing can result in the inclusion of EIIIA and EIIIB exons, generally not expressed in adult tissues. We sought to explore the regulation of FN and its splicing and their possible roles in the vascular response to disturbed flow.

APPROACH AND RESULTS

We induced low and reversing flow in mice by partial carotid ligation and assayed FN splicing in an endothelium-enriched intimal preparation. Inclusion of EIIIA and EIIIB was increased as early as 48 hours, with negligible increases in total FN expression. To test the function of EIIIA and EIIIB inclusion, we induced disturbed flow in EIIIAB(-/-) mice unable to include these exons and found that they developed focal lesions with hemorrhage and hypertrophy of the vessel wall. Acute deletion of floxed FN caused similar defects in response to disturbed flow, consistent with a requirement for the upregulation of the spliced isoforms, rather than a developmental defect. Recruited macrophages promote FN splicing because their depletion by clodronate liposomes blocked the increase in endothelial EIIIA and EIIIB inclusion in the carotid model.

CONCLUSIONS

These results uncover a protective mechanism in the inflamed intima that develops under disturbed flow, by showing that splicing of FN mRNA in the endothelium, induced by macrophages, inhibits hemorrhage of the vessel wall.

Extracellular matrix modulates angiogenesis in physiological and pathological conditions

Angiogenesis is a multistep process driven by a wide range of positive and negative regulatory factors. Extracellular matrix (ECM) plays a crucial role in the regulation of this process. The degradation of ECM, occurring in response to an angiogenic stimulus, leads to degradation or partial modification of matrix molecules, release of soluble factors, and exposure of cryptic sites with pro- and/or antiangiogenic activity. ECM molecules and fragments, resulting from proteolysis, can also act directly as inflammatory stimuli, and this can explain the exacerbated angiogenesis that drives and maintains several inflammatory diseases. In this review we have summarized some of the more recent literature data concerning the molecular control of ECM in angiogenesis in both physiological and pathological conditions.

Fibronectin matrix polymerization regulates smooth muscle cell phenotype through a Rac1 dependent mechanism

Smooth muscle cells are maintained in a differentiated state in the vessel wall, but can be modulated to a synthetic phenotype following injury. Smooth muscle phenotypic modulation is thought to play an important role in the pathology of vascular occlusive diseases. Phenotypically modulated smooth muscle cells exhibit increased proliferative and migratory properties that accompany the downregulation of smooth muscle cell marker proteins. Extracellular matrix proteins, including fibronectin, can regulate the smooth muscle phenotype when used as adhesive substrates. However, cells produce and organize a 3-dimensional fibrillar extracellular matrix, which can affect cell behavior in distinct ways from the protomeric 2-dimensional matrix proteins that are used as adhesive substrates. We previously showed that the deposition/polymerization of fibronectin into the extracellular matrix can regulate the deposition and organization of other extracellular matrix molecules in vitro. Further, our published data show that the presence of a fibronectin polymerization inhibitor results in increased expression of smooth muscle cell differentiation proteins and inhibits vascular remodeling in vivo. In this manuscript, we used an in vitro cell culture system to determine the mechanism by which fibronectin polymerization affects smooth muscle phenotypic modulation. Our data show that fibronectin polymerization decreases the mRNA levels of multiple smooth muscle differentiation genes, and downregulates the levels of smooth muscle α-actin and calponin proteins by a Rac1-dependent mechanism. The expression of smooth muscle genes is transcriptionally regulated by fibronectin polymerization, as evidenced by the increased activity of luciferase reporter constructs in the presence of a fibronectin polymerization inhibitor. Fibronectin polymerization also promotes smooth muscle cell growth, and decreases the levels of actin stress fibers. These data define a Rac1-dependent pathway wherein fibronectin polymerization promotes the SMC synthetic phenotype by modulating the expression of smooth muscle cell differentiation proteins.

Circulating fibronectin controls tumor growth

Fibronectin is ubiquitously expressed in the extracellular matrix, and experimental evidence has shown that it modulates blood vessel formation. The relative contribution of local and circulating fibronectin to blood vessel formation in vivo remains unknown despite evidence for unexpected roles of circulating fibronectin in various diseases. Using transgenic mouse models, we established that circulating fibronectin facilitates the growth of bone metastases by enhancing blood vessel formation and maturation. This effect is more relevant than that of fibronectin produced by endothelial cells and pericytes, which only exert a small additive effect on vessel maturation. Circulating fibronectin enhances its local production in tumors through a positive feedback loop and increases the amount of vascular endothelial growth factor (VEGF) retained in the matrix. Both fibronectin and VEGF then cooperate to stimulate blood vessel formation. Fibronectin content in the tumor correlates with the number of blood vessels and tumor growth in the mouse models. Consistent with these results, examination of three separate arrays from patients with breast and prostate cancers revealed that a high staining intensity for fibronectin in tumors is associated with increased mortality. These results establish that circulating fibronectin modulates blood vessel formation and tumor growth by modifying the amount of and the response to VEGF. Furthermore, determination of the fibronectin content can serve as a prognostic biomarker for breast and prostate cancers and possibly other cancers.

Non-collagenous ECM proteins in blood vessel morphogenesis and cancer

BACKGROUND

The extracellular matrix (ECM) is constituted by diverse composite structures, which determine the specific to each organ, histological architecture and provides cells with biological information, mechanical support and a scaffold for adhesion and migration. The pleiotropic effects of the ECM stem from the dynamic changes in its molecular composition and the ability to remodel in order to effectively regulate biological outcomes. Besides collagens, fibronectin and laminin are two major fiber-forming constituents of various ECM structures.

SCOPE OF REVIEW

This review will focus on the properties and the biological functions of non-collagenous extracellular matrix especially on laminin and fibronectin that are currently emerging as important regulators of blood vessel formation and function in health and disease.

MAJOR CONCLUSIONS

The ECM is a fundamental component of the microenvironment of blood vessels, with activities extending beyond providing a vascular scaffold; extremely versatile it directly or indirectly modulates all essential cellular functions crucial for angiogenesis, including cell adhesion, migration, proliferation, differentiation and lumen formation. Specifically, fibronectin and laminins play decisive roles in blood vessel morphogenesis both during embryonic development and in pathological conditions, such as cancer.

GENERAL SIGNIFICANCE

Emerging evidence demonstrates the importance of ECM function during embryonic development, organ formation and tissue homeostasis. A wealth of data also illustrates the crucial role of the ECM in several human pathophysiological processes, including fibrosis, skeletal diseases, vascular pathologies and cancer. Notably, several ECM components have been identified as potential therapeutic targets for various diseases, including cancer. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

Molecular composition and function of integrin-based collagen glues-introducing COLINBRIs

BACKGROUND

Despite detailed knowledge about the structure and signaling properties of individual collagen receptors, much remains to be learned about how these receptors participate in linking cells to fibrillar collagen matrices in tissues. In addition to collagen-binding integrins, a group of proteins with affinity both for fibrillar collagens and integrins link these two protein families together. We have introduced the name COLINBRI (COLlagen INtegrin BRIdging) for this set of molecules. Whereas collagens are the major building blocks in tissues and defects in these structural proteins have severe consequences for tissue integrity, the mild phenotypes of the integrin type of collagen receptors have raised questions about their importance in tissue biology and pathology.

SCOPE OF REVIEW

We will discuss the two types of cell linkages to fibrillar collagen (direct- versus indirect COLINBRI-mediated) and discuss how the parallel existence of direct and indirect linkages to collagens may ensure tissue integrity.

MAJOR CONCLUSIONS

The observed mild phenotypes of mice deficient in collagen-binding integrins and the relatively restricted availability of integrin-binding sequences in mature fibrillar collagen matrices support the existence of indirect collagen-binding mechanisms in parallel with direct collagen binding in vivo.

GENERAL SIGNIFICANCE

A continued focus on understanding the molecular details of cell adhesion mechanisms to collagens will be important and will benefit our understanding of diseases like tissue- and tumor fibrosis where collagen dynamics are disturbed. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

Tissue-specific and SRSF1-dependent splicing of fibronectin, a matrix protein that controls host cell invasion

Matching sets of human primary fibroblasts cocultured with placenta explants are used to compare tissue capacities to support trophoblast invasion. Substituting endometrium with dermis dramatically reduces EVCT interstitial invasion, a phenomenon related to the ECM fibronectin content, FN alternative splicing, and expression of the SR protein SRSF1.

Cell invasion targets specific tissues in physiological placental implantation and pathological metastasis, which raises questions about how this process is controlled. We compare dermis and endometrium capacities to support trophoblast invasion, using matching sets of human primary fibroblasts in a coculture assay with human placental explants. Substituting endometrium, the natural trophoblast target, with dermis dramatically reduces trophoblast interstitial invasion. Our data reveal that endometrium expresses a higher rate of the fibronectin (FN) extra type III domain A+ (EDA+) splicing isoform, which displays stronger matrix incorporation capacity. We demonstrate that the high FN content of the endometrium matrix, and not specifically the EDA domain, supports trophoblast invasion by showing that forced incorporation of plasma FN (EDA–) promotes efficient trophoblast invasion. We further show that the serine/arginine-rich protein serine/arginine-rich splicing factor 1 (SRSF1) is more highly expressed in endometrium and, using RNA interference, that it is involved in the higher EDA exon inclusion rate in endometrium. Our data therefore show a mechanism by which tissues can be distinguished, for their capacity to support invasion, by their different rates of EDA inclusion, linked to their SRSF1 protein levels. In the broader context of cancer pathology, the results suggest that SRSF1 might play a central role not only in the tumor cells, but also in the surrounding stroma.

The EDA-containing cellular fibronectin induces epithelial-mesenchymal transition in lung cancer cells through integrin α9β1-mediated activation of PI3-K/AKT and Erk1/2

Cellular fibronectin (cFN) is one of the main components of tissue extracellular matrices and is involved in multiple physiologic and pathologic processes such as embryogenesis, wound healing, inflammation and tumor progression. The function of fibronectin in regulating normal cell adhesion and migration is well documented, but its function in cancer progression is only partially unraveled. We have reported previously that fibronectin stimulates the proliferation and survival of non-small lung carcinoma cells through upregulation of pro-oncogenic signals related to cyclooxygenase-2/phosphatidylinositol-3-kinase/protein kinase B (COX-2/PI3-K/AKT)/mammalian target of rapamycin triggered by activation of the integrin α5β1. Here, we extend these studies by showing that fibronectin promotes epithelial-mesenchymal transition (EMT) in lung cancer cells. We found that cFN, but not plasma fibronectin or type 1 collagen, induces lung carcinoma cell scattering in vitro, promotes cell migration and invasion of Matrigel and stimulates the expression of the mesenchymal marker α-smooth muscle actin while decreasing the expression of the epithelial marker E-cadherin through PI3-K and Erk pathways. Interestingly, the extra domain A (EDA) within cFN was found to be crucial for this process, as confirmed by testing cells overexpressing EDA or cells exposed to EDA-containing matrices. We found that the integrin α9, but not α5, mediated cFN-induced EMT as silencing integrin α9 neutralized cFN-induced EMT. Overall, our findings show that the EDA domain within cFN induces EMT in lung carcinoma cells through integrin α9-mediated activation of PI3-K and Erk.

Fibronectin and stem cell differentiation - lessons from chondrogenesis

The extracellular matrix (ECM) is an intricate network of proteins that surrounds cells and has a central role in establishing an environment that is conducive to tissue-specific cell functions. In the case of stem cells, this environment is the stem cell niche, where ECM signals participate in cell fate decisions. In this Commentary, we describe how changes in ECM composition and mechanical properties can affect cell shape and stem cell differentiation. Using chondrogenic differentiation as a model, we examine the changes in the ECM that occur before and during mesenchymal stem cell differentiation. In particular, we focus on the main ECM protein fibronectin, its temporal expression pattern during chondrogenic differentiation, its potential effects on functions of differentiating chondrocytes, and how its interactions with other ECM components might affect cartilage development. Finally, we discuss data that support the possibility that the fibronectin matrix has an instructive role in directing cells through the condensation, proliferation and/or differentiation stages of cartilage formation.

Myocardin-like protein 2 regulates TGFβ signaling in embryonic stem cells and the developing vasculature

The molecular mechanisms that regulate and coordinate signaling between the extracellular matrix (ECM) and cells contributing to the developing vasculature are complex and poorly understood. Myocardin-like protein 2 (MKL2) is a transcriptional co-activator that in response to RhoA and cytoskeletal actin signals physically associates with serum response factor (SRF), activating a subset of SRF-regulated genes. We now report the discovery of a previously undescribed MKL2/TGFβ signaling pathway in embryonic stem (ES) cells that is required for maturation and stabilization of the embryonic vasculature. Mkl2(-/-) null embryos exhibit profound derangements in the tunica media of select arteries and arterial beds, which leads to aneurysmal dilation, dissection and hemorrhage. Remarkably, TGFβ expression, TGFβ signaling and TGFβ-regulated genes encoding ECM are downregulated in Mkl2(-/-) ES cells and the vasculature of Mkl2(-/-) embryos. The gene encoding TGFβ2, the predominant TGFβ isoform expressed in vascular smooth muscle cells and embryonic vasculature, is activated directly via binding of an MKL2/SRF protein complex to a conserved CArG box in the TGFβ2 promoter. Moreover, Mkl2(-/-) ES cells exhibit derangements in cytoskeletal organization, cell adhesion and expression of ECM that are rescued by forced expression of TGFβ2. Taken together, these data demonstrate that MKL2 regulates a conserved TGF-β signaling pathway that is required for angiogenesis and ultimately embryonic survival.

Fibronectin extra domain-A promotes hepatic stellate cell motility but not differentiation into myofibroblasts

BACKGROUND & AIMS

Myofibroblasts are the primary cell type involved in physiologic wound healing and its pathologic counterpart, fibrosis. Cellular fibronectin that contains the alternatively spliced extra domain A (EIIIA) is up-regulated during these processes and is believed to promote myofibroblast differentiation. We sought to determine the requirement for EIIIA in fibrosis and differentiation of myofibroblasts in rodent livers.

METHODS

We used a mechanically tunable hydrogel cell culture system to study differentiation of primary hepatic stellate cells and portal fibroblasts from rats into myofibroblasts. Liver fibrosis was induced in mice by bile duct ligation or administration of thioacetamide.

RESULTS

EIIIA was not required for differentiation of rat hepatic stellate cells or portal fibroblasts into fibrogenic myofibroblasts. Instead, hepatic stellate cells cultured on EIIIA-containing cellular fibronectin formed increased numbers of lamellipodia; their random motility and chemotaxis also increased. These increases required the receptor for EIIIA, the integrin α(9)β(1). In contrast, the motility of portal fibroblasts did not increase on EIIIA, and these cells expressed little α(9)β(1). Male EIIIA(-/-) mice were modestly protected from thioacetamide-induced fibrosis, which requires motile hepatic stellate cells, but not from bile duct ligation-induced fibrosis, in which portal fibroblasts are more important. Notably, myofibroblasts developed during induction of fibrosis with either thioacetamide or bile duct ligation in EIIIA(-/-) mice.

CONCLUSIONS

EIIIA is dispensable for differentiation of hepatic stellate cells and portal fibroblasts to myofibroblasts, both in culture and in mouse models of fibrosis. Our findings, however, indicate a role for EIIIA in promoting stellate cell motility and parenchymal liver fibrosis.

Extra-domain B in oncofetal fibronectin structurally promotes fibrillar head-to-tail dimerization of extracellular matrix protein

The type III extra-domain B (ED-B) is specifically spliced into fibronectin (Fn) during embryogenesis and neoangiogenesis, including many cancers. The x-ray structure of the recombinant four-domain fragment Fn(III)7B89 reveals a tightly associated, extended head-to-tail dimer, which is stabilized via pair-wise shape and charge complementarity. A tendency toward ED-B-dependent dimer formation in solution was supported by size exclusion chromatography and analytical ultracentrifugation. When amending the model with the known three-dimensional structure of the Fn(III)10 domain, its RGD loop as well as the adhesion synergy region in Fn(III)9-10 become displayed on the same face of the dimer; this should allow simultaneous binding of at least two integrins and, thus, receptor clustering on the cell surface and intracellular signaling. Insertion of ED-B appears to stabilize overall head-to-tail dimerization of two separate Fn chains, which, together with alternating homodimer formation via disulfide bridges at the C-terminal Fn tail, should lead to the known macromolecular fibril formation.

Extracellular matrix proteins in hemostasis and thrombosis

The adhesion and aggregation of platelets during hemostasis and thrombosis represents one of the best-understood examples of cell-matrix adhesion. Platelets are exposed to a wide variety of extracellular matrix (ECM) proteins once blood vessels are damaged and basement membranes and interstitial ECM are exposed. Platelet adhesion to these ECM proteins involves ECM receptors familiar in other contexts, such as integrins. The major platelet-specific integrin, αIIbβ3, is the best-understood ECM receptor and exhibits the most tightly regulated switch between inactive and active states. Once activated, αIIbβ3 binds many different ECM proteins, including fibrinogen, its major ligand. In addition to αIIbβ3, there are other integrins expressed at lower levels on platelets and responsible for adhesion to additional ECM proteins. There are also some important nonintegrin ECM receptors, GPIb-V-IX and GPVI, which are specific to platelets. These receptors play major roles in platelet adhesion and in the activation of the integrins and of other platelet responses, such as cytoskeletal organization and exocytosis of additional ECM ligands and autoactivators of the platelets.

Endothelial cell-pericyte interactions stimulate basement membrane matrix assembly: influence on vascular tube remodeling, maturation, and stabilization

Extracellular matrix synthesis and deposition surrounding the developing vasculature are critical for vessel remodeling and maturation events. Although the basement membrane is an integral structure underlying endothelial cells (ECs), few studies, until recently, have been performed to understand its formation in this context. In this review article, we highlight new data demonstrating a corequirement for ECs and pericytes to properly deposit and assemble vascular basement membranes during morphogenic events. In EC only cultures or under conditions whereby pericyte recruitment is blocked, there is a lack of basement membrane assembly, decreased vessel stability (with increased susceptibility to pro-regressive stimuli), and increased EC tube widths (a marker of dysfunctional EC-pericyte interactions). ECs and pericytes both contribute basement membrane components and, furthermore, both cells induce the expression of particular components as well as integrins that recognize them. The EC-derived factors--platelet derived growth factor-BB and heparin binding-epidermal growth factor--are both critical for pericyte recruitment to EC tubes and concomitant vascular basement membrane formation in vitro and in vivo. Thus, heterotypic EC-pericyte interactions play a fundamental role in vascular basement membrane matrix deposition, a critical tube maturation event that is altered in key disease states such as diabetes and cancer.

Fibronectin splice variants: understanding their multiple roles in health and disease using engineered mouse models

The extracellular matrix (ECM) is a highly dynamic network of proteins, glycoproteins, and proteoglycans. Numerous diseases result from mutation in genes coding for ECM proteins, but only recently it has been reported that mutations in the fibronectin (FN) gene were associated with a human disorder. FN is one of the main components of the ECM. It generates protein diversity through alternative splicing of a single pre-mRNA, having at least 20 different isoforms in humans. The precise function of these protein isoforms has remained obscure in most cases. Only in the recent few years, it was possible to shed light on the multiple roles of the alternatively spliced FN isoforms. This substantial progress was achieved basically with the knowledge derived from engineered mouse models bearing subtle mutations in specific FN domains. These data, together with a recent report associating mutations in the FN gene to a form of glomerulopathy, clearly show that mutations in constitutive exons or misregulation of alternatively spliced domains of the FN gene may have nonlethal pathological consequences. In this review, we focus on the pathological consequences of mutations in the FN gene, by connecting the function of alternatively spliced isoforms of fibronectin to human diseases.

Molecular mechanisms controlling vascular lumen formation in three-dimensional extracellular matrices

Considerable progress has been made toward a molecular understanding of how cells form lumen and tube structures in three-dimensional (3D) extracellular matrices (ECM). This progress has occurred through work performed with endothelial and epithelial cell models using both in vitro and in vivo approaches. Despite the apparent similarities between endothelial and epithelial cell lumen and tube formation mechanisms, there are clear distinctions that directly relate to their functional differences. This review will focus on endothelial cell (EC) lumen formation mechanisms which control blood vessel formation during development and postnatal life. Of great interest is that an EC lumen signaling complex has been identified which controls human EC lumen and tube formation in 3D matrices and which coordinates integrin-ECM contacts, cell surface proteolysis, cytoskeletal rearrangements, and cell polarity. This complex consists of the collagen-binding integrin α2β1, the collagen-degrading membrane-type 1 matrix metalloproteinase (MT1-MMP), junction adhesion molecule (Jam)C, JamB, polarity proteins Par3 and Par6b, and the Rho GTPase Cdc42-GTP. These interacting proteins are necessary to stimulate 3D matrix-specific signaling events (including activation of protein kinase cascades that regulate the actin and microtubule cytoskeletons) to control the formation of EC lumens and tube networks. Also, EC lumen formation is directly coupled to the generation of vascular guidance tunnels, enzymatically generated ECM conduits that facilitate EC tube remodeling and maturation. Mural cells such as pericytes are recruited along EC tubes within these tunnel spaces to control ECM remodeling events resulting in vascular basement membrane matrix assembly, a key step in tube maturation and stabilization.

Angiogenesis

Extracellular matrix (ECM) is essential for all stages of angiogenesis. In the adult, angiogenesis begins with endothelial cell (EC) activation, degradation of vascular basement membrane, and vascular sprouting within interstitial matrix. During this sprouting phase, ECM binding to integrins provides critical signaling support for EC proliferation, survival, and migration. ECM also signals the EC cytoskeleton to initiate blood vessel morphogenesis. Dynamic remodeling of ECM, particularly by membrane-type matrix metalloproteases (MT-MMPs), coordinates formation of vascular tubes with lumens and provides guidance tunnels for pericytes that assist ECs in the assembly of vascular basement membrane. ECM also provides a binding scaffold for a variety of cytokines that exert essential signaling functions during angiogenesis. In the embryo, ECM is equally critical for angiogenesis and vessel stabilization, although there are likely important distinctions from the adult because of differences in composition and abundance of specific ECM components.

Fibronectins, their fibrillogenesis, and in vivo functions

Fibronectin (FN) is a multidomain protein with the ability to bind simultaneously to cell surface receptors, collagen, proteoglycans, and other FN molecules. Many of these domains and interactions are also involved in the assembly of FN dimers into a multimeric fibrillar matrix. When, where, and how FN binds to its various partners must be controlled and coordinated during fibrillogenesis. Steps in the process of FN fibrillogenesis including FN self-association, receptor activities, and intracellular pathways have been under intense investigation for years. In this review, the domain organization of FN including the extra domains and variable region that are controlled by alternative splicing are described. We discuss how FN-FN and cell-FN interactions play essential roles in the initiation and progression of matrix assembly using complementary results from cell culture and embryonic model systems that have enhanced our understanding of this process.

Different patterns of fibronectin and tenascin-C splice variants expression in primary and metastatic melanoma lesions

We have investigated the staining patterns of primary and metastatic melanoma lesions using F8, L19 and F16. These three clinical-stage antibodies are currently being studied in clinical trials for the pharmacodelivery of cytokines or therapeutic radionuclides to neoplastic sites in patients with cancer. Frozen sections of 24 primary and 29 metastatic melanoma lesions were stained, using immunofluorescence procedures, with biotinylated preparations of the F8, L19 and F16 antibodies, which are specific to the alternatively spliced extra domain A and extra domain B domains of fibronectin and A1 domain of tenascin-C, respectively. Blood vessels were costained using von Willebrand factor-specific antibodies. In primary cutaneous melanoma lesions, F16 and F8 (but not L19) strongly stained the basal lamina at the interface between epidermis and dermis, with a strikingly complementary pattern. By contrast, metastatic melanoma lesions displayed a strong and diffuse pattern of immunoreactivity with all three antibodies. It was found that the extracellular matrix in melanoma undergoes extensive remodelling during the transition from primary to metastatic lesions. The intense staining of metastatic melanoma lesions by the F8, L19 and F16 antibodies provides a strong rationale for the use of these antibodies and their derivatives for the treatment of melanoma patients and possibly for the personalized choice of the best performing antibody in individual patients.

Extracellular matrix and heart development

The extracellular matrix (ECM) of the developing heart contains numerous molecules that form a dynamic environment that plays an active and crucial role in the regulation of cellular events. ECM molecules found in the heart include hyaluronan, fibronectin, fibrillin, proteoglycans, and collagens. Tight regulation of the spatiotemporal expression, and the proteolytic processing of ECM components by proteases including members of the ADAMTS family, is essential for normal cardiac development. Perturbation of the expression of genes involved in matrix composition and remodeling can interfere with a myriad of events involved in the formation of the four-chambered heart and result in prenatal lethality or cardiac malformations as seen in humans with congenital heart disease. In this review, we summarize what is known about the specific importance of some of the components of the ECM in relation to the cardiovascular development.

Essential roles of fibronectin in the development of the left-right embryonic body plan

Studies in Xenopus laevis suggested that cell-extracellular matrix (ECM) interactions regulate the development of the left-right axis of asymmetry; however, the identities of ECM components and their receptors important for this process have remained unknown. We discovered that FN is required for the establishment of the asymmetric gene expression pattern in early mouse embryos by regulating morphogenesis of the node, while cellular fates of the nodal cells, canonical Wnt and Shh signaling within the node were not perturbed by the absence of FN. FN is also required for the expression of Lefty 1/2 and activation of SMADs 2 and 3 at the floor plate, while cell fate specification of the notochord and the floor plate, as well as signaling within and between these two embryonic organizing centers remained intact in FN-null mutants. Furthermore, our experiments indicate that a major cell surface receptor for FN, integrin α5β1, is also required for the development of the left-right asymmetry, and that this requirement is evolutionarily conserved in fish and mice. Taken together, our studies demonstrate the requisite role for a structural ECM protein and its integrin receptor in the development of the left-right axis of asymmetry in vertebrates.

Genetic analyses of integrin signaling

The development of multicellular organisms, as well as maintenance of organ architecture and function, requires robust regulation of cell fates. This is in part achieved by conserved signaling pathways through which cells process extracellular information and translate this information into changes in proliferation, differentiation, migration, and cell shape. Gene deletion studies in higher eukaryotes have assigned critical roles for components of the extracellular matrix (ECM) and their cellular receptors in a vast number of developmental processes, indicating that a large proportion of this signaling is regulated by cell-ECM interactions. In addition, genetic alterations in components of this signaling axis play causative roles in several human diseases. This review will discuss what genetic analyses in mice and lower organisms have taught us about adhesion signaling in development and disease.

Molecular basis for endothelial lumen formation and tubulogenesis during vasculogenesis and angiogenic sprouting

Many studies reveal a fundamental role for extracellular matrix-mediated signaling through integrins and Rho GTPases as well as matrix metalloproteinases (MMPs) in the molecular control of vascular tube morphogenesis in three-dimensional (3D) tissue environments. Recent work has defined an endothelial cell (EC) lumen signaling complex of proteins that controls these vascular morphogenic events. These findings reveal a signaling interdependence between Cdc42 and MT1-MMP to control the 3D matrix-specific process of EC tubulogenesis. The EC tube formation process results in the creation of a network of proteolytically generated vascular guidance tunnels in 3D matrices that are utilized to remodel EC-lined tubes through EC motility and could facilitate processes such as flow-induced remodeling and arteriovenous EC sorting and differentiation. Within vascular guidance tunnels, key dynamic interactions occur between ECs and pericytes to affect vessel remodeling, diameter, and vascular basement membrane matrix assembly, a fundamental process necessary for endothelial tube maturation and stabilization. Thus, the EC lumen and tube formation mechanism coordinates the concomitant establishment of a network of vascular tubes within tunnel spaces to allow for flow responsiveness, EC-mural cell interactions, and vascular extracellular matrix assembly to control the development of the functional microcirculation.

Impaired splicing of fibronectin is associated with thoracic aortic aneurysm formation in patients with bicuspid aortic valve

OBJECTIVE

Thoracic aortic aneurysm is a common complication in patients with bicuspid aortic valve (BAV). Alternatively spliced extra domain A (EDA) of fibronectin (FN) has an essential role in tissue repair. Here we analyze the expression of FN spliceforms in dilated and nondilated ascending aorta of tricuspid aortic valve (TAV) and BAV patients.

METHODS AND RESULTS

The mRNA expression was analyzed in the ascending aorta by Affymetrix Exon arrays in patients with TAV (n=40) and BAV (n=69). EDA and extra domain B (EDB) expression was increased in dilated aorta from TAV patients compared with nondilated aorta (P<0.001 and P<0.05, respectively). In contrast, EDA expression was not increased in dilated aorta from BAV patients (P=0.25), whereas EDB expression was upregulated (P<0.01). The expression of EDA correlated with maximum aortic diameter in TAV (ρ=0.58) but not in BAV (ρ=0.15) patients. Protein analyses of EDA-FN showed concordant results. Transforming growth factor-β treatment influenced the splicing of FN and enhanced the formation of EDA-containing FN in cultured medial cells from TAV patients but not in cells derived from BAV patients. Gene set enrichment analysis together with multivariate and univariate data analyses of mRNA expression suggested that differences in the transforming growth factor-β signaling pathway may explain the impaired EDA inclusion in BAV patients.

CONCLUSIONS

Decreased EDA expression may contribute to increased aneurysm susceptibility of BAV patients.

Autocrine fibronectin directs matrix assembly and crosstalk between cell–matrix and cell–cell adhesion in vascular endothelial cells

Cellular fibronectin (cFN) variants harboring extra FN type 3 repeats, namely extra domains B and A, are major constituents of the extracellular matrix around newly forming blood vessels during development and angiogenesis. Their expression is induced by angiogenic stimuli and their assembly into fibrillar arrays is driven by cell-generated tension at α5β1 integrin-based adhesions. Here, we examined the role and functional redundancy of cFN variants in cultured endothelial cells by isoform-selective RNA interference. We show that FN fibrillogenesis is a cell-autonomous process whereby basally directed secretion and assembly of cellular FN are tightly coupled events that play an important role not only in signaling at cell–matrix adhesions but also at cell–cell contacts. Silencing of cFN variants differentially affects integrin usage, cell spreading, motility and capillary morphogenesis in vitro. cFN-deficient cells undergo a switch from α5β1- to αvβ3-based adhesion, accompanied by a Src-regulated disruption of adherens junctions. These studies identify a crucial role for autocrine FN in subendothelial matrix assembly and junctional integrity that provides spatially and temporally restricted control of endothelial plasticity during angiogenic blood vessel remodeling.

Assembly of fibronectin extracellular matrix

In the process of matrix assembly, multivalent extracellular matrix (ECM) proteins are induced to self-associate and to interact with other ECM proteins to form fibrillar networks. Matrix assembly is usually initiated by ECM glycoproteins binding to cell surface receptors, such as fibronectin (FN) dimers binding to α5ß1 integrin. Receptor binding stimulates FN self-association mediated by the N-terminal assembly domain and organizes the actin cytoskeleton to promote cell contractility. FN conformational changes expose additional binding sites that participate in fibril formation and in conversion of fibrils into a stabilized, insoluble form. Once assembled, the FN matrix impacts tissue organization by contributing to the assembly of other ECM proteins. Here, we describe the major steps, molecular interactions, and cellular mechanisms involved in assembling FN dimers into fibrillar matrix while highlighting important issues and major questions that require further investigation.

The extracellular matrix in development and morphogenesis: a dynamic view

The extracellular matrix (ECM) is synthesized and secreted by embryonic cells beginning at the earliest stages of development. Our understanding of ECM composition, structure and function has grown considerably in the last several decades and this knowledge has revealed that the extracellular microenvironment is critically important for cell growth, survival, differentiation and morphogenesis. ECM and the cellular receptors that interact with it mediate both physical linkages with the cytoskeleton and the bidirectional flow of information between the extracellular and intracellular compartments. This review considers the range of cell and tissue functions attributed to ECM molecules and summarizes recent findings specific to key developmental processes. The importance of ECM as a dynamic repository for growth factors is highlighted along with more recent studies implicating the 3-dimensional organization and physical properties of the ECM as it relates to cell signaling and the regulation of morphogenetic cell behaviors. Embryonic cell and tissue generated forces and mechanical signals arising from ECM adhesion represent emerging areas of interest in this field.

The extracellular matrix: not just pretty fibrils

The extracellular matrix (ECM) and ECM proteins are important in phenomena as diverse as developmental patterning, stem cell niches, cancer, and genetic diseases. The ECM has many effects beyond providing structural support. ECM proteins typically include multiple, independently folded domains whose sequences and arrangement are highly conserved. Some of these domains bind adhesion receptors such as integrins that mediate cell-matrix adhesion and also transduce signals into cells. However, ECM proteins also bind soluble growth factors and regulate their distribution, activation, and presentation to cells. As organized, solid-phase ligands, ECM proteins can integrate complex, multivalent signals to cells in a spatially patterned and regulated fashion. These properties need to be incorporated into considerations of the functions of the ECM.

Angiogenesis in differentiated placental multipotent mesenchymal stromal cells is dependent on integrin alpha5beta1

Human placental multipotent mesenchymal stromal cells (hPMSCs) can be isolated from term placenta, but their angiogenic ability and the regulatory pathways involved are not known. hPMSCs were shown to express integrins α_v, α₄, α₅, β₁, β₃, and β₅ and could be induced to differentiate into cells expressing endothelial markers. Increases in cell surface integrins α₅ and β₁, but not α₄, α_vβ₃, or α_vβ₅, accompanied endothelial differentiation. Vascular endothelial growth factor-A augmented the effect of fibronectin in enhancing adhesion and migration of differentiated hPMSC through integrin α₅β₁, but not α_vβ₃ or α_vβ₅. Formation of capillary-like structures in vitro from differentiated cells was inhibited by pre-treatment with function-blocking antibodies to integrins α₅ and β₁. When hPMSCs were seeded onto chick chorioallantoic membranes (CAM), human von Willebrand factor-positive cells were observed to engraft in the chick endothelium. CAMs transplanted with differentiated hPMSCs had a greater number of vessels containing human cells and more incorporated cells per vessel compared to CAMs transplanted with undifferentiated hPMSCs, and overall angiogenesis was enhanced more by the differentiated cells. Function-blocking antibodies to integrins α₅ and β₁ inhibited angiogenesis in the CAM assay. These results suggest that differentiated hPMSCs may contribute to blood vessel formation, and this activity depends on integrin α₅β₁.

Pericyte recruitment during vasculogenic tube assembly stimulates endothelial basement membrane matrix formation

We show that endothelial cell (EC)-generated vascular guidance tunnels (ie, matrix spaces created during tube formation) serve as conduits for the recruitment and motility of pericytes along EC ablumenal surfaces to facilitate vessel maturation events, including vascular basement membrane matrix assembly and restriction of EC tube diameter. During quail development, pericyte recruitment along microvascular tubes directly correlates with vascular basement membrane matrix deposition. Pericyte recruitment to EC tubes leads to specific induction of fibronectin and nidogen-1 (ie, matrix-bridging proteins that link together basement membrane components) as well as perlecan and laminin isoforms. Coincident with these events, up-regulation of integrins, alpha(5)beta(1), alpha(3)beta(1), alpha(6)beta(1), and alpha(1)beta(1), which bind fibronectin, nidogens, laminin isoforms, and collagen type IV, occurs in EC-pericyte cocultures, but not EC-only cultures. Integrin-blocking antibodies to these receptors, disruption of fibronectin matrix assembly, and small interfering RNA suppression of pericyte tissue inhibitor of metalloproteinase (TIMP)-3 (a known regulator of vascular tube stabilization) all lead to decreased EC basement membrane, resulting in increased vessel lumen diameter, a key indicator of dysfunctional EC-pericyte interactions. Thus, pericyte recruitment to EC-lined tubes during vasculogenesis is a stimulatory event controlling vascular basement membrane matrix assembly, a fundamental maturation step regulating the transition from vascular morphogenesis to stabilization.

Integrins as "functional hubs" in the regulation of pathological angiogenesis

It is well accepted that complex biological processes such as angiogenesis are not controlled by a single family of molecules or individually isolated signaling pathways. In this regard, new insight into the interconnected mechanisms that regulate angiogenesis might be gained by examining this process from a more global network perspective. The coordination of signaling cues from both outside and inside many different cell types is required for the successful completion of angiogenesis. Evidence is accumulating that the multifunctional integrin family of cell adhesion receptors represent an important group of molecules that play active roles in sensing, integrating, and distributing a diverse set of signals that regulate many cellular events required for angiogenesis. Given the ability of integrins to bind numerous extracellular ligands and transmit signals in a bi-directional fashion, we will discuss the multiple ways by which integrins may serve as a functional hub during pathological angiogenesis. In addition, we will highlight potential imaging and therapeutic strategies based on the expanding new insight into integrin function.

Comparative immunohistochemical staining of atherosclerotic plaques using F16, F8 and L19: Three clinical-grade fully human antibodies

OBJECTIVE

F16, F8 and L19 are three fully human monoclonal antibodies, specific to splice isoforms of tenascin-C and fibronectin, which stain sites of active tissue remodeling and which are currently in Phase I and II clinical trials as radio-immunoconjugates and immunocytokines in patients with cancer and arthritis. The characterization of atherosclerosis using these antibodies may open novel pharmacodelivery options for the imaging and treatment of cardiovascular conditions. It may also allow a better assessment of the corresponding immunoconjugates in polymorbid patients with atherosclerotic plaques.

METHODS

We performed a comparative immunohistochemical analysis with the F16, F8 and L19 antibodies in 28 freshly frozen human carotid plaques and in 11 normal arteries. Furthermore, we assessed the localization of the antibodies in relation to the infiltrating macrophages, vasa vasorum and Ki67-positive proliferating cells of the plaque.

RESULTS

The F16 antibody, specific to the extra-domain A1 of tenascin-C, stained plaques with a selective and intense pattern, while F8 and L19, specific to the EDA and EDB domains of fibronectin, respectively, exhibited a less selective and intense staining. In immunofluorescence, F16 was found to bind regions rich in macrophages, vasa vasorum and proliferating cells, while showing no detectable vs. weak staining of normal arteries and of quiescent plaque structures.

CONCLUSION

The human monoclonal antibody F16 stains areas of active tissue remodeling in atherosclerotic plaques and may thus deserve to be investigated as a suitable building block for the development of radiopharmaceuticals for plaque imaging or for the antibody-based targeted delivery of therapeutic agents to atherosclerotic lesions.

Expression of the oncofetal ED-B–containing fibronectin isoform in hematologic tumors enables ED-B–targeted 131I-L19SIP radioimmunotherapy in Hodgkin lymphoma patients

Current treatment of hematologic malignancies involves rather unspecific chemotherapy, frequently resulting in severe adverse events. Thus, modern clinical research focuses on compounds able to discriminate malignant from normal tissues. Being expressed in newly formed blood vessels of solid cancers but not in normal mature tissues, the extradomain B of fibronectin (ED-B FN) is a promising target for selective cancer therapies. Using immunohistology with a new epitope retrieval technique for paraffin-embedded tissues, ED-B FN expression was found in biopsies from more than 200 Hodgkin and non-Hodgkin lymphoma patients of nearly all entities, and in patients with myeloproliferative diseases. ED-B FN expression was nearly absent in normal lymph nodes (n = 10) and bone marrow biopsies (n = 9). The extent of vascular ED-B FN expression in lymphoma tissues was positively correlated with grade of malignancy. ED-B FN expression was enhanced in lymph nodes with severe lymphadenopathy and in some hyperplastic tonsils. The in vivo accessibility of ED-B FN was confirmed in 3 lymphoma patients, in whom the lymphoma lesions were visualized on scintigraphy with 131I-labeled L19 small immunoprotein (131I-L19SIP). In 2 relapsed Hodgkin lymphoma patients131I-L19SIP radioimmunotherapy induced a sustained partial response, qualifying ED-B FN as a promising target for antibody-based lymphoma therapies.

Fibronectins in vascular morphogenesis

Fibronectin is an extracellular matrix protein found only in vertebrate organisms containing endothelium-lined vasculature and is required for cardiovascular development in fish and mice. Fibronectin and its splice variants containing EIIIA and EIIIB domains are highly upregulated around newly developing vasculature during embryogenesis and in pathological conditions including atherosclerosis, cardiac hypertrophy, and tumorigenesis. However, their molecular roles in these processes are not entirely understood. We review genetic studies examining functions of fibronectin and its splice variants during embryonic cardiovascular development, and discuss potential roles of fibronectin in vascular disease and tumor angiogenesis.

Anastellin, the angiostatic fibronectin peptide, is a selective inhibitor of lysophospholipid signaling

Angiogenesis is regulated by integrin-dependent cell adhesion and the activation of specific cell surface receptors on vascular endothelial cells by angiogenic factors. Lysophosphatidic acid (LPA) and sphingosine-1 phosphate (S1P) are bioactive lysophospholipids that activate G protein-coupled receptors that stimulate phosphatidylinositol 3-kinase (PI3K), Ras, and Rho effector pathways involved in vascular cell survival, proliferation, adhesion, and migration. Previous studies have shown that anastellin, a fragment of the first type III module of fibronectin, functions as an antiangiogenic peptide suppressing tumor growth and metastasis. We have previously shown that anastellin blocks serum-dependent proliferation of microvessel endothelial cells (MVEC) by affecting extracellular signal-regulated kinase (ERK)-dependent G(1)-S transition. However, the mechanism by which anastellin regulates endothelial cell function remains unclear. In the present study, we mapped several lysophospholipid-mediated signaling pathways in MVEC and examined the effects of anastellin on LPA- and S1P-induced MVEC proliferation, migration, and cytoskeletal organization. Both LPA and S1P activated PI3K, Ras/ERK, and Rho/Rho kinase pathways, leading to migration, G(1)-S cell cycle progression, and stress fiber formation, respectively. Stimulation of proliferation by LPA/S1P occurred through a G(i)-dependent Ras/ERK pathway, which was independent of growth factor receptors and PI3K and Rho/Rho kinase signaling. Although LPA and S1P activated both PI3K/Akt and Ras/ERK signaling through G(i), anastellin inhibited only the Ras/ERK pathway. Stress fiber formation in response to LPA was dependent on Rho/Rho kinase but independent of G(i) and unaffected by anastellin. These results suggest that lysophospholipid mediators of G(i) activation leading to PI3K/Akt and Ras/ERK signaling bifurcate downstream of G(i) and that anastellin selectively inhibits the Ras/ERK arm of the pathway.

Fibronectin promotes differentiation of neural crest progenitors endowed with smooth muscle cell potential

The neural crest (NC) is a model system used to investigate multipotency during vertebrate development. Environmental factors control NC cell fate decisions. Despite the well-known influence of extracellular matrix molecules in NC cell migration, the issue of whether they also influence NC cell differentiation has not been addressed at the single cell level. By analyzing mass and clonal cultures of mouse cephalic and quail trunk NC cells, we show for the first time that fibronectin (FN) promotes differentiation into the smooth muscle cell phenotype without affecting differentiation into glia, neurons, and melanocytes. Time course analysis indicated that the FN-induced effect was not related to massive cell death or proliferation of smooth muscle cells. Finally, by comparing clonal cultures of quail trunk NC cells grown on FN and collagen type IV (CLIV), we found that FN strongly increased both NC cell survival and the proportion of unipotent and oligopotent NC progenitors endowed with smooth muscle potential. In contrast, melanocytic progenitors were prominent in clonogenic NC cells grown on CLIV. Taken together, these results show that FN promotes NC cell differentiation along the smooth muscle lineage, and therefore plays an important role in fate decisions of NC progenitor cells.

Quantification of fibronectin 1 (FN1) splice variants, including two novel ones, and analysis of integrins as candidate FN1 receptors in bovine preimplantation embryos

Background

Fibronectin 1 (FN1), a glycoprotein component of the extracellular matrix, exerts different functions during reproductive processes such as fertilisation, gastrulation and implantation. FN1 expression has been described to increase significantly from the morula towards the early blastocyst stage, suggesting that FN1 may also be involved in early blastocyst formation. By alternative splicing at 3 defined regions, different FN1 isoforms are generated, each with a unique biological function. The analysis of the alternative FN1 splicing on the one hand and the search for candidate FN1 receptors on the other hand during early bovine embryo development may reveal more about its function during bovine preimplantation embryo development.

Results

RT-qPCR quantification of the FN1 splice isoforms in oocytes, embryos, cumulus cells and adult tissue samples revealed a large variation in overall FN1 expression and in splice variant expression. Moreover, two new FN1 transcript variants were identified, the first one expressed in bovine preimplantation embryos and the second one expressed in cumulus cells.

In the search for candidate receptors for the new embryo specific FN1 isoform, RNA expression analysis identified 5 α integrin subunits (ITGA2B, ITGA3, ITGA5, ITGA8, ITGAV) and 2 β integrin subunits (ITGB1 and ITGB3) with a similar or overlapping RNA expression pattern as compared to FN1. But double immunofluorescent stainings could not confirm complete co-localisation between FN1 and one out of 3 selected integrins alpha subunits (ITGA3, ITGA5, ITGAV).

Conclusion

The existence of a new FN1 transcript variant, specifically expressed in morulae and blastocysts strengthens the idea that FN1 is involved in the process of compaction and blastocyst formation. Analysis of the integrin expression could not identify the binding partner for the embryo specific FN1 transcript variant making further steps necessary for the identification of the FN1 receptor and the downstream effects of FN1-receptor binding.

Comparative immunohistochemistry of L19 and F16 in non-small cell lung cancer and mesothelioma: two human antibodies investigated in clinical trials in patients with cancer

The antibody-mediated targeted delivery of therapeutics to tumor sites is an attractive avenue for combating cancer while sparing normal tissues. Indeed, five derivatives of the human monoclonal antibodies L19 and F16, specific to splice isoforms of fibronectin and tenascin-C, are currently being investigated in clinical trials in patients with malignancies. Until now, a comparative immunohistochemical analysis of these antibodies, which recognize components of the modified extracellular matrix, was missing. Here, we report that the majority of NSCLC and mesothelioma specimens are stained with both antibodies in the stroma, while non-tumoral lung and mesothelium samples rarely exhibit reactivity with either L19 or F16. In our analysis, the anti-tenascin F16 antibody was found to generally exhibit a stronger staining of desmoplastic stroma surrounding tumor. This superior performance was found to be particularly striking in the case of low-grade non-small cell lung cancer.

New insights into form and function of fibronectin splice variants

The extracellular matrix (ECM) is a highly dynamic structure that not only provides a physical framework for cells within connective tissues, but also imparts instructive signals for development, tissue homeostasis and basic cell functions through its composition and ability to exert mechanical forces. The ECM of tissues is composed of, in addition to proteoglycans and hyaluronic acid, a number of proteins, most of which are generated after alternative splicing of their pre-mRNA. However, the precise function of these protein isoforms is still obscure in most cases. Fibronectin (FN), one of the main components of the ECM, is also one of the best-known examples of a family of proteins generated by alternative splicing, having at least 20 different isoforms in humans. Over the last few years, considerable progress on elucidating the functions of the alternatively spliced FN isoforms has been achieved with the essential development of key engineered mouse strains. Here we summarize the phenotypes of the mouse strains having targeted mutations in the FN gene, which may lead to novel insights linking function of alternatively spliced isoforms of fibronectin to human pathologies.

Integrins in angiogenesis and lymphangiogenesis

Blood vessels promote tumour growth, and both blood and lymphatic vessels facilitate tumour metastasis by serving as conduits for the transport of tumour cells to new sites. Angiogenesis and lymphangiogenesis are regulated by integrins, which are members of a family of cell surface receptors whose ligands are extracellular matrix proteins and immunoglobulin superfamily molecules. Select integrins promote endothelial cell migration and survival during angiogenesis and lymphangiogenesis, whereas other integrins promote pro-angiogenic macrophage trafficking to tumours. Several integrin-targeted therapeutic agents are currently in clinical trials for cancer therapy. Here, we review the evidence implicating integrins as a family of fundamental regulators of angiogenesis and lymphangiogenesis.

The role of integrin binding sites in fibronectin matrix assembly in vivo

The extracellular matrix (ECM) glycoprotein fibronectin (FN) requires the help of cells to assemble into a functional fibrillar matrix, which then orchestrates the assembly of other ECM proteins and promotes cell adhesion, migration and signalling. Fibrillogenesis is initiated and governed by cell surface integrins that bind to specific sites in the FN molecule. Recent studies identified novel integrin binding sites in FN that can also participate in FN fibril formation and in morphogenetic events during development.