Endothelial Basement Membrane Research Articles

Identification of an active site on the laminin α4 chain globular domain that binds to αvβ3 integrin and promotes angiogenesis

Angiogenesis is important for wound healing, tumor growth, and metastasis. The laminin α4 chain, a component of laminin-8 and -9, is expressed in endothelial cell basement membranes. It mediates endothelial cell adhesion by binding with its receptors such as αvβ3 integrin and participates in new blood vessel formation. In this study, we found the recombinant laminin α4LG modules (rLG1-3, rLG1, and rLG2) mediate HUVECs adhesion. The attachment of HUVECs to the rLG2 was specifically inhibited by a function-blocking monoclonal antibody LM609 specific for αvβ3 integrin. Using deletion mutants of the α4LG2 revealed the HUVECs-adhesion site is located in amino acids 1121–1139. A synthetic G 1121–1139 peptide could be attached by HUVECs at same efficiency with the rLG2 and promoted angiogenesis in CAM. In conclusion, we have identified a new αvβ3 integrin-interacting peptide within laminin α4 G domain. This suggests that G 1121–1139 peptide-containing proteins may perform their biological functions by interacting with αvβ3 integrin.

Tunneling through cells

Staphylococcus aureus inactivates RhoA to open tunnels through endothelial cells, report Boyer et al. (page 809). These “macroapertures” may allow the pathogenic bacteria easy access to the endothelial basement membranes for tissue invasion and colonization.

Induction of transient macroapertures in endothelial cells through RhoA inhibition by Staphylococcus aureus factors

The GTPase RhoA is a major regulator of the assembly of actin stress fibers and the contractility of the actomyosin cytoskeleton. The epidermal cell differentiation inhibitor (EDIN) and EDIN-like ADP-ribosyltransferases of Staphylococcus aureus catalyze the inactivation of RhoA, producing actin cable disruption. We report that purified recombinant EDIN and EDIN-producing S. aureus provoke large transcellular tunnels in endothelial cells that we have named macroapertures (MAs). These structures open transiently, followed by the appearance of actin-containing membrane waves extending over the aperture. Disruption of actin cables, either directly or indirectly, through rhoA RNAi knockdown also triggers the formation of MAs. Intoxication of endothelial monolayers by EDIN produces a loss of barrier function and provides direct access of the endothelium basement membrane to S. aureus.

Pulmonary Function Testing in Idiopathic Interstitial Pneumonias

Diffuse parenchymal lung diseases are a group of disorders that involve the space between the epithelial and endothelial basement membranes and are generally segregated into four major categories. These include the idiopathic interstitial pneumonias, which are further categorized into seven clinical/radiologic/pathologic subsets. These disorders generally share a common pattern of physiologic abnormality characterized by a restrictive ventilatory defect and reduced diffusing capacity (DLCO). Pulmonary function testing is often used and recommended in their assessment and management. The potential clinical application of physiologic testing includes to aid in diagnosis, although its value in differential diagnosis is limited. Pulmonary function testing also aids in establishing disease severity and in defining prognosis. In nonspecific interstitial pneumonia and idiopathic pulmonary fibrosis, severely decreased DLCO has proven valuable in this regard. Similarly, exertional desaturation to less than 88% at baseline testing and a decrease in FVC (greater than 10%) over the course of short-term follow-up identify patients at particular risk of mortality. Finally, physiologic testing, especially spirometry and DLCO, have demonstrated value in monitoring response to therapy and identifying disease progression.

Collagen Subtypes in Human Vocal Folds

The collagen subtypes in human vocal folds are of particular interest, because each collagen subtype has different features that make it uniquely suited for performing specific tissue tasks and each collagen subtype can affect the tissue properties of the vocal fold lamina propria. Human vocal folds from 5 autopsy cases (less than 65 years old) were examined by immunohistochemistry for collagen types I, III, IV, and V and elastin. Collagen type III was distributed throughout the whole lamina propria. Type I was found just beneath the basal membrane, in the deep layer of the lamina propria and in the anterior and posterior maculae flavae. Types IV and V were present in the epithelial and endothelial basal membrane. Three-dimensional images from thick specimens reconstructed with confocal microscopy showed 2 distinct patterns: type III fibers were wavy, collagenous fibers, as previously observed in the vocal folds, and type I fibers were thinner than type III fibers. These results suggest that type III fibers help maintain the lamina propria structure and that type I fibers provide the tensile strength required around the basal membrane and vocal ligament to maintain the vocal fold shape while withstanding vibratory forces.

Human Prolyl-4-hydroxylase α(I) Transcription Is Mediated by Upstream Stimulatory Factors

Prolyl-4-hydroxylase alpha(I) (P4Halpha(I)) is the rate-limiting subunit for P4H enzyme activity, which is essential for procollagen hydroxylation and secretion. In the current study, we have characterized the human P4Halpha(I) promoter for transcription factors and DNA elements regulating P4Halpha(I) expression. Using a progressive deletion cloning approach, we have constructed pGL3-P4Halpha(I) recombinant plasmids. We have identified a positive regulatory region at the positions of bp -184 to -97 responsible for approximately 80% of the P4Halpha(I) promoter efficiency. Three E-boxes were located within this region, and the E-box at position bp -135 explains most of the regulatory capacity. Upstream stimulatory factors (USF1/USF2) were shown to bind on the E-box using chromatin immunoprecipitation assay. Suppression of USF1 and/or USF2 using specific short interference RNA resulted in a significant reduction in P4Halpha(I) promoter activity, and overexpressed USF1 or USF2 increased P4Halpha(I) promoter activity significantly. Although transforming growth factor beta1 increased the USF1/USF2-E-box binding and P4Halpha(I) promoter activity, this up-regulatory effect can be largely prevented by USF1/USF2-specific short interference RNA. On the other hand, cigarette smoking extracts, which have been shown to suppress P4Halpha(I) expression, inhibited the binding between the USF1/USF2 and E-box, resulting in a reduced P4Halpha(I) promoter activity. Furthermore, the E-box on the P4Halpha(I) promoter appeared to indiscriminately bind with either USF1 or USF2, with a similar outcome on the promoter efficiency. In conclusion, our study shows that USF1/USF2 plays a critical role in basal P4Halpha(I) expression, and both positive (transforming growth factor beta1) and negative (cigarette smoking extract) regulators appear to influence the USF-E-box interaction and affect P4Halpha(I) expression.

Inflammatory Biomarkers in Acute Coronary Syndromes

This final part of our review series addresses matrix metalloproteinases (MMPs) and biomarkers of platelet activation and concludes with a general discussion of biomarkers in acute coronary syndromes (ACS). ### Metalloproteinases MMPs are zinc-dependent endoproteases with collagenase and/or gelatinase activity (Table). Degradation of collagen fibrils compromises plaque stability and the integrity of the endothelial basement membrane, predisposing advanced atheromas to rupture.1 View this table: Inflammatory Biomarkers in ACS #### MMP-1, -2, and -9 MMP-1, a collagenase expressed in the interstitium, is quickly upregulated in animal models of coronary ischemia/reperfusion. MMP-2 is a gelatinase capable of degrading type IV collagen, the major collagen of the subendothelial basement membrane. MMP-9 is a gelatinase widely implicated in ventricular remodeling and the development of heart failure.2 MMPs are highly expressed in atherosclerotic plaques, with selective enrichment at the shoulder regions.3 Patients with ACS have increased plasma levels of MMP-1, -2, and -9.4 The time course of elevation is highly variable: some groups have found that MMP-1, -2, and -9 levels are not elevated at presentation but that they increase during the subsequent 7 to 14 days.5 Other groups have reported no significant elevation in MMP-2 but a rapid rise and fall of MMP-9 within the first week after ACS. Few data exist on the association, if any, between MMP levels during ACS and cardiovascular outcomes. In a study of 24 patients with ACS, elevations in MMP-1 at 7 and 14 days after ACS were negatively correlated with left ventricular ejection fraction.6 The slow elevation of MMP levels after ACS and the lack of clinical outcome data do not currently make MMPs useful biomarkers for therapeutic decision making or risk stratification in ACS. They do, however, remain an active area of investigation as a therapeutic target. #### Pregnancy-Associated Plasma Protein A Pregnancy-associated plasma protein A …

Idiopathic Interstitial Pneumonias: Usual Interstitial Pneumonia versus Nonspecific Interstitial Pneumonia

Diffuse parenchymal lung diseases (DPLDs) are a group of disorders that involve the space between the epithelial and endothelial basement membranes. Recent guidelines for the classification of DPLDs recommended separating patients into several categories, including (1) DPLDs of known cause, (2) granulomatous DPLDs, (3) rare DPLDs with well-defined clinicopathologic features, and (4) the idiopathic interstitial pneumonias (IIPs). The IIPs are further subdivided into categories that include usual interstitial pneumonia (idiopathic pulmonary fibrosis [IPF] if the usual interstitial pneumonia is idiopathic in origin) and nonspecific interstitial pneumonia. In numerous cohorts, IPF has been associated with impaired prognosis compared with nonspecific interstitial pneumonia, except in the setting of markedly impaired physiology at presentation. It is therefore imperative for the health care provider to rapidly assess the likelihood of a patient having IPF. The diagnostic approach has been refined over the past several years, with much clearer recommendations addressing the optimal clinical, radiologic, and laboratory assessments of patients with IIPs. Similarly, therapeutic interventions have rapidly evolved, with less emphasis on standard immunosuppression and increasing focus on interventions targeting fibroproliferation. Numerous therapeutic trials have recently been completed, are ongoing, or are soon to begin patient recruitment. A multicenter, National Institutes of Health-funded clinical research network has been convened to target novel therapeutic approaches in well-designed controlled trials. The next few years will be an exciting time in the evaluation and treatment of DPLDs as increasing clinically relevant biological information is translated to novel diagnostic and therapeutic approaches.

Leukocyte transmigration is decreased in mice lacking the laminin alpha‐4 chain

Leukocyte extravasation is a critical event in inflammatory reactions. In order for leukocytes to penetrate the vessel wall they need to sequentially interact with the endothelial lining and the perivascular basement membrane (BM). Laminin matrix proteins (laminins 8 and 10) are major constituents of the vascular BM. The laminin alpha-4 chain is a component of laminin-8, and serves roles as a structure protein and as a signaling molecule. The object of this study was to investigate the importance of laminin alpha-4 chain containing laminins in leukocyte transmigration. Intravital microscopy was used to visualize leukocyte adhesion and transmigration in mouse cremasteric venules of laminin alpha-4 chain deficient (Lam−/−) and wild-type mice. The cremaster muscle was stimulated with platelet activating factor (PAF) and readings were taken at early (15 min) and late (60 min) time points of stimulation. Leukocyte transmigration, but not adhesion to the endothelial lining, was markedly suppressed in Lam−/− compared to wild-type mice at 60 min indicating an impaired ability to penetrate the endothelial BM. The results suggest an essential role of interaction with vascular laminin-8 for efficient migration through the vessel wall. Supported by grants from the Swedish Research Council and the Swedish Heart-Lung Foundation.

Tissue-Engineered Blood Vessel for Adult Arterial Revascularization

There is a crucial need for alternatives to native vein or artery for vascular surgery. The clinical efficacy of synthetic, allogeneic or xenogeneic vessels has been limited by thrombosis, rejection, chronic inflammation and poor mechanical properties. Using adult human fibroblasts extracted from skin biopsies harvested from individuals with advanced cardiovascular disease, we constructed tissue-engineered blood vessels (TEBVs) that serve as arterial bypass grafts in long-term animal models. These TEBVs have mechanical properties similar to human blood vessels, without relying upon synthetic or exogenous scaffolding. The TEBVs are antithrombogenic and mechanically stable for 8 months in vivo. Histological analysis showed complete tissue integration and formation of vasa vasorum. The endothelium was confluent and positive for von Willebrand factor. A smooth muscle-specific alpha-actin-positive cell population developed within the TEBV, suggesting regeneration of a vascular media. Electron microscopy showed an endothelial basement membrane, elastogenesis and a complex collagen network. These results indicate that a completely biological and clinically relevant TEBV can be assembled exclusively from an individual's own cells.

Human tissue-engineered blood vessels for adult arterial revascularization

There is a crucial need for alternatives to native vein or artery for vascular surgery. The clinical efficacy of synthetic, allogeneic or xenogeneic vessels has been limited by thrombosis, rejection, chronic inflammation and poor mechanical properties. Using adult human fibroblasts extracted from skin biopsies harvested from individuals with advanced cardiovascular disease, we constructed tissue-engineered blood vessels (TEBVs) that serve as arterial bypass grafts in long-term animal models. These TEBVs have mechanical properties similar to human blood vessels, without relying upon synthetic or exogenous scaffolding. The TEBVs are antithrombogenic and mechanically stable for 8 months in vivo. Histological analysis showed complete tissue integration and formation of vasa vasorum. The endothelium was confluent and positive for von Willebrand factor. A smooth muscle-specific alpha-actin-positive cell population developed within the TEBV, suggesting regeneration of a vascular media. Electron microscopy showed an endothelial basement membrane, elastogenesis and a complex collagen network. These results indicate that a completely biological and clinically relevant TEBV can be assembled exclusively from an individual's own cells.

Characterizing nanoscale topography of the aortic heart valve basement membrane for tissue engineering heart valve scaffold design.

A fully effective prosthetic heart valve has not yet been developed. A successful tissue-engineered valve prosthetic must contain a scaffold that fully supports valve endothelial cell function. Recently, topographic features of scaffolds have been shown to influence the behavior of a variety of cell types and should be considered in rational scaffold design and fabrication. The basement membrane of the aortic valve endothelium provides important parameters for tissue engineering scaffold design. This study presents a quantitative characterization of the topographic features of the native aortic valve endothelial basement membrane; topographical features were measured, and quantitative data were generated using scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and light microscopy. Optimal conditions for basement membrane isolation were established. Histological, immunohistochemical, and TEM analyses following decellularization confirmed basement membrane integrity. SEM and AFM photomicrographs of isolated basement membrane were captured and quantitatively analyzed. The basement membrane of the aortic valve has a rich, felt-like, 3-D nanoscale topography, consisting of pores, fibers, and elevations. All features measured were in the sub-100 nm range. No statistical difference was found between the fibrosal and ventricular surfaces of the cusp. These data provide a rational starting point for the design of extracellular scaffolds with nanoscale topographic features that mimic those found in the native aortic heart valve basement membrane.

Localization of Cysteine Protease, Cathepsin S, to the Surface of Vascular Smooth Muscle Cells by Association with Integrin ανβ3

Smooth muscle cell (SMC) migration from the tunica media to the intima, a key event in neointimal formation, requires proteolytic degradation of elastin-rich extracellular matrix barriers. Although cathepsin S (Cat S) is overexpressed in atherosclerotic and neointimal lesions, its exact role in SMC behavior remains primarily unresolved. We examined the involvement of Cat S on SMC migration through an extracellular matrix barrier and its localization in SMCs. A selective Cat S inhibitor and the endogenous inhibitor cystatin C significantly attenuated SMC invasion across elastin gel. Western blotting and cell surface biotinylation analysis demonstrated localization of the 28-kd active form of Cat S on the SMC surface, consistent with its role in the proteolysis of subcellular matrices. Treatment with interferon-gamma or interleukin-beta1 significantly augmented the ability of SMC membranes to degrade elastin along with a significant increase in the level of active Cat S compared with controls. Immunofluorescence and confocal microscopy showed a punctuated pattern of Cat S clusters at the periphery of SMCs; further studies demonstrated partial co-localization of Cat S and integrin alphanubeta3 at the cell surfaces. These findings demonstrate that active Cat S co-localizes with integrin alphanubeta3 as a receptor on the SMC surface, playing an important role in the invasive behavior of SMCs.

Lymphocyte transcellular migration occurs through recruitment of endothelial ICAM-1 to caveola- and F-actin-rich domains

During inflammation, leukocytes bind to the adhesion receptors ICAM-1 and VCAM-1 on the endothelial surface before undergoing transendothelial migration, also called diapedesis. ICAM-1 is also involved in transendothelial migration, independently of its role in adhesion, but the molecular basis of this function is poorly understood. Here we demonstrate that, following clustering, apical ICAM-1 translocated to caveolin-rich membrane domains close to the ends of actin stress fibres. In these F-actin-rich areas, ICAM-1 was internalized and transcytosed to the basal plasma membrane through caveolae. Human T-lymphocytes extended pseudopodia into endothelial cells in caveolin- and F-actin-enriched areas, induced local translocation of ICAM-1 and caveolin-1 to the endothelial basal membrane and transmigrated through transcellular passages formed by a ring of F-actin and caveolae. Reduction of caveolin-1 levels using RNA interference (RNAi) specifically decreased lymphocyte transcellular transmigration. We propose that the translocation of ICAM-1 to caveola- and F-actin-rich domains links the sequential steps of lymphocyte adhesion and transendothelial migration and facilitates lymphocyte migration through endothelial cells from capillaries into surrounding tissue.

Involvement of Impaired Interaction with .BETA.1 Integrin in Epigallocatechin Gallate-Mediated Inhibition of Fibrosarcoma HT-1080 Cell Adhesion to Fibronectin

Animal studies have shown that tea and tea components have anti-cancer activities. Green tea and black tea catechin compounds such as (–)-epigallocatechin gallate (EGCG) and theaflavin have been investigated intensively to reveal the molecular basis for their anti-tumor activities. Adhesive interaction between tumor cells and extracellular matrix proteins such as fibronectin, laminin, and collagens is deeply involved in tumor growth, invasion, and metastasis. These extracellular proteins comprise the endothelial basement membrane, and interruption of tumor cell adhesion may be effective in the prevention of bloodborne metastasis. Interactions between tumor cells and these extracellular proteins are regulated by integrins, a family of heterodimeric transmembrane receptors. In tumor cells, integrins have been shown to mediate the organization of the extracellular matrix, adhesion to extracellular matrix proteins, and cell motility. We have reported that the binding of EGCG to Fas, presumably on the cell surface, triggers Fasmediated apoptosis in tumor cells and that EGCG impairs the adhesion of cancer cells to extracellular matrix proteins such as fibronectin and laminin by binding to fibronectin and laminin. However, it is not clear whether the binding of EGCG to tumor cells causes an impairment of cell adhesion. In the present study, we examined whether or not EGCG inhibits the adhesion of fibrosarcoma HT-1080 cells to fibronectin by binding to the fibronectin receptor, integrin β1. Involvement of Impaired Interaction with 1 Integrin in Epigallocatechin Gallate-Mediated Inhibition of Fibrosarcoma HT-1080 Cell Adhesion to Fibronectin

Megakaryocytic cells synthesize and platelets secrete α5-laminins, and the endothelial laminin isoform laminin 10 (α5β1γ1) strongly promotes adhesion but not activation of platelets

Following vascular injury, basement membrane (BM) components of the blood vessels are exposed to circulating cells and may contribute to hemostasis and/or thrombosis. Laminins 8 (LN-8) (alpha4beta1gamma1) and 10 (LN-10) (alpha5beta1gamma1) are major laminin isoforms of the endothelial BM, and LN-8 is also secreted by activated platelets. In the present study, we demonstrate synthesis of alpha5-laminins LN-10 and LN-11 (alpha5beta2gamma1) by megakaryocytic cells, and intracellular expression of these laminin isoforms in blood platelets. In contrast to platelet LN alpha4 chain that had an apparent molecular weight of 180 kDa and associated mostly to LNbeta1 chain, platelet LNalpha5 consisted of 300/350 kDa polypeptides and associated mainly to LNbeta2. Both alpha4- and alpha5-laminins were secreted by platelets following stimulation. When compared to recombinant human (rh) LN-8, rhLN-10 was much more adhesive to platelets, though adhesion to both proteins was largely mediated via alpha6beta1 integrin. In spite of their adhesive properties, rhLN-8 and rhLN-10 induced neither P-selectin expression nor cell aggregation, two signs of platelet activation. This study demonstrates synthesis/expression of heterotrimeric alpha5-laminins in hematopoietic/blood cells, and provides evidence for the adhesive, but not activating, role of endothelial laminin isoforms in platelet biology.

Laminin-10 and Lutheran blood group glycoproteins in adhesion of human endothelial cells

Laminin alpha5-chain, a constituent of laminins-10 and -11, is expressed in endothelial basement membranes. In this study we evaluated the roles of alpha5 laminins and Lutheran blood group glycoproteins (Lu), recently identified receptors of the laminin alpha5-chain, in the adhesion of human dermal microvascular and pulmonary artery endothelial cells. Field emission scanning electron microscopy and immunohistochemistry showed that the endothelial cells spread on laminin-10 and formed fibronectin-positive fibrillar adhesion structures. Immunoprecipitation results suggested that the cells produced fibronectin, which they could use as adhesion substratum, during the adhesion process. When the protein synthesis during the adhesion was inhibited with cycloheximide, the formation of fibrillar adhesions on laminin-10 was abolished, suggesting that laminin-10 does not stimulate the formation of any adhesion structures. Northern and Western blot analyses showed that the cells expressed M(r) 78,000 and 85,000 isoforms of Lu. Quantitative cell adhesion assays showed that in the endothelial cell adhesion to laminin-10, Lu acted in concert with integrins beta(1) and alpha(v)beta(3), whereas in the adhesion to laminin-10/11, Lu and integrin beta(1) were involved. In the cells adhering to the alpha5 laminins, Lu and the integrins showed uniform cell surface distribution. These findings indicate that alpha5 laminins stimulate endothelial cell adhesion but not the formation of fibrillar or focal adhesions. Lu mediates the adhesion of human endothelial cells to alpha5 laminins in collaboration with integrins beta(1) and alpha(v)beta(3).

Transendothelial Migration of Monocytes: The Underlying Molecular Mechanisms and Consequences of HIV-1 Infection

Migration of monocytes from the bloodstream across vascular endothelium is required for routine immunological surveillance of tissues and their entry into inflamed sites. Transendothelial migration of monocytes initially involves tethering of cells to the endothelium, followed by loose rolling along the vascular surface, firm adhesion to the endothelium and diapedesis between the tightly apposing endothelial cells. A number of adhesion molecules are involved in this process. Monocyte rolling can be mediated by selectins and their ligands, or alpha(4)beta(1) integrin interacting with endothelial VCAM-1. On the apical surface of the endothelial cell, bound chemokines (eg. MCP-1, MIP-1alpha/beta) can activate leukocyte beta(2) integrins for tight adhesion to ICAM-1 and -2. Diapedesis by monocytes occurs through interaction between PECAM-1 on both the monocyte and the endothelial cells, followed by similar homophilic adhesion via CD99. After penetration of the endothelial basement membrane, monocytes migrate through the extracellular matrix of the tissues where they may differentiate into tissue macrophages and/or migrate to sites of inflammation. Additionally, monocytes in the tissues may traffic to the lymphatics or back into the bloodstream, both of which involve basal to apical (reverse) transendothelial migration, possibly mediated by tissue factor and p-glycoprotein. Monocyte trafficking is of current interest in studies of the pathogenesis of HIV-infection, including establishment of viral reservoirs in tissues and sanctuary sites and the development of HIV-related dementia. This review provides insights into the most recent studies on the process of monocyte migration across the vascular endothelium, and changes in migration that can occur during HIV-infection.

Cellular changes in normal blood capillaries undergoing regression after inhibition of VEGF signaling

The vasculature of the embryo requires vascular endothelial growth factor (VEGF) during development, but most adult blood vessels lose VEGF dependence. However, some capillaries in the respiratory tract and selected other organs of adult mice regress after VEGF inhibition. The present study sought to identify the sequence of events and the fate of endothelial cells, pericytes, and vascular basement membrane during capillary regression in mouse tracheas after VEGF signaling was blocked with a VEGF-receptor tyrosine kinase inhibitor AG-013736 or soluble receptor construct (VEGF Trap or soluble adenoviral VEGFR-1). Within 1 day, patency was lost and fibrin accumulated in some tracheal capillaries. Apoptotic endothelial cells marked by activated caspase-3 were present in capillaries without blood flow. VEGF inhibition was accompanied by a 19% decrease in tracheal capillaries over 7 days and 30% over 21 days. During this period, desmin/NG2-immunoreactive pericytes moved away from regressing capillaries onto surviving vessels. Empty sleeves of basement membrane, left behind by regressing endothelial cells, persisted for about 2 wk and served as a scaffold for vascular regrowth after treatment ended. The amount of regrowth was limited by the number of surviving basement membrane sleeves. These findings demonstrate that, after inhibition of VEGF signaling, some normal capillaries regress in a systematic sequence of events initiated by a cessation of blood flow and followed by apoptosis of endothelial cells, migration of pericytes away from regressing vessels, and formation of empty basement membrane sleeves that can facilitate capillary regrowth.

Collagen XVIII/endostatin is associated with the epithelial–mesenchymal transformation in the atrioventricular valves during cardiac development

Type XVIII collagen is a multidomain protein that contains cleavable C-terminal NC1 and endostatin fragments, which have been shown to either induce or inhibit cell migration. Endostatin is being intensely studied because of its anti-angiogenic activity. Three variants of type XVIII collagen have been reported to be distributed in epithelial and endothelial basement membranes in a tissue-specific manner. The single gene encoding collagen XVIII is on chromosome 21 within the region associated with the congenital heart disease phenotype observed in Down's syndrome. In this study, we investigated the expression pattern of collagen XVIII in embryonic mouse hearts during formation of the atrioventricular (AV) valves. We found that collagen XVIII is localized not only in various basement membranes but is also highly expressed throughout the connective tissue core of the endocardial cushions and forming AV valve leaflets. It was closely associated with the epithelial-mesenchymal transformation of endothelial cells into mesenchymal cushion tissue cells and was localized around these cells as they migrated into the cardiac jelly to form the initial connective tissue elements of the valve leaflets. However, after embryonic day 17.5 collagen XVIII expression decreased rapidly in the connective tissue and thereafter remained detectable only in the basement membranes of the endothelial layer covering the leaflets. The staining pattern observed within the AV endocardial cushions suggests that collagen XVIII may have a role in cardiac valve morphogenesis. These results may help us to better understand normal heart development and the aberrant mechanisms that cause cardiac malformations in Down's syndrome.