A quantitative analysis of the incorporation of fibulin-1 into extracellular matrix indicates that fibronectin assembly is required

Identification of potential shared gene signatures between gastric cancer and type 2 diabetes: a data-driven analysis

Background

Gastric cancer (GC) and type 2 diabetes (T2D) contribute to each other, but the interaction mechanisms remain undiscovered. The goal of this research was to explore shared genes as well as crosstalk mechanisms between GC and T2D.

Methods

The Gene Expression Omnibus (GEO) database served as the source of the GC and T2D datasets. The differentially expressed genes (DEGs) and weighted gene co-expression network analysis (WGCNA) were utilized to identify representative genes. In addition, overlapping genes between the representative genes of the two diseases were used for functional enrichment analysis and protein-protein interaction (PPI) network. Next, hub genes were filtered through two machine learning algorithms. Finally, external validation was undertaken with data from the Cancer Genome Atlas (TCGA) database.

Results

A total of 292 and 541 DEGs were obtained from the GC (GSE29272) and T2D (GSE164416) datasets, respectively. In addition, 2,704 and 336 module genes were identified in GC and T2D. Following their intersection, 104 crosstalk genes were identified. Enrichment analysis indicated that "ECM-receptor interaction," "AGE-RAGE signaling pathway in diabetic complications," "aging," and "cellular response to copper ion" were mutual pathways. Through the PPI network, 10 genes were identified as candidate hub genes. Machine learning further selected BGN, VCAN, FN1, FBLN1, COL4A5, COL1A1, and COL6A3 as hub genes.

Conclusion

"ECM-receptor interaction," "AGE-RAGE signaling pathway in diabetic complications," "aging," and "cellular response to copper ion" were revealed as possible crosstalk mechanisms. BGN, VCAN, FN1, FBLN1, COL4A5, COL1A1, and COL6A3 were identified as shared genes and potential therapeutic targets for people suffering from GC and T2D.

Fibronectin isoforms in skeletal development and associated disorders

The extracellular matrix is an intricate and essential network of proteins and non-proteinaceous components that provide a conducive microenvironment for cells to regulate cell function, differentiation, and survival. Fibronectin is one key component in the extracellular matrix that participates in determining cell fate and function crucial for normal vertebrate development. Fibronectin undergoes time dependent expression patterns during stem cell differentiation, providing a unique stem cell niche. Mutations in fibronectin have been recently identified to cause a rare form of skeletal dysplasia with scoliosis and abnormal growth plates. Even though fibronectin has been extensively analyzed in developmental processes, the functional role and importance of this protein and its various isoforms in skeletal development remains less understood. This review attempts to provide a concise and critical overview of the role of fibronectin isoforms in cartilage and bone physiology and associated pathologies. This will facilitate a better understanding of the possible mechanisms through which fibronectin exerts its regulatory role on cellular differentiation during skeletal development. The review discusses the consequences of mutations in fibronectin leading to corner fracture type spondylometaphyseal dysplasia and presents a new outlook towards matrix-mediated molecular pathways in relation to therapeutic and clinical relevance.

Fibulin-4 exerts a dual role in LTBP-4L-mediated matrix assembly and function

Elastogenesis is a hierarchical process by which cells form functional elastic fibers, providing elasticity and the ability to regulate growth factor bioavailability in tissues, including blood vessels, lung, and skin. This process requires accessory proteins, including fibulin-4 and -5, and latent TGF binding protein (LTBP)-4. Our data demonstrate mechanisms in elastogenesis, focusing on the interaction and functional interdependence between fibulin-4 and LTBP-4L and its impact on matrix deposition and function. We show that LTBP-4L is not secreted in the expected extended structure based on its domain composition, but instead adopts a compact conformation. Interaction with fibulin-4 surprisingly induced a conformational switch from the compact to an elongated LTBP-4L structure. This conversion was only induced by fibulin-4 multimers associated with increased avidity for LTBP-4L; fibulin-4 monomers were inactive. The fibulin-4-induced conformational change caused functional consequences in LTBP-4L in terms of binding to other elastogenic proteins, including fibronectin and fibrillin-1, and of LTBP-4L assembly. A transient exposure of LTBP-4L with fibulin-4 was sufficient to stably induce conformational and functional changes; a stable complex was not required. These data define fibulin-4 as a molecular extracellular chaperone for LTBP-4L. The altered LTBP-4L conformation also promoted elastogenesis, but only in the presence of fibulin-4, which is required to escort tropoelastin onto the extended LTBP-4L molecule. Altogether, this study provides a dual mechanism for fibulin-4 in 1) inducing a stable conformational and functional change in LTBP-4L, and 2) promoting deposition of tropoelastin onto the elongated LTBP-4L.

Roles of fibronectin isoforms in neonatal vascular development and matrix integrity

Fibronectin (FN) exists in two forms—plasma FN (pFN) and cellular FN (cFN). Although the role of FN in embryonic blood vessel development is well established, its function and the contribution of individual isoforms in early postnatal vascular development are poorly understood. Here, we employed a tamoxifen-dependent cFN inducible knockout (cFN iKO) mouse model to study the consequences of postnatal cFN deletion in smooth muscle cells (SMCs), the major cell type in the vascular wall. Deletion of cFN influences collagen deposition but does not affect life span. Unexpectedly, pFN translocated to the aortic wall in the cFN iKO and in control mice, possibly rescuing the loss of cFN. Postnatal pFN deletion did not show a histological aortic phenotype. Double knockout (dKO) mice lacking both, cFN in SMCs and pFN, resulted in postnatal lethality. These data demonstrate a safeguard role of pFN in vascular stability and the dispensability of the individual FN isoforms in postnatal vascular development. Complete absence of FNs in the dKOs resulted in a disorganized tunica media of the aortic wall. Matrix analysis revealed common and differential roles of the FN isoforms in guiding the assembly/deposition of elastogenic extracellular matrix (ECM) proteins in the aortic wall. In addition, we determined with two cell culture models that that the two FN isoforms acted similarly in supporting matrix formation with a greater contribution from cFN. Together, these data show that pFN exerts a critical role in safeguarding vascular organization and health, and that the two FN isoforms function in an overlapping as well as distinct manner to maintain postnatal vascular matrix integrity.

Fibronectin is a protein that exists in vertebrates in two distinct forms: one present in the blood and the other in blood vessel walls. In mammals, fibronectin is important for the development of blood vessels before birth, but whether it is continuously required for blood vessel homeostasis from birth to adulthood is unknown. We present important results from three genetically modified mouse models, which show that at least one form of fibronectin is required for the proper function and integrity of blood vessels during this period. We show that fibronectin can be transferred from the blood into the vessel wall, where it can rescue the integrity of blood vessels in the absence of the vessel form. This represents an important biological mechanism to maintain the health of blood vessels. Our data also highlight the importance of both fibronectin forms in producing and organizing the microenvironment of cells, with a higher contribution from the fibronectin form residing in the blood vessel walls. Together, our findings show that fibronectin from the blood acts as a safeguard to maintain the health of blood vessels, and both fibronectin forms play crucial roles in development and support of the blood vessel microenvironment.

Insights into the key roles of epigenetics in matrix macromolecules-associated wound healing

Extracellular matrix (ECM) is a dynamic network of macromolecules, playing a regulatory role in cell functions, tissue regeneration and remodeling. Wound healing is a tissue repair process necessary for the maintenance of the functionality of tissues and organs. This highly orchestrated process is divided into four temporally overlapping phases, including hemostasis, inflammation, proliferation and tissue remodeling. The dynamic interplay between ECM and resident cells exerts its critical role in many aspects of wound healing, including cell proliferation, migration, differentiation, survival, matrix degradation and biosynthesis. Several epigenetic regulatory factors, such as the endogenous non-coding microRNAs (miRNAs), are the drivers of the wound healing response. microRNAs have pivotal roles in regulating ECM composition during wound healing and dermal regeneration. Their expression is associated with the distinct phases of wound healing and they serve as target biomarkers and targets for systematic regulation of wound repair. In this article we critically present the importance of epigenetics with particular emphasis on miRNAs regulating ECM components (i.e. glycoproteins, proteoglycans and matrix proteases) that are key players in wound healing. The clinical relevance of miRNA targeting as well as the delivery strategies designed for clinical applications are also presented and discussed.

Fibronectin-targeted drug delivery in cancer

Fibronectin is an extracellular matrix protein with pivotal physiological and pathological functions in development and adulthood. Alternative splicing of the precursor mRNA, produced from the single copy fibronectin gene, occurs at three sites coding for the EDA, EDB and IIICS domains. Fibronectin isoforms comprising the EDA or EDB domains are known as oncofetal forms due to their developmental importance and their re-expression in tumors, contrasting with restricted presence in normal adult tissues. These isoforms are also recognized as important markers of angiogenesis, a crucial physiological process in development and required by tumor cells in cancer progression. Attributed to this feature, EDA and EDB domains have been extensively used for the targeted delivery of cytokines, cytotoxic agents, chemotherapy drugs and radioisotopes to fibronectin-expressing tumors to exert therapeutic effects on primary cancers and metastatic lesions. In addition to drug delivery, the EDA and EDB domains of fibronectin have also been utilized to develop imaging strategies for tumor tissues. Furthermore, EDA and EDB based vaccines seem to be promising for the treatment and prevention of certain cancer types. In this review, we will summarize recent advances in fibronectin EDA and EDB-based therapeutic strategies developed to treat cancer.

Calumenin and fibulin-1 on tumor metastasis: Implications for pharmacology

Tumor metastasis is a key cause of cancer mortality, and inhibiting migration of cancer cells is one of the major directions of anti-metastatic drug development. Calumenin and fibulin-1 are two extracellular proteins that synergistically inhibit cell migration and tumor metastasis, and could potentially be served as targets for pharmacological research of anti-metastatic drugs. This review briefly introduces the multi-function of these two proteins, and discusses the mechanism of how they regulate cell migration and tumor metastasis.

Serotonin (5-HT) inhibits Factor XIII-A-mediated plasma fibronectin matrix assembly and crosslinking in osteoblast cultures via direct competition with transamidation

Serotonin (5-HT)--a monoamine with a variety of physiological functions--has recently emerged as a major regulator of bone mass. 5-HT is synthesized in the brain and the gut, and gut-derived 5-HT contributes to circulating 5-HT levels and is a negative modulator of bone mass and quality. 5-HT's negative effects on the skeleton are considered to be mediated via its receptors and transporter in osteoblasts and osteoclasts; however, 5-HT can also incorporate covalently into proteins via a transglutaminase-mediated serotonylation reaction, which in turn can alter protein function. Plasma fibronectin (pFN)--a major component of the bone extracellular matrix that regulates bone matrix quality in vivo--is a major transglutaminase substrate in bone and in osteoblast cultures. We have recently demonstrated that pFN assembly into osteoblast culture matrix requires a Factor XIII-A (FXIII-A) transglutaminase-mediated crosslinking step that regulates both quantity and quality of type I collagen matrix in vitro. In this study, we show that 5-HT interferes with pFN assembly into the extracellular matrix in osteoblast cultures, which in turn has major consequences on matrix assembly and mineralization. 5-HT treatment of MC3T3-E1 osteoblast cultures dramatically decreased both pFN fibrillogenesis as analyzed by immunofluorescence microscopy and pFN levels in DOC-soluble and DOC-insoluble matrix fractions. This was accompanied by an increase in pFN levels in the culture media. Analysis of the media showed covalent incorporation of 5-HT into pFN. Minor co-localization of pFN with 5-HT was also seen in extracellular fibrils. 5-HT also showed co-localization with FXIII-A on the cell surface and inhibited its transamidation activity directly. 5-HT treatment of osteoblast cultures resulted in a discontinuous pFN matrix and impaired type I collagen deposition, decreased alkaline phosphatase and lysyl oxidase activity, and delayed mineralization of the cultures. Addition of excess exogenous pFN to cultures treated with 5-HT resulted in a significant rescue of pFN fibrillogenesis as well as type I collagen deposition and mineralization. In summary, our study presents a novel mechanism on how increased peripheral extracellular 5-HT levels might contribute to the weakening of bone by directly affecting the stabilization of extracellular matrix networks.

Total and isoform-specific quantitative assessment of circulating fibulin-1 using selected reaction monitoring MS and time-resolved immunofluorometry

PURPOSE

Targeted proteomics using SRM-MS combined with stable-isotope dilution has emerged as a promising quantitative technique for the study of circulating protein biomarkers. The purpose of this study was to develop and characterize robust quantitative assays for the emerging cardiovascular biomarker fibulin-1 and its circulating isoforms in human plasma.

EXPERIMENTAL DESIGN

We used bioinformatics analysis to predict total and isoform-specific tryptic peptides for absolute quantitation using SRM-MS. Fibulin-1 was quantitated in plasma by nanoflow-LC-SRM-MS in undepleted plasma and time-resolved immunofluorometric assay (TRIFMA). Both methods were validated and compared to a commercial ELISA (CircuLex). Molecular size determination was performed under native conditions by SEC analysis coupled to SRM-MS and TRIFMA.

RESULTS

Absolute quantitation of total fibulin-1, isoforms -1C, and -1D was performed by SRM-MS. Fibulin-1C was the most abundant isoform in plasma. Circulating fibulin-1 isoforms were homo -or hetero multimeric complexes (range 318-364 kDa). Good correlation was obtained between SRM-MS and TRIFMA but not CircuLex.

CONCLUSIONS AND CLINICAL RELEVANCE

For biomarker studies using smaller cohorts, SRM-MS provides an alternative measure of total and specific fibulin-1 isoforms in undepleted plasma. For larger cohorts TRIFMA provides a faster platform for fibulin-1 quantitation in plasma. While the correlation between these methods was acceptable, low correlation was obtained between the commercial CircuLex assay and SRM-MS or TRIFMA.

Fibulin-1 is required for bone formation and Bmp-2-mediated induction of Osterix

The extracellular matrix protein Fibulin-1 (Fbln1) has been shown to be involved in numerous processes including cardiovascular and lung development. Here we have examined the role of Fbln1 in bone formation. Alizarin red staining of skulls from Fbln1-deficient mice showed reduced mineralization of both membranous and endochondral bones. MicroCT (μCT) analysis of the calvarial bones (i.e., frontal, parietal and interparietal bones collectively) indicated that bone volume in Fbln1 nulls at neonatal stage P0 were reduced by 22% (p=0.015). Similarly, Fbln1 null frontal bones showed a 16% (p=0.035) decrease in bone volume, with a reduction in the interfrontal bone, and a discontinuity in the leading edge of the frontal bone. To determine whether Fbln1 played a role in osteoblast differentiation during bone formation, qPCR was used to measure the effects of Fbln1 deficiency on the expression of Osterix (Osx), a transcription factor essential for osteoblast differentiation. This analysis demonstrated that Osx mRNA was significantly reduced in Fbln1-deficient calvarial bones at developmental stages E16.5 (p=0.049) and E17.5 (p=0.022). Furthermore, the ability of Bmp-2 to induce Osx expression was significantly diminished in Fbln1-deficient mouse embryo fibroblasts. Together, these findings indicate that Fbln1 is a new positive modulator of the formation of membranous bone and endochondral bone in the skull, acting as a positive regulator of Bmp signaling.

Prestress in the extracellular matrix sensitizes latent TGF-β1 for activation

A mild strain induced by matrix remodeling mechanically primes latent TGF-β1 for its subsequent activation and release in response to contractile forces.

Integrin-mediated force application induces a conformational change in latent TGF-β1 that leads to the release of the active form of the growth factor from the extracellular matrix (ECM). Mechanical activation of TGF-β1 is currently understood as an acute process that depends on the contractile force of cells. However, we show that ECM remodeling, preceding the activation step, mechanically primes latent TGF-β1 akin to loading a mechanical spring. Cell-based assays and unique strain devices were used to produce a cell-derived ECM of controlled organization and prestrain. Mechanically conditioned ECM served as a substrate to measure the efficacy of TGF-β1 activation after cell contraction or direct force application using magnetic microbeads. The release of active TGF-β1 was always higher from prestrained ECM as compared with unorganized and/or relaxed ECM. The finding that ECM prestrain regulates the bioavailability of TGF-β1 is important to understand the context of diseases that involve excessive ECM remodeling, such as fibrosis or cancer.

Pathological changes in the COPD lung mesenchyme--novel lessons learned from in vitro and in vivo studies

Chronic obstructive pulmonary disease (COPD) is currently the fourth leading cause of death worldwide and, in contrast to the trend for cardiovascular diseases, mortality rates still continue to climb. This increase is in part due to an aging population, being expanded by the "Baby boomer" generation who grew up when smoking rates were at their peak and by people in developing countries living longer. Sadly, there has been a disheartening lack of new therapeutic approaches to counteract the progressive decline in lung function associated with the disease that leads to disability and death. COPD is characterized by irreversible chronic airflow limitation that is caused by emphysematous destruction of lung elastic tissue and/or obstruction in the small airways due to occlusion of their lumen by inflammatory mucus exudates, narrowing and obliteration. These lesions are mainly produced by the response of the tissue to the repetitive inhalational injury inflicted by noxious gases, including cigarette smoke, which involves interaction between infiltrating inflammatory immune cells, resident cells (e.g. epithelial cells and fibroblasts) and the extra cellular matrix. This interaction leads to tissue destruction and airway remodeling with changes in elastin and collagen, such that the epithelial-mesenchymal trophic unit is dysregulated in both the disease pathologies. This review focuses on: 1--novel inflammatory and remodeling factors that are altered in COPD; 2--in vitro and in vivo models to understand the mechanism whereby the extra cellular matrix environment in altered in COPD; and 3--COPD in the context of wound-repair tissue responses, with a focus on the regulation of mesenchymal cell fate and phenotype.

Transglutaminase activity arising from Factor XIIIA is required for stabilization and conversion of plasma fibronectin into matrix in osteoblast cultures

Circulating plasma fibronectin (pFN), produced by hepatocytes, is a major component of the noncollagenous bone matrix where it was recently shown in vivo in mice to control the biomechanical quality as well as the mineral-to-matrix ratio in bone. FN fibrillogenesis is a process generally requiring FN binding to cellular integrins, and cellular tension to elongate and assemble the molecule. Whether soluble pFN undergoes cell-mediated assembly in bone is not fully established. FN is a well-known substrate for transglutaminases (TGs), which are protein-crosslinking enzymes capable of stabilizing macromolecular structures. The role of this modification regarding the function of FN in bone matrix has remained unknown. Osteoblasts express two TGs-transglutaminase 2 and Factor XIIIA-and we have shown that Factor XIIIA is the main TG active during osteoblast differentiation. In the present study, conducted using MC3T3-E1 osteoblast cultures and bone marrow stromal cells, we demonstrate that pFN requires a TG-mediated crosslinking step to form osteoblast matrix in vitro. This modification step is specific for pFN; cellular FN (EDA-FN) does not serve as a TG substrate. Inhibition of pFN assembly using a TG inhibitor, or depletion of pFN from cell culture serum, dramatically decreased total FN matrix assembly in the osteoblast cultures and affected both the quantity and quality of the type I collagen matrix, and decreased lysyl oxidase and alkaline phosphatase levels, resulting in decreased mineralization. Experiments with isozyme-specific substrate peptides showed that FXIIIA is responsible for the crosslinking of pFN. Addition of exogenous preactivated FXIIIA to osteoblast cultures promoted pFN assembly from the media into matrix. Exogenous TG2 had no effect. Analysis of pFN and EDA-FN fibrils by immunofluorescence microscopy demonstrated that they form distinct matrix network, albeit with minor overlap, suggesting different functions for the two FN forms. Further analysis using EDA-FN blocking antibody showed that it regulated preosteoblast proliferation whereas pFN depletion from the serum had no effect on this process. In conclusion, our study shows that pFN assembly into bone matrix in vitro requires FXIIIA transglutaminase activity making pFN assembly an active, osteoblast-mediated process.

Complex contributions of fibronectin to initiation and maturation of microfibrils

Fibrillins constitute the backbone of extracellular multifunctional assemblies present in elastic and non-elastic matrices, termed microfibrils. Assembly of fibrillins into microfibrils and their homoeostasis is poorly understood and is often compromised in connective tissue disorders such as Marfan syndrome and other fibrillinopathies. Using interaction mapping studies, we demonstrate that fibrillins require the complete gelatin-binding region of fibronectin for interaction, which comprises domains FNI6-FNI9. However, the interaction of fibrillin-1 with the gelatin-binding domain of fibronectin is not involved in fibrillin-1 network assembly mediated by human skin fibroblasts. We show further that the fibronectin network is essential for microfibril homoeostasis in early stages. Fibronectin is present in extracted mature microfibrils from tissue and cells as well as in some in situ microfibrils observed at the ultrastructural level, indicating an extended mechanism for the involvement of fibronectin in microfibril assembly and maturation.

Differential regulation of extracellular matrix and soluble fibulin-1 levels by TGF-β₁ in airway smooth muscle cells

Fibulin-1 (FBLN-1) is a secreted glycoprotein that is associated with extracellular matrix (ECM) formation and rebuilding. Abnormal and exaggerated deposition of ECM proteins is a hallmark of many fibrotic diseases, such as chronic obstructive pulmonary disease (COPD) where small airway fibrosis occurs. The aim of this study was to investigate the regulation of FBLN-1 by transforming growth factor beta 1 (TGF-β1) (a pro-fibrotic stimulus) in primary human airway smooth muscle (ASM) cells from volunteers with and without COPD. Human ASM cells were seeded at a density of 1×10(4) cells/cm(2), and stimulated with or without TGF-β1 (10 ng/ml) for 72 hours before FBLN-1 deposition and soluble FBLN-1 were measured. Fold change in FBLN-1 mRNA was measured at 4, 8, 24, 48, 72 hours. In some experiments, cycloheximide (0.5 µg/ml) was used to assess the regulation of FBLN-1 production. TGF-β1 decreased the amount of soluble FBLN-1 both from COPD and non-COPD ASM cells. In contrast, the deposition of FBLN-1 into the ECM was increased in ASM cells obtained from both groups. TGF-β1 did not increase FBLN-1 gene expression at any of the time points. There were no differences in the TGF-β1 induced FBLN-1 levels between cells from people with or without COPD. Cycloheximide treatment, which inhibits protein synthesis, decreased both the constitutive release of soluble FBLN-1, and TGF-β1 induced ECM FBLN-1 deposition. Furthermore, in cycloheximide treated cells addition of soluble FBLN-1 resulted in incorporation of FBLN-1 into the ECM. Therefore the increased deposition of FBLN-1 by ASM cells into the ECM following treatment with TGF-β1 is likely due to incorporation of soluble FBLN-1 rather than de-novo synthesis.

Selective integrin subunit reduction disrupts fibronectin extracellular matrix deposition and fibrillin 1 gene expression

Integrins are transmembrane receptors that can specifically bind extracellular matrix (ECM) proteins. Assembly of the ECM protein fibronectin into fibrils has been shown to be a cell-mediated process that requires integrins. Like fibronectin, fibrillin 1 is an ECM glycoprotein that can assemble into fibrils, but the role of integrins in fibril formation is not understood. To investigate the role of integrins in fibrillin 1 ECM deposition, cells that normally produce and assemble fibrillin 1 fibers in vitro were stably transfected with plasmid constructs encoding short interfering RNAs that target specific integrin subunits. Cells that were deficient in α2- and β3-integrin subunits produced and deposited fibronectin normally, but cells that were deficient for α5 and αV were unable to elaborate a fibronectin matrix, although they continued to produce and secrete the protein. Surprisingly, the cells that were unable to elaborate a fibronectin matrix also lost fibrillin 1 gene expression.

Characterization of glomerular diseases using proteomic analysis of laser capture microdissected glomeruli

The application of molecular techniques to characterize clinical kidney biopsies has the potential to provide insights into glomerular diseases that cannot be revealed by traditional renal pathology. The present work is a proof-of-concept approach to test whether proteomic analysis of glomeruli isolated from clinical biopsies by laser capture microdissection can provide unique information regarding differentially expressed proteins relevant to disease pathogenesis. The proteomes of glomeruli isolated by laser capture microdissection from biopsies of normal kidneys (living-related donor kidneys) were compared with those from patients with diabetic nephropathy, lupus nephritis, and fibronectin glomerulopathy. Glomerular proteins were extracted, trypsin digested, and subjected to liquid chromatography-tandem mass spectrometry for identification and quantitation. Relative to normal glomeruli, all disease-associated glomeruli showed an increased presence of complement components, a marked decline in podocyte-associated proteins, and a decrease in proteins associated with cellular metabolism. Additionally, fibronectin glomerulopathy glomeruli differed from all the other glomeruli because of a significant accumulation of fibronectin and fibulin. This study demonstrates that our method acquires reproducible and quantitative proteomic information from laser capture microdissection isolates that can be used to characterize the molecular features of glomerular diseases.

Fibulin-1 is required during cardiac ventricular morphogenesis for versican cleavage, suppression of ErbB2 and Erk1/2 activation, and to attenuate trabecular cardiomyocyte proliferation

BACKGROUND

Trabeculation is an integral component of cardiac ventricular morphogenesis and is dependent on the matrix metalloproteinase, ADAMTS1. A substrate of ADAMTS1 is the proteoglycan versican which is expressed in the developing ventricle and which has been implicated in trabeculation. Fibulin-1 is a versican and ADAMTS1-binding extracellular matrix protein required for ventricular morphogenesis. Here we investigated the involvement of fibulin-1 in ADAMTS1-mediated cleavage of versican in vitro, and the involvement of fibulin-1 in versican cleavage in ventricular morphogenesis.

RESULTS

We show that fibulin-1 is a cofactor for ADAMTS1-dependent in vitro cleavage of versican V1, yielding a 70-kDa amino-terminal fragment. Furthermore, fibulin-1-deficiency in mice was found to cause a significant reduction (>90%) in ventricular levels of the 70-kDa versican V1 cleavage product and a 2-fold increase in trabecular cardiomyocyte proliferation. Decreased versican V1 cleavage and augmented trabecular cardiomyocyte proliferation in fibulin-1 null hearts is accompanied by increased ventricular activation of ErbB2 and Erk1/2. By contrast, versican deficiency was found to lead to decreased cardiomyocyte proliferation and reduced ventricular trabeculation.

CONCLUSION

We conclude that fibulin-1 regulates versican-dependent events in ventricular morphogenesis by promoting ADAMTS1 cleavage of versican leading to suppression of trabecular cardiomyocyte proliferation mediated by the ErbB2-Map kinase pathway.

Plasma and cellular fibronectin: distinct and independent functions during tissue repair

Fibronectin (FN) is a ubiquitous extracellular matrix (ECM) glycoprotein that plays vital roles during tissue repair. The plasma form of FN circulates in the blood, and upon tissue injury, is incorporated into fibrin clots to exert effects on platelet function and to mediate hemostasis. Cellular FN is then synthesized and assembled by cells as they migrate into the clot to reconstitute damaged tissue. The assembly of FN into a complex three-dimensional matrix during physiological repair plays a key role not only as a structural scaffold, but also as a regulator of cell function during this stage of tissue repair. FN fibrillogenesis is a complex, stepwise process that is strictly regulated by a multitude of factors. During fibrosis, there is excessive deposition of ECM, of which FN is one of the major components. Aberrant FN-matrix assembly is a major contributing factor to the switch from normal tissue repair to misregulated fibrosis. Understanding the mechanisms involved in FN assembly and how these interplay with cellular, fibrotic and immune responses may reveal targets for the future development of therapies to regulate aberrant tissue-repair processes.

Homocysteine modifies structural and functional properties of fibronectin and interferes with the fibronectin-fibrillin-1 interaction

Homocystinuria is a genetic disorder resulting in elevated levels of homocysteine in plasma and tissues. Some of the skeletal and ocular symptoms such as long bone overgrowth, scoliosis, and ectopia lentis overlap with symptoms seen in Marfan syndrome. Marfan syndrome is caused by mutations in the extracellular matrix protein fibrillin-1. We previously showed that fibrillin-1 is a target for homocysteine and that the deposition of homocysteinylated fibrillin-1 in the extracellular matrix is compromised. Since the assembly of fibrillin-1 is critically dependent on fibronectin, we analyzed the consequences of fibronectin homocysteinylation and its interaction with fibrillin-1. Cellular fibronectin and proteolytic fragments were homocysteinylated and tested in various interaction assays with recombinant fibrillin-1 and heparin. Fibronectin homocysteinylation consistently compromised the fibronectin-fibrillin-1 interaction, while the interaction with heparin was not affected. Fibronectin homocysteinylation, but not cysteinylation, reduced the fibronectin dimers to monomers as shown by Western blotting. ELISA analyses of homocysteinylated fibronectin with three monoclonal antibodies demonstrated structural changes in the disulfide-containing FNI domains FNI(2), FNI(4), and FNI(9). Using fluorescently labeled fibronectin, we studied the consequence of fibronectin homocysteinylation on assembly in cell culture. Modified fibronectin showed deficiencies in denovo matrix incorporation and initial assembly. In conclusion, we define here characteristic structural changes of fibronectin upon homocysteinylation that translate into functional deficiencies in the fibronectin-fibrillin-1 interaction and in fibronectin assembly. Since fibronectin is a major organizer of various extracellular protein networks, these structural and functional alterations may contribute to the pathogenesis of homocystinuria and Marfan syndrome.

Ubiquitination of alpha 5 beta 1 integrin controls fibroblast migration through lysosomal degradation of fibronectin-integrin complexes

Cell migration requires endocytosis and recycling of integrins, but it is not known whether degradation of these membrane proteins is involved. Here we demonstrate that in migrating cells, a fraction of the endocytosed fibronectin receptor, alpha 5 beta 1 integrin, is sorted into multivesicular endosomes together with fibronectin and degraded in lysosomes. This sorting requires fibronectin-induced ubiquitination of the alpha 5 subunit, and the activity of the endosomal sorting complex required for transport (ESCRT) machinery, which interacts with alpha 5 beta 1 integrin. Importantly, we demonstrate that both alpha 5 ubiquitination and ESCRT functions are required for proper migration of fibroblasts. We propose that ligand-mediated degradation of alpha 5 beta 1 integrin via the ESCRT pathway is required in order to prevent endosomal accumulation of ligand-bound integrins that might otherwise form nonproductive adhesion sites. Fibronectin and alpha 5 beta 1 integrin therefore are trafficked to lysosomes in a similar way to growth factors and their receptors.

iso-DGR sequences do not mediate binding of fibronectin N-terminal modules to adherent fibronectin-null fibroblasts

Fibronectin (FN) without an RGD sequence (FN-RGE), and thus lacking the principal binding site for alpha5beta1 integrin, is deposited into the extracellular matrix of mouse embryos. Spontaneous conversion of (263)NGR and/or (501)NGR to iso-DGR possibly explains this enigma, i.e. ligation of iso-DGR by alphavbeta3 integrin may allow cells to assemble FN. Partial modification of (263)NGR to DGR or iso-DGR was detected in purified plasma FN by mass spectrometry. To test functions of the conversion, one or both NGR sequences were mutated to QGR in recombinant N-terminal 70-kDa construct of FN (70K), full-length FN, or FN-RGE. The mutations did not affect the binding of soluble 70K to already adherent fibroblasts or the ability of soluble 70K to compete with non-mutant FN or FN-RGE for binding to FN assembly sites. Non-mutant FN and FN-N263Q/N501Q with both NGRs mutated to QGRs were assembled equally well by adherent fibroblasts. FN-RGE and FN-RGE-N263Q/N501Q were also assembled equally well. Although substrate-bound 70K mediated cell adhesion in the presence of 1 mm Mn(2+) by a mechanism that was inhibited by cyclic RGD peptide, the peptide did not inhibit 70K binding to cell surface. Mutations of the NGR sequences had no effect on Mn(2+)-enhanced cell adhesion to adsorbed 70K but caused a decrease in cell adhesion to reduced and alkylated 70K. These results demonstrate that iso-DGR sequences spontaneously converted from NGR are cryptic and do not mediate the interaction of the 70K region of FN with the cell surface during FN assembly.

Matrix association of latent TGF-beta binding protein-2 (LTBP-2) is dependent on fibrillin-1

The components of the extracellular matrix (ECM) and their differential expression patterns play important roles in tissue formation. The deposition of latent TGF-beta binding proteins (LTBPs) to the ECM exhibit distinct distribution profiles. We have analyzed here the temporal and spatial ECM association of latent TGF-beta binding protein LTBP-2 in cultured human embryonic lung fibroblasts. We found that LTBP-2 was not assembled to the ECM until by confluency of cultures following the deposition of fibronectin (FN) and fibrillin-1. In 5-day-old cultures LTBP-2 was rapidly secreted from cells and it subsequently associated with the ECM as shown by metabolic labeling and immunoprecipitation. LTBP-2 colocalized transiently with fibronectin and failed to assemble to the ECM of FN deficient mouse fibroblasts. Analysis of different cultured human cell lines revealed partial colocalization of LTBP-2 and fibrillin-1 in the ECM of fibroblasts, MG-63 osteosarcoma cells and human vascular endothelial cells. Silencing of fibrillin-1 expression by lentiviral shRNAs profoundly disrupted the deposition of LTBP-2. Current results suggest that LTBP-2 is not an element of the provisional ECM of fibroblasts but is more likely a component of more mature ECM and indicate that matrix association of LTBP-2 depends on a pre-formed fibrillin-1 network.

Serum opacity factor is a streptococcal receptor for the extracellular matrix protein fibulin-1

The adhesion of bacteria to host tissues is often mediated by interactions with extracellular matrices. Herein, we report on the interactions of the group A streptococcus, Streptococcus pyogenes, with the extracellular matrix protein fibulin-1. S. pyogenes bound purified fibulin-1 in a dose-dependent manner. Genetic ablation of serum opacity factor (SOF), a virulence determinant of S. pyogenes, reduced binding by approximately 50%, and a recombinant peptide of SOF inhibited binding of fibulin-1 to streptococci by approximately 45%. Fibulin-1 bound to purified SOF2 in a dose-dependent manner with high affinity (K(d) = 1.6 nm). The fibulin-1-binding domain was localized to amino acid residues 457-806 of SOF2, whereas the fibronectin-binding domain is contained within residues 807-931 of SOF2, indicating that these two domains are separate and distinct. Fibulin-1 bound to recombinant SOF from M types 2, 4, 28, and 75 of S. pyogenes, indicating that the fibulin-1-binding domain is likely conserved among SOF from different serotypes. Mixed binding experiments suggested that gelatin, fibronectin, fibulin-1, and SOF form a quaternary molecular complex that enhanced the binding of fibulin-1. These data indicate that S. pyogenes can interact with fibulin-1 and that SOF is a major streptococcal receptor for fibulin-1 but not the only receptor. Such interactions with fibulin-1 may be involved in the adhesion of S. pyogenes to extracellular matrices of the host.

Fibrillin assembly requires fibronectin

Fibrillins constitute the major backbone of multifunctional microfibrils in elastic and nonelastic extracellular matrices. Proper assembly mechanisms are central to the formation and function of these microfibrils, and their properties are often compromised in pathological circumstances such as in Marfan syndrome and in other fibrillinopathies. Here, we have used human dermal fibroblasts to analyze the assembly of fibrillin-1 in dependence of other matrix-forming proteins. siRNA knockdown experiments demonstrated that the assembly of fibrillin-1 is strictly dependent on the presence of extracellular fibronectin fibrils. Immunolabeling performed at the light and electron microscopic level showed colocalization of fibrillin-1 with fibronectin fibrils at the early stages of the assembly process. Protein-binding assays demonstrated interactions of fibronectin with a C-terminal region of fibrillin-1, -2, and -3 and with an N-terminal region of fibrillin-1. The C-terminal half of fibrillin-2 and -3 had propensities to multimerize, as has been previously shown for fibrillin-1. The C-terminal of all three fibrillins interacted strongly with fibronectin as multimers, but not as monomers. Mapping studies revealed that the major binding interaction between fibrillins and fibronectin involves the collagen/gelatin-binding region between domains FNI(6) and FNI(9).

Maturational changes in laminin, fibronectin, collagen IV, and perlecan in germinal matrix, cortex, and white matter and effect of betamethasone

Germinal matrix is selectively vulnerable to hemorrhage in premature infants, and use of prenatal betamethasone is associated with a lower occurrence of germinal matrix hemorrhage. Because the major components of extracellular matrix of the cerebral vasculature-laminin, fibronectin, collagen IV, and perlecan-provide structural stability to blood vessels, we examined whether the expression of these molecules was decreased in the germinal matrix and affected by betamethasone. In both human fetuses and premature infants, fibronectin was significantly lower in the germinal matrix than in the cortical mantle or white matter anlagen. Conversely, laminin alpha1 gene expression was greater in the human germinal matrix compared with the cortical mantle or white matter. Expression of alpha1- and alpha2(IV) collagen chains increased with advancing gestational age. Low-dose prenatal betamethasone treatment enhanced fibronectin level by 1.5-2-fold whereas a high dose reduced fibronectin expression by 2-fold in rabbit pups. Because fibronectin provides structural stability to the blood vessels, its reduced expression in the germinal matrix may contribute to the fragility of germinal matrix vasculature and the propensity to hemorrhage in premature neonates.

Potential role for heparan sulfate proteoglycans in regulation of transforming growth factor-beta (TGF-beta) by modulating assembly of latent TGF-beta-binding protein-1

Latent transforming growth factor-beta-binding proteins (LTBPs) are extracellular matrix (ECM) glycoproteins that play a major role in storage of latent TGF-beta in the ECM and regulate its availability. We have previously identified fibronectin as a key molecule for incorporation of LTBP1 and TGF-beta into the ECM of osteoblasts and fibroblasts. Here we provide evidence that heparan sulfate proteoglycans may mediate binding between LTBP1 and fibronectin. We have localized critical domains in the N terminus of LTBP1 that are required for co-localization with fibronectin in osteoblast cultures and have identified heparin binding sites in the N terminus of LTBP1 between residues 345 and 487. Solid-phase binding assays suggest that LTBP1 does not bind directly to fibronectin but that the binding is indirect. Heparin coupled to bovine serum albumin (heparin-BSA) was able to mediate binding between fibronectin and LTBP1. Treatment of primary osteoblast cultures with heparin or heparin-BSA but not with chondroitin sulfate impaired LTBP1 deposition onto fibronectin without inhibiting expression of LTBP1. Inhibition of LTBP1 incorporation was accompanied by reduced incorporation of latent TGF-beta into the ECM, with increased amounts of soluble latent TGF-beta. Inhibition of attachment of glycosaminoglycans to the core proteins of proteoglycans by beta-d-xylosides also reduced incorporation of LTBP1 into the ECM. These studies suggest that heparan sulfate proteoglycans may play a critical role in regulating TGF-beta availability by controlling the deposition of LTBP1 into the ECM in association with fibronectin.

Engineered Cell-Adhesive Nanoparticles Nucleate Extracellular Matrix Assembly

Tissue engineering aims to regenerate new biological tissue for replacing diseased or injured tissues. We propose a new approach to accelerate the deposition of cell-secreted matrix proteins into extracellular matrix fibrils. We examined whether dynamic substrates with nanoscale ligand features allowing for alpha5beta1 integrin recruiting, cellular tension generation, and alpha5beta1 integrin mobility would enhance fibronectin matrix assembly in a ligand model system that is routinely not sufficient for its induction. To this end, we developed biodynamic substrates consisting of cell adhesive fragment from the 9th and 10th type repeats of fibronectin (FNf ) functionalized to 100 nm prefabricated albumin nanoparticles (ANPs). FNf-ANPs modulated cellular spreading processes, promoting the development of stellate or dendritic morphologies. Concomitant with the spreading, FNf-ANPs rapidly recruited beta1 integrins to focal contacts and promoted the migration of beta1 integrins centripetally from the cell periphery toward the center. FNf-ANPs stimulated the deposition of secreted fibronectin into matrix fibrils; FNf, the key ligand alone, was not sufficient for fibronectin fibrillogenesis. When FNf-ANPs were displayed from "immobilized" substrates, abolishing any mobility of ligated beta1 integrins, fibronectin matrix assembly was abrogated, implicating the role of dynamic matrix display on matrix assembly. Receptor ligation of FNf-ANPs via noncontractile adhesions was not sufficient to stimulate fibrillogenesis, and Rho-kinase inhibitors abolished fibronectin matrix deposition. Our approach highlights the possibility of engineering integrin-based extracellular matrix assembly using nanotechnology, which may have implications for improved biomaterials for wound repair and basic understanding of matrix remodeling within pathogenesis and biomedicine.

New insights into extracellular matrix assembly and reorganization from dynamic imaging of extracellular matrix proteins in living osteoblasts

The extracellular matrix (ECM) has been traditionally viewed as a static scaffold that supports cells and tissues. However, recent dynamic imaging studies suggest that ECM components are highly elastic and undergo continual movement and deformation. Latent transforming growth factor beta (TGFbeta) binding protein-1 (LTBP1) is an ECM glycoprotein that binds latent TGFbeta and regulates its availability and activity. LTBP1 initially co-distributes with fibronectin in the extracellular matrix of osteoblasts, and depends on fibronectin for its assembly. To gain further insights into the mechanisms of assembly of LTBP1 and its spatial and temporal interactions with fibronectin, we have performed dual fluorescence time-lapse imaging of these two proteins in living osteoblasts using fluorescent probes. Time-lapse movies showed surprisingly large fibril displacements associated with cellular movement as well as occasional breaking of LTBP1 or fibronectin-containing fibrils. Individual fibrils stretched to as much as 3.5 times or contracted to as much as one fourth of their original length. Motile cells appeared to actively mediate extracellular matrix assembly by adding 'globules' or 'packets' of matrix material onto existing fibrils. They also actively reorganized the extracellular matrix by shunting matrix material from one location to another and exchanging fibrillar material between fibrils. This cell-mediated matrix reorganization was primarily associated with the assembly and remodeling of the initial (early) matrix, whereas mature, established ECM was more stable. Displacement vector mapping showed that different matrix fibrillar networks within the same cultures can show different dynamic motion in response to cell movement and showed that the motion of fibrils was correlated with cell motion. These data suggest novel cell-mediated mechanisms for assembly and reorganization of the extracellular matrix and highlight a role for cell motility in the assembly process.

Extracellular matrix macroassembly dynamics in early vertebrate embryos

This chapter focuses on the in vivo macroassembly dynamics of fibronectin and fibrillin-2--two prominent extracellular matrix (ECM) components, present in vertebrate embryos at the earliest stages of development. The ECM is an inherently dynamic structure with a well-defined position fate: ECM filaments are not only anchored to and move with established tissue boundaries, but are repositioned prior to the formation of new anatomical features. We distinguish two ECM filament relocation processes-each operating on different length scales. First, ECM filaments are moved by large-scale tissue motion, which rearranges major organ primordia within the embryo. The second type of motion, on the scale of the individual ECM filaments, is driven by local motility and protrusive activity of nearby cells. The motion decomposition is made practically possible by recent advances in microscopy and high-resolution particle image velocimetry algorithms. We demonstrate that both kinds of motion contribute substantially to the establishment of normal ECM structure, and both must be taken into account when attempting to understand ECM macroassembly during embryonic morphogenesis. The tissue-scale motion changes the local amount (density) and the tissue-level structure (e.g., orientation) of ECM fibers. Local reorganization includes filament assembly and the segregation of ECM into specific patterns. Local reorganization takes place most actively at Hensen's node and around the primitive streak. These regions are also sites of active cell migration, where fibrillin-2 and fibronectin are often colocalized in ECM globules, and new fibrillin-2 foci are deposited. During filament assembly, the globular patches of ECM are joined into larger linear structures in a hierarchical process: increasingly larger structures are created by the aggregation of smaller units. A future understanding of ECM assembly thus requires the study of the complex interactions between biochemical assembly steps, local cell action, and tissue motion.

Dynamics of Assembly and Reorganization of Extracellular Matrix Proteins

This chapter will review advances in our understanding of the dynamics of assembly and reorganization of extracellular matrix (ECM) proteins and will highlight the role of fibronectin as a key orchestrator for the assembly of multiple ECM proteins. The dynamic rather than static nature of the ECM will be emphasized by reviewing time-lapse imaging studies in living cell and embryo systems, with a particular focus on fibronectin and members of the fibrillin superfamily. These studies have provided new insights into the assembly and reorganization of ECM fibrillar networks, suggesting that fibril assembly is a hierarchical process, with increasingly larger fibrillar structures formed by the progressive aggregation of smaller units. These studies have also revealed that motile cells appear to be actively involved in the assembly and reorganization of ECM fibrillar networks by shunting fibrillar material from one location to another, adding fibrillar material to the ends of growing fibrils, and exchanging material between fibrils. A common theme emerging from these studies is that cell- and tissue-generated mechanical forces are critical in the assembly and remodeling of the ECM.

Analysis of the proepicardium-epicardium transition during the malformation of the RXRalpha-/- epicardium

The epicardium of the heart originates from a cluster of mesothelial-derived cells that develop beneath the sinus venosus in the embryonic day (E) 9.0-9.5 mouse. The subsequent proepicardium-epicardium transition that forms the epicardial layer of epithelial cells covering the myocardial surface is nearly complete by E10.0-E10.5 and results in a fully covered heart by E11.0. In this study, we show that an established model of congenital heart disease, the retinoid X receptor alpha knockout (RXRalpha-/-) embryo, displays a malformed epicardium. At E10.0-E10.5, the RXRalpha-/- has several large regions of myocardium that remain bare. Furthermore, by E11.5-E12.5, when a complete epithelial layer is formed in the mutant, large regions of the epicardium become distended from the underlying myocardium. Close examination of the E9.5 mutant revealed an elevated apoptosis level within the proepicardial cluster of mesothelial cells. Additionally, among the extracellular matrix proteins analyzed, expression of fibronectin was elevated in the RXRalpha-/- as assessed by immunostaining in paraffin-embedded sections and proepicardial explants. We propose that these events contribute to a developmental delay in the formation of the epicardium, which leads to an abnormal epicardium and ultimately contributes to the cardiac malformations seen in the RXRalpha-/-.

Fibronectin regulates latent transforming growth factor-beta (TGF beta) by controlling matrix assembly of latent TGF beta-binding protein-1

Latent transforming growth factor-beta-binding proteins (LTBPs) are extracellular matrix (ECM) glycoproteins that play a major role in the storage of latent TGF beta in the ECM and regulate its availability. Here we show that fibronectin is critical for the incorporation of LTBP1 and transforming growth factor-beta (TGF beta) into the ECM of osteoblasts and fibroblasts. Immunolocalization studies suggested that fibronectin provides an initial scaffold that precedes and patterns LTBP1 deposition but that LTBP1 and fibronectin are later localized in separate fibrillar networks, suggesting that the initial template is lost. Treatment of fetal rat calvarial osteoblasts with a 70-kDa N-terminal fibronectin fragment that inhibits fibronectin assembly impaired incorporation of LTBP1 and TGFbeta into the ECM. Consistent with this, LTBP1 failed to assemble in embryonic fibroblasts that lack the gene for fibronectin. LTBP1 assembly was rescued by full-length fibronectin and superfibronectin, which are capable of assembly into fibronectin fibrils, but not by other fibronectin fragments, including a 160-kDa RGD-containing fragment that activates alpha5beta1 integrins. This suggests that the critical event for LTBP1 assembly is the formation of a fibronectin fibrillar network and that integrin ligation by fibronectin molecules alone is not sufficient. Not only was fibronectin essential for the initial incorporation of LTBP1 into the ECM, but the continued presence of fibronectin was required for the continued assembly of LTBP1. These studies highlight a nonredundant role for fibronectin in LTBP1 assembly into the ECM and suggest a novel role for fibronectin in regulation of TGF beta via LTBP1 interactions.

Fibronectin matrix turnover occurs through a caveolin-1-dependent process

Extracellular matrix remodeling occurs during development, tissue repair, and in a number of pathologies, including fibrotic disorders, hypertension, and atherosclerosis. Extracellular matrix remodeling involves the complex interplay between extracellular matrix synthesis, deposition, and degradation. Factors that control these processes are likely to play key roles in regulating physiological and pathological extracellular matrix remodeling. Our data show that fibronectin polymerization into the extracellular matrix regulates the deposition and stability of other extracellular matrix proteins, including collagen I and thrombospondin-1 (Sottile and Hocking, 2002. Mol. Biol. Cell 13, 3546). In the absence of continual fibronectin polymerization, there is a loss of fibronectin matrix fibrils, and increased levels of fibronectin degradation. Fibronectin degradation occurs intracellularly after endocytosis and can be inhibited by chloroquine, an inhibitor of lysosomal degradation, and by caveolae-disrupting agents. Down-regulation of caveolin-1 by RNAi inhibits loss of fibronectin matrix fibrils, fibronectin internalization, and fibronectin degradation; these processes can be restored by reexpression of caveolin-1. These data show that fibronectin matrix turnover occurs through a caveolin-1-dependent process. Caveolin-1 regulation of fibronectin matrix turnover is a novel mechanism regulating extracellular matrix remodeling.

Fibulins in development and heritable disease

Fibulins are a family of five extracellular glycoproteins found in a variety of tissues in association with diverse supramolecular structures, including elastic fibers, basement membrane networks, fibronectin microfibrils, and proteoglycan aggregates. Studies of the developmental expression patterns have indicated that several fibulins are prominently expressed at sites of epithelial-mesenchymal transformations during embryogenesis; among these sites, the cardiovascular system has been analyzed in more detail. Gene targeting of fibulins in mice has provided important insights into their biological roles, and has led to the identification of gene mutations in a congenital disorder of humans, cutis laxa. Genetic linkage and molecular studies have also associated several fibulin genes with various human heritable disorders that affect a wide range of organs, including limb, eye, blood, and arteries. In this review, we discuss the role of fibulins in development, with an emphasis on the cardiovascular system, and their involvement in human genetic disease.

Fibulins: a versatile family of extracellular matrix proteins

Fibulins are a newly recognized family of extracellular matrix proteins. The five known members of the family share an elongated structure and many calcium-binding sites, owing to the presence of tandem arrays of epidermal growth factor-like domains. They have overlapping binding sites for several basement-membrane proteins, tropoelastin, fibrillin, fibronectin and proteoglycans, and they participate in diverse supramolecular structures. New insights into their biological roles are now emerging from studies of transgenic mice and of some inherited human diseases.

Fibronectin polymerization regulates the composition and stability of extracellular matrix fibrils and cell-matrix adhesions

Remodeling of extracellular matrices occurs during development, wound healing, and in a variety of pathological processes including atherosclerosis, ischemic injury, and angiogenesis. Thus, identifying factors that control the balance between matrix deposition and degradation during tissue remodeling is essential for understanding mechanisms that regulate a variety of normal and pathological processes. Using fibronectin-null cells, we found that fibronectin polymerization into the extracellular matrix is required for the deposition of collagen-I and thrombospondin-1 and that the maintenance of extracellular matrix fibronectin fibrils requires the continual polymerization of a fibronectin matrix. Further, integrin ligation alone is not sufficient to maintain extracellular matrix fibronectin in the absence of fibronectin deposition. Our data also demonstrate that the retention of thrombospondin-1 and collagen I into fibrillar structures within the extracellular matrix depends on an intact fibronectin matrix. An intact fibronectin matrix is also critical for maintaining the composition of cell-matrix adhesion sites; in the absence of fibronectin and fibronectin polymerization, neither alpha5beta1 integrin nor tensin localize to fibrillar cell-matrix adhesion sites. These data indicate that fibronectin polymerization is a critical regulator of extracellular matrix organization and stability. The ability of fibronectin polymerization to act as a switch that controls the organization and composition of the extracellular matrix and cell-matrix adhesion sites provides cells with a means of precisely controlling cell-extracellular matrix signaling events that regulate many aspects of cell behavior including cell proliferation, migration, and differentiation.

The incorporation of fibrinogen into extracellular matrix is dependent on active assembly of a fibronectin matrix

Fibrinogen is a soluble protein produced by hepatocytes and secreted into plasma, where it functions in hemostasis. During inflammation, the hepatic synthesis of fibrinogen is induced 2-10 fold. Recent studies demonstrate that after an inflammatory stimulus, fibrinogen gene expression and protein production is upregulated in lung epithelial cells, where it is secreted basolaterally and consequently deposited into the extracellular matrix in fibrils that extensively colocalize with fibronectin fibrils. In this study, we show that the deposition of fibrinogen into the matrix of fibroblasts occurred rapidly and in a Rho-dependent manner in response to serum or lysophosphatidic acid; RhoA GTPase signaling is also required for fibronectin matrix assembly. Using mouse embryonic fibronectin-null cells, we show that incorporation of exogenous fibrinogen into matrix fibrils occurred only in the presence of exogenous fibronectin, which is also assembled into matrix fibrils. Furthermore, treatment of fibroblasts and fibronectin-null cells with an antibody that inhibits fibronectin matrix assembly impaired incorporation of fibrinogen into matrix fibrils. Collectively, these data suggest that incorporation of fibrinogen into the extracellular matrix requires active fibronectin polymer elongation into matrix fibrils. From these data, we hypothesize that fibrinogen deposition rapidly changes the topology of the extracellular matrix to provide a surface for cell migration and matrix remodeling during tissue repair.

Latent transforming growth factor beta-binding protein-3 and fibulin-1C interact with the extracellular domain of the heparin-binding EGF-like growth factor precursor

BACKGROUND

The membrane-bound cell-surface precursor and soluble forms of heparin-binding epidermal growth factor-like growth factor (HB-EGF) contribute to many cellular developmental processes. The widespread occurrence of HB-EGF in cell and tissue types has led to observations of its role in such cellular and tissue events as tumor formation, cell migration, extracellular matrix formation, wound healing, and cell adherence. Several studies have reported the involvement of such extracellular matrix proteins as latent transforming growth factor beta-binding protein, TGF-beta, and fibulin-1 in some of these processes. To determine whether HB-EGF interacts with extracellular matrix proteins we used the extracellular domain of proHB-EGF in a yeast two-hybrid system to screen a monkey kidney cDNA library. cDNA clones containing nucleotide sequences encoding domains of two proteins were obtained and their derived amino acid sequences were evaluated.

RESULTS

From approximately equal to 3 x 10(6) screened monkey cDNA clones, cDNA clones were recovered that contained nucleotide sequences encoding domains of the monkey latent transforming growth factor-beta binding protein-3 (MkLTBP-3) and fibulin-1C protein. The amino acid sequence derived from the MkLTBP-3 gene shared 98.6% identity with human LTBP-3 and 86.7% identity with mouse LTBP-3 amino acid sequences. The amino acid sequence derived from the monkey fibulin-1C gene shared 97.2% identity with human fibulin-1C. Yeast two-hybrid screens indicate that LTBP-3 and fibulin-1C interact with proHB-EGF through their calcium-binding EGF-like modules.

CONCLUSIONS

The interactions of the extracellular domain of proHB-EGF with LTBP-3 and fibulin-1C suggest novel functions for HB-EGF between cell and tissue surfaces.

Association of extracellular matrix proteins fibulin-1 and fibulin-2 with fibronectin in bone marrow stroma

Extracellular matrix (ECM) molecules, together with growth factors and stromal cells, regulate haematopoietic cell development in bone marrow (BM). We report here expression of ECM proteins fibulin-1 and fibulin-2 in mouse BM. In other tissues, fibulin-1 and fibulin-2 associate with fibronectin and other ECM proteins. Fibulin-2 has also been found to adhere to cells via beta3 integrins. We studied the association of fibulins with fibronectin in BM stroma. By confocal microscopy, fibulin-1 and fibulin-2 immunostainings were co-localized with fibronectin in the adherent layer of long-term BM cultures. In cell adhesion assays using recombinant proteins, mouse fibulin-2 adhered to human erythroid-megakaryocytic leukaemia cell line HEL. This adhesion was mediated by beta3 integrins. However, HEL cells did not adhere to human fibulin-2. We therefore studied a possible species-specific cell-adhesive activity of mouse fibulin-2 by using mouse megakaryocytes, obtained by culture of BM cells in the presence of thrombopoietin. These megakaryocytes did not adhere to mouse fibulin-2. Our findings suggested that the functional role of fibulin-1 and fibulin-2 in BM stroma is related to binding to the major cell adhesion protein fibronectin, whereas adhesion of mouse fibulin-2 to human cells containing the integrin beta3 chain is not related to an apparent physiological function of the protein.

Role of the latent transforming growth factor beta binding protein 1 in fibrillin-containing microfibrils in bone cells in vitro and in vivo

Latent transforming growth factor beta-binding proteins (LTBPs) are extracellular matrix (ECM) proteins that bind latent transforming growth factor beta (TGF-beta) and influence its availability in bone and other connective tissues. LTBPs have homology with fibrillins and may have related functions as microfibrillar proteins. However, at present little is known about their structural arrangement in the ECM. By using antibodies against purified LTBP1, against a short peptide in LTBP1, and against epitope-tagged LTBP1 constructs, we have shown colocalization of LTBP1 and fibrillin 1 in microfibrillar structures in the ECM of cultured primary osteoblasts. Immunoelectron microscopy confirmed localization of LTBP1 to 10- to 12-nm microfibrils and suggested an ordered aggregation of LTBP1 into these structures. Early colocalization of LTBP1 with fibronectin suggested a role for fibronectin in the initial assembly of LTBP1 into the matrix; however, in more differentiated osteoblast cultures, LTBP1 and fibronectin 1 were found in distinct fibrillar networks. Overexpression of LTBP1 deletion constructs in osteoblast-like cells showed that N-terminal amino acids 67-467 were sufficient for incorporation into fibrillin-containing microfibrils and suggested that LTBP1 can be produced by cells distant from the site of fibril formation. In embryonic long bones in vivo, LTBP1 and fibrillin 1 colocalized at the surface of newly forming osteoid and bone. However, LTBP1-positive fibrils, which did not contain fibrillin 1, were present in cartilage matrix. These studies show that in addition to regulating TGF beta 1, LTBP1 may function as a structural component of connective tissue microfibrils. LTBP1 may therefore be a candidate gene for Marfan-related connective tissue disorders in which linkage to fibrillins has been excluded.

Sequence, recombinant expression and tissue localization of two novel extracellular matrix proteins, fibulin-3 and fibulin-4

Fibulin-1 and fibulin-2 have previously been identified as basement membrane and microfibrillar proteins with a broad binding repertoire for other extracellular ligands. Here we report on the cloning and sequence analysis of human fibulin-3 (487 residues), also known as protein S1-5, and fibulin-4 (443 residues). These novel members of this protein family are most closely related to fibulin-1C. They consist of a C-terminal globular domain III, also shared by the fibrillins, a central rod-like element composed of five calcium-binding epidermal growth factor-like (EG) modules (domain II) and an N-terminal interrupted EG module (domain I) which replaces the anaphylatoxin-like modules of the other fibulins. This predicted domain structure was supported by electron microscopy of fibulin-4, which demonstrated short rods. Northern blots showed that both novel fibulins are expressed in several human tissues to a variable extent and that they are up-regulated in quiescent fibroblasts. Specific antibodies which were raised against each of the novel fibulins did not cross-react with fibulin-1. Immunohistology of adult mouse tissues showed that fibulin-3, fibulin-4 and fibulin-1 have overlapping but distinct extracellular tissue localizations. A particularly prominent feature was the staining of variable sets of large and small blood vessels.

Complete exon-intron organization of the mouse fibulin-1 gene and its comparison with the human fibulin-1 gene

Fibulin-1 is a 90 kDa calcium-binding protein present in the extracellular matrix and in the blood. Two major variants, C and D, differ in their C-termini as well as the ability to bind the basement membrane protein nidogen. Here we characterized genomic clones encoding the mouse fibulin-1 gene, which contains 18 exons spanning at least 75 kb of DNA. The two variants are generated by alternative splicing of exons in the 3' end. By searching the database we identified most of the exons encoding the human fibulin-1 gene and showed that its exon-intron organization is similar to that of the mouse gene.

Increased immunostaining of fibulin-1, an estrogen-regulated protein in the stroma of human ovarian epithelial tumors

Fibulin-1, an extracellular matrix protein, is secreted by human ovarian metastatic cancer cell lines under estrogen stimulation. Fibulin-1 expression was quantified by immunohistochemistry and computer-aided image analysis in 44 human ovarian epithelial tumors and 14 normal ovaries. The fibulin-1 staining intensity in proximal stroma, close to the surface of epithelial cells and tumor cells, progressively increased from normal ovaries to serous carcinomas. In all lesions, excluding cystadenomas, fibulin-1 accumulation was higher in proximal stroma than in distant stroma. In situ hybridization demonstrated strong fibulin-1 gene expression in epithelial cells of serous ovarian carcinomas and some cysts. The weak expression of fibulin-1 RNA in some stromal cells of these tumors could not explain the strong fibulin-1 protein accumulation in tumor stroma, which was therefore mostly produced by tumor epithelial cells. In carcinomas, fibulin-1 staining was not correlated with the percentage of estrogen receptor-alpha (ERalpha)-stained nuclei but was inversely correlated with the progesterone receptor. However, in cystadenomas and borderline tumors, both fibulin-1 and ERalpha protein levels increased, in comparison with normal ovaries, suggesting an effect of estrogens in the early steps of tumorigenesis. This fibulin-1 overexpression, demonstrated in vivo in ovarian carcinomas, might be a useful indicator for predicting cancer risk and/or aggressiveness.

Fibronectins Are Essential for Heart and Blood Vessel Morphogenesis But Are Dispensable for Initial Specification of Precursor Cells

The underlying mechanisms of lethal cardiovascular defects associated with the fibronectin-null (FN.null) mutation in mouse embryos were investigated by lineage analysis of myocardial, endocardial, and endothelial cells. A wide variation in phenotype was observed on two genetic backgrounds. In the less severe class (C57/BL6 background), FN.null embryos display a defective heart. Myocardial cells express the specific marker MF-20 and are correctly localized in the anterior trunk region, but myocardial organization is disrupted, resulting in a bulbous heart tube. Endocardial cells express the specific marker platelet-endothelial cell adhesion molecule-1 (PECAM-1) and are localized within the myocardium, but the endocardium appears collapsed. Endothelial cells of two vascular beds are specified, but the aortae are distended and lack contact with the surrounding mesenchyme, while no vessels form in the yolk sac. Defects in the more severe class suggest that FNs are essential earlier in development on the 129/Sv background. Myocardial and endocardial cells are specified, but morphogenesis of the myocardium and endocardium does not occur. Aortic endothelial cells are specified and localized normally, but remain scattered. Yolk sac endothelial cells resemble those of the less severe class. We conclude that FNs are essential for organization of heart and blood vessels, but are dispensable for cellular specification in the appropriate regions within the embryo.

Binding of fibulin-1 to nidogen depends on its C-terminal globular domain and a specific array of calcium-binding epidermal growth factor-like (EG) modules

The calcium-binding basement membrane protein fibulin-1C was shown to bind nidogen in a calcium-dependent fashion. Fibulin-1C consists of small N (domain 1) and C-terminal (domain III) globular structures connected by a central rod (domain II) composed of nine epidermal growth factor (EG) modules, eight of which possess a consensus sequence for calcium binding. Several point and deletion mutants and chimeric protein constructs were used to define the nidogen binding epitope of fibulin-1C by surface plasmon resonance and solid phase assays. All recombinant products were obtained from transfected kidney cells in a folded form as shown by CD spectroscopy, electron microscopy and proteolysis. They were used to demonstrate that calcium-binding is essentially due to the EG modules possessing the consensus binding sequence. Deletion of domain III caused a 30-fold reduction in nidogen binding, whereas deletion of domain I had no effect, yet domain III alone was also inactive. Successive deletions of two to seven EG modules of domain II also caused partial of complete inactivation of binding depending on how many were deleted or their position relative to domain III. Site-directed mutagenesis within the calcium binding consensus sequences demonstrated a similar dependence. Replacement of seven of the calcium-binding modules by a similar tandem array from a related protein showed a distinct (fibulin-2) to almost complete loss of binding (fibrillin-1). This indicates a complex epitope structure involving domains II and III, which each may provide binding epitopes or stabilize each other.

The self-association and fibronectin-binding sites of fibulin-1 map to calcium-binding epidermal growth factor-like domains

Fibulin-1 is a modular glycoprotein with amino-terminal anaphylatoxin-like modules followed by nine epidermal growth factor (EGF)-like modules and, depending on alternative splicing, four possible carboxyl termini. Fibulin-1 has been shown to self-associate as well as to bind calcium, fibronectin (FN), laminin, nidogen, and fibrinogen. To map ligand-binding sites within fibulin-1, polypeptides corresponding to various regions of fibulin-1 were expressed recombinantly and evaluated for their capacity to bind calcium, FN, or fibulin-1. A calcium-binding site(s) was mapped to EGF-like modules 5-9. A fibulin-1 self-association site was localized to EGF-like modules 5 and 6 (amino acid residues 356-440), as was a binding site for FN. The self-association interaction mediated by this pair of modules involved calcium since divalent cation chelators reduced the binding affinity of the interaction. By contrast, FN binding to EGF-like modules 5 and 6 was unaffected by the presence of divalent cation chelators. It can be concluded that EGF-like modules 5 and 6 bind calcium and mediate homotypic interaction between EGF-like modules 5 and 6 present in different fibulin-1 molecules and heterotypic interaction between EGF-like modules 5 and 6 and type III repeats 13 and 14 in FN. While additional binding sites for calcium or FN were not detected, another fibulin-1 self-association site was found within amino acid residues 30-173. However, unlike the self-association site in EGF-like modules 5 and 6, which was functional in the native protein, the amino-terminal site was cryptic and revealed only after the protein was denatured.

Human fibulin-1D: molecular cloning, expression and similarity with S1-5 protein, a new member of the fibulin gene family

Fibulin-1 is an extracellular matrix (ECM) component of basement membranes and connective tissue elastic fibers, and a blood protein. Multiple forms of fibulin-1 that differ in their C-terminal regions are produced through the process of alternative splicing of their precursor RNA. Two transcripts of 2.4 and 2.7 kb are the predominant fibulin-1 mRNAs expressed in human tissues and cultured cells. While the 2.4 kb transcript had been shown to encode fibulin-1C, the 2.7 kb transcript did not correspond to any of the previously identified human fibulin-1 variants. Herein, we report on the isolation and sequencing of cDNA corresponding to the 2.7 kb fibulin-1 transcript which encodes a novel, alternatively spliced form of human fibulin-1 that we term the D form. The deduced amino acid sequence of the D form is identical in its first 566 residues to the three known fibulin-1 variants (fibulin-1A-C); however, it has a unique 137 amino acid-C-terminal segment encoded by the alternatively spliced portion of its transcript. RNA hybridization analysis showed that the fibulin-1D transcript is coordinately expressed with that of fibulin-1C both in tissues and in cultured cells. Using antibodies specific to the unique C-terminal segment of fibulin-1D and -1C, both proteins were found to be expressed in human placenta. Recombinant fibulin-1D generated in transfected mammalian cells displayed similar ligand-binding properties as placenta-derived fibulin-1 and recombinant fibulin-1C, and it was capable of incorporating into cultured cell ECM in the absence of other fibulin-1 forms. A comparative sequence analysis revealed that the unique C-terminal region of fibulin-1D is similar to the C-terminal regions of fibulin-1C and fibulin-2. Furthermore, the C-terminal regions of fibulin-1C, -1D and -2 are similar to the C-terminal region of a recently described protein termed S1-5. In addition to this C-terminal similarity, S1-5 also contains repeated EGF-like modules and a conserved N-terminal element, thereby leading to the conclusion that S1-5 is a third member of the fibulin gene family.