Identification of a new biological function for the integrin alpha v beta 3: initiation of fibronectin matrix assembly

Matrix stiffness controls megakaryocyte adhesion, fibronectin fibrillogenesis, and proplatelet formation through Itgβ3

Megakaryocytes (MKs) are the precursor cells of platelets, located in the bone marrow (BM). Once mature, they extend elongated projections named proplatelets through sinusoid vessels, emerging from the marrow stroma into the circulating blood. Not all signals from the microenvironment that regulate proplatelet formation are understood, particularly those from the BM biomechanics. We sought to investigate how MKs perceive and adapt to modifications of the stiffness of their environment. Although the BM is one of the softest tissue of the body, its rigidification results from excess fibronectin (FN), and other matrix protein deposition occur upon myelofibrosis. Here, we have shown that mouse MKs are able to detect the stiffness of a FN-coated substrate and adapt their morphology accordingly. Using a polydimethylsiloxane substrate with stiffness varying from physiological to pathological marrow, we found that a stiff matrix favors spreading, intracellular contractility, and FN fibrils assembly at the expense of proplatelet formation. Itgb3, but not Itgb1, is required for stiffness sensing, whereas both integrins are involved in fibrils assembly. In contrast, soft substrates promote proplatelet formation in an Itgb3-dependent manner, consistent with the ex vivo decrease in proplatelet formation and the in vivo decrease in platelet number in Itgb3-deficient mice. Our findings demonstrate the importance of environmental stiffness for MK functions with potential pathophysiological implications during pathologies that deregulate FN deposition and modulate stiffness in the marrow.

Advances in targeting the extracellular matrix for glaucoma therapy: current updates

INTRODUCTION

Elevated intraocular pressure (IOP) is a well-recognized risk factor for development of primary open angle glaucoma (POAG), a leading cause of irreversible blindness. Ocular hypertension is associated with excessive extracellular matrix (ECM) deposition in trabecular meshwork (TM) resulting in increased aqueous outflow resistance and elevated IOP. Hence, therapeutic options targeting ECM remodeling in TM to lower IOP in glaucomatous eyes are of considerable importance.

AREAS COVERED

This paper discusses the complex process of ECM regulation in TM and explores promising therapeutic targets. The role of Transforming Growth Factor-β as a central player in ECM deposition in TM is discussed. We elaborate the key regulatory processes involved in its activation, release, signaling, and cross talk with other signaling pathways including Rho GTPase, Wnt, integrin, cytokines, and renin-angiotensin-aldosterone. Further, we summarize the therapeutic agents that have been explored to target ECM dysregulation in TM.

EXPERT OPINION

Targeting molecular pathways to reduce ECM deposition and/or enhance its degradation are of considerable significance for IOP lowering. Challenges lie in pinpointing specific targets and designing drug delivery systems to precisely interact with pathologically active/inactive signaling. Recent advances in monoclonal antibodies, fusion molecules, and vectored nanotechnology offer potential solutions.

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.

Cytoskeleton Response to Ionizing Radiation: A Brief Review on Adhesion and Migration Effects

The cytoskeleton is involved in several biological processes, including adhesion, motility, and intracellular transport. Alterations in the cytoskeletal components (actin filaments, intermediate filaments, and microtubules) are strictly correlated to several diseases, such as cancer. Furthermore, alterations in the cytoskeletal structure can lead to anomalies in cells’ properties and increase their invasiveness. This review aims to analyse several studies which have examined the alteration of the cell cytoskeleton induced by ionizing radiations. In particular, the radiation effects on the actin cytoskeleton, cell adhesion, and migration have been considered to gain a deeper knowledge of the biophysical properties of the cell. In fact, the results found in the analysed works can not only aid in developing new diagnostic tools but also improve the current cancer treatments.

Effects of substrate stiffness and actin velocity on in silico fibronectin fibril morphometry and mechanics

Assembly of the extracellular matrix protein fibronectin (FN) into insoluble, viscoelastic fibrils is a critical step during embryonic development and wound healing; misregulation of FN fibril assembly has been implicated in many diseases, including fibrotic diseases and cancer. We have previously developed a computational model of FN fibril assembly that recapitulates the morphometry and mechanics of cell-derived FN fibrils. Here we use this model to probe two important questions: how is FN fibril formation affected by the contractile phenotype of the cell, and how is FN fibril formation affected by the stiffness of the surrounding tissue? We show that FN fibril formation depends strongly on the contractile phenotype of the cell, but only weakly on in vitro substrate stiffness, which is an analog for in vivo tissue stiffness. These results are consistent with previous experimental data and provide a better insight into conditions that promote FN fibril assembly. We have also investigated two distinct phenotypes of FN fibrils that we have previously identified; we show that the ratio of the two phenotypes depends on both substrate stiffness and contractile phenotype, with intermediate contractility and high substrate stiffness creating an optimal condition for stably stretched fibrils. Finally, we have investigated how re-stretch of a fibril affects cellular response. We probed how the contractile phenotype of the re-stretching cell affects the mechanics of the fibril; results indicate that the number of myosin motors only weakly affects the cellular response, but increasing actin velocity results in a decrease in the apparent stiffness of the fibril and a decrease in the stably-applied force to the fibril. Taken together, these results give novel insights into the combinatorial effects of substrate stiffness and cell contractility on FN fibril assembly.

Fibronectin assembly regulates lumen formation in breast acini

Fibronectin (FN) is an extracellular matrix (ECM) glycoprotein that self-assembles into FN fibrils, forming a FN matrix contributing to the stiffness of the ECM. Stromal FN stiffness in cancer has been shown to impact epithelial functions such as migration, cancer metastasis, and epithelial-to-mesenchymal transition. The role of the FN matrix of epithelial cells in driving such processes remains less well understood and is the focus of this study. Hypoxia, defined by low oxygen tension (<5%) is one of the hallmarks of tumor microenvironments impacting fibril reorganization in stromal and epithelial cells. Here, using the MCF10 breast epithelial progression series of cell lines encompassing normal, preinvasive, and invasive states, we show that FN fibril formation decreases during hypoxia, coinciding with a decrease in migratory potential of these cells. Conversely, we find that FN fibril disruption during three-dimensional acinar growth of normal breast cells resulted in acinar luminal filling. Our data also demonstrates that the luminal filling upon fibril disruption in untransformed MCF10A cells results in a loss of apicobasal polarity, characteristic of pre-invasive and invasive breast cell lines MCF10AT and MCF10 DCIS.com. Overall this is the first study that relates fibril-mediated changes in epithelial cells as critical players in lumen clearing of breast acini and maintenance of the untransformed growth characteristic.

Fibronectin in development and wound healing

Fibronectin structure and composition regulate contextual cell signaling. Recent advances have been made in understanding fibronectin and its role in tissue organization and repair. This review outlines fibronectin splice variants and their functions, evaluates potential therapeutic strategies targeting or utilizing fibronectin, and concludes by discussing potential future directions to modulate fibronectin function in development and wound healing.

Disruption of fibronectin fibrillogenesis affects intraocular pressure (IOP) in BALB/cJ mice

Increased deposition of fibronectin fibrils containing EDA+fibronectin by TGFβ2 is thought to be involved in the reduction of aqueous humor outflow across the trabecular meshwork (TM) of the eye and the elevation in intraocular pressure (IOP) observed in primary open angle glaucoma (POAG). Using a fibronectin-binding peptide called FUD that can disrupt fibronectin fibrillogenesis, we examined if disrupting fibronectin fibrillogenesis would affect IOP in the TGFβ2 BALB/cJ mouse model of ocular hypertension. BALB/cJ mice that had been intravitreally injected with an adenovirus (Ad5) expressing a bioactive TGFβ2226/228 showed a significant increase in IOP after 2 weeks. When 1μM FUD was injected intracamerally into mice 2 weeks post Ad5-TGFβ2 injection, FUD significantly reduced IOP after 2 days. Neither mutated FUD (mFUD) nor PBS had any effect on IOP. Four days after FUD was injected, IOP returned to pre-FUD injection levels. In the absence of TGFβ2, intracameral injection of FUD had no effect on IOP. Western blotting of mouse anterior segments expressing TGFβ2 showed that FUD decreased fibronectin levels 2 days after intracameral injection (p<0.05) but not 7 days compared to eyes injected with PBS. mFUD injection had no significant effect on fibronectin levels at any time point. Immunofluorescence microscopy studies in human TM (HTM) cells showed that treatment with 2ng/ml TGFβ2 increased the amount of EDA+ and EDB+ fibronectin incorporated into fibrils and 2μM FUD decreased both EDA+ and EDB+ fibronectin in fibrils. An on-cell western assay validated this and showed that FUD caused a 67% reduction in deoxycholate insoluble fibronectin fibrils in the presence of TGFβ2. FUD also caused a 43% reduction in fibronectin fibrillogenesis in the absence of TGFβ2 while mFUD had no effect. These studies suggest that targeting the assembly of fibronectin fibrillogenesis may represent a way to control IOP.

ILK supports RhoA/ROCK-mediated contractility of human intestinal epithelial crypt cells by inducing the fibrillogenesis of endogenous soluble fibronectin during the spreading process

BACKGROUND

Fibronectin (FN) assembly into an insoluble fibrillar matrix is a crucial step in many cell responses to extracellular matrix (ECM) properties, especially with regards to the integrin-related mechanosensitive signaling pathway. We have previously reported that the silencing of expression of integrin-linked kinase (ILK) in human intestinal epithelial crypt (HIEC) cells causes significant reductions in proliferation and spreading through concomitantly acquired impairment of soluble FN deposition. These defects in ILK-depleted cells are rescued by growth on exogenous FN. In the present study we investigated the contribution of ILK in the fibrillogenesis of FN and its relation to integrin-actin axis signaling and organization.

RESULTS

We show that de novo fibrillogenesis of endogenous soluble FN is ILK-dependent. This function seemingly induces the assembly of an ECM that supports increased cytoskeletal tension and the development of a fully spread contractile cell phenotype. We observed that HIEC cell adhesion to exogenous FN or collagen-I (Col-I) is sufficient to restore fibrillogenesis of endogenous FN in ILK-depleted cells. We also found that optimal engagement of the Ras homolog gene family member A (RhoA) GTPase/Rho-associated kinase (ROCK-1, ROCK-2)/myosin light chain (MLC) pathway, actin ventral stress fiber formation, and integrin adhesion complex (IAC) maturation rely primarily upon the cell's capacity to execute FN fibrillogenesis, independent of any significant ILK input. Lastly, we confirm the integrin α5β1 as the main integrin responsible for FN assembly, although in ILK-depleted cells αV-class integrins expression is needed to allow the rescue of FN fibrillogenesis on exogenous substrate.

CONCLUSION

Our study demonstrates that ILK specifically induces the initiation of FN fibrillogenesis during cell spreading, which promotes RhoA/ROCK-dependent cell contractility and maturation of the integrin-actin axis structures. However, the fibrillogenesis process and its downstream effect on RhoA signaling, cell contractility and spreading are ILK-independent in human intestinal epithelial crypt cells.

Core Matrisome Protein Signature During Periodontal Ligament Maturation From Pre-occlusal Eruption to Occlusal Function

The pre-occlusal eruption brings the molars into functional occlusion and initiates tensional strains during mastication. We hypothesized that upon establishment of occlusal contact, the periodontal ligament (PDL) undergoes cell and extracellular matrix maturation to adapt to this mechanical function. The PDL of 12 Wistar male rats were laser microdissected to observe the proteomic changes between stages of pre-occlusal eruption, initial occlusal contact and 1-week after occlusion. The proteome was screened by mass spectrometry and confirmed by immunofluorescence. The PDL underwent maturation upon establishment of occlusion. Downregulation of alpha-fetoprotein stem cell marker and protein synthesis markers indicate cell differentiation. Upregulated proteins were components of the extracellular matrix (ECM) and were characterized with the matrisome project database. In particular, periostin, a major protein of the PDL, was induced following occlusal contact and localized around collagen α-1 (III) bundles. This co-localization coincided with organization of collagen fibers in direction of the occlusal forces. Establishment of occlusion coincides with cellular differentiation and the maturation of the PDL. Co-localization of periostin and collagen with subsequent fiber organization may help counteract tensional forces and reinforce the ECM structure. This may be a key mechanism of the PDL to adapt to occlusal forces and maintain structural integrity.

Basement Membrane Regulates Fibronectin Organization Using Sliding Focal Adhesions Driven by a Contractile Winch

We have discovered that basement membrane and its major components can induce rapid, strikingly robust fibronectin organization. In this new matrix assembly mechanism, α5β1 integrin-based focal adhesions slide actively on the underlying matrix toward the ventral cell center through the dynamic shortening of myosin IIA-associated actin stress fibers to drive rapid fibronectin fibrillogenesis distal to the adhesion. This mechanism contrasts with classical fibronectin assembly based on stable or fixed-position focal adhesions containing αVβ3 integrins plus α5β1 integrin translocation into proximal fibrillar adhesions. On basement membrane components, these sliding focal adhesions contain standard focal adhesion constituents but completely lack classical αVβ3 integrins. Instead, peripheral α3β1 or α2β1 adhesions mediate initial cell attachment but over time are switched to α5β1 integrin-based sliding focal adhesions to assemble fibronectin matrix. This basement-membrane-triggered mechanism produces rapid fibronectin fibrillogenesis, providing a mechanistic explanation for the well-known widespread accumulation of fibronectin at many organ basement membranes.

Role of Fibronectin in Primary Open Angle Glaucoma

Primary open angle glaucoma (POAG) is the most common form of glaucoma and the 2nd most common cause of irreversible vision loss in the United States. Nearly 67 million people have the disease worldwide including >3 million in the United States. A major risk factor for POAG is an elevation in intraocular pressure (IOP). The increase in IOP is believed to be caused by an increase in the deposition of extracellular matrix proteins, in particular fibronectin, in a region of the eye known as the trabecular meshwork (TM). How fibronectin contributes to the increase in IOP is not well understood. The increased density of fibronectin fibrils is thought to increase IOP by altering the compliance of the trabecular meshwork. Recent studies, however, also suggest that the composition and organization of fibronectin fibrils would affect IOP by changing the cell-matrix signaling events that control the functional properties of the cells in the trabecular meshwork. In this article, we will discuss how changes in the properties of fibronectin and fibronectin fibrils could contribute to the regulation of IOP.

Multifaced Roles of the αvβ3 Integrin in Ehlers-Danlos and Arterial Tortuosity Syndromes' Dermal Fibroblasts

The αvβ3 integrin, an endothelial cells’ receptor-binding fibronectin (FN) in the extracellular matrix (ECM) of blood vessels, regulates ECM remodeling during migration, invasion, angiogenesis, wound healing and inflammation, and is also involved in the epithelial mesenchymal transition. In vitro-grown human control fibroblasts organize a fibrillar network of FN, which is preferentially bound on the entire cell surface to its canonical α5β1 integrin receptor, whereas the αvβ3 integrin is present only in rare patches in focal contacts. We report on the preferential recruitment of the αvβ3 integrin, due to the lack of FN–ECM and its canonical integrin receptor, in dermal fibroblasts from Ehlers–Danlos syndromes (EDS) and arterial tortuosity syndrome (ATS), which are rare multisystem connective tissue disorders. We review our previous findings that unraveled different biological mechanisms elicited by the αvβ3 integrin in fibroblasts derived from patients affected with classical (cEDS), vascular (vEDS), hypermobile EDS (hEDS), hypermobility spectrum disorders (HSD), and ATS. In cEDS and vEDS, respectively, due to defective type V and type III collagens, αvβ3 rescues patients’ fibroblasts from anoikis through a paxillin-p60Src-mediated cross-talk with the EGF receptor. In hEDS and HSD, without a defined molecular basis, the αvβ3 integrin transduces to the ILK-Snail1-axis inducing a fibroblast-to-myofibroblast-transition. In ATS cells, the deficiency of the dehydroascorbic acid transporter GLUT10 leads to redox imbalance, ECM disarray together with the activation of a non-canonical αvβ3 integrin-TGFBRII signaling, involving p125FAK/p60Src/p38MAPK. The characterization of these different biological functions triggered by αvβ3 provides insights into the multifaced nature of this integrin, at least in cultured dermal fibroblasts, offering future perspectives for research in this field.

Joint inflammation related citrullination of functional arginines in extracellular proteins

We report the extent, specific sites and structural requirements of joint inflammation related citrullination in extracellular proteins. A total of 40 synovial fluid samples derived from chronically inflamed human joints were analysed by heparin-agarose fractionation and LC-MS/MS. Citrullination of 55 arginines in extracellular proteins was detected. Importantly, 20% of the sites have a characterized function related to the hallmarks of destructive joint inflammation. E.g. four arginine residues, shown here to be citrullinated, are also affected by mutations in inherited diseases causing haemolysis or blood clotting dysfunction. Citrullination of integrin ligands was selected for further studies since fibronectin R234 in isoDGR was among the most frequently citrullinated arginines in synovial fluid. Assays with synovial fibroblasts and integrin αVβ3 indicated decreased affinity to the enzymatically citrullinated integrin binding sites. To conclude, our data indicate that in inflamed joints extensive citrullination affects the functional arginine residues in extracellular proteins.

TGF-β triggers rapid fibrillogenesis via a novel TβRII-dependent fibronectin-trafficking mechanism

There is increased recycling of soluble fibronectin from the cell surface for fibrillogenesis. This recycling is regulated by TGF-β in a transcription- and SMAD-independent manner via specific TβRII and integrin α5β1 interactions. The recycling of fibronectin is Rab11 dependent and is required for TGF-β–induced cell migration.

Fibronectin (FN) is a critical regulator of extracellular matrix (ECM) remodeling through its availability and stepwise polymerization for fibrillogenesis. Availability of FN is regulated by its synthesis and turnover, and fibrillogenesis is a multistep, integrin-dependent process essential for cell migration, proliferation, and tissue function. Transforming growth factor β (TGF-β) is an established regulator of ECM remodeling via transcriptional control of ECM proteins. Here we show that TGF-β, through increased FN trafficking in a transcription- and SMAD-independent manner, is a direct and rapid inducer of the fibrillogenesis required for TGF-β–induced cell migration. Whereas TGF-β signaling is dispensable for rapid fibrillogenesis, stable interactions between the cytoplasmic domain of the type II TGF-β receptor (TβRII) and the FN receptor (α5β1 integrin) are required. We find that, in response to TGF-β, cell surface–internalized FN is not degraded by the lysosome but instead undergoes recycling and incorporation into fibrils, a process dependent on TβRII. These findings are the first to show direct use of trafficked and recycled FN for fibrillogenesis, with a striking role for TGF-β in this process. Given the significant physiological consequences associated with FN availability and polymerization, our findings provide new insights into the regulation of fibrillogenesis for cellular homeostasis.

Syndecan-1-Induced ECM Fiber Alignment Requires Integrin αvβ3 and Syndecan-1 Ectodomain and Heparan Sulfate Chains

Expression of the cell surface proteoglycan syndecan-1 (Sdc1) is frequently induced in stromal fibroblasts of invasive breast carcinomas. We have recently identified a correlation between stromal Sdc1 expression and extracellular matrix (ECM) fiber alignment, both in vitro and in vivo. ECMs derived from Sdc1-positive human mammary fibroblasts (HMF) showed an aligned fiber architecture, which contrasted markedly with the more random fiber arrangement in the ECM produced by Sdc1-negative HMFs. We further demonstrated that aligned fiber architecture promotes the directional migration and invasion of breast carcinoma cells. To decipher the molecular mechanisms governing the formation of an aligned, invasion-permissive ECM, a series of Sdc1 mutants was introduced into HMF. We found that both the ectodomain and heparan sulfate chains of Sdc1 were required for full activity of Sdc1 in regulating ECM alignment, while transmembrane and cytoplasmic domains were dispensable. Sdc1 regulates the activities of several integrins via its ectodomain. Integrins are key players in the assembly of fibronectin-rich ECM. In addition, integrins are capable of regulating cell morphology and cell shape and orientation may affect ECM architecture. Therefore, we investigated the role of integrins in Sdc1-mediated ECM fiber alignment. Sdc1-overexpressing HMF gained an enhanced spindle-shaped morphology when cultured in an overconfluent state under conditions permissive for ECM production, which was partially reversed by siRNA-mediated silencing of β3 integrin expression. Moreover, suppression of αvβ3 integrin activity by a function-blocking antibody or β3 knockdown largely abolished the aligned ECM fiber architecture and consequently the invasion-permissive properties of the ECM induced by Sdc1. The results suggest that Sdc1 may modulate fibronectin fibrillogenesis and/or alter cell morphology during ECM production through αvβ3 integrin, thereby mediating ECM fiber alignment. Understanding the mechanisms governing ECM organization may lead to the development of novel stroma-targeted therapy for breast cancer, aiming at converting an invasion-permissive to an invasion-restrictive microenvironment.

Contribution of distinct platelet integrins to binding, unfolding, and assembly of fibronectin

Fibronectin (FN) fibrillogenesis depends on the binding of FN to cellular receptors and subsequent unfolding of bound FN. Integrins αIIbβ3, αvβ3, and α5β1 are known to assemble FN fibrils on platelets. In our study, we examined the contribution of these integrins to FN binding, unfolding, and assembly on platelets in suspension and adherent platelets in the presence or absence of agonists. Phorbol 12-myristate 13-acetate (PMA), but not adenosine diphosphate (ADP), induced binding of FN to platelets in suspension. In contrast, adherent platelets were able to deposit FN on their surfaces in the absence of agonists. β3 integrins had a major impact on the interaction of FN on platelets. αvβ3 showed a similar contribution to the binding of FN as αIIbβ3 on PMA-stimulated platelets in suspension but had a lesser contribution to unfolding and deposition of FN on adherent platelets. α5β1 also participated in the interaction of FN with platelets by mediating the unfolding and assembly of FN, but to a lesser extent than β3 integrins. None of the distinct antibodies directed against one of the three integrins caused a complete inhibition of binding, unfolding, and assembly of FN by platelets. Thus, it is likely that αIIbβ3, αvβ3, and α5β1 or another still unknown receptor can be substituted.

The role of integrins in the trabecular meshwork

Integrins are a family of heterodimeric transmembrane receptors that mediate adhesion to the extracellular matrix (ECM). However, integrins are not just adhesion receptors. They can act as "bidirectional signal transducers" that coordinate a large number of cellular activities in response to the extracellular environment and intracellular signaling events. Among the activities regulated by integrins are cell adhesion, assembly of the ECM, growth factor signaling, apoptosis, organization of the cytoskeleton, and cytoskeleton-mediated processes such as contraction, endocytosis, and phagocytosis. Integrins regulate these activities through a complex network of intracellular signaling kinases and adaptor proteins that associate with the transmembrane and cytoplasmic domains of the integrin subunits. In this review, we will discuss how some of the known integrin-mediated activities can control the function of the trabecular meshwork. We will also discuss how integrin activity is a tightly regulated process that involves conformation changes within the heterodimer which are mediated by specific integrin-binding proteins.

Collagen-like polypeptide poly(Pro-Hyp-Gly) conjugated with Gly-Arg-Gly-Asp-Ser and Pro-His-Ser-Arg-Asn peptides enchances cell adhesion, migration, and stratification

Collagens are widely used in medical applications, including as a scaffold for tissue regeneration. However, animal-derived collagens have several drawbacks, such as low thermal stability, nonspecific cell adhesion, antigenicity, and contamination with pathogenic substances. To overcome these problems, we chemically synthesized the collagen-like polypeptide, poly(prolyl-hydroxyprolyl-glycyl) (poly(Pro-Hyp-Gly)), which forms a collagen-like triple-helical structure and shows biodegradability and biocompatibility. Here, we designed a novel scaffold where fibronectin-derived Gly Arg-Gly-Asp-Ser (GRGDS) and Pro-His-Ser-Arg-Asn (PHSRN) peptides were simultaneously conjugated with poly(Pro-Hyp-Gly). We assessed cell adhesion and migration activities using NIH3T3 cells in the scaffold and stratification ofimmortalized rabbit corneal epithelial cells. Cell adhesion was enhanced in scaffolds with GRGDS, increased with increasing amounts of conjugated GRGDS, and was significantly higher than bovine type I atelocollagen but lower than bovine fibronectin. Interestingly, simultaneous conjugation of GRGDS and PHSRN synergistically enhanced cell migration. Scaffolds containing almost equal amounts of GRGDS and PHSRN showed significantly higher cell migration than bovine type I atelocollagen. Addition of free GRGDS completely inhibited cell migration on the scaffold, whereas addition of free PHSRN partially inhibited cell migration. These results suggest that GRGDS plays a definitive role, and PHSRN plays an additional role, in cell migration. Conjugation of GRGDS resulted in the same level of stratification of rabbit corneal epithelial cells compared with bovine type I atelocollagen and bovine fibronectin. Because the simultaneous conjugation of GRGDS and PHSRN on poly(Pro-Hyp-Gly) enhances cell adhesion, migration, and stratification, it may be a useful scaffold for tissue regeneration.

Fibronectins, their fibrillogenesis, and in vivo functions

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

Dermatopontin interacts with fibronectin, promotes fibronectin fibril formation, and enhances cell adhesion

We report that dermatopontin (DP), an abundant dermal extracellular matrix protein, is found in the fibrin clot and in the wound fluid, which comprise the provisional matrix at the initial stage of wound healing. DP was also found in the serum but at a lower concentration than that in wound fluid. DP co-localized with both fibrin and fibronectin on fibrin fibers and interacted with both proteins. Both normal fibroblast and HT1080 cell adhesion to the fibrin-fibronectin matrix were dose-dependently enhanced by DP, and the adhesion was mediated by α5β1 integrin. The cytoskeleton was more organized in the cells that adhered to the fibrin-fibronectin-DP complex. When incubated with DP, fibronectin formed an insoluble complex of fibronectin fibrils as visualized by electron microscopy. The interacting sites of fibronectin with DP were the first, thirteenth, and fourteenth type III repeats (III(1), III(13), and III(14)), with III(13) and III(14) assumed to be the major sites. The interaction between III(2-3) and III(12-14) was inhibited by DP, whereas the interaction between I(1-5) and III(12-14) was specifically and strongly enhanced by DP. Because the interaction between III(2-3) and III(12-14) is involved in forming a globular conformation of fibronectin, and that between I(1-5) and III(12-14) is required for forming fibronectin fibrils, DP promotes fibronectin fibril formation probably by changing the fibronectin conformation. These results suggest that DP has an accelerating role in fibroblast cell adhesion to the provisional matrix in the initial stage of wound healing.

Syndecan-1 in breast cancer stroma fibroblasts regulates extracellular matrix fiber organization and carcinoma cell motility

Stromal fibroblasts of breast carcinomas frequently express the cell surface proteoglycan syndecan-1 (Sdc1). In human breast carcinoma samples, stromal Sdc1 expression correlates with an organized, parallel, extracellular matrix (ECM) fiber architecture. To examine a possible link between stromal Sdc1 and the fiber architecture, we generated bioactive cell-free three-dimensional ECMs from cultures of Sdc1-positive and Sdc1-negative murine and human mammary fibroblasts (termed ECM-Sdc1 and ECM-mock, respectively). Indeed, ECM-Sdc1 showed a parallel fiber architecture that contrasted markedly with the random fiber arrangement of ECM-mock. When breast carcinoma cells were seeded into the fibroblast-free ECMs, ECM-Sdc1, but not ECM-mock, promoted their attachment, invasion, and directional movement. We further evaluated the contribution of the structural/compositional modifications in ECM-Sdc1 on carcinoma cell behavior. By microcontact printing of culture surfaces, we forced the Sdc1-negative fibroblasts to produce ECM with parallel fiber organization, mimicking the architecture observed in ECM-Sdc1. We found that the fiber topography governs carcinoma cell migration directionality. Conversely, an elevated fibronectin level in ECM-Sdc1 was responsible for the enhanced attachment of the breast carcinoma cells. These observations suggest that Sdc1 expression in breast carcinoma stromal fibroblasts promotes the assembly of an architecturally abnormal ECM that is permissive to breast carcinoma directional migration and invasion.

Syndecans as cell surface receptors: Unique structure equates with functional diversity

An increasing number of functions for syndecan cell surface heparan sulfate proteoglycans have been proposed over the last decade. Moreover, aberrant syndecan regulation has been found to play a critical role in multiple pathologies, including cancers, as well as wound healing and inflammation. As receptors, they have much in common with other molecules on the cell surface. Syndecans are type I transmembrane molecules with cytoplasmic domains that link to the actin cytoskeleton and can interact with a number of regulators. However, they are also highly complex by virtue of their external glycosaminoglycan chains, especially heparan sulfate. This heterodisperse polysaccharide has the potential to interact with many ligands from diverse protein families. Here, we relate the structural features of syndecans to some of their known functions.

Assembly of fibronectin extracellular matrix

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

Specificities of β1 integrin signaling in the control of cell adhesion and adhesive strength

Cells exert actomyosin contractility and cytoskeleton-dependent force in response to matrix stiffness cues. Cells dynamically adapt to force by modifying their behavior and remodeling their microenvironment. This adaptation is favored by integrin activation switch and their ability to modulate their clustering and the assembly of an intracellular hub in response to force. Indeed integrins are mechanoreceptors and mediate mechanotransduction by transferring forces to specific adhesion proteins into focal adhesions which are sensitive to tension and activate intracellular signals. α(5)β(1) integrin is considered of major importance for the formation of an elaborate meshwork of fibronectin fibrils and for the extracellular matrix deposition and remodeling. Here we summarize recent progress in the study of mechanisms regulating the activation cycle of β(1) integrin and the specificity of α(5)β(1) integrin in mechanotransduction.

R-Ras regulates migration through an interaction with filamin A in melanoma cells

BACKGROUND

Changes in cell adhesion and migration in the tumor microenvironment are key in the initiation and progression of metastasis. R-Ras is one of several small GTPases that regulate cell adhesion and migration on the extracellular matrix, however the mechanism has not been completely elucidated. Using a yeast two-hybrid approach we sought to identify novel R-Ras binding proteins that might mediate its effects on integrins.

METHODS AND FINDINGS

We identified Filamin A (FLNa) as a candidate interacting protein. FLNa is an actin-binding scaffold protein that also binds to integrin beta1, beta2 and beta7 tails and is associated with diverse cell processes including cell migration. Indeed, M2 melanoma cells require FLNa for motility. We further show that R-Ras and FLNa interact in co-immunoprecipitations and pull-down assays. Deletion of FLNa repeat 3 (FLNaDelta3) abrogated this interaction. In M2 melanoma cells active R-Ras co-localized with FLNa but did not co-localize with FLNa lacking repeat 3. Thus, activated R-Ras binds repeat 3 of FLNa. The functional consequence of this interaction was that active R-Ras and FLNa coordinately increased cell migration. In contrast, co-expression of R-Ras and FLNaDelta3 had a significantly reduced effect on migration. While there was enhancement of integrin activation and fibronectin matrix assembly, cell adhesion was not altered. Finally, siRNA knockdown of endogenous R-Ras impaired FLNa-dependent fibronectin matrix assembly.

CONCLUSIONS

These data support a model in which R-Ras functionally associates with FLNa and thereby regulates integrin-dependent migration. Thus in melanoma cells R-Ras and FLNa may cooperatively promote metastasis by enhancing cell migration.

VEGFR1 (Flt1) regulates Rab4 recycling to control fibronectin polymerization and endothelial vessel branching

The cell's main receptor for VEGF, VEGFR2 (Kdr) is one of the most important positive regulators of new blood vessel growth and its downstream signalling is well characterized. By contrast, VEGFR1 (Flt1) and the mechanisms by which this VEGF receptor promotes branching morphogenesis in angiogenesis remain relatively unclear.Here we report that engagement of VEGFR1 activates a Rab4A-dependent pathway that transports alphavbeta3 Integrin from early endosomes to the plasma membrane, and that this is required for VEGF-driven fibronectin polymerization in endothelial cells. Furthermore, VEGFR1 acts to promote endothelial tubule branching in an organotypic model of angiogenesis via a mechanism that requires Rab4A and alphavbeta3 Integrin. We conclude that a recycling pathway regulated by Rab4A is a critical effector of VEGFR1 during branching morphogenesis of the vasculature.

Cadherin adhesion, tissue tension, and noncanonical Wnt signaling regulate fibronectin matrix organization

In this study we demonstrate that planar cell polarity signaling regulates morphogenesis in Xenopus embryos in part through the assembly of the fibronectin (FN) matrix. We outline a regulatory pathway that includes cadherin adhesion and signaling through Rac and Pak, culminating in actin reorganization, myosin contractility, and tissue tension, which, in turn, directs the correct spatiotemporal localization of FN into a fibrillar matrix. Increased mechanical tension promotes FN fibril assembly in the blastocoel roof (BCR), while reduced BCR tension inhibits matrix assembly. These data support a model for matrix assembly in tissues where cell-cell adhesions play an analogous role to the focal adhesions of cultured cells by transferring to integrins the tension required to direct FN fibril formation at cell surfaces.

Display of cell surface sites for fibronectin assembly is modulated by cell adherence to (1)F3 and C-terminal modules of fibronectin

BACKGROUND

Fibronectin-null cells assemble soluble fibronectin shortly after adherence to a substrate coated with intact fibronectin but not when adherent to the cell-binding domain of fibronectin (modules (7)F3-(10)F3). Interactions of adherent cells with regions of adsorbed fibronectin other than modules (7)F3-(10)F3, therefore, are required for early display of the cell surface sites that initiate and direct fibronectin assembly.

METHODOLOGY/PRINCIPAL FINDINGS

To identify these regions, coatings of proteolytically derived or recombinant pieces of fibronectin containing modules in addition to (7)F3-(10)F3 were tested for effects on fibronectin assembly by adherent fibronectin-null fibroblasts. Pieces as large as one comprising modules (2)F3-(14)F3, which include the heparin-binding and cell adhesion domains, were not effective in supporting fibronectin assembly. Addition of module (1)F3 or the C-terminal modules to modules (2)F3-(14)F3 resulted in some activity, and addition of both (1)F3 and the C-terminal modules resulted in a construct, (1)F3-C, that best mimicked the activity of a coating of intact fibronectin. Constructs (1)F3-C V0, (1)F3-C V64, and (1)F3-C Delta(V(15)F3(10)F1) were all able to support fibronectin assembly, suggesting that (1)F3 through (11)F1 and/or (12)F1 were important for activity. Coatings in which the active parts of (1)F3-C were present in different proteins were much less active than intact (1)F3-C.

CONCLUSIONS

These results suggest that (1)F3 acts together with C-terminal modules to induce display of fibronectin assembly sites on adherent cells.

Alpha v beta 3 and alpha v beta 5 integrins and their role in muscle precursor cell adhesion

BACKGROUND INFORMATION

Functional adaptation of skeletal muscle is a requirement for different muscle groups (e.g. craniofacial, ocular and limb) to undergo site-specific changes. Such tissue remodelling depends on dynamic interactions between muscle cells and their extracellular matrix, via participation of multifunctional molecules such as integrins. In view of data suggesting a role in fundamental muscle biology and muscle development in other systems, the present study has focused on expression and function of alpha v integrins, in cultured adult human craniofacial muscle (masseter) precursor cells and myotubes, and the predominantly fibroblastic IC (interstitial cells) population.

RESULTS AND CONCLUSIONS

Flow-cytometric phenotyping and immunofluorescence phenotyping show that alpha v, alpha v beta 3 and alpha v beta 5 are expressed in all mononuclear cells (muscle precursors and IC) seeded on muscle extracellular molecules such as gelatin, VN (vitronectin) and FN (fibronectin). In this system, blockade of alpha v activity using a function-perturbing antibody abrogates cell migration on VN and FN. alpha v integrins act predominantly as VN receptors as cell-substrate attachment is diminished when alpha v neutralizing agents are introduced into cultures seeded on VN, and this inhibition is reversible; these integrins also appear to be minor FN receptors. These results demonstrate that the alpha v subset of integrins present on both myogenic precursors and IC is an essential cohort of VN and, to a lesser extent, FN receptors mediating cell adhesion and, either directly or indirectly, arbiters of cell motility.

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

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

Binding of soluble fibronectin to integrin alpha5 beta1 - link to focal adhesion redistribution and contractile shape

Focal adhesions are randomly distributed across the ventral surface or along the edge of epithelial cells. In fibroblasts they orient centripetally and concentrate at a few peripheral sites connecting long F-actin stress fibers, causing a typical elongated, contractile morphology. Extensive remodeling of adhesions in fibroblasts also takes part in fibronectin fibrillogenesis, a process that depends on Rho-mediated contractility and results in the formation of a fibronectin matrix. Our current study shows that all these fibroblast characteristics are controlled by the ability of integrin alpha5 beta1 to bind soluble fibronectin molecules in their compact inactive conformation. The hypervariable region of the ligand-binding I-like domain of integrin alpha5 beta1 supports binding of soluble fibronectin. This supports the distribution of centripetally orientated focal adhesions in distinct peripheral sites, Rho activation and fibronectin fibrillogenesis through a mechanism that does not depend on Syndecan-4. Integrin alpha v beta3, even when locked in high affinity conformations for the RGD recognition motif shows no appreciable binding of soluble fibronectin and, consequently, fails to support the typical fibroblast focal adhesion distribution, Rho activity and fibronectin fibrillogenesis in the absence of integrin alpha5 beta1. The ability of alpha5 beta1 integrin to interact with soluble fibronectin may thus drive the cell-matrix adhesion and cytoskeletal organization required for a contractile, fibroblast-like morphology, perhaps explaining why alpha5 beta1 integrin, similarly to fibronectin, is essential for development.

FAK-independent alphavbeta3 integrin-EGFR complexes rescue from anoikis matrix-defective fibroblasts

Extracellular matrix (ECM) binding to integrin receptors regulates cell cycle progression and survival. In adherent cells, ECM disassembly induces anoikis, the apoptotic pathway switched on by loss of adhesion. ECM-deficient Ehlers-Danlos syndrome (EDS) fibroblasts, to adhere to rare fibronectin (FN) fibrils, and to proliferate, only organize, as FN receptor, the alphavbeta3 integrin. We report that in EDS cells the alphavbeta3 integrin is bound to talin and vinculin, but not to tensin, and that actin cytoskeleton is disorganized. Furthermore, in EDS cells Bcl-2 is down-regulated and caspases are active. We provide evidence that the antibody-mediated alphavbeta3 integrin or the FN inhibition induces anoikis in EDS cells. The alphavbeta3 integrin transduces survival signals to pp60src-mediated tyrosine phosphorylated paxillin, instead than to FAK, and interacts with EGF receptor (EGFR). This complex, when activated by EGF and FN, signals for the rescue of EDS cells from anoikis. Therefore, EDS cells, through the alphavbeta3 integrin-EGFR complexes, engage a paxillin- but not FAK-mediated pathway of cell survival.

The RGD motif in fibronectin is essential for development but dispensable for fibril assembly

Fibronectin (FN) is secreted as a disulfide-bonded FN dimer. Each subunit contains three types of repeating modules: FN-I, FN-II, and FN-III. The interactions of α5β1 or αv integrins with the RGD motif of FN-III repeat 10 (FN-III₁₀) are considered an essential step in the assembly of FN fibrils. To test this hypothesis in vivo, we replaced the RGD motif with the inactive RGE in mice. FN-RGE homozygous embryos die at embryonic day 10 with shortened posterior trunk, absent tail bud–derived somites, and severe vascular defects resembling the phenotype of α5 integrin–deficient mice. Surprisingly, the absence of a functional RGD motif in FN did not compromise assembly of an FN matrix in mutant embryos or on mutant cells. Matrix assembly assays and solid-phase binding assays reveal that αvβ3 integrin assembles FN-RGE by binding an isoDGR motif in FN-I₅, which is generated by the nonenzymatic rearrangement of asparagines (N) into an iso-aspartate (iso-D). Our findings demonstrate that FN contains a novel motif for integrin binding and fibril formation whose activity is controlled by amino acid modification.

Modulation of cell-fibronectin matrix interactions during tissue repair

Environmental signals from the extracellular matrix (ECM) are transmitted by cell surface receptors that connect to the actin cytoskeleton and to multiple intracellular signaling pathways. To dissect how the ECM regulates cell functions, we are using a three-dimensional (3D) fibrin-fibronectin matrix, resembling the wound provisional matrix. Fibroblasts adhere to fibronectin in this matrix via concomitant engagement of alpha 5 beta 1 integrin receptors and syndecan-4, a transmembrane proteoglycan. An adhesive phenotype is developed with actin stress fibers and activation of focal adhesion kinase (FAK) and Rho GTPase. Lack of syndecan-4 engagement, as occurs in the presence of the ECM protein tenascin-C, promotes a motile phenotype; FAK and Rho signaling are downregulated and filopodia are extended. Fibronectin matrices have distinct effects on two other receptors: alpha 4 beta 1 and beta v beta 3 integrins. Although alpha 4 beta 1 does not naturally support strong cell interactions with a fibrin-fibronectin matrix, binding is dramatically enhanced by proteolytic cleavage of fibronectin. Conversely, activity of alpha v beta 3 is stimulated by multimeric fibronectin fibrils showing that the organization of fibronectin differentially affects integrin functions. Thus, deposition of additional ECM components, expression of co-receptors for ECM, cleavage of adhesive proteins, and the architecture of the ECM microenvironment are different mechanisms for modulating cell responses to fibronectin matrix.

Does glycosylation of melanoma cells influence their interactions with fibronectin?

Cell surface integrins, especially those binding to fibronectin (FN), participate in processes of tumor cell invasion and metastasis. Changes in glycosylation of cell surface adhesion proteins are often associated with malignant transformation of cells. In this study we examined the influence of swainsonine (SW) on adhesion, wound healing and haptotactic migration on FN, comparing the responses of different human melanoma cell lines: primary WM35 and metastatic WM9, WM239 and A375. We also examined the role of alpha subunits in adhesion to FN. All of the antibodies inhibited adhesion to FN but with different efficiencies depending on the cell line. Adhesion was mediated mainly by integrin alpha(5)beta(1) (WM9, A375), alpha(3)beta(1) (WM35, A375, WM239). Scratch wound repair was significantly faster on FN-coated wells than on plastic for all cells except for WM9. A375 and WM9 had the greatest migration ability, both expressing the highest level of alpha(5)beta(1) integrin. It seems very likely that adhesion to FN can be accomplished by many different integrins, but for effective migration alpha(5)beta(1) integrin is responsible. Only A375 and WM239 cell lines reacted to SW treatment. In the presence of SW WM239 and A375 cells had 70% and 40% increased adhesion to FN, and their migration was decreased 40% and 50%, respectively. Interestingly, although most of the cell lines share a common profile of integrins, each line interacted with FN differently. They differed mainly in the repertoire of integrins used for adhesion, and in the manner in which glycosylation affected these processes. The influence of SW was observed in two metastatic cell lines indicating the contribution of glycosylation status to the progression of melanoma. The lack of reaction to SW in WM9 cells may suggest that there is a threshold in the expression level of the highly branched N-glycans that may influence the adhesion and migration properties of the cell.

Stimulatory effects of a three-dimensional microenvironment on cell-mediated fibronectin fibrillogenesis

The assembly of fibronectin into a fibrillar matrix is a regulated step-wise process that involves binding to integrin receptors and interactions between fibronectin molecules. This process has been studied extensively using cells in two-dimensional (2D) monolayer culture. In most situations in vivo, however, matrix assembly occurs within existing three-dimensional (3D) extracellular matrix networks. In an attempt to mimic this environment, we analyzed matrix assembly by fibroblasts cultured on a pre-assembled 3D fibronectin matrix and found significant stimulation of fibronectin fibril assembly compared to cells in 2D culture. Lower amounts of fibronectin were needed to initiate the assembly process, fibrils accumulated to higher density, and the 3D fibril organization played a key role in the stimulatory effect. Moreover, cells expressing activation-dependent integrins were able to assemble fibronectin matrix without exogenous stimulation, suggesting regulatory effects of the 3D fibronectin matrix on integrin activity. These results provide evidence for an additional level of control of fibronectin deposition through cell interactions with the local microenvironment.

Fibronectin fibrillogenesis, a cell-mediated matrix assembly process

The extracellular matrix provides a framework for cell adhesion, supports cell movement, and serves to compartmentalize tissues into functional units. Fibronectin is a core component of many extracellular matrices where it regulates a variety of cell activities through direct interactions with cell surface integrin receptors. Fibronectin is synthesized by many adherent cells which then assemble it into a fibrillar network. The assembly process is integrin-dependent and fibronectin-integrin interactions initiate a step-wise process involving conformational activation of fibronectin outside and organization of the actin cytoskeleton inside. During assembly, fibronectin undergoes conformational changes that expose fibronectin-binding sites and promote intermolecular interactions needed for fibril formation. In this review, the main steps of fibronectin assembly are described and recent studies on fibronectin conformational changes are discussed.

Smart materials as scaffolds for tissue engineering

In this review, we focused our attention on the more important natural extracellular matrix (ECM) molecules (collagen and fibrin), employed as cellular scaffolds for tissue engineering and on a class of semi-synthetic materials made from the fusion of specific oligopeptide sequences, showing biological activities, with synthetic materials. In particular, these new "intelligent" scaffolds may contain oligopeptide cleaving sequences specific for matrix metalloproteinases (MMPs), integrin binding domains, growth factors, anti-thrombin sequences, plasmin degradation sites, and morphogenetic proteins. The aim was to confer to these new "intelligent" semi-synthetic biomaterials, the advantages offered by both the synthetic materials (processability, mechanical strength) and by the natural materials (specific cell recognition, cellular invasion, and the ability to supply differentiation/proliferation signals). Due to their characteristics, these semi-synthetic biomaterials represent a new and versatile class of biomimetic hybrid materials that hold clinical promise in serving as implants to promote wound healing and tissue regeneration.

Expression and significance of integrin-linked kinase in cultured cells, normal tissue, and diseased tissue of aging rat kidneys

Integrin-linked kinase (ILK) is an integrin-binding cytoplasmic protein that has been implicated in regulating numerous cellular processes and fibronectin (Fn) deposition through mediated integrin, but the expression and significance of ILK in the aging kidney have not yet been reported. We report here that mRNA and protein expression of ILK increased in primary cultured mesangial and tubular epithelial cells, and normal and unilateral ureteral obstructed kidney tissues in 28-month-old rats but not in 3-month-old rats, moreover, accompanied by the over-expression of Fn and integrin-beta1 in the aging kidney, by means of Northern blot, Western blot, and immunofluorescent double-staining immunohistochemistry. In addition, in the primary cultured kidney cells, ILK expression was positively correlated with senescence-associated beta-gal positive staining and negatively correlated with cellular proliferation. The results suggest that ILK may be involved in the fibrotic or senescent process in the aging kidney.

Assembly of exogenous fibronectin by fibronectin-null cells is dependent on the adhesive substrate

The role of endogenously synthesized fibronectin (FN) in assembly was studied with cells lacking or expressing FN. Cells were cultured as homogeneous or mixed populations on surfaces coated with different matrix proteins. Compared with FN-expressing cells, FN-null cells poorly assembled exogenous plasma FN (pFN) when adhered to vitronectin or the recombinant cell-binding domain (III(7-10)) of FN. Vitronectin had a suppressive effect that was overcome by co-adsorbed pFN or laminin-1 but not by soluble FN. In co-cultures of FN-expressing cells and FN-null cells, endogenous FN was preferentially assembled around FN-expressing cells regardless of the adhesive ligand. If the adhesive ligand was vitronectin, exogenous pFN assembled preferentially around cells expressing cellular FN or recombinant EDa- or EDa+ FN. In co-cultures on vitronectin of FN-null cells and beta(1) integrin subunit-null cells, fibrils of cellular FN and pFN were preferentially deposited by FN-null (beta(1)-expressing) cells immediately adjacent to (FN-secreting) beta(1)-null cells. In co-cultures on vitronectin of FN-null cells and beta(1)-null cells expressing a chimera with the extracellular domain of beta(1) and the cytoplasmic domain of beta(3), preferential assembly was by the chimera-expressing cells. These results indicate that the adhesive ligand is a determinant of FN assembly by cells not secreting endogenous FN (suppressive if vitronectin, non-suppressive but non-supportive if III(7-10), supportive if pFN or laminin-1) and suggest that efficient interaction of freshly secreted cellular FN with a beta(1) integrin, presumably alpha(5)beta(1), substitutes for integrin-mediated adherence to a preformed matrix of laminin-1 or pFN to support assembly of FN.

Integrin beta3 overexpression suppresses tumor growth in a human model of gliomagenesis: implications for the role of beta3 overexpression in glioblastoma multiforme

alphaVbeta3 integrin complexes are overexpressed in the growing, invading margins of human glioblastoma multiforme (GBM) and in the GBM vasculature, suggesting a key role for alphaVbeta3 in GBM growth and invasion. The function of alphaVbeta3 complexes in tumor formation, however, has been challenged by studies showing that loss of alphaVbeta3 expression (via loss of beta3) in the host vasculature enhances, rather than suppresses, the growth of s.c. implanted carcinomas. To directly address the role of tumor-specific alphaVbeta3 overexpression in glioma formation, we increased alphaVbeta3 expression (via overexpression of a wild-type or constitutively activated beta3) in human astrocytes genetically modified to form anaplastic astrocytoma-like tumors (Ras cells) on intracranial injection in rats. Overexpression of beta3 selectively increased levels of alphaVbeta3 integrin complexes, but had no effect on anchorage-dependent or -independent growth in vitro. After intracranial injection, however, the Ras + beta3 cells formed fewer and smaller tumors than did Ras cells. Similarly, Ras-transformed mouse astrocytes that were derived from control animals formed smaller intracranial tumors than those derived from beta3 knockout animals. Although tumors formed by human Ras and Ras + beta3 cells were similar in blood vessel density, Ras + beta3 tumors had smaller, pericyte-depleted vessels and were significantly more hypoxic, suggesting a beta3-mediated vascular defect. The growth-suppressive actions of beta3, however, could be overcome by stimulation of pathways (Akt or vascular endothelial growth factor) commonly activated in GBM. These results show that tumor-specific alphaVbeta3 overexpression has growth-suppressive effects in gliomas, but that these deleterious effects are mitigated by alterations common to alphaVbeta3-overexpressing GBM.

Developmental endothelial locus-1 (Del-1), a novel angiogenic protein: its role in ischemia

BACKGROUND

Developmentally regulated endothelial locus-1 (Del-1) is an extracellular matrix protein that is expressed by endothelial cells during embryological vascular development. We speculated that Del-1 may be reexpressed in ischemia and may be involved in endogenous angiogenesis.

METHODS AND RESULTS

Del-1 protein was detected by immunohistochemistry in murine ischemic hindlimb after femoral artery excision. To determine whether exogenous Del-1 would augment angiogenesis in vivo, Del-1 or vehicle was administered for 3 weeks by intramuscular injection of murine ischemic hindlimbs. Angiogenesis was quantified by gadolinium-MRI perfusion and capillary densitometry. We used a disc angiogenesis system (DAS) to characterize the angiogenic response to vehicle (PBS), Del-1, Del-1 mutant (altered RGD domain), Del-1 minor (truncated discoidin-I-like domain), or basic fibroblast growth factor. After 14 days, the discs were extracted and sectioned to quantify vascular growth by morphometry. Endogenous Del-1 protein expression was increased in ischemic hindlimbs. Administration of Del-1 increased hindlimb vascular flow index and capillary density. In the DAS, Del-1 doubled fibrovascular growth, as did basic fibroblast growth factor. However, angiogenesis was not enhanced by the Del-1 mutant or Del-1 minor proteins.

CONCLUSIONS

Del-1 is expressed in ischemic tissue. Del-1 stimulates angiogenesis, an effect that is dependent on the RGD motif and a second signaling sequence in the discoidin-I-like domain. Exogenous intramuscular administration of Del-1 significantly enhances angiogenesis in the murine ischemic hindlimb. Del-1 may prove to be a novel therapeutic agent for patients with ischemia.

Human fibroblasts with mutations in COL5A1 and COL3A1 genes do not organize collagens and fibronectin in the extracellular matrix, down-regulate alpha2beta1 integrin, and recruit alphavbeta3 Instead of alpha5beta1 integrin

Dermal fibroblasts derived from types I and IV Ehlers-Danlos syndrome (EDS) patients, carrying mutations in COL5A1 and COL3A1 genes, respectively, synthesize aberrant types V and III collagen (COLL) and show defective organization of these proteins into the extracellular matrix (ECM) and high reduction of their functional receptor, the alpha(2)beta(1) integrin, compared with control fibroblasts. EDS cells also show reduced levels of fibronectin (FN) in the culture medium and lack an FN fibrillar network. Finally, EDS cells prevalently organize alpha(v)beta(3) integrin instead of alpha(5)beta(1) integrin. The alpha(v)beta(3) integrin, distributed on the whole EDS cell surface, shows FN binding and assembly properties when the cells are treated with purified FN. Treatment of EDS cells with purified COLLV or COLLIII, but not with FN, restores the control phenotype (COLL(+), FN(+), alpha(v)beta(3)(-), alpha(5)beta(1)(+), alpha(2)beta(1)(+)). Function-blocking antibodies to COLLV, COLLIII, or alpha(2)beta(1) integrin induce in control fibroblasts an EDS-like phenotype (COLL(-), FN(-), alpha(v)beta(3)(+), alpha(5)beta(1)(-), alpha(2)beta(1)(-)). These results show that in human fibroblasts alpha(2)beta(1) integrin organization and function are controlled by its ligand, and that the alpha(2)beta(1)-COLL interaction, in turn, regulates FN integrin receptor recruitment: high alpha(2)beta(1) integrin levels induce alpha(5)beta(1) integrin organization, while low alpha(2)beta(1) integrin levels lead to alpha(v)beta(3) integrin organization.

Matrix-fibrinogen enhances wound closure by increasing both cell proliferation and migration

Fibrinogen (FBG) assembles into matrix fibrils of fibroblasts, lung and mammary epithelial cells, but not endothelial cells. Furthermore, cryptic beta15-21 residues are exposed in FBG fibrils with no evidence of thrombin or plasmin proteolysis. Herein, the effects of FBG on migration and proliferation of wounded dermal fibroblasts were investigated. FBG preassembled into matrix prior to scrape-wounding induced 3H-thymidine incorporation 8-fold and shortened the time to wound closure 1.6-fold +/- 0.1-fold. FBG added immediately after wounding did not enhance either response. Fibroblast growth factor-2/platelet-derived growth factor (FGF-2/PDGF) stimulated cell proliferation 2.2-fold for FGF-2 and 3.2-fold for PDGF and wound closure 1.5-fold +/- 0.1-fold in the absence of matrix-FBG. Surprisingly, exogenous growth factors had negligible effect on wound closure and cell proliferation already enhanced by matrix-FBG. Matrix-FBG-enhanced wound closure required active assembly of an FBG-fibronectin matrix, engagement of alphavbeta3, and FBG Aalpha-RGDS572-575 integrin recognition sites; Aalpha-RGDF95-98 sites were not sufficient for matrix-FBG assembly, enhanced wound closure, or cell proliferation. Although Bbeta1-42 was not necessary for matrix assembly, it was required for matrix-FBG-enhanced cell migration. These data indicate that FBG serves as an important matrix constituent in the absence of fibrin formation to enhance wound repair and implicate Bbeta1-42 as a physiologic inducer of signal transduction to promote an intermediate state of cell adhesion and a migratory cell phenotype.

The ins and outs of fibronectin matrix assembly

Cell phenotype is specified by environmental cues embedded in the architecture and composition of the extracellular matrix (ECM). Much has been learned about matrix organization and assembly through analyses of the ECM protein fibronectin (FN). FN matrix assembly is a cell-mediated process in which soluble dimeric FN is converted into a fibrillar network. Binding of cell surface integrin receptors to FN converts it to an active form, which promotes fibril formation through interactions with other cell-associated FN dimers. As FN fibrils form on the outside of the cell, cytoplasmic domains of integrin receptors organize cytoplasmic proteins into functional complexes inside. Intracellular connections to the actin cytoskeletal network and stimulation of certain key intracellular signaling pathways are essential for FN-integrin interactions and propagation of FN fibril formation. Thus, assembly of native functional ECM depends on exquisite coordination between extracellular events and intracellular pathways.