Vitronectin exists in two structurally and functionally distinct forms in human plasma

Colonization and Infection of Indwelling Medical Devices by Staphylococcus aureus with an Emphasis on Orthopedic Implants

The use of indwelling medical devices has constantly increased in recent years and has revolutionized the quality of life of patients affected by different diseases. However, despite the improvement of hygiene conditions in hospitals, implant-associated infections remain a common and serious complication in prosthetic surgery, mainly in the orthopedic field, where infection often leads to implant failure. Staphylococcus aureus is the most common cause of biomaterial-centered infection. Upon binding to the medical devices, these bacteria proliferate and develop dense communities encased in a protective matrix called biofilm. Biofilm formation has been proposed as occurring in several stages-(1) attachment; (2) proliferation; (3) dispersal-and involves a variety of host and staphylococcal proteinaceous and non-proteinaceous factors. Moreover, biofilm formation is strictly regulated by several control systems. Biofilms enable staphylococci to avoid antimicrobial activity and host immune response and are a source of persistent bacteremia as well as of localized tissue destruction. While considerable information is available on staphylococcal biofilm formation on medical implants and important results have been achieved on the treatment of biofilms, preclinical and clinical applications need to be further investigated. Thus, the purpose of this review is to gather current studies about the mechanism of infection of indwelling medical devices by S. aureus with a special focus on the biochemical factors involved in biofilm formation and regulation. We also provide a summary of the current therapeutic strategies to combat biomaterial-associated infections and highlight the need to further explore biofilm physiology and conduct research for innovative anti-biofilm approaches.

The Trimeric Autotransporter Adhesin YadA of Yersinia enterocolitica Serotype O:9 Binds Glycan Moieties

Yersinia adhesin A (YadA) is a key virulence factor of Yersinia enterocolitica and Yersinia pseudotuberculosis. YadA is a trimeric autotransporter adhesin, a class of adhesins that have been shown to enable many Gram-negative pathogens to adhere to/interact with the host extracellular matrix proteins such as collagen, vitronectin, and fibronectin. Here, we show for the first time that YadA of Yersinia enterocolitica serotype O:9 not only interacts with proteinaceous surface molecules but can also attach directly to glycan moieties. We show that YadA from Y. enterocolitica serotype O:9 does not interact with the vitronectin protein itself but exclusively with its N-linked glycans. We also show that YadA can target other glycan moieties as found in heparin, for example. So far, little is known about specific interactions between bacterial autotransporter adhesins and glycans. This could potentially lead to new antimicrobial treatment strategies, as well as diagnostic applications.

Vitronectin stabilizes intravascular adhesion of neutrophils by coordinating β2 integrin clustering

The recruitment of neutrophils from the microvasculature to the site of injury or infection represents a key event in the inflammatory response. Vitronectin (VN) is a multifunctional macromolecule abundantly present in blood and extracellular matrix. The role of this glycoprotein in the extravasation process of circulating neutrophils remains elusive. Employing advanced in vivo/ex vivo imaging techniques in different mouse models as well as in vitro methods, we uncovered a previously unrecognized function of VN in the transition of dynamic to static intravascular interactions of neutrophils with microvascular endothelial cells. These distinct properties of VN require the heteromerization of this glycoprotein with plasminogen activator inhibitor-1 (PAI- 1) on the activated venular endothelium and subsequent interactions of this protein complex with the scavenger receptor low-density lipoprotein receptor-related protein-1 on intravascularly adhering neutrophils. This induces p38 mitogen-activated protein kinases-dependent intracellular signaling events which, in turn, regulates the proper clustering of the b2 integrin lymphocyte function associated antigen-1 on the surface of these immune cells. As a consequence of this molecular interplay, neutrophils become able to stabilize their adhesion to the microvascular endothelium and, subsequently, to extravasate to the perivascular tissue. Hence, endothelial-bound VN-PAI-1 heteromers stabilize intravascular adhesion of neutrophils by coordinating b2 integrin clustering on the surface of these immune cells, thereby effectively controlling neutrophil trafficking to inflamed tissue. Targeting this protein complex might be beneficial for the prevention and treatment of inflammatory pathologies.

Circulating Vitronectin Predicts Liver Injury and Mortality in Children With Sepsis: A Prospective Observational Study

Vitronectin (VTN) is a key regulator of coagulation, but clinical relevance of serum VTN in pediatric sepsis remains poorly defined. The aim of this study was to access the value of serum VTN level on pediatric intensive care unit (PICU) admission in children with sepsis. Pediatric patients with sepsis were enrolled from January 2018 to December 2018. The serum VTN levels were determined on PICU admission, and the association of serum VTN level with PICU mortality and organ dysfunction was assessed. Serum VTN levels were significantly lower in nonsurvivors compared with survivors, in patients with septic shock compared with patients with sepsis, or in patients with sepsis-associated acute liver injury (ALI) compared with patients without ALI. Serum VTN level was associated with PICU mortality (odds ratio [OR]: 0.958, 95% CI: 0.927-0.996; P = .010) or ALI (OR: 0.956, 95% CI: 0.915-0.999; P = .046), but not shock (OR: 0.996, 95% CI: 0.977-1.016; P =.716). The area under receiver operating characteristic curve for VTN in predicting the occurrence of ALI during PICU stay and PICU mortality were 0.760 (95% CI: 0.627- 0.893) and 0.737 (95% CI: 0.544-0.931), respectively. Moreover, VTN plus pediatric risk of mortality (PRISM) III had a better clinical utility according to decision curve analysis compared with VTN or PRISM III alone. These findings suggest that serum VTN level is associated with sepsis-associated ALI and PICU mortality, and VTN plus PRISM III is a powerful predictor of PICU mortality in pediatric patients with sepsis, which have a better clinical benefit compared with VTN or PRISM III alone.

Soluble Membrane Attack Complex: Biochemistry and Immunobiology

The soluble membrane attack complex (sMAC, a.k.a., sC5b-9 or TCC) is generated on activation of complement and contains the complement proteins C5b, C6, C7, C8, C9 together with the regulatory proteins clusterin and/or vitronectin. sMAC is a member of the MACPF/cholesterol-dependent-cytolysin superfamily of pore-forming molecules that insert into lipid bilayers and disrupt cellular integrity and function. sMAC is a unique complement activation macromolecule as it is comprised of several different subunits. To date no complement-mediated function has been identified for sMAC. sMAC is present in blood and other body fluids under homeostatic conditions and there is abundant evidence documenting changes in sMAC levels during infection, autoimmune disease and trauma. Despite decades of scientific interest in sMAC, the mechanisms regulating its formation in healthy individuals and its biological functions in both health and disease remain poorly understood. Here, we review the structural differences between sMAC and its membrane counterpart, MAC, and examine sMAC immunobiology with respect to its presence in body fluids in health and disease. Finally, we discuss the diagnostic potential of sMAC for diagnostic and prognostic applications and potential utility as a companion diagnostic.

Differential regulation of extracellular matrix proteins in three recurrent liver metastases of a single patient with colorectal cancer

Colorectal cancer (CRC) patients suffer from the second highest mortality among all cancer entities. In half of all CRC patients, colorectal cancer liver metastases (CRLM) can be observed. Metastatic colorectal cancer is associated with poor overall survival and limited treatment options. Even after successful surgical resection of the primary tumor, metachronous liver metastases occur in one out of eight cases. The only available curative intended treatment is hepatic resection, but metachronous CRLM frequently recur after approximately 1 year. In this study, we performed a proteome analysis of three recurrent liver metastases of a single CRC patient by mass spectrometry. Despite surgical resection of the primary CRC and adjuvant chemotherapy plus cetuximab treatment, the patient developed three metachronous CRLM which occurred consecutively after 9, 21 and 31 months. We identified a set of 1132 proteins expressed in the three metachronous CRLM, of which 481 were differentially regulated, including 81 proteins that were associated with the extracellular matrix (ECM). 56 ECM associated proteins were identified as upregulated in the third metastasis, 26 (46%) of which were previously described as negative prognostic markers in CRC, including tenascin C, nidogen 1, fibulin 1 and vitronectin. These data may reflect an ascending trend of malignancy from the first to the third metachronous colorectal cancer liver metastasis. Additionally, the results indicate different ECM phenotypes for recurrent metachronous metastasis, associated with different grades of malignancy and highlights the importance of individual analysis of molecular features in different, consecutive metastatic events in a single patient.

Control of cellular adhesiveness in hyaluronic acid-based hydrogel through varying degrees of phenol moiety cross-linking

Current hyaluronic acid-based hydrogels often cause cytotoxicity to encapsulated cells and lack the adhesive property required for effective biomedical and tissue engineering applications. Provision of the cell-adhesive surface is an important requirement to improve its biocompatibility. An aqueous solution of hyaluronic acid possessing phenolic hydroxyl (HA-Ph) moieties is gellable via a horseradish peroxidase (HRP)-catalyzed oxidative cross-linking reaction. This study evaluates the effect of different degrees of cross-linked Ph moieties on cellular adhesiveness and proliferation on the resultant enzymatically cross-linked HA-Ph hydrogels. Mechanical characterization demonstrated that the compression force of engineered hydrogels could be tuned in the range of 0.05-35 N by changing conjugated Ph moieties in the precursor formulation. The water contact angle and water content show hydrophobicity of hydrogels increased with increasing content of cross-linked Ph groups. The seeded mouse embryo fibroblast-like cell line and human cervical cancer cell line, on the HA-Ph hydrogel, proved cell attachment and spreading with a high content of cross-linked Ph groups. The HA-Ph with a higher degree of Ph moieties shows the maximum degree of cell adhesion, spreading, and proliferation which presents this hydrogel as a suitable biomaterial for biomedical and tissue engineering applications.

The iron-regulated surface determinant B (IsdB) protein from Staphylococcus aureus acts as a receptor for the host protein vitronectin

Staphylococcus aureus is an important bacterial pathogen that can cause a wide spectrum of diseases in humans and other animals. S. aureus expresses a variety of virulence factors that promote infection with this pathogen. These include cell-surface proteins that mediate adherence of the bacterial cells to host extracellular matrix components, such as fibronectin and fibrinogen. Here, using immunoblotting, ELISA, and surface plasmon resonance analysis, we report that the iron-regulated surface determinant B (IsdB) protein, besides being involved in heme transport, plays a novel role as a receptor for the plasma and extracellular matrix protein vitronectin (Vn). Vn-binding activity was expressed by staphylococcal strains grown under iron starvation conditions when Isd proteins are expressed. Recombinant IsdB bound Vn dose dependently and specifically. Both near-iron transporter motifs NEAT₁ and NEAT₂ of IsdB individually bound Vn in a saturable manner, with K_D values in the range of 16-18 nm Binding of Vn to IsdB was specifically blocked by heparin and reduced at high ionic strength. Furthermore, IsdB-expressing bacterial cells bound significantly higher amounts of Vn from human plasma than did an isdB mutant. Adherence to and invasion of epithelial and endothelial cells by IsdB-expressing S. aureus cells was promoted by Vn, and an α_vβ₃ integrin-blocking mAb or cilengitide inhibited adherence and invasion by staphylococci, suggesting that Vn acts as a bridge between IsdB and host α_vβ₃ integrin.

Two-Step Size-Exclusion Nanofiltration of Prothrombin Complex Concentrate Using Nanocellulose-Based Filter Paper

Coagulation Factor IX-rich protrhombin complex concentrate (FIX-PCC) is a therapeutic biologic product that consists of a mixture of several human plasma-derived proteins, useful for treating hemophilia B. Due to its complex composition, FIX-PCC is very challenging to bioprocess through virus removing nanofilters in order to ensure its biosafety. This article describes a two-step filtration process of FIX-PCC using a nanocellulose-based filter paper with tailored porosity. The filters were characterized with scanning electron microscopy (SEM), cryoporometry with differential scanning calorimetry, and nitrogen gas sorption. Furthermore, in order to probe the filter's cut-off size rejection threshold, removal of small- and large-size model viruses, i.e., ΦX174 (28 nm) and PR772 (70 nm), was evaluated. The feed, pre-filtrate, and permeate solutions were characterized with mass-spectrometric proteomic analysis, dynamic light scattering (DLS), sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and analytical size-exclusion high-performance liquid chromatography (SEHPLC). By sequential filtration through 11 μm pre-filter and 33 μm virus removal filter paper, it was possible to achieve high product throughput and high virus removal capacity. The presented approach could potentially be applied for bioprocessing other protein-based drugs.

Vitronectins produced by human cirrhotic liver and CCl4-treated rats differ in their glycosylation pattern and tissue remodeling activity

Liver cirrhosis (LC) is a disease characterized by pathological accumulation and alteration of extracellular matrix (ECM) proteins; the interaction between two such proteins, collagen and vitronectin (VN), is considered to be the key to controlling ECM remodeling in liver cirrhosis. If it is possible to control the modification of oligosaccharides on VN, it may be possible to retard progression of liver cirrhosis. In this study, we examined the relationship between changes in VN glycosylation and activity related to the remodeling of hepatic tissue in human LC and a rat model of LC generated using carbon tetrachloride (CCl₄). Plasma concentrations of VN in human LC declined to approximately two‐thirds that in normal plasma, but the ratio of active VN, which has collagen‐binding activities, increased 2.8 times in LC plasma. In contrast, purified LC‐VN exhibited similar binding activities toward type I, IV, and V collagens to those of normal VN. Lectin reactivities and carbohydrate analyses of LC‐VN revealed that branching, fucosylation, and sialylation of N‐glycans were higher than those of normal VN. On the other hand, the plasma level of rat CCl₄‐VN increased and the ratio of active molecules to collagen in plasma decreased. Increased fucosylation of LC‐VN was not detected in carbohydrates of CCl₄‐VN. The changes in rat VN due to CCl₄ treatment did not correspond to the changes in plasma levels of human VN caused by LC, the ratio of active molecules, or carbohydrate composition, thereby indicating that CCl₄‐treated rats are not an appropriate model for studying VNs in human LC. Glycosidase treatment of VNs supported the hypothesis that the collagen‐binding activity of VN is modulated by alterations of glycosylation during LC, which may contribute to (a) the matrix incorporation of VN and (b) tissue fibrosis.

Association of vitronectin and plasminogen activator inhibitor-1 levels with the risk of metabolic syndrome and type 2 diabetes mellitus

It was the objective of this study to investigate the relation between vitronectin and plasminogen activator inhibitor (PAI)-1 plasma levels with nine-year incidences of the metabolic syndrome (MetS) and of type 2 diabetes mellitus (T2DM). Baseline plasma concentrations of vitronectin and PAI-1 were measured in 627 healthy participants from the prospective D.E.S.I.R. cohort who subsequently developed MetS (n=487) and T2DM (n=182) over a nine-year follow-up (42 presented both) and who were matched with two healthy control subjects each by use of a nested case-control design. Parameters composing the MetS explained about 20% of plasma vitronectin levels. An increase of one standard deviation of vitronectin was associated with increased risks of both the MetS (odds ratio [OR] = 1.21 [1.07 – 1.37], p = 0.003) and T2DM (OR = 1.24 [1.01 – 1.53], p = 0.045). Corresponding ORs for PAI-1 levels were 1.46 [1.27 – 1.68] (p < 10−4) and 1.40 [1.14 – 1.72] (p = 0.0012). However, the effects of vitronectin and PAI-1 levels on outcomes were not independent. The vitronectin–MetS association was restricted to individuals with low to modest PAI-1 levels (OR = 1.33 [1.14 – 1.54], p = 0.0003) while no association was observed in individuals with high PAI-1 levels (OR = 0.87 [0.68 – 1.10], p = 0.24), the test for interaction being highly significant (p = 0.0009). In conclusion, baseline plasma vitronectin is a marker of incident MetS at nine years. Its predictive ability for MetS and T2DM should not be assessed independently of PAI-1 levels.

Vitronectin as a Micromanager of Cell Response in Material-Driven Fibronectin Nanonetworks

Surface functionalization strategies of synthetic materials for regenerative medicine applications comprise the development of microenvironments that recapitulate the physical and biochemical cues of physiological extracellular matrices. In this context, material-driven fibronectin (FN) nanonetworks obtained from the adsorption of the protein on poly(ethyl acrylate) provide a robust system to control cell behavior, particularly to enhance differentiation. This study aims at augmenting the complexity of these fibrillar matrices by introducing vitronectin, a lower-molecular-weight multifunctional glycoprotein and main adhesive component of serum. A cooperative effect during co-adsorption of the proteins is observed, as the addition of vitronectin leads to increased fibronectin adsorption, improved fibril formation, and enhanced vitronectin exposure. The mobility of the protein at the material interface increases, and this, in turn, facilitates the reorganization of the adsorbed FN by cells. Furthermore, the interplay between interface mobility and engagement of vitronectin receptors controls the level of cell fusion and the degree of cell differentiation. Ultimately, this work reveals that substrate-induced protein interfaces resulting from the cooperative adsorption of fibronectin and vitronectin fine-tune cell behavior, as vitronectin micromanages the local properties of the microenvironment and consequently short-term cell response to the protein interface and higher order cellular functions such as differentiation.

Fluid Shear Stress and Fibroblast Growth Factor-2 Increase Endothelial Cell-Associated Vitronectin

Vitronectin is a matricellular protein that plays an important role in both coagulation and angiogenesis through its effects on cell adhesion and the plasminogen system. Vitronectin is known to bind to endothelial cells upon integrin activation. However, the effect of integrin activation by shear stress and growth factors on cell-associated vitronectin and plasminogen system activity has not yet been studied. We therefore exposed human umbilical vein endothelial cells to steady laminar flow, oscillating disturbed flow, or fibroblast growth factor-2 (FGF-2) for 24 hours. We then measured cell-associated vitronectin by Western blot and plasminogen system activity using a Chromozym assay. Steady laminar flow, oscillating disturbed flow, and FGF-2 all increased cell-associated vitronectin, although the vitronectin molecular weight varied among the different conditions. FGF-2 also increased cell-associated vitronectin in microvascular endothelial cells and vascular smooth muscle cells. The increase in cell-associated vitronectin increased plasminogen system activity. Confocal microscopy showed that vitronectin was primarily located in the basal and intracellular regions. α_vβ₅ integrin inhibition via genistein, an anti-α_vβ₅ antibody, or β₅ siRNA knockdown abrogated the FGF-2-induced increase in cell-associated vitronectin and increased plasminogen system activity. These data show that shear stress and growth factors increase cell-associated vitronectin through integrin activation, which may affect coagulation and angiogenesis.

Sialylation of vitronectin regulates stress fiber formation and cell spreading of dermal fibroblasts via a heparin-binding site

Vitronectin (VN) plays an important role in tissue regeneration. We previously reported that VN from partial hepatectomized (PH) rats results in a decrease of sialylation of VN and de-sialylation of VN decreases the cell spreading of hepatic stellate cells. In this study, we analyzed the mechanism how sialylation of VN regulates the properties of mouse primary cultured dermal fibroblasts (MDF) and a dermal fibroblast cell line, Swiss 3T3 cells. At first, we confirmed that VN from PH rats or de-sialylated VN also decreased cell spreading in MDF and Swiss 3T3 cells. The de-sialylation suppressed stress fiber formation in Swiss 3T3 cells. Next, we analyzed the effect of the de-sialylation of VN on stress fiber formation in Swiss 3T3 cells. RGD peptide, an inhibitor for a cell binding site of VN, did not affect the cell attachment of Swiss 3T3 cells on untreated VN but significantly decreased it on de-sialylated VN, suggesting that the de-sialylation attenuates the binding activity of an RGD-independent binding site in VN. To analyze a candidate RGD-independent binding site, an inhibition experiment of stress fiber formation for a heparin binding site was performed. The addition of heparin and treatment of cells with heparinase decreased stress fiber formation in Swiss 3T3 cells. Furthermore, de-sialylation increased the binding activity of VN to heparin, as detected by surface plasmon resonance (SPR). These results demonstrate that sialylation of VN glycans regulates stress fiber formation and cell spreading of dermal fibroblast cells via a heparin binding site.

Glycation of vitronectin inhibits VEGF-induced angiogenesis by uncoupling VEGF receptor-2-αvβ3 integrin cross-talk

Glycation of vessel wall proteins is thought to have an important role in the pathogenesis of vascular complications in diabetes mellitus. However, no previous study has implicated glycated vitronectin (VN) in the control of vascular endothelial growth factor (VEGF) signaling. To explore whether the glycation of VN affects angiogenic signaling and to understand the molecular mechanisms involved, we synthesized glycated VN by incubating VN with methylglyoxal (MGO) in vitro and identified the formation of glycated VN by an LC–ESI–MS/MS-based method. We tested the hypothesis that glycation of VN downregulates VEGF receptor-2 (VEGFR-2) activation by uncoupling the interaction between VEGFR-2 and αvβ3. Unmodified and MGO-glycated VN were used as substrates for human umbilical vein endothelial cells (HUVECs). The effects of glycated VN on VEGF signaling in HUVECs were investigated. The glycation of VN inhibited VEGF-induced phosphorylation of VEGFR-2 and the intracellular signaling pathway downstream of VEGFR-2. Glycated VN inhibited the binding of VEGFR-2 to β3 integrin and inhibited the phosphorylation of β3 integrin. Furthermore, glycation of VN significantly decreased VEGF-induced migration of HUVECs in vitro and vessel outgrowth in an ex vivo angiogenesis model. Collectively, these data indicate that the glycation of VN inhibits VEGF-induced VEGFR-2 activation by uncoupling VEGFR-2–αvβ3 integrin cross-talk. The glycation of VN causes a reduction in the migration of endothelial cells and vessel outgrowth. This may provide a mechanism for the failure of collateral sprouting in diabetic microangiopathy.

Discovery and characterization of potential prognostic biomarkers for dengue hemorrhagic fever

Half a million patients are hospitalized with severe dengue every year, many of whom would die without timely, appropriate clinical intervention. The majority of dengue cases are uncomplicated; however, 2-5% progress to severe dengue. Severe dengue cases have been reported with increasing frequency over the last 30 years. To discover biomarkers for severe dengue, we used surface-enhanced laser desorption/ionization time-of-flight mass spectrometry to analyze dengue virus positive serum samples from the acute phase of infection. Using this method, 16 proteins were identified as candidate biomarkers for severe dengue. From these 16 biomarkers, three candidates were selected for confirmation by enzyme-linked immunosorbent assay and Western blot: vitronectin (Vtn, 55.1 kDa), hemopexin (Hx, 52.4 kDa), and serotransferrin (Tf, 79.2 kDa). Vitronectin, Hx, and Tf best differentiated between dengue and severe dengue.

Vitronectin-binding PAI-1 protects against the development of cardiac fibrosis through interaction with fibroblasts

Plasminogen activator inhibitor-1 (PAI-1) promotes or abates fibrotic processes occurring in different organs. Binding of PAI-1 to vitronectin, an extracellular matrix component, may inhibit vitronectin-integrin complex-mediated cellular responses in pathophysiological conditions. To investigate the importance of plasmin suppression vs vitronectin-binding pathways of PAI-1 in cardiac fibrosis, we studied uninephrectomized mice fed a high salt diet and infused with angiotensin II (Ang II) together with different PAI-1 variants, including PAI-1AK (AK) that inhibits plasminogen activators but does not bind vitronectin, PAI-1RR (RR) that binds vitronectin but does not have protease inhibitory effects or control PAI-1 (CPAI), the control mutant that has similar molecular backbone and half-life as AK and RR while retaining all functions of native PAI-1. Compared with RR and CPAI, non-vitronectin-binding AK significantly increased expression of cardiac fibroblast marker, periostin (Ang+AK 8.40±3.55 vs Ang+RR 2.23±0.44 and Ang+CPAI 2.33±0.12% positive area, both P<0.05) and cardiac fibrosis (Ang+AK 1.79±0.26% vs Ang+RR 0.91±0.18% and Ang+CPAI 0.81±0.12% fibrotic area, both P<0.05), as well as Col1 mRNA (Ang+AK 12.81±1.84 vs Ang+RR 4.04±1.06 and Ang+CPAI 5.23±1.21 fold increase, both P<0.05). To elucidate mechanisms underlying the protective effects of vitronectin-binding PAI-1 against fibrosis, fibroblasts from normal adult human ventricles were stimulated with Ang and different PAI-1 variants. Protease inhibitory AK and CPAI increased supernatant fibronectin, while decreasing plasminogen activator/plasmin activities and matrix metalloproteinase. RR and CPAI variants significantly reduced fibroblast expression of integrin β3, vitronectin level in the supernatant and fibroblast adhesion to vitronectin compared with the non-vitronectin-binding AK. Further, RR and CPAI preserved apoptotic, decreased anti-apoptotic and proliferative activities in fibroblasts. Thus, PAI-1 promotes or protects against development of cardiac fibrosis differentially through the protease inhibitory pathway or through its binding to vitronectin.

Cationic surface charge combined with either vitronectin or laminin dictates the evolution of human embryonic stem cells/microcarrier aggregates and cell growth in agitated cultures

The expansion of human pluripotent stem cells (hPSC) for biomedical applications generally compels a defined, reliable, and scalable platform. Bioreactors offer a three-dimensional culture environment that relies on the implementation of microcarriers (MC), as supports for cell anchorage and their subsequent growth. Polystyrene microspheres/MC coated with adhesion-promoting extracellular matrix (ECM) protein, vitronectin (VN), or laminin (LN) have been shown to support hPSC expansion in a static environment. However, they are insufficient to promote human embryonic stem cells (hESC) seeding and their expansion in an agitated environment. The present study describes an innovative technology, consisting of a cationic charge that underlies the ECM coatings. By combining poly-L-lysine (PLL) with a coating of ECM protein, cell attachment efficiency and cell spreading are improved, thus enabling seeding under agitation in a serum-free medium. This coating combination also critically enables the subsequent formation and evolution of hPSC/MC aggregates, which ensure cell viability and generate high yields. Aggregate dimensions of at least 300 μm during early cell growth give rise to ≈15-fold expansion at 7 days' culture. Increasing aggregate numbers at a quasi-constant size of ≈300 μm indicates hESC growth within a self-regulating microenvironment. PLL+LN enables cell seeding and aggregate evolution under constant agitation, whereas PLL+VN requires an intermediate 2-day static pause to attain comparable aggregate sizes and correspondingly high expansion yields. The cells' highly reproducible bioresponse to these defined and characterized MC surface properties is universal across multiple cell lines, thus confirming the robustness of this scalable expansion process in a defined environment.

Vitronectin regulation of vascular endothelial growth factor-mediated angiogenesis

BACKGROUND

Vascular endothelial growth factor (VEGF) plays a key role in regulating angiogenesis, and this process is largely dependent on the newly formed extracellular matrix (ECM). The levels of vitronectin (VN) are increased in patients with various cardiovascular diseases. A role for VN in regulating VEGF-induced angiogenesis has not been previously reported. We tested the hypothesis that VN regulates VEGFR-2 activation via effects on αvβ3, thus contributing to angiogenesis.

METHODS

We used a 3-dimensional angiogenesis assay, and examined the effects of VN on VEGF-mediated angiogenesis in aortic endothelial cells (ECs) isolated from wild-type and VN-deficient mice.

RESULTS

The addition of multimeric VN significantly enhanced VEGF-induced increases in EC migration and capillary formation. In vitro, Vn(-/-) ECs migrated significantly slower than wild-type ECs. The addition of VN to Vn(-/-) ECs increased EC migration and augmented the promigratory effect of VEGF in a manner that involved VEGFR-2 and Src signaling. Analysis of the mechanisms involved revealed that multimeric VN, but not monomeric VN, binds VEGF and enhances VEGF-induced VEGFR-2/Src activation in ECs.

CONCLUSION

These results underscore the importance of VN in the regulation of angiogenesis induced by VEGF.

Vitronectin--master controller or micromanager?

The concept that the mammalian glycoprotein vitronectin acts as a biological 'glue' and key controller of mammalian tissue repair and remodelling activity is emerging from nearly 50 years of experimental in vitro and in vivo data. Unexpectedly, the vitronectin-knockout (VN-KO) mouse was found to be viable and to have largely normal phenotype. However, diligent observation revealed that the VN-KO animal exhibits delayed coagulation and poor wound healing. This is interpreted to indicate that VN occupies a role in the earliest events of thrombogenesis and tissue repair. VN is the foundation upon which the thrombus grows in an organised structure. In addition to sealing the wound, the thrombus also serves to protect the underlying tissue from oxidation, is a reservoir of mitogens and tissue repair mediators, and provides a provisional scaffold for the repairing tissue. In the absence of VN (e.g., VN-KO animal), this cascade is disrupted before it begins. A wide variety of biologically active species associate with VN. Although initial studies were focused on mitogens, other classes of bioactives (e.g., glycosaminoglycans and metalloproteinases) are now also known to specifically interact with VN. Although some interactions are transient, others are long-lived and often result in multi-protein complexes. Multi-protein complexes provide several advantages: prolonging molecular interactions, sustaining local concentrations, facilitating co-stimulation of cell surface receptors and thereby enhancing cellular/biological responses. We contend that these, or equivalent, multi-protein complexes facilitate VN polyfunctionality in vivo. It is also likely that many of the species demonstrated to associate with VN in vitro, also associate with VN in vivo in similar multi-protein complexes. Thus, the predominant biological function of VN is that of a master controller of the extracellular environment; informing, and possibly instructing cells 'where' to behave, 'when' to behave and 'how' to behave (i.e., appropriately for the current circumstance).

A novel group of Moraxella catarrhalis UspA proteins mediates cellular adhesion via CEACAMs and vitronectin

Moraxella catarrhalis (Mx) is a common cause of otitis media and exacerbation of chronic obstructive pulmonary disease, an increasing worldwide problem. Surface proteins UspA1 and UspA2 of Mx bind to a number of human receptors and may function in pathogenesis. Genetic recombination events in the pathogen can generate hybrid proteins termed UspA2H. However, whether certain key functions (e.g. UspA1-specific CEACAM binding) can be exchanged between these adhesin families remains unknown. In this study, we have shown that Mx can incorporate the UspA1 CEACAM1-binding region not only into rare UspA1 proteins devoid of CEACAM-binding ability, but also into UspA2 which normally lack this capacity. Further, a screen of Mx isolates revealed the presence of novel UspA2 Variant proteins (UspA2V) in ∼14% of the CEACAM-binding population. We demonstrate that the expression of UspA2/2V with the CEACAM-binding domain enable Mx to bind both to cell surface CEACAMs and to integrins, the latter via vitronectin. Such properties of UspA2/2V have not been reported to date. The studies demonstrate that the UspA family is much more heterogeneous than previously believed and illustrate the in vivo potential for exchange of functional regions between UspA proteins which could convey novel adhesive functions whilst enhancing immune evasion.

Meningococcal surface fibril (Msf) binds to activated vitronectin and inhibits the terminal complement pathway to increase serum resistance

Complement evasion is an important survival strategy of Neisseria meningitidis (Nm) during colonization and infection. Previously, we have shown that Nm Opc binds to serum vitronectin to inhibit complement-mediated killing. In this study, we demonstrate meningococcal interactions with vitronectin via a novel adhesin, Msf (meningococcal surface fibril, previously NhhA or Hsf). As with Opc, Msf binds preferentially to activated vitronectin (aVn), engaging at its N-terminal region but the C-terminal heparin binding domain may also participate. However, unlike Opc, the latter binding is not heparin-mediated. By binding to aVn, Msf or Opc can impart serum resistance, which is further increased in coexpressers, a phenomenon dependent on serum aVn concentrations. The survival fitness of aVn-binding derivatives was evident from mixed population studies, in which msf/opc mutants were preferentially depleted. In addition, using vitronectin peptides to block Msf-aVn interactions, aVn-induced inhibition of lytic C5b-9 formation and of serum killing could be reversed. As Msf-encoding gene is ubiquitous in the meningococcal strains examined and is expressed in vivo, serum resistance via Msf may be of significance to meningococcal pathogenesis. The data imply that vitronectin binding may be an important strategy for the in vivo survival of Nm for which the bacterium has evolved redundant mechanisms.

Defining a threshold surface density of vitronectin for the stable expansion of human embryonic stem cells

Current methodology for pluripotent human embryonic stem cells (hESCs) expansion relies on murine sarcoma basement membrane substrates (Matrigel™), which precludes the use of these cells in regenerative medicine. To realize the clinical efficacy of hESCs and their derivatives, expansion of these cells in a defined system that is free of animal components is required. This study reports the successful propagation of hESCs (HES-3 and H1) for > 20 passages on tissue culture-treated polystyrene plates, coated from 5 μg/mL of human plasma-purified vitronectin (VN) solution. Cells maintain expression of pluripotent markers Tra1-60 and OCT-4 and are karyotypically normal after 20 passages of continuous culture. In vitro and in vivo differentiation of hESC by embryoid body formation and teratoma yielded cells from the ecto-, endo-, and mesoderm lineages. VN immobilized on tissue culture polystyrene was characterized using a combination of X-ray photoemission spectroscopy, atomic force microscopy, and quantification of the VN surface density with a Bradford protein assay. Ponceau S staining was used to measure VN adsorption and desorption kinetics. Tuning the VN surface density, via the concentration of depositing solution, revealed a threshold surface density of 250 ng/cm², which is required for hESCs attachment, proliferation, and differentiation. Cell attachment and proliferation assays on VN surface densities above this threshold show the substrate properties to be equally viable.

Neisseria meningitidis Opc invasin binds to the sulphated tyrosines of activated vitronectin to attach to and invade human brain endothelial cells

The host vasculature is believed to constitute the principal route of dissemination of Neisseria meningitidis (Nm) throughout the body, resulting in septicaemia and meningitis in susceptible humans. In vitro, the Nm outer membrane protein Opc can enhance cellular entry and exit, utilising serum factors to anchor to endothelial integrins; but the mechanisms of binding to serum factors are poorly characterised. This study demonstrates that Nm Opc expressed in acapsulate as well as capsulate bacteria can increase human brain endothelial cell line (HBMEC) adhesion and entry by first binding to serum vitronectin and, to a lesser extent, fibronectin. This study also demonstrates that Opc binds preferentially to the activated form of human vitronectin, but not to native vitronectin unless the latter is treated to relax its closed conformation. The direct binding of vitronectin occurs at its Connecting Region (CR) requiring sulphated tyrosines Y(56) and Y(59). Accordingly, Opc/vitronectin interaction could be inhibited with a conformation-dependent monoclonal antibody 8E6 that targets the sulphotyrosines, and with synthetic sulphated (but not phosphorylated or unmodified) peptides spanning the vitronectin residues 43-68. Most importantly, the 26-mer sulphated peptide bearing the cell-binding domain (45)RGD(47) was sufficient for efficient meningococcal invasion of HBMECs. To our knowledge, this is the first study describing the binding of a bacterial adhesin to sulphated tyrosines of the host receptor. Our data also show that a single region of Opc is likely to interact with the sulphated regions of both vitronectin and of heparin. As such, in the absence of heparin, Opc-expressing Nm interact directly at the CR but when precoated with heparin, they bind via heparin to the heparin-binding domain of the activated vitronectin, although with a lower affinity than at the CR. Such redundancy suggests the importance of Opc/vitronectin interaction in meningococcal pathogenesis and may enable the bacterium to harness the benefits of the physiological processes in which the host effector molecule participates.

Formation of soluble amyloid oligomers and amyloid fibrils by the multifunctional protein vitronectin

Background

The multifunctional protein vitronectin is present within the deposits associated with Alzheimer disease (AD), age-related macular degeneration (AMD), atherosclerosis, systemic amyloidoses, and glomerulonephritis. The extent to which vitronectin contributes to amyloid formation within these plaques, which contain misfolded, amyloidogenic proteins, and the role of vitronectin in the pathophysiology of the aforementioned diseases is currently unknown. The investigation of vitronectin aggregation is significant since the formation of oligomeric and fibrillar structures are common features of amyloid proteins.

Results

We observed vitronectin immunoreactivity in senile plaques of AD brain, which exhibited overlap with the amyloid fibril-specific OC antibody, suggesting that vitronectin is deposited at sites of amyloid formation. Of particular interest is the growing body of evidence indicating that soluble nonfibrillar oligomers may be responsible for the development and progression of amyloid diseases. In this study we demonstrate that both plasma-purified and recombinant human vitronectin readily form spherical oligomers and typical amyloid fibrils. Vitronectin oligomers are toxic to cultured neuroblastoma and retinal pigment epithelium (RPE) cells, possibly via a membrane-dependent mechanism, as they cause leakage of synthetic vesicles. Oligomer toxicity was attenuated in RPE cells by the anti-oligomer A11 antibody. Vitronectin fibrils contain a C-terminal protease-resistant fragment, which may approximate the core region of residues essential to amyloid formation.

Conclusion

These data reveal the propensity of vitronectin to behave as an amyloid protein and put forth the possibilities that accumulation of misfolded vitronectin may contribute to aggregate formation seen in age-related amyloid diseases.

Changes in glycosylation of vitronectin modulate multimerization and collagen binding during liver regeneration

Elucidating the mechanisms and factors regulating multimerization is biologically important in order to modulate the biological activities of functional proteins, especially adhesive proteins in the extracellular matrix (ECM). Vitronectin (VN) is a multifunctional glycoprotein present in plasma and ECM. Linkage of cellular adhesion and fibrinolysis by VN plays an essential role during tissue remodeling. Our previous study determined that the collagen-binding activity of VN was markedly enhanced with the decreased glycosylation during liver regeneration. This study demonstrated how alternations of glycans modulate the biological activity of VN. Human and rat VNs were used because of their similarities in structure and activities. The binding affinity of human VN to immobilized collagen was shown to be higher at pH 4.5 than at 7.5, at 37 degrees C than at 4 degrees C. Sedimentation velocity studies indicated that the greater the multimerization of human VN, the better it bound to collagen. The results indicate that the collagen binding of VN was modulated through its multimerization. Stepwise trimming of glycan with various exoglycosidases increased both the multimer size and the collagen binding of human VN, indicating that they are modulated by changes in glycosylation. The multimer sizes of VN purified from plasma of partially hepatectomized (PH) rats and sham-operated (SH) rats increased by about 45 and 31%, respectively, compared with those of nonoperated (NO) rats. In accordance with this, PH-VN exhibited remarkably enhanced collagen binding than SH-VN and NO-VN on surface plasmon resonance. In the PH rat sera, the multimer VN was increased in both amount and size compared with those in SH- and NO-sera. The results demonstrate that glycan alterations during tissue remodeling induce increased multimerization state to enhance the biological activity of VN.

Vitronectin stabilizes thrombi and vessel occlusion but plays a dual role in platelet aggregation

The role of vitronectin (Vn) in thrombosis is currently controversial; both inhibitory and supportive roles have been reported. To monitor directly the function of Vn in thrombotic events at the site of vascular injury, we studied Vn-deficient (Vn-/-) and wild-type (WT) control mice with two real-time intravital microscopy thrombosis models. In the mesenteric arteriole model, vessel injury was induced by ferric chloride. We observed unstable thrombi and a significantly greater number of emboli in Vn-/- mice. Vessel occlusion was also delayed and frequent vessel re-opening occurred. In the cremaster muscle arteriole model, vessel injury was induced by a nitrogen dye laser. We observed significantly fewer platelets, lower fibrin content, and unstable fibrin within the thrombi of Vn-/- mice. To define further the role of Vn in thrombus growth, we studied platelet aggregation in vitro. Consistent with our in vivo data, the second wave of thrombin-induced aggregation of gel-filtered platelets was abolished at a low concentration of thrombin in Vn-/- platelets. Interestingly, adenosine diphosphate (ADP)-induced platelet aggregation was significantly increased in Vn-/- platelet-rich plasma (PRP) and this effect was attenuated by adding purified plasma Vn. We also observed increased platelet aggregation induced by shear stress in Vn-/- whole blood. These data demonstrate that Vn is a thrombus stabilizer. However, in contrast to released platelet granule Vn which enhances platelet aggregation, plasma Vn inhibits platelet aggregation.

Direct measurement of interactions between adsorbed vitronectin layers: The influence of ionic strength and pH

Vitronectin (Vn) is an adhesive protein in the plasma serum and plays an important role in cell attachment, spreading, and proliferation. The interactions between protein bovine vitronectin layers adsorbed onto a silica probe and a mica surface have been investigated with the use of atomic force microscopy (AFM). Adsorption of vitronectin was confirmed by XPS surface analysis. The force-separation curves and pull-off forces were measured as a function of ionic strength and solution pH. The pull-off force (adhesion force) decreased as the salt concentration increased, which suggests that some binding domains of this protein may associate with the ionic species and reduce its binding ability. Discrete jumps, or discontinuities, in the separation force curve were observed to extend to a maximum of 300 nm, evidence that the protein molecules bridge between the surfaces. As a function of pH, the adhesion force on separation of the protein-coated surfaces showed a maximum at pH 5 (i.e.p. of vitronectin), decreasing in magnitude at lower and higher pH values. At pH 5, the approaching curves illustrated a jump-in force; whereas for pH values away from 5, the approaching force curves were repulsive. Correlation of the interaction forces with Vn conformational changes in different pH environments, directly visualized with the use of AFM imaging, was developed. In its i.e.p. region, the Vn molecular conformation appeared to be dense and compact. Significantly, at wounds/injured sites the pH is low (approximately 5) which this study discovered to facilitate adsorption and formation of vitronectin aggregates, known to trigger their subsequent biological functions.

Vitronectin adsorption on surfaces visualized by tapping mode atomic force microscopy

Vitronectin (Vn) is an abundant glycoprotein present in plasma and the extracellular matrix of most tissue and is an important medium required for bone cells to attach and spread on biomaterial surfaces. Hence, Vn adsorption is an initial and key step in implantation. In this study, adsorption of Vn molecules on mica and oxidized titanium substrates in Milli-Q water (pH 5.6) and in simulated body fluid (SBF, pH 7.4) has been investigated using tapping mode atomic force microscopy. Conformation of the adsorbed proteins was determined from the images. The effect of adsorption time was investigated on mica surfaces. The protein first attaches to the surface, then immediately spreads out with different lateral dimensions of adsorbed protein and becomes aggregated. After 15 min of adsorption, globular Vn molecules form clusters of aggregates in extended chains on the mica surface. Globular Vn molecules appear flatter (i.e., oblate ellipsoids) and coalesce on the ridges of the uneven oxidized titanium surface. From cross-sectional analyses of the Vn images, it was found that the contact areas of Vn molecules associated with Ti surfaces are larger than those with mica surfaces. This suggests that the different surface properties of substrates contribute to this different conformation of adsorbed Vn molecules. The larger contact areas of Vn associated with Ti substrates indicates higher affinity for the biomaterial oxidized titanium surface than for mica. In SBF, Vn interacted weakly with the mica surfaces and Vn molecules were easily desorbed during the imaging process. On oxidized titanium substrates, after 3 h of incubation time in Vn-SBF solution, images can be obtained showing adsorbed Vn aggregates on the oxidized titanium surface. In this case, the protein colloids are strongly adherent, with increased lateral dimensions compared with the adsorption on mica.

Diagnostic protein discovery using proteolytic peptide targeting and identification

Plasma protein profiling with mass spectrometry is currently being evaluated as a diagnostic tool for cancer and other diseases. These experiments consist of three steps: plasma protein fractionation, analysis with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), and comparisons of the MALDI profiles to develop diagnostic fingerprints using bioinformatic techniques. While preliminary results appear promising in small sample groups, the method is limited by the sensitivity of MALDI-MS for intact proteins, the limited mass range of MALDI-MS, and difficulties associated with isolating individual proteins for identification to validate the diagnostic fingerprint. Here we present an alternative and improved method directed toward diagnostic protein discovery, which incorporates proteolytic peptide profiling, bioinformatic targeting of ion signals, and MALDI tandem mass spectrometry (MS/MS) peptide sequencing, rather than fingerprinting. Pancreatic cancer patients, pancreatitis patients, and controls are used as the model system. Profiling peptides after enzymatic digestion improves sensitivity and extends the accessible protein molecular weight range when compared to intact protein profiling. The first step is to extract and fractionate the proteins from plasma. Each fraction is digested with trypsin and subsequently analyzed by MALDI-MS. Rather than using bioinformatic analysis as a pattern-matching technique, peptides are targeted based on the disease to control peak intensity ratios measured in the averages of all mass spectra in each group and t-tests of the intensity of each individual peak. The targeted peptide ion signals are subsequently identified using MALDI-MS/MS in quadrupole-TOF and tandem-TOF instruments. This study found not only the proteins targeted and identified by a previous protein profiling experiment, but also detected additional proteins. These initial results are consistent with the known biology of pancreatic cancer or pancreatitis, but are not specific to those diseases.

New insights into heparin binding to vitronectin: studies with monoclonal antibodies

Vitronectin is a plasma glycoprotein that binds to a variety of ligands. There is considerable debate regarding the dependency of these binding interactions upon the conformational status of vitronectin, the role of multimerization and how the binding of different ligands can change vitronectin's conformational state. We have developed a method of capturing vitronectin directly from fresh plasma using solid-phase monoclonal antibodies. Various biotin-labelled secondary monoclonal antibodies were used to quantify the bound vitronectin and to measure its degree of denaturation. Using these tools we demonstrated that one monoclonal antibody partially denatured vitronectin without direct multimerization. Treatment of vitronectin in plasma with soluble heparin produced a similar degree of denaturation. These results led to a proposed adaptation of the unfolding/refolding pathways for chemically denatured vitronectin originally presented by Zhuang and co-workers in 1996 [Zhuang, Blackburn and Peterson (1996) J. Biol. Chem. 271, 14323-14332 and Zhuang, Li, Williams, Wagner, Seiffert and Peterson (1996) J. Biol. Chem. 271, 14333-14343]. The adapted version allows for the production of a more stable partially unfolded intermediate, resulting from the binding of particular ligands. We also demonstrated that the avidity of heparin binding to vitronectin is governed by both the conformational state of the monomer and multimerization of the molecule.

Inhibition of platelet adhesion and aggregation by a defined region (Gly-486-Lys-502) of high molecular weight kininogen

Proteolytic cleavage of single chain high molecular weight kininogen (HK) by kallikrein releases the short-lived vasodilator bradykinin and leaves behind two-chain high molecular weight kininogen (HKa). HKa and particularly its His-Gly-Lys-rich domain 5 have been previously reported to exert anti-adhesive properties by binding to the extracellular matrix protein vitronectin (VN). In this study the ability of HKa and domain 5 to interfere with platelet adhesion and aggregation was investigated. In a purified system HKa and particularly domain 5 but not HK inhibited the binding of VN to the alpha(IIb)beta(3) integrin, whereas the binding of fibrinogen to this integrin was not affected. The region Gly-486-Lys-502 from the carboxyl terminus of the domain 5 was identified as responsible for inhibition of the VN-alpha(IIb)beta(3)-integrin interaction, as this portion was also found to mediate kininogen binding to VN. Through these interactions, HKa, the isolated domain 5, and the peptide Gly-486-Lys-502 abrogated the alpha(IIb)beta(3)-integrin-dependent adhesion of human platelets to VN but not to fibrinogen. The codistribution of VN and HKa at sites of ex vivo platelet aggregation was demonstrated by transmission immune electron microscopy, indicating that the described interaction is likely to take place in vivo. Moreover, domain 5 and the peptide Gly-486-Lys-502 dose-dependently blocked platelet aggregation, resembling the inhibitory effect of monoclonal antibody 13H1 against multimeric VN. Finally, treatment of mice with isolated domain 5 resulted in a significantly prolonged tail bleeding time. Taken together, our data emphasize the inhibitory role of HK domain 5 on platelet adhesion and aggregation; new anti-thrombotic compounds may become available on the basis of peptide Gly-486-Lys-502 of HK domain 5.

Incorporation of vitronectin into fibrin clots. Evidence for a binding interaction between vitronectin and gamma A/gamma' fibrinogen

Vitronectin is an abundant plasma protein that regulates coagulation, fibrinolysis, complement activation, and cell adhesion. Recently, we demonstrated that plasma vitronectin inhibits fibrinolysis by mediating the interaction of type 1 plasminogen activator inhibitor with fibrin (Podor, T. J., Peterson, C. B., Lawrence, D. A., Stefansson, S., Shaughnessy, S. G., Foulon, D. M., Butcher, M., and Weitz, J. I. (2000) J. Biol. Chem. 275, 19788-19794). The current studies were undertaken to further examine the interactions between vitronectin and fibrin(ogen). Comparison of vitronectin levels in plasma with those in serum indicates that approximately 20% of plasma vitronectin is incorporated into the clot. When the time course of biotinylated-vitronectin incorporation into clots formed from (125)I-fibrinogen is monitored, vitronectin incorporation into the clot parallels that of fibrinogen in the absence or presence of activated factor XIII. Vitronectin binds specifically to fibrin matrices with an estimated K(d) of approximately 0.6 microm. Additional vitronectin subunits are assembled on fibrin-bound vitronectin multimers through self-association. Confocal microscopy of fibrin clots reveals the globular vitronectin aggregates anchored at intervals along the fibrin fibrils. This periodicity raised the possibility that vitronectin interacts with the gamma A/gamma' variant of fibrin(ogen) that represents about 10% of total fibrinogen. In support of this concept, the vitronectin which contaminates fibrinogen preparations co-purifies with the gamma A/gamma' fibrinogen fraction, and clots formed from gamma A/gamma' fibrinogen preferentially bind vitronectin. These studies reveal that vitronectin associates with fibrin during coagulation, and may thereby modulate hemostasis and inflammation.

Vitronectin in clotting factor IX concentrates

Highly purified, plasma-derived factor IX (FIX) concentrates are produced in large part by a combination of anion exchange and heparin affinity chromatography. However, the concentrates still contain some accompanying proteins. The main impurity has turned out to be the adhesive glycoprotein, vitronectin. It occurs in concentrates exclusively in its multimeric form, in contrast to the situation in plasma. The multimeric vitronectin can be removed either by nanofiltration with a crossflow system or by size-exclusion chromatography. When these FIX concentrates are used as therapeutic agents, the fact has to be taken into account that considerable amounts of multimeric vitronectin are given to the patient. The physiological consequences of the dosage of this protein have not yet been investigated. Although no thrombogenicity has been reported in connection with the above-mentioned FIX concentrates, we recommend that the impurity should be removed from the preparation with the methods described here.

Full-length and truncated forms of vitronectin provide insight into effects of proteolytic processing on function

A genetic polymorphism in the vitronectin allele directs the production of two distinct forms of the 459 amino acid glycoprotein. A methionine present at position 381 favors production of the single-chain form of vitronectin, while threonine at this position increases the susceptibility of vitronectin to cleavage just beyond its heparin-binding domain at residue 379. This reaction gives rise to a disulfide-bonded, two-chain form of vitronectin. In order to investigate the functional significance of the vitronectin polymorphism, the baculovirus system has been used to express recombinant full-length vitronectin and a truncated form of the molecule that represents the 62-kDa fragment of two-chain vitronectin. Both forms of vitronectin bind and neutralize heparin anticoagulant activity. The proteins also bind PAI-1 and stabilize its active conformation. These experiments suggest that the C-terminal 80 amino acids do not confer a functional difference in the two allelic variants. Immunoassays and gel filtration experiments indicate that both full-length and truncated recombinant forms of vitronectin are multimeric. Together with other reports from this laboratory, these results provide information regarding the primary binding sites for two vitronectin ligands and further define regions that may be involved in multimerization of the protein.

Vitronectin Inhibits the Thrombotic Response to Arterial Injury in Mice

Vitronectin (VN) binds to plasminogen activator inhibitor-1 (PAI-1) and integrins and may play an important role in the vascular response to injury by regulating fibrinolysis and cell migration. However, the role of VN in the earliest response to vascular injury, thrombosis, is not well characterized. The purpose of this study was to test the hypothesis that variation in vitronectin expression alters the thrombotic response to arterial injury in mice. Ferric chloride (FeCl3) injury was used to induce platelet-rich thrombi in mouse carotid arteries. Wild-type (VN +/+, n = 14) and VN-deficient (VN −/−, n = 15) mice, matched for age and gender, were studied. Time to occlusion after FeCl3 injury was determined by application of a Doppler flowprobe to the carotid artery. Occlusion times of VN −/− mice were significantly shorter than those of VN +/+ mice (6.0 ± 1.2 minutesv 17.8 ± 2.3 minutes, respectively, P &lt; .001). Histologic analysis of injured arterial segments showed that thrombi from VN +/+ and VN −/− mice consisted of dense platelet aggregates. In vitro studies of murine VN +/+ andVN −/− platelets showed no significant differences in ADP-induced aggregation, but a trend towards increased thrombin-induced aggregation in VN −/− platelets. Purified, denatured VN inhibited thrombin-induced platelet aggregation, whereas native VN did not. Thrombin times of plasma from VN −/− mice (20.5 ± 2.1 seconds, n = 4) were significantly shorter than those ofVN +/+ mice (34.2 ± 6.7 seconds, n = 4, P &lt; .01), and the addition of purified VN to VN −/− plasma prolonged the thrombin time into the normal range, suggesting that VN inhibits thrombin-fibrinogen interactions. PAI-1-deficient mice (n = 6) did not demonstrate significantly enhanced arterial thrombosis compared with wild-type mice (n = 6), excluding a potential indirect antithrombin function of VN mediated by interactions with PAI-1 as an explanation for the accelerated thrombosis observed in VN−/− mice. These results suggest that vitronectin plays a previously unappreciated antithrombotic role at sites of arterial injury and that this activity may be mediated, at least in part, by inhibiting platelet-platelet interactions and/or thrombin procoagulant activity.

The role of plasminogen activator inhibitor-1 as inhibitor of platelet and megakaryoblastic cell adhesion

In the present study the ability of plasminogen activator inhibitor type-1 (PAI-1) to interfere with platelet and megakaryoblastic cell adhesion was investigated. Both cell types exhibited integrin-dependent adhesion in a static system, mediated by alphaIIb beta3 on platelets and alpha v-integrins on different megakaryoblastic cell lines, even though they also expressed alphaIIb beta3. In a concentration-dependent manner, active, but not latent or complexed, PAI-1 abrogated cell adhesion onto vitronectin but not onto fibrinogen or other matrix substrata. Urokinase as well as thrombin neutralized the anti-adhesive effect of active PAI-1. The direct binding of vitronectin, but not of other matrix proteins, to integrin alphaIIb beta3 was blocked by active PAI-1 in a purified system. Since activated platelets release active and latent PAI-1 as well as structurally and functionally distinct forms of vitronectin, the described interactions appear to be physiologically significant. Co-distribution of vitronectin and PAI-1 at sites of fibrin polymers within platelet thrombi was demonstrated by transmission electron microscopy, suggesting an extracellular functional relationship of both release products with regard to cell adhesion. Our data emphasize the regulatory role of active PAI-1 in platelet adhesion to provisional matrix proteins as found during wound healing independent of its anti-proteolytic activity. Furthermore, megakaryocyte maturation may depend on the intact vitronectin-integrin adhesion system that is influenced by PAI-1, thereby proposing a regulatory role for the inhibitor in cellular differentiation.

Degradation of distinct forms of multimeric vitronectin by human fibroblasts

The plasma protein vitronectin is thought to be an important regulator of extravascular plasminogen activation. In previous studies we have shown that a disulfide stabilized multimeric form of vitronectin is endocytosed and degraded by fibroblast cells (T.S. Panetti, P.J. McKeown-Longo, J. Biol. Chem. 268 (1993) 11988-11993; P.J. McKeown-Longo, T.S. Panetti, in: K.T. Preissner, S. Rosenblatt, C. Kost, J. Wegerhoff, D.F. Mosher (Eds.), Biology of Vitronectins and their Receptors, Elsevier Science Publishers, Amsterdam, 1993, pp. 111-118). The preparation of multimeric vitronectin used in these earlier studies was in the form of high molecular weight disulfide-bonded aggregates which were stable in sodium dodecyl sulfate (SDS). To address the question of whether vitronectin needed to be in the form of disulfide stabilized multimers in order to be endocytosed, a multimeric vitronectin, which was not disulfide stabilized, was prepared from vitronectin that had been treated with reducing agent and alkylated with iodoacetamide. The resulting protein migrated as a 65/75 kDa protein on SDS gels in the absence of reducing agent, confirming that this form of vitronectin was no longer stabilized into disulfide-bonded aggregates. However, the protein was still multimeric when analyzed by native gels and could be converted to SDS stable multimers by cross-linking agents. This result demonstrated that reduced and alkylated vitronectin aggregates into multimeric forms which are not stable in SDS. Similar to disulfide stabilized multimers, alkylated multimers of vitronectin bound to sulfated proteoglycans in the extracellular matrix and were endocytosed and degraded. Degradation of both forms of vitronectin was inhibited with arginine-glycine-aspartic acid peptides, an anti-alphavbeta5 antibody and heparin. Chloroquine and wortmannin were also able to inhibit degradation of both forms of vitronectin, indicating that both multimeric forms were following the same endocytic and degradative pathway. These results suggest that the organization of vitronectin into a multimeric form which will be recognized for endocytosis does not require disulfide bond stabilization. This study further suggests that recognition of vitronectin for endocytosis is dependent upon its conversion from a monomeric to a multivalent form (C.E. Wilkins-Port, P.J. McKeown-Longo, Mol. Biol. Cell 8:S:64A (1997).

The cell adhesion domain in plasma vitronectin is cryptic

Vitronectin (Vn) is a major adhesive glycoprotein in blood. However, many of the functions of Vn are regulated by its conformational state and degree of multimerization. Here, the ability of native and denatured Vn to bind to integrin adhesion receptors was compared. Three lines of evidence suggest that the native, plasma form of Vn is not an adhesive glycoprotein. (i) Antibodies that bind in close proximity to the cell adhesion domain of Vn fail to bind to native Vn present in unfractionated plasma. (ii) Denatured Vn binds to both glycoprotein IIb/IIIa and alphavbeta3 in a dose-dependent manner. In contrast, native Vn is unable to bind either integrin. (iii) Thermal denaturation of native Vn, or its complexation with type 1 plasminogen activator inhibitor, exposed the cell adhesion domain of Vn. Thus, while plasma Vn is unable to bind integrins and is not an adhesive glycoprotein, the conformationally altered from of the protein binds avidly to both alphavbeta3 and glycoprotein IIb/IIIa. The data presented here indicate that such conformational changes in Vn are likely to occur in areas of tissue injury and thrombosis.

The glycosaminoglycan binding site governs ligand binding to the somatomedin B domain of vitronectin

The ligand binding functions of vitronectin (Vn) are regulated by its conformational state/degree of multimerization. In the native plasma form of Vn, the C-terminal glycosaminoglycan (GAG) binding domain is believed to be cryptic. Here, evidence is provided that the addition of fucoidan or dextran sulfate to unfractionated plasma results in the formation of covalently and non-covalently stabilized Vn multimers. These multimers express conformationally sensitive antibody epitopes and ligand binding sites located in the N terminus of the Vn molecule. While heparin forms complexes with monomeric plasma Vn and induces conformational changes, a reduction in ionic strength is required for induction of multimerization. In addition, heparin serves as a template for the assembly of type 1 plasminogen activator inhibitor-induced disulfide-linked Vn multimers. These results support a new model for the structure of native Vn. The C-terminal GAG binding domain is predicted to be exposed in the native conformation, whereas the N terminus is cryptic. Ligand binding to the GAG binding site unfolds the N terminus, thereby exposing cryptic ligand binding sites.

Characterization of the binding of different conformational forms of plasminogen activator inhibitor-1 to vitronectin. Implications for the regulation of pericellular proteolysis

Plasminogen activator inhibitor type 1 (PAI-1), the primary physiologic inhibitor of plasminogen activation, is associated with the adhesive glycoprotein vitronectin (Vn) in plasma and the extracellular matrix. In this study we examined the binding of different conformational forms of PAI-1 to both native and urea-purified vitronectin using a solid-phase binding assay. These results demonstrate that active PAI-1 binds to urea-purified Vn with approximately 6-fold higher affinity than to native Vn. In contrast, inactive forms of PAI-1 (latent, elastase-cleaved, synthetic reactive center loop peptide-annealed, or complexed to plasminogen activators) display greatly reduced affinities for both forms of adsorbed Vn, with relative affinities reduced by more than 2 orders of magnitude. Structurally, these inactive conformations all differ from active PAI-1 by insertion of an additional strand into beta-sheet A, suggesting that it is the rearrangement of sheet A that results in reduced Vn affinity. This is supported by the observation that PAI-1 associated with beta-anhydrotrypsin, which does not undergo rearrangement of beta-sheet A, shows no such decrease in affinity, whereas PAI-1 complexed to beta-trypsin, which does undergo sheet A rearrangement, displays reduced affinity for Vn similar to PAI-1.plasminogen activator complexes. Together these data demonstrate that the interaction between PAI-1 and Vn depends on the conformational state of both proteins and suggest that the Vn binding site on PAI-1 is sensitive to structural changes associated with loss of inhibitory activity.

Type 1 plasminogen activator inhibitor induces multimerization of plasma vitronectin. A suggested mechanism for the generation of the tissue form of vitronectin in vivo

The conformation and degree of multimerization of vitronectin (Vn) appears to be of critical importance for its functions, but little is known about the underlying mechanisms that control Vn multimerization. We report that Vn secreted by cultured hepatoma cells is present as a mixture of monomeric and multimeric forms. A single protein of Mr 45,000 co-purified with hepatoma cell-derived Vn, which was immunologically identified as type 1 plasminogen activator inhibitor (PAI-1). The possibility that PAI-1 may modulate Vn multimerization was investigated. The addition of active PAI-1 to unfractionated plasma containing Vn monomers resulted in the formation of covalently and noncovalently associated Vn multimers and expression of conformationally sensitive epitopes. In contrast, inactive forms of PAI-1 did not efficiently induce Vn multimerization and conformational change. Gel filtration analysis revealed that Vn remained multimeric after dissociation from PAI-1. Vn multimers were also assembled using purified monomeric Vn and PAI-1, suggesting that a plasma cofactor was not required to induce Vn multimerization. This study provides insights into physiological mechanism responsible for the generation of homomultimeric Vn, a multimeric form of Vn that is not in complex with other proteins and which expresses a functional repertoire distinct from that of plasma Vn.

Evidence that conformational changes upon the transition of the native to the modified form of vitronectin are not limited to the heparin binding domain

Vitronectin (Vn) exists in vivo in at least two different conformational states, the native and the modified form, and these forms have different ligand binding properties. To characterize the molecular events associated with this conformational flexibility, modified Vn was analyzed by competitive ELISA using a panel of conformationally sensitive antibodies with known epitopes. These studies provided evidence for major molecular rearrangements upon the transition from the native to the modified form that are not limited to the C-terminal heparin binding domain, but also occur in the N-terminal part of the molecule.

Heparin-binding vitronectin up-regulates latent TGF-beta production by bovine aortic endothelial cells

Vitronectin, a serum and extracellular matrix protein, is present in vivo in two different conformations: a native form, which does not bind heparin, and a heparin-binding conformer, which results from interactions of native vitronectin with either the thrombin-antithrombin III complex or the terminal complement complex, C5b-9. We found that vitronectin stimulates the activity of the growth regulatory peptide, TGF-beta, in the conditioned media of bovine aortic endothelial cells as a result of increased production of latent TGF-beta. This effect is specific for the denatured, heparin-binding, form of vitronectin, since native vitronectin has no effect on the production of latent TGF-beta by those cells. Stimulation is time and concentration-dependent, but is independent of protease activity. Stimulation is dependent on the presence of cells, since there was no increase in TGF-beta activity observed when vitronectin was added to the conditioned media after removal from cells. Furthermore, incubation of recombinant latent TGF-beta with vitronectin in a cell-free system does not result in increased TGF-beta activity. Assays of total TGF-beta levels in heat-treated conditioned media showed that vitronectin treatment elevates the levels of total TGF-beta in the conditioned media. These results were further confirmed by western blot analysis of the conditioned media with antibodies specific for latent TGF-beta. These data suggest that vitronectin regulates expression and/or secretion of TGF-beta by bovine aortic endothelial cells. This cellular response to the heparin-binding form of vitronectin seems to be mediated by alpha v beta 3 integrins.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of vitronectin in plasma and liver tissue: relationship to chronic liver disease

To clarify the clinical significance of vitronectin, we compared the concentration of plasma vitronectin with serum fibrous markers and liver function test values in patients with chronic liver diseases. We also evaluated the vitronectin content in the liver by means of enzyme-linked immunosorbent assay and the localization of vitronectin in liver tissue with enzyme immunohistochemistry. In chronic liver disease, the concentration of plasma vitronectin was significantly lower than that in healthy controls, being related to the severity of liver disease. The plasma levels of vitronectin showed no correlation to fibrous markers but a significant correlation with those of serum albumin and prothrombin time. On the other hand, the content of vitronectin in liver tissue was significantly increased in chronic liver disease compared with that in normal controls. In the normal liver, vitronectin was observed in the portal area by light microscopy. In chronic hepatitis and cirrhosis, vitronectin was found in the connective tissue around the portal and central veins and in the areas of piecemeal and focal necrosis. These findings suggested that vitronectin is deposited in injured tissue through the process of repair and fibrosis and plays an important role as an adhesive protein. Moreover, the lower levels of plasma vitronectin in chronic liver disease may be due to its decreased synthesis, deposition or both in injured tissue.