Solution structure of human plasma fibronectin as a function of NaCl concentration determined by small-angle X-ray scattering

Probing fibronectin adsorption on chemically defined surfaces by means of single molecule force microscopy

Atomic force microscope (AFM) based single molecule force spectroscopy (SMFS) and a quartz crystal microbalance (QCM) were respectively employed to probe interfacial characteristics of fibronectin fragment FNIII^8–14 and full-length fibronectin (FN) on CH₃–, OH–, COOH–, and NH₂-terminated alkane-thiol self-assembled monolayers (SAMs). Force-distance curves acquired between hexahistidine-tagged FNIII^8–14 immobilised on trisNTA-Ni²⁺ functionalized AFM cantilevers and the OH and COOH SAM surfaces were predominantly ‘loop-like’ (76% and 94% respectively), suggesting domain unfolding and preference for ‘end-on’ oriented binding, while those generated with NH₂ and CH₃ SAMs were largely ‘mixed type’ (81% and 86%, respectively) commensurate with unravelling and desorption, and ‘side-on’ binding. Time-dependent binding of FN to SAM-coated QCM crystals occurred in at least two phases: initial rapid coverage over the first 5 min; and variably diminishing adsorption thereafter (5–70 min). Loading profiles and the final hydrated surface concentrations reached (~ 950, ~ 1200, ~ 1400, ~ 1500 ng cm⁻² for CH₃, OH, COOH and NH₂ SAMs) were consistent with: space-filling ‘side-on’ orientation and unfolding on CH₃ SAM; greater numbers of FN molecules arranged ‘end-on’ on OH and especially COOH SAMs; and initial ‘side-on’ contact, followed by either (1) gradual tilting to a space-saving ‘end-on’ configuration, or (2) bi-/multi-layer adsorption on NH₂ SAM.

Measuring the Poisson's Ratio of Fibronectin Using Engineered Nanofibers

The extracellular matrix (ECM) is a fibrillar protein-based network, the physical and chemical properties of which can influence a multitude of cellular processes. Despite having an important role in cell and tissue signaling, a complete chemo-mechanical characterization of ECM proteins such as fibronectin (FN) is lacking. In this study, we engineered monodisperse FN nanofibers using a surface-initiated assembly technique in order to provide new insight into the elastic behavior of this material over large deformations. FN nanofibers were patterned on surfaces in a pre-stressed state and when released from the surface underwent rapid contraction. We found that the FN nanofibers underwent 3.3-fold and 9-fold changes in length and width, respectively, and that the nanofiber volume was conserved. Volume was also conserved following uniaxial extension of the FN nanofibers of ~2-fold relative to the patterned state. This data suggests that the FN networks we engineered formed an incompressible material with a Poisson’s ratio of ~0.5. While the Poisson’s ratio of cells and other biological materials are widely estimated as 0.5, our experimental results demonstrate that for FN networks this is a reasonable approximation.

Properties and reactivity of polyoxazoline plasma polymer films

Polyoxazolines arise as a promising new class of polymers for biomedical applications, but creating oxzoline-based coatings via conventional methods is challenging. Herein, nanoscale polyoxazoline coatings were generated via a single step plasma deposition process. The effects of plasma deposition conditions on the film stability, structure and chemical group density were investigated. Detailed examination of the physical and chemical properties of plasma deposited polyoxazoline via XPS, FTIR, contact angle and ellipsometry unravels the complex functionality of the films. Partial retention of the oxazoline ring facilitates a covalent reaction with the carboxylic acid groups present on nanoparticles and biomolecules. Surface bound proteins effectively retain their bioactivity, therefore a vast range of potential applications unlocks for plasma deposited polyoxazoline coatings in the field of biosensing, medical arrays and diagnosis.

Adsorption and conformation behavior of biotinylated fibronectin on streptavidin-modified TiO(X) surfaces studied by SPR and AFM

It is well-known that protein-modified implant surfaces such as TiO(2) show a higher bioconductivity. Fibronectin is a glycoprotein from the extracellular matrix (ECM) with a major role in cell adhesion. It can be applied on titanium oxide surfaces to accelerate implant integration. Not only the surface concentration but also the presentation of the protein plays an important role for the cellular response. We were able to show that TiO(X) surfaces modified with biotinylated fibronectin adsorbed on a streptavidin-silane self-assembly multilayer system are more effective regarding osteoblast adhesion than surfaces modified with nonspecifically bound fibronectin. The adsorption and conformation behavior of biotinylated and nonbiotinylated (native) fibronectin was studied by surface plasmon resonance (SPR) spectroscopy and atomic force microscopy (AFM). Imaging of the protein modification revealed that fibronectin adopts different conformations on nonmodified compared to streptavidin-modified TiO(X) surfaces. This conformational change of biotinylated fibronectin on the streptavidin monolayer delivers a fibronectin structure similar to the conformation inside the ECM and therefore explains the higher cell affinity for these surfaces.

Surface oxide net charge of a titanium alloy: modulation of fibronectin-activated attachment and spreading of osteogenic cells

In the current study, we have altered the surface oxide properties of a Ti6Al4V alloy using heat treatment or radiofrequency glow discharge (RFGD) in order to evaluate the relationship between the physico-chemical and biological properties of the alloy's surface oxide. The effects of surface pretreatments on the attachment of cells from two osteogenic cell lines (MG63 and MC3T3) and a mesenchymal stem cell line (C3H10T1/2) to fibronectin adsorbed to the alloy were measured. Both heat and RFGD pretreatments produced a several-fold increase in the number of cells that attached to fibronectin adsorbed to the alloy at a range of coating concentrations (0.001-10nM FN) for each cell line tested. An antibody (HFN7.1) directed against the central integrin binding domain of fibronectin produced a 65-70% inhibition of cell attachment to fibronectin-coated disks, indicating that cell attachment to the metal discs was dependent on fibronectin binding to cell integrin receptors. Both treatments also accelerated the cell spreading response manifested by extensive flattening and an increase in mean cellular area. The treatment-induced increases in the cell attachment activity of adsorbed fibronectin were correlated with previously demonstrated increases in Ti6Al4V oxide negative net surface charge at physiological pH produced by both heat and RFGD pretreatments. Since neither treatment increased the adsorption mass of fibronectin, these findings suggest that negatively charged surface oxide functional groups in Ti6Al4V can modulate fibronectin's integrin receptor activity by altering the adsorbed protein's conformation. Our results further suggest that negatively charged functional groups in the surface oxide can play a prominent role in the osseointegration of metallic implant materials.

Enhanced surface activation of fibronectin upon adsorption on hydroxyapatite

In this study the adsorption characteristics and the structure of fibronectin adsorped on hydroxyapatite (Ha) and a reference gold substrate (Au) is examined by quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM) at the following concentrations: 20 microg/mL, 30 microg/mL, 40 microg/mL, 100 microg/mL, 200 microg/mL, and 500 microg/mL. The conformational changes of the fibronectin molecules upon surface binding were examined as well with monoclonal antibody directed against the cell binding-domain (CB domain) of fibronectin. The QCM-D and AFM results show that the fibronectin uptake is larger on Au as compared with Ha regardless of the protein bulk concentration used in the experiment, suggesting that the individual fibronectin molecules in general attach to the surfaces in a more unfolded configuration on Ha. Moreover the dissipation values obtained with QCM-D indicate that the individual fibronectin molecules bind in a more compact and rigid configuration on Au compared to the Ha surface. In particular the monoclonal antibody data show that the CB domain on fibronectin is more available on Ha, where such cell-recognizing abilities are more pronounced at low fibronectin surface coverage. The results demonstrate that the detailed molecular structure of fibronectin and its functional activity depend significantly on both the underlying surface chemistry as well as the fibronectin surface coverage.

Fibronectin adsorption on tantalum: the influence of nanoroughness

The complex mechanisms of protein adsorption at the solid-liquid interface is of great importance in many research areas, including protein purification, biocompatibility of medical implants, biosensing, and biofouling. The protein adsorption process depends crucially on both the nanoscale chemistry and topography of the interface. Here, we investigate the adsorption of the cell-binding protein fibronectin on flat and nanometer scale rough tantalum oxide surfaces using ellipsometry and quartz crystal microbalance with dissipation (QCM-D). On the flat tantalum oxide surfaces, the interfacial protein spreading causes an increase in the rigidity and a decrease in the thickness of the adsorbed fibronectin layer with decreasing bulk protein concentration. For the tantalum oxide surfaces with well-controlled, stochastic nanometer scale roughness, similar concentration effects are observed for the rigidity of the fibronectin layer and saturated fibronectin uptake. However, we find that the nanorough tantalum oxide surfaces promote additional protein conformational changes, an effect especially apparent from the QCM-D signals, interpreted as an additional stiffening of the formed fibronectin layers.

Thermal and chemical modification of titanium–aluminum–vanadium implant materials: effects on surface properties, glycoprotein adsorption, and MG63 cell attachment

The microstructure, chemical composition and wettability of thermally and chemically modified Ti-6Al-4V alloy disks were characterized and correlated with the degree of radiolabeled fibronectin-alloy surface adsorption and subsequent adhesion of osteoblast-like cells. Heating either in pure oxygen or atmosphere (atm) resulted in an enrichment of Al and V within the surface oxide. Heating (oxygen/atm) and peroxide treatment both followed by butanol treatment resulted in a reduction in content of V, but not in Al. Heating (oxygen/atm) or peroxide treatment resulted in a thicker oxide layer and a more hydrophilic surface when compared with passivated controls. Post-treatment with butanol, however, resulted in less hydrophilic surfaces than heating or peroxide treatment alone. The greatest increases in the adsorption of radiolabeled fibronectin following treatment were observed with peroxide/butanol-treated samples followed by peroxide/butanol and heat/butanol, although binding was only increased by 20-40% compared to untreated controls. These experiments with radiolabeled fibronectin indicate that enhanced adsorption of the glycoprotein was more highly correlated with changes in chemical composition, reflected in a reduction in V content and decrease in the V/Al ratio, than with changes in wettability. Despite promoting only a modest elevation in fibronectin adsorption, the treatment of disks with heat or heat/butanol induced a several-fold increase in the attachment of MG63 cells promoted by a nonadhesive concentration of fibronectin that was used to coat the pretreated disks compared to uncoated disks. Therefore, results obtained with these modifications of surface properties indicate that an increase in the absolute content of Al and/or V (heat), and/or in the Al/V ratio (with little change in hydrophilicity; heat+butanol) is correlated with an increase in the fibronectin-promoted adhesion of an osteoblast-like cell line. It would also appear that the thermal treatment-induced enhancement of cell adhesion in the presence of this integrin-binding protein is due to its increased biological activity, rather than a mass effect alone, that appear to be associated with changes in chemical composition of the metallic surface. Future studies will investigate the influence of the surface chemical composition of various implantable alloys on protein adsorption and receptor-mediated cell adhesion. In addition, by altering the properties of bound osteogenic protein enhancing exposure to cell integrin binding domains, it may be possible to develop implant surfaces which enhance the attachment, adhesion and developmental response of osteoblast precursors leading to accelerated osseointegration.

Structural changes of fibronectin adsorbed to model surfaces probed by fluorescence resonance energy transfer

Structural changes of proteins during adsorption to biomaterials affect the presentation of molecular binding sites and, ultimately, biomaterial performance. We have applied fluorescence resonance energy transfer (FRET) spectroscopy to study structural changes of the cell adhesion protein, fibronectin (Fn), following adsorption to model hydrophilic and hydrophobic surfaces. Fn was labeled with donor and acceptor fluorophores using two labeling schemes and intramolecular energy transfer was calibrated against measured structural changes of Fn in denaturing solutions. FRET was then applied to measure Fn's structure on surfaces. Based on FRET, Fn underwent greater extension of its dimer arms on hydrophilic glass than on hydrophobic fluoroalklysilane-derivatized glass (fluorosilane), and this extension was insensitive to molecular packing over a range of adsorption concentrations. Fn's conformation on glass better promoted cell attachment than on fluorosilane; the roles of both global structural changes (movements of modules) and local structural changes (disruption of secondary structure) on Fn's cell integrin binding activity are discussed. Based on previous FRET work, we compare Fn's conformations on these surfaces with its conformations in fibroblast culture. FRET is unique in allowing direct comparison of protein structure between biomaterial surfaces and cell culture.

Surface-dependent conformations of human plasma fibronectin adsorbed to silica, mica, and hydrophobic surfaces, studied with use of Atomic Force Microscopy

Human plasma fibronectin (Fn) is a large flexible protein stabilized by intermolecular ionic interactions forming a compact structure. On altering solution conditions, the structure can revert to a more expanded state, thereby exposing previously hidden domains (e.g., cell-binding sites). Electron microscopy images of Fn air-sprayed onto mica surfaces show elongated protein structures, indicating a surface-induced structural change. This makes it interesting to investigate the influence of surface properties on the structure of adsorbed Fn. We have used intermittent-contact Atomic Force Microscopy to investigate the structure of Fn adsorbed onto mica, silica, and methylated silica surfaces. We observed that on silica surfaces, which is hydrophilic, most (70%) of the molecules had an elongated structure with partial intramolecular chain interactions, compare to molecules adsorbed on hydrophobic, methylated surfaces, where a compact structure predominated (70%). On mica surfaces, both compact and elongated protein structures were observed, with a slight preference for the elongated form (53%). Results show that surface physical properties influence the molecular structure of fibronectin on adsorption, which could provide useful information in understanding surface-induced in vivo responses.

Fibronectin extension and unfolding within cell matrix fibrils controlled by cytoskeletal tension

Evidence is emerging that mechanical stretching can alter the functional states of proteins. Fibronectin (Fn) is a large, extracellular matrix protein that is assembled by cells into elastic fibrils and subjected to contractile forces. Assembly into fibrils coincides with expression of biological recognition sites that are buried in Fn's soluble state. To investigate how supramolecular assembly of Fn into fibrillar matrix enables cells to mechanically regulate its structure, we used fluorescence resonance energy transfer (FRET) as an indicator of Fn conformation in the fibrillar matrix of NIH 3T3 fibroblasts. Fn was randomly labeled on amine residues with donor fluorophores and site-specifically labeled on cysteine residues in modules FnIII(7) and FnIII(15) with acceptor fluorophores. Intramolecular FRET was correlated with known structural changes of Fn in denaturing solution, then applied in cell culture as an indicator of Fn conformation within the matrix fibrils of NIH 3T3 fibroblasts. Based on the level of FRET, Fn in many fibrils was stretched by cells so that its dimer arms were extended and at least one FnIII module unfolded. When cytoskeletal tension was disrupted using cytochalasin D, FRET increased, indicating refolding of Fn within fibrils. These results suggest that cell-generated force is required to maintain Fn in partially unfolded conformations. The results support a model of Fn fibril elasticity based on unraveling and refolding of FnIII modules. We also observed variation of FRET between and along single fibrils, indicating variation in the degree of unfolding of Fn in fibrils. Molecular mechanisms by which mechanical force can alter the structure of Fn, converting tensile forces into biochemical cues, are discussed.

Surface analysis of human plasma fibronectin adsorbed to commercially pure titanium materials

Protein binding on metallic implant surfaces, such as titanium, is governed by the physico-chemical nature of the metallic surface. Human plasma fibronectin (HPF) is an important matrix glycoprotein that mediates cell and protein attachment to each other or to the extracellular matrix present during wound healing. The objective of this study was to investigate the adsorption of HPF onto polished commercially pure titanium (cpTi) by using atomic force microscopy (AFM) and electron spectroscopy for chemical analysis (ESCA) and to measure the resultant surface contact angle before and after HPF binding. Two types of cpTi disks, one highly polished in our laboratory (HSS) and one commercially prepared (31), were reacted with HPF solutions of varying concentrations (1 microg/mL-10 ng/mL). ESCA survey spectra of samples coated with 1 microg/mL of fibronectin showed an increase in organic nitrogen and carbon compared with uncoated controls. Contact angle measurements of HSS and 31 cpTi disks showed no significant difference in average contact angle (36.3 degrees +/- 3.5 and 39.1 degrees +/- 3.1) despite differences in local root mean square (RMS) surface roughness (4.45 +/- 0.46 nm and 22.37 +/- 4.17 nm) as measured by AFM. Images obtained by AFM showed that 31 specimens were more irregular, with large parallel polishing grooves. Adsorbed HPF appeared in a globular form with an average length of 16.5 +/- 1.0 nm, a height of 2.5 +/- 0.5 nm, and a width of 9.6 +/- 1.2 nm. Fibronectin coating on both HSS and 31 cpTi specimens resulted in a significant increase in hydrophobicity compared to uncoated specimens. These results indicate the significance of HPF on cpTi and may explain how cpTi implants function in situ.

Extracellular matrix contains insulin-like growth factor binding protein-5: potentiation of the effects of IGF-I

Insulin-like growth factor binding proteins (IGFBPs) have been shown to serve as carrier proteins for the insulin-like growth factors (IGFs) and to modulate their biologic effects. Since extracellular matrix (ECM) has been shown to be a reservoir for IGF-I and IGF-II, we examined the ECM of cultured human fetal fibroblasts and found that IGFBP-5 was incorporated intact into ECM, while mostly inert proteolytic fragments were found in the medium. In contrast, two other forms of IGFBP that are secreted by these cells were either present in ECM in minimal amounts (IGFBP-3) or not detected (IGFBP-4). Likewise, when purified IGFBPs were incubated with ECM, IGFBP-5 bound preferentially. IGFBP-5 was found to bind to types III and IV collagen, laminin, and fibronectin. Increasing salt concentrations inhibited the binding of IGFBP-5 to ECM and accelerated the release of IGFBP-5 from ECM, suggesting an ionic basis for this interaction. ECM-associated IGFBP-5 had a sevenfold decrease in affinity for IGF-I compared to IGFBP-5 in solution. Furthermore, when IGFBP-5 was present in cell culture substrata, it potentiated the growth stimulatory effects of IGF-I on fibroblasts. When IGFBP-5 was present only in the medium, it was degraded to a 22-kD fragment and had no effect on IGF-I-stimulated growth. We conclude that IGFBP-5 is present in fibroblast ECM, where it is protected from degradation and can potentiate the biologic actions of IGF-I. These findings provide a molecular explanation for the association of the IGF's with the extracellular matrix, and suggest that the binding of the IGF's to matrix, via IGFBP-5, may be important in mediating the cellular growth response to these growth factors.

Lack of binding between cryoimmunoglobulins, immunoglobulins and fibronectin: implications for immune complex vasculitis

Immune-complex-mediated vasculitis is a frequent complication of rheumatoid arthritis and systemic lupus erythematosus. The mechanism of deposition of immune complexes within the vessel wall in these diseases remains unknown, but probably involves other proteins. Fibronectin is a likely candidate since it possesses the ability to bind to collagen, endothelial cells, and possibly immunoglobulins and immune complexes. In this study, the binding of fibronectin to IgG and IgM cryoglobulins, cold soluble IgM, IgG, IgG subclasses and IgG fragments was investigated in the solution phase. Static light scattering, fluorescence anisotropy, fluorescence intensity, and PEG precipitation studies were used to investigate binding under different conditions of temperature and ionic strength. These studies failed to demonstrate significant binding between fibronectin and IgM, IgG, IgG subclasses and IgG fragments under the conditions studied. These findings argue against solution phase binding of fibronectin and immunoglobulins contributing to immune complex vasculitis. The possibility of important surface interactions between these proteins has not been ruled out.