Temperature-induced beta-aggregation of fibronectin in aqueous solution

Bone surface mimicked PDMS membranes stimulate osteoblasts and calcification of bone matrix

Cellular microenvironments play a crucial role in cell behavior. In addition to the biochemical cues present in the microenvironments, biophysical and biomechanical properties on surfaces have an impact on cellular functionality and eventually cellular fate. Effects of surface topography on cell behavior are being studied extensively in the literature. However, these studies often try to replicate topographical features of tissue surfaces by using techniques such as chemical etching, photolithography, and electrospinning, which may result in the loss of crucial micro- and nano- features on the tissue surfaces such as bone. This study investigates the topographical effects of bone surface by transferring its surface features onto polydimethylsiloxane (PDMS) membranes using soft lithography from a bovine femur. Our results have shown that major features on bone surfaces were successfully transferred onto PDMS using soft lithography. Osteoblast proliferation and calcification of bone matrix have significantly increased along with osteoblast-specific differentiation and maturation markers such as osteocalcin (OSC), osterix (OSX), collagen type I alpha 1 chain (COL1A1), and alkaline phosphatase (ALP) on bone surface mimicked (BSM) PDMS membranes in addition to a unidirectional alignment of osteoblast cells compared to plain PDMS surfaces. This presented bone surface mimicking method can provide a versatile native-like platform for further investigation of intracellular pathways regarding osteoblast growth and differentiation.

Controlling Fibronectin Fibrillogenesis Using Visible Light

We previously developed a surface-assisted assay to image early steps of cell-induced plasma fibronectin (FN) fibrillogenesis by timelapse atomic force microscopy (AFM). Unexpectedly, complementary attempts to visualize FN fibrillogenesis using fluorescently labeled FN (Alexa Fluor 488 or 568) and live-cell light microscopy initially failed consistently. Further analysis revealed that fibrillar remodeling was inhibited efficiently in the focal area illuminated during fluorescence imaging, but progressed normally elsewhere on the substrate, suggesting photo sensitivity of the FN fibrillogenesis process. In agreement, active cell-driven fibrillar extension of FN could be stopped by transient illumination with visible light during AFM timelapse scanning. Phototoxic effects on the cells could be ruled out, because pre-illuminating the FN layer before cell seeding also blocked subsequent fibrillar formation. Varying the illumination wavelength range between 400 and 640 nm revealed strong inhibition across the visible spectrum up to 560 nm, and a decreasing inhibitory effect at longer wavelengths. The photo effect also affected unlabeled FN, but was enhanced by fluorophore labeling of FN. The inhibitory effect could be reduced when reactive oxygen species (ROS) were removed for the cell imaging medium. Based on these findings, FN fibrillogenesis could be imaged successfully using a labeling dye with a long excitation wavelength (Alexa Fluor 633, excitation at 632 nm) and ROS scavengers, such as oxyrase, in the imaging medium. Fibrillar remodeling of exposed cell-free FN layers by AFM scanning required higher scan forces compared to non-exposed FN, consisting with mechanical stiffing of the FN layer after illumination. In agreement with changes in FN mechanics, cells spreading on pre-exposed FN showed reduced migration speeds, altered focal adhesion arrangement, and changes in mechanosensitive signaling pathways, including reduced FAK (Y397) and paxillin (Y118) phosphorylation. Pre-exposure of FN to visible light prior to cell seeding thus provides a useful tool to delineate mechanosensitive signaling pathway related to FN fibrillogenesis. When using FN-coated cell adhesion substrates, care should be taken when comparing experimental results obtained on non-exposed FN layers in cell culture incubators, or during live-cell fluorescence imaging, as FN fibrillogenesis and mechanosensitive cellular signaling pathways may be affected differently.

Functionalized Graphene Oxide Thin Films for Anti-tumor Drug Delivery to Melanoma Cells

Since Graphene discovery, their associated derivate nanomaterials, Graphene Oxide (GO) and reduced-GO were in the forefront of continuous developments in bio-nano-technology due to unique physical-chemical properties. Although GO nano-colloids (GON) were proposed as drug release matrix for targeting cancer cells, there is still a concern regarding its cytotoxicity issues. In this study, we report on the fabrication of functional GON bio-coatings by Matrix-Assisted Pulsed Laser Evaporation (MAPLE) to be used as drug carriers for targeting melanoma cells. We first performed a thorough in vitro cytotoxicity assay for comparison between GON and protein functionalized GON coatings. As functionalization protein, Bovine Serum Albumin (BSA) was non-covalently conjugated to GO surface. Safe concentration windows were identified in cytotoxicity tests by live/dead staining and MTS assays for five different human melanoma cell lines as well as for non-transformed melanocytes and human dermal fibroblasts. Hybrid GON-BSA nano-scaled thin coatings incorporating Dabrafenib (DAB) and Trichostatin A (TSA) inhibitors for cells bearing BRAF^V600E pathway activating mutation were assembled on solid substrates by MAPLE technique. We further demonstrated the successful immobilization for each drug-containing GON-BSA assembling systems by evaluating cellular BRAF activity inhibition and histone deacetylases activity blocking, respectively. DAB activity was proven by the decreased ERK phosphorylation in primary melanoma cells (SKmel28 BRAF^V600E cell line), while TSA effect was evidenced by acetylated histones accumulation in cell's nuclei (SKmel23 BRAF WT cell line). In addition, melanoma cells exposed to GON-BSA coatings with compositional gradient of inhibitors evidenced a dose-dependent effect on target activity. Such functional bio-platforms could present high potential for cell-biomaterial interface engineering to be applied in personalized cancer therapy studies.

Fibronectin amyloid-like aggregation alters its extracellular matrix incorporation and promotes a single and sparsed cell migration

Fibronectin (Fn) is an extracellular matrix (ECM) multifunctional glycoprotein essential for regulating cells behaviors. Within ECM, Fn is found as polymerized fibrils. Apart from fibrils, Fn could also form other kind of supramolecular assemblies such as aggregates. To gain insight into the impact of Fn aggregates on cell behavior, we generated several Fn oligomeric assemblies. These assemblies displayed various amyloid-like properties but were not cytotoxic. In presence of the more amyloid-like structured assemblies of Fn, the cell-ECM networks were altered and the cell shapes shifted toward extended mesenchymal morphologies. Additionnaly, the Fn amyloid-like aggregates promoted a single-cell and sparsed migration of SKOV3 cancer cells, which was associated with a relocalization of αv integrins from plasma membrane to perinuclear vesicles. These data pointed out that the features of supramolecular Fn assemblies could represent a higher level of fine-tuning cell phenotype, and especially migration of cancer cells.

Surface Functionalization of Ti6Al4V via Self-assembled Monolayers for Improved Protein Adsorption and Fibroblast Adhesion

Although metallic biomaterials find numerous biomedical applications, their inherent low bioactivity and poor osteointegration had been a great challenge for decades. Surface modification via silanization can serve as an attractive method for improving the aforementioned properties of such substrates. However, its effect on protein adsorption/conformation and subsequent cell adhesion and spreading has rarely been investigated. This work reports the in-depth study of the effect of Ti6Al4V surface functionalization on protein adsorption and cell behavior. We prepared self-assembled monolayers (SAMs) of five different surfaces (amine, octyl, mixed [1:1 ratio of amine:octyl], hybrid, and COOH). Synthesized surfaces were characterized by Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopy, contact angle goniometry, profilometry, and field emission scanning electron microscopy (FESEM). Quantification of adsorbed mass of bovine serum albumin (BSA) and fibronectin (FN) was determined on different surfaces along with secondary structure analysis. The adsorbed amount of BSA was found to increase with an increase in surface hydrophobicity with the maximum adsorption on the octyl surface while the reverse trend was detected for FN adsorption, having the maximum adsorbed mass on the COOH surface. The α-helix content of adsorbed BSA increased on amine and COOH surfaces while it decreased for other surfaces. Whereas increasing β-turn content of the adsorbed FN with the increase in the surface hydrophobicity was observed. In FN, RGD loops are located in the β-turn and consequently the increase in Δ adhered cells (%) was predominantly increased with the increasing Δ β-turn content (%). We found hybrid surfaces to be the most promising surface modifier due to maximum cell adhesion (%) and proliferation, larger nuclei area, and the least cell circularity. Bacterial density increased with the increasing hydrophobicity and was found maximum for the amine surface (θ = 63 ± 1°) which further decreased with the increasing hydrophobicity. Overall, modified surfaces (in particular hybrid surface) showed better protein adsorption and cell adhesion properties as compared to unmodified Ti6Al4V and can be potentially used for tissue engineering applications.

Mono vs multilayer fibronectin coatings on polar/hydrophobic/ionic polyurethanes: Altering surface interactions with human monocytes

Monocyte interactions with materials that are biofunctionalized with fibronectin (Fn) are of interest because of the documented literature which associates this protein with white blood cell function at implant sites. A degradable-polar hydrophobic ionic polyurethane (D-PHI), has been reported to promote an anti-inflammatory response from human monocytes. The aim of the current work was to study the influence of intrinsic D-PHI material chemistry on Fn adsorption (mono and multi-layer structures), and to investigate the influence of such chemistry on the structural state of the Fn, as well as the latter's influence on the activity of human monocytes on the protein coated substrates. Significant differences in Fn adsorption, surface hydrophobicity and the availability of defined peptide sequences (N terminal, C terminal or Cell Binding Domain) for the Fn in mono vs multilayer structures were observed as a function of the changes in intrinsic material chemistry. A D-PHI-formulated polyurethane substrate with subtle changes in anionic and hydrophobic domain content relative to the polar non-ionic urethane/carbonate groups within the polymer matrix promoted the lowest activation of monocytes, in the presence of multi-layer Fn constructs. These results highlight the importance of chemical heterogeneity as a design parameter for biomaterial surfaces, and establishes a desired strategy for controlling human monocyte activity at the surface of devices, when these are coated with multi-layer Fn structures. The latter is an important step towards functionalizing the materials with multi-layer protein drug carriers as interventional therapeutic agents.

STATEMENT OF SIGNIFICANCE

The control of the behavior of monocytes, especially migration and activation, is of crucial interest to modulate the inflammatory response at the site of implanted biomaterial. Several studies report the influence of adsorbed serum proteins on the behavior of monocytes on biomaterials. However, few studies show the influence of surface chemical group distribution on the controlled adsorption and the subsequent induced conformation- of mono versus multi-layer assembled structures generated from specific proteins implicated in wound repair. The current research considered the role of Fn adsorption and conformation in thin films while interacting with the intrinsic chemistry of segmented block polyurethanes; and the influence of the former on modulation and activation of human monocytes.

Amyloid-like aggregates formation by blood plasma fibronectin

Fibronectin (FN) is a multifunctional glycoprotein of the extracellular matrix (ECM) playing critical roles in physiological and pathological cell processes like adhesion, migration, growth, and differentiation. These various functions of FN are modulated by its supramolecular state. Indeed, FN can polymerize into different types of assemblies like fibrils and aggregates. However, the mechanism of polymerization and the effects of such assemblies on cell behaviors still remain to be elucidated. Here we show that upon irreversible thermal denaturation, human blood plasma fibronectin forms high molecular weight aggregates. These compact and globular aggregates show amyloid features: they are stabilized by intermolecular b-sheets, they bind Thioflavin T and they are resistant to reducing and denaturing agents. Their characterization by electrospray ionization charge detection mass spectrometry shows that two populations can be distinguished according to the mass and charge density. Despite their amyloid features and the presence of hydrophobic patches on their surface, these aggregates are not toxic for cells. However, their binding abilities to gelatin and RGD are drastically decreased compare to native FN, suggesting possible effects on ECM-cell interactions.

Allosteric Regulation of Fibronectin/α5β1 Interaction by Fibronectin-Binding MSCRAMMs

Adherence of microbes to host tissues is a hallmark of infectious disease and is often mediated by a class of adhesins termed MSCRAMMs (Microbial Surface Components Recognizing Adhesive Matrix Molecules). Numerous pathogens express MSCRAMMs that specifically bind the heterodimeric human glycoprotein fibronectin (Fn). In addition to roles in adhesion, Fn-binding MSCRAMMs exploit physiological Fn functions. For example, several pathogens can invade host cells by a mechanism whereby MSCRAMM-bound Fn bridges interaction with α₅β₁ integrin. Here, we investigate two Fn-binding MSCRAMMs, FnBPA (Staphylococcus aureus) and BBK32 (Borrelia burgdorferi) to probe structure-activity relationships of MSCRAMM-induced Fn/α₅β₁integrin activation. Circular dichroism, fluorescence resonance energy transfer, and dynamic light scattering techniques uncover a conformational rearrangement of Fn involving domains distant from the MSCRAMM binding site. Surface plasmon resonance experiments demonstrate a significant enhancement of Fn/α₅β₁ integrin affinity in the presence of FnBPA or BBK32. Detailed kinetic analysis of these interactions reveal that this change in affinity can be attributed solely to an increase in the initial Fn/α₅β₁ on-rate and that this rate-enhancement is dependent on high-affinity Fn-binding by MSCRAMMs. These data implicate MSCRAMM-induced perturbation of specific intramolecular contacts within the Fn heterodimer resulting in activation by exposing previously cryptic α₅β₁ interaction motifs. By correlating structural changes in Fn to a direct measurement of increased Fn/α₅β₁ affinity, this work significantly advances our understanding of the structural basis for the modulation of integrin function by Fn-binding MSCRAMMs.

Extracellular Matrix Revisited: Roles in Tissue Engineering

The extracellular matrix (ECM) is a heterogeneous, connective network composed of fibrous glycoproteins that coordinate in vivo to provide the physical scaffolding, mechanical stability, and biochemical cues necessary for tissue morphogenesis and homeostasis. This review highlights some of the recently raised aspects of the roles of the ECM as related to the fields of biophysics and biomedical engineering. Fundamental aspects of focus include the role of the ECM as a basic cellular structure, for novel spontaneous network formation, as an ideal scaffold in tissue engineering, and its essential contribution to cell sheet technology. As these technologies move from the laboratory to clinical practice, they are bound to shape the vast field of tissue engineering for medical transplantations.

Selective Accelerated Proliferation of Malignant Breast Cancer Cells on Planar Graphene Oxide Films

Graphene nanomaterials have been actively investigated for biomedical and biological applications, including that of cancer. Despite progress made, most of such studies are conducted on dispersed graphene nanosheets in solution. Consequently, the use of planar graphene films, especially in cancer research, has not been fully explored. Here, we investigate the cellular interactions between the graphene material films and breast cancer cell lines, specifically the effects these films have on cellular proliferation, spreading area, and cytotoxicity. We demonstrate that the graphene oxide (GO) film selectively accelerates the proliferation of both metastatic (MDA-MB-231) and nonmetastatic (MCF-7) breast cancer cells, but not that of noncancer breast epithelial cells (MCF-10A). Contrastingly, this accelerated proliferation is not observed with the use of graphene (G) film. Moreover, GO induces negligible cytotoxicity on these cells. We suggest that the observed phenomena originate from the synergistic effect resulted from the high loading capacity and conformational change of cellular attachment proteins on the GO film, and the high amount of oxygenated groups present in the material. We anticipate that our findings can further shed light on the graphene-cancer cellular interactions and provide better understanding for the future design and application of graphene-based nanomaterials in cancer research.

Designing peptidic inhibitors of serum amyloid A aggregation process

Amyloid A amyloidosis is a life-threatening complication of a wide range of chronic inflammatory, infectious and neoplastic diseases, and the most common form of systemic amyloidosis worldwide. It is characterized by extracellular tissue deposition of fibrils that are composed of fragments of serum amyloid A protein (SAA), a major acute-phase reactant protein, produced predominantly by hepatocytes. Currently, there are no approved therapeutic agents directed against the formation of fibrillar SAA assemblies. We attempted to develop peptidic inhibitors based on their similarity and complementarity to the regions critical for SAA self-association, which they should interact with and block their assembly into amyloid fibrils. Inh1 and inh4 which are comprised of the residues from the amyloidogenic region of SAA1.1 protein and Aβ peptide, respectively, were found by us as capable to significantly suppress aggregation of the SAA1-12 peptide. It was chosen as an aggregation model that mimicks the amyloidogenic nucleus of SAA protein. We suppose that aromatic interactions may be responsible for inhibitory activity of both compounds. We also recognized that aromatic residues are involved in self-association of SAA1-12.

A comparison of adsorbed and grafted fibronectin coatings under static and dynamic conditions

Coatings for medical devices are expected to improve their surface biocompatibility mainly by being bioactive, i.e. stimulating healing-oriented interactions with living cells, tissues and organs.

On the potential for fibronectin/phosphorylcholine coatings on PTFE substrates to jointly modulate endothelial cell adhesion and hemocompatibility properties

The use of biomolecules as coatings on biomaterials is recognized to constitute a promising approach to modulate the biological response of the host. In this work, we propose a coating composed by 2 biomolecules susceptible to provide complementary properties for cardiovascular applications: fibronectin (FN) to enhance endothelialization, and phosphorylcholine (PRC) for its non thrombogenic properties. Polytetrafluoroethylene (PTFE) was selected as model substrate mainly because it is largely used in cardiovascular applications. Two approaches were investigated: 1) a sequential adsorption of the 2 biomolecules and 2) an adsorption of the protein followed by the grafting of phosphorylcholine via chemical activation. All coatings were characterized by immunofluorescence staining, X-Ray Photoelectron Spectroscopy and Scanning Electron Microscopy analyses. Assays with endothelial cells showed improvement on cell adhesion, spreading and metabolic activity on FN-PRC coatings compared with the uncoated PTFE. Platelets adhesion and activation were both reduced on the coated surfaces when compared with uncoated PTFE. Moreover, clotting time tests exhibited better hemocompatibility properties of the surfaces after a sequential adsorption of FN and PRC. In conclusion, FN-PRC coating improves cell adhesion and non-thrombogenic properties, thus revealing a certain potential for the development of this combined deposition strategy in cardiovascular applications.

High-performance scaffolds on titanium surfaces: osteoblast differentiation and mineralization promoted by a globular fibrinogen layer through cell-autonomous BMP signaling

Titanium has been widely used as a dental implant material. However, it takes several months for the implant body to bind with the jawbone. To develop new bioactive modification on titanium surfaces to achieve full osseointegration expeditiously, we used fibrinogen and fibronectin as bioactive scaffolds on the titanium plate, which are common extracellular matrix (ECM) proteins. We analyzed the features of the surface of ECM-modified titanium plates by atomic force microscopy and Fourier transform infrared spectrophotometry. We also evaluated the effect of ECM modification on promoting the differentiation and mineralization of osteoblasts on these surfaces. Fibrinogen had excellent adsorption on titanium surfaces even at low concentrations, due to the binding ability of fibrinogen via its RGD motif. The surface was composed of a fibrinogen monolayer, in which the ratio of β-sheets was decreased. Osteoblast proliferation on ECM-modified titanium surface was significantly promoted compared with titanium alone. Calcification on the modified surface was also accelerated. These ECM-promoting effects correlated with increased expression of bone morphogenetic proteins (BMPs) by the osteoblasts themselves and were inhibited by Noggin, a BMP inhibitor. These results suggest that the fibrinogen monolayer-modified titanium surface is recognized as bioactive scaffolds and promotes bone formation, resulting in the acceleration of osseointegration.

Fabrication of functional fibronectin patterns by nanosecond excimer laser direct write for tissue engineering applications

Laser direct write techniques represent a prospective alternative for engineering a new generation of hybrid biomaterials via the creation of patterns consisting of biological proteins onto practically any type of substrate. In this paper we report on the characterization of fibronectin features obtained onto titanium substrates by UV nanosecond laser transfer. Fourier-transform infrared spectroscopy measurements evidenced no modification in the secondary structure of the post-transferred protein. The molecular weight of the transferred protein was identical to the initial fibronectin, no fragment bands being found in the transferred protein's Western blot migration profile. The presence of the cell-binding domain sequence and the mannose groups within the transferred molecules was revealed by anti-fibronectin monoclonal antibody immunolabelling and FITC-Concanavalin-A staining, respectively. The in vitro tests performed with MC3T3-E1 osteoblast-like cells and Swiss-3T3 fibroblasts showed that the cells' morphology and spreading were strongly influenced by the presence of the fibronectin spots.

Fibronectin layers by matrix-assisted pulsed laser evaporation from saline buffer-based cryogenic targets

The deposition of fibronectin (FN) from saline buffer-based cryogenic targets by matrix-assisted pulsed laser evaporation (MAPLE) onto silicon substrates is reported. A uniform distribution of FN was revealed by Ponceau staining after control experiments on nitrocellulose paper. Well-organized particulates with heights from hundreds of nanometers up to more than 1 μm packed in homogeneous layers were evidenced by optical microscopy and profilometry on Si substrates. Atomic force microscopy images showed regions composed of buffer and FN aggregates forming a compact film. Comparison of infrared spectra of drop-cast and MAPLE-deposited FN confirmed the preservation of composition and showed no degradation of the protein. The protein deposition on Si was confirmed by antibody staining. Small aggregates and fluorescent fibrils were visualized by fluorescence microscopy. Superior attachment of human osteoprogenitor cells cultivated for 3 h proved the presence of stable and intact FN molecules after transfer.

Pre-osteoblasts on poly(L-lactic acid) and silicon oxide: Influence of fibronectin and albumin adsorption

Cell adhesion and subsequent viability are critical initial steps in biomaterial-tissue integration and are strongly dependent on the material properties and the presence of matrix proteins. In the present study MC3T3-E1 osteoblast-like cell behavior on silicon oxide (SO) and poly(L-lactic acid) (PLLA) substrates has been examined, with a focus on the influence of the adhesive protein fibronectin and the non-adhesive protein albumin adsorbed on the substrates. Quartz crystal microgravimetry showed adsorption of fibronectin and albumin to be nearly identical on SO and PLLA. Subsequent exposure a previously adsorbed fibronectin layer to albumin decreased the rigidity of the adsorbed layer without any measurable increase in adsorbed mass. Cell adhesion and spreading were significantly enhanced on both SO and PLLA substrates coated with fibronectin or with fibronectin and albumin, compared with uncoated or albumin-coated substrates. The only statistically significant difference between the two substrates in these assays was increased spreading on PLLA compared with SO in the presence of fibronectin and albumin. Cell proliferation was significantly higher on SO compared with PLLA after 7 days culture, but depended on the presence of fibronectin only in the PLLA system. In contrast, mitochondrial activity was higher on PLLA than on SO, and was enhanced by fibronectin on both substrates. PLLA substrates coated with fibronectin and subsequently exposed to albumin exhibited the highest level of cell differentiation, as assayed via alkaline phosphatase activity. These results demonstrate the importance of adsorbed proteins on osteoblast-like cell-surface interactions.

The positive influence of electrochemical cyclic potentiodynamic passivation (CPP) of a SS316LS surface on its response to fibronectin and pre-osteoblasts

The influence of an electrochemical surface passivation technique (cyclic potentiodynamic polarization, CPP) on the physico-chemical surface properties of SS316LS and its subsequent response to fibronectin (Fn) and pre-osteoblasts were investigated. Contact angle and zeta-potential measurements showed that the CPP-modified surface is more hydrophilic and more positively charged than the unmodified surface. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was used to investigate the interaction of Fn with both surfaces. The saturated surface concentration of adsorbed Fn was higher on the CPP-modified surface. As well, significant changes were identified in the secondary structure of Fn adsorbed on both surfaces, in comparison to its native state. This data also indicated a higher degree of Fn unfolding on the CPP-modified surface. Cell studies indicated that the attachment, proliferation and morphology of pre-osteoblasts were significantly improved on the CPP-modified surface, which was attributed to the more open conformation of Fn on the CPP-modified surface. Thus, the CPP surface passivation method was demonstrated to yield a SS316LS surface of enhanced biocompatibility.

Functionalization of luminescent aminated particles for facile bioconjugation

For labeling proteins (streptavidin and fibronectin) by luminescent aminated nanoparticles, an interesting strategy that requires neither activation nor chemical pre- or post-treatment was explored. Because biomolecules are easily rendered luminescent after reaction with organic dyes carrying isothiocyanate moiety, phenylene diisothiocyanate (DITC) was used for covalently binding proteins onto luminescent hybrid gadolinium oxide nanoparticles whose ability to combine imaging and therapy was recently demonstrated.

Size distribution and molecular associations of plasma fibronectin and fibronectin crosslinked by transglutaminase 2

Fibronectin (FN) is a ubiquitously expressed cell adhesion protein capable of assembling into large, extended fibrillar networks as part of an extracellular matrix (ECM) that regulates cell behavior. FN is a substrate for certain members of the transglutaminase family of protein-crosslinking enzymes-enzymes which can modify the ability of FN to support cell adhesion. In this study, we have analyzed the thermo-chemical stability of plasma FN in its noncrosslinked form, and after crosslinking by transglutaminase 2 (TG2), using dynamic light scattering. We report that FN is found in a generally globular (8.7 nm hydrodynamic radius), dimerized form in aqueous solutions, but unfolds into a linear arrangement at high ionic (1 M NaCl) and chaotropic (5 M urea) environments. FN conformation remained stable after multiple heating and cooling cycles ranging from 4 to 60 degrees C. Crosslinking of FN with TG2 formed large, multimeric complexes having high chemical stability in aqueous, high ionic and chaotropic environments, demonstrating that this covalent modification stabilizes FN. Given recent data that substrate (e.g. ECM) rigidity profoundly affects cell differentiation and behavior, we further studied how TG2 crosslinking affects the molecular rigidity of FN by obtaining atomic force microscopy nanoindentation measurements from untreated and crosslinked FN samples embedded in acrylamide gels. We demonstrate that TG2-mediated crosslinking of FN significantly increases Young's modulus (of elasticity), an observation of increased rigidity having important implications with respect to the biological role of ECM protein-crosslinking in cell signaling and guiding cell differentiation.

Fluorescein isothiocyanate-labeled human plasma fibronectin in extracellular matrix remodeling

Fluorescein isothiocyanate (FITC) is a well-known probe for labeling biologically relevant proteins. However, the impact of the labeling procedure on protein structure and biological activities remains unclear. In this work, FITC-labeled human plasma fibronectin (Fn) was developed to gain insight into the dynamic relationship between cells and Fn. The similarities and differences concerning the structure and function between Fn-FITC and standard Fn were evaluated using biochemical as well as cellular approaches. By varying the FITC/Fn ratio, we demonstrated that overlabeling (>10 FITC molecules/Fn molecule) induces probe fluorescence quenching, protein aggregation, and cell growth modifications. A correct balance between reliable fluorescence for detection and no significant modifications to structure and biological function compared with standard Fn was obtained with a final ratio of 3 FITC molecules per Fn molecule (Fn-FITC3). Fn-FITC3, similar to standard Fn, is correctly recruited into the cell matrix network. Also, Fn-FITC3 is proposed to be a powerful molecular tool to investigate Fn organization and cellular behavior concomitantly.

In vitro denaturation-renaturation of fibronectin. Formation of multimers disulfide-linked and shuffling of intramolecular disulfide bonds

It is well established that fibronectin into extracellular matrix undergoes repeated tensions applied by cells, resulting into dramatic structural changes which reflect its elastic properties. However, there is currently no study reporting with precision the consequences of this elasticity on fibronectin structure and conformation. In the present work, we investigated fibronectin structural and conformational reorganization in vitro through a denaturation-renaturation approach. The similarities and differences between "refolded fibronectin" and "native fibronectin" were investigated using various spectroscopic methods, hydrodynamic characterization, molecular imaging and biochemical characterization. In the refolded form, secondary structure elements as well as local tyrosine and tryptophan environment are identical compared to the native form. Interestingly, some differences in global tertiary structure organization and molecular conformation were observed. These differences are due to the reactivity of the two free cysteines, which are buried in the native state but become accessible during the unfolding process. First, oxidation of these residues leading to the formation of intermolecular disulfide bonds results in formation of stabilized multimer. Second, some illegitimate intramolecular disulfide bonds are formed. The presence of iodoacetamide, the sulfhydryl alkylating agent, during the unfolding-refolding process prevents all these events. This study clearly demonstrates that, under near physiological conditions, competitive renaturation pathways occur, involving free cysteines in either multimer formation or intermolecular shuffling of disulfide bonds. These findings might have important implications for future studies and be helpful to develop a deeper understanding of fibronectin morphology.

Purification of a plant cell wall fibronectin-like adhesion protein involved in plant response to salt stress

The structural role of extracellular-matrix (ECM) has been recognized in both plants and animals as a support and anchorage-inducing cell behavior. Unlike the animal ECM proteins, the proteins that have been identified in plant ECM have not yet been purified from whole plants and cell wall. As several immunological data indicate the presence of animal ECM-like proteins in plants cell wall, especially under salt stress or water deficit, we propose a protocol to purify a fibronectin-like protein from the cell wall of epicotyls of young germinating peas. The process consists of a combination of gelatin and heparin affinity chromatography, close to the classical one used for human blood plasma fibronectin purification. Proteins with affinity for gelatin and heparin, immunologically related to human fibronectin, are found in the cell wall of epicotyls grown under salt stress or not. Total amount of purified proteins is 3-4 times more enriched in salt stressed epicotyls. SDS-PAGE and Western blot with antibodies directed against human blood plasma fibronectin give evidence that the cell wall proteins purified by gelatin/heparin affinity chromatography are closely related to human fibronectin. The present protocol leads us to purify 17 (control) or 65 (salt stress) micrograms of protein per g of fresh starting material. Our results suggest that plant cell wall proteins can provide better anchorage of the cell to its cell-wall during salt stress or water deficit and could be considered not only as cell adhesion but also as signaling molecules.

Urea-Induced Sequential Unfolding of Fibronectin: A Fluorescence Spectroscopy and Circular Dichroism Study

Fibronectin (FN) is an extracellular matrix (ECM) protein found soluble in corporal fluids or as an insoluble fibrillar component incorporated in the ECM. This phenomenon implicates structural changes that expose FN binding sites and activate the protein to promote intermolecular interactions with other FN. We have investigated, using fluorescence and circular dichroism spectroscopy, the unfolding process of human fibronectin induced by urea in different ionic strength conditions. At any ionic strength, the equilibrium unfolding data are well described by a four-state equilibrium model N <= => I(1) <= =>I(2) <= => U. Fitting this model to experimental values, we have determined the free energy change for the different steps. We found that the N <= => I(1) transition corresponds to a free energy of 10.5 +/- 0.4 kcal/mol. Comparable values of free energy change are generally associated with a partial unfolding of the type III domain. For the I(1) <= => I(2) transition, the free energy change is 7.6 +/- 0.4 kcal/mol at low ionic strength but is twice as low at high ionic strength. This result is consistent with observations indicating that the complete unfolding of the type III domain from partially unfolded forms necessitates about 5 kcal/mol. The third step, I(2) <= => U, which leads to the complete unfolding of fibronectin, corresponds to a free energy change of 14.4 +/- 0.9 kcal/mol at low ionic strength whereas this energy is again twice as low under high ionic strength conditions. This hierarchical unfolding of fibronectin, as well as the stability of the different intermediates controlled by ionic strength demonstrated here, could be important for the understanding of activation of the matrix assembly.

Peculiar features of the aggregation effect of silver(I) ion on hemoglobin

Silver(I) ion has been shown to produce aggregation effect on bovine oxyhemoglobin (HbO(2)) in Tris buffer even when taken in amounts corresponding to only two or less silver ions per one HbO(2) tetramer. The extent of produced effect is comparable to those previously observed for Hg(II), Cd, Zn, and Ni in spite of significantly different electronic configurations of the ions in question. Aggregation effect of the silver is ascribed to an interaction of the reactive thiol group sulfur-bound silver atom with the carboxylate residues surrounding the reactive thiol group-bearing cysteine beta93 group of hemoglobin. Mercury ligands, in particular, Tris molecules and OH(-) anions markedly suppress the protein coagulation, thereby supporting the proposed protein aggregation mechanism.