Probing fibronectin-surface interactions: a multitechnique approach

Immunospecific analysis of in vitro and ex vivo surface-immobilized protein complex

Biomaterials used for blood contacting devices are inherently thrombogenic. Antithrombotic agents can be used as surface modifiers on biomaterials to reduce thrombus formation on the surface and to maintain device efficacy. For quality control and to assess the effectiveness of immobilization strategies, it is necessary to quantify the surface-immobilized antithrombotic agent directly. There are limited methods that allow direct quantification on device surfaces such as catheters. In this study, an enzyme immunoassay (EIA) has been developed to measure the density of a synthetic antithrombin-heparin (ATH) covalent complex immobilized on a catheter surface. The distribution of the immobilized ATH was further characterized by an immunohistochemical assay. This analyte-specific EIA is relatively simple and has high throughput, thus providing a tool for quantitative analysis of biomaterial surface modifications. These methods may be further modified to evaluate plasma proteins adsorbed and immobilized on various biomaterial surfaces of complex shapes, with a range of bioactive functionalities, as well as to assess conformational changes of proteins using specific antibodies.

A detailed guideline for the fabrication of single bacterial probes used for atomic force spectroscopy

The atomic force microscope (AFM) evolved as a standard device in modern microbiological research. However, its capability as a sophisticated force sensor is not used to its full capacity. The AFM turns into a unique tool for quantitative adhesion research in bacteriology by using "bacterial probes". Thereby, bacterial probes are AFM cantilevers that provide a single bacterium or a cluster of bacteria as the contact-forming object. We present a step-by-step protocol for preparing bacterial probes, performing force spectroscopy experiments and processing force spectroscopy data. Additionally, we provide a general insight into the field of bacterial cell force spectroscopy.

Mechanical matching between a ligand and receptor

Interactions between ligands and receptors and subsequent "locking" must involve some resistance to unbinding, manifesting itself as an interaction force. At body temperature, spontaneous unbinding will occur, however, external forces are required to accelerate this process. Bearing in mind the potential forces that the receptor-ligand complex is likely to be subjected to in a biological environment, it might be hypothesised that there is some mechanical matching between the receptor and ligand. To test this hypothesis, various receptor and ligand pairs were unfolded in their entirety in order to determine their total unfolding force. In this way, the total force to unfold the protein could be determined, allowing a comparison between ligand and receptor pairs. The interest of this work is to examine the interaction between five proteins and a mica surface by AFM without any modification to preserve the natural elastic properties of the protein molecules during the force measurements. The results showed a mechanical matching between GP120 (ligand) and CD4 (receptor) when analysing the total force required to unfold the same number of domains or events shown by the force distance curves of these proteins.

Quantitative analysis of adsorbed proteins by X-ray photoelectron spectroscopy

Protein adsorption at solid surfaces is central to many phenomena of medical and technological interest. The determination of the amount of protein attached to the surface is a critical measurement performed by using a wide range of methods. X-ray photoelectron spectroscopy (XPS) is able to provide a straightforward quantitative analysis of the amount of protein adsorbed as an overlayer on a material surface. While XPS is commonly employed to assess qualitatively the amount of adsorbed protein, this is usually expressed in terms of the elemental fraction (or at. %) of nitrogen calculated using an assumption of depth homogeneity despite the fact that this does not linearly scale with the amount of protein. In this paper, we have shown that thicknesses derived from XPS data linearly correlated with spectroscopic ellipsometry data on the same samples with a scatter of 10%. A straightforward equation to convert the concentration of nitrogen from XPS into an equivalent thickness of a protein film is presented. We highlight some discrepancies in the absolute thicknesses determined by XPS and ellipsometry on dried films and quartz crystal microbalance on wet films, which appear likely to result from the inclusion of a contribution from water in the latter two techniques.

Fibronectin conformation switch induced by coadsorption with human serum albumin

The dynamic adsorption of human serum albumin (HSA) and plasma fibronectin (Fn) onto hydrophobic poly(hydroxymethylsiloxane) (PHMS) and the structures of adsorbed protein layers from single and binary protein solutions were studied. Spectroscopic ellipsometry (SE) and quartz crystal microbalance with dissipation monitoring (QCM-D) together with atomic force microscopy (AFM) were used to measure the effective mass, thickness, viscoelastic properties, and morphology of the adsorbed protein films. Adsorbed HSA formed a rigid, tightly bound monolayer of deformed protein, and Fn adsorption yielded a thick, very viscoelastic layer that was firmly bound to the substrate. The mixed protein layers obtained from the coadsorption of binary equimolecular HSA-Fn solutions were found to be almost exclusively dominated by Fn molecules. Further sequential adsorption experiments showed little evidence of HSA adsorbed onto the predeposited Fn layer (denoted as Fn ≫ HSA), and Fn was not adsorbed onto predeposited HSA (HSA ≫ Fn). The conformational arrangement of the adsorbed Fn was analyzed in terms of the relative availability of two Fn domains. In particular, (4)F(1)·(5)F(1) binding domains in the Hep I fragment, close to the amino terminal of Fn, were targeted using a polyclonal antifibronectin antibody (anti-Fn), and the RGD sequence in the 10th segment, in the central region of the molecule, was tested by cell culture experiments. The results suggested that coadsorption with HSA induced the Fn switch from an open conformation, with the amino terminal subunit oriented toward the solution, to a close conformation, with the Fn central region oriented toward the solution.

Creating biomimetic surfaces through covalent and oriented binding of proteins

This manuscript describes a novel method for the biofunctionalization of glass surfaces with polyhistidine-tagged proteins. The main innovation of this methodology consists of the covalent binding between the nitrilotriacetic acid (NTA) moiety and the proteins, ensuring not only orientation, but also stability of the recombinant proteins on NTA-covered surfaces. In this work, as C-terminal polyhistidine tagged cadherin extracellular fragments have been used, this methodology guarantees the proper orientation of these proteins, by mimicking their insertion into cell plasma membranes. These biofunctionalized surfaces have been characterized by confocal microscopy, X-ray photoelectron spectroscopy, contact angle, and atomic force microscopy, showing a high density of cadherins on the glass surfaces and the stability of the linkage. The prepared materials exhibited a high tendency to promote cell spreading, demonstrating the functionality of the protein and the high utility of these biomaterials to promote cell adhesion events. Interestingly, differences in the cytoskeleton organization have been observed in cells adhering to surfaces with no cadherins or with nonoriented cadherins, in comparison to surfaces functionalized with well-oriented cadherins. This method, which allows the robust immobilization of polyhistidine tagged proteins due to their covalent binding and with a defined orientation, may also find particular usefulness in the making of protein biochips, for analysis of protein-protein interactions, as well as structural and single-molecule studies.

Atomic force microscopy comes of age

AFM (atomic force microscopy) analysis, both of fixed cells, and live cells in physiological environments, is set to offer a step change in the research of cellular function. With the ability to map cell topography and morphology, provide structural details of surface proteins and their expression patterns and to detect pico-Newton force interactions, AFM represents an exciting addition to the arsenal of the cell biologist. With the explosion of new applications, and the advent of combined instrumentation such as AFM-confocal systems, the biological application of AFM has come of age. The use of AFM in the area of biomedical research has been proposed for some time, and is one where a significant impact could be made. Fixed cell analysis provides qualitative and quantitative subcellular and surface data capable of revealing new biomarkers in medical pathologies. Image height and contrast, surface roughness, fractal, volume and force analysis provide a platform for the multiparameter analysis of cell and protein functions. Here, we review the current status of AFM in the field and discuss the important contribution AFM is poised to make in the understanding of biological systems.

Characterization of biomaterials polar interactions in physiological conditions using liquid-liquid contact angle measurements: relation to fibronectin adsorption

Wettability of biomaterials surfaces and protein-coated substrates is generally characterized with the sessile drop technique using polar and apolar liquids. This procedure is often performed in air, which does not reflect the physiological conditions. In this study, liquid/liquid contact angle measurements were carried out to be closer to cell culture conditions. This technique allowed us to evaluate the polar contribution to the work of adhesion between an aqueous medium and four selected biomaterials widely used in tissue culture applications: bacteriological grade polystyrene (PS), tissue culture polystyrene (tPS), poly(2-hydroxyethyl methacrylate) film (PolyHEMA), and hydroxypropylmethylcellulose-carboxymethylcellulose bi-layered Petri dish (CEL). The contributions of polar interactions were also estimated on the same biomaterials after fibronectin (Fn) adsorption. The quantity of Fn adsorbed on PS, tPS, PolyHEMA and CEL surfaces was evaluated by using the fluorescein-labeled protein. PolyHEMA and CEL were found to be hydrophilic, tPS was moderately hydrophilic and PS was highly hydrophobic. After Fn adsorption on PS and tPS, a significant increase of the surface polar interaction was observed. On PolyHEMA and CEL, no significant adsorption of Fn was detected and the polar interactions remained unchanged. Finally, an inverse correlation between the polarity of the surfaces and the quantity of adsorbed Fn was established.