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

Non-coating method for non-adherent cell culture using high molecular weight dextran sulfate and bovine serum albumin

Non-adherent cell culture surface has been widely used for producing cell spheroids and cell aggregates. The purpose of this study was to formulate a new method for non-adherent cell culture without coating or surface-modification that has been needed. We found that high-molecular-weight dextran sulfate (DS) and bovine serum albumin (BSA) synergistically prevented cell adhesion in media supplemented with no or low serum. This method worked on tissue culture-treated polystyrene surfaces as well as on commercially available low-attachment- and untreated polystyrene surfaces. Further investigation revealed that BSA may mediate the adsorption of DS to the surface. In addition, as the adsorption of fluorescently labeled fibronectin was inhibited by BSA alone, it appears that protein adsorption and cell adhesion do not always correlate. Finally, we demonstrated the successful formation of HepG2 spheroids and cardiomyocyte aggregates using this method. In conclusion, cell adhesion can be effectively suppressed by simply adding DS and BSA to the culture medium without coating or surface modification, and it may be useful for generating cell spheroids and aggregates.

Influence of Nivolumab for Intercellular Adhesion Force between a T Cell and a Cancer Cell Evaluated by AFM Force Spectroscopy

The influence of nivolumab on intercellular adhesion forces between T cells and cancer cells was evaluated quantitatively using atomic force microscopy (AFM). Two model T cells, one expressing high levels of programmed cell death protein 1 (PD-1) (PD-1^high Jurkat) and the other with low PD-1 expression levels (PD-1^low Jurkat), were analyzed. In addition, two model cancer cells, one expressing programmed death-ligand 1 (PD-L1) on the cell surface (PC-9, PD-L1⁺) and the other without PD-L1 (MCF-7, PD-L1⁻), were also used. A T cell was attached to the apex of the AFM cantilever using a cup-attached AFM chip, and the intercellular adhesion forces were measured. Although PD-1^high T cells adhered strongly to PD-L1⁺ cancer cells, the adhesion force was smaller than that with PD-L1⁻ cancer cells. After the treatment of PD-1^high T cells with nivolumab, the adhesion force with PD-L1⁺ cancer cells increased to a similar level as with PD-L1⁻ cancer cells. These results can be explained by nivolumab influencing the upregulation of the adhesion ability of PD-1^high T cells with PD-L1⁺ cancer cells. These results were obtained by measuring intercellular adhesion forces quantitatively, indicating the usefulness of single-cell AFM analysis.

Quantitative measurements of intercellular adhesion between a macrophage and cancer cells using a cup-attached AFM chip

Intercellular adhesion between a macrophage and cancer cells was quantitatively measured using atomic force microscopy (AFM). Cup-shaped metal hemispheres were fabricated using polystyrene particles as a template, and a cup was attached to the apex of the AFM cantilever. The cup-attached AFM chip (cup-chip) approached a murine macrophage cell (J774.2), the cell was captured on the inner concave of the cup, and picked up by withdrawing the cup-chip from the substrate. The cell-attached chip was advanced towards a murine breast cancer cell (FP10SC2), and intercellular adhesion between the two cells was quantitatively measured. To compare cell adhesion strength, the work required to separate two adhered cells (separation work) was used as a parameter. Separation work was almost 2-fold larger between a J774.2 cell and FP10SC2 cell than between J774.2 cell and three additional different cancer cells (4T1E, MAT-LyLu, and U-2OS), two FP10SC2 cells, or two J774.2 cells. FP10SC2 was established from 4T1E as a highly metastatic cell line, indicates separation work increased as the malignancy of cancer cells became higher. One possible explanation of the strong adhesion of macrophages to cancer cells observed in this study is that the measurement condition mimicked the microenvironment of tumor-associated macrophages (TAMs) in vivo, and J774.2 cells strongly expressed CD204, which is a marker of TAMs. The results of the present study, which were obtained by measuring cell adhesion strength quantitatively, indicate that the fabricated cup-chip is a useful tool for measuring intercellular adhesion easily and quantitatively.

Colloid-probe AFM studies of the surface functionality and adsorbed proteins on binary colloidal crystal layers

We demonstrate the applicability of colloid-probe AFM to detect different surface chemistries on binary colloidal crystal layers of different chemical and protein patterns.

Colloid-probe AFM studies of the interaction forces of proteins adsorbed on colloidal crystals

In recent years, colloid-probe AFM has been used to measure the direct interaction forces between colloidal particles of different size or surface functionality in aqueous media, as one can study different forces in symmerical systems (i.e., sphere-sphere geometry). The present study investigates the interaction between protein coatings on colloid probes and hydrophilic surfaces decorated with hexagonally close packed single particle layers that are either uncoated or coated with proteins. Controlled solvent evaporation from aqueous suspensions of colloidal particles (coated with or without lysozyme and albumin) produces single layers of close-packed colloidal crystals over large areas on a solid support. The measurements have been carried out in an aqueous medium at different salt concentrations and pH values. The results show changes in the interaction forces as the surface charge of the unmodified or modified particles, and ionic strength or pH of the solution is altered. At high ionic strength or pH, electrostatic interactions are screened, and a strong repulsive force at short separation below 5 nm dominates, suggesting structural changes in the absorbed protein layer on the particles. We also study the force of adhesion, which decreases with an increment in the salt concentration, and the interaction between two different proteins indicating a repulsive interaction on approach and adhesion on retraction.

Morphology and mechanics of chondroid cells from human adipose-derived Stem cells detected by atomic force microscopy

Chondroid cell from human adipose-derived stem cells (ADSCs) has emerged as an alternative treatment option for articular cartilage defects. Herein, we successfully compared ADSCs, normal chondrocytes, and chondroid cells. The comparative study of ADSCs and chondroid cells revealed type II collagen (COL II) and glycosaminoglycans expression of chondroid cells were similar to those in normal chondrocytes, and much higher than ADSCs. Using atomic force microscope (AFM) and laser confocal scanning microscopy (LCSM), we compared the differences in morphology, mechanical properties, and F-actin distribution between chondroid cells and normal chondrocytes. Our results showed no differences observed between these two types of cells regarding morphology, stiffness, and F-actin distribution. However, found that the adhesion force in chondroid cells was lower than that in normal chondrocytes. Taken together, our AFM and LCSM analyses suggest that the lower adhesion force in chondroid cells may contribute to the dedifferentiation of ADSC-derived chondroid cells. Future examination of surface adhesion force-related protein expression will likely provide new insight into the molecular mechanisms underlying the dedifferentiation of ADSC-derived chondroid cells.

Measurement of interaction forces between fibrinogen coated probes and mica surface with the atomic force microscope: The pH and ionic strength effect

The study of protein-surface interactions is of great significance in the design of biomaterials and the evaluation of molecular processes in tissue engineering. The authors have used atomic force microscopy (AFM) to directly measure the force of attraction/adhesion of fibrinogen coated tips to mica surfaces and reveal the effect of the surrounding solution pH and ionic strength on this interaction. Silica colloid spheres were attached to the AFM cantilevers and, after plasma deposition of poly(acrylic acid), fibrinogen molecules were covalently bound on them with the help of the cross-linker 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) in the presence of N-hydroxysulfosuccinimide (sulfo-NHS). The measurements suggest that fibrinogen adsorption is controlled by the screening of electrostatic repulsion as the salt concentration increases from 15 to 150 mM, whereas at higher ionic strength (500 mM) the hydration forces and the compact molecular conformation become crucial, restricting adsorption. The protein attraction to the surface increases at the isoelectric point of fibrinogen (pH 5.8), compared with the physiological pH. At pH 3.5, apart from fibrinogen attraction to the surface, evidence of fibrinogen conformational changes is observed, as the pH and the ionic strength are set back and forth, and these changes may account for fibrinogen aggregation in the protein solution at this pH.

pH and ionic strength effect on single fibrinogen molecule adsorption on mica studied with AFM

Although several investigations have been reported on the effect of pH or ionic strength on protein adsorption, most of them have been carried out with protein monolayers and not with single molecules. We have used atomic force microscopy to image, in phosphate buffer, single fibrinogen molecules adsorbed on mica and compare the surface coverage at variable pH (7.4, 5.8, 3.5) or ionic strength (15, 150, 500 mM) conditions. The images obtained and the statistical analysis of the surface coverage indicate adsorption enhancement at the IEP of fibrinogen (pH 5.8) and minimum adsorption at pH 3.5. On the other hand, more protein was adsorbed when the salt concentration of the buffer at pH 7.4 was increased from 15 to 150 mM. However, further increase of salt concentration up to 500 mM resulted in decreased adsorption. To confirm the aforementioned results an approaching bare Si(3)N(4) tip was used as an electrostatic analogue to a protein molecule and interaction force curves between it and the substrate were recorded. The results were in consistence with the double layer theory which justifies the screening of electrostatic repulsion as the salt concentration increases.

Colloid probe AFM investigation of interactions between fibrinogen and PEG-like plasma polymer surfaces

Interaction forces between surfaces designed to be protein resistant and fibrinogen (Fg) were investigated in phosphate-buffered saline with colloid probe atomic force microscopy. The surfaces of the silica probes were coated with a layer of fibrinogen molecules by adsorption from the buffer. The technique of low-power, pulsed AC plasma polymerization was used to make poly(ethylene glycol) (PEG)-like coatings on poly(ethylene teraphthalate) by using diethylene glycol vinyl ether as the monomer gas. The degree of PEG-like nature of the films was controlled by use of a different effective plasma power in the chamber for each coating, ranging from 0.6 to 3.6 W. This produced a series of thin films with a different number of ether carbons, as assessed by X-ray photoelectron spectroscopy. The interaction force measurements are discussed in relation to trends observed in the reduction of fibrinogen adsorption, as determined quantitatively by (125)I radio-labeling. The plasma polymer coatings with the greatest protein-repelling properties were the most PEG-like in nature and showed the strongest repulsion in interaction force measurements with the fibrinogen-coated probe. Once forced into contact, all the surfaces showed increased adhesion with the protein layer on the probe, and the strength and extension length of adhesion was dependent on both the applied load and the plasma polymer surface chemistry. When the medium was changed from buffer to water, the adhesion after contact was eliminated and only appeared at much higher loads. This indicates that the structure of the fibrinogen molecules on the probe is changed from an extended conformation in buffer to a flat conformation in water, with the former state allowing for stronger interaction with the polymer chains on the surface. These experiments underline the utility of aqueous surface force measurements toward understanding protein-surface interactions, and developing nonfouling surfaces that confer a steric barrier against protein adsorption.