Quantification of fibronectin and cell surface interactions by AFM

Machine learning-based approach for automated classification of cell and extracellular matrix using nanomechanical properties

Fibrosis characterized by excess accumulation of extracellular matrix (ECM) due to complex cell-ECM interactions plays a pivotal role in pathogenesis. Herein, we employ the pancreatic ductal adenocarcinoma (PDAC) model to investigate dynamic alterations in nanomechanical attributes arising from the cell-ECM interactions to study the fibrosis paradigm. Several segregated studies performed on cellular and ECM components fail to recapitulate their complex collaboration. We utilized collagen and fibronectin, the two most abundant PDAC ECM components, and studied their nanomechanical attributes. We demonstrate alteration in morphology and nanomechanical attributes of collagen with varying thicknesses of collagen gel. Furthermore, by mixing collagen and fibronectin in various stoichiometry, their nanomechanical attributes were observed to vary. To demonstrate the dynamicity and complexity of cell-ECM, we utilized Panc-1 and AsPC-1 cells with or without collagen. We observed that Panc-1 and AsPC-1 cells interact differently with collagen and vice versa, evident from their alteration in nanomechanical properties. Further, using nanomechanics data, we demonstrate that ML-based techniques were able to classify between ECM as well as cell, and cell subtypes in the presence/absence of collagen with higher accuracy. This work demonstrates a promising avenue to explore other ECM components facilitating deeper insights into tumor microenvironment and fibrosis paradigm.

Defining Integrin Tension Required for Chemotaxis of Metastatic Breast Cancer Cells in Confinement

Cancer metastasis is affected by chemical factors and physical cues. From cell adhesion to migration, mechanical tension applied to integrin expresses on the cell membrane and physical confinement significantly regulates cancer cell behaviors. Despite the physical interplay between integrins in cells and ligands in the tumor microenvironment, quantitative analysis of integrin tension during cancer cell migration in microconfined spaces remains elusive owing to the limited experimental tools. Herein, a platform termed microconfinement tension gauge tether to monitor spatial integrin tension with single-molecule precision by analyzing the epithelial-growth-factor-induced chemotaxis of metastatic human breast cancer cells in microfluidic channels is developed. The results reveal that the metastatic cancer cells exert the strongest integrin tension in the range of 54-100 pN at the leading edges of cells during chemokinetic migration on a planar surface, while the cells exert the strongest integrin tension exceeding 100 pN at the cell rear when entering microconfinement. Further analysis demonstrates that cells undergo mesenchymal migration under high integrin tension and less confinement, which is converted to amoeboid migration under low integrin tension or high confinement. In summary, the results identify a basic mechanism underlying the mechanical interactions between integrin tension and microenvironment that determines cancer invasion and metastasis.

Evaluation of intercellular cross-linking abilities correlated with cytotoxicities of bispecific antibodies with domain rearrangements using AFM force-sensing

Bispecific antibodies (bsAbs) are a promising engineered antibody format; thus, technologies for the fabrication and evaluation of functional bsAbs are attracting increasing attention. Here, based on atomic force microscopy (AFM) force-sensing integrated with a metal cup-attached AFM chip (cup-chip) to ensure efficient capture of a target cell on a cantilever, we established a novel method for measuring cross-linking ability that is correlated with the cytotoxicities of bsAbs targeting two cells. We previously reported that domain rearrangements of bsAbs affected their cytotoxicities; however, no differences in cross-linking ability for soluble antigens were observed by surface plasmon resonance. We predicted that there would be differences in molecular configurations to avoid steric hindrance in the cross-linking of the two whole target cells. A picked-up T cell lymphoma cell on the cantilever using a cup-chip was moved to approach a cancer cell adhered to a dish, and force-curve measurements were performed. The resulting forces mediated by the cross-linking of bsAbs with different domain orders were well-correlated with their cytotoxicities. The AFM force-sensing method established herein may reflect steric hindrance of intercellular cross-linking, and thus has the potential to evaluate the net function of bsAbs and contribute to the generation of functional bsAbs.

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.

Single-Cell Probe Force Studies to Identify Sox2 Overexpression-Promoted Cell Adhesion in MCF7 Breast Cancer Cells

The replacement of the cantilever tip by a living cell in Atomic Force Microscopy (AFM) experiments permits the direct quantification of cell-substrate and cell-cell adhesion forces. This single-cell probe force measurement technique, when complemented by microscopy, allows controlled manipulation of the cell with defined location at the area of interest. In this work, a setup based on two glass half-slides, a non-fouling one with bacterial S-layer protein SbpA from L. sphaericus CMM 2177 and the second with a fibronectin layer, has been employed to measure the adhesion of MCF7 breast cancer cells to fibronectin films (using SbpA as control) and to other cells (symmetric vs. asymmetric systems). The measurements aimed to characterize and compare the adhesion capacities of parental cells and cells overexpressing the embryonic transcription factor Sox2, which have a higher capacity for invasion and are more resistant to endocrine therapy in vivo. Together with the use of fluorescence techniques (epifluorescence, Total Internal Fluorescence Microscopy (TIRF)), the visualization of vinculin and actin distribution in cells in contact with fibronectin surfaces is enabled, facilitating the monitoring and quantification of the formation of adhesion complexes. These findings demonstrate the strength of this combined approach to assess and compare the adhesion properties of cell lines and to illustrate the heterogeneity of adhesive strength found in breast cancer cells.

Mechanical Property Changes in Breast Cancer Cells Induced by Stimulation with Macrophage Secretions in Vitro

The contribution of secretions from tumor-associated macrophage (TAM)-like cells to the stimulation of mechanical property changes in murine breast cancer cells was studied using an in vitro model system. A murine breast cancer cell line (FP10SC2) was stimulated by adding macrophage (J774.2) cultivation medium containing stimulation molecules secreted from the macrophages, and changes in mechanical properties were compared before and after stimulation. As a result, cell elasticity decreased, degradation ability of the extracellular matrix increased, and the expression of plakoglobin was upregulated. These results indicate that cancer cell malignancy is upregulated by this stimulation. Moreover, changes in intercellular adhesion strengths between pairs of cancer cells were measured before and after stimulation using atomic force microscopy (AFM). The maximum force required to separate cells was increased by stimulation with the secreted factors. These results indicate the possibility that TAMs cause changes in the mechanical properties of cancer cells in tumor microenvironments, and in vitro measurements of mechanical property changes in cancer cells will be useful to study interactions between cells in tumor microenvironments.

Enhanced Adhesion and Cohesion of Bioinspired Dry/Wet Pressure-Sensitive Adhesives

The byssus-mediated adhesion of marine mussels is a widely mimicked system for robust adhesion in both dry and wet conditions. Mussel holdfasts are fabricated from proteins that contain a significant amount of the unique catecholic amino acid dihydroxyphenylalanine, which plays a key role in enhancing interfacial adhesion to organic and inorganic marine surfaces and contributes to cohesive strength of the holdfast. In this work, pressure-sensitive adhesives (PSAs) were synthesized by copolymerization of dopamine methacrylamide (DMA) with common PSA monomers, butyl acrylate and acrylic acid, with careful attention paid to the effects of catechol on adhesive and cohesive properties. A combination of microscopic and macroscopic adhesion assays was used to study the effect of catechol on adhesion performance of acrylic PSAs. Addition of only 5% DMA to a conventional PSA copolymer containing butyl acrylate and acrylic acid resulted in 6-fold and 2.5-fold increases in work required to separate the PSA from silica and polystyrene, respectively, and a large increase in 180° peel adhesion against stainless steel after 24 h storage in both ambient and underwater conditions. Moreover, the holding power of the catechol PSAs on both steel and high-density polyethylene under shear load continuously increased as a function of catechol concentration, up to a maximum of 10% DMA. We also observed stark increases in shear and peel adhesion for the catecholic adhesives over PSAs with noncatecholic aromatic motifs, further underlining the benefits of catechols in PSAs. Overall, catechol PSAs perform extremely well on polar and metallic surfaces. The advantage of incorporating catechols in PSA formulations, however, is less straightforward for peel adhesion in nonpolar, organic substrates and tackiness of the PSAs.

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.

Increasing throughput of AFM-based single cell adhesion measurements through multisubstrate surfaces

Mammalian cells regulate adhesion by expressing and regulating a diverse array of cell adhesion molecules on their cell surfaces. Since different cell types express distinct sets of cell adhesion molecules, substrate-specific adhesion is cell type- and condition-dependent. Single-cell force spectroscopy is used to quantify the contribution of cell adhesion molecules to adhesion of cells to specific substrates at both the cell and single molecule level. However, the low throughput of single-cell adhesion experiments greatly limits the number of substrates that can be examined. In order to overcome this limitation, segmented polydimethylsiloxane (PDMS) masks were developed, allowing the measurement of cell adhesion to multiple substrates. To verify the utility of the masks, the adhesion of four different cell lines, HeLa (Kyoto), prostate cancer (PC), mouse kidney fibroblast and MDCK, to three extracellular matrix proteins, fibronectin, collagen I and laminin 332, was examined. The adhesion of each cell line to different matrix proteins was found to be distinct; no two cell lines adhered equally to each of the proteins. The PDMS masks improved the throughput limitation of single-cell force spectroscopy and allowed for experiments that previously were not feasible. Since the masks are economical and versatile, they can aid in the improvement of various assays.

Analysis of affinity maps of membrane proteins on individual human embryonic stem cells

The heterogeneity found in many cell types has greatly inspired research in single-cell gene and protein profiling for discovering the origin of heterogeneity and its role in cell fate decisions. Among the existing techniques to probe heterogeneity, atomic force microscopy (AFM) utilizes an antibody/ligand-modified tip to explore the distribution of a target membrane protein on individual cells in their native environment. In this paper, we establish a practical model to analyze the data systematically, and attempt the quantification of membrane protein abundance on single cells by taking account issues, such as the level of nonspecific interaction, the probe resolution, and the reproducibility of detecting protein distribution. We demonstrated the application in examining the heterogeneous distribution and the local protein abundance of TRA-1-81 antigen on human embryonic stem (hES) cells at the subcellular level. Heterogeneity in TRA-1-81 expression was also detected at the single cell level, suggesting the presence of subpopulation cells within an undifferentiated hES cell colony. The method provides a platform to unveiling the correlation between heterogeneity of membrane proteins and cell development in a complex cell community.

Control of osteogenic differentiation and mineralization of human mesenchymal stem cells on composite nanofibers containing poly[lactic-co-(glycolic acid)] and hydroxyapatite

We fabricated composite fibrous scaffolds from blends of poly(lactide-co-glycolide) (PLGA) and nano-sized hydroxyapatite (HA) via electrospinning. SEM-EDX and AFM analysis demonstrated that HA was homogeneously dispersed in the nanofibers, and the roughness increased along with the amount of incorporated HA. When hMSCs were cultured on these PLGA/HA composite nanofibers, we found that incorporation of HA on the nanofibers did not affect cell viability whereas increased ALP activity and expression of osteogenic genes as well as the calcium mineralization of hMSCs. Our results indicate that the composite nanofibers can be offered as a potential bone regenerative biomaterial for stem cell based therapies.

A uniaxial bioMEMS device for imaging single cell response during quantitative force-displacement measurements

A microfabricated device has been developed for imaging of a single, adherent cell while quantifying force under an applied displacement. The device works in a fashion similar to that of a displacement-controlled uniaxial tensile machine. The device was calibrated using a tipless atomic force microscope (AFM) cantilever and shows excellent agreement with the calculated spring constant. A step input was applied to a single, adherent fibroblast cell and the viscoelastic response was characterized with a mechanical model. The adherent fibroblast was imaged by use of epifluorescence and phase contrast techniques.

Distribution of olfactory marker protein on a tissue section of vomeronasal organ measured by AFM

Distribution of olfactory marker protein (OMP) on a tissue section of vomeronasal organ (VNO) was successfully measured by atomic force microscopy (AFM). Anti-OMP antibodies were covalently crosslinked with the tip of the AFM and were used as a probe to observe the distribution of OMP on a tissue section. First, force measurements were performed using a glass surface on which OMP was covalently immobilized to verify the success of tip modification. Clear differences of interaction forces were observed between a specific pair and the control experiments, indicating that the tip preparation succeeded. Next, distributions of OMP on the tissue section were observed by AFM and were compared with immunohistochemical observations. For large scale observation, a microbead was used as a probe in the AFM measurements. The results of the AFM measurements were well overlapped with that of immunohistochemistry, confirming the reliability of our method. A mapping of the AFM measurement with high resolution was also successfully obtained, which showed an advantage of the application of the AFM measurement in analysis of proteins on the tissue section.

Cell adhesion properties on photochemically functionalized diamond

The biocompatibility of diamond was investigated with a view toward correlating surface chemistry and topography with cellular adhesion and growth. The adhesion properties of normal human dermal fibroblast (NHDF) cells on microcrystalline and ultrananocrystalline diamond (UNCD) surfaces were measured using atomic force microscopy. Cell adhesion forces increased by several times on the hydrogenated diamond surfaces after UV irradiation of the surfaces in air or after functionalization with undecylenic acid. A direct correlation between initial cell adhesion forces and the subsequent cell growth was observed. Cell adhesion forces were observed to be strongest on UV-treated UNCD, and cell growth experiments showed that UNCD was intrinsically more biocompatible than microcrystalline diamond surfaces. The surface carboxylic acid groups on the functionalized diamond surface provide tethering sites for laminin to support the growth of neuron cells. Finally, using capillary injection, a surface gradient of polyethylene glycol could be assembled on top of the diamond surface for the construction of a cell gradient.

Cell dynamic adhesion and elastic properties probed with cylindrical atomic force microscopy cantilever tips

Cell adhesion is required for essential biological functions such as migration, tissue formation and wound healing, and it is mediated by individual molecules that bind specifically to ligands on other cells or on the extracellular matrix. Atomic force microscopy (AFM) has been successfully used to measure cell adhesion at both single molecule and whole cell levels. However, the measurement of inherent cell adhesion properties requires a constant cell-probe contact area during indentation, a requirement which is not fulfilled in common pyramidal or spherical AFM tips. We developed a procedure using focused ion beam (FIB) technology by which we modified silicon pyramidal AFM cantilever tips to obtain flat-ended cylindrical tips with a constant and known area of contact. The tips were validated on elastic gels and living cells. Cylindrical tips showed a fairly linear force-indentation behaviour on both gels and cells for indentations >200 nm. Cylindrical tips coated with ligands were used to quantify inherent dynamic cell adhesion and elastic properties. Force, work of adhesion and elasticity showed a marked dynamic response. In contrast, the deformation applied to the cells before rupture was fairly constant within the probed dynamic range. Taken together, these results suggest that the dynamic adhesion strength is counterbalanced by the dynamic elastic response to keep a constant cell deformation regardless of the applied pulling rate.

A uniaxial bioMEMS device for quantitative force-displacement measurements

There is a need for experimental techniques that allow the simultaneous imaging of cellular cystoskeletal components with quantitative force measurements on single cells. A bioMEMS device has been developed for the application of strain to a single cell while simultaneously quantifying its force response. The prototype device presented here allows the mechanical study of a single, adherent cell in vitro. The device works in a fashion similar to a displacement-controlled uniaxial tensile machine. The device is calibrated using an AFM cantilever and shows excellent agreement with the calculated spring constant. The device is demonstrated on a single fibroblast. The force response of the cell is seen to be linear until the onset of de-adhesion with the de-adhesion from the cell platform occurring at a force of approximately 1500 nN.

An improved hydrophilicity via electrospinning for enhanced cell attachment and proliferation

The wettability of electrospun poly(epsilon-caprolactone) (PCL) mats was improved by co-electrospinning with poly(vinyl alcohol) (PVA), by double-spinneret electrospinning method. The improved hydrophilicity of the hybrid PCL/PVA mats was confirmed by water contact angle measurement. The in vitro cell attachment on the hydrophobic PCL and hydrophilically modified PCL/PVA mats was compared by culture studies using human prostate epithelial cells (HPECs). The stability of water-soluble PVA component in the electrospun PCL/PVA mats was checked by thermogravimetric analysis and intensity of fluorescence material after immersion in water for 7 days. The images from scanning electron microscopy, field emission scanning electron microscopy, and optical microscopy showed that the attachment and proliferation rate of HPECs were improved by introducing PVA into the electrospun PCL mats.

Quantification of the number of EP3 receptors on a living CHO cell surface by the AFM

The distribution of EP3 receptors on a living cell surface was quantitatively studied by atomic force microscopy (AFM). Green fluorescent protein (GFP) was introduced to the extracellular region of the EP3 receptor on a CHO cell. A microbead was used as a probe to ensure certain contact area, whose surface was coated with anti-GFP antibody. The interactions between the antibodies and GFP molecules on the cell surface were recorded to observe the distribution of the receptors. The result indicated that EP3 receptors were distributed on the CHO cell surface not uniformly but in small patches coincident with immunohistochemical observation. Repeated measurements on the same area of cell surface gave confirmation that it was unlikely that the receptors were extracted from the cell membrane during the experiments. The measurement of single molecular interaction between GFP and the anti-GFP antibody was succeeded on the cell surface using compression-free force spectroscopy. The value of separation work required to break a single molecular pair was estimated to be about 1.5 x 10(-18)J. The number of EP3 receptor on the CHO cell surface was estimated using this value to be about 1 x 10(4) under the assumption that the area of the cell surface was about 5,000 microm(2). These results indicated that the number of receptors on a living cell surface could be quantified through the force measurement by the AFM.

Quantitative analysis of the number of antigens immobilized on a glass surface by AFM

To develop force measurements using an atomic force microscope (AFM) in a quantitative manner, it is necessary to estimate the number density of target molecules on a sample surface, and for this, the sensitivity of detection should be known. In this study, the AFM was used as a mechanical detector and an antigen and its antibody were used as a model to evaluate the sensitivity of detection. Antigens were immobilized on a glass surface and number density was estimated by monitoring optical absorbance due to product formation by the reaction of crosslinkers. The concentration of antigen was controlled by mixing control peptides. A microbead was used as a probe and antibodies were immobilized on the bead. AFM force measurements were then made for a range of number densities in the order of 10-10(6) antigen molecules per square micrometer of surface and were compared to evaluate the sensitivity of detection. Our result establishes the reliability of estimating a number of molecules like receptors on the cell surface, and indicates that the AFM is useful as a mechanical detector with high sensitivity.

Mapping of the receptor-associated protein (RAP) binding proteins on living fibroblast cells using an atomic force microscope

The distribution of the receptor-associated protein (RAP) binding protein and the adhesion forces between RAP and its binding protein on living fibroblast cells were examined using an atomic force microscope (AFM). The distribution of RAP binding protein was obtained on 256 (16x16) locations in 2x2 micro m sections over the surface of living cells. The adhesion forces between RAP and the binding protein were measured with an AFM tip functionalized with RAP. In the presence of RAP in the scanning solution, the number of force curves with large adhesion force decreased. These results indicate that the adhesive forces observed here represent specific binding between RAP and the binding protein. This method will be a useful application of AFM to examine receptors on cell surfaces in high resolution.

Quantification of cell adhesion force with AFM: distribution of vitronectin receptors on a living MC3T3-E1 cell

Distribution of vitronectin (VN) receptors on a living murine osteoblastic cell was successfully measured by atomic force microscopy (AFM). First, the distribution of the integrin beta(5) subunit which constitutes a part of the VN receptor on the cell was confirmed by conventional immunohistochemistry after fixing the cell. To visualize the distribution of the receptor on a living cell by an independent and potentially a more quantitative method, the AFM was used with a microbead attached to the cantilever tip to increase the area of contact and VN was immobilized on the microbead. Force measurements were then performed over a large area of a living murine osteoblastic cell using the microbead covered with VN.