Ligands on the string: single-molecule AFM studies on the interaction of antibodies and substrates with the Na+-glucose co-transporter SGLT1 in living cells

High-Yield Characterization of Single Molecule Interactions with DeepTipTM Atomic Force Microscopy Probes

Single molecule interactions between biotin and streptavidin were characterized with functionalized DeepTip^TM probes and used as a model system to develop a comprehensive methodology for the high-yield identification and analysis of single molecular events. The procedure comprises the covalent binding of the target molecule to a surface and of the sensing molecule to the DeepTip^TM probe, so that the interaction between both chemical species can be characterized by obtaining force-displacement curves in an atomic force microscope. It is shown that molecular resolution is consistently attained with a percentage of successful events higher than 90% of the total number of recorded curves, and a very low level of unspecific interactions. The combination of both features is a clear indication of the robustness and versatility of the proposed methodology.

Molecular Recognition of Surface Trans-Sialidases in Extracellular Vesicles of the Parasite Trypanosoma cruzi Using Atomic Force Microscopy (AFM)

Trans-sialidases (TS) are important constitutive macromolecules of the secretome present on the surface of Trypanosoma cruzi (T. cruzi) that play a central role as a virulence factor in Chagas disease. These enzymes have been related to infectivity, escape from immune surveillance and pathogenesis exhibited by this protozoan parasite. In this work, atomic force microscopy (AFM)-based single molecule-force spectroscopy is implemented as a suitable technique for the detection and location of functional TS on the surface of extracellular vesicles (EVs) released by tissue-culture cell-derived trypomastigotes (Ex-TcT). For that purpose, AFM cantilevers with functionalized tips bearing the anti-TS monoclonal antibody mAb 39 as a sense biomolecule are engineered using a covalent chemical ligation based on vinyl sulfonate click chemistry; a reliable, simple and efficient methodology for the molecular recognition of TS using the antibody-antigen interaction. Measurements of the breakdown forces between anti-TS mAb 39 antibodies and EVs performed to elucidate adhesion and forces involved in the recognition events demonstrate that EVs isolated from tissue-culture cell-derived trypomastigotes of T. cruzi are enriched in TS. Additionally, a mapping of the TS binding sites with submicrometer-scale resolution is provided. This work represents the first AFM-based molecular recognition study of Ex-TcT using an antibody-tethered AFM probe.

Atomic Force Microscopy for Tumor Research at Cell and Molecule Levels

Tumors have posed a serious threat to human life and health. Researchers can determine whether or not cells are cancerous, whether the cancer cells are invasive or metastatic, and what the effects of drugs are on cancer cells by the physical properties such as hardness, adhesion, and Young's modulus. The atomic force microscope (AFM) has emerged as a key important tool for biomechanics research on tumor cells due to its ability to image and collect force spectroscopy information of biological samples with nano-level spatial resolution and under near-physiological conditions. This article reviews the existing results of the study of cancer cells with AFM. The main foci are the operating principle of AFM and research advances in mechanical property measurement, ultra-microtopography, and molecular recognition of tumor cells, which allows us to outline what we do know it in a systematic way and to summarize and to discuss future directions.

Surface-Confined Electrochemiluminescence Microscopy of Cell Membranes

Herein is reported a surface-confined microscopy based on electrochemiluminescence (ECL) that allows to image the plasma membrane of single cells at the interface with an electrode. By analyzing photoluminescence (PL), ECL and AFM images of mammalian CHO cells, we demonstrate that, in contrast to the wide-field fluorescence, ECL emission is confined to the immediate vicinity of the electrode surface and only the basal membrane of the cell becomes luminescent. The resulting ECL microscopy reveals details that are not resolved by classic fluorescence microscopy, without any light irradiation and specific setup. The thickness of the ECL-emitting regions is ∼500 nm due to the unique ECL mechanism that involves short-lifetime electrogenerated radicals. In addition, the reported ECL microscopy is a dynamic technique that reflects the transport properties through the cell membranes and not only the specific labeling of the membranes. Finally, disposable transparent carbon nanotube (CNT)-based electrodes inkjet-printed on classic microscope glass coverslips were used to image cells in both reflection and transmission configurations. Therefore, our approach opens new avenues for ECL as a surface-confined microscopy to develop single cell assays and to image the dynamics of biological entities in cells or in membranes.

Nanoscale imaging and force probing of biomolecular systems using atomic force microscopy: from single molecules to living cells

Due to the lack of adequate tools for observation, native molecular behaviors at the nanoscale have been poorly understood. The advent of atomic force microscopy (AFM) provides an exciting instrument for investigating physiological processes on individual living cells with molecular resolution, which attracts the attention of worldwide researchers. In the past few decades, AFM has been widely utilized to investigate molecular activities on diverse biological interfaces, and the performances and functions of AFM have also been continuously improved, greatly improving our understanding of the behaviors of single molecules in action and demonstrating the important role of AFM in addressing biological issues with unprecedented spatiotemporal resolution. In this article, we review the related techniques and recent progress about applying AFM to characterize biomolecular systems in situ from single molecules to living cells. The challenges and future directions are also discussed.

Single Cell Electrochemiluminescence Imaging: From the Proof-of-Concept to Disposable Device-Based Analysis

We report here the development of coreactant-based electrogenerated chemiluminescence (ECL) as a surface-confined microscopy to image single cells and their membrane proteins. Labeling the entire cell membrane allows one to demonstrate that, by contrast with fluorescence, ECL emission is only detected from fluorophores located in the immediate vicinity of the electrode surface (i.e., 1-2 μm). Then, to present the potential diagnostic applications of our approach, we selected carbon nanotubes (CNT)-based inkjet-printed disposable electrodes for the direct ECL imaging of a labeled plasma receptor overexpressed on tumor cells. The ECL fluorophore was linked to an antibody and enabled to localize the ECL generation on the cancer cell membrane in close proximity to the electrode surface. Such a result is intrinsically associated with the unique ECL mechanism and is rationalized by considering the limited lifetimes of the electrogenerated coreactant radicals. The electrochemical stimulus used for luminescence generation does not suffer from background signals, such as the typical autofluorescence of biological samples. The presented surface-confined ECL microscopy should find promising applications in ultrasensitive single cell imaging assays.

The structure and function of cell membranes studied by atomic force microscopy

The cell membrane, involved in almost all communications of cells and surrounding matrix, is one of the most complicated components of cells. Lack of suitable methods for the detection of cell membranes in vivo has sparked debates on the biochemical composition and structure of cell membranes over half a century. The development of single molecule techniques, such as AFM, SMFS, and TREC, provides a versatile platform for imaging and manipulating cell membranes in biological relevant environments. Here, we discuss the latest developments in AFM and the progress made in cell membrane research. In particular, we highlight novel structure models and dynamic processes, including the mechanical properties of the cell membranes.

Imaging and Force Recognition of Single Molecular Behaviors Using Atomic Force Microscopy

The advent of atomic force microscopy (AFM) has provided a powerful tool for investigating the behaviors of single native biological molecules under physiological conditions. AFM can not only image the conformational changes of single biological molecules at work with sub-nanometer resolution, but also sense the specific interactions of individual molecular pair with piconewton force sensitivity. In the past decade, the performance of AFM has been greatly improved, which makes it widely used in biology to address diverse biomedical issues. Characterizing the behaviors of single molecules by AFM provides considerable novel insights into the underlying mechanisms guiding life activities, contributing much to cell and molecular biology. In this article, we review the recent developments of AFM studies in single-molecule assay. The related techniques involved in AFM single-molecule assay were firstly presented, and then the progress in several aspects (including molecular imaging, molecular mechanics, molecular recognition, and molecular activities on cell surface) was summarized. The challenges and future directions were also discussed.

The Human Sodium-Glucose Cotransporter (hSGLT1) Is a Disulfide-Bridged Homodimer with a Re-Entrant C-Terminal Loop

Na-coupled cotransporters are proteins that use the trans-membrane electrochemical gradient of Na to activate the transport of a second solute. The sodium-glucose cotransporter 1 (SGLT1) constitutes a well-studied prototype of this transport mechanism but essential molecular characteristics, namely its quaternary structure and the exact arrangement of the C-terminal transmembrane segments, are still debated. After expression in Xenopus oocytes, human SGLT1 molecules (hSGLT1) were labelled on an externally accessible cysteine residue with a thiol-reactive fluorophore (tetramethylrhodamine-C5-maleimide, TMR). Addition of dipicrylamine (DPA, a negatively-charged amphiphatic fluorescence “quencher”) to the fluorescently-labelled oocytes is used to quench the fluorescence originating from hSGLT1 in a voltage-dependent manner. Using this arrangement with a cysteine residue introduced at position 624 in the loop between transmembrane segments 12 and 13, the voltage-dependent fluorescence signal clearly indicated that this portion of the 12–13 loop is located on the external side of the membrane. As the 12–13 loop begins on the intracellular side of the membrane, this suggests that the 12–13 loop is re-entrant. Using fluorescence resonance energy transfer (FRET), we observed that different hSGLT1 molecules are within molecular distances from each other suggesting a multimeric complex arrangement. In agreement with this conclusion, a western blot analysis showed that hSGLT1 migrates as either a monomer or a dimer in reducing and non-reducing conditions, respectively. A systematic mutational study of endogenous cysteine residues in hSGLT1 showed that a disulfide bridge is formed between the C355 residues of two neighbouring hSGLT1 molecules. It is concluded that, 1) hSGLT1 is expressed as a disulfide bridged homodimer via C355 and that 2) a portion of the intracellular 12–13 loop is re-entrant and readily accessible from the extracellular milieu.

AFM-based force spectroscopy for bioimaging and biosensing

AFM-based force spectroscopy shows wide bio-related applications especially for bioimaging and biosensing.

The role of transporter ectodomains in drug recognition and binding: phlorizin and the sodium–glucose cotransporter

This article reviews the role of segments of SLCs located outside the plasma membrane bilayer (ectodomains) using the inhibition of SGLTs (SLC5 family) by the aromatic glucoside phlorizin as a model system.

Identification of novel insulin mimetic drugs by quantitative total internal reflection fluorescence (TIRF) microscopy

Background and Purpose

Insulin stimulates the transport of glucose in target tissues by triggering the translocation of glucose transporter 4 (GLUT4) to the plasma membrane. Resistance to insulin, the major abnormality in type 2 diabetes, results in a decreased GLUT4 translocation efficiency. Thus, special attention is being paid to search for compounds that are able to enhance this translocation process in the absence of insulin.

Experimental Approach

Total internal reflection fluorescence (TIRF) microscopy was applied to quantify GLUT4 translocation in highly insulin-sensitive CHO-K1 cells expressing a GLUT4-myc-GFP fusion protein.

Key Results

Using our approach, we demonstrated GLUT4 translocation modulatory properties of selected substances and identified novel potential insulin mimetics. An increase in the TIRF signal was found to correlate with an elevated glucose uptake. Variations in the expression level of the human insulin receptor (hInsR) showed that the insulin mimetics identified stimulate GLUT4 translocation by a mechanism that is independent of the presence of the hInsR.

Conclusions and Implications

Taken together, the results indicate that TIRF microscopy is an excellent tool for the quantification of GLUT4 translocation and for identifying insulin mimetic drugs.

Nanoscale monitoring of drug actions on cell membrane using atomic force microscopy

Knowledge of the nanoscale changes that take place in individual cells in response to a drug is useful for understanding the drug action. However, due to the lack of adequate techniques, such knowledge was scarce until the advent of atomic force microscopy (AFM), which is a multifunctional tool for investigating cellular behavior with nanometer resolution under near-physiological conditions. In the past decade, researchers have applied AFM to monitor the morphological and mechanical dynamics of individual cells following drug stimulation, yielding considerable novel insight into how the drug molecules affect an individual cell at the nanoscale. In this article we summarize the representative applications of AFM in characterization of drug actions on cell membrane, including topographic imaging, elasticity measurements, molecular interaction quantification, native membrane protein imaging and manipulation, etc. The challenges that are hampering the further development of AFM for studies of cellular activities are aslo discussed.

Probing fibronectin-antibody interactions using AFM force spectroscopy and lateral force microscopy

The first experiment showing the effects of specific interaction forces using lateral force microscopy (LFM) was demonstrated for lectin-carbohydrate interactions some years ago. Such measurements are possible under the assumption that specific forces strongly dominate over the non-specific ones. However, obtaining quantitative results requires the complex and tedious calibration of a torsional force. Here, a new and relatively simple method for the calibration of the torsional force is presented. The proposed calibration method is validated through the measurement of the interaction forces between human fibronectin and its monoclonal antibody. The results obtained using LFM and AFM-based classical force spectroscopies showed similar unbinding forces recorded at similar loading rates. Our studies verify that the proposed lateral force calibration method can be applied to study single molecule interactions.

Forces and dynamics of glucose and inhibitor binding to sodium glucose co-transporter SGLT1 studied by single molecule force spectroscopy

Single molecule force spectroscopy was employed to investigate the dynamics of the sodium glucose co-transporter (SGLT1) upon substrate and inhibitor binding on the single molecule level. CHO cells stably expressing rbSGLT1 were probed by using atomic force microscopy tips carrying either thioglucose, 2'-aminoethyl β-d-glucopyranoside, or aminophlorizin. Poly(ethylene glycol) (PEG) chains of different length and varying end groups were used as tether. Experiments were performed at 10, 25 and 37 °C to address different conformational states of SGLT1. Unbinding forces between ligands and SGLT1 were recorded at different loading rates by changing the retraction velocity, yielding binding probability, width of energy barrier of the binding pocket, and the kinetic off rate constant of the binding reaction. With increasing temperature, width of energy barrier and average life time increased for the interaction of SGLT1 with thioglucose (coupled via acrylamide to a long PEG) but decreased for aminophlorizin binding. The former indicates that in the membrane-bound SGLT1 the pathway to sugar translocation involves several steps with different temperature sensitivity. The latter suggests that also the aglucon binding sites for transport inhibitors have specific, temperature-sensitive conformations.

Atomic force microscopy: a multifaceted tool to study membrane proteins and their interactions with ligands

Membrane proteins are embedded in lipid bilayers and facilitate the communication between the external environment and the interior of the cell. This communication is often mediated by the binding of ligands to the membrane protein. Understanding the nature of the interaction between a ligand and a membrane protein is required to both understand the mechanism of action of these proteins and for the development of novel pharmacological drugs. The highly hydrophobic nature of membrane proteins and the requirement of a lipid bilayer for native function have hampered the structural and molecular characterizations of these proteins under physiologically relevant conditions. Atomic force microscopy offers a solution to studying membrane proteins and their interactions with ligands under physiologically relevant conditions and can provide novel insights about the nature of these critical molecular interactions that facilitate cellular communication. In this review, we provide an overview of the atomic force microscopy technique and discuss its application in the study of a variety of questions related to the interaction between a membrane protein and a ligand. This article is part of a Special Issue entitled: Structural and biophysical characterization of membrane protein-ligand binding.

Imaging and measuring the molecular force of lymphoma pathological cells using atomic force microscopy

Atomic force microscopy (AFM) provides a new technology to visualize the cellular topography and quantify the molecular interactions at nanometer spatial resolution. In this work, AFM was used to image the cellular topography and measure the molecular force of pathological cells from B-cell lymphoma patients. After the fluorescence staining, cancer cells were recognized by their special morphological features and then the detailed topography was visualized by AFM imaging. The AFM images showed that cancer cells were much rougher than healthy cells. CD20 is a surface marker of B cells and rituximab is a monoclonal antibody against CD20. To measure the CD20-rituximab interaction forces, the polyethylene glycol (PEG) linker was used to link rituximab onto the AFM tip and the verification experiments of the functionalized probe indicated that rituximab molecules were successfully linked onto the AFM tip. The CD20-rituximab interaction forces were measured on about 20 pathological cells and the force measurement results indicated the CD20-rituximab binding forces were mainly in the range of 110-120 pN and 130-140 pN. These results can improve our understanding of the topography and molecular force of lymphoma pathological cells.

Effect of the disintegrin eristostatin on melanoma-natural killer cell interactions

Malignant melanoma is difficult to treat due to its resistance to chemotherapeutic regimens. Discovery of new pharmaceuticals with inhibitory potential can be helpful in the development of novel treatments. The snake venom disintegrin eristostatin, from the viper Eristicophis macmahoni, caused immunodeficient mice to be significantly protected from development of lung colonization when melanoma cells and the disintegrin were co-injected in vivo into the lateral tail vein compared to vehicle controls. Cytotoxicity assays suggested that eristostatin makes the melanoma cells a better target for lysis by human natural killer cells. Direct binding assays using atomic force microscopy showed eristostatin does specifically bind the surface of the six melanoma cell lines tested. Eristostatin binding was partially inhibited by the addition of soluble RGDS peptide, suggesting an integrin as one likely, but not the sole, binding partner. Studies done with melanoma cells on a culture dish and natural killer cells attached to a cantilever tip in atomic force microscopy showed four major populations of interactions which exhibited altered frequency and unbinding strength in the presence of eristostatin.

SLC5 and SLC2 transporters in epithelia-cellular role and molecular mechanisms

Members of the SLC5 and SLC2 family are prominently involved in epithelial sugar transport. SGLT1 (sodium-glucose transporter) and SGLT2, as representatives of the former, mediate sodium-dependent uptake of sugars into intestinal and renal cells. GLUT2 (glucose transporter), as representative of the latter, facilitates the sodium-independent exit of sugars from cells. SGLT has played a major role in the formulation and experimental proof for the existence of sodium cotransport systems. Based on the sequence data and biochemical and biophysical analyses, the role of extramembranous loops in sugar and inhibitor binding can be delineated. Crystal structures and homology modeling of SGLT reveal that the sugar translocation involves operation of two hydrophobic gates and intermediate exofacial and endofacial occluded states of the carrier in an alternating access model. The same basic model is proposed for GLUT1. Studies on GLUT1 have pioneered the isolation of eukaryotic transporters by biochemical methods and the development of transport kinetics and transporter models. For GLUT1, results from extensive mutagenesis, cysteine substitution and accessibility studies can be incorporated into a homology model with a barrel-like structure in which accessibility to the extracellular and intracellular medium is altered by pinching movements of some of the helices. For SGLT1 and GLUT1, the extensive hydrophilic and hydrophobic interactions between sugars and binding sites of the various intramembrane helices occur and lead to different substrate specificities and inhibitor affinities of the two transporters. A complex network of regulatory steps adapts the transport activity to the needs of the body.

Single-molecule recognition force spectroscopy of transmembrane transporters on living cells

Atomic force microscopy (AFM) has proven to be a powerful tool in biological sciences. Its particular advantage over other high-resolution methods commonly used is that biomolecules can be investigated not only under physiological conditions but also while they perform their biological functions. Single-molecule force spectroscopy with AFM tip-modification techniques can provide insight into intermolecular forces between individual ligand-receptor pairs of biological systems. Here we present protocols for force spectroscopy of living cells, including cell sample preparation, tip chemistry, step-by-step AFM imaging, force spectroscopy and data analysis. We also delineate critical steps and describe limitations that we have experienced. The entire protocol can be completed in 12 h. The model studies discussed here demonstrate the power of AFM for studying transmembrane transporters at the single-molecule level.

Three powerful research tools from single cells into single molecules: AFM, laser tweezers, and Raman spectroscopy

By using three physical techniques (atomic force microscopy (AFM), laser tweezers, and Raman spectroscopy), many excellent works in single-cell/molecule research have been accomplished. In this review, we present a brief introduction to the principles of these three techniques, and their capabilities toward single-cell/molecule research are highlighted. Afterward, the advances in single-cell/molecule research that have been facilitated by these three techniques are described. Following this, their complementary assets for single-cell/molecule research are analyzed, and the necessity of integrating the functions of these three techniques into one instrument is proposed.

Atomic force microscopy: a tool to analyze the structural organization of pathogenic protozoa

The fine structure of parasitic protozoa has been the subject of intense investigation with the use of electron microscopy. The recent development of atomic force microscopy (AFM) and all of the techniques associated with AFM has created new ways to further analyze the structure of cells. In this review, the various, presently-available modalities of AFM are discussed, as well as the results obtained in analysis of: (i) the structure of intact and detergent-extracted protozoa; (ii) the surface of infected cells; (iii) the structure of parasite macromolecules; (iv) the measurement of surface potential; and (v) force spectroscopy, the measurement of elasticity and ligand-receptor interactions.

SGLT inhibitors as new therapeutic tools in the treatment of diabetes

Recently, the idea has been developed to lower blood glucose blood glucose levels in diabetes by inhibiting sugar reabsorption sugar reabsorption in the kidney kidney . The main target is thereby the early proximal tubule proximal tubule where secondary active transport secondary active transport of the sugar is mediated by the sodium-D: -glucose D-glucose cotransporter SGLT2 SGLT2 . A model substance for the inhibitors inhibitors is the O-glucoside O-glucoside phlorizin phlorizin which inhibits transport transport competitively. Its binding to the transporter involves at least two different domains: an aglucone binding aglucone binding site at the transporter surface, involving extramembranous loops extramembraneous loops , and the sugar binding sugar binding /translocation site buried in a hydrophilic pocket of the transporter. The properties of these binding sites differ between SGLT2 and SGLT1 SGLT1 , which mediates sugar absorption sugar absorption in the intestine intestine . Various O-, C-, N- and S-glucosides have been synthesized with high affinity affinity and high specificity specificity for SGLT2 SGLT2 . Some of these glucosides are in clinical trials clinical trials and have been proven to successfully increase urinary glucose excretion urinary glucose excretion and to decrease blood sugar blood sugar levels without the danger of hypoglycaemia hypoglycaemia during fasting fasting in type 2 diabetes type 2 diabetes .

Imaging and measuring the rituximab-induced changes of mechanical properties in B-lymphoma cells using atomic force microscopy

The topography and mechanical properties of single B-lymphoma cells have been investigated by atomic force microscopy (AFM). With the assistance of microfabricated patterned pillars, the surface topography and ultrastructure of single living B-lymphoma cell were visualized by AFM. The apoptosis of B-lymphoma cells induced by rituximab alone was observed by acridine orange/ethidium bromide (AO/EB) double fluorescent staining. The rituximab-induced changes of mechanical properties in B-lymphoma cells were measured dynamically and the results showed that B-lymphoma cells became dramatically softer after incubation with rituximab. These results can improve our understanding of rituximab'effect and will facilitate the further investigation of the underlying mechanisms.

Applications of atomic force microscopy in biophysical chemistry of cells

This article addresses the question of what information and new insights atomic force microscopy (AFM) provides that are of importance and relevance to cellular biophysical chemistry research. Three enabling aspects of AFM are discussed: (a) visualization of membrane structural features with nanometer resolution, such as microvilli, ridges, porosomes, lamellapodia, and filopodia; (b) revealing structural evolution associated with cellular signaling pathways by time-dependent and high-resolution imaging of the cellular membrane in correlation with intracellular components from simultaneous optical microscopy; and (c) qualitative and quantitative measurements of single cell mechanics by acquisition of force-deformation profiles and extraction of Young's moduli for the membrane as well as cytoskeleton. A future prospective of AFM is also presented.

Determination of CFTR densities in erythrocyte plasma membranes using recognition imaging

CFTR (cystic fibrosis transmembrane conductance regulator) is a cAMP-regulated chloride (Cl(-)) channel that plays an important role in salt and fluid movement across epithelia. Cystic fibrosis (CF), the most common genetic disease among Caucasians, is caused by mutations in the gene encoding CFTR. The most predominant mutation, F508del, disturbs CFTR protein trafficking, resulting in a reduced number of CFTR in the plasma membrane. Recent studies indicate that CFTR is not only found in epithelia but also in human erythrocytes. Although considerable attempts have been made to quantify CFTR in cells, conclusions on numbers of CFTR molecules localized in the plasma membrane have been drawn indirectly. AFM has the power to provide the needed information, since both sub-molecular spatial resolution and direct protein recognition via antibody-antigen interaction can be observed. We performed a quantification study of the CFTR copies in erythrocyte membranes at the single molecule level, and compared the difference between healthy donors and CF patients. We detected that the number of CFTR molecules is reduced by 70% in erythrocytes of cystic fibrosis patients.

Single-cell force spectroscopy

The controlled adhesion of cells to each other and to the extracellular matrix is crucial for tissue development and maintenance. Numerous assays have been developed to quantify cell adhesion. Among these, the use of atomic force microscopy (AFM) for single-cell force spectroscopy (SCFS) has recently been established. This assay permits the adhesion of living cells to be studied in near-physiological conditions. This implementation of AFM allows unrivaled spatial and temporal control of cells, as well as highly quantitative force actuation and force measurement that is sufficiently sensitive to characterize the interaction of single molecules. Therefore, not only overall cell adhesion but also the properties of single adhesion-receptor-ligand interactions can be studied. Here we describe current implementations and applications of SCFS, as well as potential pitfalls, and outline how developments will provide insight into the forces, energetics and kinetics of cell-adhesion processes.

Mapping specific adhesive interactions on living human intestinal epithelial cells with atomic force microscopy

Specific molecular-receptor interactions with gut epithelium cells are important in understanding bioactivity of food components and drugs, binding of commensal microflora, attachment and initiation of defense mechanisms against pathogenic bacteria and for development of targeted delivery systems to the gut. However, methods for probing such interactions are lacking. Methodology has been developed and validated to measure specific molecular-receptor interactions on living human colorectal cancer cells as in vitro models for the gut epithelium. Atomic force microscopy (AFM) was used to measure ligand-receptor interactions and to map receptor locations on cell surfaces. Measurements were made using silica beads attached to the AFM tip-cantilever assembly, which were functionalized by coupling of ligands to the bead surface. Wheat germ agglutinin (WGA) binds to the glycosylated extracellular domain III of the epidermal growth factor receptor. Methodology was tested by measuring binding of WGA to the surface of confluent monolayers of living Caco-2 human intestinal epithelial cells. The measured modal detachment force of 125 pN is typical of values expected for single molecule interactions. Adhesive events were used to map the location of binding sites on the cell surface revealing heterogeneity in their distribution within and between cells within the monolayer.

Investigation of spacer length effect on immobilized Escherichia coli pili-antibody molecular recognition by AFM

The immobilization of antibodies to sensor surfaces is critical in biochemical sensor development. In this study, Poly(ethylene glycol) (PEG) and Jeffamine spacers were employed to tether Escherichia coli K99 pilus antibody to silicon wafer surfaces for the purpose of improving the orientation of antibody as well as reducing the steric hindrance. To illustrate the effect of spacer length, a variety of linear polymers were used to covalently attach the antibodies to silicon surfaces. Atomic Force Microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the surface morphology and chemical composition at each reaction step. The effect of spacer length in improving the specificity of immobilized antibody was investigated by attaching E. coli on the end of an AFM tip. The distribution of unbinding force and rupture distance from the force-distance curves obtained by AFM showed that the introduction of PEG spacer facilitates bacterial recognition which can improve the incidence of interactions by up to 90%. J600 proved to be the most effective spacer overcoming the steric hindrance seen with direct immobilization of antibody. In addition, the force spectroscopy reveals the elementary force quantum of E. coli-antibody to be 0.3 nN.

Extending Bell's model: how force transducer stiffness alters measured unbinding forces and kinetics of molecular complexes

Forced unbinding of complementary macromolecules such as ligand-receptor complexes can reveal energetic and kinetic details governing physiological processes ranging from cellular adhesion to drug metabolism. Although molecular-level experiments have enabled sampling of individual ligand-receptor complex dissociation events, disparities in measured unbinding force F(R) among these methods lead to marked variation in inferred binding energetics and kinetics at equilibrium. These discrepancies are documented for even the ubiquitous ligand-receptor pair, biotin-streptavidin. We investigated these disparities and examined atomic-level unbinding trajectories via steered molecular dynamics simulations, as well as via molecular force spectroscopy experiments on biotin-streptavidin. In addition to the well-known loading rate dependence of F(R) predicted by Bell's model, we find that experimentally accessible parameters such as the effective stiffness of the force transducer k can significantly perturb the energy landscape and the apparent unbinding force of the complex for sufficiently stiff force transducers. Additionally, at least 20% variation in unbinding force can be attributed to minute differences in initial atomic positions among energetically and structurally comparable complexes. For force transducers typical of molecular force spectroscopy experiments and atomistic simulations, this energy barrier perturbation results in extrapolated energetic and kinetic parameters of the complex that depend strongly on k. We present a model that explicitly includes the effect of k on apparent unbinding force of the ligand-receptor complex, and demonstrate that this correction enables prediction of unbinding distances and dissociation rates that are decoupled from the stiffness of actual or simulated molecular linkers.

Comparison of the indentation and elasticity of E. coli and its spheroplasts by AFM

Atomic force microscopy (AFM) provides a unique opportunity to study live individual bacteria at the nanometer scale. In addition to providing accurate morphological information, AFM can be exploited to investigate membrane protein localization and molecular interactions on the surface of living cells. A prerequisite for these studies is the development of robust procedures for sample preparation. While such procedures are established for intact bacteria, they are only beginning to emerge for bacterial spheroplasts. Spheroplasts are useful research models for studying mechanosensitive ion channels, membrane transport, lipopolysaccharide translocation, solute uptake, and the effects of antimicrobial agents on membranes. Furthermore, given the similarities between spheroplasts and cell wall-deficient (CWD) forms of pathogenic bacteria, spheroplast research could be relevant in biomedical research. In this paper, a new technique for immobilizing spheroplasts on mica pretreated with aminopropyltriethoxysilane (APTES) and glutaraldehyde is described. Using this mounting technique, the indentation and cell elasticity of glutaraldehyde-fixed and untreated spheroplasts of E. coli in liquid were measured. These values are compared to those of intact E. coli. Untreated spheroplasts were found to be much softer than the intact cells and the silicon nitride cantilevers used in this study.

Interaction between single molecules of Mac-1 and ICAM-1 in living cells: an atomic force microscopy study

The interaction between integrin macrophage differentiation antigen associated with complement three receptor function (Mac-1) and intercellular adhesion molecule-1 (ICAM-1), which is controlled tightly by the ligand-binding activity of Mac-1, is central to the regulation of neutrophil adhesion in host defense. Several "inside-out" signals and extracellular metal ions or antibodies have been found to activate Mac-1, resulting in an increased adhesiveness of Mac-1 to its ligands. However, the molecular basis for Mac-1 activation is not well understood yet. In this work, we have carried out a single-molecule study of Mac-1/ICAM-1 interaction force in living cells by atomic force microscopy (AFM). Our results showed that the binding probability and adhesion force of Mac-1 with ICAM-1 increased upon Mac-1 activation. Moreover, by comparing the dynamic force spectra of different Mac-1 mutants, we expected that Mac-1 activation is governed by the downward movement of its alpha7 helix.