Force probing surfaces of living cells to molecular resolution

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.

Biosensors in plants

Biosensors come in an increasing array of forms and their development is defining the rate of advance for our understanding of many natural processes. Developmental biology is increasingly using mathematical models and yet few of these models are based on quantitative recordings. In particular, we know comparatively little about the endogenous concentrations or fluxes of signalling molecules such as the phytohormones, an area of great potential for new biosensors. There are extremely useful biosensors for some signals, but most remain qualitative. Other qualities sought in biosensors are temporal and spatial resolution and, usually, an ability to use them without significantly perturbing the system. Currently, the biosensors with the best properties are the genetically encoded optical biosensors based on FRET, but each sensor needs extensive specific effort to develop. Sensor technologies using antibodies as the recognition domain are more generic, but these tend to be more invasive and there are few examples of their use in plant biology. By capturing some of the opportunities appearing with advances in platform technologies it is hoped that more biosensors will become available to plant scientists.

Atomic force microscopy of fungal cell walls: an update

Although the chemical composition of many fungal cell walls is known, the spatial organization and interactions of the individual macromolecules remain mysterious. In this context, single-cell and single-molecule atomic force microscopy techniques offer new opportunities for probing the surface of fungal cells down to molecular resolution. Recent breakthroughs include the visualization of the structural dynamics of single cells as they grow or interact with drugs, the quantification and nanoscale imaging of cell surface hydrophobicity and the measurement of the molecular elasticity of cell wall polysaccharides and proteins. Here I highlight the advances made by my team in this area during the past 3 years.

Force-induced formation and propagation of adhesion nanodomains in living fungal cells

Understanding how cell adhesion proteins form adhesion domains is a key challenge in cell biology. Here, we use single-molecule atomic force microscopy (AFM) to demonstrate the force-induced formation and propagation of adhesion nanodomains in living fungal cells, focusing on the covalently anchored cell-wall protein Als5p from Candida albicans. We show that pulling on single adhesins with AFM tips terminated with specific antibodies triggers the formation of adhesion domains of 100-500 nm and that the force-induced nanodomains propagate over the entire cell surface. Control experiments (with cells lacking Als5p, single-site mutation in the protein, bare tips, and tips modified with irrelevant antibodies) demonstrate that Als5p nanodomains result from protein redistribution triggered by force-induced conformational changes in the initially probed proteins, rather than from nonspecific cell-wall perturbations. Als5p remodeling is independent of cellular metabolic activity because heat-killed cells show the same behavior as live cells. Using AFM and fluorescence microscopy, we also find that nanodomains are formed within ∼30 min and migrate at a speed of ∼20 nm·min(-1), indicating that domain formation and propagation are slow, time-dependent processes. These results demonstrate that mechanical stimuli can trigger adhesion nanodomains in fungal cells and suggest that the force-induced clustering of adhesins may be a mechanism for activating cell adhesion.

The role of physiological heterogeneity in microbial population behavior

As the ability to analyze individual cells in microbial populations expands, it is becoming apparent that isogenic microbial populations contain substantial cell-to-cell differences in physiological parameters such as growth rate, resistance to stress and regulatory circuit output. Subpopulations exist that are manyfold different in these parameters from the population average, and these differences arise by stochastic processes. Such differences can dramatically affect the response of cells to perturbations, especially stress, which in turn dictates overall population response. Defining the role of cell-to-cell heterogeneity in population behavior is important for understanding population-based research problems, including those involving infecting populations, normal flora and bacterial populations in water and soils. Emerging technological breakthroughs are poised to transform single-cell analysis and are critical for the next phase of insights into physiological heterogeneity in the near future. These include technologies for multiparameter analysis of live cells, with downstream processing and analysis.

Planar cell polarity signalling regulates cell adhesion properties in progenitors of the zebrafish laterality organ

Organ formation requires the precise assembly of progenitor cells into a functional multicellular structure. Mechanical forces probably participate in this process but how they influence organ morphogenesis is still unclear. Here, we show that Wnt11- and Prickle1a-mediated planar cell polarity (PCP) signalling coordinates the formation of the zebrafish ciliated laterality organ (Kupffer's vesicle) by regulating adhesion properties between organ progenitor cells (the dorsal forerunner cells, DFCs). Combined inhibition of Wnt11 and Prickle1a reduces DFC cell-cell adhesion and impairs their compaction and arrangement during vesicle lumen formation. This leads to the formation of a mis-shapen vesicle with small fragmented lumina and shortened cilia, resulting in severely impaired organ function and, as a consequence, randomised laterality of both molecular and visceral asymmetries. Our results reveal a novel role for PCP-dependent cell adhesion in coordinating the supracellular organisation of progenitor cells during vertebrate laterality organ formation.

Leptospiral outer membrane lipoprotein LipL32 binding on toll-like receptor 2 of renal cells as determined with an atomic force microscope

Leptopirosis is a renal disease caused by pathogenic Leptospira that primarily infects the renal proximal tubules, consequently resulting in severe tubular injuries and malfunctions. The protein extracted from the outer membrane of this pathogenic strain contains a major component of a 32 kDa lipoprotein (LipL32), which is absent in the counter membrane of nonpathogenic strains and has been identified as a crucial factor for host cell infection. Previous studies showed that LipL32 induced inflammatory responses and interacted with the extracellular matrix (ECM) of the host cell. However, the exact relationship between LipL32-mediated inflammatory responses and ECM binding is still unknown. In this study, an atomic force microscope with its tip modified by purified LipL32 was used to assess the interaction between LipL32 and cell surface receptors. Furthermore, an antibody neutralization technique was employed to identify Toll-like receptor 2 (TLR2) but not TLR4 as the major target of LipL32 attack. The interaction force between LipL32 and TLR2 was measured as approximately 59.5 +/- 8.7 pN, concurring with the theoretical value for a single-pair molecular interaction. Moreover, transformation of a TLR deficient cell line with human TLR2 brought the interaction force from the basal level to approximately 60.4 +/- 11.5 pN, confirming unambiguously TLR2 as counter receptor for LipL32. The stimulation of CXCL8/IL-8 expression by full-length LipL32 as compared to that without the N-terminal signal peptide domain suggests a significant role of the signal peptide of the protein in the inflammatory responses. This study provides direct evidence that LipL32 binds to TLR2, but not TLR4, on the cell surface, and a possible mechanism for the virulence of leptospirosis is accordingly proposed.

Is there anyone out there?--Single-molecule atomic force microscopy meets yeast genetics to study sensor functions

The ability to react to environmental stress is a key feature of microbial cells, which frequently involves the fortification of their cell wall as a primary step. In the model yeast Saccharomyces cerevisiae the biosynthesis of the cell wall is regulated by the so-called cell wall integrity signal transduction pathway, which starts with the detection of cell surface stress by a small family of five membrane-spanning sensors (Wsc1-Wsc3, Mid2, Mtl1). Although genetic evidence indicated that these proteins act as mechanosensors, direct in vivo evidence for their function remained scarce. Here, we review a new approach integrating the tools and concepts of genetics with those of nanotechnology. We show how atomic force microscopy can be combined with advanced protein design by yeast genetics, to study the function and the mechanical properties of yeast sensors in living cells down to the single molecule level. We anticipate that this novel integrated technology will enable a paradigm shift in cell biology, so that pertinent questions can be addressed, such as the nanomechanics of single sensors and receptors, and how they distribute across the cell surface when they respond to extracellular stress.

Single-molecule atomic force microscopy reveals clustering of the yeast plasma-membrane sensor Wsc1

Signalling is a key feature of living cells which frequently involves the local clustering of specific proteins in the plasma membrane. How such protein clustering is achieved within membrane microdomains (“rafts”) is an important, yet largely unsolved problem in cell biology. The plasma membrane of yeast cells represents a good model to address this issue, since it features protein domains that are sufficiently large and stable to be observed by fluorescence microscopy. Here, we demonstrate the ability of single-molecule atomic force microscopy to resolve lateral clustering of the cell integrity sensor Wsc1 in living Saccharomyces cerevisiae cells. We first localize individual wild-type sensors on the cell surface, revealing that they form clusters of ∼200 nm size. Analyses of three different mutants indicate that the cysteine-rich domain of Wsc1 has a crucial, not yet anticipated function in sensor clustering and signalling. Clustering of Wsc1 is strongly enhanced in deionized water or at elevated temperature, suggesting its relevance in proper stress response. Using in vivo GFP-localization, we also find that non-clustering mutant sensors accumulate in the vacuole, indicating that clustering may prevent endocytosis and sensor turnover. This study represents the first in vivo single-molecule demonstration for clustering of a transmembrane protein in S. cerevisiae. Our findings indicate that in yeast, like in higher eukaryotes, signalling is coupled to the localized enrichment of sensors and receptors within membrane patches.

The viscoelastic properties of microvilli are dependent upon the cell-surface molecule

We studied at nanometer resolution the viscoelastic properties of microvilli and tethers pulled from myelogenous cells via P-selectin glycoprotein ligand 1 (PSGL-1) and found that in contrast to pure membrane tethers, the viscoelastic properties of microvillus deformations are dependent upon the cell-surface molecule through which load is applied. A laser trap and polymer bead coated with anti-PSGL-1 (KPL-1) were used to apply step loads to microvilli. The lengthening of the microvillus in response to the induced step loads was fitted with a viscoelastic model. The quasi-steady state force on the microvillus at any given length was approximately fourfold lower in cells treated with cytochalasin D or when pulled with concanavalin A-coated rather than KPL-1-coated beads. These data suggest that associations between PSGL-1 and the underlying actin cytoskeleton significantly affect the early stages of leukocyte deformation under flow.

Detecting cell-adhesive sites in extracellular matrix using force spectroscopy mapping

The cell microenvironment is composed of extracellular matrix (ECM), which contains specific binding sites that allow the cell to adhere to its surroundings. Cells employ focal adhesion proteins, which must be able to resist a variety of forces to bind to ECM. Current techniques for detecting the spatial arrangement of these adhesions, however, have limited resolution and those that detect adhesive forces lack sufficient spatial characterization or resolution. Using a unique application of force spectroscopy, we demonstrate here the ability to determine local changes in the adhesive property of a fibronectin substrate down to the resolution of the fibronectin antibody-functionalized tip diameter, ~20 nm. To verify the detection capabilities of force spectroscopy mapping (FSM), changes in loading rate and temperature were used to alter the bond dynamics and change the adhesion force. Microcontact printing was also used to pattern fluorescein isothiocyanate-conjugated fibronectin in order to mimic the discontinuous adhesion domains of native ECM. Fluorescent detection was used to identify the pattern while FSM was used to map cell adhesion sites in registry with the initial fluorescent image. The results show that FSM can be used to detect the adhesion domains at high resolution and may subsequently be applied to native ECM with randomly distributed cell adhesion sites.

Cell adhesion strength is controlled by intermolecular spacing of adhesion receptors

Spatial patterning of biochemical cues on the micro- and nanometer scale controls numerous cellular processes such as spreading, adhesion, migration, and proliferation. Using force microscopy we show that the lateral spacing of individual integrin receptor-ligand bonds determines the strength of cell adhesion. For spacings > or = 90 nm, focal contact formation was inhibited and the detachment forces as well as the stiffness of the cell body were significantly decreased compared to spacings < or = 50 nm. Analyzing cell detachment at the subcellular level revealed that rupture forces of focal contacts increase with loading rate as predicted by a theoretical model for adhesion clusters. Furthermore, we show that the weak link between the intra- and extracellular space is at the intracellular side of a focal contact. Our results show that cells can amplify small differences in adhesive cues to large differences in cell adhesion strength.

Cardiomyocyte contractile status is associated with differences in fibronectin and integrin interactions

Integrins link the extracellular matrix (ECM) with the intracellular cytoskeleton and other cell adhesion-associated signaling proteins to function as mechanotransducers. However, direct quantitative measurements of the cardiomyocyte mechanical state and its relationship to the interactions between specific ECM proteins and integrins are lacking. The purpose of this study was to characterize the interactions between the ECM protein fibronectin (FN) and integrins in cardiomyocytes and to test the hypothesis that these interactions would vary during contraction and relaxation states in cardiomyocytes. Using atomic force microscopy, we quantified the unbinding force (adhesion force) and adhesion probability between integrins and FN and correlated these measurements with the contractile state as indexed by cell stiffness on freshly isolated mouse cardiomyocytes. Experiments were performed in normal physiological (control), high-K(+) (tonically contracted), or low-Ca(2+) (fully relaxed) solutions. Under control conditions, the initial peak of adhesion force between FN and myocyte alpha(3)beta(1)- and/or alpha(5)beta(1)-integrins was 39.6 +/- 1.3 pN. The binding specificity between FN and alpha(3)beta(1)- and alpha(5)beta(1)-integrins was verified by using monoclonal antibodies against alpha(3)-, alpha(5)-, alpha(3) + alpha(5)-, or beta(1)-integrin subunits, which inhibited binding by 48%, 65%, 70%, or 75%, respectively. Cytochalasin D or 2,3-butanedione monoxime (BDM), to disrupt the actin cytoskeleton or block myofilament function, respectively, significantly decreased the cell stiffness; however, the adhesion force and binding probability were not altered. Tonic contraction with high-K(+) solution increased total cell adhesion (1.2-fold) and cell stiffness (27.5-fold) compared with fully relaxed cells with low-Ca(2+) solution. However, it could be partially prevented by high-K(+) bath solution containing BDM, which suppresses contraction by inhibiting the actin-myosin interactions. Thus, our results demonstrate that integrin binding to FN is modulated by the contractile state of cardiac myocytes.

Measurement of the mechanical behavior of yeast membrane sensors using single-molecule atomic force microscopy

In Saccharomyces cerevisiae, surface stresses acting on the cell wall or plasma membrane are detected by a group of five membrane sensors: Wsc1, Wsc2, Wsc3, Mid2 and Mtl2. Here we present protocols to measure the mechanical properties of Wsc1 sensors in their native cellular environment, using the combination of genetic manipulations with single-molecule atomic-force microscopy (AFM). We describe procedures (i) for obtaining genetically modified sensors that are fully functional and suitable for AFM analysis, i.e., elongated Wsc1 derivatives terminated with a His-tag, and (ii) for detecting and stretching single Wsc1 sensors on the surface of living S. cerevisiae cells, using AFM tips functionalized with Ni(2+)-NTA groups. These procedures are multidisciplinary to implement and need competent researchers from at least two disciplines: molecular biology and nanotechnology. For experienced researchers in biological AFM, the entire protocol can be completed in approximately 3 weeks.

Application of atomic force microscopy in bacterial research

The atomic force microscope (AFM) has evolved from an imaging device into a multifunctional and powerful toolkit for probing the nanostructures and surface components on the exterior of bacterial cells. Currently, the area of application spans a broad range of interesting fields from materials sciences, in which AFM has been used to deposit patterns of thiol-functionalized molecules onto gold substrates, to biological sciences, in which AFM has been employed to study the undesirable bacterial adhesion to implants and catheters or the essential bacterial adhesion to contaminated soil or aquifers. The unique attribute of AFM is the ability to image bacterial surface features, to measure interaction forces of functionalized probes with these features, and to manipulate these features, for example, by measuring elongation forces under physiological conditions and at high lateral resolution (<1 A). The first imaging studies showed the morphology of various biomolecules followed by rapid progress in visualizing whole bacterial cells. The AFM technique gradually developed into a lab-on-a-tip allowing more quantitative analysis of bacterial samples in aqueous liquids and non-contact modes. Recently, force spectroscopy modes, such as chemical force microscopy, single-cell force spectroscopy, and single-molecule force spectroscopy, have been used to map the spatial arrangement of chemical groups and electrical charges on bacterial surfaces, to measure cell-cell interactions, and to stretch biomolecules. In this review, we present the fascinating options offered by the rapid advances in AFM with emphasizes on bacterial research and provide a background for the exciting research articles to follow.

Biofunctionalized magnetic-vortex microdiscs for targeted cancer-cell destruction

Nanomagnetic materials offer exciting avenues for probing cell mechanics and activating mechanosensitive ion channels, as well as for advancing cancer therapies. Most experimental works so far have used superparamagnetic materials. This report describes a first approach based on interfacing cells with lithographically defined microdiscs that possess a spin-vortex ground state. When an alternating magnetic field is applied the microdisc vortices shift, creating an oscillation, which transmits a mechanical force to the cell. Because reduced sensitivity of cancer cells toward apoptosis leads to inappropriate cell survival and malignant progression, selective induction of apoptosis is of great importance for the anticancer therapeutic strategies. We show that the spin-vortex-mediated stimulus creates two dramatic effects: compromised integrity of the cellular membrane, and initiation of programmed cell death. A low-frequency field of a few tens of hertz applied for only ten minutes was sufficient to achieve approximately 90% cancer-cell destruction in vitro.

Single-photon atomic force microscopy

In the last few years, an array of novel technologies, especially the big family of scanning probe microscopy, now often integrated with other powerful imaging tools such as laser confocal microscopy and total internal reflection fluorescence microscopy, have been widely applied in the investigation of biomolecular interactions and dynamics. But it is still a great challenge to directly monitor the dynamics of biomolecular interactions with high spatial and temporal resolution in living cells. An innovative method termed "single-photon atomic force microscopy" (SP-AFM), superior to existing techniques in tracing biomolecular interactions and dynamics in vivo, was proposed on the basis of the combination of atomic force microscopy with the technologies of carbon nanotubes and single-photon detection. As a unique tool, SP-AFM, capable of simultaneous topography imaging and molecular identification at the subnanometer level by synchronous acquisitions and analyses of the surface topography and fluorescent optical signals while scanning the sample, could play a very important role in exploring biomolecular interactions and dynamics in living cells or in a complicated biomolecular background.

Early T-cell activation biophysics

The T-cell is one of the main players in the mammalian immune response. It ensures antigen recognition at the surface of antigen-presenting cells in a complex and highly sensitive and specific process, in which the encounter of the T-cell receptor with the agonist peptide associated with the major histocompatibility complex triggers T-cell activation. While signaling pathways have been elucidated in increasing detail, the mechanism of TCR triggering remains highly controversial despite active research published in the past 10 years. In this paper, we present a short overview of pending questions on critical initial events associated with T-cell triggering. In particular, we examine biophysical approaches already in use, as well as future directions. We suggest that the most recent advances in fluorescence super-resolution imaging, coupled with the new classes of genetic fluorescent probes, will play an important role in elucidation of the T-cell triggering mechanism. Beyond this aspect, we predict that exploration of mechanical cues in the triggering process will provide new clues leading to clarification of the entire mechanism.

Use of self-actuating and self-sensing cantilevers for imaging biological samples in fluid

In this paper, we present a detailed investigation into the suitability of atomic force microscopy (AFM) cantilevers with integrated deflection sensor and micro-actuator for imaging of soft biological samples in fluid. The Si cantilevers are actuated using a micro-heater at the bottom end of the cantilever. Sensing is achieved through p-doped resistors connected in a Wheatstone bridge. We investigated the influence of the water on the cantilever dynamics, the actuation and the sensing mechanisms, as well as the crosstalk between sensing and actuation. Successful imaging of yeast cells in water using the integrated sensor and actuator shows the potential of the combination of this actuation and sensing method. This constitutes a major step towards the automation and miniaturization required to establish AFM in routine biomedical diagnostics and in vivo applications.

Controlled manipulation of bacteriophages using single-virus force spectroscopy

A method is described for the site-directed manipulation of single filamentous bacteriophages, by using phage display technology and atomic force microscopy. f1 filamentous bacteriophages were genetically engineered to display His-tags on their pIX tail. Following adsorption on nitrilotriacetate-terminated surfaces, force spectroscopy with tips bearing monoclonal anti-pIII antibodies was used to pull on individual phages via their pIII head. Analysis of the force-extension profiles revealed that upon pulling, the phages are progressively straightened into an extended orientation until rupture of the anti-pIII/pIII complex. The single-virus manipulation technique presented here provides new opportunities for understanding the forces driving cell-virus and material-virus interactions, and for characterizing the binding properties of polypeptide sequences or proteins selected by the phage display technology.