Atomic force microscope imaging of cells and membranes

Spectroscopic and microscopic comparisons of cell topology and chemistry analysis of mouse embryonic stem cell, somatic cell and cancer cell

While embryonic stem cells and cancer cells are known to have many similarities in signalling pathways, healthy somatic cells are known to be different in many ways. Characterization of embryonic stem cell is crucial for cancer development and cancer recurrence due to the shared signalling pathways and life course with cancer initiator and cancer stem cells. Since embryonic stem cells are the sources of the somatic and cancer cells, it is necessary to reveal the relevance between them. The past decade has seen the importance of interdisciplinary studies and it is obvious that the reflection of the physical/chemical phenomena occurring on the cell biology has attracted much more attention. For this reason, the aim of this study is to elementally and topologically characterize the mouse embryonic stem cells, mouse lung squamous cancer cells, and mouse skin fibroblast cells by using Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM) supported with Electron Dispersive Spectroscopy (EDS) techniques in a complementary way. Our AFM findings revealed that roughness data of the mouse embryonic stem cells and cancer cells were similar and somatic cells were found to be statistically different from these two cell types. However, based on both XPS and SEM-EDS results, surface elemental ratios vary in mouse embryonic stem cells, cancer cells and somatic cells. Our results showed that these complementary spectroscopic and microscopic techniques used in this work are very effective in cancer and stem cell characterization and have the potential to gather more detailed information on relevant biological samples.

Intracellular Delivery of Poorly Soluble Polyphenols: Elucidating the Interplay of Self-Assembling Nanocarriers and Human Chondrocytes

Increased molecular understanding of multifactorial diseases paves the way for novel therapeutic approaches requiring sophisticated carriers for intracellular delivery of actives. We designed and characterized self-assembling lipid-core nanocapsules for coencapsulation of two poorly soluble natural polyphenols curcumin and resveratrol. The polyphenols were identified as high-potential therapeutic candidates intervening in the intracellular inflammation cascade of chondrocytes during the progress of osteoarthritis. To elucidate the interplay between chondrocytes and nanocapsules and their therapeutic effect, we pursued a complementary analytical approach combining label-free visualization with biological assays. Primary human chondrocytes did not show any adverse effects upon nanocapsule application and coherent anti-Stokes Raman scattering images visualized their intracellular uptake. Further, by systematically blocking different uptake mechanisms, an energy independent uptake into the cells could be identified. Additionally, we tested the therapeutic effect of the polyphenol-loaded carriers on inflamed chondrocytes. Treatment with nanocapsules resulted in a major reduction of nitric oxide levels, a well-known apoptosis trigger during the course of osteoarthritis. For a more profound examination of this protective effect on joint cells, we pursued studies with atomic force microscopy investigations. Significant changes in the cell cytoskeleton as well as prominent dents in the cell membrane upon induced apoptosis were revealed. Interestingly, these effects could not be detected for chondrocytes which were pretreated with the nanocapsules. Overall, besides presenting a sophisticated carrier system for joint application, these results highlight the necessity of establishing combinatorial analytical approaches to elucidate cellular uptake, the interplay of codelivered drugs and their therapeutic effect on the subcellular level.

Artesunate attenuates glioma proliferation, migration and invasion by affecting cellular mechanical properties

Glioma is one of the most common malignant brain tumors. Current chemotherapy is far from providing satisfactory clinical outcomes for patients with glioma. More efficient drugs are urgently needed. Artesunate (ART) is clinically used as an anti-malarial agent and exhibits potent antiproliferative activity as a traditional Chinese medicine. In addition, ART has been shown to exert a profound cytotoxic effect on various tumor cell lines, presenting a novel candidate for cancer chemotherapy. However, its anticancer effect on glioma by altering cell biomechanical properties remains unclear. The present study aimed to identify the anticancer effects of ART on human glioma SHG44 cells by assessing cell proliferation, migration/invasion, the expression of claudin-1 and the biomechanical properties of ART-treated SHG44 cells. The proliferation of the SHG44 cells was assessed by MTT assay. The cell apoptosis was detected by flow cytometry. For cell migration and invasion assays, the Transwell was used. The expression of the gene claudin-1 was detected by polymerase chain reaction. The cell membrane and biomechanical properties, as targets of ART action, were investigated by atomic force microscopy (AFM). ART significantly inhibited the proliferation of SHG44 cells in a dose- and time-dependent manner. After treatment with 30 mg/l ART, the level of cell apoptosis was significantly increased (from 6.88±0.062 to 23.7±4.16%). Furthermore, the cell migration and invasion abilities of the SHG44 cells were markedly inhibited after treatment with 30 mg/l ART. Compared with the control group (0 mg/l ART), the SHG44 cells treated with 30 mg/l ART exhibited upregulated expression of claudin-1, increased adhesive force (from 2,400±300 to 3,600±500 pN), increased high connection among SHG44 cells, increased cytomembrane roughness (from 0.118±0.011 to 0.269±0.015 µm) and reduced elasticity (from 23±8 to 3.5±1.1 MPa). The present study demonstrated that ART could alter the biomechanical properties of the glioma cells to inhibit cell proliferation, migration and invasion.

Artesunate altered cellular mechanical properties leading to deregulation of cell proliferation and migration in esophageal squamous cell carcinoma

Esophageal squamous cell carcinoma (ESCC) is one of the most common types of cancer in China. Artesunate (ART) is used clinically as an anti-malarial agent and exhibits potent antiproliferative activity. In addition, ART has demonstrated remarkable antitumor activity, presenting a novel candidate for cancer chemotherapy. However, its effect on ESCC remains unknown. The present study analyzed the antitumor effects of ART in the KYSE-150 ESCC line by assessing cell proliferation, cell death, cell migration/invasion and the biomechanical properties of ART-treated KYSE-150 cells. ART treatment significantly suppressed the proliferation of KYSE-150 cells in a dose- and time-dependent manner, as assessed by MTT assay. Following treatment with 30 mg/l ART, the cell population in the G₀/G₁ phase and the level of cell apoptosis significantly increased from 54±1.5 to 68.1±0.3%, and from 4.53±0.58 to 12.45±0.62%, respectively. Furthermore, the cell migration and invasion of KYSE-150 cells treated with 30 mg/l ART was markedly inhibited. The cell membrane and biomechanical properties were investigated using atomic force microscopy, as targets of ART action. ESCC cells treated with 30 mg/l ART exhibited increased adhesive force, increased cytomembrane roughness and reduced elasticity compared with the control group (KYSE-150 cells without ART treatment). The biomechanical properties of KYSE-150 cells treated with 30 mg/l ART were similar to those of the SHEE normal human esophageal epithelial cell line. In conclusion, the present study demonstrated that ART may inhibit cell proliferation and migration in ESCC through changes in the biomechanical properties of the ESCC cells.

Preparation of DOPC and DPPC Supported Planar Lipid Bilayers for Atomic Force Microscopy and Atomic Force Spectroscopy

Cell membranes are typically very complex, consisting of a multitude of different lipids and proteins. Supported lipid bilayers are widely used as model systems to study biological membranes. Atomic force microscopy and force spectroscopy techniques are nanoscale methods that are successfully used to study supported lipid bilayers. These methods, especially force spectroscopy, require the reliable preparation of supported lipid bilayers with extended coverage. The unreliability and a lack of a complete understanding of the vesicle fusion process though have held back progress in this promising field. We document here robust protocols for the formation of fluid phase DOPC and gel phase DPPC bilayers on mica. Insights into the most crucial experimental parameters and a comparison between DOPC and DPPC preparation are presented. Finally, we demonstrate force spectroscopy measurements on DOPC surfaces and measure rupture forces and bilayer depths that agree well with X-ray diffraction data. We also believe our approach to decomposing the force-distance curves into depth sub-components provides a more reliable method for characterising the depth of fluid phase lipid bilayers, particularly in comparison with typical image analysis approaches.

Fractal properties of macrophage membrane studied by AFM

Complexity of cell membrane poses difficulties to quantify corresponding morphology changes during cell proliferation and damage. We suggest using fractal dimension of the cell membrane to quantify its complexity and track changes produced by various treatments. Glutaraldehyde fixed mouse RAW 264.7 macrophage membranes were chosen as model system and imaged in PeakForce QNM (quantitative nanomechanics) mode of AFM (atomic force microscope). The morphology of the membranes was characterized by fractal dimension. The parameter was calculated for set of AFM images by three different methods. The same calculations were done for the AFM images of macrophages treated with colchicine, an inhibitor of the microtubule polymerization, and microtubule stabilizing agent taxol. We conclude that fractal dimension can be additional and useful parameter to characterize the cell membrane complexity and track the morphology changes produced by different treatments.

Hydroxyl radical scavenging mechanism of human erythrocytes by quercetin-germanium (IV) complex

Quercetin is a popular flavonoid in plant foods, herbs, and dietary supplement. Germanium, a kind of trace elements, can enhance the body immunity. This study investigated the hydroxyl-radical-scavenging mechanism of the quercertin-germanium (IV) (Qu-Ge) complex to human erythrocytes, especially the effects on ultrastructure and mechanical properties of cell membrane, plasma membrane potential and intracellular free Ca(2+) concentration. Results showed that QuGe(2), a kind of the Qu-Ge complex, could reduce the oxidative damage of erythrocytes, change the cell-surface morphology, and partly recover the disruption of plasma membrane potential and intracellular free Ca(2+) level. Atomic force microscopy (AFM) was used to characterize the changes of the cell morphology, cell-membrane ultrastructure and biophysical properties at nanoscalar level. QuGe(2) has triggered the antioxidative factor to inhibit cellular damage. These results can improve the understanding of hydroxyl-radical-scavenging mechanism of human erythrocytes induced by the Qu-Ge complex, which can be potentially developed as a new antioxidant for treatment of oxidative damage.

Surface ultrastructure and mechanical property of human chondrocyte revealed by atomic force microscopy

OBJECTIVE

The mechanical properties of chondrocytes influence maintenance of the articular cartilage extracellular matrix. To differentiate the mechanical properties of chondrocytes between a young, normal modulus and an old, osteoarthritic (OA) modulus, we used an atomic force microscope (AFM) to probe the surface ultrastructure and to measure their adhesion force and stiffness.

METHODS

We directly visualized a single chondrocyte cell by using AFM and quantitatively measured the dimensions of the cells.

RESULTS

Profiles displayed heights of 1026+/-203 and 1668+/-352 nm for old and young cells, respectively. Contour maps revealed differences in the sizes and structures of the two groups. Mean calculated adhesion forces differed between normal and OA chondrocytes (7.06+/-3.35 and 2.97+/-1.82 nN, respectively), as did calculated stiffness values (0.0960+/-0.009 and 0.0347+/-0.005 N/m, respectively).

CONCLUSION

These findings suggested that the mechanical properties of normal chondrocytes substantially differed from those of OA chondrocytes. We believe this study represents the first direct characterization of the surface ultrastructure and mechanical measurements of human chondrocytes between normal and OA stages. This new approach could be a useful technique for investigating age-related changes in the properties of human chondrocytes.

Continuous planar phospholipid bilayer supported on porous silicon thin film reflector

Reconstituting artificial membranes for in vitro studies of cell barrier mechanisms and properties is of major interest in biology. Here, artificial membranes supported on porous silicon photonic crystal reflectors are prepared and investigated. The materials are of interest for label-free probing of supported membrane events such as protein binding, molecular recognition, and transport. The porous silicon substrates are prepared as multilayered films consisting of a periodically varying porosity, with pore dimensions of a few nanometers in size. Planar phospholipid bilayers are deposited on the topmost surface of the oxidized hydrophilic mesoporous silicon films. Atomic force microscopy provides evidence of continuous bilayer deposition at the surface, and optical measurements indicate that the lipids do not significantly infiltrate the porous region. The presence of the supported bilayer does not obstruct the optical spectrum from the porous silicon layer, suggesting that the composite structures can act as effective optical biosensors.

Mechanosensing using drag force for imaging soft biological membranes

We investigate physical processes taking place during nanoscale mechanosensing of soft biological membranes in liquid environments. Examples include tapping mode imaging by atomic force microscope (AFM) and microscopy based on the Brownian motion of a nanoparticle in an optical-tweezers-controlled trap. The softness and fluidity of the cellular membrane make it difficult to accurately detect (i.e., image) the shape of the cell using traditional mechanosensing methods. The aim of the reported work is to theoretically evaluate whether the drag force acting on the nanoscale mechanical probe due to a combined effect of intra- and extracellular environments can be exploited to develop a new imaging mode suitable for soft cellular interfaces. We approach this problem by rigorous modeling of the fluid mechanics of a complex viscoelastic biosystem in which the probe sensing process is intimately coupled to the membrane biomechanics. The effects of the probe dimensions and elastic properties of the membrane as well as intra- and extracellular viscosities are investigated in detail to establish the structure and evolution of the fluid field as well as the dynamics of membrane deformation. The results of numerical simulations, supported by predictions of the scaling analysis of forces acting on the probe, suggest that viscous drag is the dominant force dictating the probe dynamics as it approaches a biological interface. The increase in the drag force is shown to be measurable, to scale linearly with an increase in the viscosity ratio of the fluids on either side of the membrane, and to be inversely proportional to the probe-to-membrane distance. This leads to the postulation of a new strategy for lipid membrane imaging by AFM or other mechanosensing methods using a variation in the maximum drag force as an indicator of the membrane position.

Lipid rafts: at a crossroad between cell biology and physics

Membrane lateral heterogeneity is accepted as a requirement for the function of biological membranes and the notion of lipid rafts gives specificity to this broad concept. However, the lipid raft field is now at a technical impasse because the physical tools to study biological membranes as a liquid that is ordered in space and time are still being developed. This has lead to a disconnection between the concept of lipid rafts as derived from biochemical and biophysical assays and their existence in the cell. Here, we compare the concept of lipid rafts as it has emerged from the study of synthetic membranes with the reality of lateral heterogeneity in biological membranes. Further application of existing tools and the development of new tools are needed to understand the dynamic heterogeneity of biological membranes.

Adhesion force between calcium oxalate monohydrate crystal and kidney epithelial cells and possible relevance for kidney stone formation

AFM interaction force measurements have been performed between calcium oxalate monohydrate crystal (COM) colloidal probes and monolayers of renal epithelial cells (on a polymer substrate) in artificial urine (AU) solutions. The adhesion force was measured for the COM/MDCK cell interaction, while no adhesion force was found for the COM/LLC-PK(1) cell interaction. Long-range repulsive forces for both lines of cells were measured in the range of 2-3 mum. After removal of the cell from the substrate by the AU flow, the basal membrane (BM), with a thickness of 100-200 nm, remained on the substrate. In this case, the shorter-range repulsive forces were found on the extending (approaching) portion of force/indentation curves. Similar to the COM/MDCK cell interaction, the retracting portions of curves for COM/basal membranes have shown the existence of the attractive force of adhesion for the interaction of COM with a BM of MDCK cells, while no adhesion was found for COM/BM LLC-PK(1) cells interaction. No adhesion force was found for the interaction of a BM (of any cells) with the silicon nitride tip. Besides the hydrodynamic reasons, the adhesion difference between LLC-PK(1) and MDCK cells possibly explains the preferential deposition of crystals only in collecting ducts (lined with MDCK-type cells) and the lack of the crystal deposition in the proximal tubules (lined with LLC-PK(1)-type cells). Previous treatments of cells with oxalate alone increased the adhesion force COM/BM MDCK; however, even after oxalate treatment there was small or no adhesion between COM and BM LLC-PK(1) cells. Note that the adhesion force for COM/BM MDCK is practically independent of the probe velocity, i.e., does not have the viscous origin. Evaluation of the adhesion energy shows that this force should be related to the ionic or hydrogen bonds of samples.

Immunogold detection of types I and II chondrocyte collagen fibrils: An in situ atomic force microscopic investigation

Chondrocyte tissue engineering is a major challenge in the field of cartilage repair. The phenotype of chondrocytes consists of cartilage specific proteoglycan and type II collagen. During serial passages, chondrocytes dedifferentiate into cells, presenting a fibroblast-like phenotype consisting predominately of type I collagen synthesis. Observation of native collagen fibers could be visualized by atomic force microscope. Here, we developed an original and useful atomic force microscopy-based immunogold technique allowing biochemical distinction between types I and II collagen fibers. Imaging of 40-nm gold particles staining collagen fibers was performed in tapping mode. Rat 1 fibroblasts and human chondrosarcoma cells were used as positive models for types I and II collagen, respectively. As demonstrated by our data, primary rat chondrocytes adhering for 48 h on a glass substrate synthesize type II collagen native fibers. This technique allows analyses of local areas of the extracellular matrix of fixed cells, providing complementary data about cartilage phenotype. This simple approach could be of major interest for the biologist community in routine laboratory investigations, to localize in situ, macromolecules of the extracellular matrix.

Measuring cell adhesion forces of primary gastrulating cells from zebrafish using atomic force microscopy

During vertebrate gastrulation, progenitor cells of different germ layers acquire specific adhesive properties that contribute to germ layer formation and separation. Wnt signals have been suggested to function in this process by modulating the different levels of adhesion between the germ layers, however, direct evidence for this is still lacking. Here we show that Wnt11, a key signal regulating gastrulation movements, is needed for the adhesion of zebrafish mesendodermal progenitor cells to fibronectin, an abundant extracellular matrix component during gastrulation. To measure this effect, we developed an assay to quantify the adhesion of single zebrafish primary mesendodermal progenitors using atomic-force microscopy (AFM). We observed significant differences in detachment force and work between cultured mesendodermal progenitors from wild-type embryos and from slb/wnt11 mutant embryos, which carry a loss-of-function mutation in the wnt11 gene, when tested on fibronectin-coated substrates. These differences were probably due to reduced adhesion to the fibronectin substrate as neither the overall cell morphology nor the cell elasticity grossly differed between wild-type and mutant cells. Furthermore, in the presence of inhibitors of fibronectin-integrin binding, such as RGD peptides, the adhesion force and work were strongly decreased, indicating that integrins are involved in the binding of mesendodermal progenitors in our assay. These findings demonstrate that AFM can be used to quantitatively determine the substrate-adhesion of cultured primary gastrulating cells and provide insight into the role of Wnt11 signalling in modulating cell adhesion at the single cell scale.

Cell wall modification in grapevine cells in response to UV stress investigated by atomic force microscopy

Despite cell wall reinforcement being a well-known defence mechanism of plants, it remains poorly characterized from a physical point of view. The objective of this work was to further describe this mechanism. Vitis vinifera cv Gamay cells were treated with UV-light (254 nm), a well-known elicitor of defence mechanisms in grapevines, and physical cell wall modifications were observed using the atomic force microscopy (AFM) under native conditions. The grapevine cell suspensions were continuously observed in their culture medium from 30 min to 24h after elicitation. In the beginning, cellulose fibrils covered by a matrix surrounded the control and treated cells. After 3 h, the elicited cells displayed sprouted expansions around the cell wall that correspond to pectin chains. These expansions were not observed on untreated grapevine cells. The AFM tip was used to determine the average surface elastic modulus of cell wall that account for cell wall mechanical properties. The elasticity is diminished in UV-treated cells. In a comparative study, grapevine cells showed the same decrease in cell wall elasticity when treated with a fungal biotic elicitor of defence response. These results demonstrate cell wall strengthening by UV stress.

High-resolution three-dimensional imaging of the lateral plasma membrane of cochlear outer hair cells by atomic force microscopy

The outer hair cells (OHCs) from the mammalian organ of Corti are assumed to enhance the sensitivity and the selectivity of the cochlea via an electromotile response to sound stimulation. These OHC mechanical changes feed energy back into the cochlea before completion of the transduction process by inner hair cells. OHC electromotility is thought to depend on specific transmembrane motor proteins. Electron microscopy has been used previously to image the OHC lateral plasma membrane, where voltage sensors and motors are located. A very specific and regular organization of membrane particles has been described, together with an equally specific submembraneous meshwork of cytoskeleton anchored to the plasma membrane. To confirm and extend these observations, we have used, for the first time on the OHC lateral wall, atomic force microscopy (AFM). As a result of an improved tapping mode technique as well as the unique ultrastructural organization of the OHC plasma membrane, we have obtained high-resolution three-dimensional (3D) images of a markedly enhanced quality, allowing high-resolution 3D imaging. Tapping-mode AFM confirmed the presence of regularly aligned particles (presumably transmembrane proteins) on both faces of the OHC plasma membrane. It also revealed the presence of markedly different membrane domains, smooth and undulating. The differences between these zones probably are due to local differences in cytoskeleton-membrane interactions. Moreover, 3D reconstructions allowed us to distinguish between globular and pore-like particles, a distinction that may be of great functional significance.