Imaging surface and submembranous structures with the atomic force microscope: a study on living cancer cells, fibroblasts and macrophages

Nanoscopic lipid domain dynamics revealed by atomic force microscopy

Intrinsic heterogeneities, represented as domain formations in biological membranes, are important to both the structure and function of the membranes. We observed domain formations in mixed lipid bilayers of dipalmitoylphosphatidylcholine (DPPC), dilauroylphosphatidylcholine (DLPC), and cholesterol (chol) in a fluid environment using an atomic force microscope (AFM). At room temperature, we demonstrated that both microscopic and nanoscopic domains coexist and the DPPC-rich domain is approximately 1.4 nm higher than the surrounding DLPC-rich membrane areas as a consequence of intrinsic phase differences. DPPC-rich microscopic domains became larger as DPPC concentration increased. In cholesterol-free mixtures, nanoscopic DPPC-rich domain sizes ranged from 26 to 46 nm depending on phospholipid concentration. Domain size varied between 33 and 48 nm in the presence of cholesterol (0 < or = [chol] < or = 40). The nanoscopic domains were markedly fragmented near [chol] = 0.135 and appeared to fuse more readily into microscopic domains at higher and lower [chol]. By phase balance analyses we demonstrated phase behavior differences between a free-vesicle GUV system studied by confocal light microscopy and a supported membrane system studied by AFM. We propose a new three-dimensional phase diagram elucidating the effects of a solid substrate support on lipid phase behavior relevant to complex membrane phase phenomena in biological systems.

Measuring the elastic properties of living cells by the atomic force microscope

In this chapter I discussed the possibility of measuring elastic properties of living cells by AFM. One reason for using the AFM for this purpose is its ability to both measure locally the mechanics of a cell and to distinguish different regions of the cell. Since the AFM can be operated under physiological conditions cellular processes can be followed, for example, cytokinesis and the investigation of the migration of cells.

Investigating live and fixed epithelial and fibroblast cells by atomic force microscopy

PURPOSE

To investigate the use of atomic force microscopy (AFM) to image live and fixed cell in culture. Rabbit corneal fibroblasts, Chang conjunctival cells, and transformed human corneal epithelial cells were chosen so that AFM parameters could be set for future use in toxicologic and pharmacologic studies of ocular cells.

METHODS

Contact mode AFM was performed under air and in balanced salt solution (BSS) using live and fixed cells. All cell lines were imaged in the height mode for optimal resolution of cellular features.

RESULTS

Images of fixed cells showed no discernible differences in surface features when visualized in air or under physiologic solution. Structural differences were observed, however, between fixed and live cells in BSS. Although the AFM technique provides high quality images of live cells under BSS, sub-membrane features of live cells are more well-defined compared to fixed cells. It was also possible to image live cells in air if imaging was completed within 10 minutes of removal of the cells from culture medium. Images of cytoskeletal features under air were similar to those obtained under BSS.

CONCLUSION

The atomic force microscopy technique can be used to study cells and provide sub-cellular details at resolution equal to or in some situations better than the scanning electron microscopy technique. However, parameters for imaging have to be tailored for individual experimental goals.

Investigating live and fixed epithelial and fibroblast cells by atomic force microscopy

PURPOSE

To investigate the use of atomic force microscopy (AFM) to image live and fixed cell in culture. Rabbit corneal fibroblasts, Chang conjunctival cells, and transformed human corneal epithelial cells were chosen so that AFM parameters could be set for future use in toxicologic and pharmacologic studies of ocular cells.

METHODS

Contact mode AFM was performed under air and in balanced salt solution (BSS) using live and fixed cells. All cell lines were imaged in the height mode for optimal resolution of cellular features.

RESULTS

Images of fixed cells showed no discernible differences in surface features when visualized in air or under physiologic solution. Structural differences were observed, however, between fixed and live cells in BSS. Although the AFM technique provides high quality images of live cells under BSS, sub-membrane features of live cells are more well-defined compared to fixed cells. It was also possible to image live cells in air if imaging was completed within 10 minutes of removal of the cells from culture medium. Images of cytoskeletal features under air were similar to those obtained under BSS.

CONCLUSIONS

The atomic force microscopy technique can be used to study cells and provide sub-cellular details at resolution equal to or in some situations better than the scanning electron microscopy technique. However, parameters for imaging have to be tailored for individual experimental goals.

Early detection of cytotoxic events between hepatic natural killer cells and colon carcinoma cells as probed with the atomic force microscope

The atomic force microscope (AFM) is a powerful tool to investigate surface and submembranous structures of living cells under physiological conditions at high resolution. These properties enabled us to study the interaction between live hepatic natural killer (NK) cells, also called pit cells, and colon carcinoma cells in vitro by AFM. In addition, the staining for filamentous actin and DNA was performed and served as a reference, because actin and nuclear observations at the light microscopic level during the cytotoxic interaction between these two cell types have been presented earlier. In this study, we collected evidence that conjugation of hepatic NK cells with CC531s colon carcinoma cells results in a decreased binding of CC531s cells to the substratum as probed with the AFM in contact mode as early as 10 min after cell contact (n = 11). To avoid the lateral forces and smearing artefacts of contact mode AFM, non-contact imaging was performed on hepatic NK/CC531s cell conjugates, resulting in identical observations (n = 3). In contrast, the first cytotoxic signs, as determined with the nuclear staining dye Hoechst 33342, could be observed 3 h after the start of the co-culture. This study illustrates that the AFM can be used to probe early cytotoxic effects of effector to target cell contact in nearby physiological conditions. Other routine cytotoxicity tests detect the first cytotoxic effects after 1.5-3 h co-incubation at the earliest.

Structural analysis of heavy ion radiation-induced chromosome aberrations by atomic force microscopy

Heavy ion radiation (high linear energy transfer, LET, radiation) induces various types of chromosome aberration. In this report, we describe a new method employing an atomic force microscope (AFM) for nanometer-level structural analysis of chromosome damage induced by heavy ion irradiation. Metaphase mouse chromosomes with chromatid gap or chromatid breaks induced by heavy ion irradiation were marked under a light microscope. Then the detailed structure of chromosomes of Giemsa-stained or unstained samples was visualized by the AFM. The height data of chromosomes obtained by AFM provided useful information to distinguish chromatid gaps and breaks. A fibrous structure was observed on the unstained chromosome, the average width of which was about 45.8 nm in the image of AFM. These structures were considered to be 30-nm fibers on the chromosome. The structure of the break point regions induced by neon- or carbon-ion irradiation was imaged by AFM. In some cases, the fibrous structure of break points was detected by AFM imaging after carbon ion irradiation. These observations indicated that AFM is a useful tool for analysis of chromosome aberrations induced by heavy ion radiation.

Novel methods for studying lipids and lipases and their mutual interaction at interfaces. Part I. Atomic force microscopy

Mono-layers of lipids and their interaction with surface active enzymes (lipases) have been studied for more than a century. During the past decade new insight into this area has been obtained due to the development of scanning probe microscopy. This novel method provides direct microscopic information about the system in question and allows in situ investigations under near physiological conditions. In the present review the theory, experimental set-up and sample requirements of atomic force microscopy (AFM) are described. An overview of recent results is also presented with special emphasis on lipase hydrolysis and kinetics investigated in situ using AFM.

Transgene delivery with a cationic lipid in the presence of amyloid beta (betaAP) peptide

The ability of a cationic lipid to deliver plasmid DNA (pDNA) in presence of the neurotoxic fragment of amyloid beta-peptide was evaluated. Pre-treatment of cells with betaAP (25-35) peptide resulted in a modest increase in transgene expression. When betaAP (25-35) peptide was mixed with the pDNA/liposome complex and used, the complexes lost their ability to transfect. However, the reverse sequenced betaAP (35-25) peptide demonstrated no significant differences in transgene expression in pre-treated cells, and in cells where betaAP (35-25) peptide was mixed with pDNA/liposome complexes and transfected. The amount of pDNA delivered to the cells was decreased in presence of betaAP (25-35) as measured with flow cytometry using fluorescently labeled liposomes. The decreased endocytosis may be due to their rod-like structure formation as demonstrated by electron microscopy and atomic force microscopy (AFM). These results demonstrate that betaAP (25-35) peptide may interfere with gene delivery with cationic systems.

Imaging and force-distance analysis of human fibroblasts in vitro by atomic force microscopy

The structure of human fibroblasts have been characterised in vitro by atomic force microscopy (AFM) operated in the imaging or in the force versus distance (F-d) modes. The choice of cell substrate is important to ensure good adhesion. Of greater significance in the context of AFM analysis, is the observation that the substrate affects the imaging conditions for in vitro analysis of live cells. For instance, very rarely will glass coverslips lead to acceptable outcomes (i.e., resolved cytoskeletal structure). Activated tissue culture dishes, on the other hand, promote conditions that routinely result in good quality images. Those conditions are then unaffected by adoption of relatively high force loadings (more than 10 nN), large fields of view (100 x 100 microm2) and high scan speeds (up to ca. 200 microm/sec), all of which exceed values recommended in the literature. Plasma membranes are fragile in the context of AFM analysis (F-d analysis gives an equivalent Young's Modulus of ca. 5 kPa). However, the present work suggests that fragility per se need not be a problem, rather it is the adhesive interactions with the tip, which under some circumstances may exceed 20 nN, that are the source of poor imaging conditions. The present results, being supported by a qualitative model, suggest that the activated substrate acts as a preferential scavenger of cellular debris thus preventing the tip from biofouling, and will therefore promote low adhesion between tip and membrane. Good imaging conditions provide non-destructive in vitro information about cytoskeletal structure and dynamics, as shown in two examples concerned with cytochalasin treatment and with the MTT assay.