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Biomedical subjects

M Radmacher

Publications and source records attributed to M Radmacher.

12 recordsLinked to original sources
PubMed1999

Dimensional and mechanical dynamics of active and stable edges in motile fibroblasts investigated by using atomic force microscopy.

The atomic force microscope (AFM) was employed to investigate the extension and retraction dynamics of protruding and stable edges of motile 3T3 fibroblasts in culture. Such dynamics closely paralleled the results of earlier studies employing video microscopy that indicated that the AFM force-mapping technique does not appreciably perturb these dynamics. Force scans permitted height determinations of active and stable edges. Whereas the profiles of active edges are flat with average heights of 0.4-0.8 micrometer, stable edges smoothly ascend to 2-3 micrometers within about 6 micrometers of the edge. In the region of the leading edge, the height fluctuates up to 50% (SD) of the mean value, much more than the stable edge; this fluctuation presumably reflects differences in underlying cytoskeletal activity. In addition, force mapping yields an estimate of the local Young's modulus or modulus of elasticity (E, the cortical stiffness). This stiffness will be related to "cortical tension," can be accurately calculated for the stable edges, and is approximately 12 kPa in this case. The thinness of the leading edge precludes accurate estimation of the E values, but within 4 micrometers of the margin it is considerably smaller than that for stable edges, which have an upper limit of 3-5 kPa. Although blebbing cannot absolutely be ruled out as a mechanism of extension, the data are consistent with an actin polymerization and/or myosin motor mechanism in which the average material properties of the extending margin would be nearly constant to the edge. Because the leading edge is softer than the stable edge, these data also are consistent with the notion that extension preferentially occurs in regions of lower cortical tension.

3T3 Cells↗
PubMed1997

AFM imaging and elasticity measurements on living rat liver macrophages.

The authors investigated the morphology and the elastic properties of living cultured rat liver macrophages (Kupffer cells) with an atomic force microscope (AFM). Continuous imaging and elasticity mapping of individual cells in physiological buffer was carried out for several hours without damaging the cells as judged by their persistent undisturbed morphology. Dynamic events such as protrusive activity were observed in time course. The importance of the cytoskeleton for the mechanical properties of the cell has been investigated by measuring the cell's elasticity as a function of position. Chemical disassembly of the actin network by applying 10 microgram/ml cytochalasin B decreased the cell's average elastic modulus seven-fold within less than 40 minutes. Treating the cells with 0.1 micrograms/ml latrunculin A resulted in a two-fold decrease in the elastic modulus merely in the perinuclear region after 40 minutes, whereas other parts of the cell were not affected.

Animals↗
PubMed1997

Investigating the cytoskeleton of chicken cardiocytes with the atomic force microscope.

We have investigated living chicken cardiocytes with an atomic force microscope (AFM). Cytoskeletal structures like stress fibers can easily be imaged with the AFM. Here we have also measured the cell's elastic properties. By taking force curves as a function of lateral position (force mapping) we could compare the elastic properties at different locations of the same cell. In the lamellipodal region investigated here in detail, the elastic moduli range from around 10 up to 200 kPa on top of stress fibers. By degradation with cytochalasin B we can estimate to what extent the elastic properties of this type of cell are determined by the actin network.

Actins↗
PubMed1996

Measuring the viscoelastic properties of human platelets with the atomic force microscope.

We have measured force curves as a function of the lateral position on top of human platelets with the atomic force microscope. These force curves show the indentation of the cell as the tip loads the sample. By analyzing these force curves we were able to determine the elastic modulus of the platelet with a lateral resolution of approximately 100 nm. The elastic moduli were in a range of 1-50 kPa measured in the frequency range of 1-50 Hz. Loading forces could be controlled with a resolution of 80 pN and indentations of the platelet could be determined with a resolution of 20 nm.

Biophysical Phenomena↗
PubMed1996

Protein tracking and detection of protein motion using atomic force microscopy.

Height fluctuations over three different proteins, immunoglobulin G, urease, and microtubules, have been measured using an atomic force microscope (AFM) operating in fluid tapping mode. This was achieved by using a protein-tracking system, where the AFM tip was periodically repositioned above a single protein molecule (or structure) as thermal drifting occurred. Height (z-piezo signal) data were taken in 1 - or 2-s time slices with the tip over the molecule and compared to data taken on the support. The measured fluctuations were consistently higher when the tip was positioned over the protein, as opposed to the support the protein was adsorbed on. Similar measurements over patches of an amphiphile, where the noise was identical to that on the support, suggest that the noise increase is due to some intrinsic property of proteins and is not a result of different tip-sample interactions over soft samples. The orientation of the adsorbed proteins in these preliminary studies was not known; thus it was not possible to make correlations between the observed motion and specific protein structure or protein function beyond noting that the observed height fluctuations were greater for an antibody (anti-bovine IgG) and an enzyme (urease) than for microtubules.

Animals↗
PubMed1995

Imaging soft samples with the atomic force microscope: gelatin in water and propanol.

We have imaged mica coated with thin gelatin films in water, propanol, and mixtures of these two liquids by atomic force microscopy (AFM). The elastic modulus (Young's modulus) can be tuned from 20 kPa to more than 0.1 GPa depending on the ratio of propanol to water. The resolution is best in pure propanol, on the order of 20 nm, and becomes worse for the softer samples. The degradation in resolution can be understood by considering the elastic indentation of the gelatin caused by the AFM tip. This indentation becomes larger and thus the contact area becomes larger the softer the sample is. Therefore this study may be used to estimate the resolution to be expected with an AFM on other soft samples, such as cells. Nondestructive imaging was possible only by imaging at forces < 1 nN. This was difficult to achieve in contact mode because of drift in the zero load deflection of the cantilever, supposedly caused by temperature drift, but straightforward in tapping mode.

1-Propanol↗
PubMed1994

Direct observation of enzyme activity with the atomic force microscope.

The height fluctuations on top of the protein lysozyme adsorbed on mica were measured locally with an atomic force microscope operated in tapping mode in liquid. Height fluctuations of an apparent size of 1 nanometer that lasted for about 50 milliseconds were observed over lysozyme molecules when a substrate (oligoglycoside) was present. In the presence of the inhibitor chitobiose, these height fluctuations decreased to the level without the oligoglycoside. The most straightforward interpretation of these results is that the height fluctuations correspond to the conformational changes of lysozyme during hydrolysis. It is also possible, however, that the height fluctuations are, at least in part, the result of a different height or elasticity of the transient complex of lysozyme plus the substrate.

Adsorption↗
PubMed1994

Granula motion and membrane spreading during activation of human platelets imaged by atomic force microscopy.

The redistribution of platelet constituents during activation is essential for their physiological function of maintaining hemostasis. We report here about real time investigations of the activation of native human platelets under physiological conditions from the initial formation of filopodia to the fully spread form by atomic force microscopy. We followed the trafficking of granules and their interaction with the plasma membrane within single cells. Our results show movement of certain granula towards the lamellipodia. Analysis of this rearrangement and the subsequent enlargement of the platelet surface reveals details of the membrane spreading process. Images of living cells are presented that show the distribution of cytoskeletal components and membrane-bound filaments at a resolution of better than 50 nm. The local minimum forces between the tip and the platelets were estimated to be smaller than 60 pN. A model for the elastic contributions of the glycocalix to the tip/membrane interaction was developed using the theory of grafted polymers.

Biophysical Phenomena↗
PubMed1994

Mapping interaction forces with the atomic force microscope.

Force curves were recorded as the sample was raster-scanned under the tip. This opens new opportunities for imaging with the atomic force microscope: several characteristics of the samples can be measured simultaneously, for example, topography, adhesion forces, elasticity, van der Waals, and electrostatic interactions. The new opportunities are illustrated by images of several characteristics of thin metal films, aggregates of lysozyme, and single molecules of DNA.

Adsorption↗
PubMed1993

In vitro activation of human platelets triggered and probed by atomic force microscopy.

We report on the activation of human platelets probed by atomic force microscopy under nearly physiological conditions. We could image native platelets in different stages of activation at a resolution of about 100 nm. Intracellular structures within the platelets could be resolved without staining. Furthermore we could trigger and follow the activation process of individual platelets, demonstrating that the platelets are still intact and are not destroyed or severely harmed by this novel imaging process.

Blood Platelets↗
PubMed1992

From molecules to cells: imaging soft samples with the atomic force microscope.

Since its invention a few years ago, the atomic force microscope has become one of the most widely used near-field microscopes. Surfaces of hard sample are imaged routinely with atomic resolution. Soft samples, however, remain challenging. An overview is presented on the application of atomic force microscopy to organic samples ranging from thin ordered films at molecular resolution to living cells. Fundamental mechanisms of the image formation are discussed, and novel imaging modes are introduced that exploit different aspects of the tip-sample interaction for local measurements of the micromechanical properties of the sample. As examples, images of Langmuir-Blodgett films, which map the local viscoelasticity as well as the friction coefficient, are presented.

Actin Cytoskeleton↗