Probing elasticity and adhesion of live cells by atomic force microscopy indentation

Atomic force microscopy (AFM) indentation has become an important technique for quantifying the mechanical properties of live cells at nanoscale. However, determination of cell elasticity modulus from the force–displacement curves measured in the AFM indentations is not a trivial task. The present work shows that these force–displacement curves are affected by indenter-cell adhesion force, while the use of an appropriate indentation model may provide information on the cell elasticity and the work of adhesion of the cell membrane to the surface of the AFM probes. A recently proposed indentation model (Sirghi, Rossi in Appl Phys Lett 89:243118, 2006), which accounts for the effect of the adhesion force in nanoscale indentation, is applied to the AFM indentation experiments performed on live cells with pyramidal indenters. The model considers that the indentation force equilibrates the elastic force of the cell cytoskeleton and the adhesion force of the cell membrane. It is assumed that the indenter-cell contact area and the adhesion force decrease continuously during the unloading part of the indentation (peeling model). Force–displacement curves measured in indentation experiments performed with silicon nitride AFM probes with pyramidal tips on live cells (mouse fibroblast Balb/c3T3 clone A31-1-1) in physiological medium at 37°C agree well with the theoretical prediction and are used to determine the cell elasticity modulus and indenter-cell work of adhesion. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Laminar analysis of hippocampal potentials evoked by peripheral stimulation

From areas SA₁ and SA₂ of the dorsal hippocampus of unanesthetized rabbits immobilized with d-tubocurarine a laminar analysis was made of evoked potentials (EP) in response to stimulation of the sciatic nerves. Inversion of the initial surface-positive phase of the EP was observed at the level of the pyramidal layer. The subsequent surface-negative phase reached a maximum value in the layer of basal dendrites of the pyramidal cells. The initial portion was inverted somewhat above the pyramidal layer and reached its maximum value approximately at the boundary of the pyramidal and radial layers. The change in sign of the remaining portion of this component occurred 0.3–0.4 mm deeper than the pyramidal layer. It is suggested that both components of the EP picked up from the hippocampal surface are due to an excitatory postsynaptic potential (EPSP) at the apical (positive phase), and basal (negative phase) dendrites. The positivity in the region of the pyramidal somata appears to be an extracellular reflection of a composite postsynaptic potential (IPSP) generated in this region.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 4. pp. 434–438, July–August, 1970.     

Mechanical function of dystrophin in muscle cells

We have directly measured the contribution of dystrophin to the cortical stiffness of living muscle cells and have demonstrated that lack of dystrophin causes a substantial reduction in stiffness. The inferred molecular structure of dystrophin, its preferential localization underlying the cell surface, and the apparent fragility of muscle cells which lack this protein suggest that dystrophin stabilizes the sarcolemma and protects the myofiber from disruption during contraction. Lacking dystrophin, the muscle cells of persons with Duchenne muscular dystrophy (DMD) are abnormally vulnerable. These facts suggest that muscle cells with dystrophin should be stiffer than similar cells which lack this protein. We have tested this hypothesis by measuring the local stiffness of the membrane skeleton of myotubes cultured from mdx mice and normal controls. Like humans with DMD mdx mice lack dystrophin due to an x-linked mutation and provide a good model for the human disease. Deformability was measured as the resistance to indentation of a small area of the cell surface (to a depth of 1 micron) by a glass probe 1 micron in radius. The stiffness of the membrane skeleton was evaluated as the increment of force (mdyne) per micron of indentation. Normal myotubes with an average stiffness value of 1.23 +/- 0.04 (SE) mdyne/micron were about fourfold stiffer than myotubes cultured from mdx mice (0.34 +/- 0.014 mdyne/micron). We verified by immunofluorescence that both normal and mdx myotubes, which were at a similar developmental stage, expressed sarcomeric myosin, and that dystrophin was detected, diffusely distributed, only in normal, not in mdx myotubes. These results confirm that dystrophin and its associated proteins can reinforce the myotube membrane skeleton by increasing its stiffness and that dystrophin function and, therefore, the efficiency of therapeutic restoration of dystrophin can be assayed through its mechanical effects on muscle cells.     

Effects of dynamic impacts on human bones

Flat bones of human skeleton were subjected to dynamic indentation with ball indenters. The impacted surface was studied under high magnification and also by using the technique of multiple beam interferometry. The impulse caused the pile up of material at a little distance from the edge of the indent. The diameter of indent is found to increase as fourth root of the energy of impact. Bone structure also has the tendency to minimize the damage caused by external forces. There was about 90% recovery in deformation in the depth of indents due to internal stresses created inside the bone by the impact.     

Thoughts on the cerebral cortex

The cortex is often described as a network processing information in the direction from sensory to motor areas. However, the structure of the cortex is asymmetrical only in the vertical direction, suggesting an input-output transformation between layers rather than between areas. This operation must be a very generally applicable one, since the plan of the cortex is basically the same everywhere. In an attempt to understand it, a skeleton cortex of only pyramidal cells is considered. They are characterized by a double dendritic expansion, an apical one in the first layer, which is considered as the input layer, and a basal one which receives excitation from the axon collaterals of other pyramidal cells. If pyramidal cells learn (perhaps by growing dendritic spines) to respond to frequent constellations of activity in their afferents, each will learn a property of the input (through its apical dendrites) provided that it was preceded by other properties sensed by neighbouring pyramidal cells (which influences it through its basal dendrites). Thus the pyramidal cells will code the input in terms of properties which have a tendency to follow each other. This will be a coding which reflects the causal structure of the world. Various uses of a network embodying the conditional probabilities of events in the input are described, including recognition of familiar sequences and prediction. The local variation of fiber patterns in the cerebral cortex of man, described as myeloarchitectonics, is interpreted as a macroscopical expression of the different statistics of the set of conditional probabilities linking the events represented by individual pyramidal cells in different areas (in different functional contexts).     

Skeletal development in <Emphasis Type="Italic">Acropora palmata</Emphasis> (Lamarck 1816): a scanning electron microscope (SEM) comparison demonstrating similar mechanisms of skeletal extension in axial versus encrusting growth

Many Acropora palmata colonies consist of an encrusting basal portion and erect branches. Linear growth of the skeleton results in extension along the substrate (encrusting growth), lengthening of branches (axial growth) and thickening of branches and crust (radial growth). Scanning Electron Microscopy is used to compare the mechanisms of skeletal extension between encrusting growth and axial growth. In encrusting growth, the distal margin of the skeleton lacks corallites (which develop about 1 mm from the edge); in contrast, in axial growth, axial corallites along the branch tip form the distal portion of the skeleton. In both locations, the distal margin of the skeleton consists of a lattice-like structure composed of rods that extend from the body of the skeleton and bars that connect these rods. An actively extending skeleton is characterized by sharply pointed rods and partially developed bars. Distal growth of rods (and formation of bars) is effected by the formation of new sclerodermites. Each sclerodermite begins with the deposition of fusiform crystals (that range in length from 1 to 5 μm). These provide a surface for nucleation and growth of spherulitic tufts, clusters of short (<1 μm long) aragonite needles. The needles that are oriented perpendicular to the axis of the skeletal element (rod or bar), and perpendicular to the overlying calicoblastic epithelium, continue extension to appear on the surface of the skeleton as 10–15 μm wide bundles (of needle tips) called fasciculi. However, some crusts that abut competitors for space have a different morphology of skeletal elements (rods and bars). The distal edge of these crusts terminates in blunt coalescing rods, and bars that are fully formed. Absence of fusiform crystals, lack of sharply pointed rods and bars, and full development of sclerodermites characterize a skeletal region that has ceased, perhaps only temporarily, skeletal extension.     

Isolated absence of human carpal bones.

An unusual instance of congenital isolated absence of carpal bones, in a 38-year-old woman, characterized by total lack of the scaphoid, lunate, and pyramidal and a massive fusion of the distal carpal row is reported. This condition is thought to have been the result of an embryonic error exclusively involving the portion of the autopodic blastema from which the carpus originates.     

A multivariate dental sexing technique

This paper presents a method of sexing skeletal remains using dental measurements. A base sample from a population is sexed with reference to the postcranial skeleton and the dental measurements (buccal-lingual and mesial-distal diameters) are analyzed by the discriminant function technique. A linear function is derived, which will classify by sex the remaining portion of the population.     

Änderung der zeitlich-räumlichen Verteilung von228Ra und224Ra im RES und markfreien Skelet nach Inkorporierung von kolloidalem ThO2 (Thorotrast)

Summary Animal experiments were done in order toanswer the question for the variation with time of both the distribution of Th-232 and its daughters and the activity ratios between the radionuclides of the thorium decay chain immediately fter incorporation ofthorotrast.The results of these investigations indicate a continuous elimination of Ra-224 from ThO₂-aggregates in the organs of the RES, liver, spleen, and red bonemarrow. According to the variation with time of the activity ratio between Ra-224 and Th-228, even from the first day after incorporation, about 35% of the Ra-224 atoms formed in the aggregates may leave the aggregates due to recoil at -decay of Th-228 and will be eliminated from the organs of the RES. This means that at least at that time the process of aggregation of the ThO²-particles within the organs of the RES has to be finished.This portion of Ra-224 being eliminated from the RES is partly translocated into the marrow-free skeleton. The same is true for the portion of Ra-224 and R-228 in ionic form at the total activity of the suspension, that means, according to the range of the Ra-228 and Ra-224 recoil atoms within the ThO²-particles, for 80% to 90% of the total Ra-228 and Ra-224 activity of the suspension.From the variation with time of the specific Ra-228 activity of marrow-free bone it could be seen that Ra-228 and Ra-224 being fixed in the marrow-free skeleton in ionic form at the time of incorporation is eliminated from the skeleton according to a potential function. As consequence of a continuous translocation of Ra-224 to the skeleton due to decay of Th-228 within the organs of the RES the specific Ra-224 activity of marrow-free bone proved to be constant between 10 nd about 100 days fter incorporation ofthorotrast, considering decay of Ra-224 within the skeleton and its elimination from the skeleton.The experimental results were correlated to those of a theoretical model describing translocation of Ra-224 from the RES to the marrow-free skeleton and its excretion from the skeleton and the body.

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Die Untersuchungen wurden z. T. mit finanzieller Unterstützung der Europäischen Atomgemeinschaft EURATOM (Forschungsvertrag 031-67-3 PSTD) und des Bundesministeriums für Wissenschaftliche Forschung durchgeführt, wofür auch an dieser Stelle gedankt sei.

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Auszugsweise vorgetragen auf der 6. Jahrestagung der Deutschen Gesellschaft für Nuklearmedizin e.V. vom 26. bis 28. September 1968 in Wiesbaden.     

Crucial structural factors and mode of action of polyene amides as inhibitors for mitochondrial NADH-ubiquinone oxidoreductase (complex I)

Natural antibiotic polyene amides such as myxalamides are potent inhibitors of mitochondrial complex I. Because of the significant instability of this series of compounds due to an extended pi-conjugation skeleton, a detailed characterization of their inhibitory action has not been performed. To elucidate the action mechanism as well as binding manner of polyene amides with complex I, identification of the roles of each functional group in the inhibitory action is needed. We here synthesized a series of amide analogues and carried out structure-activity studies with bovine heart mitochondrial complex I. With respect to the left-hand portion, the natural pi-conjugation skeleton common to many natural products is not required for the inhibition and can be substituted with a simpler substructure such as a conjugated diene. The geometry and shape of the left-hand portion were shown to be important for the inhibition, suggesting that this portion may bind to a narrow hydrophobic pocket in the enzyme rather than merely partitioning into the lipid membrane phase. Concerning the right-hand portion of the inhibitor, the presence of the 2-methyl, amide NH, and (S)-1'-methyl groups was crucial for the activity, suggesting that both methyl groups neighboring the amide group finely adjust the hydrogen-bonding ability of the amide group. In contrast, modifications of the 2'-OH group did not significantly influence the activity, suggesting that the role of this functional group is not to serve as a hydrogen bond donor to the enzyme but to act as a hydrophilic anchor directing the right-hand portion at or near the membrane surface. Detailed characterization of the action mechanism indicated that the polyene amides share a common binding domain with other complex I inhibitors, though their binding position (or manner) within the domain may differ considerably from that of other inhibitors.     

Mechanical characterization of anisotropic planar biological soft tissues using large indentation: a computational feasibility study

Traditionally, the complex mechanical behavior of planar soft biological tissues is characterized by (multi)axial tensile testing. While uniaxial tests do not provide sufficient information for a full characterization of the material anisotropy, biaxial tensile tests are difficult to perform and tethering effects limit the analyses to a small central portion of the test sample. In both cases, determination of local mechanical properties is not trivial. Local mechanical characterization may be performed by indentation testing. Conventional indentation tests, however, often assume linear elastic and isotropic material properties, and therefore these tests are of limited use in characterizing the nonlinear, anisotropic material behavior typical for planar soft biological tissues. In this study, a spherical indentation experiment assuming large deformations is proposed. A finite element model of the aortic valve leaflet demonstrates that combining force and deformation gradient data, one single indentation test provides sufficient information to characterize the local material behavior. Parameter estimation is used to fit the computational model to simulated experimental data. The aortic valve leaflet is chosen as a typical example. However, the proposed method is expected to apply for the mechanical characterization of planar soft biological materials in general.     

Regulation Mechanism of the Lateral Diffusion of Band 3 in Erythrocyte Membranes by the Membrane Skeleton

Mechanisms that regulate the movement of a membrane spanning protein band 3 in erythrocyte ghosts were investigated at the level of a single or small groups of molecules using single particle tracking with an enhanced time resolution (0.22 ms). Two-thirds of band 3 undergo macroscopic diffusion: a band 3 molecule is temporarily corralled in a mesh of 110 nm in diameter, and hops to an adjacent mesh an average of every 350 ms. The rest (one-third) of band 3 exhibited oscillatory motion similar to that of spectrin, suggesting that these band 3 molecules are bound to spectrin. When the membrane skeletal network was dragged and deformed/translated using optical tweezers, band 3 molecules that were undergoing hop diffusion were displaced toward the same direction as the skeleton. Mild trypsin treatment of ghosts, which cleaves off the cytoplasmic portion of band 3 without affecting spectrin, actin, and protein 4.1, increased the intercompartmental hop rate of band 3 by a factor of 6, whereas it did not change the corral size and the microscopic diffusion rate within a corral. These results indicate that the cytoplasmic portion of band 3 collides with the membrane skeleton, which causes temporal confinement of band 3 inside a mesh of the membrane skeleton.     

Highly nonrandom features of synaptic connectivity in local cortical circuits

How different is local cortical circuitry from a random network? To answer this question, we probed synaptic connections with several hundred simultaneous quadruple whole-cell recordings from layer 5 pyramidal neurons in the rat visual cortex. Analysis of this dataset revealed several nonrandom features in synaptic connectivity. We confirmed previous reports that bidirectional connections are more common than expected in a random network. We found that several highly clustered three-neuron connectivity patterns are overrepresented, suggesting that connections tend to cluster together. We also analyzed synaptic connection strength as defined by the peak excitatory postsynaptic potential amplitude. We found that the distribution of synaptic connection strength differs significantly from the Poisson distribution and can be fitted by a lognormal distribution. Such a distribution has a heavier tail and implies that synaptic weight is concentrated among few synaptic connections. In addition, the strengths of synaptic connections sharing pre- or postsynaptic neurons are correlated, implying that strong connections are even more clustered than the weak ones. Therefore, the local cortical network structure can be viewed as a skeleton of stronger connections in a sea of weaker ones. Such a skeleton is likely to play an important role in network dynamics and should be investigated further.     

Elasticity of alveolar bone near dental implant-bone interfaces after one month's healing

Information is scarce about Young's modulus of healing bone surrounding an implant. The purpose of this preliminary study is to quantify elastic properties of pig alveolar bone that has healed for 1 month around titanium threaded dental implants, using the nanoindentation method. Two 2-year-old Sinclair miniswine were used for the study. Nanoindentation tests perpendicular to the bucco-lingual cross section were performed on harvested implant-bone blocks using the Hysitron TriboScope III. Nomarski differential interference contrast microscopy was used to identify pyramidal indentation measurements that were from bone. Reduced moduli, averaged for all anatomical regions, were found to start low (6.17 GPa) at the interface and gradually increase (slope=0.014) to a distance of 150 microm (7.89 GPa) from the implant surface, and then flatten to a slope of 0.001 from 150 to 1500 microm (10.13 GPa). Mean reduced modulus and its relationship to distance did not differ significantly by anatomic location (e.g., coronal, middle, and apical third; P>/=0.28 for all relevant tests) at 1 month after implantation.     

Dynamic elastic modulus of porcine articular cartilage determined at two different levels of tissue organization by indentation-type atomic force microscopy

Cartilage stiffness was measured ex vivo at the micrometer and nanometer scales to explore structure-mechanical property relationships at smaller scales than has been done previously. A method was developed to measure the dynamic elastic modulus, |E(*)|, in compression by indentation-type atomic force microscopy (IT AFM). Spherical indenter tips (radius = approximately 2.5 microm) and sharp pyramidal tips (radius = approximately 20 nm) were employed to probe micrometer-scale and nanometer-scale response, respectively. |E(*)| values were obtained at 3 Hz from 1024 unloading response curves recorded at a given location on subsurface cartilage from porcine femoral condyles. With the microsphere tips, the average modulus was approximately 2.6 MPa, in agreement with available millimeter-scale data, whereas with the sharp pyramidal tips, it was typically 100-fold lower. In contrast to cartilage, measurements made on agarose gels, a much more molecularly amorphous biomaterial, resulted in the same average modulus for both indentation tips. From results of AFM imaging of cartilage, the micrometer-scale spherical tips resolved no fine structure except some chondrocytes, whereas the nanometer-scale pyramidal tips resolved individual collagen fibers and their 67-nm axial repeat distance. These results suggest that the spherical AFM tip is large enough to measure the aggregate dynamic elastic modulus of cartilage, whereas the sharp AFM tip depicts the elastic properties of its fine structure. Additional measurements of cartilage stiffness following enzyme action revealed that elastase digestion of the collagen moiety lowered the modulus at the micrometer scale. In contrast, digestion of the proteoglycans moiety by cathepsin D had little effect on |E(*)| at the micrometer scale, but yielded a clear stiffening at the nanometer scale. Thus, cartilage compressive stiffness is different at the nanometer scale compared to the overall structural stiffness measured at the micrometer and larger scales because of the fine nanometer-scale structure, and enzyme-induced structural changes can affect this scale-dependent stiffness differently.     

Collagen deposition on a preformed grid

Appearance of collagen fibrils in the cuticle was seen by electron microscopy to be preceded by fonnation of a finely filamentous matrix material. At first, the fine filaments of the matrix are unorganized. However, signs of orthogonal ordering soon appear in the most superficial portion of the cuticle, and subsequently appear more basally and closer to the underlying epidermis. Meanwhile, fibrils of different staining properties and identifiable as collagen begin to be deposited in the superficial portion of the cuticle, the same region which first showed organized fine filaments. Then, like the fine filaments before them, the collagen fibrils polymerize more basally. Collagen appears to polymerize on the preformed skeleton of fine filaments as though the fine filaments caused the collagen to assemble. Neither the polymerization nor ordering of collagen fibrils seems to require direct cellular intervention but occur first in that portion of the cuticle which is furthest away from the underlying epidermis. The fine filaments may be self ordering, extracellular macromolecules which in turn determine the polymerization of collagen fibrils.     

Cell dynamic adhesion and elastic properties probed with cylindrical atomic force microscopy cantilever tips

Cell adhesion is required for essential biological functions such as migration, tissue formation and wound healing, and it is mediated by individual molecules that bind specifically to ligands on other cells or on the extracellular matrix. Atomic force microscopy (AFM) has been successfully used to measure cell adhesion at both single molecule and whole cell levels. However, the measurement of inherent cell adhesion properties requires a constant cell-probe contact area during indentation, a requirement which is not fulfilled in common pyramidal or spherical AFM tips. We developed a procedure using focused ion beam (FIB) technology by which we modified silicon pyramidal AFM cantilever tips to obtain flat-ended cylindrical tips with a constant and known area of contact. The tips were validated on elastic gels and living cells. Cylindrical tips showed a fairly linear force-indentation behaviour on both gels and cells for indentations >200 nm. Cylindrical tips coated with ligands were used to quantify inherent dynamic cell adhesion and elastic properties. Force, work of adhesion and elasticity showed a marked dynamic response. In contrast, the deformation applied to the cells before rupture was fairly constant within the probed dynamic range. Taken together, these results suggest that the dynamic adhesion strength is counterbalanced by the dynamic elastic response to keep a constant cell deformation regardless of the applied pulling rate.