PubMed Health⌕ Search

Biomedical subjects

Erich Sackmann

Publications and source records attributed to Erich Sackmann.

At least 19 recordsLinked to original sources
PubMed2005

Antagonist-induced deadhesion of specifically adhered vesicles.

By use of a model system consisting of giant vesicles adhering to flat substrates, we identified, both experimentally and theoretically, two new control mechanisms for antagonist-induced deadhesion. Adhesion is established by specific binding of surface-grafted E-selectin and vesicle-carrying oligosaccharide Lewis(X). Deadhesion is achieved by controlled titration of monoclonal antibodies against E-selectin. The first mechanism is characterized by a considerable retraction of the contact zone resulting in a loss of contact area between the vesicle and the substrate. Within the developed theoretical framework, the observed equilibrium state is understood as a balance between the spreading pressure of the vesicle and the antagonist-induced lateral pressure at the edge of the contact zone. In the second mechanism, the antibodies induce unbinding by penetrating the contact zone without significantly affecting its size. This process reveals the decomposition of the adhesion zone into microdomains of tight binding separated by strongly fluctuating sections of the membrane. Both experiment and theory show a sigmoidal decrease of the number of bound ligands as a function of the logarithm of antagonist concentration. The work presented herein also provides a new method for the determination of the receptor binding affinity of either the surface-embedded ligands or the competing antagonist molecules.

Animals↗
PubMed2005

Single-filament dynamics and long-range ordering of semiflexible biopolymers under flow and confinement.

We report the collective and single-filament dynamics of long semiflexible actin filaments flowing in an evaporating droplet adhering on glass and accumulating along the physical barrier constituted by the droplet triple line. The observation of fluorescent reporter filaments embedded in the entangled network enables us to relate the final collective organization of the accumulated filaments to the individual filament dynamics. Three areas corresponding to distinct filament organizations are observed in the region of the initial triple line pinning, after complete evaporation of the droplet. A nematic liquid-crystal-like alignment of the filaments is observed at the edge of the droplet because of the dynamic filament alignment, whereas a less-ordered packing is generated because of the bending and folding of most of the filaments. The latter unconventional dynamics is analyzed in terms of the amplification of undulation modes typical of semiflexible polymers. The receding regime of the droplet triple line leads finally to a remaining film of actin filaments showing random organization.

Actins↗
PubMed2005

Polymer-supported membranes as models of the cell surface.

Lipid-bilayer membranes supported on solid substrates are widely used as cell-surface models that connect biological and artificial materials. They can be placed either directly on solids or on ultrathin polymer supports that mimic the generic role of the extracellular matrix. The tools of modern genetic engineering and bioorganic chemistry make it possible to couple many types of biomolecule to supported membranes. This results in sophisticated interfaces that can be used to control, organize and study the properties and function of membranes and membrane-associated proteins. Particularly exciting opportunities arise when these systems are coupled with advanced semiconductor technology.

Cell Membrane↗
PubMed2005

A theoretical description of elastic pillar substrates in biophysical experiments.

Arrays of elastic pillars are used in biophysical experiments as sensors for traction forces. The evaluation of the forces can be complicated if they are coupled to the pillar displacements over large distances. This is the case if many of the pillars are interconnected by elastic linkages as, for example, in fiber networks that are grown on top of pillars. To calculate the traction forces in such a network, we developed a set of nonlinear inhomogeneous equations relating the forces in the linking elements to the resulting pillar deflections. We chose a homogeneous, activated two-dimensional network of cytoskeletal actin filaments to illustrate that a pillar substrate is generally not a force sensor but a force-gradient sensor. In homogeneous networks the forces acting along the filaments can be approximated by analyzing only pillar deflections in the edge zones of the substrate and by integration over the corresponding force gradients.

Actin Cytoskeleton↗
PubMed2005

Osmotic force-controlled microrheometry of entangled actin networks.

In studying a magnetic bead's creep response to force pulses in an entangled actin network we have found a novel regime where the bead motion obeys a power law x(t) approximately t(1/2) over two decades in time. It is flanked by a short-time regime with x(t) approximately t(3/4) and a viscous with x(t)approximately t. In the intermediate regime the creep compliance depends on the actin concentration c as c(-beta) with beta approximately 1.1 +/- 0.3. We explain this behavior in terms of osmotic restoring force generated by the piling up of filaments in front of the moving bead. A model based on this concept predicts intermediate x(t) approximately t(1/2) and long-time regimes x(t) approximately t in which the compliance varies as c(-4/3), in agreement with experiment.

Actins↗
PubMed2004

Internal motility in stiffening actin-myosin networks.

We present a study on filamentous actin solutions containing heavy meromyosin subfragments of myosin II motor molecules. We focus on the viscoelastic phase behavior and internal dynamics of such networks during adenosine-triphosphate depletion. By combining microrheology and fluorescence microscopy, we observed a sol-gel transition accompanied by a sudden onset of directed filament motion. We interpret the sol-gel transition in terms of myosin II enzymology, and suggest a "zipping" mechanism to explain the filament motion in the vicinity of the sol-gel transition.

Actins↗
PubMed2004

Topographical pattern dynamics in passive adhesion of cell membranes.

Strong adhesion of highly active cells often nucleates focal adhesions, synapses, and related structures. Red cells lack such complex adhesion systems and are also nonmotile, but they are shown here to dynamically evolve complex spatial patterns beyond an electrostatic threshold for strong adhesion. Spreading of the cell onto a dense, homogeneous poly-L-lysine surface appears complete in <1 s with occasional blisters that form and dissipate on a similar timescale; distinct rippled or stippled patterns in fluorescently labeled membrane components emerge later, however, on timescales more typical of long-range lipid diffusion (approximately minutes). Within the contact zone, the anionic fluorescent lipid fluorescein phosphoethanolamine is seen to rearrange, forming worm-like rippled or stippled domains of <500 nm that prove independent of whether the cell is intact and sustaining a tension or ruptured. Lipid patterns are accompanied by visible perturbations in Band 3 distribution and weaker perturbations in membrane skeleton actin. Pressing down on the membrane quenches the lipid patterns, revealing a clear topographical basis for pattern formation. Counterion screening and membrane fluctuations likely contribute, but the results primarily highlight the fact that even in adhesion of a passive red cell, regions of strong contact slowly evolve to become interspersed with regions where the membrane is more distant from the surface.

Cell Adhesion↗
PubMed2004

Pulling tethers from adhered vesicles.

The competition between adhesion and tether formation in bound vesicles is investigated. A theoretical model is developed in which tethers are induced by the application of a pulling force to the top of a strongly adhered vesicle. A critical onset force is identified where the tether spontaneously appears as part of a first order shape transition. Further growth of the tether initiates a detachment process that culminates in a continuous unbinding of the vesicle at a finite detachment force. Both critical forces, as well as all shape parameters, are calculated as a function of the reduced volume and the strength of adhesive potential.

Cell Adhesion↗
PubMed2004

Absolute interfacial distance measurements by dual-wavelength reflection interference contrast microscopy.

Dual-wavelength reflection interference contrast microscopy (DW-RICM) is established as a microinterferometric technique to measure absolute optical distances between transparent planar substrates and hard or soft surfaces such as colloidal beads or artificial and biological membranes, which hover over the substrate. In combination with a fast image processing algorithm the technique was applied to analyze the trajectories of colloidal beads sedimenting under gravity. As the beads approach the surface of the substrate, they slow down because of hydrodynamic coupling of the bead motion to the substrate. The effective surface friction coefficients were measured as a function of the absolute distance of the beads from the surface. The height dependence of the friction coefficient was found to be in quantitative agreement with previous theoretical predictions. Furthermore, we demonstrate that the DW-RICM technique allows the determination of the height of membranes above substrates and the amplitude and direction of height fluctuations. Without any further need to label the membrane the unambiguous reconstruction of the surface profile of soft surfaces is possible.

Journal Article↗
PubMed2004

Phase transformations in a model mesenchymal tissue.

Connective tissues, the most abundant tissue type of the mature mammalian body, consist of cells suspended in complex microenvironments known as extracellular matrices (ECMs). In the immature connective tissues (mesenchymes) encountered in developmental biology and tissue engineering applications, the ECMs contain varying amounts of randomly arranged fibers, and the physical state of the ECM changes as the fibers secreted by the cells undergo fibril and fiber assembly and organize into networks. In vitro composites consisting of assembling solutions of type I collagen, containing suspended polystyrene latex beads ( approximately 6 microm in diameter) with collagen-binding surface properties, provide a simplified model for certain physical aspects of developing mesenchymes. In particular, assembly-dependent topological (i.e., connectivity) transitions within the ECM could change a tissue from one in which cell-sized particles (e.g., latex beads or cells) are mechanically unlinked to one in which the particles are part of a mechanical continuum. Any particle-induced alterations in fiber organization would imply that cells could similarly establish physically distinct microdomains within tissues. Here we show that the presence of beads above a critical number density accelerates the sol-gel transition that takes place during the assembly of collagen into a globally interconnected network of fibers. The presence of this suprathreshold number of beads also dramatically changes the viscoelastic properties of the collagen matrix, but only when the initial concentration of soluble collagen is itself above a critical value. Our studies provide a starting point for the analysis of phase transformations of more complex biomaterials including developing and healing tissues as well as tissue substitutes containing living cells.

Animals↗
PubMed2004

Microviscoelasticity of the apical cell surface of human umbilical vein endothelial cells (HUVEC) within confluent monolayers.

We studied the local viscoelasticity of the apical membrane of human umbilical vein endothelial cells within confluent layers by magnetic tweezers microrheometry. Magnetic beads are coupled to various integrins by coating with fibronectin or invasin. By analyzing the deflection of beads evoked by various force scenarios we demonstrate that the cell envelope behaves as a linear viscoelastic body if forces up to 2 nN are applied for short times (<20 s) but can respond in an adaptive way if stress pulses are applied longer (>30 s). The time-dependent shear relaxation modulus G(t) exhibits three time regimes: a fast response (t < 0.05 s) where the relaxation modulus G(t) obeys a power law G(t) approximately t(-0.82+/-0.02); a plateau-like behavior (at 0.05 s < t < 0.15 s); and a slow flow-like response which is, however, partially reversible. Strain field mapping experiments with colloidal probes show that local forces induce a strain field exhibiting a range of zeta = 10 +/- 1 microm, but which could only be observed if nonmagnetic beads were coupled to the cell surface by invasin. By application of the theory of elasticity of planar bodies we estimated a surface shear modulus of 2.5 x10(-4) N/m. By assuming a thickness of the actin cortex of approximately 0.5 microm we estimate a Young modulus micro approximately 400 Pa for the apical membrane. The value agrees with a plateau modulus of an entangled or weakly cross-linked actin network of an actin concentration of 100 microM (mesh size 0.2 microm). This result together with our observation of a strong reduction of the shear modulus by the actin destabilizing agent latrunculin A suggests that the shear modulus measured by our technique is determined by the actin cortex. The effect of two ligands inducing actin stress fiber formation and centripetal contraction of cells (associated with the formation of gaps in the confluent cell monolayer) on the viscoelastic responses were studied: histamine and lysophosphatidic acid (LPA). Histamine evoked a dramatic increase of the cell stiffness by >1 order of magnitude within <30 s, which is attributed to a transient rise of the intracellular Ca(2+) level, since DMSO exerted a similar effect. The stiffening is accompanied by a concomitant rounding of the cells as observed by microinterferometry and relaxes partially in the timescale of 5 min, whereas gaps between cells close after approximately 30 min. LPA did not exert a remarkable and reproducible effect other than an occasional very weak transient increase of the shear stiffness, which shows that the gap formation activated by LPA is mediated by a different mechanism than that induced by histamine.

Cell Membrane↗
PubMed2003

Slow filament dynamics and viscoelasticity in entangled and active actin networks.

This paper deals with correlations between the viscoelastic impedance of entangled actin networks and the slow conformational dynamics and diffusive motions of single filaments. The single filament dynamics is visualized and analysed by analysing the Brownian motion of attached colloidal beads, which enables independent measurements of characteristic viscoelastic response times such as the entanglement and reptation times. We further studied the frequency-dependent viscoelastic impedance of active actin-heavy-meromyosin II networks by magnetic-tweezers microrheometry to gain insight into the effect of such highly dynamic and force-generating crosslinkers (exhibiting bond lifetimes of less than 1 s) on the rheological properties. We show that at high frequencies (higher than 1 Hz) the viscoelastic loss modulus is slightly increased relative to the entangled network (associated with an increase in the energy dissipated during mechanical excitations), while at low frequencies the plateau of the impedance spectrum becomes more pronounced as a consequence of the cross-linking of the network and the suppression of the terminal regime. Our data provide evidence that the myosin motor protein may play a role as softener of the actin cortex, enabling the adaptive reduction of the yield stress of cells and thus facilitating cellular deformations.

Actin Cytoskeleton↗
PubMed2003

Local force induced conical protrusions of phagocytic cells.

Magnetic tweezers were used to study the passive and active response of macrophages to local centripetal nanonewton forces on beta1 integrins. Superparamagnetic beads coated with the beta1-integrin-binding protein invasin were attached to J774 murine macrophages to mimic phagocytosis of bacterial pathogens. Forces exceeding approximately 0.5 nN induce the active formation of trumpet-like protrusions resembling pseudopodia after an initial elastic deflection and a response time of approximately 30 seconds. The speed of advancement of the protrusion is =0.065+/-0.020 micro m second(-1) and is force independent. After saturation (after about 100 seconds) the protrusion stops abruptly and is completely retracted again against forces exceeding 5 nN with an effective relaxation time of approximately 30 seconds. The active protrusion is tentatively attributed to the growth of the actin cortex in the direction of the force, and evidence for the involvement of actin is provided by the finding that Latrunculin A abolishes the activated cone growth. The growth is assumed to be activated by cell signaling mediated by the invasin-specific integrins (exhibiting beta1 chains) and could play a role in phagocytic and protrusive events during immune response by macrophages.

Adhesins, Bacterial↗
PubMed2003

Functional incorporation of integrins into solid supported membranes on ultrathin films of cellulose: impact on adhesion.

Biomimetic models of cell surfaces were designed to study the physical basis of cell adhesion. Vesicles bearing reconstituted blood platelet integrin receptors alpha(IIb)beta(3) were spread on ultrathin films of cellulose, forming continuous supported membranes. One fraction of the integrin receptors, which were facing their extracellular domain toward the aqueous phase, were mobile, exhibiting a diffusion constant of 0.6 micro m(2) s(-1). The functionality of receptors on bare glass and on cellulose cushions was compared by measuring adhesion strength to giant vesicles. The vesicles contained lipid-coupled cyclic hexapeptides that are specifically recognized by integrin alpha(IIb)beta(3). To mimic the steric repulsion forces of the cell glycocalix, lipids with polyethylene glycol headgroups were incorporated into the vesicles. The free adhesion energy per unit area deltag(ad) was determined by micro-interferometric analysis of the vesicle's contour near the membrane surface in terms of the equilibrium of the elastic forces. By accounting for the reduction of the adhesion strength by the repellers and from measuring the density of receptors one could estimate the specific receptor ligand binding energy. We estimate the receptor-ligand binding energy to be 10 k(B)T under bioanalogue conditions.

Biomimetic Materials↗
PubMed2003

Assembly of collagen matrices as a phase transition revealed by structural and rheologic studies.

We have studied the structural and viscoelastic properties of assembling networks of the extracellular matrix protein type-I collagen by means of phase contrast microscopy and rotating disk rheometry. The initial stage of the assembly is a nucleation process of collagen monomers associating to randomly distributed branched clusters with extensions of several microns. Eventually a sol-gel transition takes place, which is due to the interconnection of these clusters. We analyzed this transition in terms of percolation theory. The viscoelastic parameters (storage modulus G' and loss modulus G") were measured as a function of time for five different frequencies ranging from omega = 0.2 rad/s to 6.9 rad/s. We found that at the gel point both G' and G" obey a scaling law, with the critical exponent Delta = 0.7 and a critical loss angle being independent of frequency as predicted by percolation theory. Gelation of collagen thus represents a second order phase transition.

Animals↗
PubMed2002

Polymorphism of cross-linked actin networks in giant vesicles.

Actin networks cross-linked by natural linkers alpha-actinin and filamin are generated in giant vesicles by polymerization through ionophore-mediated influx of Mg2+. alpha-actinin induces the formation of randomly linked networks at 25 degrees C which transform at <15 degrees C into spiderweblike gels or ringlike bundles depending on the vesicle size. Muscle filamin forms ringlike structures under all experimental conditions which can supercoil by subsequent Mg2+ addition. The polymorphism is rationalized in terms of recent models of bivalent ion coupled semiflexible polyelectrolytes and by considering the topology of the linkers.

Actins↗
PubMed2002

Helfrich repulsion and dynamical phase separation of multicomponent lipid bilayers.

Thermal fluctuations of surfactant bilayers in an aqueous solution produce an effective, long-range repulsion that can lead to a continuous unbinding transition. We report on an optical interferometry study of the thermal fluctuations of multicomponent bilayers close to the unbinding transition. We find that, in contrast to the case of single-component bilayers, the thermal fluctuation spectrum of multicomponent bilayers does not agree with a continuous unbinding transition but instead indicates the proximity of an unbinding tricritical point.

Cholesterol↗