Institute of Fundamental Technological Research

The leukocyte-specific β2-integrin LFA-1 and its ligand ICAM-1, expressed on endothelial cells (ECs), are involved in the arrest, adhesion, and transendothelial migration of leukocytes. Although the role of mechanical forces on LFA-1 activation is well established, the impact of forces on its major ligand ICAM-1 has received less attention. Using a parallel-plate flow chamber combined with confocal and super-resolution microscopy, we show that prolonged shear flow induces global translocation of ICAM-1 on ECs upstream of flow direction. Interestingly, shear forces caused actin rearrangements and promoted actin-dependent ICAM-1 nanoclustering before LFA-1 engagement. T cells adhered to mechanically prestimulated ECs or nanoclustered ICAM-1 substrates developed a promigratory phenotype, migrated faster, and exhibited shorter-lived interactions with ECs than when adhered to non mechanically stimulated ECs or to monomeric ICAM-1 substrates. Together, our results indicate that shear forces increase ICAM-1/LFA-1 bonds because of ICAM-1 nanoclustering, strengthening adhesion and allowing cells to exert higher traction forces required for faster migration. Our data also underscore the importance of mechanical forces regulating the nanoscale organization of membrane receptors and their contribution to cell adhesion regulation.

Time traces obtained from a variety of biophysical experiments contain valuable information on underlying processes occurring at the molecular level. Accurate quantification of these data can help explain the details of the complex dynamics of biological systems. Here, we describe PLANT (Piecewise Linear Approximation of Noisy Trajectories), a segmentation algorithm that allows the reconstruction of time-trace data with constant noise as consecutive straight lines, from which changes of slopes and their respective durations can be extracted. We present a general description of the algorithm and perform extensive simulations to characterize its strengths and limitations, providing a rationale for the performance of the algorithm in the different conditions tested. We further apply the algorithm to experimental data obtained from tracking the centroid position of lymphocytes migrating under the effect of a laminar flow and from single myosin molecules interacting with actin in a dual-trap force-clamp configuration.

TETHERED PARTICLE MOTION, CHANGE-POINT DETECTION, OPTICAL TWEEZERS, CELL-ADHESION, UNKNOWN POINT, SINGLE, MYOSIN, MODEL, MIGRATION, TRACKING

The recent progress in bioengineering has created great interest in the dynamics and manipulation of long, deformable macromolecules interacting with fluid flow. We report experimental data on the cross-flow migration, bending, and buckling of extremely deformable hydrogel nanofilaments conveyed by an oscillatory flow into a microchannel. The changes in migration velocity and filament orientation are related to the flow velocity and the filament's initial position, deformation, and length. The observed migration dynamics of hydrogel filaments qualitatively confirms the validity of the previously developed worm-like bead-chain hydrodynamic model. The experimental data collected may help to verify the role of hydrodynamic interactions in molecular simulations of long molecular chains dynamics.

Essentials Fibrinogen circulates in human plasma as a complex mixture of heterogeneous molecular variants. We measured strain-stiffening of recombinantly produced fibrinogen upon clotting. Factor XIII and molecular heterogeneity alter clot elasticity at the protofibril and fiber level. This highlights the hitherto unknown role of molecular composition in fibrin clot mechanics.

blood coagulation, elasticity, fibrin, polymers, rheology, turbidimetry

Bundles of polymer filaments are responsible for the rich and unique mechanical behaviors of many biomaterials, including cells and extracellular matrices. In fibrin biopolymers, whose nonlinear elastic properties are crucial for normal blood clotting, protofibrils self-assemble and bundle to form networks of semiflexible fibers. Here we show that the extraordinary strain-stiffening response of fibrin networks is a direct reflection of the hierarchical architecture of the fibrin fibers. We measure the rheology of networks of unbundled protofibrils and find excellent agreement with an affine model of extensible wormlike polymers. By direct comparison with these data, we show that physiological fibrin networks composed of thick fibers can be modeled as networks of tight protofibril bundles. We demonstrate that the tightness of coupling between protofibrils in the fibers can be tuned by the degree of enzymatic intermolecular crosslinking by the coagulation factor XIII. Furthermore, at high stress, the protofibrils contribute independently to the network elasticity, which may reflect a decoupling of the tight bundle structure. The hierarchical architecture of fibrin fibers can thus account for the nonlinearity and enormous elastic resilience characteristic of blood clots.

During wound healing and angiogenesis, fibrin serves as a provisional extracellular matrix. We use a model system of fibroblasts embedded in fibrin gels to study how cell-mediated contraction may influence the macroscopic mechanical properties of their extracellular matrix during such processes. We demonstrate by macroscopic shear rheology that the cells increase the elastic modulus of the fibrin gels. Microscopy observations show that this stiffening sets in when the cells spread and apply traction forces on the fibrin fibers. We further show that the stiffening response mimics the effect of an external stress applied by mechanical shear. We propose that stiffening is a consequence of active myosin-driven cell contraction, which provokes a nonlinear elastic response of the fibrin matrix. Cell-induced stiffening is limited to a factor 3 even though fibrin gels can in principle stiffen much more before breaking. We discuss this observation in light of recent models of fibrin gel elasticity, and conclude that the fibroblasts pull out floppy modes, such as thermal bending undulations, from the fibrin network, but do not axially stretch the fibers. Our findings are relevant for understanding the role of matrix contraction by cells during wound healing and cancer development, and may provide design parameters for materials to guide morphogenesis in tissue engineering.

Collagen fibrils are the main structural element of connective tissues. In many tissues, these fibrils contain two fibrillar collagens (types I and V) in a ratio that changes during tissue development, regeneration, and various diseases. Here we investigate the influence of collagen composition on the structure and rheology of networks of purified collagen I and V, combining fluorescence and atomic force microscopy, turbidimetry, and rheometry. We demonstrate that the network stiffness strongly decreases with increasing collagen V content, even though the network structure does not substantially change. We compare the rheological data with theoretical models for rigid polymers and find that the elasticity is dominated by nonaffine deformations. There is no analytical theory describing this regime, hampering a quantitative interpretation of the influence of collagen V. Our findings are relevant for understanding molecular origins of tissue biomechanics and for guiding rational design of collagenous biomaterials for biomedical applications.

Fibrin gels are responsible for the mechanical strength of blood clots, which are among the most resilient protein materials in nature. Here we investigate the physical origin of this mechanical behavior by performing rheology measurements on reconstituted fibrin gels. We find that increasing levels of shear strain induce a succession of distinct elastic responses that reflect stretching processes on different length scales. We present a theoretical model that explains these observations in terms of the unique hierarchical architecture of the fibers. The fibers are bundles of semiflexible protofibrils that are loosely connected by flexible linker chains. This architecture makes the fibers 100-fold more flexible to bending than anticipated based on their large diameter. Moreover, in contrast with other biopolymers, fibrin fibers intrinsically stiffen when stretched. The resulting hierarchy of elastic regimes explains the incredible resilience of fibrin clots against large deformations.

Atomic force microscopy (AFM) is an evolution of scanning tunnelling microscopy that immediately gained popularity thanks to its ability to analyse nanomaterials. Initially, AFM was developed for nanomaterials imaging purposes, however the development of new features made it the most commonly used tool for studying the biophysical properties of biological samples. On the other hand, atomic force microscopy has limited use for examining sub-piconewton forces. Few techniques have been developed to measure forces below the AFM limit of detection. Among them, optical tweezers (OT) stand out for their high resolution, flexibility, and because they make it possible to accurately manipulate biological samples and carry out biophysics experiments without side effects thanks to their non-invasive properties. The combination of AFM with other techniques in the last decades has significantly extended its capability. The improvement of the AFM force resolution by developing a hybrid double probe instrument based on the combination of AFM and OT has great potential in cell or molecular biology. [1] We outline principles of atomic force microscopy combined with optical tweezers (AFM/OT) developed by our team underlying the techniques applied during the design, building and instrument use stages. We describe the experimental procedure for calibration of the system and we prove the achievement of a higher resolution (force: 10 fN – spatial: 0.1 nm – temporal: 10 ns) than the stand alone AFM. We show the use of the hybrid equipment in a number of different biophysics experiments performed employing both AFM and OT probes. The presented studies include the demonstration of simultaneous high-precision nanomanipulation and imaging, the evaluation of single biomolecule mechanical properties and the single cell membrane activation and probing. Finally, we show the further potential applications of our AFM/OT.

AFM, Optical Tweezers

Electrospun nanofibers are increasingly studied thanks to their potential applications in biomedical devices that include drug delivery systems and tissue engineering scaffolds [1]. Numerous synthetic and natural polymers were used to develop nanofibrous materials. Nanostructured materials high porosity, surface-to- volume ratio together with the ease in surface functionalization and drug incorporation, make them perfect candidates for the development of hemostats. Immediate hemorrhage management becomes crucial to preventing death and serious injury in emergency situations. Severe injuries caused by e.g. traffic accidents are the third leading cause of death worldwide [2]. Research on medical incidents of soldiers stationed in Iraq in 2003-2004 showed that the main cause of death was massive hemorrhage that led to death in about 51% of the rescued soldiers [3]. There is no universal dressing and despite the development of new hemostats, they fail in many preclinical studies. Therefore, there is a need to define most important nanofibrous material characteristics that are responsible for rapid and effective bleeding arrest. There is little research on nanostructured hemostats, regarding the impact of nanofibrous surface on blood and its components. Nonetheless, because of the wide use of nanofibres in wound dressings, artificial blood vessels as well as heart valves, there is knowledge helpful in determining material surface chemistry, wettability and other, which can affect blood coagulation. The very first findings appeared in the research where it was found that even polymers having excellent antiplatelet adhesion abilities, triggered increased platelet adhesion and activation when they were in the form of nanofibers. In several other studies, scaffold morphology, was found to have larger impact on platelet adhesion and activation than differences in the chemistry of the polymers used [4]. More specifically, it was found that materials with fiber diameter higher than 1 µm triggered higher platelet adhesion and aggregation than smaller fibers. In other research, nanofiber stiffness was assessed as more dominating than biological moieties and surface roughness of the nanofiber [5]. In spite of all, analyzed literature presents many contradictory results or findings that had low or no impact on blood clotting in research results of other groups. Hence, additional research and novel experimental methods are needed to find nano features that impact hemostat efficiency.

blood-biomaterial interactions, nanofibers, clotting

thermal fluctuations, lateral migration, flexible filaments

Nanofibers have received considerable attention in the past years, mainly due to their vast application in medicine [1]. One of the fastest growing areas of application are wound dressings and hemostats. Among the major causes of death from trauma, massive bleeding is responsible for 30 – 40% of mortality. In the hospital, massive bleeding are the second most common cause of death (22%) just after cardiac factors (33%) [2]. Despite a large number of experiments done in the topic of blood-biomaterial interactions, coagulation mechanisms are still not fully understood. Therefore, the main objective of our work is the analysis of protein adsorption, platelet adhesion and aggregation, and blood plasma coagulation in the contact with polymer nanofibers. Various synthetic polymers, their blends with natural polymers of confirmed hemostatic effect e.g. collagen and gelatine, and additionally nanofibers made of chitosan are investigated for their potential to stop bleeding. In the final, controlled release of drugs affecting coagulation cascade will be an important step providing accelerated blood clot formation.