Andrey Alexandrovich Butylin

Finite Platelet Size Could Be Responsible for Platelet Margination Effect

Blood flows through vessels as a segregated suspension. Erythrocytes distribute closer to the vessel axis, whereas platelets accumulate near vessel walls. Directed platelet migration to the vessel walls promotes their hemostatic function. The mechanisms underlying this migration remain poorly understood, although various hypotheses have been proposed to explain this phenomenon (e.g., the available volume model and the drift-flux model). To study this issue, we constructed a mathematical model that predicts the platelet distribution profile across the flow in the presence of erythrocytes. This model considers platelet and erythrocyte dimensions and assumes an even platelet distribution between erythrocytes. The model predictions agree with available experimental data for near-wall layer margination using platelets and platelet-modeling particles and the lateral migration rate for these particles. Our analysis shows that the strong expulsion of the platelets from the core to the periphery of the blood vessel may mainly arise from the finite size of the platelets, which impedes their positioning in between the densely packed erythrocytes in the core. This result provides what we believe is a new insight into the rheological control of platelet hemostasis by erythrocytes.

New Synthetic Thrombin Inhibitors: Molecular Design and Experimental Verification

Background The development of new anticoagulants is an important goal for the improvement of thromboses treatments. Objectives The design, synthesis and experimental testing of new safe and effective small molecule direct thrombin inhibitors for intravenous administration. Methods Computer-aided molecular design of new thrombin inhibitors was performed using our original docking program SOL, which is based on the genetic algorithm of global energy minimization in the framework of a Merck Molecular Force Field. This program takes into account the effects of solvent. The designed molecules with the best scoring functions (calculated binding energies) were synthesized and their thrombin inhibitory activity evaluated experimentally in vitro using a chromogenic substrate in a buffer system and using a thrombin generation test in isolated plasma and in vivo using the newly developed model of hemodilution-induced hypercoagulation in rats. The acute toxicities of the most promising new thrombin inhibitors were evaluated in mice, and their stabilities in aqueous solutions were measured. Results New compounds that are both effective direct thrombin inhibitors (the best KI was <1 nM) and strong anticoagulants in plasma (an IC50 in the thrombin generation assay of approximately 100 nM) were discovered. These compounds contain one of the following new residues as the basic fragment: isothiuronium, 4-aminopyridinium, or 2-aminothiazolinium. LD50 values for the best new inhibitors ranged from 166.7 to >1111.1 mg/kg. A plasma-substituting solution supplemented with one of the new inhibitors prevented hypercoagulation in the rat model of hemodilution-induced hypercoagulation. Activities of the best new inhibitors in physiological saline (1 µM solutions) were stable after sterilization by autoclaving, and the inhibitors remained stable at long-term storage over more than 1.5 years at room temperature and at 4°C. Conclusions The high efficacy, stability and low acute toxicity reveal that the inhibitors that were developed may be promising for potential medical applications.

Continuous mathematical model of platelet thrombus formation in blood flow

An injury of a blood vessel requires quick repairing of the wound in order to prevent a loss of blood. This is done by the hemostatic system. The key point of its work is the formation of an aggregate from special blood elements, namely, platelets. The construction of a mathematical model of the formation of a thrombocyte aggregate with an adequate representation of its physical, chemical, and biological processes is an extremely complicated problem. A large size of platelets compared to that of molecules, strong inhomogeneity of their distribution across the blood flow, high shear velocities, the moving boundary of the aggregate, the interdependence of its growth and the blood flux hamper the construction of closed mathematical models convenient for biologists. We propose a new PDE-based model of a thrombocyte aggregate formation. In this model, the movement of its boundary due to the adhesion and detachment of platelets is determined by the level set method. The model takes into account the distribution inhomogeneity of erythrocytes and platelets across the blood flow, the invertible adhesion of platelets, their activation, secretion, and aggregation. The calculation results are in accordance with the experimental data concerning the kinetics of the ADP-evoked growth of a thrombus in vivo for different flow velocities.

On Propagation of Excitation Waves in Moving Media: The FitzHugh-Nagumo Model

Background Existence of flows and convection is an essential and integral feature of many excitable media with wave propagation modes, such as blood coagulation or bioreactors. Methods/Results Here, propagation of two-dimensional waves is studied in parabolic channel flow of excitable medium of the FitzHugh-Nagumo type. Even if the stream velocity is hundreds of times higher that the wave velocity in motionless medium (), steady propagation of an excitation wave is eventually established. At high stream velocities, the wave does not span the channel from wall to wall, forming isolated excited regions, which we called “restrictons”. They are especially easy to observe when the model parameters are close to critical ones, at which waves disappear in still medium. In the subcritical region of parameters, a sufficiently fast stream can result in the survival of excitation moving, as a rule, in the form of “restrictons”. For downstream excitation waves, the axial portion of the channel is the most important one in determining their behavior. For upstream waves, the most important region of the channel is the near-wall boundary layers. The roles of transversal diffusion, and of approximate similarity with respect to stream velocity are discussed. Conclusions These findings clarify mechanisms of wave propagation and survival in flow.

The review of contemporary ideas about the influence of flow rate on blood clotting

Normal blood clotting is vitally important for mammals. The diffusion-convection transfer of clotting factors plays a key role in blood clot formation. Since the shear rates of blood flow are very high (up to 7000 s−1), clot formation critically depends on the flow rate. The methods of study of the flow effect on clotting are indirect and the processes are rather complex; therefore, mathematical models of this process are significant for interpretation of results and understanding of the mechanisms. The review expounds the main experimental data on the effect of flow on the blood clotting cascade, some hypotheses and mathematical models explaining different regimes of the functioning of this system. The review is focused on specific problems encountered by researchers in this field. Some of the experimental works have shown that flow decreases the size of the formed fibrin clot and that the dependence of clot formation period on the flow shear rate is a threshold function. However, there are also experimental evidence that the flow can increase production of clotting factors (factor Xa), which must be expressed in the procoagulant action of the flow. Mathematical models of different aspects of clotting give no unified predictions either. Nevertheless, the combined analysis of results of detailed modeling and experiments, in our opinion, may result in understanding of the mechanisms, which determine the behavior of clotting in a flow.

Molecular self-organization and multiple equilibrium systems

We have compared different types of natural self-organizing systems: crystal-like formation of multimolecular systems were conferred with self-organization in active media and dissipative structure formation. The comparison revealed a common feature of all such systems. They all have bi-(multi-)stable states. We propose a hypothesis that self-organization is impossible in systems that could not have bi-(multi-)stable states.

Immunological biochips for parallel detection of surface antigens and morphological analysis of cells

A biochip for detecting 26 cluster differentiation (CD), HLA-DR and IgM antigens on lymphocyte surface is described. The biochip, which represents a microarray of antibodies (IgG) against a panel of selected antigens immobilized on transparent plastic surfaces in 1.5-mm spots, was used for the study of normal and neoplastic lymphocytes and can also be used for determining percent of cells expressing definite surface antigens in lymphocyte suspensions. The results are consistent with data obtained by flow cytometry. The novel biochip technology entails a combination of conventional staining of cells immobilized on biochips and morphological analysis.

Immunological Biochips for Studies of Human Erythrocytes

Immunological microarrays (biochips) for detecting erythrocyte surface antigens, viz., blood group antigens (A, B, 0) and Rhesus system antigens (D, E, e, C, and c), are described. The biochips represent transparent plastic supports onto which 1.5-mm spots of specific immobilized antibodies (IgM) are coated in different dilutions. The volume of tested blood samples is rather small (1–2 μl). Binding of erythrocytes to antibodies immobilized on the biochips is specific and allows further morphological analysis of bound cells. Analysis of the dynamics of cell detachment from biochip spots using a microfluidic chamber at different flow rates of the washing solution showed that combination of a biochip with a microfluidic chamber is a promising approach to concentration of cells of various immunotypes even if their content in the mixture is very low.