Krystian Kubica

Membrane fluidity and the surface properties of the lipid bilayer: ESR experiment and computer simulation

Penetration of the liposome membranes formed in the gel phase from DPPC (DPPC liposomes) and in the liquid-crystalline phase from egg yolk lecithin (EYL liposomes) by the TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and 16 DOXYL (2-ethyl-2-(15-methoxy-oxopentadecyl)-4,4-dimethyl-3-oxazolidinyloxy) spin probes has been investigated. The penetration process was followed by 120 hours at 240C, using the electron spin resonance (ESR) method. The investigation of the kinetics of the TEMPO probe building into the membranes of both types of liposomes revealed differences appearing 30 minutes after the start of the experiment. The number of TEMPO particles built into the EYL liposome membranes began to clearly rise, aiming asymptotically to a constant value after about 100 minutes, whereas the number of the TEMPO particles built into the DPPC liposome membranes was almost constant in time. The interpretation of the obtained experimental results was enriched with those of computer simulation, following the behavior of the polar heads (dipoles) of the lipid particles forming a lipid layer due to the change in the value of the model parameter, k, determining the mobility of the dipoles. The possibility of the formation of an irregular ordering of the polar part of lipid membranes was proved, which leads to the appearance of spaces filled with of water for k > 0.4. The appearance of these defects enables the penetration of the bilayer by the TEMPO particles. The limited mobility of lipid polar heads (k < 0.2) prevents the appearance of such areas facilitating the penetration of the lipid membrane by alien particles in the gel phase.

Monte Carlo simulation towards ripple phase modelling

We present a novel approach to analysis of the gel-fluid transition of lipid membrane. The method is based on the Pinks model but in contrast to its standard version the dipole character of the lipid molecules polar part is considered. Moreover, less constrained movement of entire molecules is allowed. Such an approach includes into the model conditions imposed by the adjacent medium such as ionic strength, pH, and other factors affecting biological membranes via the polar part. The results obtained contribute to the explanation of the ripple phase phenomenon.

Mathematical Two-compartment Model of Human Cholesterol Transport in Application to High Blood Cholesterol Diagnosis and Treatment

Cholesterol plays a vital role in human body. Its unbalanced homeostasis, however, leads to health related problems. The elevated blood cholesterol levels are now considered a classic coronary risk factor and are suspected to lead to coronary artery diseases, causing 2.6 millions of deaths each year. Here, we develop a two-compartment mathematical model to investigate cholesterol transport in the circulatory system and its de novo synthesis in the liver. The model is described with a set of two simultaneous linear differential equations, which solutions yield changes over time of the cholesterol levels in the liver (compartment I) and bloodstream (compartment II). We show the applicability of the model to investigate the processes associated with the high blood cholesterol, e.g. lowering the cholesterol levels by inhibiting de novo cholesterol synthesis. Taking advantage of the analytically derived relationships for the steady state (equilibrium), we show how the model could aid diagnosis of high blood cholesterol by identifying whether the disturbances in the cholesterol homeostasis are due to impaired transport from the liver to the bloodstream, or vice versa.

A pore creation in a triangular network model membrane

Membrane electroporation seems to be a useful method for delivery of biological active compounds into the cell. Although it is known that this phenomenon is sensitive to the electric field intensity, duration of the electric pulse and its shape, it is not fully understood. In some theoretical descriptions it is postulated that a hydrophobic pore appears at an early stage of pore creation. Here we show how to construct a hydrophilic pore structure connecting two parallel triangular networks modeling lipid membrane. It would be useful in Monte Carlo simulation studies on electroporation. In our model the pore appearance requires only movement of one lipid molecule. At the same time the chains of the second lipid molecule should occupy two nodes, one in each network to compensate the differences in chain packing densities when electric field is applied. In consequence the hydrated polar head should be placed in a hydrophobic part of the membrane giving rise to the hydrophilic pore. We also discuss the relation between the pore diameter and its shape.

Mathematical analyses of two-compartment model of human cholesterol circulatory transport in application to high blood cholesterol prevention, diagnosis and treatment

Cholesterol plays a vital role in human body and thus its unbalanced homeostasis leads to health problems. Elevated blood cholesterol levels are now considered a classic coronary risk factor and are suspected to lead to coronary artery diseases, causing 2.6 millions of deaths each year. However, many of the mechanisms behind cholesterol-related processes remain unknown. Mathematical and computational models can aid investigation of complex biological phenomena, yet cholesterol-focused models remain rare. Here, we develop a two-compartment mathematical model to investigate cholesterol transport in the human circulatory system. We focus on the key aspects of cholesterol circulatory transport in the lipoproteins, its de novo synthesis and bile recycling and represent them by two simultaneous linear differential equations. The solutions yield changes over time of the cholesterol levels in the liver (compartment I) and bloodstream (compartment II). Drawing from the current clinical practice, we show the application of the model to personalized high blood cholesterol prevention, diagnosis and treatment.

Two-compartment model as a teaching tool for cholesterol homeostasis

Cholesterol is a vital structural and functional molecule in the human body that is only slightly soluble in water and therefore does not easily travels by itself in the bloodstream. To enable cholesterols targeted delivery to cells and tissues, it is encapsulated by different fractions of lipoproteins, complex particles containing both proteins and lipids. Maintaining cholesterol homeostasis is a highly regulated process with multiple factors acting at both molecular and tissue levels. Furthermore, to regulate the circulatory transport of cholesterol in lipoproteins, the amount of cholesterol present depends on and is controlled by cholesterol dietary intake, de novo synthesis, usage, and excretion; abnormal and/or unbalanced cholesterol levels have been shown to lead to severe outcomes, e.g., cardiovascular diseases. To investigate cholesterol transport in the circulatory system, we have previously developed a two-compartment mathematical model. Here, we show how this model can be used as a teaching tool for cholesterol homeostasis. Using the model and a hands-on approach, students can familiarize themselves with the basic components and mechanisms behind balanced cholesterol circulatory transport as well as investigate the consequences of and countermeasures to abnormal cholesterol levels. Among others, various treatments of high blood cholesterol levels can be simulated, e.g., with commonly prescribed de novo cholesterol synthesis inhibitors.

The network model of electroporation

The molecular bases of pore formation in biological membranes remain unclear. The size of the generated pore is of importance during the process of electroporation (EP) in which external electric field is applied to increase biological membranes permeability, enabling loading cells with a variety of biologically active compounds ranging ‘from ions to drugs, dyes, tracers, antibodies, and oligonucleotides to RNA and DNA’ - compounds that cannot otherwise pass the biological membranes barrier the existing natural mechanisms (e.g. by diffusion, passive or active transport). The successful outcome, e.g. being able to deliver DNA vaccination in cancer patients, depends on the ability to select the optimal EP settings (voltages, capacitances) to create the pores with desirable dimension and duration properties. This remains a challenge as relatively little is known about the molecular mechanisms behind the process of pore formation during EP, i.e. the behavior of lipid molecules or about the major components of...