Impedance Analysis of Phosphatidylcholine/α-Tocopherol System in Bilayer Lipid Membranes

Progress and Challenges in the Use of a Liver-on-a-Chip for Hepatotropic Infectious Diseases

The liver is a target organ of life-threatening pathogens and prominently contributes to the variation in drug responses and drug-induced liver injury among patients. Currently available drugs significantly decrease the morbidity and mortality of liver-dwelling pathogens worldwide; however, emerging clinical evidence reveals the importance of host factors in the design of safe and effective therapies for individuals, known as personalized medicine. Given the primary adherence of cells in conventional two-dimensional culture, the use of these one-size-fit-to-all models in preclinical drug development can lead to substantial failures in assessing therapeutic safety and efficacy. Advances in stem cell biology, bioengineering and material sciences allow us to develop a more physiologically relevant model that is capable of recapitulating the human liver. This report reviews the current use of liver-on-a-chip models of hepatotropic infectious diseases in the context of precision medicine including hepatitis virus and malaria parasites, assesses patient-specific responses to antiviral drugs, and designs personalized therapeutic treatments to address the need for a personalized liver-like model. Second, most organs-on-chips lack a monitoring system for cell functions in real time; thus, the review discusses recent advances and challenges in combining liver-on-a-chip technology with biosensors for assessing hepatocyte viability and functions. Prospectively, the biosensor-integrated liver-on-a-chip device would provide novel biological insights that could accelerate the development of novel therapeutic compounds.

Electroanalytical study of five carbosilane dendrimers at the interface between two immiscible electrolyte solutions

This work is focused on the electroanalytical study of a family of five imidazolium-terminated carbosilane dendrimers (from generation G1 to G3) at the polarized liquid-liquid interface formed between water and 1,2-dichloroethane solutions. All dendrimers with permanently and positively charged imidazolium groups located at the periphery within the branched carbosilane core were found to be electrochemically active. Based on the concentration and scan rate dependencies we have concluded that these molecules undergo interfacial ion transfer processes accompanied by interfacial adsorption/desorption rather than the electrochemically induced interfacial formation of the macromolecule-anion (tetrakis(4-chlorophenyl)borate) from the organic phase complex. Also, we report several physicochemical and electroanalytical parameters (e.g. diffusion coefficients, LODs, and detection sensitivities) for the studied family of dendrimers. Our work aims to contribute to the understating of the interaction between branched macromolecules and biomimetic interfaces.

Electrophoretic Light Scattering and Electrochemical Impedance Spectroscopy Studies of Lipid Bilayers Modified by Cinnamic Acid and Its Hydroxyl Derivatives

Pharmacological efficiency of active compounds is largely determined by their membrane permeability. Thus, identification of drug-membrane interactions seems to be a crucial element determining drug-like properties of chemical agents. Yet, knowledge of this issue is still lacking. Since chemoprevention based on natural compounds such as cinnamic acid (CinA), p-coumaric acid (p-CoA) and ferulic (FA) is becoming a strong trend in modern oncopharmacology, determination of physicochemical properties of these anticancer compounds is highly important. Here, electrophoretic light scattering and impedance spectroscopy were applied to study the effects of these phenolic acids on electrical properties of bilayers formed from 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-diacyl-sn-glycero-3-phospho-l-serine (PS) or DOPC-PS mixture. After phenolic acid treatment, the negative charge of membranes increased in alkaline pH solutions, but not in acidic ones. The impedance data showed elevated values of both the electrical capacitance and the electrical resistance. We concluded that at acidic pH all tested compounds were able to solubilize into the membrane and permeate it. At neutral and alkaline pH, the CinA could be partially inserted into the bilayers, whereas p-CoA and FA could be anchored at the bilayer surface. Our results indicate that the electrochemical methods might be crucial for predicting pharmacological activity and bioavailability of phenolic acids.

Capacitive Detection of Low-Enthalpy, Higher-Order Phase Transitions in Synthetic and Natural Composition Lipid Membranes

In-plane lipid organization and phase separation in natural membranes play key roles in regulating many cellular processes. Highly cooperative, first-order phase transitions in model membranes consisting of few lipid components are well understood and readily detectable via calorimetry, densitometry, and fluorescence. However, far less is known about natural membranes containing numerous lipid species and high concentrations of cholesterol, for which thermotropic transitions are undetectable by the above-mentioned techniques. We demonstrate that membrane capacitance is highly sensitive to low-enthalpy thermotropic transitions taking place in complex lipid membranes. Specifically, we measured the electrical capacitance as a function of temperature for droplet interface bilayer model membranes of increasing compositional complexity, namely, (a) a single lipid species, (b) domain-forming ternary mixtures, and (c) natural brain total lipid extract (bTLE). We observed that, for single-species lipid bilayers and some ternary compositions, capacitance exhibited an abrupt, temperature-dependent change that coincided with the transition detected by other techniques. In addition, capacitance measurements revealed transitions in mixed-lipid membranes that were not detected by the other techniques. Most notably, capacitance measurements of bTLE bilayers indicated a transition at ∼38 °C not seen with any other method. Likewise, capacitance measurements detected transitions in some well-studied ternary mixtures that, while known to yield coexisting lipid phases, are not detected with calorimetry or densitometry. These results indicate that capacitance is exquisitely sensitive to low-enthalpy membrane transitions because of its sensitivity to changes in bilayer thickness that occur when lipids and excess solvent undergo subtle rearrangements near a phase transition. Our findings also suggest that heterogeneity confers stability to natural membranes that function near transition temperatures by preventing unwanted defects and macroscopic demixing associated with high-enthalpy transitions commonly found in simpler mixtures.

Modulated phases of phospholipid bilayers induced by tocopherols

The influence of α-, γ- and δ-tocopherols on the structure and phase behavior of dipalmitoyl phosphatidylcholine (DPPC) bilayers has been determined from X-ray diffraction studies on oriented multilayers. In all the three cases the main-transition temperature (T(m)) of DPPC was found to decrease with increasing tocopherol concentration up to around 25 mol%. Beyond this the main transition is suppressed in the case of γ-tocopherol, whereas T(m) becomes insensitive to composition in the other two cases. The pre-transition is found to be suppressed over a narrow tocopherol concentration range between 7.5 and 10 mol% in DPPC-γ-tocopherol and DPPC-δ-tocopherol bilayers, and the ripple phase occurs down to the lowest temperature studied. In all the three cases a modulated phase is observed above a tocopherol concentration of about 10 mol%, which is similar to the P(β) phase reported in DPPC-cholesterol bilayers. This phase is found to occur even in excess water conditions at lower tocopherol concentrations, and consists of bilayers with periodic height modulation. These results indicate the ability of tocopherols to induce local curvature in membranes, which could be important for some of their biological functions.

Impedance spectroscopic investigation of the bilayer lipid membranes formed from the phosphatidylserine-ceramide mixture

Electrochemical impedance spectroscopy was used for the study of two-component lipid membranes. Phosphatidylserine and ceramide were to be investigated because they play an important biochemical role in cell membranes. The research on biolipid interaction was focused on a quantitative description of processes that take part in a bilayer. Assumed models of interaction between amphiphilic molecules and the equilibria that take place there were described by mathematical equations for the studied system. The possibility of complex formation for a two-component system forming bilayers was assumed, which could explain the deviation from the additivity rule. The molecular area and the equilibrium constant of the complex were determined.

Landscape of finite-temperature equilibrium behaviour of curvature-inducing proteins on a bilayer membrane explored using a linearized elastic free energy model

Using a recently developed multiscale simulation methodology, we describe the equilibrium behaviour of bilayer membranes under the influence of curvature-inducing proteins using a linearized elastic free energy model. In particular, we describe how the cooperativity associated with a multitude of protein-membrane interactions and protein diffusion on a membrane-mediated energy landscape elicits emergent behaviour in the membrane phase. Based on our model simulations, we predict that, depending on the density of membrane-bound proteins and the degree to which a single protein molecule can induce intrinsic mean curvature in the membrane, a range of membrane phase behaviour can be observed including two different modes of vesicle-bud nucleation and repressed membrane undulations. A state diagram as a function of experimentally tunable parameters to classify the underlying states is proposed.

Molecular associations of vitamin E

To understand how vitamin E fulfills its functions in membranes and lipoproteins, it is necessary to know how it associates with the lipid components of these structures and the effects its presence has on their structure and stability. Studies of model membrane systems containing vitamin E have proved to be an informative approach to address these questions. A review of the way vitamin E interacts with phospholipid bilayers, how it distributes within the structure, its motional diffusion characteristics, and orientation has been undertaken. The effect of vitamin E on membrane stability and permeability has been described. The tendency of vitamin E to form complexes with certain phospholipids is examined as is the way modulation of protein functions takes place. Finally, recent evidence relevant to the putative role of vitamin E in protecting membranes from free radical attack and the consequences of lipid oxidation in lipoproteins and membranes is examined.

The structure and phase behaviour of α-tocopherol-rich domains in 1-palmitoyl-2-oleoyl-phosphatidylethanolamine

The effect of alpha-tocopherol on the structure and thermotropic phase behaviour of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine dispersed in excess water was examined by synchrotron X-ray diffraction and differential scanning calorimetry. Small- and wide-angle X-ray scattering intensity profiles were recorded from mixed dispersions containing up to 20 mol% alpha-tocopherol during temperature scans over the range 10-75 degrees C. These showed that a domain enriched in alpha-tocopherol phase separated from pure phospholipid in the mixture. This domain tends to have inverted hexagonal structure which coexists with phospholipid bilayers depleted of alpha-tocopherol. The scattering intensity and dimensions of the phase are dependent on the temperature and proportion of alpha-tocopherol in the mixture. Phase separations were also manifest in calorimetric scans of the mixed dispersions evidenced from the appearance of multiple peaks at temperatures corresponding to transitions observed in the X-ray scattering experiments. The effect of alpha-tocopherol in the range 0-20 mol% on the phase behaviour and structure of the phospholipid as observed from the X-ray scattering and calorimetric results have been used to construct a partial phase diagram of the mixture in the temperature range 10-75 degrees C. This shows that alpha-tocopherol has a marked tendency to partition from bilayers of the phospholipid to form an enriched domain in which the phospholipid assumes a hexagonal-II structure.

Spectroscopic studies of D-alpha-tocopherol concentration-induced transformation in egg phosphatidylcholine vesicles

The effects of embedding up to 60 mol% of α-tocopherol (α-Toc) on the morphology and structure of the egg phosphatidylcholine (PC) membrane were studied using spectroscopic techniques. The resulting vesicles were subjected to turbidometric and dynamic light scattering measurements to evaluate their size distribution. The α-Toc intrinsic fluorescence and its quenching was used to estimate the tocopherol position in the membrane. Optical microscopy was used to visualize morphological changes in the vesicles during the inclusion of tocopherol into the 2 mg/ml PC membrane. The incorporation of up to 15 mol% of tocopherol molecules into PC vesicles is accompanied by a linear increase in the fluorescence intensity and the simultaneous formation of larger, multilamellar vesicles. Increasing the tocopherol concentration above 20 mol% induced structural and morphological changes leading to the disappearance of micrometer-sized vesicles and the formation of small unilamellar vesicles of size ranging from 30 to 120 nm, mixed micelles and non-lamellar structures.

'KMC-TDGL'-a coarse-grained methodology for simulating interfacial dynamics in complex fluids: application to protein-mediated membrane processes

In this article, we describe a new multiscale simulation algorithm (which we term the 'KMC-TDGL' method) applicable for the description of equilibrium and dynamic processes associated with a particular class of complex fluids with nanoscale inclusions, namely, biological membranes mediated by membrane-associating and membrane-bound proteins. We adopt a novel strategy of integrating two different phenomenological approaches, namely, a field theoretic (continuum) description for the membrane dynamics given by the time-dependent Ginzburg-Landau equation and a random walk on a discretized lattice description for protein diffusion dynamics. We illustrate that this integrated approach results in a unified description of protein-mediated membrane dynamics.