Dielectric Properties of Phosphatidylcholine Membranes and the Effect of Sugars

Gamma-Aminobutyric Acid Action on Membrane and Electrical Properties of Synaptosomes and Model Lipid Bilayers

Dysfunction of the main inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is the underlying reason behind many neurological disorders including Alzheimer's and Huntington's diseases, autism spectrum disorders, anxiety, depression, hypertension, and cardiovascular diseases, among others. Here, we address neurotransmitter-induced alterations of synaptosomal and model membrane electrical properties for elucidating membrane-related biophysical mechanisms of neurological disorders. We focus on membrane surface characteristics of the pinched off nerve endings synaptosomes, which for decades have been a powerful tool in neurobiology. Microelectrophoretic measurements of GABA-treated negatively charged synaptosomes from rat cerebral cortex reveal lower negative zeta potential as a result of reduced electrical charge on the membrane surface at (1-4 h) after isolation. Conversely, enhancement of the surface parameters of synaptosomes (17-22 h) post isolation is obtained due to additional negatively exposed groups on the surface of the vesicles. The electrical properties of bilayer lipid membranes are probed by electrochemical impedance spectroscopy, reporting as light increase of the membrane electrical capacitance in the presence of GABA, likely related to membrane thinning and dielectric permittivity alterations. The neurotransmitter inhibits sodium-potassium as well as the total ATPase activity and slightly enhances magnesium-ATPase of native synaptic membranes. At low (pM) GABA concentrations the activity of acetylcholinesterase (AChE) in synaptic membranes increases. AChE inhibition is reported at higher GABA concentrations. The relation between the surface electrical properties of cells and the enzymatic activity of brain ATPases and AChE, as examined here, are expected to be helpful in the elucidation of membrane-mediated molecular mechanisms relevant to neurological disorders and conditions.

Computational Tool for Determining Local Dielectric Constants in Heterogeneous Nanoscale Systems from Molecular Dynamics Trajectories

In this work, we describe a computational tool designed to determine the local dielectric constants (ε) of charge-neutral heterogeneous systems by analyzing dipole moment fluctuations from molecular dynamics (MD) trajectories. Unlike conventional methods, our tool can calculate dielectric constants for dynamically evolving selections of molecules within a defined region of space, rather than for fixed sets of molecules. We validated our approach by computing the dielectric constants of TIP3P water nanospheres, achieving results consistent with literature values for bulk water. We then applied our tool to more complex systems, the water slabs around solvated phospholipid bilayers, where we observed a lower dielectric constant of water near the bilayer headgroups (ε = 20-50) compared to nanospheres of bulk water (ε = 58-62) with the same number of molecules. Our tool also enabled us to compute the dielectric constants of water in more heterogeneous systems, where water surrounding asymmetrically distributed phospholipids on single-walled carbon nanotubes also exhibited lower dielectric constants than in bulk water nanospheres. Addition of positively charged peptides that bind to phospholipid-nanotube conjugates further lowered the dielectric constants of water in the immediate vicinity of these conjugates. Moreover, we estimated dielectric constants for lipids in symmetric bilayers, where values are well-documented, and for asymmetric phospholipid-wrapped nanotube systems, which were previously unexplored, and found that dielectric constants of phospholipids depend on their arrangement in the assembled aggregate. The results align with the literature for bilayers and provide new insights for phospholipid-nanotube systems. The ability of our tool to provide local dielectric constants for both well-studied and novel systems advances our understanding of molecular environments and interactions.

Photomanipulation of Minimal Synthetic Cells: Area Increase, Softening, and Interleaflet Coupling of Membrane Models Doped with Azobenzene-Lipid Photoswitches

Light can effectively interrogate biological systems in a reversible and physiologically compatible manner with high spatiotemporal precision. Understanding the biophysics of photo-induced processes in bio-systems is crucial for achieving relevant clinical applications. Employing membranes doped with the photolipid azobenzene-phosphatidylcholine (azo-PC), a holistic picture of light-triggered changes in membrane kinetics, morphology, and material properties obtained from correlative studies on cell-sized vesicles, Langmuir monolayers, supported lipid bilayers, and molecular dynamics simulations is provided. Light-induced membrane area increases as high as ≈25% and a ten-fold decrease in the membrane bending rigidity is observed upon trans-to-cis azo-PC isomerization associated with membrane leaflet coupling and molecular curvature changes. Vesicle electrodeformation measurements and atomic force microscopy reveal that trans azo-PC bilayers are thicker than palmitoyl-oleoyl phosphatidylcholine (POPC) bilayers but have higher specific membrane capacitance and dielectric constant suggesting an increased ability to store electric charges across the membrane. Lastly, incubating POPC vesicles with azo-PC solutions results in the insertion of azo-PC in the membrane enabling them to become photoresponsive. All these results demonstrate that light can be used to finely manipulate the shape, mechanical and electric properties of photolipid-doped minimal cell models, and liposomal drug carriers, thus, presenting a promising therapeutic alternative for the repair of cellular disorders.

Effects of Trehalose Supplementation on Lipid Composition of Rooster Spermatozoa Membranes in a Freeze/Thaw Protocol

The plasma membrane of spermatozoa plays an important role in the formation and maintenance of many functions of spermatozoa, including during cryopreservation. As a result of chromatographic analysis, the content of lipids and fatty acids in the membranes of spermatozoa of roosters of two breeds was determined under the influence of cryoprotective media containing trehalose LCM-control (0 mM), Treh20 (9.5 mM), and Treh30 (13.4 mM). The use of the cryoprotective diluent Treh20 made it possible to maintain a dynamic balance between the synthesis and degradation of phospholipids and sterols in the plasma membranes of frozen/thawed spermatozoa, close to that of native spermatozoa. This contributed to an increase in the preservation of frozen/thawed spermatozoa membranes from 48.3% to 52.2% in the egg breed and from 30.0% to 35.1% in the meat- and-egg breed. It was also noted that their kinetic apparatus (mobility indicators) remained at the level of 45.6% (egg breed) and 52.4% (meat-and-egg breed). An increase in the concentration of trehalose to 13.4 mM in a cryoprotective diluent for rooster sperm resulted in a decrease in the morphofunctional parameters of frozen/thawed spermatozoa.

Erythrocyte Membrane Biophysical Changes Mediated by Pooled Immunoglobulin G and Hematin: Electrokinetic and Lipid Peroxidation Studies

Pooled Immunoglobulin G (IgG), hematin and the membrane-disruptive amphipathic peptide melittin have received attention as powerful biomacromolecules for biomedical and pharmacology applications. Their action on surface properties, oxidation status and epifluorescence properties measured in vitro provide useful information about the functional activity of upper biomacromolecules in erythrocytes in vivo. The hemolysis of erythrocyte membranes, as well as changes in hematocrit and the morphology of erythrocytes, was investigated here via fluorescence microscopy using FITC-concanavalin A binding to cells. The effect of melittin on the membrane capacitance and resistance of model lipid bilayers was probed via electrochemical impedance spectroscopy. Lipid bilayer capacitance was higher in the presence of 0.10 g/L melittin compared to that in the control, which is likely related to bilayer thinning and alterations of the dielectric permittivity of melittin-treated membranes. The biomolecule interactions with red blood cells were probed in physiological media in which the surface of erythrocyte membranes was negatively charged. Surface parameters of erythrocytes are reported upon IgG/hematin and IgG/melittin treatment. Pooled IgG in the presence of melittin, preincubated IgG/hematin preparations promoted a significant decrease in the electrokinetic potential of erythrocytes (Rh-positive). A malondialdehyde (MDA) assay revealed a high rate of lipid peroxidation in erythrocytes treated with IgG/hematin or IgG/melittin preparations. This finding might be a result of pooled IgG interactions with the hematin molecule and the subsequent conformational changes in the protein molecule altering the electrokinetic properties of the erythrocyte membrane related to the Rh group type of erythrocytes. The pooled IgG and hematin are reported to have important consequences for the biophysical understanding of the immunopathological mechanisms of inflammatory, autoimmune and antibody-mediated pathological processes.

Assessing membrane material properties from the response of giant unilamellar vesicles to electric fields

Knowledge of the material properties of membranes is crucial to understanding cell viability and physiology. A number of methods have been developed to probe membranes in vitro, utilizing the response of minimal biomimetic membrane models to an external perturbation. In this review, we focus on techniques employing giant unilamellar vesicles (GUVs), model membrane systems, often referred to as minimal artificial cells because of the potential they offer to mimick certain cellular features. When exposed to electric fields, GUV deformation, dynamic response and poration can be used to deduce properties such as bending rigidity, pore edge tension, membrane capacitance, surface shear viscosity, excess area and membrane stability. We present a succinct overview of these techniques, which require only simple instrumentation, available in many labs, as well as reasonably facile experimental implementation and analysis.