Phase Behavior and Lipid-Membrane Structure of Phospholipid-Glycosphingolipid Liposomes and the Thermal Unfolding of Insulin

Ca2+-induced effect on mechanical properties of sulfatide-incorporated vesicles

The Ca(2+)-induced effect on the nanomechanical properties of vesicles prepared at a different ratio of dipalmitoylphosphatidylcholine (DPPC)/sulfatide was studied using atomic force microscope (AFM) on a mica surface. Vesicles were prepared by extrusion and adsorbed on the mica surface. The forces, measured between an AFM tip and the vesicle, showed that the breakthrough of the tip into the vesicles occurred two times. Force data prior to the first breakthrough were fitted well with the Hertzian model to estimate Young's modulus and bending modulus of the vesicles. Sulfatide incorporation led to a decrease of around 90% in Young's modulus and bending modulus of the vesicles due to the hydration of the headgroups, while the addition of Ca(2+) induced dehydration to recover the properties. The change of the physical properties seems to be attributed to the headgroup packing order of the vesicles, which is determined by the interference with the hydration shell.

The effects of various membrane physical-chemical properties on the aggregation kinetics of insulin

In a simplified approach to the in vivo situation, where pathogenic fibrillar protein deposits are often found associated with cellular membranes, the aggregation kinetics of insulin in the presence of various model biomembranes were investigated using the Thioflavin T (ThT) fluorescence assay. The lipid dynamics near the gel-fluid transition, the chain length of saturated lipids and the presence of DOPE or DOPS in DOPC-vesicles modulate the aggregation kinetics of insulin in an indifferent, an aggregation-accelerating or an aggregation-inhibiting manner, subtly depending on the pH-value and the presence of salt. The rate of insulin aggregation in bulk solution dominates the overall aggregation process in most cases at low pH, where the lipid additives exert no effect on the aggregation kinetics. The occurrence of dynamic line defects near the gel-fluid transition temperature of DSPC facilitates a partial membrane insertion of the protein, which in turn shields exposed hydrophobic protein patches from intermolecular association and hence inhibit aggregation. An exclusively aggregation-accelerating effect was observed in the presence of 0.1M NaCl for all lipid additives investigated, which is likely due to an enhanced surface accumulation of the protein. Apart from weak dipole-dipole, dipole-monopole and hydrogen bonding interactions, the release of curvature elastic stress in mixed DOPC/DOPE-membranes and preferred interactions of insulin with carboxylic groups in DOPC/DOPS-membranes favour an increased surface accumulation. At neutral pH, a partial insertion of insulin into the lipid bilayer is favoured, which accounts for the aggregation-inhibiting effect of all lipid bilayer systems studied except those containing DOPS. Generally, the extent of inhibition increases with the lipid chain length and the extent of curvature stress in mixed unsaturated lipid membranes and also when the gel-fluid transition temperature of pure phospholipids is approached. The accelerating effect of DOPS on the aggregation of insulin under net electrostatic repulsion at pH 7.4 remains to be elucidated, yet, it might result from increased surface accumulation and/or faster/more extensive unfolding of the protein without a subsequent membrane insertion. These results demonstrate that a delicate interplay between different physical and chemical properties of lipid membranes has to be taken into account in a detailed discussion of membrane-associated protein aggregation phenomena.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Endogenous carriers and ligands in non-immunogenic site-specific drug delivery

Targeted drug delivery has gained recognition in modern therapeutics and attempts are being made to explore the potentials and possibilities of cell biology related bioevents in the development of specific, programmed and target oriented systems. The components which have been recognized to be tools include receptors and ligands, where the receptors act as molecular targets or portals, and ligands, with receptor specificity and selectivity, are trafficked en route to the target site. Although ligands of exogenous or synthetic origin contribute to the selectivity component of carrier constructs, they may impose immunological manifestations of different magnitudes. The latter may entail a continual quest for bio-compatible, non-immunogenic and target orientated delivery. Endogenous serum, cellular and extracellular bio-ligands interact with the colloidal carrier constructs and influence their bio-fate. However, these endogenous bio-ligands can themselves serve as targeting modules either in their native form or engineered as carrier cargo. Bio-regulatory, nutrient and immune ligands are sensitive, specific and effective site directing handles which add to targeted drug delivery. The present review provides an exhaustive account of the identified bio-ligands, which are not only non-immunogenic in nature but also site-specific. The cell-related bioevents which are instrumental in negotiating the uptake of bio-ligands are discussed. Further, a brief account of ligand-receptor interactions and the set of biological events which ensures ligand-driven trafficking of the ligand-receptor complex to the cellular interior is also presented. Since ligand-receptor interaction is a critical pre-requisite for negotiating cellular uptake of endogenous ligands and anchored carrier cargo, an attempt has been made to identify differential expression of receptors and bio-ligands under normal and etiological conditions. Studies which judiciously utilized bio-ligands or their analogs in negotiating site-specific drug delivery have been reviewed and presented. Targeted delivery of bioactives using endogenous bio-ligands offers enormous options and opportunities through carrier construct engineering and could become a future reality in clinical practice.