Calcium inhibits diacylglycerol uptake by serum albumin

AbstractSerum albumin is an abundant protein in blood plasma, that is well-known for its ability to transport hydrophobic biomolecules and drugs. Recent hypotheses propose that serum albumin plays a role in the regulation of lipid metabolism in addition to its lipid transport properties. The present work explores the capacity of bovine serum albumin (BSA) to extract diacylglycerols (DAG) from phospholipid bilayers, and the inhibition of such interaction by divalent cations. Quantitative measurements using radioactive DAG and morphological evidence derived from giant unilamellar vesicles examined by confocal microscopy provide concurrent results. BSA extracts DAG from vesicles consisting of phosphatidylinositol/DAG. Long, saturated DAG species are incorporated more readily than the shorter-chain or unsaturated ones. Divalent cations hinder DAG uptake by BSA. For Ca2+, the concentration causing half-maximal inhibition is ≈10 μM; 90% inhibition is caused by 100 μM Ca2+. Sr2+ requires concentrations one order of magnitude higher, while Mg2+ has virtually no effect. As an example on how DAG uptake by BSA, and its inhibition by Ca2+, could play a regulating role in lipid metabolism, a PI-specific phospholipase C has been assayed in the presence of BSA and/or Ca2+. BSA activates the enzyme by removing the end-product DAG, but the activation is reverted by Ca2+ that inhibits DAG uptake

The Binding of Aβ42 Peptide Monomers to Sphingomyelin/Cholesterol/Ganglioside Bilayers Assayed by Density Gradient Ultracentrifugation

The binding of Aβ42 peptide monomers to sphingomyelin/cholesterol (1:1 mol ratio) bilayers containing 5 mol% gangliosides (either GM1, or GT1b, or a mixture of brain gangliosides) has been assayed by density gradient ultracentrifugation. This procedure provides a direct method for measuring vesicle-bound peptides after non-bound fraction separation. This centrifugation technique has rarely been used in this context previously. The results show that gangliosides increase by about two-fold the amount of Aβ42 bound to sphingomyelin/cholesterol vesicles. Complementary studies of the same systems using thioflavin T fluorescence, Langmuir monolayers or infrared spectroscopy confirm the ganglioside-dependent increased binding. Furthermore these studies reveal that gangliosides facilitate the aggregation of Aβ42 giving rise to more extended β-sheets. Thus, gangliosides have both a quantitative and a qualitative effect on the binding of Aβ42 to sphingomyelin/cholesterol bilayers.This work was supported in part by grants from the Spanish Ministry of Economy (grant FEDER MINECO PGC2018-099857-B-I00) and the Basque Government (grants No. IT1264-19 and IT1270-19)

The interaction of Aβ42 peptide in monomer, oligomer or fibril forms with sphingomyelin/cholesterol/ganglioside bilayers

Aβ42 peptide binds neuronal membranes and aggregates into plaques that are characteristic of Alzheimer's disease. Aβ42 peptide has been proposed to be generated in membrane (nano) domains in the liquid-ordered phase, ganglioside GM1 being a major facilitator of peptide binding to the membrane. The peptide exists in solution in various degrees of aggregation, either monomers, oligomers or fibrils, of which oligomers appear to be particularly toxic. The present study reports on the binding of Aβ42 peptide, in monomer, oligomer or fibril form, to model membranes (lipid vesicles or monolayers), composed of sphingomyelin and cholesterol in equimolar ratios, to which 1-5 mol% of different gangliosides were incorporated. Thermodynamic binding parameters obtained from calorimetric data indicate a strong tendency to bind the membrane (ΔG ≈ 7 kcal/mol peptide), in a process dominated in most cases by the increase in entropy. ΔG was virtually invariant with the ganglioside species and the aggregation state of the peptide. The Langmuir balance demonstrated the capacity of all peptide preparations to become inserted in lipid monolayers of any composition and initial π in the range 10-30 mN/m, although fibrils were less capable to do so than oligomers or monomers, their maximum initial π being ≈25 mN/m.This work was supported in part by grants from the Ministry of Economy and Competitiveness (grant FEDER MINECO PGC2018-099857-B-I00) and the Basque Government (grants No. IT1264-19 and IT1270-19), as well as by Fundación Biofísica Bizkaia and the Basque Excellence Research Centre (BERC) program of the Basque Government.Peer reviewe

β-Amyloid (1-42) peptide adsorbs but does not insert into ganglioside-containing phospholipid membranes in the liquid-disordered state: modelling and experimental studies

β-Amyloid (Aβ) is a 39-43 residue peptide involved in the pathogenesis of Alzheimer's disease. Aβ deposits onto the cells and gives rise to the plaques that are characteristic of the disease. In an effort to understand the molecular mechanism of plaque formation, we have examined the interaction of Aβ42, considered to be the most pathogenic of the peptides, with lipid bilayers consisting of 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) to which small amounts of GM1 ganglioside (1-5 mol%) were incorporated. POPC bilayers exist in the fluid, or liquid-disordered state at room temperature, mimicking the fluidity of cell membranes. An Aβ42 preparation consisting essentially of peptide monomers was used. A combination of molecular dynamics (MD), isothermal calorimetry and Langmuir balance measurements was applied. Our results show that Aβ binds POPC bilayers, and that binding increases (ΔG of binding decreases) with GM1, but only up to 3 mol% of the ganglioside, larger concentrations appearing to have a lower effect. MD and Langmuir balance measurements concur in showing that the peptide adsorbs onto the bilayer surface, but does not become inserted into it at surface pressures compatible with the cell membrane conditions. Thioflavin T measurements agree with MD in revealing a very low degree of peptide oligomerization/aggregation under our conditions. This is in contrast with previous studies showing peptide aggregation and insertion when interacting with membranes in the liquid-ordered state. The present contribution underlines the importance of bilayer lipid composition and properties for Aβ plaque formation.This work was supported in part by grants from the Spanish Ministry of Economy (grant FEDER MINECO PGC2018-099857-B-I00) and the Basque Government (grants No. IT1264-19 and IT1270-19). ABGA was supported by the University of the Basque Country. This work was partially funded by FOCEM (MERCOSUR Structural Convergence Fund), COF 03/11.Peer reviewe