Interaction of prothrombin with a phospholipid surface: evidence for a membrane-induced conformational change

Prothrombin interacts with phosphatidylserine containing platelet membranes via its N-terminal, gamma-carboxyglutamate (gla) residue-rich domain. Once bound it is cleaved to form the active protease, thrombin (factor IIa). Human prothrombin was cleaved with cathepsin G in the absence of calcium and magnesium ions. Under these conditions, the gla domain was removed. Phospholipid protected the protein from this proteolytic event, and this suggests that a conformational change may be induced by interaction with phospholipids. Binding of prothrombin to a surface containing 20% phosphatidylserine/80% phosphatidylcholine was detected by surface plasmon resonance, whereas no interaction with gla-domainless prothrombin was observed. Binding of intact prothrombin in the presence of calcium ions showed complex association kinetics, suggesting multiple modes of initial interaction with the surface. The kinetics of the dissociation phase could be fitted to a two-phase, exponential decay. This implies that there are at least two forms of the protein on the surface one of which dissociates tenfold more slowly than the other. Taken together, these data suggest that, on binding to a membrane surface, prothrombin undergoes a conformational change to a form which binds more tightly to the membrane.

A comparison of the haemagglutinating and enzymic activities of Bacteroides fragilis whole cells and outer membrane vesicles

The haemagglutinating and enzymic activities of the obligately anaerobic pathogenic bacterium Bacteroides fragilis were examined. Outer membrane vesicles are released from the surface of B. fragilis. They can be detected by electron microscopy in ultrathin sections and bacterial suspensions after negative staining. Electron microscopy and immunogold labelling with a MAb specific for surface polysaccharide of B. fragilis confirmed that the vesicles carried outer membrane associated epitopes. The haemagglutinating activity of whole cells from populations of B. fragilis strains NCTC9343, BE3 and LS66 enriched by Percoll density gradient centrifugation for a large capsule (LC), electron dense layer (EDL); non-capsulate by light microscopy) and outer membrane vesicles (OMV) which had been purified by centrifugation from EDL-enriched populations were compared using human and horse erythrocytes. The enzymic activity of OMV, LC- and EDL-enriched populations, as detected by the API ZYM kit, was compared for strains NCTC 9343 and BE3. Purified OMV from the strains examined exhibited both haemagglutinating and enzymatic activity. Haemagglutination by the EDL-enriched population was sensitive to treatment with sodium periodate. The LC-enriched population haemagglutinated only after ultrasonic removal of the capsule. This indicates that the LC masks a haemagglutinin. The results suggest a potential role for OMV in the virulence of B. fragilis.

Carboxymethylethanolamine, a biomarker of phospholipid modification during the maillard reaction in vivo

Nepsilon-(Carboxymethyl)lysine (CML) is a stable chemical modification of proteins formed from both carbohydrates and lipids during autoxidation reactions. We hypothesized that carboxymethyl lipids such as (carboxymethyl)phosphatidylethanolamine (carboxymethyl-PE) would also be formed in these reactions, and we therefore developed a gas chromatography-mass spectrometry assay for quantification of carboxymethylethanolamine (CME) following hydrolysis of phospholipids. In vitro, CME was formed during glycation of dioleoyl-PE under air and from linoleoylpalmitoyl-PE, but not from dioleoyl-PE, in the absence of glucose. In vivo, CME was detected in lipid extracts of red blood cell membranes, approximately 0.14 mmol of CME/mol of ethanolamine, from control and diabetic subjects, (n = 22, p > 0.5). Levels of CML in erythrocyte membrane proteins were approximately 0.2 mmol/mol of lysine for both control and diabetic subjects (p > 0.5). For this group of diabetic subjects there was no indication of increased oxidative modification of either lipid or protein components of red cell membranes. CME was also detected in fasting urine at 2-3 nmol/mg of creatinine in control and diabetic subjects (p = 0.085). CME inhibited detection of advanced glycation end product (AGE)-modified protein in a competitive enzyme-linked immunosorbent assay using an anti-AGE antibody previously shown to recognize CML, suggesting that carboxymethyl-PE may be a component of AGE lipids detected in AGE low density lipoprotein. Measurement of levels of CME in blood, tissues, and urine should be useful for assessing oxidative damage to membrane lipids during aging and in disease.

Respiratory Infections Cause the Release of Extracellular Vesicles: Implications in Exacerbation of Asthma/COPD

Background: Infection-related exacerbations of respiratory diseases are a major health concern; thus understanding the mechanisms driving them is of paramount importance. Despite distinct inflammatory profiles and pathological differences, asthma and COPD share a common clinical facet: raised airway ATP levels. Furthermore, evidence is growing to suggest that infective agents can cause the release of extracellular vesicle (EVs) in vitro and in bodily fluids. ATP can evoke the P2X7/caspase 1 dependent release of IL-1β/IL-18 from EVs; these cytokines are associated with neutrophilia and are increased during exacerbations. Thus we hypothesized that respiratory infections causes the release of EVs in the airway and that the raised ATP levels, present in respiratory disease, triggers the release of IL-1β/IL-18, neutrophilia and subsequent disease exacerbations.

Methods: To begin to test this hypothesis we utilised human cell-based assays, ex vivo murine BALF, in vivo pre-clinical models and human samples to test this hypothesis.

Results: Data showed that in a murine model of COPD, known to have increased airway ATP levels, infective challenge causes exacerbated inflammation. Using cell-based systems, murine models and samples collected from challenged healthy subjects, we showed that infection can trigger the release of EVs. When exposed to ATP the EVs release IL-1b/IL-18 via a P2X₇/caspase-dependent mechanism. Furthermore ATP challenge can cause a P2X₇ dependent increase in LPS-driven neutrophilia.

Conclusions: This preliminary data suggests a possible mechanism for how infections could exacerbate respiratory diseases and may highlight a possible signalling pathway for drug discovery efforts in this area.

Polycystic ovary syndrome influences the level of serum amyloid A and activity of phospholipid transfer protein in HDL2 and HDL3

The interplay between clathrin-coated vesicles and cell signalling

Internalization of cargo proteins and lipids at the cell surface occurs in both a constitutive and signal-regulated manner through clathrin-mediated and other endocytic pathways. Clathrin-coated vesicle formation is a principal uptake route in response to signalling events. Protein-lipid and protein-protein interactions control both the targeting of signalling molecules and their binding partners to membrane compartments and the assembly of clathrin coats. An emerging aspect of membrane trafficking research is now addressing how signalling cascades and vesicle coat assembly and subsequently disassembly are integrated.

An endophilin-dynamin complex promotes budding of clathrin-coated vesicles during synaptic vesicle recycling

Clathrin-mediated vesicle recycling in synapses is maintained by a unique set of endocytic proteins and interactions. We show that endophilin localizes in the vesicle pool at rest and in spirals at the necks of clathrin-coated pits (CCPs) during activity in lamprey synapses. Endophilin and dynamin colocalize at the base of the clathrin coat. Protein spirals composed of these proteins on lipid tubes in vitro have a pitch similar to the one observed at necks of CCPs in living synapses, and lipid tubules are thinner than those formed by dynamin alone. Tubulation efficiency and the amount of dynamin recruited to lipid tubes are dramatically increased in the presence of endophilin. Blocking the interactions of the endophilin SH3 domain in situ reduces dynamin accumulation at the neck and prevents the formation of elongated necks observed in the presence of GTPγS. Therefore, endophilin recruits dynamin to a restricted part of the CCP neck, forming a complex, which promotes budding of new synaptic vesicles.

The extracellular vesicles of the helminth pathogen, Fasciola hepatica: biogenesis pathways and cargo molecules involved in parasite pathogenesis.

Extracellular vesicles (EVs) released by parasites have important roles in establishing and maintaining infection. Analysis of the soluble and vesicular secretions of adult Fasciola hepatica has established a definitive characterisation of the total secretome of this zoonotic parasite. Fasciola secretes at least two sub-populations of EVs that differ according to size, cargo molecules and site of release from the parasite. The larger EVs are released from the specialised cells that line the parasite gastrodermus and contain the zymogen of the 37 kDa cathepsin L peptidase that performs a digestive function. The smaller exosome-like vesicle population originate from multivesicular bodies within the tegumental syncytium and carry many previously described immunomodulatory molecules that could be delivered into host cells. By integrating our proteomics data with recently available transcriptomic datasets we have detailed the pathways involved with EV biogenesis in F. hepatica and propose that the small exosome biogenesis occurs via ESCRT-dependent MVB formation in the tegumental syncytium before being shed from the apical plasma membrane. Furthermore, we found that the molecular machinery required for EV biogenesis is constitutively expressed across the intra-mammalian development stages of the parasite. By contrast, the cargo molecules packaged within the EVs are developmentally regulated, most likely to facilitate the parasites migration through host tissue and to counteract host immune attack.

Red cell PMVs, plasma membrane-derived vesicles calling out for standards

Plasma membrane-derived vesicles (PMVs) or microparticles are vesicles (0.1–1 μm in diameter) released from the plasma membrane of all blood cell types under a variety of biochemical and pathological conditions. PMVs contain cytoskeletal elements and some surface markers from the parent cell but lack a nucleus and are unable to synthesise macromolecules. They are also defined on the basis that in most cases PMVs express varying amounts of the cytosolic leaflet lipid phosphatidylserine, which is externalised during activation on their surface. This marks the PMV as a biologically distinct entity from that of its parent cell, despite containing surface markers from the original cell, and also explains its role in events such as phagocytosis and thrombosis. There is currently a large amount of variation between investigators with regard to the pre-analytical steps employed in isolating red cell PMVs or RPMVs (which are slightly smaller than most PMVs), with key differences being centrifugation and sample storage conditions, which often leads to result variability. Unfortunately, standardization of preparation and detection methods has not yet been achieved. This review highlights and critically discusses the variables contributing to differences in results obtained by investigators, bringing to light numerous studies of which RPMVs have been analysed but have not yet been the subject of a review.

The structural effects of divalent cations and insulin on phospholipid model membranes /

It is well accepted that structural studies with model membranes are of considerable value in understanding the structure of biological membranes. Many studies with models of pure phospholipids have been done; but the effects of divalent cations and protein on these models would make these studies more applicable to intact membrane. The present study, performed with above view, is a structural analysis of divalent io~cardio1ipin complexes using the technique of x-ray diffraction. Cardiolipin, precipitated from dilute solution by divalent ionscalcium, magnesium and barium, contains little water and the structure formed is similar to the structure of pure cardiolipin with low water content. The calcium-cardiolipin complex forms a pure hexagonal type II phase that exists from 40 to 400 C. The molar ratio of calcium and cardiolipin in the complex is 1 : 1. Cardiolipin, precipitated with magnesium and barium forms two co-existing phases, lamellar and hexagonal, the relative quantity of the two phases being dependent on temperature. The hexagonal phase type II consisting of water filled channels formed by adding calcium to cardiolipin may have a remarkable permeability property in intact membrane. Pure cardiolipin and insulin at pH 3.0 and 4.0 precipitate but form no organised structure. Lecithin/cardiolipin and insulin precipitated at pH 3.0 give a pure lamellar phase. As the lecithin/cardiolipin molar ratio changes from 93/7 to SO/50, (a) the repeat distance of the lamellar changes from 72.8 X to 68.2 A; (b) the amount of protein bound increases in such a way that cardiolipin/insulin molar ratio in the complex reaches a maximum constant value at lecithin/cardiolipin molar ratio 70/30. A structural model based on these data shows that the molecular arrangement of lipid and protein is a lipid bilayer coated with protein molecules. The lipid-protein interaction is chiefly electrostatic and little, if any, hydrophobic bonding occurs in this particular system. So, the proposed model is essentially the same as Davson-Daniellifs model of biological membrane.