The effects of membrane physical properties on the fusion of Sendai virus with human erythrocyte ghosts and liposomes. Analysis of kinetics and extent of fusion

Cholesterol Sulfate: Pathophysiological Implications and Potential Therapeutics

Cholesterol sulfate (CS) is a naturally occurring cholesterol derivative that is widely distributed across various tissues and body fluids. In humans, its biosynthesis is primarily mediated by the sulfotransferase (SULT) 2B1b (SULT2B1b). Over the years, CS has been found to play critical roles in various physiological processes, including epidermal cell adhesion, sperm capacitation, platelet adhesion, coagulation, glucolipid metabolism, bone metabolism, gut microbiota metabolism, neurosteroid biosynthesis, T-cell receptor signaling, and immune cell migration. In this review, we first introduce the endogenous regulation of CS biosynthesis and metabolism. We then highlight current advances in the understanding of the physiological roles of CS. Finally, we delve into the implications of CS in various diseases, with a particular focus on its mechanism of action and potential therapeutic applications. A comprehensive understanding of CS's physiological function, biosynthesis regulation, and role as a disease modifier offers novel insights that could pave the way for innovative therapeutic strategies targeting a wide range of conditions.

Soft matter science and the COVID-19 pandemic

Much of the science underpinning the global response to the COVID-19 pandemic lies in the soft matter domain. Coronaviruses are composite particles with a core of nucleic acids complexed to proteins surrounded by a protein-studded lipid bilayer shell. A dominant route for transmission is via air-borne aerosols and droplets. Viral interaction with polymeric body fluids, particularly mucus, and cell membranes controls their infectivity, while their interaction with skin and artificial surfaces underpins cleaning and disinfection and the efficacy of masks and other personal protective equipment. The global response to COVID-19 has highlighted gaps in the soft matter knowledge base. We survey these gaps, especially as pertaining to the transmission of the disease, and suggest questions that can (and need to) be tackled, both in response to COVID-19 and to better prepare for future viral pandemics.

Singlet oxygen effects on lipid membranes: implications for the mechanism of action of broad-spectrum viral fusion inhibitors

It was reported recently that a new aryl methyldiene rhodanine derivative, LJ001, and oxazolidine-2,4-dithione, JL103, act on the viral membrane, inhibiting its fusion with a target cell membrane. The aim of the present study was to investigate the interactions of these two active compounds and an inactive analogue used as a negative control, LJ025, with biological membrane models, in order to clarify the mechanism of action at the molecular level of these new broad-spectrum enveloped virus entry inhibitors. Fluorescence spectroscopy was used to quantify the partition and determine the location of the molecules on membranes. The ability of the compounds to produce reactive oxygen molecules in the membrane was tested using 9,10-dimethylanthracene, which reacts selectively with singlet oxygen (1O2). Changes in the lipid packing and fluidity of membranes were assessed by fluorescence anisotropy and generalized polarization measurements. Finally, the ability to inhibit membrane fusion was evaluated using FRET. Our results indicate that 1O2 production by LJ001 and JL103 is able to induce several changes on membrane properties, specially related to a decrease in its fluidity, concomitant with an increase in the order of the polar headgroup region, resulting in an inhibition of the membrane fusion necessary for cell infection.

Interaction of 3β-amino-5-cholestene with phospholipids in binary and ternary bilayer membranes

3β-Amino-5-cholestene (aminocholesterol) is a synthetic sterol whose properties in bilayer membranes have been examined. In fluid palmitoyl sphingomyelin (PSM) bilayers, aminocholesterol and cholesterol were equally effective in increasing acyl chain order, based on changes in diphenylhexatriene (DPH) anisotropy. In fluid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers, aminocholesterol ordered acyl chains, but slightly less efficiently than cholesterol. Aminocholesterol eliminated the PSM and DPPC gel-to-liquid crystalline phase transition enthalpy linearly with concentration, and the enthalpy approached zero at 30 mol % sterol. Whereas cholesterol was able to increase the thermostability of ordered PSM domains in a fluid bilayer, aminocholesterol under equal conditions failed to do this, suggesting that its interaction with PSM was not as favorable as cholesterols. In ternary mixed bilayers, containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), PSM or DPPC, and cholesterol at proportions to contain a liquid-ordered phase (60:40 by mol of POPC and PSM or DPPC, and 30 mol % cholesterol), the average lifetime of trans-parinaric acid (tPA) was close to 20 ns. When cholesterol was replaced with aminocholesterol in such mixed bilayers, the average lifetime of tPA was only marginally shorter (about 18 ns). This observation, together with acyl chain ordering data, clearly shows that aminocholesterol was able to form a liquid-ordered phase with saturated PSM or DPPC. We conclude that aminocholesterol should be a good sterol replacement in model membrane systems for which a partial positive charge is deemed beneficial.

Rigid amphipathic fusion inhibitors, small molecule antiviral compounds against enveloped viruses

Antiviral drugs targeting viral proteins often result in prompt selection for resistance. Moreover, the number of viral targets is limited. Novel antiviral targets are therefore needed. The unique characteristics of fusion between virion envelopes and cell membranes may provide such targets. Like all fusing bilayers, viral envelopes locally adopt hourglass-shaped stalks during the initial stages of fusion, a process that requires local negative membrane curvature. Unlike cellular vesicles, however, viral envelopes do not redistribute lipids between leaflets, can only use the energy released by virion proteins, and fuse to the extracellular leaflets of cell membranes. Enrichment in phospholipids with hydrophilic heads larger than their hydrophobic tails in the convex outer leaflet of vesicles favors positive curvature, therefore increasing the activation energy barrier for fusion. Such phospholipids can increase the activation barrier beyond the energy provided by virion proteins, thereby inhibiting viral fusion. However, phospholipids are not pharmacologically useful. We show here that a family of synthetic rigid amphiphiles of shape similar to such phospholipids, RAFIs (rigid amphipathic fusion inhibitors), inhibit the infectivity of several otherwise unrelated enveloped viruses, including hepatitis C and HSV-1 and -2 (lowest apparent IC(50) 48 nM), with no cytotoxic or cytostatic effects (selectivity index > 3,000) by inhibiting the increased negative curvature required for the initial stages of fusion.

Calorimetric and spectroscopic studies of the effects of cholesterol on the thermotropic phase behavior and organization of a homologous series of linear saturated phosphatidylglycerol bilayer membranes

We have examined the effects of cholesterol (Chol) on the thermotropic phase behavior and organization of aqueous dispersions of a homologous series of linear disaturated phosphatidylglycerols (PGs) by high-sensitivity differential scanning calorimetry and Fourier transform infrared and 31P NMR spectroscopy. We find that the incorporation of increasing quantities of Chol alters the temperature and progressively reduces the enthalpy and cooperativity of the gel-to-liquid-crystalline phase transition of the host PG bilayer. With dimyristoyl-PG:Chol mixtures, cooperative chain-melting phase transitions are completely or almost completely abolished at Chol concentrations near 50 mol%, whereas with the dipalmitoyl- and distearoyl-PG:Chol mixtures, cooperative hydrocarbon chain-melting phase transitions are still discernable at Chol concentrations near 50 mol%. We are also unable to detect the presence of significant populations of separate domains of the anhydrous or monohydrate forms of Chol in our binary mixtures, in contrast to previous reports. We ascribe the previously reported large scale formation of Chol crystallites to the fractional crystallization of the Chol and phospholipid phases during the removal of organic solvent from the binary mixture before the hydration of the sample. We further show that the direction and magnitude of the change in the phase transition temperature induced by Chol addition is dependent on the hydrocarbon chain length of the PG studied. This finding agrees with our previous results with phosphatidylcholine bilayers, where we found that Chol increases or decreases the phase transition temperature in a hydrophobic mismatch-dependent manner (Biochemistry 1993, 32:516-522), but is in contrast to our previous results for phosphatidylethanolamine (Biochim. Biophys. Acta 1999, 1416:119-234) and phosphatidylserine (Biophys. J. 2000, 79:2056-2065) bilayers, where no such hydrophobic mismatch-dependent effects were observed. We also show that the addition of Chol facilitates the formation of the lamellar crystalline phase in PG bilayers, as it does in phosphatidylethanolamine and phosphatidylserine bilayers, whereas the formation of such phases in phosphatidylcholine bilayers is inhibited by the presence of Chol. Moreover, the formation of the lamellar crystalline phase in PG bilayers at lower temperatures excludes Chol, resulting in an apparent Chol immiscibility in gel-state PG bilayers. We suggest that the magnitude of the effect of Chol on the thermotropic phase behavior of the host phospholipid bilayer, and its miscibility in phospholipids dispersions generally, depend on the strength of the attractive interactions between the polar headgroups and the hydrocarbon chains of the phospholipid molecule, and not on the charge of the polar headgroups per se.

Mobilization of intracellular iron by analogs of pyridoxal isonicotinoyl hydrazone (PIH) is determined by the membrane permeability of the iron-chelator complexes

In the ongoing search for an effective, orally active iron-chelator, the capacity of a series of halogenated analogs of pyridoxal isonicotinoyl hydrazone (PIH) to bind intracellular 59Fe and cause its release from cells was investigated. Reticulocytes labeled with 59Fe(2)-transferrin in which heme synthesis was inhibited by succinylacetone were used as a model of 59Fe mobilization. The kinetics of iron binding were similar for all the chelators tested (half-time of approximately 1 hr), and all bound more than twice as much 59Fe as PIH. The rate of release of the 59Fe-chelator complexes from cells depended upon the structure of the chelators. Ortho-substituted analogs were more effective at mobilizing cellular iron than meta and para isomers, due to a more efficient release of the iron complexes from the cell. The iron-chelator complexes which were released slowly from cells had a high affinity for erythrocyte ghost membranes, indicating the role of membrane permeability in the release mechanism of the complexes. The addition of BSA to the extracellular medium increased the extent of iron release by lipophilic analogs in a concentration-dependent manner, presumably by acting as a sink for the lipophilic complexes. The affinity of BSA for the chelators and their Fe(3+) complexes, determined spectrophotometrically, demonstrated that all chelators and their iron complexes bound BSA with dissociation constants ranging from 7,000 to >500,000 M(-1). Understanding the importance of the rate of release of the iron-chelator complex will direct the search for iron-chelators with improved efficacy.

Cholesteryl hemisuccinate exhibits pH sensitive polymorphic phase behavior

Cholesteryl hemisuccinate (CHEMS) is an acidic cholesterol ester that self-assembles into bilayers in alkaline and neutral aqueous media and is commonly employed in mixtures with dioleoylphosphatidylethanolamine (DOPE) to form 'pH sensitive' fusogenic vesicles. We show here that CHEMS itself exhibits pH sensitive polymorphism. This is evident from the fusogenic properties of large unilamellar vesicles (LUV) composed of CHEMS and direct visualization employing freeze-fracture electron microscopy. Below pH 4.3, LUV composed of CHEMS undergo fusion as monitored by lipid mixing assays and freeze-fracture electron micrographs reveal the characteristic striated signature of H( parallel) phase lipid. It is suggested that the pH dependent phase preferences of CHEMS contribute to the pH sensitivity of LUV composed of mixtures of CHEMS and DOPE.

Role of the N-terminal peptides of viral envelope proteins in membrane fusion

Membrane fusion is an important biological process that is observed in a wide variety of intra and intercellular events. In this review, work done in the last few years on the molecular mechanism of viral membrane fusion is highlighted, focusing in particular on the role of the fusion peptide and the modification of the lipid bilayer structure. While the Influenza hemagglutinin is currently the best understand fusion protein, there is still much to be learned about the key events in enveloped virus fusion reactions. This review compares our current understanding of the membrane fusion activity of Influenza and retrovirus viruses. We shall be concerned especially with the studies that lead to interpretations at the molecular level, so we shall concentrate on model membrane systems where the molecular components of the membrane and the environment are strictly controlled.

Reconstitution of membrane fusion between pancreatic islet secretory granules and plasma membranes: catalysis by a protein constituent recognized by monoclonal antibodies directed against glyceraldehyde-3-phosphate dehydrogenase

An isoform of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) isolated and purified from rabbit brain cytosol has previously been demonstrated to catalyze membrane fusion (Glaser and Gross, Biochemistry 33 (1994) 5805-5812; Glaser and Gross, Biochemistry 34 (1995) 12193-12203). Herein, we provide evidence suggesting that this GAPDH isoform can reconstitute in vitro protein-catalyzed fusion between naturally occurring subcellular membrane fractions involved in insulin exocytosis. Utilizing purified rat pancreatic beta-cell plasma membranes and secretory granules, we show that a brain cytosolic factor catalyzed the rapid and efficient fusion of these two purified membrane fractions which could be inhibited by a monoclonal antibody directed against the brain isoform of GAPDH. Moreover, the brain cytosolic factor also catalyzed the fusion of reconstituted vesicles prepared from lipid extracts of islet plasma membranes and secretory granules. Although the brain cytosolic factor rapidly catalyzed membrane fusion between islet plasma membranes and secretory granules, it did not catalyze fusion between one secretory granule population with another. To identify the potential importance of brain cytosolic factor catalyzed membrane fusion in islet cells, we examined extracts of hamster insulinoma tumor cells (HIT cells) for fusion-catalyzing activity. A protein constituent was present in HIT cell cytosol which was immunologically similar to the rabbit brain GAPDH isoform. Although native HIT cell cytosol did not catalyze membrane fusion, removal of an endogenous protein inhibitor unmasked the presence of the protein which catalyzed membrane fusion activity and such fusion was ablated by a monoclonal antibody directed against the brain isoform of GAPDH. Collectively, these results suggest the possibility that an isoform of brain GAPDH, also evident in HIT cells, can catalyze fusion between the two naturally occurring subcellular membrane compartments involved in insulin secretion and suggest a novel paradigm potentially coupling glycolytic flux with insulin release.

Lipid polymorphism and protein-lipid interactions

Non-lamellar-forming lipids play an important role in determining the physical properties of membranes. They affect the activity of membrane proteins and peptides. In addition, peptides which lyse membranes as well as those which promote membrane fusion facilitate the formation of non-lamellar phases, either micelles, cubic or hexagonal phases. The relationship of these diverse effects on membrane curvature is discussed in relation to the function of certain peptides and proteins. Specific examples of ionophoric peptides, cytotoxic peptides and viral fusion peptides are given. In addition, we compare the modulation of the rate of photoisomerisation of an integral membrane protein, rhodopsin, by non-lamellar-forming lipids with the effects of these lipids on an amphitropic protein, protein kinase C. Among these diverse systems it is frequently observed that the modulation of biological activity can be described in terms of the effect of the peptide or protein on the relative stability of lamellar and non-lamellar structures.

Fusion of Sendai virus and individual host cells and inhibition of fusion by lipophosphoglycan measured with image correlation spectroscopy

Fusion between Sendai virus (SV) and individual host cells was investigated with confocal laser scanning microscopy (CLSM) and image correlation spectroscopy (ICS). SV was labeled with the fluorescent probe 7-octadecylamino-4-nitrobenz-2-oxa-1,3-diazole (NBD-NH-C18) and was allowed to bind to host cells (HEp-2, BALB-3T3) at 4 degrees C. The effect of lipophosphoglycan (LPG), isolated from Leishmania donovani, on virus fusion was investigated by incorporation of LPG (0, 5, 10 or 20 microM) into the host cell membrane (HEp-2) before addition of SV. LPG did not affect the number of SV bound per cell. After incubation at 37 degrees C for 15 min without LPG, CLSM revealed a redistribution of NBD-NH-C18 from the SV envelope to the host cell membrane and an increase in average fluorescence intensity, indicating dequenching. ICS analysis of images obtained after incubation at 37 degrees C showed an increased mean cluster density to 260% of the value at 4 degrees C, reflecting the disappearance of labeled SV from the cell surface and diffusion of NBD-NH-C18 into the host cell membrane. Preincubation of the cells with LPG inhibited the temperature-induced redistribution and dequenching of NBD-NH-C18 in a concentration-dependent manner, with a total inhibition of fusion at 20 microM LPG. Together, the results demonstrate that CLSM combined with ICS is a powerful tool for studies of fusion of enveloped viruses with individual host cells and that LPG inhibits the fusion process at or before the hemifusion (lipid mixing) stage of SV interaction with cells.

Properties of lipoamino acids incorporated into membrane bilayers

Several lipoamino acids were synthesized in which palmitic acid was coupled with the alpha-amino group of an amino acid. These lipoamino acids were tested for their inhibitory action against Sendai virus fusion to liposomes composed of egg phosphatidylethanolamine and 5 mol% of the ganglioside GD1a. A commonly employed viral fusion assay based on the dilution of the fluorescent probe octadecylrhodamine (R18) exhibited an additional complication in the presence of Nalpha-palmitoyl tryptophan (palm-Trp). At higher mol fraction of palm-Trp it was observed that there was an increase in R18 quenching. Studies on the dependence of the emission wavelength of palm-Trp on excitation wavelength demonstrated that the presence of R18 alters the environment of the indole. The results illustrate one of the complexities of viral fusion assays using the R18 probe. Despite this complication it was possible to demonstrate that several of the lipoamino acids are effective at inhibiting the fusion of Sendai virus to liposomes as measured by the R18 assay. One of the most effective inhibitors of this process is palm-Trp which, at a concentration of 4 mol% in liposomes, markedly reduces the apparent rate of fusion. At pH 5.0 this amphiphile is also an inhibitor of Sendai virus fusion, indicating that the ionization of the carboxyl group of this amphiphile is not required for its antiviral activity. The inhibitory action of palm-Trp against Sendai virus was confirmed by demonstrating inhibition of Sendai-mediated cytopathic effects studied in tissue culture. A property associated with antiviral activity is the ability of amphiphiles to raise the bilayer to hexagonal phase transition temperature of dielaidoyl phosphatidylethanolamine. All of these lipoamino acids were found to possess this property, but a quantitative relationship with inhibition of viral fusion was not found.

Lipogastrins as potent inhibitors of viral fusion

The rate and extent of membrane fusion is markedly sensitive to membrane interfacial properties. Lipopeptides with hydrophilic peptide moieties will insert into membranes, leaving the peptide portion at the membrane-water interface. In this work, we have used a lipopeptide composed of the peptide [Nle15]-gastrin-(2-17)-amide covalently linked to 1,2-diacyl-3-mercaptoglycerol-N(alpha)-maleoyl-beta-alanine to give DM-gastrin or DP-gastrin having 14 or 16 carbon atom acyl chains, respectively. The fluorescence emission from the two Trp residues of these lipopeptides exhibited little or no blue shift upon addition of liposomes of egg-phosphatidylethanolamine containing 5 mol% G(D1a). Iodide quenching of DP-gastrin fluorescence was also independent of lipid. These results indicate that the peptide moiety is exposed to the aqueous environment even though the lipopeptide is firmly anchored to the membrane. Both DM and DP-gastrin markedly raise the bilayer to hexagonal phase transition temperature of dipalmitoleoyl phosphatidylethanolamine. However, DM-E5 lowers this phase transition temperature. These lipopeptides have effects on the overall fusion of Sendai virus to liposomes in accord with their opposite effects on lipid curvature. The lipogastrins are potent inhibitors of viral fusion, while DM-E5 slightly promotes this process. Truncated forms of DM-gastrin are also inhibitory to viral fusion, but are less inhibitory than the full lipopeptide. Analysis of the fusion kinetics shows that DP-gastrin causes a reduction in the final extent of fusion and a marked lowering of the fusion rate constant. Binding of Sendai virus to the ganglioside receptor-containing liposomes was not affected. Consideration of the various contributions to the mechanism of inhibition of viral fusion suggests that effects of lipogastrin on membrane intrinsic monolayer curvature is of primary importance.

Membrane anchorage brings about fusogenic properties in a short synthetic peptide

The fusogenic properties of an amphipathic net-negative peptide (wae 11), consisting of 11 amino acid residues, were studied. We demonstrate that, whereas the free peptide displays no significant fusion activity, membrane fusion is strongly promoted when the peptide is anchored to a liposomal membrane. The fusion activity of the peptide appears to be independent of pH, and membrane merging is an essentially nonleaky process. Thus, the extents of lipid mixing and contents mixing were virtually indistinguishable. Vesicle aggregation is a prerequisite for fusion. For this process to take place, the target membranes required a positive charge which was provided by incorporating lysine-coupled phosphatidylethanolamine (PElys). The coupled peptide, present in one population, could thus cause vesicle aggregation via nonspecific electrostatic interaction with PElys. However, the free peptide failed to induce aggregation of PElys vesicles, suggesting that the spatial orientation of the coupled peptide codetermined its ability to bring about vesicle aggregation and fusion. With the monitoring of changes in the intrinsic Trp fluorescence, in conjunction with KI-quenching studies, it would appear that hydrophobic interactions facilitate the fusion event, possibly involving (partial) peptide penetration. Such a penetration may be needed to trigger formation of a transient, nonbilayer structure. Since lysophosphatidylcholine inhibited while monoolein strongly stimulated peptide-induced fusion, our data indicate that wae 11-induced fusion proceeds according to a model consistent with the stalk-pore hypothesis for membrane fusion.

Cholesteryl hemisuccinate treatment protects rodents from the toxic effects of acetaminophen, adriamycin, carbon tetrachloride, chloroform and galactosamine

In addition to its use as a stabilizer/rigidifier of membranes, cholesteryl hemisuccinate, tris salt (CS) administration has also been shown to protect rats from the hepatotoxic effects of carbon tetrachloride (CCl4). To further our understanding of the mechanism of CS cytoprotection, we examined in rats and mice the protective abilities of CS and the non-hydrolyzable ether form of CS, gamma-cholesteryloxybutyric acid, tris salt (CSE) against acetaminophen-, adriamycin-, carbon tetrachloride-, chloroform- and galactosamine-induced toxicity. The results of these studies demonstrated that CS-mediated protection is not selective for a particular species, organ system or toxic chemical. A 24-h pretreatment of both rats and mice with a single dose of CS (100mg/kg, i.p.), resulted in significant protection against the hepatotoxic effects of CCl4, CHCl3, acetaminophen and galactosamine and against the lethal (and presumably cardiotoxic) effect of adriamycin administration. Maximal CS-mediated protection was observed in experimental animals pretreated 24 h prior to the toxic insult. These data suggest that CS intervenes in a critical cellular event that is an important common pathway to toxic cell death. The mechanism of CS protection does not appear to be dependent on the inhibition of chemical bioactivation to a toxic reactive intermediate (in light of the protection observed against galactosamine hepatotoxicity). However, based on the data presented, we can not exclude the possibility that CS administration inhibits chemical bioactivation. Our findings do suggest that CS-mediated protection is dependent on the action of the intact anionic CS molecule (non-hydrolyzable CSE was as protective as CS), whose mechanism has yet to be defined.

Growth inhibition of MCF-7 and MCF-10A human breast cells by alpha-tocopheryl hemisuccinate, cholesteryl hemisuccinate and their ether analogs

The growth inhibitory properties of alpha-tocopheryl hemisuccinate (vitamin E succinate) and related compounds were examined in MCF-7 human breast tumor cells and MCF-10A normal-like human breast cells since they have been suggested to be an effective antitumor compound. The data showed that both alpha-tocopherol hemisuccinate and a structurally-similar compound, cholesteryl hemisuccinate, inhibited the growth of MCF-7 and MCF-10A cells, while alpha-tocopherol, cholesterol, cholesteryl sulfate and Tris succinate had little effect on cell growth. The ether analogs of the succinate esters, alpha-tocopheryloxybutyric acid and cholesteryloxybutyric acid, also inhibited growth of MCF-7 and MCF-10A cells, indicating that hydrolysis of the succinate esters by esterases is not required for the antiproliferative effects. The antiproliferative effects of these succinate esters and ethers may be related to their physiochemical properties that allow incorporation into cell membranes.

Inhibition of cholesteryl ester transfer protein by apolipoproteins, lipopolysaccharides, and cholesteryl sulfate

Cholesteryl ester transfer protein (CETP) mediates the exchange of cholesteryl esters and triglycerides between lipoproteins in the plasma. In studies dealing with the mechanism of CETP-mediated lipid transfer, we have examined the effects of several classes of biomolecules, including apolipoproteins and related synthetic peptides, cholesteryl sulfate, and lipopolysaccharides. In all cases, the molecules were inhibitory and their effects were associated with modifications of either HDL, LDL, or both. However, the probable mechanisms were distinct for each class of inhibitor. Inhibition of lipid transfer activity by apolipoprotein A-I was correlated with an increase in the apolipoprotein A-I content of HDL but not LDL, whereas the primary effect of cholesteryl sulfate was associated with modification of LDL, and only modest alteration of HDL. Lipopolysaccharides were found to modify the size and charge properties of both LDL and HDL over the same concentration ranges that affected CETP activity, but might also interact directly with CETP. It is suggested from the present studies that a variety of biomolecules that can interact with lipoproteins under natural or pathological situations have the potential to modify CETP activity, which in turn could affect normal lipoprotein composition and distribution.

Partial fusion activity of influenza virus toward liposomes and erythrocyte ghosts is distinct from viral inactivation

Final extents of fusion of influenza virus (A/PR/8/34 strain) with neutral and partially acidic liposomes were monitored with (i) a fluorescence resonance energy-transfer assay in which the liposomes were labeled and (ii) by the dequenching of octadecylrhodamine, initially incorporated in the viral membrane. The latter assay was also employed in the fusion of influenza virus and Sendai virus with erythrocyte ghosts. In all cases, a phenomenon of partial fusion activity of the virus was observed, which is distinct from low pH inactivation. The unfused influenza or Sendai virions, which were separated by sucrose gradient centrifugation from liposomes or erythrocyte ghosts exhibited again partial fusion activity toward freshly added liposomes or ghosts, respectively. The conclusion is that the fraction of initially bound and unfused virions does not consist of defective particles, but rather of particles bound to the target membranes via inactive sites on the virus (or on cellular membranes), or else, partial fusion activity is a manifestation of a certain probability of production of fusion inactive sites by irreversible association of viral glycoproteins or peptides in the target membrane.

Non-bilayer lipids and biological fusion intermediates

Disparate biological fusion reactions and fusion of purely lipid bilayers are similarly influenced by 'non-bilayer' lipids (lipids which do not form lipid bilayers in water by themselves). Lipid composition of membranes affects biological fusion at a stage downstream of activation of fusion proteins and prior to fusion pore formation. These data suggest that actual merger of membrane lipid bilayers in different fusion reactions proceeds via the same pathway. The effects of non-bilayer lipids specifically correlate with their ability to bend lipid monolayers in different directions, and appear to be consistent with the specific hypothesis of membrane fusion suggesting that fusion proceeds through highly bent intermediates--stalks, local connections between contacting monolayers of fusing membranes.

Estimation of cholesterol sulphate in blood plasma and in erythrocyte membranes from individuals with Down's syndrome or diabetes mellitus type I

OBJECTIVES

Plasma and erythrocyte membrane cholesterol sulphate (CS) were measured in patients suffering from diabetes and Down's syndrome.

DESIGN AND METHODS

The procedure for separation and determination of CS comprised HPTLC (high-performance thin-layer chromatography) and densitometry.

RESULTS

The mean plasma and RBC membranes CS concentrations (+/- SD) of the control group (n = 16) was 188 +/- 47 micrograms/dL and 343 +/- 57 micrograms/10(12) RBC, respectively. In 15 patients with diabetes and 12 Down's syndrome patients substantially higher CS levels were found (diabetes: plasma-348 +/- 60 micrograms/dL; RBC membranes-646 +/- 113 micrograms/10(12) RBC; Down's syndrome: plasma-245 +/- 54 micrograms/dL; RBC membranes 427 +/- 74 micrograms/10(12) RBC). Analysis of variance and multiple comparison (Newman-Keuls test) show statistically significant differences between all samples both for erythrocytes, F(2.41) = 52.24, p < 0.05, and plasma, F(2.41) = 34.92, p < 0.05.

CONCLUSIONS

It is postulated that differences in CS levels may contribute to changes of erythrocyte properties in these pathological states.

Ganglioside GD1a generates domains of high curvature in phosphatidylethanolamine liposomes as determined by solid state 31P-NMR spectroscopy

We have studied the effects of gangliosides on the polymorphic behaviour of phosphatidylethanolamines. The ganglioside GD1a promotes the formation of phases which give rise to isotropic 31P-NMR resonance lines, particularly in the temperature range of the L alpha to HII transition. In addition, higher mol fractions of ganglioside raise the L alpha to HII phase transition temperature. Our results demonstrate that small mol fractions of gangliosides can have profound effects on the molecular organization of phosphatidylethanolamines.