Membrane phospholipid asymmetry in Semliki Forest virus grown in BHK cells

Early Events in Chikungunya Virus Infection-From Virus Cell Binding to Membrane Fusion

Chikungunya virus (CHIKV) is a rapidly emerging mosquito-borne alphavirus causing millions of infections in the tropical and subtropical regions of the world. CHIKV infection often leads to an acute self-limited febrile illness with debilitating myalgia and arthralgia. A potential long-term complication of CHIKV infection is severe joint pain, which can last for months to years. There are no vaccines or specific therapeutics available to prevent or treat infection. This review describes the critical steps in CHIKV cell entry. We summarize the latest studies on the virus-cell tropism, virus-receptor binding, internalization, membrane fusion and review the molecules and compounds that have been described to interfere with virus cell entry. The aim of the review is to give the reader a state-of-the-art overview on CHIKV cell entry and to provide an outlook on potential new avenues in CHIKV research.

The lipidomes of vesicular stomatitis virus, semliki forest virus, and the host plasma membrane analyzed by quantitative shotgun mass spectrometry

Although enveloped virus assembly in the host cell is a crucial step in the virus life cycle, it remains poorly understood. One issue is how viruses include lipids in their membranes during budding from infected host cells. To analyze this issue, we took advantage of the fact that baby hamster kidney cells can be infected by two different viruses, namely, vesicular stomatitis virus and Semliki Forest virus, from the Rhabdoviridae and Togaviridae families, respectively. We purified the host plasma membrane and the two different viruses after exit from the host cells and analyzed the lipid compositions of the membranes by quantitative shotgun mass spectrometry. We observed that the lipid compositions of these otherwise structurally different viruses are virtually indistinguishable, and only slight differences were detected between the viral lipid composition and that of the plasma membrane. Taken together, the facts that the lipid compositions of the two viruses are so similar and that they strongly resemble the composition of the plasma membrane suggest that these viruses exert little selection in including lipids in their envelopes.

Chemical control of phospholipid distribution across bilayer membranes

Most biological membranes possess an asymmetric transbilayer distribution of phospholipids. Endogenous enzymes expend energy to maintain the arrangement by promoting the rate of phospholipid translocation, or flip-flop. Researchers have discovered ways to modify this distribution through the use of chemicals. This review presents a critical analysis of the phospholipid asymmetry data in the literature followed by a brief overview of the maintenance and physiological consequences of phospholipid asymmetry, and finishes with a list of chemical ways to alter phospholipid distribution by enhancement of flip-flop.

Sphingolipid transport in eukaryotic cells

Sphingolipids constitute a sizeable fraction of the membrane lipids in all eukaryotes and are indispensable for eukaryotic life. First of all, the involvement of sphingolipids in organizing the lateral domain structure of membranes appears essential for processes like protein sorting and membrane signaling. In addition, recognition events between complex glycosphingolipids and glycoproteins are thought to be required for tissue differentiation in higher eukaryotes and for other specific cell interactions. Finally, upon certain stimuli like stress or receptor activation, sphingolipids give rise to a variety of second messengers with effects on cellular homeostasis. All sphingolipid actions are governed by their local concentration. The intricate control of their intracellular topology by the proteins responsible for their synthesis, hydrolysis and intracellular transport is the topic of this review.

Repair of BHK cell surface ganglioside GM3 after its degradation by extracellular sialidase

Treatment of BHK fibroblasts with V. cholerae sialidase for 20 min caused the breakdown of about 70% of total cellular ganglioside GM3 and the production of an approximately equivalent amount of lactosylceramide. On removal of the enzyme, a slow resynthesis of GM3 from lactosylceramide was observed, equivalent to about 5-6%/h of the degraded GM3. Resynthesis of degraded surface ganglioside has not previously been observed, but its magnitude is similar to previous measurements of the rate of protein resialylation after sialidase treatment. This suggests that resialylation of both lipid and protein is limited by vesicular transport of plasma membrane components through the trans-Golgi network [TGN] where sialyltransferase is thought to be localized. In contrast, resynthesis of sphingomyelin which has been degraded at the cell surface by exogenous sphinogomyelinase is about five times faster than resynthesis of GM3 and may involve non-vesicular transport of ceramide.

Mapping the lipid distribution in the membranes of BHK cells (mini-review)

An analysis of lipid distribution in the membranes of BHK cells has been carried out based on published information concerning the lipid composition of cells and subcellular fractions. This work may be useful in the quantitative analysis of cell fractionation studies and for the interpretation of the results of experiments involving the breakdown of specific pools of lipid by lipases.

Fusion of Semliki Forest virus with cholesterol-containing liposomes at low pH: a specific requirement for sphingolipids

Semliki Forest virus (SFV) utilizes a membrane fusion strategy to introduce its genome into the host cell. After binding to cell-surface receptors, virus particles are internalized through receptor-mediated endocytosis and directed to the endosomal cell compartment. Subsequently, triggered by the acid pH in the lumen of the endosomes, the viral envelope fuses with the endosomal membrane. As a result of this fusion reaction the viral RNA gains access to the cell cytosol. Low-pH-induced fusion of SFV, in model systems as well as in cells, has been demonstrated previously to be strictly dependent on the presence of cholesterol in the target membrane. In this paper, we show that fusion of SFV with cholesterol-containing liposomes depends on sphingomyelin (SM) or other sphingolipids in the target membrane, ceramide representing the sphingolipid minimally required for mediating the process. The action of the sphingolipid is confined to the actual fusion event, cholesterol being necessary and sufficient for low-pH-dependent binding of the virus to target membranes. The 3-hydroxyl group on the sphingosine backbone plays a key role in the SFV fusion reaction, since 3-deoxy-sphingomyelin does not support the process. This, and the remarkably low levels of sphingolipid required for half-maximal fusion (1-2 mol%), suggest that the sphingolipid does not play a structural role in SFV fusion, but rather acts as a cofactor, possibly through activation of the viral fusion protein. Domain formation between cholesterol and sphingolipid, although it may facilitate SFV fusion, is unlikely to play a crucial role in the process.

Transbilayer distribution of phosphatidylcholine and phosphatidylethanolamine in the vacuolar membrane of Acer pseudoplatanus cells

The distribution of phosphatidylethanolamine (PE) and phosphatidylcholine (PC) among the outer and inner monolayers of the vacuolar membrane of Acer pseudoplatanus was investigated using isolated vacuoles, chemical labelling agents (trinitrobenzene-sulfonate and fluorescamine), phospholipase A2 from bee venom, phospholipase C and phospholipase D. Treatments were performed with intact or sonicated vacuoles. Analysis of the transbilayer distribution of PC and PE in the vacuolar membrane of Acer was limited by phospholipid fractions which were inaccessible to the probes. Lipid-protein interactions and modification of the surface charge and surface pressure in the membrane layers during treatments may obviously exert a strong influence on labelling or hydrolysis of membrane phospholipids. However, simultaneous treatments carried out with phospholipase A2 and phospholipase C show that PE is approximately 20% more abundant in the outer monolayer than in the inner monolayer and PC is equally distributed between both leaflets of tonoplast. Compared to the phospholipids asymmetrical distribution observed in plasma membrane of erythrocyte, the vacuolar membrane of Acer is not characterized by a marked asymmetrical distribution of its major phospholipids.

Phospholipid asymmetry in plasma membrane vesicles derived from BHK cells

The transbilayer distribution of phospholipids in plasma membrane vesicles derived from BHK cells by treatment with iodoacetamide or fluoride and merocyanine 540 has been examined by exposing the vesicles to bee venom phospholipase A2 (PLA2) or to Bacillus cereus sphingomyelinase. The results show that almost all of the phosphatidylserine (PS) is on the inner lipid leaflet and most of the sphingomyelin is on the outer lipid leaflet. In contrast, about 50% of the phosphatidylcholine (PC) and 30-40% of the phosphatidylethanolamine (PE) is rapidly degraded by PLA2 and thus appears to be present on the surface of the vesicles. The pools of PC and PE which are accessible only slowly to PLA2 are degraded with halftimes of about 5 h and 2 h, respectively, and it is suggested that this rate reflects the rate of transbilayer migration of these lipids. We conclude that the profound energy depletion caused by treatment with iodoacetamide or fluoride does not alter the asymmetric distribution of PS across the plasma membrane but does have a marked effect on the transbilayer distribution of PE. Residual cells after treatment with fluoride and MC540 were also exposed to PLA2. The results were broadly in agreement with those obtained with vesicles, suggesting that the vesicles were representative of the BHK cell plasma membrane in terms of phospholipid asymmetry. Fluoride or MC540 added separately caused little vesicle release but did lead to significant loss of phospholipid asymmetry. When centrifuged on a sucrose density gradient, vesicles were separated into two major fractions accounting for about two thirds and about 20%, respectively, of total phospholipid but no significant differences were seen in the transbilayer phospholipid asymmetry of the two fractions.

Plasma membrane vesicles from BHK and HL60 cells treated with merocyanine 540 and iodoacetamide

Treatment of BHK or HL60 cell lines with merocyanine 540 in the presence of the sulphydryl blocker iodoacetamide caused budding of the cell surface to release vesicles about 50-100 nm in diameter which accounted for up to 25% of the total surface membrane lipid. Smaller amounts of vesicular material were released in the presence of fluoride and merocyanine 540. The vesicles had a membrane lipid composition which was characteristic of other purified plasma membranes, with large amounts of sphingomyelin, phosphatidylserine and cholesterol and low proportions of phosphatidylinositol, phosphatidylcholine, triacylglycerol and cholesterol ester. This procedure for the isolation of vesicles should be a general method for the purification of plasma membrane components from a wide range of different cell types.

An essential role for phosphatidylinositol transfer protein in phospholipase C-mediated inositol lipid signaling

Transmembrane signaling by the phospholipase C-beta (PLC-beta) pathway is known to require at least three components: the receptor, the G protein, and the PLC. Recent studies have indicated that if the cytosol is allowed to leak out of HL60 cells, then G protein-stimulated PLC activity is greatly diminished, indicating an essential role for a cytosolic component(s). We now report the complete purification of one component based on its ability to reconstitute GTP gamma S-mediated PLC activity and identify it as the phosphatidylinositol transfer protein (PI-TP). Based on the in vitro effects of PI-TP, we surmise that it is involved in transporting PI from intracellular compartments for conversion to PI bisphosphate (PIP2) prior to hydrolysis by PLC-beta 2/PLC-beta 3, the endogenous PLC isoforms present in these cells.

Two separate pools of sphingomyelin in BHK cells

In BHK cells labelled to equilibrium with [3H]choline and treated with sphingomyelinase the surface pool of sphingomyelin is degraded very rapidly (half-time 10 min) but the internal pool of sphingomyelin which accounts for about 30% of the total is only degraded slowly (half-time about 80 h) showing that the internal pool does not normally reach the surface. In [3H]choline incorporation experiments the internal pool begins to accumulate radioactivity at about the same time as phosphatidylcholine (30 min) but label does not enter the surface pool of sphingomyelin for a further 90 min. The internal and external pools reach the same specific activity only after about 20 h. Pulse-chase analysis with [3H]choline shows that radioactivity in each pool of sphingomyelin continues to increase when the specific radioactivity of phosphatidylcholine is decreasing, consistent with both pools being synthesised from a phosphatidylcholine precursor. The results suggest that sphingomyelin in BHK cells is present not only in the plasma membrane but also in a more rapidly labelling pool which does not mix with the surface pool.

Isolation of plasma membrane exovesicles from BHK cells using merocyanine 540

Treatment of cultured BHK cells with merocyanine 540 caused the non-lytic release of vesicular material having the phospholipid composition characteristic of plasma membrane. The protein composition of the vesicles closely resembled that of the soluble fraction of the cell, as expected for exovesicles budding from the cell surface. Vesicles prepared from cells surface-iodinated with 125I contained no obvious iodinated membrane polypeptides, suggesting that no major proteins in the plasma membrane of the BHK cell are free to diffuse with lipids. The procedure described should represent a general method, applicable to a wide range of cell types, for isolating plasma membrane vesicles.

The interaction of synthetic analogs of the N-terminal fusion sequence of influenza virus with a lipid monolayer. Comparison of fusion-active and fusion-defective analogs

The amino terminus of subunit-2 of influenza virus hemagglutinin (NHA2) plays a crucial role in the induction of fusion between viral and endosomal membranes leading to the infection of a cell. Three synthetic analogs with an amino acid sequence corresponding to NHA2 of variant hemagglutinins were studied in a monolayer set up. Comparison of the interaction of a fusion-active and two fusion-defective analogs with a lipid monolayer revealed a greater surface activity of the fusion-active analog. Pronounced differences were found if the pure peptides were spread at the air/water interface; the fusion-active analog showed a higher collapse pressure and a greater limiting molecular area. Circular dichroism measurements on collected lipid monolayers indicated a high content of alpha-helical structure for the fusion-active and one of the fusion-defective analogs. A simple relation between alpha-helical content and fusogenicity does not seem to exist. Instead, the extent of penetration, a defined tertiary structure or orientation of the alpha-helical peptide may be essential for its membrane perturbing activity.

Replacement of vertebrate serum with lipids and other factors in the culture of invertebrate cells, tissues, parasites, and pathogens

Culture medium supplementation with vertebrate serum results in the selection of fibroblastoid insect cell lines and a general decline during continuous subculturing of both morphologic and functional differentiation of the surviving cells. Essential lipid mixtures can substitute for vertebrate serum in the culture of insect and some vertebrate cells, tissues, parasites, and pathogens. The provision of sterols and essential (with nonessential) polyunsaturated fatty acids as phospholipids in oxidation-protected peptoliposomes or proteoliposomes allows cells in culture to duplicate in vivo specific membranes more accurately. Such lipid-corrected membranes allow cultured cells to communicate with neighboring cells through the extracellular matrix, effectively transmit hormonal signals directly and via receptor control, and respond with various tissue-specific functions and differentiation states as directed.

Transmembrane movements of lipids

Membranes allow the rapid passage of unchanged lipids. Phospholipids on the other hand diffuse very slowly from one monolayer to another with a half-time of several hours. This slow spontaneous movement in a pure lipid bilayer can be selectively modulated in biological membranes by intrinsic proteins. In microsomes, and probably in bacterial membranes, non-specific phospholipid flippases allow the rapid redistribution of newly synthesized phospholipids. In eukaryotic plasma membranes, aminophospholipid translocase selectively pumps phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the outer to the inner leaflet and establishes a permanent lipid asymmetry. The discovery of an aminophospholipid translocase in chromaffin granules proves that eukaryotic organelles may also contain lipid translocators.