Control of membrane fusion by phospholipid head groups. I. Phosphatidate/phosphatidylinositol specificity

The Exploration of the Thermococcus barophilus Lipidome Reveals the Widest Variety of Phosphoglycolipids in Thermococcales

One of the most distinctive characteristics of archaea is their unique lipids. While the general nature of archaeal lipids has been linked to their tolerance to extreme conditions, little is known about the diversity of lipidic structures archaea are able to synthesize, which hinders the elucidation of the physicochemical properties of their cell membrane. In an effort to widen the known lipid repertoire of the piezophilic and hyperthermophilic model archaeon Thermococcus barophilus, we comprehensively characterized its intact polar lipid (IPL), core lipid (CL), and polar head group compositions using a combination of cutting-edge liquid chromatography and mass spectrometric ionization systems. We tentatively identified 82 different IPLs based on five distinct CLs and 10 polar head group derivatives of phosphatidylhexoses, including compounds reported here for the first time, e.g., di-N-acetylhexosamine phosphatidylhexose-bearing lipids. Despite having extended the knowledge on the lipidome, our results also indicate that the majority of T. barophilus lipids remain inaccessible to current analytical procedures and that improvements in lipid extraction and analysis are still required. This expanded yet incomplete lipidome nonetheless opens new avenues for understanding the physiology, physicochemical properties, and organization of the membrane in this archaeon as well as other archaea.

How is the interaction of a chloride channel blocker with phospholipids influenced by divalent metal ions? Effect of unsaturation on the lipid side chain

The present study reveals the effect of various divalent ions (Ca²⁺, Mg²⁺and Zn²⁺) on the binding interaction of a prospective chloride channel blocker, 9-methylanthroate (9MA), with liposome membranes, namely, dimyristoylphosphatidylcholine (DMPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). The liposome membranes DMPC and POPC differ in the unsaturation of the side-chain. The drug (9MA) is found to experience a greater degree of partitioning into the POPC lipid bilayer (containing unsaturated side-chain) in comparison to DMPC (containing saturated side-chain). The stronger 9MA-POPC binding interaction is found to be only nominally perturbed by the presence of metal salts. On the contrary, the 9MA-DMPC binding interaction is found to be significantly perturbed by the presence of metal salts and is manifested on the environment-responsive spectroscopic properties of the drug. The steady-state and picosecond-resolved fluorescence spectroscopic results reveal the effect of metal ions on DMPC bilayer to follow the trend Ca²⁺ < Mg²⁺ < Zn²⁺. This is also quantified by evaluating the partition coefficient of the drug into DMPC lipid in the presence of various divalent ions which is found to follow the same sequence. The degree of penetration of these cations has been rationalized on the basis of adsorption of cations on DMPC headgroup region resulting in dehydration of the headgroup along with shrinking of it.

A modular platform for one-step assembly of multi-component membrane systems by fusion of charged proteoliposomes

An important goal in synthetic biology is the assembly of biomimetic cell-like structures, which combine multiple biological components in synthetic lipid vesicles. A key limiting assembly step is the incorporation of membrane proteins into the lipid bilayer of the vesicles. Here we present a simple method for delivery of membrane proteins into a lipid bilayer within 5 min. Fusogenic proteoliposomes, containing charged lipids and membrane proteins, fuse with oppositely charged bilayers, with no requirement for detergent or fusion-promoting proteins, and deliver large, fragile membrane protein complexes into the target bilayers. We demonstrate the feasibility of our method by assembling a minimal electron transport chain capable of adenosine triphosphate (ATP) synthesis, combining Escherichia coli F1Fo ATP-synthase and the primary proton pump bo3-oxidase, into synthetic lipid vesicles with sizes ranging from 100 nm to ∼10 μm. This provides a platform for the combination of multiple sets of membrane protein complexes into cell-like artificial structures.

Critical Conditions for Adsorption of Calcium Ions onto Dipolar Lipid Membranes

Dipolar lipid membranes may adsorb multivalent ions. The binding constant depends on the type of lipid and ions. In this paper, we focus on the adsorption of calcium ions onto 1,2-dilauroylphosphatidylcholine (DLPC) membrane. Using small-angle-X-ray scattering we found that at ambient room temperature ca. 0.6 mM CaCl2 is a critical concentration at which calcium ions adsorbed to 30 mg/mL (ca. 48 mM) DLPC membrane. We then determined the structure of the lamellar phases formed at CaCl2 concentrations below and above the critical concentration and characterized the effect of temperature and incubation time on the adsorption process. Our findings suggest that calcium adsorption to DLPC membranes requires an initial nucleation phase.

Therapeutic applications of reconstituted HDL: When structure meets function

Reconstituted forms of HDL (rHDL) are under development for infusion as a therapeutic approach to attenuate atherosclerotic vascular disease and to reduce cardiovascular risk following acute coronary syndrome and ischemic stroke. Currently available rHDL formulations developed for clinical use contain apolipoprotein A-I (apoA-I) and one of the major lipid components of HDL, either phosphatidylcholine or sphingomyelin. Recent data have established that quantitatively minor molecular constituents of HDL particles can strongly influence their anti-atherogenic functionality. Novel rHDL formulations displaying enhanced biological activities, including cellular cholesterol efflux, may therefore offer promising prospects for the development of HDL-based, anti-atherosclerotic therapies. Indeed, recent structural and functional data identify phosphatidylserine as a bioactive component of HDL; the content of phosphatidylserine in HDL particles displays positive correlations with all metrics of their functionality. This review summarizes current knowledge of structure-function relationships in rHDL formulations, with a focus on phosphatidylserine and other negatively-charged phospholipids. Mechanisms potentially underlying the atheroprotective role of these lipids are discussed and their potential for the development of HDL-based therapies highlighted.

Evolution of the hemifused intermediate on the pathway to membrane fusion

The pathway to membrane fusion in synthetic and biological systems is thought to pass through hemifusion, in which the outer leaflets are fused while the inner leaflets engage in a hemifusion diaphragm (HD). Fusion has been proposed to be completed by lysis of the expanded HD that matures from a localized stalklike initial connection. However, the process that establishes the expanded HD is poorly understood. Here we mathematically modeled hemifusion of synthetic vesicles, where hemifusion and fusion are most commonly driven by calcium and membrane tension. The model shows that evolution of the hemifused state is driven by these agents and resisted by interleaflet frictional and tensile stresses. Predicted HD growth rates depend on tension and salt concentration, and agree quantitatively with experimental measurements. For typical conditions, we predict that HDs expand at ~30 μm(2)/s, reaching a final equilibrium area ~7% of the vesicle area. Key model outputs are the evolving HD tension and area during the growth transient, properties that may determine whether HD lysis occurs. Applying the model to numerous published experimental studies that reported fusion, our results are consistent with a final fusion step in which the HD ruptures due to super-lysis HD membrane tensions.

Phosphatidylinositol induces fluid phase formation and packing defects in phosphatidylcholine model membranes

Liposomes consisted of phosphatidylinositol (PI) and phosphatidylcholine (PC) have been utilized as delivery vehicle for drugs and proteins. In the present work, we studied the effect of soy PI on physical properties of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes such as phase state of lipid bilayer, lipid packing and phase properties using multiple orthogonal biophysical techniques. The 6-dodecanoyl-2-dimethylamino naphthalene (Laurdan) fluorescence studies showed that presence of PI induces the formation of fluid phases in DMPC. Differential scanning calorimetry (DSC), temperature dependent fluorescence anisotropy measurements, and generalized polarization values for Laurdan showed that the presence of as low as 10mol% of PI induces substantial broadening and shift to lower temperature of phase transition of DMPC. The fluorescence emission intensity of DPH labeled, PI containing DMPC lipid bilayer decreased possibly due to deeper penetration of water molecules in lipid bilayer. In order to further delineate the effect of PI on the physico chemical properties of DMPC is due to either significant hydrophobic mismatch between the acyl chains of the DMPC and that of soy PI or due to the inositol head group, we systematically replaced soy PI with PC species of similar acyl chain composition (DPPC and 18:2 (Cis) PC) or with diacylglycerol (DAG), respectively. The anisotropy of PC membrane containing soy PI showed largest fluidity change compared to other compositions. The data suggests that addition of PI alters structure and dynamics of DMPC bilayer in that it promotes deeper water penetration in the bilayer, induces fluid phase characteristics and causes lipid packing defects that involve its inositol head group.

The structure of ions and zwitterionic lipids regulates the charge of dipolar membranes

In pure water, zwitterionic lipids form lamellar phases with an equilibrium water gap on the order of 2 to 3 nm as a result of the dominating van der Waals attraction between dipolar bilayers. Monovalent ions can swell those neutral lamellae by a small amount. Divalent ions can adsorb onto dipolar membranes and charge them. Using solution X-ray scattering, we studied how the structure of ions and zwitterionic lipids regulates the charge of dipolar membranes. We found that unlike monovalent ions that weakly interact with all of the examined dipolar membranes, divalent and trivalent ions adsorb onto membranes containing lipids with saturated tails, with an association constant on the order of ∼10 M(-1). One double bond in the lipid tail is sufficient to prevent divalent ion adsorption. We suggest that this behavior is due to the relatively loose packing of lipids with unsaturated tails that increases the area per lipid headgroup, enabling their free rotation. Divalent ion adsorption links two lipids and limits their free rotation. The ion-dipole interaction gained by the adsorption of the ions onto unsaturated membranes is insufficient to compensate for the loss of headgroup free-rotational entropy. The ion-dipole interaction is stronger for cations with a higher valence. Nevertheless, polyamines behave as monovalent ions near dipolar interfaces in the sense that they interact weakly with the membrane surface, whereas in the bulk their behavior is similar to that of multivalent cations. Advanced data analysis and comparison with theory provide insight into the structure and interactions between ion-induced regulated charged interfaces. This study models biologically relevant interactions between cell membranes and various ions and the manner in which the lipid structure governs those interactions. The ability to monitor these interactions creates a tool for probing systems that are more complex and forms the basis for controlling the interactions between dipolar membranes and charged proteins or biopolymers for encapsulation and delivery applications.

Compressibilities and volume fluctuations of archaeal tetraether liposomes

Bipolar tetraether lipids (BTLs) are abundant in crenarchaeota, which thrive in both thermophilic and nonthermophilic environments, with wide-ranging growth temperatures (4-108°C). BTL liposomes can serve as membrane models to explore the role of BTLs in the thermal stability of the plasma membrane of crenarchaeota. In this study, we focus on the liposomes made of the polar lipid fraction E (PLFE). PLFE is one of the main BTLs isolated from the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius. Using molecular acoustics (ultrasound velocimetry and densimetry), pressure perturbation calorimetry, and differential scanning calorimetry, we have determined partial specific adiabatic and isothermal compressibility, their respective compressibility coefficients, partial specific volume, and relative volume fluctuations of PLFE large unilamellar vesicles (LUVs) over a wide range of temperatures (20-85°C). The results are compared with those obtained from liposomes made of dipalmitoyl-L-α-phosphatidylcholine (DPPC), a conventional monopolar diester lipid. We found that, in the entire temperature range examined, compressibilities of PLFE LUVs are low, comparable to those found in gel state of DPPC. Relative volume fluctuations of PLFE LUVs at any given temperature examined are 1.6-2.2 times more damped than those found in DPPC LUVs. Both compressibilities and relative volume fluctuations in PLFE LUVs are much less temperature-sensitive than those in DPPC liposomes. The isothermal compressibility coefficient (β(T)(lipid)) of PLFE LUVs changes from 3.59 × 10(-10) Pa(-1) at 25°C to 4.08 × 10(-10) Pa(-1) at 78°C. Volume fluctuations of PLFE LUVs change only 0.25% from 30°C to 80°C. The highly damped volume fluctuations and their low temperature sensitivity, echo that PLFE liposomes are rigid and tightly packed. To our knowledge, the data provide a deeper understanding of lipid packing in PLFE liposomes than has been previously reported, as well as a molecular explanation for the low solute permeation and limited membrane lateral motion. The obtained results may help to establish new strategies for rational design of stable BTL-based liposomes for drug/vaccine delivery.

Surface potential of phosphoinositide membranes: comparison between theory and experiment

Surface potential of lipid membranes made of phosphatidylcholine (PC) and one of the phosphoinositides (PPI); PI, PIP or PIP(2), was studied by using the electrophoretic mobility of these lipid membrane vesicles, and a theoretical model of the surface potential developed for these membranes containing PPIs. By using the measured zeta-potential for the PI/PC membranes and a well-known surface potential theory, the inositol ring of the PI molecule was found to extend into the aqueous phase approximately normal to the membrane surface for various PI/PC ratios investigated. The outer edge of the inositol ring is located at about 5.2A from the phosphate group conjugated with the glycerol of the phospholipids. The inositol group was slightly tilted from the membrane normal direction. For both PIP/PC and PIP(2)/PC membranes, the analyses of surface potential using the measured zeta-potential values and the surface potential theory which was developed for these membranes gave consistent results with respect to the slipping layer distance from the second surface charge layer. The conclusion is that the experimental data can be fairly well resolved by using a linearized Poisson-Boltzmann surface potential equation set up for a PPI/PC membrane model up to a certain concentration of PPI in PC membranes. Our theoretical model made for these membrane surface potentials seems to be reasonable for analysis of electrical surface phenomena for these PPI/PC membranes containing small concentrations of PPI molecules.

Archaebacterial bipolar tetraether lipids: Physico-chemical and membrane properties

Bipolar tetraether lipids (BTL) are abundant in archaea and can be chemically synthesized. The structures of BTL are distinctly different from the lipids found in bacteria and eukaryotes. In aqueous solution, BTL can form extraordinarily stable liposomes with different sizes, lamellarities and membrane packing densities. BTL liposomes can serve as membrane models for understanding the structure-function relationship of the plasma membrane in thermoacidophiles and can be used for technological applications. This article reviews the separation, characterization and structures of BTL as well as the physical properties and technological applications of BTL liposomes. One of the structural features of BTL is the presence of cyclopentane rings in the lipid hydrocarbon core. Archaea use the cyclopentane ring as an adaptation strategy to cope with high growth temperature. Special attention of this article is focused on how the number of cyclopentane rings varies with environmental factors and affects membrane properties.

Myoblast fusion and inositol phospholipid breakdown: causal relationship or coincidence?

The fusion of embryonic chick myoblasts has been examined in culture. Cells were prepared from 12-day-old chick embryonic breast muscle and cultured for 50 h at a Ca2+ concentration in the medium of 10(-7) M. During this period they attain fusion competence. Addition of 1.4 mM-Ca2+ to these cells elicits rapid fusion. Changes in the metabolism of myoblast phospholipids in response to the raised Ca2+ concentration have been examined. Only the inositol phospholipids are affected. Phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate are rapidly broken down and 1,2-diacylglycerol and phosphatidic acid are synthesized. Myoblast fusion has also been found to be stimulated by a factor present in chick embryo extract, probably of neuronal origin. A receptor-mediated mechanism for myoblast fusion is proposed. This envisages the polyphosphoinositides acting as a fusion block, either themselves or by their binding to membrane proteins. The inositol phospholipid breakdown could result in a more fluid membrane and the breakdown products 1,2-diacylglycerol and phosphatidic acid, two known fusogens, could stimulate fusion.

Effect of lipid headgroup composition on the interaction between melittin and lipid bilayers

The effect of the lipid polar headgroup on melittin-phospholipid interaction was investigated by cryo-TEM, fluorescence spectroscopy, ellipsometry, circular dichroism, electrophoresis and photon correlation spectroscopy. In particular, focus was placed on the effect of the lipid polar headgroup on peptide adsorption to, and penetration into, the lipid bilayer, as well as on resulting colloidal stability effects for large unilamellar liposomes. The effect of phospholipid headgroup properties on melittin-bilayer interaction was addressed by comparing liposomes containing phosphatidylcholine, -acid, and -inositol at varying ionic strength. Increasing the bilayer negative charge leads to an increased liposome tolerance toward melittin which is due to an electrostatic arrest of melittin at the membrane interface. Balancing the electrostatic attraction between the melittin positive charges and the phospholipid negative charges through a hydration repulsion, caused by inositol, reduced this surface arrest and increased liposome susceptibility to the disruptive actions of melittin. Furthermore, melittin was demonstrated to induce liposome structural destabilization on a colloidal scale which coincided with leakage induction for both anionic and zwitterionic systems. The latter findings thus clearly show that coalescence, aggregation, and fragmentation contribute to melittin-induced liposome leakage, and that detailed molecular analyses of melittin pore formation are incomplete without considering also these colloidal aspects.

Secretion without membrane fusion: porocytosis

We have recently proposed a mechanism to describe secretion, a fundamental process in all cells. That hypothesis, called porocytosis, embodies all available data and encompasses both forms of secretion, i.e., vesicular and constitutive. The current accepted view of exocytotic secretion involves the physical fusion of vesicle and plasma membranes; however, that hypothesized mechanism does not fit all available physiological data. Energetics of apposed lipid bilayers do not favor unfacilitated fusion. We consider that calcium ions (e.g., 10(-4) to 10(-3) M calcium in microdomains when elevated for 1 ms or less), whose mobility is restricted in space and time, establish salt bridges among adjacent lipid molecules. This establishes transient pores that span both the vesicle and plasma membrane lipid bilayers; the diameter of this transient pore would be approximately 1 nm (the diameter of a single lipid molecule). The lifetime of the transient pore is completely dependent on the duration of sufficient calcium ion levels. This places the porocytosis hypothesis for secretion squarely in the realm of the physical and physical chemical interactions of calcium and phospholipids and places mass action as the driving force for release of secretory material. The porocytosis hypothesis that we propose satisfies all of the observations and provides a framework to integrate our combined knowledge of vesicular and constitutive secretion.

Synthesis and spectroscopic analysis of chromophoric lipids inducing pH-dependent liposome fusion

We design novel chromophoric amphiphiles 6a-c, which lead to pH-dependent membrane fusion of egg phosphatidylcholine (eggPC) liposome containing them. Lipids 6a-c comprise double alkyl chains, a single chain with a 2-nitrophenol group as a pH trigger, and dipeptide (Asp-Asp) between them. The pKa values of 2-nitrophenol groups of 6a-c in liposome are larger than that of hydrophilic compound 9 in an aqueous solution. Absorption spectra indicate that the fields around 2-nitrophenol of 6a-c situated in liposome membranes are more hydrophobic than that of 9 in an aqueous solution, whereas the environments around deprotonated 2-nitrophenolate of 6b and 6c are not so hydrophobic as that of 6a. This means that protonated 2-nitrophenol groups of 6a-c are embedded in bilayer membranes. Deprotonated 2-nitrophenol groups of 6b and 6c must be located in less hydrophobic circumstances, while that of 6a is still embedded in bilayer membranes because of its larger hydrophobicity. Absorption spectra and (1)H NMR spectra respectively suggest that protonated 2-nitrophenol groups of 6a and those of 6c might take face-to-face associations in bilayer membranes.

Calcium-induced aggregation of archaeal bipolar tetraether liposomes derived from the thermoacidophilic archaeon Sulfolobus acidocaldarius

Previously, we showed that the proton permeability of small unilamellar vesicles (SUVs) composed of polar lipid fraction E (PLFE) from the thermoacidophilic archaeon Sulfolobus acidocaldarius was remarkably low and insensitive to temperature (Komatsu and Chong 1998). In this study, we used photon correlation spectroscopy to investigate the time dependence of PLFE SUV size as a function of Ca2+ concentration. In the absence of Ca2+, vesicle diameter changed little over 6 months. Addition of Ca2+, however, immediately induced formation of vesicle aggregates with an irregular shape, as revealed by confocal fluorescence microscopy. Aggregation was reversible upon addition of EDTA; however, the reversibility varied with temperature as well as incubation time with Ca2+. Freeze-fracture electron microscopy showed that, after a long period of incubation (2 weeks) with Ca2+, the PLFE vesicles had not just aggregated, but had fused or coalesced. The initial rate of vesicle aggregation varied sigmoidally with Ca2+ concentration. At pH 6.6, the threshold calcium concentration (Cr) for vesicle aggregation at 25 and 40 degrees C was 11 and 17 mM, respectively. At pH 3.0, the Cr at 25 degrees C increased to 25 mM. The temperature dependence of Cr may be attributable to changes in membrane surface potential, which was -22.0 and -13.2 mV at 25 and 40 degrees C, respectively, at pH 6.6, as determined by 2-(p-toluidinyl)naphthalene-6-sulfonic acid fluorescence. The variation in surface potential with temperature is discussed in terms of changes in lipid conformation and membrane organization.

Circulating inhibitor of gonadotropin releasing hormone secretion by hypothalamic neurons in uremia

BACKGROUND

Previous studies have suggested a neuroendocrine defect underlying uremic hypogonadism, characterized by a reduced secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH).

METHODS

We studied the GnRH-producing GT1-7 cell line and the LH-producing LbetaT-2 pituitary cell line under uremic conditions to investigate whether substances circulating in uremic plasma directly affect hypothalamic or pituitary hormone secretion. The cells were incubated with serum from 5/6-nephrectomized or sham-nephrectomized castrated rats, respectively. Furthermore, GT1 cells were incubated with delipidated sera, serum subfractions separated by molecular weight, or several peptide hormones. Cellular viability, apoptosis rate and extracellular hormone degradation were assessed separately. GnRH and LH were measured by RIA in supernatants and cell lysates. GnRH gene expression was assessed by Northern blot.

RESULTS

Uremic serum caused a reduction of extracellular GnRH concentration by 31%, whereas intracellular GnRH increased by 12%. This effect was independent of serum lipids and enzymatic GnRH degradation but was abolished by trypsin digestion. Cellular viability, apoptosis rates and GnRH gene expression did not differ between the two groups. The inhibitory activity was recovered from the high-molecular weight fraction, whereas the fraction <5 kD had stimulatory activity. In contrast, uremic serum did not affect LH secretion from LbetaT-2 cells, indicating that the hypoactivity of the hypothalamo-pituitary gonadotrope unit results from an inhibition at the hypothalamic rather than the pituitary level.

CONCLUSIONS

Our results suggest that uremic serum contains macromolecular and hydrophilic peptide(s) able to specifically suppress the neurosecretion of GnRH from GT1-7 cells.

Trigger lipids inducing pH-dependent liposome fusion

Aspartic acid-derived artificial lipids (ADLs; s indicates the number of the methylene groups, s=2, 4, 6, 8, 10, 12) (Scheme 1) with various carboxyl alkyl chains as head groups are designed and synthesized, which are incorporated into liposome membranes by sonication. Fluorescence resonance energy transfer (FRET) measurements indicate that ADL6, ADL8 and ADL10 have high lipid-mixing ability in the acidic solution. The other ADLs, however, do not induce remarkable liposome fusion at acidic nor neutral pH. The hydrophobicity of the head groups of ADL6, ADL8 and ADL10 is suitable as triggers of membrane fusion.

Vac7p, a novel vacuolar protein, is required for normal vacuole inheritance and morphology

During cell division, the vacuole of Saccharomyces cerevisiae partitions between mother and daughter cells. A portion of the parental vacuole membrane moves into the bud, and ultimately membrane scission divides the vacuole into two separate structures. Here we characterize two yeast mutations causing defects in vacuole membrane scission, vac7-1 and vac14-1. A third mutant, afab1-2 strain, isolated in a nonrelated screen (A. Yamamoto et al., Mol. Biol. Cell 6:525-539, 1995) shares the vacuolar phenotypes of the vac7-1 and vac14-1 strains. Unlike the wild type, mutant vacuoles are not multilobed structures; in many cases, a single vacuole spans both the mother and bud, with a distinct gap in the mother-bud neck. Thus, even where the membranes are closely opposed, vacuole fission is arrested. Simply enlarging the vacuole does not produce this mutant phenotype. An additional common phenotype of these mutants is a defect in vacuole acidification; however, vacuole scission in most other vacuole acidification mutants is normal. An alteration in vacuole membrane lipids could account for both the vacuole membrane scission and acidification defects. Because a directed screen has not identified additional class III complementation groups, it is likely that all three genes are involved in a similar process. Interestingly, FAB1, was previously shown to encode a putative phosphatidylinositol-4-phosphate 5-kinase. Moreover, overexpression of FAB1 suppresses the vac14-1 mutation, which suggests that VAC14 and FAB1 act at a common step. VAC7 encodes a novel 128-kDa protein that is localized at the vacuole membrane. This location of Vac7p is consistent with its involvement in vacuole morphology and inheritance.

Polyphosphoinositide inclusion in artificial lipid bilayer vesicles promotes divalent cation-dependent membrane fusion

Recent studies suggest that phosphoinositide kinases may participate in intracellular trafficking or exocytotic events. Because both of these events ultimately require fusion of biological membranes, the susceptibility of membranes containing polyphosphoinositides (PPIs) to divalent cation-induced fusion was investigated. Results of these investigations indicated that artificial liposomes containing PPI or phosphatidic acid required lower Ca2+ concentrations for induction of membrane fusion than similar vesicles containing phosphatidylserine, phosphatidylinositol, or phosphatidylcholine. This trend was first observed in liposomes composed solely of one type of phospholipid. In addition, however, liposomes designed to mimic the phospholipid composition of the endofacial leaflet of plasma membranes (i.e., liposomes composed of combinations of PPI, phosphatidylethanolamine, and phosphatidylcholine) also required lower Ca2+ concentrations for induction of aggregation and fusion. Liposomes containing PPI and phosphatidic acid also had increased sensitivity to Mg(2+)-induced fusion, an observation that is particularly intriguing given the intracellular concentration of Mg2+ ions. Moreover, the fusogenic effects of Ca2+ and Mg2+ were additive in vesicles containing phosphatidylinositol bisphosphate. These data suggest that enzymatic modification of the PPI content of intracellular membranes could be an important mechanism of fusion regulation.

Plasmenylethanolamine facilitates rapid membrane fusion: a stopped-flow kinetic investigation correlating the propensity of a major plasma membrane constituent to adopt an HII phase with its ability to promote membrane fusion

A critical step in membrane fusion involves the formation of a lipid intermediate which shares a conformational similarity with an inverted hexagonal phase (HII). Since plasmenylethanolamines possess a marked propensity for hexagonal phase formation and represent a major lipid constituent of several membrane systems which undergo rapid membrane fusion (e.g., plasma membranes and synaptic vesicle membranes), we compared the relative fusogenicity of lipid vesicles containing plasmenylethanolamine to that of vesicles containing their diacyl phospholipid counterpart (i.e., phosphatidylethanolamine). Vesicles comprised of equimolar mixtures of phosphatidylcholine and phosphatidylethanolamine fused slowly with phosphatidylserine vesicles in the presence of 10 mM CaCl2, as assessed either by lipid mixing (dequenching of octadecyl rhodamine fluorescence, 7.4 Fmax% s-1) or internal contents mixing (fluorescence enhancement from the resultant Tb/dipicolinic acid charge transfer complex, 8.7Fmax% s-1). In stark contrast, vesicles comprised of equimolar mixtures of phosphatidylcholine and plasmenylethanolamine fused three times more rapidly, as assessed by both lipid mixing (22.1 Fmax% s-1) and internal contents mixing (21.4Fmax% s-1) assays. The importance of an HII-like intermediate in membrane fusion was further substantiated by demonstration that plasmenylethanolamines containing arachidonic acid at the sn-2 position (which demonstrate a greater propensity for HII phase formation) exhibited the most rapid rate of membrane fusion (five times greater than phosphatidylethanolamine containing oleic acid at the sn-2 position). Furthermore, vesicles containing plasmenylethanolamines in physiologic ratios with other phospholipids (i.e., PC/PE/PS, 45:45:10, mol/mol) underwent fusion six times more rapidly (4.4Fmax% min-1) than corresponding vesicles in which plasmenylethanolamine was replaced with phosphatidylethanolamine (0.7Fmax% min-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Surface potential regulation of phospholipid composition and in-out translocation in yeast

In yeast cells the anionic phospholipids, phosphatidylinositol and phosphatidylserine, determine to a large extent the magnitude of the negative surface charge density (sigma) [Cerbón, J. & Calderón, V. (1990) Biochim. Biophys. Acta 1028, 261-267]. We now report further findings. (a) When the yeast phi out was reduced by increasing the concentration of monovalent (C+) or divalent (C2+) cations in the culture medium, the relative amount of anionic phospholipids increased (45-52%). (b) For each such increment, a corresponding increase in the external surface charge density (sigma) was found, due to the translocation from the cytoplasmic side to the exoplasmic side of the plasma membrane. (c) These changes were reversed when the phi out was increased by reducing the concentration of cations in the culture medium. (d) When the phi out was reduced and phosphatidylserine decarboxylation or phosphatidylinositol degradation were inhibited, to measure synthesis of anionic phospholipids, a 1.4 times further increase in the anionic/zwitterionic phospholipid ratio occurred. As a consequence, a similar increase in the external surface charge (sigma) was found. (e) Under all the conditions studied, the percentage of anionic phospholipid at the external surface of the plasma membrane calculated from the sigma values was 2.3-3.0 times less than that in the cells, indicating that the asymmetric composition (more inside) was maintained. A model for the regulation of the anionic phospholipid composition of the yeast membranes is proposed.

Ion-induced fusion of phosphatidic acid vesicles and correlation between surface hydrophobicity and membrane fusion

Divalent cation (Ca2+ and Mg2+) and hydrogen ion-induced fusions of small unilamellar phosphatidic acid vesicles were studied by the use of fluorescence fusion assay. These fusogenic ions also increased interfacial tension and reduced the surface dielectric constant of phosphatidic acid membranes as the ion concentration increased. A good correlation was found between the threshold of vesicle fusion and the degree of changes in such membrane properties induced by these fusogenic ions. At the fusion threshold point, the increase in interfacial tension and the decreased surface dielectric constant of the membrane were approximately 6 and 14 dyn/cm, respectively, regardless of Ca2+, Mg2+ or H+. Such changes correspond to the increases in surface hydrophobicity of the membrane. As these fusogenic ionic concentrations increased, the electrophoretic mobility of the phosphatidic acid vesicle decreased, because of the binding of ions to negatively charged sites of the membranes. However, the quantities of the reduced negative surface charges were not necessarily in good correlation with the threshold of vesicle fusion. It is suggested that the complexes of the phosphate group with these fusogenic ions are responsible for increases in the surface hydrophobicity of the membrane, which is an important factor for membrane fusion.

Indole-3-acetic acid-mediated transport of Mn2+ and other ions across phosphatidylinositol vesicular membranes as determined by 31P-NMR

The indolic plant hormone, indole-3-acetic acid (IAA), mediated the transport of Mn2+ and other ions into small unilamellar vesicles prepared from soybean phosphatidylinositol (PI) and this process has been studied using 31P nuclear magnetic resonance spectroscopy (NMR). The rate of Mn2+ movement into PI vesicles is dependent on IAA concentration and temperature with an IAA stoichiometry of 4.1 and an activation energy of 16.8 kcal mol-1 derived for the rate-determining process. These values are altered by low concentrations of endogenous ions (which can be removed by treatment with EDTA) present in the PI. With non-EDTA-treated PI, values of 2.3 and 23.0 kcal mol-1 were obtained for the stoichiometry and activation energy, respectively. These values indicate that (a) IAA interacts with PI membranes; (b) IAA-induced changes in membrane permeability can be substantially modulated by ions and (c) IAA very significantly influences the rate of movement of some (but possibly not all) cations across PI membranes. Such effects are also modified by the oxidation state of the PI.

Accumulation of ester- and ether-linked phosphatidates by HeLa cells in response to ionophore A23187 through activation of phospholipase D

Phosphatidates seem to play an important role in the control of cell proliferation modified by ligands (M. Kaszkin et al. 1991, Cancer Res. 51, 4328-4335). In this study the potency of calcium ionophore A23187 to alter phosphatidate levels in HeLa cells as a model was studied in detail. HeLa cells prelabeled with [14C]arachidonic acid responded to calcium ionophore A23187 with a rapid accumulation of labeled 1,2-diacylglycerophosphate (acyl-PA) and 2-acyl-1-O-alkylglycerophosphate (alkyl-PA) with a first peak at 5 min and a second increase starting at 20-30 min. In cells prelabeled with [14C]oleic acid the ionophore mobilized relatively more of labeled acyl-PA. The total amount of phosphatidates mobilized was in the order of 0.2 micrograms/10(6) cells, i.e. an almost 10(-4)M concentration. The transphosphatidylation of labeled acyl- and alkyl-PA to 1-butanol in all cases showed that activation of phospholipase D had occurred. The reaction became detectable at 10(-6)M ionophore and was fully expressed at 10(-5)M. Butyl phosphatidate generated during 1 h treatment with ionophore amounted to approx. 0.5 nmol per 10(6) cells (i.e. 10(-4)M conc. within cells) as shown by the use of [14C]butanol. The 3-5-fold rise of the overall phosphatidate level is probably sufficient to alter physically cellular membranes, particularly if the new phosphatidate is restricted to certain compartment(s).

The role of diacylglycerol in the exocytosis of the sperm acrosome. Studies using diacylglycerol lipase and diacylglycerol kinase inhibitors and exogenous diacylglycerols

When ram spermatozoa were treated with Ca2+ and the ionophore A23187 to induce acrosomal exocytosis, a rise in diacylglycerol (DAG) mass was observed, concomitant with a rapid breakdown of [32P]P1-labelled phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate and a rise in [32P]Pi-labelled phosphatidate. Inclusion of the DAG lipase inhibitor RHC 80267 resulted in further but biphasic increases in DAG; there was an increasing accumulation of DAG with concentrations of RHC 80267 up to 10 microM, whereas higher concentrations produced lessening accumulation. Inclusion of RHC 80267 in the ionophore induction system also resulted in significant accelerations of the onset of exocytosis. In spermatozoa stimulated with Ca2+/A23187 and the DAG kinase inhibitor R59022, a similar increase in DAG levels together with stimulation of acrosomal exocytosis were observed. Preincubation of spermatozoa with sn-1-oleoyl-2-acetylglycerol, rac-1-oleoyl-2-acetylglycerol, sn-1,2-dioctanoylglycerol and sn-1,3-dioctanoylglycerol before treatment with Ca2+/A23187 resulted in a dose-dependent stimulation of exocytosis by all these isomers. Neomycin inhibited Ca2+/A23187-induced generation of DAG together with polyphosphoinositide breakdown, as well as acrosomal exocytosis. Inclusion of exogenous DAG, however, overcame the inhibitory effect of neomycin on exocytosis. Our results suggest that DAG has a key role in acrosomal exocytosis and that it acts as a messenger rather than as a substrate from which other active metabolites are generated. The lack of stereospecificity shown by the exogenous DAGs implies that DAG does not act by stimulating protein kinase C, but the metabolite's actual target in the sperm cell is as yet unclear.

Regulation of phospholipase D and primary granule secretion by P2-purinergic- and chemotactic peptide-receptor agonists is induced during granulocytic differentiation of HL-60 cells

We have compared the abilities of extracellular ATP (acting via P2-purinergic receptors) and formylated peptides (FMLP) to stimulate both phospholipase D (PLD)-based signal transduction and primary granule (azurophilic) secretion in HL-60 cells induced to differentiate along the granulocytic pathway. In undifferentiated HL-60 cells, neither ATP nor FMLP elicited significant PLD activation or increased secretion despite the previously documented ability of ATP to stimulate large increases in polyphosphoinositide hydrolysis and Ca2+ mobilization. Conversely, within 1 d after induction of granulocytic differentiation by dibutyryl cAMP, both ATP and FMLP induced large increases in azurophilic secretion and corresponding increases in PLD activity. ATP-activated PLD activity was near-maximal within 1 d after dibutyryl cAMP treatment, while the FMLP-induced activity increased continuously over 4 d, with a maximal level twice that stimulated by ATP. Additional experiments characterized the activation of PLD by receptor-independent pathways at different stages of differentiation; these included studies of phorbol ester action in intact cells and GTP gamma S action in electropermeabilized cells. An apparent role for guanine nucleotide-binding regulatory proteins in PLD regulation was also indicated by the significant reduction in FMLP- and ATP-stimulated PLD activity observed in cells pretreated with pertussis toxin. At all stages of differentiation, there was good correlation between the relative efficacies of ATP versus FMLP in stimulating both secretion and PLD activity. These data indicate: (a) that the receptor-regulated phospholipase D signaling pathway is induced during differentiation of myeloid progenitor cells; and (b) that differential activation of this signaling system by various Ca(2+)-mobilizing receptor agonists may underlie the differential regulation of secretion and other phagocyte functions by such agents.

Factors affecting dense and alpha-granule secretion from electropermeabilized human platelets: Ca(2+)-independent actions of phorbol ester and GTP gamma S

Electropermeabilized human platelets containing 5-hydroxy[14C]tryptamine ([14C]5-HT) were suspended in a glutamate medium containing ATP and incubated for 10 min with (in various combinations) Ca2+ buffers, phorbol 12-myristate 13-acetate (PMA), guanine nucleotides, and thrombin. Release of [14C]5-HT and beta-thromboglobulin (beta TG) were used to measure secretion from dense and alpha-granules, respectively. Ca2+ alone induced secretion from both granule types; half-maximal effects were seen at a -log [Ca2+ free] (pCa) of 5.5 and maximal secretion at a pCa of 4.5, when approximately 80% of 5-HT and approximately 50% of beta TG were released. Addition of PMA, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), GTP, or thrombin shifted the Ca2+ dose-response curves for secretion of both 5-HT and beta TG to the left and caused small increases in the maximum secretion observed. These results suggested that secretion from alpha-granules, like that from dense granules, is a Ca(2+)-dependent process stimulated by the sequential activation of a G-protein, phospholipase C, and protein kinase C (PKC). However, high concentrations of PMA and GTP gamma S had distinct effects in the absence of Ca2+ (pCa greater than 9); 100 nM PMA released approximately 20% of platelet 5-HT but little beta TG, whereas 100 microM GTP gamma S stimulated secretion of approximately 25% of each. Simultaneous addition of PMA greatly enhanced these effects of GTP gamma S. Phosphorylation of pleckstrin in permeabilized platelets incubated with [gamma-32P]ATP was used as an index of the activation of PKC during secretion. In the absence of Ca2+, 100 nM PMA caused maximal phosphorylation of pleckstrin and 100 microM GTP gamma S was approximately 50% as effective as PMA; neither GTP gamma S nor Ca2+ enhanced the phosphorylation of pleckstrin caused by 100 nM PMA. These results indicate that, although activation of PKC promoted secretion, GTP gamma S exerted additional stimulatory effects on secretion from both dense and alpha-granules that were not mediated by PKC. Measurement of [3H]inositol phosphate formation in permeabilized platelets containing [3H]phosphoinositides showed that GTP gamma S did not stimulate phosphoinositide-specific phospholipase C in the absence of Ca2+. It follows that in permeabilized platelets, GTP gamma S can both stimulate PKC and enhance secretion via G-protein-linked effectors other than this phospholipase.

Phorbol ester treatment of intact rabbit platelets greatly enhances both the basal and guanosine 5'-[gamma-thio]triphosphate-stimulated phospholipase D activities of isolated platelet membranes. Physiological activation of phospholipase D may be secondary to activation of phospholipase C

Rabbit platelets were labelled with [3H]glycerol and incubated with or without phorbol 12-myristate 13-acetate (PMA). Membranes were then isolated and assayed for phospholipase D (PLD) activity by monitoring [3H]phosphatidylethanol formation in the presence of 300 mM-ethanol. At a [Ca2+free] of 1 microM, PLD activity was detected in control membranes, but was 5.4 +/- 0.8-fold (mean +/- S.E.M.) greater in membranes from PMA-treated platelets. Under the same conditions, 10 microM-guanosine 5'-[gamma-thio]triphosphate (GTP[S]) stimulated PLD by 18 +/- 3-fold in control membranes, whereas PMA treatment and GTP[S] interacted synergistically to increase PLD activity by 62 +/- 12-fold. GTP[S]-stimulated PLD activity was observed in the absence of Ca2+, but was increased by 1 microM-Ca2+ (3.5 +/- 0.2-fold and 1.8 +/- 0.1-fold in membranes from control and PMA-treated platelets respectively). GTP exerted effects almost as great as those of GTP[S], but 20-30-fold higher concentrations were required. Guanosine 5'-[beta-thio]diphosphate inhibited the effects of GTP[S] or GTP, suggesting a role for a GTP-binding protein in activation of PLD. Thrombin (2 units/ml) stimulated the PLD activity of platelet membranes only very weakly and in a GTP-independent manner. The actions of PMA and analogues on PLD activity correlated with their ability to stimulate protein kinase C in intact platelets. Staurosporine, a potent protein kinase inhibitor, had both inhibitory and, at higher concentrations, stimulatory effects on the activation of PLD by PMA. The results suggest that PMA not only stimulates PLD via activation of protein kinase C but can also activate the enzyme by a phosphorylation-independent mechanism in the presence of staurosporine. However, under physiological conditions, full activation of platelet PLD may require the interplay of protein kinase C, increased Ca2+ and a GTP-binding protein, and may occur as a secondary effect of the activation of phospholipase C.

Diacylglycerol and phosphatidate production and the exocytosis of the sperm acrosome

We have investigated the production of diacylglycerol (DAG) and phosphatidate (PtdOH) during the exocytosis of the sperm acrosome. Ram spermatozoa treated with Ca2+ and the ionophore A23187 experienced a rapid breakdown of the polyphosphoinositides (PPIs), and a rise in [32P]Pi-labelled PtdOH and DAG mass; PtdOH mass, however, was unaffected. Treatment with Ca2+/A23187 and the DAG kinase inhibitor R59022 resulted in a dose-dependent increase in DAG mass and a concomitant decrease in [32P]PtdOH; such treatment showed a dose-dependent stimulation of acrosomal exocytosis. Pre-incubation with exogenous PtdOHs before stimulation with Ca2+/A23187 did not affect the time-course of exocytosis, whereas treatment with Ca2+/A23187 and exogenous DAGs (dioctanoylglycerol, oleoyl-acetyl-glycerol, or dioleoylglycerol) resulted in a dose-dependent stimulation of acrosomal exocytosis. Our results suggest that DAG, rather than PtdOH, is the important metabolite generated upon PPI hydrolysis; however, since spermatozoa lack protein kinase C, the target of DAG in most cells, a role for DAG in acrosomal exocytosis is as yet unclear.

Promotion of acid-induced membrane fusion by basic peptides. Amino acid and phospholipid specificities

The ability of oligo- and polymers of the basic amino acids L-lysine, L-arginine, L-histidine and L-ornithine to induce lipid intermixing and membrane fusion among vesicles containing various anionic phospholipids has been investigated. Among vesicle consisting of either phosphatidylinositol or mixtures of phosphatidic acid and phosphatidylethanolamine rapid and extensive lipid intermixing, but not complete fusion, was induced at neutral pH by poly-L-ornithine or L-lysine peptides of five or more residues. When phosphatidylcholine was included in the vesicles, the lipid intermixing was severely inhibited. Such lipid intermixing was also much less pronounced among phosphatidylserine vesicles. Poly-L-arginine provoked considerable leakage from the various anionic vesicles and caused significantly less lipid intermixing than L-lysine peptides at neutral pH. When the addition of basic amino acid polymer was followed by acidification to pH 5-6, vesicle fusion was induced. Fusion was more pronounced among vesicles containing phosphatidylserine or phosphatidic acid than among those containing phosphatidylinositol, and occurred also with vesicles whose composition resembles that of cellular membranes (i.e., phosphatidylcholine/phosphatidylethanolamine/phosphatidylserine, 50:30:20, by mol). Liposomes with this composition are resistant to fusion by Ca2+ or by acidification after lectin-mediated contact. The tight interaction among vesicles at neutral pH, resulting in lipid intermixing, does not seem to be necessary for the fusion occurring after acidification, but the basic peptides nevertheless appear to play a more active role in the fusion process than simply bringing the vesicles in contact. However, protonation of the polymer side chains and transformation of the polymer into a polycation does not explain the need for acidification, since the pH-dependence was quite similar for poly(L-histidine)- and poly(L-lysine)-mediated fusion.

Molecular mechanisms of calcium-induced membrane fusion

We have reviewed studies on calcium-induced fusion of lipid bilayer membranes and the role of synexin and other calcium-binding proteins (annexins) in membrane fusion. We have also discussed the roles of other cations, lipid phase transitions, long chain fatty acids and other fusogenic molecules. Finally, we have presented a simple molecular model for the mechanism of lipid membrane fusion, consistent with the experimental evidence and incorporating various elements proposed previously.

Trypsin inhibitors inhibit induction by interferon-gamma of HLA-DR antigen expression on human skin cells

Serine protease inhibitors with a specificity for trypsin inhibit interferon-gamma (INF-gamma)-induced HLA-DR expression on a hybrid human epidermal cell line (H12), dermal fibroblasts, and primary keratinocytes. Protease inhibitors with a specificity for chymotrypsin or papain fail to inhibit IFN-gamma. The inhibitory effect of the trypsin inhibitors is similar to that of glucocorticoids in that it is a transient event, fading with length of exposure to IFN-gamma, and is reversed by the addition of dibutyryl cyclic AMP (dbcAMP) and phospholipase C(PLC) from Clostridium perfringens. In H12 cells, dbcAMP and PLC enhance the IFN-gamma induction of HLA-DR, but do not induce in the absence of INF-gamma. Evidence suggests that the protease inhibitors, as well as dbcAMP and PLC, may modulate HLA-DR expression at a post-translational site as well as during IFN-gamma signal transduction. These results suggest that trypsin-like protease activity may be required for cellular HLA-DR antigen expression following exposure to IFN-gamma.