The effect of bilayer order and fluidity on detergent-induced liposome fusion

Regulation of the myoblast fusion reaction for muscle development, regeneration, and adaptations

Fusion of plasma membranes is essential for skeletal muscle development, regeneration, exercise-induced adaptations, and results in a cell that contains hundreds to thousands of nuclei within a shared cytoplasm. The differentiation process in myocytes culminates in their fusion to form a new myofiber or fusion to an existing myofiber thereby contributing more synthetic material to the syncytium. The choice for two cells to fuse and become one could be a dangerous event if the two cells are not committed to an allied function. Thus, fusion events are highly regulated with positive and negative factors to fine-tune the process, and requires muscle-specific fusogens (Myomaker and Myomerger) as well as general cellular machinery to achieve the union of membranes. While a unified vertebrate myoblast fusion pathway is not yet established, recent discoveries should make this pursuit attainable. Not only does myocyte fusion impact the normal biology of skeletal muscle, but new evidence indicates dysregulation of the process impacts pathologies of skeletal muscle. Here, I will highlight the molecular players and biochemical mechanisms that drive fusion events in muscle, and discuss how this key myogenic process impacts skeletal muscle diseases.

High-melting lipid mixtures and the origin of detergent-resistant membranes studied with temperature-solubilization diagrams

The origin of resistance to detergent solubilization in certain membranes, or membrane components, is not clearly understood. We have studied the solubilization by Triton X-100 of binary mixtures composed of egg sphingomyelin (SM) and either ceramide, diacylglycerol, or cholesterol. Solubilization has been assayed in the 4-50°C range, and the results are summarized in a novel, to our knowledge, form of plots, that we have called temperature-solubilization diagrams. Despite using a large detergent excess (lipid/detergent 1:20 mol ratio) and extended solubilization times (24-48 h) certain mixtures were not amenable to Triton X-100 solubilization at one or more temperatures. DSC of all the lipid mixtures, and of all the lipid + detergent mixtures revealed that detergent resistance was associated with the presence of gel domains at the assay temperature. Once the system melted down, solubilization could occur. In general adding high-melting lipids limited the solubilization, whereas the addition of low-melting lipids promoted it. Lipidomic analysis of Madin-Darby canine kidney cell membranes and of the corresponding detergent-resistant fraction indicated a large enrichment of the nonsolubilized components in saturated diacylglycerol and ceramide. SM-cholesterol mixtures were special in that detergent solubilization was accompanied, for certain temperatures and compositions, by an independent phenomenon of reassembly of the partially solubilized lipid bilayers. The temperature at which lysis and reassembly prevailed was ∼25°C, thus for some SM-cholesterol mixtures solubilization occurred both above and below 25°C, but not at that temperature. These observations can be at the origin of the detergent resistance effects observed with cell membranes, and they also mean that cholesterol-containing detergent-resistant membrane remnants cannot correspond to structures existing in the native membrane before detergent addition.

Pulsatile release from pH triggered imidazoline switchable surfactant liposomes

The incorporation of an imidazoline (IDZ) based switchable surfactant into the lipid membrane of a liposome produces a system that can be triggered to release its solute upon pH change.

Histones cause aggregation and fusion of lipid vesicles containing phosphatidylinositol-4-phosphate

In a previous article, we demonstrated that histones (H1 or histone octamers) interact with negatively charged bilayers and induce extensive aggregation of vesicles containing phosphatidylinositol-4-phosphate (PIP) and, to a lesser extent, vesicles containing phosphatidylinositol (PI). Here, we found that vesicles containing PIP, but not those containing PI, can undergo fusion induced by histones. Fusion was demonstrated through the observation of intervesicular mixing of total lipids and inner monolayer lipids, and by ultrastructural and confocal microscopy studies. Moreover, in both PI- and PIP-containing vesicles, histones caused permeabilization and release of vesicular aqueous contents, but the leakage mechanism was different (all-or-none for PI and graded release for PIP vesicles). These results indicate that histones could play a role in the remodeling of the nuclear envelope that takes place during the mitotic cycle.

Large lateral movement of transmembrane helix S5 is not required for substrate access to the active site of rhomboid intramembrane protease

Rhomboids represent an evolutionarily ancient protease family. Unlike most other proteases, they are polytopic membrane proteins and specialize in cleaving transmembrane protein substrates. The polar active site of rhomboid protease is embedded in the membrane and normally closed. For the bacterial rhomboid GlpG, it has been proposed that one of the transmembrane helices (S5) of the protease can rotate to open a lateral gate, enabling substrate to enter the protease from inside the membrane. Here, we studied the conformational change in GlpG by solving the cocrystal structure of the protease with a mechanism-based inhibitor. We also examined the lateral gating model by cross-linking S5 to a neighboring helix (S2). The crystal structure shows that inhibitor binding displaces a capping loop (L5) from the active site but causes only minor shifts in the transmembrane helices. Cross-linking S5 and S2, which not only restricts the lateral movement of S5 but also prevents substrate from passing between the two helices, does not hinder the ability of the protease to cleave a membrane protein substrate in detergent solution and in reconstituted membrane vesicles. Taken together, these data suggest that a large lateral movement of the S5 helix is not required for substrate access to the active site of rhomboid protease.

Membrane processes and biophysical characterization of living cells decorated with chromatic polydiacetylene vesicles

The structural complexity of the cell membrane makes analysis of membrane processes in living cells, as compared to model membrane systems, highly challenging. Living cells decorated with surface-attached colorimetric/fluorescent polydiacetylene patches might constitute an effective platform for analysis and visualization of membrane processes in situ. This work examines the biological and chemical consequences of plasma membrane labeling of promyelocytic leukemia cells with polydiacetylene. We show that the extent of fusion between incubated lipid/diacetylene vesicles and the plasma membrane is closely dependent upon the lipid composition of both vesicles and cell membrane. In particular, we find that cholesterol presence increased bilayer fusion between the chromatic vesicles and the plasma membrane, suggesting that membrane organization plays a significant role in the fusion process. Spectroscopic data and physiological assays show that decorating the cell membrane with the lipid/diacetylene patches reduces the overall lateral diffusion within the membrane bilayer, however polydiacetylene labeling does not adversely affect important cellular metabolic pathways. Overall, the experimental data indicate that the viability and physiological integrity of the surface-engineered cells are retained, making possible utilization of the platform for studying membrane processes in living cells. We demonstrate the use of the polydiacetylene-labeled cells for visualizing and discriminating among different membrane interaction mechanisms of pharmaceutical compounds.

Biophysics of sphingolipids I. Membrane properties of sphingosine, ceramides and other simple sphingolipids

Some of the simplest sphingolipids, namely sphingosine, ceramide, some closely related molecules (eicosasphingosine, phytosphingosine), and their phosphorylated compounds (sphingosine-1-phosphate, ceramide-1-phosphate), are potent metabolic regulators. Each of these lipids modifies in marked and specific ways the physical properties of the cell membranes, in what can be the basis for some of their physiological actions. This paper reviews the mechanisms by which these sphingolipid signals, sphingosine and ceramide in particular, are able to modify the properties of cell membranes.

Application of polyethyleneglycol (PEG)-modified liposomes for oral vaccine: effect of lipid dose on systemic and mucosal immunity

To examine the systemic and mucosal immunity towards a liposomal antigen in an oral vaccine, we prepared ovalbumin (OVA)-encapsulating polyethyleneglycol (PEG)-modified liposomes and unmodified ones, and orally administered two different concentrations of them to mice. Unmodified liposomes tended to induce a stronger systemic immune response than the PEG-modified ones especially at the higher concentration of liposomes. Whereas at the lower liposome concentration the mucosal immune response was stronger for the PEG-modified liposomes than for the unmodified ones but nearly the same at the higher concentration. The relative amount of immunoglobulin G (IgG) against OVA in the plasma was 1.7-fold higher for a 12.5 micro mol phospholipid dose of PEG-liposomes encapsulating OVA than for a 5.0 micro mol one encapsulating the same amount of OVA. On the contrary, the relative amount of IgA in the intestinal wash was 2.6-fold higher for the 5.0 micro mol phospholipid dose than for the 12.5 micro mol one. These results indicate that OVA encapsulated in a small number of liposomes, especially the PEG-modified ones, is favorable for inducing a mucosal immune response and that the same amount of OVA in a large number of liposomes tends to improve the systemic immune response. A possible explanation for this tendency is the differential release rate of OVA from the liposomes at the intestinal mucosa. Our present study suggests that the dose of liposomes containing antigen is an important factor for controlling the response of systemic and mucosal immune systems.

Surfactant effects of chlorpromazine and imipramine on lipid bilayers containing sphingomyelin and cholesterol

The surface-active drugs chlorpromazine (CPZ) and imipramine (IP) have been tested on large unilamellar vesicles composed of phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol (Ch) in different proportions. The well-characterized nonionic detergent Triton X-100 (TX) has also been used in parallel experiments. Leakage of vesicular aqueous contents and bilayer solubilization have been measured for each surfactant molecule and vesicle composition. All three surface-active molecules behave in a qualitatively similar way, irrespective of bilayer composition: they induce leakage at concentrations well below their critical micellar concentrations (cmc) and solubilization near the cmc. In these events, the potency of the three surfactants under study increases with decreasing cmc, in the order IP<CPZ<TX. With all three surfactant molecules, addition of SM to PC bilayers made the vesicles more sensitive to the detergents. The three surfactants had the same effects on PC : SM (3 : 1 mole ratio) and on PC : SM : Ch (3 : 1 : 1 mole ratio) vesicles when leakage was tested. However, the presence of cholesterol made the bilayers more resistant to solubilization. In the presence of both SM and Ch, CPZ and IP, but not TX, were able to achieve complete bilayer solubilization. Since the outer monolayer of plasma membranes is rich in SM and Ch, the present data could be relevant in understanding some of the CPZ or IP effects at the membrane level.

Membrane fluidity effects of estratrienes

Estrogens have demonstrable neuroprotective effects. This fact has lead to the proposed use of estrogens for the prevention and/or treatment of Alzheimer's disease. The exact protective mechanism estrogens provide is not fully understood. In this report, a potential non-genomic mechanism for estratrienes involving alterations in membrane fluidity was studied. Steroids, such as estrogen, are known to be membrane-active and can alter the lipid packing. In this study we used fluorescent methodologies to address the effect of naturally occurring steroids (17alpha and 17beta-estradiol, testosterone, and progesterone) and new estratriene analogs on membrane fluidity using liposomes and HT-22 hippocampal cells. The study's results indicate steroids, based on the estratriene nucleus, can modulate lipid packing as evidenced by (1) decreased membrane fusion events and (2) decreased membrane fluidity. The effects on the membrane were both time and concentration dependent. It was also demonstrated through rational design estratriene analogs can be synthesized with enhanced membrane effects. Finally, in a glutamate-induced toxicity HT-22 model, we also demonstrated cellular protection with the estratriene-based molecules and analogs. The data suggest the plethora of cellular actions of estrogens may relate to or be influenced by membrane effects of the steroid.

Interaction of membrane proteins and lipids with solubilizing detergents

Detergents are indispensable in the isolation of integral membrane proteins from biological membranes to study their intrinsic structural and functional properties. Solubilization involves a number of intermediary states that can be studied by a variety of physicochemical and kinetic methods; it usually starts by destabilization of the lipid component of the membranes, a process that is accompanied by a transition of detergent binding by the membrane from a noncooperative to a cooperative interaction already below the critical micellar concentration (CMC). This leads to the formation of membrane fragments of proteins and lipids with detergent-shielded edges. In the final stage of solubilization membrane proteins are present as protomers, with the membrane inserted sectors covered by detergent. We consider in detail the nature of this interaction and conclude that in general binding as a monolayer ring, rather than as a micelle, is the most probable mechanism. This mode of interaction is supported by neutron diffraction investigations on the disposition of detergent in 3-D crystals of membrane proteins. Finally, we briefly discuss the use of techniques such as analytical ultracentrifugation, size exclusion chromatography, and mass spectrometry relevant for the structural investigation of detergent solubilized membrane proteins.

Protein-induced fusion of phospholipid vesicles of heterogeneous sizes

We have investigated the fusion of phospholipid vesicles induced by lysozyme and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Vesicles were composed of dimyristoylphosphatidylcholine/dioleoylphosphatidylethanolamine/ cholesterol (DMPC:DOPE:Chol, 2:1:1). Small unilamellar vesicles (SUV, diameter ca. 30 nm) obtained by extensive sonication or large unilamellar vesicles (LUV, diameters ranged from 100 to 400 nm) obtained by extrusion methods were used. Fusion of LUV induced by lysozyme and GAPDH was drastically decreased when the diameter of the vesicles increased over a value of 100 nm. Lysozyme effect was stopped at the aggregation step while GAPDH effect was stopped at the fusion (lipid mixing) step. Fusion of heterogeneous vesicle populations (SUV with LUV) was observed only with GAPDH and this happened only when the lipids were in the liquid-crystalline state.

Reconstitution studies of lipid effects on insulin-receptor kinase activation

Insulin receptors extracted from human placenta were reconstituted by dialysis into well-characterized lipid vesicles. For all types of lipids studied, vesicles were shown to be unilamellar, about 120 nm in diameter. The incorporation of lectin-purified insulin receptors was assessed by cosedimentation of 125I-insulin binding and [32P]phospholipids in a sucrose gradient. The insulin-binding activity was not modified by the composition of the lipid vesicles. However, tyrosine kinase activation appeared to be more sensitive to its lipid environment. Mixtures of phosphatidylcholine/phosphatidylserine or phospholipids/phosphatidylserine, in ratios of 1-4, increased the insulin-induced tyrosine kinase activation in a dose-dependent manner. In contrast, experiments performed in the presence of phosphatidylinositol showed a decrease in the enzyme stimulation. These results indicate an opposing involvement of these two anionic phospholipids in the kinase activation. Inclusion of cholesterol (10-30%) into phosphatidylcholine vesicles reduced kinase activation, which was drastically inhibited by 30% cholesterol. The effect of a total extract of brain gangliosides was biphasic, stimulatory at low concentration (5-10%), but with a reverse effect at higher concentrations. These results stress the importance of the lipid environment for insulin-receptor signaling, particularly for the insulin-induced activation of its beta-subunit kinase.

Effects of small organic molecules on phospholipid phase transitions

Small organic molecules are known to exhibit a wide spectrum of physiological or pharmacological effects and many of them are thought to be membrane associated. Therefore a great number of studies is devoted to the interaction between these molecules and phospholipid model membranes. Results obtained for molecular species of varying hydrophobic/hydrophilic balances will be described. It will be shown that, in general, these different molecules induce similar effects on phospholipid phase transitions, although they are located differently in the membrane. Detailed studies of these interactions will help to understand these processes on a molecular level.

Interaction of surfactants with bilayer of negatively charged lipid: effect on gel-to-liquid-crystalline phase transition of dilauroylphosphatidic acid vesicle membrane

The interaction of surfactants with the vesicle membrane of the negatively charged lipid, dilauroylphosphatidic acid, was investigated through their effect on the gel-to-liquid-crystalline phase transition of the lipid bilayer. Three types of surfactants (anionic, cationic and non-ionic) with different hydrocarbon chain length were examined. (i) Anionic sodium alkylsulfates affected the phase transition temperature, Tm, only weakly. (ii) Non-ionic alkanoyl-N-methylglucamides decreased Tm monotonously with increasing concentration. The depression of Tm induced by these surfactants was analyzed by applying the van't Hoff model for the freezing-point depression, and the partition coefficients of the surfactants between bulk water and lipid membrane were estimated. (iii) Cationic alkyltrimethylammonium bromides affected Tm in a complex manner depending on the hydrocarbon chain length of the surfactants. Octyl-/tetradecyl-trimethylammonium bromide depressed/elevated Tm monotonously with increasing concentration, whereas the change in Tm induced by decyl- and dodecyltrimethylammonium bromides was not monotonous but biphasic. This complex behavior of the phase transition temperature was well explained, based on the statistical mechanical theory presented by Suezaki et al. (Biochim. Biophys. Acta, 818 (1985) 31-37), which takes into account the interaction between surfactant molecules incorporated in the lipid membrane.

Effect of proteases and other treatments on the proliferative assembly of tight junction strands in the rat prostate tissue

As already observed with the freeze-fracture technique by Kachar and Pinto da Silva (1981) incubation of excised rat ventral prostate tissue at 37 degrees C results in proliferation of tight junction strands over the lateral membranes of the epithelial cells, an effect not occurring after incubation at 4 degrees C. Our investigations have shown that a pretreatment with protease inhibitor Contrykal at 4 degrees C suppresses the proliferation of tight junction strands at 37 degrees C. On the other hand the added proteases trypsin and pronase induce the proliferation already at 4 degrees C. The effect was lowered by higher concentrations of pronase. Incubation with solutions of the non-ionic detergent Triton X-100 at 4 degrees C could also induce tight junction proliferation, whereas this effect was nearly absent after an incubation at 4 degrees C with a hyperosmotic solution of arabinose and with 50% acetone. No relation was found between proliferation of tight junction strands and cluster formation of intramembraneous particles. In conclusion of the results a small protein or a peptide resembling fusions proteins is assumed as trigger for formation of tight junction strands, acting by destabilization of the bilayer arrangement of membrane lipids.

The mechanism of liposomal damage by taurocholate

The stability of small unilamellar vesicles formed by egg-yolk phosphatidylcholine (PC) has been examined in the presence of sodium taurocholate. The permeability of the vesicular membrane changes as the total taurocholate concentration increases, until a transformation from mixed bile salt/PC vesicles to mixed micelles occurs. Based on experiments in which the bile salt-induced release of either hydrophilic (carboxyfluorescein) or hydrophobic (Bromothymol blue) probes was studied, and on fluorescence polarization of the probe 1,6-diphenyl-1,3,5-hexatriene and turbidity measurements, a two-step process for the initial stage of liposomal damage by taurocholate is postulated.

The influence of membrane composition on the solubilizing effects of Triton X-100

Multilamellar liposomes containing pure phosphatidylcholine (PC) or mixtures of PC with cholesterol, cholesteryl palmitate, beta-carotene, cardiolipin, phosphatidylethanolamine or gramicidin A have been treated with the detergent Triton X-100. Solubilization has been monitored as a decrease in turbidity of the liposome suspension, and also by determination of bilayer components in the solubilized fraction. The same solubilization pattern is found for unsaturated (egg yolk) or saturated (dimyristoyl) PC. Similar results are also found when dimyristoyl PC is solubilized above or below its gel-to-fluid transition temperature. Cholesterol solubilizes in parallel with PC; gramicidin A is solubilized preferentially to this phospholipid and the non-polar lipids cholesteryl palmitate or beta-carotene remain insoluble at detergent concentrations producing complete PC solubilization. Addition of cardiolipin or phosphatidylethanolamine does not seem to alter the general pattern of PC solubilization. Phosphatidylethanolamine is less soluble than PC, while cardiolipin solubilizes at the same detergent concentrations than PC. These results are considered in relation to previous studies with natural membranes.