Hydrolytic action of phospholipase A2 in monolayers in the phase transition region: direct observation of enzyme domain formation using fluorescence microscopy

Phospholipase A2 way to hydrolysis: Dint formation, hydrophobic mismatch, and lipid exclusion

Phospholipase A2 (PLA2) exerts a wide range of biological effects and attracts a lot of attention of researchers. Two sites are involved in manifestation of PLA2 enzymatic activity: catalytic site responsible for substrate binding and fatty acid cleavage from the sn-2 position of a glycerophospholipid, and interface binding site (IBS) responsible for the protein binding to lipid membrane. IBS is formed by positively charged and hydrophobic amino acids on the outer surface of the protein molecule. Understanding the mechanism of PLA2 interaction with the lipid membrane is the most challenging step in biochemistry of this enzyme. We used a combination of experimental and computer simulation techniques to clarify molecular details of bee venom PLA2 interaction with lipid bilayers formed by palmitoyloleoylphosphatidylcholine or dipalmitoylphosphatidylcholine. We found that after initial enzyme contact with the membrane, a network of hydrogen bonds was formed. This led to deformation of the interacting leaflet and dint formation. The bilayer response to the deformation depended on its phase state. In a gel-phase bilayer, diffusion of lipids is restricted therefore chain melting occurred in both leaflets of the bilayer. In the case of a fluid-phase bilayer, lateral diffusion is possible, and lipid polar head groups were excluded from the contact area. As a result, the bilayer became thinner and a large hydrophobic area was formed. We assume that relative ability of a bilayer to come through lipid redistribution process defines the rate of initial stages of the catalysis.

Molecular interactions of phospholipid monolayers with a model phospholipase

The intrinsic overexpression of secretory phospholipase A2 (sPLA2) in various pro-inflammatory diseases and cancers has the potential to be exploited as a therapeutic strategy for diagnostics and treatment. To explore this potential and advance our knowledge of the role of sPLA2 in related diseases, it is necessary to systematically investigate the molecular interaction of the enzyme with lipids. By employing a Langmuir trough integrated with X-ray reflectivity and grazing incidence X-ray diffraction techniques, this study examined the molecular packing structure of 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC) films before and after enzyme adsorption and enzyme-catalyzed degradation. Molecular interaction of sPLA2 (from bee venom) with the DPPC monolayer exhibited Ca2+ dependence. DPPC molecules at the interface without Ca2+ retained a monolayer organization; upon adsorption of sPLA2 to the monolayer the packing became tighter. In contrast, sPLA2-catalyzed degradation of DPPC occurred in the presence of Ca2+, leading to disruption of the ordered monolayer structure of DPPC. The interfacial film became a mixture of highly ordered multilayer domains of palmitic acid (PA) and loosely packed monolayer phase of 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (lysoPC) that potentially contained the remaining un-degraded DPPC. The redistribution of lipid degradation products into the third dimension, which produced multilayer PA domains, damaged the structural integrity of the original lipid layer and may explain the bursting of liposomes observed in other studies after a latency period of mixing liposomes with sPLA2. A quantitative understanding of the lipid packing and lipid-enzyme interaction provides an intuitive means of designing and optimizing lipid-related drug delivery systems.

How to gather useful and valuable information from protein binding measurements using Langmuir lipid monolayers

This review presents data on the influence of various experimental parameters on the binding of proteins onto Langmuir lipid monolayers. The users of the Langmuir methodology are often unaware of the importance of choosing appropriate experimental conditions to validate the data acquired with this method. The protein Retinitis pigmentosa 2 (RP2) has been used throughout this review to illustrate the influence of these experimental parameters on the data gathered with Langmuir monolayers. The methods detailed in this review include the determination of protein binding parameters from the measurement of adsorption isotherms, infrared spectra of the protein in solution and in monolayers, ellipsometric isotherms and fluorescence micrographs.

Nonspecific Binding Domains in Lipid Membranes Induced by Phospholipase A2

Phospholipase A2 (PLA2) is a peripheral membrane protein that can hydrolyze phospholipids to produce lysolipids and fatty acids. It has been found to play crucial roles in various cellular processes and is thought as a potential candidate for triggering drug release from liposomes for medical treatment. Here, we directly observed that PLA2 hydrolysis reaction can induce the formation of PLA2-binding domains at lipid bilayer interface and found that the formation was significantly influenced by the fluidity of the lipid bilayer. We prepared supported lipid bilayers (SLBs) with various molar ratios of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) to adjust the reactivity and fluidity of the lipid bilayers. A significant amount of the PLA2-induced domains was observed in mixtures of DPPC and DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) but not in either pure DPPC or pure DOPC bilayer, which might be the reason that previous studies rarely observed these domains in lipid bilayer systems. The fluorescently labeled PLA2 experiment showed that newly formed domains acted as binding templates for PLA2. The AFM result showed that the induced domain has stepwise plateau structure, suggesting that PLA2 hydrolysis products may align as bilayers and accumulate layer by layer on the support, and the hydrophobic acyl chains at the side of the layer structure may be exposed to the outside aqueous environment. The introduced hydrophobic region could have hydrophobic interactions with proteins and therefore can attract the binding of not only PLA2 but also other types of proteins such as proteoglycans and streptavidin. The results suggest that the formation of PLA2-induced domains may convert part of a zwitterionic nonsticky lipid membrane to a site where biomolecules can nonspecifically bind.

Water-binding phospholipid nanodomains and phase-separated diacylglycerol nanodomains regulate enzyme reactions in lipid monolayers

Phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) nanodomains covered with bound water as well as diacylglycerol 1-palmitoyl-2-oleoyl-sn-glycerol (POG) nanodomains separated from a lipid membrane were studied, using monolayer surfaces of POPC hydrolyzed by phospholipase C (PLC). The investigation was based on the analysis of compression isotherms and on atomic force microscope (AFM) observations of Langmuir-Blodgett (LB) films and Langmuir-Schaefer (LS) films. The results included reaction rate constants obtained by kinetic analysis of phosphocholine at surface pressures from 0.1 to 31 mN/m and determined by a luminol-enhanced chemiluminescence method. Monolayer elastic modulus values and fluorescence microscopic images confirmed that hydrolysis by PLC progressed in the intermediate monolayer between a liquid-expanded (L1) film and a liquid-condensed (L2) film at 2-17 mN/m. Furthermore, the intermediate film was confirmed to consist of L1 film and the POPC nanodomains in the L2 state are covered with bound water, conclusions based on the following AFM results: (1) nanodomains in POPC LS films were catalyzed by PLC, (2) POG nanodomains extended out from LB films of mixed POPC/POG 9/1 (mol/mol) monolayers, and (3) POPC LS films were covered with bound water, as indicated by cross-sectional analysis. At the optimal surface pressure of 10 mN/m, when POPC nanodomains (L2), with internal diameters of ∼75 nm, were hydrolyzed by PLC, they shrank down into pockets of the same size as those that appeared with POG. The resulting pocket sizes on LS films were in agreement with POG nanodomain sizes on LB films. This study demonstrated that PLC reacted with POPC nanodomains (L2) dispersed in L1/L2 mixed phase monolayers selectively and that POG nanodomains were phase-separated from the monolayer as hydrolysis proceeded.

Phospholipase A2 isolated from the venom of Crotalus durissus terrificus inactivates dengue virus and other enveloped viruses by disrupting the viral envelope

The Flaviviridae family includes several virus pathogens associated with human diseases worldwide. Within this family, Dengue virus is the most serious threat to public health, especially in tropical and sub-tropical regions of the world. Currently, there are no vaccines or specific antiviral drugs against Dengue virus or against most of the viruses of this family. Therefore, the development of vaccines and the discovery of therapeutic compounds against the medically most important flaviviruses remain a global public health priority. We previously showed that phospholipase A₂ isolated from the venom of Crotalus durissus terrificus was able to inhibit Dengue virus and Yellow fever virus infection in Vero cells. Here, we present evidence that phospholipase A₂ has a direct effect on Dengue virus particles, inducing a partial exposure of genomic RNA, which strongly suggests inhibition via the cleavage of glycerophospholipids at the virus lipid bilayer envelope. This cleavage might induce a disruption of the lipid bilayer that causes a destabilization of the E proteins on the virus surface, resulting in inactivation. We show by computational analysis that phospholipase A₂ might gain access to the Dengue virus lipid bilayer through the pores found on each of the twenty 3-fold vertices of the E protein shell on the virus surface. In addition, phospholipase A₂ is able to inactivate other enveloped viruses, highlighting its potential as a natural product lead for developing broad-spectrum antiviral drugs.

Aggregation, fusion, and leakage of liposomes induced by peptides

Biological membranes are heterogeneous systems. Their functions are closely related to the lipid lateral segregation in the presence of membrane proteins. In this work, we designed two peptides, amphiphilic cationic peptides K3L8K3 and nonamphiphilic peptides K20, and studied their interactions with binary liposomes in different phases (Lα, Lβ', and Lα/Lβ'). As mimics of membrane proteins, both K3L8K3 and K20 can cause the liposomes to aggregate, fuse, or leak. These processes were closely related to the phases of liposomes. For the liposomes in Lα phase, heavy aggregation, fusion, and leakage were observed in the presence of either K20 or K3L8K3. For the liposomes in Lβ' phase, neither K3L8K3 nor K20 can induce fusion or leakage. For the liposomes in Lα/Lβ' phase, K3L8K3 caused the liposomes to aggregate, fuse, and leak, while K20 only led to aggregation. The kinetics of aggregation, fusion, and leakage in each phase were recorded, and they were related to the lipid demixing in the presence of the peptide. Our work not only gained insight into the effect of the lipid demixing on the interactions between peptide and membrane, but also helped in developing drug delivery vehicles with liposomes as the platform.

Creating air-stable supported lipid bilayers by physical confinement induced by phospholipase A2

Supported lipid bilayer platforms have been used for various biological applications. However, the lipid bilayers easily delaminate and lose their natural structure after being exposed to an air-water interface. In this study, for the first time, we demonstrated that physical confinement can be used instead of chemical modifications to create air-stable membranes. Physical confinement was generated by the obstacle network induced by a peripheral enzyme, phospholipase A2. The enzyme and reacted lipids could be washed away from the obstacle network, which was detergent-resistant and strongly bonded to the solid support. On the basis of these properties, the obstacle framework on the solid support was reusable and lipid bilayers with the desired composition could be refilled and formed in the region confined by the obstacle framework. The results of fluorescence recovery after photobleaching (FRAP) indicate that the diffusivities of the lipid bilayers before drying and after rehydration were comparable, indicating the air stability of the physically confined membrane. In addition, we observed that the obstacles could trap a thin layer of water after the air-water interface passed through the lipid bilayer. Because the obstacles were demonstrated to be several times higher than a typical lipid membrane on a support, the obstacles may act as container walls, which can trap water above the lipid membrane. The water layer may have prevented the air-water interface from directly contacting the lipid membrane and, therefore, buffered the interfacial force, which could cause membrane delamination. The results suggest the possibility of using physical confinement to create air-stable membranes without changing local membrane rigidity or covering the membrane with protecting molecules.

Class specific peptide inhibitors for secretory phospholipases A2

Phospholipases A2 (PLA2) catalyze the hydrolytic cleavage of free fatty acids from the sn-2 OH-moiety of glycerophospholipids. These enzymes have a number of functions, from digestion to signaling and toxicity of several venoms. They have also been implicated in inflammation and are connected to diverse diseases, such as cancer, ischemia, atherosclerosis, and schizophrenia. Accordingly, there is a keen interest to develop selective inhibitors for therapeutic use. We recently proposed a novel mechanism for the control of PLA2 activity with highly active protofibrils of PLA2 existing transiently before conversion to inactive amyloid fibrils [19]. In keeping with the above mechanism several algorithms identified (85)KMYFNLI(91) and (17)AALSYGFYG(25) in bee venom (bv) and human lacrimal fluid (Lf) PLA2, respectively, as a regions potentially forming amyloid type aggregates. Interestingly, in keeping with the proposed role of these sequences in the control of the activity of these enzymes, preincubation of 2nM bvPLA2 with (85)KMYFNLI(91) caused complete inhibition of PLA2 activity while the scrambled control peptide YNFLIMK had no effect. Approximately 36% attenuation of the hydrolytic activity of LfPLA2 present in human lacrimal fluid was observed in the presence of 80nM (17)AALSYGFYG(25).

Influence of lipid heterogeneity and phase behavior on phospholipase A2 action at the single molecule level

We monitored the action of phospholipase A(2) (PLA(2)) on L- and D-dipalmitoyl-phosphatidylcholine (DPPC) Langmuir monolayers by mounting a Langmuir-trough on a wide-field fluorescence microscope with single molecule sensitivity. This made it possible to directly visualize the activity and diffusion behavior of single PLA(2) molecules in a heterogeneous lipid environment during active hydrolysis. The experiments showed that enzyme molecules adsorbed and interacted almost exclusively with the fluid region of the DPPC monolayers. Domains of gel state L-DPPC were degraded exclusively from the gel-fluid interface where the buildup of negatively charged hydrolysis products, fatty acid salts, led to changes in the mobility of PLA(2). The mobility of individual enzymes on the monolayers was characterized by single particle tracking. Diffusion coefficients of enzymes adsorbed to the fluid interface were between 3.2 microm(2)/s on the L-DPPC and 4.9 microm(2)/s on the D-DPPC monolayers. In regions enriched with hydrolysis products, the diffusion dropped to approximately 0.2 microm(2)/s. In addition, slower normal and anomalous diffusion modes were seen at the L-DPPC gel domain boundaries where hydrolysis took place. The average residence times of the enzyme in the fluid regions of the monolayer and on the product domain were between approximately 30 and 220 ms. At the gel domains it was below the experimental time resolution, i.e., enzymes were simply reflected from the gel domains back into solution.

Effect of phospholipase A2 hydrolysis products on calcium oxalate precipitation at lipid interfaces

Urinary stones are commonly composed of an inorganic component, calcium oxalate, or calcium phosphate and an organic matrix of lipids, carbohydrates, and proteinaceous matter. Of interest is the role that the organic matrix elements may play as catalysts for the heterogeneous nucleation of the calcium salts, and a number of studies have examined the role of lipids in calcium oxalate monohydrate (COM) formation. In this study, products of lipid hydrolysis from phospholipase A(2) (PLA(2)) are examined for their effect on COM formation using Langmuir monolayers as model lipid membrane assemblies. The enzyme PLA(2) hydrolyzes DPPC monolayers in the presence of a supersaturated calcium oxalate subphase, inducing the rapid and plentiful nucleation of calcium oxalate at the lipid interface. To investigate the cause of increased crystal formation in the presence of the enzyme, Langmuir monolayers modeling the hydrolysis products were investigated. Calcium oxalate crystal growth at a ternary monolayer of dipalmitoylphosphatidylcholine (DPPC), palmitic acid (PA), and a 22-carbon chain lysophospholipid (22:0 Lyso PC) dramatically increases relative to monolayers of just DPPC. Binary monolayers of DPPC with either PA or the 22:0 Lyso PC and single-component monolayers of PA were also studied. It is demonstrated that the fatty acid generated during lipid hydrolysis causes a significant increase in the extent of heterogeneous nucleation of calcium oxalate from supersaturated solutions. The results imply a possible link between increased phospholipase activity, which is associated with hyperoxaluria, and calcium oxalate precipitation.

In-plane homogeneity and lipid dynamics in tethered bilayer lipid membranes (tBLMs)

Tethered bilayer lipid membranes (tBLMs) were prepared by the self-assembly of thiolated lipidic anchor molecules on gold, followed by phospholipid precipitation via rapid solvent exchange. They were characterized by their in-plane structure, dynamics and dielectric properties. We find that the in-plane homogeneity and resistivity of the tBLMs depend critically on a well-controlled sample environment during the rapid solvent-exchange procedure. The in-plane dynamics of the systems, assessed by fluorescence correlation spectroscopy (FCS) as the diffusivity of free, labeled phospholipid dissolved in the membrane, depend on the density of the lipidic anchors in the bilayer leaflet proximal to the substrate as well as on details of the molecular structure of the anchor lipid. In DOPC tBLMs in which tethers are laterally dilute (sparsely tethered bilayer lipid membranes, stBLMs), measured diffusivities, D ≈ 4 μm(2) s(-1), are only slightly greater than those reported in physisorbed bilayers (M. Przybylo, J. Sykora, J. Humpolíckova, A. Benda, A. Zan and M. Hof, Langmuir, 2006, 22, 9096-9099). However, when we distinguish label diffusion in the proximal and in the distal bilayer leaflets, we observe distinct diffusivities, D ≈ 2 μm(2) s(-1) and 7 μm(2) s(-1), respectively. The value observed in the distal leaflet is identical to that in free membranes. stBLMs completed with phytanoyl lipids (DPhyPC) show consistently lower label diffusivity than those completed with unsaturated chains (DOPC). As the length of the tether chain increases, a reduction in the apparent diffusivity is observed, which we interpret as an increased propensity of the proximal bilayer leaflet to host free lipid. We also investigated preparation conditions that control whether the tBLMs are laterally homogeneous, as assessed by optical microscopy. In laterally heterogeneous bilayers, the label diffusivity varies only by a factor of ~2 to 4, indicating that the regions in the bilayers with different label solubilities do not correspond to distinct phases, such as a fluid phase coexisting with a gel phase.

Activation of phospholipase A2 by temporin B: formation of antimicrobial peptide-enzyme amyloid-type cofibrils

Phospholipases A2 have been shown to be activated in a concentration dependent manner by a number of antimicrobial peptides, including melittin, magainin 2, indolicidin, and temporins B and L. Here we used fluorescently labelled bee venom PLA2 (PLA2D) and the saturated phospholipid substrate 1,2-dipalmitoyl-glycero-sn-3-phosphocholine (L-DPPC), exhibiting a lag-burst behaviour upon the initiation of the hydrolytic reaction by PLA2. Increasing concentrations of Cys-temporin B and its fluorescent Texas red derivative (TRC-temB) caused progressive shortening of the lag period. TRC-temB/PLA2D interaction was observed by Förster resonance energy transfer (FRET), with maximum efficiency coinciding with the burst in hydrolysis. Subsequently, supramolecular structures became visible by microscopy, revealing amyloid-like fibrils composed of both the activating peptide and PLA2. Reaction products, palmitic acid and 1-palmitoyl-2-lyso-glycero-sn-3-phosphocholine (lysoPC, both at >8 mol%) were required for FRET when using the non-hydrolysable substrate enantiomer 2,3-dipalmitoyl-glycero-sn-1-phosphocholine (D-DPPC). A novel mechanism of PLA2 activation by co-fibril formation and associated conformational changes is suggested.

Aggregation of a peptide antibiotic alamethicin at the air-water interface and its influence on the viscoelasticity of phospholipid monolayers

The aggregation properties of an antibiotic membrane-active peptide alamethicin at the air-water interface have been studied using interfacial rheology and fluorescence microscopy techniques. Fluorescence microscopy of alamethicin monolayers revealed a coexistence of liquid expanded (LE) and solid phases at the surface concentrations studied. Interfacial oscillatory shear measurements on alamethicin monolayers indicate that its viscoelastic properties are determined by the area fraction of the solid domains. The role of zwitterionic phospholipids dioleoylphosphatidyl choline (DOPC) and dioleoylphosphatidyl ethanolamine (DOPE) on the peptide aggregation behavior was also investigated. Fluorescence microscopy of alamethicin/phospholipid monolayers revealed an intermediate phase (I) in addition to the solid and LE phase. In mixed monolayers of phospholipid (L)/alamethicin (P), with increase in L/P, the monolayer transforms from a viscoelastic to a viscous fluid with the increase in area fraction of the intermediate phase. Further, a homogeneous mixing of alamethicin/lipid molecules is observed at L/P > 4. Our studies also confirm that the viscoelasticity of alamethicin/phospholipid monolayers is closely related to the alamethicin/phospholipid interactions at the air-water interface.

Amyloid-type fiber formation in control of enzyme action: interfacial activation of phospholipase A2

The lag-burst behavior in the action of phospholipase A(2) (PLA(2)) on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine was investigated at temperatures slightly offset from the main phase transition temperature T(m) of this lipid, thus slowing down the kinetics of the activation process. Distinct stages leading to maximal activity were resolved using a combination of fluorescence parameters, including Förster resonance energy transfer between donor- and acceptor-labeled enzyme, fluorescence anisotropy, and lifetime, as well as thioflavin T fluorescence enhancement. We showed that the interfacial activation of PLA(2), evident after the preceding lag phase, coincides with the formation of oligomers staining with thioflavin T and subsequently with Congo red. Based on previous studies and our findings here, we propose a novel mechanism for the control of PLA(2), involving amyloid protofibrils with highly augmented enzymatic activity. Subsequently, these protofibrils form "mature" fibrils, devoid of activity. Accordingly, the process of amyloid formation is used as an on-off switch to obtain a transient burst in enzymatic catalysis.

Organization, structure and activity of proteins in monolayers

Many different processes take place at the cell membrane interface. Indeed, for instance, ligands bind membrane proteins which in turn activate peripheral membrane proteins, some of which are enzymes whose action is also located at the membrane interface. Native cell membranes are difficult to use to gain information on the activity of individual proteins at the membrane interface because of the large number of different proteins involved in membranous processes. Model membrane systems, such as monolayers at the air-water interface, have thus been extensively used during the last 50 years to reconstitute proteins and to gain information on their organization, structure and activity in membranes. In the present paper, we review the recent work we have performed with membrane and peripheral proteins as well as enzymes in monolayers at the air-water interface. We show that the structure and orientation of gramicidin has been determined by combining different methods. Furthermore, we demonstrate that the secondary structure of rhodopsin and bacteriorhodopsin is indistinguishable from that in native membranes when appropriate conditions are used. We also show that the kinetics and extent of monolayer binding of myristoylated recoverin is much faster than that of the nonmyristoylated form and that this binding is highly favored by the presence polyunsaturated phospholipids. Moreover, we show that the use of fragments of RPE65 allow determine which region of this protein is most likely involved in membrane binding. Monomolecular films were also used to further understand the hydrolysis of organized phospholipids by phospholipases A2 and C.

Coupling of the orientations of thermotropic liquid crystals to protein binding events at lipid-decorated interfaces

We report a study of the interactions of proteins with monolayers of phospholipids (D/L-alpha-dipalmitoyl phosphatidylcholine and L-alpha-dilauroyl phosphatidylcholine) spontaneously assembled at an interface between an aqueous phase and a 20-microm-thick film of a nematic liquid crystal (4'-pentyl-4-cyanobiphenyl). Because the orientation of the liquid crystal is coupled to the organization of the lipids, specific interactions between phospholipase A2 and the lipids (binding and/or hydrolysis) that lead to reorganization of the lipids are optically reported (using polarized light) as dynamic orientational transitions in the liquid crystal. In contrast, nonspecific interactions between proteins such as albumin, lysozyme, and cytochrome-c and the lipid-laden interface of the liquid crystal are not reported as orientational transitions in the liquid crystals. Concurrent epifluorescence and polarized light imaging of labeled lipids and proteins at the aqueous-liquid crystal interface demonstrate that spatially patterned orientations of the liquid crystals observed during specific binding of phospholipase A2 to the interface, as well as during the subsequent hydrolysis of lipids by phospholipase A2, reflect the lateral organization (micrometer-sized domains) of the proteins and lipids, respectively, at the aqueous-liquid crystal interface.

Enzyme activity to augment the characterization of tethered bilayer membranes

The rate of Ca2+ -triggered phospholipase A2 (PLA2) degradation of tethered bilayer membranes (tBLMs), composed of a synthetic lipid, beta-mercaptoethanol, and palmitoyloleoylphosphatidylcholine (POPC), is approximately 80 times greater than for those prepared with diphytanoylphosphatidylcholine (DPhyPC). Electrochemical impedance spectroscopy (EIS) and neutron reflectivity (NR) data indicate complete, water-free tBLMs that exhibit near ideal capacitive behavior and the presence of a water reservoir in the bilayer subspace proximal to the substrate (Au) surface for both tBLMs. Together these data indicate that the POPC and the DPhyPC tBLMs are structurally similar along the surface normal but markedly different at the outer leaflet/solution interface and that PLA2 is a sensitive probe of short length scale structural differences not revealed by EIS and NR.

The initial surface composition and topography modulate sphingomyelinase-driven sphingomyelin to ceramide conversion in lipid monolayers

Changes of the initial composition and topography of mixed monolayers of Sphingomyelin and Ceramide modulate the degradation of Sphingomyelin by Bacillus cereus Sphingomyelinase. The presence of initial lateral phase boundary due to coexisting condensed and expanded phase domains favors the precatalytic steps of the reaction. The amount and quality of the domain lateral interface, defined by the type of boundary undulation, appears as a modulatory supramolecular code which regulates the catalytic efficiency of the enzyme. The long range domain lattice structuring is determined by the Sphingomyelinase activity.

Impact of aluminum on the oxidation of lipids and enzymatic lipolysis in monomolecular films at the air/water interface

There is evidence that serious pathologies are associated with aluminum (Al). In the present work, the influence of Al on enzymatic lipolysis was studied with the aim to get more insight into the possible link between the Al-induced membrane modification and the cytotoxicity of the trivalent cation (AlIII). Lipid monolayers were used as model membranes. The monomolecular film technique allowed monitoring the Al-dependent modifications of the lipid monolayer properties and enzyme kinetics. Two enzymes, namely, Candida rugosa lipase and a calcium (CaII)-dependent phospholipase A2 from porcine pancreas, were used to catalyze the lipolysis of triglyceride and phosphoglyceride monolayers, respectively. The results obtained show that Al modifies both the monolayer structure and enzymatic reaction rates. While the enzymes used in this study can be considered as probes detecting lipid membrane properties, it cannot be excluded that in physiological conditions modulation of the enzyme action by the Al-bound membranes is among the reasons for Al toxicity.

Hydrolysis characterization of phospholipid monolayers catalyzed by different phospholipases at the air-water interface

Combination of some newly developed microscopic and spectroscopic techniques with conventional Langmuir monolayer method can provide more quantitative information with the molecular orientation and reorganization process of spread amphiphilic molecules at the air/water interface. These techniques are extended to investigate the hydrolysis process of spreading lipid monolayer catalyzed by different enzymes, phospholipases A2, C and D, respectively. Synchrotron X-ray diffraction and infrared reflection absorption spectroscopy are able directly to give the structural information of the assembled monolayer, interfacial activity of amphiphiles and their components at the interface. Microscopic technique such as Brewster angle microscopy (BAM), fluorescence microscopy (FM) can be used to trace the morphological changes dynamically as the spreading lipid monolayer is hydrolyzed at the air/water interface. We summary here some latest progress in this filed and give a brief review over the hydrolysis features of phospholipid monolayer catalyzed by different enzymes. It is attempted to establish a model of membrane hydrolysis process in order to better understand the mechanism of membrane metabolism and signal transduction in a living system.

Interactions of l-arginine with Langmuir monolayers of di-n-dodecyl hydrogen phosphate at the air–water interface

The surface phase behavior of di-n-dodecyl hydrogen phosphate (DDP) in Langmuir monolayer and its interactions with L-arginine (L-arg) have been investigated by measuring pi-A isotherms with a film balance and observing monolayer morphology with a Brewster angle microscopy (BAM). The DDP monolayers on pure water show a first-order liquid expanded-liquid condensed (LE-LC) phase transition and form fingering LC domains having uniform brightness at different temperatures. At 15 degrees C, the pi-A isotherms on pure water and on different concentration solutions of L-arg show a limiting molecular area at approximately 0.50 nm(2)/molecule. With increasing the subphase concentration of L-arg up to 4.0 x 10(-4)M, the LE and the LE-LC coexistence regions shift to larger molecular areas and higher surface pressures, respectively. With a further increase in the concentration of L-arg beyond this critical concentration, these isotherms show little or no more expansion. These results have been explained by considering the fact that the L-arg undergoes complexation with the DDP to form L-arg-DDP that remains in equilibrium with the components at the air-water interface. As the concentration of L-arg in the subphase increases, the equilibrium shifts towards the complex. At a concentration of L-arg > or =4.0 x 10(-4)M, the DDP monolayers get saturated and show the characteristics of the new amphiphile, L-arg-DDP. BAM is applied to confirm the above results. When the concentration of the L-arg is <4.0 x 10(-4)M, domains always start forming at an area of approximately 0.64 nm(2)/molecule, which is the critical molecular area for the phase transition in the DDP monolayers on pure water. In contrast, when the monolayers are formed on a solution containing > or =4.0 x 10(-4)M L-arg, comparatively smaller size domains are formed after the appearance of a new cusp point at approximately 0.55 nm(2)/molecule. With an increase in the concentration of L-arg in the subphase, the size of the domains decreases indicating that the fraction of the DDP gradually decreases, whereas the fraction of the complex gradually increases. In addition, a very simple procedure for determination of the stability constant, which is 2.6 x 10(4)M(-1) at 15 degrees C, has been suggested.

Externally applied electric fields on immiscible lipid monolayers: repulsion between condensed domains precludes domain migration

Lipid and protein molecules anisotropically oriented at a hydrocarbon-aqueous interface configure a dynamic array of self-organized molecular dipoles. Electrostatic fields applied to lipid monolayers have been shown to induce in-plane migration of domains or phase separation in a homogeneous system. In this work, we have investigated the effect of externally applied electrostatic fields on different lipid monolayers exhibiting surface immiscibility. In the monolayers studied, lipids in the condensed state segregate in discontinuous round-shaped domains, with the lipid in the liquid-expanded state forming the continuous phase. The use of fluorescent probes with selective phase partitioning allows analyzing by epifluorescence microscopy the migrations of the domains under the influence of inhomogeneous electric fields applied to the surface. Our observations indicate that a positive potential applied to an electrode placed over the monolayer promotes a repulsion of the domains until a steady state is reached, indicating the presence of a force opposed to the externally applied electric force. The experimental results were modeled by considering that the opposing force is generated by the dipole-dipole repulsion between the domains.

Atomic force microscope visualization of lipid bilayer degradation due to action of phospholipase A2 and Humicola lanuginosa lipase

An important application of liquid cell Atomic Force Microscopy (AFM) is the study of enzyme structure and behaviour in organized molecular media that mimic in-vivo systems. In this study we demonstrate the use of AFM as a tool to study the kinetics of lipolytic enzyme reactions occurring at the surface of a supported lipid bilayer. In particular, the time course of the degradation of lipid bilayers by Phospholipase A(2) (PLA(2)) and Humicola Lanuginosa Lipase (HLL) has been investigated. Contact mode imaging allows visualization of enzyme activity on the substrate with high lateral resolution. Lipid bilayers were prepared by the Langmuir-Blodgett technique and transferred to an AFM liquid cell. Following injection of the enzyme into the liquid cell, a sequence of images was acquired at regular time intervals to allow the identification of substrate structure, preferred sites of enzyme activation, and enzyme reaction rates.

Effect of externally applied electrostatic fields on the surface topography of ceramide-enriched domains in mixed monolayers with sphingomyelin

Lipid and protein molecules anisotropically oriented at a hydrocarbon-aqueous interface configure a dynamic array of self-organized molecular dipoles. Electrostatic fields applied to lipid monolayers have been shown to induce in-plane migration of domains or phase separation in a homogeneous system. In this work, we have investigated the effect of externally applied electrostatic fields on the distribution of the condensed ceramide-enriched domains in mixed monolayers with sphingomyelin. In these monolayers, the lipids segregate in different phases at all pressures. This allows analyzing by epifluorescence microscopy the effect of the electrostatic field at all lateral pressure because coexistence of lipid domains in condensed state are always present. Our observations indicate that a positive potential applied to an electrode placed over the monolayer promotes a repulsion of the ceramide-enriched domains which is rather insensitive to the film composition, depends inversely on the lateral pressure and exhibits threshold dependence on the in-plane elasticity.

Evidence that nystatin channels form at the boundaries, not the interiors of lipid domains

Nystatin (nys) is an antifungal agent that preferentially forms ion channels in membranes containing the sterol, ergosterol (erg). The structure of the nystatin channel is not clear, but it is known that multiple nystatin monomers must aggregate to form channels in a sterol-rich membrane. When nys/erg containing vesicles are fused to a sterol-free bilayer, characteristic spikelike changes in membrane conductance are observed. An abrupt increase in conductance is followed by a decay that is generally stepwise linear and the decay time depends strongly on [erg]. These data are inconsistent with the hypothesis that nys channels form uniformly throughout the membrane and decay independently (which would produce exponential decay). We propose that channels are located at the boundaries of lipid superlattices such that diffusion of erg out of the lattice results in correlated channel decay. This was tested using a statistical mechanical analysis and Monte Carlo simulations, which reveal details of the diffusion process and provide insight into conditions at superlattice boundaries during decay. This analysis predicts the linear decay schemes and the dramatic drop in channel decay time observed at erg mol % = 50. This interpretation also explains puzzling data relating conductance spike height to vesicle diameter.

Mechanisms of antimicrobial peptide action: Studies of indolicidin assembly at model membrane interfaces by in situ atomic force microscopy

We report here on an in situ atomic force microscopy study of the interaction of indolicidin, a tryptophan-rich antimicrobial peptide, with phase-segregated zwitterionic DOPC/DSPC supported planar bilayers. By varying the peptide concentration and bilayer composition through the inclusion of anionic lipids (DOPG or DSPG), we found that indolicidin interacts with these model membranes in one of two concentration-dependent manners. At low peptide concentrations, indolicidin forms an amorphous layer on the fluid domains when these domains contain anionic lipids. At high peptide concentrations, indolicidin appears to initiate a lowering of the gel-phase domains independent of the presence of an anionic lipid. Similar studies performed using membrane-raft mimetic bilayers comprising 30mol% cholesterol/1:1 DOPC/egg sphingomyelin revealed that indolicidin does not form a carpet-like layer on the zwitterionic DOPC domains at low peptide concentrations and does not induce membrane lowering of the liquid-ordered sphingomyelin/cholesterol-rich domains at high peptide concentration. Simultaneous AFM-confocal microscopy imaging did however reveal that indolicidin preferentially inserts into the fluid-phase DOPC domains. These data suggest that the indolicidin-membrane association is influenced greatly by specific electrostatic interactions, lipid fluidity, and peptide concentration. These insights provide a glimpse into the mechanism of the membrane selectivity of antibacterial peptides and suggest a powerful correlated approach for characterizing peptide-membrane interactions.

Lipid fluorination enables phase separation from fluid phospholipid bilayers

To probe the effect of lipid fluorination on the formation of lipid domains in phospholipid bilayers, several new fluorinated and non-fluorinated synthetic lipids were synthesised, and the extent of phase separation of these lipids from phospholipid bilayers of different compositions was determined. At membrane concentrations as low as 1% mol/mol, both fluorinated and non-fluorinated lipids were observed to phase separate from a gel-phase (solid ordered) phospholipid matrix, but bilayers in a liquid disordered state caused no phase separation; if the gel-phase samples were heated above the transition temperature, then phase separation was lost. We found incorporation of perfluoroalkyl groups into the lipid enhanced phase separation, to such an extent that phase separation was observed from cholesterol containing bilayers in the liquid ordered phase.

Protein-mediated surface structuring in biomembranes

The lipids and proteins of biomembranes exhibit highly dissimilar conformations, geometrical shapes, amphipathicity, and thermodynamic properties which constrain their two-dimensional molecular packing, electrostatics, and interaction preferences. This causes inevitable development of large local tensions that frequently relax into phase or compositional immiscibility along lateral and transverse planes of the membrane. On the other hand, these effects constitute the very codes that mediate molecular and structural changes determining and controlling the possibilities for enzymatic activity, apposition and recombination in biomembranes. The presence of proteins constitutes a major perturbing factor for the membrane sculpturing both in terms of its surface topography and dynamics. We will focus on some results from our group within this context and summarize some recent evidence for the active involvement of extrinsic (myelin basic protein), integral (Folch-Lees proteolipid protein) and amphitropic (c-Fos and c-Jun) proteins, as well as a membrane-active amphitropic phosphohydrolytic enzyme (neutral sphingomyelinase), in the process of lateral segregation and dynamics of phase domains, sculpturing of the surface topography, and the bi-directional modulation of the membrane biochemical reactivity.

Hydrolysis reaction analysis of L-alpha-distearoylphosphatidylcholine monolayer catalyzed by phospholipase A2 with polarization-modulated infrared reflection absorption spectroscopy

The hydrolysis reaction of L-alpha-distearoylphosphatidylcholine (DSPC) monolayers catalyzed by phospholipase A2 (PLA(2)) has been studied using polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) with film balance measurements. The PM-IRRAS analysis provides quantitative information about the reaction efficiency at different surface pressures. It was found that the reaction efficiency of L-DSPC monolayer hydrolysis catalyzed by PLA(2) decreased with increasing surface pressure. At zero pressure (lift-off point), the hydrolysis reaction efficiency has the highest value of 45%. Increasing surface pressure leads to the decrease of the hydrolysis efficiency. Since the surface pressure is above 20 mN/m, the hydrolysis reaction nearly stopped. PM-IRRAS technique provides a powerful means to study the hydrolysis process catalyzed by phospholipase A2 at the air/water interface.

c-Fos and phosphatidylinositol-4,5-bisphosphate reciprocally reorganize in mixed monolayers

The transcription factor c-Fos has surface thermodynamic properties that allow it to differentially interact with phospholipids, especially PIP2. It regulates phospholipid metabolism both in vivo and in vitro, and modulates degradation of phospholipid monolayers by phospholipases in a way that depends on the membrane intermolecular packing (i.e., surface lateral pressure). With the aim to understand details of the interactions of c-Fos at the membrane level, we studied the surface packing, dipole potential, compressibility and topography of mixed films of the protein with PIP2. We show that c-Fos changes the packing of liquid-expanded PIP2 monolayers, in a different manner with respect to its effect on the similarly liquid-expanded dilauroylphosphatidylcholine monolayers. The changes at the local molecular level are transduced to long-range inhomogeneities of the surface, detected by Brewster angle (BAM) and epifluorescence microscopy (EFM). Our results highlight the capacity of c-Fos to alter the packing and dipole potential of the lipid-protein interface. This involves variations of the surface in-plane elasticity and lateral segregation of phase domains. These dynamic, reversible alterations of surface organization provide a basis by which c-Fos may transduce molecular information at the membrane level.

Shape transitions and lattice structuring of ceramide-enriched domains generated by sphingomyelinase in lipid monolayers

Sphingomyelinases (SMases) hydrolyze the membrane constituent sphingomyelin (SM) to phosphocholine and ceramide (Cer). Growing evidence supports that SMase-induced SM-->Cer conversion leads to the formation of lateral Cer-enriched domains which drive structural reorganization in lipid membranes. We previously provided visual evidence in real-time for the formation of Cer-enriched domains in SM monolayers through the action of the neutral Bacillus cereus SMase. In this work, we disclose a succession of discrete morphologic transitions and lateral organization of Cer-enriched domains that underlay the SMase-generated surface topography. We further reveal how these structural parameters couple to the generation of two-dimensional electrostatic fields, based upon the specific orientation of the lipid dipole moments in the Cer-enriched domains. Advanced image processing routines in combination with time-resolved epifluorescence microscopy on Langmuir monolayers revealed: 1), spontaneous nucleation and circular growth of Cer-enriched domains after injection of SMase into the subphase of the SM monolayer; 2), domain-intrinsic discrete transitions from circular to periodically undulating shapes followed by a second transition toward increasingly branched morphologies; 3), lateral superstructure organization into predominantly hexagonal domain lattices; 4), formation of super-superstructures by the hexagonal lattices; and 5), rotationally and laterally coupled domain movement before domain border contact. All patterns proved to be specific for the SMase-driven system since they could not be observed with Cer-enriched domains generated by defined mixtures of SM/Cer in enzyme-free monolayers at the same surface pressure (pi = 10 mN/m). Following the theories of lateral shape transitions, dipolar electrostatic interactions of lipid domains, and direct determinations of the monolayer dipole potential, our data show that SMase induces a domain-specific packing and orientation of the molecular dipole moments perpendicular to the air/water interface. In consequence, protein-driven generation of specific out-of-equilibrium states, an accepted concept for maintenance of transmembrane lipid asymmetry, must also be considered on the lateral level. Lateral enzyme-specific out-of-equilibrium organization of lipid domains represents a new level of signal transduction from local (nm) to long-range (microm) scales. The cross-talk between lateral domain structures and dipolar electrostatic fields adds new perspectives to the mechanisms of SMase-mediated signal transduction in biological membranes.

Chlorpromazine interaction with phosphatidylserines: A 13C and 31P solid-state NMR study

Chlorpromazine (CPZ), a cationic, amphiphilic phenothiazine derivative is widely used as an antipsychotic drug because it antagonizes dopaminergic receptors. (13)C and (31)P solid-state NMR techniques were employed on phospholipid bilayers with and without CPZ. Phosphatidylserine from pig brain (PBPS), 1-palmitoyl-2-oleoyl phosphatidylserine (POPS), synthetic 1,2-dipalmitoyl phosphatidylcholine (DPPC) and chlorpromazineHCl were used to make phospholipid bilayers containing two types of phospholipids: DPPC (60%)/PBPS (40%) as well as POPS and PBPS bilayers without and with 10% CPZ. CPZ is found to prefer binding to the phosphate of phosphatidylserine, but also binding to the carboxyl of the serine head group in the DPPC/PBPS/CPZ bilayer is present. (31)P-NMR spectra indicate an effect of acyl chain unsaturation on the anisotropic motion of the charged serine head group. This implies that the serine head group anisotropic motion is restricted by intermolecular rather than intramolecular effects. The degree of phospholipid acyl chain unsaturation determines part of the CPZ bilayer interaction. The PBPS bilayer has the 22:6 acyl chain at 34 mol% and the C(4)?C(5) group of this acyl appears to be a determinant for CPZ bilayer interdigitation.