Activation of interfacial enzymes at membrane surfaces

Enzyme-responsive liposomes for controlled drug release

Compared to other nanovectors, liposomes exhibit unique advantages, such as good biosafety and high drug-loading capacity. However, slow drug release from conventional liposomes makes most payloads unavailable, restricting the therapeutic efficacy. Therefore, in the last ∼20 years, enzyme-responsive liposomes have been extensively investigated, which liberate drugs under the stimulation of enzymes overexpressed at disease sites. In this review, we elaborate on the research progress on enzyme-responsive liposomes. The involved enzymes mainly include phospholipases, particularly phospholipase A2, matrix metalloproteinases, cathepsins, and esterases. These enzymes can cleave ester bonds or specific peptide sequences incorporated in the liposomes for controlled drug release by disrupting the primary structure of liposomes, detaching protective polyethylene glycol shells, or activating liposome-associated prodrugs. Despite decades of efforts, there are still a lack marketed products of enzyme-responsive liposomes. Therefore, more efforts should be made to improve the safety and effectiveness of enzyme-responsive liposomes and address the issues associated with production scale-up.

Melittin-Phospholipase A2 Synergism Is Mediated by Liquid-Liquid Miscibility Phase Transition in Giant Unilamellar Vesicles

The primary constituents of honeybee venom, melittin and phospholipase A2 (PLA2), display toxin synergism in which the PLA2 activity is significantly enhanced by the presence of melittin. It has been shown previously that this is accomplished by the disruption in lipid packing, which allows PLA2 to become processive on the membrane surface. In this work, we show that melittin is capable of driving miscibility phase transition in giant unilamellar vesicles (GUVs) and that it raises the miscibility transition temperature (Tmisc) in a concentration-dependent manner. The induced phase separation enhances the processivity of PLA2, particularly at its boundaries, where a substantial difference in domain thickness creates a membrane discontinuity. The catalytic action of PLA2, in response, induces changes in the membrane, rendering it more conducive to melittin binding. This, in turn, facilitates further lipid phase separation and eventual vesicle lysis. Overall, our results show that melittin has powerful membrane-altering capabilities that activate PLA2 in various membrane contexts. More broadly, they exemplify how this biochemical system actively modulates and capitalizes on the spatial distribution of membrane lipids to efficiently achieve its objectives.

Phospholipases and Membrane Curvature: What Is Happening at the Surface?

In this revision work, we emphasize the close relationship between the action of phospholipases and the modulation of membrane curvature and curvature stress resulting from this activity. The alteration of the tridimensional structure of membranes upon the action of phospholipases is analyzed based on studies on model lipid membranes. The transient unbalance of both compositional and physical membrane properties between the hemilayers upon phospholipase activity lead to curvature tension and the catalysis of several membrane-related processes. Several proteins' membrane-bound and soluble forms are susceptible to regulation by the curvature stress induced by phospholipase action, which has important consequences in cell signaling. Additionally, the modulation of membrane fusion by phospholipase products regulates membrane dynamics in several cellular scenarios. We commented on vesicle fusion in the Golgi-endoplasmic system, synaptic vesicle fusion to the plasma membrane, viral membrane fusion to host cell plasma membrane and gametes membrane fusion upon acrosomal reaction. Furthermore, we explored the modulation of membrane fusion by the asymmetric adsorption of amphiphilic drugs. A deep understanding of the relevance of lipid membrane structure, particularly membrane curvature and curvature stress, on different cellular events leads to the challenge of its regulation, which may become a powerful tool for pharmacological therapy.

Persistent organic pollutants in model fungal membranes. Effects on the activity of phospholipases

Soils are the final sink for multiple organic pollutants emitted to the environment. Some of these chemicals which are toxic, recalcitrant and can bioaccumulate in living organism and biomagnify in trophic chains are classified persistent organic pollutants (POP). Vast areas of arable land have been polluted by POPs and the only economically possible means of decontamination is bioremediation, that is the utilization of POP-degrading microbes. Especially useful can be non-ligninolytic fungi, as their fast-growing mycelia can reach POP molecules strongly bond to soil minerals or humus fraction inaccessible to bacteria. The mobilized POP molecules are incorporated into the fungal plasma membrane where their degradation begins. The presence of POP molecules in the membranes can change their physical properties and trigger toxic effects to the cell. To avoid these phenomena fungi can quickly remodel the phospholipid composition of their membrane with employing different phospholipases and acyltransferases. However, if the presence of POP downregulates the phospholipases, toxic effects and the final death of microbial cells are highly probable. In our studies we applied multicomponent Langmuir monolayers with their composition mimicking fungal plasma membranes and studied their interactions with two different microbial phospholipases: phospholipase C (α-toxin) and phospholipase A1 (Lecitase ultra). The model membranes were doped with selected POPs that are frequently found in contaminated soils. It turned out that most of the employed POPs do not downregulate considerably the activity of phospholipases, which is a good prognostics for the application of non-ligninolytic fungi in bioremediation.

Evaluation of the Physico-Chemical Properties of Liposomes Assembled from Bioconjugates of Anisic Acid with Phosphatidylcholine

The aim of this work was the evaluation of the physico-chemical properties of a new type of liposomes that are composed of DPPC and bioconjugates of anisic acid with phosphatidylcholine. In particular, the impact of modified anisic acid phospholipids on the thermotropic parameters of liposomes was determined, which is crucial for using them as potential carriers of active substances in cancer therapies. Their properties were determined using three biophysical methods, namely differential scanning calorimetry (DSC), steady-state fluorimetry and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Moreover, temperature studies of liposomes composed of DPPC and bioconjugates of anisic acid with phosphatidylcholine provided information about the phase transition, fluidity regarding chain order, hydration and dynamics. The DSC results show that the main phase transition peak for conjugates of anisic acid with phosphatidylcholine molecules was broadened and shifted to a lower temperature in a concentration- and structure-dependent manner. The ATR-FTIR results and the results of measurements conducted using fluorescent probes located at different regions in the lipid bilayer are in line with DSC. The results show that the new bioconjugates with phosphatidylcholine have a significant impact on the physico-chemical properties of a membrane and cause a decrease in the temperature of the main phase transition. The consequence of this is greater fluidity of the lipid bilayer.

Explicit Treatment of Non-Michaelis-Menten and Atypical Kinetics in Early Drug Discovery*

Biological systems are highly regulated. They are also highly resistant to sudden perturbations enabling them to maintain the dynamic equilibrium essential to sustain life. This robustness is conferred by regulatory mechanisms that influence the activity of enzymes/proteins within their cellular context to adapt to changing environmental conditions. However, the initial rules governing the study of enzyme kinetics were mostly tested and implemented for cytosolic enzyme systems that were easy to isolate and/or recombinantly express. Moreover, these enzymes lacked complex regulatory modalities. Now, with academic labs and pharmaceutical companies turning their attention to more-complex systems (for instance, multiprotein complexes, oligomeric assemblies, membrane proteins and post-translationally modified proteins), the initial axioms defined by Michaelis-Menten (MM) kinetics are rendered inadequate, and the development of a new kind of kinetic analysis to study these systems is required. This review strives to present an overview of enzyme kinetic mechanisms that are atypical and, oftentimes, do not conform to the classical MM kinetics. Further, it presents initial ideas on the design and analysis of experiments in early drug-discovery for such systems, to enable effective screening and characterisation of small-molecule inhibitors with desirable physiological outcomes.

Tumor microenvironment-induced structure changing drug/gene delivery system for overcoming delivery-associated challenges

Nano-drug/gene delivery systems (DDS) are powerful weapons for the targeted delivery of various therapeutic molecules in treatment of tumors. Nano systems are being extensively investigated for drug and gene delivery applications because of their exceptional ability to protect the payload from degradation in vivo, prolong circulation of the nanoparticles (NPs), realize controlled release of the contents, reduce side effects, and enhance targeted delivery among others. However, the specific properties required for a DDS vary at different phase of the complex delivery process, and these requirements are often conflicting, including the surface charge, particle size, and stability of DDS, which severely reduces the efficiency of the drug/gene delivery. Therefore, researchers have attempted to fabricate structure, size, or charge changeable DDS by introducing various tumor microenvironment (TME) stimuli-responsive elements into the DDS to meet the varying requirements at different phases of the delivery process, thus improving drug/gene delivery efficiency. This paper summarizes the most recent developments in TME stimuli-responsive DDS and addresses the aforementioned challenges.

Off to a slow start: Analyzing lag phases and accelerating rates in steady-state enzyme kinetics

Steady-state enzyme kinetics typically relies on the measurement of 'initial rates', obtained when the substrate is not significantly consumed and the amount of product formed is negligible. Although initial rates are usually faster than those measured later in the reaction time-course, sometimes the speed of the reaction appears instead to increase with time, reaching a steady level only after an initial delay or 'lag phase'. This behavior needs to be interpreted by the experimentalists. To assist interpretation, this article analyzes the many reasons why, during an enzyme assay, the observed rate can be slow in the beginning and then progressively accelerate. The possible causes range from trivial artifacts to instances in which deeper mechanistic or biophysical factors are at play. We provide practical examples for most of these causes, based firstly on experiments conducted with ornithine δ-aminotransferase and with other pyridoxal-phosphate dependent enzymes that have been studied in our laboratory. On the side to this survey, we provide evidence that the product of the ornithine δ-aminotransferase reaction, glutamate 5-semialdehyde, cyclizes spontaneously to pyrroline 5-carboxylate with a rate constant greater than 3 s^-1.

Building a synthetic mechanosensitive signaling pathway in compartmentalized artificial cells

To date, reconstitution of one of the fundamental methods of cell communication, the signaling pathway, has been unaddressed in the bottom-up construction of artificial cells (ACs). Such developments are needed to increase the functionality and biomimicry of ACs, accelerating their translation and application in biotechnology. Here, we report the construction of a de novo synthetic signaling pathway in microscale nested vesicles. Vesicle-cell models respond to external calcium signals through activation of an intracellular interaction between phospholipase A2 and a mechanosensitive channel present in the internal membranes, triggering content mixing between compartments and controlling cell fluorescence. Emulsion-based approaches to AC construction are therefore shown to be ideal for the quick design and testing of new signaling networks and can readily include synthetic molecules difficult to introduce to biological cells. This work represents a foundation for the engineering of multicompartment-spanning designer pathways that can be utilized to control downstream events inside an AC, leading to the assembly of micromachines capable of sensing and responding to changes in their local environment.

The lipolytic degradation of highly structured cubic micellar nanoparticles of soy phosphatidylcholine and glycerol dioleate by phospholipase A2 and triacylglycerol lipase

The effects of different lipolytic enzymes on the structure of lipid liquid crystalline nano-particles (LCNP) have been investigated by cryogenic transmission electron microscopy (cryo-TEM) and synchrotron small angle X-ray diffraction (SAXD). Here we used highly structured cubic micellar (Fd3m) nanoparticles of 50/50 (wt%/wt%) soy phosphatidyl choline (SPC)/glycerol dioleate (GDO) as substrate. Two types of lipolytic enzymes were used, phospholipase A2 (PLA2) that catalyses degradation of the phospholipid component, SPC, and porcine pancreatic triacylglycerol lipase (TGL) that facilitate the hydrolysis of the diglyceride, GDO. Evolution of the structure was found to be very different and linked to specificity of the two types of enzymes. PLA2, which hydrolyses the lamellar forming component, SPC, induces a reversed micellar lipid phase, while TGL which hydrolysis the reverse phase forming compound, GDO, induces a lamellar phase.

Secretory phospholipase A2 responsive liposomes exhibit a potent anti-neoplastic effect in vitro, but induce unforeseen severe toxicity in vivo

The clinical use of liposomal drug delivery vehicles is often hindered by insufficient drug release. Here we present the rational design of liposomes optimized for secretory phospholipase A₂ (sPLA₂) triggered drug release, and test their utility in vitro and in vivo. We hypothesized that by adjusting the level of cholesterol in anionic, unsaturated liposomes we could tune the enzyme specificity based on membrane fluidity, thus obtaining liposomes with an improved therapeutic outcome and reduced side effects. Cholesterol is generally important as a component in the membranes of liposome drug delivery systems due to its stabilizing effects in vivo. The incorporation of cholesterol in sPLA₂ sensitive liposomes has not previously been possible due to reduced sPLA₂ activity. However, in the present work we solved this challenge by optimizing membrane fluidity. In vitro release studies revealed enzyme specific drug release. Treatment of two different cancer cell lines with liposomal oxaliplatin revealed efficient growth inhibition compared to that of clinically used stealth liposomes. The in vivo therapeutic effect was evaluated in nude NMRI mice using the sPLA₂ secreting mammary carcinoma cell line MT-3. Three days after first treatment all mice having received the novel sPLA₂ sensitive liposome formulation were euthanized due to severe systemic toxicity. Thus the present study demonstrates that great caution should be implemented when utilizing sPLA₂ sensitive liposomes and that the real utility can only be disclosed in vivo. The present studies have clinical implications, as sPLA₂ sensitive formulations are currently undergoing clinical trials (LiPlaCis®).

Enzyme-Responsive Liposomes for the Delivery of Anticancer Drugs

Liposomes are nanocarriers that deliver the payloads at the target site, leading to therapeutic drug concentrations at the diseased site and reduced toxic effects in healthy tissues. Several approaches have been used to enhance the ability of the nanocarrier to target the specific tissues, including ligand-targeted liposomes and stimuli-responsive liposomes. Ligand-targeted liposomes exhibit higher uptake by the target tissue due to the targeting ligand attached to the surface, while the stimuli-responsive liposomes do not release their cargo unless they expose to an endogenous or exogenous stimulant at the target site. In this review, we mainly focus on the liposomes that are responsive to pathologically increased levels of enzymes at the target site. Enzyme-responsive liposomes release their cargo upon contact with the enzyme through several destabilization mechanisms: (1) structural perturbation in the lipid bilayer, (2) removal of a shielding polymer from the surface and increased cellular uptake, (3) cleavage of a lipopeptide or lipopolymer incorporated in the bilayer, and (4) activation of a prodrug in the liposomes.

Interfacial activation of M37 lipase: A multi-scale simulation study

Lipases are enzymes of biotechnological importance that function at the interface formed between hydrophobic and aqueous environments. Hydrophobic interfaces can induce structural transitions in lipases that result in an increase in enzyme activity, although the detailed mechanism of this process is currently not well understood for many lipases. Here, we present a multi-scale molecular dynamics simulation study of how different interfaces affect the conformational dynamics of the psychrophilic lipase M37. Our simulations show that M37 lipase is able to interact both with anionic lipid bilayers and with triglyceride surfaces. Interfacial interactions with triglyceride surfaces promote large-scale motions of the lid region of M37, spanning residues 235-283, revealing an entry pathway to the catalytic site for substrates. Importantly, these results suggest a potential activation mechanism for M37 that deviates from other related enzymes, such as Thermomyces lanuginosus lipase. We also investigated substrate binding in M37 by using steered MD simulations, confirming the open state of this lipase. The exposure of hydrophobic residues within lid and active site flap regions (residues 94-110) during the activation process provides insights into the functional effect of hydrophobic surfaces on lipase activation.

Phospholipase A2-Induced Remodeling Processes on Liquid-Ordered/Liquid-Disordered Membranes Containing Docosahexaenoic or Oleic Acid: A Comparison Study

Vesicle cycling, which is an important biological event, involves the interplay between membrane lipids and proteins, among which the enzyme phospholipase A2 (PLA2) plays a critical role. The capacity of PLA2 to trigger the budding and fission of liquid-ordered (L(o)) domains has been examined in palmitoyl-docosahexaenoylphosphatidylcholine (PDPC) and palmitoyl-oleoylphosphatidylcholine (POPC)/sphingomyelin/cholesterol membranes. They both exhibited a L(o)/liquid-disordered (L(d)) phase separation. We demonstrated that PLA2 was able to trigger budding in PDPC-containing vesicles but not POPC ones. The enzymatic activity, line tension, and elasticity of the membrane surrounding the L(o) domains are critical for budding. The higher line tension of Lo domains in PDPC mixtures was assigned to the greater difference in order parameters of the coexisting phases. The higher amount of lysophosphatidylcholine generated by PLA2 in the PDPC-containing mixtures led to a less-rigid membrane, compared to POPC. The more elastic L(d) membranes in PDPC mixtures exert a lower counteracting force against the L(o) domain bending.

Beyond apoptosis: the mechanism and function of phosphatidylserine asymmetry in the membrane of activating mast cells

Loss of plasma membrane asymmetry is a hallmark of apoptosis, but lipid bilayer asymmetry and loss of asymmetry can contribute to numerous cellular functions and responses that are independent of programmed cell death. Exofacial exposure of phosphatidylserine occurs in lymphocytes and mast cells after antigenic stimulation and in the absence of apoptosis, suggesting that there is a functional requirement for phosphatidylserine exposure in immunocytes. In this review we examine current ideas as to the nature of this functional role in mast cell activation. Mechanistically, there is controversy as to the candidate proteins responsible for phosphatidylserine translocation from the internal to external leaflet, and here we review the candidacies of mast cell PLSCR1 and TMEM16F. Finally we examine the potential relationship between functionally important mast cell membrane perturbations and phosphatidylserine exposure during activation.

Development of a cell-based bioassay for phospholipase A2-triggered liposomal drug release

The feasibility of exploiting secretory phospholipase A₂ (sPLA₂) enzymes, which are overexpressed in tumors, to activate drug release from liposomes precisely at the tumor site has been demonstrated before. Although the efficacy of the developed formulations was evaluated using in vitro and in vivo models, the pattern of sPLA₂-assisted drug release is unknown due to the lack of a suitable bio-relevant model. We report here on the development of a novel bioluminescence living-cell-based luciferase assay for the monitoring of sPLA₂-triggered release of luciferin from liposomes. To this end, we engineered breast cancer cells to produce both luciferase and sPLA₂ enzymes, where the latter is secreted to the extracellular medium. We report on setting up a robust and reproducible bioassay for testing sPLA₂-sensitive, luciferin remote-loaded liposomal formulations, using 1,2-distearoyl-sn-glycero-3-phosphatidylcholine/1,2-distearoyl-sn-glycero-3-phosphatidylglycerol (DSPC/DSPG) 7:3 and DSPC/DSPG/cholesterol 4:3:3 as initial test systems. Upon their addition to the cells, the liposomes were degraded almost instantaneously by sPLA₂ releasing the encapsulated luciferin, which provided readout from the luciferase-expressing cells. Cholesterol enhanced the integrity of the formulation without affecting its susceptibility to sPLA₂. PEGylation of the liposomes only moderately broadened the release profile of luciferin. The provided bioassay represents a useful tool for monitoring active drug release in situ in real time as well as for testing and optimizing of sPLA₂-sensitive lipid formulations. In addition, the bioassay will pave the way for future in-depth in vitro and in vivo studies.

Alpha-helical regions of the protein molecule as organic nanotubes

An α-helical region of protein molecule was considered in a model of nanotube. The molecule is in conditions of quantum excitations. Such model corresponds to a one-dimensional molecular nanocrystal with three molecules in an elementary cell at the presence of excitation. For the analysis of different types of conformational response of the α-helical area of the protein molecule on excitation, the nonlinear response of this area to the intramolecular quantum excitation caused by hydrolysis of adenosine triphosphate (ATP) is taken into account. It has been established that in the simplest case, three types of excitation are realized. As estimates show, each of them 'serves' different kinds of protein. The symmetrical type of excitation, most likely, is realized in the reduction of traversal-striped skeletal muscles. It has the highest excitation energy. This well protects from casual actions. Antisymmetric excitations have intermediate energy (between symmetrical and asymmetrical). They, most likely, are realized in membranous and nucleic proteins. It is shown that the conformational response of the α-helical region of the protein is (in angstroms) a quantity of order N c /5, where N c is the number of spiral turns. For the number of turns typical in this case: N c  ~ 10, displacement compounds are a quantity of order 2 Å. It qualitatively corresponds to observable values. Asymmetrical excitations have the lowest energy. Therefore, most likely, they are realized in enzymatic proteins. It was shown that at this type of excitation, the bending of the α-helix is formally directed to the opposite side with respect to the antisymmetric excitations. Also, it has a greater value than the antisymmetric case for N c  ≤ 14 and smaller for N c  > 14.

PACS

92C05.

MCS

36.20.Ey.

Neutron reflectivity of supported membranes incorporating terminally anchored polymers: Protrusions vs. blisters

The effect of terminally anchored chains on the structure of lipid bilayers adsorbed at the solid/water interface was characterized by neutron reflectivity. In the studied system, the inner leaflet, closer to the substrate, consisted of head-deuterated 1,2-distearoyl-sn-glycero-3-phosphorylcholine (DSPC) and the outer leaflet comprised a mixture of DSPC and polyethylene glycol (PEG) functionalized 1,2-distearoyl-sn-glycero-3-phosphoethanolamine. The DSPC headgroups were deuterated to enhance sensitivity and demarcate the bilayer/water interface. The effect on the inner and outer headgroup layers was characterized by w(1/2), the width at half-height of the scattering length density profile. The inner headgroup layer was essentially unperturbed while w(1/2) of the outer layer increased significantly. This suggests that the anchored PEG chains give rise to headgroup protrusions rather than to blister-like membrane deformations.

Phospholipase A(2)-susceptible liposomes of anticancer double lipid-prodrugs

A novel approach to anticancer drug delivery is presented based on lipid-like liposome-forming anticancer prodrugs that are susceptible to secretory phospholipase A(2) (sPLA(2)) that is overexpressed in several cancer types. The approach provides a selective unloading of anticancer drugs at the target tissues, as well as circumvents the necessity for "conventional" drug loading. In our attempts to improve the performance of the liposomes in vivo, several PEGylated and non-PEGylated liposomal formulations composed of a retinoid prodrug premixed with the sPLA(2)-hydrolyzable DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) were prepared. Besides favorably modifying the physicochemical properties of the liposomes, the incorporation of DPPC and PEG-lipids in the liposomes should substantially enhance the enzymatic activity, as concluded from literature. In addition, one can reap benefits from the presumed permeability enhancing effect of the liberated fatty acids and lysolipids. The size distribution of the prepared liposomes as well as their phase behavior, enzymatic hydrolysis, and cytotoxicity, in the presence and absence of sPLA(2), were determined. The liposomes were around 100nm in diameter and in the gel/fluid coexistence region at 37°C. The enzymatic hydrolysis of the prodrug was pronouncedly accelerated upon the premixing with DPPC, and the hydrolysis was further enhanced by PEGylation. Interestingly, the faster hydrolysis of the prodrug and the released fatty acids and lysolipids from DPPC did not improve the cytotoxicity of the mixture; the effect of combining the prodrug with DPPC was additive and not synergistic. The data presented here question the significance of the permeability enhancing effects claimed for fatty acids and lysolipids at the target cell membrane, and whether these effects can be achieved using physiologically achievable concentrations of fatty acids and lysolipids.

Lipids, curvature, and nano-medicine

The physical properties of the lamellar lipid-bilayer component of biological membranes are controlled by a host of thermodynamic forces leading to overall tensionless bilayers with a conspicuous lateral pressure profile and build-in curvature-stress instabilities that may be released locally or globally in terms of morphological changes. In particular, the average molecular shape and the propensity of the different lipid and protein species for forming non-lamellar and curved structures are a source of structural transitions and control of biological function. The effects of different lipids, sterols, and proteins on membrane structure are discussed and it is shown how one can take advantage of the curvature-stress modulations brought about by specific molecular agents, such as fatty acids, lysolipids, and other amphiphilic solutes, to construct intelligent drug-delivery systems that function by enzymatic triggering via curvature.Practical applications: The simple concept of lipid molecular shape and how it impacts on the structure of lipid aggregates, in particular the curvature and curvature stress in lipid bilayers and liposomes, can be exploited to construct liposome-based drug-delivery systems, e.g., for use as nano-medicine in cancer therapy. Non-lamellar-forming lysolipids and fatty acids, some of which may be designed to be prodrugs, can be created by phospholipase action in diseased tissues thereby providing for targeted drug release and proliferation of molecular entities with conical shape that break down the permeability barrier of the target cells and may hence enhance efficacy.

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.

Diffusion and partitioning of fluorescent lipid probes in phospholipid monolayers

The pressure-dependent diffusion and partitioning of single lipid fluorophores in DMPC and DPPC monolayers were investigated with the use of a custom-made monolayer trough mounted on a combined fluorescence correlation spectroscopy (FCS) and wide-field microscopy setup. It is shown that lipid diffusion, which is essential for the function of biological membranes, is heavily influenced by the lateral pressure and phase of the lipid structure. Both of these may change dynamically during, e.g., protein adsorption and desorption processes. Using FCS, we measured lipid diffusion coefficients over a wide range of lateral pressures in DMPC monolayers and fitted them to a free-area model as well as the direct experimental observable mean molecular area. FCS measurements on DPPC monolayers were also performed below the onset of the phase transition (Pi < 5 mN/m). At higher pressures, FCS was not applicable for measuring diffusion coefficients in DPPC monolayers. Single-molecule fluorescence microscopy and differential scanning calorimetry clearly showed that this was due to heterogeneous partitioning of the lipid fluorophores in condensed phases. The results were compared with dye partitioning in giant lipid vesicles. These findings are significant in relation to the application of lipid fluorophores to study diffusion in both model systems and biological systems.

Activation of phospholipase A2 by ternary model membranes

Formation of liquid-ordered domains in model membranes can be linked to raft formation in cellular membranes. The lipid stoichiometry has a governing influence on domain formation and consequently, biochemical hydrolysis of specific lipids has the potential to remodel domain features. Activation of phospholipase A(2) (PLA(2)) by ternary model membranes with three components (DOPC/DPPC/Cholesterol) can potentially change the domain structure by preferential hydrolysis of the phospholipids. Using fluorescence microscopy, this work investigates the changes in domain features that occur upon PLA(2) activation by such ternary membranes. Double-supported membranes are used, which have minimal interactions with the solid support. For membranes prepared in the coexistence region, PLA(2) induces a decrease of the liquid-disordered (L(d)) phase and an increase of the liquid-ordered (L(o)) phase. A striking observation is that activation by a uniform membrane in the L(d) phase leads to nucleation and growth of L(o)-like domains. This phenomenon relies on the initial presence of cholesterol and no PLA(2) activation is observed by membranes purely in the L(o) phase. The observations can be rationalized by mapping partially hydrolyzed islands onto trajectories in the phase diagram. It is proposed that DPPC is protected from hydrolysis through interactions with cholesterol, and possibly the formation of condensed complexes. This leads to specific trajectories which can account for the observed trends. The results demonstrate that PLA(2) activation by ternary membrane islands may change the global lipid composition and remodel domain features while preserving the overall membrane integrity.

Molecular basis of phospholipase A2 activity toward phospholipids with sn-1 substitutions

We studied secretory phospholipase A(2) type IIA (sPLA(2)) activity toward phospholipids that are derivatized in the sn-1 position of the glycerol backbone. We explored what type of side group (small versus bulky groups, hydrophobic versus polar groups) can be introduced at the sn-1 position of the glycerol backbone of glycerophospholipids and at the same time be hydrolyzed by sPLA(2). The biophysical characterization revealed that the modified phospholipids can form multilamellar vesicles, and several of the synthesized sn-1 functionalized phospholipids were hydrolyzed by sPLA(2). Molecular dynamics simulations provided detailed insight on an atomic level that can explain the observed sPLA(2) activity toward the different phospholipid analogs. The simulations revealed that, depending on the nature of the side chain located at the sn-1 position, the group may interfere with an incoming water molecule that acts as the nucleophile in the enzymatic reaction. The simulation results are in agreement with the experimentally observed sPLA(2) activity toward the different phospholipid analogs.

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.

Synergy in lipofection by cationic lipid mixtures: superior activity at the gel-liquid crystalline phase transition

Some mixtures of two cationic lipids including phospholipid compounds (O-ethylphosphatidylcholines) as well as common, commercially available cationic lipids, such as dimethylammonium bromides and trimethylammonium propanes, deliver therapeutic DNA considerably more efficiently than do the separate molecules. In an effort to rationalize this widespread "mixture synergism", we examined the phase behavior of the cationic lipid mixtures and constructed their binary phase diagrams. Among a group of more than 50 formulations, the compositions with maximum delivery activity resided unambiguously in the solid-liquid crystalline two-phase region at physiological temperature. Thus, the transfection efficacy of formulations exhibiting solid-liquid crystalline phase coexistence is more than 5 times higher than that of formulations in the gel (solid) phase and over twice that of liquid crystalline formulations; phase coexistence occurring at physiological temperature thus appears to contribute significantly to mixture synergism. This relationship between delivery activity and physical property can be rationalized on the basis of the known consequences of lipid-phase transitions, namely, the accumulation of defects and increased disorder at solid-liquid crystalline phase boundaries. Packing defects at the borders of coexisting solid and liquid crystalline domains, as well as large local density fluctuations, could be responsible for the enhanced fusogenicity of mixtures. This study leads to the important conclusion that manipulating the composition of the lipid carriers so that their phase transition takes place at physiological temperature can enhance their delivery efficacy.

Structure and dynamics of crystalline protein layers bound to supported lipid bilayers

We study proteins at the surface of bilayer membranes using streptavidin and avidin bound to biotinylated lipids in a supported lipid bilayer (SLB) at the solid-liquid interface. Using X-ray reflectivity and simultaneous fluorescence microscopy, we characterize the structure and fluidity of protein layers with varied relative surface coverages of crystalline and noncrystalline protein. With continuous bleaching, we measure a 10-15% decrease in the fluidity of the SLB after the full protein layer is formed. We propose that this reduction in lipid mobility is due to a small fraction (0.04) of immobilized lipids bound to the protein layer that create obstacles to membrane diffusion. Our X-ray reflectivity data show a 40 A thick layer of protein, and we resolve an 8 A layer separating the protein layer from the bilayer. We suggest that the separation provided by this water layer allows the underlying lipid bilayer to retain its fluidity and stability.