Toward understanding interfacial activation of secretory phospholipase A2 (PLA2): membrane surface properties and membrane-induced structural changes in the enzyme contribute synergistically to PLA2 activation

The Contribution of Phospholipase A2 and Metalloproteinases to the Synergistic Action of Viper Venom on the Bioenergetic Profile of Vero Cells

Increasing concern about the use of animal models has stimulated the development of in vitro cell culture models for analysis of the biological effects of snake venoms. However, the complexity of animal venoms and the extreme synergy of the venom components during envenomation calls for critical review and analysis. The epithelium is a primary target for injected viper venom's toxic substances, and therefore, is a focus in modern toxinology. We used the Vero epithelial cell line as a model to compare the actions of a crude Macrovipera lebetina obtusa (Levantine viper) venom with the actions of the same venom with two key enzymatic components inhibited (specifically, phospholipase A2 (PLA2) and metalloproteinases) in the bioenergetic cellular response, i.e., oxygen uptake and reactive oxygen species generation. In addition to the rate of free-radical oxidation and lipid peroxidation, we measured real-time mitochondrial respiration (based on the oxygen consumption rate) and glycolysis (based on the extracellular acidification rate) using a Seahorse analyzer. Our data show that viper venom drives an increase in both glycolysis and respiration in Vero cells, while the blockage of PLA2 or/and metalloproteinases affects only the rates of the oxidative phosphorylation. PLA2-blocking in venom also increases cytotoxic activity and the overproduction of reactive oxygen species. These data show that certain components of the venom may have a different effect within the venom cocktail other than the purified enzymes due to the synergy of the venom components.

Molecular Characterization and In Silico Analyses of Maurolipin Structure as a Secretory Phospholipase A 2 ( sPLA 2 ) from Venom Glands of Iranian Scorpio maurus (Arachnida: Scorpionida)

The venom is a mixture of various compounds with specific biological activities, such as the phospholipase  A₂ (PLA₂) enzyme present in scorpion venom. PLA₂ plays a key role in inhibiting ryanodine receptor channels and has neurotoxic activity. This study is the first investigation of molecular characterization, cloning, and in silico analyses of PLA₂ from Iranian Scorpio maurus, named Maurolipin. After RNA extraction from S. maurus venom glands, cDNA was synthesized and amplified through RT-PCR using specific primers. Amplified Maurolipin was cloned in TA cloning vector, pTG19. For in silico analyses, the characterized gene was analyzed utilizing different software. Maurolipin coding gene with 432 base pair nucleotide length encoded a protein of 144 amino acid residues and 16.34 kilodaltons. Comparing the coding sequence of Maurolipin with other characterized PLA₂ from different species of scorpions showed that this protein was a member of the PLA₂ superfamily. According to SWISS-MODEL prediction, Maurolipin had 38.83% identity with bee venom PLA₂ with 100% confidence and 39% identity with insect phospholipase A₂ family, which Phyre2 predicted. According to the three-dimensional structure prediction, Maurolipin with five disulfide bonds has a very high similarity to the structure of PLA₂ that belonged to the group III subfamily. The in silico analyses showed that phospholipase A₂ coding gene and protein structure is different based on scorpion species and geographical condition in which they live.

Inhibiting spinal secretory phospholipase A2 after painful nerve root injury attenuates established pain and spinal neuronal hyperexcitability by altering spinal glutamatergic signaling

Neuropathic injury is accompanied by chronic inflammation contributing to the onset and maintenance of pain after an initial insult. In addition to their roles in promoting immune cell activation, inflammatory mediators like secretory phospholipase A₂ (sPLA₂) modulate nociceptive and excitatory neuronal signaling during the initiation of pain through hydrolytic activity. Despite having a known role in glial activation and cytokine release, it is unknown if sPLA₂ contributes to the maintenance of painful neuropathy and spinal hyperexcitability later after neural injury. Using a well-established model of painful nerve root compression, this study investigated if inhibiting spinal sPLA₂ 7 days after painful injury modulates the behavioral sensitivity and/or spinal dorsal horn excitability that is typically evident. The effects of sPLA₂ inhibition on altered spinal glutamatergic signaling was also probed by measuring spinal intracellular glutamate levels and spinal glutamate transporter (GLAST and GLT1) and receptor (mGluR5, GluR1, and NR1) expression. Spinal sPLA₂ inhibition at day 7 abolishes behavioral sensitivity, reduces both evoked and spontaneous neuronal firing in the spinal cord, and restores the distribution of neuronal phenotypes to those of control conditions. Inhibiting spinal sPLA₂ also increases intracellular glutamate concentrations and restores spinal expression of GLAST, GLT1, mGluR5, and GluR1 to uninjured expression with no effect on NR1. These findings establish a role for spinal sPLA₂ in maintaining pain and central sensitization after neural injury and suggest this may be via exacerbating glutamate excitotoxicity in the spinal cord.

Lipoprotein-associated phospholipase A2 (Lp-PLA2) - possible diagnostic and risk biomarker in chronic ischaemic heart disease

In a group of 208 patients with chronic ischaemic heart disease, the variation of A₂-associated-LDL phosphatase (Lp-PLA₂) serum concentration values was analysed in dynamics at a two-week interval. The conclusion of the study is that the values of serum concentration of Lp-PLA₂ can be accepted as a biomarker with diagnostic specificity for chronic ischaemic heart disease, a parameter of real utility in medical practice, both in situations where the patient, although clinically reporting the existence of angina pectoris, does not show specific changes on an EKG, and for the assessment of the response to personalised therapy.

Dissecting lipid metabolism alterations in SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the cause of the COVID-19 pandemic that has infected over a hundred million people globally. There have been more than two million deaths recorded worldwide, with no end in sight until a widespread vaccination will be achieved. Current research has centred on different aspects of the virus interaction with cell surface receptors, but more needs to be done to further understand its mechanism of action in order to develop a targeted therapy and a method to control the spread of the virus. Lipids play a crucial role throughout the viral life cycle, and viruses are known to exploit lipid signalling and synthesis to affect host cell lipidome. Emerging studies using untargeted metabolomic and lipidomic approaches are providing new insight into the host response to COVID-19 infection. Indeed, metabolomic and lipidomic approaches have identified numerous circulating lipids that directly correlate to the severity of the disease, making lipid metabolism a potential therapeutic target. Circulating lipids play a key function in the pathogenesis of the virus and exert an inflammatory response. A better knowledge of lipid metabolism in the host-pathogen interaction will provide valuable insights into viral pathogenesis and to the development of novel therapeutic targets.

Structural information and membrane binding of truncated RGS9-1 Anchor Protein and its C-terminal hydrophobic segment

Visual phototransduction takes place in photoreceptor cells. Light absorption by rhodopsin leads to the activation of transducin as a result of the exchange of its GDP for GTP. The GTP-bound ⍺-subunit of transducin then activates phosphodiesterase (PDE), which in turn hydrolyzes cGMP leading to photoreceptor hyperpolarization. Photoreceptors return to the dark state upon inactivation of these proteins. In particular, PDE is inactivated by the protein complex R9AP/RGS9-1/Gβ5. R9AP (RGS9-1 anchor protein) is responsible for the membrane anchoring of this protein complex to photoreceptor outer segment disk membranes most likely by the combined involvement of its C-terminal hydrophobic domain as well as other types of interactions. This study thus aimed to gather information on the structure and membrane binding of the C-terminal hydrophobic segment of R9AP as well as of truncated R9AP (without its C-terminal domain, R9AP∆TM). Circular dichroism and infrared spectroscopic measurements revealed that the secondary structure of R9AP∆TM mainly includes ⍺-helical structural elements. Moreover, intrinsic fluorescence measurements of native R9AP∆TM and individual mutants lacking one tryptophan demonstrated that W79 is more buried than W173 but that they are both located in a hydrophobic environment. This method also revealed that membrane binding of R9AP∆TM does not involve regions near its tryptophan residues, while infrared spectroscopy validated its binding to lipid vesicles. Additional fluorescence measurements showed that the C-terminal segment of R9AP is membrane embedded. Maximum insertion pressure and synergy data using Langmuir monolayers suggest that interactions with specific phospholipids could be involved in the membrane binding of R9AP∆TM.

Fluorescent Lipo-Beads for the Sensitive Detection of Phospholipase A2 and Its Inhibitors

Phospholipase A₂ (PLA₂) is a membrane lytic enzyme that is present in many organisms. Human PLA₂ has emerged as a potential biomarker as well as a therapeutic target for several diseases including cancer, cardiovascular diseases, and some inflammatory diseases. The current study focuses on the development of lipo-beads that are very reactive and highly sensitive to PLA₂. To develop the best supported lipid bilayer formulation, several lipid combinations were investigated using 10 μm porous silica beads. The reactivity of PLA₂ was monitored via the decrease in particle fluorescence because of the release of entrapped fluorescent dye from the particle pores or the disintegration of a fluorescent lipid constituted on the bilayer upon lipid hydrolysis using flow cytometry. The enzyme binding studies indicate that lipo-beads with bulky fluorescent tags in the lipid head group and anionic lipids produce a more pronounced response. The kinetic studies suggest that these lipo-beads are very reactive with PLA₂ and can generate a detectable signal in less than 5 min. The enzyme inhibition studies were also conducted with two known PLA₂ inhibitors, varespladib and quercetin. We find that quercetin can hydrolyze the supported membrane, and thus inhibition of PLA₂ is not observed; however, varespladib has shown significant PLA₂ inhibition on lipo-beads. We have demonstrated that our lipo-bead-based approach can detect annexin-3, a known disease biomarker, as low as 10 nM within 5 min after incubation.

Illustrating Interfacial Interaction between Honey Bee Venom Phospholipase A2 and Supported Negatively Charged Lipids with Sum Frequency Generation and Laser Scanning Confocal Microscopy

Phospholipase A₂ is an important enzyme species which can widely be found in animals, plants, bacteria, and so on. A large number of studies have shown that phospholipase A₂ is highly catalytic toward the lipids. Here, sum frequency generation (SFG) vibrational spectroscopy and laser scanning confocal microscopy (LSCM) were applied to study the interaction between honey bee venom phospholipase A₂ (bvPLA₂) and the negatively charged DPPG bilayer. In both cases without and with the calcium ions (Ca²⁺), the bvPLA₂ molecules were adsorbed onto the outer leaflet surface with the orientational order, and the adsorbed bvPLA₂ molecules damaged the order of the packed outer leaflet. In comparison to the case without Ca²⁺, the addition of Ca²⁺ can accelerate the attaching process of bvPLA₂ to the outer leaflet surface and decelerate the process of damaging the outer leaflet order. The experimental result also confirmed, with the help of the Ca²⁺, the DPPG molecules in the outer leaflet were hydrolyzed, with both hydrolysates, that is, lysophospholipids and fatty acids, remaining at the interface, showing a distinct difference from previous published literatures regarding neutral lipids [Phys. Chem. Chem. Phys. 2018, 20, 63-67] and PLA₁ [Langmuir 2019, 35, 12831-12838].

Inflammation and cardiovascular disease: are marine phospholipids the answer?

This review presents the latest research on the cardioprotective effects of n-3 fatty acids (FA) and n-3 FA bound to polar lipids (PL). Overall, n-3 PL may have enhanced bioavailability and potentially bioactivityversusfree FA and ester forms of n-3 FA.

Mesoporous silica/protein biocomposites: Surface, topography, thermal properties

The biocomposite systems based on mesoporous MCF silica support and protein molecules are characterized with regard to their surface, topographic, thermal properties. Mesoporous silica materials (MCF) covered by the adsorbed protein molecules (BSA and OVA) were examined and characterized by using various techniques including X-ray diffraction, the Fourier transform infrared spectroscopy with attenuated total reflectance, X-ray photoelectron spectroscopy and scanning electron microscopy with microanalysis. The results of study focused on a detailed analysis of microstructure (topography, texture), and chemistry (chemical bonds, functional groups, elemental composition) of protein/mesoporous silica biocomposite. Moreover, the thermal properties of prepared biomaterials were investigated by means of TG/DSC-FTIR-MS-coupled technique. These powerful methods provided detailed information for understanding protein adsorption on MCF. Significant differentiation in surface chemistry and topography of MCF material was observed after protein adsorption. Basing on the results of thermal analysis stronger changes of the surface properties and more stable interactions of biomolecules with MCF-d16 support were observed for larger BSA molecules compared to smaller ovalbumin ones.

Synergistic mechanisms of Sanghuang-Danshen phytochemicals on postprandial vascular dysfunction in healthy subjects: A network biology approach based on a clinical trial

With the increased risk of cardiovascular disease, the use of botanicals for vascular endothelial dysfunction has intensified. Here, we explored the synergistic mechanisms of Sanghuang–Danshen (SD) phytochemicals on the homeostatic protection against high-fat-induced vascular dysfunction in healthy subjects, using a network biology approach, based on a randomised crossover clinical trial. Seventeen differential markers identified in blood samples taken at 0, 3 and 6 h post-treatment, together with 12SD phytochemicals, were mapped onto the network platform, termed the context-oriented directed associations. The resulting vascular sub-networks illustrated associations between 10 phytochemicals with 32 targets implicated in 143 metabolic/signalling pathways. The three key events included adhesion molecule production (ellagic acid, fumaric acid and cryptotanshinone; VCAM-1, ICAM-1 and PLA2G2A; fatty acid metabolism), platelet activation (ellagic acid, protocatechuic acid and tanshinone IIA; VEGFA, APAF1 and ATF3; mTOR, p53, Rap1 and VEGF signalling pathways) and endothelial inflammation (all phytochemicals, except cryptotanshinone; 29 targets, including TP53 and CASP3; MAPK and PI3K-Akt signalling pathways, among others). Our collective findings demonstrate a potential of SD to protect unintended risks of vascular dysfunction in healthy subjects, providing a deeper understanding of the complicated synergistic mechanisms of signature phytochemicals in SD.

Giant Phospholipid Folds on Air-Water Surface: Structure Details, Formation Pathway, and Possible Recycle Mechanism

In vitro mimics recognized that the propensity of a negatively charged phospholipid, DPPS, monolayers to self-aggregate to three-dimensional (3D) giant folds under overcompression at an air-water interface. Time elapsing microscopical observations confirmed that such giant folds were able to float stably on the air-water interface for weeks or even longer. Ex situ atomic force microscopy (AFM) and transmission electronic microscopy (TEM) characterizations pointed out that such giant folds were composed of compactly stacked lipid layers. Phospholipase A₂ (PLA₂), a principal bactericide in human and animal tear secretion, was chosen to drive the in situ lipid giant folds disassembly on water and supported substrate surfaces, respectively. Our experimental results confirmed the layer-by-layer structures of the giant folds. It is noteworthy that the defect-rich areas of the giant lipid folds were eliminated quickly by PLA₂ while defect-free lipid zones were left untouched, suggesting that PLA₂ may serve as a highly effective and selective regenerator/cleaner of lipid aggregates in the physiological circumstance of certain organs.

Anti-inflammatory therapy for cardiovascular disease

Chronic subclinical inflammation is a central process in the pathogenesis of cardiovascular disease (CVD) and it has been linked with both the initiation and progression of atherosclerosis. Several pro-inflammatory cytokines, such as the C-reactive protein (CRP), tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) have been described as independent risk factors for coronary heart disease and promoters of atherogenesis. Thus, extensive research is being conducted to assess the role of anti-inflammatory therapy in the primary and secondary prevention of CVD. Our review aims to provide the clinical and scientific data pertaining to the effects of different anti-inflammatory agents administered in patients with CVD.

FTIR Analysis of Proteins and Protein-Membrane Interactions

Fourier transform infrared (FTIR) spectroscopy has become one of the major techniques of structural characterization of proteins, peptides, and protein-membrane interactions. While the method does not have the capability of providing the precise, atomic-resolution molecular structure, it is exquisitely sensitive to conformational changes occurring in proteins upon functional transitions or intermolecular interactions. The sensitivity of vibrational frequencies to atomic masses has led to development of "isotope-edited" FTIR spectroscopy, where structural effects in two proteins, one unlabeled and the other labeled with a heavier stable isotope, such as ¹³C, are resolved simultaneously based on spectral downshift (separation) of the amide I band of the labeled protein. The same isotope effect is used to identify site-specific conformational changes in proteins by site-directed or segmental isotope labeling. Negligible light scattering in the infrared region provides an opportunity to study intermolecular interactions between large protein complexes, interactions of proteins and peptides with lipid vesicles, or protein-nucleic acid interactions without light scattering problems often encountered in ultraviolet spectroscopy. Attenuated total reflection FTIR (ATR-FTIR) is a surface-sensitive version of infrared spectroscopy that has proved useful in studying membrane proteins and lipids, protein-membrane interactions, mechanisms of interfacial enzymes, the structural features of membrane pore forming proteins and peptides, and much more. The purpose of this chapter was to provide a practical guide to analyze protein structure and protein-membrane interactions by FTIR and ATR-FTIR techniques, including procedures of sample preparation, measurements, and data analysis. Basic background information on FTIR spectroscopy, as well as some relatively new developments in structural and functional characterization of proteins and peptides in lipid membranes, is also presented.

The phospholipid-repair system LplT/Aas in Gram-negative bacteria protects the bacterial membrane envelope from host phospholipase A2 attack

Secretory phospholipases A2 (sPLA2s) are potent components of mammalian innate-immunity antibacterial mechanisms. sPLA2 enzymes attack bacteria by hydrolyzing bacterial membrane phospholipids, causing membrane disorganization and cell lysis. However, most Gram-negative bacteria are naturally resistant to sPLA2 Here we report a novel resistance mechanism to mammalian sPLA2 in Escherichia coli, mediated by a phospholipid repair system consisting of the lysophospholipid transporter LplT and the acyltransferase Aas in the cytoplasmic membrane. Mutation of the lplT or aas gene abolished bacterial lysophospholipid acylation activity and drastically increased bacterial susceptibility to the combined actions of inflammatory fluid components and sPLA2, resulting in bulk phospholipid degradation and loss of colony-forming ability. sPLA2-mediated hydrolysis of the three major bacterial phospholipids exhibited distinctive kinetics and deacylation of cardiolipin to its monoacyl-derivative closely paralleled bacterial death. Characterization of the membrane envelope in lplT- or aas-knockout mutant bacteria revealed reduced membrane packing and disruption of lipid asymmetry with more phosphatidylethanolamine present in the outer leaflet of the outer membrane. Moreover, modest accumulation of lysophospholipids in these mutant bacteria destabilized the inner membrane and rendered outer membrane-depleted spheroplasts much more sensitive to sPLA2 These findings indicated that LplT/Aas inactivation perturbs both the outer and inner membranes by bypassing bacterial membrane maintenance mechanisms to trigger specific interfacial activation of sPLA2 We conclude that the LplT/Aas system is important for maintaining the integrity of the membrane envelope in Gram-negative bacteria. Our insights may help inform new therapeutic strategies to enhance host sPLA2 antimicrobial activity.

Calcium-dependent hydrolysis of supported planar lipids was triggered by honey bee venom phospholipase A2with the right orientation at the interface

Hydrolysis of planar phospholipids catalyzed by honey bee venom phospholipase A₂(bvPLA₂) was studied.

IR spectroscopy analysis of pancreatic lipase-related protein 2 interaction with phospholipids: 3. Monitoring DPPC lipolysis in mixed micelles

Usual methods for the continuous assay of lipolytic enzyme activities are mainly based on the titration of free fatty acids, surface pressure monitoring or spectrophotometry using substrates labeled with specific probes. These approaches only give a partial information on the chemistry of the lipolysis reaction and additional end-point analyses are often required to quantify both residual substrate and lipolysis products. We used transmission infrared (IR) spectroscopy to monitor simultaneously the hydrolysis of phospholipids by guinea pig pancreatic lipase-related protein 2 (GPLRP2) and the release of lipolysis products. The substrate (DPPC, 1,2-Dipalmitoyl phosphatidylcholine) was mixed with sodium taurodeoxycholate (NaTDC) to form mixed micelles in D₂O buffer at pD 6 and 8. After hydrogen/deuterium exchange, DPPC hydrolysis by GPLRP2 (100nM) was monitored at 35°C in a liquid cell by recording IR spectra and time-course variations in the CO stretching region. These changes were correlated to variations in the concentrations of DPPC, lysophospholipids (lysoPC) and palmitic acid (Pam) using calibration curves established with these compounds individually mixed with NaTDC. We were thus able to quantify each compound and its time-course variations during the phospholipolysis reaction and to estimate the enzyme activity. To validate the IR analysis, variations in residual DPPC, lysoPC and Pam were also quantified by thin-layer chromatography coupled to densitometry and similar hydrolysis profiles were obtained using both methods. IR spectroscopy can therefore be used to monitor the enzymatic hydrolysis of phospholipids and obtain simultaneously chemical and physicochemical information on substrate and all reaction products (H-bonding, hydration, acyl chain mobility).

Interfacial Enzymes: Membrane Binding, Orientation, Membrane Insertion, and Activity

Most interfacial enzymes undergo activation upon membrane binding. Interfacial activation is determined not only by the binding strength but also by the specific mode of protein-membrane interactions, including the angular orientation and membrane insertion of the enzymes. This chapter describes biophysical techniques to quantitatively evaluate membrane binding, orientation, membrane insertion, and activity of secreted phospholipase A2 (PLA2) and lipoxygenase (LO) enzymes. Procedures for recombinant production and purification of human pancreatic PLA2 and human 5-lipoxygenase (5-LO) are also presented. Several methods for measurements of membrane binding of peripheral proteins are described, i.e., fluorescence resonance energy transfer (FRET) from tryptophan or tyrosine residues of the protein to a fluorescent lipid in vesicles, changes in fluorescence of an environment-sensitive fluorescent lipid upon binding of proteins to membranes, and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. These methods produce the apparent binding constant, the protein-to-lipid binding stoichiometry, and the Hill cooperativity coefficient. Experimental procedures for segmental isotope labeling of proteins and determination of the orientation of membrane-bound proteins by polarized ATR-FTIR spectroscopy are described. Furthermore, evaluation of membrane insertion of peripheral proteins by a fluorescence quenching technique is outlined. Combination of the orientation and membrane insertion provides a unique configuration of the protein-membrane complex and hence elucidates certain details of the enzyme function, such as the modes of acquisition of a membrane-residing substrate and product release. Finally, assays for determination of the activities of secreted PLA2, soybean LO, and human 5-LO are described.

Secreted Phospholipase A2 Type IIA (sPLA2-IIA) Activates Integrins in an Allosteric Manner

Secreted phospholipase A2 type IIA (sPLA2-IIA) is a well-established pro-inflammatory protein and has been a major target for drug discovery. However, the mechanism of its signaling action has not been fully understood. We previously found that sPLA2-IIA binds to integrins αvβ3 and α4β1 in human and that this interaction plays a role in sPLA2-IIA's signaling action. Our recent studies found that sPLA2-IIA activates integrins in an allosteric manner through direct binding to a newly identified binding site of integrins (site 2), which is distinct from the classical RGD-binding site (site 1). The sPLA2-IIA-induced integrin activation may be related to the signaling action of sPLA2-IIA. Since sPLA2-IIA is present in normal human tears in addition to rheumatoid synovial fluid at high concentrations the sPLA2-IIA-mediated integrin activation on leukocytes may be involved in immune responses in normal and pathological conditions.

Immobilization of Lipid Substrates: Application on Phospholipase A2 Determination

The purpose of the study was to assess a fluorimetric assay for the determination of total phospholipase A(2) (PLA(2)) activity in biological samples introducing the innovation of immobilized substrates on crosslinked polymeric membranes. The immobilized C(12)-NBD-PtdCho, a fluorescent analogue of phosphatidylcholine, exhibited excellent stability for 3 months at 4 °C and was not desorbed in the aqueous reaction mixture during analysis. The limit of detection was 0.5 pmol FA (0.2 pg) and the linear part of the response curve extended from 1 up to 190 nmol FA/h/mL sample. The intra- and inter-day relative standard deviations (%RSD), were ≤6 and ≤9 %, respectively. Statistical comparison with other fluorescent methods showed excellent correlation and agreement. Semiempirical calculations showed a fair amount of electrostatic interaction between the NBD-labeled substrate and the crosslinked polyvinyl alcohol with the styryl pyridinium residues (PVA-SbQ) material, from the plane of which, the sn-2 acyl chain of the phospholipid stands out and is accessible by PLA(2). Atomic Force Microscopy revealed morphological alterations of the immobilized substrate after the reaction with PLA(2). Mass spectrometry showed that only C(12)-NBD-FA, the PLA(2 )hydrolysis product, was detected in the reaction mixture, indicating that PLA(2) recognizes PVA-SbQ/C(12)-NBD-PtdCho as a surface to perform catalysis.

Beyond the hydrophobic effect: Critical function of water at biological phase boundaries--A hypothesis

Many life-sustaining processes in living cells occur at the membrane-water interface. The pertinent questions that need to be asked are what is the evolutionary reason for biology to choose the membrane-water interface as the site for performing and/or controlling crucial biological reactions and what is the key physical principle that is singular to the membrane-water interface that biology exploits for regulating metabolic processes in cells? In this review, a hypothesis is developed, which espouses that cells control activities of membrane-bound enzymes and receptor activated processes via manipulating the thermodynamic activity of water at the membrane-water interfacial region. In support of this hypothesis, first we establish that the surface pressure of a lipid monolayer is a direct measure of a reduction in the thermodynamic activity of interfacial water. Second, we show that the surface pressure-dependent activation/inactivation of interfacial enzymes is fundamentally related to their dependence on interfacial water activity. We extend this argument to infer that cells might manipulate activities of membrane-associated biological processes via manipulating the activity of interfacial water via localized compression or expansion of the interface. In this paper, we critically analyze literature data on mechano-activation of large pore ion channels in Escherichia coli spheroplasts and G-proteins in reconstituted lipid vesicles, and show that these pressure-induced activation processes are fundamentally and quantitatively related to changes in the thermodynamic state of interfacial water, caused by mechanical stretching of the bilayer.

Effect of products of PLA2 catalyzed hydrolysis of DLPC on motion of rising bubbles

Local velocities of rising bubbles decrease with the increasing concentration in solution of surface-active, water-soluble species. Therefore, it is possible to use this phenomenon to monitor products of enzymatic reactions, which meet such criteria. In this study, hydrolysis of 1,2-dilauroyl-sn-glycero-3-phosphatidylcholine (DLPC) catalyzed by calcium-dependent phospholipase A2 (PLA2) (EC3.1.1.4) from porcine pancreas was used as model reaction. The products of this reaction are lauric acid (LA) and 1-lauroyl-2-hydroxy-sn-glycero-3-phosphatidylcholine (Lyso-PC). DLPC was dispersed in a chloroform/methanol mixture that was spread on a free PLA2 solution surface. Air bubbles were then formed at a capillary orifice and the local velocity of rising bubbles as a function of the distance from the capillary tip was monitored. Local velocity profiles were compared with profiles recorded for solutions of pure enzymatic reaction products and their mixtures. Our experiments showed that the product, which had a dominating effect on bubble motion retardation, was lyso-phosphatidylcholine. This can be explained by differences in the kinetics of lauric acid and lyso-phosphatidylcholine transfer from the spread layer to the solution.

Kinetic characterization, optimum conditions for catalysis and substrate preference of secretory phospholipase A2 from Glycine max in model membrane systems

Two secretory phospholipase A2 (sPLA2s) from Glycine max, GmsPLA2-IXA-1 and GmsPLA2-XIB-2, have been purified as recombinant proteins and the activity was evaluated in order to obtain the optimum conditions for catalysis using mixed micelles and lipid monolayers as substrate. Both sPLA2s showed a maximum enzyme activity at pH 7 and a requirement of Ca(2+) in the micromolar range. These parameters were similar to those found for animal sPLA2s but a surprising optimum temperature for catalysis at 60 °C was observed. The effect of negative interfacial charges on the hydrolysis of organized substrates was evaluated through initial rate measurements using short chain phospholipids with different head groups. The enzymes showed subtle differences in the specificity for phospholipids with different head groups (DLPC, DLPG, DLPE, DLPA) in presence or absence of NaCl. Both recombinant enzymes showed lower activity toward anionic phospholipids and a preference for the zwitterionic ones. The values of the apparent kinetic parameters (Vmax and KM) demonstrated that these enzymes have more affinity for phosphatidylcholine compared with phosphatidylglycerol, in contrast with the results observed for pancreatic sPLA2. A hopping mode of catalysis was proposed for the action of these sPLA2 on mixed phospholipid/triton micelles. On the other hand, Langmuir-monolayers assays indicated an optimum lateral surface pressure for activity in between 13 and 16 mN/m for both recombinant enzymes.

Modulated mechanism of phosphatidylserine on the catalytic activity of Naja naja atra phospholipase A2 and Notechis scutatus scutatus notexin

Phosphatidylserine (PS) externalization is a hallmark for apoptotic death of cells. Previous studies showed that Naja naja atra phospholipase A2 (NnaPLA2) and Notechis scutatus scutatus notexin induced apoptosis of human cancer cells. However, NnaPLA2 and notexin did not markedly disrupt the integrity of cellular membrane as evidenced by membrane permeability of propidium iodide. These findings reflected that the ability of NnaPLA2 and notexin to hydrolyze membrane phospholipids may be affected by PS externalization. To address that question, this study investigated the membrane-interacted mode and catalytic activity of NnaPLA2 and notexin toward outer leaflet (phosphatidylcholine/sphingomyelin/cholesterol, PC/SM/Chol) and inner leaflet (phosphatidylserine/phosphatidylethanolamine/cholesterol, PS/PE/Chol) of plasma membrane-mimicking vesicles. PS incorporation promoted enzymatic activity of NnaPLA2 and notexin on PC and PC/SM vesicles, but suppressed NnaPLA2 and notexin activity on PC/SM/Chol and PE/Chol vesicles. PS incorporation increased the membrane fluidity of PC vesicles but reduced membrane fluidity of PC/SM, PC/SM/Chol and PE/Chol vesicles. PS increased the phospholipid order of all the tested vesicles. Moreover, PS incorporation did not greatly alter the binding affinity of notexin and NnaPLA2 with phospholipid vesicles. Acrylamide quenching studies and trinitrophenylation of Lys residues revealed that membrane-bound mode of notexin and NnaPLA2 varied with the targeted membrane compositions. The fine structure of catalytic site in NnaPLA2 and notexin in all the tested vesicles showed different changes. Collectively, the present data suggest that membrane-inserted PS modulates PLA2 interfacial activity via its effects on membrane structure and membrane-bound mode of NnaPLA2 and notexin, and membrane compositions determine the effect of PS on PLA2 activity.

Ubiquitin activates patatin-like phospholipases from multiple bacterial species

Phospholipase A2 enzymes are ubiquitously distributed throughout the prokaryotic and eukaryotic kingdoms and are utilized in a wide array of cellular processes and physiological and immunological responses. Several patatin-like phospholipase homologs of ExoU from Pseudomonas aeruginosa were selected on the premise that ubiquitin activation of this class of bacterial enzymes was a conserved process. We found that ubiquitin activated all phospholipases tested in both in vitro and in vivo assays via a conserved serine-aspartate catalytic dyad. Ubiquitin chains versus monomeric ubiquitin were superior in inducing catalysis, and ubiquitin-like proteins failed to activate phospholipase activity. Toxicity studies in a prokaryotic dual-expression system grouped the enzymes into high- and low-toxicity classes. Toxicity measured in eukaryotic cells also suggested a two-tiered classification but was not predictive of the severity of cellular damage, suggesting that each enzyme may correspond to unique properties perhaps based on its specific biological function. Additional studies on lipid binding preference suggest that some enzymes in this family may be differentially sensitive to phosphatidyl-4,5-bisphosphate in terms of catalytic activation enhancement and binding affinity. Further analysis of the function and amino acid sequences of this enzyme family may lead to a useful approach to formulating a unifying model of how these phospholipases behave after delivery into the cytoplasmic compartment.

Gossypol increases expression of the pro-apoptotic BH3-only protein NOXA through a novel mechanism involving phospholipase A2, cytoplasmic calcium, and endoplasmic reticulum stress

Gossypol is a putative BH3 mimetic proposed to inhibit BCL2 and BCLXL based on cell-free assays. We demonstrated previously that gossypol failed to directly inhibit BCL2 in cells or induce apoptosis in chronic lymphocytic leukemia (CLL) cells or platelets, which require BCL2 or BCLXL, respectively, for survival. Here, we demonstrate that gossypol rapidly increased activity of phospholipase A2 (PLA2), which led to an increase in cytoplasmic calcium, endoplasmic reticulum (ER) stress, and up-regulation of the BH3-only protein NOXA. Pretreatment with the PLA2 inhibitor, aristolochic acid, abrogated the increase in calcium, ER stress, and NOXA. Calcium chelation also abrogated the gossypol-induced increase in calcium, ER stress, and NOXA, but not the increase in PLA2 activity, indicating that PLA2 is upstream of these events. In addition, incubating cells with the two products of PLA2 (lysophosphatidic acid and arachidonic acid) mimicked treatment with gossypol. NOXA is a pro-apoptotic protein that functions by binding the BCL2 family proteins MCL1 and BFL1. The BCL2 inhibitor ABT-199 is currently in clinical trials for CLL. Resistance to ABT-199 can occur from up-regulation of other BCL2 family proteins, and this resistance can be mimicked by culturing CLL cells on CD154(+) stroma cells. We report here that AT-101, a derivative of gossypol in clinical trials, overcomes stroma-mediated resistance to ABT-199 in primary CLL cells, suggesting that a combination of these drugs may be efficacious in the clinic.

Partitioning of lysolipids, fatty acids and their mixtures in aqueous lipid bilayers: solute concentration/composition effects

Distributions of lysopalmitoylphosphatidylcholine (LPPC), palmitic acid (PA) and their 1:1 mixtures between water and dipalmitoylphosphatidylcholine (DPPC) bilayer were determined using a fluorescence probe that selectively detects only the solutes in water. Water solute concentrations were obtained at each of several lipid concentrations. Dynamic Light Scattering experiments confirmed that the lipid/solute aggregates were vesicles in the concentration range investigated. Lipid concentration dependence of the solute component in water was fit to a thermodynamic model of solute distribution between two coexisting solvents. Water/bilayer partition coefficient and the free energy of transfer, for each of these solutes were determined from the fit. Main findings are: (1) Water/bilayer partition coefficient of solute is greater for 2 to 10% solute mole fraction than for 0 to 2%, signaling solute induced bilayer perturbation that increases bilayer solubility, beginning at 2% solute mole fraction. (2) Partition coefficients are in the order LPPC<PA<LPPC+PA at 37°C and LPPC+PA≤LPPC<PA at 50°C. This signifies synergism toward increased solute solubility in the bilayer-gel phase and lack of it in the bilayer-liquid phase when LPPC and PA are present together. Implications of the solute concentration/composition and bilayer phase dependences of the partition coefficients to the reported solute induced enhancements in transmembrane permeability are discussed.

Structural characterization of membrane proteins and peptides by FTIR and ATR-FTIR spectroscopy

Fourier transform infrared (FTIR) spectroscopy is widely used in structural characterization of proteins or peptides. While the method does not have the capability of providing the precise, atomic-resolution molecular structure, it is exquisitely sensitive to conformational changes occurring in proteins upon functional transitions or upon intermolecular interactions. Sensitivity of vibrational frequencies to atomic masses has led to development of "isotope-edited" FTIR spectroscopy, where structural effects in two proteins, one unlabeled and the other labeled with a heavier stable isotope, such as (13)C, are resolved simultaneously based on spectral downshift (separation) of the amide I band of the labeled protein. The same isotope effect is used to identify site-specific conformational changes in proteins by site-directed or segmental isotope labeling. Negligible light scattering in the infrared region provides an opportunity to study intermolecular interactions between large protein complexes, interactions of proteins and peptides with lipid vesicles, or protein-nucleic acid interactions without light scattering problems often encountered in ultraviolet spectroscopy. Attenuated total reflection FTIR (ATR-FTIR) is a surface-sensitive version of infrared spectroscopy that has proved useful in studying membrane proteins and lipids, protein-membrane interactions, mechanisms of interfacial enzymes, and molecular architecture of membrane pore or channel forming proteins and peptides. The purpose of this article was to provide a practical guide to analyze protein structure and protein-membrane interactions by FTIR and ATR-FTIR techniques, including procedures of sample preparation, measurements, and data analysis. Basic background information on FTIR spectroscopy, as well as some relatively new developments in structural and functional characterization of proteins and peptides in lipid membranes, are also presented.

Combining topographical and genetic cues to promote neuronal fate specification in stem cells

There is little remedy for the devastating effects resulting from neuronal loss caused by neural injury or neurodegenerative disease. Reconstruction of damaged neural circuitry with stem cell-derived neurons is a promising approach to repair these defects, but controlling differentiation and guiding synaptic integration with existing neurons remain significant unmet challenges. Biomaterial surfaces can present nanoscale topographical cues that influence neuronal differentiation and process outgrowth. By combining these scaffolds with additional molecular biology strategies, synergistic control over cell fate can be achieved. Here, we review recent progress in promoting neuronal fate using techniques at the interface of biomaterial science and genetic engineering. New data demonstrates that combining nanofiber topography with an induced genetic program enhances neuritogenesis in a synergistic fashion. We propose combining patterned biomaterial surface cues with prescribed genetic programs to achieve neuronal cell fates with the desired sublineage specification, neurochemical profile, targeted integration, and electrophysiological properties.

Identification of inhibitors against interaction between pro-inflammatory sPLA2-IIA protein and integrin αvβ3

Increased concentrations of secreted phospholipase A2 type IIA (sPLA2-IIA), have been found in the synovial fluid of patients with rheumatoid arthritis. It has been shown that sPLA2-IIA specifically binds to integrin αvβ3, and initiates a signaling pathway that leads to cell proliferation and inflammation. Therefore, the interaction between integrin and sPLA2-IIA could be a potential therapeutic target for the treatment of proliferation or inflammation-related diseases. Two one-bead-one-compound peptide libraries were constructed and screened, and seven target hits were identified. Herein we report the identification, synthesis, and biological testing of two pyrazolylthiazole-tethered peptide hits and their analogs. Biological assays showed that these compounds were able to suppress the sPLA2-IIA-integrin interaction and sPLA2-IIA-induced migration of monocytic cells and that the blockade of the sPLA2-IIA-integrin binding was specific to sPLA2-IIA and not to the integrin.

Quantitation of lysolipids, fatty acids, and phospholipase A2 activity and correlation with membrane polarity

Acrylodan-labeled rat-intestinal fatty acid binding protein, ADIFAB, binds both of lysophosphatidylcholines (LPC) and FA. Binding displaces Acrylodan and its fluorescence peak shifts from 432 to 505 nm. A fluorescence assay that relies on this shift is presented for quantitating LPC, FA, and phospholipase A(2) (PLA(2)) activity in phospholipid bilayers in absolute units of μM/min/mg of enzyme. This is a development over an earlier assay that took into account only FA binding. Activities of bee venom PLA(2) on dipalmitoylphosphatidylcholine (DPPC) and dioleylphosphatidylcholine (DOPC) bilayers were measured. Standard pH-Stat assays validated the present assay. Products increase linearly with time for about one minute in DOPC and five minutes in DPPC corresponding to completion of 5 to 8% hydrolysis in DOPC and 20% in DPPC. Membrane polarity and microviscosity measured using electron spin resonance (ESR) exhibited discontinuities at compositions that mimicked similar percentages of hydrolysis products in the respective bilayers. The observed hydrolysis rate decrease following the initial linear period thus correlates to changes in membrane polarity. The ability of the assay to yield actual product concentrations, reveal structure in the reaction progress curves, and interpretation in light of the ESR data bring insight into the shape of the reaction curve.

Surface pressure-dependent interactions of secretory phospholipase A2 with zwitterionic phospholipid membranes

The hydrolytic activity of secretory phospholipase A(2) (PLA(2)) is regulated by many factors, including the physical state of substrate aggregates and the chemical nature of phospholipid molecules. In order to achieve strong binding of PLA(2) on its substrates, many previous works have used anionic lipid dispersion to characterize the orientation and penetration depth of PLA(2) molecules on membrane surfaces. In this study, we applied monolayer technique with controllable surface area to investigate the PLA(2)s of Taiwan cobra venom and bee venom on zwitterionic phophatidylcholine monolayers and demonstrated an optimum hydrolytic activity at a surface pressure of 18 and 24 mN/m, respectively. By combining polarized attenuated total reflection Fourier-transform infrared spectroscopy and monolayer-binding experiments, we found that the amount of membrane-bound PLA(2) decreased markedly as the surface pressure of the monolayer was increased. Interestingly, the insertion area of the PLA(2)s decreased to near zero as the surface pressure increased to the optimum pressure for hydrolytic activity. On the basis of the measured infrared dichroic ratio, the orientation of the PLA(2)s bound to zwitterionic membranes was similar to that observed on a negatively charged membrane and was independent of the surface pressure. Our findings suggest that both PLA(2)s were located on the membrane surface rather than penetrating the membrane bilayer and that the deeply inserted mode is not a favorable condition for the hydrolysis of phospholipids in zwitterionic phospholipid membranes. The results are discussed in terms of the easy access of catalytic water for the PLA(2) activity and the mobilization of its substrate and product to facilitate the catalytic process.