Catalytic role of the active site histidine of porcine pancreatic phospholipase A2 probed by the variants H48Q, H48N and H48K

Structural and functional characterization of a thermostable secretory phospholipase A2 from Sciscionella marina and its application in liposome biotransformation

Secretory phospholipase A₂ (sPLA₂), which hydrolyzes the sn-2 acyl bond of lecithin in a Ca²⁺-dependent manner, is an important enzyme in the oil and oleochemical industries. However, most sPLA₂s are not stable under process conditions. Therefore, a thermostable sPLA₂ was investigated in this study. A marine bacterial sPLA₂ isolated from Sciscionella marina (Sm-sPLA₂) was catalytically active even after 5 h of incubation at high temperatures of up to 50°C, which is outstanding compared with a representative bacterial sPLA₂ (i.e. sPLA₂ from Streptomyces violaceoruber; Sv-sPLA₂). Consistent with this, the melting temperature of Sm-sPLA₂ was measured to be 7.7°C higher than that of Sv-sPLA₂. Furthermore, Sm-sPLA₂ exhibited an improved biotransformation performance compared with Sv-sPLA₂ in the hydrolysis of soy lecithin to lysolecithin and free fatty acids at 50°C. Structural and mutagenesis studies revealed that the Trp41-mediated anchoring of a Ca²⁺-binding loop into the rest of the protein body is directly linked to the thermal stability of Sm-sPLA₂. This finding provides a novel structural insight into the thermostability of sPLA₂ and could be applied to create mutant proteins with enhanced industrial potential.

Treatment of Mouse Sperm with a Non-Catalytic Mutant of PLA2G10 Reveals That PLA2G10 Improves In Vitro Fertilization through Both Its Enzymatic Activity and as Ligand of PLA2R1

The group X secreted phospholipase A2 (PLA2G10) is present at high levels in mouse sperm acrosome. The enzyme is secreted during capacitation and amplifies the acrosome reaction and its own secretion via an autocrine loop. PLA2G10 also improves the rate of fertilization. In in vitro fertilization (IVF) experiments, sperm from Pla2g10-deficient mice produces fewer two-cell embryos, and the absence of PLA2G10 is rescued by adding recombinant enzymes. Moreover, wild-type (WT) sperm treated with recombinant PLA2G10 produces more two-cell embryos. The effects of PLA2G10 on mouse fertility are inhibited by sPLA₂ inhibitors and rescued by products of the enzymatic reaction such as free fatty acids, suggesting a role of catalytic activity. However, PLA2G10 also binds to mouse PLA2R1, which may play a role in fertility. To determine the relative contribution of enzymatic activity and PLA2R1 binding in the profertility effect of PLA2G10, we tested H48Q-PLA2G10, a catalytically-inactive mutant of PLA2G10 with low enzymatic activity but high binding properties to PLA2R1. Its effect was tested in various mouse strains, including Pla2r1-deficient mice. H48Q-PLA2G10 did not trigger the acrosome reaction but was as potent as WT-PLA2G10 to improve IVF in inbred C57Bl/6 mice; however, this was not the case in OF1 outbred mice. Using gametes from these mouse strains, the effect of H48Q-PLA2G10 appeared dependent on both spermatozoa and oocytes. Moreover, sperm from C57Bl/6 Pla2r1-deficient mice were less fertile and lowered the profertility effects of H48Q-PLA2G10, which were completely suppressed when sperm and oocytes were collected from Pla2r1-deficient mice. Conversely, the effect of WT-PLA2G10 was not or less sensitive to the absence of PLA2R1, suggesting that the effect of PLA2G10 is polymodal and complex, acting both as an enzyme and a ligand of PLA2R1. This study shows that the action of PLA2G10 on gametes is complex and can simultaneously activate the catalytic pathway and the PLA2R1-dependent receptor pathway. This work also shows for the first time that PLA2G10 binding to gametes’ PLA2R1 participates in fertilization optimization.

PLA2G1B is involved in CD4 anergy and CD4 lymphopenia in HIV-infected patients

The precise mechanism leading to profound immunodeficiency of HIV-infected patients is still only partially understood. Here, we show that more than 80% of CD4+ T cells from HIV-infected patients have morphological abnormalities. Their membranes exhibited numerous large abnormal membrane microdomains (aMMDs), which trap and inactivate physiological receptors, such as that for IL-7. In patient plasma, we identified phospholipase A2 group IB (PLA2G1B) as the key molecule responsible for the formation of aMMDs. At physiological concentrations, PLA2G1B synergized with the HIV gp41 envelope protein, which appears to be a driver that targets PLA2G1B to the CD4+ T cell surface. The PLA2G1B/gp41 pair induced CD4+ T cell unresponsiveness (anergy). At high concentrations in vitro, PLA2G1B acted alone, independently of gp41, and inhibited the IL-2, IL-4, and IL-7 responses, as well as TCR-mediated activation and proliferation, of CD4+ T cells. PLA2G1B also decreased CD4+ T cell survival in vitro, likely playing a role in CD4 lymphopenia in conjunction with its induced IL-7 receptor defects. The effects on CD4+ T cell anergy could be blocked by a PLA2G1B-specific neutralizing mAb in vitro and in vivo. The PLA2G1B/gp41 pair constitutes what we believe is a new mechanism of immune dysfunction and a compelling target for boosting immune responses in HIV-infected patients.

Water-Intake and Water-Molecule Paths to the Active Site of Secretory Phospholipase A2 Studied Using MD Simulations and the Tracking Tool AQUA-DUCT

Secretory phospholipases A₂ (sPLA₂s) are a subclass of enzymes that catalyze the hydrolysis at the sn-2 position of glycerophospholipids, producing free fatty acids and lysophospholipids. In this study, different phospholipids with structural modifications close to the scissile sn-2 ester bond were studied to determine the effect of the structural changes on the formation of the Michaelis-Menten complex and the water entry/exit pathways using molecular dynamics simulations and the computational tracking tool AQUA-DUCT. Structural modifications include methylation, dehydrogenation, and polarization close to the sn-2 scissile bond. We found that all water molecules reaching the active site of sPLA₂-IIA pass by the aromatic residues Phe⁵ and Tyr⁵¹ and enter the active site through an active-site cleft. The relative amount of water available for the enzymatic reaction of the different phospholipid-sPLA₂ complexes was determined together with the distance between key atoms in the catalytic machinery. The results showed that (Z)-unsaturated phospholipid is a good substrate for sPLA₂-IIA. The computational results are in good agreement with previously reported experimental data on the ability of sPLA₂-IIA to hydrolyze liposomes made from the different phospholipids, and the results provide new insights into the necessary active-site solvation of the Michaelis-Menten complex and can pave the road for rational design in engineering applications.

Detection and quantification of microparticles from different cellular lineages using flow cytometry. Evaluation of the impact of secreted phospholipase A2 on microparticle assessment

Microparticles, also called microvesicles, are submicron extracellular vesicles produced by plasma membrane budding and shedding recognized as key actors in numerous physio(patho)logical processes. Since they can be released by virtually any cell lineages and are retrieved in biological fluids, microparticles appear as potent biomarkers. However, the small dimensions of microparticles and soluble factors present in body fluids can considerably impede their quantification. Here, flow cytometry with improved methodology for microparticle resolution was used to detect microparticles of human and mouse species generated from platelets, red blood cells, endothelial cells, apoptotic thymocytes and cells from the male reproductive tract. A family of soluble proteins, the secreted phospholipases A₂ (sPLA₂), comprises enzymes concomitantly expressed with microparticles in biological fluids and that catalyze the hydrolysis of membrane phospholipids. As sPLA₂ can hydrolyze phosphatidylserine, a phospholipid frequently used to assess microparticles, and might even clear microparticles, we further considered the impact of relevant sPLA2 enzymes, sPLA2 group IIA, V and X, on microparticle quantification. We observed that if enriched in fluids, certain sPLA₂ enzymes impair the quantification of microparticles depending on the species studied, the source of microparticles and the means of detection employed (surface phosphatidylserine or protein antigen detection). This study provides analytical considerations for appropriate interpretation of microparticle cytofluorometric measurements in biological samples containing sPLA2 enzymes.

Interactions of platinum and ruthenium coordination complexes with pancreatic phospholipase A(2) and phospholipids investigated by MALDI TOF mass spectrometry

Phospholipase A2 is involved in propagation of inflammatory processes and carcinogenesis through its role in phospholipid metabolism, and release of arachidonic acid and lysophospholipids. Recent findings on correlation between elevated PLA2 activity and metastatic cancer render this enzyme an attractive target for cancer therapy. On the other hand, due to a broad range of oxidation states under physiological conditions and a high affinity for protein binding, platinum and ruthenium coordination complexes are promising candidates for PLA2 inhibitors. In this article, we discuss the interactions of Pt and Ru coordination complexes with PLA2 and phospholipids, as well as the application of MALDI-TOF mass spectrometry for screening PLA2 inhibitors. Owing to the ability of this technique to simultaneously detect and monitor changes in substrate and product concentrations, the inhibitor mechanisms of both Pt and Ru complexes with various ligands were determined.

Tailoring the pH dependence of human non-pancreatic secretory phospholipase A2 by engineering surface charges

Human non-pancreatic secretory phospholipase A2 (hnpsPLA2) catalyzes the sn-2 acyl hydrolysis of phospholipids. It was reported that hnpsPLA2 is involved in various diseases like inflammation, cancer, and so on. This enzyme also exhibits anti-bacterial and anti-virus activities. It is active over a broad pH range, with higher activity at alkaline conditions. In order to make it suitable as a potential bactericide, high activity at neutral pH is preferable. We have tried to tailor the pH dependence of hnpsPLA2 activity by replacing its surface charged residues. Three surface charge replacements, Arg42Glu, Arg100Glu, and Glu89Lys, showed increased activities at neutral pH, which are 2.3, 2.8, and 2.3 times that of the wild-type enzyme at pH 7. Both the positive-to-negative and negative-to-positive mutations lowered the optimum enzymatic reaction pH of hnpsPLA2, indicating that the enzyme pH profile depends on a delicate balance of charged residues. The activity changes are in good agreement with the recently proposed calcium-coordinated catalytic triad mechanism. This study also provides a general means of enhancing hnpsPLA2 activity at low pH.

Interactions of selected indole derivatives with phospholipase A₂: in silico and in vitro analysis

Phospholipase A2 (PLA₂) is one of the key enzymes involved in the formation of inflammatory mediators. Inhibition of PLA₂ is considered to be one of the efficient methods to control inflammation. In silico docking studies of 160 selected indole derivatives performed against porcine pancreatic PLA₂ (ppsPLA2) suggested that, CID2324681, CID8617 (indolebutyric acid or IBA), CID22097771 and CID802 (indoleacetic acid or IAA) exhibited highest binding energies. In silico analysis was carried out to predict some of the ADME properties. The binding potential of these compounds with human non pancreatic secretory PLA₂ (hnpsPLA₂) was determined using molecular docking studies. In order to corroborate the in silico results, enzyme kinetics and isothermal titration calorimetric analysis of the two selected compounds, IAA and IBA were performed against ppsPLA₂. From the analysis, it was concluded that IAA and IBA can act as competitive inhibitors to the enzyme and may be used as anti inflammatory agents.

Active compound from the leaves of Vitex negundo L. shows anti-inflammatory activity with evidence of inhibition for secretory Phospholipase A(2) through molecular docking

Novel compounds with significant medicinal properties have gained much interest in therapeutic approaches for treating various inflammatory disorders like arthritis, odema and snake bites and the post-envenom (impregnating with venom) consequences. Inflammation is caused by the increased concentration of secretory Phospholipases A(2) (sPLA(2)s) at the site of envenom. A novel compound Tris(2,4-di-tert-butylphenyl) phosphate (TDTBPP) was isolated from the leaves of Vitex negundo and the crystal structure was reported recently. The acute anti-inflammatory activity of TDTBPP was assessed by Carrageenan-induced rat paw odema method. TDTBPP reduced the raw paw odema volume significantly at the tested doses of 50 mg/kg and 70 mg/kg body weight. Molecular docking studies were carried out with the X-ray crystal structures of Daboia russelli pulchella's (Vipera russelli, Indian Russell's viper) venom sPLA(2) and Human non-pancreatic secretory PLA(2) (Hnps PLA(2)) as targets to illustrate the antiinflammatory and antidote activities of TDTBPP. Docking results showed hydrogen bond (H-bond) interaction with Lys69 residue lying in the anti-coagulant loop of D. russelli's venom PLA(2), which is essential in the catalytic activity of the enzyme and hydrophobic interactions with the residues at the binding site (His48, Asp49). Docking of TDTBPP with Hnps PLA(2) structure showed coordination with calcium ion directly as well as through the catalytically important water molecule (HOH1260) located at the binding site.

Active site mutants of human secreted Group IIA Phospholipase A2 lacking hydrolytic activity retain their bactericidal effect

The Human Secreted Group IIA Phospholipase A(2) (hsPLA2GIIA) presents potent bactericidal activity, and is considered to contribute to the acute-phase immune response. Hydrolysis of inner membrane phospholipids is suggested to underlie the bactericidal activity, and we have evaluated this proposal by comparing catalytic activity with bactericidal and liposome membrane damaging effects of the G30S, H48Q and D49K hsPLA2GIIA mutants. All mutants showed severely impaired hydrolytic activities against mixed DOPC:DOPG liposome membranes, however the bactericidal effect against Micrococcus luteus was less affected, with 50% killing at concentrations of 1, 3, 7 and 9 μg/mL for the wild-type, D49K, H48Q and G30S mutants respectively. Furthermore, all proteins showed Ca(2+)-independent damaging activity against liposome membranes demonstrating that in addition to the hydrolysis-dependent membrane damage, the hsPLA2GIIA presents a mechanism for permeabilization of phospholipid bilayers that is independent of catalytic activity, which may play a role in the bactericidal function of the protein.

Putative N-acylphosphatidylethanolamine synthase from Arabidopsis thaliana is a lysoglycerophospholipid acyltransferase

AT1G78690, a gene found in Arabidopsis thaliana, has been reported to encode a N-acyltransferase that transfers an acyl chain from acyl-CoA to the headgroup of phosphatidylethanolamine (PE) to form N-acylphosphatidylethanolamine (N-acyl-PE). Our investigation suggests that At1g78690p is not a PE-dependent N-acyltransferase but is instead a lysoglycerophospholipid O-acyltransferase. We overexpressed AT1G78690 in Escherichia coli, extracted the cellular lipids, and identified the accumulating glycerophospholipid as acylphosphatidylglycerol (acyl-PG). Electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-MS) analysis yielded [M - H](-) ions, corresponding by exact mass to acyl-PG rather than N-acyl-PE. Collision-induced dissociation mass spectrometry (MS/MS) yielded product ions consistent with acyl-PG. In addition, in vitro enzyme assays using both (32)P- and (14)C-radiolabeled substrates showed that AT1G78690 acylates 1-acyllysophosphatidylethanolamine (1-acyllyso-PE) and 1-acyllysophosphatidylglycerol (1-acyllyso-PG), but not PE or phosphatidylglycerol (PG), to form a diacylated product that co-migrates with PE and PG, respectively. We analyzed the diacylated product formed by AT1G78690 using a combination of base hydrolysis, phospholipase D treatment, ESI-MS, and MS/MS to show that AT1G78690 acylates the sn-2-position of 1-acyllyso-PE and 1-acyllyso-PG.

Secretory phospholipase A2 activity toward diverse substrates

We have studied secretory phospholipase A(2)-IIA (sPLA(2)) activity toward different phospholipid analogues by performing biophysical characterizations and molecular dynamics simulations. The phospholipids were natural substrates, triple alkyl phospholipids, a prodrug anticancer etherlipid, and an inverted ester. The latter were included to study head group-enzyme interactions. Our simulation results show that the lipids are optimally placed into the binding cleft and that water molecules can freely reach the active site through a well-defined pathway; both are indicative that these substrates are efficiently hydrolyzed, which is in good agreement with our experimental data. The phospholipid analogue with three alkyl side chains forms aggregates of different shapes with no well-defined sizes due to its cone-shape structure. Phosphatidylglycerol and phosphatidylcholine head groups interact with specific charged residues, but relatively large fluctuations are observed, suggesting that these interactions are not necessarily important for stabilizing substrate binding to the enzyme.

Production of vascular endothelial growth factors from human lung macrophages induced by group IIA and group X secreted phospholipases A2

Angiogenesis and lymphangiogenesis mediated by vascular endothelial growth factors (VEGFs) are main features of chronic inflammation and tumors. Secreted phospholipases A(2) (sPLA(2)s) are overexpressed in inflammatory lung diseases and cancer and they activate inflammatory cells by enzymatic and receptor-mediated mechanisms. We investigated the effect of sPLA(2)s on the production of VEGFs from human macrophages purified from the lung tissue of patients undergoing thoracic surgery. Primary macrophages express VEGF-A, VEGF-B, VEGF-C, and VEGF-D at both mRNA and protein level. Two human sPLA(2)s (group IIA and group X) induced the expression and release of VEGF-A and VEGF-C from macrophages. Enzymatically-inactive sPLA(2)s were as effective as the active enzymes in inducing VEGF production. Me-Indoxam and RO092906A, two compounds that block receptor-mediated effects of sPLA(2)s, inhibited group X-induced release of VEGF-A. Inhibition of the MAPK p38 by SB203580 also reduced sPLA(2)-induced release of VEGF-A. Supernatants of group X-activated macrophages induced an angiogenic response in chorioallantoic membranes that was inhibited by Me-Indoxam. Stimulation of macrophages with group X sPLA(2) in the presence of adenosine analogs induced a synergistic increase of VEGF-A release and inhibited TNF-alpha production through a cooperation between A(2A) and A(3) receptors. These results demonstrate that sPLA(2)s induce production of VEGF-A and VEGF-C in human macrophages by a receptor-mediated mechanism independent from sPLA(2) catalytic activity. Thus, sPLA(2)s may play an important role in inflammatory and/or neoplastic angiogenesis and lymphangiogenesis.

Snake venoms as a source of compounds modulating sperm physiology: Secreted phospholipases A2 from Oxyuranus scutellatus scutellatus impact sperm motility, acrosome reaction and in vitro fertilization in mice

The goal of this study was to identify new compounds from venoms able to modulate sperm physiology and more particularly sperm motility. For this purpose, we screened the effects of 16 snake venoms cleared of molecules higher than 15 kDa on sperm motility. Venoms rich in neurotoxins like those from Oxyuranus scutellatus scutellatus or Daboia russelii, were highly potent inhibitors of sperm motility. In contrast, venoms rich in myotoxins like those from Echis carinatus, Bothrops alternatus and Macrovipera lebetina, were inactive. From the main pharmacologically-active fraction of the Taipan snake O. scutellatus s., a proteomic approach allowed us to identify 16 different proteins, among which OS1 and OS2, two secreted phospholipases A2 (sPLA(2)). Purified OS1 and OS2 mimicked the inhibitory effect on sperm motility and were likely responsible for the inhibitory effect of the active fraction. OS1 and OS2 triggered sperm acrosome reaction and induced lipid rearrangements of the plasma membrane. The catalytic activity of OS2 was required to modulate sperm physiology since catalytically inactive mutants had no effect. Finally, sperm treated with OS2 were less competent than control sperm to initiate in vitro normal embryo development. This is the first report characterizing sPLA(2) toxins that modulate in vitro sperm physiology.

Effective approach for calculations of absolute stability of proteins using focused dielectric constants

The ability to predict the absolute stability of proteins based on their corresponding sequence and structure is a problem of great fundamental and practical importance. In this work, we report an extensive, refinement and validation of our recent approach (Roca et al., FEBS Lett 2007;581:2065-2071) for predicting absolute values of protein stability DeltaG(fold). This approach employs the semimacroscopic protein dipole Langevin dipole method in its linear response approximation version (PDLD/S-LRA) while using the best fitted values of the dielectric constants epsilon'(p) and epsilon'(eff) for the self energy and charge-charge interactions, respectively. The method is validated on a diverse set of 45 proteins. It is found that the best fitted values of both dielectric constants are around 40. However, the self energy of internal residues and the charge-charge interactions of Lys have to be treated with care, using a somewhat lower values of epsilon'(p) and epsilon'(eff). The predictions of DeltaG(fold) reported here, have an average error of only 1.8 kcal/mole compared to the observed values, making our method very promising for estimating protein stability. It also provides valuable insight into the complex electrostatic phenomena taking place in folded proteins.

Expression of a mutated phospholipase A2 in transgenic Aedes fluviatilis mosquitoes impacts Plasmodium gallinaceum development

The genetic manipulation of mosquito vectors is an alternative strategy in the fight against malaria. It was previously shown that bee venom phospholipase A2 (PLA2) inhibits ookinete invasion of the mosquito midgut although mosquito fitness was reduced. To maintain the PLA2 blocking ability without compromising mosquito biology, we mutated the protein-coding sequence to inactivate the enzyme while maintaining the protein's structure. DNA encoding the mutated PLA2 (mPLA2) was placed downstream of a mosquito midgut-specific promoter (Anopheles gambiae peritrophin protein 1 promoter, AgPer1) and this construct used to transform Aedes fluviatilis mosquitoes. Four different transgenic lines were obtained and characterized and all lines significantly inhibited Plasmodium gallinaceum oocyst development (up to 68% fewer oocysts). No fitness cost was observed when this mosquito species expressed the mPLA2.

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.

Deuterium quadrupolar tensors of L-histidine hydrochloride monohydrate-d7

A deuterium NMR study at 14 T of a single crystal of L-histidine hydrochloride monohydrate has determined the deuteron quadrupole coupling constants CQ, asymmetry parameters eta, and electric field gradient orientation for the imidazolium and primary ammonium groups and for a water of crystallization. The imidazolium deuterons, which have very long relaxation times, have quite different coupling constants, reflecting different hydrogen bonding but nearly identical orientations, with the most distinct principal axis in both cases nearly parallel to the N-D vector. The -ND3 groups undergo 3-fold hops about the C-N bond axis and have typical quadrupole couplings; the D2O undergoes 2-fold hops, leading to a tensor with a large asymmetry parameter. With appropriate corrections for vibrational averaging, density functional cluster calculations give an excellent fit to the imidazolium tensor magnitudes and orientations.

Using bacteria to express and display anti-Plasmodium molecules in the mosquito midgut

Bacteria capable of colonizing mosquito midguts are attractive vehicles for delivering anti-malaria molecules. We genetically engineered Escherichia coli to display two anti-Plasmodium effector molecules, SM1 and phospholipase-A(2), on their outer membrane. Both molecules significantly inhibited Plasmodium berghei development when engineered bacteria were fed to mosquitoes 24h prior to an infective bloodmeal (SM1=41%, PLA2=23%). Furthermore, prevalence and numbers of engineered bacteria increased dramatically following a bloodmeal. However, E. coli survived poorly in mosquitoes. Therefore, Enterobacter agglomerans was isolated from mosquitoes and selected for midgut survival by multiple passages through mosquitoes. After four passages, E. agglomerans survivorship increased from 2 days to 2 weeks. Since E. agglomerans is non-pathogenic and widespread, it is an excellent candidate for paratransgenic control strategies.

Lysis of human immunodeficiency virus type 1 by a specific secreted human phospholipase A2

Phospholipase A2 (PLA2) proteins affect cellular activation, signal transduction, and possibly innate immunity. A specific secretory PLA2, sPLA2-X, is shown here to neutralize human immunodeficiency virus type 1 (HIV-1) through degradation of the viral membrane. Catalytic function was required for antiviral activity, and the target cells of infection were unaffected. sPLA2-X potently reduced gene transfer of HIV-1 Env-pseudotyped lentivirus vectors and inhibited the replication of both CCR5- and CXCR4-tropic HIV-1 in human CD4+ T cells. Virions resistant to damage by antibody and complement were sensitive to lysis by sPLA2-X, suggesting a novel mechanism of antiviral surveillance independent of the acquired immune system.

Upregulation of group IB secreted phospholipase A(2) and its M-type receptor in rat ANTI-THY-1 glomerulonephritis

Treatment of rat glomerular mesangial cell (GMC) cultures with pancreatic secreted phospholipase A(2) (sPLA(2)-IB) results in an enhanced expression of sPLA(2)-IIA and COX-2, possibly via binding to its specific M-type sPLA(2) receptor. In the current study, we have investigated the expression and regulation of sPLA(2)-IB and its receptor during glomerulonephritis (GN). In vivo we used the well-established rat model of anti-Thy 1.1 GN (anti-Thy 1.1-GN) to study the expression of sPLA(2)-IB and the M-type sPLA(2) receptor by immunohistochemistry. In addition, in vitro we determined the interkeukin (IL)-1beta-regulated mRNA and protein expression in primary rat glomerular mesangial and endothelial cells as well as in rat peripheral blood leukocytes (PBLs). Shortly after induction of anti-Thy 1.1-GN, sPLA(2)-IB expression was markedly upregulated in the kidney at 6-24 h. Within glomeruli, the strongest sPLA(2)-IB protein expression was detected on infiltrated granulocytes and monocytes. However, at the same time, the M-type receptor was also markedly upregulated on resident glomerular cells. In vitro, the most prominent cytokine-stimulated secretion of sPLA(2)-IB was observed in monocytes isolated from rat PBLs. Treating glomerular endothelial cells (GECs) with cytokines elicited only weak sPLA(2)-IB expression, but treatment of these cells with exogenous sPLA(2)-IB resulted in a marked expression of the endogenous sPLA(2)-IB. Mesangial cells did not express sPLA(2)-IB at all. The M-type sPLA(2) receptor protein was markedly upregulated on cytokine-stimulated mesangial and endothelial cells as well as on lymphocytes and granulocytes. During anti-Thy 1.1 rat GN, sPLA(2)-IB and the M-type sPLA(2) receptor are induced as primary downstream genes stimulated by inflammatory cytokines. Subsequently, both sPLA(2)-IB and the M-type sPLA(2) receptor are involved in the autocrine and paracrine amplification of the inflammatory process in different resident and infiltrating cells.

Neurotoxicity and other pharmacological activities of the snake venom phospholipase A2 OS2: the N-terminal region is more important than enzymatic activity

Several snake venom secreted phospholipases A2 (sPLA2s) including OS2 exert a variety of pharmacological effects ranging from central neurotoxicity to anti-HIV activity by mechanisms that are not yet fully understood. To conclusively address the role of enzymatic activity and map the key structural elements of OS2 responsible for its pharmacological properties, we have prepared single point OS2 mutants at the catalytic site and large chimeras between OS2 and OS1, a homologous but nontoxic sPLA2. Most importantly, we found that the enzymatic activity of the active site mutant H48Q is 500-fold lower than that of the wild-type protein, while central neurotoxicity is only 16-fold lower, providing convincing evidence that catalytic activity is at most a minor factor that determines central neurotoxicity. The chimera approach has identified the N-terminal region (residues 1-22) of OS2, but not the central one (residues 58-89), as crucial for both enzymatic activity and pharmacological effects. The C-terminal region of OS2 (residues 102-119) was found to be critical for enzymatic activity, but not for central neurotoxicity and anti-HIV activity, allowing us to further dissociate enzymatic activity and pharmacological effects. Finally, direct binding studies with the C-terminal chimera, which poorly binds to phospholipids while it is still neurotoxic, led to the identification of a subset of brain N-type receptors which may be directly involved in central neurotoxicity.

Activation of Cytokine Production by Secreted Phospholipase A2 in Human Lung Macrophages Expressing the M-Type Receptor

Secreted phospholipases A(2) (sPLA(2)) are enzymes released in plasma and extracellular fluids during inflammatory diseases. Because human group IB and X sPLA(2)s are expressed in the lung, we examined their effects on primary human lung macrophages (HLM). Both sPLA(2)s induced TNF-alpha and IL-6 release in a concentration-dependent manner by increasing their mRNA expression. This effect was independent of their enzymatic activity because 1) the capacity of sPLA(2)s to mobilize arachidonic acid from HLM was unrelated to their ability to induce cytokine production; and 2) two catalytically inactive isoforms of group IB sPLA(2) (bromophenacyl bromide-inactivated human sPLA(2) and the H48Q mutant of the porcine sPLA(2)) were as effective as the catalytically active sPLA(2)s in inducing cytokine production. HLM expressed the M-type receptor for sPLA(2)s at both mRNA and protein levels, as determined by RT-PCR, immunoblotting, immunoprecipitation, and flow cytometry. Me-indoxam, which decreases sPLA(2) activity as well as binding to the M-type receptor, suppressed sPLA(2)-induced cytokine production. Incubation of HLM with the sPLA(2)s was associated with phosphorylation of ERK1/2, and a specific inhibitor of this pathway, PD98059, significantly reduced the production of IL-6 elicited by sPLA(2)s. In conclusion, two distinct sPLA(2)s produced in the human lung stimulate cytokine production by HLM via a mechanism that is independent of their enzymatic activity and involves activation of the ERK1/2 pathway. HLM express the M-type receptor, but its involvement in eliciting cytokine production deserves further investigation.

Investigating porcine pancreatic phospholipase A2 action on vesicles and supported planar bilayers using a quartz crystal microbalance with dissipation

We present an investigation of the activity of porcine pancreatic phospholipase A2 towards phospholipids. The phospholipids are presented in three different ways, namely as tethered vesicles, intact surface-bound vesicles, and supported planar bilayers (SPBs). The process is followed using a quartz crystal microbalance which measures both the frequency shift and the energy dissipation factor. This technique is very sensitive not only to the mass of the material deposited on the crystal, but also to its viscoelasticity. The breakdown of the phospholipid vesicles and bilayers consequently gives rise to very large signal changes. Enzyme binding is separated from vesicle hydrolysis using nonhydrolyzable ether lipids. Intact and tethered vesicles give rise to the same profile, indicating that direct immobilization of the vesicles does not affect hydrolysis significantly. The data fit well to a Voight-based model describing the change in film structure with time. Initial enzyme binding to intact vesicles is accompanied by a significant increase in layer thickness as well as a decrease in viscosity and shear modulus. This effect, which is less pronounced in SPBs, is probably mainly due to the accumulation of hydrolysis products in the vesicle prior to rupture of the vesicles and release of bound water, since it disappears when lysolipid is included in the vesicles prior to hydrolysis.

Potentiation of tumor necrosis factor alpha-induced secreted phospholipase A2 (sPLA2)-IIA expression in mesangial cells by an autocrine loop involving sPLA2 and peroxisome proliferator-activated receptor alpha activation

In rat mesangial cells, exogenously added secreted phospholipases A2 (sPLA2s) potentiate the expression of pro-inflammatory sPLA2-IIA first induced by cytokines like tumor necrosis factor-alpha (TNFalpha) and interleukin-1 beta. The transcriptional pathway mediating this effect is, however, unknown. Because products of PLA2 activity are endogenous activators of peroxisome proliferator-activated receptor alpha (PPAR alpha, we postulated that sPLA2s mediate their effects on sPLA2-IIA expression via sPLA2 activity and subsequent PPAR alpha activation. This study shows that various sPLA2s, including venom enzymes, human sPLA2-IIA, and wild-type and catalytically inactive H48Q mutant of porcine pancreatic sPLA2-IB, enhance the TNF alpha-induced sPLA2-IIA expression at the mRNA and protein levels. In cells transfected with luciferase sPLA2-IIA promoter constructs, sPLA2s are active only when the promoter contains a functional PPRE-1 site. The effect of exogenous sPLA2s is also blocked by the PPAR alpha inhibitor MK886. Interestingly, the expression of sPLA2-IIA induced by TNF alpha alone is also attenuated by MK886, by the sPLA2-IIA inhibitor LY311727, by heparinase, which prevents the binding of sPLA2-IIA to heparan sulfate proteoglycans, and by the specific cPLA2-alpha inhibitor pyrrolidine-1. Together, these data indicate that sPLA2-IIA released from mesangial cells by TNF alpha stimulates its own expression via an autocrine loop involving cPLA2 and PPAR alpha. This signaling pathway is also used by exogenously added sPLA2s including pancreatic sPLA2-IB and is distinct from that used by TNF alpha.

Chemical denaturation of a homodimeric lysine-49 phospholipase A2: a stable dimer interface and a native monomeric intermediate

Bothropstoxin I (4BthTx-I) is a homodimeric lysine-49 (Lys49) phospholipase A(2) isolated from Bothrops jararacussu venom, which damages liposome membranes via a Ca(2+)-independent mechanism. The stability of the BthTx-I homodimer was evaluated by equilibrium chemical denaturation with guanidinium hydrochloride monitored by changes in the intrinsic tryptophan fluorescence anisotropy, far-UV circular dichroism, dynamic light scattering, and 1-anilinonaphthalene-8-sulfonate binding. Unfolding of the BthTx-I dimer proceeds via a monomeric intermediate with native-like structure, with Gibbs free energy (DeltaG(0)) values of 10.0 and 7.2 kcal mol(-1) for the native dimer-to-native monomer and native-to-denatured monomer transitions, respectively. The experimentally determined DeltaG(0) value for the dimer-to-native monomer transition is higher than the value expected for an interaction dominated by hydrophobic forces, and suggests that an unusually high propensity of hydrogen-bonded side chains found at the BthTx-I homodimer interface make a significant contribution to dimer stability.

Simple intrasequence difference (SID) analysis: an original method to highlight and rank sub-structural interfaces in protein folds. Application to the folds of bovine pancreatic trypsin inhibitor, phospholipase A2, chymotrypsin and carboxypeptidase A

We present Simple Intrasequence Difference (SID) analysis, a novel bioinformatic technique designed to help comprehend the properties of protein fold topologies. The analysis grades numerically every residue position in a given protein 3D structure according to the topological situation of the position in the folded chain. This results in an expression of the potential contribution of each residue position and its vicinity towards the integrity of the molecular conformation. Contiguous highly graded residues delineate the sub-structural interfaces that arise from the presence within the molecular fold of discrete domains and sub-domains. This comprehensive rendering of the internal arrangement of chain interfacing helps predict the potential for site-specific inductions (e.g. via mutations or ligand binding) of conformational change in the fold. Whereas SID analysis of single folds can convey an idea of the basic potential for topological adjustment in the protein family, comparative SID analysis of related folds focuses attention on those areas of the family fold where evolutionary changes, activation events and ligand binding have had the most topological impact. For demonstration, SID analysis is applied to the folds of pancreatic trypsin inhibitor (Kunitz), phospholipase A(2), chymotrypsin and carboxypeptidase A. We find that many of the potentially vulnerable sub-structural interfaces tend to be protected in the fold interior, in many cases stabilised by disulfide bridges spanning the interface. However, the most prominent interfaces tend to be externally accessible, without remedial stabilisation by disulfide bridges. These latter interfaces are associated so closely with the known functional sites that alterations to the interfacial juxtapositions should influence recognition and catalytic behaviour directly. This shows how side chain mutations, chemical modifications and binding events remote from the sites can nevertheless adjust, via interfacial realignment, the conformations and emergent properties of the sites. The close association also provides clear opportunities for interfacial rearrangements to follow intermolecular recognition events in the sites, facilitating translation of the binding into adjustment of the molecular conformation in areas distant from the sites. As a direct consequence of the topological arrangements, a large proportion of the molecular structure has the capacity to shape the character of the functional sites and, conversely, binding at these sites has the potential to radiate influence to the rest of the molecule. For the enzymes considered, the evidence is consistent with the possibility that primary and secondary binding by the substrate enhances catalytic efficiency by imposing conformational change upon the catalytic centre via adjustments to the fold. This influence may be expressed as favourable adjustment of the catalytic geometry, transition state ensemble, energy propagation pathway, or as a physical strain exerted on the substrate bond to be cleaved. The scale of the adjustments, and their importance to the mechanisms, may have been seriously underestimated.

Active-site mutagenesis of a Lys49-phospholipase A2: biological and membrane-disrupting activities in the absence of catalysis

Bothropstoxin-I (BthTx-I) is a myotoxic phospholipase A(2) variant present in the venom of Bothrops jararacussu, in which the Asp(49) residue is replaced with a lysine, which damages artificial membranes by a Ca(2+)-independent mechanism. Wild-type BthTx-I and the mutants Lys(49)-->Asp, His(48)-->Gln and Lys(122)-->Ala were expressed in Escherichia coli BL21(DE3) cells, and the hydrolytic, myotoxic and membrane-damaging activities of the recombinant proteins were compared with native BthTx-I purified from whole venom. The Ca(2+)-independent membrane-damaging and myotoxic activities of the native and wild-type recombinant BthTx-I, His(48)Gln and Lys(49)Asp mutants were similar; however, the Lys(122)Ala mutant demonstrated reduced levels of both activities. Although a low hydrolytic activity against a mixed phospholipid substrate was observed with native BthTx-I, no substrate hydrolysis was detected with the wild-type recombinant enzyme or any of the mutants. In the case of the Lys(49)Asp mutant, this demonstrates that the absence of catalytic activity in Lys(49)-PLA(2) is not a consequence of the single Asp(49)-->Lys replacement. Furthermore, these results provide unambiguous evidence that the Ca(2+)-independent membrane-damaging and myotoxic activities are maintained in the absence of hydrolysis. The evidence favours a model for a hydrolysis-independent, membrane-damaging mechanism involving an interaction of the C-terminal region of BthTx-I with the target membrane.

Secreted phospholipase A2 induces vascular endothelial cell migration

Secreted phospholipase A2 (sPLA2) regulates a variety of cellular functions. The present investigation was undertaken to elucidate the potential role of sPLA2 in endothelial cell (EC) migration. Bovine aortic endothelial cells (BAECs) exposed to sPLA2 placed in the lower compartment of a modified Boyden chamber displayed increased migration compared to cells exposed to vehicle. The effect of sPLA2 on EC migration was time and dose dependent. Migration of BAECs was observed at 30 minutes, increased over 1 to 2 hours, and declined thereafter. At 2 hours of stimulation, sPLA2 (0.01-2 μmol/L) induced 1.2- to 3-fold increased cell migration compared with media alone. Among the different sPLA2s tested, bee venom, Naja naja, and porcine and human pancreatic PLA2s all evoked a migratory response in ECs. Moreover, human synovial fluid, obtained from patients with arthritis and containing sPLA2 activity, induced EC migration. Migration of ECs was significantly reduced after exposure to a catalytic site mutant of pancreatic sPLA2with decreased lipolytic activity as compared to wild-type sPLA2. Similarly, pretreatment of human synovial fluid withp-bromophenacyl bromide, an irreversible inhibitor of sPLA2, markedly decreased the ability of human synovial fluid to stimulate EC migration. Moreover, migration of ECs was stimulated on exposure to hydrolytic products of sPLA2activity including arachidonic acid, lysophosphatidic acid, and lysophosphatidylcholine. These findings suggest that sPLA2plays a physiologic role in induction of EC migration. Moreover, the effects of sPLA2 on EC migration are mediated, at least in part, by its catalytic activity.

Secreted phospholipase A2 induces vascular endothelial cell migration

Secreted phospholipase A2 (sPLA2) regulates a variety of cellular functions. The present investigation was undertaken to elucidate the potential role of sPLA2 in endothelial cell (EC) migration. Bovine aortic endothelial cells (BAECs) exposed to sPLA2 placed in the lower compartment of a modified Boyden chamber displayed increased migration compared to cells exposed to vehicle. The effect of sPLA2 on EC migration was time and dose dependent. Migration of BAECs was observed at 30 minutes, increased over 1 to 2 hours, and declined thereafter. At 2 hours of stimulation, sPLA2 (0.01-2 μmol/L) induced 1.2- to 3-fold increased cell migration compared with media alone. Among the different sPLA2s tested, bee venom, Naja naja, and porcine and human pancreatic PLA2s all evoked a migratory response in ECs. Moreover, human synovial fluid, obtained from patients with arthritis and containing sPLA2 activity, induced EC migration. Migration of ECs was significantly reduced after exposure to a catalytic site mutant of pancreatic sPLA2with decreased lipolytic activity as compared to wild-type sPLA2. Similarly, pretreatment of human synovial fluid withp-bromophenacyl bromide, an irreversible inhibitor of sPLA2, markedly decreased the ability of human synovial fluid to stimulate EC migration. Moreover, migration of ECs was stimulated on exposure to hydrolytic products of sPLA2activity including arachidonic acid, lysophosphatidic acid, and lysophosphatidylcholine. These findings suggest that sPLA2plays a physiologic role in induction of EC migration. Moreover, the effects of sPLA2 on EC migration are mediated, at least in part, by its catalytic activity.

Enzymatic properties of rat group IIA and V phospholipases A(2) compared

Group IIA and V phospholipases A(2) (PLA(2)s) are known to play a role in inflammatory responses. We have constructed a bacterial expression vector for rat group IIA and V PLA(2)s, over-expressed, folded and purified the proteins with the aim to study and compare the properties of the enzymes in detail. For zwitterionic phospholipid micelles, both enzymes display optimum activity at pH 8. 0 and absolutely require Ca(2+) for enzymatic activity. In the presence of substrate, group V PLA(2) has a high affinity for Ca(2+) (K(Ca2+)=90 microM) while K(Ca2+) of group IIA PLA(2) was found to be 1.6 mM. The absence of substrate only marginally influences the Ca(2+) affinities. In contrast to group IIA PLA(2), group V PLA(2) does not show a jump in the activity profile at substrate concentrations around the critical micelle concentration. Direct binding studies using n-alkylphosphocholines indicate that group V PLA(2) forms protein-lipid aggregates at pre-micellar lipid concentrations in a cooperative and Ca(2+)-dependent manner. This behavior, which is comparable to that observed for the PLA(2) from Naja melanoleuca snake venom, reflects the high affinity of this enzyme for zwitterionic phospholipids. Competitive inhibition by the substrate analogues (R)-2-dodecanoylaminohexanol-1-phosphocholine and its phosphoglycol derivative was tested on zwitterionic micelles as substrate. Group IIA PLA(2) shows a preference for the phosphoglycol inhibitor whereas the phosphocholine inhibitor binds stronger to the active site of group V PLA(2). The enzymatic activity was also measured on zwitterionic liposomes which appear to be much better substrates for group V PLA(2) than for group IIA PLA(2). The overall results suggest that group V PLA(2) is better suited for action on biological membranes than group IIA PLA(2).