Calmodulin is a nonessential activator of secretory phospholipase A(2)

Secreted Phospholipases A2 - not just Enzymes: Revisited

Secreted phospholipases A₂ (sPLA₂s) participate in a very broad spectrum of biological processes through their enzymatic activity and as ligands for membrane and soluble receptors. The physiological roles of sPLA₂s as enzymes have been very well described, while their functions as ligands are still poorly known. Since the last overview of sPLA₂-binding proteins (sPLA₂-BPs) 10 years ago, several important discoveries have occurred in this area. New and more sensitive analytical tools have enabled the discovery of additional sPLA₂-BPs, which are presented and critically discussed here. The structural diversity of sPLA₂-BPs reveals sPLA₂s as very promiscuous proteins, and we offer some structural explanations for this nature that makes these proteins evolutionarily highly advantageous. Three areas of physiological engagement of sPLA₂-BPs have appeared most clearly: cellular transport and signalling, and regulation of the enzymatic activity of sPLA₂s. Due to the multifunctionality of sPLA₂s, they appear to be exceptional pharmacological targets. We reveal the potential to exploit interactions of sPLA₂s with other proteins in medical terms, for the development of original diagnostic and therapeutic procedures. We conclude this survey by suggesting the priority questions that need to be answered.

Synergy between 15-lipoxygenase and secreted PLA2 promotes inflammation by formation of TLR4 agonists from extracellular vesicles

Damage-associated endogenous molecules induce innate immune response, thus making sterile inflammation medically relevant. Stress-derived extracellular vesicles (stressEVs) released during oxidative stress conditions were previously found to activate Toll-like receptor 4 (TLR4), resulting in expression of a different pattern of immune response proteins in comparison to lipopolysaccharide (LPS), underlying the differences between pathogen-induced and sterile inflammation. Here we report that synergistic activities of 15-lipoxygenase (15-LO) and secreted phospholipase A₂ (sPLA₂) are needed for the formation of TLR4 agonists, which were identified as lysophospholipids (lysoPLs) with oxidized unsaturated acyl chain. Hydroxy, hydroperoxy, and keto products of 2-arachidonoyl-lysoPI oxidation by 15-LO were identified by mass spectrometry (MS), and they activated the same gene pattern as stressEVs. Extracellular PLA₂ activity was detected in the synovial fluid from rheumatoid arthritis and gout patients. Furthermore, injection of sPLA₂ promoted K/BxN serum-induced arthritis in mice, whereby ankle swelling was partially TLR4 dependent. Results confirm the role of oxidized lysoPL of stressEVs in sterile inflammation that promotes chronic diseases. Both 15-LO and sPLA₂ enzymes are induced during inflammation, which opens the opportunity for therapy without compromising innate immunity against pathogens.

Effect of Modifier Structure on the Activation of Leukotriene A4 Hydrolase Aminopeptidase Activity

Activation of the leukotriene A4 hydrolase (LTA4H) aminopeptidase (AP) activity with 4-methoxydiphenylmethane (4MDM) promoted resolution of neutrophil infiltration in a murine cigarette smoke-induced model for emphysematous chronic obstructive pulmonary disease. Recently, 4-(4-benzylphenyl)thiazol-2-amine (ARM1) was published as a ligand for LTA4H with potential anti-inflammatory properties. To investigate the effect of modifier structure on enzyme kinetics of LTA4H, a series of analogues bearing structural features of ARM1 and 4MDM were synthesized using trifluoroborate Suzuki coupling reactions. Following, the 2.8 Å X-ray crystal structure of LTA4H complexed with 4-OMe-ARM1, a 4MDM-ARM1 hybrid molecule, was determined. Kinetic analysis showed that ARM1 and related analogues lowered affinity for the enzyme-substrate complex, resulting in a change of mechanism from hyperbolic mixed predominately catalytic activation (HMx(Sp < Ca)A) as observed for 4MDM to a predominately specific activation (HMx(Sp > Ca)A) mechanism. 4-OMe-ARM1 was then shown to dose responsively reduce LTB4 production in human neutrophils.

Current insights into functions of phospholipase A2 receptor in normal and cancer cells: More questions than answers

Lipid signaling network was proposed as a potential target for cancer prevention and treatment. Several recent studies revealed that phospholipid metabolising enzyme, phospholipase A2 (PLA₂), is a critical regulator of cancer accelerating pathologies and apoptosis in several types of cancers. In addition to functioning as an enzyme, PLA₂ can activate a phospholipase A2 receptor (PLA2R1) in plasma membrane. While the list of PLA₂ targets extends to glucose homeostasis, intracellular energy balance, adipocyte development, and hepatic lipogenesis, the PLA2R1 downstream effectors are few and scarcely investigated. Among the most addressed PLA2R1 effects are regulation of pro-inflammatory signaling, autoimmunity, apoptosis, and senescence. Localized in glomeruli podocytes, the receptor can be identified by circulating anti-PLA2R1 autoantibodies leading to development of membranous nephropathy, a strong autoimmune inflammatory cascade. PLA2R1 was shown to induce activation of Janus-kinase 2 (JAK2) and estrogen-related receptor α (ERRα)-controlled mitochondrial proteins, as well as increasing the accumulation of reactive oxygen species, thus leading to apoptosis and senescence. These findings indicate the potential role of PLA2R1 as tumor suppressor. Epigenetic investigations addressed the role of DNA methylation, histone modifications, and specific microRNAs in the regulation of PLA2R1 expression. However, involvement of PLA2R1 in suppression of malignant growth and metastasis remains controversial. In this review, we summarize the recent findings that highlight the role of PLA2R1 in the regulation of carcinogenesis-related intracellular signaling.

The neurotoxic secreted phospholipase A2 from the Vipera a. ammodytes venom targets cytochrome c oxidase in neuronal mitochondria

The β-neurotoxic secreted phospholipases A2 (sPLA2s) block neuro-muscular transmission by poisoning nerve terminals. Damage inflicted by such sPLA2s (β-ntx) on neuronal mitochondria is characteristic, very similar to that induced by structurally homologous endogenous group IIA sPLA2 when its activity is elevated, as, for example, in the early phase of Alzheimer's disease. Using ammodytoxin (Atx), the β-ntx from the venom of the nose-horned viper (Vipera a. ammodytes), the sPLA2 receptor R25 has been detected in neuronal mitochondria. This receptor has been purified from porcine cerebral cortex mitochondria by a new Atx-affinity-based chromatographic procedure. Mass spectrometry analysis revealed R25 to be the subunit II of cytochrome c oxidase (CCOX), an essential constituent of the respiratory chain complex. CCOX was confirmed as being the first intracellular membrane receptor for sPLA2 by alternative Atx-affinity-labellings of purified CCOX, supported also by the encounter of Atx and CCOX in PC12 cells. This discovery suggests the explanation of the mechanism by which β-ntx hinders production of ATP in poisoned nerve endings. It also provides a new insight into the potential function and dysfunction of endogenous GIIA sPLA2 in mitochondria.

Neurotoxic phospholipase A2 from rattlesnake as a new ligand and new regulator of prokaryotic receptor GLIC (proton-gated ion channel from G. violaceus)

Neurotoxic phospholipases A2 (sPLA2) from snake venoms interact with various protein targets with high specificity and potency. They regulate function of multiple receptors or channels essential to life processes including neuronal or neuromuscular chemoelectric signal transduction. These toxic sPLA2 exhibit high pharmacological potential and determination of PLA2-receptor binding sites represents challenging part in the receptor-channel biochemistry and pharmacology. To investigate the mechanism of interaction of neurotoxic PLA2 with its neuronal receptor at the molecular level, we used as a model crotoxin, a heterodimeric sPLA2 from rattlesnake venom and proton-gated ion channel GLIC, a bacterial homolog of pentameric ligand-gated ion channels. The three-dimensional structures of both partners, crotoxin and GLIC have been solved by X-ray crystallography and production of full-length pentameric GLIC (with ECD and TM domains) is well established. In the present study, for the first time, we demonstrated physical and functional interaction of full-length purified and solubilized GLIC with CB, (PLA2 subunit of crotoxin). We identified GLIC as a new protein target of CB and CB as a new ligand of GLIC, and showed that this non covalent interaction (PLA2-GLIC) involves the extracellular domain of GLIC. We also determined a novel function of CB as an inhibitor of proton-gated ion channel activity. In agreement with conformational changes observed upon formation of the complex, CB appears to be negative allosteric modulator (NAM) of GLIC. Finally, we proposed a possible stoichiometric model for CB - GLIC interaction based on analytical ultracentrifugation.

Ammodytoxins efficiently release arachidonic acid and induce apoptosis in a motoneuronal cell line in an enzymatic activity-dependent manner

Secreted phospholipases A2 (sPLA2s) are phospholipolytic enzymes and receptor ligands whose action affects cell death and survival. We have previously shown that ammodytoxin A (AtxA), a snake venom sPLA2, is rapidly internalized into motoneuronal NSC34 cells, inducing characteristic neurotoxic sPLA2 cell damage and apoptosis. In this study, we have analyzed the role of sPLA2 enzymatic activity, including arachidonic acid (AA) release, in the induction of motoneuronal apoptosis by AtxA and homologous recombinant sPLA2s with different enzymatic properties: an AtxA mutant (V31W) with very high enzymatic activity, enzymatically inactive S49-sPLA2 (ammodytin L, AtnL), its mutant (LW) with restored enzymatic activity, and non-toxic, enzymatically active sPLA2 (AtnI2). Addition of AA, AtxA, AtxA-V31W and AtnL-LW, but not AtnL and AtnI2, to NSC34 cells resulted in caspase-3 activation, DNA fragmentation and disruption of mitochondrial membrane potential, leading to a significant and rapid decrease in motoneuronal cell viability that was not observed in C2C12 myoblasts and HEK293 cells. AtxA, AtxA-V31W and AtnL-LW, but not AtnL and AtnI2, also liberated large amounts of AA specifically from motoneuronal cells, and this ability correlated well with the ability to induce apoptotic changes and decrease cell viability. The enzymatic activity of AtxA and similar sPLA2s is thus necessary, but not sufficient, for inducing motoneuronal apoptosis. This suggests that specific binding to the motoneuronal cell surface, followed by internalization and enzymatic activity-dependent induction of apoptosis, possibly as a consequence of extensive extra- and intracellular AA release, is necessary for Atx-induced motoneuronal cell death.

Crystallographic characterization of functional sites of crotoxin and ammodytoxin, potent β-neurotoxins from Viperidae venom

This review will focus on a description of the three-dimensional structures of two β-neurotoxins, the monomeric PLA(2) ammodytoxin from Vipera ammodytes ammodytes, and heterodimeric crotoxin from Crotalus durissus terrificus, and a detailed structural analysis of their multiple functional sites. We have recently determined at high resolution the crystal structures of two natural isoforms of ammodytoxin (AtxA and AtxC) (Saul et al., 2010) which exhibit different toxicity profiles and different anticoagulant properties. Comparative structural analysis of these two PLA(2) isoforms, which differ only by two amino acid residues, allowed us to detect local conformational changes and delineate the role of critical residues in the anticoagulant and neurotoxic functions of these PLA(2) (Saul et al., 2010). We have also determined, at 1.35Å resolution, the crystal structure of heterodimeric crotoxin (Faure et al., 2011). The three-dimensional structure of crotoxin revealed details of the binding interface between its acidic (CA) and basic (CB) subunits and allowed us to identify key residues involved in the stability and toxicity of this potent heterodimeric β-neurotoxin (Faure et al., 2011). The precise spatial orientation of the three covalently linked polypeptide chains in the mature CA subunit complexed with CB helps us to understand the role played by critical residues of the CA subunit in the increased toxicity of the crotoxin complex. Since the CA subunit is a natural inhibitor of the catalytic and anticoagulant activities of CB, identification of the CA-CB binding interface describes residues involved in this inhibition. We propose future research directions based on knowledge of the recently reported 3D structures of crotoxin and ammodytoxin.

A recent evaluation of the lethal potencies of ammodytoxins

Ammodytoxin A (AtxA) is the most toxic secreted phospholipase A(2) of the three isotoxins with presynaptic neurotoxicity, isolated from the venom of the nose-horned viper (Vipera ammodytes ammodytes), with an LD(50) of 21 μg/kg in mice. The toxic potencies of two other isoforms have been re-evaluated using highly purified recombinant proteins, with their intraperitoneal LD(50)s determined as 960 μg/kg for AtxB and 310 μg/kg for AtxC. AtxB and AtxC differ from AtxA in only three and two amino acid residues, respectively.

Structural basis of the significant calmodulin-induced increase in the enzymatic activity of secreted phospholipases A(2)

Ammodytoxin (Atx), a neurotoxic secreted phospholipase A(2) (sPLA(2)), forms a high-affinity complex with calmodulin (CaM). The latter substantially increases the enzymatic activity of Atx under both non-reducing and reducing conditions, and the activity enhancement was accompanied, but not caused, by conformational stabilization of the enzyme. In this work, the energetically most favorable model of the complex was generated, making use of interaction site mapping, mutagenesis data and protein-docking algorithms. The model explains, in structural terms, the observed effects of stabilization and activity enhancement of the neurotoxic sPLA(2) by CaM. The structures of four mammalian sPLA(2) isoforms, groups IB, IIA, V and X, having the same fold as Atx, were superimposed on the structure of Atx in the complex with CaM. According to the generated models, the group V and X sPLA(2)s, but not the group IB and IIA enzymes, form stable complexes with CaM, which should also result in the augmentation of their enzymatic activity. By confirming the latter, the presented model is validated as a valuable tool to investigate the as yet unexplained role of CaM in the pathophysiology of snake venom and mammalian sPLA(2)s.