Complex formation with pentraxin-2 regulates factor X plasma levels and macrophage interactions.

Recently, we have identified scavenger receptor class A member I (SR-AI) as a receptor for coagulation factor X (FX), mediating the formation of an FX reservoir at the macrophage surface. Here, we demonstrate that the FX/SR-AI-complex comprises a third protein, pentraxin-2 (PTX2). The presence of PTX2 is essential to prevent internalization of FX by SR-AI, and the presence of FX is needed to interfere with internalization of PTX2. Binding studies showed that FX, SR-AI, and PTX2 independently bind to each other (KD,app: 0.2-0.7 µM). Surprisingly, immunoprecipitation experiments revealed that FX and PTX2 circulate as a complex in plasma, and complex formation involves the FX activation peptide. No binding of PTX2 to other vitamin K-dependent proteins was observed. Short hairpin RNA-mediated inhibition of PTX2 levels in mice resulted not only in reduced levels of PTX2, but also in similarly reduced FX levels. Moreover, PTX2 and FX levels were correspondingly reduced in SR-AI-deficient mice. Analysis of 71 human plasma samples uncovered a strong correlation between FX and PTX2 plasma levels. Furthermore, plasma samples of patients with reduced FX levels (congenital/acquired FX deficiency or after anti-vitamin K treatment) were characterized by concomitantly decreased PTX2 levels. In conclusion, we identified PTX2 as a novel partner for FX, and both proteins cooperate to prevent their SR-AI-mediated uptake by macrophages. Interestingly, their respective plasma levels are interdependent. These findings seem of relevance in perspective of ongoing clinical trials, in which plasma depletion of PTX2 is used as a therapeutical approach in the management of systemic amyloidosis.

Macrophage receptor SR-AI is crucial to maintain normal plasma levels of coagulation factor X.

Beside its classical role in the coagulation cascade, coagulation factor X (FX) is involved in several major biological processes including inflammation and enhancement of virus-induced immune responses. We recently reported that the long circulatory half-life of FX is linked to its interaction with liver-resident macrophages. Importantly, we now observed that macrophages, but not undifferentiated monocytes, support this interaction. Using cell biology approaches with primary and THP1-derived macrophages as well as transfected cells, we further identified the scavenger receptor type A member I (SR-AI) to be a macrophage-specific receptor for FX. This result was confirmed using SR-AI-deficient mice, which exhibit reduced circulating levels of FX in vivo and loss of FX-macrophage interactions in vitro. Binding studies using purified proteins revealed that FX binds specifically (half-maximal binding, 3 µg/mL) to the extracellular domain of SR-AI. Altogether, we demonstrate that macrophages regulate FX plasma levels in an SR-AI-dependent manner.

A Novel Single-Domain Antibody Against von Willebrand Factor A1 Domain Resolves Leukocyte Recruitment and Vascular Leakage During Inflammation-Brief Report.

OBJECTIVE: von Willebrand factor (VWF) is crucial to hemostasis, but also plays a role in inflammatory processes. Unfortunately, no proper monoclonal antibodies to study VWF function in mice are currently available. We therefore aimed to generate single-domain antibodies (sdAbs) recognizing murine VWF and blocking its function in vivo. APPROACH AND RESULTS: Llama-derived sdAbs recognizing both human and murine VWF were isolated via phage display technology. One of them (designated KB-VWF-006) recognized the VWF A1 domain with picomolar affinity. This sdAb avidity was strongly enhanced via dimerization using a triple Ala linker (KB-VWF-006bi). When administered in vivo to wild-type mice, KB-VWF-006bi dose dependently induced bleeding in a tail clip model. In 2 distinct models of inflammation, KB-VWF-006bi efficiently interfered with leukocyte recruitment and vascular leakage. CONCLUSIONS: KB-VWF-006bi is an sdAb recognizing the A1 domain of human VWF and murine VWF that interferes with VWF-platelet interactions in vivo. By using this sdAb, we now also show that the A1 domain is pertinent to the participation of VWF in the inflammatory response.

Impaired platelet activation and cAMP homeostasis in MRP4-deficient mice.

Molecules that reduce the level of cyclic adenosine 5'-monophosphate (cAMP) in the platelet cytosol, such as adenosine 5'-diphosphate (ADP) secreted from dense granules, trigger platelet activation. Therefore, any change in the distribution and/or availability of cyclic nucleotides or ADP may interfere with platelet reactivity. In this study, we evaluated the role of multidrug resistance protein 4 (MRP4, or ABCC4), a nucleotide transporter, in platelet functions in vivo and in vitro by investigating MRP4-deficient mice. MRP4 deletion resulted in a slight increase in platelet count but had no impact on platelet ultrastructure. In MRP4-deficient mice, the arterial occlusion was delayed and the tail bleeding time was prolonged. In a model of platelet depletion and transfusion mimicking a platelet-specific knockout, mice injected with MRP4(-/-) platelets also showed a significant increase in blood loss compared with mice injected with wild-type platelets. Defective thrombus formation and platelet activation were confirmed in vitro by studying platelet adhesion to collagen in flow conditions, integrin αIIbß3 activation, washed platelet secretion, and aggregation induced by low concentrations of proteinase-activated receptor 4-activating peptide, U46619, or ADP. We found no role of MRP4 in ADP dense-granule storage, but MRP4 redistributed cAMP from the cytosol to dense granules, as confirmed by increased vasodilator-stimulated phosphoprotein phosphorylation in MRP4-deficient platelets. These data suggest that MRP4 promotes platelet aggregation by modulating the cAMP-protein kinase A signaling pathway, suggesting that MRP4 might serve as a target for novel antiplatelet agents.

PIVL, a snake venom Kunitz-type serine protease inhibitor, inhibits in vitro and in vivo angiogenesis.

Development and homeostasis of the vascular system requires integrin-promoting endothelial cell adhesion, migration and survival. Nowadays, integrins represent potential targets for pharmacological agents and open new avenues for the control of metastatic spread in the treatment of tumor malignancies. We have already reported that PIVL, a serine protease inhibitor isolated from Macrovipera lebetina venom, displays an anti-tumor effect through interference with integrin receptor function. Here, we report that PIVL inhibits human vascular endothelial cell adhesion and migration onto fibrinogen and fibronectin in a dose-dependent manner without any cytotoxicity. Furthermore, we show that PIVL increases microtubule dynamic instability in HMEC-1 transfected with EGFP-tagged α-tubulin. Using Matrigel™ and chick chorioallantoic membrane assays, we demonstrate that PIVL exhibits a strong anti-angiogenic effect both in vitro and in vivo. Interestingly, results herein reveal that the potent anti-angiogenic properties of PIVL are mediated by its RGD-like motif ((41)RGN(43)).

C-type lectin protein isoforms of Macrovipera lebetina: cDNA cloning and genetic diversity.

Snake venom contains a complex protein mixture belonging to a few well-characterized protein families: disintegrins, phospholipase A2, serine protease, l-amino acid oxidase, Zn-dependent metalloproteinase, natriuretic peptides, myotoxins, cysteine-rich secretory protein (CRISP) toxins, Kunitz-type protease inhibitors and C-type lectin-like. Despite their pharmacological importance, little is known about the exact composition of each protein family. We report here the cloning of 25 complete ORFs from Macrovipera lebetina transmediterranea venom gland that encodes several isoforms and novel C-type lectins (CTLs). 16 alpha and nine beta CTL chains were identified. Based on their sequence alignment, we categorized the 16 CTL alpha subunits into five groups and the nine CTL beta subunits into four groups to deduce the phylogenetic tree of M. lebetina transmediterranea CTLs. Sequence analysis revealed that they share a high degree of similarity with each other and with other snake venom CTLs. The M. lebetina transmediterranea CTL sequences described here contain a C-lectin carbohydrate recognition domain-like fold (C-lectin CRD-like) characterized by several conserved amino acid residues in their structure, especially the cysteine. Finally, based on the comparison of some Macrovipera CTL, we propose that some new CTL gene versions should have occurred through "domains shuffling" from former genes.

Molecular cloning of disintegrins from Cerastes vipera and Macrovipera lebetina transmediterranea venom gland cDNA libraries: insight into the evolution of the snake venom integrin-inhibition system.

We report the cloning and sequence analysis of Cerastes vipera and Macrovipera lebetina transmediterranea cDNAs coding for short non-RGD (Arg-Gly-Asp) disintegrins and for dimeric disintegrin subunits. The mRNAs belong to the short-coding class, suggesting that these disintegrin mRNAs may be more widely distributed than previously thought. Our data also argue for a common ancestry of the mRNAs of short disintegrins and those coding for precursors of dimeric disintegrin chains. The Macrovipera lebetina transmediterranea dimeric disintegrin reported to inhibit the laminin-binding integrins alpha3beta1, alpha6beta1 and alpha7beta1 was analysed using a proteomic approach and was shown to bear MLD (Met-Leu-Asp) and VGD (Val-Gly-Asp) motifs. The results highlight the fact that disintegrins have evolved a restricted panel of integrin-blocking sequences that segregate with defined branches of the phylogenetic tree of the integrin alpha-chains, providing novel insights into the evolutionary adaptation of the snake venom antagonists to the ligand-binding sites of their target integrin receptors.

Soluble Siglec-5 associates to PSGL-1 and displays anti-inflammatory activity.

Interactions between endothelial selectins and the leukocyte counter-receptor PSGL1 mediates leukocyte recruitment to inflammation sites. PSGL1 is highly sialylated, making it a potential ligand for Siglec-5, a leukocyte-receptor that recognizes sialic acid structures. Binding assays using soluble Siglec-5 variants (sSiglec-5/C4BP and sSiglec-5/Fc) revealed a dose- and calcium-dependent binding to PSGL1. Pre-treatment of PSGL1 with sialidase reduced Siglec-5 binding by 79 ± 4%. In confocal immune-fluorescence assays, we observed that 50% of Peripheral Blood Mononuclear Cells (PBMCs) simultaneously express PSGL1 and Siglec-5. Duolink-proximity ligation analysis demonstrated that PSGL1 and Siglec-5 are in close proximity (<40 nm) in 31 ± 4% of PBMCs. In vitro perfusion assays revealed that leukocyte-rolling over E- and P-selectin was inhibited by sSiglec-5/Fc or sSiglec-5/C4BP, while adhesion onto VCAM1 was unaffected. When applied to healthy mice (0.8 mg/kg), sSiglec-5/C4BP significantly reduced the number of rolling leukocytes under basal conditions (10.9 ± 3.7 versus 23.5 ± 9.3 leukocytes/field/min for sSiglec-5/C4BP-treated and control mice, respectively; p = 0.0093). Moreover, leukocyte recruitment was inhibited over a 5-h observation period in an in vivo model of TNFalpha-induced inflammation following injection sSiglec-5/C4BP (0.8 mg/kg). Our data identify PSGL1 as a ligand for Siglec-5, and soluble Siglec-5 variants appear efficient in blocking PSGL1-mediated leukocyte rolling and the inflammatory response in general.

A thermoactive L-amino acid oxidase from Cerastes cerastes snake venom: purification, biochemical and molecular characterization.

A new L-amino acid oxidase (LAAO) from Cerastes cerastes snake venom, named CC-LAAO, was purified to homogeneity using a combination of size-exclusion, ion-exchange and affinity chromatography. CC-LAAO is a homodimeric glycosylated flavoprotein with a molecular mass around 58 kDa under reducing conditions and about 115 kDa in its native form when analyzed by SDS-PAGE and gel filtration chromatography, respectively. This enzyme displayed a Michaelis-Menten behavior with an optimal pH at 7.8. However, unlike known SV-LAAOs which display their maximum activity at 37 °C, CC-LAAO has an optimal temperature at 50 °C. Kinetic studies showed that the enzyme displayed high specificity towards hydrophobic L-amino acids. The best substrates were L-Phe, L-Met and L-Leu. CC-LAAO activity was inhibited by the substrate analog N-acetyl tryptophan. The N-terminal amino acid sequence of this protein was determined by automated Edman degradation. The CC-LAAO cDNA was cloned from the venom gland total RNA preparation. The cDNA sequence contained an open-reading frame (ORF) of 1551-bp, which encoded a protein of 516 amino acids comprising a signal peptide of 18 amino acids and 498-residues mature protein. CC-LAAO sequence and its tertiary model shared high similarity with other snake venom LAAOs.

Antitumoral potential of Tunisian snake venoms secreted phospholipases A2.

Phospholipases type A2 (PLA2s) are the most abundant proteins found in Viperidae snake venom. They are quite fascinating from both a biological and structural point of view. Despite similarity in their structures and common catalytic properties, they exhibit a wide spectrum of pharmacological activities. Besides being hydrolases, secreted phospholipases A2 (sPLA2) are an important group of toxins, whose action at the molecular level is still a matter of debate. These proteins can display toxic effects by different mechanisms. In addition to neurotoxicity, myotoxicity, hemolytic activity, antibacterial, anticoagulant, and antiplatelet effects, some venom PLA2s show antitumor and antiangiogenic activities by mechanisms independent of their enzymatic activity. This paper aims to discuss original finding against anti-tumor and anti-angiogenic activities of sPLA2 isolated from Tunisian vipers: Cerastes cerastes and Macrovipera lebetina, representing new tools to target specific integrins, mainly, α5ß1 and αv integrins.

PIVL, a new serine protease inhibitor from Macrovipera lebetina transmediterranea venom, impairs motility of human glioblastoma cells.

A novel Kunitz-type serine proteinase inhibitor, termed PIVL, was purified to homogeneity from the venom of the Tunisian snake Macrovipera lebetina transmediterranea. It is a monomeric polypeptide chain cross-linked by three disulfide linkages with an isotope-averaged molecular mass of 7691.7 Da. The 67-residue full-length PIVL sequence was deduced from a venom gland cDNA clone. Structurally, PIVL is built by a single Kunitz/BPTI-like domain. Functionally, it is able to specifically inhibit trypsin activity. Interestingly, PIVL exhibits an anti-tumor effect and displays integrin inhibitory activity without being cytotoxic. Here we show that PIVL is able to dose-dependently inhibit the adhesion, migration and invasion of human glioblastoma U87 cells. Our results also show that PIVL impairs the function of αvß3 and to a lesser extent, the activity of αvß6, αvß5, α1ß1 and α5ß1 integrins. Interestingly, we demonstrate that the (41)RGN(43) motif of PIVL is likely responsible for its anti-cancer effect. By using time lapse videomicroscopy, we found that PIVL significantly reduced U87 cells motility and affected cell directionality persistence by 68%. These findings reveal novel pharmacological effects for a Kunitz-type serine proteinase inhibitor.

Expression of a functional recombinant C-type lectin-like protein lebecetin in the human embryonic kidney cells.

Lebecetin is an anticoagulant C-type lectin-like protein that was previously isolated from Macrovipera lebetina venom and described to consist of two subunits (alpha and beta). It was reported to potently prevent platelet aggregation by binding to glycoprotein Ib and to exhibit a broad spectrum of inhibitory activities on various integrin-mediated functions of tumor cells, including adhesion, proliferation, and cell migration. This study aimed to investigate the structure-function of lebecetin. Accordingly, the cDNA of each subunit was cloned and separately or jointly expressed in the human embryonic kidney cells using two vectors with different selectable tags. The immunofluorescence analysis of transfected cells revealed significant expression levels and co-localization of the two lebecetin subunits. The recombinant proteins were efficiently secreted and purified using metal-chelating affinity chromatography. We found that the Lebecetin alpha and beta subunits were produced as a mixture of homodimers and heterodimers and that the heterodimerization represents a prerequisite for functioning.

MVL-PLA2, a snake venom phospholipase A2, inhibits angiogenesis through an increase in microtubule dynamics and disorganization of focal adhesions.

Integrins are essential protagonists of the complex multi-step process of angiogenesis that has now become a major target for the development of anticancer therapies. We recently reported and characterized that MVL-PLA2, a novel phospholipase A2 from Macrovipera lebetina venom, exhibited anti-integrin activity. In this study, we show that MVL-PLA2 also displays potent anti-angiogenic properties. This phospholipase A2 inhibited adhesion and migration of human microvascular-endothelial cells (HMEC-1) in a dose-dependent manner without being cytotoxic. Using Matrigel and chick chorioallantoic membrane assays, we demonstrated that MVL-PLA2, as well as its catalytically inactivated form, significantly inhibited angiogenesis both in vitro and in vivo. We have also found that the actin cytoskeleton and the distribution of alphav beta3 integrin, a critical regulator of angiogenesis and a major component of focal adhesions, were disturbed after MVL-PLA2 treatment. In order to further investigate the mechanism of action of this protein on endothelial cells, we analyzed the dynamic instability behavior of microtubules in living endothelial cells. Interestingly, we showed that MVL-PLA2 significantly increased microtubule dynamicity in HMEC-1 cells by 40%. We propose that the enhancement of microtubule dynamics may explain the alterations in the formation of focal adhesions, leading to inhibition of cell adhesion and migration.

Leberagin-C, A disintegrin-like/cysteine-rich protein from Macrovipera lebetina transmediterranea venom, inhibits alphavbeta3 integrin-mediated cell adhesion.

Leberagin-C, a new member of the disintegrin-like/cysteine-rich (D/C) family, was purified to homogeneity from the venom of Tunisian snake Macrovipera lebetina transmediterranea. It is a monomeric protein with a molecular mass of 25,787 Da. Its complete sequence of 205 amino acid residues was established by cDNA cloning. The leberagin-C shows many conserved sequences with other known D/C proteins, like the SECD binding sites and a pattern of 28 cysteines. It is the first purified protein from M. lebetina transmediterranea with only two disintegrin-like/cysteine-rich domains. Leberagin-C is able to inhibit platelet aggregation induced by thrombin and arachidonic acid with IC(50) of 40 and 50 nM respectively. It was also able to inhibit the adhesion of melanoma tumour cells on fibrinogen and fibronectin, by interfering with the function of alphavbeta3 and, to a lesser extent, with alphavbeta6 and alpha5beta1 integrins. To our knowledge, leberagin-C is the sole described D/C protein that does not specifically interact with the alpha2beta1 integrin. Structure-activity relationship study of leberagin-C suggested that there are some important amino acid differences with jararhagin, the most studied PIII metalloprotease from Bothrops jararaca, notably around the SECD motif in its disintegrin-like domain. Other regions implicated in leberagin-C specificities could not be excluded.

MVL-PLA2, a phospholipase A2 from Macrovipera lebetina transmediterranea venom, inhibits tumor cells adhesion and migration.

Here, we report the purification and characterization of an acidic Asp49 phospholipase A2, named MVL-PLA2, with a molecular mass of 13,626.64 Da. The complete MVL-PLA2 cDNA was cloned from Macrovipera lebetina transmediterranea venom gland cDNA library. MVL-PLA2 possesses 122 amino acid residues, including 14 cysteines, and belongs to group II snake venom phospholipase A2 enzymes. MVL-PLA2 was not cytotoxic up to 2 muM and completely abolished cell adhesion and migration of various human tumor cells. Chemical modification with p-bromophenacyl bromide abolished the enzymatic activity of MVL-PLA2 without affecting its anti-tumor effect, suggesting the presence of 'pharmacological sites' distinct from the catalytic site in snake venom phospholipase A2. We demonstrated for the first time that the anti-tumor effect of MVL-PLA2 was mediated by alpha5beta1 and alphav-containing integrins. This finding may serve as starting point for structure-function relationship studies leading to design a new generation of specific anti-cancer drugs.

Two purified and characterized phospholipases A2 from Cerastes cerastes venom, that inhibit cancerous cell adhesion and migration.

Two non-toxic PLA2s were purified to homogeneity from Cerastes cerastes Tunisian snake venom. The purification process employed gel filtration on Sephadex G-75 followed by C18 reverse phase high-pressure liquid chromatography. These two acidic enzymes, namely CC-PLA2-1 and CC-PLA2-2, have a molecular weight of 13,737.52 and 13,705.63 Da, respectively. These two PLA2 are the first reported glycosylated phospholipases A2 purified from snake venom. The rates of glycosylation are 2.5% and 0.5% (w/w), respectively. Specific activities of 1800 U/mg and 2400 U/mg for CC-PLA2-1 and CC-PLA2-2, respectively, were measured at optimal conditions. CC-PLA2-1 and CC-PLA2-2 strongly inhibited coagulation. They also exhibited a marked dose-dependent inhibitory effect on platelet aggregation induced by ADP and arachidonic acid in platelet-rich plasma. Interestingly, CC-PLA2-1 and CC-PLA2-2 inhibited in a dose-dependent manner adhesion of IGR39 melanoma and HT1080 fibrosarcoma cells to fibrinogen and fibronectin. Furthermore, both CC-PLA2-1 and CC-PLA2-2 abolished HT1080 cell migration towards fibrinogen and fibronectin. This activity is reported for the first time for PLA2 enzymes.

Loss of introns along the evolutionary diversification pathway of snake venom disintegrins evidenced by sequence analysis of genomic DNA from Macrovipera lebetina transmediterranea and Echis ocellatus.

Analysis of cDNAs from Macrovipera lebetina transmediterranea (Mlt) and Echis ocellatus (Eo) venom gland libraries encoding disintegrins argued strongly for a common ancestry of the messengers of short disintegrins and those for precursors of dimeric disintegrin chains. We now report the sequence analysis of disintegrin-coding genes from these two vipers. Genomic DNAs for dimeric disintegrin subunits Ml_G1 and Ml_G2 (Mlt) and Eo_D3 (Eo) contain single 1-kb introns exhibiting the 5'-GTAAG (donor)/3'-AG (acceptor) consensus intron splicing signature. On the other hand, the short RTS-disintegrins Ml_G3 (Mlt) and Eo_RTS (Eo) and the short RGD-disintegrin ocellatusin (Eo) are transcribed from intronless genomic DNA sequences, indicating that the evolutionary pathway leading to the emergence of short disintegrins involved the removal of all intronic sequences. The insertion position of the intron within Ml_G1, Ml_G2, and Eo_D3 is conserved in the genes for vertebrate ADAM (A disintegrin and metalloproteinase) protein disintegrin-like domains and within the gene for the medium-size snake disintegrins halystatins 2 and 3. However, a comparative analysis of currently available disintegrin(-like) genes outlines the view that a minimization of both the gene organization and the protein structure underlies the evolution of the snake venom disintegrin family.

Snake venomics: comparative analysis of the venom proteomes of the Tunisian snakes Cerastes cerastes, Cerastes vipera and Macrovipera lebetina.

The protein composition of the crude venoms of the three most important vipers of Tunisia was analyzed by RP-HPLC, N-terminal sequence analysis, MALDI-TOF mass determination, and in-gel tryptic digestion followed by PMF and CID-MS/MS of selected peptide ions in a quadrupole-linear IT instrument. Our results show that the venom proteomes of Cerastes cerastes, Cerastes vipera, and Macrovipera lebetina are composed of proteins belonging to a few protein families. However, each venom showed distinct degree of protein composition complexity. The three venoms shared a number of protein classes though the relative occurrence of these toxins was different in each snake species. On the other hand, the venoms of the Cerastes species and Macrovipera lebetina each contained unique components. The comparative proteomic analysis of Tunisian snake venoms provides a comprehensible catalogue of secreted proteins, which may contribute to a deeper understanding of the biological effects of the venoms, and may also serve as a starting point for studying structure-function correlations of individual toxins.