Pharmacological characterization and antithrombotic effect of agkistin, a platelet glycoprotein Ib antagonist

Snake Venom Components as Therapeutic Drugs in Ischemic Heart Disease

Ischemic heart disease (IHD), especially myocardial infarction (MI), is a leading cause of death worldwide. Although coronary reperfusion is the most straightforward treatment for limiting the MI size, it has nevertheless been shown to exacerbate ischemic myocardial injury. Therefore, identifying and developing therapeutic strategies to treat IHD is a major medical challenge. Snake venoms contain biologically active proteins and peptides that are of major interest for pharmacological applications in the cardiovascular system (CVS). This has led to their use for the development and design of new drugs, such as the first-in-class angiotensin-converting enzyme inhibitor captopril, developed from a peptide present in Bothrops jararaca snake venom. This review discusses the potential usefulness of snake venom toxins for developing effective treatments against IHD and related diseases such as hypertension and atherosclerosis. It describes their biological effects at the molecular scale, their mechanisms of action according to their different pharmacological properties, as well as their subsequent molecular pathways and therapeutic targets. The molecules reported here have either been approved for human medical use and are currently available on the drug market or are still in the clinical or preclinical developmental stages. The information summarized here may be useful in providing insights into the development of future snake venom-derived drugs.

Biomechanical thrombosis: the dark side of force and dawn of mechano-medicine

Arterial thrombosis is in part contributed by excessive platelet aggregation, which can lead to blood clotting and subsequent heart attack and stroke. Platelets are sensitive to the haemodynamic environment. Rapid haemodynamcis and disturbed blood flow, which occur in vessels with growing thrombi and atherosclerotic plaques or is caused by medical device implantation and intervention, promotes platelet aggregation and thrombus formation. In such situations, conventional antiplatelet drugs often have suboptimal efficacy and a serious side effect of excessive bleeding. Investigating the mechanisms of platelet biomechanical activation provides insights distinct from the classic views of agonist-stimulated platelet thrombus formation. In this work, we review the recent discoveries underlying haemodynamic force-reinforced platelet binding and mechanosensing primarily mediated by three platelet receptors: glycoprotein Ib (GPIb), glycoprotein IIb/IIIa (GPIIb/IIIa) and glycoprotein VI (GPVI), and their implications for development of antithrombotic 'mechano-medicine' .

Platelet sequestration and activation during GalTKO.hCD46 pig lung perfusion by human blood is primarily mediated by GPIb, GPIIb/IIIa, and von Willebrand Factor

BACKGROUND

Here, we ask whether platelet GPIb and GPIIb/IIIa receptors modulate platelet sequestration and activation during GalTKO.hCD46 pig lung xenograft perfusion.

METHODS

GalTKO.hCD46 transgenic pig lungs were perfused with heparinized fresh human blood. Results from perfusions in which αGPIb Fab (6B4, 10 mg/l blood, n = 6), αGPIIb/IIIa Fab (ReoPro, 3.5 mg/l blood, n = 6), or both drugs (n = 4) were administered to the perfusate were compared to two additional groups in which the donor pig received 1-desamino-8-d-arginine vasopressin (DDAVP), 3 μg/kg (to pre-deplete von Willebrand Factor (pVWF), the main GPIb ligand), with or without αGPIb (n = 6 each).

RESULTS

Platelet sequestration was significantly delayed in αGPIb, αGPIb+DDAVP, and αGPIb+αGPIIb/IIIa groups. Median lung "survival" was significantly longer (>240 vs. 162 min reference, p = 0.016), and platelet activation (as CD62P and βTG) were significantly inhibited, when pigs were pre-treated with DDAVP, with or without αGPIb Fab treatment. Pulmonary vascular resistance rise was not significantly attenuated in any group, and was associated with residual thromboxane and histamine elaboration.

CONCLUSIONS

The GPIb-VWF and GPIIb/IIIa axes play important roles in platelet sequestration and coagulation cascade activation during GalTKO.hCD46 lung xenograft injury. GPIb blockade significantly reduces platelet activation and delays platelet sequestration in this xenolung rejection model, an effect amplified by adding αGPIIb/IIIa blockade or depletion of VWF from pig lung.

A novel anti-platelet aggregation tripeptide from Agkistrodon acutus venom: isolation and characterization

AAP, a tripeptide that inhibited rabbit platelet aggregation, was isolated from Agkistrodon acutus venom by ion-exchange, gel filtration and reverse-phase chromatography. Amino acid sequences which determined mainly by amino acid analyses and NMR spectroscopy indicated it was a tripeptide including pyroglutamic acid, asparagine and tryptophane residues. The ESMS experiment assigned a molecular weight of 429 Da. AAP inhibited rabbit platelet aggregation induced by ADP, PAF-acether, collagen and thrombin, the IC(50)s were 178 microM, 332 microM, 179 microM and 203 microM, respectively. AAP also inhibited thrombus formation in vivo thrombosis model and prevented the combination between fibrinogen and GP IIb/IIIa. Besides, AAP was not toxic after intravenous injection into mice at a higher dose. Those studies might be helpful to delineate unknown mechanisms involved in platelet aggregation and serve as a model for developing antithrombotic agents.

A snake venom metalloproteinase, kistomin, cleaves platelet glycoprotein VI and impairs platelet functions

BACKGROUND AND OBJECTIVES

Injuries to the vessel wall and subsequent exposure of the matrix of the subendothelial layer resulted in thrombus formation. Platelet glycoprotein (GP) Ib and VI play a crucial role in matrix-induced activation and aggregation of platelets.

METHODS AND RESULTS

In the present study, we reported that the GPIb-cleaving snake venom metalloproteinase (SVMP), kistomin, inhibited collagen-induced platelet aggregation. Moreover, kistomin inhibited platelet aggregation induced by convulxin (CVX, a GPVI agonist) and a GPVI-specific antibody in a concentration and time-dependent manner. Kistomin treatment decreased platelet GPVI but not integrin alpha2beta1 and alphaIIbbeta3, accompanied with the formation of GPVI cleavage fragments, as determined by flow cytometric and Western blot analyses. In addition, intact platelet GPVI and recombinant GPVI were digested by kistomin to release 25- and 35-kDa fragments, suggesting that kistomin cleaved GPVI near the mucin-like region. We designed four synthetic peptides ranging from Leu180 to Asn249 as the substrates for kistomin and found that kistomin cleaved these synthetic peptides at FSE205/A206TA and NKV218/F219TT, as analyzed by MALDI-TOF-MS. In addition, GPVI-specific antibody-induced tyrosine kinase phosphorylation in platelets was reduced after kistomin pretreatment, and platelet adhesion to collagen but not to fibrinogen was attenuated by kistomin.

CONCLUSIONS

We provided here the first evidence that a P-I snake venom metalloproteinase, kistomin, inhibits the interaction between collagen and platelet GPVI through its proteolytic activity on GPVI, thus providing an alternative strategy for developing new anti-thrombotic agents.

Blood cells as targets of snake toxins

Snake venoms are mixtures of enzymes and peptides which exert toxicological effects by targeting their substrates or receptors upon envenomation. Snake venom proteins widely affect vascular system including circulating blood cells, coagulation factors, and vascular wall components. Many of the toxic proteins have multiple targets. For example, some metalloproteinase domain-containing snake venom protein cleaves not only fibrinogen but also receptors on platelets. Also, it is frequent that toxins from different snake venom protein families are capable of binding to a common target on cells. Most of the cytotoxic effects in the venom are usually results of the activities of metalloproteinase, C-type lectin, disintegrin, cysteine-rich protein, as well as phospholipase A(2). There has been a growing interest in studying the structure and function of these snake venom proteins because many of them have high structural homologies to proteins found in human. Therefore, the understanding of how these toxins interact with their targets may contribute to the discovery of novel physiological processes and the development of therapeutic agents for cardiovascular diseases. In this review, we summarize how snake toxins target blood cells with an emphasis on their effects on platelet function.

Molecular regulation of platelet-dependent thrombosis

Hemostasis is a normal process preventing the sequelae of uncontrolled hemorrhage. In certain settings, these same processes cause adverse clinical events due to thrombotic occlusion of a vessel. The majority of unstable coronary syndromes result from disruption of an atherosclerotic plaque, leading to the exposure of subintimal contents, which triggers coagulation and the formation of a platelet-rich thrombus. The central role of platelet activation in the events that lead to vessel occlusion is well known. However, this process is complex and influenced by a myriad of cellular and plasma-derived mediators that regulate the balance between occlusive and nonocclusive thrombosis.

Genetics of platelet glycoprotein receptors: risk of thrombotic events and pharmacogenetic implications

Platelet aggregation and coronary thrombosis have a central role in the development of acute coronary syndromes and myocardial infarction (MI). Therapies aimed at inhibiting platelet aggregation have shown great benefit in individuals with coronary disease or with multiple risk factors for coronary disease. Genetic variation in platelet surface receptors mediating thrombus formation has been suggested to be associated with platelet hyperreactivity, with increased risk of MI and possibly with the benefit received from various antithrombotic drug treatments. This review focuses on discrepancies and their likely explanations in studies on platelet glycoprotein genetics. Current knowledge on important issues concerning coronary event phenotypes and pharmacogenetics is analyzed. Possible future applicability of these data to patient treatment is also discussed.

Antiplatelet Agents: Current Drugs and Future Trends

Antiplatelet drugs in clinical use are discussed in terms of their mechanisms of action and the relevancy of that to the physiology of platelets and the pathophysiology of arterial thrombosis. Current clinical usage is outlined in detail for each drug. Experimental antiplatelet drugs also are discussed.

Inhibitory effects of human α2-macroglobulin and mouse serum on the PSGL-1 and glycoprotein Ib proteolysis by a snake venom metalloproteinase, triflamp

Triflamp, a 28 kDa snake venom metalloproteinase purified from the venom of Trimeresurus flavoviridis, possesses the proteolytic activities toward P-selectin glycoprotein ligand-1 (PSGL-1), glycoprotein Ib (GPIb) and fibrinogen. In human whole blood preparation, however, triflamp (6 microg/ml) failed to cleave neutrophil PSGL-1 and platelet GPIb. Human alpha2-macroglobulin (alpha2M) was mainly responsible for the neutralization of the proteolytic effects of triflamp on PSGL-1, GPIb and fibrinogen. Human alpha2M neutralized triflamp at a stoichiometry about 1.1:1 (molar basis) determined by azocaseinolysis. SDS-PAGE analysis revealed that triflamp cleaved the bait-region of alpha2M. Western blot demonstrated that triflamp interacted with the C-terminal half-subunits of truncated alpha2M resulting in the formation of high-molecular-weight species of alpha2M-triflamp complexes. In the presence of competing nucleophile, 0.2 M methylamine, the proteolytic activity of triflamp was conserved. In vivo we found that mice neutrophils were resistant to the cleavage of PSGL-1 by triflamp. However, mouse PSGL-1 and GPIb were susceptible to be cleaved by triflamp in washed mouse neutrophil and platelet preparation, respectively. Similarly, mouse serum was also responsible for the inactivation of the proteolytic activity of triflamp. This study provides direct evidences for the reasonable explanation regarding the reduced proteolytic activity of triflamp toward its substrates in whole blood preparation and in vivo model.

Pharmacology of platelet adhesion and aggregation

At the injured vessel wall, blood platelets become activated and adhere to the subendothelial surface as well as to each other. These cellular adhesion processes are required for primary hemostasis, but can also lead to thrombosis. Considerable progress has been made during recent years in understanding the molecular mechanisms underlying platelet activation and adhesion. This knowledge will drive future efforts towards the development of new antiplatelet drugs for the prevention and treatment of cardiovascular diseases.

Aggretin, a snake venom-derived endothelial integrin alpha 2 beta 1 agonist, induces angiogenesis via expression of vascular endothelial growth factor

Aggretin, a collagen-like alpha 2 beta 1 agonist purified from Calloselasma rhodostoma venom, was shown to increase human umbilical vein endothelial cell (HUVEC) proliferation and HUVEC migration toward immobilized aggretin was also increased. These effects were blocked by A2-IIE10, an antibody raised against integrin alpha 2. Aggretin bound to HUVECs in a dose-dependent and saturable manner, which was specifically inhibited by A2-IIE10, as examined by flow cytometry. Aggretin elicited significant angiogenic effects in both in vivo and in vitro angiogenesis assays, and incubation of HUVECs with aggretin activated phosphatidylinositol 3-kinase (PI3K), Akt, and extracellular-regulated kinase 1/2 (ERK1/2); these effects were blocked by A2-IIE10 or vascular endothelial growth factor (VEGF) monoclonal antibody (mAb). The angiogenic effect induced by aggretin may be via the production of VEGF because the VEGF level was elevated and VEGF mAb pretreatment inhibited Akt/ERK1/2 activation as well as the in vivo angiogenesis induced by aggretin. The VEGF production induced by aggretin can be blocked by A2-IIE10 mAb pretreatment. In conclusion, aggretin induces endothelial cell proliferation, migration, and angiogenesis by interacting with integrin alpha 2 beta 1 leading to activation of PI3K, Akt, and ERK1/2 pathways, and the increased expression of VEGF may be responsible for its angiogenic activity.

Antiplatelet therapy: in search of the 'magic bullet'

The central importance of platelets in the development of arterial thrombosis and cardiovascular disease is well established. No other single cell type is responsible for as much morbidity and mortality as the platelet and, as a consequence, it represents a major target for therapeutic intervention. The growing awareness of the importance of platelets is reflected in the increasing number of patients receiving antiplatelet therapy, a trend that is likely to continue in the future. There are, however, significant drawbacks with existing therapies, including issues related to limited efficacy and safety. The discovery of a 'magic bullet' that selectively targets pathological thrombus formation without undermining haemostasis remains elusive, although recent progress in unravelling the molecular events regulating thrombosis has provided promising new avenues to solve this long-standing problem.

Function-modulating human monoclonal antibodies against platelet-membrane receptors isolated from a phage-display library

Monoclonal antibodies to platelet membrane receptors have been used extensively for analysis of receptor structure and function. Function-blocking human antibodies are being used for the development of antiplatelet drugs. We isolated human monoclonal antibodies from a library of single-chain Fv (scFv) antibodies displayed on the surface of filamentous phage, by selection on whole platelets. Eight different platelet-binding clones were isolated, of which three bound to the platelet-membrane glycoprotein (GP) GPIb in an ELISA assay. Specific elution with a recombinant polypeptide of von Willebrand factor (VWF) spanning the GPIbalpha binding site, yielded the same three phage clones. Two of the three anti-GPIb clones could be purified as scFv monoclonal antibodies, and they competed with each other for binding to intact platelets, suggesting that they bind at or near the same site on GPIb. Their binding affinities differed, however, and the clone with higher affinity inhibited ristocetin-induced platelet aggregation. These data indicate that selection from a phage display library of human scFvs using whole platelets can be applied for the isolation of functional antiplatelet-GPIb antibodies useful for the development of new therapeutic and diagnostic strategies.

Pharmacological control of platelet function

Advancement in the understanding of the mechanisms of platelet activation, as well as the development of new techniques for studying platelet function, have led to the availability of new classes of platelet inhibiting drugs. Initially, characterization of arachidonic acid metabolism in platelets furthered an understanding of the utility of cyclooxygenase inhibitors, most notably aspirin. The discovery and characterization of platelet receptors such as the adenosine diphosphate (ADP) receptor and glycoprotein IIb/IIIa has been associated with the development of novel classes of anti-platelet drug, such as thienopyridine derivatives and glycoprotein IIb/IIIa receptor antagonists, respectively. Future development in receptor pathway inhibitors also includes glycoprotein Ib/IX as well as the potential use of platelet signaling pathway inhibitors.

Aggretin, a C-type lectin protein, induces platelet aggregation via integrin alpha(2)beta(1) and GPIb in a phosphatidylinositol 3-kinase independent pathway

Aggretin purified from Calloselasma rhodostoma venom was previously identified as alpha(2)beta(1) agonist in triggering platelet aggregation, and exists as a heterodimer sharing a great homologous sequence to GPIb binding proteins. We show here that binding to GPIb is also required in aggregation-inducing activity of aggretin. A2-IIE10, an anti-integrin alpha(2) monoclonal antibody, delayed platelet aggregation while agkistin, a GPIb antagonist, only slightly inhibited platelet aggregation caused by aggretin. However, the aggretin-induced platelet aggregation was completely abolished by a combination of A2-IIE10 and agkistin. Either A2-IIE10 or agkistin significantly inhibited the binding of FITC-aggretin toward fixed platelets. Aggretin and collagen induced a similar signal transduction in platelets involving a time-dependent tyrosine phosphorylation of p125(FAK) and PLCgamma2, but aggretin caused a much-delayed tyrosine-phosphorylation of PI 3-kinase compared with collagen. LY294002, a PI 3-kinase inhibitor, showed a significant inhibitory effect on collagen, but not aggretin-stimulated platelet aggregation. These findings indicate aggretin induces platelet aggregation via binding of alpha(2)beta(1) and GPIb, causing phosphorylation of p125(FAK) and PLCgamma2 leading to platelet activation without the involvement of PI 3-kinase activation.