An echistatin C-terminal peptide activates GPIIbIIIa binding to fibrinogen, fibronectin, vitronectin and collagen type I and type IV

Vitronectin stabilizes intravascular adhesion of neutrophils by coordinating β2 integrin clustering

The recruitment of neutrophils from the microvasculature to the site of injury or infection represents a key event in the inflammatory response. Vitronectin (VN) is a multifunctional macromolecule abundantly present in blood and extracellular matrix. The role of this glycoprotein in the extravasation process of circulating neutrophils remains elusive. Employing advanced in vivo/ex vivo imaging techniques in different mouse models as well as in vitro methods, we uncovered a previously unrecognized function of VN in the transition of dynamic to static intravascular interactions of neutrophils with microvascular endothelial cells. These distinct properties of VN require the heteromerization of this glycoprotein with plasminogen activator inhibitor-1 (PAI- 1) on the activated venular endothelium and subsequent interactions of this protein complex with the scavenger receptor low-density lipoprotein receptor-related protein-1 on intravascularly adhering neutrophils. This induces p38 mitogen-activated protein kinases-dependent intracellular signaling events which, in turn, regulates the proper clustering of the b2 integrin lymphocyte function associated antigen-1 on the surface of these immune cells. As a consequence of this molecular interplay, neutrophils become able to stabilize their adhesion to the microvascular endothelium and, subsequently, to extravasate to the perivascular tissue. Hence, endothelial-bound VN-PAI-1 heteromers stabilize intravascular adhesion of neutrophils by coordinating b2 integrin clustering on the surface of these immune cells, thereby effectively controlling neutrophil trafficking to inflamed tissue. Targeting this protein complex might be beneficial for the prevention and treatment of inflammatory pathologies.

The continuing saga of snake venom disintegrins

Disintegrins, a family of polypeptides released in the venoms of viperid snakes (vipers and rattlesnakes) by the proteolytic processing of multidomain metalloproteinases, selectively block the function of β(1) and β(3) integrin receptors. Few of the proteins isolated and characterized from snake venoms have proven to be more structural and functional versatile than the disintegrins. Not surprisingly, 25 years after their discovery, our knowledge on the evolutionary history and the molecular determinants modulating the integrin inhibitory activity of disintegrins still remain fragmentary. This paper highlights some seminal contributions, including personal accounts of pioneer authors, related to basic and applied research on disintegrins. Investigators have evaluated disintegrin applications in therapies for a number of pathologies in which integrin receptors play relevant roles, particularly myocardial infarction and inappropriate tumor angiogenesis. Completing the continuing story of the disintegrin family by applying novel research approaches may hold the key to learn how to use deadly toxins as therapeutic agents.

Platelet adhesion and intracellular calcium levels in antigen-challenged rats

There is considerable evidence that platelet activation occurs in allergic airways diseases. In this study we aimed to investigate platelet adhesion to immobilized fibrinogen and intracellular calcium levels in a rat model of allergic inflammation. Male Wistar rats were challenged with ovalbumin (OVA). At 30 min to 24h after OVA-challenge, assays of platelet adhesion to immobilized fibrinogen and intracellular calcium levels using fura 2-AM loaded platelets were performed. The serum levels of IgE were approximately 5-fold greater in OVA-sensitized rats. A marked eosinophil influx in bronchoalveolar lavage (BAL) fluid of OVA-challenged rats at 24h after OVA-challenge was also seen. OVA-challenge resulted in a marked thrombocytopenia, as observed within 12h after OVA-challenge. The agonists ADP (0.5-50 microM) and thrombin (30-100 mU/ml) concentration-dependently increased platelet adhesion to immobilized fibrinogen. At an early time after OVA-challenge (30 min), platelets exhibited greater platelet adhesion compared with the non-sensitized group, whereas at a late time (24h) they exhibited lower platelet adhesion to both agonists. Moreover, at 30 min after OVA-challenge, intracellular calcium levels to ADP (20 microM) and thrombin (100 mU/ml)-activated platelets were greater compared with non-challenged rats. As opposed, at 24h after OVA challenge, a lower intracellular calcium level to ADP- and thrombin-activated platelets was observed. In conclusion, OVA-challenge in rats promotes a biphasic response in platelet adhesion consisting of an increased adhesion and intracellular calcium levels at an early phase (30 min), which progress to a reduction in adhesion and intracellular calcium levels at a late time (24h) after antigen challenge.

Anti-platelet effect of cumanastatin 1, a disintegrin isolated from venom of South American Crotalus rattlesnake

Disintegrins have been previously described in the venom of several snake families inhibiting signal transduction, cell-cell interactions, and cell-matrix interactions and may have therapeutic potential in heart attacks, thrombotic diseases, and cancers. This investigation describes the first disintegrin isolated from South American Crotalus venom (Venezuelan rattlesnake Crotalus durissus cumanensis), which inhibits platelet adhesion to matrix proteins. C. d. cumanensis crude venom was first separated on a Sephadex G-100 column into 4 fractions (SI to SIV). Crude venom and SIII fraction significantly diminished platelet adhesion to fibrinogen (Fg) and to fibronectin (Fn). Anti-adhesive SIII fraction was further separated by DEAE-Sephacel followed by C-18 reverse phase high performance liquid chromatography (HPLC). The platelet anti-adhesive fraction obtained was designated as cumanastatin-1. This disintegrin has a mass of 7.442 kDa as determined by mass spectrometry (MALDI-TOF/TOF) and pI of 8.5. Cumanastatin-1 also inhibited ADP-induced platelet aggregation with an IC(50) of 158 nM. However, it did not significantly inhibit collagen and thrombin-induced platelet aggregation. Cumanastatin-1 considerably inhibited anti-alpha(IIb)beta(3) integrin binding to platelets in a dose-dependent manner; however, it did not present any effect on the alpha(5)beta(1) integrin or on P-selectin.

The “Linker” Region (Amino Acids 38-47) of the Disintegrin Elegantin Is a Novel Inhibitory Domain of Integrin α5β1-Dependent Cell Adhesion on Fibronectin

Disintegrins are a family of potent inhibitors of cell-cell and cell-matrix adhesion. In this study we have identified a region of the disintegrin elegantin, termed the "linker domain" (amino acids 38-47), with inhibitory activity toward alpha(5)beta(1)-mediated cell adhesion on fibronectin (Fn). Using a chimeric structure-function approach in which sequences of the functionally distinct disintegrin kistrin were introduced into the elegantin template at targeted sites, a loss of inhibitory function toward alpha(5)beta(1)-mediated adhesion on Fn was observed when the elegantin linker domain was substituted. Subsequent analysis comparing the inhibitory efficacies of the panel of elegantin-kistrin chimeras toward CHO alpha(5) cell adhesion on recombinant Fn III(6-10) fragments showed that the loss of inhibitory activity associated with the disruption of the elegantin linker domain was dependent upon the presence of a functional Fn III(9) synergy site within the Fn III(6-10) substrate. This suggested that the elegantin linker domain inhibits primarily the activity of the Fn synergy domain in promoting alpha(5)beta(1) integrin-mediated cell adhesion. Construction of a cyclic peptide corresponding to the entire region of the elegantin linker domain showed that this domain has intrinsic alpha(5)beta(1) inhibitory activity comparable with the activity of the RGDS peptide. These data demonstrate a novel biological function for a disintegrin domain that antagonizes integrin-mediated cell adhesion.

Conformation and concerted dynamics of the integrin-binding site and the C-terminal region of echistatin revealed by homonuclear NMR

Echistatin is a potent antagonist of the integrins alpha(v)beta3, alpha5beta1 and alpha(IIb)beta3. Its full inhibitory activity depends on an RGD (Arg-Gly-Asp) motif expressed at the tip of the integrin-binding loop and on its C-terminal tail. Previous NMR structures of echistatin showed a poorly defined integrin-recognition sequence and an incomplete C-terminal tail, which left the molecular basis of the functional synergy between the RGD loop and the C-terminal region unresolved. We report a high-resolution structure of echistatin and an analysis of its internal motions by off-resonance ROESY (rotating-frame Overhauser enhancement spectroscopy). The full-length C-terminal polypeptide is visible as a beta-hairpin running parallel to the RGD loop and exposing at the tip residues Pro43, His44 and Lys45. The side chains of the amino acids of the RGD motif have well-defined conformations. The integrin-binding loop displays an overall movement with maximal amplitude of 30 degrees . Internal angular motions in the 100-300 ps timescale indicate increased flexibility for the backbone atoms at the base of the integrin-recognition loop. In addition, backbone atoms of the amino acids Ala23 (flanking the R24GD26 tripeptide) and Asp26 of the integrin-binding motif showed increased angular mobility, suggesting the existence of major and minor hinge effects at the base and the tip, respectively, of the RGD loop. A strong network of NOEs (nuclear Overhauser effects) between residues of the RGD loop and the C-terminal tail indicate concerted motions between these two functional regions. A full-length echistatin-alpha(v)beta3 docking model suggests that echistatin's C-terminal amino acids may contact alpha(v)-subunit residues and provides new insights to delineate structure-function correlations.

Snake venom disintegrins: evolution of structure and function

Disintegrins represent a family of polypeptides present in the venoms of various vipers that selectively block the function of integrin receptors. Here, we review our current view and hypothesis on the emergence and the structural and functional diversification of disintegrins by accelerated evolution and the selective loss of disulfide bonds of duplicated genes. Research on disintegrins is relevant for understanding the biology of viper venom toxins, but also provides information on new structural determinants involved in integrin recognition that may be useful in basic and clinical research. The role of the composition, conformation, and dynamics of the integrin inhibitory loop acting in concert with the C-terminal tail in determining the selective inhibition of integrin receptors is discussed.

Concerted motions of the integrin-binding loop and the C-terminal tail of the non-RGD disintegrin obtustatin

Obtustatin is a potent and selective inhibitor of the alpha1beta1 integrin in vitro and of angiogenesis in vivo. It possesses an integrin recognition loop that harbors, in a lateral position, the inhibitory 21KTS23 motif. We report an analysis of the dynamics of the backbone and side-chain atoms of obtustatin by homonuclear NMR methods. Angular mobility has been calculated for 90 assigned cross-peaks from 22 off-resonance rotating frame nuclear Overhauser effect spectroscopy spectra recorded at three magnetic fields. Our results suggest that the integrin binding loop and the C-terminal tail display concerted motions, which can be interpreted by hinge effects. Among the integrin-binding motif, threonine 22 and serine 23 exhibit the lowest and the highest side-chain flexibility, respectively. It is noteworthy that the side chain of threonine 22 is not solvent-exposed, although based on synthetic peptides it appears to be the most critical residue for the inhibitory activity of obtustatin on the binding of integrin alpha1beta1 to collagen IV. Instead, the side chain of threonine 22 is oriented toward the loop center and hydrogen-bonded to residues Thr25 and Ser26. This network of interactions explains the restrained mobility of threonine 22 and suggests that its functional importance lies in maintaining the active conformation of the alpha1beta1 inhibitory loop.

Crystal Structure of Trimestatin, a Disintegrin Containing a Cell Adhesion Recognition Motif RGD

Disintegrins are a family of small proteins containing an Arg-Gly-Asp (RGD) sequence motif that binds specifically to integrin receptors. Since the integrin is known to serve as the final common pathway leading to aggregation via formation of platelet-platelet bridges, disintegrins act as fibrinogen receptor antagonists. Here, we report the first crystal structure of a disintegrin, trimestatin, found in snake venom. The structure of trimestatin at 1.7A resolution reveals that a number of turns and loops form a rigid core stabilized by six disulfide bonds. Electron densities of the RGD sequence are visible clearly at the tip of a hairpin loop, in such a manner that the Arg and Asp side-chains point in opposite directions. A docking model using the crystal structure of integrin alphaVbeta3 suggests that the Arg binds to the propeller domain, and Asp to the betaA domain. This model indicates that the C-terminal region is another potential binding site with integrin receptors. In addition to the RGD sequence, the structural evidence of a C-terminal region (Arg66, Trp67 and Asn68) important for disintegrin activity allows understanding of the high affinity and selectiveness of snake venom disintegrin for integrin receptors. The crystal structure of trimestatin should provide a useful framework for designing and developing more effective drugs for controlling platelet aggregation and anti-angiogenesis cancer.

Identification of the principal binding site for RGD-containing ligands in the alpha(V)beta(3) integrin: a photoaffinity cross-linking study

By superimposing data obtained by photo-cross-linking RGD-containing ligands to the human alpha(V)beta(3) integrin onto the crystal structure of the ectopic domain of this receptor (Xiong et al. (2001) Science 294, 339-345), we have identified the binding site for the RGD triad within this integrin. We synthesized three novel analogues of the 49-amino acid disintegrin, echistatin: [Bpa(21),Leu(28)]-, [Bpa(23),Leu(28)]-, and [Bpa(28)]echistatin. Each contains a photoreactive p-benzoyl-phenylalanyl (Bpa) residue in close proximity to the RGD motif which spans positions 24-26; together, the photoreactive positions flank the RGD motif. The analogues bind with high affinity to the purified recombinant alpha(V)beta(3) integrin, but very poorly to the closely related human alpha(IIb)beta(3) platelet integrin. While echistatin analogues containing Bpa in either position 23 or 28 cross-link specifically and almost exclusively to the beta(3) subunit of alpha(V)beta(3), [Bpa(21),Leu(28)]echistatin cross-links to both the alpha(V) and the beta(3) subunits, with cross-linking to the former favored. [Bpa(23),Leu(28)]echistatin cross-links 10-30 times more effectively than the other two analogues. We identified beta(3)[109-118] as the domain that encompasses the contact site for [Bpa(28)]echistatin. This domain is included in beta(3)[99-118] (Bitan et al. (2000) Biochemistry 39, 11014-11023), a previously identified contact domain for a cyclic RGD-containing heptapeptide with a benzophenone moiety in a position that is similar to the placement of the benzophenone in [Bpa(28)]echistatin relative to the RGD triad. Recently, we identified beta(3)[209-220] as the contact site for an echistatin analogue with a photoreactive group in position 45, near the C-terminus of echistatin (Scheibler et al. (2001) Biochemistry 40, 15117-14126). Taken together, these results support the hypothesis that the very high binding affinity of echistatin to alpha(V)beta(3) results from two distinct epitopes in the ligand, a site including the RGD triad and an auxiliary epitope at the C-terminus of echistatin. Combining our results from photoaffinity cross-linking studies with data now available from the recently published crystal structure of the ectopic domain of alpha(V)beta(3), we characterize the binding site for the RGD motif in this receptor.

Identification of a contact domain between echistatin and the integrin alpha(v)beta(3) by photoaffinity cross-linking

The integrin alpha(v)beta(3) is the major receptor mediating the attachment of osteoclasts to the extracellular matrix in bone and plays a critical role in bone resorption and bone remodeling. Most of the ligands interacting with the alpha(v)beta(3) receptor contain an Arg-Gly-Asp (RGD) motif. Recently, we have identified two small RGD peptides, containing a benzophenone moiety at either the carboxyl or amino terminus, that photo-cross-linked within the beta(3)[99-118] [Bitan, G., et al. (1999) Biochemistry 38, 3414-3420] or the beta(3)[167-171] [Bitan, G., et al. (2000) Biochemistry 39, 11014-11023] sequence, respectively, of the alpha(v)beta(3) receptor in a selective fashion. Here, we report the synthesis of a photoreactive analogue of echistatin (a 49-amino acid peptide), a potent RGD-containing antagonist of the alpha(v)beta(3) receptor both in vitro and in vivo. This bioactive analogue is substituted at position 45 with a p-benzoyl moiety (pBz(2)), located within the flexible C-terminal domain and removed 20 amino acid residues from the R(24)GD(26) triad. This C-terminal domain was reported to contribute to receptor binding affinity by acting as an auxiliary binding site. The radiolabeled (125)I-[Arg(35),Lys(45)(N(epsilon)-pBz(2))]-echistatin photo-cross-links effectively to a site within the beta(3)[209-220] sequence. Residues in this domain have been reported to be part of the metal ion-dependent adhesion site (MIDAS). Receptor fragments overlapping this domain were reported to bind to fibrinogen and block fibrinogen binding to alpha(IIb)beta(3), the platelet integrin receptor. Taken together, position 45 in echistatin, located within an auxiliary binding site in echistatin, cross-links to a site distinct from the two previously reported sites, beta(3)[99-118] and beta(3)[167-171], which cross-link to photophores flanking the RGD triad. These cross-linking data support the hypothesis that the ligand-bound conformation of the integrin beta(3) subunit differs from the known conformation of I domains.

Immobilised echistatin promotes platelet adhesion and protein tyrosine phosphorylation

Echistatin, a 5000-Da disintegrin, is a strong competitive inhibitor of platelet alpha(IIb)beta(3) binding to fibrinogen. In addition to its antiplatelet activity, echistatin also exhibits activating properties by inducing a switch of alpha(IIb)beta(3) conformation towards an active state. However, soluble echistatin, which is a monomeric ligand, provides only receptor affinity modulation, but it is unable to activate integrin-dependent intracellular signals. Since proteins may exhibit a multivalent functionality as a result of their absorption to a substrate, in this study we evaluated whether immobilised echistatin is able to stimulate platelet adhesion and signalling. The immobilisation process led to an increase of echistatin affinity for integrin(s) expressed on resting platelets. Unlike the soluble form, immobilised echistatin bound at comparable extent either unstimulated or ADP-activated platelets. Furthermore, echistatin presented in this manner was effective in stimulating integrin-dependent protein tyrosine phosphorylation. Platelets adhering to immobilised echistatin showed a pattern of total tyrosine phosphorylated proteins resembling that of fibrinogen-attached platelets. In particular, solid-phase echistatin induced a strong phosphorylation of tyrosine kinases pp72(syk) and pp125(FAK). Inhibitors of platelet signalling, such as apyrase, prostaglandin E(1), cytochalasin D and bisindolylmaleimide, while not affecting platelet adhesion to immobilised echistatin, abolished pp125(FAK) phosphorylation. This suggests that signals activating protein kinase C function, dense granule secretion and cytoskeleton assembly might be involved in echistatin-induced pp125(FAK) phosphorylation.

Differential recognition of snake venom proteins expressing specific Arg-Gly-Asp (RGD) sequence motifs by wild-type and variant integrin alphaIIbbeta3: further evidence for distinct sites of RGD ligand recognition exhibiting negative allostery

Several studies have demonstrated that the amino acid residues flanking the Arg-Gly-Asp (RGD) sequence of high-affinity ligands modulate their specificity of interaction with integrin complexes. Because of the absence of structural data for integrin complexes with bound ligand, the molecular basis for this specificity modulation remains obscure. In a previous paper [Rahman, Lu, Kakkar and Authi (1995) Biochem. J. 312, 223-232] we demonstrated that two genetically distinct venom-derived RGD proteins, kistrin and dendroaspin (both containing the sequence PRGDMP), were simple competitors, indicating the recognition of an identical binding site on the alpha(IIb)beta(3) complex. Furthermore, both kistrin and dendroaspin inhibited the binding of the disintegrin elegantin (containing the sequence ARGDNP) via a non-competitive mechanism, suggesting that the binding of elegantin to the alpha(IIb)beta(3) complex was at a remote site and down-regulated via an allosteric mechanism. Here we present further evidence for distinct RGD ligand recognition sites on the alpha(IIb)beta(3) complex that exhibit a negative allosteric relationship. A panel of well-characterized recombinant dendroaspin and elegantin derivatives were employed for this study. These recombinant molecules were constructed as glutathione S-transferase fusion proteins with either an Ala or Pro residue N-terminal to the RGD sequence in combination with either a Met or an Asn residue immediately C-terminal. Equilibrium competition experiments showed that elegantin binding to ADP-treated platelets was inhibited by derivatives Eleg. AM (ARGDMP) and Eleg. PM (PRGDMP) via an allosteric competitive mechanism, providing direct evidence that modulation of the RGD motif can alter competitive behaviour. In addition, recombinant kistrin and dendroaspin both inhibited elegantin binding via a non-competitive mechanism, confirming our previous observations. Further evidence for distinct binding sites employing an independent approach was obtained by analysing the binding of the panel of venom proteins to the functionally defective heterodimer alpha(IIb)beta(3) Ser(123)-->Ala expressed on Chinese hamster ovary cells. These studies demonstrated that simple competitors kistrin and dendroaspin bound with high affinity to the variant integrin complex. In contrast, the binding of elegantin and most significantly, recombinant Dendro. PN (PRGDNP) and Dendro. AN (ARGDNP) were abolished. These observations, taken together, are consistent with a model depicting the presence of distinct sites of RGD ligand recognition on the alpha(IIb)beta(3) complex that show the preferential recognition of specific RGD motifs. Competition experiments demonstrate a negative allosteric relationship between these RGD recognition sites.

Snake venoms and the hemostatic system

Snake venoms are complex mixtures containing many different biologically active proteins and peptides. A number of these proteins interact with components of the human hemostatic system. This review is focused on those venom constituents which affect the blood coagulation pathway, endothelial cells, and platelets. Only highly purified and well characterized snake venom proteins will be discussed in this review. Hemostatically active components are distributed widely in the venom of many different snake species, particularly from pit viper, viper and elapid venoms. The venom components can be grouped into a number of different categories depending on their hemostatic action. The following groups are discussed in this review: (i) enzymes that clot fibrinogen; (ii) enzymes that degrade fibrin(ogen); (iii) plasminogen activators; (iv) prothrombin activators; (v) factor V activators; (vi) factor X activators; (vii) anticoagulant activities including inhibitors of prothrombinase complex formation, inhibitors of thrombin, phospholipases, and protein C activators; (viii) enzymes with hemorrhagic activity; (ix) enzymes that degrade plasma serine proteinase inhibitors; (x) platelet aggregation inducers including direct acting enzymes, direct acting non-enzymatic components, and agents that require a cofactor; (xi) platelet aggregation inhibitors including: alpha-fibrinogenases, 5'-nucleotidases, phospholipases, and disintegrins. Although many snake venoms contain a number of hemostatically active components, it is safe to say that no single venom contains all the hemostatically active components described here. Several venom enzymes have been used clinically as anticoagulants and other venom components are being used in pre-clinical research to examine their possible therapeutic potential. The disintegrins are an interesting group of peptides that contain a cell adhesion recognition motif, Arg-Gly-Asp (RGD), in the carboxy-terminal half of their amino acid sequence. These agents act as fibrinogen receptor (integrin GPIIb/IIIa) antagonists. Since this integrin is believed to serve as the final common pathway leading to the formation of platelet-platelet bridges and platelet aggregation, blockage of this integrin leads to inhibition of platelet aggregation regardless of the stimulating agent. Clinical trials suggest that platelet GPIIb/IIIa blockade is an effective therapy for the thrombotic events and restenosis frequently accompanying cardiovascular and cerebrovascular disease. Therefore, because of their clinical poten tial, a large number of disintegrins have been isolated and characterized.

The disulphide bond pattern of bitistatin, a disintegrin isolated from the venom of the viper Bitis arietans

The disulphide bond pattern of the long disintegrin bitistatin (83 amino acids, 14 cysteines) was established using structural information gathered by amino acid analysis, N-terminal sequencing, and molecular mass determination of fragments isolated by reversed-phase HPLC after polypeptide degradation with trypsin and oxalic acid. A computer program was used to calculate all possible combinations of disulphide-bonded peptides matching the mass spectrometric data, and the output was filtered using compositional and sequence data. Disulphide bonds between cysteines 16-34, 18-29, 28-51, 42-48, 47-72, and 60-79 are conserved in medium-long disintegrins flavoridin and kistrin (70 amino acids, 12 cysteines), and the two cysteine residues at positions 5 and 24 found in bitistatin but not in other disintegrin molecules are disulphide-bridged. This linkage creates an extra, large loop, which, depending on whether the NMR structure of flavoridin or kistrin is used for modelling the structure of bitistatin, lies opposite or nearly parallel, respectively, to the biologically active RGD-containing loop.

Significance of RGD Loop and C-Terminal Domain of Echistatin for Recognition of αIIbβ3 and αvβ3 Integrins and Expression of Ligand-Induced Binding Site

Echistatin is a viper venom disintegrin containing RGD loop maintained by disulfide bridges. It binds with a high affinity to αvβ3 and αIIbβ3 and it induces extensive conformational changes in these integrins resulting in expression of ligand-induced binding site (LIBS) epitopes. We investigated the activities of echistatin and its three analogues (R24A, D27W, echistatin 1-41). R24A echistatin did not react with αIIbβ3 and αvβ3 integrins and did not cause LIBS effect. D27W echistatin showed increased binding to αIIbβ3 and decreased binding to αvβ3. This substitution impaired the ability of echistatin to induce LIBS in αvβ3 integrin. Deletion of nine C-terminal amino acids of echistatin decreased its ability to bind αIIbβ3 and inhibit platelet aggregation. Truncated echistatin failed to induce LIBS epitopes on cells transfected with αIIbβ3 and αvβ3 genes. The ability of echistatin 1-41 to compete with binding of vitronectin to immobilized αvβ3 and monoclonal antibody 7E3 to platelets and to VNRC3 cells was decreased, although this analogue, after immobilization, retained its ability to bind purified αvβ3. We propose a hypothesis in which echistatin's RGD loop determines selective recognition of αIIbβ3 and αvβ3 integrin, whereas the C-terminal domain supports its binding to resting integrin and significantly contributes to the expression of LIBS epitope and to conformational changes of the receptor, leading to a further increase of the binding affinity of echistatin and of the inhibitory effect.

Significance of RGD Loop and C-Terminal Domain of Echistatin for Recognition of αIIbβ3 and αvβ3 Integrins and Expression of Ligand-Induced Binding Site

Echistatin is a viper venom disintegrin containing RGD loop maintained by disulfide bridges. It binds with a high affinity to αvβ3 and αIIbβ3 and it induces extensive conformational changes in these integrins resulting in expression of ligand-induced binding site (LIBS) epitopes. We investigated the activities of echistatin and its three analogues (R24A, D27W, echistatin 1-41). R24A echistatin did not react with αIIbβ3 and αvβ3 integrins and did not cause LIBS effect. D27W echistatin showed increased binding to αIIbβ3 and decreased binding to αvβ3. This substitution impaired the ability of echistatin to induce LIBS in αvβ3 integrin. Deletion of nine C-terminal amino acids of echistatin decreased its ability to bind αIIbβ3 and inhibit platelet aggregation. Truncated echistatin failed to induce LIBS epitopes on cells transfected with αIIbβ3 and αvβ3 genes. The ability of echistatin 1-41 to compete with binding of vitronectin to immobilized αvβ3 and monoclonal antibody 7E3 to platelets and to VNRC3 cells was decreased, although this analogue, after immobilization, retained its ability to bind purified αvβ3. We propose a hypothesis in which echistatin's RGD loop determines selective recognition of αIIbβ3 and αvβ3 integrin, whereas the C-terminal domain supports its binding to resting integrin and significantly contributes to the expression of LIBS epitope and to conformational changes of the receptor, leading to a further increase of the binding affinity of echistatin and of the inhibitory effect.

Isolation and amino acid sequence of flavostatin, a novel disintegrin from the venom of Trimeresurus flavoviridis

Flavostatin, a novel disintegrin purified from the venom of Trimeresurus flavoviridis, consists of 68 amino acids, including an Arg-Gly-Asp sequence and 12 Cys residues at positions highly conserved among disintegrins. The N-terminal sequence of flavostatin was identical to those of triflavin and flavoridin, previously reported disintegrins from the Trimeresurus flavoviridis venom. Differences among the C-terminal sequences of these disintegrins are considered to affect their biological potencies. Isolated flavostatin inhibited ADP collage, and thrombin receptor agonist peptide-induced platelet aggregation in human platelet-rich plasma with an IC50 range of 59 to 98 nM. Contrary to expectations, these values were similar to those for triflavin.

Immunological characterization of eristostatin and echistatin binding sites on alpha IIb beta 3 and alpha V beta 3 integrins

Two disintegrins with a high degree of amino acid sequence similarity, echistatin and eristostatin, showed a low level of interaction with Chinese hamster ovary (CHO) cells, but they bound to CHO cells transfected with alpha IIb beta 3 genes (A5 cells) and to CHO cells transfected with alpha v beta 3 genes (VNRC3 cells) in a reversible and saturable manner. Scatchard analysis revealed that eristostatin bound to 816000 sites per A5 cell (Kd 28 nM) and to 200000 sites (Kd 14 nM) per VNRC3 cell respectively. However, VNRC3 cells did not bind to immobilized eristostatin. Echistatin bound to 495000 sites (Kd 53 nM) per A5 cell and to 443000 sites (Kd 20 nM) per VNRC3 cell. As determined by flow cytometry, radiobinding assay and adhesion studies, binding of both disintegrins to A5 cells and resting platelets and binding of echistatin to VNRC3 cells resulted in the expression of ligand-induced binding sites (LIBS) on the beta 3 subunit. Eristostatin inhibited, more strongly than echistatin, the binding of three monoclonal antibodies: OPG2 (RGD motif dependent), A2A9 (alpha IIb beta 3 complex dependent) and 7E3 (alpha IIb beta 3 and alpha v beta 3 complex dependent) to A5 cells, to resting and to activated platelets and to purified alpha IIb beta 3. Experiments in which echistatin and eristostatin were used alone or in combination to inhibit the binding of 7E3 and OPG2 antibodies to resting platelets suggested that these two disintegrins bind to different but overlapping sites on alpha IIb beta 3 integrin. Monoclonal antibody LM 609 and echistatin seemed to bind to different sites on alpha v beta 3 integrin. However, echistatin inhibited binding of 7E3 antibody to VNRC3 cells and to purified alpha v beta 3 suggesting that alpha v beta 3 and alpha IIb beta 3 might share the same epitope to which both echistatin and 7E3 bind. Eristostatin had no effect in these systems, providing further evidence that it binds to a different epitope on alpha v beta 3.

Substitutions of proline 42 to alanine and methionine 46 to asparagine around the RGD domain of the neurotoxin dendroaspin alter its preferential antagonism to that resembling the disintegrin elegantin

Previous studies have shown that the neurotoxin dendroaspin and the disintegrin kistrin, which show little overall sequence homology but similar residues around RGD (PRGDMP), preferentially inhibited platelet adhesion to fibrinogen. In contrast, the elegantin which has different amino acids around RGD (ARGDNP) preferentially inhibited platelet adhesion to fibronectin. To investigate further the role of amino acids around RGD in disintegrins, we have constructed the genes of a wild-type and of two mutant dendroaspins with substitutions around the RGD, namely [Asn46]- and [Ala42,Asn46]-dendroaspins. Proteins were expressed in Escherichia coli as glutathione S-transferase fusion recombinants and purified to homogeneity by affinity chromatography and reversed phase high performance liquid chromatography. Platelet aggregation studies revealed that wild-type dendroaspin showed an IC50 value similar to that of native dendroaspin, with [Ala42,Asn46]-dendroaspin showing an IC50 value similar to that of elegantin. Interestingly, in platelet adhesion assays, the mutants showed a progressive shift in inhibitory preference, in particular, [Ala42,Asn46]dendroaspin showed nearly identical behavior as elegantin when fibronectin was the immobilized ligand (IC50 = 0.33 microM and 0.6 microM, respectively, compared with 20 microM for native dendroaspin). Native and recombinant wild-type dendroaspin bound to a single class of binding site exhibiting a Kd = 67 nM; [Asn46]- and [Ala42,Asn46]dendroaspins, however, both produced biphasic isotherms with Kd values = 87 nM and 361 nM for [Asn46]dendroaspin and 33 nM and 371 nM for [Ala42,Asn46]dendroaspin, which are close to those of elegantin (Kd values = 18 nM and 179 nM). These studies prove that the amino acids flanking RGD provide an extended locus that regulate the affinity and selectivity of RGD protein dendroaspin.

Determination of the structure of two novel echistatin variants and comparison of the ability of echistatin variants to inhibit aggregation of platelets from different species

Two new variants of short disintegrins were purified from the venom of Echis carinatus leakeyi and named echistatin beta and gamma. These proteins were found to be about 85% similar in amino acid sequence to echistatin alpha which has been well studied. The disulphide pattern of echistatin gamma appeared to be identical with that of echistatin alpha. They all contain the adhesive recognition sequence Arg-Gly-Asp (RGD) but inhibit the aggregation of platelets from human and other mammals with different potencies. Echistatin beta and alpha are far more effective on platelets from humans and guinea pigs than those from rabbits and rats whereas echistatin gamma is less discriminating of the platelets of the species tested. This species-dependent platelet sensitivity to echistatin beta and gamma could be attributed to the variations in residues 15, 21, 22 and 27, which are close to or within the RGD loop, rather than to the C-terminal variations after residue 46. Taking advantage of the presence of methionine residues flanking both sides of the ARGDDM motif in echistatin gamma, we deleted this hexapeptide by CNBr cleavage to produce des-(23-28)-echistatin gamma. The modified protein showed c.d. and fluorescent spectra grossly similar to the intact echistatin but its antiplatelet potency decreased more than 200-fold. We thus propose that a favourable conformation of the RGD region is responsible mainly for the high-affinity binding of echistatin to the platelet glycoprotein IIb-IIIa as shown previously for the binding of medium-size disintegrin.

Preferential antagonism of the interactions of the integrin alpha IIb beta 3 with immobilized glycoprotein ligands by snake-venom RGD (Arg-Gly-Asp) proteins. Evidence supporting a functional role for the amino acid residues flanking the tripeptide RGD in determining the inhibitory properties of snake-venom RGD proteins

The inhibitory properties of a panel of snake-venom-derived RGD (Arg-Gly-Asp) proteins, including the disintegrins kistrin, elegantin and albolabrin, and the neurotoxin homologue dendroaspin, were investigated in a platelet-adhesion assay using three immobilized ligands of the glycoprotein IIb-IIIa complex (alpha IIb beta 3), namely fibrinogen, fibronectin and von Willebrand factor (vWF). The snake-venom proteins preferentially inhibited the adhesion of ADP-treated platelets to one or more of the immobilized ligands. Kistrin and dendroaspin exhibited similar inhibitory characteristics, abrogating platelet adhesion to fibrinogen and vWF at nanomolar concentrations, but poorly inhibiting adhesion to fibronectin. Kistrin and dendroaspin share little overall amino-acid-sequence identity, but a considerable level of sequence similarity exists around the RGD tripeptide. Synthetic cyclic peptides corresponding to these regions of kistrin and dendroaspin inhibited platelet adhesion to both fibrinogen and fibronectin with approximately equal potency, but were 100-fold weaker antagonists of the interactions of the alpha IIb beta 3 complex with fibrinogen than their parent proteins. The disintegrins elegantin and albolabrin, which share approx. 60% overall amino-acid-sequence similarity with kistrin but have different residues around the RGD tripeptide, exhibited different antagonistic preferences. Elegantin inhibited platelet adhesion to immobilized vWF and fibronectin, but was significantly less effective at disrupting adhesion to fibrinogen. Albolabrin selectively inhibited platelet adhesion to immobilized vWF and was less effective with fibrinogen and fibronectin as adhesive ligands. In contrast with the behaviour of these venom proteins, the adhesion of ADP-treated platelets to immobilized fibrinogen, fibronectin and vWF was inhibited non-selectively by a range of monoclonal antibodies with specificity for the alpha IIb beta 3 complex. These observations, therefore, define antagonistic preferences in this panel of venom proteins towards the interactions of the alpha IIb beta 3 complex with three immobilized glycoprotein ligands.

Comparison of disintegrins with limited variation in the RGD loop in their binding to purified integrins alpha IIb beta 3, alpha V beta 3 and alpha 5 beta 1 and in cell adhesion inhibition

The inhibitory capacities of six different disintegrins and one related neurotoxin analogue for the binding of RGD-dependent integrins to either fibrinogen, vitronectin or fibronectin were compared in solid phase assays. Echistatin and flavoridin were the most active inhibitors for alpha V beta 3 and alpha 5 beta 1 integrins and moderately exceeded the activity of the natural protein ligands. The same disintegrins together with eristostatin, bitistatin and barbourin were also very potent inhibitors of fibrinogen binding to alpha IIb beta 3 integrin. For all three integrins, albolabrin showed the lowest affinity, but it still clearly exceeded that of synthetic GRGDS. However, assay conditions may determine these relative affinities, as shown for the alpha IIb beta 3 and alpha V beta 3 integrins when used either in immobilized or soluble form. For alpha IIb beta 3, however, a close correlation was found between KD values determined in platelet binding assays and the concentrations required for half maximal inhibition of three disintegrins. The inhibiting capacity of disintegrins in assays with purified integrins also correlated reasonably well with their inhibition of cell attachment to RGD-dependent protein substrates. However, sequence differences in the RGD loops of the various disintegrins may not fully account for the 20-100-fold difference in their binding capacities. This was particularly evident for echistatin and albolabrin, which differ in this region only by two conservative substitutions but have considerably different inhibitory activities. More remote regions of the disintegrins and alignment of disulfide bridges are therefore likely to contribute to their affinity and selectivity.

Integrin-ligand interactions: a year in review

Many cell-cell and cell-matrix interactions depend upon the engagement of specific ligands by members of the integrin family of cell-adhesion receptors. In concert with the identification of new integrins, the number of integrin ligands continues to expand dramatically. The diversity of the integrin ligands bridges many areas of cell and molecular biology. Ligand recognition by integrins requires not only the presence of the cognate primary sequence within an appropriate secondary structure, but also the correct tertiary and quaternary structure of the ligand. Presentation of an 'activated' ligand sequence to specific contact sites within the integrin under specified divalent-cation conditions is necessary for a productive and high-affinity interaction.

Structure-activity studies of the s-echistatin inhibition of bone resorption

Synthetic Arg-Gly-Asp (RGD)-containing peptides were examined in bone resorption or attachment and detachment assays with isolated mammalian osteoclasts in an effort to elucidate the mechanistic and structural basis for the inhibition of bone resorption by s-echistatin. Bone resorption was the process most sensitive to inhibition by s-echistatin, with IC50 = 0.3 nM; inhibition of attachment to bone or detachment (lamellipodial retraction) was 30- to 70-fold less sensitive, with IC50 = 10 or 20 nM, respectively. Single amino acid substitutions within the 49-residue sequence of s-echistatin showed that although the efficacy of s-echistatin is dependent on the Arg24-Gly25-Asp26 sequence, additional residues, including Asp27, Met28, and Cys39, are also critical for potent inhibition of the resorbing activity of isolated rat osteoclasts. Because of the identification of the av beta 3 as the primary integrin on rat osteoclasts interacting the RGD peptides (Helfrich et al.), we examined the possibility of modeling bone resorption with other beta 3-mediated processes. Specifically, av beta 3 endothelial cell (human or rat) attachment to vitronectin and aIIb beta 3 platelet aggregation were compared with bone resorption for sensitivity to s-echistatin analogs, linear RGD peptides, and cyclic RGD peptides. Essentially no similarity in sensitivity to RGD peptides were observed between bone resorption, platelet aggregation, or endothelial cell attachment. Because rat osteoclasts and human giant cell tumors (osteoclastomas) shared similar sensitivity to s-echistatin and rat and human endothelial cells showed a similar sensitivity profile to RGD peptides, the dissimilarity of bone resorption to other beta 3-mediated processes cannot be explained in terms of species differences.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of disintegrins with the alpha IIb beta 3 receptor on resting and activated human platelets

Viper venom disintegrins contain the RGD/KGD motif. They inhibit platelet aggregation and cell adhesion, but show structural and functional heterogeneity. We investigated the interaction of four prototypic disintegrins with alpha IIb beta 3 expressed on the surface of resting and activated platelets. The binding affinity (Kd) of 125I-albolabrin, 125I-echistatin, 125I-bitistatin and 125I-eristostatin toward resting platelets was 294, 153, 48 and 18 nM respectively. The Kd value for albolabrin decreased 3-fold and 6-fold after ADP- or thrombin-induced activation. The Kd values for bitistatin and echistatin also decreased with ADP, but there was no further decrease with thrombin. In contrast, eristostatin bound with the same high affinity to resting and activated platelets. The pattern of fluorescein isothiocyanate (FITC)-eristostatin and FITC-albolabrin binding to resting and activated platelets was consistent with observations using radiolabelled material. Eristostatin showed faster and more irreversible binding to platelets, and greater potency compared with albolabrin in inducing conformational neo-epitopes in beta 3. The anti-alpha IIb beta 3 monoclonal antibody OP-G2 that is RGD-dependent inhibited disintegrin binding to activated platelets more strongly than binding to resting platelets and it inhibited the binding to platelets of albolabrin more strongly than eristostatin. The specificity of disintegrin interaction with alpha IIb beta 3 was confirmed by demonstrating cross-linking of these peptides to alpha IIb beta 3 on normal platelets, but not to thrombasthenic platelets deficient in alpha IIb beta 3.