Localization of the cross-linking sites of RGD and KQAGDV peptides to the isolated fibrinogen receptor, the human platelet integrin glycoprotein IIb/IIIa. Influence of peptide length

Fibrinogen binding to activated platelets and its biomimetic thrombus-targeted thrombolytic strategies

Thrombosis is associated with various fatal arteriovenous syndromes including ischemic stroke, myocardial infarction, and pulmonary embolism. However, current clinical thrombolytic treatment strategies still have many problems in targeting and safety to meet the thrombolytic therapy needs. Understanding the molecular mechanism that underlies thrombosis is critical in developing effective thrombolytic strategies. It is well known that platelets play a central role in thrombosis and the binding of fibrinogen to activated platelets is a common pathway in the process of clot formation. Based on this, a concept of biomimetic thrombus-targeted thrombolytic strategy inspired from fibrinogen binding to activated platelets in thrombosis was proposed, which could selectively bind to activated platelets at a thrombus site, thus enabling targeted delivery and local release of thrombolytic agents for effective thrombolysis. In this review, we first summarized the main characteristics of platelets and fibrinogen, and then introduced the classical molecular mechanisms of thrombosis, including platelet adhesion, platelet activation and platelet aggregation through the interactions of activated platelets with fibrinogen. In addition, we highlighted the recent advances in biomimetic thrombus-targeted thrombolytic strategies which inspired from fibrinogen binding to activated platelets in thrombosis. The possible future directions and perspectives in this emerging area are briefly discussed.

Peptide-Based Functional Biomaterials for Soft-Tissue Repair

Synthetically derived peptide-based biomaterials are in many instances capable of mimicking the structure and function of their full-length endogenous counterparts. Combine this with the fact that short mimetic peptides are easier to produce when compared to full length proteins, show enhanced processability and ease of modification, and have the ability to be prepared under well-defined and controlled conditions; it becomes obvious why there has been a recent push to develop regenerative biomaterials from these molecules. There is increasing evidence that the incorporation of peptides within regenerative scaffolds can result in the generation of structural recognition motifs that can enhance cell attachment or induce cell signaling pathways, improving cell infiltration or promote a variety of other modulatory biochemical responses. By highlighting the current approaches in the design and application of short mimetic peptides, we hope to demonstrate their potential in soft-tissue healing while at the same time drawing attention to the advances made to date and the problems which need to be overcome to advance these materials to the clinic for applications in heart, skin, and cornea repair.

Recent advances in self-assembled peptides: Implications for targeted drug delivery and vaccine engineering

Self-assembled peptides have shown outstanding characteristics for vaccine delivery and drug targeting. Peptide molecules can be rationally designed to self-assemble into specific nanoarchitectures in response to changes in their assembly environment including: pH, temperature, ionic strength, and interactions between host (drug) and guest molecules. The resulting supramolecular nanostructures include nanovesicles, nanofibers, nanotubes, nanoribbons, and hydrogels and have a diverse range of mechanical and physicochemical properties. These molecules can be designed for cell-specific targeting by including adhesion ligands, receptor recognition ligands, or peptide-based antigens in their design, often in a multivalent display. Depending on their design, self-assembled peptide nanostructures have advantages in biocompatibility, stability against enzymatic degradation, encapsulation of hydrophobic drugs, sustained drug release, shear-thinning viscoelastic properties, and/or adjuvanting properties. These molecules can also act as intracellular transporters and respond to changes in the physiological environment. Furthermore, this class of materials has shown sequence- and structure-dependent impacts on the immune system that can be tailored to non-immunogenic for drug targeting, and immunogenic for vaccine delivery. This review explores self-assembled peptide nanostructures (beta sheets, alpha helices, peptide amphiphiles, amino acid pairing, elastin like polypeptides, cyclic peptides, short peptides, Fmoc peptides, and peptide hydrogels) and their application in vaccine delivery and drug targeting.

Integrin-dependent cell behavior on ECM peptide-conjugated chitosan membranes

Extracellular matrix (ECM) plays an important role in tissue regeneration by promoting cell adhesion, migration, proliferation, and differentiation. ECM mimetics are of importance for tissue engineering because of their functions as scaffolds for cells. Previously, we developed bioactive laminin-derived peptide-conjugated chitosan membranes and demonstrated their cell- and peptide-type specific functions. Here, we conjugated twelve integrin-binding peptides derived from ECM proteins onto chitosan membranes and examined biological activity. Seven peptide-chitosan membranes promoted human foreskin fibroblast attachment. Additionally, FIB1 (YAVTGRGDSPAS; from fibronectin), A99 (AGTFALRGDNPQG; from laminin alpha1 chain), EF1zz (ATLQLQEGRLHFXFDLGKGR, X = Nle; from laminin alpha1 chain), and 531 (GEFYFDLRLKGDKY; from collagen alpha1 (IV) chain) conjugated chitosan membranes promoted integrin-dependent cell adhesion. Various integrins, including alphav, beta1, and beta3, were involved in the cell adhesion to the peptide-chitosan membranes. Further, only the FIB1- and A99-chitosan membranes promoted neurite outgrowth with PC12 rat pheochromocytoma cells. These data demonstrate that peptide-chitosan membranes can regulate specific integrin-mediated cell responses and are useful constructs as ECM mimetics.

STD and TRNOESY NMR studies for the epitope mapping of the phosphorylation motif of the oncogenic protein beta-catenin recognized by a selective monoclonal antibody

The interaction of the P-beta-Cat(19-44) peptide, a 26 amino acid peptide (K(19)AAVSHWQQQSYLDpSGIHpSGATTTAP(44)) that mimics the phosphorylated beta-Catenin antigen, has been studied with its monoclonal antibody BC-22, by transferred nuclear Overhauser effect NMR spectroscopy (TRNOESY) and saturation transfer difference NMR (STD NMR) spectroscopy. This antibody is specific to diphosphorylated beta-Catenin and does not react with the non-phosphorylated protein. Phosphorylation of beta-Catenin at sites Ser33 and Ser37 on the DSGXXS motif is required for the interaction of beta-Catenin with the ubiquitin ligase SCF(beta-TrCP). beta-TrCP is involved in the ubiquitination and proteasome targeting of the oncogenic protein beta-Catenin, the accumulation of which has been implicated in various human cancers. The three-dimensional structure of the P-beta-Cat(19-44) in the bound conformation was determined by TRNOESY NMR experiments; the peptide adopts a compact structure in the presence of mAb with formation of turns around Trp25 and Gln26, with a tight bend created by the DpS(33)GIHpS(37) motif; the peptide residues (D32-pS37) forming this bend are recognized by the antibody as demonstrated by STD NMR experiments. STD NMR studies provide evidence for the existence of a conformational epitope containing tandem repeats of phosphoserine motifs. The peptide's epitope is predominantly located in the large bend and in the N-terminal segment, implicating bidentate association. These findings are in excellent agreement with a recently published NMR structure required for the interaction of beta-Catenin with the SCF(beta-TrCP) protein.

Epitope Mapping of the Phosphorylation Motif of the HIV-1 Protein Vpu Bound to the Selective Monoclonal Antibody Using TRNOESY and STD NMR Spectroscopy

The conformational preferences of a 22-amino acid peptide (LIDRLIERAEDpSGNEpSEGEISA) that mimics the phosphorylated HIV-1-encoded virus protein U (Vpu) antigen have been investigated by NMR spectroscopy. Degradation of HIV receptor CD4 by the proteasome, mediated by the HIV-1 protein Vpu, is crucial for the release of fully infectious virions. Phosphorylation of Vpu at sites Ser52 and Ser56 on the DSGXXS motif is required for the interaction of Vpu with the ubiquitin ligase SCF(beta)(-TrCP) which triggers CD4 degradation by the proteasome. This motif is conserved in several signaling proteins known to be degraded by the proteasome. The interaction of the P-Vpu(41-62) peptide with its monoclonal antibody has been studied by transferred nuclear Overhauser effect NMR spectroscopy (TRNOESY) and saturation transfer difference NMR (STD NMR) spectroscopy. The peptide was found to adopt a bend conformation upon binding to the antibody; the peptide residues (Asp51-pSer56) forming this bend are recognized by the antibody as demonstrated by STD NMR experiments. The three-dimensional structure of P-Vpu(41-62) in the bound conformation was determined by TRNOESY spectra; the peptide adopts a compact structure in the presence of mAb with formation of several bends around Leu45 and Ile46 and around Ile60 and Ser61, with a tight bend created by the DpS(52)GNEpS(56) motif. STD NMR studies provide evidence for the existence of a conformational epitope containing tandem repeats of phosphoserine motifs. The peptide's epitope is predominantly located in the large bend and in the N-terminal segment, implicating bidentale association. These findings are in excellent agreement with a recently published NMR structure required for the interaction of Vpu with the SCF(beta)(-TrCP) protein.

Mapping the binding domains of the alpha(IIb) subunit. A study performed on the activated form of the platelet integrin alpha(IIb)beta(3)

alpha(IIb)beta(3), a member of the integrin family of adhesive protein receptors, is the most abundant glycoprotein on platelet plasma-membranes and binds to adhesive proteins via the recognition of short amino acid sequences, for example the ubiquitous RGD motif. However, elucidation of the ligand-binding domains of the receptor remains controversial, mainly owing to the fact that integrins are conformationally labile during purification and storage. In this study, a detailed mapping of the extracellular region of the alpha(IIb) subunit is presented, using overlapping 20-peptides, in order to identify the binding sites of alpha(IIb) potentially involved in the platelet-aggregation event. Regions alpha(IIb) 313-332, alpha(IIb) 265-284 and alpha(IIb) 57-64 of alpha(IIb)beta(3) were identified as putative fibrinogen-binding domains because the corresponding peptides inhibited platelet aggregation and antagonized fibrinogen association, possibly by interacting with this ligand. The latter is further supported by the finding that the above peptides did not interfere with the binding of PAC-1 to the activated form of alpha(IIb)beta(3). Furthermore, alpha(IIb) 313-332 was found to bind to fibrinogen in a solid-phase binding assay. It should be emphasized that all the experiments in this study were carried out on activated platelets and consequently on the activated form of this integrin receptor. We hypothesize that RAD and RAE adhesive motifs, encompassed in alpha(IIb) 313-332, 265-284 and 57-64, are capable of recognizing complementary domains of fibrinogen, thus inhibiting the binding of this ligand to platelets.

Agonist-specific structural rearrangements of integrin alpha IIbbeta 3. Confirmation of the bent conformation in platelets at rest and after activation

Concrete structural features of integrin alpha(IIb)beta(3) on the surface of platelets (at rest and after activation) have been obtained from epitope maps based on cross-competition among monoclonal antibodies directed against the alpha(IIb) subunit calf-2 domain and the beta(3) subunit betaA domain of alpha(IIb)beta(3). At rest, the observed intersubunit interface is formed by the sequence stretches beta(3)-(150-216), alpha(IIb) light chain-(1-92), and alpha(IIb) heavy chain-(826-856); and the alpha(IIb) interchain interface is formed by the two latter sequence stretches, disulfide-bonded between alpha(IIb) heavy chain Cys(826) and alpha(IIb) light chain Cys(9). These structural features agree with those observed in the alpha(IIb)beta(3) rudimentary connectivity map in solution and with the alpha(v)beta(3) V-shaped crystal structure (Xiong, J.-P., Zhang, R., Dunker, R., Scott, D. L., Joachimiak, A., Goodman, S. L., and Arnaout, M. A. (2001) Science 294, 339-345), but they disagree with the domain disposition suggested by the actual ultrastructural model. The epitope maps in platelets activated by ADP, thrombin receptor activation peptide, and arachidonic acid differ not only from those in platelets at rest, but also among themselves. The structural rearrangements observed confirm the presence in activated platelets of the crystallographically observed knee and argue against the switchblade mechanism proposed for activation (Beglova, N., Blacklow, S. C., Takagi, J., and Springer, T. A. (2002) Nat. Struct. Biol. 9, 282-287), demonstrate the existence of alpha(IIb)beta(3) agonist-specific activation states, explain the specificity for ligand binding and functional inhibition for some agonists, and predict the existence of agonist-specific final effectors and receptor activation mechanisms. The distinct non-reciprocal competition patterns observed at rest and after activation support the agonist-specific activation states and the existence of intrasubunit and intersubunit allosteric effects, previously proposed as the mechanism for alpha(IIb)beta(3) transmembrane activation.

Organization of the glycoprotein (GP) IIb/IIIa heterodimer on resting human platelets studied by flow cytometric energy transfer

Glycoprotein IIb/IIIa is a heterodimer of glycoproteins IIb and IIIa which serves as the inducible receptor for fibrinogen and other adhesive proteins at the surface of platelets. Although a model of the quaternary structure of the GPIIb/IIIa molecule has been constructed in solution by Calvete et al. [Biochem. J. 282 (1992) 523], a corresponding model at the surface of intact platelets is still missing. In the present work conformation and lateral distribution of the GPIIb/IIIa heterodimer were studied at a nanometer resolution on the surface of resting human platelets under physiological conditions. The experiments were based on dual wavelength flow cytometric detection of fluorescence resonance energy transfer and application of a panel of monoclonal antibodies raised against well described binding sites. Monodisperse distribution of the GPIIb/IIIa heterodimer has been observed and a detailed three-dimensional proximity map of antibody binding sites was constructed on the platelet membrane, under physiological conditions, for the first time. Our data support the view that the GPIIb subunit is in a bent conformation. A detailed analysis of the K(d)-values and the number of binding sites for a set of monoclonal antibodies was also carried out giving supplementary data for the topology of the binding sites. Our results provide a refinement of the membrane-topology of the GPIIb/IIIa heterodimer.

Tethered-TGF-beta increases extracellular matrix production of vascular smooth muscle cells

Biomaterials developed for tissue engineering and wound healing applications need to support robust cell adhesion, yet also need to be replaced by new tissue synthesized by those cells. In order to maintain mechanical integrity of the tissue, the cells must generate sufficient extracellular matrix before the scaffold is degraded. We have previously shown that materials containing cell adhesive ligands to promote or improve cell adhesion can decrease extracellular matrix production (Mann et al., Modification of surfaces with cell adhesion peptides alters extracellular matrix deposition. Biomaterials 1999;20:2281-6). Such decreased matrix production by cells in tissue engineering scaffolds may result in tissue failure. However, we have found that TGF-beta1 can be used in scaffolds to dramatically increase matrix production. Matrix production by vascular smooth muscle cells grown on adhesive ligand-modified glass surfaces and in PEG hydrogels containing covalently bound adhesive ligands was increased in the presence of 0.04 pmol/ml (1 ng/ml) TGF-beta1. TGF-beta1 can counteract the effect of these adhesive ligands on matrix production; matrix production could be increased even above that observed in the absence of adhesive peptides. Further, TGF-beta1 covalently immobilized to PEG retained its ability to increase matrix production. Tethering TGF-beta1 to the polymer scaffold resulted in a significant increase in matrix production over the same amount of soluble TGF-beta1.

The use of a thrombus-specific ultrasound contrast agent to detect thrombus in arteriovenous fistulae

RATIONALE AND OBJECTIVES

To evaluate the use of a new thrombus-specific ultrasound contrast agent, MRX-408, in the ultrasonic detection of thrombus in arteriovenous (AV) fistulae.

METHODS

Six purpose-bred mongrels with two AV fistulae each were imaged with gray-scale ultrasound 7 weeks after graft implantation before and after the intravenous bolus injection of MRX-408 (a GPIIb receptor-targeted ultrasound contrast agent). Pre- and postcontrast videotaped segments were randomized and reviewed by four radiologists blinded to the presence of thrombus in the grafts.

RESULTS

After the use of MRX-408, there was improved visualization of thrombus within the grafts (P < 0.0001). This was due to the enhancement of the thrombus (P < 0.0001). The improved visualization and contrast enhancement were more marked in the grafts that contained thrombus nonhyperechoic to surrounding soft tissues.

CONCLUSIONS

MRX-408 demonstrated better visualization of thrombus within AV fistulae. This was shown in both patent and occluded grafts. These results are encouraging and suggest that this contrast agent merits further development.

Differential effects of integrin alpha chain mutations on invasin and natural ligand interaction

To determine if recognition of the Yersinia pseudotuberculosis invasin protein and natural substrates requires identical integrin residues, a region of the human alpha3 integrin chain predicted to be involved in substrate adhesion was targeted for mutation. One point mutation located in a region of the third N-terminal repeat of the alpha3 chain, alpha3-W220A, failed to promote adhesion to the natural alpha3 beta1 substrate epiligrin but maintained near wild type levels of adhesion to invasin. A second nearby mutation, alpha3-Y218A, which showed no detectable adhesion to epiligrin, was only partially attenuated for invasin binding as well as invasin-mediated bacterial uptake. A third substitution, alpha3-D154A, predicted to be in the second N-terminal repeat not known to be implicated in cell adhesion, was competent for invasin-promoted adhesion events and appeared to encode a receptor of increased activity, as it had a higher efficiency than wild type receptor for adhesion to epiligrin. Cell lines expressing this derivative were not recognized by a function blocking anti-alpha3 antibody, indicating that the second and third repeats of the alpha3 chain are either closely linked in space or the second repeat can modulate activity of the third. Differential effects on substrate adhesion do not appear to be associated with all integrin alpha chain mutations, as alpha4 chain mutations affecting the divalent cation binding domains depressed adhesion to invasin to a significant extent.

Critical residues of integrin alphaIIb subunit for binding of alphaIIbbeta3 (glycoprotein IIb-IIIa) to fibrinogen and ligand-mimetic antibodies (PAC-1, OP-G2, and LJ-CP3)

Integrin alphaIIbbeta3 plays a critical role in platelet aggregation through its interaction with fibrinogen. Elucidation of the mechanisms of alphaIIbbeta3-fibrinogen interaction is critical to understanding hemostasis and thrombosis. Here we report that mutations of Gly-184, Tyr-189, Tyr-190, Phe-191, and Gly-193 within the predicted turn structure of the third amino-terminal repeat of alphaIIb significantly block binding of alphaIIbbeta3 to soluble fibrinogen. These mutations also block binding of alphaIIbbeta3 to ligand-mimetic monoclonal antibodies PAC-1, OP-G2, LJ-CP3, which have an RGD-related RYD sequence in their antigen-binding sites. These mutations do not significantly affect the expression of alphaIIbbeta3, in contrast to most of the natural alphaIIb mutations occurring in Glanzmann's thrombasthenic patients. The data suggest that these residues are critically involved in alphaIIbbeta3-ligand interactions.

The ligand recognition specificity of beta3 integrins

AlphaIIbbeta3 (platelet membrane glycoprotein IIb-IIIa) and alphavbeta3 are members of the beta3 subfamily of integrin adhesion receptors. A cyclic peptide, KYGC(s-s)HarGDWPC(s-s) (cHarGD), originally described by Scarborough et al. (Scarborough, R. M., Naughton, M. A., Teng, W., Rose, J. W., Phillips, D. R., Nannizzi, L., Arsten, A., Campbell, A. M., and Charo, I. F.(1993) J. Biol. Chem. 268, 1066-1073) has been employed as a high affinity ligand for alphaIIbbeta3 to examine the specificity of the beta3 integrins. cHarGD interacted with high affinity with purified alphaIIbbeta3 (Kd = 10 nM) or with platelets (Kd = 120 nM). While cHarGD was specific for alphaIIbbeta3 in the presence of Ca2+, it bound to both beta3 integrins in the presence of Mn2+. Barbourin, a snake venom disintegrin containing a reactive KGD sequence, remained alphaIIbbeta3-specific, even in the presence of Mn2+. cHarGD became cross-linked to a site in beta3 of alphaIIb beta3, which is distinct from that of RGD peptides. These results allow identification of at least four classes of beta3 ligands: Class I, represented by RGD peptides and vitronectin, react similarly with alphaIIbbeta3 and alphavbeta3; Class II, represented by cHarGD, gamma-chain peptides and fibrinogen, react with both receptors in the presence of Mn2+ but only with alphaIIbbeta3 in the presence of Ca2+; Class III, represented by barbourin, are alphaIIbbeta3-specific under all cation conditions; Class IV, represented by osteopontin, bind primarily to alphavbeta3.

Thermal stability of individual domains in platelet glycoprotein IIbIIIa

Thermal denaturation of platelet glycoprotein IIbIIIa (integrin alpha IIb beta 3) was investigated by spectrofluorimetry and differential scanning calorimetry (DSC). Two forms of the protein were compared: active IIbIIIa, i.e., that fraction that binds to RGD-Sepharose, and inactive IIbIIIa, the non-binding fraction. At pH 8.5 in the presence of octyl glucoside and Ca2+ both forms exhibited a broad complex endotherm consisting of a well expressed low-temperature heat-absorption peak in the range of 40-65 degrees C followed by a broad peak stretching over 65-110 degrees C. Each endotherm could be deconvoluted into at least eight transitions reflecting the melting of at least this many independently folded domains. The first two transitions in the region of the low-temperature peak had similar positions in both forms while at least some of the other transitions occurred at higher temperature in the active protein suggesting that some of the domains are more stable in the latter. When both fractions of IIbIIIa were heated in the fluorometer a sigmoidal transition was observed in the region of the first endothermic peak where the two thermolabile domains melt. This transition was destabilized by 15 degrees C in the presence of EDTA, suggesting that these domains are formed by the 243-468 region of the IIb subunit which contains four Ca(2+)-binding motifs. It was further stabilized by 3 degrees C upon addition of the GRGDSPK peptide in the presence of Ca2+ while in EDTA the peptide had no effect. This is consistent with the involvement of Ca(2+)-binding region in the formation of the ligand-binding site. A 66-kDa chymotryptic fragment, containing the 17-kDa NH2-terminal portion of the IIIa subunit disulfide-linked to its 50-kDa COOH-terminal portion including the cysteine-rich core, exhibited a fluorescence-detected Ca(2+)-independent transition in the region where the higher temperature DSC-detected transitions occur suggesting that some of the latter may be connected with the melting of the corresponding portions of IIbIIIa.

Binding of fibrinogen to platelet integrin alpha IIb beta 3 in solution as monitored by tracer sedimentation equilibrium

Fibrinogen showed essentially no binding (KD > 1 mM) to platelet alpha IIb beta 3 integrin in solution in the presence of Triton or octylglucoside above critical micellar concentrations. Under these conditions the integrin was an alpha beta monomer. After removal of the detergent from the Triton containing buffer (25 mM Tris/HCl;, 150 mM NaCl, 1 mM CaCl2, 1 mM MgCl2, pH 7.4) the integrin formed aggregates with hexamers as the most prominent species, as demonstrated by analytical ultracentrifugation and electron microscopy. Tracer sedimentation equilibrium experiments indicate that fibrinogen binds to the integrin aggregates, but with a surprisingly large KD (at least 3 microM). This value is 10- to 100-fold higher than values determined by solid phase assays or with integrins reconstituted onto lipid bilayers.

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.

Proteolytic degradation of the RGD-binding and non-RGD-binding conformers of human platelet integrin glycoprotein IIb/IIIa: clues for identification of regions involved in the receptor's activation

The human integrin glycoprotein (GP)IIb/IIIa plays a central role in haemostasis as an inducible receptor for fibrinogen and other RGD-containing adhesive proteins at the platelet plasma membrane. Expression of the fibrinogen receptor on platelet activation involves conformational changes in the quaternary structure of GPIIb/IIIa. Little is known, however, about the nature of this conformational transition. Given that isolated GPIIb/IIIa contains a mixture of RGD-binding and non-RGD-binding heterodimers, we used limited proteolysis as a tool for investigating the structural differences between the two conformers. Comparison of their fragmentation patterns shows that, whereas in the non-RGD-binding form of GPIIb/IIIa the N-terminal half of the heavy chain of GPIIb (GPIIbH) and the central region of GPIIIa are cleaved by endoproteinase Arg-C, these domains associate tightly with one another in the RGD-binding GPIIb/IIIa and are thus protected from proteolysis. In addition, the C-terminal half of GPIIb becomes more susceptible to degradation in the non-RGD-binding GPIIb/IIIa conformer. Our interpretation, in the context of available structural and functional data, is that a major relative reorientation of the GPIIbH and GPIIIa extracellular domains takes place along the subunit interface during the conformational transition of the platelet integrin.

Prevention of arterial thrombosis by a monoclonal antibody against the 100 to 109 amino acid sequence stretch of the beta-subunit of the human platelet fibrinogen receptor: a comparative study with low dose aspirin

OBJECTIVES

The aim of this study was to compare, in dogs, the antithrombotic activity of aspirin and the murine monoclonal antibody P37, which inhibits platelet aggregation and fibrinogen binding to activated platelets.

BACKGROUND

The antithrombotic activity of P37 has been somewhat predictable, given its in vitro platelet antiaggregating activity and localization at or very near the fibrinogen binding site in the platelet fibrinogen receptor, the glycoprotein IIb/IIIa or integrin alpha IIb-beta 3.

METHODS

The monoclonal antibody P37 of the immunogamma-globulin-1 isotype was prepared according to previously described immunization and fusion protocols and screening assays. To compare its antiaggregating capacity with that of aspirin, experimental thrombosis was induced in all dogs by means of direct current applied to the carotid artery. Autologous platelets had previously been labeled with indium-111 oxine. The dogs were assigned to three groups: group I (n = 18) was the control group; group II (n = 12) was treated orally with 5 mg of aspirin/kg body weight per day for 7 days before induction of thrombosis, and group III (n = 10) was treated intravenously with a single dose of P37 (0.8 mg/kg).

RESULTS

The indium-111 oxine activity deposited in the thrombi was 12.94 +/- 12.83% (mean +/- SD) in group I, 3.55 +/- 2.99% in group II and 0.03 +/- 0.03% in group III. The differences between groups were always statistically significant (p < 0.05).

CONCLUSIONS

We conclude that a single dose (0.8 mg/kg) of P37 in a canine model of arterial thrombosis is approximately 100 times more efficient than the administration of aspirin (5 mg/kg per day) in preventing platelet deposition during thrombus formation.

Rotational mobility of the fibrinogen receptor glycoprotein IIb/IIIa or integrin alpha IIb beta 3 in the plasma membrane of human platelets

Integrin alpha IIb beta 3 or glycoprotein IIb/IIIa (GPIIb/IIIa, 228 kDa) is a Ca(2+)-dependent, noncovalent heterodimer of glycoproteins IIb (GPIIb or alpha IIb, 136 kDa) and IIIa (GPIIIa or beta 3, 92 kDa), which serves as the receptor for fibrinogen and other adhesive proteins at the surface of activated platelets. We have determined the microsecond-range rotational motions of alpha IIb beta 3 in resting platelets, in isolated plasma membranes, and reconstituted in 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) bilayers. The measurements were based on the time-resolved phosphorescence anisotropy [r(t)] of erythrosin-labeled F(ab) fragments [Er-F(ab)] of monoclonal antibodies bound to alpha IIb beta 3. In general, the r(t) decays were satisfactorily fitted to the sum of the two exponential terms and a constant, from which the initial anisotropy (r(in) approximately 0.05-0.11), the short (phi 1 approximately 1-14 microseconds) and the long (phi 2 approximately 15-60 microseconds) rotational correlation times, and the limiting anisotropy (r infinity approximately 0.02-0.07) were obtained. The observed values depended on the platelet preparation, temperature, Ca2+ concentration, and the antibody used. In accordance with data on the order parameter and the viscosity of the lipid bilayer of the platelet plasma membrane, phi 2 and r infinity of the alpha IIb beta 3-Er-F(ab) complexes in the three preparations decreased with the increase of temperature, the r(t) curves being fully reversible within the interval from 5 to 35 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Modeling the alpha IIb beta 3 integrin solution conformation

The alpha IIb beta 3 platelet integrin is the prototypical member of a widely distributed class of transmembrane receptors formed by the noncovalent association of alpha and beta subunits. Electron microscopic (EM) images of the alpha IIb beta 3 complex show an asymmetric particle with a globular domain from which two extended regions protrude to contact the lipid bilayer. Distance constraints provided by disulfide bond patterns, epitope mapping, and ligand mimetic cross-linking studies rather suggest a somewhat more compact conformation for the alpha IIb beta 3 complex. We have studied the shape of detergent-solubilized alpha IIb beta 3 by employing a low-resolution modeling procedure in which each polypeptide has been represented as an array of interconnected, nonoverlapping spheres (beads) of various sizes. The number, size, and three-dimensional relationships among the beads were defined either solely by dimensions obtained from published EM images of integrin receptors (EM models, 21 beads), or solely by interdomain constraints derived from published biochemical data (biochemical model, 37 beads). Interestingly, although no EM data were employed in its construction, the resulting overall shape of the biochemical model was still compatible with the EM data. Both kinds of models were then evaluated for their calculated solution properties. The more elongated EM models have diffusion and sedimentation coefficients that differ, at best, by +2% and -18% from the experimental values, determined, respectively, in octyl glucoside and Triton X-100. On the other hand, the parameters calculated for the more compact biochemical model showed a more consistent agreement with experimental values, differing by -7% (octyl glucoside) to -6% (Triton X-100). Thus, it appears that using the biochemical constraints as a starting point has resulted in not only a more detailed model of the detergent-solubilized alpha IIb beta 3 complex, where the relative spatial location of specific domains the size of 5-10 kDa can be tentatively mapped, but in a model that can also reconcile the electron microscopy with the biochemical and the solution data.

Dynamic light scattering studies of alpha IIb beta 3 solution conformation

The prototypical integrin receptor, alpha IIb beta 3, isolated from the membrane fraction of human blood platelets by solubilization in Triton X-100 (reduced) and affinity chromatography on lentil lectin-agarose, has been further purified by gel filtration chromatography in octyl glucoside to obtain the intact receptor complex in a form suitable for hydrodynamic measurements. The molecular weight [(6.0 +/- 0.2) x 10(3)] and Stokes radius (2.3 +/- 0.1 nm) of detergent micelles formed in 0.03 M octyl glucoside have been determined by classical light scattering intensity and dynamic light scattering measurements, respectively. An algorithm has been developed which explicitly considers the contribution of detergent micelles to the intensity autocorrelation function of particles suspended in detergent. This procedure has been validated with polystyrene particles of known radius, as well as with the soluble protein fibrinogen. Application of these procedures to dynamic light scattering data obtained with alpha IIb beta 3 resulted in a translational diffusion coefficient (Dto(20,w)) of (2.78 +/- 0.31) x 10(-7) cm2 s-1, corresponding to a Strokes radius (Rs) of 7.67 +/- 0.85 nm for the integrin/octyl glucoside complex. Light scattering intensity measurements gave a molecular weight of (2.26 +/- 0.22) x 10(5) for the polypeptide moiety of the complex, in excellent agreement with the 2.28 x 10(5) value calculated from primary structure data. As a spherical, hydrated alpha IIb beta 3 complex, with bound detergent, would exhibit a Stokes radius of approximately 5 nm, these data indicate considerable asymmetry in the solution conformation of alpha IIb beta 3.