Ligand binding to integrin alpha(v)beta(3) requires tyrosine 178 in the alpha(v) subunit

Integrin _v3 has been implicatedin angiogenesis and other biological processes. However, theligand-binding sites in _v, a non-I-domain  subunit,remain to be identified. Recently in _IIb, the otherpartner of the ₃ subunit, several discontinuous residuesimportant for ligand binding were identified in the predicted loopsbetween repeats 2 and 3 (W3 4-1 loop) and within repeat 3 (W3 2-3 loop). Based on these findings, alanine-scanning mutagenesis in293 cells was used to investigate the role of these loops (cysteine [C]142-C155 and glycine [G]172-G181) of _v in ligandbinding. Wild-type _v3 was able to bindsoluble fibrinogen following integrin activation either by 0.5 mMmanganese dichloride (MnCl₂) or a mutation of₃ threonine (T)562 to asparagine. However, mutation oftyrosine (Y)178 to alanine in the predicted G172-G181 loop of_v abolished fibrinogen binding, and alanine (A)substitutions at adjacent residues phenylalanine (F)177 and tryptophan(W)179 had a similar effect. Cells expressing Y178A_valso failed to bind to immobilized fibrinogen. Moreover, the Y178Amutation abolished the binding of WOW-1 Fab, a monovalentligand-mimetic anti-_v3 antibody, and theexpression of ₃ ligand-induced binding sites (LIBS)induced by arginine-glycine-aspartic acid-tryptophan (RGDW). In sharpcontrast to the data obtained with _IIb, none of the mutations in the predicted W3 4-1 loop in _v impairedligand binding. These results implicate _v Y178 in ligandbinding to _v3, and they suggest thatthere are key structural differences in the adhesive ligand-bindingsites of _v3 and_IIb3.     

Beta 3 integrin-mediated fibrin clot retraction by nucleated cells: differing behavior of alpha IIb beta 3 and alpha v beta 3

Fibrin clot retraction may be important in resolution of thrombi and, in platelets, is mediated by integrin alpha IIb beta 3 (GPIIb-IIIa). Nucleated cells that lack alpha IIb beta 3 can retract fibrin clots, and we now report that integrin alpha v beta 3 can support this process. In addition, we compared the capacities of recombinant beta 3 integrins to mediate clot retraction in Chinese hamster ovary and M21 melanoma cells. We found that alpha v beta 3, but not alpha IIb beta 3, could spontaneously support retraction. Transferring the cytoplasmic domain of alpha v to alpha IIb enabled the resulting chimeric alpha IIb beta 3 to support clot retraction. The capacity of the alpha v cytoplasmic domain to support clot retraction was not caused by activation of the ligand binding function of alpha IIb beta 3 or by enhancement of alpha IIb beta 3's capacity to stimulate the formation of focal adhesions or the tyrosine phosphorylation of pp125FAK. These experiments define requirements for alpha IIb beta 3-mediating clot retraction, establish the capacity of alpha v beta 3 to mediate this process, and suggest differing functional roles of the alpha v and alpha IIb cytoplasmic domains.     

Angiotensin II and alpha(v)beta(3) integrin expression in rat neonatal cardiac fibroblasts

We recently demonstrated that alpha(v)beta(3) integrins are involved in the mechanisms of angiotensin II (Ang II)-induced DNA synthesis and collagen gel contractions in rat cardiac fibroblasts (CFBs), cellular mechanisms that are relevant for cardiac remodeling. The aim of the present study was to elucidate the effect of Ang II and other growth factors on the regulation of the alpha(v)beta(3) integrins in fibroblasts from neonatal rat hearts. The alpha(v)beta(3) integrin receptor expression was significantly increased (P<0.05) at the mRNA level after treatment with Ang II, transforming growth factor-beta(1) (TGF-beta(1)), and platelet-derived growth factor (PDGF) for 8 and 16 hours. The surface expression of the alpha(v) and beta(3) integrin subunits was elevated after 32 and 48 hours (P<0.05) as determined with flow cytometry. To investigate fibroblast motility, we performed chemotaxis experiments with transwell chambers. Ang II was chemotactic for CFBs, as tested with checkerboard experiments. The chemotactic effect was concentration dependent and was completely blocked by Ang II type 1 receptor blockers but not by Ang II type 2 receptor blocker PD 123319. Ang II- and PDGF-BB-mediated chemotaxis could be significantly inhibited by RGD peptides and the blocking antibodies against alpha(v)beta(3) integrin (both P<0.01). Adhesion of CFBs to vitronectin was partially inhibited by an antibody to alpha(v)beta(3) integrin but was mainly mediated by an alpha(v)beta(5) integrin. Relevant in vivo expression of alpha(v)beta(3) integrin by CFBs was confirmed with in situ hybridization with probes for alpha(v) and beta(3) mRNA in rat hearts. The present study demonstrates that the expression of alpha(v)beta(3) integrin is augmented by Ang II, PDGF, and TGF-beta(1) in neonatal CFBs. Furthermore, this integrin is involved in the chemotaxis, motility, and adhesion of CFBs. The present findings support the current concept that integrins participate in the control of fibroblast behavior during cardiac remodeling mechanisms.     

Autoantibodies to alpha(IIb)beta(3) in patients with chronic immune thrombocytopenic purpura bind primarily to epitopes on alpha(IIb)

Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disease caused by platelet destruction resulting from autoantibodies against platelet surface proteins, particularly platelet glycoprotein IIb/IIIa (alpha(IIb)beta(3)). To localize the auto-epitopes on platelet alpha(IIb)beta(3), the binding of autoantibodies to Chinese hamster ovary (CHO) cells expressing either alpha(IIb)beta(3) or alpha(v)beta(3) was studied. Thirteen of 14 ITP autoantibodies bound only to CHO cells expressing alpha(IIb)beta(3). Because these 2 integrins have the same beta chain (beta(3)), these results show that most epitopes in chronic ITP are dependent on the presence of glycoprotein alpha(IIb.) (Blood. 2001;97:2171-2172)     

Nitric oxide induces angiogenesis and upregulates alpha(v)beta(3) integrin expression on endothelial cells

Nitric oxide (NO) has been implicated as a mediator of angiogenesis. However, its precise role in angiogenesis and its mechanism of action have not been established. We performed in vivo and in vitro angiogenesis assays using NO donor S-nitroso-N-acetylpenicillamine (SNAP) and NO synthase inhibitor N-iminoethyl-l-ornithine (L-NIO). SNAP significantly increased and L-NIO significantly suppressed capillary ingrowth into subcutaneously implanted Matrigel plugs in mice. For the in vitro angiogenesis assay, human umbilical vein endothelial cells (HUVECs) (4 x 10(4) cells/well) were treated with placebo, SNAP (100 microM), or L-NIO (100 microM) and cultured on Matrigel for 18 h. The typical capillary networks formed on Matrigel by HUVECs as a result of cell migration and differentiation were quantified by computer-assisted image analysis as a measure of angiogenesis. Treatment of HUVECs with SNAP significantly increased the capillary network area compared with control, 8701 +/- 693 vs 6258 +/- 622 area units (P < 0.05), whereas L-NIO significantly decreased the capillary area (4540 +/- 342, P < 0.05). Furthermore, we have shown with a blocking monoclonal antibody that formation of capillary networks on Matrigel is mediated by the functional expression of the alpha(v)beta(3) integrin, which plays a role in facilitating endothelial cell adhesion to basement membrane matrix and endothelial cell migration. After an 18-h culture, flow cytometry revealed that SNAP significantly upregulated and L-NIO significantly downregulated in a concentration-dependent manner alpha(v)beta(3) integrin expression on endothelial cells. In conclusion, NO induces angiogenesis in vivo and in vitro by promoting endothelial cell migration and differentiation into capillaries. One possible mechanism might involve the upregulation of alpha(v)beta(3) integrin on endothelial cells, a critical mediator of cell-matrix adhesion and migration.     

Essential role for phosphoinositide 3-kinase in shear-dependent signaling between platelet glycoprotein Ib/V/IX and integrin alpha(IIb)beta(3).

Platelet adhesion and aggregation at sites of vascular injury are critically dependent on the interaction between von Willebrand factor (VWF) and 2 major platelet adhesion receptors, glycoprotein (GP) Ib/V/IX and integrin alpha(IIb)beta(3). GP Ib/V/IX binding to VWF mediates platelet tethering and translocation, whereas activation of integrin alpha(IIb)beta(3) promotes cell arrest. To date, the signaling pathways used by the VWF-GP Ib/V/IX interaction to promote activation of integrin alpha(IIb)beta(3), particularly under shear, have remained poorly defined. In this study, the potential involvement of type 1 phosphoinositide (PI) 3-kinases in this process was investigated. Results show that platelet adhesion and spreading on immobilized VWF results in a specific increase in the PI 3-kinase lipid product, PtdIns(3,4)P(2). Under static conditions, inhibiting PI 3-kinase with LY294002 or wortmannin did not prevent platelet adhesion, integrin alpha(IIb)beta(3) activation, or platelet spreading although it significantly delayed the onset of these events. In contrast, PI 3-kinase inhibition under shear dramatically reduced both platelet adhesion and spreading. Real-time analysis of intracellular calcium demonstrated that under static conditions inhibiting PI 3-kinase delayed the onset of intracellular fluxes in adherent platelets, but did not affect the final magnitude of the calcium response. However, under shear, inhibiting PI 3-kinase dramatically reduced intracellular calcium mobilization and integrin alpha(IIb)beta(3) activation, resulting in impaired thrombus growth. The studies demonstrate a shear-dependent role for PI 3-kinase in promoting platelet adhesion on immobilized VWF. Under static conditions, platelets appear to mobilize intracellular calcium through both PI 3-kinase-dependent and -independent mechanisms, whereas under shear PI 3-kinase is indispensable for VWF-induced calcium release.     

Integrin alpha(v)beta(3) mediates rotavirus cell entry

Rotavirus strains differ in their need for sialic acid (SA) for initial binding to the cell surface; however, the existence of a postattachment cell receptor, common to most, if not all, rotavirus strains, has been proposed. In the present study, antibodies to the alpha(v) and beta(3) integrin subunits, and the alpha(v)beta(3) ligand, vitronectin, efficiently blocked the infectivity of the SA-dependent rhesus rotavirus RRV, its SA-independent variant nar3, and the neuraminidase-resistant human rotavirus strain Wa. Vitronectin and anti-beta(3) antibodies, however, did not block the binding of virus to cells, indicating that rotaviruses interact with alpha(v)beta(3) at a postbinding step, probably penetration. This interaction was shown to be independent of the tripeptide motif arginine-glycine-aspartic acid present in the natural ligands of this integrin. Transfection of CHO cells with alpha(v)beta(3) genes significantly increased their permissiveness to all three rotavirus strains, and the increment of virus infectivity was reverted by incubation of these cells either with antibodies to beta(3) or with vitronectin. These findings implicate alpha(v)beta(3) integrin as a cellular receptor common to neuraminidase-sensitive and neuraminidase-resistant rotaviruses, and support the hypothesis that this integrin could determine, at least in part, the cellular susceptibility to rotaviruses.     

Inhibition of platelet integrin alpha(IIb)beta(3) by peptides that interfere with protein kinases and the beta(3) tail

alpha-Thrombin stimulation of human platelets initiates inside-out signaling to integrin alpha(IIb)beta(3) (glycoprotein IIb/IIIa), resulting in the exposure of ligand binding sites. In the present study, the regulation of alpha(IIb)beta(3) via protein kinases was investigated in platelets permeabilized with streptolysin O by introducing peptides that interfere with these enzymes and with possible regulatory domains in the cytosolic tail of the beta(3) subunit. Compared with intact platelets, the permeabilized platelets preserved >80% of the aggregation, secretion, and alpha(IIb)beta(3) ligand binding capacity. The peptide YIYGSFK, a substrate for Src kinases, inhibited alpha-thrombin-induced ligand binding to alpha(IIb)beta(3), but a reversed peptide with Y-->F substitutions (KFSGFIF) had no effect. Ligand binding to alpha(IIb)beta(3) was also inhibited by the peptide RKRCLRRL, which binds irreversibly to the catalytic domain of protein kinase C. Peptides corresponding to parts of the protein C inhibitor and beta(2)-glycoprotein I were used as negative controls and failed to interfere with ligand binding. Possible target domains for protein kinases are present in the cytoplasmic tail of the beta(3) subunit. The LLITIHDR peptide, matching the membrane-proximal domain of beta(3) (residues 717 to 724), had no effect, but NNPLYKEA (residues 743 to 750), EATSTFTN (residues 749 to 756), and TNITYRGT (residues 755 to 762), which mimicked overlapping domains of the carboxy-terminal part of beta(3), reduced alpha-thrombin-induced ligand binding by 60+/-4%, 97+/-1%, and 97+/-2% (n=3) at 500 micromol/L peptide, respectively. These observations indicate that Src kinases and protein kinase C take part in inside-out signaling to integrin alpha(IIb)beta(3) and identify target domains in beta(3) that contribute to the regulation of this integrin.     

A Leu262Pro mutation in the integrin beta(3) subunit results in an alpha(IIb)-beta(3) complex that binds fibrin but not fibrinogen

Platelet retraction of a fibrin clot is mediated by the platelet fibrinogen receptor, alpha(IIb)beta(3). In certain forms of the inherited platelet disorder, Glanzmann thrombasthenia (GT), mutant alpha(IIb)beta(3) may interact normally with fibrin yet fail to support fibrinogen-dependent aggregation. We describe a patient (LD) with such a form of GT. Platelets from LD supported normal clot retraction but failed to bind fibrinogen. Platelet analysis using flow cytometry and immunoblotting showed reduced but clearly detectable alpha(IIb)beta(3), findings consistent with type II GT. Genotyping of LD revealed 2 novel beta(3) mutations: a deletion of nucleotides 867 to 868, resulting in a premature stop codon at amino acid residue 267, and a T883C missense mutation, resulting in a leucine (Leu) 262-to-proline (Pro) substitution. Leu262 is highly conserved among beta integrin subunits and lies within an intrachain loop implicated in subunit association. Leu262Probeta(3) cotransfected with wild-type alpha(IIb) into COS-7 cells showed delayed intracellular maturation and reduced surface expression of easily dissociable complexes. In human embryonic kidney 293 cells, Leu262Probeta(3) formed a complex with endogenous a(v) and retracted fibrin clots similarly to wild-type beta(3). The same cells, however, were unable to bind immobilized fibrinogen. The molecular requirements for alpha(IIb)beta(3) to interact with fibrin compared with fibrinogen, therefore, appear to differ. The region surrounding beta(3) Leu262 may maintain beta(3) in a fibrinogen-binding, competent form, but it appears not to be required for receptor interactions with fibrin.     

Localization of beta 1, beta 3, alpha 5, alpha v, and alpha IIb subunits of the integrin family in spreading human erythroleukemia cells.

The localization of five integrin subunit proteins was studied in human erythroleukemia (HEL) cells spreading on various culture substrata in the presence of 12-O-tetradecanoylphorbol-13-acetate (TPA) and the absence of serum. The cells readily adhered on fibronectin, but TPA was needed for adherence on vitronectin and for the spreading of the cells on both substrata. Indirect immunofluorescence microscopy showed that in the spread cells cultured on vitronectin or fibronectin for 2 hours, beta 1, beta 3, alpha 5, and alpha IIb integrin subunits were localized at focal adhesions as identified by talin-immunoreactivity. The alpha v integrin immunoreactivity was initially found at the focal adhesions when the cells were cultured on vitronectin, but was also found later in cells cultured on fibronectin. The alpha IIb integrin immunoreactivity disappeared from focal adhesions within 24 hours. The alpha 5 and beta 1 integrin immunoreactivities disappeared from the focal adhesions in cells cultured on vitronectin, but not in cells cultured on fibronectin. When the cells were plated on glass substratum in the presence of TPA, they spread much slower than on vitronectin or fibronectin, but some cells showed focal adhesions after only 8 hours in culture. In this case, the alpha v and beta 3 integrin subunits were found at focal adhesions. After TPA treatment, HEL cells deposited thrombospondin-immunoreactive material onto their culture substratum, but synthesis of fibronectin, vitronectin, fibrinogen, or von Willebrand factor was not detected. Thus, the results suggest that TPA would activate several integrin receptors in HEL cells and also stimulate the secretion of thrombospondin, which might be used as an adhesion ligand for the integrin vitronectin receptor alpha v/beta 3 complex.     

Involvement of integrins alpha v beta 3 and alpha v beta 5 in ocular neovascular diseases.

Angiogenesis underlies the majority of eye diseases that result in catastrophic loss of vision. Recent evidence has implicated the integrins alpha v beta 3 and alpha v beta 5 in the angiogenic process. We examined the expression of alpha v beta 3 and alpha v beta 5 in neovascular ocular tissue from patients with subretinal neovascularization from age-related macular degeneration or the presumed ocular histoplasmosis syndrome or retinal neovascularization from proliferative diabetic retinopathy (PDR). Only alpha v beta 3 was observed on blood vessels in ocular tissues with active neovascularization from patients with age-related macular degeneration or presumed ocular histoplasmosis, whereas both alpha v beta 3 and alpha v beta 5 were present on vascular cells in tissues from patients with PDR. Since we observed both integrins on vascular cells from tissues of patients with retinal neovascularization from PDR, we examined the effects of a systemically administered cyclic peptide antagonist of alpha v beta 3 and alpha v beta 5 on retinal angiogenesis in a murine model. This antagonist specifically blocked new blood vessel formation with no effect on established vessels. These results not only reinforce the concept that retinal and subretinal neovascular diseases are distinct pathological processes, but that antagonists of alpha v beta 3 and/or alpha v beta 5 may be effective in treating individuals with blinding eye disease associated with angiogenesis.     

Fibrinogen binding to the integrin alpha(IIb)beta(3) modulates store-mediated calcium entry in human platelets

Effects of the occupation of integrin alpha(IIb)beta(3) by fibrinogen on Ca(++) signaling in fura-2-loaded human platelets were investigated. Adding fibrinogen to washed platelet suspensions inhibited increases in cytosolic [Ca(++)] concentrations ([Ca(++)](i)) evoked by adenosine diphosphate (ADP) and thrombin in a concentration-dependent manner in the presence of external Ca(++) but not in the absence of external Ca(++) or in the presence of the nonselective cation channel blocker SKF96365, indicating selective inhibition of Ca(++) entry. Fibrinogen also inhibited store-mediated Ca(++) entry (SMCE) activated after Ca(++) store depletion using thapsigargin. The inhibitory effect of fibrinogen was reversed if fibrinogen binding to alpha(IIb)beta(3) was blocked using RDGS or abciximab and was absent in platelets from patients homozygous for Glanzmann thrombasthenia. Fibrinogen was without effect on SMCE once activated. Activation of SMCE in platelets occurs through conformational coupling between the intracellular stores and the plasma membrane and requires remodeling of the actin cytoskeleton. Fibrinogen inhibited actin polymerization evoked by ADP or thapsigargin in control cells and in cells loaded with the Ca(++) chelator dimethyl BAPTA. It also inhibited the translocation of the tyrosine kinase p60(src) to the cytoskeleton. These results indicate that the binding of fibrinogen to integrin alpha(IIb)beta(3) inhibits the activation of SMCE in platelets by a mechanism that may involve modulation of the reorganization of the actin cytoskeleton and the cytoskeletal association of p60(src). This action may be important in intrinsic negative feedback to prevent the further activation of platelets subjected.     

Platelet HDL(3) binding sites are not related to integrin alpha(IIb)beta(3) (GPIIb-IIIa)

Early studies considered that fibrinogen receptor (glycoprotein [GP] IIb-IIIa or platelet integrin alpha(IIb)beta(3)) is the binding site for low-density lipoprotein (LDL) and high-density lipoprotein type 3 (HDL(3)). Recent data, however, do not support the hypothesis that the binding of LDL to human intact resting platelets is related to integrin alpha(IIb)beta(3). In this study we present evidence that platelet integrin alpha(IIb)beta(3) is also not involved in the interaction of HDL(3) and human intact resting platelets. Firstly, specific ligands for platelet integrin alpha(IIb)beta(3), such as fibrinogen, vitronectin, von Willebrand factor and fibronectin, were unable to inhibit the binding of HDL(3) to intact resting platelets. Secondly, the HDL(3) binding characteristics (K(d) and B(max) values), the activation of protein kinase C (PKC) and the inhibition of thrombin-induced inositoltriphosphate (IP(3)) formation and calcium (Ca(2+)) mobilization mediated by HDL(3) particles were similar in platelets from control subjects and patients with type I and type II Glanzmann's thrombasthenia, which are characterized by total and partial lack of GPIIb-IIIa and fibrinogen, respectively. In contrast, nitrosylation of tyrosine residues of HDL(3) by tetranitromethane fully abolished both the ability of particles to interact with its specific binding sites and the functional effects. Thirdly, polyclonal antibodies against the GPIIb-IIIa complex (edu-3 and 5B12), human antiserums against platelet alloantigens (anti-Bak(a/B) and anti-PL(A1/2)), anti-integrin subunits (anti-alpha(V) and anti-beta(3)), and a wide panel of monoclonal antibodies (mAbs) against well-known epitopes of GPIIb (M3, M4, M5, M6, M8 and M95-2b) and GPIIIa (P23-7, P33, P37, P40, and P97) did not affect the binding of HDL(3) particles to human intact resting platelets. Overall results show that neither the GPIIb-IIIa complex nor GPIIb or GPIIIa individually are the membrane binding proteins for HDL(3)on intact resting platelets.     

Significance of alpha(IIb)beta(3) in subacute stent thrombosis after percutaneous coronary intervention.

SUMMARY: Soluble P-selectin and whole blood aggregation (WBA) were measured after percutaneous coronary intervention in patients who then received antiplatelet therapy. One had subacute thrombosis on day 7. This patient's WBA exhibited time-dependent enhancement. In addition, the accentuation of WBA on day 3 was observed when anti-alpha(IIb)beta(3) antibody was added. The remaining 22 patients were divided according to WBA results on day 3 with anti-alpha(IIb)beta(3) antibody added (A group, WBA enhanced; B group, WBA did not enhance). WBA on day 3 was similar in the two groups. The ratio of WBA with and without anti-alpha(IIb)beta(3) antibody was higher in group A than in group B. A significant time-dependent increase of soluble P-selectin was observed in the A group. These results suggest that enhancement of WBA with anti-alpha(IIb)beta(3) antibody after percutaneous coronary intervention predicts subacute thrombosis.     

Platelet glycoprotein IIb-IIIa (alpha IIb beta 3 integrin) confers fibrinogen- and activation-dependent aggregation on heterologous cells

To analyze molecular mechanisms of platelet aggregation, we have studied the aggregation of Chinese hamster ovary (CHO) cells expressing between 1 and 4 x 10(5) recombinant human glycoprotein (GP) IIb-IIIa molecules per cell (A5 cells). These cells aggregated as measured by the disappearance of single cells during rotary agitation. Aggregation was dependent on the presence of extracellular fibrinogen (approximately 500 nmol/L) and divalent cations, and required prior activation of the GPIIb-IIIa. A synthetic peptide (GRGDSP) and monoclonal anti-GPIIb-IIIa antibody (2G12) that block platelet aggregation also blocked aggregation of these cells. Parent CHO cells or those expressing recombinant GPIIb-IIIa containing a point mutation that causes variant thrombasthenia both failed to aggregate when stimulated in the presence of fibrinogen. These data show that GPIIb-IIIa is the only unique platelet surface component required for aggregation.     

Ser-752-->Pro mutation in the cytoplasmic domain of integrin beta 3 subunit and defective activation of platelet integrin alpha IIb beta 3 (glycoprotein IIb-IIIa) in a variant of Glanzmann thrombasthenia.

Integrins are membrane receptors which mediate cell-cell or cell-matrix adhesion. Integrin alpha IIb beta 3 (glycoprotein IIb-IIIa) acts as a fibrinogen receptor of platelets and mediates platelet aggregation. Platelet activation is required for alpha IIb beta 3 to shift from noncompetent to competent for binding soluble fibrinogen. The steps involved in this transition are poorly understood. We have studied a variant of Glanzmann thrombasthenia, a congenital bleeding disorder characterized by absence of platelet aggregation and fibrinogen binding. The patient's platelets did not bind fibrinogen after platelet activation by ADP or thrombin, though his platelets contained alpha IIb beta 3. However, isolated alpha IIb beta 3 was able to bind to an Arg-Gly-Asp-Ser affinity column, and binding of soluble fibrinogen to the patient's platelets could be triggered by modulators of alpha IIb beta 3 conformation such as the Arg-Gly-Asp-Ser peptide and alpha-chymotrypsin. These data suggested that a functional Arg-Gly-Asp binding site was present within alpha IIb beta 3 and that the patient's defect was not secondary to a blockade of alpha IIb beta 3 in a noncompetent conformational state. This was evocative of a defect in the coupling between platelet activation and alpha IIb beta 3 up-regulation. We therefore sequenced the cytoplasmic domain of beta 3, following polymerase chain reaction (PCR) on platelet RNA, and found a T-->C mutation at nucleotide 2259, corresponding to a Ser-752-->Pro substitution. This mutation is likely to be responsible for the uncoupling of alpha IIb beta 3 from cellular activation because (i) it is not a polymorphism, (ii) it is the only mutation in the entire alpha IIb beta 3 sequence, and (iii) genetic analysis of the family showed that absence of the Pro-752 beta 3 allele was associated with the normal phenotype. Our data thus identify the C-terminal portion of the cytoplasmic domain of beta 3 as an intrinsic element in the coupling between alpha IIb beta 3 and platelet activation.     

Species differences in small molecule binding to alpha IIb beta 3 are the result of sequence differences in 2 loops of the alpha IIb beta propeller

Compared with human platelets, rodent platelets are less responsive to peptides and peptidomimetics containing an arginine-glycine-aspartic acid (RGD) motif. Using chimeric human-rat alphaIIbbeta3 molecules, we found that this difference in Arg-Gly-Asp-Ser (RGDS) sensitivity was the result of amino acid substitutions at residues 157, 159, and 162 in the W3:4-1 loop and an Asp-His replacement at residue 232 in the W4:4-1 loop of the alphaIIb beta propeller. Introducing the entire rat W3:4-1 and W4:4-1 loops into human alphaIIbbeta3 also decreased the inhibitory effect of the disintegrins, echistatin and eristostatin, and the alphaIIbbeta3 antagonists, tirofiban and eptifibatide, on fibrinogen binding, whereas the specific point mutations did not. This suggests that RGDS interacts with alphaIIb in a different manner than with these small molecules. None of these species-based substitutions affected the ability of alphaIIbbeta3 to interact with RGD-containing macromolecules. Thus, human von Willebrand factor contains an RGD motif and binds equally well to adenosine diphosphate-stimulated human and rodent platelets, implying that other motifs are responsible for maintaining ligand binding affinity. Many venoms contain RGD-based toxins. Our data suggest that these species amino acids differences in the alphaIIb beta-propeller represent an evolutionary response by rodents to maintain hemostasis while concurrently protecting against RGD-containing toxins.