Selection and structure of ion-selective ligands for platelet integrin alpha IIb(beta) 3

Mechanistic Basis for the Binding of RGD- and AGDV-Peptides to the Platelet Integrin αIIbβ3

Binding of soluble fibrinogen to the activated conformation of the integrin αIIbβ3 is required for platelet aggregation and is mediated exclusively by the C-terminal AGDV-containing dodecapeptide (γC-12) sequence of the fibrinogen γ chain. However, peptides containing the Arg-Gly-Asp (RGD) sequences located in two places in the fibrinogen Aα chain inhibit soluble fibrinogen binding to αIIbβ3 and make substantial contributions to αIIbβ3 binding when fibrinogen is immobilized and when it is converted to fibrin. Here, we employed optical trap-based nanomechanical measurements and computational molecular modeling to determine the kinetics, energetics, and structural details of cyclic RGDFK (cRGDFK) and γC-12 binding to αIIbβ3. Docking analysis revealed that NMR-determined solution structures of cRGDFK and γC-12 bind to both the open and closed αIIbβ3 conformers at the interface between the αIIb β-propeller domain and the β3 βI domain. The nanomechanical measurements revealed that cRGDFK binds to αIIbβ3 at least as tightly as γC-12. A subsequent analysis of molecular force profiles and the number of peptide-αIIbβ3 binding contacts revealed that both peptides form stable bimolecular complexes with αIIbβ3 that dissociate in the 60-120 pN range. The Gibbs free energy profiles of the αIIbβ3-peptide complexes revealed that the overall stability of the αIIbβ3-cRGDFK complex was comparable with that of the αIIbβ3-γC-12 complex. Thus, these results provide a mechanistic explanation for previous observations that RGD- and AGDV-containing peptides are both potent inhibitors of the αIIbβ3-fibrinogen interactions and are consistent with the observation that RGD motifs, in addition to AGDV, support interaction of αIIbβ3 with immobilized fibrinogen and fibrin.

Platelets are versatile cells: New discoveries in hemostasis, thrombosis, immune responses, tumor metastasis and beyond

Platelets are small anucleate blood cells generated from megakaryocytes in the bone marrow and cleared in the reticuloendothelial system. At the site of vascular injury, platelet adhesion, activation and aggregation constitute the first wave of hemostasis. Blood coagulation, which is initiated by the intrinsic or extrinsic coagulation cascades, is the second wave of hemostasis. Activated platelets can also provide negatively-charged surfaces that harbor coagulation factors and markedly potentiate cell-based thrombin generation. Recently, deposition of plasma fibronectin, and likely other plasma proteins, onto the injured vessel wall has been identified as a new "protein wave of hemostasis" that may occur even earlier than the first wave of hemostasis, platelet accumulation. Although no experimental evidence currently exists, it is conceivable that platelets may also contribute to this protein wave of hemostasis by releasing their granule fibronectin and other proteins that may facilitate fibronectin self- and non-self-assembly on the vessel wall. Thus, platelets may contribute to all three waves of hemostasis and are central players in this critical physiological process to prevent bleeding. Low platelet counts in blood caused by enhanced platelet clearance and/or impaired platelet production are usually associated with hemorrhage. Auto- and allo-immune thrombocytopenias such as idiopathic thrombocytopenic purpura and fetal and neonatal alloimmune thrombocytopenia may cause life-threatening bleeding such as intracranial hemorrhage. When triggered under pathological conditions such as rupture of an atherosclerotic plaque, excessive platelet activation and aggregation may result in thrombosis and vessel occlusion. This may lead to myocardial infarction or ischemic stroke, the major causes of mortality and morbidity worldwide. Platelets are also involved in deep vein thrombosis and thromboembolism, another leading cause of mortality. Although fibrinogen has been documented for more than half a century as essential for platelet aggregation, recent studies demonstrated that fibrinogen-independent platelet aggregation occurs in both gene deficient animals and human patients under physiological and pathological conditions (non-anti-coagulated blood). This indicates that other unidentified platelet ligands may play important roles in thrombosis and might be novel antithrombotic targets. In addition to their critical roles in hemostasis and thrombosis, emerging evidence indicates that platelets are versatile cells involved in many other pathophysiological processes such as innate and adaptive immune responses, atherosclerosis, angiogenesis, lymphatic vessel development, liver regeneration and tumor metastasis. This review summarizes the current knowledge of platelet biology, highlights recent advances in the understanding of platelet production and clearance, molecular and cellular events of thrombosis and hemostasis, and introduces the emerging roles of platelets in the immune system, vascular biology and tumorigenesis. The clinical implications of these basic science and translational research findings will also be discussed.

Conformational preferences and activities of peptides from the catecholamine release-inhibitory (catestatin) region of chromogranin A

Previous modeling (PDB 1cfk) of the catecholamine release-inhibitory "catestatin" region of chromogranin A (CgA) suggested a beta-strand/loop/beta-strand active conformation, displaying an electropositive Arg-rich loop (R(351)AR(353)GYGFR(358)). To explore this possibility, we studied NMR structures of linear and cyclic synthetic catestatin, bovine (bCgA(344-364)) or human (hCgA(352-372)). By 2-D (1)H-NMR, the structure of linear catestatin (hCgA(352-372)) exhibited the NOE pattern of a coiled loop (PDB 1lv4). We then constrained the structure, cyclizing the putative Arg-rich loop connecting the beta-strands: cyclic bCgA(350-362) ([C(0)]F(350)RARGYGFRGPGL(362)[C(+14)]). Favored conformations of cyclic bCgA(350-362) were determined by (1)H-NMR and (13)C-NMR spectroscopy. Cyclic bCgA(350-362) conformers (PDB 1n2y) adopted a "twisted-loop" conformation. Alignment between the homology model and the cyclic NMR structure showed that, while portions of the NMR structure's mid-molecule and carboxy-terminus were congruent with the homology model (RMSD, 1.61-1.91 A), the amino-terminal "twisted loop" coiled inward and away from the model (RMSD, 3.36 A). Constrained cyclic bCgA(350-362) did not exert nicotinic cholinergic antagonist activity (IC(50)>10 microM), when compared to full-length linear (IC(50) approximately 0.42-0.56 microM), or cyclic (IC(50) approximately 0.74 microM) catestatin. Thus, loss of activity in the small, constrained peptide did not result from either [Cys]-extension or cyclization, per se. While linear catestatin displays coiled character, a small cyclic derivative lost biological activity, perhaps because its amino-terminal domain deviated sharply from the predicted active conformation. These results refine the relationship between structure and function in catestatin, and suggest goals in future peptidomimetic syntheses, in particular attempts to constrain the correct amino-terminal shape for biological activity.