Structure-Based Cyclic Glycoprotein Ibα-Derived Peptides Interfering with von Willebrand Factor-Binding, Affecting Platelet Aggregation under Shear.

The plasmatic von Willebrand factor (VWF) circulates in a compact form unable to bind platelets. Upon shear stress, the VWF A1 domain is exposed, allowing VWF-binding to platelet glycoprotein Ib-V-IX (GPIbα chain). For a better understanding of the role of this interaction in cardiovascular disease, molecules are needed to specifically interfere with the opened VWF A1 domain interaction with GPIbα. Therefore, we in silico designed and chemically synthetized stable cyclic peptides interfering with the platelet-binding of the VWF A1 domain per se or complexed with botrocetin. Selected peptides (26-34 amino acids) with the lowest-binding free energy were: the monocyclic mono- vOn Willebrand factoR-GPIbα InTerference (ORbIT) peptide and bicyclic bi-ORbIT peptide. Interference of the peptides in the binding of VWF to GPIb-V-IX interaction was retained by flow cytometry in comparison with the blocking of anti-VWF A1 domain antibody CLB-RAg35. In collagen and VWF-dependent whole-blood thrombus formation at a high shear rate, CLB-RAg35 suppressed stable platelet adhesion as well as the formation of multilayered thrombi. Both peptides phenotypically mimicked these changes, although they were less potent than CLB-RAg35. The second-round generation of an improved peptide, namely opt-mono-ORbIT (28 amino acids), showed an increased inhibitory activity under flow. Accordingly, our structure-based design of peptides resulted in physiologically effective peptide-based inhibitors, even for convoluted complexes such as GPIbα-VWF A1.

Oxime Catalysis by Freezing.

Chemical reaction rates are generally decreased at lower temperatures. Here, we report that an oxime ligation reaction in water at neutral pH is accelerated by freezing. The freezing method and its rate effect on oxime ligation are systematically studied on a peptide model system, and applied to a larger chemokine protein, containing a single acetyl butyrate group, which is conjugated to an aminooxy-labeled ligand. Our improved ligation protocol now makes it possible to efficiently introduce oxime-bond coupled ligands into proteins under aqueous conditions at low concentrations and neutral pH.

Coated platelets function in platelet-dependent fibrin formation via integrin αIIbß3 and transglutaminase factor XIII.

Coated platelets, formed by collagen and thrombin activation, have been characterized in different ways: i) by the formation of a protein coat of α-granular proteins; ii) by exposure of procoagulant phosphatidylserine; or iii) by high fibrinogen binding. Yet, their functional role has remained unclear. Here we used a novel transglutaminase probe, Rhod-A14, to identify a subpopulation of platelets with a cross-linked protein coat, and compared this with other platelet subpopulations using a panel of functional assays. Platelet stimulation with convulxin/thrombin resulted in initial integrin α(IIb)ß3 activation, the appearance of a platelet population with high fibrinogen binding, (independently of active integrins, but dependent on the presence of thrombin) followed by phosphatidylserine exposure and binding of coagulation factors Va and Xa. A subpopulation of phosphatidylserine-exposing platelets bound Rhod-A14 both in suspension and in thrombi generated on a collagen surface. In suspension, high fibrinogen and Rhod-A14 binding were antagonized by combined inhibition of transglutaminase activity and integrin α(IIb)ß3 Markedly, in thrombi from mice deficient in transglutaminase factor XIII, platelet-driven fibrin formation and Rhod-A14 binding were abolished by blockage of integrin α(IIb)ß3. Vice versa, star-like fibrin formation from platelets of a patient with deficiency in α(IIb)ß3(Glanzmann thrombasthenia) was abolished upon blockage of transglutaminase activity. We conclude that coated platelets, with initial α(IIb)ß3 activation and high fibrinogen binding, form a subpopulation of phosphatidylserine-exposing platelets, and function in platelet-dependent star-like fibrin fiber formation via transglutaminase factor XIII and integrin α(IIb)ß3.

Protein arginine deiminase 4 inactivates tissue factor pathway inhibitor-alpha by enzymatic modification of functional arginine residues.

BACKGROUND: Tissue factor pathway inhibitor (TFPI) is an important regulator of coagulation and a link between inflammation and thrombosis. During thrombotic events, TFPI is proteolytically inactivated by neutrophil elastase while bound to neutrophil extracellular traps (NETs). Protein arginine deiminase 4 (PAD4) catalyzes the conversion of arginine to citrulline and is crucial for NET formation. OBJECTIVES: Here, we show that PAD4 inactivates full-length TFPIα by citrullination of its functional arginines. METHODS: Citrullination of TFPIα and of TFPI-constructs by PAD4 was studied using western blotting and mass spectrometry. Binding of TFPIα to PAD4 was investigated using a solid-phase assay. Functional consequences were investigated by factor Xa inhibition and thrombin generation assays. RESULTS: Nanomolar PAD4 amounts eliminated factor Xa inhibition by TFPIα. A citrullinated mutant Kunitz 2 domain did not inhibit factor Xa. Citrullination of TFPIα was found to be time- and concentration-dependent. Immunoprecipitation of citrullinated proteins from whole blood after neutrophil activation suggested the presence of TFPIα. Negatively charged phospholipids inhibited citrullination and truncated variants K1K2 and TFPI 1-161, and the isolated K2 domain were less efficiently citrullinated by PAD4. TFPIα bound to PAD4 with nanomolar affinity and involved the basic C-terminus. Thrombin generation in TFPI-deficient plasma demonstrated reduced anticoagulant activity of citrullinated TFPI. Mass spectrometry demonstrated citrullination of surface-exposed arginine residues in TFPIα after incubation with PAD4. CONCLUSION: Full-length TFPIα is sensitive to citrullination by PAD4, which causes loss of factor Xa inhibition. This process may play a role in the increased thrombosis risk associated with inflammation.

Unraveling the role of the homoarginine residue in antiplatelet drug eptifibatide in binding to the αIIbß3 integrin receptor.

Eptifibatide is an αIIbß3 inhibitor that is currently used in the clinic. More than 10 scientific communications indicate that eptifibatide has a Lys-Gly-Asp or Arg-Gly-Asp sequence, while it actually has a hArg-Gly-Asp sequence. We aimed to unravel the importance of the homoarginine residue in eptifibatide in platelet activation and aggregation. Arg- and Lys-eptifibatide were synthesized by solid-phase peptide synthesis and measured in light transmission aggregometry, flow cytometry and whole blood thrombus formation under flow. Interactions of eptifibatide and its variants with αIIbß3 integrin were studied using molecular dynamics simulations. Eptifibatide showed inhibition of collagen- and ADP-induced platelet aggregation, while Arg- and Lys-eptifibatide did not. Multiparameter assessment of thrombus formation showed suppressed platelet aggregate and fibrin formation upon eptifibatide treatment, in contrast to the other variants. Molecular dynamics simulations revealed that the hArg residue in eptifibatide is crucial to its activity, since the substitution of the hArg to Arg or Lys resulted in the inability to form double H-bonds with Asp224 in the αIIb chain of the αIIbß3 receptor. The hArg is pivotal for the interaction of eptifibatide for the αIIbß3 receptor and efficient inhibition of platelet aggregation.

Complementary roles of platelet αIIbß3 integrin, phosphatidylserine exposure and cytoskeletal rearrangement in the release of extracellular vesicles.

BACKGROUND AND AIMS: Platelets can release extracellular vesicles (EVs) upon stimulation with various agonists. Interestingly, platelets from patients with Glanzmann thrombasthenia have reduced EV release. These platelets lack functional αIIbß3 integrins, indicating that αIIbß3 integrin is critical in vesicle release. Integrin activation is central in platelet function and is associated with e.g. adhesion, aggregation and cytoskeletal rearrangement. However, while platelet activation pathways are widely known, the mechanisms underlying EV release remain uncharacterized. We investigated the role of integrin αIIbß3, phosphatidyl serine (PS) exposure, cytoskeletal rearrangement and their associated signalling pathways in EV release. METHODS: EVs were isolated from activated platelets. Platelet activation status was measured by multicolour flow cytometry. A panel of pharmacologic inhibitors was used to interfere in specific signalling pathways. EV release was quantified enzymatically based on membrane PS content and nanoparticle tracking analysis. In addition, real-time visualization of EV shedding with confocal microscopy and EVs with Cryo-TEM imaging was performed. RESULTS: Platelet activation with convulxin resulted in higher EV release than with activation by thrombin. Kinetic measurements indicated that EV release followed the pattern of αIIbß3 integrin activation and subsequent closure paralleled by PS exposure. Prevention of αIIbß3 activation with the inhibitor tirofiban dramatically suppressed EV release. Similar results were obtained using αIIbß3-deficient platelets from patients with Glanzmann thrombasthenia. Inhibition of actin cytoskeleton rearrangement decreased EV release, whereas inhibition of individual signalling targets upstream of cytoskeletal rearrangement showed no such effects. CONCLUSION: Platelet EV release requires three main events: integrin activation and closure, PS exposure, and cytoskeletal rearrangement.

Impaired processing of human pro-islet amyloid polypeptide is not a causative factor for fibril formation or membrane damage in vitro.

Human islet amyloid polypeptide (hIAPP) forms amyloid fibrils in pancreatic islets of patients with type 2 diabetes mellitus (DM2). hIAPP is synthesized by islet beta-cells initially as a preprohormone, processing of which occurs in several steps. It has been suggested that in DM2 this processing is defective and that aggregation of the processing intermediates prohIAPP and prohIAPP(1-48) may represent the initial step in formation of islet amyloid. Here we investigate this possibility by analyzing the aggregation, the structure, and the membrane interaction of mature hIAPP and its precursors, prohIAPP and prohIAPP(1-48), in vitro. Our data reveal that both precursors form amyloid fibrils in solution but not in the presence of membranes. This inhibition is in contrast to the catalyzing effect of membranes on fibril formation of mature hIAPP. Importantly, in the presence of membranes, both precursors are able to inhibit fibrillogenesis of mature hIAPP. These differences in behavior between mature hIAPP and its precursors are most likely related to differences in their mode of membrane insertion. Both precursors insert efficiently and adopt an alpha-helical structure even with a high lipid/peptide ratio, while mature hIAPP rapidly adopts a beta-sheet conformation. Furthermore, while mature hIAPP affects the barrier properties of lipid vesicles, neither of the precursors is able to induce membrane leakage. Our study suggests that the hIAPP precursors prohIAPP and prohIAPP(1-48) do not serve as amyloid initiators but rather prevent aggregation and membrane damage of mature hIAPP in early stages of its biosynthesis and intracellular transport.

Measurement of amino acid isotope enrichment by liquid chromatography mass spectroscopy after derivatization with 9-fluorenylmethylchloroformate.

An automated method is described to measure tracer-tracee-ratios (TTR) of plasma amino acids after their separation as 9-fluorenylmethylchloroformate derivatives. In a 45 min cycle, 5 microl plasma aliquots were derivatized, HPLC separated and subjected to electrospray ionization. By applying source collision induced dissociation, derivatives were dissociated at the peptide bond releasing the originating amino acids into the mass spectrometer. This approach enabled the determination of plasma amino acid TTRs with a standard deviation between 0.15 and 0.36%, which is sufficient to study the fate of infused tracers and their conversion products in an in vivo experiment in humans.

Imaging evidence for endothelin ETA/ETB receptor heterodimers in isolated rat mesenteric resistance arteries.

AIMS: In engineered cells, endothelin ETA and ETB receptors can heterodimerize. We tested whether this can also be observed in native tissue. MAIN METHODS: Rat mesenteric resistance arteries (rMRA) were maintained in organ culture for 24h to upregulate ETB-mediated contractions in addition to their normal ETA-mediated responses. They were then exposed to 100 nM linear ET-1 (ETB-agonist) labeled with Oregon Green 488 (OG488/L.-ET-1) and/or to 16nM intact ET-1 (ETA/ETB-agonist) labeled with the rhodamine dye TAMRA (TAMRA/ET-1). Two photon laser scanning microscopy (TPLSM) was used for the visualization of their binding in the tissue. Fluorescence Lifetime Imaging Microscopy (FLIM) was employed for measurements of the OG488/L.-ET-1 lifetime in the absence and presence of TAMRA/ET-1. KEY FINDINGS: After incubation with the labeled ligands, medial smooth muscle cells (SMCs) were efficiently stained and became visible under TPLSM. TAMRA/ET-1 bound to all SMCs whereas OG488/L.-ET-1 stained only groups of SMCs. Interaction of the two receptor subtypes in SMC was investigated in double staining experiments. Fluorescence lifetime of OG488/L.-ET-1 was reduced in the presence of TAMRA/ET-1, which indicates the occurrence of Fluorescence Resonant Energy Transfer (FRET) and suggests close proximity of the two receptor subtypes within the arterial wall. SIGNIFICANCE: The methodology that is introduced by these new observations may be useful to assess ET-receptor heterodimerization in biopsies from relevant experimental animal models and human patients.

The Kunitz 1 and Kunitz 3 domains of tissue factor pathway inhibitor are required for efficient inhibition of factor Xa.

Tissue factor pathway inhibitor (TFPI) is a slow tight-binding inhibitor that inhibits factor (F)Xa in a biphasic fashion: a rapid formation of loose FXa·TFPI encounter complex is followed by slow rearrangement into a tight FXa·TFPI* complex in which the Kunitz-2 (K2) domain of TFPI binds and inhibits FXa. In the current study, full-length TFPI (TFPIfl) and various truncated TFPI constructs were used to assess the importance of TFPI domains other than K2 in the inhibition of FXa. In the absence of Ca2+ ions, FXa was more effectively inhibited by TFPIfl than Gla-domain less FXa. In turn, Ca2+ ions impaired FXa inhibition by TFPIfl but not by TFPI constructs that lack the C-terminus. This suggests that, in absence of Ca2+ ions, interactions between the C-terminus of TFPI and the Gla-domain of FXa promote FXa-inhibition. TFPIfl and K2K3 had similar efficiencies for encounter complex formation. However, K2K3 showed monophasic inhibition instead of biphasic inhibition, indicating absence of rearrangement into a tight complex. K1K2 and TFPI1-161 showed biphasic inhibition, but had less efficient encounter complex formation than TFPIfl. Finally, K2K3 was a 10-fold more efficient FXa- inhibitor than K2. These results indicate that K3-C-terminus enhances the formation of encounter complex and that K1 is required for isomerisation of the encounter- into tight complex. Since TFPIfl has a 10-fold higher Ki than K2K3-C-terminus, we propose that K1 is not only required for the transition of the loose to the tight FXa·TFPI* complex, but also inhibits FXa·TFPI encounter complex formation. This inhibitory activity is counteracted by K3 and C-terminus.

Endothelin-1 and -2: two amino acids matter.

AIMS: Endothelin-1 (ET-1) and endothelin-2 (ET-2; Trp(6)Leu(7)ET-1) are expressed by different cell types, but are considered to display identical pharmacological properties on endothelin receptors. We studied agonist-dependent aspects of endothelin(A) (ET(A))-receptor function and the importance of amino acids 6 and 7 of ET-1 and ET-2 in this respect. MAIN METHODS: We used isolated rat mesenteric resistance arteries in wire myographs, in a setting that minimizes influences of endothelium and sensorimotor nerves, to study arterial smooth muscle ET(A)-receptor-mediated vasomotor responses, to ET-1, ET-2 and chimeras thereof (Trp(6)ET-1 and Leu(7)ET-1). KEY FINDINGS: ET-1 and ET-2 cause arterial contractions with comparable sensitivities and maximal responses. BQ123 (ET(A)-antagonist) reduces sensitivity to ET-1 more potently than that to ET-2 (pK(B): 7.1 ± 0.2 versus 5.6 ± 0.4). However, 1 µM BQ123 relaxes maximal contractile responses to ET-2 more markedly than those to ET-1. Leu(7)ET-1 is a contractile agonist with lower potency and similar maximal effect compared to ET-1 and greater sensitivity to BQ123 than ET-2. Up to 256 nM Trp(6)ET-1 did not cause contraction and did not antagonize arterial responses to ET-1. SIGNIFICANCE: Arterial smooth muscle ET(A)-receptor function displays agonist-dependent aspects. This involves roles of amino acids on position 6 and 7 of the endothelin sequence. Agonist-dependent pathologies may benefit from the design of specific, agonist-selective ET-receptor antagonists.