Accumulation of the β-amyloid (Aβ) peptide in extracellular senile plaques rich in copper and zinc is a defining pathological feature of Alzheimer's disease (AD). The Aβ1-x (x=16/28/40/42) peptides have been the primary focus of Cu(II) binding studies for more than 15 years; however, the N-truncated Aβ4-42 peptide is a major Aβ isoform detected in both healthy and diseased brains, and it contains a novel N-terminal FRH sequence. Proteins with His at the third position are known to bind Cu(II) avidly, with conditional log K values at pH 7.4 in the range of 11.0-14.6, which is much higher than that determined for Aβ1-x peptides. By using Aβ4-16 as a model, it was demonstrated that its FRH sequence stoichiometrically binds Cu(II) with a conditional Kd value of 3×10(-14)  M at pH 7.4, and that both Aβ4-16 and Aβ4-42 possess negligible redox activity. Combined with the predominance of Aβ4-42 in the brain, our results suggest a physiological role for this isoform in metal homeostasis within the central nervous system.

Recently we screened a combinatorial library of R(1)-(Ser/Thr)-Xaa-His-Zaa-R(2) peptides (Xaa = 17 common alpha-amino acids, except Asp, Glu, and Cys; Zaa =19 common alpha-amino acids, except Cys; R(1) = CH(3)CO-Gly-Ala, R(2) = Lys-Phe-Leu-NH(2)) and established criteria for selecting Ser/Thr, Xaa, and Zaa substitutions optimal for specific R(1)-Ser/Thr peptide bond hydrolysis in the presence of Ni(II) ions (Krezel, A.; Kopera, E.; Protas, A. M.; Poznanski, J.; Wysłouch-Cieszynska, A.; Bal, W. J. Am. Chem. Soc. 2010, 132, 3355-3366). The screening results were confirmed by kinetic studies of hydrolysis of seven peptides: R(1)-Ser-Arg-His-Trp-R(2), R(1)-Ser-Lys-His-Trp-R(2), R(1)-Ser-Ala-His-Trp-R(2), R(1)-Ser-Arg-His-Ala-R(2), R(1)-Ser-Gly-His-Ala-R(2), R(1)-Thr-Arg-His-Trp-R(2), and R(1)-Thr-His-His-Trp-R(2). In this paper, we used the same seven peptides to investigate the molecular mechanism of the hydrolysis reaction. We studied temperature dependence of the reaction rate at temperatures between 24 and 75 degrees C, measured stability constants of Ni(II) complexes with hydrolysis substrates and products, and studied the course of R(1)-Ser-Arg-His-Trp-R(2) peptide hydrolysis under a broad range of conditions. We established that the specific square planar complex containing the Ni(II) ion bonded to the His imidazole nitrogen and three preceding peptide bond nitrogens (4N complex) is required for the reaction to occur. The reaction mechanism includes the N-O acyl shift, yielding an intermediate ester of R(1) with the Ser/Thr hydroxyl group. This ester hydrolyzes spontaneously, yielding final products. The Ni(II) ion activates the R(1)-Ser peptide bond by destabilizing it directly through peptide nitrogen coordination and, indirectly, by imposing a strain in the peptide chain.

Key Points• GPVI is the major signaling receptor for fibrin in human platelets; the GPVI binding site is located in the fibrin D-dimer region.• D-dimer blocks platelet aggregation by fibrin and collagen but not by a collagen-related peptide, suggesting a distinct binding epitope.Fibrin has recently been shown to activate platelets through the immunoglobulin receptor glycoprotein VI (GPVI). In the present study, we show that spreading of human platelets on fibrin is abolished in patients deficient in GPVI, confirming that fibrin activates human platelets through the immunoglobulin receptor. Using a series of proteolytic fragments, weshow that D-dimer, but not the E fragment of fibrin, binds to GPVI and that immobilized D-dimer induces platelet spreading through activation of Src and Syk tyrosine kinases. In contrast, when platelets are activated in suspension, soluble D-dimer inhibits platelet aggregation induced by fibrin and collagen, but not by a collagen-related peptide composed of a repeat GPO sequence or by thrombin. Using surface plasmon resonance, we demonstrate that fibrin binds selectively to monomeric GPVI with a K D of 302 nM, in contrast to collagen, which binds primarily to dimeric GPVI. These results establish GPVI as the major signaling receptor for fibrin in human platelets and provide evidence that fibrin binds to a distinct configuration of GPVI. This indicates that it may be possible to develop agents that selectively block the interaction of fibrin but not collagen with the immunoglobulin receptor. Such agents are required to establish whether selective targeting of either interaction has the potential to lead to development of an antithrombotic agent with a reduced effect on bleeding relative to current antiplatelet drugs.

Essentials Glycoprotein VI (GPVI) binds collagen, starting thrombogenesis, and fibrin, stabilizing thrombi.GPVI‐dimers, not monomers, recognize immobilized fibrinogen and fibrin through their D‐domains.Collagen, D‐fragment and D‐dimer may share a common or proximate binding site(s) on GPVI‐dimer.GPVI‐dimer–fibrin interaction supports spreading, activation and adhesion involving αIIbβ3. SummaryBackgroundPlatelet collagen receptor Glycoprotein VI (GPVI) binds collagen, initiating thrombogenesis, and stabilizes thrombi by binding fibrin.ObjectivesTo determine if GPVI‐dimer, GPVI‐monomer, or both bind to fibrinogen substrates, and which region common to these substrates contains the interaction site.MethodsRecombinant GPVI monomeric extracellular domain (GPVI ex) or dimeric Fc‐fusion protein (GPVI‐Fc2) binding to immobilized fibrinogen derivatives was measured by ELISA, including competition assays involving collagenous substrates and fibrinogen derivatives. Flow adhesion was performed with normal or Glanzmann thrombasthenic (GT) platelets over immobilized fibrinogen, with or without anti‐GPVI‐dimer or anti‐αIIbβ3.ResultsUnder static conditions, GPVI ex did not bind to any fibrinogen substrate. GPVI‐Fc2 exhibited specific, saturable binding to both D‐fragment and D‐dimer, which was inhibited by mFab‐F (anti‐GPVI‐dimer), but showed low binding to fibrinogen and fibrin under our conditions. GPVI‐Fc2 binding to D‐fragment or D‐dimer was abrogated by collagen type III, Horm collagen or CRP‐XL (crosslinked collagen‐related peptide), suggesting proximity between the D‐domain and collagen binding sites on GPVI‐dimer. Under low shear, adhesion of normal platelets to D‐fragment, D‐dimer, fibrinogen and fibrin was inhibited by mFab‐F (inhibitor of GPVI‐dimer) and abolished by Eptifibatide (inhibitor of αIIbβ3), suggesting that both receptors contribute to thrombus formation on these substrates, but αIIbβ3 makes a greater contribution. Notably, thrombasthenic platelets showed limited adhesion to fibrinogen substrates under flow, which was further reduced by mFab‐F, supporting some independent GPVI‐dimer involvement in this interaction.ConclusionOnly dimeric GPVI interacts with fibrinogen D‐domain, at a site proximate to its collagen binding site, to support platelet adhesion/activation/aggregate formation on immobilized fibrinogen and polymerized fibrin.

The hallmark of fibrotic disorders is a highly cross-linked and dense collagen matrix, a property driven by the oxidative action of lysyl oxidase. Other fibrosis-associated proteins also contribute to the final collagen matrix properties, one of which is fibromodulin. Its interactions with collagen affect collagen cross-linking, packing, and fibril diameter. We investigated the possibility that a specific relationship exists between fibromodulin and lysyl oxidase, potentially imparting a specific collagen matrix phenotype. We mapped the fibromodulin-collagen interaction sites using the collagen II and III Toolkit peptide libraries. Fibromodulin interacted with the peptides containing the known collagen cross-linking sites and the MMP-1 cleavage site in collagens I and II. Interestingly, the interaction sites are closely aligned within the quarter-staggered collagen fibril, suggesting a multivalent interaction between fibromodulin and several collagen helices. Furthermore, we detected an interaction between fibromodulin and lysyl oxidase (a major collagen cross-linking enzyme) and mapped the interaction site to 12 N-terminal amino acids on fibromodulin. This interaction also increases the activity of lysyl oxidase. Together, the data suggest a fibromodulin-modulated collagen cross-linking mechanism where fibromodulin binds to a specific part of the collagen domain and also forms a complex with lysyl oxidase, targeting the enzyme toward specific cross-linking sites.