Competitive interactions of collagen and a jararhagin-derived disintegrin peptide with the integrin alpha2-I domain

Development of the Antithrombotic Peptide LEKNSTY Targeting the Collagen Surface: I. Design and Validation

Exposed collagen on the diseased vessel wall is crucial for arterial thrombosis. The currently developed antithrombotic drugs mostly target blood components such as platelets and suffer from the risk of bleeding. Therefore, anticollagen therapy of covering the collagen surface was proposed as an alternative in our previous study, and an antithrombotic peptide LWWNSYY was designed and validated. However, its application was hindered due to the poor water solubility. In the present study, in order to develop a novel antithrombotic peptide with enhanced water solubility, redesigning of LWWNSYY to LEKNSTY using the EK pattern was proposed. Improved solubility was obtained for LEKNSTY. Moreover, the binding of LEKNSTY on the collagen surface was confirmed by molecular docking, molecular dynamics simulations, and experimental validation. A K_d of 0.91 ± 0.44 μM was observed. The effective inhibition of platelet adhesion on the collagen surface by LEKNSTY was demonstrated at an IC₅₀ of 2.48 ± 0.59 μg/mL. Therefore, the successful design of the antithrombotic peptide LEKNSTY was confirmed, which would facilitate the research into the interface involving thrombus and the development of antithrombotic agents.

Development of the Antithrombotic Peptide LEKNSTY Targeting the Collagen Surface: II. Improvement of Plasma Stability

The development of antithrombotic peptides targeting collagen was proven effective, and an effective antithrombotic peptide LEKNSTY was obtained in part I. However, the plasma stability of LEKNSTY was found to be not good enough. In this part, the LEKNSTY was further optimized for improvement in plasma stability by substitution using d-amino acid residues. Two novel antithrombotic peptides LekNStY and lEKnsTy were designed, where lowercase letters represent d-amino acid residues. Improvements in plasma stability of both LekNStY and lEKnsTy were experimentally confirmed. Moreover, good binding of these antithrombotic peptides on the collagen surface was confirmed by molecular dynamics simulation and experimental validation. For example, a K_d of only 0.75 ± 0.10 μM was observed for lEKnsTy. Moreover, LekNStY and lEKnsTy were found to inhibit platelet adhesion on the collagen surface more effectively than LEKNSTY, and the IC₅₀ of lEKnsTy was only 2/5 of that of LEKNSTY. These results confirmed the successful design of LekNStY and lEKnsTy that had good plasma stability and could effectively inhibit arterial thrombosis, which would be helpful for the research into interfaces involved in thrombus formation and the development of antithrombotic nanomedicine.

Inhibition of Arterial Thrombus Formation by Blocking Exposed Collagen Surface Using LWWNSYY-Poly(l-Glutamic Acid) Nanoconjugate

Exposed collagen surface on diseased blood vessel wall is a trigger of platelet adhesion and subsequent thrombus formation, which is associated with many serious diseases such as myocardial infarction and stroke. Various antithrombotic agents have been developed, but are usually targeted on blood components such as platelet, which suffered from the risk of bleeding due to interference with hemostasis. In contrast, blocking the exposed collagen surface would prevent thrombus formation without the risk of bleeding. In the present study, an antithrombotic nanoconjugate (LWWNSYY-poly glutamic acid, L7-PGA) targeting collagen surface was designed by immobilizing heptapeptide LWWNSYY, a biomimetic inhibitor designed in our previous work, on poly(l-glutamic acid). Successful binding of L7-PGA on the collagen surface was confirmed by a negative ΔG of -5.99 ± 0.26 kcal/mol. L7-PGA was found to effectively inhibit platelet adhesion on the collagen surface, with a reduced IC₅₀ of only 1/5 of that of free LWWNSYY. The inhibition of thrombus formation by L7-PGA was also validated in vivo by a reduction of 31.2% in the weight of thrombus. These results highlight L7-PGA as an effective inhibitor of arterial thrombus formation via blocking exposed collagen surface, which would be helpful for the development of novel antithrombotic nanomedicine.

Structural Characterization of the Interaction between the αMI-Domain of the Integrin Mac-1 (αMβ2) and the Cytokine Pleiotrophin

Integrin Mac-1 (α_Mβ₂) is an adhesion receptor vital to many functions of myeloid leukocytes. It is also the most promiscuous member of the integrin family capable of recognizing a broad range of ligands. In particular, its ligand-binding α_MI-domain is known to bind cationic proteins/peptides depleted in acidic residues. This contradicts the canonical ligand-binding mechanism of αI-domains, which requires an acidic amino acid in the ligand to coordinate the divalent cation within the metal ion-dependent adhesion site (MIDAS) of αI-domains. The lack of acidic amino acids in the α_MI-domain-binding sequences suggests the existence of an as-yet uncharacterized interaction mechanism. In the present study, we analyzed interactions of the α_MI-domain with a representative Mac-1 ligand, the cationic cytokine pleiotrophin (PTN). Through NMR chemical shift perturbation analysis, cross saturation, NOESY, and mutagenesis studies, we found the interaction between the α_MI-domain and PTN is divalent cation-independent and mediated mostly by hydrophobic contacts between the N-terminal domain of PTN and residues in the α5-β5 loop of α_MI-domain. The observation that increased ionic strength weakens the interaction between the proteins indicates electrostatic forces may also play a significant role in the binding. On the basis of the results from these experiments, we formulated a model of the interaction between the α_MI-domain and PTN.

Metalloproteases Affecting Blood Coagulation, Fibrinolysis and Platelet Aggregation from Snake Venoms: Definition and Nomenclature of Interaction Sites

Snake venom metalloproteases, in addition to their contribution to the digestion of the prey, affect various physiological functions by cleaving specific proteins. They exhibit their activities through activation of zymogens of coagulation factors, and precursors of integrins or receptors. Based on their structure–function relationships and mechanism of action, we have defined classification and nomenclature of functional sites of proteases. These metalloproteases are useful as research tools and in diagnosis and treatment of various thrombotic and hemostatic conditions. They also contribute to our understanding of molecular details in the activation of specific factors involved in coagulation, platelet aggregation and matrix biology. This review provides a ready reference for metalloproteases that interfere in blood coagulation, fibrinolysis and platelet aggregation.

Vimocin and vidapin, cyclic KTS peptides, are dual antagonists of α1β1/α2β1 integrins with antiangiogenic activity

Obtustatin and viperistatin, members of the disintegrin protein family, served as lead compounds for the synthesis of linear and cyclic peptides containing the KTS binding motif. The most active linear peptide, a viperistatin analog, indicated the importance of Cys(19) and Cys(29), as well as the presence of Arg at position 24 for their biologic activity, and was used as the basic sequence for the synthesis of cyclic peptides. Vimocin (compound 6) and vidapin (compound 10) showed a high potency (IC50 = 0.17 nM) and intermediate efficacy (20 and 40%) in inhibition of adhesion of α1/α2 integrin overexpressor cells to respective collagens. Vimocin was more active in inhibition of the wound healing (53%) and corneal micropocket (17%) vascularization, whereas vidapin was more potent in inhibition of migration in the Matrigel tube formation assay (90%). Both compounds similarly inhibited proliferation (50-90%) of endothelial cells, and angiogenesis induced by vascular endothelial growth factor (80%) and glioma (55%) in the chorioallantoic membrane assay. These peptides were not toxic to endothelial cell cultures and caused no acute toxicity upon intravenous injection in mice, and were stable for 10-30 hours in human serum. The in vitro and in vivo potency of the peptides are consistent with conformational ensembles and "bioactive" space shared by obtustatin and viperistatin. These findings suggest that vimocin and vidapin can serve as dual α1β1/α2β1 integrin antagonists in antiangiogenesis and cancer therapy.

Biomimetic design of platelet adhesion inhibitors to block integrin α2β1-collagen interactions: II. Inhibitor library, screening, and experimental validation

Platelet adhesion on collagen mediated by integrin α2β1 has been proven important in arterial thrombus formation, leading to an exigent demand on development of potent inhibitors for the integrin α2β1-collagen binding. In the present study, a biomimetic design strategy of platelet adhesion inhibitors was established, based on the affinity binding model of integrin proposed in part I. First, a heptapeptide library containing 8000 candidates was designed to functionally mimic the binding motif of integrin α2β1. Then, each heptapeptide in the library was docked onto a collagen molecule for the assessment of its affinity, followed by a screening based on its structure similarity to the original structure in the affinity binding model. Eight candidates were then selected for further screening by molecular dynamics (MD) simulations. Thereafter, three candidates chosen from MD simulations were separately added into the physiological saline containing separated integrin and collagen, to check their abilities for blocking the integrin-collagen interaction using MD simulations. Of these three candidates, significant inhibition was observed in the presence of LWWNSYY. Finally, the binding affinity of LWWNSYY for collagen was demonstrated by isothermal titration calorimetry. Moreover, significant inhibition of platelet adhesion in the presence of LWWNSYY has been experimentally validated. This work has thus developed an effective strategy for the biomimetic design of peptide-based platelet adhesion inhibitors.

The structure of integrin α1I domain in complex with a collagen-mimetic peptide

We have determined the structure of the human integrin α1I domain bound to a triple-helical collagen peptide. The structure of the α1I-peptide complex was investigated using data from NMR, small angle x-ray scattering, and size exclusion chromatography that were used to generate and validate a model of the complex using the data-driven docking program, HADDOCK (High Ambiguity Driven Biomolecular Docking). The structure revealed that the α1I domain undergoes a major conformational change upon binding of the collagen peptide. This involves a large movement in the C-terminal helix of the αI domain that has been suggested to be the mechanism by which signals are propagated in the intact integrin receptor. The structure suggests a basis for the different binding selectivity observed for the α1I and α2I domains. Mutational data identify residues that contribute to the conformational change observed. Furthermore, small angle x-ray scattering data suggest that at low collagen peptide concentrations the complex exists in equilibrium between a 1:1 and 2:1 α1I-peptide complex.

Cloning and characterisation of three novel disintegrin precursors from the venoms of three Atheris species: Atheris chlorechis, Atheris nitschei and Atheris squamigera

Snake venom constitutes one of the most complex mixtures of naturally-occurring toxic proteins/polypeptides and a large number of these possess very profound biological activities. Disintegrins, that are commonly found in viper venoms, are low molecular weight proteins that usually contain an -Arg-Gly-Asp- (-RGD-) motif that is known to be involved in cell adhesion ligand recognition, binding specifically to cell surface integrin receptors and also exhibiting platelet anti-aggregation activity. Here, we report for the first time, the successful cloning of three cDNAs encoding disintegrin precursors from lyophilised venom-derived libraries of Atheris chlorechis, Atheris nitschei and Atheris squamigera, respectively. All of these disintegrins belong to the short-coding class and all exhibit high degrees of structural identity, both in their amino acid sequences and in the arrangement of their functional domains. Mass spectrometric analyses of the HPLC-separated/in-gel digested venom proteins was performed to characterise the mature disintegrins as expressed in the venom proteome. Studies on both the structures and conserved sites within these disintegrins are of considerable theoretical interest in the field of biological evolution and in the development of new research tools or novel templates for drug design.

Dynamic structural changes are observed upon collagen and metal ion binding to the integrin α1 I domain

We have applied hydrogen-deuterium exchange mass spectrometry, in conjunction with differential scanning calorimetry and protein stability analysis, to examine solution dynamics of the integrin α1 I domain induced by the binding of divalent cations, full-length type IV collagen, or a function-blocking monoclonal antibody. These studies revealed features of integrin activation and α1I-ligand complexes that were not detected by static crystallographic data. Mg(2+) and Mn(2+) stabilized α1I but differed in their effects on exchange rates in the αC helix. Ca(2+) impacted α1I conformational dynamics without altering its gross thermal stability. Interaction with collagen affected the exchange rates in just one of three metal ion-dependent adhesion site (MIDAS) loops, suggesting that MIDAS loop 2 plays a primary role in mediating ligand binding. Collagen also induced changes consistent with increased unfolding in both the αC and allosteric C-terminal helices of α1I. The antibody AQC2, which binds to α1I in a ligand-mimetic manner, also reduced exchange in MIDAS loop 2 and increased exchange in αC, but it did not impact the C-terminal region. This is the first study to directly demonstrate the conformational changes induced upon binding of an integrin I domain to a full-length collagen ligand, and it demonstrates the utility of the deuterium exchange mass spectrometry method to study the solution dynamics of integrin/ligand and integrin/metal ion interactions. Based on the ligand and metal ion binding data, we propose a model for collagen-binding integrin activation that explains the differing abilities of Mg(2+), Mn(2+), and Ca(2+) to activate I domain-containing integrins.

Jararhagin, a hemorrhagic snake venom metalloproteinase from Bothrops jararaca

Jararhagin is a metalloproteinase isolated from Bothrops jararaca snake venom, which has been extensively studied. These studies showed its involvement on most of the systemic and local damaging effects of snakebite envenomings. In this review we comment on the major targets of jararhagin as the vascular endothelium, platelets and coagulation factors and also its action on other cell systems as inflammatory cells and their mediators, cancer and cell signaling. The mechanisms of jararhagin action are discussed together with structural features essential for the expression of its biological activities. The studies reviewed here denote jararhagin as a prototype for studies of snake venom metalloproteinases, bringing new insights into cellular-matrix interactions and adding for the improvement of snakebite treatment.

Design of cyclic RKKH peptide-conjugated PEG liposomes targeting the integrin α₂β₁ receptor

Peptide conjugation to the surface of stealth liposomes has been studied for liposomal drug targeting to cells expressing specific receptors to provide a site-specific drug delivery. This study investigated the potential of peptide-conjugated liposomes for targeting cells expressing the human integrin α(2)β(1) receptor. A 12 amino acid head-to-tail cyclic peptide derived from the Jararhagin protein containing the Arg-Lys-Lys-His (RKKH)-specific binding site was conjugated to the distal ends of poly(ethylene glycol) (PEG) chains on PEGylated liposomes. Epithelial cells expressing the receptor showed increased cellular association and uptake of peptide-conjugated liposomes at 4 °C, compared to liposomes conjugated with a non-specific peptide. The interaction between cells and peptide-conjugated liposomes was significantly increased at 37 °C suggesting that a possible uptake mechanism might be energy-dependent endocytosis. In keratinocyte cell cultures, the ligand-conjugated liposomes loaded with the vitamin D(3) analogue calcipotriol induced transcription of the gene encoding the antimicrobial peptide cathelicidin, which is activated through the vitamin D(3) receptor upon binding of vitamin D(3) analogues. This suggests that the liposomes are internalized and that calcipotriol is delivered intracellularly and released in an active form. In conclusion, the 12 amino acid head-to-tail cyclic RKKH peptide seems promising for targeting of liposomes to the integrin α(2)β(1) receptor.

Haemostatically active proteins in snake venoms

Snake venom proteins that affect the haemostatic system can cause (a) lowering of blood coagulability, (b) damage to blood vessels, resulting in bleeding, (c) secondary effects of bleeding, e.g. hypovolaemic shock and organ damage, and (d) thrombosis. These proteins may, or may not, be enzymes. We review the data on the most relevant haemostatically active proteinases, phospholipases A₂, L-amino acid oxidases and 5'-nucleotidases from snake venoms. We also survey the non-enzymatic effectors of haemostasis from snake venoms--disintegrins, C-type lectins and three-finger toxins. Medical applications have already been found for some of these snake venom proteins. We describe those that have already been approved as drugs to treat haemostatic disorders or are being used to diagnose such health problems. No clinical applications, however, currently exist for the majority of snake venom proteins acting on haemostasis. We conclude with the most promising potential uses in this respect.

Determinants of the specificity of rotavirus interactions with the alpha2beta1 integrin

The human α2β1 integrin binds collagen and acts as a cellular receptor for rotaviruses and human echovirus 1. These ligands require the inserted (I) domain within the α2 subunit of α2β1 for binding. Previous studies have identified the binding sites for collagen and echovirus 1 in the α2 I domain. We used CHO cells expressing mutated α2β1 to identify amino acids involved in binding to human and animal rotaviruses. Residues where mutation affected rotavirus binding were located in several exposed loops and adjacent regions of the α2 I domain. Binding by all rotaviruses was eliminated by mutations in the activation-responsive αC-α6 and αF helices. This is a novel feature that distinguishes rotavirus from other α2β1 ligands. Mutation of residues that co-ordinate the metal ion (Ser-153, Thr-221, and Glu-256 in α2 and Asp-130 in β1) and nearby amino acids (Ser-154, Gln-215, and Asp-219) also inhibited rotavirus binding. The importance of most of these residues was greatest for binding by human rotaviruses. These mutations inhibit collagen binding to α2β1 (apart from Glu-256) but do not affect echovirus binding. Overall, residues where mutation affected both rotavirus and collagen recognition are located at one side of the metal ion-dependent adhesion site, whereas those important for collagen alone cluster nearby. Mutations eliminating rotavirus and echovirus binding are distinct, consistent with the respective preference of these viruses for activated or inactive α2β1. In contrast, rotavirus and collagen utilize activated α2β1 and show an overlap in α2β1 residues important for binding.