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Background: Platelets are essential to physiological hemostasis or pathological thrombus formation. Current antiplatelet agents inhibit platelet aggregation but leave patients at risk of systemic side-effects such as hemorrhage. Newer therapeutic strategies could involve targeting this cascade earlier during platelet adhesion or activation via inhibitory effects on specific glycoproteins, the thrombogenic collagen receptors found on the platelet surface. Aims: Glycoprotein VI (GPVI) is increasingly being recognized as the main platelet-collagen receptor involved in arterial thrombosis. This review summarizes the crucial role GPVI plays in ischemic stroke as well as the current strategies used to attempt to inhibit its activity. Summary of review: In this review, we discuss the normal hemostatic process, and the role GPVI plays at sites of atherosclerotic plaque rupture.We discuss how the unique structure of GPVI allows for its interaction with collagen and creates downstream signaling that leads to thrombus formation. We summarize the current strategies used to inhibit GPVI activity and how this could translate to a clinically viable entity that may compete with current antiplatelet therapy. Conclusion: From animal models, it is clear that GPVI inhibition leads to an abolished platelet response to collagen and reduced platelet aggregation, culminating in smaller arterial thrombi. There is now an increasing body of evidence that these findings can be translated into the development of a bleeding free pharmacological entity specific to sites of plaque rupture in humans. [ABSTRACT FROM AUTHOR]

Abstract: Microalgae are a promising source of lipids for biofuel production. To improve the economic feasibility and sustainability of this biofuel feedstock, one should create value for co-products after lipid extraction. Thus, protein isolation from the defatted biomass presents an opportunity. To extract algae protein, temperature and pH were evaluated to maximize the extraction from Nannochloropsis biomass. Maximum quantity of protein was solubilized at 60°C and pH11 and recovered at pH3.2. The isolated protein fractions contained 56.9% and 40.5% protein when using isopropanol (IPA) defatted and non-defatted biomass as the starting materials, with protein yields being 16 and 30%, respectively. The IPA-defatting treatment significantly decreased the protein extraction yield. These values are low compared with soybean protein isolates (>90% protein and ~60% yield). The relatively high protein content (>34%) in the pH11 insoluble fraction indicates needs for further extraction optimization. The nitrogen and amino acid content of the initial materials and all the fractions were determined and the calculated nitrogen to protein conversion factor was in the range of 4.06–4.70. The possibility of the presence of conjugated protein, i.e., N-containing glycoproteins, is also discussed. [Copyright &y& Elsevier]

Of the two physiologically important platelet collagen receptors, glycoprotein (GP) VI is the receptor responsible for platelet activation. However, its reactivities towards different types of vascular collagen have not been directly and quantitatively analysed with collagen preparations of defined composition, although the other major platelet collagen receptor integrin α2β1 was shown to react with collagen types I-VI and VIII under either static or flow conditions. We analysed the collagen type specificity of GPVI binding to identify the physiological contribution of the various vascular collagens and how platelet reactivity towards the various collagens may be affected by fibril size. We used two methods to analyse the binding of recombinant GPVI (GPVI-Fc2) to different types of bovine collagen: binding to collagen microparticles in suspension and binding to immobilized collagen. GPVI-Fc2 bound to type I-III collagens that can form large fibrils, but not to type V that only forms small fibrils. The apparent GPVI binding to types IV and V could be ascribed to type I collagen that was a contaminant in each of these preparations. Kinetic analyses of the binding data showed that type III collagen fibrils have both a higher Kd and Bmax than types I and II. Flow adhesion studies demonstrated that type III collagen supports the formation of larger platelet aggregates than type I. Our present results suggest that the physiological importance of type III collagen is to induce thrombus formation. Furthermore, these studies indicate that GPVI mainly binds to collagen types that can form large collagen fibrils. [ABSTRACT FROM AUTHOR]

Glycoprotein (GP) VI is a platelet membrane protein with a molecular weight of 62 kDa that was identified as a physiological collagen receptor from studies of patients deficient in this protein. GPVI-deficient platelets lacked specifically collagen-induced aggregation and the ability to form thrombi on a collagen surface under flow conditions, suggesting that GPVI makes an indispensable contribution to collagen-induced platelet activation. On the platelet surface, GPVI is present as a complex with the Fc receptor (FcR) γ-chain, probably composed of two GPVI molecules and one FcR γ-chain dimer. GPVI must form such a dimeric complex to exhibit high affinity binding to collagen. The GPVI-induced activation mechanism is initiated by tyrosine phosphorylation of the immunoreceptor tyrosine-based activation motif (ITAM) of the FcR γ-chain, and then this signal is transduced to many related proteins, mainly by tyrosine phosphorylation. GPVI is widely recognized as a requisite factor for the formation of platelet aggregates on a collagen surface under blood flow. However, individuals with GPVI-deficient or null platelets do not exhibit any strong bleeding tendency. Analyzing this apparent dichotomy should provide us with a more precise understanding of the mechanism of thrombus formation. [Copyright &y& Elsevier]

There is extensive evidence that FcR γ-chain couples to the collagen receptor glycoprotein VI (GPVI) and becomes phosphorylated on tyrosines upon receptor cross-linking. However, it is not established whether this receptor complex is sufficient to initiate the signalling cascade. We transfected GPVI and the FcR γ-chain into the human erythroleukaemia cell line K562, which lacks detectable expression of GPVI and the FcR γ-chain. The results show that GPVI is unable to signal when expressed alone, despite its surface expression, upon stimulation with the snake C-type lectin, convulxin. Coexpression of the FcR γ-chain confers signalling properties on the receptor. Furthermore, cotransfection of the FcR γ-chain and two mutant versions of GPVI shows that the transmembrane arginine and cytoplasmic tail of GPVI are necessary for association with the FcR γ-chain. These results demonstrate that reconstitution of the GPVI–FcR γ-chain complex in cells expressing the necessary signalling network is sufficient to initiate signalling events in response to convulxin and collagen-related peptide. [ABSTRACT FROM AUTHOR]

Background Glycoprotein VI (GPVI) is the essential platelet collagen receptor in atherothrombosis, but its inhibition causes only a mild bleeding tendency. Thus, targeting this receptor has selective antithrombotic potential. Objectives This study sought to compare compounds interfering with platelet GPVI–atherosclerotic plaque interaction to improve current antiatherothrombotic therapy. Methods Human atherosclerotic plaque–induced platelet aggregation was measured in anticoagulated blood under static and arterial flow conditions (550/s, 1,100/s, and 1,500/s). Inhibition by dimeric GPVI fragment crystallizable region of IgG (Fc) masking GPVI binding sites on collagen was compared with that of 3 anti-GPVI antibodies: BLO8-1, a human domain antibody; 5C4, a fragment antigen-binding (Fab fragment) of monoclonal rat immunoglobulin G; and m-Fab-F, a human recombinant sFab against GPVI dimers. Results GPVI-Fc reduced plaque-triggered platelet aggregation in static blood by 51%, BLO8-1 by 88%, and 5C4 by 93%. Under arterial flow conditions, BLO8-1 and 5C4 almost completely inhibited platelet aggregation while preserving platelet adhesion on plaque. Inhibition by GPVI-Fc, even at high concentrations, was less marked but increased with shear rate. Advanced optical imaging revealed rapid persistent GPVI-Fc binding to collagen under low and high shear flow, upstream and downstream of plaque fragments. At low shear particularly, platelets adhered in plaque flow niches to GPVI-Fc–free segments of collagen fibers and recruited other platelets onto aggregates via ADP and TxA2 release. Conclusions Anti-GPVI antibodies inhibit atherosclerotic plaque-induced platelet aggregation under static and flow conditions more effectively than GPVI-Fc. However, potent platelet inhibition by GPVI-Fc at a higher shear rate (1,500/s) suggests localized antithrombotic efficacy at denuded or fissured stenotic high-risk lesions without systemic bleeding. The compound-specific differences have relevance for clinical trials targeting GPVI-collagen interaction combined with established antiplatelet therapies in patients with spontaneous plaque rupture or intervention-associated plaque injury. [ABSTRACT FROM AUTHOR]