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  • Title: Update on collagen receptor interactions in platelets: is the two-state model still valid?
    Author: Watson S, Berlanga O, Best D, Frampton J.
    Journal: Platelets; 2000 Aug; 11(5):252-8. PubMed ID: 11030459.
    Abstract:
    This review summarises some of the key developments that have taken place in our understanding of platelet-collagen interactions within the last 18 months. Within this time, the major activatory collagen receptor glycoprotein VI (GPVI) has been sequenced and shown to reconstitute collagen responses in a megakaryocytic cell line. It is a member of the Ig superfamily of proteins, with two extracellular Ig domains, and is constitutively associated with the Fc receptor gamma-chain (FcR gamma-chain). GPVI signals through a pathway that shares many features with those of immune receptors, with critical roles for Syk and the adapters LAT and SLP-76 in the activation of PLCgamma2. Significant developments have also taken place in regard to the role of the major adhesion receptor for collagen, the integrin alpha2beta1 (also known as GPIa-IIa). An alpha2beta1-selective collagen-based peptide has been developed and co-crystallised with the I-domain of the alpha2 subunit. Polymorphisms in alpha2 have been shown to cause wide variation in expression of alpha2beta1, with the alpha2 allele T807/A873 leading to a high level of the integrin and increased risk of stroke in young people. Activation of platelets by a wide range of agonists has been shown to increase the affinity of alpha2beta1 to intermediate or high affinity states. This has important implications for the two-site, two-state model of collagen-platelet interactions. A new model is proposed in which collagen binds initially to either alpha2beta1 or GPVI, leading to subsequent binding to the other receptor and conversion of the integrin to a high affinity state. In this model, both receptors generate intracellular signals which support platelet activation.
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