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Collagen plays a critical role in hemostasis by promoting adhesion and activation of platelets at sites of vessel injury. In the present model of platelet-collagen interaction, adhesion is mediated via the inside-out regulation of integrin alpha2beta1 and activation through the glycoprotein VI (GPVI)-Fc receptor (FcR) gamma-chain complex. The present study extends this model by demonstrating that engagement of alpha2beta1 by an integrin-specific sequence from within collagen or by collagen itself generates tyrosine kinase-based intracellular signals that lead to formation of filopodia and lamellipodia in the absence of the GPVI-FcR gamma-chain complex. The same events do not occur in platelet suspensions. alpha2beta1 activation of adherent platelets stimulates tyrosine phosphorylation of many of the proteins in the GPVI-FcR gamma-chain cascade, including Src, Syk, SLP-76, and PLCgamma2 as well as plasma membrane calcium ATPase and focal adhesion kinase. alpha2beta1-mediated spreading is dramatically inhibited in the presence of the Src kinase inhibitor PP2 and in PLCgamma2-deficient platelets. Spreading is abolished by chelation of intracellular Ca2+. Demonstration that adhesion of platelets to collagen via alpha2beta1 generates intracellular signals provides a new insight into the mechanisms that control thrombus formation and may explain the unstable nature of beta1-deficient thrombi and why loss of the GPVI-FcR gamma-chain complex has a relatively minor effect on bleeding.

We investigated the adhesive behavior of mesenchymal stem cells (MSC) in blood, which might influence their fate when infused as therapy. Isolated human bone marrow MSC (BMMSC) or umbilical cord MSC (UCMSC) adhered efficiently from flow to the matrix proteins, collagen, or fibronectin, but did not adhere to endothelial selectins. However, when suspended in blood, BMMSC no longer adhered to collagen, while UCMSC adhered along with many aggregated platelets. Neither MSC adhered to fibronectin from flowing blood, although the fibronectin surface did become coated with a platelet monolayer. UCMSC induced platelet aggregation in platelet rich plasma, and caused a marked drop in platelet count when mixed with whole human or mouse blood in vitro, or when infused into mice. In contrast, BMMSC did not activate platelets or induce changes in platelet count. Interestingly, isolated UCMSC and BMMSC both adhered to predeposited platelets. The differences in behavior in blood were attributable to expression of podoplanin (an activating ligand for the platelet receptor CLEC‐2), which was detected on UCMSC, but not BMMSC. Thus, platelets were activated when bound to UCMSC, but not BMMSC. Platelet aggregation by UCMSC was inhibited by recombinant soluble CLEC‐2, and UCMSC did not cause a reduction in platelet count when mixed with blood from mice deficient in CLEC‐2. We predict that both MSC would carry platelets in the blood, but their interaction with vascular endothelium would depend on podoplanin‐induced activation of the bound platelets. Such interactions with platelets might target MSC to damaged tissue, but could also be thrombotic. Stem Cells2018;36:1062–1074 [ABSTRACT FROM AUTHOR]

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 γ-chain (FcR γ-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 PLCγ2. Significant developments have also taken place in regard to the role of the major adhesion receptor for collagen, the integrin α2β1 (also known as GPIa-IIa). An α2β1-selective collagen-based peptide has been developed and co-crystalisalised with the I-domain of the α2 subunit. Polymorphisms in α2 have been shown to cause wide variation in expression of α2β1, with the α2 allele T[sub 807]/A[sub 873] 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 α2β1 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 α2β1 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. [ABSTRACT FROM AUTHOR]