Integrin α ₆ β ₁ Is the Main Receptor for Vascular Laminins and Plays a Role in Platelet Adhesion, Activation, and Arterial Thrombosis

Abstract

Background-Laminins are major components of basement membranes, well located to interact with platelets upon vascular injury. Laminin-111 (α 1 β 1 γ 1 ) is known to support platelet adhesion but is absent from most blood vessels, which contain isoforms with the α 2 , α 4 , or α 5 chain. Whether vascular laminins support platelet adhesion and activation and the significance of these interactions in hemostasis and thrombosis remain unknown. Methods and Results-Using an in vitro flow assay, we show that laminin-411 (α 4 β 1 γ 1 ), laminin-511 (α 5 β 1 γ 1 ), and laminin-521 (α 5 β 2 γ 1 ), but not laminin-211 (α 2 β 1 γ 1 ), allow efficient platelet adhesion and activation across a wide range of arterial wall shear rates. Adhesion was critically dependent on integrin α 6 β 1 and the glycoprotein Ib-IX complex, which binds to plasmatic von Willebrand factor adsorbed on laminins. Glycoprotein VI did not participate in the adhesive process but mediated platelet activation induced by α 5 -containing laminins. To address the significance of platelet/laminin interactions in vivo, we developed a platelet-specific knockout of integrin α 6 . Platelets from these mice failed to adhere to laminin-411, laminin-511, and laminin-521 but responded normally to a series of agonists. α 6 β 1 -Deficient mice presented a marked decrease in arterial thrombosis in 3 models of injury of the carotid, aorta, and mesenteric arterioles. The tail bleeding time and blood loss remained unaltered, indicating normal hemostasis. Conclusions-This study reveals an unsuspected important contribution of laminins to thrombus formation in vivo andsuggests that targeting their main receptor, integrin α 6 β 1 , could represent an alternative antithrombotic strategy with a potentially low bleeding risk. (Circulation. 2013;128:541-552.)Key Words: blood platelets ◼ integrin α 6 β 1 ◼ laminin ◼ thrombosis

Trafficking receptor signatures define blood plasmablasts responding to tissue-specific immune challenge

“…α6β1 is a receptor for laminins (44), extracellular matrix components comprised of multiple isoforms whose expression varies in different tissues (45). α6 binding to laminins is thought to play an important role in platelet-mediated thrombosis (46). The role of α6 in lymphocyte trafficking, if any, remains to be determined; we have not seen alterations in lymphocyte homing to lymph nodes or Peyer's patches with an anti-α6 antibody that blocks laminin binding (Eugene C. Butcher, unpublished observations).…”

Proplatelet formation is selectively inhibited by collagen type I through Syk-independent GPVI signaling

“…1E,G). These isoforms were able to induce weak platelet aggregation in light transmission aggregometry (data not shown) and platelet adhesion (Schaff et al, 2013) indicating that they are, in principle, functional.…”

Platelet integrin α6β1 controls lung metastasis through direct binding to cancer cell–derived ADAM9

“…Moreover, we showed that transfusion of WT, but not α6-deficient, platelets partially restored metastasis in both thrombocytopenic and PF4-Cre-α6 -/-mice, confirming that the protective effect is platelet specific and does not rely on the absence of α6β1 integrin on any other cell type. In addition, we previously reported that platelets from PF4-Cre-α6 -/-mice expressed normal levels of the main adhesion receptors (21), further pointing towards a specific role for α6β1. Blood platelets express two additional members of the β1 integrin family, namely α2β1 and α5β1.…”

“…Of note, this blocking antibody did not provoke a thrombocytopenia, suggesting that such a pharmacological approach would preserve a normal platelet count. In addition, since platelet integrin α6β1 appears dispensable for normal hemostasis (21), one could hypothesize that targeting this integrin would also avoid bleeding complications. This is of importance since currently used antiplatelet drugs are known to increase the bleeding risk that complicates their use in clinical settings (50)(51)(52)(53).…”

“…Platelets also express other integrins, such as αvβ3 and 3 β1-containing integrins, namely α2β1, α5β1, and α6β1. Using mice with a genetic deletion of α6β1 in platelets, we previously reported that this integrin supports platelet adhesion and activation of vascular laminins and promotes experimental thrombus formation, while playing no major role in hemostatic functions (21). To date, the role of the platelet β1 integrins, notably α6β1, in physical and functional interactions with tumor cells and in metastatic dissemination is completely unknown.…”