Layilin inhibits integrin activation, and its loss results in platelet hyperactivation via Rac1 in inflammatory bowel disease

ABSTRACT: Platelets are specialized cells for hemostasis which circulate in close contact to the glycocalyx, an extracellular layer of interwoven glycoproteins, proteoglycans, and glycosaminoglycans that maintain vascular homeostasis. Platelets survey their circulating environment, balancing inhibitory signals that prevent inappropriate activation with activating signals that initiate thrombus formation. Disease can disrupt this delicate balance of endogenous inhibitory signaling, leading to an increased risk of thrombosis as in patients with inflammatory bowel disease (IBD). In this study, we demonstrate that physiological concentrations of hyaluronan (HA), an essential component of the glycocalyx, acts as an inhibitor of activation and aggregation in human platelets. Using a combination of affinity chromatography and functional assays of platelets from humans and genetically modified mice, we identify layilin as the receptor for HA and an endogenous inhibitor of platelet activation. Platelets from layilin knockout mice display agonist-induced hyperactivation of αIIbβ3 and increased adhesion to fibrinogen under venous shear. Loss of layilin results in dysregulation of Rho guanosine triphosphatase (GTPase) family members (RAC1, Cdc42, RhoA, and Ras-like Rap1) via layilin's binding partner, merlin, and downstream p21-activated kinase 1. Furthermore, platelets of patients with IBD contain reduced layilin protein levels correlating with heightened basal Rac1-guanosine triphosphate levels and increased reactivity. Finally, although IBD platelets have enhanced sensitivity to activation, pharmacologic inhibition of RAC1 effectively reduces platelet hyperactivity in platelet of patients with IBD. These findings highlight a novel role for layilin and HA in the maintenance of platelet homeostasis that becomes disrupted in patients with IBD.