Serotonylation Activates Platelets

Abstract

Platelets, small anuclear cellular fragments in the circulation, are essential to the clotting process and are implicated in vascular disorders such as thrombosis and atherosclerosis. Platelets adhere to sites of damaged vascular endothelia through a process involving von Willebrand factor (vWF). When stimulated, platelets also release granules that contain various proaggretory factors such as adenosine diphosphate, adenosine triphosphate, serotonin (5-HT), calcium, and multimeric forms of vWF. Platelets also have 5-HT receptors and 5-HT reuptake transporters (SERTs). Using a tryptophan hydroxylase 1 knockout mouse (Tph is the rate-limiting enzyme in 5-HT biosynthesis, and Tph1 is the peripheral isoform), Walther et al . analyzed the role of 5-HT in platelet aggregation and degranulation. Clotting times were prolonged in the Tph1 –/– mice, and the mice were protected from thrombotic vessel occlusion, which suggests a defect in platelet aggregation. Using pharmacological inhibitors of the plasma membrane SERT and the vesicular monoamine transporter, the authors demonstrated that cytosolic 5-HT was essential for effective aggregation of isolated platelets. Platelets treated with radioactive 5-HT showed a pool of unreleasable 5-HT in the aggregated platelets. Secretion of granule contents (vWF) was stimulated in permeabilized platelets from the Tph1 –/– mice by the addition of 5-HT, transglutaminase (TG), and skeletal muscle cytosol. Analysis of the proteins isolated from aggregated platelets showed multiple bands labeled with [ 14 C]5-HT, including the guanosine triphosphatase (GTPase) RhoA. In vitro, Rab4, which controls granule secretion in platelets, was also transamidated by purified liver TGs. Thus, the authors propose that 5-HT stimulates an increase in calcium through its activation of its G q -coupled receptor. The calcium stimulates the activity of TGs, which use 5-HT taken up by the SERT to transamidate GTPases, which renders them constitutively active, promoting changes in platelet shape (RhoA) and degranulation (Rab4). This ultimately produces irreversible platelet aggregation. D. J. Walther, J.-U. Peter, S. Winter, M. Höltje, N. Paulmann, M. Grohmann, J. Vowinckel, V. Alamo-Bethencourt, C. S. Wilhelm, G. Ahnert-Hilger, M. Bader, Serotonylation of small GTPases is a signal transduction pathway that triggers platelet α-granule release. Cell 115 , 851-862 (2003). [Online Journal]