Abstract 3733: Platelet Serotonin Receptor Stimulation Induces Shedding of the Adhesion Molecule GPIbα
Objectives Serotonin (5-hydroxytryptamine, 5HT) is not only a neurotransmitter involved in a variety of central nervous functions, but is also a peripheral hormone that is transported by platelets. Upon stimulation, platelets can release 5HT to modulate vasotone and amplify the activation of other platelets. In platelet cytoplasm, 5HT is transamidated to small GTPases leading to α-granule release, which supports primary hemostasis. We hypothesized that 5HT could also elicit other platelet functions after stimulation of the platelet membrane 5HT receptor (5HT2AR). Methods and Results After blocking the transport into mouse platelets by the selective serotonin reuptake inhibitor fluoxetine (Flx), 5HT and the specific 5HT2AR agonist DOI induced shedding of the extracellular domain of glycoprotein (GP) Ibαin a time- and dose-dependent manner (in 60 min, 20 μM Flx + 50 μM DOI decreased GPIbα by > 50% in flow cytometry, p < 0.001). The GPIbα loss was in fact due to cleavage and not internalization since we detected a considerable increase of the shed fragment glycocalicin in the supernatant by Western blot. We found the shedding to be mediated by TNFα-converting enzyme (TACE) because co-incubation with the specific TACE inhibitor TAPI-1 completely abolished the effect. Transamidation of intracellular 5HT, on the other hand, was not involved because 5HT-deficient platelets from tryptophan hydroxylase 1 knockout mice showed the same response as wild-type platelets. In flow chamber experiments, 5HT2AR-stimulated platelets in whole blood showed a significantly decreased adhesion (< 50%, p < 0.001) to collagen-bound von Willebrand factor under arterial shear rates (1500/s). In contrast, the stimulation of 5HT2AR did not change surface expression of the α-granule protein P-selectin or the activated form of integrin αIIbβ3 and did not impair ADP-induced aggregation.
Conclusion Specific stimulation of 5HT2AR induces shedding of GPIbα, the key adhesion molecule under arterial shear conditions. This mechanism might play a role in down-regulating the adhesive properties of platelets. Moreover, we suggest that this mechanism should be considered when discussing cardiovascular effects of selective serotonin reuptake inhibitors.