The bioactive lipid mediator sphingosine 1-phosphate (S1P) was recently assigned critical roles in platelet biology: whereas S1P1 receptor-mediated S1P gradient sensing was reported to be essential for directing proplatelet extensions from megakaryocytes (MKs) toward bone marrow sinusoids, MK sphingosine kinase 2 (Sphk2)–derived S1P was reported to further promote platelet shedding through receptor-independent intracellular actions, and platelet aggregation through S1P1. Yet clinical use of S1P pathway modulators including fingolimod has not been associated with risk of bleeding or thrombosis. We therefore revisited the role of S1P in platelet biology in mice. Surprisingly, no reduction in platelet counts was observed when the vascular S1P gradient was ablated by impairing S1P provision to plasma or S1P degradation in interstitial fluids, nor when gradient sensing was impaired by S1pr1 deletion selectively in MKs. Moreover, S1P1 expression and signaling were both undetectable in mature MKs in situ, and MK S1pr1 deletion did not affect platelet aggregation or spreading. When S1pr1 deletion was induced in hematopoietic progenitor cells, platelet counts were instead significantly elevated. Isolated global Sphk2 deficiency was associated with thrombocytopenia, but this was not replicated by MK-restricted Sphk2 deletion and was reversed by compound deletion of either Sphk1 or S1pr2, suggesting that this phenotype arises from increased S1P export and S1P2 activation secondary to redistribution of sphingosine to Sphk1. Consistent with clinical observations, we thus observe no essential role for S1P1 in facilitating platelet production or activation. Instead, S1P restricts megakaryopoiesis through S1P1, and can further suppress thrombopoiesis through S1P2 when aberrantly secreted in the hematopoietic niche.

Ticagrelor, an antiplatelet adenosine diphosphate (ADP)-P2Y 12 receptor antagonist, increases the risk of bleeding. Its management is challenging because platelet transfusion is ineffective and no specific antidote is currently available. Epinephrine, a vasopressor catecholamine prescribed during shock, restores platelet functions inhibited by ticagrelor through stimulation of α 2A -adrenoreceptors. It subsequently inhibits cyclic adenosine monophosphate (cAMP) pathway and PI3K signaling. However, since epinephrine may expose a patient to deleterious hemodynamic effects, we hypothesized that other α 2 -adrenoreceptor agonist drugs used in clinical practice with fewer side effects could reverse the antiplatelet effects of ticagrelor. We compared in vitro the efficacy of clonidine, dexmedetomidine, brimonidine, and norepinephrine with epinephrine to restore ADP-and PAR-1-AP-induced washed platelet aggregation inhibited by ticagrelor, as well as resulting platelet cAMP levels. In ticagrelor-free samples, none of the α 2 -adrenoreceptor agonists induced aggregation by itself but all of them potentiated ADP-induced aggregation. Compared with epinephrine, norepinephrine, and brimonidine partially restored ADP-and fully restored PAR-1-AP-induced aggregation inhibited by ticagrelor while clonidine and dexmedetomidine were ineffective. Indeed, this lack of effect resulted from a lower decrease in cAMP concentration elicited by these partial α 2 -adrenoreceptor agonists, clonidine, and dexmedetomidine, compared with full α 2 -agonists. Our results support the development of specific full and systemic α 2 -adrenoreceptor agonists for ticagrelor reversal.2 of 11 acid, and desmopressin are unlikely to reverse ticagrelor antiplatelet effects [2][3][4][5][6][7], and no specific antidote is currently available [8,9]. We recently showed that epinephrine, a vasopressor agent used to treat shock, is able in vitro to restore platelet functions inhibited by ticagrelor. Epinephrine acts through stimulation of α 2A -adrenoreceptors coupled with G z -protein on the platelet membrane surface. The α subunit of G z -protein binds to adenylate cyclase and inhibits the synthesis of the second messenger cyclic adenosine monophosphate (cAMP), which plays a central role in platelet inhibition, while the dissociated βγ subunit activates phosphoinositide 3-kinase (PI3K) pathway [10,11]. Therefore, α 2A -adrenoreceptor stimulation participates in platelet activation. In combination with ADP stimulating the P2Y 1 receptor signaling and subsequent Ca 2+ mobilization, epinephrine induces aggregation of ticagrelor-treated platelets through inhibition of the cAMP pathway and activation of the PI3K pathway. However, epinephrine is a catecholamine and cumulates α 1 -, α 2 -, β 1 -, and β 2adrenoreceptor agonist effects that induce hemodynamic changes including tachycardia, heart rhythm disorders, peripheral arterial vasoconstriction, and lactic acidosis, which are potentially deleterious in the context of acute bleeding. We therefore hypothesized that ot...