Murine platelet production is suppressed by S1P release in the hematopoietic niche, not facilitated by blood S1P sensing.

The bioactive lipid mediator sphingosine 1-phosphate (S1P) was recently assigned critical roles in platelet biology: whereas S1P 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 S1P 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 deletion selectively in MKs. Moreover, S1P expression and signaling were both undetectable in mature MKs in situ, and MK deletion did not affect platelet aggregation or spreading. When 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 deletion and was reversed by compound deletion of either or , suggesting that this phenotype arises from increased S1P export and S1P activation secondary to redistribution of sphingosine to Sphk1. Consistent with clinical observations, we thus observe no essential role for S1P in facilitating platelet production or activation. Instead, S1P restricts megakaryopoiesis through S1P, and can further suppress thrombopoiesis through S1P when aberrantly secreted in the hematopoietic niche.