sphingosine-1-phosphate and Thrombocytopenia

Topics: Animals; Brain; Brain Ischemia; Fingolimod Hydrochloride; Hemoglobins; Hemorrhage; Lymphocytes; Lysophospholipids; Male; Mice; Mice, Inbred C57BL; Mice, Transgenic; Myeloid Cells; Reperfusion Injury; RNA, Messenger; Sphingosine; Sphingosine 1 Phosphate Receptor Modulators; Sphingosine-1-Phosphate Receptors; Stroke; T-Lymphocytes; Thrombocytopenia; Up-Regulation

Millions of platelets are produced each hour by bone marrow (BM) megakaryocytes (MKs). MKs extend transendothelial proplatelet (PP) extensions into BM sinusoids and shed new platelets into the blood. The mechanisms that control platelet generation remain incompletely understood. Using conditional mutants and intravital multiphoton microscopy, we show here that the lipid mediator sphingosine 1-phosphate (S1P) serves as a critical directional cue guiding the elongation of megakaryocytic PP extensions from the interstitium into BM sinusoids and triggering the subsequent shedding of PPs into the blood. Correspondingly, mice lacking the S1P receptor S1pr1 develop severe thrombocytopenia caused by both formation of aberrant extravascular PPs and defective intravascular PP shedding. In contrast, activation of S1pr1 signaling leads to the prompt release of new platelets into the circulating blood. Collectively, our findings uncover a novel function of the S1P-S1pr1 axis as master regulator of efficient thrombopoiesis and might raise new therapeutic options for patients with thrombocytopenia.

Topics: Animals; Blood Platelets; Blotting, Western; Cell Surface Extensions; Cells, Cultured; Flow Cytometry; Lysophospholipids; Megakaryocytes; Mice; Mice, Inbred C57BL; Microscopy, Electron, Transmission; Receptors, Lysosphingolipid; Signal Transduction; Sphingosine; Sphingosine-1-Phosphate Receptors; Statistics, Nonparametric; Thrombocytopenia; Thrombopoiesis

Sphingosine 1-phosphate (S1P), an abundant lipid mediator in plasma, regulates vascular and immune cells by activating S1P receptors. In this report, we investigated the mechanisms by which high plasma S1P levels are maintained in mice. We found that plasma S1P turns over rapidly with a half-life of approximately 15 minutes, suggesting the existence of a high-capacity biosynthetic source(s). Transplantation of bone marrow from wild-type to Sphk1(-/-)Sphk2(+/-) mice restored plasma S1P levels, suggesting that hematopoietic cells are capable of secreting S1P into plasma. However, plasma S1P levels were not appreciably altered in mice that were thrombocytopenic, anemic, or leukopenic. Surprisingly, reconstitution of Sphk1(-/-)Sphk2(+/-) bone marrow cells into wild-type hosts failed to reduce plasma S1P, suggesting the existence of an additional, nonhematopoietic source for plasma S1P. Adenoviral expression of Sphk1 in the liver of Sphk1(-/-) mice restored plasma S1P levels. In vitro, vascular endothelial cells, but not hepatocytes, secreted S1P in a constitutive manner. Interestingly, laminar shear stress downregulated the expression of S1P lyase (Sgpl) and S1P phosphatase-1 (Sgpp1) while concomitantly stimulating S1P release from endothelial cells in vitro. Modulation of expression of endothelial S1P lyase with small interfering RNA and adenoviral expression altered S1P secretion, suggesting an important role played by this enzyme. These data suggest that the vascular endothelium, in addition to the hematopoietic system, is a major contributor of plasma S1P.

Topics: Adenoviridae; Aldehyde-Lyases; Anemia; Animals; Antibodies, Monoclonal; Bone Marrow Cells; Bone Marrow Transplantation; Cell Line; Cell Line, Tumor; Cells, Cultured; Disease Models, Animal; Endothelial Cells; Endothelium, Vascular; Genetic Vectors; Half-Life; Humans; Leukopenia; Liver; Lysophospholipids; Membrane Proteins; Mice; Mice, Inbred C57BL; Mice, Knockout; Phenylhydrazines; Phosphoric Monoester Hydrolases; Phosphotransferases (Alcohol Group Acceptor); Platelet Glycoprotein GPIb-IX Complex; RNA Interference; RNA, Small Interfering; Sphingosine; Stress, Mechanical; Thrombocytopenia; Time Factors; Transduction, Genetic; Whole-Body Irradiation

Topics: Aldehyde-Lyases; Anemia; Animals; Antibodies, Monoclonal; Bone Marrow Cells; Disease Models, Animal; Endothelial Cells; Endothelium, Vascular; Half-Life; Humans; Leukopenia; Liver; Lysophospholipids; Membrane Proteins; Phenylhydrazines; Phosphoric Monoester Hydrolases; Phosphotransferases (Alcohol Group Acceptor); Platelet Glycoprotein GPIb-IX Complex; Research Design; Signal Transduction; Sphingosine; Stress, Mechanical; Thrombocytopenia; Time Factors