Thrombopoietic cells and the bone marrow vascular niche.

Megakaryocytes and platelets have been known to secrete angiogenic growth factors for a long time. However, there is little in vivo data on the regulation of angiogenesis by thrombopoietic cells. Both megakaryocytes and platelets are known to carry and release a multitude of both pro- and antiangiogenic mediators. Thus, it remained unknown how the "angiogenic phenotype" of thrombopoietic cells would be determined. Our group established that platelets contribute to angiogenesis as carriers of SDF-1, which is released by platelets in response to stimulation with hematopoietic cytokines. Indeed, even the action of VEGF-A seems to be mediated in part by the release of SDF-1 from stimulated platelets, thereby attracting proangiogenic hematopoietic cells. Moreover, the analysis of murine plasma and serum showed that similar to VEGF-A, SDF-1 is almost exclusively derived from platelets, and only trace amounts are detectable in platelet poor plasma. Because tumor patients' platelets have been shown to contain lower amounts of thrombospondin (Tsp), we generated Tsp-1 and Tsp-2 double knockout mice by crossing the single knockout lines. Interestingly, megakaryocytes and platelets derived from these mice confer a proangiogenic phenotype both in the bone marrow and in reperfusion of ischemic hindlimbs, thereby verifying the hypothesis of pro- and antiangiogenic platelet constituents "in balance."