SDF-1 dynamically mediates megakaryocyte niche occupancy and thrombopoiesis at steady state and following radiation injury

Abstract

• SDF-1 acutely affects megakaryocyte spatial distribution in the bone marrow at steady state and in the setting of radiation injury.• SDF-1-directed localization of megakaryocytes into the vascular niche increases platelet output.Megakaryocyte (MK) development in the bone marrow progresses spatially from the endosteal niche, which promotes MK progenitor proliferation, to the sinusoidal vascular niche, the site of terminal maturation and thrombopoiesis. The chemokine stromal cellderived factor-1 (SDF-1), signaling through CXCR4, is implicated in the maturational chemotaxis of MKs toward sinusoidal vessels. Here, we demonstrate that both IV administration of SDF-1 and stabilization of endogenous SDF-1 acutely increase MKvasculature association and thrombopoiesis with no change in MK number. In the setting of radiation injury, we find dynamic fluctuations in marrow SDF-1 distribution that spatially and temporally correlate with variations in MK niche occupancy. Stabilization of altered SDF-1 gradients directly affects MK location. Importantly, these SDF-1-mediated changes have functional consequences for platelet production, as the movement of MKs away from the vasculature decreases circulating platelets, while MK association with the vasculature increases circulating platelets. Finally, we demonstrate that manipulation of SDF-1 gradients can improve radiation-induced thrombocytopenia in a manner additive with earlier TPO treatment. Taken together, our data support the concept that SDF-1 regulates the spatial distribution of MKs in the marrow and consequently circulating platelet numbers. This knowledge of the microenvironmental regulation of the MK lineage could lead to improved therapeutic strategies for thrombocytopenia. (Blood. 2014;124(2):277-286) Introduction Platelet-producing megakaryocytes (MKs) are derived from megakaryocyte progenitors (MKPs), which are defined functionally by their capacity to form colonies in vitro. 1,2 MKPs are thought to reside near the bone surface in an "endosteal niche," where environmental cues encourage expansion, but suppress terminal maturation.3-6 Polyploid MKs mature cytoplasmically, extrude proplatelets in association with sinusoidal vasculature, and shed platelets into the peripheral blood. [7][8][9] This process results in past-maturity "exhausted" MKs comprised of a nucleus with a thin layer of cytoplasm surrounded by a cell membrane. 10,11 Megakaryopoiesis is primarily regulated by the cytokine thrombopoietin (TPO), which signals through its receptor Mpl to promote MKP proliferation and MK maturation. [12][13][14] Although the physical association of MKs with sinusoidal vasculature was first appreciated several decades ago, 15-17 the functional significance of the "vascular niche" for MK maturation and thrombopoiesis has only more recently begun to be elucidated. 4,[18][19][20][21] Several studies have implicated the chemokine stromal cellderived factor-1 ([SDF-1] or CXCL12) signaling through receptor CXCR4 in the maturational localization of MKs to the vascular nic...

Understanding platelet generation from megakaryocytes: implications for in vitro–derived platelets

“…The SDF-1/CXC chemokine receptor type 4 signaling axis has been shown to be involved in the migration and location of megakaryocytes to appropriate bone marrow niche locations for platelet release. [88][89][90] A gradient of sphingosine-1-phosphate in the bone marrow vasculature has been shown to guide proplatelet processes for unidirectional shedding of platelets in the circulation. 91,92 Platelet generation may also be possible without PPF megakaryocytes and triggered from mature megakaryocytes utilizing the newly described IL-1a rupture method.…”

Utilization of imaging flow cytometry to define intermediates of megakaryopoiesis in vivo and in vitro

“…We have found in our studies of both megakaryopoiesis and erythropoiesis using imaging flow cytometry, that once a template is established, it can be applied with very little refinement to our control tissue, in this case bone marrow, in each experiment. Then by applying this template to the experimental samples, robust quantitation can be achieved, as we have shown for changes in erythropoiesis and megakaryopoiesis after radiation and drug treatments, or during embryonic development (Malik et al, 2013; McGrath et al, 2008a,b; Niswander et al, 2014a,b; Peslak et al, 2011, 2012). …”

Role of thrombocytopenia in radiation-induced mortality and review of therapeutic approaches targeting platelet regeneration after radiation exposure

“…1), and their presence (or absence) plays an important role in humans exposed to lethal doses of radiation. Megakaryocyte progenitor cells have varying sensitivity to radiation, depending on their state of maturation [4][5][6][7]. The duration and nadir of thrombocytopenia is radiation dose-dependent, and recovery requires the regeneration of bone marrow-derived megakaryocyte progenitors.…”