Erythroblastic island macrophages in normal erythropoiesis and disordered erythropoiesis in polycythemia vera - ABSTRACT Erythropoiesis occurs at the erythroblastic island where the central macrophage (EBI M) supports proliferation and maturation of the surrounding erythroid cells. Alterations in EBI M contribute to the disordered erythropoiesis in various hematological diseases including polycythemia vera (PV) which is characterized by erythrocytosis due to V617F mutation in JAK2 (JAK2V/F). In this application, we propose to define the mechanisms by which EBI M support normal erythropoiesis in Aim 1 and to define the mechanisms by which JAK2V/F in EBI M contributes to pathogenesis of PV in Aim 2. Our overall hypothesis is that EBI M support normal erythropoiesis via Epor-Stat5 signaling-mediated production of growth factors Igf1 and Scf as well as iron carrier transferrin and that JAK2V/F-induced constitutive activation of Epor signaling in the EBI M leads to upregulation of transferrin receptor CD71 which contributes to the hyperproliferation of EBI M along with increased production of Igf1 and Scf leading to overproduction of red blood cells that can be alleviated by interfering with CD71, Igf1-Igf1r axis or/and Scf-cKit axis. Our hypothesis is based on our findings that i) EBI M expressed EPOR; ii) EPOR+ M expressed Igf1 and Scf as well as iron carrier transferrin (Trf); iii) EPO stimulation increased the levels of Igf1, Scf and Trf by EBI M; iv) selective deletion of Epor in M led to anemia along with decreased levels of Igf1, Scf and Trf; v) selective deletion of Igf1 or Trf in M also led to anemia; vi) although knock-in of Jak2V/F in erythroid cell alone using Gypa-tdTomato-Cre or in M alone using CD169- tdTomato-Cre led to erythrocytosis in mice, knock-in of Jak2V/F in both erythroid cells and M were required to fully recapitulate characteristics of human PV; vii) knock-in of Jak2V/F in M led to increased numbers of BM EBI M and spleen red pulp M along with increased levels of transferrin receptor Trfc (CD71), Igf1 and Scf. In Aim 1, we will generate conditional knockout mice is which Epor, Stat5, Igf1, Kitl or Trf is selectively deleted in M and examine their effects on red cell parameters, formation of EBIs, cellular and molecular changes of both EBI M and erythroid cells both in vivo and in vitro. We will also examine the effects of knockdown of these genes in human EPOR+ M on human erythropoiesis using in vitro co-culture system. In Aim 2, we will examine the effects of JAK2 inhibitor on our PV mouse models in which Jak2V/F was expressed in . We will examine whether selective deletion of Trfc, Igf1 or/and Kitl in in mice can alleviate erythrocytosis of PV mice. We will also examine whether selective deletion of Igf1r and cKit (the receptor for Scf) in erythroid cells can alleviate erythrocytosis. We will use chemical inhibitors to interfere with Igf1-Igf1r axis or/and Scf-cKit axis to examine their effects on erythrocytosis. We will examine whether the changes we observed in mouse EBI are present in human BM EBI from PV patients. Findings from the proposed studies will provide novel insights into basic biology of erythropoiesis, have implications in understanding the mechanisms of anemia and erythrocytosis, help improving protocols for ex vivo red cell production and reveal novel therapeutic targets for the treatment of PV.
