The Regulation of Hematopoietic Stem Cells and Erythropoiesis
Authors: Morrison Sean 
Published: October 2019

Abstract
Hematopoietic stresses mobilize hematopoietic stem cells (HSCs) from the bone marrow to the spleen and induce extramedullary hematopoiesis (EMH), including erythropoiesis. We assessed the sources of the key niche factors, SCF and CXCL12, in the mouse spleen after EMH induction by cyclophosphamide plus granulocyte colony-stimulating factor, blood loss, or pregnancy. In each case, Scf was expressed by endothelial cells and Tcf21+ stromal cells, primarily around sinusoids in the red pulp, while Cxcl12 was expressed by a subset of Tcf21+ stromal cells (Nature 527:466, 2015). EMH induction markedly expanded the Scf-expressing endothelial cells and stromal cells by inducing their proliferation. Most splenic HSCs were adjacent to Tcf21+ stromal cells in red pulp. Conditional deletion of Scf from spleen endothelial cells or Scf or Cxcl12 from Tcf21+ stromal cells severely reduced spleen EMH and reduced blood cell counts without affecting bone marrow hematopoiesis. Endothelial cells and Tcf21+ stromal cells thus create a perisinusoidal EMH niche in the spleen, which is necessary for the physiological response to diverse hematopoietic stresses.
During pregnancy, the blood volume increases relatively quickly, requiring increased erythropoiesis to maintain red blood cell counts. We discovered that HSCs in female mice divide significantly more frequently than in males as a result of estrogen/estrogen receptor signaling in HSCs (Nature 505:555). Estrogen levels increase during pregnancy, increasing HSC division, HSC frequency, cellularity, and erythropoiesis in the spleen. Conditional deletion of Estrogen Receptor α from HSCs reduced HSC division in female, but not male, mice and attenuated the increases in HSC division, HSC frequency, and erythropoiesis during pregnancy. Treatment of mice with estradiol induces HSC proliferation in the bone marrow but not mobilization. Treatment with the alternative ERα ligand, 27-hydroxycholesterol (27HC), induces ERα-dependent HSC mobilization and EMH but not HSC division in the bone marrow (JCI 127:3392). During pregnancy, 27HC levels increased in hematopoietic stem/progenitor cells as a result of Cyp27a1, a cholesterol hydroxylase. Cyp27a1-deficient mice have significantly reduced 27HC levels, HSC mobilization, and EMH during pregnancy. Two different ERα ligands, estradiol and 27HC, thus work together to promote EMH during pregnancy.
We have also examined the bone marrow to better understand the niche for erythropoiesis. HSCs in the bone marrow are maintained in a perivascular niche created by Leptin Receptor+ (LepR) stromal cells and endothelial cells. An important question is why LepR+ cells are one hundred times more frequent than HSCs. We discovered that SCF from LepR+ cells is also necessary for the maintenance of many c-kit+ restricted hematopoietic progenitors. Conditional deletion of Scf from LepR+ cells depleted CMPs, CLPs, GMPs, MEPs, PreMegEs, and CFU-Es, depleting myeloid and erythroid cells from the blood. This was not a consequence of HSC depletion as many other restricted progenitors were unaffected. Moreover, conditional deletion of Scf from endothelial cells depleted HSCs, but not restricted progenitors. Imaging showed that early erythroid progenitors were closely associated with perisinusoidal LepR+ cells. Our data reveal cellular specialization within the niche: SCF from LepR+ cells is broadly required by HSCs and c-kit+ restricted progenitors while SCF from endothelial cells is required only by HSCs. Erythroid progenitors depend upon SCF synthesized by LepR+ cells in perisinusoidal niches.

Disclosures
Morrison: OncoMed Pharmaceuticals: Equity Ownership; GI Therapeutics: Consultancy, Equity Ownership; Frequency Therapeutics: Consultancy.