“Maturational” globin switching in primary primitive erythroid cells

Abstract

Mammals have 2 distinct erythroid lineages. The primitive erythroid lineage originates in the yolk sac and generates a cohort of large erythroblasts that terminally differentiate in the bloodstream. The definitive erythroid lineage generates smaller enucleated erythrocytes that become the predominant cell in fetal and postnatal circulation. These lineages also have distinct globin expression patterns. Our studies in primary murine primitive erythroid cells indicate that ␤H1 is the predominant ␤-globin transcript in the early yolk sac. Thus, unlike the human, murine ␤-globin genes are not up-regulated in the order of their chromosomal arrangement. As primitive erythroblasts mature from proerythroblasts to reticulocytes, they undergo a ␤H1-to ⑀y-globin switch, up-regulate adult ␤1-and ␤2-globins, and down-regulate -globin. These changes in transcript levels correlate with changes in RNA polymerase II density at their promoters and transcribed regions. Furthermore, the ⑀y-and ␤H1-globin genes in primitive erythroblasts reside within a single large hyperacetylated domain. These data suggest that this "maturational" ␤H1-to ⑀y-globin switch is dynamically regulated at the transcriptional level. Globin switching during ontogeny is due not only to the sequential appearance of primitive and definitive lineages but also to changes in globin expression as primitive erythroblasts mature in the bloodstream. IntroductionThe first blood cells to circulate during mammalian embryogenesis consist of large primitive red cells. In the mouse embryo, primitive erythroid cells are generated from a transient wave of committed progenitors found in the yolk sac between embryonic day (E) 7.25 and E9.0 of gestation. 1,2 Primitive proerythroblasts begin to emerge from blood islands beginning at E8.25, and enter the newly forming bloodstream, 3 where they undergo terminal maturation. They transition as a semisynchronous cohort of cells from proerythroblasts to enucleated erythrocytes progressively undergoing a loss of proliferative capacity, a decrease in cell size, accumulation of hemoglobin, and nuclear condensation. [4][5][6] Ultimately, they enucleate by E16.5 to become primitive erythrocytes that can circulate for several days after birth. 7 After E11.5, primitive red cells are joined by increasing numbers of smaller definitive erythrocytes that are released from the fetal liver after enucleating. Over the ensuing 5 days, definitive erythrocytes become the predominant cell type in the fetal circulation and ultimately become the exclusive red-cell lineage in the adult.The sine qua non of red cells is their accumulation of large amounts of hemoglobin composed of globin tetramers encoded by the ␣-and ␤-globin gene loci. Examination of globin expression in circulating human and mouse blood cells indicated that 5Ј members of both globin loci are expressed in the embryo, while 3Ј members are expressed in the adult, leading to the concept of globin switching during ontogeny. 8 Since this switch in globin expression coincided temporally with t...

Primitive and definitive erythropoiesis in the yolk sac: a birds eye view

“…3H). Although currently without direct evidence in the context of chicken yolk sac definitive erythropoiesis, similar hypotheses with regard to differentiation plasticity and potentials have been proposed in other systems (Dzierzak, 2002;Graf, 2002;Kingsley et al, 2006;McNagny and Graf, 2003;Orkin and Zon, 2002;Prindull and Fibach, 2007).…”

Maturation and enucleation of primitive erythroblasts during mouse embryogenesis is accompanied by changes in cell-surface antigen expression

“…Expression of ␤h1-globin was barely detectable and the adult ␤maj-globin gene was up-regulated in GFP ϩ cells by this stage of development in comparison with E10.5 blood cells, both as expected. 5 Thus, "maturational" globin gene switching 5 is seen in the GFP ϩ cells. These findings establish the ⑀-globin::KGFP transgenic mouse line as a valuable tool for investigating the differentiation of EryPs.…”

“…Cells of the 2 erythroid lineages differ in size (EryPs are larger than EryDs) and express distinct sets of ␣-and ␤-like globin genes (embryonic/fetal in EryP, adult in EryD). [1][2][3][4][5] It had long been accepted that a key distinguishing feature of primitive and definitive erythroid cells was the presence or absence of a nucleus: circulating EryPs retain their nuclei, whereas EryDs enucleate in the fetal liver or adult bone marrow, prior to entering the bloodstream. 1 Recently, however, it was discovered that primitive erythroblasts also enucleate, beginning around E12.5 in the mouse embryo and continuing throughout gestation.…”

“…7 In the yolk sac, erythroid and endothelial cells arise from mesodermal clusters to form morphologically identifiable "blood islands" 8 late in gastrulation (around E7.5). 5,[9][10][11][12] These lineages were proposed to arise from a common progenitor, the hemangioblast, based on their close temporal and spatial association in the blood islands and on their shared expression of a number of genes. [13][14][15] Some of these genes are required for normal development of both hematopoietic and endothelial cells.…”

Chromatin-binding in vivo of the erythroid kruppel-like factor, EKLF, in the murine globin loci

“…The murine β-like globin gene family consists of four functional gene members arranged and expressed during development in the order of 5'-εy-βH1-βmaj-βmin-3' [5]. In the embryonic yolk sac, higher levels of εy and βH1 are expressed.…”