During human development, distinct anatomic areas for production of erythroid cells are recruited sequentially, in a temporal succession that allows overlap. In addition, parallel changes occur in the morphologic and functional properties of the erythroid cells themselves.

During the phase of embryonic erythropoiesis in the blood islands of yolk sac, aggregates of immature erythroid cells undergo maturation synchronously as a single cohort. Before their maturation is completed, they begin to circulate, and by gestational week 5 they are found in the vascular spaces of the rudimentary liver. At about the same time, foci of immature erythroid cells emerge within the fetal liver as the fetal (or hepatic) phase of erythropoiesis commences. From week 7 onward, the liver is progressively filled with erythroid precursors and becomes the dominant site of erythroid cell production until approximately gestational week 30. Although some red cell production can be found in the thymus, the spleen, or occasionally in the lymph nodes, these other sites are never dominant. However, recently placenta has been recognized as an important local erythropoietic site. From month 6 onward, the cavities of long bones are invaded by vascular sprouts and become competent to support red cell development. However, by contrast with the liver where erythropoiesis prevails, myelopoiesis is the dominant lineage that differentiates in the fetal bone marrow. Shortly after birth, all bone cavities become actively engaged in erythroid production, and the hepatic (fetal) phase of erythropoiesis comes to an end, as the final (adult) phase of erythropoiesis unfolds exclusively within the bone marrow. This neonatal switch of erythroid site is thought to be responsible for the transient anemia of the newborn and is controlled by microenvironmental cues.

The anatomic shifts in the sites of erythropoiesis are associated with shifts in the phenotypic characteristics of the erythroid cells. Embryonic (primitive) erythroid cells (derived from the yolk sac) are large (∼200 μm), circulate as nucleated cells, and have a megaloblastic appearance. These embryonic erythrocytes eventually undergo enucleation in the fetal liver at late stages of development, once significant numbers of macrophages start to mature in this organ and after acquisition of α4-integrin expression. As BFU-E present in yolk sac, fetal liver, and fetal bone marrow have a definitive-like progeny and these progenitors were not present after ablation of core binding factor (CBF)-β, despite the presence of normal embryonic erythropoiesis, the derivation of embryonic erythroblasts from a dis tinct and unique progenitor, not present in subsequent life, remains a viable hypothesis.

Fetal liver erythroid cells are smaller than embryonic cells (∼125 μm) but have a macrocytic appearance compared with adult normocytic red cells (∼80 μm). Like adult cells and in contrast to the great majority of primitive cells, fetal liver erythroid cells eject their nuclei during maturation before being released in the blood.

Apart from variations in size and morphology, embryonic and fetal erythroid cells differ from each other and from their adult counterparts in several other properties, including growth factor requirements, proliferative status, transplantation potential, and surface antigenic expression. For example, whereas fetal erythropoiesis is under the control of EPO, the extent of EPO’s influence on embryonic erythropoiesis is disputed. Most convincing are the results of EPO/EPOR knockout mice that showed only partial effects on embryonic erythropoiesis, in contrast to fetal erythropoiesis. Single lineage transcriptomic analyses indicate that TGF-β may represent a primary regulator of embryonic erythropoiesis. EPO levels increase between weeks 9 and 32 of gestation, and fetuses respond to hypoxia or anemia with increased EPO as early as 24 weeks. Fetal erythroid progenitors when studied in vitro appear more sensitive to EPO and KL than adult progenitors. In contrast, their in vitro response to IL-3 or GM-CSF is minimal compared to that of adult erythroid progenitors. In addition, fetal erythroid progenitors are characterized by a higher proliferative potential and shorter doubling times than adult cells when cultured in vitro. The dependency of stem/progenitor cells on KL changes during ontogeny. Although generation of repopulating stem cells is minimally affected during fetal life in mice that cannot produce KL (steel-Dickie (Sl/Sld) mutant mice) adult mice which produce only soluble KL display greatly impaired erythropoiesis and hematopoiesis, suggesting that the KL/KIT pathway plays a role in the recruitment and self-renewal behavior of adult but less so of fetal stem cells in vivo.

The homing properties of fetal stem cells transplanted into adult irradiated recipients were found to be inferior to those of their adult counterparts. Whether this finding is related to their increased cycling or other reasons is unclear. However, fetal liver stem cells, despite their reduced homing potential, have higher engraftment levels, likely because of their proliferative prowess compared to adult stem cells.

The surface antigenic profiles of erythroid progenitors and precur sors are distinct at each ontogenetic stage. For example, HLA class I and class II antigens are not detected in embryonic erythroid progenitor cells but reach adult levels at approximately gestational week 9. CD34+ hematopoietic progenitors present in yolk sac express Mac-1 but are negative for stem cell antigen 1 (Sca-1), which is expressed by fetal and adult CD34+ murine progenitor cells. On the other hand, adult CD34+ progenitors lack Mac-1 and AA4.1, which are expressed by fetal CD34+ progenitor cells. Fetal BFU-E and CFU-E express similar levels of HLA class II antigens, whereas adult CFU-E are largely devoid of these antigens. β1 integrins, especially α4 β1 and α5 , are expressed widely by all hematopoietic cells, including nucleated erythroid cells. However, in the latter, they display a differentiation dependent, developmentally segregated pattern of expression, because they are absent in embryonic murine erythroblasts, and among adult cells, stem/progenitor cells express them in a constitutively active form in contrast to more mature cells. Fetal red cells display a straight, unbranched polylactosaminyl chain (i antigen) on their surface, whereas in adult cells, this structure, which bears ABH blood group determinants, is highly branched (I antigen). The enzymatic activity of several enzymes in the glycolytic pathway is greater in fetal than in adult red cells. In contrast, carbonic anhydrase levels are very low during intrauterine and early neonatal life. Distinct isozyme patterns for several enzymes (i.e., phosphoglycerate kinase, acetylcholinesterase) also distinguish fetal from adult red cells.

The most widely studied changes during red cell ontogeny are the shifts or “switches” in globin types. Embryonic erythroblasts are characterized by their avid accumulation of iron, which is stored as ferritin (0.3% to 1% of total protein) and by the synthesis of the unique hemoglobins Gower I (ζ2 ε2 ), Gower II (α2 ε2 ), and Portland (ζ2 γ2 ). The ζ- and ε-globin chains are embryonic α-like and β-like chains, respectively. These three embryonic types of hemoglobin are most likely synthesized in succession, because the concentration of Gower I is highest in smaller embryos. Thus a switch from ζ- to α- and ε- to γ-globin gene production begins during the embryonic phase of erythropoiesis but is not complete until fetal erythropoiesis is well established. During the transition from yolk sac to fetal liver erythropoiesis (6 to 9 weeks), erythroid precursors within the fetal liver co-express embryonic (ζ- or ε-) and fetal (α- or γ-) globin. The predominant type of hemoglobin synthesized during fetal liver eryth ropoiesisis HbF (α2 γ2 ), with a high proportion of γG: γA (7:3). Adult HbA (α2 β2 ), which is detectable at the earliest stages of fetal liver erythropoiesis, is synthesized as a minor component throughout this period and HbA2 (α2 δ2 ), which is a minor hemoglobin in the adult, is undetectable in these early stages. From about gestational week 30 onward, β-globin synthesis steadily increases so that, by term, 50% to 55% of hemoglobin synthesized is HbA. By 4 to 5 weeks of postnatal age, 75% of the hemoglobin is HbA. This percentage increases to 95% by 4 months as the fetal-to-adult hemoglobin switch is completed. HbF levels in circulating red cells are at a plateau for the first 2 to 3 weeks (as a result of the decline in total erythropoiesis that follows birth), but HbF levels gradually decline so that normal levels (<1 % ) are achieved by 200 days after birth.

Several in vitro and in vivo approaches have been used to study the basis of globin switching through development. Beyond its bio logic interest, rigorous research in this area was propelled by the possibility of manipulating globin switching to increase HbF production in adults and ameliorate the clinical symptoms of disorders of the β-globin locus (e.g., sickle cell anemia, thalassemia). The most important determinant of fetal-to-adult hemoglobin switching seems to be post-conceptual age, with the sharpest period for transition between 30 and 52 weeks. The fetal-to-adult switch appears to be unaffected by the time at which birth occurs or by changes in the kinetics of erythropoiesis induced by perinatal hemolysis. A delay in switching usually is observed in cases of general developmental retardation, in patients with certain chromosomal abnormalities (e.g., trisomy 13), and in diabetic infants because of increased circulating levels of α-aminobutyric acid, which directly affects HbF synthesis, inspiring the use of butyrate for increasing HbF in vivo.

Transplantation experiments and ablative endocrine maneuvers in the sheep model and transplantation of human fetal liver cells to adult recipients have failed to provide convincing support for the effects of environmental cues on the switching process. In fact, integration of data from experiments in transgenic mice and in hybrid cells (produced by fusion of human with mouse cells), as well as by isolation of stage-specific transcription factors in other erythroid systems (e.g., avian) indicates that developmental control of globin switching is mostly intrinsic to erythroid cells.

Genetic linkage and GWAS in individuals with increased levels of HbF or subjects with HPFH syndromes have provided important insights concerning the control of fetal to adult globin switching. These studies have identified elements that control the switching both in cis (deletions in β-globin cluster or mutations in the γ-globin gene promoters, i.e., in DNA regions which provide hubs for transcription factors) and in trans (genes on other chromosomes that encode the transcription factors that bind the hubs that repress the fetal globin genes) of the β-globin locus. Transcriptional factors like BCL11A, ZBTB7A (an oncogene also known as LRF), and HBS1L-Myb are examples of HbF repressors. The full-length BCL11A is expressed in adult but not fetal erythroid cells. In addition to transcriptional control, the ontogenic-specific regulation of BCL11A expression is exerted at the transcriptional level by LIN28B that suppresses the translation of BCL11A mRNA by the ribosome machinery.128 Furthermore, a group of adult individuals with high hemoglobin F harbor loss-of-function BCL11A mutations that reduce its expression in adult cells. In addition, suppression of BCL11A expression reactivates HbF expression in adult erythroid cells in vitro, and deletion of BCL11A interferes with fetal hemoglobin silencing during development and rescues the phenotype of a mouse model of sickle cell disease. These observations suggest that genome editing of BCL11A in autologous stem cells followed by transplantation may represent a useful strategy to ameliorate the clinical picture of hemoglobinopa thies such as sickle cell disease. Genome editing approaches based on zinc fingers, TALENS, and Crisper/cas technology are rapidly being developed and allow precise targeting either of BCL11A or its erythroid-specific enhancer-binding sites in g-promoter. ZBTB7A is a C2H2 zinc finger that binds at the site −200 bp of the γ-globin promoter and has been shown to effectively and independently represses the human γ-globin gene.

Myb-HBS1L is downregulated in individuals with elevated HbF levels and overexpression of Myb inhibits γ-globin in human erythroleukemia cells. The levels of Myb were found to be controlled by micro-RNA 15a and 16-1 in patients with human trisomy 13 and high HbF levels. In addition, the observation that EKLF mutations are relatively more common in a thalassemia-endemic region where they are associated with milder β-thalassemia phenotypes support a role for EKLF, the major erythroid transcriptional regulator, as a suppressor of fetal globin gene. Valuable insights of the effect of EKLF on the regulation of globin genes were obtained from families with haploinsufficiency of EKLF (missense mutations affecting DNA binding) and increased fetal hemoglobin levels. However, this was not always the case, and a compound heterozygosity was required in other families for high HbF expression. It appears that EKLF directly targets genes, such as BCL11A, EPB4.9, and CD44, which are very sensitive to its activity, whereas its effects on other genes, like γ-globin, or BCAM (carrying the Lutheran blood group antigens) are variable. Collectively, these data indicate that EKLF affects the globin gene switching both directly, by activating β-globin, and indirectly, by suppressing γ-globin through its control of BCL11A. In addition to EKLF another zinc fin ger containing protein, pogo transposable element with zinc (POGZ finger domain), positively regulates BCL11A expression in mouse and human cells by binding to BCL11A promoter and enhancer elements. Beyond that, POGZ may be a direct EKLF target.

Beyond the above transcriptional factors influencing BCL11A expression there is a multilayer control through chromatin remodeling complexes governing histone acetylation/methylation and DNA methylation which dictates differences in fetal globin expression between fetal and adult cells. Any member of the epigenetic subunits of the NuRD silencing complex may be sufficient to repress HbF and any of these (i.e., CHD4, MBD2, DNMT1, DRED complex, PRMTs, etc.) can serve as a potentially druggable target for increasing fetal globin. Furthermore redirecting the LCR to the silenced gamma-promoter by targeted tethering of the Ldb1SA domain will increase fetal globin with a reciprocal decrease in beta globin. New highly innovative base-editing approaches to either correct the sickle mutation or possibly creating novel binding sites in fetal promoters may obviate potential genotoxicity issues associated with double strand DNA brakes, although they may not be entirely free of any off-target effects of their own.

Whereas all the above-cited approaches represent potential strategies to increase fetal globin for therapeutic purposes in severe β-thalassemia and sickle cell disease, the easy administration of a small molecule addressing any of the aforementioned targets may have a greater worldwide impact.

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استعدادًا لافتتاحه.. المشرف على قسم الشؤون الطبية يجري جولة في مركز السيد جعفر الحلي للخدمات الطبية.

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لصحة القلب وتفادي السرطان.. استهلك هذا النوع من الطعام

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اختراق واسع لمنصة "إنستغرام".. ونشر بيانات على "الدارك ويب"

علماء يحذرون: هكذا سيفنى كوكب الأرض

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مواضيع ذات صلة

Cellular Dynamics in Erythropoiesis: Steady State and Stress Erythropoiesis

Erythropoiesis and Iron Regulation

Hematopoietic Microenvironment

Intrinsic Control of Erythropoiesis:Erythropoietin

Nuclear Organization

Unfolded Protein Response

Autophagy

Nonapoptotic forms of cell death

The Cell Division Cycle

Stem Cells

Proliferation and the Cell Cycle

Growth Factors and Receptors