Inhibition of erythropoiesis by Smad6 in human cord blood hematopoietic stem cells
Bone morphogenetic proteins (BMPs) that belong to the transforming growth factor-β (TGF-β) superfamily cytokines, play crucial roles in hematopoiesis. However, roles of Smad6 in hematopoiesis remained unknown in contrast to the other inhibitory Smad (I-Smad), Smad7. Here we show that Smad6 inhibits erythropoiesis in human CD34(+) cord blood hematopoietic stem cells (HSCs). Smad6 was specifically expressed in CD34(+) cord blood HSCs, which was correlated with the expression of BMP2/4/6/7 and BMP type I receptor (BMPRI). BMP-specific receptor-regulated Smads (R-Smads), Smad1 and Smad5 in cooperation with Smad4 induced transcription of the Smad6 gene. Instead of affecting cell cycle, apoptosis, self-renewal, and stemness of CD34(+) cells, Smad6 knockdown enhanced, whereas Smad6 overexpression suppressed erythropoiesis in stem cell culture and colony formation assay. Consistently, Smad6 suppressed the expression of the genes essential for erythropoiesis, such as Kruppel-like factor 1 (erythroid) (KLF1/EKLF) and GATA binding protein 2 (GATA-2). Promoter analyses showed that Smad6 repressed Smad5/4-induced transcription of the Klf1 gene. Thus, our data suggest that Smad6 indirectly maintains stemness by preventing spontaneous erythropoiesis in HSCs.
Available from: James Bieker
- "In this context, and given the importance of EKLF in controlling a number of steps in the red cell heme biosynthetic pathway (Tallack et al., 2010) ( particularly the transferrin receptor and mitoferrin), it is tempting to speculate that EKLF may also play an analogous regulatory role for iron transfer in the macrophage. EKLF expression is regulated by the BMP4 pathway during early mammalian development, particularly via Smad5 (Adelman et al., 2002; Kang et al., 2012; Lohmann and Bieker, 2008). This same pathway plays a significant role in stress erythropoiesis (Porayette and Paulson, 2008). "
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ABSTRACT: KLF1 is an erythroid specific transcription factor that is involved in erythroid lineage commitment, globin switching and terminal red blood cell maturation. Various mutations of KLF1 have been identified in humans, which have led to both benign and pathological phenotypes. The E325K mutation, within the second zinc finger of the KLF1 gene, has been shown to cause a new form of congenital dyserythropoietic anemia (CDA) now labeled as CDA type IV. We report the fourth documented case of this mutation, and propose a clinical diagnostic model to better identify this disease in other patients. Our patient is a Taiwanese child who presented to us at 8years of age with severe hemolytic anemia, splenomegaly, elevated fetal hemoglobin (HbF), iron overload, and dyserythropoiesis in the bone marrow. KLF1 sequence analysis revealed a G-to-A transition in one allele of exon 3, which resulted in the substitution of a glutamate 325 by a lysine. Flow cytometry analysis revealed decreased protein expression of CD44 on the red blood cells, and decreased red blood cell deformability as measured using an ektacytometer. Blood typing revealed his red blood cells to be Co(a-b-), In(b-), LW(ab-) and Lu(b+), even though DNA testing predicted that he would be Co(a+b-) and LW(a+b-). This newly discovered CDA combines features of a hemoglobinopathy, RBC membrane defect and hereditary persistence of HbF (HPFH) which are not seen in the previous types of CDA. Increased awareness of this phenotype may improve the more prompt and accurate diagnosis of these patients.
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ABSTRACT: Erythroid Krüppel-like factor (EKLF or KLF1) is a transcriptional regulator that plays a critical role in lineage-restricted
control of gene expression. KLF1 expression and activity are tightly controlled in a temporal and differentiation stage-specific
manner. The mechanisms by which KLF1 is regulated encompass a range of biological processes, including control of KLF1 RNA
transcription, protein stability, localization, and posttranslational modifications. Intact KLF1 regulation is essential to
correctly regulate erythroid function by gene transcription and to maintain hematopoietic lineage homeostasis by ensuring
a proper balance of erythroid/megakaryocytic differentiation. In turn, KLF1 regulates erythroid biology by a wide variety
of mechanisms, including gene activation and repression by regulation of chromatin configuration, transcriptional initiation
and elongation, and localization of gene loci to transcription factories in the nucleus. An extensive series of biochemical,
molecular, and genetic analyses has uncovered some of the secrets of its success, and recent studies are highlighted here.
These reveal a multilayered set of control mechanisms that enable efficient and specific integration of transcriptional and
epigenetic controls and that pave the way for proper lineage commitment and differentiation.
Available from: Camelia Rubin
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ABSTRACT: Red cell production is primarily determined by the action of erythropoietin. Additional erythropoiesis-regulatory factors include molecules and cellular interactions occurring within the bone marrow (BM) microenvironment. Sotatercept (ACE-011) is an activin receptor ligand trap which binds several members of the TGF-β superfamily. Treatment with ACE-011 reverses bone loss and reduces the degree of osteoporosis. Surprisingly, this was accompanied by elevated hemoglobin and hematocrit levels. The mechanisms underlying the beneficial effects of ACE-011 on red cell production remain unknown. This study explores the means by which ACE-011 promotes erythropoiesis. We showed that ACE-011 does not directly affect erythroid differentiation of human CD34(+) cells in vitro. We next tested whether ACE-011 acts indirectly by affecting BM accessory cells. Conditioned media (CM) produced by BM stromal cells (SC) inhibited erythroid differentiation of CD34(+) cells while maintained their ability to proliferate. However, CM from SC treated with ACE-011 partially restored erythropoiesis coinciding with changes in the molecular and secretory profile of SC, including the expression and secretion of erythropoiesis-modulatory factors. We conclude that inhibitory factors produced by BM-SC in vitro might control erythropoiesis in vivo and that agents that reverse these microenvironmental signals may provide an approach to attenuate anemia in clinical conditions.
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