The Role of Translation Initiation Regulation in Haematopoiesis

Organisation of RNAs into functional subgroups that are translated in response to extrinsic and intrinsic factors underlines a relatively unexplored gene expression modulation that drives cell fate in the same manner as regulation of the transcriptome by transcription factors. Recent studies on the molecular mechanisms of inflammatory responses and haematological disorders indicate clearly that the regulation of mRNA translation at the level of translation initiation, mRNA stability, and protein isoform synthesis is implicated in the tight regulation of gene expression. This paper outlines how these posttranscriptional control mechanisms, including control at the level of translation initiation factors and the role of RNA binding proteins, affect hematopoiesis. The clinical relevance of these mechanisms in haematological disorders indicates clearly the potential therapeutic implications and the need of molecular tools that allow measurement at the level of translational control. Although the importance of miRNAs in translation control is well recognised and studied extensively, this paper will exclude detailed account of this level of control

Molecular characterization of translocation (6;9) in acute nonlymphocytic leukemia

Specific chromosomal translocations are one of the defects associated with leukemia. Isolation and characterization of genes affected by these translocations may give insight into the processes of both leukemogenesis and normal hematopoiesis. When the experiments described in this thesis were started, several genes involved in translocations in lymphoid leukemia were isolated. These genes were all translocated into T-cell receptor and Immunoglobulin loci, which deregulated their expression. In myeloid leukemia only translocation (9;22) was characterized molecularly and the resulting bcr-abl gene was the only fusion gene known. Chapter 1 gives an overview of what is known to date about genes involved in leukemogenesis. To extend the research on the molecular characterization of translocations in myeloid leukemia, we decided to clone and characterize the translocation breakpoints of t(6;9) that characterizes a subtype of acute myeloid leukemia. Chapter 2 gives an introduction to t(6;9) AML and reports the results of our investigations. Chapter 3 discusses these results in relation to our current understanding of leukemogenesi

Erythropoiesis and Megakaryopoiesis in a Dish

Erythrocytes and platelets are the major cellular components of blood. Several hereditary diseases affect the production/stability of red blood cells (RBCs) and platelets (Plts) resulting in anemia or bleeding, respectively. Patients with such disorders may require recurrent transfusions, which bear a risk to develop alloantibodies and ultimately may result in transfusion product refractoriness. Cell culture models enable to unravel disease mechanisms, and to screen for alternative therapeutic products. Besides these applications, the ultimate goal is the large-scale production of blood effector cells for transfusion. Cultured RBCs that lack many of the common blood group antigens and Plts-lacking HLA expression would improve transfusion practice. Large numbers of RBCs and Plts can already be generated using hematopoietic stem cells derived from fetal liver, cord blood, peripheral blood, and bone marrow as starting material for cell culture. The recent advances to generate blood cells from induced pluripotent stem cells provide a donor-independent, immortal primary source for cell culture models. This enables us to study developmental switches during erythropoiesis/megakaryopoiesis and provides potential future therapeutic applications. In this review, we will discuss how erythropoiesis and megakaryopoiesis are mimicked in culture systems and how these models relate to the in vivo process

The Shape Shifting Story of Reticulocyte Maturation

The final steps of erythropoiesis involve unique cellular processes including enucleation and reorganization of membrane proteins and the cytoskeleton to produce biconcave erythrocytes. Surprisingly this process is still poorly understood. In vitro erythropoiesis protocols currently produce reticulocytes rather than biconcave erythrocytes. In addition, immortalized lines and iPSC-derived erythroid cell suffer from low enucleation and suboptimal final maturation potential. In light of the increasing prospect to use in vitro produced erythrocytes as (personalized) transfusion products or as therapeutic delivery agents, the mechanisms driving this last step of erythropoiesis are in dire need of resolving. Here we review the elusive last steps of reticulocyte maturation with an emphasis on protein sorting during the defining steps of reticulocyte formation during enucleation and maturation

EZH2-dependent chromatin looping controls INK4a and INK4b, but not ARF, during human progenitor cell differentiation and cellular senescence

<p>Abstract</p> <p>Background</p> <p>The <it>INK4b-ARF-INK4a </it>tumour suppressor locus controls the balance between progenitor cell renewal and cancer. In this study, we investigated how higher-order chromatin structure modulates differential expression of the human <it>INK4b-ARF-INK4a </it>locus during progenitor cell differentiation, cellular ageing and senescence of cancer cells.</p> <p>Results</p> <p>We found that <it>INK4b </it>and <it>INK4a</it>, but not <it>ARF</it>, are upregulated following the differentiation of haematopoietic progenitor cells, in ageing fibroblasts and in senescing malignant rhabdoid tumour cells. To investigate the underlying molecular mechanism we analysed binding of polycomb group (PcG) repressive complexes (PRCs) and the spatial organization of the <it>INK4b-ARF-INK4a </it>locus. In agreement with differential derepression, PcG protein binding across the locus is discontinuous. As we described earlier, PcG repressors bind the INK4a promoter, but not ARF. Here, we identified a second peak of PcG binding that is located ~3 kb upstream of the <it>INK4b </it>promoter. During progenitor cell differentiation and ageing, PcG silencer EZH2 attenuates, causing loss of PRC binding and transcriptional activation of <it>INK4b </it>and <it>INK4a</it>. The expression pattern of the locus is reflected by its organization in space. In the repressed state, the PRC-binding regions are in close proximity, while the intervening chromatin harbouring <it>ARF </it>loops out. Down regulation of EZH2 causes release of the ~35 kb repressive chromatin loop and induction of both <it>INK4a </it>and <it>INK4b</it>, whereas <it>ARF </it>expression remains unaltered.</p> <p>Conclusion</p> <p>PcG silencers bind and coordinately regulate <it>INK4b </it>and <it>INK4a</it>, but not <it>ARF</it>, during a variety of physiological processes. Developmentally regulated EZH2 levels are one of the factors that can determine the higher order chromatin structure and expression pattern of the <it>INK4b-ARF-INK4a </it>locus, coupling human progenitor cell differentiation to proliferation control. Our results revealed a chromatin looping mechanism of long-range control and argue against models involving homogeneous spreading of PcG silencers across the <it>INK4b-ARF-INK4a </it>locus.</p

Igbp1 is part of a positive feedback loop in stem cell factor–dependent, selective mRNAtranslation initiation inhibiting erythroid differentiation

The authors thank Dr Victor de Jager for assistance with the Rosetta Resolver software; Dr Ivo Touw for many fruitful discussions and critical reading of the manuscript; Liu Wing for technical assistance; Drs Peter Seither, Andreas Weith (Boehringer Ingelheim, Biberach, Germany), Helmuth Dolznig, Thomas Waerner, and Sandra Pilat (IMP, Vienna, Austria) for mRNA profiling of erythroblasts, of which the complete data will be published elsewhere; Dr Bart Aarts (Erasmus MC, Rotterdam, The Netherlands) for assistance in confocal scanning microscopy; Dr David Brautigan (University of Virginia, Charlottesville) for anti-Igbp1 antibodies; Dr Manfred Boehm (National Institutes of Health/National Heart, Lung, and Blood Institute, Bethesda, MD) for anti-Uhmk1 antibodies; and Ortho-Biotech (Tilburg, The Netherlands) for their kind gift of Eprex (erythropoietin).Stem cell factor (SCF)–induced activation of phosphoinositide-3-kinase (PI3K) is required for transient amplification of the erythroblast compartment. PI3K stimulates the activation of mTOR (target of rapamycin) and subsequent release of the cap-binding translation initiation factor 4E (eIF4E) from the 4E-binding protein 4EBP, which controls the recruitment of structured mRNAs to polysomes. Enhanced expression of eIF4E renders proliferation of erythroblasts independent of PI3K. To investigate which mRNAs are selectively recruited to polysomes, we compared SCF-dependent gene expression between total and polysome-bound mRNA. This identified 111 genes primarily subject to translational regulation. For 8 of 9 genes studied in more detail, the SCF-induced polysome recruitment of transcripts exceeded 5-fold regulation and was PI3K-dependent and eIF4E-sensitive, whereas total mRNA was not affected by signal transduction. One of the targets, Immunoglobulin binding protein 1 (Igbp1), is a regulatory subunit of protein phosphatase 2A (Pp2a) sustaining mTOR signaling. Constitutive expression of Igbp1 impaired erythroid differentiation, maintained 4EBP and p70S6k phosphorylation, and enhanced polysome recruitment of multiple eIF4E-sensitive mRNAs. Thus, PI3K-dependent polysome recruitment of Igbp1 acts as a positive feedback mechanism on translation initiation underscoring the important regulatory role of selectivemRNArecruitment to polysomes in the balance between proliferation and maturation of erythroblasts. (Blood. 2008; 112:2750-2760)peer-reviewe

The potential role of the homeobox gene, Hhex in haematopoietic progenitor expansion

Background: The decision of an erythroid progenitor to proliferate or differentiate is regulated at the level of (i) transcription; (ii) recruitment of transcripts to polysomes for protein synthesis and (iii) signal transduction activating functional effectors. We utilized a factor sensitive erythroid progenitor cell model to study the gene expression profile of cells under proliferative signals. We have shown previously that translation control is an extremely important level of regulation that controls the balance between proliferation and differentiation of erythroid progenitors. This led us to investigate those transcripts that are shifted to polysomes in cells stimulated by erythropoietin (Epo) or stem cell factor (SCF).peer-reviewe

Loss of Ercc1 Results in a Time- and Dose-Dependent Reduction of Proliferating Early Hematopoietic Progenitors

The endonuclease complex Ercc1/Xpf is involved in interstrand crosslink repair and functions downstream of the Fanconi pathway. Loss of Ercc1 causes hematopoietic defects similar to those seen in Fanconi Anemia. Ercc1−/− mice die 3-4 weeks after birth, which prevents long-term follow up of the hematopoietic compartment. We used alternative Ercc1 mouse models to examine the effect of low or absent Ercc1 activity on hematopoiesis. Tie2-Cre-driven deletion of a floxed Ercc1 allele was efficient (>80%) in fetal liver hematopoietic cells. Hematopoietic stem and progenitor cells (HSPCs) with a deleted allele were maintained in mice up to 1 year of age when harboring a wt allele, but were progressively outcompeted when the deleted allele was combined with a knockout allele. Mice with a minimal Ercc1 activity expressed by 1 or 2 hypomorphic Ercc1 alleles have an extended life expectancy, which allows analysis of HSPCs at 10 and 20 weeks of age. The HSPC compartment was affected in all Ercc1-deficient models. Actively proliferating multipotent progenitors were most affected as were myeloid and erythroid clonogenic progenitors. In conclusion, lack of Ercc1 results in a severe competitive disadvantage of HSPCs and is most deleterious in proliferating progenitor cells

Stem cell factor induces phosphatidylinositol 3'-kinase-dependent Lyn/Tec/Dok-1 complex formation in hematopoietic cells

Stem cell factor (SCF) has an important role in the proliferation, differentiation, survival, and migration of hematopoietic cells. SCF exerts its effects by binding to cKit, a receptor with intrinsic tyrosine kinase activity. Activation of phosphatidylinositol 3'-kinase (PI3-K) by cKit was previously shown to contribute to many SCF-induced cellular responses. Therefore, PI3-K-dependent signaling pathways activated by SCF were investigated. The PI3-K-dependent activation and phosphorylation of the tyrosine kinase Tec and the adapter molecule p62Dok-1 are reported. The study shows that Tec and Dok-1 form a stable complex with Lyn and 2 unidentified phosphoproteins of 56 and 140 kd. Both the Tec homology and the SH2 domain of Tec were identified as being required for the interaction with Dok-1, whereas 2 domains in Dok-1 appeared to mediate the association with Tec. In addition, Tec and Lyn were shown to phosphorylate Dok-1, whereas phosphorylated Dok-1 was demonstrated to bind to the SH2 domains of several signaling molecules activated by SCF, including Abl, CrkL, SHIP, and PLCgamma-1, but not those of Vav and Shc. These findings suggest that p62Dok-1 may function as an important scaffold molecule in cKit-mediated signaling

The miR-144/451 locus is required for erythroid homeostasis.

The process of erythropoiesis must be efficient and robust to supply the organism with red bloods cells both under condition of homeostasis and stress. The microRNA (miRNA) pathway was recently shown to regulate erythroid development. Here, we show that expression of the locus encoding miR-144 and miR-451 is strictly dependent on Argonaute 2 and is required for erythroid homeostasis. Mice deficient for the miR-144/451 cluster display a cell autonomous impairment of late erythroblast maturation, resulting in erythroid hyperplasia, splenomegaly, and a mild anemia. Analysis of gene expression profiles from wild-type and miR-144/451-deficient erythroblasts revealed that the miR-144/451 cluster acts as a "tuner" of gene expression, influencing the expression of many genes. MiR-451 imparts a greater impact on target gene expression than miR-144. Accordingly, mice deficient in miR-451 alone exhibited a phenotype indistinguishable from miR-144/451-deficient mice. Thus, the miR-144/451 cluster tunes gene expression to impart a robustness to erythropoiesis that is critical under conditions of stress