scholarly journals gata2is Required for the runx1-Independent Hematopoiesis in Zebrafish

Blood ◽  
2019 ◽  
Vol 134 (Supplement_1) ◽  
pp. 2463-2463
Author(s):  
Erica Bresciani ◽  
Blake Carrington ◽  
Erika M Kwon Kim ◽  
Kai Yu ◽  
Kevin Bishop ◽  
...  
Keyword(s):  
Stem Cells ◽  
Transcription Factor ◽  
Hematopoietic Stem Cells ◽  
Compensatory Mechanism ◽  
Loss Of Function ◽  
Wild Type ◽  
Hematopoietic Stem ◽  
Larval Stages ◽  
Independent Mechanism ◽  
Definitive Hematopoiesis

The current notion about how hematopoietic stem cells (HSCs) are generated identifies the transcription factor RUNX1 as an essential factor for the emergence of definitive hematopoietic stem cells (HSCs) from the hemogenic endothelium. Consequently, Runx1knockout mice fail to develop definitive hematopoiesis and lack all definitive blood lineages and cannot survive past embryonic day 12. However, even though zebrafish with arunx1stop codon mutation (runx1W84X/W84X) presented defects in definitive hematopoiesis during embryogenesis, runx1W84X/W84Xembryos could develop to fertile adults with blood cells of multi-lineages, raising the possibility that HSCs can emerge without RUNX1. In order to determine if a RUNX1-independent mechanism can support the generation of HSCs we have generated three new zebrafish runx1-/- with engineered deletions of the runx1gene using TALEN and CRISPR-Cas9. Our analysis shows that all three mutants have identical phenotypei.e., failure to develop definitive hematopoiesis during early embryogenesis, with later reemergence of hematopoietic cells and survivalof therunx1 mutants to adulthood, further confirming the existence of a RUNX1-independent mechanism for the emergence of HSCs. In the absence of a functional runx1, a cd41-GFP+population of hematopoietic precursors can still be detected in the aorta-gonad-mesonephros (AGM) region and in the hematopoietic tissues of the mutant embryos. Single cell RNA sequencing of the wild type and mutant HSC/HSPC at embryonic and larval stages confirmed the presence of a population of runx1- /-cd41:GFPlow cells expressing HSC signature genes at 2.5 days post fertilization. At larval stages the runx1-/-HSCs maintain their ability to generate erythroid and myeloid lineage progenitors but they present a different expression profile compared to the wild type. In order to uncover the compensatory mechanism that drives the repopulation of the hematopoietic compartment in the absence of runx1we identified the molecular signatures that separate the runx1-/-HSC/HSPCs from the wild type and subsequently focused our attention on the transcription factors differentially expressed in the runx1-/-HSC/HSPCs. Our analysis shows that the master transcription factor gata2b is strongly upregulated in the runx1- /-HSCs during the recovery of hematopoiesis and it is also upregulated in the kidney marrow of the surviving runx1-/-adults. Given the key role of GATA2 in the HSC development and maintenance in both mouse and zebrafish, gata2b represented a strong candidate gene with the potential ability to drive the rescue of the runx1-/-phenotype. Indeed, a loss of function mutation or knock-down of gata2b can significantly reduce or abolish the survivability of the runx1-/-fish, indicating that gata2bis responsible for rescuing hematopoiesis in the runx1 mutant fish. Overall our results show that even though runx1 is necessary for the normal emergence of definitive HSCs in the embryos, in the absence of runx1the transcription factor gata2 is able to support definitive hematopoiesis that is sufficient for the embryos to develop to functional adults in the zebrafish. The current notion about how hematopoietic stem cells (HSCs) are generated identifies the transcription factor RUNX1 as an essential factor for the emergence of definitive hematopoietic stem cells (HSCs) from the hemogenic endothelium. Consequently, Runx1knockout mice fail to develop definitive hematopoiesis and lack all definitive blood lineages and cannot survive past embryonic day 12. However, even though zebrafish with arunx1stop codon mutation (runx1W84X/W84X) presented defects in definitive hematopoiesis during embryogenesis, runx1W84X/W84Xembryos could develop to fertile adults with blood cells of multi-lineages, raising the possibility that HSCs can emerge without RUNX1. In order to determine if a RUNX1-independent mechanism can support the generation of HSCs we have generated three new zebrafish runx1-/- with engineered deletions of the runx1gene using TALEN and CRISPR-Cas9. Our analysis shows that all three mutants have identical phenotypei.e., failure to develop definitive hematopoiesis during early embryogenesis, with later reemergence of hematopoietic cells and survivalof therunx1 mutants to adulthood, further confirming the existence of a RUNX1-independent mechanism for the emergence of HSCs. In the absence of a functional runx1, a cd41-GFP+population of hematopoietic precursors can still be detected in the aorta-gonad-mesonephros (AGM) region and in the hematopoietic tissues of the mutant embryos. Single cell RNA sequencing of the wild type and mutant HSC/HSPC at embryonic and larval stages confirmed the presence of a population of runx1- /-cd41:GFPlow cells expressing HSC signature genes at 2.5 days post fertilization. At larval stages the runx1-/-HSCs maintain their ability to generate erythroid and myeloid lineage progenitors but they present a different expression profile compared to the wild type. In order to uncover the compensatory mechanism that drives the repopulation of the hematopoietic compartment in the absence of runx1we identified the molecular signatures that separate the runx1-/-HSC/HSPCs from the wild type and subsequently focused our attention on the transcription factors differentially expressed in the runx1-/-HSC/HSPCs. Our analysis shows that the master transcription factor gata2b is strongly upregulated in the runx1- /-HSCs during the recovery of hematopoiesis and it is also upregulated in the kidney marrow of the surviving runx1-/-adults. Given the key role of GATA2 in the HSC development and maintenance in both mouse and zebrafish, gata2b represented a strong candidate gene with the potential ability to drive the rescue of the runx1-/-phenotype. Indeed, a loss of function mutation or knock-down of gata2b can significantly reduce or abolish the survivability of the runx1-/-fish, indicating that gata2bis responsible for rescuing hematopoiesis in the runx1 mutant fish. Overall our results show that even though runx1 is necessary for the normal emergence of definitive HSCs in the embryos, in the absence of runx1the transcription factor gata2 is able to support definitive hematopoiesis that is sufficient for the embryos to develop to functional adults in the zebrafish. Disclosures No relevant conflicts of interest to declare.

The transcription factor Erg is essential for definitive hematopoiesis and the function of adult hematopoietic stem cells

2008 ◽  
Vol 9 (7) ◽  
pp. 810-819 ◽  
Author(s):  
Stephen J Loughran ◽  
Elizabeth A Kruse ◽  
Douglas F Hacking ◽  
Carolyn A de Graaf ◽  
Craig D Hyland ◽  
...  
Keyword(s):  
Stem Cells ◽  
Transcription Factor ◽  
Hematopoietic Stem Cells ◽  
Hematopoietic Stem ◽  
Definitive Hematopoiesis

Development of Multi-Lineage Hematopoiesis in Two Novel Zebrafish runx1 mutants

Blood ◽  
2015 ◽  
Vol 126 (23) ◽  
pp. 2373-2373
Author(s):  
Erica Bresciani ◽  
Blake Carrington ◽  
Erika Mijin Kwon ◽  
Marypat Jones ◽  
Stephen Wincovitch ◽  
...  
Keyword(s):  
Stem Cells ◽  
Hematopoietic Stem Cells ◽  
Blood Cells ◽  
Conflicts Of Interest ◽  
Time Lapse ◽  
Loss Of Function ◽  
Transcription Activator ◽  
Zebrafish Model ◽  
Hematopoietic Stem ◽  
Definitive Hematopoiesis

Abstract Long term hematopoietic stem cells are essential for the life-long maintenance of the hematopoietic system of an organism. The transcription factor RUNX1 is required for the emergence of definitive hematopoietic stem cells (HSCs) from the hemogenic endothelium during the embryo development. Runx1 knockout mouse embryos lack all definitive blood lineages and cannot survive past embryonic day 13. However, we previously showed that zebrafish homozygous for an ENU-induced nonsense mutation in runx1 (runx1W84X/W84X) were able to recover from a larval "bloodless" phase and develop to fertile adults with multi-lineage hematopoiesis, suggesting the formation of runx1-independent adult HSCs. However, our finding was based on a single zebrafish model, which requires verification in additional, independent models. In order to further investigate if a RUNX1-independent pathway exists for the formation of adult HSCs, we generated two new runx1 mutants, a deletion of 8 bp (runx1del8/del8) and a deletion of 25 bp (runx1del25/del25) within exon 4 of runx1, respectively, using the Transcription activator-like effector nucleases (TALENs) technology. These mutations cause frameshifts and premature terminations within the runt-homology domain,, resulting in loss of function of runx1 (runx1-/-). Both runx1del8/del8 and runx1del25/del25 mutant embryos had normal primitive hematopoiesis but failed to develop definitive hematopoiesis. Time-lapse recordings with confocal microscopy revealed that, indeed, there was no emergence of HSCs from the ventral wall of dorsal aorta in the runx1-/- embryos. The runx1-/- larvae gradually lost circulating primitive blood cells and became bloodless between 8 and 14 days post fertilization (dpf). However they gradually regained circulating blood cells between 15 and 20 dpf. Eventually, about 40% of runx1del8/del8 and runx1del25/del25 mutants developed to fertile adults with circulating blood cells of multi-lineages. Taken together, our data is consistent with the previously described runx1W84X/W84X phenotype and supports the possibility of a runx1-independent mechanism for HSC formation and definitive hematopoiesis. Disclosures No relevant conflicts of interest to declare.

scholarly journals Ing4-Deficiency Enhances Hematopoietic Stem Cell Quiescence and Confers Resistance to Inflammatory Stress

Blood ◽  
2021 ◽  
Vol 138 (Supplement 1) ◽  
pp. 1095-1095
Author(s):  
Zanshé Thompson ◽  
Georgina A Anderson ◽  
Seth Gabriel ◽  
Melanie Rodriguez ◽  
Vera Binder ◽  
...  
Keyword(s):  
Stem Cells ◽  
Stem Cell ◽  
Hematopoietic Stem Cells ◽  
Target Genes ◽  
Bone Marrow Cells ◽  
Conflicts Of Interest ◽  
Critical Role ◽  
Loss Of Function ◽  
Wild Type ◽  
Hematopoietic Stem

Abstract In a screen for epigenetic regulators of hematopoiesis in zebrafish, we identified a requirement of the tumor suppressor protein, Ing4, in hematopoietic stem and progenitor cell (HSPC) specification. Though the Ing4 mechanism of action remains poorly characterized, loss of Ing4 has been shown to promote stem cell-like characteristics in malignant cells and it is a frequent target of inactivation in various types of cancer. Mutations in Ing4 cause deregulation of both NF-kB and c-Myc target gene expression. We have also identified a requirement for Ing4 in murine hematopoiesis. Ing4-/- mice have aberrant hematopoiesis and elevated cytokine expression in bone marrow cells. Using RNA-sequencing, we found that Ing4-deficient HSPCs express high levels of c-Myc target genes and genes associated with oxidative phosphorylation and ribosomal biogenesis. Yet, Ing4 deficiency induces G 0 arrest in HSPCs and they have low levels of reactive oxygen species. This places Ing4-deficient HSPCs in a poised state, where they are quiescent, but express elevated levels of genes associated with differentiation. Under stress hematopoiesis following low-dose irradiation, Ing4-deficient long-term hematopoietic stem cells (LT-HSCs) do not expand, but short-term hematopoietic stem cells (ST-HSCs) function comparably to wild-type. Similarly, under transplantation stress, LT-HSCs fail to contribute to multilineage chimerism, while ST-HSCs contribute at levels equal to wild-type cells. These results are striking, particularly when compared to other models of enhanced NF-kB activity, where HSPCs cannot contribute to multilineage chimerism in transplantation. We sought to target the misregulated pathways in Ing4-deficient HSCs to rescue to effects of Ing4 deficiency. To this end, we chose to target the c-Myc pathway for several reasons: c-Myc target genes are over-represented in our RNA-seq data, c-Myc lies upstream of several of the misregulated pathways observed in Ing4-/- HSCs, and Ing4 has previously been reported to negatively regulate c-Myc activity directly. When treated with the c-Myc inhibitor, 10058-F4, both LT-HSCs and ST-HSCs are pushed into cycling, but this treatment also resulted in fewer cells overall. These results suggest that dampening of the c-Myc pathway can partially rescue Ing4 loss of function. Overall, our findings suggest that Ing4 plays a crucial role in the regulation of hematopoiesis and provides key tools for further identification and characterization of Ing4 pathways and functions. Given the role of Ing4 in both normal hematopoiesis and cancer, this gene likely has a critical role in regulation of stem cell self-renewal and maintenance. Disclosures No relevant conflicts of interest to declare.

scholarly journals A Novel, Myeloid Transcription Factor, C/EBPε, Is Upregulated During Granulocytic, But Not Monocytic, Differentiation

Blood ◽  
1997 ◽  
Vol 90 (7) ◽  
pp. 2591-2600 ◽  
Author(s):  
Roberta Morosetti ◽  
Dorothy J. Park ◽  
Alexey M. Chumakov ◽  
Isabelle Grillier ◽  
Masaaki Shiohara ◽  
...  
Keyword(s):  
Stem Cells ◽  
Transcription Factor ◽  
Hematopoietic Stem Cells ◽  
Cell Line ◽  
Cell Lines ◽  
Myeloid Leukemia ◽  
Leukemia Cell ◽  
Rt Pcr ◽  
Hematopoietic Stem ◽  
Nb4 Cells

Human C/EBPε is a newly cloned CCAAT/enhancer-binding transcription factor. Initial studies indicated it may be an important regulator of human myelopoiesis. To elucidate the range of expression of C/EBPε, we used reverse transcription-polymerase chain reaction (RT-PCR) analysis and examined its expression in 28 hematopoietic and 14 nonhematopoietic cell lines, 16 fresh myeloid leukemia samples, and normal human hematopoietic stem cells and their mature progeny. Prominent expression of C/EBPε mRNA occurred in the late myeloblastic and promyelocytic cell lines (NB4, HL60, GFD8), the myelomonoblastic cell lines (U937 and THP-1), the early myeloblast cell lines (ML1, KCL22, MDS92), and the T-cell lymphoblastic leukemia cell lines CEM and HSB-2. For the acute promyelocytic leukemia cell line NB4, C/EBPε was the only C/EBP family member that was easily detected by RT-PCR. No C/EBPε mRNA was found in erythroid, megakaryocyte, basophil, B lymphoid, or nonhematopoietic cell lines. Most acute myeloid leukemia samples (11 of 12) from patients expressed C/EBPε. Northern blot and RT-PCR analyses showed that C/EBPε mRNA decreased when the HL60 and KG-1 myeloblast cell lines were induced to differentiate toward macrophages. Similarly, Western blot analysis showed that expression of C/EBPε protein was either unchanged or decreased slightly as the promyelocytic cell line NB4 differentiated down the macrophage-like pathway after treatment with a potent vitamin D3 analog (KH1060). In contrast, C/EBPε protein levels increased dramatically as NB4 cells were induced to differentiate down the granulocytic pathway after exposure to 9-cis retinoic acid. Furthermore, very early, normal hematopoietic stem cells (CD34+/CD38−), purified from humans had very weak expression of C/EBPε mRNA, but levels increased as these cells differentiated towards granulocytes. Likewise, purified granulocytes appeared to express higher levels of C/EBPε mRNA than purified macrophages. Addition of phosphothiolated antisense, but not sense oligonucleotides to C/EBPε, decreased clonal growth of HL-60 and NB4 cells by about 50% compared with control cultures. Taken together, our results indicate that expression of C/EBPε is restricted to hematopoietic tissues, especially myeloid cells as they differentiate towards granulocytes and inhibition of its expression in HL-60 and NB4 myeloblasts and promyelocytes decreased their proliferative capacity. Therefore, this transcriptional factor may play an important role in the process of normal myeloid development.

scholarly journals Scl isoforms act downstream of etsrp to specify angioblasts and definitive hematopoietic stem cells

Blood ◽  
2010 ◽  
Vol 115 (26) ◽  
pp. 5338-5346 ◽  
Author(s):  
Xi Ren ◽  
Gustavo A. Gomez ◽  
Bo Zhang ◽  
Shuo Lin
Keyword(s):  
Stem Cells ◽  
Hematopoietic Stem Cells ◽  
Critical Role ◽  
Vascular Endothelial ◽  
Hematopoietic Stem ◽  
Vegf Signaling ◽  
Endothelial Growth Factor ◽  
Endothelial Development ◽  
Definitive Hematopoiesis ◽  
Runx1 Expression

Abstract Recent lineage studies suggest that hematopoietic stem cells (HSCs) may be derived from endothelial cells. However, the genetic hierarchy governing the emergence of HSCs remains elusive. We report here that zebrafish ets1-related protein (etsrp), which is essential for vascular endothelial development, also plays a critical role in the initiation of definitive hematopoiesis by controlling the expression of 2 stem cell leukemia (scl) isoforms (scl-α and scl-β) in angioblasts. In etsrp morphants, which are deficient in endothelial and HSC development, scl-α alone partially rescues angioblast specification, arterial-venous differentiation, and the expression of HSC markers, runx1 and c-myb, whereas scl-β requires angioblast rescue by fli1a to restore runx1 expression. Interestingly, when vascular endothelial growth factor (Vegf) signaling is inhibited, HSC marker expression can still be restored by scl-α in etsrp morphants, whereas the rescue of arterial ephrinb2a expression is blocked. Furthermore, both scl isoforms partially rescue runx1 but not ephrinb2a expression in embryos deficient in Vegf signaling. Our data suggest that downstream of etsrp, scl-α and fli1a specify the angioblasts, whereas scl-β further initiates HSC specification from this angioblast population, and that Vegf signaling acts upstream of scl-β during definitive hematopoiesis.

scholarly journals CRISPR genome editing of murine hematopoietic stem cells to create Npm1-Alk causes ALK+ lymphoma after transplantation

2019 ◽  
Vol 3 (12) ◽  
pp. 1788-1794 ◽  
Author(s):  
Soumya Sundara Rajan ◽  
Lingxiao Li ◽  
Mercedes F. Kweh ◽  
Kranthi Kunkalla ◽  
Amit Dipak Amin ◽  
...  
Keyword(s):  
Stem Cells ◽  
T Cell ◽  
Hematopoietic Stem Cells ◽  
Large Cell ◽  
Wild Type ◽  
Hematopoietic Stem ◽  
T Cell Lymphomas ◽  
Long Latency ◽  
Key Points ◽  
Genomic Editing

Key Points CRISPR/Cas9 genomic editing of wild-type hematopoietic stem cells generates Npm1-Alk, leading to ALK+ large-cell lymphomas in recipients. CD30+ postthymic T-cell lymphomas are polyclonal but transplantable to secondary recipients with long latency.

scholarly journals The transcription factor Foxm1 is essential for the quiescence and maintenance of hematopoietic stem cells

2015 ◽  
Vol 16 (8) ◽  
pp. 810-818 ◽  
Author(s):  
Yu Hou ◽  
Wen Li ◽  
Yue Sheng ◽  
Liping Li ◽  
Yong Huang ◽  
...  
Keyword(s):  
Stem Cells ◽  
Transcription Factor ◽  
Hematopoietic Stem Cells ◽  
Hematopoietic Stem

Erythrocyte Cytoskeletal Defects Induced in Mice by Deletion of Rac GTPases.

Blood ◽  
2004 ◽  
Vol 104 (11) ◽  
pp. 1573-1573 ◽  
Author(s):  
Theodosia A. Kalfa ◽  
Suvarnamala Pushkaran ◽  
James F. Johnson ◽  
Qian Wei ◽  
David A. Williams ◽  
...  
Keyword(s):  
Stem Cells ◽  
Confocal Microscopy ◽  
Hematopoietic Stem Cells ◽  
Peripheral Blood ◽  
Erythrocyte Ghosts ◽  
Wild Type ◽  
Rhodamine Phalloidin ◽  
Hematopoietic Stem ◽  
Post Induction ◽  
Erythrocyte Cytoskeleton

Abstract The small Rho GTPases Rac1 and Rac2 have been implicated in regulating actin structures in a variety of cells, including hematopoietic stem cells and leucocytes. Actin oligomers are a significant structural component of the erythrocyte cytoskeleton. We explored the possible role of Rac1 and Rac2 signaling molecules in the dynamic assembly of actin in the red blood cells (RBC), and thus in the regulation of morphology and function of the erythrocyte cytoskeleton. Rac1 and Rac2 GTPases have been shown to have overlapping as well as distinct roles in actin organization, cell survival, and proliferation in hematopoietic stem cells (Gu et al. Science, 2003); we focused our study on the erythrocyte phenotype of Rac2−/− and Rac1−/−;Rac2−/− mice. Cre-recombinase-induced deletion of Rac1 genomic sequence was accomplished on a Rac2-null genetic background. Deletion of Rac1 after treatment with PolyI:PolyC to induce Cre recombinase was confirmed in bone marrow cells using DNA PCR and in erythrocytes by immunoblot. Since the erythrocytes consist a population of variable age, the optimal time of the maximum Rac1 deletion in erythrocytes was determined to be three to five weeks post induction. During this period, Rac1 protein in erythrocytes was decreased by 50–80% as determined by immunoblot densitometry. Rac2−/− and wild-type mice were subjected to the same treatment to control for any effects of PolyI:PolyC independent of the Rac1 deletion. Blood samples were obtained weekly after the completion of induction and the hematologic phenotype was studied by evaluation of complete blood counts, RBC indices, and reticulocyte counts. Erythrocyte morphology was examined on Wright-Giemsa smears of peripheral blood. Intact erythrocytes and erythrocyte ghosts were stained for actin with rhodamine-phalloidin and studied by confocal microscopy. The Rac2−/− mice appeared to have a rather mild erythrocyte phenotype with no significant anemia or reticulocytosis, although they did demonstrate a mild poikilocytosis and anisocytosis at baseline. The Rac1−/−;Rac2−/− mice developed a microcytic anemia with a hemoglobin drop of up to 30% in comparison to the baseline and to the wild-type hemoglobin values, with the nadir noted at three weeks post induction. The percentage of reticulocytes increased up to threefold in comparison to the control group. The mean corpuscular volume decreased up to 20% from the baseline in the Rac1−/−;Rac2−/− mice, and remained decreased up to six weeks post induction with an elevated red blood cell distribution width. Significant anisocytosis and poikilocytosis were observed with fragmented erythrocytes in the peripheral blood smear. Filamentous actin in the RBC cytoskeleton stained with rhodamine-phalloidin appeared to have a uniform distribution in intact and ghost erythrocytes under confocal microscopy. However, Rac1−/−;Rac2−/− erythrocytes demonstrated punctuate lesions on the cell surface while Rac1−/−;Rac2−/− erythrocyte ghosts appeared to collapse into irregular shapes. These data suggest that deficiency of Rac1 and Rac2 GTPases in mice cause a microcytic hemolytic anemia with poikilocytosis and red cell fragmentation indicating a possible dynamic regulation of the erythrocyte cytoskeleton organization by these signaling molecules.

Role of the Transcription Factor LYL-1 in the Adult and Fetal Hematopoietic Stem Cells.

Blood ◽  
2004 ◽  
Vol 104 (11) ◽  
pp. 2775-2775
Author(s):  
Claude Capron ◽  
Catherine Lacout ◽  
Yann Lecluse ◽  
Isabelle Poullion ◽  
Fedor Svinarchouk ◽  
...  
Keyword(s):  
Stem Cells ◽  
Transcription Factor ◽  
Hematopoietic Stem Cells ◽  
Absolute Number ◽  
Binding Motif ◽  
Hematopoietic Stem ◽  
Cell Counts ◽  
Significant Difference ◽  
The Absolute

Abstract The hematopoietic stem cells (HSC) have the ability to self-renew and to give rise to all blood lineages. These processes occur via a hierarchy of progenitors with progressively more limited differentiation and self-renewal potential and are orchestrated by specialized protein such as transcription factors. LYL-1 protein contains a basic helix-loop-helix DNA binding motif also found in several proteins involved in the control of cellular proliferation and differentiation such as SCL/TAL-1. As LYL-1 shares an 80% homology at the protein level with SCL/TAL-1, we wanted to determine the function of LYL-1 in hematopoiesis and particularly on HSC. For this study, we used knock in lyl-1−/− mice in which exon 4 was replaced by LacZ/Neo cassette. Lyl−/− mice are viable and have normal blood cell counts as well as a normal marrow cellularity. In addition, using a hematopoietic colony forming cells (CFCs) assay, no significant difference was seen in the myeloid CFCs of either lyl-1−/− or lyl-1+/+ BM and FL cells except a 2-fold increase in the absolute number of BFU-E in lyl-1−/− FL as compared to lyl-1+/+ FL. We analyzed more primitive progenitors in details because using Fluorecein Di-beta Galactopyranoside (FDG)-staining assay, we showed that lyl-1 is mainly expressed in primitive Lin− Sca-1+ c-Kit+ cells (LSK) cells from either BM or FL (91 ± 7% and 78 ± 5% of FDG positive cells in lyl-1−/− BM and FL LSK cells, respectively). In addition, analysis of lyl-1−/− and lyl-1+/+ cells revealed a 1.8-fold and 2-fold decrease in the percentage of primitive LSK in BM and FL, respectively, as compared to wild type cells. Furthermore, using the Hoechst 33342 efflux assay, we noticed a significant decrease in the absolute number of more primitive LSK-SP (side population) cells in lyl-1−/− BM as compared to lyl-1+/+ BM cells (52800 ± 5412 cells/femur versus 91080 ± 8475 cells/femur, respectively) suggesting an important role of LYL-1 in the HSC function. In order to confirm this hypothesis, in vivo assays were performed. We observed a 1.5-fold decrease in the lyl-1−/− BM and FL day 12 CFU-S content as compared to lyl-1+/+ cells. Adoptive transfer experiments were subsequently performed using lethally irradiated Ly5.1 mice. Data showed that lyl-1−/− cells from either BM or FL displayed a hematopoietic reconstitution defect in competitive repopulation assays. Indeed, Ly5.1 recipients were injected with a mixture of 5x106 (5:1), 106 (1:1) or 0.5x106 (0.5:1) lyl-1−/− or lyl-1+/+ Ly5.2 expressing cells and 106 competitive BM Ly5.1 expressing cells. All hosts engrafted with lyl-1−/− BM cells shown a significant reduced levels of chimerism (% of circulating Ly5.2+ cells) as compared to hosts engrafted with lyl-1+/+ BM donors (4.3 ± 2.8% (5:1); 7.5 ± 5.5% (1:1); 0.6 ± 0.3% (0.5:1) in lyl-1−/− BM cells versus 66 ± 8% (5:1); 52 ± 9% (1:1); 53 ± 10% (0.5:1) in lyl-1+/+ BM cells) and similar difference was observed with FL donors (45 ± 2% (5:1); 25 ± 5% (1:1); 11 ± 5% (0.5:1) in lyl-1−/− FL cells versus 83 ± 1% (5:1); 70 ± 3% (1:1); 53 ± 6% (0.5:1) in lyl-1+/+ FL cells). This altered defect in HSC was also confirmed using LTC-IC in vitro experiments. Altogether, our results demonstrate an important role of the transcription factor LYL-1 on the maintenance of HSC properties.

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