Hematopoietic Stem Cells Increase Quiescence during Aging

Introduction: Definitive hematopoietic stem cells (HSCs) sustain blood production from fetal development throughout life. In mice, most of steady state, young adult HSCs are in the G0 phase of cell cycle (quiescence), and are estimated to divide roughly once a month. Daily hematopoietic production is thus mainly sustained by highly proliferative downstream hematopoietic progenitor cells (HPCs). Aged haematopoiesis was demonstrated to be distinct from young haematopoiesis in various aspects such as i) a shift from lymphopoiesis to myelopoiesis, ii) functional decline of HSCs (self-renewal, homing), and iii) HSCs pool expansion. While several studies attempted to address whether changes in HSCs turnover during aging can account for the distinct aging associated phenotype and function, it remained to be determined whether aged HSCs overall cycle more or less frequently than young HSCs. Methods: To construct data-based, quantitative models, we measured turnover rates and compartment sizes of populations of HSCs, HSPCs and granulopoiesis/granulocytes, i.e. a post-mitotic mature hematopoietic linage with a short half-life. We examined four age groups: 3 week, 2 month, 1 year and 2 year old mice. Mice in each group were i.p. injected every 4 hours with 1 mcg EdU up to a maximum time of 48 hours. HSC, HSPC and granulopoiesis/granoulocyte compartment sizes and snapshot cell-cycle analysis was performed by FACS at multiple sampling points in BM and peripheral blood (PB), respectively. Based on this data, we built a mathematical model of HSC turn-over and HSPC differentiation during ageing. Moreover, we evaluated HSC cycling by CFSE dilution in steady-state transplantation experiments (as described before; Takizawa et al., J Exp Med 2011). Results: In line with previous reports, the HSCs compartment size gradually increased with age from 3wk old mice to 2 year old mice. In sharp contrast, cycling activity of HSCs as determined by EdU incorporation decreased gradually and significantly with increasing age. This was driven by decreased activation from the quiescent state, while the time that actively cycling HSCs require to progress through cell-division remains constant with age. Multipotent Progenitor (MPP) cycling showed a non-significant trend towards slower turn-over. These results were confirmed by complementary CFSE-dilution experiments. Mathematical modeling of HSC proliferation and differentiation revealed a higher probability of self-renewing divisions in 3 week old mice as compared to 2 month, 1 and 2 year old mice, with the latter both having nearly equal chances of self-renewing versus differentiating divisions. Conclusions: Our data clarifies the long-standing question, how the HSC pool increases with age. Instead of an increase in active cycling, an increase in HSC quiescence is responsible for the increased size of the HSCs pool in aging. Disclosures No relevant conflicts of interest to declare.

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Microrna 199b Regulates Mouse Hematopoietic Stem Cells Maintenance

Blood ◽

10.1182/blood-2019-126283 ◽

2019 ◽

Vol 134 (Supplement_1) ◽

pp. 3704-3704

Author(s):

Aldona A Karaczyn ◽

Edward Jachimowicz ◽

Jaspreet S Kohli ◽

Pradeep Sathyanarayana

Keyword(s):

Cell Cycle ◽

Stem Cells ◽

Hematopoietic Stem Cells ◽

Conflicts Of Interest ◽

Quiescent State ◽

Differentiation Potential ◽

Self Renewal ◽

Hematopoietic Stem

The preservation of hematopoietic stem cell pool in bone marrow (BM) is crucial for sustained hematopoiesis in adults. Studies assessing adult hematopoietic stem cells functionality had been shown that for example loss of quiescence impairs hematopoietic stem cells maintenance. Although, miR-199b is frequently down-regulated in acute myeloid leukemia, its role in hematopoietic stem cells quiescence, self-renewal and differentiation is poorly understood. Our laboratory investigated the role of miR-199b in hematopoietic stem and progenitor cells (HSPCs) fate using miR-199b-5p global deletion mouse model. Characterization of miR-199b expression pattern among normal HSPC populations revealed that miR-199b is enriched in LT-HSCs and reduced upon myeloablative stress, suggesting its role in HSCs maintenance. Indeed, our results reveal that loss of miR-199b-5p results in imbalance between long-term hematopoietic stem cells (LT-HSCs), short-term hematopoietic stem cells (ST-HSCs) and multipotent progenitors (MMPs) pool. We found that during homeostasis, miR-199b-null HSCs have reduced capacity to maintain quiescent state and exhibit cell-cycle deregulation. Cell cycle analyses showed that attenuation of miR-199b controls HSCs pool, causing defects in G1-S transition of cell cycle, without significant changes in apoptosis. This might be due to increased differentiation of LT-HSCs into MPPs. Indeed, cell differentiation assay in vitro showed that FACS-sorted LT-HSCs (LineagenegSca1posc-Kitpos CD48neg CD150pos) lacking miR-199b have increased differentiation potential into MPP in the presence of early cytokines. In addition, differentiation assays in vitro in FACS-sorted LSK population of 52 weeks old miR-199b KO mice revealed that loss of miR-199b promotes accumulation of GMP-like progenitors but decreases lymphoid differentiation, suggesting that miR199b may regulate age-related pathway. We used non-competitive repopulation studies to show that overall BM donor cellularity was markedly elevated in the absence of miR-199b among HSPCs, committed progenitors and mature myeloid but not lymphoid cell compartments. This may suggest that miR-199b-null LT-HSC render enhanced self-renewal capacity upon regeneration demand yet promoting myeloid reconstitution. Moreover, when we challenged the self-renewal potential of miR-199b-null LT-HSC by a secondary BM transplantation of unfractionated BM cells from primary recipients into secondary hosts, changes in PB reconstitution were dramatic. Gating for HSPCs populations in the BM of secondary recipients in 24 weeks after BMT revealed that levels of LT-HSC were similar between recipients reconstituted with wild-type and miR-199b-KO chimeras, whereas miR-199b-null HSCs contributed relatively more into MPPs. Our data identify that attenuation of miR-199b leads to loss of quiescence and premature differentiation of HSCs. These findings indicate that loss of miR-199b promotes signals that govern differentiation of LT-HSC to MPP leading to accumulation of highly proliferative progenitors during long-term reconstitution. Hematopoietic regeneration via repopulation studies also revealed that miR-199b-deficient HSPCs have a lineage skewing potential toward myeloid lineage or clonal myeloid bias, a hallmark of aging HSCs, implicating a regulatory role for miR-199b in hematopoietic aging. Disclosures No relevant conflicts of interest to declare.

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Loss of miR-199b Impairs Active Cycling of Hematopoietic Stem Cells during Steady-State Hematopoiesis

Blood ◽

10.1182/blood-2018-99-116381 ◽

2018 ◽

Vol 132 (Supplement 1) ◽

pp. 1283-1283

Author(s):

Aldona A Karaczyn ◽

Edward Jachimowicz ◽

Jaspreet S Kohli ◽

Pradeep Sathyanarayana

Keyword(s):

Cell Cycle ◽

Stem Cells ◽

Steady State ◽

Hematopoietic Stem Cells ◽

Conflicts Of Interest ◽

Cycle Duration ◽

Mrna Levels ◽

Self Renewal ◽

Hematopoietic Stem

Abstract Several recent studies have showed that dysregulation of microRNA (miRNA) expression in hematopoietic stem cells (HSC) can affect self-renewal of HSCs, and indicated a role for miRNAs in development of acute myeloid leukemia (AML). We and others have reported a significant down-regulation of miR-199b in AML patients. Recently we found that miR-199b is enriched in long-term hematopoietic stem cells (LT-HSC), suggesting that miR199-b may regulate HSCs function. Therefore, to understand the physiologic role of miR-199 in hematopoiesis during homeostasis, we evaluated various hematopoietic stem and progenitor cells (HSPC) populations in mice harboring genetic deletion of miR199b-5p using CRISPR/Cas method. We found that ablation of miR199b resulted in markedly increased frequencies of primitive HSC and MPPs, and analyses of distribution pattern in myeloid progenitor populations showed reduced numbers of common myeloid progenitors (CMPs) biased toward granulocyte-monocyte (GMPs) linage with no changes in megakaryocytic-erythroid progenitors (MEPs). The elevated numbers of HSC and MPPs may indicate that increased proportion of HSC population is actively cycling, thus we analyzed LSK populations for expression of proliferation marker Ki67 along with DNA staining. We found that miR-199b deletion reduces proportion of primitive HSC and MPPs in cell-cycle, which may affect HSC cell self-renewal. Futher cell-cycle analyses revealed that miR-199b null HSCs leave G0 faster to accumulate in G1, but rather do not progress into mitosis, which was recovered upon 5-fluorouracil-induced cytokine burst. These results indicate that loss of miR-199b increases cell cycle duration. To verify that the absence of miR-199b influences proliferation of HSCs we pulsed miR-199b KO and WT mice with BrdU for 16 hours. We found the difference in the cell cycle distribution between HSCs and progenitors, namely reduction of BrdU-positive HSC and MPPs and progression of GMP compartment. These results show that miR-199b deletion decreases HSC active cell cycle by prolonging cell cycle transition during steady-state hematopoiesis and promotes proliferation of myeloid cells. Because quiescent cells only become susceptible to 5-FU during hematopoietic stress, driving them into cycle, we injected 5-FU into miR-199 KO and WT mice once per week until hematopoietic failure occurred. We found that miR199-b KO mice died soon after two subsequent injections, most likely due to the faster HSC exhaust as compared to WT mice. These results show that loss of miR199b produces HSC with reduced quiescence and prolonged cell cycle, however upon stress these cells progress into cell cycle, making them more susceptible for 5-FU treatment. These results demonstrate that miR-199b intrinsically regulates active cycling of HSCs. CFU-S assays showed that miR-199b KO donors showed decreased colonies in spleen, suggesting that miR-199b deletion affects short-term repopulation. In long-term repopulation assay, we observed a significant reduction of HSCs compartment, but elevated numbers of MPPs in host mice transplanted with BM from miR-199 KO mice. This data indicates that loss of miR-199b causes defects in HSC self-renewal and alters HSCs reconstitution potential. To identify potential miR-199b targets in HSCs under steady-state hematopoiesis, we performed a gene profiling in SLAM-HSCs. mRNA levels of several putative miR-199b targets were markedly elevated in miR-199b KO HSCs. These genes are known to be involved in cell adhesion, cell cycle, transcription regulation and chromatin remodeling including Klf12, Tox3 and Cdk18. Our findings reveal a novel functional role for miR-199b in governing HSC maintenance. Disclosures No relevant conflicts of interest to declare.

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Geminin Regulates Hematopoietic Stem Cell Proliferation and Differentiation.

Blood ◽

10.1182/blood.v114.22.1478.1478 ◽

2009 ◽

Vol 114 (22) ◽

pp. 1478-1478

Author(s):

Kathryn M. Shinnick ◽

Kelly A. Barry ◽

Elizabeth A. Eklund ◽

Thomas J. McGarry

Keyword(s):

Cell Cycle ◽

Stem Cells ◽

Transcription Factors ◽

Cell Division ◽

Dna Replication ◽

Hematopoietic Stem Cells ◽

Conflicts Of Interest ◽

Regulatory Protein ◽

Hematopoietic Stem ◽

Proliferation And Differentiation

Abstract Abstract 1478 Poster Board I-501 Hematopoietic stem cells supply the circulation with mature blood cells throughout life. Progenitor cell division and differentiation must be carefully balanced in order to supply the proper numbers and proportions of mature cells. The mechanisms that control the choice between continued cell division and terminal differentiation are incompletely understood. The unstable regulatory protein Geminin is thought to maintain cells in an undifferentiated state while they proliferate. Geminin is a bi-functional protein. It limits the extent of DNA replication to one round per cell cycle by binding and inhibiting the essential replication factor Cdt1. Loss of Geminin leads to replication abnormalities that activate the DNA replication checkpoint and the Fanconi Anemia (FA) pathway. Geminin also influences patterns of cell differentiation by interacting with Homeobox (Hox) transcription factors and chromatin remodeling proteins. To examine how Geminin affects the proliferation and differentiation of hematopoietic stem cells, we created a mouse strain in which Geminin is deleted from the proliferating cells of the bone marrow. Geminin deletion has profound effects on all three hematopoietic lineages. The production of mature erythrocytes and leukocytes is drastically reduced and the animals become anemic and neutropenic. In contrast, the population of megakaryocytes is dramatically expanded and the animals develop thrombocytosis. Interestingly, the number of c-Kit+ Sca1+ Lin- (KSL) stem cells is maintained, at least in the short term. Myeloid colony forming cells are also preserved, but the colonies that grow are smaller. We conclude that Geminin deletion causes a maturation arrest in some lineages and directs cells down some differentiation pathways at the expense of others. We are now testing how Geminin loss affects cell cycle checkpoint pathways, whether Geminin regulates hematopoietic transcription factors, and whether Geminin deficient cells give rise to leukemias or lymphomas. Disclosures: No relevant conflicts of interest to declare.

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Poly I:C Stimulates Sca-1 Expression in Murine Bone Marrow and Increases the Number of Phenotypic Hematopoietic Stem Cells.

Blood ◽

10.1182/blood.v114.22.2504.2504 ◽

2009 ◽

Vol 114 (22) ◽

pp. 2504-2504

Author(s):

Russell Garrett ◽

Gerd Bungartz ◽

Alevtina Domashenko ◽

Stephen G. Emerson

Keyword(s):

Cell Cycle ◽

Stem Cells ◽

Bone Marrow ◽

Hematopoietic Stem Cells ◽

Conflicts Of Interest ◽

Hematopoietic Cells ◽

Poly I:C ◽

Hematopoietic Stem ◽

Marrow Cells ◽

Myeloid Progenitors

Abstract Abstract 2504 Poster Board II-481 Polyinosinic:polycytidlyic acid (poly I:C) is a synthetic double-stranded RNA used to mimic viral infections in order to study immune responses and to activate gene deletion in lox-p systems employing a Cre gene responsive to an Mx-1 promoter. Recent observations made by us and others have suggested hematopoietic stem cells, responding to either poly I:C administration or interferon directly, enter cell cycle. Twenty-two hours following a single 100mg intraperitoneal injection of poly I:C into 10-12 week old male C57Bl/6 mice, the mice were injected with a single pulse of BrdU. Two hours later, bone marrow was harvested from legs and stained for Lineage, Sca-1, ckit, CD48, IL7R, and BrdU. In two independent experiments, each with n = 4, 41 and 33% of Lin- Sca-1+ cKit+ (LSK) IL-7R- CD48- cells from poly I:C-treated mice had incorporated BrdU, compared to 7 and 10% in cells from PBS-treated mice. These data support recently published reports. Total bone marrow cellularity was reduced to 45 and 57% in the two experiments, indicating either a rapid death and/or mobilization of marrow cells. Despite this dramatic loss of hematopoietic cells from the bone marrow of poly I:C treated mice, the number of IL-7R- CD48- LSK cells increased 145 and 308% in the two independent experiments. Importantly, the level of Sca-1 expression increased dramatically in the bone marrow of poly I:C-treated mice. Both the percent of Sca-1+ cells and the expression level of Sca-1 on a per cell basis increased after twenty-four hours of poly I:C, with some cells acquiring levels of Sca-1 that are missing from control bone marrow. These data were duplicated in vitro. When total marrow cells were cultured overnight in media containing either PBS or 25mg/mL poly I:C, percent of Sca-1+ cells increased from 23.6 to 43.7%. Within the Sca-1+ fraction of poly I:C-treated cultures, 16.7% had acquired very high levels of Sca-1, compared to only 1.75% in control cultures. Quantitative RT-PCR was employed to measure a greater than 2-fold increase in the amount of Sca-1 mRNA in poly I:C-treated cultures. Whereas the numbers of LSK cells increased in vivo, CD150+/− CD48- IL-7R- Lin- Sca-1- cKit+ myeloid progenitors almost completely disappeared following poly I:C treatment, dropping to 18.59% of control marrow, a reduction that is disproportionately large compared to the overall loss of hematopoietic cells in the marrow. These cells are normally proliferative, with 77.1 and 70.53% accumulating BrdU during the 2-hour pulse in PBS and poly I:C-treated mice, respectively. Interestingly, when Sca-1 is excluded from the analysis, the percent of Lin- IL7R- CD48- cKit+ cells incorporating BrdU decreases following poly I:C treatment, in keeping with interferon's published role as a cell cycle repressor. One possible interpretation of these data is that the increased proliferation of LSK cells noted by us and others is actually the result of Sca-1 acquisition by normally proliferating Sca-1- myeloid progenitors. This new hypothesis is currently being investigated. Disclosures: No relevant conflicts of interest to declare.

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Metabolic Regulation of Hematopoietic Stem Cells During Stress

Blood ◽

10.1182/blood.v120.21.sci-42.sci-42 ◽

2012 ◽

Vol 120 (21) ◽

pp. SCI-42-SCI-42

Author(s):

Toshio Suda

Keyword(s):

Cell Cycle ◽

Stem Cells ◽

Bone Marrow ◽

Stem Cell ◽

Hematopoietic Stem Cells ◽

Metabolic Regulation ◽

Conflicts Of Interest ◽

Hypoxia Inducible Factor ◽

Ros Generation ◽

Hematopoietic Stem

Abstract Abstract SCI-42 Tissue homeostasis over the life of an organism relies on both self-renewal and multipotent differentiation of stem cells. Hematopoietic stem cells (HSCs) are sustained in a specific microenvironment known as the stem cell niche. Adult HSCs are kept quiescent during the cell cycle in the endosteal niche of the bone marrow. Normal HSCs maintain intracellular hypoxia, stabilize the hypoxia-inducible factor-1a (HIF-1a) protein, and generate ATP by anaerobic metabolism. In HIF-1a deficiency, HSCs became metabolically aerobic, lost cell cycle quiescence, and finally became exhausted. An increased dose of HIF-1a protein in VHL-mutated HSCs and their progenitors induced cell cycle quiescence and accumulation of HSCs in the bone marrow (BM), which were not transplantable. This metabolic balance promotes HSC maintenance by limiting the production of reactive oxygen species (ROS), but leaves HSCs susceptible to changes in redox status (1). We have performed the metabolomic analysis in HSCs. Upregulation of pyruvate dehydrogenase kinases enhanced the glycolytic pathway, cell cycle quiescence, and stem cell capacity. Thus, HSCs directly utilize the hypoxic microenvironment to maintain their slow cell cycle by HIF-1a-dependent metabolism. Downregulation of mitochondrial metabolism might be reasonable, since it reduces ROS generation. On the other hand, at the time of BM transplantation, HSCs activate oxidative phosphorylation to acquire more ATP for proliferation. Autophagy also energizes HSCs by providing amino acids during transplantation. ATG (autophagy-related) 7 is essential for transplantation and metabolic homeostasis. The relationship between mitochondrial heat shock protein, mortalin, and metabolism in HSCs will also be discussed. Disclosures: No relevant conflicts of interest to declare.

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Periostin Acts As An Important Cell Cycle Regulator Of Adult Hematopoietic Stem Cells Via Binding To Integrin-αvβ3

Blood ◽

10.1182/blood.v122.21.341.341 ◽

2013 ◽

Vol 122 (21) ◽

pp. 341-341 ◽

Cited By ~ 2

Author(s):

Satish Khurana ◽

Catherine M. Verfaillie

Keyword(s):

Cell Cycle ◽

Stem Cells ◽

Hematopoietic Stem Cells ◽

Conflicts Of Interest ◽

Integrin Αvβ3 ◽

Brdu Incorporation ◽

Hematopoietic Stem ◽

Hematopoietic System ◽

Cell Cycle Regulator ◽

Blocking Antibodies

Osteoblasts are one of the important cellular components of the niche for hematopoietic stem cells (HSCs) in mammalian bone marrow (BM). Integrin receptors not only play a key role in HSC adhesion within the BM niche but also transfer regulatory signals from the microenvironment to HSCs. Periostin (Postn or osteoblast specific factor-1; OSF-1) is expressed in osteoblasts in addition to many other tissues, and acts as a ligand for Integrin-αvβ3 (ITGAV-B3). We identified POSTN as an important regulator of the cell cycle in adult murine HSCs. POSTN inhibited culture induced proliferation of HSCs thereby decreasing the total number of cells following 2-5 day culture of primitive HSCs, identified as CD150+CD48-Lin-Sca-1+c-kit+ (CD150 KLS) cells with SCF and TPO, while increasing the proportion of long-term (LT-) HSCs. Culture for 5 days with POSTN decreased the short-term (ST-) engraftment of progeny of 200 CD150 KLS cells, while significantly increasing LT- engraftment of the donor derived cells. A significant fraction of CD150 KLS cells expressed ITGAV as well as ITGB3. POSTN did not affect proliferation of HSCs in vitro following blocking of ITGAV with neutralizing antibodies. Among the important cell cycle regulators, we found an increase in p27kip1 expression in HSCs. Preliminary studies on possible signaling mechanisms involved, showed that POSTN inhibits Akt phosphorylation, known to mediate inhibition of both expression and activation of p27Kip1. Intravenous infusion of recombinant POSTN protein significantly decreased proliferation of hematopoietic progenitors as shown by Brdu incorporation and Hoechst/Pyronin staining. Interestingly, POSTN infusion also led to an increase in the number of KLS as well as CD150 KLS cells in the BM. Studies on characterization of the hematopoietic system of Postn-/- mice are underway. To further determine the role of ITGAV in HSCs, we used blocking antibodies against ITGAV and performed homing and engraftment studies. No effect on either homing potential or engraftment of ST- and LT- engraftment was seen. However, the competitive repopulation of ITGAV- CD150 KLS cells was significantly lower that that of ITGAV+ CD150 KLS cells (isolated using non-blocking antibodies). Therefore, our studies confirm the importance of ITGAV expression on primitive HSCs as well as presents POSTN as an important cell cycle regulator in the hematopoietic system. Disclosures: No relevant conflicts of interest to declare.

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Noncanonical Wnt Signaling Maintains Hematopoietic Stem Cells in Different Zones

Blood ◽

10.1182/blood.v120.21.639.639 ◽

2012 ◽

Vol 120 (21) ◽

pp. 639-639

Author(s):

Ryohichi Sugimura ◽

Linheng Li

Keyword(s):

Stem Cells ◽

Hematopoietic Stem Cells ◽

Wnt Signaling ◽

Conflicts Of Interest ◽

Quiescent State ◽

Perivascular Niche ◽

Hematopoietic Stem ◽

Knockout Mouse Model ◽

Quiescent Hscs

Abstract Abstract 639 Hematopoietic stem cells (HSCs) are maintained in a balance between quiescent state and proliferating state. While proliferating HSCs are critical for supporting the routine blood production, quiescent HSCs are essential for long term maintenance and can also be activated to replenish lost proliferating or active HSCs. How the different states of HSCs are regulated is a fundamental question. Accumulated evidence supports a model that quiescent HSCs are located in the endosteal zone and active HSCs are in the perivascular zone. The underlying signaling to regulate the quiescence and activation in different niches remains largely unknown. To address this question, we have analyzed the expression profile of Wnt receptors, Frizzleds, in HSCs. We found that noncanonical Wnt signaling via receptor Frizzled8 (Fz8) and co-receptor Flamingo presents in and functionally maintains quiescent HSCs in the endosteal zone (Sugimura, et al., Cell 2012). However, it has not been clear whether and how active HSCs in the perivascular zone are regulated by Wnt signaling. Recently, we detected that another noncanonical Wnt receptor, Frizzled5 (Fz5), is expressed in metabolically active (indicated by Mitotracker) HSCs and also in Nestin-GFP+ mesenchymal stem cells (MSCs) in the perivascular zone of central marrow. Fz5 expresses neither in H2B-GFP label-retaining quiescent HSCs nor in endosteal cells and nor in sinusoidal cells as well. Using an Mx1-Cre:Fz5 knockout mouse model, we found a 60% decrease of HSCs isolated from central marrow, but no change in the number of HSCs isolated from endosteum. Functionally, hematopoietic reconstitution was not affected in the primary transplantation, but was substantially decreased (by 80%) in the secondary transplantation compared to the control. This indicates that Fz5 maintains HSCs in the perivascular zone. To examine the role of Fz5 in Nestin+ MSCs for HSC maintenance, we examined the Nestin-Cre:Fz5 model. We observed a large loss of CD49hiHSCs that were reported to represent intermediate (IT) HSCs. We further found a correlation of the quiescent vs. active HSCs respectively to long term (LT) HSCs vs. IT-HSCs with the latter population sensitive to 5FU treatment. Mechanistically we observed that Fz5 inactivation also led to a loss of Cdc42 polarity in the HSCs residing in the perivascular niche. The results suggest that Fz5-mediated noncanonical Wnt signaling regulates polarity of active HSCs in the perivascular zones. Future study is required to see whether the Fz5-Cdc42 mediated polarity in HSCs is associated with symmetric vs. asymmetric division. We propose that noncanonical Wnt signaling maintains quiescent and active HSCs reside respectively in the endosteal zone and the perivascular zone. In these zones, Fz8 and Fz5 are differentially expressed and mediate noncanonical Wnt signaling for HSC maintenance in the endosteal niche and to regulate active HSC action in the perivascular niche. Disclosures: No relevant conflicts of interest to declare.

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Cell-Intrinsic and Cell-Extrinsic Involvement Of C/EBPβ In The Regulation Of Hematopoietic Stem Cells

Blood ◽

10.1182/blood.v122.21.1202.1202 ◽

2013 ◽

Vol 122 (21) ◽

pp. 1202-1202

Author(s):

Akihiro Tamura ◽

Hideyo Hirai ◽

Yoshihiro Hayashi ◽

Asumi Yokota ◽

Atsushi Sato ◽

...

Keyword(s):

Stem Cells ◽

Steady State ◽

Hematopoietic Stem Cells ◽

Progenitor Cells ◽

Peripheral Blood ◽

Leucine Zipper ◽

Conflicts Of Interest ◽

Chronic Phase ◽

Hematopoietic Stem ◽

The Difference

Abstract Our previous findings have revealed the requirement of CCAAT Enhancer Binding Protein β (C/EBPβ), a leucine zipper transcription factor, in emergency granulopoiesis (Hirai et al. Nat Immunol, 2006). During emergency situations such as infection, C/EBPβ is involved in the sufficient supply of granulocytes through amplification of hematopoietic stem/progenitor cells (Satake et al. J Immunol, 2012). In addition, we have shown that C/EBPβ is upregulated by downstream signaling of BCR-ABL and promotes myeloid expansion and leukemic stem cells exhaustion in chronic phase chronic myeloid leukemia (Hayashi et al. Leukemia, 2013). These observations suggested that C/EBPβ plays important roles in normal hematopoietic stem cells (HSCs). Here we investigated the cell-intrinsic and -extrinsic function of C/EBPβ in the regulation of HSCs by analyzing C/EBPβ knockout (KO) mice. At steady state, no obvious defects have been reported in hematopoiesis of C/EBPβ KO mice. Accordingly, the frequencies of long-term and short-term HSCs and various kinds of progenitor cells in bone marrows (BM) of C/EBPβ KO mice were identical to those in BM of wild type (WT) mice. To examine the functional consequences of C/EBPβ deletion, competitive repopulation assay was performed. In brief, 5x105 BM cells from WT or C/EBPβ KO mice (CD45.2+) and the same number of competitor CD45.1+ BM cells were transplanted into lethally irradiated CD45.1+ mice and the chimerisms of CD45.2+ cells in the peripheral blood of the recipient mice were monitored monthly. The chimerisms of C/EBPβ KO cells were significantly lower than that of WT cell at 1 month after transplantation and the differences were maintained thereafter (Figure A). In order to elucidate the reason for the difference, homing ability of C/EBPβ KO cells were assessed. Lineage depleted CD45.2+ WT or C/EBPβ KO BM cells together with the equal number of lineage negative CD45.1+ BM cells were transplanted into lethally irradiated CD45.1+ mice and the frequencies of CD45.2+ cells were analyzed 16 hours after transplantation. The frequencies of CD45.2+ WT and C/EBPβ KO donor cells in the recipient BMs were identical and the data indicated that the differences in the chimerisms after primary BM transplantation were due to the difference in the initial expansion of transplanted cells after equivalent levels of homing. To see the roles of C/EBPβ in hematopoiesis under stressed conditions, CD45.1+ mice were transplanted with CD45.2+ WT or C/EBPβ KO BM cells with equal numbers of CD45.1+ BM cells and these mice were administered with 150mg/kg 5-fluorouracil (5-FU) once a month and the chimerisms of peripheral blood were monitored every time before the next 5-FU administration. In consistent with the results mentioned above, the frequencies of CD45.2+ C/EBPβ KO cells were significantly lower than those of CD45.2+ WT cells 1 month after transplantation. After repetitive administration of 5-FU, however, the chimerisms of CD45.2+ C/EBPβ KO cells gradually caught up with those of CD45.2+ WT cells, suggesting that C/EBPβ is involved in the exhaustion of HSCs under stressed conditions (Figure B). To explore the functions of C/EBPβ in hematopoietic microenvironments, 1x106 CD45.1+ BM cells from WT mice were transplanted into irradiated (5Gy or 7Gy) WT or C/EBPβ KO mice (CD45.2+). All the WT recipient mice survived after 5Gy or 7Gy irradiation (4/4 and 4/4, respectively). In contrast, only 2/4 and 1/4 C/EBPβ KO recipient mice survived after 5Gy or 7Gy irradiation, respectively. We are currently trying to identify the cells expressing C/EBPβ in BM microenvironments and investigating the mechanisms for the higher sensitivity of C/EBPβ KO mice to irradiation. In summary, these data suggested that C/EBPβ is required for initial expansion of hematopoietic stem/progenitor cells at the expense of HSCs under stressed conditions, while it is dispensable for maintenance of HSCs at steady state. We are now investigating the cellular and molecular targets of C/EBPβ in HSC regulation and would like to elucidate the cell-intrinsic and cell-extrinsic mechanisms in regulation of the homeostasis of hematopoietic system by C/EBPβ. Disclosures: No relevant conflicts of interest to declare.

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Adipocyte-Derived Adiponectin Positively Regulates Exit from Quiescence of Hematopoietic Stem Cells By Potentiating mTORC1 Activation after Myelotoxic Injury

Blood ◽

10.1182/blood.v126.23.777.777 ◽

2015 ◽

Vol 126 (23) ◽

pp. 777-777 ◽

Cited By ~ 1

Author(s):

Yosuke Masamoto ◽

Shunya Arai ◽

Tomohiko Sato ◽

Iseki Takamoto ◽

Naoto Kubota ◽

...

Keyword(s):

Cell Cycle ◽

Stem Cells ◽

Adipose Tissue ◽

Steady State ◽

Hematopoietic Stem Cells ◽

Hematopoietic Stem ◽

Hematopoietic Recovery ◽

Mtorc1 Activation

Abstract Myelotoxic injury unlocks the vigorous power of hematopoietic stem cells (HSCs) to replenish the hematopoietic system, making quiescent HSCs enter the cell cycle. Microscopically, it is well known that adipose tissue replaces cellular components in bone marrow (BM) after myeloablation by chemotherapeutic agents or irradiation. In a steady-state hematopoiesis, both HSC-intrinsic and -extrinsic mechanisms enforce quiescence of HSCs, and the interaction between HSCs and BM microenvironment has been drawing much attention in maintenance of the quiescence. In this meaning, it is supposed that the drastic change in BM microenvironment by myeloablation might trigger and promote the cell cycle entry of HSCs. We have previously reported that adiponectin, adipocyte-derived anti-diabetic hormone, indirectly enhances proliferation of murine immature myeloid progenitors upon granulocyte-colony stimulating factor treatment (emergency granulopoiesis) in Socs3-Stat3 dependent fashion by suppressing TNF-α production from macrophages (ASH meeting 2013, Abstract 221), however, its direct effect against HSCs in vivo is needed to be elucidated. Additionally, we have shown that both genetic loss and high-fat diet-induced reduction of adiponectin have no impact on steady-state hematopoiesis. Considering BM adipose tissue is an endocrine organ and adipocytes are the major cellular component in ablated marrow, we hypothesized that adiponectin derived from adipocytes might be implicated in HSC activation and subsequent hematopoietic recovery. Adiponectin-null (adipo-/-) mice showed significantly delayed hematopoietic recovery after 5-fluorouracil (5-FU) administration. In 5-FU-treated BM, adipo-/- SLAM-HSCs (CD150+ CD48- Lin- Sca-1+) and CD34- SLAM-HSCs were more quiescent than adipo+/+ counterparts. Adipo-/- mice survived longer than adipo+/+ control mice after serial 5-FU treatment. Taken into account our previous data showing that impaired emergency granulopoiesis of adipo-/- mice is Socs3-Stat3 dependent and Socs3 haploinsufficiency ameliorated the defect, we further investigated whether activation of adipo-/- HSCs on 5-FU treatment was potentiated by genetic loss of Socs3. But Socs3 haploinsufficiency had no capacity to revert impaired activation of adipo-/- HSCs, suggesting some mechanisms other than that of impaired emergency granulopoiesis in adipo-/- mice. Strikingly, adipo-/- HSCs were shown to be defective in mTORC1 activation, phosphorylation of S6 and mitochondrial activity after 5-FU treatment. In vivo rapamycin treatment cancelled the effect of adiponectin upon HSC activation by 5-FU, suggesting that adiponectin enhances HSC activation through mTORC1-dependent mechanism. Physiological isoform of adiponectin (full-length adiponectin) enhanced not only 5-FU-induced mTORC1 activation in vivo but also cytokine-induced activation in vitro, shortened the time to first division, without affecting subsequent proliferation of HSCs, in contrast to the previous report using non-physiological isoform (globular adiponectin) in vitro. The concentration of adiponectin in BM had a 4-fold increase after 5-FU treatment while the level in plasma remained unchanged. In a steady state, adiponectin level in adipocyte-rich tibia is higher than femur, suggesting local production of adiponectin constitutes a significant portion of BM adiponectin. Every BM cell components examined expressed adiponectin mRNA, and adipocytes had the highest. As 5-FU treatment had little effect on adiponectin expression in adipocytes, it was suggested that increased adipocytes in BM contributed to increased adiponectin upon myelotoxic injury. Furthermore, reciprocal transplants with adipo+/+ and adipo-/- mice demonstrated that adiponectin from BM environment of recipient mice plays a major role in the activation of HSC after in vivo 5-FU treatment and in vitro cytokine stimulation. These data reveal that adiponectin, produced mainly from increased adipocytes after myelotoxic injury, positively regulates HSC activation and subsequent hematopoietic recovery. Our data also highlight adipocytes as an essential source of adiponectin to ensure the proliferative burst of hematopoietic cells in myeloablated marrow. Adiponectin treatment could be clinically applied to relieve myelosuppression by chemotherapy. Disclosures No relevant conflicts of interest to declare.

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PDK1 Controls the Differentiation of Hematopoietic Stem Cells Via Modulating ROS Levels

Blood ◽

10.1182/blood.v126.23.896.896 ◽

2015 ◽

Vol 126 (23) ◽

pp. 896-896

Author(s):

Tianyuan Hu ◽

Cong Li ◽

Le Wang ◽

Yingchi Zhang ◽

Luyun Peng ◽

...

Keyword(s):

Stem Cells ◽

Bone Marrow ◽

Hematopoietic Stem Cells ◽

Conflicts Of Interest ◽

Conditional Knockout ◽

Quiescent State ◽

Self Renewal ◽

Hematopoietic Stem ◽

Hematopoietic System ◽

Cell Counts

Abstract Hematopoietic stem cells (HSCs) exist as a rare population with two essential properties of self-renewal and differentiation. HSCs can give rise to all hematopoietic progenitor and mature cells. While critical for a full understanding of the hematopoietic process and HSC-related clinical applications, the mechanisms of self-renewal and differentiation of HSCs remain elusive. The PI3K-Akt signaling pathway plays essential roles in the regulation of hematopoiesis. 3-Phosphoinositide-dependent protein kinase 1 (PDK1) activates multiple AGC kinases including Akt and is a pivotal regulator in this pathway. PDK1 phosphorylates Akt at its T308 residue and regulates the functional development of B and T cells during hematopoiesis. However, the role of PDK1 in HSCs has not been fully defined. In this study, we generated PDK1 conditional knockout mice Vav-Cre;PDK1fl/fl (PDK1Δ/Δ) to explore the roles of PDK1 in HSCs. While PDK1Δ/Δ mice have reduced B and T cell counts as previously described, their LT-HSCs and ST-HSCs were significantly increased in comparison with WT mice while MPPs and CMPs were decreased after PDK1 deletion, indicating that the loss of PDK1 perturbed the steady-state hematopoiesis. Furthermore, although deletion of PDK1 increased the frequency of HSCs, PDK1-deficient HSCs fail to reconstitute the hematopoietic system when PDK1-deficient HSCs were used in bone marrow transplantation and competitive transplantation experiments in comparison to the WT HSCs, indicating that PDK1 is vital for hematopoiesis. To explore the mechanisms by which PDK1 regulates HSC function, we examined the cell cycle status and found the percentage of PDK1Δ/Δ HSCs was decreased significantly in G0 stage while increased in G1 and S/G2/M phases. This suggests an increase in HSC exit from a quiescent state. Since MPPs were significantly decreased in bone marrow, we examined the percentage of Annexin V+ DAPI- PDK1Δ/Δ and WT MPPs and found that they are comparable. This indicates that apoptosis did not cause the decrease in MPPs. In addition, a total of 300 LT-HSCs from PDK1Δ/Δ or WT mice and competitor cells were transplanted into lethally irradiated recipient mice to examine whether the decrease in MPPs is due to a defect in HSC differentiation. We found that less than 1% of MPPs arose from PDK1Δ/Δ HSCs 12 weeks after transplantation, indicating that PDK1 is required for the differentiation from LT-HSCs to MPPs. Because the full activation of Akt requires cooperative phosphorylation at its S473 and T308 residues by mTORC2 and PDK1, respectively, we also investigated the function of HSCs in RictorΔ/Δ PDK1Δ/Δ (DKO) mice in conjunction with RictorΔ/Δ or PDK1Δ/Δ mice to explore how mTORC2 and/or PDK1 influence Akt function in HSCs. The flow cytometric analyses of peripheral blood and bone marrow samples revealed very similar parameters of RictorΔ/Δ PDK1Δ/Δ and PDK1Δ/Δ mice. Interestingly, Rictor seemed to exert a minimal impact on HSCs and MPPs. More importantly, in contrast to RictorΔ/Δ, RictorΔ/Δ PDK1Δ/Δ HSCs failed to reconstitute the hematopoietic system after transplantation as PDK1Δ/Δ HSCs, suggesting that PDK1 plays a dominant role in the Akt-mediated regulation of HSC function. To explore the mechanism that leads to the defect in HSCs due to loss of PDK1, we assessed ROS levels in PDK1-deficient HSCs and found that PDK1-deficient LSKs and HSCs exhibit greatly reduced ROS levels when compared with the control HSCs. Treating PDK1-deficient BM cells with BSO in vitro increased cellular ROS levels and the colony counts of PDK1-deficient BM cells significantly. Notably, the recovery effect was only observed with BSO concentrations lower than 0.03 mM. This suggests that ROS levels are precisely controlled in HSCs. Higher or lower ROS levels beyond the normal range are both harmful to normal HSC functions. Since increased SDFα expression is associated with cellular ROS levels in various cells including hematopoietic cells, we also treated PDK1Δ/Δ mice with SDFα and found that it couldpartially rescue the defective differentiation ability of PDK1-deficient HSCs. In addition, we found that PDK1 deletion could significantly prolong the life span and inhibit the leukemia development in murine T-ALL model via altering leukemic cell differentiation and proliferation. Taken together, PDK1 controls HSC differentiation via regulating cellular ROS levels and regulates malignant hematopoiesis. Disclosures No relevant conflicts of interest to declare.

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