scholarly journals Reticular Dysgenesis-Associated Adenylate Kinase 2 Deficiency Impairs Hematopoietic Stem and Progenitor Cell Function through Reductive Stress

Blood ◽  
2020 ◽  
Vol 136 (Supplement 1) ◽  
pp. 33-33
Author(s):  
Wenqing Wang ◽  
Andrew Devilbiss ◽  
Thomas Mathews ◽  
Martin Arreola ◽  
Misty Martin ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Mitochondrial Respiration ◽  
Adenylate Kinase ◽  
Mitochondrial Membrane ◽  
Cell Cycle Progression ◽  
Tca Cycle ◽  
Cycle Progression ◽  
Hematopoietic Stem ◽  
Knock Out ◽  
Reductive Stress

Energy deficiency and redox stress are hallmarks of mitochondrial pathology. Reductive stress is marked by an accumulation of reducing species and can arise from defects in the electron transport chain (ETC) that prevent NAD+ regeneration from NADH. Reticular Dysgenesis (RD) is a particularly grave form of severe combined immunodeficiency (SCID), characterized by maturation arrest of both myeloid and lymphoid lineages. Unlike other forms of SCID, RD is a mitochondriopathy caused by biallelic mutations in the mitochondrial enzyme adenylate kinase 2 (AK2). AK2 catalyzes the phosphorylation of adenosine monophosphate (AMP) to adenosine diphosphate (ADP), and maintains ADP availability for ATP synthase. We hypothesize that AK2 deficiency leads to decreased ETC activities and defective NAD+ regeneration. Emerging evidence suggests that the development of hematopoietic stem and progenitor cells (HSPCs) is intricately intertwined with various aspects of mitochondrial function. Investigating the cellular and molecular consequences of AK2 deficiency during myelopoiesis provides fundamental insight into the pathology of many mitochondrial disorders. Methods: To recapitulate RD myeloid maturation defects, we developed an AK2 biallelic knock out model in human HSPCs using CRISPR gene editing. HSPCs were edited at the AK2 locus, and cells with biallelic AK2 knock out were enriched using homologous recombination-mediated dual reporters. HSPCs edited at the safe harbor locus AAVS1 were used as a control. When differentiated along the myeloid lineage in vitro, AK2-/- HSPCs showed significantly decreased proliferation, lower commitment to the granulocytic lineage, and maturation arrest at the promyelocyte stage, mimicking the presentation of RD patients. To dissect differentiation stage specific changes in metabolism, metabolomics analysis (LC-MS/MS), metabolic flux analysis (Seahorse assays) and RNA-seq were performed on FACS sorted populations of promyelocytes (PMs), metamyelocytes (MCs) and neutrophils (NPs). Additionally, mitochondrial membrane potential and ribosomal RNA (rRNA) content were quantified using TMRM and pyronin Y staining. Results: AK2-/- MCs and NPs showed higher AMP levels, and increased AMP/ADP and AMP/ATP ratios, in line with AK2's function to regenerate ADP from AMP. Mitochondrial oxygen consumption rate decreased, and mitochondrial membrane potential increased in AK2-/- MCs and NPs, indicating defective ETC function and ATP synthesis. Consistent with these results, TCA cycle metabolites were downregulated while pathways that fuel the TCA cycle, i.e. glycolysis and fatty acid oxidation, were upregulated. Interestingly, we observed a significant decrease in NAD+ levels, and an increase in NADH/NAD+ and GSH/GSSG ratios in AK2-/- MCs and NPs, indicative of reductive stress. These results suggest that AK2 deficiency compromises mitochondrial respiration, leading to NAD+ depletion and reductive stress in later stages of myeloid development. Defective mitochondrial respiration has been shown to impair NAD+-dependent aspartate and purine biosynthesis. In AK2-/- MCs and NPs, we observed a profound aspartate depletion and build-up of the purine precursor inosine monophosphate (IMP). As a building block for DNA and RNA, purine deficiency is known to block cell proliferation. Genes in cell cycle and ribosomal biogenesis pathways were down regulated in AK2-/- MCs and NPs. In addition, rRNA content was significantly decreased. These data raise the possibility that purine deficiency in AK2-/- HSPCs compromises nucleotide/protein synthesis along with cell cycle progression. Conclusions: Using an AK2 biallelic knock out HSPC model for RD, we have shown that defective mitochondrial respiration in AK2-/- HSPCs leads to reductive stress, NAD+ and purine depletion resulting in compromised nucleotide/protein synthesis and impaired cell cycle progression. Notably, these defects worsen as myeloid maturation progresses, possibly reflecting the increasing mitochondrial metabolic demand. We are currently exploring whether correcting the NADH/NAD+ ratio in AK2-/- HSPCs improves purine synthesis and restores myelopoiesis. Understanding how redox metabolism governs HSPC differentiation will not only allow us to delineate metabolic changes during development, but enable us to develop novel therapies for RD and other mitochondrial disorders. Disclosures Dever: Integral Medicines: Current Employment.

Homing efficiency, cell cycle kinetics, and survival of quiescent and cycling human CD34+ cells transplanted into conditioned NOD/SCID recipients

Blood ◽  
2002 ◽  
Vol 99 (5) ◽  
pp. 1585-1593 ◽  
Author(s):  
Anna Jetmore ◽  
P. Artur Plett ◽  
Xia Tong ◽  
Frances M. Wolber ◽  
Robert Breese ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Cell Cycle Progression ◽  
Hematopoietic Cells ◽  
Cycle Progression ◽  
Hematopoietic Stem ◽  
Human Cd34 ◽  
Nonobese Diabetic ◽  
Cd34 Cells ◽  
G1 Cells ◽  
Active Proliferation

Differences in engraftment potential of hematopoietic stem cells (HSCs) in distinct phases of cell cycle may result from the inability of cycling cells to home to the bone marrow (BM) and may be influenced by the rate of entry of BM-homed HSCs into cell cycle. Alternatively, preferential apoptosis of cycling cells may contribute to their low engraftment potential. This study examined homing, cell cycle progression, and survival of human hematopoietic cells transplanted into nonobese diabetic severe combined immunodeficient (NOD/SCID) recipients. At 40 hours after transplantation (AT), only 1% of CD34+ cells, or their G0(G0CD34+) or G1(G1CD34+) subfractions, was detected in the BM of recipient mice, suggesting that homing of engrafting cells to the BM was not specific. BM of NOD/SCID mice receiving grafts containing approximately 50% CD34+ cells harbored similar numbers of CD34+ and CD34− cells, indicating that CD34+ cells did not preferentially traffic to the BM. Although more than 64% of human hematopoietic cells cycled in culture at 40 hours, more than 92% of cells recovered from NOD/SCID marrow were quiescent. Interestingly, more apoptotic human cells were detected at 40 hours AT in the BM of mice that received xenografts of expanded cells in S/G2+M than in recipients of G0/G1 cells (34.6% ± 5.9% and 17.1% ± 6.3%, respectively; P < .01). These results suggest that active proliferation inhibition in the BM of irradiated recipients maintains mitotic quiescence of transplanted HSCs early AT and may trigger apoptosis of cycling cells. These data also illustrate that trafficking of transplanted cells to the BM is not selective, but lodgment of BM-homed cells may be specific.

scholarly journals Expression of Cell Cycle–Related Genes With Cytokine-Induced Cell Cycle Progression of Primitive Hematopoietic Stem Cells

2010 ◽  
Vol 19 (4) ◽  
pp. 453-460 ◽  
Author(s):  
Peter J. Quesenberry ◽  
Gerri J. Dooner ◽  
Michael Del Tatto ◽  
Gerald A. Colvin ◽  
Kevin Johnson ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Stem Cells ◽  
Hematopoietic Stem Cells ◽  
Cell Cycle Progression ◽  
Cycle Progression ◽  
Hematopoietic Stem

scholarly journals Loss of ISWI ATPase SMARCA5 (SNF2H) in Acute Myeloid Leukemia Cells Inhibits Proliferation and Chromatid Cohesion

2020 ◽  
Vol 21 (6) ◽  
pp. 2073
Author(s):  
Tomas Zikmund ◽  
Helena Paszekova ◽  
Juraj Kokavec ◽  
Paul Kerbs ◽  
Shefali Thakur ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Acute Myeloid Leukemia ◽  
Myeloid Leukemia ◽  
Cell Cycle Progression ◽  
Mitotic Division ◽  
Cycle Progression ◽  
Hematopoietic Stem ◽  
Chromatid Cohesion ◽  
Acute Myeloid Leukemia Cells ◽  
Acute Myeloid

ISWI chromatin remodeling ATPase SMARCA5 (SNF2H) is a well-known factor for its role in regulation of DNA access via nucleosome sliding and assembly. SMARCA5 transcriptionally inhibits the myeloid master regulator PU.1. Upregulation of SMARCA5 was previously observed in CD34+ hematopoietic progenitors of acute myeloid leukemia (AML) patients. Since high levels of SMARCA5 are necessary for intensive cell proliferation and cell cycle progression of developing hematopoietic stem and progenitor cells in mice, we reasoned that removal of SMARCA5 enzymatic activity could affect the cycling or undifferentiated state of leukemic progenitor-like clones. Indeed, we observed that CRISPR/cas9-mediated SMARCA5 knockout in AML cell lines (S5KO) inhibited the cell cycle progression. We also observed that the SMARCA5 deletion induced karyorrhexis and nuclear budding as well as increased the ploidy, indicating its role in mitotic division of AML cells. The cytogenetic analysis of S5KO cells revealed the premature chromatid separation. We conclude that deleting SMARCA5 in AML blocks leukemic proliferation and chromatid cohesion.

CCND1–CDK4–mediated cell cycle progression provides a competitive advantage for human hematopoietic stem cells in vivo

2015 ◽  
Vol 210 (2) ◽  
pp. 2102OIA144
Author(s):  
Nicole Mende ◽  
Erika E Kuchen ◽  
Mathias Lesche ◽  
Tatyana Grinenko ◽  
Konstantinos D Kokkaliaris ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Stem Cells ◽  
Hematopoietic Stem Cells ◽  
Competitive Advantage ◽  
Cell Cycle Progression ◽  
Cycle Progression ◽  
Hematopoietic Stem

Valproic Acid Accelerates Cell Cycle Progression of Hematopoietic Stem Cells Via the Activation of GSK3beta-Dependent Signaling Pathways.

Blood ◽  
2004 ◽  
Vol 104 (11) ◽  
pp. 1708-1708
Author(s):  
Gesine Bug ◽  
Hilal Gul ◽  
Kerstin Schwarz ◽  
Manuela Kampfmann ◽  
Xiaomin Zheng ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Stem Cells ◽  
Valproic Acid ◽  
Wnt Signaling ◽  
Cell Cycle Progression ◽  
Beta Catenin ◽  
Cycle Progression ◽  
Hematopoietic Stem ◽  
Cd34 Cells ◽  
Colony Assays

Abstract Histone deacetylase inhibitors (HDI) have attracted considerable attention because of their ability to overcome the differentiation block in leukemic blasts either alone, or in combination with differentiating agents such as all-trans retinoic acid (ATRA). We have previously reported favorable effects of the potent HDI valproic acid (VPA) in combination with ATRA in a small subset of patients with advanced acute myeloid leukemia (AML) leading to blast cell reduction and improvement of hemoglobin. This effect was accompanied by hypergranulocytosis most likely due to an enhancement of non-leukemic myelopoiesis and suppression of malignant hematopoiesis rather than enforced differentiation of leukemic cells. These data prompted us to investigate the impact of VPA on normal hematopoietic stem cells (HSC). Differentiation of cord blood-derived, purified CD34+ cells was assessed by FACS analysis after a 7-days suspension culture in presence of early acting cytokines and 30–150μg/mL VPA. VPA prevented differentation of CD34+ cells in a dose-dependent manner: concomitant with an increase of CD34+ cells from 17 to 47%, the proportion of monocytic CD14+ cells decreased from 27 to 3% (n=3). In addition, VPA induced a 30-fold amplification of CD34+ bone marrow (BM) cells within 10 days as determined by colony assays (n=3). To evaluate the functional capacity of VPA-treated HSC, murine Sca1+/lin−s cells were harvested from colony assays and replated. VPA treatment allowed up to four cycles of replating in contrast to VPA-naïve control cells. Further analysis demonstrated that the stimulatory effect of VPA on the in vitro growth and colony formation capacity of HSC was mainly due to accelerated cell cycle progression. VPA strongly increased the proportion of cells in S phase compared to untreated controls (38 vs. 17%, resp.), as detected by propidium iodid staining and BRDU incorporation as well as reduced expression of the CDK-inhibitor p21cip-1/waf using murine HSC after 7 days of culture. Downregulation of p21cip-1/waf was confirmed in CD34+ BM cells showing maximum inhibition after 48 hours of VPA treatment and no recovery thereafter. Recent results indicate that VPA exerts inhibitory activity on GSK3beta by phosphorylation on Ser-9 and stimulates Akt in human neuroblastoma cells. GSK3beta is an effector of the Wnt-signaling pathway located upstream of beta-catenin. Wnt-signaling can directly stimulate the proliferation of HSC, expand the HSC pool and lead to upregulation of HoxB4. Here we show that VPA increased the inhibition-associated phosphorylation of GSK3beta on Ser-9 in human CD34+ BM cells after 48 hours as well as in murine Sca1+/lin− cells after 7 days. Exposure to VPA enhanced beta-catenin and Akt activity not only in CD34+ HSC but also in KG-1 and TF-1 cells with maximum activation after 48 hours of VPA stimulation. Moreover, VPA lead to an 8-fold increase of the HoxB4 level in CD34+ BM cells as determined by real time PCR at 48 hours. In conclusion, we show that VPA i.) expands HSC as assesed by phenotype and function; ii.) accelerates cell cycle progression of HSC accompanied by the down-regulation of p21cip-1waf; iii.) activates the GSK3beta depending beta-catenin pathway and Akt and iv.) up-regulates HoxB4. Our data strongly suggest that VPA is able to influence some of the signaling pathway considered relevant for proliferation and self-renewal which might request reconsideration of their employment for the treatment of AML.

Cdk1-Dependent Phosphorylation ofNPM Overrides G2/M Checkpoint and Increases Leukemic Blasts in Mice

Blood ◽  
2008 ◽  
Vol 112 (11) ◽  
pp. 1322-1322
Author(s):  
Wei Du ◽  
Yun Zhou ◽  
Suzette Pike ◽  
Qishen Pang
Keyword(s):  
Cell Cycle ◽  
Cell Cycle Progression ◽  
Cell Cycle Control ◽  
Xenograft Model ◽  
Phosphorylation Sites ◽  
M Cell ◽  
Cycle Progression ◽  
Hematopoietic Stem ◽  
Cycle Arrest ◽  
Regulation Of Cell Cycle

Abstract An elevated level of nucleophosmin (NPM) is often found in actively proliferative cells including human tumors. To identify the regulatory role for NPM phosphorylation in proliferation and cell cycle control, a series of mutants targeting the consensus cyclin-dependent kinase (CKD) phosphorylation sites was created to mimic or abrogate either single-site or multi-site phosphorylation. Cells expressing the phosphomimetic NPM mutants showed enhanced proliferation and G2/M cell-cycle transition; whereas nonphosphorylatable mutants induced G2/M cell-cycle arrest. Simultaneous inactivation of two CKD phosphorylation sites at Ser10 and Ser70 (S10A/S70A, NPM-AA) induced phosphorylation of Cdk1 at Tyr15 (Cdc2Tyr15) and increased cytoplasmic accumulation of Cdc25C. Strikingly, stress-induced Cdk1Tyr15 and Cdc25C sequestration were completely suppressed by expression of a double phosphomimetic NPM mutant (S10E/S70E, NPM-EE). Further analysis revealed that phosphorylation of NPM at both Ser10 and Ser70 sites were required for proper interaction between Cdk1 and Cdc25C in mitotic cells. Moreover, the NPM-EE mutant directly bound to Cdc25C and prevented phosphorylation of Cdc25C at Ser216 during mitosis. Finally, NPM-EE overrided stress-induced G2/M arrest, increased peripheral-blood blasts and splenomegaly in a NOD/SCID xenograft model, and promoted leukemia development in Fanconi mouse hematopoietic stem/progenitor cells. Thus, these findings reveal a novel function of NPM on regulation of cell-cycle progression, in which Cdk1-dependent phosphorylation of NPM controls cell-cycle progression at G2/M transition through modulation of Cdc25C activity.

scholarly journals Nucleophosmin Regulates Cell Cycle Progression and Stress Response in Hematopoietic Stem/Progenitor Cells

2006 ◽  
Vol 281 (24) ◽  
pp. 16536-16545 ◽  
Author(s):  
June Li ◽  
Daniel P. Sejas ◽  
Reena Rani ◽  
Tara Koretsky ◽  
Grover C. Bagby ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Stress Response ◽  
Progenitor Cells ◽  
Cell Cycle Progression ◽  
Cycle Progression ◽  
Hematopoietic Stem

RUNX1/AML1 Is Phosphorylated at Both Its N- and C-Terminus by cdk6/cyclin D3 or cdk1/cyclin B.

Blood ◽  
2005 ◽  
Vol 106 (11) ◽  
pp. 1360-1360
Author(s):  
Florence Bernardin Fried ◽  
Alan D. Friedman
Keyword(s):  
Cell Cycle ◽  
Cell Cycle Progression ◽  
Cyclin B ◽  
Cyclin D3 ◽  
Phosphorylation Sites ◽  
Runt Domain ◽  
Cycle Progression ◽  
Hematopoietic Stem ◽  
Exogenous Dna ◽  
Cdk Phosphorylation

Abstract RUNX1/AML1 is a key transcriptional mediator of hematopoiesis and leukemogenesis. AML1 regulates myeloid and lymphoid differentiation via activation of lineage-specific genes such as those encoding myeloperoxidase or the T cell receptor δ and participates in apoptotic response pathways via its ability to tranactivate the p14/p19ARF gene. In addition, AML1 accelerates G1 to S cell cycle progression, via activation of the cyclin D3 and potentially the cdk4 genes. CBF oncoproteins such as AML1-ETO or CBFβ-SMMHC interfere with the activities of AML1 and block myeloid differentiation and slow cell cycle progression, and mutations such as loss of p16 which accelerate G1 prevent cell cycle inhibition and cooperate with CBF oncoproteins to induce acute leukemia in mice. In addition to regulation of the cell cycle by AML1, we have been interested in how AML1 activities vary and may be regulated during cell cycle progression. We recently reported that endogenous AML1 levels increase in hematopoietic cell lines as they progress from G1 to S and then diminish again at the end of mitosis (Bernardin-Fried et al J. Biol. Chem.279:15678, 2004). RNA levels did not vary, but exogenous AML1 mimicked the behaviour of the endogenous protein, suggesting regulation at the level of protein stability. Mutation of two Ras-dependent phosphorylation sites, S276 and S293, to alanine did not prevent cell cycle variation. We have therefore set out to evaluate whether AML1 stability might be regulated by cyclin-dependent kinase (cdk) phosphorylation. AML1 contains 480 amino acids and binds DNA via its N-terminal Runt domain. Both the cdk6/cyclin D3 and the cdk1/cyclin B complex, expressed from baculovirus vectors, phosphorylated GST-AML1(1-290) and GST-AML1(290–480). The Runt domain alone, in GST-AML1(86–217), was not phosphorylated. Interestingly, exogenous DNA-binding domain alone did not vary during the cell cycle. This is the first demonstration that a specific kinase phosphorylates AML1 in vitro. There are three (S/T)PX(K/R) cdk consensus sites in AML1, with serines at residues 48, 303, and 424. Mutation of S424 to alanine did not prevent phosphorylation of GST-AML1(290–480). Additional mutations of these and other serines or threonines adjacent to proline are being generated to further map the cdk phosphorylation sites and to enable in vivo experiments designed to evaluate the effects of these mutations on cell cycle-specific AML1 expression. We propose a model in which accumulated phosphorylation of AML1 during the S and G2/M cell cycle phases leads to ubiquitin-mediated AML1 destabilization at the end of mitosis. The increased stability of AML1 in the presence of proteosome inhibitors supports this model. Phosphorylation-mediated destabilization of AML1 may complement the recent finding that direct interaction of cyclin D3 with AML1 inhibits its activity as a transcriptional activator. Each of these mechanisms may help regulate the proliferation of hematopoietic stem/progenitor cells. Finally, perhaps loss of destabilizing C-terminal phosphorylation sites in the AML1-ETO oncoprotein increases its ability to dominantly repress AML1-target genes during myeloid leukemogenesis.

Nucleophosmin Regulates Differentiation, Cell Cycle Progression, and Stress Response in Hematopoietic Progenitor Cells.

Blood ◽  
2005 ◽  
Vol 106 (11) ◽  
pp. 312-312
Author(s):  
June Li ◽  
Daniel P. Sejas ◽  
Qishen Pang
Keyword(s):  
Cell Cycle ◽  
Bone Marrow ◽  
Stress Response ◽  
Progenitor Cells ◽  
Cell Cycle Progression ◽  
Proliferative Potential ◽  
Hematopoietic Progenitors ◽  
Cycle Progression ◽  
Hematopoietic Stem ◽  
Empty Vector

Abstract Nucleophosmin (NPM) is a multifunctional protein frequently overexpressed in actively proliferating cells including tumor and hematopoietic stem cells. Strong evidence indicates that NPM is involved in hematopoiesis and leukemic development. Here we report that NPM enhances the proliferative potential of hematopoietic stem/progenitor cells and increases cell survival upon stress challenge. Specifically, lin-Sca1+c-kit+ bone marrow cells transduced with retroviral vector expressing NPM exhibited higher proliferative rates in both short-term liquid culture and clonogenic progenitor cell assays, compared to the cells transduced with empty vector. Interestingly, NPM overexpression appears to inhibit differentiation of myeloid progenitors. Hematopoietic stem/progenitor cells infected with the NPM retrovirus expressed significantly lower levels of mature cell markers Gr-1 and Mac-1 compared to empty vector transduced cells, and majority of the NPM-overexpressing cells remained Sca1+C-Kit+ during the 5-day culture. Bone marrow transplantation experiments demonstrated that NPM overexpression increases long-term multi-lineage repopulating capacity of hematopoietic progenitors. We have not observed any evidence of proliferative disorders or leukemia in recipients transplanted with NPM-expressing progenitors thus far (4 months posttransplantation). Through cell-cycle profile analysis and single-cell division experiments, we showed that NPM overexpression induces rapid entry of hematopoietic progenitors into the cell cycle, probably via promoting G0/G1 to S transition. Furthermore, immunocytochemical and Western-blot analyses demonstrated that NPM-transduced cells expressed higher level of cyclin A compared to vector-transduced cells. Finally, overexpression of NPM significantly increased the survival of hematopoietic progenitors exposed to mitomycin C or hydrogen peroxide, suggesting that NPM can protect cells from DNA damage and oxidative stress. Together, these results indicate that NPM plays an important role in hematopoiesis via mechanisms involving modulation of progenitor differentiation, cell cycle progression, and stress response.

CD81 Is Essential for HSC Self-Renewal through Suppressing Proliferation

Blood ◽  
2008 ◽  
Vol 112 (11) ◽  
pp. 76-76 ◽  
Author(s):  
Kuanyin Karen Lin ◽  
Lara Rossi ◽  
Margaret A. Goodell
Keyword(s):  
Cell Cycle ◽  
Bone Marrow ◽  
Cell Cycle Progression ◽  
Background Level ◽  
Quiescent State ◽  
Self Renewal ◽  
Cycle Progression ◽  
Hematopoietic Stem ◽  
Transplantation Assay

Abstract Hematopoietic stem cells (HSCs) comprise only ~0.02% of the whole bone marrow cells but possess the capacity to extensively proliferate in order to restore hematopoietic homeostasis. Under homeostasis, HSCs are relatively quiescent with a slow cell cycle progression rate. However, upon stimulation, HSCs are able to promptly proliferate and undergo self-renewal to regenerate HSCs as daughter cells. While regulatory mechanisms involved in cell cycle progression are well characterized to be essential for HSC self-renewal, the mechanisms that facilitate the return of proliferating HSC to their quiescent state have been largely overlooked. The expression of CD81 (also called TAPA-1), a transmembrane protein that belongs to the Tetraspanin family, has been found associated with HSC proliferation. While CD81 is normally absent on HSC, it becomes markedly upregulated during HSC proliferation (Figure 1). To understand the function of CD81 in regenerating HSCs, we utilized a murine stem cell retroviral vector to deliver genes into 5-FU treated bone marrow progenitors to test the effect of enforced CD81 overexpression on HSC. The CD81-transduced proliferating progenitors were found to give rise to an increased number of phenotypically-defined HSC (SP-KLS) without significantly affecting the homeostasis in peripheral organs. In addition, we also characterized the HSCs from CD81 knock-out mice. We discovered that CD81-null HSC failed to engraft in peripheral blood of secondary recipients in serial transplantation assays (Figure 2), suggesting a role of CD81 in preserving a functional HSC compartment during proliferation-induced stress. When investigating further, we discovered that CD81 is a cell cycle suppressor for HSC, as the CD81KO HSCs are delayed in returning quiescence. In addition, clustering of CD81 on the HSC cell membrane using a monoclonal antibody rapidly induced a quiescent phenotype. This was found to be associated with an altered phosphorylation level of AKT, an inhibitor of the transcription factor FOXO1a and FOXO3a, which have been reported to be essential for HSC self-renewal through suppressing HSC proliferation. Taken together, these results demonstrate an essential role of CD81 in HSC self-renewal, and a novel mechanism that advances quiescence from a proliferating state. Figure 1. CD81 expression is upregulated at the time when HSCs (SPKLS) are proliferating in response to 5FU stimulation, a chemotheraputic agent that induces HSC to proliferate. The expression of CD81 is found at a background level in quiescent stages (5FU-Day0 and 5FU-Day11), and is upregulated during proliferation stages (starting 5FU-Day2) Figure 1. CD81 expression is upregulated at the time when HSCs (SPKLS) are proliferating in response to 5FU stimulation, a chemotheraputic agent that induces HSC to proliferate. The expression of CD81 is found at a background level in quiescent stages (5FU-Day0 and 5FU-Day11), and is upregulated during proliferation stages (starting 5FU-Day2) Figure 2. CD8KO HSCs fail to engraft in the secondary competitive transplantation assay, indicating a self-renewal defect. In this assay, 300 donor-derived HSCs (CD45.2 SPKLS) were purified from the primary recipients and transplanted along with 2×105 competitors into lethally irradiated mice (**p<0.01). Figure 2. CD8KO HSCs fail to engraft in the secondary competitive transplantation assay, indicating a self-renewal defect. In this assay, 300 donor-derived HSCs (CD45.2 SPKLS) were purified from the primary recipients and transplanted along with 2×105 competitors into lethally irradiated mice (**p<0.01).

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