Kit regulates maintenance of quiescent hematopoietic stem cells

Hematopoietic stem cell (HSC) numbers are tightly regulated and maintained in postnatal hematopoiesis. Extensive studies have supported a role of the cytokine tyrosine kinase receptor Kit in sustaining cycling HSCs when competing with wild-type HSCs posttransplantation, but not in maintenance of quiescent HSCs in steady state adult bone marrow. In this study, we investigated HSC regulation in White Spotting 41 (Kit(W41/W41)) mice, with a partial loss of function of Kit. Although the extensive fetal HSC expansion was Kit-independent, adult Kit(W41/W41) mice had an almost 2-fold reduction in long-term HSCs, reflecting a loss of roughly 10,000 Lin(-)Sca-1(+)Kit(high) (LSK)CD34(-)Flt3(-) long-term HSCs by 12 wk of age, whereas LSKCD34(+)Flt3(-) short-term HSCs and LSKCD34(+)Flt3(+) multipotent progenitors were less affected. Whereas homing and initial reconstitution of Kit(W41/W41) bone marrow cells in myeloablated recipients were close to normal, self-renewing Kit(W41/W41) HSCs were progressively depleted in not only competitive but also noncompetitive transplantation assays. Overexpression of the anti-apoptotic regulator BCL-2 partially rescued the posttransplantation Kit(W41/W41) HSC deficiency, suggesting that Kit might at least in the posttransplantation setting in part sustain HSC numbers by promoting HSC survival. Most notably, accelerated in vivo BrdU incorporation and cell cycle kinetics implicated a previously unrecognized role of Kit in maintaining quiescent HSCs in steady state adult hematopoiesis.     

A microRNA regulating adult hematopoietic stem cells

Comment on: Guo S, et al. Proc Natl Acad Sci USA 2010; 107:14229-34.     

A role for hematopoietic stem cells in promoting angiogenesis

Angiogenesis is an important event for embryonic organogenesis as well as for tissue repair in the adult. Here, we show that hematopoietic stem cells (HSCs) play important roles for angiogenesis during embryogenesis. To investigate the role of HSCs in endothelial cell (EC) development, we analyzed AML1-deficient embryos, which lack definitive hematopoiesis. These embryos showed defective angiogenesis in the head and pericardium. Para-aortic splanchnopleural (P-Sp) explant cultures on stromal cells (P-Sp culture) did not generate definitive hematopoietic cells and showed defective angiogenesis in the AML1 null embryo. Disrupted angiogenesis in P-Sp cultures from AML1 null embryos was rescued by addition of HSCs or angiopoietin-1 (Ang1). HSCs, which express Ang1, directly promoted migration of ECs in vivo and in vitro. These results indicate that HSCs are critical for angiogenesis.     

ROS与造血干细胞损伤研究进展

辐射可以通过引起造血干细胞（hematopoietic stem cell,HSC）内活性氧（reactive oxygen species,ROS）系统水平升高导致HSC损伤。HSC损伤患者出现难治性血液系统疾病,严重影响患者生存质量,甚至威胁患者生命。ROS可以通过多种机制引起组织、器官和细胞损伤。ROS的来源包括：线粒体、NOX（NADPH oxidases）、细胞色素P450酶、黄嘌呤氧化酶、非偶联NO合酶。已证实HSC内ROS来源于NOX。ROS升高后影响HSC在成骨细胞微环境定位,导致HSC与微环境相互作用减弱,从而影响HSC功能。此外,ROS升高后通过激活P38MAPK-P16Ink4途径,损伤HSC自我更新能力,并且使HSC定向分化产生更多的髓系克隆而不是红细胞系克隆;P13K—Akt-mTOR途径可能也是ROS诱导HSC损伤途径。ROS对细胞周期影响为：促使HSC离开G0期进入细胞周期,导致干细胞池的耗尽。基于NOX在氧化还原信号传递过程中的重要作用,证实辐射通过NOX产生的ROS以及鉴定产生ROS的NOX亚型,这一工作会为临床靶向治疗辐射诱发的血液系统疾病提供重要的价值。     

The mitochondrial protein OPA1 regulates the quiescent state of adult muscle stem cells

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Mll has a critical role in fetal and adult hematopoietic stem cell self-renewal

The Mixed Lineage Leukemia (Mll) gene is a homolog of Drosophila Trithorax commonly rearranged in infant leukemia. Comprehensive analysis of the role of Mll in hematopoiesis in fetal and adult knockout mice has been prevented by the lethality of Mll(-/-) mice. We have established a conditional deletion model that allows us to study adult hematopoiesis in the absence of Mll. In this study, Mll(-/-) embryos survive to E16.5 and have reduced numbers of HSCs. The quiescent fraction of these HSCs is greatly reduced, and they are unable to compete with wild-type cells in transplantation assays. Mice with Mll expression conditionally deleted in the hematopoietic system have grossly normal hematopoiesis in bone marrow, thymus, and spleen. However, transplanted Mll-deficient bone marrow cells are highly compromised in their ability to competitively reconstitute irradiated recipients. These results suggest a critical role for Mll in regulating stem cell self-renewal.     

Dexamethasone increases angiopoietin-1 and quiescent hematopoietic stem cells: a novel mechanism of dexamethasone-induced hematoprotection

Angiopoietin-1 (Ang-1) is known to have hematoprotective effects by increasing the quiescence of hematopoietic stem cells. However, it remains to be determined if the upregulation of Ang-1 and the subsequent increase in the quiescence of hematopoietic stem cells are also involved in the dexamethasone (Dex)-mediated bone marrow protection. Here Western blotting and flow cytometric analyses demonstrate that Dex increases the levels of Ang-1 in mouse bone marrow and the quiescence of hematopoietic stem cells. Our data for the first time suggest that the increased quiescence of hematopoietic stem cells provides a novel mechanism of Dex-induced hematoprotection.     

Mitophagy in hematopoietic stem cells

Hematopoietic stem cells (HSCs) are inherently quiescent and self-renewing, yet can differentiate and commit to multiple blood cell types. Intracellular mitochondrial content is dynamic, and there is an increase in mitochondrial content during differentiation and lineage commitment in HSCs. HSCs reside in a hypoxic niche within the bone marrow and rely heavily on glycolysis, while differentiated and committed progenitors rely on oxidative phosphorylation. Increased oxidative phosphorylation during differentiation and commitment is not only due to increased mitochondrial content but also due to changes in mitochondrial cytosolic distribution and efficiency. These changes in the intracellular mitochondrial landscape contribute signals toward regulating differentiation and commitment. Thus, a functional relationship exists between the mitochondria in HSCs and the state of the HSCs (i.e., stemness vs. differentiated). This review focuses on how autophagy-mediated mitochondrial clearance (i.e., mitophagy) may affect HSC mitochondrial content, thereby influencing the fate of HSCs and maintenance of hematopoietic homeostasis.     

DNA repair is crucial for maintaining hematopoietic stem cell function

Richard Cornall and collaborators recently developed a mouse model of Ligase IV syndrome with growth retardation and immunodeficiency due to a defect in nonhomologous end-joining (NHEJ) of DNA double-strand breaks. They demonstrated age-dependent loss of hematopoietic stem cell function in these mice. Simultaneously, Irving Weissman and colleagues demonstrated a similar phenomenon in Ku80(-/-) mice defective in NHEJ and telomere maintenance, Xpd(TTD) mice defective in nucleotide excision repair, and late generation mTr(-/-) missing telomerase activity. These studies strongly support the hypothesis that genomic stress causes aging by limiting the ability of stem cells to indefinitely maintain tissue homeostasis.     

Elevation of Survivin levels by hematopoietic growth factors occurs in quiescent CD34+ hematopoietic stem and progenitor cells before cell cycle entry

Survivin is a member of the inhibitor of apoptosis protein (IAP) family that is overexpressed during G(2)/M phase in most cancer cells. In contrast, we previously reported that Survivin is expressed throughout the cell cycle in normal CD34(+) hematopoietic stem and progenitor cells stimulated by the combination of Thrombopoietin (Tpo), Stem Cell Factor (SCF) and Flt3 ligand (FL). In order to address whether Survivin expression is specifically up-regulated by hematopoietic growth factors before cell cycle entry, we isolated quiescent CD34(+) cells and investigated Survivin expression in response to growth factor stimulation. Survivin is up-regulated in CD34(+) cells with 2N DNA content following growth factor addition, suggesting it becomes elevated during G(0)/G(1). Survivin is barely detectable in freshly isolated umbilical cord blood (UCB) Ki-67(negative) and Cyclin D(negative) CD34(+) cells, however incubation with Tpo, SCF and FL for 20 hrs results in up-regulation without entry of cells into cell cycle. Culture of G(0) CD34(+) cells isolated based on Hoechst 33342/PyroninY staining with Tpo, SCF and FL for 48 hrs, results in significantly elevated Survivin mRNA and protein levels. Moreover, labeling of fresh G(0) CD34(+) cells with 5-(and 6-) carboxyfluorescein diacetate succinimidyl ester (CFSE) before culture with growth factors for up to 72 hrs, revealed that Survivin expression was elevated in CFSE(bright) G(0) CD34(+) cells, indicating that up-regulation occurred before entry into G1. These results suggest that up-regulation of Survivin expression in CD34(+) cells is an early event in cell cycle entry that is regulated by hematopoietic growth factors and does not simply reflect cell cycle progression and cell division.     

Unique and independent roles for MLL in adult hematopoietic stem cells and progenitors

The Mixed Lineage Leukemia (MLL) gene is essential for embryonic hematopoietic stem cell (HSC) development, but its role during adult hematopoiesis is unknown. Using an inducible knockout model, we demonstrate that Mll is essential for the maintenance of adult HSCs and progenitors, with fatal bone marrow failure occurring within 3 weeks of Mll deletion. Mll-deficient cells are selectively lost from mixed bone marrow chimeras, demonstrating their failure to self-renew even in an intact bone marrow environment. Surprisingly, HSCs lacking Mll exhibit ectopic cell-cycle entry, resulting in the depletion of quiescent HSCs. In contrast, Mll deletion in myelo-erythroid progenitors results in reduced proliferation and reduced response to cytokine-induced cell-cycle entry. Committed lymphoid and myeloid cells no longer require Mll, defining the early multipotent stages of hematopoiesis as Mll dependent. These studies demonstrate that Mll plays selective and independent roles within the hematopoietic system, maintaining quiescence in HSCs and promoting proliferation in progenitors.     

胚胎干细胞向血液干细胞定向分化的研究进展

骨髓移植是目前治疗恶性白血病以及遗传性血液病最有效的方法之一。但是HLA相匹配的骨髓捐献者严重短缺,骨髓造血干细胞（hematopoietic stem cells,HSCs）体外培养困难,在体外修复患者骨髓造血干细胞技术不成熟,这些都大大限制了骨髓移植在临床上的应用。多能性胚胎干细胞（embryonic stem cells,ESCs）具有自我更新能力,在合适的培养条件下分化形成各种血系细胞,是造血干细胞的另一来源。在过去的二十多年里,血发生的研究是干细胞生物学中最为活跃的领域之一。小鼠及人的胚胎干细胞方面的研究最近取得了重大进展。这篇综述总结了近年来从胚胎干细胞获得造血干细胞的成就,以及在安全和技术上的障碍。胚胎干细胞诱导生成可移植性血干细胞的研究能够使我们更好地了解正常和异常造血发生的机制,同时也为造血干细胞的临床应用提供理论和实验依据。