Cell cycle regulation in hematopoietic stem cells

Hematopoietic stem cells are intrinsically protected against MLL-ENL-mediated transformation

Studies of developmental pathways of hematopoietic stem cells (HSCs) have defined lineage relationships throughout the blood system. This is relevant to acute myeloid leukemia (AML), where aggressiveness and therapeutic responsiveness can be influenced by the initial stage of transformation. To address this, we generated a mouse model in which the mixed-lineage leukemia/eleven-nineteen-leukemia (MLL-ENL) transcription factor can be conditionally activated in any cell type. We show that AML can originate from multiple hematopoietic progenitor subsets with granulocytic and monocytic potential, and that the normal developmental position of leukemia-initiating cells influences leukemic development. However, disease failed to arise from HSCs. Although it maintained or upregulated the expression of target genes associated with leukemic development, MLL-ENL dysregulated the proliferative and repopulating capacity of HSCs. Therefore, the permissiveness for development of AML may be associated with a narrower window of differentiation than was previously appreciated, and hijacking the self-renewal capacity of HSCs by a potent oncogene is insufficient for leukemic development.

Host immune response to infection and cancer: unexpected commonalities

Both microbes and tumors activate innate resistance, tissue repair, and adaptive immunity. Unlike acute infection, tumor growth is initially unapparent; however, inflammation and immunity affect all phases of tumor growth from initiation to progression and dissemination. Here, we discuss the shared features involved in the immune response to infection and cancer including modulation by commensal microbiota, reactive hematopoiesis, chronic immune responses and regulatory mechanisms to prevent collateral tissue damage. This comparative analysis of immunity to infection and cancer furthers our understanding of the basic mechanisms underlying innate resistance and adaptive immunity and their translational application to the design of new therapeutic approaches.

Transformation of quiescent adult oligodendrocyte precursor cells into malignant glioma through a multistep reactivation process

How malignant gliomas arise in a mature brain remains a mystery, hindering the development of preventive and therapeutic interventions. We previously showed that oligodendrocyte precursor cells (OPCs) can be transformed into glioma when mutations are introduced perinatally. However, adult OPCs rarely proliferate compared with their perinatal counterparts. Whether these relatively quiescent cells have the potential to transform is unknown, which is a critical question considering the late onset of human glioma. Additionally, the premalignant events taking place between initial mutation and a fully developed tumor mass are particularly poorly understood in glioma. Here we used a temporally controllable Cre transgene to delete p53 and NF1 specifically in adult OPCs and demonstrated that these cells consistently give rise to malignant gliomas. To investigate the transforming process of quiescent adult OPCs, we then tracked these cells throughout the premalignant phase, which revealed a dynamic multistep transformation, starting with rapid but transient hyperproliferative reactivation, followed by a long period of dormancy, and then final malignant transformation. Using pharmacological approaches, we discovered that mammalian target of rapamycin signaling is critical for both the initial OPC reactivation step and late-stage tumor cell proliferation and thus might be a potential target for both glioma prevention and treatment. In summary, our results firmly establish the transforming potential of adult OPCs and reveal an actionable multiphasic reactivation process that turns slowly dividing OPCs into malignant gliomas.

Exonuclease 1 is a critical mediator of survival during DNA double strand break repair in nonquiescent hematopoietic stem and progenitor cells

Hematopoietic stem cell (HSC) populations require DNA repair pathways to maintain their long-term survival and reconstitution capabilities, but mediators of these processes are still being elucidated. Exonuclease 1 (Exo1) participates in homologous recombination (HR) and Exo1 loss results in impaired 5' HR end resection. We use cultured Exo1(mut) fibroblasts and bone marrow to demonstrate that loss of Exo1 function results in defective HR in cycling cells. Conversely, in Exo1(mut) mice HR is not required for maintenance of quiescent HSCs at steady state, confirming the steady state HSC reliance on nonhomologous end joining (NHEJ). Exo1(mut) mice sustained serial repopulation, displayed no defect in competitive repopulation or niche occupancy, and exhibited no increased sensitivity to whole body ionizing radiation. However, when Exo1(mut) HSCs were pushed into cell cycle in vivo with 5-fluorouracil or poly IC, the hematopoietic population became hypersensitive to IR, resulting in HSC defects and animal death. We propose Exo1-mediated HR is dispensable for stem cell function in quiescent HSC, whereas it is essential to HSC response to DNA damage processing after cell cycle entry, and its loss is not compensated by intact NHEJ. In HSCs, the maintenance of stem cell function after DNA damage is dependent on the DNA repair capacity, segregated by active versus quiescent points in cell cycle.

Adult stem cell niches: cellular and molecular components

As stem cells (SCs) in adult organs continue to be identified and characterized, it becomes clear that their survival, quiescence, and activation depend on specific signals in their microenvironment, or niche. Although adult SCs of diverse tissues differ by their developmental origin, cycling activity, and regenerative capacity, there appear to be conserved similarities regarding the cellular and molecular components of the SC niche. Interestingly, many organs house both slow-cycling and fast-cycling SC populations, which rely on the coexistence of quiescent and inductive niches for proper regulation. In this review we present a general definition of adult SC niches in the most studied mammalian systems. We further focus on dissecting their cellular organization and on highlighting recently identified key molecular regulators. Finally, we detail the potential involvement of the SC niche in tissue degeneration, with a particular emphasis on aging and cancer.

Clonal heterogeneity as a driver of disease variability in the evolution of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are clonal hematological diseases in which cells of the myelo-erythroid lineage are overproduced and patients are predisposed to leukemic transformation. Hematopoietic stem cells are the suspected disease-initiating cells, and these cells must acquire a clonal advantage relative to nonmutant hematopoietic stem cells to perpetuate disease. In 2005, several groups identified a single gain-of-function point mutation in JAK2 that associated with the majority of MPNs, and subsequent studies have led to a comprehensive understanding of the mutational landscape in MPNs. However, confusion still exists as to how a single genetic aberration can be associated with multiple distinct disease entities. Many explanations have been proposed, including JAK2V617F homozygosity, individual patient heterogeneity, and the differential regulation of downstream JAK2 signaling pathways. Several groups have made knock-in mouse models expressing JAK2V617F and have observed divergent phenotypes, each recapitulating some aspects of disease. Intriguingly, most of these models do not observe a strong hematopoietic stem cell self-renewal advantage compared with wild-type littermate controls, raising the question of how a clonal advantage is established in patients with MPNs. This review summarizes the current molecular understanding of MPNs and the diversity of disease phenotypes and proposes that the increased proliferation induced by JAK2V617F applies a selection pressure on the mutant clone that results in highly diverse clonal evolution in individuals.

FHL2 regulates hematopoietic stem cell functions under stress conditions

FHL2, a member of the four and one half LIM domain protein family, is a critical transcriptional modulator. Here, we identify FHL2 as a critical regulator of hematopoietic stem cells (HSCs) that is essential for maintaining HSC self-renewal under regenerative stress. We find that Fhl2 loss has limited effects on hematopoiesis under homeostatic conditions. In contrast, Fhl2-null chimeric mice reconstituted with Fhl2-null bone marrow cells developed abnormal hematopoiesis with significantly reduced numbers of HSCs, hematopoietic progenitor cells (HPCs), red blood cells and platelets as well as hemoglobin levels. In addition, HSCs displayed a significantly reduced self-renewal capacity and were skewed toward myeloid lineage differentiation. We find that Fhl2 loss reduces both HSC quiescence and survival in response to regenerative stress, probably as a consequence of Fhl2-loss-mediated down-regulation of cyclin dependent kinase (CDK)-inhibitors, including p21(Cip) and p27(Kip1). Interestingly, FHL2 is regulated under control of a tissue specific promoter in hematopoietic cells and it is down-regulated by DNA hypermethylation in the leukemia cell line and primary leukemia cells. Furthermore, we find that down-regulation of FHL2 frequently occurs in myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) patients, raising a possibility that FHL2 down-regulation plays a role in the pathogenesis of myeloid malignancies.

Childhood leukaemia risks: from unexplained findings near nuclear installations to recommendations for future research

Recent findings related to childhood leukaemia incidence near nuclear installations have raised questions which can be answered neither by current knowledge on radiation risk nor by other established risk factors. In 2012, a workshop was organised on this topic with two objectives: (a) review of results and discussion of methodological limitations of studies near nuclear installations; (b) identification of directions for future research into the causes and pathogenesis of childhood leukaemia. The workshop gathered 42 participants from different disciplines, extending widely outside of the radiation protection field. Regarding the proximity of nuclear installations, the need for continuous surveillance of childhood leukaemia incidence was highlighted, including a better characterisation of the local population. The creation of collaborative working groups was recommended for consistency in methodologies and the possibility of combining data for future analyses. Regarding the causes of childhood leukaemia, major fields of research were discussed (environmental risk factors, genetics, infections, immunity, stem cells, experimental research). The need for multidisciplinary collaboration in developing research activities was underlined, including the prevalence of potential predisposition markers and investigating further the infectious aetiology hypothesis. Animal studies and genetic/epigenetic approaches appear of great interest. Routes for future research were pointed out.

Nucleotide excision repair-dependent DNA double-strand break formation and ATM signaling activation in mammalian quiescent cells

Histone H2A variant H2AX is phosphorylated at Ser(139) in response to DNA double-strand break (DSB) and single-stranded DNA (ssDNA) formation. UV light dominantly induces pyrimidine photodimers, which are removed from the mammalian genome by nucleotide excision repair (NER). We previously reported that in quiescent G0 phase cells, UV induces ATR-mediated H2AX phosphorylation plausibly caused by persistent ssDNA gap intermediates during NER. In this study, we have found that DSB is also generated following UV irradiation in an NER-dependent manner and contributes to an earlier fraction of UV-induced H2AX phosphorylation. The NER-dependent DSB formation activates ATM kinase and triggers the accumulation of its downstream factors, MRE11, NBS1, and MDC1, at UV-damaged sites. Importantly, ATM-deficient cells exhibited enhanced UV sensitivity under quiescent conditions compared with asynchronously growing conditions. Finally, we show that the NER-dependent H2AX phosphorylation is also observed in murine peripheral T lymphocytes, typical nonproliferating quiescent cells in vivo. These results suggest that in vivo quiescent cells may suffer from NER-mediated secondary DNA damage including ssDNA and DSB.

Pro-inflammatory cytokines: emerging players regulating HSC function in normal and diseased hematopoiesis

Hematopoiesis is the hierarchical process in which all lineages of blood cells are produced by self-renewing hematopoietic stem cells (HSCs) in the bone marrow (BM). While the regulatory factors that maintain proper HSC function and lineage output under normal conditions are well understood, significantly less is known about how HSC fate is regulated in response to inflammation or disease. As many blood disorders are associated with overproduction of pro-inflammatory cytokines, significant interest has emerged in understanding the impact of these factors on HSC function. In this review we highlight key advances demonstrating the impact of pro-inflammatory cytokines on the biology of HSCs and the BM niche, and address ongoing questions regarding their role in normal and pathogenic hematopoiesis.

Replication stress is a potent driver of functional decline in ageing haematopoietic stem cells

Haematopoietic stem cells (HSCs) self-renew for life, thereby making them one of the few blood cells that truly age. Paradoxically, although HSCs numerically expand with age, their functional activity declines over time, resulting in degraded blood production and impaired engraftment following transplantation. While many drivers of HSC ageing have been proposed, the reason why HSC function degrades with age remains unknown. Here we show that cycling old HSCs in mice have heightened levels of replication stress associated with cell cycle defects and chromosome gaps or breaks, which are due to decreased expression of mini-chromosome maintenance (MCM) helicase components and altered dynamics of DNA replication forks. Nonetheless, old HSCs survive replication unless confronted with a strong replication challenge, such as transplantation. Moreover, once old HSCs re-establish quiescence, residual replication stress on ribosomal DNA (rDNA) genes leads to the formation of nucleolar-associated γH2AX signals, which persist owing to ineffective H2AX dephosphorylation by mislocalized PP4c phosphatase rather than ongoing DNA damage. Persistent nucleolar γH2AX also acts as a histone modification marking the transcriptional silencing of rDNA genes and decreased ribosome biogenesis in quiescent old HSCs. Our results identify replication stress as a potent driver of functional decline in old HSCs, and highlight the MCM DNA helicase as a potential molecular target for rejuvenation therapies.

Transit-amplifying cells orchestrate stem cell activity and tissue regeneration

Transit-amplifying cells (TACs) are an early intermediate in tissue regeneration. Here, using hair follicles (HFs) as a paradigm, we show that emerging TACs constitute a signaling center that orchestrates tissue growth. Whereas primed stem cells (SCs) generate TACs, quiescent SCs only proliferate after TACs form and begin expressing Sonic Hedgehog (SHH). TAC generation is independent of autocrine SHH, but the TAC pool wanes if they can't produce SHH. We trace this paradox to two direct actions of SHH: promoting quiescent-SC proliferation and regulating dermal factors that stoke TAC expansion. Ingrained within quiescent SCs' special sensitivity to SHH signaling is their high expression of GAS1. Without sufficient input from quiescent SCs, replenishment of primed SCs for the next hair cycle is compromised, delaying regeneration and eventually leading to regeneration failure. Our findings unveil TACs as transient but indispensable integrators of SC niche components and reveal an intriguing interdependency of primed and quiescent SC populations on tissue regeneration.

Leucine-rich repeat-containing G-protein-coupled Receptor 5 marks short-term hematopoietic stem and progenitor cells during mouse embryonic development

Lgr5 is a marker for proliferating stem cells in adult intestine, stomach, and hair follicle. However, Lgr5 is not expressed in adult hematopoietic stem and progenitor cells (HSPCs). Whether Lgr5 is expressed in the embryonic and fetal HSPCs that undergo rapid proliferation is unknown. Here we report the detection of Lgr5 expression in HSPCs in the aorta-gonad-mesonephros (AGM) and fetal liver. We also found that a portion of Lgr5(+) cells expressed the Runx1 gene that is critical for the ontogeny of HSPCs. A small portion of Lgr5(+) cells also expressed HSPC surface markers c-Kit and CD34 in AGM or CD41 in fetal liver. Furthermore, the majority of Lgr5(+) cells expressed Ki67, indicating their proliferating state. Transplantation of fetal liver-derived Lgr5-GFP(+) cells (E12.5) demonstrated that Lgr5-GFP(+) cells were able to reconstitute myeloid and lymphoid lineages in adult recipients, but the engraftment was short-term (4-8 weeks) and 20-fold lower compared with the Lgr5-GFP(-) control. Our data show that Lgr5-expressing cells mark short-term hematopoietic stem and progenitor cells, consistent with the role of Lgr5 in supporting HSPCs rapid proliferation during embryonic and fetal development.

miR-99a/100~125b tricistrons regulate hematopoietic stem and progenitor cell homeostasis by shifting the balance between TGFβ and Wnt signaling

miR-99a/100, let-7, and miR-125b paralogs are encoded in two tricistrons and are highly expressed in hematopoietic stem cells (HSCs). Emmrich et al. demonstrate that miR-99a/100∼125b tricistrons are transcribed as a polycistronic message and functionally converge at the combinatorial block of the TGFβ pathway. Down-regulating tumor suppressor genes APC/APC2 stabilized active β-catenin and enhanced Wnt signaling. These tricistronic miRNAs promoted sustained expansion of murine and human HSCs by switching the balance between Wnt and TGFβ signaling.

Although regulation of stem cell homeostasis by microRNAs (miRNAs) is well studied, it is unclear how individual miRNAs genomically encoded within an organized polycistron can interact to induce an integrated phenotype. miR-99a/100, let-7, and miR-125b paralogs are encoded in two tricistrons on human chromosomes 11 and 21. They are highly expressed in hematopoietic stem cells (HSCs) and acute megakaryoblastic leukemia (AMKL), an aggressive form of leukemia with poor prognosis. Here, we show that miR-99a/100∼125b tricistrons are transcribed as a polycistronic message transactivated by the homeobox transcription factor HOXA10. Integrative analysis of global gene expression profiling, miRNA target prediction, and pathway architecture revealed that miR-99a/100, let-7, and miR-125b functionally converge at the combinatorial block of the transforming growth factor β (TGFβ) pathway by targeting four receptor subunits and two SMAD signaling transducers. In addition, down-regulation of tumor suppressor genes adenomatous polyposis coli (APC)/APC2 stabilizes active β-catenin and enhances Wnt signaling. By switching the balance between Wnt and TGFβ signaling, the concerted action of these tricistronic miRNAs promoted sustained expansion of murine and human HSCs in vitro or in vivo while favoring megakaryocytic differentiation. Hence, our study explains the high phylogenetic conservation of the miR-99a/100∼125b tricistrons controlling stem cell homeostasis, the deregulation of which contributes to the development of AMKL.

Quiescent hematopoietic stem cells accumulate DNA damage during aging that is repaired upon entry into cell cycle

Hematopoietic stem cells (HSCs) maintain homeostasis and regenerate the blood system throughout life. It has been postulated that HSCs may be uniquely capable of preserving their genomic integrity in order to ensure lifelong function. To directly test this, we quantified DNA damage in HSCs and downstream progenitors from young and old mice, revealing that strand breaks significantly accrue in HSCs during aging. DNA damage accumulation in HSCs was associated with broad attenuation of DNA repair and response pathways that was dependent upon HSC quiescence. Accordingly, cycling fetal HSCs and adult HSCs driven into cycle upregulated these pathways leading to repair of strand breaks. Our results demonstrate that HSCs are not comprehensively geno-protected during aging. Rather, HSC quiescence and concomitant attenuation of DNA repair and response pathways underlies DNA damage accumulation in HSCs during aging. These results provide a potential mechanism through which premalignant mutations accrue in HSCs.

Coupling of Hedgehog and Hippo pathways promotes stem cell maintenance by stimulating proliferation

It is essential to define the mechanisms by which external signals regulate adult stem cell numbers, stem cell maintenance, and stem cell proliferation to guide regenerative stem cell therapies and to understand better how cancers originate in stem cells. In this paper, we show that Hedgehog (Hh) signaling in Drosophila melanogaster ovarian follicle stem cells (FSCs) induces the activity of Yorkie (Yki), the transcriptional coactivator of the Hippo pathway, by inducing yki transcription. Moreover, both Hh signaling and Yki positively regulate the rate of FSC proliferation, both are essential for FSC maintenance, and both promote increased FSC longevity and FSC duplication when in excess. We also found that responses to activated Yki depend on Cyclin E induction while responses to excess Hh signaling depend on Yki induction, and excess Yki can compensate for defective Hh signaling. These causal connections provide the most rigorous evidence to date that a niche signal can promote stem cell maintenance principally by stimulating stem cell proliferation.

Surviving change: the metabolic journey of hematopoietic stem cells

Hematopoietic stem cells (HSCs) are a rare population of somatic stem cells that maintain blood production and are uniquely wired to adapt to diverse cellular fates during the lifetime of an organism. Recent studies have highlighted a central role for metabolic plasticity in facilitating cell fate transitions and in preserving HSC functionality and survival. This review summarizes our current understanding of the metabolic programs associated with HSC quiescence, self-renewal, and lineage commitment, and highlights the mechanistic underpinnings of these changing bioenergetics programs. It also discusses the therapeutic potential of targeting metabolic drivers in the context of blood malignancies.

The journey from stem cell to macrophage

Essential protectors against infection and injury, macrophages can also contribute to many common and fatal diseases. Here, we discuss the mechanisms that control different types of macrophage activities in mice. We follow the cells' maturational pathways over time and space and elaborate on events that influence the type of macrophage eventually settling a particular destination. The nature of the precursor cells, developmental niches, tissues, environmental cues, and other connecting processes appear to contribute to the identity of macrophage type. Together, the spatial and developmental relationships of macrophages compose a topo-ontogenic map that can guide our understanding of their biology.

Divisional history and hematopoietic stem cell function during homeostasis

We investigated the homeostatic behavior of hematopoietic stem and progenitor cells (HSPCs) temporally defined according to their divisional histories using an HSPC-specific GFP label-retaining system. We show that homeostatic hematopoietic stem cells (HSCs) lose repopulating potential after limited cell divisions. Once HSCs exit dormancy and accrue divisions, they also progressively lose the ability to return to G₀ and functional activities associated with quiescent HSCs. In addition, dormant HSPCs phenotypically defined as multipotent progenitor cells display robust stem cell activity upon transplantation, suggesting that temporal quiescence is a greater indicator of function than cell-surface phenotype. Our studies suggest that once homeostatic HSCs leave dormancy, they are slated for extinction. They self-renew phenotypically, but they lose self-renewal activity. As such, they question self-renewal as a characteristic of homeostatic, nonperturbed HSCs in contrast to self-renewal demonstrated under stress conditions.

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Homeostatic HSCs progressively lose self-renewal ability with cell division

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G₀ homeostatic HSCs lose functional ability in relation to their divisional history

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Temporally defined quiescence reflects HSC functional abilities better than phenotype

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Once dormant HSCs are activated without stress, they lose self-renewal activity

Regulation of stem cell function by protein ubiquitylation

Tissue homeostasis depends largely on the ability to replenish impaired or aged cells. Thus, tissue-resident stem cells need to provide functional progeny throughout the lifetime of an organism. Significant work in the past years has characterized how stem cells integrate signals from their environment to shape regulatory transcriptional networks and chromatin-regulating factors that control stem cell differentiation or maintenance. There is increasing interest in how post-translational modifications, and specifically ubiquitylation, control these crucial decisions. Ubiquitylation modulates the stability and function of important factors that regulate key processes in stem cell behavior. In this review, we analyze the role of ubiquitylation in embryonic stem cells and different adult multipotent stem cell systems and discuss the underlying mechanisms that control the balance between quiescence, self-renewal, and differentiation. We also discuss deregulated processes of ubiquitin-mediated protein degradation that lead to the development of tumor-initiating cells.

Cytogenetics in benzene-associated myelodysplastic syndromes and acute myeloid leukemia: new insights into a disease continuum

Hematopoiesis in health and disease results from complex interactions between primitive hematopoietic stem cells (HSCs) and the extrinsic influences of other cells in the bone marrow (BM) niche. Advances in stem cell biology, molecular genetics, and computational biology reveal that the immortality, self-renewal, and maintenance of blood homeostasis generally attributed to individual HSCs are functions of the cells' behavior in the normal BM environment. Here we discuss how these advances, together with results of outcomes-based clinical epidemiology studies, provide new insight into the importance of epigenetic events in leukemogenesis. For the chemical benzene (Bz), development of myeloid neoplasms depends predominantly on alterations within the microenvironments in which they arise. The primary persistent disease in Bz myelotoxicity is myelodysplastic syndrome, which precedes cytogenetic injury. Evidence indicates that acute myeloid leukemia arises as a secondary event, subsequent to evolution of the leukemia-initiating cell phenotype within the altered BM microenvironment. Further explorations into the nature of chemical versus de novo disease should consider this mechanism, which is biologically distinct from previous models of clonal cytogenetic injury. Understanding alterations of homeostatic regulation in the BM niche is important for validation of models of leukemogenesis, monitoring at-risk populations, and development of novel treatment and prevention strategies.

B-myb is an essential regulator of hematopoietic stem cell and myeloid progenitor cell development

The B-myb (MYBL2) gene is a member of the MYB family of transcription factors and is involved in cell cycle regulation, DNA replication, and maintenance of genomic integrity. However, its function during adult development and hematopoiesis is unknown. We show here that conditional inactivation of B-myb in vivo results in depletion of the hematopoietic stem cell (HSC) pool, leading to profound reductions in mature lymphoid, erythroid, and myeloid cells. This defect is autonomous to the bone marrow and is first evident in stem cells, which accumulate in the S and G2/M phases. B-myb inactivation also causes defects in the myeloid progenitor compartment, consisting of depletion of common myeloid progenitors but relative sparing of granulocyte-macrophage progenitors. Microarray studies indicate that B-myb-null LSK(+) cells differentially express genes that direct myeloid lineage development and commitment, suggesting that B-myb is a key player in controlling cell fate. Collectively, these studies demonstrate that B-myb is essential for HSC and progenitor maintenance and survival during hematopoiesis.

Re-entry into quiescence protects hematopoietic stem cells from the killing effect of chronic exposure to type I interferons

Quiescence acts as a safeguard mechanism to ensure survival of the HSC pool during chronic IFN-1 exposure

Type I interferons (IFN-1s) are antiviral cytokines that suppress blood production while paradoxically inducing hematopoietic stem cell (HSC) proliferation. Here, we clarify the relationship between the proliferative and suppressive effects of IFN-1s on HSC function during acute and chronic IFN-1 exposure. We show that IFN-1–driven HSC proliferation is a transient event resulting from a brief relaxation of quiescence-enforcing mechanisms in response to acute IFN-1 exposure, which occurs exclusively in vivo. We find that this proliferative burst fails to exhaust the HSC pool, which rapidly returns to quiescence in response to chronic IFN-1 exposure. Moreover, we demonstrate that IFN-1–exposed HSCs with reestablished quiescence are largely protected from the killing effects of IFNs unless forced back into the cell cycle due to culture, transplantation, or myeloablative treatment, at which point they activate a p53-dependent proapoptotic gene program. Collectively, our results demonstrate that quiescence acts as a safeguard mechanism to ensure survival of the HSC pool during chronic IFN-1 exposure. We show that IFN-1s can poise HSCs for apoptosis but induce direct cell killing only upon active proliferation, thereby establishing a mechanism for the suppressive effects of IFN-1s on HSC function.

Genome-wide dissection of the quorum sensing signalling pathway in Trypanosoma brucei

The protozoan parasites Trypanosoma brucei spp. cause important human and livestock diseases in sub-Saharan Africa. In mammalian blood, two developmental forms of the parasite exist: proliferative 'slender' forms and arrested 'stumpy' forms that are responsible for transmission to tsetse flies. The slender to stumpy differentiation is a density-dependent response that resembles quorum sensing in microbial systems and is crucial for the parasite life cycle, ensuring both infection chronicity and disease transmission. This response is triggered by an elusive 'stumpy induction factor' (SIF) whose intracellular signalling pathway is also uncharacterized. Laboratory-adapted (monomorphic) trypanosome strains respond inefficiently to SIF but can generate forms with stumpy characteristics when exposed to cell-permeable cAMP and AMP analogues. Exploiting this, we have used a genome-wide RNA interference library screen to identify the signalling components driving stumpy formation. In separate screens, monomorphic parasites were exposed to 8-(4-chlorophenylthio)-cAMP (pCPT-cAMP) or 8-pCPT-2'-O-methyl-5'-AMP to select cells that were unresponsive to these signals and hence remained proliferative. Genome-wide Ion Torrent based RNAi target sequencing identified cohorts of genes implicated in each step of the signalling pathway, from purine metabolism, through signal transducers (kinases, phosphatases) to gene expression regulators. Genes at each step were independently validated in cells naturally capable of stumpy formation, confirming their role in density sensing in vivo. The putative RNA-binding protein, RBP7, was required for normal quorum sensing and promoted cell-cycle arrest and transmission competence when overexpressed. This study reveals that quorum sensing signalling in trypanosomes shares similarities to fundamental quiescence pathways in eukaryotic cells, its components providing targets for quorum-sensing interference-based therapeutics.

High c-Kit expression identifies hematopoietic stem cells with impaired self-renewal and megakaryocytic bias

c-Kit^lo HSCs exhibit enhanced self-renewal and long-term reconstitution potential and give rise to c-Kit^hi HSCs that have a megakaryocytic bias.

Hematopoietic stem cells (HSCs) are heterogeneous with respect to their self-renewal, lineage, and reconstitution potentials. Although c-Kit is required for HSC function, gain and loss-of-function c-Kit mutants suggest that even small changes in c-Kit signaling profoundly affect HSC function. Herein, we demonstrate that even the most rigorously defined HSCs can be separated into functionally distinct subsets based on c-Kit activity. Functional and transcriptome studies show HSCs with low levels of surface c-Kit expression (c-Kit^lo) and signaling exhibit enhanced self-renewal and long-term reconstitution potential compared with c-Kit^hi HSCs. Furthermore, c-Kit^lo and c-Kit^hi HSCs are hierarchically organized, with c-Kit^hi HSCs arising from c-Kit^lo HSCs. In addition, whereas c-Kit^hi HSCs give rise to long-term lymphomyeloid grafts, they exhibit an intrinsic megakaryocytic lineage bias. These functional differences between c-Kit^lo and c-Kit^hi HSCs persist even under conditions of stress hematopoiesis induced by 5-fluorouracil. Finally, our studies show that the transition from c-Kit^lo to c-Kit^hi HSC is negatively regulated by c-Cbl. Overall, these studies demonstrate that HSCs exhibiting enhanced self-renewal potential can be isolated based on c-Kit expression during both steady state and stress hematopoiesis. Moreover, they provide further evidence that the intrinsic functional heterogeneity previously described for HSCs extends to the megakaryocytic lineage.

Bcl2 overexpression rescues the hematopoietic stem cell defects in Ku70-deficient mice by restoration of quiescence

DNA repair is essential for hematopoietic stem cell (HSC) maintenance. Ku70 is a key component of the nonhomologous end-joining pathway, which is the major pathway for DNA double-strand break repair. We find that HSCs from Ku70-deficient mice are severely defective in self-renewal, competitive repopulation, and bone marrow (BM) hematopoietic niche occupancy and that loss of quiescence results in a dramatic defect in the maintenance of Ku70-deficient HSCs. Interestingly, although overexpression of Bcl2 does not rescue the severe combined immunodeficiency phenotype in Ku70-deficient mice, overexpression of Bcl2 in Ku70-deficient HSCs almost completely rescued the impaired HSC quiescence, repopulation, and BM hematopoietic niche occupancy capacities. Together, our data indicate that the HSC maintenance defect of Ku70-deficient mice is due to the loss of HSC quiescent populations, whereas overexpression of Bcl2 rescues the HSC defect in Ku70-deficient mice by restoration of quiescence. Our study uncovers a novel role of Bcl2 in HSC quiescence regulation.

The ubiquitin system in immune regulation

The ubiquitin system plays a pivotal role in the regulation of immune responses. This system includes a large family of E3 ubiquitin ligases of over 700 proteins and about 100 deubiquitinating enzymes, with the majority of their biological functions remaining unknown. Over the last decade, through a combination of genetic, biochemical, and molecular approaches, tremendous progress has been made in our understanding of how the process of protein ubiquitination and its reversal deubiquitination controls the basic aspect of the immune system including lymphocyte development, differentiation, activation, and tolerance induction and regulates the pathophysiological abnormalities such as autoimmunity, allergy, and malignant formation. In this review, we selected some of the published literature to discuss the roles of protein-ubiquitin conjugation and deubiquitination in T-cell activation and anergy, regulatory T-cell and T-helper cell differentiation, regulation of NF-κB signaling, and hematopoiesis in both normal and dysregulated conditions. A comprehensive understanding of the relationship between the ubiquitin system and immunity will provide insight into the molecular mechanisms of immune regulation and at the same time will advance new therapeutic intervention for human immunological diseases.

MIF4G domain containing protein regulates cell cycle and hepatic carcinogenesis by antagonizing CDK2-dependent p27 stability

The CDK inhibitor p27(kip1) plays crucial roles in cell cycle regulation and cancer progression. Through yeast two-hybrid screening, we identified MIF4G domain containing protein (MIF4GD) as a novel binding partner for p27. The association of MIF4GD and p27 was verified using immunoprecipitation and glutathione S-transferase (GST) pull-down assays. Interaction with MIF4GD led to the stabilization of p27 both in the nucleus and in the cytoplasm in hepatocellular carcinoma (HCC) cells as a result of suppressed phosphorylation of p27 by CDK2 at threonine187. Serum stimulation decreased the levels of MIF4GD and p27 simultaneously. In addition, MIF4GD overexpression resulted in increased p27 levels and reduced cell proliferation, while knockdown of MIF4GD promoted cell cycle progression with decreased p27 levels in cells. Furthermore, overexpression of MIF4GD reduced colony formation and inhibited xenograft tumor growth in nude mice. Finally, we found that both MIF4GD and p27 were expressed at low levels in HCC tissues compared to non-cancerous tissues, and that low expression levels of MIF4GD and p27 were associated with significantly worse prognosis in HCC patients. Our results suggest that MIF4GD is a potential regulator of p27-dependent cell proliferation in HCC. These findings provide a rational framework for the development of potential HCC therapy by targeting the MIF4GD-p27 interaction.

Deficiency of GRP94 in the hematopoietic system alters proliferation regulators in hematopoietic stem cells

We have previously reported that acute inducible knockout of the endoplasmic reticulum chaperone GRP94 led to an expansion of the hematopoietic stem and progenitor cell pool. Here, we investigated the effectors and mechanisms for this phenomenon. We observed an increase in AKT activation in freshly isolated GRP94-null HSC-enriched Lin(-) Sca-1(+) c-Kit(+) (LSK) cells, corresponding with higher production of PI(3,4,5)P3, indicative of PI3K activation. Treatment of GRP94-null LSK cells with the AKT inhibitor MK2206 compromised cell expansion, suggesting a causal relationship between elevated AKT activation and increased proliferation in GRP94-null HSCs. Microarray analysis demonstrated a 97% reduction in the expression of the hematopoietic cell cycle regulator Ms4a3 in the GRP94-null LSK cells, and real-time quantitative PCR confirmed this down-regulation in the LSK cells but not in the total bone marrow (BM). A further examination comparing freshly isolated BM LSK cells with spleen LSK cells, as well as BM LSK cells cultured in vitro, revealed specific down-regulation of Ms4a3 in freshly isolated BM GRP94-null LSK cells. On examining cell surface proteins that are known to regulate stem cell proliferation, we observed a reduced expression of cell surface connexin 32 (Cx32) plaques in GRP94-null LSK cells. However, suppression of Cx32 hemichannel activity in wild-type LSK cells through mimetic peptides did not lead to increased LSK cell proliferation in vitro. Two other important cell surface proteins that mediate HSC-niche interactions, specifically Tie2 and CXCR4, were not impaired by Grp94 deletion. Collectively, our study uncovers novel and unique roles of GRP94 in regulating HSC proliferation.

Cell-intrinsic in vivo requirement for the E47-p21 pathway in long-term hematopoietic stem cells

Major regulators of long-term hematopoietic stem cell (LT-HSC) self-renewal and proliferation have been identified, but knowledge of their in vivo interaction in a linear pathway is lacking. In this study, we show a direct genetic link between the transcription factor E47 and the major cell cycle regulator p21 in controlling LT-HSC integrity in vivo under repopulation stress. Numerous studies have shown that E47 activates p21 transcription in hematopoietic subsets in vitro, and we now reveal the in vivo relevance of the E47-p21 pathway by reducing the gene dose of each factor individually (E47(het) or p21(het)) versus in tandem (E47(het)p21(het)). E47(het)p21(het) LT-HSCs and downstream short-term hematopoietic stem cells exhibit hyperproliferation and preferential susceptibility to mitotoxin compared to wild-type or single haploinsufficient controls. In serial adoptive transfers that rigorously challenge self-renewal, E47(het)p21(het) LT-HSCs dramatically and progressively decline, indicating the importance of cell-intrinsic E47-p21 in preserving LT-HSCs under stress. Transient numeric recovery of downstream short-term hematopoietic stem cells enabled the production of functionally competent myeloid but not lymphoid cells, as common lymphoid progenitors were decreased, and peripheral lymphocytes were virtually ablated. Thus, we demonstrate a developmental compartment-specific and lineage-specific requirement for the E47-p21 pathway in maintaining LT-HSCs, B cells, and T cells under hematopoietic repopulation stress in vivo.

Sox4 is a key oncogenic target in C/EBPα mutant acute myeloid leukemia

Mutation or epigenetic silencing of the transcription factor C/EBPα is observed in ∼10% of patients with acute myeloid leukemia (AML). In both cases, a common global gene expression profile is observed, but downstream targets relevant for leukemogenesis are not known. Here, we identify Sox4 as a direct target of C/EBPα whereby its expression is inversely correlated with C/EBPα activity. Downregulation of Sox4 abrogated increased self-renewal of leukemic cells and restored their differentiation. Gene expression profiles of leukemia-initiating cells (LICs) from both Sox4 overexpression and murine C/EBPα mutant AML models clustered together but differed from other types of AML. Our data demonstrate that Sox4 overexpression resulting from C/EBPα inactivation contributes to the development of leukemia with a distinct LIC phenotype.

Continuous clonal labeling reveals small numbers of functional stem cells in intestinal crypts and adenomas

Lineage-tracing approaches, widely used to characterize stem cell populations, rely on the specificity and stability of individual markers for accurate results. We present a method in which genetic labeling in the intestinal epithelium is acquired as a mutation-induced clonal mark during DNA replication. By determining the rate of mutation in vivo and combining this data with the known neutral-drift dynamics that describe intestinal stem cell replacement, we quantify the number of functional stem cells in crypts and adenomas. Contrary to previous reports, we find that significantly lower numbers of "working" stem cells are present in the intestinal epithelium (five to seven per crypt) and in adenomas (nine per gland), and that those stem cells are also replaced at a significantly lower rate. These findings suggest that the bulk of tumor stem cell divisions serve only to replace stem cell loss, with rare clonal victors driving gland repopulation and tumor growth.

Direct observation of cell cycle progression in living mouse embryonic stem cells on an extracellular matrix of E-cadherin

Self-renewal and differentiation of embryonic stem cells are tightly coordinated with cell-cycle progression and reconstructions. However, technical approach to directly visualize single embryonic stem cells still remains challenging. Here we combined two independent systems by using artificially constructed extracellular matrix that maintains embryonic stem cells in single level with cell cycle visualization reporters to directly observe cell cycle progression. Using Fucci (fluorescent ubiquitination-based cell cycle indicator) technology and computer-assisted fluorescence microscopy we were able to visualize cell cycle progression of mouse embryonic stem cells prepared from Fucci2 knock-in mice (mES/Fucci2). Imaged mES/Fucci2 cells were plated on coverslips coated with recombinant E-cadherin-IgG Fc (E-cad-Fc). This artificial extracellular matrix effectively increases adherence of cultured cells to coverslips, which is advantageous for fluorescence imaging. mES/Fucci2 cells on the E-cad-Fc maintained the typical cell cycle of mES cells with truncated G₁ phase and pluripotency. During time-lapse imaging, we were able to track these cells with dendritic-like cell morphology and many pseudopodial protrusions. By contrast, the cell cycle progression of mES/Fucci2 cells on mouse embryonic fibroblasts (MEFs) was not observable due to their compact aggregation. Cell cycle duration of mES/Fucci2 cells on the E-cad-Fc was 16 h. Thus, the unique properties of our immunocytochemical analysis have revealed that decline of pluripotency of the Fucci2 mES cells on the E-cad-Fc was coordinated with their differentiation.

Klf1, a C2H2 zinc finger-transcription factor, is required for cell wall maintenance during long-term quiescence in differentiated G0 phase

Fission yeast, Schizoaccharomyces pombe, is a model for studying cellular quiescence. Shifting to a medium that lacks a nitrogen-source induces proliferative cells to enter long-term G0 quiescence. Klf1 is a Krüppel-like transcription factor with a 7-amino acid Cys2His2-type zinc finger motif. The deletion mutant, ∆klf1, normally divides in vegetative medium, but proliferation is not restored after long-term G0 quiescence. Cell biologic, transcriptomic, and metabolomic analyses revealed a unique phenotype of the ∆klf1 mutant in quiescence. Mutant cells had diminished transcripts related to signaling molecules for switching to differentiation; however, proliferative metabolites for cell-wall assembly and antioxidants had significantly increased. Further, the size of ∆klf1 cells increased markedly during quiescence due to the aberrant accumulation of Calcofluor-positive, chitin-like materials beneath the cell wall. After 4 weeks of quiescence, reversible proliferation ability was lost, but metabolism was maintained. Klf1 thus plays a role in G0 phase longevity by enhancing the differentiation signal and suppressing metabolism for growth. If Klf1 is lost, S. pombe fails to maintain a constant cell size and normal cell morphology during quiescence.

Linking HSCs to their youth

Fetal haematopoietic stem cells (HSCs) self-renew extensively to build the blood system from scratch. The protein Lin28b negatively regulates the microRNA let-7 to keep levels of its target Hmga2 high, hence conferring high self-renewal potential to fetal HSCs. This regulatory circuit can be experimentally modulated to boost the lower self-renewal activity of quiescent adult HSCs.

Tgif1 regulates quiescence and self-renewal of hematopoietic stem cells

TG-interacting factor 1 (TGIF1) is a transcriptional repressor that can modulate retinoic acid and transforming growth factor β signaling pathways. It is required for myeloid progenitor cell differentiation and survival, and mutations in the TGIF1 gene cause holoprosencephaly. Furthermore, we have previously observed that acute myelogenous leukemia (AML) patients with low TGIF1 levels had worse prognoses. Here, we explored the role of Tgif1 in murine hematopoietic stem cell (HSC) function. CFU assays showed that Tgif1(-/-) bone marrow cells produced more total colonies and had higher serial CFU potential. These effects were also observed in vivo, where Tgif1(-/-) bone marrow cells had higher repopulation potential in short- and long-term competitive repopulation assays than wild-type cells. Serial transplantation and replating studies showed that Tgif1(-/-) HSCs exhibited greater self-renewal and were less proliferative and more quiescent than wild-type cells, suggesting that Tgif1 is required for stem cells to enter the cell cycle. Furthermore, HSCs from Tgif1(+/-) mice had a phenotype similar to that of HSCs from Tgif1(-/-) mice, while bone marrow cells with overexpressing Tgif1 showed increased proliferation and lower survival in long-term transplant studies. Taken together, our data suggest that Tgif1 suppresses stem cell self-renewal and provide clues as to how reduced expression of TGIF1 may contribute to poor long-term survival in patients with AML.

Regulation of immune responses and tolerance: the microRNA perspective

Much has been learned about the molecular and cellular components critical for the control of immune responses and tolerance. It remains a challenge, however, to control the immune response and tolerance at the system level without causing significant toxicity to normal tissues. Recent studies suggest that microRNA (miRNA) genes, an abundant class of non-coding RNA genes that produce characteristic approximately 22 nucleotides small RNAs, play important roles in immune cells. In this article, we discuss emerging knowledge regarding the functions of miRNA genes in the immune system. We delve into the roles of miRNAs in regulating signaling strength and threshold, homeostasis, and the dynamics of the immune response and tolerance during normal and pathogenic immunological conditions. We also present observations based on analyzes of miR-181 family genes that indicate the potential functions of primary and/or precursor miRNAs in target recognition and explore the impact of these findings on target identification. Finally, we illustrate that despite the subtle effects of miRNAs on gene expression, miRNAs have the potential to influence the outcomes of normal and pathogenic immune responses by controlling the quantitative and dynamic aspects of immune responses. Tuning miRNA functions in immune cells, through gain- and loss-of-function approaches in mice, may reveal novel approach to restore immune equilibrium from pathogenic conditions, such as autoimmune disease and leukemia, without significant toxicity.

Resilient and resourceful: genome maintenance strategies in hematopoietic stem cells

Blood homeostasis is maintained by a rare population of quiescent hematopoietic stem cells (HSCs) that self-renew and differentiate to give rise to all lineages of mature blood cells. In contrast to most other blood cells, HSCs are preserved throughout life, and the maintenance of their genomic integrity is therefore paramount to ensure normal blood production and to prevent leukemic transformation. HSCs are also one of the few blood cells that truly age and exhibit severe functional decline in old organisms, resulting in impaired blood homeostasis and increased risk for hematologic malignancies. In this review, we present the strategies used by HSCs to cope with the many genotoxic insults that they commonly encounter. We briefly describe the DNA-damaging insults that can affect HSC function and the mechanisms that are used by HSCs to prevent, survive, and repair DNA lesions. We also discuss an apparent paradox in HSC biology, in which the genome maintenance strategies used by HSCs to protect their function in fact render them vulnerable to the acquisition of damaging genetic aberrations.

Role of reactive oxygen species in the radiation response of human hematopoietic stem/progenitor cells

Hematopoietic stem/progenitor cells (HSPCs), which are present in small numbers in hematopoietic tissues, can differentiate into all hematopoietic lineages and self-renew to maintain their undifferentiated phenotype. HSPCs are extremely sensitive to oxidative stressors such as anti-cancer agents, radiation, and the extensive accumulation of reactive oxygen species (ROS). The quiescence and stemness of HSPCs are maintained by the regulation of mitochondrial biogenesis, ROS, and energy homeostasis in a special microenvironment called the stem cell niche. The present study evaluated the relationship between the production of intracellular ROS and mitochondrial function during the proliferation and differentiation of X-irradiated CD34⁺ cells prepared from human placental/umbilical cord blood HSPCs. Highly purified CD34⁺ HSPCs exposed to X-rays were cultured in liquid and semi-solid medium supplemented with hematopoietic cytokines. X-irradiated CD34⁺ HSPCs treated with hematopoietic cytokines, which promote their proliferation and differentiation, exhibited dramatically suppressed cell growth and clonogenic potential. The amount of intracellular ROS in X-irradiated CD34⁺ HSPCs was significantly higher than that in non-irradiated cells during the culture period. However, neither the intracellular mitochondrial content nor the mitochondrial superoxide production was elevated in X-irradiated CD34⁺ HSPCs compared with non-irradiated cells. Radiation-induced gamma-H2AX expression was observed immediately following exposure to 4 Gy of X-rays and gradually decreased during the culture period. This study reveals that X-irradiation can increase persistent intracellular ROS in human CD34⁺ HSPCs, which may not result from mitochondrial ROS due to mitochondrial dysfunction, and indicates that substantial DNA double-strand breakage can critically reduce the stem cell function.

Free iron catalyzes oxidative damage to hematopoietic cells/mesenchymal stem cells in vitro and suppresses hematopoiesis in iron overload patients

OBJECTIVES

Transfusional iron overload is of major concern in hematological disease. Iron-overload-related dyserythropoiesis and reactive oxygen species (ROS)-related damage to hematopoietic stem cell (HSC) function are major setbacks in treatment for such disorders. We therefore aim to investigate the effect of iron overload on hematopoiesis in the patients and explore the role of ROS in iron-induced oxidative damage in hematopoietic cells and microenvironment in vitro.

PATIENTS AND METHODS

The hematopoietic colony-forming capacity and ROS level of bone marrow cells were tested before and after iron chelation therapy. In vitro, we first established an iron overload model of bone marrow mononuclear cells (BMMNC) and umbilical cord-derived mesenchymal stem cells (UC-MSC). ROS level, cell cycle, and apoptosis were measured by FACS. Function of cells was individually studied by Colony-forming cell (CFC) assay and co-culture system. Finally, ROS-related signaling pathway was also detected by Western blot.

RESULTS

After administering deferoxamine (DFO), reduced blood transfusion, increased neutrophil, increased platelet, and improved pancytopenia were observed in 76.9%, 46.2%, 26.9%, and 15.4% of the patients, respectively. Furthermore, the colony-forming capacity of BMMNC from iron overload patient was deficient, and ROS level was higher, which were partially recovered following iron chelation therapy. In vitro, exposure of BMMNC to ferric ammonium citrate (FAC) for 24 h decreased the ratio of CD34(+) cell from 0.91 ± 0.12% to 0.39 ± 0.07%. Excessive iron could also induce apoptosis, arrest cell cycle, and decrease function of BMMNC and UC-MSC, which was accompanied by increased ROS level and stimulated p38MAPK, p53 signaling pathway. More importantly, N-acetyl-L-cysteine (NAC) or DFO could partially attenuate cell injury and inhibit the signaling pathway induced by excessive iron.

CONCLUSIONS

Our study shows that iron overload injures the hematopoiesis by damaging hematopoietic cell and hematopoietic microenvironment, which is mediated by ROS-related signaling proteins.

Acute myeloid leukemia following exposure to benzene more closely resembles de novo than therapy related-disease

Benzene (Bz) is widely regarded as a prototype environmental leukemogen and individuals chronically exposed are at risk for myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). It is widely assumed that initiation and pathogenesis of AML following Bz exposure (Bz-AML) is similar or identical to therapy-related AML (t-AML), in which clonal cytogenetic abnormalities, including aneuploidy, are initiating events. However, this assumption is not supported by studies reporting actual disease outcomes together with cytogenetic analyses. Here, using clinically relevant cytogenetic, hematologic, and epidemiological methods, we directly show for 722 consecutive AML cases that the pattern of clonal cytogenetic abnormalities encountered in Bz-exposed cases (n = 78) more closely resembles de novo-AML than t-AML. The prevalence of aneuploidy in Bz-exposed- and de novo-AML cases was identical (23%), and no significant increases in -5/5q- (RR = 0.79) (95% CI: 0.29-2.12) or -7/7q- (RR = 1.27) (95% CI: 0.55-2.92) abnormalities were observed between Bz- vs de novo-AML, respectively. Previous studies have suggested a role for autoimmunity in Bz related MDS including immune mediated inflammatory features and positive responses to immunosuppressive therapy which are indistinguishable from those reported in MDS with low risk of progression to AML. These observations are more consistent with an epigenetic model for initiation of Bz-AML in which altered homeostatic regulation in the bone marrow niche, not direct cytogenetic injury, predominates in the initial development of the leukemic stem cell phenotype, a mechanism biologically distinct from previous models of clonal cytogenetic injury. These findings are important for further understanding the biological basis of AML, particularly in environmental and occupational settings.

Transgenic expression of BRCA1 disturbs hematopoietic stem and progenitor cells quiescence and function

The balance between quiescence and proliferation of HSCs is an important regulator of hematopoiesis. Loss of quiescence frequently results in HSCs exhaustion, which underscores the importance of tight regulation of proliferation in these cells. Studies have indicated that cyclin-dependent kinases are involved in the regulation of quiescence in HSCs. BRCA1 plays an important role in the repair of DNA double-stranded breaks, cell cycle, apoptosis and transcription. BRCA1 is expressed in the bone marrow. However, the function of BRCA1 in HSCs is unknown. In our study, we generated BRCA1 transgenic mice to investigate the effects of BRCA1 on the mechanisms of quiescence and differentiation in HSCs. The results demonstrate that over-expression of BRCA1 in the bone marrow impairs the development of B lymphocytes. Furthermore, BRCA1 induced an increase in the number of LSKs, LT-HSCs, ST-HSCs and MPPs. A competitive transplantation assay found that BRCA1 transgenic mice failed to reconstitute hematopoiesis. Moreover, BRCA1 regulates the expression of p21(waf1)/cip1 and p57(kip2), which results in a loss of quiescence in LSKs. Together, over-expression of BRCA1 in bone marrow disrupted the quiescent of LSKs, induced excessive accumulation of LSKs, and disrupted differentiation of the HSCs, which acts through the down-regulated of p21(waf1)/cip1 and p57(kip2).