Prospective isolation and molecular characterization of hematopoietic stem cells with durable self-renewal potential

An expandable, inducible hemangioblast state regulated by fibroblast growth factor

During development, the hematopoietic and vascular lineages are thought to descend from common mesodermal progenitors called hemangioblasts. Here we identify six transcription factors, Gata2, Lmo2, Mycn, Pitx2, Sox17, and Tal1, that “trap” murine cells in a proliferative state and endow them with a hemangioblast potential. These “expandable” hemangioblasts (eHBs) are capable, once released from the control of the ectopic factors, to give rise to functional endothelial cells, multilineage hematopoietic cells, and smooth muscle cells. The eHBs can be derived from embryonic stem cells, from fetal liver cells, or poorly from fibroblasts. The eHBs reveal a central role for fibroblast growth factor, which not only promotes their expansion, but also facilitates their ability to give rise to endothelial cells and leukocytes, but not erythrocytes. This study serves as a demonstration that ephemeral progenitor states can be harnessed in vitro, enabling the creation of tractable progenitor cell lines.

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Gata2, Lmo2, Mycn, Pitx2, Sox17, and Tal1 induce and maintain a hemangioblast state

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FGF2 promotes the expansion of these progenitors and impacts their potency

The thrombopoietin receptor, MPL, is critical for development of a JAK2V617F-induced myeloproliferative neoplasm

The most frequent contributing factor in Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs) is the acquisition of a V617F mutation in Janus kinase 2 (JAK2) in hematopoietic stem cells (HSCs). Recent evidence has demonstrated that to drive MPN transformation, JAK2V617F needs to directly associate with a functional homodimeric type I cytokine receptor, suggesting that, although acquiring JAK2V617F may promote disease, there are additional cellular components necessary for MPN development. Here we show that loss of the thrombopoietin (TPO) receptor (MPL) significantly ameliorates MPN development in JAK2V617F(+) transgenic mice, whereas loss of TPO only mildly affects the disease phenotype. Specifically, compared with JAK2V617F(+) mice, JAK2V617F(+)Mpl(-/-) mice exhibited reduced thrombocythemia, neutrophilia, splenomegaly, and neoplastic stem cell pool. The importance of MPL is highlighted as JAK2V617FMpl(+/-) mice displayed a significantly reduced MPN phenotype, indicating that Mpl level may have a substantial effect on MPN development and severity. Splenomegaly and the increased neoplastic stem cell pool were retained in JAK2V617F(+)Tpo(-/-) mice, although thrombocytosis was reduced compared with JAK2V617F(+) mice. These results demonstrate that Mpl expression, but not Tpo, is fundamental in the development of JAK2V617F(+) MPNs, highlighting an entirely novel target for therapeutic intervention.

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.

Identification of regulatory networks in HSCs and their immediate progeny via integrated proteome, transcriptome, and DNA methylome analysis

In this study, we present integrated quantitative proteome, transcriptome, and methylome analyses of hematopoietic stem cells (HSCs) and four multipotent progenitor (MPP) populations. From the characterization of more than 6,000 proteins, 27,000 transcripts, and 15,000 differentially methylated regions (DMRs), we identified coordinated changes associated with early differentiation steps. DMRs show continuous gain or loss of methylation during differentiation, and the overall change in DNA methylation correlates inversely with gene expression at key loci. Our data reveal the differential expression landscape of 493 transcription factors and 682 lncRNAs and highlight specific expression clusters operating in HSCs. We also found an unexpectedly dynamic pattern of transcript isoform regulation, suggesting a critical regulatory role during HSC differentiation, and a cell cycle/DNA repair signature associated with multipotency in MPP2 cells. This study provides a comprehensive genome-wide resource for the functional exploration of molecular, cellular, and epigenetic regulation at the top of the hematopoietic hierarchy.

Distinct stromal cell factor combinations can separately control hematopoietic stem cell survival, proliferation, and self-renewal

Hematopoietic stem cells (HSCs) are identified by their ability to sustain prolonged blood cell production in vivo, although recent evidence suggests that durable self-renewal (DSR) is shared by HSC subtypes with distinct self-perpetuating differentiation programs. Net expansions of DSR-HSCs occur in vivo, but molecularly defined conditions that support similar responses in vitro are lacking. We hypothesized that this might require a combination of factors that differentially promote HSC viability, proliferation, and self-renewal. We now demonstrate that HSC survival and maintenance of DSR potential are variably supported by different Steel factor (SF)-containing cocktails with similar HSC-mitogenic activities. In addition, stromal cells produce other factors, including nerve growth factor and collagen 1, that can antagonize the apoptosis of initially quiescent adult HSCs and, in combination with SF and interleukin-11, produce >15-fold net expansions of DSR-HSCs ex vivo within 7 days. These findings point to the molecular basis of HSC control and expansion.

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HSC viability, mitogenesis, and self-renewal are differentially controlled

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Stromal cells produce nonmitogenic factors that directly sustain HSC viability

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More adult bone marrow cells can produce HSCs than display HSC activity directly

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Nerve growth factor and collagen 1 promote serially transplantable HSCs

JAK2V617F homozygosity drives a phenotypic switch in myeloproliferative neoplasms, but is insufficient to sustain disease

JAK2V617F homozygosity drives a phenotypic switch between myeloproliferative neoplasms. JAK2V617F homozygosity is insufficient to sustain clonal expansion.

Computational promoter modeling identifies the modes of transcriptional regulation in hematopoietic stem cells

Extrinsic and intrinsic regulators are responsible for the tight control of hematopoietic stem cells (HSCs), which differentiate into all blood cell lineages. To understand the fundamental basis of HSC biology, we focused on differentially expressed genes (DEGs) in long-term and short-term HSCs, which are closely related in terms of cell development but substantially differ in their stem cell capacity. To analyze the transcriptional regulation of the DEGs identified in the novel transcriptome profiles obtained by our RNA-seq analysis, we developed a computational method to model the linear relationship between gene expression and the features of putative regulatory elements. The transcriptional regulation modes characterized here suggest the importance of transcription factors (TFs) that are expressed at steady state or at low levels. Remarkably, we found that 24 differentially expressed TFs targeting 21 putative TF-binding sites contributed significantly to transcriptional regulation. These TFs tended to be modulated by other nondifferentially expressed TFs, suggesting that HSCs can achieve flexible and rapid responses via the control of nondifferentially expressed TFs through a highly complex regulatory network. Our novel transcriptome profiles and new method are powerful tools for studying the mechanistic basis of cell fate decisions.

Stem and progenitor cell subsets are affected by JAK2 signaling and can be monitored by flow cytometry

Although extremely rare, hematopoietic stem cells (HSCs) are divisible into subsets that differ with respect to differentiation potential and cell surface marker expression. For example, we recently found that CD86(-) CD150(+) CD48(-) HSCs have limited potential for lymphocyte production. This could be an important new tool for studying hematological abnormalities. Here, we analyzed HSC subsets with a series of stem cell markers in JAK2V617F transgenic (Tg) mice, where the mutation is sufficient to cause myeloproliferative neoplasia with lymphocyte deficiency. Total numbers of HSC were elevated 3 to 20 fold in bone marrow of JAK2V617F mice. Careful analysis suggested the accumulation involved multiple HSC subsets, but particularly those characterized as CD150(HI) CD86(-) CD18(L)°CD41(+) and excluding Hoechst dye. Real-Time PCR analysis of their HSC revealed that the erythropoiesis associated gene transcripts Gata1, Klf1 and Epor were particularly high. Flow cytometry analyses based on two differentiation schemes for multipotent progenitors (MPP) also suggested alteration by JAK2 signals. The low CD86 on HSC and multipotent progenitors paralleled the large reductions we found in lymphoid progenitors, but the few that were produced functioned normally when sorted and placed in culture. Either of two HSC subsets conferred disease when transplanted. Thus, flow cytometry can be used to observe the influence of abnormal JAK2 signaling on stem and progenitor subsets. Markers that similarly distinguish categories of human HSCs might be very valuable for monitoring such conditions. They could also serve as indicators of HSC fitness and suitability for transplantation.

Clonal-level responses of functionally distinct hematopoietic stem cells to trophic factors

Recent findings from several groups have identified distinct classes of hematopoietic stem cells (HSCs) in the bone marrow, each with inherent functional biases in terms of their differentiation, self-renewal, proliferation, and lifespan. It has previously been demonstrated that myeloid- and lymphoid-biased HSCs can be prospectively enriched based on their degree of Hoechst dye efflux. In the present study, we used differential Hoechst efflux to enrich lineage-biased HSC subtypes and analyzed their functional potentials. Despite similar outputs in vitro, bone marrow transplantation assays revealed contrasting lineage differentiation in vivo. To stratify the molecular differences underlying these contrasting functional potentials at the clonal level, single-cell gene expression analysis was performed using the Fluidigm BioMark system and revealed dynamic expression of genes including Meis1, CEBP/α, Sfpi1, and Dnmt3a. Finally, single-cell gene expression analysis was used to unravel the opposing proliferative responses of lineage-biased HSCs to the growth factor TGF-β1, revealing a potential role for the cell cycle inhibitor Cdkn1c as molecular mediator. This work lends further credence to the concept of HSC heterogeneity, and it presents unprecedented molecular resolution of the HSC response to trophic factors using single-cell gene expression analysis.

Discriminating cellular heterogeneity using microwell-based RNA cytometry

Discriminating cellular heterogeneity is important for understanding cellular physiology. However, it is limited by the technical difficulties of single-cell measurements. Here we develop a two-stage system to determine cellular heterogeneity. In the first stage, we perform multiplex single-cell RNA cytometry in a microwell array containing over 60,000 reaction chambers. In the second stage, we use the RNA cytometry data to determine cellular heterogeneity by providing a heterogeneity likelihood score (HLS). Moreover, we use Monte-Carlo simulation and RNA cytometry data to calculate the minimum number of cells required for detecting heterogeneity. We apply this system to characterize the RNA distributions of ageing-related genes in a highly purified mouse haematopoietic stem cell population. We identify genes that reveal novel heterogeneity of these cells. We also show that changes in expression of genes such as Birc6 during ageing can be attributed to the shift of relative portions of cells in the high-expressing subgroup versus low-expressing subgroup.

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 hematopoiesis by activators and inhibitors of Wnt signaling from the niche

Hematopoietic stem cells (HSCs) are a rare population of somatic stem cells that have the ability to regenerate the entire mature blood system in a hierarchical way for the duration of an adult life. Adult HSCs reside in the bone marrow niche. Different niche cell types and molecules regulate the balance of HSC dormancy and activation as well as HSC behavior in both normal and malignant hematopoiesis. Here, we describe the interplay of HSCs and their niche, in particular the involvement of the Wnt signaling pathway. Although the prevailing notion has been that malignant transformation of HSCs is the main cause of leukemia, evidence is mounting that disruption of niche regulation by transformed hematopoietic cells, which may overexpress Wnt signaling or intrinsic stromal defects in gene expression, is at least a collaborative factor in leukemogenesis. Thus, insights into the normal and altered functions of niche components will help to obtain a better understanding of normal and malignant hematopoiesis and how environmental factors affect these processes.

SLAM family markers resolve functionally distinct subpopulations of hematopoietic stem cells and multipotent progenitors

Hematopoietic stem cells (HSCs) and multipotent hematopoietic progenitors (MPPs) are routinely isolated using various markers but remain heterogeneous. Here we show that four SLAM family markers, CD150, CD48, CD229, and CD244, can distinguish HSCs and MPPs from restricted progenitors and subdivide them into a hierarchy of functionally distinct subpopulations with stepwise changes in cell-cycle status, self-renewal, and reconstituting potential. CD229 expression largely distinguished lymphoid-biased HSCs from rarely dividing myeloid-biased HSCs, enabling prospective enrichment of these HSC subsets. Differences in CD229 and CD244 expression resolved CD150(-)CD48(-/low)Lineage(-/low)Sca-1(+)c-Kit(+) cells into a hierarchy of highly purified MPPs that retained erythroid and platelet potential but exhibited progressive changes in mitotic activity and reconstituting potential. Use of these markers, and reconstitution assays, showed that conditional deletion of Scf from endothelial cells and perivascular stromal cells eliminated the vast majority of bone marrow HSCs, including nearly all CD229(-/low) HSCs, demonstrating that quiescent HSCs are maintained by a perivascular niche.

Transcriptional mechanisms of cell fate decisions revealed by single cell expression profiling

Transcriptional networks regulate cell fate decisions, which occur at the level of individual cells. However, much of what we know about their structure and function comes from studies averaging measurements over large populations of cells, many of which are functionally heterogeneous. Such studies conceal the variability between cells and so prevent us from determining the nature of heterogeneity at the molecular level. In recent years, many protocols and platforms have been developed that allow the high throughput analysis of gene expression in single cells, opening the door to a new era of biology. Here, we discuss the need for single cell gene expression analysis to gain deeper insights into the transcriptional control of cell fate decisions, and consider the insights it has provided so far into transcriptional regulatory networks in development.

Hematopoietic and mesenchymal stem cells for the treatment of chronic respiratory diseases: role of plasticity and heterogeneity

Chronic lung diseases, such as cystic fibrosis (CF), asthma, and chronic obstructive pulmonary disease (COPD) are incurable and represent a very high social burden. Stem cell-based treatment may represent a hope for the cure of these diseases. In this paper, we revise the overall knowledge about the plasticity and engraftment of exogenous marrow-derived stem cells into the lung, as well as their usefulness in lung repair and therapy of chronic lung diseases. The lung is easily accessible and the pathophysiology of these diseases is characterized by injury, inflammation, and eventually by remodeling of the airways. Bone marrow-derived stem cells, including hematopoietic stem/progenitor cells (HSPCs) and mesenchymal stromal (stem) cells (MSCs), encompass a wide array of cell subsets with different capacities of engraftment and injured tissue regenerating potential. Proof-of-principle that marrow cells administered locally may engraft and give rise to specialized epithelial cells has been given, but the efficiency of this conversion is too limited to give a therapeutic effect. Besides the identification of plasticity mechanisms, the characterization/isolation of the stem cell subpopulations represents a major challenge to improving the efficacy of transplantation protocols used in regenerative medicine for lung diseases.

Heterogeneity and hierarchy of hematopoietic stem cells

Hematopoietic stem cells (HSCs) are a more heterogeneous population than previously thought. Extensive analysis of reconstitution kinetics after transplantation allows a new classifications of HSCs based on lineage balance. Previously unrecognized classes of HSCs, such as myeloid- and lymphoid-biased HSCs, have emerged. However, varying nomenclature has been used to describe these cells, promoting confusion in the field. To establish a common nomenclature, we propose a reclassification of short-, intermediate-, and long-term (ST, IT, and LT) HSCs defined as: ST < 6 months, IT > 6 months, and LT > 12. We observe that myeloid-biased HSCs or α cells overlap with LT-HSCs, whereas lymphoid-biased HSCs or γ/δ cells overlap with ST-HSCs, suggesting that HSC lifespan is linked to cell differentiation. We also suggest that HSC heterogeneity prompts reconsideration of long-term (>4 months) multilineage reconstitution as the gold standard for HSC detection. In this review, we discuss relationships among ST-, IT-, and LT-HSCs relevant to stem cell heterogeneity, hierarchical organization, and differentiation pathways.

STAT5 in hematopoietic stem cell biology and transplantation

Signal transducer and activator of transcription 5 (STAT5) regulates normal lympho-myeloid development through activation downstream of early-acting cytokines, their receptors, and Janus kinases (JAKs). Despite a general understanding of the role of STAT5 in hematopoietic stem cell (HSC) proliferation, survival, and self-renewal, the transcriptional targets and mechanisms of gene regulation that control multi-lineage engraftment following transplantation for the most part remain to be understood. In this review, we focus on the role of STAT5 in HSC transplantation and recent developments toward identifying the relevant downstream target genes and their role as part of a pleiotropic STAT5 mediated signaling response.

Clonal analysis unveils self-renewing lineage-restricted progenitors generated directly from hematopoietic stem cells

Consensus holds that hematopoietic stem cells (HSCs) give rise to multipotent progenitors (MPPs) of reduced self-renewal potential and that MPPs eventually produce lineage-committed progenitor cells in a stepwise manner. Using a single-cell transplantation system and marker mice, we unexpectedly found myeloid-restricted progenitors with long-term repopulating activity (MyRPs), which are lineage-committed to megakaryocytes, megakaryocyte-erythroid cells, or common myeloid cells (MkRPs, MERPs, or CMRPs, respectively) in the phenotypically defined HSC compartment together with HSCs. Paired daughter cell assays combined with transplantation revealed that HSCs can give rise to HSCs via symmetric division or directly differentiate into MyRPs via asymmetric division (yielding HSC-MkRP or HSC-CMRP pairs). These myeloid bypass pathways could be essential for fast responses to ablation stress. Our results show that loss of self-renewal and stepwise progression through specific differentiation stages are not essential for lineage commitment of HSCs and suggest a revised model of hematopoietic differentiation.

Developmental changes in hematopoietic stem cell properties

Hematopoietic stem cells (HSCs) comprise a rare population of cells that can regenerate and maintain lifelong blood cell production. This functionality is achieved through their ability to undergo many divisions without activating a poised, but latent, capacity for differentiation into multiple blood cell types. Throughout life, HSCs undergo sequential changes in several key properties. These affect mechanisms that regulate the self-renewal, turnover and differentiation of HSCs as well as the properties of the committed progenitors and terminally differentiated cells derived from them. Recent findings point to the Lin28b-let-7 pathway as a master regulator of many of these changes with important implications for the clinical use of HSCs for marrow rescue and gene therapy, as well as furthering our understanding of the different pathogenesis of childhood and adult-onset leukemia.

Heterogeneity in hematopoietic stem cell populations: implications for transplantation

PURPOSE OF REVIEW

Transplantation of hematopoietic cells is now a well established clinical procedure, although optimal outcomes are not always obtained. This reflects insufficient knowledge of the different subsets of primitive cells required to achieve a rapid and permanent recovery of mature blood cell production. Here we review recent findings that extend our understanding of these cells and their regulation, and implications for the ex-vivo expansion of these cells.

RECENT FINDINGS

Separate subsets of platelet and neutrophil lineage-restricted human hematopoietic cells with rapid but transient repopulating activities have been identified, thus adding to previous evidence of short-term repopulating cells that generate both of these lineages. New studies also suggest intrinsically determined heterogeneity in differentiation potentialities that are sustained at the stem cell level, and have revealed new ways their self-renewal can be influenced.

SUMMARY

Hematopoietic repopulation posttransplant is highly complex both in terms of the differing numbers and types of cells required for optimal hematopoietic recoveries and the factors that will determine the composition and behavior of a given inoculum. Successful ex-vivo expansion protocols will, thus, need to incorporate conditions that will produce adequate numbers of all cell types required with retention of their full functionality.

Control of the hematopoietic stem cell state

Several studies have demonstrated the clonal transmission of distinct differentiation and self-renewal properties in hematopoietic stem cells during the regeneration of blood production in transplanted recipients. A recent publication now identifies Vwf expression as a discriminating marker of a hematopoietic stem cell state that is primed for platelet production in response to thrombopoietin, but also subject to developmental and other, as yet undefined, cues.

A novel role for factor VIII and thrombin/PAR1 in regulating hematopoiesis and its interplay with the bone structure

Analysis of hematopoietic stem cells (HSCs) in factor VIII knockout (FVIIIKO) mice revealed a novel regulatory role for the coagulation cascade in hematopoiesis. Thus, HSCs in FVIIIKO mice had reduced proportions of CD34(low) cells within Lin(-)Sca(+)Kit(+) progenitors, and exhibited reduced long-term repopulating capacity as well as hyper granulocyte-colony-stimulating factor (G-CSF)-induced mobilization. This disregulation of HSCs is likely caused by reduced levels of thrombin, and is associated with altered protease-activated receptor 1 (PAR1) signaling, as PAR1 KO mice also exhibited enhanced G-CSF-induced mobilization. Analysis of reciprocal bone marrow (BM) chimera (FVIIIKO BM into wild-type recipients and vice versa) and the detection of PAR1 expression on stromal elements indicates that this phenotype is likely controlled by stromal elements. Micro-computed tomography analysis of distal tibia metaphyses also revealed for the first time a major impact of the FVIII/thrombin/PAR1 axis on the dynamic bone structure, showing reduced bone:tissue volume ratio and trabecular number in FVIIIKO and PAR1KO mice. Taken together, these results show a critical and novel role for the coagulation cascade, mediated in part by thrombin-PAR1 interaction, and regulates HSC maintenance and a reciprocal interplay between HSCs and the dynamic bone structure.

GATA-3 regulates the self-renewal of long-term hematopoietic stem cells

The transcription factor GATA-3 is expressed and required for differentiation and function throughout the T lymphocyte lineage. Despite evidence it may also be expressed in multipotent hematopoietic stem cells (HSCs), any role for GATA-3 in these cells has remained unclear. Here we found GATA-3 was in the cytoplasm in quiescent long-term stem cells from steady-state bone marrow but relocated to the nucleus when HSCs cycled. Relocation depended on signaling via the mitogen-activated protein kinase p38 and was associated with a diminished capacity for long-term reconstitution after transfer into irradiated mice. Deletion of Gata3 enhanced the repopulating capacity and augmented the self-renewal of long-term HSCs in cell-autonomous fashion without affecting the cell cycle. Our observations position GATA-3 as a regulator of the balance between self-renewal and differentiation in HSCs that acts downstream of the p38 signaling pathway.

Platelet-biased stem cells reside at the apex of the haematopoietic stem-cell hierarchy

The blood system is maintained by a small pool of haematopoietic stem cells (HSCs), which are required and sufficient for replenishing all human blood cell lineages at millions of cells per second throughout life. Megakaryocytes in the bone marrow are responsible for the continuous production of platelets in the blood, crucial for preventing bleeding--a common and life-threatening side effect of many cancer therapies--and major efforts are focused at identifying the most suitable cellular and molecular targets to enhance platelet production after bone marrow transplantation or chemotherapy. Although it has become clear that distinct HSC subsets exist that are stably biased towards the generation of lymphoid or myeloid blood cells, we are yet to learn whether other types of lineage-biased HSC exist or understand their inter-relationships and how differently lineage-biased HSCs are generated and maintained. The functional relevance of notable phenotypic and molecular similarities between megakaryocytes and bone marrow cells with an HSC cell-surface phenotype remains unclear. Here we identify and prospectively isolate a molecularly and functionally distinct mouse HSC subset primed for platelet-specific gene expression, with enhanced propensity for short- and long-term reconstitution of platelets. Maintenance of platelet-biased HSCs crucially depends on thrombopoietin, the primary extrinsic regulator of platelet development. Platelet-primed HSCs also frequently have a long-term myeloid lineage bias, can self-renew and give rise to lymphoid-biased HSCs. These findings show that HSC subtypes can be organized into a cellular hierarchy, with platelet-primed HSCs at the apex. They also demonstrate that molecular and functional priming for platelet development initiates already in a distinct HSC population. The identification of a platelet-primed HSC population should enable the rational design of therapies enhancing platelet output.

HSC-explorer: a curated database for hematopoietic stem cells

HSC-Explorer (http://mips.helmholtz-muenchen.de/HSC/) is a publicly available, integrative database containing detailed information about the early steps of hematopoiesis. The resource aims at providing fast and easy access to relevant information, in particular to the complex network of interacting cell types and molecules, from the wealth of publications in the field through visualization interfaces. It provides structured information on more than 7000 experimentally validated interactions between molecules, bioprocesses and environmental factors. Information is manually derived by critical reading of the scientific literature from expert annotators. Hematopoiesis-relevant interactions are accompanied with context information such as model organisms and experimental methods for enabling assessment of reliability and relevance of experimental results. Usage of established vocabularies facilitates downstream bioinformatics applications and to convert the results into complex networks. Several predefined datasets (Selected topics) offer insights into stem cell behavior, the stem cell niche and signaling processes supporting hematopoietic stem cell maintenance. HSC-Explorer provides a versatile web-based resource for scientists entering the field of hematopoiesis enabling users to inspect the associated biological processes through interactive graphical presentation.

High-throughput microfluidic single-cell digital polymerase chain reaction

Here we present an integrated microfluidic device for the high-throughput digital polymerase chain reaction (dPCR) analysis of single cells. This device allows for the parallel processing of single cells and executes all steps of analysis, including cell capture, washing, lysis, reverse transcription, and dPCR analysis. The cDNA from each single cell is distributed into a dedicated dPCR array consisting of 1020 chambers, each having a volume of 25 pL, using surface-tension-based sample partitioning. The high density of this dPCR format (118,900 chambers/cm(2)) allows the analysis of 200 single cells per run, for a total of 204,000 PCR reactions using a device footprint of 10 cm(2). Experiments using RNA dilutions show this device achieves shot-noise-limited performance in quantifying single molecules, with a dynamic range of 10(4). We performed over 1200 single-cell measurements, demonstrating the use of this platform in the absolute quantification of both high- and low-abundance mRNA transcripts, as well as micro-RNAs that are not easily measured using alternative hybridization methods. We further apply the specificity and sensitivity of single-cell dPCR to performing measurements of RNA editing events in single cells. High-throughput dPCR provides a new tool in the arsenal of single-cell analysis methods, with a unique combination of speed, precision, sensitivity, and specificity. We anticipate this approach will enable new studies where high-performance single-cell measurements are essential, including the analysis of transcriptional noise, allelic imbalance, and RNA processing.

The pharmacology of regenerative medicine

Regenerative medicine is a rapidly evolving multidisciplinary, translational research enterprise whose explicit purpose is to advance technologies for the repair and replacement of damaged cells, tissues, and organs. Scientific progress in the field has been steady and expectations for its robust clinical application continue to rise. The major thesis of this review is that the pharmacological sciences will contribute critically to the accelerated translational progress and clinical utility of regenerative medicine technologies. In 2007, we coined the phrase "regenerative pharmacology" to describe the enormous possibilities that could occur at the interface between pharmacology, regenerative medicine, and tissue engineering. The operational definition of regenerative pharmacology is "the application of pharmacological sciences to accelerate, optimize, and characterize (either in vitro or in vivo) the development, maturation, and function of bioengineered and regenerating tissues." As such, regenerative pharmacology seeks to cure disease through restoration of tissue/organ function. This strategy is distinct from standard pharmacotherapy, which is often limited to the amelioration of symptoms. Our goal here is to get pharmacologists more involved in this field of research by exposing them to the tools, opportunities, challenges, and interdisciplinary expertise that will be required to ensure awareness and galvanize involvement. To this end, we illustrate ways in which the pharmacological sciences can drive future innovations in regenerative medicine and tissue engineering and thus help to revolutionize the discovery of curative therapeutics. Hopefully, the broad foundational knowledge provided herein will spark sustained conversations among experts in diverse fields of scientific research to the benefit of all.

The Lin28b-let-7-Hmga2 axis determines the higher self-renewal potential of fetal haematopoietic stem cells

Mouse haematopoietic stem cells (HSCs) undergo a postnatal transition in several properties, including a marked reduction in their self-renewal activity. We now show that the developmentally timed change in this key function of HSCs is associated with their decreased expression of Lin28b and an accompanying increase in their let-7 microRNA levels. Lentivirus-mediated overexpression of Lin28 in adult HSCs elevates their self-renewal activity in transplanted irradiated hosts, as does overexpression of Hmga2, a well-established let-7 target that is upregulated in fetal HSCs. Conversely, HSCs from fetal Hmga2(-/-) mice do not exhibit the heightened self-renewal activity that is characteristic of wild-type fetal HSCs. Interestingly, overexpression of Hmga2 in adult HSCs does not mimic the ability of elevated Lin28 to activate a fetal lymphoid differentiation program. Thus, Lin28b may act as a master regulator of developmentally timed changes in HSC programs with Hmga2 serving as its specific downstream modulator of HSC self-renewal potential.

Self-renewal of single mouse hematopoietic stem cells is reduced by JAK2V617F without compromising progenitor cell expansion

In this study, single cell assays and mathematical modeling demonstrate that a single oncogenic point mutation can negatively affect hematopoietic stem cells while leaving progenitor cell expansion intact.

Recent descriptions of significant heterogeneity in normal stem cells and cancers have altered our understanding of tumorigenesis, emphasizing the need to understand how single stem cells are subverted to cause tumors. Human myeloproliferative neoplasms (MPNs) are thought to reflect transformation of a hematopoietic stem cell (HSC) and the majority harbor an acquired V617F mutation in the JAK2 tyrosine kinase, making them a paradigm for studying the early stages of tumor establishment and progression. The consequences of activating tyrosine kinase mutations for stem and progenitor cell behavior are unclear. In this article, we identify a distinct cellular mechanism operative in stem cells. By using conditional knock-in mice, we show that the HSC defect resulting from expression of heterozygous human JAK2V617F is both quantitative (reduced HSC numbers) and qualitative (lineage biases and reduced self-renewal per HSC). The defect is intrinsic to individual HSCs and their progeny are skewed toward proliferation and differentiation as evidenced by single cell and transplantation assays. Aged JAK2V617F show a more pronounced defect as assessed by transplantation, but mice that transform reacquire competitive self-renewal ability. Quantitative analysis of HSC-derived clones was used to model the fate choices of normal and JAK2-mutant HSCs and indicates that JAK2V617F reduces self-renewal of individual HSCs but leaves progenitor expansion intact. This conclusion is supported by paired daughter cell analyses, which indicate that JAK2-mutant HSCs more often give rise to two differentiated daughter cells. Together these data suggest that acquisition of JAK2V617F alone is insufficient for clonal expansion and disease progression and causes eventual HSC exhaustion. Moreover, our results show that clonal expansion of progenitor cells provides a window in which collaborating mutations can accumulate to drive disease progression. Characterizing the mechanism(s) of JAK2V617F subclinical clonal expansions and the transition to overt MPNs will illuminate the earliest stages of tumor establishment and subclone competition, fundamentally shifting the way we treat and manage cancers.

Recent descriptions of the existence of significant heterogeneity in normal stem cells and cancers have altered our understanding of tumorigenesis, emphasizing the need to understand how single stem cells are subverted to cause tumours. In this study, we focus on understanding the stem cell defect that results from a mutation in the JAK2 tyrosine kinase gene, which is present in the majority of patients with myeloproliferative neoplasms (MPNs), a group of clonal bone marrow diseases that are characterised by the overproduction of mature blood cells and increased frequency of leukaemia development. By using single-cell assays and mathematical modeling, followed by the individual assessment of daughter cells from single HSCs, we identify a distinct cellular mechanism that differentially affects stem cell and progenitor cell expansion. Specifically, we show that this single point mutation can negatively affect HSCs while leaving progenitor cell expansion intact. Characterising the mechanisms that link JAK2 mutations with clonal expansions that eventually lead to development of MPNs will inform our understanding of the earliest stages of tumour establishment and of the competition between subclones of proliferating progenitor/stem cells. These findings have direct relevance to all cancers of a suspected stem cell origin.

Paracrine effects and heterogeneity of marrow-derived stem/progenitor cells: relevance for the treatment of respiratory diseases

Stem cell-based treatment may represent a hope for the treatment of acute lung injury and pulmonary fibrosis, and other chronic lung diseases, such as cystic fibrosis, asthma and chronic obstructive pulmonary disease (COPD). It is well established in preclinical models that bone marrow-derived stem and progenitor cells exert beneficial effects on inflammation, immune responses and repairing of damage in virtually all lung-borne diseases. While it was initially thought that the positive outcome was due to a direct engraftment of these cells into the lung as endothelial and epithelial cells, paracrine factors are now considered the main mechanism through which stem and progenitor cells exert their therapeutic effect. This knowledge has led to the clinical use of marrow cells in pulmonary hypertension with endothelial progenitor cells (EPCs) and in COPD with mesenchymal stromal (stem) cells (MSCs). Bone marrow-derived stem cells, including hematopoietic stem/progenitor cells, MSCs, EPCs and fibrocytes, encompass a wide array of cell subsets with different capacities of engraftment and injured tissue-regenerating potential. The characterization/isolation of the stem cell subpopulations represents a major challenge to improve the efficacy of transplantation protocols used in regenerative medicine and applied to lung disorders.

The ageing haematopoietic stem cell compartment

Stem cell ageing underlies the ageing of tissues, especially those with a high cellular turnover. There is growing evidence that the ageing of the immune system is initiated at the very top of the haematopoietic hierarchy and that the ageing of haematopoietic stem cells (HSCs) directly contributes to changes in the immune system, referred to as immunosenescence. In this Review, we summarize the phenotypes of ageing HSCs and discuss how the cell-intrinsic and cell-extrinsic mechanisms of HSC ageing might promote immunosenescence. Stem cell ageing has long been considered to be irreversible. However, recent findings indicate that several molecular pathways could be targeted to rejuvenate HSCs and thus to reverse some aspects of immunosenescence.

CD41 expression marks myeloid-biased adult hematopoietic stem cells and increases with age

The hematopoietic stem cell (HSC) compartment is heterogeneous, yet our understanding of the identities of different HSC subtypes is limited. Here we show that platelet integrin CD41 (αIIb), currently thought to only transiently mark fetal HSCs, is expressed on an adult HSC subtype that accumulates with age. CD41+ HSCs were largely quiescent and exhibited myeloerythroid and megakaryocyte gene priming, governed by Gata1, whereas CD41- HSCs were more proliferative and exhibited lymphoid gene priming. When isolated without the use of blocking antibodies, CD41+ HSCs possessed long-term repopulation capacity on serial transplantations and showed a marked myeloid bias compared with CD41- HSCs, which yielded a more lymphoid-biased progeny. CD41-knockout (KO) mice displayed multilineage hematopoietic defects coupled with decreased quiescence and survival of HSCs, suggesting that CD41 is functionally relevant for HSC maintenance and hematopoietic homeostasis. Transplantation experiments indicated that CD41-KO-associated defects are long-term transplantable, HSC-derived and, in part, mediated through the loss of platelet mass leading to decreases in HSC exposure to important platelet released cytokines, such as transforming growth factor β1. In summary, our data provide a novel marker to identify a myeloid-biased HSC subtype that becomes prevalent with age and highlights the dogma of HSC regulation by their progeny.

Fetal origins of hematopoietic failure in a murine model of Fanconi anemia

Hematopoietic failure is the predominant clinical manifestation of Fanconi anemia (FA), a rare, recessively inherited disorder. Mutations in 1 of 15 genes that coordinately function in a complex pathway to maintain DNA integrity also predispose patients to constitutional defects in growth and development. The hematologic manifestations have been considered to reflect the progressive loss of stem cells from the postnatal bone marrow microenvironment. Ethical concerns preclude the study of human hematopoiesis in utero. We report significant late gestational lethality and profound quantitative and qualitative deficiencies in the murine Fancc(-/-) fetal liver hematopoietic stem and progenitor cell pool. Fancc(-/-) fetal liver hematopoietic stem and progenitor cells revealed a significant loss of quiescence and decline in serial repopulating capacity, but no substantial difference in apoptosis or levels of reactive oxygen species. Our studies suggest that compromised hematopoiesis in Fancc(-/-) animals is developmentally programmed and does not arise de novo in bone marrow.

Long-term culture-initiating cell assay for mouse cells

The long-term culture-initiating cell (LTC-IC) assay is a well-established in vitro assay used to enumerate primitive mouse hematopoietic stem cells (HSCs) and relies on the two cardinal functions of HSCs: ability to self-renew and differentiation capacity. LTC-ICs present in minimally processed and purified cell suspensions and cocultured on a supportive feeder layer are detected by their sustained ability to produce hematopoietic progenitors (colony forming cells) after ≥ 4 weeks in culture. Refinements including the use of a defined stromal cell line, and extending the in vitro culture to 6 weeks allow detection of LTC-IC at similar frequencies to transplantable HSCs quantified using in vivo assays.

Comprehensive hematopoietic stem cell isolation methods

The use of flow cytometry has been critical in establishing methods to isolate and characterize hematopoietic stem cells (HSCs) and their progenitors. For more than 30 years, researchers have been uncovering novel markers that when used in combination significantly enhance the purification of HSCs from murine and human bone marrow. The complex interface between HSCs, the lymphohematopoietic system, and their niches, has made identification of HSC markers critical to understanding their biological nature, more so than other adult stem cell populations. Here we review the phenotypic markers and strategies used to purify HSCs, the appropriateness of using these markers for comparisons of HSC function at different stages of ontogeny, and their utility in defining the lineage bias in the HSC compartment.

Lineage tracing of Pf4-Cre marks hematopoietic stem cells and their progeny

The development of a megakaryocyte lineage specific Cre deleter, using the Pf4 (CXCL4) promoter (Pf4-Cre), was a significant step forward in the specific analysis of platelet and megakaryocyte cell biology. However, in the present study we have employed a sensitive reporter-based approach to demonstrate that Pf4-Cre also recombines in a significant proportion of both fetal liver and bone marrow hematopoietic stem cells (HSCs), including the most primitive fraction containing the long-term repopulating HSCs. Consequently, we demonstrate that Pf4-Cre activity is not megakaryocyte lineage-specific but extends to other myeloid and lymphoid lineages at significant levels between 15-60%. Finally, we show for the first time that Pf4 transcripts are present in adult HSCs and primitive hematopoietic progenitor cells. These results have fundamental implications for the use of the Pf4-Cre mouse model and for our understanding of a possible role for Pf4 in the development of the hematopoietic lineage.

Differential expression of surface markers in mouse bone marrow mesenchymal stromal cell subpopulations with distinct lineage commitment

Bone marrow mesenchymal stromal cells (BM MSCs) represent a heterogeneous population of progenitors with potential for generation of skeletal tissues. However the identity of BM MSC subpopulations is poorly defined mainly due to the absence of specific markers allowing in situ localization of those cells and isolation of pure cell types. Here, we aimed at characterization of surface markers in mouse BM MSCs and in their subsets with distinct differentiation potential. Using conditionally immortalized BM MSCs we performed a screening with 176 antibodies and high-throughput flow cytometry, and found 33 markers expressed in MSCs, and among them 3 were novel for MSCs and 13 have not been reported for MSCs from mice. Furthermore, we obtained clonally derived MSC subpopulations and identified bipotential progenitors capable for osteo- and adipogenic differentiation, as well as monopotential osteogenic and adipogenic clones, and thus confirmed heterogeneity of MSCs. We found that expression of CD200 was characteristic for the clones with osteogenic potential, whereas SSEA4 marked adipogenic progenitors lacking osteogenic capacity, and CD140a was expressed in adipogenic cells independently of their efficiency for osteogenesis. We confirmed our observations in cell sorting experiments and further investigated the expression of those markers during the course of differentiation. Thus, our findings provide to our knowledge the most comprehensive characterization of surface antigens expression in mouse BM MSCs to date, and suggest CD200, SSEA4 and CD140a as markers differentially expressed in distinct types of MSC progenitors.

Hematopoiesis

Enormous numbers of adult blood cells are constantly regenerated throughout life from hematopoietic stem cells through a series of progenitor stages. Accessibility, robust functional assays, well-established prospective isolation, and successful clinical application made hematopoiesis the classical mammalian stem cell system. Most of the basic concepts of stem cell biology have been defined in this system. At the same time, many long-standing disputes in hematopoiesis research illustrate our still limited understanding. Here we discuss the embryonic development and lifelong maintenance of the hematopoietic system, its cellular components, and some of the hypotheses about the molecular mechanisms involved in controlling hematopoietic cell fates.

Hematopoietic stem cells are regulated by Cripto, as an intermediary of HIF-1α in the hypoxic bone marrow niche

Cripto has been known as an embryonic stem (ES)- or tumor-related soluble/cell membrane protein. In this study, we demonstrated that Cripto has a role as an important regulatory factor for hematopoietic stem cells (HSCs). Recombinant Cripto sustained the reconstitution ability of HSCs in vitro. Flow cytometry analysis uncovered that GRP78, one of the candidate receptors for Cripto, was expressed on a subset of HSCs and could distinguish dormant/myeloid-biased HSCs and active/lymphoid-biased HSCs. Cripto is expressed in hypoxic endosteal niche cells where GRP78(+) HSCs mainly reside. Proteomics analysis revealed that Cripto-GRP78 binding stimulates glycolytic metabolism-related proteins and results in lower mitochondrial potential in HSCs. Furthermore, conditional knockout mice for HIF-1α, a master regulator of hypoxic responses, showed reduced Cripto expression and decreased GRP78(+) HSCs in the endosteal niche area. Thus, Cripto-GRP78 is a novel HSC regulatory signal mainly working in the hypoxic niche.

In situ production of innate immune cells in murine white adipose tissue

White adipose tissue (WAT) is the focus of new interest because of the presence of an abundant and complex immune cell population that is involved in key pathologies such as metabolic syndrome. Based on in vivo reconstitution assays, it is thought that these immune cells are derived from the bone marrow (BM). However, previous studies have shown that WAT exhibits specific hematopoietic activity exerted by an unknown subpopulation of cells. In the present study, we prospectively isolated a peculiar hematopoietic stem/progenitor cell population from murine WAT. The cells are phenotypically similar to BM hematopoietic stem cells and are able to differentiate into both myeloid and lymphoid lineages in vitro. In competitive repopulation assays in vivo, they reconstituted the innate immune compartment in WAT preferentially and more efficiently than BM cells, but did not reconstitute hematopoietic organs. They were also able to give rise to multilineage engraftment in both secondary recipients and in utero transplantation. Therefore, we propose that WAT hematopoietic cells constitute a population of immature cells that are able to renew innate immune cell populations. Considering the amount of WAT in adults, our results suggest that WAT hematopoietic activity controls WAT inflammatory processes and also supports innate immune responses in other organs.

Hematopoietic stem cell heterogeneity takes center stage

Over the past 10 years, increasing evidence has accumulated that heterogeneity is a feature of hematopoietic stem cell (HSC) proliferation, self-renewal, and differentiation based on examination of these properties at a clonal level. The heterogeneous behavior of HSCs reflects the operation of a complex interplay of intrinsic and extrinsic variables. In this review, we discuss key findings from the last 5 years that reveal new insights into the mechanisms involved.

Signaling from the sympathetic nervous system regulates hematopoietic stem cell emergence during embryogenesis

The first adult-repopulating hematopoietic stem cells (HSCs) emerge in the aorta-gonads-mesonephros (AGM) region of the embryo. We have recently identified the transcription factor Gata3 as being upregulated in this tissue specifically at the time of HSC emergence. We now demonstrate that the production of functional and phenotypic HSCs in the AGM is impaired in the absence of Gata3. Furthermore, we show that this effect on HSC generation is secondary to the role of Gata3 in the production of catecholamines, the mediators of the sympathetic nervous system (SNS), thus making these molecules key components of the AGM HSC niche. These findings demonstrate that the recently described functional interplay between the hematopoietic system and the SNS extends to the earliest stages of their codevelopment and highlight the fact that HSC development needs to be viewed in the context of the development of other organs.

Pharmacological targeting of the thrombomodulin-activated protein C pathway mitigates radiation toxicity

Tissue damage induced by ionizing radiation in the hematopoietic and gastrointestinal systems is the major cause of lethality in radiological emergency scenarios and underlies some deleterious side effects in patients undergoing radiation therapy. The identification of target-specific interventions that confer radiomitigating activity is an unmet challenge. Here we identify the thrombomodulin (Thbd)-activated protein C (aPC) pathway as a new mechanism for the mitigation of total body irradiation (TBI)-induced mortality. Although the effects of the endogenous Thbd-aPC pathway were largely confined to the local microenvironment of Thbd-expressing cells, systemic administration of soluble Thbd or aPC could reproduce and augment the radioprotective effect of the endogenous Thbd-aPC pathway. Therapeutic administration of recombinant, soluble Thbd or aPC to lethally irradiated wild-type mice resulted in an accelerated recovery of hematopoietic progenitor activity in bone marrow and a mitigation of lethal TBI. Starting infusion of aPC as late as 24 h after exposure to radiation was sufficient to mitigate radiation-induced mortality in these mice. These findings suggest that pharmacologic augmentation of the activity of the Thbd-aPC pathway by recombinant Thbd or aPC might offer a rational approach to the mitigation of tissue injury and lethality caused by ionizing radiation.

HOXC4 homeoprotein efficiently expands human hematopoietic stem cells and triggers similar molecular alterations as HOXB4

BACKGROUND

Expansion of hematopoietic stem cells represents an important objective for improving cell and gene therapy protocols. Retroviral transduction of the HoxB4 homeogene in mouse and human hematopoietic stem cells and hematopoietic progenitors is known to promote the cells' expansion. A safer approach consists in transferring homeobox proteins into hematopoietic stem cells taking advantage of the natural ability of homeoproteins to cross cell membranes. Thus, HOXB4 protein transfer is operative for expanding human hematopoietic cells, but such expansion needs to be improved.

DESIGN AND METHODS

To that aim, we evaluated the effects of HOXC4, a protein encoded by a HOXB4 paralog gene, by co-culturing HOXC4-producing stromal cells with human CD34(+) hematopoietic cells. Numbers of progenitors and stem cells were assessed by in vitro cloning assays and injection into immuno-deficient mice, respectively. We also looked for activation or inhibition of target downstream gene expression.

RESULTS

We show that the HOXC4 homeoprotein expands human hematopoietic immature cells by 3 to 6 times ex vivo and significantly improves the level of in vivo engraftment. Comparative transcriptome analysis of CD34(+) cells subjected or not to HOXB4 or HOXC4 demonstrated that both homeoproteins regulate the same set of genes, some of which encode key hematopoietic factors and signaling molecules. Certain molecules identified herein are factors reported to be involved in stem cell fate or expansion in other models, such as MEF2C, EZH2, DBF4, DHX9, YPEL5 and Pumilio.

CONCLUSIONS

The present study may help to identify new HOX downstream key factors potentially involved in hematopoietic stem cell expansion or in leukemogenesis.

Loss of p53 accelerates the complications of myelodysplastic syndrome in a NUP98-HOXD13-driven mouse model

The nucleoporin gene NUP98 is fused to several genes including HOXD13 in patients with myelodysplastic syndromes (MDS), acute myeloid leukemia, and chronic myeloid leukemia, blast crisis. Genetically engineered mice that express a NUP98-HOXD13 (NHD13) transgene (Tg) display the phenotypic features of MDS, including cytopenias, bone marrow dysplasia, and transformation to acute leukemia. Here we show that short-term treatment with the p53 inhibitor Pifithrin-α partially and transiently rescued the myeloid and lymphoid abnormalities found in NHD13(+) Tg mice, with no improvement in the anemia, while the genetic deletion of 2 alleles of p53 rescued both the myeloid progenitor cell and long-term hematopoietic stem cell compartments. Nonetheless, loss of one or both alleles of p53 did not rescue the MDS phenotype, but instead exacerbated the MDS phenotype and accelerated the development of acute myeloid leukemia. Our studies suggest that while targeting p53 may transiently improve hematopoiesis in MDS, over the long-term, it has detrimental effects, raising caution about abrogating its function to treat the cytopenias that accompany this disease.