Hematopoietic stem cells: concepts, definitions, and the new reality. Blood. 2015;125:2605-2613. [PMID: 25762175 DOI: 10.1182/blood-2014-12-570200]

Long-term ex vivo haematopoietic-stem-cell expansion allows nonconditioned transplantation

Multipotent self-renewing haematopoietic stem cells (HSCs) regenerate the adult blood system after transplantation¹, which is a curative therapy for numerous diseases including immunodeficiencies and leukaemias². Although substantial effort has been applied to identifying HSC maintenance factors through the characterization of the in vivo bone-marrow HSC microenvironment or niche^3-5, stable ex vivo HSC expansion has previously been unattainable^6,7. Here we describe the development of a defined, albumin-free culture system that supports the long-term ex vivo expansion of functional mouse HSCs. We used a systematic optimization approach, and found that high levels of thrombopoietin synergize with low levels of stem-cell factor and fibronectin to sustain HSC self-renewal. Serum albumin has long been recognized as a major source of biological contaminants in HSC cultures⁸; we identify polyvinyl alcohol as a functionally superior replacement for serum albumin that is compatible with good manufacturing practice. These conditions afford between 236- and 899-fold expansions of functional HSCs over 1 month, although analysis of clonally derived cultures suggests that there is considerable heterogeneity in the self-renewal capacity of HSCs ex vivo. Using this system, HSC cultures that are derived from only 50 cells robustly engraft in recipient mice without the normal requirement for toxic pre-conditioning (for example, radiation), which may be relevant for HSC transplantation in humans. These findings therefore have important implications for both basic HSC research and clinical haematology.

BRCA Genes: The Role in Genome Stability, Cancer Stemness and Therapy Resistance

Carcinogenesis is a multistep process, and tumors frequently harbor multiple mutations regulating genome integrity, cell division and death. The integrity of cellular genome is closely controlled by the mechanisms of DNA damage signaling and DNA repair. The association of breast cancer susceptibility genes BRCA1 and BRCA2 with breast and ovarian cancer development was first demonstrated over 20 years ago. Since then the germline mutations within these genes were linked to genomic instability and increased risk of many other cancer types. Genomic instability is an engine of the oncogenic transformation of non-tumorigenic cells into tumor-initiating cells and further tumor evolution. In this review we discuss the biological functions of BRCA1 and BRCA2 genes and the role of BRCA mutations in tumor initiation, regulation of cancer stemness, therapy resistance and tumor progression.

Targeting the Spleen as an Alternative Site for Hematopoiesis

Bone marrow is the main site for hematopoiesis in adults. It acts as a niche for hematopoietic stem cells (HSCs) and contains non-hematopoietic cells that contribute to stem cell dormancy, quiescence, self-renewal, and differentiation. HSC also exist in resting spleen of several species, although their contribution to hematopoiesis under steady-state conditions is unknown. The spleen can however undergo extramedullary hematopoiesis (EMH) triggered by physiological stress or disease. With the loss of bone marrow niches in aging and disease, the spleen as an alternative tissue site for hematopoiesis is an important consideration for future therapy, particularly during HSC transplantation. In terms of harnessing the spleen as a site for hematopoiesis, here the remarkable regenerative capacity of the spleen is considered with a view to forming additional or ectopic spleen tissue through cell engraftment. Studies in mice indicate the potential for such grafts to support the influx of hematopoietic cells leading to the development of normal spleen architecture. An important goal will be the formation of functional ectopic spleen tissue as an aid to hematopoietic recovery following clinical treatments that impact bone marrow. For example, expansion or replacement of niches could be considered where myeloablation ahead of HSC transplantation compromises treatment outcomes.

Human adult HSCs can be discriminated from lineage-committed HPCs by the expression of endomucin

EMCN is a novel marker of human HSCs. EMCN is a more specific marker of HSCs than CD34 as it can discriminate HSCs from lineage-committed HPCs.

New insights into hematopoietic differentiation landscapes from single-cell RNA sequencing

Single-cell transcriptomics has recently emerged as a powerful tool to analyze cellular heterogeneity, discover new cell types, and infer putative differentiation routes. The technique has been rapidly embraced by the hematopoiesis research community, and like other technologies before, single-cell molecular profiling is widely expected to make important contributions to our understanding of the hematopoietic hierarchy. Much of this new interpretation relies on inference of the transcriptomic landscape as a representation of existing cellular states and associated transitions among them. Here we review how this model allows, under certain assumptions, charting of time-resolved differentiation trajectories with unparalleled resolution and how the landscape of multipotent cells may be rather devoid of discrete structures, challenging our preconceptions about stem and progenitor cell types and their organization. Finally, we highlight how promising technological advances may convert static differentiation landscapes into a dynamic cell flux model and thus provide a more holistic understanding of normal hematopoiesis and blood disorders.

Enhancing human cord blood hematopoietic stem cell engraftment by targeting nuclear hormone receptors

PURPOSE OF REVIEW

Allogeneic hematopoietic cell transplantation (HCT) is a life-saving therapy for hematological and nonhematological diseases. Cord blood is a source of transplantable hematopoietic stem cells (HSCs), but limited numbers of HSCs in single cord blood units, which may cause delayed neutrophil, platelet, and immune cell reconstitution, is a major problem for efficient transplantation. Ex-vivo expansion and enhanced homing of cord blood HSC may overcome this disadvantage and improve its long-term engraftment. Here, we discuss the role of nuclear hormone receptors signaling in human cord blood HSC engraftment.

RECENT FINDINGS

Antagonizing retinoid acid receptor (RAR) signaling promotes human HSC expansion and increases myeloid cell production. Cord blood CD34 cells expanded by SR1 promotes efficient myeloid recovery after transplantation compared with control groups, and leads to successful engraftment. Short-term treatment of glucocorticoids enhances homing and long-term engraftment of human HSCs and HPCs in NSG mice. Peroxisome proliferator-activated receptor-γ (PPARγ) antagonism expands human HSCs and HPCs by preventing differentiation and enhancing glucose metabolism. These findings demonstrate that nuclear hormone receptor signaling components might be promising targets for improving human cord blood HCT.

SUMMARY

Better understanding of molecular mechanisms underlying human HSC expansion and homing mediated by nuclear hormone receptor signaling pathways will facilitate enhanced HCT efficacy.

Iron and iron-dependent reactive oxygen species in the regulation of macrophages and fibroblasts in non-healing chronic wounds

Chronic wounds pose a stern challenge to health care systems with growing incidence especially in the aged population. In the presence of increased iron concentrations, recruitment of monocytes from the circulation and activation towards ROS and RNS releasing M1 macrophages together with the persistence of senescent fibroblasts at the wound site are significantly enhanced. This unrestrained activation of pro-inflammatory macrophages and senescent fibroblasts has increasingly been acknowledged as main driver causing non-healing wounds. In a metaphor, macrophages act like stage directors of wound healing, resident fibroblasts constitute main actors and increased iron concentrations are decisive parts of the libretto, and - if dysregulated - are responsible for the development of non-healing wounds. This review will focus on recent cellular and molecular findings from chronic venous leg ulcers and diabetic non-healing wounds both constituting the most common pathologies often resulting in limb amputations of patients. This not only causes tremendous suffering and loss of life quality, but is also associated with an increase in mortality and a major socio-economic burden. Despite recent advances, the underlying molecular mechanisms are not completely understood. Overwhelming evidence shows that reactive oxygen species and the transition metal and trace element iron at pathological concentrations are crucially involved in a complex interplay between cells of different histogenetic origin and their extracellular niche environment. This interplay depends on a variety of cellular, non-cellular biochemical and cell biological mechanisms. Here, we will highlight recent progress in the field of iron-dependent regulation of macrophages and fibroblasts and related pathologies linked to non-healing chronic wounds.

Chasing Mavericks: The quest for defining developmental waves of hematopoiesis

Hematopoiesis is the process by which mature blood and immune cells are produced from hematopoietic stem and progenitor cells (HSCs and HSPCs). The last several decades of research have shed light on the origin of HSCs, as well as the heterogeneous pools of fetal progenitors that contribute to lifelong hematopoiesis. The overarching concept that hematopoiesis occurs in dynamic, overlapping waves throughout development, with each wave contributing to both continuous and developmentally limited cell types, has been solidified over the years. However, recent advances in our ability to track the production of hematopoietic cells in vivo have challenged several long-held dogmas on the origin and persistence of distinct hematopoietic cell types. In this review, we highlight emerging concepts in hematopoietic development and identify unanswered questions.

High-fat diet disturbs lipid raft/TGF-β signaling-mediated maintenance of hematopoietic stem cells in mouse bone marrow

Despite recent in vivo data demonstrating that high-fat diet (HFD)-induced obesity leads to major perturbations in murine hematopoietic stem cells (HSC), the direct role of a HFD is not yet completely understood. Here, we investigate the direct impact of a short-term HFD on HSC and hematopoiesis in C57BL/6J mice compared with standard diet-fed mice. We detect a loss of half of the most primitive HSC in the bone marrow (BM) cells of HFD-fed mice, which exhibit lower hematopoietic reconstitution potential after transplantation. Impaired maintenance of HSC is due to reduced dormancy after HFD feeding. We discover that a HFD disrupts the TGF-β receptor within lipid rafts, associated to impaired Smad2/3-dependent TGF-β signaling, as the main molecular mechanism of action. Finally, injecting HFD-fed mice with recombinant TGF-β1 avoids the loss of HSC and alteration of the BM’s ability to recover, underscoring the fact that a HFD affects TGF-β signaling on HSC.

High fat diets (HFD) are thought to perturb murine hematopoiesis as a result of obesity. Here the authors find that short-term HFD reduces hematopoietic stem cells (HSC), disrupts lipid rafts and TGF-β1 signalling. Injecting HFD-fed mice with recombinant TGF-β1 can rescue HSC loss.

The Impact of the Cellular Origin in Acute Myeloid Leukemia: Learning From Mouse Models

Acute myeloid leukemia (AML) is a genetically heterogeneous disease driven by a limited number of cooperating mutations. There is a long-standing debate as to whether AML driver mutations occur in hematopoietic stem or in more committed progenitor cells. Here, we review how different mouse models, despite their inherent limitations, have functionally demonstrated that cellular origin plays a critical role in the biology of the disease, influencing clinical outcome. AML driven by potent oncogenes such as mixed lineage leukemia fusions often seem to emerge from committed myeloid progenitors whereas AML without any major cytogenetic abnormalities seem to develop from a combination of preleukemic initiating events arising in the hematopoietic stem cell pool. More refined mouse models may serve as experimental platforms to identify and validate novel targeted therapeutic strategies.

Machine-Learning Approach for Modeling Myelosuppression Attributed to Nimustine Hydrochloride

PURPOSE

A major adverse effect arising from nimustine hydrochloride (ACNU) therapy for brain tumors is myelosuppression. Because its timing and severity vary among individual patients, the ACNU dose level has been adjusted in an empiric manner at individual medical facilities. To our knowledge, ours is the first study to develop a machine-learning approach to estimate myelosuppression through analysis of patient factors before treatment and attempts to clarify the relationship between myelosuppression and hematopoietic stem cells from daily clinical data. Adverse effect prediction will allow ACNU dose adjustment for patients predicted to have decreases in blood cell counts and will enable focused follow-up of patients undergoing chemoradiotherapy.

PATIENTS AND METHODS

Patients were newly pathologically diagnosed with WHO grade 2 or 3 tumors and were treated with ACNU-based chemoradiotherapy. For detailed analysis of the timing and intensity of adverse effects in patients, we developed a data-weighted support vector machine (SVM) based on adverse event criteria (nadir-weighted SVM [NwSVM]). To evaluate the estimation accuracy of blood cell count dynamics, the determination coefficient ( r²) between real and estimated data was calculated by three regression methods: polynomial, SVM, and NwSVM.

RESULTS

Only the NwSVM-based regression enabled estimation of the dynamics of all blood cell types with high accuracy (mean r² = 0.81). The mean timing of nadir arrival estimated using this regression was 35 days for platelets, 41 days for RBCs, 52 days for lymphocytes, 57 days for WBCs, and 62 days for neutrophils.

CONCLUSION

The NwSVM can be used to predict myelosuppression and clearly depicts nadir timing differences between platelets and other blood cells.

Assessment of Young and Aged Hematopoietic Stem Cell Activity by Competitive Serial Transplantation Assays

Healthy hematopoietic stem cells (HSCs) are capable to self-renew and reconstitute the complete hematopoietic system. Upon aging, there is an increased incidence of blood-related diseases. Age-related phenotypes have been widely studied by bone marrow transplantation experiments, where reconstitution of the transplanted cells is a direct measure of HSC activity. In this protocol we describe a competitive bone marrow transplantation assay to functionally test young and old HSCs.

A Unique Nonsaccharide Mimetic of Heparin Hexasaccharide Inhibits Colon Cancer Stem Cells via p38 MAP Kinase Activation

Targeting of cancer stem cells (CSC) is expected to be a paradigm-shifting approach for the treatment of cancers. Cell surface proteoglycans bearing sulfated glycosaminoglycan (GAG) chains are known to play a critical role in the regulation of stem cell fate. Here, we show for the first time that G2.2, a sulfated nonsaccharide GAG mimetic (NSGM) of heparin hexasaccharide, selectively inhibits colonic CSCs in vivo G2.2-reduced CSCs (CD133+/CXCR4+, Dual hi) induced HT-29 and HCT 116 colon xenografts' growth in a dose-dependent fashion. G2.2 also significantly delayed the growth of colon xenograft further enriched in CSCs following oxaliplatin and 5-fluorouracil treatment compared with vehicle-treated xenograft controls. In fact, G2.2 robustly inhibited CSCs' abundance (measured by levels of CSC markers, e.g., CD133, DCMLK1, LGR5, and LRIG1) and self-renewal (quaternary spheroids) in colon cancer xenografts. Intriguingly, G2.2 selectively induced apoptosis in the Dual hi CSCs in vivo eluding to its CSC targeting effects. More importantly, G2.2 displayed none to minimal toxicity as observed through morphologic and biochemical studies of vital organ functions, blood coagulation profile, and ex vivo analyses of normal intestinal (and bone marrow) progenitor cell growth. Through extensive in vitro, in vivo, and ex vivo mechanistic studies, we showed that G2.2's inhibition of CSC self-renewal was mediated through activation of p38α, uncovering important signaling that can be targeted to deplete CSCs selectively while minimizing host toxicity. Hence, G2.2 represents a first-in-class (NSGM) anticancer agent to reduce colorectal CSCs.

Colony Formation: An Assay of Hematopoietic Progenitor Cells

The colony-forming cell (CFC) assay is used to study the proliferation and differentiation pattern of each input hematopoietic progenitors by their ability to form colonies in a semisolid medium. The resulting colonies are consisting of more differentiated cells, and the number and the morphology of the colonies provide preliminary information about the ability of progenitors to differentiate and proliferate. To allow colonies to grow to a size which facilitates accurate counting and identification, about 14 days of culture is sufficient. In certain situations also shorter periods may be used.

Immune and metabolic shifts during neonatal development reprogram liver identity and function

BACKGROUND & AIMS

The liver is the main hematopoietic site in embryos, becoming a crucial organ in both immunity and metabolism in adults. However, how the liver adapts both the immune system and enzymatic profile to challenges in the postnatal period remains elusive. We aimed to identify the mechanisms underlying this adaptation.

METHODS

We analyzed liver samples from mice on day 0 after birth until adulthood. Human biopsies from newborns and adults were also examined. Liver immune cells were phenotyped using mass cytometry (CyTOF) and expression of several genes belonging to immune and metabolic pathways were measured. Mortality rate, bacteremia and hepatic bacterial retention after E. coli challenge were analyzed using intravital and in vitro approaches. In a set of experiments, mice were prematurely weaned and the impact on gene expression of metabolic pathways was evaluated.

RESULTS

Human and mouse newborns have a sharply different hepatic cellular composition and arrangement compared to adults. We also found that myeloid cells and immature B cells primarily compose the neonatal hepatic immune system. Although neonatal mice were more susceptible to infections, a rapid evolution to an efficient immune response was observed. Concomitantly, newborns displayed a reduction of several macronutrient metabolic functions and the normal expression level of enzymes belonging to lipid and carbohydrate metabolism was reached around the weaning period. Interestingly, early weaning profoundly disturbed the expression of several hepatic metabolic pathways, providing novel insights into how dietary schemes affect the metabolic maturation of the liver.

CONCLUSION

In newborns, the immune and metabolic profiles of the liver are dramatically different to those of the adult liver, which can be explained by the differences in the liver cell repertoire and phenotype. Also, dietary and antigen cues may be crucial to guide liver development during the postnatal phase.

LAY SUMMARY

Newborns face major challenges in the extra-uterine life. In fact, organs need to modify their cellular composition and gene expression profile in order to adapt to changes in both microbiota and diet throughout life. The liver is interposed between the gastrointestinal system and the systemic circulation, being the destination of all macronutrients and microbial products from the gut. Therefore, it is expected that delicately balanced mechanisms govern the transformation of a neonatal liver to a key organ in adults.

Heterogeneity in myeloproliferative neoplasms: Causes and consequences

Myeloproliferative neoplasms (MPNs) are haematopoietic stem cell-derived clonal disorders characterised by proliferation of some or all myeloid lineages, depending on the subtype. MPNs are classically categorized into three disease subgroups; essential thrombocythaemia (ET), polycythaemia vera (PV) and primary myelofibrosis (PMF). The majority (>85%) of patients carry a disease-initiating or driver mutation, the most prevalent occurring in the janus kinase 2 gene (JAK2 V617F), followed by calreticulin (CALR) and myeloproliferative leukaemia virus (MPL) genes. Although these diseases are characterised by shared clinical, pathological and molecular features, one of the most challenging aspects of these disorders is the diverse clinical features which occur in each disease type, with marked variability in risks of disease complications and progression to leukaemia. A remarkable aspect of MPN biology is that the JAK2 V617F mutation, often occurring in the absence of additional mutations, generates a spectrum of phenotypes from asymptomatic ET through to aggressive MF, associated with a poor outcome. The mechanisms promoting MPN heterogeneity remain incompletely understood, but contributing factors are broad and include patient characteristics (gender, age, comorbidities and environmental exposures), additional somatic mutations, target disease-initiating cell, bone marrow microenvironment and germline genetic associations. In this review, we will address these in detail and discuss their role in heterogeneity of MPN disease phenotypes. Tailoring patient management according to the multiple different factors that influence disease phenotype may prove to be the most effective approach to modify the natural history of the disease and ultimately improve outcomes for patients.

Adipose Stem Cell Translational Applications: From Bench-to-Bedside

During the last five years, there has been a significantly increasing interest in adult adipose stem cells (ASCs) as a suitable tool for translational medicine applications. The abundant and renewable source of ASCs and the relatively simple procedure for cell isolation are only some of the reasons for this success. Here, we document the advances in the biology and in the innovative biotechnological applications of ASCs. We discuss how the multipotential property boosts ASCs toward mesenchymal and non-mesenchymal differentiation cell lineages and how their character is maintained even if they are combined with gene delivery systems and/or biomaterials, both in vitro and in vivo.

The in vitro growth of a cord blood-derived cell population enriched for CD34+ cells is influenced by its cell cycle status and treatment with hydroxyurea

OBJECTIVE

Cell cycle plays a fundamental role in the physiology of hematopoietic stem and progenitor cells. In the present study we used a negative selection system to obtain an immature cell population-enriched for cord blood-derived CD34⁺ cells-and we determined its proliferation, expansion and differentiation patterns as a function of the cell cycle status. The effects of hydroxyurea (HU) were also assessed.

RESULTS

As compared with cells in synthesis (S)/Gap2 (G2)/mitosis (M), cells in quiescent state (G0)/Gap1 (G1) showed a higher proliferation potential in vitro. At culture onset, G0, G1 and S/G2/M cells corresponded with 63%, 33% and 4%, respectively. Treatment with HU before culture resulted in an increase in the proportion of cells in G1 with a concomitant decrease in S/G2/M cells, without affecting the proportion of cells in G0. After 3 days of culture in the presence of recombinant cytokines, the vast majority of the cells (90%) were in G1, and by day 8, G0, G1 and S/G2/M cells corresponded with 18%, 67% and 15%, respectively. HU also induced an increase in colony-forming cell (CFC) frequency, in the proliferation and expansion capacities of cultured cells under myeloid conditions, and favored the development of the erythroid lineage.

CONCLUSION

Our results show that the in vitro proliferation, expansion and differentiation potentials of immature hematopoietic cells are determined, at least in part, by their cell cycle status and that the cell cycle modifier HU significantly influences the growth of human hematopoietic cells. These results are of potential relevance for the development of ex vivo expansion protocols.

Kinetics of adult hematopoietic stem cell differentiation in vivo

The process whereby hematopoietic stem cells (HSCs) generate different blood cell types in the steady-state is poorly understood. Upadhaya et al. used inducible lineage tracing to characterize the earliest steps of adult HSC differentiation in vivo.

Adult hematopoiesis has been studied in terms of progenitor differentiation potentials, whereas its kinetics in vivo is poorly understood. We combined inducible lineage tracing of endogenous adult hematopoietic stem cells (HSCs) with flow cytometry and single-cell RNA sequencing to characterize early steps of hematopoietic differentiation in the steady-state. Labeled cells, comprising primarily long-term HSCs and some short-term HSCs, produced megakaryocytic lineage progeny within 1 wk in a process that required only two to three cell divisions. Erythroid and myeloid progeny emerged simultaneously by 2 wk and included a progenitor population with expression features of both lineages. Myeloid progenitors at this stage showed diversification into granulocytic, monocytic, and dendritic cell types, and rare intermediate cell states could be detected. In contrast, lymphoid differentiation was virtually absent within the first 3 wk of tracing. These results show that continuous differentiation of HSCs rapidly produces major hematopoietic lineages and cell types and reveal fundamental kinetic differences between megakaryocytic, erythroid, myeloid, and lymphoid differentiation.

Understanding Hematological Toxicities Using Mathematical Modeling

Balancing antitumor efficacy with toxicity is a significant challenge, and drug-induced myelosuppression is a common dose-limiting toxicity of cancer treatments. Mathematical modeling has proven to be a powerful ally in this field, scaling results from animal models to humans, and designing optimized treatment regimens. Here we outline existing mathematical approaches for studying bone marrow toxicity, identify gaps in current understanding, and make future recommendations to advance this vital field of safety research further.

Infection perturbs Bach2- and Bach1-dependent erythroid lineage 'choice' to cause anemia

Elucidation of how the differentiation of hematopoietic stem and progenitor cells (HSPCs) is reconfigured in response to the environment is critical for understanding the biology and disorder of hematopoiesis. Here we found that the transcription factors (TFs) Bach2 and Bach1 promoted erythropoiesis by regulating heme metabolism in committed erythroid cells to sustain erythroblast maturation and by reinforcing erythroid commitment at the erythro-myeloid bifurcation step. Bach TFs repressed expression of the gene encoding the transcription factor C/EBPβ, as well as that of its target genes encoding molecules important for myelopoiesis and inflammation; they achieved the latter by binding to their regulatory regions also bound by C/EBPβ. Lipopolysaccharide diminished the expression of Bach TFs in progenitor cells and promoted myeloid differentiation. Overexpression of Bach2 in HSPCs promoted erythroid development and inhibited myelopoiesis. Knockdown of BACH1 or BACH2 in human CD34⁺ HSPCs impaired erythroid differentiation in vitro. Thus, Bach TFs accelerate erythroid commitment by suppressing the myeloid program at steady state. Anemia of inflammation and myelodysplastic syndrome might involve reduced activity of Bach TFs.

Targeting the Extracellular Signal-Regulated Kinase 5 Pathway to Suppress Human Chronic Myeloid Leukemia Stem Cells

Tyrosine kinase inhibitors (TKi) are effective against chronic myeloid leukemia (CML), but their inefficacy on leukemia stem cells (LSCs) may lead to relapse. To identify new druggable targets alternative to BCR/ABL, we investigated the role of the MEK5/ERK5 pathway in LSC maintenance in low oxygen, a feature of bone marrow stem cell niches. We found that MEK5/ERK5 pathway inhibition reduced the growth of CML patient-derived cells and cell lines in vitro and the number of leukemic cells in vivo. Treatment in vitro of primary CML cells with MEK5/ERK5 inhibitors, but not TKi, strikingly reduced culture repopulation ability (CRA), serial colony formation ability, long-term culture-initiating cells (LTC-ICs), and CD26-expressing cells. Importantly, MEK5/ERK5 inhibition was effective on CML cells regardless of the presence or absence of imatinib, and did not reduce CRA or LTC-ICs of normal CD34+ cells. Thus, targeting MEK/ERK5 may represent an innovative therapeutic approach to suppress CML progenitor/stem cells.

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ERK5 is constitutively active in chronic myeloid leukemia (CML) cells

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ERK5 pathway inhibition reduces the growth of CML cells in vitro and in vivo

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ERK5 pathway inhibition strikingly reduces CML progenitor/stem cell maintenance

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The combination of ERK5i with imatinib reduces the expression of stem cell proteins

Hematopoietic stem cell and mesenchymal stem cell population size in bone marrow samples depends on patient's age and harvesting technique

Mesenchymal stem cells (MSCs) are heterogeneous population of cells with great potential for regenerative medicine. MSCs are relatively easy to expand in a cell culture, however determination of their concentration in harvested tissue is more complex and is not implemented as routine procedure. To identify MSCs collected from bone marrow we have used two combinations of cell markers (CD45^-/CD73⁺/CD90⁺/CD105⁺ and CD45^-/CD271⁺) and fibroblast colony-forming unit (CFU-F) assay. Further, in donors of various ages, mesenchymal stem cell concentration was compared with the result of CFU-F assay and with hematopoietic stem cell concentration, determined by a standardized flow cytometric assay. A positive correlation of MSC populations to the CFU-F numbers is observed, the population of the CD45^-/CD271⁺ cells correlates better with CFU-F numbers than the population of the CD45^-/CD73⁺/CD90⁺/CD105⁺ cells. The relationship between the hematopoietic CD45^dim/CD34⁺ cell concentration and mesenchymal CFU-Fs or CD45^-/CD271⁺ cells shows a positive linear regression. An age-related quantitative reduction of hematopoietic CD45^dim/CD34⁺, mesenchymal CD45^-/CD73⁺/CD90⁺/CD105⁺ and CD45^-/CD271⁺ stem cells, and CFU-F numbers were noted. Additionally, statistically significant higher CFU-F numbers were observed when bone marrow samples were harvested from three different sites from the anterior iliac crest instead of harvesting the same sample amount only from one site.

Native-mimicking in vitro microenvironment: an elusive and seductive future for tumor modeling and tissue engineering

Human connective tissues are complex physiological microenvironments favorable for optimal survival, function, growth, proliferation, differentiation, migration, and death of tissue cells. Mimicking native tissue microenvironment using various three-dimensional (3D) tissue culture systems in vitro has been explored for decades, with great advances being achieved recently at material, design and application levels. These achievements are based on improved understandings about the functionalities of various tissue cells, the biocompatibility and biodegradability of scaffolding materials, the biologically functional factors within native tissues, and the pathophysiological conditions of native tissue microenvironments. Here we discuss these continuously evolving physical aspects of tissue microenvironment important for human disease modeling, with a focus on tumors, as well as for tissue repair and regeneration. The combined information about human tissue spaces reflects the necessities of considerations when configuring spatial microenvironments in vitro with native fidelity to culture cells and regenerate tissues that are beyond the formats of 2D and 3D cultures. It is important to associate tissue-specific cells with specific tissues and microenvironments therein for a better understanding of human biology and disease conditions and for the development of novel approaches to treat human diseases.

Single-cell approaches identify the molecular network driving malignant hematopoietic stem cell self-renewal

Recent advances in single-cell technologies have permitted the investigation of heterogeneous cell populations at previously unattainable resolution. Here we apply such approaches to resolve the molecular mechanisms driving disease in mouse hematopoietic stem cells (HSCs), using JAK2V617F mutant myeloproliferative neoplasms (MPNs) as a model. Single-cell gene expression and functional assays identified a subset of JAK2V617F mutant HSCs that display defective self-renewal. This defect is rescued at the single HSC level by crossing JAK2V617F mice with mice lacking TET2, the most commonly comutated gene in patients with MPN. Single-cell gene expression profiling of JAK2V617F-mutant HSCs revealed a loss of specific regulator genes, some of which were restored to normal levels in single TET2/JAK2 mutant HSCs. Of these, Bmi1 and, to a lesser extent, Pbx1 and Meis1 overexpression in JAK2-mutant HSCs could drive a disease phenotype and retain durable stem cell self-renewal in functional assays. Together, these single-cell approaches refine the molecules involved in clonal expansion of MPNs and have broad implications for deconstructing the molecular network of normal and malignant stem cells.

Turning Stem Cells Bad: Generation of Clinically Relevant Models of Human Acute Myeloid Leukemia through Gene Delivery- or Genome Editing-Based Approaches

Acute myeloid leukemia (AML), the most common acute leukemia in the adult, is believed to arise as a consequence of multiple molecular events that confer on primitive hematopoietic progenitors unlimited self-renewal potential and cause defective differentiation. A number of genetic aberrations, among which a variety of gene fusions, have been implicated in the development of a transformed phenotype through the generation of dysfunctional molecules that disrupt key regulatory mechanisms controlling survival, proliferation, and differentiation in normal stem and progenitor cells. Such genetic aberrations can be recreated experimentally to a large extent, to render normal hematopoietic stem cells "bad", analogous to the leukemic stem cells. Here, we wish to provide a brief outline of the complementary experimental approaches, largely based on gene delivery and more recently on gene editing, employed over the last two decades to gain insights into the molecular mechanisms underlying AML development and progression and on the prospects that their applications offer for the discovery and validation of innovative therapies.

Nur77 variants solely comprising the amino-terminal domain activate hypoxia-inducible factor-1α and affect bone marrow homeostasis in mice and humans

Gene targeting via homologous recombination can occasionally result in incomplete disruption of the targeted gene. Here, we show that a widely used Nur77-deficient transgenic mouse model expresses a truncated protein encoding for part of the N-terminal domain of nuclear receptor Nur77. This truncated Nur77 protein is absent in a newly developed Nur77-deficient mouse strain generated using Cre-Lox recombination. Comparison of these two mouse strains using immunohistochemistry, flow cytometry, and colony-forming assays shows that homologous recombination-derived Nur77-deficient mice, but not WT or Cre-Lox-derived Nur77-deficient mice, suffer from liver immune cell infiltrates, loss of splenic architecture, and increased numbers of bone marrow hematopoietic stem cells and splenic colony-forming cells with age. Mechanistically, we demonstrate that the truncated Nur77 N-terminal domain protein maintains the stability and activity of hypoxia-inducible factor (HIF)-1, a transcription factor known to regulate bone marrow homeostasis. Additionally, a previously discovered, but uncharacterized, human Nur77 transcript variant that encodes solely for its N-terminal domain, designated TR3β, can also stabilize and activate HIF-1α. Meta-analysis of publicly available microarray data sets shows that TR3β is highly expressed in human bone marrow cells and acute myeloid leukemia samples. In conclusion, our study provides evidence that a transgenic mouse model commonly used to study the biological function of Nur77 has several major drawbacks, while simultaneously identifying the importance of nongenomic Nur77 activity in the regulation of bone marrow homeostasis.

Understanding human fetal pancreas development using subpopulation sorting, RNA sequencing and single-cell profiling

To decipher the populations of cells present in the human fetal pancreas and their lineage relationships, we developed strategies to isolate pancreatic progenitors, endocrine progenitors and endocrine cells. Transcriptome analysis of the individual populations revealed a large degree of conservation among vertebrates in the drivers of gene expression changes that occur at different steps of differentiation, although notably, sometimes, different members of the same gene family are expressed. The transcriptome analysis establishes a resource to identify novel genes and pathways involved in human pancreas development. Single-cell profiling further captured intermediate stages of differentiation and enabled us to decipher the sequence of transcriptional events occurring during human endocrine differentiation. Furthermore, we evaluate how well individual pancreatic cells derived in vitro from human pluripotent stem cells mirror the natural process occurring in human fetuses. This comparison uncovers a few differences at the progenitor steps, a convergence at the steps of endocrine induction, and the current inability to fully resolve endocrine cell subtypes in vitro.

High-Resolution Single-Cell DNA Methylation Measurements Reveal Epigenetically Distinct Hematopoietic Stem Cell Subpopulations

Increasing evidence of functional and transcriptional heterogeneity in phenotypically similar cells examined individually has prompted interest in obtaining parallel methylome data. We describe the development and application of such a protocol to index-sorted murine and human hematopoietic cells that are highly enriched in their content of functionally defined stem cells. Utilizing an optimized single-cell bisulfite sequencing protocol, we obtained quantitative DNA methylation measurements of up to 5.7 million CpGs in single hematopoietic cells. In parallel, we developed an analytical strategy (PDclust) to define single-cell DNA methylation states through pairwise comparisons of single-CpG methylation measurements. PDclust revealed that a single-cell epigenetic state can be described by a small (<1%) stochastically sampled fraction of CpGs and that these states are reflective of cell identity and state. Using relationships revealed by PDclust, we derive near complete methylomes for epigenetically distinct subpopulations of hematopoietic cells enriched for functional stem cell content.

A metabolic interplay coordinated by HLX regulates myeloid differentiation and AML through partly overlapping pathways

The H2.0-like homeobox transcription factor (HLX) regulates hematopoietic differentiation and is overexpressed in Acute Myeloid Leukemia (AML), but the mechanisms underlying these functions remain unclear. We demonstrate here that HLX overexpression leads to a myeloid differentiation block both in zebrafish and human hematopoietic stem and progenitor cells (HSPCs). We show that HLX overexpression leads to downregulation of genes encoding electron transport chain (ETC) components and upregulation of PPARδ gene expression in zebrafish and human HSPCs. HLX overexpression also results in AMPK activation. Pharmacological modulation of PPARδ signaling relieves the HLX-induced myeloid differentiation block and rescues HSPC loss upon HLX knockdown but it has no effect on AML cell lines. In contrast, AMPK inhibition results in reduced viability of AML cell lines, but minimally affects myeloid progenitors. This newly described role of HLX in regulating the metabolic state of hematopoietic cells may have important therapeutic implications.

HLX transcription factor regulates haematopoietic stem and progenitor cell (HSPC) differentiation and is overexpressed in acute myeloid leukemia. Here the authors show that HLX overexpression leads to myeloid differentiation block in zebrafish and human HSPCs by direct regulation of metabolic pathways.

Tracing human stem cell lineage during development using DNA methylation

Stem cell maturation is a fundamental, yet poorly understood aspect of human development. We devised a DNA methylation signature deeply reminiscent of embryonic stem cells (a fetal cell origin signature, FCO) to interrogate the evolving character of multiple human tissues. The cell fraction displaying this FCO signature was highly dependent upon developmental stage (fetal versus adult), and in leukocytes, it described a dynamic transition during the first 5 yr of life. Significant individual variation in the FCO signature of leukocytes was evident at birth, in childhood, and throughout adult life. The genes characterizing the signature included transcription factors and proteins intimately involved in embryonic development. We defined and applied a DNA methylation signature common among human fetal hematopoietic progenitor cells and have shown that this signature traces the lineage of cells and informs the study of stem cell heterogeneity in humans under homeostatic conditions.

Clonal Hematopoiesis in Aging

Purpose of Review

Clonal hematopoiesis of indeterminate potential (CHIP) is a common, age-associated condition characterized by the acquisition of somatic mutations. This concise review explores our current understanding of the mechanisms that influence the development of clonality with aging and its potential malignant and non-malignant clinical implications.

Recent Findings

Aging of the hematopoietic system results in phenotypic changes that favor clonal dominance. Cell-extrinsic factors provide additional selective pressures that further shape clonal architecture. Even so, small clones with candidate driver mutations appear to be ubiquitous with age and largely benign in the absence of strong selective pressures. Benign clonal expansion may compensate for the loss of regenerative HSC capacity as we age.

Summary

CHIP is a marker of aging that reflects the biologic interplay between HSC aging and cell-extrinsic factors. The clinical significance of CHIP is highly variable and dependent on clinical context. Distinguishing the causal relationships and confounding factors that regulate clonal behavior will be essential to define the mechanistic role of CHIP in aging and potentially mitigate its clinical consequences.

The hematopoietic stem cell diet

Hematopoietic stem cells (HSCs) are responsible for sustaining life-long blood formation or hematopoiesis and are also used clinically in a form of bone marrow transplantation, a curative cellular therapy for a range of hematological diseases. HSCs are maintained throughout adult life by a complex biological niche or microenvironment, which is thought to be composed of a range of cellular, molecular, and metabolic components. The metabolic components of the HSC niche have become of increasing interest over the past few years. It is now well-recognized that metabolic activity is intimately linked to HSC function, and dysregulation of these metabolic pathways result in hematological pathologies such as leukemia. Here, we review the recent progress in this field including our current understanding of the "dietary" requirements of HSCs and how nutrition influences HSC activity. These recent findings have suggested promising new metabolic approaches to improve clinical HSC transplantation and leukemia therapies.

Extracellular Vesicles: A New Prospective in Crosstalk between Microenvironment and Stem Cells in Hematological Malignancies

The bone marrow (BM) microenvironment in hematological malignancies (HMs) comprises heterogeneous populations of neoplastic and nonneoplastic cells. Cancer stem cells (CSCs), neoplastic cells, hematopoietic stem cells (HSCs), and mesenchymal stromal/stem cells (MSCs) are all components of this microenvironment. CSCs are the HM initiators and are associated with neoplastic growth and drug resistance, while HSCs are able to reconstitute the entire hematopoietic system; finally, MSCs actively support hematopoiesis. In some HMs, CSCs and neoplastic cells compromise the normal development of HSCs and perturb BM-MSCs. In response, "reprogrammed" MSCs generate a favorable environment to support neoplastic cells. Extracellular vesicles (EVs) are an important cell-to-cell communication type in physiological and pathological conditions. In particular, in HMs, EV secretion participates to unidirectional and bidirectional interactions between neoplastic cells and BM cells. The transfer of EV molecular cargo triggers different responses in target cells; in particular, malignant EVs modify the BM environment in favor of neoplastic cells at the expense of normal HSCs, by interfering with antineoplastic immunity and participating in resistance to treatment. Here, we review the role of EVs in BM cell communication in physiological conditions and in HMs, focusing on the effects of BM niche EVs on HSCs and MSCs.

Single-cell analysis identifies a CD33+ subset of human cord blood cells with high regenerative potential

Elucidation of the identity and diversity of mechanisms that sustain long-term human blood cell production remains an important challenge. Previous studies indicate that, in adult mice, this property is vested in cells identified uniquely by their ability to clonally regenerate detectable, albeit highly variable levels and types, of mature blood cells in serially transplanted recipients. From a multi-parameter analysis of the molecular features of very primitive human cord blood cells that display long-term cell outputs in vitro and in immunodeficient mice, we identified a prospectively separable CD33⁺CD34⁺CD38^-CD45RA^-CD90⁺CD49f⁺ phenotype with serially transplantable, but diverse, cell output profiles. Single-cell measurements of the mitogenic response, and the transcriptional, DNA methylation and 40-protein content of this and closely related phenotypes revealed subtle but consistent differences both within and between each subset. These results suggest that multiple regulatory mechanisms combine to maintain different cell output activities of human blood cell precursors with high regenerative potential.

Factors Influencing the Umbilical Cord Blood Stem Cell Industry: An Evolving Treatment Landscape

Hematopoietic stem cell transplantation (HSCT) is common practice today for life threatening malignant and non-malignant diseases of the blood and immune systems. Umbilical cord blood (UCB) is rich in hematopoietic stem cells (HSCs) and is an attractive alternative to harvesting HSCs from bone marrow or when mobilized into peripheral blood. One of the most appealing attributes of UCB is that it can be banked for future use and hence provides an off-the-shelf solution for patients in urgent need of a transplantation. This has led to the establishment of publicly funded and private UCB banks, as seen by the rapid growth of the UCB industry in the early part of this century. However, from about 2010, the release of UCB units for treatment purposes plateaued and started to decrease year-on-year from 2013 to 2016. Our interest has been to investigate the factors contributing to these changes. Key drivers influencing the UCB industry include the emergence of haploidentical HSCT and the increasing use of UCB units for regenerative medicine purposes. Further influencing this dynamic is the high cost associated with UCB transplantation, the economic impact of sustaining public bank operations and an active private UCB banking sector. We foresee that these factors will continue in a tug-of-war fashion to shape and finally determine the fate of the UCB industry. Stem Cells Translational Medicine 2018 Stem Cells Translational Medicine 2018;7:643-650.

Hematopoietic stem cells can differentiate into restricted myeloid progenitors before cell division in mice

Hematopoietic stem cells (HSCs) continuously replenish all blood cell types through a series of differentiation steps and repeated cell divisions that involve the generation of lineage-committed progenitors. However, whether cell division in HSCs precedes differentiation is unclear. To this end, we used an HSC cell-tracing approach and Ki67^RFP knock-in mice, in a non-conditioned transplantation model, to assess divisional history, cell cycle progression, and differentiation of adult HSCs. Our results reveal that HSCs are able to differentiate into restricted progenitors, especially common myeloid, megakaryocyte-erythroid and pre-megakaryocyte progenitors, without undergoing cell division and even before entering the S phase of the cell cycle. Additionally, the phenotype of the undivided but differentiated progenitors correlated with the expression of lineage-specific genes and loss of multipotency. Thus HSC fate decisions can be uncoupled from physical cell division. These results facilitate a better understanding of the mechanisms that control fate decisions in hematopoietic cells.

Dependence of hematopoietic stem cell (HSC) fate on the phase of the cell cycle has not been demonstrated in vivo. Here, the authors find that HSCs can differentiate into a downstream progenitor without physical division, even before progressing into the S phase of the cell cycle.

Isthmin 1 (ism1) is required for normal hematopoiesis in developing zebrafish

Hematopoiesis is an essential and highly regulated biological process that begins with hematopoietic stem cells (HSCs). In healthy organisms, HSCs are responsible for generating a multitude of mature blood cells every day, yet the molecular pathways that instruct HSCs to self-renew and differentiate into post-mitotic blood cells are not fully known. To understand these molecular pathways, we investigated novel genes expressed in hematopoietic-supportive cell lines from the zebrafish (Danio rerio), a model system increasingly utilized to uncover molecular pathways important in the development of other vertebrate species. We performed RNA sequencing of the transcriptome of three stromal cell lines derived from different stages of embryonic and adult zebrafish and identified hundreds of highly expressed transcripts. For our studies, we focused on isthmin 1 (ism1) due to its shared synteny with its human gene ortholog and because it is a secreted protein. To characterize ism1, we performed loss-of-function experiments to identify if mature blood cell production was disrupted. Myeloid and erythroid lineages were visualized and scored with transgenic zebrafish expressing lineage-specific markers. ism1 knockdown led to reduced numbers of neutrophils, macrophages, and erythrocytes. Analysis of clonal methylcellulose assays from ism1 morphants also showed a reduction in total hematopoietic stem and progenitor cells (HSPCs). Overall, we demonstrate that ism1 is required for normal generation of HSPCs and their downstream progeny during zebrafish hematopoiesis. Further investigation into ism1 and its importance in hematopoiesis may elucidate evolutionarily conserved processes in blood formation that can be further investigated for potential clinical utility.

The versatile nature of miR-9/9* in human cancer

miR-9 and miR-9* (miR-9/9*) were first shown to be expressed in the nervous system and to function as versatile regulators of neurogenesis. The variable expression levels of miR-9/9* in human cancer prompted researchers to investigate whether these small RNAs may also have an important role in the deregulation of physiological and biochemical networks in human disease. In this review, we present a comprehensive overview of the involvement of miR-9/9* in various human malignancies focusing on their opposing roles in supporting or suppressing tumor development and metastasis. Importantly, it is shown that the capacity of miR-9/9* to impact tumor formation is independent from their influence on the metastatic potential of tumor cells. Moreover, data suggest that miR-9/9* may increase malignancy of one cancer cell population at the expense of another. The functional versatility of miR-9/9* emphasizes the complexity of studying miRNA function and the importance to perform functional studies of both miRNA strands in a relevant cellular context. The possible application of miR-9/9* as targets for miRNA-based therapies is discussed, emphasizing the need to obtain a better understanding of the functional properties of these miRNAs and to develop safe delivery methods to target specific cell populations.

Systems for localized release to mimic paracrine cell communication in vitro

Paracrine cell communication plays a pivotal role for signal exchange between proximal cells in vivo. However, this localized, gradient type release of mediators at very low concentrations (pg/ml), relevant during physiological and pathological processes, is rarely reflected within in vitro approaches. This review gives an overview on state-of-the-art approaches, which transfer the paracrine cell-to-cell communication into in vitro cell culture model setups. The traditional methods like trans-well assays and more advanced microfluidic approaches are included. The review focusses on systems for localized release, mostly based on microparticles, which tightly mimic the paracrine interaction between single cells in 3D microenvironments. Approaches based on single microparticles, with the main focus on affinity-controlled storage and release of cytokines, are reviewed and their importance for understanding paracrine communication is highlighted. Various methods to study the cytokine release and their advantages and disadvantages are discussed. Basic principles of the release characteristics, like diffusion mechanisms, are quantitatively described, including the formation of resulting gradients around the local sources. In vitro cell experiments using such localized microparticle release systems in approaches to increase understanding of stem cell behavior within their niches and regulation of wound healing are highlighted as examples of successful localized release systems for mimicking paracrine cell communication.

Of Cytometry, Stem Cells and Fountain of Youth

Outlined are advances of cytometry applications to identify and sort stem cells, of laser scanning cytometry and ImageStream imaging instrumentation to further analyze morphometry of these cells, and of mass cytometry to classify a multitude of cellular markers in large cell populations. Reviewed are different types of stem cells, including potential candidates for cancer stem cells, with respect to their "stemness", and other characteristics. Appraised is further progress in identification and isolation of the "very small embryonic-like stem cells" (VSELs) and their autogenous transplantation for tissue repair and geroprotection. Also assessed is a function of hyaluronic acid, the major stem cells niche component, as a guardian and controller of stem cells. Briefly appraised are recent advances and challenges in the application of stem cells in regenerative medicine and oncology and their future role in different disciplines of medicine, including geriatrics.

MicroRNA Microarray Profiling during Megakaryocyte Differentiation of Cord Blood CD133+ Hematopoietic Stem Cells

Objective

In order to clarify the role of microRNAs (miRNA) in megakaryocyte differentiation, we ran a microRNA microarray experiment to measure the expression level of 961 human miRNA in megakaryocytes differentiated from human umbilical cord blood CD133+ cells.

Materials and Methods

In this experimental study, human CD133+ hematopoietic stem cells were collected from three human umbilical cord blood (UCB) samples, and then differentiated to the megakaryocytic lineage and characterized by flow cytometry, CFU-assay and ploidy analysis. Subsequently, microarray analysis was undertaken followed by quantitative polymerase chain reaction (qPCR) to validate differentially expressed miRNA identified in the microarray analysis.

Results

A total of 10 and 14 miRNAs were upregulated (e.g. miR-1246 and miR-148-a) and down-regulated (e.g. miR- 551b and miR-10a) respectively during megakaryocyte differentiation, all of which were confirmed by qPCR. Analysis of targets of these miRNA showed that the majority of targets are transcription factors involved in megakaryopoiesis.

Conclusion

We conclude that miRNA play an important role in megakaryocyte differentiation and may be used as targets to change the rate of differentiation and further our understanding of the biology of megakaryocyte commitment.

The BMP pathway: A unique tool to decode the origin and progression of leukemia

The microenvironment (niche) governs the fate of stem cells (SCs) by balancing self-renewal and differentiation. Increasing evidence indicates that the tumor niche plays an active role in cancer, but its important properties for tumor initiation progression and resistance remain to be identified. Clinical data show that leukemic stem cell (LSC) survival is responsible for disease persistence and drug resistance, probably due to their sustained interactions with the tumor niche. Bone morphogenetic protein (BMP) signaling is a key pathway controlling stem cells and their niche. BMP2 and BMP4 are important in both the normal and the cancer context. Several studies have revealed profound alterations of the BMP signaling in cancer SCs, with major deregulations of the BMP receptors and their downstream signaling elements. This was illustrated in the hematopoietic system by pioneer studies in chronic myelogenous leukemia that may now be expanded to acute myeloid leukemia and lymphoid leukemia, as reviewed here. At diagnosis, cells from the leukemic microenvironment are the major providers of soluble BMPs. Conversely, LSCs display altered receptors and downstream BMP signaling elements accompanied by altered functional responses to BMPs. These studies reveal the role of BMPs in tumor initiation, in addition to their known effects in later stages of transformation and progression. They also reveal the importance of BMPs in fueling cell transformation and expansion by overamplifying a natural SC response. This mechanism may explain the survival of LSCs independently of the initial oncogenic event and therefore may be involved in resistance processes.