Differential Expression of α2 Integrin Separates Long-Term and Short-Term Reconstituting Lin−/loThy1.1loc-kit+Sca-1+Hematopoietic Stem Cells

Hematopoietic stem cells and extramedullary hematopoiesis in the lungs

Hematopoietic stem cells are key players in hematopoiesis as the body maintains a physiologic steady state, and the signaling pathways and control mechanisms of these dynamic cells are implicated in processes from inflammation to cancer. Although the bone marrow is commonly regarded as the site of hematopoiesis and hematopoietic stem cell residence, these cells also circulate in the blood and reside in extramedullary tissues, including the lungs. Flow cytometry is an invaluable tool in evaluating hematopoietic stem cells, revealing their phenotypes and relative abundances in both healthy and diseased states. This review outlines current protocols and cell markers used in flow cytometric analysis of hematopoietic stem and progenitor cell populations. Specific niches within the bone marrow are discussed, as are metabolic processes that contribute to stem cell self-renewal and differentiation, as well as the role of hematopoietic stem cells outside of the bone marrow at physiologic steady state. Finally, pulmonary extramedullary hematopoiesis and its associated disease states are outlined. Hematopoiesis in the lungs is a new and emerging concept, and discovering ways in which the study of lung-resident hematopoietic stem cells can be translated from murine models to patients will impact clinical treatment.

Epigenetic reversal of hematopoietic stem cell aging in Phf6-knockout mice

Aging is characterized by an accumulation of myeloid-biased hematopoietic stem cells (HSCs) with reduced developmental potential. Genotoxic stress and epigenetic alterations have been proposed to mediate age-related HSC loss of regenerative and self-renewal potential. However, the mechanisms underlying these changes remain largely unknown. Genetic inactivation of the plant homeodomain 6 (Phf6) gene, a nucleolar and chromatin-associated factor, antagonizes age-associated HSC decline. Immunophenotyping, single-cell transcriptomic analyses and transplantation assays demonstrated markedly decreased accumulation of immunophenotypically defined HSCs, reduced myeloid bias and increased hematopoietic reconstitution capacity with preservation of lymphoid differentiation potential in Phf6-knockout HSCs from old mice. Moreover, deletion of Phf6 in aged mice rejuvenated immunophenotypic, transcriptional and functional hallmarks of aged HSCs. Long-term HSCs from old Phf6-knockout mice showed epigenetic rewiring and transcriptional programs consistent with decreased genotoxic stress-induced HSC aging. These results identify Phf6 as an important epigenetic regulator of HSC aging.

CD49b identifies functionally and epigenetically distinct subsets of lineage-biased hematopoietic stem cells

Hematopoiesis is maintained by functionally diverse lineage-biased hematopoietic stem cells (HSCs). The functional significance of HSC heterogeneity and the regulatory mechanisms underlying lineage bias are not well understood. However, absolute purification of HSC subtypes with a pre-determined behavior remains challenging, highlighting the importance of continued efforts toward prospective isolation of homogeneous HSC subsets. In this study, we demonstrate that CD49b subdivides the most primitive HSC compartment into functionally distinct subtypes: CD49b⁻ HSCs are highly enriched for myeloid-biased and the most durable cells, while CD49b⁺ HSCs are enriched for multipotent cells with lymphoid bias and reduced self-renewal ability. We further demonstrate considerable transcriptional similarities between CD49b⁻ and CD49b⁺ HSCs but distinct differences in chromatin accessibility. Our studies highlight the diversity of HSC functional behaviors and provide insights into the molecular regulation of HSC heterogeneity through transcriptional and epigenetic mechanisms.

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CD49b⁻ HSCs are highly enriched for durable and long-term myeloid-biased HSCs

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CD49b⁺ HSCs are enriched for less durable cells with lymphoid bias

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CD49b⁻ and CD49b⁺ HSCs are transcriptionally similar but epigenetically distinct

Integrins, anchors and signal transducers of hematopoietic stem cells during development and in adulthood

Hematopoietic stem cells (HSCs), the apex of the hierarchically organized blood cell production system, are generated in the yolk sac, aorta-gonad-mesonephros region and placenta of the developing embryo. To maintain life-long hematopoiesis, HSCs emigrate from their site of origin and seed in distinct microenvironments, called niches, of fetal liver and bone marrow where they receive supportive signals for self-renewal, expansion and production of hematopoietic progenitor cells (HPCs), which in turn orchestrate the production of the hematopoietic effector cells. The interactions of hematopoietic stem and progenitor cells (HSPCs) with niche components are to a large part mediated by the integrin superfamily of adhesion molecules. Here, we summarize the current knowledge regarding the functional properties of integrins and their activators, Talin-1 and Kindlin-3, for HSPC generation, function and fate decisions during development and in adulthood. In addition, we discuss integrin-mediated mechanosensing for HSC-niche interactions, ex vivo protocols aimed at expanding HSCs for therapeutic use, and recent approaches targeting the integrin-mediated adhesion in leukemia-inducing HSCs in their protecting, malignant niches.

N-Cadherin-Expressing Bone and Marrow Stromal Progenitor Cells Maintain Reserve Hematopoietic Stem Cells

Regulation of hematopoietic stem cells (HSCs) by bone marrow (BM) niches has been extensively studied; however, whether and how HSC subpopulations are distinctively regulated by BM niches remain unclear. Here, we functionally distinguished reserve HSCs (rHSCs) from primed HSCs (pHSCs) based on their response to chemotherapy and examined how they are dichotomously regulated by BM niches. Both pHSCs and rHSCs supported long-term hematopoiesis in homeostasis; however, pHSCs were sensitive but rHSCs were resistant to chemotherapy. Surviving rHSCs restored the HSC pool and supported hematopoietic regeneration after chemotherapy. The rHSCs were preferentially maintained in the endosteal region that enriches N-cadherin⁺ (N-cad⁺) bone-lining cells in homeostasis and post-chemotherapy. N-cad⁺ cells were functional bone and marrow stromal progenitor cells (BMSPCs), giving rise to osteoblasts, adipocytes, and chondrocytes in vitro and in vivo. Finally, ablation of N-cad⁺ niche cells or deletion of SCF from N-cad⁺ niche cells impaired rHSC maintenance during homeostasis and regeneration.

Hematopoietic Stem Cell Heterogeneity

Hematopoietic stem cells (HSCs) maintain lifelong production of mature blood cells and regenerate the hematopoietic system after cytotoxic injury. Use of expanding cell surface marker panels and advanced functional analyses have revealed the presence of several immunophenotypically different HSC subsets with distinct self-renewal and repopulating capacity and bias toward selective lineage differentiation. This chapter summarizes current understanding of the phenotypic and functional heterogeneity within the HSC pool, with emphasis on the immunophenotypes and functional features of several known HSC subsets, and their roles in steady-state and emergency hematopoiesis, and in aging. The chapter also highlights some of the future research directions to elucidate further the biology and function of different HSC subsets in health and disease states.

Obesity alters the long-term fitness of the hematopoietic stem cell compartment through modulation of Gfi1 expression

Lee et al. show that established obesity alters the composition and long-term fitness of the hematopoietic stem cell (HSC) compartment, in part through a Gfi1-dependent HSC regulatory program that is activated by the chronic oxidative stress associated with this condition.

Obesity is a chronic organismal stress that disrupts multiple systemic and tissue-specific functions. In this study, we describe the impact of obesity on the activity of the hematopoietic stem cell (HSC) compartment. We show that obesity alters the composition of the HSC compartment and its activity in response to hematopoietic stress. The impact of obesity on HSC function is progressively acquired but persists after weight loss or transplantation into a normal environment. Mechanistically, we establish that the oxidative stress induced by obesity dysregulates the expression of the transcription factor Gfi1 and that increased Gfi1 expression is required for the abnormal HSC function induced by obesity. These results demonstrate that obesity produces durable changes in HSC function and phenotype and that elevation of Gfi1 expression in response to the oxidative environment is a key driver of the altered HSC properties observed in obesity. Altogether, these data provide phenotypic and mechanistic insight into durable hematopoietic dysregulations resulting from obesity.

How One Thing Led to Another

I started research in high school, experimenting on immunological tolerance to transplantation antigens. This led to studies of the thymus as the site of maturation of T cells, which led to the discovery, isolation, and clinical transplantation of purified hematopoietic stem cells (HSCs). The induction of immune tolerance with HSCs has led to isolation of other tissue-specific stem cells for regenerative medicine. Our studies of circulating competing germline stem cells in colonial protochordates led us to document competing HSCs. In human acute myelogenous leukemia we showed that all preleukemic mutations occur in HSCs, and determined their order; the final mutations occur in a multipotent progenitor derived from the preleukemic HSC clone. With these, we discovered that CD47 is an upregulated gene in all human cancers and is a "don't eat me" signal; blocking it with antibodies leads to cancer cell phagocytosis. CD47 is the first known gene common to all cancers and is a target for cancer immunotherapy.

The Co-Expression Pattern of p63 and HDAC1: A Potential Way to Disclose Stem Cells in Interfollicular Epidermis

Stem cell markers of interfollicular epidermis (IEF) have not been established thus far. The aim of this study is to suggest a new way to disclose IFE-stem cells by combining the expression of histone deacetylases (HDAC) 1 and p63. Immunohistochemical staining of HDAC1 and p63 was performed in six normal human samples. Moreover, a skin equivalent (SE) model was treated with suberoylanilohydroxamic acid (SAHA, an HDAC inhibitor) to elucidate the role of HDAC1. Finally, rapidly adhering (RA) keratinocytes to a type IV collagen, which have been identified to represent epidermal stem cells, were subjected to Western blot analysis with antibodies against HDAC1. In normal samples, there was a minor subpopulation comprised of p63-positive and HDAC1-negative cells in the basal layers. The proportion of this subpopulation was decreased with age. In the SE model, SAHA treatment increased the epidermal thickness and number of p63-positive cells in a dose dependent manner. After SAHA treatment, the expression of differentiation markers was decreased, while that of basement membrane markers was increased. In a Western blot analysis, HDAC1 was not expressed in RA cells. In conclusion, the combination of p63-positive and HDAC1-negative expressions can be a potential new way for distinguishing epidermal stem cells.

Sphingosine-1-Phosphate Receptor-3 Supports Hematopoietic Stem and Progenitor Cell Residence Within the Bone Marrow Niche

Hematopoietic stem and progenitor cells (HSPCs) egress from bone marrow (BM) during homeostasis and at increased rates during stress; however, the mechanisms regulating their trafficking remain incompletely understood. Here we describe a novel role for lipid receptor, sphingosine-1-phosphate receptor 3 (S1PR3), in HSPC residence within the BM niche. HSPCs expressed increased levels of S1PR3 compared to differentiated BM cells. Pharmacological antagonism or knockout (KO) of S1PR3 mobilized HSPCs into blood circulation, suggesting that S1PR3 influences niche localization. S1PR3 antagonism suppressed BM and plasma SDF-1, enabling HSPCs to migrate toward S1P-rich plasma. Mobilization synergized with AMD3100-mediated antagonism of CXCR4, which tethers HSPCs in the niche, and recovered homing deficits of AMD3100-treated grafts. S1PR3 antagonism combined with AMD3100 improved re-engraftment and survival in lethally irradiated recipients. Our studies indicate that S1PR3 and CXCR4 signaling cooperate to maintain HSPCs within the niche under homeostasis. These results highlight an important role for S1PR3 in HSPC niche occupancy and trafficking that can be harnessed for both rapid clinical stem cell mobilization and re-engraftment strategies, as well as the opportunity to design novel therapeutics for control of recruitment, homing, and localization through bioactive lipid signaling. Stem Cells 2017;35:1040-1052.

Isolation and Assessment of Single Long-Term Reconstituting Hematopoietic Stem Cells from Adult Mouse Bone Marrow

Hematopoietic stem cells with long-term repopulating activity can now be routinely obtained at purities of 40% to 50% from suspensions of adult mouse bone marrow. Here we describe robust protocols for both their isolation as CD45(+) EPCR(+) CD150(+) CD48(-) (ESLAM) cells using multiparameter cell sorting and for tracking their clonal growth and differentiation activity in irradiated mice transplanted with single ESLAM cells. The simplicity of these procedures makes them attractive for characterizing the molecular and biological properties of individual hematopoietic stem cells with unprecedented power and precision. © 2016 by John Wiley & Sons, Inc.

Space constraints govern fate of hematopoietic stem and progenitor cells in vitro

Deciphering exogenous cues that determine stem cell fate decisions is a persisting challenge of cell biology and bioengineering. In an effort to unravel the role of spatial constraints in the cell-instructive characteristics of bone marrow microenvironments, murine hematopoietic stem and progenitor cells (HSPC) were exposed to fibronectin-coated microcavities in vitro. Microcavity sizes were chosen to allow for the inclusion of either individual or multiple cells. Repopulation experiments using lethally irradiated mice showed that the maintenance of functional HSPC in culture critically depends on cavity dimensions. Short-term repopulating hematopoietic stem cells (ST-HSC) were found to be best supported within single-cell sized compartments while long-term repopulating HSC (LT-HSC) were maintained within both cavity sizes. In sum, the reported data reveal spatial restriction to be a simple but powerful means for directing HSPC fate ex vivo.

Niche interactions in epidermal stem cells

Within the epidermis and dermis of the skin, cells secrete and are surrounded by the extracellular matrix (ECM), which provides structural and biochemical support. The ECM of the epidermis is the basement membrane, and collagen and other dermal components constitute the ECM of the dermis. There is significant variation in the composition of the ECM of the epidermis and dermis, which can affect “cell to cell” and “cell to ECM” interactions. These interactions, in turn, can influence biological responses, aging, and wound healing; abnormal ECM signaling likely contributes to skin diseases. Thus, strategies for manipulating cell-ECM interactions are critical for treating wounds and a variety of skin diseases. Many of these strategies focus on epidermal stem cells, which reside in a unique niche in which the ECM is the most important component; interactions between the ECM and epidermal stem cells play a major role in regulating stem cell fate. As they constitute a major portion of the ECM, it is likely that integrins and type IV collagens are important in stem cell regulation and maintenance. In this review, we highlight recent research-including our previous work-exploring the role that the ECM and its associated components play in shaping the epidermal stem cell niche.

Dynamic equilibrium of heterogeneous and interconvertible multipotent hematopoietic cell subsets

Populations of hematopoietic stem cells and progenitors are quite heterogeneous and consist of multiple cell subsets with distinct phenotypic and functional characteristics. Some of these subsets also appear to be interconvertible and oscillate between functionally distinct states. The multipotent hematopoietic cell line EML has emerged as a unique model to study the heterogeneity and interconvertibility of multipotent hematopoietic cells. Here we describe extensive phenotypic and functional heterogeneity of EML cells which stems from the coexistence of multiple cell subsets. Each of these subsets is phenotypically and functionally heterogeneous, and displays distinct multilineage differentiation potential, cell cycle profile, proliferation kinetics, and expression pattern of HSC markers and some of the key lineage-associated transcription factors. Analysis of their maintenance revealed that on a population level all EML cell subsets exhibit cell-autonomous interconvertible properties, with the capacity to generate all other subsets and re-establish complete parental EML cell population. Moreover, all EML cell subsets generated during multiple cell generations maintain their distinct phenotypic and functional signatures and interconvertible properties. The model of EML cell line suggests that interconvertible multipotent hematopoietic cell subsets coexist in a homeostatically maintained dynamic equilibrium which is regulated by currently unknown cell-intrinsic mechanisms.

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

Expression of integrin α2 receptor in human cord blood CD34+CD38-CD90+ stem cells engrafting long-term in NOD/SCID-IL2Rγ(c) null mice

Human hematopoietic stem cells reside in the CD34+CD38-CD90+ population in cord blood and bone marrow. However, this cell fraction is heterogeneous, and the phenotype of the rare primitive stem cells remains poorly defined. We here report that primitive cord blood CD34+CD38-CD90+ stem cells, with the ability to reconstitute NOD/SCID-IL2Rγ(c) null (NSG) mice long-term, at 24 weeks after transplantation, can be prospectively isolated at an increased purity by using integrin α2 receptor as an additional stem cell marker. Using a limiting dilution transplantation assay, we found a highly significant enrichment of multilineage reconstituting stem cells in the CD34+CD38-CD90+ cell fraction expressing the integrin α2 receptor, with a frequency of 1/29 cells, as compared to a frequency of 1/157 in the corresponding integrin α2- cells. In line with this, long-term reconstituting stem cells within the cord blood CD34+CD38- cell population were significantly enriched in the integrin α2+ fraction, while stem cells and progenitors reconstituting short-term, at 8-12 weeks, were heterogeneous in integrin α2 expression. Global gene expression profiling revealed that the lineage-marker negative (Lin-) CD34+CD38-CD90+CD45RA- integrin α2+ cell population was molecularly distinct from the integrin α2- cell population and the more mature Lin-CD34+CD38-CD90-CD45RA- cell population. Our findings identify integrin α2 as a novel stem cell marker, which improves prospective isolation of the primitive human hematopoietic stem cells within the CD34+CD38-CD90+ cell population for experimental and therapeutic stem cell applications.

Integrin αIIb (CD41) plays a role in the maintenance of hematopoietic stem cell activity in the mouse embryonic aorta

Integrins are transmembrane receptors that play important roles as modulators of cell behaviour through their adhesion properties and the initiation of signaling cascades. The α_IIb integrin subunit (CD41) is one of the first cell surface markers indicative of hematopoietic commitment. α_IIb pairs exclusively with β₃ to form the α_IIbβ₃ integrin. β₃ (CD61) also pairs with α_v (CD51) to form the α_vβ₃ integrin. The expression and putative role of these integrins during mouse hematopoietic development is as yet unknown. We show here that hematopoietic stem cells (HSCs) differentially express α_IIbβ₃ and α_vβ₃ integrins throughout development. Whereas the first HSCs generated in the aorta at mid-gestation express both integrins, HSCs from the placenta only express α_vβ₃, and most fetal liver HSCs do not express either integrin. By using α_IIb deficient embryos, we show that α_IIb is not only a reliable HSC marker but it also plays an important and specific function in maintaining the HSC activity in the mouse embryonic aorta.

Advances in tracking hematopoiesis at the single-cell level

PURPOSE OF REVIEW

Studying heterogeneous populations, such as hematopoietic stem cells (HSCs), requires continuous long-term observation of living cells at the single-cell level. The purpose of this review is to discuss recent advances in technologies required for continuous single-cell analysis and the contribution of this approach to find answers in hematopoiesis research.

RECENT FINDINGS

Continuous long-term imaging at the single-cell level still requires custom-made hardware, software and manual in-depth analysis of large amounts of data. Despite these technical difficulties, continuous time-lapse imaging and single-cell tracking are increasingly used in hematopoiesis research. It has already contributed to answering decades-old questions.

SUMMARY

Continuous long-term single-cell analysis is indispensable for a comprehensive analysis of dynamic processes in heterogeneous cell populations. Despite many remaining technological hurdles, this approach is increasingly used in hematopoiesis research.

Homing and long-term engraftment of long- and short-term renewal hematopoietic stem cells

Long-term hematopoietic stem cells (LT-HSC) and short-term hematopoietic stem cells (ST-HSC) have been characterized as having markedly different in vivo repopulation, but similar in vitro growth in liquid culture. These differences could be due to differences in marrow homing. We evaluated this by comparing results when purified ST-HSC and LT-HSC were administered to irradiated mice by three different routes: intravenous, intraperitoneal, and directly into the femur. Purified stem cells derived from B6.SJL mice were competed with marrow cells from C57BL/6J mice into lethally irradiated C57BL/6J mice. Serial transplants into secondary recipients were also carried out. We found no advantage for ST-HSC engraftment when the cells were administered intraperitoneally or directly into femur. However, to our surprise, we found that the purified ST-HSC were not short-term in nature but rather gave long-term multilineage engraftment out to 387 days, albeit at a lower level than the LT-HSC. The ST-HSC also gave secondary engraftment. These observations challenge current models of the stem cell hierarchy and suggest that stem cells are in a continuum of change.

Acute lymphoblastic leukemia and developmental biology: a crucial interrelationship

The latest scientific findings in the field of cancer research are redefining our understanding of the molecular and cellular basis of the disease, moving the emphasis toward the study of the mechanisms underlying the alteration of the normal processes of cellular differentiation. The concepts best exemplifying this new vision are those of cancer stem cells and tumoral reprogramming. The study of the biology of acute lymphoblastic leukemias (ALLs) has provided seminal experimental evidence supporting these new points of view. Furthermore, in the case of B cells, it has been shown that all the stages of their normal development show a tremendous degree of plasticity, allowing them to be reprogrammed to other cellular types, either normal or leukemic. Here we revise the most recent discoveries in the fields of B-cell developmental plasticity and B-ALL research and discuss their interrelationships and their implications for our understanding of the biology of the disease.

Cellular aging leads to functional heterogeneity of hematopoietic stem cells: a modeling perspective

Hematopoietic stem cells (HSCs) are the source for the life-long supply of functional cells in peripheral blood while they simultaneously maintain their own reserve pool. However, there is accumulating evidence that HSCs are themselves subject to quantitative and qualitative exhaustion. Although several processes linked to mitotic activity can potentially account for the observed aging phenomena (e.g., DNA damage, telomere shortening, epigenetic modification), a precise understanding of HSC exhaustion is still missing. It is particularly unclear how individual aging processes on the single-cell level translate on the phenotypic level of the overall tissue and whether there is a functional implication of an age-structured HSC population. We address these issues by applying a novel mathematical model of HSC organization in which division-specific, cumulative alterations of stem cell quality determine the phenotypic and functional appearance of the overall cell population. Adapting the model to a number of basic experimental findings, we quantify the level of additional heterogeneity that is introduced by a population of individually aging cells. Based on this model, we are able to conclude that division-dependent processes of cellular aging explain a wide range of phenomena on HSC exhaustion and that HSC aging needs to be considered as a highly heterogeneous process. We furthermore report that functional heterogeneity between young and old HSCs appears closely similar to the phenomena described for long- and short-term repopulating cells. We speculate whether differential, division-coupled stem cell aging introduces an intra-animal variability that also accounts for heterogeneity with respect to the repopulation ability of HSCs.

Cell-extracellular matrix interactions in normal and diseased skin

Mammalian skin comprises a multi-layered epithelium, the epidermis, and an underlying connective tissue, the dermis. The epidermal extracellular matrix is a basement membrane, whereas the dermal ECM comprises fibrillar collagens and associated proteins. There is considerable heterogeneity in ECM composition within both epidermis and dermis. The functional significance of this extends beyond cell adhesion to a range of cell autonomous and nonautonomous processes, including control of epidermal stem cell fate. In skin, cell-ECM interactions influence normal homeostasis, aging, wound healing, and disease. Disturbed integrin and ECM signaling contributes to both tumor formation and fibrosis. Strategies for manipulating cell-ECM interactions to repair skin defects and intervene in a variety of skin diseases hold promise for the future.

Mechanisms controlling hematopoietic stem cell functions during normal hematopoiesis and hematological malignancies

Hematopoiesis, the process by which all mature blood cells are generated from multipotent hematopoietic stem cells (HSCs), is a finely tuned balancing act in which HSCs must constantly decide between different cell fates: to proliferate, to self-renew or differentiate, to stay quiescent in the bone marrow niche or migrate to the periphery, to live or die. These fates are regulated by a complex interplay between cell-extrinsic cues and cell-intrinsic regulatory pathways whose function is to maintain a homeostatic balance between HSC self-renewal and life-long replenishment of lost blood cells. Improper regulation of these competing cellular programs can transform HSCs and progenitor cells into disease-initiating leukemic stem cells (LSCs). Strikingly, many of the mechanisms required for maintenance of normal HSC fate decisions are equally critical for the aberrant functions of LSCs. Because of the inherent complexities of these molecular mechanisms, a systematic approach to understanding the regulatory networks underlying HSC self-renewal is critical for uncovering the similarities and differences between HSCs and LSCs. In this review, we focus on recent developments in elucidating the regulatory networks governing normal HSC self-renewal programs and their implications for leukemic transformation. We describe the current technical and methodological limitations in isolating and characterizing HSCs and LSCs, and the emerging approaches that may afford a better understanding of the regulation of normal and leukemic hematopoiesis. Finally, we discuss how such basic mechanistic information may be of use for the design of novel therapies that will selectively reprogram and/or eliminate LSCs.

Cyclic acetal hydroxyapatite composites and endogenous osteogenic gene expression of rat marrow stromal cells

In this study, bone marrow stromal cells (BMSCs) were differentiated on cyclic acetal composites containing hydroxyapatite (HA) particles (110 or 550 nm). These composites were evaluated for their role in influencing osteogenic signalling by encapsulated BMSCs. While a number of factors exert influence on osteogenic signalling during the production of an osteogenic matrix, we hypothesize that HA particles may upregulate bone growth factor expression due to enhanced BMSC adhesion. To this end, fluorescence-activated cell sorting (FACS) analysis was performed for the evaluation of BMSC surface marker expression after culture on two-dimensional (2D) cyclic acetal/HA composites. Three-dimensional (3D) composites were then fabricated by incorporating 110 or 550 nm HA particles at 5, 10 and 50 ng/ml concentrations. Bone growth factor molecules (TGFbeta1, FGF-2 and PDGFa), bone biomarker molecules (ALP, OC, OPN and OCN) and extracellular matrix-related molecules (FN, MMP-13, Dmp1 and aggrecan) were selected for evaluation of osteogenic signalling mechanisms when in presence of these composites. FACS results at day 0 demonstrated that BMSCs were a heterogeneous population with a small percentage of cells staining positive for CD29, CD90 and CD51/61, while staining negative for CD34 and CD45. At day 3, a significant enrichment of cells staining strongly for CD29, CD90 and CD51/61 was achieved. Gene expression patterns for bone growth factors and extracellular matrix molecules were found to be largely dependent upon the size of HA particles. Bone marker molecules, except OCN, had unaltered expression patterns in response to the varied size of HA particles. Overall, the results indicate that larger-sized HA particles upregulate PDGF and these groups were also associated with the most significant increase in osteodifferentiation markers, particularly ALP. Our results suggest that endogenous signalling is dependent upon material properties. Furthermore, we propose that studying gene expression patterns induced by the surrounding biomaterials environment is a fundamental step in the creation of engineered tissues.

Stem cell plasticity revisited: the continuum marrow model and phenotypic changes mediated by microvesicles

The phenotype of marrow hematopoietic stem cells is determined by cell-cycle state and microvesicle entry into the stem cells. The stem cell population is continually changing based on cell-cycle transit and can only be defined on a population basis. Purification of marrow stem cells only addresses the heterogeneity of these populations. When whole marrow is studied, the long-term repopulating stem cells are in active cell cycle. However, with some variability, when highly purified stem cells are studied, the cells appear to be dormant. Thus, the study of purified stem cells is intrinsically misleading. Tissue-derived microvesicles enhanced by injury effect the phenotype of different cell classes. We propose that previously described stem cell plasticity is due to microvesicle modulation. We further propose a stem cell population model in which the individual cell phenotypes continually change, but the population phenotype is relatively stable. This, in turn, is modulated by microvesicle and microenvironmental influences.

Regeneration capability of Lin-/c-Kit+/Sca-1+ cells with or without radiation exposure for repopulation of peripheral blood in lethally irradiated mice monitored using Ly5.1 isotype on days 35, 90, and 270 after transplantation

OBJECTIVE

Hematopoietic stem cells are supposed to repopulate and maintain long-term regeneration of the recipient's bone marrow and peripheral blood. In this study, we evaluated the regeneration capability of Lin(-)/c-Kit(+)/Sca-1(+) (LKS) cells, the putative hematopoietic stem cells, after radiation exposure at graded doses, for long-term regeneration of peripheral blood in lethally irradiated recipients.

MATERIALS AND METHODS

LKS primitive progenitor cells, collected from the bone marrow of Ly5.1 mice that had been irradiated at graded increased doses (0.5, 1, 1.5, and 2 Gy) were transfused into lethally irradiated (9.5 Gy) Ly5.2 mice. Then, the Ly5.1 chimeric ratio in repopulated peripheral blood cells in the recipients was monitored. A reactive oxygen species (ROS)-reacting CM-H(2)DCFDA dye was used to evaluate the amount of ROS in LKS primitive progenitor cells with/without irradiation. Moreover, the amount of intracytoplasmic ROS generated after irradiation was estimated in terms of percent attenuation of cellular increase in number by the treatment with 100 microM N-acetyl-L-cysteine before irradiation.

RESULTS

Differential regeneration capability of LKS cells irradiated at graded increased doses showed a dose-dependent suppression of regeneration of peripheral blood in the recipient mice as compared with LKS cells without radiation exposure. The amount of intracytoplasmic ROS in LKS cells was much smaller than that in mature bone marrow cells, and that of ROS in LKS increased slightly after radiation exposure, as evaluated by CM-H(2)DCFDA dye fluorescence analysis. The estimated amount of ROS generated in LKS cells after radiation exposure was different between progenitor cells for early regeneration and those for late regeneration; namely, the amount of ROS in progenitors on day 270 were estimated to be smaller than that in progenitors for day 35 or day 90.

CONCLUSIONS

Because of the small amount of generated radiation-induced ROS calculated in terms of attenuation rate after N-acetyl-L-cysteine treatment, progenitor cells regenerating peripheral blood cells 270 days after transfusion were assumed to be anaerobic and more immature and radioresistant than those on day 35 or day 90. However, limited long-term regeneration capability (up to 270 days) of steady-state LKS cells than that of unfractionated rescue bone marrow cells suggests that LKS cells do not seem to be true hematopoietic stem cells.

Intermediate-term hematopoietic stem cells with extended but time-limited reconstitution potential

Sustained blood cell production depends on divisions by hematopoietic stem cells (HSCs) that yield both differentiating progeny as well as new HSCs via self-renewal. Differentiating progeny remain capable of self-renewal, but only HSCs sustain self-renewal through successive divisions securely enough to maintain clones that persist life-long. Until recently, the first identified next stage consisted of "short-term" reconstituting cells able to sustain clones of differentiating cells for only 4-6 weeks. Here we expand evidence for a numerically dominant "intermediate-term" multipotent HSC stage in mice whose clones persist for 6-8 months before becoming extinct and that are separable from both short-term as well as permanently reconstituting "long-term" HSCs. The findings suggest that the first step in stem cell differentiation consists not in loss of initial capacity for serial self-renewal divisions, but rather in loss of mechanisms that stabilize self-renewing behavior throughout successive future stem cell divisions.

New insights into the regulation of ion channels by integrins

By controlling cell adhesion to the extracellular matrix, integrin receptors regulate processes as diverse as cell migration, proliferation, differentiation, apoptosis, and synaptic stability. Because the underlying mechanisms are generally accompanied by changes in transmembrane ion flow, a complex interplay occurs between integrins, ion channels, and other membrane transporters. This reciprocal interaction regulates bidirectional signal transduction across the cell surface and may take place at all levels of control, from transcription to direct conformational coupling. In particular, it is becoming increasingly clear that integrin receptors form macromolecular complexes with ion channels. Besides contributing to the membrane localization of the channel protein, the integrin/channel complex can regulate a variety of downstream signaling pathways, centered on regulatory proteins like tyrosine kinases and small GTPases. In turn, the channel protein usually controls integrin activation and expression. We review some recent advances in the field, with special emphasis on hematology and neuroscience. Some oncological implications are also discussed.

Stem cell traits in long-term co-culture revealed by time-lapse imaging

A deeper understanding of stem cell niche engagement and subsequent behaviors would be enhanced by technologies enabling the tracking of individual stem cells at the clonal level in long-term co-culture (LTC), which mimics the complexity of the bone marrow microenvironment in vivo. Here, we report the application of time-lapse imaging with intermittent fluorescence for tracking well-defined populations of GFP(+) murine hematopoietic stem cells (HSCs) using LTC for >5 weeks. Long-term (LT) and short-term (ST) repopulating HSCs and hematopoietic progenitor cells (HPCs) were compared. The transition from cobblestone areas (CAs) under the stromal cell mantle into dispersed migrating cells on top of the stroma (COS) were directly observed. The ST-HSC and LT-HSC were able to initiate multiple waves of CA formation and COS expansion beyond 2 and 4 weeks, respectively. Retrospective tracking of individual CA forming cell (CAFC) revealed a preference for residing under stroma before the first division and a longer interval before first division for LT-HSC. Inability to maintain quiescence in subsequent divisions was revealed. Our study represents an important starting point from which the LTC system can be augmented to provide a better in vitro model for bone marrow stem cell niches.

Problems in the promised land: status of adult marrow stem cell biology

Long-term engrafting marrow hematopoietic stem cells have been considered to be a quiescent stem cell in G(0). However, there are contradictory reports on this point in the literature, showing marked variability of results over time and between mice. Furthermore, there are circadian rhythms for stem cells and progenitors. In general, most studies have not taken stochastic variability or circadian rhythms into account. In addition, stem cell purification has represented the present gold standard in stem cell research. However, evidence exists that the stem cell separations leave behind most stem cells and are not random. Thus, purified stem cells may not be representative of the stem cells in the unseparated marrow cell population. The epitope-based purification of stem cells may have misled the stem cell field. Lastly, there are interesting published studies indicating that the irradiated marrow microenvironment might be toxic to marrow stem cells, limiting self-renewal capacity, and that quantitative engraftment occurs in nonablated mice. These considerations suggest that in carrying out stem cell studies, attention needs to be directed to the appropriate number of repeat experiments, to circadian rhythms, to possible purification skewing of results, and to the most appropriate transplant assay model.

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

Hematopoietic stem cells (HSCs) are generally defined by their dual properties of pluripotency and extensive self-renewal capacity. However, a lack of experimental clarity as to what constitutes extensive self-renewal capacity coupled with an absence of methods to prospectively isolate long-term repopulating cells with defined self-renewal activities has made it difficult to identify the essential components of the self-renewal machinery and investigate their regulation. We now show that cells capable of repopulating irradiated congenic hosts for 4 months and producing clones of cells that can be serially transplanted are selectively and highly enriched in the CD150(+) subset of the EPCR(+)CD48(-)CD45(+) fraction of mouse fetal liver and adult bone marrow cells. In contrast, cells that repopulate primary hosts for the same period but show more limited self-renewal activity are enriched in the CD150(-) subset. Comparative transcriptome analyses of these 2 subsets with each other and with HSCs whose self-renewal activity has been rapidly extinguished in vitro revealed 3 new genes (VWF, Rhob, Pld3) whose elevated expression is a consistent and selective feature of the long-term repopulating cells with durable self-renewal capacity. These findings establish the identity of a phenotypically and molecularly distinct class of pluripotent hematopoietic cells with lifelong self-renewal capacity.

High level in vitro expansion of murine hematopoietic stem cells

Development of strategies to extensively expand hematopoietic stem cells (HSCs) in vitro will be a major factor in enhancing the success of a range of transplant-based therapies for malignant and genetic disorders. In addition to potential clinical applications, the ability to increase the number of HSCs in culture will facilitate investigations into the mechanisms underlying self-renewal. In this unit, we describe a robust strategy for consistently achieving over 1000-fold net expansion of HSCs in short-term in vitro culture by using novel engineered fusions of the N-terminal domain of nucleoporin 98 (NUP98) and the homeodomain of the hox transcription factor, HOXA10 (so called NUP98-HOXA10hd fusion). We also provide a detailed protocol for monitoring the magnitude of HSC expansion in culture by limiting dilution assay of competitive lympho-myeloid repopulating units (CRU Assay). These procedures provide new possibilities for achieving significant numbers of HSCs in culture, as well as for studying HSCs biochemically and genetically.

Isolation and assessment of long-term reconstituting hematopoietic stem cells from adult mouse bone marrow

Suspensions of multipotent hematopoietic stem cells with long-term repopulating activity can now be routinely isolated from adult mouse bone marrow at purities of 30%. A robust method for obtaining these cells in a single step using multiparameter cell sorting to isolate the CD45(mid)lin(-)Rho(-)SP subset is described here, together with a detailed protocol for assessing their regenerative activity in mice transplanted with single cells. These procedures provide unprecedented power and precision for characterizing the molecular and biological properties of cells with hematopoietic stem cell activity at the single cell level.

The paradoxical dynamism of marrow stem cells: considerations of stem cells, niches, and microvesicles

Marrow stem cell regulation represents a complex and flexible system. It has been assumed that the system was intrinsically hierarchical in nature, but recent data has indicated that at the progenitor/stem cell level the system may represent a continuum with reversible alterations in phenotype occurring as the stem cells transit cell cycle. Short and long-term engraftment, in vivo and in vitro differentiation, gene expression, and progenitor numbers have all been found to vary reversibly with cell cycle. In essence, the stem cells appear to show variable potential, probably based on transcription factor access, as they proceed through cell cycle. Another critical component of the stem cell regulation is the microenvironment, so-called niches. We propose that there are not just several unique niche cells, but a wide variety of niche cells which continually change phenotype to appropriately interact with the continuum of stem cell phenotypes. A third component of the regulatory system is microvesicle transfer of genetic information between cells. We have shown that marrow cells can express the genetic phenotype of pulmonary epithelial cells after microvesicle transfer from lung to marrow cells. Similar transfers of tissue specific mRNA occur between liver, brain, and heart to marrow cells. Thus, there would appear to be a continuous genetic modulation of cells through microvesicle transfer between cells. We propose that there is an interactive triangulated Venn diagram with continuously changing stem cells interacting with continuously changing areas of influence, both being modulated by transfer of genetic information by microvesicles.

Long-term propagation of distinct hematopoietic differentiation programs in vivo

Heterogeneity in the differentiation behavior of hematopoietic stem cells is well documented but poorly understood. To investigate this question at a clonal level, we isolated a subpopulation of adult mouse bone marrow that is highly enriched for multilineage in vivo repopulating cells and transplanted these as single cells, or their short-term clonal progeny generated in vitro, into 352 recipients. Of the mice, 93 showed a donor-derived contribution to the circulating white blood cells for at least 4 months in one of four distinct patterns. Serial transplantation experiments indicated that two of the patterns were associated with extensive self-renewal of the original cell transplanted. However, within 4 days in vitro, the repopulation patterns subsequently obtained in vivo shifted in a clone-specific fashion to those with less myeloid contribution. Thus, primitive hematopoietic cells can maintain distinct repopulation properties upon serial transplantation in vivo, although these properties can also alter rapidly in vitro.

Evolving views on the genealogy of B cells

Many fundamental concepts about immune system development have changed substantially in the past few years, and rapid advances with animal models are presenting prospects for further discovery. However, continued progress requires a clearer understanding of the relationships between haematopoietic stem cells and the progenitors that replenish each type of lymphocyte pool. Blood-cell formation has traditionally been described in terms of discrete developmental branch points, and a single route is given for each major cell type. As we discuss in this Review, recent findings suggest that the process of B-cell formation is much more dynamic.

Antibodies to Stem Cell Marker Antigens Reduce Engraftment of Hematopoietic Stem Cells

Hematopoietic stem cells (HSCs) have enormous potential for use in transplantation and gene therapy. However, the frequency of repopulating HSCs is often very low; thus, highly effective techniques for cell enrichment and maintenance are required to obtain sufficient cell numbers for therapeutic use and for studies of HSC physiology. Common methods of HSC enrichment use antibodies recognizing HSC surface marker antigens. Because antibodies are known to alter the physiology of other cell types, we investigated the effect of such enrichment strategies on the physiology and lineage commitment of HSCs. We sorted HSCs using a method that does not require antibodies: exclusion of Hoechst 33342 to isolate side population (SP) cells. To elucidate the effect of antibody binding on this HSC population, we compared untreated SP cells with SP cells treated with the Sca-1(+)c-Kit(+)Lin(-) (SKL) antibody cocktail prior to SP sorting. Our findings revealed that HSCs incubated with the antibody cocktail had decreased expression of the stem cell-associated genes c-Kit, Cd34, Tal-1, and Slamf1 relative to untreated SP cells or to cells treated with polyclonal isotype control antibodies. Moreover, SKL antibodies induced cycling in SP cells and diminished their ability to confer long-term hematopoietic engraftment in lethally irradiated mice. Taken together, these data suggest that antibody-based stem cell isolation procedures can have negative effects on HSC physiology.

Expression of Integrin β3 Is Correlated to the Properties of Quiescent Hemopoietic Stem Cells Possessing the Side Population Phenotype

With significant attention paid to the field of tissue-specific stem cells, the identification of stem cell-specific markers is of considerable importance. Previously, the side population (SP) phenotype, with the capacity to efflux the DNA-binding dye Hoechst 33342, has been recognized as a common feature of adult tissue-specific stem cells. In this study, we show that high expression of integrin beta(3) (CD61) is an attribute of SP cells isolated from mouse bone marrow. Additionally, we confirmed that the expression of integrin beta(3) is correlated with properties of quiescent hemopoietic stem cells (HSCs) including the strength of the SP phenotype, cell cycle arrest, expression of HSC markers, and long-term hemopoiesis. Importantly, Lineage(-) (Lin(-))/integrin beta(3)(high) (beta(3)(high)) SP cells have as strong a capacity for long-term hemopoiesis as c-Kit(+)/Sca-1(+)/Lin(-) SP cells, which are regarded as one of the most highly enriched HSC populations. Finally, the integrin beta(3) subunit that is present in SP cells having the properties of HSCs, is associated with integrin alpha(v) (CD51). Therefore, our results demonstrate that high expression of integrin beta(3) is correlated to the properties of quiescent HSCs and suggest that the integrin beta(3) subunit is available as a common surface marker of tissue-specific stem cells.

Understanding hematopoietic stem-cell microenvironments

The hematopoietic system is the paradigm for adult mammalian stem-cell research. Recent advances have improved our understanding of the cellular and molecular components of the microenvironment - or niche - that regulates hematopoietic stem cells (HSCs). Here, we summarize the molecular and cellular properties of two types of niche, namely the osteoblastic and the vascular niche, in homeostatic regulation of HSC behavior, including its maintenance, proliferation, differentiation, mobilization and homing. We highlight the most recent findings and point to an important trend to the study of niche activity in cancers. Knowledge of the basic features of the HSC niches, including physical location, cell type and various signaling pathways, should provide insights into other stem-cell systems and benefit clinical applications.

SP analysis may be used to identify cancer stem cell populations

Side populations (SP), as defined by Hoechst exclusion in flow cytometry, have been described a few years ago. While they represent only a small fraction of the whole cell population, their properties confer an important place in several investigations. SP cells express high levels of various members of ABC transporters family, such as MDR1 and BCRP, which are responsible for drug resistance. Targeting SP could improve cancer therapy by blocking these transporters. In addition, SP appear to be enriched in stem cells, cells that play a pivotal role in normal development and cancer biology. Thus, they could provide a useful tool and a readily accessible source for stem cell studies in both the normal and cancerous settings. However, these cells are poorly defined and pose challenges in their identification and isolation, particularly since they are few in number. Thus, better characterization of SP will advance our understanding of stem cells and will provide us an accessible target for drug resistance in cancer therapy.

Activation of the canonical Wnt pathway leads to loss of hematopoietic stem cell repopulation and multilineage differentiation block

Wnt signaling increases hematopoietic stem cell self-renewal and is activated in both myeloid and lymphoid malignancies, indicating involvement in both normal and malignant hematopoiesis. We report here activated canonical Wnt signaling in the hematopoietic system through conditional expression of a stable form of beta-catenin. This enforced expression led to hematopoietic failure associated with loss of myeloid lineage commitment at the granulocyte-macrophage progenitor stage; blocked erythrocyte differentiation; disruption of lymphoid development; and loss of repopulating stem cell activity. Loss of hematopoietic stem cell function was associated with decreased expression of Cdkn1a (encoding the cell cycle inhibitor p21(cdk)), Sfpi1, Hoxb4 and Bmi1 (encoding the transcription factors PU.1, HoxB4 and Bmi-1, respectively) and altered integrin expression in Lin(-)Sca-1(+)c-Kit(+) cells, whereas PU.1 was upregulated in erythroid progenitors. Constitutive activation of canonical Wnt signaling therefore causes multilineage differentiation block and compromised hematopoietic stem cell maintenance.

Integrins are markers of human neural stem cells

The identification of markers for the isolation of human neural stem cells (hNSCs) is essential for studies of their biology and therapeutic applications. This study investigated expression of the integrin receptor family by hNSCs as potential markers. Selection of alpha6(hi) or beta1(hi) cells by fluorescence-activated cell sorting led to an enrichment of human neural precursors, as shown by both neurosphere forming assays and increased expression of prominin-1, sox2, sox3, nestin, bmi1, and musashi1 in the beta1(hi) population. Cells expressing high levels of beta1 integrin also expressed prominin-1 (CD133), a marker previously used to isolate hNSCs, and selection using integrin beta1(hi) cells or prominin-1(hi) cells was found to be equally effective at enriching for hNSCs from neurospheres. Therefore, integrin subunits alpha6 and beta1 are highly expressed by human neural precursors and represent convenient markers for their prospective isolation.