Imaging hematopoietic precursor division in real time

Asymmetry-defective oligodendrocyte progenitors are glioma precursors

Postnatal oligodendrocyte progenitor cells (OPC) self-renew, generate mature oligodendrocytes, and are a cellular origin of oligodendrogliomas. We show that the proteoglycan NG2 segregates asymmetrically during mitosis to generate OPC cells of distinct fate. NG2 is required for asymmetric segregation of EGFR to the NG2(+) progeny, which consequently activates EGFR and undergoes EGF-dependent proliferation and self-renewal. In contrast, the NG2(-) progeny differentiates. In a mouse model, decreased NG2 asymmetry coincides with premalignant, abnormal self-renewal rather than differentiation and with tumor-initiating potential. Asymmetric division of human NG2(+) cells is prevalent in non-neoplastic tissue but is decreased in oligodendrogliomas. Regulators of asymmetric cell division are misexpressed in low-grade oligodendrogliomas. Our results identify loss of asymmetric division associated with the neoplastic transformation of OPC.

Distribution of CD133 reveals glioma stem cells self-renew through symmetric and asymmetric cell divisions

Malignant gliomas contain a population of self-renewing tumorigenic stem-like cells; however, it remains unclear how these glioma stem cells (GSCs) self-renew or generate cellular diversity at the single-cell level. Asymmetric cell division is a proposed mechanism to maintain cancer stem cells, yet the modes of cell division that GSCs utilize remain undetermined. Here, we used single-cell analyses to evaluate the cell division behavior of GSCs. Lineage-tracing analysis revealed that the majority of GSCs were generated through expansive symmetric cell division and not through asymmetric cell division. The majority of differentiated progeny was generated through symmetric pro-commitment divisions under expansion conditions and in the absence of growth factors, occurred mainly through asymmetric cell divisions. Mitotic pair analysis detected asymmetric CD133 segregation and not any other GSC marker in a fraction of mitoses, some of which were associated with Numb asymmetry. Under growth factor withdrawal conditions, the proportion of asymmetric CD133 divisions increased, congruent with the increase in asymmetric cell divisions observed in the lineage-tracing studies. Using single-cell-based observation, we provide definitive evidence that GSCs are capable of different modes of cell division and that the generation of cellular diversity occurs mainly through symmetric cell division, not through asymmetric cell division.

Universal patterns of stem cell fate in cycling adult tissues

In cycling tissues that exhibit high turnover, tissue maintenance and repair are coordinated by stem cells. But, how frequently stem cells are replaced following differentiation, aging or injury remains unclear. By drawing together the results of recent lineage-tracing studies, we propose that tissue stem cells are routinely lost and replaced in a stochastic manner. We show that stem cell replacement leads to neutral competition between clones, resulting in two characteristic and recurring patterns of clone fate dynamics, which provide a unifying framework for interpreting clone fate data and for measuring rates of stem cell loss and replacement in vivo. Thus, we challenge the concept of the stem cell as an immortal, slow-cycling, asymmetrically dividing cell.

Self-renewal related signaling in myeloid leukemia stem cells

A key characteristic of hematopoietic stem cells (HSC) is the ability to self-renew. Several genes and signaling pathways control the fine balance between self-renewal and differentiation in HSC and potentially also in leukemic stem cells. Besides pathways such as Wnt signaling, Hedgehog signaling and Notch signaling, transcription factors (FoxOs) and cell fate determinants may also play a role in stem cells. While some of these pathways seem to be dispensable for maintenance of adult HSC, there may be a distinct requirement in leukemia stem cells for leukemic self-renewal. Here we will focus on self-renewal related signaling in myeloid leukemia stem cells and its therapeutic relevance.

Mathematical study on kinetics of hematopoietic stem cells--theoretical conditions for successful transplantation

Numerous haematological diseases occur due to dysfunctions during homeostasis processes of blood cell production. Haematopoietic stem cell transplantation (HSCT) is a therapeutic option for the treatment of haematological malignancy and congenital immunodeficiency. Today, HSCT is widely applied as an alternative method to bone marrow transplantation; however, HSCT can be a risky procedure because of potential side effects and complications after transplantations. Although an optimal regimen to achieve successful HSCT while maintaining quality of life is to be developed, even theoretical considerations such as the evaluations of successful engraftments and proposals of clinical management strategies have not been fully discussed yet. In this paper, we construct and investigate mathematical models that describe the kinetics of hematopoietic stem cell self-renewal and granulopoiesis under the influence of growth factors. Moreover, we derive theoretical conditions for successful HSCT, primarily on the basis of the idea that the basic reproduction number R (0) represents a threshold condition for a population to successfully grow in a given steady-state environment. Successful engraftment of transplanted haematopoietic stem cells (HSCs) is subsequently ensured by employing a concept of dynamical systems theory known as 'persistence'. On the basis of the implications from the modelling study, we discuss how the conditions derived for a successful HSCT are used to link to experimental studies.

Atypical protein kinase C (aPKCzeta and aPKClambda) is dispensable for mammalian hematopoietic stem cell activity and blood formation

The stem-cell pool is considered to be maintained by a balance between symmetric and asymmetric division of stem cells. The cell polarity model proposes that the facultative use of symmetric and asymmetric cell division is orchestrated by a polarity complex consisting of partitioning-defective proteins Par3 and Par6, and atypical protein kinase C (aPKCζ and aPKCλ), which regulates planar symmetry of dividing stem cells with respect to the signaling microenvironment. However, the role of the polarity complex is unexplored in mammalian adult stem-cell functions. Here we report that, in contrast to accepted paradigms, polarization and activity of adult hematopoietic stem cell (HSC) do not depend on either aPKCζ or aPKCλ or both in vivo. Mice, having constitutive and hematopoietic-specific (Vav1-Cre) deletion of aPKCζ and aPKCλ, respectively, have normal hematopoiesis, including normal HSC self-renewal, engraftment, differentiation, and interaction with the bone marrow microenvironment. Furthermore, inducible complete deletion of aPKCλ (Mx1-Cre) in aPKCζ(-/-) HSC does not affect HSC polarization, self-renewal, engraftment, or lineage repopulation. In addition, aPKCζ- and aPKCλ-deficient HSCs elicited a normal pattern of hematopoietic recovery secondary to myeloablative stress. Taken together, the expression of aPKCζ, aPKCλ, or both are dispensable for primitive and adult HSC fate determination in steady-state and stress hematopoiesis, contrary to the hypothesis of a unique, evolutionary conserved aPKCζ/λ-directed cell polarity signaling mechanism in mammalian HSC fate determination.

Crosstalk between NOTCH and AKT signaling during murine megakaryocyte lineage specification

The NOTCH signaling pathway is implicated in a broad range of developmental processes, including cell fate decisions. However, the molecular basis for its role at the different steps of stem cell lineage commitment is unclear. We recently identified the NOTCH signaling pathway as a positive regulator of megakaryocyte lineage specification during hematopoiesis, but the developmental pathways that allow hematopoietic stem cell differentiation into the erythro-megakaryocytic lineages remain controversial. Here, we investigated the role of downstream mediators of NOTCH during megakaryopoiesis and report crosstalk between the NOTCH and PI3K/AKT pathways. We demonstrate the inhibitory role of phosphatase with tensin homolog and Forkhead Box class O factors on megakaryopoiesis in vivo. Finally, our data annotate developmental mechanisms in the hematopoietic system that enable a decision to be made either at the hematopoietic stem cell or the committed progenitor level to commit to the megakaryocyte lineage, supporting the existence of 2 distinct developmental pathways.

Regulation of asymmetric cell division in the epidermis

For proper tissue morphogenesis, cell divisions and cell fate decisions must be tightly and coordinately regulated. One elegant way to accomplish this is to couple them with asymmetric cell divisions. Progenitor cells in the developing epidermis undergo both symmetric and asymmetric cell divisions to balance surface area growth with the generation of differentiated cell layers. Here we review the molecular machinery implicated in controlling asymmetric cell division. In addition, we discuss the ability of epidermal progenitors to choose between symmetric and asymmetric divisions and the key regulatory points that control this decision.

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.

The descent of memory T cells

Our T cell repertoire is shaped by antigen encounter. From a naive T cell pool that contains millions of different T cells with unknown specificities, pathogen infection leads to selection of those T cells that can detect pathogen-derived antigens. Following clearance of infection, a population of memory T cells remains and protects the individual from severe reinfection. A central question in the field has been how the generation of long-lived memory T cells, versus short-lived ("terminally differentiated") T cells, is controlled. In this review we discuss the models that have been put forward to explain the generation of memory T cells after infection and the experimental evidence supporting these hypotheses. Based on the available data we propose a new model that stipulates that during immune responses T cells do not acquire different fates that determine their subsequent long-term survival but rather T cells assume different states that simply reflect the likelihood of future survival, states that can still be modulated by external signals.

Cell lineage and cell death: Caenorhabditis elegans and cancer research

Cancer is a complex disease in which cells have circumvented normal restraints on tissue growth and have acquired complex abnormalities in their genomes, posing a considerable challenge to identifying the pathways and mechanisms that drive fundamental aspects of the malignant phenotype. Genetic analyses of the normal development of the nematode Caenorhabditis elegans have revealed evolutionarily conserved mechanisms through which individual cells establish their fates, and how they make and execute the decision to survive or undergo programmed cell death. The pathways identified through these studies have mammalian counterparts that are co-opted by malignant cells. Effective cancer drugs now target some of these pathways, and more are likely to be discovered.

NUMB-ing down cancer by more than just a NOTCH

The protein Numb does not live up to its name. This passive-sounding protein is anything but spent. Originally identified as a cell-fate determinant in Drosophila development, Numb received a good deal of attention as an inhibitor of the Notch receptor signaling pathway. It turns out, however, that Numb does a lot more than simply regulate Notch. It has been implicated in a variety of biochemical pathways connected with signaling (it regulates Notch-, Hedgehog- and TP53-activated pathways), endocytosis (it is involved in cargo internalization and recycling), determination of polarity (it interacts with the PAR complex, and regulates adherens and tight junctions), and ubiquitination (it exploits this mechanism to regulate protein function and stability). This complex biochemical network lies at the heart of Numb's involvement in diverse cellular phenotypes, including cell fate developmental decisions, maintenance of stem cell compartments, regulation of cell polarity and adhesion, and migration. Considering its multifaceted role in cellular homeostasis, it is not surprising that Numb has been implicated in cancer as a tumor suppressor. Our major goal here is to explain the cancer-related role of Numb based on our understanding of its role in cell physiology. We will attempt to do this by reviewing the present knowledge of Numb at the biochemical and functional level, and by integrating its apparently heterogeneous functions into a unifying scenario, based on our recently proposed concept of the "endocytic matrix". Finally, we will discuss the role of Numb in the maintenance of the normal stem cell compartment, as a starting point to interpret the tumor suppressor function of Numb in the context of the cancer stem cell hypothesis.

Asymmetric cell division: recent developments and their implications for tumour biology

The ability of cells to divide asymmetrically is essential for generating diverse cell types during development. The past 10 years have seen tremendous progress in our understanding of this important biological process. We have learned that localized phosphorylation events are responsible for the asymmetric segregation of cell fate determinants in mitosis and that centrosomes and microtubules play important parts in this process. The relevance of asymmetric cell division for stem cell biology has added a new dimension to the field, and exciting connections between asymmetric cell division and tumorigenesis have begun to emerge.

Concise review: polarity in stem cells, disease, and aging

Adult somatic stem cells are central to homeostasis in tissues that present with a high cellular turnover like the skin, intestine, and the hematopoietic system. It is thought that polarity is particularly important with respect to fate decisions on stem cell division (symmetric or asymmetric) as well as for the maintenance of stem cell adhesion and quiescence (interaction with the niche). Consequently the failure to establish or regulate stem cell polarity might result in disease or tissue attrition. Members of the family of small RhoGTPases are known to exert an important role in regulating cell polarity. We summarize and discuss here recent views on the role of cell polarity in somatic stem cell function, aging, and disease, concluding that targeting cell polarity might be a novel approach to ameliorate or even revert aberrant somatic stem cell function. Stem Cells 2010; 28:1623–1629.

Investigating the link between epithelial-mesenchymal transition and the cancer stem cell phenotype: A mathematical approach

Under the cancer stem cell (CSC) hypothesis, sustained metastatic growth requires the dissemination of a CSC from the primary tumour followed by its re-establishment in a secondary site. The epithelial-mesenchymal transition (EMT), a differentiation process crucial to normal development, has been implicated in conferring metastatic ability on carcinomas. Balancing these two concepts has led researchers to investigate a possible link between EMT and the CSC phenotype-indeed, recent evidence indicates that, following induction of EMT in human breast cancer and related cell lines, stem cell activity increased, as judged by the presence of cells displaying the CD44(high)/CD24(low) phenotype and an increase in the ability of cells to form mammospheres. We mathematically investigate the nature of this increase in stem cell activity. A stochastic model is used when small number of cells are under consideration, namely in simulating the mammosphere assay, while a related continuous model is used to probe the dynamics of larger cell populations. Two scenarios of EMT-mediated CSC enrichment are considered. In the first, differentiated cells re-acquire a CSC phenotype-this model implicates fully mature cells as key subjects of de-differentiation and entails a delay period of several days before de-differentiation occurs. In the second, pre-existing CSCs experience accelerated division and increased proportion of self-renewing divisions; a lack of perfect CSC biomarkers and cell sorting techniques requires that this model be considered, further emphasizing the need for better characterization of the mammary (cancer) stem cell hierarchy. Additionally, we suggest the utility of comparing mammosphere data to computational mammosphere simulations in elucidating the growth characteristics of mammary (cancer) stem cells.

An RNAi screen identifies Msi2 and Prox1 as having opposite roles in the regulation of hematopoietic stem cell activity

In this study, we describe an in vivo RNA interference functional genetics approach to evaluate the role of 20 different conserved polarity factors and fate determinants in mouse hematopoietic stem cell (HSC) activity. In total, this screen revealed three enhancers and one suppressor of HSC-derived reconstitution. Pard6a, Prkcz, and Msi2 shRNA-mediated depletion significantly impaired HSC repopulation. An in vitro promotion of differentiation was observed after the silencing of these genes, consistent with their function in regulating HSC self-renewal. Conversely, Prox1 knockdown led to in vivo accumulation of primitive and differentiated cells. HSC activity was also enhanced in vitro when Prox1 levels were experimentally reduced, identifying it as a potential antagonist of self-renewal. HSC engineered to overexpress Msi2 or Prox1 showed the reverse phenotype to those transduced with corresponding shRNA vectors. Gene expression profiling studies identified a number of known HSC and cell cycle regulators as potential downstream targets to Msi2 and Prox1.

Cancer: evolutionary, genetic and epigenetic aspects

There exist two paradigms about the nature of cancer. According to the generally accepted one, cancer is a by-product of design limitations of a multi-cellular organism (Greaves, Nat Rev Cancer 7:213–221, 2007). The essence of the second resides in the question “Does cancer kill the individual and save the species?” (Sommer, Hum Mutat 3:166–169, 1994). Recent data on genetic and epigenetic mechanisms of cell transformation summarized in this review support the latter point of view, namely that carcinogenesis is an evolutionary conserved phenomenon—a programmed death of an organism. It is assumed that cancer possesses an important function of altruistic nature: as a mediator of negative selection, it serves to preserve integrity of species gene pool and to mediate its evolutionary adjustment. Cancer fulfills its task due apparently to specific killer function, understanding mechanism of which may suggest new therapeutic strategy.

Role of symmetric and asymmetric division of stem cells in developing drug resistance

Often, resistance to drugs is an obstacle to a successful treatment of cancer. In spite of the importance of the problem, the actual mechanisms that control the evolution of drug resistance are not fully understood. Many attempts to study drug resistance have been made in the mathematical modeling literature. Clearly, in order to understand drug resistance, it is imperative to have a good model of the underlying dynamics of cancer cells. One of the main ingredients that has been recently introduced into the rapidly growing pool of mathematical cancer models is stem cells. Surprisingly, this all-so-important subset of cells has not been fully integrated into existing mathematical models of drug resistance. In this work we incorporate the various possible ways in which a stem cell may divide into the study of drug resistance. We derive a previously undescribed estimate of the probability of developing drug resistance by the time a tumor is detected and calculate the expected number of resistant cancer stem cells at the time of tumor detection. To demonstrate the significance of this approach, we combine our previously undescribed mathematical estimates with clinical data that are taken from a recent six-year follow-up of patients receiving imatinib for the first-line treatment of chronic myelogenous leukemia. Based on our analysis we conclude that leukemia stem cells must tend to renew symmetrically as opposed to their healthy counterparts that predominantly divide asymmetrically.

Evidence that growth factor independence 1b regulates dormancy and peripheral blood mobilization of hematopoietic stem cells

Donor-matched transplantation of hematopoietic stem cells (HSCs) is widely used to treat hematologic malignancies but is associated with high mortality. The expansion of HSC numbers and their mobilization into the bloodstream could significantly improve therapy. We report here that adult mice conditionally deficient for the transcription Growth factor independence 1b (Gfi1b) show a significant expansion of functional HSCs in the bone marrow and blood. Despite this expansion, Gfi1b(ko/ko) HSCs retain their ability to self-renew and to initiate multilineage differentiation but are no longer quiescent and contain elevated levels of reactive oxygen species. Treatment of Gfi1b(ko/ko) mice with N-acetyl-cystein significantly reduced HSC numbers indicating that increased reactive oxygen species levels are at least partially responsible for the expansion of Gfi1b-deficient HSCs. Moreover, Gfi1b(-/-) HSCs show decreased expression of CXCR4 and Vascular cell adhesion protein-1, which are required to retain dormant HSCs in the endosteal niche, suggesting that Gfi1b regulates HSC dormancy and pool size without affecting their function. Finally, the additional deletion of the related Gfi1 gene in Gfi1b(ko/ko) HSCs is incompatible with the maintenance of HSCs, suggesting that Gfi1b and Gfi1 have partially overlapping functions but that at least one Gfi gene is essential for the generation of HSCs.

Epicardial spindle orientation controls cell entry into the myocardium

During heart morphogenesis, epicardial cells undergo an epithelial-to-mesenchymal transition (EMT) and migrate into the subepicardium. The cellular signals controlling this process are poorly understood. Here, we show that epicardial cells exhibit two distinct mitotic spindle orientations, directed either parallel or perpendicular to the basement membrane. Cells undergoing perpendicular cell division subsequently enter the myocardium. We found that loss of beta-catenin led to a disruption of adherens junctions and a randomization of mitotic spindle orientation. Loss of adherens junctions also disrupted Numb localization within epicardial cells, and disruption of Numb and Numblike expression in the epicardium led to randomized mitotic spindle orientations. Taken together, these data suggest that directed mitotic spindle orientation contributes to epicardial EMT and implicate a junctional complex of beta-catenin and Numb in the regulation of spindle orientation.

Understanding the role of asymmetric cell division in cancer using C. elegans

Asymmetric cell division is an important process to generate cell diversity and maintain tissue homeostasis. Recent evidence suggests that this process may also be crucial to prevent tumor formation. In the past 30 years, the embryo of the nematode Caenorhabditis elegans has proven to be a very powerful model to study the molecular and cellular basis of asymmetric cell division. Understanding this process in Caenorhabditis elegans may thus lead to a better understanding of stem cell function and tumorigenesis in humans.

Regulation of myeloid leukaemia by the cell-fate determinant Musashi

Chronic myelogenous leukemia (CML) can progress from an indolent chronic phase to an aggressive blast crisis phase1 but the molecular basis of this transition remains poorly understood. Here we have used mouse models of CML2,3 to show that disease progression is regulated by the Musashi-Numb signaling axis4,5. Specifically, we find that chronic phase is marked by high and blast crisis phase by low levels of Numb expression, and that ectopic expression of Numb promotes differentiation and impairs advanced phase disease in vivo. As a possible explanation for the decreased levels of Numb in blast crisis, we show that NUP98-HOXA9, an oncogene associated with blast crisis CML6,7, can trigger expression of the RNA binding protein Musashi2 (Msi2) which in turn represses Numb. Importantly, loss of Msi2 restores Numb expression and significantly impairs the development and propagation of blast crisis CML in vitro and in vivo. Finally, we show that Msi2 expression is not only highly upregulated during human CML progression but is also an early indicator of poorer prognosis. These data show that the Musashi-Numb pathway can control the differentiation of CML cells, and raise the possibility that targeting this pathway may provide a new strategy for therapy of aggressive leukemias.

Musashi-2 regulates normal hematopoiesis and promotes aggressive myeloid leukemia

RNA-binding proteins of the Musashi (Msi) family are expressed in stem cell compartments and in aggressive tumors, but they have not yet been widely explored in the blood. Here we demonstrate that Msi2 is the predominant form expressed in hematopoietic stem cells (HSCs), and its knockdown leads to reduced engraftment and depletion of HSCs in vivo. Overexpression of human MSI2 in a mouse model increases HSC cell cycle progression and cooperates with the chronic myeloid leukemia-associated BCR-ABL1 oncoprotein to induce an aggressive leukemia. MSI2 is overexpressed in human myeloid leukemia cell lines, and its depletion leads to decreased proliferation and increased apoptosis. Expression levels in human myeloid leukemia directly correlate with decreased survival in patients with the disease, thereby defining MSI2 expression as a new prognostic marker and as a new target for therapy in acute myeloid leukemia (AML).

Non-canonical Wnt signaling pathways in hematopoiesis

Hematopoietic stem cells (HSCs) are a rare population of cells that are responsible for life-long generation of blood cells of all lineages. In order to maintain their numbers, HSCs must establish a balance between the opposing cell fates of self-renewal and initiation of hematopoietic differentiation. Multiple signaling pathways have been implicated in the regulation of HSC cell fate. One such set of pathways are those activated by the Wnt family of ligands. The function of the canonical Wnt signaling pathway, which utilizes beta-catenin to regulate gene expression, has been extensively studied in hematopoiesis. However, there is a growing body of evidence that the other Wnt signaling pathways, termed non-canonical, also play an important role. In this review, we will discuss the regulation of hematopoiesis by the Wnt signaling pathways, focusing on the potential functions of non-canonical Wnt signaling pathways.

Radiation resistance of cancer stem cells: the 4 R's of radiobiology revisited

There is compelling evidence that many solid cancers are organized hierarchically and contain a small population of cancer stem cells (CSCs). It seems reasonable to suggest that a cancer cure can be achieved only if this population is eliminated. Unfortunately, there is growing evidence that CSCs are inherently resistant to radiation, and perhaps other cancer therapies. In general, success or failure of standard clinical radiation treatment is determined by the 4 R's of radiobiology: repair of DNA damage, redistribution of cells in the cell cycle, repopulation, and reoxygenation of hypoxic tumor areas. We relate recent findings on CSCs to these four phenomena and discuss possible consequences.

[Mechanisms of asymmetric cell division: from model organisms to tumorigenesis]

Asymmetric cell division is the process by which a single cell gives rise to two different daughter cells. This process is important to generate cell diversity during the development of multicellular organisms, as well as for stem cell self-renewal in adults. Current knowledge on so-called cancer stem cells suggests that a loss of asymmetry during their division could lead to overproliferation and favour tumorigenesis, highlighting the importance of deciphering the mechanisms governing asymmetric cell division. Two mechanisms can lead to an asymmetric cell division: asymmetry can either be governed by proximity to a given cellular environment (or niche), in which case the mechanism is referred to as extrinsic, or the mother cell polarizes itself without external intervention, in which case the mechanism is referred to as intrinsic. In the last 20 years, our understanding of intrinsic mechanisms leading to asymmetric cell division has progressed, largely after studies carried out in model organisms such as the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. These models allowed the identification of molecular complexes used by nearly all the cells that divide asymmetrically, including human cells. Here we review the main intrinsic mechanisms of asymmetric cell division as described in model organisms and discuss their relevance towards mammalian tumorigenesis.

Endothelial cells are essential for the self-renewal and repopulation of Notch-dependent hematopoietic stem cells

Bone marrow endothelial cells (ECs) are essential for reconstitution of hematopoiesis, but their role in self-renewal of long-term hematopoietic stem cells (LT-HSCs) is unknown. We have developed angiogenic models to demonstrate that EC-derived angiocrine growth factors support in vitro self-renewal and in vivo repopulation of authentic LT-HSCs. In serum/cytokine-free cocultures, ECs, through direct cellular contact, stimulated incremental expansion of repopulating CD34(-)Flt3(-)cKit(+)Lineage(-)Sca1(+) LT-HSCs, which retained their self-renewal ability, as determined by single-cell and serial transplantation assays. Angiocrine expression of Notch ligands by ECs promoted proliferation and prevented exhaustion of LT-HSCs derived from wild-type, but not Notch1/Notch2-deficient, mice. In transgenic notch-reporter (TNR.Gfp) mice, regenerating TNR.Gfp(+) LT-HSCs were detected in cellular contact with sinusoidal ECs. Interference with angiocrine, but not perfusion, function of SECs impaired repopulation of TNR.Gfp(+) LT-HSCs. ECs establish an instructive vascular niche for clinical-scale expansion of LT-HSCs and a cellular platform to identify stem cell-active trophogens.

Notch signaling and the bone marrow hematopoietic stem cell niche

Recently there has been increased interest in the regulatory interactions between osteoblasts and cells in the surrounding bone marrow microenvironment. The proximity of hematopoietic stem cells (HSCs) with osteoblastic cells first suggested regulatory interactions, and recent data have highlighted the role of osteoblastic cells in providing a HSC niche. Reports have indicated that direct contact is necessary to mediate the osteoblastic effects and that these effects could be mediated through Notch activation. Notch signaling is important throughout development and also appears to play a critical role in cellular maturation and differentiation of osteoblastic cells and hematopoietic cells as disregulation can lead to bone loss and leukemias, respectively. In this review we discuss the current understanding of Notch signaling and how it functions in hematopoiesis, osteoblastic cells, and the interactions between HSC and their osteoblastic niche.

BCR-ABL enhances differentiation of long-term repopulating hematopoietic stem cells

In a previously developed inducible transgenic mouse model of chronic myeloid leukemia, we now demonstrate that the disease is transplantable using BCR-ABL(+) Lin(-)Sca-1(+)c-kit(+) (LSK) cells. Interestingly, the phenotype is more severe when unfractionated bone marrow cells are transplanted, yet neither progenitor cells (Lin(-)Sca-1(-)c-kit(+)), nor mature granulocytes (CD11b(+)Gr-1(+)), nor potential stem cell niche cells (CD45(-)Ter119(-)) are able to transmit the disease or alter the phenotype. The phenotype is largely independent of BCR-ABL priming before transplantation. However, prolonged BCR-ABL expression abrogates the potential of LSK cells to induce full-blown disease in secondary recipients and increases the fraction of multipotent progenitor cells at the expense of long-term hematopoietic stem cells (LT-HSCs) in the bone marrow. BCR-ABL alters the expression of genes involved in proliferation, survival, and hematopoietic development, probably contributing to the reduced LT-HSC frequency within BCR-ABL(+) LSK cells. Reversion of BCR-ABL, or treatment with imatinib, eradicates mature cells, whereas leukemic stem cells persist, giving rise to relapsed chronic myeloid leukemia on reinduction of BCR-ABL, or imatinib withdrawal. Our results suggest that BCR-ABL induces differentiation of LT-HSCs and decreases their self-renewal capacity.

Asymmetric cell divisions and asymmetric cell fates

The regulation of self-renewal, cell diversity, and differentiation can occur by modulating symmetric and asymmetric cell divisions. Remarkably, asymmetric cell divisions can arise through multiple processes in which molecules in the cytoplasm and nucleus, as well as template "immortal" DNA strands, can segregate to one daughter cell during cell division. Explaining how these events direct distinct daughter cell fates is a major challenge to understanding how the organism is assembled and maintained for a lifetime. Numerous technical issues that are associated with assessing how distinct cell fates are executed in vivo have resulted in divergent interpretations of experimental findings. This review addresses some of these points and considers different developmental model systems that attempt to investigate how cell fate decisions are determined, as well as the molecules that guide these choices.

APC, T cells, and the immune synapse

CD4(+) T cells engage different activating cells during their generation in the bone marrow and thymus and during their homeostasis and activation in the periphery. During these processes, T cells or their precursors establish a molecular platform for communication in the interface between the two cells that is called immune synapse (IS). Here we review the current knowledge about those different IS. Apart from looking at the structure and signalling of the IS from the T cell region, we will also focus on the area of the IS partner, mostly antigen-presenting cells (APC). We will discuss the features of different APC and their role played in the control of the resulting activated or differentiated T cell. We will also demonstrate that despite 10 years of research into the subject, large areas of this field are yet to be explored. This will keep us busy for the years to come - new exciting results lie ahead of us.

Umbilical cord transplantation: epilogue

The field of cord blood transplantation has come a long way since the first transplant more than 20 years ago. Advancements in the field will require continuing efforts to better understand hematopoietic stem and progenitor cell function and engraftment. Cautious optimism is inherent in the potential relevance and applicability of nonhematopoietic stem and progenitor cell types found in cord blood, and induced pluripotent stem cells generated from cord blood cells. Rigorous investigations and close interactions between scientific and clinical investigators are required to translate human in vitro and animal in vivo findings into clinical utility.

Notch signaling in the regulation of stem cell self-renewal and differentiation

Stem cells are rare and unique precursor cells that participate in the building and rebuilding of tissues and organs during embryogenesis, postnatal growth, and injury repair. Stem cells are distinctively endowed with the ability to both self-renew and differentiate, such that they can replenish the stem cell pool while continuing to produce the differentiated daughter cells that are essential for tissue function. Stem cell self-renewal/differentiation decisions must be carefully controlled during organogenesis, tissue homeostasis, and regeneration, as failure in stem cell maintenance or activation can lead to progressive tissue wasting, while unchecked self-renewal is a hallmark of many cancers. Here, we review evidence implicating the Notch signaling pathway, an evolutionarily conserved cell fate determinant with widespread roles in a variety of tissues and organisms, as a crucial regulator of stem cell behavior. As discussed below, this pathway plays varied and critical roles at multiple stages of organismal development, in lineage-specific differentiation of pluripotent embryonic stem cells, and in controlling stem cell numbers and activity in the context of age-related tissue degeneration, injury-induced tissue repair, and malignancy.

Dividing cellular asymmetry: asymmetric cell division and its implications for stem cells and cancer

Cell division is commonly thought to involve the equal distribution of cellular components into the two daughter cells. During many cell divisions, however, proteins, membrane compartments, organelles, or even DNA are asymmetrically distributed between the two daughter cells. Here, we review the various types of asymmetries that have been described in yeast and in animal cells. Asymmetric segregation of protein determinants is particularly relevant for stem cell biology. We summarize the relevance of asymmetric cell divisions in various stem cell systems and discuss why defects in asymmetric cell division can lead to the formation of tumors.

Regulation of stemness and stem cell niche of mesenchymal stem cells: implications in tumorigenesis and metastasis

Human mesenchymal stem cells (MSCs) derived from adult tissues have been considered a candidate cell type for cell-based tissue engineering and regenerative medicine. These multipotent cells have the ability to differentiate along several mesenchymal lineages and possibly along non-mesenchymal lineages. MSCs possess considerable immunosuppressive properties that can influence the surrounding tissue positively during regeneration, but perhaps negatively towards the pathogenesis of cancer and metastasis. The balance between the naïve stem state and differentiation is highly dependent on the stem cell niche. Identification of stem cell niche components has helped to elucidate the mechanisms of stem cell maintenance and differentiation. Ultimately, the fate of stem cells is dictated by their microenvironment. In this review, we describe the identification and characterization of bone marrow-derived MSCs, the properties of the bone marrow stem cell niche, and the possibility and likelihood of MSC involvement in cancer progression and metastasis.

Differential destruction of stem cells: implications for targeted cancer stem cell therapy

Cancer stem cells represent a novel therapeutic target. The major challenge in targeting leukemic stem cells (LSC) is finding therapies that largely spare normal hematopoietic stem cells (HSC) while eradicating quiescent LSCs. We present a mathematical model to predict how selective a therapy must be to ensure that enough HSCs survive when LSCs have been eradicated. Stem cell population size is modeled as a birth-death process. This permits comparison of LSC and HSC eradication times under therapy and calculation of the number of HSCs at the time of LSC eradication for varied initial population sizes and stem cell death rates. We further investigate the effects of LSC quiescence and resistance mutations on our predictions. From a clinical point of view, our models suggest criteria by which cancer stem cell therapy safety can be assessed. We anticipate that in conjunction with experimental observation of cancer stem cell killing rates, our results will be useful in screening targeted therapies for both hematologic and solid tumor malignancies.

BAYESIAN INFERENCE AND MODEL CHOICE IN A HIDDEN STOCHASTIC TWO-COMPARTMENT MODEL OF HEMATOPOIETIC STEM CELL FATE DECISIONS

Despite rapid advances in experimental cell biology, the in vivo behavior of hematopoietic stem cells (HSC) cannot be directly observed and measured. Previously we modeled feline hematopoiesis using a two-compartment hidden Markov process that had birth and emigration events in the first compartment. Here we perform Bayesian statistical inference on models which contain two additional events in the first compartment in order to determine if HSC fate decisions are linked to cell division or occur independently. Pareto Optimal Model Assessment approach is used to cross check the estimates from Bayesian inference. Our results show that HSC must divide symmetrically (i.e., produce two HSC daughter cells) in order to maintain hematopoiesis. We then demonstrate that the augmented model that adds asymmetric division events provides a better fit to the competitive transplantation data, and we thus provide evidence that HSC fate determination in vivo occurs both in association with cell division and at a separate point in time. Last we show that assuming each cat has a unique set of parameters leads to either a significant decrease or a nonsignificant increase in model fit, suggesting that the kinetic parameters for HSC are not unique attributes of individual animals, but shared within a species.

Vascular niche factor PEDF modulates Notch-dependent stemness in the adult subependymal zone

We sought to address the fundamental question of how stem cell microenvironments can regulate self-renewal. We found that Notch was active in astroglia-like neural stem cells (NSCs), but not in transit-amplifying progenitors of the murine subependymal zone, and that the level of Notch transcriptional activity correlated with self-renewal and multipotency. Moreover, dividing NSCs appeared to balance renewal with commitment via controlled segregation of Notch activity, leading to biased expression of known (Hes1) and previously unknown (Egfr) Notch target genes in daughter cells. Pigment epithelium-derived factor (PEDF) enhanced Notch-dependent transcription in cells with low Notch signaling, thereby subverting the output of an asymmetrical division to the production of two highly self-renewing cells. Mechanistically, PEDF induced a non-canonical activation of the NF-kappaB pathway, leading to the dismissal of the transcriptional co-repressor N-CoR from specific Notch-responsive promoters. Our data provide a basis for stemness regulation in vascular niches and indicate that Notch and PEDF cooperate to regulate self-renewal.

Tumour-initiating cells: challenges and opportunities for anticancer drug discovery

The hypothesis that cancer is driven by tumour-initiating cells (popularly known as cancer stem cells) has recently attracted a great deal of attention, owing to the promise of a novel cellular target for the treatment of haematopoietic and solid malignancies. Furthermore, it seems that tumour-initiating cells might be resistant to many conventional cancer therapies, which might explain the limitations of these agents in curing human malignancies. Although much work is still needed to identify and characterize tumour-initiating cells, efforts are now being directed towards identifying therapeutic strategies that could target these cells. This Review considers recent advances in the cancer stem cell field, focusing on the challenges and opportunities for anticancer drug discovery.

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.

A progenitor cell origin of myeloid malignancies

All cancers rely on cells that have properties of long-term self-renewal or "stemness" to maintain and propagate the tumor, but the cell of origin of most cancers is still unknown. Here, we design a stochastic mathematical model of hematopoietic stem and progenitor cells to study the evolutionary dynamics of cancer initiation. We consider different evolutionary pathways leading to cancer-initiating cells in JAK2V617F-positive myeloproliferative neoplasms (MPN): (i) the JAK2V617F mutation may arise in a stem cell; (ii) a progenitor cell may first acquire a mutation conferring self-renewal, followed by acquisition of the JAK2V617F mutation; (iii) the JAK2V617F mutation may first emerge in a progenitor cell, followed by a mutation conferring self-renewal; and (iv) a mutation conferring self-renewal to progenitors may arise in the stem cell population without causing a change in the stem cell's phenotype, followed by the JAK2V617F mutation emerging in a progenitor cell. We find mathematical evidence that a progenitor is the most likely cell of origin of JAK2V617F-mutant MPN. These results may also have relevance to other tumor types arising in tissues that are organized as a differentiation hierarchy.

MafB restricts M-CSF-dependent myeloid commitment divisions of hematopoietic stem cells

While hematopoietic stem cell (HSC) self-renewal is well studied, it remains unknown whether distinct control mechanisms enable HSC divisions that generate progeny cells with specific lineage bias. Here, we report that the monocytic transcription factor MafB specifically restricts the ability of M-CSF to instruct myeloid commitment divisions in HSCs. MafB deficiency specifically enhanced sensitivity to M-CSF and caused activation of the myeloid master-regulator PU.1 in HSCs in vivo. Single-cell analysis revealed that reduced MafB levels enabled M-CSF to instruct divisions producing asymmetric daughter pairs with one PU.1(+) cell. As a consequence, MafB(-/-) HSCs showed a PU.1 and M-CSF receptor-dependent competitive repopulation advantage specifically in the myelomonocytic, but not T lymphoid or erythroid, compartment. Lineage-biased repopulation advantage was progressive, maintained long term, and serially transplantable. Together, this indicates that an integrated transcription factor/cytokine circuit can control the rate of specific HSC commitment divisions without compromising other lineages or self-renewal.

Cancer stem cells: a guide for skeptics

Cancer stem cells (CSC) were postulated to exist many years ago as cells within a tumor that regenerate the tumor following treatment. A stochastic clonal evolution model was used to explain observed tumor heterogeneity. Recently, xenotransplantation studies have demonstrated that prospectively identifiable subpopulations from human cancers can initiate tumors in immune deficient mice, and these results along with recent advances in stem cell biology have generated much excitement in the cancer field. The modern CSC theory posits a hierarchy of cells analogous to normal stem cell development. Some controversy remains, however, as to whether these tumor initiating cells truly represent CSC, and whether the modern CSC field can live up to the promise of providing improved cancer treatments based on a novel model of cancer biology. Recent data from CSC investigators are discussed critically.

The leukemic stem cell niche: current concepts and therapeutic opportunities

The genetic events that contribute to the pathogenesis of acute myeloid leukemia are among the best characterized of all human malignancies. However, with notable exceptions such as acute promyelocytic leukemia, significant improvements in outcome based on these insights have not been forthcoming. Acute myeloid leukemia is a paradigm of cancer stem (or leukemia initiating) cells with hierarchy analogous to that seen in hematopoiesis. Normal hematopoiesis requires complex bidirectional interactions between the bone marrow microenvironment (or niche) and hematopoietic stem cells (HSCs). These interactions are critical for the maintenance of normal HSC quiescence and perturbations can influence HSC self-renewal. Leukemia stem cells (LSCs), which also possess limitless self-renewal, may hijack these homeostatic mechanisms, take refuge within the sanctuary of the niche during chemotherapy, and consequently contribute to eventual disease relapse. We will discuss the emerging evidence supporting the importance of the bone marrow microenvironment in LSC survival and consider the physiologic interactions of HSCs and the niche that inform our understanding of microenvironment support of LSCs. Finally, we will discuss approaches for the rational development of therapies that target the microenvironment.

Hedgehog signalling is essential for maintenance of cancer stem cells in myeloid leukaemia

Although the role of Hedgehog (Hh) signalling in embryonic pattern formation is well established, its functions in adult tissue renewal and maintenance remain unclear, and the relationship of these functions to cancer development has not been determined. Here we show that the loss of Smoothened (Smo), an essential component of the Hh pathway, impairs haematopoietic stem cell renewal and decreases induction of chronic myelogenous leukaemia (CML) by the BCR-ABL1 oncoprotein. Loss of Smo causes depletion of CML stem cells--the cells that propagate the leukaemia--whereas constitutively active Smo augments CML stem cell number and accelerates disease. As a possible mechanism for Smo action, we show that the cell fate determinant Numb, which depletes CML stem cells, is increased in the absence of Smo activity. Furthermore, pharmacological inhibition of Hh signalling impairs not only the propagation of CML driven by wild-type BCR-ABL1, but also the growth of imatinib-resistant mouse and human CML. These data indicate that Hh pathway activity is required for maintenance of normal and neoplastic stem cells of the haematopoietic system and raise the possibility that the drug resistance and disease recurrence associated with imatinib treatment of CML might be avoided by targeting this essential stem cell maintenance pathway.

Cancer stem cells in brain tumor biology

Tumors are aberrant organ systems containing a complex interplay between the neoplastic compartment and recruited vascular, inflammatory, and stromal elements. Furthermore, most cancers display a hierarchy of differentiation states within the tumor cell population. Molecular signals that drive tumor formation and maintenance commonly overlap with those involved in normal development and wound responses--two processes in which normal stem cells function. It is therefore not surprising that cancers invoke stem cell programs that promote tumor malignancy. Stem-cell-like cancer cells (or cancer stem cells) need not be derived from normal stem cells but may be subjected to evolutionary pressures that select for the capacity to self-renew extensively or differentiate depending on conditions. Current cancer model systems may not fully recapitulate the cellular complexity of cancers, perhaps partially explaining the lack of power of these models in predicting clinical outcomes. New methods are enabling researchers to identify and characterize cancer stem cells. Our laboratory focuses on the roles of brain tumor stem cells in clinically relevant tumor biology, including therapeutic resistance, angiogenesis, and invasion/metastasis. We hope that these studies will translate into improved diagnostic, prognostic, and therapeutic approaches for these lethal cancers.