Rb regulates interactions between hematopoietic stem cells and their bone marrow microenvironment

Getting to the stem of chronic myeloid leukaemia

Tyrosine kinase inhibitor (TKI) therapy for chronic myeloid leukaemia (CML) is the consummate success story for targeted therapy, yet relapse is a nearly inevitable consequence of cessation or interruption of therapy. Primitive TKI-refractory CML stem cells are the likely source of these relapses, as they provide sanctuary for the Philadelphia chromosome. In advanced disease, their progressively anaplastic progeny ultimately maintain CML independently of the CML haematopoietic stem cell (HSC). Interestingly, there are at least two distinct cell types capable of self-renewal in different phases of CML: first, a primitive HSC with BCR-ABL mutation, which maintains the more indolent chronic-phase disease and, second, a coexisting mutated progenitor cell which acquires stem cell characteristics responsible for rapid cell expansion in advanced disease.

Stem cells down under-ISSCR 2007

Cairns, Queensland, Australia provided the venue for the 5th annual meeting of the International Society for Stem Cell Research (ISSCR), June 17-20, 2007. Consonant with the young society's mission to serve the international stem cell community, this meeting was the first held outside North America. The meeting drew attendees from 44 countries, with excellent representation from the Asia/Pacific region. The more than 120 presentations and 1000 posters covered virtually all aspects of the stem cell field and provided a snapshot of current areas of investigation. Here we review some of the newest findings in an effort to convey the energy and excitement of the meeting and the tempo of stem cell science.

BMP signaling in dermal papilla cells is required for their hair follicle-inductive properties

Hair follicle (HF) formation is initiated when epithelial stem cells receive cues from specialized mesenchymal dermal papilla (DP) cells. In culture, DP cells lose their HF-inducing properties, but during hair growth in vivo, they reside within the HF bulb and instruct surrounding epithelial progenitors to orchestrate the complex hair differentiation program. To gain insights into the molecular program that maintains DP cell fate, we previously purified DP cells and four neighboring populations and defined their cell-type-specific molecular signatures. Here, we exploit this information to show that the bulb microenvironment is rich in bone morphogenetic proteins (BMPs) that act on DP cells to maintain key signature features in vitro and hair-inducing activity in vivo. By employing a novel in vitro/in vivo hybrid knockout assay, we ablate BMP receptor 1a in purified DP cells. When DPs cannot receive BMP signals, they lose signature characteristics in vitro and fail to generate HFs when engrafted with epithelial stem cells in vivo. These results reveal that BMP signaling, in addition to its key role in epithelial stem cell maintenance and progenitor cell differentiation, is essential for DP cell function, and suggest that it is a critical feature of the complex epithelial-mesenchymal cross-talk necessary to make hair.

Deficiency of Src family kinases compromises the repopulating ability of hematopoietic stem cells

OBJECTIVE

Src family kinases (SFK) have been implicated in regulating growth factor and integrin-induced proliferation, migration, and gene expression in multiple cell types. However, little is known about the role of these kinases in the growth, homing, and engraftment potential of hematopoietic stem and progenitor cells.

RESULTS

Here we show that loss of hematopoietic-specific SFKs Hck, Fgr, and Lyn results in increased number of Sca-1(+)Lin(-) cells in the bone marrow, which respond differentially to cytokine-induced growth in vitro and manifest a significant defect in the long-term repopulating potential in vivo. Interestingly, a significant increase in expression of adhesion molecules, known to coincide with the homing potential of wild-type bone marrow cells is also observed on the surface of SFK(-/-) cells, although, this increase did not affect the homing potential of more primitive Lin(-)Sca-1(+) SFK(-/-) cells. The stem cell-repopulating defect observed in mice transplanted with SFK(-/-) bone marrow cells is due to the loss of Lyn Src kinase, because deficiency of Lyn, but not Hck or Fgr, recapitulated the long-term stem cell defect observed in mice transplanted with SFK(-/-) bone marrow cells.

CONCLUSIONS

Taken together, our results demonstrate an essential role for Lyn kinase in positively regulating the long-term and multilineage engraftment of stem cells, which is distinct from its role in mature B cells and myeloid cells.

Stem cells and niches: mechanisms that promote stem cell maintenance throughout life

Niches are local tissue microenvironments that maintain and regulate stem cells. Long-predicted from mammalian studies, these structures have recently been characterized within several invertebrate tissues using methods that reliably identify individual stem cells and their functional requirements. Although similar single-cell resolution has usually not been achieved in mammalian tissues, principles likely to govern the behavior of niches in diverse organisms are emerging. Considerable progress has been made in elucidating how the microenvironment promotes stem cell maintenance. Mechanisms of stem cell maintenance are key to the regulation of homeostasis and likely contribute to aging and tumorigenesis when altered during adulthood.

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.

Rb intrinsically promotes erythropoiesis by coupling cell cycle exit with mitochondrial biogenesis

Regulation of the cell cycle is intimately linked to erythroid differentiation, yet how these processes are coupled is not well understood. To gain insight into this coordinate regulation, we examined the role that the retinoblastoma protein (Rb), a central regulator of the cell cycle, plays in erythropoiesis. We found that Rb serves a cell-intrinsic role and its absence causes ineffective erythropoiesis, with a differentiation block at the transition from early to late erythroblasts. Unexpectedly, in addition to a failure to properly exit the cell cycle, mitochondrial biogenesis fails to be up-regulated concomitantly, contributing to this differentiation block. The link between erythropoiesis and mitochondrial function was validated by inhibition of mitochondrial biogenesis. Erythropoiesis in the absence of Rb resembles the human myelodysplastic syndromes, where defects in cell cycle regulation and mitochondrial function frequently occur. Our work demonstrates how these seemingly disparate pathways play a role in coordinately regulating cellular differentiation.

The hematopoietic stem cell and its niche: a comparative view

Stem cells have been identified as a source of virtually all highly differentiated cells that are replenished during the lifetime of an animal. The critical balance between stem and differentiated cell populations is crucial for the long-term maintenance of functional tissue types. Stem cells maintain this balance by choosing one of several alternate fates: self-renewal, commitment to differentiate, and senescence or cell death. These characteristics comprise the core criteria by which these cells are usually defined. The self-renewal property is important, as it allows for extended production of the corresponding differentiated cells throughout the life span of the animal. A microenvironment that is supportive of stem cells is commonly referred to as a stem cell niche. In this review, we first present some general concepts regarding stem cells and their niches, comparing stem cells of many different kinds from diverse organisms, and in the second part, we compare specific aspects of hematopoiesis and the niches that support hematopoiesis in Drosophila, zebrafish and mouse.

Cancer stem cells: the lessons from pre-cancerous stem cells

How a cancer is initiated and established remains elusive despite all the advances in decades of cancer research. Recently the cancer stem cell (CSC) hypothesis has been revived, challenging the long-standing model of "clonal evolution" for cancer development and implicating the dawning of a potential cure for cancer [1]. The recent identification of precancerous stem cells (pCSCs) in cancer, an early stage of CSC development, however, implicates that the "clonal evolution" is not contradictory to the CSC hypothesis, but is rather an aspect of the process of CSC development [2]. The discovery of pCSC has revealed and will continue to reveal the volatile properties of CSC with respects to their phenotype, differentiation and tumorigenic capacity during initiation and progression. Both pCSC and CSC might also serve as precursors of tumor stromal components such as tumor vasculogenic stem/progenitor cells (TVPCs). Thus, the CSC hypothesis covers the developing process of tumor-initiating cells (TIC) --> pCSC --> CSC --> cancer, a cellular process that should parallel the histological process of hyperplasia/metaplasia (TIC) --> precancerous lesions (pCSC) --> malignant lesions (CSC --> cancer). The embryonic stem (ES) cell and germline stem (GS) cell genes are subverted in pCSCs. Especially the GS cell protein piwil2 may play an important role during the development of TIC --> pCSC --> CSC, and this protein may be used as a common biomarker for early detection, prevention, and treatment of cancer. As cancer stem cell research is yet in its infancy, definitive conclusions regarding the role of pCSC can not be made at this time. However this review will discuss what we have learned from pCSC and how this has led to innovative ideas that may eventually have major impacts on the understanding and treatment of cancer.

The retinoblastoma tumor suppressor is a critical intrinsic regulator for hematopoietic stem and progenitor cells under stress

The retinoblastoma tumor suppressor protein (RB) plays important roles in the control of the cell division cycle. It is estimated that RB is dysfunctional/inactivated in up to 40% of human leukemias. The consequences of loss of RB on hematopoietic stem and progenitor cell (HSPC) function in vivo are incompletely understood. Here, we report that mice genetically deficient in Rb in all hematopoietic cells (Vav-Cre Rb knockout [KO] animals) showed altered contribution of distinct hematopoietic cell lineages to peripheral blood, bone marrow, and spleen; significantly increased extramedullary hematopoiesis in the spleen; and a 2-fold increase in the frequency of hematopoietic progenitor cells in peripheral blood. Upon competitive transplantation, HSPCs from Vav-Cre Rb KO mice contributed with an at least 4- to 6-fold less efficiency to hematopoiesis compared with control cells. HSPCs deficient in Rb presented with impaired cell-cycle exit upon stress-induced proliferation, which correlated with impaired function. In summary, Rb is critical for hematopoietic stem and progenitor cell function, localization, and differentiation.

Dendritic cells at the interface of innate and acquired immunity: the role for epigenetic changes

Dendritic cells (DC) are known to be essential immune cells in innate immunity and in the initiation of adaptive immunity. The shaping of adaptive immunity by innate immunity is dependent on DC unique cellular functions and DC-derived effector molecules such as cytokines and chemokines. Thus, it is not surprising that numerous studies have identified alterations in DC number, function, and subset ratios in various diseases, such as infections, cancers, and autoimmune diseases. Recent evidence has also identified that immunosuppression occurring after severe systemic inflammation, such as found in sepsis, is a result of depletion in DC numbers and a later dysfunction in DC activity. This correlation suggests that the sustained DC dysfunction initiated by life-threatening inflammation may contribute to the subsequent immunoparalysis, potentially as a result of the long-term maintenance of an abnormal gene expression pattern. In this review, we summarized the present information regarding altered DC function after a severe, acute inflammatory response and propose a mechanism, whereby epigenetic changes can influence long-term gene expression patterns by DC, thus supporting an immunosuppression phenotype.

Signaling pathways governing stem-cell fate

Hematopoietic stem cells (HSCs) are historically the most thoroughly characterized type of adult stem cell, and the hematopoietic system has served as a principal model structure of stem-cell biology for several decades. However, paradoxically, although HSCs can be defined by function and even purified to near-homogeneity, the intricate molecular machinery and the signaling mechanisms regulating fate events, such as self-renewal and differentiation, have remained elusive. Recently, several developmentally conserved signaling pathways have emerged as important control devices of HSC fate, including Notch, Wingless-type (Wnt), Sonic hedgehog (Shh), and Smad pathways. HSCs reside in a complex environment in the bone marrow, providing a niche that optimally balances signals that control self-renewal and differentiation. These signaling circuits provide a valuable structure for our understanding of how HSC regulation occurs, concomitantly with providing information of how the bone marrow microenvironment couples and integrates extrinsic with intrinsic HSC fate determinants. It is the focus of this review to highlight some of the most recent developments concerning signaling pathways governing HSC fate.

Making a tumour's bed: glioblastoma stem cells and the vascular niche

Parallel to the role that normal stem cells play in organogenesis, cancer stem cells are thought to be crucial for tumorigenesis. Understanding normal development might therefore lead to better treatments of cancer. We review recent data that stem cells of glioblastoma, a highly malignant brain tumour, seem to be dependent on cues from aberrant vascular niches that mimic the normal neural stem cell niche. These data have direct implications for cancer, highlighting the similarity between normal and malignant stem cells and identifying the tumour microenvironment as a target for new therapies.

A microenvironment-induced myeloproliferative syndrome caused by retinoic acid receptor gamma deficiency

Myeloproliferative syndromes (MPS) are a heterogeneous subclass of nonlymphoid hematopoietic neoplasms which are considered to be intrinsic to hematopoietic cells. The causes of MPS are largely unknown. Here, we demonstrate that mice deficient for retinoic acid receptor gamma (RARgamma), develop MPS induced solely by the RARgamma-deficient microenvironment. RARgamma(-/-) mice had significantly increased granulocyte/macrophage progenitors and granulocytes in bone marrow (BM), peripheral blood, and spleen. The MPS phenotype continued for the lifespan of the mice and was more pronounced in older mice. Unexpectedly, transplant studies revealed this disease was not intrinsic to the hematopoietic cells. BM from wild-type mice transplanted into mice with an RARgamma(-/-) microenvironment rapidly developed the MPS, which was partially caused by significantly elevated TNFalpha in RARgamma(-/-) mice. These data show that loss of RARgamma results in a nonhematopoietic cell-intrinsic MPS, revealing the capability of the microenvironment to be the sole cause of hematopoietic disorders.