Osteoblastic N-cadherin is not required for microenvironmental support and regulation of hematopoietic stem and progenitor cells

Building bones for blood and beyond: the growing field of bone marrow niche model development

The bone marrow (BM) niche is a complex microenvironment that provides the signals required for regulation of hematopoietic stem cells (HSCs) and the process of hematopoiesis they are responsible for. Bioengineered models of the BM niche incorporate various elements of the in vivo BM microenvironment, including cellular components, soluble factors, a three-dimensional environment, mechanical stimulation of included cells, and perfusion. Recent advances in the bioengineering field have resulted in a spate of new models that shed light on BM function and are approaching precise imitation of the BM niche. These models promise to improve our understanding of the in vivo microenvironment in health and disease. They also aim to serve as platforms for HSC manipulation or as preclinical models for screening novel therapies for BM-associated disorders and diseases.

Osteoblast Lineage Support of Hematopoiesis in Health and Disease

In mammals, hematopoiesis migrates to the bone marrow during embryogenesis coincident with the appearance of mineralized bone, where hematopoietic stem cells (HSCs) and their progeny are maintained by the surrounding microenvironment or niche, and sustain the entirety of the hematopoietic system. Genetic manipulation of niche factors and advances in cell lineage tracing techniques have implicated cells of both hematopoietic and nonhematopoietic origin as important regulators of hematopoiesis in health and disease. Among them, cells of the osteoblast lineage, from stromal skeletal stem cells to matrix-embedded osteocytes, are vital niche residents with varying capacities for hematopoietic support depending on stage of differentiation. Here, we review populations of osteoblasts at differing stages of differentiation and summarize the current understanding of the role of the osteoblast lineage in supporting hematopoiesis. © 2022 American Society for Bone and Mineral Research (ASBMR).

Peeling Back the Layers of Lymph Gland Structure and Regulation

During the past 60 years, the fruit fly, Drosophila melanogaster, has proven to be an excellent model to study the regulation of hematopoiesis. This is not only due to the evolutionarily conserved signalling pathways and transcription factors contributing to blood cell fate, but also to convergent evolution that led to functional similarities in distinct species. An example of convergence is the compartmentalization of blood cells, which ensures the quiescence of hematopoietic stem cells and allows for the rapid reaction of the immune system upon challenges. The lymph gland, a widely studied hematopoietic organ of the Drosophila larva, represents a microenvironment with similar features and functions to classical hematopoietic stem cell niches of vertebrates. Lymph gland studies were effectively supported by the unparalleled toolkit developed in Drosophila, which enabled the high-resolution investigation of the cellular composition and regulatory interaction networks of the lymph gland. In this review, we summarize how our understanding of lymph gland structure and hematopoietic cell-to-cell communication evolved during the past decades and compare their analogous features to those of the vertebrate hematopoietic stem cell niche.

Recent advances in "sickle and niche" research - Tribute to Dr. Paul S Frenette

In this retrospective, we review the two research topics that formed the basis of the outstanding career of Dr. Paul S. Frenette. In the first part, we focus on sickle cell disease (SCD). The defining feature of SCD is polymerization of the deoxygenated mutant hemoglobin, which leads to a vicious cycle of hemolysis and vaso-occlusion. We survey important discoveries in SCD pathophysiology that have led to recent advances in treatment of SCD. The second part focuses on the hematopoietic stem cell (HSC) niche, the complex microenvironment within the bone marrow that controls HSC function and homeostasis. We detail the cells that constitute this niche, and the factors that these cells use to exert control over hematopoiesis. Here, we trace the scientific paths of Dr. Frenette, highlight key aspects of his research, and identify his most important scientific contributions in both fields.

Adhesion Molecules Involved in Stem Cell Niche Retention During Normal Haematopoiesis and in Acute Myeloid Leukaemia

In the bone marrow (BM) of adult mammals, haematopoietic stem cells (HSCs) are retained in micro-anatomical structures by adhesion molecules that regulate HSC quiescence, proliferation and commitment. During decades, researchers have used engraftment to study the function of adhesion molecules in HSC's homeostasis regulation. Since the 90's, progress in genetically engineered mouse models has allowed a better understanding of adhesion molecules involved in HSCs regulation by BM niches and raised questions about the role of adhesion mechanisms in conferring drug resistance to cancer cells nested in the BM. This has been especially studied in acute myeloid leukaemia (AML) which was the first disease in which the concept of cancer stem cell (CSC) or leukemic stem cells (LSCs) was demonstrated. In AML, it has been proposed that LSCs propagate the disease and are able to replenish the leukemic bulk after complete remission suggesting that LSC may be endowed with drug resistance properties. However, whether such properties are due to extrinsic or intrinsic molecular mechanisms, fully or partially supported by molecular crosstalk between LSCs and surrounding BM micro-environment is still matter of debate. In this review, we focus on adhesion molecules that have been involved in HSCs or LSCs anchoring to BM niches and discuss if inhibition of such mechanism may represent new therapeutic avenues to eradicate LSCs.

The diverse origin of bone-forming osteoblasts

Osteoblasts are the only cells that can give rise to bones in vertebrates. Thus, one of the most important functions of these metabolically active cells is mineralized matrix production. Because osteoblasts have a limited lifespan, they must be constantly replenished by preosteoblasts, their immediate precursors. Because disruption of the regulation of bone-forming osteoblasts results in a variety of bone diseases, a better understanding of the origin of these cells by defining the mechanisms of bone development, remodeling, and regeneration is central to the development of novel therapeutic approaches. In recent years, substantial new insights into the origin of osteoblasts-largely owing to rapid technological advances in murine lineage-tracing approaches and other single-cell technologies-have been obtained. Collectively, these findings indicate that osteoblasts involved in bone formation under various physiological, pathological, and therapeutic conditions can be obtained from numerous sources. The origins of osteoblasts include, but are not limited to, chondrocytes in the growth plate, stromal cells in the bone marrow, quiescent bone-lining cells on the bone surface, and specialized fibroblasts in the craniofacial structures, such as sutures and periodontal ligaments. Because osteoblasts can be generated from local cellular sources, bones can flexibly respond to regenerative and anabolic cues. However, whether osteoblasts derived from different cellular sources have distinct functions remains to be investigated. Currently, we are at the initial stage to aptly unravel the incredible diversity of the origins of bone-forming osteoblasts. © 2021 American Society for Bone and Mineral Research (ASBMR).

Hematopoietic Stem Cells: Nature and Niche Nurture

Like all cells, hematopoietic stem cells (HSCs) and their offspring, the hematopoietic progenitor cells (HPCs), are highly sociable. Their capacity to interact with bone marrow niche cells and respond to environmental cytokines orchestrates the generation of the different types of blood and immune cells. The starting point for engineering hematopoiesis ex vivo is the nature of HSCs, and a longstanding premise is that they are a homogeneous population of cells. However, recent findings have shown that adult bone marrow HSCs are really a mixture of cells, with many having lineage affiliations. A second key consideration is: Do HSCs "choose" a lineage in a random and cell-intrinsic manner, or are they instructed by cytokines? Since their discovery, the hematopoietic cytokines have been viewed as survival and proliferation factors for lineage committed HPCs. Some are now known to also instruct cell lineage choice. These fundamental changes to our understanding of hematopoiesis are important for placing niche support in the right context and for fabricating an ex vivo environment to support HSC development.

Far from Health: The Bone Marrow Microenvironment in AML, A Leukemia Supportive Shelter

Acute myeloid leukemia (AML) is the second most common leukemia among children. Although significant progress in AML therapy has been achieved, treatment failure is still associated with poor prognosis, emphasizing the need for novel, innovative therapeutic approaches. To address this major obstacle, extensive knowledge about leukemogenesis and the complex interplay between leukemic cells and their microenvironment is required. The tremendous role of this bone marrow microenvironment in providing a supportive and protective shelter for leukemic cells, leading to disease development, progression, and relapse, has been emphasized by recent research. It has been revealed that the interplay between leukemic cells and surrounding cellular as well as non-cellular components is critical in the process of leukemogenesis. In this review, we provide a comprehensive overview of recently gained knowledge about the importance of the microenvironment in AML whilst focusing on promising future therapeutic targets. In this context, we describe ongoing clinical trials and future challenges for the development of targeted therapies for AML.

Indispensable role of Galectin-3 in promoting quiescence of hematopoietic stem cells

Hematopoietic stem cells (HSCs) in adult bone marrow (BM) are usually maintained in a state of quiescence. The cellular mechanism coordinating the balance between HSC quiescence and differentiation is not fully understood. Here, we report that galactose-binding lectin-3 (galectin-3; Gal-3) is upregulated by Tie2 or Mpl activation to maintain quiescence. Conditional overexpression of Gal-3 in mouse HSCs under the transcriptional control of Tie2 or Vav1 promoters (Gal-3 Tg) causes cell cycle retardation via induction of p21. Conversely, the cell cycle of long-term repopulating HSCs (LT-HSCs) in Gal-3-deficient (Gal-3^-/-) mice is accelerated, resulting in their exhaustion. Mechanistically, Gal-3 regulates p21 transcription by forming a complex with Sp1, thus blocking cell cycle entry. These results demonstrate that Gal-3 is a negative regulator of cell-cycling in HSCs and plays a crucial role in adult hematopoiesis to prevent HSC exhaustion.

Long term haematopoitic stem cells (LT-HSCs) are in a quiescent state during homeostasis, which is critical for their maintenance. Here, the authors show that Gal-3 expression in LT-HSCs is induced in response to Tie2 and Mpl and is both necessary and sufficient for LT-HSC quiescence through regulation of p21.

N-cadherin in osteolineage cells modulates stromal support of tumor growth

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N-cadherin in osteolineage, Osterix+ cells restrains extraskeletal tumor growth.

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Osterix+ cells are present in the stromal microenvironment of extraskeletal tumors.

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Osterix+ cells are present in normal tissues frequent sites of metastasis.

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N-cadherin modulates pro-tumorigenic signaling in tumor associated Osterix+ cells.

Tumor growth and metastases are dependent on interactions between cancer cells and the local environment. Expression of the cell–cell adhesion molecule N-cadherin (Ncad) is associated with highly aggressive cancers, and its expression by osteogenic cells has been proposed to provide a molecular “dock” for disseminated tumor cells to establish in pre-metastatic niches within the bone. To test this biologic model, we conditionally deleted the Ncad gene (Cdh2) in osteolineage cells using Osx-cre (cKO). Contrary to expectations, the metastatic breast cancer cell line PyMT-BO1 was able to form tumors in bone and to induce osteolysis in cKO as well as in control mice. Despite absence of Ncad, bone marrow stromal cells isolated from cKO mice were able to engage in direct cell–cell interactions with tumor cells expressing either N- or E-cadherin. However, subcutaneous PyMT-BO1 and B16F10 tumors grew larger in cKO relative to control littermates. Cell tracking experiments using the Ai9 reporter revealed the presence of Osx+ and Ncad+ cells in the stroma of extra-skeletal tumors and in a small population of lung cells. Gene expression analysis by RNAseq of Osx+ cells isolated from extra-skeletal tumors revealed alterations of pro-tumorigenic signaling pathways in cKO cells relative to control Osx+ cells. Thus, Ncad in Osx+ cells is not necessary for the establishment of bone metastases, but in extra-skeletal tumors it regulates pro-tumorigenic support by the microenvironment.

Hematopoietic Stem Cell Niche During Homeostasis, Malignancy, and Bone Marrow Transplantation

Self-renewal and multidirectional differentiation of hematopoietic stem cells (HSCs) are strictly regulated by numerous cellular components and cytokines in the bone marrow (BM) microenvironment. Several cell types that regulate HSC niche have been identified, including both non-hematopoietic cells and HSC-derived cells. Specific changes in the niche composition can result in hematological malignancies. Furthermore, processes such as homing, proliferation, and differentiation of HSCs are strongly controlled by the BM niche and have been reported to be related to the success of hematopoietic stem cell transplantation (HSCT). Single-cell sequencing and in vivo imaging are powerful techniques to study BM microenvironment in hematological malignancies and after HSCT. In this review, we discuss how different components of the BM niche, particularly non-hematopoietic and hematopoietic cells, regulate normal hematopoiesis, and changes in the BM niche in leukemia and after HSCT. We believe that this comprehensive review will provide clues for further research on improving HSCT efficiency and exploring potential therapeutic targets for leukemia.

Physiological Cues Involved in the Regulation of Adhesion Mechanisms in Hematopoietic Stem Cell Fate Decision

Hematopoietic stem cells (HSC) could have several fates in the body; viz. self-renewal, differentiation, migration, quiescence, and apoptosis. These fate decisions play a crucial role in maintaining homeostasis and critically depend on the interaction of the HSCs with their micro-environmental constituents. However, the physiological cues promoting these interactions in vivo have not been identified to a great extent. Intense research using various in vitro and in vivo models is going on in various laboratories to understand the mechanisms involved in these interactions, as understanding of these mechanistic would greatly help in improving clinical transplantations. However, though these elegant studies have identified the molecular interactions involved in the process, harnessing these interactions to the recipients' benefit would ultimately depend on manipulation of environmental cues initiating them in vivo: hence, these need to be identified at the earliest. HSCs reside in the bone marrow, which is a very complex tissue comprising of various types of stromal cells along with their secreted cytokines, extra-cellular matrix (ECM) molecules and extra-cellular vesicles (EVs). These components control the HSC fate decision through direct cell-cell interactions - mediated via various types of adhesion molecules -, cell-ECM interactions - mediated mostly via integrins -, or through soluble mediators like cytokines and EVs. This could be a very dynamic process involving multiple transient interactions acting concurrently or sequentially, and the adhesion molecules involved in various fate determining situations could be different. If the switch mechanisms governing these dynamic states in vivo are identified, they could be harnessed for the development of novel therapeutics. Here, in addition to reviewing the adhesion molecules involved in the regulation of HSCs, we also touch upon recent advances in our understanding of the physiological cues known to initiate specific adhesive interactions of HSCs with the marrow stromal cells or ECM molecules and EVs secreted by them.

Leukemia Stem Cell Release From the Stem Cell Niche to Treat Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a heterogeneous, complex, and deadly disease, whose treatment has hardly evolved for decades and grounds on the use of intensive chemotherapy regimens. Chemotherapy helps reduce AML bulk, but promotes relapse in the long-run by selection of chemoresistant leukemia stem cells (LSC). These may diversify and result in progression to more aggressive forms of AML. In vivo models suggest that the bone marrow stem cell niche helps LSC stay dormant and protected from chemotherapy. Here, we summarize relevant changes in stem cell niche homing and adhesion of AML LSC vs. healthy hematopoietic stem cells, and provide an overview of clinical trials aiming at targeting these processes for AML treatment and future directions within this field. Promising results with various non-mutation-targeted novel therapies directed to LSC eradication via interference with their anchoring to the stem cell niche have encouraged on-going or future advanced phase III clinical trials. In the coming years, we may see a shift in the focus of AML treatment to LSC-directed therapies if the prospect of improved cure rates holds true. In the future, AML treatment should lean toward personalized therapies using combinations of these compounds plus mutation-targeted agents and/or targeted delivery of chemotherapy, aiming at LSC eradication with reduced side effects.

Haematopoietic stem cell activity and interactions with the niche

The haematopoietic stem cell (HSC) microenvironment in the bone marrow, termed the niche, ensures haematopoietic homeostasis by controlling the proliferation, self-renewal, differentiation and migration of HSCs and progenitor cells at steady state and in response to emergencies and injury. Improved methods for HSC isolation, driven by advances in single-cell and molecular technologies, have led to a better understanding of their behaviour, heterogeneity and lineage fate and of the niche cells and signals that regulate their function. Niche regulatory signals can be in the form of cell-bound or secreted factors and other local physical cues. A combination of technological advances in bone marrow imaging and genetic manipulation of crucial regulatory factors has enabled the identification of several candidate cell types regulating the niche, including both non-haematopoietic (for example, perivascular mesenchymal stem and endothelial cells) and HSC-derived (for example, megakaryocytes, macrophages and regulatory T cells), with better topographical understanding of HSC localization in the bone marrow. Here, we review advances in our understanding of HSC regulation by niches during homeostasis, ageing and cancer, and we discuss their implications for the development of therapies to rejuvenate aged HSCs or niches or to disrupt self-reinforcing malignant niches.

The role of bone cells in immune regulation during the course of infection

Bone homeostasis depends on a balance between osteoclastic bone resorption and osteoblastic bone formation. Bone cells are regulated by a variety of biochemical factors, such as hormones and cytokines, as well as various types of physical stress. The immune system affects bone, since such factors are dysregulated under pathologic conditions, including infection. The bone marrow, one of the primary lymphoid organs, provides a special microenvironment that supports the function and differentiation of immune cells and hematopoietic stem cells (HSCs). Thus, bone cells contribute to immune regulation by modulating immune cell differentiation and/or function through the maintenance of the bone marrow microenvironment. Although osteoblasts were first reported as the population that supports HSCs, the role of osteoblast-lineage cells in hematopoiesis has been shown to be more limited than previously expected. Osteoblasts are specifically involved in the differentiation of lymphoid cells under physiological and pathological conditions. It is of critical importance how bone cells are modified during inflammation and/or infection and how such modification affects the immune system.

Acute and late effects of combined internal and external radiation exposures on the hematopoietic system

Purpose: Incidents, such as nuclear facility accidents and the release of a 'dirty bomb', might result in not only external irradiation of personnel, but additional internal exposures through concomitant inhalation and/or ingestion of radioactive particulates. The purpose of this study was to define the impact of such a combination of radiation injuries on the hematopoietic niche.Material and methods: To assess changes in the murine hematopoietic system, we used a combined exposure of total body irradiation (TBI, 6 Gy) followed immediately by an internal (intraperitoneal) administration of 100 µCi of soluble ¹³⁷Cs. We then evaluated acute survival in combined versus single modality exposure groups, as well as assessing hematopoietic function at 12 and 26 week time points.Results: Acutely, the combination of external and internal exposures led to an unexpected delay in excretion of ¹³⁷Cs, increasing the absorbed dose in the combined exposure group and leading to mortality from an acute hematopoietic syndrome. At 12 weeks, all exposure paradigms resulted in decreased numbers of phenotypic hematopoietic stem cells (HSCs), particularly the short-term HSCs (ST-HSC); long-term HSCs (LT-HSC) were depleted only in the internal and combined exposure groups. At 26 weeks, there was significant anemia in both the TBI alone and combined exposure groups. There were decreased numbers in both the LT- and ST-HSCs and decreased functionality, as measured by competitive repopulation, was seen in all radiation groups, with the greatest effects seen in the internal and combined exposure groups.Conclusions: Our data indicate that a combined injury of sublethal external irradiation with internal contamination induces significant and persistent changes in the hematopoietic system, as may have been predicted from the literature and our own group's findings. However, a novel observation was that the combined exposure led to an alteration in the excretion kinetics of the internal contamination, increasing the acute effects beyond those anticipated. As a result, we believe that a combined exposure poses a unique challenge to the medical community during both the acute and, possibly, delayed recovery stages.

Notch2 pathway mediates breast cancer cellular dormancy and mobilisation in bone and contributes to haematopoietic stem cell mimicry

Background

Recurrence after >5-year disease-free survival affects one-fifth of breast cancer patients and is the clinical manifestation of cancer cell reactivation after persistent dormancy.

Methods

We investigated cellular dormancy in vitro and in vivo using breast cancer cell lines and cell and molecular biology techniques.

Results

We demonstrated cellular dormancy in breast cancer bone metastasis, associated with haematopoietic stem cell (HSC) mimicry, in vivo competition for HSC engraftment and non-random distribution of dormant cells at the endosteal niche. Notch2 signal implication was demonstrated by immunophenotyping the endosteal niche-associated cancer cells and upon co-culture with sorted endosteal niche cells, which inhibited breast cancer cell proliferation in a Notch2-dependent manner. Blocking this signal by in vivo acute administration of the γ-secretase inhibitor, dibenzazepine, induced dormant cell mobilisation from the endosteal niche and colonisation of visceral organs. Sorted Notch2^HIGH breast cancer cells exhibited a unique stem phenotype similar to HSCs and in vitro tumour-initiating ability in mammosphere assay. Human samples confirmed the existence of a small Notch2^HIGH cell population in primary and bone metastatic breast cancers, with a survival advantage for Notch2^HIGH vs Notch2^LOW patients.

Conclusions

Notch2 represents a key determinant of breast cancer cellular dormancy and mobilisation in the bone microenvironment.

The hematopoietic stem cell niche: What's so special about bone?

Hematopoietic stem cells (HSCs) require a supportive microenvironment to regulate their function and produce sufficient hematopoietic cells over the lifetime of an individual. With the exception of fish, all vertebrates, including mammals, maintain HSCs in a complex niche within the bone marrow. Several bone specific cellular populations have been implicated as components of the HSC niche and are part of a complex network that regulates HSC functions. However, the full extent of interactions within the HSC niche, and the role of individual cell populations remain to be fully elucidated. Further, it is not clear why fish are the exception, and what advantage is gained by housing HSCs in the bone marrow. To gain a better understanding of hematopoiesis and the mechanisms that drive hematopoietic disease processes a clearer picture of the complex HSC regulatory interactions in the bone marrow microenvironment is required.

Osteoimmunology

Bone is a crucial element of the skeletal-locomotor system, but also functions as an immunological organ that harbors hematopoietic stem cells (HSCs) and immune progenitor cells. Additionally, the skeletal and immune systems share a number of regulatory molecules, including cytokines and signaling molecules. Osteoimmunology was created as an interdisciplinary field to explore the shared molecules and interactions between the skeletal and immune systems. In particular, the importance of an inseparable link between the two systems has been highlighted by studies on the pathogenesis of rheumatoid arthritis (RA), in which pathogenic helper T cells induce the progressive destruction of multiple joints through aberrant expression of receptor activator of nuclear factor (NF)-κB ligand (RANKL). The conceptual bridge of osteoimmunology provides not only a novel framework for understanding these biological systems but also a molecular basis for the development of therapeutic approaches for diseases of bone and/or the immune system.

N-cadherin in cancer metastasis, its emerging role in haematological malignancies and potential as a therapeutic target in cancer

In many types of solid tumours, the aberrant expression of the cell adhesion molecule N-cadherin is a hallmark of epithelial-to-mesenchymal transition, resulting in the acquisition of an aggressive tumour phenotype. This transition endows tumour cells with the capacity to escape from the confines of the primary tumour and metastasise to secondary sites. In this review, we will discuss how N-cadherin actively promotes the metastatic behaviour of tumour cells, including its involvement in critical signalling pathways which mediate these events. In addition, we will explore the emerging role of N-cadherin in haematological malignancies, including bone marrow homing and microenvironmental protection to anti-cancer agents. Finally, we will discuss the evidence that N-cadherin may be a viable therapeutic target to inhibit cancer metastasis and increase tumour cell sensitivity to existing anti-cancer therapies.

Germline mutations in the bone marrow microenvironment and dysregulated hematopoiesis

The relationship between the hematopoietic stem cell (HSC) population and its surrounding bone marrow microenvironment is a rapidly evolving area of research. Normal HSC processes rely heavily on a complex communication network involving various marrow niches. Although leukemogenesis largely results from abnormal genetic activity within the leukemia stem cell itself, mounting evidence indicates a significant contributory role played by marrow niche dysregulation. Furthermore, numerous instances of activating or inactivating germline mutations within marrow microenvironment cells have been shown to be sufficient for development of myelodysplastic syndrome, myeloproliferative neoplasm, and acute myeloid leukemia, even in the context of wild-type HSCs. Recent evidence suggests that targeting aberrant chemokine production from germline-mutated marrow stromal cells can potentially reverse the process of leukemogenesis. This elaborate interplay between the HSC population and the marrow microenvironment allows for a number of unique clinical possibilities in efforts to induce remission, enhance chemosensitivity, manage relapsed disease, and prevent leukemia development, both in de novo and germline mutation-associated leukemias, including the use of targeted cytokine/chemokine inhibitors, immune checkpoint blockade, CXCR4/CXCL12 axis antagonists, and combined allogeneic HSC and mesenchymal stem cell transplantation. In this review, we discuss the pathways underlying normal and abnormal bone marrow niche functioning, the relationship between germline mutations in the stem cell microenvironment and dysregulated hematopoiesis, and future clinical perspectives that may be particularly applicable to prevention and treatment of germline-associated leukemias.

Heterocellular molecular contacts in the mammalian stem cell niche

Adult tissue homeostasis and repair relies on prompt and appropriate intervention by tissue-specific adult stem cells (SCs). SCs have the ability to self-renew; upon appropriate stimulation, they proliferate and give rise to specialized cells. An array of environmental signals is important for maintenance of the SC pool and SC survival, behavior, and fate. Within this special microenvironment, commonly known as the stem cell niche (SCN), SC behavior and fate are regulated by soluble molecules and direct molecular contacts via adhesion molecules providing connections to local supporting cells and the extracellular matrix. Besides the extensively discussed array of soluble molecules, the expression of adhesion molecules and molecular contacts is another fundamental mechanism regulating niche occupancy and SC mobilization upon activation. Some adhesion molecules are differentially expressed and have tissue-specific consequences, likely reflecting the structural differences in niche composition and design, especially the presence or absence of a stromal counterpart. However, the distribution and identity of intercellular molecular contacts for adhesion and adhesion-mediated signaling within stromal and non-stromal SCN have not been thoroughly studied. This review highlights common details or significant differences in cell-to-cell contacts within representative stromal and non-stromal niches that could unveil new standpoints for stem cell biology and therapy.

The Differentiation Balance of Bone Marrow Mesenchymal Stem Cells Is Crucial to Hematopoiesis

Bone marrow mesenchymal stem cells (BMSCs), the important component and regulator of bone marrow microenvironment, give rise to hematopoietic-supporting stromal cells and form hematopoietic niches for hematopoietic stem cells (HSCs). However, how BMSC differentiation affects hematopoiesis is poorly understood. In this review, we focus on the role of BMSC differentiation in hematopoiesis. We discussed the role of BMSCs and their progeny in hematopoiesis. We also examine the mechanisms that cause differentiation bias of BMSCs in stress conditions including aging, irradiation, and chemotherapy. Moreover, the differentiation balance of BMSCs is crucial to hematopoiesis. We highlight the negative effects of differentiation bias of BMSCs on hematopoietic recovery after bone marrow transplantation. Keeping the differentiation balance of BMSCs is critical for hematopoietic recovery. This review summarises current understanding about how BMSC differentiation affects hematopoiesis and its potential application in improving hematopoietic recovery after bone marrow transplantation.

The Expanding Life and Functions of Osteogenic Cells: From Simple Bone-Making Cells to Multifunctional Cells and Beyond

During the last three decades, important progress in bone cell biology and in human and mouse genetics led to major advances in our understanding of the life and functions of cells of the osteoblast lineage. Previously unrecognized sources of osteogenic cells have been identified. Novel cellular and molecular mechanisms controlling osteoblast differentiation and senescence have been determined. New mechanisms of communications between osteogenic cells, osteocytes, osteoclasts, and chondrocytes, as well as novel links between osteogenic cells and blood vessels have been identified. Additionally, cells of the osteoblast lineage were shown to be important components of the hematopoietic niche and to be implicated in hematologic dysfunctions and malignancy. Lastly, unexpected interactions were found between osteogenic cells and several soft tissues, including the central nervous system, gut, muscle, fat, and testis through the release of paracrine factors, making osteogenic cells multifunctional regulatory cells, in addition to their bone-making function. These discoveries considerably enlarged our vision of the life and functions of osteogenic cells, which may lead to the development of novel therapeutics with immediate applications in bone disorders. © 2017 American Society for Bone and Mineral Research.

Skeletal Stem Cells: Origins, Functions and Uncertainties

The development and maintenance of the skeleton requires a steady source of skeletal progenitors to provide the osteoblasts and chondrocytes necessary for bone and cartilage growth and development. The current model for skeletal stem cells (SSCs) posits that SSC/progenitor cells are present in bone marrow (BM) and other osteogenic sites such as cranial sutures where they undergo self-renewal and differentiation to give rise to the main skeletal tissues. SSCs hold great promise for understanding skeletal biology and genetic diseases of bone as well as for the advancement of bone tissue engineering and regenerative medicine strategies. In the past few years, a considerable effort has been devoted to identifying and purifying skeletal stem cells and determining their contribution to bone formation and homeostasis. Here, we review recent progress in this area with particular emphasis on the discovery of specific SSC markers, their use in tracking the progression of cell populations along specific lineages and the regulation of SSCs in both the appendicular and cranial skeleton.

Osteoimmunology: The Conceptual Framework Unifying the Immune and Skeletal Systems

The immune and skeletal systems share a variety of molecules, including cytokines, chemokines, hormones, receptors, and transcription factors. Bone cells interact with immune cells under physiological and pathological conditions. Osteoimmunology was created as a new interdisciplinary field in large part to highlight the shared molecules and reciprocal interactions between the two systems in both heath and disease. Receptor activator of NF-κB ligand (RANKL) plays an essential role not only in the development of immune organs and bones, but also in autoimmune diseases affecting bone, thus effectively comprising the molecule that links the two systems. Here we review the function, gene regulation, and signal transduction of osteoimmune molecules, including RANKL, in the context of osteoclastogenesis as well as multiple other regulatory functions. Osteoimmunology has become indispensable for understanding the pathogenesis of a number of diseases such as rheumatoid arthritis (RA). We review the various osteoimmune pathologies, including the bone destruction in RA, in which pathogenic helper T cell subsets [such as IL-17-expressing helper T (Th17) cells] induce bone erosion through aberrant RANKL expression. We also focus on cellular interactions and the identification of the communication factors in the bone marrow, discussing the contribution of bone cells to the maintenance and regulation of hematopoietic stem and progenitors cells. Thus the time has come for a basic reappraisal of the framework for understanding both the immune and bone systems. The concept of a unified osteoimmune system will be absolutely indispensable for basic and translational approaches to diseases related to bone and/or the immune system.

HSC Niche Biology and HSC Expansion Ex Vivo

Hematopoietic stem cell (HSC) transplantation can restore a new functional hematopoietic system in recipients in cases where the system of the recipient is not functional or for example is leukemic. However, the number of available donor HSCs is often too low for successful transplantation. Expansion of HSCs and thus HSC self-renewal ex vivo would greatly improve transplantation therapy in the clinic. In vivo, HSCs expand significantly in the niche, but establishing protocols that result in HSC expansion ex vivo remains challenging. In this review we discuss current knowledge of niche biology, the intrinsic regulators of HSC self-renewal in vivo, and introduce novel niche-informed strategies of HSC expansion ex vivo.

Eviction from the sanctuary: Development of targeted therapy against cell adhesion molecules in acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is a malignant hematological disease afflicting hematopoiesis in the bone marrow. While 80%-90% of patients diagnosed with ALL will achieve complete remission at some point during treatment, ALL is associated with high relapse rate, with a 5-year overall survival rate of 68%. The initial remission failure and the high rate of relapse can be attributed to intrinsic chemoprotective mechanisms that allow persistence of ALL cells despite therapy. These mechanisms are mediated, at least in part, through the engagement of cell adhesion molecules (CAMs) within the bone marrow microenvironment. This review assembles CAMs implicated in protection of leukemic cells from chemotherapy. Such studies are limited in ALL. Therefore, CAMs that are associated with poor outcomes or are overexpressed in ALL and have been shown to be involved in chemoprotection in other hematological cancers are also included. It is likely that these molecules play parallel roles in ALL because the CAMs identified to be a factor in ALL chemoresistance also work similarly in other hematological malignancies. We review the signaling mechanisms activated by the engagement of CAMs that provide protection from chemotherapy. Development of targeted therapies against CAMs could improve outcome and raise the overall cure rate in ALL.

The Notch Ligand Jagged1 Regulates the Osteoblastic Lineage by Maintaining the Osteoprogenitor Pool

Notch signaling is critical for osteoblastic differentiation; however, the specific contribution of individual Notch ligands is unknown. Parathyroid hormone (PTH) regulates the Notch ligand Jagged1 in osteoblastic cells. To determine if osteolineage Jagged1 contributes to bone homeostasis, selective deletion of Jagged1 in osteolineage cells was achieved through the presence of Prx1 promoter-driven Cre recombinase expression, targeting mesenchymal stem cells (MSCs) and their progeny (PJag1 mice). PJag1 mice were viable and fertile and did not exhibit any skeletal abnormalities at 2 weeks of age. At 2 months of age, however, PJag1 mice had increased trabecular bone mass compared to wild-type (WT) littermates. Dynamic histomorphometric analysis showed increased osteoblastic activity and increased mineral apposition rate. Immunohistochemical analysis showed increased numbers of osteocalcin-positive mature osteoblasts in PJag1 mice. Also increased phenotypically defined Lin- /CD45- /CD31- /Sca1- /CD51+ osteoblastic cells were measured by flow cytometric analysis. Surprisingly, phenotypically defined Lin- /CD45- /CD31- /Sca1+ /CD51+ MSCs were unchanged in PJag1 mice as measured by flow cytometric analysis. However, functional osteoprogenitor (OP) cell frequency, measured by Von Kossa+ colony formation, was decreased, suggesting that osteolineage Jagged1 contributes to maintenance of the OP pool. The trabecular bone increases were not due to osteoclastic defects, because PJag1 mice had increased bone resorption. Because PTH increases osteoblastic Jagged1, we sought to understand if osteolineage Jagged1 modulates PTH-mediated bone anabolism. Intermittent PTH treatment resulted in a significantly greater increase in BV/TV in PJag1 hind limbs compared to WT. These findings demonstrate a critical role of osteolineage Jagged1 in bone homeostasis, where Jagged1 maintains the transition of OP to maturing osteoblasts. This novel role of Jagged1 not only identifies a regulatory loop maintaining appropriate populations of osteolineage cells, but also provides a novel approach to increase trabecular bone mass, particularly in combination with PTH, through modulation of Jagged1. © 2017 American Society for Bone and Mineral Research.

Extrinsic regulation of hematopoietic stem cells in development, homeostasis and diseases

Lifelong generation of blood and immune cells depends on hematopoietic stem cells (HSCs). Their function is precisely regulated by complex molecular networks that integrate and respond to ever changing physiological demands of the body. Over the past several years, significant advances have been made in understanding the extrinsic regulation of HSCs during development and in homeostasis. Propelled by technical advances in the field, the cellular and molecular components of the microenvironment that support HSCs in vivo are emerging. In addition, the interaction of HSCs with their niches is appreciated as a critical contributor to the pathogenesis of a number of hematologic disorders. Here, we review these advances in detail and highlight the extrinsic regulation of HSCs in the context of development, homeostasis, and diseases. WIREs Dev Biol 2017, 6:e279. doi: 10.1002/wdev.279 For further resources related to this article, please visit the WIREs website.

Complexity of bone marrow hematopoietic stem cell niche

Hematopoietic stem cells (HSCs) that produce a variety of hematopoietic lineage cells throughout the life reside in specialized microenvironment called "niche" in the bone marrow (BM) where they are tightly regulated. With the recent advances in experimental technologies enabling the selective deletion of molecules, various types of cells in the BM have been proposed to contribute to HSC niche activity. Among these are stromal cells closely associated with the vasculature. In this review, we provide an overview of recent advances in HSC niche research, and focus on the studies describing the functional roles of perivascular cells for HSC maintenance and mobilization. Not only for physiologic state, we also discuss the recent evidences suggesting the importance of microenvironment for emergence of malignant hematopoietic diseases.

Tissue-Engineered Model of Human Osteolytic Bone Tumor

Ewing's sarcoma (ES) is a poorly differentiated pediatric tumor of aggressive behavior characterized by propensity to metastasize to bone. Interactions between the tumor and bone cells orchestrate a vicious cycle in which tumor cells induce osteoclast differentiation and activation to cause osteolytic lesions, broken bones, pain, and hypercalcemia. The lack of controllable models that can recapitulate osteolysis in ES impedes the development of new therapies and limits our understanding of how tumor cells invade bone. In response to this need, tissue-engineered models are now being developed to enable quantitative, predictive studies of human tumors. In this study, we report a novel bioengineered model of ES that incorporates the osteolytic process. Our strategy is based on engineering human bone containing both osteoclasts and osteoblasts within three-dimensional mineralized bone matrix. We show that the bone matrix is resorbed by mature osteoclasts while the new bone matrix is formed by osteoblasts, leading to calcium release and bone remodeling. Introduction of ES cell aggregates into the bone niche induced decreases in bone density, connectivity, and matrix deposition. Additionally, therapeutic reagents, such as zoledronic acid, which have demonstrated efficacy in ES treatment, inhibited bone resorption mediated by osteoclasts in the tumor model.

Parathyroid hormone 1-34 and skeletal anabolic action: The use of parathyroid hormone in bone formation

Intermittently administered parathyroid hormone (PTH 1-34) has been shown to promote bone formation in both human and animal studies. The hormone and its analogues stimulate both bone formation and resorption, and as such at low doses are now in clinical use for the treatment of severe osteoporosis. By varying the duration of exposure, parathyroid hormone can modulate genes leading to increased bone formation within a so-called 'anabolic window'. The osteogenic mechanisms involved are multiple, affecting the stimulation of osteoprogenitor cells, osteoblasts, osteocytes and the stem cell niche, and ultimately leading to increased osteoblast activation, reduced osteoblast apoptosis, upregulation of Wnt/β-catenin signalling, increased stem cell mobilisation, and mediation of the RANKL/OPG pathway. Ongoing investigation into their effect on bone formation through 'coupled' and 'uncoupled' mechanisms further underlines the impact of intermittent PTH on both cortical and cancellous bone. Given the principally catabolic actions of continuous PTH, this article reviews the skeletal actions of intermittent PTH 1-34 and the mechanisms underlying its effect.

CITE THIS ARTICLE

L. Osagie-Clouard, A. Sanghani, M. Coathup, T. Briggs, M. Bostrom, G. Blunn. Parathyroid hormone 1-34 and skeletal anabolic action: The use of parathyroid hormone in bone formation. Bone Joint Res 2017;6:14-21. DOI: 10.1302/2046-3758.61.BJR-2016-0085.R1.

N-cadherin restrains PTH repressive effects on sclerostin/SOST by regulating LRP6-PTH1R interaction

Sclerostin/SOST is a robust negative regulator of bone formation. Loss-of-function mutations of the sclerostin gene (SOST) cause sclerosteosis and Van Buchem disease characterized by bone overgrowth. Mediated by myocyte enhancer factor 2 (MEF2) transcription factors, parathyroid hormone (PTH) suppresses SOST expression through formation of complexes of parathyroid hormone-parathyroid hormone-related peptide receptor 1 (PTH1R) and lipoprotein receptor-related protein 6 (LRP6). N-cadherin has been shown to negatively regulate Wnt/β-catenin and PTH induced, protein kinase-dependent β-catenin signaling. Here, we investigated whether N-cadherin mediates the inhibitory effects of PTH on sclerostin/SOST. In vitro, overexpression of N-cadherin resulted in blunted PTH suppressive effects on sclerostin/SOST expression, as detected by immunoblot and qPCR analysis; PTH-induced downregulation of MEF2A, C, and D was impaired by N-cadherin; and N-cadherin reduced LRP6-PTHR1 interaction and endocytosis in response to PTH. In vivo, intermittent PTH (iPTH)-induced suppression of sclerostin/SOST was accentuated in Dmp1-cre; Cdh2^f/f (Cdh2^ΔDmp1 ) mice, compared with Cdh2^f/f mice. Additionally, iPTH had greater bone anabolic effects in Cdh2^ΔDmp1 mice compared to Cdh2^f/f mice. These data indicate that N-cadherin negatively mediates PTH suppressive effects on sclerostin/SOST by regulating LRP6-PTHR1 interaction, ultimately influencing PTH anabolic effects on bone.

Bioengineering Hematopoietic Stem Cell Niche toward Regenerative Medicine

The scope of this chapter is to introduce the current consensus of hematopoietic stem cell (HSC) niche biology to bioengineering field so that can apply to regenerative medicine. A decade of research has been addressing "what is HSC niche", then next step is "how it advances medicine". The demand to improve HSC transplantation has advanced the methodology to expand HSC in vitro. Still precise modeling of bone marrow (BM) is demanded by bioengineering HSC niche in vitro. Better understanding of HSC niche is essential toward this progress. Now it would be the time to apply the knowledge of HSC niche field to the venue of bioengineering, so that a promising new approach to regenerative medicine might appear. This chapter describes the current consensus of niche that endothelial cell and perivascular mesenchymal stromal cell maintain HSC, expansion of cord blood HSC by small molecules, bioengineering efforts to model HSC niche by microfluidics chip, organoids, and breakthroughs to induce HSC from heterologous types of cells.

Tumor microenvironment for cancer stem cells

Tumor tissues consist of heterogeneous cancer cells including cancer stem cells (CSCs) that can terminally differentiate into cancer cells. Tissue-specific stem cells in normal organs maintain their stemness in a specific microenvironment, the stem cell niche; several studies have suggested that there are specific microenvironments that maintain CSCs in an immature phenotype. Cell types in a CSC niche vary from fibroblasts, to endothelial cells, immune cells, and so on; these non-cancer cells have been suggested to change their original features in the normal tissue/organ and to acquire a phenotype that protects CSCs from anticancer therapies. Therefore, to kill CSCs, we need to understand the cellular and molecular mechanisms involved in the maintenance of the immature phenotype of CSCs and in drug resistance.

The hematopoietic stem cell niche in homeostasis and disease

The bone marrow microenvironment contains a heterogeneous population of stromal cells organized into niches that support hematopoietic stem cells (HSCs) and other lineage-committed hematopoietic progenitors. The stem cell niche generates signals that regulate HSC self-renewal, quiescence, and differentiation. Here, we review recent studies that highlight the heterogeneity of the stromal cells that comprise stem cell niches and the complexity of the signals that they generate. We highlight emerging data that stem cell niches in the bone marrow are not static but instead are responsive to environmental stimuli. Finally, we review recent data showing that hematopoietic niches are altered in certain hematopoietic malignancies, and we discuss how these alterations might contribute to disease pathogenesis.

Tetraspanin CD82 regulates bone marrow homing of acute myeloid leukemia by modulating the molecular organization of N-cadherin

Communication between acute myeloid leukemia (AML) and the bone marrow microenvironment is known to control disease progression. Therefore, regulation of AML cell trafficking and adhesion to the bone marrow is of significant interest. In this study, we demonstrate that differential expression of the membrane scaffold CD82 modulates the bone marrow homing of AML cells. By combining mutational analysis and super-resolution imaging, we identify membrane protein clustering by CD82 as a regulator of AML cell adhesion and bone marrow homing. Cluster analysis of super-resolution data indicates that N-linked glycosylation and palmitoylation of CD82 are both critical modifications that control the microdomain organization of CD82 as well as the nanoscale clustering of associated adhesion protein, N-cadherin. We demonstrate that inhibition of CD82 glycosylation increases the molecular packing of N-cadherin and promotes the bone marrow homing of AML cells. In contrast, we find that inhibition of CD82 palmitoylation disrupts the formation and organization of N-cadherin clusters and significantly diminishes bone marrow trafficking of AML. Taken together, these data establish a mechanism where the membrane organization of CD82, through specific post-translational modifications, regulates N-cadherin clustering and membrane density, which impacts the in vivo trafficking of AML cells. As such, these observations provide an alternative model for targeting AML where modulation of protein organization within the membrane may be an effective treatment therapy to disrupt the bone marrow homing potential of AML cells.

Phenotypic, Morphological and Adhesive Differences of Human Hematopoietic Progenitor Cells Cultured on Murine versus Human Mesenchymal Stromal Cells

Xenogenic transplantation models have been developed to study human hematopoiesis in immunocompromised murine recipients. They still have limitations and therefore it is important to delineate all players within the bone marrow that could account for species-specific differences. Here, we evaluated the proliferative capacity, morphological and physical characteristics of human CD34⁺ hematopoietic stem and progenitor cells (HSPCs) after co-culture on murine or human bone marrow-derived mesenchymal stromal cells (MSCs). After seven days, human CD34⁺CD133^– HSPCs expanded to similar extents on both feeder layers while cellular subsets comprising primitive CD34⁺CD133⁺ and CD133⁺CD34^– phenotypes are reduced fivefold on murine MSCs. The number of migrating HSPCs was also reduced on murine cells suggesting that MSC adhesion influences cellular polarization of HSPC. We used atomic force microscopy-based single-cell force spectroscopy to quantify their adhesive interactions. We found threefold higher detachment forces of human HSPCs from murine MSCs compared to human ones. This difference is related to the N-cadherin expression level on murine MSCs since its knockdown abolished their differential adhesion properties with human HSPCs. Our observations highlight phenotypic, morphological and adhesive differences of human HSPCs when cultured on murine or human MSCs, which raise some caution in data interpretation when xenogenic transplantation models are used.

The critical and specific transcriptional regulator of the microenvironmental niche for hematopoietic stem and progenitor cells

PURPOSE OF REVIEW

It has been assumed that the special microenvironments known as niches in the marrow play an essential role in maintaining hematopoietic stem and progenitor cells (HSPCs), and the identity of the HSPC niche has been a subject of long-standing debate. Recent studies identified cells, which create microenvironments meeting the criteria for HSPC niches and the critical transcriptional regulators of their development and maintenance.

RECENT FINDINGS

Osterix as well as Ebf2 and Bmi1 are critical but not specific transcriptional regulators of HSPC niche development. The transcription factor Foxc1 is expressed preferentially in a population of adipo-osteogenic progenitors, termed CXCL12-abundant reticular (CAR) cells, which create HSPC niches and are largely equivalent to stem cell factor and Lepr-expressing cells, in developing and adult bone marrow. Foxc1 is essential for CAR cell development and maintenance of bone marrow niches for HSPCs upregulating CXCL12 and SCF expression and inhibition of adipogenic processes in CAR cell progenitors.

SUMMARY

Foxc1 is the first critical and specific transcriptional regulator that is required for development and maintenance of cells creating HSPC niches, including a specialized population of adipo-osteogenic progenitors in bone marrow.

Quantifying adhesion mechanisms and dynamics of human hematopoietic stem and progenitor cells

Using planar lipid membranes with precisely defined concentrations of specific ligands, we have determined the binding strength between human hematopoietic stem cells (HSC) and the bone marrow niche. The relative significance of HSC adhesion to the surrogate niche models via SDF1α-CXCR4 or N-cadherin axes was quantified by (a) the fraction of adherent cells, (b) the area of tight adhesion, and (c) the critical pressure for cell detachment. We have demonstrated that the binding of HSC to the niche model is a cooperative process, and the adhesion mediated by the CXCR4- SDF1α axis is stronger than that by homophilic N-cadherin binding. The statistical image analysis of stochastic morphological dynamics unraveled that HSC dissipated energy by undergoing oscillatory deformation. The combination of an in vitro niche model and novel physical tools has enabled us to quantitatively determine the relative significance of binding mechanisms between normal HSC versus leukemia blasts to the bone marrow niche.

Making sense of hematopoietic stem cell niches

The hematopoietic stem cell (HSC) niche commonly refers to the pairing of hematopoietic and mesenchymal cell populations that regulate HSC self-renewal, differentiation, and proliferation. Anatomic localization of the niche is a dynamic unit from the developmental stage that allows proliferating HSCs to expand before they reach the bone marrow where they adopt a quiescent phenotype that protects their integrity and functions. Recent studies have sought to clarify the complexity behind the HSC niche by assessing the contributions of specific cell populations to HSC maintenance. In particular, perivascular microenvironments in the bone marrow confer distinct vascular niches that regulate HSC quiescence and the supply of lineage-committed progenitors. Here, we review recent data on the cellular constituents and molecular mechanisms involved in the communication between HSCs and putative niches.

N-cadherin restrains PTH activation of Lrp6/β-catenin signaling and osteoanabolic action

Interaction between parathyroid hormone/parathyroid hormone-related peptide receptor 1 (PTHR1) and low-density lipoprotein receptor-related protein 6 (Lrp6) is important for parathyroid hormone (PTH) signaling and anabolic action. Because N-cadherin has been shown to negatively regulate canonical Wnt/β-catenin signaling, we asked whether N-cadherin alters PTH signaling and stimulation of bone formation. Ablation of the N-cadherin gene (Cdh2) in primary osteogenic lineage cells resulted in increased Lrp6/PTHR1 interaction in response to PTH1-34 , associated with enhanced PTH-induced PKA signaling and PKA-dependent β-catenin C-terminus phosphorylation, which promotes β-catenin transcriptional activity. β-catenin C-terminus phosphorylation was abolished by Lrp6 knockdown. Accordingly, PTH1-34 stimulation of Tcf/Lef target genes, Lef1 and Axin2, was also significantly enhanced in Cdh2-deficient cells. This enhanced responsiveness to PTH extends to the osteo-anabolic effect of PTH, as mice with a conditional Cdh2 deletion in Osx+ cells treated with intermittent doses of PTH1-34 exhibited significantly larger gains in trabecular bone mass relative to control mice, the result of accentuated osteoblast activity. Therefore, N-cadherin modulates Lrp6/PTHR1 interaction, restraining the intensity of PTH-induced β-catenin signaling, and ultimately influencing bone formation in response to intermittent PTH administration.

Biology of BM failure syndromes: role of microenvironment and niches

The BM microenvironment and its components regulate hematopoietic stem and progenitor cell (HSC) fate. An abnormality in the BM microenvironment and specific dysfunction of the HSC niche could play a critical role in initiation, disease progression, and response to therapy of BM failure syndromes. Therefore, the identification of changes in the HSC niche in BM failure syndromes should lead to further knowledge of the signals that disrupt the normal microenvironment. In turn, niche disruption may contribute to disease morbidity, resulting in pancytopenia and clonal evolution, and its understanding could suggest new therapeutic targets for these conditions. In this chapter, we briefly review the evidence for the importance of the BM microenvironment as a regulator of normal hematopoiesis, summarize current knowledge regarding the role of dysfunctions in the BM microenvironment in BM failure syndromes, and propose a strategy through which niche stimulation can complement current treatment for myelodysplastic syndrome.

Integrin and cadherin signaling in bone: role and potential therapeutic targets

Cell-cell and cell-matrix interactions mediated by cell adhesion molecules are important mechanisms controlling cell fate and function. Here, we review recent advances in the implication of the cell adhesion molecules integrins and cadherins in the control of osteoblastogenesis and bone formation. We discuss emerging evidence indicating that signaling pathways mediated by integrins and cadherins and their crosstalk with the Wnt/β-catenin signaling pathway regulate osteogenic differentiation and mechanotransduction. We also offer a comprehensive view of the mechanisms by which some integrins and cadherins control the differentiation of cells of the osteoblast lineage in bone marrow niches. Understanding how specific integrins or cadherins may promote osteogenic cell differentiation, bone formation, and repair may lead to novel therapeutic strategies.