Conditional ablation of the osteoblast lineage in Col2.3deltatk transgenic mice

Emerging insights into epigenetics and hematopoietic stem cell trafficking in age-related hematological malignancies

BACKGROUND

Hematopoiesis within the bone marrow (BM) is a complex and tightly regulated process predominantly influenced by immune factors. Aging, diabetes, and obesity are significant contributors to BM niche damage, which can alter hematopoiesis and lead to the development of clonal hematopoiesis of intermediate potential (CHIP). Genetic/epigenetic alterations during aging could influence BM niche reorganization for hematopoiesis or clonal hematopoiesis. CHIP is driven by mutations in genes such as Tet2, Dnmt3a, Asxl1, and Jak2, which are associated with age-related hematological malignancies.

OBJECTIVE

This literature review aims to provide an updated exploration of the functional aspects of BM niche cells within the hematopoietic microenvironment in the context of age-related hematological malignancies. The review specifically focuses on how immunological stressors modulate different signaling pathways that impact hematopoiesis.

METHODS

An extensive review of recent studies was conducted, examining the roles of various BM niche cells in hematopoietic stem cell (HSC) trafficking and the development of age-related hematological malignancies. Emphasis was placed on understanding the influence of immunological stressors on these processes.

RESULTS

Recent findings reveal a significant microheterogeneity and temporal stochasticity of niche cells across the BM during hematopoiesis. These studies demonstrate that niche cells, including mesenchymal stem cells, osteoblasts, and endothelial cells, exhibit dynamic interactions with HSCs, significantly influenced by the BM microenvironment as the age increases. Immunosurveillance plays a crucial role in maintaining hematopoietic homeostasis, with alterations in immune signaling pathways contributing to the onset of hematological malignancies. Novel insights into the interaction between niche cells and HSCs under stress/aging conditions highlight the importance of niche plasticity and adaptability.

CONCLUSION

The involvement of age-induced genetic/epigenetic alterations in BM niche cells and immunological stressors in hematopoiesis is crucial for understanding the development of age-related hematological malignancies. This comprehensive review provides new insights into the complex interplay between niche cells and HSCs, emphasizing the potential for novel therapeutic approaches that target niche cell functionality and resilience to improve hematopoietic outcomes in the context of aging and metabolic disorders.

NOVELTY STATEMENT

This review introduces novel concepts regarding the plasticity and adaptability of BM niche cells in response to immunological stressors and epigenetics. It proposes that targeted therapeutic strategies aimed at enhancing niche cell resilience could mitigate the adverse effects of aging, diabetes, and obesity on hematopoiesis and clonal hematopoiesis. Additionally, the review suggests that understanding the precise temporal and spatial dynamics of niche-HSC interactions and epigenetics influence may lead to innovative treatments for age-related hematological malignancies.

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).

Insights Into Bone Marrow Niche Stability: An Adhesion and Metabolism Route

The bone marrow microenvironment provides critical cues for hematopoietic stem cell (HSC) self-renewal and differentiation and contributes to their malignant conversion. The microenvironment comprises a complex mixture of multiple cell types, soluble factors, and extracellular matrix in specialized regions termed 'niches.' Positioning of the various cellular players within these niches depends on their repertoire of adhesion molecules and chemotactic signaling, involving integrins and chemokine receptors and the corresponding intracellular players such as kinases and GTPases. The mechanical role of adhesion is to control the strength and morphology of the cell-cell and cell-extracellular matrix contacts and thereby the energy needed for the optimal localization of cells to their surroundings. While it is clear that biomechanical adhesive bonds are energetically expensive, the crosstalk between cell adhesion and metabolic pathways in the normal and malignant microenvironment is far from understood. The metabolic profile of the various cell types within the niche includes key molecules such as AMPK, glucose, mTOR, and HIF-1α. Here, we describe our most recent understanding of how the interplay between adhesion and these metabolic components is indispensable for bone marrow niche stability. In parallel, we compare the altered crosstalk of different cell types within the bone marrow niches in hematological malignancies and propose potential therapeutic associations.

Postnatal Osterix but not DMP1 lineage cells significantly contribute to intramembranous ossification in three preclinical models of bone injury

Murine models of long-bone fracture, stress fracture, and cortical defect are used to discern the cellular and molecular mediators of intramembranous and endochondral bone healing. Previous work has shown that Osterix (Osx⁺) and Dentin Matrix Protein-1 (DMP1⁺) lineage cells and their progeny contribute to injury-induced woven bone formation during femoral fracture, ulnar stress fracture, and tibial cortical defect repair. However, the contribution of pre-existing versus newly-derived Osx⁺ and DMP1⁺ lineage cells in these murine models of bone injury is unclear. We addressed this knowledge gap by using male and female 12-week-old, tamoxifen-inducible Osx Cre_ERT2 and DMP1 Cre_ERT2 mice harboring the Ai9 TdTomato reporter allele. To trace pre-existing Osx⁺ and DMP1⁺ lineage cells, tamoxifen (TMX: 100 mg/kg gavage) was given in a pulse manner (three doses, 4 weeks before injury), while to label pre-existing and newly-derived lineage Osx⁺ and DMP1⁺ cells, TMX was first given 2 weeks before injury and continuously (twice weekly) throughout healing. TdTomato positive (TdT⁺) cell area and cell fraction were quantified from frozen histological sections of injured and uninjured contralateral samples at times corresponding with active woven bone formation in each model. We found that in uninjured cortical bone tissue, Osx Cre_ERT2 was more efficient than DMP1 Cre_ERT2 at labeling the periosteal and endosteal surfaces, as well as intracortical osteocytes. Pulse-labeling revealed that pre-existing Osx⁺ lineage and their progeny, but not pre-existing DMP1⁺ lineage cells and their progeny, significantly contributed to woven bone formation in all three injury models. In particular, these pre-existing Osx⁺ lineage cells mainly lined new woven bone surfaces and became embedded as osteocytes. In contrast, with continuous dosing, both Osx⁺ and DMP1⁺ lineage cells and their progeny contributed to intramembranous woven bone formation, with higher TdT⁺ tissue area and cell fraction in Osx⁺ lineage versus DMP1⁺ lineage calluses (femoral fracture and ulnar stress fracture). Similarly, Osx⁺ and DMP1⁺ lineage cells and their progeny significantly contributed to endochondral callus regions with continuous dosing only, with higher TdT⁺ chondrocyte fraction in Osx⁺ versus DMP1⁺ cell lineages. In summary, pre-existing Osx⁺ but not DMP1⁺ lineage cells and their progeny make up a significant amount of woven bone cells (particularly osteocytes) across three preclinical models of bone injury. Therefore, Osx⁺ cell lineage modulation may prove to be an effective therapy to enhance bone regeneration.

Gene Therapy Using Neural Stem/Progenitor Cells Derived from Human Induced Pluripotent Stem Cells: Visualization of Migration and Bystander Killing Effect

Glioblastoma is the most aggressive brain tumor characterized by diffuse infiltration into the normal brain parenchyma. Neural stem cells are known to possess the tumor-tropic migratory capacity and thus can be used as cellular vehicles for targeted delivery of therapeutic agents. In the present study, we evaluated the efficacy of herpes simplex virus thymidine kinase (HSV-TK) suicide gene therapy for glioblastoma using neural stem/progenitor cells (NS/PCs) derived from human induced pluripotent stem cells (hiPSCs). Although transduction of hiPSCs is preferable for a safe and stable supply in the clinical setting, high-level and/or constitutive HSV-TK expression was highly cytotoxic to hiPSCs. To overcome this problem, we used the tetracycline-inducible system to control the expression of HSV-TK. hiPSC-derived NS/PCs expressing HSV-TK were transplanted in an orthotopic xenograft mouse model of human glioblastoma. Glioblastoma cell growth in mice was dramatically inhibited following ganciclovir (GCV) administration. Survival of the mice was significantly prolonged with administration of GCV compared with control groups. Time-lapse imaging of organotypic brain slice cultures first demonstrated the directional migration of NS/PCs toward glioblastoma cells and the bystander killing effect upon GCV treatment. hiPSC-derived NS/PCs with HSV-TK/GCV suicide gene system may have considerable therapeutic potential for the treatment of glioblastoma. Color images are available online.

Bone marrow adiposity and the hematopoietic niche: A historical perspective of reciprocity, heterogeneity, and lineage commitment

PURPOSE

Here we review the current knowledge on bone marrow adipocytes (BMAds) as active contributors to the regulation of the hematopoietic niche, and as potentially pivotal players in the progression of hematological malignancies. We highlight the hierarchical and functional heterogeneity of the adipocyte lineage within the bone marrow, and how potentially different contexts dictate their interactions with hematopoietic populations.

RECENT FINDINGS

Growing evidence associates the adipocyte lineage with important functions in hematopoietic regulation within the BM niche. Initially proposed to serve as negative regulators of the hematopoietic microenvironment, studies have also demonstrated that BMAds positively influence the survival and maintenance of hematopoietic stem cells (HSCs). These seemingly incongruous findings may at least be partially explained by stage-specificity across the adipocytic differentiation axis and by BMAds subtypes, suggesting that the heterogeneity of these populations allows for differential context-based interactions. One such distinction relies on the location of adipocytes. Constitutive bone marrow adipose tissue (cBMAT) historically associates to the "yellow" marrow containing so-called "stable" BMAs larger in size, less responsive to stimuli, and linked to HSC quiescence. On the other hand, regulated bone marrow adipose tissue (rBMAT)-associated adipocytes, also referred to as "labile" are smaller, more responsive to hematopoietic demand and strategically situated in hematopoietically active regions of the skeleton. Here we propose a model where the effect of distinct BM stromal cell populations (BMSC) in hematopoiesis is structured along the BMSC-BMAd differentiation axis, and where the effects on HSC maintenance versus hematopoietic proliferation are segregated. In doing so, it is possible to explain how recently identified, adipocyte-primed leptin receptor-expressing, CXCL12-high adventitial reticular cells (AdipoCARs) and marrow adipose lineage precursor cells (MALPs) best support active hematopoietic cell proliferation, while adipose progenitor cells (APCs) and maturing BMAd gradually lose the capacity to support active hematopoiesis, favoring HSC quiescence. Implicated soluble mediators include MCP-1, PAI-1, NRP1, possibly DPP4 and limiting availability of CXCL12 and SCF. How remodeling occurs within the BMSC-BMAd differentiation axis is yet to be elucidated and will likely unravel a three-way regulation of the hematopoietic, bone, and adipocytic compartments orchestrated by vascular elements. The interaction of malignant hematopoietic cells with BMAds is precisely contributing to unravel specific mechanisms of remodeling.

SUMMARY

BMAds are important operative components of the hematopoietic microenvironment. Their heterogeneity directs their ability to exert a range of regulatory capacities in a manner dependent on their hierarchical, spatial, and biological context. This complexity highlights the importance of (i) developing experimental tools and nomenclature adapted to address stage-specificity and heterogeneity across the BMSC-BMAd differentiation axis when reporting effects in hematopoiesis, (ii) interpreting gene reporter studies within this framework, and (iii) quantifying changes in all three compartments (hematopoiesis, adiposity and bone) when addressing interdependency.

Integrated OMICs unveil the bone-marrow microenvironment in human leukemia

The bone-marrow (BM) niche is the spatial environment composed by a network of multiple stromal components regulating adult hematopoiesis. We use multi-omics and computational tools to analyze multiple BM environmental compartments and decipher their mutual interactions in the context of acute myeloid leukemia (AML) xenografts. Under homeostatic conditions, we find a considerable overlap between niche populations identified using current markers. Our analysis defines eight functional clusters of genes informing on the cellular identity and function of the different subpopulations and pointing at specific stromal interrelationships. We describe how these transcriptomic profiles change during human AML development and, by using a proximity-based molecular approach, we identify early disease onset deregulated genes in the mesenchymal compartment. Finally, we analyze the BM proteomic secretome in the presence of AML and integrate it with the transcriptome to predict signaling nodes involved in niche alteration in AML.

Unraveling the mystery of Gaucher bone density pathophysiology

Gaucher disease (GD) is caused by pathogenic mutations in GBA1, the gene that encodes the lysosomal enzyme β-glucocerebrosidase. Despite the existence of a variety of specific treatments for GD, they cannot completely reverse bone complications. Many studies have evidenced the impairment in bone tissue of GD, and molecular mechanisms of bone density alterations in GD are being studied during the last years and different reports emphasized its efforts trying to unravel why and how bone tissue is affected. The cause of skeletal density affection in GD is a matter of debates between research groups. and there are two opposing hypotheses trying to explain reduced bone mineral density in GD: increased bone resorption versus impaired bone formation. In this review, we discuss the diverse mechanisms of bone alterations implicated in GD revealed until the present, along with a presentation of normal bone physiology and its regulation. With this information in mind, we discuss effectiveness of specific therapies, introduce possible adjunctive therapies and present a novel model for GD-associated bone density pathogenesis. Under the exposed evidence, we may conclude that both sides of the balance of remodeling process are altered. In GD the observed osteopenia/osteoporosis may be the result of contribution of both reduced bone formation and increased bone resorption.

New insights on the reparative cells in bone regeneration and repair

Bone possesses a remarkable repair capacity to regenerate completely without scar tissue formation. This unique characteristic, expressed during bone development, maintenance and injury (fracture) healing, is performed by the reparative cells including skeletal stem cells (SSCs) and their descendants. However, the identity and functional roles of SSCs remain controversial due to technological difficulties and the heterogeneity and plasticity of SSCs. Moreover, for many years, there has been a biased view that bone marrow is the main cell source for bone repair. Together, these limitations have greatly hampered our understanding of these important cell populations and their potential applications in the treatment of fractures and skeletal diseases. Here, we reanalyse and summarize current understanding of the reparative cells in bone regeneration and repair and outline recent progress in this area, with a particular emphasis on the temporal and spatial process of fracture healing, the sources of reparative cells, an updated definition of SSCs, and markers of skeletal stem/progenitor cells contributing to the repair of craniofacial and long bones, as well as the debate between SSCs and pericytes. Finally, we also discuss the existing problems, emerging novel technologies and future research directions in this field.

Local Production of Osteoprotegerin by Osteoblasts Suppresses Bone Resorption

Osteoprotegerin (OPG) inhibits the ability of receptor activator of nuclear factor κB (NF-κB) ligand (RANKL) to stimulate the differentiation, activity, and survival of bone-resorbing osteoclasts. Genetic studies in mice show that osteocytes are an important source of RANKL, but the cellular sources of OPG are unclear. We use conditional deletion of Tnfrsf11b, which encodes OPG, from different cell populations to identify functionally relevant sources of OPG in mice. Deletion from B lymphocytes and osteocytes, two cell types commonly thought to supply OPG, has little or no impact on bone mass. By contrast, deletion of Tnfrsf11b from osteoblasts increases bone resorption and reduces bone mass to an extent similar to germline deletion, demonstrating that osteoblasts are an essential source of OPG. These results suggest that, in addition to producing new bone matrix, osteoblasts also play an active role in terminating the resorption phase of the bone remodeling cycle by suppressing RANKL activity.

Perivascular osteoprogenitors are associated with transcortical channels of long bones

Bone remodeling and regeneration are dependent on resident stem/progenitor cells with the ability to replenish mature osteoblasts and repair the skeleton. Using lineage tracing approaches, we identified a population of Dmp1+ cells that reside within cortical bone and are distinct from osteocytes. Our aims were to characterize this stromal population of transcortical perivascular cells (TPCs) in their resident niche and evaluate their osteogenic potential. To distinguish this population from osteoblasts/osteocytes, we crossed mice containing inducible DMP1CreERT2/Ai9 Tomato reporter (iDMP/T) with Col2.3GFP reporter (ColGFP), a marker of osteoblasts and osteocytes. We observed iDMP/T+;ColGFP- TPCs within cortical bone following tamoxifen injection. These cells were perivascular and located within transcortical channels. Ex vivo bone outgrowth cultures showed TPCs migrated out of the channels onto the plate and expressed stem cell markers such as Sca1, platelet derived growth factor receptor beta (PDGFRβ), and leptin receptor. In a cortical bone transplantation model, TPCs migrate from their vascular niche within cortical bone and contribute to new osteoblast formation and bone tube closure. Treatment with intermittent parathyroid hormone increased TPC number and differentiation. TPCs were unable to differentiate into adipocytes in the presence of rosiglitazone in vitro or in vivo. Altogether, we have identified and characterized a novel stromal lineage-restricted osteoprogenitor that is associated with transcortical vessels of long bones. Functionally, we have demonstrated that this population can migrate out of cortical bone channels, expand, and differentiate into osteoblasts, therefore serving as a source of progenitors contributing to new bone formation.

Strategies for elevating hematopoietic stem cells expansion and engraftment capacity

Hematopoietic stem cells (HSCs) are a rare cell population in adult bone marrow, mobilized peripheral blood, and umbilical cord blood possessing self-renewal and differentiation capability into a full spectrum of blood cells. Bone marrow HSC transplantation has been considered as an ideal option for certain disorders treatment including hematologic diseases, leukemia, immunodeficiency, bone marrow failure syndrome, genetic defects such as thalassemia, sickle cell anemia, autoimmune disease, and certain solid cancers. Ex vivo proliferation of these cells prior to transplantation has been proposed as a potential solution against limited number of stem cells. In such culture process, MSCs have also been shown to exhibit high capacity for secretion of soluble mediators contributing to the principle biological and therapeutic activities of HSCs. In addition, endothelial cells have been introduced to bridge the blood and sub tissues in the bone marrow, as well as, HSCs regeneration induction and survival. Cell culture in the laboratory environment requires cell growth strict control to protect against contamination, symmetrical cell division and optimal conditions for maximum yield. In this regard, microfluidic systems provide culture and analysis capabilities in micro volume scales. Moreover, two-dimensional cultures cannot fully demonstrate extracellular matrix found in different tissues and organs as an abstract representation of three dimensional cell structure. Microfluidic systems can also strongly describe the effects of physical factors such as temperature and pressure on cell behavior.

Exercise training differentially alters axial and appendicular marrow cellularity in old mice

Aging gradually renders bone marrow hematopoietically inactive. Endurance exercise reverses this phenotype in young mice. Here, we determine the effects in aged mice. Twenty-two month old mice (n = 6) underwent a progressive exercise training protocol. In appendicular bones, marrow cellularity increased by 51% (p < 0.05) and marrow CFU, CFU-GM, and CAFC increased by 12%, 71%, and 86%, respectively (p < 0.05). Vertebral cellularity remained unchanged. The mechanical forces associated with treadmill exercise training may be responsible for these observations.

Osteoblasts remotely supply lung tumors with cancer-promoting SiglecFhigh neutrophils

Bone marrow-derived myeloid cells can accumulate within tumors and foster cancer outgrowth. Local immune-neoplastic interactions have been intensively investigated, but the contribution of the systemic host environment to tumor growth remains poorly understood. Here, we show in mice and cancer patients (n = 70) that lung adenocarcinomas increase bone stromal activity in the absence of bone metastasis. Animal studies reveal that the cancer-induced bone phenotype involves bone-resident osteocalcin-expressing (Ocn⁺) osteoblastic cells. These cells promote cancer by remotely supplying a distinct subset of tumor-infiltrating SiglecF^high neutrophils, which exhibit cancer-promoting properties. Experimentally reducing Ocn⁺ cell numbers suppresses the neutrophil response and lung tumor outgrowth. These observations posit osteoblasts as remote regulators of lung cancer and identify SiglecF^high neutrophils as myeloid cell effectors of the osteoblast-driven protumoral response.

Comparative analysis of gene expression identifies distinct molecular signatures of bone marrow- and periosteal-skeletal stem/progenitor cells

Periosteum and bone marrow (BM) both contain skeletal stem/progenitor cells (SSCs) that participate in fracture repair. However, the functional difference and selective regulatory mechanisms of SSCs in different locations are unknown due to the lack of specific markers. Here, we report a comprehensive gene expression analysis of bone marrow SSCs (BM-SSCs), periosteal SSCs (P-SSCs), and more differentiated osteoprogenitors by using reporter mice expressing Interferon-inducible Mx1 and NestinGFP, previously known SSC markers. We first defined that the BM-SSCs can be enriched by the combination of Mx1 and NestinGFP expression, while endogenous P-SSCs can be isolated by positive selection of Mx1, CD105 and CD140a (known SSC markers) combined with the negative selection of CD45, CD31, and osteocalcinGFP (a mature osteoblast marker). Comparative gene expression analysis with FACS-sorted BM-SSCs, P-SSCs, Osterix+ preosteoblasts, CD51+ stroma cells and CD45+ hematopoietic cells as controls revealed that BM-SSCs and P-SSCs have high similarity with few potential differences without statistical significance. We also found that CD51+ cells are highly heterogeneous and have little overlap with SSCs. This was further supported by the microarray cluster analysis, where the two SSC populations clustered together but are separate from the CD51+ cells. However, when comparing SSC population to controls, we found several genes that are uniquely upregulated in endogenous SSCs. Amongst these genes, we found KDR (aka Flk1 or VEGFR2) to be most interesting and discovered that it is highly and selectively expressed in P-SSCs. This finding suggests that endogenous P-SSCs are functionally very similar to BM-SSCs with undetectable significant differences in gene expression but there are distinct molecular signatures in P-SSCs, which can be useful to specify P-SSC subset in vivo.

Brucella and Osteoarticular Cell Activation: Partners in Crime

Osteoarticular brucellosis is the most common presentation of human active disease although its prevalence varies widely. The three most common forms of osteoarticular involvement are sacroiliitis, spondylitis, and peripheral arthritis. The molecular mechanisms implicated in bone damage have been recently elucidated. B. abortus induces bone damage through diverse mechanisms in which TNF-α and the receptor activator of nuclear factor kappa-B ligand (RANKL)-the natural modulator of bone homeostasis are involved. These processes are driven by inflammatory cells, like monocytes/macrophages, neutrophils, Th17 CD4+ T, and B cells. In addition, Brucella abortus has a direct effect on osteoarticular cells and tilts homeostatic bone remodeling. These bacteria inhibit bone matrix deposition by osteoblasts (the only bone cells involved in bone deposition), and modify the phenotype of these cells to produce matrix metalloproteinases (MMPs) and cytokine secretion, contributing to bone matrix degradation. B. abortus also affects osteoclasts (cells naturally involved in bone resorption) by inducing an increase in osteoclastogenesis and osteoclast activation; thus, increasing mineral and organic bone matrix resorption, contributing to bone damage. Given that the pathology induced by Brucella species involved joint tissue, experiments conducted on synoviocytes revealed that besides inducing the activation of these cells to secrete chemokines, proinflammatory cytokines and MMPS, the infection also inhibits synoviocyte apoptosis. Brucella is an intracellular bacterium that replicates preferentially in the endoplasmic reticulum of macrophages. The analysis of B. abortus-infected synoviocytes indicated that bacteria also replicate in their reticulum suggesting that they could use this cell type for intracellular replication during the osteoarticular localization of the disease. Finally, the molecular mechanisms of osteoarticular brucellosis discovered recently shed light on how the interaction between B. abortus and immune and osteoarticular cells may play an important role in producing damage in joint and bone.

Quiescent Bone Lining Cells Are a Major Source of Osteoblasts During Adulthood

The in vivo origin of bone-producing osteoblasts is not fully defined. Skeletal stem cells, a population of mesenchymal stem cells resident in the bone marrow compartment, are thought to act as osteoprogenitors during growth and adulthood. Quiescent bone lining cells (BLCs) have been suggested as a population capable of activation into mature osteoblasts. These cells were defined by location and their morphology and studies addressing their significance have been hampered by their inaccessibility, and lack of markers that would allow for their identification and tracing. Using lineage tracing models, we have observed labeled osteoblasts at time points extending beyond the reported lifespan for this cell type, suggesting continuous reactivation of BLCs. BLCs also make a major contribution to bone formation after osteoblast ablation, which includes the ability to proliferate. In contrast, mesenchymal progenitors labeled by Gremlin1 or alpha smooth muscle actin do not contribute to bone formation in this setting. BLC activation is inhibited by glucocorticoids, which represent a well-established cause of osteoporosis. BLCs express cell surface markers characteristic of mesenchymal stem/progenitors that are largely absent in osteoblasts including Sca1 and Leptin Receptor. BLCs also show different gene expression profiles to osteoblasts, including elevated expression of Mmp13, and osteoclast regulators RANKL and macrophage colony stimulating factor, and retain osteogenic potential upon transplantation. Our findings provide evidence that bone lining cells represent a major source of osteoblasts during adulthood. Stem Cells 2016;34:2930-2942.

Behavioural changes induced by a conditional disruption of bone formation

BACKGROUND

It has been shown that the brain regulates bone remodelling through sympathetic and parasympathetic nerve fibres. However, it is unclear if signals from the skeleton also influence brain functions and animal behaviours.

METHODS

Bone formation was conditionally disrupted by daily injections of aciclovir (10 mg/kg) to transgenic mice expressing a herpes-simplex-virus thymidine kinase under the control of the osteoblast-specific promoter of the Bglap gene. Behavioural studies were conducted after 10 weeks of treatment.

RESULTS

Transgenic mice receiving aciclovir injections showed a reduced number of osteoblasts with a concomitantly reduced trabecular bone volume density, when compared to wild-type controls that were treated identically. The general health of the animals was not severely affected, as indicated by a similar increase in body weight, similar activity profiles and similar social behaviours. However, transgenic mice showed significantly increased despair behaviour and increased adrenal gland weights.

CONCLUSIONS

Specific animal behaviours can be modulated by a selective disruption of bone formation. The increased despair behaviour observed in transgenic animals indicates that these animals may be more prone to depression-related phenotypes. These findings are important in the context of the well-established clinical association between depression and reduced bone mass.

Adynamic Bone Decreases Bone Toughness During Aging by Affecting Mineral and Matrix

Adynamic bone is the most frequent type of bone lesion in patients with chronic kidney disease; long-term use of antiresorptive therapy may also lead to the adynamic bone condition. The hallmark of adynamic bone is a loss of bone turnover, and a major clinical concern of adynamic bone is diminished bone quality and an increase in fracture risk. Our current study aims to investigate how bone quality changes with age in our previously established mouse model of adynamic bone. Young and old mice (4 months old and 16 months old, respectively) were used in this study. Col2.3Δtk (DTK) mice were treated with ganciclovir and pamidronate to create the adynamic bone condition. Bone quality was evaluated using established techniques including bone histomorphometry, microcomputed tomography, quantitative backscattered electron imaging, and biomechanical testing. Changes in mineral and matrix properties were examined by powder X-ray diffraction and Raman spectroscopy. Aging controls had a natural decline in bone formation and resorption with a corresponding deterioration in trabecular bone structure. Bone turnover was severely blunted at all ages in adynamic animals, which preserved trabecular bone loss normally associated with aging. However, the preservation of trabecular bone mass and structure in old adynamic mice did not rescue deterioration of bone mechanical properties. There was also a decrease in cortical bone toughness in old adynamic mice that was accompanied by a more mature collagen matrix and longer bone crystals. Little is known about the effects of metabolic bone disease on bone fracture resistance. We observed an age-related decrease in bone toughness that was worsened by the adynamic condition, and this decrease may be due to material level changes at the tissue level. Our mouse model may be useful in the investigation of the mechanisms involved in fractures occurring in elderly patients on antiresorptive therapy who have very low bone turnover.

Bone Marrow Stress Decreases Osteogenic Progenitors

Age-related bone loss may be a result of declining levels of stem cells in the bone marrow. Using the Col2.3Δtk (DTK) transgenic mouse, osteoblast depletion was used as a source of marrow stress in order to investigate the effects of aging on osteogenic progenitors which reside in the marrow space. Five-month-old DTK mice were treated with one or two cycles of ganciclovir to conditionally ablate differentiated osteoblasts, whereas controls were saline-treated. Treatment cycles were two weeks in length followed by four weeks of recovery. All animals were sacrificed at 8 months of age; bone marrow stromal cells (BMSCs) were harvested for cell culture and whole bones were excised for bone quality assessment. Colony-forming unit (CFU) assays were conducted to investigate the osteogenic potential of BMSC in vitro, and RNA was extracted to assess the expression of osteoblastic genes. Bone quality assessments included bone histomorphometry, TRAP staining, microcomputed tomography, and biomechanical testing. Osteoblast depletion decreased CFU-F (fibroblast), CFU-ALP (alkaline phosphatase), and CFU-VK (von Kossa) counts and BMSC osteogenic capacity in cell culture. Ex vivo, there were no differences in bone mineral density of vertebrae or femurs between treatment groups. Histology showed a decrease in bone volume and bone connectivity with repeated osteoblast depletion; however, this was accompanied by an increase in bone formation rate. There were no notable differences in osteoclast parameters or observed bone marrow adiposity. We have developed a model that uses bone marrow stress to mimic age-related decrease in osteogenic progenitors. Our data suggest that the number of healthy BMSCs and their osteogenic potential decline with repeated osteoblast depletion. However, activity of the remaining osteoblasts increases to compensate for this loss in progenitor osteogenic potential.

Osteoblast-Specific Loss of IGF1R Signaling Results in Impaired Endochondral Bone Formation During Fracture Healing

Insulin-like growth factors (IGFs) are important local regulators during fracture healing. Although IGF1 deficiency is known to increase the risk of delayed union or non-union fractures in the elderly population, the underlying mechanisms that contribute to this defect remains unclear. In this study, IGF1 signaling during fracture healing was investigated in an osteoblast-specific IGF1 receptor (IGF1R) conditional knockout (KO) mouse model. A closed tibial fracture was induced in IGF1R(flox/flox) /2.3-kb α1(1)-collagen-Cre (KO) and IGF1R(flox/flox) (control) mice aged 12 weeks. Fracture callus samples and nonfractured tibial diaphysis were collected and analyzed by μCT, histology, immunohistochemistry, histomorphometry, and gene expression analysis at 10, 15, 21, and 28 days after fracture. A smaller size callus, lower bone volume accompanied by a defect in mineralization, bone microarchitectural abnormalities, and a higher cartilage volume were observed in the callus of these KO mice. The levels of osteoblast differentiation markers (osteocalcin, alkaline phosphatase, collagen 1α1) were significantly reduced, but the early osteoblast transcription factor runx2, as well as chondrocyte differentiation markers (collagen 2α1 and collagen 10α1) were significantly increased in the KO callus. Moreover, increased numbers of osteoclasts and impaired angiogenesis were observed during the first 15 days of fracture repair, but decreased numbers of osteoclasts were found in the later stages of fracture repair in the KO mice. Although baseline nonfractured tibias of KO mice had decreased trabecular and cortical bone compared to control mice, subsequent studies with mice expressing the 2.3-kb α1(1)-collagen-Cre ERT2 construct and given tamoxifen at the time of fracture and so starting with comparable bone levels showed similar impairment in fracture repair at least initially. Our data indicate that not only is the IGF1R in osteoblasts involved in osteoblast differentiation during fracture repair, but it plays an important role in coordinating chondrocyte, osteoclast, and endothelial responses that all contribute to the endochondral bone formation required for normal fracture repair.

Exposure to a youthful circulaton rejuvenates bone repair through modulation of β-catenin

The capacity for tissues to repair and regenerate diminishes with age. We sought to determine the age-dependent contribution of native mesenchymal cells and circulating factors on in vivo bone repair. Here we show that exposure to youthful circulation by heterochronic parabiosis reverses the aged fracture repair phenotype and the diminished osteoblastic differentiation capacity of old animals. This rejuvenation effect is recapitulated by engraftment of young haematopoietic cells into old animals. During rejuvenation, β-catenin signalling, a pathway important in osteoblast differentiation, is modulated in the early repair process and required for rejuvenation of the aged phenotype. Temporal reduction of β-catenin signalling during early fracture repair improves bone healing in old mice. Our data indicate that young haematopoietic cells have the capacity to rejuvenate bone repair and this is mediated at least in part through β-catenin, raising the possibility that agents that modulate β-catenin can improve the pace or quality of fracture repair in the ageing population.

The reparative capacity of tissues decreases with age. Here, Baht et al. perform parabiosis experiments and provide evidence for the existence of a soluble factor produced by young, but not old, haematopoietic cells that is capable of improving fracture repair in old mice.

Osteoblast ablation reduces normal long-term hematopoietic stem cell self-renewal but accelerates leukemia development

Hematopoietic stem cells (HSCs) reside in regulatory niches in the bone marrow (BM). Although HSC niches have been extensively characterized, the role of endosteal osteoblasts (OBs) in HSC regulation requires further clarification, and the role of OBs in regulating leukemic stem cells (LSCs) is not well studied. We used an OB visualization and ablation mouse model to study the role of OBs in regulating normal HSCs and chronic myelogenous leukemia (CML) LSCs. OB ablation resulted in increase in cells with a LSK Flt3(-)CD150(+)CD48(-) long-term HSC (LTHSC) phenotype but reduction of a more highly selected LSK Flt3(-)CD34(-)CD49b(-)CD229(-) LTHSC subpopulation. LTHSCs from OB-ablated mice demonstrated loss of quiescence and reduced long-term engraftment and self-renewal capacity. Ablation of OB in a transgenic CML mouse model resulted in accelerated leukemia development with reduced survival compared with control mice. The notch ligand Jagged-1 was overexpressed on CML OBs. Normal and CML LTHSCs cultured with Jagged-1 demonstrated reduced cell cycling, consistent with a possible role for loss of Jagged-1 signals in altered HSC and LSC function after OB ablation. These studies support an important role for OBs in regulating quiescence and self-renewal of LTHSCs and a previously unrecognized role in modulating leukemia development in CML.

Development, validation and characterization of a novel mouse model of Adynamic Bone Disease (ABD)

The etiology of Adynamic Bone Disease (ABD) is poorly understood but the hallmark of ABD is a lack of bone turnover. ABD occurs in renal osteodystrophy (ROD) and is suspected to occur in elderly patients on long-term anti-resorptive therapy. A major clinical concern of ABD is diminished bone quality and an increased fracture risk. To our knowledge, experimental animal models for ABD other than ROD-ABD have not been developed or studied. The objectives of this study were to develop a mouse model of ABD without the complications of renal ablation, and to characterize changes in bone quality in ABD relative to controls. To re-create the adynamic bone condition, 4-month old female Col2.3Δtk mice were treated with ganciclovir to specifically ablate osteoblasts, and pamidronate was used to inhibit osteoclastic resorption. Four groups of animals were used to characterize bone quality in ABD: Normal bone controls, No Formation controls, No Resorption controls, and an Adynamic group. After a 6-week treatment period, the animals were sacrificed and the bones were harvested for analyses. Bone quality assessments were conducted using established techniques including bone histology, quantitative backscattered electron imaging (qBEI), dual energy X-ray absorptiometry (DXA), microcomputed tomography (microCT), and biomechanical testing. Histomorphometry confirmed osteoblast-related hallmarks of ABD in our mouse model. Bone formation was near complete suppression in the No Formation and Adynamic specimens. Inhibition of bone resorption in the Adynamic group was confirmed by tartrate-resistant acid phosphatase (TRAP) stain. Normal bone mineral density and architecture were maintained in the Adynamic group, whereas the No Formation group showed a reduction in bone mineral content and trabecular thickness relative to the Adynamic group. As expected, the No Formation group had a more hypomineralized profile and the Adynamic group had a higher mean mineralization profile that is similar to suppressed bone turnover in human. This data confirms successful replication of the adynamic bone condition in a mouse without the complication of renal ablation. Our approach is the first model of ABD that uses pharmacological manipulation in a transgenic mouse to mimic the bone cellular dynamics observed in the human ABD condition. We plan to use our mouse model to investigate the adynamic bone condition in aging and to study changes to bone quality and fracture risk as a consequence of over-suppressed bone turnover.

Erythropoietin supports the survival of prostate cancer, but not growth and bone metastasis

Erythropoietin (Epo) is used in clinical settings to enhance hematopoietic function and to improve the quality of life for patients undergoing chemotherapy by reducing fatigue and the need for transfusions. However, several meta-analyses have revealed that Epo treatments are associated with an increased risk of mortality in cancer patients. In this study, we examined the role of Epo in prostate cancer (PCa) progression, using in vitro cell culture systems and in vivo bone metastatic assays. We found that Epo did not stimulate the proliferation of PCa cell lines, but did protect PCa cells from apoptosis. In animal models of PCa metastasis, no evidence was found to support the hypothesis that Epo enhances metastasis. Together, these findings suggest that Epo may be useful for treating severe anemia in PCa patients without increasing metastatic risk.

Mesenchymal progenitors and the osteoblast lineage in bone marrow hematopoietic niches

The bone marrow cavity is essential for the proper development of the hematopoietic system. In the last few decades, it has become clear that mesenchymal stem/progenitor cells as well as cells of the osteoblast lineage, besides maintaining bone homeostasis, are also fundamental regulators of bone marrow hematopoiesis. Several studies have demonstrated the direct involvement of mesenchymal and osteoblast lineage cells in the maintenance and regulation of supportive microenvironments necessary for quiescence, self-renewal and differentiation of hematopoietic stem cells. In addition, specific niches have also been identified within the bone marrow for maturing hematopoietic cells. Here we will review recent findings that have highlighted the roles of mesenchymal progenitors and cells of the osteoblast lineage in regulating distinct stages of hematopoiesis.

The bone marrow niche for haematopoietic stem cells

Niches are local tissue microenvironments that maintain and regulate stem cells. Haematopoiesis provides a model for understanding mammalian stem cells and their niches, but the haematopoietic stem cell (HSC) niche remains incompletely defined and beset by competing models. Recent progress has been made in elucidating the location and cellular components of the HSC niche in the bone marrow. The niche is perivascular, created partly by mesenchymal stromal cells and endothelial cells and often, but not always, located near trabecular bone. Outstanding questions concern the cellular complexity of the niche, the role of the endosteum and functional heterogeneity among perivascular microenvironments.

Cellular complexity of the bone marrow hematopoietic stem cell niche

The skeleton serves as the principal site for hematopoiesis in adult terrestrial vertebrates. The function of the hematopoietic system is to maintain homeostatic levels of all circulating blood cells, including myeloid cells, lymphoid cells, red blood cells, and platelets. This action requires the daily production of more than 500 billion blood cells. The vast majority of these cells are synthesized in the bone marrow, where they arise from a limited number of hematopoietic stem cells (HSCs) that are multipotent and capable of extensive self-renewal. These attributes of HSCs are best demonstrated by marrow transplantation, where even a single HSC can repopulate the entire hematopoietic system. HSCs are therefore adult stem cells capable of multilineage repopulation, poised between cell fate choices which include quiescence, self-renewal, differentiation, and apoptosis. While HSC fate choices are in part determined by multiple stochastic fluctuations of cell autonomous processes, according to the niche hypothesis, signals from the microenvironment are also likely to determine stem cell fate. While it had long been postulated that signals within the bone marrow could provide regulation of hematopoietic cells, it is only in the past decade that advances in flow cytometry and genetic models have allowed for a deeper understanding of the microenvironmental regulation of HSCs. In this review, we will highlight the cellular regulatory components of the HSC niche.

Genetic cell ablation

Targeted cell ablation has proven to be a valuable approach to study in vivo cell functions during organogenesis, tissue homeostasis, and regeneration. Over the last two decades, various approaches have been developed to refine the control of cell ablation. In this review, we give an overview of the distinct genetic tools available for targeted cell ablation, with a particular emphasis on their respective specificity.

Notch signaling in skeletal stem cells

The skeleton originates from stem cells residing in the sclerotome and neural crest that undergo proliferation, migration, and commitment. The development of the skeletal stem cells is influenced by many signaling pathways that govern cell fate determination, proliferation, differentiation, and apoptosis. This review will focus on Notch signaling functions in regulating the different cell types that form the skeletal system as well as the interplay between them to maintain homeostasis. Osteochondroprogenitors require Notch signaling to maintain multipotency and to prevent premature differentiation into osteoblasts. Subsequently, overactivation of Notch signaling suppresses osteoblast maturation. Moreover, Notch signaling in osteochondroprogenitors is required for chondrocyte proliferation and hypertrophy and suppresses terminal differentiation. Translational studies demonstrated a crucial role of Notch signaling in osteosarcoma and osteoarthritis, where concepts derived from developmental pathways are often recapitulated. This brings hope of taking advantage of the molecular mechanisms learned from development to approach the pathological processes underlying abnormal bone/cartilage metabolism or tumorigenesis. Pharmacological agents that target Notch receptors or ligands in a tissue-specific fashion would offer new opportunities for treating bone/cartilage diseases caused by dysregulation of Notch signaling.

Intravital imaging of hematopoietic stem cells in the mouse skull

Over the past 50 years, much insight has been gained into the biology of hematopoietic stem cells (HSCs). Much of this information has been gained though isolation of specific bone marrow populations, and transplantation into irradiated recipients followed by characterization of chimeras months later. These studies have yielded insights into the function of HSCs, but have shed little light on the interactions of individual stem cells with their environment. Characterization of the behavior of single HSCs awaited the use of relatively noninvasive intravital microscopy, which allows one to identify rare cells in real time and follow them in multiple imaging sessions. Here we describe techniques used to image transplanted HSCs in the mouse calvarium using hybrid confocal/multi-photon microscopy and second harmonic imaging. For detection, fluorescently tagged HSCs are transplanted into a recipient mouse. The architecture of the bone marrow can be delineated using a combination of fluorescent probes and vascular dyes, second harmonic generation to detect the collagen signal from bone, and transgenic recipient mice containing specific fluorescent support cell populations.

Suicide gene approach using a dual-expression lentiviral vector to enhance the safety of ex vivo gene therapy for bone repair

'Ex vivo' gene therapy using viral vectors to overexpress BMP-2 is shown to heal critical-sized bone defects in experimental animals. To increase its safety, we constructed a dual-expression lentiviral vector to overexpress BMP-2 or luciferase and an HSV1-tk analog, Δtk (LV-Δtk-T2A-BMP-2/Luc). We hypothesized that administering ganciclovir (GCV) will eliminate the transduced cells at the site of implantation. The vector-induced expression of BMP-2 and luciferase in a mouse stromal cell line (W-20-17 cells) and mouse bone marrow cells (MBMCs) was reduced by 50% compared with the single-gene vector. W-20-17 cells were more sensitive to GCV compared with MBMCs (90-95% cell death at 12 days with GCV at 1 μg ml(-1) in MBMCs vs 90-95% cell death at 5 days by 0.1 μg ml(-1) of GCV in W-20-17 cells). Implantation of LV-Δtk-T2A-BMP-2 transduced MBMCs healed a 2 mm femoral defect at 4 weeks. Early GCV treatment (days 0-14) postoperatively blocked bone formation confirming a biologic response. Delayed GCV treatment starting at day 14 for 2 or 4 weeks reduced the luciferase signal from LV-Δtk-T2A-Luc-transduced MBMCs, but the signal was not completely eliminated. These data suggest that this suicide gene strategy has potential for clinical use in the future, but will need to be optimized for increased efficiency.

Deciphering hematopoietic stem cells in their niches: a critical appraisal of genetic models, lineage tracing, and imaging strategies

In recent years, technical developments in mouse genetics and imaging equipment have substantially advanced our understanding of hematopoietic stem cells (HSCs) and their niche. The availability of numerous Cre strains for targeting HSCs and microenvironmental cells provides extensive flexibility in experimental design, but it can also pose significant challenges due to strain-specific differences in cell specificity. Here we outline various genetic approaches for isolating, detecting, and ablating HSCs and niche components and provide a guide for advantages and caveats to consider. We also discuss opportunities and limitations presented by imaging technologies that allow investigation of HSC behavior in situ.

Mechanobiology of bone marrow stem cells: from myosin-II forces to compliance of matrix and nucleus in cell forms and fates

Adult stem cells and progenitors are of great interest for their clinical application as well as their potential to reveal deep sensitivities to microenvironmental factors. The bone marrow is a niche for at least two types of stem cells, and the prototype is the hematopoietic stem cell/progenitors (HSC/Ps), which have saved many thousands of patients for several decades now. In bone marrow, HSC/Ps interact functionally with marrow stromal cells that are often referred to as mesenchymal stem cells (MSCs) or derivatives thereof. Myosin and matrix elasticity greatly affect MSC function, and these mechanobiological factors are now being explored with HSC/Ps both in vitro and in vivo. Also emerging is a role for the nucleus as a mechanically sensitive organelle that is semi-permeable to transcription factors which are modified for nuclear entry by cytoplasmic mechanobiological pathways. Since therapies envisioned with induced pluripotent stem cells and embryonic stem cells generally involve in vitro commitment to an adult stem cell or progenitor, a very deep understanding of stem cell mechanobiology is essential to progress with these multi-potent cells.

Specification of chondrocytes and cartilage tissues from embryonic stem cells

Osteoarthritis primarily affects the articular cartilage of synovial joints. Cell and/or cartilage replacement is a promising therapy, provided there is access to appropriate tissue and sufficient numbers of articular chondrocytes. Embryonic stem cells (ESCs) represent a potentially unlimited source of chondrocytes and tissues as they can generate a broad spectrum of cell types under appropriate conditions in vitro. Here, we demonstrate that mouse ESC-derived chondrogenic mesoderm arises from a Flk-1(-)/Pdgfrα(+) (F(-)P(+)) population that emerges in a defined temporal pattern following the development of an early cardiogenic F(-)P(+) population. Specification of the late-arising F(-)P(+) population with BMP4 generated a highly enriched population of chondrocytes expressing genes associated with growth plate hypertrophic chondrocytes. By contrast, specification with Gdf5, together with inhibition of hedgehog and BMP signaling pathways, generated a population of non-hypertrophic chondrocytes that displayed properties of articular chondrocytes. The two chondrocyte populations retained their hypertrophic and non-hypertrophic properties when induced to generate spatially organized proteoglycan-rich cartilage-like tissue in vitro. Transplantation of either type of chondrocyte, or tissue generated from them, into immunodeficient recipients resulted in the development of cartilage tissue and bone within an 8-week period. Significant ossification was not observed when the tissue was transplanted into osteoblast-depleted mice or into diffusion chambers that prevent vascularization. Thus, through stage-specific manipulation of appropriate signaling pathways it is possible to efficiently and reproducibly derive hypertrophic and non-hypertrophic chondrocyte populations from mouse ESCs that are able to generate distinct cartilage-like tissue in vitro and maintain a cartilage tissue phenotype within an avascular and/or osteoblast-free niche in vivo.

Defective endochondral ossification-derived matrix and bone cells alter the lymphopoietic niche in collagen X mouse models

Despite the appreciated interdependence of skeletal and hematopoietic development, the cell and matrix components of the hematopoietic niche remain to be fully defined. Utilizing mice with disrupted function of collagen X (ColX), a major hypertrophic cartilage matrix protein associated with endochondral ossification, our data identified a cytokine defect in trabecular bone cells at the chondro-osseous hematopoietic niche as a cause for aberrant B lymphopoiesis in these mice. Specifically, analysis of ColX transgenic and null mouse chondro-osseous regions via micro-computed tomography revealed an altered trabecular bone environment. Additionally, cocultures with hematopoietic and chondro-osseous cell types highlighted impaired hematopoietic support by ColX transgenic and null mouse derived trabecular bone cells. Further, cytokine arrays with conditioned media from the trabecular osteoblast cocultures suggested an aberrant hematopoietic cytokine milieu within the chondro-osseous niche of the ColX deficient mice. Accordingly, B lymphopoiesis was rescued in the ColX mouse derived trabecular osteoblast cocultures with interlukin-7, stem cell factor, and stromal derived factor-1 supplementation. Moreover, B cell development was restored in vivo after injections of interlukin-7. These data support our hypothesis that endrochondrally-derived trabecular bone cells and matrix constituents provide cytokine-rich niches for hematopoiesis. Furthermore, this study contributes to the emerging concept that niche defects may underlie certain immuno-osseous and hematopoietic disorders.

New insights into adhesion signaling in bone formation

Mineralized tissues that are protective scaffolds in the most primitive species have evolved and acquired more specific functions in modern animals. These are as diverse as support in locomotion, ion homeostasis, and precise hormonal regulation. Bone formation is tightly controlled by a balance between anabolism, in which osteoblasts are the main players, and catabolism mediated by the osteoclasts. The bone matrix is deposited in a cyclic fashion during homeostasis and integrates several environmental cues. These include diffusible elements that would include estrogen or growth factors and physicochemical parameters such as bone matrix composition, stiffness, and mechanical stress. Therefore, the microenvironment is of paramount importance for controlling this delicate equilibrium. Here, we provide an overview of the most recent data highlighting the role of cell-adhesion molecules during bone formation. Due to the very large scope of the topic, we focus mainly on the role of the integrin receptor family during osteogenesis. Bone phenotypes of some deficient mice as well as diseases of human bones involving cell adhesion during this process are discussed in the context of bone physiology.

Harnessing the potential of adult cardiac stem cells: lessons from haematopoiesis, the embryo and the niche

Across biomedicine, there is a major drive to develop stem cell (SC) treatments for debilitating diseases. Most effective treatments restore an embryonic phenotype to adult SCs. This has led to two emerging paradigms in SC biology: the application of developmental biology studies and the manipulation of the SC niche. Developmental studies can reveal how SCs are orchestrated to build organs, the understanding of which is important in order to instigate tissue repair in the adult. SC niche studies can reveal cues that maintain SC 'stemness' and how SCs may be released from the constraints of the niche to differentiate and repopulate a 'failing' organ. The haematopoietic system provides an exemplar whereby characterisation of the blood lineages during development and the bone marrow niche has resulted in therapeutics now routinely used in the clinic. Ischaemic heart disease is a major cause of morbidity and mortality in humans and the question remains as to whether these principles can be applied to the heart, in order to exploit the potential of adult SCs for use in cardiovascular repair and regeneration.

A family business: stem cell progeny join the niche to regulate homeostasis

Stem cell niches, the discrete microenvironments in which the stem cells reside, play a dominant part in regulating stem cell activity and behaviours. Recent studies suggest that committed stem cell progeny become indispensable components of the niche in a wide range of stem cell systems. These unexpected niche inhabitants provide versatile feedback signals to their stem cell parents. Together with other heterologous cell types that constitute the niche, they contribute to the dynamics of the microenvironment. As progeny are often located in close proximity to stem cell niches, similar feedback regulations may be the underlying principles shared by different stem cell systems.

The location and cellular composition of the hemopoietic stem cell niche

While it is accepted that hemopoietic stem cells (HSC) are located in a three-dimensional microenvironment, termed a niche, the cellular and extracellular composition, as well as the multifaceted effects the components of the niche have on HSC regulation, remains undefined. Over the past four decades numerous advances in the field have led to the identification of roles for some cell types and propositions of potentially a number of HSC niches. We present evidence supporting the roles of multiple cell types and extracellular matrix molecules in the HSC niche, as well as discuss the potential significant overlap and intertwining of previously proposed distinct HSC niches.

Mobilization of hematopoietic stem and leukemia cells

HSC mobilization is an essential homeostatic process during inflammation and for the maintenance of hematopoietic progenitors. It has been exploited for the therapeutic application of HSC transplantation. Recent evidence suggests that leukemic cells share surface molecules in common with stem cells and may be mobilized under similar conditions. This effect could be used for therapeutic interventions. In this review, we will provide evidence showing that leukemia cells and stem cells traffic similarly and may share a common niche. Studies are discussed comparing and contrasting the mechanism of normal stem cells and leukemic cell mobilization through the CXCR4/CXCL12 axis and other key intermediaries.

Dynamic niches in the origination and differentiation of haematopoietic stem cells

Haematopoietic stem cells (HSCs) are multipotent, self-renewing progenitors that generate all mature blood cells. HSC function is tightly controlled to maintain haematopoietic homeostasis, and this regulation relies on specialized cells and factors that constitute the haematopoietic 'niche', or microenvironment. Recent discoveries, aided in part by technological advances in in vivo imaging, have engendered a new appreciation for the dynamic nature of the niche, identifying novel cellular and acellular niche components and uncovering fluctuations in the relative importance of these components over time. These new insights significantly improve our understanding of haematopoiesis and raise fundamental questions about what truly constitutes a stem cell niche.

The positive association between peripheral blood cell counts and bone mineral density in postmenopausal women

PURPOSE

Accumulating evidence has shown a close connection between hematopoiesis and bone formation. Our aim was to evaluate the association between peripheral blood cell counts and bone mineral density (BMD) in a sample of postmenopausal women.

MATERIALS AND METHODS

Three hundreds thirty eight healthy postmenopausal women who underwent BMD measurement during their health check-up were investigated. BMD was measured by dual energy X-ray asorptiometry at L1-L4 spine, femoral neck and total proximal femur. BMD was expressed as a T-score: among T-scores obtained from three different sites (L1-L4 spine, femoral neck and total proximal femur), the lowest T-score was considered to be the subject's T-score.

RESULTS

The prevalence of osteopenia and osteoporosis diagnosed by T-score in the study participants were 49.4% (167/338) and 5.0% (17/338), respectively. Peripheral blood white blood cell (WBC), red blood cell (RBC) and platelet counts had significant positive correlations with T-scores (p<0.001) upon simple linear regression analysis. A multiple linear regression analysis, after controlling of confounders including age, body weight, systolic blood pressure, alkaline phosphatase and creatinine, showed that WBC (β=0.127; standard error=0.043; p=0.014), RBC (β=0.192; standard error=0.139; p<0.001) and platelet (β=0.097; standard error=0.001; p=0.050) counts still had significant positive association with T-scores.

CONCLUSION

The study results showed a positive relationship between blood cell counts and bone mineral density in postmenopausal women, supporting the idea of a close connection between hematopoiesis and bone formation. The study results also suggest that blood cell counts could be a putative marker for estimating BMD in postmenopausal women.

Minireview: The stem cell next door: skeletal and hematopoietic stem cell "niches" in bone

Long known to be home to hematopoietic stem cells (HSC), the bone/bone marrow organ and its cellular components are directly implicated in regulating hematopoiesis and HSC function. Over the past few years, advances on the identity of HSC "niche" cells have brought into focus the role of cells of osteogenic lineage and of marrow microvessels. At the same time, the identity of self-renewing multipotent skeletal progenitors (skeletal stem cells, also known as mesenchymal stem cells) has also been more precisely defined, along with the recognition of their own microvascular niche. The two sets of evidence converge in delineating a picture in which two kinds of stem cells share an identical microanatomical location in the bone/bone marrow organ. This opens a new view on the manner in which the skeleton and hematopoiesis can cross-regulate via interacting stem cells but also a novel view of our general concept of stem cell niches.

Bone and the hematopoietic niche: a tale of two stem cells

The revived interest in (hematopoietic) stem cell (HSC) niches has highlighted the role of multiple cellular players found in the bone environment. Initially focused on the role of osteoblasts and sinusoid endothelial cells, the quest for HSC niche cells has recently focused on a unique role for osteoprogenitor cells (skeletal stem cells, mesenchymal stem cells). Strongly validated by observations of HSC dysregulation dictated by the dysregulation of osteoprogenitors, the role of osteoprogenitors in the HSC niche integrates data from different studies into a unified view. As preosteoblastic, periendothelial cells residing at the sinusoid wall, skeletal progenitors reconcile the notions of "osteoblastic" and "sinusoidal" niches with one another. In addition, they bring into focus the cross-regulation of skeletal and hematopoietic physiology as rooted into the interplay of two stem cells (hematopoietic and skeletal) sharing a single niche. As direct regulators of hematopoietic space formation, sinusoid development, and hematopoietic function(s), as well as direct progenitors of positive and negative regulators of HSCs such as osteoblasts and adipocytes, skeletal progenitors have emerged as pivotal organizers of a complex, highly plastic niche. This development seems to represents an evolutionary advance over the deterministic stem cell niches found in archetypal invertebrate systems.

Bidirectional interactions between bone metabolism and hematopoiesis

Interactions between hematopoiesis and bone metabolism have been described in various developmental and pathological situations. Here we review this evidence from the literature with a focus on microenvironmental regulation of hematopoiesis and bone metabolism. Our hypothesis is that this process occurs by bidirectional signaling between hematopoietic and mesenchymal cells through cell adhesion molecules, membrane-bound growth factors, and secreted matrix proteins. Examples of steady-state hematopoiesis and pathologies are presented and support our view that hematopoietic and mesenchymal cell functions are modulated by specific microenvironments in the bone marrow.

Mobilization of hematopoietic stem cells into the peripheral blood

Hematopoietic stem cells can be mobilized out of the bone marrow into the blood for the reconstitution of hematopoiesis following high-dose therapy. Methods to improve mobilization efficiency and yields are rapidly emerging. Traditional methods include chemotherapy with or without myeloid growth factors. Plerixafor, a novel agent that disrupts the CXCR4-CXCL12 bond, the primary hematopoietic stem cell anchor in the bone marrow, has recently been US FDA-approved for mobilizing hematopoietic stem cells in patients with non-Hodgkin lymphoma and multiple myeloma. Plerixafor and myeloid growth factors as single agents appear safe to use in family or volunteer hematopoietic stem cells donors. Plerixafor mobilizes leukemic stem cells and is not approved for use in patients with acute leukemia. Patients failing to mobilize adequate hematopoietic stem cells with myeloid growth factors can often be successfully mobilized with chemotherapy plus myeloid growth factors or with plerixafor and granulocyte colony-stimulating factor.