Skeletal development, bone remodeling, and hematopoiesis

Construction a novel osteoporosis model in immune-deficient mice with natural ageing

Osteoporosis (OP) predominantly affects elderly individuals. Stem cells show potential for treating OP. However, animal models with normal immune function can eliminate implanted human cells. This study utilized naturally aging NOD/SCID mice, which exhibit immunodeficiency, to create a human osteoporosis model. This approach helps to minimize the premature immune clearance of transplanted allogeneic or xenogeneic cells in preclinical studies, allowing for a more accurate replication of the clinical pharmacological and pharmacokinetic processes involved in stem cell interventions for osteoporosis. NOD/SCID mice were fed until 12, 32, and 43 weeks of age, respectively, and then euthanized. We harvested lumbar vertebra for Micro-Computed Tomography (Micro-CT) scanning and pathological examination. Additionally, we performed biomechanical testing of lumbar vertebra to assess the severity of osteoporosis. We utilized real-time RT-PCR to assess gene expression changes associated with bone metabolism, aging, inflammation, oxidative stress, and the Tgf-β1/Smad3 signaling pathway. In addition, the protein expression levels of P16, Tgf-β1 and Smad3 were detected using Western Blotting (WB). In comparison to 12-week-old mice, the 32-week-old and 43-week-old mice displayed significantly sparser and fractured trabeculae in their lumbar vertebra, lower bone mineral density (BMD), and changes in bone microstructural parameters (∗∗P < 0.01, ∗∗∗P < 0.001). Additionally, compared to 12-week-old mice, the 32-week-old and 43-week-old mice exhibited decreased expression of osteogenic genes (Alp, Opg, Sp7, Col1a1), increased expression of osteoclastic gene (Rankl), the number of TRAP-positive osteoclasts significantly increased in 32-week-old and 43-week-old mice compared to 12-week-old mice. The expression of genes related to aging and inflammatory (P16, Il-1β, Tnf-α) increases with advancing age (∗P < 0.05, ∗∗P < 0.01, ∗∗∗P < 0.001). The expression of oxidative stress-related genes (Sod1, Sod2, Foxo3, Nrf2), as well as Tgf-β1 and Smad3 decreased with age (∗P < 0.05, ∗∗P < 0.01, ∗∗∗P < 0.001). As age increases, the levels of P16 protein increase, Tgf-β1 and Smad3 proteins decrease. Our study successfully replicated osteoporosis models in NOD/SCID mice at both 32 and 43 weeks, with the latter exhibiting more severe osteoporosis. This condition seems to be driven by factors such as aging, inflammation, oxidative stress, and the Tgf-β1/Smad3 signaling pathway.

Macrophage-to-osteocyte communication: Impact in a 3D in vitro implant-associated infection model

Macrophages and osteocytes are important regulators of inflammation, osteogenesis and osteoclastogenesis. However, their interactions under adverse conditions, such as biomaterial-associated infection (BAI) are not fully understood. We aimed to elucidate how factors released from macrophages modulate osteocyte responses in an in vitro indirect 3D co-culture model. Human monocyte-derived macrophages were cultured on etched titanium disks and activated with either IL-4 cytokine (anti-inflammatory M2 phenotype) or Staphylococcus aureus secreted virulence factors to simulate BAI (pro-inflammatory M1 phenotype). Primary osteocytes in collagen gels were then stimulated with conditioned media (CM) from these macrophages. The osteocyte response was analyzed by gene expression, protein secretion, and immunostaining. M1 phenotype macrophages were confirmed by IL-1β and TNF-α secretion, and M2 macrophages by ARG-1 and MRC-1.Osteocytes receiving M1 CM revealed bone inhibitory effects, denoted by reduced secretion of bone formation osteocalcin (BGLAP) and increased secretion of the bone inhibitory sclerostin (SOST). These osteocytes also downregulated the pro-mineralization gene PHEX and upregulated the anti-mineralization gene MEPE. Additionally, exhibited pro-osteoclastic potential by upregulating pro-osteoclastic gene RANKL expression. Nonetheless, M1-stimulated osteocytes expressed a higher level of the potent pro-osteogenic factor BMP-2 in parallel with the downregulation of the bone inhibitor genes DKK1 and SOST, suggesting a compensatory feedback mechanisms. Conversely, M2-stimulated osteocytes mainly upregulated anti-osteoclastic gene OPG expression, suggesting an anti-catabolic effect. Altogether, our findings demonstrate a strong communication between M1 macrophages and osteocytes under M1 (BAI)-simulated conditions, suggesting that the BAI adverse effects on osteoblastic and osteoclastic processes in vitro are partly mediated via this communication. STATEMENT OF SIGNIFICANCE: Biomaterial-associated infections are major challenges and the underlying mechanisms in the cellular interactions are missing, especially among the major cells from the inflammatory side (macrophages as the key cell in bacterial clearance) and the regenerative side (osteocyte as main regulator of bone). We evaluated the effect of macrophage polarization driven by the stimulation with bacterial virulence factors on the osteocyte function using an indirect co-culture model, hence mimicking the scenario of a biomaterial-associated infection. The results suggest that at least part of the adverse effects of biomaterial associated infection on osteoblastic and osteoclastic processes in vitro are mediated via macrophage-to-osteocyte communication.

Interactions of B-lymphocytes and bone cells in health and disease

Bone remodeling occurs through the interactions of three major cell lineages, osteoblasts, which mediate bone formation, osteocytes, which derive from osteoblasts, sense mechanical force and direct bone turnover, and osteoclasts, which mediate bone resorption. However, multiple additional cell types within the bone marrow, including macrophages, T lymphocytes and B lymphocytes influence the process. The bone marrow microenvironment, which is supported, in part, by bone cells, forms a nurturing network for B lymphopoiesis. In turn, developing B lymphocytes influence bone cells. Bone health during homeostasis depends on the normal interactions of bone cells with other lineages in the bone marrow. In disease state these interactions become pathologic and can cause abnormal function of bone cells and inadequate repair of bone after a fracture. This review summarizes what is known about the development of B lymphocytes and the interactions of B lymphocytes with bone cells in both health and disease.

How cell migration helps immune sentinels

The immune system relies on the migratory capacity of its cellular components, which must be mobile in order to defend the host from invading micro-organisms or malignant cells. This applies in particular to immune sentinels from the myeloid lineage, i.e. macrophages and dendritic cells. Cell migration is already at work during mammalian early development, when myeloid cell precursors migrate from the yolk sac, an extra embryonic structure, to colonize tissues and form the pool of tissue-resident macrophages. Later, this is accompanied by a migration wave of precursors and monocytes from the bone marrow to secondary lymphoid organs and the peripheral tissues. They differentiate into DCs and monocyte-derived macrophages. During adult life, cell migration endows immune cells with the ability to patrol their environment as well as to circulate between peripheral tissues and lymphoid organs. Hence migration of immune cells is key to building an efficient defense system for an organism. In this review, we will describe how cell migratory capacity regulates the various stages in the life of myeloid cells from development to tissue patrolling, and migration to lymph nodes. We will focus on the role of the actin cytoskeletal machinery and its regulators, and how it contributes to the establishment and function of the immune system.

The origins and roles of osteoclasts in bone development, homeostasis and repair

The mechanisms underlying bone development, repair and regeneration are reliant on the interplay and communication between osteoclasts and other surrounding cells. Osteoclasts are multinucleated monocyte lineage cells with resorptive abilities, forming the bone marrow cavity during development. This marrow cavity, essential to hematopoiesis and osteoclast-osteoblast interactions, provides a setting to investigate the origin of osteoclasts and their multi-faceted roles. This Review examines recent developments in the embryonic understanding of osteoclast origin, as well as interactions within the immune environment to regulate normal and pathological bone development, homeostasis and repair.

TLE4 Is a Critical Mediator of Osteoblast and Runx2-Dependent Bone Development

Healthy bone homeostasis hinges upon a delicate balance and regulation of multiple processes that contribute to bone development and metabolism. While examining hematopoietic regulation by Tle4, we have uncovered a previously unappreciated role of Tle4 on bone calcification using a novel Tle4 null mouse model. Given the significance of osteoblasts in both hematopoiesis and bone development, this study investigated how loss of Tle4 affects osteoblast function. We used dynamic bone formation parameters and microCT to characterize the adverse effects of Tle4 loss on bone development. We further demonstrated loss of Tle4 impacts expression of several key osteoblastogenic genes, including Runx2, Oc, and Ap, pointing toward a potential novel mechanism for Tle4-dependent regulation of mammalian bone development in collaboration with the RUNX family members.

Osteosarcoma and Metastasis Associated Bone Degradation-A Tale of Osteoclast and Malignant Cell Cooperativity

Cancer-induced bone degradation is part of the pathological process associated with both primary bone cancers, such as osteosarcoma, and bone metastases originating from, e.g., breast, prostate, and colon carcinomas. Typically, this includes a cancer-dependent hijacking of processes also occurring during physiological bone remodeling, including osteoclast-mediated disruption of the inorganic bone component and collagenolysis. Extensive research has revealed the significance of osteoclast-mediated bone resorption throughout the course of disease for both primary and secondary bone cancer. Nevertheless, cancer cells representing both primary bone cancer and bone metastasis have also been implicated directly in bone degradation. We will present and discuss observations on the contribution of osteoclasts and cancer cells in cancer-associated bone degradation and reciprocal modulatory actions between these cells. The focus of this review is osteosarcoma, but we will also include relevant observations from studies of bone metastasis. Additionally, we propose a model for cancer-associated bone degradation that involves a collaboration between osteoclasts and cancer cells and in which both cell types may directly participate in the degradation process.

Wnt16 signaling promotes osteoblast differentiation of periosteal derived cells in vitro and in vivo

Background

Periosteum plays critical roles in de novo bone formation and fracture repair. Wnt16 has been regarded as a key regulator in periosteum bone formation. However, the role of Wnt16 in periosteum derived cells (PDCs) osteogenic differentiation remains unclear. The study goal is to uncover whether and how Wnt16 acts on the osteogenesis of PDCs.

Methods

We detected the variation of Wnt16 mRNA expression in PDCs, which were isolated from mouse femur and identified by flow cytometry, cultured in osteogenic medium for 14 days, then knocked down and over-expressed Wnt16 in PDCs to analysis its effects in osteogenesis. Further, we seeded PDCs (Wnt16 over-expressed/vector) in β-tricalcium phosphate cubes, and transplanted this complex into a critical size calvarial defect. Lastly, we used immunofluorescence, Topflash and NFAT luciferase reporter assay to study the possible downstream signaling pathway of Wnt16.

Results

Wnt16 mRNA expression showed an increasing trend in PDCs under osteogenic induction for 14 days. Wnt16 shRNA reduced mRNA expression of Runx2, collage type I (Col-1) and osteocalcin (OCN) after 7 days of osteogenic induction, as well as alizarin red staining intensity after 21days. Wnt16 also increased the mRNA expression of Runx2 and OCN and the protein production of Runx2 and Col-1 after 2 days of osteogenic stimulation. In the orthotopic transplantation assay, more bone volume, trabecula number and less trabecula space were found in Wnt16 over-expressed group. Besides, in the newly formed tissue Brdu positive area was smaller and Col-1 was larger in Wnt16 over-expressed group compared to the control group. Finally, Wnt16 upregulated CTNNB1/β-catenin expression and its nuclear translocation in PDCs, also increased Topflash reporter luciferase activity. By contrast, Wnt16 failed to increase NFAT reporter luciferase activity.

Conclusion

Together, Wnt16 plays a positive role in regulating PDCs osteogenesis, and Wnt16 may have a potential use in improving bone regeneration.

The relationship between platelet count and bone mineral density: results from two independent population-based studies

In two population-based study of middle-aged and older people, we investigated if platelet count was associated with bone mineral density and determined whether the association remained over time. Highest platelet counts within the normal range are significantly associated with osteopenia and osteoporosis in middle-aged and elderly people.

PURPOSE

Recently, platelets were found to play a role in bone remodeling. However, data on the association between platelet count and osteoporosis are lacking. Our study aimed to investigate the association between platelet counts, osteopenia, and osteoporosis in middle-aged and elderly Koreans.

METHODS

We analyzed cross-sectional data from 5181 adults (postmenopausal women and men over 50 years of age) in the 2008-2011 Korea National Health and Nutrition Examination Survey (KNHANES) and longitudinal prospective data from 3312 adults over 50 years of age in the Korean Genome and Epidemiology Study (KoGES). Bone mineral density (BMD) was measured using dual-energy X-ray absorptiometry in the KNHANES and quantitative ultrasound in the KoGES. The platelet counts were categorized into quintiles within normal ranges (150-450 × 10³ μL). The associations between platelet counts, osteopenia, and osteoporosis were estimated using a multinomial logistic model.

RESULTS

BMD of the femur neck, total femur, and lumbar spine all decreased with increasing platelet counts. The cut-off points of the platelet counts to differentiate normal BMD from osteopenia and osteopenia from osteoporosis were 217 × 10³/μL and 269 × 10³/μL, respectively. The odds ratios (95% confidence intervals) in the highest platelet quartile were 1.39 (1.03-1.88) for osteopenia and 1.60 (1.07-2.37) for osteoporosis after adjusting for confounding factors. The distal radius T-score was significantly decreased in the highest platelet tertile group at a follow-up of 10 years.

CONCLUSION

Highest platelet counts within the normal range are significantly associated with osteopenia and osteoporosis in middle-aged and elderly people.

Fabrication of Bioactive Inverted Colloidal Crystal Scaffolds Using Expanded Polystyrene Beads

Inverted colloidal crystal (ICC) hydrogel scaffolds have emerged as a new class of three-dimensional cell culture matrix that represents a unique opportunity to reproduce lymphoid tissue microenvironments. ICC geometry promotes the formation of stromal cell networks and their interaction with hematopoietic cells, a core cellular process in lymphoid tissues. When subdermally implanted, ICC hydrogel scaffolds direct unique foreign body responses to form a vascularized stromal tissue with prolonged attraction of hematopoietic cells, which together resemble lymphoid tissue microenvironments. While conceptually simple, fabrication of ICC hydrogel scaffold requires multiple steps and laborious handling of delicate materials. Here, we introduce a facile route for ICC hydrogel scaffold fabrication using expanded polystyrene (EPS) beads. EPS beads shrink and fuse in a tunable manner under pressurized thermal conditions, which serves as colloidal crystal templates for ICC scaffold fabrication. Inclusion of collagen in the precursor solution greatly simplified preparation of bioactive hydrogel scaffolds. The resultant EPS-templated bioactive ICC hydrogel scaffolds demonstrate characteristic features required for lymphoid tissue modeling in both in vitro and in vivo settings. We envision that the presented method will facilitate widespread implementation of ICC hydrogel scaffolds for lymphoid tissue engineering and other emerging applications. Impact statement Inverted colloidal crystal (ICC) hydrogel scaffolds have emerged as a new class of three-dimensional cell culture matrix that represents a unique opportunity for lymphoid tissue modeling and other emerging novel bioengineering applications. While conceptually simple, fabrication of the ICC hydrogel scaffold requires multiple steps and laborious handling of delicate materials with highly toxic chemicals. The presented method for ICC hydrogel scaffold fabrication using expanded polystyrene (EPS) beads is simple, cost-effective, and involves less toxic chemicals than conventional methods, while retaining comparable biological significance. We envision that EPS bead-based hydrogel scaffold fabrication will greatly facilitate the widespread implementation of ICC hydrogel scaffolds and their practical applications.

RNA-sequencing based bone marrow cell transcriptome analysis reveals the potential mechanisms of E'jiao against blood-deficiency in mice

As a health-care food and traditional Chinese medicine, E'jiao, from the skin of Equus animus L, has been used to nourish blood in China for more than 2000 years. In modern medicine, there are also evidences indicate it has a beneficial effect on chemotherapy-caused blood deficiency. However, its mechanism of action for blood invigoration remains unclear. In the present study, we investigated the hematopoietic effect of E'jiao in 5-Fluorouracil-treated mice. In addition to the counting of bone marrow nucleated cells (BMNCs), flow cytometry was used to detect the population of hematopoietic stem cells (HSCs), and colony-forming unit (CFU) was used to assay the differentiation ability of hematopoietic progenitor cells (HPCs). Gene expression profiles of bone marrow cells were obtained from RNA sequencing (RNA-seq) and differentially expressed genes (DEGs) were analyzed with an emphasis on hematopoiesis-related pathways. The results show that E'jiao promotes the proliferation of both BMNCs and HSCs, as well as the differentiation of HPCs. By providing a hematopoiesis-related molecular regulatory network of E'jiao, we point out that the mechanism of E'jiao is associated with pathways including ECM-receptor interaction, Wnt signaling pathway, PI3K-Akt signaling pathway, TGF-beta signaling pathway, Hematopoietic cell lineage and Osteoclast differentiation, in which Ibsp, Col1a1, Col1a2, Notum, Sost, Dkk1, Irx5, Irx3 and Dcn are the key regulatory molecules. These findings provide valuable molecular basis for the mechanism of action of E'jiao.

Lipid phosphatase SHIP-1 regulates chondrocyte hypertrophy and skeletal development

SH2‐containing inositol‐5′‐phosphatase‐1 (SHIP‐1) controls the phosphatidylinositol‐3′‐kinase (PI3K) initiated signaling pathway by limiting cell membrane recruitment and activation of Akt. Despite the fact that many of the growth factors important to cartilage development and functions are able to activate the PI3K signal transduction pathway, little is known about the role of PI3K signaling in chondrocyte biology and its contribution to mammalian skeletogenesis. Here, we report that the lipid phosphatase SHIP‐1 regulates chondrocyte hypertrophy and skeletal development through its expression in osteochondroprogenitor cells. Global SHIP‐1 knockout led to accelerated chondrocyte hypertrophy and premature formation of the secondary ossification center in the bones of postnatal mice. Drastically higher vascularization and greater number of c‐kit + progenitors associated with sinusoids in the bone marrow also indicated more advanced chondrocyte hypertrophic differentiation in SHIP‐1 knockout mice than in wild‐type mice. In corroboration with the in vivo phenotype, SHIP‐1 deficient PDGFRα + Sca‐1 + osteochondroprogenitor cells exhibited rapid differentiation into hypertrophic chondrocytes under chondrogenic culture conditions in vitro. Furthermore, SHIP‐1 deficiency inhibited hypoxia‐induced cellular activation of Akt and extracellular‐signal‐regulated kinase (Erk) and suppressed hypoxia‐induced cell proliferation. These results suggest that SHIP‐1 is required for hypoxia‐induced growth signaling under physiological hypoxia in the bone marrow. In conclusion, the lipid phosphatase SHIP‐1 regulates skeletal development by modulating chondrogenesis and the hypoxia response of the osteochondroprogenitors during endochondral bone formation.

Neuropeptide Y Y2 antagonist treated ovariectomized mice exhibit greater bone mineral density

Osteoporosis, a disease characterized by progressive bone loss and increased risk of fracture, often results from menopausal loss of estrogen in women. Neuropeptide Y has been shown to negatively regulate bone formation, with amygdala specific deletion of the Y2 receptor resulting in increased bone mass in mice. In this study, ovariectomized (OVX) mice were injected once daily with JNJ-31020028, a brain penetrant Y2 receptor small molecule antagonist to determine the effects on bone formation. Antagonist treated mice had reduced weight and showed increased whole-body bone mineral density compared to vehicle-injected mice. Micro computerized tomography (micro-CT) demonstrated increased vertebral trabecular bone volume, connectivity density and trabecular thickness. Femoral micro-CT analysis revealed increased bone volume within trabecular regions and greater trabecular number, without significant difference in other parameters or within cortical regions. A decrease was seen in serum P1NP, a measure used to confirm positive treatment outcomes in bisphosphonate treated patients. C-terminal telopeptide 1 (CTX-1), a blood biomarker of bone resorption, was decreased in treated animals. The higher bone mineral density observed following Y2 antagonist treatment, as determined by whole-body DEXA scanning, is indicative of either enhanced mineralization or reduced bone loss. Additionally, our findings that ex vivo treatment of bone marrow cells with the Y2 antagonist did not affect osteoblast and osteoclast formation suggests the inhibitor is not affecting these cells directly, and suggests a central role for compound action in this system. Our results support the involvement of Y2R signalling in bone metabolism and give credence to the hypothesis that selective pharmacological manipulation of Y2R may provide anabolic benefits for treating osteoporosis.

Correlation study between osteoporosis and hematopoiesis in the context of adjuvant chemotherapy for breast cancer

This retrospective study attempts to establish if a correlation exists between osteoporosis and hematopoiesis before and after adjuvant chemotherapy in the context of non-metastatic breast cancer. Osteoporosis is interpreted both as a direct marker of osteoblastic decline and as an indirect marker of increased bone marrow adiposity within the hematopoietic microenvironment. Patients from the “Centre du Sein” at CHUV (Centre Hospitalier Universitaire Vaudois) undergoing adjuvant chemotherapy were included in this study. Evolution of blood counts was studied in correlation with the osteoporosis status. Toxicity of chemotherapy was coded according to published probability of febrile neutropenia. One hundred forty-three women were included: mean age 52.1 ± 12.5 years, mean BMI (body mass index) 24.4 ± 4.1. BMD (bone mineral density) scored osteoporotic in 32% and osteopenic in 45%. Prior to chemotherapy, BMD was positively correlated with neutrophil (p < 0.001) and thrombocyte (p = 0.01) count; TBS (trabecular bone score) was not correlated with blood count. After the first cycle of chemotherapy, an increase of one point in TBS correlated with a decrease of 57% on the time to reach leucocyte nadir (p = 0.004). There was a positive correlation between BMD and risk of infection (p < 0.001). Our data demonstrates an association between osteoporosis and lower blood counts in a younger cohort than previously published, extending it for the first time to neutrophil counts in females. Our results suggest that the healthier the bone, the earlier the lowest leucocyte count value, prompting further research on this area.

Bre Enhances Osteoblastic Differentiation by Promoting the Mdm2-Mediated Degradation of p53

Bre is a conserved cellular protein expressed in various tissues. Its major function includes DNA damage repair and anti-apoptosis. Recent studies indicate that Bre is potentially involved in stem cell differentiation although pathophysiological significance along with the molecular mechanisms is still unclear. Here, we report that Bre protein was substantially expressed in the bone tissue and its expression was highly upregulated during the osteogenic differentiation. To test a hypothesis that Bre plays functional roles in the process of osteogenic differentiation, we examined the expression of Bre in an osteoporosis mouse model. Compared with the normal bone tissue, Bre expression in osteoporotic bone was also significantly reduced. Moreover, knockdown of Bre in the mouse bone marrow mesenchymal cells significantly reduced the expression of osteogenic marker genes, the alkaline phosphatase activity, and the mineralization capacity, while overexpression of Bre greatly promoted the osteogenesis both in vitro and in vivo. Interestingly, we founded that knockdown of Bre led to activation of the p53 signaling pathways exhibited by increased p53, p21, and Mdm2. However, when we inhibited the p53 by siRNA silencing or pifithrin-α, the impaired osteogenesis caused by Bre knockdown was greatly restored. Finally, we found that Bre promoted the Mdm2-mediated p53 ubiquitination and degradation by physically interacting with p53. Taken together, our results revealed a novel function of Bre in osteoblast differentiation through modulating the stability of p53. Stem Cells 2017;35:1760-1772.

Fibrillin-1 microfibrils influence adult bone marrow hematopoiesis

We have recently demonstrated that fibrillin-1 assemblies regulate the fate of skeletal stem cells (aka, mesenchymal stem cells [MSCs]) by modulating TGFβ activity within the microenvironment of adult bone marrow niches. Since MSCs can also influence hematopoietic stem cell (HSC) activities, here we investigated adult hematopoiesis in mice with Cre-mediated inactivation of the fibrillin-1 (Fbn1) gene in the mesenchyme of the forming limbs (Fbn1(Prx1-/-) mice). Analyses of 3-month-old Fbn1(Prx1-/-) mice revealed a statistically significant increase of circulating red blood cells, which a differentiation assay correlated with augmented erythropoiesis. This finding, together with evidence of fibrillin-1 deposition in erythroblastic niches, supported the notion that this extracellular matrix protein normally restricts differentiation of erythroid progenitors. Whereas flow cytometry measurements identified a decreased HSC frequency in mutant relative to wild type mice, no appreciable differences were noted with regard to the relative abundance and differentiation potential of myeloid progenitor cells. Together these findings implied that fibrillin-1 normally promotes HSC expansion but does not influence cell lineage commitment. Since local TGFβ hyperactivity has been associated with abnormal osteogenesis in Fbn1(Prx1-/-) mice, 1-month-old mutant and wild type animals were systemically treated for 8weeks with either a pan-TGF-β-neutralizing antibody or an antibody of the same IgG1 isotype. The distinct outcomes of these pharmacological interventions strongly suggest that fibrillin-1 differentially modulates TGFβ activity in HSC vs. erythroid niches.

The Impact of Immune System in Regulating Bone Metastasis Formation by Osteotropic Tumors

Bone metastases are frequent and debilitating consequence for many tumors, such as breast, lung, prostate, and kidney cancer. Many studies report the importance of the immune system in the pathogenesis of bone metastasis. Indeed, bone and immune system are strictly linked to each other because bone regulates the hematopoietic stem cells from which all cells of the immune system derive, and many immunoregulatory cytokines influence the fate of bone cells. Furthermore, both cytokines and factors produced by immune and bone cells promote the growth of tumor cells in bone, contributing to supporting the vicious cycle of bone metastasis. This review summarizes the current knowledge on the interactions among bone, immune, and tumor cells aiming to provide an overview of the osteoimmunology field in bone metastasis from solid tumors.

Roles of osteoclasts in the control of medullary hematopoietic niches

Bone marrow is the major site of hematopoiesis in mammals. The bone marrow environment plays an essential role in the regulation of hematopoietic stem and progenitor cells by providing specialized niches in which these cells are maintained. Many cell types participate to the composition and regulation of hematopoietic stem cell (HSC) niches, integrating complex signals from the bone, immune and nervous systems. Among these cells, the bone-resorbing osteoclasts (OCLs) have been described as main regulators of HSC niches. They are not limited to carving space for HSCs, but they also provide signals that affect the molecular and cellular niche components. However, their exact role in HSC niches remains unclear because of the variety of models, signals and conditions used to address the question. The present review will discuss the importance of the implication of OCLs focusing on the formation of HSC niches, the maintenance of HSCs in these niches and the mobilization of HSCs from the bone marrow. It will underline the importance of OCLs in HSC niches.

The humerus of Eusthenopteron: a puzzling organization presaging the establishment of tetrapod limb bone marrow

Because of its close relationship to tetrapods, Eusthenopteron is an important taxon for understanding the establishment of the tetrapod body plan. Notably, it is one of the earliest sarcopterygians in which the humerus of the pectoral fin skeleton is preserved. The microanatomical and histological organization of this humerus provides important data for understanding the evolutionary steps that built up the distinctive architecture of tetrapod limb bones. Previous histological studies showed that Eusthenopteron's long-bone organization was established through typical tetrapod ossification modalities. Based on a three-dimensional reconstruction of the inner microstructure of Eusthenopteron's humerus, obtained from propagation phase-contrast X-ray synchrotron microtomography, we are now able to show that, despite ossification mechanisms and growth patterns similar to those of tetrapods, it also retains plesiomorphic characters such as a large medullary cavity, partly resulting from the perichondral ossification around a large cartilaginous bud as in actinopterygians. It also exhibits a distinctive tubular organization of bone-marrow processes. The connection between these processes and epiphyseal structures highlights their close functional relationship, suggesting that either bone marrow played a crucial role in the long-bone elongation processes or that trabecular bone resulting from the erosion of hypertrophied cartilage created a microenvironment for haematopoietic stem cell niches.

MHC class II transactivator is an in vivo regulator of osteoclast differentiation and bone homeostasis co-opted from adaptive immunity

The molecular networks controlling bone homeostasis are not fully understood. The common evolution of bone and adaptive immunity encourages the investigation of shared regulatory circuits. MHC Class II Transactivator (CIITA) is a master transcriptional co-activator believed to be exclusively dedicated for antigen presentation. CIITA is expressed in osteoclast precursors, and its expression is accentuated in osteoporotic mice. We thus asked whether CIITA plays a role in bone biology. To this aim, we fully characterized the bone phenotype of two mouse models of CIITA overexpression, respectively systemic and restricted to the monocyte-osteoclast lineage. Both CIITA-overexpressing mouse models revealed severe spontaneous osteoporosis, as assessed by micro-computed tomography and histomorphometry, associated with increased osteoclast numbers and enhanced in vivo bone resorption, whereas osteoblast numbers and in vivo bone-forming activity were unaffected. To understand the underlying cellular and molecular bases, we investigated ex vivo the differentiation of mutant bone marrow monocytes into osteoclasts and immune effectors, as well as osteoclastogenic signaling pathways. CIITA-overexpressing monocytes differentiated normally into effector macrophages or dendritic cells but showed enhanced osteoclastogenesis, whereas CIITA ablation suppressed osteoclast differentiation. Increased c-fms and receptor activator of NF-κB (RANK) signaling underlay enhanced osteoclast differentiation from CIITA-overexpressing precursors. Moreover, by extending selected phenotypic and cellular analyses to additional genetic mouse models, namely MHC Class II deficient mice and a transgenic mouse line lacking a specific CIITA promoter and re-expressing CIITA in the thymus, we excluded MHC Class II expression and T cells from contributing to the observed skeletal phenotype. Altogether, our study provides compelling genetic evidence that CIITA, the molecular switch of antigen presentation, plays a novel, unexpected function in skeletal homeostasis, independent of MHC Class II expression and T cells, by exerting a selective and intrinsic control of osteoclast differentiation and bone resorption in vivo.

Low-intensity pulsed ultrasound (LIPUS) inhibits LPS-induced inflammatory responses of osteoblasts through TLR4-MyD88 dissociation

Previous reports have shown that osteoblasts are mechano-sensitive. Low-intensity pulsed ultrasound (LIPUS) induces osteoblast differentiation and is an established therapy for bone fracture. Here we have examined how LIPUS affects inflammatory responses of osteoblasts to LPS. LPS rapidly induced mRNA expression of several chemokines including CCL2, CXCL1, and CXCL10 in both mouse osteoblast cell line and calvaria-derived osteoblasts. Simultaneous treatment by LIPUS significantly inhibited mRNA induction of CXCL1 and CXCL10 by LPS. LPS-induced phosphorylation of ERKs, p38 kinases, MEK1/2, MKK3/6, IKKs, TBK1, and Akt was decreased in LIPUS-treated osteoblasts. Furthermore, LIPUS inhibited the transcriptional activation of NF-κB responsive element and Interferon-sensitive response element (ISRE) by LPS. In a transient transfection experiment, LIPUS significantly inhibited TLR4-MyD88 complex formation. Thus LIPUS exerts anti-inflammatory effects on LPS-stimulated osteoblasts by inhibiting TLR4 signal transduction.

Activation of AMPK protects against hydrogen peroxide-induced osteoblast apoptosis through autophagy induction and NADPH maintenance: new implications for osteonecrosis treatment?

Elevated hydrogen peroxide (H2O2) causes osteoblast dysfunction and apoptosis, serving as an important contributor to the development of osteonecrosis. Here we aimed to understand the role of AMP-activated protein kinase (AMPK) in the process. We observed a high level of AMPK activation in surgery isolated patients' osteonecrosis tissues. In cultured osteoblastoma MG63 cells, H2O2 stimulation induced significant AMPK activation, oxidative stress, cell death and apoptosis. Inhibition of AMPK by its inhibitor (compound C) or by shRNA-mediated knockdown dramatically enhanced H2O2-induced MG63 cell apoptosis, while over-expression of AMPK in HEK-293 cells alleviated H2O2-induced cell damage. These results confirmed that H2O2-activated AMPK is pro-cell survival. We observed that H2O2 induced protective autophagy in MG63 cells, and AMPK-dependent Ulk1 activation and mTORC1 (mTOR complex 1) inactivation might involve autophagy activation. Further, AMPK activation inhibited H2O2-induced oxidative stress, probably through inhibiting NADPH (nicotinamide adenine dinucleotide phosphate) depletion, since more NADPH depletion and oxidative stress were induced by H2O2 in AMPK deficient MG63 cells. Finally, we observed a significant AMPK activation in H2O2-treated primary cultured and transformed (MC3T3-E1) osteoblasts, and AMPK inhibitor compound C enhanced death by H2O2 in these cells. Based on these results, we concluded that H2O2-induced AMPK activation is pro-survival and anti-apoptosis in osteoblasts. Autophagy induction and NADPH maintenance are involved in AMPK-mediated pro-survival effects. AMPK might represent a novel molecular target for osteonecrosis treatment.

Modulation of inflammasome-mediated pulmonary immune activation by type I IFNs protects bone marrow homeostasis during systemic responses to Pneumocystis lung infection

Although acquired bone marrow failure (BMF) is considered a T cell-mediated autoimmune disease, possible innate immune defects as a cause for systemic immune deviations in response to otherwise innocuous infections have not been extensively explored. In this regard, we recently demonstrated an important role of type I IFNs in protecting hematopoiesis during systemic stress responses to the opportunistic fungal pathogen Pneumocystis in lymphocyte-deficient mice. Mice deficient in both lymphocytes and type I IFN receptor (IFrag(-/-) mice) develop rapidly progressing BMF due to accelerated bone marrow (BM) cell apoptosis associated with innate immune deviations in the BM in response to Pneumocystis lung infection. However, the communication pathway between lung and BM eliciting the induction of BMF in response to this strictly pulmonary infection has been unclear. In this study, we report that absence of an intact type I IFN system during Pneumocystis lung infection not only causes BMF in lymphocyte-deficient mice but also transient BM stress in lymphocyte-competent mice. This is associated with an exuberant systemic IFN-γ response. IFN-γ neutralization prevented Pneumocystis lung infection-induced BM depression in type I IFN receptor-deficient mice and prolonged neutrophil survival time in BM from IFrag(-/-) mice. IL-1β and upstream regulators of IFN-γ, IL-12, and IL-18 were also upregulated in lung and serum of IFrag(-/-) mice. In conjunction, there was exuberant inflammasome-mediated caspase-1 activation in pulmonary innate immune cells required for processing of IL-18 and IL-1β. Thus, absence of type I IFN signaling during Pneumocystis lung infection may result in deregulation of inflammasome-mediated pulmonary immune activation, causing systemic immune deviations triggering BMF in this model.

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.

The migration of hematopoietic progenitors from the fetal liver to the fetal bone marrow: lessons learned and possible clinical applications

The ontogeny of hematopoietic stem cells (HSCs) is complex, with multiple sites of embryonic origin as well as several locations of expansion and maturation in the embryo and the adult. Hematopoietic progenitors (HPs) with diverse developmental potential are first found in the yolk sac, aorta-gonad-mesonephros region and placenta. These progenitors then colonize the fetal liver (FL), where they undergo expansion and maturation. HSCs from the FL colonize the fetal bone marrow (FBM), governed by a complex orchestration of transcription programs including migratory molecules with chemotactic activity, adhesion molecules, and molecules that modulate the extracellular matrix. Understanding the mechanisms that regulate the patterns of HSC migration between FL and FBM could improve the engraftment potential of embryonic stem cell-derived HPs, because these cells might display a migratory behavior more similar to early HPs than to adult HSCs. Understanding the changes in migratory behavior in the context of FL to FBM HSC migration could lead to new approaches in the treatment of blood malignancies. We will review the current knowledge in the field of FL to the FBM HSCs migration during development, focusing on changes in expression of molecules important for this process and exploring its clinical applications.

Population dynamics of normal and leukaemia stem cells in the haematopoietic stem cell niche show distinct regimes where leukaemia will be controlled

Haematopoietic stem cells (HSCs) are responsible for maintaining immune cells, red blood cells and platelets throughout life. HSCs must be located in their ecological niche (the bone marrow) to function correctly, that is, to regenerate themselves and their progeny; the latter eventually exit the bone marrow and enter circulation. We propose that cells with oncogenic potential-cancer/leukaemia stem cells (LSC)-and their progeny will also occupy this niche. Mathematical models, which describe the dynamics of HSCs, LSCs and their progeny allow investigation into the conditions necessary for defeating a malignant invasion of the niche. Two such models are developed and analysed here. To characterize their behaviour, we use an inferential framework that allows us to study regions in parameter space that give rise to desired behaviour together with an assessment of the robustness of the dynamics. Using this approach, we map out conditions under which HSCs can outcompete LSCs. In therapeutic applications, we clearly want to drive haematopoiesis into such regimes and the current analysis provide some guidance as to how we can identify new therapeutic targets. Our results suggest that maintaining a viable population of HSCs and their progenies in the niche may often already be nearly sufficient to eradicate LSCs from the system.

Absence of sclerostin adversely affects B-cell survival

Increased osteoblast activity in sclerostin-knockout (Sost(-/-)) mice results in generalized hyperostosis and bones with small bone marrow cavities resulting from hyperactive mineralizing osteoblast populations. Hematopoietic cell fate decisions are dependent on their local microenvironment, which contains osteoblast and stromal cell populations that support both hematopoietic stem cell quiescence and facilitate B-cell development. In this study, we investigated whether high bone mass environments affect B-cell development via the utilization of Sost(-/-) mice, a model of sclerosteosis. We found the bone marrow of Sost(-/-) mice to be specifically depleted of B cells because of elevated apoptosis at all B-cell developmental stages. In contrast, B-cell function in the spleen was normal. Sost expression analysis confirmed that Sost is primarily expressed in osteocytes and is not expressed in any hematopoietic lineage, which indicated that the B-cell defects in Sost(-/-) mice are non-cell autonomous, and this was confirmed by transplantation of wild-type (WT) bone marrow into lethally irradiated Sost(-/-) recipients. WT→Sost(-/-) chimeras displayed a reduction in B cells, whereas reciprocal Sost(-/-) →WT chimeras did not, supporting the idea that the Sost(-/-) bone environment cannot fully support normal B-cell development. Expression of the pre-B-cell growth stimulating factor, Cxcl12, was significantly lower in bone marrow stromal cells of Sost(-/-) mice, whereas the Wnt target genes Lef-1 and Ccnd1 remained unchanged in B cells. Taken together, these results demonstrate a novel role for Sost in the regulation of bone marrow environments that support B cells.

Human umbilical cord blood-derived CD34+ cells reverse osteoporosis in NOD/SCID mice by altering osteoblastic and osteoclastic activities

Background

Osteoporosis is a bone disorder associated with loss of bone mineral density and micro architecture. A balance of osteoblasts and osteoclasts activities maintains bone homeostasis. Increased bone loss due to increased osteoclast and decreased osteoblast activities is considered as an underlying cause of osteoporosis.

Methods and Findings

The cures for osteoporosis are limited, consequently the potential of CD34+ cell therapies is currently being considered. We developed a nanofiber-based expansion technology to obtain adequate numbers of CD34⁺ cells isolated from human umbilical cord blood, for therapeutic applications. Herein, we show that CD34⁺ cells could be differentiated into osteoblastic lineage, in vitro. Systemically delivered CD34⁺ cells home to the bone marrow and significantly improve bone deposition, bone mineral density and bone micro-architecture in osteoporotic mice. The elevated levels of osteocalcin, IL-10, GM-CSF, and decreased levels of MCP-1 in serum parallel the improvements in bone micro-architecture. Furthermore, CD34⁺ cells improved osteoblast activity and concurrently impaired osteoclast differentiation, maturation and functionality.

Conclusions

These findings demonstrate a novel approach utilizing nanofiber-expanded CD34⁺ cells as a therapeutic application for the treatment of osteoporosis.

Osteoclasts promote the formation of hematopoietic stem cell niches in the bone marrow

Osteoclasts promote the formation of the HSC niche by inducing the differentiation of osteoblastic cells from mesenchymal stem cells.

Formation of the hematopoietic stem cell (HSC) niche in bone marrow (BM) is tightly associated with endochondral ossification, but little is known about the mechanisms involved. We used the oc/oc mouse, a mouse model with impaired endochondral ossification caused by a loss of osteoclast (OCL) activity, to investigate the role of osteoblasts (OBLs) and OCLs in the HSC niche formation. The absence of OCL activity resulted in a defective HSC niche associated with an increased proportion of mesenchymal progenitors but reduced osteoblastic differentiation, leading to impaired HSC homing to the BM. Restoration of OCL activity reversed the defect in HSC niche formation. Our data demonstrate that OBLs are required for establishing HSC niches and that osteoblastic development is induced by OCLs. These findings broaden our knowledge of the HSC niche formation, which is critical for understanding normal and pathological hematopoiesis.

Altered matrix at the chondro-osseous junction leads to defects in lymphopoiesis

The collagen X transgenic and null (ColX-Tg/KO) mice have revealed a link between endochondral ossification (EO) and hematopoiesis, and thus serve as model systems to study hematopoietic niches. The altered collagen X function in ColX-Tg/KO mice resulted not only in skeletal defects, which included changes in growth plate ultrastructure, altered localization of heparan sulfate proteoglycans (HSPG), and reduced trabecular bone, but also in hematopoietic defects, which included reduced B lymphocyte numbers throughout life without associated increases in B cell apoptosis. Consequently, the ColX-Tg/KO mice exhibited diminished in vitro and in vivo immune responses. Moreover, reduced expression of several hematopoietic and B lymphopoietic cytokines were measured from ColX-KO-derived hypertrophic chondrocyte and trabecular osteoblast cultures. Together, these data expand the current hematopoietic niche model by including the EO-derived extracellular matrix, for example, the collagen X/HSPG network, as well as the EO-derived hypertrophic chondrocytes and trabecular osteoblasts as hematopoietic signal mediating cells.

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.

Antiosteoporotic Activity of Dioscorea alata L. cv. Phyto through Driving Mesenchymal Stem Cells Differentiation for Bone Formation

The aim of this study was to evaluate the effect of an ethanol extract of the rhizomes of Dioscorea alata L. cv. Phyto, Dispo85E, on bone formation and to investigate the mechanisms involved. Our results showed that Dispo85E increased the activity of alkaline phosphatase (ALP) and bone nodule formation in primary bone marrow cultures. In addition, Dispo85E stimulated pluripotent C3H10T1/2 stem cells to differentiate into osteoblasts rather than adipocytes. Our in vivo data indicated that Dispo85E promotes osteoblastogenesis by increasing ALP activity and bone nodule formation in both intact and ovariectomized (OVX) mice. Microcomputed tomography (μCT) analysis also showed that Dispo85E ameliorates the deterioration of trabecular bone mineral density (tBMD), trabecular bone volume/total volume (BV/TV), and trabecular bone number (Tb.N) in OVX mice. Our results suggested that Dispo85E is a botanical drug with a novel mechanism that drives the lineage-specific differentiation of bone marrow stromal cells and is a candidate drug for osteoporosis therapy.

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.

Impact of maturational status on the ability of osteoblasts to enhance the hematopoietic function of stem and progenitor cells

Osteoblasts (OBs) exert a prominent regulatory effect on hematopoietic stem cells (HSCs). We evaluated the difference in hematopoietic expansion and function in response to co-culture with OBs at various stages of development. Murine calvarial OBs were seeded directly (fresh) or cultured for 1, 2, or 3 weeks prior to seeding with 1000 Lin-Sca1 + cKit+ (LSK) cells for 1 week. Significant increases in the following hematopoietic parameters were detected when comparing co-cultures of fresh OBs to co-cultures containing OBs cultured for 1, 2, or 3 weeks: total hematopoietic cell number (up to a 3.4-fold increase), total colony forming unit (CFU) number in LSK progeny (up to an 18.1-fold increase), and percentage of Lin-Sca1+ cells (up to a 31.8-fold increase). Importantly, these studies were corroborated by in vivo reconstitution studies in which LSK cells maintained in fresh OB co-cultures supported a significantly higher level of chimerism than cells maintained in co-cultures containing 3-week OBs. Characterization of OBs cultured for 1, 2, or 3 weeks with real-time PCR and functional mineralization assays showed that OB maturation increased with culture duration but was not affected by the presence of LSK cells in culture. Linear regression analyses of multiple parameters measured in these studies show that fresh, most likely more immature OBs better promote hematopoietic expansion and function than cultured, presumably more mature OBs and suggest that the hematopoiesis-enhancing activity is mediated by cells present in fresh OB cultures de novo.

Proteomic analysis of mineralising osteoblasts identifies novel genes related to bone matrix mineralisation

Bone matrix mineralisation plays a critical role in the determination of the overall biomechanical competence of bone. However, the molecular mechanisms of bone matrix mineralisation have not been fully elucidated. We used a proteomic approach to identify proteins and genes that may play a role in osteoblast matrix mineralisation. Proteomic differential display revealed a protein band that appeared only in mineralising mouse 7F2 osteoblasts. In-gel protein digestion and mass spectrometry proteomic analysis of this protein band identified 16 proteins. Furthermore, their corresponding transcripts were upregulated. This identification of proteins that may be associated with bone matrix mineralisation presents important new information toward a better understanding of the precise mechanisms of this process.

Slug contributes to the regulation of CXCL12 expression in human osteoblasts

CXCL12/CXCR4 chemokine/receptor axis signaling has recently been found to play an important role in the remodeling of bone tissue, but little is known about the molecular mechanisms that are involved. The present study shows that CXCL12 is present at high levels both in human mesenchymal stem cells (hMSCs) and primary osteoblasts (hOBs). When osteogenesis was induced, CXCL12 expression was strictly confined to mineralized nodules. To investigate what mechanisms contribute to the maintenance of a correct expression of CXCL12 in bone cellular context, we analyzed the relationship between CXCL12 and Slug, a transcription factor recently associated with osteoblast maturation. By gene silencing and chromatin immunoprecipitation assay, we showed that both proteins are required for the mineralization process and CXCL12 is transcriptionally and functionally regulated by Slug, which is recruited at specific sites to its gene promoter in vivo. These findings showed for the first time a positive correlation between CXCL12 signaling and Slug activity, thus corroborating the role of these two proteins in bone cellular context and suggesting a new potential target for bone tissue repair and regeneration.

Runx2-mediated bcl-2 gene expression contributes to nitric oxide protection against hydrogen peroxide-induced osteoblast apoptosis

Nitric oxide (NO) can regulate osteoblast activities. This study was aimed to evaluate the protective effects of pretreatment with sodium nitroprusside (SNP) as a source of NO on hydrogen peroxide-induced osteoblast insults and its possible mechanisms. Exposure of human osteosarcoma MG63 cells to hydrogen peroxide significantly increased cellular oxidative stress, but decreased ALP activity and cell viability, inducing cell apoptosis. Pretreatment with 0.3 mM SNP significantly lowered hydrogen peroxide-induced cell insults. Treatment of human MG63 cells with hydrogen peroxide inhibited Bcl-2 mRNA and protein production, but pretreatment with 0.3 mM SNP significantly ameliorated such inhibition. Sequentially, hydrogen peroxide decreased the mitochondrial membrane potential, but increased the levels of cytochrome c and caspase-3 activity. Pretreatment with 0.3 mM SNP significantly lowered such alterations. Exposure to hydrogen peroxide decreased Runx2 mRNA and protein syntheses. However, pretreatment with 0.3 mM SNP significantly lowered the suppressive effects. Runx2 knockdown using RNA interference inhibited Bcl-2 mRNA production in human MG63 cells. Protection of pretreatment with 0.3 mM SNP against hydrogen peroxide-induced alterations in ALP activity, caspase-3 activity, apoptotic cells, and cell viability were also alleviated after administration of Runx2 small interference RNA. Thus, this study shows that pretreatment with 0.3 mM SNP can protect human MG63 cells from hydrogen peroxide-induced apoptotic insults possibly via Runx2-involved regulation of bcl-2 gene expression.

Dlx homeobox gene family expression in osteoclasts

Skeletal growth and homeostasis require the finely orchestrated secretion of mineralized tissue matrices by highly specialized cells, balanced with their degradation by osteoclasts. Time- and site-specific expression of Dlx and Msx homeobox genes in the cells secreting these matrices have been identified as important elements in the regulation of skeletal morphology. Such specific expression patterns have also been reported in osteoclasts for Msx genes. The aim of the present study was to establish the expression patterns of Dlx genes in osteoclasts and identify their function in regulating skeletal morphology. The expression patterns of all Dlx genes were examined during the whole osteoclastogenesis using different in vitro models. The results revealed that Dlx1 and Dlx2 are the only Dlx family members with a possible function in osteoclastogenesis as well as in mature osteoclasts. Dlx5 and Dlx6 were detected in the cultures but appear to be markers of monocytes and their derivatives. In vivo, Dlx2 expression in osteoclasts was examined using a Dlx2/LacZ transgenic mouse. Dlx2 is expressed in a subpopulation of osteoclasts in association with tooth, brain, nerve, and bone marrow volumetric growths. Altogether the present data suggest a role for Dlx2 in regulation of skeletal morphogenesis via functions within osteoclasts.

Congenic mice confirm that collagen X is required for proper hematopoietic development

The link between endochondral skeletal development and hematopoiesis in the marrow was established in the collagen X transgenic (Tg) and null (KO) mice. Disrupted function of collagen X, a major hypertrophic cartilage matrix protein, resulted in skeletal and hematopoietic defects in endochondrally derived tissues. Manifestation of the disease phenotype was variable, ranging from perinatal lethality in a subset of mice, to altered lymphopoiesis and impaired immunity in the surviving mice. To exclude contribution of strain specific modifiers to this variable manifestation of the skeleto-hematopoietic phenotype, C57Bl/6 and DBA/2J collagen X congenic lines were established. Comparable disease manifestations confirmed that the skeleto-hematopoietic alterations are an inherent outcome of disrupted collagen X function. Further, colony forming cell assays, complete blood count analysis, serum antibody ELISA, and organ outgrowth studies established altered lymphopoiesis in all collagen X Tg and KO mice and implicated opportunistic infection as a contributor to the severe disease phenotype. These data support a model where endochondral ossification-specific collagen X contributes to the establishment of a hematopoietic niche at the chondro-osseous junction.

Role of protein-tyrosine phosphatases in regulation of osteoclastic activity

Osteoclasts, the primary cell type mediating bone resorption, are multinucleated, giant cells derived from hematopoietic cells of monocyte-macrophage lineage. Osteoclast activity is, in a large part, regulated by protein-tyrosine phosphorylation. While information about functional roles of several protein-tyrosine kinases (PTK), including c-Src, in osteoclastic resorption has been accumulated, little is known about the roles of protein-tyrosine phosphatases (PTPs) in regulation of osteoclast activity. Recent evidence implicates important regulatory roles for four PTPs (SHP-1, cyt-PTP-epsilon, PTP-PEST, and PTPoc) in osteoclasts. Cyt-PTP-epsilon, PTP-PEST, and PTP-oc are positive regulators of osteoclast activity, while SHP-1 is a negative regulator. Of these PTPs in osteoclasts, only PTP-oc is a positive regulator of c-Src PTK through dephosphorylation of the inhibitory phosphotyrosine-527 residue. Although some information about mechanisms of action of these PTPs to regulate osteoclast activity is reviewed in this article, much additional work is required to provide more comprehensive details about their functions in osteoclasts.

Sonic hedgehog expands diaphyseal trabecular bone altering bone marrow niche and lymphocyte compartment

Bone marrow contains distinct microenvironments that regulate hematopoietic stem cells (HSCs). The endosteal HSC niche includes osteoblasts, mineral, and extracellular matrix proteins that interact through various molecular signals to control HSCs. Sonic hedgehog (Shh) is a morphogen involved in the regulation of skeletal development and hematopoiesis, but the effects of Shh on bone in relation to the HSC niche are not well understood. We demonstrate that systemic overexpression of Shh in mice increases osteoblast number with the resultant formation of new trabeculae in the femoral diaphysis. Suggestive of a functional change in the hematopoietic niche, numbers of Lin(-) Sca-1(+) c-Kit(+) cells with hematopoietic progenitor function expand, although cells with in vivo repopulating capacity in the wild-type environment do not increase. Instead, Shh mediates a decrease in number of bone marrow lymphocytes accompanied by a decreased expression of stromal-derived growth factor 1 (SDF-1) and a decrease in Flk2-expressing Lin(-) Sca-1(+) c-Kit(+) cells, indicating a modulation of early lymphopoiesis. This is caused by a microenvironment-induced mechanism as Shh treatment of bone marrow recipients, but not donors, results in a dramatic depletion of lymphocytes. Together, these data suggest that Shh mediates alterations in the bone marrow hematopoietic niche affecting the early lymphoid differentiation.

Heparan sulfate proteoglycans: a GAGgle of skeletal-hematopoietic regulators

This review summarizes our current understanding of the presence and function of heparan sulfate proteoglycans (HSPGs) in skeletal development and hematopoiesis. Although proteoglycans (PGs) comprise a large and diverse group of cell surface and matrix molecules, we chose to focus on HSPGs owing to their many proposed functions in skeletogenesis and hematopoiesis. Specifically, we discuss how HSPGs play predominant roles in establishing and regulating niches during skeleto-hematopoietic development by participating in distinct developmental processes such as patterning, compartmentalization, growth, differentiation, and maintenance of tissues. Special emphasis is placed on our novel hypothesis that mechanistically links endochondral skeletogenesis to the establishment of the hematopoietic stem cell (HSC) niche in the marrow. HSPGs may contribute to these developmental processes through their unique abilities to establish and mediate morphogen, growth factor, and cytokine gradients; facilitate signaling; provide structural stability to tissues; and act as molecular filters and barriers.

Altered endochondral ossification in collagen X mouse models leads to impaired immune responses

Disruption of collagen X function in hypertrophic cartilage undergoing endochondral ossification was previously linked to altered hematopoiesis in collagen X transgenic (Tg) and null (KO) mice (Jacenko et al., [2002] Am J Pathol 160:2019-2034). Mice displayed altered growth plates, diminished trabecular bone, and marrow hypoplasia with an aberrant lymphocyte profile throughout life. This study identifies altered B220+, CD4+, and CD8+ lymphocyte numbers, as well as CD4+/fox3P+ T regulatory cells in the collagen X mice. Additionally, diminished in vitro splenocyte responses to mitogens and an inability of mice to survive a challenge with Toxoplasma gondii, confirm impaired immune responses. In concert, ELISA and protein arrays identify aberrant levels of inflammatory, chemo-attractant, and matrix binding cytokines in collagen X mouse sera. These data link the disruption of collagen X function in the chondro-osseous junction to an altered hematopoietic stem cell niche in the marrow, resulting in impaired immune function.

Osteoclasts control osteoblast chemotaxis via PDGF-BB/PDGF receptor beta signaling

Background

Bone remodeling relies on the tightly regulated interplay between bone forming osteoblasts and bone digesting osteoclasts. Several studies have now described the molecular mechanisms by which osteoblasts control osteoclastogenesis and bone degradation. It is currently unclear whether osteoclasts can influence bone rebuilding.

Methodology/Principal Findings

Using in vitro cell systems, we show here that mature osteoclasts, but not their precursors, secrete chemotactic factors recognized by both mature osteoblasts and their precursors. Several growth factors whose expression is upregulated during osteoclastogenesis were identified by DNA microarrays as candidates mediating osteoblast chemotaxis. Our subsequent functional analyses demonstrate that mature osteoclasts, whose platelet-derived growth factor bb (PDGF-bb) expression is reduced by siRNAs, exhibit a reduced capability of attracting osteoblasts. Conversely, osteoblasts whose platelet-derived growth factor receptor β (PDGFR-β) expression is reduced by siRNAs exhibit a lower capability of responding to chemotactic factors secreted by osteoclasts.

Conclusions/Significance

We conclude that, in vitro mature osteoclasts control osteoblast chemotaxis via PDGF-bb/PDGFR-β signaling. This may provide one key mechanism by which osteoclasts control bone formation in vivo.