Adipocytes Derived from Human Bone Marrow Mesenchymal Stem Cells Exert Inhibitory Effects on Osteoblastogenesis

Novel role for alpha-2-macroglobulin (A2M) as a disease modifying protein in senile osteoporosis

Introduction: In the rapidly aging U.S. population, age-induced bone loss (senile osteoporosis) represents a major public health concern that is associated with a significant increased risk for low trauma fragility fractures, which are debilitating to patients, cause significant morbidity and mortality, and are costly to treat and manage. While various treatments exist to slow bone loss in osteoporosis patients, these suffer from poor tolerability and label restrictions that limit their overall effectiveness. Over the past decade, skeletal stem/progenitor cells (SSPCs), which are the main precursor of osteoblasts and adipocytes in adult bone marrow (BM), have emerged as important players in osteoporosis. Methods: Age-induced skeletal pathology was quantified in elderly (24-month-old) vs. mature (3-month-old) mice by micro-CT and changes in SSPC abundance in the BM of these mice was quantified by fluorescence-activated cell sorting (FACS). SSPCs from elderly vs. mature mice were also analyzed by RNA-Seq to identify differentially expressed genes (DEGs), and gain and loss-of-function studies were performed in human BM-derived mesenchymal stromal cells (BM-MSCs) to assess A2M function. Results: Elderly mice were shown to exhibit significant age-induced skeletal pathology, which correlated with a significant increase in SSPC abundance in BM. RNA-seq analysis identified alpha-2-macroglobulin (A2M), a pan-protease inhibitor that also binds inflammatory cytokines, as one of the most downregulated transcripts in SSPCs isolated from the BM of elderly vs. mature mice, and silencing of A2M expression in human BM-MSCs induced their proliferation and skewed their lineage bifurcation toward adipogenesis at the expense of osteogenesis thereby recapitulating critical aspects of age-induced stem cell dysfunction. Conclusion: These findings identify A2M as a novel disease modifying protein in osteoporosis, downregulation of which in bone marrow promotes SSPC dysfunction and imbalances in skeletal homeostasis.

Deciphering the potential pharmaceutical mechanism of Guzhi Zengsheng Zhitongwan on rat bone and kidney based on the "kidney governing bone" theory

BACKGROUND

Guzhi Zengsheng Zhitongwan (GZZSZTW) is an effective Chinese medicinal formulation for the treatment of osteoarthritis (OA) designed according to the "kidney governing bone" theory, which has been widely used as a golden guide for treating bone and cartilage diseases in traditional Chinese medicine. The aim of this study was to explore the molecular mechanism underlying its effects on the bone and kidney.

METHODS

Preparation and quality control were performed as previously described. Since GZZSZTW is orally administered in the form of pills prepared in boiled water, the Chinese materia medica (CMM) mixture of this formula was extracted with distilled water by a reflux method and was then filtered through a 0.45-μm Hollow Fiber Cartridge (GE Healthcare, USA). The filtrate was freeze-dried by a Heto PowerDry LL3000 Freeze Dryer (Thermo, USA) and stored at - 80 °C. The effects of GZZSZTW on gene expression and regulation of both kidney and bone tissues were investigated using a state-of-the-art RNA-seq technology.

RESULTS

We demonstrated that GZZSZTW could enhance kidney function and suppress bone formation and resorption by modulating the activities of osteoblast and osteoclast, and might subsequently contribute to the inhibition of osteophyte formation during the process of OA. These effects might be achieved by the synergistic interactions of various herbs and their active components in GZZSZTW, which increased the expression levels of functional genes participating in kidney function, regulation, and repair, and then decreased the expression levels of genes involved in bone formation and resorption. Thus, our findings were consistent with the "kidney governing bone" theory, which has been widely used as a guide in clinical practice for thousands of years.

CONCLUSIONS

This study has deepened the current knowledge about the molecular effects of GZZSZTW on bone and kidney regulation. Furthermore, this study might be able to provide possible strategies to further prevent and treat joint diseases by using traditional Chinese medicinal formulations following the "kidney governing bone" theory.

Emerging role of circadian rhythm in bone remodeling

The 24-h rhythm of behavioral and physiological processes is a typical biological phenomenon regulated by a group of circadian rhythm genes. Dysfunction of the circadian rhythm can cause a wide range of problems, such as cancer and metabolic diseases. In recent decades, increased understanding of the roles of circadian rhythm genes in the bone remodeling process have been documented, including osteoblastic bone formation, osteoclastic bone resorption, and osteoblast/osteoclast communication. A timely review of the current findings may help to facilitate the new field of circadian rhythmic bone remodeling research. Targeted pharmacological modulation of circadian rhythm genes is a possible therapeutic approach through which to overcome bone remodeling problems in the future.

Basal p53 expression is indispensable for mesenchymal stem cell integrity

Marrow-resident mesenchymal stem cells (MSCs) serve as a functional component of the perivascular niche that regulates hematopoiesis. They also represent the main source of bone formed in adult bone marrow, and their bifurcation to osteoblast and adipocyte lineages plays a key role in skeletal homeostasis and aging. Although the tumor suppressor p53 also functions in bone organogenesis, homeostasis, and neoplasia, its role in MSCs remains poorly described. Herein, we examined the normal physiological role of p53 in primary MSCs cultured under physiologic oxygen levels. Using knockout mice and gene silencing we show that p53 inactivation downregulates expression of TWIST2, which normally restrains cellular differentiation to maintain wild-type MSCs in a multipotent state, depletes mitochondrial reactive oxygen species (ROS) levels, and suppresses ROS generation and PPARG gene and protein induction in response to adipogenic stimuli. Mechanistically, this loss of adipogenic potential skews MSCs toward an osteogenic fate, which is further potentiated by TWIST2 downregulation, resulting in highly augmented osteogenic differentiation. We also show that p53^−/− MSCs are defective in supporting hematopoiesis as measured in standard colony assays because of decreased secretion of various cytokines including CXCL12 and CSF1. Lastly, we show that transient exposure of wild-type MSCs to 21% oxygen upregulates p53 protein expression, resulting in increased mitochondrial ROS production and enhanced adipogenic differentiation at the expense of osteogenesis, and that treatment of cells with FGF2 mitigates these effects by inducing TWIST2. Together, these findings indicate that basal p53 levels are necessary to maintain MSC bi-potency, and oxygen-induced increases in p53 expression modulate cell fate and survival decisions. Because of the critical function of basal p53 in MSCs, our findings question the use of p53 null cell lines as MSC surrogates, and also implicate dysfunctional MSC responses in the pathophysiology of p53-related skeletal disorders.

Bone marrow adipose tissue as an endocrine organ: close to the bone?

White adipose tissue (WAT) is a major endocrine organ, secreting a diverse range of hormones, lipid species, cytokines and other factors to exert diverse local and systemic effects. These secreted products, known as 'adipokines', contribute extensively to WAT's impact on physiology and disease. Adipocytes also exist in the bone marrow (BM), but unlike WAT, study of this bone marrow adipose tissue (MAT) has been relatively limited. We recently discovered that MAT contributes to circulating adiponectin, an adipokine that mediates cardiometabolic benefits. Moreover, we found that MAT expansion exerts systemic effects. Together, these observations identify MAT as an endocrine organ. Additional studies are revealing further secretory functions of MAT, including production of other adipokines, cytokines and lipids that exert local effects within bone. These observations suggest that, like WAT, MAT has secretory functions with diverse potential effects, both locally and systemically. A major limitation is that these findings are often based on in vitro approaches that may not faithfully recapitulate the characteristics and functions of BM adipocytes in vivo. This underscores the need to develop improved methods for in vivo analysis of MAT function, including more robust transgenic models for MAT targeting, and continued development of techniques for non-invasive analysis of MAT quantity and quality in humans. Although many aspects of MAT formation and function remain poorly understood, MAT is now attracting increasing research focus; hence, there is much promise for further advances in our understanding of MAT as an endocrine organ, and how MAT impacts human health and disease.

Marrow adipocytes inhibit the differentiation of mesenchymal stem cells into osteoblasts via suppressing BMP-signaling

Background

Reduced bone formation is associated with increased bone marrow fat in many bone-loss related diseases including aging, post-menopause, and anorexia nervosa. Several lines of evidence suggested the regulation of osteogenesis and adipogenesis of the bone marrow-derived mesenchymal (skeletal) stem cells (BMSCs) by paracrine mediators. This study aimed to investigate the impact of adipocytes-secreted factors on the cell proliferation and osteoblast differentiation of BMSCs.

Methods

Serum free conditioned medium (CM-Adipo) was collected from stromal ST2 cells-derived adipocytes. Cell viability, quantitative alkaline phosphatase (ALP) activity assay, Alizarin red staining for matrix mineralization and osteogenic gene array expression were performed to determine the effect of CM-Adipo on cell proliferation and osteoblast differentiation of primary murine BMSCs (mBMSCs). Regulation of BMPs and NF-κB signaling pathways by CM-Adipo were detected by Western blot analysis and gene reporter assay.

Results

CM-Adipo showed no effect on cell viability/proliferation of primary mBMSCs as compared to CM-control. On the other hand, CM-Adipo significantly inhibited the commitment of mBMSCs into osteoblastic cell lineage in dose-dependent manner. CM-Adipo was found to dramatically inhibit the BMP2-induced osteoblast differentiation and to activate the inflammatory NF-κB signaling in mBMSCs. Interestingly, treatment of mBMSCs with the selective inhibitor of NF-κB pathway, BAY11-770682, showed to retrieve the inhibitory effect of CM-Adipo on BMP2-induced osteoblast differentiation in mBMSCs.

Conclusions

Our data demonstrated that the marrow adipocytes exert paracrine inhibitory effect on the osteoblast differentiation of mBMSCs by blocking BMPs signaling in a mechanism mediated by adipokines-induced NF-κB pathway activation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-017-0321-4) contains supplementary material, which is available to authorized users.

Pyrintegrin Induces Soft Tissue Formation by Transplanted or Endogenous Cells

Focal adipose deficiency, such as lipoatrophy, lumpectomy or facial trauma, is a formidable challenge in reconstructive medicine, and yet scarcely investigated in experimental studies. Here, we report that Pyrintegrin (Ptn), a 2,4-disubstituted pyrimidine known to promote embryonic stem cells survival, is robustly adipogenic and induces postnatal adipose tissue formation in vivo of transplanted adipose stem/progenitor cells (ASCs) and recruited endogenous cells. In vitro, Ptn stimulated human adipose tissue derived ASCs to differentiate into lipid-laden adipocytes by upregulating peroxisome proliferator-activated receptor (PPARγ) and CCAAT/enhancer-binding protein-α (C/EBPα), with differentiated cells increasingly secreting adiponectin, leptin, glycerol and total triglycerides. Ptn-primed human ASCs seeded in 3D-bioprinted biomaterial scaffolds yielded newly formed adipose tissue that expressed human PPARγ, when transplanted into the dorsum of athymic mice. Remarkably, Ptn-adsorbed 3D scaffolds implanted in the inguinal fat pad had enhanced adipose tissue formation, suggesting Ptn’s ability to induce in situ adipogenesis of endogenous cells. Ptn promoted adipogenesis by upregulating PPARγ and C/EBPα not only in adipogenesis induction medium, but also in chemically defined medium specifically for osteogenesis, and concurrently attenuated Runx2 and Osx via BMP-mediated SMAD1/5 phosphorylation. These findings suggest Ptn’s novel role as an adipogenesis inducer with a therapeutic potential in soft tissue reconstruction and augmentation.

Immunosuppression for in vivo research: state-of-the-art protocols and experimental approaches

Almost every experimental treatment strategy using non-autologous cell, tissue or organ transplantation is tested in small and large animal models before clinical translation. Because these strategies require immunosuppression in most cases, immunosuppressive protocols are a key element in transplantation experiments. However, standard immunosuppressive protocols are often applied without detailed knowledge regarding their efficacy within the particular experimental setting and in the chosen model species. Optimization of such protocols is pertinent to the translation of experimental results to human patients and thus warrants further investigation. This review summarizes current knowledge regarding immunosuppressive drug classes as well as their dosages and application regimens with consideration of species-specific drug metabolization and side effects. It also summarizes contemporary knowledge of novel immunomodulatory strategies, such as the use of mesenchymal stem cells or antibodies. Thus, this review is intended to serve as a state-of-the-art compendium for researchers to refine applied experimental immunosuppression and immunomodulation strategies to enhance the predictive value of preclinical transplantation studies.

Histological analysis of in vitro co-culture and in vivo mice co-transplantation of stem cell-derived adipocyte and osteoblast

Many researchers have focused on the role of adipocytes in increasing efficient bone tissue engineering and osteogenic differentiation of stem cells. Previous reports have not reached a definite consensus on whether adipocytes positively influence in vitro osteogenic differentiation and in vivo bone formation. We investigated the adipocyte influence on osteogenic differentiation from adipose-derived stromal cells (ADSCs) and bone formation through histological analysis in vitro and in vivo. Using the direct co-culture system, we analyzed the influence of adipocytes to promote the differentiation fate of ADSCs. Using co-transplantation of ADSC-derived adipocytes and osteoblasts into the dorsal region of mice, the osteogenesis and bone quality were determined by histological morphology, radiography, and the measurement of the Ca²⁺ concentration. The adipocyte negatively affected the osteoblast differentiation of ADSCs in the in vitro system and induced osteogenesis of osteoblasts in the in vivo system through co-transplantation. Interestingly, in the co-transplanted adipocytes and osteoblasts, the bone formation areas decreased in the osteoblast only group compared with the mixed adipocytes and osteoblast group 6 weeks after transplantation. Conversely, co-transplantation and osteoblast transplantation had similar degrees of calcification as observed from radiography analysis and the measurement of the Ca²⁺ concentrations. Our results revealed that adipocytes inhibited osteoblast differentiation in vitro but enhanced the efficacy of osteogenesis in vivo. In addition, the adipocytes controlled the activity of osteoclasts in the newly formed bone tissue. Our approach can be used to reconstruct bone using stem cell-based tissue engineering and to enhance the understanding of the role adipocytes play.

Lipoaspirate fluid proteome: A preliminary investigation by LC-MS top-down/bottom-up integrated platform of a high potential biofluid in regenerative medicine

The lipoaspirate fluid (LAF) is emerging as a potentially valuable source in regenerative medicine. In particular, our group recently demonstrated that it is able to exert osteoinductive properties in vitro. This original observation stimulated the investigation of the proteomic component of LAF, by means of LC-ESI-LTQ-Orbitrap-MS top-down/bottom-up integrated approach, which represents the object of the present study. Top-down analyses required the optimization of sample pretreatment procedures to enable the correct investigation of the intact proteome. Bottom-up analyses have been directly applied to untreated samples after monodimensional SDS-PAGE separation. The analysis of the acid-soluble fraction of LAF by top-down approach allowed demonstrating the presence of albumin and hemoglobin fragments (i.e. VV- and LVV-hemorphin-7), thymosins β4 and β10 peptides, ubiquitin and acyl-CoA binding protein; adipogenesis regulatory factor, perilipin-1 fragments, and S100A6, along with their PTMs. Part of the bottom-up proteomic profile was reproducibly found in both tested samples. The bottom-up approach allowed demonstrating the presence of proteins, listed among the components of adipose tissue and/or comprised within the ASCs intracellular content and secreted proteome. Our data provide a first glance on the LAF molecular profile, which is consistent with its tissue environment. LAF appeared to contain bioactive proteins, peptides and paracrine factors, suggesting its potential translational exploitation.

Mitochondria in mesenchymal stem cell biology and cell therapy: From cellular differentiation to mitochondrial transfer

Mesenchymal stem cells (MSCs) are characterized to have the capacity of self-renewal and the potential to differentiate into mesoderm, ectoderm-like and endoderm-like cells. MSCs hold great promise for cell therapies due to their multipotency in vitro and therapeutic advantage of hypo-immunogenicity and lower tumorigenicity. Moreover, it has been shown that MSCs can serve as a vehicle to transfer mitochondria into cells after cell transplantation. Mitochondria produce most of the energy through oxidative phosphorylation in differentiated cells. It has been increasingly clear that the switch of energy supply from glycolysis to aerobic metabolism is essential for successful differentiation of MSCs. Post-translational modifications of proteins have been established to regulate mitochondrial function and metabolic shift during MSCs differentiation. In this article, we review and provide an integrated view on the roles of different protein kinases and sirtuins in the maintenance and differentiation of MSCs. Importantly, we provide evidence to suggest that alteration in the expression of Sirt3 and Sirt5 and relative changes in the acylation levels of mitochondrial proteins might be involved in the activation of mitochondrial function and adipogenic differentiation of adipose-derived MSCs. We summarize their roles in the regulation of mitochondrial biogenesis and metabolism, oxidative responses and differentiation of MSCs. On the other hand, we discuss recent advances in the study of mitochondrial dynamics and mitochondrial transfer as well as their roles in the differentiation and therapeutic application of MSCs to improve cell function in vitro and in animal models. Accumulating evidence has substantiated that the therapeutic potential of MSCs is conferred not only by cell replacement and paracrine effects but also by transferring mitochondria into injured tissues or cells to modulate the cellular metabolism in situ. Therefore, elucidation of the underlying mechanisms in the regulation of mitochondrial metabolism of MSCs may ultimately improve therapeutic outcomes of stem cell therapy in the future.

Bone Marrow Adipose Tissue: To Be or Not To Be a Typical Adipose Tissue?

Bone marrow adipose tissue (BMAT) emerges as a distinct fat depot whose importance has been proved in the bone-fat interaction. Indeed, it is well recognized that adipokines and free fatty acids released by adipocytes can directly or indirectly interfere with cells of bone remodeling or hematopoiesis. In pathological states, such as osteoporosis, each of adipose tissues - subcutaneous white adipose tissue (WAT), visceral WAT, brown adipose tissue (BAT), and BMAT - is differently associated with bone mineral density (BMD) variations. However, compared with the other fat depots, BMAT displays striking features that makes it a substantial actor in bone alterations. BMAT quantity is well associated with BMD loss in aging, menopause, and other metabolic conditions, such as anorexia nervosa. Consequently, BMAT is sensed as a relevant marker of a compromised bone integrity. However, analyses of BMAT development in metabolic diseases (obesity and diabetes) are scarce and should be, thus, more systematically addressed to better apprehend the bone modifications in that pathophysiological contexts. Moreover, bone marrow (BM) adipogenesis occurs throughout the whole life at different rates. Following an ordered spatiotemporal expansion, BMAT has turned to be a heterogeneous fat depot whose adipocytes diverge in their phenotype and their response to stimuli according to their location in bone and BM. In vitro, in vivo, and clinical studies point to a detrimental role of BM adipocytes (BMAs) throughout the release of paracrine factors that modulate osteoblast and/or osteoclast formation and function. However, the anatomical dissemination and the difficulties to access BMAs still hamper our understanding of the relative contribution of BMAT secretions compared with those of peripheral adipose tissues. A further characterization of the phenotype and the functional regulation of BMAs are ever more required. Based on currently available data and comparison with other fat tissues, this review addresses the originality of the BMAT with regard to its development, anatomy, metabolic properties, and response to physiological cues.

Overexpression of the Circadian Clock Gene Rev-erbα Affects Murine Bone Mesenchymal Stem Cell Proliferation and Osteogenesis

Bone mesenchymal stem cell (BMSC) age-related changes include decreased osteogenesis and increased adipogenesis. Rev-erbα and the Wnt/β-catenin signaling pathway were known to play important roles in BMSC aging. In this study, we have aimed to elucidate whether Rev-erbα and Wnt/β-catenin signaling interact during BMSC proliferation and osteogenesis. Our results showed that Rev-erbα expression gradually dropped during BMSC osteogenesis, and overexpression of Rev-erbα in BMSCs inhibited cell proliferation and osteogenesis. The inhibition of cell proliferation induced by Rev-erbα overexpression was partially reversed when Wnt/β-catenin signaling was activated. These results suggested that Rev-erbα could promote BMSC aging and may be the negative regulator during the late stage of osteogenesis. The clock gene Rev-erbα and Wnt/β-catenin signaling interact in the regulation of cell proliferation.

Dietary supplementation with vitamin k affects transcriptome and proteome of Senegalese sole, improving larval performance and quality

Nutritional factors strongly influence fish larval development and skeletogenesis, and may induce skeletal deformities. Vitamin K (VK) has been largely disregarded in aquaculture nutrition, despite its important roles in bone metabolism, in γ-carboxylation of Gla proteins, and in regulating gene expression through the pregnane X receptor (Pxr). Since the mechanisms mediating VK effects over skeletal development are poorly known, we investigated the effects of VK-supplementation on skeletal development in Senegalese sole larvae, aiming to identify molecular pathways involved. Larvae were fed live preys enriched with graded levels of phylloquinone (PK) (0, 50, and 250 mg kg(-1)) and survival rate, growth, VK contents, calcium content and incidence of skeletal deformities were determined, revealing an improvement of larval performance and decreasing the incidence of deformities in VK-supplemented groups. Comparative proteome analysis revealed a number of differentially expressed proteins between Control and Diet 250 associated with key biological processes including skin, muscle, and bone development. Expression analysis showed that genes encoding proteins related to the VK cycle (ggcx, vkor), VK nuclear receptor (pxr), and VK-dependent proteins (VKDPs; oc1 and grp), were differentially expressed. This study highlights the potential benefits of increasing dietary VK levels in larval diets, and brings new insights on the mechanisms mediating the positive effects observed on larval performance and skeletal development.

Curcumin Inhibits Prostate Cancer Bone Metastasis by Up-Regulating Bone Morphogenic Protein-7 in Vivo

A number of studies have focused on the beneficial properties of Curcumin (diferuloyl methane, used in South Asian cuisine and traditional medicine) such as the chemoprevention of cancer. Recent studies have also indicated that this material has significant benefits for the treatment of cancer and is currently undergoing several clinical trials. We have been interested in the application of this compound as a therapeutic agent for advanced prostate cancer, particularly the skeletal complications in this malignancy. Our earlier work indicated that this compound could inhibit the osteomimetic properties which occur in castration resistant prostate cancer cells, by interfering with the common denominators between these cancer cells and the bone cells in the metastatic tumor microenvironment, namely the osteoblasts and the osteoclast. We predicted that curcumin could break the vicious cycle of reciprocal stimulation that results in uncontrolled osteolysis in the bony matrix. In this work, we have evaluated the potential of this compound in inhibiting the bone metastasis of hormone refractory prostate cancer cells in an established animal model. Our results strongly suggest that curcumin modulates the TGF-β signaling that occurs due to bone matrix degradation by up-regulating the metastasis inhibitory bone morphogenic protein-7 (BMP- 7). This enhancement of BMP-7 in the context of TGF-βin the tumor microenvironment is shown to enhance the mesenchymal-to-epithelial transition. Most importantly, we show that as a result of BMP-7 up-regulation, a novel brown/beige adipogenic differentiation program is also up-regu- lated which plays a role in the inhibition of bone metastasis. Our results suggest that curcumin may subvert the TGF-βsignaling to an alternative adipogenic differentiation program in addition to the previously established interference with the osteomimetic properties, thus inhibiting the bone metastatic processes in a chemopreventive as well as therapeutic setting.

Roles of brain and muscle ARNT-like 1 and Wnt antagonist Dkk1 during osteogenesis of bone marrow stromal cells

OBJECTIVES

Many studies have demonstrated that the clock gene, brain and muscle ARNT-like 1 (Bmal1), is directly related to bone ageing by affecting age-related changes to mesenchymal stem cells (MSCs). As a main developmental signal, Wnt may play an important role in this process. Here, we have aimed to elucidate whether Bmal1 positively regulates osteogenesi via Wnt pathways.

MATERIALS AND METHODS

Bone marrow stromal cells were cultured in basic and in osteo-induction medium with Wnt signalling inhibitor Dkk1 and Bmal1 transfection. Proliferation and osteogenesis of MSCs, expression of Bmal1 and activation of Wnt signalling were investigated by flow cytometry, senescence-associated β-galactosidase (SA-β-gal) staining, real-time quantitative PCR and western blot analysis.

RESULTS

Expression of Bmal1 (specially after 7 days osteo-induction), activation of Wnt signalling and osteo-related factors fell significantly during osteo-induction after Dkk1 addition. When cellular Bmal1 was increased by transfection, osteogenesis inhibition by Dkk1 was rescued to a certain extent with activation of Wnt signalling. However, Dkk1 did not significantly affect proliferation or senescence of MSCs during early periods of culture.

CONCLUSION

These findings demonstrated that Bmal1 and Wnt signalling may have a synergistic effect at a particular stage of osteogenesis. Inhibition of Wnt signalling did not greatly affect ageing of MSCs through early passages.

Genome-wide approaches for identifying genetic risk factors for osteoporosis

Osteoporosis, the most common type of bone disease worldwide, is clinically characterized by low bone mineral density (BMD) and increased susceptibility to fracture. Multiple genetic and environmental factors and gene-environment interactions have been implicated in its pathogenesis. Osteoporosis has strong genetic determination, with the heritability of BMD estimated to be as high as 60%. More than 80 genes or genetic variants have been implicated in risk of osteoporosis by hypothesis-free genome-wide studies. However, these genes or genetic variants can only explain a small portion of BMD variation, suggesting that many other genes or genetic variants underlying osteoporosis risk await discovery. Here, we review recent progress in genome-wide studies of osteoporosis and discuss their implications for medicine and the major challenges in the field.

Failure to generate bone marrow adipocytes does not protect mice from ovariectomy-induced osteopenia

A reciprocal association between bone marrow fat and bone mass has been reported in ovariectomized rodents, suggesting that bone marrow adipogenesis has a negative effect on bone growth and turnover balance. Mice with loss of function mutations in kit receptor (kit(W/W-v)) have no bone marrow adipocytes in tibia or lumbar vertebra. We therefore tested the hypothesis that marrow fat contributes to the development of osteopenia by comparing the skeletal response to ovariectomy (ovx) in growing wild type (WT) and bone marrow adipocyte-deficient kit(W/W-v) mice. Mice were ovx at 4 weeks of age and sacrificed 4 or 10 weeks post-surgery. Body composition was measured at necropsy by dual-energy X-ray absorptiometry. Cortical (tibia) and cancellous (tibia and lumbar vertebra) bone architecture were evaluated by microcomputed tomography. Bone marrow adipocyte size and density, osteoblast- and osteoclast-lined bone perimeters, and bone formation were determined by histomorphometry. Ovx resulted in an increase in total body fat mass at 10 weeks post-ovx in both genotypes, but the response was attenuated in the in kit(W/W-v) mice. Adipocytes were present in bone marrow of tibia and lumbar vertebra in WT mice and bone marrow adiposity increased following ovx. In contrast, marrow adipocytes were not detected in either intact or ovx kit(W/W-v) mice. However, ovx in WT and kit(W/W-v) mice resulted in statistically indistinguishable changes in cortical and cancellous bone mass, cortical and cancellous bone formation rate, and cancellous osteoblast and osteoclast-lined bone perimeters. In conclusion, our findings do not support a causal role for increased bone marrow fat as a mediator of ovx-induced osteopenia in mice.

The influence of therapeutic radiation on the patterns of bone marrow in ovary-intact and ovariectomized mice

Background

The functional components of bone marrow (i.e., the hematopoietic and stromal populations) and the adjacent bone have traditionally been evaluated incompletely as distinct entities rather than the integrated system. We perturbed this system in vivo using a medically relevant radiation model in the presence or absence of ovarian function to understand integrated tissue interaction.

Methodology/Principal Findings

Ovary-intact and ovariectomized mice underwent either no radiation or single fractional 16 Gy radiation to the caudal skeleton (I±R, OVX±R). Marrow fat, hematopoietic cellularity, and cancellous bone volume fraction (BV/TV %) were assessed. Ovariectomy alone did not significantly reduce marrow cellularity in non-irradiated mice (OVX−R vs. I−R, p = 0.8445) after 30 days; however it impaired the hematopoietic recovery of marrow following radiation exposure (OVX+R vs. I+R, p = 0.0092). The combination of radiation and OVX dramatically increases marrow fat compared to either factor alone (p = 0.0062). The synergistic effect was also apparent in the reduction of hematopoietic marrow cellularity (p = 0.0661); however it was absent in BV/TV% changes (p = 0.2520). The expected inverse relationship between marrow adiposity vs. hematopoietic cellularity and bone volume was observed. Interestingly compared with OVX mice, intact mice demonstrated double the reduction in hematopoietic cellularity and a tenfold greater degree of bone loss for a given unit of expansion in marrow fat.

Conclusions/Significance

Ovariectomy prior to delivery of a clinically-relevant focal radiation exposure in mice, exacerbated post-radiation adipose accumulation in the marrow space but blunted bone loss and hematopoietic suppression. In the normally coupled homeostatic relationship between the bone and marrow domains, OVX appears to alter feedback mechanisms. Confirmation of this non-linear phenomenon (presumably due to differential radiosensitivity) and demonstration of the mechanism of action is needed to provide strategies to diminish the effect of radiation on exposed tissues.