52900224
Thy-1 Deficiency Augments Bone Loss in Obesity by Affecting Bone Formation and Resorption
Healthy bone remodeling results from a balanced bone formation and bone resorption realized by bone-forming osteoblasts and bone-resorbing osteoclasts, respectively. Recently, Thy-1 (CD90) was identified as positive regulator of osteoblast differentiation and activation, thus, promoting bone formation while concurrently inhibiting adipogenesis and obesity in mice. Additionally, Thy-1 did not affect bone resorption. An obesity-related co-morbidity that is increasing in prevalence is a disturbed bone formation resulting in an increased fracture risk. The underlying mechanisms of obesity-induced bone alterations are not yet fully elucidated and therefore therapy options for efficient bone-anabolic treatments are limited. Therefore, we investigated the impact of Thy-1 on bone metabolism under obese conditions. Indeed, high fat diet (HFD) induced obese mice lacking Thy-1 (Thy-1−/−) showed increased body fat mass compared to wildtype (WT) mice while bone mass (−38%) and formation (−57%) were decreased as shown by micro-computed tomography (μCT) measurement, histological analysis, and fourier-transform infrared spectroscopy (FTIR). Interestingly, under obese conditions, lack of Thy-1 affected both osteoblast and osteoclast function. Number (−30%) and activity of osteoblasts were decreased in obese Thy-1−/− mice while osteoclast number (+39%) and activity were increased. Facilitated bone marrow fat accumulation (+56%) in obese Thy-1−/− mice compared to obese WT mice was associated with upregulated tumor necrosis factor α (Tnfα, +46%) and colony stimulating factor 1 receptor (Csf1r) expression, strong promoters of osteoclast differentiation. Moreover, lack of Thy-1 was accompanied by a reduction of osteoprotegerin (Tnfrsf11b) expression (−36%), an inhibitor of osteoclast differentiation. Altered Tnfα, Csf1r, and Tnfrsf11b expression might be responsible for elevated osteoclast activity in obese Thy-1-deficient mice. In summary, our findings show that lack of Thy-1 promotes obesity under HFD conditions while concurrently decreasing bone mass and formation. Mechanistic studies revealed that under obese conditions lack of Thy-1 impairs both bone formation and bone resorption.
2269601
Bone remodelling at a glance
The bone remodelling cycle (see Poster panel “The bone remodelling cycle”) maintains the integrity of the skeleton through the balanced activities of its constituent cell types. These are the bone-forming osteoblast, a cell that produces the organic bone matrix and aids its mineralisation ([
34310269
Isolation and characterization of a molecular complex containing thy‐1 antigen from the surface of murine thymocytes and t cells
A complex containing the Thy‐1 (θ) antigen has been isolated from the surface of murine thymocytes and T cells by cell surface radioiodination, lysis by freezing and thawing, and immunoprecipitation. The specificity of the immunoprecipitation was firmly established using both congenic and noncongenic anti‐Thy‐1 sera and thymocytes from congenic and noncongenic mice.
9679911
Expression of Thy-1 on human hematopoietic progenitor cells
Expression of Thy-1 on hematopoietic cells from human fetal liver (FL), cord blood (CB), and bone marrow (BM) was studied with a novel anti-Thy- 1 antibody, 5E10. Specificity of 5E10 for human Thy-1 was demonstrated by immunoprecipitation of a 25-35-kD molecule, and the sequence of a cDNA that was cloned by immunoselection of COS cells transfected with a cDNA library derived from a 5E10+ cell line. Two- and three-color immunofluorescence staining experiments revealed that the Thy-1 expression is restricted to, an average, 1-4% of FL, CB, and BM cells, and binding to these cell types is essentially restricted to a very small subset of lymphoid cells and approximately 25% of CD34+ cells. Thy-1+ CD34+ cells were further characterized as CD38lo/CD45RO+/CD45RA- /CD71lo/c-kit(lo) and rhodamine 123dull. When CD34+ cells were sorted on the basis of Thy-1 expression, the majority of clonogenic cells were recovered in the CD34+Thy-1- fraction, whereas the majority of cells capable of producing myeloid colonies after 5-8 wk of long-term culture (long-term culture initiating cells) were recovered in the Thy-1+CD34+ fraction. In addition to CD34+ cells, Thy-1 was found to be expressed on a variable, very small number (< 1%) of CD34- mononuclear cells in BM, CB, and peripheral blood that were further characterized as CD3+ CD4+ lymphocytes. The restricted expression of Thy-1 on primitive hematopoietic cells is in agreement with a previous report (Baum et al., 1992. Proc. Natl. Acad. Sci. USA. 89:2804) in which Thy-1 expression was used to enrich for primitive hematopoietic cells from fetal tissue. Compared with those previous studies, we found Thy-1 expression on a larger proportion of CD34+ cells (25% in our study vs. 5% in Baum et al.) and furthermore performed studies on Thy-1 expression on CD34+ cells from CB, FL, and BM in relation to markers that are known to be differentially expressed on hematopoietic cells. Taken together our results indicate that Thy-1-specific antibody 5E10 is an attractive tool for further studies on the biology and purification of human stem cells.
40029707
Controlling the Balance of Fibroblast Proliferation and Differentiation: Impact of Thy-1.
Fibroblasts provide matrix and regulatory mediators to the microenvironment and thereby contribute to maintenance of tissue homeostasis, wound healing, and tumor progression. In the present study, we addressed the functional relevance of Thy-1 for fibroblast functions in vitro and in vivo. Using fibroblasts from Thy-1(-/-) and wild-type mice, recombinant expression of Thy-1, and analysis of the interaction of fibroblasts with immobilized Thy-1, we showed that Thy-1 has a crucial role in the control of cell growth by suppressing proliferation and promoting apoptosis and differentiation of dermal fibroblasts. Function-blocking studies revealed that Thy-1 mediates the control of apoptosis and proliferation via modulation of β3 integrin function. Interestingly, Thy-1-mediated growth control appears to be a more general mechanism because it also regulates growth of tumor cells of different origin dependent on their β3 integrin expression. In summary, our findings point to an important role of Thy-1 in controlling the balance between proliferation and differentiation in dermal fibroblasts.
51943049
Thy-1 (CD90) promotes bone formation and protects against obesity
Thy-1 on mesenchymal stem cells promotes osteogenesis while inhibiting adipogenesis and obesity. Stem cells’ balancing act Mesenchymal stem cells (MSCs) differentiate into multiple cell types. Picke et al. investigated how MSC differentiation is regulated to maintain homeostasis between the bone and fat lineages. Genetic deletion of Thy-1, a protein expressed on multiple cell types including MSCs, prevented MSC differentiation into osteoblasts but promoted differentiation into adipocytes. Thy-1–deficient mice had increased body fat and decreased bone mass. High-fat diet induced obesity in wild-type mice and concurrent reduction in bone formation, which was associated with decreased Thy-1 expression on MSCs. Obese human subjects and subjects with osteoporosis showed reductions in serum soluble Thy-1. This study suggests that Thy-1 regulates the balance between bone and fat lineages, with possible implications for bone and metabolic disorders. Osteoporosis and obesity result from disturbed osteogenic and adipogenic differentiation and present emerging challenges for our aging society. Because of the regulatory role of Thy-1 in mesenchyme-derived fibroblasts, we investigated the impact of Thy-1 expression on mesenchymal stem cell (MSC) fate between osteogenic and adipogenic differentiation and consequences for bone formation and adipose tissue development in vivo. MSCs from Thy-1–deficient mice have decreased osteoblast differentiation and increased adipogenic differentiation compared to MSCs from wild-type mice. Consistently, Thy-1–deficient mice exhibited decreased bone volume and bone formation rate with elevated cortical porosity, resulting in lower bone strength. In parallel, body weight, subcutaneous/epigonadal fat mass, and bone fat volume were increased. Thy-1 deficiency was accompanied by reduced expression of specific Wnt ligands with simultaneous increase of the Wnt inhibitors sclerostin and dickkopf-1 and an altered responsiveness to Wnt. We demonstrated that disturbed bone remodeling in osteoporosis and dysregulated adipose tissue accumulation in patients with obesity were mirrored by reduced serum Thy-1 concentrations. Our findings provide new insights into the mutual regulation of bone formation and obesity and open new perspectives to monitor and to interfere with the dysregulated balance of adipogenesis and osteogenesis in obesity and osteoporosis.
8761190
Thy-1-positive cells in the subodontoblastic layer possess high potential to differentiate into hard tissue-forming cells
The cells of the subodontoblastic cell-rich layer in dental pulp are speculated to contain odontoblast progenitor cells because of their positional relationship with odontoblasts as well as their high alkaline phosphatase (ALP) activity. However, it has yet to be determined whether these cells have the ability to differentiate into odontoblastic cells. In the present study, we firstly found that the majority of cells in the subodontoblastic layer expressed Thy-1, a cell-surface marker of stem and progenitor cells. Then, we evaluated the capacity of Thy-1 high- and low-expressing (Thy-1high and Thy-1low) cells separated from rat dental pulp cells by use of a fluorescence-activated cell sorter to differentiate into hard tissue-forming cells in vitro and in vivo. Following stimulation with bone morphogenetic protein-2, Thy-1high cells in vitro showed accelerated induction of ALP activity and formation of alizarin red-positive mineralized matrix compared with Thy-1low cells. Furthermore, subcutaneous implantation of Thy-1high cells efficiently induced the formation of bone-like matrix. These results collectively suggest that Thy-1-positive dental pulp cells localized in the subodontoblastic layer had the ability to differentiate into hard tissue-forming cells, and thus these cells may serve as a source of odontoblastic cells.
16906394
CD90 (Thy-1)-positive selection enhances osteogenic capacity of human adipose-derived stromal cells.
BACKGROUND
Stem cell-based bone tissue engineering with adipose-derived stromal cells (ASCs) has shown great promise for revolutionizing treatment of large bone deficits. However, there is still a lack of consensus on cell surface markers identifying osteoprogenitors. Fluorescence-activated cell sorting has identified a subpopulation of CD105(low) cells with enhanced osteogenic differentiation. The purpose of the present study was to compare the ability of CD90 (Thy-1) to identify osteoprogenitors relative to CD(105).


METHODS
Unsorted cells, CD90(+), CD90(-), CD105(high), and CD105(low) cells were treated with an osteogenic differentiation medium. For evaluation of in vitro osteogenesis, alkaline phosphatase (ALP) staining and alizarin red staining were performed at 7 days and 14 days, respectively. RNA was harvested after 7 and 14 days of differentiation, and osteogenic gene expression was examined by quantitative real-time polymerase chain reaction. For evaluation of in vivo osteogenesis, critical-sized (4-mm) calvarial defects in nude mice were treated with the hydroxyapatite-poly(lactic-co-glycolic acid) scaffold seeded with the above-mentioned subpopulations. Healing was followed using micro-CT scans for 8 weeks. Calvaria were harvested at 8 weeks postoperatively, and sections were stained with Movat's Pentachrome.


RESULTS
Transcriptional analysis revealed that the CD90(+) subpopulation was enriched for a more osteogenic subtype relative to the CD105(low) subpopulation. Staining at day 7 for ALP was greatest in the CD90(+) cells, followed by the CD105(low) cells. Staining at day 14 for alizarin red demonstrated the greatest amount of mineralized extracellular matrix in the CD90(+) cells, again followed by the CD105(low) cells. Quantification of in vivo healing at 2, 4, 6, and 8weeks postoperatively demonstrated increased bone formation in defects treated with CD90(+) ASCs relative to all other groups. On Movat's Pentachrome-stained sections, defects treated with CD90(+) cells showed the most robust bony regeneration. Defects treated with CD90(-) cells, CD105(high) cells, and CD105(low) cells demonstrated some bone formation, but to a lesser degree when compared with the CD90(+) group.


CONCLUSIONS
While CD105(low) cells have previously been shown to possess an enhanced osteogenic potential, we found that CD90(+) cells are more capable of forming bone both in vitro and in vivo. These data therefore suggest that CD90 may be a more effective marker than CD105 to isolate a highly osteogenic subpopulation for bone tissue engineering.
25454950
Thy1 (CD90) controls adipogenesis by regulating activity of the Src family kinase, Fyn
Worldwide obesity rates are at epidemic levels, and new insight into the regulation of obesity and adipogenesis are required. Thy1 (CD90), a cell surface protein with an enigmatic function, is expressed on subsets of fibroblasts and stem cells. We used a diet‐induced obesity model to show that Thy1‐null mice gain weight at a faster rate and gain 30% more weight than control C57BL/6 mice. During adipogenesis, Thy1 expression is lost in mouse 3T3‐L1 cells. Overexpression of Thy1 blocked adipocyte formation and reduced mRNA and protein expression of an adipocyte marker, fatty acid‐binding protein 4, by 5‐fold. Although preadipocyte fibroblasts expressed Thy1 mRNA and protein, adipocytes from mouse and human fat tissue had almost undetectable Thy1 levels. Thy1 decreases the activity of the adipogenic transcription factor PPARγ by more than 60% as shown by PPARγ‐dependent reporter assays. Using both genetic and pharmacologic approaches, we show Thy1 expression dampens PPARγ by inhibiting the activity of the Src‐family kinase, Fyn. Thus, these studies reveal Thy1 blocks adipogenesis and PPARγ by inhibiting Fyn and support the idea that Thy1 is a novel therapeutic target in obesity.—Woeller, C. F., O'Loughlin, C. W., Pollock, S. J., Thatcher, T. H., Feldon, S. E., Phipps, R. P., Thy1 (CD90) controls adipogenesis by regulating activity of the Src family kinase, Fyn. FASEB J. 29, 920–931 (2015). www.fasebj.org
25759295
Thy1 is a positive regulator of osteoblast differentiation and modulates bone homeostasis in obese mice
Thy1 (CD90), a glycosylated, glycophosphatidylinositol‐anchored membrane protein highly expressed by subsets of mesenchymal stem cells and fibroblasts, inhibits adipogenesis. The role of Thy1 on bone structure and function has been poorly studied and represents a major knowledge gap. Therefore, we analyzed the long bones of wild‐type (WT) and Thy1 knockout (KO) mice with micro‐computed tomography (micro‐CT) and histomorphometry to compare changes in bone architecture and overall bone structure. micro‐CT analysis of long bones revealed Thy1 KO and WT mice fed a high‐fat diet demonstrated bone structural parameters at 4 mo that differed significantly between WT and KO mice. A significant reduction in trabecular bone volume was noted in Thy1 KO mice. The most prominent differences were observed in trabecular bone volume ratio and trabecular bone connectivity density. Consistent with micro‐CT measurements, histomorphometric analysis also showed decreased bone volume in the obese Thy1 KO mice compared to obese WT mice. In vitro assays revealed that osteogenic conditions increased Thy1 expression during OB differentiation and absence of Thy1 attenuated osteoblastogenesis. Together, these findings support the concept that Thy1 serves as a major mechanistic link to regulate bone formation and negatively regulate adipogenesis.—Paine, A., Woeller, C. F., Zhang, H., Garcia‐Hernandez, M. L., Huertas, N., Xing, L., Phipps, R. P., Ritchlin, C. T. Thy1 is a positive regulator of osteoblast differentiation and modulates bone homeostasis in obese mice. FASEB J. 32, 3174–3183 (2018). www.fasebj.org
206213730
Osteoporosis prevention, screening, and treatment: a review.
Osteoporosis, defined as low bone mass leading to increased fracture risk, is a major health problem that affects approximately 10 million Americans. The aging U.S. population is predicted to contribute to as much as a 50% increase in prevalence by 2025. Although common, osteoporosis can be clinically silent, and without prevention and screening, the costs of osteoporotic fracture-related morbidity and mortality will burden the U.S. healthcare system. This is a particularly relevant concern in the context of diminishing health care resources. Dual-energy X-ray absorptiometry is the most widely used, validated technique for measuring bone mineral density (BMD) and diagnosing osteoporosis. Cost-effectiveness analyses support early detection and treatment of high-risk patients with antiresorptive medications such as bisphosphonates. Moreover, optimization of bone health throughout life can help prevent osteoporosis. Current guidelines recommend screening women by age 65 years, but because no guidelines for screening intervals exist, decisions are made on the basis of clinical judgment alone. Although the recent literature provides some guidance, this review further explores current recommendations in light of newer evidence to provide more clarity on prevention, screening, and management strategies for patients with osteoporosis in the primary care setting.
4424156
Inflammation and metabolic disorders
Metabolic and immune systems are among the most fundamental requirements for survival. Many metabolic and immune response pathways or nutrient- and pathogen-sensing systems have been evolutionarily conserved throughout species. As a result, immune response and metabolic regulation are highly integrated and the proper function of each is dependent on the other. This interface can be viewed as a central homeostatic mechanism, dysfunction of which can lead to a cluster of chronic metabolic disorders, particularly obesity, type 2 diabetes and cardiovascular disease. Collectively, these diseases constitute the greatest current threat to global human health and welfare.
34602152
Obesity: Friend or foe for osteoporosis
Osteoporosis and obesity are worldwide health problems. Interestingly, both are associated with significant morbidity and mortality. Both the diseases have common linkage as bone marrow mesenchymal stromal cells are the common precursors for both osteoblasts and adipocytes. Aging may shift composition of bone marrow by increasing adipocytes, osteoclast activity, and decreasing osteoblast activity, resulting into osteoporosis. Adipocytes secret leptin, adiponectin, adipsin, as well as proinflammatory cytokines, that contributes in pathogenesis of osteoporosis. This new concept supports the hypothesis, that the positive correlation of weight and body mass index (BMI) with bone mineral density (BMD) is not confirmed by large population-based studies. Thus, the previous concept, that obesity is protective for osteoporosis may not stand same as bone marrow fat deposition (adipogenesis) seen in obesity, is detrimental for bone health.
14739947
BMI and BMD: The Potential Interplay between Obesity and Bone Fragility
Recent evidence demonstrating an increased fracture risk among obese individuals suggests that adipose tissue may negatively impact bone health, challenging the traditional paradigm of fat mass playing a protective role towards bone health. White adipose tissue, far from being a mere energy depot, is a dynamic tissue actively implicated in metabolic reactions, and in fact secretes several hormones called adipokines and inflammatory factors that may in turn promote bone resorption. More specifically, Visceral Adipose Tissue (VAT) may potentially prove detrimental. It is widely acknowledged that obesity is positively associated to many chronic disorders such as metabolic syndrome, dyslipidemia and type 2 diabetes, conditions that could themselves affect bone health. Although aging is largely known to decrease bone strength, little is yet known on the mechanisms via which obesity and its comorbidities may contribute to such damage. Given the exponentially growing obesity rate in recent years and the increased life expectancy of western countries it appears of utmost importance to timely focus on this topic.
4439277
Relation of body composition, fat mass, and serum lipids to osteoporotic fractures and bone mineral density in Chinese men and women.
BACKGROUND
Higher fat mass may be an independent risk factor for osteoporosis and osteoporotic fractures.


OBJECTIVE
We aimed to determine the independent contribution of fat mass to osteoporosis and to estimate the risk of osteoporotic fractures in relation to body weight, lean mass, and other confounders.


DESIGN
This was a community-based, cross-sectional study of 7137 men, 4585 premenopausal women, and 2248 postmenopausal women aged 25-64 y. Total-body and hip bone mineral content (BMC) and bone mineral density (BMD) and body composition were measured by dual-energy X-ray absorptiometry. Serum lipids were measured. Sex- and menopause-specific multiple generalized linear models were applied.


RESULTS
Across 5-kg strata of body weight, fat mass was significantly inversely associated with BMC in the whole body and total hip. When we compared the highest quartile with the lowest quartile of percentage fat mass in men, premenopausal women, and postmenopausal women, the adjusted odds ratios (95% CIs) of osteoporosis defined by hip BMD were 5.2 (2.1, 13.2), 5.0 (1.7, 15.1), and 6.9 (4.3, 11.2), respectively. Significant linear trends existed for higher risks of osteoporosis, osteopenia, and nonspine fractures with higher percentage fat mass. Significant negative relations were found between whole-body BMC and cholesterol, triacylglycerol, LDL, and the ratio of HDL to LDL in all groups.


CONCLUSIONS
Risks of osteoporosis, osteopenia, and nonspine fractures were significantly higher for subjects with higher percentage body fat independent of body weight, physical activity, and age. Thus, fat mass has a negative effect on bone mass in contrast with the positive effect of weight-bearing itself.
5072335
Obesity and fracture in men and women: An epidemiologic perspective
In Western societies, mean body weight has increased dramatically in older people, and a similar trend exists in Asia. Yet insufficient attention has been directed to the problem of osteoporotic fractures in the overweight and obese. Many, if not most, osteoporotic fractures occur in overweight or obese people, and obese men may be particularly susceptible. We discuss the potential implications of these findings, including the challenge of identifying individuals at highest risk, screening and treatment strategies, and future research directions. © 2012 American Society for Bone and Mineral Research
25559853
Inhibition of osteoblast differentiation by tumor necrosis factor-alpha.
Tumor necrosis factor-alpha (TNF-alpha) has a key role in skeletal disease in which it promotes reduced bone formation by mature osteoblasts and increased osteoclastic resorption. Here we show that TNF inhibits differentiation of osteoblasts from precursor cells. TNF-alpha treatment of fetal calvaria precursor cells, which spontaneously differentiate to the osteoblast phenotype over 21 days, inhibited differentiation as shown by reduced formation of multilayered, mineralizing nodules and decreased secretion of the skeletal-specific matrix protein osteocalcin. The effect of TNF was dose dependent with an IC50 of 0.6 ng/ml, indicating a high sensitivity of these precursor cells. Addition of TNF-alpha from days 2-21, 2-14, 7-14, and 7-10 inhibited nodule formation but addition of TNF after day 14 had no effect. Partial inhibition of differentiation was observed with addition of TNF on only days 7-8, suggesting that TNF could act during a critical period of phenotype selection. Growth of cells on collagen-coated plates did not prevent TNF inhibition of differentiation, suggesting that inhibition of collagen deposition into matrix by proliferating cells could not, alone, explain the effect of TNF. Northern analysis revealed that TNF inhibited the expression of insulin-like growth factor I (IGF-I). TNF had no effect on expression of the osteogenic bone morphogenic proteins (BMPs-2, -4, and -6), or skeletal LIM protein (LMP-1), as determined by semiquantitative RT-PCR. Addition of IGF-I or BMP-6 to fetal calvaria precursor cell cultures enhanced differentiation but could not overcome TNF inhibition, suggesting that TNF acted downstream of these proteins in the differentiation pathway. The clonal osteoblastic cell line, MC3T3-E1-14, which acquires the osteoblast phenotype spontaneously in postconfluent culture, was also studied. TNF inhibited differentiation of MC3T3-E1-14 cells as shown by failure of mineralized matrix formation in the presence of calcium and phosphate. TNF was not cytotoxic to either cell type as shown by continued attachment and metabolism in culture, trypan blue exclusion, and Alamar Blue cytotoxicity assay. These results demonstrate that TNF-alpha is a potent inhibitor of osteoblast differentiation and suggest that TNF acts distal to IGF-I, BMPs, and LMP-1 in the progression toward the osteoblast phenotype.
17981205
Classical and Paradoxical Effects of TNF-α on Bone Homeostasis
Tumor necrosis factor-α (TNF-α) plays an essential role in the regulation of bone homeostasis in several chronic immune and inflammatory joint diseases, where inhibition of TNF has led to significant clinical improvement. However, TNF-activated pathways and mechanisms involved in bone remodeling remain unclear. So far, TNF-α was known as an inhibitor of osteoblast differentiation and an activator of osteoclastogenesis. Recent contradictory findings indicated that TNF-α can also activate osteoblastogenesis. The paradoxical role of TNF-α in bone homeostasis seems to depend on the concentration and the differentiation state of the cell type used as well as on the exposure time. This review aims to summarize the recent contradictory findings on the regulation of bone homeostasis by TNF-α at the isolated cell, whole bone, and whole body levels. In addition, the involvement of TNF-α in the bone remodeling imbalance is observed in inflammatory joint diseases including rheumatoid arthritis and ankylosing spondylitis, which are associated with bone destruction and ectopic calcified matrix formation, respectively. Both diseases are associated with systemic/vertebral osteoporosis.
25689037
TNFalpha potently activates osteoclasts, through a direct action independent of and strongly synergistic with RANKL.
TNFalpha is pivotal to the pathogenesis of inflammatory and possibly postmenopausal osteolysis. Much recent work has clarified mechanisms by which TNFalpha promotes osteoclastogenesis, but the means by which it activates osteoclasts to resorb bone remain uncertain. We found that very low concentrations of TNFalpha promoted actin ring formation, which correlates with functional activation in osteoclasts, both in osteoclasts formed in vitro and extracted from newborn rats. TNFalpha was equipotent with RANKL for this action. Activation by TNFalpha was unaffected by blockade of RANKL by OPG, its soluble decoy receptor, suggesting that this was due to a direct action on osteoclasts. Bone resorption was similarly directly and potently stimulated, in a RANKL-independent manner in osteoclasts, whether these were formed in vitro or in vivo. Interestingly, TNFalpha promoted actin ring formation at concentrations an order of magnitude below those required for osteoclastic differentiation. Moreover, TNFalpha strongly synergized with RANKL, such that miniscule concentrations of TNFalpha were sufficient to substantially augment osteoclast activation. The extreme sensitivity of osteoclasts to activation by TNFalpha suggests that the most sensitive osteolytic response of bone to TNFalpha is through activation of existing osteoclasts; and the strong synergy with RANKL provides a mechanism whereby increased osteolysis can be achieved without disturbance to the underlying pattern of osteoclastic localization.
12599490
Tumor Necrosis Factor-α (TNF) Stimulates RANKL-induced Osteoclastogenesis via Coupling of TNF Type 1 Receptor and RANK Signaling Pathways*
Tumor necrosis factor-α (TNF) and the ligand for receptor activator of NF-κB (RANKL) are abundant in sites of inflammatory bone erosion. Because these cytokines are potent osteoclastogenic factors and because their signaling pathways are considerably overlapping, we postulated that under pro-inflammatory conditions RANKL and TNF might synergistically orchestrate enhanced osteoclastogenesis via cooperative mechanisms. We found TNF, via TNF type 1 receptor (TNFr1), prompts robust osteoclastogenesis by osteoclast precursors pretreated with RANKL, and deletion of TNFr1 abrogates this response. Enhanced osteoclastogenesis is associated with high expression of otherwise TNF and RANKL-induced mediators, including c-Src, TRAF2, TRAF6, and MEKK-1, levels of which were notably reduced in TNFr1 knockouts. Recruitment of TRAFs and MEKK1 leads to activation of downstream pathways, primarily IκB/NF-κB, ERKs, and cJun/AP-1. Consistent with impaired osteoclastogenesis and reduced expression of TRAFs and MEKK1, we found that phosphorylation and activation of IκB, NF-κB, ERKs, and cJun/AP-1 are severely reduced in RANKL-treated TNFr1-null osteoclast precursors compared with wild type counterparts. Finally, we found that TNF and RANKL synergistically up-regulate RANK expression in wild type precursors, whereas basal and stimulated levels of RANK are significantly lower in TNFr1 knockout cells. Our data suggest that exuberant TNF-induced osteoclastogensis is the result of coupling between RANK and TNFr1 and is dependent upon signals transmitted by the latter receptor.
16471252
M-CSF mediates TNF-induced inflammatory osteolysis.
TNF-alpha is the dominant cytokine in inflammatory osteolysis. Using mice whose BM stromal cells and osteoclast precursors are chimeric for the presence of TNF receptors, we found that both cell types mediated the cytokine's osteoclastogenic properties. The greater contribution was made, however, by stromal cells that express the osteoclastogenic cytokine M-CSF. TNF-alpha stimulated M-CSF gene expression, in vivo, only in the presence of TNF-responsive stromal cells. M-CSF, in turn, induced the key osteoclastogenic cytokine receptor, receptor activator of NF-kappaB (RANK), in osteoclast precursors. In keeping with the proproliferative and survival properties of M-CSF, TNF-alpha enhanced osteoclast precursor number only in the presence of stromal cells bearing TNF receptors. To determine the clinical relevance of these observations, we induced inflammatory arthritis in wild-type mice and treated them with a mAb directed against the M-CSF receptor, c-Fms. Anti-c-Fms mAb selectively and completely arrested the profound pathological osteoclastogenesis attending this condition, the significance of which is reflected by similar blunting of the in vivo bone resorption marker tartrate-resistant acid phosphatase 5b (TRACP 5b). Confirming that inhibition of the M-CSF signaling pathway targets TNF-alpha, anti-c-Fms also completely arrested osteolysis in TNF-injected mice with nominal effect on macrophage number. M-CSF and its receptor, c-Fms, therefore present as candidate therapeutic targets in states of inflammatory bone erosion.
1129062
Functions of RANKL/RANK/OPG in bone modeling and remodeling.
The discovery of the RANKL/RANK/OPG system in the mid 1990s for the regulation of bone resorption has led to major advances in our understanding of how bone modeling and remodeling are regulated. It had been known for many years before this discovery that osteoblastic stromal cells regulated osteoclast formation, but it had not been anticipated that they would do this through expression of members of the TNF superfamily: receptor activator of NF-kappaB ligand (RANKL) and osteoprotegerin (OPG), or that these cytokines and signaling through receptor activator of NF-kappaB (RANK) would have extensive functions beyond regulation of bone remodeling. RANKL/RANK signaling regulates osteoclast formation, activation and survival in normal bone modeling and remodeling and in a variety of pathologic conditions characterized by increased bone turnover. OPG protects bone from excessive resorption by binding to RANKL and preventing it from binding to RANK. Thus, the relative concentration of RANKL and OPG in bone is a major determinant of bone mass and strength. Here, we review our current understanding of the role of the RANKL/RANK/OPG system in bone modeling and remodeling.
16754991
High-Fat Diet Causes Bone Loss in Young Mice by Promoting Osteoclastogenesis Through Alteration of the Bone Marrow Environment
Obesity is a severe health problem in children, afflicting several organ systems including bone. However, the role of obesity on bone homeostasis and bone cell function in children has not been studied in detail. Here we used young mice fed a high-fat diet (HFD) to model childhood obesity and investigate the effect of HFD on the phenotype of cells within the bone marrow environment. Five-week-old male mice were fed a HFD for 3, 6, and 12 weeks. Decreased bone volume was detected after 3 weeks of HFD treatment. After 6 and 12 weeks, HFD-exposed mice had less bone mass and increased osteoclast numbers. Bone marrow cells, but not spleen cells, from HFD-fed mice had increased osteoclast precursor frequency, elevated osteoclast formation, and bone resorption activity, as well as increased expression of osteoclastogenic regulators including RANKL, TNF, and PPAR-gamma. Bone formation rate and osteoblast and adipocyte numbers were also increased in HFD-fed mice. Isolated bone marrow cells also had a corresponding elevation in the expression of positive regulators of osteoblast and adipocyte differentiation. Our findings indicate that in juvenile mice, HFD-induced bone loss is mainly due to increased osteoclast bone resorption by affecting the bone marrow microenvironment. Thus, targeting osteoclast formation may present a new therapeutic approach for bone complications in obese children.
3645614
Osteoporosis: A Review of Treatment Options.
Approximately 10 million men and women in the U.S. have osteoporosis,1 a metabolic bone disease characterized by low bone density and deterioration of bone architecture that increase the risk of fractures.2 Osteoporosis-related fractures can increase pain, disability, nursing home placement, total health care costs, and mortality.3 The diagnosis of osteoporosis is primarily determined by measuring bone mineral density (BMD) using noninvasive dual-energy x-ray absorptiometry. Osteoporosis medications include bisphosphonates, receptor activator of nuclear factor kappa-B ligand inhibitors, estrogen agonists/antagonists, parathyroid hormone analogues, and calcitonin.3-6 Emerging therapies utilizing novel mechanisms include a cathepsin K inhibitor and a monoclonal antibody against sclerostin.7,8 While professional organizations have compiled recommendations for the management of osteoporosis in various populations, a consensus has yet to develop as to which is the gold standard; therefore, economic evaluations have been increasingly important to help guide decision-makers. A review of cost-effectiveness literature on the efficacy of oral bisphosphonates has shown alendronate and risedronate to be most cost-effective in women with low BMD without previous fractures.9 Guidelines are inconsistent as to the place in therapy of denosumab (Prolia, Amgen). In economic analyses evaluating treatment of postmenopausal women, denosumab outperformed risedronate and ibandronate; its efficacy was comparable to generic alendronate, but it cost more.10 With regard to older men with osteoporosis, denosumab was also found to be cost-effective when compared with bisphosphonates and teriparatide (Forteo, Lilly).11.
8155090
Bone defect regeneration and cortical bone parameters of type 2 diabetic rats are improved by insulin therapy.
Zucker Diabetic Fatty (ZDF) rats represent an established model of type 2 diabetes mellitus (T2DM) and display several features of human diabetic bone disease, including impaired osteoblast function, decreased bone strength, and delayed bone healing. Here, we determined whether glycemic control by insulin treatment prevents skeletal complications associated with diabetes. Subcritical femur defects were created in diabetic (fa/fa) and non-diabetic (+/+) ZDF rats. Diabetic rats were treated once daily with long-lasting insulin glargin for 12weeks for glycemic control. Insulin treatment successfully maintained serum levels of glycated hemoglobin, while untreated diabetic rats showed a 2-fold increase. Trabecular and cortical bone mass measured by μCT were decreased in diabetic rats. Insulin treatment increased bone mass of the cortical, but not of the trabecular bone compartment. Dynamic histomorphometry revealed a lower bone formation rate at the trabecular and periosteal cortical bone in diabetic animals and decreased serum procollagen type 1 N-terminal propeptide (P1NP, -49%) levels. Insulin treatment partially improved these parameters. In T2DM, serum levels of tartrate-resistant acid phosphatase (TRAP, +32%) and C-terminal telopeptide (CTX, +49%) were increased. Insulin treatment further elevated TRAP levels, but did not affect CTX levels. While diabetes impaired bone defect healing, glycemic control with insulin fully reversed these negative effects. In conclusion, insulin treatment reversed the adverse effects of T2DM on bone defect regeneration in rats mainly by improving osteoblast function and bone formation. This article is part of a Special Issue entitled Bone and diabetes.
24436664
Bone histomorphometry: Standardization of nomenclature, symbols, and units: Report of the asbmr histomorphometry nomenclature committee
RACTITIONERS OF BONE HISTOMORPHOMETRY communicate P with each other in a variety of arcane languages, which in general are unintelligible to those outside the field. Many in the bone and mineral scientific community would like to keep abreast of the contributions of histology to their subject, but are dismayed by the semantic barriers they must overcome. The need for standardization has been recognized for many years,(') during which there has been much talk but no action. To meet the needs of ASBMR members, Dr. B.L. Riggs (President, 19851986) asked the senior author to convene a committee of the Society to develop a unified system of termnology, suitable for adoption by the Journal of Bone and Mineral Research as part of its Instructions to Authors. The committee includes members from Europe and Canada as well as the U.S., and represents most existing systems of nomenclature. A circular letter seeking suggestions and information on current usage was sent to several hundred persons, with names drawn from the Society membership roster and lists of attendees at various recent conferences, to which approximately 40 replies were obtained. These confirmed the magnitude of the semantic problem (for some measurements as many as nine different terms were in use) and suggested a range of solutions likely to be generally acceptable. In formulating the new system. the committee kept in mind certain agreed general principles. First, the primary reason for change was to help other scientists understand bone histomorphometry, not to help bone histomorphometrists undcntand each other. Second. names should be self-explanatory and dcscriptive, without implicit assumptions. Third. symbols should consist mainly of abbreviations that included the first letter of each word in the same order as in the name. without subscripts or superscripts. Fourth. each symbol component should have one and only one meaning, and so eliminate ambiguity. Fifth, primary measurements should be clearly distinguished from derived indices. Finally, the chosen system should be sufficiently flexible to apply to all surfaces and all types of bone, and to accommodate any new primary measurement or derived index. The recommended system shares common elements with. but also differs substantially from. all those in current usc. was tested in practice for several months before the final forniat was chosen, and is as complex and conceptually difficult ;I\ the field with which it deals. For those within the field we hope that increased readership of their papers will be adequate conipensation for the inconvcnicncc of learning a new systcm. For those outside the field, mastering the new system will be hard work, but if we are able to secure its acceptance by all journals with an interest in bone and mineral metabolism, the effort will only have to be expended once rather than. as at present. rcpeated many times. To this end we give the reasons for our decisions in the areas of controversy and, as well as definitions, provide methods for calculation of derived indices and
21964331
High-fat diet decreases cancellous bone mass but has no effect on cortical bone mass in the tibia in mice.
Body mass has a positive effect on bone health. Whether mass derived from an obesity condition or excessive fat accumulation is beneficial to bone has not been established; neither have the mechanisms by which obesity affects bone metabolism. The aim of this study was to examine the effects of obesity on bone structure and osteoblastic expression of key markers involved in bone formation and resorption in a diet-induced obesity mouse model. Six-wk-old male C57BL/6 mice (n=21) were assigned to two groups and fed either a control (10 kcal% energy as fat) or high-fat diet (HFD, 45 kcal% energy as fat) for 14 weeks. Bone marrow stromal/osteoblastic cells (BMSC) were cultured. Osteoprogenitor activity [alkaline phosphatase (ALP) positive colonies] and mineralization (calcium nodule formation) were determined. Gene expression was measured using quantitative real-time PCR. Bone structure of proximal and midshaft tibia was evaluated by micro-computed tomography. Mice fed the HFD were 31% heavier (P<0.01) than those fed the control diet. There were more ALP positive colony forming units at d 14 and calcium nodules at d 28 of culture by BMSC from HFD mice than from control mice (P<0.01). Receptor activator of NF-kappaB ligand (RANKL) mRNA levels and the ratio of RANKL to osteoprotegerin expression in HFD animals was higher (P<0.01) than in control diet animals. Serum tartrate-resistant acid phosphatase levels were higher in HFD fed mice when compared to control diet fed mice (P<0.05). There were no significant differences in tibial fat-free weight, length, and cortical parameters of midshaft between the two groups. Compared with control mice, tibial trabecular bone volume was reduced, and trabecular separation was increased in HFD mice. Trabecular number was lower (P<0.05) and connectivity density tended to be less (P=0.07) in HFD mice than in control mice. In conclusion, our data indicate that obesity induced by a high-fat diet decreases cancellous bone mass but has no effect on cortical bone mass in the tibia in mice.
4588720
Diet‐induced obesity alters bone remodeling leading to decreased femoral trabecular bone mass in mice
Obesity‐derived body mass may be detrimental to bone health through not well‐defined mechanisms. In this study we determined changes in bone structure and serum cytokines related to bone metabolism in diet‐induced obese mice. Mice fed a high‐fat diet (HFD) had higher serum tartrate‐resistant acid phosphatase (TRAP) and leptin but lower osteocalcin concentrations than those fed the normal‐fat diet. The HFD increased multinucleated TRAP‐positive osteoclasts in bone marrow compared to the control diet. Despite being much heavier, mice fed the HFD had lower femoral bone volume, trabecular number, and connectivity density and higher trabecular separation than mice on the control diet. These findings suggest that obesity induced by a HFD increases bone resorption that may blunt any positive effects of increased body weight on bone.
19859079
Obesity-mediated inflammatory microenvironment stimulates osteoclastogenesis and bone loss in mice
Clinical evidence indicates that fat is inversely proportional to bone mass in elderly obese women. However, it remains unclear whether obesity accelerates bone loss. In this report we present evidence that increased visceral fat leads to inflammation and subsequent bone loss in 12-month-old C57BL/6J mice that were fed 10% corn oil (CO)-based diet and a control lab chow (LC) for 6 months. As expected from our previous work, CO-fed mice demonstrated increased visceral fat and enhanced total body fat mass compared to LC. The adipocyte-specific PPARγ and bone marrow (BM) adiposity were increased in CO-fed mice. In correlation with those modifications, inflammatory cytokines (IL-1β, IL-6, TNF-α) were significantly elevated in CO-fed mice compared to LC-fed mice. This inflammatory BM microenvironment resulted in increased superoxide production in osteoclasts and undifferentiated BM cells. In CO-fed mice, the increased number of osteoclasts per trabecular bone length and the increased osteoclastogenesis assessed ex-vivo suggest that CO diet induces bone resorption. Additionally, the up-regulation of osteoclast-specific cathepsin k and RANKL expression and down-regulation of osteoblast-specific RUNX2/Cbfa1 supports this bone resorption in CO-fed mice. Also, CO-fed mice exhibited lower trabecular bone volume in the distal femoral metaphysis and had reduced OPG expression. Collectively, our results suggest that increased bone resorption in mice fed a CO-enriched diet is possibly due to increased inflammation mediated by the accumulation of adipocytes in the BM microenvironment. This inflammation may consequently increase osteoclastogenesis, while reducing osteoblast development in CO-fed mice.
25683723
Increased bone resorption and impaired bone microarchitecture in short-term and extended high-fat diet-induced obesity.
Although obesity traditionally has been considered a condition of low risk for osteoporosis, this classic view has recently been questioned. The aim of this study was to assess bone microarchitecture and turnover in a mouse model of high-fat diet-induced obesity. Seven-week-old male C57BL/6J mice (n = 18) were randomized into 3 diet groups. One third (n = 6) received a low-fat diet for 24 weeks, one third was kept on an extended high-fat diet (eHF), and the remaining was switched from low-fat to high-fat chow 3 weeks before sacrifice (sHF). Serum levels of insulin, leptin, adiponectin, osteocalcin, and cross-linked telopeptides of type I collagen (CTX) were measured. In addition, bone microarchitecture was analyzed by micro-computed tomography; and lumbar spine bone density was assessed by dual-energy x-ray absorptiometry. The CTX, body weight, insulin, and leptin were significantly elevated in obese animals (sHF: +48%, +24%, +265%, and +102%; eHF: +43%, +52%, +761%, and +292%). The CTX, body weight, insulin, and leptin showed a negative correlation with bone density and bone volume. Interestingly, short-term high-fat chow caused similar bone loss as extended high-fat feeding. Bone volume was decreased by 12% in sHF and 19% in eHF. Bone mineral density was 25% (sHF) and 27% (eHF) lower when compared with control mice on low-fat diet. As assessed by the structure model index, bone microarchitecture changed from plate- to rod-like appearance upon high-fat challenge. Trabecular and cortical thickness remained unaffected. Short-term and extended high-fat diet-induced obesity caused significant bone loss in male C57BL/6J mice mainly because of resorptive changes in trabecular architecture.
51712200
Differential effects of high-fat diet and exercise training on bone and energy metabolism.
Bone microarchitecture and strength are impaired by obesity and physical inactivity, but the underlying molecular regulation of bone metabolism in response to these factors is not well understood. Therefore, we analyzed bone and energy metabolism in male mice fed a high-fat or standard chow diet for 12 weeks with or without free access to running wheels. High-fat diet (HFD) mimicked the human condition of obesity and insulin resistance, including symptoms such as elevated serum glucose and insulin levels and reduced insulin-stimulated glucose uptake into muscle and adipose tissue. Interestingly, HFD also decreased (-44%) glucose uptake into bone marrow. Bone mass was reduced (-45%) by HFD due to a diminished (-45%) bone remodeling rate. Bone matrix quality aspects, such as biomechanical stability, were additionally decreased. Concurrently, the bone marrow adiposity increased (+63%) in response to a HFD. Further, we detected elevated expression of the Wnt signaling inhibitor dickkopf-1 (Dkk-1, +42%) in mice fed a HFD, but this was not reflected in serum samples obtained from obese humans. In mice, exercise attenuated the adverse effects of HFD by reversing the glucose uptake into bone marrow, improving the bone mass and bone matrix quality while decreasing the bone marrow adiposity. This data shows that exercise prevents some, but not all of the negative effects of HFD on bone health and suggests that insulin signaling in bone marrow and Dkk-1 signaling may be involved in the pathogenesis of bone loss induced by HFD.
37633528
Relationship of obesity with osteoporosis.
CONTEXT
The relationship between obesity and osteoporosis has been widely studied, and epidemiological evidence shows that obesity is correlated with increased bone mass. Previous analyses, however, did not control for the mechanical loading effects of total body weight on bone mass and may have generated a confounded or even biased relationship between obesity and osteoporosis.


OBJECTIVE
The objective of this study was to reevaluate the relationship between obesity and osteoporosis by accounting for the mechanical loading effects of total body weight on bone mass.


METHODS
We measured whole body fat mass, lean mass, percentage fat mass, body mass index, and bone mass in two large samples of different ethnicity: 1988 unrelated Chinese subjects and 4489 Caucasian subjects from 512 pedigrees. We first evaluated the Pearson correlations among different phenotypes. We then dissected the phenotypic correlations into genetic and environmental components with bone mass unadjusted or adjusted for body weight. This allowed us to compare the results with and without controlling for mechanical loading effects of body weight on bone mass.


RESULTS
In both Chinese and Caucasian subjects, when the mechanical loading effect of body weight on bone mass was adjusted for, the phenotypic correlation (including its genetic and environmental components) between fat mass (or percentage fat mass) and bone mass was negative. Further multivariate analyses in subjects stratified by body weight confirmed the inverse relationship between bone mass and fat mass, after mechanical loading effects due to total body weight were controlled.


CONCLUSIONS
Increasing fat mass may not have a beneficial effect on bone mass.
923952
Individuals with high bone mass have an increased prevalence of radiographic knee osteoarthritis
We previously reported an association between high bone mass (HBM) and a bone-forming phenotype of radiographic hip osteoarthritis (OA). As knee and hip OA have distinct risk factors, in this study we aimed to determine (i) whether HBM is also associated with knee OA, and (ii) whether the HBM knee OA phenotype demonstrates a similar pattern of radiographic features to that observed at the hip. HBM cases (defined by DXA BMD Z-scores) from the UK-based HBM study were compared with unaffected family controls and general population controls from the Chingford and Hertfordshire cohort studies. A single blinded observer graded AP weight-bearing knee radiographs for features of OA (Kellgren–Lawrence score, osteophytes, joint space narrowing (JSN), sclerosis) using an atlas. Analyses used logistic regression, adjusting a priori for age and gender, and additionally for BMI as a potential mediator of the HBM–OA association, using Stata v12. 609 HBM knees in 311 cases (mean age 60.8 years, 74% female) and 1937 control knees in 991 controls (63.4 years, 81% female) were analysed. The prevalence of radiographic knee OA, defined as Kellgren–Lawrence grade ≥ 2, was increased in cases (31.5% vs. 20.9%), with age and gender adjusted OR [95% CI] 2.38 [1.81, 3.14], p < 0.001. The association between HBM and osteophytosis was stronger than that for JSN, both before and after adjustment for BMI which attenuated the ORs for knee OA and osteophytes in cases vs. controls by approximately 50%. Our findings support a positive association between HBM and knee OA. This association was strongest for osteophytes, suggesting HBM confers a general predisposition to a subtype of OA characterised by increased bone formation.
9701019
The role of Dickkopf-1 in bone development, homeostasis, and disease.
Wnt/beta-catenin signaling is central to bone development and homeostasis in adulthood and its deregulation is associated with bone pathologies. Dickkopf-1 (DKK1), a soluble inhibitor of Wnt/beta-catenin signaling required for embryonic head development, regulates Wnt signaling by binding to the Wnt coreceptor lipoprotein-related protein-5 (LRP5)/Arrow. LRP5 mutations causing high bone mass syndromes disrupt DKK1-mediated regulation of LRP5. Forced overexpression of Dkk1 in osteoblasts causes osteopenia, disruption of the hematopoietic stem cell (HSC) niche, and defects in HSC function. Dkk1 also inhibits fracture repair. Studies suggest that DKK1 activation in osteoblasts is the underlying cause of glucocorticoid- and estrogen deficiency-mediated osteoporosis, and at least partially underlies the teratogenic effects of thalidomide on limb development. DKK1 induces proliferation of mesenchymal stem cells (MSC) in vitro and may play a role in the development of high-grade undifferentiated pleomorphic sarcomas derived from MSC and osteosarcomas. DKK1 has been implicated in causing erosive arthritis, the osteolytic phenotypes of multiple myeloma and metastatic breast cancer, and osteoblastic metastases of prostate cancer. Preclinical studies have shown that neutralizing DKK1/Dkk1 and/or enhancing Wnt/beta-catenin signaling may prove effective in treating bone pathologies. Here, we review the rapidly growing body of literature defining a pivotal role for DKK1 in bone health and disease.
40405012
Wnt signaling in bone formation and its therapeutic potential for bone diseases
The Wnt signaling pathway plays an important role not only in embryonic development but also in the maintenance and differentiation of the stem cells in adulthood. In particular, Wnt signaling has been shown as an important regulatory pathway in the osteogenic differentiation of mesenchymal stem cells. Induction of the Wnt signaling pathway promotes bone formation while inactivation of the pathway leads to osteopenic states. Our current understanding of Wnt signaling in osteogenesis elucidates the molecular mechanisms of classic osteogenic pathologies. Activating and inactivating aberrations of the canonical Wnt signaling pathway in osteogenesis results in sclerosteosis and osteoporosis respectively. Recent studies have sought to target the Wnt signaling pathway to treat osteogenic disorders. Potential therapeutic approaches attempt to stimulate the Wnt signaling pathway by upregulating the intracellular mediators of the Wnt signaling cascade and inhibiting the endogenous antagonists of the pathway. Antibodies against endogenous antagonists, such as sclerostin and dickkopf-1, have demonstrated promising results in promoting bone formation and fracture healing. Lithium, an inhibitor of glycogen synthase kinase 3β, has also been reported to stimulate osteogenesis by stabilizing β catenin. Although manipulating the Wnt signaling pathway has abundant therapeutic potential, it requires cautious approach due to risks of tumorigenesis. The present review discusses the role of the Wnt signaling pathway in osteogenesis and examines its targeted therapeutic potential.
44143805
YAP promotes osteogenesis and suppresses adipogenic differentiation by regulating β-catenin signaling
YAP (yes-associated protein) is a transcriptional factor that is negatively regulated by Hippo pathway, a conserved pathway for the development and size control of multiple organs. The exact function of YAP in bone homeostasis remains controversial. Here we provide evidence for YAP’s function in promoting osteogenesis, suppressing adipogenesis, and thus maintaining bone homeostasis. YAP is selectively expressed in osteoblast (OB)-lineage cells. Conditionally knocking out Yap in the OB lineage in mice reduces cell proliferation and OB differentiation and increases adipocyte formation, resulting in a trabecular bone loss. Mechanistically, YAP interacts with β-catenin and is necessary for maintenance of nuclear β-catenin level and Wnt/β-catenin signaling. Expression of β-catenin in YAP-deficient BMSCs (bone marrow stromal cells) diminishes the osteogenesis deficit. These results thus identify YAP-β-catenin as an important pathway for osteogenesis during adult bone remodeling and uncover a mechanism underlying YAP regulation of bone homeostasis.Bone homeostasis: Genes play a balancing actA key regulatory gene both promotes bone formation and suppresses production of fat-storing bone marrow cells, thus supporting the key process of bone remodeling. As adults, our bones are in a constant state of flux, constantly being dissolved and rebuilt to maintain a steady state known as homeostasis. An international team led by Wen-Cheng Xiong, of Case Western Reserve University, Cleveland, Ohio, studied the role of the gene ‘yes-associated protein’ (YAP), a transcription factor (gene controlling the activity of other genes). Using mice, they found that YAP is essential for osteogenesis, and prevents adipocyte (fat-storing cell) formation. Without a functional YAP gene, mice experienced a loss of a softer type of bone known as trabecular bone. The molecular pathways controlled by YAP may thus be important for bone remodeling in vertebrates.
16524757
A reduction in CD90 (THY-1) expression results in increased differentiation of mesenchymal stromal cells
BackgroundMesenchymal stromal cells (MSCs) are multipotent progenitor cells used in several cell therapies. MSCs are characterized by the expression of CD73, CD90, and CD105 cell markers, and the absence of CD34, CD45, CD11a, CD19, and HLA-DR cell markers. CD90 is a glycoprotein present in the MSC membranes and also in adult cells and cancer stem cells. The role of CD90 in MSCs remains unknown. Here, we sought to analyse the role that CD90 plays in the characteristic properties of in vitro expanded human MSCs.MethodsWe investigated the function of CD90 with regard to morphology, proliferation rate, suppression of T-cell proliferation, and osteogenic/adipogenic differentiation of MSCs by reducing the expression of this marker using CD90-target small hairpin RNA lentiviral vectors.ResultsThe present study shows that a reduction in CD90 expression enhances the osteogenic and adipogenic differentiation of MSCs in vitro and, unexpectedly, causes a decrease in CD44 and CD166 expression.ConclusionOur study suggests that CD90 controls the differentiation of MSCs by acting as an obstacle in the pathway of differentiation commitment. This may be overcome in the presence of the correct differentiation stimuli, supporting the idea that CD90 level manipulation may lead to more efficient differentiation rates in vitro.
6420642
Long‐Term Changes in Bone Mineral and Biomechanical Properties of Vertebrae and Femur in Aging, Dietary Calcium Restricted, and/or Estrogen‐Deprived/‐Replaced Rats
To study the long‐term effect of aging, low calcium diet (LCD) and/or ovariectomy (OVX), and estrogen replacement therapy (+E) on rat bone quality of both trabecular and cortical bone, 150 female Wistar rats of 4.5 months were divided into baseline, sham‐operation (sham), sham + LCD, OVX, OVX + E, OVX + LCD, OVX + LCD + E, and were observed for 3, 6, and 9 months postsurgery. The bone mineral density (BMD) of the lumbar spine L1–L4, the femoral neck, the midshaft, and the distal metaphysis were determined using dual‐energy X‐ray absorptiometry (DXA) in vitro. Biomechanical tests of the L1 vertebral body and the left femur were performed. The right femoral midshaft and neck were processed undecalcified for determining cross‐sectional moments of inertia (CSMIs). BMD in all groups increased rapidly with aging in the femoral midshaft composed only of cortical bone at 3 months post‐OVX and stabilized or decreased thereafter, but decreased at all observation periods in the distal femoral metaphysis, consisting mostly of trabecular bone. L1 maximum compressive strength and stiffness increased as a function of aging in sham and sham + LCD but not in OVX and OVX + LCD. The order of loss in BMD at all sites and in L1 strength and stiffness was: OVX + LCD > OVX > LCD. LCD reduced while OVX improved the total femoral area, CSMIs in the femoral midshaft, and the torsional strength. Estrogen treatment preserved BMD and prevented OVX‐induced loss in L1 strength. The BMD and biomechanical properties were greater in OVX + E than in OVX + LCD + E. Loss in BMD and CSMIs was greater in the femoral neck than in the midshaft. The data suggest that rat cortical bone might not be matured until 7.5 months of age. It would be more appropriate to consider rats at peak bone mass as a model of mature rat and to perform OVX at that time. LCD and OVX have a great potential for weakening the bone quality of cortical bone and trabecular bone, respectively, and have an additive effect when combined. Estrogen prevents only OVX‐induced bone loss.
23782947
IL-6 is produced by osteoblasts and induces bone resorption.
To examine the possible involvement of IL-6 in bone metabolism, a mouse osteoblastic cell line (MC3T3-E1) and primary osteoblast-like cells from fetal mouse calvaria were cultured with several systemic and local bone-resorbing agents and their expression of IL-6 mRNA was determined. Local bone-resorbing agents such as IL-1 alpha, IL-1 beta, TNF-alpha, and LPS greatly induced IL-6 mRNA expression in both MC3T3-E1 cells and primary osteoblast-like cells. Parathyroid hormone slightly increased expression of IL-6 mRNA in primary osteoblast-like cells but not in MC3T3-E1 cells. Neither IL-6 nor 1 alpha,25-dihydroxyvitamin D3 increased expression of IL-6 mRNA in either of the osteoblast-like cells. In agreement with the expression of IL-6 mRNA, biologically active IL-6 was produced in response to the treatment with IL-1 alpha, TNF-alpha, and LPS in MC3T3-E1 cells. Adding IL-6 dose dependently stimulated the release of 45Ca from prelabeled fetal mouse calvaria. Simultaneously adding suboptimal concentrations of IL-6 and IL-1 alpha induced bone resorption cooperatively. In accord with the increase in the release of 45Ca by IL-6, there were three times as many osteoclasts in the bone sections of calvaria cultured with IL-6 for 5 days as in the controls. IL-6 slightly suppressed alkaline phosphatase activity and collagen synthesis in MC3T3-E1 cells. These results indicate that IL-6 is also produced by osteoblasts, preferentially in response to local bone-resorbing agents, and it induces bone resorption both alone and in concert with other bone-resorbing agents.
6175283
The p55 TNF receptor mediates TNF inhibition of osteoblast differentiation independently of apoptosis.
After menopause, increased tumor necrosis factor-alpha (TNF-alpha) stimulates bone resorption while inhibiting differentiation of new bone-forming osteoblasts (OB). TNF receptors, p55 and p75, signal similar intracellular pathways, but only p55 activates apoptosis. To evaluate the relationship between the TNF receptor mediating inhibition of OB differentiation and the role of apoptosis, marrow stromal cells (MSC) were cultured from mice deficient in either or both receptors. Cells grown in ascorbate and beta-glycerophosphate produce alkaline phosphatase and osteocalcin and mineralize matrix. Treatment of wild-type or p55(+/+)/p75(-/-) MSC with murine TNF (binds p55 and p75) or human TNF (binds only p55) inhibited OB differentiation. TNF did not inhibit OB differentiation in p55(-/-) MSC. Expression of p75 modestly attenuated sensitivity to TNF. To determine the role of apoptosis, changes in total DNA, cell viability, caspase 3, and percentage of annexin V-positive cells were measured in MSC and preosteoblastic MC3T3 cells. TNF treatment that reduced differentiation by 50% did not decrease cell viability or increase apoptosis, as determined by alamar blue reduction, trypan blue exclusion, and percentage of annexin V-positive cells. TNF increased caspase 3 activity 1.5-fold in MC3T3 and insignificantly in MSC cells compared with > 4-fold after 4 h actinomycin D. Treatment of MSC or MC3T3 cells with three caspase inhibitors failed to reverse the inhibitory effect of TNF on OB differentiation despite inhibition of caspase activity. These results suggest that the p55 receptor is essential, and p75 dispensable, for TNF inhibition of OB differentiation through a mechanism that does not require apoptosis.
24754641
Tumor necrosis factor inhibits mesenchymal stem cell differentiation into osteoblasts via the ubiquitin E3 ligase Wwp1
Patients with chronic inflammatory disorders, such as rheumatoid arthritis, often have osteoporosis due to a combination of Tumor necrosis factor‐induced increased bone resorption and reduced bone formation. To test if TNF inhibits bone formation by affecting the commitment and differentiation of mesenchymal stem cells (MSCs) into osteoblasts, we examined the osteogenic potential of MSCs from TNF transgenic (TNF‐Tg) mice, a model of chronic inflammatory arthritis. MSC‐enriched cells were isolated from bone marrow stromal cells using negative selection with anti‐CD45 antibody coated magnetic beads. The expression profile of MSC surface markers the osteogenic, chondrogenic, and adipogenic properties of CD45− cells were confirmed by FACS and cell differentiation assays. MSC‐enriched CD45− cells from TNF‐Tg mice formed significantly decreased numbers of fibroblast and ALP+ colonies and had a decreased expression of osteoblast marker genes. As TNF may upregulate ubiquitin ligases, which negatively regulate osteoblast differentiation, we examined the expression levels of several ubiquitin ligases and found that Wwp1 expression was significantly increased in MSC‐enriched CD45− cells of TNF‐Tg mice. Wwp1 knockdown rescued impaired osteoblast differentiation of TNF‐Tg CD45− cells. Wwp1 promotes ubiquitination and degradation of JunB, an AP‐1 transcription factor that positively regulates osteoblast differentiation. Injection of TNF into wild‐type mice resulted in decreased osteoblast differentiation of MSCs and increased JunB ubiquitination, which was completely blocked in Wwp1−/− mice. Thus, Wwp1 targets JunB for ubiquitination and degradation in MSCs after chronic exposure to TNF, and inhibition of Wwp1 in MSCs could be a new mechanism to limit inflammation‐mediated osteoporosis by promoting their differentiation into osteoblasts. STEM CELLS 2011;29:1601–1610
45108583
Participation of TNF‐α in Inhibitory Effects of Adipocytes on Osteoblast Differentiation
Mesenchymal stem cells from bone marrow (BM‐MSCs) and adipose tissue (AT‐MSCs) are attractive tools for cell‐based therapies to repair bone tissue. In this study, we investigated the osteogenic and adipogenic potential of BM‐MSCs and AT‐MSCs as well as the effect of crosstalk between osteoblasts and adipocytes on cell phenotype expression. Rat BM‐MSCs and AT‐MSCs were cultured either in growth, osteogenic, or adipogenic medium to evaluate osteoblast and adipocyte differentiation. Additionally, osteoblasts and adipocytes were indirectly co‐cultured to investigate the effect of adipocytes on osteoblast differentiation and vice versa. BM‐MSCs and AT‐MSCs exhibit osteogenic and adipogenic potential under non‐differentiation‐inducing conditions. When exposed to osteogenic medium, BM‐MSCs exhibited higher expression of bone markers compared with AT‐MSCs. Conversely, under adipogenic conditions, AT‐MSCs displayed higher expression of adipose tissue markers compared with BM‐MSCs. The presence of adipocytes as indirect co‐culture repressed the expression of the osteoblast phenotype, whereas osteoblasts did not exert remarkable effect on adipocytes. The inhibitory effect of adipocytes on osteoblasts was due to the release of tumor necrosis factor alpha (TNF‐α) in culture medium by adipocytes. Indeed, the addition of exogenous TNF‐α in culture medium repressed the differentiation of BM‐MSCs into osteoblasts mimicking the indirect co‐culture effect. In conclusion, our study showed that BM‐MSCs are more osteogenic while AT‐MSCs are more adipogenic. Additionally, we demonstrated the key role of TNF‐α secreted by adipocytes on the inhibition of osteoblast differentiation. Thus, we postulate that the higher osteogenic potential of BM‐MSCs makes them the first choice for inducing bone repair in cell‐based therapies. J. Cell. Physiol. 230: 204–214, 2016. © 2015 Wiley Periodicals, Inc.
10812215
Role of autophagy in tumor necrosis factor-α-induced apoptosis of osteoblast cells
The aim of this study is to investigate the role of tumor necrosis factor-α (TNF-α) in apoptosis and autophagy of mouse osteoblast MC3T3-E1 cells, as well as the crosstalk between autophagy and apoptosis. Mouse osteoblast MC3T3-E1 cells were cultured in vitro and treated with 5-fluorouracil (5-FU), rapamycin, 3-methyl adenine (3-MA) and TNF-α either alone or in combination, respectively. MTT assays were used to monitor the cell viability upon different treatments. Annexin-V-FITC/propidium iodide (PI) staining was used to detect the apoptotic rate of osteoblasts. Autophagic structure and apoptotic bodies were visualized by transmission electron microscopy (TEM). Western blot analysis was performed to detect the autophagic marker LC3-II/I, p62 and apoptotic marker cleaved caspase-3. TNF-α inhibits MC3T3-E1 cell viability in a dose-dependent and time-dependent manner. Annexin-V-FITC/PI staining, coupled with TEM, showed that TNF-α induced cell apoptosis and autophagy in MC3T3-E1 cells. The autophagy inducer rapamycin ameliorated TNF-α-induced apoptosis. In contrast, 3-MA, which is an autophagy inhibitor, caused an exaggerated induction of TNF-α-induced apoptosis. TNF-α upregulated autophagy marker LC3-II/I, but downregulated p62 in osteoblasts. Combined treatment of rapamycin and TNF-α further exaggerated this effect, whereas co-treatment of 3-MA and TNF-α decreased LC3-II/I, but increased p62 compared with TNF-α alone. In addition, TNF-α caused an induction of apoptotic marker cleaved caspase-3. TNF-α-mediated induction of cleaved caspase-3 was downregulated by rapamycin, but upregulated by 3-MA, respectively. TNF-α induced both autophagy and apoptosis in osteoblasts, and upregulated autophagy protects the cell by reducing TNF-α-induced apoptosis.
28557516
Cytokines, Osteoprotegerin, and RANKL In Vitro and Histomorphometric Indices of Bone Turnover in Patients With Different Bone Diseases
Cytokines are supposed to play an essential role in the regulation of the bone metabolic unit. However, information on cytokine production of primary human osteoblasts from patients with metabolic bone disease is scarce, and few attempts have been made to correlate such data to histomorphometric parameters of individual patients. We investigated 11 patients with metabolic bone disease referred to our outpatient department for bone biopsy and analyzed interleukin (IL)‐1, IL‐6, and TNF‐α protein release and gene expression in primary osteoblast cultures. Compared with four controls, five patients showed normal cytokine protein release, whereas six patients showed much higher levels of interleukin‐6 (26‐fold) and TNF‐α (84‐fold). All three cytokines were strongly correlated concerning gene expression and/or protein levels (r = 0.72–0.96). Histomorphometric analysis of the bone samples showed that eroded surface (ES/BS) as a parameter of bone resorption was significantly associated with TNF‐α. In addition, RANKL gene expression was positively associated with ES/BS and osteoclast surface (Oc.S/BS). Finally, the formation parameters osteoid volume and osteoid surface were negatively associated with TNF‐α. In conclusion, in an in vitro‐ex vivo model of bone cells obtained from a group of 11 patients with different forms of metabolic bone disease, cytokine release in conditioned medium was significantly associated with bone resorption and bone formation, as quantified by histomorphometry. TNF‐α seemed to be the more important cytokine; its effect on bone resorption could be mediated by RANKL.
10144209
Immunological Reaction in TNF-α-Mediated Osteoclast Formation and Bone Resorption In Vitro and In Vivo
Tumor necrosis factor-α (TNF-α) is a cytokine produced by monocytes, macrophages, and T cells and is induced by pathogens, endotoxins, or related substances. TNF-α may play a key role in bone metabolism and is important in inflammatory bone diseases such as rheumatoid arthritis. Cells directly involved in osteoclastogenesis include macrophages, which are osteoclast precursor cells, osteoblasts, or stromal cells. These cells express receptor activator of NF-κB ligand (RANKL) to induce osteoclastogenesis, and T cells, which secrete RANKL, promote osteoclastogenesis during inflammation. Elucidating the detailed effects of TNF-α on bone metabolism may enable the identification of therapeutic targets that can efficiently suppress bone destruction in inflammatory bone diseases. TNF-α is considered to act by directly increasing RANK expression in macrophages and by increasing RANKL in stromal cells. Inflammatory cytokines such as interleukin- (IL-) 12, IL-18, and interferon-γ (IFN-γ) strongly inhibit osteoclast formation. IL-12, IL-18, and IFN-γ induce apoptosis in bone marrow cells treated with TNF-α   in vitro, and osteoclastogenesis is inhibited by the interactions of TNF-α-induced Fas and Fas ligand induced by IL-12, IL-18, and IFN-γ. This review describes and discusses the role of cells concerned with osteoclast formation and immunological reactions in TNF-α-mediated osteoclastogenesis in vitro and in vivo.
206025659
Tumor Necrosis Factor Alpha Promotes Osteoclast Formation Via PI3K/Akt Pathway‐Mediated Blimp1 Expression Upregulation
Tumor necrosis factor alpha (TNF‐α)‐induced osteoclastogenesis have profound effects in states of inflammatory osteolysis such as rheumatoid arthritis, periprosthetic implant loosening, and periodontitis. However, the exact mechanisms by which TNF‐α promotes RANKL‐induced osteoclast formation remains poorly understood. B lymphocyte‐induced maturation protein‐1 (Blimp1) is a transcriptional repressor that plays crucial roles in the differentiation and/or function of various kinds of cells including osteoclasts. A novel mechanism was identified where TNF‐α‐mediated Blimp1 expression, which contributed to RANKL‐induced osteoclastogenesis. It is shown that TNF‐α could promote the level of Blimp1 expression during osteoclast differentiation. Silencing of Blimp1 in osteoclast precursor cells obviously attenuated the stimulatory effect of TNF‐α on osteoclastogenesis. Mechanistically, TNF‐α‐induced Blimp1 expression was markedly rescued by blocking the PI3K/Akt signaling pathway, which suggested that PI3K/Akt signaling was involved in the regulation of TNF‐α‐stimulated Blimp1 expression. Taken together, the results established a molecular mechanism of TNF‐α‐induced osteoclasts differentiation, and provided insights into the potential contribution of Blimp1 in the regulation of osteoclastogenesis by TNF‐α. J. Cell. Biochem. 118: 1308–1315, 2017. © 2016 Wiley Periodicals, Inc.
13336687
Tumor Necrosis Factor α Stimulates Osteoclast Differentiation by a Mechanism Independent of the Odf/Rankl–Rank Interaction
Osteoclast differentiation factor (ODF, also called RANKL/TRANCE/OPGL) stimulates the differentiation of osteoclast progenitors of the monocyte/macrophage lineage into osteoclasts in the presence of macrophage colony-stimulating factor (M-CSF, also called CSF-1). When mouse bone marrow cells were cultured with M-CSF, M-CSF–dependent bone marrow macrophages (M-BMMφ) appeared within 3 d. Tartrate-resistant acid phosphatase–positive osteoclasts were also formed when M-BMMφ were further cultured for 3 d with mouse tumor necrosis factor α (TNF-α) in the presence of M-CSF. Osteoclast formation induced by TNF-α was inhibited by the addition of respective antibodies against TNF receptor 1 (TNFR1) or TNFR2, but not by osteoclastogenesis inhibitory factor (OCIF, also called OPG, a decoy receptor of ODF/RANKL), nor the Fab fragment of anti–RANK (ODF/RANKL receptor) antibody. Experiments using M-BMMφ prepared from TNFR1- or TNFR2-deficient mice showed that both TNFR1- and TNFR2-induced signals were important for osteoclast formation induced by TNF-α. Osteoclasts induced by TNF-α formed resorption pits on dentine slices only in the presence of IL-1α. These results demonstrate that TNF-α stimulates osteoclast differentiation in the presence of M-CSF through a mechanism independent of the ODF/RANKL–RANK system. TNF-α together with IL-1α may play an important role in bone resorption of inflammatory bone diseases.
31229360
TNF-alpha induces osteoclastogenesis by direct stimulation of macrophages exposed to permissive levels of RANK ligand.
While TNF-alpha is pivotal to the pathogenesis of inflammatory osteolysis, the means by which it recruits osteoclasts and promotes bone destruction are unknown. We find that a pure population of murine osteoclast precursors fails to undergo osteoclastogenesis when treated with TNF-alpha alone. In contrast, the cytokine dramatically stimulates differentiation in macrophages primed by less than one percent of the amount of RANKL (ligand for the receptor activator of NF-kappaB) required to induce osteoclast formation. Mirroring their synergistic effects on osteoclast differentiation, TNF-alpha and RANKL markedly potentiate NF-kappaB and stress-activated protein kinase/c-Jun NH(2)-terminal kinase activity, two signaling pathways essential for osteoclastogenesis. In vivo administration of TNF-alpha prompts robust osteoclast formation in chimeric animals in which ss-galactosidase positive, TNF-responsive macrophages develop within a TNF-nonresponsive stromal environment. Thus, while TNF-alpha alone does not induce osteoclastogenesis, it does so both in vitro and in vivo by directly targeting macrophages within a stromal environment that expresses permissive levels of RANKL. Given the minuscule amount of RANKL sufficient to synergize with TNF-alpha to promote osteoclastogenesis, TNF-alpha appears to be a more convenient target in arresting inflammatory osteolysis.
9027428
Tumor Necrosis Factor-α Induces Differentiation of and Bone Resorption by Osteoclasts*
Osteoclast progenitors differentiate into mature osteoclasts in the presence of receptor activator of NF-κB (RANK) ligand on stromal or osteoblastic cells and monocyte macrophage colony-stimulating factor (M-CSF). The soluble RANK ligand induces the same differentiation in vitro without stromal cells. Tumor necrosis factor-α (TNF-α), a potent cytokine involved in the regulation of osteoclast activity, promotes bone resorption via a primary effect on osteoblasts; however, it remains unclear whether TNF-α can also directly induce the differentiation of osteoclast progenitors into mature osteoclasts. This study revealed that TNF-α directly induced the formation of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells (MNCs), which produced resorption pits on bone in vitro in the presence of M-CSF. The bone resorption activity of TNF-α-induced MNCs was lower than that of soluble RANK ligand-induced MNCs; however, interleukin-1β stimulated this activity of TNF-α-induced MNCs without an increase in the number of MNCs. In this case, interleukin-1β did not induce TRAP-positive MNC formation. The osteoclast progenitors expressed TNF receptors, p55 and p75; and the induction of TRAP-positive MNCs by TNF-α was inhibited completely by an anti-p55 antibody and partially by an anti-p75 antibody. Our findings presented here are the first to indicate that TNF-α is a crucial differentiation factor for osteoclasts. Our results suggest that TNF-α and M-CSF play an important role in local osteolysis in chronic inflammatory diseases.
16339983
Signaling Pathways in Osteoclast Differentiation
Osteoclasts are multinucleated cells of hematopoietic origin that are responsible for the degradation of old bone matrix. Osteoclast differentiation and activity are controlled by two essential cytokines, macrophage colony-stimulating factor (M-CSF) and the receptor activator of nuclear factor-κB ligand (RANKL). M-CSF and RANKL bind to their respective receptors c-Fms and RANK to stimulate osteoclast differentiation through regulation of delicate signaling systems. Here, we summarize the critical or essential signaling pathways for osteoclast differentiation including M-CSF-c-Fms signaling, RANKL-RANK signaling, and costimulatory signaling for RANK.
8609051
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.