Sex‐Dependent,Osteoblast Stage‐Specific Effects of Progesterone Receptor on Bone Acquisition

The role of the progesterone receptor (PR) in the regulation of sexual dimorphism in bone has yet to be determined. Here we utilized genetic fate mapping and Western blotting to demonstrate age‐dependent PR expression in the mouse femoral metaphysis and diaphysis. To define sex‐dependent and osteoblast stage–specific effects of PR on bone acquisition, we selectively deleted PR at different stages of osteoblast differentiation. We found that when Prx1‐Cre mice were crossed with PR floxed mice to generate a mesenchymal stem cell (MSC) conditional KO model (Prx1; PRcKO), the mutant mice developed greater trabecular bone volume with higher mineral apposition rate and bone formation. This may be explained by increased number of MSCs and greater osteogenic potential, particularly in males. Age‐related trabecular bone loss was similar between the Prx1; PRcKO mice and their WT littermates in both sexes. Hormone deficiency during the period of rapid bone growth induced rapid trabecular bone loss in both the WT and the Prx1; PRcKO mice in both sexes. No differences in trabecular bone mass was observed when PR was deleted in mature osteoblasts using osteocalcin‐Cre (Bglap‐Cre). Also, there were no differences in cortical bone mass in all three PRcKO mice. In conclusion, PR inactivation in early osteoprogenitor cells but not in mature osteoblasts influenced trabecular bone accrual in a sex‐dependent manner. PR deletion in osteoblast lineage cells did not affect cortical bone mass. © 2017 American Society for Bone and Mineral Research.     

Development and characterization of novel clinical grade neonatal porcine bone marrow‐derived mesenchymal stem cells

Due to recent advances in research on mesenchymal stem cells (MSCs), MSCs are expected to be used in various clinical applications. However, securing adequate cadaveric donors and safety of living donors are major issues. To solve such issues, we have examined to develop clinical grade neonatal porcine bone marrow‐derived MSCs (npBM‐MSCs). Clinical grade neonatal porcine bone marrow cells were collected, frozen, and sent to our laboratory by air. The npBM‐MSCs were isolated from thawed bone marrow cells, then frozen. The thawed npBM‐MSCs were examined for CD markers and differentiated into chondrocytes, osteocytes, and adipocytes. They were compared with human bone marrow‐derived MSCs (hBM‐MSCs) for growth rate and size. To assess the robustness of proliferation, we compared culture medium with or without gelatin. The npBM‐MSCs expressed positive MSC markers CD29, CD44, and CD90 and were differentiated into chondrocytes, osteocytes, and adipocytes. The doubling time of npBM‐MSCs was significantly shorter than that of hBM‐MSCs (17.3 ± 0.8 vs 62.0 ± 19.6 hours, P < 0.01). The size of npBM‐MSCs was also significantly smaller than that of hBM‐MSCs (13.1 ± 0.3 vs 17.5 ± 0.4 μm, P < 0.001). The npBM‐MSCs showed similar proliferation characters irrespective of with or without gelatin coating. The npBM‐MSCs secreted VEGF‐A, VEGF‐C, and TGF‐β1. We have established npBM‐MSCs which show super‐rapid growth, small size, and robust proliferation profile. The np‐MSCs might be able to solve the donor issues for MSC therapy.     

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

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

Defects in mesenchymal stem cell self‐renewal and cell fate determination lead to an osteopenic phenotype in Bmi‐1 null mice

In parathyroid hormone–related protein 1‐84 [PTHrP(1‐84)] knockin mice, expression of the polycomb protein Bmi‐1 is reduced and potentially can mediate the phenotypic alterations observed. We have therefore now examined the skeletal phenotype of Bmi‐1^?/? mice in vivo and also assessed the function of bone marrow mesenchymal stem cells (BM‐MSCs) from Bmi‐1^?/? mice ex vivo in culture. Neonatal Bmi‐1^?/? mice exhibited skeletal growth retardation, with reduced chondrocyte proliferation and increased apoptosis. Osteoblast numbers; gene expression of alkaline phosphatase, type I collagen, and osteocalcin; the mineral apposition rate; trabecular bone volume; and bone mineral density all were reduced significantly; however, the number of bone marrow adipocytes and Ppar‐γ expression were increased. These changes were consistent with the skeletal phenotype observed in the PTHrP(1‐84) knockin mouse. The efficiency of colony‐forming unit fibroblast (CFU‐F) formation in bone marrow cultures was decreased, and the percentage of alkaline phosphatase–positive CFU‐F and Runx2 expression were reduced. In contrast, adipocyte formation and Ppar‐γ expression in cultures were increased, and expression of the polycomb protein sirtuin (Sirt1) was reduced. Reduced proliferation and increased apoptosis of BM‐MSCs were associated with upregulation of senescence‐associated tumor‐suppressor genes, including p16, p19, and p27. Analysis of the skeletal phenotype in Bmi‐1^?/? mice suggests that Bmi‐1 functions downstream of PTHrP. Furthermore, our studies indicate that Bmi‐1 maintains self‐renewal of BM‐MSCs by inhibiting the expression of p27, p16, and p19 and alters the cell fate of BM‐MSCs by enhancing osteoblast differentiation and inhibiting adipocyte differentiation at least in part by stimulating Sirt1 expression. Bmi‐1 therefore plays a critical role in promoting osteogenesis. © 2010 American Society for Bone and Mineral Research     

Low‐dose BMP‐2 and MSC dual delivery onto coral scaffold for critical‐size bone defect regeneration in sheep

Tissue‐engineered constructs (TECs) combining resorbable calcium‐based scaffolds and mesenchymal stem cells (MSCs) have the capability to regenerate large bone defects. Inconsistent results have, however, been observed, with a lack of osteoinductivity as a possible cause of failure. This study aimed to evaluate the impact of the addition of low‐dose bone morphogenetic protein‐2 (BMP‐2) to MSC‐coral‐TECs on the healing of clinically relevant segmental bone defects in sheep. Coral granules were either seeded with autologous MSCs (bone marrow‐derived) or loaded with BMP‐2. A 25‐mm‐long metatarsal bone defect was created and stabilized with a plate in 18 sheep. Defects were filled with one of the following TECs: (i) BMP (n = 5); (ii) MSC (n = 7); or (iii) MSC‐BMP (n = 6). Radiographic follow‐up was performed until animal sacrifice at 4 months. Bone formation and scaffold resorption were assessed by micro‐CT and histological analysis. Bone union with nearly complete scaffold resorption was observed in 1/5, 2/7, and 3/6 animals, when BMP‐, MSC‐, and MSC‐BMP‐TECs were implanted, respectively. The amount of newly formed bone was not statistically different between groups: 1074 mm³ [970–2478 mm³], 1155 mm³ [970–2595 mm³], and 2343 mm³ [931–3276 mm³] for BMP‐, MSC‐, and MSC‐BMP‐TECs, respectively. Increased scaffold resorption rate using BMP‐TECs was the only potential side effect observed. In conclusion, although the dual delivery of MSCs and BMP‐2 onto a coral scaffold further increased bone formation and bone union when compared to single treatment, results were non‐significant. Only 50% of the defects healed, demonstrating the need for further refinement of this strategy before clinical use. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2637–2645, 2017.

     

Marfan-Like Skeletal Phenotype in the Tight Skin (Tsk) Mouse

Tight skin (Tsk) is an autosomal dominant mutation located on mouse chromosome 2 and is associated with an intragenic duplication of the fibrillin 1 (Fbn1) gene. Mutant mice (Tsk/+) display a tightness of skin in the interscapular region, lung emphysema, myocardial hypertrophy, skeletal overgrowth, and kyphosis. It is hypothesized in this study that in Tsk mice the mutation in Fbn1 alters bone cell metabolism. A detailed study of the Tsk skeletal phenotype revealed that Tsk mice have significantly longer femurs and axial skeleton as well as vertebral abnormalities. Cortical and trabecular bone volumes were significantly decreased in Tsk femurs from 2- and 4-month-old mice (13% and 39%, respectively) as well as trabecular thickness, number, connectivity, and surface area. These skeletal differences were also associated with a reduction in bone mineral density in mutant mice. Expression of the osteoblast-specific genes Col1a1, BSP and OC was examined in marrow stromal cell cultures at various time points. A decrease in the rate of maturation of the Tsk cells was indicated by a delay in the appearance of OC expression. These initial experiments demonstrated a significant role of the fibrillin 1 protein in the extracellular matrix of bone cells.     

Epidermal growth factor receptor plays an anabolic role in bone metabolism in vivo

While the epidermal growth factor receptor (EGFR)–mediated signaling pathway has been shown to have vital roles in many developmental and pathologic processes, its functions in the development and homeostasis of the skeletal system has been poorly defined. To address its in vivo role, we constructed transgenic and pharmacologic mouse models and used peripheral quantitative computed tomography (pQCT), micro–computed tomography (µCT) and histomorphometry to analyze their trabecular and cortical bone phenotypes. We initially deleted the EGFR in preosteoblasts/osteoblasts using a Cre/loxP system (Col‐Cre Egfr^f/f), but no bone phenotype was observed because of incomplete deletion of the Egfr genomic locus. To further reduce the remaining osteoblastic EGFR activity, we introduced an EGFR dominant‐negative allele, Wa5, and generated Col‐Cre Egfr^Wa5/f mice. At 3 and 7 months of age, both male and female mice exhibited a remarkable decrease in tibial trabecular bone mass with abnormalities in trabecular number and thickness. Histologic analyses revealed decreases in osteoblast number and mineralization activity and an increase in osteoclast number. Significant increases in trabecular pattern factor and structural model index indicate that trabecular microarchitecture was altered. The femurs of these mice were shorter and smaller with reduced cortical area and periosteal perimeter. Moreover, colony‐forming unit–fibroblast (CFU‐F) assay indicates that these mice had fewer bone marrow mesenchymal stem cells and committed progenitors. Similarly, administration of an EGFR inhibitor into wild‐type mice caused a significant reduction in trabecular bone volume. In contrast, Egfr^Dsk5/+ mice with a constitutively active EGFR allele displayed increases in trabecular and cortical bone content. Taken together, these data demonstrate that the EGFR signaling pathway is an important bone regulator and that it primarily plays an anabolic role in bone metabolism. © 2011 American Society for Bone and Mineral Research.     

Telomerase‐deficient mice exhibit bone loss owing to defects in osteoblasts and increased osteoclastogenesis by inflammatory microenvironment

Telomere shortening owing to telomerase deficiency leads to accelerated senescence of human skeletal (mesenchymal) stem cells (MSCs) in vitro, whereas overexpression leads to telomere elongation, extended life span, and enhanced bone formation. To study the role of telomere shortening in vivo, we studied the phenotype of telomerase‐deficient mice (Terc^?/?). Terc^?/? mice exhibited accelerated age‐related bone loss starting at 3 months of age and during 12 months of follow‐up revealed by dual‐energy X‐ray absorptiometric (DXA) scanning and by micro–computed tomography (µCT). Bone histomorphometry revealed decreased mineralized surface and bone‐formation rate as well as increased osteoclast number and size in Terc^?/? mice. Also, serum total deoxypyridinoline (tDPD) was increased in Terc^?/? mice. MSCs and osteoprogenitors isolated from Terc^?/? mice exhibited intrinsic defects with reduced proliferating cell number and impaired osteogenic differentiation capacity. In addition, the Terc^?/?‐MSC cultures accumulated a larger proportion of senescent β‐galactosidase⁺ cells and cells exhibiting DNA damage. Microarray analysis of Terc^?/? bone revealed significant overexpression of a large number of proinflammatory genes involved in osteoclast (OC) differentiation. Consistently, serum obtained from Terc^?/? mice enhanced OC formation of wild‐type bone marrow cultures. Our data demonstrate two mechanisms for age‐related bone loss caused by telomerase deficiency: intrinsic osteoblastic defects and creation of a proinflammatory osteoclast‐activating microenvironment. Thus telomerization of MSCs may provide a novel approach for abolishing age‐related bone loss. © 2011 American Society for Bone and Mineral Research.     

Combined MEK Inhibition and BMP2 Treatment Promotes Osteoblast Differentiation and Bone Healing in Nf1Osx−/− Mice

Neurofibromatosis type I (NF1) is an autosomal dominant disease with an incidence of 1/3000, caused by mutations in the NF1 gene, which encodes the RAS/GTPase‐activating protein neurofibromin. Non‐bone union after fracture (pseudarthrosis) in children with NF1 remains a challenging orthopedic condition to treat. Recent progress in understanding the biology of neurofibromin suggested that NF1 pseudarthrosis stems primarily from defects in the bone mesenchymal lineage and hypersensitivity of hematopoietic cells to TGFβ. However, clinically relevant pharmacological approaches to augment bone union in these patients remain limited. In this study, we report the generation of a novel conditional mutant mouse line used to model NF1 pseudoarthrosis, in which Nf1 can be ablated in an inducible fashion in osteoprogenitors of postnatal mice, thus circumventing the dwarfism associated with previous mouse models where Nf1 is ablated in embryonic mesenchymal cell lineages. An ex vivo–based cell culture approach based on the use of Nf1^flox/flox bone marrow stromal cells showed that loss of Nf1 impairs osteoprogenitor cell differentiation in a cell‐autonomous manner, independent of developmental growth plate–derived or paracrine/hormonal influences. In addition, in vitro gene expression and differentiation assays indicated that chronic ERK activation in Nf1‐deficient osteoprogenitors blunts the pro‐osteogenic property of BMP2, based on the observation that only combination treatment with BMP2 and MEK inhibition promoted the differentiation of Nf1‐deficient osteoprogenitors. The in vivo preclinical relevance of these findings was confirmed by the improved bone healing and callus strength observed in Nf1_osx^?/? mice receiving Trametinib (a MEK inhibitor) and BMP2 released locally at the fracture site via a novel nanoparticle and polyglycidol‐based delivery method. Collectively, these results provide novel evidence for a cell‐autonomous role of neurofibromin in osteoprogenitor cells and insights about a novel targeted approach for the treatment of NF1 pseudoarthrosis. © 2014 American Society for Bone and Mineral Research.     

Jagged1 expression by osteoblast-lineage cells regulates trabecular bone mass and periosteal expansion in mice

Loss-of-function mutations in the Notch ligand, Jagged1 (Jag1), result in multi-system developmental pathologies associated with Alagille syndrome (ALGS). ALGS patients present with skeletal manifestations including hemi-vertebrae, reduced bone mass, increased fracture incidence and poor bone healing. However, it is not known whether the increased fracture risk is due to altered bone homeostasis (primary) or nutritional malabsorption due to chronic liver disease (secondary). To determine the significance of Jag1 loss in bone, we characterized the skeletal phenotype of two Jag1-floxed conditional knockout mouse models: Prx1-Cre;Jag1^f/f to target osteoprogenitor cells and their progeny, and Col2.3-Cre;Jag1^f/f to target mid-stage osteoblasts and their progeny. Knockout phenotypes were compared to wild-type (WT) controls using quantitative micro-computed tomography, gene expression profiling and mechanical testing. Expression of Jag1 and the Notch target genes Hes1 and Hey1 was downregulated in all Jag1 knockout mice. Osteoblast differentiation genes were downregulated in whole bone of both groups, but unchanged in Prx1-Cre;Jag1^f/f cortical bone. Both knockout lines exhibited changes in femoral trabecular morphology including decreased bone volume fraction and increased trabecular spacing, with males presenting a more severe trabecular osteopenic phenotype. Prx1-Cre;Jag1^f/f mice showed an increase in marrow mesenchymal progenitor cell number and, counterintuitively, developed increased cortical thickness resulting from periosteal expansion, translating to greater mechanical stiffness and strength. Similar alterations in femoral morphology were observed in mice with canonical Notch signaling disrupted using Prx1-Cre-regulatable dominant-negative mastermind like-protein (dnMAML). Taken together, we report that 1) Jag1 negatively regulates the marrow osteochondral progenitor pool, 2) Jag1 is required for normal trabecular bone formation and 3) Notch signaling through homotypic Jag1 signaling in osteochondral progenitors, but not mature osteoblasts, inhibits periosteal expansion. Therefore, Jag1 signaling within the osteoblast lineage regulates bone metabolism in a compartment-dependent manner. Moreover, loss of Jag1 function in osteoblast lineage cells may contribute to the skeletal phenotype associated with ALGS.     

Bone marrow‐derived mesenchymal stem cell attenuates skin fibrosis development in mice

Recent studies showed that mesenchymal stem cell (MSC) transplantation significantly alleviated tissue fibrosis; however, little is known about the efficacy on attenuating cutaneous scar formation. In this study, we established a dermal fibrosis model induced by bleomycin and evaluated the benefit of bone marrow‐derived mesenchymal stem cells (BM‐MSCs) on skin fibrosis development. Tracing assay of green fluorescent protein (GFP⁺)BM‐MSCs showed that the cells disappeared gradually within 24 hours upon administration, which hinted the action of BM‐MSCs in vivo was exerted in the initial phase of repair in this model. Therefore, we repeatedly transplanted syngeneic BM‐MSCs in the process of skin fibrosis formation. After 3 weeks, it was found that BM‐MSC‐treated lesional skin demonstrated a unanimous basket‐weave organisation of collagen arrangement similar to normal skin, with few inflammatory cells. In addition, lesional skin with BM‐MSC treatment exhibited a significant down‐regulation of transforming growth factor‐β1 (TGF‐β1), type I collagen and heat‐shock protein 47 (HSP47), with higher expression of matrix metalloproteinases (MMPs)‐2, ‐9 and ‐13. Further experiments showed that α‐smooth muscle actin (α‐SMA) positive cells, the most reliable marker of myofibroblasts, apparently decreased after BM‐MSC transplantation, which revealed that BM‐MSCs could attenuate myofibroblast proliferation and differentiation as well as matrix production. Taken together, these findings suggested that BM‐MSCs can inhibit the formation process of bleomycin‐induced skin fibrosis, alleviate inflammation and favour the remodelling of extracellular matrix.     

The Effects of Androgens on Murine Cortical Bone Do Not Require AR or ERα Signaling in Osteoblasts and Osteoclasts

In men, androgens are critical for the acquisition and maintenance of bone mass in both the cortical and cancellous bone compartment. Male mice with targeted deletion of the androgen receptor (AR) in mature osteoblasts or osteocytes have lower cancellous bone mass, but no cortical bone phenotype. We have investigated the possibility that the effects of androgens on the cortical compartment result from AR signaling in osteoprogenitors or cells of the osteoclast lineage; or via estrogen receptor alpha (ERα) signaling in either or both of these two cell types upon conversion of testosterone to estradiol. To this end, we generated mice with targeted deletion of an AR or an ERα allele in the mesenchymal (AR^f/y;Prx1‐Cre or ERα^f/f;Osx1‐Cre) or myeloid cell lineage (AR^f/y;LysM‐Cre or ERα^f/f;LysM‐Cre) and their descendants. Male AR^f/y;Prx1‐Cre mice exhibited decreased bone volume and trabecular number, and increased osteoclast number in the cancellous compartment. Moreover, they did not undergo the loss of cancellous bone volume and trabecular number caused by orchidectomy (ORX) in their littermate controls. In contrast, AR^f/y;LysM‐Cre, ERα^f/f;Osx1‐Cre, or ERα^f/f;LysM‐Cre mice had no cancellous bone phenotype at baseline and lost the same amount of cancellous bone as their controls following ORX. Most unexpectedly, adult males of all four models had no discernible cortical bone phenotype at baseline, and lost the same amount of cortical bone as their littermate controls after ORX. Recapitulation of the effects of ORX by AR deletion only in the AR^f/y;Prx1‐Cre mice indicates that the effects of androgens on cancellous bone result from AR signaling in osteoblasts—not on osteoclasts or via aromatization. The effects of androgens on cortical bone mass, on the other hand, do not require AR or ERα signaling in any cell type across the osteoblast or osteoclast differentiation lineage. Therefore, androgens must exert their effects indirectly by actions on some other cell type(s) or tissue(s). © 2015 American Society for Bone and Mineral Research.     

Evaluation of adult equine bone marrow‐ and adipose‐derived progenitor cell chondrogenesis in hydrogel cultures

Bone marrow mesenchymal stem cells (BM‐MSCs) and adipose‐derived progenitor cells (ADPCs) are potential alternatives to autologous chondrocytes for cartilage resurfacing strategies. In this study, the chondrogenic potentials of these cell types were compared by quantifying neo‐tissue synthesis and assaying gene expression and accumulation of extracellular matrix (ECM) components of cartilage. Adult equine progenitor cells encapsulated in agarose or self‐assembling peptide hydrogels were cultured in the presence or absence of TGFβ1 for 3 weeks. In BM‐MSCs‐seeded hydrogels, TGFβ1 stimulated ECM synthesis and accumulation 3–41‐fold relative to TGFβ1‐free culture. In ADPC cultures, TGFβ1 stimulated a significant increase in ECM synthesis and accumulation in peptide (18–29‐fold) but not agarose hydrogels. Chromatographic analysis of BM‐MSC‐seeded agarose and peptide hydrogels cultured in TGFβ1 medium showed extensive synthesis of aggrecan‐like proteoglycan monomers. ADPCs seeded in peptide hydrogel also synthesized aggrecan‐like proteoglycans, although to a lesser extent than seen in BM‐MSC hydrogels, whereas aggrecan‐like proteoglycan synthesis in ADPC‐seeded agarose was minimal. RT‐PCR analysis of TGFβ1 cultures showed detectable levels of type II collagen gene expression in BM‐MSC but not ADPC cultures. Histological analysis of TGFβ1‐cultured peptide hydrogels showed the deposition of a continuous proteoglycan‐ and type II collagen rich ECM for BM‐MSCs but not ADPCs. Therefore, this study showed both protein and gene expression evidence of superior chondrogenesis of BM‐MSCs relative to ADPCs. © 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:322–331, 2008     

Interferon‐γ plays a role in bone formation in vivo and rescues osteoporosis in ovariectomized mice

Interferon γ (IFN‐γ) is a cytokine produced locally in the bone microenvironment by cells of immune origin as well as mesenchymal stem cells. However, its role in normal bone remodeling is still poorly understood. In this study we first examined the consequences of IFN‐γ ablation in vivo in C57BL/6 mice expressing the IFN‐γ receptor knockout phenotype (IFNγR1^?/?). Compared with their wild‐type littermates (IFNγR1^+/+), IFNγR1^?/? mice exhibit a reduction in bone volume associated with significant changes in cortical and trabecular structural parameters characteristic of an osteoporotic phenotype. Bone histomorphometry of IFNγR1^?/? mice showed a low‐bone‐turnover pattern with a decrease in bone formation, a significant reduction in osteoblast and osteoclast numbers, and a reduction in circulating levels of bone‐formation and bone‐resorption markers. Furthermore, administration of IFN‐γ (2000 and 10,000 units) to wild‐type C57BL/6 sham‐operated (SHAM) and ovariectomized (OVX) female mice significantly improved bone mass and microarchitecture, mechanical properties of bone, and the ratio between bone formation and bone resorption in SHAM mice and rescued osteoporosis in OVX mice. These data therefore support an important physiologic role for IFN‐γ signaling as a potential new anabolic therapeutic target for osteoporosis. © 2011 American Society for Bone and Mineral Research.     

Role of Trabecular Microarchitecture in Whole‐Vertebral Body Biomechanical Behavior

The role of trabecular microarchitecture in whole‐vertebral biomechanical behavior remains unclear, and its influence may be obscured by such factors as overall bone mass, bone geometry, and the presence of the cortical shell. To address this issue, 22 human T₉ vertebral bodies (11 female; 11 male; age range: 53–97 yr, 81.5 ± 9.6 yr) were scanned with μCT and analyzed for measures of trabecular microarchitecture, BMC, cross‐sectional area, and cortical thickness. Sixteen of the vertebrae were biomechanically tested to measure compressive strength. To estimate vertebral compressive stiffness with and without the cortical shell for all 22 vertebrae, two high‐resolution finite element models per specimen—one intact model and one with the shell removed—were created from the μCT scans and virtually compressed. Results indicated that BMC and the structural model index (SMI) were the individual parameters most highly associated with strength (R² = 0.57 each). Adding microarchitecture variables to BMC in a stepwise multiple regression model improved this association (R² = 0.85). However, the microarchitecture variables in that regression model (degree of anisotropy, bone volume fraction) differed from those when BMC was not included in the model (SMI, mean trabecular thickness), and the association was slightly weaker for the latter (R² = 0.76). The finite element results indicated that the physical presence of the cortical shell did not alter the relationships between microarchitecture and vertebral stiffness. We conclude that trabecular microarchitecture is associated with whole‐vertebral biomechanical behavior and that the role of microarchitecture is mediated by BMC but not by the cortical shell.