Increased glutamine catabolism mediates bone anabolism in response to WNT signaling

Authors

Courtney M. Karner, Emel Esen, Adewole L. Okunade, Bruce W. Patterson and Fanxin Long

Abstract

HIF-1 is a transcription factor typically associated with angiogenic gene transcription under hypoxic conditions. In this study, mice with HIF-1α deleted in the osteoblast lineage (ΔHIF-1α) were subjected to damaging or non-damaging mechanical loading known to produce woven or lamellar bone, respectively, at the ulnar diaphysis. By microCT, ΔHIF-1α mice produced significantly less woven bone than wild type (WT) mice 7 days after damaging loading. This decrease in woven bone volume and extent was accompanied by a significant decrease in vascularity measured by immunohistochemistry against vWF. Additionally, osteocytes, rather than osteoblasts, appear to be the main bone cell expressing HIF-1α following damaging loading. In contrast, 10 days after non-damaging mechanical loading, dynamic histomorphometry measurements demonstrated no impairment in loading-induced lamellar bone formation in ΔHIF-1α mice. In fact, both non-loaded and loaded ulnae from ΔHIF-1α mice had increased bone formation compared with WT ulnae. When comparing the relative increase in periosteal bone formation in loaded vs. non-loaded ulnae, it was not different between ΔHIF-1α mice and controls. There were no significant differences observed between WT and ΔHIF-1α mice in endosteal bone formation parameters. The increases in periosteal lamellar bone formation in ΔHIF-1α mice are attributed to non-angiogenic effects of the knockout. In conclusion, these results demonstrate that HIF-1α is a pro-osteogenic factor for woven bone formation after damaging loading, but an anti-osteogenic factor for lamellar bone formation under basal conditions and after non-damaging loading.

Link To Article

http://dx.doi.org/10.1016/j.bone.2014.12.015

HIF-1α regulates bone formation after osteogenic mechanical loading

Authors

Ryan E. Tomlinson, Matthew J. Silva

Abstract

HIF-1 is a transcription factor typically associated with angiogenic gene transcription under hypoxic conditions. In this study, mice with HIF-1α deleted in the osteoblast lineage (ΔHIF-1α) were subjected to damaging or non-damaging mechanical loading known to produce woven or lamellar bone, respectively, at the ulnar diaphysis. By microCT, ΔHIF-1α mice produced significantly less woven bone than wild type (WT) mice 7 days after damaging loading. This decrease in woven bone volume and extent was accompanied by a significant decrease in vascularity measured by immunohistochemistry against vWF. Additionally, osteocytes, rather than osteoblasts, appear to be the main bone cell expressing HIF-1α following damaging loading. In contrast, 10 days after non-damaging mechanical loading, dynamic histomorphometry measurements demonstrated no impairment in loading-induced lamellar bone formation in ΔHIF-1α mice. In fact, both non-loaded and loaded ulnae from ΔHIF-1α mice had increased bone formation compared with WT ulnae. When comparing the relative increase in periosteal bone formation in loaded vs. non-loaded ulnae, it was not different between ΔHIF-1α mice and controls. There were no significant differences observed between WT and ΔHIF-1α mice in endosteal bone formation parameters. The increases in periosteal lamellar bone formation in ΔHIF-1α mice are attributed to non-angiogenic effects of the knockout. In conclusion, these results demonstrate that HIF-1α is a pro-osteogenic factor for woven bone formation after damaging loading, but an anti-osteogenic factor for lamellar bone formation under basal conditions and after non-damaging loading.

Link To Article

http://dx.doi.org/10.1016/j.bone.2014.12.015

Absence of Dap12 and the αvβ3 integrin causes severe osteopetrosis

Authors

Wei Zou and Steven L. Teitelbaum

Abstract

In vitro, ligand occupancy of αvβ3 integrin induces phosphorylation of Dap12, which is essential for osteoclast function. Like mice deleted of only αvβ3, Dap12−/− mice exhibited a slight increase in bone mass, but Dap12−/− mice, lacking another ITAM protein, FcRγ, were severely osteopetrotic. The mechanism by which FcRγ compensates for Dap12 deficiency is unknown. We find that co-deletion of FcRγ did not exacerbate the skeletal phenotype of β3−/− mice. In contrast, β3/Dap12 double-deficient (DAP/β3−/−) mice (but not β1/Dap12 double-deficient mice) were profoundly osteopetrotic, reflecting severe osteoclast dysfunction relative to those lacking αvβ3 or Dap12 alone. Activation of OSCAR, the FcRγ co-receptor, rescued Dap12−/− but not DAP/β3−/−osteoclasts. Thus, the absence of αvβ3 precluded compensation for Dap12 deficiency by FcRγ. In keeping with this, Syk phosphorylation did not occur in OSCAR-activated DAP/β3−/− osteoclasts. Thus, FcRγ requires the osteoclast αvβ3 integrin to normalize the Dap12-deficient skeleton.

μCT-based, in vivo dynamic bone histomorphometry allows 3D evaluation of the early responses of bone resorption and formation to PTH and alendronate combination therapy

Authors

Chantal M.J. de Bakker, Allison R. Altman, Wei-Ju Tseng, Mary Beth Tribble, Connie Li, Abhishek Chandra, Ling Qin, X. Sherry Liu

Abstract

Current osteoporosis treatments improve bone mass by increasing net bone formation: anti-resorptive drugs such as bisphosphonates block osteoclast activity, while anabolic agents such as parathyroid hormone (PTH) increase bone remodeling, with a greater effect on formation. Although these drugs are widely used, their role in modulating formation and resorption is not fully understood, due in part to technical limitations in the ability to longitudinally assess bone remodeling. Importantly, it is not known whether or not PTH-induced bone formation is independent of resorption, resulting in controversy over the effectiveness of combination therapies that use both PTH and an anti-resorptive. In this study, we developed a μCT-based, in vivo dynamic bone histomorphometry technique for rat tibiae, and applied this method to longitudinally track changes in bone resorption and formation as a result of treatment with alendronate (ALN), PTH, or combination therapy of both PTH and ALN (PTH + ALN). Correlations between our μCT-based measures of bone formation and measures of bone formation based on calcein-labeled histology (r = 0.72–0.83) confirm the accuracy of this method. Bone remodeling parameters measured through μCT-based in vivo dynamic bone histomorphometry indicate an increased rate of bone formation in rats treated with PTH and PTH + ALN, together with a decrease in bone resorption measures in rats treated with ALN and PTH + ALN. These results were further supported by traditional histology-based measurements, suggesting that PTH was able to induce bone formation while bone resorption was suppressed.

Link To Article

http://dx.doi.org/10.1016/j.bone.2014.12.061

Inner ear vestibular signals regulate bone remodeling via the sympathetic nervous system

Authors

G Vignaux, JDLC Ndong, D Perrien and F Elefteriou

Abstract

The inner ear vestibular system has numerous projections on central brain centers that regulate sympathetic outflow, and skeletal sympathetic projections affect bone remodeling by inhibiting bone formation by osteoblasts and promoting bone resorption by osteoclasts. In this study, we show that bilateral vestibular lesions in mice cause a low bone mass phenotype associated with decreased bone formation and increased bone resorpt ion. This reduction in bone mass is most pronounced in lower limbs, is not associated with reduced locomotor activity or chronic inflammation, and could be prevented by the administration of the β-blocker propranolol and by genetic deletion of the β2-adrenergic receptor, globally or specifically in osteoblasts. These results provide novel experimental evidence supporting a functional autonomic link between central proprioceptive vestibular structures and the skeleton. Because vestibular dysfunction often affects the elderly, these results also suggest that age-related bone loss might have a vestibular component, and that patients with inner ear pathologies might be at risk for fracture. Lastly, these data might have relevance to the bone loss observed in microgravity, as vestibular function is altered in this condition as well.

Link To Article

http://dx.doi.org/10.1002/jbmr.2426

Gender specific differences in the skeletal response to continuous PTH in mice lacking the IGF1 receptor in mature osteoblasts

Authors

Muriel Babey (MD), Yongmei Wang (MD, PhD), Takuo Kubota (MD, PhD), Chak Fong (BS), Alicia Menendez (BS), Hashem Z. ElAlieh (BS) and Daniel D. Bikle (MD, PhD)

Abstract

The primary goal of this study was to determine whether the IGF1R in mature osteoblasts and osteocytes was required for the catabolic actions of continuous PTH (cPTH). Igf1r was deleted from male and female FVN/B mice by breeding with mice expressing cre recombinase under control of the osteocalcin promoter (OCNIgfr1-/-). Littermates lacking the cre recombinase served as controls. PTH, 60μg/kg/day, was administered continuously by Alzet minipumps for 4 weeks. Blood was obtained for indices of calcium metabolism. The femurs were examined by microcomputed tomography for structure, immunohistochemistry for IGF1R expression, histomorphometry for bone formation rates (BFR), mRNA levels by qPCR, and bone marrow stromal cell cultures (BMSC) for alkaline phosphatase activity (AP+), mineralization, and osteoblast induced osteoclastogenesis. Whereas cPTH led to a reduction in trabecular BV/TV and cortical thickness in the control females, no change was found in the control males. Although trabecular BV/TV and cortical thickness were reduced in the OCNIgfr1-/- mice of both genders, no further reduction after cPTH was seen in the females, unlike the reduction in males. BFR was stimulated by cPTH in the controls, but blocked by Igf1r deletion in the females. The OCNIgfr1-/- male mice showed a partial response. AP+ and mineralized colony formation were higher in BMSC from control males than from control females. These markers were increased by cPTH in both genders, but BMSC from male OCNIgfr1-/- also were increased by cPTH, unlike those from female OCNIgfr1-/-. cPTH stimulated RANKL and decreased osteoprotegerin and alkaline phosphatase expression more in control female bone than control male bone. Deletion of Igf1r blocked these effects of cPTH in the female but not in the male. However, PTH stimulation of osteoblast driven osteoclastogenesis was blocked by deleting Igfr1 in both genders. We conclude that cPTH is catabolic in female but not male mice. Moreover, IGF1 signaling plays a greater role in the skeletal actions of cPTH in the female mouse than the male mouse, which may underlie the gender differences in the response to cPTH.

Link To Article

http://dx.doi.org/10.1002/jbmr.2433