Central Depletion of Brain-Derived Neurotrophic Factor in Mice Results in High Bone Mass and Metabolic Phenotype

Authors

Shanil S. Juma, Zahra Ezzat-Zadeh, Dania A. Khalil, Shirin Hooshmand, Mohammed Akhter, Bahram H. Arjmandi

Abstract

Brain-derived neurotrophic factor (BDNF) plays important roles in neuronal differentiation/survival, the regulation of food intake, and the pathobiology of obesity and type 2 diabetes mellitus. BDNF and its receptor are expressed in osteoblasts and chondrocyte. BDNF in vitro has a positive effect on bone; whether central BDNF affects bone mass in vivo is not known. We therefore examined bone mass and energy use in brain-targeted BDNF conditional knockout mice (Bdnf2lox/2lox/93). The deletion of BDNF in the brain led to a metabolic phenotype characterized by hyperphagia, obesity, and increased abdominal white adipose tissue. Central BDNF deletion produces a marked skeletal phenotype characterized by increased femur length, elevated whole bone mineral density, and bone mineral content. The skeletal changes are developmentally regulated and appear concurrently with the metabolic phenotype, suggesting that the metabolic and skeletal actions of BDNF are linked. The increased bone development is evident in both the cortical and trabecular regions. Compared with control, Bdnf2lox/2lox/93 mice show greater trabecular bone volume (+50% for distal femur, P < 0.001; +35% for vertebral body, P < 0.001) and midfemoral cortical thickness (+11 to 17%, P < 0.05), measured at 3 and 6 months of age. The skeletal and metabolic phenotypes were gender dependent, with female being more affected than male mice. However, uncoupling protein-1 expression in brown fat, a marker of sympathetic tone, was not different between genotypes. We show that deletion of central BDNF expression in mice results in increased bone mass and white adipose tissue, with no significant changes in sympathetic signaling or peripheral serotonin, associated with hyperphagia, obesity, and leptin resistance.

Link to Article

http://dx.doi.org/10.1210/en.2012-1378

Soy protein with or without isoflavones failed to preserve bone density in gonadal hormone–deficient male rat model of osteoporosis

Authors

Shanil S. Juma, Zahra Ezzat-Zadeh, Dania A. Khalil, Shirin Hooshmand, Mohammed Akhter, Bahram H. Arjmandi

Abstract

Soy with its isoflavones has been shown to positively influence bone mineral density in female ovariectomized rats; hence, we hypothesized a similar effect in orchidectomized (ORX) male rats. Forty male Sprague-Dawley rats, aged 95 days, were divided into 4 groups and were either sham operated (Sham) or ORX. The ORX groups were fed a soy protein–based diet (SOY), an isoflavone-depleted soy protein diet (SOY−), or a casein based diet for 65 days after surgery. Orchidectomy increased the rate of bone turnover, resulting in reduced bone mineral density and bone mineral content by 3.5% and 14%, respectively, and compromised biomechanical properties. The mean femoral length of ORX animals was also significantly shorter than Sham animals, but ORX rats that were fed SOY diet did not experience this reduction in bone length, implicating a role for soy protein in bone growth (4.02 ± 0.02, 3.93 ± 0.01, 3.99 ± 0.02, 3.91 ± 0.01 for Sham, ORX, SOY, SOY−, respectively). The SOY and SOY− positively influenced the biomechanical properties of bone such as yield and ultimate force, the measures of bone elasticity, and plasticity. In terms of bone histomorphometry, the data indicate that SOY− tends to reduce ORX-induced increase in bone turnover as evidenced by suppressed bone formation rate/mineralized surface by about 9%. Overall, our results indicated that soy protein, regardless of its isoflavone content, was unable to prevent the ORX-induced femoral decrease in bone density and mineral content. However, soy may enhance the quality of bone as indicated by increased yield force.

Link to Article

http://dx.doi.org/10.1016/j.nutres.2012.08.001

Enhanced Periosteal and Endocortical Responses to Axial Tibial Compression Loading in Conditional Connexin43 Deficient Mice

Authors

Susan K. Grimston, Marcus P. Watkins, Michael D. Brodt, Matthew J. Silva, Roberto Civitelli

Abstract

The gap junction protein, connexin43 (Cx43) is involved in mechanotransduction in bone. Recent studies using in vivo models of conditional Cx43 gene (Gja1) deletion in the osteogenic linage have generated inconsistent results, with Gja1 ablation resulting in either attenuated or enhanced response to mechanical load, depending upon the skeletal site examined or the type of load applied. To gain further insights on Cx43 and mechanotransduction, we examined bone formation response at both endocortical and periosteal surfaces in 2-month-old mice with conditional Gja1 ablation driven by the Dermo1 promoter (cKO). Relative to wild type (WT) littermates, it requires a larger amount of compressive force to generate the same periosteal strain in cKO mice. Importantly, cKO mice activate periosteal bone formation at a lower strain level than do WT mice, suggesting an increased sensitivity to mechanical load in Cx43 deficiency. Consistently, trabecular bone mass also increases in mutant mice upon load, while it decreases in WT. On the other hand, bone formation actually decreases on the endocortical surface in WT mice upon application of axial mechanical load, and this response is also accentuated in cKO mice. These changes are associated with increase of Cox-2 in both genotypes and further decrease of Sost mRNA in cKO relative to WT bones. Thus, the response of bone forming cells to mechanical load differs between trabecular and cortical components, and remarkably between endocortical and periosteal envelopes. Cx43 deficiency enhances both the periosteal and endocortical response to mechanical load applied as axial compression in growing mice.

Link to Article

http://dx.doi.org/10.1371/journal.pone.0044222

Resorbable, amino acid-based poly(ester urea)s crosslinked with osteogenic growth peptide with enhanced mechanical properties and bioactivity

Authors

Kimberly Sloan Stakleff, Fei Lin, Laura A. Smith Callahan, Mary Beth Wade, Andrew Esterle, James Miller, Matthew Graham, Matthew L. Becker

Abstract

Materials currently used for the treatment of bone defects include ceramics, polymeric scaffolds and composites, which are often impregnated with recombinant growth factors and other bioactive substances. While these materials have seen instances of success, each has inherent shortcomings including prohibitive expense, poor protein stability, poorly defined growth factor release and less than desirable mechanical properties. We have developed a novel class of amino acid-based poly(ester urea)s (PEU) materials which are biodegradable in vivo and possess mechanical properties superior to conventionally used polyesters (<3.5 GPa) available currently to clinicians and medical providers. We report the use of a short peptide derived from osteogenic growth peptide (OGP) as a covalent crosslinker for the PEU materials. In addition to imparting specific bioactive signaling, our crosslinking studies show that the mechanical properties increase proportionally when 0.5% and 1.0% concentrations of the OGP crosslinker are added. Our results in vitro and in an in vivo subcutaneous rat model show the OGP-based crosslinkers, which are small fragments of growth factors that are normally soluble, exhibit enhanced proliferative activity, accelerated degradation properties and concentration dependent bioactivity when immobilized.

Link to Article

http://dx.doi.org/10.1016/j.actbio.2012.08.035

Hyperthyroid-associated osteoporosis is exacerbated by the loss of TSH signaling

Authors

Ramkumarie Baliram, Li Sun, Jay Cao, Jianhua Li, Rauf Latif, Amanda K. Huber, Tony Yuen, Harry C. Blair, Mone Zaidi, and Terry F. Davies

Abstract

The osteoporosis associated with human hyperthyroidism has traditionally been attributed to elevated thyroid hormone levels. There is evidence, however, that thyroid-stimulating hormone (TSH), which is low in most hyperthyroid states, directly affects the skeleton. Importantly, Tshr-knockout mice are osteopenic. In order to determine whether low TSH levels contribute to bone loss in hyperthyroidism, we compared the skeletal phenotypes of wild-type and Tshr-knockout mice that were rendered hyperthyroid. We found that hyperthyroid mice lacking TSHR had greater bone loss and resorption than hyperthyroid wild-type mice, thereby demonstrating that the absence of TSH signaling contributes to bone loss. Further, we identified a TSH-like factor that may confer osteoprotection. These studies suggest that therapeutic suppression of TSH to very low levels may contribute to bone loss in people.

Link to Article

http://dx.doi.org/10.1172/JCI63948

The ADP receptor P2RY12 regulates osteoclast function and pathologic bone remodeling

Authors

Xinming Su, Desiree H. Floyd, Alun Hughes, Jingyu Xiang, Jochen G. Schneider, Ozge Uluckan, Emanuela Heller, Hongju Deng, Wei Zou, Clarissa S. Craft, Kaiming Wu, Angela C. Hirbe, Dorota Grabowska, Mark C. Eagleton, Sarah Townsley, Lynne Collins, David Piwnica-Worms, Thomas H. Steinberg, Deborah V. Novack, Pamela B. Conley, Michelle A. Hurchla, Michael Rogers, and Katherine N. Weilbaecher

Abstract

The adenosine diphosphate (ADP) receptor P2RY12 (purinergic receptor P2Y, G protein coupled, 12) plays a critical role in platelet aggregation, and P2RY12 inhibitors are used clinically to prevent cardiac and cerebral thrombotic events. Extracellular ADP has also been shown to increase osteoclast (OC) activity, but the role of P2RY12 in OC biology is unknown. Here, we examined the role of mouse P2RY12 in OC function. Mice lacking P2ry12 had decreased OC activity and were partially protected from age-associated bone loss. P2ry12–/– OCs exhibited intact differentiation markers, but diminished resorptive function. Extracellular ADP enhanced OC adhesion and resorptive activity of WT, but not P2ry12–/–, OCs. In platelets, ADP stimulation of P2RY12 resulted in GTPase Ras-related protein (RAP1) activation and subsequent αIIbβ3 integrin activation. Likewise, we found that ADP stimulation induced RAP1 activation in WT and integrin β3 gene knockout (Itgb3–/–) OCs, but its effects were substantially blunted in P2ry12–/– OCs. In vivo, P2ry12–/– mice were partially protected from pathologic bone loss associated with serum transfer arthritis, tumor growth in bone, and ovariectomy-induced osteoporosis: all conditions associated with increased extracellular ADP. Finally, mice treated with the clinical inhibitor of P2RY12, clopidogrel, were protected from pathologic osteolysis. These results demonstrate that P2RY12 is the primary ADP receptor in OCs and suggest that P2RY12 inhibition is a potential therapeutic target for pathologic bone loss.

Link to Article

http://dx.doi.org10.1172/JCI38576