Sclerostin Blockade-A Dual Mode of Action After All?

Postnatal Skeletal Deletion of Dickkopf-1 Increases Bone Formation and Bone Volume in Male and Female Mice, Despite Increased Sclerostin Expression

The Wnt antagonist Dickkopf-1 (Dkk1) is a negative regulator of osteoblast function and bone mass. However, because of the lack of appropriate models, many aspects of its role in the regulation of postnatal bone turnover and its cellular source have remained unknown. In this study, we deleted Dkk1 postnatally and in different cell types using various Cre-drivers (Rosa26-ERT2-Cre, Osx-cre, Dmp1-Cre) and assessed to which extent cells of the osteoblastic lineage contribute to the effects of Dkk1 on bone turnover and homeostasis. Female and male mice were examined at 12 weeks of age. Mice with a global or cell type-specific deletion of Dkk1 showed a two- to threefold higher bone volume compared with their Cre-negative littermates. The mineral apposition rate and the bone formation rate were increased two- to fourfold in all three mouse lines, despite a significant increase in systemic and skeletal levels of sclerostin. Dkk1 deletion further reduced the number of osteoclasts about twofold, which was accompanied by a strong decrease in the receptor activator of nuclear factor-κB ligand/osteoprotegerin mRNA ratio in femoral bone. Despite similar increases in bone mass, the deletion of Dkk1 in osterix-expressing cells reduced circulating Dkk1 significantly (males, -79%; females, -77%), whereas they were not changed in Dkk1^fl/fl ;Dmp1-Cre mice. However, both lines showed significantly reduced Dkk1 mRNA levels in bone. In summary, we show that lack of Dkk1 in cells of the osteoblastic lineage leads to high bone mass with increased bone formation, despite increased levels of sclerostin. Moreover, the majority of systemic Dkk1 appears to originate from osteoprogenitors but not from mature osteoblasts or osteocytes. Nevertheless, the amount of Dkk1 produced locally by more mature osteogenic cells is sufficient to modulate bone mass. Thus, this study highlights the importance of local Wnt signaling on postnatal bone homeostasis. © 2018 American Society for Bone and Mineral Research.

Role of nutritional vitamin D in osteoporosis treatment

Osteoporosis is a systemic skeletal disorder characterized by a decrease in bone mass and microarchitectural deterioration of bone tissue. The World Health Organization has defined osteoporosis as a decrease in bone mass (50%) and bony quality (50%). Vitamin D, a steroid hormone, is crucial for skeletal health and in mineral metabolism. Its direct action on osteoblasts and osteoclasts and interaction with nonskeletal tissues help in maintaining a balance between bone turnover and bone growth. Vitamin D affects the activity of osteoblasts, osteoclasts, and osteocytes, suggesting that it affects bone formation, bone resorption, and bone quality. At physiological concentrations, active vitamin D maintains a normal rate of bone resorption and formation through the RANKL/OPG signal. However, active vitamin D at pharmacological concentration inhibits bone resorption at a higher rate than that of bone formation, which influences the bone quality and quantity. Nutritional vitamin D rather than active vitamin D activates osteoblasts and maintains serum 25(OH)D₃ concentration. Despite many unanswered questions, much data support nutritional vitamin D use in osteoporosis patients. This article emphasizes the role of nutritional vitamin D replacement in different turnover status (high or low bone turnover disorders) of osteoporosis together with either anti-resorptive (Bisphosphonate, Denosumab et.) or anabolic (Teriparatide) agents when osteoporosis persists.

A soluble activin type IIA receptor mitigates the loss of femoral neck bone strength and cancellous bone mass in a mouse model of disuse osteopenia

Disuse causes a rapid and substantial bone loss distinct in its pathophysiology from the bone loss associated with cancers, age, and menopause. While inhibitors of the activin-receptor signaling pathway (IASPs) have been shown to prevent ovariectomy- and cancer-induced bone loss, their application in a model of disuse osteopenia remains to be tested. Here, we show that a soluble activin type IIA receptor (ActRIIA-mFc) increases diaphyseal bone strength and cancellous bone mass, and mitigates the loss of femoral neck bone strength in the Botulinum Toxin A (BTX)-model of disuse osteopenia in female C57BL/6J mice. We show that ActRIIA-mFc treatment preferentially stimulates a dual-effect (anabolic-antiresorptive) on the periosteal envelope of diaphyseal bone, demonstrating in detail the effects of ActRIIA-mFc on cortical bone. These observations constitute a previously undescribed feature of IASPs that mediates at least part of their ability to mitigate detrimental effects of unloading on bone tissue. The study findings support the application of IASPs as a strategy to combat bone loss during disuse.

Sclerostin Blockade and Zoledronic Acid Improve Bone Mass and Strength in Male Mice With Exogenous Hyperthyroidism

Hyperthyroidism in mice is associated with low bone mass, high bone turnover, and high concentrations of sclerostin, a potent Wnt inhibitor. Here, we explored the effects of either increasing bone formation with sclerostin antibodies (Scl-Ab) or reducing bone turnover with bisphosphonates on bone mass and strength in hyperthyroid mice. Twelve-week-old C57BL/6 male mice were rendered hyperthyroid using l-thyroxine (T4; 1.2 µg/mL added to the drinking water) and treated with 20 mg/kg Scl-Ab twice weekly or 100 µg/kg zoledronic acid (ZOL) once weekly or phosphate-buffered saline for 4 weeks. Hyperthyroid mice displayed a lower trabecular bone volume at the spine (-42%, P < 0.05) and the distal femur (-55%, P < 0.05) compared with euthyroid controls. Scl-Ab and ZOL treatment of hyperthyroid mice increased trabecular bone volume at the spine by threefold and twofold, respectively. Serum bone formation and resorption markers were increased in hyperthyroid mice and suppressed by treatment with ZOL but not Scl-Ab. Trabecular bone stiffness at the lumbar vertebra was 63% lower in hyperthyroid mice (P < 0.05) and was increased fourfold by Sci-Ab (P < 0.001) and threefold by ZOL treatment (P < 0.01). Bone strength based on ultimate load, which was 10% lower in hyperthyroidism, was increased by Scl-Ab by 71% and ZOL by 22% (both P < 0.001). Increased proportion of low mineralized bone seen in hyperthyroid mice was restored by treatment with Scl-Ab and ZOL. Thus, bone-forming and antiresorptive drugs prevent bone loss in hyperthyroid mice via different mechanisms.

Chronic Mild Stress Causes Bone Loss via an Osteoblast-Specific Glucocorticoid-Dependent Mechanism

Chronic stress and depression are associated with alterations in the hypothalamic-pituitary-adrenal signaling cascade and considered a risk factor for bone loss and fractures. However, the mechanisms underlying the association between stress and poor bone health are unclear. Using a transgenic (tg) mouse model in which glucocorticoid signaling is selectively disrupted in mature osteoblasts and osteocytes [11β-hydroxysteroid-dehydrogenase type 2 (HSD2)OB-tg mice], the present study examines the impact of chronic stress on skeletal metabolism and structure. Eight-week-old male and female HSD2OB-tg mice and their wild-type (WT) littermates were exposed to chronic mild stress (CMS) for the duration of 4 weeks. At the endpoint, L3 vertebrae and tibiae were analyzed by micro-computed tomography and histomorphometry, and bone turnover was measured biochemically. Compared with nonstressed controls, exposure to CMS caused an approximately threefold increase in serum corticosterone concentrations in WT and HSD2OB-tg mice of both genders. Compared with controls, CMS resulted in loss of vertebral trabecular bone mass in male WT mice but not in male HSD2OB-tg littermates. Furthermore, both tibial cortical area and area fraction were reduced in stressed WT but not in stressed HSD2OB-tg male mice. Osteoclast activity and bone resorption marker were increased in WT males following CMS, features absent in HSD2OB-tg males. Interestingly, CMS had little effect on vertebral and long-bone structural parameters in female mice. We conclude that in male mice, bone loss during CMS is mediated via enhanced glucocorticoid signaling in osteoblasts (and osteocytes) and subsequent activation of osteoclasts. Female mice appear resistant to the skeletal effects of CMS.