Maintenance of physiologic phosphate balance is of crucial biological importance, as it is fundamental to cellular function, energy metabolism, and skeletal mineralization. Fibroblast growth factor-23 (FGF-23) is a master regulator of phosphate homeostasis, but the molecular mechanism of such regulation is not yet completely understood. Targeted disruption of the Fgf-23 gene in mice (Fgf-23−/−) elicits hyperphosphatemia, and an increase in renal sodium/phosphate co-transporter 2a (NaPi2a) protein abundance. To elucidate the pathophysiological role of augmented renal proximal tubular expression of NaPi2a in Fgf-23−/− mice and to examine serum phosphate–independent functions of Fgf23 in bone, we generated a new mouse line deficient in both Fgf-23 and NaPi2a genes, and determined the effect of genomic ablation of NaPi2a from Fgf-23−/− mice on phosphate homeostasis and skeletal mineralization. Fgf-23−/−/NaPi2a−/− double mutant mice are viable and exhibit normal physical activities when compared to Fgf-23−/− animals. Biochemical analyses show that ablation of NaPi2a from Fgf-23−/− mice reversed hyperphosphatemia to hypophosphatemia by 6 weeks of age. Surprisingly, despite the complete reversal of serum phosphate levels in Fgf-23−/−/NaPi2a−/−, their skeletal phenotype still resembles the one of Fgf23−/− animals. The results of this study provide the first genetic evidence of an in vivo pathologic role of NaPi2a in regulating abnormal phosphate homeostasis in Fgf-23−/− mice by deletion of both NaPi2a and Fgf-23 genes in the same animal. The persistence of the skeletal anomalies in double mutants suggests that Fgf-23 affects bone mineralization independently of systemic phosphate homeostasis. Finally, our data support (1) that regulation of phosphate homeostasis is a systemic effect of Fgf-23, while (2) skeletal mineralization and chondrocyte differentiation appear to be effects of Fgf-23 that are independent of phosphate homeostasis.

Estrogen is critical for skeletal homeostasis and regulates bone remodeling, in part, by modulating the expression of receptor activator of NF-κB ligand (RANKL), an essential cytokine for bone resorption by osteoclasts. RANKL can be produced by a variety of hematopoietic (e.g. T and B-cell) and mesenchymal (osteoblast lineage, chondrocyte) cell types. The cellular mechanisms by which estrogen acts on bone are still a matter of controversy. By using murine reconstitution models that allow for selective deletion of estrogen receptor-alpha (ERα) or selective inhibition of RANKL in hematopoietic vs. mesenchymal cells, in conjunction with in situ expression profiling in bone cells, we identified bone lining cells as important gatekeepers of estrogen-controlled bone resorption. Our data indicate that the increase in bone resorption observed in states of estrogen deficiency in mice is mainly caused by lack of ERα-mediated suppression of RANKL expression in bone lining cells.Estrogen is an important regulator of bone mass. The role of estrogen for bone homeostasis in humans is illustrated by the fact that estrogen deficiency is one of the major causes of postmenopausal osteoporosis 1 . Estrogen acts through two receptors, estrogen receptor-alpha (ERα) and -beta (ERβ), with ERα being more important for the regulation of bone metabolism 2 . Estrogen receptors are widely expressed in a variety of cells in bone and bone marrow. However, the actual target cell responsible for mediating the effects of estrogen on bone is still a matter of debate 3 . One of the most important downstream mediators of the action of estrogen on bone is the osteoprotegerin (OPG)/receptor activator of NF-κB ligand (RANKL) system. RANKL is an essential cytokine for osteoclast differentiation, activation, and survival 4, 5 . RANKL is produced by a variety of cells such as cells of the stromal cell lineage, activated T lymphocytes, but also B lymphocytes 5 . OPG is a soluble decoy receptor for RANKL which binds RANKL, and thereby inhibits osteoclastogenesis 6 . RANKL acts through the receptor RANK which is expressed in the cell membrane of osteoclasts and osteoclast precursor cells 7 . RANKL, RANK, and OPG are essential, non-redundant factors for osteoclast biology. Osteoclasts are entirely absent in RANK or RANKL deficient mice, leading to osteopetrosis, whereas OPG-deficient mice exhibit excessive bone resorption and severe osteoporosis 5,7,8 . RANKL exists in two biologically active forms, a membrane-bound form and a soluble form. Membrane-bound RANKL can be shed by matrix metalloproteinase 14 or by a disintegrin and metalloproteinase (ADAM) 10 9 resulting in soluble RANKL. In addition, soluble RANKL is produced by immune cells as a primary secreted form 5 . It is well established that sex steroids regulate the RANKL-OPG axis in osteoblast-like cells in vitro 10,11 . There is good evidence that OPG is regulated directly by sex hormones, whereas the sex steroid-mediated regulation of RANKL appears to be mainly indirect [10][11][12][13] . In vivo, o...

Objective. RANKL has been implicated in the pathogenesis of glucocorticoid-induced osteoporosis. This study was undertaken to evaluate the efficacy of denosumab, a neutralizing monoclonal antibody against human RANKL (hRANKL), in a murine model of glucocorticoid-induced osteoporosis.Methods. Eight-month-old male homozygous hRANKL-knockin mice expressing a chimeric RANKL protein with a humanized exon 5 received 2.1 mg/kg of prednisolone or placebo daily over 4 weeks via subcutaneous slow-release pellets and were additionally treated with phosphate buffered saline or denosumab (10 mg/kg subcutaneously twice weekly). Two groups of wild-type mice were also treated with either prednisolone or vehicle.Results. The 4-week prednisolone treatment induced loss of vertebral and femoral volumetric bone mineral density in the hRANKL-knockin mice. Glucocorticoid-induced bone loss was associated with suppressed vertebral bone formation and increased bone resorption, as evidenced by increases in the number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts, TRAP-5b protein in bone extracts, serum levels of TRAP-5b, and urinary excretion of deoxypyridinoline. Denosumab prevented prednisoloneinduced bone loss by a pronounced antiresorptive effect. Biomechanical compression tests of lumbar vertebrae revealed a detrimental effect of prednisolone on bone strength that was prevented by denosumab.Conclusion. Our findings indicate that RANKL inhibition by denosumab prevents glucocorticoidinduced loss of bone mass and strength in hRANKLknockin mice.

Excessive FGF23 has been identified as a pivotal phosphaturic factor leading to renal phosphate-wasting, and the subsequent development of rickets and osteomalacia. In contrast, loss of FGF23 in mice (Fgf23−/−) leads to high serum phosphate, calcium and 1,25-vitamin-D levels resulting in early lethality attributable to severe ectopic soft-tissue calcifications and organ-failure. Paradoxically, Fgf23−/− mice exhibit a severe defect in skeletal mineralization despite high levels of systemic mineral ions and abundant ectopic mineralization, an abnormality that remains largely unexplained. Through use of in situ hybridization, immunohistochemistry and immunogold labeling coupled with electron microscopy of bone samples we discovered that expression and accumulation of osteopontin (Opn/OPN) was markedly increased in Fgf23−/− mice. These results were confirmed by qPCR-analyses of Fgf23−/− bones and ELISA measurements of serum OPN. To investigate whether elevated OPN levels were contributing to the bone mineralization defect in Fgf23−/− mice, we generated Fgf23−/−/Opn−/− double-knockout mice (DKO). Biochemical analyses showed that the hypercalcemia and hyperphosphatemia observed in Fgf23−/− mice remained unchanged in DKO mice, however µCT and histomorphometric analyses showed a significant improvement in total mineralized bone-volume. The severe osteoidosis was markedly reduced and a normal mineral apposition rate was present in DKO mice, indicating that increased OPN levels in Fgf23−/− mice are at least in part responsible for the osteomalacia. Moreover, the increased OPN levels were significantly decreased upon lowering serum phosphate by feeding low phosphate diet or deletion NaPi2a, indicating phosphate attributes in part to the high OPN levels in Fgf23−/− mice. In summary, our results suggest that increased OPN is an important pathogenic factor mediating the mineralization defect and the alterations in bone metabolism observed in Fgf23−/− bones.

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