Control of Bone Mass and Remodeling by PTH Receptor Signaling in Osteocytes

233

223

Chronic kidney disease–mineral bone disorder (CKD‐MBD) is defined by abnormalities in mineral and hormone metabolism, bone histomorphometric changes, and/or the presence of soft‐tissue calcification. Emerging evidence suggests that features of CKD‐MBD may occur early in disease progression and are associated with changes in osteocyte function. To identify early changes in bone, we utilized the jck mouse, a genetic model of polycystic kidney disease that exhibits progressive renal disease. At 6 weeks of age, jck mice have normal renal function and no evidence of bone disease but exhibit continual decline in renal function and death by 20 weeks of age, when approximately 40% to 60% of them have vascular calcification. Temporal changes in serum parameters were identified in jck relative to wild‐type mice from 6 through 18 weeks of age and were subsequently shown to largely mirror serum changes commonly associated with clinical CKD‐MBD. Bone histomorphometry revealed progressive changes associated with increased osteoclast activity and elevated bone formation relative to wild‐type mice. To capture the early molecular and cellular events in the progression of CKD‐MBD we examined cell‐specific pathways associated with bone remodeling at the protein and/or gene expression level. Importantly, a steady increase in the number of cells expressing phosphor‐Ser33/37‐β‐catenin was observed both in mouse and human bones. Overall repression of Wnt/β‐catenin signaling within osteocytes occurred in conjunction with increased expression of Wnt antagonists (SOST and sFRP4) and genes associated with osteoclast activity, including receptor activator of NF‐κB ligand (RANKL). The resulting increase in the RANKL/osteoprotegerin (OPG) ratio correlated with increased osteoclast activity. In late‐stage disease, an apparent repression of genes associated with osteoblast function was observed. These data confirm that jck mice develop progressive biochemical changes in CKD‐MBD and suggest that repression of the Wnt/β‐catenin pathway is involved in the pathogenesis of renal osteodystrophy. © 2012 American Society for Bone and Mineral Research.

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Repression of osteocyte Wnt/β-catenin signaling is an early event in the progression of renal osteodystrophy

Sabbagh

Graciolli

O’Brien

et al. 2012

233

223

“…Measurements were done 60 mm away from the growth plates as described. (35,36) Serum biochemistry Blood was collected 2 and 4 weeks after pellet implantation from the facial vein of 3-hour fasted mice. N-terminal propeptide of type I procollagen (P1NP), C-terminal telopeptides of type I collagen (CTX), and tartrate-resistant acid phosphatase form 5b (TRAP 5b) were measured using enzyme-linked immunosorbent assays (Immunodiagnostic Systems Inc., Boldon, UK).…”

Section: Mice and Tissue Procurementmentioning

confidence: 99%

Protection From Glucocorticoid-Induced Osteoporosis by Anti-Catabolic Signaling in the Absence of Sost/Sclerostin

Sato

Cregor

Delgado‐Calle

et al. 2016

Self Cite

100

108

Excess of glucocorticoids, either due to disease or iatrogenic, increases bone resorption and decreases bone formation and is a leading cause of osteoporosis and bone fractures worldwide. Improved therapeutic strategies are sorely needed. We investigated whether activating Wnt/b-catenin signaling protects against the skeletal actions of glucocorticoids, using female mice lacking the Wnt/b-catenin antagonist and bone formation inhibitor Sost. Glucocorticoids decreased the mass, deteriorated the microarchitecture, and reduced the structural and material strength of bone in wild-type (WT), but not in Sost -/-mice. The high bone mass exhibited by Sost -/-mice is due to increased bone formation with unchanged resorption. However, unexpectedly, preservation of bone mass and strength in Sost -/-mice was due to prevention of glucocorticoid-induced bone resorption and not to restoration of bone formation. In WT mice, glucocorticoids increased the expression of Sost and the number of sclerostin-positive osteocytes, and altered the molecular signature of the Wnt/b-catenin pathway by decreasing the expression of genes associated with both anticatabolism, including osteoprotegerin (OPG), and anabolism/survival, such as cyclin D1. In contrast in Sost -/-mice, glucocorticoids did not decrease OPG but still reduced cyclin D1. Thus, in the context of glucocorticoid excess, activation of Wnt/b-catenin signaling by Sost/sclerostin deficiency sustains bone integrity by opposing bone catabolism despite markedly reduced bone formation and increased apoptosis. This crosstalk between glucocorticoids and Wnt/b-catenin signaling could be exploited therapeutically to halt resorption and bone loss induced by glucocorticoids and to inhibit the exaggerated bone formation in diseases of unwanted hyperactivation of Wnt/b-catenin signaling.

“…In mice, intermittent PTH was found to decrease sclerostin expression in bone by about 50%. (66)(67)(68) In the case of continuous PTH infusion (67) or osteocyte-specific constitutively elevated PTH signaling, (69) the effect was more pronounced, with sclerostin expression being decreased by more than 80%. Experiments using in vitro approaches have shown that PTH can downregulate sclerostin expression through the MEF2 transcription complex in a downstream enhancer region for the sclerostin gene.…”

Section: Osteocytes Mechanical Loading and Interface With The Pth Pmentioning

confidence: 99%

Sclerostin: A gem from the genome leads to bone-building antibodies

Pászty

Turner

Robinson

2010

113

Discovery of SclerostinG enomics technologies and DNA sequencing have had a transformative impact on biological research, giving us unprecedented access to the genomes of numerous organisms and ushering in such new fields as systems biology (1) and, more recently, synthetic biology. (2,3) In the 1990s, the advent of highthroughput sequencing spurred application of this powerful new technology to the challenging endeavor of trying to understand the genetic basis of various inherited human diseases. One such disease was sclerosteosis, a very rare, recessively inherited highbone-mass disorder that was thought to be caused primarily by excessive osteoblast-mediated bone formation rather than by defective osteoclast-mediated bone resorption. (4,5) Within the realm of inherited high-bone-mass disorders, (6) it was the hope for a discovery in the area of osteoblast biology that made sclerosteosis particularly interesting.Indeed, the identification of new bone-building pathways that might yield novel anabolic agents had been a long-standing goal in bone biology, with hopes for therapeutic application in fracture healing, orthopedic procedures, and low-bone-mass conditions such as osteoporosis. Against this backdrop came the exciting news, independently from Mary Brunkow's group at Celltech R&D, Inc., (7,8) and from Wim Van Hul's group at the University of Antwerp, (9) that sclerosteosis was caused by mutations in a single gene (SOST) encoding a novel secreted protein. Because these were inactivating mutations, it was clear that this protein, aptly given the name sclerostin, functioned either directly or indirectly as an inhibitor of bone formation. During this same general time period, large-scale cDNA/ expressed sequence tag (EST) (10)(11)(12) and genomic DNA sequencing efforts were being made in academia and industry to rapidly identify new human genes. A novel secreted protein of unknown function, which turned out to be sclerostin, was discovered by computational mining of large DNA sequence databases using either homology-based programs (eg, BLAST) (13,14) or a special CxGxC-class cystine-knot search pattern (C Paszty, unpublished) (15) designed to identify new families of cystine-knot proteins. (16,17) Thus sclerostin emerged during an exciting era of gene discovery that was fueled, in part, by the development of important genomics technologies and the advent of high-throughput DNA sequencing.Similar to sclerostin inactivation in humans, mice with a targeted deletion of the sclerostin gene (SOST knockout mice) were found to have high bone mass, demonstrating evolutionary conservation of sclerostin's function as a negative regulator of bone formation. (18) Analysis of bones from these mice revealed that bone formation was markedly increased on each of the key skeletal surfaces where new bone is normally formed (surface of trabecular bone and internal and external surfaces of cortical bone). Consistent with the increases in bone formation and bone mass, robust increases in bone strength also were found in these anima...