Osteocyte Wnt/β-Catenin Signaling Is Required for Normal Bone Homeostasis

Abstract

␤-Catenin-dependent canonical Wnt signaling plays an important role in bone metabolism by controlling differentiation of bone-forming osteoblasts and bone-resorbing osteoclasts. To investigate its function in osteocytes, the cell type constituting the majority of bone cells, we generated osteocyte-specific ␤-catenindeficient mice (Ctnnb1 loxP/loxP ; Dmp1-Cre). Homozygous mutants were born at normal Mendelian frequency with no obvious morphological abnormalities or detectable differences in size or body weight, but bone mass accrual was strongly impaired due to early-onset, progressive bone loss in the appendicular and axial skeleton with mild growth retardation and premature lethality. Cancellous bone mass was almost completely absent, and cortical bone thickness was dramatically reduced. The low-bone-mass phenotype was associated with increased osteoclast number and activity, whereas osteoblast function and osteocyte density were normal. Cortical bone Wnt/␤-catenin target gene expression was reduced, and of the known regulators of osteoclast differentiation, osteoprotegerin (OPG) expression was significantly downregulated in osteocyte bone fractions of mutant mice. Moreover, the OPG levels expressed by osteocytes were higher than or comparable to the levels expressed by osteoblasts during skeletal growth and at maturity, suggesting that the reduction in osteocytic OPG and the concomitant increase in osteocytic RANKL/OPG ratio contribute to the increased number of osteoclasts and resorption in osteocyte-specific ␤-catenin mutants. Together, these results reveal a crucial novel function for osteocyte ␤-catenin signaling in controlling bone homeostasis.The adult skeleton is continuously remodeled in a tightly regulated manner by the coupled activity of bone-resorbing osteoclasts and bone-forming osteoblasts to maintain skeletal integrity and bone homeostasis (24,46). A similar complex regulatory network of defined, but not necessarily coupled, osteoblast and osteoclast action is required during development and growth, when bone modeling ensures functional bone morphology and bone mass accrual, which in mice occurs throughout the first 3 to 4 months of life until peak bone mass is reached.Osteoblasts differentiate from mesenchymal bone marrow progenitor cells into bone-forming osteoblasts that reside on the bone surface and deposit new bone matrix. In contrast, osteoclasts are derived from hematopoietic bone marrow precursor cells that belong to the myeloid monocyte/macrophage lineage (63). However, the majority, i.e., more than 90 to 95% of all bone cells in the adult skeleton, are osteocytes (10), terminally differentiated cells of the mesenchymal osteoblast lineage residing in small lacunae found at regular intervals within the mineralized bone matrix. They extend long thin cellular protrusions or dendrites, which travel through small channels (canaliculi) inside the compact bone, but also reach the bone surface and bone marrow compartment (8,30). By forming gap junctions to neighboring cells, osteocytes are con...

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

“…(31) These data extend previous reports demonstrating that osteocyte-specific deletion of b-catenin is associated with an increase in the RANKL/OPG ratio, thereby supporting a role of this signaling pathway in balancing the expression of these opposing factors. (34) In vitro experiments using primary and established osteocyte cells have demonstrated that the Wnt inhibitor, sclerostin, stimulates RANKL expression, leading to an increase in the RANKL/OPG mRNA ratio favoring the formation of TRAP þ multinucleated cells. (68) These studies are also in agreement with our data demonstrating that the increased sclerostin expression observed early in CKD-MBD leads to a concomitant increase in RANKL expression that may drive osteoclastogenesis, which was associated with increased bone formation rates in the jck mouse.…”

“…(32)(33)(34) Furthermore, serum levels of the Wnt/b-catenin antagonists, sclerostin (SOST) and Dickkopft-1 (DKK1), were recently reported to be elevated in individuals on dialysis compared to non-CKD individuals, raising the possibility that repression of the b-catenin signaling pathway may also occur in the setting of CKD. (69) The importance of b-catenin signaling on bone mass and remodeling has been well studied.…”

“…(32)(33)(34) To explore the potential role of b-catenin signaling in these gene changes, we assessed the temporal expression of total and phosphorylated b-catenin, and sclerostin, a well-characterized inhibitor of the pathway, on mouse and human bone sections. Immunohistochemical analysis of jck mouse bones demonstrates that the number of osteocytes expressing phosphorylated-Ser 33/37-b-catenin relative to total b-catenin is significantly increased in jck relative to WT mice and its expression is maintained throughout disease (Fig.…”

“…In particular, new evidence suggests that the osteocyte is a prominent source of bone remodeling factors, including receptor activator of NF-kB ligand (RANKL), osteoprotegerin (OPG), and sclerostin, because these influence osteoclast and osteoblast activity. (30)(31)(32)(33)(34) Furthermore, modulation of the WNT/b-catenin pathway has recently been shown to regulate osteoclast activity by altering the osteocyte RANKL/OPG ratio. (34) Our data demonstrate that repression of the Wnt/b-catenin pathway within osteocytes occurs in parallel with an increase in the RANKL/OPG ratio and osteoclast activity.…”

“…(30)(31)(32)(33)(34) Furthermore, modulation of the WNT/b-catenin pathway has recently been shown to regulate osteoclast activity by altering the osteocyte RANKL/OPG ratio. (34) Our data demonstrate that repression of the Wnt/b-catenin pathway within osteocytes occurs in parallel with an increase in the RANKL/OPG ratio and osteoclast activity. Decreased expression of genes associated with osteoblast function is observed late in disease progression.…”