Osteocytes mediate the anabolic actions of canonical Wnt/β-catenin signaling in bone

Abstract

Osteocytes, >90% of the cells in bone, lie embedded within the mineralized matrix and coordinate osteoclast and osteoblast activity on bone surfaces by mechanisms still unclear. Bone anabolic stimuli activate Wnt signaling, and human mutations of components along this pathway underscore its crucial role in bone accrual and maintenance. However, the cell responsible for orchestrating Wnt anabolic actions has remained elusive. We show herein that activation of canonical Wnt signaling exclusively in osteocytes [dominant active (da)βcat Ot mice] induces bone anabolism and triggers Notch signaling without affecting survival. These features contrast with those of mice expressing the same daß-catenin in osteoblasts, which exhibit decreased resorption and perinatal death from leukemia. daßcat Ot mice exhibit increased bone mineral density in the axial and appendicular skeleton, and marked increase in bone volume in cancellous/trabecular and cortical compartments compared with littermate controls. daßcat Ot mice display increased resorption and formation markers, high number of osteoclasts and osteoblasts in cancellous and cortical bone, increased bone matrix production, and markedly elevated periosteal bone formation rate. Wnt and Notch signaling target genes, osteoblast and osteocyte markers, and proosteoclastogenic and antiosteoclastogenic cytokines are elevated in bones of daßcat Ot mice. Further, the increase in RANKL depends on Sost/sclerostin. Thus, activation of osteocytic β-catenin signaling increases both osteoclasts and osteoblasts, leading to bone gain, and is sufficient to activate the Notch pathway. These findings demonstrate disparate outcomes of β-catenin activation in osteocytes versus osteoblasts and identify osteocytes as central target cells of the anabolic actions of canonical Wnt/β-catenin signaling in bone.osteocytes | canonical Wnt | beta-catenin | bone anabolism | notch signaling

MEKK2 mediates an alternative β-catenin pathway that promotes bone formation

“…Although the MEKK2 and WNT pathways appear to be distinct in this regard, both pathways are dependent on reversing or preventing the basal ubiquination and degradation of β-catenin to exert their effects. Thus, the profound effect on bone seen in mice with a conditional deletion of β-catenin exon 3, which prevents this basal ubiquination of β-catenin, would be expected to reflect a decoupling from both the WNT and FGF2/ MEKK2 pathways (1,24). Notably, these findings provide a mechanistic basis for prior observations that β-catenin protein levels and activity are reduced in the absence of FGF2 (25).…”

Canonical Notch activation in osteocytes causes osteopetrosis

“…Since the expression of Sost in osteoblasts is minimal, it cannot be downregulated by Notch so that Wnt signaling cannot be enhanced by this mechanism in osteoblasts, as it can in osteocytes. The downregulation of Wnt antagonists by Notch with the consequent increase in Wnt signaling may serve as a positive feedback autocrine regulatory mechanism, since Wnt itself can activate the Notch pathway in osteocytes (51).…”

“…Indeed, Notch signaling is activated by fluid shear stress in the osteocytic cell line MLO-Y4 (S. Zanotti and E. Canalis, unpublished observations). Another possible way of Notch activation in osteocytes might be in response to Wnt signaling, which has been shown to activate the Notch pathway in these cells (51).…”

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

“…Body weight and DXA measurements were performed 2 to 4 days before (initial) and 28 days (final) after pellet implantation. (33,34) Mice were randomized to the experimental groups based on spine BMD. Percent changes in body weight, lean body mass, and BMD were calculated with the following formula: [(final -initial)/initial] multiplied by 100.…”

“…(35,40) Sclerostin was visualized in deparaffinized sections of decalcified femurs, treated with 3% H 2 O 2 to inhibit endogenous peroxidase activity, blocked with serum, and then incubated with goat polyclonal anti-mouse sclerostin antibody (1:100; Abcam, Cambridge, MA, USA; catalog Ab63097). (34) Sections were then incubated with anti-goat biotinylated secondary antibody followed by avidin conjugated peroxidase (Vectastain Elite ABC Kit; Vector Laboratories, Burlingame, CA, USA; catalog PK-6105). Color was developed with a diaminobenzidine substrate chromogen system (DAKO, Carpinteria, CA, USA).…”