CathepsinKCre mediated deletion of βcatenin results in dramatic loss of bone mass by targeting both osteoclasts and osteoblastic cells

Abstract

It is well established that activation of Wnt/βcatenin signaling in the osteoblast lineage leads to an increase in bone mass through a dual mechanism: increased osteoblastogenesis and decreased osteoclastogenesis. However, the effect of this pathway on the osteoclast lineage has been less explored. Here, we aimed to examine the effects of Wnt/βcatenin signaling in mature osteoclasts by generating mice lacking βcatenin in CathepsinK-expressing cells (Ctnnb1f/f;CtsKCre mice). These mice developed a severe low-bone-mass phenotype with onset in the second month and in correlation with an excessive number of osteoclasts, detected by TRAP staining and histomorphometric quantification. We found that WNT3A, through the canonical pathway, promoted osteoclast apoptosis and therefore attenuated the number of M-CSF and RANKL-derived osteoclasts in vitro. This reveals a cell-autonomous effect of Wnt/βcatenin signaling in controlling the life span of mature osteoclasts. Furthermore, bone Opg expression in Ctnnb1f/f;CtsKCre mice was dramatically decreased pointing to an additional external activation of osteoclasts. Accordingly, expression of CathepsinK was detected in TRAP-negative cells of the inner periosteal layer also expressing Col1. Our results indicate that the bone phenotype of Ctnnb1f/f;CtsKCre animals combines a cell-autonomous effect in the mature osteoclast with indirect effects due to the additional targeting of osteoblastic cells.

Osteocyte-specific WNT1 regulates osteoblast function during bone homeostasis

“…This last function together with its production of sclerostin shows that it can also serve as a negative and positive regulator of osteoblast differentiation and function, respectively ( Figure 6E). Recent studies also showed that Wnt signaling could directly regulate osteoclastogenesis (18)(19)(20)(21)(22). Further studies will be required to elucidate the direct regulation of osteoclasts by osteocyte-derived Wnt signaling.…”

“…Further genetic studies with β-catenin and other Wnt ligands using various mouse models provided additional evidence that corroborate the critical function of Wnt signaling in skeletal development and bone homeostasis (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17). Several recent studies also reported that Wnt signaling directly regulates osteoclast function (18)(19)(20)(21)(22). Despite the established function of Wnt signaling in bone, the role of specific Wnt ligands in human bone homeostasis was not clear.…”

Deletion of Wnt5a in osteoclasts results in bone loss through decreased bone formation

“…Ctsk ‐ Cre mice were bred with the Wnt5a F/F mice to generate Ctsk ‐ Cre ; Wnt5a F/+ mice, which were then crossed with Wnt5a F/F mice to generate experimental Ctsk ‐ Cre ; Wnt5a F/F ( Wnt5a cKO) mice. Ctsk‐Cre is a knock‐in mutation that is under the control of the endogenous promoter of the Ctsk gene and has previously been shown to effectively target floxed genes in mature osteoclasts . Conditional knockout mice were compared with Wnt5a F/+ littermate controls.…”

“…We assumed that similar to WNT5A secreted from osteoblasts, osteoclast‐derived WNT5A acts as an enhancer of osteoclastogenesis. However, upon targeting the Wnt5a F/F gene in mice utilizing the Ctsk ‐ Cre system, commonly used to target floxed genes in mature osteoclasts, we observed that Ctsk‐Cre;Wnt5a F/F mice exhibited an unexpected low bone–mass phenotype caused by decreased bone formation, not increased osteoclast number or activity. Further, our molecular mechanism analyses identified specific phosphorylation of WNT5A secreted by osteoclast‐like cells, but not in osteoblasts.…”

Constitutive Activation of β-Catenin in Differentiated Osteoclasts Induces Bone Loss in Mice

“…The number of mature osteoclasts per well was counted as multinucleated (>3 nuclei) TRAPpositive cells [19,27]. RNA expression was analysed by reverse transcription-quantitative PCR (RT-qPCR) analysis.…”