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.
Evidence accumulated during the last few years has established that the Wnt/βcatenin pathway is critical for bone formation and skeletal homeostasis1,2. The initial evidence came from the identification of loss-of-function (LOF) and gain-of-function (GOF) mutations in the human LRP5 gene (low-density-lipoprotein receptor-related protein 5; a co-receptor of the Wnt/βcatenin pathway) as responsible for the Osteoporosis-Pseudoglioma Syndrome and for the hereditary High Bone Mass trait, respectively3,4. The bone phenotypes of these mutations were later reproduced in genetically modified mouse models of Lrp5 function, further highlighting the anabolic role of Wnt/βcatenin signaling in bone5.
Genetic studies in mice manipulating βcatenin, the obligatory component of canonical Wnt signaling, have established that βcatenin in the osteoblast lineage increases bone mass through different mechanisms, depending on the specific differentiation stage of the cell