Osteocytes Produces RANKL Via Wnt-TGFβ Signaling Axis for Osteoclastogenesis

Osteocytes are derived from osteoblasts in the mineralized matrix and are the main source of RANKL required for osteoclastogenesis. We initially found osteocytes as central target cells for Wnt/β-catenin signaling that increases RANKL expression and bone resorption in mice. However, how RANKL is regulated remains unclear. Here, we demonstrated its role and molecular mechanisms using primary osteocytes isolated from long bones. Osteocyte transcriptome sequencing revealed the most associated osteoclast differentiation in KEGG pathways with upregulated expression of Tgfb1/2. In vivo data highlight the specificity of osteocytic Wnt, rather than osteoblastic Wnt, in regulating TGFβ signaling. Activation/inactivation of osteocytic TGFβ signaling stringently promotes/inhibits RANKL expression and osteoclast differentiation in dose- and time-dependent manners. Wnt signaling increases RANKL expression through TGFβ signaling via the physical interaction of its transcription factor Smad4 with the RANKL promoter region. Mice with disrupted TGFβ signaling in osteocytes recapitulate defective osteoclastogenesis and reduced RANKL expression in osteocytes. Thus, osteocytes mediate bone resorption via Wnt-TGFβ signaling axis.