Contribution of rankl regulation to bone resorption induced by PTH receptor activation in osteocytes

Abstract

PTH increases osteoclasts by upregulating RANKL in cells of the osteoblastic lineage, but the precise differentiation stage of the PTH target cell remains undefined. Recent findings demonstrate that PTH regulates gene expression in osteocytes and that these cells are an important source of RANKL. We therefore investigated whether direct regulation of the RANKL gene by PTH in osteocytes is required to stimulate osteoclastic bone resorption. To address this question, we examined bone resorption and RANKL expression in transgenic mice in which PTH receptor signaling is activated only in osteocytes (DMP1-caPTHR1) crossed with mice lacking the distal control region regulated by PTH in the RANKL gene (DCR -/-). Longitudinal analysis of circulating C-terminal telopeptide (CTX) in male mice showed elevated resorption in growing mice that progressively decreased to plateau at 3-5 month of age. Resorption was significantly higher (~100%) in DMP1-caPTHR1 mice and non-significantly lower (15-30%) in DCR -/-mice, versus wild type littermates (WT) across all ages. CTX in compound DMP1-caPTHR1; DCR -/-mice was similar to DMP1-caPTHR1 mice at 1 and 2 months of age, but by 3 months of age, was significantly lower compared to DMP1-caPTHR1 mice (50% higher than WT), and by 5 months, it was undistinguishable from WT mice. Micro-CT analysis revealed lower tissue material density in the distal femur of DMP1-caPTHR1 mice, indicative of high remodeling, and this effect was partially corrected in compound vi mice. The increased resorption exhibited by DMP1-caPTHR1 mice was accompanied by elevated RANKL mRNA in bone at 1 and 5 months of age. RANKL expression levels displayed similar patterns to CTX levels in DMP1-caPTHR1; DCR -/-compound mice at 1 and 5 month of age. The same pattern of expression was observed for M-CSF. We conclude that resorption induced by PTH receptor signaling requires direct regulation of the RANKL gene in osteocytes, but this dependence is age specific. Whereas DCR-independent mechanisms involving gp130 cytokines or vitamin D 3 might operate in the growing skeleton, DCR-dependent, cAMP/PKA/CREB-activated mechanisms mediate resorption induced by PTH receptor signaling in the adult skeleton.