Conditional deletion of the parathyroid hormone (PTH) receptor 1 from osteocytes results in decreased bone resorption and a progressive increase in cancellous bone mass.

TU XIAOLIN; EDWARDS R; OLIVOS N; BENSON J; PELLEGRINI G; BIVI N; PLOTKIN L; BELLIDO T

Congreso; American Association for Bone and MIneral Reseach; 2011

Some skeletal actions of PTH might be mediated by direct effects of the hormone on osteocytes. To explore the physiological role of the PTH receptor (PTHR1) in these cells, we generated conditional knockout mice (cKO) by mating PTHR1flox/flox mice with DMP1-8kb-Cre mice. Cre mRNA was detected in green fluorescent protein (GFP) positive cells (osteocytes) but not in GFP negative cells (osteoblasts) isolated from DMP1-8kb-Cre mice crossed with DMP1-8kb-GFP mice. PTHR1 mRNA was reduced by 54% in lumbar vertebra and 73% in tibial diaphysis of cKO mice compared to control PTHR1flox/flox littermates. cKO mice exhibited no gross skeletal abnormalities. BMD (DEXA) progressively increased in lumbar spine (5.3% to 14.8%), but not in the femur, in 2 to 8 month-old female cKO mice. Cancellous bone volume (BV/TV) and trabecular number and thickness (micro-CT) were increased in spine and distal femur, without changes in femoral cortical bone, at 4 months. At this age, the resorption marker CTX was 22% lower in plasma from cKO mice, whereas osteocalcin was similar to controls. The discrepancy with the reported reduced BMD without changes in BV/TV or CTX in PTHR1flox/flox;DMP1-10kb-CreERT2 mice might be due to the fact that the 10kb fragment directs gene expression to osteocytes and some osteoblasts, whereas the 8kb fragment only targets osteocytes. In tibial diaphysis (composed of cortical bone and enriched in osteocytes) the expression of RANKL and the Wnt target genes OPG, naked2, Cx43, Smad6 and BMP4 was lower in cKO mice. In contrast, in lumbar vertebra (mostly composed of cancellous bone), only RANKL was decreased resulting in a reduction in the RANKL/OPG ratio. This suggests that osteocytes are the main source of RANKL whereas OPG is also contributed by other osteoblastic cells relatively more abundant in cancellous versus cortical bone. Thus, removal of the PTHR1 only from osteocytes leads to a site specific reduction in the RANKL/OPG ratio resulting in increased cancellous but not cortical bone mass in the cKO mice. Reduced bone resorption induced by deletion of PTHR1 in osteocytes is consistent with earlier evidence that, conversely, osteocyte-specific activation of PTHR1 increases osteoclasts and bone resorption. We conclude that actions of the PTHR1 in osteocytes are required to maintain basal levels of Wnt signaling and RANKL, and that the primary effect of absence of the receptor in osteocytes is a reduction in osteoclast activity in cancellous bone.   Characters with spaces: 2493 (2500 max);   Categories: B. Osteocytes; or                        F. Calciotropic and Phosphotropic Hormones and Mineral Metabolism