Loss of Gsα in osteocytes leads to osteopenia due to sclerostin induced suppression of osteoblast activity

Abstract

The stimulatory subunit of G-protein, Gsα, acts as a secondary messenger of G-protein coupled receptors (GPCRs) that primarily activates cAMP-induced signaling. GPCRs, such as the parathyroid receptor (PTHR), are critical regulators of bone formation as shown by number of genetic manipulation studies targeting early osteoblast lineage cells. In this study, we have examined the role of Gsa in osteocytes, the terminally differentiated and most abundant cells of the osteoblast lineage. Mice lacking the stimulatory subunit of G-proteins (Gsα) in osteocytes (DMP1-GsαKO) have significant decrease of both trabecular and cortical bone, as assessed by μCT. Histomorphometric analysis showed that the osteopenia was mostly driven by more than 90% decrease in osteoblast numbers and activity whereas osteoclasts were only slightly decreased. The decrease in osteoblast number was associated with a striking lack of endocortical osteoblasts. We have previously shown that loss of the stimulatory subunit of G-proteins (Gsα) in osteocytes in vitro or in vivo has high expression of sclerostin. To determine if the increased sclerostin levels contributed to the decreased endosteal bone lining cells and osteopenia, we treated wild-type mice with recombinant sclerostin and the DMP1-GsαKO mice with anti-sclerostin antibody. Treatment of wild-type mice with 100mg/Kg sclerostin for 3-weeks significantly reduced the numbers of bone lining cells and led to osteopenia. Next, the DMP1-GsαKO and control littermates were treated with the anti-sclerostin antibody (25 mg/kg, 2 times per week) for 4-weeks. Upon the antibody treatment, the endocortical osteoblasts reappeared in the DMP1-GsαKO mice to a comparable level to that of the vehicle treated control littermates. In control mice, E11/gp38 positive osteocytes were observed in parallel with the endocortical osteoblasts with higher dendrite density towards the endocortical osteoblasts. In DMP1-GsαKO mice, E11/gp38 positive osteocytes were lacking dendrites and were randomly scattered throughout the bone matrix. After treatment with anti-sclerostin antibody, DMP1-GsαKO mice showed increased E11/gp38 positive osteocytes near the endosteal bone surface and endosteal osteoblasts. The anti-sclerostin antibody treatment proportionally increased the bone volume but it could not completely rescue the osteopenia in the DMP1-GsαKO mice. Taken together, this data suggests that Gsα signaling in osteocytes leads to osteopenia driven, at least in part, by increased secretion of sclerostin.

Parental origin of Gsα inactivation differentially affects bone remodeling in a mouse model of Albright hereditary osteodystrophy

“…These observations in Gnas E1+/-p mice directly correlate with the previous findings in the literature suggesting that impaired Gsα signaling leads to an osteopenic bone phenotype. (36)(37)(38)(39)(40)(41) These findings could have potential implications for patients with PPHP and have yet to be correlated.…”

Kindlin-2 regulates skeletal homeostasis by modulating PTH1R in mice

“…Notably, Fulzele et al recently reported that deleting Gsα expression utilizing the same Dmp1-Cre used in the present study led to osteopenia due to sclerostin-induced suppression of osteoblast activity. 35 …”

Osteocytes control myeloid cell proliferation and differentiation through Gsα‐dependent and ‐independent mechanisms

“…The osteopenia is associated with increased Sost/ sclerostin expression; treatment with anti-sclerostin antibodies partially rescued the skeletal phenotype, but not the myelopoiesis, thereby suggesting that additional mechanisms control the proliferation of myeloid cells. 11,12 Due to the complexity of the physiological settings and the difficulty of studying osteocytes in vivo, we used in vitro (Ocy454 osteocytic cells) and ex vivo (OEBEs) models 13 to investigate, at the molecular level, the mechanisms by which osteocytes control myelopoiesis. Here we report that osteocytes in vitro and ex vivo secrete Gsα-dependent and independent factors which support myeloid cell proliferation and osteoclast differentiation and function.…”