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Loss of Gs? in osteocytes leads to osteopenia due to sclerostin induced suppression of osteoblast activity.

ABSTRACT: The stimulatory subunit of G-protein, Gs?, acts as a secondary messenger of G-protein coupled receptors (GPCRs) that primarily activates cAMP-induced signaling. GPCRs, such as the parathyroid hormone 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 Gs? in osteocytes, the terminally differentiated and most abundant cells of the osteoblast lineage. Mice lacking the stimulatory subunit of G-proteins (Gs?) in osteocytes (DMP1-Gs?KO) have significant decrease of both trabecular and cortical bone, as assessed by ?CT. 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 (Gs?) in osteocytes in vitro or in vivo induces 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-Gs?KO mice with anti-sclerostin antibody. Treatment of wild-type mice with 100??g/kg sclerostin for 3-weeks significantly reduced the numbers of bone lining cells and led to osteopenia. Next, the DMP1-Gs?KO 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-Gs?KO 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-Gs?KO mice, E11/gp38 positive osteocytes were lacking dendrites and were randomly scattered throughout the bone matrix. After treatment with anti-sclerostin antibody, DMP1-Gs?KO 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-Gs?KO mice. Taken together, this data suggests that Gs? signaling in osteocytes leads to osteopenia driven, at least in part, by increased secretion of sclerostin.

SUBMITTER: Fulzele K

PROVIDER: S-EPMC6207374 | biostudies-literature | 2018 Dec

REPOSITORIES: biostudies-literature

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

Fulzele Keertik K Dedic Christopher C Lai Forest F Bouxsein Mary M Lotinun Sutada S Baron Roland R Divieti Pajevic Paola P

Bone 20180925

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 hormone 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 Gsα in osteocytes, the terminally differentiated and most abundant cells of the osteoblast lineage. Mice ...[more]

PMID: 30266511

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Project description:Osteocytes, the bone cells embedded in the mineralized matrix, control bone modeling, and remodeling through direct contact with adjacent cells and via paracrine and endocrine factors that affect cells in the bone marrow microenvironment or distant organs. Osteocytes express numerous G protein-coupled receptors (GPCRs) and thus mice lacking the stimulatory subunit of G-protein (Gsα) in osteocytes (Dmp1-GsαKO mice) have abnormal myelopoiesis, osteopenia, and reduced adipose tissue. We previously reported that the severe osteopenia and the changes in adipose tissue present in these mice were mediated by increased sclerostin, which suppress osteoblast functions and promote browning of white adipocytes. Inversely, the myeloproliferation was driven by granulocyte colony-stimulating factor (G-CSF) and administration of neutralizing antibodies against G-CSF only partially restored the myeloproliferation, suggesting that additional osteocyte-derived factors might be involved. We hypothesized that osteocytes secrete Gsα-dependent factor(s) which regulate the myeloid cells proliferation. To identify osteocyte-secreted proteins, we used the osteocytic cell line Ocy454 expressing or lacking Gsα expression (Ocy454-Gsαcont and Ocy454-GsαKO ) to delineate the osteocyte "secretome" and its regulation by Gsα. Here we reported that factors secreted by osteocytes increased the number of myeloid colonies and promoted macrophage proliferation. The proliferation of myeloid cells was further promoted by osteocytes lacking Gsα expression. Myeloid cells can differentiate into bone-resorbing osteoclasts, therefore, we hypothesized that osteocyte-secreted factors might also regulate osteoclastogenesis in a Gsα-dependent manner. Conditioned medium (CM) from Ocy454 (both Gsαcont and GsαKO ) significanlty increased the proliferation of bone marrow mononuclear cells (BMNC) and, at the same time, inhibited their differentiation into mature osteoclasts via a Gsα-dependent mechanism. Proteomics analysis of CM from Ocy454 Gsαcont and GsαKO cells identified neuropilin-1 (Nrp-1) and granulin (Grn) as osteocytic-secreted proteins upregulated in Ocy454-GsαKO cells compared to Ocy454-Gsαcont , whereas semaphorin3A was significantly suppressed. Treatment of Ocy454-Gsαcont cells with recombinant proteins or knockdown of Nrp-1 and Grn in Ocy454-GsαKO cells partially rescued the inhibition of osteoclasts, demonstrating that osteocytes control osteoclasts differentiation through Nrp-1 and Grn which are regulated by Gsα signaling.

Dataset Information

Loss of Gs? in osteocytes leads to osteopenia due to sclerostin induced suppression of osteoblast activity.

Publications

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

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