Loss of menin in osteoblast lineage affects osteocyte–osteoclast crosstalk causing osteoporosis

Abstract

During osteoporosis bone formation by osteoblasts is reduced and/or bone resorption by osteoclasts is enhanced. Currently, only a few factors have been identified in the regulation of bone integrity by osteoblast-derived osteocytes. In this study, we show that specific disruption of menin, encoded by multiple endocrine neoplasia type 1 (Men1), in osteoblasts and osteocytes caused osteoporosis despite the preservation of osteoblast differentiation and the bone formation rate. Instead, an increase in osteoclast numbers and bone resorption was detected that persisted even when the deletion of Men1 was restricted to osteocytes. We demonstrate that isolated Men1-deficient osteocytes expressed numerous soluble mediators, such as C-X-C motif chemokine 10 (CXCL10), and that CXCL10-mediated osteoclastogenesis was reduced by CXCL10-neutralizing antibodies. Collectively, our data reveal a novel role for Men1 in osteocyte-osteoclast crosstalk by controlling osteoclastogenesis through the action of soluble factors. A role for Men1 in maintaining bone integrity and thereby preventing osteoporosis is proposed. Cell Death and Differentiation (2017) 24, 672-682; doi:10.1038/cdd.2016.165; published online 20 January 2017The maintenance of bone mass is severely affected in osteoporotic patients and other bone disorders and can have multiple etiologies. The complex communication of boneforming osteoblasts, bone-residing osteocytes, and boneresorbing osteoclasts is still not fully understood. Two hallmark studies 1,2 have demonstrated that osteocytes are the major source of the osteoclastogenesis-stimulating factor RANKL and thus key components in the control of bone mass. This property goes beyond their previously suggested function of sensing the mechanical loading of bone. 3 PTH 4 and sclerostin 5 have been reported to stimulate osteocytes to support osteoclastogenesis via a RANKL-dependent pathway. High-mobility group box 1 (HMGB1), which is chemotactic to osteoclasts, is enhanced in apoptotic MLO-Y4 cells; 6 however, there is little information on other factors controlling osteoclast activity by osteoblast-derived osteocytes.The nuclear protein menin encoded by the gene Men1 (multiple endocrine neoplasia type 1) has recently been suggested to control bone mass at higher age through its influence on the differentiation and function of osteoblasts. 7 In vitro studies in cell lines and primary osteoblasts derived from 6-month-old mice showed that menin promotes early osteoblast differentiation in committed cells. Knockdown of Men1 with antisense oligonucleotides lead to enhanced BMP2 signaling [8][9][10] and to facilitated inhibition of the late stage of osteoblast differentiation by potentiating TGFβ-dependent Smad3 signaling. 11,12 Accordingly, reduced bone mass and reduced bone formation due to decreased osteoblast proliferation and differentiation and enhanced apoptosis have been reported using aged osteocalcin-cre mice crossed to Men1 floxed mice that leads to a conditional deletion of menin in the late stage of differentiate...

C5aR1 interacts with TLR2 in osteoblasts and stimulates the osteoclast‐inducing chemokine CXCL10

“…We analysed gene expression patterns and intracellular signalling pathways upon C5aR1 activation and found a strong modulation of genes involved for example in the mitogen‐activated protein kinase (MAPK) and insulin pathways. Furthermore, we demonstrated that C5aR1 and TLR2 interact in osteoblasts, resulting in upregulation of the immune cell chemoattractant C‐X‐C motif chemokine 10 (CXCL10), which can induce osteoclastic bone resorption . These results suggest that complement‐activated osteoblasts are able to modulate the inflammatory milieu during inflammatory bone diseases in concert with osteoclasts and immune cells.…”

“…CXCL10 attracts mainly neutrophils, macrophages and cytotoxic T cells and is generated by many cell types, including osteoblasts, which express and secrete CXCL10 in response to bacterial challenges, namely by Salmonella and P. gingivalis . Moreover, CXCL10 was found to act osteoclastogenic, either directly or indirectly by inducing RANKL expression by T cells and osteoblasts . Furthermore, CXCL10 induction via RANKL was found to crucially contribute to bone destruction at inflamed joint areas during rheumatoid arthritis .…”

“…61 Moreover, CXCL10 was found to act osteoclastogenic, either directly or indirectly by inducing RANKL expression by T cells and osteoblasts. 20,21 Furthermore, CXCL10 induction via RANKL was found to crucially contribute to bone destruction at inflamed joint areas during rheumatoid arthritis. 22,62 Here, we demonstrated that CXCL10 expression and secretion by osteoblasts was significantly enhanced under coactivation of C5aR1 and TLR2 compared to isolated receptor stimulation.…”

“…Furthermore, we demonstrated that C5aR1 and TLR2 interact in osteoblasts, resulting in upregulation of the immune cell chemoattractant C-X-C motif chemokine 10 (CXCL10), which can induce osteoclastic bone resorption. [20][21][22] These results suggest that complement-activated osteoblasts are able to modulate the inflammatory milieu during inflammatory bone diseases in concert with osteoclasts and immune cells.…”

The future: genetics advances in MEN1 therapeutic approaches and management strategies

“…Also, knockout of Men1 in the liver (using albumin promoter/ALB-Cre), a tissue not targeted in MEN1, does not lead to tumors in the liver despite the complete loss of Men1 (Scacheri, et al 2004). Other mouse models with knockout of Men1 in pancreatic endocrine cell lineages, bone cells, and intestinal cells have also helped to gain insight into the physiological actions of menin (Bonnavion, et al 2015; Kanazawa, et al 2015; Liu, et al 2017; Sundaresan, et al 2016; Veniaminova, et al 2012). These elegant studies in genetically engineered mouse models show that menin may function as a cell-type-specific tumor suppressor, and that its action is only required in MEN1-associated target tissues to prevent tumorigenesis.…”