Osteocytes, not Osteoblasts or Lining Cells, are the Main Source of the RANKL Required for Osteoclast Formation in Remodeling Bone

Abstract

The cytokine receptor activator of nuclear factor kappa B ligand (RANKL), encoded by the Tnfsf11 gene, is essential for osteoclastogenesis and previous studies have shown that deletion of the Tnfsf11 gene using a Dmp1-Cre transgene reduces osteoclast formation in cancellous bone by more than 70%. However, the Dmp1-Cre transgene used in those studies leads to recombination in osteocytes, osteoblasts, and lining cells making it unclear whether one or more of these cell types produce the RANKL required for osteoclast formation in cancellous bone. Because osteoblasts, osteocytes, and lining cells have distinct locations and functions, distinguishing which of these cell types are sources of RANKL is essential for understanding the orchestration of bone remodeling. To distinguish between these possibilities, we have now created transgenic mice expressing the Cre recombinase under the control of regulatory elements of the Sost gene, which is expressed in osteocytes but not osteoblasts or lining cells in murine bone. Activity of the Sost-Cre transgene in osteocytes, but not osteoblast or lining cells, was confirmed by crossing Sost-Cre transgenic mice with tdTomato and R26R Cre-reporter mice, which express tdTomato fluorescent protein or LacZ, respectively, only in cells expressing the Cre recombinase or their descendants. Deletion of the Tnfsf11 gene in Sost-Cre mice led to a threefold decrease in osteoclast number in cancellous bone and increased cancellous bone mass, mimicking the skeletal phenotype of mice in which the Tnfsf11 gene was deleted using the Dmp1-Cre transgene. These results demonstrate that osteocytes, not osteoblasts or lining cells, are the main source of the RANKL required for osteoclast formation in remodeling cancellous bone.

RANKL (Receptor Activator of NFκB Ligand) Produced by Osteocytes Is Required for the Increase in B Cells and Bone Loss Caused by Estrogen Deficiency in Mice

“…The Dmp1-Cre transgene used in this study also causes recombination in osteoblasts (6,30). We have shown previously that deletion of Tnfsf11 using a Sost-Cre transgene that does not delete in osteoblasts or lining cells leads to the same reduction in osteoclasts as deletion using the Dmp1-Cre transgene (30).…”

“…We have shown previously that deletion of Tnfsf11 using a Sost-Cre transgene that does not delete in osteoblasts or lining cells leads to the same reduction in osteoclasts as deletion using the Dmp1-Cre transgene (30). Based on this, we concluded that osteocytes, not osteoblasts or lining cells, are the major source of RANKL involved in oste- Osteocyte RANKL, B Cells, Estrogen, and Bone Loss NOVEMBER 25, 2016 • VOLUME 291 • NUMBER 48 oclast formation.…”

Osteocyte-specific WNT1 regulates osteoblast function during bone homeostasis

“…It was originally believed that osteocytes are quiescent cells in bone matrix, but numerous studies now show that osteocytes play essential roles in bone homeostasis by regulating osteoblasts and osteoclasts (31). For example, osteocytes can regulate osteoclastogenesis by expressing RANKL and its decoy receptor osteoprotegerin (7,(32)(33)(34)(35)(36). Osteocytes can also regulate osteoblast differentiation by secreting sclerostin (SOST), an inhibitor of Wnt signaling (37)(38)(39).…”

Papel do sistema RANKL/RANK/OPG como regulador-chave da remodelação óssea durante a movimentação ortodôntica

“…RANKL (ligante de RANK) é produzido como fator local por células do ligamento periodontal (osteoblastos e fibroblastos) em resposta a intensas forças ortodônticas. Recentemente, surgiram evidências de que os osteócitos são importante fonte de RANKL para a formação de osso esponjoso (XIONG et al, 2015). O receptor de membrana RANK (receptor activator of Nfk-B) é encontrado em células precursoras de osteoclastos.…”

“…Estudos demonstram que mudanças na tensão mecânica, provocada pelas forças ortodônticas, alteram os níveis de OPG e RANKL no ligamento periodontal (KANZAKI et al, 2006;NISHIJIMA et al, 2006;TADDEI et al, 2012) e tecido ósseo (XIONG et al, 2015). A expressão diferenciada desses mediadores, nos sítios de compressão e tensão, responderia por padrões específicos de migração celular e formação/reabsorção de tecido ósseo (LI et al, 2015;TADDEI et al, 2012;ZHANG et al, 2016;ZHOU et al, 2011).…”