title: Targeted disruption of BMP signaling through type IA receptor (BMPR1A) in osteocyte suppresses SOST and RANKL, leading to dramatic increase in bone mass, bone mineral density and mechanical strength.
abstract: Recent studies suggest a critical role of osteocytes in controlling skeletal development and bone remodeling although the molecular mechanism is largely unknown. This study investigated BMP signaling in osteocytes by disrupting Bmpr1a under the Dmp1-promoter. The conditional knockout (cKO) mice displayed a striking osteosclerotic phenotype with increased trabecular bone volume, thickness, number, and mineral density as assessed by X-ray and micro-CT. The bone histomorphometry, H&E, and TRAP staining revealed a dramatic increase in trabecular and cortical bone masses but a sharp reduction in osteoclast number. Moreover, there was an increase in BrdU positive osteocytes (2-5-fold) and osteoid volume (~4-fold) but a decrease in the bone formation rate (~85%) in the cKO bones, indicating a defective mineralization. The SEM analysis revealed poorly formed osteocytes: a sharp increase in cell numbers, a great reduction in cell dendrites, and a remarkable change in the cell distribution pattern. Molecular studies demonstrated a significant decrease in the Sost mRNA levels in bone (>95%), and the SOST protein levels in serum (~85%) and bone matrices. There was a significant increase in the β-catenin (>3-fold) mRNA levels as well as its target genes Tcf1 (>6-fold) and Tcf3 (~2-fold) in the cKO bones. We also showed a significant decrease in the RANKL levels of serum proteins (~65%) and bone mRNA (~57%), and a significant increase in the Opg mRNA levels (>20-fold) together with a significant reduction in the Rankl/Opg ratio (>95%), which are responsible for a sharp reduction in the cKO osteoclasts. The values of mechanical strength were higher in cKO femora (i.e. max force, displacement, and work failure). These results suggest that loss of BMP signaling specifically in osteocytes dramatically increases bone mass presumably through simultaneous inhibition of RANKL and SOST, leading to osteoclast inhibition and Wnt activation together. Finally, a working hypothesis is proposed to explain how BMPR1A controls bone remodeling by inhibiting cell proliferation and stimulating differentiation. It is reported that RANKL and SOST are abundantly expressed by osteocytes. Thus, BMP signaling through BMPR1A plays important roles in osteocytes.
authors.affiliation: Texas Scottish Rite Hospital for Children, Dallas, TX 75219, USA; Orthopaedic Surgery, University of Texas Southwestern Medical Center, Dallas, TX 75390-8883, USA; Sports Medicine, Tenri University, Tenri, Nara 632-0071, Japan. Electronic address: nkamiya1@sta.tenri-u.ac.jp.
authors.forename: Nobuhiro
authors.initials: N
authors.lastname: Kamiya
authors.affiliation: Department of Biomedical Sciences, Texas A&M University College of Dentistry, Dallas, TX, USA; Department of Prosthodontics, Ninth People's Hospital, Shanghai Jiao Tong University, School of Medicine, Shanghai 200011, China.
authors.forename: Lin
authors.initials: L
authors.lastname: Shuxian
authors.affiliation: Texas Scottish Rite Hospital for Children, Dallas, TX 75219, USA.
authors.forename: Ryosuke
authors.initials: R
authors.lastname: Yamaguchi
authors.affiliation: Texas Scottish Rite Hospital for Children, Dallas, TX 75219, USA.
authors.forename: Matthew
authors.initials: M
authors.lastname: Phipps
authors.affiliation: Texas Scottish Rite Hospital for Children, Dallas, TX 75219, USA.
authors.forename: Olumide
authors.initials: O
authors.lastname: Aruwajoye
authors.affiliation: Texas Scottish Rite Hospital for Children, Dallas, TX 75219, USA; Orthopaedic Surgery, University of Texas Southwestern Medical Center, Dallas, TX 75390-8883, USA.
authors.forename: Naga Suresh
authors.initials: NS
authors.lastname: Adapala
authors.affiliation: Department of Biomedical Sciences, Texas A&M University College of Dentistry, Dallas, TX, USA.
authors.forename: Hui
authors.initials: H
authors.lastname: Yuan
authors.affiliation: Texas Scottish Rite Hospital for Children, Dallas, TX 75219, USA; Orthopaedic Surgery, University of Texas Southwestern Medical Center, Dallas, TX 75390-8883, USA.
authors.forename: Harry K W
authors.initials: HK
authors.lastname: Kim
authors.affiliation: Department of Biomedical Sciences, Texas A&M University College of Dentistry, Dallas, TX, USA. Electronic address: Jfeng@bcd.tamhsc.edu.
authors.forename: Jian Q
authors.initials: JQ
authors.lastname: Feng
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D001696:Biomechanical Phenomena; D015519:Bone Density; D001853:Bone Marrow / Q000378:metabolism; D052005:Bone Morphogenetic Protein Receptors, Type I / Q000378:metabolism*; D019485:Bone Morphogenetic Proteins / Q000378:metabolism*; D001842:Bone and Bones / Q000033:anatomy & histology* / Q000000981:diagnostic imaging / Q000502:physiology*; D049109:Cell Proliferation; D006023:Glycoproteins / Q000378:metabolism*; D036341:Intercellular Signaling Peptides and Proteins; D018345:Mice, Knockout; D008954:Models, Biological; D009929:Organ Size; D010011:Osteocytes / Q000378:metabolism / Q000648:ultrastructure; D053245:RANK Ligand / Q000378:metabolism*; D015398:Signal Transduction; D055114:X-Ray Microtomography; D051176:beta Catenin / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D019485:Bone Morphogenetic Proteins; D006023:Glycoproteins; D036341:Intercellular Signaling Peptides and Proteins; D053245:RANK Ligand; C486030:Sost protein, mouse; D051176:beta Catenin; C498212:Bmpr1a protein, mouse; D052005:Bone Morphogenetic Protein Receptors, Type I
keywords: BMPR1A; Osteocyte; Proliferation; RANKL; SOST/sclerostin; Wnt/β-catenin