Nuclei were stained with hematoxylin

Nuclei were stained with hematoxylin. an increase in trabecular BV/TV and thickness of cKO adult bones, whereas osteoclast number, bone formation rate, and mineral apposition rate were decreased. Expression levels of bone formation markers (Runx2andBsp), resorption markers (Mmp9,Ctsk, andTracp), andRanklwere decreased, andOpgwas increased in Cytochrome c – pigeon (88-104) adult bones, resulting in a reduction in the ratio ofRankltoosteoprotegerin(Opg). The reduction in osteoclastogenesis through the RANKLOPG pathway was also observed in weanling stages and reproduced in newborn Cytochrome c – pigeon (88-104) calvaria culture. These results suggest thatBmpr1acKO increased endogenous bone mass primarily in trabecular bone with decreased osteoclastogenesis through the RANKLOPG pathway. We conclude that BMPRIA signaling in osteoblasts affects both bone formation and resorption to reduce endogenous bone mass in vivo. Key words:bone morphogenetic protein, knockout, modeling and remodeling, Cytochrome c – pigeon (88-104) Cytochrome c – pigeon (88-104) osteoblasts, rodent == INTRODUCTION == Bone morphogenetic proteins(BMPs) are members of the TGF- superfamily and were originally discovered as ectopic bone inducers in smooth cells.(1) The osteogenic function of BMPs has been extensively examined, mainly using osteoblasts in tradition.(2) The FDA approved some BMPs (BMP2 and BMP7) for clinical use in long bone open fractures, nonunions fractures, and spinal fusion. However, despite significant evidence of their potential for bone regeneration in animal and preclinical studies, the current medical data assisting their effectiveness are not powerful.(3,4) This may be because in part of a lack of understanding of the variable effects BMPs have in vivo about different cell types including chondrocytes, osteoblasts, and osteocytes. BMP2, BMP4, and their potent receptor BMP receptor type IA (BMPRIA or ALK3) are abundantly indicated in the skeleton; however, standard knockout (KO) mice for these genes cause early embryonic lethality.(57) We previously rescued the lethality of loss ofBmpr1ausing the Cre-loxP system under the control of an osteocalcin promoter(8) and found that bone mass (BV/TV) evaluated by bone histomorphometry was reduced in the mutant mice at 3 mo but was increased at 10 mo. This evidence suggests that BMPs have diverse effects on bone mass, bone formation, and homeostasis in an age-dependent manner. To understand age-dependent functions of BMP signaling in the skeleton, it is important to disrupt BMP signaling at different age groups in an osteoblast-specific manner. Here, we used a tamoxifen (TM)-inducible Cre-loxP system under the control of a 3.2-kb type I collagen promoter (Col1-CreERTMmice), which is definitely activated in osteoblasts, odontoblasts, and tendon fibroblasts,(9) and disrupted BMP Cytochrome c – pigeon (88-104) signaling through BMPRIA in bone. This system allows us to control the onset of the disruption ofBmpr1ain osteoblasts at any age by administration of TM. Because maximum bone mass in the mouse is definitely accomplished after 20 wk,(10) we 1st focused on bone remodeling and analyzed 22-wk mice, starting TM administration at 8 wk. Moreover, to identify age-dependent effects of BMP signaling, we also analyzed bone modeling using weanling mice starting TM administration at postnatal day time 2 (P2). In this study, we found that bone mass was improved by loss of BMP signaling in osteoblasts through BMPRIA during both phases. Osteoclastogenesis was reduced through the RANKLosteoprotegerin (OPG) pathway, even though bone formation markers were reduced or unchanged in the MAPT mutant mice, resulting in a net increase in bone mass. This evidence suggests that BMP signaling in osteoblasts restrains endogenous bone mass, which is definitely unpredicted and contrary to the current understanding of BMPs as bone inducers.(1) == MATERIALS AND METHODS == == Mice and TM administration == Mice expressing the TM-inducible Cre fusion protein Cre-ERTM(11,12) under the control of a 3.2-kb mouse procollagen1(I)promoter(9) (Col1-CreERTM) were generated by pronuclear injection and crossed with floxedBmpr1amice.(13)ROSA26Cre reporter (R26R) mice were kindly provided by Dr Philippe Soriano.(14)Col1-CreERTMmice were crossed with floxedBmpr1amice(13) to generate mice genotyped asCol1-CreERTM+:Bmpr1afx/fx andCol1-CreERTM:Bmpr1afx/fx. TM (T5648; Sigma) was dissolved in a small volume of ethanol, diluted with corn oil at a concentration of 10 mg/ml, and stored at 20C until use. After TM administration, Cre recombination was induced inBmpr1acKO mice (Col1-CreERTM+:Bmpr1afx/fx).