Bone biology
Abstract
Nearly every extracellular ligand that has been found to play a role in regulating bone biology acts, at least in part, through MAPK pathways. Nevertheless, much remains to be learned about the contribution of MAPKs to osteoblast biology in vivo. Here we report that the p38 MAPK pathway is required for normal skeletogenesis in mice, as mice with deletion of any of the MAPK pathway member–encoding genes MAPK kinase 3 (Mkk3), Mkk6, p38a, or p38b displayed profoundly reduced bone mass secondary to defective osteoblast differentiation. Among the MAPK kinase kinase (MAP3K) family, we identified TGF-β–activated kinase 1 (TAK1; also known as MAP3K7) as the critical activator upstream of p38 in osteoblasts. Osteoblast-specific deletion of Tak1 resulted in clavicular hypoplasia and delayed fontanelle fusion, a phenotype similar to the cleidocranial dysplasia observed in humans haploinsufficient for the transcription factor runt-related transcription factor 2 (Runx2). Mechanistic analysis revealed that the TAK1–MKK3/6–p38 MAPK axis phosphorylated Runx2, promoting its association with the coactivator CREB-binding protein (CBP), which was required to regulate osteoblast genetic programs. These findings reveal an in vivo function for p38β and establish that MAPK signaling is essential for bone formation in vivo. These results also suggest that selective p38β agonists may represent attractive therapeutic agents to prevent bone loss associated with osteoporosis and aging.
Authors
Matthew B. Greenblatt, Jae-Hyuck Shim, Weiguo Zou, Despina Sitara, Michelle Schweitzer, Dorothy Hu, Sutada Lotinun, Yasuyo Sano, Roland Baron, Jin Mo Park, Simon Arthur, Min Xie, Michael D. Schneider, Bo Zhai, Steven Gygi, Roger Davis, Laurie H. Glimcher
Abstract
The majority of human skeletal dysplasias are caused by dysregulation of growth plate homeostasis. As TGF-β signaling is a critical determinant of growth plate homeostasis, skeletal dysplasias are often associated with dysregulation of this pathway. The context-dependent action of TFG-β signaling is tightly controlled by numerous mechanisms at the extracellular level and downstream of ligand-receptor interactions. However, TGF-β is synthesized as an inactive precursor that is cleaved to become mature in the Golgi apparatus, and the regulation of this posttranslational intracellular processing and trafficking is much less defined. Here, we report that a cysteine-rich protein, E-selectin ligand–1 (ESL-1), acts as a negative regulator of TGF-β production by binding TGF-β precursors in the Golgi apparatus in a cell-autonomous fashion, inhibiting their maturation. Furthermore, ESL-1 inhibited the processing of proTGF-β by a furin-like protease, leading to reduced secretion of mature TGF-β by primary mouse chondrocytes and HEK293 cells. In vivo loss of Esl1 in mice led to increased TGF-β/SMAD signaling in the growth plate that was associated with reduced chondrocyte proliferation and delayed terminal differentiation. Gain-of-function and rescue studies of the Xenopus ESL-1 ortholog in the context of early embryogenesis showed that this regulation of TGF-β/Nodal signaling was evolutionarily conserved. This study identifies what we believe to be a novel intracellular mechanism for regulating TGF-β during skeletal development and homeostasis.
Authors
Tao Yang, Roberto Mendoza-Londono, Huifang Lu, Jianning Tao, Kaiyi Li, Bettina Keller, Ming Ming Jiang, Rina Shah, Yuqing Chen, Terry K. Bertin, Feyza Engin, Branka Dabovic, Daniel B. Rifkin, John Hicks, Milan Jamrich, Arthur L. Beaudet, Brendan Lee
Abstract
The modeling and remodeling of bone requires activation and polarization of osteoclasts, achieved by reorganization of the cytoskeleton. Members of the Rho subfamily of small GTPases, including Cdc42, are known regulators of cytoskeletal components, but the role of these proteins in bone physiology and pathophysiology remains unclear. Here, we examined loss-of-function mice in which Cdc42 was selectively ablated in differentiated osteoclasts and gain-of-function animals wherein Cdc42Gap, a protein that inactivates the small GTPase, was deleted globally. Cdc42 loss-of-function mice were osteopetrotic and resistant to ovariectomy-induced bone loss, while gain-of-function animals were osteoporotic. Isolated Cdc42-deficient osteoclasts displayed suppressed bone resorption, while osteoclasts with increased Cdc42 activity had enhanced resorptive capacity. We further demonstrated that Cdc42 modulated M-CSF–stimulated cyclin D expression and phosphorylation of Rb and induced caspase 3 and Bim, thus contributing to osteoclast proliferation and apoptosis rates. Furthermore, Cdc42 was required for multiple M-CSF– and RANKL-induced osteoclastogenic signals including activation and expression of the differentiation factors MITF and NFATc1 and was a component of the Par3/Par6/atypical PKC polarization complex in osteoclasts. These data suggest that Cdc42 regulates osteoclast formation and function and may represent a promising therapeutic target for prevention of pathological bone loss.
Authors
Yuji Ito, Steven L. Teitelbaum, Wei Zou, Yi Zheng, James F. Johnson, Jean Chappel, F. Patrick Ross, Haibo Zhao
Abstract
Authors
Hui Li, Hui Xie, Wei Liu, Rong Hu, Bi Huang, Yan-Fei Tan, Kang Xu, Zhi-Feng Sheng, Hou-De Zhou, Xian-Ping Wu, Xiang-Hang Luo
Abstract
Effective osteoporosis therapy requires agents that increase the amount and/or quality of bone. Any modification of osteoclast-mediated bone resorption by disease or drug treatment, however, elicits a parallel change in osteoblast-mediated bone formation because the processes are tightly coupled. Anabolic approaches now focus on uncoupling osteoblast action from osteoclast formation, for example, by inhibiting sclerostin, an inhibitor of bone formation that does not influence osteoclast differentiation. Here, we report that oncostatin M (OSM) is produced by osteoblasts and osteocytes in mouse bone and that it has distinct effects when acting through 2 different receptors, OSM receptor (OSMR) and leukemia inhibitory factor receptor (LIFR). Specifically, mouse OSM (mOSM) inhibited sclerostin production in a stromal cell line and in primary murine osteoblast cultures by acting through LIFR. In contrast, when acting through OSMR, mOSM stimulated RANKL production and osteoclast formation. A key role for OSMR in bone turnover was confirmed by the osteopetrotic phenotype of mice lacking OSMR. Furthermore, in contrast to the accepted model, in which mOSM acts only through OSMR, mOSM inhibited sclerostin expression in Osmr–/– osteoblasts and enhanced bone formation in vivo. These data reveal what we believe to be a novel pathway by which bone formation can be stimulated independently of bone resorption and provide new insights into OSMR and LIFR signaling that are relevant to other medical conditions, including cardiovascular and neurodegenerative diseases and cancer.
Authors
Emma C. Walker, Narelle E. McGregor, Ingrid J. Poulton, Melissa Solano, Sueli Pompolo, Tania J. Fernandes, Matthew J. Constable, Geoff C. Nicholson, Jian-Guo Zhang, Nicos A. Nicola, Matthew T. Gillespie, T. John Martin, Natalie A. Sims
Abstract
MicroRNAs (miRNAs) interfere with translation of specific target mRNAs and are thought to thereby regulate many cellular processes. Recent studies have suggested that miRNAs might play a role in osteoblast differentiation and bone formation. Here, we identify a new miRNA (miR-2861) in primary mouse osteoblasts that promotes osteoblast differentiation by repressing histone deacetylase 5 (HDAC5) expression at the post-transcriptional level. miR-2861 was found to be transcribed in ST2 stromal cells during bone morphogenetic protein 2–induced (BMP2-induced) osteogenesis, and overexpression of miR-2861 enhanced BMP2-induced osteoblastogenesis, whereas inhibition of miR-2861 expression attenuated it. HDAC5, an enhancer of runt-related transcription factor 2 (Runx2) degradation, was confirmed to be a target of miR-2861. In vivo silencing of miR-2861 in mice reduced Runx2 protein expression, inhibited bone formation, and decreased bone mass. Importantly, miR-2861 was found to be conserved in humans, and a homozygous mutation in pre–miR-2861 that blocked expression of miR-2861 was shown to cause primary osteoporosis in 2 related adolescents. Consistent with the mouse data, HDAC5 levels were increased and Runx2 levels decreased in bone samples from the 2 affected individuals. Thus, our studies show that miR-2861 plays an important physiological role in osteoblast differentiation and contributes to osteoporosis via its effect on osteoblasts.
Authors
Hui Li, Hui Xie, Wei Liu, Rong Hu, Bi Huang, Yan-Fei Tan, Er-Yuan Liao, Kang Xu, Zhi-Feng Sheng, Hou-De Zhou, Xian-Ping Wu, Xiang-Hang Luo
Abstract
Patients with classic fibrodysplasia ossificans progressiva, a disorder characterized by extensive extraskeletal endochondral bone formation, share a recurrent mutation (R206H) within the glycine/serine-rich domain of ACVR1/ALK2, a bone morphogenetic protein type I receptor. Through a series of in vitro assays using several mammalian cell lines and chick limb bud micromass cultures, we determined that mutant R206H ACVR1 activated BMP signaling in the absence of BMP ligand and mediated BMP-independent chondrogenesis that was enhanced by BMP. We further investigated the interaction of mutant R206H ACVR1 with FKBP1A, a glycine/serine domain–binding protein that prevents leaky BMP type I receptor activation in the absence of ligand. The mutant protein exhibited reduced binding to FKBP1A in COS-7 simian kidney cell line assays, suggesting that increased BMP pathway activity in COS-7 cells with R206H ACVR1 is due, at least in part, to decreased binding of this inhibitory factor. Consistent with these findings, in vivo analyses of zebrafish embryos showed BMP-independent hyperactivation of BMP signaling in response to the R206H mutant, resulting in increased embryonic ventralization. These data support the conclusion that the mutant R206H ACVR1 receptor in FOP patients is an activating mutation that induces BMP signaling in a BMP-independent and BMP-responsive manner to promote chondrogenesis, consistent with the ectopic endochondral bone formation in these patients.
Authors
Qi Shen, Shawn C. Little, Meiqi Xu, Julia Haupt, Cindy Ast, Takenobu Katagiri, Stefan Mundlos, Petra Seemann, Frederick S. Kaplan, Mary C. Mullins, Eileen M. Shore
Abstract
The B cell lymphoma 2 (Bcl-2) family member Bcl-xL has a well-characterized antiapoptotic function in lymphoid cells. However, its functions in other cells — including osteoclasts, which are of hematopoietic origin — and other cellular processes remain unknown. Here we report an unexpected function of Bcl-xL in attenuating the bone-resorbing activity of osteoclasts in mice. To investigate the role of Bcl-xL in osteoclasts, we generated mice with osteoclast-specific conditional deletion of Bcl-x (referred to herein as Bcl-x cKO mice) by mating Bcl-xfl/fl mice with mice in which the gene encoding the Cre recombinase has been knocked into the cathepsin K locus and specifically expressed in mature osteoclasts. Although the Bcl-x cKO mice grew normally with no apparent morphological abnormalities, they developed substantial osteopenia at 1 year of age, which was caused by increased bone resorption. Bcl-x deficiency increased the bone-resorbing activity of osteoclasts despite their high susceptibility to apoptosis, whereas Bcl-xL overexpression produced the opposite effect. In addition, Bcl-x cKO osteoclasts displayed increased c-Src activity, which was linked to increased levels of vitronectin and fibronectin expression. These results suggest that Bcl-xL attenuates osteoclastic bone-resorbing activity through the decreased production of ECM proteins, such as vitronectin and fibronectin, and thus provide evidence for what we believe to be a novel cellular function of Bcl-xL.
Authors
Mitsuyasu Iwasawa, Tsuyoshi Miyazaki, Yuichi Nagase, Toru Akiyama, Yuho Kadono, Masaki Nakamura, Yasushi Oshima, Tetsuro Yasui, Takumi Matsumoto, Takashi Nakamura, Shigeaki Kato, Lothar Hennighausen, Kozo Nakamura, Sakae Tanaka
Abstract
Regulation of the formation and function of bone-resorbing osteoclasts (OCs) is a key to understanding the pathogenesis of skeletal disorders. Gene-targeting studies have shown that the RANK signaling pathway plays a critical role in OC differentiation and function. Although pharmaceutical blockade of RANK may be a viable strategy for preventing bone destruction, RANK is implicated in multiple biological processes. Recently, a cytoplasmic motif of RANK was identified that may be specifically involved in OC differentiation. Here, we developed a cell-permeable inhibitor termed the RANK receptor inhibitor (RRI), which targets this motif. The RRI peptide blocked RANKL-induced OC formation from murine bone marrow–derived macrophages. Furthermore, RRI inhibited the resorptive function of OCs and induced OC apoptosis. Treatment with the peptide impaired downstream signaling of RANK linked to Vav3, Rac1, and Cdc42 and resulted in disruptions of the actin cytoskeleton in differentiated OCs. In addition, RRI blocked inflammation-induced bone destruction and protected against ovariectomy-induced bone loss in mice. These data may be useful in the development of selective therapeutic agents for the treatment of osteoporosis and other bone diseases.
Authors
Hyunsoo Kim, Han Kyoung Choi, Ji Hye Shin, Kyung Hee Kim, Ji Young Huh, Seung Ah Lee, Chang-Yong Ko, Han-Sung Kim, Hong-In Shin, Hwa Jeong Lee, Daewon Jeong, Nacksung Kim, Yongwon Choi, Soo Young Lee
Abstract
Authors
Christopher B. Little, Clare T. Meeker, Suzanne B. Golub, Kate E. Lawlor, Pamela J. Farmer, Susan M. Smith, Amanda J. Fosang
Copyright © 2025 American Society for Clinical Investigation
ISSN: 0021-9738 (print), 1558-8238 (online)