The discovery of the RANKL/RANK/OPG system in the mid 1990s for the regulation of bone resorption has led to major advances in our understanding of how bone modeling and remodeling are regulated. It had been known for many years before this discovery that osteoblastic stromal cells regulated osteoclast formation, but it had not been anticipated that they would do this through expression of members of the TNF superfamily: receptor activator of NF-κB ligand (RANKL) and osteoprotegerin (OPG), or that these cytokines and signaling through receptor activator of NF-κB (RANK) would have extensive functions beyond regulation of bone remodeling. RANKL/RANK signaling regulates osteoclast formation, activation and survival in normal bone modeling and remodeling and in a variety of pathologic conditions characterized by increased bone turnover. OPG protects bone from excessive resorption by binding to RANKL and preventing it from binding to RANK. Thus, the relative concentration of RANKL and OPG in bone is a major determinant of bone mass and strength. Here, we review our current understanding of the role of the RANKL/RANK/ OPG system in bone modeling and remodeling. KeywordsBone resorption; osteoclasts; RANKL; RANK; OPG Normal bone modelingWith the exception of the bones of the calvaria, all bones in the mammalian skeleton are preformed in cartilage moulds from mesenchymal progenitors, which under appropriate stimuli also have the potential to differentiate into a variety of tissue types, including fibrous tissue, fat and muscle. Chondrocytes proliferate near the ends of the cartilage moulds to drive their longitudinal growth, while others in the centers of undergo hypertrophic differentiation. The hypertrophic chondrocytes at the periphery of the centers of these moulds are invaded by blood vessels and undergo apoptosis. Some of this hypertrophic cartilage survives as thin islands of cartilage in the centers of ossification in growing bones. Osteoblasts, which differentiate from progenitors in a collar of connective tissue around the middle of the bones where vascular invasion takes place, follow the endothelial cells and lay down bone matrix on the surfaces of these islands of cartilage to form bone struts or trabeculae (1). Osteoclast precursors (OCPs), derived from progenitors in the spleen and liver are attracted from blood in the invading blood vessels close to newly formed bone trabeculae. These osteoclast precursors fuse with one

NF-B is a family of related, dimeric transcription factors that are readily activated in cells by signals associated with stress or pathogens. These factors are critical to host defense, as demonstrated previously with mice deficient in individual subunits of NF-B. We have generated mice deficient in both the p50 and p52 subunits of NF-B to reveal critical functions that may be shared by these two highly homologous proteins. We now demonstrate that unlike the respective single knockout mice, the p50/p52 double knockout mice fail to generate mature osteoclasts and B cells, apparently because of defects that track with these lineages in adoptive transfer experiments. Furthermore, these mice present markedly impaired thymic and splenic architectures and impaired macrophage functions. The blocks in osteoclast and B-cell maturation were unexpected. Lack of mature osteoclasts caused severe osteopetrosis, a family of diseases characterized by impaired osteoclastic bone resorption. These findings now establish critical roles for NF-B in development and expand its repertoire of roles in the physiology of differentiated hematopoietic cells.

The discovery of the receptor activator of nuclear factor-κB ligand (RANKL)/RANK/osteoprotegerin (OPG) system and its role in the regulation of bone resorption exemplifies how both serendipity and a logic-based approach can identify factors that regulate cell function. Before this discovery in the mid to late 1990s, it had long been recognized that osteoclast formation was regulated by factors expressed by osteoblast/stromal cells, but it had not been anticipated that members of the tumor necrosis factor superfamily of ligands and receptors would be involved or that the factors involved would have extensive functions beyond bone remodeling. RANKL/RANK signaling regulates the formation of multinucleated osteoclasts from their precursors as well as their activation and survival in normal bone remodeling and in a variety of pathologic conditions. OPG protects the skeleton from excessive bone resorption by binding to RANKL and preventing it from binding to its receptor, RANK. Thus, RANKL/OPG ratio is an important determinant of bone mass and skeletal integrity. Genetic studies in mice indicate that RANKL/RANK signaling is also required for lymph node formation and mammary gland lactational hyperplasia, and that OPG also protects arteries from medial calcification. Thus, these tumor necrosis factor superfamily members have important functions outside bone. Although our understanding of the mechanisms whereby they regulate osteoclast formation has advanced rapidly during the past 10 years, many questions remain about their roles in health and disease. Here we review our current understanding of the role of the RANKL/RANK/OPG system in bone and other tissues. IntroductionBone serves multiple functions in vertebrates, including support for muscles, protection of vital organs and hematopoietic marrow, and storage and release of vital ions, such as calcium. Unlike other durable structures, such as teeth, tendons, and cartilage, bone is continuously renewed by the process of bone remodeling in which pockets or trenches of bone are removed from the surfaces of trabecular and cortical bone by osteoclasts and subsequently replaced by new bone laid down by osteoblasts. There are at least one million of these microscopic remodeling foci at any one time in the adult skeleton, and the main function of this process is considered to be removal of effete or worn out parts of bones that have become damaged as part of normal wear and tear. It is a highly regulated process, but the molecular mechanisms that control its initiation, progression, and cessation at any given site remain poorly understood.Bone remodeling becomes perturbed in a variety of pathologic conditions that affect the skeleton, including postmenopausal osteoporosis and rheumatoid arthritis, in which there is local and/or systemic alteration in the levels of hormones or proinflammatory cytokines that are known to stimulate or inhibit bone resorption in vitro and in vivo. It has been recognized since the early 1980s, when Rodan and Martin [1] postulated that ost...

By using a cre-lox conditional knockout strategy, we report here the generation of androgen receptor knockout (ARKO) mice. Phenotype analysis shows that ARKO male mice have a female-like appearance and body weight. Their testes are 80% smaller and serum testosterone concentrations are lower than in wild-type (wt) mice. Spermatogenesis is arrested at pachytene spermatocytes. The number and size of adipocytes are also different between the wt and ARKO mice. Cancellous bone volumes of ARKO male mice are reduced compared with wt littermates. In addition, we found the average number of pups per litter in homologous and heterozygous ARKO female mice is lower than in wt female mice, suggesting potential defects in female fertility and/or ovulation. The cre-lox ARKO mouse provides a much-needed in vivo animal model to study androgen functions in the selective androgen target tissues in female or male mice

Differentiation of mesenchymal stem cells into a particular lineage is tightly regulated, and malfunction of this regulation could lead to pathological consequences. Patients with osteoporosis have increased adipocyte accumulation, but the mechanisms involved remain to be defined. In this study, we aimed to investigate if microRNAs regulate mesenchymal progenitor cells and bone marrow stromal cell (BMSC) differentiation through modulation of Runx2, a key transcription factor for osteogenesis. We found that miR-204 and its homolog miR-211 were expressed in mesenchymal progenitor cell lines and BMSCs and their expression was induced during adipocyte differentiation, whereas Runx2 protein expression was suppressed. Retroviral overexpression of miR-204 or transfection of miR-204 oligo decreased Runx2 protein levels and miR-204 inhibition significantly elevated Runx2 protein levels, suggesting that miR-204 acts as an endogenous attenuator of Runx2 in mesenchymal progenitor cells and BMSCs. Mutations of putative miR-204 binding sites upregulated the Runx2 3 0 -UTR reporter activity, suggesting that miR-204/211 bind to Runx2 3 0 -UTR. Perturbation of miR-204 resulted in altered differentiation fate of mesenchymal progenitor cells and BMSCs: osteoblast differentiation was inhibited and adipocyte differentiation was promoted when miR-204 was overexpressed in these cells, whereasosteogenesis was upregulated and adipocyte formation was impaired when miR-204 was inhibited. Together, our data demonstrated that miR-204/211 act as important endogenous negative regulators of Runx2, which inhibit osteogenesis and promote adipogenesis of mesenchymal progenitor cells and BMSCs.

No abstract

Runx (Cbfa͞AML) transcription factors are critical for tissue-specific gene expression. A unique targeting signal in the C terminus directs Runx factors to discrete foci within the nucleus. Using Runx2͞CBFA1͞AML3 and its essential role in osteogenesis as a model, we investigated the fundamental importance of fidelity of subnuclear localization for tissue differentiating activity by deleting the intranuclear targeting signal via homologous recombination. Mice homozygous for the deletion (Runx2⌬C) do not form bone due to maturational arrest of osteoblasts. Heterozygotes do not develop clavicles, but are otherwise normal. These phenotypes are indistinguishable from those of the homozygous and heterozygous null mutants, indicating that the intranuclear targeting signal is a critical determinant for function. The expressed truncated Runx2⌬C protein enters the nucleus and retains normal DNA binding activity, but shows complete loss of intranuclear targeting. These results demonstrate that the multifunctional N-terminal region of the Runx2 protein is not sufficient for biological activity. We conclude that subnuclear localization of Runx factors in specific foci together with associated regulatory functions is essential for control of Runx-dependent genes involved in tissue differentiation during embryonic development.F actors that mediate transcription, processing of gene transcripts, DNA replication, and DNA repair are organized as discrete domains within the nucleus (1-13). The osteogenic and hematopoietic Runx transcription factors contain a unique and conserved amino acid motif in the C terminus designated the nuclear matrix targeting signal that directs Runx proteins to subnuclear foci where gene regulatory complexes are assembled in situ (14)(15)(16)(17)(18)(19)(20). Thus the Runx transcription factors provide a paradigm for pursuing mechanisms that coordinate the compartmentalization of regulatory proteins in intranuclear sites. In this study we directly addressed the fundamental question of whether subnuclear targeting and the associated regulatory activities of Runx factors are obligatory for in vivo function.The Runx (CBFA͞AML͞PEBP2␣) ʈ family of transcription factors are critical for cellular differentiation and organ development (21-23). Key studies have established that Runx2͞ CBFA1͞AML3 is required for osteoblast differentiation and in vivo bone formation (24-28). Ablation of the Runx2 gene in mice results in a complete absence of intramembranous and endochondral bone that is attributed to the maturational arrest of hypertrophic chondrocytes and osteoblasts (26,29,30). Haploinsufficiency of this gene results in the human syndrome cleidocranial dysplasia, a dominantly inherited developmental disorder of bone (24,25). A basis is thereby provided for examining the biological consequences resulting from absence of the Cterminal targeting function.Runx factors share multiple functional domains. The Nterminal runt homology DNA binding domain (RHD), which interacts with many coregulatory factors and chromatin m...

Osteoclasts are multinucleated cells formed mainly on bone surfaces in response to cytokines by fusion of bone marrow-derived myeloid lineage precursors that circulate in the blood. Major advances in understanding of the molecular mechanisms regulating osteoclast formation and functions have been made in the past 20 years since the discovery that their formation requires nuclear factor-kappa B (NF-κB) signaling and that this is activated in response to the essential osteoclastogenic cytokine, receptor activator of NF-κB ligand (RANKL), which also controls osteoclast activation to resorb (degrade) bone. These studies have revealed that RANKL and some pro-inflammatory cytokines, including tumor necrosis factor, activate NF-κB and downstream signaling, including c-Fos and nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1), and inhibition of repressors of NFATc1 signaling, to positively regulate osteoclast formation and functions. However, these cytokines also activate NF-κB signaling that can limit osteoclast formation through the NF-κB signaling proteins, TRAF3 and p100, and the suppressors of c-Fos/NFATc1 signaling, IRF8, and RBP-J. This paper reviews current understanding of how NF-κB signaling is involved in the positive and negative regulation of cytokine-mediated osteoclast formation and activation.