Although active vitamin D drugs have been used for the treatment of osteoporosis, how the vitamin D receptor (VDR) regulates bone cell function remains largely unknown. Using osteoprotegerin-deficient mice, which exhibit severe osteoporosis due to excessive receptor activator of NF-κB ligand/receptor activator of NF-κB (RANKL/RANK) stimulation, we show herein that oral treatment of these mice with 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] inhibited bone resorption and prevented bone loss, suggesting that VDR counters RANKL/RANK signaling. In M-CSF–dependent osteoclast precursor cells isolated from mouse bone marrow, 1α,25(OH)2D3 potently and dose-dependently inhibited their differentiation into multinucleate osteoclasts induced by RANKL. Among signaling molecules downstream of RANK, 1α,25(OH)2D3 inhibited the induction of c-Fos protein after RANKL stimulation, and retroviral expression of c-Fos protein abrogated the suppressive effect of 1α,25(OH)2D3 on osteoclast development. By screening vitamin D analogs based on their c-Fos–suppressing activity, we identified a new analog, named DD281, that inhibited bone resorption and prevented bone loss in ovariectomized mice, more potently than 1α,25(OH)2D3, with similar levels of calcium absorption. Thus, c-Fos protein is an important target of the skeletal action of VDR-based drugs, and DD281 is a bone-selective analog that may be useful for the treatment of bone diseases with excessive osteoclastic activity.
Hisashi Takasu, Atsuko Sugita, Yasushi Uchiyama, Nobuyoshi Katagiri, Makoto Okazaki, Etsuro Ogata, Kyoji Ikeda
TNF-α is the dominant cytokine in inflammatory osteolysis. Using mice whose BM stromal cells and osteoclast precursors are chimeric for the presence of TNF receptors, we found that both cell types mediated the cytokine’s osteoclastogenic properties. The greater contribution was made, however, by stromal cells that express the osteoclastogenic cytokine M-CSF. TNF-α stimulated M-CSF gene expression, in vivo, only in the presence of TNF-responsive stromal cells. M-CSF, in turn, induced the key osteoclastogenic cytokine receptor, receptor activator of NF-κB (RANK), in osteoclast precursors. In keeping with the proproliferative and survival properties of M-CSF, TNF-α enhanced osteoclast precursor number only in the presence of stromal cells bearing TNF receptors. To determine the clinical relevance of these observations, we induced inflammatory arthritis in wild-type mice and treated them with a mAb directed against the M-CSF receptor, c-Fms. Anti–c-Fms mAb selectively and completely arrested the profound pathological osteoclastogenesis attending this condition, the significance of which is reflected by similar blunting of the in vivo bone resorption marker tartrate-resistant acid phosphatase 5b (TRACP 5b). Confirming that inhibition of the M-CSF signaling pathway targets TNF-α, anti–c-Fms also completely arrested osteolysis in TNF-injected mice with nominal effect on macrophage number. M-CSF and its receptor, c-Fms, therefore present as candidate therapeutic targets in states of inflammatory bone erosion.
Hideki Kitaura, Ping Zhou, Hyun-Ju Kim, Deborah V. Novack, F. Patrick Ross, Steven L. Teitelbaum
TNF receptor–associated factor 6 (TRAF6) associates with the cytoplasmic domain of receptor activator of NF-κB (RANK). This event is central to normal osteoclastogenesis. We discovered that TRAF6 also interacts with FHL2 (four and a half LIM domain 2), a LIM domain–only protein that functions as a transcriptional coactivator or corepressor in a cell-type–specific manner. FHL2 mRNA and protein are undetectable in marrow macrophages and increase pari passu with osteoclast differentiation in vitro. FHL2 inhibits TRAF6-induced NF-κB activity in wild-type osteoclast precursors and, in keeping with its role as a suppressor of TRAF6-mediated RANK signaling, TRAF6/RANK association is enhanced in FHL2–/– osteoclasts. FHL2 overexpression delays RANK ligand–induced (RANKL-induced) osteoclast formation and cytoskeletal organization. Interestingly, osteoclast-residing FHL2 is not detectable in naive wild-type mice, in vivo, but is abundant in those treated with RANKL and following induction of inflammatory arthritis. Reflecting increased RANKL sensitivity, osteoclasts generated from FHL2–/– mice reach maturation and optimally organize their cytoskeleton earlier than their wild-type counterparts. As a consequence, FHL2–/– osteoclasts are hyperresorptive, and mice lacking the protein undergo enhanced RANKL and inflammatory arthritis–stimulated bone loss. FHL2 is, therefore, an antiosteoclastogenic molecule exerting its effect by attenuating TRAF6-mediated RANK signaling.
Shuting Bai, Hideki Kitaura, Haibo Zhao, Ju Chen, Judith M. Müller, Roland Schüle, Bryant Darnay, Deborah V. Novack, F. Patrick Ross, Steven L. Teitelbaum
MMPs, which degrade components of the ECM, have roles in embryonic development, tissue repair, cancer, arthritis, and cardiovascular disease. We show that a missense mutation of MMP13 causes the Missouri type of human spondyloepimetaphyseal dysplasia (SEMDMO), an autosomal dominant disorder characterized by defective growth and modeling of vertebrae and long bones. Genome-wide linkage analysis mapped SEMDMO to a 17-cM region on chromosome 11q14.3–23.2 that contains a cluster of 9 MMP genes. Among these, MMP13 represented the best candidate for SEMDMO, since it preferentially degrades collagen type II, abnormalities of which cause skeletal dysplasias that include Strudwick type SEMD. DNA sequence analysis revealed a missense mutation, F56S, that substituted an evolutionarily conserved phenylalanine residue for a serine in the proregion domain of MMP13. We predicted, by modeling MMP13 structure, that this F56S mutation would result in a hydrophobic cavity with misfolding, autoactivation, and degradation of mutant protein intracellularly. Expression of wild-type and mutant MMP13s in human embryonic kidney cells confirmed abnormal intracellular autoactivation and autodegradation of F56S MMP13 such that only enzymatically inactive, small fragments were secreted. Thus, the F56S mutation results in deficiency of MMP13, which leads to the human skeletal developmental anomaly of SEMDMO.
Ann M. Kennedy, Masaki Inada, Stephen M. Krane, Paul T. Christie, Brian Harding, Carlos López-Otín, Luis M. Sánchez, Anna A.J. Pannett, Andrew Dearlove, Claire Hartley, Michael H. Byrne, Anita A.C. Reed, M. Andrew Nesbit, Michael P. Whyte, Rajesh V. Thakker
Mice heterozygous for targeted disruption of Pthrp exhibit, by 3 months of age, diminished bone volume and skeletal microarchitectural changes indicative of advanced osteoporosis. Impaired bone formation arising from decreased BM precursor cell recruitment and increased apoptotic death of osteoblastic cells was identified as the underlying mechanism for low bone mass. The osteoporotic phenotype was recapitulated in mice with osteoblast-specific targeted disruption of Pthrp, generated using Cre-LoxP technology, and defective bone formation was reaffirmed as the underlying etiology. Daily administration of the 1–34 amino-terminal fragment of parathyroid hormone (PTH 1–34) to Pthrp+/– mice resulted in profound improvement in all parameters of skeletal microarchitecture, surpassing the improvement observed in treated WT littermates. These findings establish a pivotal role for osteoblast-derived PTH-related protein (PTHrP) as a potent endogenous bone anabolic factor that potentiates bone formation by altering osteoblast recruitment and survival and whose level of expression in the bone microenvironment influences the therapeutic efficacy of exogenous PTH 1–34.
Dengshun Miao, Bin He, Yebin Jiang, Tatsuya Kobayashi, Maria A. Sorocéanu, Jenny Zhao, Hanyi Su, Xinkang Tong, Norio Amizuka, Ajay Gupta, Harry K. Genant, Henry M. Kronenberg, David Goltzman, Andrew C. Karaplis
In the developing growth plate, periarticular chondrocytes proliferate, differentiate into columnar chondrocytes, and then further differentiate into postmitotic hypertrophic chondrocytes. Parathyroid hormone–related (PTH-related) protein (PTHrP), regulated by Indian hedgehog (Ihh), prevents premature hypertrophic differentiation, thereby maintaining the length of columns. Ihh regulates cartilage development through PTHrP-independent pathways as well. Here we show that Ihh stimulates differentiation of periarticular to columnar chondrocytes (periarticular chondrocyte differentiation) and thereby regulates the length of columns independently of PTHrP. Mosaic ablation of the PTH/PTHrP receptor in the growth plate caused upregulation of Ihh action, PTHrP upregulation, acceleration of periarticular chondrocyte differentiation, and elongation of the columnar region. Decreasing Ihh action in these mice reduced elongation of columns, whereas decreasing PTHrP showed only a modest effect on column length. Overexpression of Ihh caused PTHrP upregulation, elongation of columns, and acceleration of periarticular chondrocyte differentiation. PTHrP heterozygosity in this model had a minimal effect on the elongation of columns. Moreover, the elongation of columns and stimulation of periarticular chondrocyte differentiation in these models were still observed when PTHrP signaling was maintained so that it remained constant. These results demonstrate that Ihh acts on periarticular chondrocytes to stimulate their differentiation, thereby regulating the columnar cell mass independently of PTHrP.
Tatsuya Kobayashi, Desi W. Soegiarto, Yingzi Yang, Beate Lanske, Ernestina Schipani, Andrew P. McMahon, Henry M. Kronenberg
Joint ankylosis is a major cause of disability in the human spondyloarthropathies. Here we report that this process partially recapitulates embryonic endochondral bone formation in a spontaneous model of arthritis in DBA/1 mice. Bone morphogenetic protein (BMP) signaling appears to be a key molecular pathway involved in this pathological cascade. Systemic gene transfer of noggin, a BMP antagonist, is effective both as a preventive and a therapeutic strategy in the mouse model, mechanistically interfering with enthesial progenitor cell proliferation in early stages of the disease process. Immunohistochemical staining for phosphorylated smad1/5 in enthesial biopsies of patients with spondyloarthropathy reveals active BMP signaling in similar target cells. Our data suggest that BMP signaling is an attractive therapeutic target for interfering with structural changes in spondyloarthropathy either as an alternative or complementary approach to current antiinflammatory treatments.
Rik J.U. Lories, Inge Derese, Frank P. Luyten
Infantile cortical hyperostosis (Caffey disease) is characterized by spontaneous episodes of subperiosteal new bone formation along 1 or more bones commencing within the first 5 months of life. A genome-wide screen for genetic linkage in a large family with an autosomal dominant form of Caffey disease (ADC) revealed a locus on chromosome 17q21 (LOD score, 6.78). Affected individuals and obligate carriers were heterozygous for a missense mutation (3040C↠T) in exon 41 of the gene encoding the α1(I) chain of type I collagen (COL1A1), altering residue 836 (R836C) in the triple-helical domain of this chain. The same mutation was identified in affected members of 2 unrelated, smaller families with ADC, but not in 2 prenatal cases and not in more than 300 chromosomes from healthy individuals. Fibroblast cultures from an affected individual produced abnormal disulfide-bonded dimeric α1(I) chains. Dermal collagen fibrils of the same individual were larger, more variable in shape and size, and less densely packed than those in control samples. Individuals bearing the mutation, whether they had experienced an episode of cortical hyperostosis or not, had joint hyperlaxity, hyperextensible skin, and inguinal hernias resembling symptoms of a mild form of Ehlers-Danlos syndrome type III. These findings extend the spectrum of COL1A1-related diseases to include a hyperostotic disorder.
Robert C. Gensure, Outi Mäkitie, Catherine Barclay, Catherine Chan, Steven R. DePalma, Murat Bastepe, Hilal Abuzahra, Richard Couper, Stefan Mundlos, David Sillence, Leena Ala Kokko, Jonathan G. Seidman, William G. Cole, Harald Jüppner
Short digits (Dsh) is a radiation-induced mouse mutant. Homozygous mice are characterized by multiple defects strongly resembling those resulting from Sonic hedgehog (Shh) inactivation. Heterozygous mice show a limb reduction phenotype with fusion and shortening of the proximal and middle phalanges in all digits, similar to human brachydactyly type A1, a condition caused by mutations in Indian hedgehog (IHH). We mapped Dsh to chromosome 5 in a region containing Shh and were able to demonstrate an inversion comprising 11.7 Mb. The distal breakpoint is 13.298 kb upstream of Shh, separating the coding sequence from several putative regulatory elements identified by interspecies comparison. The inversion results in almost complete downregulation of Shh expression during E9.5–E12.5, explaining the homozygous phenotype. At E13.5 and E14.5, however, Shh is upregulated in the phalangeal anlagen of Dsh/+ mice, at a time point and in a region where WT Shh is never expressed. The dysregulation of Shh expression causes the local upregulation of hedgehog target genes such as Gli1-3, patched, and Pthlh, as well as the downregulation of Ihh and Gdf5. This results in shortening of the digits through an arrest of chondrocyte differentiation and the disruption of joint development.
Michael Niedermaier, Georg C. Schwabe, Stephan Fees, Anne Helmrich, Norbert Brieske, Petra Seemann, Jochen Hecht, Volkhard Seitz, Sigmar Stricker, Gundula Leschik, Evelin Schrock, Paul B. Selby, Stefan Mundlos
Inactivation of the growth factor–regulated S6 kinase RSK2 causes Coffin-Lowry syndrome in humans, an X-linked mental retardation condition associated with progressive skeletal abnormalities. Here we show that mice lacking RSK2 develop a progressive skeletal disease, osteopenia due to impaired osteoblast function and normal osteoclast differentiation. The phenotype is associated with decreased expression of Phex, an endopeptidase regulating bone mineralization. This defect is probably not mediated by RSK2-dependent phosphorylation of c-Fos on serine 362 in the C-terminus. However, in the absence of RSK2, c-Fos–dependent osteosarcoma formation is impaired. The lack of c-Fos phosphorylation leads to reduced c-Fos protein levels, which are thought to be responsible for decreased proliferation and increased apoptosis of transformed osteoblasts. Therefore, RSK2-dependent stabilization of c-Fos is essential for osteosarcoma formation in mice and may also be important for human osteosarcomas.
Jean-Pierre David, Denis Mehic, Latifa Bakiri, Arndt F. Schilling, Vice Mandic, Matthias Priemel, Maria Helena Idarraga, Markus O. Reschke, Oskar Hoffmann, Michael Amling, Erwin F. Wagner