Osteoclast-derived activity in the coupling of bone formation to resorption

Martin, T. J.; Sims, Natalie A.

doi:10.1016/j.molmed.2004.12.004

Cited by 563 publications

(435 citation statements)

References 45 publications

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“…The resulting release of osteoblastogenic growth factors from the bone matrix may be an additional source of osteogenic signals that contribute to PTH-induced anabolism (Figure 3). RANKL may also stimulate the secretion of an osteoblastogenic factor from osteoclasts [106]. Evidence that bisphosphonates attenuated the anabolic effect of intermittent PTH in animal models and humans is consistent with the involvement of osteoclasts in the anabolic effect [107][108][109][110][111], but such inhibition was not seen in other studies [45,[112][113][114].…”

Section: Osteoclast-derived Factorsmentioning

confidence: 72%

Molecular and cellular mechanisms of the anabolic effect of intermittent PTH

Jilka

2007

Bone

623

531

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Intermittent administration of parathyroid hormone (PTH) stimulates bone formation by increasing osteoblast number, but the molecular and cellular mechanisms underlying this effect are not completely understood. In vitro and in vivo studies have shown that PTH directly activates survival signaling in osteoblasts; and that delay of osteoblast apoptosis is a major contributor to the increased osteoblast number, at least in mice. This effect requires Runx2-dependent expression of antiapoptotic genes like Bcl-2. PTH also causes exit of replicating progenitors from the cell cycle by decreasing expression of cyclin D and increasing expression of several cyclin-dependent kinase inhibitors. Exit from the cell cycle may set the stage for pro-differentiating and pro-survival effects of locally produced growth factors and cytokines, the level and/or activity of which are known to be influenced by PTH. Observations from genetically modified mice suggest that the anabolic effect of intermittent PTH requires insulin-like growth factor-I (IGF-I), fibroblast growth factor-2 (FGF-2), and perhaps Wnts. Attenuation of the negative effects of PPARγ may also lead to increased osteoblast number. Daily injections of PTH may add to the pro-differentiating and pro-survival effects of locally produced PTH related protein (PTHrP). As a result, osteoblast number increases beyond that needed to replace the bone removed by osteoclasts during bone remodeling. The pleiotropic effects of intermittent PTH, each of which alone may increase osteoblast number, may explain why this therapy reverses bone loss in most osteoporotic individuals regardless of the underlying pathophysiology.

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Section: Osteoclast-derived Factorsmentioning

confidence: 72%

Molecular and cellular mechanisms of the anabolic effect of intermittent PTH

Jilka

2007

Bone

623

531

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“…Bone remodeling is a highly orchestrated multicellular process that requires the sequential and balanced events of osteoclast-mediated bone resorption and osteoblast-mediated bone formation (38). The spatial and temporal relationships between these cells are maintained throughout the remodeling process (39).…”

Section: Discussionmentioning

confidence: 99%

“…The bone-modeling canopy cell identity and functional importance was strongly supported by the impact of partial macrophage depletion in the MAFIA mouse on the macrophage canopy architecture, osteoblast bone surface, and endosteal niche. Given the close relationship between macrophages and osteoclasts, OsteoMacs within the canopy at sites of bone modeling may simulate the coupling process that is proposed to occur between osteoclasts and osteoblasts in bone remodeling (38).…”

Section: Discussionmentioning

confidence: 99%

Osteal Tissue Macrophages Are Intercalated throughout Human and Mouse Bone Lining Tissues and Regulate Osteoblast Function In Vitro and In Vivo

Chang

Raggatt

Alexander

et al. 2008

The Journal of Immunology

608

646

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Resident macrophages are an integral component of many tissues and are important in homeostasis and repair. This study examines the contribution of resident tissue macrophages to bone physiology. Using immunohistochemistry, we showed that a discrete population of resident macrophages, OsteoMacs, was intercalated throughout murine and human osteal tissues. OsteoMacs were distributed among other bone lining cells within both endosteum and periosteum. Furthermore, OsteoMacs were coisolated with osteoblasts in murine bone explant and calvarial preparations. OsteoMacs made up 15.9% of calvarial preparations and persisted throughout standard osteoblast differentiation cultures. Contrary to previous studies, we showed that it was OsteoMacs and not osteoblasts within these preparations that responded to pathophysiological concentrations of LPS by secreting TNF. Removal of OsteoMacs from calvarial cultures significantly decreased osteocalcin mRNA induction and osteoblast mineralization in vitro. In a Transwell coculture system of enriched osteoblasts and macrophages, we demonstrated that macrophages were required for efficient osteoblast mineralization in response to the physiological remodeling stimulus, elevated extracellular calcium. Notably, OsteoMacs were closely associated with areas of bone modeling in situ, forming a distinctive canopy structure covering >75% of mature osteoblasts on diaphyseal endosteal surfaces in young growing mice. Depletion of OsteoMacs in vivo using the macrophage-Fas-induced apoptosis (MAFIA) mouse caused complete loss of osteoblast bone-forming surface at this modeling site. Overall, we have demonstrated that OsteoMacs are an integral component of bone tissues and play a novel role in bone homeostasis through regulating osteoblast function. These observations implicate OsteoMacs, in addition to osteoclasts and osteoblasts, as principal participants in bone dynamics.

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“…(4) PTH administered by daily injection is the only currently available anabolic therapy for bone. (5,6) While a number of contributing pathways, including direct prodifferentiation (7) and antiapoptotic (8) actions on osteoblasts, as well as couplingrelated activity through the osteoclast, (9) have been proposed, the full mechanism of action of anabolic PTH remains obscure. Another contributory mechanism was introduced by the discovery of sclerostin and its regulation by both PTH and PTH-related protein (PTHrP) via a distal enhancer of the SOST gene.…”

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confidence: 99%

Building bone with a SOST-PTH partnership

Sims

2010

Journal of Bone and Mineral Research

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T he discovery of the SOST gene, whose protein product, sclerostin, is an osteocyte-derived inhibitor of Wnt signalling and bone formation, (1,2) has provided a major advance in bone biology. It gives us new insights into the communicating networks among bone cells and, importantly, to new ways of restoring bone once it has been lost. In the current issue of JBMR, Kramer and colleagues complement their earlier demonstration that parathyroid hormone (PTH) inhibits sclerostin expression (3) to show that the anabolic effect of PTH is blunted both in mice overexpressing sclerostin and in SOST null mice. (4) PTH administered by daily injection is the only currently available anabolic therapy for bone. (5,6) While a number of contributing pathways, including direct prodifferentiation (7) and antiapoptotic (8) actions on osteoblasts, as well as couplingrelated activity through the osteoclast, (9) have been proposed, the full mechanism of action of anabolic PTH remains obscure. Another contributory mechanism was introduced by the discovery of sclerostin and its regulation by both PTH and PTH-related protein (PTHrP) via a distal enhancer of the SOST gene. (3,10) The experiments reported by Kramer and colleagues (4) provide further evidence that sclerostin regulation may play a part in PTH anabolic action. Mice overexpressing sclerostin have less bone as a result of decreased bone-formation rate, as described previously. (11) When these mice were treated with a high dose of PTH, sufficient to increase trabecular bone volume (BV/TV) in wild-type mice, no increase in trabecular bone volume was detected despite a robust reduction in sclerostin expression. It is worth noting that even though sclerostin mRNA levels are reduced by PTH in the transgenic model, total sclerostin expression after PTH treatment is still greater than in wild-type mice. This high level of sclerostin expression, the basal phenotype of a mild reduction in bone remodeling, or both impair the effect of PTH on BV/TV. Surprisingly, even though BV/ TV was not increased, all histomorphometric markers of bone formation, as well as the osteoclast surface, were substantially elevated by PTH treatment in the sclerostin transgenic mice to approximately the same extent as that observed in wild-type mice. Thus a question in the sclerostin transgenic model remains as to how PTH can increase bone turnover without altering BV/ TV. Is osteoclast function enhanced? No evidence was obtained for that. An alternative explanation could be that the sclerostin overexpressing mice lay down bone of reduced quality that is easier for osteoclasts to resorb. The quality of bone in the sclerostin overexpressing mouse is not yet known. Resolving this paradox may provide useful information about the interaction between osteoclasts and osteoblasts in the presence of high levels of sclerostin and, presumably, low levels of b-cateninactivated gene transcription.In SOST null mice, as expected from the skeletal abnormalities observed in van Buchem disease and sclerosteosis, (12) trabecular...

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Osteoclast-derived activity in the coupling of bone formation to resorption

Cited by 563 publications

References 45 publications

Molecular and cellular mechanisms of the anabolic effect of intermittent PTH

Molecular and cellular mechanisms of the anabolic effect of intermittent PTH

Osteal Tissue Macrophages Are Intercalated throughout Human and Mouse Bone Lining Tissues and Regulate Osteoblast Function In Vitro and In Vivo

Building bone with a SOST-PTH partnership

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