Chondrocytes-Specific Expression of Osteoprotegerin Modulates Osteoclast Formation in Metaphyseal Bone.
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ABSTRACT:
SUBMITTER: Wang B
PROVIDER: S-EPMC4556963 | biostudies-other | 2015 Sep
REPOSITORIES: biostudies-other
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Project description:Osteoprotegerin ligand (OPGL) targets osteoclast precursors and osteoclasts to enhance differentiation and activation, however, little is known about OPGL effects on osteoclast survival. In vitro, the combination of OPGL + colony-stimulating factor-1 (CSF-1) is required for optimal osteoclast survival. Ultrastructurally, apoptotic changes were observed in detached cells and culture lysates exhibited elevated caspase 3 activity, particularly in cultures lacking CSF-1. DEVD-FMK (caspase 3 inhibitor) partially protected cells when combined with OPGL, but not when used alone or in combination with CSF-1. CSF-1 maintained NF-kappaB activation and increased the expression of bcl-2 and bcl-X(L) mRNA, but had no effect on JNK activation. In contrast, OPGL enhanced both NF-kappaB and JNK kinase activation and increased the expression of c-src, but not bcl-2 and bcl-X(L) mRNA. These data suggest that aspects of both OPGL's and CSF-1's signaling/survival pathways are required for optimal osteoclast survival. In mice, a single dose of OPG, the OPGL decoy receptor, led to a >90% loss of osteoclasts because of apoptosis within 48 hours of exposure without impacting osteoclast precursor cells. Therefore, OPGL is essential, but not sufficient, for osteoclast survival and endogenous CSF-1 levels are insufficient to maintain osteoclast viability in the absence of OPGL.
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Project description:Should we believe that osteoclasts are only involved in bone resorption? What about their contribution to bone formation? In this article I will review evidence that bone formation can be regulated by osteoclasts. Why is this? Likely because in the physiologic condition of bone remodeling, bone resorption and formation are balanced, and there is no better way to control this equilibrium than through a concerted action between the two cell types. Although the influence of osteoblasts on osteoclastic bone resorption is well documented and consolidated over time, what osteoclasts do to regulate osteoblast activity is still matter of intense investigation. The original hypothesis that all is in the osteoblast-seeking factors stored in the bone matrix, released and activated during bone resorption, is now being challenged by several studies, suggesting that osteoclasts are also capable of producing 'clastokines' that regulate osteoblast performance. Indeed, several of them have been demonstrated to orchestrate osteoclast-osteoblast activities. However, we are probably still at the dawn of a new era, and future work will tell us whether any of these clastokines can be exploited to stimulate bone formation and rebalance bone remodeling in skeletal diseases.
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Project description:The pathogenesis of glucocorticoid-induced (GC-induced) bone loss is unclear. For example, osteoblast apoptosis is enhanced by GCs in vivo, but they stimulate bone formation in vitro. This conundrum suggests that an intermediary cell transmits a component of the bone-suppressive effects of GCs to osteoblasts in the intact animal. Bone remodeling is characterized by tethering of the activities of osteoclasts and osteoblasts. Hence, the osteoclast is a potential modulator of the effect of GCs on osteoblasts. To define the direct impact of GCs on bone-resorptive cells, we compared the effects of dexamethasone (DEX) on WT osteoclasts with those derived from mice with disruption of the GC receptor in osteoclast lineage cells (GRoc-/- mice). While the steroid prolonged longevity of osteoclasts, their bone-degrading capacity was suppressed. The inhibitory effect of DEX on bone resorption reflects failure of osteoclasts to organize their cytoskeleton in response to M-CSF. DEX specifically arrested M-CSF activation of RhoA, Rac, and Vav3, each of which regulate the osteoclast cytoskeleton. In all circumstances GRoc-/- mice were spared the impact of DEX on osteoclasts and their precursors. Consistent with osteoclasts modulating the osteoblast-suppressive effect of DEX, GRoc-/- mice are protected from the steroid's inhibition of bone formation.
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Project description:BackgroundOsteomyelitis (OM) is an inflammatory condition of bone characterized by cortical bone devascularization and necrosis. Dysregulation of bone remodelling is triggered by OM. Bone remodelling is precisely coordinated by bone resorption and formation via a reversal phase. However, the cellular and molecular mechanisms underlying bone remodelling failure after osteomyelitis remain elusive.MethodsTo elucidate the cellular and molecular mechanism underlying bone healing after osteomyelitis, we employed single-cell RNA sequencing (scRNA-seq) to depict the atlas of human cortical bone in normal, infected and reconstructed states. Dimensionality reduction by t-stochastic neighbourhood embedding (t-SNE) and graph-based clustering were applied to analyse the detailed clusters of osteoclast lineages. After trajectory analysis of osteoclast lineages over pseudotime, real-time PCR and immunofluorescence (IF) staining were applied to identify marker gene expression of various osteoclast lineages in the osteoclast induction model and human bone sections, respectively. The potential function and communication of osteoclasts were analysed via gene set enrichment analysis (GSEA) and CellChat. The chemotactic ability of mesenchymal stem cells (MSCs) and osteoclast lineage cells in various differentiation states was determined by transwell assays and coculture assays. The effects of various osteoclast lineages on the osteogenic differentiation potential of MSCs were also determined by using this coculture system. A normal mouse tibia fracture model and an osteomyelitis-related tibia fracture model were generated via injection of luciferase-labelled Staphylococcus aureus to verify the relationships between a novel osteoclast lineage and MSCs. Then, the infection was detected by a bioluminescence imaging system. Finally, immunofluorescence staining was used to detect the expression of markers of MSCs and novel osteoclast lineages in different remodelling phases in normal and infected bone remodelling models.ResultsIn this study, we constructed a cell atlas encompassing normal, infected, and reconstructed cortical bone. Then, we identified a novel subset at the earlier stage of the osteoclast lineage that exhibited increased expression of IDO1, CCL3, and CCL4. These IDO1highCCL3highCCL4high cells, termed osteostaticytes (OSCs), were further regarded as the reservoir of osteoclasts in the reversal phase. Notably, OSCs exhibited the highest chemotactic activity, surpassing other lineage subsets. We also discovered that cells at the earlier stage of the osteoclast lineage play a significant role in recruiting mesenchymal stem cells (MSCs). Finally, the data revealed that OSCs might be positively related to the occurrence of bone MSCs and the contribution of bone remodelling.ConclusionCollectively, our findings revealed a novel stage (OSC) within the osteoclast lineage, potentially representing elusive bone reversal cells due to its increased chemotactic ability towards MSCs and potential contribution to bone remodelling. This study provides valuable insights into the intricate mechanisms of the reversal phase during bone remodelling and unveils potential therapeutic strategies for diseases associated with bone uncoupling.Translational potential of this articleThis study identified a new subset, referred to as IDO1(plus symbol) CCL3(plus symbol) CCL4(plus symbol) osteostaticytes which displayed the highest chemotactic activity among all osteoclast lineages and may serve as reversal cells in bone remodelling. These findings offer new insights and insights for understanding bone reversal-related diseases and may serve as novel therapeutic targets for conditions such as osteomyelitis and delayed bone healing.
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