Background Rheumatoid arthritis is a common autoimmune disease characterised by the hyperplastic transformation of synovium, its infiltration with γ inflammatory cells and by stimulation of bone resorption through osteoclast activation leading to joint destruction. Posttranslational modification by SUMOylation has been described for various proteins and is involved in a wide variety of cellular processes, including protein localization and stability, transcriptional regulation, cell survival and death. Here, we investigated the role of SUMO-1 in osteoclastogenesis and studied the skeletal phenotype of SUMO-1-/- mice.
Materials and methods The skeletal phenotype of 8-week old SUMO-1-/- and wild- type (WT) mice was investigated by µCT-analysis and by van Kossa and TRAP stainings. For in vitro experiments, bone marrow-derived macrophages (BMMs) were isolated from SUMO-1-/- and WT mice, and differentiated into osteoclasts in presence of M-CSF and RANKL. Osteoclast differentiation was characterised by TRAP staining and by real-time PCR analysis of osteoclast marker genes. Activation of p38 and p44/42 MAPK as well as NF-κB signalling was investigated by western blotting. The osteoclast resorption capacity was determined using a calcium phosphate bone resorption assay. Cell motility was analysed by live cell imaging.
Results SUMO-1-/- mice demonstrated 20% higher trabecular bone volume compared with WT mice. Moreover, trabecular thickness was increased and trabecular separation was decreased in SUMO-1-/- mice. In addition, SUMO-1-/- mice display a significant reduction in osteoclasts. Of note, SUMO-1 loss was associated with impaired in vitro osteoclast differentiation and resorption capacity. Expression of DC-STAMP, cathepsin K and integrin ß3 was decreased in SUMO-1-/- osteoclasts compared to WT osteoclasts. However, no differences in activation of p38 and p44/42 MAPK as well as the NF-κB signalling pathway in BMMs and osteoclasts of SUMO-1-/- and WT mice was observed. Conversely, cell migration and fusion seemed to be affected in SUMO-1-/- osteoclasts.
Conclusions We found that SUMO-1-/- mice have a higher bone mass owing to a decreased number of functional osteoclasts. Our data suggest that SUMO-1 is involved in the regulation of bone mass by osteoclast formation and activity, and therefore may be an interesting target for treating diseases associated with bone loss.
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