Bone Abstracts

OPTN encodes a cytoplasmic protein optineurin which has been shown to play a role in autophagy. Recent GWAS studies have shown that variants within OPTN are associated with the risk of Paget’s disease of bone, a disease characterized by focal areas of increased bone turnover due to increased osteoclast activity, suggesting a possible role of OPTN in the regulation of bone metabolism.

The aim of this study was to investigate the role of optineurin in osteoclast development using in vitro knock-down experiments in primary osteoclast precursor cells derived from mouse bone marrow. We used lentiviral particles expressing either shRNA targeted against the Optn gene or a non-targeting shRNA (-ve control) and Optn knock-down was confirmed (>70%) using western blot analysis.

Optn was expressed during osteoclast formation and its expression significantly increased during later stages of osteoclast development in WT mice. The number of osteoclasts formed from Optn-depleted bone marrow cells was significantly higher compared to non-targeted cells (253±39 vs 139±41; P<0.001). We also found that the number of large osteoclasts (>10 nuclei) was higher in Optn-depleted cells (92±26) compared to non-targeted cells (37±18; P<0.001). Furthermore, Osteoclast survival after withdrawal of RANKL was 45% higher in Optn-depleted cells (P<0.05).

Quantitative assessment of NF-κB activation by reporter assays showed significantly increased NF-κB activity in the Optn-depleted cells at the basal level and 72 hrs after stimulation with RANKL compared to non-depleted cells (P<0.05).

In conclusion, Optn depletion is associated with increased NF-κB activity leading to enhanced osteoclast formation, size and survival. Our data suggest that OPTN may act as a negative regulator of osteoclast differentiation. This provides a possible mechanism by which variants in OPTN increase susceptibility to Paget’s disease of bone but further studies will be required to investigate the role of OPTN in osteoclast biology.