Bone Abstracts

Ageing is universally linked to skeletal deterioration. Common mechanisms may control both processes, where dysregulation may predispose to bone loss and osteoarthritis (OA). The epigenetic modifier SirT1 controls lifespan and decreases with age. However, the role of SirT1 in joint disease is unclear. Human tissue samples, novel genetically modified mice, a surgical disease model and advanced cellular and molecular studies were employed to explore the hypothesis that SirT1 is dysregulated in ageing and OA human cartilage, disrupting normal lifespan-protecting mechanisms such as autophagy, to predispose to OA. Autophagy degrades unwanted proteins, and is defective in ageing. Inhibition of SirT1 (pharmacological and molecular) in HTB94 chondrocytes decreased markers of autophagy (BECN1, ULK-1, LC3, P<0.01) and chondrogenesis (COL2A1, ACAN, SOX-9, P<0.01). LC3 protein conversion, essential for autophagosome formation, was positively regulated in line with manipulated SirT1 activity in chondrocytes (western blot, FACS (P<0.001)) and cartilage explants from LC3-GFP reporter mice (P<0.01). This demonstrates that SirT1 alters autophagic flux, whilst IP analysis showed direct mechanistic binding of SirT1 with autophagy proteins.

Human OA cartilage showed decreased SirT1 compared to healthy cartilage (P<0.05). Similarly, murine models with a novel articular cartilage-specific SirT1 deletion (SirT1^fl/flx Aggrecan CreER^T2 mice (SirT1^Agg)), showed significant cartilage degradation, increased OA disease score and epiphyseal volume (vs WT, P<0.001) on histomorphometric and uCT analysis at 2, 6, 12 months. SirT1Agg mice also showed decreased ACAN, COL2A1, SOX-9 (P<0.001) and autophagy markers BECN1, ULK-1, LC3, ATG5, ATG7, ATG13 (P<0.01), decreased LC3 protein in hip explants (P<0.01), and decreased autophagosome number in microdissected cartilage (SEM, P<0.05). Induction of experimental OA (destabilisation of the medial meniscus surgery) exacerbated cartilage degradation in SirT1Agg mice vs WT, increased OA disease score and decreased LC3 staining.

These studies suggest that declining SirT1 in ageing human cartilage, or SirT1 deletion in murine chondrocytes, results in OA due to dysregulated autophagy in chondrocytes.