Bone Abstracts

Duchenne muscular dystrophy (DMD) is associated with osteoporosis, and dystrophic (dystrophin-deficient) mdx mice show reduced bone mass characterised by decreased mineral apposition and elevated bone resorption. To investigate a potential role of the G-protein-coupled receptor protease-activated receptor-2 (PAR₂) in the muscle and bone pathology associated with DMD, we established a colony of PAR₂-null-mdx mice. Limb and diaphragm muscles, tibiae and serum of male PAR₂-null-mdx and littermate mdx mice were examined every 4 weeks from just after the onset of muscle pathology (4 weeks) until 20 weeks of age. By 8 weeks, serum creatine kinase activity was lower in PAR₂-null-mdx mice compared to mdx, and continued to drop in PAR₂-null-mdx mice over time (70% lower than mdx, P<0.0001, at 20 weeks). From 8 weeks, in all muscles examined histologically, the area of active inflammation and the number of damaged fibres were lower in PAR₂-null-mdx mice compared to mdx mice. Hydroxyproline content in the diaphragm (indicative of fibrosis) was lower in PAR₂-null-mdx than in mdx mice from 12 weeks onwards (27% lower at 20 weeks, P<0.01). PAR₂-null-mdx mice showed significantly higher grip strength and lower fatiguability from 8 weeks of age (41% less fatigue, P<0.001, at 20 weeks). Micro-CT evaluation of the tibial metaphysis at 20 weeks of age showed that BV/TV, trabecular number and trabecular thickness were higher (by 80, 23 and 23%, respectively; all P<0.01) and trabecular separation was lower (14%, P<0.05) in PAR₂-null-mdx than in mdx mice. The serum concentrations of IL6 (88%, P<0.01), active TGFβ (19%, P<0.01) and RANKL (37%, P<0.05), and the RANKL/OPG ratio (49%, P<0.01) were lower in PAR₂-null-mdx mice compared to mdx mice at 20 weeks. These results suggest that PAR₂ activation contributes to muscle and bone pathology in dystrophin-deficient mice and that antagonising PAR₂ may help ameliorate the effects of dystrophin deficiency.