GSK3 signalling in DBA/2J mdx mice: a comparison against the traditional C57BL/10 mdx model and investigation into its pathogenic contribution

Duchenne muscular dystrophy (DMD) is an X-linked disorder caused by an absence of dystrophin that compromises membrane integrity, ultimately resulting in muscle weakness, wasting and premature death. There is currently no cure for DMD, however, promoting the slow oxidative fibre type and reducing inflammation in muscle has become a viable therapeutic strategy. In this thesis, the role of glycogen synthase kinase 3 (GSK3) in DMD pathology, as it relates to inflammation and muscle fibre type composition, was examined. Specifically, the purpose of this thesis was to first characterize GSK3 signalling in two mdx mouse models of DMD, the traditional C57BL/10 (BL10) mdx mouse and the more severe DBA/2J (D2) mdx mouse model. Next, it was examined whether inhibiting GSK3 with a clinically relevant drug called tideglusib would promote the slow oxidative fibre type, reduce inflammation and ultimately enhance muscle structure and function in the D2 mdx mouse. In the first objective of this thesis, it was found that total GSK3 was significantly higher in extensor digitorum longus (EDL) muscles from D2 mice compared with BL10 mice. Inhibitory serine9 phosphorylation of GSK3 was also significantly lower in D2 mice compared with BL10 mice, suggestive of a strain effect whereby D2 mice had more active GSK3. In the second objective of this thesis, it was found short-term (2-4 weeks) tideglusib treatment (10 mg/kg/day) increased EDL:body mass ratio and reduced serum creatine kinase levels compared with vehicle control. Tideglusib treatment also enhanced muscle function with a significant improvement in hangwire impulse, and EDL specific force production and fatigue resistance. In the EDL muscles, tideglusib treatment reduced total GSK3, a result that was associated with an increase in the proportion of oxidative type I and IIa fibres and elevated utrophin mRNA expression. However, tideglusib treatment did not alter inflammatory cytokine expression of IL-1 and TNF-. Collectively, these results show that GSK3 activation may contribute to dystrophic pathology in the D2 mdx mouse and that short-term tideglusib treatment can inhibit GSK3 in these mice leading to a promotion of the oxidative fibres and an improvement in muscle form and function