G.P.8.07 Increased oxidative metabolism in mdx muscle treated by a combination of exon skipping and myostatin blockade

Abstract

Myostatin blockade stimulates growth of skeletal muscle and has been suggested as a potential therapy for muscular dystrophies. Previously we observed a fibre type conversion towards glycolytic fibres in myostatin deficient mice, which was associated with a decreased number of mitochondria and reduction of oxidative enzymes SDH and COX, implicating a reduced oxidative metabolism of skeletal muscle as a possible adverse effect following lack of myostatin. Here we investigated the effect on muscle metabolism when myostatin blockade was used as a supportive therapy combined with skipping of dystrophin exon 23 in mdx mouse, the animal model for Duchenne muscular dystrophy. We used a AAV construct harboring a double vector which blocks myostatin signalling by RNA interference of the activin receptor IIb (sh-AcvRIIb-3) and which rescues dystrophin expression by using a modified exon skipping strategy (U7 snRNA targeting exon 23). Remarkably, here we demonstrate that combined exon skipping and myostatin blockade increased oxidative properties of treated mdx muscle. Intramuscular injection of U7/sh-AcvRIIb-inv vector into tibialis anterior muscles of mdx mice increased the activity of COX by 78% and of respiratory chain complexes II by 104% compared to the treatment with a control vector harboring scramble sh-RNA. Furthermore, we found an increase in citrate synthase activity of 13% compared to the controls. From these results we hypothesize that myostatin blockade has a different effect on muscle metabolism and subsequently on muscle function when used in combination with dystrophin rescue than myostatin blockade on its own. Therefore, side effects resulting from myostatin blockade may be reduced if applied as a supportive therapy. However, above demonstrated data do not permit to differentiate whether observed effects resulted from myostatin blockade or from exon skipping. Experiments to compare the effect of each strategy are on course.