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The common null polymorphism (R577X) in the  ACTN3  gene is present in over 1.5 billion people worldwide and results in the absence of the protein α-actinin-3 from the Z-discs of fast-twitch skeletal muscle fibres. We have previously reported that this polymorphism is a modifier of dystrophin-deficient Duchenne Muscular Dystrophy. To investigate the mechanism underlying this, we use a double knockout (dk) Actn3KO / mdx  (dKO) mouse model, which lacks both dystrophin and sarcomere α-actinin-3. We used dKO mice and  mdx  dystrophic mice at 12 months (aged) to investigate the correlation between morphological changes to the fast-twitch dKO EDL and the reduction in force deficit produced by an  in vitro  eccentric contraction protocol. In the aged dKO mouse, we found a marked reduction in fibre branching complexity that correlated with protection from eccentric contraction induced force deficit. Complex branches in the aged dKO EDL fibres (28%) were substantially reduced compared to aged  mdx  EDL fibres (68%), and this correlates with a graded force loss over three eccentric contractions for dKO muscles (~36% after first contraction, ~66% overall) compared to an abrupt drop in  mdx  upon the first eccentric contraction (~75% after first contraction, ~89% after three contractions). In dKO, protection from eccentric contraction damage was linked with a doubling of SERCA1 pump density the EDL. We propose that the increased oxidative metabolism of fast-twitch glycolytic fibres characteristic of the null polymorphism (R577X) and increase in SR Ca 2+  pump proteins reduces muscle fibre branching and decreases susceptibility to eccentric injury in the dystrophinopathies.

human molecular genetics

Loss of α-actinin-3 confers protection from eccentric contraction damage in fast-twitch EDL muscles from aged  mdx  dystrophic mice by reducing pathological fibre branching