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PurposeGreater muscle fragility is thought to cause the exhaustion of the muscle stem cells during successive degeneration/repair cycles, leading to muscle wasting and weakness in Duchenne muscular dystrophy. Chronic voluntary exercise can partially reduce the susceptibility to contraction induced-muscle injury, i.e., muscle fragility, as shown by a reduced immediate maximal force drop following lengthening contractions, in the dystrophic mdx mice. Here, we studied the effect of Prospero-related homeobox factor 1 gene (Prox1) transfer (overexpression) on fragility in chronically exercised mdx mice, because Prox1 promotes slower type fibres in healthy mice and slower fibres are less fragile in mdx muscle.Methodsmdx mice received or not Prox1 transfer into the tibialis anterior muscle and performed voluntary running into a wheel during 1 month. We also performed Prox1 transfer in sedentary mdx mice. In situ maximal force production of the muscle in response to nerve stimulation was assessed before, during and after 10 lengthening contractions. Molecular and cellular parameters were also evaluated.ResultsInterestingly, Prox1 transfer reduced the force drop following lengthening contractions in exercised mdx mice (p < 0.05 to 0.01), but not in sedentary mdx mice. It also increased the muscle expression of Myh7 (p < 0.001), MHC-2x (p < 0.01) and Trpc1 (p < 0.01), whereas it reduced that one of Myh4 (p < 0.001) and MHC-2b (p < 0.01) in exercised mdx mice. Moreover, Prox1 transfer decreased the maximal force (p < 0.01) before lengthening contraction in exercised mdx mice (p < 0.01), and reduced muscle weight (p < 0.0001) despite increased Mstn expression (p < 0.001).ConclusionOur results indicate that the beneficial effect of Prox1 transfer on muscle fragility is only observed in chronically exercised mdx mice. Thus, Prox1 transfer combined to chronic exercise have the potential to substantially slow the progression of the dystrophic disease in the long term.