Effects of the selective inhibition of proteasome caspase-like activity by CLi a derivative of nor-cerpegin in dystrophic mdx mice

Absence of Desmin Results in Impaired Adaptive Response to Mechanical Overloading of Skeletal Muscle

Background: Desmin is a muscle-specific protein belonging to the intermediate filament family. Desmin mutations are linked to skeletal muscle defects, including inherited myopathies with severe clinical manifestations. The aim of this study was to examine the role of desmin in skeletal muscle remodeling and performance gain induced by muscle mechanical overloading which mimics resistance training.

Methods: Plantaris muscles were overloaded by surgical ablation of gastrocnemius and soleus muscles. The functional response of plantaris muscle to mechanical overloading in desmin-deficient mice (DesKO, n = 32) was compared to that of control mice (n = 36) after 7-days or 1-month overloading. To elucidate the molecular mechanisms implicated in the observed partial adaptive response of DesKO muscle, we examined the expression levels of genes involved in muscle growth, myogenesis, inflammation and oxidative energetic metabolism. Moreover, ultrastructure and the proteolysis pathway were explored.

Results: Contrary to control, absolute maximal force did not increase in DesKO muscle following 1-month mechanical overloading. Fatigue resistance was also less increased in DesKO as compared to control muscle. Despite impaired functional adaptive response of DesKO mice to mechanical overloading, muscle weight and the number of oxidative MHC2a-positive fibers per cross-section similarly increased in both genotypes after 1-month overloading. However, mechanical overloading-elicited remodeling failed to activate a normal myogenic program after 7-days overloading, resulting in proportionally reduced activation and differentiation of muscle stem cells. Ultrastructural analysis of the plantaris muscle after 1-month overloading revealed muscle fiber damage in DesKO, as indicated by the loss of sarcomere integrity and mitochondrial abnormalities. Moreover, the observed accumulation of autophagosomes and lysosomes in DesKO muscle fibers could indicate a blockage of autophagy. To address this issue, two main proteolysis pathways, the ubiquitin-proteasome system and autophagy, were explored in DesKO and control muscle. Our results suggested an alteration of proteolysis pathways in DesKO muscle in response to mechanical overloading.

Conclusion: Taken together, our results show that mechanical overloading increases the negative impact of the lack of desmin on myofibril organization and mitochondria. Furthermore, our results suggest that under these conditions, the repairing activity of autophagy is disturbed. Consequently, force generation is not improved despite muscle growth, suggesting that desmin is required for a complete response to resistance training in skeletal muscle.

Desmin prevents muscle wasting, exaggerated weakness and fragility, and fatigue in dystrophic mdx mouse

KEY POINTS

Desmin, similar to dystrophin, is associated with costameric structures bridging sarcomeres to the extracellular matrix. Deletion of the desmin gene in mdx mice [double knockout (DKO) mice] induces marked muscle weakness and fatigue resistance compared to mdx mice. Muscle fragility (higher susceptibility to contraction-induced injury) was also aggravated in DKO mice compared to mdx mice. By contrast to mdx mice, the DKO mice did not undergo muscle hypertrophy. Desmin cDNA transfer with adeno-associated virus in newborn mdx mice reduced muscle weakness. Overall, desmin plays important and beneficial roles in muscle wasting, performance and fragility in dystrophic muscle.

ABSTRACT

Duchenne muscular dystrophy (DMD) is a severe neuromuscular disease caused by dystrophin deficiency. Desmin, similar to dystrophin, is associated with costameric structures bridging sarcomeres to the extracellular matrix that contributes to muscle function. In the present study, we attempted to provide further insight into the roles of desmin, for which the expression is increased in the muscle from the mouse mdx DMD model. We show that a deletion of the desmin gene (Des) in mdx mice [double knockout (DKO) mice, mdx:desmin-/-] induces a marked muscle weakness; namely, a reduced absolute maximal force production and increased fatigue compared to that in mdx mice. Fragility (i.e. higher susceptibility to contraction-induced injury) was also aggravated in DKO mice compared to mdx mice, despite the promotion of supposedly less fragile muscle fibres in DKO mice, and this worsening of fragility was related to a decreased muscle excitability. Moreover, in contrast to mdx mice, the DKO mice did not undergo muscle hypertrophy, as indicated by smaller and fewer fibres, with a reduced percentage of centronucleated fibres, potentially explaining the severe muscle weakness. Notably, Desmin cDNA transfer with adeno-associated virus in newborn mdx mice improved specific maximal force normalized to muscle weight. Overall, desmin plays important and beneficial roles in muscle wasting, performance and fragility in dystrophic mdx mice, which differ, at least in part, from those observed in healthy muscle.