Duchenne muscular dystrophy (DMD) is characterized by myofiber death from apoptosis or necrosis, leading in many patients to fatal respiratory muscle weakness. Among other pathological features, DMD muscles show severely deranged metabolic gene regulation and mitochondrial dysfunction. Defective mitochondria not only cause energetic deficiency, but also play roles in promoting myofiber atrophy and injury via opening of the mitochondrial permeability transition pore. Autophagy is a bulk degradative mechanism that serves to augment energy production and eliminate defective mitochondria (mitophagy). We hypothesized that pharmacological activation of AMP-activated protein kinase (AMPK), a master metabolic sensor in cells and on-switch for the autophagy-mitophagy pathway, would be beneficial in the mdx mouse model of DMD. Treatment of mdx mice for 4 weeks with an established AMPK agonist, AICAR (5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside), potently triggered autophagy in the mdx diaphragm without inducing muscle fiber atrophy. In AICAR-treated mdx mice, the exaggerated sensitivity of mdx diaphragm mitochondria to calcium-induced permeability transition pore opening was restored to normal levels. There were associated improvements in mdx diaphragm histopathology and in maximal force-generating capacity, which were not linked to increased mitochondrial biogenesis or up-regulated utrophin expression. These findings suggest that agonists of AMPK and other inducers of the autophagy-mitophagy pathway can help to promote the elimination of defective mitochondria and may thus serve as useful therapeutic agents in DMD.

Our data suggest that mitochondrial dysfunction lies at the nexus between oxidative stress and the impaired diaphragmatic contractility that develops during MV. Energy substrate oversupply relative to demand, resulting from diaphragmatic inactivity during MV, could play an important role in this process.

BackgroundMitochondrial injury develops in skeletal muscles during the course of severe sepsis. Autophagy is a protein and organelle recycling pathway which functions to degrade or recycle unnecessary, redundant, or inefficient cellular components. No information is available regarding the degree of sepsis-induced mitochondrial injury and autophagy in the ventilatory and locomotor muscles. This study tests the hypotheses that the locomotor muscles are more prone to sepsis-induced mitochondrial injury, depressed biogenesis and autophagy induction compared with the ventilatory muscles.Methodology/Principal FindingsAdult male C57/Bl6 mice were injected with i.p. phosphate buffered saline (PBS) or E. coli lipopolysaccharide (LPS, 20 mg/kg) and sacrificed 24 h later. The tibialis anterior (TA), soleus (SOLD) and diaphragm (DIA) muscles were quickly excised and examined for mitochondrial morphological injury, Ca++ retention capacity and biogenesis. Autophagy was detected with electron microscopy, lipidation of Lc3b proteins and by measuring gene expression of several autophagy-related genes. Electron microscopy revealed ultrastructural injuries in the mitochondria of each muscle, however, injuries were more severe in the TA and SOL muscles than they were in the DIA. Gene expressions of nuclear and mitochondrial DNA transcription factors and co-activators (indicators of biogenesis) were significantly depressed in all treated muscles, although to a greater extent in the TA and SOL muscles. Significant autophagosome formation, Lc3b protein lipidation and upregulation of autophagy-related proteins were detected to a greater extent in the TA and SOL muscles and less so in the DIA. Lipidation of Lc3b and the degree of induction of autophagy-related proteins were significantly blunted in mice expressing a muscle-specific IκBα superrepresor.Conclusion/SignificanceWe conclude that locomotor muscles are more prone to sepsis-induced mitochondrial injury, decreased biogenesis and increased autophagy compared with the ventilatory muscles and that autophagy in skeletal muscles during sepsis is regulated in part through the NFκB transcription factor.

Comparison of these results with those of studies comparing healthy glycolytic with oxidative muscle suggests that these differences may be attributable to greater type II fiber expression in COPD muscle, as mitochondria within this fiber type have respiratory function similar to that of mitochondria from type I fibers, and yet are intrinsically prone to greater release of H(2)O(2) and more resistant to PTP opening. These results thus argue against the presence of pathological mitochondrial alterations in this category of patients with COPD.

Mitochondrial quality control plays a vital role in the maintenance of optimal mitochondrial function. However, its roles and regulation remain ill-defined in cardiac pathophysiology. Here, we tested the hypothesis that PARK2/Parkin, an E3-ligase recently described as being involved in the regulation of cardiac mitophagy, is important for (1) the maintenance of normal cardiac mitochondrial function; and (2) adequate recovery from sepsis, a condition known to induce reversible mitochondrial injury through poorly understood mechanisms. Investigations of mitochondrial and cardiac function were thus performed in wild-type and Park2-deficient mice at baseline and at 2 different times following administration of a sublethal dose of E. coli lipopolysaccharide (LPS). LPS injection induced cardiac and mitochondrial dysfunctions that were followed by complete recovery in wild-type mice. Recovery was associated with morphological and biochemical evidence of mitophagy, suggesting that this process is implicated in cardiac recovery from sepsis. Under baseline conditions, multiple cardiac mitochondrial dysfunctions were observed in Park2-deficient mice. These mild dysfunctions did not result in a visibly distinct cardiac phenotype. Importantly, Park2-deficient mice exhibited impaired recovery of cardiac contractility and constant degradation of mitochondrial metabolic functions. Interestingly, autophagic clearance of damaged mitochondria was still possible in the absence of PARK2 likely through compensatory mechanisms implicating PARK2-independent mitophagy and upregulation of macroautophagy. Together, these results thus provide evidence that in vivo, mitochondrial autophagy is activated during sepsis, and that compensation for a lack of PARK2 is only partial and/or that PARK2 exerts additional protective roles in mitochondria.