Mitochondrial Permeability Transition in Skeletal Muscle Phenocopies Muscle Alterations seen in Cancer Cachexia and other Wasting Conditions

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ABSTRACT Skeletal muscle in wasting conditions often exhibits atrophy, mitochondrial respiratory dysfunction, and fragmentation of the acetylcholine receptor (AChR) cluster at the endplate. The accompanying alterations of mitochondrial morphology suggest mitochondria may be involved in muscle pathology in these conditions. To address this gap, we tested an established pathological mechanism in ischemia-reperfusion injury and neurodegeneration but poorly studied in skeletal muscle: mitochondrial permeability transition (mPT). We tested if mPT recapitulated phenotypes common in wasting conditions, whether tumor-conditioned media (TCM) could promote mPT, and compared differentially expressed genes (DEGs) induced by mPT with DEGs observed in a mouse model of pancreatic cancer cachexia. Inducing mPT in mouse skeletal muscle bundles progressively altered mitochondrial cristae morphology, culminating in breach of the outer mitochondrial membrane. Inducing mPT in mouse muscle fibers increased mROS and Caspase 3 activity and caused atrophy. Inducing mPT caused a complex I mitochondrial respiratory impairment, increased lysosome-mitochondrion co-localization, and fragmented the AChR cluster at the muscle endplate. The Ca 2+ threshold for mPT, mitochondrial calcein colocalization and mitochondrial membrane potential were reduced by TCM in skeletal muscle or C2C12 myoblasts, respectively. Knockout of the mPT-regulating protein CypD attenuated the reduction in Ca 2+ threshold for mPT by TCM. Inhibitors of mPT attenuated atrophy with TCM in C2C12 and human primary myotubes. Finally, there was overlap between the DEGs of mPT and diaphragm muscle in a mouse model of pancreatic cancer cachexia during the muscle wasting phase. We conclude that mPT should be explored as a therapeutic target in muscle wasting disorders. Graphic Abstract Legend Skeletal muscle wasting conditions are characterized by muscle fiber atrophy, mitochondrial respiratory dysfunction, mitochondrial depletion, and fragmentation of acetylcholine receptor (AChR) clusters at the neuromuscular junction. Here, we identify mitochondrial permeability transition (mPT) as a mechanism that recapitulates these pathological features. mPT was induced pharmacologically with Bz423 or Ferutinin, or promoted by tumor-derived factors present in tumor-conditioned media. Induction of mPT increased mitochondrial reactive oxygen species (mROS) production and Caspase-3 (Casp3) activation, resulting in muscle fiber atrophy and AChR fragmentation. Furthermore, mPT induction in C2C12 and human primary myotubes caused myotube atrophy, which was attenuated by pharmacological inhibition of mPT. mPT also caused a complex I-specific mitochondrial respiratory impairment and increased mitochondrial co-localization with lysosomes. Together, these findings identify mPT as a novel mechanism of pathological phenotypes associated with muscle wasting and support mPT as a promising therapeutic target. Key Points Mitochondrial permeability transition (mPT) induces marked alterations in mitochondrial morphology and muscle phenotypes that are common in wasting conditions mPT is promoted by tumor-derived factors in a manner that depends upon the mPT-regulating protein CypD mPT generates transcriptional alterations that overlap with cachectic muscle in a mouse model of pancreatic cancer, particularly during the period of muscle wasting Pharmacological targeting of mPT attenuates or prevents atrophy in C2C12 and human primary myotubes, respectively

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