Proceedings of The Physiological Society

Mitochondria: Form and function (London, UK) (2017) Proc Physiol Soc 38, SA05

Research Symposium

Mitochondrial fusion dynamics in the skeletal muscle as a target of fibrosis

V. Eisner1

1. Pontificia Universidad Católica de Chile, Santiago, RM, Chile.

Mitochondria are key players in the patho-physiology of skeletal and cardiac muscle. The quality control of mitochondria relays on fusion and fission processes. We demonstrated that the fusion of mitochondria is frequent in the skeletal muscle and determines excitation-contraction coupling (EEC), while in the heart, EEC-derived oscillatory calcium transients prompt mitochondria to undergo fusion. We also showed that chronic alcohol consumption is detrimental to mitochondrial fusion dynamics, both in heart and muscle. One of the hallmarks of alcoholic myopathy is fibrosis, an abnormal deposition of extracellular matrix, that leads to muscle dysfunction. Currently, we are studying mitochondrial fusion dynamics in Duchene Muscular Dystrophy (DMD), a fibrotic paradigm. Particularly, we study the adult muscle of WT and mdx mice ((6-9 months old), in a CTGF +/+ or +/- background. CTGF is one of the signals leading to accumulation of extracellular matrix. Skeletal muscle from mdx CTGF+/- mice shows inhibition of fibrosis and improved muscle function. We studied mitochondrial organization and fusion frequency in FDB (Flexor Digitorum Brevis) muscle isolated fibers, previously electroporated with mtDsRed and mtPA-GFP (photoswitchable form of GFP) expressing DNA plasmids, by means of in vivo confocal microscopy. Our data demonstrate that mdx-derived fibers, displayed disorganized topology and significant inhibition of mitochondrial fusion frequency, compared to WT fibers. Strikingly, mdx CTGF+/- restored mitochondrial organization pattern and fusion frequency to WT-like levels. In addition, mitochondrial fusion protein Mfn2 was significantly inhibited in mdx mice, however, CTGF+/- mdx prevented Mfn2 depletion. Furthermore, the ultrastructure of FDB muscles, tested by Transmission Electron Microscopy (TEM), showed highly disorganized topology and altered distribution of mitochondrial size, in the adult muscle of mdx mice, that was significantly restored in the CTGF +/- mdx genotype. We then rescued Mfn2 protein in skeletal muscle of mdx mice, by acute exogenous electroporation. Expression of Mfn2 in FDB of mdx animals lead to recovery of mitochondrial fusion frequency to WT levels. Mitochondrial topology, number and size tested by TEM, was also restored. Finally, we tested the skeletal muscle strength development in tibialis anterior, finding partial restoration in mdx + Mfn2 muscles, in comparison to mdx skeletal muscle, without changes in the number of regenerative fibers or fibrosis. Thus, our data suggests that fibrosis targets mitochondrial fusion dynamics and topology in a model of DMD, by inhibition of Mfn2. Genetic inhibition of fibrosis or recue of Mfn2 restores mitochondrial fusion and skeletal muscle function.

Where applicable, experiments conform with Society ethical requirements