Skeletal Muscle Mitochondrial Morphology Negatively Affected by Loss of Xin

Altered mitochondrial structure and function are implicated in the functional decline of skeletal muscle. Numerous cytoskeletal proteins have been reported to affect mitochondrial homeostasis, but this complex network is still being unraveled. Here, we investigated alterations to mitochondrial structure and function in mice lacking the cytoskeletal adapter protein, Xin. Xin deficient (Xin-/-) and wild-type (WT) littermate mice were fed a chow or high-fat diet (HFD; 60% kcal fat) for 8 weeks before high-resolution respirometry, histology, electron microscopy and Western blot analyses of their skeletal muscles were conducted. Immuno-electron microscopy and immunofluorescence staining indicates that Xin is present in the mitochondria and peri-mitochondrial areas, as well as the myoplasm. Intermyofibrillar mitochondria in chow-fed Xin-/- mice were notably different from WT; frequently spanning a whole sarcomere and/or swollen in appearance with abnormal cristae. Succinate Dehydrogenase and Cytochrome Oxidase IV (COX) activity staining indicated greater evidence of mitochondrial enzyme activity in Xin-/- mice. HFD did not result in a difference between cohorts with respect to body mass gains or glucose handling. However, electron microscopy revealed significantly greater mitochondrial density ([~]2.1-fold) with evident structural abnormalities (swelling, reduced cristae density) in Xin-/- mice. Complex I and II-supported respiration were not different between groups per mg muscle, but when made relative to mitochondrial density, were significantly lower in Xin-/- muscles. Western blotting of fusion, fission, and autophagy proteins revealed no differences between groups. These results provide the first evidence for a role of Xin in maintaining mitochondrial morphology and function but not in regulating mitochondrial dynamics.