Xin as a novel regulator of mitochondrial morphology and bioenergetics in skeletal muscle

Abstract

Xin is a striated muscle specific, cytoskeletal adaptor protein that our lab has recently localized to the mitochondria and peri-mitochondrial regions of skeletal muscle. Further, we have identified mild mitochondrial structural and functional impairments in the skeletal muscle of Xin-/- mice. The objective of this study was to investigate the physiological effects of Xin deficiency in combination with the metabolic stress of a high fat diet, as well as the impact on skeletal muscle mitochondrial structure and function. Wild-type (WT) and Xin knockout (Xin-/-) mice were fed a high-fat diet (HFD- 60%kcal fat) for 8 weeks. HFD-fed Xin-/- mice did not gain greater body or fat mass relative to WT mice. However, Xin-/- mice had increased fasted blood glucose levels (Xin-/-: 15.81.042, WT: 10.70.652; p<0.05) and reduced glucose tolerance (AUC Xin-/-: 51.41.7, WT: 31.13.1; p<0.05) after 8 weeks of HFD feeding. Electron microscopy analysis revealed an ~1.5-fold (46.4%7.5; p<0.05) increase in intermyofibrillar mitochondrial content and a ~2.8-fold (186.3%9.6; p<0.05) increase in size, associated with mitochondrial swelling, streaming and loss of cristae. Complex I and complex II supported respiration were also impaired when corrected to mitochondrial content (complex I: JO2 Xin-/-: 130.722.3, WT: 210.238.8; p<0.05; complex II: JO2 Xin-/-: 124.413.9, WT: 175.620.9; p<0.05). We observed no changes to the protein content of either the autophagic proteins p62 or LC3, or the mitochondrial fission proteins Drp1 or Fis1 or fusion proteins Mfn1 or Mfn2. Overall, Xin-/- mice exhibited abnormal muscle mitochondrial morphology, decreased mitochondrial respiration and dysregulated glucose handling, independent of changes in body weight. Future studies are needed to identify Xin-specific binding partners and determine if Xin may be an unidentified contributor to mitochondrial myopathy and the development of metabolic disorders.