1H MRS Measurements Of Myocellular Lipids After Injury To Healthy And Dystrophic Muscles

Abstract

Duchenne muscular dystrophy (DMD) and mdx mice (an animal model for human DMD) are considered homologous based on the lack of dystrophin. The parallels in damage between injured and dystrophic skeletal muscles are striking; both are characterized by inflammation, enzyme efflux, fiber splitting, regeneration, etc. Interestingly, there is little evidence of fatty infiltration in the muscles of mdx mice when compared to patients with DMD, and the role of fat post injury is unclear. Fat is stored inside fibers as droplets (intra-myocellular lipids, IMCL) or in adipocytes between myofibers (extra-myocellular lipids, EMCL) and can be identified using magnetic resonance spectroscopy. PURPOSE: To examine the effects of injury on IMCL and EMCL in muscles of healthy mice and mdx mice. We hypothesized that muscles from mdx and injured mice would have increased fatty content after injury. METHODS: Unilateral injury to the tibialis anterior muscle (TA) was induced by 15 lengthening contractions in mdx mice (n=4) and control mice (n=4). High resolution proton density-weighted (100 μm × 100 μm × 750 µm) and in vivo localized 1H MRS were acquired on a Bruker 7T animal MRI system. Oil red O (ORO) staining of sections was used to identify lipid depositions. RESULTS: Mdx and control muscles generated a similar amount of absolute force, but mdx were significantly more susceptible to injury (85% vs 42% loss of force, respectively, p<0.01). EMCL to total metabolic peaks (EMCL/total) ratio significantly increased in the injured TA (0.23/0.60) compared to uninjured TA (0.05/0.08) (p<0.05). IMCL/total ratio also significantly increased in the injured TA (0.03/0.04) compared to the uninjured TA (0.06/0.17). For mdx muscles, the EMCL peak did not change (injured leg 0.05/0.06, uninjured 0.02/0.18). IMCL ratio was 0, but increased to 0.07/0.36 after injury. ORO staining confirmed the changes quantified by the 1H MRS findings. CONCLUSIONS: The data suggest that 1H MRS can be used as a biomarker to determine metabolic alterations in skeletal muscle associated with injury and/or disease. In the control muscles, EMCL and IMCL ratios are associated with the amount of force loss, but the mdx muscles present a more complex picture. This work was supported by grants to RML from the NIH (K01AR053235 and 1R01AR059179) and from the Muscular Dystrophy Association