Multivesicular bodies and their exosomes link poor myofiber repair with muscle degeneration in muscular dystrophy


Translational Research

Poster Number: T362


Daniel Bittel, PT, DPT, PhD, Childrens National Medical Center, Marshall Hogarth, PhD, Childrens National Medical Center, Aurelia Defour, PhD, NAMSA, Adam Bittel, DPT, PhD, Childrens National Medical Center, Sreetama Chandra, PhD, Childrens National Medical Center, James Novak, PhD, Children's National Hospital, Jyoti Jaiswal, PhD, Children's National Medical Center

Background/objective: Injury to skeletal muscle fibers is common during the course of daily activity and muscle contraction, ranging in scale from nanoruptues to micrometer-size tears of the plasma membrane. Poor repair of myofiber injuries, characteristic of several muscle diseases, leads to progressive muscle loss, chronic inflammation, and muscle degeneration. An unresolved question that has remained is whether poor repair solely leads to myofiber loss or if it also alters the muscle environment toward degeneration.
Results: We have investigated this question in Limb Girdle Muscular Dystrophy 2B (LGMD2B), where mutations in the dysferlin gene hamper myofiber repair and lead to chronic inflammation and muscle degeneration. Using live cell imaging we identified that membrane injury triggers rapid (within seconds) and robust (over 10-fold) increase in the fusion of multivesicular bodies (MVB), resulting in a bolus of released exosomes. We find MVB fusion and exosome release are regulated by calcium and Annexin A2 – a calcium-binding protein upregulated in LGM2B and other muscular dystrophies. While poor repair of dysferlin-deficient muscle cells increases MVB fusion, poor repair of Annexin A2 deficient muscle cells does not result in increased MVB fusion. This results in a greater abundance of exosomes in the endomysium of dysferlin deficient, but not in Annexin A2 deficient muscles. Improving membrane repair of dysferlin-deficient muscle reduces exosome secretion and the same can be achieved by the knockout of Annexin A2 in the dysferlin-deficient mouse model. This reduced endomysial accumulation of exosomes simultaneously reduces muscle inflammation and degeneration.
Conclusion: These studies identify myofiber injury triggers calcium-dependent exosome release, which regulates post-injury inflammatory response and highlights this as a potential target to combat chronic inflammation and muscle loss in diseases with poor myofiber repair.