In Vivo Fusion of Circulating Fluorescent Cells with Dystrophin-Deficient Myofibers Results in Extensive Sarcoplasmic Fluorescence Expression but Limited Dystrophin Sarcolemmal Expression

To investigate the therapeutic potential of bone marrow transplantation in Duchenne muscular dystrophy, green fluorescent protein-positive (GFP+) bone marrow cells were transplanted into irradiated wild-type and dystrophin-deficient mdx mice. Tibialis anterior muscles showed fivefold to sixfold more GFP+ mononucleated cells and threefold to fourfold more GFP+ myofibers in mdx than in wild-type mice. In contrast, dystrophin expression in mdx mice remained within the level of nontransplanted mdx mice, and co-expression with GFP was rare. Longitudinal sections of 5000 myofibers showed 160 GFP+ fibers, including 9 that co-expressed dystrophin. GFP was always visualized as full-length sarcoplasmic fluorescence that exceeded the span of sample length (up to 1500 μm), whereas dystrophin expression was restricted to 11 to 28% of this length. Dystrophin expression span was much shorter in GFP+ fibers (116 ± 46 μm) than in revertant fibers (654 ± 409 μm). These data suggest that soluble GFP diffuses far from the fusion site with a pre-existing dystrophin− myofiber whereas dystrophin remains mainly expressed close to the site of fusion. Because restoration of dystrophin in whole muscle fiber length is required to expect functional improvement and clinical benefits for Duchenne muscular dystrophy, future applications of cell therapies to neuromuscular disorders could be more appropriately envisaged for replacement of defective soluble sarcoplasmic proteins.