Duchenne muscular dystrophy (DMD) is characterized by progressive muscle degeneration driven in part by chronic inflammation. While loss of dystrophin leads to contraction-induced damage and secondary inflammatory responses, the mechanisms that sustain chronic immune activation within dystrophic muscle remain incompletely understood. We previously identified inflammation-induced microRNAs (inflamma-miRs), including miR-146a and miR-142-3p, that are elevated in dystrophic muscle. Here, we show that these microRNAs are packaged into exosomes in response to inflammatory stimuli and can function as endogenous ligands for the innate immune receptor Toll-like receptor 7 (TLR7). This is particularly relevant because TLR7, while normally restricted to immune cells, is mis-expressed in dystrophic myofibers.
Genetic deletion or pharmacologic inhibition of a single inflamma-miR, miR-146a, improves muscle function and histopathology in mdx mice, supporting a pathogenic role for this microRNA in dystrophic disease. To further explore mechanisms that may facilitate this immune signaling axis, we performed unbiased lipidomics in wild-type and mdx gastrocnemius muscles. Across multiple sphingolipid classes, we observed coordinated enrichment of C16:0 acyl chain–containing species in dystrophic muscle. This lipid profile was accompanied by increased expression of ceramide synthase 6 (CerS6), the enzyme responsible for C16 sphingolipid production, consistent with increased flux through the de novo sphingolipid pathway.
C16 sphingolipids are known to influence membrane organization and vesicle biogenesis. In parallel, we observed increased release of extracellular vesicles, including exosomes, from dystrophic muscle, and inhibition of exosome release improved mdx muscle histopathology and function in vivo. Together, these findings suggest that sphingolipid remodeling may contribute to a muscle environment permissive for sustained innate immune signaling by facilitating exosome-mediated delivery of inflamma-miRs to TLR7-expressing myofibers.