Role of toll-like receptors in the pathogenesis of dystrophin-deficient skeletal and heart muscle

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Although the cause of Duchenne muscular dystrophy (DMD) is known, the specific factors that initiate and perpetuate disease progression are not well understood. We hypothesized that leaky dystrophin-deficient skeletal muscle releases endogenous danger signals (TLR ligands), which bind to Toll-like receptors (TLRs) on muscle and immune cells and activate downstream processes that facilitate degeneration and regeneration in dystrophic skeletal muscle. Here, we demonstrate that dystrophin-deficient mouse muscle cells show increased expression of several cell-surface and endosomal TLRs.In vitroscreening identified ssRNA as a relevant endogenous TLR7 ligand. TLR7 activation led to myd88-dependent production of pro-inflammatory cytokines in dystrophin-deficient muscle cells, and cause significant degeneration/regenerationin vivoinmdxmouse muscle. Also, knockout of the central TLR adaptor protein, myd88 inmdxmice significantly improved skeletal and cardiac muscle function. Likewise, proof-of-concept experiments showed that treating youngmdxmice with a TLR7/9 antagonist significantly reduced skeletal muscle inflammation and increased muscle force, suggesting that blocking this pathway may have therapeutic potential for DMD.

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