The neurotrophin receptor p75NTR is induced on mature myofibres in inflammatory myopathies and promotes myotube survival to inflammatory stress.

AIMS: Recent studies propose the neurotrophin receptor p75NTR as a marker for muscle satellite cells and a key regulator of regenerative processes after injury. Here, we investigated the contribution of cellular compartments other than satellite cells and regenerating myofibres to p75NTR signal in diseased skeletal muscle. METHODS: We checked regulation of p75NTR expression in muscle biopsies from patients with inflammatory myopathies (polymyositis, dermatomyositis and inclusion body myositis), or Becker muscular dystrophy, and in nonmyopathic tissues. Quantitative PCR, immunohistochemistry, immunofluorescence or electron microscopy were used. RNA interference approaches were applied to myotubes to explore p75NTR function. RESULTS: We found p75NTR transcript and protein upregulation in all inflammatory myopathies but not in dystrophic muscle, suggesting a role for inflammatory mediators in induction of p75NTR expression. In inflamed muscle p75NTR was localized on distinct cell types, including immune cells and mature myofibres. In vitro assays on human myotubes confirmed that inflammatory factors such as IL-1 could induce p75NTR. Finally, RNA interference experiments in differentiated cells showed that, in the absence of p75NTR, myotubes were more susceptible to apoptosis when exposed to inflammatory stimuli. CONCLUSIONS: Our observations that p75NTR is upregulated on skeletal myofibres in inflammatory myopathies in vivo and promotes resistance to inflammatory mediators in vitro suggest that neurotrophin signalling through p75NTR may mediate a tissue-protective response to inflammation in skeletal myofibres.

Altered extracellular matrix transcript expression and protein modulation in primary Duchenne muscular dystrophy myotubes.

Extent of muscle fibrosis contributes to disease severity in muscular dystrophies. To investigate whether extracellular matrix (ECM) components contribute to the severe fibrosis observed in Duchenne muscular dystrophy (DMD) skeletal muscle, we quantitated several ECM components (transcripts and proteins) in primary DMD and control myotube cultures. We evaluated the fibrogenic transforming growth factor- beta1 (TGF-beta1); the small pleiotropic proteoglycan decorin, involved in collagen fibrillogenesis and TGF-beta1 modulation; metalloproteinases MMP-2 and MMP-9; tissue inhibitors of metalloproteinase (TIMP) 1, 2 and 3; collagens I and VI; and the tissue factor myostatin that inhibits muscle growth. Dystrophic myotube cultures had significantly lower levels of decorin mRNA, as also observed in DMD muscle biopsies, and significantly higher levels of TGF-beta1, myostatin, and collagens I and VI. MMP-2, TIMP-1 and TIMP-2 transcript levels were also significantly increased in DMD, but MMP-9 and TIMP-3 transcripts were unchanged. By zymography, MMP-2 activity was significantly higher in DMD than control. Protein levels were similar in DMD and controls but myostatin protein was significantly increased in DMD. We have found that transcript expression and protein modulation of several ECM components is altered in DMD muscle cells in vitro, indicating that these cells contribute fundamentally to the pathological process, since the inflammation and degeneration characterizing DMD muscle in vivo are presumably absent in culture. Our findings that myostatin-potent inhibitor of satellite cell activation and muscle renewal--is increased, and that decorin-binder and downregulator of TGFbeta1 and myostatin--is decreased, may have implications for DMD therapy to reduce muscle fibrosis.