FYN-dependent muscle-immune interaction after sciatic nerve injury.

Denervation of skeletal muscles results in loss of muscle mass and contractile force. Recent evidence suggests that local immune system activation plays a key role in these processes, but the mechanisms underlying muscle-immune system cross-talk are not understood. The purpose of this study was to address the mechanisms by which muscle responds to denervation and to elucidate the specific role played by FYN in local immune system activation. We studied initial events taking place in the gastrocnemius of wild-type and Fyn(-/-) mice following sciatic nerve transection. Discontinuous sucrose gradient centrifugation was used to prepare lipid rafts at different time-points (1, 7, and 14 days) after surgery. Activation of FYN, cytokine expression (IL-1beta and TNF-alpha), and T-cell activation (CD3 and IL-15) were followed by in vitro kinase assays, enzyme-linked immunoassay (ELISA), Western blotting, and immunoprecipitation. Sciatic nerve injury resulted in increased SRC kinase activity in gastrocnemius lipid rafts. Production of both IL-1beta and TNF-alpha was increased, peaking after 1 day, followed after 7 and 14 days by upregulation of IL-15 and CD3 expression and the development of caveolin-3 and CD3 complexes. The integrity of lipid rafts and the upregulation of SRC kinase activity, cytokine expression, and T-cell activation and cross-talk with muscle cells following denervation were abolished in Fyn(-/-) mice. The integrity of FYN-dependent lipid rafts is required for local immune system activation within denervated muscle, and lipid rafts are implicated in orchestrating muscle-immune-cell cross-talk. These results are likely to provide new insights into the therapy of neuromuscular injury.