Laminin-induced activation of Rac1 and JNKp46 is initiated by Src family kinases and mimics the effects of skeletal muscle contraction

Binding of laminin to dystroglycan in the dystrophin glycoprotein complex causes signaling through dystroglycan-syntrophin-grb2-SOS1-Rac1-PAK1-JNK. Laminin binding also causes syntrophin tyrosine phosphorylation to initiate signaling. The kinase responsible was investigated here. PP2 and SU6656, specific inhibitors of Src family kinases, decreased the amount of phosphotyrosine syntrophin and decreased the level of active Rac1 in laminin-treated myoblasts, myotubes, or skeletal muscle microsomes. c-Src and c-Fyn both phosphorylate syntrophin, and inhibition of either with specific siRNAs diminishes the level of syntrophin phosphorylation. When the rat gastrocnemius was contracted, the level of Rac1 activation increased compared to that of the relaxed control muscle and Rac1 colocalized with beta-dystroglycan. Similar results were obtained when the muscle was stretched. Contracted muscle also contained more activated c-Jun N-terminal kinase, JNKp46. E3, an expressed protein containing only laminin domains LG4 and LG5, increased the rate of proliferation of myoblasts, and PP2 prevented cell proliferation. In addition, Src family kinases colocalized with activated Rac1 and with laminin-Sepharose in solid-phase binding assays. Thus, contraction, stretching, or laminin binding causes recruitment of Src family kinase to the dystrophin glycoprotein complex, activating Rac1 and inducing downstream signaling. The DGC likely represents a mechanoreceptor in skeletal muscle-regulating muscle growth in response to muscle activity. Src family kinases play an initiating and critical role.