Expression of Eph receptors in skeletal muscle and their localization at the neuromuscular junction

The participation of ephrins and Eph receptors in guiding motor axons durin g muscle innervation has been well documented, but little is known about th eir expression and functional significance in muscle at later developmental stages. Our present study investigates the expression and localization of Eph receptors and ephrins in skeletal muscle. Prominent expression of EphA4 , EphA7, and ephrin-A ligands was detected in muscle during embryonic devel opment. More importantly, both EphA4 and EphA7, as well as ephrin-A2, were localized at the neuromuscular junction (NMJ) of adult muscle. Despite thei r relative abundance, they were not localized at the synapses during embryo nic stages. The concentration of EphA4, EphA7, and ephrin-A2 at the NMJ was observed at postnatal stages and the synaptic localization became prominen t at later developmental stages. In addition, expression of Eph receptors w as increased by neuregulin and after nerve injury. Furthermore, we demonstr ated that overexpression of EphA4 led to tyrosine phosphorylation of the ac tin-binding protein cortactin and that EphA4 was coimmunoprecipitated with cortactin in muscle. Taken together, our findings indicate that EphA4 is as sociated with the actin cytoskeleton. Since actin cytoskeleton is critical to the formation and stability of NMJ, the present findings raise the intri guing possibility that Eph receptors may have a novel role in NMJ formation and/or maintenance.