Eph receptor signaling has recently been implicated in the regulation of synapse formation and plasticity. At the neuromuscular junctions (NMJs), Eph receptors are detected at the postsynaptic sites and proposed to play roles in the formation/maintenance of the NMJs. The first part of this study characterized that ankyrin repeat-rich membrane spanning (ARMS), a substrate for Eph and Trk receptors, was concentrated at the NMJs and interacted with a PDZ domain protein, α-syntrophin. Coexpression of ARMS enhanced EphA4 signaling, which was further augmented by the presence of α-syntrophin. Moreover, the ephrin-A1 ligand-stimulated tyrosine phosphorylation of EphA4 was reduced in C2C12 myotubes after the blockade of ARMS and α-syntrophin expression. Finally, α-syntrophin-null mice exhibited...[ Read more ]

Eph receptor signaling has recently been implicated in the regulation of synapse formation and plasticity. At the neuromuscular junctions (NMJs), Eph receptors are detected at the postsynaptic sites and proposed to play roles in the formation/maintenance of the NMJs. The first part of this study characterized that ankyrin repeat-rich membrane spanning (ARMS), a substrate for Eph and Trk receptors, was concentrated at the NMJs and interacted with a PDZ domain protein, α-syntrophin. Coexpression of ARMS enhanced EphA4 signaling, which was further augmented by the presence of α-syntrophin. Moreover, the ephrin-A1 ligand-stimulated tyrosine phosphorylation of EphA4 was reduced in C2C12 myotubes after the blockade of ARMS and α-syntrophin expression. Finally, α-syntrophin-null mice exhibited a disrupted localization of ARMS and EphA4 at the NMJ. These results suggest that ARMS plays an important role in regulating postsynaptic signal transduction through the syntrophin-mediated localization of EphA4.

In the central nervous system, the morphological changes of dendritic spines are thought to be crucial for synaptic plasticity, and dendritic spines are retracted upon the activation of EphA4 receptor. The second part of this study is to explore the underlying mechanisms that control this process. Stimulation of EphA4 by ephrin-A1 induced spine retraction in hippocampal neurons, while blockade of Cdk5 activity inhibited ephrin-A1-triggered spine retraction and reduction of mEPSC frequency. Activation of EphA4 resulted in the recruitment of Cdk5 to EphA4, concomitant with increased Cdk5 activity. EphA4 and Cdk5 then cooperatively enhanced the activity of ephexin1, a guanine nucleotide exchange factor that regulated RhoA GTPase activation. Cdk5-dependent phosphorylation of ephexin1 was required for ephrin-A1-stimulated reduction of spine density. These findings demonstrate that ephrin-A1 promotes EphA4-dependent spine retraction through the regulation of Cdk5 and ephexin1.