Abstract
Agrin released by motoneurons induces and/or maintains acetylcholine receptor (AChR) clustering and other aspects of postsynaptic differentiation at the vertebrate neuromuscular junction. Agrin acts by binding and activating a receptor complex containing LDL receptor protein 4 (Lrp4) and muscle-specific kinase (MuSK). Two critical downstream components of this signaling cascade, Dox-7 and rapsyn, have been identified. However, additional intracellular essential elements remain unknown. Prior observations by others and us suggested antagonistic interactions between agrin and neuregulin-1 (Nrg-1) signaling in cultured myotubes and developing muscle fibers in vivo. A hallmark of Nrg-1 signaling in skeletal muscle cells is the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). ERK1/2 are also activated in most cells by phorbol 12-myristate 13-acetate, a classical inhibitor of agrin-induced AChR clustering in myotubes. Here, it was investigated whether agrin activates ERK1/2 directly and whether such activation modulates agrin-induced AChR clustering. Agrin induced a rapid but transient activation of ERK1/2 in myotubes that was Lrp4/MuSK-dependent. However, blocking this ERK1/2 activation did not prevent but potentiated AChR clustering induced by agrin. ERK1/2 activation was dispensable for Nrg-1-mediated inhibition of the AChR clustering activity of agrin, but was indispensable for such activity by phorbol 12-myristate 13-acetate. Together, these results suggest agrin-induced activation of ERK1/2 is a negative modulator of agrin signaling in skeletal muscle cells.