NEUREGULINS AND ERBB RECEPTORS AT NEUROMUSCULAR-JUNCTIONS AND AT AGRIN-INDUCED POSTSYNAPTIC-LIKE APPARATUS IN SKELETAL-MUSCLE

We demonstrate by immunohistochemistry that at least two isoforms of n euregulin (NRG) are concentrated at neuromuscular junctions in adult r at muscles. One is NRG beta 3, a secreted protein which is bound to ba sal lamina that occupies the synaptic cleft. The other(s), NRG-a, is i n the muscle fibers' plasma membrane. We show further that muscle NRG, including NRG-a, is concentrated at postsynaptic-like apparatus induc ed to form in the extra-junctional region of the soleus muscle by expo sure to neural agrin. The agrin-induced postsynaptic-like apparatus al so includes aggregates of the NRG receptors erbB2 and erbB3 as does po stsynaptic apparatus at neuromuscular junctions. These findings togeth er with those of others suggest a mechanism by which neural agrin indu ces the expression of epsilon-AChR subunits in postsynaptic-like appar atus, and they support the hypothesis that agrin has a similar functio n at neuromuscular junctions.