Neuregulin effect on quantal content dissociated from effect on miniature endplate potential amplitude

Rescuing Z+ agrin splicing in Nova null mice restores synapse formation and unmasks a physiologic defect in motor neuron firing

Synapse formation at the neuromuscular junction (NMJ) requires an alternatively spliced variant of agrin (Z(+) agrin) that is produced only by neurons. Here, we show that Nova1 and Nova2, neuron-specific splicing factors identified as targets in autoimmune motor disease, are essential regulators of Z(+) agrin. Nova1/Nova2 double knockout mice are paralyzed and fail to cluster AChRs at the NMJ, and breeding them with transgenic mice constitutively expressing Z(+) agrin in motor neurons rescued AChR clustering. Surprisingly, however, these rescued mice remained paralyzed, while electrophysiologic studies demonstrated that the motor axon and synapse were functional-spontaneous and evoked recordings revealed synaptic transmission and muscle contraction. These results point to a proximal defect in motor neuron firing in the absence of Nova and reveal a previously unsuspected role for RNA regulation in the physiologic activation of motor neurons.

Neuregulins at the neuromuscular synapse: Past, present, and future

At the developing vertebrate neuromuscular junction, neuregulins are growth/differentiation factors essential for terminal Schwann cell survival. Neuregulins have also been thought as the critical signals responsible for the increased transcription of acetylcholine receptor subunit genes at the neuromuscular synapse. This latter role is now highly controversial. This article reviews the evidence that has shaped the views of the neuregulins and how these views have been challenged. The most recent experiments indicate that neuregulin signaling to postsynaptic muscle fibers may modulate, rather than determine, acetylcholine receptor expression at the neuromuscular junction. Based on findings from my lab and those of others, I propose that this modulation might involve novel posttranscriptional molecular mechanisms. Finally, I also suggest that neuregulin signaling may have an important role to play in mediating the response of adult terminal Schwann cells to denervation.