Congenital myasthenic syndrome caused by novel loss-of-function mutations in the human AChR epsilon subunit gene

A Cys-loop mutation in the Caenorhabditis elegans nicotinic receptor subunit UNC-63 impairs but does not abolish channel function

The nematode Caenorhabditis elegans is an established model organism for studying neurobiology. UNC-63 is a C. elegans nicotinic acetylcholine receptor (nAChR) α-subunit. It is an essential component of the levamisole-sensitive muscle nAChR (L-nAChR) and therefore plays an important role in cholinergic transmission at the nematode neuromuscular junction. Here, we show that worms with the unc-63(x26) allele, with its αC151Y mutation disrupting the Cys-loop, have deficient muscle function reflected by impaired swimming (thrashing). Single-channel recordings from cultured muscle cells from the mutant strain showed a 100-fold reduced frequency of opening events and shorter channel openings of L-nAChRs compared with those of wild-type worms. Anti-UNC-63 antibody staining in both cultured adult muscle and embryonic cells showed that L-nAChRs were expressed at similar levels in the mutant and wild-type cells, suggesting that the functional changes in the receptor, rather than changes in expression, are the predominant effect of the mutation. The kinetic changes mimic those reported in patients with fast-channel congenital myasthenic syndromes. We show that pyridostigmine bromide and 3,4-diaminopyridine, which are drugs used to treat fast-channel congenital myasthenic syndromes, partially rescued the motility defect seen in unc-63(x26). The C. elegans unc-63(x26) mutant may therefore offer a useful model to assist in the development of therapies for syndromes produced by altered function of human nAChRs.

Acetylcholine receptors direct rapsyn clusters to the neuromuscular synapse in zebrafish

Clustering of nicotinic muscle acetylcholine receptors (AChRs) requires association with intracellular rapsyn, a protein with an intrinsic ability to self-cluster. Previous studies on sofa potato (sop), an AChR null line of zebrafish, have suggested that AChRs may play an active role in subsynaptic localization of rapsyn clusters. To test this proposal directly, we identified and cloned the gene responsible for the sop phenotype and then attempted to rescue subsynaptic localization of the receptor-rapsyn complex in mutant fish. sop contains a leucine to proline mutation at position 28, near the N terminus of the zebrafish AChR delta subunit. Transient expression of mutant delta subunit in sop fish was unable to restore surface expression of muscle AChRs. In contrast, expression of wild-type delta subunit restored the ability of muscle to assemble surface receptors along with the ability of fish to swim. Most importantly, the ability of rapsyn clusters to localize effectively to subsynaptic sites also was rescued in large part. Our results point to direct involvement of the AChR molecule in restricting receptor-rapsyn clusters to the synapse.