Limb Girdle Muscular Dystrophy type 2B (LGMD2B) is caused by loss of function mutations in the Dysferlin gene, which regulates vesicle fusion events to repair damaged muscle membranes. Exactly how loss of Dysferlin leads to LGMD2B phenotypes is unknown. For the past fifteen years, C. elegans
fer-1, the founding member of the Dysferlin gene family was thought to be sperm specific. However, we recently demonstrated that
fer-1 is also expressed in muscle and that
fer-1 mutants exhibit locomotory defects. These data suggest that C. elegans is an excellent model system to explore the function of Dysferlin in muscle and our analysis may provide new mechanistic insights into LGMD2B pathogenesis. To begin investigating the basis of the
fer-1 locomotory defect, we explored the effects of
fer-1 mutations on muscle organizational properties and found sarcomere structure to be unaffected in
fer-1 mutants. We recently identified a general defect in the localization of levamisole-sensitive acetylcholine receptors (AChRs), nicotine-sensitive AChRs and GABA receptors on the body wall muscles of
fer-1mutants. Protein levels of the AChR subunit protein UNC-29 are similar in both the wild-type and
fer-1 mutants, suggesting that defects in receptor clustering in the
fer-1 mutant are not caused by a decrease in post-synaptic receptor expression. Consistent with a loss of post-synaptic AChRs,
fer-1 mutants exhibit resistance to the cholinergic agonists levamisole, aldicarb and nicotine, and transgenic expression of wild-type
fer-1 is sufficient to rescue the mutant phenotype.
fer-1 mutants also exhibit defects in pharyngeal muscle activity and these defects can be corrected through acute inhibition of acetylcholinesterase, again suggesting that
fer-1 mutants alter cholinergic signaling. Surprisingly, in situ electrophysiological analysis reveals that loss of
fer-1 does not affect the amplitude or kinetics of the synaptically-evoked cholinergic current in body-wall muscles. In conclusion, our data suggest that loss of
fer-1 affects the clustering of multiple post-synaptic receptors at the NMJ, and we are continuing to explore possible mechanisms that may explain how receptor localization defects translate into
fer-1 mutant phenotypes. Integration of our current findings with previous models of Dysferlin-mediated vesicle fusion, suggests that loss of FER-1/Dysferlin may cause a reduction in the fusion of neurotransmitter receptor-containing vesicles at the NMJ, leading to defects in synaptic function that may contribute to LGMD2B phenotypes.