Methyllycaconitine-sensitive neuronal nicotinic receptor-operated slow Ca2+ signal by local application or perfusion of ACh at the mouse neuromuscular junction

acr-23 Encodes a monepantel-sensitive channel in Caenorhabditis elegans

Monepantel is a member of the recently identified class of anthelmintics known as the amino-acetonitrile derivatives (AADs). Monepantel controls all major gastro-intestinal nematodes in sheep including those that are resistant to the classical anthelmintics. Previous studies have shown that the Caenorhabditis elegans acr-23 and the Haemonchus contortus Hco-mptl-1 genes may be prominent targets of monepantel. With this discovery it became possible to investigate the mode of action of monepantel in nematodes at the molecular level. In the present study, we show that a C. elegans mutant acr-23 strain is fully rescued by expressing the wild-type acr-23 gene. Moreover, we present a new mutant allele, and characterize acr-23 alleles genetically. We also show that acr-23 is expressed in body wall muscle cells, and provide therefore a possible explanation for the paralysis caused by monepantel. Furthermore, genetic evidence suggests that the chaperone RIC-3 is required for expression of full monepantel resistance. Finally, we present reconstitution of the C. elegans ACR-23 receptor in Xenopus laevis oocytes and provide direct evidence of its modulation by monepantel. Conversely, co-injection of the chaperone RIC-3 had no impact for channel reconstitution in X. laevis oocytes. These results reinforce the involvement of the ACR-23 family in the mode of action of monepantel and advance our understanding of this new class of anthelmintics.

Worldwide, sheep and cattle farming are endangered by anthelmintic-resistant gastro-intestinal nematodes. Monepantel, a member of the recently identified class of anthelmintics known as the amino-acetonitrile derivatives (AADs) exhibits excellent efficacy against various species of livestock-pathogenic nematodes and, more importantly, overcomes existing resistances to the currently available anthelmintics. Previous studies conducted with the non-parasitic model nematode Caenorhabditis elegans have found that acr-23 mutants are fully resistant to monepantel. In this study, we confirm the role of acr-23 as a primary target of monepantel by 1) restoring monepantel sensitivity in the presence of extrachromosomal copies of wild-type acr-23, 2) finding that ACR-23 is mainly expressed in muscle cells, and 3) showing that acr-23 forms a monepantel-sensitive channel in Xenopus oocytes.

Inositol-1,4,5-triphosphate receptors mediate activity-induced synaptic Ca2+ signals in muscle fibers and Ca2+ overload in slow-channel syndrome

Strict control of calcium entry through excitatory synaptic receptors is important for shaping synaptic responses, gene expression, and cell survival. Disruption of this control may lead to pathological accumulation of Ca2+. The slow-channel congenital myasthenic syndrome (SCS), due to mutations in muscle acetylcholine receptor (AChR), perturbs the kinetics of synaptic currents, leading to post-synaptic Ca2+ accumulation. To understand the regulation of calcium signaling at the neuromuscular junction (NMJ) and the etiology of Ca2+ overload in SCS we studied the role of sarcoplasmic Ca2+ stores in SCS. Using fura-2 loaded dissociated fibers activated with acetylcholine puffs, we confirmed that Ca2+ accumulates around wild type NMJ and discovered that Ca2+ accumulates significantly faster around the NMJ of SCS transgenic dissociated muscle fibers. Additionally, we determined that this process is dependant on the activation, altered kinetics, and movement of Ca2+ ions through the AChR, although, surprisingly, depletion of intracellular stores also prevents the accumulation of this cation around the NMJ. Finally, we concluded that the sarcoplasmic reticulum is the main source of Ca2+ and that inositol-1,4,5-triphosphate receptors (IP3R), and to a lesser degree L-type voltage gated Ca2+ channels, are responsible for the efflux of this cation from intracellular stores. These results suggest that a signaling system mediated by the activation of AChR, Ca2+, and IP3R is responsible for localized Ca2+ signals observed in muscle fibers and the Ca2+ overload observed in SCS.

Mouse muscle denervation increases expression of an alpha7 nicotinic receptor with unusual pharmacology

Neuronal nicotinic alpha7 subunits have been found in chick and rat skeletal muscle during development and denervation. In the present study, reverse transcriptase-polymerase chain reaction was used to detect alpha7 subunit mRNA in denervated mouse muscle. To determine whether the alpha7 subunit forms functional nicotinic acetylcholine receptors (nAChRs) in muscle, choline was used to induce a membrane depolarization because choline has been considered a specific agonist of alpha7-containing (alpha7*) nAChRs. We found, however, that choline (3-10 mM) also weakly activates muscle nAChRs. After inhibiting muscle nAChRs with a specific muscle nAChR inhibitor, alpha-conotoxin GI (alphaCTxGI), choline was used to activate the alpha7* nAChRs on muscle selectively. Four weeks after denervation, rapid application of choline (10 mM) elicited a substantial depolarization in the presence of alphaCTxGI (0.1 microM). This component of the depolarization was never present in denervated muscles obtained from mutant mice lacking the alpha7 subunit (i.e. alpha7-null mice). The depolarization component that is resistant to alphaCTxGI was antagonized by pancuronium (3-10 microM) and by a 4-oxystilbene derivative (F3, 0.1-0.5 microM) at concentrations considered highly specific for alpha7* nAChRs. Another selective alpha7 antagonist, methyllycaconitine (0.05-5 microM), did not strongly inhibit this choline-induced depolarization. Furthermore, the choline-sensitive nAChRs showed little desensitization over 10 s of application with choline (10-30 mM). These results indicate that functional alpha7* nAChRs are significantly present on denervated muscle, and that these receptors display unusual functional and pharmacological characteristics.