The molecular mechanisms of levamisole (LEV) activity and expression of resistance remain largely unknown in parasitic nematodes. In contrast, genetic screens for mutants that survive exposure to LEV in the free-living nematode Caenorhabditis elegans have led to the identification of five genes (unc-38, unc-63, unc-29, lev-1 and lev-8) that encode a LEV-sensitive acetylcholine receptor (L-AChR). Loss of these genes leads to LEV resistance. In this study, orthologues of these genes were identified in three species of trichostrongylid nematodes that have a major impact on small ruminants: Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis. Polymorphism associated with LEV resistance have been investigated by comparing transcripts of these subunits in LEV susceptible and LEV-resistant isolates of the three strongylid species.
Partial sequences were identified by PCR experiments and full-length cDNA sequences corresponding to AChR subunits in the three trichostrongylid species were obtained using 3′-rapid amplification of cDNA ends-PCR and 5′ rapid amplification of cDNA ends anchored with the spliced leader sequence, SL1. Expression of L-AChR subunits was investigated in LEV-resistant and LEV-susceptible isolates of H. contortus, T. circumcincta and T. colubriformis using reverse transcription PCR.
We have identified a total of 20 full-length cDNA sequences corresponding to L-AChR subunits in three parasitic trichostrongylid species of which 14 correspond to novel sequences. Genes orthologous to unc-29, unc-63, unc-38 and lev-1 were found in each trichostrongylid species, whereas no gene corresponding to lev-8 has yet been identified. We have found 11 distinct paralogous sequences corresponding to the C. elegans unc-29 gene clustered in four groups revealing an unexpected diversity of unc-29-like genes. Complete coding sequences of the L-AChR subunits in two LEV-resistant and three susceptible isolates of H. contortus, T. circumcincta and T. colubriformis were essentially unchanged, but abbreviated transcripts of the unc-63 subunit were specifically expressed in resistant isolates of all three species.
The candidate gene strategy developed in this study revealed an unexpectedly high diversity of L-AChR subunits specific to the trichostrongylid parasites that are a principal target for the drug LEV. Abbreviated variants, predicted to produce nonfunctional unc-63, were associated with LEV resistance. This study contributes significantly to a better understanding of LEV receptor constitution in parasitic nematodes and highlights the putative role of aberrant mRNA encoding L-AChR subunits in LEV resistance.