Are Caenorhabditis elegans receptors useful targets for drug discovery: Identification of genes encoding seven potential biogenic amine receptors in the parasitic nematode Brugia malayi and pharmacological comparison of tyramine receptor homologues from C

Filarial nematodes, such as Brugia malayi, cause major health problems worldwide. At present, no vaccine against B. malayi is available and current chemotherapy is ineffective against adult parasites. Biogenic amines (BAs) regulate a number of key processes in nematodes, suggesting that nematode BA receptors may be useful targets for drug discovery. Therefore, we have used a bioinformatics approach to identify genes encoding putative B. malayi BA receptors, using BA receptors recently identified in the free-living nematode, Caenorhabditis elegans. Surprisingly, the B. malayi genome appears to contain less than half of the genes predicted to encode BA receptors in the genomes of C. elegans or C. briggsae; however, all of the predicted B. malayi receptors have clear orthologues in C. elegans. The B. malayi genes encode each of the major BA receptor subclasses, including three serotonin, two dopamine and two tyramine (TA) receptors. There is little overall synteny between the B. malayi and C. elegans genomes surrounding the genes encoding the predicted BA receptors; however, most of the intron/exon borders of orthologous BA receptor genes are conserved among the three species. Multiple BA receptor alignment and phylogenetic analysis suggests that potential ligand specificity and G-protein coupling of the individual receptors can be predicted via this bioinformatics approach. Further study of the seven B. malayi receptors identified a putative TA receptor (Bm4) and compared its pharmacology to its putative C. elegans orthologue, TYRA-2, under identical expression and assay conditions. In the present study, membranes from HEK-293 cells stably expressing Bm4 exhibited specific, saturable, high-affinity [ 3H]LSD and [3H]TA binding with Kds of 18 +/- 0.9 nM and 15 +/- 0.2 nM, respectively. More importantly, both TYRA-2 and Bm4 receptors exhibited similar rank orders of potencies for a number of potential tyraminergic ligands. However, some significant differences were noted. For example, chloropromazine exhibited an order of magnitude higher affinity for Bm4 than TYRA-2 (pKis of 7.6 +/- 0.2 and 6.49 +/- 0.1, respectively). In contrast, TYRA-2 had significantly higher affinity for phentolamine than Bm4. These results highlight the utility of the nearly completed B. malayi genome and the importance of using receptors from individual parasitic nematodes for drug discovery.