In 2010, the Apicomplexan Molecular Physiology Section made a significant contribution to drug discovery targeting the plasmodial surface anion channel (PSAC). A number of inhibitors of this channel have been available for many years, but have not been pursued for drug development because those compounds have low PSAC affinity;they also inhibit human ion channels or transporters and therefore have inadequate specificity for antimalarial development. To address these concerns, we developed a miniaturized assay for PSAC activity and carried out a high-throughput inhibitor screen. Approximately 70,000 compounds from synthetic and natural product libraries were screened, revealing inhibitors from multiple structural classes including two novel and potent heterocyclic scaffolds. Single-channel patch-clamp studies indicated that these compounds act directly on PSAC, further implicating a proposed role in transport of diverse solutes. We then used derivatives of a single thiazepinone-containing scaffold to examine whether PSAC activity serves an essential role for the intracellular parasite. This family of compounds exhibited a statistically significant correlation between channel inhibition and in vitro parasite killing, providing chemical validation of PSAC as a drug target. These new inhibitors should be important research tools and may be starting points for much-needed antimalarial drugs. The Apicomplexan Molecular Physiology Section has also contributed to the understanding of antimalarial resistance mechanisms by identifying new PSAC mutants that confer parasite resistance to water-soluble antimalarials. Previously, we found that two such toxins, blasticidin S and leupeptin, are able to select for mutant parasites with altered PSAC activities, suggesting acquired resistance via reduced channel-mediated toxin uptake. In 2010, we determined that the properties of these mutant channels depend on the applied selective pressure: the PSAC mutants generated by selection with either leupeptin or blasticidin S alone do not protect the intracellular parasite from killing by the other toxin. Leupeptin permeability in the blasticidin S-resistant mutant is relatively preserved, consistent with retained in vitro susceptibility to leupeptin. Subsequent in vitro selection with both toxins generated a double mutant parasite having additional changes in PSAC, providing additional evidence for a novel antimalarial resistance mechanism. Characterization of these mutants revealed a single conserved channel on each mutant, albeit with distinct gating properties. These findings are consistent with a shared channel that mediates uptake of ions, nutrients and toxins. This channel's gating and selectivity properties can be modified in response to in vitro selective pressure. We propose that PSAC has a complex selectivity filter different from those of model ion channels in bacteria and higher organisms.
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