Possible Role of the Ca2+/Mn2+ P-Type ATPase Pmr1p on Artemisinin Toxicity through an Induction of Intracellular Oxidative Stress

Artemisinins are widely used to treat Plasmodium infections due to their high clinical efficacy; however, the antimalarial mechanism of artemisinin remains unresolved. Mutations in P. falciparum ATPase6 (PfATP6), a sarcoplasmic endoplasmic reticulum Ca²⁺-transporting ATPase, are associated with increased tolerance to artemisinin. We utilized Saccharomyces cerevisiae as a model to examine the involvement of Pmr1p, a functional homolog of PfATP6, on the toxicity of artemisinin. Our analysis demonstrated that cells lacking Pmr1p are less susceptible to growth inhibition from artemisinin and its derivatives. No association between sensitivity to artemisinin and altered trafficking of the drug efflux pump Pdr5p, calcium homeostasis, or protein glycosylation was found in pmr1∆ yeast. Basal ROS levels are elevated in pmr1∆ yeast and artemisinin exposure does not enhance ROS accumulation. This is in contrast to WT cells that exhibit a significant increase in ROS production following treatment with artemisinin. Yeast deleted for PMR1 are known to accumulate excess manganese ions that can function as ROS-scavenging molecules, but no correlation between manganese content and artemisinin resistance was observed. We propose that loss of function mutations in Pmr1p in yeast cells and PfATP6 in P. falciparum are protective against artemisinin toxicity due to reduced intracellular oxidative damage.

SERCA plays a crucial role in the toxicity of a betulinic acid derivative with potential antimalarial activity

Malaria is one of the most significant infectious diseases that affect poor populations in tropical areas throughout the world. Plants have been shown to be a good source for the development of new antimalarial chemotherapeutic agents, as shown for the discovery of quinine and artemisinin derivatives. Our research group has been working with semisynthetic triterpene derivatives that show potential antimalarial activity toward different strains of Plasmodium falciparum by specifically modulating calcium pathways in the parasite. Promising results were obtained for nanomolar concentrations of the semisynthetic betulinic acid derivative LAFIS13 against the P. falciparum 3D7 strain in vitro, with a selectivity index of 18 compared to a mammalian cell line. Continuing these studies, we present here in vitro and in vivo toxicological evaluations of this compound, followed by docking studies with PfATP6, a sarco/endoplasmic reticulum Ca⁺²-ATPase (SERCA) protein. LAFIS13 showed an LD₅₀ between 300 and 50 mg/kg, and the acute administration of 50 mg/kg (i.p.) had no negative effects on hematological, biochemical and histopathological parameters. Based on the results of the in vitro assays, LAFIS13 not exerted significant effects on coagulation parameters of human peripheral blood, but a hemolytic activity was verified at higher concentrations. According to the molecular docking study, the PfATP6 protein may be a target for LAFIS13, which corroborates its previously reported modulatory effects on calcium homeostasis in the parasite. Notably, LAFIS13 showed a higher selectivity for the mammalian SERCA protein than for PfATP6, thus impairing the selectivity between parasite and host. In summary, the direct interaction with calcium pumps and the hemolytic potential of the compound proved to be plausible mechanism of LAFIS13 toxicity.

Design, in silico and in vitro evaluation of curcumin analogues against Plasmodium falciparum

The polyphenolic compound curcumin has been reported for its antimalarial properties in various scientific studies. Plasmodium falciparum ATP6, the parasite orthologue of mammalian sarcoplasmic Ca²⁺ ATPase (SERCA) has been identified as a key molecular target of both artemisinin and curcumin. The work was thereby undertaken to study the anti-malarial properties of two different series of curcumin analogues based on their docking interactions with PfATP6 and correlating the results with their anti-malarial activity. The compounds were designed retaining similar functional groups as that of the parent curcumin nucleus while incorporating changes in the carbon chain length, unsaturated groups and the number of ketone groups. The compounds (1E, 4E)-1,5-bis(4-methylphenyl)penta-1,4-dien-3-one (CD-9), (1E, 4E)-1,5-bis(4-methoxyphenyl)penta-1,4-dien-3-one (CD-8) and (E)-1,3-bis(4-hydroxylphenyl)prop-2-en-1-one (CD-1) showed IC₅₀ values of 1.642 μM, 1.764 μM and 2.59 μM in 3D7 strain and 3.039 μM, 7.40 μM and 11.3 μM in RKL-2 strain respectively. Detailed structure-activity relationship studies of the compounds showed that CD-9 and CD-8 had a common hydrophobic interaction with the residue Leu268 of the PfATP6 protein and has been postulated through our study to be the reason for their antimalarial activity as seen after corroborating the results with the in vitro study. The study provided valuable insight about the ligand-protein interaction of the various functional groups of curcumin and its analogues against the PfATP6 protein and their importance in imparting antimalarial action.

Expression in yeast links field polymorphisms in PfATP6 to in vitro artemisinin resistance and identifies new inhibitor classes

BACKGROUND

The mechanism of action of artemisinins against malaria is unclear, despite their widespread use in combination therapies and the emergence of resistance.

RESULTS

Here, we report expression of PfATP6 (a SERCA pump) in yeast and demonstrate its inhibition by artemisinins. Mutations in PfATP6 identified in field isolates (such as S769N) and in laboratory clones (such as L263E) decrease susceptibility to artemisinins, whereas they increase susceptibility to unrelated inhibitors such as cyclopiazonic acid. As predicted from the yeast model, Plasmodium falciparum with the L263E mutation is also more susceptible to cyclopiazonic acid. An inability to knockout parasite SERCA pumps provides genetic evidence that they are essential in asexual stages of development. Thaperoxides are a new class of potent antimalarial designed to act by inhibiting PfATP6. Results in yeast confirm this inhibition.

CONCLUSIONS

The identification of inhibitors effective against mutated PfATP6 suggests ways in which artemisinin resistance may be overcome.

Antimalarial screening via large-scale purification of Plasmodium falciparum Ca2+-ATPase 6 and in vitro studies

The most severe form of human malaria is caused by the parasite Plasmodium falciparum. Despite the current need, there is no effective vaccine and parasites are becoming resistant to most of the antimalarials available. Therefore, there is an urgent need to discover new drugs from targets that have not yet suffered from drug pressure with the aim of overcoming the problem of new emerging resistance. Membrane transporters, such as P. falciparum Ca(2+)-ATPase 6 (PfATP6), the P. falciparum sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA), have been proposed as potentially good antimalarial targets. The present investigation focuses on: (a) the large-scale purification of PfATP6 for maintenance of its enzymatic activity; (b) screening for PfATP6 inhibitors from a compound library; and (c) the selection of the best inhibitors for further tests on P. falciparum growth in vitro. We managed to heterologously express in yeast and purify an active form of PfATP6 as previously described, although in larger amounts. In addition to some classical SERCA inhibitors, a chemical library of 1680 molecules was screened. From these, we selected a pool of the 20 most potent inhibitors of PfATP6, presenting half maximal inhibitory concentration values in the range 1-9 μm. From these, eight were chosen for evaluation of their effect on P. falciparum growth in vitro, and the best compound presented a half maximal inhibitory concentration of ~ 2 μm. We verified the absence of an inhibitory effect of most of the compounds on mammalian SERCA1a, representing a potential advantage in terms of human toxicity. The present study describes a multidisciplinary approach allowing the selection of promising PfATP6-specific inhibitors with good antimalarial activity.