Retroviral protease inhibitors (RPIs) such as lopinavir (LP) and saquinavir (SQ) are active against Plasmodium parasites. However, the exact molecular target(s) for these RPIs in the Plasmodium parasites remains poorly understood. We hypothesised that LP and SQ suppress parasite growth through inhibition of aspartyl proteases. Using reverse genetics approach, we embarked on separately generating knockout (KO) parasite lines lacking Plasmepsin 4 (PM4), PM7, PM8, or DNA damage-inducible protein 1 (Ddi1) in the rodent malaria parasite Plasmodium berghei ANKA. We then tested the suppressive profiles of the LP/Ritonavir (LP/RT) and SQ/RT as well as antimalarials; Amodiaquine (AQ) and Piperaquine (PQ) against the KO parasites in the standard 4-day suppressive test. The Ddi1 gene proved refractory to deletion suggesting that the gene is essential for the growth of the asexual blood stage parasites. Our results revealed that deletion of PM4 significantly reduces normal parasite growth rate phenotype (P = 0.003). Unlike PM4_KO parasites which were less susceptible to LP and SQ (P = 0.036, P = 0.030), the suppressive profiles for PM7_KO and PM8_KO parasites were comparable to those for the WT parasites. This finding suggests a potential role of PM4 in the LP and SQ action. On further analysis, modelling and molecular docking studies revealed that both LP and SQ displayed high binding affinities (-6.3 kcal/mol to -10.3 kcal/mol) towards the Plasmodium aspartyl proteases. We concluded that PM4 plays a vital role in assuring asexual stage parasite fitness and might be mediating LP and SQ action. The essential nature of the Ddi1 gene warrants further studies to evaluate its role in the parasite asexual blood stage growth as well as a possible target for the RPIs.
Background: The human malaria parasite Plasmodium falciparum has evolved complex drug evasion mechanisms to all available antimalarials. To date, the combination of amodiaquine-artesunate is among the drug of choice for treatment of uncomplicated malaria. In this combination, a short acting, artesunate is partnered with long acting, amodiaquine for which resistance may emerge rapidly especially in high transmission settings. Here, we used a rodent malaria parasite Plasmodium berghei ANKA as a surrogate of P. falciparum to investigate the mechanisms of amodiaquine resistance. Methods: We used serial technique to select amodiaquine resistance by submitting the parasites to continuous amodiaquine pressure. We then employed the 4-Day Suppressive Test to monitor emergence of resistance and determine the cross-resistance profiles. Finally, we genotyped the resistant parasite by PCR amplification, sequencing and relative quantitation of mRNA transcript of targeted genes. Results: Submission of P. berghei ANKA to amodiaquine pressure yielded resistant parasite within thirty-six passages. The effective dosage that reduced 90% of parasitaemia (ED 90) of sensitive line and resistant line were 4.29mg/kg and 19.13mg/kg, respectively. After freezing at -80ºC for one month, the resistant parasite remained stable with an ED 90 of 18.22mg/kg. Amodiaquine resistant parasites are also resistant to chloroquine (6fold), artemether (10fold), primaquine (5fold), piperaquine (2fold) and lumefantrine (3fold). Sequence analysis of Plasmodium berghei chloroquine resistant transporter revealed His95Pro mutation. No variation was identified in Plasmodium berghei multidrug resistance gene-1 (Pbmdr1), Plasmodium berghei deubiquitinating enzyme-1 or Plasmodium berghei Kelch13 domain nucleotide sequences. Amodiaquine resistance is also accompanied by high mRNA transcripts of key transporters; Pbmdr1, V-type/H+ pumping pyrophosphatase-2 and sodium hydrogen ion exchanger-1 and Ca 2+/H + antiporter. Conclusions: Selection of amodiaquine resistance yielded stable “multidrug-resistant’’ parasites and thus may be used to study common resistance mechanisms associated with other antimalarial drugs. Genome wide studies may elucidate other functionally important genes controlling AQ resistance in P. berghei.
The human malaria parasite has evolved complex drug evasion mechanisms to all available antimalarials. To date, the combination of amodiaquine-artesunate is among the drug of choice for treatment of uncomplicated malaria. In this combination, a short acting, artesunate is partnered with long acting, amodiaquine for which resistance may emerge rapidly especially in high transmission settings. Here, we used a rodent malaria parasite ANKA as a surrogate of to investigate the mechanisms of amodiaquine resistance.: We used serial technique to select amodiaquine resistance by submitting the parasites to continuous amodiaquine pressure. We then employed the 4-Day Suppressive Test to monitor emergence of resistance and determine the cross-resistance profiles. Finally, we genotyped the resistant parasite by PCR amplification, sequencing and relative quantitation of mRNA transcript of targeted genes. Submission of ANKA to amodiaquine pressure yielded resistant parasite within thirty-six passages. The effective dosage that reduced 90% of parasitaemia (ED ) of sensitive line and resistant line were 4.29mg/kg and 19.13mg/kg, respectively. After freezing at -80ºC for one month, the resistant parasite remained stable with an ED of 18.22mg/kg. Amodiaquine resistant parasites are also resistant to chloroquine (6fold), artemether (10fold), primaquine (5fold), piperaquine (2fold) and lumefantrine (3fold). Sequence analysis of revealed His95Pro mutation. No variation was identified in or nucleotide sequences. Amodiaquine resistance is also accompanied by high mRNA transcripts of key transporters;, and and Ca /H antiporter. Selection of amodiaquine resistance yielded stable "multidrug-resistant'' parasites and thus may be used to study common resistance mechanisms associated with other antimalarial drugs. Genome wide studies may elucidate other functionally important genes controlling AQ resistance in.
The ability of the human malaria parasite, Plasmodium falciparum to develop resistance against mainstay drugs remains a public health problem. Currently, the antimalarial drugs, lumefantrine (LM), and piperaquine (PQ) are essential components of the mainstay artemisinin-based therapies used for the treatment of malaria globally. Here, we used a model parasite Plasmodium berghei, to investigate the mechanisms of LM and PQ resistance. We employed known resistance reversing agents (RA): probenecid, verapamil, or cyproheptadine to study the mechanisms of LM and PQ resistance in the standard 4-day suppressive test. We then employed reverse genetics to assess the impact of deleting or over-expressing plausible genes associated with the metabolism and transport of drugs. We show that only, cyproheptadine at 5mgkg-1 restored LM activity by above 65% against LM-resistant parasites (LMr) but failed to reinstate PQ activity against PQ-resistant parasites (PQr). Whereas the PQr had lost significant susceptibility to LM, the three RA, cyproheptadine verapamil, and probenecid restored LM potency by above 70%, 60%, and 55% respectively against the PQr. We thus focused on the mechanisms of LM resistance in PQr. Here we show the partial deletion of the cysteine desulfurase (SUFS) and overexpression of the Ferredoxin NADP+ reductase (FNR) genes in the PQr parasite achieved two results; i) abolished the impact of RA on LM activity; ii) restored the susceptibility of PQr to LM alone. Our findings associated SUFS and FNR protein with the action of LM and RA action in P. berghei. We demonstrate that the incorporation of any of the RA into an antimalarial combination that comprises LM would augment LM activity and concomitantly antagonize the emergence of LM resistance derived from PQ pressure. The impact of RA, deletion of SUFS, and overexpression of FNR on LM activity need to be tested in Plasmodium falciparum.Author summaryLumefantrine (LM) and piperaquine (PQ) are essential drugs for the treatment of malaria globally. Here, we used Plasmodium berghei, a model parasite that infects rodents to study how parasites escape killing by PQ and LM. We first used a second drug: probenecid, verapamil, or cyproheptadine to enhance the activity of LM or PQ. We show that cyproheptadine restores LM activity against LM-resistant parasites (LMr) but failed to reestablish PQ activity against PQ-resistant parasites (PQr). Since PQr is resistant to LM, combining LM with either cyproheptadine, verapamil, or probenecid reinstates LM activity against PQr. We then focused mainly on LM resistance in PQr. After genetically manipulating the PQr, we reveal that cysteine desulfurase (SUFS) and ferredoxin NADP+reductase (FNR) regulate LM capacity to kill parasites. Decreasing the level of SUFS or increasing FNR levels in the PQr makes the parasites susceptible to LM but abolishes the impact of probenecid, verapamil, and cyproheptadine on LM activity. Overall, we provide clues on the link between SUFS and FNR in the action of LM and RA in P. berghei. This study provides a basis for an in-depth analysis of how LM mediates parasites kill and how the parasite escapes LM action in Plasmodium falciparum.
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