Artemisinin resistance in P. falciparum: probing the interacting partners of Kelch13 protein in parasite

Artemisinin-resistant Plasmodium falciparum Kelch13 mutant proteins display reduced heme-binding affinity and decreased artemisinin activation

The potency of frontline antimalarial drug artemisinin (ART) derivatives is triggered by heme-induced cleavage of the endoperoxide bond to form reactive heme-ART alkoxy radicals and covalent heme-ART adducts, which are highly toxic to the parasite. ART-resistant (ART-R) parasites with mutations in the Plasmodium falciparum Kelch-containing protein Kelch13 (PfKekch13) exhibit impaired hemoglobin uptake, reduced yield of hemoglobin-derived heme, and thus decreased ART activation. However, any direct involvement of PfKelch13 in heme-mediated ART activation has not been reported. Here, we show that the purified recombinant PfKelch13 wild-type (WT) protein displays measurable binding affinity for iron and heme, the main effectors for ART activation. The heme-binding property is also exhibited by the native PfKelch13 protein from parasite culture. The two ART-R recombinant PfKelch13 mutants (C580Y and R539T) display weaker heme binding affinities compared to the ART-sensitive WT and A578S mutant proteins, which further translates into reduced yield of heme-ART derivatives when ART is incubated with the heme molecules bound to the mutant PfKelch13 proteins. In conclusion, this study provides the first evidence for ART activation via the heme-binding propensity of PfKelch13. This mechanism may contribute to the modulation of ART-R levels in malaria parasites through a novel function of PfKelch13.

Prefoldins are novel regulators of the unfolded protein response in artemisinin resistant Plasmodium falciparum malaria

Emerging Artemisinin (ART) resistance in Plasmodium falciparum (Pf) poses challenges for the discovery of novel drugs to tackle ART-resistant parasites. Concentrated efforts toward the ART resistance mechanism indicated a strong molecular link of ART resistance with upregulated expression of unfolded protein response pathways involving Prefoldins (PFDs). However, a complete characterization of PFDs as molecular players taking part in ART resistance mechanism, and discovery of small molecule inhibitors to block this process have not been identified to date. Here, we functionally characterized all Pf Prefoldin subunits (PFD1-6) and established a causative role played by PFDs in ART resistance by demonstrating their expression in intra-erythrocytic parasites along with their interactions with Kelch13 protein through immunoprecipitation coupled MS/MS analysis. Systematic biophysical interaction analysis between all subunits of PFDs revealed their potential to form a complex. The role of PFDs in ART resistance was confirmed in orthologous yeast PFD6 mutants, where PfPFD6 expression in yeast mutants reverted phenotype to ART resistance. We identified an FDA-approved drug "Biperiden" that restricts the formation of Prefoldin complex and inhibits its interaction with its key parasite protein substrates, MSP-1 and α-tubulin-I. Moreover, Biperiden treatment inhibits the parasite growth in ART-sensitive Pf3D7 and resistant Pf3D7k13R539T strains. Ring survival assays that are clinically relevant to analyze ART resistance in Pf3D7k13R539T parasites demonstrate the potency of BPD to inhibit the growth of survivor parasites. Overall, our study provides the first evidence of the role of PfPFDs in ART resistance mechanisms and opens new avenues for the management of resistant parasites.

Understanding the complex formation of falstatin; an endogenous macromolecular inhibitor of falcipains

Proteolytic activity constitutes a fundamental process essential for the survival of the malaria parasite and is thus highly regulated. Falstatin, a protease inhibitor of Plasmodium falciparum, tightly regulates the activity of cysteine hemoglobinases, falcipain-2 and 3 (FP2, FP3), by inhibiting FP2 through a single surface exposed loop. However, the multimeric nature of falstatin and its interaction with FP2 remained unexplored. Here we report that the N-terminal falstatin region is highly disordered, and needs chaperone activity (heat-shock protein 70, HSP70) for its folding. Protein-protein interaction assays showed a significant interaction between falstatin and HSP70. Further, characterization of the falstatin multimer through a series of biophysical techniques identified the formation of a falstatin decamer, which was extremely thermostable. Computational analysis of the falstatin decamer showed the presence of five falstatin dimers, with each dimer aligned in a head-to-tail orientation. Further, the falstatin C-terminal region was revealed to be primarily involved in the oligomerization process. Stoichiometric analysis of the FP2-falstatin multimer showed the formation of a heterooligomeric complex in a 1:1 ratio, with the participation of ten subunits of each protein. Taken together, our results report a novel protease-inhibitor complex and strengthens our understanding of the regulatory mechanisms of major plasmodium hemoglobinases.

Fused Enzyme Glucose-6-Phosphate Dehydrogenase::6-Phosphogluconolactonase (G6PD::6PGL) as a Potential Drug Target in Giardia lamblia, Trichomonas vaginalis, and Plasmodium falciparum

Several microaerophilic parasites such as Giardia lamblia, Trichomonas vaginalis, and Plasmodium falciparum are major disease-causing organisms and are responsible for spreading infections worldwide. Despite significant progress made in understanding the metabolism and molecular biology of microaerophilic parasites, chemotherapeutic treatment to control it has seen limited progress. A current proposed strategy for drug discovery against parasitic diseases is the identification of essential key enzymes of metabolic pathways associated with the parasite's survival. In these organisms, glucose-6-phosphate dehydrogenase::6-phosphogluconolactonase (G6PD:: 6PGL), the first enzyme of the pentose phosphate pathway (PPP), is essential for its metabolism. Since G6PD:: 6PGL provides substrates for nucleotides synthesis and NADPH as a source of reducing equivalents, it could be considered an anti-parasite drug target. This review analyzes the anaerobic energy metabolism of G. lamblia, T. vaginalis, and P. falciparum, with a focus on glucose metabolism through the pentose phosphate pathway and the significance of the fused G6PD:: 6PGL enzyme as a therapeutic target in the search for new drugs.