+Targeting Mitochondrial Functions as Antimalarial Regime, What Is Next?

Molecular mechanisms of mitochondria-mediated ferroptosis: a potential target for antimalarial interventions

Ferroptosis is an iron-dependent form of regulated cell death characterized by glutathione (GSH) depletion, glutathione peroxidase 4 (GPX4) inactivation, and the build-up of lipotoxic reactive species. Ferroptosis-targeted induction is a promising therapeutic approach for addressing antimalarial drug resistance. In addition to being the primary source of intracellular energy supply and reactive oxygen species (ROS) generation, mitochondria actively participate in diverse forms of regulated cell death, including ferroptosis. Altered mitochondrial morphology and functionality are attributed to ferroptosis. Diverse mitochondria-related proteins and metabolic activities have been implicated in fine-tuning the action of ferroptosis inducers. Herein, we review recent progress in this evolving field, elucidating the numerous mechanisms by which mitochondria regulate ferroptosis and giving an insight into the role of the organelle in ferroptosis. Additionally, we present an overview of how mitochondria contribute to ferroptosis in malaria. Furthermore, we attempt to shed light on an inclusive perspective on how targeting malaria parasites' mitochondrion and attacking redox homeostasis is anticipated to induce ferroptosis-mediated antiparasitic effects.

Analysis of Plasmodium falciparum Mitochondrial Electron Transport Chain Activity Using Seahorse XFe96 Extracellular Flux Assays

The mitochondrial electron transport chain (ETC) is a multi-component pathway that mediates the transfer of electrons from metabolic reactions that occur in the mitochondrion to molecular oxygen (O2). The ETC contributes to numerous cellular processes, including the generation of cellular ATP through oxidative phosphorylation, serving as an electron sink for metabolic pathways such as de novo pyrimidine biosynthesis and for maintaining mitochondrial membrane potential. Proper functioning of the mitochondrial ETC is necessary for the growth and survival of apicomplexan parasites including Plasmodium falciparum, a causative agent of malaria. The mitochondrial ETC of P. falciparum is an attractive target for antimalarial drugs, due to its essentiality and its differences from the mammalian ETC. To identify novel P. falciparum ETC inhibitors, we have established a real-time assay to assess ETC function, which we describe here. This approach measures the O2 consumption rate (OCR) of permeabilized P. falciparum parasites using a Seahorse XFe96 flux analyzer and can be used to screen compound libraries for the identification of ETC inhibitors and, in part, to determine the targets of those inhibitors. Key features • With this protocol, the effects of candidate inhibitors on mitochondrial O2 consumption in permeabilized asexual P. falciparum parasites can be tested in real time. • Through the sequential injection of inhibitors and substrates into the assay, the molecular targets of candidate inhibitors in the ETC can, in part, be determined. • The assay is applicable for both drug discovery approaches and enquiries into a fundamental aspect of parasite mitochondrial biology.

Chemical and Pharmacological Properties of Decoquinate: A Review of Its Pharmaceutical Potential and Future Perspectives

Decoquinate (DQ) is an antimicrobial agent commonly used as a feed additive for birds for human consumption. Its use as an additive is well established, but DQ has the potential for therapy as an antimicrobial drug for veterinary treatment and its optimized derivatives and/or formulations, mainly nanoformulations, have antimicrobial activity against pathogens that infect humans. However, DQ has a high partition coefficient and low solubility in aqueous fluids, and these biopharmaceutical properties have limited its use in humans. In this review, we highlight the antimicrobial activity and pharmacokinetic properties of DQ and highlight the solutions currently under investigation to overcome these drawbacks. A literature search was conducted focusing on the use of decoquinate against various infectious diseases in humans and animals. The search was conducted in several databases, including scientific and patent databases. Pharmaceutical nanotechnology and medicinal chemistry are the tools of choice to achieve human applications, and most of these applications have been able to improve the biopharmaceutical properties and pharmacokinetic profile of DQ. Based on the results presented here, DQ prototypes could be tested in clinical trials for human application in the coming years.

Mitochondrial perturbation reduces susceptibility to xenobiotics through altered efflux in Candida albicans

Candida albicans is a leading human fungal pathogen, which can cause superficial infections or life-threatening systemic disease in immunocompromised individuals. The ability to transition between yeast and filamentous forms is a major virulence trait of C. albicans, and a key regulator of this morphogenetic transition is the molecular chaperone Hsp90. To explore the mechanisms governing C. albicans morphogenesis in response to Hsp90 inhibition, we performed a functional genomic screen using the gene replacement and conditional expression collection to identify mutants that are defective in filamentation in response to the Hsp90 inhibitor, geldanamycin. We found that transcriptional repression of genes involved in mitochondrial function blocked filamentous growth in response to the concentration of the Hsp90 inhibitor used in the screen, and this was attributable to increased resistance to the compound. Further exploration revealed that perturbation of mitochondrial function reduced susceptibility to two structurally distinct Hsp90 inhibitors, geldanamycin and radicicol, such that filamentous growth was restored in the mitochondrial mutants by increasing the compound concentration. Deletion of two representative mitochondrial genes, MSU1 and SHY1, enhanced cellular efflux and reduced susceptibility to diverse intracellularly acting compounds. Additionally, screening a C. albicans efflux pump gene deletion library implicated Yor1 in the efflux of geldanamycin and Cdr1, in the efflux of radicicol. Deletion of these transporter genes restored sensitivity to Hsp90 inhibitors in MSU1 and SHY1 homozygous deletion mutants, thereby enabling filamentation. Taken together, our findings suggest that mitochondrial dysregulation elevates cellular efflux and consequently reduces susceptibility to xenobiotics in C. albicans.

[Fe-S] biogenesis and unusual assembly of the ISC scaffold complex in the Plasmodium falciparum mitochondrion

The malaria parasite harbors two [Fe-S] biogenesis pathways of prokaryotic origin-the SUF and ISC systems in the apicoplast and mitochondrion, respectively. While the SUF machinery has been delineated, there is little experimental evidence on the ISC pathway. We confirmed mitochondrial targeting of Plasmodium falciparum ISC proteins followed by analyses of cysteine desulfurase, scaffold, and [Fe-S]-carrier components. PfIscU functioned as the scaffold in complex with the PfIscS-PfIsd11 cysteine desulfurase and could directly assemble [4Fe-4S] without prior [2Fe-2S] formation seen in other homologs. Small angle X-ray scattering and spectral studies showed that PfIscU, a trimer, bound one [4Fe-4S]. In a deviation from reported complexes from other organisms, the P. falciparum desulfurase-scaffold complex assembled around a PfIscS tetramer instead of a dimer, resulting in a symmetric hetero-hexamer [2× (2PfIscS-2PfIsd11-2PfIscU)]. PfIscU directly transferred [4Fe-4S] to the apo-protein aconitase B thus abrogating the requirement of intermediary proteins for conversion of [2Fe-2S] to [4Fe-4S] before transfer to [4Fe-4S]-recipients. Among the putative cluster-carriers, PfIscA2 was more efficient than PfNifU-like protein; PfIscA1 primarily bound iron, suggesting its potential role as a Fe²⁺ carrier/donor. Our results identify the core P. falciparum ISC machinery and reveal unique features compared with those in bacteria or yeast and human mitochondria.

Gregarine single-cell transcriptomics reveals differential mitochondrial remodeling and adaptation in apicomplexans

BACKGROUND

Apicomplexa is a diverse phylum comprising unicellular endobiotic animal parasites and contains some of the most well-studied microbial eukaryotes including the devastating human pathogens Plasmodium falciparum and Cryptosporidium hominis. In contrast, data on the invertebrate-infecting gregarines remains sparse and their evolutionary relationship to other apicomplexans remains obscure. Most apicomplexans retain a highly modified plastid, while their mitochondria remain metabolically conserved. Cryptosporidium spp. inhabit an anaerobic host-gut environment and represent the known exception, having completely lost their plastid while retaining an extremely reduced mitochondrion that has lost its genome. Recent advances in single-cell sequencing have enabled the first broad genome-scale explorations of gregarines, providing evidence of differential plastid retention throughout the group. However, little is known about the retention and metabolic capacity of gregarine mitochondria.

RESULTS

Here, we sequenced transcriptomes from five species of gregarines isolated from cockroaches. We combined these data with those from other apicomplexans, performed detailed phylogenomic analyses, and characterized their mitochondrial metabolism. Our results support the placement of Cryptosporidium as the earliest diverging lineage of apicomplexans, which impacts our interpretation of evolutionary events within the phylum. By mapping in silico predictions of core mitochondrial pathways onto our phylogeny, we identified convergently reduced mitochondria. These data show that the electron transport chain has been independently lost three times across the phylum, twice within gregarines.

CONCLUSIONS

Apicomplexan lineages show variable functional restructuring of mitochondrial metabolism that appears to have been driven by adaptations to parasitism and anaerobiosis. Our findings indicate that apicomplexans are rife with convergent adaptations, with shared features including morphology, energy metabolism, and intracellularity.

Recombinant Toxoplasma gondii Ribosomal Protein P2 Modulates the Functions of Murine Macrophages In Vitro and Provides Immunity against Acute Toxoplasmosis In Vivo

Almost every warm-blooded animal can be an intermediate host for Toxoplasma gondii (T. gondii); there is still no efficient vaccine and medicine available for T. gondii infections. Detected on the surface of free tachyzoites of T. gondii, T. gondii ribosomal protein P2 (TgRPP2) has been identified as a target for protection against toxoplasmosis. In the present study, TgRPP2 was firstly expressed in a prokaryotic expression system, and the purified recombinant TgRPP2 (rTgRPP2) was characterized by its modulation effects on murine macrophages. Then, the purified rTgRPP2 was injected into mice to evaluate the immune protection of rTgRPP2. The results indicated that rTgRPP2 could bind to murine Ana-1 cells and showed good reactogenicity. After incubation with purified rTgRPP2, the proliferation, apoptosis, phagocytosis, nitric oxide (NO) production, and cytokines secreted by murine macrophages were modulated. Furthermore, the in vivo experiments indicated that animals immunized with rTgRPP2 could generate a significantly high level of antibodies, cytokines, and major histocompatibility complex (MHC) molecules, leading to a prolonged survival time. All of the results indicated that murine macrophages could be regulated by rTgRPP2 and are essential for the maintenance of tissue homeostasis. Immunization with rTgRPP2 triggered significant protection, with prolonged survival time in a mice model of acute toxoplasmosis. Our results lend credibility to the idea that rTgRPP2 could be a potential target for drug design and vaccine development.

Genetic ablation of the mitoribosome in the malaria parasite Plasmodium falciparum sensitizes it to antimalarials that target mitochondrial functions

The mitochondrion of malaria parasites contains several clinically validated drug targets. Within Plasmodium spp., the causative agents of malaria, the mitochondrial DNA (mtDNA) is only 6 kb long, being the smallest mitochondrial genome among all eukaryotes. The mtDNA encodes only three proteins of the mitochondrial electron transport chain and ∼27 small, fragmented rRNA genes having lengths of 22-195 nucleotides. The rRNA fragments are thought to form a mitochondrial ribosome (mitoribosome), together with ribosomal proteins imported from the cytosol. The mitoribosome of Plasmodium falciparum is essential for maintenance of the mitochondrial membrane potential and parasite viability. However, the role of the mitoribosome in sustaining the metabolic status of the parasite mitochondrion remains unclear. The small ribosomal subunit in P. falciparum has 14 annotated mitoribosomal proteins, and employing a CRISPR/Cas9-based conditional knockdown tool, here we verified the location and tested the essentiality of three candidates (PfmtRPS12, PfmtRPS17, and PfmtRPS18). Using immuno-EM, we provide evidence that the P. falciparum mitoribosome is closely associated with the mitochondrial inner membrane. Upon knockdown of the mitoribosome, parasites became hypersensitive to inhibitors targeting mitochondrial Complex III (bc1), dihydroorotate dehydrogenase (DHOD), and the F1F0-ATP synthase complex. Furthermore, the mitoribosome knockdown blocked the pyrimidine biosynthesis pathway and reduced the cellular pool of pyrimidine nucleotides. These results suggest that disruption of the P. falciparum mitoribosome compromises the metabolic capacity of the mitochondrion, rendering the parasite hypersensitive to a panel of inhibitors that target mitochondrial functions.

Identification of the Toxoplasma gondii mitochondrial ribosome, and characterisation of a protein essential for mitochondrial translation

Apicomplexan parasites cause diseases such as malaria and toxoplasmosis. The apicomplexan mitochondrion shows striking differences from common model organisms, including fundamental processes such as mitochondrial translation. Despite evidence that mitochondrial translation is essential for parasite survival, it is largely understudied. Progress has been restricted by the absence of functional assays to detect apicomplexan mitochondrial translation, a lack of knowledge of proteins involved in the process and the inability to identify and detect mitoribosomes. We report the localization of 12 new mitochondrial proteins, including 6 putative mitoribosomal proteins. We demonstrate the integration of three mitoribosomal proteins in macromolecular complexes, and provide evidence suggesting these are apicomplexan mitoribosomal subunits, detected here for the first time. Finally, a new analytical pipeline detected defects in mitochondrial translation upon depletion of the small subunit protein 35 (TgmS35), while other mitochondrial functions remain unaffected. Our work lays a foundation for the study of apicomplexan mitochondrial translation.

Mitochondrial type II NADH dehydrogenase of Plasmodium falciparum (PfNDH2) is dispensable in the asexual blood stages

The battle against malaria has been substantially impeded by the recurrence of drug resistance in Plasmodium falciparum, the deadliest human malaria parasite. To counter the problem, novel antimalarial drugs are urgently needed, especially those that target unique pathways of the parasite, since they are less likely to have side effects. The mitochondrial type II NADH dehydrogenase (NDH2) of P. falciparum, PfNDH2 (PF3D7_0915000), has been considered a good prospective antimalarial drug target for over a decade, since malaria parasites lack the conventional multi-subunit NADH dehydrogenase, or Complex I, present in the mammalian mitochondrial electron transport chain (mtETC). Instead, Plasmodium parasites contain a single subunit NDH2, which lacks proton pumping activity and is absent in humans. A significant amount of effort has been expended to develop PfNDH2 specific inhibitors, yet the essentiality of PfNDH2 has not been convincingly verified. Herein, we knocked out PfNDH2 in P. falciparum via a CRISPR/Cas9 mediated approach. Deletion of PfNDH2 does not alter the parasite’s susceptibility to multiple mtETC inhibitors, including atovaquone and ELQ-300. We also show that the antimalarial activity of the fungal NDH2 inhibitor HDQ and its new derivative CK-2-68 is due to inhibition of the parasite cytochrome bc₁ complex rather than PfNDH2. These compounds directly inhibit the ubiquinol-cytochrome c reductase activity of the malarial bc₁ complex. Our results suggest that PfNDH2 is not likely a good antimalarial drug target.

The mitochondrial ribosomal protein L13 is critical for the structural and functional integrity of the mitochondrion in Plasmodium falciparum

The phylum Apicomplexa contains a group of protozoa causing diseases in humans and livestock. Plasmodium spp., the causative agent of malaria, contains a mitochondrion that is very divergent from that of their hosts. The malarial mitochondrion is a clinically validated target for the antimalarial drug atovaquone, which specifically blocks the electron transfer activity of the bc₁ complex of the mitochondrial electron transport chain (mtETC). Most mtETC proteins are nuclear-encoded and imported from the cytosol, but three key protein subunits are encoded in the Plasmodium mitochondrial genome: cyt b, COXI, and COXIII. They are translated inside the mitochondrion by mitochondrial ribosomes (mitoribosomes). Here, we characterize the function of one large mitoribosomal protein in Plasmodium falciparum, PfmRPL13. We found that PfmRPL13 localizes to the parasite mitochondrion and is refractory to genetic knockout. Ablation of PfmRPL13 using a conditional knockdown system (TetR-DOZI-aptamer) caused a series of adverse events in the parasite, including mtETC deficiency, loss of mitochondrial membrane potential (Δψ_m), and death. The PfmRPL13 knockdown parasite also became hypersensitive to proguanil, a drug proposed to target an alternative process for maintaining Δψ_m Surprisingly, transmission EM revealed that PfmRPL13 disruption also resulted in an unusually elongated and branched mitochondrion. The growth arrest of the knockdown parasite could be rescued with a second copy of PfmRPL13, but not by supplementation with decylubiquinone or addition of a yeast dihydroorotate dehydrogenase gene. In summary, we provide first and direct evidence that mitoribosomes are essential for malaria parasites to maintain the structural and functional integrity of the mitochondrion.