The mitochondrion of Plasmodium falciparum is required for cellular acetyl-CoA metabolism and protein acetylation.

Coenzyme A (CoA) biosynthesis is an excellent target for antimalarial intervention. While most studies have focused on the use of CoA to produce acetyl-CoA in the apicoplast and the cytosol of malaria parasites, mitochondrial acetyl-CoA production is less well understood. In the current study, we performed metabolite-labeling experiments to measure endogenous metabolites in Plasmodium falciparum lines with genetic deletions affecting mitochondrial dehydrogenase activity. Our results show that the mitochondrion is required for cellular acetyl-CoA biosynthesis and identify a synthetic lethal relationship between the two main ketoacid dehydrogenase enzymes. The activity of these enzymes is dependent on the lipoate attachment enzyme LipL2, which is essential for parasite survival solely based on its role in supporting acetyl-CoA metabolism. We also find that acetyl-CoA produced in the mitochondrion is essential for the acetylation of histones and other proteins outside of the mitochondrion. Taken together, our results demonstrate that the mitochondrion is required for cellular acetyl-CoA metabolism and protein acetylation essential for parasite survival.

Development of a conditional localization approach to control apicoplast protein trafficking in malaria parasites.

Secretory proteins are of particular importance to apicomplexan parasites and comprise over 15% of the genomes of the human pathogens that cause diseases like malaria, toxoplasmosis and babesiosis as well as other diseases of agricultural significance. Here, we developed an approach that allows us to control the trafficking destination of secretory proteins in the human malaria parasite Plasmodium falciparum. Based on the unique structural requirements of apicoplast transit peptides, we designed three conditional localization domains (CLD1, 2 and 3) that can be used to control protein trafficking via the addition of a cell permeant ligand. Studies comparing the trafficking dynamics of each CLD show that CLD2 has the most optimal trafficking efficiency. To validate this system, we tested whether CLD2 could conditionally localize a biotin ligase called holocarboxylase synthetase 1 (HCS1) without interfering with the function of the enzyme. In a parasite line expressing CLD2-HCS1, we were able to control protein biotinylation in the apicoplast in a ligand-dependent manner, demonstrating the full functionality of the CLD tool. We have developed and validated a novel molecular tool that may be used in future studies to help elucidate the function of secretory proteins in malaria parasites.

Strain-specific innate immune signaling pathways determine malaria parasitemia dynamics and host mortality.

Malaria infection triggers vigorous host immune responses; however, the parasite ligands, host receptors, and the signaling pathways responsible for these reactions remain unknown or controversial. Malaria parasites primarily reside within RBCs, thereby hiding themselves from direct contact and recognition by host immune cells. Host responses to malaria infection are very different from those elicited by bacterial and viral infections and the host receptors recognizing parasite ligands have been elusive. Here we investigated mouse genome-wide transcriptional responses to infections with two strains of Plasmodium yoelii (N67 and N67C) and discovered differences in innate response pathways corresponding to strain-specific disease phenotypes. Using in vitro RNAi-based gene knockdown and KO mice, we demonstrated that a strong type I IFN (IFN-I) response triggered by RNA polymerase III and melanoma differentiation-associated protein 5, not Toll-like receptors (TLRs), binding of parasite DNA/RNA contributed to a decline of parasitemia in N67-infected mice. We showed that conventional dendritic cells were the major sources of early IFN-I, and that surface expression of phosphatidylserine on infected RBCs might promote their phagocytic uptake, leading to the release of parasite ligands and the IFN-I response in N67 infection. In contrast, an elevated inflammatory response mediated by CD14/TLR and p38 signaling played a role in disease severity and early host death in N67C-infected mice. In addition to identifying cytosolic DNA/RNA sensors and signaling pathways previously unrecognized in malaria infection, our study demonstrates the importance of parasite genetic backgrounds in malaria pathology and provides important information for studying human malaria pathogenesis.

Dissection of the in vitro developmental program of Hammondia hammondi reveals a link between stress sensitivity and life cycle flexibility in Toxoplasma gondii.

Most eukaryotic parasites are obligately heteroxenous, requiring sequential infection of different host species in order to survive. Toxoplasma gondii is a rare exception to this rule, having a uniquely facultative heteroxenous life cycle. To understand the origins of this phenomenon, we compared development and stress responses in T. gondii to those of its its obligately heteroxenous relative, Hammondia hammondi and have identified multiple H. hammondi growth states that are distinct from those in T. gondii. Of these, the most dramatic difference was that H. hammondi was refractory to stressors that robustly induce cyst formation in T. gondii, and this was reflected most dramatically in its unchanging transcriptome after stress exposure. We also found that H. hammondi could be propagated in vitro for up to 8 days post-excystation, and we exploited this to generate the first ever transgenic H. hammondi line. Overall our data show that H. hammondi zoites grow as stringently regulated, unique life stages that are distinct from T. gondii tachyzoites, and implicate stress sensitivity as a potential developmental innovation that increased the flexibility of the T. gondii life cycle.

A Plasmodium yoelii HECT-like E3 ubiquitin ligase regulates parasite growth and virulence.

Infection of mice with strains of Plasmodium yoelii parasites can result in different pathology, but molecular mechanisms to explain this variation are unclear. Here we show that a P. yoelii gene encoding a HECT-like E3 ubiquitin ligase (Pyheul) influences parasitemia and host mortality. We genetically cross two lethal parasites with distinct disease phenotypes, and identify 43 genetically diverse progeny by typing with microsatellites and 9230 single-nucleotide polymorphisms. A genome-wide quantitative trait loci scan links parasite growth and host mortality to two major loci on chromosomes 1 and 7 with LOD (logarithm of the odds) scores = 6.1 and 8.1, respectively. Allelic exchange of partial sequences of Pyheul in the chromosome 7 locus and modification of the gene expression alter parasite growth and host mortality. This study identifies a gene that may have a function in parasite growth, virulence, and host-parasite interaction, and therefore could be a target for drug or vaccine development.Many strains of Plasmodium differ in virulence, but factors that control these distinctions are not known. Here the authors comparatively map virulence loci using the offspring from a P. yoelii YM and N67 genetic cross, and identify a putative HECT E3 ubiquitin ligase that may explain the variance.

Genome-wide Analysis of Host-Plasmodium yoelii Interactions Reveals Regulators of the Type I Interferon Response.

Invading pathogens trigger specific host responses, an understanding of which might identify genes that function in pathogen recognition and elimination. In this study, we performed trans-species expression quantitative trait locus (ts-eQTL) analysis using genotypes of the Plasmodium yoelii malaria parasite and phenotypes of mouse gene expression. We significantly linked 1,054 host genes to parasite genetic loci (LOD score ≥ 3.0). Using LOD score patterns, which produced results that differed from direct expression-level clustering, we grouped host genes that function in related pathways, allowing functional prediction of unknown genes. As a proof of principle, 14 of 15 randomly selected genes predicted to function in type I interferon (IFN-I) responses were experimentally validated using overexpression, small hairpin RNA knockdown, viral infection, and/or infection of knockout mice. This study demonstrates an effective strategy for studying gene function, establishes a functional gene database, and identifies regulators in IFN-I pathways.

Genome-wide polymorphisms and development of a microarray platform to detect genetic variations in Plasmodium yoelii.

The rodent malaria parasite Plasmodium yoelii is an important model for studying malaria immunity and pathogenesis. One approach for studying malaria disease phenotypes is genetic mapping, which requires typing a large number of genetic markers from multiple parasite strains and/or progeny from genetic crosses. Hundreds of microsatellite (MS) markers have been developed to genotype the P. yoelii genome; however, typing a large number of MS markers can be labor intensive, time consuming, and expensive. Thus, development of high-throughput genotyping tools such as DNA microarrays that enable rapid and accurate large-scale genotyping of the malaria parasite will be highly desirable. In this study, we sequenced the genomes of two P. yoelii strains (33X and N67) and obtained a large number of single nucleotide polymorphisms (SNPs). Based on the SNPs obtained, we designed sets of oligonucleotide probes to develop a microarray that could interrogate ∼11,000 SNPs across the 14 chromosomes of the parasite in a single hybridization. Results from hybridizations of DNA samples of five P. yoelii strains or cloned lines (17XNL, YM, 33X, N67 and N67C) and two progeny from a genetic cross (N67×17XNL) to the microarray showed that the array had a high call rate (∼97%) and accuracy (99.9%) in calling SNPs, providing a simple and reliable tool for typing the P. yoelii genome. Our data show that the P. yoelii genome is highly polymorphic, although isogenic pairs of parasites were also detected. Additionally, our results indicate that the 33X parasite is a progeny of 17XNL (or YM) and an unknown parasite. The highly accurate and reliable microarray developed in this study will greatly facilitate our ability to study the genetic basis of important traits and the disease it causes.

Apicoplast isoprenoid precursor synthesis and the molecular basis of fosmidomycin resistance in Toxoplasma gondii.

Apicomplexa are important pathogens that include the causative agents of malaria, toxoplasmosis, and cryptosporidiosis. Apicomplexan parasites contain a relict chloroplast, the apicoplast. The apicoplast is indispensable and an attractive drug target. The apicoplast is home to a 1-deoxy-D-xylulose-5-phosphate (DOXP) pathway for the synthesis of isoprenoid precursors. This pathway is believed to be the most conserved function of the apicoplast, and fosmidomycin, a specific inhibitor of the pathway, is an effective antimalarial. Surprisingly, fosmidomycin has no effect on most other apicomplexans. Using Toxoplasma gondii, we establish that the pathway is essential in parasites that are highly fosmidomycin resistant. We define the molecular basis of resistance and susceptibility, experimentally testing various host and parasite contributions in T. gondii and Plasmodium. We demonstrate that in T. gondii the parasite plasma membrane is a critical barrier to drug uptake. In strong support of this hypothesis, we engineer de novo drug-sensitive T. gondii parasites by heterologous expression of a bacterial transporter protein. Mice infected with these transgenic parasites can now be cured from a lethal challenge with fosmidomycin. We propose that the varied extent of metabolite exchange between host and parasite is a crucial determinator of drug susceptibility and a predictor of future resistance.