VEuPathDB: the eukaryotic pathogen, vector and host bioinformatics resource center

The sulfur-related metabolic status of Aspergillus fumigatus during infection reveals cytosolic serine hydroxymethyltransferase as a promising antifungal target

Sulfur metabolism is an essential aspect of fungal physiology and pathogenicity. Fungal sulfur metabolism comprises anabolic and catabolic routes that are not well conserved in mammals, therefore is considered a promising source of prospective novel antifungal targets. To gain insight into Aspergillus fumigatus sulfur-related metabolism during infection, we used a NanoString custom nCounter-TagSet and compared the expression of 68 key metabolic genes in different murine models of invasive pulmonary aspergillosis, at 3 time-points, and under a variety of in vitro conditions. We identified a set of 15 genes that were consistently expressed at higher levels in vivo than in vitro, suggesting that they may be particularly relevant for intrapulmonary growth and thus constitute promising drug targets. Indeed, the role of 5 of the 15 genes has previously been empirically validated, supporting the likelihood that the remaining candidates are relevant. In addition, the analysis of gene expression dynamics at early (16 h), mid (24 h), and late (72 h) time-points uncovered potential disease initiation and progression factors. We further characterized one of the identified genes, encoding the cytosolic serine hydroxymethyltransferase ShmB, and demonstrated that it is an essential gene of A. fumigatus, also required for virulence in a murine model of established pulmonary infection. We further showed that the structure of the ligand-binding pocket of the fungal enzyme differs significantly from its human counterpart, suggesting that specific inhibitors can be designed. Therefore, in vivo transcriptomics is a powerful tool for identifying genes crucial for fungal pathogenicity that may encode promising antifungal target candidates.

Artificial intelligence-powered microscopy: Transforming the landscape of parasitology

Microscopy and image analysis play a vital role in parasitology research; they are critical for identifying parasitic organisms and elucidating their complex life cycles. Despite major advancements in imaging and analysis, several challenges remain. These include the integration of interdisciplinary data; information derived from various model organisms; and data acquired from clinical research. In our view, artificial intelligence-with the latest advances in machine and deep learning-holds enormous potential to address many of these challenges. This review addresses how artificial intelligence, machine learning and deep learning have been used in the field of parasitology-mainly focused on Apicomplexan, Diplomonad, and Kinetoplastid groups. We explore how gaps in our understanding could be filled by AI in future parasitology research and diagnosis in the field. Moreover, it addresses challenges and limitations currently faced in implementing and expanding the use of artificial intelligence across biomedical fields. The necessary increased collaboration between biologists and computational scientists will facilitate understanding, development, and implementation of the latest advances for both scientific discovery and clinical impact. Current and future AI tools hold the potential to revolutionise parasitology and expand One Health principles.

Reconstructing rearrangement phylogenies of natural genomes

BACKGROUND

We study the classical problem of inferring ancestral genomes from a set of extant genomes under a given phylogeny, known as the Small Parsimony Problem (SPP). Genomes are represented as sequences of oriented markers, organized in one or more linear or circular chromosomes. Any marker may appear in several copies, without restriction on orientation or genomic location, known as the natural genomes model. Evolutionary events along the branches of the phylogeny encompass large scale rearrangements, including segmental inversions, translocations, gain and loss (DCJ-indel model). Even under simpler rearrangement models, such as the classical breakpoint model without duplicates, the SPP is computationally intractable. Nevertheless, the SPP for natural genomes under the DCJ-indel model has been studied recently, with limited success.

METHODS

Building on prior work, we present a highly optimized ILP that is able to solve the SPP for sufficiently small phylogenies and gene families. A notable improvement w.r.t. the previous result is an optimized way of handling both circular and linear chromosomes. This is especially relevant to the SPP, since the chromosomal structure of ancestral genomes is unknown and the solution space for this chromosomal structure is typically large.

RESULTS

We benchmark our method on simulated and real data. On simulated phylogenies we observe a considerable performance improvement on problems that include linear chromosomes. And even when the ground truth contains only one circular chromosome per genome, our method outperforms its predecessor due to its optimized handling of the solution space. The practical advantage becomes also visible in an analysis of seven Anopheles taxa.

Dual RNA-Seq reveals strain-specific transcriptional adaptations of Trypanosoma cruzi in host cells infected with isolates from acute and chronic cases

Chagas disease, caused by the protozoan Trypanosoma cruzi, remains a major public health issue, particularly in Latin America, and is traditionally transmitted by triatomine bugs. Chagas disease progresses through two phases: an acute phase characterized by high parasitemia and a chronic phase, which may remain asymptomatic or develop into severe complications. This study utilized dual-RNAseq technology to analyze gene expression in mice fibroblast host cells infected with the strain JJ21 isolated from an acute Chagas disease case of presumptive oral transmission, comparing it with the MG strain isolated from a chronic Chagas disease human case. The results revealed that JJ21 exhibits a distinct transcriptional profile, marked by the up-regulation of immune evasion and stress response genes, suggesting specific adjustments to meet the demands of the acute phase. At 24 h post-infection, host cells exhibited increased expression of genes related to immune recognition, cargo receptor activity, and pro-inflammatory responses, indicating a robust initial defensive reaction. By 72 h, the host response shifted towards stress management and antioxidant activity, reflecting efforts to handle prolonged infection. Comparative analysis with the MG strain revealed significant differences: JJ21 showed notable up-regulation of mucin and down-regulation of dynein, indicating unique strategies for evading immune responses and altering intracellular interactions. The heightened expression of virulence factors such as trans-sialidases (Ts) and GP63 proteins in JJ21 supports this hypothesis. In conclusion, this study reveals how the JJ21 strain's unique pro-inflammatory response and adaptive strategies enhance our understanding of strain specific responses during the Chagas disease. These insights are crucial for developing targeted interventions and vaccines to more effectively manage the disease and its global impact.

De novo assembly of plasmodium interspersed repeat (pir) genes from Plasmodium vivax RNAseq data suggests geographic conservation of sub-family transcription

BACKGROUND

The plasmodium interspersed repeats (pir) multigene family is found across malaria parasite genomes, first discovered in the human-infecting species Plasmodium vivax, where they were initially named the virs. Their function remains unknown, although studies have suggested a role in virulence of the asexual blood stages. Sub-families of the P. vivax pir/virs have been identified, and are found in isolates from across the world, however their transcription at different localities and in different stages of the life cycle have not been quantified. Multiple transcriptomic studies of the parasite have been conducted, but many map the pir reads to existing reference genomes (as part of standard bioinformatic practice), which may miss members of the multigene family due to its inherent variability. This obscures our understanding of how the pir sub-families in P. vivax may be contributing to human/vector infection.

RESULTS

To overcome the issue of hidden pir diversity from utilising a reference genome, we employed de novo transcriptome assembly to construct the pir 'reference' of different parasite isolates from published and novel RNAseq datasets. For this purpose, a pipeline was written in Nextflow, and first tested on data from the rodent-infecting P. c. chabaudi parasite to ascertain its efficacy on a sample with a full, genome-based set of pir gene sequences. The pipeline assembled hundreds of pirs from the studies included. By performing BLAST sequence identity comparisons with reference genome pirs (including P. vivax and related species) we found a clustered network of transcripts which corresponded well with prior sub-family annotations, albeit requiring some updated nomenclature. Mapping the RNAseq datasets to the de novo transcriptome references revealed that the transcription of these updated pir gene sub-families is generally consistent across the different geographical regions. From this transcriptional quantification, a time course of mosquito bloodmeals (after feeding on an infected patient) highlighted the first evidence of ookinete stage pir transcription in a human-infective malaria parasite.

CONCLUSIONS

De novo transcriptome assembly is a valuable tool for understanding highly variable multigene families from Plasmodium spp., and with pipeline software these can be applied more easily and at scale. Despite a global distribution, P. vivax has a conserved pir sub-family structure-both in terms of genome copy number and transcription. We suggest that this indicates important roles of the distinct sub-families, or a genetic mechanism maintaining their preservation. Furthermore, a burst of pir transcription in the mosquito stages of development is the first glint of ookinete pir expression for a human-infective malaria parasite, suggesting a role for the gene family at a new stage of the lifecycle.

The fast-evolving FIKK kinase family of Plasmodium falciparum can be inhibited by a single compound

Of 250 Plasmodium species, 6 infect humans, with P. falciparum causing over 95% of 600,000 annual malaria-related deaths. Its pathology arises from host cell remodelling driven by over 400 exported parasite proteins, including the FIKK kinase family. About one million years ago, a bird-infecting Plasmodium species crossed into great apes and a single non-exported FIKK kinase gained an export element. This led to a rapid expansion into 15-21 atypical, exported Ser/Thr effector kinases. Here, using genomic and proteomic analyses, we demonstrate FIKK differentiation via changes in subcellular localization, expression timing and substrate motifs, which supports an individual important role in host-pathogen interactions. Structural data and AlphaFold2 predictions reveal fast-evolving loops in the kinase domain that probably enabled rapid functional diversification for substrate preferences. One FIKK evolved exclusive tyrosine phosphorylation, previously thought absent in Plasmodium. Despite divergence of substrate preferences, the atypical ATP binding pocket is conserved and we identified a single compound that inhibits all FIKKs. A pan-specific inhibitor could reduce resistance development and improve malaria control strategies.

Rapid in vitro evolution of flucytosine resistance in Candida auris

The pan-antifungal-resistant pathogen Candida auris has been causing high-mortality infection outbreaks in hospitals and healthcare settings. The prodrug 5-fluorocytosine (5FC) is one of four chemical entities, but its clinical use as an antifungal drug has been limited owing to pronounced resistance. However, antifungal combination therapy with 5FC appears as a promising strategy for treating C. auris infections. Here, we show that a C. auris clinical isolate can rapidly acquire genetic mutations to mount 5FC resistance after only one to two passages under drug selection. We exploit a new bioinformatics workflow to identify genetic polymorphisms from RNA-seq data. Strikingly, we identify several mutations in the FUR1 gene encoding the 5-fluorouracil convertase that normally generates the active drug. A single nonsense mutation truncates the enzyme at residue Q30*, leading to 5FC resistance due to inactive Fur1. Whole-genome sequencing analysis revealed that an indel mutation in FCY2 also contributes to 5FC resistance. Furthermore, at least one out of seven adapted strains acquired enhanced 5FC tolerance without mutations in the 5FC conversion pathway. Thus, we demonstrate that FUR1 mutations are critical drivers of 5FC resistance in C. auris.IMPORTANCECandida auris is a high-priority human fungal pathogen, causing infection outbreaks of high mortality in healthcare settings. Antifungal combination therapy with 5-fluorocytosine (5FC) is one of the emerging approaches in treatment. However, acquired 5FC resistance traits have been a matter of concern. 5FC is taken up by fungal cells via a cytosine permease and further metabolized by a cytosine deaminase to 5-fluorouracil (5FU). 5FU is then converted by the Fur1 uracil phosphoribosyltransferase into a toxic antimetabolite that disrupts fungal RNA and DNA syntheses. Mutations in these proteins are commonly associated with 5FC resistance in fungal species. Here, we show that C. auris can rapidly develop resistance under 5FC selective stress owing to mutational inactivation of Fur1 function. Moreover, other mechanisms that bypass mutations in the 5FC conversion pathway may also contribute to 5FC resistance traits. Finally, we have developed a tailored bioinformatics workflow that facilitates the identification of polymorphisms associated with 5FC resistance in clinical isolates.

PTRAMP, CSS and Ripr form a conserved complex required for merozoite invasion of Plasmodium species into erythrocytes

Invasion of erythrocytes by members of the Plasmodium genus is an essential step of the parasite lifecycle, orchestrated by numerous host-parasite interactions. In P. falciparum Rh5, with PfCyRPA, PfRipr, PfCSS, and PfPTRAMP, forms the essential PCRCR complex which binds basigin on the erythrocyte surface. Rh5 is restricted to P. falciparum and its close relatives; however, PTRAMP, CSS and Ripr orthologs are present across the Plasmodium genus. We investigated PTRAMP, CSS and Ripr orthologs from three species to elucidate common features of the complex. Like P. falciparum, PTRAMP and CSS form a disulfide-linked heterodimer in both P. vivax and P. knowlesi with all three species forming a complex (PCR) with Ripr by binding its C-terminal region. Cross-reactive antibodies targeting the PCR complex differentially inhibit merozoite invasion. Cryo-EM visualization of the P. knowlesi PCR complex confirmed predicted models and revealed a core invasion scaffold in Plasmodium spp. with implications for vaccines targeting multiple species of malaria-causing parasites.

SUN-domain proteins of the malaria parasite Plasmodium falciparum are essential for proper nuclear division and DNA repair

The protozoan parasite Plasmodium falciparum, which is responsible for the deadliest form of human malaria, accounts for over half a million deaths a year. These parasites proliferate in human red blood cells by consecutive rounds of closed mitoses called schizogony. Their virulence is attributed to their ability to modify the infected red cells to adhere to the vascular endothelium and to evade immunity through antigenic switches. Spatial dynamics at the nuclear periphery were associated with the regulation of processes that enable the parasites to establish long-term infection. However, our knowledge of components of the nuclear envelope (NE) in Plasmodium remains limited. One of the major protein complexes at the NE is the linker of nucleoskeleton and cytoskeleton (LINC) complex that forms a connecting bridge between the cytoplasm and the nucleus through the interaction of SUN and KASH domain proteins. Here, we have identified two SUN-domain proteins as possible components of the LINC complex of P. falciparum and show that their proper expression is essential for the parasite's proliferation in human red blood cells, and their depletion leads to the formation of membranous whorls and morphological changes of the NE. In addition, their differential expression highlights different functions at the nuclear periphery as PfSUN2 is specifically associated with heterochromatin, while PfSUN1 expression is essential for activation of the DNA damage response. Our data provide indications for the involvement of the LINC complex in crucial biological processes in the intraerythrocytic development cycle of malaria parasites.

IMPORTANCE

Plasmodium falciparum, the parasite causing the deadliest form of malaria, is able to thrive in its human host by tight regulation of cellular processes, orchestrating nuclear dynamics with cytoplasmic machineries that are separated by the nuclear envelope. One of the major protein complexes that connect nuclear and cytoplasmic processes in eukaryotes is the linker of nucleoskeleton and cytoskeleton (LINC) complex. However, while the nuclear periphery of P. falciparum was implicated in several important functions, the role of the LINC complex in Plasmodium biology is unknown. Here, we identify two components of P. falciparum LINC complex and demonstrate that they are essential for the parasites' proliferation in human blood, and their depletion leads to the formation of morphological changes in the cell. In addition, the two components have different functions in activating the DNA damage response and in their association with heterochromatin. Our data provide evidence for their essential roles in the parasites' cell cycle.

A retrospective analysis comparing metagenomic next-generation sequencing with conventional microbiology testing for the identification of pathogens in patients with severe infections

Introduction

The application value of metagenomic next-generation sequencing (mNGS) in detecting pathogenic bacteria was evaluated to promote the rational and accurate use of antibiotics. A total of 180 patients with severe infections were included in this study.

Methods

Based on their different symptoms, bronchoalveolar lavage fluid (BALF) or blood samples were collected for conventional microbiological testing (CMT) and mNGS.

Results

The results indicated that the etiological diagnosis rate of mNGS (78.89%) was significantly higher than that of CMT (20%) (p<0.001). Notably, mNGS exhibited greater sensitivity towards rare pathogens such as Chlamydia pneumoniae, Mycobacterium tuberculosis complex, and Legionella pneumophila, which were undetectable by CMT. Additionally, 64 cases underwent blood culture, BALF culture, and mNGS testing. Analysis revealed that the positive rate of blood culture (3.1%) was lower than that of BALF (25%), and the positive rate of CMT from both types was significantly lower than that of mNGS (89.1%) (p<0.001). In this study, 168 mNGS results were accepted, and 116 patients had their antibiotic therapy adjustment based on mNGS. Paired analysis indicated that white blood cell count (WBC), procalcitonin (PCT), C-reactive protein (CRP), and neutrophil (NEU) percentage provided valuable therapeutic guidance. The survival rate of patients was 55.36%, influenced by patient physical condition and age.

Discussion

Our data indicated that mNGS had significant auxiliary value in the clinical diagnosis and treatment for critically ill patients, especially for those with negative CMT results and clinically undefined infections. mNGS could broaden the detection scope, especially for special pathogens, and improve the detection rate, providing powerful assistance for early clinical diagnosis and treatment.

Reconstruction and exploitation of a dedicated Genome-Scale Metabolic Model of the human pathogen C. neoformans

C. neoformans is notorious for causing severe pulmonary and central nervous system infections, particularly in immunocompromised patients. High mortality rates, associated with its tropism and adaptation to the brain microenvironment and its drug resistance profile, makes this pathogen a public health threat and a World Health Organization (WHO) priority. In this study, we reconstructed GSMM iRV890 for C. neoformans var. grubii , providing a promising platform for the comprehensive understanding of the unique metabolic features of C. neoformans , and subsequently shedding light on its complex tropism for the brain microenvironment and potentially informing the discovery of new drug targets. The GSMM iRV890 model is openly available in the SBML format, and underwent validation using experimental data for nitrogen and carbon assimilation, as well as specific growth and glucose consumption rates. Based on the comparison with GSMMs available for other pathogenic yeasts, unique metabolic features were predicted for C. neoformans , including key pathways shaping the dynamics between C. neoformans and the human host, and underlying its adaptation to the brain environment. Finally, predicted essential genes from the validated model are explored herein as potential novel antifungal drug targets.

scRNA-seq uncovers the transcriptional dynamics of Encephalitozoon intestinalis parasites in human macrophages

Microsporidia are single-celled intracellular parasites that cause opportunistic diseases in humans. Encephalitozoon intestinalis is a prevalent human-infecting species that invades the small intestine. Macrophages are potential reservoirs of infection, and dissemination to other organ systems is also observed. The macrophage response to infection and the developmental trajectory of the parasite are not well studied. Here we use single cell RNA sequencing to investigate transcriptional changes in both the parasite and the host during E. intestinalis infection of human macrophages in vitro. The parasite undergoes large transcriptional changes throughout the life cycle, providing a blueprint for parasite development. While a small population of infected macrophages mount a response, most remain transcriptionally unchanged, suggesting that the majority of parasites may avoid host detection. The stealthy microsporidian lifestyle likely allows these parasites to harness macrophages for replication. Together, our data provide insights into the host response in primary human macrophages and the E. intestinalis developmental program.

Proteins with proximal-distal asymmetries in axoneme localisation control flagellum beat frequency

The 9 + 2 microtubule-based axoneme within motile flagella is well known for its symmetry. However, examples of asymmetric structures and proteins asymmetrically positioned within the 9 + 2 axoneme architecture have been identified. These occur in multiple different organisms, particularly involving the inner or outer dynein arms. Here, we comprehensively analyse conserved proximal-distal asymmetries in the uniflagellate trypanosomatid eukaryotic parasites. Building on the genome-wide localisation screen in Trypanosoma brucei we identify conserved proteins with an analogous asymmetric localisation in the related parasite Leishmania mexicana. Using deletion mutants, we find which are necessary for normal cell swimming, flagellum beat parameters and axoneme ultrastructure. Using combinatorial endogenous fluorescent tagging and deletion, we map co-dependencies for assembly into their normal asymmetric localisation. This revealed 15 proteins, 9 known and 6 novel, with a conserved proximal or distal axoneme-specific localisation. Most are outer dynein arm associated and show that there are multiple classes of proximal-distal asymmetry - one which is dependent on the docking complex. Many of these proteins are necessary for retaining the normal frequency of the tip-to-base symmetric flagellar waveform. Our comprehensive mapping reveals unexpected contributions of proximal-specific axoneme components to the frequency of waveforms initiated distally.

Adaptive genomic signatures of globally invasive populations of the yellow fever mosquito Aedes aegypti

In the arboviral vector Aedes aegypti, adaptation to anthropogenic environments has led to a major evolutionary shift separating the domestic Aedes aegypti aegypti (Aaa) ecotype from the wild Aedes aegypti formosus (Aaf) ecotype. Aaa mosquitoes are distributed globally and have higher vectorial capacity than Aaf, which remained in Africa. Despite the evolutionary and epidemiological relevance of this separation, inconsistent morphological data and a complex population structure have hindered the identification of genomic signals distinguishing the two ecotypes. Here we assessed the correspondence between the geographic distribution, population structure and genome-wide selection of 511 Aaf and 123 Aaa specimens and report adaptive signals in 186 genes that we call Aaa molecular signatures. Our results indicate that Aaa molecular signatures arose from standing variation associated with extensive ancestral polymorphisms in Aaf populations and have been co-opted for self-domestication through genomic and functional redundancy and local adaptation. Overall, we show that the behavioural shift of Ae. aegypti mosquitoes to live in association with humans relied on the fine regulation of chemosensory, neuronal and metabolic functions, as seen in the domestication processes of rabbits and silkworms. Our results also provide a foundation for the investigation of new genic targets for the control of Ae. aegypti populations.

Diamonds in the rif: Alignment-free comparative genomics analysis reveals strain-transcendent Plasmodium falciparum antigens amidst extensive genetic diversity

The repetitive interspersed family (rif) and subtelomeric variable open reading frames (stevor) are highly diverse multi-gene families in the malaria parasite Plasmodium falciparum. Embedded on the surface of infected erythrocytes, RIFIN and STEVOR proteins are involved in cytoadherence and immune evasion, but the extent of family-wide sequence diversity across strains has yet to be comprehensively investigated in light of improved resolution of the subtelomeric genome sequences. Using a k-mer frequency approach, we analyzed long-read genomic sequence data from 18 geographically diverse P. falciparum genome assemblies, including lab strains and clinical isolates. We hypothesized that k-mer sequence comparison can identify existing RIFIN and STEVOR subgroups, identify novel subgroups, and generate more robust and reliable estimates of family-wide sequence diversity. Full-length RIFIN and STEVOR proteins shared on average 49.5% and 61.1% amino acid k-mer similarity, respectively, which fell to 25.1% and 20% in the hypervariable regions alone. Despite this diversity, we identified 11 RIFINs and five STEVORs that were conserved across strains above expected thresholds. A subset of these strain-transcendent genes was similar and syntenic to genes in related Plasmodium species, suggesting an ancient origin. Additionally, in silico structural predictions from AlphaFold showed that three-dimensional structures of RIFIN receptor-binding regions were more conserved than their sequences suggested. Evolutionarily constrained RIFINs and STEVORs may have critical functions in parasite survival or pathogenesis. This study provides a framework for investigating diversity in highly variable multi-gene families and highlights the potential of strain-transcendent RIFIN and STEVOR proteins as vaccine candidates.

Identification of Toxoplasma gondii antigenic proteins using an in vivo approach and in silico investigation of their polymorphism

Toxoplasma gondii is a pathogen characterized by a large variety of strains whose virulence and clinical severity are likely linked to their genotype. Currently, the strains are genotyped using restriction fragment length polymorphism, multilocus sequence typing, and microsatellite markers. This typing requires the strain's DNA, which is difficult to obtain. A serotyping test could overcome the constraints of genotyping, the challenge being to identify type-specific proteins. We identified immunogenic T. gondii proteins from co-immunoprecipitations for three tachyzoite strains (strain FOU from Africa 1 type, ME49 from Type II, and VEG from Type III) with hyperimmune murine sera and conducted an in silico polymorphism search for the identified proteins. A variant calling analysis was conducted on the next-generation sequencing sequences of 117 T. gondii isolates with the objective of identifying mutations present in the genes encoding the antigenic proteins previously identified. A total of 727 immunogenic proteins were identified, including 16% dense granule protein (GRA), rhoptry protein (ROP/RON), and surface antigen protein (SAG). Genetic analysis revealed the presence of 36 single-nucleotide polymorphisms (SNPs) in over 70% of isolates belonging to the same type, while less than 30% of isolates belonging to the other types exhibited these polymorphisms. Of these, only 15 are located on coding DNA sequence regions, while four are located on genes encoding apicomplexan proteins: two SNPs on the ROP5 gene and two on the ROP7 gene. The results of this study indicate that a significant number of T. gondii immunogenic proteins can be identified using an in vivo approach. The in silico study identified SNPs that could be genotype-specific.

IMPORTANCE

Toxoplasma gondii is a unique species that exhibits genotype diversity related to clinical virulence. Currently, genotyping is restricted, which limits epidemiological knowledge of the strains. To overcome this limitation, we aimed to develop serotyping tests. First, we used a murine in vivo, non-targeted experimental approach based on proteomics techniques through which we were able to identify a panel of more than 700 antigenic proteins from T. gondii. Then, we analyzed the polymorphism of these proteins using a whole-genome sequencing database containing the genomes of 117 genotyped strains. We showed that none of the 986 non-silent SNPs detected is specific to the strain type. The in vivo approach is the first that allowed the identification of such a large panel of antigenic proteins. Moreover, the polymorphism analysis, the first based on a large next-generation sequencing database, showed the limits that currently restrict the development of a serotyping technique.

PTRAMP, CSS and Ripr form a conserved complex required for merozoite invasion of Plasmodium species into erythrocytes

Invasion of erythrocytes by members of the Plasmodium genus is an essential step of the parasite lifecycle, orchestrated by numerous host-parasite interactions. In P. falciparum Rh5, with PfCyRPA, PfRipr, PfCSS, and PfPTRAMP, forms the essential PCRCR complex which binds basigin on the erythrocyte surface. Rh5 is restricted to P. falciparum and its close relatives; however, PTRAMP, CSS and Ripr orthologs are present across the Plasmodium genus. We investigated PTRAMP, CSS and Ripr orthologs from three species to elucidate common features of the complex. Like P. falciparum, PTRAMP and CSS form a disulfide-linked heterodimer in both P. vivax and P. knowlesi with all three species forming a complex (PCR) with Ripr by binding its C-terminal region. Cross-reactive antibodies targeting the PCR complex differentially inhibit merozoite invasion. Cryo-EM visualization of the P. knowlesi PCR complex confirmed predicted models and revealed a core invasion scaffold in Plasmodium spp. with implications for vaccines targeting multiple species of malaria-causing parasites.

Targets for the diagnosis of Acanthamoeba eye infections include four cyst wall proteins and the mannose-binding domain of the trophozoite mannose-binding protein

Acanthamoebae, which are free-living amoebae, cause corneal inflammation (keratitis) and blindness, if not quickly diagnosed and effectively treated. The walls of Acanthamoeba cysts contain cellulose and have two layers connected by conical ostioles. Cysts are identified by in vivo confocal microscopy of the eye or calcofluor-white- or Giemsa-labeling of corneal scrapings, both of which demand great expertise. Trophozoites, which use a mannose-binding protein to adhere to keratinocytes, are identified in eye cultures that delay diagnosis and treatment. We recently used structural and experimental methods to characterize cellulose-binding domains of Luke and Leo lectins, which are abundant in the inner layer and ostioles. However, no antibodies have been made to these lectins or to a Jonah lectin and a laccase, which are abundant in the outer layer. Here, confocal microscopy of rabbit antibodies (rAbs) to recombinant Luke, Leo, Jonah, and laccase supported localizations of GFP-tagged proteins in walls of transfected Acanthamoebae. rAbs efficiently detected calcofluor white-labeled cysts of 10 of the 11 Acanthamoeba isolates tested, including six T4 genotypes that cause most cases of keratitis. Further, laccase shed into the medium during encystation was detected by an enzyme-linked immunoassay. Structural and experimental methods identified the mannose-binding domain (ManBD) of the Acanthamoeba mannose-binding protein, while rAbs to the ManBD efficiently detected DAPI-labeled trophozoites from all 11 Acanthamoeba isolates tested. We conclude that antibodies to four cyst wall proteins and the ManBD efficiently identify Acanthamoeba cysts and trophozoites, respectively.IMPORTANCEFree-living amoeba in the soil or water cause Acanthamoeba keratitis, which is diagnosed by identification of unlabeled cysts by in vivo confocal microscopy of the eye or calcofluor-white (CFW) labeled cysts by fluorescence microscopy of corneal scrapings. Alternatively, Acanthamoeba infections are diagnosed by the identification of trophozoites in eye cultures. Here, we showed that rabbit antibodies (rAbs) to four abundant cyst wall proteins (Jonah, Luke, Leo, and laccase) each efficiently identify CFW-labeled cysts of 10 of the 11 Acanthamoeba isolates tested. Further, laccase released into the medium by encysting Acanthamoebae was detected by an enzyme-linked immunoassay. We also showed that rAbs to the mannose-binding domain (ManBD) of the Acanthamoeba mannose-binding protein, which mediates adherence of trophozoites to keratinocytes, efficiently identify DAPI-labeled trophozoites of all 11 Acanthamoeba isolates tested. In summary, four wall proteins and the ManBD appear to be excellent targets for the diagnosis of Acanthamoeba cysts and trophozoites, respectively.

Apoptotic proteins in Leishmania donovani: in silico screening, modeling, and validation by knock-out and gene expression analysis

Visceral leishmaniasis, a life-threatening vector-borne illness that disproportionately affects children and elderly immunocompromised people, is a primary tropical neglected disease. No apoptotic partner proteins have yet been reported in Leishmania donovani, while their identification could contribute to knowledge on parasite cell death and the establishment of alternative therapeutics. We searched for mammalian Bcl-2 family protein orthologs and found one anti-apoptotic and two pro-apoptotic orthologs in L. donovani. A pro-death aquaporin protein, due to its characteristic BH3 domain known to interact with pro-apoptotic proteins in mammalian Bcl-2 family proteins, was also included in this study. Molecular docking and molecular dynamics simulations were conducted to assess protein-protein interactions between the identified apoptotic proteins and mimic mammalian intrinsic apoptotic pathways. The results showed that both pro-apoptotic proteins interacted with the hydrophobic pocket of the anti-apoptotic ortholog, forming a stable complex. This interaction may represent a critical event in an apoptotic pathway in L. donovani. To further characterise it, we used CRISPR-Cas9 approaches to target the identified proteins. Pure knocked population mutants, and episomal over-expressing mutant cells were exposed to apoptotic stimuli. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay and quantitative expression profiling suggested that these proteins are involved in the parasite's apoptosis and could play a role in its survival.

A scaleable inducible knockout system for studying essential gene function in the malaria parasite

The malaria parasite needs nearly half of its genes to propagate normally within red blood cells. Inducible ways to interfere with gene expression like the DiCre-lox system are necessary to study the function of these essential genes. However, existing DiCre-lox strategies are not well-suited to be deployed at scale to study several genes simultaneously. To overcome this, we have developed SHIFTiKO (frameshift-based trackable inducible knockout), a novel scaleable strategy that uses short, easy-to-construct, barcoded repair templates to insert loxP sites around short regions in target genes. Induced DiCre-mediated excision of the flanked region causes a frameshift mutation resulting in genetic ablation of gene function. Dual DNA barcodes inserted into each mutant enables verification of successful modification and induced excision at each locus and collective phenotyping of the mutants, not only across multiple replication cycles to assess growth fitness but also within a single cycle to identify specific phenotypic impairments. As a proof of concept, we have applied SHIFTiKO to screen the functions of malarial rhomboid proteases, successfully identifying their blood stage-specific essentiality. SHIFTiKO thus offers a powerful platform to conduct inducible phenotypic screens to study essential gene function at scale in the malaria parasite.

Supersaturation mutagenesis reveals adaptive rewiring of essential genes among malaria parasites

Malaria parasites are highly divergent from model eukaryotes. Large-scale genome engineering methods effective in model organisms are frequently inapplicable, and systematic studies of gene function are few. We generated more than 175,000 transposon insertions in the Plasmodium knowlesi genome, averaging an insertion every 138 base pairs, and used this "supersaturation" mutagenesis to score essentiality for 98% of genes. The density of mutations allowed mapping of putative essential domains within genes, providing a completely new level of genome annotation for any Plasmodium species. Although gene essentiality was largely conserved across P. knowlesi, Plasmodium falciparum, and rodent malaria model Plasmodium berghei, a large number of shared genes are differentially essential, revealing species-specific adaptations. Our results indicated that Plasmodium essential gene evolution was conditionally linked to adaptive rewiring of metabolic networks for different hosts.

Iron-mediated post-transcriptional regulation in Toxoplasma gondii

Iron is required to support almost all life; however, levels must be carefully regulated to maintain homeostasis. Although the obligate parasite Toxoplasma gondii requires iron, how it responds upon iron limitation has not been investigated. Here, we show that iron depletion triggers significant transcriptional changes in the parasite, including in iron-dependent pathways. We find that a subset of T. gondii transcripts contain stem-loop structures, which have been associated with post-transcriptional iron-mediated regulation in other cellular systems. We validate one of these (found in the 3' UTR of TGME49_261720) using a reporter cell line. We show that the presence of the stem-loop-containing UTR is sufficient to confer accumulation at the transcript and protein levels under low iron. This response is dose and time-dependent and is specific for iron. The accumulation of transcript is likely driven by an increased reporter mRNA stability under low iron. Interestingly, we find iron-mediated changes in mRNA stability in around 400 genes. To examine the potential mechanism of this stability, we tested aconitase interaction with mRNA in low iron and found 43 enriched transcripts, but no specific interaction with our reporter UTR. However, the endogenous UTR led to maintenance of protein levels and increased survival of the parasite under low iron. Our data demonstrate the existence of iron-mediated post-transcriptional regulation in Toxoplasma for the first time; and suggests iron-mediated regulation may be important to the parasite in low iron environments.

Liver stage P. falciparum antigens highly targeted by CD4+ T cells in malaria-exposed Ugandan children

T cell responses against liver stage Plasmodium help protect against reinfection, but the antigens and epitopes targeted by these T cells are largely unknown. This knowledge gap has impeded mechanistic studies to identify the effector functions most critical for protection. We performed a bioinformatic analysis of gene expression datasets to identify plasmodial genes that are highly and selectively expressed during liver stage infection and predict epitopes within them likely to bind MHC-II molecules prevalent in Uganda. We then tested their recognition by malaria-exposed Ugandan children. In over two-thirds of the children, we detected a peripheral blood CD4+ T cell response to one or more antigens. The most highly targeted antigen, LISP1, contained several epitopes, including one that was promiscuously presented and recognized by most participants. These novel liver stage P. falciparum epitopes should be explored as potential vaccine targets and will facilitate the development of tools to interrogate antimalarial immunity at the single-cell level and inform future vaccine development efforts.

A scalable CRISPR-Cas9 gene editing system facilitates CRISPR screens in the malaria parasite Plasmodium berghei

Many Plasmodium genes remain uncharacterized due to low genetic tractability. Previous large-scale knockout screens have only been able to target about half of the genome in the more genetically tractable rodent malaria parasite Plasmodium berghei. To overcome this limitation, we have developed a scalable CRISPR system called P. berghei high-throughput (PbHiT), which uses a single cloning step to generate targeting vectors with 100-bp homology arms physically linked to a guide RNA (gRNA) that effectively integrate into the target locus. We show that PbHiT coupled with gRNA sequencing robustly recapitulates known knockout mutant phenotypes in pooled transfections. Furthermore, we provide an online resource of knockout and tagging designs to target the entire P. berghei genome and scale-up vector production using a pooled ligation approach. This work presents for the first time a tool for high-throughput CRISPR screens in Plasmodium for studying the parasite's biology at scale.

The Toxoplasma gondii homolog of ATPase inhibitory factor 1 is critical for mitochondrial cristae maintenance and stress response

The production of energy in the form of ATP by the mitochondrial ATP synthase must be tightly controlled. One well-conserved form of regulation is mediated via ATPase inhibitory factor 1 (IF1), which governs ATP synthase activity and gene expression patterns through a cytoprotective process known as mitohormesis. In apicomplexans, the processes regulating ATP synthase activity are not fully elucidated. Using the model apicomplexan Toxoplasma gondii, we found that knockout and overexpression of TgIF1, the structural homolog of IF1, significantly affected gene expression. Additionally, TgIF1 overexpression resulted in the formation of a stable TgIF1 oligomer and increased the presence of higher order ATP synthase oligomers. We also show that parasites lacking TgIF1 exhibit reduced mitochondrial cristae density, and that while TgIF1 levels do not affect growth in conventional culture conditions, they are crucial for parasite survival under hypoxia. Interestingly, TgIF1 overexpression enhances recovery from oxidative stress, suggesting a mitohormetic function. In summary, while TgIF1 does not appear to play a role in ATP synthase regulation under conventional growth conditions, our work uncovers its potential role in adapting to the stressors faced by T. gondii and other apicomplexans throughout their intricate life cycles.

Toxoplasma replication is inhibited by MMV676477 without development of resistance

Protozoan parasites cause life-threatening infections in both humans and animals, including agriculturally significant livestock. Available treatments are typically narrow spectrum and are complicated by drug toxicity and the development of resistant parasites. Protozoan tubulin is an attractive target for the development of broad-spectrum antimitotic agents. The Medicines for Malaria Pathogen Box compound MMV676477 was previously shown to inhibit replication of kinetoplastid parasites, such as Leishmania amazonensis and Trypanosoma brucei, and the apicomplexan parasite Plasmodium falciparum by selectively stabilizing protozoan microtubules. In this report, we show that MMV676477 inhibits intracellular growth of the human apicomplexan pathogen Toxoplasma gondii with an EC50 value of ~50 nM. MMV676477 does not stabilize vertebrate microtubules or cause other toxic effects in human fibroblasts. The availability of tools for genetic studies makes Toxoplasma a useful model for studies of the cytoskeleton. We conducted a forward genetics screen for MMV676477 resistance, anticipating that missense mutations would delineate the binding site on protozoan tubulin. Unfortunately, we were unable to use genetics to dissect target interactions because no resistant parasites emerged. This outcome suggests that future drugs based on the MMV676477 scaffold would be less likely to be undermined by the emergence of drug resistance.

Solving the puzzle of copper trafficking in Trypanosoma cruzi: candidate genes that can balance uptake and toxicity

Trypanosoma cruzi, the causative agent of Chagas disease, depends on acquiring nutrients and cofactors, such as copper (Cu), from different hosts. Cu is essential for aerobic organisms, but it can also be toxic, and so its transport and storage must be regulated. In the present study, we characterized the effects of changes in Cu availability on growth behavior, intracellular ion content and oxygen consumption. Our results show that copper is essential for epimastigote proliferation and for the metacyclogenesis process. On the other hand, intracellular amastigotes suffered copper stress during infection. In addition, we identify gene products potentially involved in copper metabolism. Orthologs of the highly conserved P-type Cu ATPases involved in copper export and loading of secreted enzymes were identified and named T. cruzi Cu P-type ATPase (TcCuATPase). TcCuATPase transcription is upregulated during infective stages and following exposure to copper chelators in the epimastigote stage. Homolog sequences for the high affinity import protein CTR1 were not found. Instead, we propose that the T. cruzi iron transporter (TcIT), a ZIP family transporter, could be involved in copper uptake based on transcriptional response to copper availability. Further canonical copper targets (based on homology to yeast and mammals) such as the T. cruzi ferric reductase (TcFR) and the cupro-oxidase TcFet3 are upregulated during infective stages and under conditions of intracellular copper deficiency. In sum, copper metabolism is essential for the life cycle of T. cruzi. Even though cytosolic copper chaperons were not identified, we propose a previously undescribed model for copper transport and intracellular distribution in T. cruzi, including some conserved factors such as TcCuATPase, as well as others such as TcFR and TcIT, playing novel functions.

Genetic factors regulating Plasmodium falciparum gametocytogenesis identified by phenotypic screens

Successful transmission of Plasmodium falciparum from one person to another relies on the complete intraerythrocytic development of non-pathogenic sexual gametocytes infectious for anopheline mosquitoes. Understanding the genetic factors that regulate gametocyte development is vital for identifying transmission-blocking targets in the malaria parasite life cycle. Toward this end, we conducted a forward genetic study to characterize the development of gametocytes from sexual commitment to mature stage V. We described a new analysis pipeline for the piggyBac transposon-based mutagenesis phenotypic screen to identify genes that influence both early and late gametocyte stages. We classified individual mutants that increased or decreased parasite abundance as the hypoproducer and hyperproducer phenotypes, respectively, revealing distinctive temporal genetic factors early and late in the sexual development cycle. The study identifies that disruption in factors involved in transcription, protein trafficking and DNA repair are associated with decreasing gametocyte production, while modifications in phosphatase activity are linked to hyperproduction of gametocytes. Our study provides an optimized approach on genotype-phenotype evaluation, offering a new resource for understanding potential targets for therapeutic intervention strategies to disrupt transmission.

Apicomplexan mitoribosome from highly fragmented rRNAs to a functional machine

The phylum Apicomplexa comprises eukaryotic parasites that cause fatal diseases affecting millions of people and animals worldwide. Their mitochondrial genomes have been significantly reduced, leaving only three protein-coding genes and highly fragmented mitoribosomal rRNAs, raising challenging questions about mitoribosome composition, assembly and structure. Our study reveals how Toxoplasma gondii assembles over 40 mt-rRNA fragments using exclusively nuclear-encoded mitoribosomal proteins and three lineage-specific families of RNA-binding proteins. Among these are four proteins from the Apetala2/Ethylene Response Factor (AP2/ERF) family, originally known as transcription factors in plants and Apicomplexa, now repurposed as essential mitoribosome components. Cryo-EM analysis of the mitoribosome structure demonstrates how these AP2 proteins function as RNA binders to maintain mitoribosome integrity. The mitoribosome is also decorated with members of lineage-specific RNA-binding proteins belonging to RAP (RNA-binding domain abundant in Apicomplexa) proteins and HPR (heptatricopeptide repeat) families, highlighting the unique adaptations of these parasites. Solving the molecular puzzle of apicomplexan mitoribosome could inform the development of therapeutic strategies targeting organellar translation.

Elevated mutation rates in multi-azole resistant Aspergillus fumigatus drive rapid evolution of antifungal resistance

The environmental use of azole fungicides has led to selective sweeps across multiple loci in the Aspergillus fumigatus genome causing the rapid global expansion of a genetically distinct cluster of resistant genotypes. Isolates within this cluster are also more likely to be resistant to agricultural antifungals with unrelated modes of action. Here we show that this cluster is not only multi-azole resistant but has increased propensity to develop resistance to next generation antifungals because of variants in the DNA mismatch repair system. A variant in msh6-G233A is found almost exclusively within azole resistant isolates harbouring the canonical cyp51A azole resistance allelic variant TR34/L98H. Naturally occurring isolates with this msh6 variant display up to 5-times higher rate of mutation, leading to an increased likelihood of evolving resistance to other antifungals. Furthermore, unlike hypermutator strains, the G233A variant conveys no measurable fitness cost and has become globally distributed. Our findings further suggest that resistance to next-generation antifungals is more likely to emerge within organisms that are already multi-azole resistant due to close linkage between TR34/L98H and msh6-G233A, posing a major problem due to the prospect of dual use of novel antifungals in clinical and agricultural settings.

Malaria parasites require a divergent heme oxygenase for apicoplast gene expression and biogenesis

Malaria parasites have evolved unusual metabolic adaptations that specialize them for growth within heme-rich human erythrocytes. During blood-stage infection, Plasmodium falciparum parasites internalize and digest abundant host hemoglobin within the digestive vacuole. This massive catabolic process generates copious free heme, most of which is biomineralized into inert hemozoin. Parasites also express a divergent heme oxygenase (HO)-like protein (PfHO) that lacks key active-site residues and has lost canonical HO activity. The cellular role of this unusual protein that underpins its retention by parasites has been unknown. To unravel PfHO function, we first determined a 2.8 Å-resolution X-ray structure that revealed a highly α-helical fold indicative of distant HO homology. Localization studies unveiled PfHO targeting to the apicoplast organelle, where it is imported and undergoes N-terminal processing but retains most of the electropositive transit peptide. We observed that conditional knockdown of PfHO was lethal to parasites, which died from defective apicoplast biogenesis and impaired isoprenoid-precursor synthesis. Complementation and molecular-interaction studies revealed an essential role for the electropositive N-terminus of PfHO, which selectively associates with the apicoplast genome and enzymes involved in nucleic acid metabolism and gene expression. PfHO knockdown resulted in a specific deficiency in levels of apicoplast-encoded RNA but not DNA. These studies reveal an essential function for PfHO in apicoplast maintenance and suggest that Plasmodium repurposed the conserved HO scaffold from its canonical heme-degrading function in the ancestral chloroplast to fulfill a critical adaptive role in organelle gene expression.

Paracrine rescue of MYR1-deficient Toxoplasma gondii mutants reveals limitations of pooled in vivo CRISPR screens

Toxoplasma gondii is an intracellular parasite that subverts host cell functions via secreted virulence factors. Up to 70% of parasite-controlled changes in the host transcriptome rely on the MYR1 protein, which is required for the translocation of secreted proteins into the host cell. Mice infected with MYR1 knock-out (KO) strains survive infection, supporting a paramount function of MYR1-dependent secreted proteins in Toxoplasma virulence and proliferation. However, we have previously shown that MYR1 mutants have no growth defect in pooled in vivo CRISPR-Cas9 screens in mice, suggesting that the presence of parasites that are wild-type at the myr1 locus in pooled screens can rescue the phenotype. Here, we demonstrate that MYR1 is not required for the survival in IFN-γ-activated murine macrophages, and that parasites lacking MYR1 are able to expand during the onset of infection. While ΔMYR1 parasites have restricted growth in single-strain murine infections, we show that the phenotype is rescued by co-infection with wild-type (WT) parasites in vivo, independent of host functional adaptive immunity or key pro-inflammatory cytokines. These data show that the major function of MYR1-dependent secreted proteins is not to protect the parasite from clearance within infected cells. Instead, MYR-dependent proteins generate a permissive niche in a paracrine manner, which rescues ΔMYR1 parasites within a pool of CRISPR mutants in mice. Our results highlight an important limitation of otherwise powerful in vivo CRISPR screens and point towards key functions for MYR1-dependent Toxoplasma-host interactions beyond the infected cell.

Phosphorylation in the Plasmodium falciparum Proteome: A Meta-Analysis of Publicly Available Data Sets

Malaria is a deadly disease caused by Apicomplexan parasites of the Plasmodium genus. Several species of the Plasmodium genus are known to be infectious to humans, of which P. falciparum is the most virulent. Post-translational modifications (PTMs) of proteins coordinate cell signaling and hence regulate many biological processes in P. falciparum homeostasis and host infection, of which the most highly studied is phosphorylation. Phosphosites on proteins can be identified by tandem mass spectrometry (MS) performed on enriched samples (phosphoproteomics), followed by downstream computational analyses. We have performed a large-scale meta-analysis of 11 publicly available phosphoproteomics data sets to build a comprehensive atlas of phosphosites in the P. falciparum proteome, using robust pipelines aimed at strict control of false identifications. We identified a total of 26,609 phosphorylated sites on P. falciparum proteins, split across three categories of data reliability (gold/silver/bronze). We identified significant sequence motifs, likely indicative of different groups of kinases responsible for different groups of phosphosites. Conservation analysis identified clusters of phosphoproteins that are highly conserved and others that are evolving faster within the Plasmodium genus, and implicated in different pathways. We were also able to identify over 180,000 phosphosites within Plasmodium species beyond falciparum, based on orthologue mapping. We also explored the structural context of phosphosites, identifying a strong enrichment for phosphosites on fast-evolving (low conservation) intrinsically disordered regions (IDRs) of proteins. In other species, IDRs have been shown to have an important role in modulating protein-protein interactions, particularly in signaling, and thus warranting further study for their roles in host-pathogen interactions. All data have been made available via UniProtKB, PRIDE, and PeptideAtlas, with visualization interfaces for exploring phosphosites in the context of other data on Plasmodium proteins.

Evidence of γ-secretase complex involved in the regulation of intramembrane proteolysis in Entamoeba histolytica

Presenilins (PSNs) are multifunctional membrane proteins involved in signal transduction, lysosomal acidification, and certain physiological processes related to mitochondria. The aspartic protease activity of PSN and the formation of a γ-secretase complex with other subunits such as nicastrin (NCT) are required for the biological functions. Although PSN is widely conserved in eukaryotes, most studies on PSN were conducted in metazoans. Homologous genes for PSN and NCT (EhPSN and EhNCT, respectively) are encoded in the genome of Entamoeba histolytica, however, their functions remain unknown. In this study, we showed that EhPSN and EhNCT form a complex on the cell membrane, demonstrating that the parasite possesses γ-secretase. The predicted structure of EhPSN was similar to the human homolog, demonstrated by the crystal structure, and phylogenetic analysis indicated good conservation between EhPSN and human PSN, supporting the premise that EhPSN functions as a subunit of γ-secretase. By contrast, EhNCT appears to have undergone remarkable structural changes during its evolution. Blue native-polyacrylamide gel electrophoresis combined with western blotting indicated that a 150-kDa single band contains both EhPSN (estimated molecular size: 47-kDa) and EhNCT (64-kDa), suggesting that the complex also contains other unknown components or post-translational modifications. Coimmunoprecipitation from amebic lysates also confirmed that EhPSN and EhNCT formed a complex. Indirect immunofluorescence analysis revealed that the complex localized to the plasma membrane. Moreover, EhPSN exhibited protease activity, which was suppressed by a γ-secretase inhibitor. This is the first report of a γ-secretase complex in protozoan parasites.

Insights into the evolution, virulence and speciation of Babesia MO1 and Babesia divergens through multiomics analyses

Babesiosis, caused by protozoan parasites of the genus Babesia, is an emerging tick-borne disease of significance for both human and animal health. Babesia parasites infect erythrocytes of vertebrate hosts where they develop and multiply rapidly to cause the pathological symptoms associated with the disease. The identification of new Babesia species underscores the ongoing risk of zoonotic pathogens capable of infecting humans, a concern amplified by anthropogenic activities and environmental changes. One such pathogen, Babesia MO1, previously implicated in severe cases of human babesiosis in the United States, was initially considered a subspecies of B. divergens, the predominant agent of human babesiosis in Europe. Here we report comparative multiomics analyses of B. divergens and B. MO1 that offer insight into their biology and evolution. Our analysis shows that despite their highly similar genomic sequences, substantial genetic and genomic divergence occurred throughout their evolution resulting in major differences in gene functions, expression and regulation, replication rates and susceptibility to antiparasitic drugs. Furthermore, both pathogens have evolved distinct classes of multigene families, crucial for their pathogenicity and adaptation to specific mammalian hosts. Leveraging genomic information for B. MO1, B. divergens, and other members of the Babesiidae family within Apicomplexa provides valuable insights into the evolution, diversity, and virulence of these parasites. This knowledge serves as a critical tool in preemptively addressing the emergence and rapid transmission of more virulent strains.

Putative MutS2 Homologs in Algae: More Goods in Shopping Bag?

MutS2 proteins are presumably involved in either control of recombination or translation quality control in bacteria. MutS2 homologs have been found in plants and some algae; however, their actual diversity in eukaryotes remains unknown. We found putative MutS2 homologs in various species of photosynthetic eukaryotes and performed a detailed analysis of the revealed amino acid sequences. Three groups of homologs were distinguished depending on their domain composition: MutS2 homologs with full set of specific domains, MutS2-like sequences without endonuclease Smr domain, and MutS2-like homologs lacking Smr and clamp in domain IV, the extreme form of which are proteins with only a complete ATPase domain. We clarified the information about amino acid composition and set of specific motifs in the conserved domains in MutS2 and MutS2-like sequences. The models of the predicted tertiary structure were obtained for each group of homologs. The phylogenetic analysis demonstrated that all eukaryotic sequences split into two large groups. The first group included homologs belonging to species of Archaeplastida and a subset of haptophyte homologs, while the second-sequences of organisms from CASH groups (cryptophytes, alveolates, stramenopiles, haptophytes) and chlorarachniophytes. The cyanobacterial MutS2 clustered together with the first group, and proteins belonging to Deltaproteobacteria (orders Myxococcales and Bradymonadales) showed phylogenetic affinity to the CASH-including group with strong support. The observed tree pattern did not support a clear differentiation of eukaryotes into lineages with red and green algae-derived plastids. The results are discussed in the context of current conceptions of serial endosymbioses and genetic mosaicism in algae with complex plastids.

Population genomics of Plasmodium ovale species in sub-Saharan Africa

Plasmodium ovale curtisi (Poc) and Plasmodium ovale wallikeri (Pow) are relapsing malaria parasites endemic to Africa and Asia that were previously thought to represent a single species. Amid increasing detection of ovale malaria in sub-Saharan Africa, we present a population genomic study of both species across the continent. We conducted whole-genome sequencing of 25 isolates from Central and East Africa and analyzed them alongside 20 previously published African genomes. Isolates are predominantly monoclonal (43/45), with their genetic similarity aligning with geography. Pow shows lower average nucleotide diversity (1.8×10-4) across the genome compared to Poc (3.0×10-4) (p < 0.0001). Signatures of selective sweeps involving the dihydrofolate reductase gene have been found in both species, as are signs of balancing selection at the merozoite surface protein 1 gene. Differences in the nucleotide diversity of Poc and Pow may reflect unique demographic history, even as similar selective forces facilitate their resilience to malaria control interventions.

Long-term survival of asexual Zymoseptoria tritici spores in the environment

BACKGROUND

The fungal phytopathogen Zymoseptoria tritici, causal agent of the economically damaging Septoria tritici blotch of wheat, is different from most foliar fungal pathogens in that its germination occurs slowly and apparently randomly after arrival on the leaf surface and is followed by a potentially prolonged period of epiphytic growth and even reproduction, during which no feeding structures are formed by the fungus. Thus, understanding the cues for germination and the mechanisms that underpin survival in low-nutrient environments could provide key new avenues for disease control.

RESULTS

In this work, we examine survival, culturability and virulence of spores following transfer from a high nutrient environment to water. We find that a sub-population of Z. tritici spores can survive and remain virulent for at least 7 weeks in water alone, during which time multicellular structures split to single cells. The fungus relies heavily on stored lipids; however, if cell suspensions in water are dried, the cells survive without lipid utilisation. Changes in gene expression in the first hours after suspension in water reflect adaptation to stress, while longer term starvation (7 days) induces changes particularly in primary metabolism and cytochrome P450 (CYP) gene expression. Importantly, we also found that Z. tritici spores are equally or better able to survive in soil as in water, and that rain-splash occurring 49 days after soil inoculation can transfer cells to wheat seedlings growing in inoculated soil and cause Septoria leaf blotch disease.

CONCLUSIONS

Z. tritici blastospores can survive in water or soil for long periods, potentially spanning the intercrop period for UK winter wheat. They rely on internal lipid stores, with no external nutrition, and although a large proportion of spores do not survive for such an extended period, those that do remain as virulent as spores grown on rich media. Thus, Z. tritici has exceptional survival strategies, which are likely to be important in understanding its population genetics and in developing novel routes for Septoria leaf blotch control.

Characterization and functional analysis of Toxoplasma Golgi-associated proteins identified by proximity labeling

Toxoplasma gondii possesses a highly polarized secretory pathway that contains both broadly conserved eukaryotic organelles and unique apicomplexan organelles, which play essential roles in the parasite's lytic cycle. As in other eukaryotes, the T. gondii Golgi apparatus sorts and modifies proteins prior to their distribution to downstream organelles. Many of the typical trafficking factors found involved in these processes are missing from apicomplexan genomes, suggesting that these parasites have evolved unique proteins to fill these roles. Here, we identify a Golgi-localizing protein (ULP1), which is structurally similar to the eukaryotic trafficking factor p115/Uso1. We demonstrate that depletion of ULP1 leads to a dramatic reduction in parasite fitness that is the result of defects in microneme secretion, invasion, replication, and egress. Using ULP1 as bait for TurboID proximity labeling and immunoprecipitation, we identify 11 more Golgi-associated proteins and demonstrate that ULP1 interacts with the T. gondii-conserved oligomeric Golgi (COG) complex. These proteins include both conserved trafficking factors and parasite-specific proteins. Using a conditional knockdown approach, we assess the effect of each of these 11 proteins on parasite fitness. Together, this work reveals a diverse set of T. gondii Golgi-associated proteins that play distinct roles in the secretory pathway. As several of these proteins are absent outside of the Apicomplexa, they represent potential targets for the development of novel therapeutics against these parasites.

IMPORTANCE

Apicomplexan parasites such as Toxoplasma gondii infect a large percentage of the world's population and cause substantial human disease. These widespread pathogens use specialized secretory organelles to infect their host cells, modulate host cell functions, and cause disease. While the functions of the secretory organelles are now better understood, the Golgi apparatus of the parasite remains largely unexplored, particularly regarding parasite-specific innovations that may help direct traffic intracellularly. In this work, we characterize ULP1, a protein that is unique to parasites but shares structural similarity to the eukaryotic trafficking factor p115/Uso1. We show that ULP1 plays an important role in parasite fitness and demonstrate that it interacts with the conserved oligomeric Golgi (COG) complex. We then use ULP1 proximity labeling to identify 11 additional Golgi-associated proteins, which we functionally analyze via conditional knockdown. This work expands our knowledge of the Toxoplasma Golgi apparatus and identifies potential targets for therapeutic intervention.

Mining the secreted and membrane transcriptome of Hyalomma dromedarii ticks for identification of potential protective antigens

BACKGROUND

Members belonging to the tick genus Hyalomma function as a multi-host reservoir for several pathogens and important parasites infesting large animals, such as camels, goats, cattle and sheep. In Egypt, there is a high risk of pathogen transmission as camels and cattle are imported from Sudan and Ethiopia and shipped to slaughterhouses and animal markets located in populated areas. Hyalomma dromedarii ticks are semi-desert vectors and, similar to other members of the genus Hyalomma, characterized by long-term feeding. During this process, different physiological, biochemical and immunological interactions occur within both the feeding ticks and their hosts. These biological changes affect the different tick developmental phases. The aim of this study was to explore the transcriptome of mixed messenger RNAs (mRNAs) collected from H. dromedarii eggs, larvae, nymphs and fed and unfed adults, using the Gateway cDNA library prepared in pCMV sport6.1 vector METHODS: The clones were sequenced and searched for potential secreted, membrane-associated or transmembrane (SMaT) sequences. The identified SMaT sequences were compared to the National Center for Biotechnology Information (NCBI) non-redundant protein sequence database using Blastx. Annotation and functional classification were achieved by comparison to sequences in the UniProtKB/Swiss-Prot and VectorBase databases and to the publicly available annotated proteomes of six hard tick species (H. asiaticum, Rhipicephalus sanguineus sensu lato, Dermacentor silvarum, Rhipicephalus microplus, Ixodes scapularis and Haemaphysalis longicornis) in addition to the published H. dromedarii sialotranscriptome. For the common sequences, we predicted the physicochemical properties, secondary structures and antigenicity of the fragments similar to matched sequences in the UniProtKB/Swiss-Prot database using three different methods.

RESULTS

The quality-trimmed sequences from the cDNA library revealed 319 SMaT transcripts among 1248 sequenced clones. Annotation of the SMaT sequences using the UniProtKB/Swiss-Prot database revealed only 232 non-redundant sequences with at least one match. According to the UniProtKB/Swiss-Prot and Vectorbase databases, the SMaT sequences were either secreted (extracellular) (29 sequences) or cellular (transmembrane and membrane-associated) (203 sequences). These were classified into 10 functional classes: biogenesis (49 sequences), defense (9 sequences), development (36 sequences), signal transduction (28 sequences), transport (15 sequences), protein modification (33 sequences), homeostasis (6 sequences), metabolism (45 sequences) and miscellaneous/uncharacterized (11 sequences). A total of 60 sequences were shared between H. dromedarii SMaT, the sialotransciptome and six other hard tick species. The peptide fragments of these sequences that aligned to proteins from the UniProtKB/Swiss-Prot database were predicted to be promising epitopes and mapped to 10 functional classes at different ratios.

CONCLUSIONS

Our immuno-informatics analysis identified 60 sequences common among hard tick species and encoded by H. dromedarii salivary glands. These annotated SMaT sequences of H. dromedarii will pave the way for the identification and discovery of novel potential protective antigens that are either secreted, membrane-associated or transmembrane.

Establishing a dominant early larval sex-selection strain in the Asian malaria vector Anopheles stephensi

BACKGROUND

Genetic biocontrol interventions targeting mosquito-borne diseases require the release of male mosquitoes exclusively, as only females consume blood and transmit pathogens. Releasing only males eliminates the risk of increasing mosquito bites and spreading pathogens while enabling effective population control. The aim of this study is to develop robust sex-sorting methods for early larval stages in mosquitoes, enabling scalable male-only releases for genetic biocontrol interventions.

METHODS

To address the challenge of sex-sorting in the Asian malaria vector Anopheles stephensi, we engineer Sexing Element Produced by Alternative RNA-splicing of a Transgenic Observable Reporter (SEPARATOR). This dominant fluorescent-based method, previously proven effective in Aedes aegypti, exploits sex-specific alternative splicing of a reporter to ensure exclusive male-specific expression early in development. The sex-specific alternative RNA splicing of the doublesex gene was selected as a target for engineering SEPARATOR due to its evolutionary conservation in insects. To expand SEPARATOR's applicability for genetic sexing, we assessed the cross-species sex-specific RNA splicing activity of the An. gambiae doublesex (AngDsx) splicing module in An. stephensi. Male-specific enhanced green fluorescent protein (EGFP) expression was verified throughout the mosquito life cycle using a fluorescent stereomicroscope.

RESULTS

Our results confirm that SEPARATOR regulates male-specific EGFP expression in An. stephensi and enables reliable positive male selection from the first instar larval stages. Molecular analysis demonstrates that male-specific EGFP expression is dependent on doublesex sex-specific splicing events. Additionally, the splicing module from An. gambiae operates effectively in An. stephensi, demonstrating evolutionary conservation in sex-specific splicing events between these species.

CONCLUSIONS

SEPARATOR's independence from sex-chromosome linkage provides resistance to breakage that could be mediated by meiotic recombination and chromosomal rearrangements, making it highly suitable for mass male releases. By enabling precise male selection from the first instar larval stages, SEPARATOR represents a significant advancement that will aid in the genetic biocontrol for Anopheles mosquitoes.

Trypanosoma cruzi Vps34 colocalizes with Beclin1 and plays a role in parasite invasion of the host cell by modulating the expression of a sub-group of trans-sialidases

Trypanosoma cruzi, the etiological agent of Chagas' disease, can infect both phagocytic and non-phagocytic cells. T. cruzi gp82 and gp90 are cell surface proteins belonging to Group II trans-sialidases known to be involved in host cell binding and invasion. Phosphatidylinositol kinases (PIK) are lipid kinases that phosphorylate phospholipids in their substrates or in themselves, regulating important cellular functions such as metabolism, cell cycle and survival. Vps34, a class III PIK, regulates autophagy, trimeric G-protein signaling, and the mTOR (mammalian Target of Rapamycin) nutrient-sensing pathway. The mammalian autophagy gene Beclin1 interacts to Vps34 forming Beclin 1-Vps34 complexes involved in autophagy and protein sorting. In T. cruzi epimastigotes, (a non-infective replicative form), TcVps34 has been related to morphological and functional changes associated to vesicular trafficking, osmoregulation and receptor-mediated endocytosis. We aimed to characterize the role of TcVps34 during invasion of HeLa cells by metacyclic (MT) forms. MTs overexpressing TcVps34 showed lower invasion rates compared to controls, whilst exhibiting a significant decrease in gp82 expression in the parasite surface. In addition, we showed that T. cruzi Beclin (TcBeclin1) colocalizes with TcVps34 in epimastigotes, thus suggesting the formation of complexes that may play conserved cellular roles already described for other eukaryotes.

Revisiting the functional annotation of TriTryp using sequence similarity tools

Trypanosomatids are the causative agents of deadly diseases in humans and livestock. Given the high phylogenetic distance of trypanosomatids from model organisms, these organisms have ample unannotated genes. Manual functional annotation is time-consuming, highlighting the importance of automated functional annotation tools. The development of automated functional tools is a hot research topic, and multiple tools have been developed for the task. PANNZER2 is an automated functional annotation tool that merely relies on the sequence similarity of the query to the annotated proteins. We tried PANNZER2 on Trypanosoma brucei, the most studied organism among trypanosomatids, to see if it could improve our knowledge of the functions of the genes. Even with the availability of automated annotation tools like InterPro2GO in databases such as TriTrypDB, PANNZER2 has made surprisingly confident predictions for some hypothetical proteins in T. brucei. In this study, we identify gaps in such annotations because of not employing pairwise sequence alignment tools in TriTrypDB's automated annotation process. Our findings demonstrate that even the use of stringent cutoffs can successfully annotate a significant number of proteins. Additionally, we discovered that adjusting the open reading frames in certain genes leads to sequences with increased sequence signature coverage-characterized by the length covered by at least one sequence signature-compared to the original sequences. This enhanced sequence signature coverage suggests these genomic fragments could be pseudogenes. To facilitate further exploration, we developed a script to help identify potential pseudogenes within an organism's genome, offering researchers a new tool for genomic analysis and understanding. We extended all our analysis to Trypanosoma cruzi and Leishmania major to assess the impact of this approach across different species. Our study demonstrates that by utilizing pairwise sequence similarity alignment, even with stringent cutoffs, we can attribute 2986, 3953, and 3798 new GO terms to the genomes of T. brucei, T. cruzi, and L. major. Additionally, we found that 210, 239, and 29 genes exhibit increased sequence signature coverage following frame correction, suggesting the presence of pseudogenes.

Identification of new targets for the diagnosis of cysts (four) and trophozoites (one) of the eye pathogen Acanthamoeba

Acanthamoebae , which are free-living amoebae, cause corneal inflammation (keratitis) and blindness, if not diagnosed and effectively treated. While trophozoites adhere to and damage the cornea, Acanthamoeba cysts, the walls of which contain cellulose and have two layers connected by conical ostioles, are the diagnostic form by microscopy of the eye or of corneal scrapings. We recently used structural and experimental methods to characterize cellulose-binding domains of Luke and Leo lectins, which are abundant in the inner layer and ostioles. However, no antibodies have been made to these lectins or to a Jonah lectin and a laccase, which are abundant in the outer layer. Here we used confocal microscopy to show that rabbit antibodies to recombinant Luke, Leo, Jonah, and laccase generally support localizations of GFP-tagged proteins in walls of transfected Acanthamoebae. Rabbit antibodies to all four wall proteins efficiently detected calcofluor white-labeled cysts of 10 of 11 Acanthamoeba isolates obtained from the ATCC, including five T4 genotypes that cause most cases of keratitis. Laccase shed into the medium during encystation was detected by an enzyme-linked immunoassay. We also used structural and experimental methods to characterize the mannose-binding domain of an Acanthamoeba mannose-binding protein and showed that rabbit antibodies to the mannose-binding domain efficiently detected trophozoites of all 11 Acanthamoeba isolates. We conclude that four wall proteins are all excellent targets for diagnosing Acanthamoeba cysts in the eye or corneal scrapings, while the mannose-binding domain is an excellent target for identifying trophozoites in cultures of corneal scrapings.

Importance

Free-living amoeba in the soil or water cause Acanthamoeba keratitis, which is diagnosed by identification of cysts by microscopy of the eye or of corneal scrapings, using calcofluor-white that unfortunately cross-reacts with fungi and plants. Alternatively, Acanthamoeba infections are diagnosed by identification of trophozoites in cultures of scrapings. Here we showed that rabbit antibodies to four abundant cyst wall proteins (Jonah, Luke, Leo, and laccase) each efficiently detect calcofluor-white-labeled cysts of 10 of 11 Acanthamoeba isolates obtained from the ATCC. Further, laccase released into the medium by encysting Acanthamoebae was detected by an enzyme-linked immunoassay. We also showed that rabbit antibodies to the mannose-binding domain of the Acanthamoeba mannose-binding protein, which mediates adherence of trophozoites to keratinocytes, efficiently identifies trophozoites of all 11 ATCC isolates. In summary, four wall proteins and the ManBD appear to be excellent targets for diagnosis of Acanthamoeba cysts and trophozoites, respectively.

A fungal ubiquitin ligase and arrestin binding partner contribute to pathogenesis and survival during cellular stress

Cellular responses to external stress allow microorganisms to adapt to a vast array of environmental conditions, including infection sites. The molecular mechanisms behind these responses are studied to gain insight into microbial pathogenesis, which could lead to new antimicrobial therapies. Here, we explore a role for arrestin protein-mediated ubiquitination in stress response and pathogenesis in the pathogenic fungus Cryptococcus neoformans. In a previous study, we identified four arrestin-like proteins in C. neoformans and found that one of these is required for efficient membrane synthesis, likely by directing interaction between fatty acid synthases and the Rsp5 E3 ubiquitin ligase. Here, we further explore Cn Rsp5 function and determine that this single Ub ligase is absolutely required for pathogenesis and survival in the presence of cellular stress. Additionally, we show that a second arrestin-like protein, Ali2, similarly facilitates interaction between Rsp5 and some of its protein targets. Of the four postulated C. neoformans arrestin-like proteins, Ali2 appears to contribute the most to C. neoformans pathogenesis, likely by directing Rsp5 to pathogenesis-related ubiquitination targets. A proteomics-based differential ubiquitination screen revealed that several known cell surface proteins are ubiquitinated by Rsp5 and a subset also requires Ali2 for their ubiquitination. Rsp5-mediated ubiquitination alters the stability and the localization of these proteins. A loss of Rsp5-mediated ubiquitination results in cell wall defects that increase susceptibility to external stresses. These findings support a model in which arrestin-like proteins guide Rsp5 to ubiquitinate specific target proteins, some of which are required for survival during stress.

IMPORTANCE

Microbial proteins involved in human infectious diseases often need to be modified by specific chemical additions to be fully functional. Here, we explore the role of a particular protein modification, ubiquitination, in infections due to the human fungal pathogen Cryptococcus neoformans. We identified a complex of proteins responsible for adding ubiquitin groups to fungal proteins, and this complex is required for virulence. These proteins are fungal specific and might be targets for novel anti-infection therapy.

Malaria parasites require a divergent heme oxygenase for apicoplast gene expression and biogenesis

Malaria parasites have evolved unusual metabolic adaptations that specialize them for growth within heme-rich human erythrocytes. During blood-stage infection, Plasmodium falciparum parasites internalize and digest abundant host hemoglobin within the digestive vacuole. This massive catabolic process generates copious free heme, most of which is biomineralized into inert hemozoin. Parasites also express a divergent heme oxygenase (HO)-like protein (PfHO) that lacks key active-site residues and has lost canonical HO activity. The cellular role of this unusual protein that underpins its retention by parasites has been unknown. To unravel PfHO function, we first determined a 2.8 Å-resolution X-ray structure that revealed a highly α-helical fold indicative of distant HO homology. Localization studies unveiled PfHO targeting to the apicoplast organelle, where it is imported and undergoes N-terminal processing but retains most of the electropositive transit peptide. We observed that conditional knockdown of PfHO was lethal to parasites, which died from defective apicoplast biogenesis and impaired isoprenoid-precursor synthesis. Complementation and molecular-interaction studies revealed an essential role for the electropositive N-terminus of PfHO, which selectively associates with the apicoplast genome and enzymes involved in nucleic acid metabolism and gene expression. PfHO knockdown resulted in a specific deficiency in levels of apicoplast-encoded RNA but not DNA. These studies reveal an essential function for PfHO in apicoplast maintenance and suggest that Plasmodium repurposed the conserved HO scaffold from its canonical heme-degrading function in the ancestral chloroplast to fulfill a critical adaptive role in organelle gene expression.

Stability and suitability of housekeeping genes in phlebotomine sand flies

We investigated gene expression patterns in Lutzomyia and Phlebotomus sand fly vectors of leishmaniases. Using quantitative PCR, we assessed the expression stability of potential endogenous control genes commonly used in dipterans. We analyzed Lutzomyia longipalpis and Phlebotomus papatasi samples from L3 and L4 larval stages, adult sand flies of different sexes, diets, dsRNA injection, and Leishmania infection. Six genes were evaluated: actin, α-tubulin, GAPDH, 60 S ribosomal proteins L8 and L32 (RiboL8 and RiboL32), and elongation factor 1-α (EF1-α). EF1-α was among the most stably expressed along with RiboL8 in L. longipalpis larvae and RiboL32 in adults. In P. papatasi, EF1-α and RiboL32 were the top in larvae, while EF1-α and actin were the most stable in adults. RiboL8 and actin were the most stable genes in dissected tissues and infected guts. Additionally, five primer pairs designed for L. longipalpis or P. papatasi were effective in PCR with Lutzomyia migonei, Phlebotomus duboscqi, Phlebotomus perniciosus, and Sergentomyia schwetzi cDNA. Furthermore, L. longipalpis RiboL32 and P. papatasi α-tubulin primers were suitable for qPCR with cDNA from the other four species. Our research provides tools to enhance relative gene expression studies in sand flies, facilitating the selection of endogenous control for qPCR.

Analysis of Complexome Profiles with the Gaussian Interaction Profiler (GIP) Reveals Novel Protein Complexes in Plasmodium falciparum

Complexome profiling is an experimental approach to identify interactions by integrating native separation of protein complexes and quantitative mass spectrometry. In a typical complexome profile, thousands of proteins are detected across typically ≤100 fractions. This relatively low resolution leads to similar abundance profiles between proteins that are not necessarily interaction partners. To address this challenge, we introduce the Gaussian Interaction Profiler (GIP), a Gaussian mixture modeling-based clustering workflow that assigns protein clusters by modeling the migration profile of each cluster. Uniquely, the GIP offers a way to prioritize actual interactors over spuriously comigrating proteins. Using previously analyzed human fibroblast complexome profiles, we show good performance of the GIP compared to other state-of-the-art tools. We further demonstrate GIP utility by applying it to complexome profiles from the transmissible lifecycle stage of malaria parasites. We unveil promising novel associations for future experimental verification, including an interaction between the vaccine target Pfs47 and the hypothetical protein PF3D7_0417000. Taken together, the GIP provides methodological advances that facilitate more accurate and automated detection of protein complexes, setting the stage for more varied and nuanced analyses in the field of complexome profiling. The complexome profiling data have been deposited to the ProteomeXchange Consortium with the dataset identifier PXD050751.

Malate dehydrogenase in parasitic protozoans: roles in metabolism and potential therapeutic applications

The role of malate dehydrogenase (MDH) in the metabolism of various medically significant protozoan parasites is reviewed. MDH is an NADH-dependent oxidoreductase that catalyzes interconversion between oxaloacetate and malate, provides metabolic intermediates for both catabolic and anabolic pathways, and can contribute to NAD+/NADH balance in multiple cellular compartments. MDH is present in nearly all organisms; isoforms of MDH from apicomplexans (Plasmodium falciparum, Toxoplasma gondii, Cryptosporidium spp.), trypanosomatids (Trypanosoma brucei, T. cruzi) and anaerobic protozoans (Trichomonas vaginalis, Giardia duodenalis) are presented here. Many parasitic species have complex life cycles and depend on the environment of their hosts for carbon sources and other nutrients. Metabolic plasticity is crucial to parasite transition between host environments; thus, the regulation of metabolic processes is an important area to explore for therapeutic intervention. Common themes in protozoan parasite metabolism include emphasis on glycolytic catabolism, substrate-level phosphorylation, non-traditional uses of common pathways like tricarboxylic acid cycle and adapted or reduced mitochondria-like organelles. We describe the roles of MDH isoforms in these pathways, discuss unusual structural or functional features of these isoforms relevant to activity or drug targeting, and review current studies exploring the therapeutic potential of MDH and related genes. These studies show that MDH activity has important roles in many metabolic pathways, and thus in the metabolic transitions of protozoan parasites needed for success as pathogens.