Proteases as regulators of pathogenesis: examples from the Apicomplexa

Multiplexed DNA and Protease Detection with Orthogonal Energy Transfer on a Single Quantum Dot Scaffolded Biosensor

Almost all pathogens, whether viral or bacterial, utilize key proteolytic steps in their pathogenesis. The ability to detect a pathogen's genomic material along with its proteolytic activity represents one approach to identifying the pathogen and providing initial evidence of its viability. Here, we report on a prototype biosensor design assembled around a single semiconductor quantum dot (QD) scaffold that is capable of detecting both nucleic acid sequences and proteolytic activity by using orthogonal energy transfer (ET) processes. The sensor consists of a central QD assembled via peptidyl-PNA linkers with multiple DNA sequences that encode complements to genomic sequences originating from the Ebola, Influenza, and COVID-19 viruses, which we use as surrogate targets. These are hybridized to complement strands labeled with a terbium (Tb) chelate, AlexaFluor647 (AF647), and Cy5.5 dyes, giving rise to two potential FRET cascades: the first includes Tb → QD → AF647 → Cy5.5 (→ = ET step), which is detected in a time-gated modality, and QD → AF647 → Cy5.5, which is detected from direct excitation. The labeled DNA-displaying QD construct is then further assembled with a RuII-modified peptide, which quenches QD photoluminescence by charge transfer and is recognized by a protease to yield the full biosensor. Each of the labeled DNAs and peptides can be ratiometrically assembled to the QD in a controllable manner to tune each of the ET pathways. Addition of a given target DNA displaces its labeled complement on the QD, disrupting that FRET channel, while protease addition disrupts charge transfer quenching of the central QD scaffold and boosts its photoluminescence and FRET relay capabilities. Along with characterizing the ET pathways and verifying biosensing in both individual and multiplexed formats, we also demonstrate the ability of this construct to function in molecular logic and perform Boolean operations; this highlights the construct's ability to discriminate and transduce signals between different inputs or pathogens. The potential application space for such a sensor device is discussed.

CogiTx1: A novel subtilisin A inhibitor isolated from the sea anemone Condylactis gigantea belonging to the defensin 4 protein family

Subtilisin-like enzymes are recognized as key players in many infectious agents. In this context, its inhibitors are very valuable molecular lead compounds for structure based drug discovery and design. Marine invertebrates offer a great source of bioactive molecules, including protease inhibitors. In this work, we describe a new subtilisin inhibitor, from the sea anemone Condylactis gigantea (CogiTx1). CogiTx1 was purified using a combination of cation exchange chromatography, size exclusion chromatography and RP-HPLC chromatography. CogiTx1 it is a protein with 46 amino acid residues, with 4970.44 Da and three disulfide bridges. Is also able to inhibit subtilisin-like enzymes and pancreatic elastase. According to the amino acid sequence, it belongs to the defensin 4 family of proteins. The sequencing showed that CogiTx1 has an amidated C-terminal end, which was confirmed by the presence of the typical -XGR signal for amidation in the protein sequence deduced from the cDNA. This modification was described at protein level for the first time in this family of proteins. CogiTx1 is the first subtilisin inhibitor from the defensin 4 family and accordingly it has a folding consisting primarily in beta-strands in agreement with the analysis by CD and 3D modelling. Therefore, future in-depth functional studies may allow a more detailed characterization and will shed light on structure-function properties.

Protein length distribution is remarkably uniform across the tree of life

BACKGROUND

In every living species, the function of a protein depends on its organization of structural domains, and the length of a protein is a direct reflection of this. Because every species evolved under different evolutionary pressures, the protein length distribution, much like other genomic features, is expected to vary across species but has so far been scarcely studied.

RESULTS

Here we evaluate this diversity by comparing protein length distribution across 2326 species (1688 bacteria, 153 archaea, and 485 eukaryotes). We find that proteins tend to be on average slightly longer in eukaryotes than in bacteria or archaea, but that the variation of length distribution across species is low, especially compared to the variation of other genomic features (genome size, number of proteins, gene length, GC content, isoelectric points of proteins). Moreover, most cases of atypical protein length distribution appear to be due to artifactual gene annotation, suggesting the actual variation of protein length distribution across species is even smaller.

CONCLUSIONS

These results open the way for developing a genome annotation quality metric based on protein length distribution to complement conventional quality measures. Overall, our findings show that protein length distribution between living species is more uniform than previously thought. Furthermore, we also provide evidence for a universal selection on protein length, yet its mechanism and fitness effect remain intriguing open questions.

Comparison of high throughput RNA sequences between Babesia bigemina and Babesia bovis revealed consistent differential gene expression that is required for the Babesia life cycle in the vertebrate and invertebrate hosts

Bovine babesiosis caused by Babesia bigemina and Babesia bovis is an economically important disease that affects cattle worldwide. Both B. bigemina and B. bovis are transovarially transmitted by Rhipicephalus ticks. However, little is known regarding parasite gene expression during infection of the tick vector or mammalian host, which has limited the development of effective control strategies to alleviate the losses to the cattle industry. To understand Babesia gene regulation during tick and mammalian host infection, we performed high throughput RNA-sequencing using samples collected from calves and Rhipicephalus microplus ticks infected with B. bigemina. We evaluated gene expression between B. bigemina blood-stages and kinetes and compared them with previous B. bovis RNA-seq data. The results revealed similar patterns of gene regulation between these two tick-borne transovarially transmitted Babesia parasites. Like B. bovis, the transcription of several B. bigemina genes in kinetes exceeded a 1,000-fold change while a few of these genes had a >20,000-fold increase. To identify genes that may have important roles in B. bigemina and B. bovis transovarial transmission, we searched for genes upregulated in B. bigemina kinetes in the genomic datasets of B. bovis and non-transovarially transmitted parasites, Theileria spp. and Babesia microti. Using this approach, we identify genes that may be potential markers for transovarial transmission by B. bigemina and B. bovis. The findings presented herein demonstrate common Babesia genes linked to infection of the vector or mammalian host and may contribute to elucidating strategies used by the parasite to complete their life cycle.

Analysis of Volatile Secondary Metabolites in Ocimum basilicum Cell Suspensions: Inhibition, In Silico Molecular Docking, and an ADMET Analysis against Proteolytic Enzymes of Rhynchophorus ferrugineus

Our study's overarching goal was to determine which O. basilicum cell suspensions approach yielded the most insecticidal and R. ferrugineus-inhibitory volatile secondary metabolites. After inoculation with Verticillium dahliae as an activator, the growth kinetics were measured, and the extract was identified using GC-MS. Validation was achieved for the insecticidal efficacy of a volatile extract, the pure phenolic content against larva and adult R. ferrugineus, and the inhibitory effect on proteases (in vivo and in vitro). The volatile extract achieved an LC₅₀ of 1229 µg/mL and an LD₅₀ of 13.8 µg/larva. The LC₅₀ values for β-bergamotene, α-eudesmol, β-farnesene, linalool, 1,8-cineole, eugenol, α-guaiene, and β-caryophyllene were 1294, 1312, 1356, 1398, 1426, 1459, 1491, and 1523 g/mL, respectively. The LD₅₀ activities of α-eudesmol, linalool, 1,8-cineole, eugenol, and nerol were 12.4, 13.7, 13.9, 14.2, and 15.6 g/larva, respectively. Active volatile extract of O. basilicum inhibited trypsin proteinase, elastase, cysteine, overall protease, and metalloprotease activity with IC₅₀ values of 89.4, 101.7, 394.7, 112.4, and 535.2 µg/mL and 178.5, 192.4, 547.3, 208.3, and 924.8 µg/mL, in vitro and in vivo, respectively. There was evidence of action against total proteases (in vitro) with IC₅₀ values of 78.9, 81.2, 88.6, 90.7, 91.5, 97.6, 107.4, and 176.3 µg/mL for β-bergamotene, α-eudesmol, β-farnesene, linalool, 1,8-cineole, eugenol, α-guaiene, and β-caryophyllene, respectively. Total proteases (in vivo) are inhibited by the α-eudesmol, linalool, 1,8-cineole, eugenol, nerol, and (E)-β-ocimene, with IC₅₀ values of 162.3, 192.7, 193.1, 201.4, 248.6, and 273.2 µg/mL, respectively. ADMET and molecular docking modeling were the only two methods used to conduct in-depth computational analyses of compounds. The study recommended using an efficient cell suspension method to produce a volatile extract rich in useful secondary metabolites that may be utilized as a bio-insecticide.

Targeting the Plasmodium falciparum proteome and organelles for potential antimalarial drug candidates

There is an unmet need to unearth alternative treatment options for malaria, wherein this quest is more pressing in recent times due to high morbidity and mortality data arising mostly from the endemic countries coupled with partial diversion of attention from the disease in view of the SARS-Cov-2 pandemic. Available therapeutic options for malaria have been severely threatened with the emergence of resistance to almost all the antimalarial drugs by the Plasmodium falciparum parasite in humans, which is a worrying situation. Artemisinin combination therapies (ACT) that have so far been the mainstay of malaria have encountered resistance by malaria parasite in South East Asia, which is regarded as a notorious ground zero for the emergence of resistance to antimalarial drugs. This review analyzes a few key druggable targets for the parasite and the potential of specific inhibitors to mitigate the emerging antimalarial drug resistance problem by providing a concise assessment of the essential proteins of the malaria parasite that could serve as targets. Moreover, this work provides a summary of the advances made in malaria parasite biology and the potential to leverage these findings for antimalarial drug production.

Secondary Metabolites in Basil, Bio-Insecticide, Inhibition Effect, and In Silico Molecular Docking against Proteolytic Enzymes of the Red Palm Weevil (Rhynchophorus ferrugineus)

The purpose of this work was to determine the secondary metabolites generated by O. basilicum cell suspensions, as well as their insecticide and inhibitory activity against R. ferrugineus. The growth kinetics with inoculation Verticillium dahliae were determined and identified using LC-MS. Determination of total phenolic components (TFC), flavonoids (TF), and condensed tannins (TCT) were measured. Insecticidal activity of O. basilicum extract against R. ferrugineus (larva and adult) and proteolytic enzymes activity were assessed (in vitro and in vivo). The O.basilicum extract had an LC₅₀ of 1238 µg/mL and an LD₅₀ of 13.4 µg/larva. The LC₅₀ of chicoric acid, ursolic acid, salvigenin, quercetin-3-O-rutinoside, rosmarinyl glucoside, and nepetoidin B demonstrated activity at an LC₅₀ of 1132, 1167, 1189, 1214, 1275, and 1317 µg/mL, respectively. Chicoric acid, salvigenin, nepetoidin B, and rosmarinic acid demonstrated an LD₅₀ activity of 10.23, 11.4, 11.9, and 12.4 µg/larva, respectively. The active extract of O. basilicum inhibited total protease, trypsin-like serine proteinases, elastase, cysteine, and metalloprotease activity with an IC₅₀ (in vitro) of 119.4, 91, 102.4, 76.4, and 52.4 µg/mL, respectively. In silico studies of compounds were conducted, such as molecular docking and ADMET analysis. The study proposes using an efficient cell suspension technique to produce O. basilicum extract containing active secondary metabolites and accessible using as bio-insecticide.

Eimeria proteins: order amidst disorder

Apicomplexans are important pathogens that cause severe infections in humans and animals. The biology and pathogeneses of these parasites have shown that proteins are intrinsically modulated during developmental transitions, physiological processes and disease progression. Also, proteins are integral components of parasite structural elements and organelles. Among apicomplexan parasites, Eimeria species are an important disease aetiology for economically important animals wherein identification and characterisation of proteins have been long-winded. Nonetheless, this review seeks to give a comprehensive overview of constitutively expressed Eimeria proteins. These molecules are discussed across developmental stages, organelles and sub-cellular components vis-à-vis their biological functions. In addition, hindsight and suggestions are offered with intention to summarise the existing trend of eimerian protein characterisation and to provide a baseline for future studies.

In Silico Survey and Characterization of Babesia microti Functional and Non-Functional Proteases

Human babesiosis caused by the intraerythrocytic apicomplexan Babesia microti is an expanding tick-borne zoonotic disease that may cause severe symptoms and death in elderly or immunocompromised individuals. In light of an increasing resistance of B. microti to drugs, there is a lack of therapeutic alternatives. Species-specific proteases are essential for parasite survival and possible chemotherapeutic targets. However, the repertoire of proteases in B. microti remains poorly investigated. Herein, we employed several combined bioinformatics tools and strategies to organize and identify genes encoding for the full repertoire of proteases in the B. microti genome. We identified 64 active proteases and 25 nonactive protease homologs. These proteases can be classified into cysteine (n = 28), serine (n = 21), threonine (n = 14), asparagine (n = 7), and metallopeptidases (n = 19), which, in turn, are assigned to a total of 38 peptidase families. Comparative studies between the repertoire of B. bovis and B. microti proteases revealed differences among sensu stricto and sensu lato Babesia parasites that reflect their distinct evolutionary history. Overall, this data may help direct future research towards our understanding of the biology and pathogenicity of Babesia parasites and to explore proteases as targets for developing novel therapeutic interventions.

Dynamically expressed genes provide candidate viability biomarkers in a model coccidian

Eimeria parasites cause enteric disease in livestock and the closely related Cyclosporacayetanensis causes human disease. Oocysts of these coccidian parasites undergo maturation (sporulation) before becoming infectious. Here, we assessed transcription in maturing oocysts of Eimeria acervulina, a widespread chicken parasite, predicted gene functions, and determined which of these genes also occur in C. cayetanensis. RNA-Sequencing yielded ~2 billion paired-end reads, 92% of which mapped to the E. acervulina genome. The ~6,900 annotated genes underwent temporally-coordinated patterns of gene expression. Fifty-three genes each contributed >1,000 transcripts per million (TPM) throughout the study interval, including cation-transporting ATPases, an oocyst wall protein, a palmitoyltransferase, membrane proteins, and hypothetical proteins. These genes were enriched for 285 gene ontology (GO) terms and 13 genes were ascribed to 17 KEGG pathways, defining housekeeping processes and functions important throughout sporulation. Expression differed in mature and immature oocysts for 40% (2,928) of all genes; of these, nearly two-thirds (1,843) increased their expression over time. Eight genes expressed most in immature oocysts, encoding proteins promoting oocyst maturation and development, were assigned to 37 GO terms and 5 KEGG pathways. Fifty-six genes underwent significant upregulation in mature oocysts, each contributing at least 1,000 TPM. Of these, 40 were annotated by 215 GO assignments and 9 were associated with 18 KEGG pathways, encoding products involved in respiration, carbon fixation, energy utilization, invasion, motility, and stress and detoxification responses. Sporulation orchestrates coordinated changes in the expression of many genes, most especially those governing metabolic activity. Establishing the long-term fate of these transcripts in sporulated oocysts and in senescent and deceased oocysts will further elucidate the biology of coccidian development, and may provide tools to assay infectiousness of parasite cohorts. Moreover, because many of these genes have homologues in C. cayetanensis, they may prove useful as biomarkers for risk.

Human Codon Usage: The Genetic Basis of Pathogen Latency

Infectious diseases pose two main compelling issues. First, the identification of the molecular factors that allow chronic infections, that is, the often completely asymptomatic coexistence of infectious agents with the human host. Second, the definition of the mechanisms that allow the switch from pathogen dormancy to pathologic (re)activation. Furthering previous studies, the present study (1) analyzed the frequency of occurrence of synonymous codons in coding DNA, that is, codon usage, as a genetic tool that rules protein expression; (2) described how human codon usage can inhibit protein expression of infectious agents during latency, so that pathogen genes the codon usage of which does not conform to the human codon usage cannot be translated; and (3) framed human codon usage among the front-line instruments of the innate immunity against infections. In parallel, it was shown that, while genetics can account for the molecular basis of pathogen latency, the changes of the quantitative relationship between codon frequencies and isoaccepting tRNAs during cell proliferation offer a biochemical mechanism that explains the pathogen switching to (re)activation. Immunologically, this study warns that using codon optimization methodologies can (re)activate, potentiate, and immortalize otherwise quiescent, asymptomatic pathogens, thus leading to uncontrollable pandemics.

Bio-Insecticide of Thymus vulgaris and Ocimum basilicum Extract from Cell Suspensions and Their Inhibitory Effect against Serine, Cysteine, and Metalloproteinases of the Red Palm Weevil (Rhynchophorus ferrugineus)

The current study was designed to investigate the insecticide role of volatile constituents produced from cell suspensions of T. vulgaris and O. basilicum against R. ferrugineus. Constituents were extracted from cell suspension after 40 days. Growth kinetics were measured with an inoculation of Verticillium dahliae and identified by GC-MS. Total volatile phenolic constituents were measured. Insecticidal activity against R. ferrugineus (adult) and proteolytic enzyme activity in larvae were assessed. GC-MS showed that the T. vulgaris extract has higher amounts of thymol, p-cymene, γ-terpinene, β-caryophyllene, and linalool in comparison to the O. basilicum extract, which is rich in estragole, β-terpineol, (E)-β-ocimene, 1,8-cineole, germacrene D, and eugenol. The T. vulgaris extract showed an LC₅₀ of 1032 µg/mL, followed by O. basilicum with an LC₅₀ of 1246 µg/mL. The IC₅₀ values against the total proteases were 110.8 and 119.4 µg/mL for T. vulgaris and O. basilicum, respectively. The IC₅₀ for the trypsin-like serine proteinase assessment was 81.6 and 91 µg/mL for T. vulgaris and O. basilicum, respectively. Cysteine, chymotrypsin, and metalloproteinase assessment showed an IC₅₀ above 5000 µg/mL for both extracts. The study is proposed as a potential approach to use T. vulgaris and O. basilicum extract as a bio-insecticide against R. ferrugineus using an accessible and efficient cell suspension technique.

In vitro effect of a novel protease inhibitor cocktail on Toxoplasma gondii tachyzoites

Toxoplasmosis is a zoonotic disease and a global food and water-borne infection. The disease is caused by the parasite Toxoplasma gondii, which is a highly successful and remarkable pathogen because of its ability to infect almost any nucleated cell in warm-blooded animals. The present study was done to demonstrate the effect of protease inhibitors cocktail (PIC), which inhibit both cysteine and serine proteases, on in vitro cultured T. gondii tachyzoites on HepG2 cell line. This was achieved by assessing its effect on the invasion of the host cells and the intracellular development of T.gondii tachyzoites through measuring their number and viability after their incubation with PIC. Based on the results of the study, it was evident that the inhibitory action of the PIC was effective when applied to tachyzoites before their cultivation on HepG2 cells. Pre-treatment of T.gondii tachyzoites with PIC resulted in failure of the invasion of most of the tachyzoites and decreased the intracellular multiplication and viability of the tachyzoites that succeeded in the initial invasion process. Ultrastructural studies showed morphological alteration in tachyzoites and disruption in their organelles. This effect was irreversible till the complete lysis of cell monolayer in cultures. It can be concluded that PIC, at in vitro levels, could prevent invasion and intracellular multiplication of Toxoplasma tachyzoites. In addition, it is cost effective compared to individual protease inhibitors. It also had the benefit of combined therapy as it lowered the concentration of each protease inhibitor used in the cocktail. Other in vivo experiments are required to validate the cocktail efficacy against toxoplasmosis. Further studies may be needed to establish the exact mechanism by which the PIC exerts its effect on Toxoplasma tachyzoites behavior and its secretory pathway.

Fowlerstefin, a cysteine protease inhibitor of Naegleria fowleri, induces inflammatory responses in BV-2 microglial cells in vitro

Background

Naegleria fowleri is a free-living amoeba that causes an opportunistic fatal infection known as primary amoebic meningoencephalitis (PAM) in humans. Cysteine proteases produced by the amoeba may play critical roles in the pathogenesis of infection. In this study, a novel cysteine protease inhibitor of N. fowleri (fowlerstefin) was characterized to elucidate its biological function as an endogenous cysteine protease inhibitor of the parasite as well as a pathogenic molecule that induces immune responses in microglial cells.

Methods

Recombinant fowlerstefin was expressed in Escherichia coli. The inhibitory activity of fowlerstefin against several cysteine proteases, including human cathepsins B and L, papain and NfCPB-L, was analyzed. Fowlerstefin-induced pro-inflammatory response in BV-2 microglial cells was anayzed by cytokine array assay, reverse transcription polymerase chain reaction, and enzyme-linked immunosorbent assay.

Results

Fowlerstefin is a cysteine protease inhibitor with a monomeric structure, and belongs to the stefin family. Recombinant fowlerstefin effectively inhibited diverse cysteine proteases including cathepsin B-like cysteine proteases of N. fowleri (NfCPB-L), human cathepsins B and L, and papain. Expression of fowlerstefin in the amoeba was optimal during the trophozoite stage and gradually decreased in cysts. Fowlerstefin induced an inflammatory response in BV-2 microglial cells. Fowlerstefin induced the expression of several pro-inflammatory cytokines and chemokines including IL-6 and TNF in BV-2 microglial cells. Fowlerstefin-induced expression of IL-6 and TNF in BV-2 microglial cells was regulated by mitogen-activated protein kinase (MAPKs). The inflammatory response induced by fowlerstefin in BV-2 microglial cells was downregulated via inhibition of NF-κB and AP-1.

Conclusions

Fowlerstefin is a pathogenic molecule that stimulates BV-2 microglial cells to produce pro-inflammatory cytokines through NF-κB- and AP-1-dependent MAPK signaling pathways. Fowlerstefin-induced inflammatory cytokines exacerbate the inflammatory response in N. fowleri-infected areas and contribute to the pathogenesis of PAM.

Cathepsin-sensitive nanoscale drug delivery systems for cancer therapy and other diseases

Cathepsins are an important category of enzymes that have attracted great attention for the delivery of drugs to improve the therapeutic outcome of a broad range of nanoscale drug delivery systems. These proteases can be utilized for instance through actuation of polymer-drug conjugates (e.g., triggering the drug release) to bypass limitations of many drug candidates. A substantial amount of work has been witnessed in the design and the evaluation of Cathepsin-sensitive drug delivery systems, especially based on the tetra-peptide sequence (Gly-Phe-Leu-Gly, GFLG) which has been extensively used as a spacer that can be cleaved in the presence of Cathepsin B. This Review Article will give an in-depth overview of the design and the biological evaluation of Cathepsin-sensitive drug delivery systems and their application in different pathologies including cancer before discussing Cathepsin B-cleavable prodrugs under clinical trials.

Unravelling the cellular and molecular pathogenesis of bovine babesiosis: is the sky the limit?

The global impact of bovine babesiosis caused by the tick-borne apicomplexan parasites Babesia bovis, Babesia bigemina and Babesia divergens is vastly underappreciated. These parasites invade and multiply asexually in bovine red blood cells (RBCs), undergo sexual reproduction in their tick vectors (Rhipicephalus spp. for B. bovis and B. bigemina, and Ixodes ricinus for B. divergens) and have a trans-ovarial mode of transmission. Babesia parasites can cause acute and persistent infections to adult naïve cattle that can occur without evident clinical signs, but infections caused by B. bovis are associated with more severe disease and increased mortality, and are considered to be the most virulent agent of bovine babesiosis. In addition, babesiosis caused by B. divergens has an important zoonotic potential. The disease caused by B. bovis and B. bigemina can be controlled, at least in part, using therapeutic agents or vaccines comprising live-attenuated parasites, but these methods are limited in terms of their safety, ease of deployability and long-term efficacy, and improved control measures are urgently needed. In addition, expansion of tick habitats due to climate change and other rapidly changing environmental factors complicate efficient control of these parasites. While the ability to cause persistent infections facilitates transmission and persistence of the parasite in endemic regions, it also highlights their capacity to evade the host immune responses. Currently, the mechanisms of immune responses used by infected bovines to survive acute and chronic infections remain poorly understood, warranting further research. Similarly, molecular details on the processes leading to sexual reproduction and the development of tick-stage parasites are lacking, and such tick-specific molecules can be targets for control using alternative transmission blocking vaccines. In this review, we identify and examine key phases in the life-cycle of Babesia parasites, including dependence on a tick vector for transmission, sexual reproduction of the parasite in the midgut of the tick, parasite-dependent invasion and egression of bovine RBCs, the role of the spleen in the clearance of infected RBCs (IRBCs), and age-related disease resistance in cattle, as opportunities for developing improved control measures. The availability of integrated novel research approaches including "omics" (such as genomics, transcriptomics, and proteomics), gene modification, cytoadhesion assays, RBC invasion assays and methods for in vitro induction of sexual-stage parasites will accelerate our understanding of parasite vulnerabilities. Further, producing new knowledge on these vulnerabilities, as well as taking full advantage of existing knowledge, by filling important research gaps should result in the development of next-generation vaccines to control acute disease and parasite transmission. Creative and effective use of current and future technical and computational resources are needed, in the face of the numerous challenges imposed by these highly evolved parasites, for improving the control of this disease. Overall, bovine babesiosis is recognised as a global disease that imposes a serious burden on livestock production and human livelihood, but it largely remains a poorly controlled disease in many areas of the world. Recently, important progress has been made in our understanding of the basic biology and host-parasite interactions of Babesia parasites, yet a good deal of basic and translational research is still needed to achieve effective control of this important disease and to improve animal and human health.

Transciptome profiling at early infection of Elaeis guineensis by Ganoderma boninense provides novel insights on fungal transition from biotrophic to necrotrophic phase

BACKGROUND

Basal stem rot (BSR) caused by hemibiotroph Ganoderma boninense is a devastating disease resulting in a major loss to the oil palm industry. Since there is no physical symptom in oil palm at the early stage of G. boninense infection, characterisation of molecular defense responses in oil palm during early interaction with the fungus is of the utmost importance. Oil palm (Elaeis guineensis) seedlings were artificially infected with G. boninense inoculums and root samples were obtained following a time-course of 0, 3, 7, and 11 days-post-inoculation (d.p.i) for RNA sequencing (RNA-seq) and identification of differentially expressed genes (DEGs).

RESULTS

The host counter-attack was evidenced based on fungal hyphae and Ganoderma DNA observed at 3 d.p.i which became significantly reduced at 7 and 11 d.p.i. DEGs revealed upregulation of multifaceted defense related genes such as PR-protein (EgPR-1), protease inhibitor (EgBGIA), PRR protein (EgLYK3) chitinase (EgCht) and expansin (EgEXPB18) at 3 d.p.i and 7 d.p.i which dropped at 11 d.p.i. Later stage involved highly expressed transcription factors EgERF113 and EgMYC2 as potential regulators of necrotrophic defense at 11 d.p.i. The reactive oxygen species (ROS) elicitor: peroxidase (EgPER) and NADPH oxidase (EgRBOH) were upregulated and maintained throughout the treatment period. Growth and nutrient distribution were probably compromised through suppression of auxin signalling and iron uptake genes.

CONCLUSIONS

Based on the analysis of oil palm gene expression, it was deduced that the biotrophic phase of Ganoderma had possibly occurred at the early phase (3 until 7 d.p.i) before being challenged by the fungus via switching its lifestyle into the necrotrophic phase at later stage (11 d.p.i) and finally succumbed the host. Together, the findings suggest the dynamic defense process in oil palm and potential candidates that can serve as phase-specific biomarkers at the early stages of oil palm-G. boninense interaction.

A Perspective on Thiazolidinone Scaffold Development as a New Therapeutic Strategy for Toxoplasmosis

Toxoplasma gondii is one of the most successful parasites due to its ability to infect a wide variety of warm-blooded animals. It is estimated that one-third of the world's population is latently infected. The generic therapy for toxoplasmosis has been a combination of antifolates such as pyrimethamine or trimethoprim with either sulfadiazine or antibiotics such as clindamycin with a combination with leucovorin to prevent hematologic toxicity. This therapy shows limitations such as drug intolerance, low bioavailability or drug resistance by the parasite. There is a need for the development of new molecules with the capacity to block any stage of the parasite's life cycle in humans or in a different type of hosts. Heterocyclic compounds are promissory drugs due to its reported biological activity; for this reason, thiazolidinone and its derivatives are presented as a new alternative not only for its inhibitory activity against the parasite but also for its high selectivity-level with high therapeutic index. Thiazolidinones are an important scaffold known to be associated with anticancer, antibacterial, antifungal, antiviral, antioxidant, and antidiabetic activities. The molecule possesses an imidazole ring that has been described as an antiprotozoal agent with antiparasitic properties and less toxicity. Thiazolidinone derivatives have been reportedly as building blocks in organic chemistry and as scaffolds for drug discovery. Here we present a perspective of how structural modifications of the thiazolidinone core could generate new compounds with high anti-parasitic effect and less toxic results.

Metallopeptidases of Toxoplasma gondii: in silico identification and gene expression

Metallopeptidases are a family of proteins with domains that remain highly conserved throughout evolution. These hydrolases require divalent metal cation(s) to activate the water molecule in order to carry out their catalytic action on peptide bonds by nucleophilic attack. Metallopeptidases from parasitic protozoa, including Toxoplasma, are investigated because of their crucial role in parasite biology. In the present study, we screened the T. gondii database using PFAM motifs specific for metallopeptidases in association with the MEROPS peptidase Database (release 10.0). In all, 49 genes encoding proteins with metallopeptidase signatures were identified in the Toxoplasma genome. An Interpro Search enabled us to uncover their domain/motif organization, and orthologs with the highest similarity by BLAST were used for annotation. These 49 Toxoplasma metallopeptidases clustered into 15 families described in the MEROPS database. Experimental expression analysis of their genes in the tachyzoite stage revealed transcription for all genes studied. Further research on the role of these peptidases should increase our knowledge of basic Toxoplasma biology and provide opportunities to identify novel therapeutic targets. This type of study would also open a path towards the comparative biology of apicomplexans.

Cysteine Proteinase C1A Paralog Profiles Correspond with Phylogenetic Lineages of Pathogenic Piroplasmids

Piroplasmid parasites comprising of Babesia, Theileria, and Cytauxzoon are transmitted by ticks to farm and pet animals and have a significant impact on livestock industries and animal health in tropical and subtropical regions worldwide. In addition, diverse Babesia spp. infect humans as opportunistic hosts. Molecular phylogeny has demonstrated at least six piroplasmid lineages exemplified by B. microti, B. duncani, C. felis, T. equi, Theileria sensu stricto (T. annulata, T. parva, and T. orientalis) and Babesia sensu stricto (B. bovis, B. bigemina, and B. ovis). C1A cysteine-proteinases (C1A-Cp) are papain-like enzymes implicated in pathogenic and vital steps of the parasite life cycle such as nutrition and host cell egress. An expansion of C1A-Cp of T. annulata and T. parva with respect to B. bovis and B. ovis was previously described. In the present work, C1A-Cp paralogs were identified in available genomes of species pertaining to each piroplasmid lineage. Phylogenetic analysis revealed eight C1A-Cp groups. The profile of C1A-Cp paralogs across these groups corroborates and defines the existence of six piroplasmid lineages. C. felis, T. equi and Theileria s.s. each showed characteristic expansions into extensive families of C1A-Cp paralogs in two of the eight groups. Underlying gene duplications have occurred as independent unique evolutionary events that allow distinguishing these three piroplasmid lineages. We hypothesize that C1A-Cp paralog families may be associated with the advent of the schizont stage. Differences in the invertebrate tick host specificity and/or mode of transmission in piroplasmid lineages might also be associated with the observed C1A-Cp paralog profiles.

Proteases as antimalarial targets: strategies for genetic, chemical, and therapeutic validation

Malaria is a devastating parasitic disease affecting half of the world's population. The rapid emergence of resistance against new antimalarial drugs, including artemisinin-based therapies, has made the development of drugs with novel mechanisms of action extremely urgent. Proteases are enzymes proven to be well suited for target-based drug development due to our knowledge of their enzymatic mechanisms and active site structures. More importantly, Plasmodium proteases have been shown to be involved in a variety of pathways that are essential for parasite survival. However, pharmacological rather than target-based approaches have dominated the field of antimalarial drug development, in part due to the challenge of robustly validating Plasmodium targets at the genetic level. Fortunately, over the last few years there has been significant progress in the development of efficient genetic methods to modify the parasite, including several conditional approaches. This progress is finally allowing us not only to validate essential genes genetically, but also to study their molecular functions. In this review, I present our current understanding of the biological role proteases play in the malaria parasite life cycle. I also discuss how the recent advances in Plasmodium genetics, the improvement of protease-oriented chemical biology approaches, and the development of malaria-focused pharmacological assays, can be combined to achieve a robust biological, chemical and therapeutic validation of Plasmodium proteases as viable drug targets.

Comparative proteomics of hydatid fluids from two Echinococcus multilocularis isolates

The hydatid fluid (HF) that fills Echinococcus multilocularis metacestode vesicles is a complex mixture of proteins from both parasite and host origin. Here, a LC-MS/MS approach was used to compare the HF composition of E. multilocularis H95 and G8065 isolates (EmH95 and EmG8065, respectively), which present differences in terms of growth and fertility. Overall, 446 unique proteins were identified, 392 of which (88%) were from parasite origin and 54 from culture medium. At least 256 of parasite proteins were sample exclusive, and 82 of the 136 shared proteins presented differential abundance between E. multilocularis isolates. The parasite's protein repertoires in EmH95 and EmG8065 HF samples presented qualitative and quantitative differences involving antigens, signaling proteins, proteolytic enzymes, protease inhibitors and chaperones, highlighting intraspecific singularities that could be correlated to biological features of each isolate. The repertoire of medium proteins found in the HF was also differential between isolates, and the relevance of the HF exogenous protein content for the parasite's biology is discussed. The repertoires of identified proteins also provided potential molecular markers for important biological features, such as parasite growth rate and fertility, as well potential protein targets for the development of novel diagnostic and treatment strategies for alveolar echinococcosis.

BIOLOGICAL SIGNIFICANCE

E. multilocularis metacestode infection of mammal hosts involve complex interactions mediated by excretory/secretory (ES) products. The hydatid fluid (HF) that fills the E. multilocularis metacestode vesicles contains complex repertoires of parasite ES products and host proteins that mediate important molecular interactions determinant for parasite survival and development, and, consequently, to the infection outcome. HF has been also extensively reported as the main source of proteins for the immunodiagnosis of echinococcosis. The performed proteomic analysis provided a comprehensive profiling of the HF protein composition of two E. multilocularis isolates. This allowed us to identify proteins of both parasite and exogenous (medium) origin, many of which present significant differential abundances between parasite isolates and may correlate to their differential biological features, including fertility and growth rate.

Targeted Deletion of a Plasmodium Site-2 Protease Impairs Life Cycle Progression in the Mammalian Host

Site-2 proteases (S2P) belong to the M50 family of metalloproteases, which typically perform essential roles by mediating activation of membrane–bound transcription factors through regulated intramembrane proteolysis (RIP). Protease-dependent liberation of dormant transcription factors triggers diverse cellular responses, such as sterol regulation, Notch signalling and the unfolded protein response. Plasmodium parasites rely on regulated proteolysis for controlling essential pathways throughout the life cycle. In this study we examine the Plasmodium-encoded S2P in a murine malaria model and show that it is expressed in all stages of Plasmodium development. Localisation studies by endogenous gene tagging revealed that in all invasive stages the protein is in close proximity to the nucleus. Ablation of PbS2P by reverse genetics leads to reduced growth rates during liver and blood infection and, hence, virulence attenuation. Strikingly, absence of PbS2P was compatible with parasite life cycle progression in the mosquito and mammalian hosts under physiological conditions, suggesting redundant or dispensable roles in vivo.

Neospora caninum tachyzoite immunome study reveals differences among three biologically different isolates

Pathogenesis of bovine neosporosis is determined by different host- and parasite-dependent factors, including isolate virulence. A previous study identified that several Neospora caninum tachyzoite proteins were more abundant in virulent isolates, Nc-Liv and Nc-Spain7, compared with the low-virulent isolate Nc-Spain1H. Herein, we explored differences in the immunomes of these three isolates. Protein extracts from the Nc-Liv, Nc-Spain1H and Nc-Spain7 isolates were analysed in a 3×3 design by 2-DE immunoblot using sera from experimentally infected mice with these same three isolates. All isolates displayed a highly similar antigenic pattern when they were assessed using the same serum. Most of the reactive spots were located in the acidic region (pH 3-7) and grouped in 3 antigenic areas (250-70, 45-37 and 35-15 KDa). Differences found in the immunome depended on the sera used, regardless of the extract employed. In this sense, sera from Nc-Liv and Nc-Spain7 infected mice recognized the highest number of antigens, followed by Nc-Spain1H infected mice sera. In fact, 4 proteins identified by MS were not consistently detected in each isolate extract by sera from low-virulent Nc-Spain1H-infected mice: serine-threonine phosphatase 2C and superoxide dismutase (related to metabolism), gliding associated protein GAP45 (related to tachyzoites invasion), and NcGRA1 (located on dense granules). Similarly, 4 non-identified spots and another 2 spots chains located in 45-37 kDa area were not detected by this pooled sera. Variations between virulent Nc-Spain7 and Nc-Liv were limited to the absence of recognition by sera from Nc-Spain7-infected mice of GAP45 and the spot chains located in the 45-37 kDa area. These results suggest that variations in the immunome profiles rely on the immune response induced by each isolate and that these differentially recognized antigens could be investigated as putative virulence markers of neosporosis.

Chromerid genomes reveal the evolutionary path from photosynthetic algae to obligate intracellular parasites

The eukaryotic phylum Apicomplexa encompasses thousands of obligate intracellular parasites of humans and animals with immense socio-economic and health impacts. We sequenced nuclear genomes of Chromera velia and Vitrella brassicaformis, free-living non-parasitic photosynthetic algae closely related to apicomplexans. Proteins from key metabolic pathways and from the endomembrane trafficking systems associated with a free-living lifestyle have been progressively and non-randomly lost during adaptation to parasitism. The free-living ancestor contained a broad repertoire of genes many of which were repurposed for parasitic processes, such as extracellular proteins, components of a motility apparatus, and DNA- and RNA-binding protein families. Based on transcriptome analyses across 36 environmental conditions, Chromera orthologs of apicomplexan invasion-related motility genes were co-regulated with genes encoding the flagellar apparatus, supporting the functional contribution of flagella to the evolution of invasion machinery. This study provides insights into how obligate parasites with diverse life strategies arose from a once free-living phototrophic marine alga.

DOI:http://dx.doi.org/10.7554/eLife.06974.001

Single-celled parasites cause many severe diseases in humans and animals. The apicomplexans form probably the most successful group of these parasites and include the parasites that cause malaria. Apicomplexans infect a broad range of hosts, including humans, reptiles, birds, and insects, and often have complicated life cycles. For example, the malaria-causing parasites spread by moving from humans to female mosquitoes and then back to humans.

Despite significant differences amongst apicomplexans, these single-celled parasites also share a number of features that are not seen in other living species. How and when these features arose remains unclear. It is known from previous work that apicomplexans are closely related to single-celled algae. But unlike apicomplexans, which depend on a host animal to survive, these algae live freely in their environment, often in close association with corals.

Woo et al. have now sequenced the genomes of two photosynthetic algae that are thought to be close living relatives of the apicomplexans. These genomes were then compared to each other and to the genomes of other algae and apicomplexans. These comparisons reconfirmed that the two algae that were studied were close relatives of the apicomplexans.

Further analyses suggested that thousands of genes were lost as an ancient free-living algae evolved into the apicomplexan ancestor, and further losses occurred as these early parasites evolved into modern species. The lost genes were typically those that are important for free-living organisms, but are either a hindrance to, or not needed in, a parasitic lifestyle. Some of the ancestor's genes, especially those that coded for the building blocks of flagella (structures which free-living algae use to move around), were repurposed in ways that helped the apicomplexans to invade their hosts. Understanding this repurposing process in greater detail will help to identify key molecules in these deadly parasites that could be targeted by drug treatments. It will also offer answers to one of the most fascinating questions in evolutionary biology: how parasites have evolved from free-living organisms.

DOI:http://dx.doi.org/10.7554/eLife.06974.002

Ectopic expression of a Neospora caninum Kazal type inhibitor triggers developmental defects in Toxoplasma and Plasmodium

Regulated proteolysis is known to control a variety of vital processes in apicomplexan parasites including invasion and egress of host cells. Serine proteases have been proposed as targets for drug development based upon inhibitor studies that show parasite attenuation and transmission blockage. Genetic studies suggest that serine proteases, such as subtilisin and rhomboid proteases, are essential but functional studies have proved challenging as active proteases are difficult to express. Proteinaceous Protease Inhibitors (PPIs) provide an alternative way to address the role of serine proteases in apicomplexan biology. To validate such an approach, a Neospora caninum Kazal inhibitor (NcPI-S) was expressed ectopically in two apicomplexan species, Toxoplasma gondii tachyzoites and Plasmodium berghei ookinetes, with the aim to disrupt proteolytic processes taking place within the secretory pathway. NcPI-S negatively affected proliferation of Toxoplasma tachyzoites, while it had no effect on invasion and egress. Expression of the inhibitor in P. berghei zygotes blocked their development into mature and invasive ookinetes. Moreover, ultra-structural studies indicated that expression of NcPI-S interfered with normal formation of micronemes, which was also confirmed by the lack of expression of the micronemal protein SOAP in these parasites. Our results suggest that NcPI-S could be a useful tool to investigate the function of proteases in processes fundamental for parasite survival, contributing to the effort to identify targets for parasite attenuation and transmission blockage.

Vaccines against bovine babesiosis: where we are now and possible roads ahead

SUMMARY Bovine babesiosis caused by the tick-transmitted haemoprotozoans Babesia bovis, Babesia bigemina and Babesia divergens commonly results in substantial cattle morbidity and mortality in vast world areas. Although existing live vaccines confer protection, they have considerable disadvantages. Therefore, particularly in countries where large numbers of cattle are at risk, important research is directed towards improved vaccination strategies. Here a comprehensive overview of currently used live vaccines and of the status quo of experimental vaccine trials is presented. In addition, pertinent research fields potentially contributing to the development of novel non-live and/or live vaccines are discussed, including parasite antigens involved in host cell invasion and in pathogen-tick interactions, as well as the protective immunity against infection. The mining of available parasite genomes is continuously enlarging the array of potential vaccine candidates and, additionally, the recent development of a transfection tool for Babesia can significantly contribute to vaccine design. However, the complication and high cost of vaccination trials hinder Babesia vaccine research, and have so far seriously limited the systematic examination of antigen candidates and prevented an in-depth testing of formulations using different immunomodulators and antigen delivery systems.

A survey of innovation through duplication in the reduced genomes of twelve parasites

We characterize the prevalence, distribution, divergence, and putative functions of detectable two-copy paralogs and segmental duplications in the Apicomplexa, a phylum of parasitic protists. Apicomplexans are mostly obligate intracellular parasites responsible for human and animal diseases (e.g. malaria and toxoplasmosis). Gene loss is a major force in the phylum. Genomes are small and protein-encoding gene repertoires are reduced. Despite this genomic streamlining, duplications and gene family amplifications are present. The potential for innovation introduced by duplications is of particular interest. We compared genomes of twelve apicomplexans across four lineages and used orthology and genome cartography to map distributions of duplications against genome architectures. Segmental duplications appear limited to five species. Where present, they correspond to regions enriched for multi-copy and species-specific genes, pointing toward roles in adaptation and innovation. We found a phylum-wide association of duplications with dynamic chromosome regions and syntenic breakpoints. Trends in the distribution of duplicated genes indicate that recent, species-specific duplicates are often tandem while most others have been dispersed by genome rearrangements. These trends show a relationship between genome architecture and gene duplication. Functional analysis reveals: proteases, which are vital to a parasitic lifecycle, to be prominent in putative recent duplications; a pair of paralogous genes in Toxoplasma gondii previously shown to produce the rate-limiting step in dopamine synthesis in mammalian cells, a possible link to the modification of host behavior; and phylum-wide differences in expression and subcellular localization, indicative of modes of divergence. We have uncovered trends in multiple modes of duplicate divergence including sequence, intron content, expression, subcellular localization, and functions of putative recent duplicates that highlight the role of duplications in the continuum of forces that have shaped these genomes.

Ferrous iron-dependent drug delivery enables controlled and selective release of therapeutic agents in vivo

The precise targeting of cytotoxic agents to specific cell types or cellular compartments is of significant interest in medicine, with particular relevance for infectious diseases and cancer. Here, we describe a method to exploit aberrant levels of mobile ferrous iron (Fe(II)) for selective drug delivery in vivo. This approach makes use of a 1,2,4-trioxolane moiety, which serves as an Fe(II)-sensitive "trigger," making drug release contingent on Fe(II)-promoted trioxolane fragmentation. We demonstrate in vivo validation of this approach with the Plasmodium berghei model of murine malaria. Malaria parasites produce high concentrations of mobile ferrous iron as a consequence of their catabolism of host hemoglobin in the infected erythrocyte. Using activity-based probes, we successfully demonstrate the Fe(II)-dependent and parasite-selective delivery of a potent dipeptidyl aminopeptidase inhibitor. We find that delivery of the compound in its Fe(II)-targeted form leads to more sustained target inhibition with greatly reduced off-target inhibition of mammalian cathepsins. This selective drug delivery translates into improved efficacy and tolerability. These findings demonstrate the utility of a purely chemical means to achieve selective drug targeting in vivo. This approach may find useful application in parasitic infections and more broadly in any disease state characterized by aberrant production of reactive ferrous iron.

Direct proteolytic cleavage of NLRP1B is necessary and sufficient for inflammasome activation by anthrax lethal factor

Inflammasomes are multimeric protein complexes that respond to infection by recruitment and activation of the Caspase-1 (CASP1) protease. Activated CASP1 initiates immune defense by processing inflammatory cytokines and by causing a rapid and lytic cell death called pyroptosis. Inflammasome formation is orchestrated by members of the nucleotide-binding domain and leucine-rich repeat (NLR) or AIM2-like receptor (ALR) protein families. Certain NLRs and ALRs have been shown to function as direct receptors for specific microbial ligands, such as flagellin or DNA, but the molecular mechanism responsible for activation of most NLRs is still poorly understood. Here we determine the mechanism of activation of the NLRP1B inflammasome in mice. NLRP1B, and its ortholog in rats, is activated by the lethal factor (LF) protease that is a key virulence factor secreted by Bacillus anthracis, the causative agent of anthrax. LF was recently shown to cleave mouse and rat NLRP1 directly. However, it is unclear if cleavage is sufficient for NLRP1 activation. Indeed, other LF-induced cellular events have been suggested to play a role in NLRP1B activation. Surprisingly, we show that direct cleavage of NLRP1B is sufficient to induce inflammasome activation in the absence of LF. Our results therefore rule out the need for other LF-dependent cellular effects in activation of NLRP1B. We therefore propose that NLRP1 functions primarily as a sensor of protease activity and thus could conceivably detect a broader spectrum of pathogens than just B. anthracis. By adding proteolytic cleavage to the previously established ligand-receptor mechanism of NLR activation, our results illustrate the remarkable flexibility with which the NLR architecture can be deployed for the purpose of pathogen-detection and host defense.

Recognition of pathogens by the innate immune system is necessary for initiating an appropriate immune response. The innate immune system must distinguish pathogens from abundant harmless microbes present within the host and the environment, and scale the response appropriately. It has been proposed that the host can respond specifically to pathogens by monitoring common virulence-associated activities, previously termed “patterns of pathogenesis,” that are used by pathogens to survive and replicate within their hosts. For example, pathogens can manipulate host functions by delivering toxins into host cells. In response, the host encodes dedicated cytosolic sensors to detect these toxins, but the molecular basis for how the sensors recognize the toxins is poorly understood. Here we define the molecular mechanism by which a mouse sensor, NLRP1B, directly recognizes the activity of a bacterial toxin, lethal factor. Lethal factor is a protease secreted by Bacillus anthracis, the causative agent of anthrax. We show that anthrax lethal factor cleaves NLRP1B and this cleavage event is both necessary and sufficient for the activation of this sensor. Our findings raise the possibility that NLRP1B could sense the activity of other proteases encoded by diverse pathogens.

Target deconvolution techniques in modern phenotypic profiling

The past decade has seen rapid growth in the use of diverse compound libraries in classical phenotypic screens to identify modulators of a given process. The subsequent process of identifying the molecular targets of active hits, also called 'target deconvolution', is an essential step for understanding compound mechanism of action and for using the identified hits as tools for further dissection of a given biological process. Recent advances in 'omics' technologies, coupled with in silico approaches and the reduced cost of whole genome sequencing, have greatly improved the workflow of target deconvolution and have contributed to a renaissance of 'modern' phenotypic profiling. In this review, we will outline how both new and old techniques are being used in the difficult process of target identification and validation as well as discuss some of the ongoing challenges remaining for phenotypic screening.

Stage-specific expression of protease genes in the apicomplexan parasite, Eimeria tenella

Background

Proteases regulate pathogenesis in apicomplexan parasites but investigations of proteases have been largely confined to the asexual stages of Plasmodium falciparum and Toxoplasma gondii. Thus, little is known about proteases in other Apicomplexa, particularly in the sexual stages. We screened the Eimeria tenella genome database for proteases, classified these into families and determined their stage specific expression.

Results

Over forty protease genes were identified in the E. tenella genome. These were distributed across aspartic (three genes), cysteine (sixteen), metallo (fourteen) and serine (twelve) proteases. Expression of at least fifteen protease genes was upregulated in merozoites including homologs of genes known to be important in host cell invasion, remodelling and egress in P. falciparum and/or T. gondii. Thirteen protease genes were specifically expressed or upregulated in gametocytes; five of these were in two families of serine proteases (S1 and S8) that are over-represented in the coccidian parasites, E. tenella and T. gondii, distinctive within the Apicomplexa because of their hard-walled oocysts. Serine protease inhibitors prevented processing of EtGAM56, a protein from E. tenella gametocytes that gives rise to tyrosine-rich peptides that are incorporated into the oocyst wall.

Conclusion

Eimeria tenella possesses a large number of protease genes. Expression of many of these genes is upregulated in asexual stages. However, expression of almost one-third of protease genes is upregulated in, or confined to gametocytes; some of these appear to be unique to the Coccidia and may play key roles in the formation of the oocyst wall, a defining feature of this group of parasites.

Validation of the proteasome as a therapeutic target in Plasmodium using an epoxyketone inhibitor with parasite-specific toxicity

The Plasmodium proteasome has been suggested to be a potential antimalarial drug target; however, toxicity of inhibitors has prevented validation of this enzyme in vivo. We report a screen of a library of 670 analogs of the recent US Food and Drug Administration-approved inhibitor, carfilzomib, to identify compounds that selectively kill parasites. We identified one compound, PR3, that has significant parasite killing activity in vitro but dramatically reduced toxicity in host cells. We found that this parasite-specific toxicity is not due to selective targeting of the Plasmodium proteasome over the host proteasome, but instead is due to a lack of activity against one of the human proteasome subunits. Subsequently, we used PR3 to significantly reduce parasite load in Plasmodium berghei infected mice without host toxicity, thus validating the proteasome as a viable antimalarial drug target.

Screening for small molecule inhibitors of Toxoplasma gondii

INTRODUCTION

Toxoplasma gondii, the agent that causes toxoplasmosis, is an opportunistic parasite that infects many mammalian species. It is an obligate intracellular parasite that causes severe congenital neurological and ocular disease mostly in immunocompromised humans. The current regimen of therapy includes only a few medications that often lead to hypersensitivity and toxicity. In addition, there are no vaccines available to prevent the transmission of this agent. Therefore, safer and more effective medicines to treat toxoplasmosis are urgently needed.

AREAS COVERED

The author presents in silico and in vitro strategies that are currently used to screen for novel targets and unique chemotypes against T. gondii. Furthermore, this review highlights the screening technologies and characterization of some novel targets and new chemical entities that could be developed into highly efficacious treatments for toxoplasmosis.

EXPERT OPINION

A number of diverse methods are being used to design inhibitors against T. gondii. These include ligand-based methods, in which drugs that have been shown to be efficacious against other Apicomplexa parasites can be repurposed to identify lead molecules against T. gondii. In addition, structure-based methods use currently available repertoire of structural information in various databases to rationally design small-molecule inhibitors of T. gondii. Whereas the screening methods have their advantages and limitations, a combination of methods is ideally suited to design small-molecule inhibitors of complex parasites such as T. gondii.

Passage of parasites across the blood-brain barrier

The blood-brain barrier (BBB) is a structural and functional barrier that protects the central nervous system (CNS) from invasion by blood-borne pathogens including parasites. However, some intracellular and extracellular parasites can traverse the BBB during the course of infection and cause neurological disturbances and/or damage which are at times fatal. The means by which parasites cross the BBB and how the immune system controls the parasites within the brain are still unclear. In this review we present the current understanding of the processes utilized by two human neuropathogenic parasites, Trypanosoma brucei spp and Toxoplasma gondii, to go across the BBB and consequences of CNS invasion. We also describe briefly other parasites that can invade the brain and how they interact with or circumvent the BBB. The roles played by parasite-derived and host-derived molecules during parasitic and white blood cell invasion of the brain are discussed.