Intracellular eukaryotic parasites have a distinct unfolded protein response

Single-cell quantitative bioimaging of P. berghei liver stage translation

IMPORTANCE

Plasmodium parasites cause malaria in humans. New multistage active antimalarial drugs are needed, and a promising class of drugs targets the core cellular process of translation, which has many potential molecular targets. During the obligate liver stage, Plasmodium parasites grow in metabolically active hepatocytes, making it challenging to study core cellular processes common to both host cells and parasites, as the signal from the host typically overwhelms that of the parasite. Here, we present and validate a flexible assay to quantify Plasmodium liver stage translation using a technique to fluorescently label the newly synthesized proteins of both host and parasite followed by computational separation of their respective nascent proteomes in confocal image sets. We use the assay to determine whether a test set of known compounds are direct or indirect liver stage translation inhibitors and show that the assay can also predict the mode of action for novel antimalarial compounds.

Single-cell quantitative bioimaging of P. berghei liver stage translation

Plasmodium parasite resistance to existing antimalarial drugs poses a devastating threat to the lives of many who depend on their efficacy. New antimalarial drugs and novel drug targets are in critical need, along with novel assays to accelerate their identification. Given the essentiality of protein synthesis throughout the complex parasite lifecycle, translation inhibitors are a promising drug class, capable of targeting the disease-causing blood stage of infection, as well as the asymptomatic liver stage, a crucial target for prophylaxis. To identify compounds capable of inhibiting liver stage parasite translation, we developed an assay to visualize and quantify translation in the P. berghei-HepG2 infection model. After labeling infected monolayers with o-propargyl puromycin (OPP), a functionalized analog of puromycin permitting subsequent bioorthogonal addition of a fluorophore to each OPP-terminated nascent polypetide, we use automated confocal feedback microscopy followed by batch image segmentation and feature extraction to visualize and quantify the nascent proteome in individual P. berghei liver stage parasites and host cells simultaneously. After validation, we demonstrate specific, concentration-dependent liver stage translation inhibition by both parasite-selective and pan-eukaryotic active compounds, and further show that acute pre-treatment and competition modes of the OPP assay can distinguish between direct and indirect translation inhibitors. We identify a Malaria Box compound, MMV019266, as a direct translation inhibitor in P. berghei liver stages and confirm this potential mode of action in P. falciparum asexual blood stages.

Aorta- and liver-generated TMAO enhances trained immunity for increased inflammation via ER stress/mitochondrial ROS/glycolysis pathways

We determined whether gut microbiota-produced trimethylamine (TMA) is oxidized into trimethylamine N-oxide (TMAO) in nonliver tissues and whether TMAO promotes inflammation via trained immunity (TI). We found that endoplasmic reticulum (ER) stress genes were coupregulated with MitoCarta genes in chronic kidney diseases (CKD); TMAO upregulated 190 genes in human aortic endothelial cells (HAECs); TMAO synthesis enzyme flavin-containing monooxygenase 3 (FMO3) was expressed in human and mouse aortas; TMAO transdifferentiated HAECs into innate immune cells; TMAO phosphorylated 12 kinases in cytosol via its receptor PERK and CREB, and integrated with PERK pathways; and PERK inhibitors suppressed TMAO-induced ICAM-1. TMAO upregulated 3 mitochondrial genes, downregulated inflammation inhibitor DARS2, and induced mitoROS, and mitoTEMPO inhibited TMAO-induced ICAM-1. β-Glucan priming, followed by TMAO restimulation, upregulated TNF-α by inducing metabolic reprogramming, and glycolysis inhibitor suppressed TMAO-induced ICAM-1. Our results have provided potentially novel insights regarding TMAO roles in inducing EC activation and innate immune transdifferentiation and inducing metabolic reprogramming and TI for enhanced vascular inflammation, and they have provided new therapeutic targets for treating cardiovascular diseases (CVD), CKD-promoted CVD, inflammation, transplantation, aging, and cancer.

The endoplasmic reticulum of trypanosomatids: An unrevealed road for chemotherapy

The endoplasmic reticulum (ER) of higher eukaryotic cells forms an intricate membranous network that serves as the main processing facility for folding and assembling of secreted and membrane proteins. The ER is a highly dynamic organelle that interacts with other intracellular structures, as well as endosymbiotic pathogenic and non-pathogenic microorganisms. A strict ER quality control (ERQC) must work to ensure that proteins entering the ER are folded and processed correctly. Unfolded or misfolded proteins are usually identified, selected, and addressed to Endoplasmic Reticulum-Associated Degradation (ERAD) complex. Conversely, when there is a large demand for secreted proteins or ER imbalance, the accumulation of unfolded or misfolded proteins activates the Unfold Protein Response (UPR) to restore the ER homeostasis or, in the case of persistent ER stress, induces the cell death. Pathogenic trypanosomatids, such as Trypanosoma cruzi, Trypanosoma brucei and Leishmania spp are the etiological agents of important neglected diseases. These protozoans have a complex life cycle alternating between vertebrate and invertebrate hosts. The ER of trypanosomatids, like those found in higher eukaryotes, is also specialized for secretion, and depends on the ERAD and non-canonical UPR to deal with the ER stress. Here, we reviewed the basic aspects of ER biology, organization, and quality control in trypanosomatids. We also focused on the unusual way by which T. cruzi, T. brucei, and Leishmania spp. respond to ER stress, emphasizing how these parasites' ER-unrevealed roads might be an attractive target for chemotherapy.

The HSP90 inhibitor 17-AAG induced calcium-mediated apoptosis in filarial parasites

The available antifilarial medications are effective only against the larval stage of the filarial parasite. As a result, there is a pressing need for an adulticidal drug. The development of drugs requires the identification of molecular targets that are critical for parasite life. In this study, we observed the effect of 17-N-allyl-17-demethoxygeldanamycin on the survival of adult filarial parasites. The 17-N-allyl-17-demethoxygeldanamycin (17-AAG) is a derivative of geldanamycin (GA), which is an inhibitor of heat shock protein (HSP)90. It is less toxic as compared to geldanamycin. The motility and viability of the adult filarial parasite Setaria cervi were decreased on exposure to 17-AAG at 2.5 and 5.0 μM/ml concentrations. The 17-AAG treated parasites showed induction of oxidative stress as evidenced by decreased activity of various antioxidant enzymes like glutathione s-transferase, glutathione reductase, thioredoxin reductase, and an increase in ROS production in comparison to control. Oxidative stress may lead to altered calcium homeostasis. Indeed, in 17-AAG treated worms, there was a rise in calcium in the cytosol and mitochondria, as well as a decrease in the ER. We also observed enhanced activity of phospholipase C in the treated parasite, suggesting the opening of calcium channels located on the ER membrane. ER stress is marked by a reduced level of protein disulfide isomerase. Further, 17-AAG treated worms showed an increase in apoptotic marker enzyme activities like calpain, cyt-c, and caspase-3. The 2D-gel electrophoresis technique showed 142 protein spots in the control and 112 spots in the 17-AAG treated parasite. Thus, 17-AAG induced oxidative stress, and altered calcium, and proteostasis of parasites, which led to apoptosis.

Effect of the endoplasmic reticulum stressor tunicamycin in Angomonas deanei heat-shock protein expression and on the association with the endosymbiotic bacterium

The endoplasmic reticulum (ER) presents unique properties to establishing bacterium symbiosis in eukaryotic cells since it synthesizes and glycosylates essential molecules like proteins and lipids. Tunicamycin (TM) is an antibiotic that inhibits the first step in the N-linked glycosylation in eukaryotes and has been used as an ER stress inducer to activate the Unfolded Protein Response (UPR). Mutualistic symbiosis in trypanosomatids is characterized by structural adaptations and intense metabolic exchanges, thus we investigated the effects of TM in the association between Angomonas deanei and its symbiotic bacterium, through ultrastructural and proteomic approaches. Cells treated with the inhibitor showed a decrease in proliferation, enlargement of the ER and Golgi cisternae and an increased distance between the symbiont and the ER. TM proved to be an important tool to better understand ER stress in trypanosomatids, since changes in protein composition were observed in the host protozoan, especially the expression of the Hsp90 chaperone. Furthermore, data obtained indicates the importance of the ER for the adaptation and maintenance of symbiotic associations between prokaryotes and eukaryotes, considering that this organelle has recognized importance in the biogenesis and division of cell structures.

Identification of glucose regulated protein94 (GRP94) in filarial parasite S. cervi and its expression under ER stress

GRP94, a member of HSP90 family, is involved in folding and degradation of endoplasmic reticulum proteins. The proteome analysis of Setaria cervi, a bovine filarial parasite showed that a 91 kDa protein was over expressed, after the parasites were maintained in glucose deprived medium. The MALDI- LC/MS analysis of the 91 kDa band confirmed it as endoplasmin precursor (GRP94). Amino acid sequence alignment of S.cervi GRP94 exhibited maximum similarity with human filarial parasite Wuchereria bancrofti, Brugia malayi and Loa loa GRP94. Tunicamycin treatment of S. cervi worms revealed that the expression of GRP94 is associated with ER stress. Transcription of S. cervi grp94 as well as igf is regulated by transcription factors ATF-6 and XBP-1S which was confirmed by Real Time PCR. Moreover, marked alteration in the expression of igf after 3 h and 6 h of drug treatment suggested propagation of survival pathway under ER stress. The activities of ER stress markers protein disulphide isomerase and glycosyltransferase were significantly reduced after 6 h of tunicamycin treatment. The present findings thus indicate that the expression of GRP94 and regulation of its expression is under ER stress in Setaria cervi. To our knowledge this is the first report of identification of GRP94, in any filarial parasite till date.

Endoplasmic Reticulum Stress, a Target for Drug Design and Drug Resistance in Parasitosis

Endoplasmic reticulum stress (ER stress) can be induced when cellular protein homeostasis is damaged, and cells can activate the unfolded protein response (UPR) to restore protein homeostasis or induce cell death to facilitate the survival of the whole system. Globally, parasites are a constant threat to human health and are therefore considered a serious public health problem. Parasitic infection can cause ER stress in host cells, and parasites also possess part or all of the UPR under ER stress conditions. In this review, we aim to clarify the role of ER stress pathways and related molecules in parasites for their survival and development, the pathogenesis of parasitosis in hosts, and the artemisinin resistance of Plasmodium, which provides some potential drug design targets to inhibit survival of parasites, relieves pathological damage of parasitosis, and solves the problem of artemisinin resistance.

The development of an endoplasmic reticulum-targeting fluorescent probe for the imaging of 1,4-dithiothreitol (DTT) in living cells

1,4-Dithiothreitol (DTT) is a robust reducing agent that contributes significantly to the folding process of proteins and maintaining endoplasmic reticulum (ER) homeostasis. Abnormally high levels of DTT can lead to severe endoplasmic reticulum stress (ERS), which induces cell death. In addition, DTT can also hinder cell growth and enhance reactive oxygen species (ROS) production in the ER. Herein, an effective turn-on ER-targeting fluorescent probe, ER-DTT, was designed to image DTT for the first time. The probe ER-DTT was based upon naphthalimide as a fluorophore, p-toluenesulfonamide as an exceptional unit for ER-targeting, and sulfoxide as a response site for imaging DTT based on an intramolecular charge transfer (ICT) mechanism. Optical-response experiments showed that the probe ER-DTT had good selectivity and sensitivity for DTT. Furthermore, confocal microscopy indicated that ER-DTT was suitable for selectively targeting ER in living cells and could be implemented to recognize cellular DTT.

Violacein-Induced Chaperone System Collapse Underlies Multistage Antiplasmodial Activity

Antimalarial drugs with novel modes of action and wide therapeutic potential are needed to pave the way for malaria eradication. Violacein is a natural compound known for its biological activity against cancer cells and several pathogens, including the malaria parasite, Plasmodium falciparum (Pf). Herein, using chemical genomic profiling (CGP), we found that violacein affects protein homeostasis. Mechanistically, violacein binds Pf chaperones, PfHsp90 and PfHsp70-1, compromising the latter's ATPase and chaperone activities. Additionally, violacein-treated parasites exhibited increased protein unfolding and proteasomal degradation. The uncoupling of the parasite stress response reflects the multistage growth inhibitory effect promoted by violacein. Despite evidence of proteotoxic stress, violacein did not inhibit global protein synthesis via UPR activation-a process that is highly dependent on chaperones, in agreement with the notion of a violacein-induced proteostasis collapse. Our data highlight the importance of a functioning chaperone-proteasome system for parasite development and differentiation. Thus, a violacein-like small molecule might provide a good scaffold for development of a novel probe for examining the molecular chaperone network and/or antiplasmodial drug design.

Role of anti-filarial drugs in inducing ER stress mediated signaling in bovine filarial parasitosis Setaria cervi

In this ex vivo study, S. cervi parasitoses were treated with Ivermectin (50 μM), Albendazole (200 μM) alone and Ivermectin + Albendazole (50 + 200 μM) at 37°C for 8 h and the motility and viability of the parasitoses were evaluated. Individually both drugs Ivermectin (Iver) and Albendazole (Alb) are reported to affect the function and integrity of ER, however till date, no reports are available on the functional changes in ER due to a combined Iver and Alb treatment of bovine helminth parasitosis. Here, we report the lethal effect of a combination treatment of Iver and Alb against adult bovine filarial parasitosis Setaria cervi. The underlying mechanism of drug action was elucidated by performing a systematic biochemical, molecular and proteomics based study. Altered calcium homeostasis in drug treated parasitoses lead to reduction in levels of total Endoplasmic Reticulum (ER) calcium by 50 % and 61 % and elevation by 50 % and 63 % in cytosol in Iver alone and Iver + Alb treated parasitoses respectively. Further, it was found that upregulated expression of ER localized GRP94, galactosyltransferase and glycosyltransferase activity in addition to reduction in activity of PDI indicated ER stress mechanisms being operative under combined drug treatment. Marked rise of 79 % reactive oxygen species and reduced antioxidant levels induced oxidative stress in drug treated parasitosis. The collective effect of both ER and oxidative stress might have triggered apoptosis, as evidenced by the elevated calpain activity, reduction of 67 % in cytochrome c oxidase and 83 % rise in caspase-3 activity in the Iver + Alb treated parasitoses respectively. The ER proteome analysis by 2D gel electrophoresis revealed 76 spots in the control and 56 spots in the treated proteome. A MALDI-MS/MS analysis of some of the differentially expressed spots of the combination drug treated parasitoses identified glucuronosyltransferase as a major upregulated protein with a fold change of 1.81. Trafficking protein, acyl transferase, MATH involved in protein folding were also found to be downregulated. Thus, this study based on biochemical and proteomic approaches indicates that a combination of anti-filarial drugs Iver and Alb can alter calcium homeostasis in bovine filarial parasitosis leading to induction of ER stress culminating into apoptosis.

Plasmodium falciparum Artemisinin Resistance: The Effect of Heme, Protein Damage, and Parasite Cell Stress Response

Despite a significant decline in morbidity and mortality over the last two decades, in 2018 there were 228 million reported cases of malaria and 405000 malaria-related deaths. Artemisinin, the cornerstone of artemisinin-based combination therapies, is the most potent drug in the antimalarial armamentarium against falciparum malaria. Heme-mediated activation of artemisinin and its derivatives results in widespread parasite protein alkylation, which is thought to lead to parasite death. Alarmingly, cases of decreased artemisinin efficacy have been widely detected across Cambodia and in neighboring countries, and a few cases have been reported in the Guiana Shield, India, and Africa. The grim prospect of widespread artemisinin resistance propelled a concerted effort to understand the mechanisms of artemisinin action and resistance. The identification of genetic markers and the knowledge of molecular mechanisms underpinning artemisinin resistance allow prospective surveillance and inform future drug development strategies, respectively. Here, we highlight recent advances in our understanding of how parasite vesicle trafficking, hemoglobin digestion, and cell stress responses contribute to artemisinin resistance.

The effects of endoplasmic reticulum stressors, tunicamycin and dithiothreitol on Trypanosoma cruzi

In higher eukaryotic cells, pertubations in ER environment, called ER stress, usually activate unfolded protein response (UPR) pathway in an attempt to re-stablish the ER homeostasis and prevent cell death. Because trypanosomatids appear to lack the classical UPR, it is not clear how these parasites respond to ER stress. Thus, the aim of this work was to evaluate the effects of ER stressors tunicamycin (TM) or dithiothreitol (DTT) on Trypanosoma cruzi. The TM treatment showed strong trypanostatic effect. At 2.5 μg/mL of TM, the mRNA levels of both binding protein (BiP) and calreticulin (CRT) increased significantly, whereas the protein levels of BiP remained stable. TM treatment induced ultrastructural changes compatible with an autophagic process. The DTT treatment inhibited the cell growth, induced drastic morphological changes, mitochondrial membrane depolarization and increased ROS production. The expression of BiP apparently was not affected by DTT, whereas the mRNA levels of BiP and CRT were significantly reduced. Our results suggest that TM induces autophagy/ER-phagy without causing substantial injury to the parasite. Conversely, the DTT treatment seems to rupture the mitochondrion homeostasis leading to parasite death. The comprehension of the mechanisms behind the susceptibility of T. cruzi to ER stress open perspectives for the development of chemotherapeutic agents addressed to these pathways.

Emerging therapeutic targets for treatment of leishmaniasis

INTRODUCTION

Parasitic diseases that pose a threat to human life include leishmaniasis - caused by protozoan parasite Leishmania species. Existing drugs have limitations due to deleterious side effects like teratogenicity, high cost and drug resistance. This calls for the need to have an insight into therapeutic aspects of disease. Areas covered: We have identified different drug targets via. molecular, imuunological, metabolic as well as by system biology approaches. We bring these promising drug targets into light so that they can be explored to their maximum. In an effort to bridge the gaps between existing knowledge and prospects of drug discovery, we have compiled interesting studies on drug targets, thereby paving the way for establishment of better therapeutic aspects. Expert opinion: Advancements in technology shed light on many unexplored pathways. Further probing of well established pathways led to the discovery of new drug targets. This review is a comprehensive report on current and emerging drug targets, with emphasis on several metabolic targets, organellar biochemistry, salvage pathways, epigenetics, kinome and more. Identification of new targets can contribute significantly towards strengthening the pipeline for disease elimination.

Oxidative stress and protein damage responses mediate artemisinin resistance in malaria parasites

Due to their remarkable parasitocidal activity, artemisinins represent the key components of first-line therapies against Plasmodium falciparum malaria. However, the decline in efficacy of artemisinin-based drugs jeopardizes global efforts to control and ultimately eradicate the disease. To better understand the resistance phenotype, artemisinin-resistant parasite lines were derived from two clones of the 3D7 strain of P. falciparum using a selection regimen that mimics how parasites interact with the drug within patients. This long term in vitro selection induced profound stage-specific resistance to artemisinin and its relative compounds. Chemosensitivity and transcriptional profiling of artemisinin-resistant parasites indicate that enhanced adaptive responses against oxidative stress and protein damage are associated with decreased artemisinin susceptibility. This corroborates our previous findings implicating these cellular functions in artemisinin resistance in natural infections. Genomic characterization of the two derived parasite lines revealed a spectrum of sequence and copy number polymorphisms that could play a role in regulating artemisinin response, but did not include mutations in pfk13, the main marker of artemisinin resistance in Southeast Asia. Taken together, here we present a functional in vitro model of artemisinin resistance that is underlined by a new set of genetic polymorphisms as potential genetic markers.

Participation of dengue virus NS4B protein in the modulation of immune effectors dependent on ER stress in insect cells

Organisms' reactions to adverse events result in the generation of immune effectors, which, in the case of insects, may be produced from the direct activation of pathways such as Toll, Jak-STAT, Imd, or RNAi or may be derived from the crosstalk of different intracellular pathways. One such pathway, the unfolded protein response (UPR), has the primary objective of restoring homeostasis in the endoplasmic reticulum. In addition, the UPR participates in signaling crosstalk with the immune pathways, generating protection against pathogenic organisms. Dengue virus is a plus-strand RNA virus belonging to the Flavivirus genus that uses the ER as a replication site; during the infection, there are indicators of the activation of the UPR, which in turn, induces the synthesis of internal membranes and preferential translation of viral proteins enhancing the replication. One of the dengue virus proteins, the NS4B can block the pathway of α/β interferon in mammals. However, what happen in insects is interesting because the lack of the main antiviral pathway, the interferon and the role of the NS4B protein in the UPR-immunity relationship can be better understood. Thus, in this study, we demonstrated that the DENV2/16681 NS4B protein is capable of modulating the immune effectors that result from the activation of the UPR in insect cells.

Endoplasmic reticulum stress and unfolded protein response in infection by intracellular parasites

Perturbations of the physiological status of the endoplasmic reticulum (ER) trigger a specific response known as the ER stress response or unfolded protein response (UPR). In mammalian cells, the UPR is mediated by three ER transmembrane proteins (IRE1, PERK and ATF6) which activate three signaling cascades to restore ER homeostasis. In recent years, a cross-talk between UPR, inflammatory and microbial sensing pathways has been elucidated. Pathogen infection can lead to UPR activation; moreover, several pathogens subvert the UPR to promote their survival and replication. While the UPR in viral and bacterial infection has been characterized, little is known about the role of UPR in intracellular parasite infection. Here, we review recent findings on UPR induction/modulation by intracellular parasites in host cells.

Phosphorylation of Translation Initiation Factor 2-Alpha in Leishmania donovani under Stress Is Necessary for Parasite Survival

The transformation of Leishmania donovani from a promastigote to an amastigote during mammalian host infection displays the immense adaptability of the parasite to survival under stress. Induction of translation initiation factor 2-alpha (eIF2α) phosphorylation by stress-specific eIF2α kinases is the basic stress-perceiving signal in eukaryotes to counter stress. Here, we demonstrate that elevated temperature and acidic pH induce the phosphorylation of Leishmania donovani eIF2α (LdeIF2α). In vitro inhibition experiments suggest that interference of LdeIF2α phosphorylation under conditions of elevated temperature and acidic pH debilitates parasite differentiation and reduces parasite viability (P < 0.05). Furthermore, inhibition of LdeIF2α phosphorylation significantly reduced the infection rate (P < 0.05), emphasizing its deciding role in successful invasion and infection establishment. Notably, our findings suggested the phosphorylation of LdeIF2α under H₂O₂-induced oxidative stress. Inhibition of H₂O₂-induced LdeIF2α phosphorylation hampered antioxidant balance by impaired redox homeostasis gene expression, resulting in increased reactive oxygen species accumulation (P < 0.05) and finally leading to decreased parasite viability (P < 0.05). Interestingly, exposure to sodium antimony glucamate and amphotericin B induces LdeIF2α phosphorylation, indicating its possible contribution to protection against antileishmanial drugs in common use. Overall, the results strongly suggest that stress-induced LdeIF2α phosphorylation is a necessary event for the parasite life cycle under stressed conditions for survival.

Phosphorylation of Eukaryotic Initiation Factor-2α during Stress and Encystation in Entamoeba Species

Entamoeba histolytica is an enteric pathogen responsible for amoebic dysentery and liver abscess. It alternates between the host-restricted trophozoite form and the infective environmentally-stable cyst stage. Throughout its lifecycle E. histolytica experiences stress, in part, from host immune pressure. Conversion to cysts is presumed to be a stress-response. In other systems, stress induces phosphorylation of a serine residue on eukaryotic translation initiation factor-2α (eIF2α). This inhibits eIF2α activity resulting in a general decline in protein synthesis. Genomic data reveal that E. histolytica possesses eIF2α (EheIF2α) with a conserved phosphorylatable serine at position 59 (Ser⁵⁹). Thus, this pathogen may have the machinery for stress-induced translational control. To test this, we exposed cells to different stress conditions and measured the level of total and phospho-EheIF2α. Long-term serum starvation, long-term heat shock, and oxidative stress induced an increase in the level of phospho-EheIF2α, while short-term serum starvation, short-term heat shock, or glucose deprivation did not. Long-term serum starvation also caused a decrease in polyribosome abundance, which is in accordance with the observation that this condition induces phosphorylation of EheIF2α. We generated transgenic cells that overexpress wildtype EheIF2α, a non-phosphorylatable variant of eIF2α in which Ser⁵⁹ was mutated to alanine (EheIF2α-S59A), or a phosphomimetic variant of eIF2α in which Ser⁵⁹ was mutated to aspartic acid (EheIF2α-S59D). Consistent with the known functions of eIF2α, cells expressing wildtype or EheIF2α-S59D exhibited increased or decreased translation, respectively. Surprisingly, cells expressing EheIF2α-S59A also exhibited reduced translation. Cells expressing EheIF2α-S59D were more resistant to long-term serum starvation underscoring the significance of EheIF2α phosphorylation in managing stress. Finally, phospho-eIF2α accumulated during encystation in E. invadens, a model encystation system. Together, these data demonstrate that the eIF2α-dependent stress response system is operational in Entamoeba species.

Entamoeba histolytica is the causative agent of amoebic dysentery and liver abscess and is prevalent in underdeveloped countries that lack proper sanitation. Infection is acquired by ingestion of the cyst form in contaminated food or water. During infection, the parasite experiences stress including demanding growth conditions and host immune pressure. Conversion to the infective cyst may be induced by such stress. In other organisms, stress causes a decrease in protein biosynthesis by inducing phosphorylation of eIF2α, which participates in translation initiation. We exposed E. histolytica to six different stress conditions and observed that some of these conditions (long-term serum starvation, long-term heat shock, and oxidative stress) induced an increase in the level of phospho-eIF2α. Long-term serum starvation was also accompanied by a decrease in mRNA translation. A cell line expressing a mutant version of eIF2α that behaves as a phosphomimetic exhibited decreased translation and increased survival during long-term serum starvation. Finally, phospho-eIF2α accumulated in cysts of E. invadens, a reptilian pathogen that readily encysts in vitro. Together, these data demonstrate that the eIF2α-dependent stress response system is operational in Entamoeba and may regulate encystation.

High resolution microscopy reveals an unusual architecture of the Plasmodium berghei endoplasmic reticulum

To fuel the tremendously fast replication of Plasmodium liver stage parasites, the endoplasmic reticulum (ER) must play a critical role as a major site of protein and lipid biosynthesis. In this study, we analysed the parasite's ER morphology and function. Previous studies exploring the parasite ER have mainly focused on the blood stage. Visualizing the Plasmodium berghei ER during liver stage development, we found that the ER forms an interconnected network throughout the parasite with perinuclear and peripheral localizations. Surprisingly, we observed that the ER additionally generates huge accumulations. Using stimulated emission depletion microscopy and serial block-face scanning electron microscopy, we defined ER accumulations as intricate dense networks of ER tubules. We provide evidence that these accumulations are functional subdivisions of the parasite ER, presumably generated in response to elevated demands of the parasite, potentially consistent with ER stress. Compared to higher eukaryotes, Plasmodium parasites have a fundamentally reduced unfolded protein response machinery for reacting to ER stress. Accordingly, parasite development is greatly impaired when ER stress is applied. As parasites appear to be more sensitive to ER stress than are host cells, induction of ER stress could potentially be used for interference with parasite development.

Cellular stress associated with the differentiation of Plasmodium berghei ookinetes

For malaria transmission, Plasmodium parasites must develop in the mosquito vector. Oxidative stress in the insect midgut, triggered by environmental changes (e.g., pH and temperature), influences the cellular signaling involved in differentiation from gametocytes to mobile ookinetes for the purpose of parasite survival. Oxidative stress activates the homeostatic response to stress characterized by the phosphorylation eIF2α, the attenuation of protein synthesis, and the transcription of genes participating in the unfolded protein response and antioxidant processes, forming a part of an integrated stress response (ISR). We hypothesized that ISR operates during the differentiation of gametocytes to ookinetes to assure Plasmodium survival. Using in-vitro conditions resembling the mosquito midgut conditions, we cultured Plasmodium berghei gametocytes to ookinetes and evaluated the redox balance by detecting reactive oxygen species and superoxide dismutase activity. Additionally, we evaluated the phosphorylation of eIF2α, the attenuation of the global protein synthesis, and the gene expression of cellular stress markers (e.g., endoplasmic reticulum chaperones and antioxidant molecules, measured by reverse-transcription quantitative polymerase chain reaction), finding that these processes were all taking place, probably to improve survival during the differentiation of Plasmodium berghei ookinetes.

Unfolded protein response regulates yeast small GTPase Arl1p activation at late Golgi via phosphorylation of Arf GEF Syt1p

ADP ribosylation factor (Arf) GTPases are key regulators of membrane traffic at the Golgi complex. In yeast, Arf guanine nucleotide-exchange factor (GEF) Syt1p activates Arf-like protein Arl1p, which was accompanied by accumulation of golgin Imh1p at late Golgi, but whether and how this function of Syt1p is regulated remains unclear. Here, we report that the inositol-requiring kinase 1 (Ire1p)-mediated unfolded protein response (UPR) modulated Arl1p activation at late Golgi. Arl1p activation was dependent on both kinase and endo-RNase activities of Ire1p. Moreover, constitutively active transcription factor Hac1p restored the Golgi localization of Arl1p and Imh1p inIRE1-deleted cells. Elucidating the mechanism of Ire1p-Hac1p axis actions, we found that it regulated phosphorylation of Syt1p, which enhances Arl1p activation, recruitment of Imh1p to the Golgi, and Syt1p interaction with Arl1p. Consistent with these findings, the induction of UPR by tunicamycin treatment increases phosphorylation of Syt1p, resulting in Arl1p activation. Thus, these findings clarify how the UPR influences the roles of Syt1p, Arl1p, and Imh1p in Golgi transport.

Plasmodium falciparum: multifaceted resistance to artemisinins

Plasmodium falciparum resistance to artemisinins, the most potent and fastest acting anti-malarials, threatens malaria elimination strategies. Artemisinin resistance is due to mutation of the PfK13 propeller domain and involves an unconventional mechanism based on a quiescence state leading to parasite recrudescence as soon as drug pressure is removed. The enhanced P. falciparum quiescence capacity of artemisinin-resistant parasites results from an increased ability to manage oxidative damage and an altered cell cycle gene regulation within a complex network involving the unfolded protein response, the PI3K/PI3P/AKT pathway, the PfPK4/eIF2α cascade and yet unidentified transcription factor(s), with minimal energetic requirements and fatty acid metabolism maintained in the mitochondrion and apicoplast. The detailed study of these mechanisms offers a way forward for identifying future intervention targets to fend off established artemisinin resistance.

Leishmania infantum Asparagine Synthetase A Is Dispensable for Parasites Survival and Infectivity

A growing interest in asparagine (Asn) metabolism has currently been observed in cancer and infection fields. Asparagine synthetase (AS) is responsible for the conversion of aspartate into Asn in an ATP-dependent manner, using ammonia or glutamine as a nitrogen source. There are two structurally distinct AS: the strictly ammonia dependent, type A, and the type B, which preferably uses glutamine. Absent in humans and present in trypanosomatids, AS-A was worthy of exploring as a potential drug target candidate. Appealingly, it was reported that AS-A was essential in Leishmania donovani, making it a promising drug target. In the work herein we demonstrate that Leishmania infantum AS-A, similarly to Trypanosoma spp. and L. donovani, is able to use both ammonia and glutamine as nitrogen donors. Moreover, we have successfully generated LiASA null mutants by targeted gene replacement in L. infantum, and these parasites do not display any significant growth or infectivity defect. Indeed, a severe impairment of in vitro growth was only observed when null mutants were cultured in asparagine limiting conditions. Altogether our results demonstrate that despite being important under asparagine limitation, LiAS-A is not essential for parasite survival, growth or infectivity in normal in vitro and in vivo conditions. Therefore we exclude AS-A as a suitable drug target against L. infantum parasites.

Depletion of UDP-Glucose and UDP-Galactose Using a Degron System Leads to Growth Cessation of Leishmania major

Interconversion of UDP-glucose (UDP-Glc) and UDP-galactose (UDP-Gal) by the UDP-Glc 4´-epimerase intimately connects the biosynthesis of these two nucleotide sugars. Their de novo biosynthesis involves transformation of glucose-6-phosphate into glucose-1-phosphate by the phosphoglucomutase and subsequent activation into UDP-Glc by the specific UDP-Glc pyrophosphorylase (UGP). Besides UGP, Leishmania parasites express an uncommon UDP-sugar pyrophosphorylase (USP) able to activate both galactose-1-phosphate and glucose-1-phosphate in vitro. Targeted gene deletion of UGP alone was previously shown to principally affect expression of lipophosphoglycan, resulting in a reduced virulence. Since our attempts to delete both UGP and USP failed, deletion of UGP was combined with conditional destabilisation of USP to control the biosynthesis of UDP-Glc and UDP-Gal. Stabilisation of the enzyme produced by a single USP allele was sufficient to maintain the steady-state pools of these two nucleotide sugars and preserve almost normal glycoinositolphospholipids galactosylation, but at the apparent expense of lipophosphoglycan biosynthesis. However, under destabilising conditions, the absence of both UGP and USP resulted in depletion of UDP-Glc and UDP-Gal and led to growth cessation and cell death, suggesting that either or both of these metabolites is/are essential.

Leishmaniases are a set of tropical and sub-tropical diseases caused by protozoan parasites of the genus Leishmania. They affect about 12 million people and cause a high morbidity. Since treatments against all forms of leishmaniasis are limited in number and efficacy, many efforts are made to identify potential drug targets and develop new therapies. Although considerable progress in genetic manipulation of Leishmania parasites have been made, it remains difficult to study molecules or metabolic pathways essential for parasite viability and growth. In the present work, we used a combination of gene deletion and conditional protein destabilization to demonstrate that biosynthesis of the nucleotide sugar UDP-glucose and its derivative UDP-galactose is essential for parasite growth. Addition of a specific ligand to the culture medium of the engineered parasite protected the targeted enzyme from degradation and enabled cell growth and viability. However, removal of the stabilizing compound led to depletion of UDP-glucose and UDP-galactose, growth arrest and cell death. This work thus opens a new possibility for the study of essential proteins.

Unfolded Protein Response Pathways in Bloodstream-Form Trypanosoma brucei?

The unfolded protein response (UPR) is a stress mechanism to cope with misfolded proteins in the early secretory pathway, the hallmark being transcriptional upregulation of endoplasmic reticulum (ER) molecular chaperones such as BiP and protein disulfide isomerase. Despite the lack of transcriptional regulation and the absence of the classical UPR machinery, African trypanosomes apparently respond to persistent ER stress by a UPR-like response, including upregulation of BiP, and a related spliced leader silencing (SLS) response whereby SL RNA transcription is shut down. Initially observed by knockdown of the secretory protein translocation machinery, both responses are also induced by chemical agents known to elicit UPR in mammalian cells (H. Goldshmidt, D. Matas, A. Kabi, A. Carmi, R. Hope, S. Michaeli, PLoS Pathog 6:e1000731, 2010, http://dx.doi.org/10.1371/journal.ppat.1000731). As these findings were generated primarily in procyclic-stage trypanosomes, we have investigated both responses in pathogenic bloodstream-stage parasites. RNA interference (RNAi) silencing of the core translocon subunit Trypanosoma brucei Sec61α (TbSec61α) failed to induce either response. Interestingly, cell growth halted within 16 h of silencing, but sufficient TbSec61α remained to allow full competence for translocation of nascent secretory proteins for up to 24 h, indicating that replication is finely coupled with the capacity to synthesize and transport secretory cargo. Tunicamycin and thapsigargin at concentrations compatible with short-term (4 h) and long-term (24 h) viability also failed to induce any of the indicators of UPR-like or SLS responses. Dithiothreitol (DTT) was lethal at all concentrations tested. These results indicate that UPR-like and SLS responses to persistent ER stress do not occur in bloodstream-stage trypanosomes.

The response of trypanosomes and other eukaryotes to ER stress and the spliced leader RNA silencing (SLS) pathway in Trypanosoma brucei

The unfolded protein response (UPR) is induced when the quality control machinery of the cell is overloaded with unfolded proteins or when one of the functions of the endoplasmic reticulum (ER) is perturbed. Here, I describe UPR in yeast and mammals, and compare it to what we know about pathogenic fungi and the parasitic protozoans from the order kinetoplastida, focusing on the novel pathway the spliced leader silencing (SLS) in Trypanosoma brucei. Trypanosomes lack conventional transcription regulation, and thus, lack most of the UPR machinery present in other eukaryotes. Trypanosome genes are transcribed in polycistronic units that are processed by trans-splicing and polyadenylation. In trans-splicing, which is essential for processing of each mRNA, an exon known as the spliced leader (SL) is added to all mRNAs from a small RNA, the SL RNA. Under severe ER stress, T. brucei elicits the SLS pathway. In SLS, the transcription of the SL RNA gene is extinguished, and the entire transcription complex dissociates from the SL RNA promoter. Induction of SLS is mediated by an ER-associated kinase (PK3) that migrates to the nucleus, where it phosphorylates the TATA-binding protein (TRF4), leading shut-off of SL RNA transcription. As a result, trans-splicing is inhibited and the parasites activate a programmed cell death (PCD) pathway. Despite the ability to sense the ER stress, the different eukaryotes, especially unicellular parasites and pathogenic fungi, developed a variety of unique and different ways to sense and adjust to this stress in a manner different from their host.

Targeting the cell stress response of Plasmodium falciparum to overcome artemisinin resistance

Successful control of falciparum malaria depends greatly on treatment with artemisinin combination therapies. Thus, reports that resistance to artemisinins (ARTs) has emerged, and that the prevalence of this resistance is increasing, are alarming. ART resistance has recently been linked to mutations in the K13 propeller protein. We undertook a detailed kinetic analysis of the drug responses of K13 wild-type and mutant isolates of Plasmodium falciparum sourced from a region in Cambodia (Pailin). We demonstrate that ART treatment induces growth retardation and an accumulation of ubiquitinated proteins, indicative of a cellular stress response that engages the ubiquitin/proteasome system. We show that resistant parasites exhibit lower levels of ubiquitinated proteins and delayed onset of cell death, indicating an enhanced cell stress response. We found that the stress response can be targeted by inhibiting the proteasome. Accordingly, clinically used proteasome inhibitors strongly synergize ART activity against both sensitive and resistant parasites, including isogenic lines expressing mutant or wild-type K13. Synergy is also observed against Plasmodium berghei in vivo. We developed a detailed model of parasite responses that enables us to infer, for the first time, in vivo parasite clearance profiles from in vitro assessments of ART sensitivity. We provide evidence that the clinical marker of resistance (delayed parasite clearance) is an indirect measure of drug efficacy because of the persistence of unviable parasites with unchanged morphology in the circulation, and we suggest alternative approaches for the direct measurement of viability. Our model predicts that extending current three-day ART treatment courses to four days, or splitting the doses, will efficiently clear resistant parasite infections. This work provides a rationale for improving the detection of ART resistance in the field and for treatment strategies that can be employed in areas with ART resistance.

Resistance to artemisinin antimalarial drugs is jeopardizing malaria control. This study shows that proteasome-mediated stress responses can be targeted to overcome artemisinin resistance and suggests alternate therapeutic regimens and monitoring strategies.

Resistance to artemisinin antimalarials, some of the most effective antimalarial drugs, has emerged in Southeast Asia, jeopardizing malaria control. We have undertaken a detailed study of artemisinin-sensitive and-resistant strains of Plasmodium falciparum, the parasite responsible for malaria, taken directly from the field in a region where resistance is developing. We compared these strains to lab strains engineered with either mutant or wild-type resistance alleles. We demonstrate that in sensitive P. falciparum, artemisinin induces growth retardation and accumulation of ubiquitinated proteins, indicating that the drugs activate the cellular stress response. Resistant parasites, on the other hand, exhibit reduced protein ubiquitination and delayed onset of cell death following drug exposure. We show that proteasome inhibitors strongly synergize artemisinin activity, offering a means of overcoming artemisinin resistance. We have developed a detailed model of parasite responses and have modelled in vivo clearance profiles. Our data indicate that extending artemisinin treatment from the standard three-day treatment to a four-day treatment will clear resistant parasites, thus preserving the use of this critical therapy in areas experiencing artemisinin resistance.

LACK, a RACK1 ortholog, facilitates cytochrome c oxidase subunit expression to promote Leishmania major fitness

Leishmania are kinetoplastid parasites that cause the sandfly-transmitted disease leishmaniasis. To maintain fitness throughout their infectious life cycle, Leishmania must undergo rapid metabolic adaptations to the dramatically distinct environments encountered during transition between sandfly and vertebrate hosts. We performed proteomic and immunoblot analyses of attenuated L. major strains deficient for LACK, the Leishmania ortholog of the mammalian receptor for activated c kinase (RACK1), that is important for parasite thermotolerance and virulence. This approach identified cytochrome c oxidase (LmCOX) subunit IV as a LACK-dependent fitness protein. Consistent with decreased levels of LmCOX subunit IV at mammalian temperature, and in amastigotes, LmCOX activity and mitochondrial function were also impaired in LACK-deficient L. major under these conditions. Importantly, overexpression of LmCOX subunit IV in LACK-deficient L. major restored thermotolerance and macrophage infectivity. Interestingly, overexpression of LmCOX subunit IV enhanced LmCOX subunit VI expression at mammalian temperature. Collectively, our data suggest LACK promotes Leishmania adaptation to the mammalian host environment by sustaining LmCOX subunit IV expression and hence energy metabolism in response to stress stimuli such as heat. These findings extend the repertoire of RACK1 protein utility to include a role in mitochondrial function.

Characterization of metabolically quiescent Leishmania parasites in murine lesions using heavy water labeling

Information on the growth rate and metabolism of microbial pathogens that cause long-term chronic infections is limited, reflecting the absence of suitable tools for measuring these parameters in vivo. Here, we have measured the replication and physiological state of Leishmania mexicana parasites in murine inflammatory lesions using ²H₂O labeling. Infected BALB/c mice were labeled with ²H₂O for up to 4 months, and the turnover of parasite DNA, RNA, protein and membrane lipids estimated from the rate of deuterium enrichment in constituent pentose sugars, amino acids, and fatty acids, respectively. We show that the replication rate of parasite stages in these tissues is very slow (doubling time of ~12 days), but remarkably constant throughout lesion development. Lesion parasites also exhibit markedly lower rates of RNA synthesis, protein turnover and membrane lipid synthesis than parasite stages isolated from ex vivo infected macrophages or cultured in vitro, suggesting that formation of lesions induces parasites to enter a semi-quiescent physiological state. Significantly, the determined parasite growth rate accounts for the overall increase in parasite burden indicating that parasite death and turnover of infected host cells in these lesions is minimal. We propose that the Leishmania response to lesion formation is an important adaptive strategy that minimizes macrophage activation, providing a permissive environment that supports progressive expansion of parasite burden. This labeling approach can be used to measure the dynamics of other host-microbe interactions in situ.

Microbial pathogens can adapt to changing conditions in their hosts by switching between different growth and physiological states. However, current methods for measuring microbial physiology in vivo are limited, hampering detailed dissection of host-pathogen interactions. Here we have used heavy water labeling to measure the growth rate and physiological state of Leishmania parasites in murine lesions. Based on the rate of in situ labeling of parasite DNA, RNA, protein, and lipids, we show that the growth rate of intracellular parasite stages is very slow, and that these stages enter a semi-quiescent state characterized by very low rates of RNA, protein, and membrane turnover. These changes in parasite growth and physiology are more pronounced than in in vitro differentiated parasites, suggesting that they are induced in part by the lesion environment. Despite their slow growth, the parasite burden in these lesions progressively increases as a result of low rates of parasite death and host cell turnover. We propose that these changes in Leishmania growth and physiology contribute to the development of a relatively benign tissue environment that is permissive for long term parasite expansion. This approach is suitable for studying the dynamics of other host-pathogen systems.

Proteomic analysis of posttranslational modifications using iTRAQ in Leishmania

iTRAQ is a high coverage quantitative proteomics technique identifies and quantitates abundance changes of multiple (up to eight) distinct protein samples. To date, one iTRAQ-MS/MS assay can identify up to quarter of cells proteome. Each of the eight tags covalently binds to the N-terminus as well as arginine and lysine side chains of peptides, enabling labeling of the entire peptide population in each sample. Following tagging, the various protein samples are mixed and subjected to LC-MS/MS analysis. In the first round identical peptides from the different protein populations focus in a single pick. Subsequently, sequence of each peptide is determined. The tags whose m/z is similar to that of natural amino acids are used to determine relative abundance. To date, iTRAQ enabled identification of almost 2,000 Leishmania proteins. Here, we provide protocols for protein abundance changes and for phosphoproteomics analysis in Leishmania parasites.

ER stress: how trypanosomes deal with it

An efficient response to endoplasmic reticulum (ER) stress is essential for the viability of eukaryotic cells. The causative agent of African sleeping sickness, Trypanosoma brucei, responds to such stress by inducing spliced leader RNA silencing (SLS), resulting in shutdown of mRNA biogenesis. A new study elucidates the activation cascade and its molecular components, which are unique to the ER stress response in trypanosomes.

Endoplasmic reticulum stress responses in Leishmania

Perturbation of endoplasmic reticulum (ER) homeostasis can lead to an accumulation of misfolded proteins within the ER lumen causing initiation of ER stress. To reestablish homeostasis and mitigate the stress, a series of adaptive intracellular signaling pathways termed the unfolded protein response (UPR) are activated. ER stress is of considerable interest to parasitologists because it takes place in parasites subjected to adverse environmental conditions. During a digenetic lifestyle, Leishmania parasites encounter and adapt to harsh environmental conditions that provide potential triggers of ER stress. These include nutrient deficiency, hypoxia, oxidative stress, changing pH, and shifts in temperature. Protozoan human pathogens, including the causative agents of trypanosomiasis, leishmaniasis, toxoplasmosis and malaria, contain a minimal conventional UPR network relative to higher eukaryotic cells. Three different signaling pathways in the ER stress response have been described in trypanosomatids: these pathways involve (i) the down-regulation of translation by a protein kinase RNA-like ER kinase (PERK), (ii) the ER-associated degradation (ERAD) pathway, and (iii) the spliced leader silencing (SLS) pathway and its target mRNAs. Under short-term ER stress, signaling from PERK activates autophagy, a cell survival response. But both chronic and unresolved ER stresses lead to initiation of apoptotic events and eventual cell death. This review presents the current understanding of the ER stress response in Leishmania with an emphasis on protein folding and ER quality control, unfolded protein response, autophagy as well as apoptosis in reference to the mammalian system.

Phosphorylation of the TATA-binding protein activates the spliced leader silencing pathway in Trypanosoma brucei

The parasite Trypanosoma brucei is the causative agent of human African sleeping sickness. T. brucei genes are constitutively transcribed in polycistronic units that are processed by trans-splicing and polyadenylation. All mRNAs are trans-spliced to generate mRNAs with a common 5' exon derived from the spliced leader RNA (SL RNA). Persistent endoplasmic reticulum (ER) stress induces the spliced leader silencing (SLS) pathway, which inhibits trans-splicing by silencing SL RNA transcription, and correlates with increased programmed cell death. We found that during ER stress induced by SEC63 silencing or low pH, the serine-threonine kinase PK3 translocated from the ER to the nucleus, where it phosphorylated the TATA-binding protein TRF4, leading to the dissociation of the transcription preinitiation complex from the promoter of the SL RNA encoding gene. PK3 loss of function attenuated programmed cell death induced by ER stress, suggesting that SLS may contribute to the activation of programmed cell death.

Endoplasmic reticulum stress triggers gametocytogenesis in the malaria parasite

The malaria parasite experiences a significant amount of redox stress during its growth in human erythrocytes and heavily relies on secretory functions for pathogenesis. Most certainly, the parasite is equipped with machinery to tackle perturbations in the secretory pathway, like the unfolded protein response pathway in higher eukaryotes. Our bioinformatics analysis revealed the complete absence of genes involved in the canonical unfolded protein response pathway in Plasmodium falciparum. Accordingly, the parasite was unable to up-regulate endoplasmic reticulum (ER) chaperones or ER-associated degradation in response to DTT-mediated ER stress. Global profiling of gene expression upon DTT treatment revealed a network of AP2 transcription factors and their targets being activated. The overall outcome was up-regulation of genes involved in protein export and the sexual stage of the parasite life cycle culminating in gametocytogenesis. Our results suggest that the malaria parasite uses ER stress as a cue to switch to the transmissible sexual stages.

A2 and other visceralizing proteins of Leishmania: role in pathogenesis and application for vaccine development

Visceral leishmaniasis is a re-emergent disease and a significant cause of morbidity worldwide. Amongst the more than 20 Leishmania species, Leishmania donovani, Leishmania infantum and more rarely Leishmania amazonensis are associated with visceral leishmaniasis. A major question in leishmaniasis research is how these species migrate to and infect visceral organs whereas other species such as Leishmania major and Leishmania braziliensis remain in the skin, causing tegumentary leishmaniasis. Here we present the more recent advances and approaches towards the identification of species-specific visceralizing factors of Leishmania, such as the A2 protein, leading to a better understanding of parasite biology. We also discuss their potential use for the development of a vaccine for visceral leishmaniasis.

The unfolded protein response in the protozoan parasite Toxoplasma gondii features translational and transcriptional control

The unfolded protein response (UPR) is an important regulatory network that responds to perturbations in protein homeostasis in the endoplasmic reticulum (ER). In mammalian cells, the UPR features translational and transcriptional mechanisms of gene expression aimed at restoring proteostatic control. A central feature of the UPR is phosphorylation of the α subunit of eukaryotic initiation factor-2 (eIF2) by PERK (EIF2AK3/PEK), which reduces the influx of nascent proteins into the ER by lowering global protein synthesis, coincident with preferential translation of key transcription activators of genes that function to expand the processing capacity of this secretory organelle. Upon ER stress, the apicomplexan parasite Toxoplasma gondii is known to induce phosphorylation of Toxoplasma eIF2α and lower translation initiation. To characterize the nature of the ensuing UPR in this parasite, we carried out microarray analyses to measure the changes in the transcriptome and in translational control during ER stress. We determined that a collection of transcripts linked with the secretory process are induced in response to ER stress, supporting the idea that a transcriptional induction phase of the UPR occurs in Toxoplasma. Furthermore, we determined that about 500 gene transcripts showed enhanced association with translating ribosomes during ER stress. Many of these target genes are suggested to be involved in gene expression, including JmjC5, which continues to be actively translated during ER stress. This study indicates that Toxoplasma triggers a UPR during ER stress that features both translational and transcriptional regulatory mechanisms, which is likely to be important for parasite invasion and development.

Evolution of the unfolded protein response

The unfolded protein response (UPR) is a network of signaling pathways that responds to stress in the endoplasmic reticulum (ER). The general output of the UPR is to upregulate genes involved in ER function, thus restoring and/or increasing the capacity of the ER to fold and process proteins. In parallel, many organisms have mechanisms for limiting the load on the ER by attenuating translation or degrading ER-targeted mRNAs. Despite broad conservation of these signaling pathways across eukaryotes, interesting variations demonstrate a variety of mechanisms for managing ER stress. How do early-diverging protozoa respond to stress when they lack traditional transcriptional regulation? What is the role of the ER stress sensor Ire1 in fungal species that are missing its main target? Here I describe how diverse species have optimized the UPR to fit their needs. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Determinants for the development of visceral leishmaniasis disease

Leishmaniasis is a vector-borne neglected tropical disease associated with a spectrum of clinical manifestations, ranging from self-healing cutaneous lesions to fatal visceral infections. Among the most important questions in Leishmania research is why some species like L. donovani infect visceral organs, whereas other species like L. major remain in the skin. The determinants of visceral leishmaniasis are still poorly understood, although genomic, immunologic, and animal models are beginning to provide important insight into this disease. In this review, we discuss the vector, host, and pathogen factors that mediate the development of visceral leishmaniasis. We examine the progression of the parasite from the initial site of sand fly bite to the visceral organs and its ability to survive there. The identification of visceral disease determinants is required to understand disease evolution, to understand visceral organ survival mechanisms, and potentially to develop better interventions for this largely neglected disease.

Targeting the ERAD pathway via inhibition of signal peptide peptidase for antiparasitic therapeutic design

Early secretory and endoplasmic reticulum (ER)-localized proteins that are terminally misfolded or misassembled are degraded by a ubiquitin- and proteasome-mediated process known as ER-associated degradation (ERAD). Protozoan pathogens, including the causative agents of malaria, toxoplasmosis, trypanosomiasis, and leishmaniasis, contain a minimal ERAD network relative to higher eukaryotic cells, and, because of this, we observe that the malaria parasite Plasmodium falciparum is highly sensitive to the inhibition of components of this protein quality control system. Inhibitors that specifically target a putative protease component of ERAD, signal peptide peptidase (SPP), have high selectivity and potency for P. falciparum. By using a variety of methodologies, we validate that SPP inhibitors target P. falciparum SPP in parasites, disrupt the protein's ability to facilitate degradation of unstable proteins, and inhibit its proteolytic activity. These compounds also show low nanomolar activity against liver-stage malaria parasites and are also equipotent against a panel of pathogenic protozoan parasites. Collectively, these data suggest ER quality control as a vulnerability of protozoan parasites, and that SPP inhibition may represent a suitable transmission blocking antimalarial strategy and potential pan-protozoan drug target.

Involvement of the Leishmania donovani virulence factor A2 in protection against heat and oxidative stress

Leishmania is an obligate intracellular protozoan parasite that infects cells of the reticulo-endothelial system. Host defences against Leishmania include fever and oxidant production, and the parasite has developed a number of defence mechanisms to neutralize the host response. The Leishmania donovani A2 family of proteins has been shown to be essential for survival in mammalian visceral organs. Here we provide evidence that A2 proteins protect the parasite against host defences, namely heat stress (fever) and oxidative stress. A2 is however unable to protect the cells from endoplasmic reticulum stress induced by dithiothreitol. To downregulate A2 protein expression, L. donovani was transfected with an A2 antisense RNA expressing-vector, resulting in significant reduction of A2 levels. The resulting A2-deficient cells were more sensitive to heat shock and this was associated with increased production of internal oxidants during heat shock. Moreover, axenic amastigotes with downregulated A2 expression had increased internal oxidants and decreased viability following treatment with hydrogen peroxide or a nitric oxide donor when compared to control cells. Overall, these results suggest that A2 protects L. donovani from a variety of stresses, thereby allowing it to survive in the internal organs of the mammalian host and to cause visceral disease.

What has proteomics taught us aboutLeishmaniadevelopment?

Leishmaniaare obligatory intracellular parasitic protozoa that cycle between sand fly mid-gut and phagolysosomes of mammalian macrophages. They have developed genetically programmed changes in gene and protein expression that enable rapid optimization of cell function according to vector and host environments. During the last two decades, host-free systems that mimic intra-lysosomal environments have been devised in which promastigotes differentiate into amastigotes axenically. These cultures have facilitated detailed investigation of the molecular mechanisms underlyingLeishmaniadevelopment inside its host. Axenic promastigotes and amastigotes have been subjected to transcriptome and proteomic analyses. Development had appeared somewhat variable but was revealed by proteomics to be strictly coordinated and regulated. Here we summarize the current understanding ofLeishmaniapromastigote to amastigote differentiation, highlighting the data generated by proteomics.

Endoplasmic reticulum stress-sensing mechanism is activated in Entamoeba histolytica upon treatment with nitric oxide

The Endoplasmic Reticulum stores calcium and is a site of protein synthesis and modification. Changes in ER homeostasis lead to stress responses with an activation of the unfolded protein response (UPR). The Entamoeba histolytica endomembrane system is simple compared to those of higher eukaryotes, as a canonical ER is not observed. During amoebiasis, an infection of the human intestine and liver by E. histolytica, nitric oxide (NO) triggers an apoptotic-like event preceded by an impairment of energy production and a loss of important parasite pathogenic features. We address the question of how this ancient eukaryote responds to stress induced by immune components (i.e. NO) and whether stress leads to ER changes and subsequently to an UPR. Gene expression analysis suggested that NO triggers stress responses marked by (i) dramatic up-regulation of hsp genes although a bona fide UPR is absent; (ii) induction of DNA repair and redox gene expression and iii) up-regulation of glycolysis-related gene expression. Enzymology approaches demonstrate that NO directly inhibits glycolysis and enhance cysteine synthase activity. Using live imaging and confocal microscopy we found that NO dramatically provokes extensive ER fragmentation. ER fission in E. histolytica appears as a protective response against stress, as it has been recently proposed for neuron self-defense during neurologic disorders. Chronic ER stress is also involved in metabolic diseases including diabetes, where NO production reduces ER calcium levels and activates cell death. Our data highlighted unique cellular responses of interest to understand the mechanisms of parasite death during amoebiasis.