TcSCA complements yeast mutants defective in Ca2+ pumps and encodes a Ca2+-ATPase that localizes to the endoplasmic reticulum of Trypanosoma cruzi

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 old and the new about the contractile vacuole of Trypanosoma cruzi

Osmoregulation is a conserved cellular process required for the survival of all organisms. In protists, the need for robust compensatory mechanisms that can maintain cell volume and tonicity within physiological range is even more relevant, as their life cycles are often completed in different environments. Trypanosoma cruzi, the protozoan pathogen responsible for Chagas disease, is transmitted by an insect vector to multiple types of mammalian hosts. The contractile vacuole complex (CVC) is an organelle that senses and compensates osmotic changes in the parasites, ensuring their survival upon ionic and osmotic challenges. Recent work shows that the contractile vacuole is also a key component of the secretory and endocytic pathways, regulating the selective targeting of surface proteins during differentiation. Here we summarize our current knowledge of the mechanisms involved in the osmoregulatory processes that take place in the vacuole, and we explore the new and exciting functions of this organelle in cell trafficking and signaling.

Gene expression study to elucidate the anti-trypanosomal activity of quinapyramine methyl sulphate (QPS)

The kinetoplastid protozoan parasite, Trypanosoma evansi causes a fatal disease condition known as Surra in equines throughout the globe. Disease condition being acute in nature, entrust a huge economic and health impact on the equine industry. Till date, quinapyramine methyl sulphate (QPS) is the first line of treatment and a panacea for the T. evansi infection in equines. Still after the >70 years of its discovery, there is no clue about the mode of action of QPS in T. evansi. The establishment of in vitro cultivation of T. evansi in HMI-9 media has provided opportunity to study the alteration in mRNA expression of parasite on exposure to the drug. With this research gap, the present study aimed to investigate the relative mRNA expression of 13 important drug target genes to elucidate the anti-trypanosomal activity of QPS against T. evansi. The IC₅₀ of QPS against a pony isolate of T. evansi was determined as 276.4 nM(147.21 ng/ mL) in the growth inhibitory assay. The in vitro cultured T. evansi population were further exposed to IC₅₀ of QPS and their relative mRNA expression was studied at 12 h, 24 h and 48 h interval.The mRNA expression of several genes such as hexokinase, trypanothione reductase, aurora kinase, oligopeptidase B and ribonucleotide reductase II were found refractory (non-significant, p > 0.1234) to the exposure of QPS. Significant up-regulation of trans-sialidase (p < 0.0001), ESAG8 (p < 0.0021), ribonucleotide reductase I (p < 0.0001), ornithine decarboxylase (p < 0.0001), topoisomerase II (p < 0.0021) and casein kinase I (p < 0.0021) were recorded after exposure with QPS. The arginine kinase 1 and calcium ATPase I showed highly significant (p < 0.0001) down-regulation in the drug kinetics. Therefore, the arginine kinase 1 and calcium ATPase I can be explored further to elucidate the trypanocidal activity of QPS. The preliminary data generated provide the potential of arginine kinase 1 and calcium ATPase I mRNA mediated pathway of trypanocidal action of QPS. Further, transcriptomics approach is required to investigate the possible mechanism of action of drugs at molecular level against the targeted organism.

The Kinetoplastid-Specific Protein TcCAL1 Plays Different Roles During In Vitro Differentiation and Host-Cell Invasion in Trypanosoma cruzi

In the pathogen Typanosoma cruzi, the calcium ion (Ca2+) regulates key processes for parasite survival. However, the mechanisms decoding Ca2+ signals are not fully identified or understood. Here, we investigate the role of a hypothetical Ca2+-binding protein named TcCAL1 in the in vitro life cycle of T. cruzi. Results showed that the overexpression of TcCAL1 fused to a 6X histidine tag (TcCAL1-6xHis) impaired the differentiation of epimastigotes into metacyclic trypomastigotes, significantly decreasing metacyclogenesis rates. When the virulence of transgenic metacyclic trypomastigotes was explored in mammalian cell invasion assays, we found that the percentage of infection was significantly higher in Vero cells incubated with TcCAL1-6xHis-overexpressing parasites than in controls, as well as the number of intracellular amastigotes. Additionally, the percentage of Vero cells with adhered metacyclic trypomastigotes significantly increased in samples incubated with TcCAL1-6xHis-overexpressing parasites compared with controls. In contrast, the differentiation rates from metacyclic trypomastigotes to axenic amastigotes or the epimastigote proliferation in the exponential phase of growth have not been affected by TcCAL1-6xHis overexpression. Based on our findings, we speculate that TcCAL1 exerts its function by sequestering intracellular Ca2+ by its EF-hand motifs (impairing metacyclogenesis) and/or due to an unknown activity which could be amplified by the ion binding (promoting cell invasion). This work underpins the importance of studying the kinetoplastid-specific proteins with unknown functions in pathogen parasites.

Interaction With the Extracellular Matrix Triggers Calcium Signaling in Trypanosoma cruzi Prior to Cell Invasion

Trypanosoma cruzi, the etiological agent of Chagas disease in humans, infects a wide variety of vertebrates. Trypomastigotes, the parasite infective forms, invade mammalian cells by a still poorly understood mechanism. Adhesion of tissue culture- derived trypomastigotes to the extracellular matrix (ECM) prior to cell invasion has been shown to be a relevant part of the process. Changes in phosphorylation, S-nitrosylation, and nitration levels of proteins, in the late phase of the interaction (2 h), leading to the reprogramming of both trypomastigotes metabolism and the DNA binding profile of modified histones, were described by our group. Here, the involvement of calcium signaling at a very early phase of parasite interaction with ECM is described. Increments in the intracellular calcium concentrations during trypomastigotes-ECM interaction depends on the Ca²⁺ uptake from the extracellular medium, since it is inhibited by EGTA or Nifedipine, an inhibitor of the L-type voltage gated Ca²⁺ channels and sphingosine-dependent plasma membrane Ca²⁺ channel, but not by Vanadate, an inhibitor of the plasma membrane Ca²⁺-ATPase. Furthermore, Nifedipine inhibits the invasion of host cells by tissue culture- derived trypomastigotes in a dose-dependent manner, reaching 95% inhibition at 100 µM Nifedipine. These data indicate the importance of both Ca²⁺ uptake from the medium and parasite-ECM interaction for host-cell invasion. Previous treatment of ECM with protease abolishes the Ca²⁺ uptake, further reinforcing the possibility that these events may be connected. The mitochondrion plays a relevant role in Ca²⁺ homeostasis in trypomastigotes during their interaction with ECM, as shown by the increment of the intracellular Ca²⁺ concentration in the presence of Antimycin A, in contrast to other calcium homeostasis disruptors, such as Cyclopiazonic acid for endoplasmic reticulum and Bafilomycin A for acidocalcisome. Total phosphatase activity in the parasite decreases in the presence of Nifedipine, EGTA, and Okadaic acid, implying a role of calcium in the phosphorylation level of proteins that are interacting with the ECM in tissue culture- derived trypomastigotes. In summary, we describe here the increment of Ca²⁺ at an early phase of the trypomastigotes interaction with ECM, implicating both nifedipine-sensitive Ca²⁺ channels in the influx of Ca²⁺ and the mitochondrion as the relevant organelle in Ca²⁺ homeostasis. The data unravel a complex sequence of events prior to host cell invasion itself.

Deletion of a Golgi protein in Trypanosoma cruzi reveals a critical role for Mn2+ in protein glycosylation needed for host cell invasion and intracellular replication

Trypanosoma cruzi is a protist parasite and the causative agent of American trypanosomiasis or Chagas disease. The parasite life cycle in its mammalian host includes an intracellular stage, and glycosylated proteins play a key role in host-parasite interaction facilitating adhesion, invasion and immune evasion. Here, we report that a Golgi-localized Mn²⁺-Ca²⁺/H⁺ exchanger of T. cruzi (TcGDT1) is required for efficient protein glycosylation, host cell invasion, and intracellular replication. The Golgi localization was determined by immunofluorescence and electron microscopy assays. TcGDT1 was able to complement the growth defect of Saccharomyces cerevisiae null mutants of its ortholog ScGDT1 but ablation of TcGDT1 by CRISPR/Cas9 did not affect the growth of the insect stage of the parasite. The defect in protein glycosylation was rescued by Mn²⁺ supplementation to the growth medium, underscoring the importance of this transition metal for Golgi glycosylation of proteins.

Trypanosoma cruzi is the etiologic agent of Chagas disease, which is endemic from North to South America and the most important cause of heart disease in Latin America. T. cruzi can infect most mammalian nucleated cells and its glycoproteins are needed for its adhesion to cells, and for host cell invasion. Efficient glycosylation of proteins in the Golgi complex requires cations as cofactors. In this work, we found that ablation of a Golgi localized cation transporter prevents normal protein glycosylation, host cell invasion, and intracellular replication, and that protein glycosylation can be rescued by Mn²⁺ but not by Ca²⁺, Mg²⁺, or Zn²⁺, revealing the importance of Mn²⁺ for host parasite interaction.

Disruption of Intracellular Calcium Homeostasis as a Therapeutic Target Against Trypanosoma cruzi

There is no effective cure for Chagas disease, which is caused by infection with the arthropod-borne parasite, Trypanosoma cruzi. In the search for new drugs to treat Chagas disease, potential therapeutic targets have been identified by exploiting the differences between the mechanisms involved in intracellular Ca²⁺ homeostasis, both in humans and in trypanosomatids. In the trypanosomatid, intracellular Ca²⁺ regulation requires the concerted action of three intracellular organelles, the endoplasmic reticulum, the single unique mitochondrion, and the acidocalcisomes. The single unique mitochondrion and the acidocalcisomes also play central roles in parasite bioenergetics. At the parasite plasma membrane, a Ca²⁺-⁻ATPase (PMCA) with significant differences from its human counterpart is responsible for Ca²⁺ extrusion; a distinctive sphingosine-activated Ca²⁺ channel controls Ca²⁺ entrance to the parasite interior. Several potential anti-trypansosomatid drugs have been demonstrated to modulate one or more of these mechanisms for Ca²⁺ regulation. The antiarrhythmic agent amiodarone and its derivatives have been shown to exert trypanocidal effects through the disruption of parasite Ca²⁺ homeostasis. Similarly, the amiodarone-derivative dronedarone disrupts Ca²⁺ homeostasis in T. cruzi epimastigotes, collapsing the mitochondrial membrane potential (ΔΨ_m), and inducing a large increase in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) from this organelle and from the acidocalcisomes in the parasite cytoplasm. The same general mechanism has been demonstrated for SQ109, a new anti-tuberculosis drug with potent trypanocidal effect. Miltefosine similarly induces a large increase in the [Ca²⁺]_i acting on the sphingosine-activated Ca²⁺ channel, the mitochondrion and acidocalcisomes. These examples, in conjunction with other evidence we review herein, strongly support targeting Ca²⁺ homeostasis as a strategy against Chagas disease.

P-type transport ATPases in Leishmania and Trypanosoma

P-type ATPases are critical to the maintenance and regulation of cellular ion homeostasis and membrane lipid asymmetry due to their ability to move ions and phospholipids against a concentration gradient by utilizing the energy of ATP hydrolysis. P-type ATPases are particularly relevant in human pathogenic trypanosomatids which are exposed to abrupt and dramatic changes in their external environment during their life cycles. This review describes the complete inventory of ion-motive, P-type ATPase genes in the human pathogenic Trypanosomatidae; eight Leishmania species (L. aethiopica, L. braziliensis, L. donovani, L. infantum, L. major, L. mexicana, L. panamensis, L. tropica), Trypanosoma cruzi and three Trypanosoma brucei subspecies (Trypanosoma brucei brucei TREU927, Trypanosoma brucei Lister strain 427, Trypanosoma brucei gambiense DAL972). The P-type ATPase complement in these trypanosomatids includes the P_1B (metal pumps), P_2A (SERCA, sarcoplasmic-endoplasmic reticulum calcium ATPases), P_2B (PMCA, plasma membrane calcium ATPases), P_2D (Na⁺ pumps), P_3A (H⁺ pumps), P₄ (aminophospholipid translocators), and P_5B (no assigned specificity) subfamilies. These subfamilies represent the P-type ATPase transport functions necessary for survival in the Trypanosomatidae as P-type ATPases for each of these seven subfamilies are found in all Leishmania and Trypanosoma species included in this analysis. These P-type ATPase subfamilies are correlated with current molecular and biochemical knowledge of their function in trypanosomatid growth, adaptation, infectivity, and survival.

Glucose Transporters and Virulence in Leishmania mexicana

Glucose transport plays important roles for in vitro growth of insect-stage promastigotes and especially for viability of intramacrophage mammalian host-stage amastigotes of Leishmania mexicana. However, the roles of the three distinct glucose transporters, GT1, GT2, and GT3, in parasite viability inside macrophages and virulence in mice have not been fully explored. Parasite lines expressing GT1 or GT2 alone were strongly impaired in growth inside macrophages, but lines expressing GT3 alone infected macrophages and caused lesions in mice as robustly as wild-type parasites. Notably, GT3 localizes to the endoplasmic reticulum of intracellular amastigotes, suggesting a potential role for salvage of glucose from that organelle for viability of infectious amastigotes. This study establishes the unique role of GT3 for parasite survival inside host macrophages and for robust virulence in infected animals.

Glucose transporters are important for viability and infectivity of the disease-causing amastigote stages of Leishmania mexicana. The Δgt1-3 null mutant, in which the 3 clustered glucose transporter genes, GT1, GT2, and GT3, have been deleted, is strongly impaired in growth inside macrophages in vitro. We have now demonstrated that this null mutant is also impaired in virulence in the BALB/c murine model of infection and forms lesions considerably more slowly than wild-type parasites. Previously, we established that amplification of the PIFTC3 gene, which encodes an intraflagellar transport protein, both facilitated and accompanied the isolation of the original Δgt1-3 null mutant generated in extracellular insect-stage promastigotes. We have now isolated Δgt1-3 null mutants without coamplification of PIFTC3. These amplicon-negative null mutants are further impaired in growth as promastigotes, compared to the previously described null mutants containing the PIFTC3 amplification. In contrast, the GT3 glucose transporter plays an especially important role in promoting amastigote viability. A line that expresses only the single glucose transporter GT3 grows as well inside macrophages and induces lesions in animals as robustly as do wild-type amastigotes, but lines expressing only the GT1 or GT2 transporters replicate poorly in macrophages. Strikingly, GT3 is restricted largely to the endoplasmic reticulum in intracellular amastigotes. This observation raises the possibility that GT3 may play an important role as an intracellular glucose transporter in the infectious stage of the parasite life cycle.

IMPORTANCE Glucose transport plays important roles for in vitro growth of insect-stage promastigotes and especially for viability of intramacrophage mammalian host-stage amastigotes of Leishmania mexicana. However, the roles of the three distinct glucose transporters, GT1, GT2, and GT3, in parasite viability inside macrophages and virulence in mice have not been fully explored. Parasite lines expressing GT1 or GT2 alone were strongly impaired in growth inside macrophages, but lines expressing GT3 alone infected macrophages and caused lesions in mice as robustly as wild-type parasites. Notably, GT3 localizes to the endoplasmic reticulum of intracellular amastigotes, suggesting a potential role for salvage of glucose from that organelle for viability of infectious amastigotes. This study establishes the unique role of GT3 for parasite survival inside host macrophages and for robust virulence in infected animals.

Membrane Proteins in Trypanosomatids Involved in Ca2+ Homeostasis and Signaling

Calcium ion (Ca2+) serves as a second messenger for a variety of cell functions in trypanosomes. Several proteins in the plasma membrane, acidocalcisomes, endoplasmic reticulum, and mitochondria are involved in its homeostasis and in cell signaling roles. The plasma membrane has a Ca2+ channel for its uptake and a plasma membrane-type Ca2+-ATPase (PMCA) for its efflux. A similar PMCA is also located in acidocalcisomes, acidic organelles that are the primary Ca2+ store and that possess an inositol 1,4,5-trisphosphate receptor (IP₃R) for Ca2+ efflux. Their mitochondria possess a mitochondrial calcium uniporter complex (MCUC) for Ca2+ uptake and a Ca2+/H⁺ exchanger for Ca2+ release. The endoplasmic reticulum has a sarcoplasmic-endoplasmic reticulum-type Ca2+-ATPase (SERCA) for Ca2+ uptake but no Ca2+ release mechanism has been identified. Additionally, the trypanosomatid genomes contain other membrane proteins that could potentially bind calcium and await further characterization.

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.

BAPTA-AM decreases cellular pH, inhibits acidocalcisome acidification and autophagy in amino acid-starved T. brucei

To investigate the role of Ca²⁺ signaling in starvation-induced autophagy in Trypanosoma brucei, the causative agent of human African trypanosomiasis, we used cell-permeant Ca²⁺ chelator BAPTA-AM and cell impermeant chelator EGTA, and examined the potential involvement of several intracellular Ca²⁺ signaling pathways in T. brucei autophagy. The results showed an unexpected effect of BAPTA-AM in decreasing cellular pH and inhibiting acidocalcisome acidification in starved cells. The implication of these results in the role of Ca²⁺ signaling and cellular/organellar pH in T. brucei autophagy is discussed.

Molecular Modeling of Fluorescent SERCA Biosensors

Molecular modeling and simulation are useful tools in structural biology, allowing the formulation of functional hypotheses and interpretation of spectroscopy experiments. Here, we describe a method to construct in silico models of a fluorescent fusion protein construct, where a cyan fluorescent protein (CFP) is linked to the actuator domain of the Sarco/Endoplasmic Reticulum Ca(2+)-ATPase (SERCA). This CFP-SERCA construct is a biosensor that can report on structural dynamics in the cytosolic headpiece of SERCA. Molecular modeling and FRET experiments allow us to generate new structural and mechanistic models that better describe the conformational landscape and regulation of SERCA. The methods described here can be applied to the creation of models for any fusion protein constructs and also describe the steps needed to simulate FRET results using molecular models.

Calcium signaling in trypanosomatid parasites

Calcium ion (Ca(2+)) is an important second messenger in trypanosomatids and essential for their survival although prolonged high intracellular Ca(2+) levels lead to cell death. As other eukaryotic cells, trypanosomes use two sources of Ca(2+) for generating signals: Ca(2+) release from intracellular stores and Ca(2+) entry across the plasma membrane. Ca(2+) release from intracellular stores is controlled by the inositol 1,4,5-trisphosphate receptor (IP3R) that is located in acidocalcisomes, acidic organelles that are the primary Ca(2+) reservoir in these cells. A plasma membrane Ca(2+)-ATPase controls the cytosolic Ca(2+) levels and a number of pumps and exchangers are responsible for Ca(2+) uptake and release from intracellular compartments. The trypanosomatid genomes contain a wide variety of signaling and regulatory proteins that bind Ca(2+) as well as many Ca(2+)-binding proteins that await further characterization. The mitochondrial Ca(2+) transporters of trypanosomatids have an important role in the regulation of cell bioenergetics and flagellar Ca(2+) appears to have roles in sensing the environment. In trypanosomatids in which an intracellular life cycle is present, Ca(2+) signaling is important for host cell invasion.

Golgi UDP-GlcNAc:polypeptide O-α-N-Acetyl-d-glucosaminyltransferase 2 (TcOGNT2) regulates trypomastigote production and function in Trypanosoma cruzi

All life cycle stages of the protozoan parasite Trypanosoma cruzi are enveloped by mucin-like glycoproteins which, despite major changes in their polypeptide cores, are extensively and similarly O-glycosylated. O-Glycan biosynthesis is initiated by the addition of αGlcNAc to Thr in a reaction catalyzed by Golgi UDP-GlcNAc:polypeptide O-α-N-acetyl-d-glucosaminyltransferases (ppαGlcNAcTs), which are encoded by TcOGNT1 and TcOGNT2. We now directly show that TcOGNT2 is associated with the Golgi apparatus of the epimastigote stage and is markedly downregulated in both differentiated metacyclic trypomastigotes (MCTs) and cell culture-derived trypomastigotes (TCTs). The significance of downregulation was examined by forced continued expression of TcOGNT2, which resulted in a substantial increase of TcOGNT2 protein levels but only modestly increased ppαGlcNAcT activity in extracts and altered cell surface glycosylation in TCTs. Constitutive TcOGNT2 overexpression had no discernible effect on proliferating epimastigotes but negatively affected production of both types of trypomastigotes. MCTs differentiated from epimastigotes at a low frequency, though they were apparently normal based on morphological and biochemical criteria. However, these MCTs exhibited an impaired ability to produce amastigotes and TCTs in cell culture monolayers, most likely due to a reduced infection frequency. Remarkably, inhibition of MCT production did not depend on TcOGNT2 catalytic activity, whereas TCT production was inhibited only by active TcOGNT2. These findings indicate that TcOGNT2 downregulation is important for proper differentiation of MCTs and functioning of TCTs and that TcOGNT2 regulates these functions by using both catalytic and noncatalytic mechanisms.

The streamlined genome of Phytomonas spp. relative to human pathogenic kinetoplastids reveals a parasite tailored for plants

Members of the family Trypanosomatidae infect many organisms, including animals, plants and humans. Plant-infecting trypanosomes are grouped under the single genus Phytomonas, failing to reflect the wide biological and pathological diversity of these protists. While some Phytomonas spp. multiply in the latex of plants, or in fruit or seeds without apparent pathogenicity, others colonize the phloem sap and afflict plants of substantial economic value, including the coffee tree, coconut and oil palms. Plant trypanosomes have not been studied extensively at the genome level, a major gap in understanding and controlling pathogenesis. We describe the genome sequences of two plant trypanosomatids, one pathogenic isolate from a Guianan coconut and one non-symptomatic isolate from Euphorbia collected in France. Although these parasites have extremely distinct pathogenic impacts, very few genes are unique to either, with the vast majority of genes shared by both isolates. Significantly, both Phytomonas spp. genomes consist essentially of single copy genes for the bulk of their metabolic enzymes, whereas other trypanosomatids e.g. Leishmania and Trypanosoma possess multiple paralogous genes or families. Indeed, comparison with other trypanosomatid genomes revealed a highly streamlined genome, encoding for a minimized metabolic system while conserving the major pathways, and with retention of a full complement of endomembrane organelles, but with no evidence for functional complexity. Identification of the metabolic genes of Phytomonas provides opportunities for establishing in vitro culturing of these fastidious parasites and new tools for the control of agricultural plant disease.

Some plant trypanosomes, single-celled organisms living in phloem sap, are responsible for important palm diseases, inducing frequent expensive and toxic insecticide treatments against their insect vectors. Other trypanosomes multiply in latex tubes without detriment to their host. Despite the wide range of behaviors and impacts, these trypanosomes have been rather unceremoniously lumped into a single genus: Phytomonas. A battery of molecular probes has been used for their characterization but no clear phylogeny or classification has been established. We have sequenced the genomes of a pathogenic phloem-specific Phytomonas from a diseased South American coconut palm and a latex-specific isolate collected from an apparently healthy wild euphorb in the south of France. Upon comparison with each other and with human pathogenic trypanosomes, both Phytomonas revealed distinctive compact genomes, consisting essentially of single-copy genes, with the vast majority of genes shared by both isolates irrespective of their effect on the host. A strong cohort of enzymes in the sugar metabolism pathways was consistent with the nutritional environments found in plants. The genetic nuances may reveal the basis for the behavioral differences between these two unique plant parasites, and indicate the direction of our future studies in search of effective treatment of the crop disease parasites.

Intracellular calcium channels in protozoa

Ca(2+)-signaling pathways and intracellular Ca(2+) channels are present in protozoa. Ancient origin of inositol 1,4,5-trisphosphate receptors (IP3Rs) and other intracellular channels predates the divergence of animals and fungi as evidenced by their presence in the choanoflagellate Monosiga brevicollis, the closest known relative to metazoans. The first protozoan IP3R cloned, from the ciliate Paramecium, displays strong sequence similarity to the rat type 3 IP3R. This ciliate has a large number of IP3- and ryanodine(Ry)-like receptors in six subfamilies suggesting the evolutionary adaptation to local requirements for an expanding diversification of vesicle trafficking. IP3Rs have also been functionally characterized in trypanosomatids, where they are essential for growth, differentiation, and establishment of infection. The presence of the mitochondrial calcium uniporter (MCU) in a number of protozoa indicates that mitochondrial regulation of Ca(2+) signaling is also an early appearance in evolution, and contributed to the discovery of the molecular nature of this channel in mammalian cells. There is only sequence evidence for the occurrence of two-pore channels (TPCs), transient receptor potential Ca(2+) channels (TRPCs) and intracellular mechanosensitive Ca(2+)-channels in Paramecium and in parasitic protozoa.

Inositol 1,4,5-trisphosphate receptor regulates replication, differentiation, infectivity and virulence of the parasitic protist Trypanosoma cruzi

In animals, inositol 1,4,5-trisphosphate receptors (IP3 Rs) are ion channels that play a pivotal role in many biological processes by mediating Ca(2+) release from the endoplasmic reticulum. Here, we report the identification and characterization of a novel IP3 R in the parasitic protist, Trypanosoma cruzi, the pathogen responsible for Chagas disease. DT40 cells lacking endogenous IP3 R genes expressing T. cruzi IP3 R (TcIP3 R) exhibited IP3 -mediated Ca(2+) release from the ER, and demonstrated receptor binding to IP3 . TcIP3 R was expressed throughout the parasite life cycle but the expression level was much lower in bloodstream trypomastigotes than in intracellular amastigotes or epimastigotes. Disruption of two of the three TcIP3 R gene loci led to the death of the parasite, suggesting that IP3 R is essential for T. cruzi. Parasites expressing reduced or increased levels of TcIP3 R displayed defects in growth, transformation and infectivity, indicating that TcIP3 R is an important regulator of the parasite's life cycle. Furthermore, mice infected with T. cruzi expressing reduced levels of TcIP3 R exhibited a reduction of disease symptoms, indicating that TcIP3 R is an important virulence factor. Combined with the fact that the primary structure of TcIP3 R has low similarity to that of mammalian IP3 Rs, TcIP3 R is a promising drug target for Chagas disease.

Endoplasmic reticulum calcium regulates the retrotranslocation of Trypanosoma cruzi calreticulin to the cytosol

For most secretory pathway proteins, crossing the endoplasmic reticulum (ER) membrane is an irreversible process. However, in some cases this flow can be reversed. For instance, misfolded proteins retained in the ER are retrotranslocated to the cytosol to be degraded by the proteasome. This mechanism, known as ER associated degradation (ERAD), is exploited by several bacterial toxins to gain access to the cytosol. Interestingly, some ER resident proteins can also be detected in the cytosol or nucleus, calreticulin (CRT) being the most studied. Here we show that in Trypanosoma cruzi a minor fraction of CRT localized to the cytosol. ER calcium depletion, but not increasing cytosolic calcium, triggered the retrotranslocation of CRT in a relatively short period of time. Cytosolic CRT was subsequently degraded by the proteasome. Interestingly, the single disulfide bridge of CRT is reduced when the protein is located in the cytosol. The effect exerted by ER calcium was strictly dependent on the C-terminal domain (CRT-C), since a CRT lacking it was totally retained in the ER, whereas the localization of an unrelated protein fused to CRT-C mirrored that of endogenous CRT. This finding expands the regulatory mechanisms of protein sorting and may represent a new crossroad between diverse physiological processes.

Characterization of Ca2+uptake in a subcellular membrane fraction ofHerpetomonassp. promastigotes

ATP-dependent Ca2+uptake was studied in a subcellular fraction fromHerpetomonassp. prepared by mechanical disruption and using45Ca2+as a tracer. The uptake was stimulated by Ca2+with a K0·5of 0·1 μmand a Hill number (nH)=2·8±0·4. The Ca2+-dependent ATP hydrolysis was optimal at pH 7·0 and had a Ca2+dependence identical to uptake. The uptake was highly stimulated by oxalate whereas calmodulin had no activating effect. ATP stimulated Ca2+uptake with a biphasic pattern that resembled the curves described for the purified preparations of rabbit sarcoplasmic reticulum. The ATP stimulation is described as the sum of two Michaelis-Menten curves with Km1=0·25±0·19 μmand Km2=29·6±6·8 μm. GTP or UTP could also promote Ca2+uptake, but with less efficiency than ATP. Vanadate inhibited the uptake with low apparent affinity. Thapsigargin and cyclopiazonic acid were almost ineffective. The Ca2+uptake was insensitive to H+ionophores and to bafilomycin suggesting no participation of acidocalcisomes. The results are comparable to those obtained using cells permeabilized with digitonin and using arsenaze III as Ca2+indicator. The Ca2+uptake activity described here seems to belong to the endoplasmic reticulum ofHerpetomonassp. and is suitable for further studies on the mechanisms of calcium homeostasis in parasites.

Proteomics in Trypanosoma cruzi--localization of novel proteins to various organelles

The completion of the genome sequence of Trypanosoma cruzi has been followed by several studies of protein expression, with the long-term aim to obtain a complete picture of the parasite proteome. We report a proteomic analysis of an organellar cell fraction from T. cruzi CL Brener epimastigotes. A total of 396 proteins were identified by LC-MS/MS. Of these, 138 were annotated as hypothetical in the genome databases and the rest could be assigned to several metabolic and biosynthetic pathways, transport, and structural functions. Comparative analysis with a whole cell proteome study resulted in the validation of the expression of 173 additional proteins. Of these, 38 proteins previously reported in other stages were not found in the only large-scale study of the total epimastigote stage proteome. A selected set of identified proteins was analyzed further to investigate gene copy number, sequence variation, transmembrane domains, and targeting signals. The genes were cloned and the proteins expressed with a c-myc epitope tag in T. cruzi epimastigotes. Immunofluorescence microscopy revealed the localization of these proteins in different cellular compartments such as ER, acidocalcisome, mitochondrion, and putative cytoplasmic transport or delivery vesicles. The results demonstrate that the use of enriched subcellular fractions allows the detection of T. cruzi proteins that are undetected by whole cell proteomic methods.

Artemisinin induces calcium-dependent protein secretion in the protozoan parasite Toxoplasma gondii

Intracellular calcium controls several crucial cellular events in apicomplexan parasites, including protein secretion, motility, and invasion into and egress from host cells. The plant compound thapsigargin inhibits the sarcoplasmic-endoplasmic reticulum calcium ATPase (SERCA), resulting in elevated calcium and induction of protein secretion in Toxoplasma gondii. Artemisinins are natural products that show potent and selective activity against parasites, making them useful for the treatment of malaria. While the mechanism of action is uncertain, previous studies have suggested that artemisinin may inhibit SERCA, thus disrupting calcium homeostasis. We cloned the single-copy gene encoding SERCA in T. gondii (TgSERCA) and demonstrate that the protein localizes to the endoplasmic reticulum in the parasite. In extracellular parasites, TgSERCA partially relocalized to the apical pole, a highly active site for regulated secretion of micronemes. TgSERCA complemented a calcium ATPase-defective yeast mutant, and this activity was inhibited by either thapsigargin or artemisinin. Treatment of T. gondii with artemisinin triggered calcium-dependent secretion of microneme proteins, similar to the SERCA inhibitor thapsigargin. Artemisinin treatment also altered intracellular calcium in parasites by increasing the periodicity of calcium oscillations and inducing recurrent, strong calcium spikes, as imaged using Fluo-4 labeling. Collectively, these results demonstrate that artemisinin perturbs calcium homeostasis in T. gondii, supporting the idea that Ca2+-ATPases are potential drug targets in parasites.

Artemisinins inhibit Trypanosoma cruzi and Trypanosoma brucei rhodesiense in vitro growth

Artemisinin compounds inhibit in vitro growth of cultured Trypanosoma cruzi and Trypanosoma brucei rhodesiense at concentrations in the low micromolar range. Artemisinin also inhibits calcium-dependent ATPase activity in T. cruzi membranes, suggesting a mode of action via membrane pumps. Artemisinins merit further investigation as chemotherapeutic options for these pathogens.

P-type Proton ATPases are Involved in Intracellular Calcium and Proton Uptake in the Plant Parasite Phytomonas francai

The use of digitonin to permeabilize the plasma membrane of promastigotes of Phytomonas francai allowed the identification of two non-mitochondrial Ca(2+) compartments; one sensitive to ionomycin and vanadate (neutral or alkaline), possibly the endoplasmic reticulum, and another sensitive to the combination of nigericin plus ionomycin (acidic), possibly the acidocalcisomes. A P-type (phospho-intermediate form) Ca(2+)-ATPase activity was found to be responsible for intracellular Ca(2+) transport in these cells, with no evidence of a mitochondrial Ca(2+) transport activity. ATP-driven acidification of internal compartments in cell lysates and cells mechanically permeabilized was assayed spectrophotometrically with acridine orange. This activity was inhibited by low concentrations of vanadate and digitonin, was insensitive to bafilomycin A(1), and stimulated by Na(+) ions. Taken together, our results indicate that P-type ATPases are involved in intracellular Ca(2+) and H(+) transport in promastigotes of P. francai.

Evaluation of the Presence of a Thapsigargin-Sensitive Calcium Store in Trypanosomatids Using Trypanosoma evansi as a Model

Ca2+ plays an important role in the regulation of several important activities in different trypanosomatids. These parasites possess a Ca2+ transport system in the endoplasmic reticulum (ER) involved in Ca2+ homeostasis, which has been reported to be insensitive to thapsigargin, a classical inhibitor of the sarcoplasmic-ER Ca2+ adenosine triphosphatase (ATPase) (SERCA) in most eukaryotic cells. However, currently there is a controversy regarding the existence of a thapsigargin-sensitive ER Ca2+ store in these parasites. Therefore, we decided to explore the effect of this inhibitor using different methodological approaches. First, we selected Trypanosoma evansi as a parasite model to warrant the homogeneity of the population because this parasite has only a single life cycle, i.e., bloodstream-form trypomastigotes. Second, we compared the thapsigargin effect on Ca2+ homeostasis by spectrophotometrical Ca2+ measurements using 3 different approaches: whole-cell populations, cells that have been permeabilized by treatment with digitonin, and intact single cells. Our results demonstrate that a low concentration of thapsigargin induces Ca2+ release from intracellular Ca2+ stores in this parasite, which can be observed independently of the method used. Furthermore, the addition of thapsigargin before or after nigericin did not abolish its effect, showing that thapsigargin acts specifically on the ER. In conclusion, our results indicate the presence of a nonmitochondrial thapsigargin-sensitive Ca2+ store in T. evansi.

Functional expression of heterologous proteins in yeast: insights into Ca2+signaling and Ca2+-transporting ATPases

The baker's yeast Saccharomyces cerevisiae is a well-developed, versatile, and widely used model organism. It offers a compact and fully sequenced genome, tractable genetics, simple and inexpensive culturing conditions, and, importantly, a conservation of basic cellular machinery and signal transducing pathways with higher eukaryotes. In this review, we describe recent technical advances in the heterologous expression of proteins in yeast and illustrate their application to the study of the Ca2+homeostasis machinery, with particular emphasis on Ca2+-transporting ATPases. Putative Ca2+-ATPases in the newly sequenced genomes of organisms such as parasites, plants, and vertebrates have been investigated by functional complementation of an engineered yeast strain lacking endogenous Ca2+pumps. High-throughput screens of mutant phenotypes to identify side chains critical for ion transport and selectivity have facilitated structure-function analysis, and genomewide approaches may be used to dissect cellular pathways involved in Ca2+transport and trafficking. The utility of the yeast system is demonstrated by rapid advances in the study of the emerging family of Golgi/secretory pathway Ca2+,Mn2+-ATPases (SPCA). Functional expression of human SPCA1 in yeast has provided insight into the physiology, novel biochemical characteristics, and subcellular localization of this pump. Haploinsufficiency of SPCA1 leads to Hailey-Hailey disease (HDD), a debilitating blistering disorder of the skin. Missense mutations, identified in patients with HHD, may be conveniently assessed in yeast for loss-of-function phenotypes associated with the disease.

Rotenone-sensitive mitochondrial potential in Phytomonas serpens: electrophoretic Ca(2+) accumulation

Phytomonas sp. are flagellated trypanosomatid plant parasites that cause diseases of economic importance in plantations of coffee, oil palm, cassava and coconuts. Here we investigated Ca(2+) uptake by the vanadate-insensitive compartments using permeabilized Phytomonas serpens promastigotes. This uptake occurs at a rate of 1.13+/-0.23 nmol Ca(2+) mg x protein(-1) min(-1). It is completely abolished by the H(+) ionophore FCCP and by valinomycin and nigericin. It is also inhibited by 2 microM ruthenium red, which, at this low concentration, is known to inhibit the mitochondrial calcium uniport. Furthermore, salicylhydroxamic acid (SHAM) and propylgallate, specific inhibitors of the alternative oxidase in plant and parasite mitochondria, are also effective as inhibitors of the Ca(2+) transport. These compounds abolish the membrane potential that is monitored with safranine O. Rotenone, an inhibitor of NADH-CoQ oxidoreductase, can also dissipate 100% of the membrane potential. It is suggested that the mitochondria of P. serpens can be energized via oxidation of NADH in a pathway involving the NADH-CoQ oxidoreductase and the alternative oxidase to regenerate the ubiquinone. The electrochemical H(+) gradient can be used to promote Ca(2+) uptake by the mitochondria.

Calcium regulation in protozoan parasites

The calcium ion (Ca(2+)) is used as a major signaling molecule in a diverse range of eukaryotic cells including several human parasitic protozoa, such as Trypanosoma cruzi, Trypanosoma brucei, Leishmania spp, Plasmodium spp, Toxoplasma gondii, Cryptosporidium parvum, Entamoeba histolytica, Giardia lamblia and Trichomonas vaginalis. Ca(2+) is critical for invasion of intracellular parasites, and its cytosolic concentration is regulated by the concerted operation of several transporters present in the plasma membrane, endoplasmic reticulum, mitochondria and acidocalcisomes. Recent findings have shed light on the function of these transporters, the roles that they play in cellular metabolism and their potential use for targeting them for new therapies.

Calcium signaling in excystation of the early diverging eukaryote, Giardia lamblia

Excystation of Giardia lamblia, which initiates infection, is a poorly understood but dramatic differentiation induced by physiological signals from the host. Our data implicate a central role for calcium homeostasis in excystation. Agents that alter cytosolic Ca(2+) levels (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-tetra(acetyloxymethyl) ester, a Ca(2+) channel blocker, Ca(2+) ionophores, and thapsigargin) strongly inhibit excystation. Treatment of Giardia with thapsigargin raised intracellular Ca(2+) levels, and peak Ca(2+) responses increased with each stage of excystation, consistent with the kinetics of inhibition. Fluorescent thapsigargin localized to a likely Ca(2+) storage compartment in cysts. The ability to sequester ions in membrane-bounded compartments is a hallmark of the eukaryotic cell. These studies support the existence of a giardial thapsigargin-sensitive Ca(2+) storage compartment resembling the sarcoplasmic/endoplasmic reticulum calcium ATPase pump-leak system and suggest that it is important in regulation of differentiation and appeared early in the evolution of eukaryotic cells. Calmodulin antagonists also blocked excystation. The divergent giardial calmodulin localized to the eight flagellar basal bodies/centrosomes, like protein kinase A. Inhibitor kinetics suggest that protein kinase A signaling triggers excystation, whereas calcium signaling is mainly required later, for parasite activation and emergence. Thus, the basal bodies may be a cellular control center to coordinate the resumption of motility and cytokinesis in excystation.

An atypical protein disulfide isomerase from the protozoan parasite Leishmania containing a single thioredoxin-like domain

In higher eukaryotes, secretory proteins are under the quality control of the endoplasmic reticulum for their proper folding and release into the secretory pathway. One of the proteins involved in the quality control is protein disulfide isomerase, which catalyzes the formation of protein disulfide bonds. As a first step toward understanding the endoplasmic reticulum quality control of secretory proteins in lower eukaryotes, we have isolated a protein disulfide isomerase gene from the protozoan parasite Leishmania donovani. The parasite enzyme shows high sequence homology with homologs from other organisms. However, unlike the four thioredoxin-like domains found in most protein disulfide isomerases, of which two contain an active site, the leishmanial enzyme possesses only one active site present in a single thioredoxin-like domain. When expressed in Escherichia coli, the recombinant parasite enzyme shows both oxidase and isomerase activities. Replacement of the two cysteins with alanines in its active site results in loss of both enzymatic activities. Further, overexpression of the mutated/inactive form of the parasite enzyme in L. donovani significantly reduced their release of secretory acid phosphatases, suggesting that this single thioredoxin-like domain protein disulfide isomerase could play a critical role in the Leishmania secretory pathway.

Expression of calreticulin P-domain results in impairment of secretory pathway in Leishmania donovani and reduced parasite survival in macrophages

The secretory proteins of Leishmania are thought to be involved in the parasite survival inside the insect vector or mammalian host. It is clear from studies in higher eukaryotes that proper folding in the endoplasmic reticulum and targeting out of the endoplasmic reticulum is critical for the function of secretory proteins. The endoplasmic reticulum chaperones such as calreticulin play an important role in the quality control of secretory proteins. However, very little is known about the secretory pathway of trypanosomatid parasites such as Leishmania. In the present study, we show that overexpression of the P-domain of Leishmania donovani calreticulin in transfected L. donovani resulted in a significant reduction in the secretion of the parasite secretory acid phosphatases. This effect is associated with an intracellular accumulation of active enzyme in these transfected parasites. In addition, parasites expressing the P-domain calreticulin showed a significant decrease in survival inside human macrophages. This study suggests that altering the function of an endoplasmic reticulum chaperone such as calreticulin in Leishmania may affect the targeting of proteins that are associated with the virulence of the parasite during their trafficking through the parasite secretory pathway.

Secretory pathway of trypanosomatid parasites

The Trypanosomatidae comprise a large group of parasitic protozoa, some of which cause important diseases in humans. These include Trypanosoma brucei (the causative agent of African sleeping sickness and nagana in cattle), Trypanosoma cruzi (the causative agent of Chagas' disease in Central and South America), and Leishmania spp. (the causative agent of visceral and [muco]cutaneous leishmaniasis throughout the tropics and subtropics). The cell surfaces of these parasites are covered in complex protein- or carbohydrate-rich coats that are required for parasite survival and infectivity in their respective insect vectors and mammalian hosts. These molecules are assembled in the secretory pathway. Recent advances in the genetic manipulation of these parasites as well as progress with the parasite genome projects has greatly advanced our understanding of processes that underlie secretory transport in trypanosomatids. This article provides an overview of the organization of the trypanosomatid secretory pathway and connections that exist with endocytic organelles and multiple lytic and storage vacuoles. A number of the molecular components that are required for vesicular transport have been identified, as have some of the sorting signals that direct proteins to the cell surface or organelles in the endosome-vacuole system. Finally, the subcellular organization of the major glycosylation pathways in these parasites is reviewed. Studies on these highly divergent eukaryotes provide important insights into the molecular processes underlying secretory transport that arose very early in eukaryotic evolution. They also reveal unusual or novel aspects of secretory transport and protein glycosylation that may be exploited in developing new antiparasite drugs.