Calcium homeostasis in Trypanosoma brucei. Identification of a pH-sensitive non-mitochondrial calcium pool

Vacuolar ATPase depletion contributes to dysregulation of endocytosis in bloodstream forms of Trypanosoma brucei

Background

Vacuolar H⁺-ATPase (V-ATPase) is a highly conserved protein complex which hydrolyzes ATP and pumps protons to acidify vacuolar vesicles. Beyond its role in pH maintenance, the involvement of V-ATPase in endocytosis is well documented in mammals and plants but is less clear in Trypanosoma brucei.

Methods

In this study, the subcellular localization of V-ATPase subunit B (TbVAB) of T. brucei was assessed via in situ N-terminal YFP-tagging and immunofluorescence assays. Transgenic bloodstream forms (BSF) of T. brucei were generated which comprised either a V-ATPase subunit B (TbVAB) conditional knockout or a V-ATPase subunit A (TbVAA) knockdown. Acridine orange and BCECF-AM were employed to assess the roles of V-ATPase in the pH regulation of BSF T. brucei. The endocytic activities of three markers were also characterized by flow cytometry analyses. Furthermore, trypanosomes were counted from trypanolysis treatment groups (either containing 1% or 5% NHS) and endocytosed trypanosome lytic factor (TLF) was also analyzed by an immunoblotting assay.

Results

TbVAB was found to localize to acidocalcisomes, lysosomes and probably also to endosomes of BSF of T. brucei and was demonstrated to be essential for cell growth. TbVAB depletion neutralized acidic organelles at 24 hours post-tetracycline depletion (hpd), meanwhile the steady state intracellular pH increased from 7.016 ± 0.013 to 7.422 ± 0.058. Trypanosomes with TbVAB depletion at 24 hpd were found to take up more transferrin (2.068 ± 0.277 fold) but less tomato lectin (49.31 ± 22.57%) by endocytosis, while no significant change was detected in dextran uptake. Similar endocytic dysregulated phenotypes were also observed in TbVAA knockdown cells. In addition, TbVAB depleted trypanosomes showed a low uptake of TLF and exhibited less sensitive to lysis in both 1% and 5% NHS treatments.

Conclusions

TbVAB is a key component of V-ATPase and was found to play a key function in endocytosis as well as exhibiting different effects in a receptor/cargo dependent manner in BSF of T. brucei. Besides vacuolar alkalinization, the dysregulation of endocytosis in TbVAB depleted T. brucei is considered to contribute to the reduced sensitivity to lysis by normal human serum.

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.

Trypanosoma brucei: trypanocidal and cell swelling activities of lasalocid acid

Chemotherapeutic treatment of human and animal trypanosomiasis is unsatisfactory because only a few drugs are available. As these drugs have poor efficacy and cause adverse reactions, more effective and tolerable medications are needed. As the polyether ionophore antibiotic lasalocid acid is used as medicated feed additive in cattle, the compound was tested for its trypanocidal and cytotoxic activity against bloodstream forms of Trypanosoma brucei and human myeloid HL-60 cells. The concentrations required of lasalocid acid to reduce the growth rate of trypanosomes by 50% and to kill the parasites were 1.75 and 10 μM, respectively. The ionophore displayed also cytotoxic activity against HL-60 cells but the human cells were about 10 to 14 times less sensitive indicating moderate selectivity. As the trypanocidal mechanism of action of polyether ionophore antibiotics is due to a sodium influx-induced cell swelling, the effect of lasalocid acid on cell volume change in bloodstream-form trypanosomes was investigated. Interestingly, lasalocid acid induced a much faster cell swelling in trypanosomes than the more trypanocidal related ionophore salinomycin. These results support further investigations of lasalocid acid and derivatives thereof as potential agents against African trypanosomiasis.

Dissecting the catalytic mechanism of Trypanosoma brucei trypanothione synthetase by kinetic analysis and computational modeling

In pathogenic trypanosomes, trypanothione synthetase (TryS) catalyzes the synthesis of both glutathionylspermidine (Gsp) and trypanothione (bis(glutathionyl)spermidine (T(SH)₂)). Here we present a thorough kinetic analysis of Trypanosoma brucei TryS in a newly developed phosphate buffer system at pH 7.0 and 37 °C, mimicking the physiological environment of the enzyme in the cytosol of bloodstream parasites. Under these conditions, TryS displays K_m values for GSH, ATP, spermidine, and Gsp of 34, 18, 687, and 32 μm, respectively, as well as K_i values for GSH and T(SH)₂ of 1 mm and 360 μm, respectively. As Gsp hydrolysis has a K_m value of 5.6 mm, the in vivo amidase activity is probably negligible. To obtain deeper insight in the molecular mechanism of TryS, we have formulated alternative kinetic models, with elementary reaction steps represented by linear kinetic equations. The model parameters were fitted to the extensive matrix of steady-state data obtained for different substrate/product combinations under the in vivo-like conditions. The best model describes the full kinetic profile and is able to predict time course data that were not used for fitting. This system's biology approach to enzyme kinetics led us to conclude that (i) TryS follows a ter-reactant mechanism, (ii) the intermediate Gsp dissociates from the enzyme between the two catalytic steps, and (iii) T(SH)₂ inhibits the enzyme by remaining bound at its product site and, as does the inhibitory GSH, by binding to the activated enzyme complex. The newly detected concerted substrate and product inhibition suggests that TryS activity is tightly regulated.

New insights into roles of acidocalcisomes and contractile vacuole complex in osmoregulation in protists

While free-living protists are usually subjected to hyposmotic environments, parasitic protists are also in contact with hyperosmotic habitats. Recent work in one of these parasites, Trypanosoma cruzi, has revealed that its contractile vacuole complex, which usually collects and expels excess water as a mechanism of regulatory volume decrease after hyposmotic stress, has also a role in cell shrinking when the cells are submitted to hyperosmotic stress. Trypanosomes also have an acidic calcium store rich in polyphosphate (polyP), named the acidocalcisome, which is involved in their response to osmotic stress. Here, we review newly emerging insights on the role of acidocalcisomes and the contractile vacuole complex in the cellular response to hyposmotic and hyperosmotic stresses. We also review the current state of knowledge on the composition of these organelles and their other roles in calcium homeostasis and protein trafficking.

In vitro and in vivo antitrypanosomal activities of three peptide antibiotics: leucinostatin A and B, alamethicin I and tsushimycin

In the course of our screening for antitrypanosomal compounds from soil microorganisms, as well as from the antibiotics library of the Kitasato Institute for Life Sciences, we found three peptide antibiotics, leucinostatin (A and B), alamethicin I and tsushimycin, which exhibited potent or moderate antitrypanosomal activity. We report here the in vitro and in vivo antitrypanosomal properties and cytotoxicities of leucinostatin A and B, alamethicin I and tsushimycin compared with suramin. We also discuss their possible mode of action. This is the first report of in vitro and in vivo trypanocidal activity of leucinostatin A and B, alamethicin I and tsushimycin.

Optical and pharmacological tools to investigate the role of mitochondria during oxidative stress and neurodegeneration

Mitochondria are critical for cellular adenosine triphosphate (ATP) production; however, recent studies suggest that these organelles fulfill a much broader range of tasks. For example, they are involved in the regulation of cytosolic Ca(2+) levels, intracellular pH and apoptosis, and are the major source of reactive oxygen species (ROS). Various reactive molecules that originate from mitochondria, such as ROS, are critical in pathological events, such as ischemia, as well as in physiological events such as long-term potentiation, neuronal-vascular coupling and neuronal-glial interactions. Due to their key roles in the regulation of several cellular functions, the dysfunction of mitochondria may be critical in various brain disorders. There has been increasing interest in the development of tools that modulate mitochondrial function, and the refinement of techniques that allow for real time monitoring of mitochondria, particularly within their intact cellular environment. Innovative imaging techniques are especially powerful since they allow for mitochondrial visualization at high resolution, tracking of mitochondrial structures and optical real time monitoring of parameters of mitochondrial function. The techniques discussed include classic imaging techniques, such as rhodamine-123, the highly advanced semi-conductor nanoparticles (quantum dots), and wide field microscopy as well as high-resolution multiphoton imaging. We have highlighted the use of these techniques to study mitochondrial function in brain tissue and have included studies from our laboratories in which these techniques have been successfully applied.

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.

Physiological and morphological evidences for the presence acidocalcisomes in Trypanosoma evansi: single cell fluorescence and 31P NMR studies

A new Ca(2+) intracellular store, the acidocalcisome, has been reported in trypanosomatids. It has been characterized physiologically as a Ca(2+) store sensitive to nigericin. The Ca(2+)/H(+)-ATPase is the system responsible for Ca(2+) accumulation, which depends on a pH gradient formed by ATP- and PPi-dependent proton pumps. In this work we present physiological and morphological evidences for the presence of acidocalcisomes in Trypanosoma evansi. The parasites were purified and loaded with the fluorescent dye Fura 2-AM in order to detect the intracellular changes of Ca(2+) levels in individual cells. The simultaneous incubation of T. evansi cells with ionomycin and nigericin led to large release of Ca(2+) (ca. 200 nM) from intracellular stores, which was not observed with either agent alone. On the other hand, no enhancement of the nigericin-induced Ca(2+) release was observed in the presence of oligomycin. Additionally, the pretreatment with bafilomycin decreases the nigericin-induced Ca(2+) release. These results confirm the presence of an intracellular non-mitochondrial acidic Ca(2+) storage compartment. These results suggest that H(+)-ATPase is involved in the process of Ca(2+) accumulation into the acidocalcisomes. Furthermore, the cells loaded with acridine orange exhibited abundant fluorescent vacuoles, which were sensitive to nigericin or bafilomycin A(1). Electronic transmission microscopy observations demonstrated the presence of electron dense particles in the parasites. High levels of inorganic pyrophosphate and triphosphate were detected in perchloric acid extracts of T. evansi by high resolution 31P NMR. Taken together, these results present the first evidence for the presence of acidocalcisomes in T. evansi.

Significant differences between procyclic and bloodstream forms of Trypanosoma brucei in the maintenance of their plasma membrane potential

The plasma membrane potential (deltapsi) of procyclic and bloodstream trypomastigotes of Trypanosoma brucei was studied using the potentiometric fluorescent dye bisoxonol. Our results suggest that a proton pump plays a significant role in the regulation of deltapsi in procyclic and bloodstream forms, as evidenced by depolarization of the plasma membrane by H(+)-ATPase inhibitors (e.g. dicyclohexylcarbo-diimide, N-ethylmaleimide, diethylstilbestrol, and bafilomycin A1). In bloodstream stages the plasma membrane was significantly depolarized by ouabain only when the cells were incubated in sodium-rich buffers indicating that a sodium pump was being inhibited. In contrast, ouabain had no effect on the deltapsi of the procyclic stages in a sodium-rich buffer. However, it induced an additional significant depolarization in these stages when their plasma membrane was already partially depolarized by the H(+)-ATPase inhibitor dicyclohexylcarbo-diimide, indicating the presence of an ouabain-sensitive sodium pump whose activity is masked by the H(+)-ATPase. Unlike procyclic forms, the deltapsi of bloodstream-stage trypomastigotes was markedly sensitive to extracellular Na+ and K+ concentrations. Thus, there are significant differences between procyclic and bloodstream forms in the maintenance of the deltapsi and in their permeability to cations.

Trypanosoma evansi: a convenient model for studying intracellular Ca(2+) homeostasis using fluorometric ratio imaging from single parasites

The aim of this work was to measure, for the first time, the basal cytosolic Ca(2+) levels of Trypanosoma evansi and to explore the possibility of observing changes in the intracellular Ca(2+) concentration ([Ca(2+)](i)) using fluorescence ratio imaging techniques in single isolated parasites of this species. Under appropriate loading conditions, the high intracellular levels of the Ca(2+) fluorescence probe Fura-2 permits resolution, in real time, of single parasite [Ca(2+)](i) signals. Measurements of the basal [Ca(2+)](i) indicate that homeostatic mechanisms maintain [Ca(2+)](i) at 106 +/- 38 (n = 32) nM in the presence of 2 mM extracellular calcium. The resting [Ca(2+)](i) was unaffected by changes in extracellular Ca(2+) in the range from 0 to 10 mM. The Ca(2+) ionophore A23187 induced a large increase in [Ca(2+)](i) which (i) reached a steady state value even in the simultaneous presence of both external calcium and ionophore and (ii) returned to base line upon removal of extracellular Ca(2+). A dose-response curve of the protonophore nigericin shows that T. evansi contains an important pH-sensitive intracellular pool which may be released by this drug with a K(1/2) of 8 microM. These data demonstrate that this parasite contains highly efficient systems to control [Ca(2+)](i). Finally, our results, with the use of sera as source of an antibody-complement to induce Ca(2+) entry, demonstrate that it is possible to resolve fast [Ca(2+)](i) signals in single parasites from T. evansi.

Hydrogen peroxide induces apoptosis-like death in Leishmania donovani promastigotes

Leishmania donovani promastigotes introduced into the bloodstream by sandfly vectors, are exposed to reactive oxygen species like H2O2 during phagocytosis by the host macrophages. H2O2 can induce promastigote death, but the mechanism of induction of this death is not known. Studies presented in this paper demonstrate that exposure to 4 mM H2O2 results in a pattern of promastigote death that shares many features with metazoan apoptosis. Motility and cell survival in these parasites show a gradual decline with increasing doses of H2O2. Features common to metazoan apoptosis, such as nuclear condensation, DNA fragmentation with accompanying DNA ladder formation and loss of cell volume, are observed after exposure to 4 mM H2O2. Within 30 minutes of the exposure, there is a significant increase in the ability of the cell lysates to cleave the fluorogenic tetrapeptide acetyl-Asp-Glu-Val-Asp-7-amino-4-trifluoromethyl coumarin, which is a substrate for the CED-3/CPP32 group of proteases. Pretreatment of cells with a specific inhibitor of CED-3/CPP32 group of proteases, Z-DEVD-FMK, reduces the number of cells showing apoptosis-like features, prevents DNA breakage and inhibits cleavage of a PARP-like protein. Activation of the caspase-like proteases is followed at 2 hours by the cleavage of a poly(ADP)ribose-polymerase-like protein and a reduction in intracellular glutathione concentration. DNA breakdown as detected by TdT labelling of cells and agarose gel electrophoresis is visible at 6 hours. Taken together, the above data show for the first time that there is a distinct pathway for apoptosis-like death in L. donovani.

Leishmania plasma membrane Mg2+-ATPase is a H+/K+-antiporter involved in glucose symport. Studies with sealed ghosts and vesicles of opposite polarity

Experiments from other laboratories conducted with Leishmania donovani promastigote cells had earlier indicated that the plasma membrane Mg2+-ATPase of the parasite is an extrusion pump for H+. Taking advantage of the pellicular microtubular structure of the plasma membrane of the organism, we report procedures for obtaining sealed ghost and sealed everted vesicle of defined polarity. Rapid influx of H+ into everted vesicles was found to be dependent on the simultaneous presence of ATP (1 mm) and Mg2+ (1 mm). Excellent correspondence between rate of H+ entry and the enzyme activity clearly demonstrated the Mg2+-ATPase to be a true H+ pump. H+ entry into everted vesicle was strongly inhibited by SCH28080 (IC50 = approximately 40 microm) and by omeprazole (IC50 = approximately 50 microm), both of which are characteristic inhibitors of mammalian gastric H+,K+-ATPase. H+ influx was completely insensitive to ouabain (250 microm), the typical inhibitor of Na+,K+-ATPase. Mg2+-ATPase activity could be partially stimulated with K+ (20 mm) that was inhibitable (>85%) with SCH28080 (50 microm). ATP-dependent rapid efflux of 86Rb+ from preloaded vesicles was completely inhibited by preincubation with omeprazole (150 microm) and by 5,5'-dithiobis-(2-nitrobenzoic acid) (1 mm), an inhibitor of the enzyme. Assuming Rb+ to be a true surrogate for K+, an ATP-dependent, electroneutral stoichiometric exchange of H+ and K+(1:1) was established. Rapid and 10-fold active accumulation of [U-(14)C]2-deoxyglucose in sealed ghosts could be observed when an artificial pH gradient (interior alkaline) was imposed. Rapid efflux of [U-(14)C]d-glucose from preloaded everted vesicles could also be initiated by activating the enzyme, with ATP. Taken together, the plasma membrane Mg2+-ATPase has been identified as an electroneutral H+/K+ antiporter with some properties reminiscent of the gastric H+,K+-ATPase. This enzyme is possibly involved in active accumulation of glucose via a H+-glucose symport system and in K+ accumulation.

Two special organelles found in Trypanosoma cruzi

We review here two unique organelles from Trypanosoma cruzi. One of them is the acidocalcisome, cytoplasmic vacuoles containing a very high Ca2+ concentration and a Ca2+ - H+ translocating ATPase activity, present in all trypanosomatids. The other organelle is the reservosome, site of accumulation of endocytosed macromolecules, very rich in cysteine proteinase, that is present only in epimastigote forms of trypanosomes belonging to the Schyzotrypanum sub-genus.

Hydrogen ion gradients across the mitochondrial, endosomal and plasma membranes in bloodstream forms of trypanosoma brucei solving the three-compartment problem

Conditions for the use of both [14C]methylamine and 5, 5-dimethyl[14C]oxa-azolidine-2,4-dione (DMO) to measure the H+ concentration of intracellular compartments of monomorphic long thin bloodstream forms of Trypanosoma brucei were established. Neither probe was actively transported or bound to internal components of the cell and both probes equilibrated passively with a t1/2 close to 8 min. DMO was excluded from cells, while methylamine was accumulated but not metabolized. Solution of the three-compartment problem revealed that, when cells were respiring aerobically on glucose at an external pH of 7.5, the cytoplasmic pH was in the range 6.99-7.03, the pH of the mitochondrial matrix was 7.71-7.73, and the algebraic average pH of the various endosomal compartments was 5.19-5.50. Similar values were found when cells were respiring aerobically on glycerol. However, bloodstream forms of T. brucei could not maintain a constant internal H+ concentration outside the external pH range 7.0-7.5, and no evidence for the presence of an H+/Na+ exchanger was found. Full motility and levels of pyruvate production were maintained as the external pH was raised as high as 9.5, suggesting that these cells tolerate significant internal alkalinisation. However, both motility and pyruvate production were severely inhibited under acidic conditions, and the cells deteriorated rapidly below an external pH of 6.5. Physiologically, the plasma membrane of T. brucei had low permeability to H+ and the internal pH was unaffected by changes in Deltapsip, which is dominated by the potassium diffusion potential. However, in the presence of FCCP, the internal pH fell rapidly about 0.5 pH unit and came into equilibrium with Deltapsip. Oligomycin abolished the mitochondrial pH gradient (DeltapHm) selectively, whereas chloroquine abolished only the endosomal pH gradient (DeltapHe). The pH gradient across the plasma membrane (DeltapHp) alone could be abolished by careful osmotic swelling of cells. The plasma membrane had an inwardly directed proton-motive force (DeltaPp) of -52 mV and an inwardly directed sodium-motive force (DeltaNp) of -149 mV, whereas the mitochondrial inner membrane had only an inwardly directed DeltaPm of -195 mV. The pH gradient across the endosomal membranes was not accompanied by an electrical gradient. Consequently, endosomal membranes had an algebraically average outwardly directed DeltaPl within the range + 89 to +110 mV, depending on the measurement method.

Calcium mobilization by arachidonic acid in trypanosomatids

A recent report (Eintracht J, Maathai R, Mellors A, Ruben L. Calcium entry in Trypanosoma brucei is regulated by phospholipase A, and arachidonic acid, Biochem J 1998:336:659-66) provided evidence that calcium entry in Trypanosoma brucei bloodstream trypomastigotes is regulated via a signaling pathway involving phospholipase A2-mediated generation of arachidonic acid and stimulation of a plasma membrane-located calcium channel. Here we show that Ca2+ influx in T. brucei procyclic trypomastigotes, Leishmania donovani promastigotes and T. cruzi amastigotes was also stimulated in a dose-dependent manner (50-400 nM) by the amphiphilic peptide melittin. This effect was blocked by the phospholipase A, inhibitor 3-(4-octadecyl)-benzoylacrylic acid. The unsaturated fatty acid arachidonic acid, in the range of 10-75 microM, induced Ca2+ entry by a mechanism sensitive to LaCl3. However, both melittin and arachidonic acid induced an increase in [Ca2+]i in T. brucei procyclic trypomastigotes incubated in Ca2+-free medium implying Ca2+ mobilization from intracellular stores. This hypothesis was supported by experiments showing that arachidonic acid promoted Ca2+ release from the acidocalcisomes of these cells. The results showing changes in mitochondrial membrane potential, release of acridine orange and Ca2+ from the acidocalcisomes and Ca2+ transport across the plasma membrane suggest that in addition to the possible stimulation of a Ca2+ channel-mediated process, arachidonic acid, in the range of concentrations used here, have other nonspecific effects on the trypanosomatids membranes.

Acidocalcisome: A novel Ca2+ storage compartment in trypanosomatids and apicomplexan parasites

Acidocalcisomes are novel acidic Ca2+ storage organelles found in trypanosomatids and apicomplexan parasites, abundant in the intracellular stages of these parasites, and characterized by their high electron density, and high content of phosphorus, Ca2+, Mg2+, Na+ and Zn2+. A number of energy-utilizing pumps and exchangers have been found in these organelles, which underlines their importance in the homeostasis of different elements, as discussed here by Roberto Docampo and Silvia Moreno.

Acidic calcium pools in intraerythrocytic malaria parasites

Calcium uptake by permeabilized P. chabaudi malaria parasites was measured at the trophozoite stage to assess calcium accumulation by the parasite organelles. As determined with 45Ca2+, the total calcium in the parasite was found to be 11 pmoles/10(7) cells. When the K+/H+ uncoupling agent, nigericin was present, this level fell to 6.5 pmoles/10(7) cells. A similar regulatory mechanism operates in P. falciparum, since addition of nigericin to intact parasites in calcium free-medium resulted in a transient elevation of free calcium in the parasite cytosol, as judged by fluorescent imaging of single cells loaded with the calcium indicator fluo-3,AM. 7-Chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD-Cl) and monensin, inhibitors of H+ ATPases and K+/H+ ionophore respectively, induced calcium elevation in fluo-3, AM-labeled intact P. chabaudi parasites. We conclude that malaria parasites utilize acidic intracellular compartments to regulate their cytosolic free calcium concentration.

Trypanosoma brucei: the dynamics of calcium movement between the cytosol, nucleus, and mitochondrion of intact cells

Targeted aequorins (CYT-AEQ, NUC-AEQ, and MT-AEQ) were used to measure Ca2+ concentrations within organelles of live trypanosomes. We determined that the nuclear envelope is a slight barrier to the free diffusion of Ca2+. This situation was especially evident when Ca2+ influx across the plasma membrane was stimulated with 200 nM melittin ([Ca2+]cyt = 1.2 +/- 0.4 microM and [Ca2+]nuc = 0.85 +/- 0.15 microM). By contrast, the ionophores nigericin (2.7 microM) or monensin (2 microg/ml) were used to induce Ca2+ efflux from the acidic storage compartment. Small transient elevations in [Ca2+]cyt were observed (peaking at 660 +/- 200 and 580 +/- 120 nM, respectively). Parallel and equivalent changes in [Ca2+1]nuc were recorded. Active accumulation of Ca2+ into the nucleus was not observed. Nigericin or monensin did not disrupt mitochondrial Ca2+ transport in vivo. Instead, the mitochondrion actively sequestered large quantities of Ca2+ in the presence of these ionophores, with peak values of 2.7 +/- 1.4 and 4.4 +/- 1.1 microM, respectively. Overall, these data demonstrate that significant quantities of Ca2+ enter the nucleus following influx across the plasma membrane or following efflux from an intracellular acidic storage compartment. However, the magnitude of change for [Ca2+]cyt and [Ca2+]nuc is small compared to the total amount of exchangeable Ca2+ since the majority of released Ca2+ is actively sequestered by the mitochondrion.

Selective transfer of calcium from an acidic compartment to the mitochondrion of Trypanosoma brucei. Measurements with targeted aequorins

Organelle compartments are used by cells as reservoirs of exchangeable Ca2+ and as Ca2+ buffers. The following study uses recombinant aequorins (CYT-AEQ and MT-AEQ) to measure the dynamics of Ca2+ flux between organelles in procyclic forms of the pathogenic protozoan, Trypanosoma brucei. Emphasis is placed on the exchange between an acidic Ca2+ reservoir and the mitochondrion. The mammalian mitochondrial targeting sequence was functional in trypanosomes as determined by immunoblots, immunolocalizations, and the observation that MT-AEQ was in a compartment whose Ca2+ uptake was inhibited 82% with carbonyl cyanide p-trifluoromethoxyphenylhydrazone and KCN. The resting level of free calcium ion concentration in the mitochondrion ([Ca2+]mit) was slightly higher than that in the cytoplasm ([Ca2+]cyt) (400 +/- 50 nM and 290 +/- 40 nM, respectively). Melittin (125 nM) disrupted Ca2+ homeostasis by inducing Ca2+ influx across the plasma membrane. [Ca2+]cyt became slightly elevated to 410 +/- 100 nM, whereas [Ca2+]mit was selectively increased approximately 12-fold, with a broad peak at 4.8 +/- 1.9 microM. At the peak, the mitochondrion contained approximately three times more free Ca2+ than the cytosol. However, mitochondrial retention of the Ca2+ was transient. Similar selective transport into the mitochondrion was observed when Ca2+ efflux from an acidic compartment was induced with monensin (2 microg/ml) in the presence of 5 mM EGTA. [Ca2+]cyt was transiently elevated to 400 +/- 50 nM, whereas [Ca2+]mit was elevated to 3.3+/-1.3 microM. When cells were treated sequentially with monensin (2 microg/ml) and then melittin (200 nM), mitochondrial Ca2+ transport was normal. However, [Ca2+]cyt became elevated to a level that was 1.4-fold higher than with melittin alone. Overall, these data demonstrate that the trypanosome mitochondrion is not a reservoir of exchangeable Ca2+ in the resting cell. However, Ca2+ is selectively channeled to the mitochondrion from the plasma membrane or acidic Ca2+ storage compartment. Additionally, the acidic compartment contributes to maintenance of Ca2+ homeostasis in response to melittin.

Regulation of the intracellular pH in the protozoan parasite Trypanosoma brucei brucei

The mechanisms regulating the intracellular pH (pHi) in both forms of Trypanosoma brucei brucei (cultured cells) were investigated using the fluorescent probe 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF). The pHi values measured were 7.22+/-0.03 in the procyclics and 7.40+/-0.05 in the bloodstream form. In the presence of 24mM HCO3-, pHi values were slightly higher in both forms of trypanosomes suggesting a bicarbonate-linked pH regulation. pHi was more stable in procyclics (between 7.15 and 7.30 in the external pH range 6.4-7.6) than in the bloodstream forms. The amiloride analogue tested decreased pHi, suggesting Na+-driven Na+/H+ antiporters. H+-ATPases also seem to be involved in pHi regulation since the inhibitors N-ethylmaleimide (1 mM) and N,N'-dicyclohexylcarbodiimide (80 microM) induced a rapid acidification in both forms of trypanosomes. Addition of pyruvate caused a cytosol acidification in the bloodstream form only confirming the existence of a diffusion-facilitated carrier for pyruvate, with the cotransport of H+. Our results show that, although similar pH regulation mechanisms seem to exist in both forms of trypanosomes, the procyclics can regulate efficiently their pHi and consequently their plasma membrane potential whereas the bloodstream forms cannot always maintain their pHi and are easily depolarized following a small acid load.

Nuclear calcium flux in Trypanosoma brucei can be quantified with targeted aequorin

The following study was undertaken to determine if calcium ions move from the plasma membrane to the nucleus of Trypanosoma brucei. Nuclear and cytosolic calcium flux was measured with the calcium sensitive photoprotein, aequorin which was targeted to various locations in stably transformed procyclic cells. Immunoblots revealed that the recombinant proteins, CYT-AEQ and NUC-AEQ were translated in transformants, and that CYT-AEQ was contained in a soluble fraction. Immunolocalization demonstrated that NUC-AEQ was contained within the trypanosome nucleus. To evaluate calcium movement from the plasma membrane to the nucleus in live trypanosomes, aequorin was reconstituted in vivo with coelenterazine and luminescence was recorded. The resting levels of [Ca2+]cyt and [Ca2+]nuc were similar (314 +/- 43 and 287 +/- 28 nM, respectively). When calcium influx across the plasma membrane was initiated with 2 microM ionomycin, [Ca2+]cyt and [Ca2+]nuc each became elevated in parallel to a new steady state which was approximately 2-fold above the resting level. Compound 48/80 initiated a calcium flux across the plasma membrane by a different mechanism from ionomycin, and in a manner that was inhibited by the calcium channel antagonist, La3+. Compound 48/80 (8 micrograms/ml) transiently elevated [Ca2+]cyt to 1.73 +/- 0.3 microM over the course of 20 s, and also generated a transient rise in [Ca2+]nuc which peaked at 1.32 + 0.29 microM over the same time course. Overall, these data demonstrate that calcium moves into and out of the trypanosome nucleus in a manner which closely parallels changes in [Ca2+]cyt. A small calcium ion gradient between nucleus and cytoplasm was also observed.

Calcium influx in Trypanosoma brucei can be induced by amphiphilic peptides and amines

The following study was undertaken to determine whether an inducible calcium influx pathway is present in intact bloodstream forms of Trypanosoma brucei. Fura-2 fluorescence was used to demonstrate that amphiphilic peptides and amines, including melittin, mastoparan and compound 48/80, each produced a dose dependent calcium influx across the plasma membrane. Calcium influx did not result from general disruption of membrane integrity, since a corresponding influx of ethidium bromide or other divalent cations was not observed. Instead, the calcium influx was selectively blocked by the calcium channel antagonists, La3+, Cd2+ or Ni2+, and was not affected by the Na+ channel antagonists, tetrodotoxin or amiloride. Activation of the trypanosome calcium influx pathway was dependent upon an intact membrane potential, and the rise in intracellular free calcium concentration ([Ca2+]i) was reversed upon membrane depolarization with gramicidin D. Changes in Ins(1,4,5)P3 did not accompany the calcium influx. Overall, these data provide the first evidence of an inducible calcium influx pathway in T. brucei, and describe methods to selectively manipulate this pathway.

The requirements for G1 checkpoint progression of Trypanosoma brucei S 427 clone 1

Trypanosoma brucei S 427 clone 1 accumulated in G1 when incubated under growth-limiting conditions. Further incubation of the G1-restricted organisms in medium containing 10% fetal bovine serum (FBS) and 2 mM hydroxyurea resulted in their reversible arrest after a G1 checkpoint beyond which serum was not required for progress into and through S. Progress of the G1-restricted T. brucei through the G1 checkpoint was linear and required continuous incubation with exogenous serum growth factors. These were principally low and high density lipoproteins; both lipoproteins triggered G1 progression in a dose- and time-dependent manner whilst their removal by immunoaffinity chromatography severely reduced the capacity of FBS to stimulate G1 progression. Serum-induced progress of T. brucei through G1 was Ca(2+)-independent, but required gene transcription, protein synthesis, and continuous kinase activity that was inhibited by tyrphostin 51 and DAPH 1 which typically inhibit epidermal growth factor receptor protein tyrosine kinase activity. The tyrphostin 51-sensitive catalytic activity was not required for T. brucei protein synthesis, glycolysis, or S phase progression but was required for tyrosine phosphorylation of several polypeptides, none of which was specifically associated with serum-induced G1 progression.

Simultaneous but independent activation of adenylate cyclase and glycosylphosphatidylinositol-phospholipase C under stress conditions in Trypanosoma brucei

Previous observations suggested a concomitant relationship between the release of the variant surface glycoprotein (VSG) and the activation of adenylate cyclase in the bloodstream form of the parasitic protozoan Trypanosoma brucei. In order to evaluate this hypothesis, adenylate cyclase activity was measured in live trypanosomes subjected to different treatments known to induce the shedding of the VSG coat, namely low pH and trypsin digestion. In both cases adenylate cyclase activation occurred in parallel with the release of the VSG. The latter was found to be mediated by the glycosylphosphatidylinositol-specific phospholipase C that cleaves the glycosylphosphatidylinositol anchor of the protein (VSG lipase). Furthermore, both adenylate cyclase and VSG release were activated by the incubation of trypanosomes with specific inhibitors of protein kinase C, suggesting a repressive role for protein kinase C on both VSG lipase and adenylate cyclase activities. Significantly, in mutant trypanosomes lacking VSG lipase, adenylate cyclase was activated under conditions where VSG release did not occur. Moreover,VSG release was also found to occur in the absence of activation of the cyclase, as observed in the presence of low concentration of the thiol modifying reagent p-chloromercuriphenylsulfonic acid. These observations provide the first demonstration that release of the VSG in response to cellular stress is mediated by the VSG lipase and that while both release of the VSG and activation of adenylate cyclase occur in response to the same stimuli they are not obligatorily coupled.

Temperature-induced rapid increase in cytoplasmic free Ca2+ in pathogenic Leishmania donovani promastigotes

We demonstrate that the cytoplasmic free Ca2+ concentration in three virulent Leishmania donovani promastigote strains is maintained at 20-30 nM level whereas the avirulent promastigotes maintain [Ca2+]i at 80-100 nM. Rapid exposure of virulent promastigotes to higher physiological temperature increases [Ca2+]i many-fold. Use of CaCl2, EGTA and Mn2+ confirmed that both release of Ca2+ from internal pool(s) and influx from outside milieu are responsible for this increase in [Ca2+]i. Application of oligomycin, CN-, and nigericin indicated the non-mitochondrial pHi sensitive pool as the possible intracellular origin for internal Ca2+ release. Exposure of these cells to acidic environment had no influence on [Ca2+]i. In contrast, the avirulent promastigotes or freshly isolated amastigotes had shown no effect of heat-shock or pH shock on [Ca2+]i of these cells. Our results indicate that the upward shift in temperature may play a role in signal transduction events in morphogenetic transformation of L. donovani promastigotes that involves mobilization of Ca2+ in cytoplasm.

Identification of a 44 kDa protein localized within the endoplasmic reticulum of Trypanosoma brucei brucei

Immunoaffinity chromatography and gel electrophoresis were used to isolate a 44 kDa protein that was bound to a 72 kDa chaperone in Trypanosoma brucei brucei. A polyclonal antiserum to the 44 kDa protein was raised in rats and employed in conjunction with chromatography using DEAE-cellulose, Sephacryl S-300, and hydroxyapatite to purify the protein from membranes of bloodstream forms of the trypanosomes. Immunoblot analysis using this antiserum revealed a protein doublet of 44/45 kDa in T. b. brucei and a single protein band of 53 kDa in almost equivalent amounts throughout the life-cycle stages of T. congolense. Indirect immunofluorescence using affinity-purified antibodies specific for the 44 kDa protein showed labelling of the perinuclear area and reticular system extending throughout the parasites, suggesting that this protein was located in the endoplasmic reticulum. Localization of the 44 kDa molecule in the endoplasmic reticulum was confirmed by immunoelectron microscopy. Protease protection experiments demonstrated that the epitopes bound by antibody were buried within the membrane or towards the lumenal face of the endoplasmic reticulum. Ruthenium Red overlay of nitrocellulose blots containing the 44/45 kDa doublet suggested that the molecules have the potential to bind calcium. The N-terminal amino acid sequence of the 44 kDa protein showed no sequence similarity to any proteins in the database.

Membrane-related processes and overall energy metabolism in Trypanosoma brucei and other kinetoplastid species

An electrochemical proton gradient exists across the plasma membrane and the mitochondrial membrane of the bloodstream form of Trypanosoma brucei. The membrane potential across the plasma membrane and the regulation of the internal pH depend on the temperature. Leishmania donovani regulates its internal pH and maintains a constant electrochemical proton gradient across its plasma membrane under all conditions examined. The mitochondrion of the T. brucei bloodstream form is energized, even though the reactions taking place in it do not result in net ATP synthesis and the Kreb's cycle and the respiratory chain are absent. Glucose is transported across the plasma membrane of T. brucei by a facilitated diffusion carrier, that can transport a wider range of substrates than its mammalian counterparts. Pyruvate exits the cell via a facilitated diffusion transporter as well. Conflicting evidence exists for the mechanism of glucose transport in L. donovani; biochemical evidence suggests proton/glucose symport, while facilitated diffusion is indicated by physiological data.

Evaluation of rhodamine 123 as a probe for monitoring mitochondrial function in Trypanosoma brucei spp

Rhodamine 123, a membrane potential-specific dye, has been evaluated as a probe to monitor the function of the mitochondrion in long slender bloodstream and procyclic trypomastigotes of several Trypanosoma brucei spp. By epifluorescence microscopy, mitochondrial development has been followed in long slender bloodstream and procyclic organisms stained with rhodamine 123. To photograph stained long slender bloodstream forms, it was necessary to develop a method to completely immobilize viable organisms. In both parasite forms, as the cell cycle progressed, the mitochondrion developed from a thread-like structure to a highly branched organelle. A dramatic reorganization occurred preceding cytokinesis to produce two progeny thread-like structures which were partitioned into newly formed daughter cells. The organelle within the long slender trypomastigote was found to stain optimally at 0.3 microgram/ml of rhodamine 123, while the procyclic form required 3.0 micrograms/ml. The results suggest that the plasma membrane potential is higher in the long slender parasite than in the procyclic form. The effects of inhibitors that disrupt mitochondrial function were examined in long slender and procyclic parasites, and some of these agents were shown to affect rhodamine 123 accumulation and retention. In long slender trypomastigotes the trypanosome alternative oxidase does not appear to be coupled to proton pumping, whereas in procyclic organisms the effects of inhibitors indicate that this oxidase may be coupled to a pathway that is branched preceding an antimycin A1-sensitive site.

Trypanosoma brucei: the tumor promoter thapsigargin stimulates calcium release from an intracellular compartment in slender bloodstream forms

Maintenance of calcium homeostasis is a critical activity of eukaryotic cells. Homeostatic pathways stabilize intracellular free calcium concentrations ([Ca2+]i) at the resting level and provide the source of mobilized calcium for cellular activation. We have measured calcium release from intracellular pools within bloodstream forms of Trypanosoma brucei to better understand homeostatic pathways which operate in these organisms. Fura-2 and 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein were used to quantitate [Ca2+]i and intracellular pH (pHi), respectively. We report that the tumor promoter, thapsigargin, elevated [Ca2+]i by 50-75 nM. Mn2+ quench experiments demonstrated that the source of calcium was intracellular. No change in pHi was associated with the release of calcium from this compartment. In contrast, nigericin released approximately three-fold more calcium than thapsigargin from a pH-sensitive, intracellular pool. The nigericin-sensitive pool was nonmitochondrial. The effects of thapsigargin and nigericin on [Ca2+]i were additive, regardless of the order in which the treatment was given. We conclude that at least two pools of exchangeable calcium occur in bloodstream forms of T. brucei. One pool is sensitive to thapsigargin and apparently resides within the endoplasmic reticulum, while the nigericin-sensitive pool is nonmitochondrial and is of unknown origin.

Purification of novel calcium binding proteins from Trypanosoma brucei: properties of 22-, 24- and 38-kilodalton proteins

The present study was undertaken to systematically purify calcium binding proteins (CaBPs) from homogenates of Trypanosoma brucei. This work is important since CaBPs either serve as intracellular calcium buffers or mediate cellular response to calcium signals. Disruption of either process should be lethal to trypanosomes. We report that the 45Ca-gel overlay assay can be used to detect CaBPs following fractionation on DE-52, phenyl-Sepharose, Mono-Q, and Superose 12. Specific CaBPs of 22, 24, and 38 kDa were purified. Each of these proteins associated with 45Ca under denaturing and non-denaturing conditions. An approximate Kd for calcium of 8 microM was calculated for 22-kDa CaBP. None of the trypanosome CaBPs were related to known calcium binding protein families. They did not associate with hydrophobic interaction columns or cellular membranes in a calcium-dependent way, nor cross-react with 2 separate antibodies against annexin consensus sequences. A synthetic peptide corresponding to amino terminal residues 16-30 of 22-kDa CaBP was used to generate polyclonal antibodies. Immunoblots identified 22-kDa CaBP in African trypanosomes but not in other Kinetoplastidae or mammalian cells. Nonetheless, significant homology (58%) was observed between the amino terminal 37 residues of 22-kDa CaBP and the amino terminus of translationally controlled p21 from mammalian tumor cells. The present study is the first to apply systemic fractionation techniques to identify the complement of CaBPs in T. brucei. We conclude that novel CaBPs other than calmodulin and annexin family members contribute towards calcium pathways in these organisms.