Regulation of Intracellular Calcium in Promastigotes of the Human Protozoan Parasite Leishmania donovani

The emerging paradigm of calcium homeostasis as a new therapeutic target for protozoan parasites

Calcium channels (CCs), a group of ubiquitously expressed membrane proteins, are involved in many pathophysiological processes of protozoan parasites. Our understanding of CCs in cell signaling, organelle function, cellular homeostasis, and cell cycle control has led to improved insights into their structure and functions. In this article, we discuss CCs characteristics of five major protozoan parasites Plasmodium, Leishmania, Toxoplasma, Trypanosoma, and Cryptosporidium. We provide a comprehensive review of current antiparasitic drugs and the potential of using CCs as new therapeutic targets. Interestingly, previous studies have demonstrated that human CC modulators can kill or sensitize parasites to antiparasitic drugs. Still, none of the parasite CCs, pumps, or transporters has been validated as drug targets. Information for this review draws from extensive data mining of genome sequences, chemical library screenings, and drug design studies. Parasitic resistance to currently approved therapeutics is a serious and emerging threat to both disease control and management efforts. In this article, we suggest that the disruption of calcium homeostasis may be an effective approach to develop new anti-parasite drug candidates and reduce parasite resistance.

Allicin Induces Calcium and Mitochondrial Dysregulation Causing Necrotic Death in Leishmania

Background

Allicin has shown antileishmanial activity in vitro and in vivo. However the mechanism of action underlying its antiproliferative effect against Leishmania has been virtually unexplored. In this paper, we present the results obtained in L.infantum and a mechanistic basis is proposed.

Methodology/Principal Finding

Exposure of the parasites to allicin led to high Ca²⁺ levels and mitochondrial reactive oxygen species (ROS), collapse of the mitochondrial membrane potential, reduced production of ATP and elevation of cytosolic ROS. The incubation of the promastigotes with SYTOX Green revealed that decrease of ATP was not associated with plasma membrane permeabilization. Annexin V and propidium iodide (PI) staining indicated that allicin did not induce phospholipids exposure on the plasma membrane. Moreover, DNA agarose gel electrophoresis and TUNEL analysis demonstrated that allicin did not provoke DNA fragmentation. Analysis of the cell cycle with PI staining showed that allicin induced cell cycle arrest in the G₂/M phase.

Conclusions/Significance

We conclude that allicin induces dysregulation of calcium homeostasis and oxidative stress, uncontrolled by the antioxidant defense of the cell, which leads to mitochondrial dysfunction and a bioenergetic catastrophe leading to cell necrosis and cell cycle arrest in the premitotic phase.

Leishmaniasis is a vectorial parasitic disease caused by flagellate organisms from the genus Leishmania. Infection is present in over 80 countries and visceral forms are the second most fatal human parasitic disease. Control relies on chemotherapy but available drugs have important shortcomings such as toxicity, side effects, unaffordable price of the safest presentations and increasing reports of parasite resistance and clinical failures. Thus, new drugs are needed. Allicin, a molecule obtained from garlic, has shown antiproliferative effect against different cancer cells, bacteria, fungi and Protista including Leishmania. Insofar its mechanism of action is poorly known. Our results with L.infantum point towards allicin inducing high levels of intracellular calcium, redox inbalance, and mitochondrial dysfunction with reduction of ATP. These events lead to cell necrosis without evidence of apoptotic-like markers. The proposed model suggests the potential use of allicin against leishmaniasis, alone or in combination with other drugs with different mechanisms of action.

Calmodulin regulates dimerization, motility, and lipid binding of Leishmania myosin XXI

Myosin XXI is the only myosin expressed in Leishmania parasites. Although it is assumed that it performs a variety of motile functions, the motor's oligomerization states, cargo-binding, and motility are unknown. Here we show that binding of a single calmodulin causes the motor to adopt a monomeric state and to move actin filaments. In the absence of calmodulin, nonmotile dimers that cross-linked actin filaments were formed. Unexpectedly, structural analysis revealed that the dimerization domains include the calmodulin-binding neck region, essential for the generation of force and movement in myosins. Furthermore, monomeric myosin XXI bound to mixed liposomes, whereas the dimers did not. Lipid-binding sections overlapped with the dimerization domains, but also included a phox-homology domain in the converter region. We propose a mechanism of myosin regulation where dimerization, motility, and lipid binding are regulated by calmodulin. Although myosin-XXI dimers might act as nonmotile actin cross-linkers, the calmodulin-binding monomers might transport lipid cargo in the parasite.

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.

Identification of intracellular and plasma membrane calcium channel homologues in pathogenic parasites

Ca²⁺ channels regulate many crucial processes within cells and their abnormal activity can be damaging to cell survival, suggesting that they might represent attractive therapeutic targets in pathogenic organisms. Parasitic diseases such as malaria, leishmaniasis, trypanosomiasis and schistosomiasis are responsible for millions of deaths each year worldwide. The genomes of many pathogenic parasites have recently been sequenced, opening the way for rational design of targeted therapies. We analyzed genomes of pathogenic protozoan parasites as well as the genome of Schistosoma mansoni, and show the existence within them of genes encoding homologues of mammalian intracellular Ca²⁺ release channels: inositol 1,4,5-trisphosphate receptors (IP₃Rs), ryanodine receptors (RyRs), two-pore Ca²⁺ channels (TPCs) and intracellular transient receptor potential (Trp) channels. The genomes of Trypanosoma, Leishmania and S. mansoni parasites encode IP₃R/RyR and Trp channel homologues, and that of S. mansoni additionally encodes a TPC homologue. In contrast, apicomplexan parasites lack genes encoding IP₃R/RyR homologues and possess only genes encoding TPC and Trp channel homologues (Toxoplasma gondii) or Trp channel homologues alone. The genomes of parasites also encode homologues of mammalian Ca²⁺influx channels, including voltage-gated Ca²⁺ channels and plasma membrane Trp channels. The genome of S. mansoni also encodes Orai Ca²⁺ channel and STIM Ca²⁺ sensor homologues, suggesting that store-operated Ca²⁺ entry may occur in this parasite. Many anti-parasitic agents alter parasite Ca²⁺ homeostasis and some are known modulators of mammalian Ca²⁺ channels, suggesting that parasite Ca²⁺ channel homologues might be the targets of some current anti-parasitic drugs. Differences between human and parasite Ca²⁺ channels suggest that pathogen-specific targeting of these channels may be an attractive therapeutic prospect.

Calcium-dependent proteolytic activity of a cysteine protease caldonopain is detected during Leishmania infection

A calcium-activated protease caldonopain in the cytosolic fraction of Leishmania donovani has been found to digest different endogenous proteins when subjected to SDS-PAGE. Gelatin-embedded gel electrophoresis confirms presence of calcium-dependent protease activity. Ca(2+) affects proteolytic activity after 10 h. When host-parasite interaction was conducted in vitro, caldonopain was found to be active after 10 h of incubation with calcium. A 67-kDa protein is specifically digested during this time and two new proteins of 45 and 36 kDa appeared in SDS-PAGE electrophoregram. This belated action of calcium towards protease activity may be pre-requisite to facilitate invasion of host tissues and thereby mediate protein metabolism during survival of this pathogen both independently and intracellularly. It is likely that calcium metabolism in promastigotes and amastigotes does not propagate in the same manner. Involvement of calcium to initiate caldonopain activity may be critically associated with signal transduction pathways which may be responsible for the pathobiological action of this parasite. We propose that caldonopain could be a potential target to develop new chemotherapeutic approach against leishmaniasis.

An intracellular calcium store is present in Leishmania donovani glycosomes

A fourth intracellular Ca2+ pool in Leishmania donovani was identified by permeabilizing plasma membrane with digitonin. In Fura 2 loaded cells Ca2+ was released synergistically when mitochondrial function was blocked by antimycin and oligomycin. Vanadate did not have any effect if applied before incorporation of these mitochondrial poisons. However, the same inhibitor which inhibits Ca2+-ATPase activity of endoplasmic reticulum was able to release Ca2+ at a slow rate when added after antimycin and oligomycin. Alkalization of cytoplasmic pH allowed further release of Ca2+ essentially from the acidocalcisome. Purified glycosomes could mediate Ca2+ uptake mechanism in presence of vanadate whereas bafilomycin, a specific and potent inhibitor of vacuolar proton pump did not have any effect. Glycosomal Ca2+-ATPase activity was optimum at pH 7.5. The apparent Km for calciumin presence of vanadate was 12 nM. Taken together, it may be suggested that a vanadate-insensitive Ca2+-ATPase is present in the membrane of this microbody. Presence of glycosomal Ca2+ was further confirmed by imaging of Ca2+ activity in the Fura 2 loaded purified organelle using confocal laser. Results reveal that newly localized glycosomal calcium may essentially be an effective candidate to play a significant role in cellular function.

Leishmania species: evidence for transglutaminase activity and its role in parasite proliferation

Albeit transglutaminase (TGase) activity has been reported to play crucial physiological roles in several organisms including parasites; however, there was no previous report(s) whether Leishmania parasites exhibit this activity. We demonstrate herein that TGase is functionally active in Leishmania parasites by using labeled polyamine that becomes conjugated into protein substrates. The parasite enzyme was about 2- to 4-fold more abundant in Old World species than in New World ones. In L. amazonensis, comparable TGase activity was found in both promastigotes and amastigotes. TGase activity in either parasite stage was optimal at the basic pH, but the enzyme in amastigote lysates was more stable at higher temperatures (37-55 degrees C) than that in promastigote lysates. Leishmania TGase differs from mouse macrophage (M Phi) TGase in two ways: (1) the parasite enzyme is Ca(2+)-independent, whereas the mammalian TGase depends on the cation for activity, and (2) major protein substrates for L. amazonensis TGase were found within the 50-75 kDa region, while those for the M Phi TGase were located within 37-50 kDa. The potential contribution of TGase-catalyzed reactions in promastigote proliferation was supported by findings that standard inhibitors of TGase [e.g., monodansylcadaverine (MDC), cystamine (CS), and iodoacetamide (IodoA)], but not didansylcadaverine (DDC), a close analogue of MDC, had a profound dose-dependent inhibition on parasite growth. Myo-inositol-1-phosphate synthase and leishmanolysin (gp63) were identified as possible endogenous substrates for L. amazonensis TGase, implying a role for TGase in parasite growth, development, and survival.

Peroxisome is a reservoir of intracellular calcium

We have examined fura 2-loaded purified peroxisomes under confocal microscope to prove that this mammalian organelle is a store of intracellular calcium pool. Presence of calcium channel and vanadate sensitive Ca(2+)-ATPase in the purified peroxisomal membrane has been demonstrated. We have further observed that machineries to maintain calcium pool in this mammalian organelle are impaired during infection caused by Leishmania donovani. Results reveal that peroxisomes have a merit to play a significant role in the metabolism of intracellular calcium.

Influence of calcium ion on host cell invasion and intracellular replication by Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular protozoan parasite, which invades a wide range of hosts including humans. The exact mechanisms involved in its invasion are not fully understood. This study focused on the roles of Ca2+ in host cell invasion and in T. gondii replication. We examined the invasion and replication of T. gondii pretreated with several calcium modulators, the conoid extrusion of tachyzoites. Calmodulin localization in T. gondii were observed using the immunogold method, and Ca2+ levels in tachyzoites by confocal microscopy. In light microscopic observation, tachyzoites co-treated with A23187 and EGTA showed that host cell invasion and intracellular replication were decreased. The invasion of tachyzoites was slightly inhibited by the Ca2+ channel blockers, bepridil and verapamil, and by the calmodulin antagonist, calmidazolium. We observed that calcium saline containing A23187 induced the extrusion of tachyzoite conoid. By immunoelectron microscopy, gold particles bound to anti-calmodulin or anti-actin mAb, were found to be localized on the anterior portion of tachyzoites. Remarkably reduced intracellular Ca2+ was observed in tachyzoites treated with BAPTA/AM by confocal microscopy. These results suggest that host cell invasion and the intracellular replication of T. gondii tachyzoites are inhibited by the calcium ionophore, A23187, and by the extracellular calcium chelator, EGTA.

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.

The sterol composition of Trypanosoma cruzi changes after growth in different culture media and results in different sensitivity to digitonin-permeabilization

Respiration, oxidative phosphorylation. and the corresponding changes in membrane potential (deltapsi) of Trypanosoma cruzi epimastigotes grown either in liver infusion-tryptose (LIT) or brain heart infusion (BHI) culture medium were assayed in situ using digitonin to render their plasma membrane permeable to succinate, ADP, safranine O, and other small molecules. When the cells were permeabilized with 64 microM digitonin, a concentration previously used with epimastigotes, the ability of the cells grown in LIT medium to sustain oxidative phosphorylation was demonstrated by the detection of an oligomycin-sensitive decrease in mitochondrial membrane potential induced by ADP. In contrast, the cells grown in BHI medium were not able to sustain a stable membrane potential and did not respond to ADP addition. Analyses of oxygen consumption by these permeabilized cells indicated that the rate of basal respiration, which was similar in both cell types, was significantly decreased by 64 microM digitonin. Addition of ADP to the permeabilized cells grown in LIT medium promoted an oligomycin-sensitive transition from resting to phosphorylating respiration in contrast to the cells grown in BHI medium, whose respiration decreased steadily and did not respond either to ADP or CCCP. Titration of the cells grown in BHI medium with different digitonin concentrations indicated that their mitochondria have higher sensitivity to digitonin than those grown in LIT medium. Analysis of the sterol composition of epimastigotes grown in the two different media showed a higher percentage of cholesterol in total and mitochondrial extracts of epimastigotes grown in BHI medium as compared to those grown in LIT medium, suggesting the involvement of this sterol in their increased sensitivity to digitonin-permeabilization.

Signal transduction in red bloodcells of the lizards Ameiva ameiva and Tupinambis merianae (Squamata, Teiidae)

The fluorescent calcium probe, Fluo-3, AM was used to measure the intracellular calcium concentration in red blood cells (RBCs) of the teiid lizards Ameiva ameiva and Tupinambis merianae. The cytosolic [Ca2+] is maintained around 20 nM and the cells contain membrane-bound Ca2+ pools. One pool appears to be identifiable with the endoplasmic reticulum (ER) inasmuch as addition of the sarco-endoplasmic reticulum Ca2+ ATPase, SERCA, inhibitor thapsigargin induces an increase in cytosolic [Ca2+ both in the presence and in the absence of extracellular Ca2+. In addition to the ER, an acidic compartment appears to be involved in Ca2+ storage, as collapse of intracellular pHgradients by monensin, a Na+ -H+ exchanger, and nigericin, a K+ -H+ exchanger, induce the release of Ca2+ from internal pools. A vacuolar H+ pump, sensitive to NBD-Cl and bafilomycin appears to be necessary to load the acidic Ca2+ pools. Finally, the purinergic agonist ATP triggers a rapid and transient increase of [Ca2+]c in the cells from both lizard species, mostly by mobilization of the cation from internal stores.

Binding of nicotinic ligands to and nicotine-induced calcium signaling in Trypanosoma cruzi

This work provides evidence that nicotine (1 x 10(-5) M) can cause changes in the intracellular calcium concentration of Trypanosoma cruzi epimastigotes, which can be blocked by alpha-bungarotoxin but not by atropine. Moreover, parasite membranes also bind such nicotinic acetylcholine receptor antagonist as well as agonists such as carbamylcholine (IC(50): 7.6 x 10(-7) M) and nicotine (IC(50): 1 x 10(-7) M). Results suggest that there is a molecular species in the surface of the parasite able to bind nicotinic ligands; therefore, nicotine interaction could lead to the activation of the mechanisms involved in intracellular calcium concentration increase in the parasite.

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.

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.

Peroxynitrite affects Ca2+ transport in Trypanosoma cruzi

Macrophages play an important role against Trypanosoma cruzi infection, via superoxide, nitric oxide, and peroxynitrite production. Peroxynitrite has been shown to be highly cytotoxic against Trypanosoma cruzi epimastigotes. Calcium is involved in many vital functions of the parasites, being its intracellular concentration governed by several transport systems, involving mitochondrial and non-mitochondrial compartments. In this paper, we report the effect of peroxynitrite on the calcium uptake systems, as studied by digitonin-permeabilized trypanosomes in the presence of arsenazo III. Peroxynitrite, at biologically relevant concentrations produced within phagosomes (250-750 microM), inhibited calcium uptake in a dose-dependent manner. Peroxynitrite decreased the mitochondrial membrane potential obtained in the presence of tetramethyl-p-phenylenediamine (TMPD)/ascorbate. In addition, a decrease of the non-mitochondrial Ca(2+)-uptake, concomitant with the inactivation of a Ca(2+)-dependent ATPase activity, was observed. HPLC analyses of the cellular adenine nucleotide pool showed a time-dependent decrease of ATP content and energy charge of the parasite; however this drop in ATP levels was significantly delayed with respect to decrease of the ATP-dependent Ca(2+)-transport. We conclude that the disruption of calcium homeostasis by peroxynitrite may contribute to the observed cytotoxic effects of macrophages against T. cruzi.

Overexpression of HSP-70 attenuates increases in [Ca2+]i and protects human epidermoid A-431 cells after chemical hypoxia

This laboratory previously reported that thermotolerance diminishes the NaCN-induced increase in intracellular free calcium concentrations ([Ca2+]i) in human epidermoid A-431 cells and that blocking this increase protects the cells from NaCN toxicity. In this study, we report that cell viability after exposure to NaCN (10 mM, 1 h) is enhanced by the overexpression of HSP-70 resulting from heat shock (45 degrees C, 10 min), treatment with a protein kinase C activator phorbol 12 myristate 13-acetate (PMA; 1 microM, 4 h), or HSP-70 cDNA transfection. Because the toxicity of NaCN is mediated by increases in [Ca2+]i, we sought to determine whether the overexpression of HSP-70 might protect the cells by altering the [Ca2+]i response induced by NaCN. Basal [Ca2+]i in vector-, HSF1 cDNA-, and HSP-70 cDNA-transfected cells was 114 +/- 11 (n = 11), 95 +/- 5 (n = 6), and 151 +/- 11 (n = 15) nM, respectively, suggesting that HSP-70 metabolism is associated with maintenance of resting [Ca2+]i. Removal of external Ca2+ reduced the resting [Ca2+]i in all of these cells. With external Ca2+ reduced the resting [Ca2+]i by 97 +/- 21% in vector-transfected cells and 111 +/- 5% in HSF1 vector-transfected cells but by only 27 +/- 8% in HSP-70 cDNA-transfected cells. Heat shock or PMA treatment of vector- or HSF1 cDNA-transfected cells to induce HSP-70 also attenuated the NaCN-induced increase in [Ca2+]i, perhaps because of a decrease in Vmax for the uptake of external Ca2+. Removal of external Ca2+ or treatment with inhibitors of Na+/Ca2+ exchangers eliminated the NaCN-induced increase in [Ca2+]i in HSP-70 cDNA-transfected cells, but ryanodine treatment did not. HSP-70 cDNA transfection also reduced Ca2+ mobilization stimulated by various Ca(2+)-mobilizing agents. The results suggest that HSP-70 overexpression protects cells from NaCN cytotoxicity, perhaps by attenuating the [Ca2+]i response.

Leishmania sp.: growth and survival are impaired by ion channel blockers

In the present work we examined the effect of ion transport blockers on the growth and viability of Leishmania sp. and on the infection of macrophages by the parasite. 4-aminopyridine and glibenclamide block voltage-dependent and K+ ATP channels, respectively; amiloride is used to detect Na+ channels and Na+/H+ antiporters; and anthracene-9-carboxylic acid affects chloride channels. The EC50 for promastigote cultures of three strains of the Leishmania subgenus, namely, Leishmania (Leishmania) NR, Leishmania (Leishmania) amazonensis LTB0016, and Leishmania (Leishmania) major, at their stationary phase of growth, were, respectively, 39, 46, and 464 microM for 4-aminopyridine; 7, 0.8, and 10 microM for glibenclamide and 66, 170, and 10 microM for anthracene-9-carboxylic acid. The amiloride EC50 for NR was 264 microM and 10 microM for L. (L.) major, but was never reached for LTB0016. Higher concentrations of the drugs impaired the exponential growth of Leishmania promastigotes. These results suggest the susceptibility of Leishmania sp. to blockers associated with K+ and Cl- and to Na+ or Na+/H+ transport systems. Blockade of such systems might have impaired the survival of the parasites as promastigotes. In addition, it affected the persistence of parasites in host cells. Although the infection of the macrophage cell line J774 and peritoneal-exudate macrophages was not significantly decreased by concentrations of the drugs around the promastigotes' EC50, the survival of intracellular parasites decreased significantly in the presence of these drugs without affecting the viability of the macrophages. Some blockers consistently gave small EC50 and significantly decreased the infection process as well as the survival of intracellular parasites. Thus, elucidation of their mechanism of action in Leishmania is relevant, since they could represent a potential subject for the development of leishmanicidal drugs.

Intracellular Ca2+ homeostasis in trypomastigotes of Trypanosoma cruzi

Trypomastigotes of Trypanosoma cruzi maintain an intracellular Ca2+ concentration ([Ca2+]i) of 64 +/- 30 nM. Equilibration of trypomastigotes in an extracellular buffer containing 0.5 mM [Ca2+]o (preloaded cells) increased [Ca2+]i < 20 nM whereas total cell Ca2+ increased by 1.5 to 2.0 pmole/cell. This amount of Ca2+ would be expected to increase [Ca2+]i to > 10 microM suggesting active sequestration of Ca2+. We tested the hypothesis that maintenance of [Ca2+]i involved both the sequestration into intracellular storage sites and extrusion into the extracellular space. Pharmacological probes known to influence [Ca2+]i through well characterized pathways in higher eukaryotic cells were employed. [Ca2+]i responses in the presence or absence of [Ca2+]o were measured to asses the relative contribution of sequestration or extrusion processes in [Ca2+]i homeostasis. In the presence of 0.5 mM [Ca2+]o, the ability of several agents to increase [Ca2+]i was magnified in the order ionomycin >>> nigericin > thapsigargin > monensin > valinomycin. In contrast, preloading markedly enhanced the increase in [Ca2+]i observed only in response to monensin. Manoalide, an inhibitor of phospholipase A2, enhanced the accumulation of [Ca2+]i due to all agents tested, particularly ionomycin and thapsigargin. Our results suggest that sequestration of [Ca2+]i involved storage sites sensitive to monensin and ionomycin whereas extrusion of Ca2+ may involve phospholipase A2 activity. A Na+/Ca2+ exchange mechanism did not appear to contribute to Ca2+ homeostasis.

Modulation of the swelling-activated amino acid channel of Leishmania major promastigotes by protein kinases

Leishmania promastigotes respond to hypotonic challenges by a mechanism of regulatory volume decrease (RVD), whereby anionic amino acid channels (HAAC) are hypotonically-activated and intracellular amino acids are released from the cells. Irrespective of the experimental conditions, restoration of isotonicity triggered an immediate blockage of the amino acid release. Both the speed and amplitude of the response depended on the hypotonic stimulus and on the operation of intracellular signaling mechanisms. The initial (5 s) hypotonic-induced release of amino acids (ri) and the steady state levels of amino acids attained (5 min) or amplitude (A), were markedly affected by modulators of protein kinase C: phorbol 12-myristate 13-acetate, 1-oleoyl-2-acetylglycerol and phorbol 12,13-diacetate whereas staurosporine and the related analog, bis-indolylmaleimide I (GF-109203.X) inhibited the RVD response. Agonists of cAMP-dependent protein kinase A such as forskolin or (8-(4-chlorophenylthio))-adenosine-3',5'cyclic-monophosphate enhanced the speed of the response but had little effect on its amplitude. Neither 4alpha-phorbol 12,13-didecanoate,1,9-dideoxyforskolin nor genistein, tamoxifen or thapsigargin had any apparent effect on either parameter tested. The most striking stimulation of hypotonic-induced amino acid release was exerted by arachidonic acid or by its non-metabolizable analog, 5,8,11,14-eicosatetraynoic acid (ETYA). These agents caused a major increase in the initial rate of amino acid release as well as a higher amplitude of the response, both of which were markedly inhibited by an anion channel blocker. The present studies indicate not only that hypotonicity is an obligatory and dominant component in HAAC activation, but implicate specific second messengers in the modulation of the RVD response. The modes of activation or attenuation of HAAC activity apparently differ for PKC and PKA modulators as well as for arachidonic acid. The involvement of Ca2+ in HAAC was studied in hypotonic challenged cells which were treated with intracellular Ca2+-chelators or Ca2+-free medium. These cells showed a lag in AA release and a modest inhibition of the amplitude. The inhibition of HAAC was markedly increased when cells were treated with the ionophore A23187 in Ca2+-free media. The HAAC activity was accompanied by a significant increase in internal Ca2+ when performed in Ca2+-containing medium (from 88+/-9 to 179+/-22 nM) but by no significant change when measured in Ca2+-free medium. These studies indicate that although Ca2+ might be involved in the early activation phase of HAAC, it is either not absolutely required or its action might be associated with localized events.

Characterization of mitochondrial electron-transfer in Leishmania mexicana

Some general features of the respiratory chain and respiratory control were characterized in coupled mitochondrial preparations from Leishmania mexicana promastigotes. O2 uptake was sensitive to the electron-transfer inhibitors rotenone, flavone, malonate, 4,4,4-trifluoro-1-(2-thienyl) 1.3 butanedione (TTFA), antimycin A, 2n-nonyl-4-hydroxyquinoline-N-oxide (HQNO), myxothiazol, cyanide and azide. A high concentration of rotenone (60 microM) was required to inhibit O2 uptake effectively. Difference spectra revealed the presence of cytochromes (a + a3), b and c. Respiratory control was stimulated 2-fold by ADP with different exogenous oxidizable substrates. Calculated ADP/O ratios were consistent with the notion that ascorbate/N,N,N',N'-tetramethylphenylenediamine (TMPD)-linked and FAD-linked respiration proceeds, respectively, with one third and two thirds of the ATP producing capacity of NADH-linked respiration. State 3 was suppressed by the ATP synthase inhibitors oligomycin and aurovertin and by the adenine nucleotide translocator inhibitors atractyloside and carboxy atractyloside. The protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) provoked state 3u respiration. The mitochondrial preparation was capable of Ca2+ uptake and Ca2+ stimulated respiration. Data obtained suggests strongly that mitochondrial complexes I, II, III and IV are present in a major pathway of electron-transfer and that oxidative phosphorylation might proceed with high bioenergetic efficiency.

Intracellular Ca2+ pool content and signaling and expression of a calcium pump are linked to virulence in Leishmania mexicana amazonesis amastigotes

Virulent and avirulent clones of Leishmania mexicana amazonensis promastigotes or amastigotes were loaded with the fluorescent reagent fura 2/AM to measure intracellular free calcium ([Ca2+]i). When the cells were treated with the calcium ionophore ionomycin in the nominal absence of extracellular Ca2+, there was an increase of [Ca2+]i that was further elevated by addition of either NH4Cl, nigericin, or the vacuolar H+-ATPase inhibitor bafilomycin A1. Similar results were obtained when the order of additions was reversed. Taking into account the relative importance of the ionomycin-releasable and the ionomycin plus NH4Cl-releasable Ca2+ pools, it is apparent that a significant amount of the Ca2+ stored in L. mexicana amazonensis promastigotes and amastigotes is present in an acidic compartment rich in Ca2+ (acidocalcisome). Results indicated that more releasable Ca2+ is stored intracellularly in virulent amastigotes than in virulent promastigotes or avirulent cells of both stages. This higher amount of releasable Ca2+ was correlated with the presence of Ca2+ signals in the virulent amastigotes during invasion of macrophages. Ca2+ signals and invasion were reduced by preloading the parasites with intracellular Ca2+ chelators (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid/AM) and quin 2/AM) but not by a non-Ca2+-chelating analog (N-(2-methoxyphenyl)imidoacetic acid/AM). The gene encoding an organelle-type Ca2+-ATPase was cloned and sequenced and found overexpressed in virulent amastigotes as compared with all other forms. Together, these results demonstrate a significant link between expression of a Ca2+-ATPase, intracellular Ca2+ pool content and signaling, and virulence.

Heat shock induction of apoptosis in promastigotes of the unicellular organism Leishmania (Leishmania) amazonensis

Apoptosis and/or programmed cell death have been described in examples ranging from fungi to man as gene-regulated processes with roles in cell and tissue physiopathology. These processes require the operation of an intercellular communicating network able to deliver alternative signals for cells with different fates and is thus considered a prerogative of multicellular organisms. Promastigotes from Leishmania (Leishmania) amazonensis, when shifted from their optimal in vitro growth temperature (22 degrees C) to the temperature of the mammalian host (37 degrees C), die by a calcium-modulated mechanism. More parasites die in the presence of this ion than in its absence, as detected by a colorimetric assay based on the activity of mitochondrial and cytoplasmic dehydrogenases which measures cell death, independently of the process by which it occurs. A heat shock, unable to induce detectable parasite death (34 degrees C for 1 h), is able to significantly raise the concentration of intracellular free calcium in these cells. Heat-shocked parasites present ultrastructural and molecular features characteristic of cells dying by apoptosis. Morphological changes, observed only in the presence of calcium, are mainly nuclear. Cytoplasmic organelles are preserved. Heat shock is also able to induce DNA cleavage into an oligonucleosomal ladder detected in agarose gels by ethidium bromide staining and autoradiography of [alpha 32P]ddATP-labeled fragments. These results indicate that death by apoptosis is not exclusive of multicellular organisms.

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.

Characterization of Ca2+ transport in Euglena gracilis mitochondria

The present study was designed to establish the characteristics of the Ca2+ fluxes in isolated mitochondria of the protist Euglena gracilis. Uptake of Ca2+ and Sr2+ was supported by succinate and lactate oxidation. Ca2+ influx was slightly inhibited by 5 microM Ruthenium red and completely blocked by La3+ with a half-maximal inhibition attained at 50 microM. The addition of inorganic phosphate induced a 3-fold stimulation of Ca2+ uptake. Ca2+ uptake was inhibited by Mg2+ only in the absence of phosphate. Ca2+ efflux was induced by Na+, Li+ and K+ through a diltiazem-insensitive reaction. Ca2+ release, collapse of membrane potential and swelling were induced by Hg2+ and Cd2+ but not by carboxyatractyloside; cyclosporin A did not prevent the Ca2+ release induced by the heavy metal ions. Ca2+ uptake was achieved in the presence of 3 microM antimycin or 0.1 mM cyanide; this finding indicates that the alternative respiratory chain present in Euglena mitochondria can support this energy-dependent reaction. The data obtained suggest similar pathways, but different regulatory mechanisms, for Ca2+ transport between protist and mammalian mitochondria.

Calcium dependent thiol protease caldonopain and its specific endogenous inhibitor in Leishmania donovani

A calcium dependent proteolytic enzyme was detected in the lysed promastigotes of Leishmania donovani, the causative agent of kala-azar. The protease was able to hydrolyse an added substrate, azocasein and showed high affinity for calcium. Rate of azocasein digestion was primarily slow but boosted up after eight hours. It was not inactivated when heated at 55 degrees C for 15 min at pH 7.4. Sulfhydryl reagents significantly reduced the enzymic activity but trypsin-like protease inhibitors hardly had any effect. The enzyme was not sensitive to calmodulin from a heterologous source but registered low activity when treated with chlorpromazine. The caseinolytic activity was stimulated when leishmanial cells were preincubated with ionophore A23187 in presence of 1 mM Ca2+. The enzyme is named caldonopain due to its similarity with a general class of calcium dependent protease calpain present in different tissues and cells. Caldonopain was found to be localized in cytosol along with its specific endogenous inhibitor caldonostatin. The ratio of caldonopain-caldonostatin unit was higher in the infected macrophage compared to the parasitic protozoa and Balb/c macrophage alone. It may be postulated that the amount of both calcium and its protein inhibitor may have a direct impact on the caldonopain-induced biological process to regulate cellular action of this pathogen.

Studies on mitochondrial ATPase of Leishmania donovani using digitonin-permeabilized promastigotes

Mitochondrial ATPase of Leishmania donovani was characterized using digitonin-permeabilized promastigotes and the results were compared with those from isolated mitochondria. Maximum mitochondrial ATPase activity was obtained in promastigotes permeabilized with digitonin at a final concentration of 20 microM and the specific activity of the enzyme was 46% and 57% higher than that of homogenized and sonicated promastigotes, respectively. At concentrations above 20 microM digitonin inhibited ATPase activity and the degree of inhibition increased with increasing concentrations of the detergent. The ATPase activity of promastigotes remained DCCD-sensitive when permeabilized with digitonin at concentrations up to 120 microM but the enzyme became increasingly resistant to this inhibitor as digitonin concentrations were increased to 140 microM and more, indicating the loss of functional activity of the enzyme. The pH and temperature optima for mitochondrial ATPase were determined to be 7.5 and 30 degrees C, respectively. Mg2+ ions were essential for ATPase activity but free Mg2+ ions were found to be inhibitory. A Mg2+/ATP ratio of 1:3 supported the optimum ATPase activity. Sulfite and hexanol activated the enzyme but failed to prevent the inhibition by free Mg2+ ions. The results indicate that digitonin-permeabilized promastigotes provide an ideal system for studying the mitochondrial ATPase of L. donovani.

Effect of thapsigargin on calcium homeostasis in Trypanosoma cruzi trypomastigotes and epimastigotes

By using the fluorescent calcium indicator fura-2, it was found that the concentration of free Ca2+ in the cytoplasm of Trypanosoma cruzi trypomastigotes incubated in the presence or absence of external calcium was maintained at very low levels (10-20 nM). When trypomastigotes were incubated in the presence of succinate and ATP and permeabilized with digitonin, they lowered the medium calcium concentration to a submicromolar level. In the presence of 1 microM FCCP the initial rate of Ca2+ sequestration by these permeabilized cells was very slow. When succinate alone was present, the initial rate of Ca2+ accumulation was slower than with ATP plus succinate, and the calcium set point was about 0.6 microM. The succinate dependence and FCCP sensitivity of the later Ca2+ uptake indicate that it may be exerted by the mitochondria. High concentrations of the tumor promoter thapsigargin slightly increased cytosolic Ca2+ in the presence of extracellular Ca2+ but had no effect on the FCCP- and oligomycin/antimycin A-insensitive Ca2+ pool. In addition, when used at those concentrations (4-20 microM), thapsigargin was shown to release Ca2+ from the mitochondria and to decrease the inner mitochondrial membrane potential of trypomastigotes and epimastigotes as measured using safranine O. Despite the presence of inositol phosphates as determined by [3H]inositol incorporation, no IP3-sensitive Ca2+ release could be detected in trypomastigotes.

High-affinity calcium-stimulated, magnesium-dependent adenosine triphosphatase in Trypanosoma cruzi

1. A high-affinity (Ca2+ + Mg2+)-ATPase and a low-affinity Mg(2+)-ATPase were identified in the 105,000 g fraction from epimastigote forms of Trypanosoma cruzi, the agent of Chagas' disease (Tulahuen strain). 2. Activities were conserved after enzyme solubilization with deoxycholate. 3. The Ca(2+)-stimulated ATPase activity was (a) lower than that of the Mg(2+)-ATPase; (b) inhibited by p-chloromercurobenzoate and orthovanadate and (c) insensitive to oligomycin. 4. Optimal stimulation by Ca2+ was observed at pH 6.5-6.8 in the presence of 1 mM MgCl2 and 0.1 M KCl. 5. The Mg(2+)-ATPase was insensitive to p-chloromercurobenzoate and orthovanadate and did not require KCl for activity. 6. Kinetic analysis of the (Ca2+ + Mg2+)-ATPase yielded a half-maximal stimulating concentration of 1.1 microM for Ca2+ and a Km of 66 microM for ATP. 7. The (Ca2+ + Mg2+)-ATPase clearly differed from the Ca(2+)- or Mg(2+)-ATPases previously characterized in the same strain of T. cruzi (Frasch et al., 1978; Comp. Biochem. Physiol. 60B, 271-275).

Calcium homeostasis in Trypanosoma cruzi amastigotes: presence of inositol phosphates and lack of an inositol 1,4,5-trisphosphate-sensitive calcium pool

The permeabilization of Trypanosoma cruzi amastigotes with digitonin allowed the study of Ca2+ fluxes between intracellular organelles in situ. In addition, fura-2 was used to determine the cytosolic Ca2+ concentration in the intact cells. When amastigotes were permeabilized in a reaction medium containing MgATP, succinate and 3.5 microM Ca2+, they lowered the medium Ca2+ concentration to the submicromolar level, a range which correlates favorably with that detected in the intact cells with fura-2. The presence of 1 microM FCCP strongly decreased the initial rate of Ca2+ sequestration by these permeabilized cells. This FCCP-insensitive Ca2+ uptake, probably represented by the endoplasmic reticulum, was completely inhibited by 500 microM vanadate. On the other hand, when vanadate instead of FCCP was present, the initial rate of Ca2+ accumulation was decreased and the Ca2+ set point was increased to about 0.8 microM. The succinate dependence and FCCP sensitivity of the later Ca2+ uptake indicate that it may be exerted by the mitochondria. Despite the presence of inositol phosphates, as determined by [3H]inositol incorporation, and of a large extramitochondrial Ca2+ pool, no IP3-sensitive or thapsigargin-sensitive Ca2+ release could be detected in either amastigotes or epimastigotes.

Comparative physiology of two protozoan parasites, Leishmania donovani and Trypanosoma brucei, grown in chemostats

Cultures of the insect stage of the protozoan parasites Leishmania donovani and Trypanosoma brucei were grown in chemostats with glucose as the growth rate-limiting substrate. L. donovani has a maximum specific growth rate (mu max) of 1.96 day-1 and a Ks for glucose of 0.1 mM; the mu max of T. brucei is 1.06 day-1 and the Ks is 0.06 mM. At each steady state (specific growth rate, mu, equals D, the dilution rate), the following parameters were measured: external glucose concentration (Glcout), cell density, dry weight, protein, internal glucose concentration (Glcin), cellular ATP level, and hexokinase activity. L. donovani shows a relationship between mu and yield that allows an estimation of the maintenance requirement (ms) and the yield per mole of ATP (YATP). Both the ms and the YATP are on the higher margin of the range found for prokaryotes grown on glucose in a complex medium. L. donovani maintains the Glcin at a constant level of about 50 mM as long as it is not energy depleted. T. brucei has a decreasing yield with increasing mu, suggesting that it oxidizes its substrate to a lesser extent at higher growth rates. Glucose is not concentrated internally but is taken up by facilitated diffusion, while phosphorylation by hexokinase is probably the rate-limiting step for glucose metabolism. The Ks is constant as long as glucose is the rate-limiting substrate. The results of this study demonstrate that L. donovani and T. brucei have widely different metabolic strategies for dealing with varying external conditions, which reflect the conditions they are likely to encounter in their respective insect hosts.

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.

Leishmania donovani: characterization of a 38-kDa membrane protein that cross-reacts with the mammalian G-protein transducin

We investigated the presence in Leishmania donovani promastigotes of proteins with homology to the G-proteins known to mediate signal transduction in other organisms. [alpha 32P]GTP binding experiments revealed the presence in the promastigote membrane of GTP-binding sites with high affinity and specificity. Experiments with antisera directed against mammalian G-proteins showed that the promastigotes possess a 38-kDa protein (p38) which strongly reacts with an antiserum directed against a decapeptide containing the C-terminal sequence of transducin, the G-protein that mediates visual signal transduction. The interaction of p38 with the antiserum is specifically blocked by the decapeptide antigen. p38 is enriched in plasma membranes and is absent in cytosol and in a mitochondria-enriched fraction. p38 was also detected in two other Leishmania species, L. mexicana and L. major. The migration of p38 upon sucrose gradient centrifugation of detergent extract of L. donovani membranes corresponded to Mr of approximately 70,000, indicating that p38 is part of an oligomeric structure. The findings suggest that p38 may be a component of a transmembrane signal transduction system in Leishmania.

Effects of osmotic pressure on the oxidative metabolism of Leishmania major promastigotes

Leishmania major promastigotes were washed and resuspended in an iso-osmotic buffer. The rate of oxidation of 14C-labeled substrates was then measured as a function of osmolality. An acute decrease in osmolality (achieved by adding H2O to the cell suspension) caused an increase in the rates of 14CO2 production from [6-14C]glucose and, to a lesser extent, from [1,(3)-14C]glycerol. An acute increase in osmolality (achieved by adding NaCl, KCl, or mannitol) strongly inhibited the rates of 14CO2 production from [1-14C]alanine,[1-14C]glutamate, and [1,(3)-14C]glycerol. The rates of 14CO2 formation from [1-14C]laurate,[1-14C]acetate, and [2-14C]glucose (all of which form [1-14C]acetyl CoA prior to oxidation) were also inhibited, but less strongly, by increasing osmolality. These data suggest that with increasing osmolality there is an inhibition of mitochondrial oxidative capacity, which could facilitate the increase in alanine pool size that occurs in response to hyper-osmotic stress. Similarly, an increase in oxidative capacity would help prevent a rebuild up of the alanine pool after its rapid loss to the medium in response to hypo-osmotic stress.

Regulation of intracellular calcium homeostasis in Trypanosoma cruzi. Effects of calmidazolium and trifluoperazine

Trypanosoma cruzi epimastigotes maintained an intracellular free calcium concentration of about 0.15 microM, as measured with the fluorescent indicator Fura-2. The maintenance of low [Ca2+]i is energy-dependent since it is disrupted by KCN and FCCP. When the cells were permeabilized with digitonin, the steady-state free Ca2+ concentration in the absence of ATP was about 0.7 microM. The additional presence of ATP resulted in a steady-state level close to 0.1-0.2 microM which compares favorably with the concentration detected in intact cells. Intracellular Ca2+ uptake at high levels of free Ca2+ (greater than 1 microM) was due to energy-dependent mitochondrial uptake as indicated by its FCCP-sensitivity. However, as the free Ca2+ concentration was lowered from 1 microM, essentially all uptake was due to the ATP-dependent Ca2+ sequestration by the endoplasmic reticulum as indicated by its stimulation by ATP, and its inhibition by sodium vanadate. High concentrations of the calmodulin antagonist trifluoperazine, inhibited both the Ca2+ uptake by the endoplasmic reticulum and by the mitochondria, while calmidazolium released Ca2+ from both compartments. In addition, trifluoperazine and calmidazolium inhibited respiration and collapsed the mitochondrial membrane potential of T. cruzi, thus indicating non-specific effects unrelated to calmodulin.

A calcium pump in plasma membrane vesicles from Leishmania braziliensis

A subcellular fraction highly enriched in plasma membrane vesicles was prepared from Leishmania promastigotes. This fraction showed (Ca2+ + Mg2+)-ATPase activity. This, however, represented a small fraction (about 25%) of the overall ATPase activity. The Ca2(+)-ATPase showed general characteristics common to plasma membrane ATPases involved in Ca2+ transport. Thus, the Ca2(+)-ATPase was activated by Ca2+ with a high affinity (Km about 0.7 microM), saturating at about 5 microM Ca2+. Furthermore, it was stimulated by calmodulin (about 70-80% with 5 micrograms/ml) and almost fully inhibited by trifluoperazine (100 microM). The above vesicles accumulated Ca2+ against a concentration gradient and released it after the addition of A23187, as shown independently by 45Ca2+ and Arsenazo III studies. The transport mechanism showed the same kinetics parameters as described for the enzyme, indicating a single molecular entity. In addition, Ca2(+)-ATPase activity and Ca2+ uptake were completely inhibited by vanadate (20 microM), indicating that an E1-E2 type mechanism is involved. The results clearly demonstrate the presence of a Ca2+ pump in the plasma membrane of Leishmania which is capable of maintaining a low cytoplasmic Ca2+ concentration.

Ca2+ transport in digitonin-permeabilized trypanosomatids

The use of digitonin to permeabilize Leishmania mexicana mexicana, Leishmania agamae, and Crithidia fasciculata plasma membranes enabled us to study Ca2+ transport in situ. The present results show that the mitochondria of these trypanosomatids are able to build up and retain a membrane potential as indicated by a tetraphenylphosphonium-sensitive electrode. Ca2+ uptake caused membrane depolarization compatible with the existence of an electrogenically mediated Ca2+ transport mechanism in these mitochondria. Ca2+ uptake was partially inhibited by ruthenium red, almost totally inhibited by carbonyl cyanide p-trifluoromethoxyphenylhydrazone, and stimulated by inorganic phosphate. Large amounts of Ca2+ were retained by C. fasciculata mitochondria even after addition of thiols and NAD(P)H oxidants such as t-butylhydroperoxide and diamide. In contrast, Ca2+ was not retained in the matrix of Leishmania sp. mitochondria for long periods of time. In addition to the mitochondrial Ca2+ uptake, a vanadate-sensitive Ca2(+)-transporting system was also detectable in these trypanosomatids.

Tricyclic drugs reduce proton motive force in Leishmania donovani promastigotes

Tricyclic compounds have been suggested as potential anti-leishmanial drugs. We have studied the effect of tricyclic drugs on several cellular functions in L. donovani promastigotes. Imipramine inhibits proline transport and reduces delta pH and cellular ATP at relatively high concentrations (IC50 = 50-80 microM). High concentrations of imipramine are also required to kill L. donovani promastigotes (LD50 greater than 50 microM). The presence of a chlorine atom in the side ring of either imipramine or promazine results in a three-fold increase in both IC50 and LD50 values. Tricyclic compounds in which the nitrogen in the middle ring was substituted with a carbon atom (amitryptyline and chlorprothixene) are most effective in causing cell death and in decreasing proline transport and delta pH (IC50 congruent to 5 microM), whereas depletion of cellular ATP requires a higher drug concentration (IC50 = 12 microM). Transchlorprothixene has IC50 values for proline transport, delta pH and cellular ATP that are similar to those of amitriptyline, whereas the cis isomer is less active. Imipramine, chlomipramine and chlorpromazine decrease the membrane potential in promastigotes. There is a direct correlation between inhibition of membrane transport of proline and the size of the membrane potential at various concentrations of the drugs. Taken together, the multiple effects of the tricyclic drugs on cellular functions in Leishmania suggest that the drugs cause cellular death by non-specific mechanisms, probably involving a general increase in membrane permeability.