Calcium signaling in a low calcium environment: how the intracellular malaria parasite solves the problem

Cytoplasmic free Ca2+ is essential for multiple steps in malaria parasite egress from infected erythrocytes

Background

Egress of Plasmodium falciparum, from erythrocytes at the end of its asexual cycle and subsequent parasite invasion into new host cells, is responsible for parasite dissemination in the human body. The egress pathway is emerging as a coordinated multistep programme that extends in time for tens of minutes, ending with rapid parasite extrusion from erythrocytes. While the Ca²⁺ regulation of the invasion of P. falciparum in erythrocytes is well established, the role of Ca²⁺ in parasite egress is poorly understood. This study analysed the involvement of cytoplasmic free Ca²⁺ in infected erythrocytes during the multistep egress programme of malaria parasites.

Methods

Live-cell fluorescence microscopy was used to image parasite egress from infected erythrocytes, assessing the effect of drugs modulating Ca²⁺ homeostasis on the egress programme.

Results

A steady increase in cytoplasmic free Ca²⁺ is found to precede parasite egress. This increase is independent of extracellular Ca²⁺ for at least the last two hours of the cycle, but is dependent upon Ca²⁺ release from internal stores. Intracellular BAPTA chelation of Ca²⁺ within the last 45 minutes of the cycle inhibits egress prior to parasitophorous vacuole swelling and erythrocyte membrane poration, two characteristic morphological transformations preceding parasite egress. Inhibitors of the parasite endoplasmic reticulum (ER) Ca²⁺-ATPase accelerate parasite egress, indicating that Ca²⁺ stores within the ER are sufficient in supporting egress. Markedly accelerated egress of apparently viable parasites was achieved in mature schizonts using Ca²⁺ ionophore A23187. Ionophore treatment overcomes the BAPTA-induced block of parasite egress, confirming that free Ca²⁺ is essential in egress initiation. Ionophore treatment of immature schizonts had an adverse effect inducing parasitophorous vacuole swelling and killing the parasites within the host cell.

Conclusions

The parasite egress programme requires intracellular free Ca²⁺ for egress initiation, vacuole swelling, and host cell cytoskeleton digestion. The evidence that parasitophorous vacuole swelling, a stage of unaffected egress, is dependent upon a rise in intracellular Ca²⁺ suggests a mechanism for ionophore-inducible egress and a new target for Ca²⁺ in the programme liberating parasites from the host cell. A regulatory pathway for egress that depends upon increases in intracellular free Ca²⁺ is proposed.

Synthesis and antiplasmodial activity of betulinic acid and ursolic acid analogues

More than 40% of the World population is at risk of contracting malaria, which affects primarily poor populations in tropical and subtropical areas. Antimalarial pharmacotherapy has utilised plant-derived products such as quinine and artemisinin as well as their derivatives. However, worldwide use of these antimalarials has caused the spread of resistant parasites, resulting in increased malaria morbidity and mortality. Considering that the literature has demonstrated the antimalarial potential of triterpenes, specially betulinic acid (1) and ursolic acid (2), this study investigated the antimalarial activity against P. falciparum chloroquine-sensitive 3D7 strain of some new derivatives of 1 and 2 with modifications at C-3 and C-28. The antiplasmodial study employed flow cytometry and spectrofluorimetric analyses using YOYO-1, dihydroethidium and Fluo4/AM for staining. Among the six analogues obtained, compounds 1c and 2c showed excellent activity (IC₅₀ = 220 and 175 nM, respectively) while 1a and b demonstrated good activity (IC₅₀ = 4 and 5 μM, respectively). After cytotoxicity evaluation against HEK293T cells, 1a was not toxic, while 1c and 2c showed IC₅₀ of 4 μM and a selectivity index (SI) value of 18 and 23, respectively. Moreover, compound 2c, which presents the best antiplasmodial activity, is involved in the calcium-regulated pathway(s).

Genome-wide association study indicates two novel resistance loci for severe malaria

Malaria causes approximately one million fatalities per year, mostly among African children. Although highlighted by the strong protective effect of the sickle-cell trait, the full impact of human genetics on resistance to the disease remains largely unexplored. Genome-wide association (GWA) studies are designed to unravel relevant genetic variants comprehensively; however, in malaria, as in other infectious diseases, these studies have been only partly successful. Here we identify two previously unknown loci associated with severe falciparum malaria in patients and controls from Ghana, West Africa. We applied the GWA approach to the diverse clinical syndromes of severe falciparum malaria, thereby targeting human genetic variants influencing any step in the complex pathogenesis of the disease. One of the loci was identified on chromosome 1q32 within the ATP2B4 gene, which encodes the main calcium pump of erythrocytes, the host cells of the pathogenic stage of malaria parasites. The second was indicated by an intergenic single nucleotide polymorphism on chromosome 16q22.2, possibly linked to a neighbouring gene encoding the tight-junction protein MARVELD3. The protein is expressed on endothelial cells and might therefore have a role in microvascular damage caused by endothelial adherence of parasitized erythrocytes. We also confirmed previous reports on protective effects of the sickle-cell trait and blood group O. Our findings underline the potential of the GWA approach to provide candidates for the development of control measures against infectious diseases in humans.

Insights into Duffy binding-like domains through the crystal structure and function of the merozoite surface protein MSPDBL2 from Plasmodium falciparum

Invasion of human red blood cells by Plasmodium falciparum involves interaction of the merozoite form through proteins on the surface coat. The erythrocyte binding-like protein family functions after initial merozoite interaction by binding via the Duffy binding-like (DBL) domain to receptors on the host red blood cell. The merozoite surface proteins DBL1 and -2 (PfMSPDBL1 and PfMSPDBL2) (PF10_0348 and PF10_0355) are extrinsically associated with the merozoite, and both have a DBL domain in each protein. We expressed and refolded recombinant DBL domains for PfMSPDBL1 and -2 and show they are functional. The red cell binding characteristics of these domains were shown to be similar to full-length forms of these proteins isolated from parasite cultures. Futhermore, metal cofactors were found to enhance the binding of both the DBL domains and the parasite-derived full-length proteins to erythrocytes, which has implications for receptor binding of other DBL-containing proteins in Plasmodium spp. We solved the structure of the erythrocyte-binding DBL domain of PfMSPDBL2 to 2.09 Å resolution and modeled that of PfMSPDBL1, revealing a canonical DBL fold consisting of a boomerang shaped α-helical core formed from three subdomains. PfMSPDBL2 is highly polymorphic, and mapping of these mutations shows they are on the surface, predominantly in the first two domains. For both PfMSPDBL proteins, polymorphic variation spares the cleft separating domains 1 and 2 from domain 3, and the groove between the two major helices of domain 3 extends beyond the cleft, indicating these regions are functionally important and are likely to be associated with the binding of a receptor on the red blood cell.

Regulation of Plasmodium falciparum glideosome associated protein 45 (PfGAP45) phosphorylation

The actomyosin motor complex of the glideosome provides the force needed by apicomplexan parasites such as Toxoplasma gondii (Tg) and Plasmodium falciparum (Pf) to invade their host cells and for gliding motility of their motile forms. Glideosome Associated Protein 45 (PfGAP45) is an essential component of the glideosome complex as it facilitates anchoring and effective functioning of the motor. Dissection of events that regulate PfGAP45 may provide insights into how the motor and the glideosome operate. We found that PfGAP45 is phosphorylated in response to Phospholipase C (PLC) and calcium signaling. It is phosphorylated by P. falciparum kinases Protein Kinase B (PfPKB) and Calcium Dependent Protein Kinase 1 (PfCDPK1), which are calcium dependent enzymes, at S89, S103 and S149. The Phospholipase C pathway influenced the phosphorylation of S103 and S149. The phosphorylation of PfGAP45 at these sites is differentially regulated during parasite development. The localization of PfGAP45 and its association may be independent of the phosphorylation of these sites. PfGAP45 regulation in response to calcium fits in well with the previously described role of calcium in host cell invasion by malaria parasite.

Generation of second messengers in Plasmodium

Signalling in malaria parasites is a field of growing interest as its components may prove to be valuable drug targets, especially when one considers the burden of a disease that is responsible for up to 500 million infections annually. The scope of this review is to discuss external stimuli in the parasite life cycle and the upstream machinery responsible for translating them into intracellular responses, focussing particularly on the calcium signalling pathway.

Calcium dependent protein kinase 1 and calcium fluxes in the malaria parasite

Calcium dependent protein kinases (CDPKs) are found only in plants and alveolates and are distinguished from other kinases by an activation domain that binds calcium directly. Plants contain families of these kinases and their functions are modulated by post translational modifications as well as calcium activation. Apicomplexan parasites also contain CDPK families and this review is focused on CDPK1 in Plasmodium spp. This enzyme has been implicated in parasite motility and host cell invasion and at least two substrates associated with the actomyosin motor complex have been identified. By analogy with the plant CDPKs we propose that its activity is modulated both by post translational modifications and by its subcellular location in a compartment within the parasite's pellicle, which may regulate the calcium concentration required for activation.

Extracellular ATP triggers proteolysis and cytosolic Ca²⁺ rise in Plasmodium berghei and Plasmodium yoelii malaria parasites

BACKGROUND

Plasmodium has a complex cell biology and it is essential to dissect the cell-signalling pathways underlying its survival within the host.

METHODS

Using the fluorescence resonance energy transfer (FRET) peptide substrate Abz-AIKFFARQ-EDDnp and Fluo4/AM, the effects of extracellular ATP on triggering proteolysis and Ca²⁺ signalling in Plasmodium berghei and Plasmodium yoelii malaria parasites were investigated.

RESULTS

The protease activity was blocked in the presence of the purinergic receptor blockers suramin (50 μM) and PPADS (50 μM) or the extracellular and intracellular calcium chelators EGTA (5 mM) and BAPTA/AM (25, 100, 200 and 500 μM), respectively for P. yoelii and P. berghei. Addition of ATP (50, 70, 200 and 250 μM) to isolated parasites previously loaded with Fluo4/AM in a Ca²⁺-containing medium led to an increase in cytosolic calcium. This rise was blocked by pre-incubating the parasites with either purinergic antagonists PPADS (50 μM), TNP-ATP (50 μM) or the purinergic blockers KN-62 (10 μM) and Ip5I (10 μM). Incubating P. berghei infected cells with KN-62 (200 μM) resulted in a changed profile of merozoite surface protein 1 (MSP1) processing as revealed by western blot assays. Moreover incubating P. berghei for 17 h with KN-62 (10 μM) led to an increase in rings forms (82% ± 4, n = 11) and a decrease in trophozoite forms (18% ± 4, n = 11).

CONCLUSIONS

The data clearly show that purinergic signalling modulates P. berghei protease(s) activity and that MSP1 is one target in this pathway.

Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic Apicomplexa (Plasmodium, Toxoplasma)

The importance of Ca2+-signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives, the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca2+ signaling in the two closely related groups. Acidic Ca2+ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca2+-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca2+-ATPases (PMCA, SERCA), respectively. Only recently was it possible to identify in Paramecium a number of homologs of ryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP3R) and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP3Rs are constitutively active in the contractile vacuole complex; RyR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca2+ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.

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.

The potent antimalarial peptide cyclosporin A induces the aggregation and permeabilization of sphingomyelin-rich membranes

Cyclosporin A (CsA) is a hydrophobic peptide drug produced by the fungus Tolypocladium inflatum. CsA is commonly used as an immunosuppressive drug, but it also has antimalarial activity. The immunosuppressive activity of CsA is clearly due to its association with specific proteins of immune cells such as cyclophilins. By contrast, the antimalarial properties of this peptide are completely independent of the association with a parasite's cyclophilins. Because of its hydrophobicity, CsA may interact with biological membranes, which may participate in its therapeutic effect. Recently, we have shown a marked preference of CsA for insertion into sphingomyelin (SM) monolayers. In this article, we measure for the first time the ability of CsA to induce permeabilization and aggregation and to change the lipid order, especially in the presence of SM. Calcein-release experiments permitted us to show that CsA causes the leakage of the fluorescent probe from SM-rich liposomes by 40% and PC liposomes by 11%, suggesting a lipid-selective effect. Electron microscopy and dynamic light scattering experiments confirmed the different interaction of CsA with SM and PC vesicles: it formed much larger aggregates with SM than with PC. Our results taken together suggest that CsA could specifically weaken and aggregate SM-rich membranes, which could in turn explain why CsA is efficient in the treatment of malaria. Indeed, CsA could inhibit the development of Plasmodium by permeabilizing and aggregating the SM-rich membrane network formed by the parasite during its intraerythrocytic growth cycle.

Melatonin triggers PKA activation in the rodent malaria parasite Plasmodium chabaudi

Calcium (Ca(2+) ) is a critical regulator of many aspects of the Plasmodium reproductive cycle. In particular, intra-erythrocyte Plasmodium parasites respond to circulating levels of the melatonin in a process mediated partly by intracellular Ca(2+) . Melatonin promotes the development and synchronicity of parasites, thereby enhancing their spread and worsening the clinical implications. The signalling mechanisms underlying the effects of melatonin are not fully established, although both Ca(2+) and cyclic AMP (cAMP) have been implicated. Furthermore, it is not clear whether different strains of Plasmodium use the same, or divergent, signals to control their development. The aim of this study was to explore the signalling mechanisms engaged by melatonin in P. chabaudi, a virulent rodent parasite. Using parasites at the throphozoite stage acutely isolated from mice erythrocytes, we demonstrate that melatonin triggers cAMP production and protein kinase A (PKA) activation. Interestingly, the stimulation of cAMP/PKA signalling by melatonin was dependent on elevation of Ca(2+) within the parasite, because buffering Ca(2+) changes using the chelator BAPTA prevented cAMP production in response to melatonin. Incubation with melatonin evoked robust Ca(2+) signals within the parasite, as did the application of a membrane-permeant analogue of cAMP. Our data suggest that P. chabaudi engages both Ca(2+) and cAMP signalling systems when stimulated by melatonin. Furthermore, there is positive feedback between these messengers, because Ca(2+) evokes cAMP elevation and vice versa. Melatonin more than doubled the observed extent of parasitemia, and the increase in cAMP concentration and PKA activation was essential for this effect. These data support the possibility to use melatonin antagonists or derivates in therapeutic approach.

FRET peptides reveal differential proteolytic activation in intraerythrocytic stages of the malaria parasites Plasmodium berghei and Plasmodium yoelii

Malaria is still a major health problem in developing countries. It is caused by the protist parasite Plasmodium, in which proteases are activated during the cell cycle. Ca(2+) is a ubiquitous signalling ion that appears to regulate protease activity through changes in its intracellular concentration. Proteases are crucial to Plasmodium development, but the role of Ca(2+) in their activity is not fully understood. Here we investigated the role of Ca(2+) in protease modulation among rodent Plasmodium spp. Using fluorescence resonance energy transfer (FRET) peptides, we verified protease activity elicited by Ca(2+) from the endoplasmatic reticulum (ER) after stimulation with thapsigargin (a sarco/endoplasmatic reticulum Ca(2+)-ATPase (SERCA) inhibitor) and from acidic compartments by stimulation with nigericin (a K(+)/H(+) exchanger) or monensin (a Na(+)/H(+) exchanger). Intracellular (BAPTA/AM) and extracellular (EGTA) Ca(2+) chelators were used to investigate the role played by Ca(2+) in protease activation. In Plasmodium berghei both EGTA and BAPTA blocked protease activation, whilst in Plasmodium yoelii these compounds caused protease activation. The effects of protease inhibitors on thapsigargin-induced proteolysis also differed between the species. Pepstatin A and phenylmethylsulphonyl fluoride (PMSF) increased thapsigargin-induced proteolysis in P. berghei but decreased it in P. yoelii. Conversely, E64 reduced proteolysis in P. berghei but stimulated it in P. yoelii. The data point out key differences in proteolytic responses to Ca(2+) between species of Plasmodium.

Melatonin and IP3-induced Ca2+ release from intracellular stores in the malaria parasite Plasmodium falciparum within infected red blood cells

IP(3)-dependent Ca(2+) signaling controls a myriad of cellular processes in higher eukaryotes and similar signaling pathways are evolutionarily conserved in Plasmodium, the intracellular parasite that causes malaria. We have reported that isolated, permeabilized Plasmodium chabaudi, releases Ca(2+) upon addition of exogenous IP(3). In the present study, we investigated whether the IP(3) signaling pathway operates in intact Plasmodium falciparum, the major disease-causing human malaria parasite. P. falciparum-infected red blood cells (RBCs) in the trophozoite stage were simultaneously loaded with the Ca(2+) indicator Fluo-4/AM and caged-IP(3). Photolytic release of IP(3) elicited a transient Ca(2+) increase in the cytosol of the intact parasite within the RBC. The intracellular Ca(2+) pools of the parasite were selectively discharged, using thapsigargin to deplete endoplasmic reticulum (ER) Ca(2+) and the antimalarial chloroquine to deplete Ca(2+) from acidocalcisomes. These data show that the ER is the major IP(3)-sensitive Ca(2+) store. Previous work has shown that the human host hormone melatonin regulates P. falciparum cell cycle via a Ca(2+)-dependent pathway. In the present study, we demonstrate that melatonin increases inositol-polyphosphate production in intact intraerythrocytic parasite. Moreover, the Ca(2+) responses to melatonin and uncaging of IP(3) were mutually exclusive in infected RBCs. Taken together these data provide evidence that melatonin activates PLC to generate IP(3) and open ER-localized IP(3)-sensitive Ca(2+) channels in P. falciparum. This receptor signaling pathway is likely to be involved in the regulation and synchronization of parasite cell cycle progression.

Purinergic signalling is involved in the malaria parasite Plasmodium falciparum invasion to red blood cells

Plasmodium falciparum, the most important etiological agent of human malaria, is endowed with a highly complex cell cycle that is essential for its successful replication within the host. A number of evidence suggest that changes in parasite Ca(2+) levels occur during the intracellular cycle of the parasites and play a role in modulating its functions within the RBC. However, the molecular identification of Plasmodium receptors linked with calcium signalling and the causal relationship between Ca(2+) increases and parasite functions are still largely mysterious. We here describe that increases in P. falciparum Ca(2+) levels, induced by extracellular ATP, modulate parasite invasion. In particular, we show that addition of ATP leads to an increase of cytosolic Ca(2+) in trophozoites and segmented schizonts. Addition of the compounds KN62 and Ip5I on parasites blocked the ATP-induced rise in [Ca(2+)](c). Besides, the compounds or hydrolysis of ATP with apyrase added in culture drastically reduce RBC infection by parasites, suggesting strongly a role of extracellular ATP during RBC invasion. The use of purinoceptor antagonists Ip5I and KN62 in this study suggests the presence of putative purinoceptor in P. falciparum. In conclusion, we have demonstrated that increases in [Ca(2+)](c) in the malarial parasite P. falciparum by ATP leads to the modulation of its invasion of red blood cells.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s11302-010-9202-y) contains supplementary material, which is available to authorized users.

The survival strategies of malaria parasite in the red blood cell and host cell polymorphisms

Parasite growth within the erythrocyte causes dramatic alterations of host cell which on one hand facilitates nutrients acquisition from extracellular environment and on other hand contributes to the symptoms of severe malaria. The current paper focuses on interactions between the Plasmodium parasite and its metabolically highly reduced host cell, the natural selection of numerous polymorphisms in the genes encoding hemoglobin and other erythrocyte proteins.

Biochemical markers of nutritional status and childhood malaria severity in Cameroon

To investigate the part played by undernutrition in malaria severity, some biomarkers of nutritional status were assessed in children with severe malarial anaemia (MA) and cerebral malaria (CM) in comparison with healthy children or those with uncomplicated malaria. Undernutrition was assessed using the weight-for-age Z score (WAZ). Retinol was determined by HPLC; lipid profile, Ca, Mg and albumin were determined by spectrophotometry. Severe and moderate undernutritions were more prevalent in children with MA and those with the combined symptoms of CM and MA, but not in those with CM alone. Some perturbations were noticed in the lipid profile, but most of the values remained within the normal ranges. The risk of vitamin A deficiency, as assessed by plasma retinol concentration, was noteworthy in children with severe malaria: 0.48 × 10(-6) and 0.50 × 10(-6) mol/l, respectively, in children with MA and CM (reference value: >0.7 × 10(-6) mol/l). A significant difference was obtained for retinol values after an ANOVA of all the groups (P = 0.0029), with the value in the MA group being significantly low than that in the control group (P < 0.05); likewise, a significant difference was obtained after comparison of all the groups for Mg and albumin (P = 0.0064 and 0.0082, respectively). Despite their low number (n 6), fatal cases of CM had a normal mean WAZ on admission, but low values of retinol, albumin and HDL:LDL ratio. Despite these associations, undernutrition itself did not appear to be a primary factor associated with fatal outcome.

PfCHA is a mitochondrial divalent cation/H+ antiporter in Plasmodium falciparum

The human malaria parasite Plasmodium falciparum is capable of adapting to vastly different extracellular Ca(2+) environments while maintaining tight control of its intracellular Ca(2+) concentration. The mechanisms underpinning Ca(2+) homeostasis in this important pathogen are only partly understood. Here we have functionally expressed the putative Ca(2+)/H(+) antiporter PfCHA in Xenopus laevis oocytes. Our data suggest that PfCHA mediates H(+)-coupled Ca(2+) and Mn(2+) exchange. The apparent dissociation constant K(M) for Ca(2+) of 2.2 +/- 0.7 mM and the maximal velocity V(max) of 0.6 +/- 0.1 nmol per oocyte per hour are consistent with PfCHA being a low-affinity high-capacity Ca(2+) carrier. In the parasite, PfCHA was found to localize to the mitochondrion. Physiological studies conducted with live parasitized erythrocytes, and using Fluo-4 and Rhod-2 to monitor cytoplasmic and mitochondrial Ca(2+) dynamics, suggest that the mitochondrion serves as a dynamic Ca(2+) store and that PfCHA functions as a Ca(2+) efflux system expelling excess Ca(2+) from the mitochondrion. PfCHA lacks appreciable homologies to the human mitochondrial Ca(2+)/H(+) exchanger and might represent an evolutionary divergent class of mitochondrial cation antiporter, which, in turn, might provide novel opportunities for intervention.

The plasmodium receptor for activated C kinase protein inhibits Ca(2+) signaling in mammalian cells

Plasmodium falciparum, the most lethal malarial parasite, expresses an ortholog for the protein kinase C (PKC) activator RACK1. However, PKC has not been identified in this parasite, and the mammalian RACK1 can interact with the inositol 1,4,5-trisphosphate receptor (InsP3R). Therefore we investigated whether the Plasmodium ortholog PfRACK also can affect InsP3R-mediated Ca(2+) signaling in mammalian cells. GFP-tagged PfRACK and endogenous RACK1 were expressed in a similar distribution within cells. PfRACK inhibited agonist-induced Ca(2+) signals in cells expressing each isoform of the InsP3R, and this effect persisted when expression of endogenous RACK1 was reduced by siRNA. PfRACK also inhibited Ca(2+) signals induced by photorelease of caged InsP3. These findings provide evidence that PfRACK directly inhibits InsP3-mediated Ca(2+) signaling in mammalian cells. Interference with host cell signaling pathways to subvert the host intracellular milieu may be an important mechanism for parasite survival.

Unlike the synchronous Plasmodium falciparum and P. chabaudi infection, the P. berghei and P. yoelii asynchronous infections are not affected by melatonin

We have previously reported that Plasmodium chabaudi and P. falciparum sense the hormone melatonin and this could be responsible for the synchrony of malaria infection. In P. chabaudi and P. falciparum, melatonin induces calcium release from internal stores, and this response is abolished by U73122, a phospholipase C inhibitor, and luzindole, a melatonin-receptor competitive antagonist. Here we show that, in vitro, melatonin is not able to modulate cell cycle, nor to elicit an elevation in intracellular calcium concentration of the intraerythrocytic forms of P. berghei or P. yoelii, two rodent parasites that show an asynchrononous development in vivo. Interestingly, melatonin and its receptor do not seem to play a role during hepatic infection by P. berghei sporozoites either. These data strengthen the hypothesis that host-derived melatonin does not synchronize malaria infection caused by P. berghei and P. yoelii. Moreover, these data explain why infections by these parasites are asynchronous, contrary to what is observed in P. falciparum and P. chabaudi infections.

Antimalarials and the fight against malaria in Brazil

Malaria, known as the “fevers,” has been treated for over three thousand years in China with extracts of plants of the genus Artemisia (including Artemisia annua, A. opiacea, and A. lancea) from which the active compound is artemisin, a sesquiterpene that is highly effective in the treatment of the disease, especially against young forms of the parasite. South American Indians in the seventeenth century already used an extract of the bark of chinchona tree, commonly named “Jesuits’ powder.” Its active compound was isolated in 1820 and its use spread all over the world being used as a prophylactic drug during the construction of the Madeira–Mamoré railroad in the beginning of the twentieth century. During the 1920s to the 1940s, new antimalarial drugs were synthesized to increase the arsenal against this parasite. However, the parasite has presented systematic resistence to conventional antimalarial drugs, driving researchers to find new strategies to treat the disease. In the present review we discuss how Brazil treats Plasmodium-infected patients.

Dissection of mechanisms involved in the regulation of Plasmodium falciparum calcium-dependent protein kinase 4

Recent studies have demonstrated that calcium-dependent protein kinases (CDPKs) are used by calcium to regulate a variety of biological processes in the malaria parasite Plasmodium. CDPK4 has emerged as an important enzyme for parasite development, because its gene disruption in rodent parasite Plasmodium berghei causes major defects in sexual differentiation of the parasite ( Billker, O., Dechamps, S., Tewari, R., Wenig, G., Franke-Fayard, B., and Brinkmann, V. (2004) Cell 117, 503-514 ). Despite these findings, it is not very clear how PfCDPK4 or any other PfCDPK is regulated by calcium at the molecular level. We report the biochemical characterization and elucidation of molecular mechanisms involved in the regulation of PfCDPK4. PfCDPK4 was detected on gametocyte periphery, and its activity in the parasite was regulated by phospholipase C. Even though the Junction Domain (JD) of PfCDPK4 shares moderate sequence homology with that of the plant CDPKs, it plays a pivotal role in PfCDPK4 regulation as previously reported for some plant CDPKs. The regions of the J-domain involved in interaction with both the kinase domain and the calmodulin-like domain were mapped. We propose a model for PfCDPK regulation by calcium, which may also prove useful for design of inhibitors against PfCDPK4 and other members of the PfCDPK family.

Molecular machinery of signal transduction and cell cycle regulation in Plasmodium

The regulation of the Plasmodium cell cycle is not understood. Although the Plasmodium falciparum genome is completely sequenced, about 60% of the predicted proteins share little or no sequence similarity with other eukaryotes. This feature impairs the identification of important proteins participating in the regulation of the cell cycle. There are several open questions that concern cell cycle progression in malaria parasites, including the mechanism by which multiple nuclear divisions is controlled and how the cell cycle is managed in all phases of their complex life cycle. Cell cycle synchrony of the parasite population within the host, as well as the circadian rhythm of proliferation, are striking features of some Plasmodium species, the molecular basis of which remains to be elucidated. In this review we discuss the role of indole-related molecules as signals that modulate the cell cycle in Plasmodium and other eukaryotes, and we also consider the possible role of kinases in the signal transduction and in the responses it triggers.

Ortholog-based protein-protein interaction prediction and its application to inter-species interactions

BACKGROUND

The rapid growth of protein-protein interaction (PPI) data has led to the emergence of PPI network analysis. Despite advances in high-throughput techniques, the interactomes of several model organisms are still far from complete. Therefore, it is desirable to expand these interactomes with ortholog-based and other methods.

RESULTS

Orthologous pairs of 18 eukaryotic species were expanded and merged with experimental PPI datasets. The contributions of interologs from each species were evaluated. The expanded orthologous pairs enable the inference of interologs for various species. For example, more than 32,000 human interactions can be predicted. The same dataset has also been applied to the prediction of host-pathogen interactions. PPIs between P. falciparum calmodulin and several H. sapiens proteins are predicted, and these interactions may contribute to the maintenance of host cell Ca2+ concentration. Using comparisons with Bayesian and structure-based approaches, interactions between putative HSP40 homologs of P. falciparum and the H. sapiens TNF receptor associated factor family are revealed, suggesting a role for these interactions in the interference of the human immune response to P. falciparum.

CONCLUSION

The PPI datasets are available from POINT http://point.bioinformatics.tw/ and POINeT http://poinet.bioinformatics.tw/. Further development of methods to predict host-pathogen interactions should incorporate multiple approaches in order to improve sensitivity, and should facilitate the identification of targets for drug discovery and design.

Host control of malaria infections: constraints on immune and erythropoeitic response kinetics

The two main agents of human malaria, Plasmodium vivax and Plasmodium falciparum, can induce severe anemia and provoke strong, complex immune reactions. Which dynamical behaviors of host immune and erythropoietic responses would foster control of infection, and which would lead to runaway parasitemia and/or severe anemia? To answer these questions, we developed differential equation models of interacting parasite and red blood cell (RBC) populations modulated by host immune and erythropoietic responses. The model immune responses incorporate both a rapidly responding innate component and a slower-responding, long-term antibody component, with several parasite developmental stages considered as targets for each type of immune response. We found that simulated infections with the highest parasitemia tended to be those with ineffective innate immunity even if antibodies were present. We also compared infections with dyserythropoiesis (reduced RBC production during infection) to those with compensatory erythropoiesis (boosted RBC production) or a fixed basal RBC production rate. Dyserythropoiesis tended to reduce parasitemia slightly but at a cost to the host of aggravating anemia. On the other hand, compensatory erythropoiesis tended to reduce the severity of anemia but with enhanced parasitemia if the innate response was ineffective. For both parasite species, sharp transitions between the schizont and the merozoite stages of development (i.e., with standard deviation in intra-RBC development time <or=2.4 h) were associated with lower parasitemia and less severe anemia. Thus tight synchronization in asexual parasite development might help control parasitemia. Finally, our simulations suggest that P. vivax can induce severe anemia as readily as P. falciparum for the same type of immune response, though P. vivax attacks a much smaller subset of RBCs. Since most P. vivax infections are nonlethal (if debilitating) clinically, this suggests that P. falciparum adaptations for countering or evading immune responses are more effective than those of P. vivax.

Gene expression signatures and small-molecule compounds link a protein kinase to Plasmodium falciparum motility

Calcium-dependent protein kinases play a crucial role in intracellular calcium signaling in plants, some algae and protozoa. In Plasmodium falciparum, calcium-dependent protein kinase 1 (PfCDPK1) is expressed during schizogony in the erythrocytic stage as well as in the sporozoite stage. It is coexpressed with genes that encode the parasite motor complex, a cellular component required for parasite invasion of host cells, parasite motility and potentially cytokinesis. A targeted gene-disruption approach demonstrated that pfcdpk1 seems to be essential for parasite viability. An in vitro biochemical screen using recombinant PfCDPK1 against a library of 20,000 compounds resulted in the identification of a series of structurally related 2,6,9-trisubstituted purines. Compound treatment caused sudden developmental arrest at the late schizont stage in P. falciparum and a large reduction in intracellular parasites in Toxoplasma gondii, which suggests a possible role for PfCDPK1 in regulation of parasite motility during egress and invasion.

Genome-wide detection of serpentine receptor-like proteins in malaria parasites

Serpentine receptors comprise a large family of membrane receptors distributed over diverse organisms, such as bacteria, fungi, plants and all metazoans. However, the presence of serpentine receptors in protozoan parasites is largely unknown so far. In the present study we performed a genome-wide search for proteins containing seven transmembrane domains (7-TM) in the human malaria parasite Plasmodium falciparum and identified four serpentine receptor-like proteins. These proteins, denoted PfSR1, PfSR10, PfSR12 and PfSR25, show membrane topologies that resemble those exhibited by members belonging to different families of serpentine receptors. Expression of the pfsrs genes was detected by Real Time PCR in P. falciparum intraerythrocytic stages, indicating that they potentially code for functional proteins. We also found corresponding homologues for the PfSRs in five other Plasmodium species, two primate and three rodent parasites. PfSR10 and 25 are the most conserved receptors among the different species, while PfSR1 and 12 are more divergent. Interestingly, we found that PfSR10 and PfSR12 possess similarity to orphan serpentine receptors of other organisms. The identification of potential parasite membrane receptors raises a new perspective for essential aspects of malaria parasite host cell infection.

Plasmodium in the postgenomic era: new insights into the molecular cell biology of malaria parasites

In this review, we bring together some of the approaches toward understanding the cellular and molecular biology of Plasmodium species and their interaction with their host red blood cells. Considerable impetus has come from the development of new methods of molecular genetics and bioinformatics, and it is important to evaluate the wealth of these novel data in the context of basic cell biology. We describe how these approaches are gaining valuable insights into the parasite-host cell interaction, including (1) the multistep process of red blood cell invasion by the merozoite; (2) the mechanisms by which the intracellular parasite feeds on the red blood cell and exports parasite proteins to modify its cytoadherent properties; (3) the modulation of the cell cycle by sensing the environmental tryptophan-related molecules; (4) the mechanism used to survive in a low Ca(2+) concentration inside red blood cells; (5) the activation of signal transduction machinery and the regulation of intracellular calcium; (6) transfection technology; and (7) transcriptional regulation and genome-wide mRNA studies in Plasmodium falciparum.

Role of Ca2+/calmodulin-PfPKB signaling pathway in erythrocyte invasion by Plasmodium falciparum

Molecular mechanisms by which signaling pathways operate in the malaria parasite and control its development are promiscuous. Recently, we reported the identification of a signaling pathway in Plasmodium falciparum, which involves activation of protein kinase B-like enzyme (PfPKB) by calcium/calmodulin (Vaid, A., and Sharma, P. (2006) J. Biol. Chem. 281, 27126-27133). Studies carried out to elucidate the function of this pathway suggested that it may be important for erythrocyte invasion. Blocking the function of the upstream activators of this pathway, calmodulin and phospholipase C, resulted in impaired invasion. To evaluate if this signaling cascade controls invasion by regulating PfPKB, inhibitors against this kinase were developed. PfPKB inhibitors dramatically reduced the ability of the parasite to invade erythrocytes. Furthermore, we demonstrate that PfPKB associates with actin-myosin motor and phosphorylates PfGAP45 (glideosome-associated protein 45), one of the important components of the motor complex, which may help explain its role in erythrocyte invasion.

Human malarial parasite, Plasmodium falciparum, displays capacitative calcium entry: 2-aminoethyl diphenylborinate blocks the signal transduction pathway of melatonin action on the P. falciparum cell cycle

The malarial parasite senses the environment to modulate its own cycle. Knowledge of the mechanisms for regulation signaling processes at the invasion, maturation, as well as division of Plasmodium falciparum before reinvasion would represent a major breakthrough and, therefore, might open new avenues for therapy. We have previously reported that melatonin modulates the circadian rhythm of malarial parasites through the activation of phospholipase C (PLC), production of InsP3, and induction of calcium release from intracellular stores. To further investigate the molecular mechanism of melatonin's action, we have used the InsP3 modulator 2-aminoethyl diphenylborinate (2-APB) given in a culture of P. falciparum parasites. Here we show that the melatonin acts on Plasmodium cell cycle through InsP3 signaling as 2-APB blocks melatonin's effect on calcium release. The function of the InsP3 signaling can be regarded as an important event for parasite invasion and maturation process, since addition of the PLC inhibitor, U73122 into Plasmodium-infected red blood cells impairs parasite invasion in vitro. By using 8BrcAMP, we also report here that Plasmodia displays a 'capacitative calcium entry' mechanism for amplification of calcium signals throughout the cytoplasm.

Antimalarial drugs disrupt ion homeostasis in malarial parasites

Plasmodium chabaudi malaria parasite organelles are major elements for ion homeostasis and cellular signaling and also target for antimalarial drugs. By using confocal imaging of intraerythrocytic parasites we demonstrated that the dye acridine orange (AO) is accumulated into P. chabaudi subcellular compartments. The AO could be released from the parasite organelles by collapsing the pH gradient with the K+/H+ ionophore nigericin (20 microM), or by inhibiting the H+-pump with bafilomycin (4 microM). Similarly, in isolated parasites loaded with calcium indicator Fluo 3-AM, bafilomycin caused calcium mobilization of the acidic calcium pool that could also be release with nigericin. Interestingly after complete release of the acidic compartments, addition of thapsigargin at 10 microM was still effective in releasing parasite intracellular calcium stores in parasites at trophozoite stage. The addition of antimalarial drugs chloroquine and artemisinin resulted in AO release from acidic compartments and also affected maintenance of calcium in ER store by using different drug concentrations.

Cyclic AMP and calcium interplay as second messengers in melatonin-dependent regulation of Plasmodium falciparum cell cycle

The host hormone melatonin increases cytoplasmic Ca(2+) concentration and synchronizes Plasmodium cell cycle (Hotta, C.T., M.L. Gazarini, F.H. Beraldo, F.P. Varotti, C. Lopes, R.P. Markus, T. Pozzan, and C.R. Garcia. 2000. Nat. Cell Biol. 2:466-468). Here we show that in Plasmodium falciparum melatonin induces an increase in cyclic AMP (cAMP) levels and cAMP-dependent protein kinase (PKA) activity (40 and 50%, respectively). When red blood cells infected with P. falciparum are treated with cAMP analogue adenosine 3',5'-cyclic monophosphate N6-benzoyl/PKA activator (6-Bz-cAMP) there is an alteration of the parasite cell cycle. This effect appears to depend on activation of PKA (abolished by the PKA inhibitors adenosine 3',5'-cyclic monophosphorothioate/8 Bromo Rp isomer, PKI [cell permeable peptide], and H89). An unexpected cross talk was found to exist between the cAMP and the Ca(2+)-dependent signaling pathways. The increases in cAMP by melatonin are inhibited by blocker of phospholipase C U73122, and addition of 6-Bz-cAMP increases cytosolic Ca(2+) concentration, through PKA activation. These findings suggest that in Plasmodium a highly complex interplay exists between the Ca(2+) and cAMP signaling pathways, but also that the control of the parasite cell cycle by melatonin requires the activation of both second messenger controlled pathways.

Divergent calcium signaling in RBCs from Tropidurus torquatus (Squamata--Tropiduridae) strengthen classification in lizard evolution

Background

We have previously reported that a Teiid lizard red blood cells (RBCs) such as Ameiva ameiva and Tupinambis merianae controls intracellular calcium levels by displaying multiple mechanisms. In these cells, calcium stores could be discharged not only by: thapsigargin, but also by the Na⁺/H⁺ ionophore monensin, K⁺/H⁺ ionophore nigericin and the H⁺ pump inhibitor bafilomycin as well as ionomycin. Moreover, these lizards possess a P2Y-type purinoceptors that mobilize Ca²⁺ from intracellular stores upon ATP addition.

Results

Here we report, that RBCs from the tropidurid lizard Tropidurus torquatus store Ca²⁺ in endoplasmic reticulum (ER) pool but unlike in the referred Teiidae, these cells do not store calcium in monensin-nigericin sensitive pools. Moreover, mitochondria from T. torquatus RBCs accumulate Ca²⁺. Addition of ATP to a calcium-free medium does not increase the [Ca²⁺]_c levels, however in a calcium medium we observe an increase in cytosolic calcium. This is an indication that purinergic receptors in these cells are P2X-like.

Conclusion

T. torquatus RBCs present different mechanisms from Teiid lizard red blood cells (RBCs), for controlling its intracellular calcium levels. At T. torquatus the ion is only stored at endoplasmic reticulum and mitochondria. Moreover activation of purinergic receptor, P2X type, was able to induce an influx of calcium from extracelullar medium. These studies contribute to the understanding of the evolution of calcium homeostasis and signaling in nucleated RBCs.

Calcium signaling in lizard red blood cells

The ion calcium is a ubiquitous second messenger, present in all eukaryotic cells. It modulates a vast number of cellular events, such as cell division and differentiation, fertilization, cell volume, decodification of external stimuli. To process this variety of information, the cells display a number of calcium pools, which are capable of mobilization for signaling purposes. Here we review the calcium signaling on lizards red blood cells, an interesting model that has been receiving an increasing notice recently. These cells possess a complex machinery to regulate calcium, and display calcium responses to extracellular agonists. Interestingly, the pattern of calcium handling and response are divergent in different lizard families, which enforces the morphological data to their phylogenetic classification, and suggest the radiation of different calcium signaling models in lizards evolution.

N1-acetyl-N2-formyl-5-methoxykynuramine modulates the cell cycle of malaria parasites

We previously reported that intraerythrocytic malaria parasites have their development synchronized by melatonin and other products of tryptophan catabolism (i.e. serotonin, N-acetylserotonin and tryptamine). Here, we show that N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK), a product of melatonin degradation, synchronizes Plasmodium chabaudi and Plasmodium falciparum. The synchronization is abrogated with a melatonin receptor antagonist, luzindole. We established quantitatively that a differential AFMK production occurred within the intraerythrocytic stages of rodent malaria parasite Plasmodium chabaudi (ring, trophozoite and schizont), when the infected erythrocytes were previously incubated with melatonin. Measurement of AFMK formation in P. chabaudi after incubation with melatonin at a concentration of 500 nmol/L revealed the following values for AFMK production: ring 0.1 +/- 0.1 nmol/L, trophozoite 22.9 +/- 0.5 nmol/L, schizont 29 +/- 5 nmol/L. Confocal and spectrofluorophotometer experiments with isolated parasites and infected-RBC, loaded with calcium indicator Fluo-4 showed that AFMK elicits an increase in the cytosol calcium concentration in these parasites. Our data suggest that AFMK could have an important role in modulating the cell cycle of malaria parasites mainly in the late stages (trophozoite and schizont).

Inhibition of eryptosis and intraerythrocytic growth of Plasmodium falciparum by flufenamic acid

Non-selective (NSC) cation channels participate in the Ca(2+) leak of human erythrocytes. Sustained activity of these channels triggers suicidal erythrocyte death (eryptosis), which is characterized by Ca(2+)-stimulated cell shrinkage and phosphatidylserine (PS) exposure. PS-exposing erythrocytes are rapidly cleared from circulating blood. PGE(2) activates the NSC channels, and erythrocyte PGE(2) formation is stimulated by a decrease in intra- or extracellular Cl(-) concentration. In addition, the intraerythrocytic malaria parasite Plasmodium falciparum activates the NSC channels, most probably to accomplish Na(+) and Ca(2+) entry into the erythrocyte cytosol required for parasite development. By Ca(2+) uptake the parasite maintains a low Ca(2+) concentration in the erythrocyte cytosol and thus delays the suicidal death of the host erythrocyte. Flufenamic acid has previously been shown to inhibit NSC channels. The present study thus explored the effect of flufenamic acid on erythrocyte Ca(2+) entry, on suicidal erythrocyte death and on intraerythrocytic growth of P. falciparum. Within 48 h, replacement of extracellular Cl(-) with gluconate or application of PGE(2) (50 microM) increased Fluo3 fluorescence reflecting cytosolic Ca(2+) activity, decreased forward scatter reflecting cell volume and increased annexin V binding reflecting PS exposure in FACS analysis. All those effects were significantly blunted in the presence of flufenamic acid (10 microM). Flufenamic acid (25 microM) further significantly delayed the intraerythrocytic growth of P. falciparum and the PS exposure of the infected erythrocytes. The present observations disclose a novel effect of flufenamic acid, which may allow the pharmacological manipulation of erythrocyte survival and the course of malaria.

PfPKB, a protein kinase B-like enzyme from Plasmodium falciparum: II. Identification of calcium/calmodulin as its upstream activator and dissection of a novel signaling pathway

Intracellular cell signaling cascades of protozoan parasite Plasmodium falciparum are not clearly understood. We have reported previously (Kumar, A., Vaid, A., Syin, C., and Sharma, P. (2004) J. Biol. Chem. 279, 24255-24264) the identification and characterization of a protein kinase B-like enzyme in P. falciparum (PfPKB). PfPKB lacks the phosphoinositide-interacting pleckstrin homology domain present in mammalian protein kinase B. Therefore, the mechanism of PfPKB regulation was expected to be different from that of the host and had remained unknown. We have identified calmodulin (CaM) as the regulator of PfPKB activity. A CaM binding domain was mapped in the N-terminal region of PfPKB. CaM, in a calcium-dependent manner, interacts with this domain and activates PfPKB. CaM associates with PfPKB in the parasite and regulates its activity. Furthermore phospholipase C acts as an upstream regulator of this cascade as it facilitates the release of calcium from intracellular stores. This is one of the first multicomponent signaling pathways to be dissected in the malaria parasite.

Calcium-ions are involved in erythrocyte invasion by equine Babesia parasites

Ethylene glycol bis (β-aminoethylether)-N,N,N,N-tetraacetic acid (EGTA) is a chelating agent capable of binding to positively-charged metal ions, including a calcium-ion (Ca2+). Here, we demonstrated the inhibitory effect of the chemical on the in vitro asexual growth of the equine protozoan parasites, Babesia caballi and Babesia equi. The growth of both B. caballi and B. equi was significantly inhibited in the presence of EGTA (IC50=1·27 and 2·25 mM, respectively). Under microscopical observation, increased percentages of extracellular merozoites in the total parasites were detected in both of the cultures treated with high concentrations of EGTA. In contrast, further addition of Ca2+ to the EGTA-treated cultures prevented the parasites from clearing and the percentages of extracellular merozoites from increasing. As for B. caballi, an invasion test using high-voltage pulsing proved that EGTA has an inhibitory effect to their erythrocyte invasion. These results suggest that Ca2+ is involved in erythrocyte invasion by equine Babesia parasites.

Dense granules: are they key organelles to help understand the parasitophorous vacuole of all apicomplexa parasites?

Together with micronemes and rhoptries, dense granules are specialised secretory organelles of Apicomplexa parasites. Among Apicomplexa, Plasmodium represents a model of parasites propagated by way of an insect vector, whereas Toxoplasma is a model of food borne protozoa forming cysts. Through comparison of both models, this review summarises data accumulated over recent years on alternative strategies chosen by these parasites to develop within a parasitophorous vacuole and explores the role of dense granules in this process. One of the characteristics of the Plasmodium erythrocyte stages is to export numerous parasite proteins into both the host cell cytoplasm and/or plasma membrane via the vacuole used as a step trafficking compartment. Whether this feature can be correlated to few storage granules and a restricted number of dense granule proteins, is not yet clear. By contrast, the Toxoplasma developing vacuole is decorated by abundantly expressed dense granule proteins and is characterised by a network of membranous nanotubes. Although the exact function of most of these proteins remains currently unknown, recent data suggest that some of these dense granule proteins could be involved in building the intravacuolar membranous network. Conserved expression of the Toxoplasma dense granule proteins throughout most of the parasite stages suggests that they could also be key elements of the cyst formation.

Plasmodium permeomics: membrane transport proteins in the malaria parasite

Membrane transport proteins are integral membrane proteins that mediate the passage across the membrane bilayer of specific molecules and/or ions. Such proteins serve a diverse range of physiological roles, mediating the uptake of nutrients into cells, the removal of metabolic wastes and xenobiotics (including drugs), and the generation and maintenance of transmembrane electrochemical gradients. In this chapter we review the present state of knowledge of the membrane transport mechanisms underlying the cell physiology of the intraerythrocytic malaria parasite and its host cell, considering in particular physiological measurements on the parasite and parasitized erythrocyte, the annotation of transport proteins in the Plasmodium genome, and molecular methods used to analyze transport protein function.

Products of tryptophan catabolism induce Ca2+ release and modulate the cell cycle of Plasmodium falciparum malaria parasites

Intraerythrocytic malaria parasites develop in a highly synchronous manner. We have previously shown that the host hormone melatonin regulates the circadian rhythm of the rodent malaria parasite, Plasmodium chabaudi, through a Ca2+-based mechanism. Here we show that melatonin and other molecules derived from tryptophan, i.e. N-acetylserotonin, serotonin and tryptamine, also modulate the cell cycle of human malaria parasite P. falciparum by inducing an increase in cytosolic free Ca2+. This occurs independently of the extracellular Ca2+ concentration, indicating that these molecules induce Ca2+ mobilization from intracellular stores in the trophozoite. This in turn leads to an increase in the proportion of schizonts. The effects of the indolamines in increasing cytosolic free Ca2+ and modulating the parasite cell cycle are both abrogated by an antagonist of the melatonin receptor, luzindole, and by the phospholipase inhibitor, U73122.

Cysteine-protease activity elicited by Ca2+ stimulus in Plasmodium

Bloodstage malaria parasites require proteolytic activity for key processes as invasion, hemoglobin degradation and merozoite escape from red blood cells (RBCs). We investigated by confocal microscopy the presence of cysteine-protease activity elicited by calcium stimulus in Plasmodium chabaudi and Plasmodium falciparum in free trophozoites or for the later parasite within RBC using fluorescence resonance energy transfer (FRET) peptides. Peptide probes access, to either free or intraerythrocytic parasites, was also tested by selecting a range of fluorescent peptides (653-3146 Da molecular mass) labeled with Abz or FITC. In the present work we show that Ca2+ stimulus elicited by treatment with either melatonin, thapsigargin, ionomicin or nigericin, promotes an increase of substrate hydrolysis, which was blocked by the specific cysteine-protease inhibitor E-64 and the intracellular Ca2+ chelator, BAPTA. When parasites were treated with cytoplasmic Ca2+ releasing compounds, a cysteine-protease was labeled in the parasite cytoplasm by the fluorescent specific irreversible inhibitor, Ethyl-Eps-Leu-Tyr-Cap-Lys(Abz)-NH2, where Ethyl-Eps is Ethyl-(2S,3S)-oxirane-2,3-dicarboxylate. In summary, we demonstrate that P. chabaudi and P. falciparum have a cytoplasmic dependent cysteine-protease activity elicited by Ca2+.

Export of Plasmodium falciparum calcium-dependent protein kinase 1 to the parasitophorous vacuole is dependent on three N-terminal membrane anchor motifs

Calcium-dependent protein kinases play a pivotal role in calcium signalling in plants and some protozoa, including the malaria parasites. They are found in various subcellular locations, suggesting an involvement in multiple signal transduction pathways. Recently, Plasmodium falciparum calcium-dependent protein kinase 1 (PfCDPK1) has been found in the membrane and organelle fraction of the parasite. The kinase contains three motifs for membrane binding at its N-terminus, a consensus sequence for myristoylation, a putative palmitoylation site and a basic motif. Endogenous PfCDPK1 and the in vitro translated kinase were both shown to be myristoylated. The supposed membrane attachment function of the basic cluster was experimentally verified and shown to participate together with N-myristoylation in membrane anchoring of the kinase. Using immunogold electron microscopy, the protein was detected in the parasitophorous vacuole and the tubovesicular system of the parasite. Mutagenesis of the predicted acylated residues and the basic motif confirmed that dual acylation and the basic cluster are required for correct targeting of Aequorea victoria green fluorescent protein to the parasitophorous vacuole, suggesting that PfCDPK1 as the leishmanial hydrophilic acylated surface protein B is a representative of a novel class of proteins whose export is dependent on a 'non-classical' pathway involving N-myristoylation/palmitoylation.

Quantitative Calcium Measurements in Subcellular Compartments of Plasmodium falciparum-infected Erythrocytes

The acidic food vacuole exerts several important functions during intraerythrocytic development of the human malarial parasite Plasmodium falciparum. Hemoglobin taken up from the host erythrocyte is degraded in the food vacuole, and the heme liberated during this process is crystallized to inert hemozoin. Several anti-malarial drugs target food vacuolar pathways, such as hemoglobin degradation and heme crystallization. Resistance and sensitization to some antimalarials is associated with mutations in food vacuolar membrane proteins. Other studies suggest a role of the food vacuole in ion homeostasis, and release of Ca2+ from the food vacuole may mediate adopted physiological responses. To investigate whether the food vacuole is an intracellular Ca2+ store, which in turn may affect other physiological functions in which this organelle partakes, we have investigated total and exchangeable Ca2+ within the parasite's food vacuole using x-ray microanalysis and quantitative confocal live cell Ca2+ imaging. Apparent free Ca2+ concentrations of approximately 90, approximately 350, and approximately 400 nM were found in the host erythrocyte cytosol, the parasite cytoplasm, and the food vacuole, respectively. In our efforts to determine free intracellular Ca2+ concentrations, we evaluated several Ca2+-sensitive fluorochromes in a live cell confocal setting. We found that the ratiometric Ca2+ indicator Fura-Red provides reliable determinations, whereas measurements using the frequently used Fluo-4 are compromised due to problems arising from phototoxicity, photobleaching, and the strong pH dependence of the dye. Our data suggest that the food vacuole contains only moderate amounts of Ca2+, disfavoring a role as a major intracellular Ca2+ store.

Of malaria, metabolism and membrane transport

With the sequencing of the Plasmodium falciparum genome now complete, increasing attention is turning to the function of gene products and to cell-regulatory processes. The combination of in silico analyses with modern molecular and biophysical methods is leading to rapid advances in our understanding of the mechanisms underlying the biochemistry and physiology of the parasite and its host cell. In this brief review, we present a "snap shot" of recent work in this area, with particular emphasis on aspects relevant to the development of new antimalarial drugs.