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

Antimalarial and Plasmodium falciparum serpentine receptor 12 targeting effect of FDA approved purinergic receptor antagonist

Intraerythrocytic stages of Plasmodium falciparum responsible for all clinical manifestations of malaria are regulated by array of signalling cascades that represent attractive targets for antimalarial therapy. G-protein coupled receptors (GPCRs) are druggable targets in the treatment of various pathological conditions, however, there is limited understanding about the role of GPCRs in malaria pathogenesis. In Plasmodium, serpentine receptors (PfSR1, PfSR10, PfSR12 and PfSR25) with GPCR-like membrane topology have been reported with the finite knowledge about their potential as antimalarial targets. We analyzed the localization of these receptors in malaria parasite by immunofluorescence assays. All four receptors were expressed in blood stages with PfSR12 expressing more in late intraerythrocytic stages. Further, we evaluated the druggability of PfSR12 using FDA-approved P2Y purinergic receptor antagonist, Prasugrel and its active metabolite R138727, which is proposed to be specific towards PfSR12. Interestingly, biophysical analysis indicated strong binding between PfSR12 and R138727 as compared to the prodrug Prasugrel. This binding interaction was further confirmed by thermal shift assay. Treatment of parasite with Prasugrel and R138727 resulted in growth inhibition of P. falciparum indicating an important role of purinergic signalling and PfSR12 in parasite survival. Next, progression studies indicated the inhibitory effect of Prasugrel begins in late erythrocyte stages corroborating with PfSR12 expression at these stages. Furthermore, Prasugrel also blocked in vivo growth of malaria parasite in a mouse experimental model. This study indicates the presence of P2Y type of purinergic signalling in growth and development of malaria parasite and suggests PfSR12, putative purinergic receptor druggability through Prasugrel.Communicated by Ramaswamy H. Sarma.

Contribution of Transcriptome to Elucidate the Biology of Plasmodium spp

In the present review, we discuss some of the new technologies that have been applied to elucidate how Plasmodium spp escape from the immune system and subvert the host physiology to orchestrate the regulation of its biological pathways. Our manuscript describes how techniques such as microarray approaches, RNA-Seq and single-cell RNA sequencing have contributed to the discovery of transcripts and changed the concept of gene expression regulation in closely related malaria parasite species. Moreover, the text highlights the contributions of high-throughput RNA sequencing for the current knowledge of malaria parasite biology, physiology, vaccine target and the revelation of new players in parasite signaling.

Evidence of G-Protein-Coupled Receptors (GPCR) in the Parasitic Protozoa Plasmodium falciparum-Sensing the Host Environment and Coupling within Its Molecular Signaling Toolkit

Throughout evolution, the need for single-celled organisms to associate and form a single cluster of cells has had several evolutionary advantages. In complex, multicellular organisms, each tissue or organ has a specialty and function that make life together possible, and the organism as a whole needs to act in balance and adapt to changes in the environment. Sensory organs are essential for connecting external stimuli into a biological response, through the senses: sight, smell, taste, hearing, and touch. The G-protein-coupled receptors (GPCRs) are responsible for many of these senses and therefore play a key role in the perception of the cells' external environment, enabling interaction and coordinated development between each cell of a multicellular organism. The malaria-causing protozoan parasite, Plasmodium falciparum, has a complex life cycle that is extremely dependent on a finely regulated cellular signaling machinery. In this review, we summarize strong evidence and the main candidates of GPCRs in protozoan parasites. Interestingly, one of these GPCRs is a sensor for K+ shift in Plasmodium falciparum, PfSR25. Studying this family of proteins in P. falciparum could have a significant impact, both on understanding the history of the evolution of GPCRs and on finding new targets for antimalarials.

Evidences of G Coupled-Protein Receptor (GPCR) Signaling in the human Malaria Parasite Plasmodium falciparum for Sensing its Microenvironment and the Role of Purinergic Signaling in Malaria Parasites

The nucleotides were discovered in the early 19th century and a few years later, the role of such molecules in energy metabolism and cell survival was postulated. In 1972, a pioneer work by Burnstock and colleagues suggested that ATP could also work as a neurotransmitter, which was known as the "purinergic hypothesis". The idea of ATP working as a signaling molecule faced initial resistance until the discovery of the receptors for ATP and other nucleotides, called purinergic receptors. Among the purinergic receptors, the P2Y family is of great importance because it comprises of G proteincoupled receptors (GPCRs). GPCRs are widespread among different organisms. These receptors work in the cells' ability to sense the external environment, which involves: to sense a dangerous situation or detect a pheromone through smell; the taste of food that should not be eaten; response to hormones that alter metabolism according to the body's need; or even transform light into an electrical stimulus to generate vision. Advances in understanding the mechanism of action of GPCRs shed light on increasingly promising treatments for diseases that have hitherto remained incurable, or the possibility of abolishing side effects from therapies widely used today.

The Knockout for G Protein-Coupled Receptor-Like PfSR25 Increases the Susceptibility of Malaria Parasites to the Antimalarials Lumefantrine and Piperaquine but Not to Medicine for Malaria Venture Compounds

Previously we have reported that the G protein-coupled receptor (GPCR)-like PfSR25 in Plasmodium falciparum is a potassium (K⁺) sensor linked to intracellular calcium signaling and that knockout parasites (PfSR25-) are more susceptible to oxidative stress and antimalarial compounds. Here, we explore the potential role of PfSR25 in susceptibility to the antimalarial compounds atovaquone, chloroquine, dihydroartemisinin, lumefantrine, mefloquine, piperaquine, primaquine, and pyrimethamine and the Medicine for Malaria Venture (MMV) compounds previously described to act on egress/invasion (MMV006429, MMV396715, MMV019127, MMV665874, MMV665878, MMV665785, and MMV66583) through comparative assays with PfSR25- and 3D7 parasite strains, using flow cytometry assays. The IC₅₀ and IC₉₀ results show that lumefantrine and piperaquine have greater activity on the PfSR25- parasite strain when compared to 3D7. For MMV compounds, we found no differences between the strains except for the compound MMV665831, which we used to investigate the store-operated calcium entry (SOCE) mechanism. The results suggest that PfSR25 may be involved in the mechanism of action of the antimalarials lumefantrine and piperaquine. Our data clearly show that MMV665831 does not affect calcium entry in parasites after we depleted their internal calcium pools with thapsigargin. The results demonstrated here shed light on new possibilities on the antimalarial mechanism, bringing evidence of the involvement of the GPCR-like PfSR25.

Inhibition of Hemoglobin Degrading Protease Falcipain-2 as a Mechanism for Anti-Malarial Activity of Triazole-Amino Acid Hybrids

BACKGROUND

Novel drug development against malaria parasite over old conventional antimalarial drugs is essential due to rapid and indiscriminate use of drugs, which led to the emergence of resistant strains.

METHODS

In this study, previously reported triazole-amino acid hybrids (13-18) are explored against Plasmodium falciparum as antimalarial agents. Among six compounds, 15 and 18 exhibited antimalarial activity against P. falciparum with insignificant hemolytic activity and cytotoxicity towards HepG2 mammalian cells. In molecular docking studies, both compounds bind into the active site of PfFP-2 and block its accessibility to the substrate that leads to the inhibition of target protein further supported by in vitro analysis.

RESULTS

Antimalarial half-maximal inhibitory concentration (IC50) of 15 and 18 compounds were found to be 9.26 μM and 20.62 μM, respectively. Blood stage specific studies showed that compounds, 15 and 18 are effective at late trophozoite stage and block egress pathway of parasites. Decreased level of free monomeric heme was found in a dose dependent manner after the treatment with compounds 15 and 18, which was further evidenced by the reduction in percent of hemoglobin hydrolysis. Compounds 15 and 18 hindered hemoglobin degradation via intra- and extracellular cysteine protease falcipain-2 (PfFP-2) inhibitory activity both in in vitro and in vivo in P. falciparum.

CONCLUSION

We report antimalarial potential of triazole-amino acid hybrids and their role in the inhibition of cysteine protease PfFP-2 as its mechanistic aspect.

Tinkering with targeting nucleotide signaling for control of intracellular Leishmania parasites

Nucleotides are one of the most primitive extracellular signalling molecules across all phyla and regulate a multitude of responses. The biological effects of extracellular nucleotides/sides are mediated via the specific purinergic receptors present on the cell surface. In mammalian system, adenine nucleotides are the predominant nucleotides found in the extracellular milieu and mediate a constellation of physiological functions. In the context of host-pathogen interaction, extracellular ATP is recognized as a danger signal and potentiates the release of pro-inflammatory mediators from activated immune cells, on the other hand, its breakdown product adenosine exerts potential anti-inflammatory and immunosuppressive actions. Therefore, it is increasingly apparent that the interplay between extracellular ATP/adenosine ratios has a significant role in coordinating the regulation of the immune system in health and diseases. Several pathogens express ectonucleotidases on their surface and exploit the purinergic signalling as one of the mechanisms to modulate the host immune response. Leishmania pathogens are one of the most successful intracellular pathogens which survive within host macrophages and manipulate protective Th1 response into disease promoting Th2 response. In this review, we discuss the regulation of extracellular ATP and adenosine levels, the role of ATP/adenosine counter signalling in regulating the inflammation and immune responses during infection and how Leishmania parasites exploit the purinergic signalling to manipulate host response. We also discuss the challenges and opportunities in targeting purinergic signalling and the future prospects.

ATPe Dynamics in Protozoan Parasites. Adapt or Perish

In most animals, transient increases of extracellular ATP (ATPe) are used for physiological signaling or as a danger signal in pathological conditions. ATPe dynamics are controlled by ATP release from viable cells and cell lysis, ATPe degradation and interconversion by ecto-nucleotidases, and interaction of ATPe and byproducts with cell surface purinergic receptors and purine salvage mechanisms. Infection by protozoan parasites may alter at least one of the mechanisms controlling ATPe concentration. Protozoan parasites display their own set of proteins directly altering ATPe dynamics, or control the activity of host proteins. Parasite dependent activation of ATPe conduits of the host may promote infection and systemic responses that are beneficial or detrimental to the parasite. For instance, activation of organic solute permeability at the host membrane can support the elevated metabolism of the parasite. On the other hand ecto-nucleotidases of protozoan parasites, by promoting ATPe degradation and purine/pyrimidine salvage, may be involved in parasite growth, infectivity, and virulence. In this review, we will describe the complex dynamics of ATPe regulation in the context of protozoan parasite⁻host interactions. Particular focus will be given to features of parasite membrane proteins strongly controlling ATPe dynamics. This includes evolutionary, genetic and cellular mechanisms, as well as structural-functional relationships.

Differential expression pattern of co-inhibitory molecules on CD4+ T cells in uncomplicated versus complicated malaria

The immune response of malaria patients is a main factor influencing the clinical severity of malaria. A tight regulation of the CD4+ T cell response or the induction of tolerance have been proposed to contribute to protection from severe or clinical disease. We therefore compared the CD4+ T cell phenotypes of Ghanaian children with complicated malaria, uncomplicated malaria, asymptomatic Plasmodium falciparum (Pf) infection or no infection. Using flow cytometric analysis and automated multivariate clustering, we characterized the expression of the co-inhibitory molecules CTLA-4, PD-1, Tim-3, and LAG-3 and other molecules implicated in regulatory function on CD4+ T cells. Children with complicated malaria had higher frequencies of CTLA-4+ or PD-1+ CD4+ T cells than children with uncomplicated malaria. Conversely, children with uncomplicated malaria showed a higher proportion of CD4+ T cells expressing CD39 and Granzyme B, compared to children with complicated malaria. In contrast, asymptomatically infected children expressed only low levels of co-inhibitory molecules. Thus, different CD4+ T cell phenotypes are associated with complicated versus uncomplicated malaria, suggesting a two-sided role of CD4+ T cells in malaria pathogenesis and protection. Deciphering the signals that shape the CD4+ T cell phenotype in malaria will be important for new treatment and immunization strategies.

Sequential Membrane Rupture and Vesiculation during Plasmodium berghei Gametocyte Egress from the Red Blood Cell

Malaria parasites alternate between intracellular and extracellular stages and successful egress from the host cell is crucial for continuation of the life cycle. We investigated egress of Plasmodium berghei gametocytes, an essential process taking place within a few minutes after uptake of a blood meal by the mosquito. Egress entails the rupture of two membranes surrounding the parasite: the parasitophorous vacuole membrane (PVM), and the red blood cell membrane (RBCM). High-speed video microscopy of 56 events revealed that egress in both genders comprises four well-defined phases, although each event is slightly different. The first phase is swelling of the host cell, followed by rupture and immediate vesiculation of the PVM. These vesicles are extruded through a single stabilized pore of the RBCM, and the latter is subsequently vesiculated releasing the free gametes. The time from PVM vesiculation to completion of egress varies between events. These observations were supported by immunofluorescence microscopy using antibodies against proteins of the RBCM and PVM. The combined results reveal dynamic re-organization of the membranes and the cortical cytoskeleton of the erythrocyte during egress.

Ned-19 inhibition of parasite growth and multiplication suggests a role for NAADP mediated signalling in the asexual development of Plasmodium falciparum

Background

Although malaria is a preventable and curable human disease, millions of people risk to be infected by the Plasmodium parasites and to develop this illness. Therefore, there is an urgent need to identify new anti-malarial drugs. Ca²⁺ signalling regulates different processes in the life cycle of Plasmodium falciparum, representing a suitable target for the development of new drugs.

Results

This study investigated for the first time the effect of a highly specific inhibitor of nicotinic acid adenine dinucleotide phosphate (NAADP)-induced Ca²⁺ release (Ned-19) on P. falciparum, revealing the inhibitory effect of this compound on the blood stage development of this parasite. Ned-19 inhibits both the transition of the parasite from the early to the late trophozoite stage and the ability of the late trophozoite to develop to the multinucleated schizont stage. In addition, Ned-19 affects spontaneous intracellular Ca²⁺ oscillations in ring and trophozoite stage parasites, suggesting that the observed inhibitory effects may be associated to regulation of intracellular Ca²⁺ levels.

Conclusions

This study highlights the inhibitory effect of Ned-19 on progression of the asexual life cycle of P. falciparum. The observation that Ned-19 inhibits spontaneous Ca²⁺ oscillations suggests a potential role of NAADP in regulating Ca²⁺ signalling of P. falciparum.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-2013-7) contains supplementary material, which is available to authorized users.

Plasmodium falciparum GPCR-like receptor SR25 mediates extracellular K+ sensing coupled to Ca2+ signaling and stress survival

The malaria parasite Plasmodium falciparum is exposed, during its development, to major changes of ionic composition in its surrounding medium. We demonstrate that the P. falciparum serpentine-like receptor PfSR25 is a monovalent cation sensor capable of modulating Ca²⁺ signaling in the parasites. Changing from high (140 mM) to low (5.4 mM) KCl concentration triggers [Ca²⁺]_cyt increase in isolated parasites and this Ca²⁺ rise is blocked either by phospholipase C (PLC) inhibition or by depleting the parasite’s internal Ca²⁺ pools. This response persists even in the absence of free extracellular Ca²⁺ and cannot be elicited by addition of Na⁺, Mg²⁺ or Ca²⁺. However, when the PfSR25 gene was deleted, no effect on [Ca²⁺]_cyt was observed in response to changing KCl concentration in the knocked out (PfSR25⁻) parasite. Finally, we also demonstrate that: i) PfSR25 plays a role in parasite volume regulation, as hyperosmotic stress induces a significant decrease in parasite volume in wild type (wt), but not in PfSR25⁻ parasites; ii) parasites lacking PfSR25 show decreased parasitemia and metacaspase gene expression on exposure to the nitric oxide donor sodium nitroprusside (SNP) and iii), compared to PfSR25⁻ parasites, wt parasites showed a better survival in albumax-deprived condition.

Proteolytic Post-Translational Processing of Adhesins in a Pathogenic Bacterium

Mollicutes, including mycoplasmas and spiroplasmas, have been considered as good representatives of the « minimal cell » concept: these wall-less bacteria are small in size and possess a minimal genome and restricted metabolic capacities. However, the recent discovery of the presence of post-translational modifications unknown so far, such as the targeted processing of membrane proteins of mycoplasma pathogens for human and swine, revealed a part of the hidden complexity of these microorganisms. In this study, we show that in the phytopathogen, insect-vectored Spiroplasma citri GII-3 adhesion-related protein (ScARP) adhesins are post-translationally processed through an ATP-dependent targeted cleavage. The cleavage efficiency could be enhanced in vitro when decreasing the extracellular pH or upon the addition of polyclonal antibodies directed against ScARP repeated units, suggesting that modification of the surface charge and/or ScARP conformational changes could initiate the cleavage. The two major sites for primary cleavage are localized within predicted disordered regions and do not fit any previously reported cleavage motif; in addition, the inhibition profile and the metal ion requirements indicate that this post-translational modification involves at least one non-conventional protease. Such a proteolytic process may play a role in S. citri colonization of cells of the host insect. Furthermore, our work indicates that post-translational cleavage of adhesins represents a common feature to mollicutes colonizing distinct hosts and that processing of surface antigens could represent a way to make the most out of a minimal genome.

P. falciparum RH5-Basigin interaction induces changes in the cytoskeleton of the host RBC

The successful invasion of Plasmodium is an essential step in their life cycle. The parasite reticulocyte-binding protein homologues (RHs) and erythrocyte-binding like proteins are two families involved in the invasion leading to merozoite-red blood cell (RBC) junction formation. Ca²⁺ signaling has been shown to play a critical role in the invasion. RHs have been linked to Ca²⁺ signaling, which triggers the erythrocyte-binding like proteins release ahead of junction formation, consistent with RHs performing an initial sensing function in identifying suitable RBCs. RH5, the only essential RHs, is a highly promising vaccine candidate. RH5-basigin interaction is essential for merozoite invasion and also important in determining host tropism. Here, we show that RH5 has a distinct function from the other RHs. We show that RH5-Basigin interaction on its own triggers a Ca²⁺ signal in the RBC resulting in changes in RBC cytoskeletal proteins phosphorylation and overall alterations in RBC cytoskeleton architecture. Antibodies targeting RH5 that block the signal prevent invasion before junction formation consistent with the Ca²⁺ signal in the RBC leading to rearrangement of the cytoskeleton required for invasion. This work provides the first time a functional context for the essential role of RH5 and will now open up new avenues to target merozoite invasion.

Plasmodium falciparum GFP-E-NTPDase expression at the intraerythrocytic stages and its inhibition blocks the development of the human malaria parasite

Plasmodium falciparum is the causative agent of the most dangerous form of malaria in humans. It has been reported that the P. falciparum genome encodes for a single ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase), an enzyme that hydrolyzes extracellular tri- and di-phosphate nucleotides. The E-NTPDases are known for participating in invasion and as a virulence factor in many pathogenic protozoa. Despite its presence in the parasite genome, currently, no information exists about the activity of this predicted protein. Here, we show for the first time that P. falciparum E-NTPDase is relevant for parasite lifecycle as inhibition of this enzyme impairs the development of P. falciparum within red blood cells (RBCs). ATPase activity could be detected in rings, trophozoites, and schizonts, as well as qRT-PCR, confirming that E-NTPDase is expressed throughout the intraerythrocytic cycle. In addition, transfection of a construct which expresses approximately the first 500 bp of an E-NTPDase-GFP chimera shows that E-NTPDase co-localizes with the endoplasmic reticulum (ER) in the early stages and with the digestive vacuole (DV) in the late stages of P. falciparum intraerythrocytic cycle.

The effect of suramin on inhibiting fibroblast proliferation and preventing epidural fibrosis after laminectomy in rats

Background

Epidural fibrosis often causes serious complications in patients after lumbar laminectomy and discectomy and is associated with the proliferation of fibroblasts. Suramin is known to have an obvious inhibitory effect on the coactions of many growth factors and their receptors, but little was previously known about the effect of suramin on fibroblast proliferation and the progress of epidural fibrosis.

Methods

We illustrated the effect of suramin on cultured fibroblasts of rats with different concentrations (0, 200, 400, 600 mg/l). The proliferation of suramin-treated fibroblasts was evaluated by CCK-8 and western blot analysis. Additionally, in a rat model of laminectomy, different concentrations of suramin (100, 200, and 300 mg/ml) and saline were applied to the laminectomy sites locally. The effect of suramin on preventing epidural fibrosis was detected by the Rydell classification, hydroxyproline content, histological analysis, and collagen density analyses.

Results

The results of CCK-8 shown that suramin could significantly inhibit fibroblasts proliferation in a dose-dependent manner. The western blotting shown that the expression levels of the cell proliferation markers such as cyclin D1, cyclin E, and PCNA were down-regulated. Moreover, in a rat model, we found that suramin could reduce epidural fibrosis as well as inhibit fibroblast proliferation, and 300 mg/ml suramin had better effect.

Conclusions

Topical application of suramin could reduce epidural fibrosis after laminectomy, and the application of suramin could inhibit the proliferation of fibroblasts in rats. This study indicates that suramin is a potent antifibrotic agent that may have therapeutic potential for patients with epidural fibrosis.

Tumor necrosis factor reduces Plasmodium falciparum growth and activates calcium signaling in human malaria parasites

Background

Plasmodium has a complex biology including the ability to interact with host signals modulating their function through cellular machinery. Tumor necrosis factor (TNF) elicits diverse cellular responses including effects in malarial pathology and increased infected erythrocyte cytoadherence. As TNF levels are raised during Plasmodium falciparum infection we have investigated whether it has an effect on the parasite asexual stage.

Methods

Flow cytometry, spectrofluorimetric determinations, confocal microscopy and PCR real time quantifications were employed for characterizing TNF induced effects and membrane integrity verified by wheat germ agglutinin staining.

Results

TNF is able to decrease intracellular parasitemia, involving calcium as a second messenger of the pathway. Parasites incubated for 48 h with TNF showed reduced erythrocyte invasion. Thus, TNF induced rises in intracellular calcium concentration, which were blocked by prior addition of the purinergic receptor agonists KN62 and A438079, or interfering with intra- or extracellular calcium release by thapsigargin or EGTA (ethylene glycol tetraacetic acid). Importantly, expression of PfPCNA1 which encodes the Plasmodium falciparum Proliferating-Cell Nuclear Antigen 1, decreased after P. falciparum treatment of TNF (tumor necrosis factor) or 6-Bnz cAMP (N⁶-benzoyladenosine-3′,5′-cyclic monophosphate sodium salt).

Conclusions

This is potentially interesting data showing the relevance of calcium in downregulating a gene involved in cellular proliferation, triggered by TNF.

General significance

The data show that Plasmodium may subvert the immunological system and use TNF for the control of its proliferation within the vertebrate host.

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TNF is able to decrease parasitemia in P. falciparum‐infected RBCs.

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TNF induced rises in intracellular calcium concentration, which were blocked by the purinergic receptor agonists KN62 and A438079.

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Interfering with intra‐ or extracellular calcium release by thapsigargin or EGTA also block TNF‐induce calcium release in P. falciparum.

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Expression of the P. falciparum Proliferating‐Cell Nuclear Antigen 1 (PfPCNA1) decreased after P. falciparum treatment with TNF or 6‐Bnz cAMP.

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The data show that Plasmodium may subvert the immunological system and use TNF for the control of its proliferation within the vertebrate host.

P2X and P2Y receptor signaling in red blood cells

Purinergic signaling involves the activation of cell surface P1 and P2 receptors by extracellular nucleosides and nucleotides such as adenosine and adenosine triphosphate (ATP), respectively. P2 receptors comprise P2X and P2Y receptors, and have well-established roles in leukocyte and platelet biology. Emerging evidence indicates important roles for these receptors in red blood cells. P2 receptor activation stimulates a number of signaling pathways in progenitor red blood cells resulting in microparticle release, reactive oxygen species formation, and apoptosis. Likewise, activation of P2 receptors in mature red blood cells stimulates signaling pathways mediating volume regulation, eicosanoid release, phosphatidylserine exposure, hemolysis, impaired ATP release, and susceptibility or resistance to infection. This review summarizes the distribution of P2 receptors in red blood cells, and outlines the functions of P2 receptor signaling in these cells and its implications in red blood cell biology.

High-throughput matrix screening identifies synergistic and antagonistic antimalarial drug combinations

Drug resistance in Plasmodium parasites is a constant threat. Novel therapeutics, especially new drug combinations, must be identified at a faster rate. In response to the urgent need for new antimalarial drug combinations we screened a large collection of approved and investigational drugs, tested 13,910 drug pairs, and identified many promising antimalarial drug combinations. The activity of known antimalarial drug regimens was confirmed and a myriad of new classes of positively interacting drug pairings were discovered. Network and clustering analyses reinforced established mechanistic relationships for known drug combinations and identified several novel mechanistic hypotheses. From eleven screens comprising >4,600 combinations per parasite strain (including duplicates) we further investigated interactions between approved antimalarials, calcium homeostasis modulators, and inhibitors of phosphatidylinositide 3-kinases (PI3K) and the mammalian target of rapamycin (mTOR). These studies highlight important targets and pathways and provide promising leads for clinically actionable antimalarial therapy.

Propensity of red blood cells to undergo P2X7 receptor-mediated phosphatidylserine exposure does not alter during in vivo or ex vivo aging

BACKGROUND

Phosphatidylserine (PS) exposure facilitates the removal of red blood cells (RBCs) from the circulation, potentially contributing to the loss of stored RBCs after transfusion, as well as senescent RBCs. Activation of the P2X7 receptor by extracellular adenosine 5'-triphosphate (ATP) can induce PS exposure on freshly isolated human RBCs, but whether this process occurs in stored RBCs or changes during RBC aging is unknown.

STUDY DESIGN AND METHODS

RBCs were processed and stored according to Australian blood banking guidelines. PS exposure was determined by annexin V binding and flow cytometry. Efficacy of P2X antagonists was assessed by flow cytometric measurements of ATP-induced ethidium+ uptake in RPMI 8226 cells. Osmotic fragility was assessed by lysis in hypotonic saline. RBCs were fractionated by discontinuous density centrifugation.

RESULTS

ATP (1 mmol/L) induced PS exposure on RBCs stored for less than 1 week. This process was near-completely inhibited by the P2X7 antagonists A438079 and AZ10606120 and the P2X1/P2X7 antagonist MRS2159 but not the P2X1 antagonist NF499. ATP-induced PS exposure on RBCs was not dependent on K+, Na+, or Cl- fluxes. ATP did not alter the osmotic fragility of stored RBCs. ATP-induced PS exposure was similar between RBCs of different densities. ATP-induced PS exposure was also similar between RBCs stored for less than 1 week or for 6 weeks.

CONCLUSION

The propensity of RBCs to undergo P2X7-mediated PS exposure does not alter during in vivo and ex vivo aging. Thus, P2X7 activation is unlikely to be involved in the removal of senescent RBCs or stored RBCs after transfusion.

An interplay between 2 signaling pathways: melatonin-cAMP and IP3-Ca2+ signaling pathways control intraerythrocytic development of the malaria parasite Plasmodium falciparum

Plasmodium falciparum spends most of its asexual life cycle within human erythrocytes, where proliferation and maturation occur. Development into the mature forms of P. falciparum causes severe symptoms due to its distinctive sequestration capability. However, the physiological roles and the molecular mechanisms of signaling pathways that govern development are poorly understood. Our previous study showed that P. falciparum exhibits stage-specific spontaneous Calcium (Ca(2+)) oscillations in ring and early trophozoites, and the latter was essential for parasite development. In this study, we show that luzindole (LZ), a selective melatonin receptor antagonist, inhibits parasite growth. Analyses of development and morphology of LZ-treated P. falciparum revealed that LZ severely disrupted intraerythrocytic maturation, resulting in parasite death. When LZ was added at ring stage, the parasite could not undergo further development, whereas LZ added at the trophozoite stage inhibited development from early into late schizonts. Live-cell Ca(2+) imaging showed that LZ treatment completely abolished Ca(2+) oscillation in the ring forms while having little effect on early trophozoites. Further, the melatonin-induced cAMP increase observed at ring and late trophozoite stage was attenuated by LZ treatment. These suggest that a complex interplay between IP3-Ca(2+) and cAMP signaling pathways is involved in intraerythrocytic development of P. falciparum.

Suramin inhibits hepatic tissue damage in hepatocellular carcinoma through deactivation of heparanase enzyme

Hepatocellular carcinoma (HCC) is resistant to conventional chemotherapy, and is rarely amenable to radiotherapy. Heparanase, enzyme attacks heparan sulfate proteoglycans (HSPGs), is preferentially expressed in human tumors and its overexpression in low-metastatic tumor confers a highly invasive phenotype in experimental animals. Meanwhile, high doses of suramin dramatically increase tissue glycosaminoglycans due, in part, to inhibition of heparanase enzymes. Therefore, the following study was conducted to evaluate the chemopreventive and hepatoprotective effects of suramin in in-vivo model of HCC. Therefore, HCC was induced in SD rats by thioacetamide (200mg/kg) in presence/absence of suramin (20mg/kg). Liver impairment was assessed by measuring serum α-fetoprotein and investigating liver sections stained with Hematoxylin/Eosin. Hepatic HSPGs and heparanse were measured by ELISA. Glucosamine and glucuronic acid were measured by chemical methods. Gene expression of fibroblast growth factor (FGF)-2 and caspase-3 was measured. Apoptotic pathway was evaluated by measuring the activity of caspase-3/8/9. Suramin increased the animal survival and decreased serum α-fetoprotein. In addition, suramin ameliorated fibrosis and massive hepatic tissue breakdown. Suramin restored hepatic HSPGs and reduced the activity of hepatic heparanase leading to decreased hepatic levels of glucosamine and glucuronic acid. Moreover, suramin reduced the gene expression of FGF-2 and caspase-3. Finally, suramin blocked the elevated activity of caspase-3/8/9. In conclusion, surmain showed antitumor activity as well as hepatoprotective effects. Besides its antioxidant activity, other mechanisms are involved including restoration of HSPGs and inhibition of heparanase and FGF-2. Suramin inhibits intrinsic and extrinsic apoptotic pathway. Targeting HSPGs expression is potential therapeutic target for HCC.

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.

The role of the reticulocyte-binding-like protein homologues of Plasmodium in erythrocyte sensing and invasion

Malaria remains a serious public health problem with significant morbidity and mortality accounting for nearly 20% of all childhood deaths in Africa. The cyclical invasion, cytoadherence and destruction of the host's erythrocyte by the parasite are responsible for the observed disease pathology. The invasive form of the parasite, the merozoite, uses a complex set of interactions between parasite ligands and erythrocyte receptors that leads to the formation of a tight junction and ultimately successful erythrocyte invasion. Understanding the molecular mechanism underlying host cell recognition and invasion is crucial for the development of a targeted intervention strategy. Two parasite protein families termed reticulocyte-binding-like protein homologues (RBL) and the erythrocyte-binding-like (EBL) protein family are conserved in all Plasmodium species and have been shown to play an important role in host cell recognition and invasion. Over the last few years significant new insights have been gained in understanding the function of the RBL family and this review attempts to provide an update with a specific focus on the role of RBL in signal transduction pathways during invasion.

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.