Flow cytometry as a tool for analyzing changes in Plasmodium falciparum cell cycle following treatment with indol compounds

Trio fluorophore-based phenotypic assay for the detection of artemisinin-induced growth-arrested Plasmodium falciparum in human erythrocytes

Artemisinin combination therapy remains effective for the treatment of falciparum malaria. However, Plasmodium falciparum can escape the effects of artemisinin by arresting their growth. The growth-arrested parasites cannot be distinguished from nonviable parasites with standard microscopy techniques due to their morphological similarities. Here, we demonstrated the efficacy of a new laboratory assay that is compatible with the artemisinin susceptibility test. As a result of the differential cell permeabilities of two DNA-binding fluorophores, growth-arrested P. falciparum can be distinguished from parasites killed by artemisinin, since the latter lose cell membrane permeability. This fluorescence-based assay increased the sensitivity and specificity of the ring survival assay in the assessment of artemisinin susceptibility. When combined with a third fluorophore-conjugated anti-human leukocyte antibody, this trio fluorophore assay became more useful in identifying growth-arrested parasites in mock human blood samples. This novel assay is a simple and rapid technique for monitoring artemisinin resistance with greater sensitivity and accuracy compared with morphology-based observations under a light microscope.

Overexpression of Plasmodium falciparum M1 Aminopeptidase Promotes an Increase in Intracellular Proteolysis and Modifies the Asexual Erythrocytic Cycle Development

Plasmodium falciparum, the most virulent of the human malaria parasite, is responsible for high mortality rates worldwide. We studied the M1 alanyl-aminopeptidase of this protozoan (PfA-M1), which is involved in the final stages of hemoglobin cleavage, an essential process for parasite survival. Aiming to help in the rational development of drugs against this target, we developed a new strain of P. falciparum overexpressing PfA-M1 without the signal peptide (overPfA-M1). The overPfA-M1 parasites showed a 2.5-fold increase in proteolytic activity toward the fluorogenic substrate alanyl-7-amido-4-methylcoumarin, in relation to the wild-type group. Inhibition studies showed that overPfA-M1 presented a lower sensitivity against the metalloaminopeptidase inhibitor bestatin and to other recombinant PfA-M1 inhibitors, in comparison with the wild-type strain, indicating that PfA-M1 is a target for the in vitro antimalarial activity of these compounds. Moreover, overPfA-M1 parasites present a decreased in vitro growth, showing a reduced number of merozoites per schizont, and also a decrease in the iRBC area occupied by the parasite in trophozoite and schizont forms when compared to the controls. Interestingly, the transgenic parasite displays an increase in the aminopeptidase activity toward Met-, Ala-, Leu- and Arg-7-amido-4-methylcoumarin. We also investigated the potential role of calmodulin and cysteine proteases in PfA-M1 activity. Taken together, our data show that the overexpression of PfA-M1 in the parasite cytosol can be a suitable tool for the screening of antimalarials in specific high-throughput assays and may be used for the identification of intracellular molecular partners that modulate their activity in P. falciparum.

Progress and challenges in the use of fluorescence-based flow cytometric assays for anti-malarial drug susceptibility tests

Drug-resistant Plasmodium is a frequent global threat in malaria eradication programmes, highlighting the need for new anti-malarial drugs and efficient detection of treatment failure. Plasmodium falciparum culture is essential in drug discovery and resistance surveillance. Microscopy of Giemsa-stained erythrocytes is common for determining anti-malarial effects on the intraerythrocytic development of cultured Plasmodium parasites. Giemsa-based microscopy use is conventional but laborious, and its accuracy depends largely on examiner skill. Given the availability of nucleic acid-binding fluorescent dyes and advances in flow cytometry, the use of various fluorochromes has been frequently attempted for the enumeration of parasitaemia and discrimination of P. falciparum growth in drug susceptibility assays. However, fluorochromes do not meet the requirements of being fast, simple, reliable and sensitive. Thus, this review revisits the utility of fluorochromes, notes previously reported hindrances, and highlights the challenges and opportunities for using fluorochromes in flow cytometer-based drug susceptibility tests. It aims to improve drug discovery and support a resistance surveillance system, an essential feature in combatting malaria.

A nuclear protein, PfMORC confers melatonin dependent synchrony of the human malaria parasite P. falciparum in the asexual stage

The host hormone melatonin is known to modulate the asexual cell-cycle of the human malaria parasite Plasmodium falciparum and the kinase PfPK7 is fundamental in the downstream signaling pathways. The nuclear protein PfMORC displays a histidine kinase domain and is involved in parasite cell cycle control. By using a real-time assay, we show a 24 h (h) rhythmic expression of PfMORC at the parasite asexual cycle and the expression is dramatically changed when parasites were treated with 100 nM melatonin for 17 h. Moreover, PfMORC expression was severely affected in PfPK7 knockout (PfPK7⁻) parasites following melatonin treatment. Parasites expressing 3D7^morc-GFP shows nuclear localization of the protein during the asexual stage of parasite development. Although the PfMORC knockdown had no significant impact on the parasite proliferation in vitro it significantly changed the ratio of the different asexual intraerythrocytic stages of the parasites upon the addition of melatonin. Our data reveal that in addition to the upstream melatonin signaling pathways such as IP₃ generation, calcium, and cAMP rise, a nuclear protein, PfMORC is essential for the hormone response in parasite synchronization.

A simple monochromatic flow cytometric assay for assessment of intraerythrocytic development of Plasmodium falciparum

Background

Gold standard microscopic examination of Plasmodium falciparum intraerythrocytic stage remains an important process for staging and enumerating parasitized erythrocytes in culture; however, microscopy is laborious and its accuracy is dependent upon the skill of the examiner.

Methods

In this study, ViSafe Green (VSG), which is a nucleic acid-binding fluorescent dye, was used for assessing in vitro development of P. falciparum using flow cytometry.

Results

Fluorescence intensity of VSG was found to depend on the developmental stage of parasites. Specifically, multiple-nuclei-containing schizonts were observed in the VSG^high population, and growing trophozoites and ring-shaped forms were observed in the VSG^intermediate and VSG^low populations. The efficacy of VSG-based assay was found to be comparable to the microscopic examination method, and it demonstrated an ability to detect as low as 0.001% of the parasitaemia estimated by Giemsa staining. Moreover, when applying VSG for anti-malarial drug test, it was able to observe the growth inhibitory effect of dihydroartemisinin, the front-line drug for malaria therapy.

Conclusions

Taken together, the results of this study suggest the VSG-based flow cytometric assay to be a simple and reliable assay for assessing P. falciparum malaria development in vitro.

Blocking IP3 signal transduction pathways inhibits melatonin-induced Ca2+ signals and impairs P. falciparum development and proliferation in erythrocytes

Inositol 1,4,5 trisphosphate (IP₃) signaling plays a crucial role in a wide range of eukaryotic processes. In Plasmodium falciparum, IP₃ elicits Ca²⁺ release from intracellular Ca²⁺ stores, even though no IP₃ receptor homolog has been identified to date. The human host hormone melatonin plays a key role in entraining the P. falciparum life cycle in the intraerythrocytic stages, apparently through an IP₃-dependent Ca²⁺ signal. The melatonin-induced cytosolic Ca²⁺ ([Ca²⁺]_cyt) increase and malaria cell cycle can be blocked by the IP₃ receptor blocker 2-aminoethyl diphenylborinate (2-APB). However, 2-APB also inhibits store-operated Ca²⁺ entry (SOCE). Therefore, we have used two novel 2-APB derivatives, DPB162-AE and DPB163-AE, which are 100-fold more potent than 2-APB in blocking SOCE in mammalian cells, and appear to act by interfering with clustering of STIM proteins. In the present work we report that DPB162-AE and DPB163-AE block the [Ca²⁺]_cyt rise in response to melatonin in P. falciparum, but only at high concentrations. These compounds also block SOCE in the parasite at similarly high concentrations suggesting that P. falciparum SOCE is not activated in the same way as in mammalian cells. We further find that DPB162-AE and DPB163-AE affect the development of the intraerythrocytic parasites and invasion of new red blood cells. Our efforts to episomally express proteins that compete with native IP₃ receptor like IP₃-sponge and an IP₃ sensor such as IRIS proved to be lethal to P. falciparum during intraerythrocytic cycle. The present findings point to an important role of IP₃-induced Ca²⁺ release in intraerythrocytic stage of P. falciparum.

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.

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.

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.

High-throughput tri-colour flow cytometry technique to assess Plasmodium falciparum parasitaemia in bioassays

BACKGROUND

Unbiased flow cytometry-based methods have become the technique of choice in many laboratories for high-throughput, accurate assessments of malaria parasites in bioassays. A method to quantify live parasites based on mitotracker red CMXRos was recently described but consistent distinction of early ring stages of Plasmodium falciparum from uninfected red blood cells (uRBC) remains a challenge.

METHODS

Here, a high-throughput, three-parameter (tri-colour) flow cytometry technique based on mitotracker red dye, the nucleic acid dye coriphosphine O (CPO) and the leucocyte marker CD45 for enumerating live parasites in bioassays was developed. The technique was applied to estimate the specific growth inhibition index (SGI) in the antibody-dependent cellular inhibition (ADCI) assay and compared to parasite quantification by microscopy and mitotracker red staining. The Bland-Altman analysis was used to compare biases between SGI estimated by the tri-colour staining technique, mitotracker red and by microscopy.

RESULTS

CPO allowed a better separation between early rings and uRBCs compared to mitotracker red resulting in a more accurate estimate of total parasitaemia. The tri-colour technique is rapid, cost effective and robust with comparable sensitivity to microscopy and capable of discriminating between live and dead and/or compromised parasites. Staining for CD45 improved parasitaemia estimates in ADCI assay since high numbers of leucocytes interfered with the accurate identification of parasitized RBC. The least bias (-1.60) in SGI was observed between the tri-colour and microscopy.

CONCLUSION

An improved methodology for high-throughput assessment of P. falciparum parasitaemia under culture conditions that could be useful in different bioassays, including ADCI and growth inhibition assays has been developed.

Investigation of antimalarial activity, cytotoxicity and action mechanism of piperazine derivatives of betulinic acid

OBJECTIVES

To semisynthesise piperazine derivatives of betulinic acid to evaluate antimalarial activity, cytotoxicity and action mechanism.

METHODS

The new derivatives were evaluated against the CQ-sensitive Plasmodium falciparum 3D7 strain by flow cytometry (FC) using YOYO-1 as stain. Cytotoxicity of 4a and 4b was performed with HEK293T cells for 24 and 48 h by MTT assay. The capability of compound 4a to modulate Ca(2+) in the trophozoite stage was investigated. The trophozoites were stained with Fluo4-AM and analysed by spectrofluorimetry. Effect on mitochondrial membrane potential (ΔΨm) was tested for 4a by FC with DiOC6 (3) as stain. For β-haematin assay, 4a was incubated for 24 h with reagents such as haemin, and the fluorescence was measured by FlexStation at an absorbance of 405 nm.

RESULTS

Antimalarial activity of 4a and 4b was IC50 = 1 and 4 μm, respectively. Compound 4a displayed cytotoxicity with IC50 = 69 and 29 μm for 24 and 48 h, respectively, and 4b was not cytotoxic at the tested concentrations. Addition of 4a leads to an increase in cytosolic Ca(2+) . We have measured ΔΨm after treating parasites with the compound. Data on Figure 4a show that mitochondria were not affected. The action mechanism for 4a, inhibition of β-haematin formation (17%), was lower than CQ treatment (83%; IC50 = 3 mm).

CONCLUSION

Compound 4a showed excellent antimalarial activity, and its action mechanism is involved in Ca(2+) pathway(s).

Effects of melatonin on oxidative stress, and resistance to bacterial, parasitic, and viral infections: a review

Melatonin, a hormone secreted by the pineal gland, works directly and indirectly as a free radical scavenger. Its other physiological or pharmacological activities could be dependent or independent of receptors located in different cells, organs, and tissues. In addition to its role in promoting sleep and circadian rhythms regulation, it has important immunomodulatory, antioxidant, and neuroprotective effects suggesting that this indole must be considered as a therapeutic alternative against infections. The aim of this review is to describe the effects of melatonin on oxidative stress and the resistance to bacterial (Klebsiella pneumoniae, Helicobacter pylori, Mycobacterium tuberculosis, and Clostridium perfringens), viral (Venezuelan equine encephalomyelitis virus and respiratory syncytial virus), and parasitic (Plasmodium spp., Entamoeba histolytica, Trypanosoma cruzi, Toxoplasma gondii, and Opisthorchis viverrini) infections.

The GCaMP3 - A GFP-based calcium sensor for imaging calcium dynamics in the human malaria parasite Plasmodium falciparum

Calcium (Ca²⁺) signaling pathways are vital for all eukaryotic cells. It is well established that changes in Ca²⁺ concentration can modulate several physiological processes such as muscle contraction, neurotransmitter secretion and metabolic regulation (Giacomello et al. (2007) [1], Rizzuto and Pozzan (2003) [2]). In the complex life cycle of Plasmodium falciparum, the causative agent of human malaria, Ca²⁺ is involved in the processes of protein secretion, motility, cell invasion, cell progression and parasite egress from red blood cells (RBCs) (Koyama et al. (2009) [3]).

The generation of P. falciparum expressing genetically encoded calcium indicators (GECIs) represents an innovation in the study of calcium signaling. This development will provide new insight on calcium homeostasis and signaling in P. falciparum. In addition, these novel transgenic parasites, PfGCaMP3, is a useful tool for screening and identifying new classes of compounds with anti-malarial activity. This represents a possibility of interfering with signaling pathways controlling parasite growth and development. Our new method differs from previous loading protocols (Garcia et al. (1996) [4]; Beraldo et al. (2007) [5]) since:•

It provides a novel method for imaging calcium fluctuations in the cytosol of P. falciparum, without signal interference from the host cell and invasive loading protocols.

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This technique could also be expanded for imaging calcium in different subcellular compartments.

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It will be helpful in the development of novel antimalarials capable of disrupting calcium homeostasis during the intraerythrocytic cycle of P. falciparum.

The PfNF-YB transcription factor is a downstream target of melatonin and cAMP signalling in the human malaria parasite Plasmodium falciparum

Plasmodium falciparum causes the most severe form of malaria and is responsible for the majority of deaths worldwide. The mechanism of cell cycle control within intra-erythrocytic stages has been examined as a potential means of a promising way to identifying how to stop parasite development in red blood cells. Our group determined that melatonin increases parasitemia in P. falciparum and P. chabaudi through a complex signalling cascade. In vertebrates, melatonin controls the expression of transcription factors, leading us to postulate rather that the indoleamine would affect PfNF-YB expression in human malaria parasites. We show here that PfNF-YB transcription factor is highly expressed and colocalized in the nucleus in mature parasites during intra-erythrocytic stages, thus suggesting an important role in cell division. Moreover, we demonstrate for the first time that melatonin and cAMP modulate the PfNF-YB transcription factor expression in P. falciparum at erythrocytic stages. In addition, PfNF-YB is found to be more ubiquitinated in the presence of melatonin. Finally, the proteasome inhibitor bortezomib is able to modulate PfNF-YB expression as well. Taken together, our dada reinforce the role played by melatonin in the cell cycle control of P. falciparum and point this indolamine as a target to develop new antimalarial drugs.

Isolation of erythrocytes infected with viable early stages of Plasmodium falciparum by flow cytometry

The erythrocytic life cycle of Plasmodium falciparum is highly associated with severe clinical symptoms of malaria that causes hundreds of thousands of death each year. The parasite develops within human erythrocytes leading to the disruption of the infected red blood cell (iRBC) prior to the start of a new cycle of erythrocyte infection. Emerging mechanisms of resistance against antimalarial drugs require improved knowledge about parasite's blood stages to facilitate new alternative antimalarial strategies. For the analysis of young blood stages of Plasmodium at the molecular level, the isolation of ring stages is essential. However, early stages can hardly be separated from both, late stages and non-infected red blood cells using conventional methods. Here, iRBCs were stained with the DNA-binding dyes Vybrant® DyeCycle™ Violet and SYBR® Green I. Subsequently, cells were subjected to flow-cytometric analysis. This enabled the discrimination of early stage iRBCs as well as late-stage iRBCs from non-infected erythrocytes and the properties of the used dyes were evaluated. Moreover, early stage iRBCs were isolated with high purity (>98%) by FACS. Subsequently, development of sorted early stages of the parasite was monitored over time and compared with control cultures. The described flow cytometry method, based on staining with Vybrant DyeCycle Violet, allows the isolation of viable ring stages of the malarial agent P. falciparum, and thereby provides the basis for new, broad-range molecular investigations of the parasite.