ATP and luciferase assays to determine the rate of drug action in in vitro cultures of Plasmodium falciparum

Real-time measurements of ATP dynamics via ATeams in Plasmodium falciparum reveal drug-class-specific response patterns

Malaria tropica, caused by the parasite Plasmodium falciparum (P. falciparum), remains one of the greatest public health burdens for humankind. Due to its pivotal role in parasite survival, the energy metabolism of P. falciparum is an interesting target for drug design. To this end, analysis of the central metabolite adenosine triphosphate (ATP) is of great interest. So far, only cell-disruptive or intensiometric ATP assays have been available in this system, with various drawbacks for mechanistic interpretation and partly inconsistent results. To address this, we have established fluorescent probes, based on Förster resonance energy transfer (FRET) and known as ATeam, for use in blood-stage parasites. ATeams are capable of measuring MgATP2- levels in a ratiometric manner, thereby facilitating in cellulo measurements of ATP dynamics in real-time using fluorescence microscopy and plate reader detection and overcoming many of the obstacles of established ATP analysis methods. Additionally, we established a superfolder variant of the ratiometric pH sensor pHluorin (sfpHluorin) in P. falciparum to monitor pH homeostasis and control for pH fluctuations, which may affect ATeam measurements. We characterized recombinant ATeam and sfpHluorin protein in vitro and stably integrated the sensors into the genome of the P. falciparum NF54attB cell line. Using these new tools, we found distinct sensor response patterns caused by several different drug classes. Arylamino alcohols increased and redox cyclers decreased ATP; doxycycline caused first-cycle cytosol alkalization; and 4-aminoquinolines caused aberrant proteolysis. Our results open up a completely new perspective on drugs' mode of action, with possible implications for target identification and drug development.

Next Generation Chemiluminescent Probes for Antimalarial Drug Discovery

Malaria is caused by parasites of the Plasmodium genus and remains one of the most pressing human health problems. The spread of parasites resistant to or partially resistant to single or multiple drugs, including frontline antimalarial artemisinin and its derivatives, poses a serious threat to current and future malaria control efforts. In vitro drug assays are important for identifying new antimalarial compounds and monitoring drug resistance. Due to its robustness and ease of use, the [3H]-hypoxanthine incorporation assay is still considered a gold standard and is widely applied, despite limited sensitivity and the dependence on radioactive material. Here, we present a first-of-its-kind chemiluminescence-based antimalarial drug screening assay. The effect of compounds on P. falciparum is monitored by using a dioxetane-based substrate (AquaSpark β-D-galactoside) that emits high-intensity luminescence upon removal of a protective group (β-D-galactoside) by a transgenic β-galactosidase reporter enzyme. This biosensor enables highly sensitive, robust, and cost-effective detection of asexual, intraerythrocytic P. falciparum parasites without the need for parasite enrichment, washing, or purification steps. We are convinced that the ultralow detection limit of less than 100 parasites of the presented biosensor system will become instrumental in malaria research, including but not limited to drug screening.

Construction of luciferase-expressing Neospora caninum and drug screening

BACKGROUND

Neospora caninum is an apicomplexan parasite that is particularly responsible for abortions in cattle and neuromuscular disease in dogs. Due to the limited effectiveness of currently available drugs, there is an urgent need for new therapeutic approaches to control neosporosis. Luciferase-based assays are potentially powerful tools in the search for antiprotozoal compounds, permitting the development of faster and more automated assays. The aim of this study was to construct a luciferase-expressing N. caninum and evaluate anti-N. caninum drugs.

METHODS

Luciferase-expressing N. caninum (Nc1-Luc) was constructed using clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein 9 (CRISPR/Cas9). After testing the luciferase expression and phenotype of the Nc1-Luc strains, the drug sensitivity of Nc1-Luc strains was determined by treating them with known positive or negative drugs and calculating the half-maximal inhibitory concentration (IC50). The selective pan-rapidly accelerated fibrosarcoma (pan-RAF) inhibitor TAK-632 was then evaluated for anti-N. caninum effects using Nc1-Luc by luciferase activity reduction assay and other in vitro and in vivo studies.

RESULTS

The phenotypes and drug sensitivity of Nc1-Luc strains were consistent with those of the parental strains Nc1, and Nc1-Luc strains can be used to determine the IC50 for anti-N. caninum drugs. Using the Nc1-Luc strains, TAK-632 showed promising activity against N. caninum, with an IC50 of 0.6131 μM and a selectivity index (SI) of 62.53. In vitro studies demonstrated that TAK-632 inhibited the invasion, proliferation, and division of N. caninum tachyzoites. In vivo studies showed that TAK-632 attenuated the virulence of N. caninum in mice and significantly reduced the parasite burden in the brain.

CONCLUSIONS

In conclusion, a luciferase-expressing N. caninum strain was successfully constructed, which provides an effective tool for drug screening and related research on N. caninum. In addition, TAK-632 was found to inhibit the growth of N. caninum, which could be considered as a candidate lead compound for new therapeutics for neosporosis.

Paving the Way: Contributions of Big Data to Apicomplexan and Kinetoplastid Research

In the age of big data an important question is how to ensure we make the most out of the resources we generate. In this review, we discuss the major methods used in Apicomplexan and Kinetoplastid research to produce big datasets and advance our understanding of Plasmodium, Toxoplasma, Cryptosporidium, Trypanosoma and Leishmania biology. We debate the benefits and limitations of the current technologies, and propose future advancements that may be key to improving our use of these techniques. Finally, we consider the difficulties the field faces when trying to make the most of the abundance of data that has already been, and will continue to be, generated.

Assay methods for in vitro and in vivo anti-Babesia drug efficacy testing: Current progress, outlook, and challenges

Absence of an effective high-throughput drug-screening system for Babesia parasites is considered one of the main causes for the presence of a wide gap in the treatment of animal babesiosis when compared with other hemoprotozoan diseases, such as malaria. Recently, a simple, accurate, and automatic fluorescence assay was established for large-scale anti-Babesia (B. bovis, B. bigemina, B. divergens, B. caballi and T. equi) drug screening. Such development will facilitate anti-Babesia drug discovery, especially in the post-genomic era, which will bring new chemotherapy targets with the completion of the Babesia genome sequencing project currently in progress. In this review, we present the current progress in the various assays for in vitro and in vivo anti-Babesia drug testing, as well as the challenges, highlighting new insights into the future of anti-Babesia drug screening.

Inducing controlled cell cycle arrest and re-entry during asexual proliferation of Plasmodium falciparum malaria parasites

The life cycle of the malaria parasite Plasmodium falciparum is tightly regulated, oscillating between stages of intense proliferation and quiescence. Cyclic 48-hour asexual replication of Plasmodium is markedly different from cell division in higher eukaryotes, and mechanistically poorly understood. Here, we report tight synchronisation of malaria parasites during the early phases of the cell cycle by exposure to DL-α-difluoromethylornithine (DFMO), which results in the depletion of polyamines. This induces an inescapable cell cycle arrest in G₁ (~15 hours post-invasion) by blocking G₁/S transition. Cell cycle-arrested parasites enter a quiescent G₀-like state but, upon addition of exogenous polyamines, re-initiate their cell cycle. This ability to halt malaria parasites at a specific point in their cell cycle, and to subsequently trigger re-entry into the cell cycle, provides a valuable framework to investigate cell cycle regulation in these parasites. We subsequently used gene expression analyses to show that re-entry into the cell cycle involves expression of Ca²⁺-sensitive (cdpk4 and pk2) and mitotic kinases (nima and ark2), with deregulation of the pre-replicative complex associated with expression of pk2. Changes in gene expression could be driven through transcription factors MYB1 and two ApiAP2 family members. This new approach to parasite synchronisation therefore expands our currently limited toolkit to investigate cell cycle regulation in malaria parasites.

Development in Assay Methods for in Vitro Antimalarial Drug Efficacy Testing: A Systematic Review

The emergence and spread of drug resistance are the major challenges in malaria eradication mission. Besides various strategies laid down by World Health Organization, such as vector management, source reduction, early case detection, prompt treatment, and development of new diagnostics and vaccines, nevertheless the need for new and efficacious drugs against malaria has become a critical priority on the global malaria research agenda. At several screening stages, millions of compounds are screened (1,000–2,000,000 compounds per screening campaign), before pre-clinical trials to select optimum lead. Carrying out in vitro screening of antimalarials is very difficult as different assay methods are subject to numerous sources of variability across different laboratories around the globe. Despite this, in vitro screening is an essential part of antimalarial drug development as it enables to resource various confounding factors such as host immune response and drug–drug interaction. Therefore, in this article, we try to illustrate the basic necessity behind in vitro study and how new methods are developed and subsequently adopted for high-throughput antimalarial drug screening and its application in achieving the next level of in vitro screening based on the current approaches (such as stem cells).

Cell based assays for anti-Plasmodium activity evaluation

Malaria remains one of the most common and deadly infectious diseases worldwide. The severity of this global public health challenge is reflected by the approximately 198 million people, who were reportedly infected in 2013 and by the more than 584,000 related deaths in that same year. The rising emergence of drug resistance towards the once effective artemisinin combination therapies (ACTs) has become a serious concern and warrants more robust drug development strategies, with the objective of eradicating malaria infections. The intricate biology and life cycle of Plasmodium parasites complicate the understanding of the disease in such a way that would enhance the development of more effective chemotherapies that would achieve radical clinical cure and that would prevent disease relapse. Phenotypic cell based assays have for long been a valuable approach and involve the screening and analysis of diverse compounds with regards to their activities towards whole Plasmodium parasites in vitro. To achieve the Millennium Development Goal (MDG) of malaria eradication by 2020, new generation drugs that are active against all parasite stages (erythrocytic (blood), exo-erythrocytic (liver stages and gametocytes)) are needed. Significant advances are being made in assay development to overcome some of the practical challenges of assessing drug efficacy, particularly in the liver and transmission stage Plasmodium models. This review discusses primary screening models and the fundamental progress being made in whole cell based efficacy screens of anti-malarial activity. Ongoing challenges and some opportunities for improvements in assay development that would assist in the discovery of effective, safe and affordable drugs for malaria treatments are also discussed.

Multidrug ATP-binding cassette transporters are essential for hepatic development of Plasmodium sporozoites

Multidrug resistance-associated proteins (MRPs) belong to the C-family of ATP-binding cassette (ABC) transport proteins and are known to transport a variety of physiologically important compounds and to be involved in the extrusion of pharmaceuticals. Rodent malaria parasites encode a single ABC transporter subfamily C protein, whereas human parasites encode two: MRP1 and MRP2. Although associated with drug resistance, their biological function and substrates remain unknown. To elucidate the role of MRP throughout the parasite life cycle, Plasmodium berghei and Plasmodium falciparum mutants lacking MRP expression were generated. P. berghei mutants lacking expression of the single MRP as well as P. falciparum mutants lacking MRP1, MRP2 or both proteins have similar blood stage growth kinetics and drug-sensitivity profiles as wild type parasites. We show that MRP1-deficient parasites readily invade primary human hepatocytes and develop into mature liver stages. In contrast, both P. falciparum MRP2-deficient parasites and P. berghei mutants lacking MRP protein expression abort in mid to late liver stage development, failing to produce mature liver stages. The combined P. berghei and P. falciparum data are the first demonstration of a critical role of an ABC transporter during Plasmodium liver stage development.

Enlightening the malaria parasite life cycle: bioluminescent Plasmodium in fundamental and applied research

The unicellular protozoan parasites of the genus Plasmodium impose on human health worldwide the enormous burden of malaria. The possibility to genetically modify several species of malaria parasites represented a major advance in the possibility to elucidate their biology and is now turning laboratory lines of transgenic Plasmodium into precious weapons to fight malaria. Amongst the various genetically modified plasmodia, transgenic parasite lines expressing bioluminescent reporters have been essential to unveil mechanisms of parasite gene expression and to develop in vivo imaging approaches in mouse malaria models. Mainly the human malaria parasite Plasmodium falciparum and the rodent parasite P. berghei have been engineered to express bioluminescent reporters in almost all the developmental stages of the parasite along its complex life cycle between the insect and the vertebrate hosts. Plasmodium lines expressing conventional and improved luciferase reporters are now gaining a central role to develop cell based assays in the much needed search of new antimalarial drugs and to open innovative approaches for both fundamental and applied research in malaria.

Plasmodium falciparum transfected with ultra bright NanoLuc luciferase offers high sensitivity detection for the screening of growth and cellular trafficking inhibitors

Drug discovery is a key part of malaria control and eradication strategies, and could benefit from sensitive and affordable assays to quantify parasite growth and to help identify the targets of potential anti-malarial compounds. Bioluminescence, achieved through expression of exogenous luciferases, is a powerful tool that has been applied in studies of several aspects of parasite biology and high throughput growth assays. We have expressed the new reporter NanoLuc (Nluc) luciferase in Plasmodium falciparum and showed it is at least 100 times brighter than the commonly used firefly luciferase. Nluc brightness was explored as a means to achieve a growth assay with higher sensitivity and lower cost. In addition we attempted to develop other screening assays that may help interrogate libraries of inhibitory compounds for their mechanism of action. To this end parasites were engineered to express Nluc in the cytoplasm, the parasitophorous vacuole that surrounds the intraerythrocytic parasite or exported to the red blood cell cytosol. As proof-of-concept, these parasites were used to develop functional screening assays for quantifying the effects of Brefeldin A, an inhibitor of protein secretion, and Furosemide, an inhibitor of new permeation pathways used by parasites to acquire plasma nutrients.

From the genome to the phenome: tools to understand the basic biology of Plasmodium falciparum

Malaria plagues one out of every 30 humans and contributes to almost a million deaths, and the problem could worsen. Our current therapeutic options are compromised by emerging resistance by the parasite to our front line drugs. It is thus imperative to better understand the basic biology of the parasite and develop novel drugs to stem this disease. The most facile approach to analyse a gene's function is to remove it from the genome or inhibit its activity. Although genetic manipulation of the human malaria parasite Plasmodium falciparum is a relatively standard procedure, there is no optimal method to perturb genes essential to the intraerythrocytic development cycle--the part of the life cycle that produces the clinical manifestation of malaria. This is a severe impediment to progress because the phenotype we wish to study is exactly the one that is so elusive. In the absence of any utilitarian way to conditionally delete essential genes, we are prevented from investigating the parasite's most vulnerable points. This review aims to focus on the development of tools identifying essential genes of P. falciparum and our ability to elicit phenotypic mutation.