An exported protein-interacting complex involved in the trafficking of virulence determinants in Plasmodium-infected erythrocytes

The malaria parasite, Plasmodium falciparum, displays the P. falciparum erythrocyte membrane protein 1 (PfEMP1) on the surface of infected red blood cells (RBCs). We here examine the physical organization of PfEMP1 trafficking intermediates in infected RBCs and determine interacting partners using an epitope-tagged minimal construct (PfEMP1B). We show that parasitophorous vacuole (PV)-located PfEMP1B interacts with components of the PTEX (Plasmodium Translocon of EXported proteins) as well as a novel protein complex, EPIC (Exported Protein-Interacting Complex). Within the RBC cytoplasm PfEMP1B interacts with components of the Maurer\u27s clefts and the RBC chaperonin complex. We define the EPIC interactome and, using an inducible knockdown approach, show that depletion of one of its components, the parasitophorous vacuolar protein-1 (PV1), results in altered knob morphology, reduced cell rigidity and decreased binding to CD36. Accordingly, we show that deletion of the Plasmodium berghei homologue of PV1 is associated with attenuation of parasite virulence in vivo

Knockdown of the translocon protein EXP2 in Plasmodium falciparum reduces growth and protein export.

Malaria parasites remodel their host erythrocytes to gain nutrients and avoid the immune system. Host erythrocytes are modified by hundreds of effector proteins exported from the parasite into the host cell. Protein export is mediated by the PTEX translocon comprising five core components of which EXP2 is considered to form the putative pore that spans the vacuole membrane enveloping the parasite within its erythrocyte. To explore the function and importance of EXP2 for parasite survival in the asexual blood stage of Plasmodium falciparum we inducibly knocked down the expression of EXP2. Reduction in EXP2 expression strongly reduced parasite growth proportional to the degree of protein knockdown and tended to stall development about half way through the asexual cell cycle. Once the knockdown inducer was removed and EXP2 expression restored, parasite growth recovered dependent upon the length and degree of knockdown. To establish EXP2 function and hence the basis for growth reduction, the trafficking of an exported protein was monitored following EXP2 knockdown. This resulted in severe attenuation of protein export and is consistent with EXP2, and PTEX in general, being the conduit for export of proteins into the host compartment

Malaria in pregnancy (MiP) studies assessing the clinical performance of highly sensitive rapid diagnostic tests (HS-RDT) for Plasmodium falciparum detection.

Background: Rapid diagnostic tests (RDTs) are effective tools to diagnose and inform the treatment of malaria in adults and children. The recent development of a highly sensitive rapid diagnostic test (HS-RDT) for Plasmodium falciparum has prompted questions over whether it could improve the diagnosis of malaria in pregnancy and pregnancy outcomes in malaria endemic areas. Methods: This landscape review collates studies addressing the clinical performance of the HS-RDT. Thirteen studies were identified comparing the HS-RDT and conventional RDT (co-RDT) to molecular methods to detect malaria in pregnancy. Using data from five completed studies, the association of epidemiological and pregnancy-related factors on the sensitivity of HS-RDT, and comparisons with co-RDT were investigated. The studies were conducted in 4 countries over a range of transmission intensities in largely asymptomatic women. Results: Sensitivity of both RDTs varied widely (HS-RDT range 19.6 to 85.7%, co-RDT range 22.8 to 82.8% compared to molecular testing) yet HS-RDT detected individuals with similar parasite densities across all the studies including different geographies and transmission areas [geometric mean parasitaemia around 100 parasites per µL (p/µL)]. HS-RDTs were capable of detecting low-density parasitaemias and in one study detected around 30% of infections with parasite densities of 0-2 p/µL compared to the co-RDT in the same study which detected around 15%. Conclusion: The HS-RDT has a slightly higher analytical sensitivity to detect malaria infections in pregnancy than co-RDT but this mostly translates to only fractional and not statistically significant improvement in clinical performance by gravidity, trimester, geography or transmission intensity. The analysis presented here highlights the need for larger and more studies to evaluate incremental improvements in RDTs. The HS-RDT could be used in any situation where co-RDT are currently used for P. falciparum diagnosis, if storage conditions can be adhered to

<i>Plasmodium falciparum</i> Transfected with Ultra Bright NanoLuc Luciferase Offers High Sensitivity Detection for the Screening of Growth and Cellular Trafficking Inhibitors

<div><p>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 <i>Plasmodium falciparum</i> 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.</p></div

<i>Plasmodium falciparum</i> stably expressing Nluc.

<p>(A) The Nluc gene was cloned in the pEF vector for stable expression in <i>P. falciparum</i>. (B) Aliquots (100 µL) of cultures at 1% hematocrit and 5% parasitemia corresponding to 500,000 <i>P. falciparum</i> infected RBCs transfected with pEF-Nluc were mixed with 1 volume of Nano-Glo Luciferase Assay Reagent and reporter activity measured (1∶1). Nano-Glo Luciferase Assay Reagent was further diluted in 10-fold increments in Luciferase Cell Culture Lysis Reagent and used to determine reporter activity of the same culture. As a negative control, wild type parasites (wt) were mixed 1∶1 with Nano-Glo Luciferase Assay Reagent. (C) Parasites stably transfected with pEF-Nluc were diluted in 2-fold increments in RPMI + RBC maintaining 0.5% hematocrit. For each sample, 10 µL of the culture dilutions were mixed with 40 µL of Nano-Glo diluted 1∶400 in water and measured in the luminometer for 2 s with the gain adjusted 10% below saturation for the brightest sample. The solid line represents the mean RLU after linear regression. The dashed line represents the background +3 standard deviations. (D) Nluc expressing parasites were cultured in varying concentrations of chloroquine (CQ) and their growth determined by the LDH standard method or by measuring reporter activity using Nano-Glo Luciferase Assay Reagent at its standard dilution (1∶1) or diluted 1∶1000 as described in (C). Similarly, wild type parasites were transiently transfected with pPfNluc and their growth determined using Nano-Glo Luciferase Assay Reagent (Transient). IC₅₀ was calculated by non-linear regression and represents the mean of 3 experiments. (E) The IC₅₀s determined in D were plotted with 95% confidence intervals (CI).</p

Summary of the assay parameters from the growth assays.

1<p>refers to luminescence measured from transiently transfected parasites using the standard Nano-Glo dilution.</p><p>Summary of the assay parameters from the growth assays.</p

NanoLuc is targeted to the PV and to the RBC.

<p>Diagrams of gene constructs and the IFA images are shown on the left and right respectively. Nluc fused at its N-terminus to (A) the N-terminal region of an exported protein (PEXEL), (B) to a secretion signal peptide (SP) or (C) original (cytosolic). The gene encoding each fusion protein was cloned in the pEF vector. Transfected parasites were analysed by IFA using antibodies to detect Nluc and the PVM marker Exp2. DAPI was used for nuclear staining. Size bar = 5 µm.</p

Quantification of Nluc in cellular compartments of infected RBCs.

<p>(A) Schematic of the iRBC’s compartments where RBC represents the exported fraction that is released after Equinatoxin treatment. The PV compartment was then released following treatment with 0.01% saponin and finally, the Parasite fraction was lysed by Nano-Glo Luciferase Assay Reagent. (B) Trophozoite stage parasites transfected with either the original Nluc, secreted SP-Nluc or the exported PEXEL-Nluc fusions were fractionated as shown in (A) and luciferase activity measured. Nluc activities as a percentage of the total for each parasite line are shown and represent the mean of 3 experiments +/−SEM. Similar to (B), (C) Schizonts and (D) Ring stage parasites were also fractionated. Statistical significance (* p&lt;0.05) was determined by 2 way ANOVA test comparing the percentage of reporter activity of each sub-cellular fraction among the 3 cell lines.</p

NanoLuc (Nluc) luminescence in <i>Plasmodium falciparum.</i>

<p>(A) Diagrams of firefly and Nluc reporter vectors. (B) pPf86 and pPfNluc were transfected in trophozoite stage parasites and luciferase activity in relative light units (RLU) determined 4 days later. Note that the RLU of each luciferase was measured in its own optimal substrate ie, Nluc with Nano-Glo and Firefly with D-luciferin. Mock-transfected parasites were used as negative control and to determine background luminescence, which was then subtracted from firefly and Nluc activities. The result represents the mean of 3 independent transfections ± standard deviation.</p

Summary of the assay parameters from the NPP assay.

<p>Summary of the assay parameters from the NPP assay.</p