Dual fluorescence labeling of surface-exposed and internal proteins in erythrocytes infected with the malaria parasite Plasmodium falciparum

Bisphenol AF elevates procoagulant platelets by inducing necroptosis via RIPK1-inflammasome axis

Bisphenol AF, an analogue of Bisphenol A, is an important raw material used in the production of plastic and rubber substances like plastic bottles and containers, toys, and medical supplies. Increased contamination of air, water, dust, and food with BPA/BPAF, poses an enormous threat to humans, globally. BPAF/BPA are endocrine-disrupting chemicals that mimic estrogen hormone, thus increasing the risks of various metabolic and chronic disorders. Exposure of human blood cells to BPA/BPAF induces oxidative stress and genotoxicity. However, its effects on platelets, which play central roles in hemostasis and thrombosis, are not well-documented. In this study, we demonstrate that BPAF induces RIPK1-inflammasome axis-mediated necroptosis in platelets, increasing procoagulant platelet levels in vivo and in vitro. We also show that BPAF-induced rise in procoagulant platelets worsens pulmonary thromboembolism in vivo. The elevated procoagulant platelets are shown to increase platelet-neutrophil/monocyte aggregates that mediate pathogenesis of CVD, thrombosis, and chronic inflammatory diseases. Our results demonstrate the toxic effects of BPAF on platelets and how it propagates the clinical complications by elevating procoagulant platelet numbers. Altogether, our study sends a cautionary message against extensive use of BPAF in the plastic and rubber industries, resulting in frequent human exposure to it, thus endangering platelet functions.

Live and Let Dye: Visualizing the Cellular Compartments of the Malaria Parasite Plasmodium falciparum

Malaria remains one of the deadliest diseases worldwide and it is caused by the protozoan parasite Plasmodium spp. Parasite visualization is an important tool for the correct detection of malarial cases but also to understand its biology. Advances in visualization techniques promote new insights into the complex life cycle and biology of Plasmodium parasites. Live cell imaging by fluorescence microscopy or flow cytometry are the foundation of the visualization technique for malaria research. In this review, we present an overview of possibilities in live cell imaging of the malaria parasite. We discuss some of the state-of-the-art techniques to visualize organelles and processes of the parasite and discuss limitation and advantages of each technique. © 2019 International Society for Advancement of Cytometry.

Para-tertiary butyl catechol induces eryptosis in vitro via oxidative stress and hemoglobin leakage in human erythrocytes

Exposure of human population to industrial chemicals is believed as a significant contributing factor to the outgrowth of occupational diseases especially in developing countries due to improper safety measures and sanitary conditions. Para-tertiary butylcatechol (PTBC) widely employed in petrochemical, thermofax and phototypesetting industries, induces melanocytotoxicity and contact dermatitis leading to occupational leukoderma/vitiligo. Few vitiligo patients were reported for oxidative stress-induced hemolytic anemia and thrombocytopenia, however its impact on blood components is still not clear. Erythrocytes are the major cell population in circulation and play a prominent role in various diseases. In this work, the effect of PTBC on human erythrocytes is evaluated in vitro. PTBC induces oxidative stress-mediated eryptosis (erythrocyte death) causing detrimental changes such as depleted antioxidant levels, altered surface morphology, hemoglobin denaturation and heinz body formation. These findings validate that PTBC could induce toxic effects on human erythrocytes. Exposure of humans to toxic chemicals constitutes an important issue in various industries; one such issue is the exposure of PTBC at work place resulting in a spectrum of dermal complications. Therefore, it is imperative to appraise the long-term toxicities in order to further delineate the mechanisms of resultant disorders associated with PTBC and to establish the therapeutic interventions.

Synthetic peptides derived from the C-terminal 6kDa region of Plasmodium falciparum SERA5 inhibit the enzyme activity and malaria parasite development

BACKGROUND

Plasmodium falciparum serine repeat antigen 5 (PfSERA5) is an abundant blood stage protein that plays an essential role in merozoite egress and invasion. The native protein undergoes extensive proteolytic cleavage that appears to be tightly regulated. PfSERA5 N-terminal fragment is being developed as vaccine candidate antigen. Although PfSERA5 belongs to papain-like cysteine protease family, its catalytic domain has a serine in place of cysteine at the active site.

METHODS

In the present study, we synthesized a number of peptides from the N- and C-terminal regions of PfSERA5 active domain and evaluated their inhibitory potential.

RESULTS

The final proteolytic step of PfSERA5 involves removal of a C-terminal ~6kDa fragment that results in the generation of a catalytically active ~50kDa enzyme. In the present study, we demonstrate that two of the peptides derived from the C-terminal ~6kDa region inhibit the parasite growth and also cause a delay in the parasite development. These peptides reduced the enzyme activity of the recombinant protein and co-localized with the PfSERA5 protein within the parasite, thereby indicating the specific inhibition of PfSERA5 activity. Molecular docking studies revealed that the inhibitory peptides interact with the active site of the protein. Interestingly, the peptides did not have an effect on the processing of PfSERA5.

CONCLUSIONS

Our observations indicate the temporal regulation of the final proteolytic cleavage step that occurs just prior to egress.

GENERAL SIGNIFICANCE

These results reinforce the role of PfSERA5 for the intra-erythrocytic development of malaria parasite and show the role of carboxy terminal ~6kDa fragments in the regulation of PfSERA5 activity. The results also suggest that final cleavage step of PfSERA5 can be targeted for the development of new anti-malarials.

Expression of a type B RIFIN in Plasmodium falciparum merozoites and gametes

BACKGROUND

The ability of Plasmodium falciparum to undergo antigenic variation, by switching expression among protein variants encoded by multigene families, such as var, rif and stevor, is key to the survival of this parasite in the human host. The RIFIN protein family can be divided into A and B types based on the presence or absence of a 25 amino acid motif in the semi-conserved domain. A particular type B RIFIN, PF13_0006, has previously been shown to be strongly transcribed in the asexual and sexual stages of P. falciparum in vitro.

METHODS

Antibodies to recombinant PF13_0006 RIFIN were used in immunofluorescence and confocal imaging of 3D7 parasites throughout the asexual reproduction and sexual development to examine the expression of PF13_0006. Furthermore, reactivity to recombinant PF13_0006 was measured in plasma samples collected from individuals from both East and West African endemic areas.

RESULTS

The PF13_0006 RIFIN variant appeared expressed by both released merozoites and gametes after emergence. 7.4% and 12.1% of individuals from East and West African endemic areas, respectively, carry plasma antibodies that recognize recombinant PF13_0006, where the antibody responses were more common among older children.

CONCLUSIONS

The stage specificity of PF13_0006 suggests that the diversity of RIFIN variants has evolved to provide multiple specialized functions in different stages of the parasite life cycle. These data also suggest that RIFIN variants antigenically similar to PF13_0006 occur in African parasite populations.

Proteome analysis reveals a large merozoite surface protein-1 associated complex on the Plasmodium falciparum merozoite surface

Plasmodium merozoite surface protein-1 (MSP-1) is an essential antigen for the merozoite invasion of erythrocytes. A key challenge to the development of an effective malaria vaccine that can block the erythrocyte invasion is to establish the molecular interaction(s) among the parasite surface proteins as well as with the host cell encoded receptors. In the present study, we applied molecular interactions and proteome approaches to identify PfMSP-1 associated complex on the merozoite surface. Proteomic analysis identified a major malaria surface protein, PfRhopH3 interacting with PfMSP-1(42). Pull-down experiments with merozoite lysate using anti-PfMSP-1 or anti-PfRhopH3 antibodies showed 16 bands that when identified by tandem mass spectrometry corresponded to11 parasite proteins: PfMSP-3, PfMSP-6, PfMSP-7, PfMSP-9, PfRhopH3, PfRhopH1, PfRAP-1, PfRAP-2, and two RAP domain containing proteins. This MSP-1 associated complex was specifically seen at schizont/merozoite stages but not the next ring stage. We could also identify many of these proteins in culture supernatant, suggesting the shedding of the complex. Interestingly, the PfRhopH3 protein also showed binding to the human erythrocyte and anti-PfRhopH3 antibodies blocked the erythrocyte invasion of the merozoites. These results have potential implications in the development of PfMSP-1 based blood stage malaria vaccine.

The progression of the intra-erythrocytic cell cycle of Plasmodium falciparum and the role of the centriolar plaques in asynchronous mitotic division during schizogony

The cell division cycle and mitosis of intra-erythrocytic (IE) Plasmodium falciparum are poorly understood aspects of parasite development which affect malaria molecular pathogenesis. Specifically, the timing of the multiple gap (G), DNA synthesis (S) and chromosome separation (M) phases of parasite mitosis are not well defined, nor whether genome divisions are immediately followed by cleavage of the nuclear envelope. Curiously, daughter merozoite numbers do not follow the geometric expansion expected from equal numbers of binary divisions, an outcome difficult to explain using the standard model of cell cycle regulation. Using controlled synchronisation techniques, confocal microscopy to visualise key organelles and fluorescence in situ hybridization (FISH) to follow the movements and replication of genes and telomeres, we have re-analysed the timing and progression of mitotic events. The asynchronous duplications of the P. falciparum centrosome equivalents, the centriolar plaques, are established and these are correlated with chromosome and nuclear divisions in a new model of P. falciparum schizogony. Our results improve the resolution of the cell cycle and its phases during P. falciparum IE development, showing that asynchronous, independent nuclear division occurs during schizogony, with the centriolar plaques playing a major role in regulating mitotic progression.

The kinetics of antibody binding to Plasmodium falciparum VAR2CSA PfEMP1 antigen and modelling of PfEMP1 antigen packing on the membrane knobs

Background

Infected humans make protective antibody responses to the PfEMP1 adhesion antigens exported by Plasmodium falciparum parasites to the erythrocyte membrane, but little is known about the kinetics of this antibody-receptor binding reaction or how the topology of PfEMP1 on the parasitized erythrocyte membrane influences antibody association with, and dissociation from, its antigenic target.

Methods

A Quartz Crystal Microbalance biosensor was used to measure the association and dissociation kinetics of VAR2CSA PfEMP1 binding to human monoclonal antibodies. Immuno-fluorescence microscopy was used to visualize antibody-mediated adhesion between the surfaces of live infected erythrocytes and atomic force microscopy was used to obtain higher resolution images of the membrane knobs on the infected erythrocyte to estimate knob surface areas and model VAR2CSA packing density on the knob.

Results

Kinetic analysis indicates that antibody dissociation from the VAR2CSA PfEMP1 antigen is extremely slow when there is a high avidity interaction. High avidity binding to PfEMP1 antigens on the surface of P. falciparum-infected erythrocytes in turn requires bivalent cross-linking of epitopes positioned within the distance that can be bridged by antibody. Calculations of the surface area of the knobs and the possible densities of PfEMP1 packing on the knobs indicate that high-avidity cross-linking antibody reactions are constrained by the architecture of the knobs and the large size of PfEMP1 molecules.

Conclusions

High avidity is required to achieve the strongest binding to VAR2CSA PfEMP1, but the structures that display PfEMP1 also tend to inhibit cross-linking between PfEMP1 antigens, by holding many binding epitopes at distances beyond the 15-18 nm sweep radius of an antibody. The large size of PfEMP1 will also constrain intra-knob cross-linking interactions. This analysis indicates that effective vaccines targeting the parasite's vulnerable adhesion receptors should primarily induce strongly adhering, high avidity antibodies whose association rate constant is less important than their dissociation rate constant.

Detecting the surface localization and cytoplasmic cleavage of membrane-bound proteins

Polycystin-1 (PC1) is a large, membrane-bound protein that localizes to the cilia and is implicated in the common ciliopathy autosomal-dominant polycystic kidney disease. The physiological function of PC1 is dependent upon its subcellular localization as well as specific cleavages that release soluble fragments of its C-terminal tail. The techniques described here allow visualization and quantification of these aspects of the biology of the PC1 protein. To visualize PC1 at the plasma membrane, a live-cell surface labeling immunofluorescence protocol paired with the labeling of an internal antigen motif allows a robust detection of the surface population of this protein. This technique is modified to generate a surface enzyme-linked immunosorbent assay (ELISA), which quantitatively measures the amount of surface protein as a fraction of the total amount of the protein expressed in that cell population. These assays are powerful tools in the assessment of the small but biologically important pool of PC1 that reaches the cell surface. The C-terminal tail cleavage of PC1 constitutes an interesting modification that allows PC1 to extend its functional role into the nucleus. A reporter assay based on Gal4/VP16 luciferase can be used to quantitate the amount of PC1 C-terminal tail that reaches the nucleus. This assay can be paired with quantitative measurement of the protein expression in the cell, allowing a more complete understanding of the pattern of PC1 cleavage and the nuclear localization of the resultant.