Reliable transfection of Plasmodium falciparum using non-commercial plasmid mini preparations

Molecular make-up of the Plasmodium parasitophorous vacuolar membrane

Plasmodium, the causative agent of malaria, is an obligate, intracellular, eukaryotic cell that invades, replicates, and differentiates within hepatocytes and erythrocytes. Inside a host cell, a second membrane delineates the developing pathogen in addition to the parasite plasma membrane, resulting in a distinct cellular compartment, termed parasitophorous vacuole (PV). The PV membrane (PVM) constitutes the parasite-host cell interface and is likely central to nutrient acquisition, host cell remodeling, waste disposal, environmental sensing, and protection from innate defense. Over the past two decades, a number of parasite-encoded PVM proteins have been identified. They include multigene families and protein complexes, such as early-transcribed membrane proteins (ETRAMPs) and the Plasmodium translocon for exported proteins (PTEX). Nearly all Plasmodium PVM proteins are restricted to this genus and display transient and stage-specific expression. Here, we provide an overview of the PVM proteins of Plasmodium blood and liver stages. Biochemical and experimental genetics data suggest that some PVM proteins are ideal targets for novel anti-malarial intervention strategies.

Transfection of Plasmodium falciparum

Genetic manipulation of Plasmodium falciparum remains very challenging, mainly due to the parasite genome's high A/T-richness and low transfection efficiency. This chapter includes methods for generating transient and stable transfections by electroporation, allelic replacement with tagged genes, gene deletion, and the analysis of all the above.

MAHRP2, an exported protein of Plasmodium falciparum, is an essential component of Maurer's cleft tethers

Upon invasion into erythrocytes, the malaria parasite Plasmodium falciparum must refurbish the host cell. The objective of this study was to elucidate the location and function of MAHRP2 in these processes. Using immunofluorescence and immunoelectron microscopy we showed that the membrane-associated histidine-rich protein-2 (MAHRP2) is exported during this process to novel cylindrical structures in the erythrocyte cytoplasm. We hypothesize that these structures tether organelles known as Maurer's clefts to the erythrocyte skeleton. Live cell imaging of parasite transfectants expressing MAHRP2-GFP revealed both mobile and fixed populations of the tether-like structures. Differential centrifugation allowed the enrichment of these novel structures. MAHRP2 possesses neither a signal peptide nor a PEXEL motif, and sequences required for export were determined using transfectants expressing truncated MAHRP2 fragments. The first 15 amino acids and the histidine-rich N-terminal region are necessary for correct trafficking of MAHRP2 together with a predicted hydrophobic region. Solubilization studies showed that MAHRP2 is membrane associated but not membrane spanning. Several attempts to delete the mahrp2 gene failed, indicating that the protein is essential for parasite survival.

Development and host cell modifications of Plasmodium falciparum blood stages in four dimensions

Blood stages of Plasmodium falciparum cause the pathology of malaria; however, the progression of the parasite through this complex part of the life cycle has never been visualized. In this study, we use four-dimensional imaging to show for the first time the development of individual parasites in erythrocytes and the concomitant host cell modifications. Our data visualize an unexpectedly dynamic parasite, provide a reference for this life cycle stage and challenge the model that protein export in P. falciparum is linked to the biogenesis of host cell modifications termed Maurer's clefts. Our results provide a novel view of the blood-stage development, Maurer's cleft development and protein export in malaria parasites, and open the door to study dynamic processes, drug effects and the phenotype of mutants.

Interaction of an atypical Plasmodium falciparum ETRAMP with human apolipoproteins

BACKGROUND

In order to establish a successful infection in the human host, the malaria parasite Plasmodium falciparum must establish interactions with a variety of human proteins on the surface of different cell types, as well as with proteins inside the host cells. To better understand this aspect of malaria pathogenesis, a study was conducted with the goal of identifying interactions between proteins of the parasite and those of its human host.

METHODS

A modified yeast two-hybrid methodology that preferentially selects protein fragments that can be expressed in yeast was used to conduct high-throughput screens with P. falciparum protein fragments against human liver and cerebellum libraries. The resulting dataset was analyzed to exclude interactions that are not likely to occur in the human host during infection.

RESULTS

An initial set of 2,200 interactions was curated to remove proteins that are unlikely to play a role in pathogenesis based on their annotation or localization, and proteins that behave promiscuously in the two-hybrid assay, resulting in a final dataset of 456 interactions. A cluster that implicates binding between P. falciparum PFE1590w/ETRAMP5, a putative parasitophorous vacuole membrane protein, and human apolipoproteins ApoA, ApoB and ApoE was selected for further analysis. Different isoforms of ApoE, which are associated with different outcomes of malaria infection, were shown to display differential interactions with PFE1590w.

CONCLUSION

A dataset of interactions between proteins of P. falciparum and those of its human host was generated. The preferential interaction of the P. falciparum PFE1590w protein with the human ApoE epsilon3 and ApoE epsilon4 isoforms, but not the ApoE epsilon2 isoform, supports the hypothesis that ApoE genotype affects risk of malaria infection. The dataset contains other interactions of potential relevance to disease that may identify possible vaccine candidates and drug targets.

Characterization of a conserved rhoptry-associated leucine zipper-like protein in the malaria parasite Plasmodium falciparum

One of the key processes in the pathobiology of the malaria parasite is the invasion and subsequent modification of the human erythrocyte. In this complex process, an unknown number of parasite proteins are involved, some of which are leading vaccine candidates. The majority of the proteins that play pivotal roles in invasion are either stored in the apical secretory organelles or located on the surface of the merozoite, the invasive stage of the parasite. Using transcriptional and structural features of these known proteins, we performed a genomewide search that identified 49 hypothetical proteins with a high probability of being located on the surface of the merozoite or in the secretory organelles. Of these candidates, we characterized a novel leucine zipper-like protein in Plasmodium falciparum that is conserved in Plasmodium spp. This protein is expressed in late blood stages and localizes to the rhoptries of the parasite. We demonstrate that this Plasmodium sp.-specific protein has a high degree of conservation within field isolates and that it is refractory to gene knockout attempts and thus might play an important role in invasion.

The M18 aspartyl aminopeptidase of the human malaria parasite Plasmodium falciparum

A member of the M18 family of aspartyl aminopeptidases is expressed by all intra-erythrocytic stages of the human malaria parasite Plasmodium falciparum (PfM18AAP), with highest expression levels in rings. Functionally active recombinant enzyme, rPfM18AAP, and native enzyme in cytosolic extracts of malaria parasites are 560-kDa octomers that exhibit optimal activity at neutral pH and require the presence of metal ions to maintain enzymatic activity and stability. Like the human aspartyl aminopeptidase, the exopeptidase activity of PfM18AAP is exclusive to N-terminal acidic amino acids, glutamate and aspartate, making this enzyme of particular interest and suggesting that it may function alongside the malaria cytosolic neutral aminopeptidases in the release of amino acids from host hemoglobin-derived peptides. Whereas immunocytochemical studies using transgenic P. falciparum parasites show that PfM18AAP is expressed in the cytosol, immunoblotting experiments revealed that the enzyme is also trafficked out of the parasite into the surrounding parasitophorous vacuole. Antisense-mediated knockdown of PfM18AAP results in a lethal phenotype as a result of significant intracellular damage and validates this enzyme as a target at which novel antimalarial drugs could be directed. Novel phosphinic derivatives of aspartate and glutamate showed modest inhibition of rPfM18AAP but did not inhibit malaria growth in culture. However, we were able to draw valuable observations concerning the structure-activity relationship of these inhibitors that can be employed in future inhibitor optimization studies.

Characterization of the Plasmodium falciparum M17 Leucyl Aminopeptidase

Amino acids generated from the catabolism of hemoglobin by intra-erythrocytic malaria parasites are not only essential for protein synthesis but also function in maintaining an osmotically stable environment, and creating a gradient by which amino acids that are rare or not present in hemoglobin are drawn into the parasite from host serum. We have proposed that a Plasmodium falciparum M17 leucyl aminopeptidase (PfLAP) generates and regulates the internal pool of free amino acids and therefore represents a target for novel antimalarial drugs. This enzyme has been expressed in insect cells as a functional 320-kDa homo-hexamer that is optimally active at neutral or alkaline pH, is dependent on metal ions for activity, and exhibits a substrate preference for N-terminally exposed hydrophobic amino acids, particularly leucine. PfLAP is produced by all stages in the intra-erythrocytic developmental cycle of malaria but was most highly expressed by trophozoites, a stage at which hemoglobin degradation and parasite protein synthesis are elevated. The enzyme was located by immunohistochemical methods and by transfecting malaria cells with a PfLAP-green fluorescent protein construct, to the cytosolic compartment of the cell at all developmental stages, including segregated merozoites. Amino acid dipeptide analogs, such as bestatin and its derivatives, are potent inhibitors of the protease and also block the growth of P. falciparum malaria parasites in culture. This study provides a biochemical basis for the antimalarial activity of aminopeptidase inhibitors. Availability of functionally active recombinant PfLAP, coupled with a simple enzymatic readout, will aid medicinal chemistry and/or high throughput approaches for the future design/discovery of new antimalarial drugs.