Maurer's clefts of Plasmodium falciparum are secretory organelles that concentrate virulence protein reporters for delivery to the host erythrocyte

A conserved guided entry of tail-anchored pathway is involved in the trafficking of a subset of membrane proteins in Plasmodium falciparum

Tail-anchored (TA) proteins are defined by the absence of N-terminus signal sequence and the presence of a single transmembrane domain (TMD) proximal to their C-terminus. They play fundamental roles in cellular processes including vesicular trafficking, protein translocation and quality control. Some of the TA proteins are post-translationally integrated by the Guided Entry of TA (GET) pathway to the cellular membranes; with their N-terminus oriented towards the cytosol and C-terminus facing the organellar lumen. The TA repertoire and the GET machinery have been extensively characterized in the yeast and mammalian systems, however, they remain elusive in the human malaria parasite Plasmodium falciparum. In this study, we bioinformatically predicted a total of 63 TA proteins in the P. falciparum proteome and revealed the association of a subset with the P. falciparum homolog of Get3 (PfGet3). In addition, our proximity labelling studies either definitively identified or shortlisted the other eligible GET constituents, and our in vitro association studies validated associations between PfGet3 and the corresponding homologs of Get4 and Get2 in P. falciparum. Collectively, this study reveals the presence of proteins with hallmark TA signatures and the involvement of evolutionary conserved GET trafficking pathway for their targeted delivery within the parasite.

Tail-anchored (TA) membrane proteins are known to play essential cellular functions in the eukaryotes. These proteins are trafficked to their respective destinations by post-translational translocation pathways that are evolutionarily conserved from yeast to human. However, they remain unidentified in the malaria parasite Plasmodium falciparum. We have used bioinformatic prediction algorithms in conjunction with functional validation studies to identify the candidate TA repertoire and some of the homologs of the trafficking machinery in P. falciparum. Initially, we predicted the presence of 63 putative TA proteins localized to distinct compartments within this parasite, including a few confirmed TA homologs in other eukaryotic systems. We then identified and characterized PfGet3 as a central component in the Guided-Entry of TA (GET) translocation machinery, and our bacterial co-expression and pulldown assays with two selected recombinant TA proteins, PfBOS1 and PfUSE1, showed co-association with PfGet3. We also identified PfGet2 and PfGet4 as the other two components of the GET machinery in P. falciparum using proximity biotinylation followed by mass spectrometry. Interestingly, we also found six TA proteins in the parasite enriched in this fraction. We further validated the direct interactions between a few TA candidates, PfGet4 and PfGet2 with PfGet3 using recombinant-based pulldown studies. In conclusion, this study classified a subset of membrane proteins with the TA nomenclature and implicated a previously unidentified GET pathway for their translocation in this apicomplexan parasite.

A network-based approach reveals novel invasion and Maurer's clefts-related proteins in Plasmodium falciparum

Malaria continues to be a major concern in developing countries despite continuous efforts to find a cure for the disease. Understanding the pathogenesis mechanism is necessary to identify more effective drug targets against malaria. Many years of experimental research have generated a large amount of data for the malarial parasite, Plasmodium falciparum. These data are useful to understand the importance of certain parasite proteins, but it often remains unclear how these proteins come together, interact with other proteins and carry out their function. Identification of all proteins involved in pathogenesis is an important step towards understanding the molecular mechanism of pathogenesis. In this study, dynamic stage-specific protein-protein interaction networks were created based on gene expression data during the parasite's intra-erythrocytic stages and static protein-protein interaction data. Using previously known proteins of a biological event as seed proteins, the random walk with restart (RWR) method was used on the dynamic protein-protein interaction networks to identify novel proteins related to that event. Two screening procedures namely, permutation test and GO enrichment test were performed to increase the reliability of the RWR predictions. The proposed method was first validated on Plasmodium falciparum proteins related to invasion, where it could reproduce the existing knowledge from a small set of seed proteins. It was then used to identify novel Maurer's clefts resident proteins, where it could identify 152 parasite proteins. We show that the current approach can annotate conserved proteins with unknown function. The predicted proteins can help build a mechanistic model for disease pathogenesis, which will be useful in identifying new drug targets.

Plasmodium falciparum gametocyte transit through the cutaneous microvasculature: A new target for malaria transmission blocking vaccines?

Malaria remains one of the most significant infectious diseases worldwide. Concordant with scaled intervention efforts and the emphasis of elimination and eradication on the agenda of many malaria control programs, the development of a malaria vaccine that reduces transmission of the parasite from human host to mosquito vector has been incorporated as an important new strategic goal. Transmission of malaria from man to mosquito relies on gametocytes, highly specialized sexual-stage parasites, that once mature, circulate in the peripheral vasculature of the human host. The complex interplay between mature gametocytes, their uptake in the mosquito bloodmeal and forward maturation/fertilization events provide unique opportunities for intervention. Although recent advances have yielded greater understanding into the mechanisms that mediate sequestration of immature gametocytes in the human host, the spatial dynamics of circulating mature gametocytes in the cutaneous microvaculature remains far less defined, which is the focus of this review.

The aspartyl protease TgASP5 mediates the export of the Toxoplasma GRA16 and GRA24 effectors into host cells

Toxoplasma gondii and Plasmodium species are obligatory intracellular parasites that export proteins into the infected cells in order to interfere with host-signalling pathways, acquire nutrients or evade host defense mechanisms. With regard to export mechanism, a wealth of information in Plasmodium spp. is available, while the mechanisms operating in T. gondii remain uncertain. The recent discovery of exported proteins in T. gondii, mainly represented by dense granule resident proteins, might explain this discrepancy and offers a unique opportunity to study the export mechanism in T. gondii. Here, we report that GRA16 export is mediated by two protein elements present in its N-terminal region. Because the first element contains a putative Plasmodium export element linear motif (RRLAE), we hypothesized that GRA16 export depended on a maturation process involving protein cleavage. Using both N- and C-terminal epitope tags, we provide evidence for protein proteolysis occurring in the N-terminus of GRA16. We show that TgASP5, the T. gondii homolog of Plasmodium plasmepsin V, is essential for GRA16 export and is directly responsible for its maturation in a Plasmodium export element-dependent manner. Interestingly, TgASP5 is also involved in GRA24 export, although the GRA24 maturation mechanism is TgASP5-independent. Our data reveal different modus operandi for protein export, in which TgASP5 should play multiple functions.

The Role of Extracellular Vesicles in Modulating the Host Immune Response during Parasitic Infections

Parasites are the cause of major diseases affecting billions of people. As the inflictions caused by these parasites affect mainly developing countries, they are considered as neglected diseases. These parasitic infections are often chronic and lead to significant immunomodulation of the host immune response by the parasite, which could benefit both the parasite and the host and are the result of millions of years of co-evolution. The description of parasite extracellular vesicles (EVs) in protozoa and helminths suggests that they may play an important role in host-parasite communication. In this review, recent studies on parasitic (protozoa and helminths) EVs are presented and their potential use as novel therapeutical approaches is discussed.

Protein trafficking in Plasmodium falciparum-infected red cells and impact of the expansion of exported protein families

SUMMARY Erythrocytes are extensively remodelled by the malaria parasite following invasion of the cell. Plasmodium falciparum encodes numerous virulence-associated and host-cell remodelling proteins that are trafficked to the cytoplasm, the cell membrane and the surface of the infected erythrocyte. The export of soluble proteins relies on a sequence directing entry into the secretory pathways in addition to an export signal. The export signal consisting of five amino acids is termed the Plasmodium export element (PEXEL) or the vacuole transport signal (VTS). Genome mining studies have revealed that PEXEL/VTS carrying protein families have expanded dramatically in P. falciparum compared with other malaria parasite species, possibly due to lineage-specific expansion linked to the unique requirements of P. falciparum for host-cell remodelling. The functional characterization of such genes and gene families may reveal potential drug targets that could inhibit protein trafficking in infected erythrocytes. This review highlights some of the recent advances and key knowledge gaps in protein trafficking pathways in P. falciparum-infected red cells and speculates on the impact of exported gene families in the trafficking pathway.

Characterization of the small exported Plasmodium falciparum membrane protein SEMP1

Survival and virulence of the human malaria parasite Plasmodium falciparum during the blood stage of infection critically depend on extensive host cell refurbishments mediated through export of numerous parasite proteins into the host cell. The parasite-derived membranous structures called Maurer's clefts (MC) play an important role in protein trafficking from the parasite to the red blood cell membrane. However, their specific function has yet to be determined. We identified and characterized a new MC membrane protein, termed small exported membrane protein 1 (SEMP1). Upon invasion it is exported into the RBC cytosol where it inserts into the MCs before it is partly translocated to the RBC membrane. Using conventional and conditional loss-of-function approaches we showed that SEMP1 is not essential for parasite survival, gametocytogenesis, or PfEMP1 export under culture conditions. Co-IP experiments identified several potential interaction partners, including REX1 and other membrane-associated proteins that were confirmed to co-localize with SEMP1 at MCs. Transcriptome analysis further showed that expression of a number of exported parasite proteins was up-regulated in SEMP1-depleted parasites. By using Co-IP and transcriptome analysis for functional characterization of an exported parasite protein we provide a new starting point for further detailed dissection and characterisation of MC-associated protein complexes.

Inward cholesterol gradient of the membrane system in P. falciparum-infected erythrocytes involves a dilution effect from parasite-produced lipids

Plasmodium falciparum (Pf) infection remodels the human erythrocyte with new membrane systems, including a modified host erythrocyte membrane (EM), a parasitophorous vacuole membrane (PVM), a tubulovesicular network (TVN), and Maurer's clefts (MC). Here we report on the relative cholesterol contents of these membranes in parasitized normal (HbAA) and hemoglobin S-containing (HbAS, HbAS) erythrocytes. Results from fluorescence lifetime imaging microscopy (FLIM) experiments with a cholesterol-sensitive fluorophore show that membrane cholesterol levels in parasitized erythrocytes (pRBC) decrease inwardly from the EM, to the MC/TVN, to the PVM, and finally to the parasite membrane (PM). Cholesterol depletion of pRBC by methyl-β-cyclodextrin treatment caused a collapse of this gradient. Lipid and cholesterol exchange data suggest that the cholesterol gradient involves a dilution effect from non-sterol lipids produced by the parasite. FLIM signals from the PVM or PM showed little or no difference between parasitized HbAA vs HbS-containing erythrocytes that differed in lipid content, suggesting that malaria parasites may regulate the cholesterol contents of the PVM and PM independently of levels in the host cell membrane. Cholesterol levels may affect raft structures and the membrane trafficking and sorting functions that support Pf survival in HbAA, HbAS and HbSS erythrocytes.

Plasmodial Hsp40 and Hsp70 chaperones: current and future perspectives

Plasmodium falciparumdisplays a large and remarkable variety of heat shock protein 40 family members (PfHsp40s). The majority of the PfHsp40s are poorly characterized, and although the functions of some of them have been suggested, their exact mechanism of action is still elusive and their interacting partners and client proteins are unknown. TheP. falciparumheat shock protein 70 family members (PfHsp70s) have been more extensively characterized than the PfHsp40s, with certain members shown to function as molecular chaperones. However, little is known about the PfHsp70-PfHsp40 chaperone partnerships. There is mounting evidence that these chaperones are important not only in protein homoeostasis and cytoprotection, but also in protein trafficking across the parasitophorous vacuole (PV) and into the infected erythrocyte. We propose that certain members of these chaperone families work together to maintain exported proteins in an unfolded state until they reach their final destination. In this review, we critically evaluate what is known and not known about PfHsp40s and PfHsp70s.

Microvesicles and intercellular communication in the context of parasitism

There is a rapidly growing body of evidence that production of microvesicles (MVs) is a universal feature of cellular life. MVs can incorporate microRNA (miRNA), mRNA, mtDNA, DNA and retrotransposons, camouflage viruses/viral components from immune surveillance, and transfer cargo between cells. These properties make MVs an essential player in intercellular communication. Increasing evidence supports the notion that MVs can also act as long-distance vehicles for RNA molecules and participate in metabolic synchronization and reprogramming eukaryotic cells including stem and germinal cells. MV ability to carry on DNA and their general distribution makes them attractive candidates for horizontal gene transfer, particularly between multi-cellular organisms and their parasites; this suggests important implications for the co-evolution of parasites and their hosts. In this review, we provide current understanding of the roles played by MVs in intracellular pathogens and parasitic infections. We also discuss the possible role of MVs in co-infection and host shifting.

Circulating microparticles: square the circle

Background

The present review summarizes current knowledge about microparticles (MPs) and provides a systematic overview of last 20 years of research on circulating MPs, with particular focus on their clinical relevance.

Results

MPs are a heterogeneous population of cell-derived vesicles, with sizes ranging between 50 and 1000 nm. MPs are capable of transferring peptides, proteins, lipid components, microRNA, mRNA, and DNA from one cell to another without direct cell-to-cell contact. Growing evidence suggests that MPs present in peripheral blood and body fluids contribute to the development and progression of cancer, and are of pathophysiological relevance for autoimmune, inflammatory, infectious, cardiovascular, hematological, and other diseases. MPs have large diagnostic potential as biomarkers; however, due to current technological limitations in purification of MPs and an absence of standardized methods of MP detection, challenges remain in validating the potential of MPs as a non-invasive and early diagnostic platform.

Conclusions

Improvements in the effective deciphering of MP molecular signatures will be critical not only for diagnostics, but also for the evaluation of treatment regimens and predicting disease outcomes.

Spatial and temporal mapping of the PfEMP1 export pathway in Plasmodium falciparum

The human malaria parasite, Plasmodium falciparum, modifies the red blood cells (RBCs) that it infects by exporting proteins to the host cell. One key virulence protein, P. falciparum Erythrocyte Membrane Protein-1 (PfEMP1), is trafficked to the surface of the infected RBC, where it mediates adhesion to the vascular endothelium. We have investigated the organization and development of the exomembrane system that is used for PfEMP1 trafficking. Maurer's cleft cisternae are formed early after invasion and proteins are delivered to these (initially mobile) structures in a temporally staggered and spatially segregated manner. Membrane-Associated Histidine-Rich Protein-2 (MAHRP2)-containing tether-like structures are generated as early as 4 h post invasion and become attached to Maurer's clefts. The tether/Maurer's cleft complex docks onto the RBC membrane at ~20 h post invasion via a process that is not affected by cytochalasin D treatment. We have examined the trafficking of a GFP chimera of PfEMP1 expressed in transfected parasites. PfEMP1B-GFP accumulates near the parasite surface, within membranous structures exhibiting a defined ultrastructure, before being transferred to pre-formed mobile Maurer's clefts. Endogenous PfEMP1 and PfEMP1B-GFP are associated with Electron-Dense Vesicles that may be responsible for trafficking PfEMP1 from the Maurer's clefts to the RBC membrane.

A HT/PEXEL motif in Toxoplasma dense granule proteins is a signal for protein cleavage but not export into the host cell

Apicomplexan parasites, such as Toxoplasma gondii and Plasmodium, secrete proteins for attachment, invasion and modulation of their host cells. The host targeting (HT), also known as the Plasmodium export element (PEXEL), directs Plasmodium proteins into erythrocytes to remodel the host cell and establish infection. Bioinformatic analysis of Toxoplasma revealed a HT/PEXEL-like motif at the N-terminus of several hypothetical unknown and dense granule proteins. Hemagglutinin-tagged versions of these uncharacterized proteins show co-localization with dense granule proteins found on the parasitophorous vacuole membrane (PVM). In contrast to Plasmodium, these Toxoplasma HT/PEXEL containing proteins are not exported into the host cell. Site directed mutagenesis of the Toxoplasma HT/PEXEL motif, RxLxD/E, shows that the arginine and leucine residues are permissible for protein cleavage. Mutations within the HT/PEXEL motif that prevent protein cleavage still allow for targeting to the PV but the proteins have a reduced association with the PVM. Addition of a Myc tag before and after the cleavage site shows that processed HT/PEXEL protein has increased PVM association. These findings suggest that while Toxoplasma and Plasmodium share similar HT/PEXEL motifs, Toxoplasma HT/PEXEL containing proteins interact with but do not cross the PVM.

PI(3)P-independent and -dependent pathways function together in a vacuolar translocation sequence to target malarial proteins to the host erythrocyte

Malaria parasites export 'a secretome' of hundreds of proteins, including major virulence determinants, from their endoplasmic reticulum (ER), past the parasite plasma and vacuolar membranes to the host erythrocyte. The export mechanism is high affinity (nanomolar) binding of a host (cell) targeting (HT) motif RxLxE/D/Q to the lipid phosphatidylinositol 3-phosphate (PI(3)P) in the ER. Cleavage of the HT motif releases the secretory protein from the ER membrane. The HT motif is thought to be the only export signal resident in an N-terminal vacuolar translocation sequence (VTS) that quantitatively targets green fluorescent protein to the erythrocyte. We have previously shown that the R to A mutation in the HT motif, abrogates VTS binding to PI(3)P (K(d)>5 μM). We now show that remarkably, the R to A mutant is exported to the host erythrocyte, for both membrane and soluble reporters, although the efficiency of export is reduced to ~30% of that seen with a complete VTS. Mass spectrometry indicates that the R to A mutant is cleaved at sites upstream of the HT motif. Antibodies to upstream sequences confirm that aberrantly cleaved R to A protein mutant is exported to the erythrocyte. These data suggest that export mechanisms, independent of PI(3)P as well as those dependent on PI(3)P, function together in a VTS to target parasite proteins to the host erythrocyte.

Functional analysis of Plasmodium vivax dihydrofolate reductase-thymidylate synthase genes through stable transformation of Plasmodium falciparum

Mechanisms of drug resistance in Plasmodium vivax have been difficult to study partially because of the difficulties in culturing the parasite in vitro. This hampers monitoring drug resistance and research to develop or evaluate new drugs. There is an urgent need for a novel method to study mechanisms of P. vivax drug resistance. In this paper we report the development and application of the first Plasmodium falciparum expression system to stably express P. vivax dhfr-ts alleles. We used the piggyBac transposition system for the rapid integration of wild-type, single mutant (117N) and quadruple mutant (57L/58R/61M/117T) pvdhfr-ts alleles into the P. falciparum genome. The majority (81%) of the integrations occurred in non-coding regions of the genome; however, the levels of pvdhfr transcription driven by the P. falciparum dhfr promoter were not different between integrants of non-coding and coding regions. The integrated quadruple pvdhfr mutant allele was much less susceptible to antifolates than the wild-type and single mutant pvdhfr alleles. The resistance phenotype was stable without drug pressure. All the integrated clones were susceptible to the novel antifolate JPC-2067. Therefore, the piggyBac expression system provides a novel and important tool to investigate drug resistance mechanisms and gene functions in P. vivax.

Moving "Forward" in Plasmodium Genetics through a Transposon-Based Approach

The genome sequence of the human malaria parasite, Plasmodium falciparum, was released almost a decade ago. A majority of the Plasmodium genome, however, remains annotated to code for hypothetical proteins with unknown functions. The introduction of forward genetics has provided novel means to gain a better understanding of gene functions and their associated phenotypes in Plasmodium. Even with certain limitations, the technique has already shown significant promise to increase our understanding of parasite biology needed for rationalized drug and vaccine design. Further improvements to the mutagenesis technique and the design of novel genetic screens should lead us to some exciting discoveries about the critical weaknesses of Plasmodium, and greatly aid in the development of new disease intervention strategies.

Endoplasmic reticulum PI(3)P lipid binding targets malaria proteins to the host cell

Hundreds of effector proteins of the human malaria parasite Plasmodium falciparum constitute a "secretome" carrying a host-targeting (HT) signal, which predicts their export from the intracellular pathogen into the surrounding erythrocyte. Cleavage of the HT signal by a parasite endoplasmic reticulum (ER) protease, plasmepsin V, is the proposed export mechanism. Here, we show that the HT signal facilitates export by recognition of the lipid phosphatidylinositol-3-phosphate (PI(3)P) in the ER, prior to and independent of protease action. Secretome HT signals, including those of major virulence determinants, bind PI(3)P with nanomolar affinity and amino acid specificities displayed by HT-mediated export. PI(3)P-enriched regions are detected within the parasite's ER and colocalize with endogenous HT signal on ER precursors, which also display high-affinity binding to PI(3)P. A related pathogenic oomycete's HT signal export is dependent on PI(3)P binding, without cleavage by plasmepsin V. Thus, PI(3)P in the ER functions in mechanisms of secretion and pathogenesis.

Visualizing the 3D architecture of multiple erythrocytes infected with Plasmodium at nanoscale by focused ion beam-scanning electron microscopy

Different methods for three-dimensional visualization of biological structures have been developed and extensively applied by different research groups. In the field of electron microscopy, a new technique that has emerged is the use of a focused ion beam and scanning electron microscopy for 3D reconstruction at nanoscale resolution. The higher extent of volume that can be reconstructed with this instrument represent one of the main benefits of this technique, which can provide statistically relevant 3D morphometrical data. As the life cycle of Plasmodium species is a process that involves several structurally complex developmental stages that are responsible for a series of modifications in the erythrocyte surface and cytoplasm, a high number of features within the parasites and the host cells has to be sampled for the correct interpretation of their 3D organization. Here, we used FIB-SEM to visualize the 3D architecture of multiple erythrocytes infected with Plasmodium chabaudi and analyzed their morphometrical parameters in a 3D space. We analyzed and quantified alterations on the host cells, such as the variety of shapes and sizes of their membrane profiles and parasite internal structures such as a polymorphic organization of hemoglobin-filled tubules. The results show the complex 3D organization of Plasmodium and infected erythrocyte, and demonstrate the contribution of FIB-SEM for the obtainment of statistical data for an accurate interpretation of complex biological structures.

The exported Plasmodium berghei protein IBIS1 delineates membranous structures in infected red blood cells

The importance of pathogen-induced host cell remodelling has been well established for red blood cell infection by the human malaria parasite Plasmodium falciparum. Exported parasite-encoded proteins, which often possess a signature motif, termed Plasmodium export element (PEXEL) or host-targeting (HT) signal, are critical for the extensive red blood cell modifications. To what extent remodelling of erythrocyte membranes also occurs in non-primate hosts and whether it is in fact a hallmark of all mammalian Plasmodium parasites remains elusive. Here we characterize a novel Plasmodium berghei PEXEL/HT-containing protein, which we term IBIS1. Temporal expression and spatial localization determined by fluorescent tagging revealed the presence of IBIS1 at the parasite/host interface during both liver and blood stages of infection. Targeted deletion of the IBIS1 protein revealed a mild impairment of intra-erythrocytic growth indicating a role for these structures in the rapid expansion of the parasite population in the blood in vivo. In red blood cells, the protein localizes to dynamic, punctate structures external to the parasite. Biochemical and microscopic data revealed that these intra-erythrocytic P. berghei-induced structures (IBIS) are membranous indicating that P. berghei, like P. falciparum, creates an intracellular membranous network in infected red blood cells.

Toxoplasma gondii Sis1-like J-domain protein is a cytosolic chaperone associated to HSP90/HSP70 complex

Toxoplasma gondii is an obligate intracellular protozoan parasite in which 36 predicted Hsp40 family members were identified by searching the T. gondii genome. The predicted protein sequence from the gene ID TGME49_065310 showed an amino acid sequence and domain structure similar to Saccharomyces cerevisiae Sis1. TgSis1 did not show differences in its expression profile during alkaline stress by microarray analysis. Furthermore, TgSis1 showed to be a cytosolic Hsp40 which co-immunoprecipitated with T. gondii Hsp70 and Hsp90. Structural modeling of the TgSis1 peptide binding fragment revealed structural and electrostatic properties different from the experimental model of human Sis1-like protein (Hdj1). Based on these differences; we propose that TgSis1 may be a potentially attractive drug target for developing a novel anti-T. gondii therapy.

Functional analysis of the exported type IV HSP40 protein PfGECO in Plasmodium falciparum gametocytes

During Plasmodium falciparum infection, host red blood cell (RBC) remodeling is required for the parasite's survival. Such modifications are mediated by the export of parasite proteins into the RBC that alter the architecture of the RBC membrane and enable cytoadherence. It is probable that some exported proteins also play a protective role against the host defense response. This may be of particular importance for the gametocyte stage of the life cycle that is responsible for malaria transmission, since the gametocyte remains in contact with blood as it proceeds through five morphological stages (I to V) during its 12-day maturation. Using microarray analysis, we identified several genes with encoded secretory or export sequences that were differentially expressed during early gametocytogenesis. One of these, PfGECO, encodes a predicted type IV heat shock protein 40 (HSP40) that we show is expressed in gametocyte stages I to IV and is exported to the RBC cytoplasm. HSPs are traditionally induced under stressful conditions to maintain homeostasis, but PfGECO expression was not increased upon heat shock, suggesting an alternate function. Targeted disruption of PfGECO indicated that the gene is not essential for gametocytogenesis in vitro, and quantitative reverse transcriptase PCR (RT-PCR) showed that there was no compensatory expression of the other type IV HSP40 genes. Although P. falciparum HSP40 members are implicated in the trafficking of proteins to the RBC surface, removal of PfGECO did not affect the targeting of other exported gametocyte proteins. This work has expanded the repertoire of known gametocyte-exported proteins to include a type IV HSP40, PfGECO.

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.

Rethinking cerebral malaria pathology

PURPOSE OF REVIEW

Intense interventions are ongoing to combat malaria. Malaria mortality investigation remains as an intense area of study with controversies, competing models of pathogenesis, and a few carefully proceeding clinical trials. This review suggests a reframing of the question of cerebral malaria pathology in light of recent findings to focus on dissection of pathogenesis that will lead to effective treatments.

RECENT FINDINGS

Pediatric and adult manifestations of cerebral malaria within the retina allows for intense study of the clinical defined patients including the advent of multiple imaging modalities in endemic regions. Basic pathogenesis in mouse models and human studies, focused on cytokines, inflammation, cytoadherence, and endothelial activation, continues to be elucidated molecule by molecule. Coagulation is variably important and may serve as one of several unifying principles of current pathogenesis models. Parasite-derived molecules - surface or soluble - remain necessary but not sufficient to explain pathologic manifestations.

SUMMARY

As we close the gaps in the fight against global malaria, the question of cerebral malaria mortality remains a source of great concern. We currently have no effective means of reversal of coma or impacting mortality in the comatose patient. As transmission is broken, cerebral malaria will spread to older age groups in Africa where we expect mortality will be higher. Continued directed study of pathogenesis with the primary goal of efficacious interventions in the comatose is a necessity.

The survival strategies of malaria parasite in the red blood cell and host cell polymorphisms

Parasite growth within the erythrocyte causes dramatic alterations of host cell which on one hand facilitates nutrients acquisition from extracellular environment and on other hand contributes to the symptoms of severe malaria. The current paper focuses on interactions between the Plasmodium parasite and its metabolically highly reduced host cell, the natural selection of numerous polymorphisms in the genes encoding hemoglobin and other erythrocyte proteins.

Translocation of Magnaporthe oryzae effectors into rice cells and their subsequent cell-to-cell movement

Knowledge remains limited about how fungal pathogens that colonize living plant cells translocate effector proteins inside host cells to regulate cellular processes and neutralize defense responses. To cause the globally important rice blast disease, specialized invasive hyphae (IH) invade successive living rice (Oryza sativa) cells while enclosed in host-derived extrainvasive hyphal membrane. Using live-cell imaging, we identified a highly localized structure, the biotrophic interfacial complex (BIC), which accumulates fluorescently labeled effectors secreted by IH. In each newly entered rice cell, effectors were first secreted into BICs at the tips of the initially filamentous hyphae in the cell. These tip BICs were left behind beside the first-differentiated bulbous IH cells as the fungus continued to colonize the host cell. Fluorescence recovery after photobleaching experiments showed that the effector protein PWL2 (for prevents pathogenicity toward weeping lovegrass [Eragrostis curvula]) continued to accumulate in BICs after IH were growing elsewhere. PWL2 and BAS1 (for biotrophy-associated secreted protein 1), BIC-localized secreted proteins, were translocated into the rice cytoplasm. By contrast, BAS4, which uniformly outlines the IH, was not translocated into the host cytoplasm. Fluorescent PWL2 and BAS1 proteins that reached the rice cytoplasm moved into uninvaded neighbors, presumably preparing host cells before invasion. We report robust assays for elucidating the molecular mechanisms that underpin effector secretion into BICs, translocation to the rice cytoplasm, and cell-to-cell movement in rice.

New insights into protein export in malaria parasites

In order to survive and promote its virulence the malaria parasite must export hundreds of its proteins beyond an encasing vacuole and membrane into the host red blood cell. In the last few years, several major advances have been made that have significantly contributed to our understanding of this export process. These include: (i) the identification of sequences that direct protein export (a signal sequence and a motif termed PEXEL), which have allowed predictions of the exportomes of Plasmodium species that are the cause of malaria, (ii) the recognition that the fate of proteins destined for export is already decided within the parasite's endoplasmic reticulum and involves the PEXEL motif being recognized and cleaved by the aspartic protease plasmepsin V and (iii) the discovery of the Plasmodium translocon of exported proteins (PTEX) that is responsible for the passage of proteins across the vacuolar membrane. We review protein export in Plasmodium and these latest developments in the field that have now provided a new platform from which trafficking of malaria proteins can be dissected.

Comparative transcriptional and genomic analysis of Plasmodium falciparum field isolates

Mechanisms for differential regulation of gene expression may underlie much of the phenotypic variation and adaptability of malaria parasites. Here we describe transcriptional variation among culture-adapted field isolates of Plasmodium falciparum, the species responsible for most malarial disease. It was found that genes coding for parasite protein export into the red cell cytosol and onto its surface, and genes coding for sexual stage proteins involved in parasite transmission are up-regulated in field isolates compared with long-term laboratory isolates. Much of this variability was associated with the loss of small or large chromosomal segments, or other forms of gene copy number variation that are prevalent in the P. falciparum genome (copy number variants, CNVs). Expression levels of genes inside these segments were correlated to that of genes outside and adjacent to the segment boundaries, and this association declined with distance from the CNV boundary. This observation could not be explained by copy number variation in these adjacent genes. This suggests a local-acting regulatory role for CNVs in transcription of neighboring genes and helps explain the chromosomal clustering that we observed here. Transcriptional co-regulation of physical clusters of adaptive genes may provide a way for the parasite to readily adapt to its highly heterogeneous and strongly selective environment.

Describing commonalities in microbial effector delivery using the Gene Ontology

Myriad symbiotic microbes, ranging from mutualistic through to pathogenic, deliver 'effector' molecules into the cytoplasm or cellular milieu of their hosts to facilitate colonization. Among ecologically and evolutionarily diverse taxa, analogous processes and structures exist to facilitate effector delivery. These include syringe-like injection (bacteria and nematodes), common host-targeting signals (oomycetes and protozoans) and specialized intercellular structures (fungi and oomycetes). Here, we briefly introduce readers to the Gene Ontology (GO), a controlled vocabulary to facilitate comparative genomics of diverse taxa. We also summarize and compare selected mechanisms of effector delivery from various organisms and show how careful annotation of gene products with GO can reveal underlying similarities among diverse taxa.

The role of the Maurer's clefts in protein transport in Plasmodium falciparum

Maurer's clefts (MCs) are membranous structures that are formed by Plasmodium falciparum and used by the parasite for protein sorting and protein export. Virulence proteins, as well as other proteins used to remodel the erythrocyte, are exported. Discontinuity between major membrane compartments within the infected erythrocyte cytoplasm suggests multiple traffic routes for exported proteins. The sequences of the conserved Plasmodium export element seem insufficient for export of all parasite proteins. The parasite displays remarkable versatility in the types of proteins exported to the MCs and in the functions of the proteins within the MCs. In this Review, protein export to the MCs and the role of the MCs in the transport of proteins to the erythrocyte membrane are summarized.

piggyBac is an effective tool for functional analysis of the Plasmodium falciparum genome

BACKGROUND

Much of the Plasmodium falciparum genome encodes hypothetical proteins with limited homology to other organisms. A lack of robust tools for genetic manipulation of the parasite limits functional analysis of these hypothetical proteins and other aspects of the Plasmodium genome. Transposon mutagenesis has been used widely to identify gene functions in many organisms and would be extremely valuable for functional analysis of the Plasmodium genome.

RESULTS

In this study, we investigated the lepidopteran transposon, piggyBac, as a molecular genetic tool for functional characterization of the Plasmodium falciparum genome. Through multiple transfections, we generated 177 unique P. falciparum mutant clones with mostly single piggyBac insertions in their genomes. Analysis of piggyBac insertion sites revealed random insertions into the P. falciparum genome, in regards to gene expression in parasite life cycle stages and functional categories. We further explored the possibility of forward genetic studies in P. falciparum with a phenotypic screen for attenuated growth, which identified several parasite genes and pathways critical for intra-erythrocytic development.

CONCLUSION

Our results clearly demonstrate that piggyBac is a novel, indispensable tool for forward functional genomics in P. falciparum that will help better understand parasite biology and accelerate drug and vaccine development.

Malaria parasite proteins that remodel the host erythrocyte

Exported proteins of the malaria parasite Plasmodium falciparum interact with proteins of the erythrocyte membrane and induce substantial changes in the morphology, physiology and function of the host cell. These changes underlie the pathology that is responsible for the deaths of 1-2 million children every year due to malaria infections. The advent of molecular transfection technology, including the ability to generate deletion mutants and to introduce fluorescent reporter proteins that track the locations and dynamics of parasite proteins, has increased our understanding of the processes and machinery for export of proteins in P. falciparum-infected erythrocytes and has provided us with insights into the functions of the parasite protein exportome. We review these developments, focusing on parasite proteins that interact with the erythrocyte membrane skeleton or that promote delivery of the major virulence protein, PfEMP1, to the erythrocyte membrane.

Plasmodium falciparum and Hyaloperonospora parasitica effector translocation motifs are functional in Phytophthora infestans

The oomycete potato late blight pathogen, Phytophthora infestans, and the apicomplexan malaria parasite Plasmodium falciparum translocate effector proteins inside host cells, presumably to the benefit of the pathogen or parasite. Many oomycete candidate secreted effector proteins possess a peptide domain with the core conserved motif, RxLR, located near the N-terminal secretion signal peptide. In the Ph. infestans effector Avr3a, RxLR and an additional EER motif are essential for translocation into host cells during infection. Avr3a is recognized in the host cytoplasm by the R3a resistance protein. We have exploited this cytoplasmic recognition to report on replacement of the RxLR-EER of Avr3a with the equivalent sequences from the intracellular effectors ATR1NdWsB and ATR13 from the related oomycete pathogen, Hyaloperonospora parasitica, and the host targeting signal from the Pl. falciparum virulence protein PfHRPII. Introduction of these chimeric transgenes into Ph. infestans and subsequent virulence testing on potato plants expressing R3a demonstrated the alternative motifs to be functional in translocating Avr3a inside plant cells. These results suggest common mechanisms for protein translocation in both malaria and oomycete pathosystems.

Interactions of the malaria parasite and its mammalian host

A hallmark of Plasmodium development inside its mammalian victim is the remarkable restriction to the host species. Adaptation to an intracellular life style in specific target cells is determined by multiple parasite-host interactions. The first line of crosstalk occurs during intradermal sporozoite injection by an Anopheles mosquito. The following expansion in the liver is highly efficient and leads to successful establishment of the parasite population. During the periodic waves of fevers and chills the parasite destroys and re-infects red blood cells. Recent advances in experimental genetics and imaging techniques begin to expose the complex interactions at the changing parasite-host interfaces. Understanding the cellular and molecular mechanisms of target cell recognition, nutrient acquisition, and hijacking of cellular and immune functions may ultimately explain the elaborate biology of a medically important single cell eukaryote.

Functional alteration of red blood cells by a megadalton protein of Plasmodium falciparum

Proteins exported from Plasmodium falciparum parasites into red blood cells (RBCs) interact with the membrane skeleton and contribute to the pathogenesis of malaria. Specifically, exported proteins increase RBC membrane rigidity, decrease deformability, and increase adhesiveness, culminating in intravascular sequestration of infected RBCs (iRBCs). Pf332 is the largest (>1 MDa) known malaria protein exported to the RBC membrane, but its function has not previously been determined. To determine the role of Pf332 in iRBCs, we have engineered and analyzed transgenic parasites with Pf332 either deleted or truncated. Compared with RBCs infected with wild-type parasites, mutants lacking Pf332 were more rigid, were significantly less adhesive to CD36, and showed decreased expression of the major cytoadherence ligand, PfEMP1, on the iRBC surface. These abnormalities were associated with dramatic morphologic changes in Maurer clefts (MCs), which are membrane structures that transport malaria proteins to the RBC membrane. In contrast, RBCs infected with parasites expressing truncated forms of Pf332, although still hyperrigid, showed a normal adhesion profile and morphologically normal MCs. Our results suggest that Pf332 both modulates the level of increased RBC rigidity induced by P falciparum and plays a significant role in adhesion by assisting transport of PfEMP1 to the iRBC surface.

The Salmonella virulence protein SifA is a G protein antagonist

Salmonella's success at proliferating intracellularly and causing disease depends on the translocation of a major virulence protein, SifA, into the host cell. SifA recruits membranes enriched in lysosome associated membrane protein 1 (LAMP1) and is needed for growth of Salmonella induced filaments (Sifs) and the Salmonella containing vacuole (SCV). It directly binds a host protein called SKIP (SifA and kinesin interacting protein) which is critical for membrane stability and motor dynamics at the SCV. SifA also contains a WxxxE motif, predictive of G protein mimicry in bacterial effectors, but whether and how it mimics the action of a host G protein is not known. We show that SKIP's pleckstrin homology domain, which directly binds SifA, also binds to the late endosomal GTPase Rab9. Knockdown studies suggest that both SKIP and Rab9 function to maintain peripheral LAMP1 distribution in cells. The Rab9:SKIP interaction is GTP-dependent and is inhibited by SifA binding to the SKIP pleckstrin homology domain, suggesting that SifA may be a Rab9 antagonist. SifA:SKIP binding is significantly tighter than Rab9:SKIP binding and may thus allow SifA to bring SKIP to the SCV via SKIP's Rab9-binding site. Rab9 can measurably reverse SifA-dependent LAMP1 recruitment and the perinuclear location of the SCV in cells. Importantly, binding to SKIP requires SifA residues W197 and E201 of the conserved WxxxE signature sequence, leading to the speculation that bacterial G protein mimicry may result in G protein antagonism.

An erythrocyte vesicle protein exported by the malaria parasite promotes tubovesicular lipid import from the host cell surface

Plasmodium falciparum is the protozoan parasite that causes the most virulent of human malarias. The blood stage parasites export several hundred proteins into their host erythrocyte that underlie modifications linked to major pathologies of the disease and parasite survival in the blood. Unfortunately, most are ‘hypothetical’ proteins of unknown function, and those that are essential for parasitization of the erythrocyte cannot be ‘knocked out’. Here, we combined bioinformatics and genome-wide expression analyses with a new series of transgenic and cellular assays to show for the first time in malaria parasites that microarray read out from a chemical perturbation can have predictive value. We thereby identified and characterized an exported P. falciparum protein resident in a new vesicular compartment induced by the parasite in the erythrocyte. This protein, named Erythrocyte Vesicle Protein 1 (EVP1), shows novel dynamics of distribution in the parasite and intraerythrocytic membranes. Evidence is presented that its expression results in a change in TVN-mediated lipid import at the host membrane and that it is required for intracellular parasite growth, but not invasion. This exported protein appears to be needed for the maintenance of an essential tubovesicular nutrient import pathway induced by the pathogen in the host cell. Our approach may be generalized to the analysis of hundreds of ‘hypothetical’ P. falciparum proteins to understand their role in parasite entry and/or growth in erythrocytes as well as phenotypic contributions to either antigen export or tubovesicular import. By functionally validating these unknowns, one may identify new targets in host–microbial interactions for prophylaxis against this major human pathogen.

Plasmodium falciparum, the most virulent form of human malaria, causes disease when it invades a red blood cell. It sends proteins beyond its borders into the host, changing the red cell to make it a suitable environment to live in and to interact with the host immune system. Recent findings have predicted that hundreds of parasite proteins are released into the host red cell. However, most of these proteins have no known function. One major challenge is to understand what role these proteins play in parasite growth in order to design drugs or vaccines that block protein function. In this study, we designed a strategy to characterize ‘hypothetical’ proteins and use one as an example to illustrate the principle. We show that this protein resides within a novel compartment within the red cell and changes lipid transport at the host surface to stabilize a major nutrient pathway formed within the human cell. In principle, this strategy is applicable in determining the functions of other parasite genes involved in pathogen–host interactions.

A conditional export system provides new insights into protein export in Plasmodium falciparum-infected erythrocytes

The human malarial parasite Plasmodium falciparum exports determinants of virulence and pathology to destinations within its host erythrocyte, including the cytoplasm, the plasma membrane and membrane profiles of parasite origin termed Maurer's clefts. While there is some information regarding the signals that allot proteins for export, the trafficking route itself has remained largely obscure, partly due to technical limitations in following protein trafficking with time. To overcome these shortcomings, we have established a conditional protein export system in P. falciparum, based on the previously described conditional aggregation domain (CAD domain) that self-aggregates in the endoplasmic reticulum in a manner that is reversible by the addition of a small molecule. By fusing the CAD domain to the first 80 amino acids of STEVOR and full-length PfSBP1, we were able to control export of a soluble and a transmembrane protein to the erythrocyte cytosol and the Maurer's clefts respectively. The conditional export system allowed us to study the temporal sequence of events of protein export and identify intermediate steps. We further explored the potential of the conditional export system in identifying factors that interact with exported proteins en route. Our data provide evidence for a physical interaction of exported proteins with the molecular chaperone PfBiP during early export steps.

Oomycete RXLR effectors: delivery, functional redundancy and durable disease resistance

To manipulate host defences, plant pathogenic oomycetes secrete and translocate RXLR effectors into plant cells. Recent reports have indicated that RXLR effectors are translocated from the extrahaustorial matrix during the biotrophic phase of infection and that they are able to suppress PAMP-triggered immunity. Oomycete genomes contain potentially hundreds of highly diverse RXLR effector genes, providing the potential for considerable functional redundancy and the consequent ability to readily shed effectors that are recognised by plant surveillance systems without compromising pathogenic fitness. Understanding how these effectors are translocated, their precise roles in virulence, and the extent to which functional redundancy exists in oomycete RXLR effector complements, are major challenges for the coming years.