Proteases in host cell invasion by the malaria parasite

Genome-Wide Analysis of Haemonchus contortus Proteases and Protease Inhibitors Using Advanced Informatics Provides Insights into Parasite Biology and Host-Parasite Interactions

Biodiversity within the animal kingdom is associated with extensive molecular diversity. The expansion of genomic, transcriptomic and proteomic data sets for invertebrate groups and species with unique biological traits necessitates reliable in silico tools for the accurate identification and annotation of molecules and molecular groups. However, conventional tools are inadequate for lesser-known organismal groups, such as eukaryotic pathogens (parasites), so that improved approaches are urgently needed. Here, we established a combined sequence- and structure-based workflow system to harness well-curated publicly available data sets and resources to identify, classify and annotate proteases and protease inhibitors of a highly pathogenic parasitic roundworm (nematode) of global relevance, called Haemonchus contortus (barber's pole worm). This workflow performed markedly better than conventional, sequence-based classification and annotation alone and allowed the first genome-wide characterisation of protease and protease inhibitor genes and gene products in this worm. In total, we identified 790 genes encoding 860 proteases and protease inhibitors representing 83 gene families. The proteins inferred included 280 metallo-, 145 cysteine, 142 serine, 121 aspartic and 81 "mixed" proteases as well as 91 protease inhibitors, all of which had marked physicochemical diversity and inferred involvements in >400 biological processes or pathways. A detailed investigation revealed a remarkable expansion of some protease or inhibitor gene families, which are likely linked to parasitism (e.g., host-parasite interactions, immunomodulation and blood-feeding) and exhibit stage- or sex-specific transcription profiles. This investigation provides a solid foundation for detailed explorations of the structures and functions of proteases and protease inhibitors of H. contortus and related nematodes, and it could assist in the discovery of new drug or vaccine targets against infections or diseases.

Characterization of Extracellular Vesicles Secreted by a Clinical Isolate of Naegleria fowleri and Identification of Immunogenic Components within Their Protein Cargo

Extracellular vesicles (EVs) are small lipid vesicles released by both prokaryotic and eukaryotic cells, involved in intercellular communication, immunomodulation and pathogenesis. In this study, we performed a characterization of the EVs produced by trophozoites of a clinical isolate of the free-living amoeba Naegleria fowleri (N. fowleri). Size distribution, zeta potential, protein profile and protease activity were analyzed. Under our incubation conditions, EVs of different sizes were observed, with a predominant population ranging from 206 to 227 nm. SDS-PAGE revealed protein bands of 25 to 260 KDa. The presence of antigenic proteins was confirmed by Western blot, which evidenced strongest recognition by rat polyclonal antibodies raised against N. fowleri in the region close to 80 KDa and included peptidases, as revealed by zymography. Proteins in selected immunorecognized bands were further identified using nano-ESI-MS/MS. A preliminary proteomic profile of the EVs identified at least 184 proteins as part of the vesicles' cargo. Protease activity assays, in combination with the use of inhibitors, revealed the predominance of serine proteases. The present characterization uncovers the complexity of EVs produced by N. fowleri, suggesting their potential relevance in the release of virulence factors involved in pathogenicity. Owing to their cargo's diversity, further research on EVs could reveal new therapeutic targets or biomarkers for developing rapid and accurate diagnostic tools for lethal infections such as the one caused by this amoeba.

Immunogenicity Analysis of the Recombinant Plasmodium falciparum Surface-Related Antigen in Mice

Plasmodium falciparum, mainly distributed in tropical and subtropical regions of the world, has received widespread attention owing to its severity. As a novel protein, P. falciparum surface-related antigen (PfSRA) has the structural and functional characteristics to be considered as a malaria vaccine candidate; however, limited information is available on its immunogenicity. Here, we expressed three fragments of recombinant PfSRA in an Escherichia coli system and further analyzed its immunogenicity. The results showed that rPfSRA-immunized mice produced specific antibodies with high endpoint titers (1:10,000 to 1:5,120,000) and affinity antibodies (i.e., rPfSRA-F1a (97.70%), rPfSRA-F2a (69.62%), and rPfSRA-F3a (91.87%)). In addition, the sera of immunized mice recognized both the native PfSRA and recombinant PfSRA, the rPfSRA antibodies inhibited the invasion of P. falciparum into the erythrocytes, and they were dose-dependent in vitro. This study confirmed PfSRA could be immunogenic, especially the F1a at the conserved region N-terminal and provided further support for it as a vaccine candidate against P.falciparum.

Phylogeny of Anopheles darlingi (Diptera:Culicidae) based on the antimicrobial peptide genes cecropin and defensin

Cecropins and defensins are the main classes of antimicrobial peptides in the mosquito innate immune system, acting against bacteria, fungi and protozoa. There is a knowledge gap concerning these peptide genes in anopheline mosquitoes from the Brazilian Amazon. Thus, this work aimed to describe molecular techniques for detecting the genes encoding the antimicrobial peptides cecropin A (CecA) and defensin in Anopheles darlingi mosquitoes and to perform molecular phylogeny of the sequenced genes using the maximum likelihood method and Bayesian inference with other species from different geographic areas. Our results show, for the first time, a molecular biology method for detecting CecA and defensin in Anopheles darlingi that allows for the use of these molecular markers for phylogenetic analysis in anopheline species, separating the species into single and monophyletic clades.

A Comparative Review on Current and Future Drug Targets Against Bacteria & Malaria

Long before the discovery of drugs like 'antibiotic and anti-parasitic drugs', the infectious diseases caused by pathogenic bacteria and parasites remain as one of the major causes of morbidity and mortality in developing and underdeveloped countries. The phenomenon by which the organism exerts resistance against two or more structurally unrelated drugs is called multidrug resistance (MDR) and its emergence has further complicated the treatment scenario of infectious diseases. Resistance towards the available set of treatment options and poor pipeline of novel drug development puts an alarming situation. A universal goal in the post-genomic era is to identify novel targets/drugs for various life-threatening diseases caused by such pathogens. This review is conceptualized in the backdrop of drug resistance in two major pathogens i.e. "Pseudomonas aeruginosa" and "Plasmodium falciparum". In this review, the available targets and key mechanisms of resistance of these pathogens have been discussed in detail. An attempt has also been made to analyze the common drug targets of bacteria and malaria parasite to overcome the current drug resistance scenario. The solution is also hypothesized in terms of a present pipeline of drugs and efforts made by scientific community.

Molecular mechanisms that mediate invasion and egress of malaria parasites from red blood cells

PURPOSE OF REVIEW

Malaria parasites invade and multiply in diverse host cells during their complex life cycle. Some blood stage parasites transform into male and female gametocytes that are transmitted by female anopheline mosquitoes. The gametocytes are activated in the mosquito midgut to form male and female gametes, which egress from RBCs to mate and form a zygote. Here, we will review our current understanding of the molecular mechanisms that mediate invasion and egress by malaria parasites at different life cycle stages.

RECENT FINDINGS

A number of key effector molecules such as parasite protein ligands for receptor-engagement during invasion as well as proteases and perforin-like proteins that mediate egress have been identified. Interestingly, these parasite-encoded effectors are located in internal, vesicular organelles and are secreted in a highly regulated manner during invasion and egress. Here, we will review our current understanding of the functional roles of these effectors as well as the signaling pathways that regulate their timely secretion with accurate spatiotemporal coordinates.

SUMMARY

Understanding the molecular basis of key processes such as host cell invasion and egress by malaria parasites could provide novel targets for development of inhibitors to block parasite growth and transmission.

Molecular Signaling Involved in Entry and Exit of Malaria Parasites from Host Erythrocytes

During the blood stage, Plasmodium spp. merozoites invade host red blood cells (RBCs), multiply, exit, and reinvade uninfected RBCs in a continuing cycle that is responsible for all the clinical symptoms associated with malaria. Entry into (invasion) and exit from (egress) RBCs are highly regulated processes that are mediated by an array of parasite proteins with specific functional roles. Many of these parasite proteins are stored in specialized apical secretory vesicles, and their timely release is critical for successful invasion and egress. For example, the discharge of parasite protein ligands to the apical surface of merozoites is required for interaction with host receptors to mediate invasion, and the timely discharge of proteases and pore-forming proteins helps in permeabilization and dismantling of limiting membranes during egress. This review focuses on our understanding of the signaling mechanisms that regulate apical organelle secretion during host cell invasion and egress by malaria parasites. The review also explores how understanding key signaling mechanisms in the parasite can open opportunities to develop novel strategies to target Plasmodium parasites and eliminate malaria.

Multiple Plasmodium falciparum Merozoite Surface Protein 1 Complexes Mediate Merozoite Binding to Human Erythrocytes

Successful invasion of human erythrocytes byPlasmodium falciparummerozoites is required for infection of the host and parasite survival. The early stages of invasion are mediated via merozoite surface proteins that interact with human erythrocytes. The nature of these interactions are currently not well understood, but it is known that merozoite surface protein 1 (MSP1) is critical for successful erythrocyte invasion. Here we show that the peripheral merozoite surface proteins MSP3, MSP6, MSPDBL1, MSPDBL2, and MSP7 bind directly to MSP1, but independently of each other, to form multiple forms of the MSP1 complex on the parasite surface. These complexes have overlapping functions that interact directly with human erythrocytes. We also show that targeting the p83 fragment of MSP1 using inhibitory antibodies inhibits all forms of MSP1 complexes and disrupts parasite growthin vitro.

Characterization of Plasmodium vivax Early Transcribed Membrane Protein 11.2 and Exported Protein 1

In Plasmodium, the membrane of intracellular parasites is initially formed during invasion as an invagination of the red blood cell surface, which forms a barrier between the parasite and infected red blood cells in asexual blood stage parasites. The membrane proteins of intracellular parasites of Plasmodium species have been identified such as early-transcribed membrane proteins (ETRAMPs) and exported proteins (EXPs). However, there is little or no information regarding the intracellular parasite membrane in Plasmodium vivax. In the present study, recombinant PvETRAMP11.2 (PVX_003565) and PvEXP1 (PVX_091700) were expressed and evaluated antigenicity tests using sera from P. vivax-infected patients. A large proportion of infected individuals presented with IgG antibody responses against PvETRAMP11.2 (76.8%) and PvEXP1 (69.6%). Both of the recombinant proteins elicited high antibody titers capable of recognizing parasites of vivax malaria patients. PvETRAMP11.2 partially co-localized with PvEXP1 on the intracellular membranes of immature schizont. Moreover, they were also detected at the apical organelles of newly formed merozoites of mature schizont. We first proposed that these proteins might be synthesized in the preceding schizont stage, localized on the parasite membranes and apical organelles of infected erythrocytes, and induced high IgG antibody responses in patients.

Structural basis for epitope masking and strain specificity of a conserved epitope in an intrinsically disordered malaria vaccine candidate

Merozoite surface protein 2 (MSP2) is an intrinsically disordered, membrane-anchored antigen of the malaria parasite Plasmodium falciparum. MSP2 can elicit a protective, albeit strain-specific, antibody response in humans. Antibodies are generated to the conserved N- and C-terminal regions but many of these react poorly with the native antigen on the parasite surface. Here we demonstrate that recognition of a conserved N-terminal epitope by mAb 6D8 is incompatible with the membrane-bound conformation of that region, suggesting a mechanism by which native MSP2 escapes antibody recognition. Furthermore, crystal structures and NMR spectroscopy identify transient, strain-specific interactions between the 6D8 antibody and regions of MSP2 beyond the conserved epitope. These interactions account for the differential affinity of 6D8 for the two allelic families of MSP2, even though 6D8 binds to a fully conserved epitope. These results highlight unappreciated mechanisms that may modulate the specificity and efficacy of immune responses towards disordered antigens.

Antimicrobial peptides: a new class of antimalarial drugs?

A range of antimicrobial peptides (AMP) exhibit activity on malaria parasites, Plasmodium spp., in their blood or mosquito stages, or both. These peptides include a diverse array of both natural and synthetic molecules varying greatly in size, charge, hydrophobicity, and secondary structure features. Along with an overview of relevant literature reports regarding AMP that display antiplasmodial activity, this review makes a few considerations about those molecules as a potential new class of antimalarial drugs.

Chemical biology approaches for the study of apicomplexan parasites

Chemical biology and the techniques the field encompasses provide scientists with the means to address biological questions in ever-evolving and technically sophisticated ways. They facilitate the dissection of molecular mechanisms of cell phenomena on timescales not achievable by other means. Libraries of small molecules, bioorthogonal chemistries and technical advances in mass-spectrometry techniques enable the modern chemical biologist to tackle even the most difficult of biological questions. It is because of their broad applicability that these approaches are well suited to systems less tractable to more classical genetic methods. As such, the parasite community has embraced them with great success. Some of these successes and the continuing evolution of chemical biology applied to apicomplexans will be discussed.

Sequential processing of merozoite surface proteins during and after erythrocyte invasion by Plasmodium falciparum

Plasmodium falciparum causes malaria disease during the asexual blood stages of infection when merozoites invade erythrocytes and replicate. Merozoite surface proteins (MSPs) are proposed to play a role in the initial binding of merozoites to erythrocytes, but precise roles remain undefined. Based on electron microscopy studies of invading Plasmodium merozoites, it is proposed that the majority of MSPs are cleaved and shed from the surface during invasion, perhaps to release receptor-ligand interactions. In this study, we demonstrate that there is not universal cleavage of MSPs during invasion. Instead, there is sequential and coordinated cleavage and shedding of proteins, indicating a diversity of roles for surface proteins during and after invasion. While MSP1 and peripheral surface proteins such as MSP3, MSP7, serine repeat antigen 4 (SERA4), and SERA5 are cleaved and shed at the tight junction between the invading merozoite and erythrocyte, the glycosylphosphatidylinositol (GPI)-anchored proteins MSP2 and MSP4 are carried into the erythrocyte without detectable processing. Following invasion, MSP2 rapidly degrades within 10 min, whereas MSP4 is maintained for hours. This suggests that while some proteins that are shed upon invasion may have roles in initial contact steps, others function during invasion and are then rapidly degraded, whereas others are internalized for roles during intraerythrocytic development. Interestingly, anti-MSP2 antibodies did not inhibit invasion and instead were carried into erythrocytes and maintained for approximately 20 h without inhibiting parasite development. These findings provide new insights into the mechanisms of invasion and knowledge to advance the development of new drugs and vaccines against malaria.

Molecular determinants for subcellular trafficking of the malarial sheddase PfSUB2

The malaria merozoite invades erythrocytes in the vertebrate host. Iterative rounds of asexual intraerythrocytic replication result in disease. Proteases play pivotal roles in erythrocyte invasion, but little is understood about their mode of action. The Plasmodium falciparum malaria merozoite surface sheddase, PfSUB2, is one such poorly characterized example. We have examined the molecular determinants that underlie the mechanisms by which PfSUB2 is trafficked initially to invasion-associated apical organelles (micronemes) and then across the surface of the free merozoite. We show that authentic promoter activity is important for correct localization of PfSUB2, likely requiring canonical features within the intergenic region 5' of the pfsub2 locus. We further demonstrate that trafficking of PfSUB2 beyond an early compartment in the secretory pathway requires autocatalytic protease activity. Finally, we show that the PfSUB2 transmembrane domain is required for microneme targeting, while the cytoplasmic domain is essential for surface translocation of the protease to the parasite posterior following discharge from micronemes. The interplay of pre- and post-translational regulatory elements that coordinate subcellular trafficking of PfSUB2 provides the parasite with exquisite control over enzyme-substrate interactions.

Structure-activity relationships and blood distribution of antiplasmodial aminopeptidase-1 inhibitors

Malaria is a severe infectious disease that causes between 655,000 and 1.2 million deaths annually. To overcome the resistance to current drugs, new biological targets are needed for drug development. Aminopeptidase M1 (PfAM1), a zinc metalloprotease, has been proposed as a new drug target to fight malaria. Herein, we disclosed the structure-activity relationships of a selective family of hydroxamate PfAM1 inhibitors based on the malonic template. In particular, we performed a "fluoro-scanning" around hit 1 that enlightened the key positions of the halogen for activity. The docking of the best inhibitor 2 is consistent with in vitro results. The stability of 2 was evaluated in microsomes, in plasma, and toward glutathione. The in vivo distribution study performed with the nanomolar hydroxamate inhibitor 2 (BDM14471) revealed that it reaches its site of action. However, it fails to kill the parasite at concentrations relevant to the enzymatic inhibitory potency, suggesting that killing the parasite remains a challenge for potent and druglike catalytic-site binding PfAM1 inhibitors. In all, this study provides important insights for the design of inhibitors of PfAM1 and the validity of this target.

Solution NMR structure of a sheddase inhibitor prodomain from the malarial parasite Plasmodium falciparum

Plasmodium subtilisin 2 (Sub2) is a multidomain protein that plays an important role in malaria infection. Here, we describe the solution NMR structure of a conserved region of the inhibitory prodomain of Sub2 from Plasmodium falciparum, termed prosub2. Despite the absence of any detectable sequence homology, the protozoan prosub2 has structural similarity to bacterial and mammalian subtilisin-like prodomains. Comparison with the three-dimensional structures of these other prodomains suggests a likely binding interface with the catalytic domain of Sub2 and provides insights into the locations of primary and secondary processing sites in Plasmodium prodomains.

Proteases as regulators of pathogenesis: examples from the Apicomplexa

The diverse functional roles that proteases play in basic biological processes make them essential for virtually all organisms. Not surprisingly, proteolysis is also a critical process required for many aspects of pathogenesis. In particular, obligate intracellular parasites must precisely coordinate proteolytic events during their highly regulated life cycle inside multiple host cell environments. Advances in chemical, proteomic and genetic tools that can be applied to parasite biology have led to an increased understanding of the complex events centrally regulated by proteases. In this review, we outline recent advances in our knowledge of specific proteolytic enzymes in two medically relevant apicomplexan parasites: Plasmodium falciparum and Toxoplasma gondii. Efforts over the last decade have begun to provide a map of key proteotolyic events that are essential for both parasite survival and propagation inside host cells. These advances in our molecular understanding of proteolytic events involved in parasite pathogenesis provide a foundation for the validation of new networks and enzyme targets that could be exploited for therapeutic purposes. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Plasmodium vinckei: infectivity of arteether-sensitive and arteether-resistant parasites in different strains of mice

Malaria is one of the most lethal parasitic infections in the world. The lethality of the parasite depends on the rate of multiplication of the parasite within host erythrocytes. Different strains of the malaria parasite often respond in a different way to the same strain of mice or vice versa. In the present study, we investigated the course of infection of the arteether-sensitive and arteether-resistant Plasmodium vinckei parasites in Swiss albino AKR (inbred) and AJ (outbred) mice. The higher parasite burden and mortality were observed in the sensitive parasite-infected mice, whereas the infection with the resistant parasite was non-lethal. Resistant parasite-infected mice developed a moderate level of parasitemia that decreased gradually throughout the infection. The microscopic examination suggests that the resistant parasite invades reticulocytes more efficiently than normocytes, regardless of the mouse strain examined. Since the reticulocytes are rare in blood circulation, it limits the increase in parasite proliferations, while arteether-sensitive parasites can invade both mature normocytes and reticulocytes, resulting in the mortality of the mice. However, treatment with phenylhydrazine in Swiss mice results in reticulocytosis, which transforms the non-lethal resistant parasites to produce lethal infections. Our findings demonstrate that the characteristic response during infections with the arteether-resistant strain is dependent on the availability of reticulocytes in peripheral blood circulation. We can use this model for identifying the interaction between host and artemisinin derivative-resistant parasites.

Plasmodium falciparum reticulocyte binding-like homologue protein 2 (PfRH2) is a key adhesive molecule involved in erythrocyte invasion

Erythrocyte invasion by Plasmodium merozoites is a complex, multistep process that is mediated by a number of parasite ligand-erythrocyte receptor interactions. One such family of parasite ligands includes the P. falciparum reticulocyte binding homologue (PfRH) proteins that are homologous with the P. vivax reticulocyte binding proteins and have been shown to play a role in erythrocyte invasion. There are five functional PfRH proteins of which only PfRH2a/2b have not yet been demonstrated to bind erythrocytes. In this study, we demonstrated that native PfRH2a/2b is processed near the N-terminus yielding fragments of 220 kDa and 80 kDa that exhibit differential erythrocyte binding specificities. The erythrocyte binding specificity of the 220 kDa processed fragment of native PfRH2a/2b was sialic acid-independent, trypsin resistant and chymotrypsin sensitive. This specific binding phenotype is consistent with previous studies that disrupted the PfRH2a/2b genes and demonstrated that PfRH2b is involved in a sialic acid independent, trypsin resistant, chymotrypsin sensitive invasion pathway. Interestingly, we found that the smaller 80 kDa PfRH2a/2b fragment is processed from the larger 220 kDa fragment and binds erythrocytes in a sialic acid dependent, trypsin resistant and chymotrypsin sensitive manner. Thus, the two processed fragments of PfRH2a/2b differed with respect to their dependence on sialic acids for erythrocyte binding. Further, we mapped the erythrocyte binding domain of PfRH2a/2b to a conserved 40 kDa N-terminal region (rPfRH2₄₀) in the ectodomain that is common to both PfRH2a and PfRH2b. We demonstrated that recombinant rPfRH2₄₀ bound human erythrocytes with the same specificity as the native 220 kDa processed protein. Moreover, antibodies generated against rPfRH2₄₀ blocked erythrocyte invasion by P. falciparum through a sialic acid independent pathway. PfRH2a/2b thus plays a key role in erythrocyte invasion and its conserved receptor-binding domain deserves attention as a promising candidate for inclusion in a blood-stage malaria vaccine.

FRET peptides reveal differential proteolytic activation in intraerythrocytic stages of the malaria parasites Plasmodium berghei and Plasmodium yoelii

Malaria is still a major health problem in developing countries. It is caused by the protist parasite Plasmodium, in which proteases are activated during the cell cycle. Ca(2+) is a ubiquitous signalling ion that appears to regulate protease activity through changes in its intracellular concentration. Proteases are crucial to Plasmodium development, but the role of Ca(2+) in their activity is not fully understood. Here we investigated the role of Ca(2+) in protease modulation among rodent Plasmodium spp. Using fluorescence resonance energy transfer (FRET) peptides, we verified protease activity elicited by Ca(2+) from the endoplasmatic reticulum (ER) after stimulation with thapsigargin (a sarco/endoplasmatic reticulum Ca(2+)-ATPase (SERCA) inhibitor) and from acidic compartments by stimulation with nigericin (a K(+)/H(+) exchanger) or monensin (a Na(+)/H(+) exchanger). Intracellular (BAPTA/AM) and extracellular (EGTA) Ca(2+) chelators were used to investigate the role played by Ca(2+) in protease activation. In Plasmodium berghei both EGTA and BAPTA blocked protease activation, whilst in Plasmodium yoelii these compounds caused protease activation. The effects of protease inhibitors on thapsigargin-induced proteolysis also differed between the species. Pepstatin A and phenylmethylsulphonyl fluoride (PMSF) increased thapsigargin-induced proteolysis in P. berghei but decreased it in P. yoelii. Conversely, E64 reduced proteolysis in P. berghei but stimulated it in P. yoelii. The data point out key differences in proteolytic responses to Ca(2+) between species of Plasmodium.

Plasmodium falciparum PfA-M1 aminopeptidase is trafficked via the parasitophorous vacuole and marginally delivered to the food vacuole

Background

The Plasmodium falciparum PfA-M1 aminopeptidase, encoded by a single copy gene, displays a neutral optimal activity at pH 7.4. It is thought to be involved in haemoglobin degradation and/or invasion of the host cells. Although a series of inhibitors developed against PfA-M1 suggest that this enzyme is a promising target for therapeutic intervention, the biological function(s) of the three different forms of the enzyme (p120, p96 and p68) are not fully understood. Two recent studies using PfA-M1 transfections have also provided conflicting results on PfA-M1 localization within or outside the food vacuole. Alternative destinations, such as the nucleus, have also been proposed.

Methods

By using a combination of techniques, such as cellular and biochemical fractionations, biochemical analysis, mass-spectrometry, immunofluorescence assays and live imaging of GFP fusions to various PfA-M1 domains, evidence is provided for differential localization and behaviour of the three different forms of PfA-M1 in the infected red blood cell which had not been established before.

Results

The high molecular weight p120 form of PfA-M1, the only version of the protein with a hydrophobic transmembrane domain, is detected both inside the parasite and in the parasitophorous vacuole while the processed p68 form is strictly soluble and localized within the parasite. The transient intermediate and soluble p96 form is localized at the border of parasitophorous vacuole and within the parasite in a compartment sensitive to high concentrations of saponin. Upon treatment with brefeldin A, the PfA-M1 maturation is blocked and the enzyme remains in a compartment close to the nucleus.

Conclusions

The PfA-M1 trafficking/maturation scenario that emerges from this data indicates that PfA-M1, synthesized as the precursor p120 form, is targeted to the parasitophorous vacuole via the parasite endoplasmic reticulum/Golgi, where it is converted into the transient p96 form. This p96 form is eventually redirected into the parasite to be converted into the processed p68 form that is only marginally delivered to the parasite food vacuole. These results provide insights on PfA-M1 topology regarding key compartments of the infected red blood cells that have important implications for the development of inhibitors targeting this plasmodial enzyme.

Antibodies to reticulocyte binding protein-like homologue 4 inhibit invasion of Plasmodium falciparum into human erythrocytes

Plasmodium falciparum invasion into human erythrocytes relies on the interaction between multiple parasite ligands and their respective erythrocyte receptors. The sialic acid-independent invasion pathway is dependent on the expression of P. falciparum reticulocyte binding protein-like homologue 4 (PfRh4), as disruption of the gene abolishes the ability of parasites to switch to this pathway. We show that PfRh4 is present as an invasion ligand in culture supernatants as a 160-kDa proteolytic fragment. We confirm that PfRh4 binds to the surfaces of erythrocytes through recognition of an erythrocyte receptor that is neuraminidase resistant but trypsin and chymotrypsin sensitive. Serum antibodies from malaria-exposed individuals show reactivity against the binding domain of PfRh4. Purified immunoglobulin G raised in rabbits against the binding domain of PfRh4 blocked the binding of native PfRh4 to the surfaces of erythrocytes and inhibited erythrocyte invasion of parasites using sialic acid-independent invasion pathways and grown in neuraminidase-treated erythrocytes. Our results suggest PfRh4 is a potential vaccine candidate.

Role of CpSUB1, a subtilisin-like protease, in Cryptosporidium parvum infection in vitro

The apicomplexan parasite Cryptosporidium is a significant cause of diarrheal disease worldwide. Previously, we reported that a Cryptosporidium parvum subtilisin-like serine protease activity with furin-type specificity cleaves gp40/15, a glycoprotein that is proteolytically processed into gp40 and gp15, which are implicated in mediating infection of host cells. Neither the enzyme(s) responsible for the protease activity in C. parvum lysates nor those that process gp40/15 are known. There are no furin or other proprotein convertase genes in the C. parvum genome. However, a gene encoding CpSUB1, a subtilisin-like serine protease, is present. In this study, we cloned the CpSUB1 genomic sequence and expressed and purified the recombinant prodomain. Reverse transcriptase PCR analysis of RNA from C. parvum-infected HCT-8 cells revealed that CpSUB1 is expressed throughout infection in vitro. In immunoblots, antiserum to the recombinant CpSUB1 prodomain revealed two major bands, of approximately 64 kDa and approximately 48 kDa, for C. parvum lysates and proteins "shed" during excystation. In immunofluorescence assays, the antiserum reacted with the apical region of sporozoites and merozoites. The recombinant prodomain inhibited protease activity and processing of recombinant gp40/15 by C. parvum lysates but not by furin. Since prodomains are often selective inhibitors of their cognate enzymes, these results suggest that CpSUB1 may be a likely candidate for the protease activity in C. parvum and for processing of gp40/15. Importantly, the recombinant prodomain inhibited C. parvum infection of HCT-8 cells. These studies indicate that CpSUB1 plays a significant role in infection of host cells by the parasite and suggest that this enzyme may serve as a target for intervention.

Formation of the food vacuole in Plasmodium falciparum: a potential role for the 19 kDa fragment of merozoite surface protein 1 (MSP1(19))

Plasmodium falciparum Merozoite Surface Protein 1 (MSP1) is synthesized during schizogony as a 195-kDa precursor that is processed into four fragments on the parasite surface. Following a second proteolytic cleavage during merozoite invasion of the red blood cell, most of the protein is shed from the surface except for the C-terminal 19-kDa fragment (MSP1₁₉), which is still attached to the merozoite via its GPI-anchor. We have examined the fate of MSP1₁₉ during the parasite's subsequent intracellular development using immunochemical analysis of metabolically labeled MSP1₁₉, fluorescence imaging, and immuno-electronmicroscopy. Our data show that MSP1₁₉ remains intact and persists to the end of the intracellular cycle. This protein is the first marker for the biogenesis of the food vacuole; it is rapidly endocytosed into small vacuoles in the ring stage, which coalesce to form the single food vacuole containing hemozoin, and persists into the discarded residual body. The food vacuole is marked by the presence of both MSP1₁₉ and the chloroquine resistance transporter (CRT) as components of the vacuolar membrane. Newly synthesized MSP1 is excluded from the vacuole. This behavior indicates that MSP1₁₉ does not simply follow a classical lysosome-like clearance pathway, instead, it may play a significant role in the biogenesis and function of the food vacuole throughout the intra-erythrocytic phase.

Roles of proteases during invasion and egress by Plasmodium and Toxoplasma

Apicomplexan pathogens replicate exclusively within the confines of a host cell. Entry into (invasion) and exit from (egress) these cells requires an array of specialized parasite molecules, many of which have long been considered to have potential as targets of drug or vaccine-based therapies. In this chapter the authors discuss the current state of knowledge regarding the role of parasite proteolytic enzymes in these critical steps in the life cycle of two clinically important apicomplexan genera, Plasmodium and Toxoplasma. At least three distinct proteases of the cysteine mechanistic class have been implicated in egress of the malaria parasite from cells of its vertebrate and insect host. In contrast, the bulk of the evidence indicates a prime role for serine proteases of the subtilisin and rhomboid families in invasion by both parasites. Whereas proteases involved in egress may function predominantly to degrade host cell structures, proteases involved in invasion probably act primarily as maturases and 'sheddases', required to activate and ultimately remove ligands involved in interactions with the host cell.

Plasmodium in the postgenomic era: new insights into the molecular cell biology of malaria parasites

In this review, we bring together some of the approaches toward understanding the cellular and molecular biology of Plasmodium species and their interaction with their host red blood cells. Considerable impetus has come from the development of new methods of molecular genetics and bioinformatics, and it is important to evaluate the wealth of these novel data in the context of basic cell biology. We describe how these approaches are gaining valuable insights into the parasite-host cell interaction, including (1) the multistep process of red blood cell invasion by the merozoite; (2) the mechanisms by which the intracellular parasite feeds on the red blood cell and exports parasite proteins to modify its cytoadherent properties; (3) the modulation of the cell cycle by sensing the environmental tryptophan-related molecules; (4) the mechanism used to survive in a low Ca(2+) concentration inside red blood cells; (5) the activation of signal transduction machinery and the regulation of intracellular calcium; (6) transfection technology; and (7) transcriptional regulation and genome-wide mRNA studies in Plasmodium falciparum.

Whole-genome natural histories of apicomplexan surface proteins

The natural histories of free-living and pathogenic protozoans have been described in over a century of studies, spanning a range of disciplines such as microscopic, cellular, taxonomic, pathological, clinical and molecular. Only in the last decade has this landscape of work benefited from the availability of whole-genome nucleotide sequence data. For many pathogens, it is now possible to overlay analyses of protein repertoires onto the current spectrum of knowledge. This article illuminates protozoan natural histories, particularly the rapidly evolving and highly adaptive direct physical interface of apicomplexan parasites and their hosts, by providing a brief introduction to the origin and phylogenetic distribution of parasite-encoded surface proteins and their component domains.

Novel Selective Inhibitors of the Zinc Plasmodial Aminopeptidase PfA-M1 as Potential Antimalarial Agents

Proteases that are expressed during the erythocytic stage of Plasmodium falciparum are newly explored drug targets for the treatment of malaria. We report here the discovery of potent inhibitors of PfA-M1, a metallo-aminopeptidase of the parasite. These compounds are based on a malonic hydroxamic template and present a very good selectivity toward neutral aminopeptidase (APN-CD13), a related protease in mammals. Structure-activity relationships in these series are described. Further optimization of the best inhibitor yielded a nanomolar, selective inhibitor of PfA-M1. This inhibitor displays good physicochemical and pharmacokinetic properties and a promising antimalarial activity.

A conserved subtilisin protease identified in Babesia divergens merozoites

Invasion of erythrocytes is an integral part of the Babesia divergens life cycle. Serine proteases have been shown to play an important role in invasion by related Apicomplexan parasites such as the malaria parasite Plasmodium falciparum. Here we demonstrate the presence of two dominant serine proteases in asexual B. divergens using a biotinylated fluorophosphonate probe. One of these active serine proteases (p48) and its precursors were recognized by anti-PfSUB1 antibodies. These antibodies were used to clone the gene encoding a serine protease using a B. divergens cDNA library. BdSub-1 is a single copy gene with no introns. The deduced gene product (BdSUB-1) clearly belongs to the subtilisin superfamily and shows significant homology to Plasmodium subtilisins, with the highest degree of sequence identity around the four catalytic residues. Like subtilisin proteases in other Apicomplexan parasites, BdSUB-1 undergoes two steps of processing during activation in the secretory pathway being finally converted to an active form (p48). The mature protease is concentrated in merozoite dense granules, apical secretory organelles involved in erythrocyte invasion. Anti-PfSUB1 antibodies have a potent inhibitory effect on erythrocyte invasion by B. divergens merozoites in vitro. This report demonstrates conservation of the molecular machinery involved in erythrocyte invasion by these two Apicomplexan parasites and paves the way for a comparative analysis of other molecules that participate in this process in the two parasites.

Effect of metalloprotease inhibitors on invasion of red blood cell by Plasmodium falciparum

For successful invasion, the malaria merozoite needs to attach to the red blood cell membrane, undergo reorientation, form a junction of the apical end with the host membrane, and internalize. Malaria proteases have been implicated in the invasion process, but their specific cellular functions remain unclear. To demonstrate the involvement of metalloprotease in the process of Plasmodium falciparum merozoite entry into host red blood cell, schizont-infected red blood cells and parasitophorous vacuolar membrane-enclosed merozoite structures were treated with 1,10-phenanthroline, a metal chelator, resulting in a reduction of invasion with IC50 value of 25 and 29 microM, respectively. Absence of an accumulation of schizont stages after treatment with 1,10-phenanthroline indicated that the inhibitory effect was not due to suppression of merozoite release from red blood cells, but on the invasion step. Although treatment with GM6001, a well-known inhibitor of the mammalian matrix and disintegrin metalloprotease family, was less effective, nevertheless this study points to the importance of metal-requiring protease in the process of invasion of host red blood cell by the malaria parasite.

Distinct mechanisms govern proteolytic shedding of a key invasion protein in apicomplexan pathogens

Apical membrane antigen-1 (AMA1) is a conserved apicomplexan protein that plays an important but undefined role in host cell invasion. We have studied the fate of Plasmodium falciparum AMA1 (PfAMA1) during erythrocyte invasion by the malaria merozoite, and compared it with that of the Toxoplasma gondii orthologue, TgAMA1. Shedding of the PfAMA1 ectodomain goes essentially to completion during invasion, and occurs predominantly or exclusively via juxtamembrane cleavage at the previously identified sheddase cleavage site, Thr517. Only the resulting juxtamembrane stub of the ectodomain is efficiently carried into the host cell, and this remains distributed around the plasma membrane of the intracellular ring-stage parasite. Inhibition of normal shedding, however, results in proteolysis at an intramembrane, rhomboid-like cleavage site, and PfAMA1 is susceptible to cleavage by Drosophila rhomboid-1, showing that it can be a substrate for intramembrane cleavage but is not normally processed in this manner. In contrast, shedding of TgAMA1 from the surface of extracellular tachyzoites occurs exclusively via cleavage within the luminal half of its transmembrane domain by a rhomboid-like protease. Also unlike PfAMA1, complete TgAMA1 shedding does not accompany Toxoplasma invasion as the intact protein was readily detected on the surface of newly invaded tachyzoites. This work reveals unexpected differences in the manner in which Plasmodium and Toxoplasma shed AMA1 from the surface of invasive zoites, and demonstrates the presence at the malaria merozoite surface of a rhomboid-like protease.

Unique insertions within Plasmodium falciparum subtilisin-like protease-1 are crucial for enzyme maturation and activity

Parasite serine proteases play essential roles in the asexual erythrocytic life cycle of the malaria parasite. The timing and location of expression of Plasmodium falciparum subtilisin-like protease-1 (PfSUB-1) are consistent with a role in erythrocyte invasion. Maturation of PfSUB-1 involves two autocatalytic processing events in which an 82 kDa precursor is converted to a 54 kDa form, followed by further cleavage to produce a 47 kDa form. Here we have compared PfSUB-1 with a number of Plasmodium orthologues and the most closely related bacterial subtilase sequences and find that, like many malarial proteins, PfSUB-1 possesses both low and high complexity insertions. The latter take the form of six surface-associated strands or loops which are conserved in all SUB-1 orthologues but not present in any other subtilase. Several mutants of PfSUB-1 with deletions of all, or part, of each of the six loop insertions were produced in an insect cell expression system. Aside from loop III, which was dispensable, individual deletion of the loop insertions revealed a role in protein maturation and/or stability. Specific substitutions within loop II inhibited maturation and enzyme activity. Mutations in loops V and VI specifically inhibited the second step of autocatalytic maturation providing evidence that the two processing steps have distinct structural requirements and that conversion to p47 is not a prerequisite for proteolytic activity in trans.

Evaluation of three Pichia pastoris-expressed Plasmodium falciparum merozoite proteins as a combination vaccine against infection with blood-stage parasites

Because invasion of erythrocytes by Plasmodium falciparum merozoites involves multiple receptor-ligand interactions, it may be necessary to develop a multivalent malaria vaccine that is comprised of distinct parasite ligands. PfAMA-1, PfMSP1, and PfEBA-175 are merozoite proteins that play important roles in invasion. We have constructed a PfCP-2.9 chimeric protein consisting of PfAMA-1 and PfMSP1 and tested it for immunogenicity in animal models and humans. The F2 subdomain of PfEBA-175 (PfEBA-175II F2) was identified as the binding domain for glycophorin A on erythrocytes. In this study, we used the codon frequencies of the yeast Pichia pastoris to redesign and synthesize a gene encoding the F2 domain. We found that the codon-optimized gene was expressed at a high level in P. pastoris as a soluble protein with a yield of about 300 mg/liter. The expressed protein was able to bind normal erythrocytes but not those treated with neuraminidase or trypsin. Moreover, the protein was recognized by the sera of malaria patients and was highly immunogenic in mice, rabbits, and rhesus monkeys. Immunoglobulin G isolated from both immunized rabbits and monkeys inhibited in vitro parasite growth. Immunization of animals with a combination of PfEBA-175II F2 and PfCP-2.9 did not result in antigen (Ag) competition in animals. Moreover, antibodies to both PfEBA-175II F2 and PfCP-2.9, isolated from rabbits immunized with both constructs, inhibited parasite growth in vitro. The combination of high yield, functional folding, antibody inhibition, and lack of Ag competition provides support for inclusion of these merozoite proteins in a combination vaccine against infection with blood-stage parasites.

Location, Location, Location: Trafficking and Function of Secreted Proteases of Toxoplasma and Plasmodium

The Apicomplexan parasites Toxoplasma gondii and Plasmodium species are obligate intracellular parasites that rely upon unique secretory organelles for invasion and other specialized functions. Data is emerging that proteases are critical for the biogenesis of micronemes and rhoptries, regulated secretory organelles reminiscent of dense core granules and secretory lysosomes of higher eukaryotes. Proteases targeted to the Plasmodium food vacuole, a unique organelle dedicated to hemoglobin degradation, are also critical to parasite survival. Thus study of the targeting and function of the proteases of the Apicomplexa provides a fascinating model system to understand regulated secretion and secretory organelle biogenesis.