A conserved molecular motor drives cell invasion and gliding motility across malaria life cycle stages and other apicomplexan parasites

Secretion of an acid phosphatase provides a possible mechanism to acquire host nutrients by Plasmodium falciparum

As an intracellular proliferating parasite, Plasmodium falciparum exploits the human host to acquire nutrients. However, nutrients such as nucleotides and cofactors are mostly phosphorylated in the host cell cytosol and thus have to be dephosphorylated in order to be taken up by the parasite. Here we report the functional characterization of a unique secreted phosphatase in P. falciparum, which is expressed throughout the developmental stages in the red blood cell. We show that this enzyme, formerly described as anchoring glideosome-associated protein 50 (GAP50), reveals a broad substrate profile with preference for di- and triphosphates at pH 5-7. Bioinformatic studies of the protein sequence identified an N-terminal signal anchor (SA) as well as a C-terminal transmembrane domain. By means of live microscopy of parasites transfected with GFP-fusions of this secreted acid phosphatase (PfSAP), we demonstrate that PfSAP enters the secretory pathway en route to the parasite periphery - mediated by SA - and is subsequently engulfed into the food vacuole. We corroborate this with independent data where acid phosphatase activity is visualized in close proximity to hemozoin. The biochemical as well as the trafficking results support the proposed role of PfSAP in the acquisition of host nutrients by dephosphorylation.

Cryptosporidium: genomic and biochemical features

Recent progress in understanding the unique biochemistry of the two closely related human enteric pathogens Cryptosporidium parvum and Cryptosporidium hominis has been stimulated by the elucidation of the complete genome sequences for both pathogens. Much of the work that has occurred since that time has been focused on understanding the metabolic pathways encoded by the genome in hopes of providing increased understanding of the parasite biology, and in the identification of novel targets for pharmacological interventions. However, despite identifying the genes encoding enzymes that participate in many of the major metabolic pathways, only a hand full of proteins have actually been the subjects of detailed scrutiny. Thus, much of the biochemistry of these parasites remains a true mystery.

Transcriptional profiling of growth perturbations of the human malaria parasite Plasmodium falciparum

Functions have yet to be defined for the majority of genes of Plasmodium falciparum, the agent responsible for the most serious form of human malaria. Here we report changes in P. falciparum gene expression induced by 20 compounds that inhibit growth of the schizont stage of the intraerythrocytic development cycle. In contrast with previous studies, which reported only minimal changes in response to chemically induced perturbations of P. falciparum growth, we find that approximately 59% of its coding genes display over three-fold changes in expression in response to at least one of the chemicals we tested. We use this compendium for guilt-by-association prediction of protein function using an interaction network constructed from gene co-expression, sequence homology, domain-domain and yeast two-hybrid data. The subcellular localizations of 31 of 42 proteins linked with merozoite invasion is consistent with their role in this process, a key target for malaria control. Our network may facilitate identification of novel antimalarial drugs and vaccines.

Effects of calcium signaling on Plasmodium falciparum erythrocyte invasion and post-translational modification of gliding-associated protein 45 (PfGAP45)

Plasmodium falciparum erythrocyte invasion is powered by an actin/myosin motor complex that is linked both to the tight junction and to the merozoite cytoskeleton through the Inner Membrane Complex (IMC). The IMC association of the myosin motor, PfMyoA, is maintained by its association with three proteins: PfMTIP, a myosin light chain, PfGAP45, an IMC peripheral membrane protein, and PfGAP50, an integral membrane protein of the IMC. This protein complex is referred to as the glideosome, and given its central role in erythrocyte invasion, this complex is likely the target of several specific regulatory effectors that ensure it is properly localized, assembled, and activated as the merozoite prepares to invade its target cell. However, little is known about how erythrocyte invasion as a whole is regulated, or about how or whether that regulation impacts the glideosome. Here we show that P. falciparum erythrocyte invasion is regulated by the release of intracellular calcium via the cyclic-ADP Ribose (cADPR) pathway, but that inhibition of cADPR-mediated calcium release does not affect PfGAP45 phosphorylation or glideosome association. By contrast, the serine/threonine kinase inhibitor, staurosporine, affects both PfGAP45 isoform distribution and the integrity of the glideosome complex. This data identifies specific regulatory elements involved in controlling P. falciparum erythrocyte invasion and reveals that the assembly status of the merozoite glideosome, which is central to erythrocyte invasion, is surprisingly dynamic.

Novel putative glycosylphosphatidylinositol-anchored micronemal antigen of Plasmodium falciparum that binds to erythrocytes

We have identified a new Plasmodium falciparum erythrocyte binding protein that appears to be located in the micronemes of the merozoite stage of the parasite and membrane linked through a glycosylphosphatidylinositol (GPI) anchor. The protein is designated GPI-anchored micronemal antigen (GAMA) and was identified by applying a set of selection criteria to identify previously uncharacterized merozoite proteins that may have a role in cell invasion. The protein is also present in the proteomes of the sporozoite and ookinete micronemes and is conserved throughout the genus. GAMA contains a novel domain that may be constrained by disulfide bonds and a predicted C-terminal hydrophobic sequence that is presumably replaced by the GPI. The protein is synthesized late during schizogony, processed into two fragments that are linked by a disulfide bond, and translocated to an apical location, which is probably the micronemes. In a proportion of free merozoites GAMA can also be detected on the parasite surface. Following erythrocyte invasion the bulk of the protein is shed in a soluble form, although a short C-terminal fragment may be carried into the newly invaded red blood cell. The protein was shown to bind reversibly to erythrocytes and therefore represents a new example of a host cell binding protein.

A cyclic GMP signalling module that regulates gliding motility in a malaria parasite

The ookinete is a motile stage in the malaria life cycle which forms in the mosquito blood meal from the zygote. Ookinetes use an acto-myosin motor to glide towards and penetrate the midgut wall to establish infection in the vector. The regulation of gliding motility is poorly understood. Through genetic interaction studies we here describe a signalling module that identifies guanosine 3′, 5′-cyclic monophosphate (cGMP) as an important second messenger regulating ookinete differentiation and motility. In ookinetes lacking the cyclic nucleotide degrading phosphodiesterase δ (PDEδ), unregulated signalling through cGMP results in rounding up of the normally banana-shaped cells. This phenotype is suppressed in a double mutant additionally lacking guanylyl cyclase β (GCβ), showing that in ookinetes GCβ is an important source for cGMP, and that PDEδ is the relevant cGMP degrading enzyme. Inhibition of the cGMP-dependent protein kinase, PKG, blocks gliding, whereas enhanced signalling through cGMP restores normal gliding speed in a mutant lacking calcium dependent protein kinase 3, suggesting at least a partial overlap between calcium and cGMP dependent pathways. These data demonstrate an important function for signalling through cGMP, and most likely PKG, in dynamically regulating ookinete gliding during the transmission of malaria to the mosquito.

Malaria parasites are single celled organisms, which must alternate between vertebrate and mosquito hosts to survive and spread. In both hosts, certain parasite stages can glide through tissues and invade cells. Many components of the molecular motor that powers gliding and invasion are known and we have a good idea how these may interact to generate force. It is less well understood how the motor is assembled and how its component parts are regulated to switch it on and off. We have begun to address these questions in the ookinete, a parasite stage, which forms in the blood meal of a mosquito and relies on gliding to penetrate the gut wall. Using a malaria parasite of rodents, we have examined the effect of deleting candidate genes involved in controlling levels of the intracellular signalling molecule cyclic guanosine monophosphate (cGMP). We show that the right balance between cGMP production and degradation is important for ookinetes to glide, while also maintaining their typical cell shape. Overall levels of cGMP are not much affected in the mutants, though, and we therefore believe the messenger exerts its effect either locally within the cell or only while the parasite is gliding.

A novel family of Apicomplexan glideosome-associated proteins with an inner membrane-anchoring role

The phylum Apicomplexa are a group of obligate intracellular parasites responsible for a wide range of important diseases. Central to the lifecycle of these unicellular parasites is their ability to migrate through animal tissue and invade target host cells. Apicomplexan movement is generated by a unique system of gliding motility in which substrate adhesins and invasion-related proteins are pulled across the plasma membrane by an underlying actin-myosin motor. The myosins of this motor are inserted into a dual membrane layer called the inner membrane complex (IMC) that is sandwiched between the plasma membrane and an underlying cytoskeletal basket. Central to our understanding of gliding motility is the characterization of proteins residing within the IMC, but to date only a few proteins are known. We report here a novel family of six-pass transmembrane proteins, termed the GAPM family, which are highly conserved and specific to Apicomplexa. In Plasmodium falciparum and Toxoplasma gondii the GAPMs localize to the IMC where they form highly SDS-resistant oligomeric complexes. The GAPMs co-purify with the cytoskeletal alveolin proteins and also to some degree with the actin-myosin motor itself. Hence, these proteins are strong candidates for an IMC-anchoring role, either directly or indirectly tethering the motor to the cytoskeleton.

Do apicomplexan parasite-encoded proteins act as both ligands and receptors during host cell invasion?

Apicomplexan parasites are responsible for a wide range of diseases in animals, including humans, in whom Plasmodium species cause the devastating disease malaria. Several recent discoveries now indicate that these intracellular parasites may use a conserved mechanism to infect their host cells by using parasite-encoded proteins as both parasite ligands and receptors anchored to the host cells.

Molecular analyses of Toxoplasma gondii calmodulin-like domain protein kinase isoform 3

Ca(2+) signaling is thought to play an important role in Toxoplasma gondii motility, including invasion of and egress from host cells. Recently, it has been reported that phosphorylation of the glideosome apparatus components of T. gondii occurs during invasion. To elucidate the role of T. gondii calmodulin-like domain protein kinase in the signaling pathway that bridges Ca(2+) stimulation and motility, we characterized T. gondii calmodulin-like domain protein kinase isoform 3 (TgCDPKif3). TgCDPKif3 is homologous to Plasmodium falciparum calcium-dependent protein kinase 1, which has been reported to phosphorylate P. falciparum glideosome components. TgCDPKif3 was purified as a fusion protein that was labeled with [gamma-(32)P]ATP, and the label was subsequently removed by phosphatase treatment. Phosphorylation was eliminated when the putative catalytic lysine residue of TgCDPKif3 was replaced with alanine. TgCDPKif3 phosphorylated Histone II(AS) as a representative substrate in a Ca(2+)-dependent manner at a high Ca(2+) concentration. TgCDPKif3 was localized to the apical ends of tachyzoites. TgCDPKif3 showed the translocation between intra- and extracellular tachyzoites. TgCDPKif3 could phosphorylate T. gondii aldolase 1 (TgALD1) in vitro. The interaction between TgCDPKif3 and TgALD1 was confirmed by the co-immunoprecipitation assay in mammal cells. We suggested that TgCDPKif3 could participate in the motility of T. gondii through the phosphorylation of glideosome complex member.

The malaria merozoite, forty years on

The invasive blood stage of malaria parasites, merozoites, are complex entities specialized for the capture and entry of red blood cells. Their potential for vaccination and other anti-malaria strategies have attracted much research attention over the last 40 years, and there is now a considerable body of data relating to their biology. In this article some of the major advances over this period and remaining challenges are reviewed.

Plasmodium falciparum enolase: stage-specific expression and sub-cellular localization

Background

In an earlier study, it was observed that the vaccination with Plasmodium falciparum enolase can confer partial protection against malaria in mice. Evidence has also build up to indicate that enolases may perform several non-glycolytic functions in pathogens. Investigating the stage-specific expression and sub-cellular localization of a protein may provide insights into its moonlighting functions.

Methods

Sub-cellular localization of P. falciparum enolase was examined using immunofluorescence assay, immuno-gold electron microscopy and western blotting.

Results

Enolase protein was detected at every stage in parasite life cycle examined. In asexual stages, enolase was predominantly (≥85–90%) present in soluble fraction, while in sexual stages it was mostly associated with particulate fraction. Apart from cytosol, enolase was found to be associated with nucleus, food vacuole, cytoskeleton and plasma membrane.

Conclusion

Diverse localization of enolase suggests that apart from catalyzing the conversion of 2-phosphoglycericacid into phosphoenolpyruvate in glycolysis, enolase may be involved in a host of other biological functions. For instance, enolase localized on the merozoite surface may be involved in red blood cell invasion; vacuolar enolase may be involved in food vacuole formation and/or development; nuclear enolase may play a role in transcription.

New innovations for an old infection: antimalarial lead discovery from marine natural products during the period 2003–2008

Malaria remains one of the most serious global infectious diseases, with an estimated 2 billion people at risk and 1 million deaths annually. Drug resistance is hampering the effectiveness of many current antimalarial therapies and resistant strains of the parasite are now known for almost all classes of antimalarial compounds. Owing to a lack of concerted drug-discovery efforts over the last 30 years, the development pipeline is limited and the identification of new antimalarial lead compounds is a pressing concern. The development of new antimalarials that exhibit novel modes of action is of critical importance if the devastating effects of malaria are to be controlled. Natural products have traditionally played an important role in antimalarial drug development and the marine environment represents an underexplored resource in this regard. This review covers developments in the field of antimalarial drug discovery from marine sources between January 2003 and December 2008 and offers a comprehensive overview of all marine-derived compounds from this period. Marine natural products represent an emerging opportunity in the development of new antimalarial lead compounds. This review provides examples of several recent lead discovery projects that show promise in this regard and presents a perspective on areas of possible future study.

Molecular genetics and comparative genomics reveal RNAi is not functional in malaria parasites

Techniques for targeted genetic disruption in Plasmodium, the causative agent of malaria, are currently intractable for those genes that are essential for blood stage development. The ability to use RNA interference (RNAi) to silence gene expression would provide a powerful means to gain valuable insight into the pathogenic blood stages but its functionality in Plasmodium remains controversial. Here we have used various RNA-based gene silencing approaches to test the utility of RNAi in malaria parasites and have undertaken an extensive comparative genomics search using profile hidden Markov models to clarify whether RNAi machinery exists in malaria. These investigative approaches revealed that Plasmodium lacks the enzymology required for RNAi-based ablation of gene expression and indeed no experimental evidence for RNAi was observed. In its absence, the most likely explanations for previously reported RNAi-mediated knockdown are either the general toxicity of introduced RNA (with global down-regulation of gene expression) or a specific antisense effect mechanistically distinct from RNAi, which will need systematic analysis if it is to be of use as a molecular genetic tool for malaria parasites.

Migration and motility of spermatozoa in the female reproductive tract of the soft tick Ornithodoros moubata (Acari, Argasidae)

The spermatozoa of ticks are anomalous in many respects: they are very large, cytoplasm-rich cells which lack a flagellum but move with a peculiar gliding motility. Their metamorphosis after deposition in the female has been well documented, but many of the subsequent events in the career of the spermatozoa are controversial or poorly documented. Our observations of motility imply that the many types of motility that have been reported (up to 5 different types in several reports) can be reduced to 2 apparently independent types of active motility: (1) gliding motility generated along the whole spermatozoon and (2) contortions of the anterior tip of the head. These types of motility appear as a consequence of sperm maturation after transfer to the female, but only become pronounced if the female has taken a recent bloodmeal. A consequence of this enhanced gliding motility after feeding is the movement of the spermatozoa out of the naturally ruptured neck of the spermatophore and up the female genital tract. This occurs without any apparent assistance from the female's musculature and likely is the prime mechanism of movement of the spermatozoa to the site of fertilization.

Distinct roles of Plasmodium rhomboid 1 in parasite development and malaria pathogenesis

Invasion of host cells by the malaria parasite involves recognition and interaction with cell-surface receptors. A wide variety of parasite surface proteins participate in this process, most of which are specific to the parasite's particular invasive form. Upon entry, the parasite has to dissociate itself from the host-cell receptors. One mechanism by which it does so is by shedding its surface ligands using specific enzymes. Rhomboid belongs to a family of serine proteases that cleave cell-surface proteins within their transmembrane domains. Here we identify and partially characterize a Plasmodium berghei rhomboid protease (PbROM1) that plays distinct roles during parasite development. PbROM1 localizes to the surface of sporozoites after salivary gland invasion. In blood stage merozoites, PbROM1 localizes to the apical end where proteins involved in invasion are also present. Our genetic analysis suggests that PbROM1 functions in the invasive stages of parasite development. Whereas wild-type P. berghei is lethal to mice, animals infected with PbROM1 null mutants clear the parasites efficiently and develop long-lasting protective immunity. The results indicate that P. berghei Rhomboid 1 plays a nonessential but important role during parasite development and identify rhomboid proteases as potential targets for disease control.

Malaria is one of the major infectious diseases and is responsible for the death of more than a million people, mostly children under the age of five. Plasmodium, the causative agent of malaria, is transmitted by female Anopheles mosquitoes. Successful development of the parasite requires efficient recognition, attachment, and invasion of host cells. Several parasite cell-surface molecules have been implicated in these processes and may require proteolytic processing in order for the parasite to complete invasion. Rhomboid family proteins are serine proteases that cleave within the transmembrane region of their substrates. Here, we use a genetic approach to study the function of Plasmodium berghei rhomboid 1 (PbROM1). PbROM1 is expressed in both vertebrate and mosquito stages of parasite development, and the protein is present in secretory organelles that contain other parasite molecules required for invasion. We find that PbROM1 is required for efficient infection of both the mosquito and the vertebrate host. Interestingly, we also find that mice infected with ROM1(−) parasites clear the infection efficiently and are protected upon subsequent wild-type parasite challenge. Our study suggests a role for PbROM1 throughout parasite development and identifies ROM1 as a target for disease intervention.

Formin' an invasion machine: actin polymerization in invading apicomplexans

Apicomplexan parasites are motile and invade host cells. The force required for this is generated by an actomyosin motor. In a recent paper, Baum and colleagues suggest that the protein formin regulates the polymerization of actin at the moving junction between parasite and host cell. This finding provides novel insight into the mechanism of host cell invasion.

No TRAP, no invasion

Host-cell invasion by apicomplexan parasites is a unique process that is powered by the gliding motility motor and requires a transmembrane link between the parasite cytoskeleton and the host cell. The thrombospondin-related anonymous protein (TRAP) from Plasmodium plays such a part during sporozoite invasion by linking to actin through its cytoplasmic tail while binding to hepatocytes via its extracellular portion. In recent years, there have been major advances in the identification and characterization of TRAP-family proteins in the other invasive stages of Plasmodium as well as other Apicomplexa. This review summarizes the recent experimental data on these TRAP-family proteins, focusing on their structure and function.

Actin/myosin-based gliding motility in apicomplexan parasites

Apicomplexan parasites move and actively enter host cells by substrate-dependent gliding motility, an unusual form of eukaryotic locomotion that differs fundamentally from the motility of prokaryotic and viral pathogens. Recent research has uncovered some of the cellular and molecular mechanisms underlying parasite motility, transmigration, and cell invasion during life cycle progression. The gliding motor machinery is embedded between the plasma membrane and the inner membrane complex, a unique double membrane layer. It consists ofimmobilized unconventional myosins, short actin stubs, and TRAP-family invasins. Assembly of this motor machinery enables force generation between parasite cytoskeletal components and an extracellular substratum. Unique properties of the individual components suggest that the rational design of motility inhibitors may lead to new intervention strategies to combat some of the most devastating human and livestock diseases.

Role of the Plasmodium export element in trafficking parasite proteins to the infected erythrocyte

The intracellular survival of Plasmodium falciparum within human erythrocytes is dependent on export of parasite proteins that remodel the host cell. Most exported proteins require a conserved motif (RxLxE/Q/D), termed the Plasmodium export element (PEXEL) or vacuolar targeting sequence (VTS), for targeting beyond the parasitophorous vacuole membrane and into the host cell; however, the precise role of this motif in export is poorly defined. We used transgenic P. falciparum expressing chimeric proteins to investigate the function of the PEXEL motif for export. The PEXEL constitutes a bifunctional export motif comprising a protease recognition sequence that is cleaved, in the endoplasmic reticulum, from proteins destined for export, in a PEXEL arginine- and leucine-dependent manner. Following processing, the remaining conserved PEXEL residue is required to direct the mature protein to the host cell. Furthermore, we demonstrate that N acetylation of proteins following N-terminal processing is a PEXEL-independent process that is insufficient for correct export to the host cell. This work defines the role of each residue in the PEXEL for export into the P. falciparum-infected erythrocyte.

The motor complex of Plasmodium falciparum: phosphorylation by a calcium-dependent protein kinase

Calcium-dependent protein kinases (CDPKs) of Apicomplexan parasites are crucial for the survival of the parasite throughout its life cycle. CDPK1 is expressed in the asexual blood stages of the parasite, particularly late stage schizonts. We have identified two substrates of Plasmodium falciparum CDPK1: myosin A tail domain-interacting protein (MTIP) and glideosome-associated protein 45 (GAP45), both of which are components of the motor complex that generates the force required by the parasite to actively invade host cells. Indirect immunofluorescence shows that CDPK1 localizes to the periphery of P. falciparum merozoites and is therefore suitably located to act on MTIP and GAP45 at the inner membrane complex. A proportion of both GAP45 and MTIP is phosphorylated in schizonts, and we demonstrate that both proteins can be efficiently phosphorylated by CDPK1 in vitro. A primary phosphorylation of MTIP occurs at serine 47, whereas GAP45 is phosphorylated at two sites, one of which could also be detected in phosphopeptides purified from parasite lysates. Both CDPK1 activity and host cell invasion can be inhibited by the kinase inhibitor K252a, suggesting that CDPK1 is a suitable target for antimalarial drug development.

Apicomplexan cytoskeleton and motors: key regulators in morphogenesis, cell division, transport and motility

Protozoan parasites of the phylum Apicomplexa undergo a lytic cycle whereby a single zoite produced by the previous cycle has to encounter a host cell, invade it, multiply to differentiate into a new zoite generation and escape to resume a new cycle. At every step of this lytic cycle, the cytoskeleton and/or the gliding motility apparatus play a crucial role and recent results have elucidated aspects of these processes, especially in terms of the molecular characterization and interaction of the increasing number of partners involved, and the signalling mechanisms implicated. The present review aims to summarize the most recent findings in the field.

In silico identification of specialized secretory-organelle proteins in apicomplexan parasites and in vivo validation in Toxoplasma gondii

Apicomplexan parasites, including the human pathogens Toxoplasma gondii and Plasmodium falciparum, employ specialized secretory organelles (micronemes, rhoptries, dense granules) to invade and survive within host cells. Because molecules secreted from these organelles function at the host/parasite interface, their identification is important for understanding invasion mechanisms, and central to the development of therapeutic strategies. Using a computational approach based on predicted functional domains, we have identified more than 600 candidate secretory organelle proteins in twelve apicomplexan parasites. Expression in transgenic T. gondii of eight proteins identified in silico confirms that all enter into the secretory pathway, and seven target to apical organelles associated with invasion. An in silico approach intended to identify possible host interacting proteins yields a dataset enriched in secretory/transmembrane proteins, including most of the antigens known to be engaged by apicomplexan parasites during infection. These domain pattern and projected interactome approaches significantly expand the repertoire of proteins that may be involved in host parasite interactions.

Reticulocyte-binding protein homologue 5 - an essential adhesin involved in invasion of human erythrocytes by Plasmodium falciparum

Invasion of erythrocytes is a prerequisite in the life history of the malaria parasite. Members of the reticulocyte-binding homologue family (PfRh) have been implicated in the invasion process and in some cases have been shown to act as adhesins, binding to specific receptors on the erythrocyte surface. We have identified a further, putatively essential, PfRh family member in the most virulent human malaria Plasmodium falciparum, called PfRh5, which binds to an unknown class of glycosylated receptors on the erythrocyte surface. This protein is an atypical PfRh family member, being much smaller than others and lacking a transmembrane and cytosolic region at the C-terminus. This suggests it may be part of a functional protein complex. PfRh5 localises to the rhoptries in merozoites and follows the tight junction during the process of erythrocyte invasion. Furthermore, rabbit immune serum raised against a portion of the ecto-domain, inhibits parasite invasion in vitro. We hypothesise an essential role for the PfRh5 adhesin in erythrocyte selection and commitment to invasion. Given its small size, we believe PfRh5 may prove to be a valuable candidate for inclusion in a multi-component anti-malarial vaccine.

TREP, a novel protein necessary for gliding motility of the malaria sporozoite

The invasive stages of parasites of the protozoan phylum Apicomplexa have the capacity to traverse host tissues and invade host cells using a unique type of locomotion called gliding motility. Gliding motility is powered by a sub-membranous actin-myosin motor, and the force generated by the motor is transduced to the parasite surface by transmembrane proteins of the apicomplexan-specific thrombospondin-related anonymous protein (TRAP) family. These proteins possess short cytoplasmic tails that interact with the actin-myosin motor via the glycolytic enzyme aldolase. Gliding motility of the Plasmodium sporozoite, the stage of the malaria parasite that is transmitted by the mosquito to the mammalian host, depends on the TRAP protein. We describe a second protein, herein termed TREP, which also plays a role in the gliding motility of the Plasmodium sporozoite. TREP is a transmembrane protein that possesses a short cytoplasmic tail typical of members of the TRAP family of proteins, as well as a large extracellular region that contains a single thrombospondin type 1 repeat domain. TREP transcripts are expressed predominantly in oocyst stage sporozoites. Plasmodium berghei sporozoites harbouring a disrupted TREP gene have a highly diminished capacity to invade mosquito salivary glands and display a severe defect in gliding motility. We conclude that the gliding motility of the Plasmodium sporozoite in the mosquito depends on at least two proteins, TRAP and TREP.

Host-cell invasion by malaria parasites: insights from Plasmodium and Toxoplasma

Recent years have seen tremendous progress in our understanding of malaria parasite molecular biology. To a large extent, this progress follows significant developments in genetic, molecular and chemical tools available to study the malaria parasites and related Apicomplexa, in particular Toxoplasma gondii. One area of major advancement has been in understanding parasite host-cell invasion, a process that utilizes several essential molecular mechanisms that are conserved across the different lifecycle stages. Here, we summarize some of the most recent experimental data that shed light on the events underlying preparation and execution of malaria parasite invasion and how these insights might relate to the development of new antimalarial drugs.

Switching Plasmodium falciparum genes on and off for erythrocyte invasion

Culture-adapted lines of the malaria parasite Plasmodium falciparum use alternative pathways for the invasion of erythrocytes. The expression of parasite ligands that are involved in the different pathways varies among parasite lines. Recently, several studies have attempted to characterize the use of different invasion pathways and the expression of specific invasion ligands in field isolates, opening the way to understand how invasion occurs in natural infections. In this review, these findings are discussed in the context of the most recent data on invasion by culture-adapted parasites to describe the current understanding of how wild parasites invade, how the variant expression of invasion ligands relates to switching between alternative invasion pathways and why so many different pathways are needed.

TRAP-like protein of Plasmodium sporozoites: linking gliding motility to host-cell traversal

To reach its final destination in the liver, the sporozoite (the stage of the malaria parasite that is transmitted by the mosquito vector) needs to glide through tissues and traverse host cells. Although the molecular bases of these behaviors are typically considered separately, two recent reports suggest the first molecular link between the two via a novel protein called 'TRAP-like protein'.

Genetic characterization of Hawaiian isolates of Plasmodium relictum reveals mixed-genotype infections

Background

The relatively recent introduction of a highly efficient mosquito vector and an avian pathogen (Plasmodium relictum) to an isolated island ecosystem with naïve, highly susceptible avian hosts provides a unique opportunity to investigate evolution of virulence in a natural system. Mixed infections can significantly contribute to the uncertainty in host-pathogen dynamics with direct impacts on virulence. Toward further understanding of how host-parasite and parasite-parasite relationships may impact virulence, this study characterizes within-host diversity of malaria parasite populations based on genetic analysis of the trap (thrombospondin-related anonymous protein) gene in isolates originating from Hawaii, Maui and Kauai Islands.

Methods

A total of 397 clones were produced by nested PCR amplification and cloning of a 1664 bp fragment of the trap gene from two malarial isolates, K1 (Kauai) and KV115 (Hawaii) that have been used for experimental studies, and from additional isolates from wild birds on Kauai, Maui and Hawaii Islands. Diversity of clones was evaluated initially by RFLP-based screening, followed by complete sequencing of 33 selected clones.

Results

RFLP analysis of trap revealed a minimum of 28 distinct RFLP haplotypes among the 397 clones from 18 birds. Multiple trap haplotypes were detected in every bird evaluated, with an average of 5.9 haplotypes per bird. Overall diversity did not differ between the experimental isolates, however, a greater number of unique haplotypes were detected in K1 than in KV115. We detected high levels of clonal diversity with clear delineation between isolates K1 and KV115 in a haplotype network. The patterns of within-host haplotype clustering are consistent with the possibility of a clonal genetic structure and rapid within-host mutation after infection.

Conclusion

Avian malaria (P. relictum) and Avipoxvirus are the significant infectious diseases currently affecting the native Hawaiian avifauna. This study shows that clonal diversity of Hawaiian isolates of P. relictum is much higher than previously recognized. Mixed infections can significantly contribute to the uncertainty in host-pathogen dynamics with direct implications for host demographics, disease management strategies, and evolution of virulence. The results of this study indicate a widespread presence of multiple-genotype malaria infections with high clonal diversity in native birds of Hawaii, which when coupled with concurrent infection with Avipoxvirus, may significantly influence evolution of virulence.

Reviewers

This article was reviewed by Joseph Schall (nominated by Laura Landweber), Daniel Jeffares (nominated by Anthony Poole) and Susan Perkins (nominated by Eugene Koonin).

A new model for hemoglobin ingestion and transport by the human malaria parasite Plasmodium falciparum

The current model for hemoglobin ingestion and transport by intraerythrocytic Plasmodium falciparum malaria parasites shares similarities with endocytosis. However, the model is largely hypothetical, and the mechanisms responsible for the ingestion and transport of host cell hemoglobin to the lysosome-like food vacuole (FV) of the parasite are poorly understood. Because actin dynamics play key roles in vesicle formation and transport in endocytosis, we used the actin-perturbing agents jasplakinolide and cytochalasin D to investigate the role of parasite actin in hemoglobin ingestion and transport to the FV. In addition, we tested the current hemoglobin trafficking model through extensive analysis of serial thin sections of parasitized erythrocytes (PE) by electron microscopy. We find that actin dynamics play multiple, important roles in the hemoglobin transport pathway, and that hemoglobin delivery to the FV via the cytostomes might be required for parasite survival. Evidence is provided for a new model, in which hemoglobin transport to the FV occurs by a vesicle-independent process.

Gene expression signatures and small-molecule compounds link a protein kinase to Plasmodium falciparum motility

Calcium-dependent protein kinases play a crucial role in intracellular calcium signaling in plants, some algae and protozoa. In Plasmodium falciparum, calcium-dependent protein kinase 1 (PfCDPK1) is expressed during schizogony in the erythrocytic stage as well as in the sporozoite stage. It is coexpressed with genes that encode the parasite motor complex, a cellular component required for parasite invasion of host cells, parasite motility and potentially cytokinesis. A targeted gene-disruption approach demonstrated that pfcdpk1 seems to be essential for parasite viability. An in vitro biochemical screen using recombinant PfCDPK1 against a library of 20,000 compounds resulted in the identification of a series of structurally related 2,6,9-trisubstituted purines. Compound treatment caused sudden developmental arrest at the late schizont stage in P. falciparum and a large reduction in intracellular parasites in Toxoplasma gondii, which suggests a possible role for PfCDPK1 in regulation of parasite motility during egress and invasion.

Functional characterization of a redundant Plasmodium TRAP family invasin, TRAP-like protein, by aldolase binding and a genetic complementation test

Efficient and specific host cell entry is of exquisite importance for intracellular pathogens. Parasites of the phylum Apicomplexa are highly motile and actively enter host cells. These functions are mediated by type I transmembrane invasins of the TRAP family that link an extracellular recognition event to the parasite actin-myosin motor machinery. We systematically tested potential parasite invasins for binding to the actin bridging molecule aldolase and complementation of the vital cytoplasmic domain of the sporozoite invasin TRAP. We show that the ookinete invasin CTRP and a novel, structurally related protein, termed TRAP-like protein (TLP), are functional members of the TRAP family. Although TLP is expressed in invasive stages, targeted gene disruption revealed a nonvital role during life cycle progression. This is the first genetic analysis of TLP, encoding a redundant TRAP family invasin, in the malaria parasite.

What can transgenic parasites tell us about the development of Plasmodium-specific immune responses?

Malaria infects 500 million people and kills an estimated 2.7 million annually, representing one of the most significant diseases in the world. However, efforts to develop effective vaccines have met with limited success. One reason is our lack of basic knowledge of how and where the immune system responds to parasite antigens. This is important as the early events during induction of an immune response influence the acquisition of effector function and development of memory responses. Our knowledge of the interactions of Plasmodia with the host immune system has largely been derived through in vitro study. This is a significant issue as the component parts of the immune system do not work in isolation and their interactions occur in distinct and specialized micro- and macro-anatomical locations that can only be assessed in the physiological context, in vivo. In this context, the availability of transgenic malaria parasites over the last 10 years has greatly enhanced our ability to understand and evaluate factors involved in host-parasite interactions in vivo. In this article, we review the current status of this area and speculate on what parasite transgenesis approaches will tell us about the development of Plasmodium-specific immune responses in the future.

Plasmodium possesses dynein light chain classes that are unique and conserved across species

Plasmodium belongs to the phylum Apicomplexa. Within the Apicomplexa, Plasmodium, Toxoplasma and Cryptosporidium are parasites of considerable medical importance while Theileria and Eimeria are animal pathogens. P. falciparum is particularly important as it causes malaria, resulting in more than 1 million deaths each year. The malaria parasite actively invades the host cell in which it propagates and several proteins associated with the apical organelles have been implicated to be crucial in the invasion process. The biogenesis of the apical organelles is not well understood, but several studies indicate that microtubule-based vesicular transport is involved. Vesicular transport proteins are also present in Plasmodium and are presumed to be involved in transcellular transport in infected erythrocytes. Dynein is a multi-subunit motor protein involved in microtubule-based vesicular transport. In this study, we analyzed the cytoplasmic dynein light chains (Dlcs) of P. falciparum since they provide adaptor surface to the cargoes and are likely to be involved in differential transport. Dlcs consist of three different families: TcTex1/2, LC8 and LC7/roadblock. The data presented demonstrate that P. falciparum Dlcs sequences and functional domains show high sequence similarity within the species, but that only the Dlc group 1 (LC8) has a high similarity to human orthologues. TcTex1 and LC7/roadblock have low similarity to human orthologues. This sequence variation could be targeted for vaccine or drug development.

The Plasmodium TRAP/MIC2 family member, TRAP-Like Protein (TLP), is involved in tissue traversal by sporozoites

In the apicomplexan protozoans motility and cell invasion are mediated by the TRAP/MIC2 family of transmembrane proteins, members of which link extracellular adhesion to the intracellular actomyosin motor complex. Here we characterize a new member of the TRAP/MIC2 family, named TRAP-Like Protein (TLP), that is highly conserved within the Plasmodium genus. Similar to the Plasmodium sporozoite protein, TRAP, and the ookinete protein, CTRP, TLP possesses an extracellular domain architecture that is comprised of von Willebrand factor A (vWA) and thrombospondin type 1 (TSP1) domains, plus a short cytoplasmic domain. Comparison of the vWA domain of TLP genes from multiple Plasmodium falciparum isolates showed relative low sequence diversity, suggesting that the protein is not under selective pressures of the host immune system. Analysis of transcript levels by quantitative reverse transcription polymerase chain reaction (RT-PCR) showed that TLP is predominantly expressed in salivary gland sporozoites of P. falciparum and P. berghei. Targeted disruption of P. berghei TLP resulted in a decreased capacity for cell traversal by sporozoites, and reduced infectivity of sporozoites in vivo, whereas in vitro sporozoite motility and hepatocyte invasion were unaffected. These results indicate a role of TLP in cell traversal by sporozoites.

A Presenilin-like protease associated with Plasmodium falciparum micronemes is involved in erythrocyte invasion

We describe identification of a Plasmodium falciparum microneme protease involved in RBC invasion. From the yeast two-hybrid screening of a P. falciparum cDNA library, we have identified a 47 kDa membrane protein that interacted with the 5ABC domain of human RBC band 3. This protein shared homology with a Presenilin-type aspartyl protease, the signal peptide peptidase (SPP). An antibody raised against a predicted exposed region of this protein reacted specifically to a single band of approximately 47 kDa in the P. falciparum protein extract. Immunofluorescence microscopy suggested that this protein co-localized with the microneme protein EBA-175 in schizonts, and immunoelectron microscopy established that it is primarily localized to micronemes in merozoites. Functional characterization of Plasmodium falciparum signal peptide peptidase (PfSPP), demonstrates that an antibody to PfSPP blocks RBC invasion by P. falciparumin vitro. Native and recombinant PfSPP bound directly to the 5ABC domain of band 3 in solution and the binding of PfSPP to RBCs was chymotrypsin-sensitive, but trypsin and neuraminidase-resistant. Together, these results suggest that host band 3 interacts with PfSPP during RBC invasion presumably following parasite microneme discharge. PfSPP is the first microneme-associated intramembrane aspartyl protease identified in the apicomplexan parasites that interacts with a major transmembrane receptor on host erythrocytes.

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

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

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.

Role of Ca2+/calmodulin-PfPKB signaling pathway in erythrocyte invasion by Plasmodium falciparum

Molecular mechanisms by which signaling pathways operate in the malaria parasite and control its development are promiscuous. Recently, we reported the identification of a signaling pathway in Plasmodium falciparum, which involves activation of protein kinase B-like enzyme (PfPKB) by calcium/calmodulin (Vaid, A., and Sharma, P. (2006) J. Biol. Chem. 281, 27126-27133). Studies carried out to elucidate the function of this pathway suggested that it may be important for erythrocyte invasion. Blocking the function of the upstream activators of this pathway, calmodulin and phospholipase C, resulted in impaired invasion. To evaluate if this signaling cascade controls invasion by regulating PfPKB, inhibitors against this kinase were developed. PfPKB inhibitors dramatically reduced the ability of the parasite to invade erythrocytes. Furthermore, we demonstrate that PfPKB associates with actin-myosin motor and phosphorylates PfGAP45 (glideosome-associated protein 45), one of the important components of the motor complex, which may help explain its role in erythrocyte invasion.

Comparative genomics of protists: new insights into the evolution of eukaryotic signal transduction and gene regulation

Data from protist genomes suggest that eukaryotes show enormous variability in their gene complements, especially of genes coding regulatory proteins. Overall counts of eukaryotic signaling proteins show weak nonlinear scaling with proteome size, but individual superfamilies of signaling domains might show vast expansions in certain protists. Alteration of domain architectural complexity of signaling proteins and repeated lineage-specific reshaping of architectures might have played a major role in the emergence of new signaling interactions in different eukaryotes. Lateral transfer of various signaling domains from bacteria or from hosts, in parasites such as apicomplexans, appears to also have played a major role in the origin of new functional networks. Lineage-specific expansion of regulatory proteins, particularly of transcription factors, has played a critical role in the adaptive radiation of different protist lineages. Comparative genomics allows objective reconstruction of the ancestral conditions and subsequent diversification of several regulatory systems involved in phosphorylation, cyclic nucleotide signaling, Ubiquitin conjugation, chromatin remodeling, and posttranscriptional gene silencing.

High affinity interactions between red blood cell receptors and synthetic Plasmodium thrombospondin-related apical merozoite protein (PTRAMP) peptides

Plasmodium falciparum thrombospondin-related apical merozoite protein (PTRAMP) has a thrombospondin related (TSR) domain which in many proteins has been reported as a fragment involved in pathogen-host and cell-interactions. Receptor-ligand studies using eighteen non-overlapping 20-aminoacid-long synthetic peptides from this protein were carried out to determine regions involved in parasite invasion of red blood cells (RBC). Two high activity binding peptides (HABPs) were determined, 33405 (21YISSNDLTSTNLKVRNNWEH40) and 33413 (180LEGPIQFSLGKSSGAFRINY199), presenting high dissociation constants and positive cooperativity. One of the HABPs displayed a modified Plasmodium export element (PEXEL), suggesting that this protein could be involved in the merozoite cytoplasmic reticulum, parasitophorous vacuole, red blood cell (RBC) cytosol, and probably infected RBC (iRBC) membrane transport of some other molecules and nutrients. Enzymatic treatment of RBCs increased HABP 33405 binding to them whilst it decreased HABP 33413 binding. Merozoite invasion assays revealed that HABPs have around 57% ability to inhibit new RBC invasion. Circular dichroism revealed the presence of possible alpha-helical elements in both HABPs structures. RBC binding interaction specificity and the presence of a PEXEL motif make these 2 HABPs good candidates for being included in further studies to develop a new multi-antigenic, multi-stage, subunit-based, chemically-synthesised, anti-malarial vaccine.

Building the perfect parasite: cell division in apicomplexa

Apicomplexans are pathogens responsible for malaria, toxoplasmosis, and crytposporidiosis in humans, and a wide range of livestock diseases. These unicellular eukaryotes are stealthy invaders, sheltering from the immune response in the cells of their hosts, while at the same time tapping into these cells as source of nutrients. The complexity and beauty of the structures formed during their intracellular development have made apicomplexans the darling of electron microscopists. Dramatic technological progress over the last decade has transformed apicomplexans into respectable genetic model organisms. Extensive genomic resources are now available for many apicomplexan species. At the same time, parasite transfection has enabled researchers to test the function of specific genes through reverse and forward genetic approaches with increasing sophistication. Transfection also introduced the use of fluorescent reporters, opening the field to dynamic real time microscopic observation. Parasite cell biologists have used these tools to take a fresh look at a classic problem: how do apicomplexans build the perfect invasion machine, the zoite, and how is this process fine-tuned to fit the specific niche of each pathogen in this ancient and very diverse group? This work has unearthed a treasure trove of novel structures and mechanisms that are the focus of this review.

The solution structure of the adhesion protein Bd37 from Babesia divergens reveals structural homology with eukaryotic proteins involved in membrane trafficking

Babesia divergens is the Apicomplexa agent of the bovine babesiosis in Europe: this infection leads to growth and lactation decrease, so that economical losses due to this parasite are sufficient to require the development of a vaccine. The major surface antigen of B. divergens has been described as a 37 kDa protein glycosyl phosphatidyl inositol (GPI)-anchored at the surface of the merozoite. The immuno-prophylactic potential of Bd37 has been demonstrated, and we present here the high-resolution solution structure of the 27 kDa structured core of Bd37 (Delta-Bd37) using NMR spectroscopy. A model for the whole protein has been obtained using additional small angle X-ray scattering (SAXS) data. The knowledge of the 3D structure of Bd37 allowed the precise epitope mapping of antibodies on its surface. Interestingly, the geometry of Delta-Bd37 reveals an intriguing similarity with the exocyst subunit Exo84p C-terminal region, an eukaryotic protein that has a direct implication in vesicle trafficking. This strongly suggests that Apicomplexa have developed in parallel molecular machines similar in structure and function to the ones used for endo- and exocytosis in eukaryotic cells.

Illuminating Plasmodium falciparum-infected red blood cells

The malaria parasite undergoes a remarkable series of morphological transformations, which underpin its life in both human and mosquito hosts. The advent of molecular transfection technology coupled with the ability to introduce fluorescent reporter proteins that faithfully track and expose the activities of parasite proteins has revolutionized our view of parasite cell biology. The greatest insights have been realized in the erythrocyte stages of Plasmodium falciparum. P. falciparum invades and remodels the human erythrocyte: it feeds on haemoglobin, grows and divides, and subverts the physiology of its hapless host. Fluorescent proteins have been employed to track and dissect each of these processes and have revealed details and exposed new paradigms.

Immobilization of the type XIV myosin complex in Toxoplasma gondii

The substrate-dependent movement of apicomplexan parasites such as Toxoplasma gondii and Plasmodium sp. is driven by the interaction of a type XIV myosin with F-actin. A complex containing the myosin-A heavy chain, a myosin light chain, and the accessory protein GAP45 is attached to the membranes of the inner membrane complex (IMC) through its tight interaction with the integral membrane glycoprotein GAP50. For the interaction of this complex with F-actin to result in net parasite movement, it is necessary that the myosin be immobilized with respect to the parasite and the actin with respect to the substrate the parasite is moving on. We report here that the myosin motor complex of Toxoplasma is firmly immobilized in the plane of the IMC. This does not seem to be accomplished by direct interactions with cytoskeletal elements. Immobilization of the motor complex, however, does seem to require cholesterol. Both the motor complex and the cholesterol are found in detergent-resistant membrane domains that encompass a large fraction of the inner membrane complex surface. The observation that the myosin XIV motor complex of Toxoplasma is immobilized within this cholesterol-rich membrane likely extends to closely related pathogens such as Plasmodium and possibly to other eukaryotes.