Proteolysis and Toxoplasma invasion

Toxoplasma gondii infection triggers ongoing inflammation mediated by increased intracellular Cl- concentration in airway epithelium

Toxoplasma gondii is a widespread parasitic protozoan causing toxoplasmosis including pulmonary toxoplasmosis. As the first line of host defense, airway epithelial cells play critical roles in orchestrating pulmonary innate immunity. However, the mechanism underlying the airway inflammation induced by the T. gondii infection remains largely unclear. This study demonstrated that after infection with T. gondii, the major anion channel located in the apical membranes of airway epithelial cells, cystic fibrosis transmembrane conductance regulator (CFTR), was degraded by the parasite-secreted cysteine proteases. The intracellular Cl^- concentration ([Cl^-]_i) was consequently elevated, leading to activation of nuclear factor-κB (NF-κB) signaling via serum/glucocorticoid regulated kinase 1. Furthermore, the heightened [Cl^-]_i and activated NF-κB signaling could be sustained in a positive feedback regulatory manner resulting from decreased intracellular cAMP level through NF-κB-mediated up-regulation of phosphodiesterase 4. Conversely, the sulfur-containing compound allicin conferred anti-inflammatory effects on pulmonary toxoplasmosis by decreasing [Cl^-]_i via activation of CFTR. These results suggest that the intracellular Cl^- dynamically modulated by T. gondii mediates sustained airway inflammation, which provides a potential therapeutic target against pulmonary toxoplasmosis.

Absence of Toxoplasma gondii in 100% Iberian products from experimentally infected pigs cured following a specific traditional process

Infection with Toxoplasma gondii in humans has usually been related to the consumption of raw, undercooked or cured meat. Our study is based on the detection of T. gondii in cured legs and shoulders made from 100% Iberian sows fed mainly with acorn and raised as outdoor livestock in Aracena (Spain), which having been elaborated following a specific curing process (time period and location). An outdoor farm with a total of 636 animals was studied, showing a seroprevalence of 10% for the parasite T. gondii. Twenty individuals were chosen to be experimentally infected and slaughtered 60 days post-infection. Their legs and shoulders were processed to make 100% Iberian ham legs and shoulders. The meat ready to be eaten was analyzed by quantification and viability assays using magnetic capture real-time qPCR and bioassay techniques proving that this specific traditional "Cinco Jotas" curing process 100% Iberian ham is strong enough to eliminate the parasite T. gondii, resulting in a safe product for consumers.

Profiling of myristoylation in Toxoplasma gondii reveals an N-myristoylated protein important for host cell penetration

N-myristoylation is a ubiquitous class of protein lipidation across eukaryotes and N-myristoyl transferase (NMT) has been proposed as an attractive drug target in several pathogens. Myristoylation often primes for subsequent palmitoylation and stable membrane attachment, however, growing evidence suggests additional regulatory roles for myristoylation on proteins. Here we describe the myristoylated proteome of Toxoplasma gondii using chemoproteomic methods and show that a small-molecule NMT inhibitor developed against related Plasmodium spp. is also functional in Toxoplasma. We identify myristoylation on a transmembrane protein, the microneme protein 7 (MIC7), which enters the secretory pathway in an unconventional fashion with the myristoylated N-terminus facing the lumen of the micronemes. MIC7 and its myristoylation play a crucial role in the initial steps of invasion, likely during the interaction with and penetration of the host cell. Myristoylation of secreted eukaryotic proteins represents a substantial expansion of the functional repertoire of this co-translational modification.

A systematic review of Toxoplasma gondii antigens to find the best vaccine candidates for immunization

At present, there is not any available accepted vaccine for prevention of Toxoplasma gondii (T. gondii) in human and animals. We conducted literature search through English (Google Scholar, PubMed, Science Direct, Scopus, EBSCO, ISI Web of Science) scientific paper databases to find the best vaccine candidates against toxoplasmosis among T. gondii antigens. Articles with information on infective stage, pathogenicity, immunogenicity and characterization of antigens were selected. We considered that the ideal and significant vaccines should include different antigens and been expressed in all infective stages of the parasite with a high pathogenicity and immunogenicity. Evaluation within this systematic review indicates that MIC 3, 4, 13, ROP 2, RON 5, GRA 1, 6, 8, 14 are expressed in all three infective stages and have pathogenicity and immunogenicity. MIC 5, ROM 4, GRA 2, 4, 15, ROP 5, 16, 17, 38, RON 4, MIC 1, GRA 10, 12, 16, SAG 3 are expressed in only tachyzoites and bradyzoites stages of T. gondii with pathogenicity/immunogenicity. Some antigens appeared to be expressed in a single stage (tachyzoites) but have high pathogenicity and induce immune response. They include enolase2 (ENO2), SAG 1, SAG5D, HSP 70, ROM 1, ROM 5, AMA 1, ROP 18, RON2 and GRA 24. In conclusion, current vaccination against T. gondii infection is not satisfactory, and with the increasing number of high-risk individuals, the development of an effective and safe specific vaccine is greatly valuable for toxoplasmosis prevention. This systematic review reveals prepare candidates for immunization studies.

Antigens to detect the acute phase of toxoplasmosis in pregnant women: standardized comparison

A key element in any diagnostic technique is the antigen (Ag), a biomarker, but this is usually a protein that has a function to the parasite. Some biological aspects of the Ags and of the Toxoplasma gondii can influence the effectiveness of the diagnosis, as well as the antibody isotype and the characteristics of the assay. A large number of papers have assessed different proteins to distinguish the phases of infection, but the ‘indices of effectiveness’ differ among reports. This work presents for the first time a summary of all the Ags that have been evaluated, with standardized measurements of sensitivity and specificity. These values were calculated with information presented in the papers on Ag evaluations to differentiate the infection phases.

Identification and characterization of peptidases secreted by quahog parasite unknown (QPX), the protistan parasite of hard clams

Quahog parasite unknown (QPX) is a protistan parasite capable of causing deadly infections in the hard clam Mercenaria mercenaria, one of the most valuable shellfish species in the USA. QPX is an extracellular parasite found mostly in the connective tissue of clam mantle and, in more severe cases of infection, other clam organs. Histopathologic examinations revealed that QPX cells within clam tissues are typically surrounded by hollow areas that have been hypothesized to be, at least in part, a result of extracellular digestion of clam proteins by the parasite. We investigated peptidase activity in QPX extracellular secretions using sodium dodecyl sulfate-polyacrylamide gels containing gelatin as a co-polymerized substrate. Multiple peptidase activity bands of molecular weights ranging from 20 to 100 kDa were detected in QPX secretions derived from a variety of culture media. One major band of approximately 35 kDa was composed of subtilisin-like peptidases that were released by QPX cells in all studied media, suggesting that these are the most common peptidases used by QPX for nutrient acquisition. PCR quantification of mRNA encoding QPX subtilisins revealed that their expression changes with the protein substrate used in the culture media. A fast protein liquid chromatography (FPLC) was used to fractionate QPX extracellular secretions. An FPLC-fraction containing a subtilisin-type serine peptidase was able to digest clam plasma proteins, suggesting that this peptidase might be involved in the disease process, and making it a good candidate for further investigation as a possible virulence factor of the parasite.

Functional analysis of rhomboid proteases during Toxoplasma invasion

Host cell invasion by Toxoplasma gondii and other apicomplexan parasites requires transmembrane adhesins that mediate binding to receptors on the substrate and host cell to facilitate motility and invasion. Rhomboid proteases (ROMs) are thought to cleave adhesins within their transmembrane segments, thus allowing the parasite to disengage from receptors and completely enter the host cell. To examine the specific roles of individual ROMs during invasion, we generated single, double, and triple knockouts for the three ROMs expressed in T. gondii tachyzoites. Analysis of these mutants demonstrated that ROM4 is the primary protease involved in adhesin processing and host cell invasion, whereas ROM1 or ROM5 plays negligible roles in these processes. Deletion of ROM4 blocked the shedding of adhesins such as MIC2 (microneme protein 2), causing them to accumulate on the surface of extracellular parasites. Increased surface adhesins led to nonproductive attachment, altered gliding motility, impaired moving junction formation, and reduced invasion efficiency. Despite the importance of ROM4 for efficient invasion, mutants lacking all three ROMs were viable and MIC2 was still efficiently removed from the surface of invaded mutant parasites, implying the existence of ROM-independent mechanisms for adhesin removal during invasion. Collectively, these results suggest that although ROM processing of adhesins is not absolutely essential, it is important for efficient host cell invasion by T. gondii.

Apicomplexan parasites such as Toxoplasma gondii express surface proteins that bind host cell receptors to aid invasion. Many of these adhesins are subject to cleavage by rhomboid proteases (ROMs) within their transmembrane segments during invasion. Previous studies have demonstrated the importance of adhesin cleavage for parasite invasion and proposed that the ROMs responsible for processing would be essential for parasite survival. In T. gondii, ROM5 was thought to be the critical ROM for adhesin shedding due to its robust protease activity in vitro and posterior localization on the parasite surface. Here, we knocked out all three ROMs in T. gondii tachyzoites and found that ROM4, but not ROM5, was key for adhesin cleavage. However, none of the ROMs individually or in combination was essential for cell entry, further emphasizing that essential pathways such as invasion typically rely on redundant pathways to ensure survival.

Molecular dissection of novel trafficking and processing of the Toxoplasma gondii rhoptry metalloprotease toxolysin-1

Toxoplasma gondii utilizes specialized secretory organelles called rhoptries to invade and hijack its host cell. Many rhoptry proteins are proteolytically processed at a highly conserved SΦXE site to remove organellar targeting sequences that may also affect protein activity. We have studied the trafficking and biogenesis of a secreted rhoptry metalloprotease with homology to insulysin that we named toxolysin-1 (TLN1). Through genetic ablation and molecular dissection of TLN1, we have identified the smallest rhoptry targeting domain yet reported and expanded the consensus sequence of the rhoptry pro-domain cleavage site. In addition to removal of its pro-domain, TLN1 undergoes a C-terminal cleavage event that occurs at a processing site not previously seen in Toxoplasma rhoptry proteins. While pro-domain cleavage occurs in the nascent rhoptries, processing of the C-terminal region precedes commitment to rhoptry targeting, suggesting that it is mediated by a different maturase, and we have identified residues critical for proteolysis. We have additionally shown that both pieces of TLN1 associate in a detergent-resistant complex, formation of which is necessary for trafficking of the C-terminal portion to the rhoptries. Together, these studies reveal novel processing and trafficking events that are present in the protein constituents of this unusual secretory organelle.

Toxoplasma gondii aspartic protease 1 is not essential in tachyzoites

Aspartic proteases are important virulence factors for pathogens and are recognized as attractive drug targets. Seven aspartic proteases (ASPs) have been identified in Toxoplasma gondii genome. Bioinformatics and phylogenetic analyses regroup them into five monophyletic groups. Among them, TgASP1, a coccidian specific aspartic protease related to the food vacuole plasmepsins, is associated with the secretory pathway in non-dividing cells and relocalizes in close proximity to the nascent inner membrane complex (IMC) of daughter cells during replication. Despite a potential role for TgASP1 in IMC formation, the generation of a conventional knockout of the TgASP1 gene revealed that this protease is not required for T. gondii tachyzoite survival or for proper IMC biogenesis.

Toxoplasma gondii protease TgSUB1 is required for cell surface processing of micronemal adhesive complexes and efficient adhesion of tachyzoites

Host cell invasion by Toxoplasma gondii is critically dependent upon adhesive proteins secreted from the micronemes. Proteolytic trimming of microneme contents occurs rapidly after their secretion onto the parasite surface and is proposed to regulate adhesive complex activation to enhance binding to host cell receptors. However, the proteases responsible and their exact function are still unknown. In this report, we show that T. gondii tachyzoites lacking the microneme subtilisin protease TgSUB1 have a profound defect in surface processing of secreted microneme proteins. Notably parasites lack protease activity responsible for proteolytic trimming of MIC2, MIC4 and M2AP after release onto the parasite surface. Although complementation with full-length TgSUB1 restores processing, complementation of Δsub1 parasites with TgSUB1 lacking the GPI anchor (Δsub1::ΔGPISUB1) only partially restores microneme protein processing. Loss of TgSUB1 decreases cell attachment and in vitro gliding efficiency leading to lower initial rates of invasion. Δsub1 and Δsub1::ΔGPISUB1 parasites are also less virulent in mice. Thus TgSUB1 is involved in micronemal protein processing and regulation of adhesive properties of macromolecular adhesive complexes involved in host cell invasion.

Cathepsin proteases in Toxoplasma gondii

Cysteine proteases are important for the growth and survival of apicomplexan parasites that infect humans. The apicomplexan Toxoplasma gondii expresses five members of the C1 family of cysteine proteases, including one cathepsin L-like (TgCPL), one cathepsin B-like (TgCPB) and three cathepsin C-like (TgCPC1, 2 and 3) proteases. Recent genetic, biochemical and structural studies reveal that cathepsins function in microneme and rhoptry protein maturation, host cell invasion, replication and nutrient acquisition. here, we review the key features and roles of T. gondii cathepsins and discuss the therapeutic potential for specific inhibitor development.

Bovipain-2, the falcipain-2 ortholog, is expressed in intraerythrocytic stages of the tick-transmitted hemoparasite Babesia bovis

BACKGROUND

Cysteine proteases have been shown to be highly relevant for Apicomplexan parasites. In the case of Babesia bovis, a tick-transmitted hemoparasite of cattle, inhibitors of these enzymes were shown to hamper intraerythrocytic replication of the parasite, underscoring their importance for survival.

RESULTS

Four papain-like cysteine proteases were found to be encoded by the B. bovis genome using the MEROPS database. One of them, the ortholog of Plasmodium falciparum falcipain-2, here named bovipain-2, was further characterized. Bovipain-2 is encoded in B. bovis chromosome 4 by an ORF of 1.3 kb, has a predicted molecular weight of 42 kDa, and is hydrophilic with the exception of a transmembrane region. It has orthologs in several other apicomplexans, and its predicted amino acid sequence shows a high degree of conservation among several B. bovis isolates from North and South America. Synteny studies demonstrated that the bovipain-2 gene has expanded in the genomes of two related piroplasmids, Theileria parva and T. annulata, into families of 6 and 7 clustered genes respectively. The bovipain-2 gene is transcribed in in vitro cultured intra-erythrocyte forms of a virulent and an attenuated B. bovis strain from Argentina, and has no introns, as shown by RT-PCR followed by sequencing. Antibodies against a recombinant form of bovipain-2 recognized two parasite protein bands of 34 and 26 kDa, which coincide with the predicted sizes of the pro-peptidase and mature peptidase, respectively. Immunofluorescence studies showed an intracellular localization of bovipain-2 in the middle-rear region of in vitro cultured merozoites, as well as diffused in the cytoplasm of infected erythrocytes. Anti-bovipain-2 antibodies also reacted with B. bigemina-infected erythrocytes giving a similar pattern, which suggests cross-reactivity among these species. Antibodies in sera of two out of six B. bovis-experimentally infected bovines tested, reacted specifically with recombinant bovipain-2 in immunoblots, thus demonstrating expression and immunogenicity during bovine-infecting stages.

CONCLUSIONS

Overall, we present the characterization of bovipain-2 and demonstrate its in vitro and in vivo expression in virulent and attenuated strains. Given the involvement of apicomplexan cysteine proteases in essential parasite functions, bovipain-2 constitutes a new vaccine candidate and potential drug target for bovine babesiosis.

Rhomboid homologs in mycobacteria: insights from phylogeny and genomic analysis

BACKGROUND

Rhomboids are ubiquitous proteins with diverse functions in all life kingdoms, and are emerging as important factors in the biology of some pathogenic apicomplexa and Providencia stuartii. Although prokaryotic genomes contain one rhomboid, actinobacteria can have two or more copies whose sequences have not been analyzed for the presence putative rhomboid catalytic signatures. We report detailed phylogenetic and genomic analyses devoted to prokaryotic rhomboids of an important genus, Mycobacterium.

RESULTS

Many mycobacterial genomes contained two phylogenetically distinct active rhomboids orthologous to Rv0110 (rhomboid protease 1) and Rv1337 (rhomboid protease 2) of Mycobacterium tuberculosis H37Rv, which were acquired independently. There was a genome-wide conservation and organization of the orthologs of Rv1337 arranged in proximity with glutamate racemase (mur1), while the orthologs of Rv0110 appeared evolutionary unstable and were lost in Mycobacterium leprae and the Mycobacterium avium complex. The orthologs of Rv0110 clustered with eukaryotic rhomboids and contained eukaryotic motifs, suggesting a possible common lineage. A novel nonsense mutation at the Trp73 codon split the rhomboid of Mycobacterium avium subsp. Paratuberculosis into two hypothetical proteins (MAP2425c and MAP2426c) that are identical to MAV_1554 of Mycobacterium avium. Mycobacterial rhomboids contain putative rhomboid catalytic signatures, with the protease active site stabilized by Phenylalanine. The topology and transmembrane helices of the Rv0110 orthologs were similar to those of eukaryotic secretase rhomboids, while those of Rv1337 orthologs were unique. Transcription assays indicated that both mycobacterial rhomboids are possibly expressed.

CONCLUSIONS

Mycobacterial rhomboids are active rhomboid proteases with different evolutionary history. The Rv0110 (rhomboid protease 1) orthologs represent prokaryotic rhomboids whose progenitor may be the ancestors of eukaryotic rhomboids. The Rv1337 (rhomboid protease 2) orthologs appear more stable and are conserved nearly in all mycobacteria, possibly alluding to their importance in mycobacteria. MAP2425c and MAP2426c provide the first evidence for a split homologous rhomboid, contrasting whole orthologs of genetically related species. Although valuable insights to the roles of rhomboids are provided, the data herein only lays a foundation for future investigations for the roles of rhomboids in mycobacteria.

Identification and characterization of a surface-associated, subtilisin-like serine protease inTrichomonas vaginalis

Trichomonas vaginalisis a protozoan parasite causing trichomonosis, a sexually transmitted infection in humans. This parasite has numerous proteases, most of which are cysteine proteases that appear to be involved in adherence and cytotoxicity of host cells. In this report we identify and characterize a putative subtilisin-like serine protease (SUB1). Thesub1gene encodes a 101-kDa protein.In silicoanalyses predict signal and pro-peptides at the N-terminus, and a transmembrane helix at the carboxy-terminal region. Thesub1gene was found as single copy by Southern analysis, albeit additional serine protease related genes are annotated in theT. vaginalisgenome. The expression ofsub1could only be detected by RT-PCR and Ribonuclease Protection Assays, suggesting a low abundant mRNA. Thesub1gene transcription start site was correctly assigned by RPA. The transcript abundance was found to be modulated by the availability of iron in the growth medium. Antibodies raised to a specific SUB1 peptide recognized a single protein band (~82 kDa) in Western blots, possibly representing the mature form of the protein. Immunofluorescence showed SUB1 on the trichomonad surface, and in dispersed vesicles throughout the cytoplasm. A bioinformatic analysis of genes annotated as serine proteases in theT. vaginalisgenome is also presented. To our knowledge this is the first putative serine protease experimentally described forT. vaginalis.

Action of a pentacyclic triterpenoid, maslinic acid, against Toxoplasma gondii

The action of maslinic acid (2alpha,3beta-dihydroxyolean-12-en-28-oic acid) (1), a pentacyclic derivative present in the pressed fruits of the olive (Olea europaea), has been studied against the tachyzoites of Toxoplasma gondii. The capability of tachyzoites to infect Vero cells treated with 1 was affected. The LD(50) values were 58.2 muM for the isolated tachyzoites and 236 muM for the noninfected Vero cells. Zymograms of the T. gondii proteases incubated with 1 showed a dosage-dependent inhibition of some of the proteases. The parasites treated with 1 showed gliding motility and ultrastructural alterations. The present findings suggest that protease activity of the parasite required for cell invasion is the action target for maslinic acid (1).

Toxoplasma gondii proteomics

Toxoplasma gondii is a ubiquitous, Apicomplexan parasite that, in humans, can cause several clinical syndromes, including encephalitis, chorioretinitis and congenital infection. T. gondii was described a little over 100 years ago in the tissues of the gundi (Ctenodoactylus gundi). There are a large number of applicable experimental techniques available for this pathogen and it has become a model organism for the study of intracellular pathogens. With the completion of the genomes for a type I (GT-1), type II (ME49) and type III (VEG) strains, proteomic studies on this organism have been greatly facilitated. Several subcellular proteomic studies have been completed on this pathogen. These studies have helped elucidate specialized invasion organelles and their composition, as well as proteins associated with the cytoskeleton. Global proteomic studies are leading to improved strategies for genome annotation in this organism and an improved understanding of protein regulation in this pathogen. Web-based resources, such as EPIC-DB and ToxoDB, provide proteomic data and support for studies on T. gondii. This review will summarize the current status of proteomic research on T. gondii.

Export of a Toxoplasma gondii rhoptry neck protein complex at the host cell membrane to form the moving junction during invasion

One of the most conserved features of the invasion process in Apicomplexa parasites is the formation of a moving junction (MJ) between the apex of the parasite and the host cell membrane that moves along the parasite and serves as support to propel it inside the host cell. The MJ was, up to a recent period, completely unknown at the molecular level. Recently, proteins originated from two distinct post-Golgi specialised secretory organelles, the micronemes (for AMA1) and the neck of the rhoptries (for RON2/RON4/RON5 proteins), have been shown to form a complex. AMA1 and RON4 in particular, have been localised to the MJ during invasion. Using biochemical approaches, we have identified RON8 as an additional member of the complex. We also demonstrated that all RON proteins are present at the MJ during invasion. Using metabolic labelling and immunoprecipitation, we showed that RON2 and AMA1 were able to interact in the absence of the other members. We also discovered that all MJ proteins are subjected to proteolytic maturation during trafficking to their respective organelles and that they could associate as non-mature forms in vitro. Finally, whereas AMA1 has previously been shown to be inserted into the parasite membrane upon secretion, we demonstrated, using differential permeabilization and loading of RON-specific antibodies into the host cell, that the RON complex is targeted to the host cell membrane, where RON4/5/8 remain associated with the cytoplasmic face. Globally, these results point toward a model of MJ organization where the parasite would be secreting and inserting interacting components on either side of the MJ, both at the host and at its own plasma membranes.

A unique feature of apicomplexan parasites is the formation of an intimate contact between the apex of the parasite and the host cell membrane called the moving junction that moves along the parasite during invasion. Proteins originated from two distinct secretory organelles, the microneme for AMA1 and the rhoptry neck for RON2/4/5 proteins, are associated to form the junction. Here, we have furthered the characterization of the MJ complex by describing RON8, an additional protein component. AMA1 has previously been shown to be inserted into the parasite membrane upon secretion. Our study demonstrates that all the RON proteins are translocated into the host cell, where RON4/5/8 remain associated with the cytoplasmic face of the host cell plasma membrane. Furthermore, we identified a privileged interaction between transmembrane MJ proteins AMA1 and RON2 in vitro. Overall, this led us to propose the first model describing the putative MJ organisation at the interface between the host cell and Toxoplasma. In this original concept, the parasite would export its own receptor (RON2) and ligand (AMA1) on either side of the MJ.

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.

Molecular signals in the trafficking of Toxoplasma gondii protein MIC3 to the micronemes

The protozoan parasite Toxoplasma gondii is equipped with a sophisticated secretory apparatus, including three distinct exocytic organelles, named micronemes, rhoptries, and dense granules. We have dissected the requirements for targeting the microneme protein MIC3, a key component of T. gondii infection. We have shown that MIC3 is processed in a post-Golgi compartment and that the MIC3 propeptide and epidermal growth factor (EGF) modules contain microneme-targeting information. The minimal requirement for microneme delivery is defined by the propeptide plus any one of the three EGF domains. We have demonstrated that the cleavage of the propeptide, the dimerization of MIC3, and the chitin binding-like sequence, which are crucial for host cell binding and virulence, are dispensable for proper targeting. Finally, we have shown that part of MIC3 is withheld in the secretory pathway in a cell cycle-dependent manner.

Adenylyl cyclase alpha and cAMP signaling mediate Plasmodium sporozoite apical regulated exocytosis and hepatocyte infection

Malaria starts with the infection of the liver of the host by Plasmodium sporozoites, the parasite form transmitted by infected mosquitoes. Sporozoites migrate through several hepatocytes by breaching their plasma membranes before finally infecting one with the formation of an internalization vacuole. Migration through host cells induces apical regulated exocytosis in sporozoites. Here we show that apical regulated exocytosis is induced by increases in cAMP in sporozoites of rodent (P. yoelii and P. berghei) and human (P. falciparum) Plasmodium species. We have generated P. berghei parasites deficient in adenylyl cyclase alpha (ACalpha), a gene containing regions with high homology to adenylyl cyclases. PbACalpha-deficient sporozoites do not exocytose in response to migration through host cells and present more than 50% impaired hepatocyte infectivity in vivo. These effects are specific to ACalpha, as re-introduction of ACalpha in deficient parasites resulted in complete recovery of exocytosis and infection. Our findings indicate that ACalpha and increases in cAMP levels are required for sporozoite apical regulated exocytosis, which is involved in sporozoite infection of hepatocytes.

Proteomic analysis of calcium-dependent secretion in Toxoplasma gondii

Toxoplasma gondii is an intracellular protozoan parasite that invades a wide range of nucleated cells. In the course of intracellular parasitism, the parasite releases a large variety of proteins from three secretory organelles, namely, micronemes, rhoptries and dense granules. Elevation of intracellular Ca(2+) in the parasite causes microneme discharge, and microneme secretion is essential for the invasion. In this study, we performed a proteomic analysis of the Ca(2+)-dependent secretion to evaluate the protein repertoire. We found that Ca(2+)-mobilising agents, such as thapsigargin, NH(4)Cl, ethanol and a Ca(2+) ionophore, A23187, promoted the secretion of the parasite proteins. The proteins, artificially secreted by A23187, were used in a comparative proteomic analysis by 2-DE followed by PMF analysis and/or N-terminal sequencing. Major known microneme proteins (MICs), such as MIC2, MIC4, MIC6 and MIC10 and apical membrane antigen 1 (AMA1), were identified, indicating that the proteomic analysis worked accurately. Interestingly, new members of secretory proteins, namely rhoptry protein 9 (ROP9) and Toxoplasma SPATR (TgSPATR), which was a homologue of a Plasmodium secreted protein with an altered thrombospondin repeat (SPATR), were detected in Ca(2+)-dependent secretion. Thus, we succeeded in detecting Ca(2+)-dependent secretory proteins in T. gondii, which contained novel secretory proteins.

N-linked glycosylation of proteins in the protozoan parasite Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite of animal cells. Infection of humans is common and may result in devastating disease, especially in immunocompromised individuals. Despite previous reports that N-glycosylation of proteins may be a rare post-translational modification in this and related organisms, we demonstrate that it is actually quite prevalent in Toxoplasma. N-Glycosylation is completely inhibited by treatment of parasites with tunicamycin, but this does not appear to exert its major effect on the parasites until they have egressed from their host cells. Although the tunicamycin-treated parasites appear structurally normal at this time they are not motile and mostly incapable of invading new host cells. The few tunicamycin-treated parasites that do invade are severely affected in their ability to replicate and accumulate with a distended endoplasmic reticulum, deformed nuclei, and without recognizable late secretory organelles. We provide experimental evidence that indicate that Toxoplasma N-glycans differ structurally from those in other eukaryotes.

Neospora caninum emerges from the shadow of Toxoplasma gondii

It is sometimes easy to make the mistake of assuming that everything that holds true for Toxoplasma gondii is also true for its relative Neospora caninum. However, a recurring theme in the recent review by Hemphill et al. is not the similarities but the striking differences between the two parasites.