Conformationally correct expression of membrane-anchored Toxoplasma gondii SAG1 in the primitive protozoan Giardia duodenalis

Expression of in vivo biotinylated recombinant antigens SAG1 and SAG2A from Toxoplasma gondii for improved seroepidemiological bead-based multiplex assays

BACKGROUND

Few bead-based multiplex assays have been described that detect antibodies against the protozoan parasite Toxoplasma gondii in large-scale seroepidemiological surveys. Moreover, each multiplex assay has specific variations or limitations, such as the use of truncated or fusion proteins as antigens, potentially masking important epitopes. Consequently, such an assay must be developed by interested groups as none is commercially available.

RESULTS

We report the bacterial expression and use of N-terminal fusion-free, soluble, in vivo biotinylated recombinant surface antigens SAG1 and SAG2A for the detection of anti-T. gondii IgG antibodies. The expression system relies on three compatible plasmids. An expression construct produces a fusion of maltose-binding protein with SAG1 (or SAG2A), separated by a TEV protease cleavage site, followed by a peptide sequence recognized by E. coli biotin ligase BirA (AviTag), and a terminal six histidine tag for affinity purification. TEV protease and BirA are encoded on a second plasmid, and their expression leads to proteolytic cleavage of the fusion protein and a single biotinylated lysine within the AviTag by BirA. Correct folding of the parasite proteins is dependent on proper disulfide bonding, which is facilitated by a sulfhydryl oxidase and a protein disulfide isomerase, encoded on the third plasmid. The C-terminal biotinylation allowed the oriented, reproducible coupling of the purified surface antigens to magnetic Luminex beads, requiring only minute amounts of protein per determination. We showed that an N-terminal fusion partner such as maltose-binding protein negatively influenced antibody binding, confirming that access to SAG1's N-terminal epitopes is important for antibody recognition. We validated our bead-based multiplex assay with human sera previously tested with commercial diagnostic assays and found concordance of 98-100% regarding both, sensitivity and specificity, even when only biotinylated SAG1 was used as antigen.

CONCLUSIONS

Our recombinant in vivo-biotinylated T. gondii antigens offer distinct advantages compared to previously described proteins used in multiplex serological assays for T. gondii. They offer a cheap, specific and sensitive alternative to either parasite lysates or eukaryotic-cell expressed SAG1/SAG2A for BBMA and other formats. The described general expression strategy can also be used for other antigens where oriented immobilization is key for sensitive recognition by antibodies and ligands.

A cytonaut's guide to protein trafficking in Giardia lamblia

Over the past years, the subcellular organization of the Excavata member Giardia lamblia (syn. duodenalis, intestinalis) has been investigated in considerable detail. There are several reasons for this endeavour which go beyond this parasite's medical importance and are mostly concerned with its reduced subcellular complexity and debated evolutionary status. One may say that simplification has emerged as a paradigm for the evolution of Giardia's subcellular architecture. However, a complete appreciation of the evolutionary and ecological significance of this phenomenon is far from complete. In this chapter, we present and discuss the most recent data on the main trafficking pathways in G. lamblia which include endo- and exo-cytosis, organellar import and function. We provide perspectives on open questions concerning organelle replication and inheritance and include a technical outlook on methods and approaches to genetic manipulations in G. lamblia. A better understanding of G. lamblia subcellular organization at the morphological and molecular level complements any effort aimed at elucidating this parasitic species' evolutionary status and could provide us with the basis for novel strategies to interfere with parasite transmission and/or pathogenesis.

Export of cyst wall material and Golgi organelle neogenesis in Giardia lamblia depend on endoplasmic reticulum exit sites

Giardia lamblia parasitism accounts for the majority of cases of parasitic diarrheal disease, making this flagellated eukaryote the most successful intestinal parasite worldwide. This organism has undergone secondary reduction/elimination of entire organelle systems such as mitochondria and Golgi. However, trophozoite to cyst differentiation (encystation) requires neogenesis of Golgi-like secretory organelles named encystation-specific vesicles (ESVs), which traffic, modify and partition cyst wall proteins produced exclusively during encystation. In this work we ask whether neogenesis of Golgi-related ESVs during G. lamblia differentiation, similarly to Golgi biogenesis in more complex eukaryotes, requires the maintenance of distinct COPII-associated endoplasmic reticulum (ER) subdomains in the form of ER exit sites (ERES) and whether ERES are also present in non-differentiating trophozoites. To address this question, we identified conserved COPII components in G. lamblia cells and determined their localization, quantity and dynamics at distinct ERES domains in vegetative and differentiating trophozoites. Analogous to ERES and Golgi biogenesis, these domains were closely associated to early stages of newly generated ESV. Ectopic expression of non-functional Sar1 GTPase variants caused ERES collapse and, consequently, ESV ablation, leading to impaired parasite differentiation. Thus, our data show how ERES domains remain conserved in G. lamblia despite elimination of steady-state Golgi. Furthermore, the fundamental eukaryotic principle of ERES to Golgi/Golgi-like compartment correspondence holds true in differentiating Giardia presenting streamlined machinery for secretory organelle biogenesis and protein trafficking. However, in the Golgi-less trophozoites ERES exist as stable ER subdomains, likely as the sole sorting centres for secretory traffic.

Membrane trafficking and organelle biogenesis in Giardia lamblia: use it or lose it

The secretory transport capacity of Giardia trophozoites is perfectly adapted to the changing environment in the small intestine of the host and is able to deploy essential protective surface coats as well as molecules which act on epithelia. These lumen-dwelling parasites take up nutrients by bulk endocytosis through peripheral vesicles or by receptor-mediated transport. The environmentally-resistant cyst form is quiescent but poised for activation following stomach passage. Its versatility and fidelity notwithstanding, the giardial trafficking systems appear to be the product of a general secondary reduction process geared towards minimization of all components and machineries identified to date. Since membrane transport is directly linked to organelle biogenesis and maintenance, less complexity also means loss of organelle structures and functions. A case in point is the Golgi apparatus which is missing as a steady-state organelle system. Only a few basic Golgi functions have been experimentally demonstrated in trophozoites undergoing encystation. Similarly, mitochondrial remnants have reached a terminally minimized state and appear to be functionally restricted to essential iron-sulfur protein maturation processes. Giardia's minimized organization combined with its genetic tractability provides unique opportunities to study basic principles of secretory transport in an uncluttered cellular environment. Not surprisingly, Giardia is gaining increasing attention as a model for the investigation of gene regulation, organelle biogenesis, and export of simple but highly protective cell wall biopolymers, a hallmark of all perorally transmitted protozoan and metazoan parasites.

Expression of Cryptosporidium parvum Cpa135/CpCCP1 chimeras in Giardia duodenalis: organization of the protein domains affects the protein secretion pathway

Cpa135 is a multidomain antigenic protein secreted at the sporozoite stage of the Apicomplexa protozoan Cryptosporidium parvum. Previous studies have shown that the protozoan flagellate parasite Giardia duodenalis is a suitable system for the heterologous expression of secreted proteins of Apicomplexa. Here, we designed three different Cpa135 variants fused to a C-terminal HA tag in order to test their expression in G. duodenalis under the control of the inducible promoter of the cyst wall protein 1 gene (cwp1). The three Cpa135 chimeras encompassed different portions of the protein; CpaG encodes the entire polypeptide of 1574 amino acids (aa); CpaGΔC includes the first 826 aa at the N-terminus; and CpaGΔN consists in of the final 833 aa at the C-terminus. Immunoblot experiments showed that CpaG and CpaGΔN maintained the epitopes recognized by anti-C. parvum-specific human serum. The intracellular localization and transport of the three Cpa135 variants were studied by immunofluorescence in combination with G. duodenalis-specific antibodies. CpaGΔC was mainly accumulated in the endoplasmic reticulum and the intact form was also excreted in the medium. Differently, the Cpa135 chimeras possessing an intact C-terminus (CpaG and CpaGΔN) were transported towards the forming cyst wall possibly and were not detected in the medium. Furthermore, the full-length CpaG was incorporated into the cyst wall. The data presented suggest that the C-terminus of Cpa135, which includes a cysteine reach domain, could influence the secretion of the chimeric proteins.

Assembly and export of a Toxoplasma microneme complex in Giardia lamblia

The microneme proteins of Toxoplasma gondii belong to a large family of adhesins of apicomplexan parasites involved in motility and host cell invasion. During secretory transport, soluble micronemes associate with membrane-bound carriers/escorters and become exposed on the parasite surface as complexes with an array of adhesive domains. Previously, we have exploited the intestinal protozoan Giardia lamblia as an expression system to produce correctly folded and unglycosylated monomeric surface proteins of T. gondii. Here, we report assembly and export of a trimeric microneme (MIC1/4/6) adhesin complex from Toxoplasma. Co-expressed, recombinant microneme proteins were used to investigate structural requirements for microneme complex formation. In addition, export of a microneme subunit induced development of novel Golgi-like compartments demonstrating the existence of post endoplasmic reticulum structures involved in constitutive secretion in this 'Golgi-less' cell. Recreation of the trimeric microneme escorter-cargo system in Giardia is a versatile tool to analyse universal requirements for complex assembly, receptor-ligand interactions and Golgi neogenesis in the basal Giardia secretory system.

Production of the main surface antigen of Toxoplasma gondii in tobacco leaves and analysis of its antigenicity and immunogenicity

We adapted a previously described Agrobacterium-mediated transient expression system to test the expression level of three constructs carrying the surface antigen 1 (SAG1) of Toxoplasma gondii. Two constructs were based in a Potato virus X (PVX) amplicon. In one of them, the PVX movement protein genes were replaced by the sag1 gene. In the other, the sag1 gene was placed under the control of an additional coat protein subgenomic promoter. In the third construct, the sag1 gene was fused to an apoplastic peptide signal under the CaMV 35S promoter. Western blot analysis of leaf extracts infiltrated with each construct revealed a protein of 35 kDa. SAG1 accumulation in leaves ranged from 0.1 to 0.06% of total soluble protein (equivalent to 10 microg and 6 microg of SAG1 per gram of fresh leaf tissue, respectively). Three of five human seropositive samples reacted with tobacco-expressed SAG1 in Western blot analysis. The C3H mice were immunized with SAG-expressing leaf extracts and perorally challenged with a nonlethal dose of the T. gondii Me49 strain. Mice vaccinated with SAG1 showed significantly lower brain cyst burdens compared to those from the control group. Immunization with SAG1-expressing leaves elicited a specific humoral response with predominant participation of type IgG2a. In conclusion, a functional SAG1 version could be transiently expressed in tobacco leaves.

Secretory protein trafficking in Giardia intestinalis

Early diverged extant organisms, which may serve as convenient laboratory models to look for and study evolutionary ancient features of eukaryotic cell biology, are rare. The diplomonad Giardia intestinalis, a protozoan parasite known to cause diarrhoeal disease, has become an increasingly popular object of basic research in cell biology, not least because of a genome sequencing project nearing completion. Commensurate with its phylogenetic status, the Giardia trophozoite has a very basic secretory system and even lacks hallmark structures such as a morphologically identifiable Golgi apparatus. The cell's capacity for protein sorting is nevertheless unimpeded, exemplified by its ability to cope with massive amounts of newly synthesized cyst wall proteins and glycans, which are sorted to dedicated Golgi-like compartments termed encystation-specific vesicles (ESVs) generated from endoplasmic reticulum (ER)-derived transport intermediates. This soluble bulk cargo is kept strictly separate from constitutively transported variant surface proteins during export, a function that is dependent on the stage-specific recognition of trafficking signals. Encysting Giardia therefore provide a unique system for the study of unconventional, Golgi-independent protein trafficking mechanisms in the broader context of eukaryotic endomembrane organization and evolution.

An ancestral secretory apparatus in the protozoan parasite Giardia intestinalis

The protozoan parasite Giardia intestinalis belongs to one of the earliest diverged eukaryotic lineages. This is also reflected in a simple intracellular organization, as Giardia lacks common subcellular compartments such as mitochondria, peroxisomes, and apparently also a Golgi apparatus. During encystation, developmentally regulated formation of large secretory compartments containing cyst wall material occurs. Despite the lack of any morphological similarities, these encystation-specific vesicles (ESVs) show several biochemical characteristics of maturing Golgi cisternae. Previous studies suggested that Golgi structure and function are induced only during encystation in Giardia, giving rise to the hypothesis that ESVs, as a Giardia Golgi equivalent, are generated de novo. Alternatively, ESV compartments could be built on the template structure of a cryptic Golgi in trophozoites in response to ER export of cyst wall material during encystation. We addressed this question by defining the molecular framework of the Giardia secretory apparatus using a comparative genomic approach. Analysis of the corresponding transcriptome during growth and encystation revealed surprisingly little stage-specific regulation. A panel of antibodies was generated against selected marker proteins to investigate the developmental dynamics of the endomembrane system. We show evidence that Giardia accommodates the export of large amounts of cyst wall material through re-organization of membrane compartment(s) in trophozoites with biochemical similarities to ESVs. This suggests that ESVs are selectively stabilized Golgi-like compartments in a unique and archetypical secretory system, which arise from a structural template in trophozoites rather than being generated de novo.

Protein trafficking in Giardia lamblia

Giardia, a protozoan parasite of humans and other vertebrates, is a common cause of intestinal disease worldwide. Besides its medical importance, Giardia is considered an excellent system to study the evolution of fundamental cellular processes because it belongs to the earliest branches of the eukaryotic lineage of descent. Giardia trophozoites lack organelles typical of higher eukaryotes such mitochondria, peroxisomes and compartments involved in intracellular protein trafficking and secretion, such as the Golgi apparatus and secretory granules. Nevertheless, the minimal machinery for protein transport and sorting is present in this parasite. When Giardia undergoes encystation, the biogenesis of secretory organelles necessary to transport cyst wall constituents to the cell surface takes place. Recent studies in both vegetative and encysting trophozoites have provided interesting information regarding the secretory pathway of this important human pathogen.

The secretory apparatus of an ancient eukaryote: protein sorting to separate export pathways occurs before formation of transient Golgi-like compartments

Transmission of the protozoan parasite Giardia intestinalis to vertebrate hosts presupposes the encapsulation of trophozoites into an environmentally resistant and infectious cyst form. We have previously shown that cyst wall proteins were faithfully sorted to large encystation-specific vesicles (ESVs), despite the absence of a recognizable Golgi apparatus. Here, we demonstrate that sorting to a second constitutively active pathway transporting variant-specific surface proteins (VSPs) to the surface depended on the cytoplasmic VSP tail. Moreover, pulsed endoplasmic reticulum (ER) export of chimeric reporters containing functional signals for both pathways showed that protein sorting was done at or very soon after export from the ER. Correspondingly, we found that a limited number of novel transitional ER-like structures together with small transport intermediates were generated during encystation. Colocalization of transitional ER regions and early ESVs with coat protein (COP) II and of maturing ESVs with COPI and clathrin strongly suggested that ESVs form by fusion of ER-derived vesicles and subsequently undergo maturation by retrograde transport. Together, the data supported the hypothesis that in Giardia, a primordial secretory apparatus is in operation by which proteins are sorted in the early secretory pathway, and the developmentally induced ESVs carry out at least some Golgi functions.

Sorting of encystation-specific cysteine protease to lysosome-like peripheral vacuoles in Giardia lamblia requires a conserved tyrosine-based motif

Encystation-specific cysteine protease (ESCP) was the first membrane-associated protein described to be part of the lysosome-like peripheral vacuoles in the intestinal parasite Giardia lamblia. ESCP is homologous to cathepsin C enzymes of higher eukaryotes, but is distinguished from other lysosomal cysteine proteases because it possesses a transmembrane domain and a short cytoplasmic tail. Tyrosine-based motifs within tails of membrane proteins are known to participate in endosomal/lysosomal protein sorting in higher eukaryotes. In this study, we show that a YRPI motif within the ESCP cytoplasmic tail is necessary and sufficient to mediate ESCP sorting to peripheral vacuoles in Giardia. Deletion and point mutation analysis demonstrated that the tyrosine residue is critical for ESCP sorting, whereas amino acids located at the Y+1 (Arg), Y+2 (Pro), and Y+3 (Ile) positions show minimal effect. Loss of the motif resulted in surface localization, whereas addition of the motif to a variant-specific surface protein resulted in lysosomal localization. Although Giardia trophozoites lack a morphologically discernible Golgi apparatus, our findings indicate that this parasite directs proteins to the lysosomes using a conserved sorting signal similar to that used by yeast and mammalian cells. Because Giardia is one of the earliest branching protist, these results demonstrate that sorting motifs for specific protein traffic developed very early during eukaryotic evolution.