Transient transfection and expression of firefly luciferase in Giardia lamblia

Nourseothricin as a novel drug for selection of transgenic Giardia lamblia

Functional gene and protein characterizations in parasitic protists are often limited by their genetic tractability. Despite the development of CRISPR-Cas9-derived or inspired approaches for a handful of protist parasites, the overall genetic tractability of these organisms remains limited. The intestinal parasite Giardia lamblia is one such species, with the added challenge of a paucity of reliable selection markers. To address this limitation, we tested the feasibility of using Nourseothricin as an effective selection agent in Giardia. Here, we report that axenically-grown WB Giardia cells are sensitive to Nourseothricin and that engineering expression of the streptothricin acetyltransferase (SAT-1) gene from Streptomyces rochei in transgenic parasites confers resistance to this antibiotic. Furthermore, we determine that SAT-1-expressing parasites are cross-resistant neither to Neomycin nor Puromycin, which are widely used to select for transgenic parasites. Consequently, we show that Nourseothricin can be used in sequential combination with both Neomycin and Puromycin to select for dual transfection events. This work increases the number of reliable selection agents and markers for Giardia genetic manipulation, expanding the limited molecular toolbox for this species of global medical importance.

Harnessing the power of new genetic tools to illuminate Giardia biology and pathogenesis

Giardia is a prevalent single-celled microaerophilic intestinal parasite causing diarrheal disease and significantly impacting global health. Double diploid (essentially tetraploid) Giardia trophozoites have presented a formidable challenge to the development of molecular genetic tools to interrogate gene function. High sequence divergence and the high percentage of hypothetical proteins lacking homology to proteins in other eukaryotes have limited our understanding of Giardia protein function, slowing drug target validation and development. For more than 25 years, Giardia A and B assemblages have been readily amenable to transfection with plasmids or linear DNA templates. Here, we highlight the utility and power of genetic approaches developed to assess protein function in Giardia, with particular emphasis on the more recent clustered regularly interspaced palindromic repeats/Cas9-based methods for knockdowns and knockouts. Robust and reliable molecular genetic approaches are fundamental toward the interrogation of Giardia protein function and evaluation of druggable targets. New genetic approaches tailored for the double diploid Giardia are imperative for understanding Giardia's unique biology and pathogenesis.

Antibodies to variable surface antigens induce antigenic variation in the intestinal parasite Giardia lamblia

The genomes of most protozoa encode families of variant surface antigens. In some parasitic microorganisms, it has been demonstrated that mutually exclusive changes in the expression of these antigens allow parasites to evade the host's immune response. It is widely assumed that antigenic variation in protozoan parasites is accomplished by the spontaneous appearance within the population of cells expressing antigenic variants that escape antibody-mediated cytotoxicity. Here we show, both in vitro and in animal infections, that antibodies to Variant-specific Surface Proteins (VSPs) of the intestinal parasite Giardia lamblia are not cytotoxic, inducing instead VSP clustering into liquid-ordered phase membrane microdomains that trigger a massive release of microvesicles carrying the original VSP and switch in expression to different VSPs by a calcium-dependent mechanism. This novel mechanism of surface antigen clearance throughout its release into microvesicles coupled to the stochastic induction of new phenotypic variants not only changes current paradigms of antigenic switching but also provides a new framework for understanding the course of protozoan infections as a host/parasite adaptive process.

Application of electric field treatment (EFT) for microbial control in water and liquid food

Water disinfection and food pasteurization are critical to reducing waterborne and foodborne diseases, which have been a pressing public health issue globally. Electrified treatment processes are emerging and have become promising alternatives due to the low cost of electricity, independence of chemicals, and low potential to form by-products. Electric field treatment (EFT) is a physical pathogen inactivation approach, which damages cell membrane by irreversible electroporation. EFT has been studied for both water disinfection and food pasteurization. However, no study has systematically connected the two fields with an up-to-date review. In this article, we first provide a comprehensive background of microbial control in water and food, followed by the introduction of EFT. Subsequently, we summarize the recent EFT studies for pathogen inactivation from three aspects, the processing parameters, its efficacy against different pathogens, and the impact of liquid properties on the inactivation performance. We also review the development of novel configurations and materials for EFT devices to address the current challenges of EFT. This review introduces EFT from an engineering perspective and may serve as a bridge to connect the field of environmental engineering and food science.

Comprehensive characterization of Cysteine-rich protein-coding genes of Giardia lamblia and their role during antigenic variation

Giardia lamblia encodes several families of cysteine-rich proteins, including the Variant-specific Surface Proteins (VSPs) involved in the process of antigenic variation. Their characteristics, definition and relationships are still controversial. An exhaustive analysis of the Cys-rich families including organization, features, evolution and levels of expression was performed, by combining pattern searches and predictions with massive sequencing techniques. Thus a new classification for Cys-rich proteins, genes and pseudogenes that better describes their involvement in Giardia's biology is presented. Moreover, three novel characteristics exclusive to the VSP genes, comprising an Initiator element/Kozak-like sequence, an extended polyadenylation signal and a unique pattern of mutually exclusive transcript accumulation is presented as well as the finding that High Cysteine Membrane Proteins, upregulated under stress, may protect the parasite during VSP switching. These results allow better interpretation of previous reports providing the basis for further studies of the biology of this early-branching eukaryote.

Efficient CRISPR/Cas9-mediated gene disruption in the tetraploid protist Giardia intestinalis

CRISPR/Cas9-mediated genome editing has become an extremely powerful technique used to modify gene expression in many organisms, including parasitic protists. Giardia intestinalis, a protist parasite that infects approximately 280 million people around the world each year, has been eluding the use of CRISPR/Cas9 to generate knockout cell lines due to its tetraploid genome. In this work, we show the ability of the in vitro assembled CRISPR/Cas9 components to successfully edit the genome of G. intestinalis. The cell line that stably expresses Cas9 in both nuclei of G. intestinalis showed effective recombination of the cassette containing the transcription units for the gRNA and the resistance marker. This highly efficient process led to the removal of all gene copies at once for three independent experimental genes, mem, cwp1 and mlf1. The method was also applicable to incomplete disruption of the essential gene, as evidenced by significantly reduced expression of tom40. Finally, testing the efficiency of Cas9-induced recombination revealed that homologous arms as short as 150 bp can be sufficient to establish a complete knockout cell line in G. intestinalis.

Activity of the Giardia intestinalis proteasome during encystation and its connection with the expression of the cyst wall protein 1 (CWP1)

Giardia is a parasite whose life cycle is composed of two stages: replicative trophozoites, responsible for the symptoms of the disease, and infective cysts, resistant to adverse environments outside of hosts. Proteasomes are multicatalytic peptidase complexes responsible for the specific degradation of proteins in eukaryotic cells. This study assessed the proteasome activity in the trophozoite and during encystation. Strong activation of the proteasome was observed during the differentiation of trophozoites into cysts, reaching its maximum level 24 h after the stimulus. We also found that the Giardia proteasome presents unusual characteristics related to higher eukaryotic proteasomes, making it an eventual therapeutic target. Here we tested the effects on the synthesis of a cyst wall protein by chemical inactivation of the proteasome and by overexpression or partial inhibition of the deubiquitinating protein RPN11 in transfected cells. Moreover, an analysis of the intracellular localization of RPN11 (an integral part of the proteasome regulatory particle) revealed major changes associated with the differentiation of trophozoites into cysts. This evidence further supports the important role of the proteasome in Giardia encystation.

Giardia duodenalis: Biology and Pathogenesis

Giardia duodenalis captured the attention of Leeuwenhoek in 1681 while he was examining his own diarrheal stool, but, ironically, it did not really gain attention as a human pathogen until the 1960s, when outbreaks were reported. Key technological advances, including in vitro cultivation, genomic and proteomic databases, and advances in microscopic and molecular approaches, have led to an understanding that this is a eukaryotic organism with a reduced genome rather than a truly premitochondriate eukaryote. This has included the discovery of mitosomes (vestiges of mitochondria), a transport system with many of the features of the Golgi apparatus, and even evidence for a sexual or parasexual cycle. Cell biology approaches have led to a better understanding of how Giardia survives with two nuclei and how it goes through its life cycle as a noninvasive organism in the hostile environment of the lumen of the host intestine. Studies of its immunology and pathogenesis have moved past the general understanding of the importance of the antibody response in controlling infection to determining the key role of the Th17 response. This work has led to understanding of the requirement for a balanced host immune response that avoids the extremes of an excessive response with collateral damage or one that is unable to clear the organism. This understanding is especially important in view of the remarkable ranges of early manifestations, which range from asymptomatic to persistent diarrhea and weight loss, and longer-term sequelae that include growth stunting in children who had no obvious symptoms and a high frequency of postinfectious irritable bowel syndrome (IBS).

Transfection With Plasmid Causing Stable Expression of a Foreign Gene Affects General Proteome Pattern in Giardia lamblia Trophozoites

Giardia lamblia is an important causative agent of persistent diarrhea in humans, domestic animals, and cattle. Basic research is usually performed with the strain WBC6 and includes genetic manipulations such as transfections. Here, we investigate how transfection with a plasmid causing stable expression of a foreign gene affects the whole proteome pattern. Using shotgun mass spectrometry, we compare the proteomes of untransfected trophozoites to trophozoites transfected with Escherichia coli glucuronidase A (GusA). Besides GusA, which is detected in the transfected trophozoites only, the proteomes of untransfected and transfected trophozoites differ by 132 differentially expressed proteins. In particular, transfection induces antigenic variation. Since transfection causing stable expression affects the proteome pattern, transfection experiments should take into account this effect. Due to a unique peptide panel, GusA is an example for a suitable internal standard for experiments involving transfected cells. Data are available via ProteomeXchange with identifier PXD022565.

Recent advances in functional research in Giardia intestinalis

This review considers current advances in tools to investigate the functional biology of Giardia, it's coding and non-coding genes, features and cellular and molecular biology. We consider major gaps in current knowledge of the parasite and discuss the present state-of-the-art in its in vivo and in vitro cultivation. Advances in in silico tools, including for the modelling non-coding RNAs and genomic elements, as well as detailed exploration of coding genes through inferred homology to model organisms, have provided significant, primary level insight. Improved methods to model the three-dimensional structure of proteins offer new insights into their function, and binding interactions with ligands, other proteins or precursor drugs, and offer substantial opportunities to prioritise proteins for further study and experimentation. These approaches can be supplemented by the growing and highly accessible arsenal of systems-based methods now being applied to Giardia, led by genomic, transcriptomic and proteomic methods, but rapidly incorporating advanced tools for detection of real-time transcription, evaluation of chromatin states and direct measurement of macromolecular complexes. Methods to directly interrogate and perturb gene function have made major leaps in recent years, with CRISPr-interference now available. These approaches, coupled with protein over-expression, fluorescent labelling and in vitro and in vivo imaging, are set to revolutionize the field and herald an exciting time during which the field may finally realise Giardia's long proposed potential as a model parasite and eukaryote.

Evidence of nuclear transport mechanisms in the protozoan parasite Giardia lamblia

Nuclear-cytoplasmic trafficking of proteins is a highly regulated process that modulates multiple biological processes in eukaryotic cells. In Giardia lamblia, shuttling has been described from the cytoplasm to nuclei of proteins during the biological cell cycle of the parasite. This suggests that a mechanism of nucleocytoplasmic transport is present and functional in G. lamblia. By means of computational biology analyses, we found that there are only two genes for nuclear transport in this parasite, named Importin α and Importin β. When these transporters were overexpressed, both localized close to the nuclear envelope, and no change was observed in trophozoite growth rate. However, during the encystation process, both transporters induced an increase in the number of cysts produced. Importazole and Ivermectin, two known specific inhibitors of importins, separately influenced the encysting process by inducing an arrest in the trophozoite stage that prevents the production of cysts. This effect was more noticeable when Ivermectin, an anti-parasitic drug, was used. Finally, we tested whether the enzyme arginine deiminase, which shuttles from the cytoplasm to the nuclei during encystation, was influenced by these transporters. We found that treatment with each of the inhibitors abrogates arginine deiminase nuclear translocation and favors perinuclear localization. This suggests that Importin α and Importin β are key transporters during the encystation process and are involved, at least, in the transport of arginine deiminase into the nuclei. Considering the effect produced by Ivermectin during growth and encystation, we postulate that this drug could be used to treat giardiasis.

Development of CRISPR/Cas9-mediated gene disruption systems in Giardia lamblia

Giardia lamblia becomes dormant by differentiation into a water-resistant cyst that can infect a new host. Synthesis of three cyst wall proteins (CWPs) is the fundamental feature of this differentiation. Myeloid leukemia factor (MLF) proteins are involved in cell differentiation, and tumorigenesis in mammals, but little is known about its role in protozoan parasites. We developed a CRISPR/Cas9 system to understand the role of MLF in Giardia. Due to the tetraploid genome in two nuclei of Giardia, it could be hard to disrupt a gene completely in Giardia. We only generated knockdown but not knockout mutants. We found that knockdown of the mlf gene resulted in a significant decrease of cwp gene expression and cyst formation, suggesting a positive role of MLF in encystation. We further used mlf as a model gene to improve the system. The addition of an inhibitor for NHEJ, Scr7, or combining all cassettes for gRNA and Cas9 expression into one plasmid resulted in improved gene disruption efficiencies and a significant decrease in cwp gene expression. Our results provide insights into a positive role of MLF in inducing Giardia differentiation and a useful tool for studies in Giardia.

Successful Genetic Transfection of the Colonic Protistan Parasite Blastocystis for Reliable Expression of Ectopic Genes

The microbial parasite Blastocystis colonizes the large intestines of numerous animal species and increasing evidence has linked Blastocystis infection to enteric diseases with signs and symptoms including abdominal pain, constipation, diarrhea, nausea, vomiting, and flatulence. It has also recently been reported to be an important member of the host intestinal microbiota. Despite significant advances in our understanding of Blastocystis cell biology and host-parasite interactions, a genetic modification tool is absent. In this study, we successfully established a robust gene delivery protocol for Blastocystis subtype 7 (ST7) and ectopic protein expression was further tested using a high sensitivity nano-luciferase (Nluc) reporter system, with promoter regions from several genes. Among them, a strong promoter encompassing a region upstream of the legumain 5' UTR was identified. Using this promoter combined with the legumain 3' UTR, which contains a conserved, precise polyadenylation signal, a robust transient transfection technique was established for the first time in Blastocystis. This system was validated by ectopic expression of proteins harbouring specific localization signals. The establishment of a robust, reproducible gene modification system for Blastocystis is a significant advance for Blastocystis research both in vitro and in vivo. This technique will spearhead further research to understand the parasite's biology, its role in health and disease, along with novel ways to combat the parasite.

Calcium-binding proteins that are type B″ regulatory subunits of phosphatase 2A in Giardia intestinalis

Giardia intestinalis is a protozoan parasite that colonizes the upper part of the small intestine of its mammalian hosts. The trophozoite, which is the replicative stage, has a complex cytoskeleton that allows it to move and adhere to intestinal cells. It has been proposed that protein phosphatase 2A (PP2A) participates in the regulation of changes to the parasite cytoskeleton during its life cycle. However, how PP2A is involved in this regulation remains unclear since its substrates and regulators have not been characterized. In this work, we report the bioinformatic and experimental analysis of two potential regulatory B″ subunits of PP2A in Giardia, both of which are calcium-binding proteins. In this work, in silico and experimental evidence of the binding of both proteins to calcium is presented; the proteins are shown to interact with the catalytic PP2A subunit in the trophozoite stage, and they exhibit different subcellular localization patterns. Because PP2A is a heterotrimer, homology analysis of the different subunits of PP2A indicates that fewer holoenzyme combinations can be formed in this parasite than in other organisms. Our results suggest that the localization of PP2A may be associated with calcium-dependent signaling through its B″ type regulatory subunits.

RNAi-Mediated Specific Gene Silencing as a Tool for the Discovery of New Drug Targets in Giardia lamblia; Evaluation Using the NADH Oxidase Gene

The microaerophilic protozoan Giardia lamblia is the agent causing giardiasis, an intestinal parasitosis of worldwide distribution. Different pharmacotherapies have been employed against giardiasis; however, side effects in the host and reports of drug resistant strains generate the need to develop new strategies that identify novel biological targets for drug design. To support this requirement, we have designed and evaluated a vector containing a cassette for the synthesis of double-stranded RNA (dsRNA), which can silence expression of a target gene through the RNA interference (RNAi) pathway. Small silencing RNAs were detected and quantified in transformants expressing dsRNA by a stem-loop RT-qPCR approach. The results showed that, in transformants expressing dsRNA of 100-200 base pairs, the level of NADHox mRNA was reduced by around 30%, concomitant with a decrease in enzyme activity and a reduction in the number of trophozoites with respect to the wild type strain, indicating that NADHox is indeed an important enzyme for Giardia viability. These results suggest that it is possible to induce the G. lamblia RNAi machinery for attenuating the expression of genes encoding proteins of interest. We propose that our silencing strategy can be used to identify new potential drug targets, knocking down genes encoding different structural proteins and enzymes from a wide variety of metabolic pathways.

Giardia's primitive GPL biosynthesis pathways with parasitic adaptation 'patches': implications for Giardia's evolutionary history and for finding targets against Giardiasis

Giardia is a worldwide spread protozoan parasite colonizing in small intestines of vertebrates, causing Giardiasis. The controversy about whether it is an extremely primitive eukaryote or just a highly evolved parasite has become a fetter to its uses as a model for both evolutionary and parasitological studies for years. Glycerophospholipid (GPL) synthesis is a conserved essential cellular process, and thus may retain some original features reflecting its evolutionary position, and this process should also have undergone parasitic adaptation to suit Giardia’s dietary lipid-rich environment. Thus, GPL synthesis pathways may be a perfect object to examine the controversy over Giardia. Here, we first clarified Giardia’s previously confusing GPL synthesis by re-identifying a reliable set of GPL synthesis genes/enzymes. Then using phylogenetic and comparative genomic analyses, we revealed that these pathways turn out to be evolutionarily primitive ones, but with many secondary parasitic adaptation ‘patches’ including gene loss, rapid evolution, product relocation, and horizontal gene transfer. Therefore, modern Giardia should be a mosaic of ‘primary primitivity’ and ‘secondary parasitic adaptability’, and to make a distinction between the two categories of features would restart the studies of eukaryotic evolution and parasitic adaptation using Giardia as a model system.

Expression and secretion of the Giardia duodenalis variant surface protein 9B10A by transfected trophozoites causes damage to epithelial cell monolayers mediated by protease activity

Giardia duodenalis is the protozoan parasite responsible for most cases of parasitic diarrhea worldwide. The pathogenic mechanisms of giardiasis have not yet been fully characterized. In this context parasite's excretory/secretory products have been related to the damage induced by the parasite on enterocytes. Among these is the Variable Surface Proteins (VSPs) family involved in antigenic variation and in the induction of protective response. In proteomic analyses carried out to identify the proteases with high molecular weight secreted by Giardia trophozoites during the initial phase of interaction with IEC-6 cell monolayers we identified the VSP9B10A protein. In silico bioinformatics analyses predicted a central region in residues 324-684 displaying the catalytic triad and the substrate binding pocket of cysteine proteases. The analysis of the effect of the VSP9B10A protein on epithelial cell monolayers using trophozoites that were transfected with a plasmid carrying the vsp9b10a gene sequence under the control of a constitutive promoter showed that transfected trophozoites expressing the VSP9B10A protein caused cytotoxic damages on IEC-6 and MDCK cell monolayers. This was characterized by loss of cell-cell contacts and cell detachment from the substrate while no damage was observed with trophozoites that did not express the VSP9B10A protein. The same cytotoxic effect was detected when IEC-6 cell monolayers were incubated only with supernatants from co-cultures of IEC-6 cell monolayers with VSP9B10A transfected trophozoites and this effect was not observed when transfected trophozoites were incubated with a monospecific polyclonal antibody anti-VSP9B10A previous to interaction with IEC-6 monolayers. These results demonstrate that the VSP9B10A protein secreted upon interaction with epithelial cells caused damage in these cells. Thus this protein might be considered as a conditional virulence factor candidate. To our knowledge this is the first report on the proteolytic activity from a Giardia VSP opening new research lines on these proteins.

Protein SUMOylation is Involved in Cell-cycle Progression and Cell Morphology in Giardia lamblia

The unicellular protozoa Giardia lamblia is a food- and waterborne parasite that causes giardiasis. This illness is manifested as acute and self-limited diarrhea and can evolve to long-term complications. Successful establishment of infection by Giardia trophozoites requires adhesion to host cells and colonization of the small intestine, where parasites multiply by mitotic division. The tight binding of trophozoites to host cells occurs by means of the ventral adhesive disc, a spiral array of microtubules and associated proteins such as giardins. In this work we show that knock down of the Small Ubiquitin-like MOdifier (SUMO) results in less adhesive trophzoites, decreased cell proliferation and deep morphological alterations, including at the ventral disc. Consistent with the reduced proliferation, SUMO knocked-down trophozoites were arrested in G1 and in S phases of the cell cycle. Mass spectrometry analysis of anti-SUMO immunoprecipitates was performed to identify SUMO substrates possibly involved in these events. Among the identified SUMOylation targets, α-tubulin was further validated by Western blot and confirmed to be a SUMO target in Giardia trophozoites.

Cyst-Wall-Protein-1 is fundamental for Golgi-like organelle neogenesis and cyst-wall biosynthesis in Giardia lamblia

The genome of the protozoan parasite Giardia lamblia is organized in two diploid nuclei, which has so far precluded complete analysis of gene function. Here we use a previously developed Cre/loxP-based knock-out and selection marker salvage strategy in the human-derived isolate WB-C6 to eliminate all four copies of the Cyst-Wall-Protein-1 locus (CWP1). Because these loci are silenced in proliferating trophozoites and highly expressed only in encysting cells, CWP1 ablation allows functional characterization of a conditional phenotype in parasites induced to encyst. We show that encysting Δcwp1 cells are unable to establish the stage-regulated trafficking machinery with Golgi-like encystation-specific vesicles required for cyst-wall formation but show morphological hallmarks of cyst development and karyokinesis. This ‘pseudocyst' phenotype is rescued by transfection of Δcwp1 cells with an episomally maintained CWP1 expression vector. Genome editing in genera Giardia and Trypanosoma are the only reported examples addressing questions on pathogen transmission within the Excavata supergroup.

Giardia lamblia is a human protozoan parasite with two diploid nuclei, which makes complete knock-out of a gene of interest challenging. Here the authors use a Cre/loxP-based approach to knock-out cyst-wall protein 1 (cwp1) and show that CWP1 is essential for cyst-wall biosynthesis.

The giardial ENTH protein participates in lysosomal protein trafficking and endocytosis

In the protozoa parasite Giardia lamblia, endocytosis and lysosomal protein trafficking are vital parasite-specific processes that involve the action of the adaptor complexes AP-1 and AP-2 and clathrin. In this work, we have identified a single gene in Giardia encoding a protein containing an ENTH domain that defines monomeric adaptor proteins of the epsin family. This domain is present in the epsin or epsin-related (epsinR) adaptor proteins, which are implicated in endocytosis and Golgi-to-endosome protein trafficking, respectively, in other eukaryotic cells. We found that GlENTHp (for G. lamblia ENTH protein) localized in the cytosol, strongly interacted with PI3,4,5P3, was associated with the alpha subunit of AP-2, clathrin and ubiquitin and was involved in receptor-mediated endocytosis. It also bonded PI4P, the gamma subunit of AP-1 and was implicated in ER-to-PV trafficking. Alteration of the GlENTHp function severely affected trophozoite growth showing an unusual accumulation of dense material in the lysosome-like peripheral vacuoles (PVs), indicating that GlENTHp might be implicated in the maintenance of PV homeostasis. In this study, we showed evidence suggesting that GlENTHp might function as a monomeric adaptor protein supporting the findings of other group indicating that GlENTHp might be placed at the beginning of the ENTH family.

Identification of Giardia lamblia DHHC proteins and the role of protein S-palmitoylation in the encystation process

Protein S-palmitoylation, a hydrophobic post-translational modification, is performed by protein acyltransferases that have a common DHHC Cys-rich domain (DHHC proteins), and provides a regulatory switch for protein membrane association. In this work, we analyzed the presence of DHHC proteins in the protozoa parasite Giardia lamblia and the function of the reversible S-palmitoylation of proteins during parasite differentiation into cyst. Two specific events were observed: encysting cells displayed a larger amount of palmitoylated proteins, and parasites treated with palmitoylation inhibitors produced a reduced number of mature cysts. With bioinformatics tools, we found nine DHHC proteins, potential protein acyltransferases, in the Giardia proteome. These proteins displayed a conserved structure when compared to different organisms and are distributed in different monophyletic clades. Although all Giardia DHHC proteins were found to be present in trophozoites and encysting cells, these proteins showed a different intracellular localization in trophozoites and seemed to be differently involved in the encystation process when they were overexpressed. dhhc transgenic parasites showed a different pattern of cyst wall protein expression and yielded different amounts of mature cysts when they were induced to encyst. Our findings disclosed some important issues regarding the role of DHHC proteins and palmitoylation during Giardia encystation.

Giardiasis is a major cause of non-viral/non-bacterial diarrheal disease worldwide and has been included within the WHO Neglected Disease Initiative since 2004. Infection begins with the ingestion of Giardia lamblia in cyst form, which, after exposure to gastric acid in the host stomach and proteases in the duodenum, gives rise to trophozoites. The inverse process is called encystation and begins when the trophozoites migrate to the lower part of the small intestine where they receive signals that trigger synthesis of the components of the cyst wall. The cyst form enables the parasite to survive in the environment, infect a new host and evade the immune response. In this work, we explored the role of protein S-palmitoylation, a unique reversible post-translational modification, during Giardia encystation, because de novo generation of endomembrane compartments, protein sorting and vesicle fusion occur in this process. Our findings may contribute to the design of therapeutic agents against this important human pathogen.

SUMOylation and deimination of proteins: two epigenetic modifications involved in Giardia encystation

SUMOylation, a posttranslational modification of proteins, has been recently described as vital in eukaryotic cells. In a previous work, we analyzed the role of SUMO protein and the genes encoding the putative enzymes of the SUMOylation pathway in the parasite Giardia lamblia. Although we observed several SUMOylated proteins, only the enzyme Arginine Deiminase (ADI) was confirmed as a SUMOylated substrate. ADI is involved in the survival of the parasite and, besides its role in ATP production, it also catalyzes the modification of arginine residues to citrulline in the cytoplasmic tail of surface proteins. During encystation, however, ADI translocates to the nuclei and downregulates the expression of the Cyst Wall Protein 2 (CWP2). In this work, we made site-specific mutation of the ADI SUMOylation site (Lys101) and observed that transgenic trophozoites did not translocate to the nuclei at the first steps of encystation but shuttled in the nuclei late during this process through classic nuclear localization signals. Inside the nuclei, ADI acts as a peptidyl arginine deiminase, being probably involved in the downregulation of CWPs expression and cyst wall formation. Our results strongly indicate that ADI plays a regulatory role during encystation in which posttranslational modifications of proteins are key players.

The giardial VPS35 retromer subunit is necessary for multimeric complex assembly and interaction with the vacuolar protein sorting receptor

The retromer is a pentameric protein complex that mediates the retrograde transport of acid hydrolase receptors between endosomes and the trans-Golgi network and is conserved across all eukaryotes. Unlike other eukaryotes, the endomembrane system of Giardia trophozoite is simple and is composed only of the endoplasmic reticulum and peripheral vesicles (PVs), which may represent an ancient organellar system converging compartments such as early and late endosomes and lysosomes. Sorting and trafficking of membrane proteins and soluble hydrolases from the endoplasmic reticulum to the PVs have been described as specific and conserved but whether the giardial retromer participates in receptor recycling remains elusive. Homologs of the retromer Vacuolar Protein Sorting (Vps35p, Vps26p, and Vps29p) have been identified in this parasite. Cloning the GlVPS35 subunit and antisera production enabled the localization of this protein in the PVs as well as in the cytosol. Tagged expression of the subunits was used to demonstrate their association with membranes, and immunofluorescence confocal laser scanning revealed high degrees of colabeling between the retromer subunits and also with the endoplasmic reticulum and PV compartment markers. Protein-protein interaction data revealed interaction between the subunits of GlVPS35 and the cytosolic domain of the hydrolase receptor GlVps. Altogether our data provide original information on the molecular interactions that mediate assembly of the cargo-selective retromer subcomplex and its involvement in the recycling of the acid hydrolase receptor in this parasite.

Vacuolar protein sorting receptor in Giardia lamblia

In Giardia, lysosome-like peripheral vacuoles (PVs) need to specifically coordinate their endosomal and lysosomal functions to be able to successfully perform endocytosis, protein degradation and protein delivery, but how cargo, ligands and molecular components generate specific routes to the PVs remains poorly understood. Recently, we found that delivering membrane Cathepsin C and the soluble acid phosphatase (AcPh) to the PVs is adaptin (AP1)-dependent. However, the receptor that links AcPh and AP1 was never described. We have studied protein-binding to AcPh by using H6-tagged AcPh, and found that a membrane protein interacted with AcPh. This protein, named GlVps (for Giardia lamblia Vacuolar protein sorting), mainly localized to the ER-nuclear envelope and in some PVs, probably functioning as the sorting receptor for AcPh. The tyrosine-binding motif found in the C-terminal cytoplasmic tail domain of GlVps was essential for its exit from the endoplasmic reticulum and transport to the vacuoles, with this motif being necessary for the interaction with the medium subunit of AP1. Thus, the mechanism by which soluble proteins, such as AcPh, reach the peripheral vacuoles in Giardia appears to be very similar to the mechanism of lysosomal protein-sorting in more evolved eukaryotic cells.

SUMOylation in Giardia lamblia: A Conserved Post-Translational Modification in One of the Earliest Divergent Eukaryotes

Post-translational modifications are able to regulate protein function and cellular processes in a rapid and reversible way. SUMOylation, the post-translational modification of proteins by the addition of SUMO, is a highly conserved process that seems to be present in modern cells. However, the mechanism of protein SUMOylation in earlier divergent eukaryotes, such as Giardia lamblia, is only starting to become apparent. In this work, we report the presence of a single SUMO gene encoding to SUMO protein in Giardia. Monoclonal antibodies against recombinant Giardia SUMO protein revealed the cytoplasmic localization of native SUMO in wild-type trophozoites. Moreover, the over-expression of SUMO protein showed a mainly cytoplasmic localization, though also neighboring the plasma membrane, flagella, and around and even inside the nuclei. Western blot assays revealed a number of SUMOylated proteins in a range between 20 and 120 kDa. The genes corresponding to putative enzymes involved in the SUMOylation pathway were also explored. Our results as a whole suggest that SUMOylation is a process conserved in the eukaryotic lineage, and that its study is significant for understanding the biology of this interesting parasite and the role of post-translational modification in its evolution.

The ubiquitin-activating enzyme (E1) of the early-branching eukaryote Giardia intestinalis shows unusual proteolytic modifications and play important roles during encystation

Giardia intestinalis is considered an early-branching eukaryote and is therefore a valuable model for studying primordial cellular processes. This work reports the characterization of the ubiquitin-activating enzyme (E1) during growth and different stages of trophozoite differentiation into cysts. We found that in Giardia E1 expression (both at mRNA and protein levels) is regulated during encystation. The enzyme is proteolytically processed mainly into two fragments of 68kDa (N-terminal) and 47kDa (C-terminal). This phenomenon has not been described for any other E1. In trophozoites, this enzyme localized at spots within the cytoplasm as detected by using polyclonal antibodies against either E1 N- or C-terminal fragments. This pattern changed during encystation into a diffuse localization throughout the cytoplasm of encysting cells. E1 localizes in mature cysts at cytoplasmic spots and in the cyst wall. Our antisense silencing experiments suggested that E1 is an essential gene for parasite viability. On the other hand, E1 over-expression greatly increased the encystation rate, indicating a relationship between E1 and Giardia differentiation.

Establishment of a transient transfection system and expression of firefly luciferase in Entamoeba invadens

Entamoeba invadens is used as a model system to study trophozoite to cyst differentiation since Entamoeba histolytica, the causative agent of amoebiasis cannot encyst in culture. However, a system for introduction of cloned genes in E. invadens is not available. Here we report an electroporation-based method for transfection of E. invadens tophozoites and demonstrate the expression of firefly luciferase reporter gene driven from the E. invadens ribosomal protein L3 promoter. The efficiency of luciferase expression driven from the promoters of three different E. invadens genes (rpl3, rps10 and h2b) was tested and found to correlate with the in vivo expression levels of the respective gene. This system will permit the analysis of regulatory elements required for gene expression in E. invadens.

Giardia lamblia low-density lipoprotein receptor-related protein is involved in selective lipoprotein endocytosis and parasite replication

As Giardia lamblia is unable to synthesize cholesterol de novo, this steroid might be obtained from the host's intestinal milieu by endocytosis of lipoproteins. In this work, we identified a putative Giardia lamblia low-density lipoprotein receptor-related proteins (GlLRP), a type I membrane protein, which shares the substrate N-terminal binding domain and a FXNPXY-type endocytic motif with human LRPs. Expression of tagged GlLRP showed that it was localized predominantly in the endoplasmic reticulum, lysosomal-like peripheral vacuoles and plasma membrane. However, the FXNPXY-deleted GlLRP was retained at the plasma membrane suggesting that it is abnormally transported and processed. The low-density lipoprotein and chylomicrons interacted with GlLRP, with this interaction being necessary for lipoprotein internalization and cell proliferation. Finally, we show that GlLRP binds directly to the medium subunit of Giardia adaptor protein 2, indicating that receptor-mediated internalization occurs through an adaptin mechanism.

Characterization of SNAREs determines the absence of a typical Golgi apparatus in the ancient eukaryote Giardia lamblia

Giardia is a eukaryotic protozoal parasite with unusual characteristics, such as the absence of a morphologically evident Golgi apparatus. Although both constitutive and regulated pathways for protein secretion are evident in Giardia, little is known about the mechanisms involved in vesicular docking and fusion. In higher eukaryotes, soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) of the vesicle-associated membrane protein and syntaxin families play essential roles in these processes. In this work we identified and characterized genes for 17 SNAREs in Giardia to define the minimal set of subcellular organelles present during growth and encystation, in particular the presence or not of a Golgi apparatus. Expression and localization of all Giardia SNAREs demonstrate their presence in distinct subcellular compartments, which may represent the extent of the endomembrane system in eukaryotes. Remarkably, Giardia SNAREs, homologous to Golgi SNAREs from other organisms, do not allow the detection of a typical Golgi apparatus in either proliferating or differentiating trophozoites. However, some features of the Golgi, such as the packaging and sorting function, seem to be performed by the endoplasmic reticulum and/or the nuclear envelope. Moreover, depletion of individual genes demonstrated that several SNAREs are essential for viability, whereas others are dispensable. Thus, Giardia requires a smaller number of SNAREs compared with other eukaryotes to accomplish all of the vesicle trafficking events that are critical for the growth and differentiation of this important human pathogen.

Arginine deiminase has multiple regulatory roles in the biology of Giardia lamblia

The protozoan parasite Giardia lamblia uses arginine deiminase (ADI) to produce energy from free L-arginine under anaerobic conditions. In this work, we demonstrate that, in addition to its known role as a metabolic enzyme, it also functions as a peptidylarginine deiminase, converting protein-bound arginine into citrulline. G. lamblia ADI specifically binds to and citrullinates the arginine in the conserved CRGKA tail of variant-specific surface proteins (VSPs), affecting both antigenic switching and antibody-mediated cell death. During encystation, ADI translocates from the cytoplasm to the nuclei and appears to play a regulatory role in the expression of encystation-specific genes. ADI is also sumoylated, which might modulate its activity. Our findings reveal a dual role played by ADI and define novel regulatory pathways used by Giardia for survival.

Giardia lamblia aurora kinase: a regulator of mitosis in a binucleate parasite

Giardia lamblia is a major cause of diarrhoeal disease worldwide. Since it has no known toxin, the ability of trophozoites to colonise the human small intestine is required for its pathogenesis. Mitosis in this protozoan parasite is a unique challenge because its two equivalent nuclei and complex cytoskeleton must be duplicated and segregated accurately. Giardial mitosis is a complex and rapid event that is poorly understood at the cellular and molecular levels. Higher eukaryotes have one to three members of the highly conserved Ser/Thr aurora kinase (AK) family that regulate key aspects of mitosis and cytokinesis. Giardia has a single AK orthologue (gAK) with 61% similarity to human AK A. In addition to the conserved active site residues, activation loop and destruction-box motifs characteristic of AKs, gAK contains a unique insert near the active site region. We epitope-tagged gAK at its C-terminus and expressed it under its own promoter. During interphase, gAK localises exclusively to the nuclei, but is not phosphorylated as shown by lack of staining with an antibody specific to phosphorylated AK A (pAK). In contrast, during mitosis pAK localises to the basal bodies/centrosomes and co-localises with tubulin to the spindle. During specific stages of mitosis, giardial pAK also localised dynamically to cytoskeletal structures unique to Giardia: the paraflagellar dense rods of the anterior flagella and the median body, whose functions are unknown, as well as to the parent attachment disc. Two AK inhibitors significantly decreased giardial growth and increased the numbers of cells arrested in cytokinesis. These inhibitors appeared to increase microtubule nucleation and cell-ploidy. Our data show that gAK is phosphorylated in mitosis and suggest that it plays an important role in the Giardia cell cycle. The pleiotropic localisation of AK suggests that it may co-ordinate the reorganisation and segregation of tubulin-containing structures in mitosis. We believe this is the first report of a signalling protein regulating cell division in Giardia.

Core histone genes of Giardia intestinalis: genomic organization, promoter structure, and expression

Background

Giardia intestinalis is a protist found in freshwaters worldwide, and is the most common cause of parasitic diarrhea in humans. The phylogenetic position of this parasite is still much debated. Histones are small, highly conserved proteins that associate tightly with DNA to form chromatin within the nucleus. There are two classes of core histone genes in higher eukaryotes: DNA replication-independent histones and DNA replication-dependent ones.

Results

We identified two copies each of the core histone H2a, H2b and H3 genes, and three copies of the H4 gene, at separate locations on chromosomes 3, 4 and 5 within the genome of Giardia intestinalis, but no gene encoding a H1 linker histone could be recognized. The copies of each gene share extensive DNA sequence identities throughout their coding and 5' noncoding regions, which suggests these copies have arisen from relatively recent gene duplications or gene conversions. The transcription start sites are at triplet A sequences 1–27 nucleotides upstream of the translation start codon for each gene. We determined that a 50 bp region upstream from the start of the histone H4 coding region is the minimal promoter, and a highly conserved 15 bp sequence called the histone motif (him) is essential for its activity. The Giardia core histone genes are constitutively expressed at approximately equivalent levels and their mRNAs are polyadenylated. Competition gel-shift experiments suggest that a factor within the protein complex that binds him may also be a part of the protein complexes that bind other promoter elements described previously in Giardia.

Conclusion

In contrast to other eukaryotes, the Giardia genome has only a single class of core histone genes that encode replication-independent histones. Our inability to locate a gene encoding the linker histone H1 leads us to speculate that the H1 protein may not be required for the compaction of Giardia's small and gene-rich genome.

Bidirectional transcription is an inherent feature of Giardia lamblia promoters and contributes to an abundance of sterile antisense transcripts throughout the genome

A prominent feature of transcription in Giardia lamblia is the abundant production of sterile antisense transcripts (Elmendorf et al. The abundance of sterile transcripts in Giardia lamblia. Nucleic Acids., 29, 4674-4683). Here, we use a computational biology analysis of SAGE data to assess the abundance and distribution of sense and antisense messages in the parasite genome. Sterile antisense transcripts are produced at approximately 50% of loci with detectable transcription, yet their abundance at a given locus does not correlate to the abundance of the complementary sense transcripts at that locus or to transcription levels at neighboring loci. These data suggest that sterile antisense transcripts are not simply a local effect of open chromatin structure. Using 5'RACE, we demonstrate that Giardia promoters are a source of antisense transcripts through bidirectional transcription, producing both downstream coding sense and upstream sterile antisense transcripts. We use a dual reporter system to explore roles of specific promoter elements in this bidirectional initiation of transcription and suggest that the degenerate AT-rich nature of TATA and Inr elements in Giardia permits them to function interchangeably. The phenomenon of bidirectional transcription in G. lamblia gives us insight into the interaction between transcriptional machinery and promoter elements, and may be the prominent source of the abundant antisense transcription in this parasite.

Molecular tools for analysis of gene function in parasitic microorganisms

With the completion of several genome sequences for parasitic protozoa, research in molecular parasitology entered the "post-genomic" era. Accompanied by global transcriptome and proteome analysis, huge datasets have been generated that have added many novel candidates to the list of drug and vaccine targets. The challenge is now to validate these factors and to bring science back to the bench to perform a detailed characterization. In some parasites, like Trypanosoma brucei, high-throughput genetic screens have been established using RNA interference [for a detailed review, see Motyka and Englund (2004)]. In most protozoan parasites, however, more time-consuming approaches have to be employed to identify and characterize the function of promising candidates in detail. This review aims to summarize the status of molecular genetic tools available for a variety of protozoan pathogens and discuss how they can be implemented to advance our understanding of parasite biology.

A novel palmitoyl acyl transferase controls surface protein palmitoylation and cytotoxicity inGiardia lamblia

The intestinal protozoan parasite Giardia lamblia undergoes surface antigenic variation whereby one of a family of structurally related variant-specific surface proteins (VSPs) is replaced in a regulated process by another antigenically distinct VSP. All VSPs are type I membrane proteins that have a conserved hydrophobic sequence terminated by the invariant hydrophilic amino acids, CRGKA. Using transfected Giardia constitutively expressing HA-tagged VSPH7 and incubated with radioactive [3H]palmitate, we demonstrate that the palmitate is attached to the Cys in the conserved CRGKA tail. Surface location of mutant VSPs lacking either the CRGKA tail or its Cys is identical to that of wild-type VSPH7 but non-palmitoylated mutants fail to undergo complement-independent antibody specific cytotoxicity. In addition, membrane localization of non-palmitoylated mutant VSPH7 changes from a pattern similar to rafts to non-rafts. Palmitoyl transferases (PAT), responsible for protein palmitoylation in other organisms, often possess a cysteine-rich domain containing a conserved DHHC motif (DHHC-CRD). An open reading frame corresponding to a putative 50 kDa Giardia PAT (gPAT) containing a DHHC-CRD motif was found in the Giardia genome database. Expression of epitope-tagged gPAT using a tetracycline inducible vector localized gPAT to the plasma membrane, a pattern similar to that of VSPs. Transfection with gPAT antisense producing vectors inhibits gPAT expression and palmitoylation of VSPs in vitro confirming the function of gPAT. These results show that VSPs are palmitoylated at the cysteine within the conserved tail by gPAT and indicate an essential function of palmitoylation in control of VSP-mediated signalling and processing.

Giardia lamblia: stable expression of green fluorescent protein mediated by giardiavirus

Giardia lamblia, an early diverging eukaryote that infects several species including humans and a major agent of water-borne diarrhea throughout the world, can be infected with a double-stranded RNA virus, giardiavirus (GLV). A chimeric GLV cDNA and green fluorescent protein (GFP) according to the cis-acting signals of the GLV genome required for expression of foreign gene was constructed and its in vitro transcript was electroporated into GLV-infected G. lamblia trophozoites, GFP was expressed transiently. pGDH5/NEO/GLV was constructed by combining the neomycin resistance cassette in which the neomycin phosphotransferase gene was flanked by Giardia glutamate dehydrogenase (GDH) uncoding regions and the transcription cassette in which the chimera of GLV cDNA and GFP was located downstream from GDH gene promoter on a single plasmid. This plasmid was electroporated into G. lamblia and the transfectants persistently expressed GFP under G418 selection. This stable transfection system should provide a valuable tool for genetic study of G. lamblia.

Adaptor protein complex 1 mediates the transport of lysosomal proteins from a Golgi-like organelle to peripheral vacuoles in the primitive eukaryote Giardia lamblia

Giardia lamblia is an early branching protist that possesses peripheral vacuoles (PVs) with characteristics of lysosome-like organelles, located underneath the plasma membrane. In more evolved cells, lysosomal protein trafficking is achieved by cargo recognition involving adaptor protein (AP) complexes that recognize specific amino acid sequences (tyrosine and/or dileucine motifs) within the cytoplasmic tail of membrane proteins. Previously, we reported that Giardia has a tyrosine-based sorting system, which mediates the targeting of a membrane-associated cysteine protease (encystation-specific cysteine protease, ESCP) to the PVs. Here, we show that Giardia AP1 mediates the transport of ESCP and the soluble acid phosphatase (AcPh) to the PVs. By using the yeast two-hybrid assay we found that the ESCP tyrosine-based motif interacts specifically with the medium subunit of AP1 (Gimicroa). Hemagglutinin-tagged Gimicroa colocalizes with ESCP and AcPh and coimmunoprecipitates with clathrin, suggesting that protein trafficking toward the PVs is clathrin-adaptin dependent. Targeted disruption of Gimicroa results in mislocalization of ESCP and AcPh but not of variant-specific surface proteins. Our results suggest that, unlike mammalian cells, only AP1 is involved in anterograde protein trafficking to the PVs in Giardia. Moreover, even though Giardia trophozoites lack a morphologically discernible Golgi apparatus, the presence of a clathrin-adaptor system suggests that this parasite possess a primitive secretory organelle capable of sorting proteins similar to that of more evolved cells.

Encystation-specific regulation of the cyst wall protein 2 gene in Giardia lamblia by multiple cis-acting elements

Giardia lamblia, a worldwide cause of diarrhoea, must differentiate into environmentally resistant cysts for dissemination and completion of its life cycle. Although G. lamblia is an early diverging eukaryote, encystation involves many complex cellular changes including formation of the cyst wall that contains at least two cyst wall proteins, cyst wall proteins 1 and 2. Cwp genes are transcribed only during encystation. In this study, we examine the regulatory elements for the encystation-specific gene cwp2. The 64 bp immediately upstream of the cwp2 open reading frame (-64 to -1 relative to ATG) was shown to be sufficient for the encystation-specific expression of luciferase. To determine which region(s) within this 64 bp contributed to encystation-specific expression in vivo, a series of deletions were cloned into a Giardia luciferase expression vector and their ability to control encystation-specific expression of luciferase was assessed. Deletion of elements in the -64 to -23 region of the cwp2 promoter significantly increased expression of luciferase in vegetative trophozoites, suggesting that this area contains a negative cis-acting element. Deletions of elements from -23 to -10 led to decreased expression in encysting cells, suggesting that this region may contain positive cis-acting elements. When the A/T-rich initiator was deleted but the cis-acting elements (-64 to -10) were retained, encystation-specific expression of luciferase was maintained but an aberrant transcriptional start site was utilised. These results indicate that Giardia has developed a classic repressor mechanism(s) that allows tight, encystation-specific control by the cwp2 promoter.

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.

Genetic manipulation of Giardia lamblia

Giardia lamblia is a flagellated protozoan that infects several species including humans and is a major agent of waterborne outbreaks of diarrhea. G. lamblia is also important in the study of basic eukaryotic molecular biology and evolution; however, it has been difficult to employ standard genetic methods in the study of Giardia. Over the past 6 years, two transfection systems were developed and used for the genetic manipulation of G. lamblia. Both systems allow transient or stable transfection of Giardia and/or foreign genes. The DNA-based transfection system allows electroporation of circular or linear plasmid DNA into trophozoites. The RNA virus-based transfection system requires electroporation of in vitro transcribed RNA into GLV-infected trophozoites. Because G. lamblia is one of the most rudimentary eukaryotes, its processes of transcription, translation and protein transport, as well as its metabolic and biochemical pathways, are of interest. Study of these areas will continue to be advanced using transfection in combination with cellular and molecular tools. Several groups have combined these technologies with other techniques to study protein transport and the transcriptional and post-transcriptional regulation of Giardia genes, including encystation-specific and variant surface protein genes. In addition, coupling antisense techniques with transfection has permitted functional knockout of Giardia metabolic genes, allowing Giardia metabolic pathways to be studied. In the near future, both transfection systems will be potent tools in our investigations of the perplexing questions in Giardia biology.

The activity of a developmentally regulated cysteine proteinase is required for cyst wall formation in the primitive eukaryote Giardia lamblia

Giardia is an intestinal parasite that belongs to the earliest diverging branch of the eukaryotic lineage of descent. Giardia undergoes adaptation for survival outside the host's intestine by differentiating into infective cysts. Encystation involves the synthesis and transport of cyst wall constituents to the plasma membrane for release and extracellular organization. Nevertheless, little is known about the molecular events related to cyst wall biogenesis in Giardia. Among the components of the cyst wall there are two proteins that we have previously identified and characterized: CWP1 (26 kDa) and CWP2 (39 kDa). Expression of these proteins is coordinately induced, and both concentrated within encystation-specific secretory vesicles before their extracellular polymerization. Although highly similar to each other at the amino terminus, CWP2 includes a COOH-terminal 121-amino acid extension. Here, we show that this extension, rich in basic residues, is cleaved from CWP2 before cyst wall formation by an intracellular cysteine proteinase activity, which is induced during encystation like CWPs. Specific inhibitors prevent release of cyst wall materials, abolishing cyst wall formation. We also report the purification, cloning, and characterization of the encystation-specific cysteine proteinase responsible for the proteolytic processing of CWP2, which is homologue to lysosomal cathepsin C. Encystation-specific cysteine proteinase ESCP possesses unique characteristics compared with cathepsins from higher eukaryotes, such as a transmembrane domain and a short cytoplasmic tail. These features make this enzyme the most divergent cathepsin C identified to date and provide new insights regarding cyst wall formation in Giardia.