Development of a tetracycline controlled gene expression system in the parasitic protozoan Giardia lamblia

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.

Implementation of a tunable t-CRISPRi system for gene regulation in Giardia duodenalis

Giardia duodenalis, is a binuclear and microaerophilic protozoan that causes giardiasis. Up to date, several molecular approaches have been taken to understand the molecular mechanisms of diverse cellular processes in this parasitic protozoan. However, the role of many genes involved in these processes needs further analysis. The CRISPR interference (CRISPRi) system has been widely used, as a constitutive expression system for gene silencing purposes in several parasites, including Giardia. The aim of this work was to implement a tunable t-CRISPRi system in Giardia to silence abundant, moderately and low expressed genes, by constructing an optimized and inducible plasmid for the expression of both gRNA and dCas9. A doxycycline inducible pRan promoter was used to express dCas9 and each gRNA, consistently dCas9 expression and nuclear localization were confirmed by Western-blot and immunofluorescence in transfected trophozoites. The transcriptional repression was performed on α-tubulin (high expression), giardipain-1 (moderate expression) and Sir2 and Sir4 (low expression) genes. The α-tubulin gene knock-down caused by dCas9 doxycycline-induction was confirmed by a decrease in its protein expression which was of 50% and 60% at 24 and 48 h, respectively. This induced morphological alterations in flagella. The giardipain-1 knock down, showed a decrease in protein expression of 40 and 50% at 12 and 24 h, respectively, without affecting trophozoites viability, consistent with this a zymogram analysis on giardipain-1 knock down revealed a decrease in giardipain-1 protease activity. When repressing sirtuins expression, a total repression was obtained but trophozoites viability was compromised. This approach provides a molecular tool for a tailored repression to produce specific gene knockdowns.

Sirtuin GdSir2.4 participates in the regulation of rRNA transcription in the Giardia duodenalis parasite

Giardia duodenalis is a parasite of great medical interest due to the number of infections it causes worldwide each year. Although research on epigenetic mechanisms in this protist has only begun recently, epigenetic regulation has already been shown to have important roles in encystation, antigenic variation, and resistance to antibiotics in Giardia. In this work, we show that a Giardia ortholog of Sir2, GdSir2.4, is involved in the silencing of rRNA expression. Our results demonstrate that GdSir2.4 localizes to the nucleolus, and its binding to the intergenic spacer region of the rDNA is associated with the deacetylation of the chromatin in this region. Given the importance of the regulation of rRNA expression to maintain adequate levels of ribosomes and genomic stability within the cells, GdSir2.4 can be considered a target to create new therapeutic agents against this parasite.

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.

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.

Characterization of microtubule-binding and dimerization activity of Giardia lamblia end-binding 1 protein

End-binding 1 (EB1) proteins are evolutionarily conserved components of microtubule (MT) plus-end tracking protein that regulate MT dynamics. Giardia lamblia, with two nuclei and cytoskeletal structures, requires accurate MT distribution for division. In this study, we show that a single EB1 homolog gene of G. lamblia regulates MT dynamics in mitosis. The haemagglutinin-tagged G. lamblia EB1 (GlEB1) localizes to the nuclear envelopes and median bodies, and is transiently present in mitotic spindles of dividing cells. Knockdown of GlEB1 expression using the morpholinos-based anti-EB1 oligonucleotides, resulted in a significant defect in mitosis of Giardia trophozoites. The MT-binding assays using recombinant GlEB1 (rGlEB1) proteins demonstrated that rGlEB1_102–238, but not rGlEB1_1–184, maintains an MT-binding ability comparable with that of the full length protein, rGlEB1_1–238. Size exclusion chromatography showed that rGlEB1 is present as a dimer formed by its C-terminal domain and a disulfide bond. In vitro-mutagenesis of GlEB1 indicated that an intermolecular disulfide bond is made between cysteine #13 of the two monomers. Complementation assay using the BIM1 knockout mutant yeast, the yeast homolog of mammalian EB1, indicated that expression of the C13S mutant GlEB1 protein cannot rescue the mitotic defect of the BIM1 mutant yeast. These results suggest that dimerization of GlEB1 via the 13th cysteine residues plays a role during mitosis in Giardia.

The Cre/loxP system in Giardia lamblia: genetic manipulations in a binucleate tetraploid protozoan

The bacteriophage-derived Cre/loxP system is a valuable tool that has revolutionised genetic and cell biological research in many organisms. We implemented this system in the intestinal parasite Giardia lamblia, an evolutionarily diverged protozoan whose binucleate and tetraploid genome organisation severely limits the application of reverse genetic approaches. We show that Cre-recombinase is functionally expressed in G. lamblia and demonstrate "recycling" of selectable markers. Providing the means for more complex and versatile genetic modifications, this technique massively increases the scope of functional investigations in G. lamblia and other protozoa with similar limitations with respect to genetic manipulation.

Identification of obscure yet conserved actin-associated proteins in Giardia lamblia

Consistent with its proposed status as an early branching eukaryote, Giardia has the most divergent actin of any eukaryote and lacks core actin regulators. Although conserved actin-binding proteins are missing from Giardia, its actin is utilized similarly to that of other eukaryotes and functions in core cellular processes such as cellular organization, endocytosis, and cytokinesis. We set out to identify actin-binding proteins in Giardia using affinity purification coupled with mass spectroscopy (multidimensional protein identification technology [MudPIT]) and have identified >80 putative actin-binding proteins. Several of these have homology to conserved proteins known to complex with actin for functions in the nucleus and flagella. We validated localization and interaction for seven of these proteins, including 14-3-3, a known cytoskeletal regulator with a controversial relationship to actin. Our results indicate that although Giardia lacks canonical actin-binding proteins, there is a conserved set of actin-interacting proteins that are evolutionarily indispensable and perhaps represent some of the earliest functions of the actin cytoskeleton.

Plasmid vectors for proteomic analyses in Giardia: purification of virulence factors and analysis of the proteasome

In recent years, proteomics has come of age with the development of efficient tools for purification, identification, and characterization of gene products predicted by genome projects. The intestinal protozoan Giardia intestinalis can be transfected, but there is only a limited set of vectors available, and most of them are not user friendly. This work delineates the construction of a suite of cassette-based expression vectors for use in Giardia. Expression is provided by the strong constitutive ornithine carbamoyltransferase (OCT) promoter, and tagging is possible in both N- and C-terminal configurations. Taken together, the vectors are capable of providing protein localization and production of recombinant proteins, followed by efficient purification by a novel affinity tag combination, streptavidin binding peptide-glutathione S-transferase (SBP-GST). The option of removing the tags from purified proteins was provided by the inclusion of a PreScission protease site. The efficiency and feasibility of producing and purifying endogenous recombinant Giardia proteins with the developed vectors was demonstrated by the purification of active recombinant arginine deiminase (ADI) and OCT from stably transfected trophozoites. Moreover, we describe the tagging, purification by StrepTactin affinity chromatography, and compositional analysis by mass spectrometry of the G. intestinalis 26S proteasome by employing the Strep II-FLAG-tandem affinity purification (SF-TAP) tag. This is the first report of efficient production and purification of recombinant proteins in and from Giardia, which will allow the study of specific parasite proteins and protein complexes.

A microRNA derived from an apparent canonical biogenesis pathway regulates variant surface protein gene expression in Giardia lamblia

We have previously shown that a snoRNA-derived microRNA, miR2, in Giardia lamblia potentially regulates the expression of 22 variant surface protein (VSP) genes. Here, we identified another miRNA, miR4, also capable of regulating the expression of several VSPs but derived from an unannotated open reading frame (ORF) rather than a snoRNA, suggesting a canonical miRNA biogenesis pathway in Giardia. miR4 represses expression of a reporter containing two miR4 antisense sequences at the 3' UTR without causing a corresponding decrease in the mRNA level. This repression requires the presence of the Giardia Argonaute protein (GlAgo) and is reversed by 2' O-methylated antisense oligo to miR4, suggesting an RNA-induced silencing complex (RISC)-mediated mechanism. Furthermore, in vivo and in vitro evidence suggested that the Giardia Dicer protein (GlDcr) is required for miR4 biogenesis. Coimmunoprecipitation of miR4 with GlAgo further verified miR4 as a miRNA. A total of 361 potential target sites for miR4 were bioinformatically identified in Giardia, out of which 69 (32.7%) were associated with VSP genes. miR4 reduces the expression of a reporter containing two copies of the target site from VSP (GL50803_36493) at the 3' UTR. Sixteen of the 69 VSP genes were further found to contain partially overlapping miR2 and miR4 targeting sites. Expression of a reporter carrying the two overlapping sites was inhibited by either miR2 or miR4, but the inhibition was neither synergistic nor additive, suggesting a complex mechanism of miRNA regulation of VSP expression and the presence of a rich miRNAome in Giardia.

Giardia flagellar motility is not directly required to maintain attachment to surfaces

Giardia trophozoites attach to the intestinal microvilli (or inert surfaces) using an undefined “suction-based” mechanism, and remain attached during cell division to avoid peristalsis. Flagellar motility is a key factor in Giardia's pathogenesis and colonization of the host small intestine. Specifically, the beating of the ventral flagella, one of four pairs of motile flagella, has been proposed to generate a hydrodynamic force that results in suction-based attachment via the adjacent ventral disc. We aimed to test this prevailing “hydrodynamic model” of attachment mediated by flagellar motility. We defined four distinct stages of attachment by assessing surface contacts of the trophozoite with the substrate during attachment using TIRF microscopy (TIRFM). The lateral crest of the ventral disc forms a continuous perimeter seal with the substrate, a cytological indication that trophozoites are fully attached. Using trophozoites with two types of molecularly engineered defects in flagellar beating, we determined that neither ventral flagellar beating, nor any flagellar beating, is necessary for the maintenance of attachment. Following a morpholino-based knockdown of PF16, a central pair protein, both the beating and morphology of flagella were defective, but trophozoites could still initiate proper surface contacts as seen using TIRFM and could maintain attachment in several biophysical assays. Trophozoites with impaired motility were able to attach as well as motile cells. We also generated a strain with defects in the ventral flagellar waveform by overexpressing a dominant negative form of alpha2-annexin::GFP (D122A, D275A). This dominant negative alpha2-annexin strain could initiate attachment and had only a slight decrease in the ability to withstand normal and shear forces. The time needed for attachment did increase in trophozoites with overall defective flagellar beating, however. Thus while not directly required for attachment, flagellar motility is important for positioning and orienting trophozoites prior to attachment. Drugs affecting flagellar motility may result in lower levels of attachment by indirectly limiting the number of parasites that can position the ventral disc properly against a surface and against peristaltic flow.

Giardia is a widespread, single-celled, intestinal parasite that infects millions of people and animals each year. Colonization of the small intestine is a critical part of Giardia's life cycle in any host. This colonization is initiated when cells attach to the intestinal wall via a specialized suction cup-like structure, the ventral disc. In the host, Giardia moves by beating four pairs of flagella; movement of the ventral pair has been implicated in attachment. This study shows that the beating of the flagella is not important for attachment, but rather for positioning Giardia close to the intestinal wall prior to attachment, and thus disproves the commonly held model of giardial attachment. This work implies that drugs targeting Giardia motility could prevent or slow attachment, leading to lower rates of infection.

A La autoantigen homologue is required for the internal ribosome entry site mediated translation of giardiavirus

Translation of Giardiavirus (GLV) mRNA is initiated at an internal ribosome entry site (IRES) in the viral transcript. The IRES localizes to a downstream portion of 5′ untranslated region (UTR) and a part of the early downstream coding region of the transcript. Recent studies indicated that the IRES does not require a pre-initiation complex to initiate translation but may directly recruit the small ribosome subunit with the help of a number of trans-activating protein factors. A La autoantigen homologue in the viral host Giardia lamblia, GlLa, was proposed as one of the potential trans-activating factors based on its specific binding to GLV-IRES in vitro. In this study, we further elucidated the functional role of GlLa in GLV-IRES mediated translation in Giardia by knocking down GlLa with antisense morpholino oligo, which resulted in a reduction of GLV-IRES activity by 40%. An over-expression of GlLa in Giardia moderately stimulated GLV-IRES activity by 20%. A yeast inhibitory RNA (IRNA), known to bind mammalian and yeast La autoantigen and inhibit Poliovirus and Hepatitis C virus IRES activities in vitro and in vivo, was also found to bind to GlLa protein in vitro and inhibited GLV-IRES function in vivo. The C-terminal domain of La autoantigen interferes with the dimerization of La and inhibits its function. An over-expression of the C-terminal domain (200–348aa) of GlLa in Giardia showed a dominant-negative effect on GLV-IRES activity, suggesting a potential inhibition of GlLa dimerization. HA tagged GlLa protein was detected mainly in the cytoplasm of Giardia, thus supporting a primary role of GlLa in translation initiation in Giardiavirus.

Rapid tagging and integration of genes in Giardia intestinalis

We developed a series of plasmids that allow C-terminal tagging of any gene in its endogenous locus in Giardia intestinalis, with different epitope tags (triple hemagglutinin [3HA] and triple Myc [3Myc]) and selection markers (puromycin, neomycin, and a newly developed marker, blasticidin). Using these vectors, cyclin B and aurora kinase were tagged, expressed, and localized.

Imaging and analysis of the microtubule cytoskeleton in giardia

Giardia intestinalis, a common parasitic protist, possesses a complex microtubule cytoskeleton critical for cellular function and transitioning between the cyst and trophozoite life cycle stages. The giardial microtubule cytoskeleton is comprised of highly dynamic and stable structures. Novel microtubule structures include the ventral disc that is essential for the parasite's attachment to the intestinal villi to avoid peristalsis. The completed Giardia genome combined with new molecular genetic tools and live imaging will aid in the characterization and analysis of cytoskeletal dynamics in Giardia. Fundamental areas of giardial cytoskeletal biology remain to be explored and knowledge of the molecular mechanisms of cytoskeletal functioning is needed to better understand Giardia's unique biology and pathogenesis.

Neogenesis and maturation of transient Golgi-like cisternae in a simple eukaryote

The highly reduced protozoan parasite Giardia lamblia has minimal machinery for cellular processes such as protein trafficking. Giardia trophozoites maintain diverse and regulated secretory pathways but lack an identifiable Golgi complex. During differentiation to cysts, however, they produce specialized compartments termed encystation-specific vesicles (ESVs). ESVs are hypothesized to be unique developmentally regulated Golgi-like organelles dedicated to maturation and export of pre-sorted cyst wall proteins. Here we present a functional analysis of this unusual compartment by direct interference with the functions of the small GTPases Sar1, Rab1 and Arf1. Conditional expression of dominant-negative variants revealed an essential role of Sar1 in early events of organelle neogenesis, whilst inhibition of Arf1 uncoupled morphological changes and cell cycle progression from extracellular matrix export. The latter led to development of ;naked cysts', which lacked water resistance and thus infectivity. Time-lapse microscopy and photobleaching experiments showed that putative Golgi-like cisternae in Giardia develop into a network capable of exchanging soluble cargo at a high rate via dynamic, tubular connections, presumably to synchronize maturation. The minimized and naturally pulsed trafficking machinery for export of the cyst wall biopolymer in Giardia is a simple model for investigating basic principles of neogenesis and maturation of Golgi compartments.

Kinesin-13 regulates flagellar, interphase, and mitotic microtubule dynamics in Giardia intestinalis

Microtubule depolymerization dynamics in the spindle are regulated by kinesin-13, a nonprocessive kinesin motor protein that depolymerizes microtubules at the plus and minus ends. Here we show that a single kinesin-13 homolog regulates flagellar length dynamics, as well as other interphase and mitotic dynamics in Giardia intestinalis, a widespread parasitic diplomonad protist. Both green fluorescent protein-tagged kinesin-13 and EB1 (a plus-end tracking protein) localize to the plus ends of mitotic and interphase microtubules, including a novel localization to the eight flagellar tips, cytoplasmic anterior axonemes, and the median body. The ectopic expression of a kinesin-13 (S280N) rigor mutant construct caused significant elongation of the eight flagella with significant decreases in the median body volume and resulted in mitotic defects. Notably, drugs that disrupt normal interphase and mitotic microtubule dynamics also affected flagellar length in Giardia. Our study extends recent work on interphase and mitotic kinesin-13 functioning in metazoans to include a role in regulating flagellar length dynamics. We suggest that kinesin-13 universally regulates both mitotic and interphase microtubule dynamics in diverse microbial eukaryotes and propose that axonemal microtubules are subject to the same regulation of microtubule dynamics as other dynamic microtubule arrays. Finally, the present study represents the first use of a dominant-negative strategy to disrupt normal protein function in Giardia and provides important insights into giardial microtubule dynamics with relevance to the development of antigiardial compounds that target critical functions of kinesins in the giardial life cycle.

Neomycin and puromycin affect gene expression in Giardia lamblia stable transfection

Two systems for stable transfection of Giardia have been established using selection either by neomycin or by puromycin. We asked if these selection systems themselves influenced expression of endogenous giardial genes. Northern blot analysis showed a approximately 1.4 to approximately 7-fold increase in the encystation-induced cyst wall protein 1 (cwp1), cwp2, and gmyb2 gene transcripts in the drug selected cell lines during vegetative growth, compared with untransfected cells. However, the levels of the constitutive ran, lrp3, or alpha2-tubulin gene transcripts decreased slightly or did not change in these stably transfected cell lines. Part of the effect could be due to drug selection, since treatment of untransfected cells with G418 or puromycin also had similar effects. Nuclear run-on assays showed that part of the effect comes from an increase in transcription initiation rate. The levels of CWP and cyst formation during vegetative growth also increased in the transfected cell lines. Using proteomic technologies, we identified eight genes whose expression is upregulated in neomycin selected cell lines, including phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase, ornithine carbamoyltransferase, carbamate kinase, orf 16424, cyclophilin, co-chaperone-like p21, and bip. Six of these are also upregulated in puromycin selected cell lines. Our results indicate that transfection and drug selection, per se, can alter expression of genes involved in metabolism, protein folding, and differentiation status in Giardia.

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.

Characterization, kinetics, and crystal structures of fructose-1,6-bisphosphate aldolase from the human parasite, Giardia lamblia

Class I and class II fructose-1,6-bisphosphate aldolases (FBPA), glycolytic pathway enzymes, exhibit no amino acid sequence homology and utilize two different catalytic mechanisms. The mammalian class I FBPA employs a Schiff base mechanism, whereas the human parasitic protozoan Giardia lamblia class II FBPA is a zinc-dependent enzyme. In this study, we have explored the potential exploitation of the Giardia FBPA as a drug target. First, synthesis of FBPA was demonstrated in Giardia trophozoites by using an antibody-based fluorescence assay. Second, inhibition of FBPA gene transcription in Giardia trophozoites suggested that the enzyme is necessary for the survival of the organism under optimal laboratory growth conditions. Third, two crystal structures of FBPA in complex with the transition state analog phosphoglycolohydroxamate (PGH) show that the enzyme is homodimeric and that its active site contains a zinc ion. In one crystal form, each subunit contains PGH, which is coordinated to the zinc ion through the hydroxamic acid hydroxyl and carbonyl oxygen atoms. The second crystal form contains PGH only in one subunit and the active site of the second subunit is unoccupied. Inspection of the two states of the enzyme revealed that it undergoes a conformational transition upon ligand binding. The enzyme cleaves d-fructose-1,6-bisphosphate but not d-tagatose-1,6-bisphosphate, which is a tight binding competitive inhibitor. The essential role of the active site residue Asp-83 in catalysis was demonstrated by amino acid replacement. Determinants of catalysis and substrate recognition, derived from comparison of the G. lamblia FBPA structure with Escherichia coli FBPA and with a closely related enzyme, E. coli tagatose-1,6-bisphosphate aldolase (TBPA), are described.

Tight control of transcription in Toxoplasma gondii using an alternative tet repressor

Fusion of yellow fluorescent protein (YFP) to the N-terminus of the Escherichia coli Tn10 tet repressor (TetR) created a functional YFP-TetR repressor with the capacity of 88-fold repression of transcription when expressed in Toxoplasma gondii. As a test promoter we used the T. gondii ribosomal protein RPS13 promoter for which we provide experimental evidence of having a single major transcriptional start site, a condition favourable to the design of inducible expression systems. Integration of four tet operator (tetO) elements, 23-43 bp upstream of the RPS13 transcriptional start site, resulted in maximal repression of transcription (88-fold). Moreover, integration of these four tetO elements reduced the promoter activity only 20% in comparison with the wildtype promoter. Regulation was six-fold higher compared with an inducible expression system employing wildtype TetR. Importantly, only 0.1 microg/ml tetracycline was required for maximal induction demonstrating a higher affinity of tetracycline for YFP-TetR than for wildtype TetR which required 1 microg/ml tetracycline for maximal induction. The use of 0.1 microg/ml tetracycline allows prolonged continuous culturing of T. gondii for which levels of 1 microg/ml tetracycline are toxic. Our results show that YFP-TetR is superior to TetR for transcriptional regulation in T. gondii and we expect that its improved characteristics will be exploitable in other parasites or higher eukaryotes.

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.

The transcription machinery and the molecular toolbox to control gene expression in Toxoplasma gondii and other protozoan parasites

The phylum of Apicomplexa groups a large variety of obligate intracellular protozoan parasites that exhibit complicated life cycles, involving transmission and differentiation within and between different hosts. Little is known about the level of regulation and the nature of the factors controlling gene expression throughout their life stages. Unravelling the mechanisms that govern gene regulation is critical for the development of adequate tools to manipulate these parasites and modulate gene expression, in order to study their function in molecular terms in vivo. A comparative analysis of the transcriptional machinery of several apicomplexan genomes and other protozoan parasites has revealed the existence of a primitive eukaryotic transcription apparatus consisting only of a subset of the general transcription factors found in higher eukaryotes. These findings have some direct implications on development of tools.

Influence of 5' sequences on expression of the Tet repressor in Giardia lamblia

Gene expression is poorly understood in Giardia lamblia. Previously we utilized the Escherichia coli tetracycline regulatory elements to develop a giardial-inducible gene expression system. In this study, we tested the hypothesis that regions flanking the tet repressor (tet R) may influence its expression and affect inducibility of the regulatory system. We found that addition of a 6-His tag or nuclear localization signal (NLS) at the N- but not C-terminus of tet R, increased the induction ratios >100-fold. A non-specific sequence also increased the induction ratio. Fusing NLS at the N-terminus, also led to exclusively nuclear tet R localization. Changing the promoter from gdh or alpha-giardin to alpha2-tubulin increased the induction ratio slightly. Tet R expression at both RNA and protein levels correlated with repression efficiency, indicating that coding sequences of the 6-His tag or NLS may contribute to transcriptional activation of the exotic tet R gene in Giardia. In addition, we found that the tet R system mediated gene repression and induction during encystation. Previous studies used an artificial reporter gene. In this study, we were able to induce overexpression of epitope-tagged cyst wall protein 1 (CWP1) in vegetatively growing Giardia trophozoites. Moreover, we could repress or induce expression of exogenous CWP1 in encysting cells. Taken together, our data suggest that expression of tet R in Giardia is complex and can be strongly influenced by additional sequences, especially at its N-terminus. This system provides insights into expression of an alien gene and can be exploited to regulate gene expression and study important functions in 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.

Gene regulation by tetracyclines

Gene regulation by tetracyclines has become a widely-used tool to study gene functions in pro- and eukaryotes. This regulatory system originates from Gram-negative bacteria, in which it fine-tunes expression of a tetracycline-specific export protein mediating resistance against this antibiotic. This review attempts to describe briefly the selective pressures governing the evolution of tetracycline regulation, which have led to the unique regulatory properties underlying its success in manifold applications. After discussing the basic mechanisms we will present the large variety of designed alterations of activities which have contributed to the still growing tool-box of components available for adjusting the regulatory properties to study gene functions in different organisms or tissues. Finally, we provide an overview of the various experimental setups available for pro- and eukaryotes, and touch upon some highlights discovered by the use of tetracycline-dependent gene regulation.

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.

Gene regulation by tetracyclines. Constraints of resistance regulation in bacteria shape TetR for application in eukaryotes

The Tet repressor protein (TetR) regulates transcription of a family of tetracycline (tc) resistance determinants in Gram-negative bacteria. The resistance protein TetA, a membrane-spanning H+-[tc.M]+ antiporter, must be sensitively regulated because its expression is harmful in the absence of tc, yet it has to be expressed before the drugs' concentration reaches cytoplasmic levels inhibitory for protein synthesis. Consequently, TetR shows highly specific tetO binding to reduce basal expression and high affinity to tc to ensure sensitive induction. Tc can cross biological membranes by diffusion enabling this inducer to penetrate the majority of cells. These regulatory and pharmacological properties are the basis for application of TetR to selectively control the expression of single genes in lower and higher eukaryotes. TetR can be used for that purpose in some organisms without further modifications. In mammals and in a large variety of other organisms, however, eukaryotic transcriptional activator or repressor domains are fused to TetR to turn it into an efficient regulator. Mechanistic understanding and the ability to engineer and screen for mutants with specific properties allow tailoring of the DNA recognition specificity, the response to inducer tc and the dimerization specificity of TetR-based eukaryotic regulators. This review provides an overview of the TetR properties as they evolved in bacteria, the functional modifications necessary to transform it into a convenient, specific and efficient regulator for use in eukaryotes and how the interplay between structure--function studies in bacteria and specific requirements of particular applications in eukaryotes have made it a versatile and highly adaptable regulatory system.

Tetracycline-inducible gene expression in Trichomonas vaginalis

Transient and stable gene delivery systems are available for Trichomonas vaginalis, however, they do not allow regulated expression of target genes. To study essential genes or proteins that are toxic to the cells when over expressed, we have developed an inducible/repressible gene expression system in this parasite, which is driven by the tet-operator (tetO) and regulated tetracycline-responsive Tet repressor (TetR). Inducible chloramphenicol acetyl transferase (CAT) gene expression is observed using a concentration of tetracycline (Tc) as low as 0.1 microg x ml(-1). Expression increases with drug dose with a maximum level of CAT induction achieved in stable transfectants using 5 microg x ml(-1) Tc. CAT protein expression is detectable within 12 h and reaches a maximum level at 48 h, demonstrating that inducible expression is time and dose-dependent. In an inverse experiment, parasites previously cultivated with 1 microg x ml(-1) of Tc for 48 h, were grown in the absence of drug to determine the kinetics of repression. A significant decrease in protein concentration is detected after 48 h, and no detectable protein is observed after 72 h. Experiments replacing the CAT gene with the puromycin N-acetyltransferase (PAC) gene in the Tet regulated expression construct have demonstrated the use of this system for testing putative toxic and essential genes. The establishment of regulated gene expression of exogenous genes in T. vaginalis represents a crucial step towards determining the function of proteins in this divergent parasite.

Calcium signaling in excystation of the early diverging eukaryote, Giardia lamblia

Excystation of Giardia lamblia, which initiates infection, is a poorly understood but dramatic differentiation induced by physiological signals from the host. Our data implicate a central role for calcium homeostasis in excystation. Agents that alter cytosolic Ca(2+) levels (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-tetra(acetyloxymethyl) ester, a Ca(2+) channel blocker, Ca(2+) ionophores, and thapsigargin) strongly inhibit excystation. Treatment of Giardia with thapsigargin raised intracellular Ca(2+) levels, and peak Ca(2+) responses increased with each stage of excystation, consistent with the kinetics of inhibition. Fluorescent thapsigargin localized to a likely Ca(2+) storage compartment in cysts. The ability to sequester ions in membrane-bounded compartments is a hallmark of the eukaryotic cell. These studies support the existence of a giardial thapsigargin-sensitive Ca(2+) storage compartment resembling the sarcoplasmic/endoplasmic reticulum calcium ATPase pump-leak system and suggest that it is important in regulation of differentiation and appeared early in the evolution of eukaryotic cells. Calmodulin antagonists also blocked excystation. The divergent giardial calmodulin localized to the eight flagellar basal bodies/centrosomes, like protein kinase A. Inhibitor kinetics suggest that protein kinase A signaling triggers excystation, whereas calcium signaling is mainly required later, for parasite activation and emergence. Thus, the basal bodies may be a cellular control center to coordinate the resumption of motility and cytokinesis in excystation.

The cytoskeleton of Giardia lamblia

Giardia lamblia is a ubiquitous intestinal pathogen of mammals. Evolutionary studies have also defined it as a member of one of the earliest diverging eukaryotic lineages that we are able to cultivate and study in the laboratory. Despite early recognition of its striking structure resembling a half pear endowed with eight flagella and a unique ventral disk, a molecular understanding of the cytoskeleton of Giardia has been slow to emerge. Perhaps most importantly, although the association of Giardia with diarrhoeal disease has been known for several hundred years, little is known of the mechanism by which Giardia exacts such a toll on its host. What is clear, however, is that the flagella and disk are essential for parasite motility and attachment to host intestinal epithelial cells. Because peristaltic flow expels intestinal contents, attachment is necessary for parasites to remain in the small intestine and cause diarrhoea, underscoring the essential role of the cytoskeleton in virulence. This review presents current day knowledge of the cytoskeleton, focusing on its role in motility and attachment. As the advent of new molecular technologies in Giardia sets the stage for a renewed focus on the cytoskeleton and its role in Giardia virulence, we discuss future research directions in cytoskeletal function and regulation.

A novel Myb-related protein involved in transcriptional activation of encystation genes in Giardia lamblia

Giardia lamblia is an important human intestinal parasite that survives outside of the host by differentiation of trophozoites into infectious cysts. Transcriptional regulation is key for encystation gene expression, but the mechanisms are unknown. Giardia genome database searches identified a myb-like gene (gmyb2) whose expression increased during encystation. Epitope-tagged gMyb2 localized to both nuclei. DNA binding and mutation analysis showed that gMyb2 binds specifically to C(T/A)ACAG, a c-Myb-like target sequence in the promoters of encystation-induced genes encoding gMyb2, three cyst wall proteins and G6PI-B, a key enzyme in cyst wall polysaccharide biosynthesis. gMyb2 binding sites were not found in the upstream regions of 31 other giardial genes. Deletion of the putative gMyb2 binding site greatly reduced encystation-specific promoter activity of g6pi-b. Fusion of gMyb2 binding sites to the constitutive ran promoter or g6pi-b promoter deletion lacking the gMyb2 binding site in-duced encystation-specific expression. gMyb2 may play an important role in transcriptional regulation of encystation genes, and may help co-ordinate synthesis of cyst wall proteins and polysaccharide. gMyb2 is the first giardial transcription factor to be functionally identified and the first that is associated with upregulation of encystation genes. This work provides a model for study of differential gene regulation in early diverging eukaryotic organisms.

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.

Modulation of myosin A expression by a newly established tetracycline repressor-based inducible system in Toxoplasma gondii

We have developed a control system for regulating gene activation in Toxoplasma gondii. The elements of this system are derived from the Escherichia coli tetracycline resistance operon, which has been widely used to tightly control gene expression in eukaryotes. The tetracycline repressor (tetR) interferes with transcription initiation while the chimeric transactivator, composed of the tetR fused to the activating domain of VP16 transcriptional factor, allows tet-dependent transcription. Accordingly, tetracycline derivatives such as anhydrotetracycline, which we found to be well tolerated by T.gondii, can serve as effector molecules, allowing control of gene expression in a reversible manner. As a prerequisite to functionally express the tetR in T.gondii, we used a synthetic gene with change of codon frequency. Whereas no activation of transcription was achieved using the synthetic tetracycline-controlled transactivator, tTA2(s), the TetR(s )modulates parasite transcription over a range of approximately 15-fold as measured for several reporter genes. We show here that the tetR-dependent induction of the T.gondii myosin A transgene expression drastically down-regulates the level of endogenous MyoA. This myosin is under the control of a tight feedback mechanism, which occurs at the protein level.

Evaluation of the tetracycline- and ecdysone-inducible systems for expression of neurotransmitter receptors in mammalian cells

Establishing a stable cell line that expresses a particular protein of interest is often a laborious and time-consuming experience. With constitutive expression systems, a gradual loss of the highly expressing clones over a given time span and/or a severe counter-selection due to toxicity of the expressed protein for the host cell line are major drawbacks. In both cases, inducible expression systems offer a valuable alternative. Over the years, many regulated expression systems have been developed and evaluated. In the present study, we compare the efficiency, the advantages and the drawbacks of a tetracycline- and an ecdysone-inducible system for expression of the reporter protein chloramphenicol acetyltransferase and of different G-protein-coupled serotonin (5-HT) receptors. A high level of expression of different 5-HT receptors was obtained with the tetracycline-inducible system. In the cell line L929, which stably expresses the tetracycline-responsive transactivator, a maximum ligand binding of 20,000 and 9500 fmol/mg protein was measured for the h5-HT(1B) and h5-ht(1F) receptors, respectively. In the HEK293rtTA cell line, levels of 15,700, 3000, and 9100 fmol bound ligand/mg protein were obtained for the h5-HT(1B), h5-ht(1F) and h5-HT(4b) receptors, respectively. These high expression levels remained stable for several months of continuous culture. Although the ecdysone-inducible expression system was useful for tightly regulated expression, the levels were far lower than those obtained with the tetracycline system (e.g. 640 fmol bound ligand/mg protein for the h5-ht(1F) receptor in HEK293EcR).

Initiator and upstream elements in the alpha2-tubulin promoter of Giardia lamblia

Giardia lamblia, one of the earliest diverging eukaryotes and a major cause of diarrhea world-wide, has unusually short intergenic regions, raising questions concerning its regulation of gene expression. We have approached this issue through examination of the alpha2-tubulin promoter and in particular investigated the function of an AT-rich element surrounding the transcription start site. Its placement and the ability of this sequence to direct transcription initiation in the absence of any other promoter elements is similar to the initiator element in higher eukaryotes. However, the sequence diversity of extremely short (8-10 bp) initiator elements is surprising, as is their ability to independently direct substantial levels of transcription. We also identified a large AT-rich element located between -64 and -29 bp upstream of the transcriptional start site and show using both deletions and site-specific mutations of this region that sequences between -60 and the start of transcription are important for promoter strength; interestingly this AT-rich sequence is not highly conserved among different Giardia promoters. These data suggest that while the overall structure of the core promoter has been conserved throughout eukaryotic evolution, significant variation and flexibility is allowed in element consensus sequences and roles in transcription. In particular, the short and diverse sequences that function in transcription initiation in Giardia suggest the potential for relaxed transcriptional regulation.

The development of genetic tools for dissecting the biology of malaria parasites

Plasmodium parasites are haploid unicellular organisms that cause malaria. In the last decade, transfection systems have been developed for both human and animal model species of Plasmodium, providing a broad range of genetic tools for the study of malaria parasite biology. Transient transfection has been used to provide insight into the regulation of gene expression by Plasmodium spp. The development of stable transfection technologies has provided the opportunity to express transgenes in Plasmodium spp., as well as elucidate the function of proteins by disrupting, modifying, or replacing the genes encoding them. These genetic tools represent an important breakthrough for malaria research and will significantly contribute to our understanding of the biology of the parasite. However, further developments in this technology are still required, especially because the full genome sequence of the major human malaria parasite Plasmodium falciparum will shortly be available. Ultimately, the biological information obtained through genetic manipulation of Plasmodium spp. will facilitate a more rational approach to vaccine and drug design.

Multistage neoplastic transformation of Syrian hamster embryo cells cultured at pH 6.70

MicroRNAs are major post-transcriptional regulators of gene expression. Here we show in the ancient protozoan Giardia lamblia a snoRNA-derived 26-nucleotide microRNA, miR3, which represses the translation of histone H2A mRNA containing an imperfect target but enhances translation when the target is made fully complementary. A stepwise mutational analysis of the fully complementary target showed that the activating effect of miR3 was significantly reduced when a single nucleotide at the 5′-end of the target was altered. The effect of miR3 became repressive when 12 of the nucleotides lost their complementation to miR3 with maximum repression reached when only 8 base-pairs remained between the miR3 seed sequence and the target. A synthetic 8-nucleotide RNA oligomer of the miR3 seed sequence was found capable of exerting a similar Argonaute-dependent translational repression. This is the first report showing a correlation between the extent of base-pairing with the target and a change in miRNA function.