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

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.

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.

Experimental verification of the identity of variant-specific surface proteins in Giardia lamblia trophozoites

The cell membrane of a Giardia lamblia trophozoite is covered with a single species of variant-specific surface protein (VSP) that is replaced by another VSP every 6 to 13 generations of cell growth, possibly for an evasion of host immunity. Experimentally, only six VSP species have been verified to localize to the cell membrane thus far. By assuming that VSP contains multiple CXXC motifs, 219 vsp genes were annotated in GiardiaDB of the WB isolate. By further assuming that VSP possesses both CXXC motifs and a CRGKA tail at the C terminus, Adam et al. (BMC Genomics 11:424, 2010) identified a total of 303 potential vsp genes in Giardia WB. The discrepancies between these two assumed VSP identities have caused some confusion. Here, we used experimental approaches to further verify what is required of the structures of a VSP to localize to the surface of cell membrane. The data led to the following conclusions. (i) The C-terminal CRGKA sequence is not essential for localizing VSPs to the cell membrane. (ii) A “motif 1” of 45 residues, consisting of two CXXCs separated by 12 to 15 amino acid residues, located close to the C terminus and a hydrophobic “motif 2” of 38 residues at the C terminus are both essential and sufficient for localizing the protein to the cell membrane. (ii) An N-terminal sequence upstream from motif 1 is not required for targeting VSPs to the cell membrane. By these criteria, we are able to identify 73 open reading frames as the putative vsp genes in Giardia.

The intestinal pathogen Giardia lamblia expresses only one variant-specific surface protein (VSP) on the cell membrane surface at a given time, but it changes spontaneously every 6 to 13 generations of growth, presumably for evading the host immunity. Only 6 VSPs have been empirically shown to localize to the cell membrane surface thus far. Here, we used mutations of VSPs and methods of identifying their locations in Giardia cells and found that a “motif 1” of 45 residues, consisting of two CXXCs separated by 12 to 15 amino acid residues, located close to the C terminus and a hydrophobic “motif 2” of 38 residues at the C terminus are the only essential and sufficient structural requirements for localizing a protein to the cell membrane. By these criteria, 73 genes are identified in the Giardia WB strain genome database as the putative repertoire of VSPs.

The profile of snoRNA-derived microRNAs that regulate expression of variant surface proteins in Giardia lamblia

In the current investigation, we analysed all the known small nucleolar RNAs (snoRNAs) in the deeply branching protozoan parasite Giardia lamblia for potential microRNAs (miRNAs) that might be derived from them. Two putative miRNAs have since been identified by Northern blot, primer extension, 3' RACE and co-immunoprecipitation with Giardia Argonaute (GlAgo), and designated miR6 and miR10. Giardia Dicer (GlDcr) is capable of processing the snoRNAs into the corresponding miRNAs in vitro. Potential miR6 and miR10 binding sites in Giardia genome were predicted bio-informatically. A miR6 binding site was found at the 3' untranslated regions (UTR) of 44 variant surface protein (vsp) genes, whereas a miR10 binding site was identified at the 3' end of 159 vsp open-reading frames. Thirty-three of these vsp genes turned out to contain binding sites for both miR6 and miR10. A reporter mRNA tagged with the 3' end of vsp1267, which contains the target sites for both miRNAs, was translationally repressed by both miRNAs in Giardia. Episomal expression of an N-terminal c-myc tagged VSP1267 was found significantly repressed by introducing either miR6 or miR10 into the cells and the repressive effects were additive. When the 2'-O-methyl antisense oligos (ASOs) of either miR6 or miR10 was introduced, however, there was an enhancement of tagged VSP1267 expression suggesting an inhibition of the repressive effects of endogenous miR6 or miR10 by the ASOs. Of the total 220 vsp genes in Giardia, we have now found 178 of them carrying putative binding sites for all the miRNAs that have been currently identified, suggesting that miRNAs are likely the regulators of VSP expression in Giardia.

Regulation of a Myb transcription factor by cyclin-dependent kinase 2 in Giardia lamblia

The protozoan Giardia lamblia parasitizes the human small intestine to cause diseases. It undergoes differentiation into infectious cysts by responding to intestinal stimulation. How the activated signal transduction pathways relate to encystation stimulation remain largely unknown. During encystation, genes encoding cyst wall proteins (CWPs) are coordinately up-regulated by a Myb2 transcription factor. Because cell differentiation is linked to cell cycle regulation, we tried to understand the role of cell cycle regulators, cyclin-dependent kinases (Cdks), in encystation. We found that the recombinant Myb2 was phosphorylated by Cdk-associated complexes and the levels of phosphorylation increased significantly during encystation. We have identified a putative cdk gene (cdk2) by searching the Giardia genome database. Cdk2 was found to localize in the cytoplasm with higher expression during encystation. Interestingly, overexpression of Cdk2 resulted in a significant increase of the levels of cwp gene expression and cyst formation. In addition, the Cdk2-associated complexes can phosphorylate Myb2 and the levels of phosphorylation increased significantly during encystation. Mutations of important catalytic residues of Cdk2 resulted in a significant decrease of kinase activity and ability of inducing cyst formation. Addition of a Cdk inhibitor, purvalanol A, significantly decreased the Cdk2 kinase activity and the levels of cwp gene expression and cyst formation. Our results suggest that the Cdk2 pathway may be involved in phosphorylation of Myb2, leading to activation of the Myb2 function and up-regulation of cwp genes during encystation. The results provide insights into the use of Cdk inhibitory drugs in disruption of Giardia differentiation into cysts.

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.

Tetracycline alters drug susceptibility in Candida albicans and other pathogenic fungi

The tetracycline (TET) promoter has been used in several systems as an inducible regulator of gene expression. In control analyses, the standard Candida albicans laboratory strain SC5314 was found to have altered susceptibility to a variety of antifungal drugs in the presence of relatively high concentrations (50-200 microg ml(-1)) of TET. Altered susceptibility was most notable with exposure to amphotericin B (AMB), with a 32-fold increase in susceptibility, and terbinafine (TRB), with a 32-fold decrease in susceptibility. The TET/AMB synergy was observed in several clinical isolates of C. albicans and in the distantly related species Aspergillus fumigatus and Cryptococcus neoformans. The TET/AMB synergy is not related to efflux pump activity, as determined by FACS analyses and by analysis of a strain containing efflux pump deletions. Gene expression analyses by luciferase and by quantitative real-time reverse transcriptase PCR failed to identify significant alterations in expression of any genes associated with resistance. C. albicans grown with TET for 48 h does show a reduction in total cellular ergosterol. Analysis of growth curves suggests that the TET effect is associated with lack of a diauxic shift, which is related to a loss of mitochondrial function. MitoTracker fluorescent dye was used to demonstrate that TET has a direct effect on mitochondrial function. These results demonstrate the need for careful analysis of TET effects when using a TET-inducible promoter, especially in studies that involve antifungal drugs. This study defines some limits to the use of the TET-inducible promoter, and identifies effects on cells that are the result of TET exposure alone, not the result of expression of a targeted gene.

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.

A novel ARID/Bright-like protein involved in transcriptional activation of cyst wall protein 1 gene in Giardia lamblia

The capability of protozoan parasite Giardia lamblia to encyst is critical for survival outside the host and its transmission. AT-rich interaction domain (ARID) or Bright homologs constitute a large family of transcription factors in higher eukaryotes that regulate cell proliferation, development, and differentiation. We asked whether Giardia has ARID-like genes and whether they influence gene expression during Giardia encystation. Blast searches of the Giardia genome data base identified two genes with putative ARID/Bright domains (gARID1 and 2). Epitope-tagged gARID1 was found to localize to nuclei. Recombinant gARID1 specifically bound to the encystation-induced cyst wall protein (cwp) gene promoters. Mutation analysis revealed that AT-rich initiators were required for binding of gARID1 to the cwp promoters. gARID1 contains several key residues for DNA binding, and its binding sequences are similar to those of the known ARID family proteins. The gARID1 binding sequences were positive cis-acting elements of the cwp1 promoter during both vegetative growth and encystation. We also found that gARID1 transactivated the cwp1 promoter through its binding sequences in vivo. Our results suggest that the ARID family has been conserved during evolution and that gARID1 is an important transactivator in regulation of the Giardia cwp1 gene, which is key to Giardia differentiation into cysts.

Leishmania chagasi: a tetracycline-inducible cell line driven by T7 RNA polymerase

Trypanosomatid protozoa lack consensus promoters for RNA polymerase (RNAP) II. However, the artificial insertion of the T7 promoter (P(T7)) and the tetracycline repressor into Trypanosoma brucei cell lines expressing T7RNAP allows P(T7)-driven gene expression to be tetracycline-inducible. These cell lines provide a molecular tool to address protein function by several recombinant approaches. We describe here the development of an analogous Leishmania chagasi cell line bearing the genes for exogenous T7RNAP and the tetracycline repressor inserted in the multi-gene alpha-tubulin locus. A plasmid construct with P(T7) and the tetracycline operator upstream of a reporter gene, when introduced into this cell line as episomal plasmids or chromosomal insertion into the non-coding strand of an 18SrRNA gene, resulted in tetracycline-inducible expression of the reporter as much as 16- and 150-fold, respectively. The reporter was under a much tighter control when chromosomally inserted than extra-chromosomally born. Furthermore, P(T7) augmented the reporter's expression 2-fold more in comparison to P(T7)-less constructs. This cell line is the first Leishmania spp. that allows the exogenous T7RNAP-driven gene expression to be tetracycline-inducible; and may provide a useful tool for addressing protein function by manipulating expression levels of Leishmania endogenous genes.