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

Diclazuril-induced expression of CDK-related kinase 2 in the second-generation merozoites of Eimeria tenella

Diclazuril is a classic anticoccidial drug. The key molecules of diclazuril in anticoccidial action allows target screening for the development of anticoccidial drugs. Cyclin-dependent kinases (CDK) are prominent target proteins in apicomplexan parasites. In this study, a diclazuril anticoccidiosis animal model was established, and the transcription and translation levels of the CDK-related kinase 2 of Eimeria tenella (EtCRK2) were detected. mRNA and protein expression levels of EtCRK2 decreased in the infected/diclazuril group compared with those in the infected/control group. In addition, immunofluorescence analysis showed that EtCRK2 was localised in the cytoplasm of the merozoites. The fluorescence intensity of EtCRK2 in the infected/diclazuril group was significantly weaker than that in the infected/control group. The anticoccidial drug diclazuril against E.tenella affects the expression pattern of EtCRK2 molecule, and EtCRK2 is a potential target for new drug development.

A myeloid leukemia factor homolog is involved in tolerance to stresses and stress-induced protein metabolism in Giardia lamblia

BACKGROUND

The eukaryotic membrane vesicles contain specific sets of proteins that determine vesicle function and shuttle with specific destination. Giardia lamblia contains unknown cytosolic vesicles that are related to the identification of a homolog of human myeloid leukemia factor (MLF) named MLF vesicles (MLFVs). Previous studies suggest that MLF also colocalized with two autophagy machineries, FYVE and ATG8-like protein, and that MLFVs are stress-induced compartments for substrates of the proteasome or autophagy in response to rapamycin, MG132, and chloroquine treatment. A mutant protein of cyclin-dependent kinase 2, CDK2m3, was used to understand whether the aberrant proteins are targeted to degradative compratments. Interestingly, MLF was upregulated by CDK2m3 and they both colocalized within the same vesicles. Autophagy is a self-digestion process that is activated to remove damaged proteins for preventing cell death in response to various stresses. Because of the absence of some autophagy machineries, the mechanism of autophagy is unclear in G. lamblia.

RESULTS

In this study, we tested the six autophagosome and stress inducers in mammalian cells, including MG132, rapamycin, chloroquine, nocodazole, DTT, and G418, and found that their treatment increased reactive oxygen species production and vesicle number and level of MLF, FYVE, and ATG8-like protein in G. lamblia. Five stress inducers also increased the CDK2m3 protein levels and vesicles. Using stress inducers and knockdown system for MLF, we identified that stress induction of CDK2m3 was positively regulated by MLF. An autophagosome-reducing agent, 3-methyl adenine, can reduce MLF and CDK2m3 vesicles and proteins. In addition, knockdown of MLF with CRISPR/Cas9 system reduced cell survival upon treatment with stress inducers. Our newly developed complementation system for CRISPR/Cas9 indicated that complementation of MLF restored cell survival in response to stress inducers. Furthermore, human MLF2, like Giardia MLF, can increase cyst wall protein expression and cyst formation in G. lamblia, and it can colocalize with MLFVs and interact with MLF.

CONCLUSIONS

Our results suggest that MLF family proteins are functionally conserved in evolution. Our results also suggest an important role of MLF in survival in stress conditions and that MLFVs share similar stress-induced characteristics with autophagy compartments.

Functional Differentiation of Cyclins and Cyclin-Dependent Kinases in Giardia lamblia

Cyclin-dependent kinases (CDKs) are serine/threonine kinases that control the eukaryotic cell cycle. Limited information is available on Giardia lamblia CDKs (GlCDKs), GlCDK1 and GlCDK2. After treatment with the CDK inhibitor flavopiridol-HCl (FH), division of Giardia trophozoites was transiently arrested at the G1/S phase and finally at the G2/M phase. The percentage of cells arrested during prophase or cytokinesis increased, whereas DNA synthesis was not affected by FH treatment. Morpholino-mediated depletion of GlCDK1 caused arrest at the G2/M phase, while GlCDK2 depletion resulted in an increase in the number of cells arrested at the G1/S phase and cells defective in mitosis and cytokinesis. Coimmunoprecipitation experiments with GlCDKs and the nine putative G. lamblia cyclins (Glcyclins) identified Glcyclins 3977/14488/17505 and 22394/6584 as cognate partners of GlCDK1 and GlCDK2, respectively. Morpholino-based knockdown of Glcyclin 3977 or 22394/6584 arrested cells in the G2/M phase or G1/S phase, respectively. Interestingly, GlCDK1- and Glcyclin 3977-depleted Giardia showed significant flagellar extension. Altogether, our results suggest that GlCDK1/Glcyclin 3977 plays an important role in the later stages of cell cycle control and in flagellar biogenesis. In contrast, GlCDK2 along with Glcyclin 22394 and 6584 functions from the early stages of the Giardia cell cycle. IMPORTANCE Giardia lamblia CDKs (GlCDKs) and their cognate cyclins have not yet been studied. In this study, the functional roles of GlCDK1 and GlCDK2 were distinguished using morpholino-mediated knockdown and coimmunoprecipitation. GlCDK1 with Glcyclin 3977 plays a role in flagellum formation as well as cell cycle control of G. lamblia, whereas GlCDK2 with Glcyclin 22394/6584 is involved in cell cycle control.

Kinesin-13, a Motor Protein, is Regulated by Polo-like Kinase in Giardia lamblia

Kinesin-13 (Kin-13), a depolymerizer of microtubule (MT), has been known to affect the length of Giardia. Giardia Kin-13 (GlKin-13) was localized to axoneme, flagellar tips, and centrosomes, where phosphorylated forms of Giardia polo-like kinase (GlPLK) were distributed. We observed the interaction between GlKin-13 and GlPLK via co-immunoprecipitation using transgenic Giardia cells expressing Myc-tagged GlKin-13, hemagglutinin-tagged GlPLK, and in vitro-synthesized GlKin-13 and GlPLK proteins. In vitro-synthesized GlPLK was demonstrated to auto-phosphorylate and phosphorylate GlKin-13 upon incubation with [γ-32P]ATP. Morpholino-mediated depletion of both GlKin-13 and GlPLK caused an extension of flagella and a decreased volume of median bodies in Giardia trophozoites. Our results suggest that GlPLK plays a pertinent role in formation of flagella and median bodies by modulating MT depolymerizing activity of GlKin-13.

A cell-cycle-dependent GARP-like transcriptional repressor regulates the initiation of differentiation in Giardia lamblia

Transcriptional regulation of differentiation is critical for parasitic pathogens to adapt to environmental changes and regulate transmission. In response to encystation stimuli, Giardia lamblia shifts the distribution of the cell cycle toward G2 and induces the expression of cyst wall proteins (CWPs) within 2 to 4 h, indicating that key regulatory steps occur within the first 4 h of encystation. However, the role of transcription factors (TFs) in encystation has primarily been investigated at later time points. How TFs initiate encystation and link it to the cell cycle remains enigmatic. Here, we systematically screened six putative early up-regulated TFs for nuclear localization, established their dynamic expression profiles, and determined their functional role in regulating encystation. We found a critical repressor, Golden2, ARR-B, Psr-1–like protein 1 (GARP)–like protein 4 (GLP4), that increases rapidly after 30 min of encystation stimuli and down-regulates encystation-specific markers, including CWPs and enzymes in the cyst N-acetylgalactosamine pathway. Depletion of GLP4 increases cyst production. Importantly, we observe that G2+M cells exhibit higher levels of CWP1, resulting from the activation of myeloblastosis domain protein 2 (MYB2), a TF previously linked to encystation in Giardia. GLP4 up-regulation occurs in G1+S cells, suggesting a role in repressing MYB2 and encystation-specific genes in the G1+S phase of the cell cycle. Furthermore, we demonstrate that depletion of GLP4 up-regulates MYB2 and promotes encystation while overexpression of GLP4 down-regulates MYB2 and represses encystation. Together, these results suggest that Giardia employs a dose-dependent transcriptional response that involves the cell-cycle–regulated repressor GLP4 to orchestrate MYB2 and entry into the encystation pathway.

A Detailed Gene Expression Map of Giardia Encystation

Giardia intestinalis is an intestinal protozoan parasite that causes diarrheal infections worldwide. A key process to sustain its chain of transmission is the formation of infectious cysts in the encystation process. We combined deep RNAseq of a broad range of encystation timepoints to produce a high-resolution gene expression map of Giardia encystation. This detailed transcriptomic map of encystation confirmed a gradual change of gene expression along the time course of encystation, showing the most significant gene expression changes during late encystation. Few genes are differentially expressed early in encystation, but the major cyst wall proteins CWP-1 and -2 are highly up-regulated already after 3.5 h encystation. Several transcription factors are sequentially up-regulated throughout the process, but many up-regulated genes at 7, 10, and 14 h post-induction of encystation have binding sites in the upstream regions for the Myb2 transcription factor, suggesting that Myb2 is a master regulator of encystation. We observed major changes in gene expression of several meiotic-related genes from 10.5 h of encystation to the cyst stage, and at 17.5 h encystation, there are changes in many different metabolic pathways and protein synthesis. Late encystation, 21 h to cysts, show extensive gene expression changes, most of all in VSP and HCMP genes, which are involved in antigenic variation, and genes involved in chromatin modifications. This high-resolution gene expression map of Giardia encystation will be an important tool in further studies of this important differentiation process.

An update on cell division of Giardia duodenalis trophozoites

Giardia duodenalis is a flagellated protozoan that is responsible for many cases of diarrheal disease worldwide and is characterized by its great divergence from the model organisms commonly used in studies of basic cellular processes. The life cycle of Giardia involves an infectious cyst form and a proliferative and mobile trophozoite form. Each Giardia trophozoite has two nuclei and a complex microtubule cytoskeleton that consists of eight flagellar axonemes, basal bodies, the adhesive disc, the funis and the median body. Since the success of Giardia infecting other organisms depends on its ability to divide and proliferate efficiently, Giardia must coordinate its cell division to ensure the duplication and partitioning of both nuclei and the multiple cytoskeletal structures. The purpose of this review is to summarize current knowledge about cell division and its regulation in this protist.

A myeloid leukemia factor homolog involved in encystation-induced protein metabolism in Giardia lamblia

BACKGROUND

Giardia lamblia differentiates into resistant cysts as an established model for dormancy. Myeloid leukemia factor (MLF) proteins are important regulators of cell differentiation. Giardia possesses a MLF homolog which was up-regulated during encystation and localized to unknown cytosolic vesicles named MLF vesicles (MLFVs).

METHODS

We used double staining for visualization of potential factors with role in protein metabolism pathway and a strategy that employed a deletion mutant, CDK2m3, to test the protein degradation pathway. We also explored whether autophagy or proteasomal degradation are regulators of Giardia encystation by treatment with MG132, rapamycin, or chloroquine.

RESULTS

Double staining of MLF and ISCU or CWP1 revealed no overlap between their vesicles. The aberrant CDK2m3 colocalized with MLFVs and formed complexes with MLF. MG132 increased the number of CDK2m3-localized vesicles and its protein level. We further found that MLF colocalized and interacted with a FYVE protein and an ATG8-like (ATG8L) protein, which were up-regulated during encystation and their expression induced Giardia encystation. The addition of MG132, rapamycin, or chloroquine, increased their levels and the number of their vesicles, and inhibited the cyst formation. MLF and FYVE were detected in exosomes released from culture.

CONCLUSIONS

The MLFVs are not mitosomes or encystation-specific vesicles, but are related with degradative pathway for CDK2m3. MLF, FYVE, and ATG8L play a positive role in encystation and function in protein clearance pathway, which is important for encystation and coordinated with Exosomes.

GENERAL SIGNIFICANCE

MLF, FYVE, and ATG8L may be involved an encystation-induced protein metabolism during Giardia differentiation.

Eukaryote-conserved histone post-translational modification landscape in Giardia duodenalis revealed by mass spectrometry

Diarrheal disease caused by Giardia duodenalis is highly prevalent, causing over 200 million cases globally each year. The processes that drive parasite virulence, host immune evasion and transmission involve coordinated gene expression and have been linked to epigenetic regulation. Epigenetic regulatory systems are eukaryote-conserved, including in deep branching excavates such as Giardia, with several studies already implicating histone post-translational modifications in regulation of its pathogenesis and life cycle. However, further insights into Giardia chromatin dynamics have been hindered by a lack of site-specific knowledge of histone modifications. Using mass spectrometry, we have provided the first known molecular map of histone methylation, acetylation and phosphorylation modifications in Giardia core histones. We have identified over 50 previously unreported histone modifications including sites with established roles in epigenetic regulation, and co-occurring modifications indicative of post-translational modification crosstalk. These demonstrate conserved histone modifications in Giardia which are equivalent to many other eukaryotes, and suggest that similar epigenetic mechanisms are in place in this parasite. Further, we used sequence, domain and structural homology to annotate putative histone enzyme networks in Giardia, highlighting representative chromatin modifiers which appear sufficient for identified sites, particularly those from H3 and H4 variants. This study is to our knowledge the first and most comprehensive, complete and accurate view of Giardia histone post-translational modifications to date, and a substantial step towards understanding their associations in parasite development and virulence.

DNA topoisomerase IIIβ promotes cyst generation by inducing cyst wall protein gene expression in Giardia lamblia

Giardia lamblia causes waterborne diarrhoea by transmission of infective cysts. Three cyst wall proteins are highly expressed in a concerted manner during encystation of trophozoites into cysts. However, their gene regulatory mechanism is still largely unknown. DNA topoisomerases control topological homeostasis of genomic DNA during replication, transcription and chromosome segregation. They are involved in a variety of cellular processes including cell cycle, cell proliferation and differentiation, so they may be valuable drug targets. Giardia lamblia possesses a type IA DNA topoisomerase (TOP3β) with similarity to the mammalian topoisomerase IIIβ. We found that TOP3β was upregulated during encystation and it possessed DNA-binding and cleavage activity. TOP3β can bind to the cwp promoters in vivo using norfloxacin-mediated topoisomerase immunoprecipitation assays. We also found TOP3β can interact with MYB2, a transcription factor involved in the coordinate expression of cwp1-3 genes during encystation. Interestingly, overexpression of TOP3β increased expression of cwp1-3 and myb2 genes and cyst formation. Microarray analysis confirmed upregulation of cwp1-3 and myb2 genes by TOP3β. Mutation of the catalytically important Tyr residue, deletion of C-terminal zinc ribbon domain or further deletion of partial catalytic core domain reduced the levels of cleavage activity, cwp1-3 and myb2 gene expression, and cyst formation. Interestingly, some of these mutant proteins were mis-localized to cytoplasm. Using a CRISPR/Cas9 system for targeted disruption of top3β gene, we found a significant decrease in cwp1-3 and myb2 gene expression and cyst number. Our results suggest that TOP3β may be functionally conserved, and involved in inducing Giardia cyst formation.

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.

DNA topoisomerase II is involved in regulation of cyst wall protein genes and differentiation in Giardia lamblia

The protozoan Giardia lamblia differentiates into infectious cysts within the human intestinal tract for disease transmission. Expression of the cyst wall protein (cwp) genes increases with similar kinetics during encystation. However, little is known how their gene regulation shares common mechanisms. DNA topoisomerases maintain normal topology of genomic DNA. They are necessary for cell proliferation and tissue development as they are involved in transcription, DNA replication, and chromosome condensation. A putative topoisomerase II (topo II) gene has been identified in the G. lamblia genome. We asked whether Topo II could regulate Giardia encystation. We found that Topo II was present in cell nuclei and its gene was up-regulated during encystation. Topo II has typical ATPase and DNA cleavage activity of type II topoisomerases. Mutation analysis revealed that the catalytic important Tyr residue and cleavage domain are important for Topo II function. We used etoposide-mediated topoisomerase immunoprecipitation assays to confirm the binding of Topo II to the cwp promoters in vivo. Interestingly, Topo II overexpression increased the levels of cwp gene expression and cyst formation. Microarray analysis identified up-regulation of cwp and specific vsp genes by Topo II. We also found that the type II topoisomerase inhibitor etoposide has growth inhibition effect on Giardia. Addition of etoposide significantly decreased the levels of cwp gene expression and cyst formation. Our results suggest that Topo II has been functionally conserved during evolution and that Topo II plays important roles in induction of the cwp genes, which is key to Giardia differentiation into cysts.

Giardia lamblia becomes infective by differentiation into water-resistant cysts. During encystation, cyst wall proteins (CWPs) are highly synthesized and are targeted to the cyst wall. However, little is known about the regulation mechanisms of these genes. DNA topoisomerases can resolve the topological problems and are needed for a variety of key cellular functions, including cell proliferation, cell differentiation and organ development in higher eukaryotes. We found that giardial Topo II was highly expressed during encystation. Topo II is present in Giardia nuclei and is associated with the encystation-induced cwp gene promoters. Topo II has typical DNA cleavage activity of type II topoisomerases. Interestingly, overexpression of Topo II can induce cwp gene expression and cyst formation. Addition of a type II topoisomerase inhibitor, etoposide, significantly decreased the levels of cwp gene expression and cyst formation. Etoposide also has growth inhibition effect on Giardia. Our results suggest that Topo II plays an important role in induction of encystation by up-regulation of the cwp gene expression. Our results provide insights into the function of Topo II in parasite differentiation into cysts and help develop ways to interrupt the parasite life cycle.