Giardia intestinalis: aphidicolin influence on the trophozoite cell cycle

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.

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 content in Acanthamoeba during two stress defense reactions: Encystation, pseudocyst formation and cell cycle

During environmental stress, the vegetative cells of the facultative pathogenic amoeba Acanthamoeba castellanii reversibly differentiate into resistant dormant stages, namely, cysts or pseudocysts. The type of resistant stage depends on the nature and duration of the stressor. Cell differentiation is accompanied by changes in morphology and cellular metabolism. Moreover, cell differentiation is also expected to be closely linked to the regulation of the cell cycle and, thus, to cellular DNA content. While the existence of the resistant stages in A. castellanii is well known, there is no consensus regarding the relationship between differentiation and cell cycle progression. In the present work, we used flow cytometry analysis to explore the changes in the DNA content during Acanthamoeba encystation and pseudocyst formation. Our results strongly indicate that A. castellanii enters encystation from the G2 phase of the cell cycle. In contrast, differentiation into pseudocysts can begin in the G1 and G2 phases. In addition, we present a phylogenetic analysis and classification of the main cell cycle regulators, namely, cyclin-dependent kinases and cyclins that are found in the genome of A. castellanii.

Role of gamma-giardin in ventral disc formation of Giardia lamblia

Background

Giardia lamblia, a protozoan pathogen causing diarrheal outbreaks, has characteristic cytoskeletal structures including eight flagella, a median body and a ventral disc. Gamma-giardin is a unique component protein of the cytoskeleton of this protozoan.

Results

Through comparative proteomic analysis between different stages of the cell cycle, G. lamblia γ-giardin (Glγ-giardin) was identified as an upregulated protein in the G2-phase. Increased Glγ-giardin expression in G2 was confirmed by western blot and real-time polymerase chain reaction analyses. Knockdown of this protein using a morpholino affected the formation of ventral discs, especially the microribbons of the discs, but exerted little effect on the binding ability of G. lamblia. The number of cells with four nuclei was increased in Glγ-giardin-knockdown cells. Expression of Glγ-giardin was decreased during encystation, in contrast with the G2-phase.

Conclusions

Knockdown experiments demonstrated that Glγ-giardin is a component of the trilaminar structure of the ventral disc. Expression of Glγ-giardin is induced in the G2-phase prior to active cell division, whereas its expression decreases during encystation, a dormant stage of G. lamblia.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3478-8) contains supplementary material, which is available to authorized users.

Genetic variation in metronidazole metabolism and oxidative stress pathways in clinical Giardia lamblia assemblage A and B isolates

Purpose: Treatment-refractory Giardia cases have increased rapidly within the last decade. No markers of resistance nor a standardized susceptibility test have been established yet, but several enzymes and their pathways have been associated with metronidazole (MTZ) resistant Giardia. Very limited data are available regarding genetic variation in these pathways. We aimed to investigate genetic variation in metabolic pathway genes proposed to be involved in MTZ resistance in recently acquired, cultured clinical isolates.

Methods: Whole genome sequencing of 12 assemblage A2 and 8 assemblage B isolates was done, to decipher genomic variation in Giardia. Twenty-nine genes were identified in a literature search and investigated for their single nucleotide variants (SNVs) in the coding/non-coding regions of the genes, either as amino acid changing (non-synonymous SNVs) or non-changing SNVs (synonymous).

Results: In Giardia assemblage B, several genes involved in MTZ activation or oxidative stress management were found to have higher numbers of non-synonymous SNVs (thioredoxin peroxidase, nitroreductase 1, ferredoxin 2, NADH oxidase, nitroreductase 2, alcohol dehydrogenase, ferredoxin 4 and ferredoxin 1) than the average variation. For Giardia assemblage A2, the highest genetic variability was found in the ferredoxin 2, ferredoxin 6 and in nicotinamide adenine dinucleotide phosphate (NADPH) oxidoreductase putative genes. SNVs found in the ferredoxins and nitroreductases were analyzed further by alignment and homology modeling. SNVs close to the iron-sulfur cluster binding sites in nitroreductase-1 and 2 and ferredoxin 2 and 4 could potentially affect protein function. Flavohemoprotein seems to be a variable-copy gene, due to higher, but variable coverage compared to other genes investigated.

Conclusion: In clinical Giardia isolates, genetic variability is common in important genes in the MTZ metabolizing pathway and in the management of oxidative and nitrosative stress and includes high numbers of non-synonymous SNVs. Some of the identified amino acid changes could potentially affect the respective proteins important in the MTZ metabolism.

Roles of end-binding 1 protein and gamma-tubulin small complex in cytokinesis and flagella formation of Giardia lamblia

Giardia lamblia is a unicellular organism with two nuclei, a median body, eight flagella, and an adhesive disk. γ‐Tubulin is a microtubule (MT)‐nucleating protein that functions in the γ‐tubulin small complex (γ‐TuSC) in budding yeast. In this study, G. lamblia γ‐tubulin (Glγ‐tubulin) was found to bind to another MT‐binding protein, namely G. lamblia end‐binding protein 1 (GlEB1), via both in vivo and in vitro assays. Hemagglutinin (HA)‐tagged Glγ‐tubulin localized to the basal bodies, axonemes, and median bodies of G. lamblia trophozoites. The knockdown of Glγ‐tubulin expression using an anti‐Glγ‐tubulin morpholino resulted in a decreased growth rate and an increased failed cytokinesis cells of Giardia. The formation of median bodies was affected, and the central pair of MTs in flagella was frequently missing in the Giardia treated with an anti‐Glγ‐tubulin morpholino. G. lamblia γ‐tubulin complex protein 2 (GlGCP2) and GlGCP3, which are putative components of γ‐TuSC, were co‐immunoprecipitated with HA‐tagged Glγ‐tubulin in Giardia extracts. The knockdown of GlGCP2 and GlGCP3 expression also resulted in decreased formation of both the median body and flagella MTs. Knockdown of Glγ‐tubulin, GlGCP2, and GlGCP3 expression affected localization of GlEB1 in G. lamblia. In addition, decreased level of GlEB1 caused reduced formation of median body and the central pair of flagella MTs. These results indicated that Glγ‐tubulin plays a role in MT nucleation for median body formation and flagella biogenesis as a component of Glγ‐TuSC in Giardia and GlEB1 may be involved in this process.

DNA damage induced by metronidazole in Giardia duodenalis triggers a DNA homologous recombination response

The mechanisms underlying metronidazole (MTZ) resistance in Giardia duodenalis have been associated with decreased activity of the enzymes implicated in its activation including nitroductase-1, thioredoxin reductase and pyruvate-ferredoxin oxidoreductase (PFOR). MTZ activation generates radicals that can form adducts with proteins such as thioredoxin reductase and α- and -β giardins as well as DNA damage resulting in trophozoite's death. The damage induced in DNA requires a straight forward response that may allow parasite survival. Here, we studied changes in histone H2A phosphorylation to evaluate the DNA repair response pathway after induction of double strand break (DSB) by MTZ in Giardia DNA. Our results showed that the DNA repair mechanisms after exposure of Giardia trophozoites to MTZ, involved a homologous recombination pathway. We observed a significant increase in the expression level of proteins GdDMC1B, which carries out Rad51 role in G. duodenalis, and GdMre11, after 12 h of exposure to 3.2 μM MTZ. This increase was concomitant with the generation of DSB in the DNA of trophozoites treated MTZ. Altogether, these results suggest that MTZ-induced DNA damage in Giardia triggers the DNA homologous recombination repair (DHRR) pathway, which may contribute to the parasite survival in the presence of MTZ.

Drug-Free Approach To Study the Unusual Cell Cycle of Giardia intestinalis

Giardia intestinalis is a protozoan parasite that causes giardiasis, a form of severe and infectious diarrhea. Despite the importance of the cell cycle in the control of proliferation and differentiation during a giardia infection, it has been difficult to study this process due to the absence of a synchronization procedure that would not induce cellular damage resulting in artifacts. We utilized counterflow centrifugal elutriation (CCE), a size-based separation technique, to successfully obtain fractions of giardia cultures enriched in G₁, S, and G₂. Unlike drug-induced synchronization of giardia cultures, CCE did not induce double-stranded DNA damage or endoreplication. We observed increases in the appearance and size of the median body in the cells from elutriation fractions corresponding to the progression of the cell cycle from early G₁ to late G₂. Consequently, CCE could be used to examine the dynamics of the median body and other structures and organelles in the giardia cell cycle. For the cell cycle gene expression studies, the actin-related gene was identified by the program geNorm as the most suitable normalizer for reverse transcription-quantitative PCR (RT-qPCR) analysis of the CCE samples. Ten of 11 suspected cell cycle-regulated genes in the CCE fractions have expression profiles in giardia that resemble those of higher eukaryotes. However, the RNA levels of these genes during the cell cycle differ less than 4-fold to 5-fold, which might indicate that large changes in gene expression are not required by giardia to regulate the cell cycle. IMPORTANCE Giardias are among the most commonly reported intestinal protozoa in the world, with infections seen in humans and over 40 species of animals. The life cycle of giardia alternates between the motile trophozoite and the infectious cyst. The regulation of the cell cycle controls the proliferation of giardia trophozoites during an active infection and contains the restriction point for the differentiation of trophozoite to cyst. Here, we developed counterflow centrifugal elutriation as a drug-free method to obtain fractions of giardia cultures enriched in cells from the G₁, S, and G₂ stages of the cell cycle. Analysis of these fractions showed that the cells do not show side effects associated with the drugs used for synchronization of giardia cultures. Therefore, counterflow centrifugal elutriation would advance studies on key regulatory events during the giardia cell cycle and identify potential drug targets to block giardia proliferation and transmission.

Semisynthesis of new aphidicolin derivatives with high activity against Trypanosoma cruzi

Chagas disease continues to be a difficult disease to eradicate, largely because of the widespread populations it affects as well as the highly toxic effects of current therapies. Thus, the exploration of innovative scaffolds, ideally with distinct mechanisms of action, is urgently needed. The natural product aphidicolin and its effects on cell cycle division have been widely studied; it is a potent inhibitor of parasitic cells. In the present study, we report for the first time the semisynthesis of a series of aphidicolin derivatives, their unique structural features, and demonstration of their activity against Trypanosoma cruzi cells. Two demonstrated high potency and selectivity against parasitic amastigote cells, and thus show promise as new leads for Chagas disease treatment.

Albendazole induces oxidative stress and DNA damage in the parasitic protozoan Giardia duodenalis

The control of Giardia duodenalis infections is carried out mainly by drugs, among these albendazole (ABZ) is commonly used. Although the cytotoxic effect of ABZ usually involves binding to β-tubulin, it has been suggested that oxidative stress may also play a role in its parasiticidal mechanism. In this work the effect of ABZ in Giardia clones that are susceptible or resistant to different concentrations (1.35, 8, and 250 μM) of this drug was analyzed. Reactive oxygen species (ROS) were induced by ABZ in susceptible clones and this was associated with a decrease in growth that was alleviated by cysteine supplementation. Remarkably, ABZ-resistant clones exhibited partial cross-resistance to H₂O_2, whereas a Giardia H₂O₂-resistant strain can grow in the presence of ABZ. Lipid oxidation and protein carbonylation in ABZ-treated parasites did not show significant differences as compared to untreated parasites; however, ABZ induced the formation of 8OHdG adducts and DNA degradation, indicating nucleic acid oxidative damage. This was supported by observations of histone H2AX phosphorylation in ABZ-susceptible trophozoites treated with 250 μM ABZ. Flow cytometry analysis showed that ABZ partially arrested cell cycle in drug-susceptible clones at G2/M phase at the expense of cells in G1 phase. Also, ABZ treatment resulted in phosphatidylserine exposure on the parasite surface, an event related to apoptosis. All together these data suggest that ROS induced by ABZ affect Giardia genetic material through oxidative stress mechanisms and subsequent induction of apoptotic-like events.

UV irradiation responses in Giardia intestinalis

The response to ultraviolet light (UV) radiation, a natural stressor to the intestinal protozoan parasite Giardia intestinalis, was studied to deepen the understanding of how the surrounding environment affects the parasite during transmission. UV radiation at 10 mJ/cm(2) kills Giardia cysts effectively whereas trophozoites and encysting parasites can recover from UV treatment at 100 mJ/cm(2) and 50 mJ/cm(2) respectively. Staining for phosphorylated histone H2A showed that UV treatment induces double-stranded DNA breaks and flow cytometry analyses revealed that UV treatment of trophozoites induces DNA replication arrest. Active DNA replication coupled to DNA repair could be an explanation to why UV light does not kill trophozoites and encysting cells as efficiently as the non-replicating cysts. We also examined UV-induced gene expression responses in both trophozoites and cysts using RNA sequencing (RNA seq). UV radiation induces small overall changes in gene expression in Giardia but cysts show a stronger response than trophozoites. Heat shock proteins, kinesins and Nek kinases are up-regulated, whereas alpha-giardins and histones are down-regulated in UV treated trophozoites. Expression of variable surface proteins (VSPs) is changed in both trophozoites and cysts. Our data show that Giardia cysts have limited ability to repair UV-induced damage and this may have implications for drinking- and waste-water treatment when setting criteria for the use of UV disinfection to ensure safe water.

The effect of metronidazole on the cell cycle and DNA in metronidazole-susceptible and -resistant Giardia cell lines

Metronidazole (MTZ) is used as the drug of choice to treat Giardia infections. It is believed that the prodrug is transformed intracellularly into toxic intermediates that interact with cellular components, leading to cell death. The present study aimed to describe the effects of MTZ treatment on DNA and cell cycle progression in MTZ-sensitive and in vitro-derived MTZ-resistant cell lines. Detection of the phosphorylated form of histone H2A in cell nuclei together with electrophoresis of genomic DNA, flow cytometry analysis and incubation of cells with other drugs (albendazole or neomycin) demonstrated that DNA damage in MTZ-treated cells is clearly conditioned by the presence of this drug. The flow cytometry analysis and a BrdU labeling assay showed that the sublethal drug concentration affects the replication phase of the cell cycle. Cells incubated with lethal drug concentration exhibit unchanged DNA profile, only about 50% of cells are positive for γH2A and lose an ability to attach to a surface after few hours of incubation. It is likely that the early reaction of cells to lethal concentration of MTZ is not primarily initiated by the reaction to DNA damage but rather by the immediate interaction of MTZ with biomolecules where activated MTZ is generated. Interestingly, in MTZ-resistant lines incubated in the presence of the drug, about 40% of cells remain permanently positive for γH2A without any effects on the cell cycle progression suggesting that DNA damage caused by MTZ treatment persists in these cells. Accelerated mutagenesis caused by MTZ-induced DNA damage may therefore be a new factor contributing to the development of natural resistance.

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.

Evaluation of drugs and stationary growth on the cell cycle of Giardia intestinalis

We examined the effect of aphidicolin, colchicine, demecolcine, fluorouracil, hydroxyurea, and nocodazole, as well as nutrient deprivation on the Giardia intestinalis cell cycle. Aphidicolin was the only drug that was able to block the cell cycle at a specific stage (G1/S), and permit cells to resume growth at a high rate upon its removal. Nutrient deprivation resulted in a portion of G2/M cells completing mitosis and cytokinesis in synchrony during the recovery period, but this synchrony was shortly lost and a sample containing a predominance of G1 cells could not be obtained. Flow cytometry analysis of normal and untreated Giardia cultures showed the occasional appearance of a small percentage of cells with a DNA content of 16C, which is twice the DNA content of G2 cells. However, this 16C peak is larger and more frequently observed in drug-treated Giardia. These 16C are likely produced from endoreplication of 8C/G2 cells, and we propose that they represent a pre-encystation stage that is induced by drug treatments and other stressors.

How nuclei of Giardia pass through cell differentiation: semi-open mitosis followed by nuclear interconnection

Differentiation into infectious cysts (encystation) and multiplication of pathogenic trophozoites after hatching from the cyst (excystation) are fundamental processes in the life cycle of the human intestinal parasite Giardia intestinalis. During encystation, a bi-nucleated trophozoite transforms to a dormant tetra-nucleated cyst enveloped by a protective cyst wall. Nuclear division during encystation is not followed by cytokinesis. In contrast to the well-studied mechanism of cyst wall formation, information on nuclei behavior is incomplete and basic cytological data are lacking. Here we present evidence that (1) the nuclei divide by semi-open mitosis during early encystment; (2) the daughter nuclei coming from different parent nuclei are always arranged in pairs; (3) in both pairs, the nuclei are interconnected via bridges formed by fusion of their nuclear envelopes; (4) each interconnected nuclear pair is associated with one basal body tetrad of the undivided diplomonad mastigont; and (5) the interconnection between nuclei persists through the cyst stage being a characteristic feature of encysted Giardia. Based on the presented results, a model of nuclei behavior during Giardia differentiation is proposed.

The minimal kinome of Giardia lamblia illuminates early kinase evolution and unique parasite biology

Background

The major human intestinal pathogen Giardia lamblia is a very early branching eukaryote with a minimal genome of broad evolutionary and biological interest.

Results

To explore early kinase evolution and regulation of Giardia biology, we cataloged the kinomes of three sequenced strains. Comparison with published kinomes and those of the excavates Trichomonas vaginalis and Leishmania major shows that Giardia's 80 core kinases constitute the smallest known core kinome of any eukaryote that can be grown in pure culture, reflecting both its early origin and secondary gene loss. Kinase losses in DNA repair, mitochondrial function, transcription, splicing, and stress response reflect this reduced genome, while the presence of other kinases helps define the kinome of the last common eukaryotic ancestor. Immunofluorescence analysis shows abundant phospho-staining in trophozoites, with phosphotyrosine abundant in the nuclei and phosphothreonine and phosphoserine in distinct cytoskeletal organelles. The Nek kinase family has been massively expanded, accounting for 198 of the 278 protein kinases in Giardia. Most Neks are catalytically inactive, have very divergent sequences and undergo extensive duplication and loss between strains. Many Neks are highly induced during development. We localized four catalytically active Neks to distinct parts of the cytoskeleton and one inactive Nek to the cytoplasm.

Conclusions

The reduced kinome of Giardia sheds new light on early kinase evolution, and its highly divergent sequences add to the definition of individual kinase families as well as offering specific drug targets. Giardia's massive Nek expansion may reflect its distinctive lifestyle, biphasic life cycle and complex cytoskeleton.