Small-intestinal factors promote encystation of Giardia lamblia in vitro

Giardia lamblia: Laboratory Maintenance, Lifecycle Induction, and Infection of Murine Models

Giardia lamblia is a protozoan parasite that is found ubiquitously throughout the world and is a major contributor to diarrheal disease. Giardia exhibits a biphasic lifestyle existing as either a dormant cyst or a vegetative trophozoite. Infections are typically initiated through the consumption of cyst-contaminated water or food. Giardia was first axenized in the 1970s and can be readily maintained in a laboratory setting. Additionally, Giardia is one of the few protozoans that can be induced to complete its complete lifecycle using laboratory methods. In this article, we outline protocols to maintain Giardia and induce passage through its lifecycle. We also provide protocols for infecting and quantifying parasites in an animal infection model. © 2020 Wiley Periodicals LLC. Basic Protocol 1: In vitro maintenance and growth of Giardia trophozoites Basic Protocol 2: In vitro encystation of Giardia cysts Basic Protocol 3: In vivo infections using Giardia trophozoites.

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.

Lactoferrin and lactoferricin endocytosis halt Giardia cell growth and prevent infective cyst production

Lactoferrin (LF) is an 80 KDa iron-binding glycoprotein that plays a significant role in the innate immune system and is considered to be an important microbicide molecule. It has been suggested to be effective in the treatment of giardiasis, an intestinal disease caused by the protozoan parasite G. lamblia. However, the molecular mechanisms by which LF exerts its effect on this parasite are unknown. Most of the microbicidal activity of human or bovine LF (hLF or bLF) has been associated with the N-terminal region of the mature LF - lactoferricin (LFcin). LFcin is produced by pepsin cleavage of the native protein in vitro and likely in vivo. In this work, we analyse the participation of the endocytic machinery of G. lamblia in the internalization of bLF and bLFcin and their effects on cell homeostasis. Our results show that, when bLF or bLFcin are internalized by receptor-mediated endocytosis, cell growth stops, and morphological changes are produced in the trophozoites, which ultimately will produce immature cysts. Our findings contribute to disclose the fine mechanism by which bLF and bLFcin may function as an antigiardial molecule and why they have therapeutic potential to eradicate giardiasis.

Characterization of the Giardia intestinalis secretome during interaction with human intestinal epithelial cells: The impact on host cells

BACKGROUND

Giardia intestinalis is a non-invasive protozoan parasite that causes giardiasis in humans, the most common form of parasite-induced diarrhea. Disease mechanisms are not completely defined and very few virulence factors are known.

METHODOLOGY

To identify putative virulence factors and elucidate mechanistic pathways leading to disease, we have used proteomics to identify the major excretory-secretory products (ESPs) when Giardia trophozoites of WB and GS isolates (assemblages A and B, respectively) interact with intestinal epithelial cells (IECs) in vitro.

FINDINGS

The main parts of the IEC and parasite secretomes are constitutively released proteins, the majority of which are associated with metabolism but several proteins are released in response to their interaction (87 and 41 WB and GS proteins, respectively, 76 and 45 human proteins in response to the respective isolates). In parasitized IECs, the secretome profile indicated effects on the cell actin cytoskeleton and the induction of immune responses whereas that of Giardia showed anti-oxidation, proteolysis (protease-associated) and induction of encystation responses. The Giardia secretome also contained immunodominant and glycosylated proteins as well as new candidate virulence factors and assemblage-specific differences were identified. A minor part of Giardia ESPs had signal peptides (29% for both isolates) and extracellular vesicles were detected in the ESPs fractions, suggesting alternative secretory pathways. Microscopic analyses showed ESPs binding to IECs and partial internalization. Parasite ESPs reduced ERK1/2 and P38 phosphorylation and NF-κB nuclear translocation. Giardia ESPs altered gene expression in IECs, with a transcriptional profile indicating recruitment of immune cells via chemokines, disturbances in glucose homeostasis, cholesterol and lipid metabolism, cell cycle and induction of apoptosis.

CONCLUSIONS

This is the first study identifying Giardia ESPs and evaluating their effects on IECs. It highlights the importance of host and parasite ESPs during interactions and reveals the intricate cellular responses that can explain disease mechanisms and attenuated inflammatory responses during giardiasis.

Giardia Colonizes and Encysts in High-Density Foci in the Murine Small Intestine

Giardia is a single-celled parasite causing significant diarrheal disease in several hundred million people worldwide. Due to limited access to the site of infection in the gastrointestinal tract, our understanding of the dynamics of Giardia infections in the host has remained limited and largely inferred from laboratory culture. To better understand Giardia physiology and colonization in the host, we developed imaging methods to quantify Giardia expressing bioluminescent physiological reporters in two relevant animal models. We discovered that parasites primarily colonize and encyst in the proximal small intestine in discrete, high-density foci. We also show that high parasite density contributes to encystation initiation.

Giardia lamblia is a highly prevalent yet understudied protistan parasite causing significant diarrheal disease worldwide. Hosts ingest Giardia cysts from contaminated sources. In the gastrointestinal tract, cysts excyst to become motile trophozoites, colonizing and attaching to the gut epithelium. Trophozoites later differentiate into infectious cysts that are excreted and contaminate the environment. Due to the limited accessibility of the gut, the temporospatial dynamics of giardiasis in the host are largely inferred from laboratory culture and thus may not mirror Giardia physiology in the host. Here, we have developed bioluminescent imaging (BLI) to directly interrogate and quantify the in vivo temporospatial dynamics of Giardia infection, thereby providing an improved murine model to evaluate anti-Giardia drugs. Using BLI, we determined that parasites primarily colonize the proximal small intestine nonuniformly in high-density foci. By imaging encystation-specific bioreporters, we show that encystation initiates shortly after inoculation and continues throughout the duration of infection. Encystation also initiates in high-density foci in the proximal small intestine, and high density contributes to the initiation of encystation in laboratory culture. We suggest that these high-density in vivo foci of colonizing and encysting Giardia likely result in localized disruption to the epithelium. This more accurate visualization of giardiasis redefines the dynamics of the in vivo Giardia life cycle, paving the way for future mechanistic studies of density-dependent parasitic processes in the host.

IMPORTANCEGiardia is a single-celled parasite causing significant diarrheal disease in several hundred million people worldwide. Due to limited access to the site of infection in the gastrointestinal tract, our understanding of the dynamics of Giardia infections in the host has remained limited and largely inferred from laboratory culture. To better understand Giardia physiology and colonization in the host, we developed imaging methods to quantify Giardia expressing bioluminescent physiological reporters in two relevant animal models. We discovered that parasites primarily colonize and encyst in the proximal small intestine in discrete, high-density foci. We also show that high parasite density contributes to encystation initiation.

Transcriptomic Profiling of High-Density Giardia Foci Encysting in the Murine Proximal Intestine

Giardia is a highly prevalent, understudied protistan parasite causing significant diarrheal disease worldwide. Its life cycle consists of two stages: infectious cysts ingested from contaminated food or water sources, and motile trophozoites that colonize and attach to the gut epithelium, later encysting to form new cysts that are excreted into the environment. Current understanding of parasite physiology in the host is largely inferred from transcriptomic studies using Giardia grown axenically or in co-culture with mammalian cell lines. The dearth of information about the diversity of host-parasite interactions occurring within distinct regions of the gastrointestinal tract has been exacerbated by a lack of methods to directly and non-invasively interrogate disease progression and parasite physiology in live animal hosts. By visualizing Giardia infections in the mouse gastrointestinal tract using bioluminescent imaging (BLI) of tagged parasites, we recently showed that parasites colonize the gut in high-density foci. Encystation is initiated in these foci throughout the entire course of infection, yet how the physiology of parasites within high-density foci in the host gut differs from that of cells in laboratory culture is unclear. Here we use BLI to precisely select parasite samples from high-density foci in the proximal intestine to interrogate in vivo Giardia gene expression in the host. Relative to axenic culture, we noted significantly higher expression (>10-fold) of oxidative stress, membrane transporter, and metabolic and structural genes associated with encystation in the high-density foci. These differences in gene expression within parasite foci in the host may reflect physiological changes associated with high-density growth in localized regions of the gut. We also identified and verified six novel cyst-specific proteins, including new components of the cyst wall that were highly expressed in these foci. Our in vivo transcriptome data support an emerging view that parasites encyst early in localized regions in the gut, possibly as a consequence of nutrient limitation, and also impact local metabolism and physiology.

Rac Regulates Giardia lamblia Encystation by Coordinating Cyst Wall Protein Trafficking and Secretion

Encystation of the common intestinal parasite Giardia lamblia involves the production, trafficking, and secretion of cyst wall material (CWM). However, the molecular mechanism responsible for the regulation of these sequential processes remains elusive. Here, we examined the role of GlRac, Giardia’s sole Rho family GTPase, in the regulation of endomembrane organization and cyst wall protein (CWP) trafficking. Localization studies indicated that GlRac is associated with the endoplasmic reticulum (ER) and the Golgi apparatus-like encystation-specific vesicles (ESVs). Constitutive GlRac signaling increased levels of the ER marker PDI2, induced ER swelling, reduced overall CWP1 production, and promoted the early maturation of ESVs. Quantitative analysis of cells expressing constitutively active hemagglutinin (HA)-tagged GlRac (HA-Rac^CA) revealed fewer but larger ESVs than control cells. Consistent with the phenotype of premature maturation of ESVs in HA-Rac^CA-expressing cells, constitutive GlRac signaling resulted in increased CWP1 secretion and, conversely, morpholino depletion of GlRac blocked CWP1 secretion. Wild-type cells unexpectedly secreted large quantities of CWP1 into the medium, and free CWP1 was used cooperatively during cyst formation. These results, in part, could account for the previously reported observation that G. lamblia encysts more efficiently at high cell densities. These studies of GlRac show that it regulates encystation at several levels, and our findings support its coordinating role as a regulator of CWP trafficking and secretion. The central role of GlRac in regulating membrane trafficking and the cytoskeleton, both of which are essential to Giardia parasitism, further suggests its potential as a novel target for drug development to treat giardiasis.

The encystation process is crucial for the transmission of giardiasis and the life cycle of many protists. Encystation for Giardia lamblia involves the assembly of a protective cyst wall via sequential production, trafficking, and secretion of cyst wall material. However, the regulatory pathways that coordinate cargo maturation and secretion remain unknown. Here, we asked whether the signaling activities of G. lamblia’s single Rho family GTPase, GlRac, might have a regulatory role in the encystation process. We show that GlRac localizes to endomembranes and its signaling activities regulate the production of cyst wall protein 1 (CWP1), the maturation of encystation-specific vesicles (ESVs), and secretion of CWP1. We also show that secreted CWP1 is available for the development of cysts at the population level, a finding that in part could explain why Giardia encystation proceeds more efficiently at high cell densities.

Infection Strategies of Intestinal Parasite Pathogens and Host Cell Responses

Giardia lamblia, Cryptosporidium sp., and Entamoeba histolytica are important pathogenic intestinal parasites and are amongst the leading causes worldwide of diarrheal illness in humans. Diseases caused by these organisms, giardiasis, cryptosporidiosis, and amoebiasis, respectively, are characterized by self-limited diarrhea but can evolve to long-term complications. The cellular and molecular mechanisms underlying the pathogenesis of diarrhea associated with these three pathogens are being unraveled, with knowledge of both the strategies explored by the parasites to establish infection and the methods evolved by hosts to avoid it. Special attention is being given to molecules participating in parasite–host interaction and in the mechanisms implicated in the diseases’ pathophysiologic processes. This review focuses on cell mechanisms that are modulated during infection, including gene transcription, cytoskeleton rearrangements, signal transduction pathways, and cell death.

A systematic review and meta-analysis of the association between Giardia lamblia and endemic pediatric diarrhea in developing countries

We performed a systematic literature review and meta-analysis examining the association between diarrhea in young children in nonindustrialized settings and Giardia lamblia infection. Eligible were case/control and longitudinal studies that defined the outcome as acute or persistent (>14 days) diarrhea, adjusted for confounders and lasting for at least 1 year. Data on G. lamblia detection (mainly in stools) from diarrhea patients and controls without diarrhea were abstracted. Random effects model meta-analysis obtained pooled odds ratios (ORs) and 95% confidence intervals (CIs). Twelve nonindustrialized-setting acute pediatric diarrhea studies met the meta-analysis inclusion criteria. Random-effects model meta-analysis of combined results (9774 acute diarrhea cases and 8766 controls) yielded a pooled OR of 0.60 (95% CI, .38-.94; P = .03), indicating that G. lamblia was not associated with acute diarrhea. However, limited data suggest that initial Giardia infections in early infancy may be positively associated with diarrhea. Meta-analysis of 5 persistent diarrhea studies showed a pooled OR of 3.18 (95% CI, 1.50-6.76; P < .001), positively linking Giardia with that syndrome. The well-powered Global Enteric Multicenter Study (GEMS) is prospectively addressing the association between G. lamblia infection and diarrhea in children in developing countries.

Nuclear inheritance and genetic exchange without meiosis in the binucleate parasite Giardia intestinalis

The protozoan parasite Giardia intestinalis (also known as Giardia lamblia) is a major waterborne pathogen. During its life cycle, Giardia alternates between the actively growing trophozoite, which has two diploid nuclei with low levels of allelic heterozygosity, and the infectious cyst, which has four nuclei and a tough outer wall. Although the formation of the cyst wall has been studied extensively, we still lack basic knowledge about many fundamental aspects of the cyst, including the sources of the four nuclei and their distribution during the transformation from cyst into trophozoite. In this study, we tracked the identities of the nuclei in the trophozoite and cyst using integrated nuclear markers and immunofluorescence staining. We demonstrate that the cyst is formed from a single trophozoite by a mitotic division without cytokinesis and not by the fusion of two trophozoites. During excystation, the cell completes cytokinesis to form two daughter trophozoites. The non-identical nuclear pairs derived from the parent trophozoite remain associated in the cyst and are distributed to daughter cells during excystation as pairs. Thus, nuclear sorting (such that each daughter cell receives a pair of identical nuclei) does not appear to be a mechanism by which Giardia reduces heterozygosity between its nuclei. Rather, we show that the cyst nuclei exchange chromosomal genetic material, perhaps as a way to reduce heterozygosity in the absence of meiosis and sex, which have not been described in Giardia. These results shed light on fundamental aspects of the Giardia life cycle and have implications for our understanding of the population genetics and cell biology of this binucleate parasite.

Glycogen storage and degradation during in vitro growth and differentiation of Giardia intestinalis

Giardia intestinalis is the causative agent of human giardiasis, a common diarrheal illness worldwide. Despite its global distribution and prevalence, many questions regarding its basic biology and metabolism remain unanswered. In this study, we examine the accumulation and degradation of glycogen, an important source of stored carbon and energy, during the in vitro growth and differentiation of G. intestinalis . We report that, as G. intestinalis progresses through its growth cycle, cultures of trophozoites accumulate glycogen during the lag and early logarithmic phases of growth and then utilize this compound during their remaining logarithmic growth. As cultures enter the stationary phase of growth, they re-accumulate glycogen stores. The activity of glycogen phosphorylase, an enzyme involved in glycogen metabolism, also varied throughout in vitro trophozoite growth. During the in vitro induction of trophozoite differentiation into water-resistant cyst forms, the cultures initially accumulated stores of glycogen which diminished throughout transition to the cyst form. This observation is suggestive of a role for glycogen in the differentiation process. These studies represent the first thorough analysis of changes in glycogen content and glycogen phosphorylase activity during G. intestinalis growth and differentiation.

Lipid metabolism in Giardia: a post-genomic perspective

Giardia lamblia, a protozoan parasite, infects a wide variety of vertebrates, including humans. Studies indicate that this anaerobic protist possesses a limited ability to synthesize lipid molecules de novo and depends on supplies from its environment for growth and differentiation. It has been suggested that most lipids and fatty acids are taken up by endocytic and non-endocytic pathways and are used by Giardia for energy production and membrane/organelle biosynthesis. The purpose of this article is to provide an update on recent progress in the field of lipid research of this parasite and the validation of lipid metabolic pathways through recent genomic information. Based on current cellular, biochemical and genomic data, a comprehensive pathway has been proposed to facilitate our understanding of lipid and fatty acid metabolism/syntheses in this waterborne pathogen. We envision that the current review will be helpful in identifying targets from the pathways that could be used to design novel therapies to control giardiasis and related diseases.

Encystation process of Giardia lamblia: morphological and regulatory aspects

One important step in the life cycle of the pathogenic protozoan Giardia lamblia is the transformation of the proliferative form, the trophozoite, into the non-proliferative cyst. This process, known as encystation, can be triggered in vitro. Morphological analysis showed that during trophozoite-cyst transformation, major changes take place: modification of the protozoan shape, internalization of the flagella, fragmentation of the adhesive disk, and appearance of encystation vesicles (ESVs), which later on fuse with the plasma membrane forming the cell wall. Sites of attachment of these vesicles to the inner portion of the protozoan plasma membrane were observed 6 h after the beginning of the encystation process. These sites were only visible when we used high-resolution scanning electron microscopy to study Giardia surface. In order to analyze the involvement of protein kinases and phosphatases on the encystation process, inhibitors of these enzymes were added to the culture medium, and their effect on the differentiation process was determined using light, immunofluorescence, and electron microscopy. Significant inhibition was observed with LY294002, an inhibitor of PI3 kinase; genistein, an inhibitor of tyrosine kinase; and staurosporine, at concentrations, which inhibit protein kinase C. Okadaic acid, an inhibitor or protein phosphatase, and wortmannin, an inhibitor of PI3K, did not interfere with the encystation process. However, they induced the appearance of large and pleomorphic forms where several nuclei and disorganization of the peripheral vesicles were observed.

Changes in the N-glycome, glycoproteins with Asn-linked glycans, of Giardia lamblia with differentiation from trophozoites to cysts

Giardia lamblia is present in the intestinal lumen as a binucleate, flagellated trophozoite or a quadranucleate, immotile cyst. Here we used the plant lectin wheat germ agglutinin (WGA), which binds to the disaccharide di-N-acetyl-chitobiose (GlcNAc(2)), which is the truncated Asn-linked glycan (N-glycan) of Giardia, to affinity purify the N-glycomes (glycoproteins with N-glycans) of trophozoites and cysts. Fluorescent WGA bound to the perinuclear membranes, peripheral acidified vesicles, and plasma membranes of trophozoites. In contrast, WGA bound strongly to membranes adjacent to the wall of Giardia cysts and less strongly to the endoplasmic reticulum and acidified vesicles. WGA lectin-affinity chromatography dramatically enriched secreted and membrane proteins of Giardia, including proteases and acid phosphatases that retain their activities. With mass spectroscopy, we identified 91 glycopeptides with N-glycans and 194 trophozoite-secreted and membrane proteins, including 42 unique proteins. The Giardia oligosaccharyltransferase, which contains a single catalytic subunit, preferred N glycosylation sites with Thr to those with Ser in vivo but had no preference for flanking amino acids. The most-abundant glycoproteins in the N-glycome of trophozoites were lysosomal enzymes, folding-associated proteins, and unique transmembrane proteins with Cys-, Leu-, or Gly-rich repeats. We identified 157 secreted and membrane proteins in the Giardia cysts, including 20 unique proteins. Compared to trophozoites, cysts were enriched in Gly-rich repeat transmembrane proteins, cyst wall proteins, and unique membrane proteins but had relatively fewer Leu-rich repeat proteins, folding-associated proteins, and unique secreted proteins. In summary, there are major changes in the Giardia N-glycome with the differentiation from trophozoites to cysts.

Use of Giardia, which appears to have a single nucleotide-sugar transporter for UDP-GlcNAc, to identify the UDP-Glc transporter of Entamoeba

Nucleotide-sugar transporters (NSTs) transport activated sugars (e.g. UDP-GlcNAc) from the cytosol to the lumen of the endoplasmic reticulum or Golgi apparatus where they are used to make glycoproteins and glycolipids. UDP-Glc is an important component of the N-glycan-dependent quality control (QC) system for protein folding. Because Entamoeba has this QC system while Giardia does not, we hypothesized that transfected Giardia might be used to identify the UDP-Glc transporter of Entamoeba. Here we show Giardia membranes transport UDP-GlcNAc and have apyrases, which hydrolyze nucleoside-diphosphates to make the antiporter nucleoside-monophosphate. The only NST of Giardia (GlNst), which we could identify, transports UDP-GlcNAc in transfected Saccharomyces and is present in perinuclear and peripheral vesicles and increases in expression during encystation. Entamoeba membranes transport three nucleotide-sugars (UDP-Gal, UDP-GlcNAc, and UDP-Glc), and Entamoeba has three NSTs, one of which has been shown previously to transport UDP-Gal (EhNst1). Here we show recombinant EhNst2 transports UDP-Glc in transfected Giardia, while recombinant EhNst3 transports UDP-GlcNAc in transfected Saccharomyces. In summary, all three NSTs of Entamoeba and the single NST of Giardia have been molecularly characterized, and transfected Giardia provides a new system for testing heterologous UDP-Glc transporters.

Transcriptional analysis of three major putative phosphatidylinositol kinase genes in a parasitic protozoan, Giardia lamblia

The current investigation evaluates the expression of phosphatidylinositol kinase (PIK) genes in the parasitic protozoan, Giardia lamblia. The G. lamblia Genome Database revealed the presence of two putative phosphatidylinositol-3-kinase (gPI3K) and one phosphatidylinositol-4-kinase (gPI4K) genes resembling the catalytic subunit of eukaryotic PIKs. Primers, designed to amplify mRNA of these three genes, were used to measure transcription by quantitative reverse-transcriptase polymerase chain reactions. Results suggest that all three PIK genes are expressed in non-encysting and encysting trophozoites. The relative levels of the mRNA were highest in parasites cultured in pre-encysting medium that contained no bile. Two inhibitors of PI3K, LY 294002 and wortmannin were found to inhibit the growth of the trophozoite in culture. However, wortmannin was more effective than LY294002. Altogether, the present study indicates that Giardia is capable of expressing PIKs that are necessary for the growth and differentiation of this pathogen.

Fine structure of the biogenesis of Giardia lamblia encystation secretory vesicles

Synthesis, transport, and assembly of the extracellular cyst wall is the hallmark of Giardia lamblia encystation. Much is known of the biochemical pathways and their regulation. However, from a cell biology point of view, the biogenesis of the encystation specific vesicles (ESVs) that transport cyst wall proteins to the periphery of the cell is poorly understood. Therefore, we exploited a number of complementary ultrastructural approaches to test the hypothesis that the formation of ESVs utilizes a novel regulated secretory pathway. We analyzed parasites at different stages of encystation in vitro by electron microscopy of thin sections, freeze fracture replicas, and three-dimensional reconstruction from serial sections of cells fixed for cytochemical localization of the endoplasmic reticulum (ER) marker, glucose 6-phosphatase. We also used a stereological approach to determine the area occupied by the ER, clefts, ESVs, and cyst wall. Taken together, our kinetic data suggest that some ER cisternae first dilate to form clefts, which enlarge into the ESVs. Living non-encysting and early-encysting trophozoites were labeled around the periphery of both nuclei with C(6)-NBD-ceramide. At 18-21 h, outward migration of some ESVs frequently caused protrusions at the periphery of encysting trophozoites. The presence of lysosome-like peripheral vesicles between the ESV and plasma membrane of the cell was confirmed using acridine orange, an acidic compartment marker. Our data suggest that G. lamblia has a novel secretory pathway in which certain functions of the ER and Golgi co-localize spatially and temporally. These studies will increase understanding of the evolutionary appearance of regulated secretory pathways for assembly of a primitive extracellular matrix in an early diverging eukaryote.

Lipid metabolism in mucous-dwelling amitochondriate protozoa

Entamoeba, Giardia, and trichomonads are the prominent members of a group known as 'mucosal parasites'. While Entamoeba and Giardia trophozoites colonise the small intestine, trichomonads inhabit the genitourinary tracts of humans and animals. These protozoa lack mitochondria, well-developed Golgi complexes, and other organelles typical of higher eukaryotes. Nonetheless, they have developed unique metabolic pathways that allow them to survive and multiply in the small intestine and reproductive tracts by scavenging nutrients from the host. Various investigators have shown that these protozoa are unable to synthesise the majority of their own lipids and cholesterol de novo; rather, they depend mostly on supplies from outside sources. Therefore, questions of how they transport and utilise exogenous lipids for metabolic purposes are extremely important. There is evidence suggesting that these parasites can take up the lipids and cholesterol they need from lipoprotein particles present in the host and/or in the growth medium. Studies also support the idea that individual lipid and fatty acid molecules can be transported without the help of lipoproteins. Exogenous phospholipids have been shown to undergo fatty acid remodelling (by deacylation/reacylation reactions), which allows these protozoa to alter lipids, bypassing the synthesis of entirely new phospholipid molecules. In addition, many of these amitochondriates are, however, capable of elongating/desaturating long-chain fatty acids, and assembling novel glycophospholipid molecules. In this review, progress in various aspects of lipid research on these organisms is discussed. Attempts are also made to identify steps of lipid metabolic pathways that can be used to develop chemotherapeutic agents against these and other mucosal parasites.

How Giardia swim and divide

To determine how binuclear giardia swim, we used video microscopy to observe trophozoites of Giardia intestinalis, which were labeled with an amino-specific Alexa Fluor dye that highlighted the flagella and adherence disc. Giardia swam forward by means of the synchronous beating of anterior, posterolateral, and ventral flagella in the plane of the ventral disc, while caudal flagella swam in a plane perpendicular to the disc. Giardia turned in the plane of the disc by means of a rudder-like motion of its tail, which was constant rather than beating. To determine how giardia divide, we used three-dimensional confocal microscopy, the same surface label, nuclear stains, and antitubulin antibodies. Giardia divided with mirror-image symmetry in the plane of the adherence disc, so that the right nucleus of the mother became the left nucleus of the daughter. Pairs of nuclei were tethered together by microtubules which surrounded nuclei and prevented mother or daughter giardia from receiving two copies of the same nucleus. New adherence discs formed upon a spiral backbone of microtubules, which had a clockwise rotation when viewed from the ventral surface. These dynamic observations of the parasite begin to reveal how giardia swim and divide.

Biology of Giardia lamblia

Giardia lamblia is a common cause of diarrhea in humans and other mammals throughout the world. It can be distinguished from other Giardia species by light or electron microscopy. The two major genotypes of G. lamblia that infect humans are so different genetically and biologically that they may warrant separate species or subspecies designations. Trophozoites have nuclei and a well-developed cytoskeleton but lack mitochondria, peroxisomes, and the components of oxidative phosphorylation. They have an endomembrane system with at least some characteristics of the Golgi complex and encoplasmic reticulum, which becomes more extensive in encysting organisms. The primitive nature of the organelles and metabolism, as well as small-subunit rRNA phylogeny, has led to the proposal that Giardia spp. are among the most primitive eukaryotes. G. lamblia probably has a ploidy of 4 and a genome size of approximately 10 to 12 Mb divided among five chromosomes. Most genes have short 5' and 3' untranslated regions and promoter regions that are near the initiation codon. Trophozoites exhibit antigenic variation of an extensive repertoire of cysteine-rich variant-specific surface proteins. Expression is allele specific, and changes in expression from one vsp gene to another have not been associated with sequence alterations or gene rearrangements. The Giardia genome project promises to greatly increase our understanding of this interesting and enigmatic organism.

Ultrastructural and biochemical characterization of promastigote and cystic forms of Leptomonas wallacei n. sp. isolated from the intestine of its natural host Oncopeltus fasciatus (Hemiptera: Lygaeidae)

Promastigote forms of a trypanosomatid were isolated from the third and fourth ventricles of the midgut and from the hindgut of the phytophagous hemipteran Oncopeltus fasciatus. Some individuals had adhered to its anterior region, close to the flagellar pocket, or to the flagellum up to four rounded aflagellated forms known as straphangers cysts. Scanning electron microscopy revealed that the flagellated forms presented a twisted cell body and a long flagellum, and the cysts, smaller than the parental promastigote, had a nascent flagellum. Transmission electron microscopy showed that promastigotes were typical, while cystic forms were ovoid dense cells devoid of a cyst wall, but presenting a cell coat, a special subpellicular region limited by a membrane unit, and a condensed cytoplasm. The kinetoplast-DNA fibrils appeared as dense spots and the condensed chromatin was arranged in a labyrinthic structure. Desmosome-like structures, observed in the region of adhesion of the precystic forms to the parental promastigote, could explain how cysts remain attached to the mother cell during the encystation process. Release of membranes from the surface of promastigotes and cysts seems to be correlated with the condensation of the cytoplasm during encystment. Morphological and isozyme analyses indicated that this trypanosomatid belongs to the genus Leptomonas. The molecular karyotype of this isolate was compared with that of a strain of Leptomonas oncopelti obtained from Oncopeltus varicolor by contour-clamped homogeneous electric field (CHEF) electrophoresis and revealed similar DNA banding patterns between 2,200-825 Kb, but not in lower bands (825-225 Kb). This suggested that the isolate from O. fasciatus and that from O. varicolor were not identical. Based on our findings we are describing Leptomonas wallacei n. sp. for our isolate from O. fasciatus.

Giardia lamblia: incorporation of free and conjugated fatty acids into glycerol-based phospholipids

Giardia lamblia trophozoites are flagellated protozoa that inhabit the human small intestine, where they are exposed to various dietary lipids and fatty acids. It is believed that G. lamblia, which colonizes a lipid-rich environment of the human small intestine, is unable to synthesize phospholipids, long-chain fatty acids, and sterols de novo. Therefore, it is possible that this protozoan has developed a special process for acquiring lipids from its host. We have previously shown that G. lamblia can take up saturated fatty acids and incorporate them into phosphatidylglycerol (PG) and other glycerol-based phospholipids (Stevens et al., Experimental Parasitology, 86, 133-143, 1997). In the present study, an attempt has been made to investigate the underlying mechanisms of transesterification and interesterification reactions of giardial phospholipids by free and conjugated fatty acids. Results show that exogenously supplied, unsaturated, fatty acids were taken up by Giardia and incorporated into various phosphoglycerides, including PG. To test whether this intestinal pathogen can utilize conjugated fatty acids, live trophozoites were exposed to either [3]H;cbphosphatidylcholine (PC), where the fatty acid was 3H-labeled at its sn2 position, or to [14C]lyso-PC (fatty acid was 14C-labeled at the sn1 position) for 90 min, followed by phospholipid analysis using thin-layer chromatography. The results suggest that conjugated fatty acids, like free fatty acids, were incorporated into PG. It was also observed that aristolochic acid, an inhibitor of Ca2+-ionophore-stimulated phospholipase A2, decreased the transfer of fatty acids from [3H]PC to PG, indicating that giardial phospholipases were involved in these esterification reactions. Additional experiments, which include culturing trophozoites in serum-supplemented and serum-deprived medium, along with numerous biochemical analyses suggest that (i) PG is a major transesterified and interesterified product, (ii) it is likely that giardial phospholipases are involved in esterification reactions, (iii) in G. lamblia, PG is localized in perinuclear membranes, as well as intracellularly, but not in the plasma membrane, and (iv) various synthetic analogs of PG inhibit the growth of the parasite in vitro. These studies suggest that PG is an important phospholipid of Giardia and a potential target for lipid-based chemotherapy against giardiasis.

Uptake and cellular localization of exogenous lipids by Giardia lamblia, a primitive eukaryote

Giardia lamblia trophozoites are unable to carry out de novo lipid synthesis. It is therefore likely that lipids are acquired from the small intestine of the host, in which the trophozoites are exposed to free and conjugated fatty acids, various sterols, phospholipids, bile acids, and bile-lipid mixed micelles. Here we show that G. lamblia is capable of taking up exogenous phosphatidylcholine (PC), phosphatidylinositol (PI), sphingomyelin (SM), cholesterol, ceramide (Cer), and fatty acids. Results from epifluorescence and high-resolution confocal microscopy suggest that fluorescent analogs of SM and PC were accumulated in the plasma membranes, whereas palmitic acid and Cer were localized intracellularly. Interestingly, many of these analogs were also concentrated in perinuclear regions. Similar labeling patterns were observed when the fluorescent analogs were delivered to the parasite via liposomes. To test whether G. lamblia was capable of esterifying exogenous fatty acids into membrane or cellular phospholipids, trophozoites were pulse-labeled with 3H-labeled palmitic or myristic acids and the phospholipids analyzed by thin-layer chromatography. Results document that G. lamblia was able to incorporate exogenous fatty acids into various phospholipids, i.e., PI, PC, PE, and PG. Interestingly, a major portion of radiolabeled fatty acids was incorporated into PG, a phospholipid characteristic of prokaryotic membranes.

The efficacy of a Giardia lamblia vaccine in kittens

Twenty kittens were vaccinated with a Giardia lamblia vaccine prepared on a commercial scale on day 0 and boosted on day 21 (group 1); while 10 kittens received only saline (group 2). These kittens were challenged on day 35 with 10(6) Giardia lamblia trophozoites by a surgical intraduodenal injection. Three control kittens were not vaccinated and not challenged (group 3). Following challenge, Giardia vaccinated kittens had significantly fewer days in which abnormal stools were observed and reduced food intake occurred compared to saline injected animals. The rate of weight gain between group 1 and group 2 animals was not different in the prechallenge period (day 0 to day 35), but vaccinated animals had a significantly higher weight gain in the postchallenge period (P < 0.05). On day 56, all vaccinated animals were not passing cysts in their feces, while 40% of saline injected kittens had Giardia cysts in their feces. In vaccinated kittens, cysts were never demonstrated in 45% of the animals, while cysts were detected in 90% of the saline injected kittens. Viability of the cysts in vaccinated kittens was 38% while the cysts viability in saline injected kittens was 99%. On postmortem examination, trophozoites could be detected in 5% of vaccinated kittens and 60% of saline injected kittens. Vaccination produced an elevated Giardia specific serum IgG and IgA response prior to challenge and throughout the postinfection period. The Giardia infection in the saline injected group did not induce an elevated specific serum response. Giardia vaccination of kittens provides protection in kittens from an experimental challenge by reducing or eliminating intestinal trophozoites and fecal cyst excretion.

Cell biology of the primitive eukaryote Giardia lamblia

Giardia lamblia is an extremely primitive or early-diverging eukaryote that has been considered to have no typical ER or Golgi apparatus, although it is a complex and highly developed cell. Both the trophozoite and cyst have unusual surface proteins that enable these stages to survive in very different and hostile environments. We found that G. lamblia forms novel encystation-specific secretory vesicles and can sort cyst wall proteins to a regulated secretory pathway distinct from the constitutive pathway used to transport the variable cysteine-rich protein to the trophozoite surface. Our studies, utilizing novel ultrastructural methods that preserve the endomembranes, as well as IEM, support the idea that G. lamblia has many of the endomembrane protein transport elements and sorting functions of higher cells and that these appeared very early in the evolution of eukaryotic cells.

Giardiasis

Despite rapid progress in understanding the biology of Giardia, several questions remain unanswered. First, there is no adequate explanation for the diverse clinical spectrum of giardiasis. Second, the mechanisms by which Giardia produces diarrhea and malabsorption are poorly understood, although some progress has been made. Finally, despite extensive studies in animal models and human infections, the key immunologic determinants for clearance of acute infection and development of protective immunity remain ill defined. This article discusses the epidemiology, pathology, diagnosis, treatment, and prevention of giardiasis.

A distinct array of proinflammatory cytokines is expressed in human colon epithelial cells in response to bacterial invasion

Pathogenic bacteria that penetrate the intestinal epithelial barrier stimulate an inflammatory response in the adjacent intestinal mucosa. The present studies asked whether colon epithelial cells can provide signals that are important for the initiation and amplification of an acute mucosal inflammatory response. Infection of monolayers of human colon epithelial cell lines (T84, HT29, Caco-2) with invasive strains of bacteria (Salmonella dublin, Shigella dysenteriae, Yersinia enterocolitica, Listeria monocytogenes, enteroinvasive Escherichia coli) resulted in the coordinate expression and upregulation of a specific array of four proinflammatory cytokines, IL-8, monocyte chemotactic protein-1, GM-CSF, and TNF alpha, as assessed by mRNA levels and cytokine secretion. Expression of the same cytokines was upregulated after TNF alpha or IL-1 stimulation of these cells. In contrast, cytokine gene expression was not altered after infection of colon epithelial cells with noninvasive bacteria or the noninvasive protozoan parasite, G. lamblia. Notably, none of the cell lines expressed mRNA for IL-2, IL-4, IL-5, IL-6, IL-12p40, IFN-gamma, or significant levels of IL-1 or IL-10 in response to the identical stimuli. The coordinate expression of IL-8, MCP-1, GM-CSF and TNF alpha appears to be a general property of human colon epithelial cells since an identical array of cytokines, as well as IL-6, also was expressed by freshly isolated human colon epithelial cells. Since the cytokines expressed in response to bacterial invasion or other proinflammatory agonists have a well documented role in chemotaxis and activation of inflammatory cells, colon epithelial cells appear to be programmed to provide a set of signals for the activation of the mucosal inflammatory response in the earliest phases after microbial invasion.

Encystation of Giardia lamblia leads to expression of antigens recognized by antibodies against conserved heat shock proteins

During in vitro encystation, Giardia lamblia expresses several stage-specific proteins which are recognized in immunoblots by antisera raised against antigens from three different pathogens. The antigens belong to two different families of conserved stress proteins: (i) HSP60 purified from Legionella pneumophila and recombinant HSP60 from Mycobacterium bovis BCG and (ii) recombinant HSP70 from Plasmodium falciparum.

Giardia--diagnosis, clinical course and epidemiology. A review

Infection with giardia may be associated with significant ill-health and while the reported incidence of infection is increasing in the United Kingdom, the true prevalence of infection and extent of morbidity due to this organism is unknown. Diagnosis is made difficult by non-specificity of symptoms and low sensitivity of traditional diagnostic techniques. Immunological methods of diagnosis hold promise for the future, but in the meantime, more routine testing by laboratories and multiple faecal testing by clinicians may prevent unnecessary morbidity. The late summer/autumn peak in reported infection is difficult to explain while the age distribution is typical of an organism which is spread faeco-orally. The importance of potable water supplies as a source of infection in this country is not clear, nor is the role of zoonotic spread. The apparent susceptibility to infection of certain population groups requires further exploration as does the role of the asymptomatically infected in transmission.

Human secretory and serum antibodies recognize environmentally induced antigens of Giardia lamblia

The variability in duration and severity of infection with Giardia lamblia is likely to be due to trophozoite interactions with immune and nonimmune components of the small intestinal milieu. Despite its potential importance, nothing is known of the isotype or the specificity of the secretory antibody response to G. lamblia. In the present study, we show that serum and secretory antibodies recognize many Giardia antigens whose expression is induced by exposure to selected intestinal conditions. Isotype-specific immunoblots of antigens from trophozoites grown at pH 7.0 without bile or at the intestinal pH of 7.8 with bile were reacted with milk or serum antibodies from subjects with or without histories of giardiasis. While the results were complex, several key observations emerged. Serum and secretory immunoglobulin A (IgA), IgM, and IgG antibodies reacted with many regulated antigens. Antigen recognition patterns varied with isotype and between milk and serum antibodies of the same isotype. Antigen recognition also differed among subjects. Antibodies from virtually every patient recognized some G. lamblia antigens. Furthermore, milk and/or serum samples from putative controls without histories of giardiasis were positive more frequently than would be predicted from published prevalence studies, suggesting either that these antibodies may be cross-reactive or that undiagnosed infections with G. lamblia may be more common than previously thought. Thus, recognition of neoantigens induced by host conditions may be due to conserved or cross-reactive epitopes which could constitute a form of immune evasion by G. lamblia.

The biology of Giardia spp

Gardia spp. are flagellated protozoans that parasitize the small intestines of mammals, birds, reptiles, and amphibians. The infectious cysts begin excysting in the acidic environment of the stomach and become trophozoites (the vegetative form). The trophozoites attach to the intestinal mucosa through the suction generated by a ventral disk and cause diarrhea and malabsorption by mechanisms that are not well understood. Giardia spp. have a number of unique features, including a predominantly anaerobic metabolism, complete dependence on salvage of exogenous nucleotides, a limited ability to synthesize and degrade carbohydrates and lipids, and two nuclei that are equal by all criteria that have been tested. The small size and unique sequence of G. lamblia rRNA molecules have led to the proposal that Giardia is the most primitive eukaryotic organism. Three Giardia spp. have been identified by light lamblia, G. muris, and G. agilis, but electron microscopy has allowed further species to be described within the G. lamblia group, some of which have been substantiated by differences in the rDNA. Animal models and human infections have led to the conclusion that intestinal infection is controlled primarily through the humoral immune system (T-cell dependent in the mouse model). A major immunogenic cysteine-rich surface antigen is able to vary in vitro and in vivo in the course of an infection and may provide a means of evading the host immune response or perhaps a means of adapting to different intestinal environments.

Giardia lamblia: regulation of secretory vesicle formation and loss of ability to reattach during encystation in vitro

Encystation of Giardia lamblia is required for survival outside the host, as well as for initiation of new infections. Previously, we induced cultured G. lamblia trophozoites to encyst in vitro for the first time. During encystation, we observed the appearance of a new class of large secretory vesicle (encystation-specific vesicle; ESV) within which cyst antigens are concentrated and transported to the nascent wall. The present kinetic and physiologic studies now show that ESV are the earliest morphologic change observed in encystation. Expression of ESV, as well as subsequent encystation, are regulated by exposure to bile at the slightly alkaline pH which is typical of the human intestinal tract. ESV formation appears to be less stringently regulated than formation of water-resistant cysts because omission of either encystation stimuli or alkaline pH preferentially inhibits encystation. Since cysts do not attach, we asked when in encystation this physiologic transition occurs. We found that most encysting trophozoites remain attached until they begin to round up (greater than 24 hr). However, if they are made to detach, as early as 12 hr in encystation, well before they round up, they are defective in the ability to reattach. If trophozoites also become less able to reattach to the intestinal epithelium early in encystation in vivo, this would increase their exposure to lumenal encystation stimuli and promote encystation. These studies have provided new insights into the complex sequence of morphologic and physiologic alterations which occur during encystation of G. lamblia in vitro and their regulation by host intestinal factors.

Acquired resistance to Giardia muris in X-linked immunodeficient mice

A previous study from this laboratory (D. P. Snider, D. Skea, and B. J. Underdown, Infect. Immun. 56:2838-2842, 1988) indicated that immunodeficient mice expressing the xid gene develop prolonged infections with Giardia muris, unlike immunocompetent mice, which eliminate the intestinal protozoan parasite in 8 to 10 weeks. In this study, CBA/N (xid) and CBA/Ca mice were infected with G. muris cysts and at various times following this primary infection were cured by treatment with metronidazole. In contrast to the marked differences in the ability of xid and normal mice to eliminate a primary infection, mice of both strains were resistant to a secondary challenge of G. muris cysts. These data imply that the mechanism(s) responsible for elimination of a primary infection is not identical to those required to resist a secondary challenge infection. Splenocytes from immunocompetent CBA/Ca mice (but not immunodeficient CBA/N mice) could transfer the ability to eliminate a primary G. muris infection to irradiated mice of either strain. In contrast, splenocytes from previously infected CBA/Ca mice could not transfer resistance to a challenge infection, further supporting the hypothesis that there are differences between mechanisms required to eliminate a primary infection and those necessary to resist a second challenge infection.

Identification of developmentally regulated Giardia lamblia cyst antigens using GCSA-1, a cyst-specific monoclonal antibody

GCSA-1, a monoclonal antibody raised against cysts generated in vitro was shown to be Giardia cyst-specific by immunoblot analysis and immunofluorescence. GCSA-1 recognized four polypeptides ranging from 29-45 kD present in the cyst wall. These antigens appeared within eight hours of exposure of trophozoites to encystation medium and were shown to be synthesized by encysting parasites by means of metabolic labelling with [35S]-cysteine. Trophozoites were not stained by the antibody. GCSA-1 also reacted with in vivo cysts obtained from faeces of infected humans, gerbils and mice. These data demonstrate that the determinants recognized by GCSA-1 are early cyst antigens which are developmentally regulated and conserved components of the cyst wall. The actual role of the antigens detected by GCSA-1 in encystation are unknown, but they represent a potential target for strategies directed at inhibiting this process.

Excystation of in vitro-derived Giardia lamblia cysts

This is the first in-depth analysis of the excystation of Giardia lamblia cysts prepared in vitro. Its goals were both to achieve efficient excystation and to gain insights into this crucial but poorly understood process. To identify the critical elements of excystation, we tested the sequential low-pH induction and protease treatments which had been reported to be important for excystation of fecal cysts. The optimal pH for induction of excystation was 4.0. Emergence was greatly (approximately 10-fold) stimulated by subsequent exposure of in vitro-derived cysts to chymotrypsin, trypsin, or human pancreatic fluid. The stimulatory activity of each was abolished by soybean trypsin inhibitor, demonstrating that the activity of pancreatic fluid was due to these proteases. Excystation of in vitro-derived cysts was approximately 10 to 38%. Although the walls of in vitro-derived cysts were partially digested by protease treatment, trophozoites emerged only from one pole, as observed with fecal cysts. The conditions of encystation also determined the efficiency of excystation. Specifically, encystation in the presence of lactic acid, a major metabolite of colonic bacteria, stimulated excystation approximately fourfold, although it did not increase the total numbers of cysts. These experiments have shown that excystation of in vitro-derived cysts reflects that of cysts purified from human feces in that it is dependent upon conditions which simulate the passage of cysts through the human stomach (low pH) and into the small intestine (pancreatic proteases).

Giardia lamblia: the roles of bile, lactic acid, and pH in the completion of the life cycle in vitro

Large numbers (10(4) to greater than 10(5)/ml) of Type I water-resistant Giardia lamblia cysts were produced in vitro under conditions that are characteristic of the human intestinal lumen. We define Type I cyst morphology as oval shaped, smooth, and refractile, with cyst wall, axostyle, and median body visible in relief by Normarski differential interference contrast optics. Human and porcine bile induced higher levels of encystation than bovine bile at the alkaline pH (7.8) which occurs in the human lower small intestine. High-pressure liquid chromatography analysis showed that the porcine bile had a preponderance of hyocholate, rather than cholate, while bovine bile had less chenodeoxycholate and more deoxycholate than human bile. Lactic acid, a major product of bacterial metabolism in the human colon, further stimulated encystation. Growth of the preencystation culture without bile also increased subsequent encystation. More than 90% of Type I cysts produced with porcine bile plus lactic acid were viable as indicated by the uptake and retention of fluorescein diacetate and exclusion of propidium iodide. Biological activity of in vitro-derived water-resistant cysts was demonstrated by the observation that 1 to 9.5% excysted in vitro. The percentage of excystation was greatly decreased following encystation at pH 7.0 or by omission of bile or lactic acid. This is the first quantitative in vitro demonstration of the complete life cycle of G. lamblia from humans.

Identification and localization of cyst-specific antigens of Giardia lamblia

We induced Giardia lamblia trophozoites to encyst in vitro by exposure to conditions which are specific to the human small intestinal milieu. We now show that encystation entails the appearance of two new groups of antigens detected in Western blots by rabbit antiserum against cysts which had been purified from human feces. A heterodisperse group of lower-molecular-mass antigens (approximately 21 to 39 kilodaltons) was expressed relatively early (less than 19 h) in encystation. In contrast, discrete bands at approximately 66, 78, 92, and 103 kilodaltons only appeared after 24 h of incubation under conditions which lead to production of large numbers of water-resistant cysts. We also describe for the first time the appearance of prominent cytoplasmic vesicles, which were the earliest morphologic change in encysting trophozoites observable by light microscopy. Early in encystation, cyst wall antigens were concentrated in these vesicles, as shown by immunocytochemistry, suggesting that the vesicles function in export of cyst wall constituents.