Identification and characterization of taglin, a mannose 6-phosphate binding, trypsin-activated lectin from Giardia lamblia

Herbal-based compounds: A review on treatments of cryptosporidiosis

Cryptosporidium, a monoxenous apicomplexan coccidia, is a prevalent diarrhetic and an opportunistic agent, mainly in immunocompromised individuals. As there are few chemotherapeutic compounds that have limited efficacy, we need to identify new compounds or specific parasite targets for designing more potent drugs to treat cryptosporidiosis. Herbal products with low toxicity, environmental compatibility, wide therapeutic potential, and abundant resources can be considered alternatives for treatment. The current review tried to summarize the studies on plants or herbal bioactive constituents with anti-cryptosporidial activities. Based on constituents, plants act via different mechanisms, and further investigations are needed to clarify the exact mechanisms by which they act on the developmental stages of the parasite or host-parasite relationships.

Repurposing Terfenadine as a Novel Antigiardial Compound

Giardia lamblia is a highly infectious protozoan that causes giardiasis, a gastrointestinal disease with short-term and long-lasting symptoms. The currently available drugs for giardiasis treatment have limitations such as side effects and drug resistance, requiring the search for new antigiardial compounds. Drug repurposing has emerged as a promising strategy to expedite the drug development process. In this study, we evaluated the cytotoxic effect of terfenadine on Giardia lamblia trophozoites. Our results showed that terfenadine inhibited the growth and cell viability of Giardia trophozoites in a time-dose-dependent manner. In addition, using scanning electron microscopy, we identified morphological damage; interestingly, an increased number of protrusions on membranes and tubulin dysregulation with concomitant dysregulation of Giardia GiK were observed. Importantly, terfenadine showed low toxicity for Caco-2 cells, a human intestinal cell line. These findings highlight the potential of terfenadine as a repurposed drug for the treatment of giardiasis and warrant further investigation to elucidate its precise mechanism of action and evaluate its efficacy in future research.

Identification of Conserved Candidate Vaccine Antigens in the Surface Proteome of Giardia lamblia

Giardia lamblia, one of the most common protozoal infections of the human intestine, is an important worldwide cause of diarrheal disease, malabsorption, malnutrition, delayed cognitive development in children, and protracted postinfectious syndromes. Despite its medical importance, no human vaccine is available against giardiasis. A crude veterinary vaccine has been developed, and experimental vaccines based on expression of multiple variant-specific surface proteins have been reported, but poorly defined vaccine components and excessive antigen variability are problematic for pharmaceutical vaccine production. To expand the repertoire of antigen candidates for vaccines, we reasoned that surface proteins may provide an enriched source of such antigens since key host effectors, such as secretory IgA, can directly bind to such antigens in the intestinal lumen and interfere with epithelial attachment. Here, we have applied a proteomics approach to identify 23 novel surface antigens of G. lamblia that show >90% amino acid sequence identity between the two human-pathogenic genetic assemblages (A and B) of the parasite. Surface localization of a representative subset of these proteins was confirmed by immunostaining. Four selected proteins, uridine phosphorylase-like protein-1, protein 21.1 (GL50803_27925), α1-giardin, and α11-giardin, were subsequently produced in recombinant form and shown to be immunogenic in mice and G. lamblia-infected humans and confer protection against G. lamblia infection upon intranasal immunization in rodent models of giardiasis. These results demonstrate that identification of conserved surface antigens provides a powerful approach for overcoming a key rate-limiting step in the design and construction of an effective vaccine against giardiasis.

Curcumin alters the cytoskeleton and microtubule organization on trophozoites of Giardia lamblia

Giardia lamblia is a worldwide protozoan responsible for a significant number of intestinal infections. There are several drugs for the treatment of giardiasis, but they often cause side effects. Curcumin, a component of turmeric, has antigiardial activity; however, the molecular target and mechanism of antiproliferative activity are not clear. The effects of curcumin on cellular microtubules have been widely investigated. Since tubulin is the most abundant protein in the cytoskeleton of Giardia, to elucidate whether curcumin has activity against the microtubules of this parasite, we treated trophozoites with curcumin and the cells were analyzed by scanning electron microscopy and confocal microscopy. Curcumin inhibited Giardia proliferation and adhesion in a time-concentration-dependent mode. The higher inhibitory concentrations of curcumin (3 and 15μM) disrupted the cytoskeletal structures of trophozoites; the damage was evident on the ventral disk, flagella and in the caudal region, also the membrane was affected. The immunofluorescence images showed altered distribution of tubulin staining on ventral disk and flagella. Additionally, we found that curcumin caused a clear reduction of tubulin expression. By docking analysis and molecular dynamics we showed that curcumin has a high probability to bind at the interface of the tubulin dimer close to the vinblastine binding site. All the data presented indicate that curcumin may inhibit Giardia proliferation by perturbing microtubules.

Comparative Cell Biology and Evolution of Annexins in Diplomonads

Annexins are proteins that associate with phospholipids in a Ca²⁺-dependent fashion. These proteins have been intensely studied in animals and plants because of their importance in diverse cellular processes, yet very little is known about annexins in single-celled eukaryotes, which represent the largest diversity of organisms. The human intestinal parasite Giardia intestinalis is known to have more annexins than humans, and they contribute to its pathogenic potential. In this study, we investigated the annexin complement in the salmon pathogen Spironucleus salmonicida, a relative of G. intestinalis. We found that S. salmonicida has a large repertoire of annexins and that the gene family has expanded separately across diplomonads, with members showing sequence diversity similar to that seen across kingdom-level groups such as plants and animals. S. salmonicida annexins are prominent components of the cytoskeleton and membrane. Two annexins are associated with a previously unrecognized structure in the anterior of the cell.

Annexins are multifunctional, calcium-binding proteins found in organisms across all kingdoms. Most studies of annexins from single-celled eukaryotes have focused on the alpha-giardins, proteins assigned to the group E annexins, expressed by the diplomonad Giardia intestinalis. We have characterized the annexin gene family in another diplomonad parasite, Spironucleus salmonicida, by phylogenetic and experimental approaches. We constructed a comprehensive phylogeny of the diplomonad group E annexins and found that they are abundant across the group with frequent gene duplications and losses. The annexins of S. salmonicida were found to be related to alpha-giardins but with better-preserved type II Ca²⁺ coordination sites. Two annexins were confirmed to bind phospholipids in a Ca²⁺-dependent fashion but with different specificities. Superresolution and confocal microscopy of epitope-tagged S. salmonicida annexins revealed localization to distinct parts of the cytoskeleton and membrane. The ultrastructural details of the localization of several annexins were determined by proximity labeling and transmission electron microscopy. Two annexins localize to a novel cytoskeletal structure in the anterior of the cell. Our results show that the annexin gene family is expanded in diplomonads and that these group E annexins are associated mostly with cytoskeletal and membrane structures.

IMPORTANCE Annexins are proteins that associate with phospholipids in a Ca²⁺-dependent fashion. These proteins have been intensely studied in animals and plants because of their importance in diverse cellular processes, yet very little is known about annexins in single-celled eukaryotes, which represent the largest diversity of organisms. The human intestinal parasite Giardia intestinalis is known to have more annexins than humans, and they contribute to its pathogenic potential. In this study, we investigated the annexin complement in the salmon pathogen Spironucleus salmonicida, a relative of G. intestinalis. We found that S. salmonicida has a large repertoire of annexins and that the gene family has expanded separately across diplomonads, with members showing sequence diversity similar to that seen across kingdom-level groups such as plants and animals. S. salmonicida annexins are prominent components of the cytoskeleton and membrane. Two annexins are associated with a previously unrecognized structure in the anterior of the cell.

Purification and characterization of hemagglutinating proteins from Poker-chip Venus (Meretrix lusoria) and Corbicula clam (Corbicula fluminea)

Hemagglutinating proteins (HAPs) were purified from Poker-chip Venus (Meretrix lusoria) and Corbicula clam (Corbicula fluminea) using gel-filtration chromatography on a Sephacryl S-300 column. The molecular weights of the HAPs obtained from Poker-chip Venus and Corbicula clam were 358 kDa and 380 kDa, respectively. Purified HAP from Poker-chip Venus yielded two subunits with molecular weights of 26 kDa and 29 kDa. However, only one HAP subunit was purified from Corbicula clam, and its molecular weight was 32 kDa. The two Poker-chip Venus HAPs possessed hemagglutinating ability (HAA) for erythrocytes of some vertebrate animal species, especially tilapia. Moreover, HAA of the HAP purified from Poker-chip Venus was higher than that of the HAP of Corbicula clam. Furthermore, Poker-chip Venus HAPs possessed better HAA at a pH higher than 7.0. When the temperature was at 4°C–10°C or the salinity was less than 0.5‰, the two Poker-chip Venus HAPs possessed better HAA compared with that of Corbicula clam.

Giardia lamblia Nek1 and Nek2 kinases affect mitosis and excystation

The NIMA-related serine/threonine kinases (Neks) function in the cell cycle and regulate ciliary and flagellar length. The Giardia lamblia genome encodes 198 Neks, of which 56 are predicted to be active. Here we believe that we report the first functional analysis of two G. lamblia Neks. The GlNek1 and GlNek2 kinase domains share 57% and 43% identity to the kinase domains of human Nek1 and Nek2, respectively. Both GlNeks are active in vitro, have dynamic relocalisation during the cell cycle, and are expressed throughout the life cycle, with GlNek1 being upregulated in cysts. Over-expression of inactive GlNek1 delays disassembly of the parental attachment disc and cytokinesis, whilst over-expression of either wild type GlNek1 or inactive mutant GlNek2 inhibits excystation.

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.

Excreted/secreted glycoproteins of G. intestinalis play an essential role in the antibody response

In the present work, glycoproteins in the excretory/secretory products of G. intestinalis were identified and the reactivity in serum of immunized mice with these molecules was evaluated by western blotting before and after chemical treatment or enzymatic deglycosylation. Glycoproteins of 58 and 63 kDa were revealed in E/S products after periodic acid-Schiff (PAS) stain. Studies of carbohydrate specificity using digoxigenin-labeled lectins, revealed the presence of O-glycans and N-glycans. Chemical treatment of excretory/secretory products with sodium meta-periodate or enzymatic deglycosylation with N-glycosidase F reduced the reactivity in serum for proteins of 36, 58 and 63 kDa, respectively. These results show the presence of glycoproteins in E/S products of G. intestinalis and suggest that the antibody response is directed against glycoepitopes. The expression of carbohydrate moieties in the E/S-G. intestinalis may play an essential role in the antibody response and may be a target for serodiagnosis or immune intervention in human giardiasis.

Characterization of the N-linked glycans of Giardia intestinalis

This article reports the first rigorous evidence for the existence of N-glycans in Giardia intestinalis, a parasite that is a widespread human pathogen, being a major cause of enteric disease in the world. Excreted/secreted molecules of G. intestinalis are known to stimulate the immune system. Structural strategies based on MALDI and electrospray mass spectrometry were employed to examine the excreted/secreted molecules for their N-glycan content. These revealed that the major oligosaccharides released by peptide N-glycosidase F are complex-type structures and correspond to bi-, and triantennary structures without core (alpha1,6) fucosylation. The major nonreducing epitopes in these complex-type glycans are: Galbeta1-4GlcNAc (LacNAc) and NeuAc alpha2-6Galbeta1-4GlcNAc (sialylated LacNAc).

Microbial-gut interactions in health and disease. Mucosal immune responses

The host gastrointestinal tract is exposed to countless numbers of foreign antigens and has embedded a unique and complex network of immunological and non-immunological mechanisms, often termed the gastrointestinal 'mucosal barrier', to protect the host from potentially harmful pathogens while at the same time 'tolerating' other resident microbes to allow absorption and utilization of nutrients. Of the many important roles of this barrier, it is the distinct responsibility of the mucosal immune system to sample and discriminate between harmful and beneficial antigens and to prevent entry of food-borne pathogens through the gastrointestinal (GI) tract. This system comprises an immunological network termed the gut-associated lymphoid tissue (GALT) that consists of unique arrangements of B cells, T cells and phagocytes which sample luminal antigens through specialized epithelia termed the follicle associated epithelia (FAE) and orchestrate co-ordinated molecular responses between immune cells and other components of the mucosal barrier. Certain pathogens have developed ways to bypass and/or withstand defence by the mucosal immune system to establish disease in the host. Some 'opportunistic' pathogens (such as Clostridium difficile) take advantage of host or other factors (diet, stress, antibiotic use) which may alter or weaken the response of the immune system. Other pathogens have developed mechanisms for invading gastrointestinal epithelium and evading phagocytosis/destruction by immune system defences. Once cellular invasion occurs, host responses are activated to limit local mucosal damage and repel the foreign influence. Some pathogens (Shigella spp, parasites and viruses) primarily establish localized disease while others (Salmonella, Yersinia, Listeria) use the lymphatic system to enter organs or the bloodstream and cause more systemic illness. In some cases, pathogens (Helicobacter pylori and Salmonella typhi) colonize the GI tract or associated lymphoid structures for extended periods of time and these persistent pathogens may also be potential triggers for other chronic or inflammatory diseases, including inflammatory bowel disease and malignancies. The ability of certain pathogens to avoid or withstand the host's immune assault and/or utilize these host responses to their own advantage (i.e. enhance further colonization) will dictate the pathogen's success in promoting illness and furthering its own survival.

Evidence for Adhesive Activity of the Ventrolateral Flange in Giardia lamblia

Giardia lamblia is an intestinal protozoan that inhabits the intestinal tract of man and other mammals by attaching to the mucosal surface via the contractile activity of an attachment organelle called the ventral adhesive disk. We have investigated the presence of other attachment mechanisms in G. lamblia trophozoites by using microfabricated substrates that sterically interfere with formation of the hypothesized "negative pressure" under the ventral adhesive disk that would mediate attachment to a substratum. Pillars measuring 1 microm high and 2 microm in diam. were constructed in microarrays with spacings smaller than the diameter of the ventral adhesive disk. Using high resolution field emission scanning electron microscopy, the attachment of trophozoites to the tops of pillars in the microfabricated substrates was investigated. Firm adhesion of trophozoites was observed to be mediated by direct attachment of the ventrolateral flange membrane to the tops of microfabricated pillars. Attachment to microfabricated surfaces was 16% of that observed for attachment mediated by the ventral adhesive disk (4.4 +/- 1.5 cells/100 micro2 micropillar surface vs. 25.9 +/- 3.1 cells/100 micro2 flat substrate, p < 0.0001) This is the first report of trophozoite adhesion to a substratum by a mechanism other than the direct attachment of the ventral adhesive disk, and provides experimental evidence that the ventrolateral flange may play a role in trophozoite adhesion. A hypothesis is presented describing how the adhesive nature of the ventrolateral flange might be involved in normal attachment of G. lamblia trophozoites to a substratum.

Characterisation of alpha-1 giardin: an immunodominant Giardia lamblia annexin with glycosaminoglycan-binding activity

Alpha-1 giardin is an immunodominant protein in the intestinal protozoan parasite Giardia lamblia. The Triage((R)) parasite panel, used to detect copro-antigens in stool from giardiasis patients, reacts with an epitope between amino acids 160 and 200 in alpha-1 giardin. This region of the protein is also highly immunogenic during human infections. Alpha-1 giardin is related to annexins and like many other annexins it was shown to be plasma membrane associated. Immunoelectron and immunofluorescence microscopy revealed that some alpha-1 giardin are displayed on the surface of recently excysted cells. Recombinant alpha-1 giardin displayed a Ca(2+)-dependent binding to glycosaminoglycans (GAGs), in particular heparan sulphate, a common GAG in the intestinal tract. Recombinant alpha-1 giardin bound to thin sections of human small intestine, a binding which could be inhibited by adding increasing concentrations of sulphated sugars. A surface associated trypsin activated Giardia lectin (taglin) has been suggested to be important for G. lamblia attachment. In this study we show that a monoclonal antibody that inhibits taglin recognises alpha-1 and alpha-2 giardin. Thus, alpha-1 giardin is a highly immunoreactive GAG-binding protein, which may play a key role in the parasite-host interaction. Our results further show a conserved function of annexins from lower to higher eukaryotes.

Structural and biochemical alterations in Giardia lamblia cysts exposed to ozone

Because of its efficacy in inactivating waterborne protozoan cysts and oocysts, ozone is frequently used for disinfection of drinking water. The effect of ozone on cysts of Giardia lamblia was investigated in gerbils (Meriones unguiculatus), using an infectivity assay by scanning electron microscopy, immunoblotting, and flow cytometry. Cysts recovered from experimentally infected gerbils were exposed to an initial ozone concentration of 1.5 mg/L for 0, 30, 60, and 120 sec. This treatment resulted in a dose-dependent reduction in cysts concentration, loss of infectivity in gerbils, and profound structural modifications to the cyst wall. Exposure for 60 sec or longer resulted in extensive protein degradation and in the disappearance of a cyst wall and a trophozoite antigen.

Parasitic infections of the gastrointestinal tract

This article updates recent advances in the body of knowledge of diagnosis and treatment of intestinal parasites. The articles focus on the manifestations of disease in the immunocompetent adult host from developed countries. Specific pathogens discussed are Giardia lamblia and Dientamoeba fragilis, Entamoeba histolytica, Entamoeba dipar, Blastocystis hominis, Cyclospora cayetanensis, and Cryptosporidium parvum.

Reversible interruption of Giardia lamblia cyst wall protein transport in a novel regulated secretory pathway

To survive in the environment and infect a new host, Giardia lamblia secretes an extracellular cyst wall using a poorly understood pathway. The two cyst wall proteins (CWPs) form disulphide-bonded heterodimers and are exported via novel encystation-specific secretory vesicles (ESVs). Exposure of eukaryotic cells to dithiothreitol (DTT) blocks the formation of disulphide bonds in nascent proteins that accumulate in the endoplasmic reticulum (ER) and induces an unfolded protein response (UPR). Proteins that have exited the ER are not susceptible. Exposure to DTT inhibits ESV formation by > 85%. Addition of DTT to encysting cells causes rapid (t1/2 < 10 min), reversible disappearance of ESVs, correlated with reduction of CWPs to monomers and reformation of CWP oligomers upon removal of DTT. Neither CWPs nor ESVs are affected by mercaptoethanesulphonic acid, a strong reducing agent that does not penetrate cells. DTT does not inhibit the overall protein secretory pathway, and recovery does not require new protein synthesis. We found evidence of protein disulphide isomerases in the ESV and the surface of encysting cells, in which they may catalyse initial CWP folding and recovery from DTT. This is the first suggestion of non-CWP proteins in ESVs and of enzymes on the giardial surface. DTT treatment did not stimulate a UPR, suggesting that Giardia may have diverged before the advent of this conserved form of ER quality control.

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.

Adherence of Giardia lamblia trophozoites to Int-407 human intestinal cells

Attachment of Giardia lamblia trophozoites to enterocytes is essential for colonization of the small intestine and is considered a prerequisite for parasite-induced enterocyte dysfunction and clinical disease. In this work, coincubation of Giardia with Int-407 cells, was used as an in vitro model to study the role of cytoskeleton and surface lectins involved in the attachment of the parasite. This interaction was also studied by scanning and transmission electron microscopy. Adherence was dependent on temperature and was maximal at 37 degrees C. It was reduced by 2.5 mM colchicine (57%), mebendazole (10 microg/ml) (59%), 100 mM glucose (26%), 100 mM mannose (22%), 40 mM mannose-6-phosphate (18%), and concanavalin A (100 microg/ml) (21%). No significant modification was observed when Giardia was pretreated with cytochalasins B and D and with EDTA. Giardia attachment was also diminished by preincubating Int-407 cells with cytochalasin B and D (5 microg/ml) (16%) and by glutaraldehyde fixation of intestinal cells and of G. lamblia trophozoites (72 and 100%, respectively). Ultrastructural studies showed that Giardia attaches to the Int-407 monolayer predominantly by its ventral surface. Int-407 cells contact trophozoites with elongated microvilli, and both trophozoite imprints and interactions of Giardia flagella with intestinal cells were also observed. Transmission electron microscopy showed that Giardia lateral crest and ventrolateral flange were important structures in the adherence process. Our results suggest a combination of mechanical and hydrodynamic forces in trophozoite attachment; surface lectins also seem to mediate binding and may be involved in specific recognition of host cells.

Immune response to Giardia duodenalis

The intestinal protozoan Giardia duodenalis is a widespread opportunistic parasite of humans and animals. This parasite inhabits the upper part of the small intestine and has a direct life cycle. After ingestion of cysts, which are the infective stage, the trophozoites emerge from the cysts in the duodenum and attach to the small intestinal mucosa of the host. Since the migration of trophozoites from the lumen of the intestine into surrounding tissues is an unusual occurrence, the immune response to Giardia remains localized. The identification of antigens that play a role in acquired immunity has been difficult because of the occurrence of antigenic variation and because, Giardia being an ubiquitous organism, it is possible that the antigenic profiles of isolates from different geographic areas will vary. Innate-immunity mechanisms play a role in the control and/or severity of the infection. Both humoral and cell-mediated immune responses play a role in acquired immunity, but the mechanisms involved are unknown. A variety of serological assays have been used to detect circulating antibodies in serum. Because of the biological characteristics of the parasite and the lack of suitable antigens, the sensitivity of serological assays remains poor. On the other hand, detection of antigens in feces of infected patients has met with success. Commercial kits are available, and they are reported to be more sensitive than microscopic examination for the detection of giardiasis on a single specimen.

Nitric oxide production by human intestinal epithelial cells and competition for arginine as potential determinants of host defense against the lumen-dwelling pathogen Giardia lamblia

Giardia lamblia infection of the human small intestine is a common protozoan cause of diarrheal disease worldwide. Although infection is luminal and generally self-limiting, and secretory Abs are thought to be important in host defense, other defense mechanisms probably affect the duration of infection and the severity of symptoms. Because intestinal epithelial cells produce NO, and its stable end products, nitrite and nitrate, are detectable mainly on the apical side, we tested the hypothesis that NO production may constitute a host defense against G. lamblia. Several NO donors, but not their control compounds, inhibited giardial growth without affecting viability, suggesting that NO is cytostatic rather than cytotoxic for G. lamblia. NO donors also inhibited giardial differentiation induced by modeling crucial environmental factors, i. e., encystation induced by bile and alkaline pH, and excystation in response to gastric pH followed by alkaline pH and protease. Despite the potent antigiardial activity of NO, G. lamblia is not simply a passive target for host-produced NO, but has strategies to evade this potential host defense. Thus, in models of human intestinal epithelium, G. lamblia inhibited epithelial NO production by consuming arginine, the crucial substrate used by epithelial NO synthase to form NO. These studies define NO and arginine as central components in a novel cross-talk between a luminal pathogen and host intestinal epithelium.

Immune response to Giardia duodenalis

The intestinal protozoan Giardia duodenalis is a widespread opportunistic parasite of humans and animals. This parasite inhabits the upper part of the small intestine and has a direct life cycle. After ingestion of cysts, which are the infective stage, the trophozoites emerge from the cysts in the duodenum and attach to the small intestinal mucosa of the host. Since the migration of trophozoites from the lumen of the intestine into surrounding tissues is an unusual occurrence, the immune response to Giardia remains localized. The identification of antigens that play a role in acquired immunity has been difficult because of the occurrence of antigenic variation and because, Giardia being an ubiquitous organism, it is possible that the antigenic profiles of isolates from different geographic areas will vary. Innate-immunity mechanisms play a role in the control and/or severity of the infection. Both humoral and cell-mediated immune responses play a role in acquired immunity, but the mechanisms involved are unknown. A variety of serological assays have been used to detect circulating antibodies in serum. Because of the biological characteristics of the parasite and the lack of suitable antigens, the sensitivity of serological assays remains poor. On the other hand, detection of antigens in feces of infected patients has met with success. Commercial kits are available, and they are reported to be more sensitive than microscopic examination for the detection of giardiasis on a single specimen.

Review article: the management of Giardiasis

Giardiasis is the intestinal infection resulting from infestation with the human parasite Giardia intestinalis, also called Giardia lamblia. The infection may be asymptomatic or present with a variety of symptoms such as diarrhoea, weight loss, abdominal cramps, and failure to thrive. Giardiasis is most often diagnosed after recent travel or in day care centres. The organism has two stages in its life cycle. It is usually ingested as a cyst with as few as 10-25 cysts being sufficient to cause infection. After excystation, the organism is a replicative trophozoite which may attach to the small bowel wall. Giardia intestinalis does not invade the bowel wall. Trophozoites may encyst and be shed in faeces for future ingestion by another host. Diagnosis of infection is by stool examination which may also eliminate other possible infectious agents. Small bowel biopsy may be necessary in difficult individual cases or to rule out non-infectious illnesses, and stool ELISA may serve for large population screening examinations. The mainstay of treatment is metronidazole 250-400 mg three times per day by mouth for 5 days.

Differentiation-associated surface antigen variation in the ancient eukaryote Giardia lamblia

Encystation of Giardia lamblia is required for survival outside the host, whereas excystation initiates infection. The dormant cyst was considered an adaptation to external survival and passage through the stomach. However, we found previously that trophozoites which had recovered after completion of the life cycle had switched their major variant surface protein (VSP), called TSA 417, but neither the timing nor the molecular mechanism of switching had been elucidated. Here we demonstrate that TSA 417 predominates in cysts, but is downregulated during the stage of excystation that models cyst arrival in the small intestine. Transcripts of new VSPs appear late in encystation, and during and after excystation. Trophozoites appear to prepare for switching during encystation, when the major VSP on the cell surface diminishes and is internalized in lysosome-like vacuoles. As short-range DNA rearrangements were not detected, giardial VSP switching during differentiation appears to resemble the in situ switching of surface glycoproteins in African trypanosomes. We also report a unique extended 15 nucleotide polyadenylation signal in all VSP transcripts, but not in other known giardial genes. Antigenic variation during encystation-excystation may be a novel form of immune evasion that could help explain the common occurrence of reinfection by Giardia and other parasites with similar life cycles.

Binding of mannose-6-phosphate and heparin by boar seminal plasma PSP-II, a member of the spermadhesin protein family

PSP-I/PSP-II, a heterodimer of glycosylated spermadhesins, is the major component of boar seminal plasma. Similarly to other spermadhesins, the PSP-II subunit is a lectin which displays heparin- and zona pellucida glycoprotein-binding activities. We have investigated the ligand binding capabilities of the heterodimer and the isolated subunits using several polysaccharides, glycoproteins, and phospholipids. PSP-II binds the sulfated polysaccharides heparin and fucoidan in a dose-dependent and seemingly-specific manner. In addition, PSP-II binds oligosaccharides containing exposed mannose-6-phosphate monoester groups and the binding is selectively inhibited by mannose-6-phosphate and glucose-6-phosphate. Inhibition experiments indicate that binding of PSP-II to sulfated polysaccharides and mannose-6-phosphate-containing oligosaccharides involves distinct but possibly overlapping binding sites. Heterodimer formation with PSP-I abolishes both the heparin and the mannose-6-phosphate binding capabilities, suggesting that the corresponding sites may be located at the dimer interface. Using the crystal structure of PSP-I/PSP-II heterodimer as a template, we have explored possible binding sites which satisfy the observed binding characteristics. In the proposed models, PSP-II Arg43 appears to play a pivotal role in both heparin- and mannose-6-phosphate-complexation as well as in heterodimer formation.

Cellular and transcriptional changes during excystation of Giardia lamblia in vitro

Excystation of Giardia lamblia entails differentiation of dormant cysts into parasitic trophozoites. Despite its importance for infection, this transformation is not understood at the cellular or molecular levels. In these studies, we report that excystation entails detection of environmental stimuli across the tough extracellular cyst wall leading to highly coordinated physiological, structural, and molecular responses. We found that novel cytoplasmic rearrangements and changes in individual species of mRNA and in cytoplasmic pH occur within the cyst wall in the earliest stage of excystation, in response to conditions modeling cyst ingestion and passage into the human stomach. This suggests that cysts do not contain all the mRNA needed for excystation and emergence and supports our hypothesis that external stimuli, including hydrogen ions, may penetrate or be perceived across the cyst wall. In contrast, changes in cyst wall structure or proteins were detected only later in excystation, in the stage that models passage into the human small intestine, where trophozoites can emerge and survive. These findings show that excystation of G. lamblia is a highly complex and active process and provide important insights into its cellular and molecular components.

A mitochondrial-like chaperonin 60 gene in Giardia lamblia: evidence that diplomonads once harbored an endosymbiont related to the progenitor of mitochondria

Diplomonads, parabasalids, as represented by trichomonads, and microsporidia are three protist lineages lacking mitochondria that branch earlier than all other eukaryotes in small subunit rRNA and elongation factor phylogenies. The absence of mitochondria and plastids in these organisms suggested that they diverged before the origin of these organelles. However, recent discoveries of mitochondrial-like heat shock protein 70 and/or chaperonin 60 (cpn60) genes in trichomonads and microsporidia imply that the ancestors of these two groups once harbored mitochondria or their endosymbiotic progenitors. In this report, we describe a mitochondrial-like cpn60 homolog from the diplomonad parasite Giardia lamblia. Northern and Western blots reveal that the expression of cpn60 is independent of cellular stress and, except during excystation, occurs throughout the G. lamblia life cycle. Phylogenetic analyses position the G. lamblia cpn60 in a clade that includes mitochondrial and hydrogenosomal cpn60 proteins. The most parsimonious interpretation of these data is that the cpn60 gene was transferred from the endosymbiotic ancestors of mitochondria to the nucleus early in eukaryotic evolution, before the divergence of the diplomonads and trichomonads from other extant eukaryotic lineages. A more complicated explanation requires that these genes originated from distinct alpha-proteobacterial endosymbioses that formed transiently within these protist lineages.

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.

Identification of a 148-kDa surface lectin from Giardia lamblia with specificity for alpha-methyl-D-mannoside

A lectin specific for alpha-methyl-D-mannoside was purified from the membrane extract of Giardia lamblia by a combination of gel filtration chromatography on Sephadex G-75 and Superose 6-HR 10/30. The homogeneity of the lectin was established by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The molecular mass of the native protein was 148 kDa. The lectin agglutinated rabbit erythrocytes in the presence of Ca2+ at 37 degrees C and pH 7.0. The maximum activity of the lectin was obtained after trypsin treatment. The inhibition study clearly suggests that the binding site of the lectin recognizes alpha-methyl-D-mannoside as the immunodominant sugar.

Cell adhesion to proteins separated by lithium dodecyl sulfate-polyacrylamide gel electrophoresis and blotted onto a polyvinylidene difluoride membrane: a new cell-blotting technique

Cell blotting, although conceptually simple, has failed to achieve wide practical application. Described here is a new cell-blotting technique which involves cell adhesion to protein bands after separation by lithium dodecyl sulfate-polyacrylamide gel electrophoresis (LDS-PAGE) and blotting onto polyvinylidene difluoride (PVDF) membrane at 4 degrees C. Cell bands adherent on PVDF are detected using hematoxylin, or propidium iodide (PI) staining followed by viewing under ultraviolet (UV) light. The technique allows quick microscopic visualization of adherent cells composing the bands, without requiring clearing of the membrane. Representative cell adhesion proteins from different sources, i.e., plant lectins (e.g., phytohemagglutinin, PHA; concanavalin A, ConA; and wheat germ agglutinin, WGA); extracellular matrix (ECM) proteins; and integral membrane proteins (e.g., recombinant soluble vascular cell adhesion molecule-1, rs VCAM-1) were tested for cell binding by the new cell-blotting technique using human lymphoid progenitor (NALM-6) and myeloid progenitor (KG1a) cell lines. Cell adhesion proteins retained their adhesion function in all cases tested. Specificity of cell binding on PVDF blot was demonstrated by inhibition of cell adhesion to WGA protein bands using an appropriate sugar, i.e., N-acetyl D-glucosamine. The cell blotting assay was comparable in sensitivity to Coomassie blue staining of protein bands. The ability to conduct protein extraction, separation and blotting at low temperature avoids thermal denaturation, thereby preserving the adhesion properties of the proteins. The electrophoretic/blotting system has unique detergent removal/protein renaturation properties and the ability to preserve functionally active adhesion protein complexes. The cell-blotting technique described is sufficiently robust for routine application in the investigation of novel cell adhesion proteins.

Growth inhibition of the intestinal parasite Giardia lamblia by a dietary lectin is associated with arrest of the cell cycle

Giardia lamblia, a cause of diarrheal disease throughout the world, is a protozoan parasite that thrives in the small intestine. It is shown here that wheat germ agglutinin (WGA), a naturally occurring lectin widely consumed in normal human diets, reversibly inhibits the growth of G. lamblia trophozoites in vitro, and reduces infection by G. muris in the adult mouse model of giardiasis. The inhibitory effect was dose related, not associated with cytotoxicity and reversed by N-acetyl-D-glucosamine in accordance with the known specificity of the lectin and in agreement with the presence of GlcNAc residues on the surface membrane of G. lamblia trophozoites. Cell cycle analysis revealed that parasites grown in the presence of WGA are arrested in the G2/M phase, providing an explanation for the lectin-induced inhibition of cell proliferation. Comparison of electrophoretic profiles by lectin blot analysis revealed both glycoprotein induction and suppression in growth-arrested organisms. Our findings raise the possibility that blocking trophozoite growth with naturally occurring dietary lectins may influence the course of giardiasis. In addition, the study of cell cycle arrest by WGA may provide a model to study the regulation of cell division in lower eukaryotes.

Attachment of Cryptosporidium parvum sporozoites to MDCK cells in vitro

The initial attachment of Cryptosporidium parvum sporozoites to host cells in vivo may be a critical event in the pathogenesis of this infection. The molecular basis of attachment and the conditions influencing this host-parasite interaction have not been studied systematically. Therefore, we have developed a sporozoite attachment model by using paraformaldehyde-fixed Madin-Darby canine kidney (MDCK) cells. Attachment of sporozoites to fixed MDCK cells was quantitated by indirect immunofluorescence and confirmed by transmission electron microscopy. Attachment in this system was time, temperature (37 degrees C), and pH (7.2 to 7.6) dependent. Dose-response studies demonstrated that the attachment of sporozoites to fixed MDCK cells was a saturable process. Attachment was enhanced in the presence of 10 mM manganese, 1 mM calcium, and 1 to 10 mM zinc. Attachment of sporozoites to MDCK cells was inhibited in a dose-dependent manner by polyclonal anti-Cryptosporidium antisera and by purified immunoglobulin G (IgG). This model will be useful for the study of parasite and host cell molecules involved in the initial interaction of C. parvum sporozoites with their target cell.

Role of surface components in the process of interaction of Giardia duodenalis with epithelial cells in vitro

Monosaccharides, lectins, periodate, trypsin and neuraminidase were used to analyse the process of adhesion of Giardia duodenalis trophozoites to IEC cells, an intestinal epithelial cell line. Addition of N-acetyl-glucosamine, N-acetyl-galactosamine, galactose and fucose to the interaction medium inhibited attachment of the parasites to the epithelial cells. Experiments in which the parasites or epithelial cells were treated before interaction showed that these monosaccharides interfered with both cell surfaces. Trypsin-sensitive, but not neuraminidase-sensitive, groups exposed on the cell surface are important for the parasite-epithelial cell association. Fluorescein isothiocyanate (FITC)- or colloidal gold-labeled lectins were used to analyse the distribution of carbohydrates on the surface of G. duodenalis and epithelial cells. It is important to stress here the presence of fucose on the parasite surface. Treatment of the cells with lectins was also used to analyse the role of carbohydrate-containing macromolecules in the parasite-cell interaction.

Purification and characterization of a sialic acid-specific lectin from Tritrichomonas mobilensis

New sialic acid-specific lectin has been isolated from culture supernatant of the protozoan Tritrichomonas mobilensis. It was purified by adsorption by erythrocytes or bovine submaxillary gland mucin (BSM)-Sepharose affinity chromatography. The T. mobilensis lectin (TML) does not require bivalent cations for activity and agglutinates all human erythrocytes. The lectin forms multimeric complexes with molecular mass 556 and 491 kDa as determined by size-exclusion chromatography. SDS/PAGE under reducing conditions disclosed a large band of 343 kDa and three bands of 246, 265 and 286 kDa which, after denaturation with urea, were split into three subunits of 56, 61 and 66 kDa; under non-reducing conditions there were two bands, of 360 and 260 kDa. Western blots performed with anti-TML monoclonal antibodies revealed bands identical with those in the silver-stained gels, suggesting homogeneity of the BSM -Sepharose-purified lectin. TML is a highly glycosylated protein with approx. 8% of N-linked glycosides found by protein-N-glycanase F treatment; the total amount of saccharides revealed by chemical deglycosylation was 20%. Haemagglutination-inhibition studies documented exclusive specificity for sialic acid (NeuAc). Both (alpha 2-->6)- and (alpha 2-->3)-linked and free NeuAc were eight times more potent inhibitors than N-glycolylneuraminic acid. The lectin does not require O-acetyl groups on NeuAc for recognition. A spectrum of mono- and oligo-saccharides other than sialic acid had no inhibitory effect at 200 mM. Anti-TML monoclonal antibodies strongly inhibited the lectin activity. TML was stable at temperatures below 4 degrees C and lyophilized with 3% (w/w) glycerol.

Diarrhoeal disease: current concepts and future challenges. Pathogenesis of giardiasis

Giardiasis is the most common small intestinal protozoal infection and is found worldwide. The mechanisms by which Giardia duodenalis (= G. lamblia) produces chronic diarrhoea and malabsorption have still not been clearly defined. Many infections are associated with mild to moderate mucosal damage which, in animal models of infection, have functional correlates. Possible mechanisms include direct physical injury, release of parasite products such as proteinases or lectin, and mucosal inflammation associated with T cell activation and cytokine release. Other possible mechanisms of malabsorption include associated bacterial overgrowth and bile salt deconjugation, bile salt uptake by the parasite with depletion of intraluminal bile salts, and inhibition of pancreatic hydrolytic enzymes. Thus, there is no single mechanism to explain the diarrhoea and malabsorption caused by Giardia, which currently should be regarded as a multifactorial process.

A cytopathic effect of Trichomonas vaginalis probably mediated by a mannose/N-acetyl-glucosamine binding lectin

The pathogenic effect of a highly pathogenic strain of Trichomonas vaginalis on McCoy cell monolayers was investigated. Specific inhibition of the cytopathic effect by monosaccharides, such as N-acetyl-glucosamine (GlcNAc) and mannose (Man), was observed. Our preliminary results suggest that the pathogenicity of T. vaginalis depends on a lectin specifically sensitive to GlcNAc and to a lesser extent to Man. Although N-acetyl-mannosamine was found to be the most efficient inhibitor, this effect seems to be unrelated to the natural biological behaviour of the infested host.

Giardia duodenalis: a freeze-fracture, fracture-flip and cytochemistry study

The freeze-fracture technique was used to study the structural organization of the membranes of trophozoites of the protozoon Giardia duodenalis. No special array of intramembranous particles was observed in the membrane lining the protozoon body or the flagella. A large globular protuberance located in the ventral region displayed several small circular indentations similar to those seen in the dorsal region. These also occurred on the parasite surface as revealed in fracture-flip replicas. A large number of vesicles were observed below the plasma membrane; they corresponded to an acidic compartment as indicated by fluorescence microscopy of acridine orange-stained cells and contained acid phosphatase as indicated by cytochemistry. In addition, gold-labeled macromolecules (albumin, peroxidase, transferrin, and low-density lipoprotein) accumulated in the vesicles. These observations suggest that the peripheral vesicles of trophozoites are part of the endosomal-lysosomal system of G. duodenalis.

Isolate and epitope variability in susceptibility of Giardia lamblia to intestinal proteases

The surface antigens of Giardia lamblia differ. To determine whether the unique surface antigens found in variants and isolates could differentially protect the parasite from digestion by intestinal protease, G. lamblia clones WB-2X (WB), GS/M-H7 (GS/M), and B6, each of which expresses a unique surface variant antigen, were exposed to alpha-chymotrypsin and trypsin at concentrations up to 20 mg/ml in culture medium. The number of surviving trophozoites and morphologic changes were assessed over time. After 24 h, there was a significant decrease in the number of surviving trophozoites of WB (80.5 and 94.2% for trypsin and alpha-chymotrypsin treatments, respectively, compared with controls) and B6 (78.9 and 95.5% for trypsin and alpha-chymotrypsin treatments, respectively, compared with controls) at 10 mg of enzyme per ml compared with culture medium alone. Cytotoxicity was prevented by the presence of soybean trypsin inhibitor, indicating the effects were due to protease activity. In contrast, there was no significant cytotoxicity after exposure of GS/M to either enzyme at the same enzyme concentration. After exposure to alpha-chymotrypsin, susceptible G. lamblia became rounded and then lysed, but after exposure to trypsin, G. lamblia appeared plastered onto the surface of the well and was intertwined and surrounded by finely granular material. Effects were concentration and time dependent; at least 6 h of treatment was required to observe changes 12 to 18 h later. Trophozoites surviving alpha-chymotrypsin or trypsin exposure became stably resistant to protease treatment. In vitro, the variant surface antigen of GS/M, but not those of WB or B6, resisted digestion by trypsin or alpha-chymotrypsin, suggesting that the variant surface antigens impart susceptibility or resistance to digestion. The initial surface variant antigens of WB and B6 were replaced in resistant cultures. Trophozoites differ in their ability to survive after exposure to intestinal proteases, which may enable certain G. lamblia isolates or isolates possessing certain surface variant antigens to survive in the small intestine.

Role of cytoskeleton and surface lectins in Giardia duodenalis attachment to Caco2 cells

An in vitro model of Giardia duodenalis and the Caco2 cell line enable the study of parameters that could play a part in trophozoite attachment. We explored the role of membranous lectins of G. duodenalis in attachment-inhibition studies using carbohydrates in solution. Attachment rates were reduced by 14% and 23% in the presence of 100 mmol/l mannose-6-phosphate and glucose, respectively, as compared with control values. No significant modification was observed after trophozoite trypsinization at room temperature or at 37 degrees C. The inhibitory effects of colchicine (35%) and nocodazole (70%) suggest a primordial role of the cytoskeleton; microtubules appear to be the principal effectors of trophozoite fixation. Scanning electron microscopy revealed circular imprints on the Caco2 brush border after trophozoite detachment. The mechanisms of attachment of G. duodenalis to intestinal enterocyte-like cells in culture are thus essentially of the mechanical or hydrodynamic type; surface lectins would appear to intervene in the specificity for duodenal cells.

The interaction between intestinal mucus glycoproteins and enteric infections

Adherence of pathogenic enteric organisms to specific receptors on mucosal surfaces is widely recognized as an important first step in the initiation of infectious diseases. The specific interactions whereby parasites and bacteria exploit mucus substrates for colonization, and the host uses them as a nonimmunological defense mechanism, is only now being unravelled. In this review, Sil-King Tse and Kris Chadee discuss various hypothetical models for interaction, including the role of the immune system in the regulation of mucus secretion.

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).

The surface free energy of Leishmania mexicana amazonensis

Surface charge of Leishmania mexicana amazonensis was investigated by direct zeta-potential determination and ultrastructural cytochemistry, and its surface tension was studied by measurements of the advancing contact angle formed by the parasite monolayers with drops of liquids of different polarities. Both virulent and avirulent promastigotes exhibited negatively charged surfaces with a zeta-potential of about -15 mV. Treatment of these cells with trypsin, alkaline phosphatase, or phospholipase C rendered their surfaces less negatively charged, whereas neuraminidase did not alter the parasite negativeness. Cytochemically, we could observe a reduction in the cationized ferritin binding after the parasite treatment with each of the former enzymes, but not with neuraminidase. The surface free energy of parasites was calculated by taken to account the London dispersion, the Keeson dipole-dipole, and the Debye dipole-induced forces, as well as the surface polarity of the parasites and their zeta-potentials, by considering their adhesion to polystyrene surfaces. The delta G values of -6.4 and -18.1 mJ.m-2 were obtained for avirulent and virulent promstigotes, respectively.