Purification of a variant-specific surface protein of Giardia lamblia and characterization of its metal-binding properties

Conserved Candidate Antigens and Nanoparticles to Develop Vaccine against Giardia intestinalis

Giardia intestinalis (Giardia lambia, Giardia duodenalis) infections in humans may be asymptomatic or symptomatic and associated with diarrhea (without blood), abdominal cramps, bloating, flatulence, and weight loss. The protozoan Giardia is the third most common cause of diarrhea and death in children under five, preceded only by rotavirus and by Cryptosporidium parvum and C. hominis infections. Antimicrobial drugs, particularly 5-nitroimidazole (5-NIs), are used to treat giardiasis in humans. Immunologically naive or immunocompromised host are more vulnerable to Giardia infection, whereas a degree of resistance to this protozoan is present in humans living in endemic areas. This suggests that vaccination may be a potential and appropriate means to control this parasitic disease outbreak and protect the human population. This review discusses Giardia antigens related to vaccine development. Additionally, based on the latest development of nanoparticle technology, a combination of methods for future research and development is proposed for the design of the next generation of powerful immunogens and an effective vaccine against Giardia.

Comprehensive characterization of Cysteine-rich protein-coding genes of Giardia lamblia and their role during antigenic variation

Giardia lamblia encodes several families of cysteine-rich proteins, including the Variant-specific Surface Proteins (VSPs) involved in the process of antigenic variation. Their characteristics, definition and relationships are still controversial. An exhaustive analysis of the Cys-rich families including organization, features, evolution and levels of expression was performed, by combining pattern searches and predictions with massive sequencing techniques. Thus a new classification for Cys-rich proteins, genes and pseudogenes that better describes their involvement in Giardia's biology is presented. Moreover, three novel characteristics exclusive to the VSP genes, comprising an Initiator element/Kozak-like sequence, an extended polyadenylation signal and a unique pattern of mutually exclusive transcript accumulation is presented as well as the finding that High Cysteine Membrane Proteins, upregulated under stress, may protect the parasite during VSP switching. These results allow better interpretation of previous reports providing the basis for further studies of the biology of this early-branching eukaryote.

Efficient oral vaccination by bioengineering virus-like particles with protozoan surface proteins

Intestinal and free-living protozoa, such as Giardia lamblia, express a dense coat of variant-specific surface proteins (VSPs) on trophozoites that protects the parasite inside the host’s intestine. Here we show that VSPs not only are resistant to proteolytic digestion and extreme pH and temperatures but also stimulate host innate immune responses in a TLR-4 dependent manner. We show that these properties can be exploited to both protect and adjuvant vaccine antigens for oral administration. Chimeric Virus-like Particles (VLPs) decorated with VSPs and expressing model surface antigens, such as influenza virus hemagglutinin (HA) and neuraminidase (NA), are protected from degradation and activate antigen presenting cells in vitro. Orally administered VSP-pseudotyped VLPs, but not plain VLPs, generate robust immune responses that protect mice from influenza infection and HA-expressing tumors. This versatile vaccine platform has the attributes to meet the ultimate challenge of generating safe, stable and efficient oral vaccines.

Giardia lamblia express a dense coat of variant-specific surface proteins (VSPs) on trophozoites that protects the parasite inside the host´s intestine. Here the authors show that stability and immunomodulatory properties of VSPs can be exploited to both protect and adjuvant vaccine antigens for oral administration.

Expression and secretion of the Giardia duodenalis variant surface protein 9B10A by transfected trophozoites causes damage to epithelial cell monolayers mediated by protease activity

Giardia duodenalis is the protozoan parasite responsible for most cases of parasitic diarrhea worldwide. The pathogenic mechanisms of giardiasis have not yet been fully characterized. In this context parasite's excretory/secretory products have been related to the damage induced by the parasite on enterocytes. Among these is the Variable Surface Proteins (VSPs) family involved in antigenic variation and in the induction of protective response. In proteomic analyses carried out to identify the proteases with high molecular weight secreted by Giardia trophozoites during the initial phase of interaction with IEC-6 cell monolayers we identified the VSP9B10A protein. In silico bioinformatics analyses predicted a central region in residues 324-684 displaying the catalytic triad and the substrate binding pocket of cysteine proteases. The analysis of the effect of the VSP9B10A protein on epithelial cell monolayers using trophozoites that were transfected with a plasmid carrying the vsp9b10a gene sequence under the control of a constitutive promoter showed that transfected trophozoites expressing the VSP9B10A protein caused cytotoxic damages on IEC-6 and MDCK cell monolayers. This was characterized by loss of cell-cell contacts and cell detachment from the substrate while no damage was observed with trophozoites that did not express the VSP9B10A protein. The same cytotoxic effect was detected when IEC-6 cell monolayers were incubated only with supernatants from co-cultures of IEC-6 cell monolayers with VSP9B10A transfected trophozoites and this effect was not observed when transfected trophozoites were incubated with a monospecific polyclonal antibody anti-VSP9B10A previous to interaction with IEC-6 monolayers. These results demonstrate that the VSP9B10A protein secreted upon interaction with epithelial cells caused damage in these cells. Thus this protein might be considered as a conditional virulence factor candidate. To our knowledge this is the first report on the proteolytic activity from a Giardia VSP opening new research lines on these proteins.

Factors associated with anemia in children under three years of age in Perú: analysis of the Encuesta Demográfica y de Salud Familiar, ENDES, 2007-2013

INTRODUCTION

Despite the reduction of poverty in Perú, the prevalence of anemia in the country remains high. 

OBJECTIVE

To identify socio-demographic, child and maternal-child care factors associated with anemia in children between 6 and 35 months in Perú. 

MATERIALS AND METHODS

We conducted an analytical and descriptive study that included registered data from the national survey on demography and family health, 2007-2013, on children between 6 and 35 months old, including the measurement of blood hemoglobin. Anemia was confirmed by hemoglobin-altitude corrected values below 11 mg/dl. We used multivariate logistic regression models to assess potential associated factors for anemia. 

RESULTS

Anemia prevalence was high (47.9%). Twelve factors were independently associated with anemia in children: Socio-demographic factors such as living outside Lima and Callao, in a low socioeconomic household, and having an adolescent mother with low education level; child-related factors as being male, younger than 24 months of age, and having fever in the previous two weeks, and maternal-child care factors such as lack of prenatal control in the first trimester of pregnancy, lack or short period of iron supplementation during pregnancy, house delivery, anemia detection at the moment of the survey, and lack of intestinal anti-parasite preventive treatment in the child. 

CONCLUSIONS

The analysis of survey data provided valuable information about factors associated with anemia in children between 6 and 35 months, which can be used to increase the coverage and effectiveness of maternal-child care practices.

Intraepithelial giardia intestinalis: a case report and literature review

The giardiasis is a neglected parasitic disease. The WHO has estimated more than 280 million of human infections each year; however, intraepithelial giardiasis is a rare entity, there are only 5 reports showing invasive giardiasis. A pediatric female patient with chronic abdominal pain, diarrhea, or pasty stools, without fever, was seen in the Gastroenterology and Nutrition Service. The stool studies were negative for pathogens and lactose hydrogen breath test was positive. The presumptive clinical diagnosis was giardiasis and the patient was empirically treated with nitazoxanide. But, the patient persisted with abdominal pain and pasty stools. Endoscopy was indicated to search for Helicobacter and Giardia. Guardian and patient gave written informed consent. Hematological profile was normal. The endoscopy was performed under general anesthesia and the biopsies and duodenal aspirate were obtained. The microscopic analyses of duodenal fluid showed Giardia trophozoites. Electron microscopic analysis was negative for Helicobacter pylori, but Giardia trophozoites with a typical crescent shape within the tissue were found. The patient was treated with tinidazole, subsequent tests showed that lactose absorption was normal, stool examinations were negative for Giardia and abdominal pain had stopped. This case suggest that intraepithelial giardiasis could be a common entity but unseen because the giardiasis diagnosis is usually made on fecal samples. Future studies are necessary to determine the role of intraepithelial trophozoites in giardiasis pathogenic mechanisms.

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

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

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

Effect of cryptosporidial and giardial diarrhoea on social maturity, intelligence and physical growth in children in a semi-urban slum in south India

BACKGROUND

Early childhood diarrhoea is a major cause of infant morbidity and mortality in developing countries. Recurrent and persistent diarrhoea affect growth and cognition in children as young as 6 years.

OBJECTIVES

To evaluate the effect of early childhood cryptosporidial and giardial diarrhoea on growth and development in children in a semi-urban slum in India. This is the first report of such assessment at 3 years of age.

METHODS

This study was undertaken on 116 children who were part of an ongoing birth cohort study (n=452) of rotaviral and cryptosporidial diarrhoea between June and December 2005. Social quotients (SQ) assessed by the Vineland Social Maturity Scale, intelligence quotients (IQ) assessed by the Seguin Form Board Test, physical growth parameters and sociodemographic data in 84 children with a history of cryptosporidial or giardial diarrhoea were compared with those of 32 without diarrhoea.

RESULTS

Children with a past history of giardial diarrhoea showed a trend towards lower SQ (p=0.09) and had significantly lower IQ (p=0.04) and increased wasting (p=0.04). Cryptosporidial diarrhoea was not associated with poor IQ, SQ or physical growth.

CONCLUSION

This study demonstrates the long-term effect of protozoan diarrhoea, especially that caused by giardia, on both intelligence and physical growth in Indian children as early as 3 years of age and re-inforces the need for early detection and prevention of early childhood protozoan diarrhoea.

The Giardia lamblia vsp gene repertoire: characteristics, genomic organization, and evolution

Background

Giardia lamblia trophozoites colonize the intestines of susceptible mammals and cause diarrhea, which can be prolonged despite an intestinal immune response. The variable expression of the variant-specific surface protein (VSP) genes may contribute to this prolonged infection. Only one is expressed at a time, and switching expression from one gene to another occurs by an epigenetic mechanism.

Results

The WB Giardia isolate has been sequenced at 10× coverage and assembled into 306 contigs as large as 870 kb in size. We have used this assembly to evaluate the genomic organization and evolution of the vsp repertoire. We have identified 228 complete and 75 partial vsp gene sequences for an estimated repertoire of 270 to 303, making up about 4% of the genome. The vsp gene diversity includes 30 genes containing tandem repeats, and 14 vsp pairs of identical genes present in either head to head or tail to tail configurations (designated as inverted pairs), where the two genes are separated by 2 to 4 kb of non-coding DNA. Interestingly, over half the total vsp repertoire is present in the form of linear gene arrays that can contain up to 10 vsp gene members. Lastly, evidence for recombination within and across minor clades of vsp genes is provided.

Conclusions

The data we present here is the first comprehensive analysis of the vsp gene family from the Genotype A1 WB isolate with an emphasis on vsp characterization, function, evolution and contributions to pathogenesis of this important pathogen.

Iron deficiency in international adoptees from Eastern Europe

OBJECTIVE

To assess iron deficiency (ID) in international adoptees after adoption.

STUDY DESIGN

Participants (n = 37) were adopted into the United States from Eastern Europe before they were 24 months of age. Baseline (within 1 month post-adoption) and follow-up (6 months post-adoption) assessments included routine post-adoption clinical evaluations, anthropometrics, dietary intakes, and iron measures (hemogram and serum analysis).

RESULTS

At adoption and follow-up, mean percent transferrin saturation and mean corpucuscular volume were low compared with the US population. Mean serum ferritin concentration became lower than the US population at follow-up, although the mean daily iron intake was more than the Recommended Dietary Allowance. Participants with Giardia lamblia at baseline had more compromised iron status at baseline and follow-up. Growth rate (change in z-score/months between assessments) was negatively correlated with change in serum ferritin concentrations between baseline and follow-up (r = -0.34; P < .05).

CONCLUSIONS

International adoptees had compromised iron status, with ID more prevalent in participants with G lamblia, a parasite that may interfere with iron absorption. The persistent ID at follow-up was likely caused by the erythropoietic demands of catch-up growth.

Goblet cells: are they an unspecific barrier against Giardia intestinalis or a gate?

Giardiosis is one of the major intestinal parasitic diseases of human beings as well as wild and domesticated animals. Several protective mechanisms against infection have been described. However, specific information about relationship between giardiosis and the increased proliferation of goblet cells (GC) in patients infected with Giardia intestinalis (Syn. G. duodenalis, G. lamblia) is scarce. In this work, we compare and quantify the number of GC, and have inferred their metabolic state in the small intestine of dogs parasitized with Giardia intestinalis compared to dogs without parasites. Small intestine segments were processed using routine methods for histology and electron microscopy; areas and cells were screened with an Axiovision Ver. 4.0 system. Data were analyzed by ANOVA and comparison of averages. Parasitized dogs showed higher GC numbers than nonparasitized ones. Averages were: 20+/-0.81 GC/25 microm(2) with independent mucin granules and 11+/-1.53 GC/25 microm(2) that were expelling mucus, compared to 11+/-0.94 GC/25 microm(2) and 1+/-0.27 GC/25 microm(2), respectively, in nonparasitized dogs (Tukey, p<0.001). The increases in GC number seem to be an unspecific defensive mechanism against Giardia trophozoites. However, we found some evidence supporting that GC hyperplasia could be a prejudicial to epithelial barrier that gives rise to gates allowing for Giardia-tissue invasion.

Metalloproteomics: High-Throughput Structural and Functional Annotation of Proteins in Structural Genomics

A high-throughput method for measuring transition metal content based on quantitation of X-ray fluorescence signals was used to analyze 654 proteins selected as targets by the New York Structural GenomiX Research Consortium. Over 10% showed the presence of transition metal atoms in stoichiometric amounts; these totals as well as the abundance distribution are similar to those of the Protein Data Bank. Bioinformatics analysis of the identified metalloproteins in most cases supported the metalloprotein annotation; identification of the conserved metal binding motif was also shown to be useful in verifying structural models of the proteins. Metalloproteomics provides a rapid structural and functional annotation for these sequences and is shown to be approximately 95% accurate in predicting the presence or absence of stoichiometric metal content. The project's goal is to assay at least 1 member from each Pfam family; approximately 500 Pfam families have been characterized with respect to transition metal content so far.

Assembly and export of a Toxoplasma microneme complex in Giardia lamblia

The microneme proteins of Toxoplasma gondii belong to a large family of adhesins of apicomplexan parasites involved in motility and host cell invasion. During secretory transport, soluble micronemes associate with membrane-bound carriers/escorters and become exposed on the parasite surface as complexes with an array of adhesive domains. Previously, we have exploited the intestinal protozoan Giardia lamblia as an expression system to produce correctly folded and unglycosylated monomeric surface proteins of T. gondii. Here, we report assembly and export of a trimeric microneme (MIC1/4/6) adhesin complex from Toxoplasma. Co-expressed, recombinant microneme proteins were used to investigate structural requirements for microneme complex formation. In addition, export of a microneme subunit induced development of novel Golgi-like compartments demonstrating the existence of post endoplasmic reticulum structures involved in constitutive secretion in this 'Golgi-less' cell. Recreation of the trimeric microneme escorter-cargo system in Giardia is a versatile tool to analyse universal requirements for complex assembly, receptor-ligand interactions and Golgi neogenesis in the basal Giardia secretory system.

Giardia duodenalis: direct experimental evidence for the absence of a glycosylphosphatidylinositol anchor in a variant surface protein

The trophozoites of Giardia duodenalis express variant surface proteins (VSPs) that cover the entire surface of the cell and can be altered by antigenic variation. In the present study, a VSP (VSPH7) expressed by the Giardia GS isolate was purified using Triton-X-114 extraction/phase partitioning and a combination of column chromatography methods. The purified VSP was typed by mass spectrometric fingerprint mapping and peptide sequencing and found to share 58-99.8% peptide identity with the VSPH7 protein sequence previously deduced from the cloned cDNA. Carbohydrate compositional analyses consistently showed the presence of galactose in the VSP preparations but a direct association of carbohydrate with the VSPH7 could not be established. Analysis of the C-terminal part of the purified VSPH7 by off-blot myo-inositol analysis provided for the first time direct experimental evidence that this protein is not modified via a GPI lipid.

High-throughput computational and experimental techniques in structural genomics

Structural genomics has as its goal the provision of structural information for all possible ORF sequences through a combination of experimental and computational approaches. The access to genome sequences and cloning resources from an ever-widening array of organisms is driving high-throughput structural studies by the New York Structural Genomics Research Consortium. In this report, we outline the progress of the Consortium in establishing its pipeline for structural genomics, and some of the experimental and bioinformatics efforts leading to structural annotation of proteins. The Consortium has established a pipeline for structural biology studies, automated modeling of ORF sequences using solved (template) structures, and a novel high-throughput approach (metallomics) to examining the metal binding to purified protein targets. The Consortium has so far produced 493 purified proteins from >1077 expression vectors. A total of 95 have resulted in crystal structures, and 81 are deposited in the Protein Data Bank (PDB). Comparative modeling of these structures has generated >40,000 structural models. We also initiated a high-throughput metal analysis of the purified proteins; this has determined that 10%-15% of the targets contain a stoichiometric structural or catalytic transition metal atom. The progress of the structural genomics centers in the U.S. and around the world suggests that the goal of providing useful structural information on most all ORF domains will be realized. This projected resource will provide structural biology information important to understanding the function of most proteins of the cell.

Sorting of encystation-specific cysteine protease to lysosome-like peripheral vacuoles in Giardia lamblia requires a conserved tyrosine-based motif

Encystation-specific cysteine protease (ESCP) was the first membrane-associated protein described to be part of the lysosome-like peripheral vacuoles in the intestinal parasite Giardia lamblia. ESCP is homologous to cathepsin C enzymes of higher eukaryotes, but is distinguished from other lysosomal cysteine proteases because it possesses a transmembrane domain and a short cytoplasmic tail. Tyrosine-based motifs within tails of membrane proteins are known to participate in endosomal/lysosomal protein sorting in higher eukaryotes. In this study, we show that a YRPI motif within the ESCP cytoplasmic tail is necessary and sufficient to mediate ESCP sorting to peripheral vacuoles in Giardia. Deletion and point mutation analysis demonstrated that the tyrosine residue is critical for ESCP sorting, whereas amino acids located at the Y+1 (Arg), Y+2 (Pro), and Y+3 (Ile) positions show minimal effect. Loss of the motif resulted in surface localization, whereas addition of the motif to a variant-specific surface protein resulted in lysosomal localization. Although Giardia trophozoites lack a morphologically discernible Golgi apparatus, our findings indicate that this parasite directs proteins to the lysosomes using a conserved sorting signal similar to that used by yeast and mammalian cells. Because Giardia is one of the earliest branching protist, these results demonstrate that sorting motifs for specific protein traffic developed very early during eukaryotic evolution.

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.

Glycoconjugate structures of parasitic protozoa

Glycoconjugates are abundant and ubiquitious on the surface of many protozoan parasites. Their tremendous diversity has implicated their critical importance in the life cycle of these organisms. This review highlights our current knowledge of the major glycoconjugates, with particular emphasis on their structures, of representative protozoan parasites, including Leishmania, Trypanosoma, Giardia, Plasmodia, and others.

Trends of amino acid usage in the proteins from the unicellular parasite Giardia lamblia

Correspondence analysis of amino acid frequencies was applied to 75 complete coding sequences from the unicellular parasite Giardia lamblia, and it was found that three major factors influence the variability of amino acidic composition of proteins. The first trend strongly correlated with (a) the cysteine content and (b) the mean weight of the amino acids used in each protein. The second trend correlated with the global levels of hydropathy and aromaticity of each protein. Both axes might be related with the defense of the parasite to oxygen free radicals. Finally, the third trend correlated with the expressivity of each gene, indicating that in G. lamblia highly expressed sequences display a tendency to preferentially use a subset of the total amino acids.

Antigenic variation and the murine immune response to Giardia lamblia

The protozoan parasite Giardia lamblia is an important causative agent of acute or chronic diarrhoea in humans and various animals. During infection, the parasite survives the host's reactions by undergoing continuous antigenic variation of its major surface antigen, named VSP (variant surface protein). The VSPs form a unique family of cysteine-rich proteins that are extremely heterogeneous in size. The relevance of antigenic variation for the survival in the host has been most successfully studied by performing experimental infections in a combined mother/offspring mouse system and by using the G. lamblia clone GS/M-83-H7 (human isolate) as model parasite. In-vivo antigenic variation of G. lamblia clone GS/M-83-H7 is characterised by a diversification of the intestinal parasite population into a complex mixture of different variant antigen types. It could be shown that maternally transferred lactogenic anti-VSP IgA antibodies exhibit cytotoxic activity on the Giardia variant-specific trophozoites in suckling mice, and thus express a modulatory function on the proliferative parasite population characteristics. Complementarily, in-vitro as well as in-vivo experiments in adult animals indicated that non-immunological factors such as intestinal proteases may interfere into the process of antigen variation in that they favour proliferation of those variant antigen-type populations which resist the hostile physiological conditions within the intestine. These observations suggest that an interplay between immunological and physiological factors, rather than one of these two factor alone, modulates antigenic diversification of a G. lamblia population within an experimental murine host and thus influences the survival rate and strategy of the parasite.

Efficacies of zinc-finger-active drugs against Giardia lamblia

Twenty-nine of 34 (85%) Zn-finger-active compounds at 300 microM or less inhibited the growth of Giardia lamblia. The most active compound, disulfiram (Antabuse), was cidal at 1.23 +/- 0.32 microM. In the adult mouse model, significant in vivo activity was demonstrated by increased cure rates and decreased parasite burdens.

Antigenic variation in Giardia lamblia and the host's immune response

Giardia lamblia, a protozoan parasite of the small intestine of humans and other animals, undergoes surface antigenic variation. The antigens involved belong to a family of variant-specific surface proteins (VSPs), which are unique, cysteine-rich zinc finger proteins. The patterns of infection in humans and animals fail to show the expected cyclical waves of increasing and decreasing numbers of parasites expressing unique VSPs. Nevertheless, changes in VSP expression occur within the population in vivo owing to selection of VSPs by both immune and non-immune mechanisms. After inoculation of a single G. lamblia clone (able to persist in the absence of immune pressure) expressing one VSP (> or = 90%) into mice or humans, the original VSP continues to be expressed until 2 weeks post inoculation (p.i.), when many other VSPs gradually replace it. Selection by immune-mediated processes is suggested because switching occurs at the same time that humoral responses are first detected. In most mouse strains, switching also occurs at about two weeks. Almost all trophozoites are eliminated at three weeks (p.i.), but a barely detectable infection persists over months. In neonatal mice, apparent self-cure is delayed until the sixth or seventh week. Antigenic switching does not occur in adult or neonatal severe combined immunodeficiency disease (SCID) mice, but does occur in neonatal nude mice, thus implicating B-cell-mediated mechanisms in immune switching. Not all VSPs are expressed to the same degree in vivo. Some VSPs appear to be preferentially selected whereas others are eliminated on a non-immune basis. In infections in which immunity does not play a role, such as in SCID mice, and during the first week of infection in immunocompetent mice or gerbils, persisting VSPs are preferentially expressed and maintained whereas non-persisting VSPs are replaced within the first week of infection. The purpose of antigenic variation may be presentation of a wide assortment of VSPs to hosts, increasing the chance of a successful initial infection or reinfection. Immune selection of variants comes into play following biological selection.

Mechanisms of Giardia lamblia differentiation into cysts

Microbiologists have long been intrigued by the ability of parasitic organisms to adapt to changes in the environment. Since most parasites occupy several niches during their journey between vectors and hosts, they have developed adaptive responses which allow them to survive under adverse conditions. Therefore, the life cycles of protozoan and helminthic parasites are excellent models with which to study numerous mechanisms involved in cell differentiation, such as the regulation of gene expression, signal transduction pathways, and organelle biogenesis. Unfortunately, many of these studies are very difficult because the conditions needed to elicit developmental changes in parasites remain undetermined in most cases. Recently, several interesting findings were reported on the process of differentiation of Giardia lamblia trophozoites into cysts. G. lamblia is a flagellated protozoan that inhabits the upper small intestine of its vertebrate host and is a major cause of enteric disease worldwide. It belongs to the earliest identified lineage among eukaryotes and therefore offers a unique insight into the progression from primitive to more complex eukaryotic cells. The discovery of a specific stimulus that induces trophozoites to differentiate into cysts, the identification and characterization of encystation-specific molecules, the elucidation of novel biochemical pathways, and the development of useful reagents and techniques have made this parasite an excellent model with which to study differentiation in eukaryotic cells. In this review, we summarize the most recent fundings on several aspects of Giardia differentiation and discuss the significance of these findings within the context of current knowledge in the field.

Kinetoplastid glucose transporters

Protozoa of the order kinetoplastida have colonized many habitats, and several species are important parasites of humans. Adaptation to different environments requires an associated adaptation at a cell's interface with its environment, i.e. the plasma membrane. Sugar transport by the kinetoplastida as a phylogenetically related group of organisms offers an exceptional model in which to study the ways by which the carrier proteins involved in this process may evolve to meet differing environmental challenges. Seven genes encoding proteins involved in glucose transport have been cloned from several kinetoplastid species. The transporters all belong to the glucose transporter superfamily exemplified by the mammalian erythrocyte transporter GLUT1. Some species, such as the African trypanosome Trypanosoma brucei, which undergo a life cycle where the parasites are exposed to very different glucose concentrations in the mammalian bloodstream and tsetse-fly midgut, have evolved two different transporters to deal with this fluctuation. Other species, such as the South American trypanosome Trypanosoma cruzi, multiply predominantly in conditions of relative glucose deprivation (intracellularly in the mammalian host, or within the reduviid bug midgut) and have a single, relatively high-affinity type, transporter. All of the kinetoplastid transporters can also transport d-fructose, and are relatively insensitive to the classical inhibitors of GLUT1 transport cytochalasin B and phloretin.

Giardia lamblia variant surface protein H7 stimulates a heterogeneous repertoire of antibodies displaying differential cytological effects on the parasite

Previous investigations had shown that the Giardia lamblia clone GS/M-83-H7-specific variant surface protein (VSP) H7 consists of at least two antigenically distinct parts: (i) a variable 314-aa N-terminal region which contains one, or more, variant-specific epitopes eliciting a transient and consequently low-level antibody response preferentially detectable during the early phase of G. lamblia infection in mice; and (ii) a 171-aa C-terminal region which contains relatively conserved epitope(s) causing a persistent and consequently high-level antibody response during the later phase of an infection. The present study indicated that monoclonal antibody G10/4 and polyclonal antibodies from early-phase infected or hyperimmunized mice, directed against the variant-specific N-terminal regional exclusively recognized conformational cysteine-containing epitopes. These antibodies caused detachment and aggregation of trophozoites, and exhibited complement-independent cytotoxic effect towards the parasite. In contrast, polyclonal antibodies from late-phase infected mice, directed against the semi-conserved peptidyl structures in the C-terminal region, preferentially reacted with non-conformational epitopes. Such antibodies had no cytotoxic effect, but provoked parasite-detachment and -aggregation. These findings indicated that infection of mice with G. lamblia clone GS/M-83-H7 generates a heterogeneous repertoire of cytologically active anti-VSP antibodies which may have a direct influence on the course of the parasite infection.

The variant-specific surface protein of Giardia, VSP4A1, is a glycosylated and palmitoylated protein

The variant-specific surface proteins (VSPs) of the ancient protist Giardia duodenalis (syn.: Giardia intestinalis, Giardia lamblia) are cysteine- and threonine-rich polypeptides that can vary considerably in sequence and size. In the present study, we have purified a VSP (VSP4A1, formerly called CR1SP-90) from a cloned Giardia isolate, derived from a sheep, by Triton X-114 phase partitioning and anion-exchange chromatography. Analysis of the purified VSP4A1 showed that this protein is posttranslationally modified with both glycans and lipid. The glycans of VSP4A1 were detected and partially characterized by (1) compositional analysis, which indicated the presence of GlcNAc and Glc (0.5 and 1.0 mol/mol of protein respectively), and (2) the specific labelling of VSP4A1 with galactosyltransferase/UDP-[3H]Gal. The glycans were released by beta-elimination, suggesting that they are O-linked to the protein. Bio-Gel P4 chromatography of the released galactosylated glycans and further compositional analysis suggested that the major glycan on the VSP is a trisaccharide with Glc at the reducing terminus. These and other results indicate the absence of any N-linked glycans on the VSP and suggest instead that it is elaborated with a novel type of short O-linked glycan. Compositional analysis and radiolabelling experiments also indicated that VSP4A1 is modified with covalently linked palmitate (1 mol/mol of protein). Hydroxylamine treatment at neutral pH of[3H]palmitate-labelled VSP4A1 indicated that the acyl chain may be attached by a thioester linkage. A likely location for the lipid modification appears to be in the region of the C-terminal domain where it may facilitate association of the protein with the plasma membrane.

Ultrastructural effects of lactoferrin binding on Giardia lamblia trophozoites

Lactoferrin and its derived N-terminal peptide may be important host defenses against Giardia lamblia. We showed earlier that lactoferrin and the derived peptides have potent giardicidal activity in vitro. Using indirect immunofluorescence, we now demonstrate binding of lactoferrin and the peptides to the live trophozoite surface. Iron strongly inhibited binding of lactoferrin, and decreased binding of the peptides, while certain divalent metal ions decreased binding of all forms by about half. Lactoferrin and the peptides caused striking and complex morphologic changes in the trophozoite plasmalemma, endomembranes and cytoskeleton, and increased the electron density of the lysosome-like peripheral vacuoles.

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.

Primary structure and phylogenetic relationships of glyceraldehyde-3-phosphate dehydrogenase genes of free-living and parasitic diplomonad flagellates

Complete nucleotide sequences have been established for two genes (gap1 and gap2) coding for glyceraldehyde-3-phosphate dehydrogenase (GAPDH, EC 1.2.1.12) homologs in the diplomonad Giardia lamblia. In addition, almost complete sequences of the GAPDH open reading frames were obtained from PCR products for two free-living diplomonad species, Trepomonas agillis and Hexamita inflata, and a parasite of Atlantic salmon, an as yet unnamed species with morphological affinities to Spironucleus. Giardia lamblia gap1 and the genes from the three other diplomonad species show high similarity to each other and to other glycolytic GAPDH genes. All amino-acyl residues known to be highly conserved in this enzyme are also conserved in these sequences. Giardia lamblia gap2 gene is more divergent and its putative translation reveals the presence of a cysteine and serine-rich insertion resembling a metal binding finger. This motif has not yet been noted in other GAPDH molecules. All sequences contain an S-loop signature with characteristics close to those of eukaryotes. In phylogenetic reconstructions based on the derived amino acid sequences with neighbor-joining, parsimony and maximum-likelihood methods the four typical GAPDH sequences of diplomonads cluster into a single clade. Within this clade, G. lambia gap1 shares a common ancestor with the rest of the genes. The latter are more closely related to each other, indicating an early separation of the lineage leading to the genus Giardia from the lineage encompassing the morphologically less differentiated genera, Trepomonas, Hexamita and that of the unnamed species. This result is discordant with the orthogonal evolution of diplomonads suggested on the basis of comparative morphology. In neighbor-joining reconstructions G. lamblia gap2 occupies a variable position, due to its great divergence. In parsimony and maximum likelihood analysis however, it shares a most recent common ancestor with the typical G. lamblia gap1 gene, suggesting that it diverged after the separation of the Giardia lineage. The position of the diplomonad clade in broader phylogenetic reconstructions is firmly within the typical cytosolic glycolytic representatives of GAPDH of eukaryotes.

Identification of a novel Giardia lamblia cyst wall protein with leucine-rich repeats. Implications for secretory granule formation and protein assembly into the cyst wall

Giardia lamblia trophozoites, like most intestinal parasitic protozoa, undergo fundamental biological changes to survive outside the intestine of their mammalian host by differentiating into infective cysts. This complex process entails the coordinated production, processing, and transport of cyst wall constituents for assembly into a protective cyst wall. Yet, little is known about this process and the identity of cyst wall constituents. We previously identified a 26-kDa cyst wall protein, CWP1. In the present work, using monoclonal antibodies to cyst wall antigens, we cloned the gene that encodes a novel 39-kDa cyst wall protein, CWP2. Expression of CWP1 and CWP2 was induced during encystation with identical kinetics. Soon after synthesis, these two proteins combine to form a stable complex, which is concentrated within the encystation-specific secretory granules before incorporation into the cyst wall. Both proteins contain five tandem copies of a 24-residue leucine-rich repeat, a motif implicated in protein-protein interactions. Unlike CWP1, CWP2 has an extremely basic 121-residue COOH-terminal extension that might be involved in the sorting of these proteins to the secretory granules.