Novel role of sphingolipid synthesis genes in regulating giardial encystation

The Sphinx and the egg: Evolutionary enigmas of the (glyco)sphingolipid biosynthetic pathway

In eukaryotes, the de novo synthesis of sphingolipids (SLs) consists of multiple sequential steps which are compartmentalized between the endoplasmic reticulum and the Golgi apparatus. Studies over many decades have identified the enzymes in the pathway, their localization, topology and an array of regulatory mechanisms. However, little is known about the evolutionary forces that underly the generation of this complex pathway or of its anteome, i.e., the metabolic pathways that converge on the SL biosynthetic pathway and are essential for its activity. After briefly describing the pathway, we discuss the mechanisms by which the enzymes of the SL biosynthetic pathway are targeted to their different subcellular locations, how the pathway per se may have evolved, including its compartmentalization, and the relationship of the pathway to eukaryogenesis. We discuss the circular interdependence of the evolution of the SL pathway, and comment on whether current Darwinian evolutionary models are able to provide genuine mechanistic insight into how the pathway came into being.

Protists: Eukaryotic single-celled organisms and the functioning of their organelles

Organelles are membrane bound structures that compartmentalize biochemical and molecular functions. With improved molecular, biochemical and microscopy tools the diversity and function of protistan organelles has increased in recent years, providing a complex panoply of structure/function relationships. This is particularly noticeable with the description of hydrogenosomes, and the diverse array of structures that followed, having hybrid hydrogenosome/mitochondria attributes. These diverse organelles have lost the major, at one time, definitive components of the mitochondrion (tricarboxylic cycle enzymes and cytochromes), however they all contain the machinery for the assembly of Fe-S clusters, which is the single unifying feature they share. The plasticity of organelles, like the mitochondrion, is therefore evident from its ability to lose its identity as an aerobic energy generating powerhouse while retaining key ancestral functions common to both aerobes and anaerobes. It is interesting to note that the apicoplast, a non-photosynthetic plastid that is present in all apicomplexan protozoa, apart from Cryptosporidium and possibly the gregarines, is also the site of Fe-S cluster assembly proteins. It turns out that in Cryptosporidium proteins involved in Fe-S cluster biosynthesis are localized in the mitochondrial remnant organelle termed the mitosome. Hence, different organisms have solved the same problem of packaging a life-requiring set of reactions in different ways, using different ancestral organelles, discarding what is not needed and keeping what is essential. Don't judge an organelle by its cover, more by the things it does, and always be prepared for surprises.

Commensal protist Tritrichomonas musculus exhibits a dynamic life cycle that induces extensive remodeling of the gut microbiota

Commensal protists and gut bacterial communities exhibit complex relationships, mediated at least in part through host immunity. To improve our understanding of this tripartite interplay, we investigated community and functional dynamics between the murine protist Tritrichomonas musculus and intestinal bacteria in healthy and B-cell-deficient mice. We identified dramatic, protist-driven remodeling of resident microbiome growth and activities, in parallel with Tritrichomonas musculus functional changes, which were accelerated in the absence of B cells. Metatranscriptomic data revealed nutrient-based competition between bacteria and the protist. Single-cell transcriptomics identified distinct Tritrichomonas musculus life stages, providing new evidence for trichomonad sexual replication and the formation of pseudocysts. Unique cell states were validated in situ through microscopy and flow cytometry. Our results reveal complex microbial dynamics during the establishment of a commensal protist in the gut, and provide valuable data sets to drive future mechanistic studies.

The commensal protist Tritrichomonas musculus exhibits a dynamic life cycle that induces extensive remodeling of the gut microbiota

Commensal protists and gut bacterial communities exhibit complex relationships, mediated at least in part through host immunity. To improve our understanding of this tripartite interplay, we investigated community and functional dynamics between the murine protist Tritrichomonas musculus ( T. mu ) and intestinal bacteria in healthy and B cell-deficient mice. We identified dramatic, protist-driven remodeling of resident microbiome growth and activities, in parallel with T. mu functional changes, accelerated in the absence of B cells. Metatranscriptomic data revealed nutrient-based competition between bacteria and the protist. Single cell transcriptomics identified distinct T. mu life stages, providing new evidence for trichomonad sexual replication and the formation of pseudocysts. Unique cell states were validated in situ through microscopy and flow cytometry. Our results reveal complex microbial dynamics during the establishment of a commensal protist in the gut, and provide valuable datasets to drive future mechanistic studies.

Giardial lipid rafts share virulence factors with secreted vesicles and participate in parasitic infection in mice

Giardia lamblia, a protozoan parasite, is a major cause of waterborne infection, worldwide. While the trophozoite form of this parasite induces pathological symptoms in the gut, the cyst form transmits the infection. Since Giardia is a noninvasive parasite, the actual mechanism by which it causes disease remains elusive. We have previously reported that Giardia assembles cholesterol and GM1 glycosphingolipid-enriched lipid rafts (LRs) that participate in encystation and cyst production. To further delineate the role of LRs in pathogenesis, we isolated LRs from Giardia and subjected them to proteomic analysis. Various cellular proteins including potential virulence factors—e.g., giardins, variant surface proteins, arginine deaminases, elongation factors, ornithine carbomyltransferases, and high cysteine-rich membrane proteins—were found to be present in LRs. Since Giardia secretes virulence factors encapsulated in extracellular vesicles (EVs) that induce proinflammatory responses in hosts, EVs released by the parasite were isolated and subjected to nanoparticle tracking and proteomic analysis. Two types of EV—i.e., small vesicles (SVs; <100 nm, exosome-like particles) and large vesicles (LVs; 100–400 nm, microvesicle-like particles)—were identified and found to contain a diverse group of proteins including above potential virulence factors. Although pretreatment of the parasite with two giardial lipid raft (gLR) disruptors, nystatin (27 μM) and oseltamivir (20 μM), altered the expression profiles of virulence factors in LVs and SVs, the effects were more robust in the case of SVs. To examine the potential role of rafts and vesicles in pathogenicity, Giardia-infected mice were treated with oseltamivir (1.5 and 3.0 mg/kg), and the shedding of cysts were monitored. We observed that this drug significantly reduced the parasite load in mice. Taken together, our results suggest that virulence factors partitioning in gLRs, released into the extracellular milieu via SVs and LVs, participate in spread of giardiasis and could be targeted for future drug development.

The sphingolipid anteome: implications for evolution of the sphingolipid metabolic pathway

Modern cell membranes contain a bewildering complexity of lipids, among them sphingolipids (SLs). Advances in mass spectrometry have led to the realization that the number and combinatorial complexity of lipids, including SLs, is much greater than previously appreciated. SLs are generated de novo by four enzymes, namely serine palmitoyltransferase, 3-ketodihydrosphingosine reductase, ceramide synthase and dihydroceramide Δ4-desaturase 1. Some of these enzymes depend on the availability of specific substrates and cofactors, which are themselves supplied by other complex metabolic pathways. The evolution of these four enzymes is poorly understood and likely depends on the co-evolution of the metabolic pathways that supply the other essential reaction components. Here, we introduce the concept of the 'anteome', from the Latin ante ('before') to describe the network of metabolic ('omic') pathways that must have converged in order for these pathways to co-evolve and permit SL synthesis. We also suggest that current origin of life and evolutionary models lack appropriate experimental support to explain the appearance of this complex metabolic pathway and its anteome.

Giardia intestinalis and its Endomembrane System

Giardia intestinalis has unique characteristics, even in the absence of certain organelles. For instance, Golgi and mitochondria are not found. On the other hand, there is a network of peripheral vacuoles (PVs) and mitosomes. The endoplasmic reticulum (ER), nuclear membrane, peroxisomes, and lipid bodies are present. The peripheral vacuole system seems to play several simultaneous roles. It is involved in the endocytic activity of the trophozoite but also has characteristics of early and late endosomes and even lysosomes, establishing a connection with the ER. Some of the PVs contain small vesicles, acting as multivesicular bodies, including the release of exosomes. The mitosomes are surrounded by two membranes, divide during mitosis, and are distributed throughout the cell. They do not contain DNA, enzymes involved in the citric acid cycle, respiratory chain, or ATP synthesis. However, they contain the iron-sulfur complex and transporters as TOM and TIM. Some mitosomes are linked to flagellar axonemes through a fibrillar connection. During encystation, two types of larger cytoplasmic vesicles appear. One originating from the ER contains the cyst wall proteins. Another contains carbohydrates. Both migrate to the cell periphery and fuse with plasma membrane secreting their contents to give rise to the cell wall.

Multimodal regulation of encystation in Giardia duodenalis revealed by deep proteomics

Cyst formation in the parasitic protist Giardia duodenalis is critical to its transmission. Existing proteomic data quantifies only 17% of coding genes transcribed during encystation and does not cover the complete process from trophozoite to mature cyst. Using high-resolution mass spectrometry, we have quantified proteomic changes across encystation and compared this with published transcriptomic data. We reproducibly identified 3863 (64.5% of Giardia proteins) and quantified 3382 proteins (56.5% of Giardia proteins) over standard trophozoite growth (TY), during low-bile encystation priming (LB), 16 h into encystation (EC), and at cyst maturation (C). This work provides the first known expanded observation of encystation at the proteomic level and triples the coverage of previous encystation proteomes. One-third (1169 proteins) of the quantified proteome is differentially expressed in the mature cyst relative to the trophozoite, including proteasomal machinery, metabolic pathways, and secretory proteins. Changes in lipid metabolism indicated a shift in lipid species dependency during encystation. Consistent with this, we identified the first, putative lipid transporters in this species, representing the steroidogenic acute regulatory protein-related lipid transfer (StARkin), oxysterol binding protein related protein (ORP/Osh) and glycosphingolipid transfer protein (GLTP) families, and follow their differential expression over cyst formation. Lastly, we undertook correlation analyses of the transcriptome and proteome of trophozoites and cysts, and found evidence of post-transcriptional regulation of key protein classes (RNA binding proteins) and stage-specific genes (encystation markers) implicating translation-repression in encystation. We provide the most extensive proteomic analysis of encystation in Giardia to date and the first known exploration across its complete duration. This work identifies encystation as highly coordinated, involving major changes in proteostasis, metabolism and membrane dynamics, and indicates a potential role for post-transcriptional regulation, mediated through RNA-binding proteins. Together our work provides a valuable resource for Giardia research and the development of transmission-blocking anti-giardials.

Development of a Multilocus Sequence Typing Scheme for Giardia intestinalis

Giardia intestinalis is an intestinal protozoan most commonly found in humans. It has been grouped into 8 assemblages (A-H). Markers such as the glutamate dehydrogenase gene, triose phosphate isomerase and beta-giardin (β-giardin) have been widely used for genotyping. In addition, different genetic targets have been proposed as a valuable alternative to assess diversity and genetics of this microorganism. Thus, our objective was to evaluate new markers for the study of the diversity and intra-taxa genetic structure of G. intestinalis in silico and in DNA obtained from stool samples. We analysed nine constitutive genes in 80 complete genome sequences and in a group of 24 stool samples from Colombia. Allelic diversity was evaluated by locus and for the concatenated sequence of nine loci that could discriminate up to 53 alleles. Phylogenetic reconstructions allowed us to identify AI, AII and B assemblages. We found evidence of intra- and inter-assemblage recombination events. Population structure analysis showed genetic differentiation among the assemblages analysed.

Triazoxins: Novel nucleosides with anti-Giardia activity

Novel nucleoside analogues named "triazoxins" were synthesized. Of these, two analogues were found to be highly effective against Giardia lamblia, an intestinal parasite and a major cause of waterborne infection, worldwide. While compound 7 reduced the growth of trophozoites in culture (IC50, ~5 μM), compound 21 blocked the in vitro cyst production (IC50 ~5 μM). Compound 21 was also effective against trophozoites (IC50, ~36 μM). A third analogue (compound 8) was effective against both trophozoites (IC50, ~36 μM) and cysts (IC50, ~20 μM) although at higher concentration. Thus triazoxin analogues are unique and exhibit morphology (i.e., trohozoites or cysts) -specific effects against Giardia.

Exosome Biogenesis in the Protozoa Parasite Giardia lamblia: A Model of Reduced Interorganellar Crosstalk

: Extracellular vesicles (EVs) facilitate intercellular communication and are considered a promising therapeutic tool for the treatment of infectious diseases. These vesicles involve microvesicles (MVs) and exosomes and selectively transfer proteins, lipids, mRNAs, and microRNAs from one cell to another. While MVs are formed by extrusion of the plasma membrane, exosomes are a population of vesicles of endosomal origin that are stored inside the multivesicular bodies (MVBs) as intraluminal vesicles (ILVs) and are released when the MVBs fuse with the plasma membrane. Biogenesis of exosomes may be driven by the endosomal sorting complex required for transport (ESCRT) machinery or may be ESCRT independent, and it is still debated whether these are entirely separate pathways. In this manuscript, we report that the protozoan parasite, Giardia lamblia, although lacking a classical endo-lysosomal pathway, is able to produce and release exosome-like vesicles (ElV). By using a combination of biochemical and cell biology analyses, we found that the ElVs have the same size, shape, and protein and lipid composition as exosomes described for other eukaryotic cells. Moreover, we established that some endosome/lysosome peripheral vacuoles (PVs) contain ILV during the stationary phase. Our results indicate that ILV formation and ElV release depend on the ESCRT-associated AAA+-ATPase Vps4a, Rab11, and ceramide in this parasite. Interestingly, EIV biogenesis and release seems to occur in Giardia despite the fact that this parasite has lost most of the ESCRT machinery components during evolution and is unable to produce ceramide de novo. The differences in protozoa parasite EV composition, origin, and release may reveal functional and structural properties of EVs and, thus, may provide information on cell-to-cell communication and on survival mechanisms.

A Targeted Mass Spectrometric Analysis Reveals the Presence of a Reduced but Dynamic Sphingolipid Metabolic Pathway in an Ancient Protozoan, Giardia lamblia

Giardia lamblia, a single-celled eukaryote, colonizes and thrives in the small intestine of humans. Because of its compact and reduced genome, Giardia has adapted a “minimalistic” life style, as it becomes dependent on available resources of the small intestine. Because Giardia expresses fewer sphingolipid (SL) genes—and glycosphingolipids are critical for encystation—we investigated the SL metabolic cycle in this parasite. A tandem mass spectrometry (MS/MS) analysis reveals that major SLs in Giardia include sphingomyelins, sphingoid bases, ceramides, and glycosylceramides. Many of these lipids are obtained by Giardia from the growth medium, remodeled at their fatty acyl chains and end up in the spent medium. For instance, ceramide-1-phosphate, a proinflammatory molecule that is not present in the culture medium, is generated from sphingosine (abundant in the culture medium) possibly by remodeling reactions. It is then subsequently released into the spent medium. Thus, the secretion of ceramide-1-phospate and other SL derivatives by Giardia could be associated with inflammatory bowel disease observed in acute giardiasis. Additionally, we found that the levels of SLs increase in encysting Giardia and are differentially regulated throughout the encystation cycle. We propose that SL metabolism is important for this parasite and, could serve as potential targets for developing novel anti-giardial agents.

De novo synthesis of sphingolipids plays an important role during in vitro encystment of Entamoeba invadens

Entamoeba invadens is a protozoan, which causes multiple damages in reptiles and is considered a prototype for the study of the Entamoeba encystment in vitro. Here we report for the first time the role of the de novo synthesis pathway of sphingolipids during the encystment of E. invadens. In silico analysis showed that this parasite has six putative genes coding for ceramide synthases (CerS), all of them coding for proteins containing the Lag1p motif, a region conserved in the ceramide synthases of multiple organisms, suggesting that they might be bona fide CerS. The six genes of E. invadens are differentially expressed at different time intervals in both stages trophozoite and cyst, based on the results obtained through qRT-PCR assays, the genes involved in the synthesis of sphingolipids with long-chain fatty acids CerS 2,3,4 (EIN_046610, EIN_097030, EIN_130350) have maximum points of relative expression in both stages of the E. invadens life cycle, which strongly suggest that the signaling exerted from the synthesis pathway of sphingolipids is essential for the encystment of E. invadens, since the generation of the more abundant sphingomyelin (SM) subspecies with long-chain fatty acids are fundamental for the parasite to reach its conversion from trophozoite to cyst. When myriocin was used as an inhibitor of serine palmitoyl CoA transferase (SPT), first enzyme in the de novo biosynthesis of sphingolipids, the trophozoites of E. invadens were unable to reach the encystment. Since the effect of myriocin was reversed with exogenous d-erythrosphingosine (DHS), it was demonstrated that the inhibition was specific and it was confirmed that the synthesis of sphingolipids play an essential role during the encystment process of E. invadens.

Contribution of neutral sphingomyelinases to in vitro virulence of Entamoeba histolytica

Amoebiasis is a worldwide health problem caused by the pathogen Entamoeba histolytica. Several virulence factors have been implicated in host invasion, immune evasion, and tissue damage. There are still new factors that remain to be elucidated and characterized. In this work, we obtained amoebic transfectants overexpressing three of the neutral sphingomyelinase enzymes encoded in the E. histolytica genome. The EhnSM3 overexpression induced an increase in hemolytic and cytotoxic activities, besides an increase in gene expression of amoebapore A, B, and C. Meanwhile the EhnSM1 and EhnSM2 overexpression caused an increase in cytopathic activity. In all the neutral sphingomyelinases overexpressing strains, the gene expression levels for cysteine proteinase 5, adhesin 112 and, heavy and light Gal/GalNAc lectin subunits were not affected. We propose that the increase of cytotoxic and lytic effect of EhnSM3 overexpressed strain can be related to the sum of the effect of EhnSM3 plus amoebapores, in a process cell contact-dependent or as mediator by inducing the gene expression of amoebapores enabling a link between EhnSM3 with the virulence phenotype in E. histolytica. Our results suggest a differential role for neutral sphingomyelinases in E. histolytica virulence.

Glucosylceramide transferase in Giardia preferentially catalyzes the synthesis of galactosylceramide during encystation

The stage differentiation from trophozoite to cyst (i.e., encystation) is an essential step for Giardia to survive outside its human host and spread the infection via the fecal-oral route. We have previously shown that Giardia expresses glucosylceramide transferase 1 (GlcT1) enzyme, the activity of which is elevated during encystation. We have also reported that blocking the activity of gGlcT1 interferes with the biogenesis of encystation-specific vesicles (ESVs) and cyst viability in Giardia. To further understand the role of this enzyme and how it regulates encystation, we overexpressed, knocked down, and rescued the giardial GlcT1 (gGlcT1) gene and measured its enzymatic activity in live parasites as well as in isolated membrane fractions using NBD-ceramide and UDP-glucose or UDP-galactose. We observed that gGlcT1 is able to catalyze the synthesis of both glucosylceramide (GlcCer) and galactosylceramide (GalCer), however the synthesis of GalCer is 2-3 fold higher than of GlcCer. Although both activities follow Michaelis-Menten kinetics, the bindings of UDP-glucose and UDP-galactose with the enzyme appear to be non-competitive and independent of each other. The modulation of gGlcT1 synthesis concomitantly influenced the expression cyst-wall protein (CWP) and overall encystation. We propose that gGlcT1 is a unique enzyme and that Giardia uses this enzyme to synthesize both GlcCer and GalCer to facilitate the process of encystation/cyst production.

Sphingolipids, Lipid Rafts, and Giardial Encystation: The Show Must Go On

Sphingolipids are sphingosine-based phospholipids, which are present in the plasma and endomembranes of many eukaryotic cells. These lipids are involved in various cellular functions, including cell growth, differentiation, and apoptosis. In addition, sphingolipid and cholesterol-enriched membrane microdomains (also called "lipid rafts") contain a set of proteins and lipids, which take part in the signaling process in response to intra- or extracellular stimuli. Recent findings suggest that sphingolipids, especially glucosylceramide, play a critical role in inducing encystation and maintaining the cyst viability in Giardia. Similarly, the assembly/disassembly of lipid rafts modulates the encystation and cyst production of this ubiquitous enteric parasite. In this review article, we discuss the overall progress in the field and examine whether sphingolipids and lipid rafts can be used as novel targets for designing therapies to control infection by Giardia, which is rampant in developing countries, where children are especially vulnerable.

Host-lipidome as a potential target of protozoan parasites

Host-lipidome caters parasite interaction by acting as first line of recognition, attachment on the cell surface, intracellular trafficking, and survival of the parasite inside the host cell. Here, we summarize how protozoan parasites exploit host-lipidome by suppressing, augmenting, engulfing, remodeling and metabolizing lipids to achieve successful parasitism inside the host.

Glucosylceramide transferase activity is critical for encystation and viable cyst production by an intestinal protozoan, Giardia lamblia

The production of viable cysts by Giardia is essential for its survival in the environment and for spreading the infection via contaminated food and water. The hallmark of cyst production (also known as encystation) is the biogenesis of encystation-specific vesicles (ESVs) that transport cyst wall proteins to the plasma membrane of the trophozoite before laying down the protective cyst wall. However, the molecules that regulate ESV biogenesis and maintain cyst viability have never before been identified. Here, we report that giardial glucosylceramide transferase-1 (gGlcT1), an enzyme of sphingolipid biosynthesis, plays a key role in ESV biogenesis and maintaining cyst viability. We find that overexpression of this enzyme induced the formation of aggregated/enlarged ESVs and generated clustered cysts with reduced viability. The silencing of gGlcT1 synthesis by antisense morpholino oligonucleotide abolished ESV production and generated mostly nonviable cysts. Interestingly, when gGlcT1-overexpressed Giardia was transfected with anti-gGlcT1 morpholino, the enzyme activity, vesicle biogenesis, and cyst viability returned to normal, suggesting that the regulated expression of gGlcT1 is important for encystation and viable cyst production. Furthermore, the overexpression of gGlcT1 increased the influx of membrane lipids and fatty acids without altering the fluidity of plasma membranes, indicating that the expression of gGlcT1 activity is linked to lipid internalization and maintaining the overall lipid balance in this parasite. Taken together, our results suggest that gGlcT1 is a key player of ESV biogenesis and cyst viability and therefore could be targeted for developing new anti-giardial therapies.

Lipid metabolism in Giardia: a post-genomic perspective

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

Glucosylceramide synthesis inhibition affects cell cycle progression, membrane trafficking, and stage differentiation in Giardia lamblia

Synthesis of glucosylceramide via glucosylceramide synthase (GCS) is a crucial event in higher eukaryotes, both for the production of complex glycosphingolipids and for regulating cellular levels of ceramide, a potent antiproliferative second messenger. In this study, we explored the dependence of the early branching eukaryote Giardia lamblia on GCS activity. Biochemical analyses revealed that the parasite has a GCS located in endoplasmic reticulum (ER) membranes that is active in proliferating and encysting trophozoites. Pharmacological inhibition of GCS induced aberrant cell division, characterized by arrest of cytokinesis, incomplete cleavage furrow formation, and consequent block of replication. Importantly, we showed that increased ceramide levels were responsible for the cytokinesis arrest. In addition, GCS inhibition resulted in prominent ultrastructural abnormalities, including accumulation of cytosolic vesicles, enlarged lysosomes, and clathrin disorganization. Moreover, anterograde trafficking of the encystations-specific protein CWP1 was severely compromised and resulted in inhibition of stage differentiation. Our results reveal novel aspects of lipid metabolism in G. lamblia and specifically highlight the vital role of GCS in regulating cell cycle progression, membrane trafficking events, and stage differentiation in this parasite. In addition, we identified ceramide as a potent bioactive molecule, underscoring the universal conservation of ceramide signaling in eukaryotes.

The role of ATP-binding cassette (ABC) proteins in protozoan parasites

The ATP-binding cassette (ABC) superfamily is one of the largest protein families with representatives in all kingdoms of life. Members of this superfamily are involved in a wide variety of transport processes with substrates ranging from small ions to relatively large polypeptides and polysaccharides, but also in cellular processes such as DNA repair, translation or regulation of gene expression. For many years, the role of ABC proteins was mainly investigated for their implication in drug resistance. However, recent studies focused rather on their physiological functions for the parasite. In this review, we present an overview of ABC proteins in major protozoan parasites including Leishmania, Trypanosoma, Plasmodium, Toxoplasma, Cryptosporidium and Entamoeba species. We will also discuss the role of characterized ABC transporters in the biology of the parasite and in drug resistance.

Lipidomic analysis reveals that phosphatidylglycerol and phosphatidylethanolamine are newly generated phospholipids in an early-divergent protozoan, Giardia lamblia

The pathogenic protozoan Giardia lamblia is known to not synthesize membrane lipids de novo. Therefore, it is possible that lipids in the small intestine, where trophozoites colonize, play key roles in regulating the growth and differentiation of this important pathogen. The focus of the current study is to conduct a complete lipidomic analysis and to test the hypothesis that Giardia has some ability to generate new phospholipids (PLs). Using mass spectrometry, now we show that phosphatidylglycerols (PGs) are major PLs followed by phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs) in non-encysting and encysting trophozoites, as well in cysts. The fatty acids attached to these PLs consist mostly of palmitate, palmitoleate, oleate, and linoleate. Results also indicate that PGs and PEs, unlike PCs, are not present in bovine bile and serum, the major sources of lipids of the culture medium, and that they could therefore be produced by fatty acid and headgroup remodeling reactions, circumventing the synthesis of entirely new PLs via de novo pathways. Genomic and transcriptional analyses show the presence of giardial phosphatidylglycerolphosphate synthase (gpgps) and phosphatidylserine decarboxylase (gpsd) genes, which are expressed throughout the life cycle. Bioinformatic and phylogenetic analyses further indicated that both genes are of prokaryotic origin and that they have undergone duplication in the course of evolution. Our studies suggest that the abundance of PG in Giardia is unique among eukaryotes and that its synthesis thus could serve as a potential target for developing new therapies against this waterborne parasite.