Characterization of SNAREs determines the absence of a typical Golgi apparatus in the ancient eukaryote Giardia lamblia

A core UPS molecular complement implicates unique endocytic compartments at the parasite-host interface in Giardia lamblia

Unconventional protein secretion (UPS) plays important roles in cell physiology. In contrast to canonical secretory routes, UPS does not generally require secretory signal sequences and often bypasses secretory compartments such as the ER and the Golgi apparatus. Giardia lamblia is a protist parasite with reduced subcellular complexity which releases several proteins, some of them virulence factors, without canonical secretory signals. This implicates UPS at the parasite-host interface. No dedicated machinery nor mechanism(s) for UPS in Giardia are currently known, although speculations on the involvement of endocytic organelles called PV/PECs, have been put forth. To begin to address the question of whether PV/PECs are implicated in virulence-associated UPS and to define the composition of molecular machinery involved in protein release, we employed affinity purification and mass spectrometry, coupled to microscopy-based subcellular localization and signal correlation quantification to investigate the interactomes of 11 reported unconventionally secreted proteins, all predicted to be cytosolic. A subset of these are associated with PV/PECs. Extended and validated interactomes point to a core PV/PECs-associated UPS machinery, which includes uncharacterized and Giardia-specific coiled-coil proteins and NEK kinases. Finally, a subset of the alpha-giardin protein family was enriched in all PV/PECs-associated protein interactomes, highlighting a previously unappreciated role for these proteins at PV/PECs and in UPS. Taken together, our results provide the first characterization of a virulence-associated UPS protein complex in Giardia lamblia at PV/PECs, suggesting a novel link between these primarily endocytic and feeding organelles and UPS at the parasite-host interface.

Phosphoinositide-binding proteins mark, shape and functionally modulate highly-diverged endocytic compartments in the parasitic protist Giardia lamblia

Phosphorylated derivatives of phosphatidylinositol (PIPs) are key membrane lipid residues involved in clathrin-mediated endocytosis (CME). CME relies on PIP species PI(4,5)P₂ to mark endocytic sites at the plasma membrane (PM) associated to clathrin-coated vesicle (CCV) formation. The highly diverged parasitic protist Giardia lamblia presents disordered and static clathrin assemblies at PM invaginations, contacting specialized endocytic organelles called peripheral vacuoles (PVs). The role for clathrin assemblies in fluid phase uptake and their link to internal membranes via PIP-binding adaptors is unknown. Here we provide evidence for a robust link between clathrin assemblies and fluid-phase uptake in G. lamblia mediated by proteins carrying predicted PX, FYVE and NECAP1 PIP-binding modules. We show that chemical and genetic perturbation of PIP-residue binding and turnover elicits novel uptake and organelle-morphology phenotypes. A combination of co-immunoprecipitation and in silico analysis techniques expands the initial PIP-binding network with addition of new members. Our data indicate that, despite the partial conservation of lipid markers and protein cohorts known to play important roles in dynamic endocytic events in well-characterized model systems, the Giardia lineage presents a strikingly divergent clathrin-centered network. This includes several PIP-binding modules, often associated to domains of currently unknown function that shape and modulate fluid-phase uptake at PVs.

In well-characterized model eukaryotes, clathrin-mediated endocytosis is a key process for uptake of extracellular material and is regulated by more than 50 known proteins. A large number of these carry phosphoinositide (PIP)-binding domains and play a central role in the regulation of endocytosis. Here, we report on the detailed functional characterization of PIP-binding proteins in the intestinal parasitic protist Giardia lamblia. We show evidence that proteins carrying specific PIP-binding domains are directly involved in fluid-phase uptake. Furthermore, using co-immunoprecipitation assays, we confirm these proteins’ association to G. lamblia’s clathrin assemblies. In addition, using state-of-the-art imaging strategies, we demonstrate a previously unappreciated level of complexity involving PIPs and their partner proteins in marking and shaping G. lamblia’s unique endocytic compartments. Our data contribute substantially to an updated working model for G. lamblia’s host-parasite interface, demonstrating how uptake in this parasite is directly regulated by a variety of PIP residues and PIP-binding modules, which have been re-routed from conserved pathways, likely as a result of host-parasite co-evolution.

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 cytonaut's guide to protein trafficking in Giardia lamblia

Over the past years, the subcellular organization of the Excavata member Giardia lamblia (syn. duodenalis, intestinalis) has been investigated in considerable detail. There are several reasons for this endeavour which go beyond this parasite's medical importance and are mostly concerned with its reduced subcellular complexity and debated evolutionary status. One may say that simplification has emerged as a paradigm for the evolution of Giardia's subcellular architecture. However, a complete appreciation of the evolutionary and ecological significance of this phenomenon is far from complete. In this chapter, we present and discuss the most recent data on the main trafficking pathways in G. lamblia which include endo- and exo-cytosis, organellar import and function. We provide perspectives on open questions concerning organelle replication and inheritance and include a technical outlook on methods and approaches to genetic manipulations in G. lamblia. A better understanding of G. lamblia subcellular organization at the morphological and molecular level complements any effort aimed at elucidating this parasitic species' evolutionary status and could provide us with the basis for novel strategies to interfere with parasite transmission and/or pathogenesis.

Multiple paralogues of α-SNAP in Giardia lamblia exhibit independent subcellular localization and redistribution during encystation and stress

BACKGROUND

The differently-diverged parasitic protist Giardia lamblia is known to have minimal machinery for vesicular transport. Yet, it has three paralogues of SNAP, a crucial component that together with NSF brings about disassembly of the cis-SNARE complex formed following vesicle fusion to target membranes. Given that most opisthokont hosts of this gut parasite express only one α-SNAP, this study was undertaken to determine whether these giardial SNAP proteins have undergone functional divergence.

RESULTS

All three SNAP paralogues are expressed in trophozoites, encysting trophozoites and cysts. Even though one of them clusters with γ-SNAP sequences in a phylogenetic tree, functional complementation analysis in yeast indicates that all the three proteins are functionally orthologous to α-SNAP. Localization studies showed a mostly non-overlapping distribution of these α-SNAPs in trophozoites, encysting cells and cysts. In addition, two of the paralogues exhibit substantial subcellular redistribution during encystation, which was also seen following exposure to oxidative stress. However, the expression of the three genes remained unchanged during this redistribution process. There is also a difference in the affinity of each of these α-SNAP paralogues for GlNSF.

CONCLUSIONS

None of the genes encoding the three α-SNAPs are pseudogenes and the encoded proteins are likely to discharge non-redundant functions in the different morphological states of G. lamblia. Based on the difference in the interaction of individual α-SNAPs with GlNSF and their non-overlapping pattern of subcellular redistribution during encystation and under stress conditions, it may be concluded that the three giardial α-SNAP paralogues have undergone functional divergence. Presence of one of the giardial α-SNAPs at the PDRs of flagella, where neither GlNSF nor any of the SNAREs localize, indicates that this α-SNAP discharges a SNARE-independent role in this gut pathogen.

Membrane-Associated Proteins in Giardia lamblia

The manner in which membrane-associated proteins interact with the membrane defines their subcellular fate and function. This interaction relies on the characteristics of the proteins, their journey after synthesis, and their interaction with other proteins or enzymes. Understanding these properties may help to define the function of a protein and also the role of an organelle. In the case of microorganisms like protozoa parasites, it may help to understand singular features that will eventually lead to the design of parasite-specific drugs. The protozoa parasite Giardia lamblia is an example of a widespread parasite that has been infecting humans and animals from ancestral times, adjusting itself to the changes of the environment inside and outside the host. Several membrane-associated proteins have been posted in the genome database GiardiaDB, although only a few of them have been characterized. This review discusses the data regarding membrane-associated proteins in relationship with lipids and specific organelles and their implication in the discovery of anti-giardial therapies.

The single cyclic nucleotide-specific phosphodiesterase of the intestinal parasite Giardia lamblia represents a potential drug target

Background

Giardiasis is an intestinal infection correlated with poverty and poor drinking water quality, and treatment options are limited. According to the Center for Disease Control and Prevention, Giardia infections afflict nearly 33% of people in developing countries, and 2% of the adult population in the developed world. This study describes the single cyclic nucleotide-specific phosphodiesterase (PDE) of G. lamblia and assesses PDE inhibitors as a new generation of anti-giardial drugs.

Methods

An extensive search of the Giardia genome database identified a single gene coding for a class I PDE, GlPDE. The predicted protein sequence was analyzed in-silico to characterize its domain structure and catalytic domain. Enzymatic activity of GlPDE was established by complementation of a PDE-deficient Saccharomyces cerevisiae strain, and enzyme kinetics were characterized in soluble yeast lysates. The potency of known PDE inhibitors was tested against the activity of recombinant GlPDE expressed in yeast and against proliferating Giardia trophozoites. Finally, the localization of epitope-tagged and ectopically expressed GlPDE in Giardia cells was investigated.

Results

Giardia encodes a class I PDE. Catalytically important residues are fully conserved between GlPDE and human PDEs, but sequence differences between their catalytic domains suggest that designing Giardia-specific inhibitors is feasible. Recombinant GlPDE hydrolyzes cAMP with a K_m of 408 μM, and cGMP is not accepted as a substrate. A number of drugs exhibit a high degree of correlation between their potency against the recombinant enzyme and their inhibition of trophozoite proliferation in culture. Epitope-tagged GlPDE localizes as dots in a pattern reminiscent of mitosomes and to the perinuclear region in Giardia.

Conclusions

Our data strongly suggest that inhibition of G. lamblia PDE activity leads to a profound inhibition of parasite proliferation and that GlPDE is a promising target for developing novel anti-giardial drugs.

Cellular signaling by the cyclic nucleotides cAMP and cGMP is ubiquitously found in organisms from human to unicellular parasites. Cyclic nucleotide-specific phosphodiesterases (PDEs) are pivotal regulators of these signaling processes and these enzymes represent important drug targets for a variety of diseases. Eleven PDE families are distinguished in humans and selective inhibition of a single human PDE family without targeting others is feasible. In parasites, interference in the signaling mechanism by PDE inhibition may be fatal. The diarrhea-causing parasite Giardia lamblia contains only one single PDE, named GlPDE. GlPDE activity is highly impaired by a range of PDE inhibitors, which also suppress parasite proliferation in vitro. Thus, there is a good agreement between PDE inhibition and parasite drug susceptibility. We demonstrate molecular differences between human PDEs and GlPDE that can be exploited for the development of GlPDE-selective inhibitors. Finally, our data may suggest localization of GlPDE to mitosome organelles, which are absent in human cells and thus are in the focus as possible targets for the treatment of giardiasis. This may add to the notion that GlPDE represents a potential target for the development of novel anti-giardial drugs.

Giardia intestinalis mitosomes undergo synchronized fission but not fusion and are constitutively associated with the endoplasmic reticulum

BACKGROUND

Mitochondria of opisthokonts undergo permanent fission and fusion throughout the cell cycle. Here, we investigated the dynamics of the mitosomes, the simplest forms of mitochondria, in the anaerobic protist parasite Giardia intestinalis, a member of the Excavata supergroup of eukaryotes. The mitosomes have abandoned typical mitochondrial traits such as the mitochondrial genome and aerobic respiration and their single role known to date is the formation of iron-sulfur clusters.

RESULTS

In live experiments, no fusion events were observed between the mitosomes in G. intestinalis. Moreover, the organelles were highly prone to becoming heterogeneous. This suggests that fusion is either much less frequent or even absent in mitosome dynamics. Unlike in mitochondria, division of the mitosomes was absolutely synchronized and limited to mitosis. The association of the nuclear and the mitosomal division persisted during the encystation of the parasite. During the segregation of the divided mitosomes, the subset of the organelles between two G. intestinalis nuclei had a prominent role. Surprisingly, the sole dynamin-related protein of the parasite seemed not to be involved in mitosomal division. However, throughout the cell cycle, mitosomes associated with the endoplasmic reticulum (ER), although none of the known ER-tethering complexes was present. Instead, the ER-mitosome interface was occupied by the lipid metabolism enzyme long-chain acyl-CoA synthetase 4.

CONCLUSIONS

This study provides the first report on the dynamics of mitosomes. We show that together with the loss of metabolic complexity of mitochondria, mitosomes of G. intestinalis have uniquely streamlined their dynamics by harmonizing their division with mitosis. We propose that this might be a strategy of G. intestinalis to maintain a stable number of organelles during cell propagation. The lack of mitosomal fusion may also be related to the secondary reduction of the organelles. However, as there are currently no reports on mitochondrial fusion in the whole Excavata supergroup, it is possible that the absence of mitochondrial fusion is an ancestral trait common to all excavates.

Cyst-Wall-Protein-1 is fundamental for Golgi-like organelle neogenesis and cyst-wall biosynthesis in Giardia lamblia

The genome of the protozoan parasite Giardia lamblia is organized in two diploid nuclei, which has so far precluded complete analysis of gene function. Here we use a previously developed Cre/loxP-based knock-out and selection marker salvage strategy in the human-derived isolate WB-C6 to eliminate all four copies of the Cyst-Wall-Protein-1 locus (CWP1). Because these loci are silenced in proliferating trophozoites and highly expressed only in encysting cells, CWP1 ablation allows functional characterization of a conditional phenotype in parasites induced to encyst. We show that encysting Δcwp1 cells are unable to establish the stage-regulated trafficking machinery with Golgi-like encystation-specific vesicles required for cyst-wall formation but show morphological hallmarks of cyst development and karyokinesis. This ‘pseudocyst' phenotype is rescued by transfection of Δcwp1 cells with an episomally maintained CWP1 expression vector. Genome editing in genera Giardia and Trypanosoma are the only reported examples addressing questions on pathogen transmission within the Excavata supergroup.

Giardia lamblia is a human protozoan parasite with two diploid nuclei, which makes complete knock-out of a gene of interest challenging. Here the authors use a Cre/loxP-based approach to knock-out cyst-wall protein 1 (cwp1) and show that CWP1 is essential for cyst-wall biosynthesis.

An Interactome-Centered Protein Discovery Approach Reveals Novel Components Involved in Mitosome Function and Homeostasis in Giardia lamblia

Protozoan parasites of the genus Giardia are highly prevalent globally, and infect a wide range of vertebrate hosts including humans, with proliferation and pathology restricted to the small intestine. This narrow ecological specialization entailed extensive structural and functional adaptations during host-parasite co-evolution. An example is the streamlined mitosomal proteome with iron-sulphur protein maturation as the only biochemical pathway clearly associated with this organelle. Here, we applied techniques in microscopy and protein biochemistry to investigate the mitosomal membrane proteome in association to mitosome homeostasis. Live cell imaging revealed a highly immobilized array of 30–40 physically distinct mitosome organelles in trophozoites. We provide direct evidence for the single giardial dynamin-related protein as a contributor to mitosomal morphogenesis and homeostasis. To overcome inherent limitations that have hitherto severely hampered the characterization of these unique organelles we applied a novel interaction-based proteome discovery strategy using forward and reverse protein co-immunoprecipitation. This allowed generation of organelle proteome data strictly in a protein-protein interaction context. We built an initial Tom40-centered outer membrane interactome by co-immunoprecipitation experiments, identifying small GTPases, factors with dual mitosome and endoplasmic reticulum (ER) distribution, as well as novel matrix proteins. Through iterative expansion of this protein-protein interaction network, we were able to i) significantly extend this interaction-based mitosomal proteome to include other membrane-associated proteins with possible roles in mitosome morphogenesis and connection to other subcellular compartments, and ii) identify novel matrix proteins which may shed light on mitosome-associated metabolic functions other than Fe-S cluster biogenesis. Functional analysis also revealed conceptual conservation of protein translocation despite the massive divergence and reduction of protein import machinery in Giardia mitosomes.

Organelles with endosymbiotic origin are present in virtually all extant eukaryotes and have undergone considerable remodeling during > 1 billion years of evolution. Highly diverged organelles such as mitosomes or plastids in some parasitic protozoa are the product of extensive secondary reduction. They are sufficiently unique to generate interest as targets for pharmacological intervention, in addition to providing a rich ground for evolutionary cell biologists. The so-called mitochondria-related organelles (MROs) comprise mitosomes and hydrogenosomes, with the former having lost any role in energy metabolism along with the organelle genome. The mitosomes of the intestinal pathogen Giardia lamblia are the most highly reduced MROs known and have proven difficult to investigate because of their extreme divergence and their unique biophysical properties. Here, we implemented a novel strategy aimed at systematic analysis of the organelle proteome by iterative expansion of a protein-protein interaction network. We combined serial forward and reverse co-immunoprecipitations with mass spectrometry analysis, data mining, and validation by subcellular localization and/or functional analysis to generate an interactome network centered on a giardial Tom40 homolog. This iterative ab initio proteome reconstruction provided protein-protein interaction data in addition to identifying novel organelle proteins and functions. Building on this data we generated information on organelle replication, mitosome morphogenesis and organelle dynamics in living cells.

Probing the Biology of Giardia intestinalis Mitosomes Using In Vivo Enzymatic Tagging

Giardia intestinalis parasites contain mitosomes, one of the simplest mitochondrion-related organelles. Strategies to identify the functions of mitosomes have been limited mainly to homology detection, which is not suitable for identifying species-specific proteins and their functions. An in vivo enzymatic tagging technique based on the Escherichia coli biotin ligase (BirA) has been introduced to G. intestinalis; this method allows for the compartment-specific biotinylation of a protein of interest. Known proteins involved in the mitosomal protein import were in vivo tagged, cross-linked, and used to copurify complexes from the outer and inner mitosomal membranes in a single step. New proteins were then identified by mass spectrometry. This approach enabled the identification of highly diverged mitosomal Tim44 (GiTim44), the first known component of the mitosomal inner membrane translocase (TIM). In addition, our subsequent bioinformatics searches returned novel diverged Tim44 paralogs, which mediate the translation and mitosomal insertion of mitochondrially encoded proteins in other eukaryotes. However, most of the identified proteins are specific to G. intestinalis and even absent from the related diplomonad parasite Spironucleus salmonicida, thus reflecting the unique character of the mitosomal metabolism. The in vivo enzymatic tagging also showed that proteins enter the mitosome posttranslationally in an unfolded state and without vesicular transport.

Proteomics of secretory and endocytic organelles in Giardia lamblia

Giardia lamblia is a flagellated protozoan enteroparasite transmitted as an environmentally resistant cyst. Trophozoites attach to the small intestine of vertebrate hosts and proliferate by binary fission. They access nutrients directly via uptake of bulk fluid phase material into specialized endocytic organelles termed peripheral vesicles (PVs), mainly on the exposed dorsal side. When trophozoites reach the G2/M restriction point in the cell cycle they can begin another round of cell division or encyst if they encounter specific environmental cues. They induce neogenesis of Golgi-like organelles, encystation-specific vesicles (ESVs), for regulated secretion of cyst wall material. PVs and ESVs are highly simplified and thus evolutionary diverged endocytic and exocytic organelle systems with key roles in proliferation and transmission to a new host, respectively. Both organelle systems physically and functionally intersect at the endoplasmic reticulum (ER) which has catabolic as well as anabolic functions. However, the unusually high degree of sequence divergence in Giardia rapidly exhausts phylogenomic strategies to identify and characterize the molecular underpinnings of these streamlined organelles. To define the first proteome of ESVs and PVs we used a novel strategy combining flow cytometry-based organelle sorting with in silico filtration of mass spectrometry data. From the limited size datasets we retrieved many hypothetical but also known organelle-specific factors. In contrast to PVs, ESVs appear to maintain a strong physical and functional link to the ER including recruitment of ribosomes to organelle membranes. Overall the data provide further evidence for the formation of a cyst extracellular matrix with minimal complexity. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium with the dataset identifier PXD000694.

The ubiquitin-activating enzyme (E1) of the early-branching eukaryote Giardia intestinalis shows unusual proteolytic modifications and play important roles during encystation

Giardia intestinalis is considered an early-branching eukaryote and is therefore a valuable model for studying primordial cellular processes. This work reports the characterization of the ubiquitin-activating enzyme (E1) during growth and different stages of trophozoite differentiation into cysts. We found that in Giardia E1 expression (both at mRNA and protein levels) is regulated during encystation. The enzyme is proteolytically processed mainly into two fragments of 68kDa (N-terminal) and 47kDa (C-terminal). This phenomenon has not been described for any other E1. In trophozoites, this enzyme localized at spots within the cytoplasm as detected by using polyclonal antibodies against either E1 N- or C-terminal fragments. This pattern changed during encystation into a diffuse localization throughout the cytoplasm of encysting cells. E1 localizes in mature cysts at cytoplasmic spots and in the cyst wall. Our antisense silencing experiments suggested that E1 is an essential gene for parasite viability. On the other hand, E1 over-expression greatly increased the encystation rate, indicating a relationship between E1 and Giardia differentiation.

Live imaging of mitosomes and hydrogenosomes by HaloTag technology

Hydrogenosomes and mitosomes represent remarkable mitochondrial adaptations in the anaerobic parasitic protists such as Trichomonas vaginalis and Giardia intestinalis, respectively. In order to provide a tool to study these organelles in the live cells, the HaloTag was fused to G. intestinalis IscU and T. vaginalis frataxin and expressed in the mitosomes and hydrogenosomes, respectively. The incubation of the parasites with the fluorescent Halo-ligand resulted in highly specific organellar labeling, allowing live imaging of the organelles. With the array of available ligands the HaloTag technology offers a new tool to study the dynamics of mitochondria-related compartments as well as other cellular components in these intriguing unicellular eukaryotes.

Sculpting the endomembrane system in deep time: high resolution phylogenetics of Rab GTPases

The presence of a nucleus and other membrane-bounded intracellular compartments is the defining feature of eukaryotic cells. Endosymbiosis accounts for the origins of mitochondria and plastids, but the evolutionary ancestry of the remaining cellular compartments is incompletely documented. Resolving the evolutionary history of organelle-identity encoding proteins within the endomembrane system is a necessity for unravelling the origins and diversification of the endogenously derived organelles. Comparative genomics reveals events after the last eukaryotic common ancestor (LECA), but resolution of events prior to LECA, and a full account of the intracellular compartments present in LECA, has proved elusive. We have devised and exploited a new phylogenetic strategy to reconstruct the history of the Rab GTPases, a key family of endomembrane-specificity proteins. Strikingly, we infer a remarkably sophisticated organellar composition for LECA, which we predict possessed as many as 23 Rab GTPases. This repertoire is significantly greater than that present in many modern organisms and unexpectedly indicates a major role for secondary loss in the evolutionary diversification of the endomembrane system. We have identified two Rab paralogues of unknown function but wide distribution, and thus presumably ancient nature; RabTitan and RTW. Furthermore, we show that many Rab paralogues emerged relatively suddenly during early metazoan evolution, which is in stark contrast to the lack of significant Rab family expansions at the onset of most other major eukaryotic groups. Finally, we reconstruct higher-order ancestral clades of Rabs primarily linked with endocytic and exocytic process, suggesting the presence of primordial Rabs associated with the establishment of those pathways and giving the deepest glimpse to date into pre-LECA history of the endomembrane system.

Lysosomal protein trafficking in Giardia lamblia: common and distinct features

Giardia is a flagellated protozoan parasite that has to face different microenvironments during its life cycle in order to survive. All cells exchange materials with the extracellular medium through the reciprocal processes of endocytosis and secretion. Unlike more evolved cells, Giardia lacks a defined endosomal/lysosomal system, but instead possesses peripheral vacuoles that play roles in endocytosis, degradation, recycling, and secretion of proteins during growth and differentiation of the parasite. This review focuses on recent reports defining the role of different molecules involved in protein trafficking to the peripheral vacuoles, and discusses possible mechanisms of receptor recycling. Since Giardia is an early-branching protist, the study of this parasite may lead to a clearer understanding of the minimal machinery required for protein transport in eukaryotic cells.

Mechanisms of adaptation in the intestinal parasite Giardia lamblia

Giardia lamblia, a parasite of humans, is a major source of waterborne diarrhoeal disease. Giardia is also an excellent system to study basic biochemical processes because it is a single-celled eukaryote with a small genome and its entire life cycle can be replicated in vitro. Giardia trophozoites undergo fundamental changes to survive outside the intestine of their host by differentiating into infective cysts. Encystation entails the synthesis, processing, transport, secretion and extracellular assembly of cyst wall components. To survive within the intestine, Giardia undergoes antigenic variation, a process by which the parasite continuously switches its major surface molecules, allowing the parasite to evade the host's immune response and produce chronic and recurrent infections. The objective of the present chapter is to provide a better understanding of the molecular mechanisms involved in adaptation and differentiation in Giardia, with a particular focus on the process of encystation and antigenic variation of this interesting micro-organism.

Membrane trafficking and organelle biogenesis in Giardia lamblia: use it or lose it

The secretory transport capacity of Giardia trophozoites is perfectly adapted to the changing environment in the small intestine of the host and is able to deploy essential protective surface coats as well as molecules which act on epithelia. These lumen-dwelling parasites take up nutrients by bulk endocytosis through peripheral vesicles or by receptor-mediated transport. The environmentally-resistant cyst form is quiescent but poised for activation following stomach passage. Its versatility and fidelity notwithstanding, the giardial trafficking systems appear to be the product of a general secondary reduction process geared towards minimization of all components and machineries identified to date. Since membrane transport is directly linked to organelle biogenesis and maintenance, less complexity also means loss of organelle structures and functions. A case in point is the Golgi apparatus which is missing as a steady-state organelle system. Only a few basic Golgi functions have been experimentally demonstrated in trophozoites undergoing encystation. Similarly, mitochondrial remnants have reached a terminally minimized state and appear to be functionally restricted to essential iron-sulfur protein maturation processes. Giardia's minimized organization combined with its genetic tractability provides unique opportunities to study basic principles of secretory transport in an uncluttered cellular environment. Not surprisingly, Giardia is gaining increasing attention as a model for the investigation of gene regulation, organelle biogenesis, and export of simple but highly protective cell wall biopolymers, a hallmark of all perorally transmitted protozoan and metazoan parasites.

Adaptor protein 2 regulates receptor-mediated endocytosis and cyst formation in Giardia lamblia

The parasite Giardia lamblia possesses PVs (peripheral vacuoles) that function as both endosomes and lysosomes and are implicated in the adaptation, differentiation and survival of the parasite in different environments. The mechanisms by which Giardia traffics essential proteins to these organelles and regulates their secretion have important implications in the control of parasite dissemination. In the present study, we describe the participation of the heterotetrameric clathrin-adaptor protein gAP2 (Giardia adaptor protein 2) complex in lysosomal protein trafficking. A specific monoclonal antibody against the medium subunit (gmu2) of gAP2 showed localization of this complex to the PVs, cytoplasm and plasma membrane in the growing trophozoites. gAP2 also co-localized with clathrin in the PVs, suggesting its involvement in endocytosis. Uptake experiments using standard molecules for the study of endocytosis revealed that gAP2 specifically participated in the endocytosis of LDL (low-density lipoprotein). Targeted down-regulation of the gene encoding gmu2 in growing and encysting trophozoites resulted in a large decrease in the amount of cell growth and cyst wall formation, suggesting a distinct mechanism in which gAP2 is directly involved in both endocytosis and vesicular trafficking.

Behind the smile: cell biology and disease mechanisms of Giardia species

The eukaryotic intestinal parasite Giardia intestinalis was first described in 1681, when Antonie van Leeuwenhoek undertook a microscopic examination of his own diarrhoeal stool. Nowadays, although G. intestinalis is recognized as a major worldwide contributor to diarrhoeal disease in humans and other mammals, the disease mechanisms are still poorly understood. Owing to its reduced complexity and proposed early evolutionary divergence, G. intestinalis is used as a model eukaryotic system for studying many basic cellular processes. In this Review we discuss recent discoveries in the molecular cell biology and pathogenesis of G. intestinalis.

Seroepidemiology of human Toxoplasma gondii infection in China

BACKGROUND

Toxoplasmosis is an important zoonotic parasitic disease worldwide. In immune competent individuals, Toxoplasma gondii preferentially infects tissues of central nervous systems, which might be an adding factor of certain psychiatric disorders. Congenital transmission of T. gondii during pregnancy has been regarded as a risk factor for the health of newborn infants. While in immune-compromised individuals, the parasite can cause life-threatening infections. This study aims to investigate the prevalence of T. gondii infection among clinically healthy individuals and patients with psychiatric disorders in China and to identify the potential risk factors related to the vulnerability of infection in the population.

METHODS

Serum samples from 2634 healthy individuals and 547 patients with certain psychiatric disorders in Changchun and Daqing in the northeast, and in Shanghai in the south of China were examined respectively for the levels of anti-T. gondii IgG by indirect ELISA and a direct agglutination assay. Prevalence of T. gondii infection in the Chinese population in respect of gender, age, residence and health status was systematically analyzed.

RESULTS

The overall anti-T. gondii IgG prevalence in the study population was 12.3%. In the clinically healthy population 12.5% was sero-positive and in the group with psychiatric disorders 11.3% of these patients were positive with anti-T. gondii IgG. A significant difference (P = 0.004) was found between male and female in the healthy population, the seroprevalence was 10.5% in men versus 14.3% in women. Furthermore, the difference of T. gondii infection rate between male and female in the 20-19 year's group was more obvious, with 6.4% in male population and 14.6% in female population.

CONCLUSION

A significant higher prevalence of T. gondii infection was observed in female in the clinically healthy population. No correlation was found between T. gondii infection and psychiatric disorders in this study. Results suggest that women are more exposed to T. gondii infection than men in China. The data argue for deeper investigations for the potential risk factors that threat the female populations.

Neogenesis and maturation of transient Golgi-like cisternae in a simple eukaryote

The highly reduced protozoan parasite Giardia lamblia has minimal machinery for cellular processes such as protein trafficking. Giardia trophozoites maintain diverse and regulated secretory pathways but lack an identifiable Golgi complex. During differentiation to cysts, however, they produce specialized compartments termed encystation-specific vesicles (ESVs). ESVs are hypothesized to be unique developmentally regulated Golgi-like organelles dedicated to maturation and export of pre-sorted cyst wall proteins. Here we present a functional analysis of this unusual compartment by direct interference with the functions of the small GTPases Sar1, Rab1 and Arf1. Conditional expression of dominant-negative variants revealed an essential role of Sar1 in early events of organelle neogenesis, whilst inhibition of Arf1 uncoupled morphological changes and cell cycle progression from extracellular matrix export. The latter led to development of ;naked cysts', which lacked water resistance and thus infectivity. Time-lapse microscopy and photobleaching experiments showed that putative Golgi-like cisternae in Giardia develop into a network capable of exchanging soluble cargo at a high rate via dynamic, tubular connections, presumably to synchronize maturation. The minimized and naturally pulsed trafficking machinery for export of the cyst wall biopolymer in Giardia is a simple model for investigating basic principles of neogenesis and maturation of Golgi compartments.