The giardial VPS35 retromer subunit is necessary for multimeric complex assembly and interaction with the vacuolar protein sorting receptor

Genomic survey maps differences in the molecular complement of vesicle formation machinery between Giardia intestinalis assemblages

Giardia intestinalis is a globally important microbial pathogen with considerable public health, agricultural, and economic burden. Genome sequencing and comparative analyses have elucidated G. intestinalis to be a taxonomically diverse species consisting of at least eight different sub-types (assemblages A-H) that can infect a great variety of animal hosts, including humans. The best studied of these are assemblages A and B which have a broad host range and have zoonotic transmissibility towards humans where clinical Giardiasis can range from asymptomatic to diarrheal disease. Epidemiological surveys as well as previous molecular investigations have pointed towards critical genomic level differences within numerous molecular pathways and families of parasite virulence factors within assemblage A and B isolates. In this study, we explored the necessary machinery for the formation of vesicles and cargo transport in 89 Canadian isolates of assemblage A and B G. intestinalis. Considerable variability within the molecular complement of the endolysosomal ESCRT protein machinery, adaptor coat protein complexes, and ARF regulatory system have previously been reported. Here, we confirm inter-assemblage, but find no intra-assemblage variation within the trafficking systems examined. This variation includes losses of subunits belonging to the ESCRTIII as well as novel lineage specific duplications in components of the COPII machinery, ARF1, and ARFGEF families (BIG and CYTH). Since differences in disease manifestation between assemblages A and B have been controversially reported, our findings may well have clinical implications and even taxonomic, as the membrane trafficking system underpin parasite survival, pathogenesis, and propagation.

Identification of Actin Filament-Associated Proteins in Giardia lamblia

The deep-branching protozoan parasite Giardia lamblia is the causative agent of the intestinal disease giardiasis. Consistent with its proposed evolutionary position, many pathways are minimalistic or divergent, including its actin cytoskeleton. Giardia is the only eukaryote known to lack all canonical actin-binding proteins. Previously, our lab identified a number of noncanonical Giardia lamblia actin (GlActin) interactors; however, these proteins appeared to interact only with monomeric or globular actin (G-actin) rather than with filamentous actin (F-actin). To identify F-actin interactors, we used a chemical cross-linker to preserve native interactions followed by an anti-GlActin antibody, protein A affinity chromatography, and liquid chromatography coupled to mass spectrometry. We found 46 putative actin interactors enriched under the conditions favoring F-actin. Data are available via ProteomeXchange with identifier PXD026067. None of the proteins identified contain known actin-interacting motifs, and many lacked conserved domains. Each potential interactor was then tagged with the fluorescent protein mNeonGreen and visualized in live cells. We categorized the proteins based on their primary localization; localizations included ventral disc, marginal plate, nuclei, flagella, plasma membrane, and internal membranes. One protein from each of the six categories was colocalized with GlActin using immunofluorescence microscopy. We also co-immunoprecipitated one protein from each category and confirmed three of the six potential interactions. Most of the localization patterns are consistent with previously demonstrated GlActin functions, but the ventral disc represents a new category of actin interactor localization. These results suggest a role for GlActin in ventral disc function, which has previously been controversial. IMPORTANCE Giardia lamblia is an intestinal parasite that colonizes the small intestine and causes diarrhea, which can lead to dehydration and malnutrition. Giardia actin (GlActin) has a conserved role in Giardia cells, despite being a highly divergent protein with none of the conserved regulators found in model organisms. Here, we identify and localize 46 interactors of polymerized actin. These putative interactors localize to a number of places in the cell, underlining GlActin's importance in multiple cellular processes. Surprisingly, eight of these proteins localize to the ventral disc, Giardia's host attachment organelle. Since host attachment is required for infection, proteins involved in this process are an appealing target for new drugs. While treatments for Giardia exist, drug resistance is becoming more common, resulting in a need for new treatments. Giardia and human systems are highly dissimilar, thus drugs specifically tailored to Giardia proteins would be less likely to have side effects.

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.

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.

Phosphoinositide binding profiles of the PX domains of Giardia lamblia

The membrane trafficking machinery that functions at the endomembrane system of Giardia lamblia appears to be significantly different from that present in most model eukaryotes. This machinery is important for encystation as cyst wall material is trafficked to the cell surface via encystation-specific vesicles. Since proteins containing the phosphoinositide-binding PX domains are known regulators of vesicular trafficking, BLAST search was used to identify the PX domains of G. lamblia. Six putative PX domain-containing ORFs were identified. Some of the encoded PX domains contained non-canonical amino acid residues in the highly conserved ligand binding pocket. In vitro and in vivo binding studies indicate that these domains have the ability to bind to diverse phosphoinositides. Also, coincidence detection is likely to play a significant role in ligand binding in vivo since domains that bind to the same lipid in vitro, exhibit differences in subcellular localization. Analyses of the expression of these six genes in trophozoites, encysting trophozoites and cysts showed that while the expression of four of the genes were downregulated in cysts, the other two were upregulated. The variation in ligand preference of the individual PX domains and the differential expression of most of the PX-domain encoding genes indicate that these PX domain-containing proteins are likely to perform diverse cellular functions.

Resolving the homology-function relationship through comparative genomics of membrane-trafficking machinery and parasite cell biology

With advances in DNA sequencing technology, it is increasingly common and tractable to informatically look for genes of interest in the genomic databases of parasitic organisms and infer cellular states. Assignment of a putative gene function based on homology to functionally characterized genes in other organisms, though powerful, relies on the implicit assumption of functional homology, i.e. that orthology indicates conserved function. Eukaryotes reveal a dazzling array of cellular features and structural organization, suggesting a concomitant diversity in their underlying molecular machinery. Significantly, examples of novel functions for pre-existing or new paralogues are not uncommon. Do these examples undermine the basic assumption of functional homology, especially in parasitic protists, which are often highly derived? Here we examine the extent to which functional homology exists between organisms spanning the eukaryotic lineage. By comparing membrane trafficking proteins between parasitic protists and traditional model organisms, where direct functional evidence is available, we find that function is indeed largely conserved between orthologues, albeit with significant adaptation arising from the unique biological features within each lineage.

Significantly Diverged Did2/Vps46 Orthologues from the Protozoan Parasite Giardia lamblia

The endosomal compartment performs extensive sorting functions in most eukaryotes, some of which are accomplished with the help of the multivesicular body (MVB) sorting pathway. This pathway depends on the sequential action of complexes, termed the endosomal sorting complex required for transport (ESCRT). After successful sorting, the crucial step of recycling of the ESCRT complex components requires the activation of the AAA ATPase Vps4, and Did2/Vps46 plays an important role in this activation event. The endolysosomal system of the protozoan parasite Giardia lamblia appears to lack complexity, for instead of having distinct early endosomes, late endosomes and lysosomes, there are only peripheral vesicles (PVs) that are located close to the cell periphery. Additionally, comparative genomics studies predict the presence of only a subset of the ESCRT components in G. lamblia. Thus, it is possible that the MVB pathway is not functional in G. lamblia. To address this issue, the present study focused on the two putative orthologues of Did2/Vps46 of G. lamblia as their function is likely to be pivotal for a functional MVB sorting pathway. In spite of considerable sequence divergence, compared to other eukaryotic orthologues, the proteins encoded by both these genes have the ability to function as Did2/Vps46 in the context of the yeast ESCRT pathway. Furthermore, they also localized to the cellular periphery, where PVs are also located. Thus, this report is the first to provide experimental evidence indicating the presence of a functional ESCRT component in G. lamblia by characterizing the putative Did2/Vps46 orthologues.