A new set of vesicles in Giardia lamblia

Observations on the transmission of Dientamoeba fragilis and the cyst life cycle stage

Little is known about the life cycle and mode of transmission of Dientamoeba fragilis. Recently it was suggested that fecal–oral transmission of cysts may play a role in the transmission of D. fragilis. In order to establish an infection, D. fragilis is required to remain viable when exposed to the pH of the stomach. In this study, we investigated the ability of cultured trophozoites to withstand the extremes of pH. We provide evidence that trophozoites of D. fragilis are vulnerable to highly acidic conditions. We also investigated further the ultrastructure of D. fragilis cysts obtained from mice and rats by transmission electron microscopy. These studies of cysts showed a clear cyst wall surrounding an encysted parasite. The cyst wall was double layered with an outer fibrillar layer and an inner layer enclosing the parasite. Hydrogenosomes, endoplasmic reticulum and nuclei were present in the cysts. Pelta-axostyle structures, costa and axonemes were identifiable and internal flagellar axonemes were present. This study therefore provides additional novel details and knowledge of the ultrastructure of the cyst stage of D. fragilis.

Combined nanometric and phylogenetic analysis of unique endocytic compartments in Giardia lamblia sheds light on the evolution of endocytosis in Metamonada

Background

Giardia lamblia, a parasitic protist of the Metamonada supergroup, has evolved one of the most diverged endocytic compartment systems investigated so far. Peripheral endocytic compartments, currently known as peripheral vesicles or vacuoles (PVs), perform bulk uptake of fluid phase material which is then digested and sorted either to the cell cytosol or back to the extracellular space.

Results

Here, we present a quantitative morphological characterization of these organelles using volumetric electron microscopy and super-resolution microscopy (SRM). We defined a morphological classification for the heterogenous population of PVs and performed a comparative analysis of PVs and endosome-like organelles in representatives of phylogenetically related taxa, Spironucleus spp. and Tritrichomonas foetus. To investigate the as-yet insufficiently understood connection between PVs and clathrin assemblies in G. lamblia, we further performed an in-depth search for two key elements of the endocytic machinery, clathrin heavy chain (CHC) and clathrin light chain (CLC), across different lineages in Metamonada. Our data point to the loss of a bona fide CLC in the last Fornicata common ancestor (LFCA) with the emergence of a protein analogous to CLC (GlACLC) in the Giardia genus. Finally, the location of clathrin in the various compartments was quantified.

Conclusions

Taken together, this provides the first comprehensive nanometric view of Giardia’s endocytic system architecture and sheds light on the evolution of GlACLC analogues in the Fornicata supergroup and, specific to Giardia, as a possible adaptation to the formation and maintenance of stable clathrin assemblies at PVs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01402-3.

Inorganic phosphate transporter in Giardia duodenalis and its possible role in ATP synthesis

Giardia duodenalis is a flagellated protozoan that inhabits vertebrate host intestines, causing the disease known as giardiasis. Similar to other parasites, G. duodenalis must take advantage of environmental resources to survive, such as inorganic phosphate (P_i) availability. P_i is an anionic molecule and an essential nutrient for all organisms because it participates in the biosynthesis of biomolecules, energy storage, and cellular structure formation. The first step in Pi metabolism is its uptake through specific transporters on the plasma membrane. We identified a symporter H⁺:P_i-type ORF sequence in the G. duodenalis genome (GenBank ID: GL50803_5164), named GdPho84, which is homologous to Saccharomyces cerevisiae PHO84. In trophozoites, P_i transport was linear for up to 15 min, and the cell density was 3 × 10⁷ cells/ml. Physiological variations in pH (6.4-8.0) did not influence P_i uptake. This P_i transporter had a high affinity, with K_0.5 = 67.7 ± 7.1 µM P_i. SCH28080 (inhibitor of H⁺, K⁺-ATPase), bafilomycin A₁ (inhibitor of vacuolar H⁺-ATPase), and FCCP (H⁺ ionophore) were able to inhibit P_i transport, indicating that an H⁺ gradient in the cell powered uphill P_i movement. PAA, an H⁺-dependent P_i transport inhibitor, reduced cell proliferation, P_i transport activity, and GdPHO48 mRNA levels. P_i starvation stimulated membrane potential-sensitive P_i uptake coupled to H⁺ fluxes, increased GdPho84 expression, and reduced intracellular ATP levels. These events indicate that these cells had an increased capacity to internalize P_i as a compensatory mechanism compared to cells maintained in control medium conditions. Internalized P_i can be used in glycolytic metabolism once iodoacetamide (GAPDH inhibitor) inhibits P_i influx. Together, these results reinforce the hypothesis that P_i is a crucial nutrient for G. duodenalis energy metabolism.

Variation in Giardia: implications for taxonomy and epidemiology

The taxonomy, life cycle patterns and zoonotic potential of Giardia infecting mammals and birds have been poorly understood and controversial for many years. The development of molecular tools for characterising isolates of Giardia directly from faeces or environmental samples has made an enormous contribution to resolving these issues. It is now clear that the G. duodenalis morphological group is a species complex comprising a series of what appear to be largely host-adapted species, and at least two zoonotic species for which humans are the major host, but which are also capable of infecting other mammals. It is proposed that this new information be reflected in the redesignation of several species of Giardia described previously. The molecular epidemiological tools that are now available need to be applied in different endemic foci of Giardia transmission, as well as in outbreak situations, in order to understand better the frequency of zoonotic transmission as well as to develop more effective approaches to controlling giardiasis.

Giardia lamblia: behavior of the nuclear envelope

Giardia Lamblia is a flagellar parasite possessing the unusual morphology of bearing two nuclei. New morphological observations on trophozoites and encysting Giardia nuclei using routine transmission electron microscopy, freeze fracture and cytochemistry are presented. Nuclear pores of both nuclei in the same cells were assessed on freeze-fracture replicas from different cell cycle phases, and compared. These techniques showed that (1) both nuclei in the same cell are distinct in nuclear pore number and distribution; (2) nuclear pore complexes are frequently clustered in nuclear envelope domains; (3) dividing nuclei display very few nuclear pores; (4) few ribosomes are found on the outer nuclear envelope of the trophozoite form; (5) nuclear membranes present spots of closely apposed membranes, which are different from the typical diaphragm nuclear pore complexes; (6) in addition to the nuclear pores, membrane blebs are also present in the nuclear envelope; (7) encysting cells show intranuclear inclusions, morphologically similar to the ESV (encystation-specific vesicles) and to the ER membranes, which may be the result of nuclear envelope folding. It is proposed that the two nuclei in Giardia are dissimilar in morphology and activity.

Secretory protein trafficking in Giardia intestinalis

Early diverged extant organisms, which may serve as convenient laboratory models to look for and study evolutionary ancient features of eukaryotic cell biology, are rare. The diplomonad Giardia intestinalis, a protozoan parasite known to cause diarrhoeal disease, has become an increasingly popular object of basic research in cell biology, not least because of a genome sequencing project nearing completion. Commensurate with its phylogenetic status, the Giardia trophozoite has a very basic secretory system and even lacks hallmark structures such as a morphologically identifiable Golgi apparatus. The cell's capacity for protein sorting is nevertheless unimpeded, exemplified by its ability to cope with massive amounts of newly synthesized cyst wall proteins and glycans, which are sorted to dedicated Golgi-like compartments termed encystation-specific vesicles (ESVs) generated from endoplasmic reticulum (ER)-derived transport intermediates. This soluble bulk cargo is kept strictly separate from constitutively transported variant surface proteins during export, a function that is dependent on the stage-specific recognition of trafficking signals. Encysting Giardia therefore provide a unique system for the study of unconventional, Golgi-independent protein trafficking mechanisms in the broader context of eukaryotic endomembrane organization and evolution.

Mitosomes of Entamoeba histolytica are abundant mitochondrion-related remnant organelles that lack a detectable organellar genome

The existence of mitochondrion-related relict organelles (mitosomes) in the amitochondrial human pathogen Entamoeba histolytica and the detection of extranuclear DNA-containing cytoplasmic structures (EhKOs) has led to the suggestion that a remnant genome from the original mitochondrial endosymbiont might have been retained in this organism. This study reports on the mutually exclusive distribution of Cpn60 and extranuclear DNA in E. histolytica and on the distribution of Cpn60-containing mitosomes in this parasite. In situ nick-translation coupled to immunofluorescence microscopy failed to detect the presence of DNA in mitosomes, either in fixed parasite trophozoites or in partially purified organellar fractions. These results indicate that a remnant organellar genome has not been retained in E. histolytica mitosomes and demonstrate unequivocally that EhKOs and mitosomes are distinct and unrelated cellular structures.