Endocytosis in anaerobic parasitic protists

Abstract

The incorporation of different molecules by eukaryotic cells occurs through endocytosis, which is critical to the cell's survival and ability to reproduce. Although this process has been studied in greater detail in mammalian and yeast cells, several groups working with pathogenic protists have made relevant contributions. This review analysed the most relevant data on the endocytic process in anaerobic protists (Entamoeba histolytica, Giardia intestinalis, Trichomonas vaginalis, and Tritrichomonas foetus). Many protozoa can exert endocytic activity across their entire surface and do so with great intensity, as with E. histolytica. The available data on the endocytic pathway and the participation of PI-3 kinase, Rab, and Rho molecular complexes is reviewed from a historical perspective.

Fig. 1:. squeme (A) of Giardia intestinalis as seen by scanning electron microscopy (SEM) in a ventral view. D: disc; F: Flagella; Fu: funis; MB: median body; N: nucleus. Benchimol (unpublished data).

Fig. 2:. thin-section (A-A’) and freeze-fracture (B-B`) of Giardia intestinalis. The peripheral vesicles (PVs) (arrows and asterisks). (A) PVs are artificially coloured. (A) Arrows point PVs; in (B, B’), PVs are pointed by asterisks. Note that PVs are located just beneath the plasma membrane. N: nucleus; D: disc; MB: median body. Benchimol (unpublished data).

Fig. 3:. peripheral vesicles (PVs) of Giardia intestinalis. Some PVs are polymorphic (arrows in A), whereas others are tubular (arrow in B). The tridimensional reconstruction shows the polymorphism of the PVs (magenta). Endoplasmic reticulum (white), nuclear envelope (yellow). A-B: Benchimol (unpublished data); C: after.

Fig. 4:. focused ion beam (FIB) and three-dimensional reconstruction of peripheral vesicles (PVs) (green) of Giardia intestinalis trophozoites. Notice nanovesicles inside PVs (arrows). The number of intra-vesicular bodies varied. After.

Fig. 5:. scanning (A) and transmission electron microscopy (B-D) of Giardia intestinalis in interaction with uncoated latex beads (A), latex beads coated with albumin (B), yeast (C), and bacteria (D). In (A), the latex beads are in the process of internalisation, just in the region of the ventral flagella exit. (B) plasma membrane expansions, like pseudopods, are in direction and contact with latex beads (L) (the inset shows it better). (C) A yeast (Y) is seen in the process of endocytosis by G. intestinalis. (D) A bacterium (B) is attached to a membrane expansion of G. intestinalis. Notice that the parasite has drastically changed its shape, presenting a long membrane extension, like a pseudopod, which presents filamentous structures actin-like (asterisks). N: nucleus. After.

Fig. 6:. scheme (A), scanning electron microscopy (SEM) (B), and transmission electron microscopy (TEM) (C) of the endocytic activity of Trichomonas vaginalis when in contact with bacteria (arrows in A). The asterisk points to an endocytic pit. Notice the digestive vacuole (DV), where organic material is digested. Ax: axostyle; c: costa; G: Golgi; H: hydrogenosome; F: flagella; N: nucleus; RF: recurrent flagellum. Benchimol (unpublished data).

Fig. 7:. transmission electron microscopy (TEM) of the endocytic activity of Trichomonas vaginalis when in contact with ferritin-coated latex beads (arrows). Note that many látex beads are inside vacuoles and on the cell surface. Benchimol (unpublished data).

Fig. 8:. scanning electron microscopy (SEM) of the sinking process. Trichomonas vaginalis ingests the yeast Saccharomyces cerevisiae (orange) without any apparent participation of plasma membrane extensions (A-B). Notice that several yeasts (asterisks) can be ingested by the same cell (C). After.

Fig. 9:. scanning electron microscopy (SEM) shows the attachment of Saccharomyces cerevisiae (Y) to the anterior (A) and recurrent flagellum (B) before the endocytic process. After.

Fig. 10:. trogocytosis. (A) scanning electron microscopy (SEM) of a 1-h interaction between Trichomonas vaginalis (green) with a confluent monolayer of bovine oviduct epithelial cells (red). Notice that the parasite is pulling up the epithelial cells. (B) transmission electron microscopy (TEM) of trogocytosis: it is possible to note that large cell organelles, such as a nucleus, can be phagocytosed. After.

Fig. 11:. scheme (A) and scanning electron microscopy (SEM) of the interaction between Tritrichomonas foetus (T), K strain, and horse erythrocytes (E). The arrow points to a pseudopod formation. AF: anterior flagellum; N: nucleus; S: sigmoid filament; Ax: axostyle; H: hydro genome; Er: endoplasmic reticulum; UM: undulating membrane; L: lysosome; P: pelta; BB: basal body; PF: parabasal filament; F: cytoskeletal filaments. (A) Benchimol (unpublished data); (B) After.

Fig. 12:. scanning electron microscopy (SEM) of a bovine sperm cell (S) in close contact with Tritrichomonas foetus after 30 min of interaction. (B) Transmission electron microscopy: after phagocytosis, remains of the sperm cell are seen inside an intracellular vacuole. Modified after.

Fig. 13:. Entamoeba histolytica is seen by scanning electron microscopy (SEM) (A) and transmission electron microscopy (TEM) (B). Notice the stoma (S) formed during the phagocytic process (A) and the ingested red cells (H) in the vacuoles. Benchimol (unpublished data).

Fig. 14:. morphological characteristics of Entamoeba histolytica endocytic pathway. (A) Scanning electron micrograph (SEM) of an erythrocyte (E) phagocytosed by E. histolytica. After. (B) The overall appearance of the trophozoite forms is observed by transmission electron microscopy (TEM), revealing numerous vesicles, vacuoles, and glycogen particles within the cytoplasm. The nucleus and the presence of certain chromatoid bodies are also visible. Bars: 1.6 µm. (C-E) Endocytosis of lactoferrin coupled to gold particles shows the presence of the protein at tubular plasma membrane invaginations (C), inside vesicles in the cytosol (D), and in vacuoles at the central region of the cell, after 30 minutes of endocytosis (E). Bars: (C) 0,5 µm; (D) 0,17 µm; (E) 0,25 µm. (F) Fluid-phase endocytosis of HRP shows labelling in many vacuoles inside the cell (star), with some fusing with unlabeled ones (arrowheads). Bar: 1,65 µm. (B-F) After (G), FITC-Ferritin endocytosis is mediated by clathrin (upper panel), and ferritin is delivered to lysosomes after 30 min, marked by lysosomal-associated membrane protein 2 (LAMP-2) positive compartments (lower panel). Bars - upper panel: 24 µm; lower panel: 8 µm. After.

Fig. 15:. cytoskeleton participation in Entamoeba histolytica endocytosis. (A) Fluorescence images of EhGEF (green) with F-actin (red) and EhRho1 (blue) during different steps of erythrophagocytosis. Arrowheads indicate phagocytic cups, and asterisks mark actin enrichment in phagosomes. After. (B) Serum-starved GFP-EhRho5 trophozoites were stimulated in the presence and absence of 15 μM LPA. After.. Bars: 10 µm.

Fig. 16:. endosomes-mitosomes contact sites in Entamoeba histolytica. E. histolytica trophozoites expressing HA-tagged EHD1. (A) Immunofluorescence of fixed HA-EHD1-expressing cells double-stained with anti-HA (green) and anti-APSK (red). The arrow and arrowheads indicate proximity and colocalisation between anti-HA and anti-APSK signals, respectively. (B) Immunogold of HA-EHD1 trophozoites labeling anti-HA 5 nm gold and anti-APSK 15 nm gold. c: cytosol; e: endosome; m: mitosome. The arrow points to the structure where the membranes of the mitosome and endosome are in close contact. (C) Double labelling immunofluorescence of HA-EHD1 trophozoites with anti-HA and anti-endosomal markers antibodies. (D) Immunogold of HA-EHD1 trophozoites labelled with 15-nm gold-anti-HA. c: cytosol; MVB: multivesicular body; ILV: intraluminal vesicle. Bars: (A,C) 10 µm; (B,D) 200 nm. After.