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The endocytic pathway

Trypanosomatids are highly polarized cells, and their endocytic activity is restricted to the flagellar pocket and cytostome regions.2 Studies of T. cruzi have shown that this protozoan exhibits certain peculiarities in its endocytic pathway that distinguish it from other cells. First, endocytosis only occurs at high levels in the epimastigote. Second, epimastigotes have two sites in which macromolecule uptake takes place: the flagellar pocket, and a highly specialized structure known as the cytostome. Third,

the cargo of the endocytic vesicles is delivered to unusual structures called reservo- somes, which are located at the posterior end of the cell. The cytostome is a plasma membrane invagination coupled to a few special microtubules that penetrate the cell almost to the nucleus. The opening of this complex has a diameter of up to 0.3 pm. There is a specialized region of the membrane lining the parasite that starts in the opening of the cytostome and projects toward the flagellar pocket. The cystostome is the most responsible for ingestion of macromolecules51 and has an acidic nature.52 In relation to the ultrastructure of the cytostome and the cytopharynx, it was observed that cytostome presents a cytoskeleton composed of two microtubule sets (a triplet that starts underneath the cytostome membrane and a quartet that originates underneath the flagellar-pocket membrane and follows the preoral ridge before reaching the cytopharynx).34 Following binding to the cytostome and flagellar pocket, macromolecules are rapidly internalized and appear in small endocytic vesicles, which bud from regions of these structures.35,36

The fusion of endocytic vesicles with the tubule-vesicular network can be observed from the perinuclear region to the posterior tip of the cell.

Macromolecules from the extracellular medium or from the ER-Golgi system are concentrated in structures known as reservosomes.53 The main function of reser- vosomes in T. cruzi epimastigotes is to store macromolecules, although they also contain high concentrations of lysosomal hydrolases. In fact, reservosomes are also considered the main site of protein degradation and regulation. Each epimastigote has several reservosomes, primarily in the posterior region of the cell (Fig. 18.13).

Reservosomes have been described as a structure unique to epimastigotes and several studies demonstrates thar typical reservosomes disappear during the transformation of epimastigotes into metacyclic trypomastigotes in vitro. While lipid and protein uptake has never been demonstrated in either trypomastigotes or amasti- gotes, intracellular organelles that share many reservosomal features were recently described in the T. cruzi mammalian stages.54 Like reservosomes, these organelles

Transmission electron microscopy of T

Figure 18.13 Transmission electron microscopy of T. cruzi reservosomes. (A) Ultrathin section of an epimastigote showing reservosomes (R) in situ, with their typical morphology and position, between nucleus (N) and posterior end of the cell. Bar, 1 pm.

Source: After Sant’Anna et al. (2009).

are concentrated in the parasite’s posterior region. Additionally, they accumulate cruzipain, as well as its natural inhibitor chagasin and serine carboxypetidase. The organelles are acidic and have a P-type H+-ATPase.

 
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