Abstract
The Coccidia are a subclass of the Apicomplexa and include several genera of protozoan parasites that cause important diseases in humans and animals, with Toxoplasma gondii becoming the ‘model organism’ for research into the coccidian molecular and cellular processes. The amenability to the cultivation of T. gondii tachyzoites and the wide availability of molecular tools for this parasite have revealed many mechanisms related to their cellular trafficking and roles of parasite secretory organelles, which are critical in parasite-host interaction. Nevertheless, the extrapolation of the T. gondii mechanisms described in tachyzoites to other coccidian parasites should be done carefully. In this review, we considered published data from Eimeria parasites, a coccidian genus comprising thousands of species whose infections have important consequences in livestock and poultry. These studies suggest that the Coccidia possess both shared and diversified mechanisms of protein trafficking and secretion potentially linked to their lifecycles. Whereas trafficking and secretion appear to be well conversed prior to and during host-cell invasion, important differences emerge once endogenous development commences. Therefore, further studies to validate the mechanisms described in T. gondii tachyzoites should be performed across a broader range of coccidians (including T. gondii sporozoites). In addition, further genus-specific research regarding important disease-causing Coccidia is needed to unveil the individual molecular mechanisms of pathogenesis related to their specific lifecycles and hosts.
Highlights
The mechanisms for this trafficking pathway described in T. gondii are likely to be conserved in the Eimeria species, since the gene orthologues to essential molecules for trans-Golgi network (TGN) to endosome-like compartments (ELC) trafficking are present in different Eimeria genomes (Table 1; www.toxodb.org)
Other GRA genes, such as TgGRA 2, 4, and 6, were >8-fold lower in T. gondii than in tachyzoites. This supports the previous finding of Ferguson and collaborators [97], which showed that TgGRA proteins 1 to 6 were not detected by specific antibodies in merozoites, while TgNTPase and TgGRA7 were expressed in merozoites [98]. These results suggest the possibility that dense granules are of biological importance, primarily in the acute and tissue cyst stages of T. gondiii and not in the gut stages, which correlates well with observations in the Eimeria species that they develop only in the intestine
After reviewing the studies related to protein trafficking in T. gondii and Eimeria spp., we considered these studies in the context of the described events that occur during the host-cell invasion and subsequent parasite endogenous development (Figure 1)
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Previous studies in Apicomplexan parasites, such as Toxoplasma gondii and Plasmodium species, have established that apicomplexans have repurposed the exocytic and endocytic pathways of higher eukaryotes to evolve their specialized regulatory secretory organelles [2,3,4]. This rearrangement has led apicomplexans to possess a reduced endomembrane system compared to higher eukaryotes, which is efficient enough to perform the protein trafficking, targeting, processing and recycling needed for their obligate intracellular lifestyles. Whereas work on T. gondii provides valuable insights into the mechanisms of coccidian invasion, differences in the specific infection biology between parasites, such as host-range, tissue- and site-specificity, PV formation, the mechanism of asexual cellular division, and pathological interaction with the host, suggests that it is not always possible to directly infer functions in Eimeria species from the information gathered in T. gondii [7]
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