Protozoan phagotrophy from predators to parasites: An overview of the enigmatic cytostome‐cytopharynx complex of Trypanosoma cruzi

Abstract

Eating is fundamental and from this basic principle, living organisms have evolved innumerable strategies to capture energy and nutrients from their environment. As part of the world's aquatic ecosystems, the expansive family of heterotrophic protozoans uses self‐generated currents to funnel prokaryotic prey into an ancient, yet highly enigmatic, oral apparatus known as the cytostome‐cytopharynx complex prior to digestion. Despite its near ubiquitous presence in protozoans, little is known mechanistically about how this feeding organelle functions. Intriguingly, one class of these flagellated phagotrophic predators known as the kinetoplastids gave rise to a lineage of obligate parasitic protozoa, the trypanosomatids, that can infect a wide variety of organisms ranging from plants to humans. One parasitic species of humans, Trypanosoma cruzi, has retained this ancestral organelle much like its free‐living relatives and continues to use it as its primary mode of endocytosis. In this review, we will highlight foundational observations made regarding the cytostome‐cytopharynx complex and examine some of the most pressing questions regarding the mechanistic basis for its function. We propose that T. cruzi has the potential to serve as an excellent model system to dissect the enigmatic process of protozoal phagotrophy and thus enhance our overall understanding of fundamental eukaryotic biology.

Endocytosis inTrypanosoma cruziDepends on Proper Recruitment and Regulation of Functionally Redundant Myosin Motors

“…This technique also allowed us to examine the localization of MyoF and MyoC and we found both motors also localized on these root fibers beginning at the cytostome entrance. The targeting of MyoF to these microtubules validates prior observations by Alves et al who, by using immuno-EM of epitope tagged MyoF on membrane extracted T. cruzi cytoskeletons, demonstrated that MyoF associates with SPC rootlet microtubules ((28) and reviewed in (12)). Our subsequent direct deletion of both MyAP paralogs using CRISPR/Cas9, resulted in endocytic-null parasites that phenocopied our myosin double deletion mutants ( ΔFΔC ) and we were able to completely restore this endocytic defect via complementation.…”

“…In the absence of a clear in vitro growth defect exhibited by our endocytic null mutants, we continue to be faced with the central question regarding the ultimate role of endocytosis in this parasite's life cycle. Although we have speculated extensively in our prior reports as to why the SPC was retained specifically in those trypanosomatids that are transmitted via the insect feces, we still lack definitive evidence showing when or where endocytosis is essential in the parasite life cycle (12,19). However, what we have been able to demonstrate here using TEM and MSbased lipidomics analyses, is that these mutants have a dramatically reduced capacity to take in and store extracellular material such as host-derived cholesterol in their reservosome structures.…”

“…which have repurposed their flagellar pocket membrane to be the sole location for endocytosis and exocytosis (9)(10)(11), T. cruzi utilizes an ancestral form of phagocytosis operating via a flagellar pocket adjacent organelle known as the cytostome-cytopharynx complex (SPC) which is still used by its free-living relatives (e.g. Bodo saltans) to capture and consume bacterial prey (reviewed in (12)). The SPC begins as an opening on the parasite surface (cytostome) and is followed by a dynamic tubular membrane invagination (cytopharynx) through which captured and endocytosed material is brought into the cell and ultimately digested (13).…”

“…Among single celled protozoans, these microtubule rootlets often facilitate the proper positioning and function of various subcellular organelles (reviewed in (17)). Up until recently, the vast majority of our understanding of the SPC apparatus was gleaned from structural examinations using electron microscopy-based techniques (12). These methods, however, were unable to provide mechanistic insight into how this organelle is able to capture and pull in endocytosed material.…”

Phagocytosis underpins the biotrophic lifestyle of intracellular parasites in the class Phytomyxea (Rhizaria)

“…Although not common, intracellular endocytosis has been documented in intracellular parasites spanning the taxa Apicomplexa (Spielmann et al , 2020 and references therein), Cryptomycota (Torruella et al , 2018) and Euglenozoa (Etheridge, 2022). The reduced dataset of phagotrophy-related proteins from Rozella allomycis correctly describes Trypanosoma cruzii and Leishmania braziliensis as capable of intracellular phagotrophy (Chasen et al , 2020; Halliday et al , 2020); whilst fails to assign Plasmodium falciparum and Toxoplasma gondii to this category.…”