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

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.

The Functional Characterization of TcMyoF Implicates a Family of Cytostome-Cytopharynx Targeted Myosins as Integral to the Endocytic Machinery of Trypanosoma cruzi

The parasite Trypanosoma cruzi is the etiological agent of Chagas disease and chronically infects upwards of 7 million people in the Americas. Current diagnostics and treatments remain grossly inadequate due, in part, to our general lack of understanding of this parasite’s basic biology. One aspect that has resisted detailed scrutiny is the mechanism employed by this parasite to extract nutrient resources from the radically different environments that it encounters as it transitions between its invertebrate and mammalian hosts. These parasites engulf food via a tubular invagination of its membrane, a strategy used by many protozoan species, but how this structure is formed or functions mechanistically remains a complete mystery. The significance of our research is in the identification of the mechanistic underpinnings of this feeding organelle that may bring to light new potential therapeutic targets to impede parasite feeding and thus halt the spread of this deadly human pathogen.

Of the pathogenic trypanosomatids, Trypanosoma cruzi alone retains an ancient feeding apparatus known as the cytostome-cytopharynx complex (SPC) that it uses as its primary mode of endocytosis in a manner akin to its free-living kinetoplastid relatives who capture and eat bacterial prey via this endocytic organelle. In a recent report, we began the process of dissecting how this organelle functions by identifying the first SPC-specific proteins in T. cruzi. Here, we continued these studies and report on the identification of the first enzymatic component of the SPC, a previously identified orphan myosin motor (MyoF) specifically targeted to the SPC. We overexpressed MyoF as a dominant-negative mutant, resulting in parasites that, although viable, were completely deficient in measurable endocytosis in vitro. To our surprise, however, a full deletion of MyoF demonstrated only a decrease in the overall rate of endocytosis, potentially indicative of redundant myosin motors at work. Thereupon, we identified three additional orphan myosin motors, two of which (MyoB and MyoE) were targeted to the preoral ridge region adjacent to the cytostome entrance and another (MyoC) which was targeted to the cytopharynx tubular structure similar to that of MyoF. Additionally, we show that the C-terminal tails of each myosin are sufficient for targeting a fluorescent reporter to SPC subregions. This work highlights a potential mechanism used by the SPC to drive the inward flow of material for digestion and unveils a new level of overlapping complexity in this system with four distinct myosin isoforms targeted to this feeding structure.

IMPORTANCE The parasite Trypanosoma cruzi is the etiological agent of Chagas disease and chronically infects upwards of 7 million people in the Americas. Current diagnostics and treatments remain grossly inadequate due, in part, to our general lack of understanding of this parasite’s basic biology. One aspect that has resisted detailed scrutiny is the mechanism employed by this parasite to extract nutrient resources from the radically different environments that it encounters as it transitions between its invertebrate and mammalian hosts. These parasites engulf food via a tubular invagination of its membrane, a strategy used by many protozoan species, but how this structure is formed or functions mechanistically remains a complete mystery. The significance of our research is in the identification of the mechanistic underpinnings of this feeding organelle that may bring to light new potential therapeutic targets to impede parasite feeding and thus halt the spread of this deadly human pathogen.

Observations on the oral organelle, cytopharynx, and subpellicular structure of a Dileptus sp

We used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to observe the oral organelle, cytopharynx, and subpellicular structure of a Dileptus sp. The main results were as follows: (a) the cytostome was located on the ventral surface of the base of the beak, surrounded by a periportal matrix that integrated 135 microtube bundles. When these microtube bundles contract, radially arranged into a disk, the cytostome was closed. When these microtube bundles were stretch, they fell into the cytostome and opens. The diameter of the cytostome was about 16 μm regardless of its closure or opening, indicating that the contraction or elongation of these microtube bundles did not change the size of the cytostome, which was only related to whether it blocked the cytostome, thus determining the opening and closing of the cytostome. There were many microtube bundles on two sides of the feeding trough, which could widen or narrow the feeding trough and facilitate beak feeding. (b) The cytopharynx was basket-like without a bottom with a diameter of about 6 μm and was woven from two kind fibers about 0.08 and 0.19 μm. (c) There were two types of extrusomes under the pellicle. Using transmission electron microscopy，the Type I extrusomes showed narrow and long egg shape, its cross section was circular which is composed by various electronic density of concentric. Using the scanning electron microscope, they were two slightly thin clavate, the length was about 5 μm, the diameter of the middle section was about 0.75 μm, and the diameter of the two ends was about 0.32 μm, they were distributed abundantly between the microtubule fasciculi which were located on both sides of the gap on the feeding groove. Using transmission electron microscopy, the Type II extrusomes showed egg shape. Using the scanning electron microscopy, they were about 1.6 × 0.8 μm in size, they were distributed abundantly under the body pellicle while rarely the proboscis. In addition, many different of developmental stages two types of extrusomes could be also seen in the cytoplasm. (d) There were very well-developed fibrous systems under the pellicle that were woven from fibers about 0.14 μm in diameter that attached to the pellicle and bound some organelles in the cytoplasm (e.g., mitochondria, extrusomes) and other structures to the cytoplasm and maintained cell morphology. The results of this study not only supplement and enrich the morphological contents of the Dileptus sp., but also provide the basis for the study of the taxonomy of the Dileptus sp. It also provides a new method for researchers to explore the morphology and structure of ciliate cells under the cortex by SEM.

Identification and Localization of the First Known Proteins of the Trypanosoma cruzi Cytostome Cytopharynx Endocytic Complex

The etiological agent of Chagas disease, Trypanosoma cruzi, is an obligate intracellular parasite that infects an estimated 7 million people in the Americas, with an at-risk population of 70 million. Despite its recognition as the highest impact parasitic infection of the Americas, Chagas disease continues to receive insufficient attention and resources in order to be effectively combatted. Unlike the other parasitic trypanosomatids that infect humans (Trypanosoma brucei and Leishmania spp.), T. cruzi retains an ancestral mode of phagotrophic feeding via an endocytic organelle known as the cytostome-cytopharynx complex (SPC). How this tubular invagination of the plasma membrane functions to bring in nutrients is poorly understood at a mechanistic level, partially due to a lack of knowledge of the protein machinery specifically targeted to this structure. Using a combination of CRISPR/Cas9 mediated endogenous tagging, fluorescently labeled overexpression constructs and endocytic assays, we have identified the first known SPC targeted protein (CP1). The CP1 labeled structure co-localizes with endocytosed protein and undergoes disassembly in infectious forms and reconstitution in replicative forms. Additionally, through the use of immunoprecipitation and mass spectrometry techniques, we have identified two additional CP1-associated proteins (CP2 and CP3) that also target to this endocytic organelle. Our localization studies using fluorescently tagged proteins and surface lectin staining have also allowed us, for the first time, to specifically define the location of the intriguing pre-oral ridge (POR) surface prominence at the SPC entrance through the use of super-resolution light microscopy. This work is a first glimpse into the proteome of the SPC and provides the tools for further characterization of this enigmatic endocytic organelle. A better understanding of how this deadly pathogen acquires nutrients from its host will potentially direct us toward new therapeutic targets to combat infection.