Integrated morphological and transcriptome profiles reveal a highly-developed extrusome system associated to virulence in the notorious fish parasite, Ichthyophthirius multifiliis

Ichthyophthirius multifiliis is an obligate parasitic ciliate that causes severe economic damage in aquaculture. The parasite contains numerous extrusive organelles (extrusomes) that assist in its pathogenesis and reproduction. However, the structure of these extrusomes and the molecular profiles involved in exocytosis remain unclear. In the present study, through comparative ultrastructural observations across the life cycle of I. multifiliis, we demonstrated that all three of its life stages (theront, trophont, and tomont) exhibited an abundance of extrusomes. In addition, two different types of extrusomes were identified according to their unique structures. Type I extrusomes (mucocysts) are crystalline, oval-shaped, 0.7-1.4 × 0.6-1.1 μm, and distributed as "rosettes" below the trophont membrane. Type II extrusomes, 2.0-3.0 × 0.2-0.3 μm, are rod-shaped with tubular cores and identified as toxicysts, the aggregation of which in the anterior part of the theront and cortex of the trophont revealed their potential roles in I. multifiliis invasion. This was confirmed by our transcriptome investigations of the three stages of I. multifiliis, which revealed that a set of genes involved in proteolysis and DNA/protein biogenesis was highly expressed in the theront and trophont. Furthermore, to map the molecular mechanisms of extrusome release, we characterized 25 Rab family genes in I. multifiliis and determined their expression profiles across the life cycle, reflecting the distribution patterns of the two extrusomes. Collectively, our data revealed that a highly developed extrusome system could play a potential role in the virulence of I. multifiliis, which facilitates a better understanding of the parasite's development.

Chemical Defence by Sterols in the Freshwater Ciliate Stentor polymorphus

Heterotrich ciliates typically retain toxic substances in specialized ejectable organelles, called extrusomes, which are used in predator-prey interactions. In this study, we analysed the chemical defence strategy of the freshwater heterotrich ciliate Stentor polymorphus against the predatory ciliate Coleps hirtus, and the microturbellarian flatworm Stenostomum sphagnetorum. The results showed that S. polymorphus is able to defend itself against these two predators by deploying a mix of bioactive sterols contained in its extrusomes. Sterols were isolated in vivo and characterized by liquid chromatography-mass spectrometry (LC-MS), and nuclear magnetic resonance (NMR), as ergosterol, 7-dehydroporiferasterol, and their two peroxidized analogues. The assessment of the toxicity of ergosterol and ergosterol peroxide against various organisms, indicated that these sterols are essential for the effectiveness of the chemical defence in S. polymorphus.

Evidence for the role of extrusomes in evading attack by the host immune system in a scuticociliate parasite

Like other ciliates, Philasterides dicentrarchi, the scuticociliate parasite of turbot, produces a feeding-only or growing stage called a trophont during its life cycle. Exposure of the trophonts to heat-inactivated serum extracted from the turbot host and containing specific antibodies that induce agglutination/immobilization leads to the production of a mucoid capsule from which the trophonts later emerge. We investigated how these capsules are generated, observing that the mechanism was associated with the process of exocytosis involved in the release of a matrix material from the extrusomes. The extruded material contains mucin-like glycoproteins that were deposited on the surface of the cell and whose expression increased with time of exposure to the heat-inactivated immune serum, at both protein expression and gene expression levels. Stimulation of the trophonts with the immune serum also caused an increase in discharge of the intracellular storage compartments of calcium necessary for the exocytosis processes in the extrusomes. The results obtained suggest that P. dicentrarchi uses the extrusion mechanism to generate a physical barrier protecting the ciliate from attack by soluble factors of the host immune system. Data on the proteins involved and the potential development of molecules that interfere with this exocytic process could contribute to improving the prevention and control of scuticociliatosis in turbot.

Chemical Defense by Erythrolactones in the Euryhaline Ciliated Protist, Pseudokeronopsis erythrina

Pseudokeronopsis erythrina produces three new secondary metabolites, erythrolactones A2, B2 and C2, and their respective sulfate esters (A1, B1, C1), the structures of which have been recently elucidated on the basis of NMR spectroscopic data coupled to high resolution mass measurements (HR-MALDI-TOF). An analysis of the discharge of the protozoan pigment granules revealed that the non-sulfonated erythrolactones are exclusively stored in these cortical organelles, which are commonly used by a number of ciliates as chemical weapons in offense/defense interactions with prey and predators. We evaluated the toxic activity of pigment granule discharge on a panel of free-living ciliates and micro-invertebrates, and the activity of each single purified erythrolactone on three ciliate species. We also observed predator-prey interactions of P. erythrina with unicellular and multicellular predators. Experimental results confirm that only P. erythrina cells with discharged pigment granules were preferentially or exclusively hunted and eaten by at least some of its predators, whereas almost all intact (fully pigmented) cells remained alive. Our results indicate that erythrolactones are very effective as a chemical defense in P. erythrina.