How Ciliated Protists Survive by Cysts: Some Key Points During Encystment and Excystment

Forming cysts is a common and important bionomic strategy for microorganisms to persist in harsh environments. In ciliated protists, many species have been reported to form cysts when facing unfavorable conditions. Despite traditional studies on the morphological features of cysts and the chemical composition of cyst wall, recent research has focused more on the molecular mechanisms of encystment. The present work reviews studies on developmental features and molecular information of resting cysts in ciliates, and pays more attention to the following questions: what are the inducing factors of encystment and excystment? How does the cell change morphologically during these dynamic processes? And what molecular mechanisms underlie those changes? We also present and summarize the characteristics of cysts from diverse ciliate lineages in a phylogenetic framework, aiming to provide new perspectives for studies on adaptive evolution of unicellular eukaryotes.

Signaling in temperature-induced resting cyst formation in the ciliated protozoan Colpoda cucullus

The terrestrial ciliated protozoan Colpoda cucullus inhabits soil. When the habitat conditions become unfavorable, the vegetative cells of C. cucullus quickly transform into resting cysts. C. cucullus culture is established in our laboratory, and encystment is routinely induced by the addition of Ca²⁺ to overpopulated vegetative cells. However, an increase in Ca²⁺ concentration and overpopulation of vegetative cells do not always occur in natural. We investigated the effect of temperature and found that cyst formation was induced by a rapid increase of 5 °C within 2 min but not by a decrease. Moreover, an increase in intracellular Ca²⁺ concentrations is essential, but Ca²⁺ inflow does not necessarily occur during encystment. Ca²⁺ image analysis showed that Ca²⁺ is stored in vesicular structures and released into the cytoplasm within 60 s after temperature stimulation. Multiple signaling pathways are activated after the release of Ca²⁺ from vesicles, and cAMP is a candidate second messenger with a crucial role in the process of temperature-induced encystment. Further studies are needed to clarify the mechanism underlying the sensing of temperature and release of Ca²⁺ from vesicles.

Early signaling pathways mediating dormant cyst formation in terrestrial unicellular eukaryote Colpoda

Dormant (resting) cyst formation (encystment) in unicellular eukaryotes is the process of a large-scale digestion of vegetative cell structures and reconstruction into the dormant form, which is performed by cell signaling pathways accompanied by up- or down-regulation of protein expression, and by posttranslational modification such as phosphorylation. In this review, the author describes the morphogenetic events during encystment of Colpoda and the early molecular events in the Ca2+/calmodulin-triggered signaling pathways for encystment, based mainly on our research results of the past 10 years; especially, the author discusses the role of c-AMP dependently phosphorylated proteins (ribosomal P0 protein, ribosomal S5 protein, Rieske iron-sulfur protein, actin and histone H4) and encystment-dependently upregulated (EF-1α-HSP60, actin-related protein) and downregulated proteins (ATP synthase β-chain). In addition, the roles of AMPK, a key molecule in the signaling pathways leading to Colpoda encystment, and differentially expressed genes and proteins during encystment of other ciliates are discussed.