Effect of femtomolar concentrations of hormones on insulin binding by Tetrahymena, as a function of time

Provocation of life functions at a unicellular eukaryote level by extremely low doses of mammalian hormones: Evidences of hormesis

Hormones, characteristic to higher ranked animals, are synthesized, stored, and secreted by unicellular eukaryote animals. The unicells also have receptors for recognizing these materials and transmit the message into the cells for provoking response. The hormones are effective in very low concentrations (down to 10-21 M) and opposite effects of lower and higher concentrations can be observed. However, sometimes linear concentration effects can be found, which means that hormesis exists, nevertheless uncertain, as it is in the phase of formation (evolutionary experimentation). Hormesis, by transformation (fixation) of cytoplasmic receptor-like membrane components to receptors in the presence of the given hormone, likely helps the development of unicellular endocrine character and by this the evolution of endocrine system. The effect by extremely low concentrations of hormones had been forced by the watery way of unicellular life, which could establish the physiological concentrations of hormones in the blood of higher ranked animals. This means that hormetic low doses are the normal, effective concentrations and the high concentrations are artificial, consequently could be dangerous.

Entry and killing of Tetrahymena thermophila by bacterially produced Shiga toxin

Phage-encoded Shiga toxin (Stx) acts as a bacterial defense against the eukaryotic predator Tetrahymena thermophila. It is unknown how Stx enters Tetrahymena protozoa or how it kills them. Tetrahymena protozoa are phagocytotic; hence, Stx could gain entry to the cytoplasm through the oral apparatus or via endocytosis. We find that Stx2 can kill T. thermophila protozoa that lack an oral apparatus, indicating that Stx2 can enter these cells via endocytosis. As opposed to the lack of effect on mammalian phagocytes, Stx2 produced by bacteria encapsulated within phagocytotic vesicles is also capable of killing Tetrahymena. Addition of an excess of the carbohydrate binding subunits of Stx2 (StxB) and/or ricin (ricin B) blocks Stx2 cytotoxicity. Thus, regardless of whether Stx2 enters the cytoplasm by endocytosis or from the phagocytotic vesicle, this transport is mediated by a putative glycoconjugate receptor. Bacteriophage-mediated lysis of Stx-encoding bacteria is necessary for Stx toxicity in Tetrahymena; i.e., toxin released as a consequence of digestion of bacteria by Tetrahymena is harmless to the cell. This finding provides a rationale as to why the genes encoding Stx are found almost exclusively on bacteriophages; Stx must be released from the bacteria prior to the digestion of the cell, or it will not be able to exert its cytotoxic effect. It also suggests a reason why other bacterial exotoxins are also found only on temperate bacteriophages. Incubation of Tetrahymena with purified Stx2 decreases total protein synthesis. This finding indicates that, similar to mammalian cells, Stx2 kills Tetrahymena by inactivating its ribosomes.

Tetrahymena is a bacterial predator and a model for mammalian phagocytosis and intracellular vesicular trafficking. Phage-encoded exotoxins apparently have evolved for the purpose of bacterial antipredator defense. These exotoxins kill mammalian cells by inactivating universally conserved factors and/or pathways. Tetrahymena and susceptible mammalian cells are killed when exposed to bacteriophage-encoded Shiga toxin (Stx). Stx toxicity in mammalian cells requires Stx binding to the globotriaosyl ceramide (Gb3) receptor, followed by receptor-mediated endocytosis (RME). We show that, similar to mammalian cells, internalized Stx inhibits protein synthesis in Tetrahymena. Although Tetrahymena lacks Gb3, our results suggest that the cytotoxic effect of Stx on Tetrahymena is apparently mediated by a receptor, thereby arguing for the existence of RME in Tetrahymena. As opposed to the case with mammalian phagocytes, Stx produced by bacteria inside Tetrahymena is cytotoxic, suggesting that these cells may represent a “missing link” between unicellular eukaryotic bacterial predators and phagocytotic mammalian cells.

The hormonal system of the unicellular Tetrahymena: a review with evolutionary aspects

The unicellular ciliate, Tetrahymena has receptors for hormones of the higher ranked animals, these hormones (e.g. insulin, triiodothyronine, ACTH, histamine, etc.) are also produced by it and it has signal pathways and second messengers for signal transmission. These components are chemically and functionally very similar to that of mammalian ones. The exogenously given hormones regulate different functions, as movement, phagocytosis, chemotaxis, cell growth, secretion, excretion and the cells' own hormone production. The receptors are extremely sensitive, certain hormones are sensed (and response is provoked) at 10-21 M concentration, which makes likely that the function could work by the effect of hormones produced by the Tetrahymena itself. The signal reception is selective, it can differentiate between closely related hormones. The review is listing the hormones produced by the Tetrahymena, the receptors which can receive signals and the signal pathways and second messengers as well, as the known effects of mammalian hormones to the life functions of Tetrahymena. The possible and justified role of hormonal system in the Tetrahymena as a single cell and inside the Tetrahymena population, as a community is discussed. The unicellular hormonal system and mammalian endocrine system are compared and evolutionary conclusions are drawn.