Chemotactic range fitting

Abstract: Migratory responses of cells belong to the most basic cell physiological activities of evolution of intercellular communication. In this process we distinguish two main phases: (i) in the first period a large set of “pure” physical effects were only present therefore several very essential locomotor responses developed to these physical factors; (ii) much later the complexity of chemical and biological signals appeared and these responses (e.g. chemotaxis) were/are far enough to perform the project. In the present chapter our objective was to discuss the following problems with a special respect to the chemotactic activity of the eukaryotic ciliate Tetrahymena pyriformis, a well known model-cell of chemotaxis: (i) characterization of chemotactic ligands (inorganic, amino acids, oligo- and polypeptides, lipids and carbohydrates) of Tetrahymena; (ii) signalling mechanisms of Tetrahymena used in chemotaxis (ligand-‘chemotaxis receptor’ relations, second messengers, kinase-cascades); (iii) significance of paracrine and autocrine activity in chemotaxis; (iv) special phenomena based on chemotaxis (chemotactic range fitting; chemotactic selection; chemotactic drug targeting). Practical approaches (test systems, bioreactors) based on protozoan motility (e.g. bioindicator of the freshwater quality and production organic compounds on industrial level) are also discussed.

Abstract: Tetrahymena pyriformis is one of the most frequently used unicellular, eukaryotic models in molecular and cell biology. The suitability of the model is supported by several molecular level homologies to higher ranked vertebrates, like (i) identical and inducible receptor pools of the surface membrane (e.g., insulin receptor) and cytoplasm (e.g., steroids); (ii) similar elements of signaling pathways (e.g., cyclic nucleotide phosphates, Ca 2+ -calmodulin system, phosphatidylinositol metabolism); (iii) homologous cell physiological responsiveness (e.g., chemotaxis, phagocytosis, proliferation, and metabolic processes) induced by natural ligands (e.g., peptide hormones, chemokines, drugs, and artificial signals); (iv) high-level sensitivity and molecular level distinctiveness are shown by diverse activity or structurally closely related molecules (e.g., bradykinins, crystalline, and amorphous insulins). Practical properties like the size of the cells (20 × 50 μm), short generation time (about 150 min), chemically defined media, fast and easy handling as well as the potential of accurate application of test substances all support using Tetrahymena cells not only as an advantageous model in biology and medicine but also as a good candidate of microbiorobotics too. Motion itself and the wide range of random and vectorial forms of locomotion represent one of the most essential and physiological responses of T. pyriformis . However, these cells – as regular ciliates – show no cell adhesion. The axis of “cell adhesion – chemotaxis – phagocytosis” is still observed as creeping on solid surfaces represent a functionally analogous process to adhesion, which provides the possibility of reorientation followed by chemotaxis which directs cells to nourishment and finished by phagocytosis, the target reaction of chemotaxis. The structural backgrounds of motion in Tetrahymena are well detectable both on microscopic and molecular biological levels. In small scale, about 600 cilia beating on the surface of the cells cover the force generation requirements, while coordinated interaction of several families of cytoskeletal proteins (e.g., tubulins, nexins, dyneins, etc.) support the structural basis of migration. The changes in swimming behavior of our model provide a good index to be analyzed. Computer-based path evaluation of consecutive swimming patterns (run, rest, and turn) and mathematical simulations of chemoattractant induced, concentration dependent migration (e.g., time-delayed model) are sensible and high-tech investigations of the classic phenomena of motion (chemotaxis, chemokinesis, and necrotaxis), while the model is also a proper one to evaluate a list of other migratory responses elicited by transient or definitive shifts of environmental stimuli (e.g., phototaxis, magnetotaxis, galvanotaxis). It is obvious that majority of the above listed migratory responses are elicited via specific signaling pathways. Investigation of these processes was more detailed in the case of chemotaxis, where groups of professional signal molecules were identified as strong chemoattractants (e.g., fMLF and IL-8) or chemorepellents (e.g., serotonin); however, several other non-professional chemotaxis inducer molecules are also capable of eliciting strong migratory responses (e.g., vasoactive peptides, melatonin). It is worth mentioning that classes of membrane-receptors identical to vertebrate ones were demonstrated by tools of molecular genetics as significant ones in evolving chemotactic responses of Tetrahymena and more intracellular signaling pathways (e.g., PI3K pathway) were also described as triggered ones. On the basis of processes described above, more migration-specific phenomena like (i) chemotactic range fitting, (ii) chemotactic selection, and (iii) chemotactic drug-targeting were also reported using Tetrahymena chemotaxis as a referent one. In each case, the increased ability for molecular (membrane) level discrimination of the individual cells provides the possibility to gain distinct groups of cells with increased responsiveness to a specific concentration of chemicals. However, the possibility to form long-term sub-populations of cells by the help of their migratory responses shows that genetic/nuclear level mechanisms have also significant role in development of migratory responsiveness of cells as individual ones and in populations, too. A theoretically underlined aspect of experiments on Tetrahymena motion is that, due to its basic cell physiological properties, investigations of chemotaxis provide data significant in respect of phylogenetics. Evaluation of chemotactic properties of amino acids and comparison of these data with consensus sequences describing order of appearance of amino acids in the primordial soup show that the first ones in molecular phylogeny (e.g., G, E, P) are chemoattractant while the latest ones (e.g., W, F, Y) are chemorepellent.