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Signalling in Ciliates : Long- and Short-Range Signals and Molecular

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Signalling in Ciliates : Long- and Short-Range Signals and Molecular Erschienen in: Biological Reviews ; 92 (2017), 1. - S. 60-107 https://dx.doi.org/10.1111/brv.12218 60 Signalling in ciliates: long- and short-range signals and molecular determinants for cellular dynamics Helmut Plattner∗ Department of Biology, University of Konstanz, PO Box M625, 78457 Konstanz, Germany ABSTRACT In ciliates, unicellular representatives of the bikont branch of evolution, inter- and intracellular signalling pathways have been analysed mainly in Paramecium tetraurelia, Paramecium multimicronucleatum and Tetrahymena thermophila and in part also in Euplotes raikovi. Electrophysiology of ciliary activity in Paramecium spp. is a most successful example. Established signalling mechanisms include plasmalemmal ion channels, recently established intracellular Ca2+-release channels, as well as signalling by cyclic nucleotides and Ca2+.Ca2+-binding proteins (calmodulin, centrin) and Ca2+-activated enzymes (kinases, phosphatases) are involved. Many organelles are endowed with specific molecules cooperating in signalling for intracellular transport and targeted delivery. Among them are recently specified soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), monomeric GTPases, H+-ATPase/pump, actin, etc. Little specification is available for some key signal transducers including mechanosensitive Ca2+-channels, exocyst complexes and Ca2+-sensor proteins for vesicle–vesicle/membrane interactions. The existence of heterotrimeric G-proteins and of G-protein-coupled receptors is still under considerable debate. Serine/threonine kinases dominate by far over tyrosine kinases (some predicted by phosphoproteomic analyses). Besides short-range signalling, long-range signalling also exists, e.g. as firmly installed microtubular transport rails within epigenetically determined patterns, thus facilitating targeted vesicle delivery. By envisaging widely different phenomena of signalling and subcellular dynamics, it will be shown (i) that important pathways of signalling and cellular dynamics are established already in ciliates, (ii) that some mechanisms diverge from higher eukaryotes and (iii) that considerable uncertainties still exist about some essential aspects of signalling. Key words: ciliates, Euplotes, Paramecium, protozoa, signalling, Tetrahymena, traffic, transport. CONTENTS I. Introduction – an overview .............................................................................. 61 (1) Principles of signalling ................................................................................ 61 (2) Background information about signalling from ciliates to man ....................................... 62 (3) Some characteristics of the ciliate species under scrutiny ............................................. 64 (4) Why study signalling in ciliates? ...................................................................... 64 II. Subcompartmentalisation of signalling pathways ......................................................... 64 (1) Relevance of local Ca2+ signalling ................................................................... 65 (2) Local Ca2+ signalling in Paramecium .................................................................. 65 III. Ciliary beat activity ....................................................................................... 65 (1) Aspects pertinent to biogenesis of cilia ................................................................ 66 (2) Functional aspects of Ca2+ increase in cilia .......................................................... 66 (a) Hyperpolarisation response ....................................................................... 67 (b) Depolarisation response .......................................................................... 68 (3) Subsequent regulation steps in cilia ................................................................... 68 (4) Organismal aspects ................................................................................... 68 *Address for correspondence (Tel: (0)7531 88 2228; Fax: (0)7531 88 2168; E-mail: [email protected]). Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-0-305185 61 IV. Organellar trafficking signals – general rules and implications for vesicle transport in ciliates ............ 69 (1) Support by microtubules ............................................................................. 69 (2) Molecular signals ..................................................................................... 70 (3) Support by F-actin, SNAREs and Ca2+-dependent proteins ......................................... 72 V. The phagocytotic cycle ................................................................................... 73 (1) Role of H+-ATPase, GTPases and SNARE proteins ................................................. 73 (2) Role of actin, Ca2+ and Ca2+-binding proteins ...................................................... 74 (3) Proteins at the cytoproct .............................................................................. 75 VI. Signalling in the contractile vacuole complex ............................................................ 75 (1) Signals for contractile vacuole biogenesis ............................................................. 75 (2) Signalling in the mature contractile vacuole complex ................................................ 76 VII. Additional determinants for vesicle trafficking in ciliates ................................................. 77 VIII. Why this multiplicity of Ca2+-release channels (CRCs) in ciliates? ....................................... 78 (1) InsP3- and RyR-type CRCs .......................................................................... 78 (2) Additional types of CRCs ............................................................................ 79 IX. Protein phosphorylation involved in signalling ........................................................... 80 (1) Kinases and the phosphoproteome ................................................................... 81 (2) Tyrosyl phosphorylation and mitogen-activated protein kinases in ciliates? .......................... 81 X. Surface receptors for intracellular signalling .............................................................. 82 (1) Different types of surface receptors ................................................................... 82 (2) G-protein-coupled receptors in ciliates? .............................................................. 84 XI. Graviperception and gravikinesis/gravitaxis ............................................................. 85 XII. Metabolic aspects of signalling in ciliates ................................................................. 86 (1) Ca2+-flux, ATP decay and ATP recovery during synchronous trichocyst exocytosis ................. 86 (2) ATP decay – unlikely due to second messenger formation, SNARE rearrangement or exocytosis-coupled endocytosis ....................................................................... 87 (3) ATP decay due to superimposed ciliary reversal? .................................................... 88 (4) Effects of massive exocytosis on ATP synthesis ....................................................... 88 XIII. Feedback from cytoplasmic functional states, from mitochondria and from endosymbionts to the macronucleus ............................................................................................ 89 (1) Feedback from the cytoplasm to the macronucleus ................................................... 90 (2) Feedback from mitochondria and endosymbionts .................................................... 90 (a) Mitochondria ..................................................................................... 90 (b) Symbionts ........................................................................................ 91 XIV. Downregulation of the most common activation mechanisms ............................................ 91 XV. Epigenetic signalling, encystment and ‘programmed nuclear death’ in ciliates ........................... 92 (1) Feedback from intracellular epigenetic signals ........................................................ 92 (2) Feedback from extracellular epigenetic signals, encystment and ‘programmed nuclear death’ ....... 93 (a) Epigenetics ........................................................................................ 93 (b)Cysts .............................................................................................. 93 (c) Programmed nuclear death ....................................................................... 94 XVI. Conclusions .............................................................................................. 94 XVII. Acknowledgements ....................................................................................... 95 XVIII. References ................................................................................................ 95 XIX. APPENDIX: ............................................................................................. 106 I. INTRODUCTION – AN OVERVIEW may mediate signals via step-by-step processes. In its broadest sense, therefore, signalling includes not only primary signals (1) Principles of signalling and second messengers, but also molecular components on As cells became ‘eukaryotic’ by sequestration of an increased membrane surfaces which are required as short-range signals genome in a nucleus, a dialogue between the nucleus and for recognition and specific organelle interaction,
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