The molecular mechanisms of intraciliary energy-supply system and ciliary movements revealed by the studies on Paramecium cilia

Abstract

Most eukaryotes have motile cilia/flagella as cell organelles for swimming, locomotion, or generating extracellular fluid flow. Recent studies have revealed that dysfunctions in ciliary/flagellar motility engender human disease. Most motile cilia/flagella possess the inner structure called the axoneme with “9 + 2” pattern, in which the nine doublet microtubules surround two central singlet microtubules. This structural pattern is evolutionally conserved. The axoneme comprises many structural components aligned on the microtubules, including axonemal dyneins, radial spokes, and projections on the central pair microtubules. Ciliary/flagellar movements are generated by dynein-driven microtubule sliding, and are controlled by second messengers such as Ca²⁺ and cAMP. However, molecular mechanisms of ciliary/flagellar movements in response to Ca²⁺ and cAMP, and the individual roles of the axonemal components in the mechanisms remain unclear. Furthermore, mechanisms by which the energy is supplied for ciliary/flagellar movement are not well defined. Paramecium has long been used as a model organism for studying ciliary motility, because of its valuable experimental systems. For example, cell excitement can be analyzed elecrophysiologically, and cilia on demembranated cell models and cortical sheets can be reactivated in vitro. Furthermore, protocols for RNAi depletion of specific genes, as well as the genome and the ciliary proteome databases, became available recently. This review describes recent studies on molecular mechanisms of ciliary movements in Paramecium, highlighting intraciliary energy-supply systems and regulatory systems by Ca²⁺ and cAMP.