ABSTRACT
In Paramecium, a mechanical stimulus applied to the posterior portion of the cell causes a transient increase in membrane permeability to potassium ions, transiently rendering the membrane in a hyperpolarized state. Hyperpolarization causes a transient increase in Cyclic adenosine monophosphate (cAMP) concentration in the cilia, resulting in a transient fast-forward swimming of the cell. Schultz and coworkers (1992) reported that a unique adenylate cyclase (AC)-coupled potassium channel is involved in the reaction underlying this response, which is known as the "escape response." However, the AC responsible for this reaction remains to be identified. Moreover, the molecular linkage between mechanoreception and AC activation has not been elucidated adequately. Currently, we can perform an efficient and simple gene-knockdown technique in Paramecium using RNA interference (RNAi). Paramecium is one of the several model organisms for which whole-genome sequences have been elucidated. The RNAi technique can be applied to whole genome sequences derived from the Paramecium database (ParameciumDB) to investigate the types of proteins that elicit specific biological responses and compare them with those of other model organisms. In this review, we describe the applications of the RNAi technique in elucidating the molecular mechanism underlying the escape response and identifying the AC involved in this reaction. The findings of this study highlight the advantages of the RNAi technique and ParameciumDB.
ABSTRACT
Paramecium shows rapid forward swimming due to increased beat frequency of cilia in normal (forward swimming) direction in response to various kinds of stimuli applied to the cell surface that cause K+ -outflow accompanied by a membrane hyperpolarization. Some adenylate cyclases are known to be functional K+ channels in the membrane. Using gene-specific knockdown methods, we examined nine paralogues of adenylate cyclases in P. tetraurelia to ascertain whether and how they are involved in the mechanical stimulus-induced hyperpolarization-coupled acceleration of forward swimming. Results demonstrated that knockdown of the adenylate cyclase 1 (ac1)-gene and 2 (ac2)-gene inhibited the acceleration of forward swimming in response to mechanical stimulation of the cell, whereas that spared the acceleration response to external application of 8-Br-cAMP and dilution of extracellular [K+ ] induced hyperpolarization. Electrophysiological examination of the knockdown cells revealed that the hyperpolarization-activated inward K+ current is smaller than that of a normal cell. Our results suggest that AC1 and AC2 are involved in the mechanical stimulus-induced acceleration of ciliary beat in Paramecium.
