Membrane control of ciliary movement in ciliates

Abstract

Ciliary movement is generated in the axoneme by the unidirectional sliding of the outer doublets of microtubules produced by the adenosine triphosphate (ATP)-energized dynein arms. It is composed of an effective stroke phase and a passive recovery stroke phase. Two parameters are modulated to determine swimming characteristics of the cell (speed and direction): beat frequency; direction of the effective stroke. They are linked to the internal Ca++ level and to the membrane potential. The membrane governs the internal Ca++ level by regulating Ca++ influx and efflux. It contains voltage-sensitive Ca++ channels through which a passive Ca++ influx, driven by the electrochemical gradient, occurs during step depolarization. The rise of the Ca++ level, up to 6.10-7M triggers ciliary reversal and enhances beat frequency. Ca+ is extruded from cilia by active transport. Ca++ also activates a multistep enzymatic process, the first component of which is a membrane calmodulin-dependent guanylate cyclase. cGMP interacts with Ca++ to modulate the parameters of the ciliary beat. The phosphorylation-dephosphorylation cycle of axoneme and membrane proteins seems to play a major role in controlling ciliary movement. Hyperpolarization of the membrane enhances beat frequency by an unknown mechanism. It could be a modification of the ratio of axonemal bound Ca++ and Mg++, or activation by cyclic adenosine monophosphate (cAMP) produced by a membrane adenylate cyclase. The ciliary membrane behaves as a receptor able to detect modifications of external parameters, and as a transductor transmitting the detected signal by a second or third messengers toward the interior of the cilia. These messengers. acting at different levels, modulate the parameters of the mechanism that generates ciliary movement.