Motile statocyst cilia transmit rather than directly transduce mechanical stimuli

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Abstract
The role of motile cilia in mechanotransduction by statocysts of the nudibranch mollusk Hermissenda crassicornis. Movement of the cilia that experience the weight of statoconia causes increased variance of voltage noise and membrane depolarization of the statocyst hair cell. Two complementary approaches were used to immobilize the cilia. Vanadate was iontophoretically injected into hair cells. This reversible inhibitor of dynein cross-bridge cycling initially caused the cilia to lose their normally upright, rigid, vibratile form and to assume a more classic, pliable beat pattern. Voltage noise decreased as the cilia slowed and bent more extremely, nearly disappearing as motility was lost. When the intracellular vanadate concentration approached 10-5 M, the cilia were arrested in an effective stroke against the cell membrane. The cell no longer depolarized upon gravitational or local mechanical stimulation. Rapid reversal of ciliary inhibition by norepinephrine or slow reversal with time restored both the voltage noise and depolarization response. Cilia were rendered rigid and upright by covalent cross-linkage of the membrane sleeve to the 9 + 2 axoneme using the photoactivated, lipophilic, bifunctional agent 4,4''-dithiobisphenyl azide. In the initial stages of cross-linkage, the cilia remained vibratile but slowed and moved through wider excursions. Voltage noise decreased in frequency but increased in amplitude. When the cilia were fully arrested, voltage noise was minimized while the resting potential and membrane resistance remained essentially constant. Mechanical stimulation of the rigid cilia, normal to the cell membrane, elicited a generator potential of the same amplitude but of greater duration than before treatment. Because cilia that are partially arrested by vanadate undergo increased bending, although the hair cell shows decreased noise, neither the axoneme nor the ciliary membrane proper would appear to be sites of direct transduction. In cells with beating but stiffened cilia, voltage noise becomes amplified, implying an increased efficiency of transduction. Active but rigid flexure of the axoneme may be involved in amplification and continuous signal detection. The basal insertion area is the most likely transduction site, being the terminal leverage point through which force is applied to the plasma membrane via the flexing ciliary shaft.