Inner arm dynein 1 is essential for Ca++-dependent ciliary reversals in Tetrahymena thermophila

Cilia in many organisms undergo a phenomenon called ciliary reversal during which the cilia reverse the beat direction, and the cell swims backwards. Ciliary reversal is typically caused by a depolarizing stimulus that ultimately leads to a rise in intraciliary Ca++ levels. It is this increase in intraciliary Ca++ that triggers ciliary reversal. However, the mechanism by which an increase in intraciliary Ca++ causes ciliary reversal is not known. We have previously mutated the DYH6 gene of Tetrahymena thermophila by targeted gene knockout and shown that the knockout mutants (KO6 mutants) are missing inner arm dynein 1 (I1). In this study, we show that KO6 mutants do not swim backward in response to depolarizing stimuli. In addition to being unable to swim backwards, KO6 mutants swim forward at approximately one half the velocity of wild-type cells. However, the ciliary beat frequency in KO6 mutants is indistinguishable from that of wild-type cells, suggesting that the slow forward swimming of KO6 mutants is caused by an altered waveform rather than an altered beat frequency. Live KO6 cells are also able to increase and decrease their swim speeds in response to stimuli, suggesting that some aspects of their swim speed regulation mechanisms are intact. Detergent-permeabilized KO6 mutants fail to undergo Ca++-dependent ciliary reversals and do not show Ca++-dependent changes in swim speed after MgATP reactivation, indicating that the axonemal machinery required for these responses is insensitive to Ca++ in KO6 mutants. We conclude that Tetrahymena inner arm dynein 1 is not only an essential part of the Ca++-dependent ciliary reversal mechanism but it also may contribute to Ca++-dependent changes in swim speed and to the formation of normal waveform during forward swimming.