Mutations in genes encoding inner arm dynein heavy chains in Tetrahymena thermophila lead to axonemal hypersensitivity to Ca2+.

Calcium-dependent ciliary reversals are seen in ciliated protozoans such as Tetrahymena in response to depolarizing stimuli, but the axonemal mechanisms responsible for this response are not well understood. The model is that the outer arm dyneins (OADs) control the beating frequency while the inner arm dyneins (IADs) regulate ciliary waveform. Since ciliary reversal is a type of waveform change, the model would predict that IAD mutations could affect ciliary reversal. We have used gene disruption techniques to generate several behavioral mutants of Tetrahymena with functional disruptions of various IADs. One such mutant, called KO-6, is missing I1 (the two-headed IAD) and is unable to show ciliary reversals in response to any stimuli due to a loss of axonemal Ca2+ sensitivity [Eur J Cell Biol 80 (2001) 486-497; Cell Motil Cytoskeleton 53 (2002) 281-288.]. In contrast, disruption of 3 one-headed IADs [Liu et al., Cell Motil Cytoskeleton 59 (2004), 201-214] produced mutants, which showed over-responsiveness in bioassays measuring either their depolarization-induced avoiding reactions (AR) in Na+ and Ba2+ solutions or their duration of backward swimming (continuous ciliary reversal or CCR) in K+ solutions. Detergent-extracted and reactivated mutants also showed increased probabilities of CCR at lower Ca2+ concentrations suggesting that the behavioral over-responsiveness of these three mutants in vivo is due to increased axonemal Ca2+ sensitivity. Our data suggest the possibility that the one-headed IADs and the two-headed IAD act antagonistically in vivo and that loss of any one of the one-headed IADs leads to behavioral over-responsiveness due to less resistance to I1-induced reversals.