Investigating the Force-transduction Mechanism Driving Gravi-kinesis in a Swimming Ciliated Protist

Ciliated protists often exhibit negative gravi-kinesis. Paramecium caudatum, for example, propel themselves more strongly when swimming against gravity and less when swimming with gravity. While the difference between up and down propulsions scales linearly with the apparent weight (w), it is not clearly established how paramecia sense this very small force (w ~ 100 pN in water). According to the prevailing mechanism, the full body statocyst model (FBS), the cytoplasm acts like a statocyst that distorts the cell membrane with the force w. The distortions, depend on orientation altering ion channel conduction to change the cell membrane potential, which controls ciliary beating. Concerns about whether the w-induced stresses are sufficiently large to overcome thermal noise have led us to consider an alternative model.  In the alternative Sedimentation Stress Sensing Model (SSS), sedimentation-induced shear stresses (which are also orientation-dependent and proportional to w) cause the changes in motile cilia beating.*  We will describe how studies of gravi-kinesis in confining geometries are promising for discerning between the FBS and SSS models, as well as available results.

*D. B. Hill et al., Biophysical Journal 98, 57 (2010).