Chemoattraction to lysophosphatidic acid does not require a change in membrane potential in Tetrahymena thermophila

LPA (lysophosphatidic acid), a known chemoattractant for many types of eukaryotic cells, is also a reliable chemoattractant for Tetrahymena. Since LPA receptors are GPCRs (G-protein coupled receptors) in many cell types and several putative GPCR sequences can be found in the Tetrahymena Genome Database, we are interested to determine whether similar GPCR pathways can be used for chemosensory transduction in Tetrahymena. To confirm our procedures, we tested the known chemoattractant proteose peptone (at 1.0 mg/ml), which caused hyperpolarization and increased forward swimming speed in Tetrahymena, consistent with the current model for ciliate chemoattraction. Although 10 µM LPA did not produce these same responses, it was still an effective chemoattractant. PTX (pertussis toxin) blocked attraction to both of these compounds, suggesting a possible G-protein involvement in chemoattraction. Both of these chemoattractants also decreased the basal percent of cells showing direction changes [PDC (percent directional change)] and the duration of backward swimming in 0.5 mM Ba2+ (a general excitability assay). LPA probably causes chemoattraction in Tetrahymena by decreasing the basal PDC without changing either membrane potential or swim speed. Since a pertussis-sensitive G-protein might modulate the ciliate voltage-dependent Ca2+ channels, we propose that LPA acts through an uncharacterized GPCR to lower the PDC by decreasing cellular excitability. These combined behavioural and electrophysiological analyses support the novel hypothesis that chemoattraction to some attractants, like LPA, can occur without hyperpolarization and increased swim speed in Tetrahymena.