David L Nelson

We study signal transductions in the protozoan Paramecium, using biochemical, genetic, molecular genetic, cell biological, and electrophysiological tools. Paramecium propels itself through the surrounding medium by the coordinated beating of the cilia that cover its surface. In response to chemical, mechanical, and thermal stimuli the cell changes direction or speed by changing the orientation or frequency of its ciliary beat. Stimuli initially register as a change in membrane potential, and then Ca²⁺, cyclic AMP, and cyclic GMP serve as second messengers that regulate the ciliary beat. We have purified the protein kinases of Paramecium that are regulated by these second messengers, and have cloned the genes that encode them, and are now using them to study the mechanism by which ciliary motion is controlled. We are characterizing ciliary proteins that are phosphorylated by these kinases, and exploring their role in the ciliary beat.

One of the extracellular compounds to which Paramecium responds is GTP, apparently through a plasma membrane receptor that may be related to the purinoceptors found in many animal cells that respond to extracellular ATP. Extracellular GTP elicits slow oscillations in the membrane potential of Paramecium, which correspond to alternating periods of forward and backward swimming. Mutants specifically defective in their behavioral response to GTP also lack this membrane response to GTP. The mechanism by which the extracellular signal (GTP) is transduced into a change in swimming behavior are central interests of the laboratory. The mechanisms of intracellular sequestration of Ca²⁺ are also under investigation by genetic and biochemical approaches. As an experimental organism, Paramecium offers the advantages of large size (easy electrophysiology and microinjection), good classical genetics, and easily quantifiable behavior in a unicellular organism that can be cultured in the laboratory. The long-term goal of this research is to describe a behavioral response in molecular terms, from reception of stimulus to change in ciliary beat.