Date of Completion

Spring 5-19-2021

Thesis Advisor(s)

David Knecht

Honors Major

Biological Sciences

Disciplines

Biology | Cell Anatomy | Cell Biology

Abstract

Many types of cells crawl on solid surfaces by amoeboid locomotion. Membrane protrusions, such as pseudopods, are generated by outward directed forces and the cell body retracts to allow the cell to migrate on the surface. The movement can be random, or can be directional in response to diffusible (chemotaxis) or surface associated signals (haptotaxis). It has been known for some time that chemotactic signals also lead to an increase in overall cell speed (chemokinesis), however the mechanism of this speed increase is unknown. This project investigates the cellular signaling pathways involved in the regulation of cell speed by ligands of cell surface receptors in the soil amoeba Dictyostelium discoideum. The mechanisms and pathways that allow Dictyostelium to polarize and move in response to diffusional gradients of folic acid in vegetative cells and cAMP in developing cells has been extensively investigated. A chemokinesis assay was developed that measures the speed of cells before and after uniform stimulation with folic acid. This assay was used to measure the response of wild-type Ax2 cells, cells treated with chemical inhibitors, and genetic knockout mutants lacking critical components of the cAMP chemotactic signaling pathways. Mutants lacking the IP3 receptor for calcium release (iplA) and phospholipase A2 (pla2) had robust chemokinetic speed increases in response to folic acid stimulation, indicating that the signaling pathways related to changes in cytoplasmic calcium concentration were not required for chemokinesis. PI3K-null and PTEN-null mutants both had a reduced response to folic acid, indicating these gene products may play some role. Plc-null and Gβ-null cells both had no measurable chemokinetic response, indicating that folic acid chemokinesis is indeed GPCR-dependent, and that PLC activity, though not essential for chemotaxis to cAMP, is essential for upregulation of speed in cells stimulated with folic acid. The data shows that chemokinesis is a distinct process with its own regulatory framework. Further work will allow us to define how the speed of cell movement is controlled.

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