In this proposal the PIs will investigate how highly motile cells such as the model organism Dictyostelium discoideum determine its forward direction of motion. The PIs will investigate this question taking into account all information about their surrounding and will integrate signals of different type - surface topography, mechanical force exerted by their surrounding as well as biochemical signals. The PIs will focus on the study of topography, mechanical and biochemical signals together in the same cell. Their goal is to develop quantitative experiments amenable to comparison to simple, tractable models for the coupling between the mechanical and biochemical pathways of the cell. They will apply small local perturbations to both biochemical and mechanical pathways using functionalized beads held by holographic laser tweezers, and measure both putative upstream and downstream responses of the cell. In addition, they will fabricate surfaces of controlled topography to generate large scale perturbations. The measured cell response will be compared with simulations of reaction diffusion based model systems of the key signaling molecules exposed to similar perturbations. Such perturbation analysis is sensitive to the structure of the signaling pathways, and will allow the validation of the coupled topographical, mechanical and biochemical pathways. This work will train quantitative scientists for interdisciplinary work at the interface of physics and biology. As part of this project the PIs will develop demonstration materials on cell motility. These demos will be used in undergraduate courses, university open houses, and public lectures. Undergraduate students will be involved in the proposed research. The PIs will actively recruit and aid in retention of underrepresented minorities through mentoring of local high school students, lab tours, and programs supported by the College of Math and Physical Sciences. The understanding of the integration of the mechanical and biochemical signals will help in designing better drugs and new approaches to control cell differentiation, tissue formation, organ development, or other processes where biochemical and mechanical signals are tightly coupled.
The controlled migration of cells is key to life – from development to immune response to cancer. Migration is guided by a variety of cues from chemical signals to mechanical cues to simply cues based on the topography of the surface the cells are moving on. Our first outcome was the development of a tool to analyze random and directed motion of cells by automatically tracking large numbers of cells. The first accompanying image shows an example of cell traces of hundreds of cells that move randomly without directional cues. The main finding was that cells exhibit persistence in their direction of motion, and that chemical signals do not lead to straighter cell tracks or faster cell motion. This work was reported in an article in the Journal of Cell Science in 2010. Based on these findings our work focused on elucidating how cells are able to maintain speed and directional persistence. We developed new quantitative assays for measuring cell shape dynamics and reported the results in two publications. These papers demonstrated that cell protrusions do not simply push out and retract, but instead travel along the surface in a wave-like way. Such protrusion waves that travel from the leading edge to the back of the cells can best be seen when parametrizing cell shapes based on their local curvature. Regularly repeating waves are a natural method for cells to maintain direction and move at constant speed under a wide range of conditions. This wave like migration mechanism has significant consequences for how a cell responds to the topography of the surface. We discovered that the wave-like character of protrusions is particularly prominent when cells are protruding significantly over the edge of a boundary on the surface. Specifically we guided cells over the edge of microfabricated cliffs using a chemotactic signal. Rather than falling off the edge of the cliff cells - protruding with a vast majority of their body over the edge of the cliff - hang onto the edge and swing significantly with pronounced internal waves. These waves appear to help cells in reestablishing a new migration direction with better surface contact.
