Title

Biomechanics and molecular mechanisms of amoeboid cell movement

Date of Completion

January 2008

Keywords

Biology, Cell

Degree

Ph.D.

Abstract

Cell movement is a multistep cycle that is essential for many biological events and involves the integration of biochemical and biophysical signals. This process requires the coordination of protrusion and adhesion formation at the cell front with retraction and detachment of adhesions at the rear edge. Many systems, such as fish epithelial keratocytes, leukocytes, fibroblasts and Dictyostelium, have been used with fluorescence microscopy, molecular biology, and biochemistry to provide information about the major proteins and molecular mechanisms underlying these processes. However, our knowledge of the biomechanics of cell migration, i.e. whether these proteins and mechanisms are able to generate the necessary forces for movement, is still limited. ^ Here we have used a gelatin-based traction force assay to detect the traction forces produced by wild-type (NC4A2), myosin II essential light chain mutant (mlcE-), and myosin II heavy chain mutant (mhcA-) Dictyostelium cells, and GFP-myosin II expressing Dictyostelium. We found that for each cell type, the most rapid movement occurred when an asymmetric distribution of traction stress exists, in which forces at the rear are significantly greater than at the front, irrespective of the absolute value of traction stress magnitude. We propose that cell speed for each cell type is determined by the rate and extent to which traction stress asymmetry develops, and can be related to the distinct roles of myosin II motor and actin cross-linking activity. ^ We also gathered evidence that rapid movement in Dictyostelium cells can be regulated by mechano-chemical signaling. By performing high resolution Ca2+ imaging, we found that these cells display transient increases in Ca2+ that are dependent on both intracellular and extracellular calcium and can be inhibited by gadolinium (Gd3+ ). The data provide evidence for the role of stretch activated calcium channels (SACs) in stimulating these Ca2+ transients initiating calcium-dependent retraction. ^ Taken together, these results emphasize the importance of the integration of the biomechanics and cellular pathways in the regulation of rapid movement in Dictyostelium cells. These studies can be applied with other cell types, such as leukocytes, to understand cell motility in healthy and abnormal diseased states and for the development of novel drug treatments. ^