Distribution Of Traction Forces Associated With Shape Changes In Migrating Amoeboid Cells

Abstract

Amoeboid motility results from the cyclic repetition of a repertoire of shape changes leading to periodic oscillations of cell area (motility cycle). This study aimed to identify the dominant shape changes and their association to the regulated activity and localization of molecular motors. For this purpose, we applied Principal Component Analysis (PCA) to time-lapse measurements of cell shape, traction forces and fluorescence from the F-actin-binding protein limEΔcoil-GFP in migrating Dictyostelium cells. This method provides the most significant cell shape changes of the motility cycle, together with maps of the traction forces and F-actin distribution associated with each shape change mode. It also sorts these modes according to their contribution to the variance of the cell area oscillations observed during the motility cycle. Using wild-type cells (wt) as reference, we investigated myosin II activity by studying myosin II null cells (mhcA-) and essential light chain null cells (mlcE-). The results revealed that wt, mlcE- and mhcA- cells implement similar shape changes during their motility cycle, although they are implemented at a slower pace in myosin mutants. The repertoire of shape changes is surprisingly reduced as only three modes are enough to represent 67% of the variance in cell area in wt, mlcE- and mhcA- cells. The three principal shape modes are dilation/elongation, a half-moon shape and bulging of the front/back. The second of these modes represents sideways protrusion/retraction, is associated to lateral asymmetries in the cell traction forces / F-actin distribution, and is significantly less important in mhcA- cells. These results indicate that the mechanical cycle of traction stresses and cell shape remains similar but is slowed down when myosin function is lost, probably due to a reduced control on the spatial organization of the traction stresses.