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Distribution of Traction Forces and Intracellular Markers Associated with Shape Changes During Amoeboid Cell Migration
Juan C. Lasheras, Baldomero Alonso-Latorre, Ruedi Meili, Effie Bastounis, Juan C. del Álamo, Richard A. Firtel

During migration, amoeboid cells perform a cycle of quasi-periodic repetitive events (motility cycle). The cell length and the strain energy exchanged with the substrate oscillate in time with an average frequency, f, on top of which are imposed smaller random fluctuations. The fact that a considerable portion of the changes in cell shape are due to periodic repetitive events enables the use of conditional statistics methods to analyze the network of biochemical processes involved in cell motility. Taking advantage of this cyclic nature, we apply Principal Component Analysis (PCA) and phaseaverage statistics to analyze the dominant modes of shape change and their association to the activity and localization of molecular motors. We analyze time-lapse measurements of cell shape, traction forces and fluorescence from green fluorescent protein (GFP) reporters for F-actin in Dictyostelium cells undergoing guided chemotactic migration. Using wild-type cells (wt) as reference, we investigated the contractile and actin crosslinking functions of Myosin II by studying Myosin II heavy chain null mutant cells (mhcA-) and Myosin II essential light chain null cells (mlcE-). We found that the mechanical cycle of generation of traction stresses and cell shape changes remains remarkably similar for all cell lines, although these shape changes are implemented at a slower pace in Myosin II null mutants, probably due to a reduced control on the spatial organization of the traction stresses. We found that wt, mlcE- and mhcA- cells utilize similar modes of shape changes during their motility cycle. The number of these dominant modes of shape changes is surprisingly few, with only three modes accounting for almost 70% of the variance in all cases. These principal shape modes are dilation/elongation, bending, and bulging of the front/back. The first mode is associated with forward pseudopod protrusion, while the second mode, resulting from sideways protrusion/ retraction, is associated to lateral asymmetries in both the cell traction forces and the distribution of F-actin, and is significantly less important in mhcA- cells.

Keywords: Cell Mechanics, Cell migration

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