Coupling Step-Wise Motility and Traction Force Patterns in chemotaxing cells

Chemotaxing Dictyostelium discoideum cells migrate in a step-wise fashion characterized by periodic protrusion, contraction, and rear retraction cycles accompanied by distinct traction force patterns. Traction force microscopy reveals two stationary force spots that exchange identity as the cell advances and generate a convergent stress pattern with both axial and lateral components. To investigate the physical origin of these traction patterns, we developed a continuum, phase-field model that couples cytosolic flow, active stresses, and substrate friction within a cell with a deformable morpholgy. The model incorporates protrusive forces at the front, contractile stresses at the rear and sides, and spatially localized adhesive regions that undergo cyclic activation. While this baseline model reproduces persistent motion, it fails to capture the experimentally observed traction force patterns and cell morphology. Guided by new experiments visualizing myosin dynamics, we extended the model to include a localized contractile myosin patch positioned between the two adhesive regions. This modification yields cell shapes, speeds, and convergent traction patterns consistent with experimental measurements. The results demonstrate that a centrally positioned myosin patch is sufficient to generate the step-wise migration cycle and the characteristic convergent traction pattern of Dictyostelium cells, providing a mechanistic link between intracellular contractility, cytosolic flow, and force transmission during amoeboid motility.