Leukocytes and
tumor cells migrate via rapid shape changes in an amoeboid-like manner, distinct from mesenchymal cells such as fibroblasts. However, the mechanisms of how rapid shape changes are caused and how they lead to migration in the amoeboid mode are still unclear. In this study, we confined differentiated human promyelocytic
leukemia cells between opposing surfaces of two pieces of
polyacrylamide gels and characterized the mechanics of
fibronectin-dependent mesenchymal versus
fibronectin-independent amoeboid migration. On
fibronectin-coated
gels, the cells form lamellipodia and migrate mesenchymally. Whereas in the absence of cell-substrate adhesions through
fibronectin, the same cells migrate by producing
blebs and "chimneying" between the gel sheets. To identify the orientation and to quantify the magnitude of the
traction forces, we found by
traction force microscopy that expanding
blebs push into the
gels and generate anchoring stresses whose magnitude increases with decreasing gap size while the resulting migration speed is highest at an intermediate gap size. To understand why there exists such an optimal gap size for migration, we developed a computational model and showed that the chimneying speed depends on both the magnitude of intracellular pressure as well as the distribution of
blebs around the cell periphery. The model also predicts that the optimal gap size increases with weakening cell membrane to actin cortex adhesion strength. We verified this prediction experimentally, by weakening the membrane-cortex adhesion strength using the
ezrin inhibitor,
baicalein. Thus, the chimneying mode of amoeboid migration requires a balance between intracellular pressure and membrane-cortex adhesion strength.