Tracking traction in amoeboid cells

Bastounis et al. examine how chemotaxing Dictyostelium cells generate traction.

Dictyostelium cells move rapidly toward a chemoattractant using an amoeboid mode of migration in which cycles of actin polymerization and contraction drive protrusion at the cell front and retraction of the cell’s rear. The resulting forces must be transmitted to the underlying substrate in order to move the cell forward. In contrast to slower-moving fibroblasts, however, which contact the extracellular matrix via large, stable focal adhesions, Dictyostelium cells form small, transient attachments to their substrate. Bastounis et al. used Fourier traction force microscopy to measure where and when Dictyostelium cells transmit force to their surroundings.

Chemotaxing Dictyostelium cells tended to transmit force at 2–3 stable sites aligned along the cell’s front-to-back axis. The researchers named these sites “traction adhesions.” Cells formed new traction adhesions underneath their leading edge protrusions and lost them at the rear as their trailing edge retracted. The traction adhesions exerted strong contractile forces along the anterior–posterior axis of the cell, as well as perpendicularly aligned lateral forces that appeared to squeeze the cell and facilitate the formation of leading edge protrusions. Lateral forces were particularly important in cells migrating on sticky substrates and in cells lacking key actin-binding proteins. Myosin II–null cells couldn’t contract along their anterior–posterior axis and therefore relied on lateral contractions for their limited motility, whereas cells lacking the actin cross-linker filamin used lateral forces to form leading edge protrusions in the absence of F-actin assembly.

Human neutrophils, which also use an amoeboid mode of migration, formed similar traction adhesions, the researchers found. The authors now want to examine how these sites are organized in cells migrating through 3D environments.