Probing Intracellular Force Gradient with Soft Polymers

Iwasaki and Wang, Biophys. J. 94:L35-L37 (2008)

Cytoplasmic streaming in amoeboid movement is believed to be driven by a gradient of pressure, which in turn may be generated by localized contractions in the tail region. In cultured adherent cells, neither cytoplasmic streaming nor force gradient has been reported, due in part to the difficulties in detecting intracellular forces. This problem was addressed by microinjecting non-crosslinked, high molecular weight, linear polyacrylamide (PAA) polymers into migrating 3T3 fibroblasts. The polymers form tangled aggregates, which move and change shapes in response to the distribution of surrounding forces.

Preferential Localization of PAA Aggregates in the Anterior Region of Migrating Cells

Fluorescent dextran was added to the solution of PAA to facilitate the detection of the PAA aggregate. When injected cells resume migration, the aggregate moves to and remains in the anterior region. Upon spontaneous reversal of the direction of cell migration, the aggregate rapidly repositions to the new anterior region, suggesting that there is a pressure gradient in migrating fibroblasts with strong pressure in the rear. Recording time, 370 min.

Dependence of Anterior Positioning of PAA Aggregates on Myosin II

A 3T3 fibroblast was injected with the PAA solution. After the resumption of directional migration, the cell was treated with blebbistatin, a potent inhibitor of myosin II (indicated by a blank frame). While cell migration per se is not inhibited by the treatment, cell shape becomes grossly distorted. In contrast to control cells shown above, the PAA aggregate becomes localized to the posterior region. Recording time, 193 min.

Disorganization of Intracellular Forces upon the Disassembly of Microtubules

Microtubules are known to be involved in maintaining cell polarity. To determine if the effect involves the coordination of intracellular forces, a 3T3 fibroblast loaded with the force probe was treated with nocodazole, which causes microtubule disassembly (indicated by a blank frame). The treatment causes cell migration to become disorganized. The polymers appear as small aggregates scattered over a wide region, indicating the absence of an organized force gradient. Recording time, 45 min.