Regulation of polarity in cells devoid of actin bundle system after treatment with inhibitors of myosin II activity.

Interplay of two cytoskeletal systems--microfilaments and microtubules is essential for directional cell movement. To better understand the role of those cytoskeletal systems in polarization of cells, rat fibroblasts were incubated with drugs inhibiting activity of myosin II: blebbistatin and Y-27632. Both drugs led to disappearance of actin-myosin bundles and mature focal cell-matrix adhesions but did not affect polarization and directional motility. The rate of motility even increased after inhibitor treatment. The characteristic feature of inhibitor-treated fibroblasts was collapse of the cytoplasm accompanied by bundling of microtubules that led to transformation of lamellae into long immobile tails. The only exception was the leading anterior lamella which was not transformed into the tail and supported directional movement of the cell. The tail at the cell rear determined the position of anterior lamella and direction of locomotion. Depolymerization of microtubules by colcemid stopped directional locomotion of inhibitor-treated cells. These data show that integrity of the microtubular system provides the basic mechanism of polarization and orientation which is only modified by interactions with actin-myosin system and cell-substrate adhesions. We suggest that the position of bundled tail microtubules and dispersed microtubules in leading lamella determine polarization in cells lacking stress fibers and focal adhesions. Thus, polarization is based on microtubule-dependent mechanisms both in non-contractile and contractile cells. These mechanisms could switch dependent on circumstances as fibroblasts may acquire non-contractile phenotype, not only after direct inhibition of myosin II but also in certain conditions of microenvironment.