Mechanisms of regulation of pseudopodial activity by the microtubule system.

Polarization of pseudopodial activity may develop spontaneously or be induced by external signals; this polarization is stabilized by cytoskeletal mechanisms. We have studied the mechanisms of microtubule-dependent control of the polarization of pseudopodial activity. Experiments with cultured fibroblasts exposed to drugs specifically inhibiting or enhancing polymerization of microtubules show that an intact microtubule system is essential, not only for restricting pseudopodial activity to certain sites at the cell edge, but also for enhancing this polarized activity. In other experiments, extension of pseudopods and polarization of cultured fibroblasts was enhanced by N-ras proto-oncogene over-expression or by phorbol ester induced activation of protein kinase C. This enhancement of polarization was accompanied in both cases by significant activation of the motility of vesicular organelles. Microtubules in the elongated processes of these cells were enriched in detyrosinated (Glu) alpha tubulin. Colcemid inhibited both organelle motility and cell process extension in this cell system. Intracellular injection of antibody to kinesin, the protein that moves vesicles toward the plus (distal) end of microtubules, mimicked some effects of microtubule-depolymerizing drugs on cell shape and pseudopodial activity. On the basis of these data it is suggested that, at least in fibroblasts, microtubules direct and enhance the outward component of cortical flow essential for pseudopod extension. This control may be associated with the organelle transport function of microtubules. A model of the stabilization of polarization by reorganization of both the actin cortex and the microtubule system is proposed and discussed.