The idea that the spa­tial reg­u­la­tion of con­trac­til­ity in a cell has a pri­mary impact on cell polar­ity is sup­ported by exper­i­ments with fibrob­lasts made apo­lar and immo­bile by the depoly­meri­sa­tion of micro­tubules. When such cells are exposed to inhibitors of con­trac­til­ity on one edge, the retrac­tion induced is accom­pa­nied by pro­tru­sion on the oppo­site edge (next movie). Con­tin­ued expo­sure of inhibitor to the retract­ing edge leads to the devel­op­ment of polar­ity and direc­tional loco­mo­tion.

Asym­met­ric relax­ation can restore polar­i­sa­tion and motil­ity to cells lack­ing micro­tubules. Two exam­ples are shown of cells trans­fected with GFP-zyxin that were treated and main­tained in noco­da­zole to dis­as­sem­ble micro­tubules. After noco­da­zole treat­ment, the cells become sym­met­ric and develop large focal adhe­sions at their periph­ery. The video shows the result of expos­ing the cells on one edge with the myosin relax­ant, ML-7, through a micropipette. Upper pan­els show flu­o­res­cence images and lower pan­els the phase con­trast images. Note that the release of adhe­sions at one edge, induced by local relax­ation, is fol­lowed by pro­tru­sion at the oppo­site edge. (From Kave­rina et al., 2000)

One con­clu­sion from this exper­i­ment is that micro­tubules influ­ence cell polar­ity by induc­ing an asym­me­try in con­trac­til­ity in the actin cytoskele­ton. As already sug­gested from the pre­ced­ing exper­i­ments, this effect on con­trac­til­ity is medi­ated by the inter­ac­tion of micro­tubules with the sites of link­age of con­trac­tile actin bun­dles with the sub­strate, namely with the focal adhe­sions.

It is worth com­par­ing the char­ac­ter­is­tics of focal adhe­sion turnover in cells lack­ing micro­tubules and forced to move with myosin inhibitors (pre­vi­ous movie), with that in nor­mal cells mov­ing spon­ta­neously (next movie). Note that in the con­trol cell, focal adhe­sions formed behind the cell front dis­solve in the wake of the cell body. Not so in the cells lack­ing micro­tubules; here they per­sist as the cell body moves over them. We con­clude that in nor­mally mov­ing cells, micro­tubules play a role in dis­as­sem­bling the older adhe­sions behind the cell front, thus pro­mot­ing turnover of the cytoskele­ton to facil­i­tate fur­ther pro­tru­sion.

The turnover of adhe­sion sites in a motile fish fibrob­last, with an intact micro­tubule net­work. The cell was trans­fected with GFP-zyxin. Adhe­sion sites formed behind the cell front dis­solve before the cell body reaches them. This con­trasts with cells lack­ing micro­tubules that are forced to move as shown in the first movie.

Related Pub­li­ca­tions

  • Kave­rina, I., Krylyshk­ina, O., Gimona, M., Beningo, K., Wang, Y. L., Small, J. V. (2000). Enforced polar­i­sa­tion and loco­mo­tion of fibrob­lasts lack­ing micro­tubules. Curr Biol. 10, 739742. PDF

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