Sev­eral stud­ies have shown that the depoly­meri­sa­tion of micro­tubules in fibrob­lasts leads to an increase in con­trac­til­ity of the actin cytoskele­ton (Small et al., 2002). This is shown most vividly by cul­ti­vat­ing cells on a suit­ably flex­i­ble sub­strate: after micro­tubule depoly­meri­sa­tion the sub­strate is pulled into creases (Danowski, 1989).

As we have already seen, an increase in con­trac­til­ity in the actin cytoskele­ton leads to an increased bundling of actin fil­a­ments and to the growth of focal adhe­sions. This is what is also observed in cells treated with micro­tubule inhibitors. This fig­ure shows the same cell before and after treat­ment with noco­da­zole for 3h. After treat­ment, the cell is no longer polar and there are fewer and larger focal adhe­sions:

Change in cell polar­ity and adhe­sion pat­terns in response to micro­tubule dis­rup­tion. A Xeno­pus fibrob­last was trans­fected with GFP-zyxin and imaged before (A) and after (B) treat­ment with 2.5µM noco­da­zole for 3h. Note the loss of cell polar­ity and an increase in size of focal adhe­sions, in response to noco­da­zole. The increase in focal adhe­sion size is diag­nos­tic of an increase in con­trac­til­ity in the actin cytoskele­ton. (From Krylyshk­ina et al., 2002)

The ampli­fi­ca­tion of con­trac­til­ity by micro­tubule dis­rup­tion is con­firmed by the obser­va­tion that myosin inhibitors pre­vent the aug­men­ta­tion of focal adhe­sions by noco­da­zole (Ber­shad­sky et al., 1996).

These find­ings sug­gest (see also Ber­shad­sky et al, 1996) that the micro­tubule cytoskele­ton medi­ates a sup­pres­sion of con­trac­til­ity in the actin cytoskele­ton.

We have pro­posed that micro­tubules pro­vide a “fibre deliv­ery sys­tem” that allows this supres­sion of con­trac­til­ity to be directed pre­cisely to selected focal adhe­sions. The deliv­ery of highly localised “relax­ing” sig­nals to sin­gle adhe­sion sites would then retard their growth or pro­mote their dis­as­sem­bly.

Cir­cum­stan­tial evi­dence in sup­port of this idea comes from exper­i­ments in which cells were chal­lenged locally with inhibitors of myosin con­trac­til­ity. Fil­a­men­tous myosin II, which gen­er­ates con­trac­til­ity in the actin cytoskele­ton, is acti­vated by phos­pho­ry­la­tion of the myosin reg­u­la­tory light chain in the neck of the mol­e­cule. This phos­pho­ry­la­tion is medi­ated by the myosin light chain kinase (MLCK) and can be blocked by inhibitors of MLCK, such as ML-7. When ML-7 is applied locally to a cell edge, the retrac­tion of the edge is induced, mim­ic­k­ing that seen at the flanks of a migrat­ing cell:

A local inhi­bi­tion of con­trac­til­ity induces the release of sub­strate adhe­sions. A fish fibrob­last was injected with rhodamine-vinculin to mark adhe­sion sites and then exposed on one edge to the myosin relaxant-ML-7– through a micropipette. (From Kave­rina et al., 2000)

This effect is due to local relax­ation of myosin at the cell periph­ery, as shown using cells injected with flu­o­res­cent myosin. The con­trac­tile activ­ity in dif­fer­ent regions of a cell are reflected by the change in spac­ing of myosin assem­blies. As seen in the fig­ure below, myosin assem­blies do not decrease their spac­ing at the cell edge exposed to ML-7, but they become con­cen­trated fur­ther away from the edge in the body of the cytoskele­ton. Thus, this local relax­ation at the periph­ery suf­fices to release adhe­sions, but does not block the overal con­trac­til­ity in the actin bun­dles that dri­ves retrac­tion of the cell edge.

Exper­i­ment as in the pre­vi­ous fig­ure, but with a cell injected with rhodamine-labelled smooth mus­cle myosin. A reduc­tion in spac­ing between the myosin „spots“ gives a qual­i­ta­tive mea­sure of con­trac­til­ity. The local appli­ca­tion of the myosin inhibitor causes cell edge retrac­tion, whereby the retrac­tion is due to adhe­sion release together with con­trac­tion of the medial zones of the actin fil­a­ment bun­dles. At the cell edge, no reduc­tion in myosin spac­ing is seen, con­sis­tent with a local relax­ation, which pro­motes adhe­sion release. (From Kave­rina et al., 2000)

Related Pub­li­ca­tions

  • Ber­shad­sky, A., Chausovsky, A., Becker, E., Lyu­bi­mova, A., Geiger, B. (1996). Involve­ment of micro­tubules in the con­trol of adhesion-dependent sig­nal trans­duc­tion. Curr. Biol. 10, 12791289. NCBI PubMed
  • Danowski, B., A. (1989). Fibrob­last con­trac­til­ity and actin orga­ni­za­tion are stim­u­lated by micro­tubule inhibitors. J. Cell. Sci­ence. 93, 255266. NCBI PubMed
  • 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
  • Krylyshk­ina, O., Kave­rina, I., Kranewit­ter, W., Stef­fen, W., Alonso, M. C., Cross, R. A., Small, J. V. (2002). Mod­u­la­tion of sub­strate adhe­sion dynam­ics via micro­tubule tar­get­ing requires kinesin-1. J. Cell Biol. 156, 349359. PDF
  • Small, J. V., Geiger, B., Kave­rina, I., Ber­shad­sky, A. (2002). How do micro­tubules guide migrat­ing cells? Nat. Rev. Mol. Cell Biol. 3, 95764. PDF