A gen­eral scheme indi­cat­ing how micro­tubules may deter­mine cell polar­ity is pre­sented in the ani­ma­tion below.

Schematic model of the cross-talk between the micro­tubule and actin cytoskele­tons. Focal com­plexes at the cell front (cir­cles) can turnover or develop into focal adhe­sions (ellipses). Micro­tubules tar­get focal adhe­sions to inhibit growth or to pro­mote their turnover. Tar­get­ing involves sig­nal­ing (stars) between com­po­nents at micro­tubule tips and in focal adhe­sions. Cell polar­ity is induced and main­tained by a spa­tial, polarised con­trol of adhe­sion site turnover. The fre­quency of adhe­sion site tar­get­ing is higher at sites of retrac­tion, sug­gest­ing that trail­ing adhe­sions require higher sig­nal­ing „doses“ to pro­mote release. The turnover of focal adhe­sions behind the cell front is required to remodel the actin cytoskele­ton, involv­ing the recy­cling of com­po­nents, for fur­ther pro­tru­sion.

Dif­fer­ent cell types show a dif­fer­en­tial depen­dence on micro­tubules for polar­i­sa­tion. At one extreme, fibrob­lasts are com­pletely depen­dent on micro­tubules for polar­i­sa­tion and at the other, ker­a­to­cytes remain polarised with­out them. This dif­fer­ence appears to relate to the extent of for­ma­tion of focal adhe­sions; namely, focal adhe­sion for­ma­tion sig­nals the engage­ment of micro­tubules (for fur­ther dis­cus­sion, see Small et al., 2002).

A migrat­ing cell must release adhe­sions at the flanks and rear. Mea­sure­ments show that the fre­quency of tar­get­ing of focal adhe­sions by micro­tubules is high­est in these retract­ing regions; pre­sum­ably, micro­tubules deliver mul­ti­ple “relax­ing pulses” in these zones to pro­mote adhe­sion dis­as­sem­bly.

As already dis­cussed above, focal adhe­sions behind the advanc­ing cell front must also be dis­as­sem­bled to allow cytoskele­ton turnover to facil­i­tate con­tin­ued pro­tru­sion. Micro­tubules are thus also engaged in tar­get­ing focal adhe­sions in the ante­rior part of the cell.

Related Pub­li­ca­tions

  • Small, JV., Geiger, B., Kave­rina, I., Ber­shad­sky, A. (2002). How do micro­tubules guide migrat­ing cells? Nat. Rev. Mol. Cell Biol. 3, 957964. PDF