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.