As we have seen, cell motility requires the development of a protruding front and a retracting rear, namely a polarisation of cell shape. Most cells are unable to polarise in the absence of microtubules, pointing to a cross-talk between the microtubule and the actin cytoskeleton for the development of polarity.
The loss of polarity of fibroblasts when microtubules are disassembled by drugs like nocodazole is illustrated in the following movie:
The effect of the microtubule inhibitor nocodazole on a goldfish fibroblast. Note the loss of polarisation of the cell following the depolymerisation of microtubules by this drug.
Live cell imaging has provided compelling evidence about where the cross talk between microtubules and actin occurs, namely at the focal adhesions where the actin cytoskeleton is linked via transmembrane receptors to matrix ligands.
In video sequences of cells in which both microtubules and substrate adhesions were labelled with fluorescent probes a persistent interaction was revealed between microtubules and substrate adhesion sites. Fish fibroblasts have proved a useful cell model for these studies as they are thin and can be cultivated and viewed on the microscope at room temperature. The same interactions between microtubules and adhesion sites have also been observed in other cell types.
Examples of the directed growth of microtubules into adhesion foci (referred to as targeting) are presented in the next three videos:
Targeting of substrate adhesions by microtubules. Video of a fish fibroblast that was co-injected with Cy-3 tubulin and rhodamine vinculin to render both microtubules and substrate adhesions visible in the same fluorescent channel. Note that the microtubules grow towards the advancing cell front. As they do so, some pass over, or into adhesion foci. (From Kaverina et al., 1998)
Inset region of the previous movie, showing details of microtubule-adhesion site targeting interactions. Three adhesion sites are numbered (1,2,3). Note that the microtubules specfically target the adhesion sites by growing into them. In some cases, microtubules depolymerise away from one adhesion site and are then redirected into another adhesion site. (From Kaverina et al., 1998)
Video of a fish fibroblast that was transfected with GFP-tubulin and Ds-Red zyxin (to label adhesion sites) and imaged alternately in the green and red fluorescent channels. Microtubule-adhesion site targeting interactions are circled. (From Small et al., 2002)
Adhesion sites can experience multiple targeting interactions, either involving several microtubules, repeated targeting by the same microtubule, or a combination of these events. Microtubules can also withdraw from one adhesion and redirect their growth into another (see videos).
- Kaverina, I., Rottner, K., Small, J. V. (1998). Targeting, capture, and stabilization of microtubules at early focal adhesions. J. Cell Biol. 142, 181–190.
- Small, J. V., Geiger, B., Kaverina, I., Bershadsky, A. (2002). How do microtubules guide migrating cells? Nat. Rev. Mol. Cell Biol. 3, 957–964.