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number of downstream effector molecules that must somehow relay
information to the cytoskeleton, which is still poorly understood on a
molecular level. For example, it only recently became evident that the
induction of actin polymerization downstream of Rac1 and Cdc42 might be
due to WASp family proteins, which stimulate actin nucleation through
activation of the Arp2/3 complex (Higgs and Pollard, 2001; Takenawa and
Miki, 2001; see chapters by Pollard and Insall). Furthermore, it is still an open
question how the activity of Rho GTPases is regulated spatially inside the cell
to achieve functional polarization of the cytoskeleton to generate directed
motility. The discovery that certain membrane lipids that act as activators of
Rac1 are localized in a polarized fashion in cells migrating in a chemotactic
gradient is a first step towards understanding how cytoskeletal polarity is
generated (Firtel and Chung, 2000; Servant et al., 2000; see chapter by Firtel).
The role of microtubules in migrating cells
In contrast to actin, the role of the microtubule cytoskeleton in migrating cells
is less well defined. While some small cell types such as neutrophils can
migrate in the absence of microtubules, tissue cells such as fibroblasts,
endothelial (Figure 13.1) and epithelial cells, for example, require micro-
tubules for directed migration and the regulation of protrusion formation
(Bershadsky et al., 1991; Goldman, 1971; Vasiliev et al., 1970; Waterman-
Figure 13.1 Microtubules are required for endothelial cell motility. (A) Human
microvascular endothelial cells were injected with control buffer (upper row; arrowhead
indicates the injected cell) or phosphorylation-site-deficient, dominant active Op18/
stathmin resulting in microtubule depolymerization (lower row) and observed by time-
lapse phase contrast microscopy. The box on the right shows the track of the centre of the
nucleus at 10-min intervals over a total time of 6 h. Elapsed time is indicated in minutes.
Bar, 20 mm. (B) Quantitation of experiments as shown in (A). The average migration speed
was calculated from tracks of nuclear movement over a total time of 6 h and is significantly
decreased in cells injected with Op18(S16A). n, number of cells analysed
