Biology Reference
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microtubule-associated proteins and whether
this affects microtubule
dynamics during cell motility is unclear.
Regulation of microtubule dynamic instability
downstream of Rac1
In addition to the polarized organization of the microtubule cytoskeleton as a
whole, microtubule polymerization dynamics are polarized in migrating cells.
Microtubules in the lamella behind the leading edge are moved backwards by
actin retrograde flow, indicating a coupling between the actin and microtubule
networks (Salmon et al., 2002; Waterman-Storer and Salmon, 1997; Yvon and
Wadsworth, 2000). Since microtubule plus ends are often found close to the
leading edge, they must undergo net growth as they are continuously swept
backwards. Further, as the leading edge protrudes and the rear edge retracts,
microtubules grow forward and fill in the advancing cellular space
(Waterman-Storer and Salmon, 1997). Thus, although microtubules undergo
dynamic instability throughout the cell, microtubule growth is biased towards
the leading edge. Indeed, quantitative analysis of the behaviour of individual
microtubules indicates that microtubule plus ends in protruding cell edges
spend more time growing and undergo far fewer catastrophes than
microtubules in central cell regions or at quiescent cell edges (Wadsworth,
1999; Waterman-Storer and Salmon, 1997).
These observations pose the question of how such regional differences in
microtubule dynamics are generated. There has so far been no documentation
of regional localization or regulation of microtubule-stabilizing factors, such
as microtubule-associated proteins, or catastrophe-promoting factors in
migrating cells. However, we have found in PtK1 epithelial cells expressing
dominant active Rac1(Q61L) that many microtubules reach the very edge of
the cell although Rac1(Q61L)-induced actin retrograde flow constantly carries
them towards the cell centre (Figure 13.4). In addition, in these cells,
exceptionally long microtubules often form extensive bundles that run parallel
to the cell edge. This is in contrast to cells expressing dominant negative
Rac1(T17N) where few microtubule ends reach the cell edge, and retrograde
flow is inhibited (Figure 13.5). Careful measurements of microtubule dynamic
instability parameters in cells expressing active Rac1(Q61L) demonstrated a
fourfold increase in microtubule net growth, due to a decrease in catastrophe
frequency and an increase of the time individual microtubules spend growing
as compared with cells expressing dominant negative Rac1(T17N) (Wittmann
et al., 2003).
Interestingly, recent data suggest a pathway by which Rac1 and Cdc42
could influence microtubule dynamics. Growth-factor-induced activation of
