Ludwig Institute for Cancer Research, Royal Free and University College School of Medicine, 91 Riding House Street, London W1W 7BS, UK
(e-mail: anne{at}ludwig.ucl.ac.uk )
![]() |
SUMMARY |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: Rho GTPases, Actin, Cytoskeleton, Migration, Cell adhesion
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Cell migration is a multistep process involving changes in the cytoskeleton, cell-substrate adhesions and the extracellular matrix. Many cell types migrate as individual cells, including leukocytes, lymphocytes, fibroblasts and neuronal cells, but epithelial cells and endothelial cells often move as sheets or groups of cells - for example, in duct development, in healing a wound and in angiogenesis.
Cell migration is usually initiated in response to extracellular cues,
which can be diffusible factors, signals on neighbouring cells, and/or signals
from the extracellular matrix. These cues then stimulate transmembrane
receptors to initiate intracellular signalling. Many different intracellular
signalling molecules have been implicated in cell migration, including small
GTPases, Ca2+-regulated proteins, mitogen-activated protein kinase
(MAPK) cascades, protein kinases C, phosphatidylinositide kinases,
phospholipases C and D, and tyrosine kinases. The involvement of all these
signalling molecules is not surprising given the diversity of extracellular
signals that affect cell migration, and the number of cellular responses that
have to be coordinated. The concept that Rho family GTPases could regulate
cell migration stems from observations that they mediate the formation of
specific actincontaining structures (Van Aelst and D'Souza-Schorey,
1997; Hall,
1998
). Subsequently, Rho
proteins have been found to regulate several other processes relevant to cell
migration, including cell-substrate adhesion, cell-cell adhesion, protein
secretion, vesicle trafficking and transcription. Here, I discuss recent
insights into how Rho proteins and their signalling partners contribute to
cell migration, focusing in particular on the cytoskeleton and cell-substrate
adhesion.
![]() |
Rho proteins: the tools |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
Rho proteins generally cycle between an active, GTP-bound, conformation and
an inactive GDP-bound conformation (Fig.
1). In the GTP-bound form, they interact with downstream target
proteins to induce cellular responses (for a recent extensive review of Rho
targets see Schmitz et al.,
2000). Rho proteins can
exchange nucleotide and hydrolyse GTP at slow rates in vitro, and these
reactions are catalysed by guanine-nucleotide-exchange factors (GEFs) and
GTPase-activating proteins (GAPs), respectively. In addition, Rho proteins can
bind to proteins known as GDIs (guanine-nucleotide-dissociation inhibitors;
Fig. 1), which prevent their
interaction with the plasma membrane but not necessarily with downstream
targets (Carpenter et al.,
1999
; Hansen and Nelson,
2001
). Most Rho proteins are
post-translationally modified at their C-termini by prenylation of a conserved
cysteine (Fig. 1), and this is
required for their interaction with membranes (Seabra,
1998
). Two classes of point
mutant have been used extensively to analyse Rho protein function: first,
activated mutants, which are constitutively GTP-bound because the GTPase
activity is inhibited (Fig. 1);
second, dominant negative mutants, which generally have reduced affinity for
nucleotides (Self and Hall,
1995
) and are likely to
titrate out GEFs (Feig, 1999
).
It is interesting that dominant negative mutants appear to be selective in
inhibiting the action of one Rho family member, at least when Rho, Rac and
Cdc42 are compared, and this is likely to be because each protein has a
different localization in cells. So far, however, it is not known whether
dominant negative Cdc42 can inhibit, for example, the activation of a closely
related protein such as TC10 (Table
1).
|
A number of bacterial toxins covalently modify some of the Rho family
members and either activate (deamidation) or inactivate (ribosylation,
glucosylation) them (Lerm et al.,
2000). Although these toxins
have been very useful for assessing Rho protein involvement in cellular
responses, the majority of Rho family proteins have not been tested for their
ability to be modified by each of these toxins, and so it is not possible to
assign effects of a toxin to inhibition or activation of a particular family
member. C3 transferase from Clostridium botulinum is an exception in
that so far it appears to be specific for RhoA, B and C (Wilde et al.,
2000
).
Members of the Rnd subfamily of Rho proteins appear to have a mode of
regulation distinct from that of other Rho proteins and indeed GTPases in
general, because their affinity for GDP is below detection levels, and they
have very low or undetectable rates of GTP hydrolysis in vitro (Chardin,
1999). It is not known how Rnd
proteins are regulated, although recent evidence shows that Ras can upregulate
RhoE/Rnd3 expression (Hansen et al.,
2000
).
![]() |
Steps of cell migration |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
Lamellipodium extension
Lamellipodium extension involves actin polymerization, and it is widely
believed that lamellipodia consist of branching filament networks formed
through the actin-nucleating activity of the Arp2/3 complex
(Fig. 3; Pollard et al.,
2000). Rac is required for
lamellipodium extension induced by growth factors, cytokines and extracellular
matrix components, and videomicroscopy experiments show that when Rac is
inhibited cells cannot migrate (Allen et al.,
1998
; Nobes and Hall,
1999
; Knight et al.,
2000
). A similar requirement
for Rac is observed in Boyden chamber experiments, in which cells are plated
on filters and the passage of cells through the filters is measured
(Anand-Apte et al., 1997
; Leng
et al., 1999
; Banyard et al.,
2000
). Effects of
constitutively active Rac1 on migration vary, probably reflecting differences
in cell type, stimulus, expression level and timecourse of expression. For
example, constitutively active Rac1 inhibits growth-factor-induced macrophage
migration, because lamellipodia extend all around the cells and they do not
polarize (Allen et al., 1998
).
However, activated Rac1 can either promote or reduce translocation of cells
across Boyden chambers, depending on the conditions (Leng et al.,
1999
; Banyard et al.,
2000
). Effects of
overexpressing wild-type Rac1 on cell migration have not been extensively
investigated, but in one study it was shown to increase PDGF-induced cell
migration in Boyden chambers (Hooshmand-Rad et al.,
1997
).
|
Recent analysis of cells derived from Rac1- and Rac2-null mice supports a
central role for Rac in cell migration. Rac2 is specifically expressed only in
haematopoeitic cells, and neutrophils derived from mice lacking Rac2 show
reduced migration speed in vitro as well as reduced F-actin polymerization in
response to chemoattractants (Roberts et al.,
1999). In addition,
recruitment of neutrophils to inflammatory sites is much reduced in vivo. In
contrast to Rac2, Rac1 is ubiquitously expressed, and Rac1-deficient embryos
die before E9.5 (Sugihara et al.,
1998
). However, cultured
epiblast cells derived from early embryos showed reduced migration rate
(determined by videomicroscopy). Whether Rac3 is also important for cell
migration remains to be determined, but interestingly Rac3 is highly expressed
in the brain (Haataja et al.,
1997
; Malosio et al.,
1997
) and hyperactivated in
some breast cancer cell lines (Mira et al.,
2000
). Rac proteins are also
clearly required for cell migration in Drosophila and C.
elegans (Reddien and Horvitz,
2000
; Blelloch et al.,
1999
; Lu and Settleman,
1999
), and Rac1 proteins in
Dictyostelium have recently been shown to regulate pseudopod
extension (equivalent to lamellipodia) and cell migration (Chung et al.,
2000
; Dumontier et al.,
2000
).
These studies all indicate that Rac is a key regulator of migration because
of its ability to stimulate lamellipodium extension, but recent reports
suggest that Rac can be bypassed in certain circumstances. For example,
expression of active Rab5 (which regulates endocytosis) can induce
lamellipodium extension independently of Rac (Spaargaren and Bos
1999), and membrane ruffling
in immature dendritic cells does not require Rac (West et al.,
2000
). In colon carcinoma
cells plated on laminin, dominant negative Rac does not inhibit membrane
ruffling or cell spreading, and instead Rho is implicated in lamellipodium
extension (O'Connor et al.,
2000
). In the latter case, Rho
might link to the Arp2/3 complex, which is consistent with the observation
that Rho and the Arp2/3 complex but not Rac/Cdc42 are required for
integrin-mediated phagocytosis (May et al.,
2000
).
How does Rac get activated? Rac activation by both tyrosine kinases and
G-protein-coupled receptors is often dependent on phosphoinositide 3-kinase
(PI 3-kinase) activity, and inhibitors of PI 3-kinase block Rac activation
(Sander et al., 1998; Rickert
et al., 2000
; Royal et al.,
2000
). In addition, the
products of PI 3-kinase,
PtdIns(3,4,5)P3/PtdIns(3,4)P2, appear
to be enriched at the leading edge of migrating neutrophils and
Dictyostelium (Meili et al.,
1999
; Haugh et al.,
2000
; Servant et al.,
2000
), which suggests that
they activate Rac specifically here. Indeed, a novel technique for detecting
where Rac is active in living cells has revealed that active Rac is localized
preferentially towards the front of migrating cells (Kraynov et al.,
2000
). The
PtdIns(3,4,5)P3-responsive exchange factors at the leading
edges of cells have not yet been identified, although the Rac exchange factors
Tiam1 and Vav1 are likely candidates because they can be regulated by PI
3-kinase products (Das et al.,
2000
; Fleming et al.,
2000
). Another route to Rac
activation has been delineated in C. elegans and Drosophila,
in which genetic studies have revealed that Rac acts downstream of the
Crk/DOCK180 adaptor proteins during developmental processes involving cell
migration (Nolan et al., 1998
;
Reddien and Horvitz, 2000
).
Studies in mammalian cells have also shown the importance of Crk/DOCK180
together with Cas, another adaptor protein, for Rac activation and
lamellipodium extension (Kiyokawa et al.,
1998
; Cheresh et al.,
1999
). Given that none of
these proteins is an exchange factor for Rac, it is still not clear how this
complex of proteins activates Rac, although Sos1 (an exchange factor for Ras
and Rac) could be recruited through Crk (Cheresh et al.,
1999
). Sos1 is also implicated
in linking Ras to Rac activation through two further adaptor proteins, Eps8
and E3b1 (Scita et al.,
1999
). Finally, Rac can be
activated by integrin engagement (del Pozo et al.,
2000
), and Vav family exchange
factors have been implicated in this process (Moores et al.,
2000
; Schwartz and Shattil,
2000
). In the case of ß3
integrins, Rac activation appears to depend on the initial formation of small
Rac-independent integrin clusters and the action of the protease calpain
(Bialkowska et al., 2000
).
How does activated Rac coordinate lamellipodium extension? Several Rac
targets are likely to be involved in this process
(Fig. 3). First, Rac stimulates
new actin polymerization, and one way it can do this is to stimulate the
Arp2/3 complex, which initiates the formation of new actin filaments on the
sides of existing filaments to form a branching actin filament network
(Pollard et al., 2000). Recent
evidence suggests that the Arp2/3 complex is activated by Rac through its
target IRSp53 (also known as IRS-58; Fig.
3; Miki et al.,
2000
). Rac interacts with
IRSp53, which in turn interacts through an SH3 domain with WAVE, which then
binds to and activates the Arp2/3 complex
(Fig. 3). There are three WAVE
isoforms (also known as SCARs) so far identified, but WAVE2 seems to be the
predominantly expressed isoform in fibroblasts (Miki et al.,
2000
). Interestingly, IRSp53
binds through a different domain to Cdc42 (Govind et al.,
2001
), so it could act as a
direct link between Cdc42 and Rac, which would explain the observation that
Cdc42 can induce Rac-dependent lamellipodium extension (Hall,
1998
). IRSp53 can also bind to
a Rho target, Dial (Fujiwara et al.,
2000
), and this might underlie
the ability of Rho to contribute to lamellipodium extension in the cases
described above.
In addition to activating the Arp2/3 complex, Rac can stimulate actin
polymerization by promoting the uncapping of actin filaments at the plasma
membrane. In resting cells, existing actin filaments are capped at their
barbed (+) ends with capping proteins to prevent spontaneous actin
polymerization. In platelets, Rac acts via a phosphatidylinositide 4-phosphate
5-kinase (PIP 5-kinase) to induce the formation of
PtdIns(4,5)P2, which then binds to capping proteins and
removes them from the barbed ends of actin filaments (Carpenter et al.,
1999; Tolias et al.,
2000
).
As well as inducing actin polymerization, Rac may affect the rate of actin
depolymerization. Rac has been reported to stimulate the activity of
LIM-kinase via the Rac/Cdc42 target PAK
(Fig. 3; Edwards et al.,
1999; Stanyon and Bernard,
1999
). LIM-kinase then
phosphorylates and inactivates cofilin, a protein that can promote actin
depolymerization (Stanyon and Bernard,
1999
). This suggests that Rac
would inhibit cofilin-induced depolymerization. However, there is also strong
evidence that cofilin is required for and promotes lamellipodium extension and
cell migration (Aizawa et al.,
1996
; Chen et al.,
2001
), either by promoting
release of actin monomers that can then be reincorporated into growing actin
filaments at the plasma membrane and/or by severing actin filaments and
thereby providing more barbed ends for actin polymerization (Chan et al.,
2000
; Zebda et al.,
2000
). The effect of Rac on
cofilin activity therefore warrants further investigation.
A number of myosins have been implicated in cell migration (Mermall et al.,
1998), and Rac can affect the
phosphorylation of both myosin II heavy chain (MHC; van Leeuwen et al.,
1999
) and myosin light chain
(MLC) via PAK (Daniels and Bokoch,
1999
; Kiosses et al.,
1999
). Precisely how these
effects of Rac on myosins could contribute to lamellipodium extension has not
been established. On the one hand, MLC phosphorylation is enhanced in the
lamellipodial region of cells (Matsumura et al.,
1998
), which suggests a role
for myosins in lamellipodium extension, but on the other hand Rac-induced MHC
phosphorylation correlates with loss of cortical MHC, which presumably leads
to decreased cortical actomyosin tension and thereby cell spreading (van
Leeuwen et al., 1999
). To
complicate the issue, expression of mutant PAKs can enhance or inhibit cell
migration and have different effects on MLC phosphorylation depending on the
cell type (Kiosses et al.,
1999
; Sells et al.,
1999
). Myosins also regulate
membrane traffic (Mermall et al.,
1998
), and new membrane and
proteins need to move to the front of migrating cells. In fact it has been
suggested that receptors are selectively exocytosed at the leading edge of
cells by a Rac-dependent mechanism (Bretscher and Aguado-Velasco,
1998
), although a recent
report has disputed this model (Bailly et al.,
2000
). Interestingly, myosins
might also act upstream of Rac: a novel myosin in Dictyostelium has
an exchange factor domain for Rac1, and can promote cell protrusion (Geissler
et al., 2000
). Clearly,
Rac-mediated effects on myosins are important for lamellipodium extension, but
further work is needed to determine which myosins are affected under specific
conditions and whether they themselves feed back to regulate Rac activity.
Formation and turnover of new cell-substrate adhesions
Small focal complex structures are localized in the lamellipodia of most
migrating cells, and are believed to be important in mediating the attachment
of the extending lamellipodium to the extracellular matrix
(Fig. 2; Lauffenburger and
Horwitz, 1996). Rac is
required for focal complex assembly (Nobes and Hall,
1995
; Allen et al.,
1997
; Rottner et al.,
1999
), but whether this
reflects an active Rac-induced process or that focal complexes only form in
lamellipodia is not clear. Cell adhesion to the extracellular matrix itself
activates Rac and Cdc42: for example, plating cells on fibronectin induces Rac
and Cdc42 activation, and this in turn is required for cell spreading (Price
et al., 1998
). It is therefore
possible that continuous formation of new interactions between integrins and
the extracellular matrix at the leading edge of cells maintains Rac active
here and that this could provide a positive feedback loop allowing cells to
carry on migrating even when receptor signalling is downregulating (Allen et
al., 1998
; Bailly et al.,
2000
). Rac-mediated recruitment
of activated integrins to the leading edge of cells may also contribute to
maintaining this cell migration feedback loop (Kiosses et al.,
2001
). The speed of cell
migration is dependent on substrate composition, and indeed the relative
levels of Rho, Rac and Cdc42 activation vary with extracellular matrix
composition (Adams and Schwartz,
2000
; Ridley,
2000
; Wenk et al.,
2000
). There is thus
continuous crosstalk between integrins and Rac to allow cells to respond to
changing extracellular matrix composition (Schwartz and Shattil,
2000
).
Focal complexes can be disassembled as the cell lamella moves over them, or
in slowly migrating cells such as fibroblasts they can mature into Rho-induced
focal adhesions (Rottner et al.,
1999). In either case, it is
important that focal complexes/adhesions turn over for cells to migrate: a
high level of integrin-mediated adhesion inhibits cell migration because of
the strength of attachment to the extracellular matrix, and this correlates
with high levels of Rho activity (Cox et al.,
2001
). As well as being
involved in focal complex formation in lamellipodia, Rac can induce focal
complex/adhesion turnover both directly, through PAK (Zhao et al.,
2000
), and indirectly, by
antagonizing Rho activation (Sander et al.,
1999
). PAK interacts with a
complex of the exchange factor PIX, paxillin and a GIT family protein to
localize to focal complexes/adhesions, and once there induces their
disassembly, presumably by phosphorylating one or more focal adhesion
components (Zhao et al.,
2000
). Interestingly,
integrin-mediated adhesion appears to be required for growth-factor-activated
Rac to couple to its downstream target PAK (del Pozo et al.,
2000
), which suggests that a
critical level of integrin engagement triggers PAK activation and thus
ultimately focal complex/adhesion disassembly. However, other signals,
including Src and FAK, can also induce focal adhesion disassembly (Jones et
al., 2000
), and recently it
has been suggested that microtubules target focal adhesions to deliver
proteins that induce their breakdown (Kaverina et al.,
1999
).
Cell body contraction
Cell body contraction is dependent on actomyosin contractility (Mitchison
and Cramer, 1996) and can be
regulated by Rho. For example, when Rho is inhibited, macrophages continue to
extend processes, but the cell body does not translocate significantly (Allen
et al., 1997
; Allen et al.,
1998
). Rho acts via ROCKs (also
known as Rho-kinases) to affect MLC phosphorylation
(Fig. 4), both by inhibiting
MLC phosphatase and by phosphorylating MLC (Kaibuchi et al.,
1999
; Amano et al.,
2000
). MLC phosphorylation is
also regulated by MLC kinase (MLCK), which is activated by calcium, and
stimulated by the ERK MAPKs (Hansen et al.,
2000
). It is likely that ROCKs
and MLCK act in concert to regulate different aspects of cell contractility,
because ROCKs appear to be required for MLC phosphorylation associated with
actin filaments in the cell body, whereas MLCK is required at the cell
periphery (Totsukawa et al.,
2000
).
|
As in the case of PAK, the effect of inhibiting Rho on cell migration rate
depends on the cell type, and this probably reflects the basal level of stress
fibres and focal adhesions in cells. In cells that have stress fibres, such as
cultured fibroblasts, the high level of substrate adhesion through
stress-fibre-associated focal adhesions inhibits cell migration (Cox and
Huttenlocher 1998; Cox et al.,
2001
). Reducing Rho activity
therefore has two opposing effects: it promotes migration by lowering
adhesion, but decreases cell migration by inhibiting cell body contraction. In
less adherent cells that lack focal adhesions, such as macrophages,
neutrophils and various cancer cell lines, Rho does not affect adhesion but
induces cell body contraction, and here Rho and ROCK are clearly required for
cell polarization and migration (Allen et al.,
1998
; Niggli,
1999
; Wicki and Niggli,
2001
). Rho activity therefore
does not necessarily correlate with stress fibre levels, as illustrated by the
observation that Ras-transformed fibroblasts have high levels of active Rho
but no stress fibres (Sahai et al.,
2001
). It has been suggested
that the lack of stress fibres reflects a Ras-induced decrease in the coupling
of Rho to ROCK (Sahai et al.,
2001
). Alternatively, Ras
could act via Raf to induce increased RhoE expression (Hansen et al.,
2000
), which can concomitantly
reduce the level of stress fibres and enhance cell migration rate (Guasch et
al., 1998
).
In addition to MLC phosphorylation, Rho has other effects on the actin
cytoskeleton that are relevant to cell migration
(Fig. 4). Like PAK, ROCK
activates LIMK to inhibit cofilin-mediated actin depolymerization, thereby
promoting F-actin accumulation (Maekawa et al.,
1999; Sumi et al.,
1999
). Another way for Rho to
induce F-actin accumulation is via PIP 5-kinases (see above; Ren and
Schwartz, 1998
), and recently
ROCK has been reported to stimulate PIP 5-kinase activity (Oude
Weernink et al., 2000
),
although whether this involves direct phosphorylation of a PIP
5-kinase by ROCK is not known. Rho is also linked via Dia proteins to the
actin cytoskeleton, and expressing constitutively active Dia together with
ROCK effectively mimics Rho-induced stress fibre formation (Ridley
1999
; Tominaga et al.,
2000
). Dia interacts directly
with Src and is dependent on Src kinases for its effect on stress fibres
(Tominaga et al., 2000
). This
is seemingly at odds with the ability of oncogenic Src to induce loss of focal
adhesions (Jones et al.,
2000
), which would be expected
to correlate with a decrease in stress fibres, but it is possible that what
Dia really does is to act via Src to limit the size of ROCK-induced focal
adhesions, thereby allowing the formation of many stress fibres rather than
the few large ROCK-induced actin bundles. Dia proteins have also been reported
to interact with IRSp53 (Fujiwara et al.,
2000
), which, as described
above, can interact with WAVEs to induce actin polymerization, although so far
whether Dia interaction with IRSp53 affects actin polymerization is not
known.
Tail detachment
Tail detachment can often be the rate-limiting step of cell migration
(Palecek et al., 1998). The
mechanisms regulating tail detachment depend on the type of cell and strength
of adhesion to the extracellular matrix (Cox and Huttenlocher,
1998
; Palecek et al.,
1998
). In slowly moving cells
tail detachment appears to depend on the action of the protease calpain, which
degrades focal adhesion components at the rear of cells (Palecek et al.,
1998
; Glading et al.,
2000
). Interestingly, calpain
activity can be regulated by ERK in EGF-stimulated cells (Glading et al.,
2000
), but so far there is no
indication that it is regulated by Rho family members. A reduction in Rho
activity could inhibit tail detachment, however, through decreased actomyosin
contractility (Cox and Huttenlocher,
1998
). Conversely, in cells
such as fibroblasts that have large focal adhesions, inhibition of Rho would
reduce adhesion and this could actually promote tail detachment. As described
above, other signals, including Src, FAK and PAK, can induce focal adhesion
disassembly (Jones et al.,
2000
; Zhao et al.,
2000
) and may contribute to
tail detachment (Cox and Huttenlocher,
1998
).
![]() |
Other Rho-regulated cellular responses contributing to cell migration |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Filopodia: sensing the extracellular environment
In neuronal growth cones, filopodia are classically regarded as sensors for
the extracellular milieu, steering the direction of growth cones during axon
pathfinding (Kater and Rehder,
1995), and the role of Rho
GTPases in regulating growth cone guidance has been recently reviewed
(Dickson, 2001
). Increasing
evidence indicates that Cdc42, which regulates filopodium formation, is
required for direction sensing during chemotaxis in other cell types as well
as neurons (Allen et al., 1998
;
Nobes and Hall, 1999
). By
extending out from cells into the surroundings, receptors on filopodia could
detect changes in extracellular signals that would then be transmitted back
into cells. The role of Cdc42 in this process is presumably to initiate the
actin polymerization required for filopodium extension, and indeed the ability
of Cdc42 to stimulate actin polymerization via its interaction with WASp and
N-WASp, leading to activation of the Arp2/3 complex, is well-characterized
(Fig. 3). Interestingly, Cdc42
localizes predominantly to the Golgi complex, although it is also present on
the plasma membrane (Erickson et al.,
1996
), and possibly on other
intracellular vesicles (Michaelson et al.,
2001
). This suggests that
Cdc42 has another, or overlapping, function in regulating vesicle trafficking
(Wu et al., 2000
; Ridley,
2001
).
As well as Cdc42, its close relatives TC10 and TCL can interact with WASp
proteins and thus have the potential to stimulate actin polymerization
(Neudauer et al., 1998; Vignal
et al., 2000
). TC10 also
induces filopodium extension (Neudauer et al.,
1998
), and it would be
interesting to know how it affects directional cell migration. Interestingly,
TC10 expression is upregulated following nerve injury, and it is able to
promote neurite extension, which suggests that it may play an important role
in growth cone extension (Tanabe et al.,
2000
). Similarly, RhoG can
also promote neurite extension in PC12 cells (Katoh et al.,
2000
).
Microtubule dynamics
Although the majority of studies on Rho GTPases have concentrated on their
roles in regulating the actin cytoskeleton, recent evidence has indicated that
they can also affect the organization of microtubules (Waterman-Storer and
Salmon, 1999). The extent of
microtubule involvement in cell migration varies depending on the cell type.
Some rapidly migrating cells, such as neutrophils and keratocytes, are not
affected by microtubule polymerization inhibitors (e.g. nocadazole), and
fragments of keratocytes can migrate in the absence of microtubule-organizing
centres. In contrast, nocadazole rapidly inhibits the migration of fibroblasts
and epithelial cells (reviewed by Waterman-Storer and Salmon,
1999
). Why are microtubules
important for cell migration in some cells but not in others? Microtubules
appear to be required for tail retraction (Ballestrem et al.,
2000
), which possibly reflects
the fact that microtubules can target focal adhesions and induce their
turnover in slow-moving cells (Kaverina et al.,
1999
; Kaverina et al.,
2000
). In this scenario,
fast-moving cells such as neutrophils that do not have focal adhesions are not
dependent on microtubule-mediated delivery of adhesion-disassembling proteins.
The size of lamellipodial extensions may also contribute to the requirement
for microtubules: larger extensions may rely more on microtubule infiltration
to stabilize the extension.
How do Rho GTPases link with microtubules? First, microtubule
depolymerization induced by nocadazole activates Rho (Liu et al.,
1998), whereas washout of
nocadazole activates Rac (Waterman-Storer et al.,
1999
). These effects could be
mediated by changes in the activity of the several GEFs that have been
reported to associate with microtubules (Ren et al.,
1998
; Glaven et al.,
1999
; van Horck et al.,
2001
). Second, Rho has been
shown to be required for stabilization of microtubules induced by
lysophosphatidic acid in fibroblasts (Cook et al.,
1998
; reviewed by Gundersen
and Cook, 1999
), and this may
reflect a requirement for stable microtubules to counteract the contractility
induced by Rho via ROCK (Pletjushkina et al.,
2001
). The Rho target Dia
could provide a link between Rho and microtubules, because it regulates
microtubule and F-actin polarization (Ishizaki et al.,
2001
). Cdc42 and Rac could
also affect microtubule stability through stathmin, which can be
phosphorylated by PAK (Daub et al.,
2001
). PAK phosphorylation of
stathmin would inhibit its action in destabilizing microtubules, and this
could be important for ensuring that microtubules grow into the lamellal
region of migrating cells (Waterman-Storer and Salmon,
1999
).
Cdc42 can mediate the polarization of the microtubule network observed in
migrating cells (Nobes and Hall,
1999). Cdc42 may be directly
linked to the microtubule network via its target CIP4, which interacts with
both WASp and microtubules (Linder et al.,
2000
; Tian et al.,
2000
). Interestingly,
microtubules, Cdc42 and WASp are required for the formation of podosomes,
which are unique, rapidly turning over adhesion structures found at the
leading edge of dendritic cells and macrophages (Linder et al.,
1999
; Linder et al.,
2000
), although whether Cdc42
is involved in linking microtubules to podosomes is not known. A recently
identified target for Cdc42, PAR6, is involved in generating cell polarity and
asymmetric cell division in C. elegans (Kim,
2000
), and it will therefore
be interesting to see whether Cdc42 acts via PAR6 to induce microtubule
polarization.
Intermediate filaments
As in the case of microtubules, the role of intermediate filaments in cell
migration depends on the circumstances. For example, although lack of vimentin
has no discernable effect on mouse development, cultured fibroblasts derived
from vimentin-null mice show impaired migration and contraction in collagen
gels (Eckes et al., 1998), and
the mice show impaired wound healing, owing to a failure of fibroblast
migration into the wound site and subsequent mesenchymal contraction (Eckes et
al., 2000
). Intermediate
filament protein phosphorylation is important for regulating the intermediate
filament network, both during interphase and at cytokinesis (Herrmann and
Aebi, 2000
). Expression of
activated RhoA induces reorganization of vimentin filaments (Paterson et al.,
1990
; Sin et al.,
1998
), and ROCK
phosphorylates vimentin (Goto et al.,
1998
; Sin et al.,
1998
) and the intermediate
filament protein GFAP (Kosako et al.,
1997
; Matsuzawa et al.,
1997
). These effects of ROCK
have been linked to reorganization of intermediate filaments at cytokinesis
(Herrmann and Aebi, 2000
), and
whether there is also a link with cell migration is not yet known. Activated
RhoG, Rac and Cdc42 induce collapse of vimentin filaments into the perinuclear
region, as do growth factors such as PDGF and bradykinin (Valgeirsdottir et
al., 1998
; Meriane et al.,
2000
), and this response could
be important for remodelling the intermediate filament network during cell
migration.
![]() |
Conclusions |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Adams, J. C. and Schwartz, M. A. (2000).
Stimulation of fascin spikes by thrombospondin-1 is mediated by the GTPases
Rac and Cdc42. J. Cell Biol.
150,807
-822.
Aizawa, H., Sutoh, K. and Yahara, I. (1996). Overexpression of cofilin stimulates bundling of actin filaments, membrane ruffling, and cell movement in Dictyostelium. J. Cell Biol. 132,335 -344.[Abstract]
Allen, W. E., Jones, G. E., Pollard, J. W. and Ridley, A. J.
(1997). Rho, Rac and Cdc42 regulate actin organization and cell
adhesion in macrophages. J. Cell Sci.
110,707
-720.
Allen, W. E., Zicha, D., Ridley, A. J. and Jones, G. E.
(1998). A role for Cdc42 in macrophage chemotaxis. J.
Cell Biol. 141,1147
-1157.
Amano, M., Fukata, Y. and Kaibuchi, K. (2000). Regulation and functions of Rho-associated kinase. Exp. Cell Res. 261,44 -51.[Medline]
Anand-Apte, B., Zetter, B. R., Viswanathan, A., Qiu, R. G.,
Chen, J., Ruggieri, R. and Symons, M. (1997).
Platelet-derived growth factor and fibronectin-stimulated migration are
differentially regulated by the Rac and extracellular signal-regulated kinase
pathways. J. Biol. Chem.
272,30688
-30692.
Bailly, M., Wyckoff, J., Bouzahzah, B., Hammerman, R.,
Sylvestre, V., Cammer, M., Pestell, R. and Segall, J. E.
(2000). Epidermal growth factor receptor distribution during
chemotactic responses. Mol. Biol. Cell
11,3873
-3883.
Ballestrem, C., Wehrle-Haller, B., Hinz, B. and Imhof, B. A.
(2000). Actin-dependent lamellipodia formation and
microtubule-dependent tail retraction control-directed cell migration.
Mol. Biol. Cell 11,2999
-3012.
Banyard, J., Anand-Apte, B., Symons, M. and Zetter, B. R. (2000). Motility and invasion are differentially modulated by Rho family GTPases. Oncogene 19,580 -591.[Medline]
Bialkowska, K., Kulkarni, S., Du, X., Goll, D. E., Saido, T. C.
and Fox, J. E. (2000). Evidence that beta3 integrin-induced
Rac activation involves the calpain-dependent formation of integrin clusters
that are distinct from the focal complexes and focal adhesions that form as
Rac and RhoA become active. J. Cell Biol.
151,685
-696.
Blelloch, R., Newman, C. and Kimble, J. (1999). Control of cell migration during Caenorhabditis elegans development. Curr. Opin. Cell Biol. 11,608 -613.[Medline]
Bretscher, M. S. and Aguado-Velasco, C. (1998). EGF induces recycling membrane to form ruffles. Curr. Biol. 8,721 -724.[Medline]
Carpenter, C. L., Tolias, K. F., Van Vugt, A. and Hartwig, J. (1999). Lipid kinases are novel effectors of the GTPase Rac1. Adv. Enzyme Regul. 39,299 -312.[Medline]
Chan, A. Y., Bailly, M., Zebda, N., Segall, J. E. and Condeelis,
J. S. (2000). Role of cofilin in epidermal growth
factor-stimulated actin polymerization and lamellipod protrusion.
J. Cell Biol. 148,531
-542.
Chardin, P. (1999). Rnd proteins: a new family of Rho-related proteins that interfere with the assembly of filamentous actin structures and cell adhesion. Prog. Mol. Subcell. Biol. 22,39 -50.[Medline]
Chen, J., Godt, D., Gunsalus, K., Kiss, I., Goldberg, M. and Laski, F. A. (2001). Cofilin/ADF is required for cell motility during Drosophila ovary development and oogenesis. Nat. Cell Biol. 3,204 -209.[Medline]
Cheresh, D. A., Leng, J. and Klemke, R. L.
(1999). Regulation of cell contraction and membrane ruffling by
distinct signals in migratory cells. J. Cell Biol.
146,1107
-1116.
Chung, C. Y., Lee, S., Briscoe, C., Ellsworth, C. and Firtel, R.
A. (2000). Role of Rac in controlling the actin cytoskeleton
and chemotaxis in motile cells. Proc. Natl. Acad. Sci.
USA 97,5225
-5230.
Cook, T. A., Nagasaki, T. and Gundersen, G. G.
(1998). Rho guanosine triphosphatase mediates the selective
stabilization of microtubules induced by lysophosphatidic acid. J.
Cell Biol. 141,175
-185.
Cox, E. A. and Huttenlocher, A. (1998). Regulation of integrin-mediated adhesion during cell migration. Microsc. Res. Tech. 43,412 -419.[Medline]
Cox, E. A., Sastry, S. K. and Huttenlocher, A.
(2001). Integrin-mediated adhesion regulates cell polarity and
membrane protrusion through the Rho family of GTPases. Mol. Biol.
Cell 12,265
-277.
Daniels, R. H. and Bokoch, G. M. (1999). p21-activated protein kinase: a crucial component of morphological signaling? Trends Biochem. Sci. 24,350 -355.[Medline]
Das, B., Shu, X., Day, G. J., Han, J., Krishna, U. M., Falck, J.
R. and Broek, D. (2000). Control of intramolecular
interactions between the pleckstrin homology and Dbl homology domains of Vav
and Sos1 regulates Rac binding. J. Biol. Chem.
275,15074
-15081.
Daub, H., Gevaert, K., Vandekerckhove, J., Sobel, A. and Hall,
A. (2001). Rac/Cdc42 and p65PAK regulate the
microtubule-destabilizing protein stathmin through phosphorylation at serine
16. J. Biol. Chem. 276,1677
-1680.
del Pozo, M. A., Price, L. S., Alderson, N. B., Ren, X. D. and
Schwartz, M. A. (2000). Adhesion to the extracellular matrix
regulates the coupling of the small GTPase Rac to its effector PAK.
EMBO J. 19,2008
-2014.
Dickson, B. J. (2001). Rho GTPases in growth cone guidance. Curr. Opin. Neurobiol. 11,103 -110.[Medline]
Dumontier, M., Hocht, P., Mintert, U. and Faix, J.
(2000). Rac1 GTPases control filopodia formation, cell motility,
endocytosis, cytokinesis and development in Dictyostelium. J. Cell
Sci. 113,2253
-2265.
Eckes, B., Dogic, D., Colucci-Guyon, E., Wang, N., Maniotis, A.,
Ingber, D., Merckling, A., Langa, F., Aumailley, M., Delouvee, A.,
Koteliansky, V., Babinet, C. and Krieg, T. (1998). Impaired
mechanical stability, migration and contractile capacity in vimentin-deficient
fibroblasts. J. Cell Sci.
111,1897
-1907.
Eckes, B., Colucci-Guyon, E., Smola, H., Nodder, S., Babinet,
C., Krieg, T. and Martin, P. (2000). Impaired wound healing
in embryonic and adult mice lacking vimentin. J. Cell
Sci. 113,2455
-2462.
Edwards, D. C., Sanders, L. C., Bokoch, G. M. and Gill, G. N. (1999). Activation of LIM-kinase by Pak1 couples Rac/Cdc42 GTPase signalling to actin cytoskeletal dynamics. Nat. Cell Biol. 1,253 -259.[Medline]
Erickson, J. W., Zhang, C., Kahn, R. A., Evans, T. and Cerione,
R. A. (1996). Mammalian Cdc42 is a brefeldin A-sensitive
component of the Golgi apparatus. J. Biol. Chem.
271,26850
-26854.
Feig, L. A. (1999). Tools of the trade: use of dominant-inhibitory mutants of Ras-family GTPases. Nat. Cell Biol. 1,E25 -27.[Medline]
Fleming, I. N., Gray, A. and Downes, C. P. (2000). Regulation of the Rac1-specific exchange factor Tiam1 involves both phosphoinositide 3-kinase-dependent and -independent components. Biochem. J. 351,173 -182.[Medline]
Fujiwara, T., Mammoto, A., Kim, Y. and Takai, Y. (2000). Rho small G-protein-dependent binding of mDia to an Src homology 3 domain-containing IRSp53/BAIAP2. Biochem. Biophys. Res. Commun. 271,626 -629.[Medline]
Geissler, H., Ullmann, R. and Soldati, T. (2000). The tail domain of myosin M catalyses nucleotide exchange on Rac1 GTPases and can induce actin-driven surface protrusions. Traffic 1,399 -410.[Medline]
Glading, A., Chang, P., Lauffenburger, D. A. and Wells, A.
(2000). Epidermal growth factor receptor activation of calpain is
required for fibroblast motility and occurs via an ERK/MAP kinase signaling
pathway. J. Biol. Chem.
275,2390
-2398.
Glaven, J. A., Whitehead, I., Bagrodia, S., Kay, R. and Cerione,
R. A. (1999). The Dbl-related protein, Lfc, localizes to
microtubules and mediates the activation of Rac signaling pathways in cells.
J. Biol. Chem. 274,2279
-2285.
Goto, H., Kosako, H., Tanabe, K., Yanagida, M., Sakurai, M.,
Amano, M., Kaibuchi, K. and Inagaki, M. (1998).
Phosphorylation of vimentin by Rho-associated kinase at a unique
amino-terminal site that is specifically phosphorylated during cytokinesis.
J. Biol. Chem. 273,11728
-11736.
Govind, S., Kozma, R., Monfries, C., Lim, L. and Ahmed, S.
(2001). Cdc42hs facilitates cytoskeletal reorganization and
neurite outgrowth by localizing the 58-kD insulin receptor substrate to
filamentous actin. J. Cell Biol.
152,579
-594.
Guasch, R. M., Scambler, P., Jones, G. E. and Ridley, A. J.
(1988). RhoE regulates actin cytoskeleton organization and cell
migration. Mol. Cell. Biol.
18,4761
-4771.
Gundersen, G. G. and Cook, T. A. (1999). Microtubules and signal transduction. Curr. Opin. Cell Biol. 11,81 -94.[Medline]
Haataja, L., Groffen, J. and Heisterkamp, N.
(1997). Characterization of RAC3, a novel member of the Rho
family. J. Biol. Chem.
272,20384
-20388.
Hall, A. (1998). Rho GTPases and the actin
cytoskeleton. Science
279,509
-514.
Hansen, M. D. and Nelson, W. J. (2001). Serum-activated assembly and membrane translocation of an endogenous Rac1:effector complex. Curr. Biol. 11,356 -360.[Medline]
Hansen, S. H., Zegers, M. M., Woodrow, M., Rodriguez-Viciana,
P., Chardin, P., Mostov, K. E. and McMahon, M. (2000).
Induced expression of Rnd3 is associated with transformation of polarized
epithelial cells by the Raf-MEK-extracellular signal-regulated kinase pathway.
Mol. Cell. Biol. 20,9364
-9375.
Haugh, J. M., Codazzi, F., Teruel, M. and Meyer, T.
(2000). Spatial sensing in fibroblasts mediated by 3'
phosphoinositides. J. Cell Biol.
151,1269
-1280.
Herrmann, H. and Aebi, U. (2000). Intermediate filaments and their associates: multi-talented structural elements specifying cytoarchitecture and cytodynamics. Curr. Opin. Cell Biol. 12,79 -90.[Medline]
Hooshmand-Rad, R., Claesson-Welsh, L., Wennstrom, S., Yokote, K., Siegbahn, A. and Heldin, C. H. (1997). Involvement of phosphatidylinositide 3'-kinase and Rac in platelet-derived growth factor-induced actin reorganization and chemotaxis. Exp. Cell Res. 234,434 -441.[Medline]
Ishizaki, T., Morishima, Y., Okamoto, M., Furuyashiki, T., Kato, T. and Narumiya, S. (2001). Coordination of microtubules and the actin cytoskeleton by the Rho effector mDial. Nat. Cell Biol. 3,8 -14.[Medline]
Jin, G., Sah, R. L., Li, Y. S., Lotz, M., Shyy, J. Y. and Chien, S. (2000). Biomechanical regulation of matrix metalloproteinase-9 in cultured chondrocytes. J. Orthop. Res. 18,899 -908.[Medline]
Jones, R. J., Brunton, V. G. and Frame, M. C. (2000). Adhesion-linked kinases in cancer; emphasis on src, focal adhesion kinase and PI3-kinase. Eur. J. Cancer 36,1595 -1606.[Medline]
Kaibuchi, K., Kuroda, S. and Amano, M. (1999). Regulation of the cytoskeleton and cell adhesion by the Rho family GTPases in mammalian cells. Annu. Rev. Biochem. 68,459 -486.[Medline]
Kater, S. B. and Rehder, V. (1995). The sensory-motor role of growth cone filopodia. Curr. Opin. Neurobiol. 5,68 -74.[Medline]
Katoh, H., Yasui, H., Yamaguchi, Y., Aoki, J., Fujita, H., Mori,
K. and Negishi, M. (2000). Small GTPase RhoG is a key
regulator for neurite outgrowth in PC12 cells. Mol. Cell.
Biol. 20,7378
-7387.
Kaverina, I., Krylyshkina, O. and Small, J. V.
(1999). Microtubule targeting of substrate contacts promotes
their relaxation and dissociation. J. Cell Biol.
146,1033
-1044.
Kaverina, I., Krylyshkina, O., Gimona, M., Beningo, K., Wang, Y. L. and Small, J. V. (2000). Enforced polarisation and locomotion of fibroblasts lacking microtubules. Curr. Biol. 10,739 -742.[Medline]
Kheradmand, F., Werner, E., Tremble, P., Symons, M. and Werb,
Z. (1998). Role of Rac1 and oxygen radicals in collagenase-1
expression induced by cell shape change. Science
280,898
-902.
Kim, S. K. (2000). Cell polarity: new PARtners for Cdc42 and Rac. Nat. Cell Biol. 2,E143 -145.[Medline]
Kiosses, W. B., Daniels, R. H., Otey, C., Bokoch, G. M. and
Schwartz, M. A. (1999). A role for p21-activated kinase in
endothelial cell migration. J. Cell Biol.
147,831
-844.
Kiosses, W. B., Shattil, S. J., Pampori, N. and Schwartz, M.
A. (2001). Rac recruits high-affinity integrin
vß3 to lamellipodia in endothelial cell migration. Nat.
Cell Biol. 3,316
-320.[Medline]
Kiyokawa, E., Hashimoto, Y., Kobayashi, S., Sugimura, H.,
Kurata, T. and Matsuda, M. (1998). Activation of Rac1 by a
Crk SH3-binding protein, DOCK180. Genes Dev.
12,3331
-3336.
Knight, B., Laukaitis, C., Akhtar, N., Hotchin, N. A., Edlund, M. and Horwitz, A. R. (2000). Visualizing muscle cell migration in situ. Curr. Biol. 10,576 -585.[Medline]
Kosako, H., Amano, M., Yanagida, M., Tanabe, K., Nishi, Y.,
Kaibuchi, K. and Inagaki, M. (1997). Phosphorylation of glial
fibrillary acidic protein at the same sites by cleavage furrow kinase and
Rho-associated kinase. J. Biol. Chem.
272,10333
-10336.
Kraynov, V. S., Chamberlain, C., Bokoch, G. M., Schwartz, M. A.,
Slabaugh, S. and Hahn, K. M. (2000). Localized Rac activation
dynamics visualized in living cells. Science
290,333
-337.
Lauffenburger, D. A. and Horwitz, A. F. (1996). Cell migration: a physically integrated molecular process. Cell 84,359 -369.[Medline]
Leng, J., Klemke, R. L., Reddy, A. C. and Cheresh, D. A.
(1999). Potentiation of cell migration by adhesion-dependent
cooperative signals from the GTPase Rac and Raf kinase. J. Biol.
Chem. 274,37855
-37861.
Lerm, M., Schmidt, G. and Aktories, K. (2000). Bacterial protein toxins targeting rho GTPases. FEMS Microbiol. Lett. 188,1 -6.[Medline]
Linder, S., Nelson, D., Weiss, M. and Aepfelbacher, M.
(1999). Wiskott-Aldrich syndrome protein regulates podosomes in
primary human macrophages. Proc. Natl. Acad. Sci. USA
96,9648
-9653.
Linder, S., Hufner, K., Wintergerst, U. and Aepfelbacher, M.
(2000). Microtubule-dependent formation of podosomal adhesion
structures in primary human macrophages. J. Cell Sci.
113,4165
-4176.
Liu, B. P., Chrzanowska-Wodnicka, M. and Burridge, K. (1998). Microtubule depolymerization induces stress fibers, focal adhesions, and DNA synthesis via the GTP-binding protein Rho. Cell Adhes. Commun. 5,249 -255.[Medline]
Lu, Y. and Settleman, J. (1999). The role of rho family GTPases in development: lessons from Drosophila melanogaster.Mol. Cell Biol. Res. Commun. 1, 87-94.[Medline]
Maekawa, M., Ishizaki, T., Boku, S., Watanabe, N., Fujita, A.,
Iwamatsu, A., Obinata, T., Ohashi, K., Mizuno, K. and Narumiya, S.
(1999). Signaling from Rho to the actin cytoskeleton through
protein kinases ROCK and LIM-kinase. Science
285,895
-898.
Malosio, M. L., Gilardelli, D., Paris, S., Albertinazzi, C. and
de Curtis, I. (1997). Differential expression of distinct
members of Rho family GTP-binding proteins during neuronal development:
identification of Rac1B, a new neural-specific member of the family.
J. Neurosci. 17,6717
-6728.
Matsumura, F., Ono, S., Yamakita, Y., Totsukawa, G. and
Yamashiro, S. (1998). Specific localization of serine 19
phosphorylated myosin II during cell locomotion and mitosis of cultured cells.
J. Cell Biol. 140,119
-129.
Matsuzawa, K., Kosako, H., Inagaki, N., Shibata, H., Mukai, H., Ono, Y., Amano, M., Kaibuchi, K., Matsuura, Y., Azuma, I. and Inagaki, M. (1997). Domain-specific phosphorylation of vimentin and glial fibrillary acidic protein by PKN. Biochem. Biophys. Res. Commun. 234,621 -625.[Medline]
May, R. C., Caron, E., Hall, A. and Machesky, L. M.
(2000). Involvement of the Arp2/3 complex in phagocytosis
mediated by FcR or CR3. Nat. Cell Biol.
2, 246-248.[Medline]
Meili, R., Ellsworth, C., Lee, S., Reddy, T. B., Ma, H. and
Firtel, R. A. (1999). Chemoattractant-mediated transient
activation and membrane localization of Akt/PKB is required for efficient
chemotaxis to cAMP in Dictyostelium. EMBO J.
18,2092
-2105.
Meriane, M., Mary, S., Comunale, F., Vignal, E., Fort, P. and
Gauthier-Rouviere, C. (2000). Cdc42Hs and rac1 GTPases induce
the collapse of the vimentin intermediate filament network. J.
Biol. Chem. 275,33046
-33052.
Mermall, V., Post, P. L. and Mooseker, M. S.
(1998). Unconventional myosins in cell movement, membrane
traffic, and signal transduction. Science
279,527
-533.
Michaelson, D., Silletti, J., Murphy, G., D'Eustachio, P., Rush,
M. and Philips, M. R. (2001). Differential localization of
Rho GTPases in live cells: regulation by hypervariable regions and RhoGDI
binding. J. Cell Biol.
152,111
-126.
Miki, H., Yamaguchi, H., Suetsugu, S. and Takenawa, T. (2000). IRSp53 is an essential intermediate between Rac and WAVE in the regulation of membrane ruffling. Nature 408,732 -735.[Medline]
Mira, J. P., Benard, V., Groffen, J., Sanders, L. C. and Knaus,
U. G. (2000). Endogenous, hyperactive Rac3 controls
proliferation of breast cancer cells by a p21-activated kinase-dependent
pathway. Proc. Natl. Acad. Sci. USA
97,185
-189.
Mitchison, T. J. and Cramer, L. P. (1996). Actin-based cell motility and cell locomotion. Cell 84,371 -379.[Medline]
Moores, S. L., Selfors, L. M., Fredericks, J., Breit, T.,
Fujikawa, K., Alt, F. W., Brugge, J. S. and Swat, W. (2000).
Vav family proteins couple to diverse cell surface receptors. Mol.
Cell. Biol. 20,6364
-6373.
Murphy, G. and Gavrilovic, J. (1999). Proteolysis and cell migration: creating a path? Curr. Opin. Cell Biol. 11,614 -621.[Medline]
Neudauer, C. L., Joberty, G., Tatsis, N. and Macara, I. G. (1998). Distinct cellular effects and interactions of the Rho-family GTPase TC10. Curr. Biol. 8,1151 -1160.[Medline]
Niggli, V. (1999). Rho-kinase in human neutrophils: a role in signalling for myosin light chain phosphorylation and cell migration. FEBS Lett. 445, 69-72.[Medline]
Nobes, C. D. and Hall, A. (1995). Rho, rac, and cdc42 GTPases regulate the assembly of multimolecular focal complexes associated with actin stress fibers, lamellipodia, and filopodia. Cell 81,53 -62.[Medline]
Nobes, C. D. and Hall, A. (1999). Rho GTPases
control polarity, protrusion, and adhesion during cell movement. J.
Cell Biol. 144,1235
-1244.
Nolan, K. M., Barrett, K., Lu, Y., Hu, K. Q., Vincent, S. and
Settleman, J. (1998). Myoblast city, the Drosophila
homolog of DOCK180/CED-5, is required in a Rac signaling pathway utilized for
multiple developmental processes. Genes Dev.
12,3337
-3342.
O'Connor, K. L., Nguyen, B. K. and Mercurio, A. M.
(2000). RhoA function in lamellae formation and migration is
regulated by the alpha6beta4 integrin and cAMP metabolism. J. Cell
Biol. 148,253
-258.
Oude Weernink, P. A., Schulte, P., Guo, Y., Wetzel, J., Amano,
M., Kaibuchi, K., Haverland, S., Voss, M., Schmidt, M., Mayr, G. W. and
Jakobs, K. H. (2000). Stimulation of
phosphatidylinositol-4-phosphate 5-kinase by Rho-kinase. J. Biol.
Chem. 275,10168
-10174.
Palecek, S. P., Huttenlocher, A., Horwitz, A. F. and
Lauffenburger, D. A. (1998). Physical and biochemical
regulation of integrin release during rear detachment of migrating cells.
J. Cell Sci. 111,929
-940.
Paterson, H. F., Self, A. J., Garrett, M. D., Just, I., Aktories, K. and Hall, A. (1990). Microinjection of recombinant p21rho induces rapid changes in cell morphology. J. Cell Biol. 111,1001 -1007.[Abstract]
Pletjushkina, O. J., Rajfur, Z., Pomorski, P., Oliver, T. N., Vasiliev, J. M. and Jacobson, K. A. (2001). Induction of cortical oscillations in spreading cells by depolymerization of microtubules. Cell Motil. Cytoskel. 48,235 -244.[Medline]
Pollard, T. D., Blanchoin, L. and Mullins, R. D. (2000). Molecular mechanisms controlling actin filament dynamics in nonmuscle cells. Annu. Rev. Biophys. Biomol. Struct. 29, 545-576.[Medline]
Price, L. S., Leng, J., Schwartz, M. A. and Bokoch, G. M.
(1998). Activation of Rac and Cdc42 by integrins mediates cell
spreading. Mol. Biol. Cell
9,1863
-1871.
Reddien, P. W. and Horvitz, H. R. (2000). CED-2/CrkII and CED-10/Rac control phagocytosis and cell migration in Caenorhabditis elegans. Nat. Cell Biol. 2, 131-136.[Medline]
Ren, X. D. and Schwartz, M. A. (1998). Regulation of inositol lipid kinases by Rho and Rac. Curr. Opin. Genet. Dev. 8,63 -67.[Medline]
Ren, Y., Li, R., Zheng, Y. and Busch, H.
(1998). Cloning and characterization of GEF-H1, a
microtubule-associated guanine nucleotide exchange factor for Rac and Rho
GTPases. J. Biol. Chem.
273,34954
-34960.
Rickert, P., Weiner, O. D., Wang, F., Bourne, H. R. and Servant,
G. (2000). Leukocytes navigate by compass: roles of
PI3K and its lipid products. Trends Cell Biol.
10,466
-473.[Medline]
Ridley, A. (2000). Rho GTPases. Integrating
integrin signaling. J. Cell Biol.
150,F107
-109.
Ridley, A. J. (1999). Stress fibres take shape. Nat. Cell Biol. 1,E64 -66.[Medline]
Ridley, A. J. (2001). Rho proteins: linking signalling with membrane trafficking. Traffic 2, 303-310.[Medline]
Rivero, F., Dislich, H., Glockner, G. and Noegel, A. A.
(2001). The Dictyostelium discoideum family of
Rho-related proteins. Nucl. Acids Res.
29,1068
-1079.
Roberts, A. W., Kim, C., Zhen, L., Lowe, J. B., Kapur, R., Petryniak, B., Spaetti, A., Pollock, J. D., Borneo, J. B., Bradford, G. B. et al. (1999). Deficiency of the hematopoietic cell-specific Rho family GTPase Rac2 is characterized by abnormalities in neutrophil function and host defense. Immunity 10,183 -196.[Medline]
Rottner, K., Hall, A. and Small, J. V. (1999). Interplay between Rac and Rho in the control of substrate contact dynamics. Curr. Biol. 9,640 -648.[Medline]
Royal, I., Lamarche-Vane, N., Lamorte, L., Kaibuchi, K. and
Park, M. (2000). Activation of cdc42, rac, PAK, and
rho-kinase in response to hepatocyte growth factor differentially regulates
epithelial cell colony spreading and dissociation. Mol. Biol.
Cell 11,1709
-1725.
Sahai, E., Olson, M. F. and Marshall, C. J.
(2001). Cross-talk between Ras and Rho signalling pathways in
transformation favours proliferation and increased motility. EMBO
J. 20,755
-766.
Sander, E. E., van Delft, S., ten Klooster, J. P., Reid, T., van
der Kammen, R. A., Michiels, F. and Collard, J. G. (1998).
Matrix-dependent Tiam1/Rac signaling in epithelial cells promotes either
cell-cell adhesion or cell migration and is regulated by phosphatidylinositol
3-kinase. J. Cell Biol.
143,1385
-1398.
Sander, E. E., ten Klooster, J. P., van Delft, S., van der
Kammen, R. A. and Collard, J. G. (1999). Rac downregulates
Rho activity: reciprocal balance between both GTPases determines cellular
morphology and migratory behavior. J. Cell Biol.
147,1009
-1022.
Schmitz, A. A., Govek, E. E., Bottner, B. and Van Aelst, L. (2000). Rho GTPases: signaling, migration, and invasion. Exp. Cell Res. 261,1 -12.[Medline]
Schwartz, M. A. and Shattil, S. J. (2000). Signaling networks linking integrins and rho family GTPases. Trends Biochem. Sci. 25,388 -391.[Medline]
Scita, G., Nordstrom, J., Carbone, R., Tenca, P., Giardina, G., Gutkind, S., Bjarnegard, M., Betsholtz, C. and Di Fiore, P. P. (1999). EPS8 and E3B1 transduce signals from Ras to Rac. Nature 401,290 -293.[Medline]
Seabra, M. C. (1998). Membrane association and targeting of prenylated Ras-like GTPases. Cell Signal. 10,167 -172.[Medline]
Self, A. J. and Hall, A. (1995). Purification of recombinant Rho/Rac/G25K from Escherichia coli. Methods Enzymol. 256,3 -10.[Medline]
Sells, M. A., Boyd, J. T. and Chernoff, J.
(1999). p21-activated kinase 1 (Pak1) regulates cell motility in
mammalian fibroblasts. J. Cell Biol.
145,837
-849.
Servant, G., Weiner, O. D., Herzmark, P., Balla, T., Sedat, J.
W. and Bourne, H. R. (2000). Polarization of chemoattractant
receptor signaling during neutrophil chemotaxis.
Science 287,1037
-1040.
Sin, W. C., Chen, X. Q., Leung, T. and Lim, L.
(1998). RhoA-binding kinase alpha translocation is facilitated by
the collapse of the vimentin intermediate filament network. Mol.
Cell. Biol. 18,6325
-6339.
Spaargaren, M. and Bos, J. L. (1999). Rab5
induces Rac-independent lamellipodia formation and cell migration.
Mol. Biol. Cell 10,3239
-3250.
Stanyon, C. A. and Bernard, O. (1999). LIM-kinase1. Int. J. Biochem. Cell Biol. 31,389 -394.[Medline]
Sugihara, K., Nakatsuji, N., Nakamura, K., Nakao, K., Hashimoto, R., Otani, H., Sakagami, H., Kondo, H., Nozawa, S., Aiba, A. and Katsuki, M. (1998). Rac1 is required for the formation of three germ layers during gastrulation. Oncogene 17,3427 -3433.[Medline]
Sumi, T., Matsumoto, K., Takai, Y. and Nakamura, T.
(1999). Cofilin phosphorylation and actin cytoskeletal dynamics
regulated by rho- and Cdc42-activated LIM-kinase 2. J. Cell
Biol. 147,1519
-1532.
Tanabe, K., Tachibana, T., Yamashita, T., Che, Y. H., Yoneda,
Y., Ochi, T., Tohyama, M., Yoshikawa, H. and Kiyama, H.
(2000). The small GTP-binding protein TC10 promotes nerve
elongation in neuronal cells, and its expression is induced during nerve
regeneration in rats. J. Neurosci.
20,4138
-4144.
Tian, L., Nelson, D. L. and Stewart, D. M.
(2000). Cdc42-interacting protein 4 mediates binding of the
Wiskott-Aldrich syndrome protein to microtubules. J. Biol.
Chem. 275,7854
-7861.
Tolias, K. F., Hartwig, J. H., Ishihara, H., Shibasaki, Y., Cantley, L. C. and Carpenter, C. L. (2000). Type I alpha phosphatidylinositol-4-phosphate 5-kinase mediates Rac-dependent actin assembly. Curr. Biol. 10,153 -156.[Medline]
Tominaga, T., Sahai, E., Chardin, P., McCormick, F., Courtneidge, S. A. and Alberts, A. S. (2000). Diaphanous-related formins bridge Rho GTPase and Src tyrosine kinase signaling. Mol. Cell 5,13 -25.[Medline]
Totsukawa, G., Yamakita, Y., Yamashiro, S., Hartshorne, D. J.,
Sasaki, Y. and Matsumura, F. (2000). Distinct roles of ROCK
(Rho-kinase) and MLCK in spatial regulation of MLC phosphorylation for
assembly of stress fibers and focal adhesions in 3T3 fibroblasts.
J. Cell Biol. 150,797
-806.
Valgeirsdottir, S., Claesson-Welsh, L., Bongcam-Rudloff, E.,
Hellman, U., Westermark, B. and Heldin, C. H. (1998). PDGF
induces reorganization of vimentin filaments. J. Cell
Sci. 111,1973
-1980.
Van Aelst, L. and D'Souza-Schorey, C. (1997).
Rho GTPases and signaling networks. Genes Dev.
11,2295
-2322.
van Horck, F. P., Ahmadian, M. R., Haeusler, L. C., Moolenaar,
W. H. and Kranenburg, O. (2001). Characterization of
p190RhoGEF: a RhoA-specific guanine nucleotide exchange factor that interacts
with microtubules. J. Biol Chem.
276,4948
-4956.
van Leeuwen, F. N., van Delft, S., Kain, H. E., van der Kammen, R. A. and Collard, J. G. (1999). Rac regulates phosphorylation of the myosin-II heavy chain, actinomyosin disassembly and cell spreading. Nat. Cell Biol. 1, 242-248.[Medline]
Vignal, E., De Toledo, M., Comunale, F., Ladopoulou, A.,
Gauthier-Rouviere, C., Blangy, A. and Fort, P. (2000).
Characterization of TCL, a new GTPase of the Rho family related to TC10 and
Cdc42. J. Biol. Chem.
275,36457
-36464.
Waterman-Storer, C. M. and Salmon, E. (1999). Positive feedback interactions between microtubule and actin dynamics during cell motility. Curr. Opin. Cell Biol. 11, 61-67.[Medline]
Waterman-Storer, C. M., Worthylake, R. A., Liu, B. P., Burridge, K. and Salmon, E. D. (1999). Microtubule growth activates Rac1 to promote lamellipodial protrusion in fibroblasts. Nat. Cell Biol. 1,45 -50.[Medline]
Wenk, M. B., Midwood, K. S. and Schwarzbauer, J. E.
(2000). Tenascin-C suppresses Rho activation. J. Cell
Biol. 150,913
-920.
West, M. A., Prescott, A. R., Eskelinen, E. L., Ridley, A. J. and Watts, C. (2000). Rac is required for constitutive macropinocytosis by dendritic cells but does not control its downregulation. Curr. Biol. 10,839 -848.[Medline]
Westermarck, J. and Kahari, V. M. (1999).
Regulation of matrix metalloproteinase expression in tumor invasion.
FASEB J. 13,781
-792.
Wicki, A. and Niggli, V. (2001). The Rho/Rho-kinase and the phosphatidylinositol 3-kinase pathways are essential for spontaneous locomotion of Walker 256 carcinosarcoma cells. Int. J. Cancer 91,763 -771.[Medline]
Wilde, C., Chhatwal, G. S., Schmalzing, G., Aktories, K. and
Just, I. (2000). A novel C3-like ADP-ribosyltransferase from
Staphylococcus aureus modifying RhoE and Rnd3. J. Biol.
Chem. 276,9537
-9542.
Winge, P., Brembu, T., Kristensen, R. and Bones, A. M.
(2000). Genetic structure and evolution of RAC-GTPases in
Arabidopsis thaliana. Genetics
156,1959
-1971.
Wu, W. J., Erickson, J. W., Lin, R. and Cerione, R. A. (2000). The gamma-subunit of the coatomer complex binds Cdc42 to mediate transformation. Nature 405,800 -804.[Medline]
Zebda, N., Bernard, O., Bailly, M., Welti, S., Lawrence, D. S.
and Condeelis, J. S. (2000). Phosphorylation of ADF/cofilin
abolishes EGF-induced actin nucleation at the leading edge and subsequent
lamellipod extension. J. Cell Biol.
151,1119
-1128.
Zhao, Z. S., Manser, E., Loo, T. H. and Lim, L.
(2000). Coupling of PAK-interacting exchange factor PIX to GIT1
promotes focal complex disassembly. Mol. Cell. Biol.
20,6354
-6363.