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First published online 7 February 2007
doi: 10.1242/dev.000513
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Review |
1 Research Institute for Molecular Pathology, Dr Bohr-Gasse 7, A-1030 Vienna,
Austria.
2 Max Planck Institute - Molecular Cell Biology and Genetics, Pfotenhauerstrasse
108, D-01307 Dresden, Germany.
e-mails: cowan{at}imp.univie.ac.at; hyman{at}mpi-cbg.de
SUMMARY
The symmetry-breaking event during polarization of C. elegans embryos is an asymmetric rearrangement of the acto-myosin network, which dictates cell polarity through the differential recruitment of PAR proteins. The sperm-supplied centrosomes are required to initiate this cortical reorganization. Several questions about this event remain unanswered: how is the acto-myosin network regulated during polarization and how does acto-myosin reorganization lead to asymmetric PAR protein distribution? As we discuss, recent studies show that C. elegans embryos use two GTPases, RHO-1 and CDC-42, to regulate these two steps in polarity establishment. Although RHO-1 and CDC-42 control distinct aspects of polarization, they function interdependently to regulate polarity establishment in C. elegans embryos.
Introduction
The signals that induce cell polarization are as varied as the functions of
polarized cells themselves. Epithelial cells polarize in response to their
neighbors, whereas migrating neutrophils respond to longrange
chemoattractants. Mating yeast cells respond to each other, and alga zygotes
polarize according to light. Despite the variety of signals, the polarizing
signal leads to a seemingly conserved outcome: modulation of the cell cortex
to generate distinct cortical domains
(Gonczy and Hyman, 1996
;
Nelson, 2003). The cell
cortex, an acto-myosin-rich layer underlying the plasma membrane, provides
both mechanical stability and the capacity for force generation. Spatial
differences in the properties of the acto-myosin cortex allow the asymmetric
localization of effector molecules and thus provide polarity to a cell.
Cortical acto-myosin reorganization is central to generating polarized
domains, but it remains largely unclear how asymmetry in an acto-myosin cortex
is generated in response to a signal, and how changes in the cortex can be
translated into asymmetric function in a polarized cell.
Although the cortex is a fundamental part of cell polarity, we have only a
basic idea of its composition and mechanical properties. The cortex is rich in
actin and myosin, which interact to generate contractility. Actin assembles
into a linear polar polymer, which can provide structure or force for movement
(Stossel, 1984). Myosin is an
actin-dependent ATPase; it uses energy to undergo a conformational change that
allows movement along actin filaments
(Warrick and Spudich, 1987).
The myosin found in the cortex (type II myosin) can form tail-to-tail
multimers, called myosin filaments, which cross-link actin
(Niederman and Pollard, 1975;
Pollard et al., 1978;
Yumura and Fukui, 1985). The
classical view of acto-myosin contraction is based largely on the mechanics of
muscle sarcomeres, which contain highly ordered and polarized arrays of actin
filaments cross-linked by myosin filaments
(Fig. 1A). The acto-myosin
arrays interact to generate anti-parallel sliding of actin, and thus either
uniform contraction or uniform relaxation
(Huxley, 2000).
In the cell cortex, by contrast, actin filaments do not seem to have an
ordered arrangement and thus the mechanics of cortical contraction are less
clear. However, a simple model of the cortex has emerged
(Fig. 1B). The cortical
acto-myosin network consists of actin filaments, myosin filaments and
additional cross-linking proteins (Adelman
and Taylor, 1969; Hatano and
Oosawa, 1966; Hatano and
Tazawa, 1968; Pollard,
1981). The actin filaments are arranged isotropically within the
cortex (Kane, 1983;
Pollard and Korn, 1973;
Stossel, 1984), but without
any particular orientation, and are connected to each other by myosin and
other cross-linking proteins. Myosin-driven motility slides the filaments
against each other (Condeelis and Taylor,
1977; Taylor et al.,
1976), as in sarcomeres, but because of the random filament
orientation, the cortex undergoes local contraction complemented by local
relaxation (Fig. 1B). The
result of cortical contraction is regions of high acto-myosin density
surrounded by holes in the network (Kane,
1983). Modifications of cortical contractility can result in both
reorganization of the cell and remodeling of the cortex
(Hellewell and Taylor, 1979;
Janson et al., 1991;
Janson and Taylor, 1993;
Stendahl and Stossel, 1980;
Taylor and Fechheimer, 1982),
both of which are essential aspects of domain formation.
Several studies in 2006 revealed important insights into the mechanisms that generate an asymmetric acto-myosin network in C. elegans embryos and how the acto-myosin network, in turn, contributes to the asymmetric distribution of PAR proteins. The mechanics of cortical polarization in C. elegans embryos appear to be conserved in a range of biological contexts. These recent studies suggest that the molecules that regulate cortical polarization in C. elegans embryos are also conserved. In this review, we summarize current ideas on how the activity of the acto-myosin network is related to the establishment of polarity in C. elegans embryos.
Polarity establishment in C. elegans
The C. elegans embryo is an excellent model system in which to
address polarity establishment. The anterior-posterior (AP) axis in one-cell
C. elegans embryos is defined by two coordinated cortical polarities:
PAR polarity and contractile polarity
(Cowan and Hyman, 2004a;
Munro, 2006). PAR polarity
involves the formation of two complementary cortical domains, each occupying
half of the one-cell embryo (Fig.
2B). The conserved anterior PAR complex (PAR-3-PAR-6-PKC-3)
localizes to one half; PAR-1 and PAR-2 localize to the other half, defining
the posterior. PAR polarity correlates with contractile polarity. The anterior
cortex of one-cell embryos is highly dynamic, undergoing cycles of cortical
contraction and regression, whereas the posterior cortex is quiescent
(Fig. 2). How are these
polarities established? C. elegans oocytes lack developmental
polarity, and fertilization itself does not induce polarization of the zygote.
Following fertilization, the oocyte nucleus undergoes two rounds of meiotic
divisions, and upon entry into the first mitotic cell cycle the embryo cortex
remains unpolarized. Cortical ingressions/regressions occur throughout the
surface, corresponding to contractility generated by a cortical acto-myosin
network (Fig. 2C). The anterior
PAR complex localizes uniformly around the cortex
(Fig. 2B). Within the first two
minutes of the cell cycle, polarity is initiated in the embryo. Polarity
establishment requires centrosomes, which lie near the cortex at the time of
polarization (Fig. 2A)
(Cowan and Hyman, 2004b;
Cuenca et al., 2003;
Hamill et al., 2002;
O'Connell et al., 2000). The
earliest visual indication of polarity establishment is that contractions
cease in the small region of the cortex overlying the centrosomes, where the
acto-myosin network disassembles (Fig.
2C) (Munro et al.,
2004). This region correlates with the site of sperm entry
(Goldstein and Hird, 1996),
although whether this coincidence is an active or passive consequence of
fertilization remains unknown. The local change in contractility breaks the
symmetry of the egg. The acto-myosin network, contractility, and the anterior
PAR complex proceed to shrink in a coordinated manner until they occupy half
of the embryo (Fig. 2B,C)
(Cuenca et al., 2003;
Munro et al., 2004;
Strome, 1986). This
establishes the anterior domain. The complementary cortical domain is
non-contractile and accumulates posterior PAR proteins (PAR-1 and PAR-2)
(Cuenca et al., 2003). Thus,
roughly eight minutes after the initiation of polarity, the embryo consists of
anterior and posterior cortical domains that exhibit different properties of
acto-myosin contractility: the anterior domain is contractile and the
posterior domain is non-contractile.
|
;
Hill and Strome, 1988;
Hill and Strome, 1990;
Severson et al., 2002;
Severson and Bowerman, 2003;
Shelton et al., 1999). The
initiation of contractile polarity is independent of PAR polarity: acto-myosin
contractility becomes asymmetrically localized in Par mutant embryos
(Fig. 3B,C)
(Kirby et al., 1990;
Munro et al., 2004). PAR-3,
however, contributes to cortical contractility, and thus par-3
mutants fail to completely segregate the acto-myosin network
(Fig. 3C). Contractile polarity
appears to be sufficient for some aspects of PAR polarity: PAR-6 is
asymmetrically distributed during polarity establishment in par-2
mutants (Fig. 3B)
(Cuenca et al., 2003). It has
been proposed that the anterior PAR complex could be physically linked to the
contractile acto-myosin cortex, such that segregation of acto-myosin
contractility pulls the anterior PAR proteins with it
(Munro et al., 2004). How
posterior PAR proteins recognize the non-contractile cortex is less clear. The
absence of the anterior PAR complex from the non-contractile cortex may allow
PAR-2 to localize to the cortex (Hao et
al., 2006), although the precise mechanism is likely to be more
complex.
|
Actin and myosin provide the structural basis of contractility, and changes in the acto-myosin network change the contractile properties of the cortex. The modulation of contractility appears to play a central role in polarity establishment in C. elegans embryos. Prior to polarization, the entire cortex undergoes contractions. After polarization, half the cortex is contractile and the other half is non-contractile. Thus, polarity establishment requires the inhibition of contractility at the posterior cortex. How is contractility regulated?
A crucial feature of an acto-myosin network is the ability to transition
between a `gel' and a `sol'. Gels consist of highly cross-linked actin
filaments (Fig. 1B). The sol
(solation) state comprises short, non-cross-linked actin filaments
(Fig. 1C). The spatial
regulation of these two states can create distinct domains
(Hellewell and Taylor, 1979;
Janson et al., 1991;
Janson and Taylor, 1993;
Stendahl and Stossel, 1980;
Taylor and Fechheimer, 1982).
For instance, localized solation of an acto-myosin gel in vitro causes the
remaining network to contract away from the weakened point, resulting in a
contractile and a non-contractile domain
(Fig. 1D)
(Janson and Taylor, 1993).
Classically, it was proposed that such an asymmetric contraction could
polarize one-cell C. elegans embryos
(Hird and White, 1993;
White, 1990;
White and Borisy, 1983).
Solation can occur in two ways: reducing actin filament length, and
eliminating myosin activity (Taylor and
Fechheimer, 1982). Actin filament length can be influenced by the
number of nucleators, actinsevering proteins or actin-capping proteins
(Pollard and Cooper, 1986;
Stossel et al., 1985).
Currently, there is no evidence that such molecules are required to modulate
contractility in C. elegans embryos, although redundancy may
complicate the analysis of phenotypes. Myosin appears to be a more likely
control point in C. elegans embryos. Myosin activity is
regulated positively by phosphorylation
(Adelstein and Conti, 1975;
Craig et al., 1983;
Umemoto et al., 1989), and the
phosphorylation state is determined by the balance of kinase and phosphatase
activities (Amano et al., 1996;
Frearson and Perry, 1975;
Kureishi et al., 1997;
Morgan et al., 1976;
Pires and Perry, 1977). Myosin
regulation is highly conserved, and the small GTPase Rho provides the central
point of control (Kimura et al.,
1996). Recent work has uncovered that Rhodependent signaling is
central to the regulation of contractility in C. elegans embryos.
|
Three recently published papers have shown that the activity of Rho is
required for contractility and PAR polarity in C. elegans embryos
(Jenkins et al., 2006;
Motegi and Sugimoto, 2006;
Schonegg and Hyman, 2006). The
studies show that if Rho (RHO-1) is uniformly active, contractility occurs
over the entire surface (Fig.
3G); if Rho is uniformly inactive, contractility is inhibited
throughout (Fig. 3E,F). In both
cases, the anterior PAR complex does not shrink to the anterior, remaining
uniformly distributed over the entire embryo surface. The activity of Rho
during polarity establishment is controlled by two factors, which also
regulate Rho during cytokinesis (Glotzer,
2005): the RhoGEF ECT-2 and the RhoGAP CYK-4. GTPase-activating
proteins (GAPs) promote GTP hydrolysis, leading to inactivation of signaling.
Guanidine nucleotide exchange factors (GEFs) accelerate the loading of GTP,
leading to activation of signaling. Therefore, CYK-4 and ECT-2 are
antagonistic with respect to contractility: RhoGAP CYK-4 inhibits Rho and thus
inhibits contractility, whereas RhoGEF ECT-2 activates Rho and thus activates
contractility.
Polarity requires the local regulation of contractility - specifically,
suppression of contractility at the posterior. This change in contractility
could be mediated by local inactivation of Rho, in principle provided either
by downregulation of the GEF or upregulation of the GAP. Indeed, Motegi and
Sugimoto (Motegi and Sugimoto,
2006) show that RhoGEF ECT-2 is absent from the non-contractile
region of the cortex (Fig. 2E).
On the other hand, Jenkins et al. (Jenkins
et al., 2006) show that RhoGAP CYK-4 localizes to a limited
cortical region adjacent to the sperm-supplied centrosomepronucleus complex
(Fig. 2E). RhoGAP CYK-4 and
RhoGEF ECT-2 might show reciprocal localizations when polarization is
initiated, although this has not been demonstrated. Thus, the initial
contractile asymmetry of C. elegans embryos could be generated, at
least in part, as follows: a local region of the cortex becomes
non-contractile through local downregulation of Rho signaling, mediated by the
presence of RhoGAP CYK-4 and exclusion of RhoGEF ECT-2 from this region.
|
;
Janson and Taylor, 1993), as
discussed above, the initial break in contractility could cause the remaining
contractile network to contract away from the break
(Fig. 1D). In theory, however,
in the context of the cell, contractility could be re-established in the
nascent posterior. Thus, there should be a mechanism that ensures that
contractility remains suppressed in the posterior but continues in the
anterior. Motegi and Sugimoto (Motegi and
Sugimoto, 2006) provide evidence for such a mechanism. Rho, and to
a lesser extent its activator RhoGEF ECT-2, shrink with the contractile
acto-myosin network (Fig.
2D,E), leaving the non-contractile domain devoid of the regulatory
molecules that support contractility. Thus, the asymmetric segregation of both
contractility and the capacity for contractility ensure that only the anterior
domain is contractile. Furthermore, the continued contractility of the
anterior domain seems to support continued shrinking of the domain, resulting
in a feedback loop. The contractile domain may stop shrinking either when
further collapse is limited by physical constraints of the contractile
network, or when contractility is eliminated, for instance by the
downregulation of Rho, which could be mediated by the disappearance of RhoGEF
ECT-2. Spatial and temporal regulation of Rho activity
The initial event in polarity establishment comes in the form of a signal
that dictates where and when the cell should polarize. In many polarized
embryos, there is a general consensus regarding the signal: the site of
fertilization and the sperm-contributed centrosomes establish the initial axis
(Astrow et al., 1989;
Carre and Sardet, 1984;
Dondua et al., 1997;
Eckberg, 1981;
Fernandez et al., 1998;
Freeman, 1978;
Hable and Kropf, 2000;
Hasegawa et al., 2004;
Luetjens and Dorresteijn,
1998; Roegiers et al.,
1995; Ubbels et al.,
1983). Likewise, in C. elegans embryos, centrosomes are
essential for polarity establishment
(Cowan and Hyman, 2004b). How
does the centrosome relate to the regulation of Rho activity? The initial
asymmetric regulation of Rho activity may require centrosomes. RhoGEF ECT-2
exclusion from the presumptive posterior depends on centrosomes
(Motegi and Sugimoto, 2006);
the centrosome-dependency of asymmetric RhoGAP CYK-4 has not been
investigated. Two populations of RhoGAP CYK-4 appear to exist in one-cell
C. elegans embryos: sperm- and oocyte-derived. Whereas sperm-derived
CYK-4 is required for polarity establishment, oocyte-derived CYK-4 is not
(Jenkins et al., 2006). This
difference between paternally- and maternally-contributed RhoGAP CYK-4
suggests that the two populations may be differentially regulated. One
significant difference between the two CYK-4 pools may be their proximity to
centrosomes. The sperm delivers both paternal CYK-4 and centrosomes, and thus
they are likely to be near each other within the egg. Centrosomes, however,
can travel significant distances in the cytoplasm prior to the establishment
of polarity (Cowan and Hyman,
2004b), and whether the ultimate position of centrosomes at the
time of polarity establishment is random or predetermined is not known.
A second level of control during polarity establishment comes in the form
of temporal regulation. About thirty minutes pass from fertilization until
entry into the first mitotic cell cycle, a time during which the polarizing
signal is kept inactive. Upon entry into the cell cycle, the entire embryo
cortex is contractile, suggesting that the polarity establishment mechanism is
not yet active. After approximately one and a half minutes, however,
contractions in the presumptive posterior cease, indicative of polarity
establishment and the activation of RhoGAP CYK-4. During this initial one and
a half minutes of the first cell cycle, the centrosome recruits a number of
centrosomal proteins that are required for polarity
(Cowan and Hyman, 2004b). A
delay in centrosomal protein recruitment - for instance, in embryos depleted
of cyclin E-CDK-2 - leads to a failure to establish polarity
(Cowan and Hyman, 2006). Thus,
one possibility is that the recruitment of the centrosomal proteins may be a
prerequisite for activation of RhoGAP CYK-4. A second possibility is that
centrosome assembly and Rho activity may be regulated in parallel by a common
upstream pathway. A common regulator could depend on global signals, such as
the `egg-to-embryo' transition pathways
(Pellettieri et al.,
2003).
|
In order for polarity to be established in the embryo, downstream polarity
effectors - in this case the PAR proteins - must recognize contractile
polarity. A parallel analysis of the shrinking myosin network and the anterior
PAR domain during polarization has shown that these two cortical structures
move with similar kinetics (Munro et al.,
2004). Thus, it was suggested that the anterior PAR complex is
physically connected to the acto-myosin cortex. When the contractile cortex
moves, so too would the PAR complex. This simple idea could account for how
anterior PAR proteins localize specifically to the contractile cortex, i.e.
they are attached. But how are they attached? Two recent studies implicate the
small GTPase CDC-42 in helping to mediate the linkage of anterior PAR proteins
to the acto-myosin network (Aceto et al.,
2006; Schonegg and Hyman,
2006). CDC-42 moves away from the non-contractile posterior during
polarity establishment (Motegi and
Sugimoto, 2006; Schonegg and
Hyman, 2006), similar to the acto-myosin network and the anterior
PAR complex (Fig. 4A), and the
cortical localization of PAR-6 depends, in part, on CDC-42
(Aceto et al., 2006;
Gotta et al., 2001;
Schonegg and Hyman, 2006).
The extent to which CDC-42 is required for cortical localization of the
anterior PAR proteins remains a matter of debate. Complete CDC-42
loss-of-function embryos have not been examined; RNAimediated CDC-42 depletion
most likely represents a partial loss-of-function phenotype, perhaps
explaining the varying interpretations of the role of CDC-42 in anterior PAR
protein localization. CDC-42 binds to the anterior PAR complex in many
polarized cell types (Gotta et al.,
2001; Hutterer et al.,
2004; Joberty et al.,
2000; Johansson et al.,
2000; Lin et al.,
2000; Qiu et al.,
2000), and it is well established that CDC-42 interacts with the
CRIB domain of its binding partners. Likewise, in C. elegans, PAR-6
contains a CRIB domain, which indeed is required for CDC-42-PAR-6 interactions
in vitro (Aceto et al., 2006).
Aceto et al. (Aceto et al.,
2006) have shown that PAR-6 that lacks the CRIB domain
(PAR-6-
CRIB) shows reduced cortical localization as compared with
wild-type PAR-6; the reduction in cortical PAR-6-CRIB localization is
enhanced if endogenous PAR-6 is not present
(Fig. 4C). These results
demonstrate that the CRIB domain of PAR-6, which interacts with CDC-42, is
required for efficient PAR-6 cortical localization, but that PAR-6-PAR-6
interactions may contribute to cortical localization independently of
CDC-42-PAR-6-CRIB binding. However, the data do not exclude the possibility
that PAR-6 and CDC-42 can interact at the cortex through PAR-6-CRIBindependent
mechanisms. Finally, a recent report (Beers
and Kemphues, 2006) suggests that PAR-3 is required for the
efficient localization of PAR-6 to the cortex independently of CDC-42. This
study supports the idea that multiple pathways contribute to the cortical
localization of the anterior PAR complex, but the relative contributions of
these pathways or the existence of additional pathways remain unknown.
In embryos depleted of Rho or RhoGEF ECT-2, myosin can shrink
asymmetrically despite the apparent lack of contractility; neither the
anterior PAR complex nor CDC-42, however, follows the acto-myosin network and
PAR polarity is not established (Fig.
4E) (Motegi and Sugimoto,
2006; Schonegg and Hyman,
2006). Why don't CDC-42 and the anterior PAR proteins segregate
with the acto-myosin network? One possibility is that the opportunity for
segregation of CDC-42 and the anterior PAR complex is temporally regulated.
The asymmetric segregation of myosin in rho-1(RNAi) or
ect-2(RNAi) embryos occurs approximately ten minutes after the normal
time of polarity establishment. Perhaps CDC-42, and with it the anterior PAR
complex, can only follow the acto-myosin network during the polarity
establishment phase (Cuenca et al.,
2003). A second possibility is that the segregation of CDC-42 and
the anterior PAR complex requires a specific acto-myosin network property. The
myosin network in rho-1(RNAi) or ect-2(RNAi) embryos appears
much less contractile and exhibits fewer interconnected foci than in wild-type
embryos (Fig. 4A,D,E)
(Jenkins et al., 2006;
Motegi and Sugimoto, 2006;
Schonegg and Hyman, 2006).
Perhaps CDC-42 can only respond to a highly contractile, interlinked
acto-myosin network. A third possibility is that CDC-42 is directly regulated
by Rho signaling. PAR-6 and CDC-42 localize to the cortex in
rho-1(RNAi) and ect-2(RNAi) embryos
(Fig. 4D), suggesting overall
CDC-42 function is not disrupted when Rho signaling is inhibited. However, Rho
signaling may control a CDC-42 activity that is required specifically for its
segregation with the acto-myosin cortex. Thus, CDC-42 appears to function in
two aspects of polarization: CDC-42 helps link PAR-6 to the cortex, and CDC-42
coordinates segregation of the anterior PAR complex with the acto-myosin
cortex.
Supporting continued contractility: the anterior domain
Following the initial symmetry-breaking event that establishes contractile
and non-contractile cortical regions, a feedback loop appears to take over,
mediated in part by the anterior segregation of Rho and its activator RhoGEF
ECT-2. Further promotion of contractility in the anterior comes from two
additional sources, which also segregate exclusively with the anterior: the
anterior PAR complex and CDC-42 (Kirby et
al., 1990; Motegi and
Sugimoto, 2006; Munro et al.,
2004; Schonegg and Hyman,
2006). Several lines of evidence suggest that acto-myosin
contractility is aberrant in embryos lacking PAR-3 or CDC-42 function.
par-3 mutants exhibit altered anterior and posterior domain sizes,
such that the anterior domain does not segregate completely
(Fig. 3C)
(Kirby et al., 1990;
Munro et al., 2004). This
phenotype may be consistent with reduced contractility. par-3(RNAi)
embryos also exhibit inefficient CDC-42 segregation toward the anterior
(Motegi and Sugimoto, 2006), a
likely correlative to the reduced acto-myosin network movement. Embryos
depleted of CDC-42 have altered contractility dynamics, such that the cortical
ingressions that result from acto-myosin contraction in the anterior are
shallower and more static than in wild-type embryos
(Fig. 4B)
(Schonegg and Hyman, 2006).
Finally, in both par-3(RNAi) and cdc-42(RNAi) embryos,
RhoGEF ECT-2 movement out of the non-contractile region is also reduced, a
phenotype similar to that in embryos depleted of myosin activity, suggesting
again that PAR-3 and CDC-42 regulate contraction
(Motegi and Sugimoto, 2006).
It remains unresolved whether the cdc-42(RNAi) contractility defects
result from reduced cortical localization of the anterior PAR complex, or
reflect a direct role of CDC-42 in contractility. In both cases, contractility
is not abolished, consistent with the ability of par-3 and
cdc-42 mutant embryos to initiate contractile polarity; the dynamics
of contractility, however, appear to be altered. Thus, several molecules
associated with the anterior cortex, including RHO-1, ECT-2, PAR-3 and CDC-42,
promote cortical contractility and help generate the necessary mechanical
force for continued anterior-directed segregation of the contractile domain
during polarity establishment.
Polarity establishment and the cortex: a model
A possible model of polarity establishment in C. elegans embryos emerges from the insight offered by these recent studies, although many of the steps remain speculative (Fig. 5). RHO-1, RhoGEF ECT-2, the acto-myosin network, CDC-42 and the anterior PAR complex behave as a unit that promotes contractility. RhoGEF ECT-2 activates RHO-1, RHO-1 activates acto-myosin network contractility, CDC-42 helps link the anterior PAR complex to the cortex, and CDC-42 and the anterior PAR complex promote acto-myosin network contractility. Sperm-supplied RhoGAP CYK-4 and centrosomes cause this contractile unit to be displaced from a small region of the cortex, resulting in a break in the contractile network (Fig. 1D). This break in the network results in a progressive collapse of the remaining contractile unit in a sort of `snow ball' effect: once contractile asymmetry is initiated, it cannot be reversed, as the positive regulators of contractility are found only within the contractile unit.
Conclusion
Testing this model and filling in both its molecular and mechanical gaps will be the next challenges in understanding the relationship between cortical activity and cell polarization. Is downregulation of myosin activity sufficient for the symmetry-breaking event during polarization? GTPases can modulate multiple cellular pathways; perhaps RHO-1 (or another GTPase regulated by RhoGAP CYK-4) controls additional polarity establishment pathways. How is RhoGAP CYK-4 temporally and spatially regulated? Phosphorylation states modulate the activity of many GEFs and GAPs, and several kinases are required for polarization. How do centrosomes integrate into the Rho signaling pathway? Centrosomes are essential for polarity establishment but their precise role remains unknown. How is the anterior PAR complex linked to the cortex? CDC-42 has an important role, but alternative pathways also exist. How does Rho ensure that CDC-42 follows the acto-myosin network as it shrinks? Different properties mediate segregation of CDC-42 compared with acto-myosin. How do CDC-42 and the anterior PAR complex regulate contractility? Downstream targets of these molecules may modulate the cortex through Rho or through alternative pathways, such as actin polymerization. As more details of the molecular control of polarity establishment are uncovered, it will become possible to understand which of the activities in C. elegans represent general principles in polarity establishment, and which are specialized to the particular case of embryonic polarity establishment.
ACKNOWLEDGMENTS
The recent C. elegans papers referred to in this review are not summarized here in full. All of the studies offer significant insights that we have not covered, and we therefore encourage interested readers to consult the original publications. We thank Ed Munro (University of Washington) for insightful discussions and to three reviewers for helpful suggestions and for pointing out omissions.
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