Cellular Symmetry Breaking during Caenorhabditis elegans Development

21 Pages·2009·1.23 MB·English

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Cellular Symmetry Breaking during
Caenorhabditis elegans Development
EdwinMunro1andBruceBowerman2
1CenterforCellDynamics,FridayHarborLabs,620UniversityRd,FridayHarborWA98250
2InstituteofMolecularBiology,UniversityofOregon,1370FranklinBoulevard,Eugene,Oregon97403
Correspondence:[email protected]
The nematode worm Caenorhabditis elegans has produced a wellspring of insights into
mechanismsthatgoverncellularsymmetrybreakingduringanimaldevelopment.Herewe
focuson twohighly conserved systemsthat underliemanyof thekeysymmetry-breaking
events that occur during embryonic and larval development in the worm. One involves
the interplay between Par proteins, Rho GTPases, and the actomyosin cytoskeleton and
mediatesasymmetriccelldivisionsthatestablishthegermline.Theotheruseselementsof
theWnt signalingpathwayand ahighlyreiterativemechanism that distinguishesanterior
fromposteriordaughtercellfates.Muchofwhatweknowaboutthesesystemscomesfrom
intensive study of a few key events—Par/Rho/actomyosin-mediated polarization of the
zygoteinresponsetoasperm-derivedcueandtheWnt-mediatedinductionofendoderm
atthefour-cellstage.However,agrowingbodyofworkisrevealinghowC.elegansexploits
elements/variants of these systems to accomplish a diversity of symmetry-breaking tasks
throughoutembryonicandlarvaldevelopment.
Over the past few decades, the C. elegans then ensures differential inheritance of these
embryohasbecomeapremieresystemfor potentials. With respect to cell fates, the
studyingcellularsymmetrybreakinginadevel- output of these asymmetric divisions is amaz-
opmental context. During C. elegans develop- ingly diverse, yet the embryo seemsto accom-
ment, nearlyeverydivision producesdaughter plish this diversity through variants of a few
cells with different developmental trajectories. conserved symmetry-breaking systems. Thus
In some cases, these differences are imposed the C. elegans embryo provides an exceptional
on daughters before or after division through opportunitytoexplorenotonlythecoremech-
inductive signals, but many of these divi- anismsunderlyingcellularsymmetrybreaking,
sions are intrinsically asymmetric—an initial but also how evolution can reconfigure these
symmetry-breaking step creates polarized mechanismstododifferentbutrelated jobsin
distributions or activities of factors that multiplecontexts.
control developmental potential. Registration In this review, we focus most of ouratten-
of the cleavage plane with the axis of polarity tion on two conserved systems that together
Editors:RongLiandBruceBowerman
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E.MunroandB.Bowerman
account for much of the cellular asymmetry center to form the first mitotic spindle. The
observed during C. elegans embryogenesis. asymmetrically localized Par proteins in turn
The first, which is best known for its role shape the distributions of cytoplasmic factors
in the early asymmetric cell divisions that thatspecifydifferentdevelopmentalpotentials.
segregate germline from the soma, involves a They also shape the distributions of cortical
complex interplay between Par proteins, Rho- factors that position the mitotic spindle, and
familyGTPases,andtheactomyosincytoskele- thusthefirstcleavagefurrow,tosegregatecor-
ton.Interestingly,theembryoexploitselements rectly developmental potentials to unequally
of this same system to break symmetryduring sizeddaughters. Theselatereventsarecausally
cleavage furrow specification and to establish downstream of the symmetry-breaking mech-
apicobasal polarity in early embryonic cells anismandhavebeenextensivelyreviewedelse-
and in the first true embryonic epithelia. The where (Cowan and Hyman 2004a; Munro
second system we focus on involves an 2006; Goldstein and Macara 2007; Galli and
unusual application of WNT signaling path- van den Heuvel 2008). Here we focus on the
way components and is used reiteratively initial symmetry-breaking events that shape
throughoutembryonicandlarvaldevelopment cortical polarity in response to the sperm
to distinguish anterior and posterior daughter cue.WefocusonC.elegansbutalsociteregula-
cell fates. Rather than comprehensively review tory interactions known from work in other
these systems, we highlight topics not exten- organisms.
sivelycoveredinotherreviews.
AsymmetricContractionandCorticalFlows
SYMMETRYBREAKINGINP0 SegregateCorticalFactorsto
EstablishCellPolarity
The study of cellular symmetry breaking in
the C. elegans zygote (called P ) dates back to The outline of a mechanism for symmetry
0
the discoveries of germline P granule segre- breaking in P has emerged in the last few
0
gation and the partitioning-defective mutants years (Figs. 1 and 2). Near the end of meiosis,
(Strome and Wood 1983; Kemphues et al. the cortex is essentially unpolarized. The ulti-
1988)in the 1980s. The P0 zygote polarizes in mately “anterior” Par proteins (Par-3, Par-6,
responseto atransient cueassociatedwiththe and Pkc-3) are distributed throughout the
centrosomes/microtubule organizing center cortex, where they are required to prevent
(thespermMTOC)thatformsnear thesiteof accumulation of Par-1 and Par-2, probably
sperm entry about 25min after fertilization, through phosphorylation of Par-1 and Par-2
following meiosis (Goldstein and Hird 1996). by Pkc-3 (Benton and St Johnston 2003; Hao
Acting on an essentially unpolarized cell, the et al. 2006). Actomyosin contractility drives
sperm cue triggers a rapid reorganization of transient focal contractions throughout the
the cell cortex, accompanied by the de novo cortex, which concentrate F-actin and myosin
enrichmentofconservedpolaritydeterminants intofociwithacharacteristicspacingof (cid:2)2–3
known as Par proteins within complementary mm that then disassemble or disperse, with
anterior and posterior cortical domains. The lifetimes of (cid:2)60 s (Munro et al. 2004). These
PDZ domain proteins Par-3 and Par-6 and focalcontractionsareassociatedwithandpre-
the atypical protein kinase PKC-3 localize to sumably drive local membrane invaginations.
the anterior cortex, whereas the serine/threo- Movement of the sperm MTOC toward the
ninekinasePar-1andtheringdomainprotein cortex causes a rapid local cessation of focal
Par-2 localize to the posterior. These cortical contractility and an asymmetrical contraction
asymmetries are subsequently maintained as oftheremainingactomyosinnetwork,causing
the female pronucleus migrates to meet the a flow of cortical actomyosin toward the
malepronucleusintheposteriorofthezygote, opposite (future anterior) pole. These flows
and asthey move together toward the zygote’s transport Par-3/Par-6/Pkc-3, causing their
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CellularSymmetryBreakingduringCaenorhabditiselegansDevelopment
Fertilization
t ~ 25 minutes
Myosin II Par-3/Par-6/Pkc-3 Rho-1
F-actin Par-1 Par-2 Cdc-42
Establishment
of cortical
asymmetries
A B C
Figure1.Corticalrearrangementsunderlyingsymmetry-breakingandpolarityestablishmentinP0.(A)The
symmetry-breaking cue is associated with the microtubule organizing center (the sperm MTOC: Gray,
nucleus; cyan, centrosomes; yellow, microtubules) that forms near the site of sperm entry at the end of
meiosis (cid:2)25 min after fertilization. Near the end of meiosis, a cortical network containing F-actin (blue)
andnonmusclemyosinII(orange)drivesfocalcontractionsandcorticalruffling.Signal(s)fromtheMTOC
locally inhibit contractility, inducing asymmetrical contractions and cortical flow that segregate focal
contractionsandrufflingintoananteriorcapboundedontheposteriorbyadeeptransientfurrowcalledthe
pseudocleavage furrow.(B)ThesameflowconcentratesPar-3/Par-6/Pkc-3(green)withintheanteriorcap,
removinganinhibitoryinfluence(seetextandFig.2)thatallowsPar-1andPar-2(red)toconcentratewithin
a complementary posterior domain. (C) Cortical flows also segregate Rho-1 and Cdc-42, leading to their
concentrationwithinananteriorcap.Anterioristotheleftinthisandallsubsequentfigures.
enrichment within an anterior cap and their assemblyinvitro(seevanderGuchtandSykes
depletion from the posterior cortex. This in 2009 and Mullins 2009). It may also govern
turn allows Par-1 and Par-2 to associate with symmetry-breakingprocessesunderlyingdirec-
the cortex in a complementary posterior tional actin-dependent motility of endosomes
domain. In addition to cross-regulating one (Taunton et al. 2000), chromosomes (Li et al.
another, the Par proteins also feedback to 2008), and the episodic myosin-dependent
modulateactomyosindynamics,corticalflows, collapse of the cell cortex that underlies shape
andthustheirownredistributionsinwaysthat oscillations in cellular fragments (Paluch et al.
arestillnotwellunderstood(Fig.2). 2005).
How does the sperm cue work mechani- However, acute mechanical rupture seems
cally? One possibility is that it triggers the anunlikelyexplanationforsymmetrybreaking
mechanical rupture of a prestressed cortical in P . Direct observations fail to reveal a pro-
0
network, leading to the rapid discharge of nounced depletion of F-actin near the sperm
storedmechanicalstress.Thistypeofmechan- MTOC. This is inconsistent with mechanical
ical instability is thought to drive symmetry rupture of the actin networkandsuggeststhat
breaking and directional motility of beads new actin filaments must rapidly assemble to
coatedwithagentsthatpromoteactinfilament replace any that flow away from the sperm
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E.MunroandB.Bowerman
Passive cortical
transport
Cortical
flow
Contractile
Par-3
actomyosin
Par-1/Par-2 Rho-GTP Ect-2
Par-6 Pkc-3
Cortex/PM
Cytoplasm
Par-3 Par-1/Par-2 Myosin II Actin Rho-GDP Ect-2
Par-6 Pkc-3
MTOC
MTOC
Figure2.FeedbackinteractionsunderlyingsymmetrybreakingduringpolarityestablishmentinP0.Parproteins
and GTPases associate dynamically with the cortex/plasma membrane. F-actin and myosin II organize a
contractilenetworkthatgeneratescorticalflows,whichtransportactomyosin,Par-3/Par-6/Pkc-3,andsmall
GTPases(onlyRho-1isshownhere).The“anterior”and“posterior”Parproteinscross-inhibitoneanother’s
cortical association to help enforce complementarity. The Par proteins and small GTPases also feedback to
modulatecorticalcontractilityandthustheirownredistribution.
MTOC. Likewise, focal contractions persist as longer than the lifetimes of its elements.
the cortex flows toward the posterior (Munro Therefore cortical flows that establish polarity
etal.2004).Theaveragedurationofafocalcon- probably represent the response of a viscous
traction((cid:2)60s)isshortrelativetotheperiodof material to a persistent gradient of contractile
corticalflow((cid:2)8min),andtheactinfilaments tension, rather than the one-time discharge of
and myosin II that accumulate locally during aprestressedelasticmaterial.
each contraction disappear rapidly when the A likelysourceof thistension gradient is a
contractionterminates.Thusthecorticalacto- sustained gradient of Rho activity. Indeed,
myosin network turns over rapidly relative to severalrecentstudieshaveshownthatfocalcon-
the timescale of cortical flow, consistent with tractility and cortical flows require RhoA, and
observations in other cells (Theriot and that the onset of cortical flows is associated
Mitchison 1992; Watanabe and Mitchison with, and requires, a local inhibition of RhoA
2002). Such a network would dissipate stored in the vicinity of the sperm MTOC (Jenkins
elastic stress as its stress-bearing elements— et al. 2006; Motegi and Sugimoto 2006;
actin filaments, cross-linkers, and myosin Schonegg and Hyman 2006). This inhibition
motors—turnover, and therefore should is associated with local cortical depletion of a
behave like a viscous material on timescales Rho GEF called Ect-2 in the vicinity of the
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CellularSymmetryBreakingduringCaenorhabditiselegansDevelopment
sperm cue (Motegi and Sugimoto 2006). occurrence and kinetics of polarity establish-
Depletion of Ect-2 requires functional centro- ment (Tsai and Ahringer 2007); zygotes with
somes,butnotactivecontractility,makingita smaller asters took longer to establish cortical
prime candidate for mediating the initial polarity. An attractive scenario is that centro-
symmetry-breaking cue. It has also been somes(ormeioticchromosomesinthecaseof
suggested that this inhibition requires a Rho polarity reversal) produce a polarity-inducing
GAPcalledCyk-4,whoseactivitymaybelocal- signal,andthatmicrotubulearrayshelptotrans-
izedbythedepositionofpaternallysynthesized mitthatsignaltothenearbycortex,suggesting
mRNA/protein during sperm entry (Jenkins thattheasterisnormallyinvolvedintransmit-
etal.2006). ting a signal to the cortex, if not in producing
that signal. Although it is also possible that
microtubules could exert a direct mechanical
WHATISTHECUE?
effectonthe cortex,this remainsto beshown.
The identity of the cue(s) that trigger polari- Clearly, further progress in this area will
zation, and whether this cue involves micro- depend on identifying molecular components
tubules,hasbeenanongoingcontroversy.With of the cue beyond those factors required to
the possible exception of Cyk-4/RhoGAP form mature centrosomes and nucleate a
(Jenkinsetal.2006),mutantsthatfailinpolarity spermaster.Thesehavenotturnedupthusfar
establishmenthaveturnedouttoencodemater- in genome-wide screens, perhaps because of
nally supplied factors associated with centro- pleiotropicorredundantgenerequirements.
some assembly and maturation (O’Connell
et al. 2000; Hamill et al. 2002; Cowan and
ATHRESHOLDFORTRIGGERING
Hyman2004b),andUVlaser-inducedablation
POLARITYESTABLISHMENT?
of centrosomes can prevent polarity establish-
ment (CowanandHyman2004).Thuscentro- Some symmetry-breaking systems require a
somes, or something closely associated with polarizing cue. Others can spontaneously
them,arerequired.However,theonsetofpolar- break symmetry in the absence of any cues,
izationcoincideswiththeoutgrowthofacentro- oftenbyamplifyinglocalstochasticfluctuations
somallynucleatedmicrotubulearray(thesperm in the concentrations of participating mol-
aster),andthusithasbeendifficulttoseparatea ecules.Inthesecases,polarizingcues“choose”
requirementforcentrosomesthemselvesfroma the axis of symmetry breaking. C. elegans
requirement for the microtubule aster they zygotes in which centrosomes are ablated
produce. Indeed, several groups have shown genetically (Munro et al. 2004) or with a UV
that when meiosis failsor is slow tocomplete, laser(CowanandHyman 2004b)show normal
the acentriolar meiotic spindle can induce the focal contractility but do not spontaneously
formationofreversed,butstillcomplementary, breaksymmetry.Localcorticalflowsassociated
distributions of the Par proteins, actomyosin, withfocalcontractionsimplylocalgradientsof
and some, but not necessarily all, cytoplas- cortical tension (i.e., that spaces between foci
mic determinants (O’Connell et al. 2000; represent “weak spots”), and yet these do not
Wallenfang and Seydoux 2000; Sonneville and spontaneously self-amplify to produce global
Gonczy 2004; Cowan and Hyman 2006; Tsai contractileasymmetry.Thissuggeststhatthere
andAhringer2007).Initialstudiesofembryos is a threshold level of inhibition that the cue
depleted of b-tubulin by RNAi suggested that must exceed to trigger symmetry breaking.
neithernormalnorreversedpolarityinduction This could either be a threshold in the inhi-
requires intact microtubules (Cowan and bitiondomainsize,thedurationofinhibition,
Hyman 2004b; Sonneville and Gonczy 2004). or both. In wild-type embryos, polarityestab-
However,recentworkfromtheAhringergroup lishment is associated with the appearance
documented a strong correlation between the and progressive growth of a smooth posterior
presence and size of the sperm aster and the domain, which lacks focal contractions and
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E.MunroandB.Bowerman
cortical ruffling and contains high levels of Measuring these quantities should be an im-
Par-2. Interestingly, when centrosomes are portantgoalforfuturestudies.
laser-ablated after the smooth domain has Like Rho, the small GTPase Cdc-42 be-
formed,thedomaincontinuestogrow—again comes anteriorly enriched during polarity
supporting the existence of a threshold establishment in a manner that depends on
(CowanandHyman2004b). cortical flows (Fig. 1C) (Aceto and Kemphues
Whatisthenatureofthisthreshold?Growth 2006; Motegi and Sugimoto 2006; Schoenegg
of the smooth domain coincides with a pro- and Hyman 2006). Cdc-42 is subsequently
gressive depletion of both Rho and Ect-2/ required to maintain cortical actomyosin and
RhoGEF from the posterior (Jenkins et al. Par protein asymmetries (Gotta et al. 2001;
2006; Motegi and Sugimoto 2006), implying Aceto and Kemphues 2006; Schonegg and
thegrowthofazoneofrelativemyosininactiv- Hyman 2006; Motegi and Sugimoto 2006,
ity. This depletion itself depends on myosin Velardeet al. 2007). However, Cdc-42’s role in
contractility and cortical flow, implying a polarityestablishmentitselfhasbeenlessclear,
feedback loop: Local inhibition/depletion of in part because of a difficulty in assessing the
Rho promotes cortical flows that transport complete loss of function phenotype. Unlike
Ect-2 and Rho away from the posterior, for Rho, strong depletions of Cdc-42 only
leadingtothefurthergrowthofazoneofRho weaklyeffectcorticalflowduringpolarityestab-
depletion/inhibition (Fig. 2). Likewise, in lishment, but strongly inhibit cortical recruit-
wild-type embryos, the smooth Par-2-rich ment of Par-6/Pkc-3 (Motegi and Sugimoto
domain grows progressively during polarity 2006; Schonegg and Hyman 2006; Aceto and
establishment. Previous work suggests that Kemphues2006).ThusCdc-42maybeinvolved,
Par-2 acts locally to inhibit myosin II recruit- atleastindirectly,incouplingcorticalflowsand
ment (Munro et al. 2004), again implying a Par protein transport (Schonegg and Hyman,
feedback loop: Local accumulation of Par-2 2006), but how this works physically remains
promotes contractile asymmetries that drive tobedetermined.
flows and transport factors (Par-3/Par-6/
Pkc-3) that inhibit Par-2 accumulation, thus
WHATPOSITIONSANDSTABILIZES
promoting growth of an inhibitory Par-2
THEAPBOUNDARY?
domain.
These observations suggest the attractive A key feature of polarization in P is that the
0
possibility that the threshold for triggering a response to the sperm cue is self-limiting—
self-amplifying asymmetry is associated with rather than contract itself right off the end of
the formation kinetics of local Rho-depleted the egg, the asymmetrical contraction “stalls”
and/orPar-2-rich/myosininhibitorydomains. about halfway across to form stable anterior
Simple mathematical analyses show that such andposteriordomains.Howmightthiswork?
feedback loops could endow the cortex with a Studies in a variety of cells suggest that cross-
thresholdresponse(Munroetal.unpublished). inhibitory interactions between anterior
Whetherornottheyactuallydosodependson (Par-3/Par-6/Pkc-3) and posterior (Par-1/
quantitative details: Cortical flow speeds will Par-2) Par proteins implement a kind of
depend on the relative magnitudes of contrac- winner-take-all competition, in which either
tileforcesthatdrivecorticalflowandtheeffec- Par-3/Par-6/Pkc-3 or Par-1/Par-2, but not
tiveviscousresistancetoflow;corticalexchange both, can occupy the cortex locally (Benton
rates for key factors and the efficiencyof their andStJohnston2003;Haoetal.2006;reviewed
couplingtocorticalflowwilljointlydetermine in Munro 2006). Such a mechanism could dy-
how rapidly cortical flows either concentrate namically maintain a local boundary between
inhibitors (Par-2) or deplete activators (Rho/ complementaryanteriorandposteriorPardo-
Ect-2), and whether and how rapidly this mains. By transporting Par-3/Par-6/Pkc-3,
zone of inhibition becomes self-amplifying. cortical flows would tend to move this
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CellularSymmetryBreakingduringCaenorhabditiselegansDevelopment
boundary toward the anterior. Thus the posi- cross-inhibitory interactions, the Par proteins
tion of the boundary will be shaped by the mustfeedbacktoregulatethebalanceofcortical
magnitude andextent of cortical flow.Indeed, forcesandtheirownredistribution.Thenature
inhibiting contractility during polarity estab- ofthisfeedbackisnotyetknown,butamecha-
lishment causes the AP Par boundary to shift nistic explanation of how the zygote converts
to the posterior (Shelton et al. 1999; Cuenca local inhibition of contractility into a pair of
et al. 2003), whereas increasing Rho activity stable cortical domains must integrate a sys-
and presumably contractile forces causes the tem of feedback loops involving the interplay
boundary to shift to the anterior (Schmutz between local biochemical regulation, cyto-
etal.2007;Schoneggetal.2007).Interestingly, skeletaldynamics,andcytomechanics.
if centrosomes are ablated after a symmetry-
breaking threshold is reached, then the AP
CORTICALACTOMYOSINSYMMETRY
boundary never extends as far as it does in a
BREAKINGDURINGCYTOKINESIS
wild-typeembryo;iftheboundaryhasreached
its maximal extent before ablation, then it Recent work highlights interesting parallels
shifts back posteriorly (Cowan and Hyman betweenpolarizationoftheanterior–posterior
2004b).Thissuggeststhatapersistentlocalinhi- axis and cleavage furrow specification in P .
0
bition of contractility near the sperm cue may During cytokinesis, the mitotic apparatus
alsocontributetopositioningtheboundary. signals to the cell cortex to specify the site of
However, biasing the strength of contrac- contractile ring assembly. In P0, this involves
tility cannot alone explain the ability of the two classically defined modes of signaling—
cortex to adopt a stable boundary position. A equatorial stimulation and polar relaxation.
mechanicallystablepositionfortheAPbound- The key pathway for equatorial stimulation
ary implies either an absence of force or a involves the highly conserved centralspindlin
balance of equal and opposite forces. If the complex, formed by a kinesin-6 motor
anteriorcortexismorecontractilethanthepos- (Zen-4/Mklp-1) and a Rho GAP (Cyk-4/
teriorcortex,asimpliedbytheasymmetricdis- MgcRAC-GAP). In many other cells, central-
tribution of myosin II, focal contractility, and spindlin locallyactivatesthe Rho GEF (Ect-2/
cortical ruffling, then some other mechanism RhoGEF)tospecifyanequatorialzoneofhigh
must supply a balancing force. One general Rho activity (Somers and Saint 2003; Yuce
possibilityisthattheanteriorand/orposterior et al. 2005; Nishimura and Yonemura 2006).
cortex contains elastic or viscoelastic material The molecular mechanisms underlying polar
that passively resists deformation. Initially, the relaxation have been more elusive, but recent
contractile forces must dominate resistive work in C. elegans (Werner et al. 2007) and
forces to allow symmetry breaking to occur, othercells(FoeandvonDassow2008;Murthy
buttheneithertheresistiveforcemustincrease, and Wadsworth 2008) points to a key role for
orthedifferencebetweenanteriorandposterior astralmicrotubulesinshapingthedistribution
contractileforcesmustattenuate,astheanterior ofsignal(s)thatlocallyinhibitRho-dependent
domain contracts and the posterior domain myosin recruitment/activation at anaphase,
expands. There are many variants of this biasing cortical contractility to the equator,
scenario, and it is premature to speculate where the density of astral microtubule plus
on the underlying molecular mechanisms. endsmaybelow(DeschantandGlotzer,2003).
Significantly, mutating anterior Par proteins InC.elegans,theastralandcentralspindle
leads to a posterior shift in the AP boundary, pathways function redundantlyduring normal
whereas mutating posterior Par proteins leads cytokinesis, but it has been possible through
to a posterior shift in par-2 mutants and an physical,genetic,andpharmacologicalmanip-
anterior shift in par-1 mutants (Kirby et al. ulations to isolate and resolve their individual
1990; Cuenca et al. 2003; Munro et al. 2004; actions within the same cell (Hird and White
Velarde et al. 2007). Thus in addition to 1993; Bringmann and Hyman 2005; Motegi
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E.MunroandB.Bowerman
etal.2006;Bringmannetal.2007;Werneretal. (Fig. 3E). This is followed by an asymmetric
2007; Baruni et al. 2008). Studies of cortical contractionofthecortex,corticalflows,andthe
actomyosin dynamics and furrowing in these formationofasingletransientdeeplyingressing
manipulated embryos reveal parallels between anterior furrow that resembles, at least super-
cortical responses to astral signals during ficially, the pseudocleavage furrow that forms
furrow induction and the sperm cue during during polarity establishment (Werner et al.
polarity establishment (Fig. 3) (Hird and 2007).Interestingly,corticalflowsandfurrowing
White 1993; Werner et al. 2007). In wild-type inresponsetothespermMTOCcueandtopos-
C. elegans embryos, and in mutant embryos teriorized spindles at anaphase share similar
lacking functional centralspindlin, anaphase requirements for Rho and Ect-2, and similar
onsetcoincideswithaburstofcorticalmyosin dependencies on factors that regulate myosin
recruitment (Werneretal.2007).Thisrecruit- II activity (e.g., let-502/Rho Kinase and
ment is biased to regions that are relatively Mel-ll/phosphatase) oractin assembly(Cyk-1/
devoidofmicrotubuleplusends—theanterior Formin)downstreamofRho.Moreover,pertur-
cortexandabroadequatorialdomainthatpre- bationsthatinhibitpseudocleavageoranterior
figuresthe contractile ring (Fig. 3C) (Dechant furrow formation in embryos with posterior-
and Glotzer 2003; Werner et al. 2007)—and ized spindles also inhibit furrowing during
newly recruited myosin concentrates within normal cytokinesis in embryos lacking Zen-4
foci that are similar in size and spacing to (Bringmannetal.2007;Werneretal.2007).
those observed during polarity establishment. These datasuggest that underlying normal
When partial inhibition of astral microtubule cytokinesis, as proposed by White and Borisy
assembly leads to the formation of a small more than 25 years ago (White and Borisy
posteriorly localized spindle, anaphase myosin 1983),isanabilityoftheactomyosincortexto
recruitment occurs everywhere except in a re- amplify crude asymmetries imposed by astral
gion surrounding the posterior spindle inhibition into more sharply focused zones of
Polarity Normal Posteriorized
establishment cytokinesis spindles
A C E
B D F
Figure 3. Similarities between polarity establishment, normal cytokinesis, and cytokinesis in embryos with
posteriorized spindles. (A) During polarity establishment, signals from the sperm aster inhibit focal
actomyosincontractions,triggeringcorticalflowsthatresult(B)informationofanactomyosin-richanterior
cap, bounded by a single deep transient pseudocleavage furrow. (C,D) At anaphase in wild-type embryos,
myosin-rich foci accumulate in regions that are relatively devoid of astral microtubules—on the extreme
anterior cortex, and in an equatorial zone that prefigures the contractile ring. (D) As furrowing proceeds,
corticalflowscarrycorticalmaterialtowardandintothedeepeningfurrow.(E)Atanaphaseinembryoswith
posteriorized spindles, astral signals inhibit accumulation of myosin-rich foci at the posterior cortex,
resulting in cortical flows, formation of an anterior cap, and formation of a single transient “cleavage
furrow.”(F)Asecondcentralspindlin-dependentfurrowformsmidwaybetweenthemitoticasters.
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CellularSymmetryBreakingduringCaenorhabditiselegansDevelopment
contractile activity. In wild-type embryos, this which produces one daughter that contributes
ability is supplemented by direct equatorial only to soma, and another that produces the
stimulation via the central spindle pathway. germline. Interestingly, the Par proteins segre-
Although Borisyand White envisioned ampli- gateinto complementarydomains beforeeach
fication through flow-mediated reorientation of these divisions (Fig. 4A–C) (Boyd et al.
ofcorticalmicrofilaments,itisalsopossible— 1996; Etemad-Moghadam and Kemphues
perhaps more likely—that amplification is 1995; Guo and Kemphues 1995; Hung and
mediated through cortical transport of fac- Kemphues 1999; Tabuse et al. 1998). At least
tors (e.g., Ect-2/GEF, RhoA, Anillin, Cyk-1/ for the secondgermlineprecursordivision (of
formins) that promote local myosin recruit- P1) (see Fig. 4B), this is preceded by a move-
ment/activation or actin filament assembly ment of the centrosomes and nucleus of P1
andcross-linking. towardtheposteriorpole,andisaccompanied
by cortical flows of actomyosin and Par-3/
Par-6/Pkc-3towardtheoppositeanteriorpole
SYMMETRYBREAKINGAND
(HirdandWhite1993;Munroetal.2004),sug-
ESTABLISHMENTOFSIMPLEAPICO-BASAL
gestingareiterativeuseofthesamesymmetry-
POLARITYINEARLYSOMATIC
breakingmechanisms.
BLASTOMERES
Incontrast,beginningatthefour-cellstage,
The asymmetric division of P is the first in a thesomaticdaughtersmanifestanewasymme-
0
series of four asymmetric divisions, each of try in which Par-1 and Par-2 accumulate on
P0 AB P1 P2 P3
EMS MS E
A B C D
GTP
cell–cell GTP Cdc42
contact Pac-1 Cdc42 Par-6 Pkc-3 Par-2
Cortex/PM
Cytoplasm
Pac-1 Cdc42 Par-6 Pkc-3 Par-2
E GDP
Figure4.Establishmentofapico-basalParasymmetriesonsomaticblastomeres.(A–D)Schematicoverviewof
Parasymmetriesinearlyblastomeres.Intheone-cell(A)andtwo-cell(B)stages,Par1/Par2areenrichedonthe
posteriorcortexofgermlineprecursorsP0andP1,whereasPar-3/Par-6/Pkc-3areenrichedontheanterior
cortex. At the early four-cell stage (C), Par-1/Par-2 are enriched in newly born somatic blastomeres on
domains of cell–cell contact and at the cortex of the germline precursor P2, whereas Par-6/Pkc-3 are
enriched on all somatic cell boundaries. By the seven-cell stage (D), Par-6/Pkc-3 have disappeared from
domains of somatic cell contact. (E) A pathway for establishment of apico-basal polarity downstream of
cell–cell contact (based on Anderson et al. 2008). The Rho GAP Pac-1 accumulates on somatic cell–cell
contacts, where it promotes the conversion of Cdc-42 from an active membrane-bound to an inactive
cytoplasmicform.RemovalofCdc-42fromcellcontactsresultsinalossofcorticalPar-6/Pkc-3;thisinturn
allowsPar-1/Par-2toaccumulateatsomaticcell–cellcontacts(seeAndersonetal.2008fordetails).
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E.MunroandB.Bowerman
“basolateral”domainsofcontactbetweenadja- zygote) does not abolish cortical localization
cent blastomeres, whereas Par-3/Par-6/Pkc-3 of Par-3 (Gotta et al. 2001; Beers and
accumulate on the outer noncontacting Kemphues 2006). One possibility isthat Pac-1
“apical” domains (Nance and Priess 2002). acts through additional GTPases, such as Rho
Failure to properly establish and maintain itself.Interestingly,theestablishmentofapico-
these asymmetries is associated with defects basalpolarity isaccompaniedbycorticalflows
in the ingression of endoderm progenitors of actomyosin toward the center of the apical
during early gastrulation (Nance et al. 2003). domainwherePar-3/Par-6/Pkc-3becomecon-
Blastomereisolationandrecombinationexper- centrated(Gottaetal.2001;Nanceetal.2003;
iments showed that these asymmetries depend Munro et al. 2004). Thus in principle, cortical
on, and can be induced by, cell–cell contacts. flows triggered by local inhibition of Rho
As in P0, Par-3/Par-6/Pkc-3 are required to could establish Par-3 asymmetries as observed
exclude Par-1 and Par-2 from noncontacting in the zygote,but this remainsto be tested. In
surfaces (Nance and Priess 2002; Nance et al. summary, the establishment of apico-basal
2003). However, whereas removal of Par-1 or polarityinsomaticcellsappearstouseadiffer-
Par-2 results in loss of cortical Par-3/Par-6/ ent cue but an overlapping set of molecules
Pkc-3asymmetriesinP0, ithasnodiscernible and interactions, compared with anterior–
effect on these later apico-basal asymmetries posteriorpolarityestablishmentinP .
0
ofPar-3/Par-6/Pkc-3.
Recently, a Rho-GAP called PAC-1 was
APICO-BASALSYMMETRYBREAKINGIN
identified thataccumulatesspecificallyon sur-
EMBRYONICANDPOSTEMBRYONIC
faces of cell–cell contact and is required for
EPITHELIA
apico-basalasymmetry(Andersonetal.2008).
In pac-1 mutants, Par asymmetries in P are The mechanisms that establish apico-basal
0
normal but in somatic blastomeres Par-3/ polarity in C. elegans epithelia are beginning
Par-6/Pkc-3 are uniformly cortical, whereas toreceiveattention(Labouesse2006).Epithelial
Par-2 and Par-1 are absent from domains of structuresinC.elegansincludethegastrointes-
cell–cell contact. Although Pac-1’s Rho GAP tinaltubethatrunsthelengthofthebodyand
domain has activity toward Rho, Rac-1, and the spermatheca, a polarized epithelium that
Cdc-42 in vitro, it appears to act through formsa highlydistensible valve andlinks each
Cdc-42.LossofCdc-42insomaticblastomeres gonadal arm to acommon uterus in the adult
results in complete loss of Par-6/Pkc-3 from, hermaphrodite (reviewed in Hubbard and
and the accumulation of Par-2 onto, domains Greenstein 2000; Schneider and Bowerman
ofcell–cellcontact.Expressingaconstitutively 2003). Sperm reside in the spermatheca, and
active form of Cdc-42 in the same cells forces oocytes sequentially transit through them
accumulation of Par-6/Pkc-3 (and loss of during ovulation and fertilization. Finally, the
Par-2) at cell–cell contacts without affecting embryonic epidermis polarizes and ultimately
Pac-1 localization. This latter effect depends encloses the embryo, after the completion of
onasemi-CRIBdomaininPar-6thatmediates most cell proliferation at the so-called bean
binding of Par-6to activeCdc-42(Acetoetal. stage. Contractile forces generated by the
2006).Together,thesedatasupportamodelin actomyosin cytoskeleton within the epidermis
which Pac-1 inactivates Cdc-42, releasing then promote the morphogenetic squeezing
Par-6/Pkc-3 from cell–cell contacts, and thus and elongation of an initially oval mass of
allowing Par-1 and Par-2 to accumulate there embryoniccellsintoalong,thinworm(Priess
(Fig.4E). andHirsh1986).
HowPac-1accumulatesspecificallyatcell– In each of these polarized C. elegans
cell contacts, and how Pac-1 excludes Par-3 epithelia, we are only beginning to identify
from cell–cell contacts, remains unclear mechanisms that break symmetry during the
because removal of Cdc-42 (at least in the establishment of apico-basal polarity. Proper
10 AdvancedOnlineArticle.CitethisarticleasColdSpringHarbPerspectBioldoi:10.1101/cshperspect.a003400