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Theactincortex:abridgebetweencellshapeandfunctionKevinJ.Chalut1,EwaK.Paluch2,31WellcomeTrust/MedicalResearchCouncilStemCellInstitute,UniversityofCambridge,UnitedKingdom;2MRC-LMCB,UniversityCollegeLondon,London,UnitedKingdom;3InstituteforthePhysicsofLivingSystems,UniversityCollegeLondon,London,UnitedKingdom.Correspondence:kc370@cam.ac.uke.paluch@ucl.ac.ukPrecisecontrolofcellmorphogenesisisakeytohealthycellphysiology,andcellshape

deregulationisattheheartofmanypathologicaldisorders.Changesincellshapestrongly

correlatewith,ifnotcause,processessuchascellmigration,tissuehomeostasis,epithelial-

amoeboid-mesenchymaltransitionsandcellulardifferentiation(Figure1).Infact,early

embryologistsdefinedmanycellfatechangesbasedoncellmorphology,andusedcell

shapeasaprimaryidentifierofdifferentnascenttissues.Here,wediscussthecontrolofcell

shapeandmechanics,andtheemergingrelationshipbetweenshape,biochemicalsignaling

andcellularfunction,highlightingremaininggapsinourunderstandingandpotential

directionsoffutureinvestigations.

Cellshapeisdefinedbycellularmechanicalpropertiesandbythecell’sphysicalinteractions

withitsenvironment.Mostcelldeformationsaredrivenbychangesinthephysical

propertiesofthecellsurface,whicharedominatedbythemechanicsofthecellularcortex.

Thecortexisathinnetworkofactinthatliesunderandistetheredtotheplasma

membraneinmostanimalcells.Corticalactinfilamentsareorganizedinameshworkcross-

linkedbyspecificproteinsandbymyosinmotors,whichgeneratecontractilestressesinthe

network(Clarketal.,2014).Thesestressesgiverisetocorticaltension,whichdetermines

globalcellsurfacemechanics.Gradientsincorticaltensionresultincorticalflowsand

cellularcontractions,suchasthosedrivingcleavagefurrowingression,cellbodyretraction

duringcellmigrationandepithelialcontractionsunderlyingtissueconstrictionevents

(LevayerandLecuit,2012).

Overthepastdecade,anincreasingnumberofbiologicalandbiophysicalinvestigations

havefocusedontheactincortex.Cortexcompositionhasbeencharacterizedbymass

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spectrometryusingisolatedcellularblebstocollectsufficientamountsofcorticalmaterial

(Bovellanetal.,2014).Corticalactinnucleatorsandvariousregulatorsofcortical

contractilityhavebeenidentified(Bovellanetal.,2014;Luoetal.,2013),andtoolsare

availabletomeasurephysicalcharacteristics,suchascorticaltensionandthickness

(reviewedin(Clarketal.,2014)).Themechanicsofcorticalcontractionsandflowsinmany

morphogeneticeventshavebeendissected,includingC.eleganszygotepolarization(Mayer

etal.,2010),mouseembryocompaction(Maitreetal.,2015)andepithelialconstrictionsin

theDrosophilaembryo(LevayerandLecuit,2012).However,mostpaststudieshave

focusedonthecortexinitself.Incontrast,muchlessisknownabouthowthecortexis

dynamicallyregulatedbyspecificsignalingpathways,howitinturntriggersbiochemical

signalingevents,andhowcorticalprocessesareintegratedwithinthecelltodrive

morphogenesis.

Manymorphogeneticprocessesappeartobedrivenbytransitionsbetweenacorticalanda

stressfiberdominatedorganizationofintracellularactinnetworks.Stressfibersarequasi

one-dimensionalbundlesofactinfilamentsusuallyconnectingtwoadhesionpoints,

whereasinthecortex,actinformsaroughlyisotropicmeshworkundertheplasma

membrane(Figure2).Whilethemolecularregulationofboththecortexandstressfibers

arereasonablywellunderstood,howtransitionsbetweenthesetwotypesofnetworksare

controlledremainselusive.Forexample,duringdevelopmentaltransitionssuchasexitfrom

naïvepluripotencyorduringepithelialtomesenchymaltransitions(EMT),actinreorganizes

fromamostlycorticalarrangementintostressfibersandlamellipodia(Figure1).Howthis

reorganization,whichdrivescellspreading,isregulatedbythesignalingpathwaysdriving

cellstatechangessuchascelldifferentiationisnotunderstood.

Transitionsinactinnetworkorganizationarealsoassociatedwithchangesinthewaythe

cellinteractswiththeenvironment.Stressfibersareusuallyconnectedtoandpromotethe

formationofintegrin-basedfocaladhesions(LivneandGeiger,2016),whereascorticalactin

isoftenassociatedwithcadherin-basedcell-celladhesions(Engletal.,2014).Interestingly,

cell-cellcontactformationisconcomitantwithcorticalclearinginthecontactzone(Maitre

etal.,2012)whileactindynamicsandtensioninturninfluencecadherinrecruitment(Engl

etal.,2014).Thus,thecell’sinteractionswithneighborsandmatrixarecontrolledviaa

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subtlecross-talkbetweenactinorganization,contractility,dynamicsandcontactswith

integrinsand/orcadherins.Onefuturechallengewillbetofullyunderstandthiscross-talk,

andhowitismodulatedduringcellularshapechangesassociatedwithdevelopmentalfate

transitions.Extendingthealreadybroadlibrariesofactinbindingproteinsandidentifying

connectionstoadhesionsmayprovideonepathtowardsthisgoal.Theseextensionscanin

turninstructgain-andloss-of-functionstudiestoshedlightonthechangestocellshapeand

fatethataccompanydevelopmentalprocesses.

Anotherkeyyetpoorlyunderstoodaspectoftheactincytoskeletonfunctionin

developmentisitsroleincellularsignaling.Indeed,actinnetworksnotonlycontrolcell

shape,theyalsoareacenterofbothmechanicalandbiochemicalsignaling.Forexample,

focaladhesionmaturationisdrivenbytensioninstressfibers,whichupregulatesfocal

adhesionkinaseandSrcfamilykinasebasedsignaling.Thesekinasesareupstream

mediatorsofmitogenactivatedproteinkinase(MAPK)signaling,whichisessentialfora

multitudeofcellularprocessesincludingdifferentiation.Moreover,thereisfeedbackfrom

MAPKsignalingtotheactincytoskeleton:actinorganizationandcontractilityisregulatedby

interactionsbetweenErkandRhoGTPases(Vialetal.,2003).Thoughlessstudied,similar

interactionsarepossiblebetweencadherin-basedadhesionsandWntsignaling,both

canonicalandnon-canonical(HeubergerandBirchmeier,2010).Itislikelythatthe

connectionbetweenactinnetworksandcellularadhesionsdrivesmanyotherbiochemical

signalingfeedbackloops,raisingthetantalizingpossibilitythatactinorganizationandcell

shapearemorethanpassivedownstreamplayersincellulartransformationssuchas

differentiation.

Thepossibilitythatactinactivelyregulatescellfunctionisbuttressedbythefactthatthere

areatleasttwootherconnectionsbetweenactinorganizationandsignaling.First,actin

dynamicsitselfcaninfluencegeneexpression.Changesinactinorganizationarelikelyto

modifytheintracellularfilamentous(F)tomonomeric(G)actinratiobothinthecytoplasm

and,becauseactinisactivelyshuttledintoandoutofthenucleus,inthenucleus.Increasing

levelsofG-actininthenucleusaffectstranscription,bothduetointeractionswithallthree

typesofRNApolymeraseandalsobecauseitisinvolvedinthenuclearexportofmyocardin-

relatedtranscriptionfactors(MRTFs).Forexample,iflessG-actinisavailableinthenucleus

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(perhapsbecauseofincreasedlevelsofpolymerizedactininthecell)thenMRTFfamily

membersinturnactivatetheserumresponsefactor(SRF)pathway.SRFactivates

immediateearlygenessuchasc-fos(PosernandTreisman,2006),whichplaymajorrolesin

cellulartransformationssuchasdifferentiationandoncogenesis.Second,thecortexis

physicallyconnectedtothenucleusviacytoplasmicstructuralcomponents,andstressesat

thecellsurfacecantranslatetostrainonthenucleus(Pagliaraetal.,2014).Increasedstress

intheactincytoskeletoncanactthroughtheLINCcomplexonthenuclearmembraneand

thenuclearlaminstofurthertunebiochemicalsignaling.Forexample,increased

cytoskeletalstresscanstabilizelaminA/CwhichactivatesSRFandtheretinoicacid

pathways,andfurthermoreactsasamediatorofMAPKsignaling(Swiftetal.,2013)and

activationofimmediateearlygenes.Thereisalsonewevidencethatstressthroughthe

actincytoskeletoncanactdirectlythroughemerins(alsonuclearmembraneproteins)to

facilitatepolycomb-mediatedgenesilencingatthenuclearenvelope(Leetal.,2016).

Itistemptingtogiveintodespairwhenconsideringthemyriadofwaysinwhichtheactin

cytoskeletonaffectssignalingandviceversa.However,itappearsthatthenexusofactin-

regulatedsignalingandshapemaybeactinnetworkorganization.Thus,tofullyunderstand

therelationshipbetweencellshapeandcellfunction,wemustfirstunderstandthe

transitionsbetweendifferenttypesofactinnetworks.Then,cytoskeletalinvestigations

mustbefullyintegratedwithstudiesofsignalingpathwaysthatdrivecellstatechanges.

Truecomprehensionoftheinterplaybetweenactinnetworktransitionsandcellstate

transitionswillrequireaninterdisciplinarypushinvolvingbiophysicists,molecularandcell

biologistsstudyingthecytoskeleton,developmentalbiologistsandstemcellbiologists.

Acknowledgements:

KJCacknowledgessupportfromtheRoyalSociety,MedicalResearchCouncilUKand

WellcomeTrust(forcorefundingtotheCambridgeStemCellInstitute).EKPacknowledges

supportfromtheMedicalResearchCouncilUK(corefundingtotheLMCB).WethankCéline

LabouesseandMurielleSerresforprovidingtheimagesforthefigures.

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Figure1:Embryonicstemcellsinanaïvephaseofpluripotency(A)andafterexitfromnaïve

pluripotency(B).Duringthistime,actin(incyan)transitionsfromcorticalactintostress

fibersasthecellsspread.Nucleiarelabeledinred.

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Figure2:Differenttypesofactinnetworksininterphase(A)andmitotic(B)HeLacells

stainedwithDAPItodetectDNA(red)andphalloidintomarkF-actin(cyan).Interphasecells

arespreadandactinisprimarilyorganizedinstressfibers.Mitoticcellsareroundedand

actinispredominantlycortical.

A B

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