Principles of progenitor temporal patterning in the developing invertebrate and vertebrate nervous system

Principles

of

progenitor

temporal

patterning

in

the

developing

invertebrate

and

vertebrate

nervous

system

Polina

Oberst

,

Gulistan

Agirman

and

Denis

Jabaudon

Duringthedevelopmentofthecentralnervoussystem, progenitorssuccessivelygeneratedistincttypesofneurons whichassembleintothecircuitsthatunderlieourabilityto interactwiththeenvironment.Spatialandtemporalpatterning mechanismsarepartiallyevolutionarilyconservedprocesses thatallowgenerationofneuronaldiversityfromalimitedsetof progenitors.Here,wereviewexamplesoftemporalpatterning inneuronalprogenitorsintheDrosophilaventralnervecordand inthemammaliancerebralcortex.Wediscuss

cell-autonomousmechanismsandenvironmentalinfluencesonthe temporaltransitionsofneuronalprogenitors.Identifyingthe principlescontrollingthetemporalspecificationofprogenitors acrossspecies,ashighlightedhere,mayhelpunderstandthe evolutionaryconstraintsoverbraincircuitdesignandfunction. Addresses

1

DepartmentofBasicNeurosciences,UniversityofGeneva,Switzerland 2_{GIGA-Neurosciences,UniversityofLie`ge,C.H.U.Sart-Tilman,Lie`ge,} Belgium

3_{DepartmentofNeurology,GenevaUniversityHospital,Geneva,} Switzerland

Correspondingauthor:Jabaudon,Denis(denis.jabaudon@unige.ch) 4_{Contributedequallytothiswork.}

CurrentOpinioninNeurobiology2019,56:185–193 ThisreviewcomesfromathemedissueonNeuronalidentity EditedbySachaNelsonandOliverHobert

ForacompleteoverviewseetheIssueandtheEditorial Availableonline15thApril2019

https://doi.org/10.1016/j.conb.2019.03.004 0959-4388/ã2019ElsevierLtd.Allrightsreserved.

Introduction

Neurons are the building blocks of the circuits of the

central nervoussystem(CNS). Assuch,initialneuronal

diversity setstheframefor thediversityof circuitsthat

canbebuilt,andhenceforananimal’sbehavioral

reper-toire. The last few years have provided us with an

increasinglydetailedcensusofthedistinctneuronalcell

typesthatpopulatetheCNS,andparticularlythecerebral

cortex,thanksinparticulartotheadventof

high-through-put single-cell technologies (reviewed in Ref. [1]).

Despitethis expandedcellulartaxonomy,theoriginsof

neuronaldiversityremainpoorlyunderstood.

Twomainprocesseshavebeeninvolvedinthe

develop-mental generationof diversetypesof cells oftheCNS:

spatialpatterningandtemporalpatterning.The

pattern-ingofmolecularlydistinctprogenitorsanddaughtercells

into separate spatial domains (‘spatial patterning’) is

widespreadthroughouttheCNS,includingintheretina

[2],spinal cord [3],cerebellum[4], and ventralpallium

[5].Inadditionto thisprocess,however,inmanycases,

neuronal diversity also emerges from ‘temporal

patterning’,thatis,in thesuccessiveemergenceof

pro-genitors andneurons with distinctmolecular properties

withinconfinedbrainregions.Inthisreview,wewillfocus

on this latter process and highlight select aspects of

temporal progression in neural progenitor identity and

their ability to sequentially generate diverse neuronal

subtypesinthedevelopingDrosophilaventralnervecord

(VNC) and mouse neocortex.Our review isfocused on

selectexamplesof temporalpatterningin bothof these

species, and interested readers can refer to previous

reviewsaddressing otheraspectsofnervoussystem

pat-terning(inparticularspatialpatterning)[6–9].

Temporal

patterning

in

Drosophila

neuroblasts

Much of ourunderstanding of themechanisms

control-lingneuronalspecificationbytemporalpatterningcomes

fromstudiesperformedinthecommonfruitflyDrosophila

melanogaster. Temporal patterning in Drosophila occurs

throughoutthedevelopingCNS(comprisingthecentral

brain,theopticlobes,andtheVNC)duringboth

embry-onicandlarvalstages.Here,wereviewselectexamplesof

temporal patterning in the embryonic VNC, since

exhaustive reviews onthetemporalspecification in the

Drosophila CNS across developmental stages have

recently beenpublished[10–12].

NeuronsandglialcellsoftheDrosophilaVNCare

gener-atedbyneuralstemcellscalledneuroblasts.Each

neuro-blast has a unique spatial identity (resulting in around

100uniqueneuroblastsineachlobeofthecentralbrain,

andaround30uniqueneuroblastsineachhemisegment

oftheVNC(reviewedinRef.[6])andproducesa

stereo-typed series of progeny over time [13].Drosophila

neu-rogenesis occurs in two sequential waves; a first wave

occursduringembryogenesis(contributingtoaround10%

of adult neurons),whichis followedbyalongersecond

wavespanninglarvalandpupalstages,duringwhichthe

vast majority of neurons and glia of the adult CNS is

Temporalpatterninghasbeenfirstandbestdescribedin

DrosophilaembryonicVNC [16–18].TheVNC contains

typeIneuroblasts,whichhaveshortlineagesanddividea

total of about five times, within a single day. Type I

neuroblastsgenerateaneuroblastandaganglionmother

cell(GMC)ateachdivision;thelatterdividesoncemore

to produce a pair of neurons or glial cells. Early-born

progeny are displaced by later-born progeny, resulting

in a laminar organization of the VNC reflecting birth

order: early-born neurons are located in deep layers

and late-born neurons in more superficial layers [2], as

isthecaseinthemammaliancerebralcortex[19,20].

Each neuroblast in the embryonic VNC sequentially

expressesaseriesoftemporaltranscriptionfactors(tTFs),

thatis, transcriptionfactors whichspecify thetemporal

identity of neurons born during their time window of

expression.Hunchback(Hb)isexpressedfirst,followed

byKruppel (Kr), POUdomain proteins 1 and2 (Pdm),

Castor and Grainy head [16–18] (Figure 1b). tTF

Figure1

Embryo Larva

NB delamination Cell cycle reentry

Pupa

Quiescence Cell cycle exit

Early E E E Ea Eararararlylylyyyy Late Hbhigh _Hblow _{Kr Pdm} Pdm Cas Motorneurons Interneurons NB GMC Neuron or glia Motorneuron competence Dan hb locus available hb locus silenced NB Neuroepithelium central brain VNC _optic lobe (a) (b) (c)

Type I Neuroblasts Type II Neuroblasts

NB INP NB INP INP INP INP INP NB NB GMC Neuron or glia

Current Opinion in Neurobiology

NeuraldevelopmentandtemporalpatterninginDrosophila.

(a)Neuroblastsdelaminatefromtheneuroepitheliumearlyinembryogenesisandpassthroughsequentialtemporalstatestogeneratedifferent typesofneuronsbeforeenteringquiescenceattheendofembryogenesis.Duringlarvalstages,neuroblastsre-enterthecellcycleandpass throughadditionaltemporalstatestogeneratedistincttypesofneurons.Terminalcellcycleexitoccursduringpupalstages.(b)EmbryonicVNC neuroblaststransitionthroughaneuroblast-intrinsictTFcascadeandgiverisetodistinctneuronsduringeachtemporalwindow.IntheNB7-1and NB3-1lineages,neuroblastsfirstgeneratefivedistinctmotorneuronsandthenswitchtoproducinginterneurons.Competencetogenerate motorneuronsislostatthetransitiontointerneurongenerationthroughepigeneticsilencingofdevelopmentalgenes,hereexemplifiedbythe silencingofthehblocusintheabsenceoftheneuroblastnuclearproteinDan.(c)Divisionmodesofneuroblastsinthelarvalcentralbrain.TypeI neuroblastsdivideasymmetricallyandproduceaGMCateachdivision,whichdividesoncemoretogenerateapairofneuronsorglia.TypeII neuroblastsgiverisetoINPs,whichprogressthroughtheirowntemporalseriesanddivideseveraltimestogiverisetodistinctGMCsateach division,generatinganadditionallayerofneuronaldiversity.

Abbreviations:GMC,ganglionmothercell;Hb,Hunchback;INP,intermediateneuralprogenitor;NB,neuroblast;tTF,temporaltranscriptionfactor; VNC,ventralnervecord.

expressionprogressionoccursatapproximatelyeach

neu-roblast divisionandthesame seriesof tTFs isactivein

differentneuroblastlineages,ineachofwhichitspecifies

distinct neuronalfates.Thus,thesame tTFcanspecify

an interneuron in one lineage, and a motor neuron in

another[2,17].Aswillbediscussedbelow,some

Drosoph-ila tTF have mammalian orthologs (e.g. Hunchback –

Ikzf1(Ikaros)andCastor–Casz1),whichappeartoalsobe

involvedintheprogressionoftemporalidentityinretinal

and corticalprogenitors [21–23].Interestingly,daughter

neurons continueto express thetTF thattheir mother

cellwasexpressing,althoughtheroleofthisexpressionis

unknown[2].

Of note, the Drosophila central brain contains an

addi-tional, lessabundant typeof neuroblast (type II

neuro-blasts),whichgivesrisetotransitamplifyingintermediate

neural progenitors (INPs) with a limited proliferative

capacity[24,25].INPsprogressthroughtheirownseries

oftTFs(Dichaete!Grainyhead!Eyeless)andspecify

distinctcelltypesduringeachtemporalwindow.Thus,in

the Drosophila central brain, the two parallel axes of

temporal progression in type II neuroblasts and INPs

combinatorially specifycellidentitiesandincrease

neu-ronaldiversity[26](Figure1c).

Controloftemporaltransitions

How is the successive expression of tTFs regulated?

StudiesinembryonicVNCneuroblastssuggestthat

tran-scriptionalcross-regulation betweentTFs together with

additionalindependentmechanismsacttoregulate

tem-poraltransitions[2,16–18,27].Prolongedexpressionofthe

early-onset tTFs Hb or Kr blocks the progression of

neuroblast neurogenic competence and results in the

excessive production of early-born neurons at the

expense of later-born ones[17,27].Hb andKr promote

expressionofthefollowingtTFintheseriesandrepress

the next-plus-one factor [17],but removal of Hb or Kr

leads to loss of only one temporal identity window,

without affecting subsequent temporal transitions

[2,17]. This suggests that tTFs do not alone account

for temporaltransitions, but that other factors, possibly

includingextrinsicsignalingasoccursinotherpartsofthe

nervoussystem(seebelow),areatplay.Transitionfrom

Hb to Krexpression requires repression of Hbthrough

the orphan nuclear receptor Svp. Translation of Svp

proteiniscoupledtocytokinesis,andthusthetransition

from Hb to Kr requires cell division [27–30].

Interest-ingly, the mammalian Svp homologs COUP-TF1/2

(Nr2f1andNr2f2)actin mammaliancorticalprogenitors

topromotetheswitchfromearly-borntolate-bornneuron

production, and from neurogenesis to gliogenesis [31],

suggestinganevolutionarilyconservedroleinregulating

temporaltransitions.Allothertransitionsexamined,

how-ever, have been shown to occur even in G2-arrested

neuroblasts [27], as also seems to be the case for

cell-cyclearrestedmammaliancorticalprogenitors[32],such

that ‘counting’of celldivisionsdoes notseem tobe an

obligatory process for temporal progression in identity.

Finally,clonallyculturedVNCneuroblastsprogress

nor-mallythroughthetemporalTFcascade,suggestingthat

lineage-intrinsic cuesare sufficienttomediate temporal

progression[16,27],althoughfeedbackcuesfromneural

progenymightplayarole.Ofnote,extrinsicfactorshave

been implicated in the temporal progression of larval

central braintypeIIneuroblasts:for example,ecdysone

signalingviatheEcR-B1receptorinitiatesamajor

early-to-lategeneexpressiontransition,andlackofthis

signal-ing leads to maintained expression of early temporal

factors[33,34].

TemporalplasticityofDrosophilaneuroblast competence

The competence of neuroblasts to successively produce

distinctneuronaltypesatsuccessivestagesoftheirlineage

has been best studied using ectopic (i.e. heterochronic)

expressionoftTFs[35,36].Ectopicexpressionoftheearly

tTF Hb atlater stagesin the NB7-1 neuroblast lineage

inducesthegenerationofearly-bornneuronaltypesthatare

normally specified during the Hb expression window

[35,36].However,competenceto respondto ectopicHb

islostafterthe fifthdivisionof thisneuroblast,atatime

pointwhendaughtercellfateswitchesfrommotorneurons

tointerneurons.Thislossofcompetencetorespondto Hbis

thoughttobeduetoarepositioningofHbtargetgenesclose

tothenuclearlamina,whichrenderstheminaccessible,as

exemplified bythe silencingof Hbitself throughsucha

process [37] (Figure 1b). This genomic reorganization

occursinnearsynchronywithintheentireneuroblast

pop-ulation,suggesting that an extrinsic global signal maytrigger

this process [2,37]. Similarly, in the NB7-1 and NB3-1

lineages, Kr specifies third-born U3 motor neurons and

itsmis-expressionbetweenthe thirdandfifthneuroblast

divisioninducesthegenerationofsuchmotorneurons[36],

but the competence to respond to Kr is lost when the

neuroblast transitsto generatinginterneurons. Polycomb

repressive complexes(PRCs),which aremulti-protein

com-plexes thatinhibittranscription via epigenetic silencing,

restrictthecompetenceofNB7-1andNB3-1neuroblaststo

respondtoKr,suchthatdecreasedPRCactivityextendsthe

competence window for motor neuron generation [38].

PRCs are also found in mammalian neural progenitors,

where they regulate progenitor identity and control the

switch fromneurogenesis to gliogenesis [39,40]. Finally,

progressive restriction in the competence to respond to

specific signalsisnot limited toneuroblastsbut hasalso

been observed in intermediate progenitors, which lose

competencetorespondtoNotch-signalingastheyage[41].

Temporal

patterning

in

the

mammalian

cerebral

cortex

AsisthecaseinDrosophila,atleastsomeneural

progeni-tors in vertebrates also generate distinct neuronal

wellstudied in the mouse neocortex. The neocortex is

organizedinsixlayers,eachenrichedinspecificsubtypes

ofneuronswithdistinctmolecular identities,

morpholo-gies,andconnectivity[19,20].Inthedeveloping

neocor-tex, excitatory neurons are generated from apical

pro-genitors(APs)locatedinadeepgerminalzoneadjacentto

thelateralventricles(ventricularzone,VZ).FromE11.5

toE16.5,APsdividetoself-renewandtoproduce

daugh-terneurons anddaughter intermediateprogenitors (IPs,

alsocalledbasalprogenitors).Thelattercellsmoveaway

fromtheventricularzonetoformasecondgerminalzone

(subventricular zone, SVZ) and undergo only a few

roundsofneurogenicdivisions.Corticalneuronscanthus

be born directly from APs or indirectly from IPs, and

laminarly distinct subtypes of neurons are sequentially

generatedfromthesecells,withdeep-layerneuronsbeing

bornfirstandsuperficiallayerneuronslast,asisthecase

inDrosophilaVNC[2](Figure2a).Towardtheendofthe

neurogenicperiod, aroundE17.5,APsundergoterminal

divisionstogenerateglialcells[19,42].Thecompetence

ofAPstogeneratetemporallydefineddaughtercelltypes

resultsfromtheinterplayofbothcell-autonomous

mech-anismsandlocalandlong-rangeenvironmentalcues.The

transcriptional,environmental,andepigeneticinfluences

onAPstemporalpatterningaredescribedinthefollowing

sections.

APneurogeniccompetenceacrossneurogenesis

Clonalanalysisof E12.5APs usingtheMosaic Analysis

with Double Markers (MADM) technology has shown

that the majority of APs produce approximately 8–9

neurons (range from 3 to 16) that settle in both deep

and superficial layers[42].This indicates thatas isthe

caseforDrosophilaneuroblasts,APsprogressivelyacquire

competencetogeneratedistinctneuronalsubtypes.

Sup-portingtheseobservations,invivogeneticfatemapping

of early APs expressing thedeep layer marker FEZF2

showedthat these progenitors existthroughout

cortico-genesis andsequentially generate deep thensuperficial

layer projection neurons[43]. Subsets of fate-restricted

progenitorsmay,however,exist,sinceCUX2-expressing

progenitors have been proposed to exclusively produce

superficiallayerneurons.Thesecellswerefoundinthe

ventricular zone as early as E12.5, and would undergo

severalroundsofproliferative divisionsbefore

undergo-ingneurogenicdivisions atthetime of superficiallayer

neurongeneration[44].Suchfate-restrictedprogenitors,

if present atall, are probablyrare, however, since they

have notyet conclusively been identified in single-cell

RNAsequencingdatasets[45,46].

Cortical progenitors cultured in vitro recapitulate the

normal courseof corticogenesis and produce early-born

deeplayerneuronsbeforegeneratinglate-bornsuperficial

layer neurons [47]. However, several studies have

reportedan underrepresentationof late-born superficial

layerneuronsininvitrosystems[47,48],andthe

propor-tion of early-born to late-born fates is influenced by

culture conditions [32,49]. Supporting these findings,

geneexpressionstudieshaveshownthatwhile

progeni-torsculturedasaggregatesororganoidsareprogressively

changing their temporal gene expression profile over

time, isolated progenitors where cell–cell contacts are

prevented show only limited progression of temporal

geneexpression[32,49].Similarly,whileonlyalimited

numberof late-bornneuronswasdetectedinaninvitro

systemusinghumanembryonicstemcell-derivedcortical

progenitor cells, transplantation into a neonatal mouse

significantlyincreasedtheproductionoflate-born

super-ficiallayerneurons[48].Together,thesefindingssuggest

that temporal progression of cortical progenitors may

require additional cell-extrinsic cues to express their

competencetogeneratesuperficiallayerneurons.

An important question is whether fate progression of

cortical progenitors necessarily implies fate restriction.

Spatially parcellated progenitors belong to distinct

lineagesandarethusrelativelyindependentintheirfate

progression. Temporal parcellation instead requires a

mechanism to repress past competences and induce

new ones, but whether this mechanism is reversible or

nothasnotbeen systematically examinedwith modern

analytictools.Seminalworkintheferrethasinvestigated

theplasticity in theneurogenic competence of cortical

progenitorsatdifferentdevelopmentalstagesusing

het-erochronictransplantations. Thesestudiesrevealedthat

earlyprogenitorstransplantedintoalateenvironmentare

ableto produce late-bornsuperficiallayerneurons,

sug-gesting that early progenitors are multipotent and

respond to cues present in alater environment [50,51].

In contrast, late progenitors transplanted into younger

hosts did not reset their neurogenic competence and

invariably gave rise to superficial layer neurons [52],

suggestingthatprogenitorsatlatestagesofcorticogenesis

becomefate-restricted. A different interpretation,

how-ever,isthatatlatestagesofcorticogenesis,and

particu-larlyintheferret,transplantedprogenitorsconsistmostly

oftransitamplifyingcells,includingIPs,ratherthanAPs.

Thus,assuggestedbyarecentpreprintfromour

labora-tory,thelackofplasticityinneurogeniccompetenceupon

transplantationinto younger hosts may reflectcell-type

specificdifferencesinAPandIPcompetenceratherthan

aprogressive restrictioninthecompetenceof APs[53].

ControloftemporaltransitionsinAPs

Incontrastto thewell-characterizedtemporalsequence

of tTFs in Drosophila, the temporal transcriptional

dynamics in mammalian cortical progenitors are still

relativelypoorlydescribedwithonlyfewgenesidentified

FOXG1 during early stages of corticogenesis, as they

transit fromCajal Retzius cellproductionto deeplayer

neuron production. Loss of FOXG1 at mid stages of

corticogenesisleadstoheterochronicgenerationofCajal

Retziusneurons,thussuggestingthatcontinuedFOXG1

expressionisnecessarytosuppressCajalRetzius

produc-tion[54].Acoretranscriptionalnetworkoflayer-enriched

transcription factors including FEZF2, CTIP2, TBR1

and SATB2 has been identified over recent years [55].

These factors cross-regulate each other’s expression,

which is thought to allow thesequential acquisition of

deepthensuperficiallayeridentitiesinnewbornneurons

(Figure2b).However,noneofthesetranscriptionfactors

are clearlytemporallyregulatedand someof them(e.g.

CTIP2)areexpressedinpost-mitoticneuronsbutnotin

APs [56–59]. Interestingly, late-born neurons initially

express a combination of lamina-specific markers, and

onlylatertheiridentityisrefinedtoincludeonly

superfi-cialneuronmarkers.Thisprocess,whichhasbeentermed

‘transcriptional priming’ and which isalso found in the

hematopoieticsystem,suggeststhatfinalneuronal

iden-tity is progressively acquired in the courseof

develop-ment [46,60,61].

DespitethelackofunequivocaltTFsintheneocortex,the

mammalianhomologofHunchbackIkaros(Ikzf1)provides

apotentialexampleof evolutionaryfunctional

conserva-tion,asitishighlyexpressedinAPsduringearly

cortico-genesis and promotes early-born deep layer fates [22].

InductionofIkarosexpressioninlatecorticalprogenitors

(where it is normally downregulated) is not, however,

sufficienttoinduceectopicgenerationofdeeplayer

neu-rons.ThissuggeststhatcompetencetorespondtoIkarosis

lostovertime,reminiscentoftheprogressivelossof

com-petencetorespondtoHunchbackandKruppelin

Drosoph-ila neuroblasts [35–38]. FEZF2, which is expressed by

early-born,deep-layerneuronsisabletogiverisetosuch

neurons whenoverexpressed laterin corticogenesis,but

cannotpersebecalledatTF,sincetheprogenyofFEZF2

expressingAPsarefoundinallcorticallayers[43,57].

Figure2 (a) Dorsal pallium VI V IV Embryo VI V IV II/III Superficial layers Deep layers II/III Neurogenesis Gliogenesis CSF Developmental time AP IP Astrocytes Adult Neurons (b) Ctip2 T Tbr1 Fezf2 S Saatb22 Foxg1 Ctip2p2 Tb Tbr111 Fez Fezf2f2 Satb2 b b Foxg1 Early Late (c) Deep layer genesis Superficial layer genesis Gliogenesis AP Neurons Sip1 Factors Fgf9,Ntf3,Wnt7 Hyperpolarization SVZ VZ LV LV

Current Opinion in Neurobiology

Overviewofthemammaliancorticaldevelopment.

(a)Inthedevelopingdorsalpallium,excitatoryglutamatergicneuronsborndirectlyfromAPsorindirectlyfromIPsaregeneratedinsequential wavesthatorganizeoneabovetheotherandformsixdistinctneuronallayersintheadultneocortex.Atlatecorticogenesis,APsundergo self-consumingsymmetricdivisiontogenerateglialcells.(b)Cross-talkbetweenacoretranscriptionfactornetworkregulatingdeepversussuperficial layeridentity.Ctip2andFezf2instructdeeplayeridentityandareexpressedatearlycorticogenesis.Laterindevelopment,Satb2expressionis triggeredandinstructsuperficiallayerneuronidentity.(c)IllustrationofintrinsicandextrinsicinfluencesonthetemporaltransitionsofAPs. SignalingfactorsfrompostmitoticneuronsfeedbacktoAPsandinstructthetransitionfromdeeplayertosuperficiallayergenesis.Progressive hyperpolarizationofAPsmembranepotentialconstitutesanadditionalmechanismoftemporalregulation.Finally,dynamicchangesinthe compositionoftheCSFcouldmodulateAPsbehaviorthroughoutcorticogenesis.

Abbreviations:AP,apicalprogenitors;CSF,cerebrospinalfluid;IP,intermediateprogenitors;LV,lateralventricles;SVZ,subventricularzone;VZ, ventricularzone.

Beyond individualtTF candidates, recent studieshave

usedsingle-celltranscriptomicstoinvestigatethe

tempo-ral diversityand transcriptional dynamics of APs across

neurogenesis [32,45,46]. In a recent study [46], we

foundthattype-specificneuronalidentityemergesfrom

the apposition of generic differentiation programs onto

groundstate,embryonicage-dependenttemporal

identi-ties.The coincidencebetweentheinitially shared

tem-poral identity between newborn neurons and their

motherprogenitorissimilarto howtheprogeny of

Dro-sophila neuroblasts are temporally patterned by their

mothercells (seeRefs.[2,17]),with thedifferencethat

in themammalian brain, interactions betweenmultiple

transcriptionalprogramsratherthansingletTFsappearto

beatplay.

Epigeneticregulationplaysacriticalroleinthe

progres-sionofneocorticalprogenitoridentity.Thishasbeenthe

topic of a detailed recent review [62] and will only be

brieflydiscussedhere.ArecentstudyanalyzingtheDNA

methylation status of progenitors at different stages of

corticogenesisreported that APsare regulated bythree

successivewaves of demethylation,coinciding withthe

periodofneurogenesis,astrogenesisand

oligodendrogen-esis [63].In linewith this, in neurogenicAPsthe

pro-moters of core astrocytic genes are hypermethylated,

preventingAPstorespondtogliogenicextracellularcues

thatare already present earlyin corticogenesis [64–66].

Similarly, in late APs, the Polycomb group complex

(whichisalsoinvolvedinidentityprogressionin

Drosoph-ila,seeRef.[38]anddiscussionabove)hasbeenreported

torepressthepromoterofproneuralgenessuchasNgn1,

thusfavoring thetransitionto gliogenesis[67].

Non-cellautonomouscontrolsoverAPtemporalidentity

Ashasbeen reported for Drosophilalarval neurogenesis

[33,34], cell-extrinsic factors are involved in the

pro-gressionoftemporalidentityinthemammalianneocortex

(Figure2c).

Wehaverecentlyshownthatprogressive

hyperpolariza-tionisrequiredforprogressionintheneurogenic

compe-tenceofAPs,throughamechanisminvolvingregulation

ofWntsignaling[68].Supportingapathophysiological

relevance of these findings, mutation in the sodium

channelSCN3A,which isexpressedin cortical

progeni-tors, leads to cortical folding defects in humans [69].

Thus,environmentalsignals,byregulatingAPmembrane

potential, mayaffectthecourseof neurogenesis.Given

theiranatomicallocationliningthelateralventricles,APs

aredirectlyinfluencedbythecerebrospinalfluid(CSF),

whichcontainsalargeandhighlydynamicsetof

diffus-ible proteins including regulators of cell survival and

proliferation [70–73]. Neuron-derived signals may also

beinvolved,andthalamicaxons,inparticular,mayserve

asasourceofsignalingfactorstomodulatethecellcycle

length,proliferationrateandneuronaloutputofcortical

progenitorsandfine-tunepost-mitoticneuronalidentities

inanarea-specificmanner [74–76].

IllustratingaroleforneuronalprogenyincontrollingAP

behavior,deletionofthetranscriptionfactorSIP1

specifi-callyinnewbornneuronsleadstoaprecociousgeneration

of superficial layer neurons and increased gliogenesis

through feedback Fgf9, Ntf3 and Wnt signaling from

newly born neurons to cortical progenitor cells [77,78].

Inaddition,theembryonicgeneticablationofdeeplayer

neuronsusingNeurog2CreER/+micelengthenstheperiodof

deeplayerneuronproductionattheexpenseofsuperficial

layer generation, suggesting that feedback cues from

post-mitoticdeep layerneurons are transmitted to APs

to allow their temporal transitions and generation of

superficiallayerneurons [79].

Finally,as mentionedabove, post-mitotic controls over

thetemporalidentityof neuronsare also atplayin the

developing neocortex, including through interactions

with subplate and thalamocortical afferents [80,81],

whichtogethersculptdevelopingneuronsintotheirfinal

stage-specificidentity.

Perspectives

To sequentially generate distinct neuronal cell types,

neuronalprogenitorsprogressivelychangetheirtemporal

identity by integrating cell-autonomous transcriptional

dynamics and environmental cues. This progression

determines the identity of the neuronal progeny, and

hencesubsequentcircuit assemblyand function.Given

theoverarchingroleoftemporalpatterninginthe

assem-blyofthenervoussystem,itwillbeinterestingto

exam-inewhether abnormalmolecularpatterning,oncebetter

characterized,isacommonprocessattherootof

seem-inglydisparate neurodevelopmentaldisorders.

Conflict

of

interest

statement

Nothingdeclared.

Acknowledgements

WorkintheJabaudonlaboratoryissupportedbytheSwissNationalScience FoundationandtheCarigestFoundation.GAissupportedbyaPhD fellowshipfromthebelgianF.N.R.S-F.R.I.AandbytheFondsLeon Fredericq.

References

and

recommended

reading

Papersofparticularinterest,publishedwithintheperiodofreview, havebeenhighlightedas:

 ofspecialinterest ofoutstandinginterest

1. JeongH,TiwariVK:Exploringthecomplexityofcortical developmentusingsingle-celltranscriptomics.FrontNeurosci 2018,12:31.

2. KohwiM,DoeCQ:Temporalfatespecificationandneural progenitorcompetenceduringdevelopment.NatRevNeurosci 2013,14:823-838.

3. GuillemotF:Spatialandtemporalspecificationofneuralfates bytranscriptionfactorcodes.Development2007, 134:3771-3780.

4. DastjerdiFV,ConsalezGG,HawkesR:Patternformationduring developmentoftheembryoniccerebellum.FrontNeuroanat 2012,6:10.

5. HuJS,VogtD,SandbergM,RubensteinJL:Corticalinterneuron development:ataleoftimeandspace.Development2017, 144:3867-3878.

6. UrbachR,TechnauGM:Neuroblastformationandpatterning duringearlybraindevelopmentinDrosophila.Bioessays2004, 26:739-751.

7. BorelloU,PieraniA:Patterningthecerebralcortex:traveling withmorphogens.CurrOpinGenetDev2010,20:408-415. 8. AzzarelliR,HardwickLJA,PhilpottA:Emergenceofneuronal

diversityfrompatterningoftelencephalicprogenitors.Wiley InterdiscipRevDevBiol2015,4:197-214.

9. HolgueraI,DesplanC:Neuronalspecificationinspaceand time.Science2018,362:176-180.

10. DoeCQ:TemporalpatterningintheDrosophilaCNS.AnnuRev CellDevBiol2017,33:219-240.

11. LiX,ChenZ,DesplanC:Temporalpatterningofneural progenitorsinDrosophila.CurrTopDevBiol2013,105:69-96. 12. MiyaresRL,LeeT:TemporalcontrolofDrosophilacentral

nervoussystemdevelopment.CurrOpinNeurobiol2018, 56:24-32.

13. SkeathJB,ThorS:GeneticcontrolofDrosophilanervecord development.CurrOpinNeurobiol2003,13:8-15.

14. TrumanJW,BateM:Spatialandtemporalpatternsof neurogenesisinthecentralnervoussystemofDrosophila melanogaster.DevBiol1988,125:145-157.

15. ProkopA,TechnauGM:Theoriginofpostembryonic neuroblastsintheventralnervecordofDrosophila melanogaster.Development1991,111:79-88.

16. BrodyT,OdenwaldWF:Programmedtransformationsin neuroblastgeneexpressionduringDrosophilaCNSlineage development.DevBiol2000,226:34-44.

17. IsshikiT,PearsonB,HolbrookS,DoeCQ:Drosophila neuroblastssequentiallyexpresstranscriptionfactorswhich specifythetemporalidentityoftheirneuronalprogeny.Cell 2001,106:511-521.

18. KambadurR,KoizumiK,StiversC,NagleJ,PooleSJ, OdenwaldWF:RegulationofPOUgenesbycastorand hunchbackestablisheslayeredcompartmentsinthe DrosophilaCNS.GenesDev1998,12:246-260.

19. JabaudonD:Fateandfreedomindevelopingneocortical circuits.NatCommun2017,8:16042.

20. GreigLC,WoodworthMB,GalazoMJ,PadmanabhanH, MacklisJD:Molecularlogicofneocorticalprojectionneuron specification,developmentanddiversity.NatRevNeurosci 2013,14:755-769.

21. ElliottJ,JolicoeurC,RamamurthyV,CayouetteM:Ikarosconfers earlytemporalcompetencetomouseretinalprogenitorcells. Neuron2008,60:26-39.

22. AlsioJM,TarchiniB,CayouetteM,LiveseyFJ:Ikarospromotes early-bornneuronalfatesinthecerebralcortex.ProcNatlAcad SciUSA2013,110:E716-E725.

23. MattarP,EricsonJ,BlackshawS,CayouetteM:Aconserved regulatorylogiccontrolstemporalidentityinmouseneural progenitors.Neuron2015,85:497-504.

24. BelloBC,IzerginaN,CaussinusE,ReichertH:Amplificationof neuralstemcellproliferationbyintermediateprogenitorcells inDrosophilabraindevelopment.NeuralDev2008,3:5. 25. BooneJQ,DoeCQ:IdentificationofDrosophilatypeII

neuroblastlineagescontainingtransitamplifyingganglion mothercells.DevNeurobiol2008,68:1185-1195.

26. BayraktarOA,DoeCQ:Combinatorialtemporalpatterningin progenitorsexpandsneuraldiversity.Nature2013, 498:449-455.

27. GrosskortenhausR,PearsonBJ,MarusichA,DoeCQ:Regulation oftemporalidentitytransitionsinDrosophilaneuroblasts.Dev Cell2005,8:193-202.

28. KanaiMI,OkabeM,HiromiY:Seven-upcontrolsswitchingof transcriptionfactorsthatspecifytemporalidentitiesof Drosophilaneuroblasts.DevCell2005,8:203-213.

29. MettlerU,VoglerG,UrbanJ:Timingofidentity:spatiotemporal regulationofhunchbackinneuroblastlineagesofDrosophila bySeven-upandprospero.Development2006,133:429-437. 30. KohwiM,HiebertLS,DoeCQ:Thepipsqueak-domainproteins

DistalantennaandDistalantenna-relatedrestrictHunchback neuroblastexpressionandearly-bornneuronalidentity. Development2011,138:1727-1735.

31. NakaH,NakamuraS,ShimazakiT,OkanoH:Requirementfor COUP-TFIandIIinthetemporalspecificationofneuralstem cellsinCNSdevelopment.NatNeurosci2008,11:1014-1023. 32.

 OkamotoHashimotoM,M,MiyataMatsuzakiT,KonnoF,KawaguchiD,UedaHR,A:Cell-cycle-KasukawaT, independenttransitionsintemporalidentityofmammalian neuralprogenitorcells.NatCommun2016,7:11349.

Thisstudyidentifiedsubsetsoftemporallyregulatedgenesincortical progenitorsandshowedthattemporaltransitionsoccurevenincellcycle arrestedprogenitors.Italsoshowedthattemporaltransitionsofinvitro culturedprogenitorsstronglydependoncultureconditions,highlighting theimportanceofextrinsicfeedbackcuestotemporalprogressionof progenitors.

33.

 SyedtemporalMH,geneMarkexpressionB,DoeCQ:inSteroidDrosophilahormonebraininductionneuroblastsof generatesneuronalandglialdiversity.eLife2017,6.

ThisstudyidentifiednoveltTFsinlarvalcentralbrainneuroblastsandwas the first to show that extrinsic signaling via ecdysone receptors is necessaryforamajortransitionfromearlytolateneuroblasttemporal state.

34. SyedMH,MarkB,DoeCQ:Playingwellwithothers:extrinsic cuesregulateneuralprogenitortemporalidentitytogenerate neuronaldiversity.TrendsGenet2017,33:933-942.

35. PearsonBJ,DoeCQ:Regulationofneuroblastcompetencein Drosophila.Nature2003,425:624-628.

36. ClearyMD,DoeCQ:Regulationofneuroblastcompetence: multipletemporalidentityfactorsspecifydistinctneuronal fateswithinasingleearlycompetencewindow.GenesDev 2006,20:429-434.

37. KohwiM,LuptonJR,LaiS-L,MillerMR,DoeCQ: Developmentallyregulatedsubnucleargenome

reorganizationrestrictsneuralprogenitorcompetencein Drosophila.Cell2013,152:97-108.

38. ToumaJJ,WeckerleFF,ClearyMD:Drosophilapolycomb complexesrestrictneuroblastcompetencetogenerate motoneurons.Development2012,139:657-666.

39. PereiraJD,SansomSN,SmithJ,DobeneckerM-W,

TarakhovskyA,LiveseyFJ:Ezh2,thehistonemethyltransferase ofPRC2,regulatesthebalancebetweenself-renewaland differentiationinthecerebralcortex.ProcNatlAcadSciUSA 2010,107:15957-15962.

40. Morimoto-SuzkiN,HirabayashiY,TyssowskiK,ShingaJ,VidalM, KosekiH,GotohY:ThepolycombcomponentRing1Bregulates thetimedterminationofsubcerebralprojectionneuron productionduringmouseneocorticaldevelopment. Development2014,141:4343-4353.

41. FarnsworthDR,BayraktarOA,DoeCQ:Agingneuralprogenitors losecompetencetorespondtomitogenicnotchsignaling. CurrBiol2015,25:3058-3068.

42. GaoP,PostiglioneMP,KriegerTG,HernandezL,WangC,HanZ, StreicherC,PapushevaE,InsoleraR,ChughKetal.:

Deterministicprogenitorbehaviorandunitaryproductionof neuronsintheneocortex.Cell2014,159:775-788.

43. GuoC,EcklerMJ,McKennaWL,McKinseyGL,RubensteinJLR, ChenB:Fezf2expressionidentifiesamultipotentprogenitor forneocorticalprojectionneurons,astrocytes,and oligodendrocytes.Neuron2013,80:1167-1174.

44. FrancoSJ,Gil-SanzC,Martinez-GarayI,EspinosaA, Harkins-PerrySR,RamosC,Mu¨llerU:Fate-restrictedneuralprogenitors inthemammaliancerebralcortex.Science2012,337:746-749. 45. YuzwaSA,BorrettMJ,InnesBT,VoronovaA,KetelaT,KaplanDR, BaderGD,MillerFD:Developmentalemergenceofadultneural stemcellsasrevealedbysingle-celltranscriptionalprofiling. CellRep2017,21:3970-3986.

46. TelleyL,AgirmanG,PradosJ,FievreS,OberstP,VitaliI,NguyenL, DayerA,JabaudonD:Single-celltranscriptionaldynamicsand originsofneuronaldiversityinthedevelopingmouse neocortex.bioRxiv2018http://dx.doi.org/10.1101/409458. 47. GaspardN,BouschetT,HourezR,DimidschsteinJ,NaeijeG,van

denAmeeleJ,Espuny-CamachoI,HerpoelA,PassanteL, SchiffmannSNetal.:Anintrinsicmechanismofcorticogenesis fromembryonicstemcells.Nature2008,455:351-357. 48. Espuny-CamachoI,MichelsenKA,GallD,LinaroD,HascheA,

BonnefontJ,BaliC,OrduzD,BilheuA,HerpoelAetal.:Pyramidal neuronsderivedfromhumanpluripotentstemcellsintegrate efficientlyintomousebraincircuitsinvivo.Neuron2013, 77:440-456.

49. CampJG,BadshaF,FlorioM,KantonS,GerberT, Wilsch-Bra¨uningerM,LewitusE,SykesA,HeversW,LancasterMetal.: Humancerebralorganoidsrecapitulategeneexpression programsoffetalneocortexdevelopment.ProcNatlAcadSciU SA2015,112:15672-15677.

50. McConnellSK:Fatesofvisualcorticalneuronsintheferret afterisochronicandheterochronictransplantation.JNeurosci 1988,3:945-974.

51. McconnellSK,KaznowskiCE:Cellcycledependenceoflaminar determinationindevelopingneocortex.Science1991, 254:282-285.

52. FrantzGD,McConnellSK:Restrictionoflatecerebralcortical progenitorstoanupper-layerfate.Neuron1996,17:55-61. 53. OberstP,FievreS,BaumannN,ConcettiC,JabaudonD:Apical

progenitorsremainmultipotentthroughoutcortical neurogenesis.bioRxiv2018http://dx.doi.org/10.1101/478891. 54. HanashimaC,LiSC,ShenL,LaiE,FishellG:Foxg1suppresses

earlycorticalcellfate.Science2004,303:56-59. 55. FameRM,MacDonaldJL,MacklisJD:Development,

specification,anddiversityofcallosalprojectionneurons. TrendsNeurosci2011,34:41-50.

56. McKennaWL,BetancourtJ,LarkinKA,AbramsB,GuoC, RubensteinJLR,ChenB:Tbr1andFezf2regulatealternate corticofugalneuronalidentitiesduringneocortical development.JNeurosci2011,31:549-564.

57. ChenB,WangSS,HattoxAM,RayburnH,NelsonSB, McConnellSK:TheFezf2-Ctip2geneticpathwayregulatesthe fatechoiceofsubcorticalprojectionneuronsinthe developingcerebralcortex.ProcNatlAcadSciUSA2008, 105:11382-11387.

58. HanW,KwanKY,ShimS,LamMMS,ShinY,XuX,ZhuY,LiM, SestanN:TBR1directlyrepressesFezf2tocontrolthelaminar originanddevelopmentofthecorticospinaltract.ProcNatl AcadSciUSA2011,108:3041-3046.

59. SrinivasanK,LeoneDP,BatesonRK,DobrevaG,KohwiY, Kohwi-ShigematsuT,GrosschedlR,McConnellSK:Anetworkof geneticrepressionandderepressionspecifiesprojection fatesinthedevelopingneocortex.ProcNatlAcadSciUSA 2012,109:19071-19078.

60.

 ZahrKaplanSK,DR,YangMillerG,FD:KazanAtranslationalH,BorrettMJ,repressionYuzwaSA,complexVoronovainA, developingmammalianneuralstemcellsthatregulates neuronalspecification.Neuron2018,97:520-537.e6.

Thisstudyshowedthatapicalprogenitorsaretranscriptionallybimodalas theyexpressidentifiersofbothdeepandsuperficiallayerneuronsatthe

mRNAlevel,butthetranslationoftheinappropriatemarkerisinhibitedby atranslationalrepressivecomplex.

61. AzimE,ShniderSJ,CederquistGY,SohurUS,MacklisJD:Lmo4 andClim1progressivelydelineatecorticalprojectionneuron subtypesduringdevelopment.CerebCortex2009,19:62-69. 62. YoonK-J,VissersC,MingG-L,SongH:Epigeneticsand

epitranscriptomicsintemporalpatterningofcorticalneural progenitorcompetence.JCellBiol2018,217:1901-1914. 63.

 OtsukaSanosakaM,T,MiuraImamuraF,ItoT,T,HamazakiFujiiN,IkeoN,KChaietal.:M,DNAIgarashimethylomeK, Ideta-analysisidentifiestranscriptionfactor-basedepigenomic signaturesofmultilineagecompetenceinneuralstem/ progenitorcells.CellRep2017,20:2992-3003.

ThisstudyreportedtheDNAmethylationdynamicsofcorticalprogenitors alongcorticogenesisandidentifiedwavesofdemethylationcoinciding withtheneurogenicandgliogenicperiod.

64. TakizawaT,NakashimaK,NamihiraM,OchiaiW,UemuraA, YanagisawaM,FujitaN,NakaoM,TagaT:DNAmethylationisa criticalcell-intrinsicdeterminantofastrocytedifferentiationin thefetalbrain.DevCell2001,1:749-758.

65. FanG,MartinowichK,ChinMH,HeF,FouseSD,HutnickL, HattoriD,GeW,ShenY,WuHetal.:DNAmethylationcontrols thetimingofastrogliogenesisthroughregulationofJAK-STAT signaling.Development2005,132:3345-3356.

66. HeF,GeW,MartinowichK,Becker-CataniaS,CoskunV,ZhuW, WuH,CastroD,GuillemotF,FanGetal.:Apositive

autoregulatoryloopofJak-STATsignalingcontrolstheonset ofastrogliogenesis.NatNeurosci2005,8:616-625.

67. HirabayashiY,SuzkiN,TsuboiM,EndoTA,ToyodaT,ShingaJ, KosekiH,VidalM,GotohY:Polycomblimitstheneurogenic competenceofneuralprecursorcellstopromoteastrogenic fatetransition.Neuron2009,63:600-613.

68.

 VitaliBaumannI,Fie`vreN,McMahonS,TelleyL,JJ,OberstKlinglerP,E,BariselliBocchiS,RetFrangeulal.:ProgenitorL, hyperpolarizationregulatesthesequentialgenerationof neuronalsubtypesinthedevelopingneocortex.Cell2018, 174:1264-1276.e15.

Thisstudyshowedthatbioelectricmembranepropertiesarepermissive fortemporalprogressionofcorticalapicalprogenitorsandthat hyper-polarizationleadstoaforwardshiftinapicalprogenitortemporalstatevia inhibitionofWnt-beta-cateninsignaling.

69.

 PartlowSmithRS,JN,KennyHillRS,CJ,ShinGaneshT,ChenV,JangAY,A,DoanBorges-MonroyRNetal.:SodiumR, channelSCN3A(NaV1.3)regulationofhumancerebralcortical foldingandoralmotordevelopment.Neuron2018,99:905-913. e7.

Thisstudyshowedthatthevoltage-gatedsodiumchannelNaV1.3plays animportantroleintheprenataldevelopmentofhumancorticalareasand thatmutationsinNaV1.3disruptscorticalfolding.

70. ZappaterraMD,LisgoSN,LindsayS,GygiSP,WalshCA,BallifBA: Acomparativeproteomicanalysisofhumanandrat embryoniccerebrospinalfluid.JProteomeRes2007, 6:3537-3548.

71. LehtinenMK,ZappaterraMW,ChenX,YangYJ,HillAD,LunM, MaynardT,GonzalezD,KimS,YePetal.:Thecerebrospinal fluidprovidesaproliferativenicheforneuralprogenitorcells. Neuron2011,69:893-905.

72. LehtinenMK,WalshCA:Neurogenesisatthe brain-cerebrospinalfluidinterface.AnnuRevCellDevBiol2011, 27:653-679.

73. ChauKF,SpringelMW,BroadbeltKG,ParkH-Y,TopalS,LunMP, MullanH,MaynardT,SteenH,LaMantiaASetal.:Progressive differentiationandinstructivecapacitiesofamnioticfluidand cerebrospinalfluidproteomesfollowingneuraltubeclosure. DevCell2015,35:789-802.

74. PouchelonG,FrangeulL,RijliFM,JabaudonD:Patterningof pre-thalamicsomatosensorypathways.EurJNeurosci2012, 35:1533-1539.

75. VueTY,LeeM,TanYE,WerkhovenZ,WangL,NakagawaY: Thalamiccontrolofneocorticalareaformationinmice.J Neurosci2013,33:8442-8453.

76. DehayC,SavatierP,CortayV,KennedyH:Cell-cyclekineticsof neocorticalprecursorsareinfluencedbyembryonicthalamic axons.JNeurosci2001,21:201-214.

77. SeuntjensE,NityanandamA,MiquelajaureguiA,DebruynJ, StryjewskaA,GoebbelsS,NaveK-A,HuylebroeckD,TarabykinV: Sip1regulatessequentialfatedecisionsbyfeedbacksignaling frompostmitoticneuronstoprogenitors.NatNeurosci2009, 12:1373-1380.

78. ParthasarathyS,SrivatsaS,NityanandamA,TarabykinV:Ntf3 actsdownstreamofSip1incorticalpostmitoticneuronsto controlprogenitorcellfatethroughfeedbacksignaling. Development2014,141:3324-3330.

79. TomaK,KumamotoT,HanashimaC:Thetimingofupper-layer neurogenesisisconferredbysequentialderepressionand

negativefeedbackfromdeep-layerneurons.JNeurosci2014, 34:13259-13276.

80.

 OzairmodelingMZ,KirstrevealsC,vanthatdenfateBergselectionBL,RuzoofA,corticalRitoT,BrivanloudeepprojectionAH:hPSC neuronsoccursinthesubplate.CellStemCell2018,23:60-73.e6. This study highlights the importance of the transient expansion of the subplate in primatecorticogenesisassubplateneuronscanbemodulatedpostmitotically byWnt-signalingtospecifyintoallclassesofdeeplayerprojectingneurons. 81.

 Ohtaka-MaruyamaYuraK,OkadoH,MiyataC,OkamotoT,MaedaM,EndoN:SynapticK,OshimatransmissionM,KanekoN, fromsubplateneuronscontrolsradialmigrationofneocortical neurons.Science2018,360:313-317.

This study showed that in the developing cortex, subplate neurons developtransientsynapsestocontactmigratingneurons andinstruct theirmultipolartobipolarmorphologicaltransition.