This document describes a study that challenges the classic model of neuronal migration in the adult mammalian brain. Using clonal lineage tracing in mice, the study found that precursor cells for excitatory principal neurons in the dentate gyrus exhibit significant tangential migration away from their parental stem cells, rather than strictly radial migration as predicted by the classic model. This represents a previously unidentified mode of glutamatergic neuronal migration under physiological conditions in the adult mammalian brain. The results reveal a two-step migration process, with initial significant tangential migration of neuroblasts followed by limited radial migration of immature neurons into the granule cell layer.

1. NovelMigraonModeofNeuronalPrecursorsintheAdultDentateGyrus
a
InstuteforCellEngineering,JohnsHopkinsUniversitySchoolofMedicine,Balmore,MD,21205
b
TheSolomonH.SnyderDepartmentofNeuroscience,JohnsHopkinsUniversitySchoolofMedicine,Balmore,MD21205
c
DepartmentofFundamentalNeurosciences,UniversityofLaussane,1005Laussane,Switzerland
d
CenterforSensoryBiology,JohnsHopkinsUniversitySchoolofMedicine,Balmore,MD21205
e
DepartmentofMolecularBiologyandGenecs,JohnsHopkinsUniversitySchoolofMedicine,Balmore,MD,21205
f
DepartmentofNeurology,JohnsHopkinsUniversitySchoolofMedicine,Balmore,MD,21205
gg
DepartmentofPsychiatryandBehavioralSciences,JohnsHopkinsUniversitySchoolofMedicine,Balmore,MD21205
References
Figure1.SampleprojectedconfocalimageofanAscl1CreERT2
;Rosa-
YFPf/+
–labeledRGLinthedentategyrusat1dpi.Asshowninahigh-
magnificaonimage(Bo om),thisGFAP+
RGL(closedarrowheads)
underwentanasymmetricdivisiontoproduceaGFAP−
neuronal
progeny(openarrowheads;onadifferentfocalplane).
[Scalebars,100μm(Top)and10μm(Bo om).]
Figure2.Sampleprojeconconfocalimageofalabeledclone
inanAscl1CreERT2
;Rosa-YFPf/+
mouseat1mpishowingtangenal
distribuonofProx1+
NeuN+
matureglutamatergicgranule
neuronsandtheirparentalGFAP+
RGL.(Inset)Two-dimensional
SGZplaneprojeconin200-μm-squarewindowsofallclonally
relatedcelllocaons.TheRGLisrepresentedasthecenteropen
cicircle;neuralprogenyarerepresentedasclosedcircles.
Figure3.(Le)DistribuonplotofthetangenaldistancebetweenalabeledRGLand
itsprogenywithineachcloneat3dpi,7dpi,and1–2mpi.Rawdistribuonsareshown
asbargraphs;curvedlinescorrespondtosmootheddistribuons.(Right)Dotplotof
thetangenalversusradialdistancefromtheparentalRGLforeachneuralprogenyin
alllabeledRGL-containingclonesat3dpi,7dpi,and1–2mpi.(Inset)Tangenaldistance
ofeachneuralprogenyfromitsparentalRGL.
Figure4.(Top)Schemacdiagramfor
measurementsoftangenalandradialdistances
ofneuronalprogenyfromtheparentalRGL.
(Bo om)EnlargedconfocalimagesforaGFAP+
RGL(1)andProx1+
NeuN+
matureneuronal
progeny(2).(Scalebars,10μm.)GCL,granule
celllcelllayer.
Figure5.Summaryofmolecularmarkersusedforidenficaonofeachcelltype
duringadulthippocampalneurogenesis.Newborncellsaregeneratedfrom
GFAP+
Nesn+
RGLsthatundergoasymmetricdivisions,whichdevelopintoTbr2+
intermediateprogenitorcellswithshort,mulpolarprocesses.Within3–7d,
newborncellspossesslong,bipolarprocessesandelongatedsomasinaTbr2+/−
DCX+
neuroblaststagebeforepenetrangthegranulecelllayerandbecominga
polaripolarizedProx1+
DCX+
immatureneuronwithaxonanddendrite.Overthenext
month,newborncellsmatureintoProx1+
NeuN+
neuronswithspinydendritesand
longaxonsthatprojecttoCA3.
Figure6.(Le)Summaryofthetangenaldistanceofeachneuralprogenyfromits
parentalRGLateachdevelopmentalstage.IN,immatureneuron;N,maturegranule
neuron;NB,neuroblast.(Right)Histogramandcumulavedistribuonplotofthe
maximumdistancebetweenclonallyrelatedneuralprogenyforallclones,including
non–RGL-containingclones.
Abstract
Mammalianbraindevelopmentisacomplex,orderedprocesswherebynewborn
neuronsfollowstereotypedmigraonmodestoorganizeintospecificpaerns
requiredforcomplicatedneuralcircuitformaon.Classically,principalexcitatory
neuronsarethoughttoorganizeintoradialcolumnsthatunderliethebasicbrain
circuits,whereasinhibitoryneuronsdispersetangenallyacrossthesecolumnsto
modulatetheprincipalcircuits.Theseprinciplesarethoughttobefundamentaltothe
ggenesisofthecomplexmammalianbrain.Surprisingly,wefoundthatprecursorsfor
excitatoryprincipalneuronsexhibittangenalmigraonintheadultmammalian
brain.Ourfindingsenrichourunderstandingofneurodevelopmentandlayimportant
conceptualgroundworkforstudiesofbrainplascity,disease,andrepair.
Introducon
Conclusions
Thenervoussystemisformedbymigraonofneuronalprecursorsand
immatureneuronstospecificlocaonsduringdevelopment.Theclassic
radialunithypothesisofmammalianbraindevelopmentpostulatesthatin
thedevelopingneocortex,glutamatergic,excitatory,principalneurons
migrateradiallytoformdiscreteinformaon-processingcolumnsof
ontogenecorigin,whereasGABAergic,inhibitory,modulatory
iinterneuronsmigratetangenallyacrosscolumns.Neurogenesispersistsin
theadultmammalianbrainintwoprimaryregionsandisthoughttofollow
theclassicmigraonmodel.Inthesubventricularzone(SVZ)ofthelateral
ventricles,newneuronsgeneratedfromneuralprecursorsmigrate
tangenallytotheolfactorybulbtobecomeGABAergicinterneurons.In
contrast,inthesubgranularzone(SGZ)ofthedentategyrus,newneurons
generatedfromradialglia-likeneuralstemcells(RGLs)migrateradiallyinto
thegthegranulecelllayertobecomeprincipalglutamatergicgranulecells.Due
totechnicalchallenges,migratorypaernshaveonlybeenexaminedatthe
cell-populaonlevel,andthusweslllackdetailedinformaonaboutthe
spaalrelaonshipbetweenindividualprecursorsandtheirprogenyinvivo.
Contrarytotheclassicmodel,ourrecentclonallineagetracingofindividual
quiescentRGLsshowedtangenaldistribuonofglutamatergicgranule
neuronswithrespecttotheirparentalRGLintheadultdentategyrus.We
thereforesystemacallyinvesgatedthemigraonpaernandtrajectoryof
thesenewborncells.Usingaclonallineage-tracingapproachthat
preferenallytargetsacveRGLsintheadultmousedentategyrus,thereby
birthdbirthdangtheirnewbornprogenyinvivo,wefoundsignificanttangenal
distribuonofnewbornneuroblastsfromtheirparentalRGL.Ourresults
revealapreviouslyunidenfiedmodeofglutamatergicneuronalmigraon
underphysiologicalcondionsintheadultmammalianbrain.
Results
Incontrasttotheprevailingmodelthatglutamatergicneuronsmigrate
radially,whereasinterneuronsmigratetangenallyduringdevelopment,
wedemonstratedforthefirst meintheadultmammalianbrainthat
neuroblastprecursorsofprincipalglutamatergicneuronsexhibitsignificant
tangenaldistribuonawayfromtheirparentalstemcellsunder
physiologicalcondions.Wefoundatwo-stepmigraonprocessduring
adulthippoadulthippocampalneurogenesisinwhichsignificanttangenalmigraon
ofneuroblastsisfollowedbylimitedradialmigraonofimmatureneurons.
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