AsinglecellWntsignalingnichemaintainsstemnessofalveolartype2cellsAhmadNabhan1,DouglasG.Brownfield1,MarkA.Krasnow*,1,andTusharJ.Desai*,2,31DepartmentofBiochemistryandHowardHughesMedicalInstitute,StanfordUniversitySchoolofMedicine,StanfordCA,94305-53072DepartmentofInternalMedicine,DivisionofPulmonaryandCriticalCare,StanfordUniversitySchoolofMedicine,StanfordCA,94305-53073InstituteforStemCellBiology&RegenerativeMedicine,StanfordUniversitySchoolofMedicine,StanfordCA,94305-5307AbstractLungalveoliarelinedbysquamousalveolarepithelialtype1(AT1)epithelialcellsthatfacilitategasexchange,andneighboringAT2cellsthatsynthesizeandsecretesurfactant.Alveoliaremaintainedbyintermittentactivationofrare‘bifunctional’AT2cellsthatretainsurfactantbiosynthesisfunctionbutalsoserveasstemcells,generatingnewAT1cellsandself-renewingthroughoutadultlife.WhilestemcellproliferationiscontrolledbyEGFR/KRASsignaling,howthestemcellsareselected,maintained,andthefatesoftheirdaughtercellscontrolledareunknown.HereweshowthatexpressionoftheWnttargetgeneAxin2inmouselungidentifiesarare,stablesubpopulationofAT2cellswithstemcellactivity.ManylienearsinglefibroblaststhatexpressWnt5aandotherWntgenes,andgeneticallytargetingWntsecretionbyfibroblastsdepletestheAxin2+AT2stemcellpopulation.Axin2turnsoffwhendaughtercellsleavetheWntnicheandtransdifferentiateintoAT1cells,andsustainingWntsignalingblockstransdifferentiationwhereasabrogationofWntsignalingpromotesit,bothinvivoandinvitro.Uponseverealveolarepithelialinjury,Axin2isinducedthroughouttheAT2population,recruiting‘ancillary’AT2cellsintoaprogenitorrole.NicheexpressionofWnt5aandtheWntsecretionmediatorPorcupineisunchangedbyinjury,butWnt7bandseveralotherWntgenesarebroadlyinducedalongwithPorcupineinAT2cells,andpharmacologicorgeneticinhibitionofthisautocrineWntsignalingimpairstheAT2proliferativeresponse.TheresultssupportamodelinwhichindividualAT2cellsresideinsinglecellfibroblastnichesthatprovideashort-rangeparacrine(or"juxtacrine")Wntsignalthatselectsandmaintainsalveola...
