ARTICLESPUBLISHEDONLINE:31OCTOBER2016|DOI:10.1038/NMAT4780MicroRNA-21preservesthefibroticmechanicalmemoryofmesenchymalstemcellsChenXiLi1†,NileshP.Talele1†,StellarBoo1†,AnneKoehler1,ErickaKnee-Walden1,JennaL.Balestrini2,PamSpeight3,AndrasKapus3andBorisHinz1*Expansiononstifculturesubstratesactivatespro-fibroticcellprogramsthatareretainedbymechanicalmemory.Here,weshowthatprimingonphysiologicallysoftsiliconesubstratessuppressesfibrogenesisanddesensitizesmesenchymalstemcells(MSCs)againstsubsequentmechanicalactivationinvitroandinvivo,andidentifythemicroRNAmiR-21asalong-termmemorykeeperofthefibrogenicprograminMSCs.Duringstifpriming,miR-21levelsweregraduallyincreasedbycontinuedregulationthroughtheacutelymechanosensitivemyocardin-relatedtranscriptionfactor-A(MRTF-A/MLK-1)andremainedhighover2weeksafterremovalofthemechanicalstimulus.KnockingdownmiR-21oncebytheendofthestif-primingperiodwassufcienttoerasethemechanicalmemoryandsensitizeMSCstosubsequentexposuretosoftsubstrates.SoftpriminganderasingmechanicalmemoryfollowingcellcultureexpansionprotectsMSCsfromfibrogenesisinthehostwoundenvironmentandincreasesthechancesforsuccessofMSCtherapyintissue-repairapplications.Wehavepreviouslycoinedtheterm‘mechanicalmemory’toestablishthatcellspermanentlyimprintinformationregardingsubstratemechanicalconditions1.Forexample,continuedcultureonpathologicallystiffculturesubstratesactivatesnormallungfibroblastsintopro-fibroticmyofibroblaststhatbecomeresistanttodeactivationbysubsequentexposuretophysiologically(lung-)softculturesubstratesevenafterweeks1.Mechanicalmemorywasalsoreportedtoinfluencelineagechoiceofmesenchymalstemcells(MSCs),althoughonashortertimescale2.Severalessentialquestionsremainunanswered:Doessubstrate-imprintedmechanicalmemoryaffectthefibrogenicbehaviourofculturedMSCs?Doesmemorypersistaftertransplantationinvivo?Ifso,canmechanicalmemorybemanipulatedtoenhancethesuccessofMSCtherapyforwoundrepair?Finally,whatarethemolecularmechanismsthatregulatelong-termmemoryoffibrogenicprogramsi...
