In vitro models of the blood-brain barrier.

ReviewActaNeurobiolExp2011,71:113–128©2011byPolishNeuroscienceSociety-PTBUN,NenckiInstituteofExperimentalBiologyINTRODUCTIONTheblood-brainbarrier(BBB)isanactiveinterfacebetweenthecirculationandthecentralnervoussystem(CNS)whichrestrictsthefreemovementofdifferentsubstancesbetweenthetwocompartmentsandplaysacrucialroleinthemaintenanceofthehomeostasisoftheCNS.TheBBBhasadual,abarrierandacarrierfunction.ThebarrierfunctionmeansthattheBBBrestrictsthetransportofpotentiallytoxicorharmfulsubstancesfromthebloodtothebrainwhichisachievedthroughafourfolddefenseline:1)Theparacellularbarrierformedbyinterendothe-lialjunctionsrestrictsthefreemovementofwatersolublecompoundsbetweentwoadjacentcells.2)Thetranscellularbarrierismadepossiblebythelowlevelofendocytosisandtranscytosischaracteristicforbrainendothelialcellsandinhibitstransportofsubstancesthroughthecytoplasm.3)Theenzymaticbarrierisprovidedbyacomplexsetofenzymes,includingacetylcholinesterase,alka-linephosphatase,gamma-glutamyltranspeptidase,monoamineoxidases,andotherdrugmetabolizingenzymescapabletodegradedifferentchemicalcom-pounds.4)Inadditiontothese,thecerebralendotheliumexpressesalargenumberofeffluxtransporters(ABC,ATP-bindingcassettetransporters)likeABCB1(P-glycoprotein),ABCC1,ABCC4andABCG2(BCRP).BesidesthebarrierfunctiontheBBBpossessesanimportantcarrierfunctionwhichisresponsibleforthetransportofnutrientstothebrainandremovalofmetab-olites.Smalllipid-solublemoleculesandbloodgaseslikeoxygenandcarbondioxidediffusepassivelytheBBB,whileessentialpolarnutrientslikeglucoseandaminoacidsrequirespecifictransportproteins(solutecarriers,SLCtransporters)inordertoreachthebrain.TheBBBplaysacrucialroleintheclinicalpracticeaswell.Ontheonesidethereisalargenumberofneu-rologicaldisordersincludingcerebralischemia,brainInvitromodelsoftheblood-brainbarrierImolaWilhelm,CsillaFazakas,andIstvanA.Krizbai*InstituteofBiophysics,BiologicalResearchCenter,HungarianAcademyofSciences,Szeged,Hungary,*Email:[e-mail]-brainbarrier(BBB)isanactiveinterfacebetweenthecirculationandthecentralnervoussystem(CNS)withadualfunction:thebarrierfunctionrestrictsthetransportfromthebloodtothebrainofpotentiallytoxicorharmfulsubstances;thecarrierfunctionisresponsibleforthetransportofnutrientstothebrainandremovalofmetabolites.TheBBBplaysacrucialroleintheclinicalpracticeaswell.Ontheonesidethereisalargenumberofneurologicaldisordersincludingcerebralischemia,braintraumaandtumors,neurodegenerativedisorders,inwhichthepermeabilityoftheBBBisincreased.OntheotherhandduetotherelativeimpermeabilityofthebarriermanydrugsareunabletoreachtheCNSintherapeuticallyrelevantconcentration,makingtheBBBoneofthemajorimpedimentsinthetreatmentofCNSdisorders.ThesignificantscientificandindustrialinterestinthephysiologyandpathologyoftheBBBledtothedevelopmentofseveralinvitromodelsoftheBBB.Thesemodelsaremainlybasedonthecultureofcerebralendothelialcells.Thebestinvitromodelswhichmimicthebestwaytheinvivoanatomicalconditionsaretheco-culturemodelsinwhichbrainendothelialcellsareco-culturedwithastrocytesand/orpericytes.OurinvitroBBBmodelischaracterizedbyhightransendothelialelectricalresistance(TEERregularilyabove200Ohm×cm2),lowpermeabilityandexpressionofseveraltransporters.OurexperimentshaveproventhatthemodelissuitableforbasicresearchandfortestingtheinteractionbetweentheBBBandpotentialdrugcandidates(toxicity,permeability,interactionwitheffluxtransporters)aswell.Keywords:cerebralendothelialcells,blood-brainbarrier,invitromodels,permeability,tightjunctionCorrespondenceshouldbeaddressedto:I.A.KrizbaiE-mail:[e-mail]20December2010,accepted26January2011114I.Wilhelmetal.trauma,tumors,neurodegenerativedisordersinwhichthepermeabilityoftheBBBisincreased(Weissetal.2009).Ontheotherhand,duetotherelativeimperme-abilityofthebarriermanydrugsareunabletoreachtheCNSintherapeuticallyrelevantconcentration,makingtheBBBoneofthemajorimpedimentsinthetreatmentofCNSdisorders(JeffreyandSummerfield2010).CELLULARCOMPONENTSOFTHEBBBTheprincipalcomponentsoftheBBBaretheendothelialcells,astrocytes,pericytes(Fig.1).SomeothercellularelementslikeneuronsormicrogliamayalsoplayasignificantroleinthefunctionofBBB.EndothelialcellsFromthepointofviewofthepermeabilitythemostimportantcelltypesoftheBBBarecerebralendothelialcells(CECs)whichformacontinuoussheetcoveringtheinnersurfaceofthecapillaries.Endothelialcellsareinterconnectedbytightjunctionswhichformabelt-likestructureattheapicalregionofthecells.Brainendothe-lialcellshavebothendothelial-likefeatures(i.e.,expres-sionofvonWillebrandfactor,uptakeofacetylatedLDL,highactivityofalkalinephosphataseandgamma-glu-tamyltranspeptidase)andepithelial-likefeatureslikecontinuouslineoftightjunctions,lowlevelofpinocyto-sis,hightransendothelialelectricalresistance(TEER).PericytesEndothelialcellsaresittingonthebasalmembranewhichconsistsmainlyofcollagenIV,fibronectin,lami-ninandproteoglycans.Engulfedinthebasalmembranearethepericyteswhichcoverapproximately22-32%oftheendothelium.Pericytesplayanimportantroleintheregulationofendothelialproliferation,angio