Apoptosisorprogrammedcelldeath(PCD)isageneticallycontrolledcellularsuicidemechanismforselectivelyeliminatingunwantedcells(forreviewssee1-4).PCDisanobligatoryprocessinavarietyofBIOLOGicalprocessesincludingembryonicandneuronaldevelopment,immunesystemregulation,organogenesis,tissuehomeostasisandinthepreventionofmalignanciessuchastumorgrowthandviralinfection.Apoptosisischaracterisedbyplasmamembraneblebbing,cellshrinkage,nuclearcondensation,endonucleolyticcleavageofgenomicDNAintointernucleosomallengthfragmentsandtheformationofapoptoticbodies.
Currentmethodsforstudyingapoptosisincludetheassessmentofmorphologicalchangesatthecellularlevelbylight-,electron-ortime-lapsemicroscopyincombinationwithvitalfluorescentdyes(reviewedin5,6),theuseofAnnexinVtomonitorthelossofmembranephospholipidasymmetryduringapoptosis(7)orassaystodetectDNAfragmentationbyagarosegelelectrophoresis(8-10),byinsitunick-endlabelling(TUNEL)(11),orbyusinganenzyme-linkedimmunoabsorbentassay(12).Mostofthesemethods,however,arecumbersome,inappropriateor,inthecaseoftheTUNELmethod,expensiveforstudyingtheeffectofputativecelldeathgenesintransienttransfectionassays.
Wehavedevelopedamethodsuitablefortheflowcytometricdeterminationofapoptosisinalargenumberofsamplesbypropidiumiodide(PI)inapopulationofcellstransientlyco-transfectedwithaplasmidencodingenhancedgreenfluorescentprotein(eGFP)(13)andaplasmidcarryingageneofinterest.ThisassaycombinestheFACS-optimisedGFPmutant(eGFP)(13)asaMarkertodetecttransientlytransfectedcellsandthereducedfluorescenceoftheDNA-bindingdyePIintheapoptoticsubpopulationasamarkerforapoptosis(14-17).ThereducedfluorescenceofPIinapoptosingcellsresultsintheappearanceofacharacteristicsub-2NfluorescencepeakwithrespecttotheG0/G1cellcycleregion.
Inordertovalidatethismethod,weinitiallyusedtheestablishedmurinetumorcelllines([beta]TCand[beta]HC)whichoriginatefrom[beta]celltumors(18)oftransgenicmiceinwhichtheinsulingeneregulatoryregion(Rip)targetstheexpressionofthesimianvirus40largeT-antigen(Tag)tothe[beta]cellsofthepancreaticislets(Rip1Tag2)(19).
Approximately80000cellswereseededina3cmdiameterwellofasix-welldishandgrowninDMEMsupplementedwith10%FCS(v/v),2mMglutamine,100IUpenicillinand100µg/mlstreptomycinto70%confluency.Cellsweretransfectedwith1µgofaplasmidencodingenhancedGFP(pEGFP-C1;Clontech)togetherwith1µgofeitheracontrolplasmid(pMEX),orapCMVplasmidcarryingeitherthepro-apoptoticadenovirusE1Aortheanti-apoptoticadenovirusE1B-19Kgene(20,21)using10µlLipofectAMINEReagent(Gibco-BRL)accordingtothemanufacturer"srecommendations.Followingtransfection,thecellswereallowedtorecoverincompletemediumfor16handweretheneitherleftuntreatedoranapoptoticstimulus(800ng/mlstaurosporine;Sigma)(22,23)wasaddedforanadditional16h.At32haftertransfection,floatingcellswerecollectedandcombinedwithtrypsinised,adherentcells,washedtwicewith4mlPBSandfixed[2%paraformaldehyde(PFA),100mMNaCl,300mMsucrose,3mMMgCl2,1mMEGTA,10mMPIPESpH6.8]atroomtemperaturefor30min,washedtwicewith4mlPBSandpost-fixedinice-cold70%EtOHfor10-14h.
Followingfixation,cellswerewashedtwicewith4mlPBSanddividedtoallowcomparisonofourmethodtothecommonlyusedmodifiedTUNELmethodusingfluorescentCy5-CTP.OnehalfofthesamplewastreatedwithRNaseA(50µg/ml)inPBSfor30min,washedtwicewith4mlPBSandstainedwithPI(50µg/ml)inPBSfor30minbeforeFACSanalysis.Theotherhalfofthesamplewasincubatedin50µlofterminaldeoxynucleotidyltransferase(TdT)reactionmixture(BoehringerMannheim;200mMpotassiumcacodylate,25mMTris-HClpH6.6,0.25mg/mlbovineserumalbumin,1mMCoCl2,0.25nmolofFluorOlinkCy5AP3-dCTP[Amersham],12.5UTdT)for1hat37°C.Thesamplewasthenwashedtwicewith4mlPBS,treatedwithRNaseA(50µg/ml)inPBSfor30min,washedtwicewith4mlHBS(Note:HBSisusedfromthissteponwardssinceDAPItendstocausemicro-precipitatesinPBS),stainedwithDAPI(10µg/ml;BoehringerMannheim)inHBSfor20minandanalysedonaBectonDickinsonFACSVantagemachine.FlowcytometricanalysisofthePI-stainedcellswascarriedoutonaBectonDickinsonFACScanequippedwithadoubletdiscriminationmodule.
AsshowninFigure1A,ourmethodreliablydetectstheexpectedapoptoticoranti-apoptoticeffectoftheadenovirusE1AorE1B-19Kgeneproducts,respectively(20,21).Theadditionofstaurosporinefollowingtransfectionfurtherexacerbatestheobservedeffects.SimilarresultsareobtainedusingfluorescentCy5-CTPandtheTdTreactiontomeasureapoptosis.AlthoughthesensitivityoftheGFP/PImethodisbelowthatoftheCy5-CTP/TdTmethod(Fig.1A),thenormalisedamountofapoptosis,expressedasaratiooftheapoptosismaximaobtainedwiththeE1Ageneproductwithineachdetectionmethod,isnearlyidentical(Fig.1B).
