(Mito ) 线粒体衰老循环理论的证据和挑战 mitochondrial vicious cycle theory of aging

Evidencesandchallengesforthemitochondrialviciouscycletheoryofaging

1.Thetheoey2.Specialtiesformitochondrioninaging3.Evidence4..Chanllege5.RecentAgingisassociatedwithmitochondrialDNA(mtDNA)mutations,mitochondrialdysfunctionandtheactivationofmitochondrialmediatedapoptosis.Themitochondrialgenomeisdenselypackedandclosetothemaingeneratorofreactiveoxygenspecies(ROS)inthecell,theelectrontransportchain(ETC),animportantroleformtDNAmutationsinaginghasbeenproposed.themitochondrialviciouscycletheoryofaging

ROSproducedatETCleadtotheinhibitionofmtDNAtranscriptionandETC

activity,resultinginevenhigherlevelsofROSproduction.Age-relatedchangesinmitochondria

MarkK.Shigenaga,July27,1994November1994.Sci.USASpecialitiesformtDNA

greaterexposureofmtDNAtoreactants:mtDNAisincloseproximitytotheETC,whosecomplexesIandIIIarebelievedtobethepredominantsitesofROSproductioninsidethecell2)thelackofprotectivehistones,3)thelackofintronsandthecompactnessofitsgeneticinformation,sothatdamageatanypointinthegenomewilllikelyoccurinagenemitochondriaarebelievedtoentirelylacknucleotideexcisionrepair(NER)havelesssophisticatedmismatchrepair(MMR)systemFigure1MapofhumanmtDNAOHandOL,originsofheavyandlightstrandreplicationrespectively;ND1–ND6,NADHdehydrogenase(ETCcomplexI)subunits1–6;Cox1–Cox3,cytochromeoxidasesubunits1–3(ETCcomplexIV);ATP6andATP8,subunits6and8ofmitochondrialATPase(complexV);Cytb,cytochromeb(complexIII).TheobservedalterationsofmtDNAincludeoxidativedamagetoDNAbases,pointmutationsandlargescaledeletionsorduplications.MtDNAmutationsareknowntohavedeleteriouseffectsonoxidativephosphorylation,especiallyinpatientswithmitochondrialdiseasesandtissuesthatrelyheavilyonoxidativephosphorylationareexpectedtobemoreaffected.(skeletalmuscle).ROSasimportantsignalingmolecules,maydeterminetheratingofagingMitochondrialDNAMutations,DeletionsandElectronTransportChainAbnormalitiesinAgingCommonlyusedmarkersformitochondrialETCabnormalitiesincludethelossofcytochromecoxidase(COX)activityandtheconcomitantincreaseinsuccinatedehydrogenase(SDH)activity(COX-/SDH++)(COX-/SDH++regions,alsoknownasraggedredfibers(RRF).

In

Aiken'sgrouptherewerenoETCabnormalfiberswiththeRRFphenotypeobservedinthe5-month-oldmuscles,whereas42%ofthetotalETSabnormalfibersinthe38-month-oldanimalsdisplayedtheRRFphenotype.Previouscytochemical-immunocytochemicalstudiesofoxidativephosphorylationenzymesinmonkeys(10-25yearsofage)showedcomplexIII,complexIVandcomplexVdefectsinskeletalmuscles,diaphragm,myocardiumandextraocularmusclesof25-year-oldanimals.DecreasedactivityofcomplexIwithagewasalsoreportedingastrocnemiusmuscleofmice.ThesedefectswererandomlydistributedandnotassociatedwithalossofcomplexII,whichisallnuclearencoded.Fayetetal.identifiedhighlevelsofclonallyexpandedmtDNApointmutationsincytochromecoxidasedeficientmusclefibers,fromoldindividualswithoutmuscledisease,whilenopointmutationsweredetectedinanyofthenormalfibersImmunohistochemicalexperimentsshowedthatthemajorityofthecytochromecoxidasedeficientmusclefibersexpressedreducedlevelsofsubunitIIofcytochromecoxidase,whichisencodedbymitochondrialDNA,whereastherewasnormalorincreasedexpressionofsubunitIVofcytochromecoxidase,whichisencodedbynuclearDNA.TheauthorsconcludedthatmtDNApointmutationsareassociatedwithcytochromecoxidasedeficientmusclefibersegmentsinaging,thefocalaccumulationofwhichmaycausesignificantimpairmentofmitochondrialfunctioninindividualcellsinspiteoflowoveralllevelsofmitochondrialDNAmutationsinmuscle.inskeletalmuscleofagedindividuals,normalmtDNAdevoidofdeletionsorpointmutationsmayrepresentaminorityofthetotalmtDNApool.Asdiscussedabove,thereisanevergrowingbodyofresearchthatsupportsanimportantroleformtDNAmutationsinagingbyprovidingexperimentalsupportforanassociationbetweenmtDNAmutations,ETCabnormalitiesandtissuedysfunction,particularlyinlong-livedpostmitoticcells.EvidenceforaCausalRoleofmtDNAMutationsinAgingInordertotesttheinvivoeffectsofsomaticmtDNAmutationaccumulation,theyconstructedknock-inmicethatexpressedaproof-reading-deficientversionofPolgA,thenucleus-encodedcatalyticsubunitofthemitochondrialDNApolymeraseγ.ThemaintenanceofmitochondrialDNA(mtDNA)iscriticallydependentuponpolymerase-gammaencodedbythenucleargenePOLG.

ITISdemonstratedthatmtDNAmutationsanddeletionsareresponsibleforaprogressivedeclineinrespiratoryfunctionofmitochondriallyencodedcomplexes,thatwasevidentasearlyas12weeks,resultingindecreasedoxygenconsumptionandATPproduction。ModelofPOLGmice(mutantmice)(POLGmicewithaD257Amutationexpressaproof-reading-deficientversionofPolgA,thenucleus-encodedcatalyticsubunitofthemitochondrialDNApolymeraseγ,resultingintheaccumulationofsomaticmtDNAmutationsInskeletalmuscleofmutantmice,wefoundprofounddecreasesinmitochondrialO2consumptionduringstate3,andsignificantlyreducedATPcontencomparedtowildtypemice(WT)at∼11-monthofage.SomeoneshowedthattheaccumulationofmtDNAmutationswasnotassociatedwithincreasedlevelsofoxidativestressbutcorrelatedwiththeinductionofapoptoticmarkersAmutationalmechanismwithCOXdeficiencyM.G.Hanna,identifiedthefirststop-codonmutationinhumanmtDNAandhaveprovidedevidencethatitisassociatedwithadult-onsetisolatedCOXdeficiency.orseemtotriggerapoptosis.ThismutationislikelytoinduceCOXdeficiencybyanovelmechanism,possiblybyaffectingtheassemblyofcomplexIV.(interesting)ChallengingtheMitochondrial‘ViciousCycle’TheoryofAgingKujothetal.haveshownthatin∼9montholdPOLGmice,whenacceleratedaginghascommencedandagingphenotypesareevident,therewerenodifferencesinmitochondrialhydrogenperoxideproduction,inmitochondrialandcytosolicproteincarbonylation,orintotalDNAoxidationinheartandlivertissues,comparedtoage-matchedWTanimals.RNAoxidationmeasuredbythelevelsof7,8-dihydro-8-oxoguanine(8-oxoG)wasactuallylowerinliverRNAofmutantmicetheyalsofoundnormalaconitaseactivity,nodifferencesinproteinoxidationandnoup-regulationofmanganesesuperoxidedismutase(MnSOD)andglutathioneperoxidase1(GPX1)intheheartandliverofPOLGmicecomparedtoage-matchedWTmiceZassenhausandcolleaguesalsotestedthehypothesisthattheproductionofreactiveoxygenspeciescausesthepathogeniceffectsofmtDNAmutationsbyusingtransgenicmicethatdevelopcardiomyopathyduetotheaccumulationofmitochondrialDNAmutationsspecificallyintheheart(58).Inagreementwiththeabovefindings,theyhavedemonstratednoelevationsinproteincarbonyls,nodifferencesinmtDNAoxidativedamagemeasuredby8-oxo-7,8-dihydro-2′-deoxyguanine(8-oxodG)levels,noup-regulationofantioxidantdefensesystems,thisdysfunctionwasnotaccompaniedbyanincreaseinmitochondrialROSproductionoroxidativedamage.Infact,wedetectedadecreaseintherateofhydrogenperoxideproductionbyintactPOLGmitochondriaandnodifferenceinmtDNAoxidativemodificationmeasuredby8-oxodG,comparedtoWT.Furthermore,wedetectedDNAladderingandanincreaseintheamountofcytosolicmono-andoligo-nucleosomesinPOLGmicecomparedtoWT,indicativeofapoptosis.Theactivitiesofboththeinitiatorcaspase-9,andthefinaleffectorcaspase-3ViewpointsofAleksandraTrifunovicandNils-GoranLarsson(Stockholm,INSTITUTEOFAGING),bycomparingWTandMTinaging

RespirationIsSeverelyImpairedinmtDNAMutatorMEFSNoIncreaseinHydrogenPeroxide-InducedCellDeathinmtDNAMutatorMEFs.NoDifferenceinOxidativeDamagetoProteins.NoIncreasedExpressionofAntioxidantDefenseEnzymes.H2O2inducednecrosisandapoptosisinwild-type(bluebars)andmtDNAmutator(redbars)primaryMEFcultures.CellswerestainedwithpropidiumiodideandannexinVandassayedforviability(meanvaluesSD)byFACSanalysisafter1hincubationincultu