THE JOURNAL OF CELL BIOLOGY

tiation andintumorcells. death andtheapoptosomeareregulatedduringdifferen- with earlierones,deepenourunderstandingofhowcell (Wright etal.,2004).Theseresults,together tain survival ability oftheinhibitorapoptosisproteins(IAPs)tosus- and theactivityofapoptosomewithanincreased isaccompaniedbyadecreaseinApaf-1 differentiation the currentissue,ithasbeenshowninneuronsthatcell haveremainedunclear.combat celldeathandinjury In cells The molecularmechanismsbywhichdifferentiated and theinhibitorofapoptosisproteins Cell differentiation:reciprocalregulationofApaf-1 pase-9, andcytochrome from apoptoticproteaseactivatingfactor-1(Apaf-1),procas- In thecytosol,acomplexknownasapoptosomeisformed membrane spacetriggeringcelldemise(Lindholmetal.,2004). c crease inthepermeabilityofoutermembrane,cytochrome organelle (DanialandKorsmeyer,2004).Followinganin- formation inducedbydifferentdeathsignalsconvergingonthe cell viability,controllingthemitochondrialmembranepore drial level. possibility forinterventionbothatthepre-andpost-mitochon- allows forthestrictregulationofcelldeathandprovides downstream oftheorganelle.Thecomplexitythispathway ing withtheparticipationofmoleculesbothupstreamand intrinsic pathwayinvolvingmitochondriaisespeciallyinterest- cellular factorsthatactivateconservedprocesses.The disorders. Cellsuicideisregulatedbybothextra-andintra- human diseases,suchascancerandneurologicaldegenerative anditsdysfunctionplayacrucialroleindifferent 3 2 1 Dan Lindholm recruitment domain(CARD)thatallowsittobindthe worm (Lietal.,1997;Zhou1997).Apaf-1hasacaspase eleg linked IAP. recruitment domain;IAP,inhibitorofapoptosis protein;XIAP,Xchromosome– totic proteaseactivatingfactor-1;BIR, baculovirus IAPrepeat;CARD, Abbreviations usedinthispaper:AIP,Apaf-1 interactingprotein;Apaf-1,apop- Correspondence toDanLindholm:[email protected] http://www.jcb.org/cgi/doi/10.1083/jcb.200409171 The JournalofCell Biology ©

Institute ofBiotechnology, UniversityofHelsinki,FIN-00014Finland Minerva ResearchInstitute,Biomedicum,FIN-00290Helsinki,Finland Minerva Department ofNeuroscience,UppsalaUniversity,Department BiomedicalCentre,S-75123Uppsala,Sweden

andotherpro-apoptoticproteinsarereleasedfromtheinter- TeRceelrUiest rs $8.00 TheRockefellerUniversity Press

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Proteins oftheBcl-2familyaremajordeterminants

protein CED-4thatisrequiredforapoptosisinthe 1,2 and Urmas Arumäe andUrmas , Vol. 167,No.2,October25,2004 193–195 c /dATP. Apaf-1 3 resemblesthe C. in thelysatesofdifferentiated primary fibroblasts,asisseenin (2004) showthatcytochrome that isIAPdependent. ferentiated PC12cellshaveincreased resistancetocelldeath with thisoutcome,Vyasetal.(2004)recentlyshowedthatdif- allowing formoreeffectiveregulationbytheIAPs.Consistent pase-9, Apaf-1levelsandapoptosomeactivitywerereduced, Although therewasnochangeintheamountsofIAPsorcas- Apaf-1 isaccompaniedbyincreasedprotectiontheIAPs. apoptosis. Theauthorsalsoshowthatreductioninthelevel of explains earlierobservationsthatcytoplasmiccytochrome duced andtheprotectiveactivitiesofIAPsareincreased.This cells orprimarysympatheticneurons,thelevelofApaf-1isre- (2004) nowdemonstratethatduringdifferentiationofPC12 in cell-freesystems,andlesslivingcells.Wrightetal. et al.,2001;Okada2002). or Smac/Diabloshowedanyabnormalityinapoptosis(Harlin endogenous IAPsisunclearasneithergenedeletionofXIAP stimuli (Perreletetal.,2002;Trapp2003),buttheroleof expression ofXIAPcanprotectcellsagainstdifferentdeath mitochondria duringcelldeath(Duetal.,2000;Fig.1).Over- inhibited byproteins,suchasSmac/Diabloreleasedfromthe (XIAP) canbindprocessedcaspase-9and-3,inturnbe repeats(BIR).ViaitsBIRs,Xchromosome–linkedIAP IAP to threewell-conserveddomains,thesocalledbaculovirus both Apaf-1andtheefficacyofIAPs.IAPscontainone some changeswithneuronaldifferentiationalterationsin “death wheel”(Fig.1). 2004). Thestructureoftheapoptosomethusconstitutesacellular ultimately causingcelldeath(Acehanetal.,2002;Hill autoactivation ofprocaspase-9thatinturncleavescaspase-3, renders thematureneuronsresistanttocytochrome dramatically duringneuronaldifferentiation,andthisreduction differentiated PC-12cellsandsympatheticneurons,butdrop 2002; Pottsetal.,2003).ThelevelsofApaf-1arehighinnon- it wassufficienttokillnonneuronalcells(Deshmukhetal., was notabletotriggerdeathofsympatheticneurons,although cytochrome and attheCOOHterminusareWD40repeatsthatbindto CARD incaspase9,acentralPloopmotifthatbindsATP, Does thisonlyapplytoneuronal cells?Wrightetal. The biologyoftheapoptosomehasbeenstudiedmainly Wright etal.(2004)showthattheactivityofapopto- c (Fig.1).OligomerizationofApaf-1leadsto c inducesrapidcaspaseactivation JCB: c –triggered COMMENT JCB c

193 Downloaded from http://rupress.org/jcb/article-pdf/167/2/193/1315967/jcb1672193.pdf by guest on 29 September 2021 September 29 on guest by http://rupress.org/jcb/article-pdf/167/2/193/1315967/jcb1672193.pdf from Downloaded

Figure 1. The death wheel. Apaf-1 together with procaspase-9 form the wheel-shaped apoptosome complex. Cyto- chrome c binds to Apaf-1 and drives cell death through the activation of caspase 3. The IAPs, such as XIAP, can counteract caspase activation, but this brake on cell death is di- minished by Smac/Diablo released from the mitochondria. The steady-state levels of the various interacting proteins may vary between cell types and with cell differentiation. As well as the apoptosome, other cellular pathways can be activated and probably interact during cell demise. Downloaded from http://rupress.org/jcb/article-pdf/167/2/193/1315967/jcb1672193.pdf by guest on 29 September 2021

undifferentiated nerve cells. Thus, in dividing nonneuronal eventually died following death stimuli was unaltered (Ekert et cells, the high Apaf-1 levels keep the activity of the IAPs low. al., 2004). Deprivation of neurons of trophic factors usually Also, regulation of cell death by the IAPs seems blunted in triggers the mitochondrial death pathway and activation of the many mitotic compared with postmitotic cells. In Apaf-1 het- apoptosome. Differentiated neurons with low levels of Apaf-1 erozygous fibroblasts, however, the lowered levels of Apaf-1 and reduced apoptosome activity die also when challenged by enhanced the activity of IAPs, as in the differentiated neurons. different factors or in neurodegenerative diseases. Whether this Although the findings refer to postmitotic normal cells, still involves Apaf-1–mediated cell death or contributions from they may have implications for abnormal cells. Small molecu- other factors is unknown. Cellular pathways, not primarily lar compounds targeting XIAP can lead to activation of cas- involving mitochondria and the apoptosome complex, are pases and induction of cell death of tumors (Schimmer et al., probably also activated in differentiated brain cells. Thus, 2004). In many cancer cells, the brake exerted by the IAPs is sympathetic neurons die through a nonmitochondrial, but still strong enough to counteract cell death. In view of the present caspase-dependent, pathway in the absence of glial cell line– findings of a strict correlation between Apaf-1 and the action of derived neurotrophic factor (Yu et al., 2003). XIAP in cell differentiation, it would be interesting to study In conclusion, Wright et al. (2004) reveal many important whether Apaf-1 might be an additional target to consider in aspects of neuronal apoptosis, but also raise further questions. cancer therapies. In this case, Apaf-1 may be elevated by en- It would be interesting to know what makes the IAPs more effi- hancing transcription of the gene or possibly by influencing the cient protectors in the differentiated neurons. Is this achieved stability/post-transcriptional modifications of the protein. by the mere reduction in Apaf-1 levels, or is there a closer asso- Apart from Apaf-1/IAPs, other molecules may influence ciation between these proteins in the cell involving some feed- cell death at the level of the apoptosome. In this respect, a back mechanisms? The work nicely connects the regulation of novel factor was recently cloned, named Apaf-1 interacting Apaf-1 and the functional state of the IAPs with cell differenti- protein (AIP). It is expressed in brain tissue and is able to pro- ation. However, there is certainly more to be learned about how tect neurons after ischemia/stroke (Cao et al., 2004). AIP con- cell death is regulated in normal, differentiated, and highly pro-

tains only the NH2-terminal CARD domain and constitutes a liferating tumor cells. In view of this, the wheel to more in- dominant-negative inhibitor of the activity of the Apaf-1–cas- sights is still spinning. pase-9 complex. Changes in expression of AIP by cell differen- We thank Dr. Laura Korhonen for artwork. tiation or in tumor cells have so far not been reported. The authors are supported by Uppsala University, Cancerfonden, Sigrid Although there is a partial requirement for Apaf-1 for de- Juselius, Minerva Foundation, Finnish Academy, and Institute of Biotechnology. velopmental neuronal apoptosis (Cecconi et al., 1998; Yoshida Submitted: 28 September 2004 et al., 1998; Cozzolino et al., 2004), loss of Apaf-1 does not Accepted: 1 October 2004 prevent all types of cell death, as is evident in lymphoid cells lacking both Apaf-1 and caspase-9 (Marsden et al., 2002). In References the absence of the apoptosome, apoptosis did not occur in inter- leukin-3–dependent myeloid cells but the number of cells that Acehan, D., X. Jiang, D. Gene Morgan, J.E. Heuser, X. Wang, and C.W. Akey.

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