1、IY36CH19_NagataARI23 March 20188:38Annual Review of ImmunologyApoptosis and Clearanceof Apoptotic CellsShigekazu NagataLaboratory of Biochemistry and Immunology,World Premier International Research CenterInitiative Immunology Frontier Research Center,Osaka University,Osaka 5650871,Japan;email:snagat
2、aifrec.osaka-u.ac.jpAnnu.Rev.Immunol.2018.36:489517First published as a Review in Advance onFebruary 5,2018The Annual Review of Immunology is online atimmunol.annualreviews.orghttps:/doi.org/10.1146/annurev-immunol-042617-053010Copyright c?2018 by Annual Reviews.All rights reservedKeywordsapoptosis,
3、efferocytosis,macrophages,phosphatidylserine,flippase,scramblase,caspase,TIM4,TAM receptorsAbstractThe human body generates 10100 billion cells every day,and the samenumber of cells die to maintain homeostasis in our body.Cells infected bybacteria or viruses also die.The cell death that occurs under
4、 physiologicalconditions mainly proceeds by apoptosis,which is a noninflammatory,orsilent,process,while pathogen infection induces necroptosis or pyropto-sis,which activates the immune system and causes inflammation.Dead cellsgenerated by apoptosis are quickly engulfed by macrophages for degrada-tio
5、n.Caspases are a large family of cysteine proteases that act in cascades.Acascade that leads to caspase 3 activation mediates apoptosis and is respon-sible for killing cells,recruiting macrophages,and presenting an“eat me”signal(s).When apoptotic cells are not efficiently engulfed by macrophages,the
6、y undergo secondary necrosis and release intracellular materials that rep-resent a damage-associated molecular pattern,which may lead to a systemiclupus-like autoimmune disease.489Click here to view this articles online features:Download figures as PPT slides Navigate linked references Download cita
7、tions Explore related articles Search keywordsANNUAL REVIEWSFurtherAnnu.Rev.Immunol.2018.36:489-517.Downloaded from www.annualreviews.org Access provided by University of California-Santa Barbara on 06/22/18.For personal use only.IY36CH19_NagataARI23 March 20188:38INTRODUCTIONPeople die for various
8、reasonsaging,infection,accidents,organ malfunction,suicide.We buryor cremate corpses swiftly to prevent the spread of pathogens.Cells,too,die for various reasonsand are swiftly cleared by other cells acting as morticians.The adult human body consists of 3.721013cells of various types(70.1%erythrocyt
9、es,6.8%endothelial cells,5.0%skin fibroblasts,8.0%glial cells,etc.)(1).When the body is shaped duringembryogenesis,many cells are overproduced,and the excessive or harmful cells are developmen-tallyprogrammedtoundergocelldeath(2).Thedeadcellsareswiftlyrecognizedandphagocytosedbytissue-residentmacrop
10、hages.Thisprocessoccursininterdigitformation,sexualdifferentiation,neural-network formation,and immune-system development.Different cell types have differentlifespans(for example,neutrophils,less than 10 h;enterocytes,57 days;erythrocytes,120 days;hepatocytes,150 days;cortical neurons,the lifetime o
11、f the body)(36).Thus,several hundredbillion cells die daily and are replaced by newly generated cells.How they die depends on theircell type.Senescent neutrophils die and are then phagocytosed by macrophages in the bone mar-row,liver,and spleen(7,8),while senescent red blood cells are recognized and
12、 phagocytosed bymacrophages in the spleen and liver(9).Senescent enterocytes in the small intestine are shed intothe lumen of the intestine(10).Cells infected by viruses or bacteria undergo cell autonomousnecrosis or are killed by the immune system.These cells may not be recognized by macrophagesas
13、whole,and necroticcells release materials that may activate the immunesystem.The inflamma-tion caused by bacterial or viral infection involves a massive production of white blood cells,andwhen the infection ceases,these cells quickly die.In addition,various anticancer drugs kill tumorcells by inhibi
14、ting the synthesis of purines,DNA,or RNA or by cross-linking or intercalatingDNA.In this review,I briefly describe the process of cell death and then discuss how dead cellsare engulfed and cleared by macrophages.PROGRAMMED CELL DEATHLockshin&Williams(11)originally defined programmed cell death in th
15、e context of insectdevelopment.Subsequently,Kerr et al.(12)noticed by ultrastructural analysis two morpho-logically different types of cell death in humans,apoptosis and necrosis.In necrosis,the cellsswell,plasma membranes rupture,and cellular components are released;in apoptosis,the cellsshrink wit
16、h integral but ruffling plasma membranes,and nuclei are condensed and fragmented.Apoptotic cells are swiftly phagocytosed by macrophages to prevent the release of intracellularcomponents.This process prevents the release of inflammatory factors and is thus called cleancell death.Since apoptosis occu
17、rs in developing embryos or in cells that die under physiologicalconditions,“programmed cell death”and“apoptosis”are often used synonymously.Further-more,when apoptosis was discovered to be mediated by gene products,it was regarded as beingprogrammed.Thus,the term programmed cell death has been used
18、,confusingly,with two dif-ferent meanings:the cell death programmed into animal development,and the cellular deathprocess elicited by a molecular mechanism.Meanwhile,cell death with a necrotic morphologythat occurs during inflammation or infection was also found to be programmed or regulated bygene
19、products and was categorized as necroptosis and pyroptosis(13).In addition,nonapop-totic cell death was observed during Caenorhabditis elegans development(14)and Drosophila meta-morphosis(15),indicating that cell death in animal development can occur by a nonapoptoticmechanism.Thus,programmed cell d
20、eath should not be used as a synonym for apoptosis;itshould be reserved for the cell death that takes place in animal development,as originally defined(11).490NagataAnnu.Rev.Immunol.2018.36:489-517.Downloaded from www.annualreviews.org Access provided by University of California-Santa Barbara on 06/
21、22/18.For personal use only.IY36CH19_NagataARI23 March 20188:38APOPTOSISApoptosis plays an important role in animal development.For example,interdigital cells,non-functional nerve cells,and activated lymphocytes are removed by apoptosis.The involution ofmammary glands is also mediated by apoptosis.I
22、n contrast,apoptosis may not play a major rolein clearing senescent cells(red blood cells,and intestinal enterocytes).Apoptosis is mediated byspecific sets of caspases that act in cascades,at the end of which caspase 3 or 7 is responsible forkilling the cells.Apoptotic cells are engulfed by macropha
23、ges in a process called efferocytosis(16).Because efferocytosis is efficient and swift,it is difficult to find free apoptotic cells in vivo,even intissues where large numbers of cells undergo apoptosis(17).Thus,apoptosis involves mechanismsnot only for killing cells but also for recruiting macrophag
24、es(“find me”)and presenting a signal(s)(“eat me”)to the macrophages for cell engulfment(18,19).CASPASESIn the early 1990s,the Horvitz group(20)found that CED-3(cell death abnormal 3),whichis essential for programmed cell death in C.elegans,is homologous to mammalian ICE(IL-1-converting enzyme),a pro
25、tease responsible for producing mature IL-1.The human genomeproject,started in 1990,revealed many genes homologous to ICE,some of which were found tobe involved in apoptosis.Biochemical and cellular analyses indicated that these ICE homologsare proteases with a motif consisting of histidine and cyst
26、eine residues at the active site(21).These proteases recognize at least five amino acid residues on their target proteins and cleavepeptide bonds strictly after aspartate(position P1).This protease family was called caspases,forcysteine-proteases that cleave after aspartic acid,or cysteine aspartyl
27、proteases(22).Caspases aresynthesized as precursors,or zymogens.Specific cleavages at two positions remove the prodomainat the N terminus and generate an active enzyme consisting of two large and two small subunits(22).There are 12 human and 11 mouse caspases.Some caspases have different recognition
28、sequences(21),and those that recognize a certain sequence often cleave different substrates withdifferent efficiency(23),indicating that the substrate specificity of caspases is determined by boththe target sequence and the tertiary structure of the substrate.One group of caspases(humancaspases 2,3,
29、and 610)mediates apoptosis,while another group(human caspases 1,4,and 5)mediates pyroptosis,which causes inflammation.Consistent with these distinct caspase groups,the proteins cleaved during inflammation are different from those cleaved during apoptosis(24).INTRINSIC DEATH PATHWAYSTwo apoptosis pat
30、hways,intrinsic and extrinsic,have been identified in mammalian systems(25)(Figure 1).The intrinsic pathway,also called the mitochondrial pathway,operates in develop-mentally controlled and genotoxic agentmediated apoptosis and is regulated by Bcl-2 familymembers.The Bcl-2 family consists of three s
31、ubfamilies:proapoptotic BH3-only members(Bim,Bid,Puma,Noxa,Hrk,Bmf,and Bad),proapoptotic effector molecules(Bax and Bak),and an-tiapoptotic Bcl-2 family proteins(Bcl-2,Bcl-xL,Mcl1,A1,and Bcl-B)(26).In healthy cells,theBax/Bak action is arrested by antiapoptotic Bcl-2 family members.In response to an
32、 apoptoticstimulus(developmental cue,lack of growth or survival factor,or genotoxic agent),BH3-onlymembers are transcriptionally or posttranscriptionally upregulated.The activated BH3-only pro-teins act on Bak and Bax or antagonize the antiapoptotic Bcl-2 family members.Bax and Bak thenstimulate the
33、 release of cytochrome c from mitochondria.Cytochrome c,together with Apaf-1,forms a heptametrical complex called an apoptosome in a dATP/ATP-dependent manner(27).www.annualreviews.orgClearance of Apoptotic Cells491Annu.Rev.Immunol.2018.36:489-517.Downloaded from www.annualreviews.org Access provide
34、d by University of California-Santa Barbara on 06/22/18.For personal use only.IY36CH19_NagataARI23 March 20188:38Cleavage of more than 1,300 substratesDeath receptors(Fas,TNF-R1,DR4/5)Death factors(FasL,TRAIL,TNF)Caspase 8Death domainFADDCaspase 3Caspase 9Glucocorticoid,DNA damage,cytokine withdrawa
35、l Apaf-1Bcl-2/BclxLBH3-only(Bim,Bid,Puma,Noxa,Hrk,Bid,Bmf,Bad)Bax/BakCytochrome cIntrinsic death pathwayDeath factor pathwayMitochondrionICAD(inhibitor of CAD):DNA fragmentationATP11A and 11C,and XKR8:PtdSer exposureROCK1:membrane blebbingDeath effector domainFigure 1Signal transduction of apoptosis
36、.Two distinct apoptotic signaling pathways converge.The death factor pathway,or extrinsic deathpathway,is activated by death factors such as FasL,TNF-,and TRAIL.Binding of a death factor to its specific receptor generatesDISC(death-inducing signaling complex)and causes processing of procaspase 8 int
37、o mature active caspase 8.DISC is formed viahomologous associations between death domains of Fas and FADD(blue)and between death effector domains(DEDs)of FADD andprocaspase 8(orange).Procaspase 8 carries two DEDs that are self-associated.In another pathway triggered by genotoxic agents,cytokinewithd
38、rawal,or developmental clues,BH3-only proteins(Bim,Bid,Bad,Bmf,Hrk,Puma,and Noxa)are activated transcriptionallyor posttranscriptionally.They directly(or indirectly,by antagonizing Bcl-2)induce Bax/Bak-oligomerization,and the oligomerizedBax/Bak acts on mitochondria to release cytochrome c.The cytoc
39、hrome c binds APAF-1 to form a heptameric complex that actsas a scaffold to mediate the conversion of procaspase 9 to active caspase 9.The caspase 8 activated in the extrinsic pathway and caspase9 in the intrinsic pathway cleave procaspase 3 to mature caspase 3,which cleaves more than 1,300 cellular
40、 substrates to execute theapoptosis.For example,ICAD(inhibitor of caspase-activated DNase)is cleaved to release CAD(caspase-activated DNase)for apoptoticDNA fragmentation(241),ATP11A/11C and XKR8 for PtdSer exposure(Figure 2),and ROCK1 for apoptotic membrane blebbing(242).Black arrows indicate direc
41、t conversion,while blue arrows indicate catalytic actions.Figure modified from References 29 and 243.The X-ray structure of the apoptosome,together with the biochemical analysis of its assembly,revealed that Apaf-1 exists in an autoinhibitory form in a complex with ADP(28).The binding ofcytochrome c
42、 to Apaf-1 replaces the ADP with dATP(ATP),and the dATP(ATP)-bound Apaf-1forms a heptameric complex with cytochrome c.This complex recruits monomeric procaspase9 and helps it to form a dimer,which then undergoes autocatalytic cleavage to form an activeheterotetrameric complex.The activated caspase 9
43、 then cleaves and activates caspase 3.Although492NagataAnnu.Rev.Immunol.2018.36:489-517.Downloaded from www.annualreviews.org Access provided by University of California-Santa Barbara on 06/22/18.For personal use only.IY36CH19_NagataARI23 March 20188:38cytochrome c is not required for the apoptosome
44、 formation in nematodes and flies(27),the othermolecules(Bcl-2,Apaf-1,and caspase)are required for apoptosis in these organisms,suggestingthat the intrinsic death pathway is conserved in metazoans.THE DEATH RECEPTOR PATHWAYDeath FactorsThe extrinsic apoptosis pathway is triggered by death factors of
45、 the TNF family(FasL,Fas ligand;TNF-,tumornecrosisfactor;TRAIL,TNF-relatedapoptosis-inducingligand)(2931).Thesefactors are synthesized as type II membrane proteins with a homotrimeric structure and can becleaved from the membrane to generate a soluble form.TNF-is predominantly cleaved andshed by a m
46、etalloprotease called ADAM17(a disintegrin and metalloprotease)or TACE(TNF-converting enzyme),while FasL is cleaved by ADAM10(32,33).FasLs apoptosis-inducing activity is executed by its membrane-bound form(3436).Thesoluble trimeric form of FasL generally acts as an antagonist against the membrane-bo
47、und FasLin vitro and in vivo(3537).Fusing a Flag tag and leucine(or isoleucine)-zipper motif(LZ-FasL)(38,39)or HA-Flag-ACRP30 the trimeric domain of ACRP30,a complement factor(C1q)homolog(Mega-FasL)(40)to the N terminus of soluble FasL generates a hexameric FasL,whichdramatically gains apoptosis-ind
48、ucing activity.These findings agreed with a previous observationthat cross-linking Apo1(Fas)to an agonistic anti-Apo1 antibody strengthens its killing activity(41),and confirmed that a high-order oligomeric structure is necessary for efficient FasL-induceddeath signaling.Note that there are three ty
49、pes of soluble FasL in the field:one corresponding to the solubleform generated by shedding from membrane-bound FasL,one engineered to form an oligomer(LZ-FasL or ACRP-FasL),and one secreted as an exosome-bound form.The shed form of FasLis commercially available and usually acts as an antagonist aga
50、inst membrane-bound FasL(35),although it can induce apoptosis in cells that are very sensitive to Fas-mediated apoptosis,forexample,due to a high expression level of Fas(42).In contrast,LZ-FasL,ACRP-FasL,andexosome-bound FasL appear to mimic membrane-bound FasL(38,43).Death ReceptorsThe specific rec
