Cell$Death$$$Today:$$Cell$aging$and$cell$death!Cells!age.!!Old!cells!have!different!characteris3cs!than!young!ones.!!Cells!die.!‘Immortalized’!cell!lines!can!die!too.!!But!cell!death!is!not!always!bad.!!In!fact,!cell!death!plays!a!crucial!role!in!animal!development,!in!the!func3on!of!the!immune!system,!and!in!the!preven3on!of!cancer.!!Cells!can!die!by!several!mechanisms,!including!those!listed!below.!!Necrosis!refers!to!premature!death!due!to!infec3on!or!injury.!!Apoptosis!is!a!form!of!programmed!cell!death!(cell!suicide).!!This!process!is!some3mes!influenced!by!an!intracellular!degrada3on!mechanism!termed!autophagy!(self-ea3ng).!!Phagocytosis!involves!one!cell!(a!phagocyte!like!a!macrophage)!‘ea3ng’!a!dying!cell!(such!as!an!apopto3c!or!infected!cell).!!(Phagocytes!func3on!to!kill!free-living!bacteria!as!well.)!!!Today!we!will!discuss!cell!aging!and!then!focus!on!apoptosis!and!autophagy!(all!hot!top ics) .!!PLEASE$TAKE$THE$TIME$TO$FILL$OUT$YOUR$COURSE$EVALUATIONS$AT$THE$END$OF$CLASS$!$$ALSO,$COME$TO$THE$IN-CLASS$REVIEW$SESSION$ON$FRIDAY$!!!$Telomerase$is$an$enzyme$with$roles$in$cell$aging$and$limiMng$replicaMon$potenMal!Telomeres!are!the!physical!ends!of!linear!chromosomes!that!consist!of!tandem!arrays!of!a!short!DNA!sequence!TTAGGG.!Telomerase!is!a!reverse!transcriptase!enzyme!that!adds!these!telomere!sequences!to!the!ends!of!chromosomes!and!maintains!3-20kb!of!DNA!repeats!in!humans.!Germ-line!cells!and!stem!cells!produce!high!levels!of!telomerase,!but!most!soma3c!cells!produce!only!low!levels!of!telomerase.!!As!a!result,!telomeres!in!soma3c!cells!shorten!with!each!cell!cycle!–!a!sign!of!cell!aging.!!ASer!many!cell!genera3ons,!chromosomes!will!not!have!normal!ends!and!cells!will!withdraw!from!the!cell!cycle!and!stop!dividing,!a!process!called!cell!senescence.!Telomere!repeats!are!proposed!to!provide!each!cell!with!a!coun3ng!mechanism!to!prevent!unlimited!prolifera3on.!In!fact,!deple3on!of!telomerase!in!HeLa!cells!using!RNAi!causes!them!to!stop!growing!within!a!month.!And!some!mice!that!lack!telomerase!show!signs!of!premature!aging.!Telomerase$is$an$enzyme$with$roles$in$cell$aging$and$cancer!Telomere!shortening!over!many!cell!genera3ons!could!provide!a!safeguard!against!uncontrolled!prolifera3on!of!abnormal!cells!–!cancer!cells.!Extensive!telomere!shortening!can!be!recognized!by!the!cell!as!a!form!of!DNA!damage.!Under!these!circumstances,!p53$becomes!ac3vated!and!can!trigger!apoptosis.!!However,!mice!that!lack!telomerase!have!also!been!shown!to!develop!tumors!because!of!gene3c!instability!–!complete!loss!of!telomeres!can!lead!to!end-to-end!chromosome!fusions.!Importantly,!many!tumor!cells!overcome!telomere!shortening!by!re-expressing!high!levels!of!telomerase.!In!fact,!the!lifespan!of!cultured!human!cells!that!lack!telomerase!and!have!a!limited!replica3on!poten3al!in#vitro!can!be!increased!by!>20!doublings!by!re-introducing!telomerase!into!these!cells.!(hTERT=!!the!enzyma3c!subunit!of!human!telomerase)!p53$Telomerase$in$the$news!h\p://www.youtube.com/watch?v=r9SHSAXqdtw!Figure 18-1 Molecular Biology of the Cell (© Garland Science 2008) Apoptosis$and$Necrosis:$Two$major$forms$of$cell$death!An$apoptoMc$cell$ A$necroMc$cell$An$apoptoMc$cell$digested$by$a$phagocyMc$cell$h\p://www.youtube.com/watch?v=xkASer3gEnc!Apoptosis$visualized$in$live$cells$(as$they$die)!ApoptoMc$cells$have$disMnct$features!1.!Chromosome!condensa3on!at!the!nuclear!periphery!!2.!Shrinking!of!the!cell!body!!3.!Blebbing!of!the!cell!membrane!(not!well-depicted!in!this!figure)!!4.!Endonuclease!cleavage!of!nuclear!DNA!(can!be!visualized!upon!agarose!gel!electrophoresis)!!5.!Flipping!of!phospha3dylserine!(PS;!a!phospholipid)!from!the!inner!leaflet!of!the!plasma!membrane!to!the!outer!leaflet!–!this!can!serve!as!an!“eat!me”!signal!for!macrophages!Figure 18-2 Molecular Biology of the Cell (© Garland Science 2008) Apoptosis$is$crucial$for$normal$development$and$Mssue$homeostasis!Mouse!paw!Some!benefits!of!apoptosis!include:!-elimina3ng!abnormal!cells!during!development!in!a!quality!control!process!-”sculp3ng”!the!embryo!during!embryonic!morphogenesis!(below).!!-elimina3on!of!cells!that!could!be!dangerous!to!the!organism,!such!as!self-reac3ve!immune!cells!-elimina3on!of!cells!with!the!poten3al!to!cause!cancer,!including!those!with!damaged!DNA!-the!killing!of!virus-infected!cells!by!cytotoxic!T-lymphocytes!(CTLs)!-maintenance!of!organs!at!a!constant!proper!size!(e.g.!maintaining!normal!liver!size!or!adjus3ng!!the!number!of!developing!neurons!to!the!amount!of!target!3ssue)!C.elegans$as$a$model$for$studying$apoptosis!The!developmental!fate!of!every!single!soma3c!cell!of!C.elegans#has!been!mapped!out.!These!pa\erns!of!cell!lineage!are!largely!invariant!between!individuals.!In!hermaphrodites,!131!cells!are!eliminated!by!programmed!cell!death.!Many!of!the!genes!that!control!apoptosis!in!worms!are!conserved!in!humans.!!MCB!(Lodish)!–!Chapter!21!Extra$cells$can$be$visualized$in$a$C.elegans$mutant$that$lacks$a$pro-apoptoMc$gene!WT$ced+Apoptosis$is$driven$by$signaling$cascades$catalyzed$by$proteases$called$caspases$!The!intracellular!machinery!responsible!for!apoptosis!is!comprised!of!proteases!called!caspases!that!have!a!cysteine!residue!at!their!ac3ve!site!and!cleave!their!target!proteins!at!specific!aspartate!residues.!Caspases!are!synthesized!as!inac3ve!precursors!called!procaspases!that!are!ac3vated!upon!proteoly3c!cleavage.!Apopto3c!signals!lead!to!ac3va3on!of!“ini3ator”!caspases!that!in!turn!ac3vate!“execu3oner”!caspases!in!an!intracellular!caspase$cascade$that!drives!apoptosis.!Apoptosis$can$be$triggered$by$intrinsic$or$extr insic $pathways!Intrinsic$pathways$to$apoptosis$are!triggered!from!within!the!cell,!usually!in!response!to!stresses!such!as!DNA!damage,!hypoxia,!low!nutrients,!or!a!lack!of!proper!extracellular!survival!signals.!This!pathway!depends!on!the!release!of!mitochondrial!proteins!from!the!intermembrane!space!into!the!cytosol.!A!par3cularly!crucial!protein!that!is!released!into!the!cytosol!is!cytochrome$c,!a!component!of!the!electron!transport!chain.!Cytochrome!c!release!in!the!intrinsic!pathways!is!regulated!by!the!Bcl2!family!of!proteins.!!In$the$intrinsic$pathway,$c ytosolic