The Journal of Cell Biology
JCB
Anthony Brown destined fortheaxonaretransported anterogradely,toward body forthesynthesisofmanytheircomponents.Materials meter inlargeanimals,yettheyaredependentonthecell drical processescanextendfordistancesinexcessofone axons ofnervecellsistrulyanepicone.Theseslendercylin- In theworldofintracellulartransport,journeyalong Fast andslowaxonaltransport messenger RNAs. polymers, cytosolicproteincomplexes,ribosomes,and tal cle, aswellnonmembranouscargoessuchcytoskele- type ofmembranousorganelleandspeciestransportvesi- are farmorevariedthanpreviouslyimagined,includingevery (Vale andMilligan,2000).Moreover,thecargoesthemselves in aparticulardirectionalongcytoskeletalpolymertracks motor proteinsthatbindspecificcargoesandconveythem transport isorchestratedbyadiverseconstellationofmolecular molecular mechanisms.Wenowknowthatintracellular the extentofthesemovementsinsidecellsandtheirunderlying years wehavewitnessedarevolutioninourunderstandingof of intracellularcomponentsfromplacetoplace.Inrecent plasm, nothingcanfunctionwithouttheconstantshuttling cell. Inthebustlingmacromolecularenvironmentofcyto- Intracellular movementisthelifebloodofeukaryotic Introduction panies www.jcb.org/cgi/content/full/jcb.200212017/DC1) accom- the timethatthey spendmoving. A Flashfeature (http:// diverse cargostructures thatdifferintheproportion of and slow axonaltransport basedonrapid movements of longed pauses. This supportsaunified perspective forfast slow rate isduetorapid movements interrupted by pro- axonal transport isgenerated by fastmotors andthatthe ment ofcytoskeletal polymers inaxonssuggestthatslow transport, respectively. Recentobservations onthemove- along axonsinthefastandslow componentsofaxonal Membranous andnonmembranous cargoesare transported Neurobiotechnology ofNeuroscience, CenterandDepartment The Ohio State University, Columbus,OH43210 cargoes: aunifiedperspective Axonal transport ofmembranous andnonmembranous E-mail: [email protected] Tel.: (614)292-1205. Fax:(614)292-5379. 43210. OH Columbus, sity, NeurobiotechnologyCenter,Rightmire Hall,1060CarmackRoad, Address correspondencetoDr.Anthony Brown,TheOhioStateUniver- http://www.jcb.org/cgi/doi/10.1083/jcb.200212017 The Journal ofCellBiology
The Rockefeller University Press, 0021-9525 Mini-Review
this Mini-Review. ,
Volume 160, Number 6,March 17,2003817–821 /2003/03/817/5 $8.00 se, butratherthemannerinwhichmovementisregulated. between fastandslowtransportisnottherateofmovementper transported byfastmotorsandthattheprincipledifference these cargoesinlivingcellsnowindicatethattheyareall mechanisms, butdirectobservationsonthemovementof and nonmembranouscargoesmovebyfundamentallydistinct transport haslongbeenassumedtoindicatethatmembranous nents. Thedifferenceintherateoffastandslowaxonal protein complexesmovemoreslowly,intheslowcompo- axonal transport,whereascytoskeletalpolymersandcytosolic organelles movemostrapidly,inthefastcomponentsof move atdifferentrates(Tytelletal.,1981).Membranous membranous andnonmembranouscargostructuresthat are transportedalongaxonsinassociationwithdistinct digm hasdemonstratedthatproteinsandothermolecules 1980). Severaldecadesofworkusingthisexperimentalpara- using radioisotopicpulselabeling(GrafsteinandForman, of axonaltransportcomesfromstudiesonlaboratoryanimals axon throughoutthelifeofneuron. cytoplasm aretransportedalongtheentirelengthof cules andmembranouscompartmentsthatcompriseaxonal is remarkableforitsscale:essentiallyallofthemacromole- and membranetrafficthatoccurinalleukaryoticcells,butit mentally differentfromthepathwaysofmacromolecular transport process,knownasaxonaltransport,isnotfunda- ported retrogradely,towardthecellbody.Thisbidirectional the axontip,andmaterialsdestinedtoreturnaretrans- conveying auniquemolecular consignment,andeachthe movement ofadiversefleetorganelles, eachtypeoforganelle of thesecomponentsfastaxonal transportrepresentsthe transport vesiclesmoveanterogradelyatmaximalratesof in itsmembrane,orboundtosurface.Golgi-derived either becausetheyaresequesteredwithinitslumen,embedded association withoneormoresubclassesoforganellevesicle, that movealongtheaxonatfastratesdosobyvirtueoftheir transport. Themanyproteins,lipids,andpolysaccharides Membranous organellesaretheprincipalcargoesoffastaxonal Movement ofmembranousorganelles rates of lysosomes, andautophagosomes moveretrogradelyatmaximal 200–400 mm/d(or Most ofwhatweknowaboutthecompositionandkinetics 100–250 mm/d(or 2–5 m/s), whereasendocyticvesicles, 1–3 m/s) (TableI).Each 817 The Journal of Cell Biology 818 Microtubules, neurofilaments organelles move lessrapidlythanmostGolgi-derived and ganelles aremitochondria (Hollenbeck,1996, Fig.1).These ganelles ischaracterizedbyahigh dutyratio. say thattheaxonaltransport ofmanymembranousor- that acargostructurespends actually moving.Thus,wecan tracellular transport,theduty ratioistheproportionoftime efficiency isintermsoftheduty ratio.Inthecontextofin- infrequently duringtheirjourney.Onewaytoexpressthis move alongaxonsinahighlyefficientmanner,pausingonly observations indicatethatmanymembranousorganelles motors invitro(e.g.,WoehlkeandSchliwa,2000).These I) andalsotothemaximalratesreportedformicrotubule transport determinedbyradioisotopicpulselabeling(Table rates thatarecomparabletothemaximalrateoffastaxonal in acontinuousandunidirectionalmanneratinstantaneous ganelles inlivingaxonsindicatethatmanyorganellesmove folds ofcytosoliclinkerproteins(Karcheretal.,2002). or byindirectinteractionwiththeorganellesurfaceviascaf- tion withtransmembraneproteinsontheorganellesurface, Yang, 2000).Eachmotorbindsitscargobydirectinterac- sin motors)propelorganellesretrogradely(Goldsteinand whereas dynein(andpossiblyalsominusend–directedkine- motors propelorganellesalongmicrotubulesanterogradely, ments alongtheaxisofaxon.Plusend–directedkinesin bules appeartobetheprincipaltracksforlong-rangemove- motor proteins(kinesins,dyneins,andmyosins).Microtu- crotubule andmicrofilamenttrackspoweredbymolecular established. Membranousorganellesmovealongbothmi- principles underlyingthistransportprocessarenowclearly mechanisms oforganellemovementinaxons,thegeneral nar-Queralt andGoldstein,2001). product ofadistinctpackagingandsortingpathway(Alme- <70mm/d move inslowcomponentasomeneurons,andtubulinmayalsob(Oblingeretal.,1987),butthesignif particles arenotincludedinthistablebecausethereisinsufficientinformationabouttheiroverallrateofmovementat comparison oftheoverallandinstantaneousrates.Otheraxonallytransportedcargos,suchasendoplasmicreticulum,mRNAs, structures (betweenpauses)determinedbylightmicroscopy.Thedutyratioistheproportionoftimethatsp variation betweendifferentcelltypesandstagesofdevelopmentmaturation.Theinstantaneousrateistheactual rates, whereastheratesofslowaxonaltransportaregenerallyquotedasaveragerates).Notethattheseapproximate movements andpauses)determinedbyradioisotopicpulselabeling(fortechnicalreasons,theratesoffastaxonaltransportare Overallrate Each ratecomponentofaxonaltransportcorrespondstoadistinctgroupcargostructures.Theoverallistheaverageor Microfilaments, cytosolicproteincomplexes Mitochondria Endocytic vesicles,lysosomes,autophagosomes Golgi-derived vesicles Cargo structures Table I. f e d c b a Roy etal.,2000; Wangetal.,2000; WangandBrown,2001,2002. Lasek etal.,1984. Lorenz andWillard,1978; GrafsteinandForman,1980. Grafstein andForman,1980. Morris andHollenbeck,1993; LigonandSteward,2000. Breuer etal.,1987; ViancourandKreiter,1993; Nakataetal.,1998; Kaetheretal.,2000. (slow componenta) (slow componentb) (fast retrograde) (fast anterograde) One exceptiontothehighduty ratioofmembranousor- Direct observationsonthemovementofmembranousor- Though muchremainstobelearnedaboutthemolecular
The JournalofCellBiology
Motile behaviorofaxonallytransportedcargoes
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Volume 160,Number6,2003 0.2–1 mm/d 2–8 mm/d 100–250 mm/d 200–400 mm/d (pulse labeling) e c (.200 ms nnw nnw Unknown Unknown Unknown (0.02–0.09µm/s) (08µ/)0.3–0.7µm/s (<0.8µm/s) e (0.002–0.01 µm/s) 0.3–1 µm/s 0.3–1 (0.002–0.01µm/s) a a 13µ/)1–3µm/s 1–5µm/s (1–3 µm/s) (2–5 µm/s) chondrial movementisgenerallyquotedtobe of axonaltransport(TableI).Themaximalratemito- endocytic vesicles,formingakineticallydistinctcomponent controversy has centeredontheforminwhich thesepro- focused onthe cytoskeletal subunitproteins, andmostofthe rectly inlivingcells(Brown,2000). Mostattentionhasbeen of numerousattemptstoobserve slowaxonaltransportdi- troversy formorethantwodecades, duelargelytothefailure teins movehasbeenthesubject ofintenseinterestandcon- component b(TableI).Themechanism bywhichthesepro- and Lindsey,1983). also betransportedalongaxonsinthismanner(Ellisman sively studied,itappearsthatendoplasmicreticulummay rates ofmovement(Blakeretal.,1981).Thoughnotexten- hibit frequentpausesandreversals,resultinginmuchslower tances, resultinginthemaximalrateofmovement,mostex- chondria maymoverapidlyandcontinuouslyforlongdis- Ligon andSteward,2000).Althoughsomeaxonalmito- broad rangeofoverallrates(MorrisandHollenbeck,1993; grade andretrogrademovementspausesgivingrisetoa rectional manner,withdifferencesinthebalanceofantero- to bethatmitochondriamoveinanintermittentandbidi- 1980). Theexplanationforthistransportbehaviorappears slower (LorenzandWillard,1978;GrafsteinForman, living cells,buttheaveragerateappearstobeconsiderably which iscomparabletotheinstantaneousratesobservedin generally moveatslightlyfaster ratesof a, whereasactinandseveralhundredothercytosolicproteins generally moveatratesof proteins, tubulin,andanumberofothercytosolicproteins groups (Laseketal.,1984).Neuronalintermediatefilament solic proteinscanberesolvedintotwokineticallydistinct In radioisotopicpulselabelingstudies,cytoskeletalandcyto- Movement ofcytoskeletalpolymers (light microscopy) Instantaneous rate b b f d 0.2–1 mm/dinslowcomponent iietoa Low Bidirectional Intermediate High Bidirectional High Retrograde Anterograde ietoaiyDutyratio Directionality time.Notethatactinmayalso rateofmovementthecargo andthatthereisconsiderable generallyquotedasmaximal maximumrate(including icance ofthisisnotclear. end moving,inferredby 2-8 mm/dinslow ribonucleoprotein 20–70 mm/d, The Journal of Cell Biology Remarkably, the keytoobservingthismovement wassimply et al.,2000;Wang etal.,2000;Wangand Brown,2001). the movementsarebothinfrequent andbidirectional(Roy ganelles, buttheaveragerate ofmovementisslowbecause approaching therateofmovement ofmembranousor- Both neurofilamentsandmicrotubules moveatfastrates, in axons,buttheirmovements arenotslowafterall(Fig.1). versy isdisarminglysimple.Cytoskeletal polymersdomove dicate thatthesolutiontoslowaxonaltransportcontro- bules hasbeenobservedinaxons,andtheseobservationsin- al., 1997). the formoffreesubunitsorsmalloligomers(Hirokawa et mers arestationaryandthatcytoskeletalproteinsmovein 1997), whereasothershavearguedthatcytoskeletalpoly- move intheformofassembledpolymers(BaasandBrown, teins move.Somehavearguedthatcytoskeletalproteins permission fromAmericanSocietyforCellBiology. 12:3257–3267, Wang,L.,andA.Brown,Copyright2001,with Images inBarereprintedfrom Images inAwereprovidedbySunitaR.ChadaandPeterJ.Hollenbeck. ment ofmembranousandnonmembranouscargoes.Bar,5 underscores thesimilarityinunderlyingmechanismofmove- move inarapid,intermittent,andbidirectionalmanner,which Brown (2001)fordetails.Bothneurofilamentsandmicrotubules The timeintervalbetweenthesetwoimagesis8s.SeeWangand small arrowheadsmarktheleadingandtrailingendsoffilament. arrowheads marktheedgesofphotobleachedregion,and filament inaphotobleachedaxonofculturedneuron.Thelarge images is3s.(B)AnterogrademovementofaGFP-taggedneuro- field ofviewremainpaused.Thetimeintervalbetweenthesetwo the positionofmovingorganelle.Theothermitochondriain 123 intheaxonofaculturedneuron.Thesmallarrowheadsmark RetrogrademovementofamitochondrionstainedwithRhodamine- (A) Figure 1. Recently themovementofneurofilamentsandmicrotu- Movement ofmitochondriaandneurofilamentsinaxons. Molecular BiologyoftheCell