Figure3.RepeatedTransientBurstsinMessengerRibonucleicAcidPrecedeIncreasesinTranscriptionalandMitochondrialProteinsinResponsetoShort-TermTraining
Subjects(n=9)completedsevensessionsofhigh-intensityintervaltrainingduringa2weekintervention.Skeletalmusclebiopsiesfromthevastuslateraliswereobtained4and24hrafterthe?rst,third,?fth,andseventhtrainingsession.PGC-1a,peroxisome-proliferator-activatedreceptorgcoactivatora;CS,citratesynthase.DataareredrawnfromPerryetal.(2010).
potentialforoxidationthatmarkedlysurpassestheaerobicca-pacityoftypeI?bersofuntrainedindividuals(Saltinetal.,1977).Indeed,theabsolutelevelfortheactivitiesofbothoxida-tiveandglycolyticenzymesinall?bertypesinhumansislargeenoughtoaccommodateasubstantialrangeofaerobicandanaerobicmetabolism.
Whetherendurance-orresistance-basedexercisetraininginhumanscanresultin?bertype‘‘reprogramming’’remainsopentodebate.Certainlyendurancetraininginduceschangesinthemetabolicpropertiesofskeletalmusclebyconferringanincreasedoxidativepro?letothetrainedmyo?bers(Saltinetal.,1977).Sucheffectsarelikelytoinvolveaplethoraofsignalingcascadesandtranscriptionfactorsincluding,butnotlimitedto,calciumsignalingpathwaysinvolvingcalcineurin,calcium-calmodulin-dependentkinase,andthetranscriptionalcofactorsperoxisomeproliferator-activatedreceptorgcoactiva-tor1a(PGC-1a)andPPARd(OlsonandWilliams2000;Linetal.,2002;Wangetal.,2004;Wuetal.,2002).However,thespeci?ctranscriptionalfactorsdirectlyinvolvedinthecontrolofthemus-clephenotypeand?ber-speci?ccontractilepropertiesremaintobefullycharacterized.
Humanskeletalmuscledisplaysremarkableplasticity,withthecapacitytoalterboththetypeandamountofproteininresponsetodisruptionsincellularhomeostasisinducedbythehabituallevelofcontractileactivity,theprevailingsubstrateavailability,andenvironmentalconditions(Hawleyetal.,2011;ZierathandHawley2004).Thoughthisphenomenonof‘‘adapta-tionplasticity’’iscommontoallvertebrates,alargevariationinthedegreeofadaptabilitybetweenhumansisevident.Thispartlyexplainsthelargeinterindividualresponsestostandardizedexercisetraininginterventionsandthestrikingdifferencesinper-formancebetweenindividuals(Bouchardetal.,2011).Thefunc-tionalconsequencesofadaptationplasticityarespeci?ctothemodeofexerciseandarein?uencedbythevolume,intensity,andfrequencyofthecontractilestimulialongwiththehalf-lifeofspeci?cexercise-inducedproteins(Hawley2002).Prolonged
endurance-basedexercisetrainingelicitschangesthatincreasethemitochondrialproteincontentandrespiratorycapacityofthetrainedmyo?bers.Theseadaptationsunderpinthealteredpat-ternsofsubstrateoxidationduringsubmaximalexercise(fromcarbohydrate-tofat-basedfuels)thatresultinlesslactatepro-ductionatagivensubmaximalpoweroutputorspeed(Holloszy1967).Incontrast,strengthandresistance-basedtrainingstimu-latesthemyo?brillarproteinsresponsibleformusclehypertro-phy,culminatinginincreasesinmaximalcontractileforceoutput(Phillips2014)withoutsubstantialchangesinfueluseduringexercise.Concomitantwiththevastlydifferentfunctionaloutcomesinducedbythesediverseexercisemodes,thege-neticandmolecularmachineryaffectingtheseadaptationsaredistinct.
AdaptationstoExerciseTraining:TheCumulativeEffectofRepeatedExerciseBouts
Theconversionofvariouschemical,electrical,andmechanicalsignalsgeneratedduringmusclecontractiontomoleculareventspromotingphysiologicalresponsesandsubsequentadaptationsinvolvesacascadeofeventsresultinginactivationand/orrepressionofspeci?csignalingpathwaysregulatingexercise-inducedgeneexpressionandproteinsynthesis/degradation.Thesepathwaysarenumerousandhavebeenreviewedelse-where(Bassel-DubyandOlson2006;CoffeyandHawley2007;EganandZierath,2013;Hoodetal.,2006).Potentialsignalsdur-ingcontractileactivityinclude,butarenotlimitedto,increasedsarcoplasmic[Ca2+],increasedAMPand/oranincreasedADP/ATPratio,reducedcreatinephosphateandglycogenlevels,increasedfattyacidandROSlevels,acidosis,andalteredredoxstate,includingNAD/NADH,andhyperthermia(Hawleyetal.,2006).Redundancyandcompensatoryregulationarekeycharacteristicsofbiologicalsystemsthatacttopreservephysi-ologicalresponsesandadaptationstoavarietyof‘‘threats’’tocellularhomeostasis.Indeed,somegenedeletionsormutationshavelittleeffectonmetabolicadaptation,highlightingthepoten-tialcaveatsinvolvedinutilizingtransgenicorknockoutmodelstoexaminemechanismsofmuscleadaptation(McGeeetal.,2014).KeysignalingpathwaysinvolveCa2+/calmodulin-dependentki-nases(CaMK),calcineurin,AMP-activatedproteinkinase(AMPK),mitogen-activatedproteinkinases(ERK1/2,p38MAPK),andmammaliantargetofrapamycin(mTOR).Thetargetsofthesesignalingpathwaysincludemanytranscriptionfactors,coactiva-tors,andrepressors.ExerciseincreasesactivationofCaMKII(RoseandHargreaves,2003),AMPK(WinderandHardie1996;Fujiietal.,2000),andMAPKs(Widegrenetal.,1998).Aspreviouslynoted,contraction-inducedalterationsinintracellular[Ca2+]arelinkedtodistinctiveprogramsofgeneexpressionthatestablishphenotypicdiversityamongskeletalmyo?bers(Chin,2005;TaviandWesterblad,2011).Inaddition,activationofAMPKbyexer-cise-inducedalterationsinmuscleenergystatusincreasesgenetranscriptioninskeletalmuscle(McGeeandHargreaves,2010).
Adaptationstoexercisetrainingresultfromthecumulativeef-fectoftransientincreasesinmRNAtranscriptsthatencodeforvariousproteinsaftereachsuccessiveexercisebout.TheserepeatedburstsinmRNAexpressionappeartobeessentialtodrivetheintracellularadaptiveresponsetoexercisetraining(NeuferandDohm1993;Perryetal.,2010)(Figure3).Thetiming
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Figure4.SchematicoftheMajorSignalingPathwaysInvolvedintheControlofSkeletalMuscleHypertrophyandMitochondrialBiogenesis
Multipleprimarysignals,including,butnotlimitedto,mechanicalstretch,calcium,pH,redoxstate,hypoxia,andmuscleenergystatus,arealteredwithvoluntarydynamicexercise.Followinginitiationofoneormoreofthesepri-marysignals,additionalkinases/phosphatasesareactivatedtomediateaspeci?cexercise-inducedsignal.Inmammaliancells,numeroussignalingcascadesexist.Thesepathwaysareregulatedatmultiplesites,withsub-stantialcrosstalkbetweenpathwaysproducingahighlysensitive,complextransductionnetwork.
andresponsivenessofindividualmRNAspeciestodifferenttypesofcontractileactivityisvariable,butpeakinductionforboth‘‘metabolic’’and‘‘myogenic’’genesgenerallyoccur4–8hrafteranexercisebout,withmRNAlevelsreturningtopre-ex-erciselevelswithin24hr(Yangetal.,2005).AnotherlevelofregulationofmRNAandproteinabundancebyexerciseinvolves
alterationsinDNAmethylationstatus(Barre
`setal.,2012),his-tonemodi?cations(McGeeandHargreaves,2011),andmicro-RNAexpression(Zacharewiczetal.,2013).Ultimatelytheabilityofagivenmusclecelltoalterthetypeandquantityofproteinisafunctionofitshalf-life.Proteinsthatturnoverrapidlyandhavehighratesofsynthesisarecapableofattaininganewsteady-statelevelfasterthanthosethatturnoverslowlyduringadapta-tiontocontractileandotherstimuli.
MitochondrialBiogenesisandEnduranceTrainingAdaptation
Mitochondrialbiogenesisrequiresthecoordinationofmultiplecellularevents,includingtranscriptionoftwogenomes,synthe-744Cell159,November6,2014a2014ElsevierInc.
sisoflipidsandproteins,andthestoichiometricassemblyofmultisubunitproteincomplexesintoafunctionalrespiratorychain(Hoodetal.,2006).Impairmentsatanystepcanleadtodefectiveelectrontransport,failureofATPproduction,andaninabilitytomaintainenergyhomeostasis.SincetheseminalworkofHolloszy(1967),whodiscoveredthatmusclesoftread-mill-trainedratsexhibitedhigherlevelsofmitochondrialproteinsthanthoseofuntrainedanimals,majorbreakthroughsinunravel-ingthecellulareventscontrollingskeletalmusclemitochondrialbiogenesishaveoccurred.Severaltranscriptionfactorsthatregulatetheexpressionofthenucleargenesencodingmito-chondrialproteinswerediscovered(Scarpulla2006).Theseincludenuclearrespiratoryfactors1and2(NRF-1,NRF-2)thatbindtothepromotersandactivatetranscriptionofgenesthatencodemitochondrialrespiratorychainproteins(KellyandScar-pulla2004).NRF-1alsoactivatesexpressionofthenucleargenethatencodesmitochondrialtranscriptionfactorA(TFAM),whichmovestothemitochondriaandregulatestranscriptionofthemitochondrialDNA(i.e.,themitochondrialgenome).BecausenotallpromotersofgenestranscribingmitochondrialproteinshaveNRF-1-bindingsites,othertranscriptionfactorsareinvolvedincontractile-modulatedmitochondrialbiogenesis,includingtheestrogen-receptor-relatedreceptors(ERR)aanddandtheperoxisomeproliferator-activatedreceptorcoactiva-tors(PPARs),whichregulateexpressionofthemitochondrialfattyacidoxidativeenzymes(KellyandScarpulla,2004;Scar-pulla,2006).
AnothermajorbreakthroughinunravelingthecellulareventsthatpromotemitochondrialbiogenesiswasthediscoveryofPGC-1a,aninduciblecoactivatorthatregulatesthecoordinatedexpressionofmitochondrialproteinsencodedinthenuclearandmitochondrialgenomes(Linetal.,2005).AcriticalfeatureofthePGC-1coactivatorsisthattheyarehighlyversatileandinteractwithmanydifferenttranscriptionfactorstoactivatedistinctbio-logicalprogramsindifferenttissues(Linetal.,2005).Inskeletalmuscle,PGC-1ahasemergedasakeyregulatorofmitochon-drialbiogenesisthatrespondstoneuromuscularinputandtheprevailingcontractileactivity.Asingleboutofenduranceexer-ciseinducesarapidandsustainedincreaseinPGC-1ageneandproteininskeletalmuscle(Mathaietal.,2008),whereasmuscle-speci?coverexpressionofPGC-1aresultsinalargein-creaseinfunctionalmitochondria(Linetal.,2002),improve-mentsinwhole-bodyVO2max,ashiftfromcarbohydratetofatfuelsduringsubmaximalexercise,andimprovedenduranceper-formance(Calvoetal.,2008).Gain-of-functionstudiesrevealthatexpressionofPGC-1aatornearphysiologicallevelsleadstoactivationofgeneticprogramscharacteristicofslow-twitchmuscle?bers(Linetal.,2002),withthemusclesofthesetrans-genicmiceresistanttocontraction-inducedfatigue.LossoffunctionstudieschallengetheabsoluterequirementofPGC-1aforexercisetraining-inducedchangesinmusclemitochondrialbiogenesis,angiogenesis,and?bertypechanges(Gengetal.,2010;Roweetal.,2012).Onbalance,currentobservationsplacePGC-1aasacentralplayerinorchestratingmanyoftheoxidativeadaptationstoexercise.
AMPKandp38MAPKaretwoimportantsignalingcascadesthatconvergeupontheregulationofPGC-1aandconsequentlytheregulationofmitochondrialbiogenesis(Figure4).AMPK
inducesmitochondrialbiogenesispartlybydirectlyphosphory-latingandactivatingPGC-1a(Ja
¨geretal.,2007),butalsobyphosphorylatingthetranscriptionalrepressorHDAC5,whichrelievesinhibitionofthetranscriptionfactormyocyteenhancerfactor2(MEF2),aknownregulatorofPGC-1a(McGeeandHar-greaves,2010).Ofnote,MEF2activationisassociatedwithincreasedmuscleoxidativecapacityandrunningendurance(Potthoffetal.,2007).p38MAPKphosphorylatesandactivatesPGC-1a(Puigserveretal.,2001)andalsoincreasesPGC-1aexpressionbyphosphorylatingthetranscriptionfactorATF-2,whichinturnincreasesPGC-1aproteinabundancebybindingtoandactivatingtheCREBsiteonthePGC-1apromoter(Aki-motoetal.,2005).Thetumorsuppressorproteinp53,presum-ablyactivatedbyAMPKand/orp38MAPK,isemergingasanothertranscriptionfactorinvolvedinexercise-inducedmito-chondrialbiogenesisinskeletalmuscle.p53knockoutmicedisplayreducedenduranceexercisecapacitycomparedwithwild-typemice,alongwithreducedsubsarcolemmalandinter-myo?brillarmitochondrialcontentandPGC-1aexpression.p53mayregulateexercise-inducedmitochondrialbiogenesisthroughinteractionswithTFAMinthemitochondria,whereitfunctionstoco-ordinateregulationofthemitochondrialgenome(Bartlettetal.,2014).
MuscleHypertrophyandMyogenicPathways
Strengthtrainingincreasesmuscle?bersize(hypertrophy)andmaximaltensionoutput.Theseadaptationsareattainedbypos-itivemuscleproteinbalanceandsatellitecelladditiontopre-ex-isting?bers.Positivemuscleproteinbalanceoccurswhentherateofnewmuscleproteinsynthesisexceedsthatofbreakdown.Althoughresistanceexerciseandpostprandialhyper-aminoaci-demiabothstimulatemuscleproteinsynthesis,itisthroughthesynergisticeffectsofthesestimulithatanetgaininmusclepro-teinoccursand?berhypertrophytakesplace(Phillips2014).ActivationofmTORappearstobeimportantforcontraction-inducedincreasesinmuscleproteinsynthesis(Drummondetal.,2009).Onceactivated,mTORexistsastwodistinctcomplexes,mTORcomplex1(TORC1)andmTORcomplex2(TORC2).TORC1ischaracterizedbythepresenceofregulato-ry-associatedproteinofmTOR(RAPTOR),whereasTORC2bindsrapamycin-insensitivecompanionofmTOR(RICTOR).Thesetwoproteincomplexessensediversesignalsandproduceamultitudeofresponses,includingmRNAtranslation,ribosomalbiogenesis,andnutrientmetabolism(CoffeyandHawley2007;EgermanandGlass,2014).IGF-1haslongbeenconsideredakeyupstreamregulatorofmTOR.SignalingactivatedbyIGF-1positivelyregulatesskeletalmusclemassviainductionofproteinsynthesisdownstreamofproteinkinaseB/AktandthemTORpathway(Bodineetal.,2001).IGF-1transmitssignalingalongthePI3K/Aktpathway(Figure4),resultingintheparallelactiva-tionofthemTORpathway,producingamultitudeofresponses,includingmRNAtranslation,ribosomalbiogenesis,andnutrientmetabolism(CoffeyandHawley2007).Growth-factor-indepen-dent,mechanosensitiveactivationofmTORalsocontributestomuscleproteinsynthesis(Philpetal.,2011).
Themostwell-de?nedeffectorsofmTORsignalingarepro-teinsimplicatedintranslationalcontrol:ribosomalproteinS6ki-nase(p70S6K)andeukaryoticinitiationfactor4E-bindingprotein(4E-BP1).Indeed,afteractivationbyAkt,TORC1controlspro-teinsynthesisbyphosphorylatingp70S6kinase1andthe4E-BP1,andtheTORC2multiproteincomplexcontributestotheprolongedactivationofAkt.Phosphorylationofp70S6KandsubsequentactivationofribosomalproteinS6enhancestransla-tionofmRNAs,encodingelongationfactorsandribosomalproteinsandtherebyincreasingtranslationalcapacity.p70S6Kplaysafundamentalroleinskeletalmusclehypertrophy(Liuetal.,2002).Asingleboutofresistanceexerciseinhumansleadstoincreasesinp70S6Kphosphorylation,whichiscorrelatedwiththechronicincreaseinmusclemassandstrengthobservedafterchronicresistancetraining.Thus,theacuteresponsestoexer-cise,includingdynamicchangesinmuscleproteinturnoverandtheearlyactivationofsignalingproteins,mayactassurro-gatesoflong-termphenotypicchangesinmusclemassandstrength.
PGC-1a4,atranscriptfromthePGC-1agene,isabundantlyexpressedinskeletalmuscleandappearstoplayaroleintheadaptiveresponsetoexercise,particularlyinthesettingofresis-tancetraining(Ruasetal.,2012).ThisproteindoesnotappeartoregulatethesamesetofoxidativegenesinducedbyPGC-1abut,rather,activatestheexpressionofIGF-1whileconcomi-tantlysuppressingmyostatin(aninhibitorofmusclecelldifferen-tiationandgrowth)pathways.Aftertrainingconsistingofeitherenduranceexercise,resistanceexercise,oracombinationofbothenduranceandresistanceexercise,increasesinPGC-1a4werecon?nedtoresistance-onlyandcombinedexercisetrainingprograms,withnochangesinthistranscriptafterendur-ance-onlytraining(Ruasetal.,2012).Thoughtheseresultsarenotable,theproposalthatskeletalmusclehypertrophyfollowingresistanceexerciseismediatedthroughPGC-1a4remainsamatterofdebate.
Skeletalmusclefromendurance-andstrength-trainedindivid-ualsrepresentsdiverseadaptivestates(Figure4).Thus,itishardlysurprisinglythatsimultaneouslytrainingforbothenduranceandstrengthresultsinacompromisedadaptationcomparedwithtrainingforeitherexercisemodalityalone(Hick-son1980),aphenomemonknownasthe‘‘interferenceeffect.’’Theseobservationsmademorethan30yearsago(Hickson1980)raisedthepossibilitythatthegeneticandmolecularmechanismsofadaptationinducedbyenduranceandresis-tancetrainingaredistinct,witheachmodeofexerciseactivatingand/orrepressingspeci?csubsetsofgenesandcellularsignalingpathways.Preliminaryevidenceforselectiveactivationand/ordownregulationoftheAMPK-PGC-1aorAkt-mTORsignalingpathwayswasreportedinrodentskeletalmuscleinresponsetoeitherlow-frequency(tomimicendurancetraining)orhigh-frequency(tomimicresistancetraining)electricalstimu-lationinvitro(Athertonetal.,2005).However,inwell-trainedhu-mans,littleevidenceexistsforanAMPK-Akt‘‘masterswitch.’’Usinghighlytrainedathleteswithahistoryofeitherenduranceorstrengthtrainingwhoperformedbothanacuteboutofexerciseintheirspecializeddisciplineandthen‘‘crossedover’’andundertookaboutofunfamiliarexercise,ahighdegreeof‘‘responseplasticity’’isconservedatoppositeendsoftheendurance-hypertrophicadaptationcontinuum(Coffeyetal.,2006).Giventhatgenotypeswereoriginallyselectedtosupportdiversephysicalactivitypatternsobligatoryforhumansurvivalandthatmoderndaysuccessinmanysportingendeavors
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requiresahighendurancecapacitycoupledwithsuperiorexplo-sivepower,theconservationofmultiplesignalingnetworkstomeetdivergentphysiologicaldemandsseemstomakesoundevolutionaryandbiologicalsense!
SpreadingtheMessage:SkeletalMuscleCrosstalk
Morethan50yearsago,Goldsteinproposedthatskeletalmus-clecellspossesseda‘‘humoral’’factorthatcontributedtothemaintenanceofglucosehomeostasisduringexercise(Gold-stein,1961).Duringthepastdecade,skeletalmusclehasbeencon?rmedasanendocrineorgan.Cytokinesandotherpeptidesthatareexpressed,produced,expressed,and/orreleasedbymuscle?bersandexerttheirautocrine,paracrine,orendocrineeffectsarenowclassi?edas‘‘myokines’’(Pedersenetal.,2003).The?ndingofmuscle‘‘crosstalk’’withotherorgans,includingadiposetissue,liver,pancreas,bone,andthebrain,providesaframeworkforunderstandinghowexercisemediatesmanyofitsbene?cial‘‘whole-body’’effects.Althoughsomemy-okinesexerttheiractionsonotherorgansinanendocrinefashion,manyoperatelocallyonskeletalmuscleandtherebyprovideafeedbackloopforthemuscletoregulateitsowngrowthandregenerationforadaptationtoexercisetraining.The?rstcytokinefoundtobereleasedintothebloodstreaminresponsetomusclecontractionwasinterleukin6(IL-6).HumanskeletalmuscleisuniqueinthatitcanproduceIL-6duringexer-ciseindependentlyoftumornecrosisfactor,suggestingthatmuscle-derivedIL-6hasaroleinmetabolismratherthaninin?ammation.IL-6increasesbothmuscleandwhole-bodyratesoflipidoxidation(possiblythoughactivationofAMPK)andalsocontributestohepaticglucoseproductionduringexercise.
Contractingmuscle?bersproducemanycirculatingfactors.ThecurrentlistofpotentialmyokinesincludesbutisnotlimitedtoIL-8,IL-15,decorin,follistatin-like1,?broblastgrowthfac-tor-21(FGF21),irisin,chemokineCXCmotifligand-1(CXCL-1)alsoknownasKC(keratinocyte-derivedchemokine),andmete-orin-like(Metrnl)(Bostro
¨metal.,2012;Raoetal.,2014;Pedersenetal.,2012;Wrannetal.,2013).Asmall-moleculemyokine,b-aminoisobutyricacid(BAIBA),witheffectsonadiposetissueandliver,hasalsobeendescribed(Robertsetal.,2014).Mostrecently,exercise-inducedalterationsinkynureninemetabolism,mediatedviaincreasedskeletalmusclePGC-1a1expression,in-creaseresiliencetostress-induceddepressioninmice(Agudeloetal.,2014).Althoughthepotentialtherapeuticbene?tsofmus-cle-derivedmoleculesfortreatingobesityandotherinactivity-relateddisordersareappealing,thereislittleclinicalevidenceinhumanstodate.
Can‘‘ExerciseMimetics’’EverReplaceExercise?
Manyoftheadaptiveresponsesofskeletalmuscletoexercisetrainingcanbemimickedbygeneticmanipulationand/ordrugtreatment,atleastinanimalmodels(Narkaretal.,2008).Conse-quently,giventhenumerousbene?tsofexerciseongeneralhealth,ithasbeenstatedthat‘‘theidenti?cationofgeneticand/ororallyactiveagentsthatmimicorpotentiatetheeffectsofenduranceexerciseisalongstandingalbeitelusivemedicalgoal’’(Narkaretal.,2008).Recognizingtheprovenbene?tsofexercisetrainingonhealthoutcomesandthetrendtowardincreasinginactivityatthepopulationlevel,effortsareunderwaytodiscoverorallyactivecompoundsthatmimicorpotentiatetheeffectsofexercisetraining,so-called‘‘exercisemimetics.’’
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Althoughtheconceptoftakingapilltoobtainthebene?tsofexercisewithoutactuallyexpendinganyenergyhasmassap-pealforalargemajorityofsedentaryindividuals,suchanapproachislikelytofail.Exercisetrainingprovokeswidespreadperturbationsinnumerouscells,tissues,andorgan,conferringmultiplehealth-promotingbene?ts,anditisthemultiplicityandcomplexityoftheseresponsesandadaptationsthatmakeithighlyimprobablethatanysinglepharmacologicalapproachcouldevermimicsuchwide-rangingeffects.Thougha‘‘polypill’’containingseveralagonistsaimedatselectedexercise-inducedtargetsisapossibility,suchanapproachislikelytobeassoci-atedwithmultipleoff-targetandpotentialdeleterioussideeffects.Amoreachievablegoalwillbetoidentifytissue-speci?ctargetsthroughadeeperunderstandingofthemolecularpath-waysactivatedbyexerciseinvariousorgansystems,enablinglimitedaspectsoftheexerciseresponsetobepharmacologi-callymimicked.Althoughsuchagentsmaybeusefuladjuvantsinsomesettings,exerciseitselfremainsthebest‘‘polypill’’toimprovehealthandwellbeing(Fiuza-Lucesetal.,2013).Inouropinion,?ndingwaystomotivatepeopletoadoptandmaintainaphysicallyactivelifestylewillhaveagreaterimpactonindivid-ualandpopulationhealththansearchingforpotentialpharmaco-logicaltreatments.If?ndingorallyactiveexercisemimeticsisreallya‘‘longstanding’’medicalgoal,webelieveitwillcontinuetobeelusiveforreasonsevidentinthisReview.
BeyondtheFinishLine:TheNext40Years
Duringthelast40years,withtheapplicationofmoleculartech-niquestoexercisebiology,multipleandapparentlyredundantmolecularpathwaysengagedinmanykeyacuteandchronicresponsestoexercisehavebeenelucidatedinskeletalmuscleandothertissues.Althoughmajorbreakthroughsintheknowl-edgeofhowexerciseactivatesnumerouscellular,molecular,andbiochemicalpathwayshavebeenwitnessed,directevi-dencelinkingsucheffectstospeci?chealthoutcomesandun-derstandinghowtheseeffectsexerttheirbene?tsindifferentpopulationsremainselusiveandachallengeforfutureresearch.Duringthepasttwodecades,thelong-hypothesizedcrosstalkbetweenmuscleandotherorgansviathereleaseofsubstancesbythecontractingmuscleshasbeencon?rmed.However,inmanycases,normalresponsesandadaptationstobothacuteexerciseandchronicexercisetrainingcanbeseenwhenoneormorekeypathwaysareabsent,areblockedwithdrugs,orareotherwiseattenuated.Thisbiologicalredundancyindicatesthatperhapstheonlyobligatoryresponsetoexerciseisthede-fenseofhomeostasisitself.Clearlyabigchallengeforexercisebiologistsinthenext40yearswillbetoconnectdistinctsignalingcascadestode?nedmetabolicresponsesandspeci?cchangesingeneexpressioninskeletalmusclethatoccurafterexercise.Thiswillbecomplicatedbecausemanyofthesepath-waysarenotlinear,but,rather,theyconstituteacomplexnetwork,withahighdegreeofcrosstalk,feedbackregulation,andtransientactivation.Thevarious‘‘omics’’technologiesandtheapplicationofcomputationalandsystemsbiologyap-proachestoproblemsinexercisebiologyshouldfacilitatefutureprogress.
Futureresearchinthe?eldofexercisebiologyrequiresincreasinglysophisticatedapproachestounderstandthecritical
nodesofenergyhomeostasisandhowthesepathwaysaredis-ruptedinanumberofinactivity-relateddisorders.Variousstrainsofmicehavelongbeenusedtoexamineresearchquestionsinexercisebiology.Recentstudiesusingtheworm(Caenorhabdi-tiselegans),?y(Drosophilamelanogaster),andzebra?sh(Daniorerio)indicatethatthese‘‘lower’’metazoansalsopossessuniqueattributesthatcouldprovevaluableinthestudyofmeta-bolicdiseases,includingtheeffectsofexercise/musclecontrac-tion.Detailedcharacterizationoftheknownpathwaysregulatinglowermetazoanenergymetabolismmayaidinidentifyingandcharacterizingnovelcandidategenesforhumandiseasessuchasobesityandtype2diabetesmellitus,andthefunctionofsuchgenesmaybemoreamenabletostreamlinedcharacteriza-tioninlowerorganisms.Althoughtherearelimitationsofeachmodelsystemthatneedtoberecognizedwhendeployingtheseorganismsfortargetvalidationand,ultimately,translationintohumans,theabilitytoperformsophisticatedandmechanisticstudiesindicatesthatsuchanapproachcouldyieldtransforma-tiveresearchoutcomesinthecomingdecades.Inthe?nalanal-ysis,itistheorganism’sphenotypeasawholethatinteractswithandadaptstotheexternalworld.Thestudyofexercisebiologyshowsthattheneedtointegrateobservationsfromgenes,mol-ecules,andcellsinaphysiologicalcontexthasneverbeengreater.
ACKNOWLEDGMENTS
Duringourcareers,wehavebeenfortunatetoworkalongsidesomeofthepi-oneersinthe?eldofexercisebiology,includingDr.DavidL.Costill,Dr.JohnO.Holloszy,andthelateDr.BengtSaltin.WehopethatthisReview,insomesmallpart,doesjusticetoyourlegacies.Duetorestrictionsonthenumberofrefer-ences,wehavebeenunabletoincludeimportantworkbysomeofourpeers,forwhichweapologizeinadvance.
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