TRP Channels and Pain

33 Pages·2014·5.8 MB·English

by  David Julius

CB29CH13-Julius ARI 1September2013 10:5
TRP Channels and Pain
David Julius
DepartmentofPhysiology,UniversityofCalifornia,SanFrancisco,California94158;
email:[email protected]
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55-gh Annu.Rev.CellDev.Biol.2013.29:355–84 Keywords
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13.2Pitts TBiholeogAyninsuoanllRineeviaetwhotftpC:/e/llcaenlldbiDo.eavnenloupamlreenvtaielws.org somatosensation,nociception,thermosensation,inflammatorypain,
Cell Dev. Biol. 20by University of AT1C0lohl.p1irs1yigar4hri6gtt/ihscatlnree(cid:2)n’cssuedrr2evo0vei1:-d3cebllybAion-1n0u1al0R11e-v1ie5w58s.33 nANmabaottucsoirtcsareelanppcstrtiooordnyuiscsytstsht,eeamnpardlogeceteessiccstswnohxeiroeubsysptirmimualir.yPauffnegreennttnirerritvaenfitsbferrosmofptehpepseor-,
v. mint,andmustardplantshaveservedaspowerfulpharmacologicaltoolsfor
e
R identifying molecules and mechanisms underlying this initial step of pain
u.
n sensation.Thesenaturalproductshaverevealedthreemembersofthetran-
n
A
sientreceptorpotential(TRP)ionchannelfamily—TRPV1,TRPM8,and
TRPA1—asmoleculardetectorsofthermalandchemicalstimulithatacti-
vatesensoryneuronstoproduceacuteorpersistentpain.AnalysisofTRP
channelfunctionandexpressionhasvalidatedtheexistenceofnociceptors
as a specialized group of somatosensory neurons devoted to the detection
ofnoxiousstimuli.Thesestudiesarealsoprovidinginsightintothecoding
logic of nociception and how specification of nociceptor subtypes under-
liesbehavioraldiscriminationofnoxiousthermal,chemical,andmechanical
stimuli.Biophysicalandpharmacologicalcharacterizationofthesechannels
hasprovidedtheintellectualandtechnicalfoundationfordevelopingnew
classesofanalgesicdrugs.
355
CB29CH13-Julius ARI 1September2013 10:5
Contents
SENSINGNOXIOUSSTIMULI:NOCICEPTIONASADISTINCT
SOMATOSENSORYMODALITY............................................ 356
NATURALPRODUCTSASPROBESOFTHEPAINPATHWAY............... 358
TRPCHANNELS:ABRIEFPRIMER............................................ 358
TRPV1:AHEAT-ACTIVATEDIONCHANNELANDINTEGRATOR
OFINFLAMMATORYSIGNALS............................................. 361
AnEmergingFunctionalMapofTRPV1........................................ 362
TRPM8:ACOLD-ACTIVATEDCHANNEL.................................... 369
TRPM8AccountsforColdSensationInVivo.................................... 369
TheCoolingAgentSite(s)...................................................... 370
g
or TRPM8Modulation............................................................ 370
s.
w HowDoTRPV1andTRPM8DetectThermalStimuli?......................... 371
e
vi TRPA1:ACHEMORECEPTORFORENVIRONMENTALIRRITANTSAND
e
ualry. INFLAMMATORYAGENTS................................................. 371
w.annse onl ATcRtiPvAat1ioInsAbylsCooavRaleecnetpMtoro-dOifipceartaitoend.C..h.a.n.n..e.l....................................................................... 337743
wu
m wonal EvolutionaryTuningofTRPA1:ChemosensationVersusThermosensation....... 374
ded froor pers COTNRCPLCUhDanINneGlsRanEdMNAeRurKaSlS.p.e.c.i.fi.c.i.t.y....................................................................................... 337755
84. Downloan 01/30/14. F SETNhSeIrNapGeutNicOPoXteInOtiUal.S..S.T..I.M...U..L..I.:.N..O..C..I..C..E.P..T..I.O..N...A..S..A...D..I.S.T..I.N..C..T.... 375
3o
55-gh SOMATOSENSORYMODALITY
9:3bur
2s Somatosensation,colloquiallyreferredtoastouch,actuallyencompassesseveralsubmodalitiesthat
v. Biol. 2013.versity of Pitt ip(ndlacecltueedcmteieotnontuoocffhmn(douexsticeolceutsaionmndeojcofhilnaigntshi)ct,amtlh,eetcrhhmearonmsiecanal,lsasottriimocnhue(ldim)e,itpceracoltpisotrniimoocufelcpiottioholanatn(gddievwteeacrrtimisoetnhtoo),fpamnaidencnhsoeacnniisccaeatpilotdinoissn-)
DeUni (Gardneretal.2000).Exceptforchemicalsensitivity,nociceptioncouldsimplybeconsideredan
Cell by extremeversionoftouchandtemperaturesensation.Indeed,oneofthegreatdebatesinthisfield
v. hasrevolvedaroundthequestionofwhether(andtowhatextent)nociceptionrepresentsnotjust
e
R apsychophysicallydistinctaspectofsomatosensationbutamechanisticallydistinctone,aswell.
nu. Noxiousstimuliareinitiallydetectedbyprimaryafferentsensorynervefibersthatinnervate
n
A aperipheraltargetandtransmitthisinformationtoneuronswithinthedorsalhornofthespinal
cord,andthencetothebrain,viaascendingneuralcircuits(Figure1)(Basbaum&Jessell2000).
Twoopposingmodelshavebeenputforthtoexplainhowprimaryafferentneuronsencodenoxious
percepts.Accordingtothepatterntheory,painisproducedwhenastimulusofsufficientintensity
elicits a pattern of activity across functionally indistinct sensory nerve fibers; the resulting pat-
ternisthendeconvolutedwithinthecentralnervoussystem(spinalcordandbrain)togenerate
aspecificperceptrepresentingthenoxiousmechanical,thermal,orchemicalstimulusappliedto
the peripheral receptive field (Melzack & Wall 1965). In contrast, the specificity theory posits
thatpainisproducedbyactivationofsensoryneuronsubtypesthataretunedtodetectastimulus
of a specific quality and/or intensity (e.g., noxious heat versus cold) (Bessou & Perl 1969, Perl
2007,Sherrington1903).Assuch,informationaboutthemodalityandintensityofastimulusis
encoded,atleastinpart,bytheprimaryafferentnervefiberitself,evenbeforesignalsreachthe
356 Julius
CB29CH13-Julius ARI 1September2013 10:5
Basophil
Neutrophil
Mast cell Substance P
Histamine DRG
Serotonin cell body
NGF
Lesion Proteases To spinal cord
Bradykinin
Prostaglandin
5-HT
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384. Downloaon 01/30/14. F anpiSnreoeernxtchiasoeicetputiiritzsvaaoastbitfigeoialndnictaubylotsyftetonnodooaisslxcesciiococooeumspnisgftdtoonharraretslyrobamnrnyeatputliai,drsimrosnoun.emesIcnhiiinncaaajnudtlhlirdycyei,atlsasi,opunonicdnrhtaiconlthhflcteahoamtmerdtsihcmedaecaloyetsrinrtsoietamnrllea.hualPllosyirer.itnmOtrr,aaanannrcnsysemdmaatificthfttteteeirevnderacnstseetingdtnono,otattcohlhsinec,eselnyebportfncoareioircnrsmev,p(eepctlfioueicbrnrisepttrlriaenasrlgnetreuaaunnroisqnmus)iet
55-gh terminalsinthespinalcordbutalsofromstimulatedperipheralterminals.Inparticular,somenociceptors
9:3bur releasethepeptidessubstancePandCGRP,whichelicitvasodilation,vascularleakage,andotherresponses
2s
13.Pitt fromnearbyperipheralcelltypes.Theseactionsproduceorreleaseapanoplyoflocalsignalingmolecules,a
Cell Dev. Biol. 20by University of pnecpharaecoieunhcmreohoistcyfsrapowrlepehsrfhtseiiciemrnhnrseus,idilntpii.ttrveoToirtsaahytcs.aitsnAnspoebwhiubdiertrsonheagovrneimeadncteiieoconptnoihtn:noeflDrrrsaebRmopiGnormea,taschdetteiinoovtrnensso.alaiclTpikrcihoedeeyosp,titpneogexflrartaintrpemaghrclmmeieorlailnanutl.oalmalrryteopcsroheouantnphoianssnomscapewnridohtdsenursueeccbnelydseiottiiitnvsicdsituleuyesd,tieoansnjpudhrmyyspoicrnaoalmmoorinteess,
v.
Re centralnervoussystem.Thatistosay,thespecificitytheorypredictstheexistenceofthenocicep-
u. torasafunctionallydistinctsubtypeofsomatosensoryneuronthatisdevotedtothedetectionand
n
n
A transductionofnoxious,high-intensitystimulicapableofcausingtissuedamageandelicitingpain
(Sherrington 1906). This controversy is not merely academic because each side makes predic-
tionsastohownoxiousstimuliareencoded;whetherreceptorsandotherfunctionallyimportant
moleculesareexpressedbyspecificsubsetsofsomatosensorynervefibers;and,thus,whetherand
towhatextentprimaryafferentnervefibersconstituteeffectivetargetsforanalgesictherapy.
Asissoclearlydemonstratedinothersensorysystems(e.g.,vision,olfaction,andtaste),the
cloning and functional characterization of sensory receptors provides the definitive molecular
tools with which to address such questions and elucidate the logic of stimulus detection and
perception (Julius & Nathans 2012). Indeed, the study of transient receptor potential (TRP)
channelshashelpedresolvelong-standingcontroversiesinthepainfieldbyvalidatingtheexistence
of the nociceptor (and thus the specificity theory), while providing a molecular framework for
understandingperipheralmechanismsunderlyingstimulusdetection,injury-evokedsensitization,
www.annualreviews.org • TRPChannelsandPain 357
CB29CH13-Julius ARI 1September2013 10:5
andpsychophysicalcoding(Basbaumetal.2009,Caterina&Julius1999,Patapoutianetal.2009,
Woolf&Ma2007).
NATURALPRODUCTSASPROBESOFTHEPAINPATHWAY
Naturalproductsfrommedicinalplantshaveproventobeamongthemostpowerfulandimportant
toolsforidentifyingandmanipulatingkeyelementsofthepainpathway.Twonotableexamples
includemorphinefromtheopiumpoppyandsalicylatefromthewillowbark,pharmacological
agentsthatwerekeytodiscoveringopioidreceptorsandcyclooxygenases,respectively(Snyder
1977,Vane&Botting1998)(Figure2a).Theprofoundinfluenceofthesenaturalproductson
painresearchandtherapeuticsishighlightedbythefactthattheyrepresentthetwomainclasses
ofpainmedicinesstillinusetoday,namely,opiateanalgesicsandnonsteroidalanti-inflammatory
drugs(Jancso´ etal.1977).
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s.or Morphine and aspirin suppress pain, but some plants generate compounds that elicit pain,
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w.annualrese only. Tcsehhlieleictmpiveoepslpyteefraxsmcitithleieapirrriomzfinathrgye(saFeffiepgrrueornaetlgs2eeansi)sc.ocrCyoamnpespuaoircuoinnndsas(nJiasdncrcaespol´asaeteitcdailn.v,1at9nh7iel7lo)p,iudenlicgcoeitmnintpgaogauesnnedntsstahrtoaiotbnugsiovtfleyascahunotdet
wwal u burningpainaccompaniedbylocalvasodilationandinflammation,followedbyhypersensitivity
m on toheat(thermalhyperalgesia)and,tosomedegree,touch(mechanicalallodynia)(Jancso´ etal.
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ded fror per 1p9re6s7u)m.Tpthiuves,nsoecniscietipvtiotyrst,oigcnaiptsinaigciandheacsadloens-globnegenseraergcahrdfoerdiatsssaitdeeafinndinmgefcuhnacntiisomnaolfhaaclltmioanr.kof
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5-3h o (Figure2a).Likecapsaicin,thesecompoundshavebeenusedextensivelyincellularandbehavioral
9:35burg studiesofacuteandpersistentpainand,mostrecently,toidentifyproteinsandsensoryneurons
2s thatcontributetotheseprocesses(Basbaumetal.2009).Remarkably,eachofthesestructurally
v. Biol. 2013.versity of Pitt dmniesertcvihneacfintbicselmarsss;ientshweoshfeiicrihrnicttalhunedtysethTaarRsgeePtvVosp1lve(ectdihfietcocmeapleiscmaitibcdeinirsscrooefcmetfhpoetrotTra)R,nTPdRpchaPiaMnnnt8he(rltofhuaemgmhileayncotonhmoplmrrimoecnaermpytooalfref)ec,ruaelnnadrt
DeUni TRPA1(thewasabireceptor)(Figure2b).Functionalandhistologicalanalyseshaveshownthat
Cell by TRPV1, TRPM8, and TRPA1 exhibit distinct patterns of expression within primary sensory
v. ganglia,providingpharmacologicandgeneticstrategiesforidentifyingandmanipulatingsensory
e
R neuronsubtypesthatcontributetoparticularaspectsofnociceptionandpain.
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TRPCHANNELS:ABRIEFPRIMER
TRP channels were first discovered in the fly eye, where light-activated rhodopsin stimulates
phospholipase C (PLC) to hydrolyze the minor plasma membrane lipid, phosphatidyl-inositol
bisphosphate(PIP ).This,inturn,promotesgatingofTRPchannelstodepolarizethephotore-
2
ceptorcell(Figure 3)(Hardie2007,Minke&Parnas2006,Montell2012).Althoughthebasic
outlinesofthisreceptor-operatedTRPchannelsignalingpathwaywerelaidoutseveraldecades
ago, the mechanism whereby PIP hydrolysis leads to TRP channel gating remains enigmatic.
2
Geneticandelectrophysiologicalstudiessuggestthatchannelactivationinvolvesacombinatorial
processrequiringbothdepletionofPIP andconsequentgenerationofaphosphoinositide-derived
2
secondmessenger(s),suchaspolyunsaturatedfattyacidand/orintracellularprotons(Estacionetal.
2001,Huangetal.2010).Accordingtothismodel,TRPchannelsintheflyeyearenegatively
358 Julius
CB29CH13-Julius ARI 1September2013 10:5
Aspirin Morphine
a CH
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Menthol
Capsaicin
CH
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Figure2
Naturalproductsasprobesofthepainpathway.(a)Naturalplantproductshaveprovidedthemostpowerful
pharmacologicalprobesforidentifyingandstudyingmoleculesandsignalingmechanismsunderlying
nociceptionandpain.Theseincludesalicylicacid(relatedtoaspirin)fromwillowbark;morphinefrom
opiumpoppies;mentholfrommintleaves;capsaicinfromchilipeppers;thiosulfinatesfromgarlic,onions,
andotheralliumplants;andisothiocyanatesfromwasabiandothermustardplants.(b)Capsaicinelicitsacute
painandneurogenicinflammationbyselectivelyactivatingexcitatorytransientreceptorpotential(TRP)ion
channelsonnociceptornerveendings,promotingthereleaseofneurotransmittersandpeptidesfromboth
centralandperipheralnerveterminals.Similarmechanismsunderlietheactionsofmenthol,isothiocyanates,
thiosulfinates,andrelatedpungentirritants.
www.annualreviews.org • TRPChannelsandPain 359
CB29CH13-Julius ARI 1September2013 10:5
Light
T R P
PPIIPP
22
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PLC
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C
A N
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or
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e
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w.annualrse only. FDirgousorpehi3latransientreceptorpotential(TRP)—amodelforthecanonicalreceptor-operatedTRPchannel.
wu TRPchannelswerefirstcharacterizedintheflyeye,inwhichtheydepolarizethephotoreceptorcellsin
m wonal responsetolight.Photon-evokedactivationofGprotein–coupledrhodopsin(purple)stimulates
aded froFor pers pbdhoiswopshnposhtsropelhaipamatespe(oPClIyPu(P2n)Ls,aCytiu)erltadotiencdagtfasaeltyctzyoenatdchidmeshe(syPsdeUrnoFglAyesrsis)s,ionafnodpsiltiaonsltmrtaracipmehlleoumslapbrhrpaatnreoet(poIPhn3os.)s,Hpdhoiaawtciydtlyhglel-ysienceoarscoittlioo(lDn-4sA,p5Gr-o)manodte
84. Downlon 01/30/14. TihammeRtpiePnororoctthatrenairmncmeneieronlifcualscGi.ptiivpdar-tocihoteaninnrn;eeImNlainAintDesr,aascmctaiaoftfntoeslr.dAionfbgbdprisercovuitasestiiinoonnfosar:nAfld,ydaTnebkRyaPtreian,nardletphaesosauotgsc;hiaCtme,docsastribgenovixadylientnegcremmpoinoleiuncstu;slGetos,;tNhe,
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v. Biol. 2013.2versity of Pitts rmperogoVvudelueascrttteeheddbisbmriaynethePtaiIbbTPiot2Roli(Prtaeynsccdoohprnaonsostntrsheiabelisnlrytpraoehntpyhdrseeiaroslellpoonhwgtoiscsatpnshhteioemcxinhtueoalnins.idnteieddletsop),rtohsuteecnihnbtehffaaumtllPiylLyaCcct-oivmnasetiedsdtiiabnteygdsoihmfyudmlrtooalrnyeesoitsuhrsaelny-
DeUni 30 distinct subtypes (Ramsey et al. 2006). Genetic studies have highlighted the importance of
Cell by TRPchannelsinnumerousbiologicalprocesses(e.g.,calciumadsorption,neuronalgrowthcone
v. guidance, keratinocyte development, and sensory transduction), and TRP channelopathies are
e
R associatedwitharangeofhumandisorders(e.g.,polycystickidneydisease,skeletaldysplasia,and
u.
n familialepisodicpainsyndrome)(Nilius2007).Thus,understandinghowthesechannelsrespond
n
A
tophysiologicalstimulianddrugsisofdirectclinicalandtherapeuticrelevancetodiseasesthat
affectvirtuallyeverymajororgansysteminthebody.However,whencomparedwithmostother
ionchannelsuperfamilies,suchasthoseofthevoltage-orligand-gatedilk,thestudyofTRPchan-
nelsisstillinitsinfancy,andendogenousstimuliorphysiologicalrolesformanyfamilymembers
remain unknown. Moreover, whereas functional TRP channels likely resemble voltage-gated
potassium channels in regard to their overall transmembrane topology and tetrameric subunit
composition, relatively little is known about their structure or structure-activity relationships.
These gaps in knowledge reflect several limitations, including (a) a dearth of pharmacological
agentsformanipulatingchannelfunctioninvitroorinvivo;(b)alackofdetailed(i.e.,crystallo-
graphic)structuralinformation(exceptforsomesolubleintracellulardomains);and(c)relatively
low sequence similarity among TRP family members, which makes identification of conserved
motifs and analysis of subtype chimeras challenging. As such, recent efforts have focused on
360 Julius
CB29CH13-Julius ARI 1September2013 10:5
discovering novel, subtype-selective pharmacophores (including toxins and other natural prod-
ucts);developingunbiased,high-throughputscreensforpinpointingfunctionallyimportantchan-
neldomains;andapplyingproteinpurificationandreconstitutionmethodstoprobechannelfunc-
tionindefinedsystemsusingbiophysicalandstructuralapproaches.Theselimitationsnotwith-
standing, somewhat rudimentary structure-function maps are beginning to emerge for certain
TRPchannelsubtypes,whicharediscussedhereprimarilyinthecontextofTRPV1andTRPA1.
AlthoughTRPchannelsarewidelydivergentintheirprimarystructuresand(knownorpre-
sumed) physiologic functions, many resemble ancestral fly channels in their ability to be acti-
vatedormodulatedbyPLC-coupledreceptors(Venkatachalam&Montell2007).Asinthefly
eye, mechanisms underlying these receptor-operated pathways remain enigmatic, but existing
results suggest that vertebrate TRPs have evolved to recognize distinct consequences of PLC
activation,withsomesubtypesrespondingtolipidmetabolismperseandothersrespondingto
inositol-triphosphate(IP3)-mediatedCa2+ release.Inthisway, neurotransmittersorhormones
g
s.or that activate PLC-coupled receptors can activate TRP channels or enhance their sensitivity to
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e otherphysiologicalstimuli.Indeed,someTRPchannelsrespondtomultiplephysiologicalinputs,
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w.annualrese only. etthhneeabccloeilnnltgoetrxhtteiosmfsunteoocefinucvneipcrttoiiononmn,aebsnepta.orDlsysemisgponidtiafielctsahingetnplayhloyinnsitopelagotrghaiwtcoaarlysrscelalepeavadabinnlecgeotfooafnsssoueccsishcienrpgetgocuorlmasetpinolsenixt—iczhawtahinoigcnheas,niinnd
wwal u consequentpainhypersensitivity—themechanisticandstructuraldetailsunderlyingPLC-evoked
m on channelmodulationremainlargelyunresolved.
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ded fror per knoAwsnm, heanmtiopnereidngaboouvre,abpihliytsyiotologuinccostvimeruilni fvoivromroosltesm, memecbhearnsiosmf tshoefTacRtiPvaftaimonil,yorresmtrauicntuurna-l
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84. Downlon 01/30/14. updnaetrdhievwrepadyiniranrriientagnnsotsotafabnsltedimesxpucildeupesrtidoteontxseicnbtsei)ocntahuaastnedetlhiccehiytanadrniesecltoagmragfteointregtd.obTryRpadPienfc,ehnpasrniovnveeidlnsianotgufrutahnleipqrusooedmauanctdtossp(eponlwsaonerrty--
5-3h o ful pharmacological tools with which to manipulate and probe channel function in a variety of
9:35burg biologicalsystems.Anotheradvantagederivesmoregenerallyfromthefactthatsensorysystems
2s mustevolvetodetectandtransducesignalsthataremostsalientforthesurvivalandreproduc-
v. Biol. 2013.versity of Pitt tftrihooanmtsopdfeiafcfniefryoersngtitamnspuiselmucsie(dsJeurtleeipuctrsieo&snenNotrsaotahthatnreersmf2ue0nn1cd2toi)o.unAsalsylsauptcotrhwi,becurotfuemsl.pwIanardayteitevode,iadtnheainsltyaisfpyisprroeofsasicdehuneshosarsoybrredeeconempqatuoiintress
DeUni fruitfulinthestudyofsensoryTRPchannelsandstandsincontrastwiththerelativelymeagersuc-
Cell by cessincomparingTRPchannelsubtypeswithinagivenorganism,wherelowsequencesimilarity
v. amongmembersofthesuperfamilyhasstymiedidentificationofconservedfunctionaldomains
e
R ortheanalysisofchimerasgeneratedfromTRPchannelhomologswithinagivenspecies.For
nu. thesereason,somatosensoryTRPshavebecomefavoredsubjectsforelucidatingbasicprinciples
n
A underlyingTRPchannelpharmacology,structure,andregulation.Whatfollowsisacurrentview
ofphysiologicalroles,activationmechanisms,andstructure-functionrelationshipsforthenoci-
ceptiveTRPchannels,TRPV1,TRPM8,andTRPA1.Thisisnotmeanttobecomprehensive
but,rather,tohighlightafewareasthatareparticularlyinterestingandrelevanttothestudyof
nociceptionandpain.
TRPV1:AHEAT-ACTIVATEDIONCHANNELANDINTEGRATOR
OFINFLAMMATORYSIGNALS
TRPV1 (also known as VR1) is the founding member of a subfamily of thermosensitive TRP
channels that enable primary afferent nociceptors, or other cells, to detect changes in ambient
temperature over a wide physiologic range (Julius 2005, Patapoutian et al. 2003). TRPV1 is a
www.annualreviews.org • TRPChannelsandPain 361
CB29CH13-Julius ARI 1September2013 10:5
noxious heat–activated channel with a steep temperature coefficient (Q > 20) and a thermal
10
activation threshold of ∼43◦C (Caterina et al. 1997). Genetically engineered (knockout) mice
lacking TRPV1 show partial deficits in acute thermal nociception, which attests to a role for
thischannelinheat-evokedpain.Importantly,productsoftissuedamageandinflammationcan
dramatically decrease this threshold, and genetic and pharmacologic studies have shown that
TRPV1 is an essential component of the cellular signaling mechanisms through which injury
produces thermal hyperalgesia and pain hypersensitivity (Basbaum et al. 2009, Caterina et al.
2000,Davisetal.2000).
TRPV1marksapopulationofunmyelinated,slowlyconductingneurons(C-fibers)thatexpress
theneuropeptidessubstanceP,neurokininA,andCGRPandconstituteapproximately30–50%
of all somatosensory neurons within rodent sensory ganglia (Kobayashi et al. 2005, Tominaga
etal.1998).Activationofthesefiberspromotespeptide-mediatedvascularleakage,vasodilation,
andotherparacrineactionsonbothexcitableandnonexcitablecellsinthevicinityofthetrauma,
g
s.or culminatingintheproductionandreleaseofacomplexmixtureofproinflammatoryandproal-
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e gesic factors (including neurotrophins, bioactive lipids, extracellular protons and nucleotides,
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w.annualrese only. ptthorueoscteehaf(saFecstig,ourasnraedc1tm)b.oaTnckohaiosmnpilrnooeccsae)l,sssceoonlfsloencreytrivvneeelrfiyvbereeerfn–eidrnrinietdgiasttteoodae(snnhtehuaenrocigenefltnhaiemci)rmisnaefltnoasrmityimvsiotayutiptoo.nOt,etmnocgpeeetrrhaeetleurarwseeiotdhr,
wwal u immunologicallyinitiatedinflammation,accountsforanimportantcomponentoftissueinjury–
m on evokedpainhypersensitivity(McMahonetal.2006,Meyeretal.2006).Indeed,TRPV1hascome
os
ded fror per tcoourenptsrefsoernatctuhteepsreenesmitiivnietynttomnoolexciouulasrhmeaatr,knereufororgdeenfiinciningfltahmemnoatciiocnep,taonrdsuthbeprompaullahtyiopnertahlgatesaica-.
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84. Downlon 01/30/14. Awrictitathibvlaaetwibooindweoerflastnhygneesderoofpmeppaett,ihdpoeaprnhgciyrcesiaCotli-otfiigsb,icemarsligclirokaneindlyeitichooennastdrianicbvhuoetle,vsianntgodiacncaflunatcmeermanpadatiinochn(rS,osznuoiclcchspa´aansiynaira&thssroSitcaiinsa,dtieord-r
5-3h o 2012).Importantly,neurogenicinflammationhelpstoinitiateacycleinwhichinjury-evokedno-
9:35burg ciceptor activation initiates or exacerbates the inflammatory response, leading to, for instance,
2s furtheractivationandsensitizationofthenociceptorandongoinginflammation.Thus,drugstar-
v. Biol. 2013.versity of Pitt gneetuiRrnoegimnthflareakmnabomlcyai,ctoesorpymtocarytcmolseae.ynsnoortyonnelryvedifimbienrisshexpparienssbruetcaelpsotofrascfiolirtamteohste,ailfinngotbaylli,nctoermruppotninengttshoisf
DeUni theinflammatorysoup,renderingthemcompetenttorespondtothesefactorsthroughavarietyof
Cell by signalingmechanisms(Julius&McCleskey2006).TRPV1isanimportanttargetformanyofthese
v. factors,whichenhancesensitivitytoheatbyloweringthechannel’sthermalactivationthreshold.
e
R TRPV1sensitizationcanoccurintwoways:Someproalgesicagents(e.g.,extracellularprotons,
nu. anandamide and other bioactive lipids, capsaicin, and peptide toxins) interact directly with the
n
A channeltoserveaspositiveallostericmodulators,whereasothers(e.g.,bradykinin,ATP,andnerve
growthfactor)stimulatetheirownreceptors(oftheGprotein–coupledortyrosinekinasevariety)
toactivatePLCorothersecondmessenger–signalingpathways.Inthisway,TRPV1functions
asapolymodalintegratorofthermalandchemicalsignals,modulatingnociceptorexcitabilityin
responsetochangesinthelocaltissueenvironment(Caterina&Julius2001,Huangetal.2006,
Tominagaetal.1998).Elucidatingstructuralandbiophysicalunderpinningsoftheseinteractions
remainsafascinatingandimportantgoalfrombothbasicandtranslationalstandpoints.
AnEmergingFunctionalMapofTRPV1
One relatively straightforward but important goal has been to identify regions of TRPV1 that
specify sensitivity to direct-acting chemical ligands, including exogenous irritants, endogenous
362 Julius
CB29CH13-Julius ARI 1September2013 10:5
proalgesicagents,andsyntheticpharmacophores.Thishasbeenaccomplishedthroughstandard
mutationalanalyses,guidedwherepossiblebyspecies-specificdifferencesinligandsensitivity.
Thevanilloidsite. Oneofthefirstsuchgoalswastoidentifyelementsgoverningsensitivityto
capsaicinandothervanilloidcompounds.Thiswasfacilitatedbyabitofneuroethologicalinsight
thatrelatestoaphenomenoncalleddirecteddeterrence,whereinmammalsaresensitivetothe
noxiouseffectsofcapsaicinandthusrepelledfromthechiliplant,whereasbirdsareindifferent
to capsaicin and therefore are evolutionarily favored as vectors for seed dispersal (Tewksbury
& Nabhan 2001). Analysis of chimeric rat/chicken TRPV1 channels, together with other site-
directedmutationalstudies,suggeststhatcapsaicinsensitivityisspecifiedbyresidueswithinthe
secondandthirdtransmembrane(TM2andTM3)regionsandtheinterveningcytoplasmicloop
(Jordt & Julius 2002), consistent with patch-clamp-recording studies suggesting that vanilloid
agonistspenetrateorcrossthemembranetoactivateTRPV1fromtheintracellularfaceofthe
g
s.or bilayer(Jungetal.1999).Inasimilarvein,speciesdifferencesinefficacyand/orpotencyofother
w
e vanilloidcompounds,suchasresiniferatoxin,anultrapotentTRPV1agonistfromEuphorbiacactus,
vi
w.annualrese only. osheringchsailptiigsvahizttyeTp(iGMnea2,vavtahsyrenottuhagleh.ti2cT0aM0n4t4,agJaooshntnihssteo,nhreaegvteioaanll.sco2o0bn0et6ean)i.neiTnxpgalkoaeintpeudtottagoteimvtheaepvra,rntehislieldosueidessbttuihndaditeisnspgceocpniofsyciskltieegtna,tnilndy
wwal u whichthevanillylmoietyorthealiphaticsidechainisproposedtointeractwitharomaticand/or
m on otherhydrophobicresiduesinthecytoplasmicloopbetweenTM2andTM3(Figure4a).Atthis
os
ded fror per paohiingth,-hroewsoelvuetrio,anmstorurectaucrceuoraftaelaingdandde-tcahilaendnpeilcctuormepolfetxh.evanilloidbindingsitewilllikelyrequire
aF
84. Downlon 01/30/14. raesaasFnoranonsmd.aFamirpisdth,eytsahinoedlvooagntihcileallroisbdtiaobnaidcnptdiovinienglti,spisitduesclhitkheianltyfobsrepmaercasitfitiroeunscitwnuitrlealrlbasceitmihoiilngahrwiltyiythstiogenncdiafipocsgaaeninctionfuo(srsofaa-ccctaoolurlespdlseuecnoh-f
5-3h o dovanilloids)(Jordt&Julius2002).Thus,thevanilloidsiterepresentsanimportantlocusatwhich
9:35burg certaincomponentsoftheinflammatorysoupenhancethe(mammalian)channel’ssensitivityto
2s heat.Second,weknowthatforsomespecies,thevanilloidantagonistcapsazepineblocksactiva-
v. Biol. 2013.versity of Pitt tsmitoreuncchntauonrtiesomann(lysd)brbeyylacwtaiphvsiecahilcoiccnhatebimounticaoallsfoatnhbdiysphkheeyyastaic(lTlaolosstmteimrinicuaglsiiatgeeattweaillt.lh1lei9kc9eh8lay)n.pnTreohlv.uisd,edgerlienaetaitninsigghthteinattoomthiec
DeUni
Cell by Theprotonsite. Localtissueacidosisisahallmarkofinjuryandinflammation,andextracellular
v. protonsconstituteaprominentcomponentoftheinflammatorysoup.Protonsexciteorsensitize
e
R nociceptors by interacting with numerous molecular targets, including members of the acid-
nu. sensingionchannel(ASIC)andTRPchannelfamilies(Holzer2009,McMahonetal.2006,Meyer
n
A etal.2006).GeneticandphysiologicalstudiessuggestthatASICscontributesignificantlytoacute
acid-evokedischemicpaininskeletalorcardiacmuscle,whereasTRPV1playsaprominentrole
in acid-evoked sensitization of cutaneous and visceral nociceptors (Leffler et al. 2006, Sugiura
et al. 2007, Yagi et al. 2006). ASIC channels can be activated by relatively mild decreases in
extracellularpHandaccountforthelarge,transientcomponentofproton-evokedcurrentsseenin
avarietyofsensoryneuronsubtypes.TRPV1isactivatedatlowerpHvalues(threshold∼pH6at
roomtemperature)andaccountsforthesmaller,persistentphaseofproton-evokedcurrentsseen
inmanysmall,unmyelinatedneurons(Leffleretal.2006).Whereasprotonsserveasbonafide
TRPV1agonistsunderconditionsofextremeacidosis(pH<6),theyfunctionaspositiveallosteric
modulators under more moderate acidic conditions (pH 6.5), greatly enhancing sensitivity to
heat,capsaicin,orotherinflammatoryagents.Thesedualactionslikelycontributetonociceptor
sensitizationandpainhypersensitivityunderavarietyofpathologicalconditions(e.g.,infection,
www.annualreviews.org • TRPChannelsandPain 363
CB29CH13-Julius ARI 1September2013 10:5
a
A PLC
A PIP2
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v. Biol. 20versity of TRPV1
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Cell by
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e
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n C C
A N N
Figure4
Anemergingstructure-functionmapforthecapsaicinreceptor,TRPV1.(a)Pharmacologicalandmutagenesisstudieshaveidentified
TRPV1domainsthatconfersensitivitytovariousstimuliorcontributetophysiologicalmodulationofthechanneldownstreamof
metabotropicreceptors.Theseincludesitesofactionforcapsaicin(chilipepper)andrelatedvanilloidligands,extracellularprotons
(lemon),orpeptidetoxinsfromtarantula(spider).Regionsimplicatedinchannelmodulationbycellularproteinsandcytoplasmicsecond
messengersarealsoindicated.(b)Thebivalentdoubleknottoxin(DkTx)fromtheEarthTigertarantulabindstotheouterporeregion
ofTRPV1,lockingitinitsopenstate.Fourresidueswithinthisregiondefineaputativefootprintfortoxinbinding,illustrated
schematicallyinacrosssectionofthechannelshowingjusttwodiagonallyopposedsubunitsforclarity.(c)Top-downviewofproposed
DkTxbindingsiteonahomotetramericTRPV1channel.Orangeandredspheresrepresentlocationsofaminoacids(onneighboring
channelsubunits)thathavebeenimplicatedintoxinbinding.TheTRPV1poredomainwasmodeledafterthestructureofthebacterial
KcsApotassiumchannel.PanelsbandcwereadaptedwithpermissionfromBohlenetal.(2010).Abbreviations:A,ankyrinrepeats;
C,carboxyterminus;N,aminoterminus;PLC,phospholipaseC.
364 Julius