CellSignaling 1- CellBiology

DrKirstyWaltersk.walters@unsw.edu.au

• Understandmaintypesofsignaling mechanisms

• Understandconceptofreceptorandligand

• Understandmembrane,cytoplasmicandnuclearreceptors

• Briefunderstandingofsteroidhormonepathway

• Briefunderstandingproteingrowthfactorpathway

• Understanddifferencesbetweenselectmembranereceptors

(Gproteinstructure,tyrosinekinases)

Objectives

Cellsignaling basicsSignalmolecule

Targetcell

Specificreceptor

Initiatesaresponseinthetargetcell

• Manydifferentkindsofmoleculestransmitinformationbetweenthecellsofmulticellularorganisms.

Signaling moleculesandtheirreceptors

• Somemoleculescarrysignalsoverlongdistances,whereasothersactlocallytoconveyinformationbetweenneighbouringcells.

• Thesemoleculesactasligandsthatbindtoreceptorsexpressedbytheirtargetcells.

• Thereisconsiderablevariationinthestructureandfunctionofthedifferenttypesofmoleculesthatserveassignaltransmitters.

• Somesignaling moleculesareabletocrosstheplasmamembraneandbindtointracellularreceptorsinthecytoplasmornucleus,whereasothersbindtoreceptorsexpressedonthetargetcellsurface.

ContactMediated Communication• displaymoleculesoncellsurface,recognizedbyreceptoron

anothercellNon-ContactMediatedCommunication• chemicalsignal,nearbyoratadistance

CellCommunication• Cellsinhigheranimalscommunicatebymeansofhundredsofkindsofsignalmolecules.• Theseincludeproteins,smallpeptides,aminoacids,nucleotides,steroids,retinoids,fattyacid derivatives,andevendissolvedgasessuchasnitricoxideandcarbonmonoxide.

DirectCell-CellSignalingDirectcellcontact

Example- integrins andcadherins arecelladhesionmolecules (CAMs).CAMs arelocatedonthecellsurfaceandare involvedinbinding withothercellsorwiththeextracellularmatricintheprocesscalledcelladhesion.

Butintegrins andcadherins functionnotonlyasCAMsbutalsoassignaling moleculesthatregulatecellproliferationandsurvivalinresponsetocell-cellandcell-matrixcontacts.

ModesofSignaling

ModesofSignaling

AutocrinesignalingCellproducesasignaling moleculetowhichitalsorespondsExample:Theresponseofcellsofthevertebrateimmunesystemtoforeignantigens.Tlymphocytescandrivetheirownproliferation.

ParacrinesignalingAmoleculereleasedfromonecellactslocallytoaffectnearbytargetcells.Example:Theactionofneurotransmittersincarryingsignalsbetweennervecellsatasynapse.

Signaling viatheactionofsecretedsignaling moleculesEndocrinesignalingHormones(atadistance)arecarriedthroughthecirculatorysystemtoactondistanttargetcells.Example:Estrogenproducedbyovary,transportedtothebrainwhereisactstoregulatefemalereproductivesystem.

• Receptorscanbelocatedonthetargetcellsurfaceorinsidethetargetcell.

• Ifinsidecell,signalmoleculehastoenterthecelltoactivateit.

WatersolublesignalmoleculesWater-solublesignaling moleculescannotdiffuseacrosstheplasmamembranesoallbindtocell-surfacereceptors.

ChemicalSignalTypes

LipidsolublesignalmoleculesMany lipid-solublehormonesdiffuseacrosstheplasmamembraneandinteractwithreceptorsinthecytosolornucleus.

Signaling Molecule• Synthesis• Releasebysignaling cell• Transporttotargetcell• Detectionbyaspecificreceptorprotein• Changebyreceptor-signalcomplex(trigger)

- Thebindingofaligand toitsreceptorcausesaconformationalchangeinthereceptorthatinitiatesasequenceofreactionsleadingtoaspecificcellularresponse.

• Therecanalsobeafeedbackmechanismfollowingsignaling.

ExtracellularSignalSteps

• Differentcellsmayhavedifferentsetsofreceptorsforthesameligand,eachofwhichinducesadifferentresponse.

• Orthesamereceptormayoccuronvariouscells,andbindingofthesameligandmaytriggeradifferentresponseineachtypeofcell.

Fourmainclasses• Gprotein - coupledreceptors- LigandbindingactivatesaGproteinwhichactivatesorinhibitsanenzyme thatgeneratesaresponse.

CellSurfaceReceptors

• Receptorswithintrinsiccatalyticactivity - Severaltypesofreceptorshaveintrinsiccatalyticactivity,whichisactivatedbybindingofligand.

• Tyrosinekinases- linkedreceptors - Thesereceptorslackintrinsiccatalyticactivity,butligandbindingstimulatesformationofadimericreceptor,whichtheninteractswithandactivatesoneormorecytosolicprotein-tyrosinekinases.

• Ion-channelreceptors - Ligandbindingchangestheconformationofthereceptor sothatspecificionsflowthroughit;thisalterstheelectricpotentialacrossthecellmembrane.

CellSurfaceReceptorsLigandbindingtocell-surfacereceptor

• triggerintracellularpathways• modulatecellularmetabolism,function,ordevelopment

Removalofthesignal• Oftenterminatescellularresponse

SecondMessengers

• Cyclicnucleotides- cAMP,cGMP(seeGproteinsignallinglecture)• CalciumIons• Inositol1,4,5-trisphosphate(IP3)IP3receptorplaysintheconversionofnumerousexternalstimulitointracellularCa2+signal• Diacylglycerol(DAG)- modifiedlipidactivatesPKC• ProteinKinaseA- PKA,B,C(phosphorylateotherproteins)

• Acttoregulatetheactivityofcellularproteins• Theelevatedintracellularconcentrationofoneormoresecondmessengersfollowingligand bindingtriggersarapidalterationintheactivityofoneormoreenzymesornon-enzymaticproteins.

Thebindingofligandstomanycell-surfacereceptorsleadstoashort-livedincrease(ordecrease)intheconcentrationoftheintracellularsignaling moleculestermedsecondmessengers.

CellSurfaceReceptors/SecondMessengerThetwocell-two-gonadotrophinsystemforestradiol synthesisintheovarianfollicle.

LHandFSHareshowntostimulateadenylatecyclaseviaG-protein-couplesreceptors.

ThecyclicAMP(cAMP)generatedfromATPactivatesproteinkinaseAtostimulateexpressionoftherespectivesteroidgenicenzymesinthecaandgranulosacells.

GDP,guanosinediphosphate,GTP,guanosinetriphosphate

• SteroidHormones(androgens,glucocorticoids,mineralocorticoids)• thyroxine(thyroid,notasteroid)• vitaminD3• retinoicacid• cortisol(adrenal)• estrogensandtestosterone(gonad)• (longestactingofhormoneclasses,hours-days)

• IntracellularReceptorsrespondtosignaling moleculesthatareabletodiffuseacrosstheplasmamembrane.

IntracellularSignaling/SteroidResponses

Canallentercellsbydiffusingacrosstheplasmamembrane

• Steroidhormonesarelesssolubleinaqueoussolution,mostaretransportedinthebloodbycarrierproteinssuchasspecificplasmaglobulinsoralbumin(synthesisedintheliver)

Example- DHT,Tbindtosexhormone-bindingglobulin(SHBG)Mainfunction:transportsteroidwithinbloodstreamtotargettissues

• Allsteroidhormonesaresynthesizedfromcholesterol.

SteroidSynthesis

2 cell, 2 gonadotrophin theorySteroidactions

Cytosollocation• receptorboundtoinhibitor• ligandbindingactivatesreceptor• translocatestonucleusonligandbinding

SteroidHormoneReceptors(SHRs)Whensteroidhormonesbindtoreceptors,theyactivatethereceptors,whichbindtoDNA toregulatethetranscriptionofspecificgenes.

Nuclearlocation• bindsligandandDNA• becomes transcriptionfactor

receptorinactivewhenboundtoinhibitory complex

ligandbindsandalterstheconformation ofthe receptor protein

inhibitorycomplexdissociates

receptorbindstocoactivatorproteinsthatinducegenetranscription

SteroidReceptorStructureNuclearreceptorscandirectlybindtoDNA andregulatetheexpressionofadjacentgenes,hencetheyareclassifiedastranscriptionfactors.

Mostmembershaveacommondomainstructureconsistingof:- amino-terminalactivationdomain(AF-1)

- DNA-bindingdomain- controlswhichgenewillbeactivated

- hingeregion- controlsmovementofreceptortothenucleus

- acarboxy-terminal ligand-bindingdomain(LBD).

LBDspecifiestheligand-bindingpropertiesandligand-regulatedinteractionswithco-activatorsandco-repressors(regionknownasAF-2).

Steroidsignaling

Nuclearreceptor(NR),intheabsenceofligand,islocatedinthecytosol.HormonebindingtotheNRtriggersdissociationofheatshockproteins(HSP),dimerization,andtranslocation tothenucleus,wheretheNRbindstoaspecificsequenceofDNA knownasahormoneresponseelement(HRE).ThenuclearreceptorDNAcomplexinturnrecruitsotherproteins thatareresponsiblefortranscription ofdownstreamDNAintomRNA,whichiseventuallytranslatedintoprotein,whichresultsinachangeincellfunction.

48humannuclearreceptortypesSteroidhormonereceptorfamily

TypeIReceptors- locatedinthecytosol• Sexhormonereceptors(sexhormones)- Androgenreceptor,

Estrogenreceptor,Progesteronereceptor• Glucocorticoidreceptor(glucocorticoids)• Mineralocorticoidreceptor(mineralocorticoids)

ReceptorTypes

TypeIIReceptors- locatedinthenucleus• VitaminAreceptor• VitaminDreceptor• Retinoidreceptor• Thyroidhormonereceptor

Orphanreceptors- receptorthathasasimilarstructuretootheridentifiedreceptorsbutwhoseligand hasnotyetbeenidentified.

• Severaltypesoflipids serveassignaling moleculesthat,incontrasttothesteroidhormones,actbybindingtocellsurfacereceptors.

• Themostimportantofthesemoleculesaremembersofaclassoflipidscalledtheeicosanoids,whichincludes prostaglandins,prostacyclins,thromboxanes andleukotrienes

Eicosanoids

• lipidsactingassignaling molecules- madefromessentialfattyacids(EFAs)oxidation(arachidonicacid)

• actbybindingtocellsurfacereceptors• rapidlybrokendown• actlocallyinautocrineorparcine signaling pathways

• Stimulatevarietyofresponsesintargetcellincludingbloodplateletaggregationandinflammation(anti-inflammatorydrugsactbydownregulatingeicosanoidsynthesis)

• Thegasnitricoxide(NO)isasimplegas,veryquickshort-livedsignal• Synthesisedfromargininebytheenzymenitricoxidesynthase• NOdiffusesoutofthecellandcanactlocallytoaffectnearbycells.

NitricOxide

• Majorparacrinesignaling moleculeinthenervous,immune,andcirculatorysystems.• LikesteroidhormonesNOisabletodiffusedirectlyacrosstheplasmamembrane ofitstargetcells.• Ratherthanbindingtoareceptorthatregulatestranscription,NOalterstheactivityofintracellulartargetenzymesExampleofNOactionissignaling thedilationofbloodvessels.(Themedicaluseofnitroglycerin intreatmentofheartdiseaseisbasedonitsconversiontoNO,whichdilatescoronarybloodvesselsandincreasesbloodflowtotheheart)

SummaryofactionofproteinsandsteroidsP SProtein

hormoneSteroidhormoneBindstocell

surfacereceptor

Activationofeffectorenzymes

Generationofsecondmessengers

Changesinenzymeactivity

Biologicaloutput

Activationoftranscriptionfactors

RegulationofGenetranscription

Diffusesacrosscellmembrane

P

SCytoplasmicornuclearreceptor

Hormone-activatedreceptor

ReceptorbindstoDNAelementsIntargetgenes

asadimer

Plasmamembrane

Nucleus

Cytoplasm

Proteinsynthesis

mRNAsynthesisCytoplasm

RegulationofGenetranscription

Proteinsynthesis

mRNAsynthesis