• Understandmaintypesofsignaling mechanisms
• Understandconceptofreceptorandligand
• Understandmembrane,cytoplasmicandnuclearreceptors
• Briefunderstandingofsteroidhormonepathway
• Briefunderstandingproteingrowthfactorpathway
• Understanddifferencesbetweenselectmembranereceptors
(Gproteinstructure,tyrosinekinases)
Objectives
• 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
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