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صبا المسیمي

تسنیم الرواضیة

دیاال

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ExtraCellularMatrix(ECM)

*TheECMisaspacethatsurroundsthecellandisoccupiedbyagroupofmoleculesthatfacilitateinteractionsbetweencellsaswellasthesurroundingenvironment.

*Insometissues,theECMisthemajorcomponent,forexampletheconnectivetissues(e.g.,cartilage,boneandtendon),unlikeepithelium,epithelialcellshaveaminimalamountofECMwhichisfoundmostlyinthebasallamina(basementmembrane).

*TypesofECM:

1-basallamina:

-isathinsheetlikestructureuponwhichlayersofepitheliallayersrest

-itseparatesepithelialcellsfromunderlyingconnectivetissue.

-itsupportssheetsofepithelialcells

-itsurroundsmusclecells,adiposecellsandperipheralnerves.

2-Connectivetissue(Bone,Tendon,Cartilage)

-Loosenetworkofproteinsandcarbohydratesunderneathepithelialcelllayerswherefibroblastsaredistributed

DifferenttissueshavedifferentECMcomponentshowever,therearecertaincomponentsthatarefoundinallECMs:

-Proteins:

1-fibrousproteins:”toughfibrousproteinsembeddedinagellikepolysaccharidegroundsubstance”FROMSLIDES

2-adhesionproteins:connectdifferenttypesofmolecules

-polysaccharides:interactwithcoreproteinstoform(proteoglycans=glycosaminoglycans(GAGs)+coreproteins)

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DifferentECMCompositions

A)Bone:

Osteocytessitinlacunaeandarewidelyseparated

MostofthebonetissueisECM.ECMishardenedbydepositionofinorganicmolecules(calciumphosphatecrystals)whichisimportantforthenature,propertiesandfunctionofthebonetissue.

B)Cartilage:

ItsECMisflexiblebecauseitconsistsoflargenumberofproteoglycans,proteoglycansaremainlyofsugarswhicharepolarmoleculesthathavetheabilitytointeractwithwatermolecules,producinggel-likestructurethatallowsthecartilagetoperformitsfunctionasshockabsorbent(cushion).Oneofthemostabundantproteoglycanspresentinthecartilageishyaluronicacid.

C)ConnectiveTissue:

Tendonscontainahighproportionoffibrousproteins(elastic-thefinefibersandcollagen-thethickfibers,theygivethemechanicalstrength).

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Tendonsattachmusclestobonessoitisexposedtobigforcesespeciallyincontractionofthemusclemustbemechanicallystrongandhavesomesortofflexibilityatthesametimebecauseitmustmovewiththemuscle.

Nowlet’stalkindetailsaboutthegeneralcomponentswe’vementionedabove:1)Fibrousproteins:

a)Collagens

-themostabundantproteinpresentinourbodies,it’sastructuralproteinarrangedintriplehelices,it’scomposedofhighlevelsof(Proline13%,hydroxyproline9%andGlycine33%).Bothprolineandlysinegetintohydroxylationprocess,inwhichweaddOHgroup,thismodificationisposttranslational

-Hydroxylysinespecificallyservesasattachmentsiteforsugar,makingitaglycoprotein.Also,therearecrosslinkingprocessesthathappenbetweenHydroxylysineandLysinetoattachcollagenmolecules.

Therearemanytypesofcollagenmolecules

1)Fibrilformingcollagen(type1)

2)Fibrilassociatedcollagen(type9,12)

3)Networkformingcollagen(mostlyinthebasallamina,Type4)

4)Anchoringfibers(associatenetwork-formingcollagentofibril-formingcollagen)

5)Transmembrane(theyparticipateintheinteractionbetweenthecellanditsECM).

Likeanyotherprotein,collagenisencodedbyageneandisgoingtobetranscribedandsynthesizedbyribosomes,interactionbetweenthreepolypeptidechains,toformthebasicunitsofmaturecollagenwhichiscalled(TropoCollagen)thisbasicunitisgoingtoberepeatedmanytimestoformfibrilsandthenfibers.Fibrilsare

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tropocollagensthatarepartiallyoverlappedtodistributethespacesinbetweenthemindifferentregionstoavoidcreatingweakpoints.Associationoffibrilsformstructurescalledfibirswhichareabout10timesthickerthanfibrils.

Nowe'regoingtotalkaboutsomedetails,

Insidethecell,proteinaresynthesizedbyribosomestheygettranslocatedintotheER,wherehydroxylationofprolinehappens

Then,hydroxylysinesareglycosylatedandthen3polypeptidechainsaregoingtointeracttoformastructurecalledprocollagen,thenitisexcretedbyexocytosisandfurthermodificationsaregoingtohappeninsideECMincludingcleavageofthegreenareasthatarenotmadeintoheliceslikepropeptidesbyanenzymecalledpropeptidase,nowthetropocollagenisreadytobeassembledintofibrilsandthenfibers.

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Applications:

1)Scurvy:adiseaserelatedtodeficiencyinvitaminC.VitaminCisessentialinhydroxylationofproline.hydroxylationistheadditionofOHgroupwhichformsthehydrogenbondsinthecollagenmolecule,makingitastrongmolecule,soifthereisnoOHgroup,thecollagenstructurebecomesweakercausinglacerationsinthetissuesthatcontaincollagenlikegums,skin,andbloodvessels.

2)Osteogenesisimperfect:imperfectboneformation,ageneticdiseasewhichisautosomaldominant(onecopyoftheabnormalgeneisenoughtocauseit.CausedbymutationsinseveralcollagentypeslikeCOL1A1andCOL1A2resultingindifferenttypeswithdifferentseverities.

Type1isthemildestformandtype2isthemostsevereandmightcausedeathofthefoetusinthewomb.itcausesdeformitiesintheskeletonlikethecurveshowninthex-ray,weakstructureintheirbones(brittlecanbebrokeneasily).

3)Chondrodysplasia:relatedtocartilageduetomutationsintype2collagen.Cartilageisfoundintheepiphysealplateswheregrowthandthenossificationofthebonehappen.so,abnormalstructureofthecartilageisgoingtoresultindwarfism(thetypeofdwarfismwherethewholebodyisshortbutthefaceisnormal.Itcanalsocausejointproblemsbecauseabnormalquantityorqualityifcartilageinthejoint.

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4)EhlersDanlossyndrome:Causedbymutationsinmorethanonetypeofcollagen(ex:TYPE1,3,5)orinenzymesthatmodifyandassemblecollagen,sotheyaffectthestructureofcollagen.Signsandsymptomsdependontheunderlyingmutation.Someoftheuniquefeaturestheyhaveisthehyperextensibilityoftheirskinandhypermobilityoftheirjoints

b)ELASTIN

ThesecondtypeoffibrousproteinintheECMiselastinwhichformselasticfibers,theyaddelasticityandflexibilitytothetissue,sotheresizeissmallerthancollagen.Theyhavecommonfeatureswithcollagenandsomedifferencesalso.

Comparisonbetweenfeaturesofelastinandcollagen:

1-Theyarerichinlysineandprolinelikecollagen,buttheydon’thaveasmuchhydroxyprolineascollagen.

2-Theydon’thydroxylysinewhichmeansthatthereisnoglycosylationontheirproteins.

Bystudyingtheprimaryfeaturesofelastinthefoundthattheresegmentsintheaminoacidsequencethatgivethepropertiesofelastin(ex:hydrophobicstretchesgiveelastinitselasticityandalanine-lysinerichstretchescontributetothecrosslinkingofmoleculestogether.

Synthesisandassembly:

1-Synthesisstartsinribosomes

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2-TheyentertheERmodificationstartsincludinghydroxylationofproteins

3-stilltheelastinhaspropeptidesthataregoingtobecutintheECMusingaproteincalledtropoelastin,itcutstheextrapart

4-Assembly(association)oftropoelastinwithmicrofibrillarproteins(othertypeofproteins)toformastructurethatisthencrosslinkedtoformthestructureofelastin.

Thereisadifferencebetweenelastinandcollagenformationespeciallyintheend(inassembly):thereismicrofibrillarproteinsandtheirstructureisgoingtogetassembledinadifferentwaythancollagen.

Application

Elastinpresenceinthewallsofarteriestunica(layer)internaisimportantingivingflexibilitytothearteriesandmakingthemreturnbacktotheirnormalpositionaftereatingsaltwhichincreasestensioninthearterialwallsesspeciallythatthearterialwallsarethick.Ifthereisamutationinelastinordeficiencyorabnormalstructurewhathappensisthatsmoothmusclestrytocompensatesotheyconstrictbutbecausetheyarenotelastic,theycannotgetbacktotheirnormalpositioncausingnarrowing,construction,highpressure…..

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Microfibrils

Anelastincoreiscoveredwithasheetofmicrofibrils.Microfibrilsarecomposedofanumberofdistinctglycoproteinsincludingthelargeglycoproteinfibrillin,whichbindstoelastinandisessentialforthe

integrityofelasticfibers.Itispresentinhairandspermtail.

Clinicalapplicationsofdiseasesrelatedtoelastinfiber

1-Marfun’ssyndrome:ageneticdiseasemutationsinfibrillin(themicrofibrillarprotein)causesabnormalstructureofelasticfibersleadingtoruptureofaorta,longextremities,abdomenisretorted.

2-Emphysema(destructivelungdisease)

Mostlycausedbycigarettesmoking,thelunghaslungsacksfilledwithairduringinhalationforgasexchange,thenduringexhalationitbecomesempty.Itisflexible(itexpanselikeaballoon)Ifwehaveanormalstructureofelasticfibersthatmeansthattheylosttheirelasticity,causingdeficiencyinbreathing.ToxinsincigarettesinhibitanenzymecalledAlphaoneantitrypsinonceAlphaoneantitrypsinhaslowactivity,overActivationofanenzymecalledelastasehappenswhichdestructselasticfiberssowelosethatqualityofthelungsacs,EmphysemamightbecausedbyageneticmutationinAlphaoneantitrypsincausingit’sdeficiency.

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Polysaccharides

PolysaccharidearethesecondcomponentoftheECMwearegoingtotalkabout,theyareglycosaminoglycans(GAGs)whicharedisaccharideunitsrepeatedsomanytimestoformapolysaccharidestructure,thedisaccharideiscomposedofmodifiedsugars(oneofthesugarsisaminatedlikeN-AcetylglucoseamineorN-Acetylgalactoseamineandtheothersugariseitheroxidizedwhichmeansthatoneofitsgroupsbecomesacarboxylgroupandthesugarbecomesacidicorsulphatedwhichmeansithasanegativechargefromthenegativesulphategroup.)sothesedisaccharidesarehighlypolar,andhighlychargedtheyattractwatermoleculestothemtheyformgellikestructuresandtheyactascushions(shockabsorbers)

Gagsalwaysassociatewithcoproteinsasaminorcomponenttoformthestructureofproteoglycans.

Therearemanykindsofproteoglycans,somecanbefoundfloatingintheECMandothersarecellsurfaceproteinswitheithertransmembranedomains(syndecans)orGPIanchors(glycipans)interactingwithintegrins.

Examplesonmatrixpolysaccharides

1-Aggrecanandhyalronan:hyalronanisthemajorproteoglycanofcartilageECM.

Agrecaniscomposedofcoreproteinandchondroitinsulphate(hairlikestructure)anditcanaggregatewithhyaluronantomakeahighlypolarwaterattractingstructure.

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2-Perlecan:foundinthebasallaminaspecificallyanditinteractswithothercomponentsoftheECMlikecollagentype4andlaminin,soitcontributesinthestructureofbasallamina.

Matrixadhesionproteins

ThesearethirdcomponentoftheECM.Therefunctionisadhesionbetweenfibrousproteinsandpolysaccharidesexamples:

1- Fibronectin:itisaproteinthatformsdimers,thesedimersarecrosslinkedbydisulphidebridges.theyhaveasiteforbindingproteoglycans,siteforbindingcollagen(afibrousprotein),andsiteforbindingtothecells.

2-Laminin:itconnectsbetweencollagenandperlecan(aproteoglycan)itsshapeislikeacrossmadefrommorethanonechain:alpha,beta1andbeta2chainsareinterwindaroundeachother.Intheareawherethesechainsareinterwind,thereisasiteforcellbinding.Onthestructurethereareothercellbindingsites,collagenbindingsitesentactinbindingsite,andproteoglycanbindingsite.Sotheyhavebindingsitesforfibrousproteins,proteoglycans,aswellascells.

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HowdoestheECMinteractwithcells?

BycellularadhesionsbetweencellandECM(notbetweencells)like:

1-Focaladhesions:formedfromintegrins+talin+vinculin.Connectedtoactinfilament.

2-Hemidesmosomes:formedfromintegrins+plectin(aconnectorproteinthatisconnectedtointermediatefilamentsfromtheinsideandtotheECMfromtheoutside).

INTEGRIN(foundinbothfocaladhesionsandhemidesmosomes)iscomposedoftwoproteins:alphaandbeta,eachproteinhasadifferentbindingsitetothematrixprotein.Integrinhas2confirmations,bindinganddetachment.Whenitisinactive,itisbentandawayfromtheECM.Onceitisactive,itcanbindtotheECMandformfocaladhesions,theseconformationalchangeshappenwhilethecellsmove.Integrinisdeactivatedinthesitewheredetachmentisneededandisactivatedintheleadingedgewherebindingandfocaladhesionformationisneeded.

TypesofECMinteractions

InteractionsintheECMaredividedintotwotypesaccordingtotheirtype:

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1-Homophilicinteractions:interactionbetweentwoproteinsfromthesamekind.

2-Heterophilicinteractions:interactionbetweentwodifferentkindsofprotein.Forexampleintegrinsarefoundinfocal

adhesionsaswellashemidesmosomes;theyinteractwiththeECMortheymightinteractwithagroupofproteinscalledIGsuperfamilyimmunoglobulin.Igsuperfamilyisdifferentthanintegrinsoitisconsideredthetroublethinksoitisconsideredheterophilicinteraction.ImmunoglobulinIGsuperfamilyalsointeractwithintegrinsbuttheydon'tproduceanycellularjunctionsandtheymightmakehomophilicinteractions.Cadherinscanmakehomophilicinteractionsandtheyproduceadherencejunctionsandsomekindsarefoundindesmosomes(theyarecalleddesmosomalcadherins).Thelastoneisselectin,itinteractwithsugarsfoundonglycoproteins.

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Inthefollowingpicture,don’tmemorizetheexamplesonlyknowthatthefirsttwoshowahomophilicinteractionandthesecondtwoshowa

heterophilicinteraction,thelastoneshowsthattheinteractionisnotnecessarilybetweentwomembraneproteins,itcanbebetweenamembraneproteinanfibronectinintheECM.

Nowe'regoingtotalkabouthowselectinsinteractwithcarbohydratesandhowthiswouldhelpusintheextravasationofleukocytesfrombloodvesselstothesiteofinjury(howitgetsoutbetweentheendothelialcellsuntilitreachesthesiteofinjury.)inthepicture:

Bluebloodstream

Pinkendothelialcells(liningofbloodvessels)

Thecirclesaretheleucocytesthataregoingtogetoutofthebloodvessels.

Onthesurfaceoftheleucocytesthereisatypeofselectin,theLselectin

OntheepithelialcellthereareE-selectinsandP-selectins.

*selectinsdon'tinteractwitheachother,interactionhappensbetweenselectinandsugarsofglycoproteins.

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1-InteractionbetweenPandE-selectinandglycoproteinsonthesurfaceofleukocytes.

2-PandE-selectins‘grab’aleukocyteandpullitnearthecellmembrane.

3-Signalingpathwaysareactivatedinsidetheleukocyte,whichwillinduceaccumulationofintegrinsinoneregion,theseintegrinsbindtootherproteinsonthesurfaceofendothelialcellscalledintracellularadhesionmolecules(ICAM),ICAMisapartifIGSF,IGSFinteractwithintegrinsinaheterophilicinteraction.

4-Westartedwithaninteractionbetweenselectinandcarbohydrates,thenintegrinsandIGSF.Nowthattheleukocyteisveryclosetothecellmembrane,itcanbepulledinsidethebloodvesselandallowedtotraveltothesiteofinjury.

GOODLUCK!!!