BIPN 144 Week 7 Notes (part1)

BIPN 144 Week 7 Notes (part1) - BIPN144Week7Notes(Part1)

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Unformatted text preview: BIPN144Week7Notes(Part1) Note:ThishasnotbeenreviewedbyDr.Bierandmayhaveerrors.Always trusttheprofessoroverthesenotes. DrBierstartedlecturebyreviewingthestructureofthedrosophilaCNSandventral midline(VML).Itessentiallylookslikealadder.Theverticalpiecesarecalled longitudinasandtherungsarecalledcommisures.TheVMLisparalleltoand exactlybetweenthetwolongitudinals. TheVMListhesourceofnetrin.Netrinisasignalwhichinteractswiththeunc40 (worm)receptorandtheunc5receptor(worm). Netrinattractsunc40expressingaxonsandrepulsesunc5expressingaxons. Mutantwormreview: Unc6mutantproducesnoNetrin,nervoussystemgoescrazy. Unc40mutantaxonsthatareusuallyattractedtonetrinarenowseverelyconfused astheylackthereceptorsthatinteractwithnetrin,butaxonswithunc5arefine Unc5mutantaxonswhicharegenerallyattractedtonetrinstilgotherightway,but axonswhichareusuallyrepulsedbynetrinarenolongerrepulsed. Manycreatureshave2differenttypesofnetrin.Andothermidlineattractantsdo exist. DrBierthenintroducedtwonewgenesRoundabout(Robo)andSlit. Robomutantsareodd,theaxonscrossthemidlinethenheadbacktowardsthe midlineagain Slitmutantallaxonsthatarenormallyattractedtothemidlinegetstuckatthe midline. Roboisthereceptorforslit,buttherearethreedifferentRobos(robo,robo2and robo3).Ifyouknockoutall3robos,yougetthesamephenotypeasslit. Itturnsoutthatroboisrepulsedbyslit,andslitispresentatthemidline.Sowhydo theseaxonsapproachthemidlineinthefirstplace? AsignalcalledCommisureless(comm)preventsRobofrombeexpressedonthecell surfaceuntiltheaxoncrossestheVML.OncetheaxoncrossestheVML,comm.Stops repressingrobo,andtheaxonsarethenrepulsedbyslitintheVML.Repulsionwins overattraction. SeeImagebelow. Next,DrBierexplainedhowaxonswhichprojecttomusclesintheflyfindtheir targets. WEfocusedontheventralmusclesthataretargetedbytheSNbaxons.Normally,the SNbtractdivergesfromthemainaxonfreewayandheadsofftowardstheventral muscles,andthencontinuestosendbranchestoeachmuscleasitcontinuesto grow.However,amutantcalledbeatenpathnevergetsofftheFreeway.The scientiststheorizedthatthishadsomethingtodowiththewaybeatenpath interactswithoneofthefasciclins,specificallyfasII.Fasciclinsarestickymolecules thatkeepaxonstogether.KnockingoutfasIIdoesn'treallyworkbecausethereistoo muchredundancyinthesystem,sothescientistsoverexpressedFasIIinstead.This resultedinthesamephenotypeasbeatenpath. Dr.BierthentalkedaboutsomeexperimentsbyrogerSperry,thefrogtorturer. Sperrry,cutoutafrogeye,rotatedit180,andstuckitbackin.Theopticnervesre attachedtothesamelocationstowhichtheyhadpreviouslybeenattached.This causedthefrogtospitoutitstongueintheoppositedirectionasanapproachingfly. Whowouldthinktodothis?ThisleadtotheSperryChemoaffinitymodelwhich theorizedthatretinalfibershaveachemicalcodethatmatchesareciprocalcodein thetectum. Howeverwedon'thaveenoughgenestoprovideasinglechemicalforeachnerve fiber.Sperrywasontherighttrack,buthadn'tquitegottentherealanswer. Peoplefurtherinvestigatedwithtwoexperiments:thehemisectedeyeexperiment andthenemisectedtecumexperiment.Butfirst,hereisarepresentationoftheeye andtectum: Hemisectedeyeexpt: Cuttheeyedownthemiddlewatchedtoseewhereopticnerveswouldattachtothe tectum.Earlyon,theattachedtothenormalregionsofthetectum.However,over time,theseopticnervesspreadapartandtookuptheentiretectum Hemisectedtectumexperiment.Cuttectumdownthemiddle.Interestingly,theoptic nervesinthisscenariomaintainedthecorrectorder,butsharedtheremaining tectum. What'sgoingon: TheopticnervesprojectingfromtheeyehaveEphreceptors.Axonsprojectingfrom theAnteriorsideoftheeyehavemore,theposterioroneshavefewer.Thetectum hasEphligandsexpressedinagradient,withfewerontheanteriorsidandmoreon theposteriorside.Astheaxonsprojectfromtheeyetothetectum,theybinda certainamountofeph,andthenstop.Thissystemseemstoberelativelyplastic. ...
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