Seite - 4 - in Advances in Neuroimmunology

BrainSci. 2016,6, 19 Thedifferences insignal transductionpathwaysutilizedbyIL-6andchemokinescould indicate differentbiologicalactions.However, signal transductionpathwaysdownstreamof theG-proteinand tyrosinekinasestepcanmergeatcommonpathwaypartnersor targetsandleadtosimilarbiological actions. Thus, it isnotsurprisingthatall threeneuroimmunefactorshaveneuronalorsynapticactions, althoughtheactionsarenot identical. Both neurons and glial cells express receptors and signal transduction pathways utilized by IL-6R [41,43], CCR2 [44–46], andCXCR3 [47,48], and arepotential downstreamcellular targets of the astrocyte producedneuroimmune factors. Because of the close association of astrocyteswith neuronsandsynapses [39], actionsofcytokinesorchemokinesoneithercell typecouldpotentially alterneuronalandsynaptic function.Downstreammolecular targetsofGPCRandIL-6Rpathwayscan regulategeneexpression,whichmaybe instrumental indirectingneuroadaptivechangesassociated withelevatedexpressionof IL-6,CCL2,andCXCL10 in theCNSof the transgenicmice. 4. IL-6,CCL2,orCXCL10TransgenicMice All three linesof transgenicmicewith increasedastrocyteexpressionof IL-6,CCL2,orCXCL10 were generated by a similar approach, insertion of the transgene (mouse or human) for the neuroimmunefactorunder transcriptional controlof theglialfibrillaryacidicprotein (GFAP)gene promoter [29,34,49,50]. GFAP is an intermediatefilamentprotein expressedalmost exclusivelyby astrocytes in theadultCNSandcommonlyusedasamarker forastrocytes [50,51]. More thanone linewasgeneratedforeachneuroimmunefactor.Heterozygotes fromthefollowinglineswereused for the studiesdiscussed in this review: IL-6 transgenic line 167 (IL-6 tg), CXCL10 transgenic line CXCL10-10 (CXCL10tg),CCL2transgenic lineonaSJLbackground(CCL2-tgSJLmice),andCCL2 transgenic lineonaC57Bl/6Jbackground(CCL2-tg),whichweredevelopedfromtheCCL2-tgSJL mice.Non-transgenic littermatesof therespective transgenic linewereusedascontrols. Ingeneral, elevatedexpressionofotherneuroimmunefactorswasnotevident,orat lowlevel in these transgenic lines [29,34,52], enabling investigationof theconsequencesof elevatedexpressionof the transgene aloneor incombinationwithotherexperimentalmanipulations. 4.1. Expressionof IL-6,CCL2,orCXCL10 in theTransgenicMice Because transgene expression in the transgenicmice is under control of theGFAPpromoter, elevated expressionof IL-6, CCL2, orCXCL10 is linked toGFAPexpression. GFAPexpression in astrocytesis initiatedduringthedevelopmentalperiod,whichoccursprimarilyduringthefirst3weeks ofpostnatal life inmice.GFAPexpression in themousehippocampus isevidentat1daypostnatal, increaseswithageuntil6dayspostnatal,andthenlevelsoffandremainsstablethroughadulthood[53]. Thus,neuronal/synapticexposure to theseneuroimmunefactors in the transgenicmiceoccursduring an importantperiodofstructuralandsynapticdevelopmentandcouldaffectdevelopmentalpatterns. Evidence is limitedonthis topic,but ingeneral,neuropathologyinthehippocampusof theIL-6,CCL2, andCXCL10heterozygousmice isabsentorminimalupto3–6monthsofage,althoughhomozygous micecanshowpathologyatearlyages [29,32,54,55]. Thus, if theelevatedexpressionof IL-6,CCL2, orCXCL10alteredCNSdevelopment in the transgenicmice, theeffectsondevelopmentwerenot pathologicalorwerecompensatedforbyotherchanges. In this review,discussionof the transgenic micerefers to theheterozygotes. CNSexpressionof IL-6,CCL2,orCXCL10hasbeenquantifiedin therespective transgenicmice at themRNAand/orprotein levels. Studiesof IL-6-tgmice showed that IL-6mRNAwasevident in theCNSat7dayspostnatal, increasedwithageandreachedapeakat3monthspostnatal (adult stage), afterwhich a declinewas observed [52]. IL-6 transgene expressionwas demonstrated in hippocampalastrocytesbyexpressionof the lacZreportergeneandimmunohistochemicaldetection ofβ-gal [55].Constitutivesecretionof IL-6 fromastrocyteswasdemonstrated instudiesofastrocyte culturespreparedfromCNSof the IL-6 tgmice [49]. IL-6 levelswere~150pg/mLinthesupernatant fromastrocyteculturespreparedfromCNSof IL-6 tgmice, comparedwith<5pg/mLforsupernatant 4