The FMRP/ICAM5 Interaction Uncovers A New Mechanism Of Modulating Dendritic Spine Development In Fragile X Syndrome

Objective The absence of fragile X mental retardation protein(FMRP)results in fragile X syndrome(FXS)that is the most common form of inherited intellectual disability and the leading monogenic cause of autism spectrum disorders(ASD).Dendrite and spine abnormalities have been correlated with impaired cognitive abilities in FXS,but the causes of these malformations are not yet well understood.Intercellular adhesion molecule 5(ICAM5;Telencephalin),a protein of the Ig superfamily,has been implicated in neuronal and spine development as well as the formation and maintenance of dendritic filopodia.The present study is to examine a possible link for ICAM5 in Fmr1 knockout(KO)mice,the animal model of FXS and to futher investigate the direct interaction of RNA-binding protein FMRP and ICAM5 mRNA.Methods Fmr1 knock out(KO)(FVB.129P2-Pde6b+ Tyrc-ch Fmr1tm1Cgr/J)and control(FVB.129P2-Pde6b+Tyrc-ch/Ant J),here-after referred to as wild-type(WT)strain mice were used as the animal model of FXS.Primary mouse neuronal cultures were generated from prefrontal cerebral cortex of WT or Fmr1 KO embryos(E17–E18).The pattern of postnatal spine development and ICAM5 expression was profiled by Golgi staining and western bloting respectively.The interaction between FMRP and ICAM5 mRNA was identified by RNA binding protein immunoprecipitation and quantitative real-time PCR.The alteration of dendritic spine in neurons was measured by live cell imaging.Results 1)ICAM5 protein is up-regulated in the postnatal brains of Fmr1 KO mice and cultured KO neurons,but ICAM5 mRNA is less affected in cultured KO neurons and even is decreased in later postnatal KO brains.2)The developmental alterations of ICAM5 protein parallels the time of high dense spines in Fmr1 KO mice.3)The reduction of FMRP by using FMR1 sh RNA in WT neurons alters the production of ICAM5 protein,whereas the level of ICAM5 mRNA was unchanged,and increse immature filamentous dendritic spines was.and affects spine maturation.4)The overexpression of FMRP in WT neuronsalters the production of ICAM5 protein,and the level of ICAM5 mRNA was unchanged,and the growth of immature filamentous dendritic spines.and affects spine maturation.5)Overexpression of FMRP normalizes ICAM5 expression and further ameliorates dendritic spine morphology in Fmr1 KO neurons.6)FMRP directly binds with the coding sequence of ICAM5 mRNA,and regulate ICAM5 expression to affect dendritic spine maturation.7)Excessive production of ICAM5 protein is implicated in the behavior impairments including spatial and fear memory in Fmr1 KO mice,Reducing the expression of ICAM5 can improve behavioral abnormalities.Conclusion These results demonstrate that FMRP regulates synthesis of ICAM5 and that in FXS,lack of FMRP results in overexpression of ICAM5,leading to defects in dendritic spine morphology and behaviors.Thus,regulation of ICAM5 may provide a functional link among deficient neuronal morphology and cognition impairment in FXS.It is possible that depressing ICAM5 activity in FXS model mice could reverse the exaggerated protein synthesis and thereby correct multiple phenotypes displayed by FXS mice.