Absence of FMRP causes fragile X syndrome, the most common inherited form of mental retardation.Significant advances have been made in understanding the function of FMRP, and numerous mRNA targets ofthis RNA binding protein have been identified. Stemming from the observation that hippocampal long-termdepression (LTD) is enhanced in Fmr1 knockout mice, Huber, Bear and Warren proposed a theory to explainmuch of the phenotype of fragile X syndrome1. This 'mGluR theory' proposes that abnormally elevatedresponses to glutamate signaling through the metabotropic glutamate receptor(s) (mGluRs) results fromabsence of FMRP in the dendritic compartment of glutamatergic synapses. This theory suggests newdirections in treatment of fragile X patients using inhibitors of mGluR signaling. Very encouraging data havebeen developed in preclinical testing with both mouse and fly models using MPEP, a specific antagonist of themGluRS receptor2'4, and additional effort to define this pathway and potential drugs for intervention isproceeding in numerous laboratories.The principal goal of this project is to elucidate the requirements for Fmrp during development. Fragile Xsyndrome is widely considered to be a developmental disorder; yet direct evidence to support this assertion islimited. We propose to use mouse models that allow expression of Fmrp to be manipulated in a timedependentmanner to assess the requirement for Fmrp during development. Temporal deletion of Fmr1 in miceduring embryonic and postnatal development will be achieved by in vivo expression of a Tamoxifen-inducibleOre recombinase to ablate the Fmr1 promoter and first exon flanked by /ox P sites. A similar approach willallow restoration of full Fmr1 expression from a gene that has an interfering neomycin cassette in intron 1 thatcan be removed with Ore recombination. Using these inducible Fmr1 alleles, we will determine whether thephenotypes observed in fragile X mice result from a lack of postnatal FMRP expression or are due to lastingconsequences of the absence of FMRP during development.While mGluR antagonists have great promise as a potential treatment for some symptoms seen in fragile Xadults, it remains unclear whether the disease results primarily from a lack of postnatal expression of Fmrp oris due to the consequences of early brain development in the absence of Fmrp. The outcome of our proposedstudies will allow better definition of expectations with therapeutic treatments, as they should set a 'bestoutcome' target for phenotypic changes. In combination with the projects proposed by Paylor and Warren,which seek to identify and study additional drugs and to understand effects of background mutations, this effortwill significantly advance our understanding of the consequences of absence of Fmrp and the potential fortreatment in fragile X syndrome.We propose to utilize inducible ablation and restoration models of Fmr1 allowing temporal alterations to thegene by induction of Ore recombinase using tamoxifen to study the potential to rescue phenotypes previouslydemonstrated in knockout animals.
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