Fragile X syndrome (FXS) is a developmental intellectual disability caused by the loss of fragile X mental retardation protein (FMRP) function. Altered synaptic plasticity and downstream protein synthesis has been implicated in the pathogenesis of FXS, contributing to typical FXS phenotypes. Although the loss of FMRP in neurons abolishes its repressive function on protein synthesis, how synaptic activation and plasticity (especially in cortex) is altered in FXS remains largely unclear. Interestingly, previou in vivo experiments to selectively delete neuronal FMRP showed partial FXS-related phenotypes. This implies that the selective loss of neuronal FMRP may not be sufficient to induce full FXS pathology and therefore that the loss of FMRP in other brain cells may also contribute to pathogenesis of FXS. Previous studies have found FMRP is expressed in astroglial cells. However, pathogenic roles of the in vivo loss of the astroglial FMRP in FXS remain essentially unexplored. We found significantly reduced glutamate transporter GLT1 and EAAT2 (human analog of rodent GLT1) expression in the cortex of the mouse model (fmr1 knock-out, KO mice) of FXS and human FXS post-mortem samples, and decreased glutamate uptake in fmr1 KO mice. We recently generated inducible astroglia-specific fmr1 conditional knock-out (i-astro-fmr1-cKO) and restoration (i-astro-fmr1-cON) mouse models in which the fmr1 allele is selectively disrupted or restored in astroglia, respectively. We showed that selective deletion of the astroglial FMRP plays a primary role in GLT1 reduction in FXS in vitro and in vivo. Selective deletion of the astroglial FMRP also leads to decreased synaptic AMPA/NMDA current ratio and FXS-like behavior phenotypes (hyperactivity and exaggerated memory extinction). Based on previous observations and our preliminary results, we propose to investigate astroglial dysfunctions in FXS. Specifically, we will 1) Test if the loss of astroglial FMRP contributes to th pathogenesis of FXS in vivo; 2) Investigate mechanisms for GLT1 dysregulation in FXS mouse models; 3) Test if GLT1 up-regulation attenuates FXS phenotypes in FXS mouse models We have generated a large amount of preliminary data to support our rationales and to demonstrate feasibility for proposed aims. Results from this project will determine if the selective loss of astroglial FMRP contributes to the FXS pathogenesis by potentially reducing GLT1 expression and impairing extracellular glutamate uptake. These results may demonstrate a conceptually novel astroglia-mediated pathogenic pathway in FXS. In addition, the effects of GLT1 up-regulation on FXS-related phenotypes in mouse models of FXS will potentially validate astroglial GLT1 as a new therapeutic target for treating FXS.
Fragile X Syndrome (FXS) is a developmental intellectual disability that shares many clinical features with autism, including hyperactivity, anxiety, intellectual disability, and increased susceptibility to seizure. FXS is considered the most commonly inherited form of autism. Currently, there is no effective treatment available for FXS or autism, making it one of the significant unmet medical challenges in our society. Although FXS is caused by the loss of fragile mental retardation protein (FMRP), it is unknown whether the loss of FMRP in astroglial cells, a type of abundant non-neuronal glial cell in the central nervous system, contributes to neuronal and behavioral changes observed in FXS. This project will specifically investigate the mechanisms how the loss of astroglial FMRP is involved in the pathogenesis of FXS. Especially, this project will investigate if the glutamate transporter, a cell surface protein that is highly and selectively expressed in astroglial cells and transports the important brain messenger glutamate, is altered and contributes to the development of typical FXS phenotypes.
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