Dysregulated Metabotropic Glutamate Receptor-Dependent Translation of AMPA Receptor and Postsynaptic Density-95 mRNAs at Synapses in a Mouse Model of Fragile X Syndrome

Abstract

Fragile X syndrome, a common form of inherited mental retardation, is caused by the loss of fragile X mental retardation protein (FMRP), an mRNA binding protein that is hypothesized to regulate local mRNA translation in dendrites downstream of gp1 metabotropic glutamate receptors (mGluRs). However, specific FMRP-associated mRNAs that localize to dendritesin vivoand show altered mGluR-dependent translation at synapses ofFmr1knock-out mice are unknown so far. Using fluorescencein situhybridization, we discovered that GluR1/2 and postsynaptic density-95 (PSD-95) mRNAs are localized to dendrites of cortical and hippocampal neuronsin vivo. Quantitative analyses of their dendritic mRNA levels in cultured neurons and synaptoneurosomes did not detect differences between wild-type andFmr1knock-out (KO) mice. In contrast, PSD-95, GluR1/2, and calcium/calmodulin-dependent kinase IIα (CaMKIIα) mRNA levels in actively translating polyribosomes were dysregulated in synaptoneurosomes fromFmr1knock-out mice in response to mGluR activation. [³⁵S]methionine incorporation into newly synthesized proteins similarly revealed impaired stimulus-induced protein synthesis of CaMKIIα and PSD-95 in synaptoneurosomes fromFmr1KO mice. Quantitative analysis of mRNA levels in FMRP-specific immunoprecipitations from synaptoneurosomes demonstrated the association of FMRP with CaMKIIα, PSD-95, and GluR1/2 mRNAs. These findings suggest a novel mechanism whereby FMRP regulates the local synthesis AMPA receptor (AMPAR) subunits, PSD-95, and CaMKIIα downstream of mGluR-activation. Dysregulation of local translation of AMPAR and associated factors at synapses may impair control of the molecular composition of the postsynaptic density and consequently alter synaptic transmission, causing impairments of neuronal plasticity observed inFmr1knock-out mice and fragile X syndrome.