The fragile X mental retardation protein–RNP granules show an mGluR-dependent localization in the post-synaptic spines

The localization of RNA/mRNA in dendrites plays a role in both local and temporal regulation of protein synthesis, which is required for certain forms of synaptic plasticity. A key molecule in these processes is the fragile X mental retardation protein (FMRP). Using in situ hybridization coupled to immunofluorescence confocal microscopy, we find that the FMRP–RNP particle contains αCaMKII and BC1 RNAs as well as Staufen and CPEB proteins. Furthermore, following mGluR activation, the FMRP–mRNP complex moves into spines as shown by co-localization with the PSD-95 and Shank proteins. This study shows, for the first time, that the translationally inactive FMRP–mRNP complex relocates into neuronal spines after stimulation and that de novo protein synthesis mainly contributes to the pool of FMRP at synapses.