The fragile X mental retardation protein is required for type-I metabotropic glutamate receptor-dependent translation of PSD-95

Abstract

Fragile X syndrome (FXS) is a common inherited cause of mental retardation resulting from the absence of the fragile X mental retardation protein (FMRP). FMRP is thought to regulate the translation of target mRNAs, including its own transcript. Here we show that the levels of FMRP are rapidly up-regulated in primary cortical neurons in response to the type-I metabotropic glutamate receptor (mGluR) agonist S-3,5-dihydrophenylglycine. These changes require new protein synthesis but not transcription and are specific to mGluR activation. We also demonstrate that the mRNA for PSD-95, a scaffolding protein involved in synaptic plasticity, contains a highly conserved canonical binding site for FMRP within its 3 UTR. Furthermore, PSD-95 is rapidly translated in response to S-3,5-dihydrophenylglycine. Finally, we show that these mGluR-dependent changes in PSD-95 expression are lost in neurons derived from FMRP knockout mice, a model of FXS. Taken together, these studies suggest that FMRP is required for mGluRdependent translation of PSD-95 and provide insights into the pathophysiology of FXS.F ragile X syndrome is the most common inherited cause of mild to moderate mental retardation (1, 2). The syndrome typically results from transcriptional silencing of the fragile X mental retardation gene, FMR1, and loss of the fragile X mental retardation protein (FMRP) (for a review, see ref.3). Through RGG and KH domains, FMRP binds up to 4% of total brain mRNA, including its own transcript (4-7). FMRP is highly expressed in the cytoplasm and dendrites of neurons (8), often in large messenger ribonucleoprotein particles, implicating FMRP in translational regulation (9-12).FMRP is translated in response to activation of metabotropic glutamate receptors (mGluRs) (13,14) and may be required for mGluR-dependent translation (15). Moreover, FMRP is required for normal type-I mGluR-dependent long-term depression, a process that requires protein synthesis (16). Thus, we hypothesized that FMRP-bound mRNAs would be translated in cultured neurons in response to application of a type-I mGluR agonist.Here we show that application of the type-I mGluR agonist S-3,5-dihydrophenylglycine (DHPG) onto cortical cultures leads to the rapid and robust translation of both FMRP and the synaptic scaffolding protein PSD-95, whose mRNA contains a canonical FMRP binding site. Furthermore, the mGluRdependent translation of PSD-95 is lost in cultures derived from FMR1 knockout (KO) mice. Taken together, these findings support a role for FMRP in activity-dependent translation and may provide insights into mGluR-dependent synaptic plasticity. Figs. 1-3) and used at 14-15 days in vitro. Western blots were performed and analyzed as described (14) with anti-FMRP (mAb 2160, Chemicon), anti-PSD-95 (7E3-1B8, Affinity BioReagents, Neshanic Station, NJ), anti-mGluR 1 and 5 (Upstate Biotechnology, Lake Placid, NY), and anti-␤-actin (Sigma). For FMRP, only the top band was quantitated and normalized to ␤-actin. MethodsImmunofluorescence and Immunoblotting. For immunoflu...

On BC1 RNA and the fragile X mental retardation protein

“…5A). In contrast, FMRP target PSD-95 mRNA (27) was readily detectable in both F-IP and P-IP (SI Fig. 7).…”

The fragile X protein controls microtubule-associated protein 1B translation and microtubule stability in brain neuron development

“…In fragile X patient cells and synaptoneurosomes isolated from the Fmr1 KO brain, the lack of FMRP causes abnormal polyribosome-association of several mRNAs that normally bind to FMRP (18,21). Consistent with the proposed function of FMRP in regulating translation in synaptic plasticity, activity-dependent production of the synaptic protein PSD95 is abrogated in the Fmr1 KO primary neuronal cultures (22). In addition, the mRNA of the microtubuleassociated protein 1B (MAP1B), a neuronal MAP playing principle roles in neurite and synapse development (23), is a predicted target of FMRP (18,19,21).…”

A balance between excitatory and inhibitory synapses is controlled by PSD-95 and neuroligin

“…Moreover, chromosomal rearrangements that harbor NLG1 and NLG2 and PSD-95 genes have been associated with autism (42)(43)(44). Abnormal expression of PSD-95 is also altered in fragile-X syndrome (45). We propose a model in which improper expression and͞or targeting of molecules that control synaptic specificity, such as PSD-95 and NLG, may trigger an imbalance in neuronal excitability.…”