A new function for the fragile X mental retardation protein in regulation of PSD-95 mRNA stability.

Fragile X syndrome (FXS) results from the loss of the fragile X mental retardation protein (FMRP), an RNA-binding protein that regulates a variety of cytoplasmic mRNAs. FMRP regulates mRNA translation and may be important in mRNA localization to dendrites. We report a third cytoplasmic regulatory function for FMRP: control of mRNA stability. In mice, we found that FMRP binds, in vivo, the mRNA encoding PSD-95, a key molecule that regulates neuronal synaptic signaling and learning. This interaction occurs through the 3' untranslated region of the PSD-95 (also known as Dlg4) mRNA, increasing message stability. Moreover, stabilization is further increased by mGluR activation. Although we also found that the PSD-95 mRNA is synaptically localized in vivo, localization occurs independently of FMRP. Through our functional analysis of this FMRP target we provide evidence that dysregulation of mRNA stability may contribute to the cognitive impairments in individuals with FXS.

Induction and maintenance of late-phase long-term potentiation in isolated dendrites of rat hippocampal CA1 pyramidal neurones.

Expression of N-methyl-d-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) in the CA1 region of the hippocampus can be divided into an early (1-2 h), protein synthesis-independent phase and a late (>4 h), protein synthesis-dependent phase. In this study we have addressed whether the de novo protein synthesis required for the expression of late-LTP can be sustained solely from the translation of mRNAs located in the dendrites of CA1 pyramidal neurones. Our results show that late-LTP, lasting at least 5 h, can be maintained in hippocampal slices where the dendrites located in stratum radiatum have been isolated from their cell bodies by a microsurgical cut. The magnitude of the potentiation of the slope of field EPSPs in these 'isolated' slices was similar to that recorded in 'intact' slices. Incubation of the slices with the mRNA translation inhibitor cycloheximide or the mammalian target of rapamycin (mTOR) inhibitor rapamycin blocked late-LTP in both 'intact' and 'isolated' slice preparations. In contrast, incubation of slices with the transcription inhibitor, actinomycin D, resulted in a reduction of sustained potentiation, at 4 h, in 'intact' slices while in 'isolated' slices the magnitude of potentiation was similar to that seen in untreated slices. These results indicate that late-LTP can be induced and maintained in 'isolated' dendritic preparations via translation of pre-existing mRNAs.

Analyses of murine postsynaptic density-95 identify novel isoforms and potential translational control elements.

Postsynaptic density-95 (PSD-95) is an evolutionarily conserved synaptic adaptor protein that is known to bind many proteins including the NMDA receptor. This observation has implicated it in many NMDA receptor-dependent processes including spatial learning and synaptic plasticity. We have cloned and characterised the murine PSD-95 gene. In addition, we have identified two previously uncharacterised splice variants of the major murine PSD-95 transcript (PSD-95alpha): PSD-95alpha-2b results from an extension of exon 2 and PSD-95alpha-Delta18 from the temporal exclusion of exon 18. The presence of PSD-95alpha-2b sequences in other PSD-95 family members implicates this peptide stretch as functionally significant. Another potential transcript (PSD-95gamma) was also identified based on examination of EST databases. Immunoprecipitation assays demonstrate that proteins corresponding in size to PSD-95alpha-Delta18 and PSD-95gamma interact with the NMDA receptor, suggesting an important biological role for these isoforms. Finally, we have performed bioinformatics analyses of the PSD-95 mRNA untranslated regions, identifying multiple translational control elements that suggest protein production could be regulated post-transcriptionally. The variety of mRNA isoforms and regulatory elements identified provides for a high degree of diversity in the structure and function of PSD-95 proteins.

Regulation of mRNA translation by 5'- and 3'-UTR-binding factors.

The translational regulation of specific mRNAs is important for controlling gene expression. The past few years have seen a rapid expansion in the identification and characterization of mRNA regulatory elements and their binding proteins. For the majority of these examples, the mechanism by which translational regulation is achieved is not well understood. Nevertheless, detailed analyses of a few examples show that almost every event in the initiation pathway, from binding of the cap complex to the joining of the 60S ribosomal subunit, is subject to regulation.