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.

Poly(A) polymerase and the regulation of cytoplasmic polyadenylation

Translational activation in oocytes and embryos is often regulated via increases in poly(A) length. Cleavage and polyadenylation specificity factor (CPSF), cytoplasmic polyadenylation element binding protein (CPEB), and poly(A) polymerase (PAP) have each been implicated in cytoplasmic polyadenylation in Xenopus laevis oocytes. Cytoplasmic polyadenylation activity first appears in vertebrate oocytes during meiotic maturation. Data presented here shows that complexes containing both CPSF and CPEB are present in extracts of X. laevis oocytes prepared before or after meiotic maturation. Assessment of a variety of RNA sequences as polyadenylation substrates indicates that the sequence specificity of polyadenylation in egg extracts is comparable to that observed with highly purified mammalian CPSF and recombinant PAP. The two in vitro systems exhibit a sequence specificity that is similar, but not identical, to that observed in vivo, as assessed by injection of the same RNAs into the oocyte. These findings imply that CPSFs intrinsic RNA sequence preferences are sufficient to account for the specificity of cytoplasmic polyadenylation of some mRNAs. We discuss the hypothesis that CPSF is required for all polyadenylation reactions, but that the polyadenylation of some mRNAs may require additional factors such as CPEB. To test the consequences of PAP binding to mRNAs in vivo, PAP was tethered to a reporter mRNA in resting oocytes using MS2 coat protein. Tethered PAP catalyzed polyadenylation and stimulated translation approximately 40-fold; stimulation was exclusively cis-acting, but was independent of a CPE and AAUAAA. Both polyadenylation and translational stimulation required PAPs catalytic core, but did not require the putative CPSF interaction domain of PAP. These results demonstrate that premature recruitment of PAP can cause precocious polyadenylation and translational stimulation in the resting oocyte, and can be interpreted to suggest that the role of other factors is to deliver PAP to the mRNA.

Multiple portions of poly(A)-binding protein stimulate translation in vivo

Translational stimulation of mRNAs during early development is often accompanied by increases in poly(A) tail length. Poly(A)-binding protein (PAB) is an evolutionarily conserved protein that binds to the poly(A) tails of eukaryotic mRNAs. We examined PAB's role in living cells, using both Xenopus laevis oocytes and Saccharomyces cerevisiae, by tethering it to the 3'-untranslated region of reporter mRNAs. Tethered PAB stimulates translation in vivo. Neither a poly(A) tail nor PAB's poly(A)-binding activity is required. Multiple domains of PAB act redundantly in oocytes to stimulate translation: the interaction of RNA recognition motifs (RRMs) 1 and 2 with eukaryotic initiation factor-4G correlates with translational stimulation. Interaction with Paip-1 is insufficient for stimulation. RRMs 3 and 4 also stimulate, but bind neither factor. The regions of tethered PAB required in yeast to stimulate translation and stabilize mRNAs differ, implying that the two functions are distinct. Our results establish that oocytes contain the machinery necessary to support PAB-mediated translation and suggest that PAB may be an important participant in translational regulation during early development.

Determination of the free-energy change for repair of a DNA phosphodiester bond

The repair of phosphodiester bonds in nicked DNA is catalyzed by DNA ligases. Ligation is coupled to cleavage of a phosphoanhydride bond in a nucleotide cofactor resulting in a thermodynamically favorable process. A free energy value for phosphodiester bond formation was calculated using the reversibility of the T4 DNA ligase reaction. The relative number of DNA nicks to phosphodiester bonds in a circular plasmid DNA, formed during this reaction at fixed concentrations of ATP to AMP and PP(i), was quantified. At 25 degrees C, pH 7, the equilibrium constant (K(eq)) for the ligation reaction is 3.89 x 10(4) m. This value corresponds to a standard free energy (DeltaG degrees ') of -6.3 kcal mol(-1). By subtracting the known energy contribution due to hydrolysis of ATP to AMP and PP(i), DeltaG degrees ' for the hydrolysis of a DNA phosphodiester bond is -5.3 kcal mol(-1).

Translational control of cyclin B1 mRNA during meiotic maturation: coordinated repression and cytoplasmic polyadenylation

Translational control is prominent during meiotic maturation and early development. In this report, we investigate a mode of translational repression in Xenopus laevis oocytes, focusing on the mRNA encoding cyclin B1. Translation of cyclin B1 mRNA is relatively inactive in the oocyte and increases dramatically during meiotic maturation. We show, by injection of synthetic mRNAs, that the cis-acting sequences responsible for repression of cyclin B1 mRNA reside within its 3'UTR. Repression can be saturated by increasing the concentration of reporter mRNA injected, suggesting that the cyclin B1 3'UTR sequences provide a binding site for a trans-acting repressor. The sequences that direct repression overlap and include cytoplasmic polyadenylation elements (CPEs), sequences known to promote cytoplasmic polyadenylation. However, the presence of a CPE per se appears insufficient to cause repression, as other mRNAs that contain CPEs are not translationally repressed. We demonstrate that relief of repression and cytoplasmic polyadenylation are intimately linked. Repressing elements do not override the stimulatory effect of a long poly(A) tail, and polyadenylation of cyclin B1 mRNA is required for its translational recruitment. Our results suggest that translational recruitment of endogenous cyclin B1 mRNA is a collaborative effect of derepression and poly(A) addition. We discuss several molecular mechanisms that might underlie this collaboration.

The cleavage and polyadenylation specificity factor in Xenopus laevis oocytes is a cytoplasmic factor involved in regulated polyadenylation

During early development, specific mRNAs receive poly(A) in the cytoplasm. This cytoplasmic polyadenylation reaction correlates with, and in some cases causes, translational stimulation. Previously, it was suggested that a factor similar to the multisubunit nuclear cleavage and polyadenylation specificity factor (CPSF) played a role in cytoplasmic polyadenylation. A cDNA encoding a cytoplasmic form of the 100-kDa subunit of Xenopus laevis CPSF has now been isolated. The protein product is 91% identical at the amino acid sequence level to nuclear CPSF isolated from Bos taurus thymus. This report provides three lines of evidence that implicate the X. laevis homologue of the 100-kDa subunit of CPSF in the cytoplasmic polyadenylation reaction. First, the protein is predominantly localized to the cytoplasm of X. laevis oocytes. Second, the 100-kDa subunit of X. laevis CPSF forms a specific complex with RNAs that contain both a cytoplasmic polyadenylation element (CPE) and the polyadenylation element AAUAAA. Third, immunodepletion of the 100-kDa subunit of X. laevis CPSF reduces CPE-specific polyadenylation in vitro. Further support for a cytoplasmic form of CPSF comes from evidence that a putative homologue of the 30-kDa subunit of nuclear CPSF is also localized to the cytoplasm of X. laevis oocytes. Overexpression of influenza virus NS1 protein, which inhibits nuclear polyadenylation through an interaction with the 30-kDa subunit of nuclear CPSF, prevents cytoplasmic polyadenylation, suggesting that the cytoplasmic X. laevis form of the 30-kDa subunit of CPSF is involved in this reaction. Together, these results indicate that a distinct, cytoplasmic form of CPSF is an integral component of the cytoplasmic polyadenylation machinery.