Short-Term Memory Deficit Associates with miR-153-3p Upregulation in the Hippocampus of Middle-Aged Mice

The early stages of ageing are a critical time window in which the ability to detect and identify precocious molecular and cognitive markers can make the difference in determining a healthy vs unhealthy course of ageing. Using the 6-different object task (6-DOT), a highly demanding hippocampal-dependent recognition memory task, we classified a population of middle-aged (12-month-old) CD1 male mice in Impaired and Unimpaired based on their short-term memory. This approach led us to identify a different microRNAs expression profile in the hippocampus of Impaired mice compared to Unimpaired ones. Among the dysregulated microRNAs, miR-153-3p was upregulated in the hippocampus of Impaired mice and appeared of high interest for its putative target genes and their possible implication in memory-related synaptic plasticity. We showed that intra-hippocampal injection of the miR-153-3p mimic in adult (3-month-old) mice is sufficient to induce a short-term memory deficit similar to that observed in middle-aged Impaired mice. Overall, these findings unravel a novel role for hippocampal miR-153-3p in modulating short-term memory that could be exploited to prevent early cognitive deficits in ageing.

One-step nested PCR for detection of 2 LTR circles in PBMCs of HIV-1 infected patients with no detectable plasma HIV RNA

A highly sensitive nested PCR was carried out in order to detect 2 LTR circles as a marker of recent and ongoing viral replication in HIV-1 infected patients with HIV plasma RNA undetectable. This "in house" two-step nested PCR is very sensitive, but it is not feasible for routine tests and presents a high risk of contamination. In order to reduce the time of reactions and crossover contamination, the possibility was explored to carry out a single step nested PCR, in which the two successive amplification rounds are carried out in the same tube. This single step nested PCR has the same sensitivity of the two-step nested, is easy to conduct and requires a short time of reaction. The two different PCR methods were compared and the clinical use of monitoring 2 LTR DNA circles in HIV-1 infected patients with undetectable plasma viral load is discussed.

Identification of a regulated pathway for nuclear pre-mRNA turnover

We have identified a nuclear pathway that rapidly degrades unspliced pre-mRNAs in yeast. This involves 3'-->5' degradation by the exosome complex and 5'-->3' degradation by the exonuclease Rat1p. 3'-->5' degradation is normally the major pathway and is regulated in response to carbon source. Inhibition of pre-mRNA degradation resulted in increased levels of pre-mRNAs and spliced mRNAs. When splicing was inhibited by mutation of a splicing factor, inhibition of turnover resulted in 20- to 50-fold accumulation of pre-mRNAs, accompanied by increased mRNA production. Splicing of a reporter construct with a 3' splice site mutation was also increased on inhibition of turnover, showing competition between degradation and splicing. We propose that nuclear pre-mRNA turnover represents a novel step in the regulation of gene expression.

Mex67p mediates nuclear export of a variety of RNA polymerase II transcripts

Mex67p is essential for nuclear poly(A)(+) RNA export in yeast, but which specific transcripts are transported by Mex67p is not known. We observed that thermosensitive mex67-5 cells do not produce a heat shock response at 37 degrees C but will induce heat shock proteins (Hsp) (e.g. Hsp104p and Hsp70p) when shifted back from the restrictive to permissive temperature (30 degrees C). This memory of a previous heat stress in mex67-5 cells could be explained if HSP mRNAs accumulated inside the nucleus during heat shock and were exported and translated in the cytoplasm on return to the permissive temperature. To test this hypothesis, nuclear export of heat shock mRNAs was directly analyzed by in situ hybridization using fluorescent-labeled oligonucleotide probes specific for SSA transcripts. This revealed that Mex67p is required for nuclear export of heat shock mRNAs. Furthermore, other polymerase II transcripts encoding the transcriptional repressor ASH1 and the glycolytic enzyme PGK1 are shown to require Mex67p for their export into the cytoplasm. Thus, Mex67p is an mRNA export factor for a broad range of polymerase II transcripts.

Processing of the intron-encoded U18 small nucleolar RNA in the yeast Saccharomyces cerevisiae relies on both exo- and endonucleolytic activities

Many small nucleolar RNAs (snoRNAs) are encoded within introns of protein-encoding genes and are released by processing of their host pre-mRNA. We have investigated the mechanism of processing of the yeast U18 snoRNA, which is found in the intron of the gene coding for translational elongation factor EF-1beta. We have focused our analysis on the relationship between splicing of the EF-1beta pre-mRNA and production of the mature snoRNA. Mutations inhibiting splicing of the EF-1beta pre-mRNA have been shown to produce normal U18 snoRNA levels together with the accumulation of intermediates deriving from the pre-mRNA, thus indicating that the precursor is an efficient processing substrate. Inhibition of 5'-->3' exonucleases obtained by insertion of G cassettes or by the use of a rat1-1 xrn1Delta mutant strain does not impair U18 release. In the Exo- strain, 3' cutoff products, diagnostic of an endonuclease-mediated processing pathway, were detected. Our data indicate that biosynthesis of the yeast U18 snoRNA relies on two different pathways, depending on both exonucleolytic and endonucleolytic activities: a major processing pathway based on conversion of the debranched intron and a minor one acting by endonucleolytic cleavage of the pre-mRNA.

Self-cleaving motifs are found in close proximity to the sites utilized for U16 snoRNA processing

A class of small nucleolar RNAs (snoRNAs) is encoded in introns of protein-coding genes. The U16 snoRNA belongs to this class; it is encoded in the third intron of the Xenopus laevis (Xl) L1 ribosomal protein encoding gene and is released from the pre-mRNA by processing both in vivo and in vitro systems. In this paper, we show that in close proximity to the U16 snoRNA processing sites, sequences displaying self-cleaving activity are present. These elements are conserved in the two copies of the Xl L1 and in the single copy of the X. tropicalis L1. The catalytic activity corresponds to that already described for the minimal hairpin ribozyme [Dange et al., Science 242 (1990) 585-588]; it is Mn(2+)-dependent, produces 2'-3' cyclic phosphate and 5'-OH termini and comprises an essential GAAA element. Here we show that the 2'-OH group of the G residue is essential for catalysis.

Identification of the cis-elements mediating the autogenous control of ribosomal protein L2 mRNA stability in yeast

The ribosomal protein L2 (rpL2) of Saccharomyces cerevisiae regulates the accumulation of its own mRNA by a feedback mechanism. An RNA sequence is responsible for this control, initially characterized as a 360 nucleotide-long region, localized at the 5' end of the transcript. This region, fused to an unrelated coding sequence, is able to down-regulate the accumulation of the chimeric transcript when increased levels of rpL2 are induced in the cell. The target regulatory region also responds to regulation when inserted inside an intron, demonstrating that the control process can take place inside the nucleus. Deletion analysis from the 5' and 3' borders have restricted the responsive region to approximately 200 nt. The insertion of a poly-G cassette downstream of the regulatory region allowed the identification of truncated 3' cut-off poly(A)+ RNA molecules. The parallel identification of cut-off molecules containing the 5' portion of the transcript allowed us to deduce that the truncated products originate by endonucleolytic cleavage. Altogether, these results are consistent with a mechanism by which the presence of excess amounts of rpL2 in the cell triggers its own mRNA to a degradative pathway; this involves an initial endonucleolytic cleavage that is followed by exonucleolytic trimming. Such a regulatory mechanism shows interesting analogies with the translational regulation of r-proteins in Escherichia coli.

Two different snoRNAs are encoded in introns of amphibian and human L1 ribosomal protein genes

We previously reported that the third intron of the X.laevis L1 ribosomal protein gene encodes for a snoRNA called U16. Here we show that four different introns of the same gene contain another previously uncharacterized snoRNA (U18) which is associated with fibrillarin in the nucleolus and which originates by processing of the pre-mRNA. The pathway of U18 RNA release from the pre-mRNA is the same as the one described for U16: primary endonucleolytic cleavages upstream and downstream of the U18 coding region produce a pre-U18 RNA which is subsequently trimmed to the mature form. Both the gene organization and processing of U18 are conserved in the corresponding genes of X.tropicalis and H.sapiens. The L1 gene thus has a composite structure, highly conserved in evolution, in which sequences coding for a ribosomal protein are intermingled with sequences coding for two different snoRNAs. The nucleolar localization of these different components suggests some common function on ribosome biosynthesis.

The mechanisms controlling ribosomal protein L1 pre-mRNA splicing are maintained in evolution and rely on conserved intron sequences

Sequences corresponding to the third intron of the X.laevis L1 ribosomal protein gene were isolated from the second copy of the X.laevis gene and from the single copy of X.tropicalis. Sequence comparison revealed that the three introns share an unusual sequence conservation which spans a region of 110 nucleotides. In addition, they have the same suboptimal 5' splice sites. The three introns show similar features upon oocyte microinjection: they have very low splicing efficiency and undergo the same site specific cleavages which lead to the accumulation of truncated molecules. Computer analysis and RNAse digestions have allowed to assign to the conserved region a specific secondary structure. Mutational analysis has shown that this structure is important for conferring the cleavage phenotype to these three introns. Competition experiments show that the cleavage phenotype can be prevented by coinjection of excess amounts of homologous sequences.

The ribosomal protein L2 in S. cerevisiae controls the level of accumulation of its own mRNA

The expression of the yeast L2 r-protein gene is controlled at the level of mRNA accumulation. The product of the gene appears to participate in this regulation by an autogenous feedback mechanism. This control does not operate at the level of transcription but instead affects L2 mRNA accumulation. This autogenous regulation of mRNA accumulation provides an interesting analogy to the autogenous translational regulation of r-proteins in Escherichia coli.

The ABF1 factor is the transcriptional activator of the L2 ribosomal protein genes in Saccharomyces cerevisiae

The same factor, ABF1, binds to the promoters of the two gene copies (L2A and L2B) coding for the ribosomal protein L2 in Saccharomyces cerevisiae. In vitro binding experiments and in vivo functional analysis showed that the different affinities of the L2A and L2B promoters for the ABF1 factor are responsible for the differential transcriptional activities of the two gene copies. The presence of ABF1-binding sites in front of many housekeeping genes suggests a general role for ABF1 in the regulation of gene activity.

Gene dosage alteration of L2 ribosomal protein genes in Saccharomyces cerevisiae: effects on ribosome synthesis

In Saccharomyces cerevisiae, the genes coding for the ribosomal protein L2 are present in two copies per haploid genome. The two copies, which encode proteins differing in only a few amino acids, contribute unequally to the L2 mRNA pool: the L2A copy makes 72% of the mRNA, while the L2B copy makes only 28%. Disruption of the L2B gene (delta B strain) did not lead to any phenotypic alteration, whereas the inactivation of the L2A copy (delta A strain) produced a slow-growth phenotype associated with decreased accumulation of 60S subunits and ribosomes. No intergenic compensation occurred at the transcriptional level in the disrupted strains; in fact, delta A strains contained reduced levels of L2 mRNA, whereas delta B strains had almost normal levels. The wild-type phenotype was restored in the delta A strains by transformation with extra copies of the intact L2A or L2B gene. As already shown for other duplicated genes (Kim and Warner, J. Mol. Biol. 165:79-89, 1983; Leeret al., Curr. Genet. 9:273-277, 1985), the difference in expression of the two gene copies could be accounted for via differential transcription activity. Sequence comparison of the rpL2 promoter regions has shown the presence of canonical HOMOL1 boxes which are slightly different in the two genes.

Ribosomal protein L2 in Saccharomyces cerevisiae is homologous to ribosomal protein L1 in Xenopus laevis. Isolation and characterization of the genes

By cross-hybridization with a cDNA probe for the Xenopus laevis ribosomal protein L1 we have been able to isolate the homologous genes from a Saccharomyces cerevisiae genomic library. We have shown that these genes code for a ribosomal protein which was previously named L2. In yeast, like in X. laevis, these genes are present in two copies per haploid genome and, unlike the vertebrate counterpart, they do not contain introns. Amino acid comparison of the X. laevis L1 and S. cerevisiae L2 proteins has shown the presence of a highly conserved protein domain embedded in very divergent sequences. Although these sequences are very poorly homologous, they confer an overall secondary structure and folding highly conserved in the two species.