Control of messenger RNA stability during development

Dinucleoside polyphosphates and uremia

Dinucleoside polyphosphates constitute a group of endogenous vasoregulatory purines and pyrimidines with a strong impact on physiologic and pathophysiologic processes of the cardiovascular system. Recently, the importance of dinucleoside polyphosphates in chronic kidney disease (CKD) and uremia gained increasing interest. Although our knowledge about the impact of dinucleoside polyphosphates in CKD and uremia is just at the beginning, this article reviews the current knowledge of the physiologic and pathophysiologic role of dinucleoside polyphosphates in CKD and uremia.

Drosophila gene tazman, an orthologue of the yeast exosome component Rrp44p/Dis3, is differentially expressed during development

The major 3'-5' pathway of RNA degradation in eukaryotic cells involves the exosome, which is a multi-protein complex of exoribonucleases. The exoribonucleases within this complex are highly conserved and are closely related to prokaryotic ribonucleases. We have identified and characterised the expression pattern of Drosophila tazman (taz), a component of the exosome which is closely related to Escherichia coli RNaseR and yeast Rrp44p. The tazman transcripts are differentially expressed during development, with maximum expression levels in 6-8 hr embryos. In situ hybridisation and immunolocalisation experiments show that tazman transcripts and protein are maternally derived, and are expressed ubiquitously throughout the embryo, with high levels in germ band and head structures. Differential expression of TAZ is likely to reflect changes in the activity of the 3'-5' mRNA turnover pathway which could have a major impact of the expression of target RNAs.

The beta-globin C-->G mutation at 6 bp 3' to the termination codon causes beta-thalassaemia by decreasing the mRNA level

We have studied the expression of the silent beta-thalassaemia term+6 (C-->G) mutation, at nucleotide 6 after the stop codon within the human beta-globin 3' untranslated regions (3'UTR), by stable transfection in murine erythroleukaemia (MEL) cells. Steady state mRNA levels from transfected MEL cells containing the term+6 mutant allele were reduced by 52-60%, compared with those obtained from the normal beta-globin gene, in both total and cytoplasmic RNA fractions, showing that the mutation itself is responsible for the similar data obtained from patients. Upon analysis of nuclear RNA, the term+6 mutation was found to also lower the ratio of cleaved/uncleaved transcripts by 22-30%, thus revealing that it interferes with correct 3'-end formation of beta-globin mRNA. The term+6 mutation lies within a polypyrimidine track, similar to that in the beta-intervening sequence II (beta-IVSII), which is known to be an important contributor to the promotion of premRNA 3'-end formation. We propose that the two polypyrimidine tracks flanking the translated region of exon III of the human beta-globin gene may co-operate during beta-globin mRNA biogenesis.

Mitogen-activated protein kinase p38 controls the expression and posttranslational modification of tristetraprolin, a regulator of tumor necrosis factor alpha mRNA stability

Signal transduction pathways regulate gene expression in part by modulating the stability of specific mRNAs. For example, the mitogen-activated protein kinase (MAPK) p38 pathway mediates stabilization of tumor necrosis factor alpha (TNF-alpha) mRNA in myeloid cells stimulated with bacterial lipopolysaccharide (LPS). The zinc finger protein tristetraprolin (TTP) is expressed in response to LPS and regulates the stability of TNF-alpha mRNA. We show that stimulation of RAW264.7 mouse macrophages with LPS induces the binding of TTP to the TNF-alpha 3' untranslated region. The p38 pathway is required for the induction of TNF-alpha RNA-binding activity and for the expression of TTP protein and mRNA. Following stimulation with LPS, TTP is expressed in multiple, differentially phosphorylated forms. We present evidence that phosphorylation of TTP is mediated by the p38-regulated kinase MAPKAPK2 (MAPK-activated protein kinase 2). Our findings demonstrate a direct link between a specific signal transduction pathway and a specific RNA-binding protein, both of which are known to regulate TNF-alpha gene expression at a posttranscriptional level.

Functional mapping of destabilizing elements in the protein-coding region of the Drosophila fushi tarazu mRNA

The instability of the fushi tarazu (ftz) mRNA is essential for the proper development of the Drosophila embryo. Previously, we identified a 201-nucleotide instability element (FIE3) in the 3' untranslated region (UTR) of the ftz mRNA. Here we report on the identification of two additional elements in the protein-coding region of the message: the 63-nucleotide-long FIE5-1 and the 69-nucleotide-long FIE5-2. The function of both elements was position-dependent; the same elements destabilized RNAs when present within the coding region but did not when embedded in the 3' UTR of the hybrid mRNAs. We conclude that ftz mRNA has three redundant instability elements, two in the protein-coding region and one in the 3' UTR. Although each instability element is sufficient to destabilize a heterologous mRNA, the destabilizing activity of the two 5'-elements depended on their position within the message.

Developmental regulation of an instability element from the Drosophila fushi tarazu mRNA

The Drosophila fushi tarazu (ftz) mRNA is one of the shortest-lived metazoan mRNAs, and its instability is crucial for proper development of the embryo. Previously, we identified two cis-acting elements that are required for ftz mRNA degradation, one within the 5' one-third and another in the 3'UTR of the message. Here we focus on the 3'UTR element termed FIE3 (ftz instability element in the 3'UTR). To investigate the developmental regulation of the FIE3-dependent degrading activity we measured the abundance of an FIE3-containing mRNA in ovaries, unfertilized eggs, and different larval and adult tissues. We found that FIE3-degrading activity is present at all developmental stages and tissues examined, except in the ovary. Activation of the FIE3-dependent mRNA decay is independent of fertilization because it could be triggered by egg activation. Finally, we provide evidence that mutation of conserved elements within FIE3 had no effect on mRNA instability.

Reporter-linked monitoring of transgene expression in living cells using the ecdysone-inducible promoter system

Inducible promoter systems such as the ecdysone-inducible system or the tetracycline-regulated expression systems have proven to be powerful tools in studying gene function. In practice, such systems have met with the difficulty that either the vector expressing the transactivator gene or the vector carrying the response element are frequently silenced by flanking genomic sequences after stable integration. In order to identify those cells in a heterogeneous population in which a transgene is expressed from an ecdysone-inducible promoter, we have created the vector p2ER-EGFP/mcs that contains two ecdysone-inducible expression cassettes in tandem. Using two reporter genes, lacZ and green fluorescent protein (EGFP), we demonstrate that the expression of both genes can be co-induced from a very low baseline in CHO cells expressing the modified ecdysone receptor and the retinoid X receptor. The expression of EGFP and lacZ from vector p2ER-EGFP/lacZ follows the same Muristerone A concentration-dependence as that of EGFP from vector pER-EGFP, indicating that the juxtaposition of the two inducible promoters in vector p2ER-EGFP/mcs does not cause cross interference between them. We suggest that this modification of the ecdysone-inducible promoter system will allow for the visual control of the induced expression of other genes by Muristerone A.

Regulation of mda-7 gene expression during human melanoma differentiation

Induction of irreversible growth arrest and terminal differentiation in human melanoma cells following treatment with recombinant human fibroblast interferon (IFN-beta) and mezerein (MEZ) results in elevated expression of a specific melanoma differentiation associated gene, mda-7. Experiments were conducted to define the mechanism involved in the regulation of mda-7 expression in differentiating human melanoma cells. The mda-7 gene is actively transcribed in uninduced HO-1 human melanoma cells and the rate of transcription of mda-7 is not significantly enhanced by treatment with IFN-beta, MEZ or IFN-beta+MEZ. The high basal activity of the mda-7 promoter in uninduced melanoma cells and the absence of enhancing effect upon treatment with differentiation inducers is corroborated by transfection studies using the promoter region of mda-7 linked to a luciferase reporter gene containing the SV40 polyadenylation signal sequence. RT - PCR analysis detects the presence of low levels of mda-7 transcripts in uninduced and concomitant increases in differentiation inducer treated HO-1 cells. However, steady-state mda-7 mRNA is detected only in IFN-beta+MEZ and to a lesser degree in MEZ treated cells. We show that induction of terminal differentiation of HO-1 cells with IFN-beta+MEZ dramatically increases the half-life of mda-7 mRNA while treatment with cycloheximide results in detectable mda-7 mRNA in control and inducer treated cells. These observations confirm constitutive activity of the mda-7 promoter in HO-1 cells irrespective of differentiation status suggesting posttranscriptional processes as important determinants of mda-7 expression during terminal differentiation. The 3' UTR region of mda-7 contains AU-rich elements (ARE) that contribute to rapid mda-7 mRNA turnover during proliferation and reversible differentiation, a process controlled by a labile protein factor(s). Substitution of the SV40 polyadenylation signal sequence in the luciferase reporter plasmid with the mda-7-ARE-3'-UTR renders the Luciferase message unstable when expressed in proliferating and reversibly differentiated melanoma cells. In contrast, the luciferase message is stabilized when the mda-7-ARE-3'-UTR construct is expressed in terminally differentiated HO-1 cells. These results provide compelling evidence that mda-7 expression during terminal differentiation in human melanoma cells is regulated predominantly at a posttranscriptional level.