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Person in charge of the Unit : Oui
Regulation of gene expression is fundamental for cell homeostasis and invariably leads to severe human pathogenesis when defective. Therefore, novel treatment strategies for a number of different diseases may depend on our ability to exploit mechanisms that normally alter the expression of endogenous genes. While historically, gene regulation studies have mostly focused on transcription, it has recently become evident that post-transcriptional levels of control play an equally important role. From the very onset of transcription, mRNAs have a complex existence: they are bound by the abundant shuttling hnRNPs proteins, processed at their 5'- and 3' ends (capping, poly-adenylation), internally processed and modified (splicing, editing,...), subjected to various nuclear quality controls (tested for the absence of premature stops,...), routed to the nuclear pore complex, translocated to the cytoplasm, translated (and/or stored) and eventually degraded. All these events are intimately fine-tuned and co-ordinated to ensure that Only 'proper' mRNAs are translated at the correct time and place. Our laboratory studies different aspects of postranscriptional gene regulation with a particular focus on the translation and stability control of mRNA containing so called AU-rich elements.
Secretion of antimicrobial peptides (AMP) is a central defense mechanism used by invertebrates to combat infections. In Drosophila, the synthesis of these peptides is a highly regulated process allowing their rapid release in the hemolymph upon contact with pathogens and the arrest of their production after pathogen clearance. We observed that AMP genes have either a transient or sustained expression profile in S2 drosophila cells treated with peptidoglycan. Moreover, AMP genes containing AU-rich elements (ARE) in their 3' untranslated region (UTR) are subject to a post-transcriptional control affecting mRNA stability, thereby contributing to their transient expression profile. Cecropin A1 (CecA1) constitutes the prototype of this latter class of AMPs.
The AAu-rich sequences present in the 3' non transslated region of the mRNA of several cytokines, protooncogenes and growth factors are playing a crucial role in the stability and translation of the mRNA. The mechanisms of these reualtions are analysed.
TIAR and its homolog, TIA-1, belong to the family of RNA-binding proteins containing RNA-Recognition Motifs (RRM). As many RNA-binding proteins, TIAR and TIA-1 accumulate both in the nucleus and the cytoplasm. In the nucleus, they are involved in the alternative splicing of several hnRNAs (Förch et al., 2000). In vitro studies revealed that both TIA-1 and TIAR bind AU-rich elements (ARE) located in the 3' untranslated regions of several mRNAs, such that encoding Tumor Necrosis Factor-; (TNF), Granulocyte Macrophage Colony Stimulating Factor (GM-CSF) and cyclooxygenase-2 (COX-2) (Gueydan et al., 1999; Cok et al., 2003). Furthermore, in vivo studies demonstrated that TIA-1 acts as a translational repressor of TNF and COX-2 mRNAs. Indeed, tia-1 gene knock out induces an enhanced production of TNF and COX-2 in monocyte/macrophages, resulting from an increased association of the mRNAs to the translational machinery (Piecyk et al., 2000; Dixon et al., 2003). Finally, both TIA-1 and TIAR co-localize with poly(A)+ RNA in cytoplasmic granules induced by environmental stresses (Kedersha et al., 1999; 2001). The importance of TIAR in the post-transcriptional regulation of ARE-containing mRNAs has not been established. Indeed, the knock out of tiar gene is partially lethal and induces infertility (Beck et al., 1998), thereby precluding the loss-of-function approach to analyze TIAR function in vivo. Therefore, we generated transgenic mice overexpressing TIAR. However, ubiquitous and constitutive expression of TIAR leads to embryonic lethality (unpublished results).
Post-transcriptional regulation mediated by the AU-rich elements located in the 3'untranslated region of cytokines messenger RNA
Characterization of TIAR and TIA-1 nucleo-cytoplasmic trafficking