Translational symphony in (hnRNP) C major for APP

Fragile X and APP: a Decade in Review, a Vision for the Future

Fragile X syndrome (FXS) is a devastating developmental disability that has profound effects on cognition, behavior, and seizure susceptibility. There are currently no treatments that target the underlying cause of the disorder, and recent clinical trials have been unsuccessful. In 2007, seminal work demonstrated that amyloid-beta protein precursor (APP) is dysregulated in Fmr1KO mice through a metabotropic glutamate receptor 5 (mGluR5)-dependent pathway. These findings raise the hypotheses that: (1) APP and/or APP metabolites are potential therapeutic targets as well as biomarkers for FXS and (2) mGluR5 inhibitors may be beneficial in the treatment of Alzheimer's disease. Herein, advances in the field over the past decade that have reproduced and greatly expanded upon these original findings are reviewed, and required experimentation to validate APP metabolites as potential disease biomarkers as well as therapeutic targets for FXS are discussed.

miR-195 competes with HuR to modulate stim1 mRNA stability and regulate cell migration

Stromal interaction molecule 1 (Stim1) functions as a sensor of Ca2+ within stores and plays an essential role in the activation of store-operated Ca2+ entry (SOCE). Although lowering Stim1 levels reduces store-operated Ca2+ entry and inhibits intestinal epithelial repair after wounding, the mechanisms that control Stim1 expression remain unknown. Here, we show that cellular Stim1 abundance is controlled posttranscriptionally via factors that associate with 3'-untranslated region (3'-UTR) of stim1 mRNA. MicroRNA-195 (miR-195) and the RNA-binding protein HuR competed for association with the stim1 3'-UTR and regulated stim1 mRNA decay in opposite directions. Interaction of miR-195 with the stim1 3'-UTR destabilized stim1 mRNA, whereas the stability of stim1 mRNA increased with HuR association. Interestingly, ectopic miR-195 overexpression enhanced stim1 mRNA association with argonaute-containing complexes and increased the colocalization of tagged stim1 RNA with processing bodies (P-bodies); the translocation of stim1 mRNA was abolished by HuR overexpression. Moreover, decreased levels of Stim1 by miR-195 overexpression inhibited cell migration over the denuded area after wounding but was rescued by increasing HuR levels. In sum, Stim1 expression is controlled by two factors competing for influence on stim1 mRNA stability: the mRNA-stabilizing protein HuR and the decay-promoting miR-195.

Regulation of cyclin-dependent kinase 4 translation through CUG-binding protein 1 and microRNA-222 by polyamines

The amino acid-derived polyamines are organic cations that are essential for growth in all mammalian cells, but their exact roles at the molecular level remain largely unknown. Here we provide evidence that polyamines promote the translation of cyclin-dependent kinase 4 (CDK4) by the action of CUG-binding protein 1 (CUGBP1) and microRNA-222 (miR-222) in intestinal epithelial cells. Both CUGBP1 and miR-222 were found to bind the CDK4 mRNA coding region and 3'-untranslated region and repressed CDK4 translation synergistically. Depletion of cellular polyamines increased cytoplasmic CUGBP1 abundance and miR-222 levels, induced their associations with the CDK4 mRNA, and inhibited CDK4 translation, whereas increasing the levels of cellular polyamines decreased CDK4 mRNA interaction with CUGBP1 and miR-222, in turn inducing CDK4 expression. Polyamine-deficient cells exhibited an increased colocalization of tagged CDK4 mRNA with processing bodies; this colocalization was abolished by silencing CUGBP1 and miR-222. Together, our findings indicate that polyamine-regulated CUGBP1 and miR-222 modulate CDK4 translation at least in part by altering the recruitment of CDK4 mRNA to processing bodies.