Cytoplasmic polyadenylation plays a key role in the translational control of mRNAs driving biological processes such as gametogenesis, cell-cycle progression, and synaptic plasticity. What determines the distinct time of polyadenylation and extent of translational control of a given mRNA, however, is poorly understood. The polyadenylation-regulated translation is controlled by the cytoplasmic polyadenylation element (CPE) and its binding protein, CPEB, which can assemble both translational repression or activation complexes. Using a combination of mutagenesis and experimental validation of genome-wide computational predictions, we show that the number and relative position of two elements, the CPE and the Pumilio-binding element, with respect to the polyadenylation signal define a combinatorial code that determines whether an mRNA will be translationally repressed by CPEB, as well as the extent and time of cytoplasmic polyadenylation-dependent translational activation.

Beyond the well-known function of poly(A) tail length in mRNA stability, recent years have witnessed an explosion of information about how changes in tail length and the selection of alternative polyadenylation sites contribute to the translational regulation of a large portion of the genome. The mechanisms and factors mediating nuclear and cytoplasmic changes in poly(A) tail length have been studied in great detail, the targets of these mechanisms have been identified--in some cases by genome-wide screenings--and changes in poly(A) tail length are now implicated in a number of physiological and pathological processes. However, in very few cases have all three levels--mechanisms, targets and functions--been studied together.

Protein synthesis is regulated in response to environmental stimuli by covalent modification, primarily phosphorylation, of components of the tranelational machinery. Phosphorylation of the α subunit of eIF‐2 is one of the best‐characterized mechanisms for down‐regulating protein synthesis in higher eukaryotes in response to various stress conditions. Three distinct protein kinases regulate protein synthesis in eukaryotic cells by phosphorylating the α subunit of eIF‐2 at serine‐51. There are two mammalian eIF‐2α kinases: the double‐stranded RNA‐dependent kinase (PKR) and heme‐regulated inhibitor kinase (HRI), and the yeast GCN2. The regulatory mechanisms and the molecular sizes of these eIF‐2α kinases are different. The expression of PKR is induced by interferon, and the kinase activity is stimulated by low concentrations of double‐stranded RNA. HRI is activated under heme‐defi‐cient conditions. Yeast GCN2 is activated by amino acid starvation. The phosphorylation of eIF‐2α results in the shutdown of protein synthesis. Nevertheless, the eIF‐2α kinases can regulate both global as well as specific mRNA translation. Inhibition of protein synthesis correlates with eIF‐2α phosphorylation in response to a wide variety of different stimuli, including heat shock, serum deprivation, glucose starvation, amino acid starvation, exposure to heavy metal ions, and viral infection. Finally, recent studies suggest a role for eIF‐2α phosphorylation in the control of cell growth and differentiation.—de Haro, C., Méndez, R., Santoyo, J. The eIF‐2α kinases and the control of protein synthesis. FASEB J. 10, 1378‐1387(1996)

Blocking mitotic progression has been proposed as an attractive therapeutic strategy to impair proliferation of tumour cells. However, how cells survive during prolonged mitotic arrest is not well understood. We show here that survival during mitotic arrest is affected by the special energetic requirements of mitotic cells. Prolonged mitotic arrest results in mitophagy-dependent loss of mitochondria, accompanied by reduced ATP levels and the activation of AMPK. Oxidative respiration is replaced by glycolysis owing to AMPK-dependent phosphorylation of PFKFB3 and increased production of this protein as a consequence of mitotic-specific translational activation of its mRNA. Induction of autophagy or inhibition of AMPK or PFKFB3 results in enhanced cell death in mitosis and improves the anti-tumoral efficiency of microtubule poisons in breast cancer cells. Thus, survival of mitotic-arrested cells is limited by their metabolic requirements, a feature with potential implications in cancer therapies aimed to impair mitosis or metabolism in tumour cells.

Malignant transformation, invasion and angiogenesis rely on the coordinated reprogramming of gene expression in the cells from which the tumor originated. Although deregulated gene expression has been extensively studied at genomic and epigenetic scales, the contribution of the regulation of mRNA-specific translation to this reprogramming is not well understood. Here we show that cytoplasmic polyadenylation element binding protein 4 (CPEB4), an RNA binding protein that mediates meiotic mRNA cytoplasmic polyadenylation and translation, is overexpressed in pancreatic ductal adenocarcinomas and glioblastomas, where it supports tumor growth, vascularization and invasion. We also show that, in pancreatic tumors, the pro-oncogenic functions of CPEB4 originate in the translational activation of mRNAs that are silenced in normal tissue, including the mRNA of tissue plasminogen activator, a key contributor to pancreatic ductal adenocarcinoma malignancy. Taken together, our results document a key role for post-transcriptional gene regulation in tumor development and describe a detailed mechanism for gene expression reprogramming underlying malignant tumor progression.

Common genetic contributions to autism spectrum disorder (ASD) reside in risk gene variants that individually have minimal effect sizes. As environmental factors that perturb neurodevelopment also underlie idiopathic ASD, it is crucial to identify altered regulators that can orchestrate multiple ASD risk genes during neurodevelopment. Cytoplasmic polyadenylation element binding proteins 1-4 (CPEB1-4) regulate the translation of specific mRNAs by modulating their poly(A)-tails and thereby participate in embryonic development and synaptic plasticity. Here we find that CPEB4 binds transcripts of most high-confidence ASD risk genes. The brains of individuals with idiopathic ASD show imbalances in CPEB4 transcript isoforms that result from decreased inclusion of a neuron-specific microexon. In addition, 9% of the transcriptome shows reduced poly(A)-tail length. Notably, this percentage is much higher for high-confidence ASD risk genes, correlating with reduced expression of the protein products of ASD risk genes. An equivalent imbalance in CPEB4 transcript isoforms in mice mimics the changes in mRNA polyadenylation and protein expression of ASD risk genes and induces ASD-like neuroanatomical, electrophysiological and behavioural phenotypes. Together, these data identify CPEB4 as a regulator of ASD risk genes.

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Meiotic progression is driven by the sequential translational activation of maternal messenger RNAs stored in the cytoplasm. This activation is mainly induced by the cytoplasmic elongation of their poly(A) tails, which is mediated by the cytoplasmic polyadenylation element (CPE) present in their 3 0 untranslated regions. Although polyadenylation in prophase I and metaphase I is mediated by the CPE-binding protein 1 (CPEB1), this protein is degraded during the first meiotic division. Thus, raising the question of how the cytoplasmic polyadenylation required for the second meiotic division is achieved. In this work, we show that CPEB1 generates a positive loop by activating the translation of CPEB4 mRNA, which, in turn, replaces CPEB1 and drives the transition from metaphase I to metaphase II. We further show that CPEB1 and CPEB4 are differentially regulated by phase-specific kinases, generating the need of two sequential CPEB activities to sustain cytoplasmic polyadenylation during all the meiotic phases. Altogether, this work defines a new element in the translational circuit that support an autonomous transition between the two meiotic divisions in the absence of DNA replication.