5'-UTR mediated translational control of splicing assembly factor RNP-4F expression during development of the Drosophila central nervous system

EIF4E regulates STEAP1 expression in peritoneal metastasis

Gastric cancer is the most prominent form of malignancy in China, and the high mortality associated with it is mostly due to peritoneal metastasis. We have previously elucidated that the RNA-binding protein poly r(C) binding protein 1 (PCBP1) and miR-3978 function as repressors of peritoneal metastasis, partially by downregulation of six-transmembrane epithelial antigen of the prostate 1 (STEAP1). We now show that STEAP1 is regulated at the level of cap-dependent translation initiation by phosphorylated eIF4E. Chemically inhibiting phosphorylation of eIF4E or genetic ablation of phosphorylated eIF4E inhibit translational upregulation of STEAP1 in the peritoneal metastasis mimicking cell line MKN45 in comparison to the normal mesothelial cell line HMrSV5. Thus phosphorylation of eIF4E is required for peritoneal metastasis of gastric cancer via translational control of STEAP1. Chemical inhibitors targeting phosphorylation of eIF4E or its interaction with the translation initiation complex thus might prove effective in treating patients with peritoneal metastasis.

CELF1 is a central node in post-transcriptional regulatory programmes underlying EMT

The importance of translational regulation in tumour biology is increasingly appreciated. Here, we leverage polyribosomal profiling to prospectively define translational regulatory programs underlying epithelial-to-mesenchymal transition (EMT) in breast epithelial cells. We identify a group of ten translationally regulated drivers of EMT sharing a common GU-rich cis-element within the 3'-untranslated region (3'-UTR) of their mRNA. These cis-elements, necessary for the regulatory activity imparted by these 3'-UTRs, are directly bound by the CELF1 protein, which itself is regulated post-translationally during the EMT program. CELF1 is necessary and sufficient for both mesenchymal transition and metastatic colonization, and CELF1 protein, but not mRNA, is significantly overexpressed in human breast cancer tissues. Our data present an 11-component genetic pathway, invisible to transcriptional profiling approaches, in which the CELF1 protein functions as a central node controlling translational activation of genes driving EMT and ultimately tumour progression.

Non-coding functions of alternative pre-mRNA splicing in development

A majority of messenger RNA precursors (pre-mRNAs) in the higher eukaryotes undergo alternative splicing to generate more than one mature product. By targeting the open reading frame region this process increases diversity of protein isoforms beyond the nominal coding capacity of the genome. However, alternative splicing also frequently controls output levels and spatiotemporal features of cellular and organismal gene expression programs. Here we discuss how these non-coding functions of alternative splicing contribute to development through regulation of mRNA stability, translational efficiency and cellular localization.

A phylogenetic study of Drosophila splicing assembly chaperone RNP-4F associated U4-/U6-snRNA secondary structure

The rnp-4f gene in Drosophila melanogaster encodes nuclear protein RNP-4F. This encoded protein is represented by homologs in other eukaryotic species, where it has been shown to function as an intron splicing assembly factor. Here, RNP-4F is believed to initially bind to a recognition sequence on U6-snRNA, serving as a chaperone to facilitate its association with U4-snRNA by intermolecular hydrogen bonding. RNA conformations are a key factor in spliceosome function, so that elucidation of changing secondary structures for interacting snRNAs is a subject of considerable interest and importance. Among the five snRNAs which participate in removal of spliceosomal introns, there is a growing consensus that U6-snRNA is the most structurally dynamic and may constitute the catalytic core. Previous studies by others have generated potential secondary structures for free U4- and U6-snRNAs, including the Y-shaped U4-/U6-snRNA model. These models were based on study of RNAs from relatively few species, and the popular Y-shaped model remains to be systematically re-examined with reference to the many new sequences generated by recent genomic sequencing projects. We have utilized a comparative phylogenetic approach on 60 diverse eukaryotic species, which resulted in a revised and improved U4-/U6-snRNA secondary structure. This general model is supported by observation of abundant compensatory base mutations in every stem, and incorporates more of the nucleotides into base-paired associations than in previous models, thus being more energetically stable. We have extensively sampled the eukaryotic phylogenetic tree to its deepest roots, but did not find genes potentially encoding either U4- or U6-snRNA in the Giardia and Trichomonas data-bases. Our results support the hypothesis that nuclear introns in these most deeply rooted eukaryotes may represent evolutionary intermediates, sharing characteristics of both group II and spliceosomal introns. An unexpected result of this study was discovery of a potential competitive binding site for Drosophila splicing assembly factor RNP-4F to a 5’-UTR regulatory region within its own premRNA, which may play a role in negative feedback control.