Monitoring the 5`UTR landscape reveals 5`terminal oligopyrimidine (TOP) motif switches to drive translational efficiencies

Transcriptional and translational control are key determinants of gene expression, however, to what extent these two processes can be collectively coordinated is still poorly understood. Here we use long-read sequencing to document the 5`and 3`untranslated region (UTR) isoform landscape of epidermal stem cells, wild-type keratinocytes and squamous cell carcinomas. Focusing on squamous cell carcinomas, we show that a small cohort of genes with alternative 5`UTR isoforms exhibit overall increased translational efficiencies and are enriched in ribosomal proteins and splicing factors. These 5`UTR isoforms with identical coding sequences either include or exclude 5`terminal oligopyrimidine (TOP) motifs and result in vastly altered translational efficiencies of the mRNA. Our findings suggest that switching between TOP and non-TOP motif-containing 5`UTR isoforms is an elegant and simple way to alter protein synthesis rates, set their sensitivity to the mTORC1-dependent nutrient-sensing pathway and direct the translational potential of an mRNA by the precise 5`UTR sequence.

The mTOR regulated RNA-binding protein LARP1 requires PABPC1 for guided mRNA interaction

The mammalian target of rapamycin (mTOR) is a critical regulator of cell growth, integrating multiple signalling cues and pathways. Key among the downstream activities of mTOR is the control of the protein synthesis machinery. This is achieved, in part, via the co-ordinated regulation of mRNAs that contain a terminal oligopyrimidine tract (TOP) at their 5′ends, although the mechanisms by which this occurs downstream of mTOR signalling are still unclear. We used RNA-binding protein (RBP) capture to identify changes in the protein-RNA interaction landscape following mTOR inhibition. Upon mTOR inhibition, the binding of LARP1 to a number of mRNAs, including TOP-containing mRNAs, increased. Importantly, non-TOP-containing mRNAs bound by LARP1 are in a translationally-repressed state, even under control conditions. The mRNA interactome of the LARP1-associated protein PABPC1 was found to have a high degree of overlap with that of LARP1 and our data show that PABPC1 is required for the association of LARP1 with its specific mRNA targets. Finally, we demonstrate that mRNAs, including those encoding proteins critical for cell growth and survival, are translationally repressed when bound by both LARP1 and PABPC1.

Upregulation of 5′-terminal oligopyrimidine mRNA translation upon loss of the ARF tumor suppressor

Tumor cells require nominal increases in protein synthesis in order to maintain high proliferation rates. As such, tumor cells must acquire enhanced ribosome production. How the numerous mutations in tumor cells ultimately achieve this aberrant production is largely unknown. The gene encoding ARF is the most commonly deleted gene in human cancer. ARF plays a significant role in regulating ribosomal RNA synthesis and processing, ribosome export into the cytoplasm, and global protein synthesis. Utilizing ribosome profiling, we show that ARF is a major suppressor of 5′-terminal oligopyrimidine mRNA translation. Genes with increased translational efficiency following loss of ARF include many ribosomal proteins and translation factors. Knockout of p53 largely phenocopies ARF loss, with increased protein synthesis and expression of 5′-TOP encoded proteins. The 5′-TOP regulators eIF4G1 and LARP1 are upregulated in Arf- and p53-null cells.

Multifaceted deregulation of gene expression and protein synthesis with age

Protein synthesis represents a major metabolic activity of the cell. However, how it is affected by aging and how this in turn impacts cell function remains largely unexplored. To address this question, herein we characterized age-related changes in both the transcriptome and translatome of mouse tissues over the entire life span. We showed that the transcriptome changes govern those in the translatome and are associated with altered expression of genes involved in inflammation, extracellular matrix, and lipid metabolism. We also identified genes that may serve as candidate biomarkers of aging. At the translational level, we uncovered sustained down-regulation of a set of 5′-terminal oligopyrimidine (5′-TOP) transcripts encoding protein synthesis and ribosome biogenesis machinery and regulated by the mTOR pathway. For many of them, ribosome occupancy dropped twofold or even more. Moreover, with age, ribosome coverage gradually decreased in the vicinity of start codons and increased near stop codons, revealing complex age-related changes in the translation process. Taken together, our results reveal systematic and multidimensional deregulation of protein synthesis, showing how this major cellular process declines with age.

capCLIP: a new tool to probe protein synthesis in human cells through capture and identification of the eIF4E-mRNA interactome

SummaryTranslation of eukaryotic mRNAs starts with binding of the m7G cap to the protein eIF4E followed by recruitment of other translation initiation factors. eIF4E’s essential role in translation suggests the cellular eIF4E-mRNA interactome (or ‘eIF4E cap-ome’) may serve as a faithful proxy of cellular translational activity. Here we describe capCLIP, a novel method to systematically capture and quantify the eIF4E cap-ome. To validate capCLIP, we identified the cap-omes in human cells ± the partial mTORC1 inhibitor rapamycin. As expected, TOP (terminal oligopyrimidine) mRNA representation is systematically reduced in rapamycin-treated cells. capCLIP tag data permits refinement of a 7-nucleotide TOP motif (5′-CUYUYYC-3′). We also apply capCLIP to probe the consequences of phosphorylation of eIF4E, whose function had remained unclear. eIF4E phosphorylation drives an overall reduction in eIF4E-mRNA association; strikingly, mRNAs most sensitive to phosphorylation possess short 5′-UTRs. capCLIP provides a sensitive and comprehensive measure of cellular translational activity. We foresee its application as a high-throughput way to assess translation in contexts not amenable to existing methodologies.

Global analysis of LARP1 translation targets reveals tunable and dynamic features of 5′ TOP motifs

Terminal oligopyrimidine (TOP) motifs are sequences at the 5′ ends of mRNAs that link their translation to the mTOR Complex 1 (mTORC1) nutrient-sensing signaling pathway. They are commonly regarded as discrete elements that reside on ∼100 mRNAs that mostly encode translation factors. However, the full spectrum of TOP sequences and their prevalence throughout the transcriptome remain unclear, primarily because of uncertainty over the mechanism that detects them. Here, we globally analyzed translation targets of La-related protein 1 (LARP1), an RNA-binding protein and mTORC1 effector that has been shown to repress TOP mRNA translation in a few specific cases. We establish that LARP1 is the primary translation regulator of mRNAs with classical TOP motifs genome-wide, and also that these motifs are extreme instances of a broader continuum of regulatory sequences. We identify the features of TOP sequences that determine their potency and quantify these as a metric that accurately predicts mTORC1/LARP1 regulation called a TOPscore. Analysis of TOPscores across the transcriptomes of 16 mammalian tissues defines a constitutive “core” set of TOP mRNAs, but also identifies tissue-specific TOP mRNAs produced via alternative transcription initiation sites. These results establish the central role of LARP1 in TOP mRNA regulation on a transcriptome scale and show how it connects mTORC1 to a tunable and dynamic program of gene expression that is tailored to specific biological contexts.

ARF suppresses 5’-terminal oligopyrimidine mRNA translation

Tumor cells require nominal increases in protein synthesis in order to maintain high proliferation rates. As such, tumor cells must acquire enhanced ribosome production. How many of the mutations in tumor cells ultimately achieve this aberrant production is largely unknown. The gene encoding ARF is the most commonly deleted gene in human cancer. ARF plays a significant role in regulating ribosomal RNA synthesis and processing, ribosome export into the cytoplasm, and global protein synthesis. Utilizing ribosome profiling, we show that ARF is a major suppressor of 5’-terminal oligopyrimidine mRNA translation. Genes with increased translational efficiency following loss of ARF include many ribosomal proteins and translation factors. Knockout of p53 caused a similar increase in 5’-TOP mRNA translation. The 5’-TOP regulators mTORC1, eIF4G1 and LARP1 are dysregulated in ARF and p53 null cells.

Inhibition of cytoplasmic cap methylation identifies 5′ TOP mRNAs as recapping targets and reveals recapping sites downstream of native 5′ ends

Cap homeostasis is the cyclical process of decapping and recapping that maintains the translation and stability of a subset of the transcriptome. Previous work showed levels of some recapping targets decline following transient expression of an inactive form of RNMT (ΔN-RNMT), likely due to degradation of mRNAs with improperly methylated caps. The current study examined transcriptome-wide changes following inhibition of cytoplasmic cap methylation. This identified mRNAs with 5′-terminal oligopyrimidine (TOP) sequences as the largest single class of recapping targets. Cap end mapping of several TOP mRNAs identified recapping events at native 5′ ends and downstream of the TOP sequence of EIF3K and EIF3D. This provides the first direct evidence for downstream recapping. Inhibition of cytoplasmic cap methylation was also associated with mRNA abundance increases for a number of transcription, splicing, and 3′ processing factors. Previous work suggested a role for alternative polyadenylation in target selection, but this proved not to be the case. However, inhibition of cytoplasmic cap methylation resulted in a shift of upstream polyadenylation sites to annotated 3′ ends. Together, these results solidify cap homeostasis as a fundamental process of gene expression control and show cytoplasmic recapping can impact regulatory elements present at the ends of mRNA molecules.

Multi-faceted deregulation of gene expression and protein synthesis with age

Protein synthesis represents a major metabolic activity of the cell. However, how it is affected by aging and how this in turn impacts cell function remains largely unexplored. To address this question, herein we characterized age-related changes in both the transcriptome and translatome of mouse tissues over the entire lifespan. Expression of the majority of differentially expressed genes followed a U-shaped curve with the turning point around 3-months-old. We showed that transcriptome changes govern changes in the translatome and are associated with altered expression of genes involved in inflammation, extracellular matrix and lipid metabolism. We also identified genes that may serve as candidate biomarkers of aging. At the translational level, we uncovered sustained down-regulation of a set of 5’ terminal oligopyrimidine (5’TOP) transcripts encoding protein synthesis and ribosome biogenesis machinery and regulated by the mTOR pathway. For many of them, ribosome occupancy dropped 3-fold or even more. Moreover, with age, ribosome coverage gradually decreased in the vicinity of start codons and increased near stop codons, revealing complex age-related changes in the translation process. Taken together, our results reveal systematic and multi-dimensional deregulation in protein synthesis, showing how this major cellular process declines with age.