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Nonsense-mediated mRNA decay (NMD) is one of the best characterized and most evolutionarily conserved cellular quality control mechanisms. Although NMD was first found to target one-third of mutated, disease-causing mRNAs, it is now known to also target ~10% of unmutated mammalian mRNAs to facilitate appropriate cellular responses — adaptation, differentiation or death — to environmental changes. Mutations in NMD genes in humans are associated with intellectual disability and cancer. In this Review, we discuss how NMD serves multiple purposes in human cells by degrading both mutated mRNAs to protect the integrity of the transcriptome and normal mRNAs to control the quantities of unmutated transcripts.Nonsense-mediated mRNA decay (NMD) is a quality and quantity control mechanism for degrading mutated mRNAs to protect the integrity of the transcriptome and proteome and unmutated mRNAs to control their quantity. NMD dysfunction in humans is associated with intellectual disability and cancer.

Alternative cleavage and polyadenylation (APA) is a widespread mechanism to generate mRNA isoforms with alternative 3′ untranslated regions (UTRs). The expression of alternative 3′ UTR isoforms is highly cell type specific and is further controlled in a gene-specific manner by environmental cues. In this Review, we discuss how the dynamic, fine-grained regulation of APA is accomplished by several mechanisms, including cis-regulatory elements in RNA and DNA and factors that control transcription, pre-mRNA cleavage and post-transcriptional processes. Furthermore, signalling pathways modulate the activity of these factors and integrate APA into gene regulatory programmes. Dysregulation of APA can reprogramme the outcome of signalling pathways and thus can control cellular responses to environmental changes. In addition to the regulation of protein abundance, APA has emerged as a major regulator of mRNA localization and the spatial organization of protein synthesis. This role enables the regulation of protein function through the addition of post-translational modifications or the formation of protein–protein interactions. We further discuss recent transformative advances in single-cell RNA sequencing and CRISPR–Cas technologies, which enable the mapping and functional characterization of alternative 3′ UTRs in any biological context. Finally, we discuss new APA-based RNA therapeutics, including compounds that target APA in cancer and therapeutic genome editing of degenerative diseases.Alternative cleavage and polyadenylation (APA) generates mRNA isoforms with alternative 3′ untranslated regions; these isoforms modulate protein abundance and functionality, including through subcellular localization of mRNA and translation. APA is modulated by signalling pathways that control co-transcriptional and post-transcriptional processes, and its dysregulation affects cell responses to environmental changes.

Key Points The EJC is deposited 24 nucleotides upstream of spliced junctions during splicing. It accompanies mRNAs from the nucleus to the cytoplasm, where it is removed by the first round of translation, and recycled back into the nucleus. The core of the EJC consists of four proteins. Structural studies revealed that the DEAD-box RNA helicase eIF4A3 functions as a clamp that binds RNA in a sequence-unspecific manner. MAGOH and Y14 form a heterodimer to lock eIF4A3 onto the mRNA, whereas MLN51 contacts eIF4A3 and the mRNA and provides further stability. The core complex acts as a binding platform for peripheral factors involved in splicing, transport, translation and nonsense-mediated decay (NMD). The composition of peripheral factors depends on the different stages of mRNA processing. The EJC has several functions in regulating different post-transcriptional processes, including splicing, cellular localization, translation and NMD. The EJC is not present at every exon junction, and it does not always bind at the canonical position. This differential loading could impact the composition and functions of different EJCs. The EJC acts as a central node of post-transcriptional gene regulation, and changes in EJC protein expression levels lead to several developmental defects and diseases. In addition to its known roles in nonsense-mediated mRNA decay, recent findings show that the exon junction complex (EJC) participates in diverse mRNA maturation processes, including splicing, transport and translation. This multi-functionality is reflected by an increasing number of EJC-related disorders being discovered. The exon junction complex (EJC) is deposited onto mRNAs following splicing and adopts a unique structure, which can both stably bind to mRNAs and function as an anchor for diverse processing factors. Recent findings revealed that in addition to its established roles in nonsense-mediated mRNA decay, the EJC is involved in mRNA splicing, transport and translation. While structural studies have shed light on EJC assembly, transcriptome-wide analyses revealed differential EJC loading at spliced junctions. Thus, the EJC functions as a node of post-transcriptional gene expression networks, the importance of which is being revealed by the discovery of increasing numbers of EJC-related disorders.

Many steps of RNA processing occur during transcription by RNA polymerases. Co-transcriptional activities are deemed commonplace in prokaryotes, in which the lack of membrane barriers allows mixing of all gene expression steps, from transcription to translation. In the past decade, an extraordinary level of coordination between transcription and RNA processing has emerged in eukaryotes. In this Review, we discuss recent developments in our understanding of co-transcriptional gene regulation in both eukaryotes and prokaryotes, comparing methodologies and mechanisms, and highlight striking parallels in how RNA polymerases interact with the machineries that act on nascent RNA. The development of RNA sequencing and imaging techniques that detect transient transcription and RNA processing intermediates has facilitated discoveries of transcription coordination with splicing, 3′-end cleavage and dynamic RNA folding and revealed physical contacts between processing machineries and RNA polymerases. Such studies indicate that intron retention in a given nascent transcript can prevent 3′-end cleavage and cause transcriptional readthrough, which is a hallmark of eukaryotic cellular stress responses. We also discuss how coordination between nascent RNA biogenesis and transcription drives fundamental aspects of gene expression in both prokaryotes and eukaryotes.Methodological advances have enabled discoveries of RNA polymerase interactions with RNA processing machineries, such as the splicing and 3′-end cleavage machineries. This Review discusses the roles of these interactions in gene regulation and eukaryotic cellular stress responses, and highlights parallels between co-transcriptional RNA processing in eukaryotes and prokaryotes.

Nonsense-mediated mRNA decay (NMD) is an mRNA degradation pathway that eliminates transcripts containing premature termination codons (PTCs). Half-lives of the mRNAs containing PTCs demonstrate that a small percent escape surveillance and do not degrade. It is not known whether this escape represents variable mRNA degradation within cells or, alternatively cells within the population are resistant. Here we demonstrate a single-cell approach with a bi-directional reporter, which expresses two β-globin genes with or without a PTC in the same cell, to characterize the efficiency of NMD in individual cells. We found a broad range of NMD efficiency in the population; some cells degraded essentially all of the mRNAs, while others escaped NMD almost completely. Characterization of NMD efficiency together with NMD regulators in single cells showed cell-to-cell variability of NMD reflects the differential level of surveillance factors, SMG1 and phosphorylated UPF1. A single-cell fluorescent reporter system that enabled detection of NMD using flow cytometry revealed that this escape occurred either by translational readthrough at the PTC or by a failure of mRNA degradation after successful translation termination at the PTC. Here the author developed a single-cell reporter system to identify cell-to-cell variability of nonsense-mediated mRNA decay (NMD). This approach provides a sensitive tool to investigate cellular heterogeneity of NMD in various physiological conditions.

Steady-state RNA levels are controlled by the balance between RNA synthesis and RNA turnover. A selective RNA turnover mechanism that has received recent attention in neurons is nonsense-mediated RNA decay (NMD). NMD has been shown to influence neural development, neural stem cell differentiation decisions, axon guidance and synaptic plasticity. In humans, NMD factor gene mutations cause some forms of intellectual disability and are associated with neurodevelopmental disorders, including schizophrenia and autism spectrum disorder. Impairments in NMD are linked to neurodegenerative disorders, including amyotrophic lateral sclerosis. We discuss these findings, their clinical implications and challenges for the future.

Eukaryotic gene expression is constantly controlled by the translation-coupled nonsense-mediated mRNA decay (NMD) pathway. Aberrant translation termination leads to NMD activation, resulting in phosphorylation of the central NMD factor UPF1 and robust clearance of NMD targets via two seemingly independent and redundant mRNA degradation branches. Here, we uncover that the loss of the first SMG5-SMG7-dependent pathway also inactivates the second SMG6-dependent branch, indicating an unexpected functional connection between the final NMD steps. Transcriptome-wide analyses of SMG5-SMG7-depleted cells confirm exhaustive NMD inhibition resulting in massive transcriptomic alterations. Intriguingly, we find that the functionally underestimated SMG5 can substitute the role of SMG7 and individually activate NMD. Furthermore, the presence of either SMG5 or SMG7 is sufficient to support SMG6-mediated endonucleolysis of NMD targets. Our data support an improved model for NMD execution that features two-factor authentication involving UPF1 phosphorylation and SMG5-SMG7 recruitment to access SMG6 activity. Degradation of nonsense mediated mRNA decay (NMD) substrates is carried out by two seemingly independent pathways, SMG6-mediated endonucleolytic cleavage and/or SMG5-SMG7-induced accelerated deadenylation. Here the authors show that SMG5-SMG7 maintain NMD activity by permitting SMG6 activation.

Translation of messenger RNAs lacking a stop codon results in the addition of a carboxy-terminal poly-lysine tract to the nascent polypeptide, causing ribosome stalling. Non-stop proteins and other stalled nascent chains are recognized by the ribosome quality control (RQC) machinery and targeted for proteasomal degradation. Failure of this process leads to neurodegeneration by unknown mechanisms. Here we show that deletion of the E3 ubiquitin ligase Ltn1p in yeast, a key RQC component, causes stalled proteins to form detergent-resistant aggregates and inclusions. Aggregation is dependent on a C-terminal alanine/threonine tail that is added to stalled polypeptides by the RQC component, Rqc2p. Formation of inclusions additionally requires the poly-lysine tract present in non-stop proteins. The aggregates sequester multiple cytosolic chaperones and thereby interfere with general protein quality control pathways. These findings can explain the proteotoxicity of ribosome-stalled polypeptides and demonstrate the essential role of the RQC in maintaining proteostasis. Defects in the ribosome quality control (RQC) complex, which clears proteins that stalled during translation, can cause neurodegeneration; here it is shown that in RQC-defective cells a peptide tail added by the RQC subunit 2 to stalled polypeptides promotes their aggregation and the sequestration of chaperones in these aggregates, affecting normal protein quality control processes. Defective surveillance underlies proteotoxicity Translation of proteins lacking a stop codon is prevented by a surveillance mechanism that utilizes the ribosome quality control (RQC) complex. Failure of quality control can result in proteotoxic stress and neurodegeneration. Ulrich Hartl and colleagues show that in the absence of Ltn1, an RQC subunit with E3 ubiquitin ligase activity, another RQC subunit, Rqc2p, adds a tail to stalled polypeptides and promotes their aggregation. Chaperones become sequestered in these aggregates, which affects normal protein quality control processes. These findings suggest a possible general mechanism underlying aggregate toxicity and proteostasis impairment.

Oxidation and alkylation of nucleobases are known to disrupt their base-pairing properties within RNA. It is, however, unclear whether organisms have evolved general mechanism(s) to deal with this damage. Here we show that the mRNA-surveillance pathway of no-go decay and the associated ribosome-quality control are activated in response to nucleobase alkylation and oxidation. Our findings reveal that these processes are important for clearing chemically modified mRNA and the resulting aberrant-protein products. In the absence of Xrn1, the level of damaged mRNA significantly increases. Furthermore, deletion of LTN1 results in the accumulation of protein aggregates in the presence of oxidizing and alkylating agents. This accumulation is accompanied by Hel2-dependent regulatory ubiquitylation of ribosomal proteins. Collectively, our data highlight the burden of chemically damaged mRNA on cellular homeostasis and suggest that organisms evolved mechanisms to counter their accumulation. The mRNA-surveillance pathway of no-go decay (NGD) is a eukaryotic ribosome-based-quality-control process that targets transcripts that stall the ribosome. Here the authors show no-go decay (NGD) and ribosome-quality control (RQC) pathways are activated by mRNAs damaged by alkylation and oxidation stress.

Cells have evolved so-called mRNA surveillance mechanisms to monitor mRNAs as they are translated and to degrade troublesome transcripts. Studies of mRNA surveillance have traditionally focused on mRNA fate. In this Perspective, the authors explore mRNA surveillance from the viewpoint of its origins on the ribosome, which should lead to new and unanticipated insights that inform future studies. Cells have evolved so-called mRNA surveillance mechanisms to monitor mRNAs as they are translated and to degrade troublesome transcripts. Studies of mRNA surveillance have traditionally focused on mRNA fate. In this Perspective, the authors explore mRNA surveillance from the viewpoint of its origins on the ribosome, which should lead to new and unanticipated insights that inform future studies. There are three predominant forms of co-translational mRNA surveillance: nonsense-mediated decay (NMD), no-go decay (NGD) and nonstop decay (NSD). Although discussion of these pathways often focuses on mRNA fate, there is growing consensus that there are other important outcomes of these processes that must be simultaneously considered. Here, we seek to highlight similarities between NMD, NGD and NSD and their probable origins on the ribosome during translation.