ATP hydrolysis by UPF1 is required for efficient translation termination at premature stop codons

Northern Blotting: Protocols for Radioactive and Nonradioactive Detection of RNA

Northern blotting is a common technique in RNA biology, allowing to detect and quantify RNAs of interest following separation by gel electrophoresis, transfer to a membrane, and hybridization of specific anti-complementary labelled probes. In this chapter, we describe our protocol for efficient RNA extraction from yeast, separation on agarose gel, and capillary transfer to a membrane. We provide two different methods for strand-specific detection of several types of RNAs using oligonucleotide probes, the first using radioactive 32P-labelled probes, the second based on nonradioactive digoxigenin-labelled probes.

Nonsense-Mediated mRNA Decay in Human Health and Diseases: Current Understanding, Regulatory Mechanisms and Future Perspectives

Nonsense-mediated mRNA decay (NMD) is a surveillance mechanism that is conserved across all eukaryotes ensuring the quality of transcripts by targeting messenger RNA (mRNA) harbouring premature stop codons. It regulates the gene expression by targeting aberrant mRNA carrying pre-termination codons (PTCs) and eliminates C-terminal truncated proteins. NMD distinguishes aberrant and non-aberrant transcript by looking after long 3' UTRs and exon-junction complex (EJC) downstream of stop codon that indicate the presence of PTC. Therefore, NMD modulates cellular surveillance and eliminates the truncated proteins but if the PTC escapes the surveillance pathway it can lead to potential negative phenotype resulting in genetic diseases. The alternative splicing also contributes in formation of NMD-sensitive isoforms by introducing PTC. NMD plays a complex role in cancer, it can either aggravate or downregulates the tumour. Some tumours agitate NMD to deteriorate mRNAs encoding tumour suppressor proteins, stress response proteins and neoantigens. In other case, tumours suppress the NMD to encourage the expression of oncoproteins for tumour growth and survival. This mechanism augmented in the development of new therapeutics by PTC read-through mechanism and personalized medicine. Detailed studies on NMD surveillance will possibly lead towards development of strategies for improving human health aligning with United Nations sustainable development goals (SDG 3: Good health and well-being). The potential therapeutic applications of NMD pose a challenge in terms of safe and effective modulation. Understanding the complexities of NMD regulation and its interaction with other cellular processes can lead to the development of new interventions for various diseases.

Structure of the Nmd4-Upf1 complex supports conservation of the nonsense-mediated mRNA decay pathway between yeast and humans

The nonsense-mediated mRNA decay (NMD) pathway clears eukaryotic cells of mRNAs containing premature termination codons (PTCs) or normal stop codons located in specific contexts. It therefore plays an important role in gene expression regulation. The precise molecular mechanism of the NMD pathway has long been considered to differ substantially from yeast to metazoa, despite the involvement of universally conserved factors such as the central ATP-dependent RNA-helicase Upf1. Here, we describe the crystal structure of the yeast Upf1 bound to its recently identified but yet uncharacterized partner Nmd4, show that Nmd4 stimulates Upf1 ATPase activity and that this interaction contributes to the elimination of NMD substrates. We also demonstrate that a region of Nmd4 critical for the interaction with Upf1 in yeast is conserved in the metazoan SMG6 protein, another major NMD factor. We show that this conserved region is involved in the interaction of SMG6 with UPF1 and that mutations in this region affect the levels of endogenous human NMD substrates. Our results support the universal conservation of the NMD mechanism in eukaryotes.

UPF1 ATPase autoinhibition and activation modulate RNA binding kinetics and NMD efficiency

The RNA helicase UPF1 interacts with mRNAs, mRNA decay machinery, and the terminating ribosome to promote nonsense-mediated mRNA decay (NMD). Structural and biochemical data have revealed that UPF1 exists in an enzymatically autoinhibited 'closed' state. Upon binding the NMD protein UPF2, UPF1 undergoes an extensive conformational change into a more enzymatically active 'open' state, which exhibits enhanced ATPase and helicase activity. However, mechanically deficient UPF1 mutants (i.e. poorly processive, slow, and mechanochemically uncoupled) can support efficient NMD, bringing into question the roles of UPF1 enzymatic autoinhibition and activation in NMD. Here, we identify two additional important features of the activated open state: slower RNA binding kinetics and enhanced ATP-stimulated RNA dissociation kinetics. Computational modeling based on empirical measurements of UPF1, UPF2 and RNA interaction kinetics predicts that the majority of UPF1-RNA binding and dissociation events in cells occur independently of UPF2 binding. We find that UPF1 mutants with either reduced or accelerated dissociation from RNA have NMD defects, whereas UPF1 mutants that are more dependent on UPF2 for catalytic activity remain active on well-established NMD targets. These findings support a model in which the kinetics of UPF1-mRNA interactions are important determinants of cellular NMD efficiency.

Pervasive translation of Xrn1-sensitive unstable long noncoding RNAs in yeast

Despite being predicted to lack coding potential, cytoplasmic long noncoding (lnc)RNAs can associate with ribosomes. However, the landscape and biological relevance of lncRNA translation remain poorly studied. In yeast, cytoplasmic Xrn1-sensitive unstable transcripts (XUTs) are targeted by nonsense-mediated mRNA decay (NMD), suggesting a translation-dependent degradation process. Here, we report that XUTs are pervasively translated, which impacts their decay. We show that XUTs globally accumulate upon translation elongation inhibition, but not when initial ribosome loading is impaired. Ribo-seq confirmed ribosomes binding to XUTs and identified ribosome-associated 5'-proximal small ORFs. Mechanistically, the NMD-sensitivity of XUTs mainly depends on the 3'-untranslated region length. Finally, we show that the peptide resulting from the translation of an NMD-sensitive XUT reporter exists in NMD-competent cells. Our work highlights the role of translation in the posttranscriptional metabolism of XUTs. We propose that XUT-derived peptides could be exposed to natural selection, while NMD restricts XUT levels.

Proinflammatory cytokines suppress nonsense-mediated RNA decay to impair regulated transcript isoform processing in pancreatic β cells

Introduction

Proinflammatory cytokines are implicated in pancreatic ß cell failure in type 1 and type 2 diabetes and are known to stimulate alternative RNA splicing and the expression of nonsense-mediated RNA decay (NMD) components. Here, we investigate whether cytokines regulate NMD activity and identify transcript isoforms targeted in ß cells.

Methods

A luciferase-based NMD reporter transiently expressed in rat INS1(832/13), human-derived EndoC-ßH3, or dispersed human islet cells is used to examine the effect of proinflammatory cytokines (Cyt) on NMD activity. The gain- or loss-of-function of two key NMD components, UPF3B and UPF2, is used to reveal the effect of cytokines on cell viability and function. RNA-sequencing and siRNA-mediated silencing are deployed using standard techniques.

Results

Cyt attenuate NMD activity in insulin-producing cell lines and primary human ß cells. These effects are found to involve ER stress and are associated with the downregulation of UPF3B. Increases or decreases in NMD activity achieved by UPF3B overexpression (OE) or UPF2 silencing raise or lower Cyt-induced cell death, respectively, in EndoC-ßH3 cells and are associated with decreased or increased insulin content, respectively. No effects of these manipulations are observed on glucose-stimulated insulin secretion. Transcriptomic analysis reveals that Cyt increases alternative splicing (AS)-induced exon skipping in the transcript isoforms, and this is potentiated by UPF2 silencing. Gene enrichment analysis identifies transcripts regulated by UPF2 silencing whose proteins are localized and/or functional in the extracellular matrix (ECM), including the serine protease inhibitor SERPINA1/α-1-antitrypsin, whose silencing sensitizes ß-cells to Cyt cytotoxicity. Cytokines suppress NMD activity via UPR signaling, potentially serving as a protective response against Cyt-induced NMD component expression.

Conclusion

Our findings highlight the central importance of RNA turnover in ß cell responses to inflammatory stress.

Proinflammatory Cytokines Suppress Nonsense-Mediated RNA Decay to Impair Regulated Transcript Isoform Processing in Pancreatic β-Cells

Proinflammatory cytokines are implicated in pancreatic β-cell failure in type 1 and type 2 diabetes and are known to stimulate alternative RNA splicing and the expression of Nonsense-Mediated RNA Decay (NMD) components. Here, we investigate whether cytokines regulate NMD activity and identify transcript isoforms targeted in β-cells. A luciferase-based NMD reporter transiently expressed in rat INS1(832/13), human-derived EndoC-βH3 or dispersed human islet cells is used to examine the effect of proinflammatory cytokines (Cyt) on NMD activity. Gain- or loss-of function of two key NMD components UPF3B and UPF2 is used to reveal the effect of cytokines on cell viability and function. RNA-sequencing and siRNA-mediated silencing are deployed using standard techniques. Cyt attenuate NMD activity in insulin-producing cell lines and primary human β-cells. These effects are found to involve ER stress and are associated with downregulation of UPF3B. Increases or decreases in NMD activity achieved by UPF3B overexpression (OE) or UPF2 silencing, raises or lowers Cyt-induced cell death, respectively, in EndoC-βH3 cells, and are associated with decreased or increased insulin content, respectively. No effects of these manipulations are observed on glucose-stimulated insulin secretion. Transcriptomic analysis reveals that Cyt increase alternative splicing (AS)-induced exon skipping in the transcript isoforms, and this is potentiated by UPF2 silencing. Gene enrichment analysis identifies transcripts regulated by UPF2 silencing whose proteins are localized and/or functional in extracellular matrix (ECM) including the serine protease inhibitor SERPINA1/α-1-antitrypsin, whose silencing sensitises β-cells to Cyt cytotoxicity. Cytokines suppress NMD activity via UPR signalling, potentially serving as a protective response against Cyt-induced NMD component expression. Our findings highlight the central importance of RNA turnover in β-cell responses to inflammatory stress.

An alternative spliced UPF2 transcript in pancreatic inflammatory myofibroblastic tumors

BACKGROUND

Inflammatory myofibroblastic tumors (IMTs) are characterized by myofibroblast proliferation and an inflammatory cell infiltrate. Our previous study on IMTs reveals that disrupt NMD pathway causes to lower the threshold for triggering the immune cell infiltration, thereby resulting in inappropriate immune activation. However, myofibroblast differentiation and proliferation is not yet known.

METHODS

RT-PCR, RT-qPCR, DNA sequence, western bolt, 5'race analysis and site-specific mutagenesis were used in this study.

RESULTS

Here, an alternative spliced (ALS) UPF2 mRNA skipping exon 2 and 3 and corresponding to the truncated UPF2 protein were found in 2 pancreatic IMTs. We showed that the uORF present in the 5'UTR of UPF2 mRNA is responsible for the translation inhibition, whiles ALS UPF2 is more facilitated to be translated into the truncated UPF2 protein. Several mRNA targets of the NMD were upregulated in IMT samples, indicating that the truncated UPF2 function is strongly perturbed, resulted in disrupted NMD pathway in IMTs. These upregulated NMD targets included cdkn1a expression and the generation of high levels of p21 (waf1/cip1), which may contribute to triggering IMTs.

CONCLUSION

The disrupt UPFs/NMD pathway may link to molecular alteration associated with differentiation and proliferation for IMTs.

UPF1 ATPase autoinhibition and activation modulate RNA binding kinetics and NMD efficiency

The RNA helicase UPF1 interacts with mRNAs, mRNA decay machinery, and the terminating ribosome to promote nonsense-mediated mRNA decay (NMD). Structural and biochemical data have revealed that UPF1 exists in an enzymatically autoinhibited "closed" state. Upon binding the NMD protein UPF2, UPF1 undergoes an extensive conformational change into a more enzymatically active "open" state, which exhibits enhanced ATPase and helicase activity. However, mechanically deficient UPF1 mutants can support efficient NMD, bringing into question the roles of UPF1 enzymatic autoinhibition and activation in NMD. Here, we identify two additional important features of the activated open state: slower nucleic acid binding kinetics and enhanced ATP-stimulated nucleic acid dissociation kinetics. Computational modeling based on empirical measurements of UPF1, UPF2, and RNA interaction kinetics predicts that the majority of UPF1-RNA binding and dissociation events in cells occur independently of UPF2 binding. We find that UPF1 mutants with either reduced or accelerated dissociation from RNA have NMD defects, whereas UPF1 mutants that are more dependent on UPF2 for catalytic activity remain active on well-established NMD targets. These findings support a model in which the kinetics of UPF1-mRNA interactions are important determinants of cellular NMD efficiency.

Eukaryotic mRNA decapping factors: molecular mechanisms and activity

Decapping is the enzymatic removal of 5' cap structures from mRNAs in eukaryotic cells. Cap structures normally enhance mRNA translation and stability, and their excision commits an mRNA to complete 5'-3' exoribonucleolytic digestion and generally ends the physical and functional cellular presence of the mRNA. Decapping plays a pivotal role in eukaryotic cytoplasmic mRNA turnover and is a critical and highly regulated event in multiple 5'-3' mRNA decay pathways, including general 5'-3' decay, nonsense-mediated mRNA decay (NMD), AU-rich element-mediated mRNA decay, microRNA-mediated gene silencing, and targeted transcript-specific mRNA decay. In the yeast Saccharomyces cerevisiae, mRNA decapping is carried out by a single Dcp1-Dcp2 decapping enzyme in concert with the accessory activities of specific regulators commonly known as decapping activators or enhancers. These regulatory proteins include the general decapping activators Edc1, 2, and 3, Dhh1, Scd6, Pat1, and the Lsm1-7 complex, as well as the NMD-specific factors, Upf1, 2, and 3. Here, we focus on in vivo mRNA decapping regulation in yeast. We summarize recently uncovered molecular mechanisms that control selective targeting of the yeast decapping enzyme and discuss new roles for specific decapping activators in controlling decapping enzyme targeting, assembly of target-specific decapping complexes, and the monitoring of mRNA translation. Further, we discuss the kinetic contribution of mRNA decapping for overall decay of different substrate mRNAs and highlight experimental evidence pointing to the functional coordination and physical coupling between events in mRNA deadenylation, decapping, and 5'-3' exoribonucleolytic decay.

Nonsense-Mediated mRNA Decay Factor Functions in Human Health and Disease

Nonsense-mediated mRNA decay (NMD) is a cellular surveillance mechanism that degrades mRNAs with a premature stop codon, avoiding the synthesis of C-terminally truncated proteins. In addition to faulty mRNAs, NMD recognises ~10% of endogenous transcripts in human cells and downregulates their expression. The up-frameshift proteins are core NMD factors and are conserved from yeast to human in structure and function. In mammals, NMD diversified into different pathways that target different mRNAs employing additional NMD factors. Here, we review our current understanding of molecular mechanisms and cellular roles of NMD pathways and the involvement of more specialised NMD factors. We describe the consequences of mutations in NMD factors leading to neurodevelopmental diseases, and the role of NMD in cancer. We highlight strategies of RNA viruses to evade recognition and decay by the NMD machinery.

Ribosome-bound Upf1 forms distinct 80S complexes and conducts mRNA surveillance

Upf1, Upf2, and Upf3, the central regulators of nonsense-mediated mRNA decay (NMD), appear to exercise their NMD functions while bound to elongating ribosomes, and evidence for this conclusion is particularly compelling for Upf1. Hence, we used selective profiling of yeast Upf1:ribosome association to define that step in greater detail, understand whether the nature of the mRNA being translated influences Upf1:80S interaction, and elucidate the functions of ribosome-associated Upf1. Our approach has allowed us to clarify the timing and specificity of Upf1 association with translating ribosomes, obtain evidence for a Upf1 mRNA surveillance function that precedes the activation of NMD, identify a unique ribosome state that generates 37-43 nt ribosome footprints whose accumulation is dependent on Upf1's ATPase activity, and demonstrate that a mutated form of Upf1 can interfere with normal translation termination and ribosome release. In addition, our results strongly support the existence of at least two distinct functional Upf1 complexes in the NMD pathway.

UPF1 mutants with intact ATPase but deficient helicase activities promote efficient nonsense-mediated mRNA decay

The conserved RNA helicase UPF1 coordinates nonsense-mediated mRNA decay (NMD) by engaging with mRNAs, RNA decay machinery and the terminating ribosome. UPF1 ATPase activity is implicated in mRNA target discrimination and completion of decay, but the mechanisms through which UPF1 enzymatic activities such as helicase, translocase, RNP remodeling, and ATPase-stimulated dissociation influence NMD remain poorly defined. Using high-throughput biochemical assays to quantify UPF1 enzymatic activities, we show that UPF1 is only moderately processive (<200 nt) in physiological contexts and undergoes ATPase-stimulated dissociation from RNA. We combine an in silico screen with these assays to identify and characterize known and novel UPF1 mutants with altered helicase, ATPase, and RNA binding properties. We find that UPF1 mutants with substantially impaired processivity (E797R, G619K/A546H), faster (G619K) or slower (K547P, E797R, G619K/A546H) unwinding rates, and/or reduced mechanochemical coupling (i.e. the ability to harness ATP hydrolysis for work; K547P, R549S, G619K, G619K/A546H) can still support efficient NMD of well-characterized targets in human cells. These data are consistent with a central role for UPF1 ATPase activity in driving cycles of RNA binding and dissociation to ensure accurate NMD target selection.

Features and factors that dictate if terminating ribosomes cause or counteract nonsense-mediated mRNA decay

Nonsense-mediated mRNA decay (NMD) is a quality control pathway in eukaryotes that continuously monitors mRNA transcripts to ensure truncated polypeptides are not produced. The expression of many normal mRNAs that encode full-length polypeptides is also regulated by this pathway. Such transcript surveillance by NMD is intimately linked to translation termination. When a ribosome terminates translation at a normal termination codon, NMD is not activated, and mRNA can undergo repeated rounds of translation. On the other hand, when translation termination is deemed abnormal, such as that on a premature termination codon, it leads to a series of poorly understood events involving the NMD pathway, which destabilizes the transcript. In this review, we summarize our current understanding of how the NMD machinery interfaces with the translation termination factors to initiate NMD. We also discuss a variety of cis-acting sequence contexts and trans-acting factors that can cause readthrough, ribosome reinitiation, or ribosome frameshifting at stop codons predicted to induce NMD. These alternative outcomes can lead to the ribosome translating downstream of such stop codons and hence the transcript escaping NMD. NMD escape via these mechanisms can have wide-ranging implications on human health, from being exploited by viruses to hijack host cell systems to being harnessed as potential therapeutic possibilities to treat genetic diseases.

Elucidation of TRIM25 ubiquitination targets involved in diverse cellular and antiviral processes

The tripartite motif (TRIM) family of E3 ubiquitin ligases is well known for its roles in antiviral restriction and innate immunity regulation, in addition to many other cellular pathways. In particular, TRIM25-mediated ubiquitination affects both carcinogenesis and antiviral response. While individual substrates have been identified for TRIM25, it remains unclear how it regulates diverse processes. Here we characterized a mutation, R54P, critical for TRIM25 catalytic activity, which we successfully utilized to "trap" substrates. We demonstrated that TRIM25 targets proteins implicated in stress granule formation (G3BP1/2), nonsense-mediated mRNA decay (UPF1), nucleoside synthesis (NME1), and mRNA translation and stability (PABPC4). The R54P mutation abolishes TRIM25 inhibition of alphaviruses independently of the host interferon response, suggesting that this antiviral effect is a direct consequence of ubiquitination. Consistent with that, we observed diminished antiviral activity upon knockdown of several TRIM25-R54P specific interactors including NME1 and PABPC4. Our findings highlight that multiple substrates mediate the cellular and antiviral activities of TRIM25, illustrating the multi-faceted role of this ubiquitination network in modulating diverse biological processes.

SMG-6 mRNA cleavage stalls ribosomes near premature stop codons in vivo

Nonsense-mediated mRNA decay (NMD) protects cells from the toxic and potentially dominant effects of truncated proteins. Targeting of mRNAs with early stop codons is mediated by the ribosome and spatiotemporally aligned with translation termination. Previously we identified a novel NMD intermediate: ribosomes stalled on cleaved stop codons, raising the possibility that NMD begins even prior to ribosome removal from the stop codon. Here we show that this intermediate is the result of mRNA cleavage by the endonuclease SMG-6. Our work supports a model in which ribosomes stall secondary to SMG-6 mRNA cleavage in Caenorhabditis elegans and humans, i.e. that the novel NMD intermediate occurs after a prior ribosome elicits NMD. Our genetic analysis of C. elegans' SMG-6 supports a central role for SMG-6 in metazoan NMD, and provides a context for evaluating its function in other metazoans.

Translational suppression via IFG-1/eIF4G inhibits stress-induced RNA alternative splicing in Caenorhabditis elegans

Splicing of precursor mRNA is an essential process for dividing cells, and splicing defects have been linked to aging and various chronic diseases. Environmental stress has recently been shown to modify alternative splicing, and molecular mechanisms that influence stress-induced alternative splicing remain unclear. Using an in vivo RNA splicing reporter, we performed a genome-wide RNAi screen in Caenorhabditis elegans and found that protein translation suppression via silencing of the conserved eukaryotic initiation factor 4G (IFG-1/eIF4G) inhibits cadmium-induced alternative splicing. Transcriptome analysis of an ifg-1-deficient mutant revealed an overall decrease in intronic and intergenic reads and prevented cadmium-induced alternative splicing compared to the wild type. We found that the ifg-1 mutant up-regulates >80 RNA splicing regulatory genes controlled by the TGF-β transcription factor SMA-2. The extended lifespan of the ifg-1 mutant is partially reduced upon sma-2 depletion and completely nullified when core spliceosome genes including snr-1, snr-2, and uaf-2 are knocked down. Depletion of snr-1 and snr-2 also diminished the enhanced cadmium resistance of the ifg-1 mutant. Together, these data describe a molecular mechanism through which translation suppression inhibits stress-induced alternative splicing and demonstrate an essential role for RNA splicing in promoting longevity and stress resistance in a translation-compromised mutant.

The broader sense of nonsense

The term 'nonsense-mediated mRNA decay' (NMD) was initially coined to describe the translation-dependent degradation of mRNAs harboring premature termination codons (PTCs), but it is meanwhile known that NMD also targets many canonical mRNAs with numerous biological implications. The molecular mechanisms determining on which RNAs NMD ensues are only partially understood. Considering the broad range of NMD-sensitive RNAs and the variable degrees of their degradation, we highlight here the hallmarks of mammalian NMD and point out open questions. We review the links between NMD and disease by summarizing the role of NMD in cancer, neurodegeneration, and viral infections. Finally, we describe strategies to modulate NMD activity and specificity as potential therapeutic approaches for various diseases.

Unusual SMG suspects recruit degradation enzymes in nonsense-mediated mRNA decay

Degradation of eukaryotic RNAs that contain premature termination codons (PTC) during nonsense-mediated mRNA decay (NMD) is initiated by RNA decapping or endonucleolytic cleavage driven by conserved factors. Models for NMD mechanisms, including recognition of PTCs or the timing and role of protein phosphorylation for RNA degradation are challenged by new results. For example, the depletion of the SMG5/7 heterodimer, thought to activate RNA degradation by decapping, leads to a phenotype showing a defect of endonucleolytic activity of NMD complexes. This phenotype is not correlated to a decreased binding of the endonuclease SMG6 with the core NMD factor UPF1, suggesting that it is the result of an imbalance between active (e.g., in polysomes) and inactive (e.g., in RNA-protein condensates) states of NMD complexes. Such imbalance between multiple complexes is not restricted to NMD and should be taken into account when establishing causal links between gene function perturbation and observed phenotypes.

Nuclear mRNA Quality Control and Cytoplasmic NMD Are Linked by the Guard Proteins Gbp2 and Hrb1

Pre-mRNA splicing is critical for cells, as defects in this process can lead to altered open reading frames and defective proteins, potentially causing neurodegenerative diseases and cancer. Introns are removed in the nucleus and splicing is documented by the addition of exon-junction-complexes (EJCs) at exon-exon boundaries. This “memory” of splicing events is important for the ribosome, which translates the RNAs in the cytoplasm. In case a stop codon was detected before an EJC, translation is blocked and the RNA is eliminated by the nonsense-mediated decay (NMD). In the model organism Saccharomyces cerevisiae, two guard proteins, Gbp2 and Hrb1, have been identified as nuclear quality control factors for splicing. In their absence, intron-containing mRNAs leak into the cytoplasm. Their presence retains transcripts until the process is completed and they release the mRNAs by recruitment of the export factor Mex67. On transcripts that experience splicing problems, these guard proteins recruit the nuclear RNA degradation machinery. Interestingly, they continue their quality control function on exported transcripts. They support NMD by inhibiting translation and recruiting the cytoplasmic degradation factors. In this way, they link the nuclear and cytoplasmic quality control systems. These discoveries are also intriguing for humans, as homologues of these guard proteins are present also in multicellular organisms. Here, we provide an overview of the quality control mechanisms of pre-mRNA splicing, and present Gbp2 and Hrb1, as well as their human counterparts, as important players in these pathways.

From Yeast to Mammals, the Nonsense-Mediated mRNA Decay as a Master Regulator of Long Non-Coding RNAs Functional Trajectory

The Nonsense-Mediated mRNA Decay (NMD) has been classically viewed as a translation-dependent RNA surveillance pathway degrading aberrant mRNAs containing premature stop codons. However, it is now clear that mRNA quality control represents only one face of the multiple functions of NMD. Indeed, NMD also regulates the physiological expression of normal mRNAs, and more surprisingly, of long non-coding (lnc)RNAs. Here, we review the different mechanisms of NMD activation in yeast and mammals, and we discuss the molecular bases of the NMD sensitivity of lncRNAs, considering the functional roles of NMD and of translation in the metabolism of these transcripts. In this regard, we describe several examples of functional micropeptides produced from lncRNAs. We propose that translation and NMD provide potent means to regulate the expression of lncRNAs, which might be critical for the cell to respond to environmental changes.

Ribosome Recycling by ABCE1 Links Lysosomal Function and Iron Homeostasis to 3' UTR-Directed Regulation and Nonsense-Mediated Decay

Nonsense-mediated decay (NMD) is a pathway that degrades mRNAs containing premature termination codons. Here we describe a genome-wide screen for NMD factors that uncovers an unexpected mechanism that broadly governs 3' untranslated region (UTR)-directed regulation. The screen reveals that NMD requires lysosomal acidification, which allows transferrin-mediated iron uptake, which, in turn, is necessary for iron-sulfur (Fe-S) cluster biogenesis. This pathway maintains the activity of the Fe-S cluster-containing ribosome recycling factor ABCE1, whose impaired function results in movement of ribosomes into 3' UTRs, where they displace exon junction complexes, abrogating NMD. Importantly, these effects extend beyond NMD substrates, with ABCE1 activity required to maintain the accessibility of 3' UTRs to diverse regulators, including microRNAs and RNA binding proteins. Because of the sensitivity of the Fe-S cluster of ABCE1 to iron availability and reactive oxygen species, these findings reveal an unanticipated vulnerability of 3' UTR-directed regulation to lysosomal dysfunction, iron deficiency, and oxidative stress.

Nuclear SR-protein mediated mRNA quality control is continued in cytoplasmic nonsense-mediated decay

One important task of eukaryotic cells is to translate only mRNAs that were correctly processed to prevent the production of truncated proteins, found in neurodegenerative diseases and cancer. Nuclear quality control of splicing requires the SR-like proteins Gbp2 and Hrb1 in S. cerevisiae, where they promote the degradation of faulty pre-mRNAs. Here we show that Gbp2 and Hrb1 also function in nonsense mediated decay (NMD) of spliced premature termination codon (PTC)-containing mRNAs. Our data support a model in which they are in a complex with the Upf-proteins and help to transmit the Upf1-mediated PTC recognition to the transcripts ends. Most importantly they appear to promote translation repression of spliced transcripts that contain a PTC and to finally facilitate degradation of the RNA, presumably by supporting the recruitment of the degradation factors. Therefore, they seem to control mRNA quality beyond the nuclear border and may thus be global surveillance factors. Identification of SR-proteins as general cellular surveillance factors in yeast will help to understand the complex human system in which many diseases with defects in SR-proteins or NMD are known, but the proteins were not yet recognized as general RNA surveillance factors.

Readthrough of stop codons under limiting ABCE1 concentration involves frameshifting and inhibits nonsense-mediated mRNA decay

To gain insight into the mechanistic link between translation termination and nonsense-mediated mRNA decay (NMD), we depleted the ribosome recycling factor ABCE1 in human cells, resulting in an upregulation of NMD-sensitive mRNAs. Suppression of NMD on these mRNAs occurs prior to their SMG6-mediated endonucleolytic cleavage. ABCE1 depletion caused ribosome stalling at termination codons (TCs) and increased ribosome occupancy in 3′ UTRs, implying enhanced TC readthrough. ABCE1 knockdown indeed increased the rate of readthrough and continuation of translation in different reading frames, providing a possible explanation for the observed NMD inhibition, since enhanced readthrough displaces NMD activating proteins from the 3′ UTR. Our results indicate that stalling at TCs triggers ribosome collisions and activates ribosome quality control. Collectively, we show that improper translation termination can lead to readthrough of the TC, presumably due to ribosome collisions pushing the stalled ribosomes into the 3′ UTR, where it can resume translation in-frame as well as out-of-frame.

Nonsense-Mediated RNA Decay Factor UPF1 Is Critical for Posttranscriptional and Translational Gene Regulation in Arabidopsis

Nonsense-mediated RNA decay (NMD) is an RNA control mechanism that has also been implicated in the broader regulation of gene expression. Nevertheless, a role for NMD in genome regulation has not yet been fully assessed, partially because NMD inactivation is lethal in many organisms. Here, we performed an in-depth comparative analysis of Arabidopsis (Arabidopsis thaliana) mutants lacking the NMD-related proteins UPF3, UPF1, and SMG7. We found different impacts of these proteins on NMD and the Arabidopsis transcriptome, with UPF1 having the biggest effect. Transcriptome assembly in UPF1-null plants revealed genome-wide changes in alternative splicing, suggesting that UPF1 functions in splicing. The inactivation of UPF1 led to translational repression, as manifested by a global shift in mRNAs from polysomes to monosomes and the downregulation of genes involved in translation and ribosome biogenesis. Despite these global changes, NMD targets and mRNAs expressed at low levels with short half-lives were enriched in the polysomes of upf1 mutants, indicating that UPF1/NMD suppresses the translation of aberrant RNAs. Particularly striking was an increase in the translation of TIR domain-containing, nucleotide binding, leucine-rich repeat (TNL) immune receptors. The regulation of TNLs via UPF1/NMD-mediated mRNA stability and translational derepression offers a dynamic mechanism for the rapid activation of TNLs in response to pathogen attack.

Inhibition of post-termination ribosome recycling at premature termination codons in UPF1 ATPase mutants

Recognition and rapid degradation of mRNA harboring premature translation termination codons (PTCs) serves to protect cells from accumulating non-functional and potentially toxic truncated polypeptides. Targeting of PTC-containing transcripts is mediated by the nonsense-mediated mRNA decay (NMD) pathway and requires a conserved set of proteins including UPF1, an RNA helicase whose ATPase activity is essential for NMD. Previously, we identified a functional interaction between the NMD machinery and terminating ribosomes based on 3’ RNA decay fragments that accrue in UPF1 ATPase mutants. Herein, we show that those decay intermediates originate downstream of the PTC and harbor 80S ribosomes that migrate into the mRNA 3’ UTR independent of canonical translation. Accumulation of 3’ RNA decay fragments is determined by both RNA sequence downstream of the PTC and the inactivating mutation within the active site of UPF1. Our data reveal a failure in post-termination ribosome recycling in UPF1 ATPase mutants.

UPF1-Mediated RNA Decay-Danse Macabre in a Cloud

Nonsense-mediated RNA decay (NMD) is the prototype example of a whole family of RNA decay pathways that unfold around a common central effector protein called UPF1. While NMD in yeast appears to be a linear pathway, NMD in higher eukaryotes is a multifaceted phenomenon with high variability with respect to substrate RNAs, degradation efficiency, effector proteins and decay-triggering RNA features. Despite increasing knowledge of the mechanistic details, it seems ever more difficult to define NMD and to clearly distinguish it from a growing list of other UPF1-mediated RNA decay pathways (UMDs). With a focus on mammalian, we here critically examine the prevailing NMD models and the gaps and inconsistencies in these models. By exploring the minimal requirements for NMD and other UMDs, we try to elucidate whether they are separate and definable pathways, or rather variations of the same phenomenon. Finally, we suggest that the operating principle of the UPF1-mediated decay family could be considered similar to that of a computing cloud providing a flexible infrastructure with rapid elasticity and dynamic access according to specific user needs.

The RNA-binding protein PTBP1 promotes ATPase-dependent dissociation of the RNA helicase UPF1 to protect transcripts from nonsense-mediated mRNA decay

The sequence-specific RNA-binding proteins PTBP1 (polypyrimidine tract-binding protein 1) and HNRNP L (heterogeneous nuclear ribonucleoprotein L) protect mRNAs from nonsense-mediated decay (NMD) by preventing the UPF1 RNA helicase from associating with potential decay targets. Here, by analyzing in vitro helicase activity, dissociation of UPF1 from purified mRNPs, and transcriptome-wide UPF1 RNA binding, we present the mechanistic basis for inhibition of NMD by PTBP1. Unlike mechanisms of RNA stabilization that depend on direct competition for binding sites among protective RNA-binding proteins and decay factors, PTBP1 promotes displacement of UPF1 already bound to potential substrates. Our results show that PTBP1 directly exploits the tendency of UPF1 to release RNA upon ATP binding and hydrolysis. We further find that UPF1 sensitivity to PTBP1 is coordinated by a regulatory loop in domain 1B of UPF1. We propose that the UPF1 regulatory loop and protective proteins control kinetic proofreading of potential NMD substrates, presenting a new model for RNA helicase regulation and target selection in the NMD pathway.

CTELS: A Cell-Free System for the Analysis of Translation Termination Rate

Translation termination is the final step in protein biosynthesis when the synthesized polypeptide is released from the ribosome. Understanding this complex process is important for treatment of many human disorders caused by nonsense mutations in important genes. Here, we present a new method for the analysis of translation termination rate in cell-free systems, CTELS (for C-terminally extended luciferase-based system). This approach was based on a continuously measured luciferase activity during in vitro translation reaction of two reporter mRNA, one of which encodes a C-terminally extended luciferase. This extension occupies a ribosomal polypeptide tunnel and lets the completely synthesized enzyme be active before translation termination occurs, i.e., when it is still on the ribosome. In contrast, luciferase molecule without the extension emits light only after its release. Comparing the translation dynamics of these two reporters allows visualization of a delay corresponding to the translation termination event. We demonstrated applicability of this approach for investigating the effects of cis- and trans-acting components, including small molecule inhibitors and read-through inducing sequences, on the translation termination rate. With CTELS, we systematically assessed negative effects of decreased 3' UTR length, specifically on termination. We also showed that blasticidin S implements its inhibitory effect on eukaryotic translation system, mostly by affecting elongation, and that an excess of eRF1 termination factor (both the wild-type and a non-catalytic AGQ mutant) can interfere with elongation. Analysis of read-through mechanics with CTELS revealed a transient stalling event at a "leaky" stop codon context, which likely defines the basis of nonsense suppression.

Insights into the Effects of Cancer Associated Mutations at the UPF2 and ATP-Binding Sites of NMD Master Regulator: UPF1

Nonsense-mediated mRNA decay (NMD) is a quality control mechanism that recognizes post-transcriptionally abnormal transcripts and mediates their degradation. The master regulator of NMD is UPF1, an enzyme with intrinsic ATPase and helicase activities. The cancer genomic sequencing data has identified frequently mutated residues in the CH-domain and ATP-binding site of UPF1. In silico screening of UPF1 stability change as a function over 41 cancer mutations has identified five variants with significant effects: K164R, R253W, T499M, E637K, and E833K. To explore the effects of these mutations on the associated energy landscape of UPF1, molecular dynamics simulations (MDS) were performed. MDS identified stable H-bonds between residues S152, S203, S205, Q230/R703, and UPF2/AMPPNP, and suggest that phosphorylation of Serine residues may control UPF1-UPF2 binding. Moreover, the alleles K164R and R253W in the CH-domain improved UPF1-UPF2 binding. In addition, E637K and E833K alleles exhibited improved UPF1-AMPPNP binding compared to the T499M variant; the lower binding is predicted from hindrance caused by the side-chain of T499M to the docking of the tri-phosphate moiety (AMPPNP) into the substrate site. The dynamics of wild-type/mutant systems highlights the flexible nature of the ATP-binding region in UPF1. These insights can facilitate the development of drug discovery strategies for manipulating NMD signaling in cell systems using chemical tools.

The ribosomal stalk protein is crucial for the action of the conserved ATPase ABCE1

The ATP-binding cassette (ABC) protein ABCE1 is an essential factor in ribosome recycling during translation. However, the detailed mechanochemistry of its recruitment to the ribosome, ATPase activation and subunit dissociation remain to be elucidated. Here, we show that the ribosomal stalk protein, which is known to participate in the actions of translational GTPase factors, plays an important role in these events. Biochemical and crystal structural data indicate that the conserved hydrophobic amino acid residues at the C-terminus of the archaeal stalk protein aP1 binds to the nucleotide-binding domain 1 (NBD1) of aABCE1, and that this binding is crucial for ATPase activation of aABCE1 on the ribosome. The functional role of the stalk•ABCE1 interaction in ATPase activation and the subunit dissociation is also investigated using mutagenesis in a yeast system. The data demonstrate that the ribosomal stalk protein likely participates in efficient actions of both archaeal and eukaryotic ABCE1 in ribosome recycling. The results also show that the stalk protein has a role in the function of ATPase as well as GTPase factors in translation.

Nonsense-mediated mRNA decay: The challenge of telling right from wrong in a complex transcriptome

The nonsense-mediated mRNA decay pathway selects and degrades its targets using a dense network of RNA-protein and protein-protein interactions. Together, these interactions allow the pathway to collect copious information about the translating mRNA, including translation termination status, splice junction positions, mRNP composition, and 3'UTR length and structure. The core NMD machinery, centered on the RNA helicase UPF1, integrates this information to determine the efficiency of decay. A picture of NMD is emerging in which many factors contribute to the dynamics of decay complex assembly and disassembly, thereby influencing the probability of decay. The ability of the NMD pathway to recognize mRNP features of diverse potential substrates allows it to simultaneously perform quality control and regulatory functions. In vertebrates, increased transcriptome complexity requires balance between these two functions since high NMD efficiency is desirable for maintenance of quality control fidelity but may impair expression of normal mRNAs. NMD has adapted to this challenge by employing mechanisms to enhance identification of certain potential substrates, while using sequence-specific RNA-binding proteins to shield others from detection. These elaborations on the conserved NMD mechanism permit more sensitive post-transcriptional gene regulation but can have severe deleterious consequences, including the failure to degrade pathogenic aberrant mRNAs in many B cell lymphomas. This article is categorized under: RNA Evolution and Genomics > RNA and Ribonucleoprotein Evolution RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms.

The temporal geographically-explicit network of public transport in Changchun City, Northeast China

The vehicle trajectory data is a feasible way for us to understand and reveal urban traffic conditions and human mobility. Therefore, it is extremely valuable to have a fine-grained picture of large-scale vehicle trajectory data, particularly in two different modes, taxis and buses, over the same period at an urban scale. This paper integrates the trajectory data of approximately 7,000 taxis and 1,500 buses in Changchun City, China and accesses the temporal geographically-explicit network of public transport via sequential snapshots of vehicle trajectory data every 30 seconds of the first week in March 2018. In order to reveal urban traffic conditions and human mobility, we construct two-layer urban traffic network (UTN) between these two different transport modes, take crossings as nodes and roads as edges weighted by the volume or average speed of vehicles in each hour. We released this temporal geographically-explicit network of public transport and the dynamics, weighted and directed UTN in simple formats for easy access.

ALS mutations of FUS suppress protein translation and disrupt the regulation of nonsense-mediated decay

Amyotrophic lateral sclerosis (ALS) is an incurable neurodegenerative disease characterized by preferential motor neuron death. Approximately 15% of ALS cases are familial, and mutations in the fused in sarcoma (FUS) gene contribute to a subset of familial ALS cases. FUS is a multifunctional protein participating in many RNA metabolism pathways. ALS-linked mutations cause a liquid-liquid phase separation of FUS protein in vitro, inducing the formation of cytoplasmic granules and inclusions. However, it remains elusive what other proteins are sequestered into the inclusions and how such a process leads to neuronal dysfunction and degeneration. In this study, we developed a protocol to isolate the dynamic mutant FUS-positive cytoplasmic granules. Proteomic identification of the protein composition and subsequent pathway analysis led us to hypothesize that mutant FUS can interfere with protein translation. We demonstrated that the ALS mutations in FUS indeed suppressed protein translation in N2a cells expressing mutant FUS and fibroblast cells derived from FUS ALS cases. In addition, the nonsense-mediated decay (NMD) pathway, which is closely related to protein translation, was altered by mutant FUS. Specifically, NMD-promoting factors UPF1 and UPF3b increased, whereas a negative NMD regulator, UPF3a, decreased, leading to the disruption of NMD autoregulation and the hyperactivation of NMD. Alterations in NMD factors and elevated activity were also observed in the fibroblast cells of FUS ALS cases. We conclude that mutant FUS suppresses protein biosynthesis and disrupts NMD regulation, both of which likely contribute to motor neuron death.

Nonsense-mediated mRNA decay involves two distinct Upf1-bound complexes

Nonsense-mediated mRNA decay (NMD) is a translation-dependent RNA degradation pathway involved in many cellular pathways and crucial for telomere maintenance and embryo development. Core NMD factors Upf1, Upf2 and Upf3 are conserved from yeast to mammals, but a universal NMD model is lacking. We used affinity purification coupled with mass spectrometry and an improved data analysis protocol to characterize the composition and dynamics of yeast NMD complexes in yeast (112 experiments). Unexpectedly, we identified two distinct complexes associated with Upf1: Upf1-23 (Upf1, Upf2, Upf3) and Upf1-decapping Upf1-decapping contained the mRNA decapping enzyme, together with Nmd4 and Ebs1, two proteins that globally affected NMD and were critical for RNA degradation mediated by the Upf1 C-terminal helicase region. The fact that Nmd4 association with RNA was partially dependent on Upf1-23 components and the similarity between Nmd4/Ebs1 and mammalian Smg5-7 proteins suggest that NMD operates through conserved, successive Upf1-23 and Upf1-decapping complexes. This model can be extended to accommodate steps that are missing in yeast, to serve for further mechanistic studies of NMD in eukaryotes.

Translation Termination and Ribosome Recycling in Eukaryotes

Termination of mRNA translation occurs when a stop codon enters the A site of the ribosome, and in eukaryotes is mediated by release factors eRF1 and eRF3, which form a ternary eRF1/eRF3-guanosine triphosphate (GTP) complex. eRF1 recognizes the stop codon, and after hydrolysis of GTP by eRF3, mediates release of the nascent peptide. The post-termination complex is then disassembled, enabling its constituents to participate in further rounds of translation. Ribosome recycling involves splitting of the 80S ribosome by the ATP-binding cassette protein ABCE1 to release the 60S subunit. Subsequent dissociation of deacylated transfer RNA (tRNA) and messenger RNA (mRNA) from the 40S subunit may be mediated by initiation factors (priming the 40S subunit for initiation), by ligatin (eIF2D) or by density-regulated protein (DENR) and multiple copies in T-cell lymphoma-1 (MCT1). These events may be subverted by suppression of termination (yielding carboxy-terminally extended read-through polypeptides) or by interruption of recycling, leading to reinitiation of translation near the stop codon.

UPF1-like helicase grip on nucleic acids dictates processivity

Helicases are molecular engines which translocate along nucleic acids (NA) to unwind double-strands or remodel NA–protein complexes. While they have an essential role in genome structure and expression, the rules dictating their processivity remain elusive. Here, we developed single-molecule methods to investigate helicase binding lifetime on DNA. We found that UPF1, a highly processive helicase central to nonsense-mediated mRNA decay (NMD), tightly holds onto NA, allowing long lasting action. Conversely, the structurally similar IGHMBP2 helicase has a short residence time. UPF1 mutants with variable grip on DNA show that grip tightness dictates helicase residence time and processivity. In addition, we discovered via functional studies that a decrease in UPF1 grip impairs NMD efficiency in vivo. Finally, we propose a three-state model with bound, sliding and unbound molecular clips, that can accurately predict the modulation of helicase processivity.

UPF1 is a highly processive helicase that plays an essential role in nonsense-mediated mRNA decay. Here the authors use single molecule binding assays to establish a functionally important relationship between helicase grip to nucleic acids, binding lifetime and the duration of translocation.

Uncovering the Role of RNA-Binding Proteins in Gene Expression in the Immune System

Fighting external pathogens requires an ever-changing immune system that relies on tight regulation of gene expression. Transcriptional control is the first step to build efficient responses while preventing immunodeficiencies and autoimmunity. Post-transcriptional regulation of RNA editing, location, stability, and translation are the other key steps for final gene expression, and they are all controlled by RNA-binding proteins (RBPs). Nowadays we have a deep understanding of how transcription factors control the immune system but recent evidences suggest that post-transcriptional regulation by RBPs is equally important for both development and activation of immune responses. Here, we review current knowledge about how post-transcriptional control by RBPs shapes our immune system and discuss the perspective of RBPs being the key players of a hidden immune cell epitranscriptome.

New insights into the interplay between the translation machinery and nonsense-mediated mRNA decay factors

Faulty mRNAs with a premature stop codon (PTC) are recognized and degraded by nonsense-mediated mRNA decay (NMD). Recognition of a nonsense mRNA depends on translation and on the presence of NMD-enhancing or the absence of NMD-inhibiting factors in the 3'-untranslated region. Our review summarizes our current understanding of the molecular function of the conserved NMD factors UPF3B and UPF1, and of the anti-NMD factor Poly(A)-binding protein, and their interactions with ribosomes translating PTC-containing mRNAs. Our recent discovery that UPF3B interferes with human translation termination and enhances ribosome dissociation in vitro, whereas UPF1 is inactive in these assays, suggests a re-interpretation of previous experiments and modification of prevalent NMD models. Moreover, we discuss recent work suggesting new functions of the key NMD factor UPF1 in ribosome recycling, inhibition of translation re-initiation and nascent chain ubiquitylation. These new findings suggest that the interplay of UPF proteins with the translation machinery is more intricate than previously appreciated, and that this interplay quality-controls the efficiency of termination, ribosome recycling and translation re-initiation.

Nonsense-mediated mRNA decay factors cure most [PSI+] prion variants

The yeast prion [PSI+] is a self-propagating amyloid of Sup35p with a folded in-register parallel β-sheet architecture. In a genetic screen for antiprion genes, using the yeast knockout collection, UPF1/NAM7 and UPF3, encoding nonsense-mediated mRNA decay (NMD) factors, were frequently detected. Almost all [PSI+] variants arising in the absence of Upf proteins were eliminated by restored normal levels of these proteins, and [PSI+] arises more frequently in upf mutants. Upf1p, complexed with Upf2p and Upf3p, is a multifunctional protein with helicase, ATP-binding, and RNA-binding activities promoting efficient translation termination and degradation of mRNAs with premature nonsense codons. We find that the curing ability of Upf proteins is uncorrelated with these previously reported functions but does depend on their interaction with Sup35p and formation of the Upf1p-Upf2p-Upf3p complex (i.e., the Upf complex). Indeed, Sup35p amyloid formation in vitro is inhibited by substoichiometric Upf1p. Inhibition of [PSI+] prion generation and propagation by Upf proteins may be due to the monomeric Upf proteins and the Upf complex competing with Sup35p amyloid fibers for available Sup35p monomers. Alternatively, the association of the Upf complex with amyloid filaments may block the addition of new monomers. Our results suggest that maintenance of normal protein-protein interactions prevents prion formation and can even reverse the process.

HTLV-1 Tax plugs and freezes UPF1 helicase leading to nonsense-mediated mRNA decay inhibition

Up-Frameshift Suppressor 1 Homolog (UPF1) is a key factor for nonsense-mediated mRNA decay (NMD), a cellular process that can actively degrade mRNAs. Here, we study NMD inhibition during infection by human T-cell lymphotropic virus type I (HTLV-1) and characterise the influence of the retroviral Tax factor on UPF1 activity. Tax interacts with the central helicase core domain of UPF1 and might plug the RNA channel of UPF1, reducing its affinity for nucleic acids. Furthermore, using a single-molecule approach, we show that the sequential interaction of Tax with a RNA-bound UPF1 freezes UPF1: this latter is less sensitive to the presence of ATP and shows translocation defects, highlighting the importance of this feature for NMD. These mechanistic insights reveal how HTLV-1 hijacks the central component of NMD to ensure expression of its own genome.

UPF1 is a central protein in nonsense-mediated mRNA decay (NMD), but contribution of its RNA processivity to NMD is unclear. Here, the authors show how the retroviral Tax protein interacts with and inhibits UPF1, and demonstrate that UPF1’s translocase activity contributes to NMD.

Upf proteins: highly conserved factors involved in nonsense mRNA mediated decay

Over 10% of genetic diseases are caused by mutations that introduce a premature termination codon in protein-coding mRNA. Nonsense-mediated mRNA decay (NMD) is an essential cellular pathway that degrades these mRNAs to prevent the accumulation of harmful partial protein products. NMD machinery is also increasingly appreciated to play a role in other essential cellular functions, including telomere homeostasis and the regulation of normal mRNA turnover, and is misregulated in numerous cancers. Hence, understanding and designing therapeutics targeting NMD is an important goal in biomedical science. The central regulator of NMD, the Upf1 protein, interacts with translation termination factors and contextual factors to initiate NMD specifically on mRNAs containing PTCs. The molecular details of how these contextual factors affect Upf1 function remain poorly understood. Here, we review plausible models for the NMD pathway and the evidence for the variety of roles NMD machinery may play in different cellular processes.

An RNA decay factor wears a new coat: UPF3B modulates translation termination

Nonsense-mediated RNA decay (NMD) is a highly conserved and selective RNA turnover pathway that has been subject to intense scrutiny. NMD identifies and degrades subsets of normal RNAs, as well as abnormal mRNAs containing premature termination codons. A core factor in this pathway—UPF3B—is an adaptor protein that serves as an NMD amplifier and an NMD branch-specific factor. UPF3B is encoded by an X-linked gene that when mutated causes intellectual disability and is associated with neurodevelopmental disorders, including schizophrenia and autism. Neu-Yilik et al. now report a new function for UPF3B: it modulates translation termination. Using a fully reconstituted in vitro translation system, they find that UPF3B has two roles in translation termination. First, UPF3B delays translation termination under conditions that mimic premature translation termination. This could drive more efficient RNA decay by allowing more time for the formation of RNA decay-stimulating complexes. Second, UPF3B promotes the dissociation of post-termination ribosomal complexes that lack nascent peptide. This implies that UPF3B could promote ribosome recycling. Importantly, the authors found that UPF3B directly interacts with both RNA and the factors that recognize stop codons—eukaryotic release factors (eRFs)—suggesting that UPF3B serves as a direct regulator of translation termination. In contrast, a NMD factor previously thought to have a central regulatory role in translation termination—the RNA helicase UPF1—was found to indirectly interact with eRFs and appears to act exclusively in post-translation termination events, such as RNA decay, at least in vitro. The finding that an RNA decay-promoting factor, UFP3B, modulates translation termination has many implications. For example, the ability of UPF3B to influence the development and function of the central nervous system may be not only through its ability to degrade specific RNAs but also through its impact on translation termination and subsequent events, such as ribosome recycling.

Dual function of UPF3B in early and late translation termination

Nonsense-mediated mRNA decay (NMD) is a cellular surveillance pathway that recognizes and degrades mRNAs with premature termination codons (PTCs). The mechanisms underlying translation termination are key to the understanding of RNA surveillance mechanisms such as NMD and crucial for the development of therapeutic strategies for NMD-related diseases. Here, we have used a fully reconstituted in vitro translation system to probe the NMD proteins for interaction with the termination apparatus. We discovered that UPF3B (i) interacts with the release factors, (ii) delays translation termination and (iii) dissociates post-termination ribosomal complexes that are devoid of the nascent peptide. Furthermore, we identified UPF1 and ribosomes as new interaction partners of UPF3B. These previously unknown functions of UPF3B during the early and late phases of translation termination suggest that UPF3B is involved in the crosstalk between the NMD machinery and the PTC-bound ribosome, a central mechanistic step of RNA surveillance.